Environ. Sci. Technol. 2006, 40, 7919-7924
Simultaneous Removal of SO2 and
Trace SeO2 from Flue Gas: Effect of
SO2 on Selenium Capture and
Kinetics Study
YUZHONG LI,* HUILING TONG,
YUQUN ZHUO, SHUJUAN WANG, AND
XUCHANG XU
Key Laboratory for Thermal Science and Power Engineering of
Ministry of Education, Department of Thermal Engineering,
Tsinghua University, Beijing 100084, China
Sulfur dioxide (SO2) and trace elements are all pollutants
derived from coal combustion. This study relates to the
simultaneous removal of SO2 and trace selenium dioxide
(SeO2) from flue gas by calcium oxide (CaO) adsorption in
the moderate temperature range, especially the effect
of SO2 presence on selenium capture. Experiments performed
on a thermogravimetric analyzer (TGA) can reach the
following conclusions. When the CaO conversion is relatively
low and the reaction rate is controlled by chemical
kinetics, the SO2 presence does not affect the selenium
capture. When the CaO conversion is very high and the
reaction rate is controlled by product layer diffusion, the SO2
presence and the product layer diffusion resistance
jointly reduce the selenium capture. On the basis of the
kinetics study, a method to estimate the trace selenium
removal efficiency using kinetic parameters and the sulfur
removal efficiency is developed.
Introduction
Following SO2 and NOx, trace elements such as mercury,
selenium, arsenic, and lead emitted from coal combustion
have become a major concern for coal-burning utilities.
Technologies to control SO2 and NOx have been maturely
developed and widely used. Some research on trace element
control has been conducted in recent years, but few practical
technologies are brought into application. Till now, most
technologies to control coal combustion pollutants are
performed individually, that is, one technology controls only
one pollutant. With more and more pollutants needing to be
eliminated, the divide-and-conquer approach will face
challenges. Simultaneous removal technology for multipollutants will be more and more appealing. Low-cost calciumbased sorbent presents an attractive option for the technology
because it has the ability to capture both sulfur and trace
elements such as selenium, arsenic, and lead species.
Many studies have been performed on Ca-based sorbents
adsorbing SO2 from hot flue gases (1-7). The following
reaction scheme has been proposed for SO2 capture under
this condition:
CaO + SO2 + 1/2O2 ) CaSO4
(1)
Zhang et al. (8) have reported the results of the experiments
performed on a pilot-scale circulating fluidized bed flue gas
* Corresponding author e-mail:
10.1021/es061709u CCC: $33.50
Published on Web 11/17/2006
[email protected].
 2006 American Chemical Society
desulfurization (CFB-FGD) experimental facility at 600-800
°C. They found that the desulfurization efficiency increases
rapidly with increasing temperature above 600 °C. The
removal efficiency can reach 95% in this pilot device at 750
°C with the Ca/S ratio of 2.
This desulfurization technology is called moderate temperature dry FGD (MTD-FGD). The MTD-FGD technology
is performed in the moderate temperature range neither as
high as the desulfurization technology of lime injection into
furnace nor as low as wet FGD or semidry FGD. The facility
of this technology, the CFB-FGD system, can be fitted into
the down stream of the coal-fired boilers in the moderate
temperature range.
Although the retrofit of boiler system is the most obvious
obstacle for its application, the MTD-FGD technology is still
attractive for the following two reasons: (1) The MTD-FGD
technology has the advantages of low capital expense, low
operating costs, no water consumption, and high desulfurization efficiency. Thus, it is a feasible technology for SO2
removal in very arid regions. (2) Trace elements such as
selenium (9, 10) and arsenic (11, 12) can be absorbed by CaO
in the moderate temperature window; therefore, the simultaneous removal of SO2 and trace elements may be performed
through the CFB-FGD system. Matsushima et al. (5) also
support that the MTD-FGD technology has great potentials.
