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High-temperature sequestration of elemental mercury by noncarbon based sorbents.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2010; 5: 259–265
Published online 16 March 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.263
Special Theme Research Article
High-temperature sequestration of elemental mercury
by noncarbon based sorbents
Sung Jun Lee,1 Jost O. L. Wendt1 * and Joep Biermann2
1
2
Department of Chemical Engineering, University of Utah, Salt Lake City, UT, 84108, USA
MinPlus, BV., Arnhem, The Netherlands
Received 13 October 2008; Accepted 11 December 2008
ABSTRACT: This work is concerned with sequestration of elemental Hg at high temperatures (900–1100 ◦ C) on a
sorbent that is mineral based, rather than carbon based. This sorbent consists of an intimate mixture of CaO, CaCO3 ,
and Al2 O3 –2SiO2 , and is manufactured in industrially relevant quantities (metric tons) from residues produced in paper
recycling processes. In contrast to activated carbon (AC), this noncarbon based sorbent has special advantages in that,
it can actually enhance fly ash utilization for cement manufacture, rather than diminish it, as is the case for AC.
Disperse phase experiments have been conducted, using an externally heated quartz tube reactor, with sorbent
feeding rates ranging from 1 to 6 g/h. Preliminary results indicate that Hg removal efficiency is sensitive to sorbent
feed rates and to furnace temperature. The Hg removal percentage increased with both these variables. Two mechanisms
come into play: an in-flight Hg sorption mechanism, and an Hg sorption mechanism related to sorbent deposits on
the reactor wall. A maximum total (in-flight plus deposit-related) Hg removal efficiency of 83–90% was obtained
at temperatures of 900–1100 ◦ C. There was negligible sorption by either mechanism at temperatures below 600 ◦ C.
Results for the in-flight mechanism alone showed a maximum sorption efficiency at ∼900 ◦ C, whereas that on the
reactor surface increased monotonically with temperature. This suggests that sorbent deactivation can occur in-flight
at high temperatures, which is in agreement with other fixed bed results obtained in this laboratory. Deactivation was
not apparent for the sorbent-related substance formed on the reactor wall. Raw and spent sorbents were analyzed by
X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectrophotometer (SEM-EDS) to
identify the sorbent mineral transitions that seem to activate the process. The in-flight mechanisms appear to involve
(1) activation of the sorbent, caused most probably by an internal solid–solid reaction, followed by (2) Hg sorption,
and (3) possible deactivation, if the temperatures are too high for longer period. Reactor surface mechanisms still
remain to be elucidated.  2009 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: mercury control; coal combustion and gasification; adsorption
INTRODUCTION
Coal is the most abundant fossil energy source in the
world as well as in the United States. Use of coal
causes emission of global pollutants such as sulfur
oxide (SOx), Nitrogen Oxide (NOx), carbon dioxide
(CO2 ), particulate matter (PM), as well as mercury
(Hg). Hg control technologies have been the subject
of considerable research because of that metal’s high
toxicity to human health and within the ecosystem.[1]
The United States environmental protection agency
(EPA), enacted the Clean Air Mercury Rule (CAMR) on
March 15, 2005 to reduce total mercury emission from
coal utilizing facilities through a ‘cap and trade’ system,
which should reduce national utility emission of Hg
*Correspondence to: Jost O. L. Wendt, Department of Chemical
Engineering, University of Utah, Salt Lake City, UT, 84112, USA.
E-mail: Jost.Wendt@utah.edu
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
from 48 to 38 ton/yr by 2010, and to 15 ton/yr by 2018,
when about 70% of the 2005 Hg emission would have
been removed.[2] Although CAMR was struck down
by the District of Columbia Circuit Court of Appeals
in February 2008, individual states and regions have
enacted Hg control rules that are often more stringent
than CAMR. Therefore, Hg control technology remains
an important issue within the United States.
