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Application of microwave energy in the control of DPM, oxides of nitrogen and VOC emissions

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APPLICATION OF MICROWAVE ENERGY IN THE CONTROL OF DPM, NOx AND
VOC EMISSIONS
A Dissertation
Presented to
The Faculty of the College of Graduate Studies
Lamar University
In Partial Fulfillment
of the Requirements for the Degree
Doctorate of Philosophy
by
Sameer M. Pallavkar
August 2011
UMI Number: 3515423
All rights reserved
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APPLICATION OF MICROWAVE ENERGY IN THE CONTROL OF DPM, NOx AND
VOC EMISSIONS
SAMEER M. PALLAVKAR
Approved:
Tomas C. Ho
Supervising Professor
Tae Hoon Kim
Committee Member
Rafael Tadmc u
Committee W
Che-J^n^in
C^fmttee Member
$
(2jL
Paul Chiou
Committee Member
Thomas C. Ho
Chair, Department of Chemical Engineering
Jack BL. Hopper
Deffll, College of Engineering
Victor A. Zaloor
Interim Dean, College of Graduate Studies
© 2011 by Sameer M. Pallavkar
No part of this dissertation can be reproduced without permission except as indicated by
the "Fair Use" clause of the copyright law. Passages, images, or ideas taken from this
work must be properly credited in any written or published materials.
ABSTRACT
APPLICATION OF MICROWAVE ENERGY IN THE CONTROL OF DPM, NOx AND
VOC EMISSIONS
Sameer M. Pallavkar
The emissions of DPM (diesel particulate matter), NOx (oxides of nitrogen), and
toxic VOCs (volatile organic compounds) from diesel engine exhaust gases and other
sources such as chemical process industry and manufacturing industry have been a great
environmental and health concern. Most control technologies for these emissions require
elevated temperatures. The use of microwave energy as a source of heat energy, however,
has not been fully explored.
In this study, the microwave energy was used as the energy source in three separate
emission control processes, namely, the regeneration of diesel particulate filter (DPF) for
DPM control, the NOx reduction using a platinum catalyst, and the VOC destruction
involving a ceramic based material. The study has demonstrated that microwave heating
is an effective method in providing heat for the studied processes. The control
efficiencies associated with the microwave-assisted processes have been observed to be
high and acceptable. Further research, however, is required for the commercial use of
these technologies.
DEDICATION
I take this opportunity filled with deep sense of joy and pure happiness to dedicate this
dissertation to my beloved mother Mrs. Kamlesh Patil Pallavkar, my ever loving father
Mr. MuzafFar S. Pallavkar, and my adorable sweet sister Ms. Seema M. Pallavkar, whose
words of encouragement, support, and patience acted as the beacon of guidance and
helped me keep my composure, and steadfastness through troubling times.
In the same breath, it's a honor of high esteem, that I get to dedicate my dissertation to
my advisor, Dr Thomas C. Ho and my co-advisor Dr. Tae-Hoon Kim, whose
commitment in my project and in me as a person, the much needed guidance at every step,
and the training that was imparted to me during the entire period of my Ph.D. candidacy
has strengthened me as a professional and as a person of high caliber.
iii
ACKNOWLEDGEMENTS
The author wishes to express his deep sense of gratitude and appreciation to his
advisor, Dr. Thomas C. Ho and co-advisor, Dr. Tae-Hoon Kim, for their guidance,
training and sense of professionalism, which has always motivated the author to be a
better professional. Furthermore, the author appreciates the support shown by Dr. CheJen Lin, and for the critical thinking imparted by him during the course of this
dissertation. The author is also thankful to Dr. Rafael Tadmor and Dr. Paul Chiou for
their valuable inputs and for serving as members of his dissertation committee.
The author would like to thank Dr. John L. Gossage, Dr. Carl L. Yaws, and Dr. John
Zhanhu Guo for their valuable suggestions, which were incorporated in this dissertation.
The author would like to specially thank Dr. Daniel H. Chen, Dr. Tracy J. Benson and Mr.
Randall Bramston-Cook from Lotus Consulting for their valuable inputs, suggestions,
and much needed help which directly resulted in successfully trouble shooting various
design and operational issues related to the GC/TCD and GC/MS. The author is indebted
by the extended assistance, guidance, and help imparted by Mrs. Dewanna Campbell, the
Administrative Associate of the Department of Chemical Engineering. Much
acknowledgement is also to be given to Dr. Andrew Gomes and Mr. Dan Rutman of the
Materials Instrumentation Center, and Mr. Mathew Hall, the System Administrator with
the Department of Chemical Engineering for their prompt help at crucial times.
iv
On a personal note, the author wishes to acknowledge the support, and everlasting
company of some of his friends, Dr. Kayzad Vajifdar, Mr. Ashwin Juneja, Mr. Suraj
Shetty, Mr. Sudhir Kutty, Mr. Manpreet Singh Johal, Dr. Prashant Bahadur, Mr. Vaibhav
Desai, and Mr. Kunal Vighne who always were there with the author as friends,
companions and fellow colleagues.
The author is grateful for the financial support of this study from U.S. EPA through
the University of Houston (EPA Project Number X833306). The author would also like
to acknowledge the support form the National Science Foundation for a 2004 MRI
(Major Research Instrumentation) award (NSF Award No. 0320818) for a GC/MS system
for the project. The support from TCEQ (Texas Commission on Environmental Quality)
for the microwave experimental facilities through a 2005 NTRD (New Technology and
Research Development) project is also acknowledged.
Last but definitely not the least, I would like to acknowledge Dr. Randy Pausch who's
famous lecture titled "Achieving Your Childhood Dreams" was instrumental in giving
me a second perspective about things that are around us.
v
TABLE OF CONTENTS
List of Tables
xiii
List of Figures
xiv
Chapter
1. Introduction
01
1.1
Diesel Engine Emissions
01
1.2
VOC Emissions
03
1.3
Issues Associated with Current DPM Control Technologies
03
1.4
Issues Associated with Current NOx Control Technologies
04
1.4.1
Engine Design Modification
04
1.4.2
Exhaust Gas After-Treatment
05
1.5
Issues Associated with Current VOC Control Technologies
06
1.6
Microwave Heating Technology
07
1.7
Objectives
08
1.8
1.7.1
Microwave-Assisted DPF Regeneration
08
1.7.2
Microwave-Assisted NOx Abatement
09
1.7.3
Microwave-Assisted VOC Destruction
09
Organization of the Dissertation
10
References
11
2. Literature Survey
15
2.1
The Diesel Engine
15
vi
2.2
Soot Control
18
2.2.1
Active Regeneration Methods
19
2.2.1.1
Additional Fuel Burning
19
2.2.1.2
Electric Heating
20
2.2.1.3
Microwave Heating
20
2.2.2
2.3
Passive Regeneration Methods
NOx Control
22
2.3.1
Thermal NOx
23
2.3.2
Prompt NOx
24
2.3.3
N2O Pathway
25
2.3.4
Fuel Nitrogen
25
2.3.5
Engine Design
27
2.3.5.1
Charge Air System Modifications
27
2.3.5.2
Exhaust Gas Recirculation
29
2.3.6
Exhaust Gas After-Treatment
2.3.6.1
2.3.6.2
2.3.6.3
31
Hydrocarbon based Selective Catalytic
Reduction
36
NOx Storage and Reduction Catalyst
41
VOC Destruction
2.4.1
29
Ammonia based Selective Catalytic
Reduction
2.4
22
44
VOC Emission Control
vii
44
2.5
Microwave Heating Technology
46
2.5.1
Microwave Radiation
46
2.5.2
Classification of Materials
47
2.5.3
Applications of Microwave Technology
48
2.5.4
Advantages of Microwave Heating
51
2.5.5
Safety Considerations
52
References
53
3. Active Regeneration Of Diesel Particulate Filter Loaded With DPM
Employing Microwave Heating
62
3.1
Introduction
63
3.2
Objective
66
3.3
Experimental Details
67
3.3.1
Diesel Generator and Exhaust Flow System
68
3.3.2
Diesel Particulate Filter and Microwave
69
3.4
DPF Temperature Measurement
72
3.5
DPM Sampling
73
3.6
DPF Regeneration Experiments
73
3.7
Results and Discussion
74
3.7.1
DPF Temperature Profile
75
3.7.2
DPF Particulate Loading
77
3.7.3
Off-Line DPF Regeneration
84
3.7.4
On-Line DPF Regeneration
89
viii
3.8
Conclusions
91
Nomenclature
93
References
94
4. Characterization Of Microwave-Assisted De-NOx Catalytic
Reactions With Hydrogen and Hydrocarbons Serving As
97
The Reducing Agent
4.1
Introduction
98
4.2
Objective
101
4.3
Microwave Heating Technology
101
4.3.1
Dipolar Polarization
4.3.2
Conversion of Microwave Energy to Heat
Energy
4.4
4.5
Experimental
102
103
104
4.4.1
Experimental Set-Up
104
4.4.2
Microwave Application System
106
4.4.3
Design of Packed Bed Reactor
108
4.4.4
Gas Measurement System
110
4.4.5
Experimental Procedure
Ill
4.4.6
Establishment of Experimental Parameters
Ill
4.4.7
Application of Microwave Energy
112
4.4.8
Measurement of Product Concentrations
112
Results and Discussion
ix
113
4.6
4.5.1
General Observations
113
4.5.2
Effect of Reactor Design
116
4.5.3
Effect of Reducing Agent
117
4.5.4
Formation of Reaction By-Products
119
Conclusions
121
Nomenclature
122
References
123
5. Microwave-Assisted Noncatalytic Destruction Of Volatile Organic
Compounds Using Ceramic-Based Microwave Absorbing Media
127
5.1
Introduction
128
5.2
Objective
130
5.3
Microwave Heating Technology
131
5.3.1
5.4
Experimental Setup
5.4.1
5.5
Conversion of Microwave Energy
Experimental Procedure
Results and Discussions
5.5.1
133
136
136
Dynamic Temperature Profiles during
Microwave Heating
5.5.3
132
Net Spent Microwave Power and Stead State
Temperature
5.5.2
132
138
Destruction and Removal Efficiency
ofVOC
x
142
5.5.4
Destruction Byproducts with Nitrogen
being the Carrier Gas
5.6
Conclusions
145
145
Nomenclature
149
References
150
6. Conclusions And Recommendations
153
6.1
Conclusions
6.1.1
153
Conclusions from Microwave-Assisted
DPF Regeneration
6.1.2
Conclusions from Microwave-Assisted
Catalytic NOx Abatement
6.1.3
Recommendations
6.2.1
Recommendations for Microwave-Assisted
156
Recommendations for Microwave-Assisted
Catalytic NOx Abatement
6.2.3
155
156
DPF Regeneration
6.2.2
154
Conclusions from Microwave-Assisted
Non-Catalytic VOC Destruction
6.2
153
156
Recommendations for Microwave-Assisted NonCatalytic VOC Destruction
xi
157
Appendix A. Calibration Charts For The Brooks Mass Flow Controller
For Individual Gas Components And Gas Standard
158
Appendix B. Sample Flow Sequence, Pressure Equilibration And
Injection For The GC/TCD
xii
161
LIST OF TABLES
Table number
Table Name
Page
Table 2.1
Diesel Engine Exhaust Gas Composition
Table 2.2
Dielectric Properties of Some Common Materials used in Soot
Control
Table 2.3
21
Summary of Various Catalysts used in Hydrocarbon Assisted
SCR
Table 2.4
18
37
Effect of Various Hydrocarbons on NOx Reduction (Catalyst Ag/Al203)
38
Table 2.5
Effect of H2 on Light-off Temperature
38
Table 3.1
DPF Filter Specifications
69
Table 3.2
Constants used for Equations 1 and 2 Calculations
81
Table 5
Steady State SiC Temperatures under Different Experimental
Conditions
140
xiii
LIST OF FIGURES
Figure number
Figure 2.1
Figure Name
Page
Schematic of combustion cylinder with
major components
16
Figure 2.2
Schematic of four stroke-cycle
17
Figure 2.3
Block diagram of charge air system
28
Figure 2.4
Charge air system with inter-cooling
28
Figure 2.5
Trade off between hydrocarbon and NOx level with
respect to the extent of EGR
Figure 2.6
30
Trade off between PM and NOx level with respect to
the extent of EGR
30
Figure 2.7
Schematics of basic ammonia based SCR
32
Figure 2.8
Schematics of improved ammonia based SCR with
preoxidation catalyst
Figure 2.9
Effect of NH3/NOX ratio on NOx conversion at
various temperatures
Figure 2.10
35
Schematic representation of a passive lean NOx
catalytic system
Figure 2.12
34
Effect of NH3/NOX ratio on NH3 slippage at
various temperatures
Figure 2.11
33
40
Schematic representation of an active lean NOx
catalytic system
xiv
40
Figure 2.13
NOx storage and reduction mechanism for NOx
adsorber
42
Figure 2.14
Electromagnetic radiation
47
Figure 3.1
Schematic diagram of the diesel test facilities
67
Figure 3.2
Assembled microwave diesel emission test unit
68
Figure 3.3
Schematic diagram of the quartz filter holder
70
Figure 3.4
Installed waveguide in the microwave oven
71
Figure 3.5
Schematic diagram of the waveguide assembly
72
Figure 3.6
DPF temperature profiles in different radial directions
(without a waveguide)
Figure 3.7
75
DPF temperature profiles in different radial directions
(with a waveguide)
Figure 3.8
76
DPF temperature profiles in different vertical locations
I
(with a waveguide)
Figure 3.9
DPF pressure drop vs. time in three consecutive stages
(engine exhaust flow rate: 83.3 L/min)
Figure 3.10
77
78
Wall layer permeability during DPM loading at four
filtration/regeneration cycles corresponding to the
differential pressure drop results shown in Figure 3.11
(exhaust flow rate: 83.3 L/min)
Figure 3.11
80
Plot of CO concentration/temperature/pressure drop
vs. time for an off-line four-cycle filtration/regeneration
operation (exhaust flow: 83.3 L/min)
xv
81
Figure 3.12
Wall porosity during DPM loading at four
filtration/regeneration cycles corresponding to the
differential pressure drop results shown in Figure
3.9 (exhaust flow: 83.3 L/min)
Figure 3.13
82
Plot of concentration of C^/NOx/CO temperature drop
vs. time for an off-line single-cycle filtration/
regeneration operation (exhaust flow: 83.3 L/min)
Figure 3.14
Plot of concentration of (VCO/NOx vs. time for an
off-line four-cycle filtration/regeneration
operation
(exhaust flow: 83.3 L/min)
Figure 3.15
85
86
Plot of concentration of C^/NOx/CO/
temperature/pressure drop vs. time for one
regeneration cycle of an off-line multicycle
filtration/regeneration operation with 5-min
microwave heating (exhaust flow: 83.3 L/min)
Figure 3.16
88
Plot of concentration of O2/NOX/CO/
temperature/pressure drop vs time for one
regeneration cycle of an off-line multicycle
filtration/regeneration operation with 10-min
microwave heating (exhaust flow: 83.3 L/min)
Figure 3.17
89
Plot of concentration of C^/NOx/CO/temperature/
pressure drop vs. time for an on-line regeneration
experiment (exhaust flow: 16.7 L/min)
xvi
90
Figure 4.1
Schematic view of the NOx abatement experimental
set-up
Figure 4.2
105
The block diagram view of the microwave
heating unit
Figure 4.3
106
Holes drilled through the applicator for infrared
temperature measurements
Figure 4.4
The De-NOx catalyst packs used in the NOx
abatement tests
Figure 4.5
108
109
Low power NOx abatement test. De-NOx
catalyst pack: PT-9. Feed gas flowrate: 200 cc/min.
Feed gas composition: NO ~ 1000 ppm,
H2 ~ 3000 ppm, balance N2. Microwave power
absorbed: 70 W
Figure 4.6
114
Low power NOx abatement test in the
presence of O2. De-NOx catalyst pack: PT-9.
Feed gas flowrate: 200 cc/min. Feed gas composition:
NO ~ 702 ppm, N02 ~ 51, H2 - 7000 ppm, 02 - 5 %,
balance N2. Microwave power absorbed: 80 W
Figure 4.7
115
Low power NOx abatement test in the absence of O2.
De-NOx catalyst pack: PT-0, Feed gas flowrate: 200
cc/min. Feed gas composition: NO ~ 1000 ppm,
H2 ~ 3000 ppm, balance N2 Test A: Microwave
power absorbed ~ 110 W, Temperature ~ 467 °C
xvu
116
Figure 4.8
Low power NOx abatement test in the presence of O2.
De-NOx catalyst pack: PT-P. Feed gas flowrate: 200
cc/min. Feed gas composition: NO ~ 559 ppm,
NO2 -213 ppm, H2 ~ 7000 ppm, O2 ~ 5 %,
balance N2. Microwave power absorbed: 70 W
Figure 4.9
118
NOx abatement test in the presence of O2
using n-C6H}4 as the reducing agent. De-NOx
catalyst pack: PT-P. Feed gas flowrate: 200 cc/min.
Feed gas composition: NO ~ 372 ppm, NO2 ~ 140 ppm,
n-CeHi4 ~ 10000 ppm, O2 ~ 5 %, balance N2
Microwave power absorbed: 60 W
119
Figure 5.1
Schematic diagram of the experimental set up
133
Figure 5.2
Block diagram and the corresponding setup of the
microwave application system
Figure 5.3
134
Plot of forward power, net power, and the
corresponding steady state temperature of SiC
under different gas flow rates (carrier gas, nitrogen)
Figure 5.4
137
Temperature profiles of SiC during experiments with
different levels of microwave irradiation (carrier gas,
nitrogen; carrier gas flow-rate, 500 mL/min)
Figure 5.5
138
Distribution of net microwave power during the
heating up of SiC (carrier gas, nitrogen;
gas flow rate, 500 mL/min)
xviii
141
Figure 5.6
DRE for p-xylene destruction (carrier gas,
nitrogen; gas flow rate, 200 mL/min)
Figure 5.7
DRE for p-xylene destruction (carrier gas,
nitrogen; gas flow rate, 350 mL/min)
Figure 5.8
144
DRE for p-xylene destruction (carrier gas,
nitrogen; gas flow rate, 500 mL/min)
Figure 5.9
143
144
GC/MS mass spectrum data for p-xylene destruction
(carrier gas, nitrogen; gas flow rate, 200 mL/min; net
power level, (A) 318 W, (B) 380 W, (C) 458 W;
temperature, (A) 971 °C, (B) 1067 °C,
(C) 1127 °C)
Figure 5.10
147
10 GC/MS mass spectrum data for p-xylene
destruction (carrier gas, nitrogen; gas flow rate, 500
mL/min; net power level, 374 W;
temperature, 995 °C)
Figure A.l
148
Air flow calibration using two different Brooks Mass
Flow Controller, 5850-E series Air Tank Pressure 60 psig; Series-A: De-NOx experiments; Series-B: VOC
Destruction experiments
Figure A.2
158
Make-up N2 flow calibration using Brooks Mass
Flow Controller, 5850-E series, N2 Tank Pressure 40 psig
159
xix
Figure A.3
NOx in N2 standard flow calibration using Brooks
Mass Flow Controller, 5850-E series, Tank Pressure 75 psig
Figure B.l
160
Sample injection sequence, A: Initial stage with
sample by-passed, B: Sample routed through the
sample loop
Figure B.2
161
Sample injection sequence, C: Pressure Equilibration
stage, D: Sample injection stage
Figure B.3
162
Sample injection sequence, E: Stage after sample
injection, F: Vent line between SV-A and
SV-B opened
Figure B.4
163
Sample injection sequence, G: System back to the
initial stage, sample by-passed, and purge gas flowing
through the sample loop
xx
164
PALLAVKAR1
CHAPTER 1
INTRODUCTION
1.1 Diesel Engine Emissions
Diesel engines have been established as the main type of power plant for heavy-duty
trucks, buses, off-road vehicles and machineries. The use of diesel engine in light-duty
applications has been followed by the importance and public awareness of issues such as
energy conservation and greenhouse gas emissions as they are attributed with high fuel
combustion efficiency, low VOCs (volatile organic compounds) and CO (carbon
monoxide) emissions. The most common view of public about diesel engines, however,
is rather negative, with the image of black smoke emitted in plain view being the major
obstacle to their wide acceptance. As the usage and importance of diesel engine gradually
increased, so did the public pressure to mandate increasing stringent diesel emission
limits, namely DPM (diesel particulate matter) and NOx (oxides of nitrogen) [1],
The current focus of diesel engine legislation has been on emissions of NOx along
with DPM due to their sizeable contributions to global emission inventories. DPM is
found to be responsible for reduction in visibility and damage of materials and properties
due to their corrosive and erosive effects. Also, studies observing the effects of
particulates on human health have shown that traffic related particulates may adversely
affect human health and plant growth. NOx, on the other hand, causes a wide variety of
environment and health hazards. For instance, ground level ozone (smog) is known to be
formed when NOx and VOCs (volatile organic compounds) react in the presence of
sunlight. This ozone thus formed is known to bring about adverse health effects such as
PALLAVKAR 2
tissue damage and reduction in lung functions. They can also impact vegetation and result
in low crop yields. NOx and SO2 (sulfur dioxide) can react with other substances in the
air and can form acid rains which fall onto the earths surface as rain, fog, snow, or dry
particles. NOx are also known to react with common organic chemicals, and even ozone
in the air, which in-tum form wide variety of toxic products such as nitrate radical,
nitroarenes, and nitrosamines which are known to cause biological mutations. NOx react
with ammonia, moisture, and other compounds to form nitric acid vapor and related
particles which are known to cause respiratory problems such as emphysema, and
bronchitis.
At present, EPA has set stringent emissions regulations for the heavy-duty highway
engines (for model 2007 and later) which would bring about more than 90% reduction in
their emissions level. The current regulation consists of two main components; the
emission standard, and the diesel fuel regulations. The first component of the regulation
deals with emission level of the pollutants at hand and in-view of which DPM, and NOx
emissions from the diesel engine exhaust are to be 0.01 g/bhp-hr and 0.2 g/bhp-hr
respectively, where as the emission level for NMHC (non methane based hydrocarbon) in
the exhaust is set at 0.14 g/bhp-hr. The NOx and NMHC standards are to be phased in the
diesel engines between 2007 and 2010. Since some of the after exhaust treatment devices
being pursued are extremely sensitive to sulfur poisoning, the second component of this
regulation deals with the diesel fuel quality and EPA has proposed to improve the diesel
fuel quality available 2006 onwards and limit its sulfur content to about 15 ppm from the
previous limit of 500 ppm [2]. Thus, refineries are to start producing the 15 ppm sulfur
PALLAVKAR 3
diesel fuel beginning June of 2006. The implementation of these stringent regulations
would result in reduction of NOx emissions by 2.8 million tons per year in 2030.
1.2 VOC Emissions
Volatile organic compounds (VOCs) emitting from various industrial operations and
automobiles are organic chemical species that readily volatilize in ambient air with many
of them having great potential to pose serious long-term health and environmental
impacts. Studies in the past have shown that prolonged exposure to VOCs such as toluene
and /?-xylene may affect central nervous functions and induce reproductive and
developmental toxicity [3, 4]. Toluene in particular is recognized for its neurotoxicity
effect on liver, heart, and kidney as well [4]. Additionally, apart from being a potential
health hazard, many of these VOCs are photochemically active and, along with oxides of
nitrogen and in the presence of sunlight, they have the potential to form ground level
ozone which is a well-known secondary air pollutant [5, 6].
1.3 Issues Associated with Current DPM Control Technologies
DPM removal from the diesel engine exhaust is achieved by means of wall flow DPF
(diesel particulate filter) which act as a trapping media for the particulates. However, as
the filter pores are blocked by the DPM during the filtration process, the pressure drop
across these DPF increase in-turn affecting the efficiency of the engine performance.
Thus in order to maintain practical and economic feasibility, periodic regeneration of
these DPFs are a must, necessitating the removal of accumulated DPM and regenerating
the filter pores for subsequent cycles of filtration. One of the biggest challenges in DPF
regeneration lies in the fact that the diesel exhaust temperatures are not high enough to
PALLAVKAR 4
reach soot ignition temperatures. Essentially, filter regeneration is brought about by
means of active regeneration scheme which involves supplying some sort of additional
energy source in the form of heat to promote soot combustion and filter regeneration.
