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Catalytic microwave torrefaction and pyrolysis of douglas fir pellet to improve biofuel quality

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CATALYTIC MICROWAVE TORREFACTION AND
PYROLYSIS OF DOUGLAS FIR PELLET TO
IMPROVE BIOFUEL QUALITY
By
SHOUJIE REN
A dissertation submitted in partial fulfillment of
the requirements for the degree of
DOCTOR OF PHILOSOPHY
WASHINGTON STATE UNIVERSITY
Department of Biological Systems Engineering
DECEMBER 2012
UMI Number: 3554589
All rights reserved
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UMI 3554589
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To the Faculty of Washington State University:
The members of the Committee appointed to examine the dissertation of
SHOUJIE REN find it satisfactory and recommend that it be accepted.
___________________________________
Hanwu Lei, Ph.D., Chair
___________________________________
Shulin Chen, Ph.D.
___________________________________
Joan Wu, Ph.D.
ii
ACKNOWLEDGEMENTS
I would like to express my deepest appreciation to my committee chair, Dr. Hanwu Lei, for his
encouragement and guidance on my study and research. Without his consistent advice and
support, I would have not had the chance to pursue and accomplish my study in this attractive
research area. I also thank him for his great support and help to my family during my study. I am
grateful to my committee members, Drs. Shunlin Chen and Joan Wu, for their time, effort,
encouragement, and enlightening advice.
I thank Dr. Manual Garcia, Jonathan Lomber, Zhouhong Wang, and Shuai Zhou for their kind
help and assistance in using TGA. Thanks go to John Anderson, Joan Hagedorn, and Patricia
Huggins for their administrative support.
I am thankful to Dr. Birgitte Ahring, Dr. Keith Thomsen, Dr. Bin Yang, Dr. Xiao Zhang, Dr.
Aftab ahamed, Dr. Junfeng Xue, Shara, Soike and Weiqun Zhong for their advice, suggestions,
and assistances during my doctoral research. The contribution and help with sample analysis
from the scientists of Pacific Northwest National Laboratory (PNNL), Alan Zacher and Todd
Hart, and the encouragement and help from Dr. Roger Ruan at University of Minnesota and Dr.
James Julson at South Dakota State University are greatly appreciated. I am in debt to all my
group colleagues and friends for their support and friendship during my PhD study.
The Office of Research and Department of Biological System Engineering at Washington State
University and Pacific Northwest National Laboratory provided funding support to this research.
I owe my deepest appreciation to my wife Bing Xia, my son George Ren, and my daughter
Kaylee Ren for their everlasting love, understanding, and encouragement. My wholehearted
thanks go to my parents, my wife’s parents, and my brother for their love and support.
iii
CATALYTIC MICROWAVE TORREFACTION AND
PYROLYSIS OF DOUGLAS FIR PELLET TO
IMPROVE BIOFUEL QUALITY
ABSTRACT
By Shoujie Ren, Ph.D.
Washington State University
December 2012
Chair: Hanwu Lei
The aims of this dissertation were to understand the effects of torrefaction as pretreatment on
biomass pyrolysis and catalytic pyrolysis for improving biofuel quality, and the feasibility of
biochar as a cheap catalyst for hydrocarbons production in biomass catalytic pyrolysis and biooil upgrading. The process conditions for microwave torrefaction and pyrolysis of Douglas fir
sawdust pellets were optimized. Microwave pyrolysis of Douglas fir sawdust pellet produced a
comparative bio-oil yield with those from fluidized-bed pyrolysis at the optimization conditions.
The phenols and guaiacols accounted for the largest amount of chemicals in the bio-oil. The
specific phenolic chemicals are highly related to the reaction temperature. The torrefaction
conditions, such as reaction temperature and time, significantly influenced the yields of products.
The bio-oils from torrefaction contained valuable chemicals. The energy yields of torrefied
biomass ranging 67.03−90.06% implied that most energy was retained in the torrefied biomass.
Torrefaction as pretreatment in biomass pyrolysis favored the phenols and sugar production,
iv
producing about 3.21 to 7.50 area% hydrocarbons while reducing organic acids and furans in
bio-oils. Torrefaction also altered the compositions of syngas by reducing CO2 and increasing H2
and CH4. Torrefaction improved the phenols, hydrocarbons, and hydrogen production in catalytic
microwave pyrolysis. The phenols, hydrocarbons, and H2 obtained from torrefied biomass
catalytic pyrolysis over biochar were up to 46 area%, 16 area%, and 27.02 vol%, respectively.
These results indicated that torrefaction as pretreatment can greatly improve the quality of bio-oil
and syngas in biomass pyrolysis and catalytic pyrolysis. Upgraded bio-oil was dominated by
phenols (37.23 area%) and hydrocarbons (42.56 area%) at higher biochar catalyst loadings. The
biochar catalyst may be as a cheap catalyst in biomass conversion and bio-oil upgrading. The
two step-reaction model fits well for Douglas fir sawdust torrefaction with the activation
energies of about 112 kJ/mol and 150 kJ/mol for the first and second reaction stages, respectively.
Derivative thermogravimetric (DTG) curves showed that the shoulder of hemicelluloses
decomposition in torrefied biomass pyrolysis was eliminated. The first-order one-step global
model fitted well for the raw and torrefied biomass pyrolysis with the average activation energies
in the range of 203.94 −195.13 kJ/mol.
v
TABLE OF CONTENTS
ACKNOWLEDGEMENTS...........................................................................................................iii
ABSTRACT...................................................................................................................................iv
TABLE OF CONTENTS...............................................................................................................vi
LIST OF TABLES..........................................................................................................................ix
LIST OF FIGURES ........................................................................................................................x
CHAPTER ONE: Introduction........................................................................................................1
1.1. Torrefaction and pyrolysis........................................................................................................2
1.2. Biomass decomposition by thermal chemical process..............................................................4
1.3 Microwave pyrolysis .................................................................................................................5
1.4 Crude bio-oil properties.............................................................................................................7
1.5 Catalytic biomass pyrolysis and bio-oil refinery...........................................................….…...8
1.6 Objective..................................................................................................................................10
1.7 The contents of this dissertation..............................................................................................11
1.8 References................................................................................................................................13
CHAPTER TWO: MICROWAVE PYROLYSIS OF DOUGLAS FIR SAWDUST PELLETS .......19
2.1. Abstract...................................................................................................................................19
2.2. Introduction.............................................................................................................................20
2.3. Materials and methods............................................................................................................22
2.4. Results and discussion............................................................................................................26
2.5. Conclusion..............................................................................................................................37
2.6. References...............................................................................................................................39
vi
CHAPTER THREE: INTEGRATION OF MICROWAVE TORREFACTION AND PYROLYSIS
TO IMPROVE BIOFUEL QUALITY.............................................................................................45
3.1. Abstract…...............................................................................................................................45
3.2. Introduction.............................................................................................................................46
3.3. Materials and Methods............................................................................................................49
3.4. Results and discussion............................................................................................................52
3.5. Conclusion..............................................................................................................................72
3.6 References................................................................................................................................74
CHAPTER FOUR: BIOMASS AND TORREFIED BIOMASS CATALYTIC PYROLYSIS AND
BIO-OIL UPGRADING OVER BIOCHAR FOR HYDROCARBONS AND HYDROGEN-RICH
SYNGAS PRODUCTION ..............................................................................................................77
4.1. Abstract ..................................................................................................................................77
4.2. Introduction.............................................................................................................................78
4.3. Materials and methods............................................................................................................80
4.4 Results and discussion.............................................................................................................83
4.6. Conclusion..............................................................................................................................95
4.7. References..............................................................................................................................96
CHAPTER FIVE: THERMAL BEHAVIOR AND KINETIC STUDY FOR WOODY BIOMASS
TORREFACTION AND TORREFIED BIOMASS PYROLYSIS BY TGA....................................100
5.1. Abstract ................................................................................................................................100
5.2. Introduction...........................................................................................................................101
5.3. Materials and methods..........................................................................................................103
5.4. Results and discussion..........................................................................................................105
vii
5.5. Conclusion............................................................................................................................117
5.6. References.............................................................................................................................119
CHAPTER SIX: Conclusions and future research.....................................................................122
6.1. Conclusions...........................................................................................................................122
6.2. Future research......................................................................................................................125
viii
LIST OF TABLES
Table 1.1 Physical property of crude bio-oil and heavy fuel oil......................................................7
Table 2.1 Proximate and elemental analyses of Douglas fir pellet................................................22
Table 2.2 Summary of experimental design and results................................................................25
Table 2.3 Optimum conditions and maximum yield of specific chemicals in furans and phenolics
........................................................................................................................................................35
Table 3.1 Coded levels of independent variables in the experimental design...............................50
Table 3.2 Summary of experimental design and results................................................................53
Table 3.3 Summary of torrefaction conditions and results............................................................65
Table 5.1 kinetic parameters from the least –square evaluation..................................................109
Table 5.2 Average apparent activation energy, log A, and R2 of raw and torrefied Douglas fir
sawdust calculated by Friedman method in the reaction order of n=1........................................117
ix
LIST OF FIGURES
Figure1.1 Physical-chemical phenomena happened in torrefaction................................................2
Figure 2.1 Diagram of lab-scale microwave oven setting.............................................................24
Figure 2.2 Response surface and contour line of bio-oil yields (a), syngas yield (b), and biochar
yield (c) as a function of reaction time and reaction temperature..................................................28
Figure 2.3 Product yield distribution from microwave pyrolysis..................................................30
Figure 2.4 Chemical composition distribution of bio-oil from microwave pyrolysis...................32
Figure 2.5 Proposed reaction pathway of Douglas fir pellet pyrolysis..........................................36
Figure 3.1 3D response surface profiles for the yield of bio-oil, non-condensable gases, and
torrefied biomass versus torrefaction conditions......................................................................….54
Figure 3.2 Yield distributions of water, liquid chemicals, non-condensable gases, and torrefied
biomass..........................................................................................................................................57
Figure 3.3 Chemicals distributions in bio-oil........................................................................……58
Figure 3.4 Compositions of non-condensable gases at different reaction temperatures….……...60
Figure 3.5 Higher heating values (HHVs) of torrefied biomass....................................................61
Figure 3.6 Comparison of mass yield and energy yield of torrefied biomass…………...............62
Figure 3.7 Response surface profiles for the energy yield of torrefied biomass versus torrefaction
conditions.......................................................................................................................................63
Figure 3.8 The effects of torrefaction temperatures on the products yield (based on torrefied
biomass) of torrefied biomass pyrolysis……………………........................................................66
Figure 3.9 Chemical composition distributions of bio-oils from microwave pyrolysis of torrefied
biomass..........................................................................................................................................68
x
Figure 3.10 The effects of torrefaction on the chemical composition of bio-oils from torrefied
biomass pyrolysis……………………………………………………….......................................69
Figure 3.11 The effects of torrefaction on the chemical composition of syngas from torrefied
biomass pyrolysis...........................................................................................................................71
Figure 4.1 The effects of biochar loading on product yields of raw Douglas fir pellets (DF)
catalytic pyrolysis ………………….…………………………....................................................84
Figure 4.2 The effects of biochar loading on product yields of torrefied Douglas fir pellets (TDF)
catalytic pyrolysis…………………..............................................................................................85
Figure 4.3 Chemicals distribution of bio-oil from raw (DF) and torrefied (TDF) Douglas fir
pellets pyrolysis under different biochar loading……………………….......................................87
Figure 4.4 The effects of biochar loading on the chemical composition of syngas from raw
Douglas fir pellets (DF) catalytic pyrolysis………………….......................................................89
Figure 4.5 The effects of biochar loading on the chemical composition of syngas from torrefied
Douglas fir pellets (TDF) catalytic pyrolysis…………………….................................................90
Figure 4.6 TGA profiles of raw biochar and recycled biochars.......................………...……….91
Figure 4.7 Chemicals compositions of upgraded bio-oil..............................................................92
Figure 4.8 Proposed reaction pathway of biomass catalytic pyrolysis and bio-oil upgrading for
hydrocarbon and syngas production using biochar catalyst under microwave heating................94
Figure 5.1 TG curves of Douglas fir sawdust torrefaction at different temperatures..................106
Figure 5.2 DTG curves of Douglas fir sawdust torrefaction at different temperatures...............107
Figure 5.3 Two-step reaction model of biomass torrefaction……..............................................108
Figure 5.4 TG curves for raw and torrefied Douglas fir sawdust pyrolysis…………................110
Figure 5.5 DTG curves of raw and torrefied Douglas fir sawdust pyrolysis...............................111
xi
Fig.5.6 Iso-conversional plot of Friedman method for raw (a) and torrefied Douglas fir sawdust b:
from the torrefaction of 250°C and10min, c: from the torrefaction of 275°C and10min, and d:
from the torrefaction of 300°C and10min....................................................................................114
xii
Dedication
This dissertation is dedicated to my wife, my son, and my daughter who
provided encouragement and support during the study and before.
xiii
CHAPTER ONE
INTRODUCTION
With the increasing consumption of energy the price of fossil oils sharply rose in the last several
years. The high price of fossil energy influences the economic development and living properties
significantly. The application of fossil oils also brings the environmental problems as fossil
carbons are not considered into the carbon cycle of atmosphere. The combustion of fossil oils
emits lots of carbon dioxide which contributes to the global warming. The fossil oil also contains
large amount of sulfuric and nitrogen that will convert to sulfuric oxides and nitrogen oxides in
the combustion and form acid rain to destroy the forest. These problems and reducing of the
fossil energy motivate scientists and researchers to look for the renewable energy sources.
Biomass is one of important and large amount renewable fuel sources. Unlike the fossil oil,
biomass is planted and collected annually that can provide a continuous energy supply. Biomass
is considered carbon neutral as its carbon is recycled from the atmosphere (Ragauskas, 2006).
Compared with other renewable resources such as wind energy and nuclear, biomass is the
unique source that can produce the carbon-based fuels and chemicals. The bio-fuels from
biomass can help reduce the dependence for foreign fuels. The potential capacity of biomass is
up to 1.3 billion dry tons annually in US which can replace a large portion of current petroleum
consumption (Perlack, 2005).
However, biomass has some disadvantages such as low bulk density, high water content, and low
energy content to retard its utilization. The low bulk density and high water content in the
1
biomass lead to the cost increasing for transportation and storage. The energy contents of
biomass are much lower than those of other fuel sources such as crude oils (McKendry, 2002).
So biomass cannot be directly applied as a fuel. The pretreatment and treatment for biomass are
required for converting to high energy bio-fuels.
1.1 Torrefaction and pyrolysis
There are several technologies such as biological, mechanical, and thermal chemical conversions
of biomass to high energy fuels. Torrefaction and pyrolysis are widely known thermal chemical
technologies that can produce high energy solid and liquid fuels.
Figure1.1 Physical-chemical phenomena happened in torrefaction (Bergman, 2005a)
2
Torrefaction is a mild thermal chemical method which is conducted at the temperature ranged
from 200 to300 °C in absence of oxygen (Bergman, 2005c). The aim of torrefaction is to
increase the energy density of biomass. During torrefaction, water in biomass is removed
significantly, hemicelluloses are decomposed deeply, and cellulose and lignin are dehydrated and
partially decomposed. The structure of cellulose and lignin are modified. The main product from
torrefaction is torrefied biomass with co-products of bio-oil and syngas. The physical-chemical
phenomena happened in torrefaction is showed in figure 1.1.
There are some advantages of torrefied biomass which has very lower moisture content and high
energy content. The moisture content of torrefied biomass is generally about 1−6wt%. The C/O
ratio of torrefied biomass is increased after torrefaction. The high heating value (HHV) of
torrefied biomass is increased to 22 to 25MJ/kg which is 20−30% higher than that of fresh
biomass. About 90% of energy content in raw biomass is maintained in torrefied biomass which
is about 70wt% of raw biomass. The energy density of torredied biomass is significantly
increased. Torrefied biomass also has high grindability as biomass structure is changed during
torrefaction (Lipinsky et al. 2002; Prins et al., 2006; Arias et al., 2008; Bridgeman et al., 2008).
It will reduce the energy consumption for biomass grinding. According to Bergman (2005b) the
power consumption in size reduction is decreased about 80−90%.
Torrefaction upgrades the energy density, hydrophobic nature and grindability properties of
biomass. These characteristics of torrefied biomass benefit further processes such as combustion
and gasification (Bergman et al., 2005a; Prins et al., 2006; Bergman et al., 2008). Torrefied
biomass can generates electricity with a similar efficiency to coal. It also can be co-fired with
coal. Prins (2006a) investigated gasification for torrified biomass and pointed out that the quality
3
of syngas was improved compared to direct biomass gasfication. These results indicate that
torrefaction is a good pretreatment method for biomass conversion processes.
However, about 10-20% of biomass energy is maintained in two co-products, bio-oil and syngas
after torrefaction process. It is important to investigate the characterization of bio-oil and syngas
from torrefaction and it will be helpful to understand the mechanism of biomass torrefaction and
recover the energy by investigating their potential utilization.
Pyrolysis is a strong thermal chemical technology conducted at temperature ranged from 350 to
800°C in absence of oxygen (Scott et al., 1984). During pyrolysis biomass is heated and
decomposed to produce bio-oil, biochar, and syngas. The heating rate, residence time and final
temperature during pyrolysis significantly influence biofuel production (Babu, 2008). At low
temperature and low heating rate, biomass pyrolysis tends to produce high yield of biochar and
low yield of bio-oils and syngas. At high reaction temperature, the biomass pyrolysis tends to
produce syngas. At high heating rate and low residence time, biomass pyrolysis tends to produce
high yield of bio-oils. In recently years, most of researchers are focusing on biomass pyrolysis to
produce high yield liquids as they are promising hydrocarbon biofuels which can be easily
transported, burnt directly in thermal power stations, or upgraded to obtain a hydrocarbon fuel.
1.2 Biomass decomposition by thermal chemical process
Biomass is mainly composed of three components, hemicelluloses, cellulose and lignin with the
compositions about 25-35%, 40-50% and 20-30% respectively. The reaction mechanism for
three components decomposition was widely investigated in the previous research. These three
components are decomposed at different temperature and rate. The hemicelluloses and cellulose
4
decomposed quickly and in the narrow temperature range. Hemicellulose is decomposed to
produce acetic acid and furans (Ponder, et al., 1991; Sharizadeh et al., 1972; Antal et al., 1991;
Shen et al., 2010) at the temperature from 200 to 300°C. Cellulose is decomposed into ketones,
aldehydes, furans, and sugars (Shen et al., 2009; Lu et al., 2011) at the temperature of
240−350°C. The yields of chemicals from hemicelluloses and cellulose decomposition are
increased with the increase of reaction temperature and time. Lignin is decomposed slowly in a
large range temperature from 280 to 500°C. The chemicals from lignin decomposition are
mainly phenolic compounds (Nowakowski et al., 2010; Pandey et al., 2011; Windt et al., 2009;
Klein et al., 2008).
1.3 Microwave pyrolysis
Traditional pyrolysis processes such as fixed and fluidized bed reactors, use heating provided by
heated surface, sands, and hot gas (Meier et al., 1999; Czernik et al., 2004; Mohan et al., 2006).
The particle size of biomass feed material is an important parameter in determining the efficacy
of pyrolysis. Very fine feedstock is required by conventional pyrolysis in order to obtain high
heating rates (Moghtaderi et al., 2004; Chen et al., 2008). Pyrolysis oil and char yields were
found to be largely dependent on particle sizes (Chen et al., 2008). In the fluidized bed pyrolysis
system, large size particles tend to settle to the bottom of the bed where heat transfer and speed
of thermal processing are reduced. This has a negative effect on the efficiency of production of
bio-oil. The small particle size in the traditional heating needs fine grinding for biomass that will
increase the energy consumption. According to Mani et al. (2004), the energy consumption can
be increased about 50 to 80% when biomass is ground from 3.2mm to 0.6mm. Therefore, the
total cost of process will be increased.
5
Microwave pyrolysis is a thermal chemical technology by heating biomass with microwave
irradiation.
The fundamental mechanism of microwave heating is the agitation of polar
molecules which oscillate under the influence of an oscillating electric and magnetic field.
Microwave heating process with its advantages over conventional heating methods is the nature
of internal fast and uniform heating by microwave irradiation (Miura et al., 2000; McKendry et
al., 2002).
Microwave pyrolysis technology offers a number of advantages over conventional pyrolysis. The
main advantage is that microwave pyrolysis can be occurred for large size particle feedstocks as
the heating is agitated by the polar molecules which oscillate under the influence of an
oscillating electric and magnetic field. Some literatures reported that microwave heating were
used in wood log, wood pellets, and large size biomass pyrolysis (Miura et al., 2004; Lei et al.,
2009; Robinson et al., 2010). Compared to fast pyrolysis like fluidized bed, pre-dried biomass is
not required in microwave pyrolysis. The moisture in biomass needs to be removed for obtaining
high heating rate in conventional fast pyrolysis. But in microwave pyrolysis the moisture is a
good adsorption material for irradiation that induces the pyrolysis (Robinson et al., 2010).
Microwave pyrolysis also produces clean products like bio-oil and syngas because the process
does not have to use biomass powder and does not require agitation and fluidization. The syngas
produced by microwave pyrolysis has higher heating value since it is not diluted by the carrying
gas. Microwave pyrolysis has been successfully applied to process plant residues, wood, and
sewage sludge to produce bio-oil, gas, and bio-char (Yu et al., 2007; Miura et al., 2000;
Dominguez et al., 2005; Wang et al., 2008; Miura et al., 2004; Lei et al., 2009; Huang et al.,
2010; Salema wt al., 2011; Dominguez et al., 2006; Tian et al., 2010).
6
1.4 Crude bio-oil properties
Table 1.1 Physical property of crude bio-oil and heavy fuel oil (Czernik et al., 2004)
Physical property
Bio-oil
moisture content, wt %
15−30
Heavy fuel oil
0.1
pH
~2.5
specific gravity
1.2
0.94
C
54−58
85
H
5.5−7.0
11
O
35−40
1
N
0−0.2
0.3
ash
0−0.2
0.1
HHV, MJ/kg
16−19
40
viscosity (at 50 °C), cP
40−100
180
0.2−1
0.1
elemental composition, wt %
solids, wt %
The bio-oil is a main product from biomass pyrolysis. Bio-oils are carbon based liquid fuels that
have some similar properties such as low solid content and low viscosity compared to petroleum
products. They can be stored, pumped and transported like petroleum products. Bio-oils have
high carbon content and can be combusted directly in boilers, gas turbines, and slow and
medium-speed diesel engines for heat and power applications. Bio-oils also have low nitrogen
and sulfuric content. Therefore, bio-oils are considered to be very promising hydrocarbon fuels.
The physical properties of crude bio-oil and heavy fuel oil are showed in table 1.1.
However, bio-oils have some disadvantages. Compared to the heavy fuel oil, the crude bio-oils
contain higher moisture content which is up to 15-30wt%. The elemental analysis showed that
7
the bio-oils contains about 35-40wt% of oxygen which is much larger than that of heavy fuel oil.
Bio-oil is miscible with ethanol and methanol but immiscible with hydrocarbons. The pH value
of crude bio-oils is around 2.5 due to the existence of organic acids. So bio-oils need a special
container to be stored and transported. The heating value of bio-oil is ~17MJ/kg which is lower
than half of energy from heavy fuel oil.
