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Ind. Eng. Chem. Res. 1999, 38, 812-819
Sorbent/Ash Reactivation for Enhanced SO2 Capture Using a Novel Carbonation Technique Rajeev Agnihotri, Shriniwas S. Chauk, S. K. Mahuli, and L.-S. Fan* Department of Chemical Engineering, The Ohio State University, Columbus Ohio 43210
A novel process has been developed for reactivation of partially utilized calcium-based sorbents for increased SO2 removal and sorbent utilization from coal-fired boilers/combustors. Spent sorbent and combustor ash samples are treated under specific conditions to modify their internal structure and expose the under-utilized calcium for further SO2 capture. The reactivated sorbent shows a significant improvement in utilization, increasing from less than 45% to nearly 100%. Application of this novel reactivation process to ash samples obtained from a circulating fluidizedbed combustor also shows a marked improvement in utilization of available calcium, nearly doubling the amount of sulfur captured. The reactivation process involves carbonation of the unsulfated calcium. Better redistribution and exposure of the available calcium by the carbonation reactivation process, as compared to reactivation via hydration, is proposed as the main factor in increasing the sorbent utilization. The increased ultimate sorbent utilization obtained by this reactivation process could significantly improve the sorbent-based flue gas desulfurization technology. Introduction Limestone (CaCO3) or hydrated lime (Ca(OH)2) used in pulverized or fluidized-bed combustors for SO2 removal suffer from low reactivity and under-utilization. In spite of being economical and easily retrofittable in the existing utility units, dry sorbent processes fail to compete with other more expensive SO2 control technologies because of their poor SO2 removal efficiency and low sorbent utilization. Typically, less than 50% of the available calcium is converted to a high molar volume calcium sulfate product which causes pore blocking and pore mouth plugging and renders the sorbent ineffective for any further SO2 capture. The spent sorbent from pulverized combustors (PCs) exhibits less than 35% calcium utilization, while for circulating fluidized-bed (CFB) combustors, up to 45% calcium utilization is realized.1 The spent sorbent exhibits negligible reactivity toward SO2 and, in order to increase the sorbent utilization, the sorbent needs to be reactivated to expose the unreacted CaO. Reactivation of the under-utilized sorbent would necessarily require reexposing and/or redistribution of the CaO from the interior of the sorbent particle and reactivation of the sintered CaO by converting it into a more reactive form. The fundamental challenge and goal of the reactivation process is to redistribute the CaSO4 predominantly from the surface of the particle to a more uniform distribution. One of the most common methods for reactivating partially utilized sorbents is by the process of hydration.2 In this process, the unsulfated CaO is reacted with water to form Ca(OH)2. Because of higher molar volume of Ca(OH)2 (33 cm3/gmol), compared to CaO (17 cm3/ gmol), the sorbent particle expands and the nonporous CaSO4 shell cracks, thereby exposing the inner calcium hydroxide. Moreover, once this reactivated sorbent is reintroduced into the combustor, calcination of Ca(OH)2 further increases the porosity and provides added * To whom correspondence should be addressed.
exposure of CaO to SO2. Hydration has been known to increase the utilization of spent sorbent from 35% up to 70%.1 It is known that the effectiveness of the hydration reactivation process is dictated by the duration of hydration, the hydration temperature, and the solids concentration in the process. The temperature for drying the hydration products has also been indicated to markedly affect the activity of the reactivated product.3,4 Researchers5 have studied the slurry-based hydration (3% solids concentration) of calcined dolomitic particles to produce an effective sorbent for SO2 removal. They observed a 1.3-1.6-fold increase in SO2 capture ability at 900 °C in a thermogravimetric setup. In their study, increasing the hydration time and temperature had a favorable effect on the hydration reactivation. Several researchers have reported that a recycle of spent sorbent and fly ash mixture into the spray dryer results in a substantial improvement in sorbent utilization and SO2 removal.6-8 It has been suggested that substantial reactions take place between the fresh Ca(OH)2 and recycled fly ash from a spray dryer, resulting in the formation of hydrated calcium silicates and their subsequent reaction with SO2 which leads to increased removal efficiency. Reactivation of spent limestone samples from a circulating fluidized-bed combustor via hydration has been found to cause particle expansion with an increase in their internal volume.1,9,10 Couturier et al. (1994) determined that the conversion of available calcium to CaSO4 in the treated/reactivated sorbent particles increased from 32% to 80%. They suggested that hydration creates new pores and increases the volume of the particle. The water permeates through the partially sulfated layer and reacts with the inner CaO core to form calcium hydroxide. The hydroxide having the higher molar volume swells and cracks the partially sulfated shell. Marquis (1992) studied the correlation between the extent of conversion of CaO to Ca(OH)2 in fly ash during hydration and the utilization of Ca upon
10.1021/ie980368t CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999
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Figure 2. Schematic of reactivation experimental setup.