Most of these active regeneration scheme use thermal methods for DPF regeneration such
as additional fuel burning, electrical heating of the exhaust gases, and microwave heating.
The second category of DPF regeneration is known as the passive regeneration scheme,
which involves the use of catalyst coated filters for soot combustion in the exhaust gas
temperature range. These passive systems in-turn reduce the soot oxidation temperatures
but they remain virtually ineffective under high load conditions where the oxygen content
in the exhaust is not high enough for soot combustion. Hence catalyzed DPF may not be
suitable at all operating conditions of the diesel engine.
1.4 Issues Associated with Current NOx Control Technologies
NOx abatement remains a daunting task as the conventional TWC (three-waycatalyst) remains ineffective in minimizing nitric oxide emissions from the diesel engine
exhaust due to the lean nature (high oxygen content) of the diesel engine exhaust [7].
Invariably, NOx abatement technology is mainly divided into two groups; engine design
modifications, and exhaust gas after-treatment.
1.4.1 Engine Design Modification
The engine design modifications mainly avoid peak combustion temperature, thus
limiting the formation of NOx in the combustion chamber. The two most widely used
practices involving engine design modifications are charge air system modifications, and
PALLAVKAR 5
exhaust gas recirculation. Turbo charging the intake air before it enters the combustion
chamber is an effective way of increasing the mass of air entering the combustion
chamber, and which in-turn results in additional power output. Inadvertently, the process
of turbo charging also heats the intake air, which itself becomes a source of increased
NOx formation during the combustion process due to high temperature. Thus turbo
chargers are often followed by charged air cooling system where the intake air after
getting turbo charged is cooled thus limiting the formation of NOx. Exhaust gas
recirculation on the other hand, displaces the intake air charge with inert materials with
higher heat capacity. This mode of operation assists in avoiding peak combustion
temperatures and thus minimizes the formation of NOx during the combustion process.
However both of these abatement schemes come with a trade off resulting in increased
PM and VOC emissions from the diesel engine.
1.4.2 Exhaust Gas After-Treatment
The diesel engine exhaust leaving the exhaust port of the combustion chamber
contains NOx which can be abated only by means of exhaust gas after-treatment.
However, the temperature of the diesel engine exhaust is considerably low and is high in
oxygen content. Hence, formulating a catalytic system which would promote the
reduction of NOx over oxidation reactions remains a challenging task as far as NOx
emissions control from the diesel engine exhaust is concerned. Some of the most
effective catalytic system as applied to exhaust gas after-treatment are NH3 (ammonia)
based SCR (selective catalytic reduction), hydrocarbon based SCR, and the most
extensively researched LNT (lean NOx trap) also known as the NOx adsorbers.
PALLAVKAR 6
Ammonia based SCR is an established NOx abatement technology for stationary diesel
generators. Here, ammonia is used as the reducing agent and the scheme exhibits greater
than 90% NOx reduction potential. But the major drawback of this technology is
ammonia slippage with increased NH3/NOX ratio coupled with on-board storage
problems and refueling station distribution issues. Hydrocarbon assisted SCR system on
the other hand use hydrocarbon as the reducing agent. However, the amount of
hydrocarbon in the diesel engine exhaust is very low to bring about any effective NOx
reduction and hence the system requires hydrocarbon enrichment.
The lean NOx trap incorporates NOx trapping material which traps NOx in the form
of nitrates and nitrites during the lean operating conditions of the diesel engine and when
the trap needs to be regenerated, short duration spikes of rich fuel-to-air mixture is
introduced into the system, which advertently releases the trapped NOx and reduces it
catalytically into N2 and O2. However, lean NOx trap is highly sensitive to sulfur
poisoning and it essentially needs ULSD (ultra low sulfur diesel) to operate efficiently.
1.5 Issues Associated with Current VOC Control Technologies
Control of VOC emissions from various industrial operations at an acceptable level
and with minimum energy usage constitutes a challenging task for the industry [8].
Various VOC control devices, such as chilled water/refrigerated brine condensers,
carbon/zeolite/polymer adsorbers, and membrane separation systems, are employed in
many industrial applications to recover VOCs and achieve low VOC emissions [9-11].
Other available VOC control devices also include thermal oxidizers, catalytic oxidizers,
flares, and plasma/electron beam devices, where the first three involve the use of thermal
PALLAVKAR 7
energy generated from fossil fuel to destroy VOCs [12 - 17]. Among them, thermal
oxidizers are designed to treat waste streams with VOC concentrations ranging from 100
to 2000 ppm and are considered to have the broadest VOC control applicability with a
high destruction and removal efficiency (DRE) ranging from 95 to 99% [18]. The
catalytic oxidizers are generally more energy efficient than the thermal oxidizers.
However, they are more restricted in the application ranges and require additional
maintenance for reliable operations.
1.6 Microwave Heating Technology
Microwaves are electromagnetic waves with frequencies ranging from 0.3 to 300 GHz
in the vicinity of the high-frequency range of these radio waves [19 - 29]. In order to
avoid interference with telecommunication devices, particular frequencies have been
allocated for purely industrial and domestic microwave systems. The standard frequency
used in various microwave heating applications is 915 and 2450 MHz [21], with the
corresponding wavelengths for the above two frequencies, being 32.77 and 12.24 cm,
respectively. The microwave heating of a potential microwave absorber depends on its
dipolar polarization. It is worth pointing out that microwave heating technology is highly
selective, with some materials absorbing microwave energy selectively, and getting
heated up mainly through dielectric heating. Thus microwave energy becomes an energy
efficient heating tool for addressing problems associated with DPF regeneration by
supplying heat energy, NOx abatement by enhancing the temperature window of
operation and VOC destruction processes by providing heat energy efficiently. With the
PALLAVKAR 8
source of microwave energy coming from non-greenhouse gas energy sources,
microwave-assisted abatement technologies becomes an attractive option.
1.7 Objectives
The objective of this dissertation was to demonstrate that microwave can be an energy
efficient heat source to raise the temperature of catalysts to achieve high control
efficiencies. Specifically, the objective was accomplished in three separate emission
control processes: (1) Regeneration of DPF using microwave heating; (2) NOx reduction
using a microwave-assisted De-NOx catalytic system; and (3) VOC destruction using a
ceramic based material and microwave heating. They are described below.
1.7.1 Microwave-Assisted DPF Regeneration
With distinct disadvantages in using catalyzed DPF or additional fuel burning for filter
regeneration, regenerating the loaded DPF can be achieved efficiently if the control
strategy formulated involves selectively heating the filter substrate or the soot directly.
Microwave heating technology is evidently selective in nature and with the choice of
proper filter material; it can readily attain temperatures as high as 700 °C. With the diesel
engine exhaust being lean, deposition of microwave energy to the soot can easily trigger
off the DPF regeneration without the requirement of heating exhaust gases, and thus
avoiding added fuel penalty. However, the use of microwave heating has issues with
uneven temperature distribution pattern and thermal cracking of the filter medium.
This task of microwave heating for DPF regeneration had the following objectives:
•
High filtration and regeneration efficiency of both fresh and regenerated DPF.
PALLAVKAR 9
•
Even distribution of microwave energy throughout the filter substrate so as to
eliminate any formation of hot spots.
•
Waveguide installation so as to enhance the microwave heating efficiency.
•
Demonstration of filter loading and regeneration for both single cycle and multiple
cycles.
1.7.2 Microwave-Assisted Catalytic NOx Abatement
The catalysts used in the previously mentioned exhaust gas after-treatment systems for
NOx abatement are formulated so as to exhibit its potential in the diesel engine exhaust
temperature range. Conventionally this temperature window of operation is very small
and an effective way to enhance this temperature window is to actively supply energy to
the catalyst for the entire range of operating conditions.
The current research objectives for NOx abatement using microwave heating can be
summarized as follows:
•
Studying the effect of microwave heating on De-NOx catalyst system for NOx
abatement.
•
Studying the effect of reducing agent enrichment on NOx abatement.
•
Speciation of the off gases evolved during NOx abatement.
1.7.3 Microwave-Assisted VOC Destruction
With the emerging emphasis on using green technologies to minimize greenhouse gas
emissions, [19] the employment of thermal oxidizers and catalytic oxidizers for VOC
destruction with thermal energy generated from fossil fuels is becoming undesirable due
PALLAVKAR10
to its potential generation of additional greenhouse gas, e.g., carbon dioxide. Instead, the
use of microwave energy to achieve the control with its electric power coming from
nongreenhouse-related energy sources, such as wind, geothermal, solar, or even nuclear
energy, becomes an attractive option [8, 14-29]. The research proposal for VOC
Destruction experiments could be summarizes as follows:
•
To conduct experiments to characterize the VOC destruction using microwave energy
with SiC being the microwave absorbing media.
•
To measure the DRE of the chosen VOC and to identify the destruction byproduct
using a gas chromatograph/mass-spectrometer (GC/MS).
•
To study the effect of power level on VOC destruction.
•
To describe the energy balances in the system based on the temperature profiles of the
absorbing media during microwave heating.
1.8 Organization of the Dissertation
In this dissertation, chapter 2 is the literature survey which reviews literature related to
the current study. Chapter 3 describes the study on the microwave-assisted regeneration
of diesel particulate filter for DPM control. Chapter 4 describes the NOx abatement
process using microwave and a platinum based catalyst system. Chapter 5 describes the
study on VOC destruction of p-xylene employing microwave energy as the heat source.
Chapter 6 concludes the findings of these individual studies and makes recommendations
for fiiture studies.
PALLAVKAR 11
References
1. Walsh, M.P. Global trends in diesel emissions control - A 1999 update. Soc. Auto.
Eng. 1999, 1999-01-0107.
2. Regulatory Announcement; Proposed Heavy-Duty Engine and Vehicle Standards and
Highway Diesel Fuel Sulfur Control Requirements. United States Environmental
Protection Agency. May 2000, EPA420-F-00-022.
3. Olson, B. A.; Gamberale, F.; Iregren, A. Coexposure to toluene and p-xylene in man:
central nervous functions. Br. J. Ind. Med. 1985, 42, 117.
4. Donald, J. M.; Hooper, K.; Claudia, H. R. Reproductive and developmental toxicity
of toluene: A review. Environ. HI. Pers. 1991, 94, 237.
5. Cooper, D. C.; Alley, F. C. Air Pollution Control: A Design Approach-, Waveland
Press Inc.: Prospect Heights, IL, 2002.
6. Zalel, A. Y.; Broday, D. M. Revealing source signatures in ambient BTEX
concentrations. Environ. Pollut. 2008,156, 533.
7. Brogan, M.S.; Brisley, R.J.; Moore, J.S.; Clark, A.D. Evaluation of NOx adsorber
catalysts systems to reduce emissions of lean running gasoline engines. Soc. Auto.
Eng. 1996, 962045.
8. Kim, T. H.; Rupani, H.; Pallavkar, S.; Hopper, J.; Ho, T.; Lin, C. J. Destruction of
toxic volatile organic compounds (VOCs) in a microwave-assisted catalyst bed. J.
Chin. Inst. Chem. Eng. 2006, 37, 519.
9. Baker, R. W.; Yoshioka, N.; Mohr, J. M.; Khan, A. J. Separation of organic vapors
from air. J. Membr. Sci. 1987, 31, 259.
PALLAVKAR 12
10. Behling, R. D.; Ohlrogge, K.; Peinemann, K. V.; Kyburz, E. The separation of
hydrocarbons from waste vapor streams. AIChE Symp. Ser. 1988, 48, 68.
11. Busca, G.; Berardinelli, S.; Resini, C.; Arrighi, L. Technologies for the removal of
phenol from fluid streams: A short review of recent developments. J. Hazard. Mater.
2008,160, 265.
12. Jol, A.; Dragt, A. J. Biotechnological elimination of volatile organic compounds in
waste gases. Proc. Bio. Downstrm. Proc. 1995, 2, 373.
13. Kiared, K.; Bieau, L.; Brzezinski, R.; Viel, G.; Heitz, M. Biological elimination of
VOCs in bio-filter. Environ. Prog. 1996,15, 148.
14. Spivey, J. J. Recovery of volatile organics from small industrial sources. Environ.
Prog. 1988, 7, 31.
15. Environmental Protection Agency (EPA). Control Technologies for Fugitive VOC
Emissions from Chemical Process Facilities. EPA Handbook, EPA/625/R-93-003,
1994.
16. Ottenger, S. P.; Van den Ocver, A. H. C. Kinetics of organic compound removal from
waste gases with a biological filter. Biotechnol. Bioeng. 1983,12, 3089.
17. Khan, F. I.; Ghoshal, A. K. Review: Removal of volatile organic compounds from
polluted air. J. Loss Prev. Proc. Ind. 2000, 13, 527.
18. Ryan, M. A.; Tinnesand, M. Introduction to Green Chemistry, American Chemical
Society: Washington, DC, 2002.
19. Hashisho, Z.; Rood, M. Microwave-swing adsorption to capture and recover vapors
from air streams with activated carbon fiber cloth. Environ. Sci. Technol. 2005, 39,
6851.
PALLAVKAR13
20. Hashisho, Z.; Emamipour, H.; Rood, M. J.; Hay, J.; Kim, B. J.; Thurston, D.
Concomitant adsorption and desorption of organic vapor in dry and humid air streams
using microwave and direct electrothermal swing adsorption. Environ. Sci. Technol.
2008, 42, 9317.
21. Metaxas, A. C.; Meredith, R. J. Industrial Microwave Heating-, Power Engineering
Series 4; P. Peregrinus: London, U.K, 1983.
22. Plazl, I.; Pipus, G.; Koloini, T. Microwave heating of the continuous flow catalytic
reactor in a nonuniform electric field. AIChE J. 1997, 43, 754.
23. Turner, M. D.; Laurence, R. L.; Conner, W. C. Microwave radiation's influence on
sorption and competitive sorption in zeolites. AIChE J. 2000, 46, 758.
24. Curtis, W. M.; Tompsett, G.; Lee, K. H.; Yngvesson, K. S. Microwave synthesis of
zeolites. J. Phys. Chem. B 2004,108, 13913.
25. Valle, S. J.; Conner, W. C. Microwaves and sorption on oxides: A surface
temperature investigation. J. Phys. Chem. B 2006,110- 15459.
26. Panzarella, B.; Tompsett, G. A.; Yngvesson, K. S.; Conner, W. C. Microwave
synthesis of zeolites. 2. Effect of vessel size, precursor volume, and irradiation
method. J. Phys. Chem. B 2007, 111, 12657.
27. Conner, W. C.; Tompsett, G. A. How could and do microwaves influence chemistry
at interfaces. J. Phys. Chem. B 2008,112, 2110.
28. Gharibeh, M.; Tompsett, G. A.; Yngvesson, K. S.; Conner, W. C. Microwave
synthesis of zeolites: Effect of power delivery. J. Phys. Chem. B 2009,113, 8930.
PALLAVKAR 14
29. Cherbanski, R.; Molga, E. Intensification of desorption process by use of microwaves
- An overview of possible applications and industrial perspectives. Chem. Eng. Proc.
2009, 48, 48.
PALLAVKAR 15
CHAPTER 2
LITERATURE SURVEY
Diesel engines are well known for their low fuel consumption, reliability, and
durability characteristics. They are characterized by low emissions of unburned
hydrocarbons and carbon monoxide emissions. However, they are associated with high
levels of DPM and NOx emissions. In particular, the formation of NOx in the diesel
engine combustion chamber is quite a complicated process and mainly formed due to
excess charged air used in the combustion chamber.
2.1 The Diesel Engine
The combustion of diesel fuel is unique. The source of power in the diesel engine
system is the diesel fuel that is burned, and the transformation of the fuel's chemical
energy into vehicle motion proceeds as follows:
Chemical Energy -> Thermal Energy -> Mechanical Energy -> Shaft Energy
The combustion process is mainly the main distinguishing feature between a diesel
engine and a gasoline engine. In the diesel engine, air is compressed by vertical piston
movement, raising the air temperature. This temperature must be high enough to ignite
the fuel sprayed in the combustion chamber thus causing its combustion. The key
controlling element is the compression ratio which signifies the amount of air compressed
in the combustion chamber. The greater the compression ratio, the more the air is
compressed and the greater will be the air's temperature and pressure. However, the
compression ratio which in-turn depends on the amount of air charged in the combustion
PALLAVKAR 16
chamber depends on the Top Dead Center (TDC) and the Bottom Dead Center (BDC) of
the chamber and limits the amount of air that can be charged in one sweep, as shown in
Figure 2.1. For diesel engines this compression ratio varies from 14:1 to 24:1.
Cylinder
TDC
' Piston
Connecting Rod
Crankshaft
TDC - Top Dead Center
BDC - Bottom Dead Center
Fig. 2.1 Schematic of combustion cylinder with major components [1]
By definition, compression ratio (r) is given by equation 1:
r = (volume at TDC) / (volume at BDC)
(1)
The combustion process in a typical diesel fuel powered automobile comprises of a four
stroke combustion cycle. The four strokes comprising the combustion cycle may be
classified as follows:
•
Intake stroke: The intake valve is opened; the exhaust valve is closed, and the piston
PALLAVKAR17
moves down, bringing in a fresh air into the combustion cylinder.
•
Compression stroke: Both intake and exhaust valves are now closed and the air inside
the combustion chamber is compressed by the upward piston movement to very high
pressure and temperature conditions.
•
Power stroke: Both intake and exhaust valves are still closed; diesel fuel is injected in
the combustion chamber and due to high temperatures in the combustion chamber,
combustion of the injected fuel occurs, resulting in increase in the resultant pressure
thus forcing the piston downward.
•
Exhaust stroke: The exhaust valve is now opened, the intake valve is closed, and the
upward movement of the piston forces the products of the combustion (exhaust) from
the engine.
The schematic of the four stroke cycle is illustrated in Figure 2.2.
intake
Compression
Power/Work/
Expansion
Exhaust
i
Air only
Compressed air
Fuel injected
nsar TDC
Fig. 2.2 Schematic of four stroke cycle [1]
The combustion of diesel fuel theoretically would result only carbon dioxide (CO2) and
water vapor (H2O) formation and unused portion of air charged in the combustion
PALLAVKAR18
chamber. Concentrations of the gases in the diesel engine exhaust however, vary
depending on the engine, its load and speed conditions. A typical diesel engine exhaust
composition (Exhaust flow - 50 m3/hr; 70% of maximum load) of our test unit is shown
in Table 2.1.
Table 2.1 Diesel Engine Exhaust Gas Composition
Components
Concentration
o2
NO
NO2
CO
8.51%
607 ppm
22.9 ppm
660 ppm
so2
co2
9 ppm
6.94%
2.2 Soot Control
With new stringent regulations imposed by EPA with an aim of eliminating 90% of
the DPM from the engine exhaust [1 - 3], the DPM emissions from the engine exhaust is
to be restricted to 0.01 g/bhp-hr incase of heavy duty vehicles. The new regulations
essentially facilitates the need for units capable of removing DPM from the diesel engine
exhaust without adversely affecting the engine performance, and the most popular choice
for this task is the wall-flow diesel particulate filter (DPF). The filtration mechanism of
the DPFs involves forcing the exhaust gases to flow through the porous walls as they
enter the trap through one end, trapping the soot and the clean exhaust gases are emitted
out through the open channels at the other end. However with the accumulation of the
PALLAVKAR19
soot in the trapping media, the pressure drop across the filter increases which in-turn
exerts a back pressure on the engine exhaust, resulting in poor performance of the diesel
engine in terms of increased CO emissions, poor fuel efficiency and increased soot
production. A commercial DPM control system consists of filtration and regeneration
scheme and the regeneration methods are mainly divided into two main classes, active
regeneration and passive regeneration.
2.2.1 Active Regeneration Methods
Majority of these active regeneration methods use thermal means of DPF regeneration
and in principle involve supplying additional heat energy so as to raise the filter substrate
temperatures high enough to facilitate soot oxidation process. There are two challenging
issues related to these regeneration methods. First and foremost, temperatures as high as
650 °C have to be achieved to cause rapid DPM oxidation and secondly, because this
oxidation process is highly exothermic, it can result in severe thermal gradients thus
causing filter damages [4]. Hence a suitable DPF should be resilient to severe thermal
gradients and yet exhibit high filtration efficiencies. Thermal regeneration methods such
as additional fuel burning, electrical heating of the exhaust gases and use of microwave to
heat the filter or soot directly are some of the means for active regeneration of DPF.
2.2.1.1 Additional Fuel Burning
Additional fuel burning mainly involves hydrocarbon source such as the diesel fuel
itself which is then mixed with the atomized air from a pressure reservoir of the vehicle,
introduced in a retrofitted combustion chamber and ignited by means of ignition
PALLAVKAR 20
electrodes. The key feature of this technology is development of soot free flame
irrespective of the engine operating conditions which involves compressed air as the
source of oxygen for combustion in the burner. However, this method essentially results
in an added fuel penalty, and increases the complexity of the system in terms of Engine
Control Unit, retrofitting, and sensor management.
2.2.1.2 Electric Heating
Electric heating regeneration method involves heating of the exhaust gases or the
regeneration air by a resistive heater, to reach soot ignition temperatures and trigger the
regeneration of the soot filled DPF. The heater is placed upstream to the DPF where the
energy from the heating element is deposited to the exhaust stream thus elevating its
temperature to the soot ignition temperatures. For automotive applications applying this
strategy on-board means additional power consumption for heating the electric resistive
element which in-turn has to be drawn from the automobile's battery. This puts extra load
on the engine and hence results in relatively higher fuel penalty.
2.2.1.3 Microwave Heating
Microwave heating technology is very selective in nature and energy efficient. With
the choice of proper material as the filter substrate, it can easily provide high
temperatures needed for soot combustion and DPF regeneration [4-8]. The extent of
microwave energy absorption by the filter material depends on its dielectric properties
(especially the dielectric loss factor). Table 2.2 illustrates the various dielectric properties
of some of the common materials used in the DPF regeneration.
PALLAVKAR 21
Table 2.2 Dielectric Properties of Some Common Materials used in Soot Control
Diesel soot23
dielectric constant
s'
10.695
dielectric loss factor
e"
3.561
Quartz30
3.78
0.001
Cordierite 23
2.873
0.138
8.9
0.009
30
11
Material
Alumina ceramic AI2O320
Silicon carbide SiC
20
With high dielectric loss factor, the absorbed microwave energy can be readily
converted into heat energy enabling the filters to reach soot ignition temperatures.
Perhaps the single most advantage of using microwave-assisted DPF regeneration is its
ability to deposit energy directly to the soot which is collected in the DPF. With the
diesel engine exhaust being lean (high in oxygen content), deposition of microwave
energy to the soot can easily trigger off the DPF regeneration without the requirement of
heating exhaust gases. Microwave-assisted DPF regeneration however has been
associated with uneven energy distribution and regeneration pattern, which if not radiated
evenly, would result in hotspots during the exothermic soot oxidation reactions. The
distribution of microwave energy mainly depends on the medium geometry (radio
frequency cavity or waveguide). Metallic objects are reflectors of microwave energy and
thus when reflected by the walls of a metallic container, the reflected energy sets up a
pattern of electric and magnetic standing waves. This electric field distribution in a
PALLAVKAR 22
microwave cavity can be visualized as an array of two-dimensional electric field resulting
in peaks of high intensities and valleys of low intensities. Materials with dielectric loss
properties would be heated only when they are placed in the high intensity peaks of
electric field.
In order to distribute this high intensity energy throughout the volume of the DPF, a
metallic waveguide is required surrounding the filter body with an open cavity for the
microwave energy to be inline with the DPF assembly. This enables the microwave
energy to be guided onto the filter substrate, and with the waveguide distributing the
energy all throughout the filter body.
2.2.2 Passive Regeneration Methods
Another variant for DPF regeneration constitutes the use of catalyst coated filters
where the exhaust gases are treated passively without any additional heat energy. These
systems in-turn reduce the soot oxidation temperatures by more than 200 °C. However,
they remain virtually ineffective under high load conditions where the oxygen content in
the exhaust is not high enough for soot combustion. Hence catalyzed DPF may not be
suitable at all operating conditions of the diesel engine [4].
