Hydrated Lime

Jul 31, 2009 - Kinetics of the Reaction of Iron Blast Furnace Slag/Hydrated Lime Sorbents with SO2 at Low Temperatures: ... E-mail: [email protected]...
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Ind. Eng. Chem. Res. 2009, 48, 8335–8340

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of the Reaction of Iron Blast Furnace Slag/Hydrated Lime Sorbents with SO2 at Low Temperatures: Effects of the Presence of CO2, O2, and NOx Chiung-Fang Liu† and Shin-Min Shih*,‡ Energy and EnVironment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, and Department of Chemical Engineering, National Taiwan UniVersity, Taipei 106, Taiwan

The effects of the presence of CO2, O2, and NOx in the flue gas on the kinetics of the sulfation of blast furnace slag/hydrated lime sorbents at low temperatures were studied using a differential fixed-bed reactor. When O2 and NOx were not present simultaneously, the reaction kinetics was about the same as that under the gas mixtures containing SO2, H2O, and N2 only, being affected mainly by the relative humidity. The sulfation of sorbents can be described by the surface coverage model and the model equations derived for the latter case. When both O2 and NOx were present, the sulfation of sorbents was greatly enhanced, forming a great amount of sulfate in addition to sulfite. The surface coverage model is still valid in this case, but the model equations obtained show a more marked effect of relative humidity and negligible effects of SO2 concentration and temperature on the reaction. The effect of sorbent composition on the reaction kinetics was entirely represented by the effects of the initial specific surface area (Sg0) and the Ca molar content (M-1) of sorbent. The initial conversion rate of sorbent increased linearly with increasing Sg0, and the ultimate conversion increased linearly with increasing Sg0M-1. The model equations obtained in this work are applicable to describe the kinetics of the sulfation of the sorbents in the low-temperature dry and semidry flue gas desulfurization processes either with an upstream NOx removal unit or without. Introduction The low-temperature spray-drying (semidry) and dry flue gas desulfurization (FGD) processes are effective means of reducing SO2 emission from coal-fired power plants. Hydrated lime is commonly used as the sorbent in these processes. The economy of these processes can be improved if sorbents with higher reactivity are used. Sorbents made of iron blast furnace slag and hydrated lime (BFS/HL) have been found to be more reactive toward SO2 than hydrated lime alone under the conditions similar to those prevailing in the bag filters of a dry or semidry FGD process.1-3 The increase in sorbent reactivity is attributed to the formation of high surface area hydration products such as calcium silicate hydrates during the slurrying process in preparation of the BFS/HL sorbents.1-3 The kinetic analysis of the reaction of the BFS/HL sorbents with SO2 in the gas mixtures composed of SO2, H2O, and N2 at low temperatures has been reported in our previous work.2,4 However, in practice, besides SO2, H2O, and N2, the flue gas from a coal-fired boiler also contains CO2, O2, and NOx,5 which may affect the reaction of a calcium-based sorbent with SO2. The influences of the latter gaseous species on the sulfation of hydrated lime at low temperatures have been studied by many investigators.6-12 Some investigators focused on the dry scrubbing technology for combined SO2 and NOx removal.13,14 Liu and Shih12 reviewed the results reported in the literature and performed experiments to study the reactions of hydrated lime with gas mixtures containing these gas species and SO2 at their typical concentrations in the flue gas evolved from a conventional coal-fired boiler. They found that the ultimate extent of * To whom correspondence should be addressed. E-mail: smshih@ ntu.edu.tw. Tel.: 886-2-23633974. Fax: 886-2-23623040. † Industrial Technology Research Institute. ‡ National Taiwan University.

