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Preparation and Characterization of Lime-Silica Solids Gi-Hune Jung,† Hotae Kim,‡ and Sun-Geon Kim*,‡ Department of Chemical Engineering, Chung Ang University, 221 Huksuk-Dong, Dongjak-Ku, Seoul 156-756, Korea, and Polymer Technology Research Center, Korea Engineering Plastics Co. Ltd., 183-2 Hoge-2Dong, Dongan-Ku, Anyang-Si, Kyungki-Do 431-082, Korea
Silica-enhanced lime sorbents for flue gas desulfurization have been prepared by pozzolanic hydration of Ca(OH)2 with excess SiO2. As the hydration proceeded, new particles grew in clearcut features with regular micropores, consisting of chainlike aggregates. The increase in the specific surface area of the product particles with hydration was closely related to the conversion to calcium silicate. Even after the completion of the pozzolanic reaction, self-hydration of excess SiO2 continued to make the particles with more clear-cut features but prevent the BrunauerEmmett-Teller (BET) surface area from further increasing. The mole ratio of Ca/Si in the calcium silicate prepared was independent of its initial mole ratio and converged to 0.9-1.5, depending on the hydration temperature. Pressure hydration at temperatures above 100 °C dramatically accelerated the reaction, resulting in a less compact structure composed of smaller primary particles with the ultimate BET surface area unchanged. Introduction The Ca(OH)2 sorbent, one of the calcium-based sorbents, has been widely used for flue gas desulfurization (FGD). However, dry desulfurization with Ca(OH)2 ended up with a significant amount of Ca(OH)2 unreacted (60-90%) inside the sorbent particles by pore plugging.1,2 This occurred because of the volume expansion in the pore mouth caused by the conversion of Ca(OH)2 to CaSO3. Higher and more effective utilization of the sorbents would be necessary to meet both the source reduction and strict SO2 control levels (at least 50% removal efficiency of SO2). To improve the utilization of Ca(OH)2, Jozewicz and other researchers have introduced the silica-enhanced lime sorbent prepared from the hydration of Ca(OH)2 with the siliceous materials.1,3,4 It was proposed that the enhancement of its utilization came from the pozzolanic reaction between siliceous materials and Ca(OH)2 to form various types of calcium silicate hydrates, (CaO)x(SiO2)y(H2O)z, which have poor crystallinity and high surface area.5-7 Fly ash, containing 28-52 wt % SiO2 and 15-34 wt % Al2O3, is an example of a man-made pozzolan. Natural pozzolans include diatomaceous earth, which contains about 87 wt % SiO2 and is more reactive than fly ash, and some volcanic ashes.3,7 The mechanism of pozzolanic reaction between Ca(OH)2 and SiO2 in aqueous media starts with the dissolution of two reagents. The dissolution of SiO2 in water, the rate-limiting step of the pozzolanic reaction, yields Si(OH)4.8 The nuclei of calcium silicate are formed by the chemical reaction among dissolved Si(OH)4 monomer and Ca2+ and OH- ions near undissolved SiO2. The Ca/Si mole ratio in calcium silicate varies from 0.8 to 2.0 by the conditions of the reaction.9 Though high surface area has been known to be the main source for the enhancement of sorbent utilization, there have been few publications to investigate how the * To whom correspondence should be addressed. Phone: 812-820-5272. Fax: 81-2-824-3495. E-mail:
[email protected]. † Korea Engineering Plastics Co., Ltd. ‡ Chung Ang University.