This study relates to the second reason above. We tried
to reveal some regularities about the simultaneous removal
of sulfur and trace selenium by CaO in the moderate
temperature range. Previous literature (13, 14) reported that
selenium exists as SeO2 for its entire course in combustion
environment. Ghosh-Dastidar et al. (9) found that selenium
can be removed by CaO through following reaction in the
range of 400-1000 °C.
CaO + SeO2 ) CaSeO3
(2)
Since there is a common temperature window in which
both SO2 and SeO2 can be captured by CaO, the simultaneous
removal of sulfur and trace selenium by CaO is studied in
the moderate temperature range of about 700 °C in our
research scheme. The species such as SO2 and CO2 which are
coexistent in flue gas may bring competition in the trace
selenium capture process via sulfate reaction and carbonate
reaction. The effect of the SO2 presence on the ability of CaO
to absorb trace SeO2 is especially a concern in this paper,
while the effect of CO2 will be reported elsewhere.
For the effect of the SO2 presence, the concentration of
SO2 is several magnitudes higher than that of SeO2 in flue
gases, and then the sulfate reaction will be the main reaction
in the simultaneous sorption process. When these two gases
with such a wide concentration gap are simultaneously
absorbed by CaO, it is not very clear how the high concentration of SO2 affects the selenium capture. As for this
problem, only Agnihotri et al. (10) reported a minor conclusion that SO2 in the gas phase along with SeO2 can decrease
the ability of CaO to capture SeO2. The possible explanation
is that the sorbent pore plugging/blocking due to the
formation of a high molar volume CaSO4 product. Other
reports gave some conclusions about the effect of SO2 on the
capture of other trace elements such as arsenic (11) and lead
(15, 16).
On the basis of former research, we have developed a
study on this problem. As we all know, the SO2-CaO reaction
includes two stages: One is the initial stage in which the
CaO conversion is relatively low and the reaction rate, which
is controlled by chemical kinetics, is constantly high. The
VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic of thermogravimetric analyzer (TGA) reactor
system. 1. trace element vapor generator; 2. main TGA reactor; 3.
scrubbers; x. inlet of TGA protection gas (N2); y. inlet of carrier gas
(N2); z. inlet of SO2 standard gas and diluent gases.
other is the later stage in which the CaO conversion is high
and the reaction rate, which is controlled by product layer
diffusion, is diminishingly small. For short, we call the stage
controlled by chemical kinetics the CK stage and the stage
controlled by product layer diffusion the PD stage. In this
research, the effect of the SO2 presence on trace selenium
capture is studied in the CK stage and the PD stage,
respectively. Different conclusions are made so that SO2 does
not affect selenium capture in the CK stage and does affect
the PD stage.
As for the removal efficiency, the sulfur removal efficiency
can be easily obtained by concentration measurement
devices. However, the trace selenium removal efficiency is
difficult to be determined because its trace concentration is
difficult to measure. To find a method to estimate the trace
selenium removal efficiency, kinetics studies on sulfate
reaction and selenite reaction are carried out in this research.
Finally, through general analyses combining the experimental
results and kinetic data, a method is developed to estimate
the trace selenium removal efficiency using kinetic parameters and the sulfur removal efficiency.
Experimental Section
Apparatus. The reaction rate and capability of CaO adsorbing
SO2 and SeO2 are obtained by measuring the mass change
of a fixed amount of solid reactant in a gas-solid reactor
system for a specific experimental time. The schematic of
the experimental assemblies is shown in Figure 1. It consists
of three parts: trace element vapor generator, main TGA
reactor, and gas scrubber.
The trace element vapor generator, with reference to
Sterling et al.’s method (17), consists of a 24 mm o.d.
vaporization quartz tube housed in a horizontal furnace. A
boat is used to hold the solid selenium (SeO2) inside the
heated quartz tube. A 9 mm o.d. quartz pipe wrapped by
heat tapes connects the outlet of the quartz tube with the
main TGA reactor. The temperature of heat tapes is controlled
to be higher than that of the vaporization tube to avoid SeO2
(g) condensation.
