Novel Preparation Method of Macroporous Lime from Limestone for

In order to develop a highly active calcium oxide high-temperature desulfurization sorbent, macroporous calcium oxides were directly prepared from lim...
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Ind. Eng. Chem. Res. 1997, 36, 3639-3646

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Novel Preparation Method of Macroporous Lime from Limestone for High-Temperature Desulfurization Eiji Sasaoka*,† Faculty of Health and Welfare Science, Okayama Prefectural University, Kuboki-111, Soja, Okayama 719-11, Japan

Md. Azhar Uddin Faculty of Engineering, Okayama University, Tsushima-naka, Okayama 700, Japan

Shigeru Nojima Mitsubisi Heavy Industries, Ltd., Kan-on-shin-machi, Nishi-ku, Hiroshima 733, Japan

In order to develop a highly active calcium oxide high-temperature desulfurization sorbent, macroporous calcium oxides were directly prepared from limestone. This method is composed of two steps: swelling of the limestone in the gas phase followed by drying and calcination of the swelled samples. The swelling was found when limestone was exposed to a vapor of aqueous acetic acid. The swelling of the sample resulted from an increase of calcium acetate formation in the sample. It was then converted to macroporous calcium oxides by heating the sample to 850 °C. The reactivity of the macroporous calcium oxide for the removal of SO2 or H2S under coexisting H2O vapor was higher than that of the calcined raw limestone. In particular, its SO2 removal capacity and oxidative character of CaS to CaSO4 and CaO were greatly improved by the swelling method. Introduction Limestone is a very important material as a hightemperature desulfurization sorbent: limestone is used for in-bed SO2 capture in fluidized bed combustors of coal and can be used in coal gasifiers for the in-bed removal of H2S. In coal combustors, limestone usually decomposes into CaO and CO2 and then reacts with SO2. The reactions are expressed as follows:

CaCO3 S CaO + CO2

(1)

CaO + 1/2O2 + SO2 S CaSO4

(2)

In gasifiers of coal, decomposition of limestone into CaO and CO2 depends on the concentration and/or pressure of CO2 and the temperature (Fenouil and Lynn, 1995; Yrjas et al., 1996; Khinast et al., 1996). Therefore, two sulfidation reactions have to be considered:

CaO + H2S S CaS + H2O

(3)

CaCO3 + H2S S CaS + H2O + CO2

(4)

Furthermore, the product CaS has to be converted to CaSO4 before disposal because H2S is released from the reaction between CaS and water (Ninomiya et al., 1995). The conversion of CaS to CaSO4 is expressed as follows:

CaS + 2O2 f CaSO4

(5)

If any of the gas-solid reactions of eqs 1-5 occur, the internal structure of the lime particle (solid) changes because a solid product is produced. As the molecular * Author to whom correspondence should be addressed. † Present address: Department of Environmental Chemistry and Materials, Okayama University, 2-1-1, Tsushima, Okayama 700, Japan. Telephone: 086-251-8442. Fax: 086-251-8442. S0888-5885(97)00134-6 CCC: $14.00

volume of CaCO3, CaO, CaS, and CaSO4 are 36.9, 16.9, 28.9, and 46.0 cm3/mol, respectively (Zevenhoven et al., 1996; Yrjas et al., 1996), it is apparent that the intraparticle pores of lime may become plugged by the product of reactions 2, 3, and 5, even if the reactant solid particle is highly porous material. Concerning the reaction of eq 2, Hartman and Coughlin (1974) and Zevenhoven et al. (1995) indicated the occurrence of pore plugging due to the molar volume change of CaO to CaSO4 (molecular volume ratio CaSO4/CaO, ca. 2.7/1); as a resolution of the plugging, Naruse et al. (1995) proposed the use of macroporous shells although the percentage conversion of CaO to CaSO4 was still under 50%. In the case of eq 3, the molar volume change accompanying the conversion of CaO to CaS is relatively small compared with that in eq 2 (molecular volume ratio CaS/CaO, ca. 1.7/1). Thus, the problem of pore plugging with sulfidation has not been reported. Efthimiadis and Sotirchos (1992) reported that the reaction rate was affected by the connectivity of the intraparticle pores; Fenouil and Lynn (1995) reported that the reaction kinetics is controlled by the diffusion of H2S through the pores of the CaS product layer around the lime particle; Yrjas et al. (1996) supposed that the high conversion of lime to CaS induced by the calcination of limestone resulted in an increased particle porosity. In the case of eq 4, the molar volume change accompanying the reaction (molecular volume ratio CaS/ CaCO3, ca. 0.8/1) is the smallest and is favorable to the reaction. However, the porosity of limestone is usually very low, and this has a low reactivity compared with that of the calcined limestones (Yrjas et al. 1996). The molecular volume ratio of CaS to CaSO4 in eq 5 is ca. 1.6/1. This large volume change may induce the plugging of the intraparticle pore of CaS if the pore size is not sufficiently large. If a layer of CaSO4 covers the inside of the CaS particle, the CaS is not converted to CaSO4 until the CaSO4 decomposes into CaO, SO2, © 1997 American Chemical Society