2.3 NOx Control
The combustion process being of complex in nature, exhibits different mechanism for
NOx formation under different engine operating conditions. Some of the key parameters
affecting formation of NOx are temperature, pressure, residence time and concentration
of reacting species. The reacting components proceed through a number of different
PALLAVKAR 23
chemical paths which lead to the NOx formation. Some of the known mechanisms for
NOx formation are discussed briefly in the following sections.
2.3.1 Thermal NOx
The thermal mechanism of NOx formation, also known as the extended Zeldovich
mechanism, is responsible for the majority of NOx formation in the diesel engine
combustion process. The peak combustion temperatures attained during the said engine
operating conditions exceed 2000 K which is the order of temperature required to
maximize engine operating efficiency [9]. The three chemical reactions associated with
this mechanism are as follows:
O + N2
->
NO + N
(2)
N + O2
-»
NO + O
(3)
N + OH
->
NO + H
(4)
The overall rate of NO formation via this mechanism is slow and is very temperature
sensitive. Hence, thermal NOx appears in significant quantities in the post combustion
process where temperatures in the combustion chamber exceed 2000 K. The temperature
sensitivity of this mechanism indicates that thermal NOx appears in significant quantities
only after the start of the heat release in the combustion process but thereafter formation
freezes when temperatures in the combustion chamber drops during the expansion
(exhaust) stroke.
PALLAVKAR 24
2.3.2 Prompt NOx
The prompt NO mechanism, also known as the Fenimore mechanism is very rapid and
is carried out by the rapid reaction of hydrocarbon radicals from the combustible fuel
with molecular nitrogen available in abundance in the combustion chamber. This rapid
reaction leads to the formation of amines or cyano compounds that in turn react to form
NO.
CH + N2
->
HCN + N
(5)
HCN + 0
-»
NCO + H
(6)
NH + CO
(7)
NCO + H
NH + H
->
N + H2
(8)
N + OH
-»
NO + H
(9)
Hence subsequent rapid conversion of these compounds to NO is strongly influenced
by O and OH. This mechanism is more evident when the fuel concentrations are higher
than stoichiometry and hence result in the formation of hydrocarbon radicals to form
HCN which in-turn leads to NO formation.
PALLAVKAR 25
2.3.3 N2O Pathway
This is commonly known as the three body mechanism where it involves formation of
N2O as follows:
0 + N2 + M
->
N20 + M
(10)
While N2O in general degenerates back to molecular nitrogen, this is not the case
under lean operating conditions. Under these operating conditions, NO is formed via any
of the following two possible reaction pathways:
N20 + 0
H + N20
->
NO + NO
(11)
NO + NH
(12)
It is to be noted that without the third body (M), the energy liberated in the formation
of N20 would however high enough to decompose it back to the original reactants. Thus
at higher pressures and lower temperatures, the three body mechanism becomes
competitive with the thermal mechanism.
2.3.4 Fuel Nitrogen
The diesel fuels themselves contain organic nitrogen linked to the fuel molecule. This
nitrogen can too react to form NO and the pathway taken by the fuel nitrogen in forming
NO depends on whether this nitrogen is bound to an aromatic ring or an amine. In the
case where the fuel nitrogen is bound to the aromatic ring, it tends to form HCN which
PALLAVKAR 26
essentially takes the path of Prompt NOx mechanism. On the other hand, if the fuel
nitrogen is linked to an amine, it leads to the formation of NH3 which in turn results in
the formation of NH2. The reactions involved in this mechanism are as follows:
NH2 + O
->
NHO + H
(13)
NH2 + O2
-»
NHO + OH
(14)
HNO + X
->
NO + XH
(15)
HNO + M
-»
NO + H + M
(16)
It is evident that one of the most important factors influencing NOx formation in the
combustion chamber is temperature. High temperatures are required to maximize engine
operating efficiency but also result in NOx formation during the power stroke of the
combustion process. At the same time, the NOx formation and reduction process is
paused during the exhaust cycle of the combustion process during which temperatures
drop down to values which is insignificant for NOx formations [10, 11]. The net process
involves formation of NOx which when exit the combustion chamber need to be
controlled. This controlling strategy is divided into two main categories, engine design
modifications, and exhaust gas after-treatment. The following sections cover the various
technologies that are available for controlling NOx emissions. •
PALLAVKAR 27
2.3.5 Engine Design
The control of temperature and avoiding peak combustion temperatures is an effective
remedial solution for attaining low NOx emission levels from the engine exhaust. This
can be attempted by applying various modifications to engine design such as Charge air
inter-cooling, Fuel delivery modifications, Combustion chamber modifications, Exhaust
Gas Recirculation (EGR). While each modifications used alone typically has some
penalty, such as higher cost of operation or lower engine efficiency, these modifications
can be used together to optimize their individual benefits and minimize penalties. The
two most widely used approaches to limit NOx emissions from the diesel engine exhaust
are discussed in the following two sections.
2.3.5.1 Charge Air System Modifications
A common method in engine design to achieve an increase in power output without an
increase in engine displacement and weight is the use of charge air compression, typically
through turbo charging the incoming air. This method increases the mass of air entering
the engine's combustion chamber, which allows more fuel to be used, thus increasing the
power output. However, charge air compression also heats the intake air, which in-turn
increases the level of NOx formation. A block diagram of a conventional charge air
system is shown in Figure 2.3.
PALLAVKAR 28
Turbochargar
Engine
Fig. 2.3 Block diagram of charge air system
This effect can be minimized by cooling the charged air after compression, thus
keeping the temperature of the charged air low enough to keep the NOx formation
minimal. However, in-view of over cooling, PM formation is expected to increase in the
diesel engine exhaust. A typical block diagram of a charge air system with inter-cooling
is shown in Figure 2.4.
Exhaust
In-Take Air
Turbochargar
r
Coolant In /^*N Coolant Out
L
Engine
Fig. 2.4 Charge air system with inter-cooling
PALLAVKAR 29
2.3.5.2 Exhaust Gas Recirculation
Displacing some of engine's intake air with inert materials is another NOx reduction
strategy which is the main NOx controlling scheme in the current diesel engines [12].
The inert material lowers combustion temperatures by mode of diluting effect and
thermal effect. The dilution effect involves replacing the oxygen entering into the
combustion chamber with inert materials thus assisting in NOx abatement. The thermal
effect involves replacing the incoming air with constituents such as H2O and CO2 which
have a higher specific heat value and thus heat absorption capacity. The inert mixture
thus absorbs heat from the burning fuel and reduces peak combustion temperature
conditions [13]. However, exhaust gas recirculation comes with a trade off with
hydrocarbon (HC) and PM emissions [14]. Thus the use of EGR in controlling NOx
emissions from the diesel engine exhaust although effective is limited because of the PM
and HC emissions from the exhaust. As seen in both Figures 2.5 and 2.6 with the increase
in EGR dilution, NOx level tend to decrease, however, it essentially leads to incomplete
combustion as a result of which, both the hydrocarbon and PM emissions increase and
out weigh the advantage of using EGR dilution in the intake.
2.3.6 Exhaust Gas After-Treatment
NOx formed in the combustion chamber and thus leaving the exhaust port of the
combustion chamber cannot be destroyed using engine design modifications. Hence there
is a need for exhaust gas after-treatment. The conventional three-way catalyst (TWC)
technology used in our regular gasoline engines requires a near stoichiometric air-to-fuel
ratio for reducing NOx, CO and the hydrocarbons. The fact that diesel powered engines
PALLAVKAR 30
500
500
HC
NO,
450
400
400
300
350
300
200
250
100
200
150
0
10
20
30
40
50
EGR, %
Fig. 2.5 Trade off between hydrocarbon and NOx level with respect to the extent of EGR [1]
500
Mass Rate
a
400
0.35
300
£
a.
CL
200
100
EGR. %
Fig. 2.6 Trade off between PM and NOx level with respect to the extent of EGR [1]
PALLAVKAR 31
emit pollutants under lean operating conditions, where the oxygen content is in excess of
the stoichiometric requirements makes it impossible for employing the conventional
TWC catalytic system for treating exhausts of a diesel powered engine [15]. At present,
some of the most important exhaust gas after-treatments schemes being researched are as
follows:
•
NH3 based selective catalytic reduction.
•
Hydrocarbon based selective catalytic reduction also known as the Lean NOx
catalyst.
•
NOx storage and reduction catalytic system also known as the NOx adsorbers or Lean
NOx Trap (LNT).
2.3.6.1 Ammonia based Selective Catalytic Reduction
Selective Catalytic Reduction (SCR) technology utilizing ammonia or urea to reduce
NOx is a well established technology for stationary power plants. Applying this
technology to transient and dynamic operating conditions to abate NOx emitting from the
exhaust of an automobile proved to be a challenging task. Yet, a SCR system based on an
aqueous urea/water solution was demonstrated for vehicle application [16]. In its simplest
form, ammonia based selective catalytic reduction of NOx involves using nitrogen based
compounds as reducing agents. Conventional ammonia based SCR is depicted in the
Figure 2.7. This scheme comprises of three different catalysts in series after the urea
injection point, the hydrolysis catalyst (H), the SCR catalyst (S) and a guard oxidation
catalyst (O).
PALLAVKAR 32
Una
(NHihCO
Engine Exhaust Out
Eugine Exhaust In
H
(NH&CO + H2O 4 2NH3 + CQ2
s
4NH3 + 4NO + O2
6H20
4HC + 302 ->4C0 + 2H20
4HC + 5C>2 4C02 + 2H20
0
4NH3 + 3Q2 ->21^ + 6H20
4HC + 5C>2 4CO2 + 2H20
2CO + O2 2CO2
Fig. 2.7 Schematics of basic ammonia based SCR
The urea is converted into ammonia by the hydrolysis catalyst and yields carbon
dioxide as the by product. The ammonia then reacts on the SCR catalyst with the NOx
present in the exhaust to form nitrogen. This stream is then directed to the
guard catalyst where all secondary emissions are minimized. A variation of this scheme
involves a preoxidation catalyst which significantly improves the NOx conversion in the
low temperature region of the engine operating conditions [17]. A schematic version of
this improved SCR is shown in Figure 2.8. In the improved version of this SCR, a
preoxidation catalyst (V) is installed upstream to the urea injection point. This
preoxidation catalyst increases the NO2 fraction of the NOx in the engine exhaust so as to
PALLAVKAR 33
improve NOx conversion efficiency at lower temperatures. The biggest advantage that
the ammonia/urea based SCR has on all the other NOx reduction technology is that it is
resilient to sulfur poisoning.
Ur*a
(NHrijCO
Kugine Exhaust In
Engiue Exbanit Chit
H
V
V
H
O
S
$
0
2N0 + 0i -> 2NCb
(NHj))C0+ HjO 2NH3+ COi «H3 + 4NO + Oj4Nj+ 6H3O 4NH3+302 -> 2N3 + 6H30
2NH3 + NO + NOj 2Nj + 3HjO
4HC + 50j ^ 4COj + 2HjO
SNH}+6NOi-»7Ni+12HjO
2C0 + Oj -> CQi
Fig. 2.8 Schematics of improved ammonia based SCR with preoxidation catalyst
At present, the diesel fuel sulfur levels are in the range of 350 to 500 ppm and SCR
seems to be the most promising after-treatment system for meeting the stringent
regulations for NOx emissions regulated by US-EPA [18]. However, as shown in Figure
2.9 at lower temperatures, the conversion increases only linearly with the increase in
NH3/NOX ratio up to a certain value above which any further increase in ammonia
injection doesn't results in any significant NOx conversion. Moreover, it is also evident
from Figure 2.10 that subsequent increase in this ratio results in NH3 slippage [17] which
itself becomes a detrimental emissions issue. The most popular catalyst for this scheme is
PALLAVKAR 34
V20s/W03/Ti02, however the V2O5 containing SCR catalysts get deactivated
permanently at temperatures above 700 °C due to the phase change of the Ti02 support
oxide from anatase to rutile, loss of surface area and evaporation of V2O5 and WO3 [19].
Operational difficulties pertaining to on-vehicle storage and replenishing the reducing
agent in due time are yet to be overcome. At the same time, increasing the DeNOx
activity at low temperatures represents a major developmental goal [20] due to poor
catalytic activity.
120
400 °C
100
350 °C
90
275 #C
CO
40-
Figure A - % NOx conversion vs. alpha
30-
0
02
04
OJS
OS
«pM(HH3M0Kr*IO)
1
u
Fig. 2.9 Effect of NH3/NOX ratio on NOx conversion at various temperatures [1]
PALLAVKAR 35
275 *C
300 #C
&
/ 350 #C
5>
/
Figure B - % NHj slippage vs. alpha
400 #C
a.
•
02
0.4
0.6
0.8
1.2
1.4
«pft«(NH3jNOx ratio)
Fig. 2.10 Effect of NH3/NOX ratio on NH3 slippage at various temperatures [1]
PALLAVKAR 36
2.3.6.2 Hydrocarbon based Selective Catalytic Reduction
Hydrocarbon based selective catalytic reduction catalyst system also known as the
Lean NOx catalyst utilizes hydrocarbons to reduce NOx from the engine exhaust into
nitrogen and oxygen. The reaction is essentially a redox process resulting in reduction of
nitric oxide on the catalytic site, leading to the formation of nitrogen and adsorbed
oxygen [21]. The oxygen adsorbed is then removed by the hydrocarbon which then
regenerates the catalyst active site. The two main reactions summarizing NOx reduction
using hydrocarbon as the reducing agent are as follows. Both of these reactions are
temperature sensitive.
CmHn + (2m + l/2n) NO
CmHn + (m + l/4n) 02
->
(m + l/4n) N2 + mC02 + l/2nH20
(17)
mC02 + l/2nH20
(18)
The two main catalytic group assisting in hydrocarbon based selective catalytic
reduction of NOx are precious metal catalyst and base metal catalyst supported over
zeolites [21]. Table 2.3 lists the peak temperature conditions and conversions for the
various catalysts.
PALLAVKAR 37
Table 2.3 Summary of Various Catalysts used in Hydrocarbon Assisted SCR [21]
Catalyst
metal
Hydrocarbon
used
Temperature
conditions
°C
Cu
Cu
Co
Co
Rh
Rh
Pt
Pt
Methane
Ethane
Methane
Ethane
Methane
Ethane
Methane
Ethane
550
400
550
450
450
400
400
400
%NO
conversion
(without
oxygen)
0
0
66
70
100
100
93
95
%NO
conversion
(with oxygen)
6
18
10
51
30
72
3
1
It is evident from the table above that only Pt and Rh show appreciable NOx
conversion at temperatures which match the temperatures of the diesel engine exhaust.
However, the diesel engine exhaust is predominantly lean and none of the catalyst
mentioned show any effective NOx reduction in the presence of oxygen. A NOx
reduction mechanism essentially is that of nitric oxide being oxidized to nitrogen dioxide
on certain catalytic sites and on other sites hydrocarbon reacts with oxygen forming
reaction intermediates of the form CxHy02Z. The nitrogen dioxide and this reaction
intermediate then react to form products such as CO2, H2O, N2 and N2O [22]. A second
catalytic system studied for NOx reduction using hydrocarbons was silver over alumina.
Here the gas phase reactions seem to play a significant role in achieving high NOx
conversion to N2 with NOx reduction potential as high as 90% [23]. Extensive studies
PALLAVKAR 38
were made to study the effect of different hydrocarbons as reductant for NOx reduction
with this catalytic system and are shown in Table 2.4. It is observed from this table that
peak NOx conversion can be obtained if the reaction temperatures are high enough.
Table 2.4 Effect of Various Hydrocarbons on NOx Reduction (Catalyst - Ag/AhOi) [21]
Hydrocarbon used
propane + propene
octane
iso-octane
1-octene
octanal
octanol
octanoic acid
Temperature °C
525
525
500
500
350
400
400
% NO conversion
88
90
80
60
85
82
75
Studies have also shown that the hydrocarbon assisted NOx reduction activity
reduction activity improves in the presence of H2 [24, 25]. Here H2 was added to the
reaction gas, resulting in lower light-off temperature. H2 promotes formation of gas-phase
species such as NH at high concentrations of H2 where as at lower concentrations, NO2 is
produced instead which can be readily reduced to N2 and O2 [25]. Table 2.5 enumerates
the findings of that study.
Table 2.5 Effect of H2 on Light-off Temperature [24, 25]
H2 concentration added
(ppm)
0
227
451
909
1818
Temperature °C
% NOx conversion
500
500
475
450
450
45
45
55
60
60
PALLAVKAR 39
A variant of this reduction scheme involves a mechanical mixture of CoFER and
HZSM-5 zeolites for a continuous reduction of NOx under lean operating conditions
[26]. In this study, the degree of NO conversion to N2 was found to be dependent on O2
concentration in the gas mixture. Here again it is suggested that NO is oxidized into NO2
which then is reduced to N2 with hydrocarbons on a catalytic system which has low
activity for oxidation of hydrocarbon with oxygen. The reaction summarizing this
mechanism is as follows:
CmHn + (m + l/4n) N02
(l/2m + l/4n) N2 + mC02 + l/2nH20
(19)
However, the individual zeolites themselves are not active for NOx reduction. Also, there
is an optimum concentration of oxygen for the reduction of NOx. This is so because too
high oxygen level leads to the consumption of hydrocarbon by the oxygen instead of
reacting with NOx.
The most attractive source of hydrocarbon for selective catalytic reduction of NOx is
the hydrocarbon from the diesel engine exhaust. Depending on the strategy employed in
providing the necessary hydrocarbon for NOx reduction, two schemes are devised for
NOx reduction; the passive lean NOx reduction and the active lean NOx reduction [12].
Under passive condition, the hydrocarbons as the reducing agent is drawn from the
engine exhaust itself. However, the amount of hydrocarbon emitted from the engine
exhaust is not sufficient enough to bring about an acceptable conversion levels of NOx
[27]. At the same time, the catalyst is not active on account of low exhaust gas
temperature thus rendering low catalytic activity [12]. On the other hand, a high level of
PALLAVKAR 40
NOx is produced under high-load conditions, for example during vehicle acceleration.
However, the HC/NOx ratio then is small and thus high reduction of NOx cannot be
expected. Thus under high-load conditions, NOx reduction strategy calls for injecting the
necessary amount of hydrocarbon reductant externally to bring about the necessary NOx
conversion extent [27]. This strategy is commonly known as active lean NOx catalytic
system. The schematic representations of both the passive and active schemes of lean
NOx catalytic system are shown in Figure 2.11 and Figure 2.12.
Engine
Lean NOx catalyst
Passive Lean NOx catalytic System
Fig. 2.11 Schematic representation of a passive lean NOx catalytic system [1]
Fual
Tank
1
Injection
I Controller
Lean NOx catalyst
Active Lean NOx catalytic System
Fig. 2.12 Schematic representation of an active lean NOx catalytic system [1]
PALLAVKAR 41
Having advantages of its own, the hydrocarbon based selective catalytic reduction has
its own share of demerits. For instance, in the presence of water vapor, the zeolite-based
catalytic systems have limited hydrothermal stability and are readily poisoned by SO2
[12, 28, 29]. With platinum as the catalyst, there is a detrimental potential of converting
NOx to N2O rather than N2 and once formed, it is not readily decomposed in the
temperature range of the diesel engine exhaust (200 - 300 °C) [29, 30].
2.3.6.3 NOx Storage and Reduction Catalyst
The NOx storage and reduction catalytic system, also known as the Lean NOx Trap
(LNT) is perhaps the most extensively researched NOx reduction system which
incorporates NOx trapping materials in the catalyst washcoat, which adsorbs nitrogen
oxides. The simplest way to describe this LNT is a chemical reaction scheme where NOx
from the engine exhaust is trapped in the trapping material as nitrates and eventually the
same nitrates are reduced to N2 on the catalyst. The adsorber requires frequent, short
duration spikes of rich fuel-to-air mixture for regeneration of the stored NOx, which is
catalytically reduced to nitrogen under the rich exhaust conditions. NOx adsorbers were
demonstrated to have as high as 90% NOx reduction potential in par with the ammonia
based selective catalytic reduction [3, 31, 32]. A typical mechanistic overview for NOx
storage and reduction process is shown in Figure 2.13.
PALLAVKAR 42
mzzzM
m
R: Reducing agents
Fig. 2.13 NOx storage and reduction mechanism for NOx adsorber
While the engine is running under lean operating conditions, the NO fraction of NOx
in the engine exhaust is oxidized to form NO2 on an oxidizing catalyst such as platinum.
NO2 thus formed is then trapped on the storage material such as BaO, in the form of
nitrite or nitrate. The choice of the NOx storage material is based on it's basicity with
high basicity implying larger quantity of NOx stored [31 - 39]. The next step is to inject a
reductant externally or operate engine so as to obtain rich operating conditions. During
the regeneration phase of the cycle, the reductants first react with the barium nitrites and
nitrates, thereby returning the barium to its initial form and secondly, react with oxygen
present in the exhaust to create a temporary fuel rich environment, freeing up sites on the
platinum for NOx reduction to occur. A typical cycle would include a lean period of
about 60 seconds and a rich phase of about 10 seconds [31]. NOx adsorber exhibits high
NOx reduction potential in the presence of large amount of oxygen which is the most
important advantage of employing the technique for NOx reduction. However, the
technology is prone to sulfur poisoning and studies have shown that its NOx storage and
reducing capacity deteriorates drastically in the presence of high amount of SO2 in the
engine exhaust [33, 35, 39]. This is because barium posses a high affinity to store sulfur
PALLAVKAR 43
as sulfates which in-turn are thermally more stable than nitrates. SO2 present in the
engine exhaust is oxidized to SO3 by the noble metal catalyst such as platinum and reacts
with the NOx storing compounds forming a sulfate on the trapping material. Sulfate
species thus formed cannot be desorbed during the rich periodic pluses where the
temperature of the system remains as low as 500 °C [36]. The most conducive conditions
for desulfurization of the NOx trapping materials is to obtain temperatures as high as 700
°C, combined with rich fuel/air mixture which is difficult in the case of the diesel engine
exhaust [37]. This is because sulfates are thermally more stable than nitrates. Thus this
requires regenerating the trap material from the sulfates by inducing rich operating
conditions for the diesel engine exhaust to elevate temperatures for periods of several
minutes, which essentially means more fuel penalty [40]. Also, the catalyst material itself
should withstand such high temperatures of operation. Other NOx trapping materials such
as strontium (Pt/SrO/A^Os) have shown better sulfur regeneration behavior than barium
based trapping materials due to the fact that strontium sulfates show lower stability than
barium sulfates; however both in strontium and barium, NOx reduction reactions are
suppressed by sulfate formation. NOx trapping materials have found to form mix oxides
such as BaAl204 at elevated temperatures which then lower the storage capacity of the
trapping material. These mix oxides can then be regenerated back to their original form
only when subjected at the atmosphere of H2O/NO2 to regain their NOx storage activity
[41 ]. By trapping the NOx material and releasing them when the engine operates at rich
conditions, NOx can be effectively reduced to N2 and O2 with reducing agent such as H2
or CO and more effectively by both H2 and CO [40]. However, excess H2 may lead to the
PALLAVKAR 44
formation ofNH3 [31, 34, 41 - 45] due to the reaction between N and H species which
itself is a secondary pollutant.
2.4 VOC Destruction
Volatile organic compounds (VOCs) emitting from various industrial operations and
automobiles are organic chemical species that readily volatilize in ambient air with many
of them having great potential to pose serious long-term health and environmental
impacts. Studies in the past have shown that prolonged exposure to VOCs such as toluene
and /^-xylene may affect central nervous functions and induce reproductive and
developmental toxicity [46, 47]. Toluene in particular is recognized for its neurotoxicity
effect on liver, heart, and kidney as well [47]. Additionally, apart from being potential
health hazards, many of these VOCs are photochemically active and, along with oxides
of nitrogen and in the presence of sunlight, they have the potential to form ground level
ozone which is a well-known secondary air pollutant [48]. An urgent need, therefore, is
to develop effective VOC treatment technologies to control VOC emissions to reduce
potential health and environmental problems caused by these VOCs [48, 49].
2.4.1 VOC Emissions Control
Control of VOC emissions from various industrial operations at an acceptable level
and with minimum energy usage constitutes a challenging task for the industry
[50] .Various VOC control devices, such as chilled water/refrigerated brine condensers,
carbon/zeolite/polymer adsorbers, and membrane separation systems, are employed in
many industrial applications to recover VOCs and achieve low VOC emissions [51 - 53].