Figure 1. Schematic of differential bed reactor system for sulfation studies.
resulfation. They observed that with increasing the conversion of CaO to Ca(OH)2 the extent of calcium utilization increased upon sulfating the reactivated sorbent. As suggested by several researchers,12-14 the key to the high reactivity of a sorbent, fresh or partially utilized, lies in its open initial internal structure and subsequent pore structure evolution under high-temperature conditions. In this paper, a novel reactivation technique is developed from a fundamental understanding of the pore structural properties of both CaCO3 and Ca(OH)2 and the evolution of pore structure with calcination and sintering. Understanding the solubility and precipitation characteristics of various calcium compounds in partially utilized sorbents is essential in developing a process capable of successfully reactivating the under-utilized sorbent. This new technique for reactivating the partially utilized sorbent is based on a suspension-based carbonation process.15 This process involves converting the unreacted CaO into calcium carbonate (CaCO3) instead of calcium hydroxide (Ca(OH)2). Along with reactivation of unreacted CaO, this process also provides a better distribution/exposure of available calcium.16 The novel carbonation reactivation process is successfully applied to the reactivation of two partially utilized sorbents generated in the laboratory and has been further extended to the reactivation of two commercial ash samples. Experimental Section Partially Utilized Sorbents and Ashes: Generation and Characterization. Partially utilized sorbents are generated by sulfating commercially available CaCO3 [Linwood carbonate (LC)] and Ca(OH)2 [Linwood hydrate (LH)] obtained from Linwood Minerals and
Mining Co. This step, called the first cycle of sulfation, is carried out in a differential fixed-bed reactor assembly shown in Figure 1. The sulfation reactor is a 2.54 cm o.d. ceramic tube housed in a single-zone vertical furnace (Thermcraft). The reactor accommodates a custom-designed 1.27 cm o.d. sorbent-bed holder. A small amount of sorbent (∼20 mg) is dispersed on quartz wool placed in the sorbent-bed holder. Differential conditions are maintained by using a high-reactant gas flow rate of 1.6 slpm which corresponds to a velocity of about 15 cm/s through the sorbent bed, ensuring minimal external transport resistances. Prior to sulfation, the sorbent is calcined in situ by subjecting it to a calcination temperature of 900 °C under inert nitrogen flow for 10 min. Sulfation is conducted by exposing the calcined sorbent to the reactant gas stream, consisting of 3900 ppmv SO2, 5.5% O2 and balance N2, for 30 min at a flow rate of 1.6 slpm and temperature of 900 °C. The fly ash (FA) and bottom ash (BA) samples are obtained from a commercial CFB combustor unit utilizing petroleum coke. Since the fly ash and bottom ash samples have already undergone at least one cycle of sulfation, they are not subjected to the first sulfation cycle as described above for the Ca-based sorbents. The extent of sulfation is analyzed from SO42- concentration, using ion chromatography (Alltech). The particle size distribution is measured by Sedigraph 5100 (Micromeritics). The surface area and pore volume are analyzed by low-temperature N2 adsorption using the BET technique (Quantachrome). The X-ray diffractograms (XRD) are used to obtain the chemical composition of the sorbent and ash, while a thermogravimetric analyzer (TGA) is employed to obtain the distribution of available calcium in the form of oxide, hydroxide, or carbonate in various samples. The physical characteristics, such as crystal structure, surface morphology, and elemental quantification of calcium and sulfur in the vicinity of the surface, are investigated using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. Reactivation of Partially Utilized Sorbent and Ashes. The partially utilized sorbents generated after