3640 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997

Figure 1. Schematic presentation of the granulation method and the swelling method. Table 1. Chemical Corporation of Limestones components (wt %) limestone

CaO

MgO

SiO2

Al2O3

Fe2O3

Ig. loss

Okayama Chubu

54.7 55.1

0.23 0.75

0.05 0.02

0.02 0.02

0.03 0.03

44.3 43.6

and O2 at a high temperature (Torres-Ordonez et al., 1989). The purpose of this work is to develop a preparation method for a highly active macroporous CaO from limestone and thus prevent pore plugging. Macroporous materials are usually prepared by granulation using fine particles as shown schematically in Figure 1 (Miura et al., 1995): the space among the particles act as macropores. In this work, coarse limestone particles were swelled as shown in Figure 1. Acetic acid was selected as the swelling reagent, and the vapor of the aqueous solution was used in this study. The swelled sample was dried and calcined to CaO. In practical application, the conversion should be done in-bed via a combustor or a gasifier. In this paper, the swelling of limestone and the pore structure of the macroporous lime thus produced are reported. The reactivity of the representative lime produced by the swelling method is also reported. Experimental Section Swelling and Calcination Procedure. Table 1 shows the properties of limestones used in this study. The two limestones originated from different areas of Japan (Okayama and Chubu). Approximately 2 cm3 of the sample limestone particles (0.7 mm in diameter) were placed in a glass sample tube (i.d. ) 27 mm) which was placed unplugged in a cylindrical polystyrene sample tube (i.d. ) 60 mm; volume ) 200 cm3). An acetic acid aqueous solution, ca. 30 cm3, was put into the polystyrene sample tube, and this tube was plugged. This polystyrene sample tube was placed in an incubator at a controlled setting temperature. The limestone particle was exposed to the vapor of acetic-water and stirred occasionally. The sample volume was measured by a tapping method using a measuring cylinder. The fractional swelling of the sample was calculated according to following equation:

fractional swelling of sample ) volume of swelled sample (6) volume of nonswelled sample The swelled samples were dried (60 °C, 4 h + 110 °C, 21 h) and then calcined in a muffle oven under an air atmosphere. The calcination temperature of 850 °C was