The chemical constitutions of bio-oils are well investigated. More than hundreds of chemical
compounds are identified in bio-oils. The content of each compound in bio-oils is relatively low,
generally smaller than 5wt% or 10 area%. The compounds found in bio-oils can be classified to
organic acids, aldehydes/ketones, sugars and dehydrosugars, and phenolic compounds which
represent about 5%, 5−10%, 10% and 10−20% respectively (Czernik et al., 2004; Mohan et al.,
2006; Zhang et al., 2007). The chemical compounds are significantly different with those of
petroleum fuel. The petroleum is mainly composed of hydrocarbons such as aromatics, paraffins
and naphthennes. There are few hydrocarbons observed in the bio-oil. The organic acid and high
oxygen content make bio-oil unstable and reduce the heating value of bio-oil. The bio-oils need
to be upgraded to obtain transportation fuels by removing organic acids and reducing oxygen
content.
1.5 Catalytic biomass pyrolysis and bio-oil refinery
Although the properties of bio-oils can be improved partially through pretreated biomass, the
bio-oils still contain high content of oxygen which makes bio-oil unstable and immiscible with
gasoline. And the hydrocarbon content in bio-oil is also very low. For obtaining stable and low
oxygen content bio-oil, some researchers investigated catatlytic biomass pyrolysis. Demiral et al.
8
(2008) using activated alumina as a catalyst pyrolyzed the olive and hazelnut bagasse biomass
samples and reported that the oxygen contents of the bio-oils were markedly reduced. Lappas et
al. (2002) used silica sand, a commercial fluid catalytic cracking catalyst, and a ZSM-5 additive
on a circulation fluid bed. They noted that these catalysts increased the production of water, coke
and gases compared to conventional pyrolysis. However, the obtained liquid product quality and
composition were improved. Aho et al. (2008) investigated a series of acid catalysts of H-Beta25 (CP 814E), H-Y-12 (CBV-712), H-ZSM-5-23(CBV-2314), and H-MOR-20 (CBV-21A) on
pine wood pyrolysis in a fluidized bed reactor. They noted that these catalysts favored to produce
ketones and phenols. Zhang et al. (2009b) also investigated HZSM-5 zeolite catalysts on corncob
pyrolysis in a fluidized bed reactor. They reported that HZSM-5 zeolite catalyst reduced more
than 25% oxygen in bio-oil compared to uncatalyzed process.
Zhang et al. (2009a) also
investigated the effects of different percentage of fresh fluidized catalytic cracking (FCC)
catalyst (FC) and spent FCC catalyst (SC) on corncob pyrolysis in a fluidized bed reactor. They
observed that the hydrocarbon content was increased with the increase of the catalyst
percentages, and this contributed to the decrease of the oxygen content of the bio-oil.
The bio-oils can be refined under catalysts to reduce oxygen and increase hydrocarbons.
Catalytic cracking is a mature and widely used technology to break down heavy and large
molecular fractions to produce light hydrocarbons in petroleum refinery. The catalysts used in
catalytic cracking are mainly based on solid acid supported by alumina-silica such as zeolite and
silica-alumina synthesis. Researchers have paid more attention to bio-oil catalytic cracking as
they need not consume hydrogen and can be conducted at atmosphere or low pressure conditions.
Samolada et al. (2000) evaluated zeolites (HZSM-5), fluid catalytic cracking (FCC) catalysts,
transition metal catalysts (Fe/Cr), and aluminas in a fixed bed catalytic reactor for pyrolysis bio-
9
oil vapors. Their results indicated that HZSM-5 catalysts had high selectivity for aromatic
hydrocarbons while transition metal catalysts (Fe/Cr) had selectivity for phenol and light
phenolics. Vitolo et al. (1999) studied the upgrading of wood pyrolysis oils using HZSM-5 and
H-Y zeolites in a fixed-bed laboratory scale reactor and found out that catalytic upgrading
produced highly deoxygenated oil which had a quite elevated heating value and a good
combustibility. Aho et al. (2010) using beta, Y and ferrierite zeolites, and their iron modified
counterparts upgraded the woody biomass pyrolysis vapours in a dual-fluidized bed reactor and
found out that these catalysts deoxygenated bio-oils and increased selectivity. Adjaye et al. (1995)
also investigated the bio-oil upgrading over the catalysts of HZSM-5, H-mordenite H-Y,
silicalite and silica-alumina. They concluded that the HZSM-5 was the most effective catalyst for
the production of hydrocarbons and aromatics hydrocarbons.
1.6 Objectives
The objectives of this research are: (1) to investigate the effects of torrefaction as pretreatment
on biomass pyrolysis and catalytic pyrolysis to improve the quality of bio-oil and syngas; (2) to
understand the thermal decomposition behaviors and reaction kinetics of biomass torrefaction
and torrefied biomass pyrolysis; (3) to investigate the feasibility of biochar as a cost competitive
catalyst in biomass catalytic pyrolysis and bio-oil upgrading for hydrocarbons and hydrogen-rich
syngas production.
1.7 The contents of this dissertation
This dissertation includes six chapters. The first chapter is a general introduction providing the
background of biomass thermal conversions. The second to fifth chapters are four main parts of
10
the research studies. The chapter sixth is the summary of the studies and future research
directions. The summaries of four main chapters are introduced below.
Chapter two: Microwave pyrolysis of Douglas fir sawdust pellets. This chapter optimized the
reaction conditions of Douglas fir pellet microwave pyrolysis for biofuel prodcution and
developed the models to predict the product yields. The bio-oil and syngas from different
pyrolysis conditions were characterized and compared by GC/MS and GC. The reaction
mechanisms of woody biomass microwave pyrolysis related to reaction temperature were also
proposed based on the bio-oil and syngas analysis.
Chapter three: Integration of microwave torrefaction and pyrolysis to improve biofuel
quality. In this chapter, the process conditions for microwave torrefaction of Douglas fir pellets
were optimized and the mathematic models to predict torrefied biomass yields were developed.
The integrated process by combination of torrefaction and pyrolysis were developed to
investigate the effects of torrefaction as pretreatment on biomass pyrolysis. The bio-oil and
syngas from torrefied biomass pyrolysis were determined and compared with those from raw
biomass pyrolysis.
Chapter four: Biomass and torrefied biomass catalytic pyrolysis and bio-oil upgrading over
biochar catalyst for hydrocarbons and hydrogen-rich syngas production. In this chapter, the
effects of loading amounts of biochar catalyst on products yields were determined. The
characterizations of products from the biomass catalytic pyrolysis over biochar catalyst were
investigated to determine the effects of biochar catalyst and possible reaction mechanisms. The
comparison of products from raw and torrefied biomass microwave catalytic pyrolysis was also
conducted. The recycling of biochar catalyst and their thermal behaviors were also discussed in
11
this chapter. In addition, bio-oil upgrading under biochar catalysis was studied and upgraded
bio-oil were characterized.
Chapter five: Thermal behavior and kinetic study for woody biomass torrefaction and
torrefied woody biomass pyrolysis by TGA. In this chapter, the thermal decomposition
behaviors of Douglas fir sawdust biomass torrefaction and pyrolysis were investigated using
TGA. The isothermal model for biomass torrefaction was developed and the parameters were
determined. One-step global reaction model for torrefied biomass pyrolysis was also developed
and the parameters were determined. The parameters of models for biomass torrefaction and
torrefied biomass pyrolysis were compared with the findings from others.
12
1.8 References
Adjaye J.D., Bakhshi N.N., 1995. Production of hydrocarbons by catalytic upgrading of a fast
pyrolysis bio-oil.1. conversion over various catalysts, Fuel Process. Technol. 45, 161−183
Aho A., Kumar N., Eranen K., Salmi T., Hupa M., Murzin D.Y., 2008. Catalytic pyrolysis of
woody biomass in a fluidized bed reactor: Influence of the zeolite structure, Fuel 87, 2493−2501
Aho A., Kumar N., Lashkul A.V., Eranen K., Ziolek M., Decyk P., Salmi T., Holmbomb B.,
Hupa M., Murzin D. Y., 2010. Catalytic upgrading of woody biomass derived pyrolysis vapours
over iron modified zeolites in a dual-fluidized bed reactor, Fuel 89, 1992–2000
Antal M.J., Leesomboon Jr. T., Mok W.S. Richards G.N., 1991. Mechanism of formation of 2furaldehyde from D-xylose, Carbohydr. Res. 217, 71–85
Arias B.R, Pevida C.G., Fermoso J.D., Plaza M.G., Rubiera F.G., Martinez J.J.P., 2008.
Influence of torrefaction on the grindability and reactivity of woody bBiomass, Fuel Process.
Technol. 89, 169–175
Babu B.V., 2008. Biomass pyrolysis: a state-of-the-art review, Biofuel Bioprod. Bior. 2, 393–
414
Bergman P.C.A., Boersma A.R., Kiel J.H.A., Prins M.J., Ptasinski K.J., Janssen F.J.J.G.
Torrefied biomass for entrained-flow gasification of biomass, Report ECN-C--05-026, 2005a
Bergman P.C.A., Boersma A.R., Zwart R.W.H., Kiel J.H.A. Development of torrefaction for
biomass co-firing in existing coal-fired power stations, ECN report, 2005b
13
Bergman P.C.A., Kiel J.H.A. Torrefaction for biomass upgrading, 14th European Biomass
Conference & Exhibition, France, 2005c
Bridgeman T.G., Jones J.M., Shield I., Williams P.T., 2008. Torrefaction of reed canary grass,
wheat straw and willow to enhance solid fuel qualities and combustion properties, Fuel 87, 844–
856
Chen M., Wang J., Zhang M., Chen M., Zhu X., Min F., Tan Z., 2008. Catalytic effects of eight
inorganic additives on pyrolysis of pine wood sawdust by microwave heating, J. Anal. Appl.
Pyrolysis 82, 145–150
Czernik S., Bridgwater A.V., 2004. Overview of applications of biomass fast pyrolysis oil,
Energy Fuels 18, 590–598
Demiral I., Sensoz S., 2008. The effects of different catalysts on the pyrolysis of industrial
wastes (olive and hazelnut bagasse), Bioresour. Technol. 99, 8002–8007
Dominguez A., Menendez J.A., Inguanzo M., Pis J.J., 2005. Investigations into the
characteristics of oils produced from microwave pyrolysis of sewage sludge, Fuel Process.
Technol. 86, 1007–1020
Dominguez A., Menendez J.A., Inguanzo M., Pis J.J., 2006. Production of bio-fuels by high
temperature pyrolysis of sewage sludge using conventional and microwave heating, Bioresour.
Technol. 97, 1185–1193
Huang Y., Kuan W., Lo S., Lin C., 2010. Hydrogen-rich fuel gas from rice straw via microwaveinduced pyrolysis, Bioresour. Technol. 101, 1968–1973
Klein M.T., Virk P.S., 2008. Modeling of lignin thermolysis, Energy Fuels 22, 2175–2182.
14
Lappas A.A., Samolada M.C., Iatridis D.K., Voutetakis S.S., Vasalos I.A., 2002. Biomass
pyrolysis in a circulating fluid bed reactor for the production of fuels and chemicals, Fuel 81,
2087–2095
Lei H., Ren S., Julson J., 2009. The effects of reaction temperature and time and particle size of
corn stover on microwave pyrolysis, Energy Fuels 23, 3254–3261
Lipinsky E.S., Arcate J.R., Reed T.B., 2002. Enhanced wood fuels via torrefaction, Fuel Chem.
Div. Preprints 47, 408–410
Lu Q., Yang X., Dong C., Zhang Z., Zhang X., Zhu X., 2011. Influence of pyrolysis temperature
and time on the cellulose fast pyrolysis products: Analytical Py-GC/MS study, J. Anal. Appl.
Pyrolysis 92, 430–438
Mani, S., Tabil L.G., Sokhansanj S., 2004. Grinding performance and physical properties of
wheat and barley straws, corn stover and switchrgrass, Biomass Bioenergy 27, 339–352
McKendry P., 2002. Energy production from biomass (part 1): Overview of biomass, Bioresour.
Technol. 8, 37–46
Meier D., Faix O., 1999. State of the art of applied fast pyrolysis of lignocellulosic materials: A
review, Bioresour. Technol. 68, 71–77
Miura M., Kaga H., Tanaka S., Takanashi K., Ando K., 2000. Rapid microwave pyrolysis of
wood, J. Chem. Eng. Jpn. 33, 299–302
Miura M., Kaga H., Sakurai A., Kakuchi T., Takahashi K., 2004. Rapid pyrolysis of wood block
by microwave heating, J. Anal. Appl. Pyrolysis 71, 187–199
15
Moghtaderi B., Meesri C., Wall T.F., 2004. Pyrolytic characteristics of blended coal and woody
biomass, Fuel 83, 745–750
Mohan D., Pittman C.U., Steele P.H., 2006. Pyrolysis of wood/biomass for bio-oil: A critical
review, Energy Fuels 20, 848–889
Nowakowski D.J., Bridgwater A.V., Elliott D.C., Meier D., de Wild P., 2010. Lignin fast
pyrolysis: Results from an international collaboration, J. Anal. Appl. Pyrolysis 88, 53–72
Pandey M.P., Kim C.S., 2011. Lignin depolymerization and conversion: A review of
thermochemical methods, Chem. Eng. Technol. 34, 29–41
Perlack R.D., Wright L.L., Turhollow A.F., Graham R.L., Stockes B.J., Erbach D.C. Biomass as
feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion-ton
annual supply, US Department of Energy and the US Department of Agriculture by Oak Ridge
NationalLaboratory, Oak Ridge, TN, April 2005
Ponder G.R., Richards G.N., 1991. Thermal synthesis and pyrolysis of a xylan, Carbohydr. Res.
218, 143–155
Prins M.J., Ptasinski K.J., Janssen F.J.J.G., 2006.
More efficient biomass gasification via
torrefaction, Energy 31, 3458–3470
Ragauskas A.J., Williams C.K., Davison B.H., Britovsek G., Cairney J., Eckert C.A., Frederick
Jr. W.J., Hallett J.P., Leak D.J., Liotta C.L., Mielenz J.R., Murphy R., Templer R., Tschaplinski
T., 2006. The path forward for biofuels and biomaterials, Science 311, 484−489
Robinson, J.P., Kingman, S.W., Barranco, R., Snape, C.E., Al-Sayegh, H., 2010. Microwave
pyrolysis of wood pellets, Ind. Eng. Chem. Res. 49, 459–463
16
Salema A.A., Ani F.N., 2011. Microwave induced pyrolysis of oil palm biomass, Bioresour.
Technol. 102, 3388–3395
Samolada M.C., Papafotica A., Vasalos I.A., 2000. Catalyst evaluation for catalytic biomass
pyrolysis, Energy Fuels 14, 1161−1167
Scott D.S., Piskorz J., 1984. The continuous flash pyrolysis of biomass, Can. J. Chem. Eng. 62,
404– 412
Sharizadeh F., Mcginnis G.D., Philpot C.W., 1972. Thermal degradation of xylan and related
model compounds, Carbohydr. Res. 25, 23–33
Shen D.K., Gu S., Bridgwater A.V., 2010. Study on the pyrolytic behaviour of xylan-based
hemicellulose using TG–FTIR and Py–GC–FTIR, J. Anal. Appl. Pyrolysis 87, 199–206
Shen D.K., Gu S., 2009. The mechanism for thermal decomposition of cellulose and its main
products, Bioresour. Technol. 100, 6496–6504
Tian Y., Zuo W., Ren Z., Chen D., 2010. Estimation of a novel method to produce bio-oil from
sewage sludge by microwave pyrolysis with the consideration of efficiency and safety, Bioresour.
Technol. 102, 2053–2061
Vitolo S., Seggiani M., Frediani P., Ambrosini G., Politi L., 1999. Catalytic upgrading of
pyrolytic oils to fuel over different zeolites, Fuel 78, 1147−1159
Wang X., Chen H., Luo K., Shao J., Yang H., 2008. The influence of microwave drying on
biomass pyrolysis, Energy Fuels 22, 67–74
17
Windt M., Meier D., Marsman J.H., Heeres H.J., de Koning S., 2009. Micro-pyrolysis of
technical lignins in a new modular rig and product analysis by GC–MS/FID and GC X GC–
TOFMS/FID, J. Anal. Appl. Pyrolysis 85, 38–46
Yu F., Deng S., Chen P., Liu Y., Wan Y., Olson A., Kittelson D., Ruan R., 2007. Physical and
chemical properties of bio-oils from microwave pyrolysis of corn stover, Appl. Biochem.
Biotechnol. 136–140, 957–970
Zhang Q., Chang J., Wang T., Xu Y., 2007. Review of biomass pyrolysis oil properties and
upgrading research, Energy Convers. Manage. 48, 87–92
Zhang H., Xiao R., Huang H., Wang D., Zhong Z., Song M., Pan Q., He G., 2009a. Catalytic fast
pyrolysis of biomass in a fluidized bed with fresh and spent fluidized catalytic cracking (FCC)
catalysts, Energy Fuels 23, 6199–6206
Zhang H., Xiao R., Huang H., Xiao G., 2009b. Comparison of non-catalytic and catalytic fast
pyrolysis of corncob in a fluidized bed reactor, Bioresour. Technol. 100, 1428–1434
18
CHAPTER TWO
MICROWAVE PYROLYSIS OF DOUGLAS FIR SAWDUST
PELLETS
2.1 Abstracts
Microwave pyrolysis of Douglas fir sawdust pellet was investigated to determine the effects of
reaction temperature and time on the yields of bio-oil, syngas, and biochar using a central
composition design (CCD) and response surface analysis. The research results indicated that
thermo-chemical conversion reactions can take place rapidly in large-sized biomass pellet by
using microwave pyrolysis. The yields of bio-oil and syngas were increased with the reaction
temperature and time. The highest yield of bio-oil was 57.8% (dry biomass basis) obtained at
471°C and 15min. GC/MS analysis indicated that the bio-oils were mainly composed of phenols,
guaiacols, furans, ketones/aldehydes, and organic acids. The yield of specific chemicals such as
furans and phenolic compounds were highly relatived to the reaction temperature. The syngas
contained high value chemicals, such as carbon monoxide, methane, and short chain
hydrocarbons.
Keywords: Douglas fir sawdust pellet; biofuels; microwave pyrolysis
19
2.2 Introduction
Biomass pyrolysis is a thermo-chemical process that conducted at 400–600°C. During pyrolysis
biomass is heated and decomposed in the absence of oxygen to produce biofuels, biochar, and
other chemicals (Scott and Piskorz, 1984). Traditional pyrolysis processes such as fixed and
fluidized bed reactors, use heating provided by heated surface, sands, and hot gas (Meier and
Faix, 1999; Czernik and Bridgwater, 2004; Mohan et al., 2006). Microwave pyrolysis is one of
the novel thermo-chemical technologies by heating biomass with microwave irradiation. The
major advantage of the microwave heating process over conventional heating methods is the
nature of internal fast and uniform heating by microwave irradiation (Miura et al., 2000;
McKendry, 2002). At present microwave pyrolysis is successfully applied to processing plant
residues (Yu et al., 2007; Dominguez et al., 2007; Lei et al., 2009; Huang et al., 2010; Salema
and Ani, 2011), wood (Miura et al., 2000; McKendry, 2002; Miura et al., 2004; Wang et al.,
2008), and sewage sludge (Dominguez et al., 2006; Tian et al., 2010)to produce bio-oil, gas, and
bio-char.
The particle size of biomass feed material, as an important parameter in determining the efficacy
of pyrolysis, significantly effects pyrolysis oil and char yields (Sensoz et al., 2000). In
conventional pyrolysis system like fluidized bed, very fine feedstock is used to obtain high
heating rates and liquid yield because large–sized particles are difficult to agitate and process
(Sensoz et al., 2000; Moghtaderi et al., 2004; Yi et al., 2008). In the fluidized bed pyrolysis
system, large size particles tend to settle to the bottom of the bed where heat transfer and speed
of thermal processing are reduced. This has a negative effect on the efficiency of production of
bio-oil, which is increased when the particle size is reduced. But in microwave pyrolysis system,
previous research results from biomass microwave pyrolysis indicate that thermochemical
20
conversion reactions can take place rapidly in relatively large–sized biomass materials (Lei et al.,
2009).
Compared to the fast pyrolysis, microwave pyrolysis of biomass produced low liquid yield. In
biomass fast pyrolysis using conventional heating reactors such as fluidized bed, the liquid yield
is pretty high, up to 60-70wt% (Meier and Faix, 1999; Mohan et al., 2006). However, in
microwave assisted pyrolysis of biomass the liquid yield was generally lower than 30wt%
(Miura et al., 2004; Lei et al., 2009; Salema and Ani, 2011). In some reports the microwave
absorption materials or catalysts were added to increase heating rate and liquid yield production
during microwave pyrolysis (Chen et al., 2008; Moen et al., 2009; Du et al., 2010; Salema and
Ani, 2011). The liquid yield can be increased to about 40wt%, but it is still much lower than that
from fluidized bed pyrolysis. It indicates that the high liquid yield production is a big challenge
in microwave pyrolysis.
Douglas fir is one of the widespread and abundant species in western North America. It is a soft
wood and belongs to the coniferous family. The Douglas fir is an important commercial wood; it
can be used for structural timbers, lumber, and furniture, which generate large amounts of
sawdust and wood residues every year. Wood pellets offer a renewable energy source for power
generation and residential heating. Densification of wood residues into pellets has been practiced
since several decades ago (Vinterback, 2002). In North America there were estimated 800,000
wood pellet stoves in use with a total of about 1,500,000 tons of annual wood pellet consumption
in 2008 (Spelter and Toth, 2009). Although microwave pyrolysis of wood pellets has been
previously reported (Robinson et al., 2010), data on the pyrolysis process optimization and
characteristics of products such as bio-oil and syngas, has not been reported.
21
The objective of this study was to investigate microwave pyrolysis of Douglas fir sawdust pellets,
to determine the effects of pyrolytic conditions on the yields of bio-oil, syngas, and biochar, and
to establish models to predict the product yields. The compositions of bio-oil and syngas were
characterized by GC/MS and GC respectively. The reaction kinetics of wood pellet microwave
pyrolysis was investigated.
2.3 Materials and methods
2.3.1 Materials
Table 2.1 Proximate and elemental analyses of Douglas fir pellet
Characteristics
Proximate analysis (wt %)
Elemental analysis (wt %)
Douglas fir pellet
Moisture
4.82
Volatile matter
76.08
Fixed carbon
18.89
Ash
0.21
Carbon
47.9
Hydrogen
6.55
Nitrogen
0.08
Oxygen
45.57
HHV (MJ/kg)
19.4
Douglas fir sawdust pellets were purchased from Bear Mountain Forest Products Inc. (USA).
The pellets were made from 100% natural Douglas fir wood sawdust with a heating value of
19.4MJ/kg and a water content of 7 wt%. The pellets had an average diameter of 6mm and an
22
average length of 10mm. Proximate and elemental analysis of Douglas fir pellet were showed in
Table 2.1.
2.3.2 Microwave apparatus
A Sineo MAS-II batch microwave oven (Shanghai, China) with a rated power of 1000W was
used at the 700W power setting. 400g Douglas fir pellets were placed in a 1-liter quartz flask
inside the microwave oven. The experimental setting was showed in Figure 2.1. Five parallel
bulb condensers, each one-half meter long, were used for condensation. The temperature of
cooling water in condensers was 0–5 °C. The system was purged with nitrogen on a flow rate of
1000 mL/min for 20 min prior to microwave torrefaction to create an oxygen free background.
The temperature of biomass was measured by an infrared sensor through a dead end quartz tube
which was penetrated to the central of the flask. After reaching desired reaction temperatures,
the microwave reactor equipped with automatic temperature/power control used a minimum
power (e.g. 0-100 W) to maintain the desired reaction temperatures. During pyrolysis the
heavier volatiles were condensed into liquids as bio-oils and the lighter volatiles escaped as
syngasES at the end of the condensers where they were either burned or collected for analysis.