area is generated and to quantitatively correlate the area and other properties to sorbent utilization. We prepared the silica-enhanced lime particles under various conditions by atmospheric and pressure hydrations and characterized their physical and chemical properties. The reactants were Ca(OH)2, SiO2, and water, and the main variables for preparation of the sorbents were the time, temperature, and/or pressure of hydration and the initial mole ratio of the reactants. Properties to be characterized include the morphology and specific surface area of the sorbents, the conversion of mixed solid reactants to silicate, and the Si/Ca ratio in the silicate produced. The correlation of the sorbent properties with the sorbent utilization for desulfurization is under study. Experimental Section Sorbent Preparation. The sorbents were prepared by atmospheric or pressure hydration of Ca(OH)2 [Yakuri Pure Chemical, Japan, 7.07 m2/g Brunauer-EmmettTeller (BET) surface area] with quartz SiO2 (SigmaAldrich Co., S-5631, 5.91 m2/g BET surface area) in the presence of water. The mass ratio of water to solid reactants (SiO2 + Ca(OH)2) in slurry was fixed at 20.10,11 The total amount of slurry was also kept constant at 250 g for all cases. The reference value in the initial mole ratio of SiO2/ Ca(OH)2 was chosen as 1.5 by considering the moderate reaction rate and waste reduction. Variation in the ratio was achieved by changing the mole ratio of the two solid reactants with their total mass fixed. All of the slurry thus consisted of 12 g of the solid reactants and 238 g of water. For the initial mole ratio of 1.5, the solid reactant in the slurry would consist of 6.54 g of SiO2 and 5.37 g of Ca(OH)2. The reference hydration temperatures were 90 and 150 °C, for atmospheric and pressure hydration, respectively. The latter was the boiling temperature of water at 4 bar. The atmospheric hydration was performed in a 500cm3 flask with a reflux condenser in a thermostated water bath. The slurry was stirred vigorously throughout the entire hydration period which varied from 1 to
10.1021/ie990511y CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000
Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1265 Table 1. Process Variables and Their Values Used in Preparing Sorbentsa process variables water to solid weight ratio SiO2/Ca(OH)2 mole ratio hydration temperature (°C) atmospheric pressure hydration time (h) a
values 20 (fixed) 1 1.5 3
6
40 60 90 120 150 1 ∼ 48 ∼
86
The bold letters are the values under the reference condition.
86 h, while the reference hydration time was selected as 48 h. The pressure hydration was performed in a stainless steel autoclave for the mixture with an initial SiO2/Ca(OH)2 fixed mole ratio of 1.5. The temperatures for pressure hydration were 120 and 150 °C, with the boiling temperatures corresponding to 200 and 480 kPa, respectively. The process variables used are shown in Table 1. After completion of the hydration, the slurry was dried by two steps: It was first vacuum-dried for 1 day at a temperature of 35 °C, because in the preexperiment the pozzolanic reactivity was found to be sufficiently low below 40 °C. After milling of the partially cracked cake from vacuum drying, the powder was further dried in the oven at 85 °C for 2-3 days, for complete removal of the remaining moisture. Characterization. The specific surface area of the sorbent was measured by a BET sorptometer (model: PMI automated BET meter, PMI) using nitrogen as the adsorbate. The multipoint BET method was used instead of the single-point method to increase the accuracy of the measurement. Prior to each measurement, the sample was outgassed at a temperature of 85 °C for 30 min. The crystalline phases were detected with an X-ray diffractometer (XRD; model XDS 2000, Scintag). It is believed that the product particles consist of calcium silicate hydrate in a stoichiomeric ratio of Ca to Si and the excess SiO2 in a hydrated form. The conversion to silicate was defined as the ratio of the total moles of Ca(OH)2 + SiO2 converted to silicate to the total moles of the initial solid mixture. The Ca/Si mole ratio in the silicate was defined as the ratio of the elements in the silicate phase produced by the hydration. Both the conversion to silicate and the Ca/Si mole ratio in silicate for the sample given were determined by comparing the relative peak intensities of Ca(OH)2 and SiO2, respectively, from its XRD pattern with those in calibration curves. The curves had been prepared by plotting the relative peak intensities corresponding to