Many studies have been focused on the development and evaluation of cost-effective and widely useable sorbents for Hg compounds from coal-fired power
plant and gasification systems. To control the mercury compounds from coal utilizing facilities, including coal-fired power plant and gasification systems,
adsorption process have been widely employed. These
commonly used activated carbon (AC), which was
injected as a fine powder, upstream of ash particle control devices’ such as electrostatic precipitators
260
S. J. LEE, J. O. L. WENDT AND J. BIERMANN
Asia-Pacific Journal of Chemical Engineering
(ESP), or fabric filters (FF).[1,3 – 7] However, the use
of carbon-based sorbents is limited by their applicable
temperature zone (below 300 ◦ C) and by the ensuing
deterioration of the quality of the fly ash for cement
manufacturing.[1,5,6]
Integrated gasification combined cycle (IGCC) is one
of the alternatives to advance coal use for efficient
electricity generation, and for production of useful
by-products, such as hydrogen to be used in fuel
cells.[8,9]
However, trace elements such as As, Se, V, Ni,
Cd, and especially Hg compounds are quite difficult
to control, because the gasification process is operated at high temperature/pressure conditions and under
reducing atmospheres, resulting in most Hg compounds
existing in their elemental form (Hg0 ).[10,11] In general, Hg0 is more difficult to control than ionic mercury
(Hg2+ ) because of its high volatility, low solubility
in water, and low reactivity with conventional adsorbents.
MATERIALS AND EXPERIMENTAL SETUP
Properties of MinPlus sorbent
This study is concerned with a specially manufactured
noncarbon based sorbent, marketed under the trade
name MinPlus. This is an inorganic, mineral-based
substance derived from a unique thermal process in
which sludge from paper recycling is converted into
the desired material. This material also serves as a
special mineral additive to cement in order to enhance
quality. Therefore, one of the advantages of this sorbent
is that its use in coal-fired power plants leads to no
adverse effects to the use of the coal fly ash for cement
manufacturing.
A detailed chemical composition of MinPlus sorbents
is shown in Table 1. A Malvern particle size distribution analysis showed a mean particle size of about
10 µm.
Entrained-flow reactor and Hg analysis setup
A schematic diagram of the experimental set up is
shown in Fig. 1 and pictured in Fig. 2. Two furnaces
(Thermcraft, Inc.USA, three zone split) are installed
vertically in series, to control the reaction temperature;
the maximum furnace temperature is 1200 ◦ C.
The quartz reactor (50-mm O.D., 47-mm I.D, 190 cm
in length) consists of three parts, (1) the top part for
introducing sorbents, simulated flue gas, and elemental mercury; (2) the main body, which has two sampling ports used for different residence times expected;
and (3) the bottom part for particle/gas separation,
collection, and ventilation. A preheater is set over
850 ◦ C to heat carrier gas before entering into the reactor. The reaction temperature range is 600–1100 ◦ C.
The mercury concentration was monitored by a cold
vapor atomic fluorescence (CVAF) type Hg analyzer
(Tekran 2537A, Canada) with a four-channel sampling
port. Elemental mercury was generated by temperaturecontrolled Hg0 generator (Cavkit Hg calibration, P S
Analytical), which produces about 25 µg/m3 , at a total
flow rate 2.2–4.7 l/min with compressed air. The flow
of all gas components was controlled by calibrated mass
flow controllers (MFC). Flow rates were checked and
verified before each experiment.
Hg speciation was determined on-line by a twin set of
wet chemical impinger systems. One set of impingers
contained SnCl2 –HCl and NaOH solution in series,
yielding total mercury [Hg (T)], while the other set
of impingers contained KCl–Na2 S2 O3 and NaOH solutions in series to yield elemental Hg (Hg0 ) only. The
difference between Hg(T) and Hg0 represents oxidized
mercury (Hg2+ ) concentration. Powdered MinPlus sorbent was injected into the reactor by a sorbent feeder
wherein an aliquot of several grams of sorbents was
moved from a glass tube (1/2 O.D), into the reactor
with 0.5-l/min nitrogen gas (N2 ) as a carrier by means
of a syringe pump, and vibration. The applied sorbent
feeding rate of 2–7 g/h was controlled by the forward motion velocity of the syringe pump, the carrier
Table 1. Chemical composition of MinPlus sorbent.
