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The Effects of Solid Sorbents on Heavy Metal Emission under Various Combustion.

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Dev. Chem. Eng. Mineral Process. 13(3/4), pp. 495-509, 2005.
The Effects of Solid Sorbents on Heavy
Metal Emission under Various Combustion
Conditions
J. Han, M.H. Xu*, H. Zeng, Z. Zhang and F.Yi
State Key Laboratory of Coal Combustion, Huazhong University of
Science and Technology, Wuhan 430074, P.R.China
Experimental studies on heavy metals captured by solid sorbents were carried out in
a downfow reactor. The effects of various sorbents on heavy metal emissions under
different combustion conditions were investigated. The operating parameters
evaluated included: (1) different ratios of sorbent/coal(2%, 4% and 6%); (2) elevated
temperatures (950 C,1100 C and 1250 C);(3) un-staged and staged combustion.
The experimental results indicated that the best metal capture efficiency occurs when
the ratio of bauxitekoal is 2%, and 4% for both limestone/coal and calcium
sulfate/coal systems. It was found that increasing temperature has a negative effect on
heavy metal capture by sorbents. The experimental results also showed that the
staged combustion has no effect on heavy metal removal, although it is effective in the
control of NOx emissions. The leaching tests indicated that the mechanisms of heavy
metal capture by solid sorbents include both physical adsorption and chemical
reaction.
Introduction
Coal combustion produces not only energy, but also the formation of gaseous and
particulate pollutants such as SOX,NOx, heavy metals, and particulate matter. These
pollutants may be emitted into the atmosphere with the flue gas if no emission control
device is employed. Some of the pollutants, such as SOXand NOx, have long been of
concern because of their amount and well-documented harmful effects. However, the
potential adverse effects of the heavy metals on human health are recent concerns.
Title III of the 1990 Clean Air Act Amendments by the USA government stipulated
emission controls on 11 toxic metals and their species. All of these metallic species
are encountered in coal combustion environments. It has been established that some
* Author for correspondence (mhxu@mail.hust.edu.cn)
495
J.Nan, M.H. Xu, H. Zeng, 2.Zhang and F.Yi
of these species are enriched in the sub-micrometer particles in combustion exhausts.
The Conventional Air Pollution Control Devices (APCDs) such as wet scrubbers,
baghouses, and electrostatic precipitators (ESPs) are very effective in collecting the
particles, but the semi-volatile and volatile metals which are associated with submicrometer particles can easily escape from these devices in the flue gas [l].
As mentioned above, it is very difficult to control heavy metal emissions by
conventional APCDs during coal combustion. Some experimental results [2-61 have
shown that the injection of solid sorbents into the combustion chamber or flue duct, in
which the heavy metals were removed from flue gas by physical adsorption or
chemical reaction, was an attractive technology for the control of emissions of those
heavy metals or their compounds. In some cases, the mineral sorbents can transform
some toxic heavy metal compounds to harmless species. Chen [7] experimentally
studied the adsorption efficiency of different sorbents for heavy metals under various
conditions, and the results showed that the emission of heavy metals can be controlled
by adding solid sorbents into the combustion chamber during incineration. The
adsorption efficiency of sorbents is related to the species of heavy metals, the
sorbents, and combustion conditions. Each sorbent has its own optimum operating
temperature. Kaolinite and aluminum oxide have their best efficiency of adsorption at
800°C, and at 700°C for bauxite. The adsorption efficiency of the three sorbents for
lead, copper, chromium and cadmium follows the sequence of Pb > Cu > Cr > Cd.
The presence of inorganic chloride and sulfate improves the adsorption efficiency of
the sorbents. However, the organic chloride reduces the adsorption efficiency.
Agnihotri [8] found that the sorption of selenium oxide decreased when SO2is present
in the flue gas, while the sulfation of sorbents increased the adsorption efficiency.
Uberoi et al. [2-51 performed studies on the effectiveness of different mineral sorbents
for the removal of cadmium, lead, and alkali metal compounds from hot flue gas. The
effectiveness of calcium-based sorbents, especially hydrated lime (Ca(OH)*), was
demonstrated to be effective for the removal of selenium. The results by GhoshDastidar et al. [6] and Agnihotri [8] indicated that the mechanisms of selenium
captured by Ca(OH)2were not a simple physical adsorption process, but also involved
chemical reaction between CaO and Se02.It was also found that the rate of the above
reaction varied with the initial sorbent surface area. They found that the temperature
range 400-600°C was the most favourable for the reaction between the sorbent and
selenium, and high selenium sorption can also be obtained at these conditions. At
higher temperatures, thermodynamic equilibrium tended to dissociate the reaction
product, and the amount of metal captured reduced significantly with increasing
temperature.