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Table 1. Chemical Composition and Initial Structural Properties of Pure Ca-Based Sorbents Investigated Linwood hydratea (LH) composition (wt %) Ca(OH)2 CaCO3 SiO2 Al2O3 CaO Fe2O3 MgCO3 Mg(OH)2 MgO MgSO4 CaSO4 sorbent utilizationb (%) BET S.A. (m2/g) pore volume (cm3/g) median diam., d50 (µm) a
virgin
1st cycle
95.0 1.0 0.9 0.6 1.0 0.5
hydration
Linwood carbonatea (LC) carbonation
virgin
1st cycle
34.5
hydration
carbonation
49.3
0.6 0.4 26.9 0.3
0.4 0.3
41.6 0.4 0.2
97.0 0.8 0.6
0.2
0.2
0.6 1.0
2.0 69.8
1.2 63.4
1.0 56.5
51
48
49
0.9 0.7 43.0 0.7
0.8 0.6
56.1 0.7 0.5
0.6
0.5
1.7 53
1.5 47.1
1.3 40.9
35
33
33
1.0
16.9 0.06 1.2
1.4
1.3
0.9
1.9 0.004 5.6
6.1
5.8
1.2
b
Linwood Mining and Minerals Co. Sorbent utilization is defined as the ratio of gram moles of sorbent reacted to the total initial gram moles of sorbent. Following reactivation the sorbent utilization is determined assuming that CaSO4 is obtained from Ca(OH)2 and CaCO3 for hydration and carbonation, respectively.
Figure 3. Primary particle size distribution of bottom ash (BA), fly ash (FA), Linwood hydroxide (LH), and Linwood carbonate (LC).
the first cycle of sulfation and the as-received FA and BA samples are reactivated in a slurry bubble column reactor. The same reactor system is used for reactivation via hydration as well as carbonation. The operating conditions are identical for both reactivation processes except that pure nitrogen is used for hydration while pure carbon dioxide is used for carbonation. A schematic of the reactor setup used for reactivation is shown in Figure 2. The Pyrex reactor is 6.4 cm in diameter and 38 cm in height. A sintered glass plate with a pore opening of 25-50 µm (ASTM Por C) is used as the gas distributor. An aqueous suspension with a solids concentration of 2.5 wt % is prepared and reacted batchwise with either pure N2 or CO2. A small amount of anionic surfactant, Dispex N40V (Georgia Pacific), is used in a concentration of 2 wt % (based on the weight of calcium hydroxide formed in the slurry) during both reactivation procedures. Such ionic surfactant additives are known to act as dispersing agents in aqueous systems, leading to reduced agglomeration of crystallites. Dispex N40V produces a stabilizing and dispersing action by ionizing
Figure 4. Scanning electron micrographs of (a) Ca(OH)2 after the first cycle of sulfation, and (b) CaCO3 after the first cycle of sulfation. Reaction temp.: 900 °C. SO2 conc.: 3900 ppm. Reaction time: 30 min.
in water to give sodium cations together with a polyanion. These polyanions adsorb irreversibly on the particle surface, causing the particle to become negatively charged. Adjacent particles then repel each other to maintain a state of dispersion. The reactivation is
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Figure 5. Scanning electron micrographs of hydration-reactivated (a) partially sulfated calcines from Ca(OH)2 and (b) partially sulfated calcines from CaCO3.