Char was left in the quartz flask. The weight of syngas product was calculated using following
equation:
Weight of syngas = initial wood pellet mass – bio-oil mass – biochar mass
23
(1)
Figure 2.1 Diagram of lab-scale microwave oven setting
2.3.3 Experiment design
Central composite experimental design (CCD) was used in the optimization of volatile (bio-oil
and syngas) and biochar production (Box and Wilson, 1951). Reaction temperature (X1, °C) and
reaction residence time (X2, min) were chosen as the independent variables and are shown in
Table 1. Reaction residence time was recorded after the desired temperature was reached.
Volatile yield (Yi, %) was the dependent output variable. For statistical calculations, the variables
Xi were coded as xi according to Eq. 2:
xi=(Xi−X0)/∆X
(2)
where xi is dimensionless, Xi is the real value of an independent variable, X0 is real value of the
independent variable at the center point, and ∆X is step change.
24
A 22-factorial CCD, with 4 axial points (α=1.414) and 5 replications at the center points (n0=5)
leading to a total number of 13 experiments, was employed (Table 2.2) for the optimization of
the conditions of pyrolysis process. The second–degree polynomials (Eq. 3) were calculated with
the statistical package (SAS Institute Inc., USA) to estimate the response of the dependent
variable.
Yi = b0 + b1X1 + b2X2 + b11X12 + b21X2X1 + b22X22
(3)
where Yi is predicted response, X1 and X2, are independent variables, b0 is the offset term, b1 and
b2 are linear effects, b11 and b22 are squared effects, and b21 are interaction terms.
Table 2.2 Summary of experimental design and results
Code value
Run#
Yield (wt%)
Reaction
Reaction
Bio-oil
Syngas
Biochar
DF1
temperature
x1
–1
time
–1 x2
35.0
9.8
55.2
DF2
1
–1
52.2
13.3
34.4
DF3
–1
1
38.6
10.6
50.8
DF4
1
1
53.8
14.5
31.8
DF5
–1.414
0
31.4
7.9
60.7
DF6
1.414
0
53.9
15.0
31.2
DF7
0
–1.414
45.2
11.8
42.9
DF8
0
1.414
49.6
12.9
37.6
DF9
0
0
49.8
12.9
37.3
DF10
0
0
50.2
13.1
36.7
DF11
0
0
49.9
12.2
37.9
DF12
0
0
48.9
11.9
39.4
DF13
0
0
49.0
12.0
39.0
25
2.3.4 GC/MS analysis for bio-oil
Chemical compositions of the bio-oil were determined using an Agilent 7890A GC/MS with a
DB-5 capillary column. The GC was programmed at 45°C for 3 min and then increased at
10°C/min to 300°C and finally held isothermal for 10 min. The injector temperature was 300°C
and the injection size was 1 L. The flow rate of the carrier gas (helium) was 0.6 mL/min. The
ion source temperature was 230°C for the mass selective detector. The compounds were
identified by comparing the spectral data with the NIST Mass Spectral library.
2.3.5 GC analysis for syngas
The chemical compositions of syngas were determined by a Carle 400 gas chromatography (GC)
system with a thermal conductivity detector (TCD). The details of experiment setting were
described in previous report (Elliott et al., 2006).
2.4 Results and discussion
2.4.1 Response surface analysis
The detailed experimental design and observed results were shown in Table 2.2. Thirteen
experiments were performed using different combinations of the variables as per the CCD. The
suitable levels for these parameters were also determined using statistical CCD. Previous
research indicated that the most important physical factors that affected bio-oil and syngas
production from corn stover microwave pyrolysis were the reaction time and reaction
temperature (Lei et al., 2009). In this research, these two parameters have significant effects on
microwave pyrolysis of Douglas fir pellets. Using the results of the experiments obtained the
26
following second–order polynomial equations for the bio-oil yield (Eq. 4), syngas yield (Eq. 5),
and biochar yield (Eq. 6) as a function of reaction temperature (X1, °C) and reaction time (X2,
min):
Ybio-oil = -250.01 + 1.27 X1 + 1.59 X2 – 0.0014X12 – 0.043 X22
(4)
Ysyngasl = -36.29 + 0.19 X1 + 0.086 X2 – 0.00019X12
(5)
Ybiochar = 385.04 – 1.45 X1 – 1.67 X2 + 0.0016X12 + 0.044 X22
(6)
The model terms X12 (P-value=0.079, 0.355) were insignificant (P-values > 0.05) when Eq. 3
was used to fit the data for bio-oil and biochar. The model term X12 (P-value = 0.783) and X22 (Pvalue = 0.974) were not significant (P-value > 0.05) when Eq. 3 was used to fit the data for
syngas. Eq. 3 was reduced by using backward statistical analysis, and Eq. 4–6 were obtained
with its significant terms (P-value < 0.0001). The correlation coefficients of determination, R2
was 1.00, 0.95, and 0.99 respectively for bio-oil, syngas, and biochar, implying that the reduced
quadratic regression models can be used to explain the pyrolysis reaction, and the biofuel yield
variations were attributed to the independent variables of reaction time, reaction temperature, and
their squared effects. Hence, these models can adequately represent the experimental data and
can be used to predict the production yields for biomass microwave pyrolysis processes.
The volatile (bio-oil and syngas) and bio-oil yields increased with increasing reaction
temperature and time. The volatile yields were found to range from 39.3 to 68.8wt% depending
on pyrolysis conditions, while the yield of bio-oils was from 33.8 to 57.8wt% based on dry
biomass. The syngas yield ranged from 7.9 to 15.0wt% and increased with the reaction
27
temperature. The biochar yield varied from 31.2 to 60.7wt%. Figure 2.2 represents the response
contour and surface plots for the pyrolysis conditions of product yields.
28
Figure 2.2 Response surface and contour line of bio-oil yields (a), syngas yield (b), and biochar
yield (c) as a function of reaction time and reaction temperature
29
The water contents of bio-oils were determined by Mettler Toledo V30 volumetric Karl-Fisher
Titrator. The water contents of bio-oils ranged from 36.8 to 52.9%, 17.2 to 24.7wt% based on
dry biomass. Liquid chemicals in bio-oils ranged 16.1 to 34.4wt% based on dry biomass. The
highest yield of liquid chemicals in bio-oil was observed at the pyrolysis condition of 450°C and
20 min. The yield distributions of water, liquid chemicals, syngas and biochar are showed in
figure 2.3.
100
Product yields, wt% on biomass
90
80
70
60
50
40
30
20
10
0
DF1
DF2
DF3
Charcoal
DF4
Syngas
DF5
DF6
DF7
Liquid Chemicals
Water
DF8
DF9
Figure 2.3 Product yield distribution from microwave pyrolysis
Biomass microwave pyrolysis produced low yield bio-oil in previous reports. 22.6wt% bio-oil
yield was obtained from rice straw microwave pyrolysis at temperature of 407°C with power
input of 300W (Huang et al., 2008). 7.9–9.2wt% bio-oil yield on dry basis was obtained from
30
coffee hulls microwave pyrolysis at the temperature ranged from 500°C to 1000°C with the
reaction time of 15min (Dominguez et al., 2007). 31.5wt% bio-oil yield was obtained from wood
block microwave pyrolysis with the power consumption of 1.11 kWh kg−1 and reaction time of
11min (Miura et al., 2004). To increase the liquid yield of microwave pyrolysis, microwave
absorption was added in the microwave pyrolysis. About 25wt% bio-oil yield was obtained from
oil palm biomass with char absorber microwave pyrolysis with the power input of 450W and
reaction time of 25min (Salema and Ani, 2011). About 34wt% bio-oil yield was obtained from
wood sawdust with ionic liquid microwave pyrolysis (Du et al., 2010). But in fluidized bed about
60–70% bio-oil yield on dry basis was obtained (Meier and Faix, 1999; Mohan et al., 2006).
Compared to the fluidized bed, microwave pyrolysis produces much lower liquid yield. In this
research the highest bio-oil was 57.8wt% based on dry biomass obtained at the optimization
conditions with the reaction temperature of 470.7°C and reaction time of 15 min. This result
indicates that Douglas fir pellet microwave pyrolysis can produce the high yield liquid close to
conventional pyrolysis in optimum conditions (Meier and Faix, 1999; Bridgwater et al., 1999;
Mohan et al., 2006; Garcia-Perez et al., 2007; Mullen and Boateng, 2008).
2.4.2 GC/MS analysis for bio-oil
To further understand the chemical reactions in microwave pyrolysis, we carried out GC/MS
analysis to determine the composition of bio-oils. The main ingredients of bio-oils were
guaiacols, phenols, furans, ketones/aldehydes, and organic acids (Figure 2.4). The guaiacols
accounted for the largest amounts in the bio-oil which represented 49.0–72.7% in area. The
guaiacols were mainly made up of 2-methoxy-4-methylphenol, 2-methoxyphenol, 4-ethyl-2methoxy-phenol and phenol, 2-methoxy-4-(1-propenyl)-, (E)- which represented 14.0–19.8%,
5.3–12.0%, 5.0–12.8% and 5.5–14.2% of the bio-oil in area, respectively. The phenols in the bio31
oils ranged from 3.6–11.0% in area. The phenols were mainly made up of 1,2-penzenediol,
phenol, 2-methyl- and phenol which represented 1.9–5.0%, 0.1–1.4% and 0.2–0.9% in area,
respectively. The furans in the bio-oils ranged from 10.1–17.7% in area. The furans were
primarily composed of furfural, tetrahydro-2, 5-dimethoxy- furan, ß-methoxy-(S) - 2furanethanol and, 5-methyl-2-furancarboxaldehyde. Furfural represented 2.8–6.6% of the bio-oil
in area. The organic acids in the bio-oil ranged 0.3–6.2% in area. Levoglucosan sugars had very
low yield except in run#DF7 which was processed in the condition of 400°C and 7.9min. The
reason of this is that the levoglucosan can be decomposed to gas and volatiles at high
temperature and long reaction residence time (Thangalazhy-Gopakumar et al., 2011).
Figure 2.4 Chemical composition distribution of bio-oil from microwave pyrolysis
32
Douglas fir is mainly composed of three components, hemicelluloses, cellulose and lignin with
the compositions of 21%, 44% and 32% respectively (Pettersen, 1984). The reaction mechanism
and characteristics of bio-oil for three components decomposition were widely investigated in
previous research (Sharizadeh et al., 1972; Ponder and Richards, 1991; Antal et al., 1991; Klein
and Virk, 2008; Shen and Gu, 2009; Windt et al., 2009; Shen et al., 2010; Nowakowski et al.,
2010; Pandey and Kim, 2011; Lu et al., 2011). Hemicelluloses can be decomposed to acetic
acid and furfural and cellulose can be decomposed to ketones, aldehydes, furans. The yields of
chemicals derived from hemicelluloses and cellulose decomposition are increased with the
increase of reaction temperature and time. Our results are in good agreement with the results
from previous reports. The total yield of organic acid, ketones, aldehydes, and furans were
increase from 17.5 to 30.9% in area with the temperature and reaction time increasing. The
maximum yield was obtained at the temperature 400°C and reaction time 22min. Phenolic
compounds mainly from lignin decomposition are wide useful chemicals in pharmacy, synthesis
and food industry (Proestos et al., 2005; Effendi et al., 2008; Pandey and Kim, 2011). Therefore,
some researchers focus on in lignin pyrolysis to produce phenolic compounds (Murwanashyaka
et al., 2001; Klein and Virk, 2008; Windt et al., 2009; Nowakowski et al., 2010; Pandey and Kim,
2011). In this research we produced very high yield of phenolic compounds, from 60 to 78% in
area in which guaiacols contributed about 49 to 72% in area. The relatively high yields of
phenolic compounds were obtained at temperatures of 350°C and 450°C with reaction time of
10min.
The chemical ingredients of furans and phenolic compounds were further analyzed as they were
large amount chemicals in bio-oil which represented up to 62.6 to 84.3% in area. In furans the
large amount chemicals were furfural and 2-furancarboxaldehyde, 5-methyl- which decreased
33
with the increase of reaction temperature and increased with the increase of reaction time. The
amounts of other two main furans, ß-methoxy-(S) - 2-furanethanol and tetrahydro-2, 5dimethoxy- furan, increased with the reaction temperature and time. It was found that the yield
of guaiacols first increased and then decrease with the increase of reaction time at the same
temperature while the yield of phenols were not significantly changed. The guaiacols were
mainly made up of eight specific chemicals, phenol, 2-methoxy-, phenol, 2-methoxy-4-methyl-,
phenol, 4-ethyl-2-methoxy-, phenol, 2-methoxy-4-(1-propenyl)-, (E)- 2-pethoxy-4-vinylphenol,
phenol, 2-methoxy-4-propyl-, phenol, 2-methoxy-3-(2-propenyl)- , and phenol, 2-methoxy-4-(1propenyl)-. The first four chemicals had high percentage ranged from 5% to 20% in area, and the
last four chemicals had the percentage ranged from 1% to 5% in area. The yield of 2-methoxyphenol, 2-methoxy-4-vinylphenol, and 2-methoxy-4-propyl-phenol, were very stable in different
reaction temperature. The yield of 4-ethyl-2-methoxy-phenol, 2-methoxy-3-(2-propenyl)-phenol,
and 2-methoxy-4-(1-propenyl)-phenol, decreased with the increasing reaction temperature.
However, the yield of 2-methoxy-4-methyl-phenol, increased with the increasing reaction
temperature in this research. The yield of 2-methoxy-4-methyl-phenol was also related to the
reaction time. At temperature 400°C, the yield significantly increased from 7.9 to 15min and
then significantly decreased from 15min to 22min. The maximum yield of 19.8% in area was
obtained at condition of 400°C and 15min. Furthermore, it was found that the yield of 1, 2benzenediol increased with the increase of reaction temperature and time. This result indicated
that the second reaction occurred in which the guaiacols were converted to 1, 2-benzenediol
which is in a good agreement with the previous reports (Thangalazhy-Gopakumar et al., 2011;
Hosoya et al., 2008).The optimum conditions and maximum yield for main furans and phenolic
34
compounds are showed in Table 2.3 These results indicated that the selectivity of specific
compounds can be improved through controlling the reaction temperature and time.
Table 2.3 Optimum conditions and maximum yield of specific chemicals in furans and phenolics
Category
Compound name
Reaction
Reaction
Yields
Furfural
15
time
20
(min)
22
6.56
(% 3.64
in area)
2-Furanethanol, ß-methoxy-(S)-
329
temperature
350
(°C)
400
Furan, tetrahydro-2,5-dimethoxy-
470
15
3.27
2-Furancarboxaldehyde, 5-methylFurans
Total
Phenolic
chemicals
3.46
16.93
Phenol, 2-methoxy-3-(2-propenyl)-
329
15
4.47
Phenol, 2-methoxy-4-(1-propenyl)-
329
15
3.34
Phenol, 2-methoxy-4-(1-propenyl)-
350
10
14.17
Phenol, 4-ethyl-2-methoxy, (E)-
350
20
12.79
Phenol, 2-methoxy-4-propyl-
400
15
3.47
Phenol, 2-methoxy-4-methyl-
400
15
19.83
Phenol, 2-methoxy-
450
10
11.98
2-Methoxy-4-vinylphenol
471
15
5.41
1,2-Benzenediol
471
15
5.03
Total
80.49
The proposed reaction pathway of Douglas fir pellet microwave pyrolysis is showed in Figure
2.5. We proposed that hemicellulose and cellulose decomposition mainly include two steps. The
first step is that hemicelluloses and cellulose are depolymerized and dehydrated to furfural and 2furancarboxaldehyde at low temperature ranged from 329 to 350°C. The second step is that C=O
35
linkage is broken and recombinated to ß-methoxy-(S) - 2-furanethanol and tetrahydro-2, 5dimethoxy- furan at high reaction temperature ranged from 400 to 471°C. Compared to
hemicelluloses and cellulose, lignin decomposition involving depolymerization, dehydration,
cracking and hydrogenation is much complex. At low temperatures from 329 to 350°C, lignin is
primarily depolymerized and dehydrated to produce propenyl- guaiacols. The propenyl-guaiacols
are further hydrogenated to propyl-guaiacols at the temperature 350 to 400°C. Cracking of lignin
and guaiacols occurred at temperatures from 350 to 471°C and the positions of C–C bond broken
are highly related to the temperature. The cracking of Cβ–C γ bond occurs at 350°C, followed by
the cracking of Cα–Cβ bound at 400°C and C4–Cα bond at 450°C. The cleavage of C–OCH3
occurs at the temperature 471°C.
Figure 2.5 Proposed reaction pathway of Douglas fir pellet pyrolysis
36
2.4.3 GC analysis for syngas
Syngas was one of the main products of Douglas fir pellet pyrolysis. The yield of syngas ranged
from 7.9 to 15.0wt%. Knowing the compositions of the syngas will help better understand the
reaction of microwave pyrolysis and explore its potential utilization.
The uncondensed gas was mainly composed of CO, CH4, CO2, and short-chain hydrocarbons.
The amounts of these chemicals in the gas varied with reaction conditions. Carbon monoxide is
the largest amount of the chemicals in the syngases from the Douglas fir pellet pyrolysis. The
highest content of carbon monoxide was around 64% (v/v) of the total amount of gas at the
conditions of 329°C and 15 min. The content of carbon dioxide was about 25% (v/v). The
contents of methane and short chain hydrocarbons were relatively low, which contributed to
about 1.5-7.7% (v/v) and 0.4-3.5% (v/v) of the syngases respectively. There was small amount of
hydrogen detected in the gas products of Douglas fir pellet. The reason is that the hydrogen
might be involved in the secondary reaction such as hydrogenation during pyrolysis. The syngas
had up to 70% (v/v) of usable gas components that can be burned or utilized as a syngas.
Therefore, the gas products from microwave pyrolysis of Douglas fir pellets have high values for
potential utilizations.
2.5 Conclusions
In this study, the microwave pyrolysis of Douglas fir sawdust pellet was investigated and the
effects of reaction temperature and reaction time on the yields of products (bio-oil, syngas, and
biochar) were determined using the central composition design (CCD) and response surface
analysis. The fast pyrolysis of Douglas fir pellet was achieved by microwave heating with high
37
bio-oil yields. The yields of bio-oil was 33.8–57.8wt% on dry biomass basis. The yields of coproducts, syngas and biochar, were7.9–15.0wt%, and 31.2%–60.7wt%, respectively. The bio-oil
and syngas yields increased with the reaction temperature and time increasing. The highest yields
of bio-oil and syngas were obtained at 470.7°C and 15min. Second-order polynomial models
were obtained to predict the yields of products with high accuracy.
The bio-oil was mainly composed of aromatics phenols, guaiacols, furans, ketones/aldehydes,
and organic acids. The phenols and guaiacols accounted for the largest amount of chemicals in
the bio-oil which represented 59.7–78.6% in area depending on different conditions. The specific
phenolic chemicals are highly related to the reaction temperature. The high selectivity and
maximum yield of specific phenolic chemicals was achieved through controlling the reaction
temperature and reaction time. The syngas contained carbon monoxide, methane, and short–
chain hydrocarbons, which accounted for 70% (v/v) of the total gas. The results showed that both
the bio-oils and syngas contained high–value chemicals.
38
2.6 References
Antal M.J., Leesomboon Jr., T.. Mok W.S., Richards G.N., 1991. Mechanism of formation of 2furaldehyde from D-xylose, Carbohydr. Res., 217, 71–85.
Box G.E.P., Wilson K.B., 1951. On the experimental attainment of optimum conditions, J. R.
Statist. Soc. B 13, 1–45.
Bridgwater A.V., Meier D., Radlein D., 1999. An overview of fast pyrolysis of biomass, Org.
Geochem. 30, 1479–1493.
Chen M., Wang J., Zhang M., Chen M., Zhu X., Min F., Tan Z., 2008. Catalytic effects of eight
inorganic additives on pyrolysis of pine wood sawdust by microwave heating, J. Anal. Appl.
Pyrolysis 8, 145–150
Czernik S., Bridgwater A.V., 2004. Overview of applications of biomass fast pyrolysis oil,
Energy Fuels 18, 590–598.
Dominguez A., Menendez J.A., Fernandez Y., Pis J.J., Valente Nabais J.M., Carrott P.J.M.,
Ribeiro Carrott M.M.L., 2007. Conventional and microwave induced pyrolysis of coffee hulls
for the production of a hydrogen rich fuel gas, J. Anal. Appl. Pyrolysis 79, 128–135.
Dominguez A., Menendez J.A., Inguanzo M., Pis J.J., 2006. Production of bio-fuels by high
temperature pyrolysis of sewage sludge using conventional and microwave heating, Bioresour.
Technol. 97, 1185–1193.
Du J., Liu P., Liu Z., Sun D., Tao C., 2010. Fast pyrolysis of biomass for bio-oil with ionic liquid
and microwave irradiation, J. Fuel Chem. Technol. 38, 554–559.
39
Effendi A., Gerhauser H., Bridgwater A.V., 2008. Production of renewable phenolic resins by
thermochemical conversion of biomass: a review. Renew, Sust. Energ. Rev. 12, 2092–2116.
Elliott D. C.; Hart T. R.; Neuenschwander G. G., 2006. Chemical processing in high-pressure
aqueous environments. 8. Improved Catalysts for Hydrothermal Gasification, Ind. Eng. Chem.
Res. 45, 3776-3781.
Garcia-Perez M., Chaala A., Pakdel H., Kretschmer D., Roy C., 2007. Vacuum pyrolysis of
softwood and hardwood biomass: comparison between product yields and bio-oil properties, J.
Anal. Appl. Pyrolysis 78, 104–116.
Hosoya T., Kawamoto H., Saka S., 2008. Secondary reactions of lignin-derived primary tar
components, J. Anal. Appl. Pyrolysis 83, 78–87.
Huang Y., Kuan W., Lo S., Lin C., 2008. Total recovery of resources and energy from rice straw
using microwave-induced pyrolysis, Bioresour. Technol. 99, 8252–8258.
Huang Y., Kuan W., Lo S., Lin C., 2010. Hydrogen-rich fuel gas from rice straw via microwaveinduced pyrolysis, Bioresour. Technol. 101, 1968–1973.
Klein M.T., Virk P. S., 2008. Modeling of lignin thermolysis, Energy Fuels 22, 2175–2182.
Lei H., Ren S., Julson J., 2009. The effects of reaction temperature and time and particle size of
corn stover on microwave pyrolysis, Energy Fuels 23, 3254–3261.
Lu Q., Yang X., Dong C., Zhang Z., Zhang X., Zhu X., 2011. Influence of pyrolysis temperature
and time on the cellulose fast pyrolysis products: Analytical Py-GC/MS study, J. Anal. Appl.
Pyrolysis 92, 430–438.
40
McKendry P., 2002. Energy production from biomass (part 1): overview of biomass, Bioresour.
Technol. 83, 37–46.
Meier D., Faix O., 1999. State of the art of applied fast pyrolysis of lignocellulosic materials: a
review, Bioresour. Technol. 68, 71–77.
Miura M., Kaga H., Tanaka S., Takanashi K., Ando K., 2000. Rapid microwave pyrolysis of
wood, J. Chem. Eng. Jpn. 33, 299–302.
Miura M., Kaga H., Sakurai A., Kakuchi T., Takahashi K., 2004. Rapid pyrolysis of wood block
by microwave heating, J. Anal. Appl. Pyrolysis 71, 187–199.