sulfation of hydrated lime increased only a little when O2 and NOx were not present together in the gas mixture but increased markedly when O2 and NOx or when CO2, O2, and NOx were present simultaneously. This finding indicates that the kinetics of the sulfation of hydrated lime or BFS/HL sorbents with the simultaneous presence of O2 and NOx would be different from that without their simultaneous presence. The kinetics of the sulfation of hydrated lime with the presence of CO2, O2, and NOx has been reported recently by Liu and Shih.15 However, for BFS/HL sorbents, prior to this study, no work conducted with the presence of CO2/O2/NOx had been found. Because of the lack of prior research and the importance to the design and operation of FGD processes, it is worthwhile to undertake a kinetic study of the sulfation of these sorbents with the presence of CO2, O2, and NOx. In this work, the effects of the presence of CO2, O2, and NOx in different combinations on the kinetics of the sulfation of BFS/ HL sorbents at low temperatures were studied, and the kinetic model equations pertinent to the sulfation of these sorbents under different flue gas conditions were derived. Experimental Section The sorbents were prepared by slurrying high-fineness iron blast furnace slag (HBFS) and HL in deionized water with a liquid/solid (L/S) ratio of 10/1 at 25 °C for 16 h. The compositions of HBFS and HL and the details of the sorbent preparation procedure were reported in our previous work.2 The sorbents prepared were porous, and some hydration products such as foillike calcium silicate hydrates were formed in sorbents containing HBFS. The appearances of particles of sorbents prepared with different BFS/HL weight ratios were similar to those shown in our previous work.3 The BFS/HL weight ratios, specific surface areas (Sg0), and sorbent weights per mole of Ca (M) for the sorbents used in this study are listed in Table 1.

10.1021/ie900379c CCC: $40.75  2009 American Chemical Society Published on Web 07/31/2009

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Table 1. BFS/HL Weight Ratios, Sorbent Weights per Mole of Ca (M, g of Sorbent/(mol of Ca)), and Specific Surface Areas (Sg0, m2/g) of Sorbents (Slurrying Conditions: 25 °C, L/S ) 10/1, and 16 h) BFS/HL wt. ratio

M

Sg0

100/0 70/30 50/50 30/70 0/100

143 123 110 97 75

12.8 28.1 23.4 22.3 10.0

The same differential fixed-bed reactor and experimental procedure as those employed in our previous work12 were used for the sulfation tests of sorbents. About 40 mg of sample was used for each run. The CO2, O2, NOx, SO2, and N2 gases supplied from cylinders and the H2O vapor from a water evaporator were mixed to form the simulated flue gas. The NOx gas consisted of NO (>97%) and NO2. The composition of the gas mixture was adjusted by controlling the flow rate of each component. The CO2, O2, NOx, and SO2 concentrations were controlled at 12.6%, 5%, 600 ppm, and 1000-3000 ppm, respectively, which are their typical concentrations in the flue gas evolved from a conventional coal-fired boiler; the relative humidity (30-70%) and the temperature (60-80 °C) of the gas mixture were in the ranges typical in the bag filters of a semidry FGD system.5 The longest reaction time was 1 h, which was taken to see whether the conversion of a sorbent reached an ultimate value. The conversion due to sulfation, X, for a reacted sample was defined as the molar ratio of the amount of total sulfur captured to that of Ca contained in the reacted sample. When the sample was sulfated without O2 and NOx present, the amount of total sulfur was that of sulfite determined by iodimetric titration of the sample dissolved in acid. When the sample was sulfated with O2 and/or NOx present, the amounts of total sulfur and sulfite were determined by ion chromatography (IC) and iodimetric titration, respectively. H2O2 was added to the sample solution to oxidize any S(IV) to S(VI) before injecting it into the IC. The amount of sulfate was obtained by subtracting the amount of sulfite from that of total sulfur. The amount of Ca was determined by ethylenediaminetetraacetic acid (EDTA) titration.7 The fractions of sulfite, XS1, and sulfate, XS2,, were calculated with respect to the moles of Ca. At least two repeated runs were performed for each set of reaction conditions, and the average of the results was taken. The absolute errors of the mole fractions measured were about 0.03. Results and Discussion Effect of Gas Composition. Figures 1-3 are plots of conversion X versus reaction time t for the sorbent with BFS/ HL ) 30/70 reacted under various sulfation conditions. In spite of the experimental conditions being different, these plots show the same reaction pattern that the reaction was rapid in the initial period but stopped after about 15 min, leaving the sorbent incompletely converted. This reaction pattern is resulted from the formation of an impervious product layer of calcium salts. The solid products were mainly calcium sulfite and sulfate hemihydrates, when the sorbent was sulfated with O2 and/or NOx present. Figure 1 shows the effect of gas composition on the sulfation reaction of the sorbent with BFS/HL ) 30/70 at 60 °C, 70% RH, and 1000 ppm SO2. Comparing with the base case without the presence of CO2, O2, and NOx and taking into account the experimental errors in X (about 0.03), one can see that the reaction was unaffected by the presence of CO2 and was slightly enhanced by the presence of O2 or NOx or O2/CO2 in the gas mixture when O2 and NOx were not present together, whereas

Figure 1. Conversion versus time for the reaction of HBFS/HL (30/70 weight ratio) sorbent at 60 °C, 70% RH, 1000 ppm SO2, and various CO2, NOx, and O2 concentrations.

