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 7920
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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 7922
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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
Literature Cited (1) Jozewicz, W.; Chang, C. S. J.; Brna, G. T.; Sedman, B. C. Reactivation of solids from furnace injection of limestone for SO2 control. Environ. Sci. Technol. 1987, 21, 664-670. (2) Fernandez, I.; Garea, A.; Irabien, A. SO2 reaction with Ca(OH)2 at medium temperatures (300-450°C): Kinetic behavior. Chem. Eng. Sci. 1998, 53, 1869-1881. (3) Li, Y.; Nishioka, M.; Sadakata, M. High calcium utilization and gypsum formation for dry desulfurization process. Energy Fuels 1999, 13, 1015-1020. (4) Li, Y.; Loh, B. C.; Matsushima, N.; Nishioka, M.; Sadakata, M. Chain reaction mechanism by NOx in SO2 removal process. Energy Fuels 2002, 16, 155-160.
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Received for review July 18, 2006. Revised manuscript received September 8, 2006. Accepted October 4, 2006. ES061709U