Primary constituents
Limestone (CaCO3 )
Meta-kaolinite (Al2 O3 2 SiO2 )
Lime (CaO)
Inert
Trace constituents
MgO
2%
1%
TiO2
0.7%
Fe2 O3
0.6%
SO3
0.4%
K2 O
0.5%
P2 O5
CuO
0.1%
41%
29%
23%
7%
Na2 O
BaO
Cl
MnO
ZrO2
ZnO
NiO
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
<0.1%
<0.1%
<0.1%
<0.1%
<0.1%
0.03%
0.002%
PbO
Cr2 O3
As2 O3
HgO
CdO
0.005%
0.002%
<0.001%
<0.0001%
0.00005%
Asia-Pac. J. Chem. Eng. 2010; 5: 259–265
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
HIGH TEMPERATURE SEQUESTRATION OF ELEMENTAL MERCURY
Hg(0)
N2
CO2
CH4
Other Mixture gas
Pre-Heating
system
N2
Sorbents feeding
With vibrator
Furnace
#1
24” (L)
3 zone
Quartz Reactor
182 cm in Length
5 cm O.D
Sampling
Port #1
Tekran
2537A
Hg(0)
&
Hg(T)
Hg Analyzer
Hg treatment
system
Furnace
#2
24” (L)
3 zone
Sampling
Port #2
Vent
Sorbent
Separation
Cyclone
Figure 1. Schematic of disperse phase, entrained-flow reactor. This figure is available
in colour online at www.apjChemEng.com.
Sorbent in
Hg° in
Pre-heater
Sorbents
Feeder
Furnace #1
24”, 3 zone
Sampling
Port #1
Elemental Hg Source
Mixture Gas
Manifold
Tekran 2537A Hg Analyzer
Furnace #2
24”, 3 zone
Buck Hg
Analyzer
Sampling
Port #2
Temp.
Controller
OHM wet Hg
Treatment
v Bath
Oil
Sorbents
collection
& Vent
Reactor Specification
- Material: Quartz
- O.D: 5cm
- Length: 180cm
- Max. temp.1200C
- Two sampling ports
Figure 2. Picture of entrained-flow reactor and mercury analyzer setup. This figure is available
in colour online at www.apjChemEng.com.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 259–265
DOI: 10.1002/apj
261
S. J. LEE, J. O. L. WENDT AND J. BIERMANN
Asia-Pacific Journal of Chemical Engineering
gas flow rate, coupled with the frequency of vibration
applied.
removal for tests conducted in the disperse phase reactor described above. Overall Hg removal efficiency is
shown in Fig. 3, which increased with increasing sorbent feed rate and with reaction temperature, suggesting
an overall chemi-sorption, rather than physi-sorption
mechanism. Figure 3 suggests that for this reactor configuration, an overall maximum Hg removal of about
83–90% was achievable at a temperature range of
900–1100 ◦ C.
Figure 4 shows a temporal trace of the mercury outlet concentrations as a function of run time during the
RESULTS AND DISCUSSION
Effects of reaction temperature and sorbents
feeding rate
Figures 3–5 show the various effects of different reaction temperatures and sorbent feeding rates on total Hg
Hg Removal Transition with Air flow
(Temperature v/s Sorbents feeding rate)
Feeding
set#1
Avg. 2.65 g/hr
S.D 0.23
Feeding
set#2
Avg. 3.85 g/hr
S.D 0.25
Feeding
set#3
Avg. 5.37 g/hr
S.D 0.19
Feeding
set#4
Avg. 6.47 g/hr
S.D 0.41
100.0
90.0
80.0
Hg Removal %
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
500
600
700
800
900
1000
1100
1200
Temperature (C)
Figure 3. Total Hg removal efficiency at the different reaction temperatures
and sorbent feeding rates. This figure is available in colour online at
www.apjChemEng.com.
1.20
600 C
change
sorbent tube stop feeding
700 C
1.00
800 C
900 C
1000 C
Hg Conc. (C/C0)
262
0.80
1100 C
In-flight
capture
of Hg
0.60
0.40
0.20
0.00
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
Time (min.)
Figure 4. Temporal Hg traces: comparison of in-flight capture and on-surface
effect for Hg removal at different temperature – see text. (total flow rate 2.2
l/min, average feeding rate 6.87 g/h, s.d. 0.21). This figure is available in
colour online at www.apjChemEng.com.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 259–265
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
HIGH TEMPERATURE SEQUESTRATION OF ELEMENTAL MERCURY
100.0
90.0
51.0
Wall effect
80.0
In-flight capture
92.0
18.1
Hg Removal (%)
70.0
60.0
13.1
50.0
7.1
40.0
5.7
30.0
20.0
10.0
24.3
34.1
33.7
52.8
40.9
0.1
0.0
600
700
800
900
1000
1100
Temperature (°C)
Hg removal efficiency from in-flight capture of Hg by dispersed
sorbent (lighter shading) and (maximum) capture by deposits on reactor (darker
shading), at various temperatures. This figure is available in colour online at
www.apjChemEng.com.