Figure 2. Conversion versus time for the reaction of HBFS/HL (30/70 weight ratio) sorbent at 12.6% CO2, 5% O2, and various other reaction conditions.

the reaction was greatly enhanced when CO2, O2, and NOx were present together in the gas mixture. The enhancement effects due to the simultaneous presence of O2 and NOx at other relative humidities can be observed by comparing Figure 2 with Figure 3a. Moreover, both Figures 2 and 3 show that the reaction was significantly enhanced by increasing the relative humidity but was affected little by the reaction temperature and SO2 concentration. Similar effects of relative humidity, temperature, and SO2 concentration on the sulfation reaction of sorbents have been reported for the case without the presence of CO2, O2, and NOx.2 The conversions and the mole fractions of sulfite and sulfate for sorbents which had reacted with various gas mixtures for 1 h are listed in Table 2. In experiments with O2 and/or NOx present, sulfate was formed in addition to sulfite. When O2 and NOx were not present together, the fractions of sulfate were small and less than the fractions of sulfite. Whereas, when O2 and NOx were present together, the fractions of sulfate became comparable to those of sulfite; CO2 had a positive effect when it was also present with O2/NOx. Furthermore, at each gas composition, the 1 h conversions for sorbents with BFS/HL ratios of 30/70 to 70/30 were much greater than that for HL or BFS alone. Without the presence of CO2/O2/NOx, the conver-

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Table 2. Results for HBFS/HL Sorbents Reacted at 60 °C, 70% RH, 1000 ppm SO2, and Various CO2, NOx, and O2 Concentrations for 1 h BFS/HL wt ratio

NOx, ppm

CO2, %

O 2, %

XS1

XS2

X

100/0

0 0 0 0 600 600 600 0 0 0 0 600 600 600 0 0 0 0 600 600 600 0 0 0 0 600 600 600 0 0 0 0 600 600 600

0 12.6 0 12.6 0 0 12.6 0 12.6 0 12.6 0 0 12.6 0 12.6 0 12.6 0 0 12.6 0 12.6 0 12.6 0 0 12.6 0 12.6 0 12.6 0 0 12.6

0 0 5 5 0 5 5 0 0 5 5 0 5 5 0 0 5 5 0 5 5 0 0 5 5 0 5 5 0 0 5 5 0 5 5

0.08 0.07 0.06 0.08 0.09 0.11 0.16 0.45 0.43 0.42 0.40 0.42 0.39 0.40 0.42 0.39 0.38 0.42 0.41 0.38 0.41 0.40 0.42 0.41 0.43 0.39 0.40 0.43 0.22 0.22 0.20 0.22 0.20 0.28 0.30

0 0 0.04 0.03 0.04 0.09 0.15 0 0 0.07 0.09 0.09 0.34 0.33 0 0 0.06 0.04 0.07 0.30 0.30 0 0 0.04 0.05 0.08 0.31 0.40 0 0 0.02 0.04 0.04 0.24 0.32

0.08 0.07 0.10 0.11 0.13 0.20 0.31 0.45 0.43 0.49 0.49 0.51 0.73 0.73 0.42 0.39 0.44 0.46 0.48 0.68 0.71 0.40 0.42 0.45 0.48 0.47 0.71 0.83 0.22 0.22 0.22 0.26 0.24 0.52 0.62

70/30

50/50

30/70

0/100

Figure 3. Conversion versus time for the reaction of HBFS/HL (30/70 weight ratio) sorbent: (a) at 60 °C, 1000 ppm SO2, 12.6% CO2, 5% O2, 600 ppm NOx, and various relative humidities; (b) at 80 °C and 1000 ppm SO2 and at 60 °C and 3000 ppm SO2 with 70% RH, 12.6% CO2, 5% O2, and 600 ppm NOx.