each compound in XRD patterns of the simple mixtures of Ca(OH)2 and SiO2 with respect to its mole fraction in them.12 For example, if the mole fractions of unreacted Ca(OH)2 and SiO2 were found to be 0.2 and 0.4, respectively, for the sample started with the initial SiO2/ Ca(OH)2 mole ratio of 1.5, or 0.6 to 0.4, the conversion to silicate and the Ca/Si mole ratio in silicate would be 1.0 - (0.2 + 0.4) ) 0.4 and (0.4-0.2):(0.6-0.4) ) 1.0, respectively. All of the data points on the curve were well within (5% error bound. The X-ray fluorescence spectrometer (XRF; model QuanX, Spectrace) was used to identify the elements in noncrystalline compounds that were not detected with XRD. The surface morphology, the pore structure of the product sorbents, and the shape and size of their primary particles were characterized using a scanning electron microscope (SEM; model S-2700, Hitachi). Some typical single particles were line-scanned with an energy-dispersive spectrometer (EDS; model Voyager, Noran) in the SEM to find out the spatial distribution of Ca and Si elements across the sorbent particle. In addition, a thermogravimetric analyzer (TGA; model TGA-50H, Shimadzu) and a differential thermal analyzer (DTA; model DTA-50, Shimadzu) were used to determine the thermal stability of the sorbent particles up to a temperature of 1000 °C. The particle size was also determined by a laser particle size analyzer (LPA; model LPA-3100, Otsuka). Results and Discussion Observation of the Sorbent Particles Prepared. Figure 1 shows the SEM micrographs of Ca(OH)2 and SiO2 particles used as reactants. Although all of the particles seemed aggregated with each other, the average LPA diameters of Ca(OH)2 and SiO2 were 3.7 and 0.4 µm, respectively. However, the solubilities of SiO2 are not quite affected, because the pH of the reaction mixture remained almost constant despite the variation in the Ca ion concentration8 and the BET areas of SiO2 and Ca(OH)2 at the start were not so different (7.07 and 5.91 m2/g, respectively). Figure 2 shows the growth of the product particles during atmospheric hydration with the initial SiO2/Ca(OH)2 mole ratio of 1.5 at 90 °C. The hydrated particles had significantly different morphologies compared to the two-reactant particles. As the hydration proceeded, the particles had a clearer envelope and became more spherical with a highly regular and microporous structure up to 48 h, while further hydration resulted in extremely clear-cut features with
Figure 1. SEM micrographs of SiO2 and Ca(OH)2 particles: (a) Ca(OH)2 at ×5000; (b) SiO2 at ×5000.
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Figure 2. Variation in morphology of product particles hydrated with a SiO2/Ca(OH)2 mole ratio of 1.5 at 90 °C with respect to hydration times: (a) 24 h at ×5000; (b) 48 h at ×5000; (c) 74 h at ×5000.
pore disappearance. The average particle size converged to about 10 µm. This transformation of the morphology would well enhance the fluidity of the sorbent powder. Figure 3, micrographs with low magnification (×1000), shows more clearly the compactness and even monodispersity of the product particles, which is probably the result of Ostwald ripening.13 Identification of the Sorbent Particles. Figure 4 shows the XRD patterns of the atmospherically hydrated particles at the start of hydration and at 48 h of hydration, respectively. The latter had no new peaks compared to the former (designated as 0 h). In the 48-h sample, the diffraction patterns of Ca(OH)2 (designated as c) all disappeared, while the intensity of SiO2 was significantly reduced. Instead, two mild humps at 2θ
angles of about 12.6° and 29.4°, respectively, appeared in the patterns of the hydrated sorbent. These humps might be indicative of the amorphism of the hydration product (the lower the intensity of the hump, the more amorphous the product).3 Through these XRD results, it was proposed that all of Ca(OH)2 and a stoichiometric amount of SiO2 were combined to form amorphous calcium silicate hydrate. In TG/DT analysis, which are not shown, unbound water was released up to 200 °C with a 6% loss of weight and the water of hydration released up to 600 °C with an additional 9% loss. The silica-enhanced lime sorbent had an excellent thermal stability, compared to pure Ca(OH)2 sorbent decomposing at a temperature of about 350 °C, though a very small exothermic peak was observed at a temperature of about 830 °C. By heat treatment at 250 °C, the BET area of sorbent particles varied merely from 147.1 to 145 m2/g. Furthermore, the XRD patterns of the particles treated thermally at 800 °C remained almost unchanged with respect to the sharpness as well as the position of the peaks. As described previously, the ultimate product consisted of the silicate phase and unreacted SiO2. The Ca/ Si mole ratio in the silicate phase varied with respect to the time of hydration, as shown in Figure 5, for the two products produced at 90 °C with initial SiO2/ Ca(OH)2 mole ratios of 1.5 and 6, respectively. As the hydration proceeded, the Ca/Si mole ratio in the silicate phase converged to 1.4-1.5, independent of the initial mole ratio. The limiting value was, however, approached faster in the case of SiO2/Ca(OH)2 ) 6, because of the enhanced rate of SiO2 dissolution, the limiting