reached after about 36 min of heating and was maintained for 1 h. The samples were then crushed and sieved to an average diameter of 0.7 mm. The bulk density of the samples was measured and compared with that of the nonswelled sample. Characterization of the Sample. The thermal decomposition characteristics (under He flow) of the swelled samples were examined using temperature programmed decomposition (TPD) apparatus equipped with a quadrupole mass spectrometer. The pore volume and pore size distribution of the calcined samples were measured using a mercury penetration porosimeter. The pore of the calcined samples were observed directly using a field emission scanning electron microscope (FESEM). The reactivities of the macroporous CaO prepared by the swelling method were measured using representative samples. For the reactivity of the sample with SO2, a flow-type packed-bed tubular reactor system was used under atmospheric pressure at a constant temperature of 800 °C. The microreactor consisted of a quartz tube, 1.5 cm i.d., in which 0.2 g of a sorbent was packed. In these sulfation experiments, a mixture of SO2 (1500 ppm), O2 (3%), CO2 (10%), H2O (9.2%), and the rest of He was fed into the reactor at 500 cm3/min at STP. SO2 concentration of inlet and outlet gases were measured using a wet absorption method {Arusenazo III method (JIS K103) (Sasaoka et al., 1994)}. The reactivity of a sample with H2S and oxidation of the CaS formed were examined using a flow-type thermogravimetric apparatus (Sasaoka et al., 1995). About 25 mg of the sample particles was placed in a platinum net sample holder. The sample in the reactor could be directly observed from the outside (from the upper site) during the reaction. The sulfation experiments were done at 800 °C under atmospheric pressure and a gas flow of 500 cm3/min at STP. The inlet gas composition during sulfidation was 1500 ppm of SO2, 9.2% H2O, and the remainder of N2. In the oxidation experiments, the inlet gas was 1% O2, 9.2% H2O, and the remainder of N2. The concentration of O2 was determined from the results of primary experiments. In those experiments, if the O2 concentration was set at 10%, the sample shined brightly for a short time after the injection of the O2-containing gas, and the temperature monitor responded a little (the change was not followed by the monitor because it was too quick). This brightening was thought to be caused by the rapid exothermic oxidation of CaS. To prevent an overheating of the sample, the concentration of O2 was decreased from 10% to 1%. The amount of SO2 evolved from the sample during the oxidation was measured using the wet absorption method, as in the case of the sulfation of CaO. Results and Discussion Swelling Rate of Limestone. The swelling rates of the two limestones were examined by exposing them to aqueous acetic acid vapors at different concentrations at 18 °C. As shown in Figures 2 and 3, the swelling rate reached a maximum value at around 50 vol % CH3COOH for both samples. The two samples were also exposed to a vapor of ca. 100% CH3COOH (reagent grade: 97% and up), but neither expanded. Thus, it was concluded that water vapor is indispensable for the swelling of the limestones. The temperature dependency of the the limestone’s swelling rate was examined using 50 and 80 vol % CH3COOH aqueous solutions. The swelling rate increased

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3641

Figure 2. Effect of concentration of CH3COOHaq on the swelling of the limestone (Okayama) at 18 °C.

Figure 3. Effect of concentration of CH3COOHaq on the swelling of the limestone (Chubu) at 18 °C.

Figure 4. Effect of exposure temperature of the sample (Okayama) on the swelling using 50 vol % CH3COOHaq.

at higher temperatures as shown in Figures 4 and 5. The data for 80 vol % CH3COOHaq is omitted, but the results showed the same dependency as that in the case of 50 vol % CH3COOHaq, although the rate was smaller even at 40 °C. Using the limestone (Okayama), the limit of the swelling was examined in the vapors of 50 and 80 vol

Figure 5. Effect of exposure temperature of the sample (Chubu) on the swelling using 50 vol % CH3COOHaq.

% CH3COOHaq at 40 °C. After ca. 1 month of exposure at 40 °C, the swelling rate of the two samples reached ca. zero. The fractional swelling is ca. 9.2 for the 50 vol % and ca. 7.4 for 80 vol %. Characterization of the Swelled Limestone by TPD. As it is well-known that calcium acetate Ca(CH3COO)2 is produced from the reaction of Ca(OH)2 and CH3COOH, it can be supposed that Ca(CH3COO)2 is formed as a result of the exposure of CaCO3 to the vapor of CH3COOH and H2O. Figure 6 shows the TPD spectra of the reagent Ca(CH3COO)2‚H2O, the limestone (Okayama) CaCO3, and a partially swelled sample of the limestone. Figures 7 and 8 show the TPD spectra of the partially and fully swelled sample under different concentration solutions at 40 °C. From the reagent Ca(CH3COO)2, CH3COCH3 and CO2 were evolved according to the following reactions:

Ca(CH3COO)2 f CH3COCH3 + CaCO3

(7)

CaCO3 f CaO + CO2

(1)