sions of these BFS/HL sorbents were in the range of 0.40-0.45, while the conversions of HL and BFS were 0.22 and 0.08, respectively. With the presence of CO2/O2/NOx, the conversions of these BFS/HL sorbents were in the range of 0.71-0.83 with the highest conversion at BFS/HL ) 30/70, while the conversions of HL and BFS were 0.62 and 0.31, respectively. At the reaction conditions given in Table 1 and when CO2 was present in the gas mixture, calcium carbonate was detected, but its fraction was very small, less than 0.03. It has been reported that calcium carbonate was formed in the early stage of sulfation reaction, but it was consumed by its reaction with SO2 in the latter stage.7 Furthermore, when NOx was present, calcium nitrite and nitrate were formed, but their fractions were also rather small, less than 0.08 and 0.04, respectively. Kinetic Analysis. For Ca-containing alkaline sorbents reacted under humid and low-temperature conditions, it has been known that the reaction of the solid reactants proceeds through the reactions with the acids formed from the molecules of the reactive gases and water adsorbed on the solid surface and the reaction rate increases significantly with increasing RH and decreases as the solid products accumulate on the surface.16-19 The reaction behavior of sorbent shown in Figures 1-3 is the same as that predicted by the surface coverage model first proposed by Shih et al.:16 X ) [1 - exp(-k1k2t)]/k2

(1)

where k1 is the initial rate of conversion and k2 is the reciprocal of the ultimate conversion, and both of them are functions of sorbent properties and reaction conditions. This model has been reported to well-describe the sulfation of hydrated lime17 and BFS/HL sorbents2 in gas mixtures containing SO2, H2O, and N2 only. This model also has been found to well-describe the sulfation of hydrated lime with the presence of CO2, O2, and NOx.15 The hypotheses of the surface coverage model are that the sorbent is made up of plate grains and that the reaction rate is controlled by chemical reaction on the surface of a grain and the reacting surface area of the grain decreases with the deposition of solid products. According to this model, the reaction of a sorbent reaches an ultimate conversion when its reacting surface is fully covered by the products.16-19 The foregoing results of sulfation tests indicate that whether O2 and NOx are present simultaneously or not in the gas phase would significantly affect the values of k1 and k2 in eq 1. Therefore, the equations representing k1 and k2 as functions of reaction conditions were obtained by analyzing the results for the following two cases: sulfation with CO2 and O2 present and sulfation with CO2, O2, and NOx present in the gas mixture. These two cases correspond to the flue gas that has been treated by a NOx removal process and that has not, respectively. Sulfation with CO2 and O2 Present. As mentioned above, the presence of CO2 and O2 affected the sulfation of sorbents only slightly as long as NOx was not present simultaneously in the gas phase. Therefore, the equations for k1 and k2-1 obtained in our previous study2 for the reaction of sorbents under the gas mixtures composed of SO2, H2O, and N2 were tested for the validity in this case:

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k1 ) 0.00779Sg0e0.0124RHe-9500/RTy0.31

(2)

k2-1 ) 0.0344Sg0M-1RH0.96

(3)

where RH is the relative humidity (%), R is the gas constant ((J/mol)/K)), T is the reaction temperature (K), and y is the SO2 concentration (ppm). The curves in Figures 1 and 2 were plotted using eqs 1-3. One can see that the model predictions indeed agree well with the experimental data; the average deviation of the experimental X value from the prediction was about the same as the average experimental error in X, 0.03. Furthermore, the comparisons between the predictions and the data shown in Figure 2 indicate that the effect of relative humidity on the sulfation reaction did not change due to the presence of CO2 and O2. The data, however, were not accurate enough to reveal the slight effects of temperature and SO2 concentration as predicted. Nevertheless, eqs 1-3 are valid for describing the sulfation kinetics of sorbents in the case in which CO2 and O2 are present together in the gas phase. Sulfation with CO2, O2, and NOx Present. The X-t plots for the sorbent with BFS/HL ) 30/70 sulfated under the simulated flue gases containing CO2, O2, and NOx in addition to SO2, H2O, and N2 are shown in Figure 3. The values of k1 and k2 were estimated by fitting eq 1 to the X-t data; the curves in Figure 3 are the best fitting curves. From the values of k1 and k2 obtained at different reaction conditions and the values of Sg0 and M for sorbents (Table 1), the best equations representing k1 and k2 as functions of sorbent properties and reaction conditions were searched and were found to be k1 ) 0.00082Sg0e0.044RH

(4)

k2-1 ) 0.0059Sg0M-1RH1.53

(5)