Gullett and Raghunathan [9] canied out experiments to examine the effect of solid
sorbents (limestone, kaolinite) injection into the combustion chamber on the emission
of the heavy metals arsenic, cadmium and lead. Their results showed that the injection
of kaolinite, bauxite and limestone can effectively reduce the emission of As, Cd and
Pb w i h n the temperature range 1000-13OO0C. Wu et al. [lo-151 demonstrated the
effectiveness of mineral sorbents for the capture of mercury. Ho [16] performed
experiments to compare the adsorption efficiencies of limestone, sand and alumina for
Pb and Cd in a fluidized-bed combustor. The experimental results indicated that the
efficiency of lead captured by limestone in a furnace was as high as 95%, while the
efficiency by sand and alumina was only 47% and 43%, respectively. The effect of
496
Efects of Solid Sorbents on Heavy Metal Emission under Combustion Conditions
different temperatures on heavy metals adsorption was also studied. The optimum
operating temperature for limestone capturing Cd was 600°C, and for Pb was 750°C.
Scotto et al. [ 171 conducted experiments using a bench-scale therrno-gravimetric
reactor and a pilot-scale combustor. In the bench-scale test, both kaolinite and bauxite
beds were shown to be effective for lead chloride removal at 800°C, while only
bauxite was effective for cadmium chloride removal. They also showed that the
combined usage of silica with alumina greatly enhanced the overall capacity. In their
pilot-scale tests, lead acetate was used as the feed and the effect of chlorine on the
removal by sorbent injection was evaluated. The presence of chlorine significantly
reduced the effectiveness of the sorbent. Linak et al. [ 181 found that nickel, lead, and
cadmium can be removed by sorbent injection in a laboratory swirl-flame incinerator.
They indicated that there were two high-temperature mechanisms in the scavenging of
metal by sorbent at combustion temperatures. The first mechanism involved reactions
between metal vapor and sorbent surface, and the rate of reaction was controlled by
surface reaction or pore diffusion. For the second mechanism, metal vapor was
scavenged by the metal melting onto the sorbent at high temperatures in the chamber.
In recent years, research has been focused towards finding multi-functional
sorbents that were suitable for removing a number of air toxins [6, 19, 201. Cheng et
al. [21] extended their earlier studies to study the emission control of heavy metals,
NOx and SO2 using different sorbents (CaS04, kaolinite, limestone and bauxite)
during a coal combustion process. Their results showed that Pb and Cd could be
captured by these sorbents. The adsorption efficiency of sorbents with several metals
was quite different. The effect of kaolinite for lead was better than other sorbents.
Bauxite was the best sorbent for Cd. Kaolinite and bauxite were not very effective in
capturing Cr. Sorbents can decrease the emission of SOXby the chemical reaction of
sorbent with SO2 during coal combustion, but had no influence on the emission of
NOx. Capturing and immobilizing the heavy metals by sorbent injection was
effective, or a sorbent premixed with coal during the combustion process. Most
research concerning the removal of heavy metal vapors by sorbents did not involve
coal combustion [6, 14, 15, 22-25], The concentrations found in other materials may
be several times higher than those found in coals, and also operate very differently
from the case of coal combustion.
It is well known that air staging is effective in reducing the emission of NOx
during coal combustion, but the effect on emission of heavy metals is not well
understood, especially for the case of solid sorbent injection. In this paper, the capture
of the heavy metals using various sorbents under different conditions and different
combustion temperatures is investigated. Air-staged combustion and various
sorbentlcoal ratios were considered.
Experimental Details
A downflow reactor (0.175 m inside diameter, approximately 3.5 m high) was used
for all of the combustion experiments in this work. A schematic of the combustion
system is shown in Figure 1. The chamber is heated electrically to maintain a constant
temperature. Coal is premixed with the combustion air and fed into a burner located
on the top of the furnace. The flow rate of the pulverized coal is 1.5 kg/h. The overall
497
J. Han, M.H.Xu, H. Zeng, 2.Zhang and F. Yi
excess air ratio for all the experiments is kept constant at 1.2. Combustion gases and
solid particles travel down the funace chamber. An ash trap located at the bottom of
the furnace is used to collect bottom ash particles. A series of sample ports are used to
investigate the conditions within the furnace using various sampling equipment. The
temperature inside the furnace is primarily monitored by a thermocouple. Gas
composition (02,CO, C02,and NOx) is continuously monitored on-line by analysers.