The peak temperature of CO2 evolved from Ca(CH3COO)2 was lower than that of the limestone. The cause of the shift is unknown, but it may be supposed that the CaCO3 formed from the decomposition of Ca(CH3COO)2 was more unstable than that of the limestone. From both the partially and fully swelled samples CH3COOH was evolved at low temperature in addition to CH3COCH3 and CO2. This CH3COOH may be due to adsorbed CH3COOH on the samples, and its amount increased with the CH3COOH concentration of the solution and/or increased the fractional swelling of the sample. From a comparison of Figures 8 and 6, the fully swelled sample seemed to be almost completely converted from CaCO3 to Ca(CH3COO)2. From the results of Figures 6, 7, and 8, the simplest detailed mechanism necessary for a rational understanding of the swelling process may be given as follows: (a) H2O and CH3COOH physically coadsorb on the exposed surface of the sample particles and may condense. The desorption of CH3COOH at a low-temperature range in Figures 6, 7, and 8 indicate physical adsorption. The desorption of H2O was also observed in the same temperature range as that of acetic acid in TPD spectra (not shown).

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Figure 6. Temperature-programmed decomposition of the limestone (Okayama), the partially swelled sample, and reagent Ca(CH3COO)2‚H2O. The partially swelled sample was prepared using 80 vol % CH3COOHaq at 40 °C and its fractional swelling (f) was 2.2.

(b) In the liquid phase of adsorbed H2O, ionization of CH3COOH, which is a relatively fast process can be described by the following equation:

CH3COOH f H+ + CH3COO-

(8)

(c) H+ ions attack the particles of the calcareous material in the sample:

nH+ + CaCO3 f H2CO3 + (n - 2)H+ + Ca2+ (9) The H+ ions participating in this reaction may have originated from acetic acid as well as from the carbonic acid in the system. (d) If carbonic acid is formed via eq 9, the ionization, which is a fast process, may be described by the following reactions:

H2CO3 f H+ + HCO3-

(10)

HCO3- + H+ S 2H+ + CO32-

(11)

2H+ + CO32- f CO2 + H2O

(12)

(e) Ca2+ may react with CH3COO-:

Figure 7. Temperature-programmed decomposition of the partially swelled limestone (Okayama). The partially swelled sample was prepared using 50 vol % CH3COOHaq and 80 vol % CH3COOHaq at 40 °C. Fractional swelling, f.

Ca2+ + 2CH3COO- f Ca(CH3COO)2

(13)

The formation of calcium acetate will depend on various parameters, such as the concentration of the acid, reaction time, temperature, nature, and grain size of the raw sample. Therefore, the details of the reaction mechanism need much more study. The data from the TPD of the swelled limestone (Chubu) are not shown, but almost the same results as that of the swelled limestone (Okayama) were obtained. Drying and Calcination of the Swelled Limestone. The volume of the swelled samples after the calcination changed, and the particles partially connected to each other. The placement of the sample particles in a ceramic dish for the calcination was neither isolated nor monolayered, so the particles connected to each other by sintering. The bulk density of the 0.7 mm calcined samples was measured. The values of the bulk density of the samples prepared using the 50 and 80 vol % CH3COOHaq are shown in Figure 9 in comparison with their fractional swelling. In Figure 9, a solid line and a broken line show the theoretical relationship between the bulk density and fractional swelling, assuming that the swelled sample was unaltered by the dry calcination: the lines were calculated using the value of the bulk density of the nonswelled sample, that is, if the value of the bulk density of the sample, in which fractional swelling is 2, is calculated

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3643

Figure 10. Integral pore volume of the swelled-calcined sample (Okayama). The samples were swelled using 50 vol % CH3COOHaq at 40 °C and calcined at 850 °C. ( ), fractional swelling of sample; [ ], ratio of bulk density ()nonswelled-calcined sample/swelledcalcined sample).

Figure 8. Temperature-programmed decomposition of the fully swelled limestone (Okayama). The fully swelled sample was prepared using 50 vol % CH3COOHaq and 80 vol % CH3COOHaq at 40 °C. Fractional swelling, f.

Figure 11. Pore size distribution of the swelled-calcined sample (Okayama). The samples were swelled using 50 vol % CH3COOHaq at 40 °C and calcined at 850 °C. ( ), fractional swelling of sample; [ ], ratio of bulk density.