The main TGA reactor is Dupont 951 type with a 24 mm
o.d. quartz tube in its horizontal furnace. A platinum boat
is used to hold the sorbent. The weight signal can be recorded
every 6 s.
At the exit of the TGA reactor, gases pass through a latex
pipe to a scrubbing assembly in which all residual toxic gases
are removed by 7% HNO3 solution. Then the clean gas is
vented to the atmosphere.
x, y, and z are the three gas inlets. Fifty mL/min of pure
N2 is introduced through the inlet x as the TGA protection
gas and 200 mL/min N2 is introduced through the inlet y as
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 24, 2006
FIGURE 2. TGA program for individual sorption of SeO2, simultaneous
sorption of SO2 and SeO2, and individual sorption of SO2 by fresh
CaO with whole steps at 700 °C. Gases: (1) 36 ppm SeO2; (2) 36 ppm
SeO2 and 36 ppm SO2; (3) 36 ppm SO2; (4) and (5): N2; diluent gases
in steps (1)-(3): 5% O2 and balanced N2.
the carrier of SeO2 vapor. The inlet z is for other gases such
as SO2 standard gas, 5% O2, and balanced pure N2. To avoid
the effect of CO2, it is not included in the stream. The total
flow rate is 400 mL/min which is proved to be able to neglect
the effect of outside diffusion.
Materials. Analytic pure solid of SeO2 is used as the vapor
source. The desired SeO2 concentration in the gas can be
attained by regulating the temperature of the vapor generator.
As calibrated previously, the temperature has been set as
160-195 °C to give the desired SeO2 concentrations. The
method to quantify the concentration of SeO2 vapor is
explained in the Supporting Information.
The CaO sample is obtained from calcination of analytic
pure Ca(OH)2 at 600 °C for 30 min. Its BET specific area is
39 m2/g and the mean particle size is 47 µm. The dosages of
CaO samples are 3 mg for kinetics experiments and 10 mg
for other experiments.
Postsorption Sorbent Analyses The amounts of selenium
captured, except those that can be obtained by the TGA
signals, are determined by measuring the selenium content
of the sorbent after experiments. The postsorption samples
are dissolved in 1:1 hydrochloric acid which is found to be
able to leach out all the selenium. The selenium contents of
the solutions are determined by inductively coupled plasmaatomic emission spectrometry (ICP-AES).
Results and Discussions
Effect of Low Concentration Coexisting SO2. First of all,
experiments are performed to find out whether the coexisting
SO2 affects SeO2 capture when the concentrations of these
two gases are in the same low level. And then the concentration of SO2 will be heightened to find out the change of the
effect.
In the experiment corresponding to Figure 2, all conditions
are uniform: the concentrations of SO2 and SeO2 are both
36 ppm, every sorption time is 30 min, the temperature is
700 °C, and the individual sorption processes and the
simultaneous one are designed in a whole TGA program.
The experiment steps are as follows: (1) individual sorption
of SeO2; (2) simultaneous sorption of SO2 and SeO2; (3)
individual sorption of SO2; (4) remaining in the balanced gas
for 15 min; and (5) desorption of the product of absorbed
SeO2, CaSeO3, at 860 °C for 30 min. CaSeO3 can be
decomposed at 860 °C (9), while CaSO4 cannot (7). In step
5, the TG curve becoming horizontal means the decomposition of CaSeO3 is complete. Herein, time of the three sorption
stages sums up to 1.5 h. Many pre-experiments are performed
to make clear whether the product layer diffusion resistance
TABLE 1. Description of the Experiments with Different
Concentration Ratios of SeO2 and SO2
concn ratio
SeO2 (ppm)
SO2 (ppm)
Figure no.