PALLAVKAR 45
Other available VOC control devices also include thermal oxidizers, catalytic oxidizers,
flares, and plasma/electron beam devices, where the first three involve the use of thermal
energy generated from fossil fuel to destroy VOCs [54 - 59]. Among them, thermal
oxidizers are designed to treat waste streams with VOC concentrations ranging from 100
to 2000 ppm and are considered to have the broadest VOC control applicability with a
high destruction and removal efficiency (DRE) ranging from 95 to 99% [60]. The
catalytic oxidizers are generally more energy efficient than the thermal oxidizers.
However, they are more restricted in the application ranges and require additional
maintenance for reliable operations.
With the emerging emphasis on using green technologies to minimize greenhouse gas
emissions,[61] the employment of thermal oxidizers and catalytic oxidizers for VOC
destruction with thermal energy generated from fossil fuels is becoming undesirable due
to its potential generation of additional greenhouse gas, e.g., carbon dioxide. Instead, the
use of microwave energy to achieve the control with its electric power coming from
nongreenhouse-related energy sources, such as wind, geothermal, solar, or even nuclear
energy, becomes an attractive option [50, 62 - 73]. Recent studies pertaining to VOC
control technologies using activated carbon fiber cloths [62, 63] have shown desirable
results in promoting microwave-assisted regeneration of these spent VOC adsorbents.
These studies have indicated that microwave heating has an advantage over conventional
heating as it allows selective heating depending on the dielectric properties of the
substrate. It is worth pointing out that microwave heating is volumetric heating [62] with
all of the infinitesimal volume elements within the object getting heated simultaneously,
which is in contrast to surface heating such as from a hot gas stream where the direction
PALLAVKAR 46
of the heat flux was from the substrate surface inward. However, because of the
absorption of microwave energy at the outer surfaces of the absorbing medium, the
microwave field strength may be substantially reduced in the interior depending on the
physical properties of the medium.
2.5 Microwave Heating Technology
Microwave energy has unique heating properties for dielectric compounds and which
may be used as heating source for catalytic reactions, which require high temperature of
operation, for instance, destruction of NOx [63, 64]. Microwave energy interacts with
matter in a way different than all other thermal treatment processes [64]. The main
advantage is energy saving for low flow rate or low concentrated NOx stream since no
supplement fuels are required for this technique [4 - 8],
2.5.1 Microwave Radiation
Microwave radiation is an energy composed of an electric field and a magnetic field.
It is a form of electromagnetic radiation, with the frequencies ranging from 100
MHz to 300 GHz shown in Figure 2.14. Industrial heating application uses
microwave with 2450MHz. The Federal Communications Commission (FCC) has
reserved 915 MHz and 2450 MHz, among other frequencies, for industrial applications.
2450 MHz is a wavelength of 4.8" in air where as 915 MHz is about 13" in air.
Microwave power is usually measured in kilowatts. At room temperature, and 1
atmosphere pressure, 1 kilowatt of microwave energy will evaporate approximately 2.5
pounds of water in 1 hour.
PALLAVKAR 47
Frequency. H?
10*
10®
. !lll; III i
A-c power i
3X10'
1012
10»
'Radio,, jMicrowave
I !lll HrvH !
3 X 104
30
Wavelength,
10 ,!
Infra l mi
red {Visible
O.
3 x 10"'
10"
X rays
_1_JL
3 x 10"*
cm
Fig. 2.14 Electromagnetic radiation [74]
The electric field interacts with polar materials and the magnetic field reacts with
magnetic or charged materials. Polar molecules and free ions in receptive materials
respond to these fields by creating a molecular friction, which results in heat throughout
the mass of the material. Hence microwave heating is also known as volume heating.
2.5.2 Classification of Materials
There are three classes of materials in a microwave radiation processing system:
•
Conductors
•
Insulators
•
Dielectrics
Most good conductors are metallic materials such as copper, brass, aluminum, silver, etc.
These conductors reflect microwaves except the ones with sharp edges. This reflective
property is used to contain and direct microwaves. For example, the wave-guide of a
microwave oven is usually made of brass or aluminum. Insulators either reflect or
transmit microwaves and generally absorb only a little amount of microwave energy.
Teflon, polypropylene, etc. play the insulator part in a microwave system. Dielectrics
PALLAVKAR 48
have properties in between conductors and insulators. In the microwave radiation field,
dielectrics materials absorb microwave energy in varying degrees. Properties of materials
determine the absorption of microwave radiation energy. It is known that the dielectric
constant of a material plays an important role. The dielectric constant in-turn depends on
moisture content, temperature, and geometric factors.
2.5.3 Applications of Microwave Technology
Microwave energy represents a unique heating source with the potential of providing a
highly flexible means for (a) minimizing generation of selected future wastes, (b)
reducing existing wastes and hazardous components, and (c) reclaiming or recycling
reusable and sometimes valuable components in waste products [65]. Microwave
treatments include remediation of discarded electronic circuitry and reclamation of the
precious metals within, incinerator ashes, medical and infectious wastes, industrial
wastes/sludge, rubber products including tires, asbestos, groundwater, volatile organic
compounds (VOCs), shipboard wastes, contaminated soils and sediments, and radioactive
wastes and sludge (high, low and intermediate level wastes, and mixed wastes). The
demonstrated applications of microwave in environmental applications are
discussed as follows:
•
Soil Vapor Remediation: This system captures and recovers VOCs. Overall, the
process uses granular activated carbon (GAC) to remove the VOCs from the air
stream, continuously regenerates the used GAC with microwaves, and recovers the
VOCs, desorbed from the GAC, by condensation. This project was funded by an
SBIR granted by the National Institute of Environmental Health of NIH. The process
PALLAVKAR 49
has been demonstrated at McClellan Air Force Base in 2003. The system can be used
to recover vapor discharged from Soil Vapor Extraction (SVE) treatment. It can also
recover vapor discharged from dry cleaners, paint booths and recover vapor
discharged from gasoline loading stations.
•
Microwave Destruction of Waste Rocket Fuels: This system was developed to
eliminate waste hydrazine-based rocket fuel and waste oxidizer, separately. In rocket
propulsion, fuel and oxidizer are combined to achieve self-ignition. The hydrazine is
passed through a microwave carbon-bed reactor and then through a microwave
oxidizing catalyst reactor. There is no adsorption and all the contaminants are
destroyed. This project is directly funded by the Air Force.
•
Treatment of PCB and Secondary Gases: A wide array of electronic components
was successfully treated by a relatively simple and flexible, one-step, hybrid-heated
microwave process. Significant volume reductions were achieved in all studies. In
this process, circuitry was completely converted into two waste form products; a glass
and a metal form, both of which can be reused and recycled. The metal form
produced was subsequently processed to effectively reclaim precious and valuable
metals, including gold and silver. The separation of glass and metal waste products
was very effective and efficient. A new "tandem microwave" system successfully
handled both primary as well as secondary wastes in an integrated unit, i.e., the offgases produced were simultaneous treated along with the electronic circuitry. The
entire hybrid microwave process and specialized equipment (now being patented) can
also be mocked-up to a larger scale and made mobile, if desired.
PALLAVKAR 50
•
VOC Abatement at Air Force Paint Workshop: The U.S. Department of Air
Force sponsored CHA Corporation under a Phase I and Phase II Small Business
Innovation Research program to investigate a feasibility of using a novel microwavebased process for the removal and destruction of volatile organic compounds in
effluents from non-combustion sources such as paint booth ventilation streams. The
CHA microwave-based gas cleanup process is designed to capture and destroy a wide
variety of both chlorinated and non-chlorinated VOCs, many of which are commonly
found at industrial and military sites. In addition, the CHA process also removes the
pollutants SO2 and NOx from flue gases. The technology has been successfully
demonstrated in March 1998 at McClellan Air Force Base where NOx and VOCs
were removed from diesel engine exhaust gas. This microwave process was
successfully demonstrated on a pilot-scale at McClellan Air force Base.
•
Recycling of Rubber: Microwave energy is used to selectively break S-S and S-C
bonds in the rubber compound, while leaving the backbone of the rubber (C-C bonds)
relatively intact, thus de-vulcanizing, but not de-polymerizing, the rubber to be
recycled. This also produces activated surfaces that when combined with new rubber,
yields a composite with excellent properties. This produces new type of "crumb
rubber" that can be combined with "new rubber" to produce high-quality tires, having
25% or more recycled rubber than obtained previously
•
Treatment of Medical Waste: The tandem microwave system provides a unique
means for treating both the primary and secondary medical waste, and provides a
system that is compact and can be made portable. It has been used successfully, on a
laboratory scale, for destruction of a variety of mixed simulated infectious medical
PALLAVKAR 51
wastes, including plastics, clothing, sharps and other metal components, etc. This
technology has the potential of a) disinfecting b) sterilizing and c) destroying
discarded medical waste products.
2.5.4 Advantages of Microwave Heating
Microwave energy interacts with matter is a way different from all other thermal
treatment processes. As a result of these unique features, the advantages of using
microwave energy for treating a vast array of hazardous wastes can include many
potential advantages. The advantages ultimately realized will depend both on the type
and characteristics of the wastes to be treated [4 - 8, 65]. The major advantages of
microwave heating are listed below.
•
Selective and Rapid heating.
•
Quick startup and shutting.
•
No need of supplement fuel.
•
Ease of control.
•
Overall reduction of treatment costs.
•
Portability of equipment and process.
•
Rapid and flexible process that can also be made remote
•
Ability to treat wastes in-situ.
•
Treatment or immobilization of hazardous components to meet regulatory
requirements of transportation or disposal.
•
Improved safety, including reductions in personnel exposure of potentially hazardous
chemicals or materials for processing and disposition.
PALLAVKAR 52
2.5.5 Safety Considerations
Microwave heating equipment is designed, constructed and operated to provide
adequate protection against radiation hazards due to microwave leakage. All equipment
where accessibility to the applicator by a part of the human body are possible is to be
provided with means of access. Such means of access may be omitted if other types of
protective measures such as doors or barriers are arranged with required interlocking.
The microwave leakage power density shall not exceed a power density of 50 W/m2
(5 mW/cm2) at any accessible location 0.05 m from any portion of the equipment under
conditions designated as "normal operation". In addition, the microwave leakage shall not
exceed a power density of 100 W/m2 at any accessible location 0.05 m from any portion
of the equipment under conditions designated as "abnormal operation". These levels shall
not be exceeded at any point located at distances greater than 0.05 m. For normal
environmental conditions and for incident electromagnetic energy of frequencies from 10
MHz to 100 GHz, the radiation protection guideline is 10 mW/cm2 (milliwatts per square
centimeter) as averaged over any possible 0.1 hour period. This means the following
Power density 10 mW/cm2 for periods of 0.1 hour or more and Energy density: 1 mWh/cm2 (milliwatt-hour per square centimeter) during any 0.1-hour period. This applies to
all radiation organizing from radio stations, radar equipment and other possible sources
of electromagnetic radiation such as used for communication, radio navigation and
industrial and scientific purposes.
PALLAVKAR 53
References
1. Majewski, W.A.; Khair, M.K. Diesel Emissions and Their Control; Society of
Automotive Engineers, International: Warrendale, PA., 2006.
2. Regulatory Announcement; Proposed Heavy-Duty Engine and Vehicle Standards and
Highway Diesel Fuel Sulfur Control Requirements. United States Environmental
Protection Agency. May 2000, EPA420-F-00-022.
3. Brogan, M.S.; Brisley, R.J.; Moore, J.S.; Clark, A.D.; Evaluation ofNOx adsorber
catalysts systems to reduce emissions of lean running gasoline. Soc. Auto. Eng. Intl.
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PALLAVKAR 62
CHAPTER 3
ACTIVE REGENERATION OF DIESEL PARTICULATE FILTER LOADED
WITH DPM EMPLOYING MICROWAVE HEATING
Wall-flow diesel particulate filters are considered the most effective devices for the
control of diesel particulate emissions. A requirement for the reliable operation of the
DPFs, however, is the periodic and/or continuous regeneration of the filters. While
microwave heating has been considered a potential active regeneration method for the
DPFs, past studies on the technology have identified several technical problems leading
to filter failure. The problems are mainly associated with the use of inappropriate filter
materials for the microwave system and the generation of local hotspots due to uneven
microwave heating, resulting in the physical damage to the filters. The objective of this
study was to develop and demonstrate the technology employing a microwave-absorbing
filter material coupled with an effective waveguide design for the reliable regeneration of
DPFs.
In this study, a well-equipped diesel emission control laboratory was established to
conduct the experiments. The experimental facilities included a 6 kW diesel generator, an
exhaust flow control system, a diesel particulate filter system, microwave energy supply
system, a soot sampling system, a differential-pressure measurement system, and a
temperature measurement system. The DPF was a silicone carbide wall-flow monolith
filter enclosed in a quartz filter holder. A commercial 1.4 kW microwave oven was
modified to accommodate the quartz holder and a waveguide was engineered to evenly
supply the microwave energy to the enclosed filter to achieve filter regeneration. In the
PALLAVKAR 63
experiments, the diesel engine exhaust was lined up to flow through the filter with a fixed
flow rate. The microwave regeneration was triggered after a specific amount of soot
loading was reached based on the differential pressure drop reading. The results have
indicated that the designed system has been able to achieve uniform temperature profiles
both in the radial and the vertical DPF positions. The off-line regeneration of DPF by
microwave energy has been observed to be highly efficient in terms of energy
consumption and regeneration efficiency. The DPM filtration efficiency has remained
comparably high after 150 cycles of filtration/regeneration
with no apparent physical
damage to the DPF being observed. The on-line microwave regeneration of the DPF,
however, is not as efficient as the off-line regeneration due to the insufficient oxygen
concentration in the engine exhaust stream.
3.1 Introduction
Diesel engines are superior to conventional gasoline engines with regard to fuel
consumption, which reduces CO2 emissions and helps suppress global warming [1].
These engines, however, produce more NOx as well as particulate matter emissions as
compared to the gasoline engines. In response to the environmental and health concern
from the emission of diesel particulate matter (DPM) [2, 3] the United States
Environmental Protection Agency (US EPA) has continued to impose more stringent
regulations restricting its emissions. The new 2007 standard for on-highway heavy duty
diesel engines sets an emission limit of 0.01 g/bhp-h, which is 10 times more stringent
than the 2005 standard of 0.1 g/bhp-h [4]. Control of DPM emissions from diesel engines
has been a challenging issue. Wall-flow diesel particulate filters (DPFs) are currently
PALLAVKAR 64
considered the most efficient control devices for DPM [5], The filtration mechanism of
the DPFs involves forcing the DPM containing exhaust gases to flow through the porous
walls as they enter the DPF channels from the front end, trapping the DPM (or soot) in
the porous walls or depositing it on the wall surfaces, and exiting out as cleaned exhaust
gases through the open channels at the back end [6], However, with the accumulation of
the DPM in the wall media, the pressure drop across the filter increases which in-turn
exerts a back pressure on the engine exhaust, resulting in gradually poorer performance
of the diesel engine in terms of increased CO emissions, poor fuel efficiency, and
increased DPM production. Therefore, a requirement for reliable operation of DPFs is the
effective regeneration of the filters by continuously or periodically burning off the
trapped DPM [7-10]. Two methods have been developed to achieve DPF regeneration
[11]. One is the passive regeneration which requires the use of oxidation catalysts
upstream from the DPF to convert NO to NO2 and uses the converted NO2 to oxidize the
trapped DPM in the DPF in the presence of catalysts [12, 13]. Since the DPM oxidation
reaction can occur below 270 °C with the presence of the catalysts, no additional energy
is needed to achieve the regeneration. However, the method is ineffective under high load
conditions where the oxygen content in the exhaust is not sufficient enough for sustained
DPM combustion [14]. The other DPF regeneration method, termed active regeneration,
involves the supply of additional energy to burn off the trapped DPM at a temperature
higher than 450 °C [15-17]. Since the diesel exhaust system cannot generate such a high
temperature continuously, additional energy must be used to raise the DPF temperature to
accomplish the burning. The energy may come from additional fuel combustion, electric
furnaces, electric heating elements, or microwave irradiation. Although effective, two
PALLAVKAR 65
concerns are associated with active regeneration, one is the consumption of additional
fuel and the other is the potential damage to the DPFs due to high thermal gradients
induced by rapid DPM oxidation. Additional fuel combustion mainly involves the
hydrocarbon source such as the diesel fuel itself which is mixed with the atomized air
from a pressure reservoir of the vehicle, introduced in a retrofitted combustion chamber,
and ignited by means of ignition electrodes [18]. The key feature of this technology is the
development of DPM-free flame irrespective of the engine operating conditions, which
involves compressed air as the source of oxygen for clean combustion in the burner.
However, this method essentially results in an added fuel penalty and increases the
complexity of the system in terms of the engine control unit, retrofitting, and sensor
management [19]. The electric regeneration method involves the heating of the exhaust
gases or the regeneration air by a resistive heater to reach DPM ignition temperatures and
trigger the regeneration. The heater is placed upstream to the DPF where the energy from
the heating element is deposited to the exhaust stream thus elevating its temperature to
the DPM ignition temperatures. For automotive applications applying this strategy on­
board means additional power consumption for heating the electric resistive element
which in-turn has to be drawn from the battery of the automobile. This puts extra load on
the engine and hence results in a relative higher fuel penalty [19]. Microwave heating is
very selective and can be an efficient energy source in many applications [20]. With the
choice of proper material as the filter substrate, it can easily provide high temperatures
needed for DPM combustion and DPF regeneration [21 - 23]. The extent of microwave
energy absorption by the filter material depends on its dielectric properties (especially the
dielectric loss factor). Table 3.1 illustrates the various dielectric properties of some of the
PALLAVKAR 66
common materials. With high dielectric constant and loss factor such as SiC, the material
can readily absorb the microwave energy and convert it into heat energy.
As indicated in Table 2.2 (Chapter 2), an additional advantage of using microwaveassisted DPF regeneration is its ability to deposit energy directly to the DPM (or soot)
which is collected in the DPF. With the SiC being the filter material, the microwave
energy can heat up both the DPM and the SiC DPF and trigger the regeneration process
without the requirement of heating up the exhaust gases. Microwave-assisted DPF
regeneration, however, has been experienced with uneven energy distribution and
regeneration patterns, which results in hotspots during the exothermic DPM oxidation
reactions [24].
3.2 Objective
The objective of this study was to carry out experiments to demonstrate the use of
microwave heating for DPF regeneration involving SiC as the filter material for its
excellent dielectric properties in converting microwave energy to heat energy. A metallic
waveguide was designed to ensure uniform distribution of the microwave energy
throughout the volume of the DPF.
Specifically, the objectives of this study were to conduct experiments to (1) characterize
the designed waveguide for the efficient use and uniform distribution of the supplied
microwave energy, and (2) demonstrate the efficiency of the microwave DPF
regeneration process involving SiC as the filter material.
PALLAVKAR 67
3.3 Experimental Details
A diesel emission test unit equipped with a diesel generator, an exhaust flow system, a
SiC DPF, a DPM sampling system, a differential pressure measurement system, a
temperature measurement system, and an on-line data acquisition system was established
to conduct the experiments. A 1.4-kW commercial microwave oven was modified and
used as the microwave generator. A schematic diagram of the experimental facilities is
shown in Figure 3.1.
M
a-io
S-7
S-1
S-2
PT - Pressure transmitter
TT - Temperature transmitter
FT - Flow transmitter
OP - Pressure drop transmitter
AX - Emissions analyzer
FC - Mass flow controller
V1.V2.V3.V4-Valves
S1- Diesel fuel
S2 - Air
S3 - Regeneration air
S4 - Engine exhaust
89 - Exhaust by pass line
S6 - Exhaust routed to the DPF
37 - Exhaust leaving the DPF
St - Exhaust routed to the PM sampler
39, S10 - Exhaust vent
Fig. 3.1 Schematic diagram of the diesel test facilities
PALLAVKAR 68
3.3.1 Diesel Generator and Exhaust Flow System
A 6 kW generator driven by a single-cycle diesel engine (Lombardi model 15LD 400)
was used in the experiments. The engine exhaust pipe was connected to the DPF through
a flow control valve (Badger Meter) and a flow meter (Asea Brown Boveri). A picture
showing the generator and the flow control system is shown in Figure 3.2. The system
allowed various resistive loads in terms of back pressure and various generator loads. The
default generator load was 36.4 A, which was equivalent to 60% of generator capacity.
During the regeneration experiments, the engine exhaust was routed through the DPF
until the pressure drop across the filter reached a preset critical value for regeneration.
The microwave was then turned on for on-line regeneration.
Prttaur* transmitter
DPM
AAaMflklhhM
MII jMing
pump
Dlaaal angina
Exhaust flow
Control vtlvt
Fig. 3.2 Assembled microwave diesel emission test unit
PALLAVKAR 69
3.3.2 Diesel Particulate Filter and Microwave
A silicon carbide (SiC, Ceramic Techniques et Industrielles) wall-flow monolith filter
(50 mm diameter x 150 mm length, cell Density =150 cpsi, pore size = 20 jum) was
enclosed in a custom-made quartz holder (Technical Glass Products), which was
insulated and sealed by using Fiberfax alumina blanket and Interam mat (3M). Some of
the important properties of the SiC filter used in our experiments are listed in Table 3.1.
A schematic diagram of the DPF filter/holder assembly is shown in Figure 3.3. Two high
temperature flange gaskets were also used as a seal between the filter element and the
holder. The microwave oven employed was a 1.4 kW Sharp consumer microwave oven.
It was modified with holes drilled in its top and bottom surfaces to accommodate the
filter assembly. A waveguide was designed and installed in the oven to effectively direct
microwave energy to the SiC filter. Figure 3.4 shows a picture of the microwave oven
with the waveguide assembly in it. A schematic diagram of the waveguide assembly is
shown in Figure 3.5.
Table 3.1 DPF Filter Specifications
Properties
Material
CPSI coding
Cell size, wall width (mm)
Wall thickness (mm/1000/inch)
Filtration efficiency (%) clean filter - PM10
Filtration efficiency (%) 10% loaded filter
Specific SiC weight, massive material density (kg/dm3)
Specific DPF weight, porous wall density (kg/dmJ)
Monolith weight, bulk density (kg/dm3)
Value
100% SiC
150
1.6 x 1.6
12-15
>98
>99
3.2
1.8
0.85
PALLAVKAR 70
Diesel Engine Exhaust - Out
•
Thermocoupte
\
n
High Temparatur* Gasket
Ctramlc Wool I Interim Mat
Qfasal Particulate Filter(SIC)
Faraday Screen
§
Diaaal Engine Exhaust - In
Fig. 3.3 Schematic diagram of the quartz filter holder
PALLAVKAR 71
For off-line regeneration, the engine exhaust to the DPF was switched to an air stream
and the microwave was turned on for DPM oxidation. In such regeneration, the engine
exhaust was routed to bypass the DPF and vent out through the exhaust hood. The diesel
fuel (Citgo Clear No. 2) for the experiments was purchased in bulk from a local supplier
to ensure the consistency of fuel quality.
Wawflufd*
Fig. 3.4 Installed waveguide in the microwave oven
PALLAVKAR 72
RkUkMb catty
(Son 110cm)
18 an
Lateral view
10cai
MWNracttan
1*4 an
18.San
Fig. 3.5 Schematic diagram of the waveguide assembly
3.4 DPF Temperature Measurement
The DPF filter was fitted with three thermocouples composed of three thin gauge
thermocouple wires (J-type with 0.8 mm in diameter) with the wire leads placed inside
the SiC channels both in the vertical and radial directions. For the vertical temperature
profile, the three thermocouple leads were placed at depths of 50, 75, and 100 mm,
respectively, at the center of the filter. In the case of radial temperature profile, the three
leads were placed at three radial locations at a fixed depth of 75 mm.
PALLAVKAR 73
3.5 DPM Sampling
The DPM sampling system consisted of sample selector valves, a sample holder (SKC
model LS-47), and a sampling pump (SKC model Hi Lite 30) with a rotameter to
maintain a sample flow rate of 30 cm3/min. The valves and the pump were integratedcontrolled by the switches located on the control panel to ensure the right sampling
sequence. The sample lines and the sample holder were wrapped with electric heating
tapes to maintain temperature at about 90 °C to avoid the condensation of water
in the lines and the PM holder. After collecting PM for a preset time, the sample holder
was allowed to dry in a desiccator for 24 h and the filter paper was weighed by a
micro balance.