A confidence level of 95% was used in estimating the parameters. The detailed procedure of the derivation of eqs 4 and 5 can be found in our previous work.2,4 The conversions calculated using eqs 1, 4, and 5 are compared with the experimental values in Figure 4. The model predictions are in good agreement with the experimental values; the average deviation of the experimental value from the prediction was about 0.07. Sorbents with different BFS/HL ratios have different compositions and structural properties and thus have different values of Sg0 and M. The effect of BFS/HL ratio on the sulfation

reaction of sorbent can be entirely represented by the effects of Sg0 and M, as shown in equations for k1 and k2-1, eqs 2 and 3 or eqs 4 and 5, respectively. These equations indicate that k1 increases linearly with increasing Sg0 but is independent of M and that k2-1 increases linearly with increasing Sg0M-1. From the values of Sg0 and M given in Table 1, one can see that the sorbent with BFS/HL ) 70/30 would have the highest value of k1 or the initial conversion rate and that the values of k2-1 or the ultimate conversion for BFS/HL sorbents with ratios of 30/ 70, 50/50, and 70/30 would be about the same and greater than that for HL or BFS alone. Comparing eqs 4 and 5 with the model equations obtained without the presence of CO2, O2, and NOx, eqs 2 and 3, one can see that the effects of SO2 concentration and temperature on the reaction became negligible and the effect of RH became more marked when CO2, O2, and NOx were present simultaneously in the gas phase. The data obtained with the presence of CO2/O2/NOx were not accurate enough to reveal any inherently slight effect of SO2 concentration or temperature as that observed without the presence of CO2/O2/NOx. It is well-known that the reactions of calcium-based alkaline sorbents with SO2 at low temperatures proceed through the reactions involving water adsorbed on the solid surface and the amount of water should be large enough for the reactions of sorbents to be appreciable:2,17-19 SO2 + H2O a SO2 · H2O a HSO3- + H+

(6)

HSO3- a SO3- + H+

(7)

The amount of water or the thickness of water layer adsorbed on the solid surface increases with increasing relative humidity. The presence of deliquescent salts in a sorbent also can enhance the collection of water by the sorbent.20 As the water layer becomes thicker, more molecules of SO2 can be absorbed into the water layer and greater quantities of SO2 · H2O and/or H+, which can attack the alkaline solid reactants, will be produced. Therefore, the rate and the extent of the reaction of a sorbent increase with increasing amount of water adsorbed. Liu and Shih12 pointed out that the great enhancement of the sulfation of Ca(OH)2 by the presence of O2/NOx in the gas mixture results from the rise in the NO2 concentration in the gas phase. When NOx (mostly NO) is mixed with O2, NO is oxidized to NO2 and the concentration of NO2 is thus raised. As NO2 is a much stronger oxidant and more soluble in water than NO and O2, there are more NO2 molecules absorbed into the water layer adsorbed on the solid surface, and the oxidation of bisulfite and sulfite ions to sulfate ions is hence enhanced, which induces more SO2 molecules to be captured into the water layer. The reactions that take place in the water layer involving NO2 can be represented by the following stoichiometric equations:14,21,22 2NO2 + HSO3- + H2O a SO42- + 3H+ + 2NO2(8) 2NO2 + SO32- + H2O a SO42- + 2H+ + 2NO2(9) 2NO2 + O2 + 3HSO3- + H2O a 3SO42- + 5H+ + 2NO2(10)

Figure 4. Comparison of the calculated and experimental conversion values.

2NO2 + O2 + 3SO32- + H2O a 3SO42- + 2H+ + 2NO2(11)

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-

2NO2 + H2O a NO2 + NO3 + 2H 3HNO2 a HNO3 + 2NO + H2O

+

(12) (13)