4
1.fan 2. valve 3.flowmeter 4. primary air 5. coal feeder 6. increaser
7. resister wire 8. chamber 9. samplingport 10. upper secondary air
I I . middle secondary air 12. lower secondary air 13. secondary air
14. baghouse
Figure 1. Schematic diagram of the downjlow reactor.
Flue gases are pumped to a baghouse for removal of the remaining fly ash
particulates before being emission to the atmosphere. For the air staged combustion
test, the ratio of primary to secondary air is kept constant. The staging intensity is
calculated as the ratio of the middle secondary air to the total combustion air flow
rate. Staging intensity was varied by changing the middle and upper secondary air
flow rates.
498
Efiects of Solid Sorbents on Heavy Metal Emission under Combustion Conditions
o n e Chinese Coal (wad 0.46%, Aad 32.22%, Vad 22.18%, FCad 45.14%) and Six
sorbents were used in this study. The compositions and characteristics of the sorbents
are given in Tables 1 and 2. Because the residence time of the pulverized coal in the
combustion tube is very short (about 0.5-1.0second), the limestone cannot be
decomposed completely after injection into the chamber during coal combustion. In
order to use the limestone effectively, it was calcined for 1.5 hours at 750°C in a
Morphine furnace, and was decomposed completely to CaO before mixing with coal.
The sorbents and coal are mixed in various ratios before a test. Then the coal and
sorbent are fed into chamber with the combustion air. The fly ash is collected at the
sample port through a sampling tube, and digested in HN03-HF-HC104 solution
for fbrther analysis by Inductively Coupled Plasma-Atomic Emission Spectrometry
(ICP-AES).
Sorbent
Limestone Dolomite
BET su$ace
area (m2/&
Total pore
volume
HZKaolinite
dolomite
Bauxite
Calcium
sulphate
6.50
1.92
3.57
20.86
4.21
4.8 1
0.004
0.001
0.003
0.02
0.02
0.01
27.53
22.02
31.48
42.79
15.49
11.34
Dolomite
HZdolomite
Kaolinite
I Bauxite
CaO
Si02
(ml/g;)
Average
pore
diameter (A)
~~
Sorbent
Composition
PA)
I
Limestone
CaC03
88.33
1
Calcium
sulphate
~
CaO
29.12
Si02
MgO
7.65
1.88
39.20
MgO
11.68
33.68
CaS04
99.0
A1203
34.61
MgC03
Fez03
3.41
1.36
495
J. Han, M.H.Xu, H. Zeng, Z. Zhang and F. Yi
Results and Discussion
I Effects of sorbenUcoa1ratio on metal capture
Figures 2 and 3 are experimental results of lead and cadmium captured by sorbents at
various sorbentlcoal ratios at 1250°C. In general, the percent of metals captured
increases with the sorbentlcoal ratio. Figure 2 shows the results of Cd capture at
various sorbentlcoal ratios. The concentration of Cd in fly ash increases from
0.20 ppm to 0.35 ppm with increased sorbentlcoal ratios. For example, the
concentration of Cd in fly ash increases by 25% when the bauxitekoal ratio increases
from 0% to 2%. However, only a minor increase occurs if the bauxitekoal ratio
increases above 2%. For example, the concentration of Cd is 0.25 ppm at a
bauxite/coal ratio of 2%, and 0.275 ppm at 4%, which suggests that increasing the
sorbentlcoal ratio does not lead to a significant change in Cd concentration. For a
ii.uther increase in the bauxitekoal ratio, e.g. to 6%, then the concentration of Cd
increases to 0.34 ppm (the highest at 1250°C). However, a high percent of sorbent in
coal has a negative effect on the stability of combustion. Considering both the
efficiency of Cd capture and the stability of combustion, the best ratio of bauxitehoal
is suggested to be 2%. Similar results can be found in Figure 2 for limestone and
calcium sulfate, but the corresponding marked change of Cd concentration occurs at
higher sorbentlcoal ratios. For example, the concentration of Cd in samples increases
30% and 32.5% for limestone and calcium sulfate as the sorbentlcoal ratio increases
from 2% to 4%, respectively. Further increase of the limestone/coal ratio only causes
a minor increase in the Cd concentration, about 15% and 22% higher if the
sorbentlcoal ratio increases from 4% to 6% for limestone and calcium sulfate,
respectively. The best ratio of sorbentlcoal for Cd capture by limestone and calcium
sulfate is suggested to be 4%.