1:1.0
1:1.6
1:2.8
1:3.4
36
46
51
43
36
73
145
145
Figure 2
Figure S1
Figure S2
Figure S3
takes effect within 1.5 h. In these pre-experiments, the
conditions such as temperature and concentration of SeO2
and SO2 are the same with those concerning Figure 2. The
process of individual sorption of SeO2 lasts more than 1.5 h,
and the TG curve is always straight suggesting that no obvious
product layer diffusion resistance occurs within 1.5 h because
the reaction rate is small due to the low concentration of
SeO2. The similar pre-experiments dealing with SO2 give the
same result. Therefore, all these sorption stages are carried
out in the CK stage.
In Figure 2, the weights changed in steps 1-3 are denoted
by a-c, respectively, and the weight loss in step 5 is denoted
by d. According to the experimental data, two equations can
be obtained: a + c ≈ b and d ≈ 2a. Due to the equations,
conclusion can be drawn that the amount of selenium
captured in the simultaneous sorption (step 2) is the same
with that in the individual sorption (step 1) and that the
amount of sulfur captured in the simultaneous sorption (step
2) is the same with that in the individual sorption (step 3).
Therefore, these two gases, SO2 and SeO2, whose concentrations are in the same low level, do not affect each other when
they are simultaneously absorbed by CaO.
The concentration ratio of SeO2 and SO2 is 1:1 in the above
experiment. More experiments with other concentration
ratios have been performed. These experiments also include
the same steps 1-5 with the ratios of 1:1.6, 1:2.8, and 1:3.4.
All ratios are described in detail in Table 1. In these
experiments, the concentration of SO2 is heightened, and all
sorption processes are also performed in the CK stage. The
results of these experiments are shown in Figures S1-S3 in
the Supporting Information.
All these experiments can get the result of a + c ≈ b. As
for d ≈ 2a, the experiment with the SO2 concentration of 73
ppm can get it, while those with 145 ppm, the result is d <
2a. It has been proved that the presence of CaSO4 can hinder
the decomposition of CaSeO3 (18). Maybe in the experiments
with 145 ppm SO2, enough of an amount of CaSO4 is produced
to present this effect and then d < 2a is produced. To sum
up, it can be concluded that although the concentration of
SO2 is a little higher than that of SeO2, when they are
simultaneously absorbed by CaO at 700 °C, SO2 and SeO2 do
not affect each other under the conditions of these experiments.
Effect of High Concentration Coexisting SO2. As for the
above experiments, the concentration difference between
SeO2 and SO2 is not as big as that in the actual flue gases.
Otherwise, these experiments are only performed in the CK
stage, and the results cannot reflect the status in the PD
stage.
To make up these limitations, experiments with a higher
concentration of SO2 should be carried out. Because of the
short time of the CK stage with respect to the reaction between
CaO and high concentration SO2, these experiments cannot
follow steps 1-5 shown in Figure 2. Then a new experiment
scheme including three cases is designed as follows. The
concentration of SeO2 is set as 15 ppm and that of SO2 is 700
ppm and 1400 ppm, which provides a wider concentration
gap. Three cases are performed, respectively: case 1,
individual sorption of SeO2 by fresh CaO; case 2, simultaneous
sorption of SO2 and SeO2 by fresh CaO; case 3, simultaneous
sorption of SO2 and SeO2 by used CaO with about 45%
conversion caused by sulfate reaction. The sorption time of
each case is 30 min with respect to 700 ppm SO2 and 20 min
with respect to 1400 ppm SO2. Case 1 is performed to get the
blank value. In order to explain cases 2 and 3, Figure 3a,c is
provided. The method to determine the CaO conversion in
Figure 3a,c is explained in the Supporting Information. As
a fact, case 2 is carried out in stage A or C of the sulfate
reaction which is approximately regarded as a part of the CK
stage and case 3 is performed in stage B or D which is thought
of as a part of the PD stage. Otherwise, the detailed processes
to get Figure 3 are provided in the Supporting Information.
The amounts of selenium captured in cases 1-3 are
determined by ICP-AES, and the results are given in Figure
3b,d.