Moen J., Yang C., Zhang B., Lei H., Hennessy K., Wan Y., Le Z, Liu Y., Chen P., . Ruan R,
2009. Catalytic microwave assisted pyrolysis of aspen, Int. J. Agric. & Biol. Eng. 2, 70–75.
Moghtaderi B., Meesri C., Wall T.F., 2004. Pyrolytic characteristics of blended coal and woody
biomass, Fuel 83, 745–750.
Mohan D., Pittman C.U., Steele P.H., 2006. Pyrolysis of wood/biomass for bio-oil: a critical
review, Energy Fuels 20, 848–889.
Mullen C.A., Boateng A.A., 2008. Chemical composition of bio-oils produced by fast pyrolysis
of two energy crops, Energy Fuels 22, 2104–2109.
Murwanashyaka J.N., Pakdel H., Roy C., 2001. Step-wise and one-step vacuum pyrolysis of
birch-derived biomass to monitor the evolution of phenols, J. Anal. Appl. Pyrolysis 60, 219–231.
Nowakowski D.J., Bridgwater A.V., Elliott D.C., Meier D., de Wild P., 2010. Lignin fast
pyrolysis: Results from an international collaboration, J. Anal. Appl. Pyrolysis 88, 53–72.
41
Pandey M.P., Kim C.S., 2011. Lignin depolymerization and conversion: a review of
thermochemical methods, Chem. Eng. Technol. 34, 29–41.
Pettersen R.C., Chapter 2: The chemical composition of wood, in: R.M. Rowell, (Eds.), The
chemistry of solid wood, Amer. Chem. Soc., Washington, DC, 1984.
Ponder G. R., Richards G. N., 1991. Thermal synthesis and pyrolysis of a xylan, Carbohydr. Res.,
218, 143–155.
Proestos C., Chorianopoulos N., Nychas J.-G.E., Komaitis M., 2005. RP-HPLC analysis of the
phenolic compounds of plant extracts. Investigation of their antioxidant capacity and
antimicrobial activity, J. Agric. Food. Chem. 53, 1190–1195.
Robinson J.P., Kingman S.W., Barranco R., Snape C.E., Al-Sayegh H., 2010. Microwave
pyrolysis of wood pellets, Ind. Eng. Chem. Res. 49, 459–463.
Salema A.A., Ani F.N., 2011. Microwave induced pyrolysis of oil palm biomass, Bioresour.
Technol. 102, 3388–3395.
Scott D.S., Piskorz J., 1984. The continuous flash pyrolysis of biomass, Can. J. Chem. Eng. 62,
404– 412.
Sensoz S., Angın D., Yorgun S., 2000. Influence of particle size on the pyrolysis of rapeseed
(Brassica napus L.): fuel properties of bio-oil Biomass Bioenergy, 19, 271–279.
Sharizadeh F., Mcginnis G.D., Philpot C.W., 1972. Thermal degradation of xylan and related
model compounds, Carbohydr. Res., 25, 23–33.
42
Shen D.K., Gu S., Bridgwater A.V., 2010. Study on the pyrolytic behaviour of xylan-based
hemicellulose using TG–FTIR and Py–GC–FTIR, J. Anal. Appl. Pyrolysis 87, 199–206.
Shen D.K., Gu S., 2009. The mechanism for thermal decomposition of cellulose and its main
products, Bioresour. Technol. 100, 6496–6504.
Spelter H., Toth D., 2009. North America’s Wood Pellet Sector, Res. Pap. FPLRP-656, Madison,
WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 21 p.
Thangalazhy-Gopakumar S., Adhikari S., Gupta R.B., Fernando S.D., 2011. Influence of
pyrolysis operating conditions on bio-oil components: a microscale study in a pyroprobe, Energy
Fuels 25, 1191–1199
Tian Y., Zuo W., Ren Z., Chen D., 2010. Estimation of a novel method to produce bio-oil from
sewage sludge by microwave pyrolysis with the consideration of efficiency and safety, Bioresour.
Technol. 102, 2053–2061.
Vinterback J., 2004. Pellets 2002: the first world conference on pellets, Biomass Bioenergy 27,
513–20.
Wang X., Chen H., Luo K., Shao J., Yang H., 2008. The influence of microwave drying on
biomass pyrolysis, Energy Fuels 22, 67–74.
Windt M., Meier D., Marsman J.H., Heeres H.J., de Koning S., 2009. Micro-pyrolysis of
technical lignins in a new modular rig and product analysis by GC–MS/FID and GC X GC–
TOFMS/FID, J. Anal. Appl. Pyrolysis 85, 38–46.
Yi W., Bai X., Li Z., Wang L., Wang N., Yang Y., 2008. Laboratory and pilot scale studies on
fast pyrolysis of corn stover, Int. J. Agric. & Biol. Eng. 1, 57–63.
43
Yu F., Deng S., Chen P., Liu Y., WanY., Olson A., Kittelson D., Ruan R., 2007. Physical and
chemical properties of bio-oils from microwave pyrolysis of corn stover, Appl. Biochem.
Biotechnol. 136–140, 957–970.
44
CHAPTER THREE
INTEGRATION OF MICROWAVE TORREFACTION AND
PYROLYSIS TO IMPROVE BIOFUEL QUALITY
3.1 Abstracts
The focus of this study is to investigate the effects of torrefaction as pretreatment on the
compositions of bio-oil and syngas from pyrolysis. The effects of process conditions of
microwave torrefaction on the yields of products were first determined. The reaction temperature
and time significantly influenced the yields of torrefied biomass, bio-oil, and non-condensable
gases. Three linear models were developed to predict the product yield as a function of reaction
temperature and time.
GC/MS analysis for bio-oils showed that the bio-oils were mainly
composed of furans, phenolics, sugars, ketones/aldehydes, and organic acids. The amounts of
each compound varied with the reaction conditions. Over the reaction temperature of 275 °C,
non-condensable gases were mainly composed of CO2 and CO and its yields and compositions
were significantly influenced by the temperature. Higher heating values (HHV) of torrefied
biomass were from 20.90−25.07 MJ/kg, about 6−31 % increase compared to HHV of raw
biomass. The energy yields of torrefied biomass from 67.03−90.06 % implied that most energy
was retained in the torrefied biomass. One linear model as a function of reaction temperature and
time was developed to predict the energy yield. Torrefied Douglas fir sawdust pellet was further
investigated to determine the effects of torrefaction on the biofuel production. Compared to the
pyrolysis of raw biomass, the increased concentrations of phenols and sugars and reduced
concentrations of guaiacols and furans were obtained from pyrolysis of torrefied biomass,
45
indicating that torrefaction as a pretreatment favoured the phenols and sugars production.
Additionally, about 3.21 to 7.50 area% hydrocarbons and the reduced concentration of organic
acids were obtained from pyrolysis of torrefied biomass. Torrefaction also altered the
compositions of syngas by reducing CO2 and increasing H2 and CH4. The syngas was rich in H2,
CH4, and CO implying that the syngas quality was significantly improved by torrefaction process.
Keywords: Microwave torrefaction, Douglas fir pellet, GC/MS, torrefied biomass, bio-oil
syngas
3.2 Introduction
Biomass including wood, crop residues and energy grass are enormous and renewable energy
sources. Over 1.3 billion dry tons of potential biomass is available annually and more than 6 %
of total energy consumption is from renewable sources in which biomass contributes about 47 %
in US (Perlack et al., 2005). With reducing fossil energy and increasing energy demand, biomass
is playing an important role in energy supplies.
However, biomass has some disadvantages to retarding its utilization such as low bulk density,
high moisture content, low energy content. These properties lead to increased cost for
transportation and storage and influence biomass conversion efficiency. Therefore, it is difficult
to efficiently and consistently apply biomass as a source of fuel. Biomass pretreatment and
treatment is required before converting to high energy bio-fuels.
Biomass pyrolysis has received interests to produce high yield liquids, called bio-oil. Bio-oils
are carbon based liquid which has some similar properties to the petroleum fuel such as low solid
46
content and low viscosity (Czernik and Bridgwater, 2004; Mohan et al., 2006). Bio-oils have
high carbon content and can be combusted directly in boilers, gas turbines, and slow and
medium-speed diesel engines for heat and power applications. Bio-oils also have low nitrogen
and sulfuric content. Therefore, bio-oils are considered to be very promising hydrocarbon fuels.
However, the crude bio-oils contain high moisture content which is up to 15−30 wt%. The
elemental analysis showed that the bio-oils contain about 35−40 wt% of oxygen which is much
higher than that of petroleum fuels. The pH value of crude bio-oils is around 2.5 due to the
existence of organic acids. The heating value of bio-oil is ~17 MJ/kg which is lower than half of
energy from petroleum fuels. These properties make the bio-oil immiscible with gasoline and
difficult to integrate into the current petroleum refinery system.
Torrefaction is a mild thermal chemical technology at relatively low reaction temperature of 200
to 300 °C to improve the biomass quality (Bergman and Kiel, 2005). The mechanisms analysis
for biomass torrefaction indicates that the hemicelluloses are deeply decomposed and cellulose
and lignin are partially decomposed. Below the temperature of 250 °C the limited decomposition
and carbonization of hemicelluloses were mainly involved accompanied with the minor lignin
and cellulose decomposition. But at the temperatures over 270 °C, further carbonization of
hemicelluloses and limited decomposition and carbonization of lignin and cellulose occurred.
The strong torrefaction with a serious weight loss occurred in the temperature regime above
275 °C (Bridgeman et al., 2008; Chen and Kuo, 2011; Prins et al., 2006a). Thus, during
torrefaction the moisture of biomass will be removed by evaporation and some organic acids
such as acetic acid will be driven off from biomass by the hemicelluloses decomposition. The
O/C ratio of torrefied biomass will be significantly decreased as the decomposition of
hemicelluloses and dehydration of cellulose and lignin occurred during torrefaction (Prins et al.,
47
2006b). Therefore, torrefied biomass contains less moisture and high carbon content. These
characteristics benefit the further process such as combustion and gasification. It has been proved
that torrefied biomass can generate electricity with a similar efficiency to coal and improve the
syngas quality by gasification (Lipinsky et al., 2002; Bergman et al., 2005a, 2005b). Prins et al.
(2006c) investigated gasification for torrefied biomass and concluded that the quality of noncondensable gases was improved compared with direct biomass gasification.
In addition, in torrefaction process about 10−20 % energy of biomass is maintained in two coproducts, torrefaction bio-oil and non-condensable gases. Characterization of bio-oil and noncondensable gases from torrefaction is important. It will be helpful to understand the mechanism
of biomass torrefaction and recover energy by investigating the potential utilization for bio-oil
and non-condensable gases.
There were two main objectives in this study. First objective was to investigate microwave
torrefaction of Douglas fir pellet. The effects of torrefaction conditions on yields of torrefied
biomass, non-condensable gases and torrefaction bio-oil were determined and regression models
as functions of reaction temperature and time were established to predict products yields.
Heating values of torrefied biomass were investigated to determine the energy yield.
Compositions of bio-oil and noncondensable non-condensable gases were determined by GC/MS
and GC respectively. The kinetics evaluation, mass and energy balances were also studied.
Second objective of this study was to investigate microwave pyrolysis of torrefied Douglas fir
sawdust pellets. The effects of torrefaction conditions on yields of bio-oil, syngas, and biochar
were determined. The characteristics of bio-oil and syngas from torrefied biomass microwave
pyrolysis were also determined by GC/MS and GC respectively.
48
3.3. Materials and Methods
3.3.1 Materials
Douglas fir sawdust pellets were also used in this study and the properties can be found in
section 2.3.1.
3.3.2 Douglas fir pellets torrefaction and torrefied biomass preparation
The same batch microwave oven with the section 2.3.2 was employed to conduct the torrefaction
and pyrolysis process. The details of experimental setting also can be found at the section 2.3.2.
The torrefaction was processed at a 600W power input setting. 200g Douglas fir pellets were
placed in a half liter quartz flask inside of the microwave oven. The oven reached desired
temperatures after about 6 min with the average heating rate of 40 °C/min. During torrefaction
condensable volatiles were condensed into liquids as bio-oils and non-condensable gases escaped
at the end of the condensers where they were either burned or collected for analysis. Torrefied
biomass was left in the quartz flask. The weight of non-condensable gases was calculated by
difference using the following equation:
Weight of non−condensable gases = initial wood pellet mass – bio-oil mass – torrefied biomass
mass
(1)
3.3.3 Experiment design
A central composite experimental design (CCD) was used to optimize the process conditions in
microwave torrefaction as well for torrefied biomass preparation (Box and Wilson, 1951). Two
independent variables, reaction temperature (X1, °C) and reaction residence time (X2, min), and
three dependent variables, the yields of bio-oil, non-condensable gases, and torrefied biomass
49
were chosen. For statistical calculations, the variables Xi were coded as xi according to the
Equation 2:
xi=(Xi−X0)/∆X
(2)
where xi is dimensionless value of an independent variable, Xi is real value of an independent
variable, X0 is real value of the independent variable at the center point, and ∆X is the step
change. The central point of independent variables was set at 275 °C and 15 min and the step
changes were chosen at 25 °C and 5 min. Total 11 experiments with 4 axial points (α=1.41) and
3 replications at the center points was employed for the optimization of microwave torrefaction
(Tables 3.1 and 3.2). The second degree polynomial (Equation 3) was calculated to estimate the
response of dependent variables.
Yi = b0 + b1X1 + b2X2 + b11X12 + b21X2X1 + b22X22
(3)
where Yi is predicted responses, X1 and X2 are independent variables, b0, b1, b2, b11, b22 and b21are
regression coefficients.
Table 3.1 Coded levels of independent variables in the experimental design
Level
Reaction temperature (°C)
Reaction time (min)
− α = −1.41
240
8
−1
250
10
0
275
15
1
300
20
= 1.41
310
22
∆X
25
5
α
50
Total 11 samples from the torrefaction process were collected to analyze the properties and
further for pyrolysis.
3.3.4 Torrefied biomass pyrolysis
Torrefied biomass pyrolysis was performed in the batch microwave oven which was the same
apparatus used for the biomass torrefaction process. The power input for microwave pyrolysis
was used at the 700 W. About 100 g of torrefied biomass was placed in a half liter quartz flask
inside of the microwave oven. Microwave pyrolysis of torrefied biomass was conducted at the
reaction temperature of 480 °C and reaction time of 15 min as in this reaction condition the
highest bio-oil yield of biomass microwave pyrolysis can be obtained according to the previous
experiment in chapter 2. During pyrolysis the heavier volatiles were condensed into liquids as
bio-oils and the lighter volatiles escaped as syngases at the end of the condensers where they
were either burned or collected for analysis. Char was left in the quartz flask. The weight of
syngas product was calculated by difference using following equation:
Weight of syngas = initial torrefied wood pellet mass – bio-oil mass – biochar mass
(1)
3.3.5 GC/MS analysis for bio-oil and GC analysis for non-condensable gases and syngas
The same instruments and methods with the chapter 2 were used to analyze bio-oil, noncondensable gases, and syngases.
3.3.6 Heating value determination
The heating values of torrefied Douglas fir pellet and bio-oils were determined by the Poultry lab
at the University of Arkansas.
51
3.4 Results and Discussion
3.4.1 Response surface analysis
Reaction temperature and time were chosen as independent variables to investigate their effects
on products yields as they have significant influences on biomass thermal conversions. The
detailed experimental conditions and results were showed in Table 3.2. Using these results of
experiments obtained the following three linear model equations for bio-oil yield (Equation 4),
non-condensable gases yield (Equation 5), and torrefied biomass yield (Equation 6) as a function
of reaction temperature (X1, °C) and reaction time (X2, min) which ranged from 240 to 310 °C
and 7.9 to 22 min respectively:
Ybio-oil = −69.72 + 0.32X1 + 0.38X2
(4)
Ynon-condensable gases = −24.37 + 0.10X1 + 0.18X2
(5)
Ytorrefied biomass = 194.09 – 0.43X1 – 0.55X2
(6)
It was found that the model terms X12 (P-value=0.86, 0.65, 0.98), X11 (P-value = 0.80, 0.079,
0.76) and X22 (P-value = 0.61, 0.72, 0.76) were insignificant (P-values > 0.05) when the
Equation (3) was used to fit the data for bio-oil, non-condensable gases and torrefied biomass.
The Equation (3) was reduced by using backward statistical analysis, and the Equations (4, 5,
and 6) were obtained with its significant terms (P-value < 0.0001). The correlation coefficients of
determination, R2 was 0.96, 0.93, and 0.97 respectively for bio-oil, non-condensable gases, and
torrefied biomass, implying that the linear regression models accurately represented the
experiment data and can be used to predict the production yields for microwave torrefaction of
biomass.
52
Table 3.2 Summary of experimental design and results
Run#
Code value
Actual value
Yield (%)
Reaction
Reaction
Reaction
Reaction
Bio-oil
Non-
Torrefied
DFT1
–1
temperature
–1 x2
time
250
temperature
10 X2
time
13.12
3.74
condensable
83.15
biomass
DFT2
x11
–1
300
X1
10
32.57
8.33
gases
59.11
DFT3
–1
1
250
20
17.66
5.82
76.52
DFT4
1
1
300
20
36.35
11.04
52.61
DFT5
–1.41
0
240
15
14.06
3.55
82.39
DFT6
1.41
0
310
15
32.62
11.32
56.07
DFT7
0
–1.41
275
7.93
22.08
5.61
72.31
DFT8
0
1.41
275
22
26.79
7.29
65.93
DFT9
0
0
275
15
23.41
5.75
70.85
DFT10
0
0
275
15
25.56
6.25
68.20
DFT11
0
0
275
15
24.56
7.15
68.29
Three dimensional surface response profiles were developed for the yields of bio-oil, noncondensable gases, and torrefied biomass versus torrefaction conditions (Figure 3.1). The yields
of bio-oil and non-condensable gases were ranged from 13.12 to 36.35 wt% and 3.55 to 11.32 wt%
respectively and increased with the increase of reaction temperature and time. The maximum
yield of bio-oil was observed at a temperature of 300 °C and reaction time of 20 min. The
maximum yield of non-condensable gases was found at 310 °C and 15 min. On the contrary, the
yield of torrefied biomass was ranged from 52.61 to 83.15 wt% and decreased with the increase
53
of reaction temperature and time. The minimum and maximum torrefied biomass yields were
observed at a condition of 300 °C and 20 min, and 250 °C and 10 min respectively.
Torrefaction bio-oil yield (%)
34.35
29.39
24.43
19.47
14.51
20
300
18
288
15
Reaction time (min)
275
13
263
10
250
Reaction temperature (C)
10.39
Syngas yield (%)
8.64
6.89
5.15
3.40
20
300
18
288
15
Reaction time (min)
275
13
263
10
250
54
Reaction temperature (C)
Torrefied biomass yield (%)
82.09
75.38
68.67
61.97
55.26
10
250
13
263
15
Reaction time (min)
275
18
288
20
300
Reaction temperature (C)
Figure 3.1 3D response surface profiles for the yield of bio-oil, non-condensable gases, and
torrefied biomass versus torrefaction conditions
The amount of mass loss is an important parameter in torrefaction as too much mass loss has
negative effects on torrefied biomass production. The mass loss of biomass during torrefaction is
highly correlated to the reaction temperature according to previous reports. At the temperature
below 250 °C during torrefaction mass loss occurs with the decomposition of hemicelluloses
mainly involved. At the temperature over 270 °C, the torrefaction conditions cause large mass
loss of biomass as cellulose and lignin are involved in the reactions (Bridgeman et al., 2008;
Chen and Kuo, 2011; Prins et al., 2006a). Chen et al. (2011) did the thermogravimetry analysis
to investigate the behaviors of hemicellulose, cellulose, lignin, xylan and dextran in torrefaction
55
at the temperatures of 230, 260 and 290 °C. They noted that the severe damage of hemicelluloses
and cellulose with large mass weight loss (>35 wt%) and light damage of lignin with small
weight mass loss (<10 wt%) were observed when the torrefaction temperature was increased to
290 °C. The mass weight loss of biomass also related to the types of biomass due to the
differences of biomass compositions. Prins et al. (2006a) indicated that the mass weight loss of
softwood is less than hard wood and straw as the hemicelluloses content in softwood is lower
than those in hard wood and straw. In this research, the mass loss of Douglas fir pellet increased
with the increase of temperatures. The mass weight loss was observed from 16.85 to 34.07 wt%
at the temperatures ranging from 240 to 275 °C. A very large weight loss, up to 40.9 to 47.39 wt%
was observed at the temperatures over 300°C. These results were similar to the previous report of
corn stover torrefaction at the temperatures of 250 and 300 °C (Medic et al., 2012). However,
the mass weight loss of biomass observed in this research was larger than other reports at the
similar torrefaction conditions for wood and grass especially at the temperature over 275 °C
(Bridgeman et al., 2008; Prins et al., 2006a).
The water contents of torrefaction bio-oils were determined by Mettler Toledo V30 volumetric
Karl-Fisher Titrator (Mettler Toledo International Inc., USA). The water contents of bio-oils
ranged from 48.43 to 60.94 wt%, which was much higher than those of bio-oils from biomass
pyrolysis. The yield of water ranging from 7.99 to 20.82 wt% increased with the increase of
reaction temperature and time. The lowest water yield was observed at the reaction temperature
and time of 250 °C and 10 min. The highest water yield was observed at the reaction temperature
and time of 300 °C and 20 min. The similar results were reported by Medic et al. (2012) and
Prins et al. (2006a). The yield of liquid chemicals ranging from 5.12 to 16.8 wt% also increased
56
with the increase of reaction temperature but was not significantly changed with the reaction
time at the same reaction temperature. The highest yield of liquid chemicals in bio-oil was
observed at the condition of 300 °C and 10 min. The yield distributions of water, liquid
chemicals, non-condensable gases and torrefied biomass were showed in Figure 3.2.
100
Products yields, wt% on biomass
90
80
70
60
50
40
30
20
10
0
DFT1
DFT2
Syngas
DFT3
DFT4
Water
DFT5
DFT6
Liquid chemicals
DFT7
DFT8
DFT9
DFT10 DFT11
Torrefied biomass
Figure 3.2 Yield distributions of water, liquid chemicals, non-condensable gases, and torrefied
biomass
3.4.2 GC/MS analysis for bio-oil
The bio-oil yield in Douglas fir pellet torrefaction was about 13.12 to 36.35 wt%. The
composition of bio-oil was determined by GC/MS to obtain an insight of the torrefaction
57
reactions and can be used to investigate potential utilization for bio-oil. The main components of
bio-oils were furans, guaiacols, ketones/aldehydes, sugars, and organic acids (Figure 3.3).
100
90
80
GC/MS Area (%)
70
60
50
40
30
20
10
0
DFT1
DFT2
DFT3
DFT4
DFT5
DFT6
DFT7
DFT8
DFT9
Other
Sugars
Esters
Furans
Guaiacols
Phenols
Hydrocarbons
Alcohols
Ketones/aldehydes
Acids
Figure 3.3 Chemicals distributions in bio-oil
The furans were ranged from 18.49–29.07 area% and primarily composed of ß-methoxy-(S) - 2furanethanol, furfural, 2-furanmethanol, and tetrahydro-2, 5-dimethoxy- furan. The largest
amount chemical in furans was ß-methoxy-(S) - 2-furanethanol which contributed about 10–
20.55 area% in bio-oils. Sugars in bio-oils were ranged from 2.74–17.9 area% and mainly made
up of 1, 6-anhydro-ß-D-glucopyranose, 1, 4:3, 6-dianhydro-α-D-glucoppyranose, and Dmannose, and the amounts of these sugars varied with the reaction conditions. The organic acids
58
were ranged from 3.23–5.59 area% and increased with the increase of reaction temperature.