0
B
0
2
4
6
The ratio of sorbentdcoal
8
Figure 2. Eflects of sorbentkoal ratio on concentration of Cd in fly ash at 1250°C.
500
Efects of Solid Sorbents on Heavy Metal Emission under Combustion Conditions
0
0 CaS04
n
f
25
--
A Bauxite
&
U
2e 20 --
8
4
0
.C
0
4
6
The ratio of sorbent/coal(ppm)
2
8
Figure 3. Eflects of sorbentkoal ratio on concentration of Pb injly ash at 1250°C.
Similar results for lead capture by different sorbents are shown in Figure 3. The
suggested sorbent/coal ratios for Pb capture by bauxite, limestone and calcium sulfate
are 2%, 4% and 4%, respectively. Figures 2 and 3 show that the sorbent preferred for
Cd capture (at sorbentkoal ratio of 2%) follows the sequence, bauxite > calcium
sulfate > limestone, while the sequence for Pb capture is bauxite > limestone >
calcium sulfate. At higher sorbentkoal ratio of 4%, the most effective sorbent for Cd
is calcium sulfate, while limestone is best for Pb.
11 Eflects of temperature on metal capture
Linak et al. [ 181 reported that toxic metal capture by sorbents was more practical in
high temperature environments, while the experimental results of Ho et al. [16]
indicated that each sorbent has an optimum reaction temperature. In order to
determine the effect of temperature on metal capture by different solid sorbents
(bauxite, limestone and calcium sulfate), experiments were carried out in the
downflow reactor at 950, 1100 and 1250°C at constant sorbentkoal ratio of 2%. The
results are given in Table 3.
The enrichment factor is defined as follows:
where E , is the enrichment factor of element i; [XIjis the concentration of element
i in the fly ash; [Aj'o is the concentration of element i in coal.
501
J.Hun, M.H. Xu, H. Zeng, Z. Zhang and F. Yi
Table 3. Enrichmentfactors of metals in fly ash at diferent temperatures.
Figures 4 and 5 show that the enrichment factor would decrease if temperature
increases, which is consistent with the conclusions of Chen [7] and Ho et al. [ 161. For
the bauxite/coal system, the enrichment factor of Cd decreases fiom 1.76 to 1.20
when the combustion temperature increases fiom 950 to 1250°C. The temperature has
a similar effect on the metals capture by limestone and CaS04. Figures 4 and 5 show
that fine sorbents are more effective than coarse ones in removing heavy metals. The
main reason for the difference of enrichment factor between the fine and coarse
sorbents is that fine sorbent particles have more surface area making it easier for the
reaction with metals. The causes of low capture efficiency at high temperatures are
speculated as follows. Increasing temperature can improve the reaction rate of sorbent
and the vapor of heavy metals, and theoretically the capture capability of sorbent
would increase. At the same time, the metal vapors are scavenged by liquid melting
onto the sorbents [18]. However, there are two factors hindering metal capture by
sorbents at high temperatures. First, the rate and the amount of metal vaporization
would increase with temperature and more metal vapors exit in the combustion
chamber or the flue duct. The vapors would experience chemical reaction and
physical transformation (including nucleation, condensation and coagulation) when
the flue gas temperature is below their dew point temperature, hence more heavy
metals occur as fine particles in the flue gas and escape directly to atmosphere. The
second factor is high temperature promoting sintering of the sorbents and causing
blockage of the fine pores, thus inhibiting reaction of metal vapors with the sorbents.
Figures 4 and 5 also show that the effects of temperature on metal capture by
solid sorbents can vary. A preferred sorbent is one which has a relative stable capture
capability at different temperatures. The best sorbent is bauxite, especially for the
capture of Pb, then calcium sulfate, and the least effective is limestone. Note that
limestone has a sintering temperature of 950°C and is relatively easy to sinter at high
temperatures.
502
Eflects of Solid Sorbents on Heavy Metal Emission under Combustion Conditions
A.
1.5
1 --
rfi
0.5
.'
--
'&-.
.
.
- - - 11r-4-- --*:qJ
0
4
'E:
\
EL,_
8
c.,
4
\
--
- +- calcium sulphate
- -D- limestone
-A-
bauxite
0
I
1
I
I
950
1100
1250
Tm-CC)
Figure 4. Enrichmentfactor of Cd at different temperatures.