Figure 5.
experiment. These data are for sorbent feed rates of
∼6.9 g/h. The ordinate depicts the concentration ratio
of the outlet total mercury over the inlet total mercury
in the gas phase. Events such as changing the sorbent
feeding tube (because of limited capacity) and shutting
off the sorbent stream are noted in the figure. Figure 4
shows that capture (sorption) of mercury continues to
increase over long time scales (i.e. 40 min), whereas the
mean residence time of the sorbent in-flight in the reactor was of the order of 8 s. Furthermore, a white scaly
deposit on the reactor walls was observed at the conclusion of the experiment. Clearly, the interaction between
MinPlus and the quartz wall contributed to Hg sorption, and to the total capture reported from Fig. 3. The
aggregate capture is the result of two different mechanisms, (1) an in-flight mechanism involving MinPlus
sorbent (or a thermal derivative thereof) and Hg, and
(2) a mechanism involving Hg and deposits formed on
the reactor walls. Therefore, in Fig. 4, the temporal trace
of outlet Hg contains the results of both in-flight capture and capture on the reactor wall deposits. When the
sorbent feed is turned off, the Hg outlet level rapidly
increases. This rapid increase of outlet Hg, upon eliminating the in-flight sorbent is attributed to the disappearance of in-flight capture of Hg. Hence, the magnitude
of the sudden increase can be taken to be representative
of in-flight Hg capture, and the on-surface capture can
be calculated by the difference (Fig. 5).
With this interpretation of the temporal traces on
Fig. 4, the in-flight capture of Hg is shown also to
depend on reaction temperature, but unlike the total
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
capture (in-flight plus on-surface), it shows a maximum
in-flight capture at a temperature of about 900 ◦ C. In
these experiments, with a high-temperature residence
time of 8 s, the in-flight sorbent thus appears to
undergo a deactivation at high temperatures. The onsurface capture, (Fig. 5) on the other hand, increases
monotonically with temperature. At 1100 ◦ C, the inflight capture is almost zero, but the on-surface capture
is 92% (Fig. 5).
On the basis of test results from this study, the following overall mechanism of sorbent activation and
deactivation for the in-flight mechanism might be
hypothesized.
−→
Rxn (1) MinPlus sorbent −
Heat active site (Activation)
Rxn (2) active site + Hg −−→ Hg . . . sorbent (active
site) (Adsorption)
−−−−−→
Rxn (3) active site on sorbent Over heat de-activated
sorbent (Deactivation)
An hypothetical mechanistic understanding of the
process might be as follows: At low experiment temperatures (600–900 ◦ C) in this study, the reaction Rxn
(1) occurs slowly and controls the rate of Rxn (2) as
temperature increases. However, at high temperatures
(1000–1100 ◦ C), Rxn (1) proceeds rapidly, the reaction
rate of Rxn (3) is greater than Rxn (2), and the sorbent loose its Hg adsorption ability and is deactivated.
The mechanism on deposits on the reactor wall does
not appear to exhibit deactivation.
Asia-Pac. J. Chem. Eng. 2010; 5: 259–265
DOI: 10.1002/apj
263
S. J. LEE, J. O. L. WENDT AND J. BIERMANN
(a)
Asia-Pacific Journal of Chemical Engineering
Sample 1-Full
C
C – Calcite(CaCO3)
L – Lime (CaO)
K – Kaolinite (Al2SiO2O5(OH)4)
400
Counts
L
K
C
LKCC C L
C
C
100
0
(b)
K
20
CCC
L
K
L K
C KL C
K
30
40
50
60
70
80
L
L
90
100
110
120
XRD patterns of raw sorbents (before experiment)
and spent in-flight (after experiment at 1100 ◦ C) are
shown in Fig. 6. Calcite (CaCO3), lime (CaO), and
kaolinite [Al2 SiO2 O6 (OH)4 ] peaks are dominant in the
raw sorbent, while gehlenite (Ca2 Al2 SiO7 ) and calcium
silicate peaks appear in the spent sorbent (test at
1100 ◦ C).
In this regard, Okada et al .[12] and Traore et al .[13]
have observed the mineral transition between calcium
compound and kaolinite at high temperature and suggest
that the following reaction is possible.