Acetic acid was the largest amount acid which was about 1.10–2.06 area% in bio-oils.
Previous reports indicated that the lignin of biomass was slightly decomposed during
torrefaction and small amount of phenolic chemicals was detected in bio-oils (Prins et al., 2006b;
Medic et al., 2012). In this research, GC/MS analysis of torrefaction bio-oils showed that a large
amount of phenolic chemicals such as 2-methoxyphenol, 2-methoxy-4-(1-propenyl)-phenol, 2methoxy-4-methylphenol, and 4-ethyl-2-methoxyphenol was observed. The total phenolic
chemicals in bio-oil ranged from 33.66 to 49.02 area%. The smallest amount of phenolics was
observed at the reaction temperature of 240 °C and reaction time of 15 min. The amount of
phenolics was not significantly changed with the reaction temperature and time over 250 °C.
The large amount of phenolics observed in bio-oil indicates that the lignin of Douglas fir pellet
was partially damaged and decomposed during microwave torrefaction.
3.4.3 GC analysis for non-condensable gases
Non-condensable gases were one of the main by-products in the microwave torrefaction. The
yields of non-condensable gases were from 3.55 to 11.32 wt% and increased with the increase of
reaction temperature and time. The non-condensable gaseses were collected at the end of
condensers and analyzed by a GC. The composition of non-condensable gases was highly
correlated with the reaction temperature. At the reaction temperature of 240 °C and 250 °C, only
small amount of CO2 was observed. At the torrefaction temperature over 275 °C, the
composition of noncondensable non-condensable gases was mainly composited of CO2 and CO.
The amounts of CO2 and CO were ranged from 30 to 52 % (v/v) and 18 to 28 % (v/v)
respectively and increased significantly with the increase of reaction temperatures. The methane
59
and short-chain hydrocarbons like ethylene, ethane and propane were also observed in the
reaction temperature over 275 °C with small amounts (0.5 to 1 %v/v) observed. These
observations were consistent with the previous reports (5, 27). Hydrogen in all torrefaction
processes there was not detected. It might due to the mild biomass decomposition with the
relatively low reaction temperature in torrefaction. The compositions of non-condensable gases
at different reaction temperatures were showed in Figure 3.4.
60
Amount (%)
50
40
250°C
30
275°C
20
300°C
10
0
CO2
CO
short-chain
hydrocarbons
Methane
Figure 3.4 Compositions of non-condensable gases at different reaction temperatures
3.4.4 Heating value analysis for torrefied Douglas fir pellet
Torrefaction can remarkably improve the heating value of biomass. The higher heating value
(HHV) was 30% improved compared to the raw material in previous reports (Pimchuai et al.,
2010; Bridgeman et al., 2008). In this research, the HHVs of torrefied biomass were ranged from
20.3 to 25.4 MJ/kg and increased with the increase of reaction temperature and time (Figure 3.5).
The highest HHV of torrefied biomass was 31 % more than that of raw biomass, which was
60
observed at the reaction temperature of 300 °C and reaction time of 20 min. The heating values
of torrefied biomass were significantly greater than those of raw biomass and the wood torrefied
biomass, but the similar to those of TOP pellets and coals (Tumuluru et al., 2011; Bergman et al.,
2005c; van der Stelt et al., 2011). The mass density (bulk) of torrefied biomass ranged from 488
to 621 kg/m3 was lower than the raw pellet but much higher than the wood biomass (Tumuluru et
al., 2011; Bergman et al., 2005c; van der Stelt et al., 2011). The mass density (bulk) also
decreased with the severity of torrefaction. The energy density (bulk) was about 11.6 to 13.0
GJ/m3 slightly lower than the raw pellet but significantly higher than wood biomass and torrefied
wood biomass coals (Tumuluru et al., 2011; Bergman et al., 2005c; van der Stelt et al., 2011).
30
25
HHV (MJ/kg)
20
15
10
5
0
Figure 3.5 Higher heating values (HHVs) of torrefied biomass
The energy yields of torrefied biomass were calculated according to the HHVs and mass yields.
The energy yields of torrefied biomass were 67.03−90 % (Figure 3.6). The highest energy yield
61
was observed at the reaction temperature of 239 °C and reaction time of 15 min. The lowest
energy yield was observed at the reaction temperature of 310 °C and reaction time of 15 min.
The energy yields of torrefied biomass were used to fit the Equation 3 to estimate the effect of
process conditions. The following linear model equation for energy yield as a function of
reaction temperature (X1, °C) and reaction time (X2, min) which ranged from 240 to 310 °C and
7.9 to 22 min respectively was obtained:
Yenergy = 176.87 – 8.56X1 – 0.98X2
(7)
The correlation coefficients of determination, R2 was 0.96, implying that the linear regression
models accurately represented the experiment data and can be used to predict the energy yield
for microwave torrefaction of Douglas fir pellet.
100
90
80
Yield (%)
70
60
50
40
30
20
10
0
DFT1 DFT2 DFT3 DFT4 DFT5 DFT6 DFT7 DFT8 DFT9 DFT10 DFT11
Torrefied biomass yield (wt%)
Energy yield(%)
Figure 3.6 Comparison of mass yield and energy yield of torrefied biomass
62
The surface response profile was developed for the yields of energy versus torrefaction
conditions (Figure 3.7). The energy yields of torrefied biomass decreased with the increase of
reaction temperature, but they were not significantly influenced by the reaction time. At the
temperature range of 239 to 250 °C, the energy yields of torrefied biomass were 85.46 to 90.6 %.
At the reaction temperature of 275 °C, the energy yields of torrefied biomass decreased to
80.24−82.05 %. Further increasing temperatures over 300 °C, the energy yields of torrefied
biomass significantly reduced to 67.69−71.02 %. The energy loss of biomass was from 9.4 to
32.31 %, which increased with the increase of the reaction temperature.
89.30
Energy yield (%)
84.53
79.76
74.99
70.22
250
20
263
18
275
15
Reaction temperature (C)288
13
300
10
Reaction time (min)
Figure 3.7 Response surface profiles for the energy yield of torrefied biomass versus torrefaction
conditions
63
The degree of carbonization was calculated by the ratio of the HHVs of torrefied biomass to the
HHVs of raw biomass. The degree of carbonization significantly increased with the reaction
temperature and time. The highest degree of carbonization was 1.3 obtained at the reaction
temperature of 300 °C and reaction time of 20 min. The lowest degree of carbonization was 1.08
obtained at the reaction temperature of 250 °C and 10 min. These results showed that most
energy of raw biomass was maintained in less mass of torrefied biomass.
3.4.5 Product yields in torefied biomass pyrolysis
Total 11 samples from torrefaction pretreatment plus one raw biomass were pyrolyzed. The
products yield distributions were showed in Table 3.3. The products yield of torrefied biomass
pyrolysis had high relationship with the torrefaction process. The bio-oil yields from pyrolysis of
torrefied biomass were ranged from 21.17 to 40.25 wt% based on torrefied biomass which was
much less than that from pyrolysis of raw biomass; while the syngas and biochar yields were
ranged from 24.13 to 34.37 wt% and 31.13 to 49.61 wt% based on torrefied biomass respectively,
which were much higher than those from pyrolysis of raw biomass. However, the total bio-oil
yields from both torrefaction and pyrolysis based on raw biomass were ranged from 42.32 to
51.18 wt% (Table 3.3) while the bio-oil yields from pyrolysis of raw biomass were from 31.5 to
53.5 % based raw biomass depending on pyrolysis conditions (Ren et al., 2012), indicating that
the total bio-oils yield from combined torrefaction and pyrolysis processes was comparable with
the bio-oil yield from pyrolysis of raw biomass. The total syngas yields based on raw biomass
were increased compared to that from pyrolysis of raw biomass, suggesting that the combined
processes tend to produce more syngas.
64
Table 3.3 Summary of torrefaction conditions and results
Run
#
Torrefaction
Weight loss
HHVs
Products yield from
Products yield from
conditions
Temperature Time
in
of
torrefaction
torrefied
pyrolysis (wt% based on
BioSyngas Biochar
torrefied biomass)
oil
torrefaction and pyrolysis
BioSyngas Biochar
(wt% based on raw
oil
(wt%)
biomass
(°C)
(min)
biomass)
1
250
10
16.85
20.9
(MJ/kg)
37.35
32.42
30.23
44.17
30.69
25.14
2
300
10
40.90
23.25
22.56
34.37
43.07
45.91
28.64
25.45
3
250
20
23.48
21.61
32.23
34.02
33.76
42.32
31.85
25.83
4
300
20
47.39
25.07
17.14
33.25
49.61
45.37
28.53
26.10
5
240
15
17.62
21.28
39.42
29.45
31.13
46.54
27.82
25.65
6
310
15
43.93
23.36
21.17
32.03
46.80
44.48
29.27
26.24
7
275
8
27.69
21.59
40.25
24.13
35.63
51.18
23.05
25.76
8
275
22
34.07
23.55
28.28
31.26
40.47
45.43
27.89
26.68
9
275
15
29.16
22.41
30.97
32.33
36.70
45.35
28.65
26.00
10
275
15
31.81
23.01
31.49
30.08
38.43
47.03
26.76
26.21
11
275
15
31.71
22.82
32.13
28.62
39.25
46.50
26.69
26.80
19.6
53.42
19.48
27.1
12
Raw biomass
Further analysis showed that the torrefaction temperature and time had significantly effects on
the products yield of torrefied biomass pyrolysis. The bio-oil yields based on torrefied biomass
were significantly decreased with the increase of the torrefaction temperature and time (Fig. 3.8).
It might be attributed to the removing of components such as hemicelluloses and partial cellulose
and lignin during the torrefaction. The syngas yields based on torrefied biomass from pyrolysis
of torrefied biomass were slightly increased with the increase of torrefaction temperature while
65
the biochar yields (based on torrefied biomass) were greatly increased with the increase of
torrefaction temperature. The torreaction time had minor effects on syngas and biochar yields
(Table 3.3). However, the biochar yields based on raw biomass from combined processes were
slightly lower than that from biomass direct pyrolysis and relatively stable at different
torrefaction conditions (Table 3.3).
60
Yield distribution (wt%)
50
40
Raw biomass
240°C, 15min
250°C, 10min
30
275°C, 15min
300°C, 10min
20
310°C, 15min
10
0
Bio-oil
Syngas
Biochar
Figure 3.8 The effects of torrefaction temperatures on the products yield (based on torrefied
biomass) of torrefied biomass pyrolysis
3.4.6 The effects of torrefaction on bio-oil compositions
To further understand the chemical reactions of microwave pyrolysis of torrefied biomass and
obtain more insight of the effects of torrefaction on bio-oils, the compositions of bio-oils were
66
characterized by GC/MS and categorized into ten groups based on bio-oil’s functional groups
(Fig. 3.9). The compositions of the bio-oil from pyrolysis of torrefied biomass were obviously
different from that obtained from pyrolysis of raw biomass. The guaiacols from pyrolysis of raw
biomass were the largest amount chemicals which contributed to 50.90 area% of the total.
Comparing to that from pyrolysis of raw biomass, the guaiacols in the bio-oil from pyrolysis of
torrefied biomass ranged from 27.82 to 38.26 % were significantly decreased. The phenols from
pyrolysis of torrefied biomass were ranged from 16.16 to 29.95 area% which was significantly
increased compared to that from pyrolysis of raw biomass which was observed at 8.73 area%.
The other two large amount chemicals observed in the bio-oil from pyrolysis of torrefied
biomass were furans and sugars. The amounts of furans from pyrolysis of torrefied biomass were
ranged from 7.96 to 14.15 area% which was obviously lower than that from pyrolysis of raw
biomass. On the contrary, the amounts of sugars from pyrolysis of torrefied biomass ranged from
5.9 to 20.71 area% were significantly higher than that from pyrolysis of raw biomass which was
2.83 area%. These results indicate that the torrefaction process favoured the phenols and sugars
production by torrefied biomass pyrolysis. The acids from torrefied biomass pyrolysis ranged
from 1.6 to 2.78 area% were also decreased compared to that from pyrolysis of raw biomass. The
same findings were reported in previous research that investigated the characteristics of bio-oil
from torrefied biomass using fluidized bed pyrolysis (Meng et al., 2012). Hydrocarbons are
important chemicals for transportation fuels. No hydrocarbons were detected in the bio-oil from
pyrolysis of raw biomass. But in this research, about 3.21 to 7.5 area% hydrocarbons were
observed which was not reported in previous work (Meng et al., 2012). These hydrocarbons
might have benefits for further bio-oil refinery process such as reducing consumption of
hydrogen.
67
100.00
90.00
80.00
GC/MS Area (%)
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
1
2
3
Other
Furans
Hydrocarbons
4
5
6
Sugars
Guaiacols
Alcohols
7
8
9
Raw
biomass
Esters
Phenols
ketones/aldehydes
Figure 3.9 Chemical composition distribution of bio-oils from microwave pyrolysis of torrefied
biomass
The changes of chemical compositions of bio-oils at the different torrefaction temperature and
time were showed in Fig.3.10 The guaiacols from torrefied biomass were not significantly
changed with the torrefaction temperature below 300 °C; but guaiacols were greatly decreased at
the torrefaction temperature of 310 °C. However, the guaiacols were significantly decreased with
the increase of reaction time at the same torrefaction temperature. The amounts of phenols from
torrefied biomass pyrolysis were relatively stable as only slight changes were observed at
different torrefaction processes.
The total phenolic compounds showed a decreasing trend
especially at the high torrefaction temperature and long reaction time. This result was consistent
68
with the very large weight loss observed at the temperature over 300 °C in which lignin might be
partially decomposed. The sugars were increased with the increase of torrefaction temperature
and time while the furans were decreased. The hydrocarbons were also increased with the
increase of torrefaction temperature and time. The maximum sugars and hydrocarbons were
20.71 area% and 7.5 area% observed at the torrefaction temperature of 275 °C and time of 22
min and 300 °C and 20 min, respectively.
60.00
50.00
Raw biomass
GC/MS Area (%)
40.00
250°C, 10min
250°C, 20min
30.00
275°C, 15min
300°C, 10min
20.00
300°C, 20min
310°C, 15min
10.00
0.00
Guaiacols
Phenols
Sugars
Furans
Hydrocarbons
Figure 3.10 The effects of torrefaction on the chemical composition of bio-oils from torrefied
biomass pyrolysis
3.4.7 The effects of torrection on syngas compositions
Syngas was one of important co-products from microwave pyrolysis of biomass as the high
contents of valuable gases were detected in previous reports (Lei et al., 2009; Ren et al., 2012).
69
Thus, characterizing the compositions of the syngas will help better understand the reaction
mechanism of torrefied biomass microwave pyrolysis and explore its potential utilization.
The syngas from pyrolysis of raw biomass was mainly composed of carbon dioxide, carbon
monoxide, methane, and short-chain hydrocarbons. Carbon monoxide was the largest amount of
the chemicals in the syngas which contributed to about 44 % (v/v). The content of carbon
dioxide was about 26 % (v/v). The contents of methane and short chain hydrocarbons were
relatively low which contributed to about 6 % (v/v) and 5 % (v/v) of the syngas respectively.
There was no hydrogen detected in the gas products of raw Douglas fir pellet microwave
pyrolysis. Comparing to syngas from pyrolysis of raw biomass, the compositions of syngas from
pyrolysis of torrefied biomass were significantly different. Carbon monoxide was still the largest
amount of chemical in the syngas from pyrolysis of torrefied biomass which was ranged from
25.98 to 45.30 % (v/v) varied with the torrefaction condition significantly. The contents of
carbon dioxide from pyrolysis of torrefied biomass ranged from 8.75 to 18.18 % (v/v) were
much lower than that from pyrolysis of raw biomass which is consistent with previous reports
(Wannapeera et al., 2011). The contents of methane from pyrolysis of torrefied biomass ranged
from 10.99 to 25.54 % (v/v) were much higher than that from pyrolysis of raw biomass. And the
large amount of hydrogen which was ranged from 2.38 to 20.61 % (v/v) was detected in
pyrolysis of torrefied biomass.
The torrefaction conditions also had significant effects on the syngas compositions (Fig. 3.11).
The content of carbon monoxide from pyrolysis of torrefied biomass was obviously increased at
the torrefaction temperature over 275 °C. The contents of carbon dioxide were mildly increased
with the increase of torrefaction temperatures from 240 to 250 °C.
At the torrefaction
temperature over 275 °C, the contents of carbon dioxide were stable. The contents of methane
70
from pyrolysis of torrefied biomass were significantly increased with the increase of torrefaction
temperatures with the highest content of 25.54 % (v/v) at the torrefaction temperature of 310 °C.
The contents of hydrogen were first greatly increased with the increase of the torrefaction
temperature then decreased at the torrefaction temperature of 310 °C. Total carbon monoxide,
methane, and hydrogen were up to 68 % (v/v) at the torrefaction temperature over 275 °C which
indicates that the syngas from pyrolysis of torrefied biomass had high values for potential
utilizations.
50.00
45.00
40.00
Amount (%)
35.00
Raw biomass
30.00
240°C, 15min
25.00
250 °C, 20min
20.00
275°C, 15min
15.00
300°C, 20min
10.00
310°C, 15min
5.00
0.00
CO
CO2
Methane
Hydrogen
Figure 3.11 The effects of torrefaction on the chemical composition of syngas from torrefied
biomass pyrolysis
3.4.8 Mechanism analysis of torrefied biomass pyrolysis
Torrefaction can modify the structure and chemical components of biomass by removing
hemicelluloses and dehydrating and partially reducing cellulose and lignin.
71
The torrefied
biomass was rich in cellulose and lignin. These changes may affect the reaction pathways of
biomass pyrolysis. In the previous report the interaction between hemicelluloses and cellulose
has negative effect on the formation of sugars and positive effect on the formation of furans
(Wang et al., 2011). In pyrolysis of torrefied biomass this interaction might be eliminated or
reduced as the hemicelluloses were removed during torrefaction. This might explain our findings
in analysis of bio-oil that the pyrolysis of torrefied biomass had the selectivity for sugars
production (Fig. 3.9 and 3.10). The bio-oil analysis also showed that some guaiacols were
replaced by phenols in the torrefied biomass pyrolysis (Fig. 3.9 and 3.10). This implied that the
torrefaction also affected the mechanism of lignin decomposition. In our previous report, the
small amount of phenols in woody biomass pyrolysis was produced by the cleavage of methyl
from O−CH3 at the temperature over 471 °C (Ren et al., 2012). But in the pyroysis of torrefied
biomass this cleavage was enhanced resulting in the large amount of phenols. Hosoya et al.
(2009) described the reaction pathway involving the homolysis O−CH3 and methane production
under the effects of cellulose-derived products on the lignin pyrolysis. This mechanism might be
applied to this study to illustrate the significant increase of methane found in the syngas (Fig.
3.11).
3.5 Conclusions
In this study, microwave torrefaction of Douglas fir sawdust pellets was conducted to investigate
the effects of process conditions on yields and characteristics of products. The yield of torrefied
biomass decreased with the increase of reaction temperature and time while the yield of bio-oil
and non-condensable gases were increased. Three linear equations as a function of reaction
72
temperature and time were developed to predict the yield of torrefied biomass, bio-oil, and noncondensable gases based on the central composite experimental design and surface response
analysis. The bio-oils from microwave torrefaction of Douglas fir pellets contained some
valuable chemical compounds such as furans, guaiacols, ketones/aldehydes, and sugars. The
non-condensable gases were mainly composed of CO2 and CO, and their amounts were
significantly influenced by the reaction temperature. The HHVs of Douglas fir pellets were
significantly increased which were ranged from 20.3 to 25.4 MJ/kg and increased with the
increase of reaction temperature and time. The energy yields of torrefied biomass were from
67.03−90 % and decreased with the increase of reaction temperature. The analysis for torrefied
biomass pyrolysis indicated that the torrefaction has significant effects on the yields of products,
and the compositions of bio-oil and syngas. The bio-oil yields were decreased with the severity
of torrefaction. The compositional analysis of bio-oils showed that the torrefaction improved the
sugars, phenols, and hydrocarbons production and reduced organic acids which might has
potential benefits for bio-oil storage and refinery such as less hydrogen consumption.
Additionally, the torrefaction enhanced the CH4 and H2 in syngas and reduced the CO2
production, suggesting that the torrefaction significantly improved the syngas quality during
pyrolysis of torrefied biomass.
73
3.6 References
Bergman, C.A.P., Kiel, H.A.J., Torrefaction for biomass upgrading. 2005. 14th European
Biomass Conference & Exhibition, France.
Bergman, P.C.A., Boersma, A.R., Zwart, R.W.H., Kiel, J.H.A., 2005a. Development of
torrefaction for biomass co-firing in existing coal-fired power stations. ECN report.
Bergman, P.C.A., Boersma, A.R., Kiel, J.H.A., Prins, M.J., Ptasinski, K.J., Janssen, F.J.J.G.,
2005b. Torrefied biomass for entrained-flow gasification of biomass. ECN report.
Bergman P.C.A. Combined torrefaction and pelletisation: The TOP process. ECN report 2005c.
Prins M.J., Ptasinski K.J., Janssen F.J.J.G., 2006c. More efficient biomass gasification via
torrefaction, Energy 31, 3458–3470
Box G.E.P., Wilson K.B., 1951. On the experimental attainment of optimum conditions, J. R.
Statist. Soc. B 13, 1–45.
Bridgeman, T.G., Jones, J.M., Shield, I., Williams, P.T., 2008. Torrefaction of reed canary grass,
wheat straw and willow to enhance solid fuel qualities and combustion properties. Fuel 87, 844–
856.
Chen, W., Kuo, P., 2011. Torrefaction and co-torrefaction characterization of hemicellulose,
cellulose and lignin as well as torrefaction of some basic constituents in biomass. Energy 36,
803–811.
Czernik, S., Bridgwater, A.V., 2004. Overview of applications of biomass fast pyrolysis oil.
Energy Fuels 18, 590–598.
74
Hosoya, T., Kawamoto, H., Saka, S., 2009. Solid/liquid- and vapor-phase interactions between
cellulose- and lignin-derived pyrolysis products. J. Anal. Appl. Pyrol. 85, 237–246.
Lei, H., Ren, S., Julson, J., 2009. The effects of reaction temperature and time and particle size
of corn stover on microwave pyrolysis. Energy Fuels 23, 3254–3261.
Lipinsky, E.S., Arcate, J.R., Reed, T.B., 2002. Enhanced wood fuels via torrefaction. Fuel Chem.
Div. Preprints 47, 408–410.
Medic, D., Darr, M., Shah,A., Potter, B., and Zimmerman, J., 2012. Effects of torrefaction
process parameters on biomass feedstock upgrading. Fuel 91, 147–154.
Meng, J., Park, J., Tilotta, D., Park, S., 2012. The effect of torrefaction on the chemistry of fastpyrolysis bio-oil. Bioresour. Technol. 11, 439–446.
Mohan, D., Pittman, C.U., Steele, P.H., 2006. Pyrolysis of wood/biomass for bio-oil: a critical
review. Energy Fuels 20, 848–889.
Perlack, R.D.; Wright, L.L.; Turhollow, A.F.; Graham, R.L.; Stockes, B.J.;
Erbach, D.C.
Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a
billion-ton annual supply, US Department of Energy and the US Department of Agriculture by
Oak Ridge NationalLaboratory, Oak Ridge, TN, April 2005.
Pimchuai, A., Dutta, A., and Basu, P., 2010. Torrefaction of agriculture residue to enhance
combustible properties. Energy Fuels 24, 4638–4645.