W1, W2, and W3 are introduced to indicate the amounts
of selenium captured in cases 1-3, respectively. Through
comparison, it can be found that W2 is a little lower than W1.
They seem to be equal, which suggests that the presence of
SO2 does not affect the ability of CaO to adsorb trace SeO2
obviously in the CK stage. It also can be found that W3 is
much lower than W1, which indicates that the presence of
SO2 decreases the ability of CaO to adsorb trace SeO2
obviously in the PD stage. As a matter of fact, in the PD stage,
the ability of CaO to adsorb SO2 decreases too, which is clearly
shown in Figure 3a,c.
Mechanism Analyses. The theory of active site is introduced to the analyses mechanism. There are many active
sites on CaO surface. Once the active SO2/SeO2 molecules
diffuse to the sorbent surface and collide with the active
sites effectively, they will be absorbed.
When the CaO conversion is relatively low and the reaction
is in the CK stage, the reaction rate is high and increases with
the increasing SO2 concentration, which suggests that the
active sites on the CaO surface are abundant for all active
SO2 molecules. Now that the active sites are abundant in the
CK stage, the little increase of the amount of active molecules,
which is caused by trace SeO2 being mixed into the stream
containing a high concentration of SO2, cannot change the
abundant active site status. In this status, all active SO2/SeO2
molecules which diffuse to the sorbent surface can be farthest
absorbed. Therefore, the conclusion that the presence of
SO2 does not affect the selenium capture when the simultaneous sorption is carried out in the CK stage can be made.
In the high CaO conversion range, i.e., in the PD stage,
the sorbent is covered by a layer of CaSO4, and the product
layer diffusion is probably in “outward growth mode” (19,
20), that is, Ca2+ and O2- ions migrate outward through the
product layer to react with SO2/SeO2 on the surface. Because
the ionic diffusion rate is slow, the number of the ions
reaching the outside product surface in unit time is limited.
Namely, the amount of the active sites provided by the
sorbent is small in the PD stage. Maybe the small amount
of active sites are absent for a relatively high concentration
of SO2; therefore, the sulfate reaction rate is low in the PD
stage. However, under this condition, if only trace SeO2 is
absorbed with the absent of SO2, it has been proved that the
selenium sorption is not weakened because the small amount
of active sites are still abundant for trace SeO2 due to its low
concentration (21). When the simultaneous sorption is
performed in the PD stage, there are two disadvantageous
factors for trace selenium sorption: the first is that the amount
of active sites is not so much as that in the CK stage; the
second is that the high concentration of SO2 contest with
trace SeO2 for the absent active sites. Therefore, the conclusion that the SO2 presence and the product layer diffusion
resistance jointly reduce the selenium capture when simultaneous sorption is carried out in the PD stage can be
made.
VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Comparison of selenium capture by CaO sorbent in different cases. Case 1: individual sorption of SeO2 using fresh CaO (blank
value); Case 2: simultaneous sorption of SeO2 and SO2 using fresh CaO performed in the CK stage (stage A or C); Case 3: simultaneous
sorption of SeO2 and SO2 using used CaO with the sulfate conversion of about 45% performed in the PD stage (stage B or D); experimental
conditions: temperature: 700 °C; SeO2 concentration: 15 ppm; SO2 concentration: 700 ppm (a) and (b) and 1400 ppm (c) and (d); sorption
time of each case: 30 min (b) and 20 min (d); diluent stream: O2:5% and N2: balance; flow rate: 400 mL/min. (a) Typical curve of
desulfurization by CaO in TGA with 700 ppm SO2 at 700 °C. (b) Amounts of absorbed selenium in different cases. (The selenium contents
are calculated according to the masses of original sorbents.) (c) Typical curve of desulfurization by CaO in TGA with 1400 ppm SO2 at
700 °C. (d) Amounts of absorbed selenium in different cases. (The selenium contents are calculated according to the masses of original
sorbents.)