step throughout the entire hydration. For the sample prepared at SiO2/Ca(OH)2 ) 1.5, though all of the diffraction patterns of Ca(OH)2 were found to disappear in 24 h of hydration, the Ca/Si mole ratio in the silicate was still 2.7 and kept on decreasing to the limiting value, as shown in the figure. Dissolved SiO2 would be, therefore, supposed to continue participating in the formation of calcium silicate hydrate even in the absence of Ca(OH)2 in the aqueous phase to meet the limiting Ca/Si ratio. Compared to the SEM pictures in Figure 2, the change in particle morphologies from 24 to 48 h would thus reflect the additional SiO2 participation to the final stoichiometry of the product silicate. The conversion to silicate is plotted with respect to the time of hydration, as shown in Figure 6. The figure also includes the plot of BET surface area vs the hydration time, which will be explained in the next section. The conversion to silicate gradually increased up to the hydration time of 48 h, and it converges to 0.64 thereafter. Because, as already mentioned, all of Ca(OH)2 finished participating in the hydration reaction at 24 h, it was clear again that the increase of the conversion from 24 to 48 h would be caused by additional inclusion of SiO2 in calcium silicate hydrate. Specific Surface Area of the Sorbent Produced. In Figure 6, the BET specific surface area of the product particles dramatically increased from 6.4 m2/g (averaged value of those for SiO2 and Ca(OH)2) at the start of hydration to 171.6 m2/g at 74 h of hydration. The figure thus implies that the increase in the BET area is closely related with the conversion of reactants to silicate at least up to 48 h. In other words, the formation of calcium silicate brought a new surface area, supporting the theory that the new particles were generated by the aqueous reaction between Ca(OH)2 and SiO2. To inves-
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Figure 3. SEM micrographs of product particles hydrated with a SiO2/Ca(OH)2 mole ratio of 1.5 at 90 °C for different hydration times: (a) 48 h at ×1000; (b) 74 h at ×1000.
Figure 4. XRD patterns of a SiO2/Ca(OH)2 mixture and hydrated sorbent (SiO2/Ca(OH)2 ) 1.5:1, 90 °C, 48 h).
Figure 5. Ca/Si mole ratio in silicate with respect to hydration time starting with different SiO2/Ca(OH)2 mole ratios and temperatures.
tigate the particle growth after 48 h, line scanning with EDS was performed for isolated 10-µm particles hydrated for 48 and 74 h, respectively, and the results are shown in Figure 7 for the elements Ca and Si present in the two samples. In the case of the 48-h sample, the element Si was found more in the center of the particle, while Ca was evenly distributed throughout the particle. On the other hand, both Ca and Si were uniformly distributed across the particle for the 74-h sample. It was postulated that because of the extremely low solubility of SiO2, compared to Ca(OH)2, the reaction
Figure 6. Conversion to silicate and BET surface area vs hydration time for different temperatures (SiO2/Ca(OH)2 ) 1.5, 90 °C).
would occur in the aqueous phase very close to the surfaces of the SiO2 aggregates and the product silicate would either nucleate or precipitate on their surfaces. As the hydration proceeded, the surface area would increase and the voids in the aggregates become micropores with a more regular outlook, as shown in the previous SEM micrograph. As shown in Figure 5, after 48 h, the Ca/Si mole ratio in the silicate was not varied significantly, which implied that SiO2 dissolved thereafter would participate not in the formation of calcium silicate hydrate but in simple hydration of the remaining SiO2 itself. This would be accomplished by diffusing out SiO2 from the center of the particle, reacting with water in the aqueous phase, and finally precipitating the product on the existing silicate surface and thus sometimes plugging the pores to stop the BET area from increasing, as shown in Figure 6. The average pore diameter monotonically decreased from 22 nm at the BET area of 60 m2/g to 16 nm at that of 170 m2/g. This was further supported by the SEM pictures in Figures 2 and 3. Figure 8 shows the effect of the initial mole ratio of SiO2/Ca(OH)2 on the BET area in atmospheric hydration at 90 °C for different hydration times of 0, 8, and 48 h. When the hydration proceeded for 8 h, the surface area of the product increased proportionally to the initial mole ratio. Probably, at this early stage of hydration, the pozzolanic reaction would increase with the initial moles of SiO2, as described before. On the other hand, for the samples prepared for 48 h, sufficient time for
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Figure 7. EDS line scanning on Ca and Si elements for the sorbent particles hydrated at 90 °C for different hydration times (SiO2/ Ca(OH)2 mole ratio ) 1.5): (a) 48 h (top, Ca; bottom, Si); (b) 74 h (top, Ca; bottom, Si).