Prins, M.J., Ptasinski, K.J., Janssen, F.J.J.G., 2006a. Torrefaction of wood: Part 1. Weight loss
kinetics. J. Anal. Appl. Pyrol. 77, 28–34.
75
Prins, M.J., Ptasinski, K.J., Janssen, F.J.J.G., 2006b. Torrefaction of wood: Part 2. Analysis of
products. J. Anal. Appl. Pyrol. 77, 35–40.
Ren, S., Lei, H., Wang, L., Bu, Q., Chen, S., Wu, J., Julson, J., Ruan, R., 2012. Biofuel
production and kinetics analysis for microwave pyrolysis of Douglas fir sawdust pellet. J. Anal.
Appl. Pyrol. 94, 163–169.
Tumuluru, J.S., Sokhansanj, S., Hess, J.R., Wright, C.T., Boardman R.D., 2011. A review on
biomass torrefaction process and product properties for energy applications. Ind. Biotechnol. 7,
384–401.
Van der Stelt M.J.C., Gerhauser H., Kiel J.H.A., Ptasinski K.J., 2011. Biomass upgrading by
torrefaction for the production of biofuels: A review. 35, 3748–3762.
Wannapeera, J., Fungtammasan, B., Worasuwannarak, N., 2011. Effects of temperature and
holding time during torrefaction on the pyrolysis behaviors of woody biomass. J. Anal. Appl.
Pyrol. 92, 99–105.
Wang, S., Guo, X., Wang, K., Luo, Z., 2011. Influence of the interaction of components on the
pyrolysis behavior of biomass. J. Anal. Appl. Pyrol. 91, 183–189.
76
CHAPTER FOUR
BIOMASS AND TORREFIED BIOMASS CATALYTIC
PYROLYSIS AND BIO-OIL UPGRADING OVER
BIOCHAR FOR HYDROCARBONS AND
HYDROGEN-RICH SYNGAS
PRODUCTION
4.1 Abstract
The focus of this study is to investigate the influences of biochar as a catalyst in biomass
pyrolysis and bio-oil upgrading to biofuels production. The biochar catalyst enhanced the syngas
and improved the bio-oil quality in biomass pyrolysis. The phenols and hydrocarbons in bio-oil
increased with the increase of biochar catalyst loadings while the guaiacol decreased. The high
concentrations of phenols (46 area%) and hydrocarbons (16 area%) were obtained from torrefied
biomass catalytic pyrolysis over biochar catalysts. High-quality syngas richened in H2, CO, and
CH4 was observed. The amounts of H2 and CO in syngas were up to 20.43 vol% and 43.03 vol%
in raw biomass catalytic pyrolysis, and 27.02 vol% and 38.34 vol% in torrefied biomass catalytic
pyrolysis. Thermal gravimetric analysis (TGA) showed that the raw and recycled biochar
catalysts had little weight loss from the temperature of 150 to 800°C, indicating good thermal
stability of the biochar catalysts. Upgraded bio-oil was dominated by phenols (37.23 area%) and
77
hydrocarbons (42.56 area%) at higher biochar catalyst loadings. The biochar catalyst might be
used as a cost-competitive catalyst in biomass conversion and bio-oil upgrading.
Keywords: torrefied biomass; biochar catalyst; bio-oil; syngas: upgrading
4.2 Introduction
Biomass pyrolysis is a thermo-chemical process conducted at the temperature of 350–600°C in
inert condition (Scott and Piskorz, 1984). The primary product of biomass fast pyrolysis is biooils that are carbon-based liquids with some properties similar to those of the petroleum fuel,
such as low solid content and low viscosity (Mohan et al., 2006). Bio-oils also have low nitrogen
and sulfuric contents. Therefore, bio-oils are considered as promising alternative transportation
fuels for displacing gasoline and other petroleum fuels.
Still, biomass pyrolysis has certain disadvantages. For instance, the crude bio-oils from biomass
pyrolysis contain high content of oxygen, up to 35−40 wt% and much higher than that of
petroleum. The crude bio-oils are acidic and reactive due to the existence of organic acids. The
high contents of oxygen and organic acids make the bio-oils unstable and immiscible with
gasoline. Furthermore, the bio-oils contain few hydrocarbons that are dominant chemicals in
petroleum, and, the heating value of bio-oils is about 17MJ/kg, which is lower than half of that of
crude oil. Hence, the bio-oils need to be upgraded to transportation fuels by removing organic
acids, reducing oxygen content, and improving hydrocarbon content.
A number of technologies for bio-oil upgrading have been developed and reported, including
hydrotreating, hydrocracking, esterification, emulsification, and steam reforming. Among these
78
technologies, hydrotreating and hydrocracking should be operated under pressure conditions and
consume large amount of hydrogen (Elliott et al., 2009; Wright et al., 2010). Esterification and
emulsification need solvents that will increase the cost of process (Hilten et al., 2010; Ikura et al.,
2003). Steam reforming is a complicated process with high reaction temperature (Rioche et al.,
2005; Wang et al., 2007). Recently, biomass catalytic pyrolysis and bio-oil catalytic cracking are
drawing more interest as they do not consume hydrogen and can be conducted at atmospheric or
low-pressure conditions. In previous studies, scientists have investigated biomass catalytic
pyrolysis and catalytic cracking for bio-oil and vapors using different catalysts, such as zeolitebased catalysts (Lappas et al., 2002; Aho et al., 2010), activated alumina (Demiral et al., 2008),
fluid catalytic cracking (FCC) catalysts (Zhang et al., 2009; Samolada et al., 2000), and
transition metal catalysts (Fe/Cr) (Samolada et al., 2000). They reported that the oxygen content
was significantly reduced and the selectivity for hydrocarbons was increased. However, the
catalysts used in biomass catalytic pyrolysis and catalytic cracking are expensive as they need to
be purchased or synthesized by using metals or expensive precious metals. Therefore, it is
necessary to develop cost-competitive catalysts to improve the efficiency of biomass conversion.
Biochar is one of the main co-products in biomass pyrolysis and gasification. With the
development of biomass thermo-chemical conversion, abundant biochar will be produced. The
analysis of biochar properties indicated that biochar has relatively high porosity and surface area,
contains high minerals such as K, Ca, and P and functional groups on the surface (Chun et al.,
2004; Lei et al., 2009; Mullen et al., 2010). These properties make the biochar as adsorbent,
catalyst supports and catalyst. Application of biochar has been widely investigated in areas of
soil amendment (Chan et al., 2007), water treatment (Mohan et al., 2012), activated carbon
79
production (Azargohar et al., 2006), and catalyst support (Dehkhoda et al., 2010). Recently,
biochar-supported catalysts were developed for biodiesel production (Dehkhoda et al., 2010;
Chen et al., 2011), catalytic esterification (Kastner et al., 2012), biogas reforming (Muradov et al.,
2012), and biomass hydrolysis (Ormsby et al., 2012). However, studies using biochar as a
catalyst in biomass pyrolysis and bio-oil upgrading have been lacking.
In our previous research, woody pellets biomass has been successfully pyrolyzed, producing
comparable liquid yield by microwave heating (Ren et al., 2012). The torrefied woody pellet
biomass through microwave pyrolysis showed the enhancement of sugar, phenolic chemicals,
and hydrocarbons in bio-oil and improvement in syngas quality (Ren et al., 2012). This paper
present the continued study on conversions of raw and torrefied woody pellet biomass to biofuels
based on our previous findings.
The main purpose of this study was to investigate the feasibility of using biochar as a low cost
catalyst in raw and torrefied biomass microwave pyrolysis. The effects of loadings of the biochar
catalysts on product yields were determined. The characterization of the products from the
biomass catalytic pyrolysis using biochar catalyst was evaluated and the effects of biochar
catalyst and possible reaction mechanisms determined. The products from raw and torrefied
biomass microwave catalytic pyrolysis were compared, and the recycling of biochar catalysts
and their thermal behaviors were discussed. In addition, bio-oil upgrading under biochar catalyst
was studied.
4.3. Materials and Methods
80
4.3.1 Materials
Douglas fir sawdust pellets (DF) and torrefied Douglas fir sawdust pellets (TDF) were used in
this study. The Douglas fir sawdust pellets were purchased from Bear Mountain Forest Products
Inc. (USA) and their properties can be found in section 2.3.1. Torrefied Douglas fir sawdust
pellets were prepared by a lab bench-scale microwave reactor at the reaction temperature of
275°C for a total reaction time of 15min with the power input of 600W.
4.3.2 Catalyst
Corn stover biochar produced from microwave pyrolysis were used as a catalyst. The corn stover
(collected from Brookings, South Dakota, USA and dried in air at room temperature) was ground
to 2 mm and pyrolyzed at the temperature of 650°C with the power input of 700W. The cold
biochar catalyst was collected after microwave pyrolysis and used for subsequent catalysis
experiments. Mineral analysis of the biochar catalyst was previously reported (Lei et al., 2009).
4.3.3 Biomass catalytic pyrolysis
A lab-scale Sineo MAS-II batch microwave oven (Sineo Microwave Chemistry Technology
Company, Shanghai, China) was used to conduct the biomass catalytic pyrolysis at the reaction
temperature of 480°C for 10min with 700W power setting. The ratios of biochar catalyst to
biomass were 1:4, 1:2, and 1:1 with a fixed biomass loading of 25g. The Douglas fir pellets or
torrefied pellets were first introduced in a half-liter quartz flask, and the corn stover biochar in a
certain ratio to pellets was then added to the flask to cover the feedstock. The detailed
experimental setting, including the nitrogen purging, temperature measurement, power input
control, and condensers can be found in our previous papers (Lei et al., 2009; Ren et al., 2012).
The catalyst and biochar formed from pellet biomass pyrolysis were separated according to their
81
difference in particle size. All experiments were carried out in triplicate. The weight of syngas
from biomass catalytic pyrolysis was calculated using the following equation:
Weight of syngas = initial pellet mass – bio-oil mass – pellet char mass
(1)
4.3.4 Thermal behavior analysis for corn stover biochar by TGA
The thermal behavior of raw biochar catalyst and biochar catalysts after 1 and 10 recycles were
analyzed by TGA (Mettler Toledo 188 TGA/SDTA 851, Switzerland). The TGA analysis was
performed at the nitrogen atmosphere with the flow rate of 20ml/min and the temperature
increased from 25°C to 105°C with a heating rate of 50°C, held for 10min at 105°C, increased to
800°C with a heating rate of 20°C, and then held for 10min at 800°C.
4.3.5 Crude bio-oil upgrading by biochar catalyst
Crude bio-oil from pine wood fludized bed pyrolysis was used in bio-oil upgrading over biochar
catalyst. The same reactor and process setting as described section 2.3 were employed. The ratios
of biochar catalyst to crude bio-oil were 1:2 and 3:1. The upgraded bio-oil was collected in a
condensation system described section 2.3.
4.3.6 GC/MS analysis for bio-oil and upgraded bio-oil and GC analysis for syngas
Chemical compositions of bio-oils were determined using an Agilent 7890A GC/MS (Agilent
Technologies, USA) with a DB-5 capillary column. The detailed experimental analysis process
was presented in Lei et al. (2009).
82
Chemical compositions of syngas were determined by a Carle AGC 400 gas chromatography
(GC) system with a thermal conductivity detector (TCD). The details of experimental settings
can be found in Elliott et al. (2006).
4.4. Results and discussion
4.4.1 Products yields
The biochar catalyst in biomass microwave pyrolysis has both functions of microwave receptor
and catalyst to promote pyrolysis by increasing the heating rate and inducing self-gasification by
forming “micro plasma” (Menendez et al., 2007; Salema et al., 2011). In this study the average
heating rate observed was about 100 °C/min. The product yields had a close relationship with the
loadings of the biochar catalyst (Fig. 4.1 and 4.2). The syngas yield ranged 44.18–46.06 wt% in
catalytic pyrolysis of raw Douglas fir pellets, and 41.37–44.19 wt% in catalytic pyrolysis of
torrified Douglas fir pellets, both much higher than reported in previous findings for the same
pyrolysis conditions without catalyst (Ren et al., 2012). Hence, the biochar catalyst favored the
syngas production as found in other study (Menendez et al., 2007). The bio-oil yield was within
the range of 25.84–37.11 wt% in catalytic pyrolysis of raw Douglas fir pellets, and 18.07–25.00
wt% in catalytic pyrolysis of torrefied Douglas fir pellets. The biochar yields were in the range
of 18.71–28.1 wt% and 27.18–40.55 wt%, respectively, in catalytic pyrolysis of raw and
torrefied Douglas fir pellets. Both bio-oil and biochar yields were reduced compared to those
from pyrolysis without catalyst. In this study, catalytic pyrolysis of torrefied Douglas fir pellets
produced less bio-oil and higher pellet biochar compared to the catalytic pyrolysis of raw
83
Douglas fir pellets (Fig. 4.1 and 4.2). This phenomenon might be due to the carbonization of
biomass in the torrefaction process.
50.00
45.00
Products Yields (wt%)
40.00
35.00
30.00
Catalyst:DF=1:4
25.00
Catalyst:DF=1:2
20.00
Catalyst:DF=1:1
15.00
10.00
5.00
0.00
Syngas
Bio-oil
DF Biochar
Figure 4.1 The effects of biochar catalyst loading on product yields from catalytic pyrolysis of
raw Douglas fir pellets (DF)
The ratio of biochar catalyst to biomass significantly influenced the product yields, especially the
bio-oil and biochar yields (Fig. 4.1 and 4.2). The bio-oil yield significantly decreased with the
increase of the ratio of biochar catalyst to biomass in both raw and torrefied Douglas fir pellets
catalytic pyrolysis but the DF biochar yields were increased. Syngas yield was about 41 to 46
wt%, which was relatively stable under different ratios of biochar catalyst to biomass.
84
50.00
45.00
Products yields (wt%)
40.00
35.00
30.00
Catalyst:TDF=1:4
25.00
Catalyst:TDF=1:2
20.00
Catalyst:TDF=1:1
15.00
10.00
5.00
0.00
Syngas
Bio-oil
TDF Biochar
Figure 4.2 The effects of biochar catalyst loading on product yields from catalytic pyrolysis of
torrefied Douglas fir pellets (TDF)
4.4.2 Bio-oil analysis by GC/MS
Fig. 4.3 shows the chemical distribution in bio-oils based on the functional groups. The
compositions of bio-oils were greatly influenced by the biochar catalyst and its loadings. Few
organic acids were detected in the bio-oils from both the raw and torrefied Douglas fir pellets
catalytic pyrolysis over biochar catalyst. Phenols and guaiacols were significantly changed
compared to those from the biomass pyrolysis without catalyst in previous studies (Ren et al.,
2012). The phenols were 30 to 38 area% and slightly increased with the increase of the biochar
catalyst loading in raw Douglas fir pellets catalytic pyrolysis while the guaiacols slightly
decreased. The same trends were observed at the low loadings of the biochar catalyst in torrefied
Douglas fir pellets catalytic pyrolysis. However, at high loadings of the biochar catalyst (ratio of
biochar catalyst to biomass=1:1), the phenols and guaiacols were 46 and 14 area%, respectively,
85
indicating that most guaiacols were converted to phenols. Few hydrocarbons were found in
Douglas fir pellets pyrolysis without catalyst or with activated carbon (AC) catalyst from
previous studies (Ren et al., 2012; Bu et al., 2012). In this study, the hydrocarbons were about
4.25 to 8.31 area% and increased with the increase of biochar catalyst loading in catalytic
pyrolysis of raw Douglas fir pellets. The hydrocarbons were also increased in catalytic pyrolysis
of torrefied Douglas fir pellets with the increase of biochar catalyst loading. The amount of
hydrocarbons at low biochar catalyst loadings was close to that from torrefied Douglas fir pellets
pyrolysis without catalyst. The yield of hydrocarbons at the biochar catalyst to biomass ratio of
1:1 (up to 15.61 area%) was much higher than the yields from torrefied biomass pyrolysis and
catalytic pyrolysis of the raw Douglas fir pellets. These results indicated that the loading of
biochar catalyst is an important factor affecting the bio-oil composition and hydrocarbons’
production. Further, it was easier for the torrefied biomass, compared to the raw biomass, to be
converted to phenols and hydrocarbons under biochar catalyst.
Since the biochar catalyst was placed on top of the biomass, the volatiles from biomass
decomposition first passed through the biochar zone prior to condensation. Within this catalysis
zone, the cracking and reforming of the volatiles possibly occurred, involving heterogeneous
solid-gas reactions and gas-gas reactions. One reaction mechanism could be the guaiacols
cracking by the cleavage of methyl from O−CH3 to phenols over biochar catalyst. This
explanation was supported by our observations that the guaiacols decreased while the phenols
increased with the increase of catalyst loading. However, the concentration of hydrocarbons
increased with the increased biochar catalyst loading and more than 15 area% was obtained at
86
the high catalyst loadings. The hydrocarbons might have been produced from both phenolics and
aliphatic compounds as they were all noticeably reduced.
100.00
90.00
Chemical distribution (Area%)
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
Catalyst:DF=1:4 Catalyst:DF=1:2 Catalyst:DF=1:1 Catalyst:TDF=1:4 Catalyst:TDF=1:2 Catalyst:TDF=1:1
Other
Sugars
Esters
Furans
Guaiacols
Phenols
Hydrocarbons (linear, branched, aromatic, no oxygen)
Alcohols
ketones/aldehydes
Acids
Figure 4.3 Chemicals distribution of bio-oil from raw (DF) and torrefied (TDF) Douglas fir
pellets pyrolysis under different biochar catalyst loading
4.4.3 Syngas analysis by GC
The compositions of syngas and their variations under different biochar catalyst loadings are
shown in Fig. 4.4 and 4.5. The syngas were mainly composed of CO, H2, CO2, CH4, and short87
chain hydrocarbons, such as C2H4, C2H6, and C3H8. CO was the predominant chemical in the
syngas, which first increased and then decreased with the increase of biochar catalyst loading in
both raw and torrefied biomass catalytic pyrolysis. The highest concentrations of CO were 43.03
and 38.34 vol%, respectively, detected at the biochar catalyst to biomass ratio of 1:2 in both raw
and torrefied biomass catalytic pyrolysis. The concentrations of CO in syngas were similar to
those from biomass microwave pyrolysis without catalyst (Ren et al., 2012).
In previous research no hydrogen was observed in the Douglas fir pellets pyrolysis without
catalyst (Ren et al., 2012). But in raw Douglas fir pellets catalytic pyrolysis over biochar catalyst
about 11.86 to 20.43% (v/v) hydrogen was observed, and 21.36 to 27.02% (v/v) hydrogen was
also obtained in torrefied pellets catalytic pyrolysis over biochar catalyst, much higher than in
torrefied Douglas fir pellet pyrolysis without catalyst. These results indicated that biochar as
catalyst favored the hydrogen production as also found by Menendez et al. (2007). In contrast,
the concentration of hydrogen decreased first and then increased with the increase of biochar
catalyst loading. The relatively low amount of CO2, less than 25% (v/v), was produced in this
study. CO2 from catalytic pyrolysis of raw Douglas fir pellets was increased with the increase of
biochar catalyst loading. In comparison, the concentration of CO2 from catalytic pyrolysis of
torrefied Douglas fir pellets was stable at the biochar catalyst to biomass ratios of 1:2 and 1:1.
The concentrations of CH4 in syngas from catalytic pyrolysis of raw and torrefied biomass were
higher than 10% (v/v) and decreased with the increase of biochar catalyst loading.
Mominguez et al. (2007) reported that CH4 and CO2 can be reformed to syngas under biochar
catalyst by microwave heating. This mechanism can partly explain our findings that the CH 4
88
decreased and hydrogen increased with the increase of biochar catalyst loading. However, the
concentration of CO2 increased with the increase of biochar catalyst loading in this study (Fig.
4.4 and 4.5). The deoxygenation of bio-oils might have contributed to the CO2 production.
Noticeable differences in the concentrations of syngas components between raw and torrefied
Douglas fir pellets catalytic pyrolysis were observed. More CO and CO2 were produced in the
former process than in the latter; however, the H2 production in the latter was increased by up to
27.02% (v/v) at the biochar catalyst to biomass ratio of 1:1.
50.00
45.00
40.00
Catalyst:DF=1:4
Amounts (v/v %)
35.00
Catalyst:DF=1:2
30.00
Catalyst:DF=1:1
25.00
20.00
15.00
10.00
5.00
0.00
CO
H2
CO2
CH4
C2H4
C2H6
C3H8
Figure 4.4 The effects of biochar catalyst loading on the chemical composition of syngas from
catalytic pyrolysis of raw Douglas fir pellets (DF)
89
45.00
40.00
Catalyst:TDF=1:4
Amounts (v/v %)
35.00
Catalyst:TDF=1:2
30.00
Catalyst:TDF=1:1
25.00
20.00
15.00
10.00
5.00
0.00
CO
H2
CO2
CH4
C2H4
C2H6
C3H8
Figure 4.5 The effects of biochar catalyst loading on the chemical composition of syngas from
catalytic pyrolysis of torrefied Douglas fir pellets (TDF)
4.4.4 TGA analysis for biochar catalyst
The thermal behavior of corn stover raw biochar and biochar catalysts after 1 and 10 recycles in
Douglas fir pellets catalytic pyrolysis were analyzed by TGA under N2 flow of 20ml/min. The
TGA curves of these three samples are shown in Fig. 4.6. The small weight loss observed at the
temperature below 150°C was about 2–3% for the raw and recycled biochars. This weight loss
was mainly due to evaporation of water and low molecular volatiles. At the temperature over
150°C, there was a continuous, slight weight loss in raw and recycled biochar catalysts. A larger
weight loss of the raw biochar catalyst occurred at the temperatures above 300°C, suggesting the
presence of volatiles in the raw biochar, detectable even at the high temperature of 650°C. Still,
90
the total weight loss in these three samples was only 4 to 6% at the temperature from 150 to
800 °C, indicating the stability of both the raw and recycled biochars.
In biomass catalytic pyrolysis coke deposits on the catalyst are generally aromatic compounds
and they can be removed at the temperatures over 350 °C (Guo et al., 2009). In this study, the
weight loss from temperature 350 to 800°C was only 2–3 wt% in recycled biochar and there was
no significant weight loss change among different recycled biochar catalysts. It might be due to
low coke deposits on the biochar catalyst in microwave assisted process and this result was also
consistent with the findings that the weight of biochar did not change after the pyrolysis
(Dominguez et al., 2007).
100
Raw catalyst
Catalyst after 1 recycle
98
Catalyst after 10 recycles
Weight (%)
96
94
92
90
88
86
84
25
105
273
473
673
800
Temperature (ºC)
Figure 4.6 TGA profiles of raw biochar and recycled biochar catalysts
91
4.4.5 Crude bio-oil upgrading using biochar as catalyst
The chemical compositions of upgraded bio-oil using biochar as catalyst were analyzed using
GC/MS. Fig. 4.7 reveals the changes of main chemical groups with different biochar catalyst
loadings. The concentrations of hydrocarbons and phenols were increased with the increase of
biochar catalyst loading while those of the guaiacols and aliphatic compounds decreased. The
hydrocarbons and phenols were up to 42.56 area% and 37.23 area%, respectively, at the biochar
to bio-oil ratio of 3:1. Meanwhile, the concentrations of guaiacols and aliphatic compounds were
reduced from about 40 area% to 6.11 area% and 13.76 area%, respectively. Hence, the biochar as
catalyst facilitated the upgrading of the crude bio-oil to phenols and hydrocarbons.