As seen in the above equations, when NO2 reacts with HSO3-, SO32-, and water, ions of H+, NO2-, and NO3- are formed, leading to the formation of Ca(NO2)2 and Ca(NO3)2. These salts are deliquescent; their deliquescence will collect a great quantity of water and thus will enhance the reaction of the sorbent with the reactive gases. The preceding explanation set forth by Liu and Shih12 on the enhancement effect of O2/NOx on the sulfation of hydrated lime is considered also valid for the case of BFS/ HL sorbents. Furthermore, the marked difference in X between 50 and 60% RH as shown in Figure 3 may indicate that Ca(NO2)2 or Ca(NO3)2 started to deliquesce in that range of RH at 60 °C. Also, the greater effect of RH on the reaction of sorbents is thought to be due to the deliquescence of these salts at RH > 50%. The effects of the concentrations of the gas components on the ultimate sulfation extents of sorbents were also studied by varying the SO2 concentration from 100 to 1000 ppm, NOx from 300 to 600 ppm, CO2 from 3.2 to 12.6%, and O2 from 1.0 to 5.4%, and negligible effects were found. However, flue gases with compositions greatly different from those studied in this work may be encountered in other coal combustion processes, such as the O2/CO2 recycle coal combustion.23,24 How the flue gas components affect the sulfation reaction of the sorbents in these cases requires further investigations. Practical Implications. The above results reveal the fact that the presence of H2O, CO2, O2, and NOx with SO2 in the flue gas has positive effects on the SO2 removal efficiencies and calcium utilizations for the low-temperature dry and semidry FGD processes which use BFS/HL as the sorbent. Because SO2 and NOx are air pollutants, they have to be removed from the flue gas in order to reduce their concentrations to meet the emission standards. According to the findings of this work, removing SO2 before removing NOx can take the advantage of the enhancement effect of O2/NOx on the sulfation of sorbents. The SO2 removal efficiency can be raised or the amount of sorbent required can be reduced. However, it is ineffective to achieve simultaneous SO2/NOx removal because the amounts of NOx captured by the sorbents are small when both SO2 and NOx are present. In practice, depending on the design of the air pollution control system, the flue gas entering an FGD unit may or may not have been treated with a NOx removal unit. In the former case the sorbent is sulfated with CO2 and O2 present, and in the latter case the sorbent is sulfated with CO2, O2, and NOx present. These two cases are the two cases discussed in preceding sections. Therefore, for the practical application of the kinetic model proposed, eqs 1, 2, and 3 and eqs 1, 4, and 5 are applicable to describe the sulfation of a sorbent in an FGD unit with and without an upstream NOx removal unit, respectively. Conclusion The presence of CO2, O2, and NOx with SO2 in gas phase at their typical concentrations in the flue gas had slight effects on the sulfation of BFS/HL sorbents, if O2 and NOx were not present simultaneously. The reaction kinetics, being affected mainly by the relative humidity, can be described by the surface coverage model and the model equations derived previously for the reaction under the gas mixtures containing SO2, H2O, and N2 only. When O2 and NOx were present simultaneously, the sulfation of sorbents was greatly enhanced and a great amount

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of sulfate was formed in addition to sulfite. The surface coverage model is still valid in this case, but the model equations obtained show a more marked effect of relative humidity and negligible effects of SO2 concentration and temperature. In either case, the effect of sorbent composition on the reaction kinetics was entirely represented by the effects of the initial specific surface area (Sg0) and the Ca molar content (M-1) of sorbent. The initial conversion rate of sorbent increased linearly with increasing Sg0, and the ultimate conversion increased linearly with increasing Sg0M-1. The enhancement effect of the presence of O2/ NOx is due to the rise in the concentration of NO2 in the gas mixture. Higher concentration of NO2 enhances the oxidation of HSO3- and SO32- to form SO42- in the water layer on the solid surface as well as the formation of calcium nitrite and nitrate that deliquesce and collect water at high relative humidity. The presence of H2O, CO2, O2, and NOx in the flue gas has positive effects on the SO2 capture in the lowtemperature dry and semidry FGD processes which use BFS/ HL as the sorbent. The model equations obtained in this work are applicable to describe the kinetics of the sulfation of BFS/ HL sorbents in the FGD processes either with an upstream NOx removal unit or without. Acknowledgment This research was supported by the National Science Council, Taiwan. Abbreviations BFS ) iron blast furnace slag HBFS ) high fineness iron blast furnace slag HL ) hydrated lime k1 ) parameter defined by eq 1, min-1 k2 ) parameter defined by eq 1 M ) initial weight of sorbent per mole of Ca, g/(mol of Ca) R ) gas constant, 8.314 (J/mol)/K RH ) relative humidity, % Sg0 ) initial specific surface area, m2/g T ) reaction temperature, K X ) conversion XS1 ) mole fraction of calcium sulfite XS2 ) mole fraction of calcium sulfate y ) SO2 concentration, ppm

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ReceiVed for reView March 8, 2009 ReVised manuscript receiVed June 24, 2009 Accepted July 14, 2009 IE900379C