Al2 Si2 O7 + (2 + n)CaO −−→ Ca2 Al2 SiO7
130
Full Scan_2
+ nCaOSiO2
G – Gehlenite (Al2Ca2SiO7)
G
S – Calcium Silicate Oxide (Ca2SiO4)
400
S
Counts
264
G G
100
G S G
SG S
S S
SG
SG
G G
GS
SS
G S
G
0
20
30
40
50
60
70
80
90 100 110 120 130
Position [o2 Theta]
Figure 6. XRD analysis for raw (a) and spent in-flight
sorbents (b) tested at 1100 ◦ C. This figure is available in
colour online at www.apjChemEng.com.
Sorbent deactivation and mineral transition at
high temperature
It has been shown above that at a high temperature
of 1100 ◦ C, the sorbent ability to capture Hg was
greatly diminished. In order to identify the reason
of deactivation and deterioration of sorbent, X-ray
diffraction (XRD) and scanning electron microscopy
with energy dispersive spectrophotometer (SEM-EDS)
analysis were employed and the results are indicated in
Figs 6 and 7.
After several months of high-temperature experimentation, a white unknown material was collected from the
inside of reactor, which was analyzed by SEM-EDS.
SEM-EDS results of surface morphology (a) and
line scanning (b) of vertical cross section of a small
fragment of coated material are shown in Fig. 7. Some
molten materials and the major elementals O, Si, Ca,
Al, etc. could be observed in (a). Line scanning results
also indicated that O, Si, Au (from coating required
for SEM), Hg, and Ca were major elements in (b).
Although some mercury (Hg) peaks appeared in SEMEDS analysis, it is hard to identify final products
of mercury compounds and its mass balance. The
Hg levels automatically generated and shown on the
left hand micrograph (b) seem abnormally high, and
additional work may be required to confirm these
values. However, the qualitative presence of Hg in the
wall deposits is unambiguous and important. Future
work involving X-ray absorption fine structure (XAFS)
analysis might shed light on the nature of the mercury
compounds formed on the spent sorbents.
CONCLUSIONS
Specially manufactured MinPlus-based sorbent was
introduced into a bench scale entrained-flow reactor
Top
(b)
(a)
(1)
Elem Wt %
OK
AsL
AlK
SiK
CaK
FeK
Total
At %
32.89 50.52
0.53
1.62
2.88
3.16
31.02 27.14
29.75 18.24
0.68
1.56
100.00 100.00
Coated
materials
layer
Bottom
(quartz wall side)
Elem
Wt % At %
CK
22.24
42.78
OK
17.94
Si K
9.04
Au M
6.72
Hg M
Ca K
1.28
Total 100.00
36.3
53.9
46.7
19.5
17.4
16.9
-
Figure 7. SEM-EDS surface morphology (a), and vertical line scanning (b) materials
coating inside wall of reactor after many experiments.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 259–265
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
HIGH TEMPERATURE SEQUESTRATION OF ELEMENTAL MERCURY
to evaluate Hg adsorption ability at high temperatures (600–1100 ◦ C). The total Hg removal efficiency
depended on temperature and sorbent feeding rate with
total Hg removal increasing with both increasing temperature and sorbent feeding rate. However, the total Hg
removal observed in this experiment was the result of
two different mechanisms, namely, an in-flight mechanism that had maximum effectiveness at approximately
900 ◦ C, and an on-surface mechanism whose effectiveness increased monotonically with temperature. This
suggests that the active sorbent substance for the inflight mechanism, which exhibited an high temperature
deactivation, was different from the on-surface mechanism, which did not.
In either case, the temperature dependent reaction
suggests that chemi-sorption occurred between the surface of the sorbent (product) and elemental mercury.
The spent in-flight sorbent which was deactivated,
showed large quantities of gehlenite, which were not
present in the original MinPlus. The role of this substance on either the activation or deactivation process
for the in-flight mechanism is not yet clear. Future work
will focus on quantifying the pertinent activation and
deactivation mechanisms for the in-flight mechanism,
and also on the nature of the activated product for the
on-surface mechanism.
Acknowledgements
This research was supported by the United States
Department of Energy through Co-operative Agreement
DE-FC26-04NT42313. The Technical Project Officer
was Pieri Noceti, DOE, National Energy Technology
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Laboratory, Pittsburgh. The authors are very grateful
to Brydger Cauch, JoAnn Lighty, and Geoff Silcox of
the University of Utah, for providing a fully debugged
Tekran Analyzer and Cavkit Hg supply system for the
experimental analysis system.
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Asia-Pac. J. Chem. Eng. 2010; 5: 259–265
DOI: 10.1002/apj
265
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