45.00
40.00
Crude bio-oil
35.00
Catalyst:oil=1:2
GC/MS (area%)
30.00
Catalyst:oil=3:1
25.00
20.00
15.00
10.00
5.00
0.00
Hydrocarbons
Phenols
Guaiacols
Figure 4.7 Chemicals compositions of upgraded bio-oil
92
Aliphatic
coumpounds
4.4.6 Mechanism analysis of biomass catalytic pyrolysis and bio-oil upgrading
The reaction mechanism of biomass catalytic pyrolysis and bio-oil upgrading for hydrocarbons
and syngas production using biochar catalyst was shown in Fig. 4.8. As we found in the previous
analysis, aliphatic compounds also contributed to the aromatics production. The reaction
pathway of glucose catalytic pyrolysis to aromatics proposed by Carlson et al. (2010) can explain
the aromatic hydrocarbon production from aliphatic compounds in this study. The intermediate
chemicals mainly composed of anhydrosugars and furans are first formed from hemicelluloses
and cellulose decomposition. The carboxylic acid sites presented on the biochar surface catalyze
the anhydrosugars dehydration to furans and furans oligomerization, decarboxylation and
decarbonylation to aromatics. Another pathway of aromatic production is from lignin. The lignin
predominantly decomposes to guaiacols, and then guaiacols are cracked to phenols by cleavage
of bond O-CH3. The phenols are further cracked to aromatics and by cleavage of –OH. The
biochar catalyst with carboxylic function groups on the surface promotes the cracking occurred.
In this study, the syngas with high concentration of H2 and CO were obtained and two main
reaction mechanisms can reveal the H2 and CO production. One reaction mechanism is water
gas shift reaction for the small molecular compounds such H2O, CO formed from the biomass
decomposition, intermediate compounds dehydration, decarboxylation, decarbonylation and
cracking. This reaction might be triggered by the metals like Cu and Fe presented in the biochar
catalyst and enhanced by the microwave irradiation (Chen et al., 2008). Another reaction
mechanism for H2 and CO production is dry reforming of methane. Dominguez et al. (2007) and
Muradov et al. (2012) reported that the biochar catalyzed biogas conversion to syngas and K
presented in biochar and microwave irradiation both favored the biogas conversion. The
93
observed decrease of methane with the increase of biochar catalyst loading in this study
confirmed the occurrence of dry reforming of methane in this study. Dominguez et al. (2007)
pointed out that the self-gasification of the char occurred in microwave pyrolysis and K
presented in biochar favored this gasification and reduced the coke formation. This might explain
the low coke observed in the recycled biochar catalyst.
Figure 4.8 Proposed reaction pathway of biomass catalytic pyrolysis and bio-oil upgrading for
hydrocarbon and syngas production using biochar catalyst under microwave heating
94
4.5. Conclusions
We investigated the effects of biochar as a catalyst in biomass catalytic pyrolysis and bio-oil
upgrading. The biochar catalyst favored the syngas production and had positive influence on the
bio-oil quality. The bio-oil chemical profile from catalytic pyrolysis and bio-oil upgrading over
biochar catalyst was simplified to phenols and hydrocarbons, and their concentrations were
increased with the increase of biochar catalyst loading. High-quality syngas richened in H2, CO,
and CH4 was obtained for biomass catalytic pyrolysis over biochar catalysts. These results
indicated that biochar might be a cheap catalyst in biomass conversion and bio-oil upgrading.
95
4.6 References
1. Aho A., Kumar N., Lashkul A.V., Eranen K., Ziolek M., Decyk P., Salmi T., Holmbomb B.,
Hupa M., Murzin D. Y., 2010. Catalytic upgrading of woody biomass derived pyrolysis vapours
over iron modified zeolites in a dual-fluidized bed reactor. Fuel 89, 1992–2000
2. Azargohar R., Dalai A.K., 2006. Biochar as a precursor of activated carbon. Appl. Biochem.
Biotech. 129–132
3. Bu Q., Lei H., Ren S., Wang L., Zhang Q., Tang J., Ruan R., 2012. Production of phenols and
biofuels by catalytic microwave pyrolysis of lignocellulosic biomass. Bioresour. Technol. 108,
274–279
4. Carlson T.R., Jae J., Lin Y, Tompsett G.A., Huber G.W., 2010. Catalytic fast pyrolysis of
glucose with HZSM-5: The combined homogeneous and heterogeneous reactions. J. of Catal.
270, 110–124
5. Chan K.Y., Van Zwieten L., Meszaros I., Downie A., Joseph S., 2007. Agronomic values of
green waste biochar as a soil amendment. Aust. J. Soil Res. 45, 629–634
6. Chen G., Fang B., 2011. Preparation of solid acid catalyst from glucose–starch mixture for
biodiesel production. Bioresour. Technol. 102, 2635–2640
7. Chen W., Jheng J, Yu A.B., 2008. Hydrogen generation from a catalytic water gas shift
reaction under microwave irradiation. Int. J. Hydrogen Energ. 33, 4789–4797
8. Chun Y., Sheng G., Chiou C.T., Xing B., 2004. Compositions and sorptive properties of crop
residue-derived chars. Environ. Sci. Technol. 38, 4649−4655
9. Dehkhoda A.M., West A.H., Ellis N., 2010. Biochar based solid acid catalyst for biodiesel
production. Appl. Catal. A-Gen. 382, 197–204
10. Demiral I., Sensoz S., 2008. The effects of different catalysts on the pyrolysis of industrial
96
wastes (olive and hazelnut bagasse). Bioresour. Technol. 99, 8002–8007
11. Dominguez A., Fernandez Y., Fidalgo B., Pis J.J., Menendez J.A., 2007. Biogas to syngas by
microwave-assisted dry reforming in the presence of char. Energy Fuels 21, 2066–2071
12. Elliott D.C.; Hart T.R.; Neuenschwander G.G., 2006. Chemical Processing in High-Pressure
Aqueous Environments. 8. Improved Catalysts for Hydrothermal Gasification. Ind. Eng. Chem.
Res. 45, 3776–3781
13. Elliott D.C., Hart T.R., Neuenschwander G.G., Rotness L.J., Zacher A.H., 2009. Catalytic
hydroprocessing of biomass fast pyrolysis bio-oil to produce hydrocarbon products. Environ
Prog Sustain Energy 28, 441–449
14. Guo X., Zheng Y., Zhang B., Chen J., 2009. Analysis of coke precursor on catalyst and study
on regeneration of catalyst in upgrading of bio-oil. Biomass Bioenerg. 33, 1469–1473
15. Hilten R.N., Bibens B.P., Kastner J.R., Das K.C., 2010. In-Line Esterification of Pyrolysis
Vapor with Ethanol Improves Bio-oil Quality. Energy Fuels 24, 673–682
16. Ikura M., Stanciulescu M., Hogan E., 2003. Emulsification of pyrolysis derived bio-oil in
diesel fuel. Biomass Bioenerg. 24, 221–232
17. Kastner J.R., Miller J., Geller D.P., Locklin J., Keith L.H., Johnson T., 2012. Catalytic
esterification of fatty acids using solid acid catalysts generated from biochar and activated
carbon. Catal. Today 190, 122–132
18. Lappas A.A., Samolada M.C., Iatridis D.K., Voutetakis S.S., Vasalos I.A., 2002. Biomass
pyrolysis in a circulating fluid bed reactor for the production of fuels and chemicals. Fuel 81,
2087–2095
19. Lei, H., Ren, S., Julson, J., 2009. The effects of reaction temperature and time and particle
size of corn stover on microwave pyrolysis. Energy Fuels 23, 3254–3261
97
20. Menendez, J.A., Dominguez, A., Fernandez, Y., Pis, J.J., 2007. Evidence of selfgasification
during the microwave-induced pyrolysis of coffee hulls. Energy Fuels 21, 373–378
21. Mohan, D., Pittman, C.U., Steele, P.H., 2006. Pyrolysis of wood/biomass for bio-oil: a
critical review. Energy Fuels 20, 848–889
22. Mohan D., Sharma R., Singh V.K., Steele P., Jr. Pittman C.U., 2012. Fluoride Removal from
Water using Bio-Char, a Green Waste, Low-Cost Adsorbent: Equilibrium Uptake and Sorption
Dynamics Modeling. Ind. Eng. Chem. Res. 51, 900–914
23. Mullen C.A., Boateng A.A., Goldberg N.M., Lima I.M., Laird D.A., Hicks K.B., 2010. Biooil and bio-char production from corn cobs and stover by fast pyrolysis. Biomass Bioenerg. 34,
67−74
24. Muradov N., Fidalgo B., Gujar A.C., Garceau N., T-Raissi A., 2012. Production and
characterization of Lemna minor bio-char and its catalytic application for biogas reforming.
Biomass Bioenerg. 42, 123−131
25. Ormsby R., Kastner J.R., Miller J., 2012. Hemicellulose hydrolysis using solid acid catalysts
generated from biochar. Catal. Today 190, 89– 97
26. Ren, S., Lei, H., Wang, L., Bu, Q., Chen, S., Wu, J., Julson, J., Ruan, R., 2012. Biofuel
production and kinetics analysis for microwave pyrolysis of Douglas fir sawdust pellet. J. Anal.
Appl. Pyrol. 94, 163–169
27. Ren, S., Lei, H., Wang, L., Bu, Q., Chen, S., Wu, J., Julson, J., Ruan, R., 2012. The effects of
torrefaction on compositions of bio-oil and syngas from biomass pyrolysis by microwave heating.
Bioresource Technology Available online 4 July 2012
28. Rioche C., Kulkarni S., Meunier F. C., Breen J. P., Burch R., 2005. Steam reforming of
model compounds and fast pyrolysis bio-oil on supported noble metal catalysts. Appl. Catal. B-
98
Environ. 61, 130–139
29. Salema A.A., Ani F.N., 2011. Microwave induced pyrolysis of oil palm biomass. Bioresour.
Technol. 102, 3388–3395
30. Samolada M.C., Papafotica A., Vasalos I.A., 2000. Catalyst evaluation for catalytic biomass
pyrolysis. Energy Fuels 14, 1161−1167
31. Scott D.S., Piskorz J., 1984. The continuous flash pyrolysis of biomass. Can. J. Chem. Eng.
62, 404–412
32. Vitolo S., Seggiani M., Frediani P., Ambrosini G., Politi L., 1999. Catalytic upgrading of
pyrolytic oils to fuel over different zeolites. Fuel 78, 1147−1159
33. Wright M.M., Daugaardc D.E., Satriob J.A., Brown R.C., 2010. Techno-economic analysis
of biomass fast pyrolysis to transportation fuels. Fuel 89, S2–10
34. Wang Z., Pan Y., Dong T., Zhu X., Kan T., Yuan L., Torimoto Y., Sadakata M., Li Q., 2007.
Production of hydrogen from catalytic steam reforming of bio-oil using C12A7-O-based
catalysts. Appl. Catal. A-Gen. 320, 24–34
35. Zhang H., Xiao R., Huang H., Wang D., Zhong Z., Song M., Pan Q., He G., 2009. Catalytic
fast pyrolysis of biomass in a fluidized bed with fresh and spent fluidized catalytic cracking
(FCC) catalysts. Energy Fuels 23, 6199–6206
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CHAPTER FIVE
THERMAL BEHAVIOR AND KINETIC STUDY FOR WOODY
BIOMASS TORREFACTION AND TORREFIED BIOMASS
PYROLYSIS BY TGA
5.1 Abstract
This study was focues on investigating the thermal decompostion behaviors and kinetics of
Douglas fir sawdust torrefaction and torrefied Douglas fir sawdust pyrolysis using a
thermogravimetric analyzer (TGA). The weight loss of samples in torrefaction was highly related
to the torrefaction temperature. The two step-reaction model fitted well for Douglas fir sawdust
torrefaction. The activation energies of the first and second reaction stages were 112 kJ/mol and
150 kJ/mol, respectively. Torrefied biomass exhibited the different thermo decomposition
behaviors as compared to untreated biomass. The start point of torrefied biomass decomposition
was shifted and the degree of shift increased with the severity of torrefaction. The final biochar
yield of torrefied biomass was also increased with the increase of torrefaction temperature.
Derivative thermogravimetric (DTG) curves showed that the shoulder of hemicelluloses
decomposition in torrefied biomass pyrolysis was eliminated. The decomposition rate of
torrefied biomass has a decreasing trend due to the mass depletion in torrefaction. The first-order
one-step global model fitted well for the raw and torrefied biomass pyrolysis with the average
activation energies in the range of 195−204 kJ/mol. The kinetic analysis also showed that the
torrefied biomass pyrolysis from high terrefaction temperature might be multiple-step reactions.
100
Keywords: TGA; Kinetics; torrefied biomass; pyrolysis
5.2 Introduction
Fossil fuels play an important role in the past and today’s transportation fuel supplies. However,
fossil fuels are not renewable. The declining of limited reserve and increasing demand for fossil
fuels has led to the global energy crisis. The contribution of the greenhouse gases emission by
burning fossil fuels has brought a major environmental challenge. These problems motive
scientists to search for alternative energy. Biomass is an abundant and carbon-based renewable
fuel resource that can provide a continuous energy supply by annually planting and harvesting.
Biomass is also a clean and carbon-neutral energy source as its carbon is recycled from the
atmosphere (Ragauskas et al., 2006).
Biomass has high water and oxygen content as well as low bulk density and heating value. These
properties hinder its broad utilization. Several technologies have been used to convert biomass to
high-energy fuels in the form of solids, liquids, and gases. These technologies include
torrefaction, pyrolysis, liquefaction, digestion, fermentation, and gasification, aming which
torrefaction and pyrolysis are thermo-chemical methods. Torrefaction, also called mild pyrolysis,
is conducted at the temperature of 200–300°C in inert environment to convert biomass to solid
fuels (Bergman et al., 2005). Compared to torrefaction, biomass pyrolysis is a strong thermochemical method conducted at the temperature of 350−600°C in inert environment (Scott et al.,
1984). The primary product of biomass pyrolysis is bio-oil. During the thermo-chemical
processes, thermal decompositions of biomass are involved.
101
Biomass is complex, composed primarily of hemicelluloses, cellulose, and lignin. In pyrolysis,
these three components of biomass are decomposed at different temperatures and rates. The
hemicelluloses and cellulose can be decomposed relatively fast at low temperatures of 200 to
350°C while lignin are decomposed slowly in a wide range of temperatures from 280 to 600°C
(Mohan et al., 2006). As a result, the thermal decomposition and reaction kinetics of biomass are
complicated. The developed models in biomass pyrolysis are generally categorized as one-step
global or semi-global models and multiple-step reaction models (Blasi, 2008; White et al., 2011).
Shafizadeh and Chin (1977) proposed the one-step reaction mechanism for biomass pyrolysis by
postulating one component of biomass. Blasi (1998) proposed semi-global reaction models for
woody biomass pyrolysis in which three parallel reactions for the main components of the
biomass would take place in pyrolysis. Bradbury (1979) proposed the multi-step reactions for
cellulose pyrolysis in which cellulose is assumed to be first converted to active cellulose and
then decomposed to volatiles and char. The char would be further degraded. Alves et al. (1989)
proposed another mechanism, which consists of three first-order consecutive reactions.
Torrefaction as a mild pyrolysis technology at relatively low reaction temperatures of 200 to 300
°C shows different decomposition mechanisms from biomass pyrolysis. Biomass torrefaction
mainly decomposes hemicelluloses and partially degrades cellulose and lignin. Prins et al. (2006)
studied the weight loss kinetics of willow biomass torrefaction with their results conforming to
the two-stage mechanism proposed by Blasi and Lanzetta (1997). They showed that the first step
of torrefaction was fast with the hemicellulose decomposition and the second step was relatively
slow with cellulose decomposition. Chen and Kuo (2011a) investigated the isothermal
torrefaction kinetics of the basic constituents of biomass and demonstrated their developed
model could properly describe their experiment.
102
Recently biomass torrefaction as a pretreatment in pyrolysis has drawn increasing attention. The
torrefied biomass pyrolysis exhibits different decomposition behaviors due to the removal of
hemicelluloses and modification of cellulose and lignin in torrefaction (Wannapeera et al., 2011;
Chen and Kuo, 2010). The cross-linking reaction also occurs in torrefied biomass thermal
decomposition leading to more biochar produced (Wannapeera et al., 2011; Broströma et al.,
2012). However, studies on the kinetics of torrefied biomass pyrolysis have been lacking.
Therefore, the aim of this study was to investigate torrefied woody biomass decomposition
behavior and to develop kinetic models from termogravimetric analysis.
5.3 Materials and methods
5.3.1 Materials
Douglas fir sawdust pellets were used in this study and their properties can be found in section
2.3.1.
Three samples of torrefied Douglas fir sawdust pellets were prepared using a bench-scale
microwave reactor with different reaction temperatures of 250, 275, and 300°C, but the same
reaction time of 10min at the power input of 600W. All samples were ground to 2 mm before the
thermogravimetric analysis.
5.3.2 Thermogravimetric analysis of Douglas fir sawdust torrefaction
The thermal degradation behavior of Douglas fir sawdust torrefaction was analyzed by TGA
(Mettler Toledo 188 TGA/SDTA 851, Switzerland). The TG analysis was performed at the
nitrogen atmosphere with a flow rate of 20ml/min. For each test, about 10mg sample was loaded
103
into the crucible. The heating rate was 40°C/min. The torrefactions of samples were performed at
temperatures of 250, 275, and 300°C. After the desired temperature was reached, the samples
were held for 1 hr.
5.3.3 Thermogravimetric analysis of torrefied Douglas fir sawdust pyrolysis
The same thermogravimetric analyzer was used to perform the thermogravimetric analysis for
torrefid biomass pyrolysis. The selected heating rates were 10, 20, 30, 40, and 50 °C/min. The
analysis for torrefied biomass was programmed from 25 to 600°C with a nitrogen flow rate of
20ml/min. For each test, 10mg samples were used.
5.3.4 Biomass decomposition kinetics
In general the reaction decomposition rate in kinetics analysis is a function of the remaining raw
material (Varhegyi and Antal, 1989)
d
dt
k (1
)n
(1)
where k is reaction rate, and the fractional reaction α is defined in terms of the change in mass of
samples:
X0 X
(2)
X0 X f
where X0 is the initial sample weight, X is the sample weight at time t, and Xf is the final sample
weight.
The rate of decomposition follows the Arrhenius law dependent on temperature:
k
Aexp( E / RT )
(3)
104
and, Eq. 1 can be expressed as:
d
dt
A exp( E / RT )(1
)n
(4)
where A is frequency or pre-exponential factor of the torrefaction process (s–1), E is apparent
activation energy (J/mol), T is temperature (K), R is universal gas constant, 8.3145 (J/mol · K), n
is the order of reaction, and t is time (s).
5.4 Results and discussion
5.4.1 Termal decomposition behavior of Douglas fir sawdust torrefaction
Figure 5.1 shows the thermogravimetric (TG) curves for different torrefaction processes. The
curves in Fig. 5.1 can be divided into two stages (Prins et al., 2006). At the first stage the fast
weight loss occurred during the heating period and first several minutes of the isothermal process.
After that, the weight loss showed a linearly proportional at the isothermal torrefaction with slow
weight loss. The sample weight loss in torrefaction was significantly influenced by the reaction
temperature. At 250°C, the weight loss was about 12wt%, indicating mild decomposition. At
275°C, the weight loss was about 26wt%, much greater than at 250°C. At 300°C, the sample
weight loss was up to 48wt%, suggesting the decomposition of cellulose and lignin in biomass in
addition to the hemicelluloses. These results were consistent with the findings of Prins et al.
(2006) and Chen and Kuo (2010, 2011b).
The peaks of the derivative thermogravimetric (DTG) were close to the temperature of the
setting point of torrefaction (Fig.5.2) and the height of peaks significantly increased with the
increase of the torrefaction temperature. Chen and Kuo (2011b) studied the hemicelluloses,
105
cellulose, and lignin decomposition behavior by TGA and reported that most of hemicelluloses
were decomposed between 250 and 275°C, and the rapid weight drops of cellulose occurred at
the torrefaction of 300°C. In our research, we observed a rather low weight loss and peak of
DTG curve at the torrefaction temperature of 250°C, most likely a result of the predominant
decomposition of hemicelluloses. At 275°C, the weight loss and the height of the DTG peak
were doubled compared to those at 250°C, suggesting the decomposition of both hemicelulose
and cellulose of the biomass. At 300°C, a very high weight loss and DTG peak were observed,
indicating that the three components of biomass, especially hemicellulose and cellulose, were all
largely decomposed.
100
90
80
TG (wt%)
70
60
50
250°C
40
275°C
30
300°C
20
10
0
0
600
1200
1800
Time (s)
2400
3000
3600
Figure 5.1 TG curves of Douglas fir sawdust torrefaction at different temperatures
106
DTG (wt%/min)
10
9
8
7
6
5
4
3
2
1
0
250°C
275°C
300°C
0
500
1000
1500
2000
Time (s)
2500
3000
3500
Figure 5.2 DTG curves of Douglas fir sawdust torrefaction at different temperatures
5.4.2 Isothermal kinetics of Douglas fir sawdust torrefaction
In the thermogravimetric analysis of torrefaction, the temperature program consisted of two
temperature heating periods. One was the dynamic heating period in which the sample was
heated from 25°C to the desired temperature. Another was the isothermal heating period in
which the sample was held at the desired temperature to complete the torrefaction. In this study,
the relatively high heating rate of 40°C/min was used to minimize the dynamic heating period to
about 6 min. Considering the long isothermal heating period ( 60min) compared to the dynamic
heating period, the torrefaction process may be considered an isothermal reaction.
Di Blasi and Lanzetta (1997) proposed a two-step reaction mechanism for xylan torrefaction
using isothermal TGA (Fig. 5.3). In their model volatiles and intermediate components are
formed in the first-setp reaction, and the intermediate components are decomposed to volatiles
and biochar in the second-step reaction. Based on this model, Prins et al. (2006) investigated the
107
weight loss kinetics of willow wood torrefaction in isothermal conditions and determined the
parameters of kinetics.
Figure 5.3 Two-step reaction model of biomass torrefaction (Di Blasi and Lanzetta, 1997; Prins
et al. 2006)
As described previously, Douglas fir sawdust exhibited two-step weight losses. Hence, the twostep reaction model developed by Di Blasi and Lanzetta was used to describe Douglas fir
sawdust torrefaction in this study. Both reactions were assumed to be first-order. The parameters
were estimated by the least-squares method by minimizing the sum S:
N
S
(( X t ) exp,i ( X t ) calc,i ) 2
(5)
i
where i is an index, from 1 to the total number of points N, ( X t ) exp,i is the ith observed mass value
and ( X t )calc,i the calculated mass value obtained by numerical solution with the given parameters.
The parameters of initial set were based on Prins’s findings.
108
Table 5.1 Kinetic parameters determined from the least-square evaluation
Parameters
Douglas fir sawdust
A1 (min−1)
3.02 x 102
A1v (min−1)
5.38 x 105
A2 (min−1)
1.8 x 108
A2v (min−1)
2.66 x 108
E1 (kJ/mol)
113
E1v(kJ/mol)
111
E2 (kJ/mol)
151
E2v (kJ/mol)
148
The percent average deviation was calculated as
Deviation (%) 100
S /i
(6)
The percent average deviation was 2.41, implying that the model fits the data well. The
activation energy for the first stage of reaction was 112 kJ/mol, which was slightly higher than
those values in Blasi and Lanzetta (1997) and Prins et al (2006) for willow and xylan torrefaction.