conducted for 60 min, using a gas flow rate (N2/CO2) of 2.25 slpm. Carbonation reactivation of the spent sorbent and ashes is dependent on various parameters such as CO2 gas flow rate, solids mass fraction in the slurry, pH, dispersant concentration, solution/slurry temperature, and so forth. Reactivation of the spent sorbents is conducted at the optimum values of these parameters and details of how these parameters are optimized are given elsewhere.13,15,16 The slurry bubble column is designed to facilitate sampling of the slurry at various axial locations; however, in this study the entire slurry is decanted from the slurry bubble column reactor. After decanting, the slurry is filtered through no. 1 filter paper and the cake (containing 50% water) thus obtained is dried in a vacuum oven at 110 °C for 24 h to remove all the surface moisture. More than 90% solid mass recovery was achieved. The extent of conversion of CaO to Ca(OH)2/CaCO3 during reactivation is found by TGA analyses. Sulfation of Reactivated Sorbents and Ashes. The reactivated sorbents and ash samples are tested for their SO2 removal capability and sorbent utilization. This, called the second cycle of sulfation, is conducted in the differential bed reactor system. The in situ calcination and subsequent sulfation are performed as described under the experimental conditions explained earlier.
Figure 6. Scanning electron micrographs of carbonation-reactivated (a) partially sulfated calcines from Ca(OH)2 and (b) partially sulfated calcines from CaCO3.
Results and Discussion “Life Cycle” of Ca-Based Sorbents: Sulfation and Reactivation. The commercial sorbents, Linwood CaCO3 and Ca(OH)2, are analyzed to give a surface area of 1.9 and 16.9 m2/g and a pore volume of 0.004 and 0.06 cm3/g, respectively. The chemical composition of the sorbents is given in Table 1. Figure 3 shows the particle size distributions of CaCO3 and Ca(OH)2, which indicate a mass median diameter (d50) of 7 µm for carbonate and 1.2 µm for hydroxide. The first cycle of sulfation of pure CaCO3 and Ca(OH)2 shows the ultimate sorbent utilization of 35% for CaCO3, while a sorbent conversion of 51% is obtained for Ca(OH)2 after 30 min of sulfation at 900 °C. The XRD analyses of sulfated samples show the presence of CaSO4 and CaO only, thus confirming that all the available/unsulfated calcium is in the form of CaO. The SEM pictographs of the samples after the first sulfation cycles are illustrated in Figure 4. These micrographs indicate a highly sintered nonporous surface structure. This is due to high levels of sintering and the buildup of the high molar volume CaSO4 product layer, which envelops the CaO particle and renders it ineffective in capturing any further SO2. The EDS analyses of the sulfated samples exhibit a calcium to sulfur atomic ratio of about 1.0, for both Ca(OH)2 and CaCO3, in the vicinity
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Ind. Eng. Chem. Res., Vol. 38, No. 3, 1999 Table 2. Chemical Composition and Initial Structural Properties of Fly Ash and Bottom Ash composition (wt %)
fly asha (FA)
bottom asha (BA)
Ca S CaSO4 CaO mineral matter utilization (% Ca as CaSO4)
28 8.5 36.1 24.3 39.6 38
40 12.7 54 33.6 12.4 40
BET surface area (m2/g) pore volume, (cm3/g)
9.7 negligible
1.4 negligible
a Obtained from a circulating fluidized-bed combustor utilizing petroleum coke.
Figure 7. Sorbent utilization arfter the first cycle and second cycle of sulfation. Reaction temp.: 900 °C. SO2 conc.: 3900 ppm. Reaction time: 30 min.