Previous Research Related. Agnihotri et al. (10) carried
out the simultaneous sorption of SO2 and SeO2 at 600 °C for
2 h. Their conclusion was that the selenium capture was
drastically reduced in the presence of SO2. The simultaneous
sorption time of Agnihotri’s experiment was relatively long.
Through the result figure provided in their paper, it can be
found that about 3/4 sorption time was in the PD stage.
Therefore, their conclusion only reflects the status in the PD
stage. Their conclusion is the same as ours drawn in the PD
stage.
Seames (22) performed a comprehensive study to investigate the partitioning of selenium during pulverized coal
combustion. The partitioning of selenium to fly ash surfaces
is dependent on the availability of active cation sites. For
coals with relatively low Se/Ca ratios, selenium is expected
to react with calcium surface sites to form calcium selenite
complexes. If the Se/Ca ratio is relatively high and the sulfur
content is moderate to high, cationic surface sites will not
be available for selenium partitioning, and most of the
selenium is expected to exit the furnace in the vapor phase
or as fly ash surface-based SeO2. These conclusions are similar
to ours, and the methodology of mechanism analyses is the
same as ours.
Kinetics Study. We apply the kinetic model developed by
Agnihotri et al. as follows (10). In the initial stages, the reaction
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rate can be given by the following general reaction rate
equation.
(dxdt )
dx
)( )
dt
-rSe,init )
-rS,init
Se,init
S,init
) kSeCSeO2m1
(3)
) kSCSO2m2
(4)
Using above kinetic model, kSe and kS are determined in
the range of 600-850 °C through experiments. The flow rate,
400 mL/min, has been proved to be sufficiently high to
eliminate the external mass transfer. The product layer
diffusion resistance can be neglected for very small conversions as is the case in initial rate studies. Under appropriate
conditions, the initial reaction rate can be applied to calculate
the kinetic parameters.
For a given reaction temperature, the linear relationship
between ln(dx/dt) and ln(CSeO2) can be represented as
(dxdt )|
ln
tf0
) ln(kSe) + m1ln(CSeO2)
(5)
Many experiment results attained under different gas
concentration or temperature conditions are gathered in
Figure 4. The slop of the line fitted to the data is the reaction
order for the selenite reaction with respect to SeO2 concen-
FIGURE 4. Estimation of the reaction order, m1, with respect to SeO2
concentration and rate constant kSe.
TABLE 2. Reaction Order with Respect to SeO2 Concentration
Depending on Temperature
temp (˚C)
m1
600
1.02
700
1.05
720
1.06
740
1.09
750
1.12
760
1.27
780
1.40
800
1.52
tained, which suggests that the fresh sorbent is the main
contributor to the desulfurization efficiency, i.e., most of SO2
is removed by CaO in the CK stage. Combining this standpoint
and our conclusion that the coexisting SO2 does not affect
the ability of CaO to adsorb trace SeO2 when the simultaneous
sorption is carried out in the CK stage, it can be concluded
that there is a great potential that trace SeO2 can be efficiently
removed in the MTD-FGD technology.
Now that SO2 and trace SeO2 can be simultaneously
absorbed by fresh CaO, the removal efficiency of each gas
will be determined by the rate constant, k, of each reaction
respectively. This viewpoint will be inferred through the
following modeling analyses.
Given that there is a dimensional mesh in the CFB-FGD
reactor. The gas containing SO2, trace SeO2, and particles of
CaO flows across the mesh. The sorption happens there.
Two pieces of the hypothesis are made in this modeling: (1)
CaO is fresh; (2) when the particles leave the mesh, the sulfate
reaction is still in the CK stage. These two pieces of the
hypothesis are made according to the conclusion that the
fresh sorbent is the main contributor to the desulfurization
efficiency. After sorbent particles flow across the mesh, the
amount of gas pollutant changed can be described as
-dng ) -dnCaO )
m0
m0
dx )
kCm dt
MCaO
MCaO g
(6)
The original SO2 or SeO2 molar mass in the mesh is
ng,0 ) V0Cg
(7)
Therefore, the removal efficiency after these sorbent
particles flow across the mesh can be expressed as
dηg )
-dng
m0
)
kC m-1 dt
ng,0
MCaOV0 g
(8)
In the range of 600-740 °C, the sulfate reaction and the
selenite reaction can be approximately looked at as the firstorder chemical reactions. Therefore, the following equations
can be obtained:
ηSeO2 ≈
FIGURE 5. The reaction rate constants of these two reactions, kSe
and kS.
tration, m1. The exponential value of the intercept of each
line is the rate constant of the reaction, kSe.