3.6. DPF Regeneration Experiments
The facilities described in Figure 3.1 were involved in the DPF regeneration
experiments, including both off-line and on-line regeneration. In a regeneration
experiment, the diesel engine was turned on and the DPF was loaded with DPM under
constant exhaust flow rate conditions, e.g., 83.3 L/min. The DPF differential pressure
drop was continuously recorded during the loading process. The regeneration process was
triggered when the recorded pressure drop reached a preset critical value, for example, 50
in. of water (12.451 kPa).
In the case of on-line regeneration, the microwave oven was turned on at this time and
the regeneration was initialized while the DPM loading on the DPF was continued during
the regeneration process. The microwave was then turned off when the differential
pressure of the DPF returned to the clean status. In the case of off-line regeneration, when
PALLAVKAR 74
the differential pressure drop reached the preset regeneration pressure, the microwave
oven was turned on and the engine exhaust gas was switched to a pure air stream at a
smaller flow rate (10 L/min) for DPM oxidation.
In this off-line regeneration, the engine exhaust stream was routed to bypass the DPF
and vent to the hood without being treated. The microwave was turned off after the
regeneration was completed, and the air stream was switched back to the engine exhaust
for another cycle of DPM loading. The experiment may continue for many filtrationregeneration cycles depending on the design of the experiment. For a typical set of
design, the experiment involved 10 filtration/regeneration
cycles. An emissions analyzer
(Testo 350-XL) was used to measure simultaneously the concentrations of O2, NOx, and
CO during the regeneration process. In addition, on-line CO2 measurements were taken
with a CEA Instruments GD444 portable analyzer. Both single and multiple cycles of
filtration and regeneration data were logged every second using a data acquisition system
designed for the experiment (N.I. LabVIEW).
3.7 Results and Discussion
The experimental results are reported in this section, which include DPF Temperature
Profile, Characteristics of DPM Loading, OfF-Line DPF Regeneration, and On-Line DPF
Regeneration.
PALLAVKAR 75
3.7.1 DPF Temperature Profile
The temperature profiles along the radial and vertical directions of the DPF were
measured when the microwave system was turned on with and without the designed
waveguide. Typical results are shown in Figures 3.6 - 3.8.
600
1 - Front «nd thwmoeoupto
2 - Cantor thcrmocoupls
3 — Rear end thermocouple
300
o 400
a
0
| 300
a
a
E
£ 200
MW
100
0
0
2
4 ft
8
10
12
U
Tiim (min}
Fig. 3.6 DPF temperature profiles in different radial directions (without a waveguide)
The results shown in Figure 3.6 indicate that SiC is a good conducting material capable
of absorbing microwave energy and distributing the absorbed energy efficiently across
the entire DPF body as observed in its relatively uniform temperature profiles at three
radial locations.
PALLAVKAR 76
890
710
MO
1 - Front and thermocouple
2 - Center thermocouple
3 - Rear end thermocouple
4*0
3M
2M
MW
110
6
Tine (milt)
I
1®
12
14
Fig. 3.7 DPF temperature profiles in different radial directions (with a waveguide)
With the installation of the designed waveguide, the results shown in Figures 3.7 and 3.8
strongly indicate that the recorded temperatures are substantially higher than those
without the waveguide as shown in Figure 3.6. It is obvious that the designed waveguide
has achieved its purpose by raising the energy efficiency by at least 30% while the
temperatures across the entire DPF body remain relatively uniform.
PALLAVKAR 77
900
800
1 - Top tfMimocoupia
2 -ftMddta theonoeoupto
3 - Bottom MwwaflaHi
700
>
1
3
3
MW
100
/
0 £
0
2
4
€
9
10
12
14
Trrw (min)
Fig. 3.8 DPF temperature profiles in different vertical locations (with a waveguide)
3.7.2 DPF Particulate Loading
The loading of DPM in the DPF is a three-stage dynamic process as shown in Figure
3.9. The first stage (region 1) is the initial loading phase during which the DPM starts to
fill the voids inside the DPF filtration walls, resulting in a fast increase of differential
pressure drop across the DPF. Here, the fast-rising of the pressure drop is mainly caused
by the filter wall permeability and porosity [6]. Particulates disperse deep inside the
PALLAVKAR 78
filter-wall pores and form pore bridges thus causing a significant decrease in filter-wall
porosity and increase in the pressure drop.
ao
A - Mi* ctaga afDPM loading (fefian 1)
• - TruMltlanal mm> aTOm l»4lita (R*0«n 1)
C - Third (tag* of DPM lorttoa
3}
SO
90
20
0
6
10
16
20
26
30
Tim*
Fig. 3.9 DPF pressure drop vs. time in three consecutive stages
(engine exhaust flow rate: 83.3 L/min)
During this stage, filter properties such as permeability and porosity change
dynamically. With the wall filtration approaching its saturation state, the wall porosity
and permeability in turn approach their saturated values and the filtration process starts to
develop to the next stage where the DPM layer is forming on the filter-wall surfaces. This
second stage is termed the transitional DPM layer formation stage where the DPM
permeability is changing with time slowly and the DPM layer starts to form on the wall
surfaces (see region 2 in Figure 3.9) [6, 8]. Once a critical mass of DPM has accumulated
on the wall surfaces of the filter channels, a definite particulate layer of DPM starts to
build up and this itself acts as a filter (constant DPM layer filtration). At this stage (see
PALLAVKAR 79
region 3 in Figure 3.9), the filter-wall permeability and porosity have reached their
saturation values and the filter is in a steady-state filtration stage where the DPM layer
continues to grow with a relatively constant density.
For the first two stages of DPF filtration, the following equation was proposed to describe
the pressure drop across the DPF [6].
APW= (|iUHww/4Lkw) + (2nFUL/3H2)
(1)
In the above equation, the first term calculates the pressure drop due to the porous
filter wall, which depends on the wall layer permeability (kw), filter wall thickness (*vw),
cell entrance velocity (U), and cell dimensions, and the second term calculates the
pressure drop due to the frictional losses of the flow of the exhaust gases through the
filter channels. This correlation implies that the filter pressure drop is a function of the
physical properties of the filter as well as the properties of the engine exhaust gas such as
its viscosity, temperature and flow rate. Given the measured pressure drop, the above
equation can be used to calculate the wall layer permeability, that is, kw, during the two
filtration stages. The corresponding filter-wall porosity can be calculated by using the
following empirical correlation in the literature [6,7].
Mt))[e(t)55Dp2]/5.6
(2)
where the wall layer permeability, kw, was empirically proposed to be a function of the
wall layer porosity (e) with respect to time and the pore diameter, Dp.
PALLAVKAR 80
S3"
J,
1.6C-13 1
1.4E-13
J
£
1.2E-13
3
16-13
-e-Cycle 1
-•-Cycle 2
8E-14
&
1
I
-*-Cycto3
6C-14
-»-Cycki4
4C-14
2E-14
"fgefihgniimL
0
10
15
Time Imin)
20
25
30
Fig. 3.10 Wall layer permeability during DPM loading at four filtration/regeneration
cycles corresponding to the differential pressure drop results shown in Figure 3.11
(exhaust flow: 83.3 L/min)
Figure 3.10 shows the variation of filter-wall permeability for multiple cycles of DPF
loading experiments corresponding to the results of the measured pressure drop shown in
Figure 3.11. These permeability values were calculated based on equation 1 described
above and the constants used in the calculations are summarized in Table 3.2.
PALLAVKAR 81
CO —Ttmptfatum — Flnr prMturadrap
e
o
M
Fa
o
CL
<
d * la
e
e
S« !«
Tim (min)
Fig. 3.11 Plot of CO concentration/temperature/pressure drop vs. time for an off-line
four-cycle filtration/regeneration operation (exhaust flow: 83.3 L/min).
Table 3.2 Constants usedfor Equations 1 and 2 Calculations
Constant
D
Dp
F
H
L
U
Ww
Description
DPF diameter
Wall pore diameter
Friction factor
Cell width
Channel length
Inlet cell entrance velocity
DPF wall thickness
Exhaust gas viscosity
Value
0.05 m
2.0 x 10"5m
14.23
1.6 x 10*3m
0.15 m
2.377 m/s
0.305 x 10"3 m
2.95 x 10'8 kPa-s
PALLAVKAR 82
The calculated wall permeability values shown in Figure 3.10 indicate that the
permeability reduces by 94% during the initial stage (region 1) of DPM loading and
reaches its saturation value during the second stage (region 2) of loading. It remains
constant during the third stage (region 3) of loading.
The corresponding values of filter-wall porosity calculated on the basis of equation 2
are shown in Figure 3.12.
0.3S
0.3
0.25
II
0.2
O.OS
0
9
10
15
2S
Tim© <mln)
Fig. 3.12 Wall porosity during DPM loading at four filtration/regeneration
cycles corresponding to the differential pressure drop results shown in Figure
3.9 (exhaust flow: 83.3 L/min)
PALLAVKAR 83
The results indicate that the wall porosity drops from 0.33 to 0.22 during the initial
stage of DPM loading and reaches a saturated value of about 0.18 during the filtration
process. It is worth pointing out that, in the third stage of the DPM loading, the pressure
drop across the filter increases linearly with the DPM layer thickness, which is a function
of time. The following equations were proposed to describe the pressure drop as a
function of the DPM layer thickness (wp) for this stage [6].
APS = OUHwp/4Lkp)
(3)
wp = M(t)/(Af ps)
(4)
It should be noted that, during this stage, the DPM layer permeability (k p ) does not vary
appreciably with time and the DPF pressure drop depends mainly on the DPM layer
density (ps), DPM layer thickness (wp), mass of DPM trapped in the filter (M), and the
filtration area (Aj). In this stage, the DPF is considered to have reached its steady
filtration operation, and it is predominantly the stage associated with DPF regeneration. It
should be noted that the total pressure drop across the DPF (APT) at any given time is the
sum of the clean filter pressure drop (APC), pressure drop due to wall flow filtration
(AP„), and pressure drop due to DPM layer filtration (APS) given below:
PALLAVKAR 84
APt - APW+APS+APC
(5)
where APC depends on the clean filter permeability, wall layer pore diameter, cell density,
and channel length. It can be measured experimentally.
3.7.3 Off-Line DPF Regeneration
As described in the experimental section, the experiments for microwave DPF
regeneration involved both off-line and on-line operations. In the offline regeneration,
when the pressure drop across the DPF reached a predesignated value, the engine exhaust
stream was switched to bypass the DPF and replaced with an air stream for DPM
oxidation, while the microwave oven was simultaneously turned on to heat up the DPF
and the trapped DPM. The two streams were switched back after the regeneration was
completed. In the on-line regeneration, when the pressure drop across the DPF reached a
predesignated value, the microwave oven was turned on and both the regeneration and
the filtration were simultaneously occurring in the DPF. The microwave oven was turned
off after the regeneration was completed. Typical experimental observations
corresponding to the off-line regeneration are reported below. Figure 3.13 shows a typical
set of results for an off-line microwave regeneration of a cold DPM-loaded DPF in an air
stream.
The figure shows profiles of five measured parameters, namely, the concentrations of
O2, NOx, and CO, temperature, and pressure drop plotted against operation time after the
microwave was turned on. The recorded temperature profile indicates that, because of the
PALLAVKAR 85
microwave energy, the DPF temperature rises from the initial temperature of 25 °C to
about 350 °C where a CO peak appears indicating the onset of the DPM oxidation
process. Although it is not shown, the CO2 peak follows the CO peak when the
temperature is higher. Also shown in the figure is the formation of NOx after the CO
peak due to an increase in temperature. The temperature is seen to continue to climb up to
about 660 °C after 10 min of the regeneration process mainly because of the heat of
02
NOx
Twnpwihiw
Filter piMwir* drop
o
x5
If
a*
U
s
a
1
s
m
u
V
Z 5 Si
o* u
e
o
01 M
°i S3
4
i
«
Tim* (min)
Fig. 3.13 Plot of concentration of 02/N0x/C0/temperature/pressure drop vs time for an
off-line single-cycle filtration/regeneration
operation (exhaust flow:
83.3 L/min)
combustion due to the DPM oxidation as well as the continuous addition of microwave
energy. It then cools down after the combustion is completed and the microwave turned
PALLAVKAR 86
off. In the mean time, the pressure curve indicates that it increases slightly at the
beginning of the microwave heating process due to the increase in temperature.
The pressure continues to rise with the temperature until after it passes the onset of
DPM oxidation, where it reaches its peak and then starts to decrease. The decrease is
mainly because of the combustion of solid diesel particulates into gaseous CO and CO2,
while in the process, opens up the blocked DPM pores and results in the lower pressure
drop. The pressure curve is seen to drop down to the initial pressure reading after the
regeneration is completed.
The off-line regeneration experiments associated with multicycle
filtration/regeneration operations were also performed. A typical set of such results are
shown in Figures 3.11 (referred previously) and 3.14.
I
02
NO*
CO
i
fl
*n
:§
—
~1
[I
i
!'
og
o • i*I X
I
-V
v
[
\
1
V
k
*-
-f-
S
»
J—
a
a
a
M
a
I
•si
Ttn*(mln)
Fig. 3.14 Plot of concentration of O2/CO/NOX vs. time for an off-line four-cycle
filtration/regeneration
operation (exhaust flow: 83.3 L/min)
PALLAVKAR 87
The results shown in Figure 3.11 indicate that the differential pressure across the DPF
increases during the filtration process where the temperature is around 260-280 °C and no
CO is observed. When the differential pressure reaches the designated readings (64 in. of
water or 15.937 kPa for the first regeneration and 50 in. or 12.451 kPa thereafter as
indicated in the figure), the exhaust gas stream was switched off and the air stream
switched on with the observed pressure immediately dropped down to about 4 in. of
water (0.996 kPa) mainly because of the much slower air flow rate, 10 L/min for the air
stream vs. 83.3 L/min for the exhaust gas stream. At these moments, for example, at 33,
75, 128, and 130 min, the microwave oven was simultaneously turned on and the
regeneration operation was initialized as shown in Figure 3.11. The temperature and CO
peaks are observed in the figure similar to those shown in Figure 3.13 and the pressure
reading is seen to reduce to about 2 in. of water (or 0.498 kPa) after the regeneration is
completed within about 5 min. The engine exhaust stream was then switched back on,
and the filtration process for the next cycle was started. It has been observed that the
DPM filtration efficiency has remained to be comparably high (>90%) after 150
multicycles of filtration/regeneration
operations and no apparent physical damages to the
DPF have been observed. It should be noted that the corresponding experiment results
shown in Figure 3.14 indicate that the NOx peaks are also observed, similar to that shown
in Figure 3.13 during the single-cycle experiment. It is worth pointing out that, during the
multicycle off-line regeneration, the time required for microwave heating for completing
a regeneration cycle is only 5 min because of the DPF being at a relatively high
temperature already heated by the engine exhaust gas.
PALLAVKAR 88
An attempt was made to compare the microwave regeneration processes with 5 and 10
min of microwave heating and the results indicated that they are almost identical as
shown in Figures 3.15 and 3.16.
0
s
€L "
<
r
O
fj
*
§! St
M
o
o* o
1
&
l2
i. .
•
t
1
J
4
5
Fig. 3.15 Plot of concentration of Ch/NOx/CO/temperature/pressure drop vs. time for one
regeneration cycle of an off-line multicycle filtration/regeneration
operation with 5-min microwave heating (exhaust flow: 83.3 L/min)
The observation appears to suggest that the microwave energy is only needed to heat up
the DPM to the ignition temperature, and after that, the released heat from the DPM
combustion will support the continuous burning of the remaining DPM without the need
for additional microwave energy. This observation is significant in the efficient use of
microwave energy for DPF regeneration.
PALLAVKAR 89
OJ
NOx
CO
_ mm praam drop
*
«
o
*
*
I
2
4
Tkv»(min)
I
I
II
Fig. 3.16 Plot of concentration of C^/NOx/CO/temperature/pressure drop vs time for one
regeneration cycle of an off-line multicycle filtration/regeneration
operation with 10-min microwave heating (exhaust flow: 83.3 L/min)
3.7.4 On-Line DPF Regeneration
Although the off-line microwave regeneration appeared to be effective as reported
above, the on-line microwave regeneration has not shown to be as promising. A typical
set of such results is shown in Figure 3.17, where O2, NOx, CO, temperature, and
pressure drop are plotted against the operation time. As indicated, during the operation
from 80 to 124 min, the pressure increases steadily from 40 to 50 in. of water (or 9.961 to
12.451 kPa) with temperature remaining at about 128 °C and CO at a low value. When
the pressure reaches 50 in. of water (12.451 kPa) after 124 min, the microwave was
turned on, and the temperature started to rise while the pressure also increased because of
PALLAVKAR 90
8
I
i
-9i
t
a
?s } ! si
s
i
I I1
8
M
v*
&
m
m
Tlmt (mln)
m
Fig. 3.17 Plot of concentration of C^/NOx/CO/temperature/pressure drop vs. time for an
on-line regeneration experiment (exhaust flow: 16.7 L/min)
the increase in temperature. The first CO peak then appeared indicating the onset of DPM
oxidation at a relatively lower temperature followed by another CO peak at a higher
temperature due to additional DPM oxidation at a higher temperature but without
sufficient supply of oxygen. The temperature was seen to peak at about 780 °C and then
it dropped because of less intensity in DPM combustion, although the microwave was
still on. The pressure appeared to have a step drop corresponding to the DPM
combustion; however, the drop was not significant indicating the incomplete regeneration
due to insufficient oxygen supply in the engine exhaust gas, that is, 5% in on-line
regeneration (see Figure 3.17) as compared to 18% in off-line regeneration (Figure 3.15).
PALLAVKAR 91
When the microwave was turned off after 149 min, the temperature started to drop
substantially but the pressure only decreased slightly confirming that the regeneration
was not as complete as desired. The lack of oxygen in the engine exhaust stream is
considered the main reason for this incomplete regeneration. Further research is needed
to investigate this problem associated with the on-line microwave regeneration of DPF.
3.8 Conclusions
An experimental study has been carried out to demonstrate the effectiveness of DPF
regeneration employing microwave energy. A well-equipped diesel emission control
laboratory was established to conduct the experiments. The experimental facilities
included a 6 kW diesel generator, an exhaust flow control system, a diesel particulate
filter system, a microwave energy supply system, a soot sampling system, a differential
pressure measurement system, and a temperature measurement system. The DPF tested
was a silicone carbide wall-flow monolith filter (50 mm diameter * 150 mm length, cell
density = 150 cpsi, pore size = 20 fjm) enclosed in a quartz filter holder.
A commercial 1.4 kW microwave oven was modified to accommodate the quartz
holder, and a waveguide was engineered to evenly supply the microwave energy to the
enclosed filter to achieve filter regeneration. In the experiments, the diesel engine exhaust
was lined up to flow through the filter with a fixed flow rate. The microwave
regeneration was triggered after a specific soot loading was reached based on the
differential pressure drop reading. The results have indicated that the designed system has
been able to achieve uniform temperature profiles both in the radial and the vertical DPF
positions. The off-line regeneration of DPF by microwave energy has been observed to
PALLAVKAR 92
be highly efficient. The DPM filtration efficiency has remained to be comparably high
after 150 cycles of filtration/regeneration
with no apparent physical damage to the DPF
being observed. The on-line microwave regeneration of the DPF, however, is not as
efficient as the off-line regeneration due to the insufficient oxygen concentration in the
engine exhaust stream. Further research to address the low oxygen concentration problem
is required to develop the on-line microwave regeneration process.
PALLAVKAR 93
Nomenclature
Af = Filtration area, m2
Dp = pore diarrteter, m
F = friction factor, dimensionless
H = cell width, m
kp = soot layer permeability, m2
kw = wall layer permeability, m2
L = channel length, m
M = mass of soot trapped, kg
APC = clean filter pressure drop, kPa
APS = filter pressure drop due to soot layer filtration, kPa
APt = total filter pressure drop, kPa
APW = filter pressure drop due to wall filtration, kPa
U = cell entrance velocity, m/s
e' = dielectric constant, dimensionless
s" = dielectric loss factor, dimensionless
s = filter wall porosity, dimensionless
\i = exhaust gas viscosity, kPa-s
ww = cell wall thickness, m
wp = DPM (soot) layer thickness, m
Fs = DPM (soot) layer density, kg/m3
PALLAVKAR 94
References
1. Ohara, E.; Mizuno, Y.; Miyairi, Y.; Mizutani, T.; Yuuki, K.; Noguchi, Y.;
Hiramatsu, T.; Makino, A.; Sakai, H.; Tanaka, M.; Martin, A.; Fujii, S.; Busch,
P.; Toyoshima, T.; Ito, T.; Lappas, I.; Vogt, C. D. Filtration behavior of diesel
particulate filters. Soc. Auto. Eng. 2007, 2007-01-0921.
2. Hoek, G.; Brunekreef, S.; Goldbohm, S.; Fischer, P. The association between
mortality and indicators of traffic related air pollution in a dutch cohort study.
Lancet 2002, 360, 1203-1209.
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association between air pollution and mortality in six US cities. New Eng. J. Med.
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4. Cowland, C.; Gutmann, P.; Herzog, P. L. Passenger vehicle diesel engines for the
U.S. Soc. Auto. Eng. 2004, 2004-01-1452.
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control technology and emissions effects. Soc. Auto. Eng. 1994, 940233.
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cordierite traps - pressure drop and permeability of clean and particulate loaded
traps. Soc. Auto. Eng. 2000, 2000-01-0476.
7. Ohno, K.; Shimato, K.; Taoka, N.; Santae, H.; Ninomiya, A.; Ninomiya, T.;
Komori, T.; Salvat, O. Characterization of SiC-DPF for passenger car. Soc. Auto.
Eng. 2000, 2000-01-0185.
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Eng. 2006, 2006-01-0261.
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filtration characteristics of a ceramic diesel particulate trap. Soc. Auto. Eng. 1998,
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pressure drop of diesel particulate filters. Soc. Auto. Eng. 2001,2001-01-0909.
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Zarvalis, D.; Kladopoulou, E. Fundamental studies of diesel particulate filters:
transient loading, regeneration and aging. Soc. Auto. Eng. 2000, 2000-01-1016.
PALLAVKAR 95
12. Bakeman, A. G.; Chiffey, A. F.; Phillips, P. R.; Twigg, M. V.; Walker, A. P.
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catalyzed, uncatalyzed systems. Soc. Auto. Eng. 2002, 2002-01-0322.
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PALLAVKAR 96
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PALLAVKAR 97
CHAPTER 4
CHARACTERIZATION OF MICROWAVE-ASSISTED DE-NOX CATALYTIC
REACTIONS WITH HYDROGEN AND HYDROCARBONS SERVING AS THE
REDUCING AGENT
Automobile engines and power generators run by directly ignited diesel fuel is far
more superior compared to the spark ignited gasoline powered engines in terms of fuel
efficiency, CO2 emissions and power generating capacity. However, the diesel engine
operation demands lean conditions (excess oxygen) and high compression ratios, which
is one of the main reasons for formation of nitric oxides (NOx), which being a critical
pollutant has received a high priority attention by US-EPA. The lean nature of the diesel
engine exhaust and low exhaust gas temperatures renders a conventional three-waycatalytic system ineffective in controlling NOx emissions from the exhaust. The NOx
abatement technologies currently employed in the case of diesel engine emissions control
include NH3 based selective catalytic reduction (SCR), the Lean-NOx Trap (LNT), and
the Lean-NOx catalyst, also known as the De-NOx catalyst, of which only the SCR has
demonstrated greater than 90% NOx reduction potential but is marred with operational
challenges and detrimental emission issues such as ammonia slippage. The operation of
the LNT on the other hand is extremely complicated, and highly prone to sulfur
poisoning. The De-NOx catalyst is similar to the SCR where-in the reducing agent is
hydrocarbon such as the diesel fuel itself. However, this technology has its limitations
due to lesser amount of hydrocarbon compared to the amount of NOx in the engine
exhaust. Moreover, the temperature window of operation for this catalytic system is in
PALLAVKAR 98
the range of 200 °C to 400 °C, which is too narrow compared to the exhaust gas
temperature of the diesel engine exhaust to achieve any significant NOx reduction
potential. With temperature being a critical parameter, achieving a wider temperature
window of operation is essential for high NOx reduction potential. The present study
elucidates the effectiveness of the microwave heating technology in assisting NOx
reduction both in the presence and absence of oxygen employing a microwave assisted
De- NOx catalytic system. The method on the lab-scale involves a microwave heating
system with a microwave applicator, and a De- NOx catalyst incorporating a microwave
absorbing substrate such as silicon carbide (SiC) and a platinum based catalyst. A nitric
oxide (NO) standard gas was used as the source of NOx for our lab-scale tests. Surface
temperatures were monitored using infrared temperature measuring units at a fixed
microwave power. The NOx reduction potential in our lab-scale experimental set-up was
studied both in the presence and absence of excess oxygen, with H2 being the reducing
agent, followed by high NOx reduction potential (> 90%).