Figure 8. Effect of the SiO2/Ca(OH)2 mole ratio on the BET surface area of sorbents hydrated at 90 °C as a parameter of hydration time.
hydration, the increase in the surface area ceased only at the ratio of 1.5, reaching 147.1 m2/g, and the area gradually decreased thereafter. With the initial ratio above 1.5, because the higher mole ratio meant that the amount of SiO2 became larger at the expense of Ca(OH)2, the amount of calcium silicate hydrate produced would be smaller and smaller, resulting in the decrease in the BET area. Figure 9 shows the effect of hydration temperature on the BET surface area with the initial SiO2/Ca(OH)2 mole ratio of 1.5 as a parameter of hydration time. BET
Figure 9. Effect of the hydration temperature on the BET surface area of sorbents hydrated with a SiO2/Ca(OH)2 mole ratio of 1.5.
area increased from ∼10 to ∼150 m2/g as the hydration temperature increased from 40 to 150 °C, where the data at 120 and 150 °C were obtained from the pressure hydration to be described in the next session. From the figure, it was observed that the effect of the temperature on the area was less significant at the temperatures both lower than 60 °C and higher than 150 °C. Specifically at 60 °C, the BET surface area increased from 6.4 m2/g for the product hydrated for 8 h to 20.3 m2/g for that for 48 h, while at 40 °C the BET surface area increased only from 8.8 to 10.5 m2/g. XRD diffraction patterns for the product particles hydrated at 40 °C for
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Figure 10. Morphologies of sorbents prepared at different hydration temperatures: (a) 90 °C, 1 bar, ×5000; (b) 90 °C, 1 bar, ×50 000; (c) 150 °C, 4 bar, ×5000; (d) 150 °C, 4 bar, ×50 000.
8 h were almost the same as those of the starting mixture sample. This resulted from the fact that the rate of the pozzolanic reaction was significantly reduced below 60 °C. In contrast, the hydration would proceed so fast above 150 °C that the increase in the area was insignificant after 8 h of hydration. Pressure Hydration. In general, the pressure hydration, which naturally proceeds at higher temperatures, could promote the solubility and dissolution rate of SiO2, which is the rate-limiting step of the pozzolanic reaction, but slightly demotes the solubility of Ca(OH)2. Jozewicz et al.3,14 reported that the time for sorbent preparation was significantly reduced and the sorbent reactivity toward SO2 was remarkably enhanced by pressure hydration. However, the reactivity was reported3 to decrease with the sorbents prepared at temperatures of above 150 °C. Figure 10 compares the particles prepared at 4 bar (150 °C) with those at 1 bar and 90 °C. The former particles seemed to have larger and more irregular pores than the latter ones, as shown in the picture with lower magnification (×5000). The pictures with higher magnification (×50 000) show that both consist of small primary particles aggregated in chains. The particles hydrated at 150 °C, however, consisted of much longer chains aggregated with much smaller primary particles than those at 90 °C. It was clear that the elevated temperature (and pressure) accelerated the hydration; more nuclei for the hydrated particles were supposed to be generated with smaller size.15 This would enhance the rate of coagulation, thus making longer chain aggregates, with more loose structure. The reason the chain aggregates, instead of random structures being produced, was not clear yet.
Figure 5 shows the Ca/Si mole ratio in silicate with respect to the time of hydration. As the hydration proceeded, the limiting values were obtained as 1.31.4 and 0.9-1.0, at 120 and 150 °C, respectively. Therefore, the stoichiometric ratio decreased with the reaction temperature. The ratio at 150 °C is close to 0.8, reported in the literature9 for elevated temperatures. Figure 6 shows the evolution of both the BET surface area and conversion to silicate with respect to the hydration temperature for pressure-hydrated products prepared at 150 °C. The BET area and conversion increase dramatically within 2 h. Especially, the conversion to silicate of 0.6 was obtained within 2 h by pressure hydration at 150 °C, compared to 48 h at 90 °C. The ultimate conversion to silicate increases from 0.64 at 90 °C to 0.73 at 150 °C. After 5 h, both the conversion and BET area almost saturated simultaneously unlike the hydration below 150 °C. This was probably because the pozzolanic reaction and SiO2 hydration would occur more or less simultaneously at this elevated temperature. As shown in Figure 9, the maximum BET surface area remained at about 170 m2/ g, similar to that prepared at atmospheric hydration at 90 °C. The phenomenon was probably due to the competing effects from smaller primary particles and larger pores in the sorbents prepared at 150 °C. The pressure hydration at 120 °C, not shown here, resulted in the intermediate behavior between the atmospheric hydration at 90 °C and the pressure hydration at 150 °C. The ultimate conversion to silicate was 0.68, while the conversion of 0.6 was obtained after a hydration time of about 8 h. The maximum BET surface area was, however, again about 170 m2/g.