3
2.5
5
Y
u
&
2
1.5
0.5
- €I-- limestone
0
950
1100
1250
Temperature( "C)
Figure 5. Enrichmentfactor of Pb at diflerent temperatures.
503
J. Han, M.H. Xu, H. Zeng, Z. Zhang and F. Yi
111 Effects of staged combustion on metal capture
It is well known that NOx has a very serious effect on both human health and the
environment, and pollutant emission regulations are becoming more stringent in
developed countries. Currently, the measures used to control the NOx emission of
large-scale coal-fired boilers can be classified as: the use of low nitrogen fbel;
combustion modification; and the utilization of flue gas cleaning. Combustion
modification measures, especially air-staged combustion, are preferred due to the
economics. It has been shown that staged combustion is very effective in reducing the
emission of NOx and has already been comprehensively adopted in both developed
and developing countries. However, the effect of staged combustion on heavy metal
emission has not yet been reported. In this study, experiments are conducted on the
effects of staged combustion on metal emission. The temperature is maintained at
1250°C and the sorbendcoal ratio is 2%, other experimental conditions are the same
as previous described. The results are shown in Table 4 and Figures 6 to 9.
Table 4 and Figures 6-9 indicate that kaolinite is the best sorbent for metals
capture under unstaged combustion. The concentration of Co, Cr and Ni in the sorbent
is 18.14, 104.02 and 133.56 pg/g, respectively; while HZ-dolomite is most effective
for capturing Cr and Ni during staged combustion. The results show that kaolinite and
HZ-dolomite are better than the other two sorbents for the capture of Co, Cr, Cu and
Ni. There appear to be two reasons for these results. The first reason is that HZdolomite and kaolinite have more BET surface than other sorbents, which means they
have more surface area to react with the vaporised metals. The second reason is that
they have more active sites at their surface. Limestone has smaller pores and more
BET surface, but it is easy to sinter during the combustion process which may hinder
the reaction between sorbent and metal vapors. The experimental results also indicate
that the adsorption efficiency of the four sorbents for the capture of the four tested
heavy metals under unstaged combustion follows the sequence of Cu > Cr > Co > Ni,
while the sequence changes to Cu > Co > Cr > Ni when air staged combustion is used.
Table 4. Trace metal concentrations (pg.g-1) in coal, sorbents andfly ash under
different conditions.
Metal
Coal
Combustion
HZLimestone
Kaolinite
co
Cr
cu
Ni
504
7.24
12.27
20.36
20.78
Fly ash
Dolomiie
condition
Dolomite
Unstaged
15.99
18.14
16.39
18.13
15.74
Staged
14.70
16.55
13.57
15.35
14.20
Unstaged
99.80
104.02
101.29
89.65
90.52
Staged
76.49
84.67
87.83
89.67
82.42
Unstaged
148.95
146.04
144.60
143.42
130.52
Staged
131.83
135.94
127.11
129.67
120.24
Unstaged
116.90
133.56
116.93
116.93
120.52
Staged
72.11
85.29
83.58
89.57
81.25
Effects of Solid Sorbents on Heavy Metal Emission under Combustion Conditions
18
16
14
g 12
‘2 10
-
3
.
$ 8
:8 6
4
2
0
limestone kaolinite
dolomite
HZ-
coal
do lo m ite
Figure 6. Effect of staged and unstaged combustion on Co concentration.
1
0 unstaging W staging
estone kaolinite d rnolite
HZ-
coal
domolite
Figure 7. Effects of staged and unstaged combustion on Cr concentration.
1601
E60
20
:stone kaolinite Qmolite
HZ-
coal
domolite
Figure 8. Efects of staged and unstaged combustion on Cu concentration.
505
J. Hun, M.H.Xu,H. Zeng, Z. Zhang and F. Yz
0unstaging
-2
140
120
2
100
a
2b
8
$
80
60
40
20
0
limestone
kaolinite domolmite
HZ-
staging
coal
domolite
Figure 9. Efects of staged and unstaged combustion on Ni concentration.
The effects of staged combustion on the adsorption efficiency of solid sorbents for
the capture of Co, Cr, Cu and Ni are shown in Figures 6-9. The results show that the
concentration of the metals collected in the fly ash decreases when air staged
combustion is adopted, which means that the emission of heavy metals increases.