of the sorbent particle surface which indicates that, near the surface, CaSO4 is the predominant species. The surface area and pore volume of the sulfated sorbent samples are found to be negligible. The sulfated sorbents are reactivated via both carbonation (using CO2) and hydration (using N2) in the slurry bubble column reactor. The scanning electron micrographs of the hydration-reactivated sorbent samples are shown in Figure 5. It can be seen that the hydrated spent sorbent samples show the development of cracks on the surface due to the formation of a high molar volume Ca(OH)2. The hydration process rarely leads to the breakage of the particles, and hence, the hydrated sorbent particles show a size distribution similar to that of the parent spent sorbent. Figure 6 shows the scanning electron micrographs of carbonation-reactivated sorbent samples. Unlike reactivation via hydration, carbonated sorbent samples have a predominantly calcitic CaCO3 structure with dimensions smaller than the parent spent sorbent.13 The crystalline structure shown by the carbonation-reactivated sorbents is very similar to the structure shown by the high-reactivity CaCO3 sorbent.13 The EDS analysis is performed on both hydration- and carbonation-reactivated sorbent samples and the results are given in Table 3. After reactivation, the Ca/S ratio in the vicinity of the surface is significantly higher for carbonated samples than for hydrated samples when compared to the Ca/S ratio of an untreated partially utilized sorbent. Thus, on the basis of microscopy and surface composition analyses, reactivation via carbonation is found to redistribute the available calcium within the sorbent particle more effectively than hydration reactivation, therefore, making the free calcium available for reaction with SO2. The second cycle of sulfation of reactivated sorbents is carried out under experimental conditions described earlier. Figure 7 shows the results of sulfation of sorbents reactivated via both carbonation and hydration. It can be seen that carbonation-reactivated sorbents show an ultimate sorbent utilization of more than 95%, while the hydration-reactivated sorbents demonstrate a sorbent utilization of about 70% for partially
sulfated calcine from Ca(OH)2. The calcine from CaCO3 shows about 80% and 55% sorbent utilization following a second cycle of sulfation after reactivation via carbonation and hydration, respectively. The theoretical basis for the difference in the reactivity of the sorbents reactivated via carbonation and hydration can be postulated on the basis of a difference in solubilities for Ca(OH)2 and CaCO3 in water. During hydration, more than the stoichiometric amount of water (water needed for complete hydration of all the unreacted CaO) is added. All the CaO is converted to Ca(OH)2, as found from the TGA results. As reported by previous researchers, the reactivation due to hydration occurs because of the formation of a high molar volume Ca(OH)2 which generates cracks in the outer nonporous CaSO4 layer. Upon calcination these cracks are further developed and provide access to the unreacted calcium.9,10 Thus, reactivation via hydration exposes the unreacted calcium by generating “fissures” and “canyons” in the outer CaSO4 layer (otherwise nonporous) but does not assure a complete redistribution of available calcium. Moreover, the solubilities of Ca(OH)2 and CaSO4 (1.85 and 2.9 g/L, respectively) are such that in order to dissolve and reconstitute the sorbent for the desired calcium and sulfur redistribution, enormous amounts of water would be needed. For the reactivation of a partially utilized sorbent via carbonation a “source and sink” mechanism is proposed which involves the formation of Ca(OH)2, as an intermediate, which subsequently is dissolved in water and acts as an incessant source of Ca2+ ions. CO2 is constantly bubbled through the suspension and provides the necessary CO32- ions which react with Ca2+ to form CaCO3. The negligible solubility of CaCO3 ensures that the freshly formed carbonate is precipitated out. Thus, the formation of CaCO3 acts as a sink for calcium ions and provides the necessary “gradient” for Ca(OH)2 to constantly dissolve in water. In the absence of CO2, Ca(OH)2 in water would reach an equilibrium concentration determined by its solubility limit, thus preventing any further dissolution of Ca(OH)2. The source and sink mechanism allows the reformulation of the sorbent particle structure and thus makes the unconverted calcium from the sorbent (free Ca) accessible to SO2. The increased sorbent utilization shown by carbonation reactivation is due to better redistribution of unsulfated calcium. In order to ensure that the majority of CaCO3 formed during the process is due to the carbonation of Ca2+ ions contributed by CaO (unreacted core), the amount of CaSO4 present in the solid sample before and after the reactivation was determined using ion chromatography (IC). These analyses determined that the CaSO4 content of the two samples did not change appreciably (Table 1). Furthermore, in order to get
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Figure 8. Scanning electron micrographs of as-received (a) fly ash and (b) bottom ash.