Figure 9. Relationship between the fractional swelling of the sample after the swelling and the bulk density of the sample after the calcination. Lines are theoretical value: s, Okayama; - - -, Chubu.

to 1/2 of the value of that of the nonswelled and calcined sample. The positions of the data of the samples which were exposed to the 50 vol % CH3COOHaq lie on the right side of the lines: this means that the samples shrank during the drying-calcination. The positions of the samples exposed to the 80 vol % CH3COOHaq were

on or near the lines: the volume of these samples were not greatly changed by the drying-calcination. This effect of the concentration of the solutions might be induced by the formation of hydrated Ca(CH3COO)2 in the presence of a high concentration H2O vapor; however, there is no other evidence for this. Pore Size Distribution of Lime Produced from the Swelled Sample. The pore size distributions were examined using calcined samples which were prepared using 50 and 80 vol % CH3COOHaq. Figure 10 shows the integral pore volume (Hg intrusion volume) of the samples (Okayama) prepared using the 50 vol % CH3COOHaq: the volume of the samples prepared by the swelling method increased with the increase of the fractional swelling but the change of pore size is not clear in this expression. Figure 11 shows the differential pore volume of the samples shown in Figure 10: the pore size distribution of the nonswelled sample observed almost agree with that previously reported (for example, Efthimiadis and Sotirchos, 1992; Naruse et al., 1995); the size of macropore formed by the swelling

3644 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997

Figure 12. Pore size distribution of the swelled-calcined sample (Okayama). The samples were swelled using 80 vol % CH3COOHaq at 40 °C and calcined at 850 °C. ( ), fractional swelling of sample; [ ], ratio of bulk density.

Figure 13. Pore size distribution of the swelled-calcined sample (Chubu). The samples were swelled using 80 vol % CH3COOHaq at 40 °C and calcined at 850 °C. ( ), fractional swelling of sample; [ ], ratio of bulk density.

method is distributed into two classes: one of them ranging from ca. 0.2 µm to ca. 4 µm and another class larger than this range. Pore of the larger class may be affected by the spaces among the sample particles (Naruse et al., 1995). Figures 12 and 13 show the pore size distributions of the samples prepared using the 80 vol % CH3COOHaq: the pore size distributions of the samples prepared from the two kinds of limestone were almost the same as each other and the data of the limestone (Okayama) were similar to that of the sample prepared using the 50 vol % CH3COOHaq. The values of the fractional swelling of the samples and the ratio of the bulk density of the nonswelled-calcined sample to that of the swelledcalcined sample are shown in Figures 10, 11, 12, and

13. The pore size distribution seemed to be more affected by the bulk density than by the fractional swelling. The pores formed by the swelling method were directly confirmed by SEM using the swelled sample (Okayama) and compared with that of the nonswelled sample. Figures 14 and 15 show the photographs: the pores in the nonswelled sample can be observed in the higher magnification photograph (Figure 14B) and its size was almost the same as that measured with the porosimeter. The smaller size macropores could be observed clearly in the swelled sample (Figure 15A); the pores which were observed in the nonswelled sample were also observed in the swelled sample but they were not clear (Figure 15B). Naruse et al. (1995) reported that pores with average diameter between 1 and 1.6 µm are responsible for the high SO2 capture capacity of calcined shells. Hartman and Coughlin (1974) also reported that pores with diameters larger than 0.796 µm were responsible for the high capacity of calcined limestone. This pore size range curiously coincided with that of the smaller class macropore produced by the swelling in this study. Therefore, it was expected that the capacity of SO2 capture of the limestone was improved by the swelling method. Reactivity of the Macroporous Lime Prepared. The nonswelled sample (Okayama) and a representative sample were used for the reactivity test in this study. The representative sample was prepared using 50 vol % CH3COOHaq at 40 °C (fractional swelling, 9.2; bulk density ratio, 6.2). The reactivity of the representative sample for the removal of SO2 is shown in Table 2 in comparison with that of the nonswelled sample: the fractional conversion of the sample was increased from 0.33 to 0.81 by the swelling. The average SO2 concentration in the experiment using the swelled sample or the nonswelled sample could be calculated from the total amount of the breakthrough SO2 during the experiment. The values for the swelled sample and the nonswelled sample were ca. 465 and 1080 ppm, respectively. As the average SO2 concentration of the gas in contact with the two samples was not the same, an accurate reactivity comparison of the two samples was difficult. However, it could be concluded that the swelling method is highly effective for improving the reactivity of the limestone. The reactivity test for H2S and the oxidative character test of formed CaS were serially done at 800 °C. Figure 16 shows the results of the representative, in comparison with that of the nonswelled sample. The reaction rate of the swelled sample was considerably improved by the swelling and the final conversion, which was measured during the 2 h reaction, also increased from 0.81 to 0.94. The oxidation character was drastically changed: the weight gain of the swelled sample by the oxidation was ca. 2.2 times larger than that of the nonswelled sample. In this oxidation, a considerable amount of SO2 evolution was measured. The evolution of SO2 was explained by the following reaction:

CaS + 3/2O2 f CaO + SO2

(14)

The sigmoid curve of the weight gain in Figure 16 may be explained by the reaction of eq 10: if the reactions of eq 10 and eq 5 simultaneously occur, the weight loss by the reaction of eq 10 affects the weight gain of the reaction of eq 5. Miura et al. (1996) reported the SO2

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3645

Figure 14. Electron micrographs of the nonswelled-calcined sample (Okayama). (A) ×5000; (B) ×20 000.

Figure 15. Electron micrographs of the swelled-calcined sample (Okayama). The sample was fully swelled using 50 vol % CH3COOHaq at 40 °C. (A) ×5000; (B) ×20 000. Table 2. Reactivity of Nonswelled and Swelled Sample (Okayama) with SO2

sample

fractional conversion of CaO to CaSO4

average fractional removal of SO2 during reaction for 2 h

nonswelled sample swelled samplea

0.33 0.81

0.28 0.69

a The sample was exposed to 50% CH COOH 3 aq at 40 °C; the fractional swelling was 9.2. The calcination temperature of the two samples was 850 °C.

evolution just after the injection of air into the reactor, in which the CaS sample was set, at 900 °C. From the weight gain and the amount of evolved SO2, the composition of the oxidized sample could be calculated: from the amount of the evolved SO2, the amount of the formed CaO and the weight loss due to the reaction of eq 10 can be calculated. The weight gain of CaS to CaSO4 is equal to the total of the experimental weight gain and the amount of the weight loss due to the conversion of CaS to CaO; the rest of the formed CaO and the formed CaSO4 is CaS. Figure 17 shows the composition of the samples after the oxidation. In this figure, the composition was calculated on the basis of the CaS formed from the calcined sample during the sulfidation: the unreacted part (CaO) of the sample was not counted in the calculation of the composition. The oxidation of the CaS was accelerated by swelling: the

Figure 16. Weight gain of the fully swelled-calcined and the nonswelled-calcined sample during the sulfurization-oxidation. The fully swelled-calcined sample was prepared using 50 vol % CH3COOHaq at 40 °C.

ratio of the remained CaS was drastically decreased by the swelling. It could be supposed that this difference in the two samples was caused by the different character of the pore structure: the CaS formed from the macroporous CaO was easily converted to CaSO4 and CaO, because the formed CaSO4 could not plug the large

3646 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997

acetate decomposed into CaCO3 and CH3COCH3, and then the produced CaCO3 decomposed into CaO and CO2 by heating. In-bed decomposition of the calcium acetate is a significant problem for saving energy in CaO preparation; therefore study of the decomposition in combustion gas and/or coal-derived fuel gas is needed for the practical application of the calcium acetate. Literature Cited

Figure 17. Composition of the sample after the oxidation. The samples were sulfurized and then oxidized as shown in Figure 16. The composition was calculated on the basis of the CaS formed from the calcined samples (Okayama). The fully swelled-calcined sample was prepared using 50 vol % CH3COOHaq at 40 °C.