There are two reasons which may affect the emission of heavy metals when air staged
combustion is used. First, staged combustion produces a reducing atmosphere in some
part of the chamber where the metals oxide would be reduced by carbon oxide, which
induces the formation of more volatile sub-oxides or metals. Then these metal vapors
would be enriched in fine particles. Second, the temperature of the reducing area may
decrease because of the delayed mixture of coal and air, which is very helpful for the
heavy metals capture by sorbents. Considering these two aspects, the atmosphere is
more significant than temperature for metal capture in this study.
N
The leaching of the metals
In order to understand the adsorption mechanisms, leaching tests were performed.
Samples were collected from the bottom ash at 1250°C, and sorbentlcoal ratio was
2%. One gram of each sample was loaded in a leaching tube, then the sequential
extraction procedure was used for the samples and partitioning the species of trace
elements present. The extraction method is given in Table 5 . Finally, the trace
elements concentration in each leaching solution was determined by ICP-AES.
Table 6 lists the morphologies of Pb and Cd in coal and the coahorbent system.
Comparing metals morphologies in a coaVsorbent system with those in coal shows
that the percent that are H20soluble and ion exchangeable in the coaYsorbent system
are slightly higher than in coal. These differences suggest that physical adsorption
may have occurred between sorbent and metals during coal combustion, although the
physical adsorption has only a minor effect on the metal compound morphology and
is associated with the properties of the sorbent surface. Many experiments were
performed and show that the effects of physical adsorption increase with the sorbent
surface area. Table 6 shows that physical adsorption is greater for limestone, followed
by calcium sulphate, and bauxite is the worst.
506
Eflects of Solid Sorbents on Heavy Metal Emission under Combustion Conditions
Table 5. Conditions of sequential extraction.
~~~
~~
Morphology
Leaching conditions
H20 soluble
Add 10 ml distilled water for 1 hour at room temperature.
Ion exchangeable
Sediments were extracted at room temperature for 1 hour
with magnesium chloride solution (1 mom, pH 7.0).
Acetic acid soluble
Residues (from ii) leached at room temperature with 10 ml
of 1 mol/L NaAc-Hac (pH 5.0) for 5 hours at 22 f 5°C.
HC1 soluble
Residues (fromiii) were added to 20 ml of 5 moVL HC1
and the sample was heated to 100 f 5°C for 1 hour.
Residues
Residues were digested with HF : HNO3 : HC104 solution.
Table 6. Morphologies of Pb and Cd in fly ash PA).
[Note: Stable species
= HCl
soluble + residues]
507
f.Han, M.H. Xu, H. Zeng, Z. Zhang and F. Yi
For Pb and Cd, the main forms in the fly ash are HCl soluble and residues. In
other words, the main form is stable species. After the sorbents injection, the fraction
of stable species increases, which indicates the result of the chemical reaction
between the sorbents and metals. The detailed reactions are listed as follows:
Al,O, .2SiO, + PbCl, + H,O + PbO. Al,O, 2Si0, + 2HCl
A1,0, a2Si0, + CdC1, + H,O + CdO ' Al,O, .2SiO, + 2HC1
A1,0, - 2Si0, + CdO + CdO. Al,O, + 2HC1
CaSO, + PbO + CaSO, . PbSO,
CaSO, + PbO, + CaSO, - Pb(SO,),
The percent of stable species for Pb increases by 13.2% when bauxite is injected,
while the increases for limestone and calcium sulfate are 2.8% and 3.3%,
respectively. The results are consistent with the experimental results of Chen [7], and
indicate that bauxite is the best sorbent for Pb capture.
Conclusions
1. The ratio of sorbentkoal influences the capture of heavy metals. The capture
efficiency increases with this ratio. However, too much sorbent causes unstable
combustion. It is suggested that the sorbentlcoal ratio is 2% for bauxite, and 4%
for both limestone and calcium sulfate.
2. Each sorbent has an optimum operating temperature for metal capture. High
temperature has a negative effect on heavy metal removal during a combustion
process. Effects of temperature on metal capture are different depending upon the
sorbents used. The capture efficiency at various temperatures is realistic for
bauxite, especially for the capture of Pb, while it is unrealistic for limestone and
possible for calcium sulfate.
3. The staged combustion has a minor effect on heavy metal removal by solid
sorbents, although it is effective for reducing emissions of NOx.
4. The reaction between solid sorbents and trace metals is a complex process
involving physical adsorption and chemical reaction. The physical adsorption has
only a limited effect on the control of heavy metals during coal combustion, while
chemical reaction can not only remove the metals but also immobilize them.
Acknowledgments
This work was sponsored by the National Key Basic Research and Development
Program of China (2002CB211602) and the Natural Science Foundation of China
(50325621).
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