4.1 Introduction
The emissions of nitrogen oxides (NOx) from combustion processes, including motor
vehicles and industrial operations, have continued to be a great environmental concern
due to its harmful effects to human health and its potential to form ground level ozone.
The current NOx control technologies include NH3-based selective catalytic reduction
(SCR), the Lean-NOx Catalyst (also known as the De- NOx Catalyst), the Lean- NOx
Trap (LNT), and Exhaust Gas Recirculation. Among them, the first three involve the
use of various catalysts to convert NOx in the engine exhaust into non-harmful gases [1].
PALLAVKAR 99
The SCR technology utilizing ammonia or urea to reduce NOx is a well established
technology for stationary power plants and has recently been successfully applied to
diesel engine exhausts with an operating temperature range of between 180 - 450°C,
which varies with the catalyst used such as Pt or V20s/W03/Ti02 [1, 2]. A conventional
NH3-based SCR for diesel exhausts is composed of three different catalysts in series after
urea injection, i.e., a hydrolysis catalyst, a SCR catalyst and a guard oxidation catalyst. A
variation of this scheme involves a preoxidation catalyst which significantly improves
the NOx conversion in the low temperature region of the engine operating conditions [3].
This preoxidation catalyst essentially increases the NO2 fraction of the NOx in the engine
exhaust so as to improve the overall NOx conversion efficiency at lower temperatures.
The biggest advantage that the ammonia/urea-based SCR has on all the other NOx
reduction technologies is that the NOx removal efficiency can be consistently higher
than 90% with the catalyst resilient to sulfur poisoning [4, 5]. The demerit of this
technology is the issue pertaining to ammonia slippage from this catalytic system at high
NH3/NOx injection rates, which in-turn becomes a detrimental pollution issue.
The Lean NOx Catalyst is essentially an SCR system utilizing hydrocarbons to
convert NOx in the engine exhaust to nitrogen and oxygen involving Pt, Cu or other
catalysts. It can be a reduction/oxidation process resulting in the reduction of nitric oxide
with the hydrocarbon on the catalytic site leading to the formation of nitrogen and
adsorbed oxygen [6]. It can also be a hydrocarbon-based SCR involving the oxidation of
nitric oxide to nitrogen dioxide on certain catalytic sites and hydrocarbons react with
oxygen forming reaction intermediates of the form CxHy02Z on other catalytic sites, with
the formed nitrogen dioxide and this reaction intermediate reacting to form products such
as CO2, H2O, N2 and N2O [7]. Studies have shown that the hydrocarbon-assisted NOx
PALLAVKAR 100
reduction activity improves in the presence of H2 [8, 9]. However, since the amount of
hydrocarbon present in the diesel engine exhaust is not sufficient to bring about an
acceptable NOx conversion level, an injection of additional hydrocarbons in the exhaust
stream is required [10]. Further, the NOx reduction efficiency associated with the Lean
NOx Catalyst is relatively low in the 60% range since the catalyst may not be at its most
active state under the normal engine exhaust temperatures of between 200 - 300°C [2,
11]. Concurrently, with platinum as the catalyst, there is a detrimental potential of
converting NOx to N2O rather than N2 and, once formed, it is not readily decomposed in
the normal temperature range of the diesel engine exhaust of between 200 - 300°C
[12,13].
The Lean NOx Trap (LNT) incorporates NOx trapping materials in the catalyst washcoat to adsorb NOx, trapped as nitrates, and periodically reduced to N2 on the catalyst
with the supply of reducing agents such as hydrocarbons during cyclic operations of
NOx -trapping in lean conditions and NOx -reducing in rich conditions [2, 14, 15].
Although the technology may accomplish a NOx reduction efficiency of up to 80%, the
requirement for cyclic operations appears to be cumbersome. In addition, the technology
is prone to sulfur poisoning [16-18] which practically deactivates the LNT permanently
[19 - 21]. It is worth pointing out that most of the above described NOx control
technologies use extra fuel burning to produce the needed elevated temperatures for the
reactions. The practice is neither energy-efficient nor reliable for achieving a specific
temperature. In addition, although hydrogen has been reported to enhance the De- NOx
activities, the use of hydrogen as a reducing agent especially at elevated temperatures has
not been systemically studied.
PALLAVKAR 101
4.2 Objective
The objective of this study was to conduct experiments to characterize the
effectiveness of hydrogen as the reducing agent for NOx reduction in a Pt/SiC catalyst
bed under microwave heating. Experiments were carried out in a 10.25 cm ID quartz
glass reactor with various reactor designs involving SiC serving as the microwave
absorbing medium and Pt serving as the De- NOx catalyst. In the experiments, a NOcontaining gas with and without oxygen in a nitrogen stream was mixed with hydrogen
or a hydrocarbon and the mixture was sent through a microwave-heated packed bed
reactor composed of a Pt/SiC catalyst for De- NOx reactions at a medium reactor
temperature below 500°C. Dynamic temperature profiles and concentrations of NO and
NO2 were simultaneously measured for an on-line evaluation of De- NOx efficiency
during the experiments. The experimental parameters also included various reactor
designs as well as the use of n-hexane in addition to hydrogen.
4.3 Microwave Heating Technology
The existence of electromagnetic waves was first predicted by Maxwell's equation in
1864, and their rapid technological developments were realized during the Second World
War era because of the development of RADAR [22, 23]. Microwaves are
electromagnetic waves with frequencies ranging from 0.3 GHz to 300 GHz in the
vicinity of high frequency range of these radio waves. In order to avoid interference with
telecommunication devices, particular frequencies have been allocated for purely
industrial and domestic microwave systems. The standard frequency used in various
microwave heating applications is 915 and 2450 MHz [22],
PALLAVKAR 102
The common equation relating the frequency (f) of the microwave and its wavelength (X.)
is:
k=c/f
(1)
where "c" in this equation is the speed of light, i.e., 3 x 1010 cm/s. According to the
above equation, the corresponding wavelengths for the above two frequencies, i.e., 915
and 2450 MHz, are 32.77 and 12.24 cm, respectively. It is worth pointing out that some
materials are capable of absorbing microwave and getting heated up mainly through
dielectric heating. The dielectric heating for a potential microwave absorber depends on
its dipolar polarization which is briefly described below [22 - 24].
4.3.1 Dipolar Polarization
Dipolar polarization refers to materials which exhibit dipole moments, e.g. water,
methanol, and ethanol. Under low frequency irradiation, the dipoles of these materials
are able to align themselves at these low frequencies and able to -keep in phase with the
alternating electric field. However, at the higher microwave frequencies, these dipole
moments lag behind the rapid changing electric fields resulting in increased molecular
friction and collisions, and the heating of the irradiated material. The extent to which
these polar materials can convert the irradiated microwave energy into heat energy
depends on their dielectric properties, namely the relative dielectric constant (e') and the
relative dielectric loss factor (e"). The relative dielectric constant represents the ability
of the material to get polarized and the relative dielectric loss factor is the measure of the
extent to which a dielectric material can convert the absorbed microwave energy to heat
PALLAVKAR 103
energy. Typical dielectric values [25] for some of the materials are given in Table 2.2
(Chapter 2) where silicon carbide is seen to have high values on both the two properties,
i.e., 30 for e' and 11 for e", respectively. It is, therefore, expected to be an excellent
microwave absorbing media as well as an excellent material to convert the absorbed
microwave energy to heat energy. On the contrary, quartz glass, with its e' value being
at 3.78, is not considered a good microwave absorbing material. In addition, since its e"
value is very low, i.e., 0.001, it is not expected to be heated up at all by microwave
irradiation.
4.3.2 Conversion of Microwave Energy to Heat Energy
The conversion of microwave energy to heat energy depends on the dielectric
properties of the material (e' and e"), frequency of the microwave (co), and the electric
field intensity (Ems) [22 - 26] given by the following Equation:
Qavg
= (0))(Eo)(8")(Erms)2
(2)
where Qavg is the average heating potential, a> is the angular frequency, £o is the
permittivity of free space (= 8.85 x 10"12 F/m), and Em,s is the electric field intensity.
It should be noted that the electric field intensity (Enns) is a complex function of the
microwave intensity and the physical properties of the material including e\
PALLAVKAR 104
4.4 Experimental
This section describes the experimental aspect of the study. It includes the following
five sub-sections: Experimental Set-Up, Microwave Application System, Design of
Packed Bed Reactor, Gas Measurement System, and Experimental Procedure.
4.4.1 Experimental Set-Up
A schematic diagram of the experimental set-up is shown in Figure 4.1. The
experimental facilities included various gas cylinders, mass flow controllers, a packed
bed reactor enclosed in a microwave applicator, and a gas measurement system
composed of an emission analyzer, a GC-TCD (Gas Chromatograph - Thermal
Conductivity Detector) and a GC/MS (Gas Chromatograph/Mass Spectrometer). The
gas cylinders involved included air, nitrogen, nitric oxide, and hydrogen. The mass flow
controllers (Brooks Instrument Model No. 5850-E) were calibrated for the flow rates of
respective gases using a gas-volume flow calibrator (American Meter, model AL-17-1).
Individual gas streams of air (zero grade air from Airgas) and certified nitric oxide
standard (1000 ppm of NO with balance N2 from Airgas) were mixed at constant flow
rates to achieve the desired premix stream concentration of NO, O2 andN2.
This premixed gas stream was fed through a stainless steel gas sample holder (300 cc
Volume, 316SS) followed by an in-line filter (Swagelok, F series, sintered, 7fim pore
size) and mixed with a fixed flow rate of pure hydrogen gas (ultra high purity grade from
Airgas). The hydrogen gas flow rate was controlled by using a variable area flow-meter
(correlated Tube Cube-600 from Matheson Tri-Gas) so as to achieve the desired stream
concentration of NO, O2, H2, and N2 in the final gas stream entering the packed bed
PALLAVKAR 105
reactor. For the De- NOx experiments in the absence of oxygen, the premix gas stream
was prepared without the use of air.
Vent
Microwave Applicator
Packed Bed Reactor
GC/TCD
GSH
GSB
GFM
V
GC/MS
TWV
M - Mass flow controller
GSH - Gas sample holder
GFM -Gas flow meter
V - Shutt off valve
TWV - Three-way-valve
EA - Emissions analyzer
GC -Gas chromatograph
TCD - Thermal conductivity detector
GSB -Gas sample bag
MS - Mass spectrometer
W - Water bubbler
Fig. 4.1 Schematic view of the NOx abatement experimental set-up
PALLAVKAR 106
It is worth pointing out that all the De- NOx experiments (with or without O2) were
carried out at a fixed final stream flow rate of 200 cc/min. The product stream from the
packed bed reactor was then sent to the gas measurement system for concentration
measurement of NO, NO2 and other reactant/product species.
4.4.2 Microwave Application System
Figure 4.2 shows a schematic diagram of the microwave application System used in
the experiments.
1 - Power Supply
2-Magnetron Head
3-Dummy Load
4-3-Port Circulator
5 -Directional Power Coupler!
6 -Power Met*
7 -4-Stub T uner
8-3-StubTumr
9-App*calor
10-SMIng Short Clrci*
11 -Cooling MMer In
12-Cooling Water Out
13 -Proem Strom In
14-Proem Strom Out
Fig. 4.2 The block diagram view of the microwave heating unit
It was composed of a Variable-Rate Power Supply Unit (60-3000 W Cober Muegge
Model MS3000D-111EE) as the power source, a Magnetron Head (Cober Muegge
Model MH3000S-210BA) to generate microwaves of 2.45 GHz, and other system
PALLAVKAR 107
components supplied by Gerling Applied Engineering, Inc. including a Short Dummy
Load (Gerling Model GA1204) to absorb excess/reflected microwave, a 3-Port
Circulator (Gerling Model GA1105A) to regulate microwave, a Directional Power
Coupler (Gerling Model No. 3301-2.0) to measure the intensity of both the forward and
the reflected microwave, a 4-Stub Tuner (Gerling Model GL405) to tune the microwave
transmitted to the dummy load, a 3-Stub Tuner (Gerling Model GA1009) to tune the
microwave transmitted to the Microwave Applicator, a microwave applicator to house
the packed bed reactor, and a Sliding Short Circuit (Gerling Model GA1205-A) to reflect
the unabsorbed microwave back to the Applicator. Both the magnetron head and the
short dummy load were cooled continuously with cooling water.
The packed bed reactor, which was essentially a fused quartz glass holder (10.5
mm OD x 300 mm L from Technical Glass Products Inc.) packed with various designs of
catalyst packing materials, was enclosed in the microwave applicator to be irradiated and
heated up by the microwaves for enhanced De-NOx reactions at a higher temperature for
the gas stream flowing through the packed bed reactor. The reactor temperature during
the microwave heating process was measured by two infrared temperature measurement
systems sensing through holes drilled at the front side of the microwave applicator (see
Figure 4.3). The two systems were: (1) a low temperature infrared measuring system
(Raytek, MI25) for a temperature range from 22°C to 500°C; and (2) a high temperature
infrared measuring system (Mikron, M-90) for a range from 700°C to 2000°C. The
measured data were recorded by a data acquisition system (labVIEW).
PALLAVKAR 108
Bole* for infrared
temperature
measurements
Fig. 4.3 Holes drilled through the applicator for infrared temperature measurements
4.4.3 Design of Packed Bed Reactor
The packed bed reactor where the De-NOxreactions occur used silicon carbide (SiC)
as the microwave absorbing material and platinum as the catalyst in four different packed
bed designs abbreviated as PT-0, PT-1, PT-9 and PT-P illustrated in Figure 4.4. In the
PT-0 design, the bed was made up from two SiC (10.25 mm OD x 25 mm L, 80 ppi,
OBSiC from Hi-Tech Ceramics) foams stacked together for a height of 5 cm with no
platinum being involved. In PT-1, a single-layer of platinum wire gauze with a diameter
of 10.25 mm was inserted in between the two SiC forms described in PT-0. In PT-9, nine
single-layers of platinum wire gauze used in PT-1 were inserted in between ten SiC
PALLAVKAR 109
Quartz glass holder
A - De-NOx catalyst pack PT-0
B - De-NOx catalyst pack PT-1
C - De-NOx catalyst pack PT-9
D - De-NOx catalyst pack PT-P
E - SiC disc
F - SiC foam
P - platinum gauze
a - SiC granule
b - platinum based powder particle
Fig. 4.4 The De-NOx catalyst packs used in the NOx abatement tests
PALLAVKAR 110
foams of 0.5 cm thickness each to make it a 5 cm bed. While in PT-P, instead of SiC
foams and platinum wire gauze, SiC granules of 14 mesh in size were mixed with a
mixture of platinum/SiC powder of 80 mesh in size containing 2% mass of platinum to
produce a mixture of 0.2% mass fraction of platinum in the final mixture with a total bed
height of 5 cm. All the platinum catalysts used were activated in air in a high temperature
tube furnace (Lindberg Model 55035) equipped with a temperature controller
(Eurotherm Model 847). In an experiment, a particular packed bed design, i.e., PT-0,
PT-1, PT-9 or PT-P, was assembled in the quartz glass holder with the open ends
plugged with alumina wool (interim mat from 3M). The assembled packed bed was then
placed inside the microwave applicator to be irradiated and heated by microwaves for
carrying out the De- NOx reactions.
4.4.4 Gas Measurement System
The gas measurement system included an emission analyzer (Testo Model 350-XL), a
GC-TCD (Gow-Mac Series 350 GC retrofitted with a stand-alone TCD) and a GC/MS
(Varian CP-3800/Saturn 2200). The concentrations of NO, NO2, and O2 in the gas stream
leaving the packed bed reactor were measured on-line by the emissions analyzer. Since
the emissions analyzer required a high sampling flow rate of greater than 1000 cc/min, a
make-up flow of 800 cc/min of nitrogen gas (ultra high purity grade from Airgas) was
added to the reactor gas stream to achieve the required minimum sampling flow rate for
the emissions analyzer.
The hydrogen gas concentration in the gas stream was measured on-line by the GCTCD with a dual filament assembly of tungsten/rhenium filaments (TCD2-WRE from
VICI Valco Instruments Co. Inc.). The TCD enabled gas chromatograph was modified
PALLAVKAR 111
so that the sample injection sequence and the data acquisition process were fully
automated using data acquisition cards (E-Series 6014E and 6023E from National
Instruments) and the corresponding data acquisition software (labVIEW from National
Instruments). The GC-TCD was operated under the non-referenced mode and argon gas
was used as both the purge gas and the carrier gas during the operation.
The identification of other gas species in the gas stream leaving the reactor, including
ammonia and hydrocarbons, was performed off-line using the GC/MS. In the practice,
the gas sample was first collected in a 5 liter gas sample bag (Cat.No.232-05 from SKC
Inc.) and then injected into the GC/MS with the column oven temperature kept at 50 °C.
The sample injection was eluted through a capillary column which was a wall-coated
fused silica column (50m x 0.32mm ID, Model CP-WAX 52CB from Varian Inc.).
4.4.5 Experimental Procedure
The experimental procedure in the carrying out of the De- NOx experiments involved
the following three steps:
Step 1: Establishment of Experimental Parameters;
Step 2: Application of Microwave Energy; and
Step 3: Measurement of Product Concentrations.
4.4.6
Establishment of Experimental Parameters
The first step was to assemble the packed bed reactor with a specific catalyst bed
design, i.e., PT-0, PT-1, PT-9, or PT-P, up to the desired bed height in the glass quartz
holder. The assembled packed bed reactor was then purged with ultra high purity
nitrogen gas at a controlled flow-rate for at least 15 minutes. After purging, the
PALLAVKAR 112
temperature and gas measurement systems were turned on and individual feed gas
components were fed at controlled flow rates through the gas sample holder. The feed
gas stream was first operated under a by-pass mode (by-passing the reactor) until a
steady state is reached. Once the steady state is reached in terms of constant feed gas
flow rates and compositions, the operation was switched to the reaction mode where the
feed gas was directed to the packed bed reactor for De- NOx reactions. In the
experiments, the flow rate of the feed gas was 200 cc/min with the concentration of nitric
oxide ranging between 750-1,000 ppm, hydrogen ranging between 3,000-7,000 ppm,
oxygen ranging between 0 - 5 %, and balanced nitrogen. In selected experiment runs,
instead of hydrogen, n-hexane (n-CeHu) was used as the reducing agent for the De- NOx
reactions.
4.4.7 Application of Microwave Energy
Once the feed gas was directed to the packed bed reactor, the microwave power was
turned on and the power level was adjusted to the pre-established settings for both the 3Stub Tuner and the 4-Stub Tuner corresponding to a designated reactor temperature. The
pre-established settings for the 3-Stub Tuner and the 4-Stub Tuner corresponding to a
particular reactor temperature for different bed designs, e.g., PT-0, PT-1, PT-9, or PT-P,
were obtained through a series of blank tests prior to the reaction experiments. In the
experiments, the reactor temperature ranged between 290°C and 810°C.
4.4.8 Measurement of Product Concentrations
The concentrations of NO, NO2, and CO were measured on-line and continuously
throughout each experiment by the emission analyzer. The concentrations of CO2 and
PALLAVKAR 113
H2 were measured on-line by the GC-TCD once the De- NOx reactions reached the
steady state. In addition, once the steady De- NOx reaction was recorded, the product
gas leaving the GC-TCD was collected in a gas sample bag for an off-line measurement
of product species using the GC/MS. The GC/MS was run under the fixed electron
ionization mode (El-fixed) with the scanning mass-to-charge ratio (m/z) range of 10 to
50, which resembled the molecular weight of various species of interest including
ammonia and various hydrocarbons.
4.5 Results and Discussion
The observed results are reported in the following four sub-sections: General
Observations, Effect of Reactor Design, Effect of Reducing Agent, and Formation of
Reaction By-Products.
4.5.1 General Observations
Typical experimental observations on the dynamic behavior of temperature and NOx
concentrations during the experiment are shown in Figures 5 and 6. The results shown in
Figure 4.5 were associated with an experiment involving PT-9 as the catalyst bed design
without the presence of oxygen and those shown in Figure 4.6 were associated with the
same reactor design but with the presence of oxygen. The results shown in Figure 4.5
indicate that, without the presence of oxygen, as soon as the microwave was turned on at
a power level of 70 W, the reactor temperature was raised quickly from the initial 25°C
to near its steady state temperature of about 290°C within about two minutes. During the
process, the NO concentration was seen to drop from the initial 980 ppm to its steady
state concentration of about 150 ppm within about six minutes, indicating a NO
PALLAVKAR 114
reduction efficiency of 84.7%. The NO concentration was seen to return to its initial
concentration once the microwave was turned off and the reactor temperature dropped
back to its initial room temperature.
1 - NO
2 • Twnparature
E
/*
u o1
co
s!
/
r
I
*£
z
J
\—L?J
/
Imworrl
J
r-
p.. w.|
J/
ai
Tim* (min)
isj
M
32J
Fig. 4.5 Low power NOx abatement test. De-NOx catalyst pack: PT-9. Feed
gas flowrate: 200 cc/min. Feed gas composition: NO ~ 1000 ppm, H2 ~ 3000
ppm, balance N2. Microwave power absorbed: 70 W
With the presence of oxygen in the feed gas, the results shown in Figure 6 indicate
that, similar to those observed in Figure 4.5, the reactor temperature was seen to rise
quickly to its steady state temperature of about 410°C once the microwave power was
turned on. Two different observations, however, can be observed regarding the NOx
concentrations as compared to those observed in Figure 4.5. First, as indicated in Figure
4.6, approximately 6.8% (51 ppm) of the initial NO concentration (753 ppm) in the feed
gas was converted to NO2 prior to the application of microwave energy with the presence
PALLAVKAR 115
of oxygen in the feed gas. Second, with the steady state NO concentration of 320 ppm as
indicated in Figure 4.6, the efficiency of NO reduction in this experiment with the
presence of oxygen in the feed gas was much lower than that without the presence of
oxygen, e.g., 57.5% vs 84.7% as indicated in Figure 4.5.
=•>
53 o 5
S
h
°X 8*
Oz
MwOn
I
•J
SJ
MJ
If J
TinM(min)
KJ
BJ
Fig. 4.6 Low power NOx abatement test in the presence of O2. De-NOx
catalyst pack: PT-9. Feed gas flowrate: 200 cc/min. Feed gas composition:
NO ~ 702 ppm, NO2 ~ 51, H2 ~ 7000 ppm, O2 ~ 5 %, balance N2. Microwave
power absorbed: 80 W
The effect of oxygen appears to be significant due to the competing reactions between
the De-NOx reactions involving hydrogen and NO/NO2 and the oxidation reactions
involving hydrogen and oxygen.
PALLAVKAR 116
4.5.2 Effect of Reactor Design
An attempt was made to investigate the effect of various reactor designs on the DeNOx operation, i.e., the effect of PT-0, PT-1, PT-9, and PT-P. It was observed that,
when no platinum was involved as the catalyst, i.e., the design of PT-0, the De- NOx
reactions were not effective. A typical set of such results shown in Figure 4.7 indicate
that the steady state NO concentration was approximately 750 ppm corresponding to a
De- NOx efficiency of approximately 23.5% at the steady state temperature of about
467°C when PT-0 was used.