pores and O2 easily diffused into the inside of the sample particle through the pores. The SO2 formed inside the particle may react with the formed CaO via the reaction of eq 2, if O2 is fed. Therefore, the actual amount of the SO2 formed via eq 10 could not be evaluated. The molar ratio of formed SO2 and CaSO4 ()CaO/CaSO4) of the nonswelled and swelled sample was ca. 0.50 and 0.55, respectively. If the selectivity of the formation of CaSO4 via eq 5 over the two samples is the same and the SO2 formed in the large size pore exits the pore more easily than the SO2 formed in the small size pore, the molar ratio for the swelled sample should be larger than that of the nonswelled sample. The experimental results accorded with this assumption, but the difference between the two value was too small to confirm. The clarification of the mechanism of the oxidation of CaS needs a more detailed study. Conclusion In this study, it was found that limestone could be swelled by exposure to the vapor of aqueous acetic acid solution and that the swelled limestone converted to macroporous lime upon calcination. Furthermore, it was confirmed that the reactivity of the limestone for the removal of SO2 or H2S under coexisting H2O vapor was improved by the swelling method. In particular, its SO2 removal capacity and the oxidative character of CaS to CaSO4 and CaO were greatly improved. From the TPD studies, it was found that the swelling of limestone was induced by the formation of calcium acetate from the reaction between limestone CaCO3 with the vapor of acetic acid and H2O. The calcium

Efthimiadis, E. A.; Sotirchos, S. V. Sulfidation of LimestoneDerived Calcines. Ind. Eng. Chem. Res. 1992, 31, 2311-2321. Fenouil, L. A.; Lynn, S. Study of Calcium-Based Sorbent for HighTemperature H2S Removal. 2. Kinetics of H2S Sorption by Calcined Limestone. Ind. Eng. Chem. Res. 1995, 34, 2334-2342. Hartman, M.; Coughlin, R. W. Reaction of Sulfur Dioxide with Limestone and Influence of Pore Structure. Ind. Eng. Chem., Process Des. Develop. 1974, 13, 248-253. Khinast, J.; Krammer, G. F.; Brunner, Ch.; Staudinger, G. Decomposition of Limestone: The Influence of CO2 and Particle Size on The Reaction Rate. Chem. Eng. Sci. 1996, 51, 623634. Miura, K.; Mae, K.; Inatomi, J. Study of Macroporous CaO Particles for High-Temperature H2S Removal. Preprint of the 61st Annual Meeting of the Society of Chemical Engineering, Japan, 1995; M204, p 167. Naruse, I.; Nishimura, K.; Otake, K. Characteristics of Desulfurization Reaction by Shells. Kagaku Kogaku Ronbunshu 1995, 21, 904-908. Ninomiya,Y.; Sato, A.; Watkinson, A. P. Oxidation of Calcium Sulfide in Fluidized Bed Combustion/Regeneration Conditions. 13th Int. Conf. Fluid. Bed Combust. 1995, 1027-1033. Sasaoka, E.; Tanaka, K.; Inami, Y.; Sakata, Y.; Kasaoka, S. Development of Catalyst for Simultaneous Oxidative Adsorption of SO2 and NO. Kagaku Kogaku Ronbunshu 1994, 20, 880888. Sasaoka, E.; Iwamoto, Y.; Hirano, S.; Uddin. M. A.; Sakata, Y. Soot Formation over Zinc Ferrite High-Temperature Desulfurization Sorbent. Energy Fuels 1995, 9, 344-353. Torres-Ordonez, R. J.; Wall, F. T.; Longwell, P. J.; Sarofim, A. F. Sulfur Retention as CaS(s) during Coal Combustion: a Modelling Study to Define Mechanisms and Possible Technologies. Fuel 1993, 72, 633-643. Yrjas, K. P.; Cornelis, A. P.; Hupa, M. M. Hydrogen Sulfide Capture by Limestone and Dolomite at Elevated Pressure. 1. Sorbent Performance. Ind. Eng. Chem. Res. 1996, 35, 176-183. Zevenhoven, R.; Yrjas, P.; Hupa, M. How Does Sorbent Particle Structure Influence Sulfur Capture under PFBC Conditions? 13th Int. Conf. Fluid. Bed Combust. 1995, 1381-1392. Zevenhoven, C. A. P.; Yrjas, K. P.; Hupa, M. M. Hydrogen Sulfide Capture by Limestone and Dolomite at Elevated Pressure. 2. Sorbent Particle Conversion Modeling. Ind. Eng. Chem. Res. 1996, 35, 943-949.

Received for review February 6, 1997 Revised manuscript received April 28, 1997 Accepted May 2, 1997X IE970134U

X Abstract published in Advance ACS Abstracts, July 1, 1997.