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Conclusions Silica-enhanced lime particles were prepared under varying conditions such as the temperature, and the time of hydration, and the initial SiO2/Ca(OH)2 mole ratio. The silica-enhanced lime had a BET surface area of 170 m2/g, much higher than 6 m2/g for pure Ca(OH)2. The enhancement of the BET surface area was correlated quite well with the formation of new particles of calcium silicate hydrate from the pozzolanic reaction between dissolved reactants. The silica-enhanced lime sorbent had regular micropores at the highest conversion to silicate, whose average diameter was about 10 µm. Excess hydration resulted in more clear-cut features with some pore plugging. The mole ratio of Ca/Si in calcium silicate hydrate gradually decreased and then converged to 1.5-0.9 depending on the temperature of hydration, independently of the initial mole ratio of SiO2/Ca(OH)2. The hydration at elevated temperature and pressure was accelerated to reduce the size of primary particles but to make a less compact structure, yielding a maximum surface area similar to that obtained at lower temperature and pressure. Acknowledgment This work was supported by BK 21 Plan of Ministry of Education, Korea, and partly by Chung Ang University. Literature Cited (1) Jorgensen, C.; Chang, J. C. S.; Brna, T. G. Evaluation of Sorbents and Additives for Dry SO2 Removal Environ. Prog. 1987, 6, 26. (2) Milne, C. R.; Silcox, G. D.; Pershing, D. W. High-Temperature, Short-Time Sulfation of Calcium-Based Sorbents. 1. Theoretical Sulfation Model. Ind. Eng. Chem. Res. 1990, 29, 2192.
(3) Jozewicz, W.; Chang, J. C. S.; Sedman, C. B.; Brna, T. G. Silica-Enhanced Sorbents for Dry Injection Removal of SO2 from Flue Gas. J. Air Waste Manage. Assoc. 1988, 38, 1027. (4) Jozewicz, W.; Jorgensen, C.; Chang, J. C. S. Development and Pilot Plant Evaluation of Silica-Enhanced Lime Sorbents for Dry Flue Gas Desulfurization. J. Air Waste Manage. Assoc. 1988, 38, 796. (5) Shen, D.; Liu, G.; He, Z.; Wang, Y. Development of Pilot Scale Test of Activation Calcium Hydroxide Sorbents for Dry Flue Gas Desulfurization. Proc. Asia-Pac. Conf. Sustainable Energy Environ. Technol. 1996, 194. (6) Peterson, J. R.; Rochelle, G. T. Aqueous Reaction of Fly Ash and Ca(OH)2 to Produce Calcium Silicate Absorbent for Flue Gas Desulfurization. Environ. Sci. Technol. 1988, 22, 1299. (7) Sanders, J. F.; Keener, T. C.; Wang, J. Heated Fly Ash/ Hydrated Lime Slurries for SO2 Removal in Spray Dryer Absorbers. Ind. Eng. Chem. Res. 1995, 34, 302. (8) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (9) Runova, R. F.; Maistrenko, A. A. Interaction of Dispersed Polyvinyl Acetate with Calcium Silicate Hydrates. Proc. 8th ICPIC Congr. Polym. Concr. 1995, 369. (10) Davini, P. Investigation of SO2 Adsorption Properties Ca(OH)2-Fly Ash Systems. Fuel 1996, 75, 713. (11) Ho, C.; Shih, S. Ca(OH)2/Fly Ash Sorbents for SO2 Removal. Ind. Eng. Chem. Res. 1992, 31, 1130. (12) Jung, G. H. Preparation and Characterization of SilicaEnhanced Lime Powder as Dry FGD Sorbents. M.S. Thesis, Chung Ang University, Seoul, Korea, 1998. (13) Pierre, A. C. Introduction to Sol-Gel Processing; Kluwer Academic Publishers: Boston, 1998. (14) Jozewicz, W.; Chang, J. C. S.; Brna, T. G.; Sedman, C. B. Reactivation of Solids from Furnace Injection of Limestone for SO2 Removal. Environ. Sci. Technol. 1987, 21, 664. (15) Friedlander, K. Smoke, Dust and Haze; Wiley: New York, 1977.
Received for review July 12, 1999 Revised manuscript received January 10, 2000 Accepted January 11, 2000 IE990511Y