1 -NO
2 • Temperature
t6
80
o
X
OZ
Time(min)
Fig. 4.7 Low power NOx abatement test in the absence of O2. De-NOx
catalyst pack: PT-0, Feed gas flowrate: 200 cc/min. Feed gas composition:
NO ~ 1000 ppm, H2 ~ 3000 ppm, balance N2 Test A: Microwave power
absorbed ~ 110 W, Temperature ~ 467 0 C
PALLAVKAR 117
The observation strongly indicates that the use of platinum as the De- NOx catalyst is
essential in promoting the De-NOx reactions at a moderate reaction temperature of
around 500°C. Although not shown, the De- NOx efficiency associated with the PT-1
design was observed to be slightly lower than that associated with PT-9. The
observation was expected since more platinum was involved in the PT-9 design than in
the PT-1 design.
Regarding the use of PT-P as the reactor design, a typical set of results shown in
Figure 4.8 indicates that, once the power was turned on at approximately the same power
level as that used in Figure 6 (associated with the PT-9 design), the temperature was
quickly raised to its steady state temperature of about 440°C and the NOx (NO + NO2)
concentration was gradually reduced to its steady state level of about 20 ppm,
corresponding to a De- NOx efficiency of 97.4%. Since the observed steady state
temperature of440°C for PT-P is higher than that of 410°C for PT-9 (shown in Figure
4.6) and the steady state NOx concentration of 20 ppm for PT-P is lower than that of 320
ppm for PT-9 (shown in Figure 4.5), the observed results strongly suggest that the PT-P
design is more effective than PT-9 in terms of both the microwave absorbing capability
and the De- NOx reaction efficiency.
4.5.3 Effect of Reducing Agent
Another attempt was made to investigate the effect of different reducing agents, e.g.,
n-hexane vs. hydrogen, on the De- NOx reactions. A typical set of results is shown in
Figure 9 where the results suggest that the use of n-hexane as a reducing agent is much
less efficient as compared to that of hydrogen for the De- NOx reactions under the
experimental conditions in the presence of oxygen.
PALLAVKAR 118
With a low microwave power of 60 W, the results in Figure 9 indicate that the reactor
temperature was raised to a temperature higher than 800°C due to the absorption of both
the applied microwave energy as well as the generated heat of combustions associated
with both the complete and the incomplete combustion reactions of n-hexane with
oxygen. The NOx concentration, however, was reduced only slightly from 512 ppm to
280 ppm, corresponding to a much lower NO reduction efficiency of 45.3% as compared
to that of 97.4% when hydrogen was used as the reducing agent (see Figure 4.8).
1 - NO
2-NO2
3 • Tamperature
J
jj
Ln
1
I
sc
0
Q
o
y
/
Sq
I8
I
•q
y1
/
/
mrorr
\
1
z
%
M
\
MVVOn
1+
3J (4
j
9J
12J
15J
U
21J
241
ZTM
33J
XJ
42J
Tirrw (min)
Fig. 4.8 Low power NOx abatement test in the presence of O2. De-NOx
catalyst pack: PT-P. Feed gas flowrate: 200 cc/min. Feed gas composition:
NO ~ 559 ppm, NO2 -213 ppm, H2 ~ 7000 ppm, O2 ~ 5 %, balance N2.
Microwave power absorbed: 70 W
PALLAVKAR 119
4 - Tmpwature
IS"
*
§
!!
c
o
h
O2
o
u
aar m,nn,jr
i2jb U4
Time (min)
tu
n»
m
n&
m
Fig. 4.9 NOxabatement test in the presence of O2 using n-CeHn as the
reducing agent. De-NOx catalyst pack: PT-P. Feed gas flowrate: 200 cc/min.
Feed gas composition: NO ~ 372 ppm, NO2 - 140 ppm, n-CeHu ~ 10000
ppm, O2 ~ 5 %, balance N2 Microwave power absorbed: 60 W
4.5.4
Formation of Reaction By-Products
It is essential to report that the formation of ammonia (NH3) was observed in all
experiments involving hydrogen as the reducing agent as that reported in the literature
[27 - 29]. The reported reaction mechanisms on the reduction of NO and NO2 and the
formation of NH3 can be summarized as follows:
PALLAVKAR 120
N02 + S
N02-S
(3)
NO + S -> NO-S
(4)
2S + 02 -> 20-S
(5)
NO-S + O-S -> NO2-S + S
(6)
N02-S^02+N-S
(7)
NO-S + S -> N-S + O-S
(8)
2N-S -» N2 + 2S
(9)
2S + H2->2H-S
(10)
2NO-S + 2H-S -> N2 + H20 + 4S
(11)
O-S + 2H-S -> H20 + 3S
(12)
N-S + H-S
(13)
NH-S + S
NH-S + H-S
NH2-S + S
(14)
NH2-S + H-S
NH3 + 2S
(15)
Note that, in the above reaction equations, S is the vacant active catalyst site on the
platinum-base catalyst (platinum gauze or platinum based powdered particle). It should
be noted that, in a typical experiment with the PT-P design involving 753 ppm of NOx,
7,000 ppm of H2, 5 % of 02 and balanced N2, it was estimated that roughly 55% of the
supplied H2 remained un-reacted, more than 40% reacted to form H20, and less than 5%
reacted to form NH3.
It is worth pointing out that, in the current study, the measured speciation in the
product stream did not indicate the existence of N20 even in the experiments involving
oxygen. The observation was expected since, in all the experiments involving oxygen, a
strong reducing environment, i.e., H2/ NOx = 7, was maintained in the feed gas [29, 30].
PALLAVKAR 121
It should be pointed out that the oxidation of NO to NO2 prior to the heating up of the
reactor was expected since platinum is capable of oxidizing NO to NO2 under lean
conditions at a low temperature [31].
4.6 Conclusions
A SCR process involving microwave heating of Pt/SiC-based catalyst for NOx
emission control has been experimentally studied with hydrogen or a hydrocarbon
serving as the reducing agent. The results have indicated that microwave energy is an
effective method for the heating of Pt/SiC catalyst and hydrogen is an effective reducing
agent for the De- NOx reactions even with the presence of oxygen. The PT-P design of
the catalyst bed has been observed to be the most effective reactor for carrying out the
De- NOx reactions with the observed NOx destruction and removal efficiency greater
than 97% under certain experimental conditions. Although insignificant, ammonia has
been observed to form during the experiments. The study has demonstrated that the
microwave-heated and H2-based SCR process can be an option for the control of NOx
emissions if the use of NH3 is prohibited. However, further research is needed to
investigate the effects of reactor temperature as well as the effects of the existence of
other gases such as CO, CO2 and SO2 on the De- NOx process to establish optimal
reactor design and operation conditions for achieving the maximum NOx destruction and
removal efficiency while minimizing the formation of ammonia.
PALLAVKAR 122
Nomenclature
X
Wavelength, cm
c
Speed of light (=3 x 108), ctn/s
f
Frequency, Hz
Qavg
Average heat potential of the absorbing media, W/m3
CO
Angular frequency, rad/s
eo
Permittivity of free space (=8.85 x 10"12), F/m
e
Relative dielectric constant of the absorbing media, -
8"
Relative dielectric loss factor of the absorbing media, -
Erms
Electric field intensity, V/m
PALLAVKAR 123
References
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PALLAVKAR 124
9. Breen, J.P.; Burch, R.; Hardacre, C.; Hill, C.J.; Rioche, C. A fast transient kinetic
study of the effect of H2 on the selective catalytic reduction of NOx with octane
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11. Kitahara, Y.; Akama, H.; Kamikubo, M.; Shinzawa, M. Passive and active
performance characteristics of NOx catalysts for direct-injection diesel engines for
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of NOx storage and reduction on Pt/Ba0/Al203. Catal. Today 2004, 98, 393 - 402.
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17. Takeuchi, M.; Matsumoto, S. NOx storage-reduction catalysts for gasoline engines.
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PALLAVKAR 125
18. Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.;
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catalyst geometry and fuel sulfur content on NOx adsorber poisoning, Soc. Auto. Eng.
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PALL AVICAR 126
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PALLAVKAR 127
CHAPTER 5
MICROWAVE-ASSISTED NONCATALYTIC DESTRUCTION OF VOLATILE
ORGANIC COMPOUNDS USING CERAMIC-BASED MICROWAVE
ABSORBING MEDIA
Volatile organic compound (VOC) emissions from various sources such as chemical
process industry, manufacturing industry, and automobiles have been an environmental
and health concern. With the emerging emphasis on using green technologies to
minimize greenhouse gas emissions, the use of microwave energy to achieve VOC
emissions control with its electric power coming from nongreenhouse-related energy
sources, such as wind, geothermal, solar, or even nuclear energy, becomes an attractive
option. In this study, an experimental investigation involving the use of microwave
energy to accomplish high temperature destruction of p-xylene in a packed bed reactor
was performed using SiC (silicon carbide) foam as the microwave absorbing media with
air or nitrogen being the carrier gas. The experimental facilities consisted of a gas
cylinder, a mass flow controller, a p-xylene vaporizer, a packed bed reactor packed with
a SiC foam, a microwave applicator, and a gas chromatograph/mass spectrometer
(GC/MS) for gas analysis. The SiC was found to be an excellent microwave absorber,
which efficiently converts the microwave energy into heat energy. It was observed that
the SiC temperature rises rapidly upon microwave irradiation and reaches a steady state
temperature of higher than 800 °C within 2-3 min depending on the experimental
conditions. A semiempirical energy balance model was formulated to describe the
dynamic temperature profiles of the SiC in the reactor, and the model was found to
PALLAVKAR 128
simulate the observed profiles reasonably well. The destruction and removal efficiencies
(DREs) for p-xylene were observed to reach 100% for all the experiments conducted
with air being the carrier gas; however, the DREs never reached 100% with nitrogen
being the carrier gas and the major destruction byproducts were observed to be benzene,
toluene, styrene, biphenyl, and the unreacted p-xylene. The study has demonstrated that
the microwave technology can be effectively developed to control the emissions of low
concentrations of VOCs, especially in air.
5.1 Introduction
Volatile organic compounds (VOCs) emitting from various industrial operations and
automobiles are organic chemical species that readily volatilize in ambient air with many
of them having great potential to pose serious long-term health and environmental
impacts. Studies in the past have shown that prolonged exposure to VOCs such as
toluene and p-xylene may affect central nervous functions and induce reproductive and
developmental toxicity [1,2]. Toluene in particular is recognized for its neurotoxicity
effect on liver, heart, and kidney as well [2]. Additionally, apart from being potential
health hazards, many of these VOCs are photochemically active and, along with oxides
of nitrogen and in the presence of sunlight, they have the potential to form ground level
ozone which is a well-known secondary air pollutant [3]. An urgent need, therefore, is to
develop effective VOC treatment technologies to control VOC emissions to reduce
potential health and environmental problems caused by these VOCs [3, 4], Control of
VOC emissions from various industrial operations at an acceptable level and with
minimum energy usage constitutes a challenging task for the industry [5]. Various VOC
control devices, such as chilled water/refrigerated brine condensers,
PALLAVKAR 129
carbon/zeolite/polymer adsorbers, and membrane separation systems, are employed in
many industrial applications to recover VOCs and achieve low VOC emissions [6-8],
Other available VOC control devices also include thermal oxidizers, catalytic oxidizers,
flares, and plasma/electron beam devices, where the first three involve the use of thermal
energy generated from fossil fuel to destroy VOCs [9-14]. Among them, thermal
oxidizers are designed to treat waste streams with VOC concentrations ranging from 100
to 2000 ppm and are considered to have the broadest VOC control applicability with a
high destruction and removal efficiency (DRE) ranging from 95 to 99% [15]. The
catalytic oxidizers are generally more energy efficient than the thermal oxidizers.
However, they are more restricted in the application ranges and require additional
maintenance for reliable operations.
With the emerging emphasis on using green technologies to minimize greenhouse gas
emissions [16], the employment of thermal oxidizers and catalytic oxidizers for VOC
destruction with thermal energy generated from fossil fuels is becoming undesirable due
to its potential generation of additional greenhouse gas, e.g., carbon dioxide. Instead, the
use of microwave energy to achieve the control with its electric power coming from
nongreenhouse-related energy sources, such as wind, geothermal, solar, or even nuclear
energy, becomes an attractive option [5, 17-26]. Recent studies pertaining to VOC
control technologies using activated carbon fiber cloths [17, 18] have shown desirable
results in promoting microwave-assisted regeneration of these spent VOC adsorbents.
These studies have indicated that microwave heating has an advantage over conventional
heating as it allows selective heating depending on the dielectric properties of the
substrate. It is worth pointing out that microwave heating is volumetric heating [17] with
all of the infinitesimal volume elements within the object getting heated simultaneously,
PALLAVKAR 130
which is in contrast to surface heating such as from a hot gas stream where the direction
of the heat flux was from the substrate surface inward. However, because of the
absorption of microwave energy at the outer surfaces of the absorbing medium, the
microwave field strength may be substantially reduced in the interior depending on the
physical properties of the medium.
Note that the use of microwave heating for VOC destruction had not been systematically
investigated and was the objective of this study.
5.2 Objective
The objective of the study was to conduct experiments to characterize the destruction
of VOCs using microwave energy with silicon carbide (SiC) being the microwave
absorbing media and p-xylene being the tested VOC. In the experiments, a specific
concentration of p-xylene vapor in air or nitrogen was sent through a microwave-assisted
packed-bed reactor containing a SiC foam and was destroyed in the reactor by the
elevated temperatures generated by microwave irradiation. A gas chromatograph/mass
spectrometer (GC/MS) was used to measure the DRE of p-xylene and to identify the
destruction byproduct. The experimental parameters also included the carrier gas flow
rate and the power level of the microwave energy. A model was developed to describe
the energy balances in the system and simulate the temperature profiles of the silicon
carbide foams during microwave heating.
PALLAVKAR 131
5.3 Microwave Heating Technology
The existence of electromagnetic waves was first predicted by Maxwell's equation in
1864, and their rapid technological developments were realized during the Second World
War era because of the development of radar [19,27, 28]. Microwaves are
electromagnetic waves with frequencies ranging from 0.3 to 300 GHz in the vicinity of
the high- frequency range of these radio waves. In order to avoid interference with
telecommunication devices, particular frequencies have been allocated for purely
industrial and domestic microwave systems. The standard frequency used in various
microwave heating applications is 915 and 2450 MHz [19]. The common equation
relating the frequency (f) of the microwave and its wavelength (X) is
X = c/f
(1)
where "c" in this equation is the speed of light, i.e., 3 x 1010cm/s. According to the
above equation, the corresponding wavelengths for the above two frequencies, i.e., 915
and 2450 MHz, are 32.77 and 12.24 cm, respectively. It is worth pointing out that some
materials are capable of absorbing microwave and getting heated up mainly through
dielectric heating. The dielectric heating for a potential microwave absorber depends
on its dipolar polarization [19,20,27], which has been briefly described in chapter 4
(section 4.3).
PALLAVKAR 132
5.3.1 Conversion of Microwave Energy
The conversion of microwave energy to heat energy depends on the dielectric
properties of the material (e' and e"), frequency of the microwave (co), and the electric
field intensity (.firms) [17,20,21,27] given by the following equation:
Qavg=(ffl)(80)(e")(Erms)2
(2)
where Qavg is the average heating potential, to is the angular frequency, eo is the
permittivity of free space (= 8.85 * 10"12F/m), and Erms is the electric field intensity. It
should be noted that the electric field intensity (Erms) is a complex function of the
microwave intensity and the physical properties of the material including e.
5.4 Experimental Setup
A schematic diagram of the experimental setup is shown in Figure 5.1. It consists of a
gas cylinder, a mass flow controller, a p-xylene vaporizer, a packed bed reactor, a
microwave applicator, and a GC/MS (Varian model CP-3800 GC coupled with Varian
Saturn 2200 MS) for gas analysis. The GC/MS was equipped with a built-in high
performance sample concentrator for parts-per-billion level detection capability designed
by Lotus Consulting. The packed bed reactor was a 10.5 mm inside diameter (i.d.) and
304.8 mm long quartz tube from Technical Glass Products, Inc. It was packed with
microwave absorbing material of silicon carbide foam supported on glass wool. The
dimensions of the silicon carbide foam, supplied from Hi-Tech Ceramics, were 10 mm in
outside diameter (o.d.) and 50 mm in height, and the reactor was placed inside a
microwave applicator as shown in Figure 5.1. The applicator was a microwave
PALLAVKAR 133
application system shown in Figure 5.2 made up of a power supply, a microwave
generator, a dummy load, a detector, a monitor, a three-stub turner, and a terminator. The
system was safe-guarded and thoroughly examined using a microwave survey meter
(ETS.Lindgren, Holaday EMF Measurement, model 1501) to ensure that no leaking of
microwaves occurred during the experiments.
5.4.1 Experimental Procedure
The experimental procedure for each set of experiment involved the following
sequential steps. The first was to assemble the packed bed with the designed bed material,
i.e., silicon carbide, to the designated bed height. The next was to purge the assembled
system with ultrahigh purity nitrogen (ultra high purity from Airgas) for about 15 min at
a specific flow rate for purging by means of a mass flow controller (Brooks instrument
v
Mlcrowwi AppMcator
PvctorfBarf DndM
GC/MS
-OZH
vQ
r
e
V
G
1
6 - 6M cyttndar
M - Maaa flow eoMroUar
P - p-Xy<ana vaportear
W
V - 3-way valv*
T - Ttwcmocoopto
W-Water bubbter
Fig. 5.1 Schematic diagram of the experimental set up
PALLAVKAR 134
VWMrto
Fig. 5.2 Block diagram and the corresponding setup of the microwave application system
5850-e series). The gas flow rate was then adjusted to a designated flow rate for
experiments with nitrogen or switched to air for experiments with air (zero grade purity
from Airgas). The procedure was followed by turning on the cooling water supply for the
magnetron head and the short dummy load (Gerling Applied Engineering model
PALLAVKAR 135
GA1204). The data acquisition system (LabVIEW) was then turned on to record the
temperature of the exit gas stream using a J type thermocouple (Omega Engineering Inc.)
and the bulk surface temperature of the SiC foam using an infrared temperature (IR)
measuring unit (Mikron model M90 with a temperature range of 700-2000 °C) through
holes drilled through the applicator (see Figure 4.3, Chapter 4). It is worth pointing out
that the measured temperature based on the described IR system might not represent the
true SiC temperature in the reactor because the SiC foam was enclosed in a quartz
reactor. However, since quartz glass is considered IR transparent with a transparency
index of 0.93 and since the quartz glass used in the experiment was thin (1.2 mm in
thickness) and was in close contact with the SiC foam, the measured temperatures were
interpreted as the SiC temperatures. The procedure was then followed by turning on the
microwave generator and setting up the microwave power to the desired power level.
Once the microwave power was turned on, both the forward power and the reverse
power were monitored by two respective crystal detectors (Gerling Applied Engineering
model GA3104/0015) and recorded on a dual microwave power meter (Gerling Applied
Engineering model GA3004-2). The forward microwave power was then fine-tuned to
the designed level by adjusting a three-stub tuner. This setting of the three stub tuner was
left undisturbed for the entire duration of a particular experimental run. With the
microwave power being on, the temperatures of both the SiC foam and the exit gas were
observed to rise rapidly. Once a steady temperature of the SiC foam in the reactor was
achieved, the carrier gas (either nitrogen or air) was switched to the p-xylene vaporizer
by a three-way control valve. The gas carried the vaporized p-xylene to the packed bed
reactor containing the microwave-heated high temperature SiC foam. Upon contact with
the high temperature SiC foam, the majority of the inlet p-xylene vapor was destructed.
PALLAVKAR 136
The effluent stream containing the potentially undestructed p-xylene and the destruction
products was sampled and analyzed online using a GC/MS. This allowed the DRE to be
calculated based on the results from the GC/MS, which provided the mole concentration
of p-xylene in the gas stream. The remaining effluent gas was directed to a water bubbler
to ensure that positive pressure was maintained throughout the experiment, and it was
then sent to the exhaust vent. Once the experiment was completed, the system was
switched back to the purge mode with high-purity nitrogen gas flowing through the
system to clean up the p-xylene residue and bring the system back to the starting
conditions. A series of experiments was performed with the experimental parameters
being the flow rate of the carrier gas and the power level of the microwave energy. In
this study, three carrier gas flow rates were used, i.e., 200, 350, and 500 mL/min and the
corresponding gas residence times in the SiC reactor were 1.30, 0.74, and 0.52 s,
respectively.
5.5 Results and Discussions
5.5.1 Net Spent Microwave Power and Steady State Temperature
It was described in the Experimental Section that the designed microwave system was
capable of measuring both the forward power and the reverse power with the net spent
microwave power being the difference between the two. While the forward power was a
set value and was constant throughout an experiment, the reverse power was observed to
increase steadily and reached a steady value once the system reached its steady state
temperature.
PALLAVKAR 137
The relationships among the forward power, the net power, and the SiC temperature at
the steady state under various operating conditions are plotted in Figure 5.3.
1300j0
1300.00
1000J)
1000AO
MXM>
MOJOO
—*—T0m(>«rai*« - 200 ecAnh
» Temperalm -350 ceAn tn
—•—T«mp«rttu™ - 900 ccMIn
•604
400M
400.0
200.0
OJO
SOOjO
tOOJOO
—•— Na) Pcwmr - 200 cc/mir
» Nat Power - 350 coTrtn
I Power-900 ccrtrin
JL.
390.0
400J)
480.0
JL
_l_
800.0
S90.0
200X>0
0.00
OOOJO
forward IW«r
Fig. 5.3 Plot of forward power, net power, and the corresponding steady state
temperature of SiC under different gas flow rates (carrier gas, nitrogen)
The results in the figure indicate that the percentages of the net spent microwave power
at a steady state are approximately 84%, 75%, and 68% of the forward power for the
three gas flow rates studied, i.e., 200, 350, and 500 mL/min, respectively. The results
also indicate that the steady state temperature is higher at a lower gas flow rate for all the
power levels studied. The observed results are expected since the net power input is
higher at a lower gas flow rate, which results in a higher steady state temperature.
PALLAVKAR 138
5.5.2 Dynamic Temperature Profiles during Microwave Heating
Besides the steady state temperatures, the dynamic nature of the SiC temperature
during microwave heating as also investigated in this study. Since SiC is an excellent
microwave absorber and has an excellent dielectric loss factor to convert the microwave
energy to heat energy, the temperature of the SiC foam was expected to rise rapidly and
reach steady state values within a short period of time.
UN
Tim* (aae)
Fig. 5.4 Temperature profiles of SiC during experiments with different levels of
microwave irradiation (carrier gas, nitrogen; carrier gas flow-rate, 500
mL/min)
Typical dynamic SiC temperature profiles during selected experiments are shown in
Figure 5.4 where the results indicate that the steady state temperatures of higher than
800 °C are reached within about 2-3 min and that higher steady state temperatures were
observed with higher microwave powers. In addition, the results also indicate that the
PALLAVKAR 139
introduction of p-xylene in the stream raises the steady state temperature slightly due to
the exothermic nature of the reaction related to p-xylene combustion or decomposition.
Table 1 summarizes the observed steady state temperatures with or without p-xylene
under different experimental conditions. As expected, higher steady state temperatures
are associated with higher microwave powers and lower gas flow rates.
An attempt was made to simulate the observed dynamic temperature profiles by using
the following energy balance equation, i.e,
[(m)(Cp)(dT/dt)] = Ps(l - ept) - [(h)(Ac)(T - T0)/2 + (0)(Ar)(T4 - Ts4)]
(3)
where m is the mass of the absorbing media, Cp is the specific heat of the absorbing
media, (h)(Ac) is the convective heat transfer, (o)(Ar) is the radiative heat transfer, Ps is
the net microwave power consumed, and T0and Ts are the gas inlet temperature and the
surrounding temperature, respectively. In the above equation, the term on the left-handside represents the rate of increase of SiC temperature and the terms on the right-handside represent the net microwave energy contributed to the increase in SiC temperature
with the first term representing the net microwave power being converted to heat energy,
the second term representing the loss of heat energy to the gas through convective heat
«
loss, and the third term representing the loss of heat energy to the surroundings through
radiation heat loss. It should be noted that the term ept appearing in eq 3 is an empirical
expression representing the amount of net microwave energy not contributing to the heat
energy balance of SiC, e.g., the energy absorbed by SiC but not converted to heat energy
and/or the energy absorbed by other components of the system, e.g., the quartz glass or
reactor walls. It is worth pointing out that the term Ps(l - ept) is theoretically a function of
PALLAVKAR 140
the dielectric properties of SiC, which in turn, are a complex function of temperature
and other physical properties such as density, mass, and specific heat.[17,19].