The difference might be due to the different properties of samples. The activation energy for the
second stage of reaction was 150 kJ/mol, which was very close to the values obtained in the
aforementioned two studies, possibly because the decomposition of intermediate components to
biochar in different samples tends to be similar.
5.4.3 Thermal decomposition behavior of torrefied Douglas fir sawdust
109
Raw Douglas fir sawdust
100
90
Torrefied Douglas fir sawdust
(250°C, 10min)
80
Torrefied Douglas fir sawdust
(275°C, 10min)
70
Torrefied Douglas fir sawdust
(300°C, 10min)
TG (wt%)
60
50
40
30
20
10
0
100
200
300
400
500
600
Temperature (°C)
Figure 5.4 Thermogravimetric (TG) curves for raw and torrefied Douglas fir sawdust pyrolysis
As the TG curves (Fig. 5.4) reveal, the decompositions of torrefied biomass shifted to a higher
temperature and the degree of the shift increased with the severity of torrefaction. For raw
biomass, 5wt% weight loss was observed at the temperature of 278°C, but for the torrefied
biomass the same loss was obtained at 300°C, and the temperature of decomposition was
increased to 313°C. This phenomenon might be due to the hemicelluloses removal in torrefaction
process. The final solid (biochar yield) of raw and torrefied biomass pyrolysis also showed
significant differences (Fig.5.4). The biochar yield increased substantially with the torrefction
temperature. The raw biomass pyrolysis yielded 21wt% of biochar, and the torrefied biomass
110
pyrolysis yielded 30wt%, 35wt%, and 49wt% of biochar at torrefaction temperatures of 250, 275,
and 300°C, respectively. The high biochar yield obtained for the torrfied biomass with
torrefaction temperature of 300°C was due to the large weight loss along with the hemicelluloses
and cellulose removal during torrefaction. The torrefied biomass formed more biochar in
pyrolysis by cross-linkage reactions (Wannapeera et al., 2011).
14
Raw Douglas fir sawdust
Torrefied Douglas fir sawdust (250°C,
10min)
Torrefied Douglas fir sawdust (275°C,
10min)
Torrefied Douglas fir sawdust (300°C,
10min)
12
10
DTG (wt%)
8
6
4
2
0
100
200
300
400
500
600
Temperature (°C)
Figure 5.5 Derivative thermogravimetric (DTG) curves of raw and torrefied Douglas fir sawdust
pyrolysis
The hemicelluloses and cellulose are decomposed fast at relatively low temperatures of 200 to
350°C while lignin are decomposed slowly in a large range of temperatures from 280 to 600°C.
The maximum weight loss of hemicelluloses and cellulose occurred at the temperature 268 and
111
355°C in a previous study for pure hemicelluloses and cellulose analysis (Yang et al., 2007).
Generally, for biomass, the shoulder that is contributed by the decomposition of hemicelluloses
can be observed at the temperature about 300°C in DTG analysis (Muller-Hagedorn et al., 2003;
Yang et al., 2007). In our research, this shoulder was also observed for the raw Douglas fir
sawdust pyrolysis (Fig. 5.5). However, for the torrefied biomass pyrolysis, the shoulder was
eliminated especially for the sample subjected to the high temperature torrefaction. The peak of
DTG along with the maximum weight loss was mainly contributed by cellulose (Yang et al.,
2007). In our study the peak of DTG curve for raw biomass pyrolysis was near 365°C. The peaks
of DTG curve for torrefied biomass pyrolysis were observed at the same temperature, indicating
that the torrefaction had less effect on the temperatures of cellulose decomposition. However, the
height of peak in torrefied biomass revealed the decreasing trend with the severity of torrefaction.
The DTG peak of torrefied biomass with terrefaction temeperature of 250°C showed slight
reduction compared to that of raw biomass, suggesting insignificant effects on cellulose in
torrefaction. However, the DTG peaks of torrefied biomass were reduced significantlywhen the
torrefaction temperature was increaded to 300 °Cdue to the depletion of the majority of the
biomass in the torrefaction. The increase in the peaks of the DTG curves for torrefied biomass
was observed and the increase grew larger with the severity of torrefaction. This may be
attributed to the decomposition of lignin and second reaction of biochar.
5.4.4. One-step global kinetics of torrefied Douglas fir sawdust pyrolysis
A one-step global kinetic model was used to describe the thermal decomposition of raw and
torrefied Douglas fir sawdust pyrolysis, which can be schematized as:
Raw/torrefied Douglas fir sawdust
Bio-oil + Syngas + biochar
112
(7)
The rate of decomposition of raw and torrefied Douglas fir sawdust pyrolysis can be expressed
by Eq. 4. Eq. 4 was solved with the Friedman method to determine the values of kinetic
parameters.
The Friedman method is an isoconversional method based on the assumption that the rate of
reaction ( /dt) at a constant conversion ( ) is only a function of temperature (Friedman, 1964).
The equation 4 can be rewritten as the following equation:
da/dt = A• exp (-E/RT) •f ( )
(8)
Converting Eq. 8 into a logarithmic expression we have:
ln (da/dt )= (-E/RT )+ ln (Af(
))
(9)
Therefore, for the given value of conversion rate ( ) the plot of ln (da/dt) versus 1/T gives a
straight line with the slope of −E/R and intercept of ln (Af( )).
Fig. 5.6 shows the plots of ln (d
/dt) vs. 1/T for raw and torrefied Douglas fir sawdust. The fitted
lines for raw Douglas fir sawdust were nearly parallel, indicating that the activation energies
were rather close for different conversion rates and the mechanism of biomass pyrolysis was a
one-step global reaction. The similar phenomenon was observed for the torrefied biomass at
torrefaction of 250°C and 10min, implying that the torrefaction at the relatively low temperature
did not change the reaction mechanism of pyrolysis. Fig. 5.6c shows the plots for torrefied
biomass at the torrefaction temperature of 275 °C. The fitted lines in the conversion rate of 0.1 to
0.7 were nearly parallel. For the very low (< 0.1) or high (≥ 0.8) conversion rates, the fitted lines
are slightly diverged from the parallel lines, implying a change in the trend with the increase in
torrefaction temperature due to the different reaction mechanism for torrefied biomass. The plots
113
for the torrefied biomass with the torrefaction temperature of 300°C illustrated that the fitted,
nearly parallel lines can only be observed at the conversion rate of 0.2 to 0.5 (Fig. 5.6 d). It
confirmed that the torrefaction processes at relatively high temperatures substantially influenced
the reaction mechanism. The multiple-step reactions, instead of one-step global reactions, might
occur in such torrefied biomass. These observations were consistent with the findings on DTG
curves analysis.
0
-0.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
LN (da/dt) (min-1)
-1
-1.5
-2
-2.5
-3
-3.5
-4
1.4
a
1.5
1.6
1.7
1.8
1000/T (K)
114
1.9
2
0
-0.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
LN (da/dt) (min-1)
-1
-1.5
-2
-2.5
-3
-3.5
-4
1.4
1.5
1.6
b
1.7
1.8
1.9
2
1000/T (K)
0
0.1
0.5
-0.5
0.2
0.6
0.3
0.7
0.4
0.8
-1
LN (da/dt) (min-1)
-1.5
-2
-2.5
-3
-3.5
-4
-4.5
1.4
c
1.5
1.6
1.7
1000/T (K)
115
1.8
1.9
2
0
-0.5
LN (da/dt) (min-1)
-1
0.1
0.2
0.3
0.4
0.5
0.6
-1.5
-2
-2.5
-3
-3.5
-4
1.4
d
1.5
1.6
1.7
1.8
1.9
2
1000/T (K)
Figure 5.6 Isoconversional plot for (a) raw and torrefied Douglas fir sawdust, (b) torrefaction of
250°C and10min, (c) torrefaction of 275°C and10min, and (d) torrefaction of 300°C and10min
The average activation energies were summarized in Table 5.2. The average activation energy
for the raw biomass was 204 kJ/mol, slightly higher than those reported in previous studies for
woody biomass pyrolysis (Blasi and Branca, 2001; Grønli et al., 2002; Blasi et al., 2008). The
average activation energies of torrefied biomass ranged from 198 to 195kJ/mol, which were
slightly lower than for the raw biomass. The reduction was increased with the severity of
torrefaction. This may be attributed to the concentration of mass and modified structure in the
cellulose and lignin in torrefied biomass. These changes in the biomass may have eased the
decomposition process during pyrolysis. The high R-square value of the fitted model suggests
116
that the one-step, first-order reaction model can adequately describe the raw and torrefied
Douglas fir sawdust pyrolysis.
Table 5.2 Average apparent activation energy, logA and R2 of the models for raw and torrefied
Douglas fir sawdust calculated by the Friedman method for first-order reactions
Activation
logA
energy (kJ/mol)
(min−1)
Raw Douglas fir sawdust
204 (10. 8)
16.6 (1.22)
0.988 (0.012)
Torrefied Douglas fir sawdust (250, 10min)
198 (15.2)
15.8 (1.49)
0.980 (0.022)
Torrefied Douglas fir sawdust (275, 10min)
197 (33.9)
15.5 (2.17)
0.9911 (0.009)
Torrefied Douglas fir sawdust (300, 10min)a
195 (23.7)
15.5 (1.93)
0.8976 (0.143)
R2
Biomass
a
:
=0.1–0.5
5.5 Conclusion
In this study the thermal decompostion behaviors and kinetics of Douglas fir sawdust
torrefaction and raw and torrefied Douglas fir sawdust pyrolysis were investigated using
thermogravimetric analysis. The thermogravimetric analysis of different torrefaction processes
showed that biomass torrefaction contained two reaction stages. The first stage involved fast
weight loss during the heating period and first several minutes of the isothermal process. The
second stage signified an isothermal torrefaction process with the slow weight loss. The sample
117
weight loss in torrefaction was highly related to the torrefaction temperature. The peaks of
derivative thermogravimetric (DTG) indicated that the reaction rate increased with the increase
of the torrefaction temperature. At high torrefaction temperatures, the three components of
biomass, especially hemicellulose and cellulose, were mostlygreatly decomposed. The two-step
model fits well for Douglas fir sawdust torrefaction. The activation energies of the first and
second reaction stages were112 kJ/mol and 150 kJ/mol, respectively.
Raw and torrefied Douglas fir sawdust illustrated different thermal decomposition behaviors.
The decomposition of torrefied biomass shifted to high temperature and the degree of the shift
increased with the severity of torrefaction. The final biochar yield of the torrefied biomass was
much higher than from the raw biomass pyrolysis and also increased with the increase of
torrefaction temperature. DTG analysis revealed that the decomposition of hemicelluloses in
torrefied biomass was reduced or even eliminated at the high torrefaction temperature. However,
the decomposition rate of torrefied biomass has a decreasing trend due to the mass depletion in
torrefaction. The first-order, one-step global model fits well for the raw and torrefied biomass
pyrolysis. The average activation energies for the raw and torrefied biomass pyrolysis were in
the range of 203.94−195.13 kJ/mol. The slightly decreasing trend of activation energy was
observed for the torrefied biomass pyrolysis compared to the raw biomass. The kinetic analysis
showed that the torrefied biomass subject to with high terrefaction temperatures may involve
multiple-step reactions.
118
5.6 References
Alves S.S., Figueiredo J.L., 1989. Kinetics of cellulose pyrolysis modeled by three consecutive
first-order reactions, J. Anal. Appl. Pyrolysis, 17, 37–46
Bergman P.C.A., Kiel J.H.A. Torrefaction for biomass upgrading, 14th European Biomass
Conference & Exhibition, France, 2005
Blasi C.D., Lanzetta M., 1997. Intrinsic kinetics of isothermal xylan degradation in inert
atmosphere, J. Anal. Appl. Pyrolysis, 40–41, 287–303
Blasi C.D., 1998. Comparison of semi-global mechanisms for primary pyrolysis of
lignocellulosic fuels, J Anal. Appl. Pyrolysis, 47, 43–64
Blasi C.D., Branca C., 2001. Kinetics of Primary Product Formation from Wood Pyrolysis, Ind.
Eng. Chem. Res., 40, 5547–5556
Blasi C.D., 2008. Modelingchemical and physicalprocesses of wood and biomass pyrolysis.
Progress in Energy and Combustion Science, 34, 47–90
Broström M., Nordin A., Pommer L., Branca C., Blasi C.D, 2012. Influence of torrefaction on
the devolatilization and oxidation kinetics of wood, J. Anal. Appl. Pyrolysis, 96, 100–109
Chen W.H., Kuo P.C., 2011a. Isothermaltorrefactionkinetics of hemicellulose, cellulose, lignin
and xylan using thermogravimetric analysis, Energy, 36, 6451–6460
Chen W.H., Kuo P.C., 2010b. A study on torrefaction of various biomass materials and its
impact on lignocellulosic structure simulated by a thermogravimetry, Energy, 35, 2580-2586
119
Chen W.H., Kuo P.C., 2011b. Torrefaction and co-torrefaction characterization of hemicellulose,
cellulose and lignin as well as torrefaction of some basic constituents in biomass, Energy, 36,
803−811
Friedman , H.L. 1964. Kinetics of thermal degradation of char-forming plastics from
thermogravimetry. Application to phenolic plastic, J. Pol. Sci. 6, 183-195.
Grønli M.G., Varhegyi G., Blasi C.D., 2002. Thermogravimetric Analysis and Devolatilization
Kinetics of Wood, Ind. Eng. Chem. Res., 41, 4201-4208
Mohan D., Pittman C.U., Steele P.H., 2006. Pyrolysis of wood/biomass for bio-oil: a critical
review, Energy Fuels, 20, 848–889
Muller-Hagedorn M., Bockhorn H., Krebs L., Muller U., 2003. A comparative kinetic study on
the pyrolysis of three different wood species, J. Anal. Appl. Pyrolysis, 68-69, 231-249
Prins M.J., Ptasinski K.J., Janssen F.J.J.G., 2006. Torrefaction of wood: Part 1. Weight loss
kinetics, J. Anal. Appl. Pyrolysis, 77, 28–34
Ragauskas A.J., Williams C.K., Davison B.H., Britovsek G., Cairney J., Eckert C.A., Frederick
Jr. W.J., Hallett J.P., Leak D.J., Liotta C.L., Mielenz J.R., Murphy R., Sharfizadeh F., Chin P.S.,
1977. Thermal Deterioration of Wood. Chap. 5, 57–81
Bradbury A.G.W., Sakai Y., Shafizadeh F.J., 1979. A kinetic model for pyrolysis of cellulose, J.
Appl. Polym. Sci., 23, 3271–3280
Scott D.S., Piskorz J., 1984. The continuous flash pyrolysis of biomass, Can. J. Chem. Eng. 62,
404– 412
120
Templer R., Tschaplinski T., 2006. The path forward for biofuels and biomaterials, Science 311,
484−489
Varhegyi G., Antal M.J., 1989. Kinetics of the thermal decomposition of cellulose, hemicellulose
and sugar can bagasse, Energy Fuels, 3, 329–335
Wannapeera J., Fungtammasan B., Worasuwannarak N., 2011. Effects of temperature and
holdingtime during torrefaction on the pyrolysis behaviors of woody biomass, J. Anal. Appl.
Pyrolysis, 92, 99-105
White J.E., Catallo W.J., Legendre B.L., 2011. Biomass pyrolysis kinetics: A comparative
critical review with relevant agricultural residue case studies. J. Anal. Appl. Pyrolysis, 91, 1–33
Yang H., Yan R., Chen H., Lee D.H., Zheng C., 2007. Characteristics of hemicellulose, cellulose
and lignin pyrolysis, Fuel, 86, 1781–1788
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CHAPTER SIX
CONCLUSIONS AND FUTURE RESEARCH
6.1 Conclusions
The main purpose of this study was to understand the effects of torrefaction as pretreatment on
improving the biofuel quality via biomass pyrolysis and catalytic pyrolysis. The process
conditions for microwave torrefaction and pyrolysis of Douglas fir sawdust pellets were first
optimized. The torrefied biomass microwave pyrolysis and catalytic pyrolysis was then studied
and compared with the untreated biomass pyrolysis. The thermal decomposition behaviors and
reaction kinetics of torrefaction and torrefied biomass pyrolysis were also investigated using
TGA to study their reaction mechanisms. Major conclusions from this doctoral research are
summarized below.
1. The optimization of process conditions based on CCD implied that the yields of the bio-oil
and syngas increased with the reaction temperature and time. The optimized condition for the
highest bio-oil yield was at 470.7°C and 15min. The bio-oil was mainly composed of aromatic
phenols, guaiacols, furans, ketones/aldehydes, and organic acids. The phenols and guaiacols
accounted for the largest amounts of chemicals in the bio-oil representing 59.7–78.6% in area
depending on reaction conditions. The specific phenolic chemicals were highly related to the
reaction temperature. These phenolic compounds were from lignin decomposition with the
consequent reactions including dehydration of hydroxyl, Rearrangement of side chain,
hydrogenation and cracking the side chain of methoxy by cleavage of O-C bond. The syngas
contained carbon monoxide, methane, and short-chain hydrocarbons, which accounted for 70%
122
(v/v) of the total syngas. The results showed that both the bio-oils and syngas contained highvalue chemicals. Microwave pyrolysis of Douglas fir sawdust pellet produced a high bio-oil
yield which is comparable with the bio-oil yield from other pyrolysis processes, such as fludized
bed pyrolysis. It might be contributed by the uniform and fast heating of microwave irradiation
especially for the high density biomass. This study revealed that pelletized biomass might be a
promising feedstock in microwave pyrolysis. The production and selectivity of specific
chemicals, such as phenols and guaiacols from woody pellet biomass pyrolysis can be improved
by controlling the process conditions.
2. It was believed that it was the first observation of development of a two-step process by
integrating torrefaction and pyrolysis to improve the fuel quality. The reaction temperature and
time of torrefaction significantly influenced the yields of torrefied biomass, bio-oil, and noncondensable gases. The energy yields of torrefied biomass ranging from 67.03−90.06 % implied
that most energy of original biomass was retained in the torrefied biomass.
Microwave pyrolysis of torrefied Douglas fir sawdust pellet revealed that the torrefaction had
significant effects on the compositions of bio-oils and syngases. The decomposition of
hemicelluloses in torrefaction helped reduce the concentration of organic acids in pyrolytic biooils. The interaction between hemicelluloses and cellulose in torrefied biomass pyrolysis was
reduced along with the removal of hemicelluloses in the torrefaction that promoted the reaction
and conversion of cellulose to anhydrosugars. Therefore, the increased anhydrosugars and
reduced concentrations of furans were obtained from pyrolysis of torrefied biomass. Additionally,
the biochar formed from hemicelluloses and cellulose might be as catalyst in the cracking
reaction for the conversion of guaiacols to phenols which were further cracked to produce
hydrocarbons. 3.21 to 7.50 area% hydrocarbons were observed proving this reaction mechanism.
123
Meanwhile, these reactions also altered the compositions of syngas by reducing CO2 and
increasing H2 and CH4. These findings indicate that this two-step process can significantly
improve the quality of bio-oils and syngases.
3. A novel and cheap biochar catalyst was developed and studied in biomass catalytic pyrolysis
and bio-oil upgrading. The concentrations of phenols and hydrocarbons from torrefied biomass
catalytic pyrolysis over biochar catalysts were 46 area% and 16 area%, respectively. The amount
of H2 was up to 27.02 vol% in torrefied biomass catalytic pyrolysis. The biochar as catalyst
increased the phenols and hydrocarbons in bio-oil and the increase was related to the increase of
the biochar catalyst loading. High-quality syngas rich in H2, CO, and CH4 was observed.
Upgraded bio-oil was dominated by phenols (37.23 area %) and hydrocarbons (42.56 area %) at
the high biochar catalyst loadings. This biochar catalyst simplified bio-oil to phenols and
hydrocarbons and improved the syngas quality by richening H2, CO, and CH4. Two reaction
pathways can explain the hydrocarbons production in biochar catalytic pyrolysis and bio-oil
upgrading. One is that aliphatic compounds such as sugars and furans were converted to
hydrocarbons via oligomerization, decarboxylation, and decarbonlation reactions under biochar
catalyst. Another pathway is that lignin or lignin oligomers were cracked and converted to
hydrocarbons via the cleavage of methoxy and hydroxyl under biochar catalysts. The high
concentration of hydrogen and CO might be due to the water gas shift reaction and dry reforming
of methane under the microwave irradiation over the biochar catalyst.
Thermal gravimetric analyzer (TGA) analysis showed that the raw and recycled biochar catalysts
had little weight loss when the temperatures was increased from 150 to 800°C, implying good
thermal stability of the biochar catalyst. This study is the first observation to reveal that the
124
biochar catalyst can be used as a cheap and effective catalyst in biomass pyrolysis and bio-oil
upgrading to produce hydrocarbons fuels.
4. The weight loss of samples in torrefaction was highly related to the torrefaction temperature.
Two stages of weight loss were observed in torrefaction. The step-reaction model fits well for
Douglas fir sawdust torrefaction with the activation energies of the first and second reaction
stages being 112 kJ/mol and 150 kJ/mol, respectively. Torrefied biomass illustrated a different
thermo decomposition behavior compared to untreated biomass. The torrefied biomass
postponed the start point of decomposition and increased the biochar yield. Derivative
thermogravimetric (DTG) curves showed that the shoulder of hemicelluloses decomposition in
torrefied biomass pyrolysis was eliminated. The decomposition rate of torrefied biomass had a
decreasing trend due to the mass depletion in torrefaction. The first-order one-step global model
fitted well for the raw and torrefied biomass pyrolysis with the average activation energies in the
range of 203.94 −195.13 kJ/mol. Compared to raw biomass pyrolysis, the need of less activation
energy for the torrefied biomass pyrolysis was suggested by this study. These findings provide
the fundamental knowledge of reaction kinetics of torrefied biomass pyrolysis and will be
helpful for the process design.
6.2 Future research
Based on the studies of this dissertation several future research efforts may be attempted and
they are disscued below.
1. Assessment of the effects of torrefaction as pretreatment on the recycling of catalyst. The
recycling of catalysts used in the biomass catalytic pyrolysis and bio-oil upgrading is critical to
125
determining the efficiency of biomass conversion process. Catalysts used in biomass conversion
and bio-oil upgrading are coked and deactivated due to the organic acids and small molecular
compounds partially. Torrefaction as pretreatment reduces the organic acids and furans, and
favors the production of phenols and sugars in the bio-oil from torrefied biomass pyrolysis as
observed in this dissertation study. The reduction of organic acids and furans may slow the
deactivation and extend the recycling of the catalysts, which will help to reduce the cost of
regeneration and improve the process efficiency. Hence, it is worthy to investigate the effects of
torrefaction as pretreatment on the behavior of catalysts.
2. Developing biochar-based catalyst and assessing the effects of these catalysts in biomass
pyrolysis and bio-oil upgrading
Catalysts play a critical role in biomass conversion. Inexpensive, high-efficiency catalysts and
catalytic technologies will help overcome the complexity of biomass in hydrocarbon fuels and
chemicals production. In this doctoral study, untreated biochar catalyst improved the bio-oil
quality by increasing the hydrocarbons and phenols of bio-oil in biomass catalytic pyrolysis and
bio-oil refinery. However, biochar as catalyst in catalytic pyrolysis and bio-oil refinery reduced
the liquid yield and increased the syngas. The main reason is that the biochar contains high ash
and alkali metals, which gasify the carbohydrates. These ash and alkali metals also reduce the
selectivity of specific chemical production by multiplying the reactions of conversion and
influence the syngas properties. Therefore, it is necessary to develop methods to remove these
chemicals and contaminants in biochar and test the biochar-based catalyst in biomass pyrolysis
and bio-oil upgrading.
126
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