The reaction order values at different temperatures are
listed in Table 2, and the values of kSe are shown in Figure
5. With the increase of temperature, the value of m1 increases
and kSe decreases, which is due to the decomposition of the
reaction product, CaSeO3. It has been found that the
decomposition rate of CaSeO3 increases with the increasing
temperature above 700 °C (9).
As for the sulfate reaction, the values of m2 and kS are
obtained in the same way. The reaction order with respect
to SO2 concentration is 1, and the values of kS are shown in
Figure 5 too. It can be found that when the temperature is
below 740 °C, kSe is bigger than kS.
General Analyses. It can be found in Figure 3a,c that the
sulfate reaction rate in the CK stage is much higer than that
in the PD stage, which implies that most of SO2 will be
removed by CaO in the CK stage in the MTD-FGD technology.
This standpoint has also been proved by Zhang et al. (23)
through experiments performed in a pilot scale CFB-FGD
experimental system. They found that after the fresh sorbent
supply stopped, the desulfurization efficiency decreased
rapidly even though the sorbent recirculation was main-
(
(
)
)
kSe
kSe
η
η < 100%
kS SO2 kS SO2
ηSeO2 f 100%
kSe
η g100%
kS SO2
(9)
(10)
Because the ratio of kSe/kS is bigger than 1 in the range
of 600-740 °C, it can be concluded that the selenium removal
efficiency will be higher than the sulfur removal efficiency
when simultaneous removal is performed in this temperature
range. Otherwise, the sulfur removal efficiency can be easily
obtained by measuring SO2 concentrations at the inlet and
the outlet of the FGD reactor, while the selenium removal
efficiency can hardly be measured accurately because its
concentration is at trace level. The selenium removal
efficiency can be estimated according to the sulfur removal
efficiency and the ratio of kSe/kS.
Acknowledgments
This work is supported by the State Key Development
Program for Basic Research of China (2006CB200301). The
experiments are funded by Open Fund of the Laboratory
AdministrationofTsinghuaUniversity(LF20050489,LF20060797).
Supporting Information Available
Methods to quantify the concentration of SeO2 and SO2 and
to determine the CaO conversion, the processes to get Figures
VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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3-5, and results of the experiments with SeO2/SO2 concentration ratios of 1:1.6, 1:2.8, and 1:3.4 (Figures S1-S3). This
material is available free of charge via the Internet at http://
pubs.acs.org.
Nomenclature
CK stage
reaction stage in which rate is controlled by
chemical kinetics
C
concentration of SO2 or SeO2
CSO2
concentration of SO2
CSeO2
concentration of SeO2
k
rate constant
kS
rate constant of sulfate reaction
kSe
rate constant of selenite reaction
MCaO
CaO molecular weight
m0
CaO flow rate
m1
reaction order with respect to SeO2
m2
reaction order with respect to SO2
mCaO
CaO mass
nCaO
CaO molar mass
ng
SO2 or SeO2 molar mass
PD stage
reaction stage in which rate is controlled by
product layer diffusion
rS,init
initial sulfate reaction rate
rSe,init
initial selenite reaction rate
t
reaction time
V0
flow rate
x
CaO conversion
ηg
removal efficiency of gas pollutant
ηSO2
sulfur removal efficiency
ηSeO2
selenium removal efficiency
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Received for review July 18, 2006. Revised manuscript received September 8, 2006. Accepted October 4, 2006.
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