Table 5 Steady State SiC Temperatures under Different Experimental Conditions
Flow rate of
nitrogen gas
(mL/min)
Microwave
forward power
(watt)
Net microwave
power
(watt)
200
350
500
200
350
500
200
350
500
350
350
350
450
450
450
550
550
550
318
271
238
380
355
306
458
440
374
T
(no
p-xylene)
°C
967.8
815.0
808.6
1065.8
910.0
880.0
1087.0
996.0
993.4
T
(with
p-xylene)
°C
971.0
849.6
810.0
1067.0
931.9
882.9
1127.0
1010.6
995.0
However, since its correlation with temperature is not available at this time, the term is
treated as a constant, i.e., independent of T, in the equation. In the simulation, the
constant was determined through trial and error to best-fit the simulation results to the
experimentally observed values. It was assumed to be a constant for experiments with
different microwave power levels but with the same gas and the same gas flow rate.
Typical simulation results based on equation 3 are also plotted in Figure 5 (without
considering the effect of p-xylene), and good agreement between the simulated and
observed results is observed. It should be noted that, in Figure 5.4, measurement
temperatures below 700 °C were not available due to the limitations of our infrared
temperature unit which was designed for measuring temperatures between 700 and
2000 °C.
PALLAVKAR 141
The proposed equation for energy balance calculations, i.e., eq 3, was also used to
provide insights into the distribution of the net spent microwave energy in various
energy terms.
100
1 • HMt ibsocfetd by Um mdi
to*t to ttw system
—S - RadMtw haatloam
'4-ComMtWllMl
0
100
200
300
400
Tim* (MM)
Fig. 5.5 Distribution of net microwave power during the heating up of SiC (carrier gas,
nitrogen; gas flow rate, 500 mL/min)
A typical set of such simulation results is shown in Figure 5.5 where the results indicate
that, at the beginning of the heating process, the majority of the net spent microwave
energy is wasted with less than 25% being used to raise the temperature of the SiC. As
the process is progressed, the wasted energy gradually diminishes and the majority of the
microwave energy is converted to heat energy and lost to the surrounding via radiation
PALLAVKAR 142
loss. The convective heat loss to the carrier gas remains to be low throughout the entire
VOC destruction process. It is worth pointing out that the proposed semiempirical model
serves to describe the observed temperature profiles well. However, a more theoretical
approach should be used for a more serious analysis. Although it was not involved in the
current study, it should be pointed out that the microwave VOC destruction process can
be improved in its energy efficiency through the use of reflective materials to reduce the
radiative heat losses. Further study will be conducted to demonstrate the practice.
5.5.3 Destruction and Removal Efficiency of VOC
The DRE of a VOC is defined to be the amount of the VOC destroyed, i.e., the
difference between VOCin and VOCout, over the input amount of the VOC with the
equation given below:
DRE = [(VOQn - VOCou,)/VOCin] * 100%
(4)
In this study, the VOC tested was p-xylene; the VOCjnwas the p-xylene mole
concentration in the inlet stream, and the VOCoutwas the p-xylene mole concentration in
the outlet stream. Both the VOCjn and VOCout were measured by the GC/MS after the
system reached the steady state during an experiment. It was generally observed that the
destruction of p-xylene was always complete during our experiments when air was the
carrier gas, i.e., the p-xylene concentration was undetectable in the outlet stream; and the
DREs were always greater than 90% depending on the operating conditions when
nitrogen was the carrier gas. It should be pointed out that, when nitrogen gas was the
carrier gas, the main products were methane and ethane with trace amounts of
PALLAVKAR 143
destruction byproducts including benzene, toluene, styrene, biphenyl, and the unreacted
p-xylene, which are to be discussed in the next subsection. However, when air was the
carrier gas, the main products from this destruction process were carbon dioxide and
water with a trace amount of carbon monoxide.
Results from equilibrium calculations have indicated that, although the equilibrium
trend favors CO over CO2 at high temperatures, the equilibrium ratios of CO/ CO2
remain to be extremely low in the temperature range of our experiments, i.e.,
approximately 2.0 x 10"8 at 900 °C to 4.0 x 10"5 at 1300 °C. Figures 5.6 - 5.8 plot DREs
against the net microwave power with nitrogen being the carrier gas under different
nitrogen flow rates, i.e., 200, 350, and 500 mL/min. Also plotted on the figures are the
corresponding steady state temperatures.
P I
MOO 1
100 JO
200.0 200 O 300 O *094 400.0 4*0.0 MOO
j
J
KM hHMir (watt)
Fig. 5.6 DRE for p-xylene destruction (carrier gas, nitrogen; gas flow
rate, 200 mL/min)
PALLAVKAR 144
«i
m-i
100JM
•O.OQ
*
©
uoo
—m—% ORE
TempOTtuni
ooo.o
1IOJ
1.0
I
2MJO MIjO MOJO 400J9
Mm* Poww (w«tt)
I
400J>
BOO*
Fig. 5.7 DRE for p-xylene destruction (carrier gas, nitrogen; gas flow
rate, 350 mL/min)
78.00
•0.00
2L00
100.0
Net Pewer (watt)
Fig. 5.8 DRE for p-xylene destruction (carrier gas, nitrogen; gas flow
rate, 500 mL/min)
PALLAVKAR 145
The results strongly indicate that the DREs are greater than 99.9% when the reactor
temperatures are higher than 900 °C. However, the DRE can only be 90% when the
reactor temperature is low at around 800 °C as indicated in Figure 5.8. It is worth
pointing out that the DREs for the corresponding experiments with air being the carrier
gas were all 100%, i.e., no destruction byproducts were detected.
5.5.4 Destruction Byproducts with Nitrogen being the Carrier Gas
As discussed previously, when nitrogen was the carrier gas, the destruction of pxylene was greater than 99.9%at high temperatures with trace amounts of less than 0.1%
of destruction byproducts including benzene, toluene, styrene, biphenyl, and the
unreacted p-xylene. Typical GC/MS mass spectrum results showing the existence of
these destruction byproducts are displayed in Figures 9 and 10. The results shown in
Figure 10 indicate that, under the nitrogen flow rate of200 mL/min, five major
destruction byproducts, namely, benzene, toluene, styrene, biphenyl, and p-xylene, were
detected at a lower microwave power level. The detected byproducts were reduced to
two, i.e., p-xylene and biphenyl, at higher microwave power levels. The same two
destruction byproducts were also detected at a higher nitrogen flow rate (500 mL/min)
and a high microwave power level as indicated in Figure 10.
5.6 Conclusions
An experimental study involving the use of microwave energy to accomplish hightemperature destruction of p-xylene in a packed bed reactor has been performed using a
SiC foam as the microwave absorbing media with air and nitrogen being the carrier gas.
PALLAVKAR 146
The SiC has been found to be an excellent microwave absorber which efficiently
converts the microwave energy into heat energy. It has been observed that the SiC
temperature rises rapidly upon microwave irradiation and reaches a steady state
temperature of higher than 800 °C within 2-3 min depending on the experimental
conditions. A semiempirical energy balance model has been formulated to describe the
dynamic temperature profiles of the SiC in the reactor, and the model has been found to
simulate the observed profiles reasonably well. The DREs for p-xylene have been
observed to reach 100% for all the experiments conducted with air being the carrier gas;
however, the DREs have never been observed to reach 100% with nitrogen being the
carrier gas and destruction byproducts being benzene, toluene, styrene, biphenyl, and the
unreacted p-xylene.
The study has indicated that the microwave technology can be effectively developed
to control the emissions of low concentration of VOCs especially in air. The use of
microwave energy to achieve the control with its electric power coming from
nongreenhouse-related energy sources, such as wind, geothermal, solar, or even nuclear
energy, becomes an attractive option.
PALLAVKAR 147
1 - Benzene
2 - Toluene
3- p-Xylene
4-Stymie
5 - Biphenyl
£
s
10
4
I
.1 i . l
ii
1
|
'
B
"E
3
s
c
o
1®
o
«0l
1 -
i
u
S
aoH
5
g
JL
i
10
t»
Data collection ihno
Fig. 5.9 GC/MS mass spectrum data for p-xylene destruction (carrier
gas, nitrogen; gas flow rate, 200 mL/min; net power level, (A) 318 W, (B)
380 W, (C) 458 W; temperature, (A) 971 °C, (B) 1067 °C, (C) 1127 °C)
PALLAVKAR 148
40
1- P-XYBN*
2-r
30
20
| 10
5
10
15
20
pM« cahetM Vim (rein)
Fig. 5.10 GC/MS mass spectrum data for p-xylene destruction (carrier
gas, nitrogen; gas flow rate, 500 mL/min; net power level, 374 W;
temperature, 995 °C)
Nomenclature
Ac = convective heat transfer area, m2
Ar = radiative heat transfer area, m2
Cp = heat capacity of the absorbing media, J/kg K
C = speed of light, cm/s
Erms= electric field intensity, V/m
f = frequency of irradiated microwave energy, Hz
h = convective heat transfer coefficient, W/m K
m = mass of the absorbing media, kg
Ps = steady state microwave power, W
Qavg = average heat potential of the absorbing media, W/m3
T = temperature of the absorbing media, K or °C
T0 = gas inlet temperature, K
Ts = surrounding temperature, K
CT = Stefan-Bo ltzmann constant, W/m2 K4
p = empirical coefficient appearing in eq.3, 1/s
so = permittivity of free space (=8.85 x 10"12), F/m
e = relative dielectric constant of the absorbing media, e = relative dielectric loss factor of the absorbing media, -
X = wavelength of the irradiated microwave, cm
co = angular frequency of microwave energy, rad/s
PALLAVKAR 150
References
1. Olson, B. A.; Gamberale, F.; Iregren, A. Coexposure to toluene and p-xylene in man:
central nervous functions. Br. J. Ind. Med. 1985, 42, 117.
2. Donald, J. M.; Hooper, K.; Claudia, H. R. Reproductive and developmental toxicity
of toluene: A review. Environ. Health Perspect. 1991, 94, 237.
3. Cooper, D. C.; Alley, F. C. Air Pollution Control: A Design Approach; Waveland
Press Inc.: Prospect Heights, IL, 2002.
4. Zalel, A. Y.; Broday, D. M. Revealing source signatures in ambient BTEX
concentrations. Environ. Pollut. 2008,156, 533.
5. Kim, T. H.; Rupani, H.; Pallavkar, S.; Hopper, J.; Ho, T.; Lin, C. J. Destruction of
toxic volatile organic compounds (VOCs) in a microwave-assisted catalyst bed. J.
Chin. Inst. Chem. Eng. 2006, 37, 519.
6. Baker, R. W.; Yoshioka, N.; Mohr, J. M.; Khan, A. J. Separation of organic vapors
from air. J. Membr. Sci. 1987, 31,259.
7. Behling, R. D.; Ohlrogge, K.; Peinemann, K. V.; Kyburz, E. The separation of
hydrocarbons from waste vapor streams. AIChE Symp. Ser. 1988, 48, 68.
8. Busca, G.; Berardinelli, S.; Resini, C.; Arrighi, L. Technologies for the removal of
phenol from fluid streams: A short review of recent developments. J. Hazard. Mater.
2008,160, 265.
9. Jol, A.; Dragt, A. J. Biotechnological elimination of volatile organic compounds in
waste gases. Proc. Bioreactor Downstream Process. 1995, 2, 373.
PALLAVKAR 151
10. Kiared, K.; Bieau, L.; Brzezinski, R.; Viel, G.; Heitz, M. Biological elimination of
VOCs in bio-filter. Environ. Prog. 1996, 15, 148.
11. Deng, S.; Sourirajan, A.; Matsuura, T. Study of volatile hydrocarbon emission
control by an aromatic poly(Ether Imide) membrane. Ind. Eng. Chem. Res. 1995, 34,
4494.
12. Spivey, J. J. Recovery of volatile organics from small industrial sources. Environ.
Prog. 1988, 7, 31.
13. Environmental Protection Agency (EPA). Control Technologies for Fugitive VOC
Emissions from Chemical Process Facilities. EPA Handbook, EPA/625/R-93-003,
1994.
14. Ottenger, S. P.; Van den Ocver, A. H. C. Kinetics of organic compound removal
from waste gases with a biological filter. Biotechnol. Bioeng. 1983,12, 3089.
15. Khan, F. I.; Ghoshal, A. K. Review: Removal of volatile organic compounds from
polluted air. J. Loss Prev. Process Ind. 2000,13, 527.
16. Ryan, M. A.; Tinnesand, M. Introduction to Green Chemistry, American Chemical
Society: Washington, DC, 2002.
17. Hashisho, Z.; Rood, M. Microwave-swing adsorption to capture and recover vapors
from air streams with activated carbon fiber cloth. Environ. Sci. Technol. 2005, 39,
6851.
18. Hashisho, Z.; Emamipour, H.; Rood, M. J.; Hay, J.; Kim, B. J.; Thurston, D.
Concomitant adsorption and desorption of organic vapor in dry and humid air
streams using microwave and direct electrothermal swing adsorption. Environ. Sci.
Technol. 2008, 42, 9317.
PALLAVKAR 152
19. Metaxas, A. C.; Meredith, R. J. Industrial Microwave Heating; Power Engineering
Series 4; P. Peregrinus: London, U.K., 1983.
20. Plazl, I.; Pipus, G.; Koloini, T. Microwave heating of the continuous flow catalytic
reactor in a nonuniform electric field. AIChE J. 1997, 43, 754.
21. Turner, M. D.; Laurence, R. L.; Conner, W. C. Microwave radiation's influence on
sorption and competitive sorption in zeolites. AIChE J. 2000,46, 758.
22. Curtis, W. M.; Tompsett, G.; Lee, K. H.; Yngvesson, K. S. Microwave synthesis of
zeolites. J. Phys. Chem. B 2004,108, 13913.
23. Valle, S. J.; Conner, W. C. Microwaves and sorption on oxides: A surface
temperature investigation. J. Phys. Chem. B 2006,110- 15459.
24. Panzarella, B.; Tompsett, G. A.; Yngvesson, K. S.; Conner, W. C. Microwave
synthesis of zeolites. 2. Effect of vessel size, precursor volume, and irradiation
method. J. Phys. Chem. B 2007, 111, 12657.
25. Conner, W. C.; Tompsett, G. A. How could and do microwaves influence chemistry
at interfaces. J. Phys. Chem. B 2008,112, 2110.
26. Gharibeh, M.; Tompsett, G. A.; Yngvesson, K. S.; Conner, W. C. Microwave
synthesis of zeolites: Effect of power delivery. J. Phys. Chem. B 2009,113, 8930.
27. Cherbanski, R.; Molga, E. Intensification of desorption process by use of
microwaves - An overview of possible applications and industrial perspectives.
Chem. Eng. Process. 2009, 48, 48.
28. Meredith, R. Engineer's Handbook of Industrial Microwave Heating; IEE Power
Engineering Series 25; Institution of Electrical Engineers: London, U.K., 1998.
PALLAVKAR 153
CHAPTER 6
CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
In this dissertation research, microwave heating has been applied to three emissions
control processes, namely DPF regeneration, NOx reduction, and VOC destruction. The
research has generated valuable conclusions from the three case-studies presented in
chapters 3, 4, and 5, respectively. These conclusions are summarized in the following
sub-sections.
6.1.1 Conclusions from Microwave-Assisted DPF Regeneration
Chapter 3 describes the study on microwave-assisted DPF regeneration. The
conclusions from the study are listed below:
1. Microwave energy is an efficient heat source to raise the temperature of the DPF
made out of SiC.
2. The designed waveguide has been able to achieve relatively uniform temperature
profiles both in the radial and vertical directions of the DPF.
3. The designed waveguide has enhanced the soot oxidation temperature and eliminated
formation of hotspots.
4. The high thermal conductivity of SiC has contributed towards reduced temperature
gradients.
5. The offline regeneration method has been observed to be highly efficient in-terms of
high filtration and regeneration efficiencies after 150 cycles of DPF loading and
regeneration.
PALLAVKAR 154
6. The on-line regeneration of the DPF has not been observed to be efficient due to the
insufficient oxygen concentration in the engine exhaust stream and the heat losses
from the DPF to the surrounding exhaust gases causing low gas temperatures.
6.1.2 Conclusions from Microwave-Assisted Catalytic NOx Abatement
Chapter 4 describes the study of the microwave-assisted De-NOx process using
Pt/SiC based catalyst pack. The conclusions from this study are listed below:
1. The microwave-assisted NOx abatement process using H2 as the reducing agent and
PT-P as the catalyst pack is the most efficient combination in-terms of NOx
reduction.
2. The microwave-assisted De-NOx process is more efficient compared to NOx
adsorber with respect to platinum content and the amount of H2 needed for NOx
abatement.
3. The use of SiC as the microwave absorbing material results in sustained and
controlled heating process, which, in-turn heats the platinum based catalyst during
the NOx abatement process even in the presence of O2.
4. The NOx abatement process results in the formation of trace amount of NH3 which is
an indication of over reduction of NOx.
5. The NOx abatement process using the microwave-assisted De-NOx catalyst system
can be an effective alternate NOx controlling technology compared to NOx adsorber
or NH3 based SCR.
PALLAVKAR 155
6.1.3 Conclusions from Microwave-Assisted Non-Catalytic VOC Destruction
Chapter 5 describes the study on the use of microwave energy as a heating tool for
achieving high temperature destruction of p-xylene in a packed bed reactor with SiC
being the packed material for absorbing microwave energy. The conclusions from this
study are listed below:
1. The choice of SiC as the microwave absorbing media has exhibited excellent
microwave absorbing properties, with rapid conversion of absorbed microwave
energy into heat energy.
2. The steady-state temperature is reached in about 2 to 3 minutes of the microwave
irradiation process.
3. The semiempirical energy balance model describes the dynamic temperature profiles
of the absorbing media and has been found to simulate the experimental data
reasonably well.
4. The DREs of p-xylene with nitrogen as the carrier gas is high, but results in the
formation of other detrimental off-gas species such as Biphenyl, Styrene, Benzene,
and Toluene.
5. The choice of air as the carrier gas is ideal since the by-products of the VOC
destruction process will mainly comprise of CO and CO2.
PALLAVKAR 156
6.2 Recommendations
The recommendations for future studies related to DPF regeneration, NOx abatement
and VOC destruction are listed in the following sub-sections.
6.2.1 Recommendations for Microwave-Assisted DPF Regeneration
The recommendations for the study on microwave-assisted DPF regeneration are
listed below:
1. The use of additional oxygen supply during the regeneration process can be
considered to eliminate the incomplete soot oxidation.
2. The minimization of heat losses from the DPF to the engine exhaust can be
considered by bringing the important components of the microwave and the DPF
system closer to the exhaust pipe and using additional insulation to minimize heat
losses throughout the exhaust line of the diesel engine test unit.
6.2.2 Recommendations for Microwave-Assisted Catalytic NOx Abatement
The recommendations for the study on microwave-assisted NOx abatement process
are listed as follows:
1. The optimum temperature window of operation for NOx abatement using the novel
microwave-assisted De-NOx catalyst system can be studied at different microwave
power levels, and the off-gas species may be indentified for each of these
temperature conditions during the NOx abatement process.
2. The concentration profiles for both H2 consumption and NH3 formation along with
the NOx concentration profile in transient may be measured during the NOx
PALLAVKAR 157
abatement process. The use of online H2 and NH3 analyzer is recommended to
achieve this objective.
3. The effect of other exhaust gas components such as CO, CO2, and SO2 on De-NOx
activity may be studied using H2 as the reducing agent.
4. The use of conventional ammonia guard catalyst as that used in the NH3-SCR
technology may be considered to eliminate the formation of NH3.
6.2.3 Recommendations for Microwave-Assisted Non-Catalytic VOC Destruction
The microwave-assisted VOC destruction process needs further research, and the
recommendations pertaining to this technology are listed below:
1. The measurement of off-gas species such as CO and CO2 may be performed online
during the VOC oxidation process with air being the carrier gas to determine the
carbon balance.
2. The ignition temperature may be determined by measuring temperature, CO, and
CO2 online simultaneously.
3. The effect of an additional catalyst such as platinum on the formation of CO, CO2,
and the corresponding ignition temperature may be studied.
PALLAVKAR 158
APPENDIX A
CALIBRATION CHARTS FOR THE BROOKS MASS FLOW CONTROLLER
FOR INDIVIDUAL GAS COMPONENTS AND GAS STANDARD
600
500
ASeries-A
c
|
o
o
S
400
300
2
S
o
iZ 200
y 2.0218x
R2 • 0.9999
100
0
20
40
60
80
100
120
Scale reading
Fig A.l Air flow calibration using two different Brooks Mass Flow Controller, 5850-E
series Air Tank Pressure - 60 psig; Series-A: De-NOx experiments; Series-B: VOC
Destruction experiments
PALLAVKAR 159
1200
y137.005x
R2* 0.9998
1000
800
£
E
o
o
«
to
i
o
600
200
0
5
10
15
20
25
30
Scale reading
Fig. A.2 Make-up N2 flow calibration using Brooks Mass Flow Controller, 5850-E series,
N2 Tank Pressure - 40 psig
PALLAVKAR 160
2000
1800
1600
1400
| 800
u.
600
400
200
0
10
20
30
40
SO
60
70
80
90
100
Scale reading
Fig. A.3 NOx inN2 standard flow calibration using Brooks Mass Flow Controller, 5850E series, Tank Pressure - 75 psig
PALLAVKAR 161
APPENDIX B
SAMPLE FLOW SEQUENCE, PRESSURE EQUILIBRATION AND INJECTION
FOR THE GC/TCD
initial stag*
A
,„
\V A
V-A: Sample inlet/by-pass valve
V-B, V-C: Sample injection valves
SV-A, SV-B, SV-C: Solenoid valves
^—0-
sv-c
Vent
To column A
To co umn B
(too pi)
Carrier gas
Carrier gas
Sample flow
V-A
i—Q.
SV-A
SV-B
V«nt
To column A
To column B
V-B
Sample loc
(100 Ml)
V-C
Sample loop
(200 Ml)
Carrier gas
Fig. B.l Sample injection sequence, A: Initial stage with sample by-passed,
B: Sample routed through the sample loop
PALLAVKAR 162
Excess sample vent to the atmosphere (pressure equilibration)
1
©SV-B
San^ttnpptd 1 I SV'C
'l insictethaiira ' ' ^
Vent
To column A
column B
Sample loo|
V-B
(100 Ml)
Carrier gas
Carrier gas
Sample Injection stage
V-A
1—G>
: SV-A
sv-c !
Vent
To column A
To column B
Sample loot
V-B
Carrier gas
Sample loof
(1Q0 pi)
V-C
(200 MO
Carrier ga*
Fig. B.2 Sample injection sequence, C: Pressure Equilibration stage,
D: Sample injection stage
PALLAVKAR 163
Stage after sample injection
SV-A
SV-B
I|
'I
Sample trapped
inside the
loop
> I
SV-C
Vent
To column A
To column B
Sample loo|
(200 pi)
Sample loo|
V-B
(100 Ml)
Carrier gas
Carrier gas
Vent line between SV-A and SV-B are opened
Ttia sampla trappad InsMathaloop I* not part of
sampla
InjaeHon stags and is always vantad at any glvan Instanea of tima
SV-A
SV-B
To column A
Sample trapped
inside theloop
SV-C
To column B
Sample loo
(200 m0
Sample
(100MI)
Carrier gas
Carrier gas
Fig. B.3 Sample injection sequence, E: Stage after sample injection,
F: Vent line between SV-A and SV-B opened
PALLAVKAR 164
Purge gas flowing through the system
SV-A
SV-B
SV-C
Vent
To column A
To column B
V-B
Carrier gas
Sample loo|
(100 pi)
V-C
Sample loo|
(200 Ml)
Carrier gas
Fig. B.4 Sample injection sequence, G: System back to the initial stage, sample
by-passed, and purge gas flowing through the sample loop
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