Figure 9. Scanning electron micrographs of hydration-reactivated (a) fly ash and (b) bottom ash.
mass-balance closure for SO42- ions, the filtrate solution was also analyzed for SO42- ions and minute amounts of sulfate ions were indeed determined. The contribution of CaSO4 toward Ca2+ ions for the formation of CaCO3 was thus determined to be negligible. Another factor contributing to the increased reactivity shown by the reactivated sorbents is the reduction of particle size following the reactivation process (Table 1). However, reduction in particle size alone is not enough to reactivate the spent sorbent particles. Redistribution of available calcium on the surface (internal as well as external) accompanied by reduction in particle size is observed to be the reason for increased reactivity. Reduction in the particle dimension would not alleviate the problem of inaccessibility of SO2 to the available unreacted CaO core of the particle. Redistribution of the calcium is indeed necessary to increase the accessibility to SO2. Reactivation of Fly Ash and Bottom Ash. The petroleum coke ash samples obtained from a commercial circulating fluidized-bed combustor are analyzed to give a surface area of 9.7 m2/g for FA and 1.4 m2/g for BA and negligible pore volumes. Approximately 10% of the BA particles are larger than 1 mm. Only a small quantity of ash (less than 25 mg) is used for a given experiment, and in order to avoid nonrepresentative sampling, particles larger than 500 µm are discarded.
Figure 3 shows the particle size distributions of FA and sieved BA, which indicate a mass median diameter (d50) of 15 µm for FA and 190 µm for BA. The chemical compositions of the FA and sieved BA are given in Table 2. The free calcium contents of the ash samples are calculated assuming that all sulfur is in the form of calcium sulfate (determined from IC) and all calcium not in the form of hydroxide, carbonate (determined from TGA), or sulfate is free calcium. These assumptions, although not strictly valid, make a very good approximation.3 The ion chromatography (IC) analyses show a sulfation conversion of 38% for FA and 40% for BA. The scanning electron micrographs of FA and BA are shown in Figure 8. It can be seen from these micrographs that the ash surface has a nonporous molten texture, which is indicative of high levels of sintering. The EDS results show a relatively low Ca/S ratio in the vicinity of ash surface (Table 3). The as-received FA and BA are found to exhibit negligible further SO2 capture and are concluded to be completely spent or deactivated. These as-received ash samples are reactivated via both carbonation (using CO2) and hydration (using N2) in the slurry bubble column reactor. The scanning electron micrographs of the hydration-reactivated ash samples are shown in Figure 9. It can be seen that, similar to Figure 5, hydrated spent sorbent/ash samples show the develop-
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Table 3. Summary of Results Obtained from Energy-Dispersive Spectroscopy (Ca/S Ratio on the Surface) and Structural Properties after Reactivation as-received Ca/S Linwood hydroxide (LH) Linwood carbonate (LC) fly ash (FA) bottom ash (BA)
1st sulfation Ca/S
after reactivation by hydration surf. area (m2/g) Ca/S
0.98 0.95 4.7 1.4
2.1 3.7 8.1 2.2
1.5 1.5 5.3 1.7
after reactivation via carbonation surf. area (m2/g) Ca/S 3.0 4.7 14.1 4.7
13.4 30 11.8 4.6
Figure 11. Extent of sulfation of available calcium in the ash samples following reactivation. Reaction temp.: 900 °C. SO2 conc.: 3900 ppm. Reaction time: 30 min.
of sulfation following hydration and carbonation reactivation are shown in Figure 11. Carbonation-reactivated ash shows the ultimate sulfation extent of 85% for FA and 82% for BA, while the hydration-reactivated sorbent/ash demonstrates a sorbent utilization of only 65% for FA and 60% for BA. The reproducibility of sulfation results shown by the reactivated ash samples is confirmed by conducting multiple experiments under identical conditions. Conclusions Figure 10. Scanning electron micrographs of carbonationreactivated (a) fly ash and (b) bottom ash.
ment of cracks on the surface due to the formation of high molar volume Ca(OH)2. Scanning electron microscopy was performed on carbonation reactivated samples and the results are presented in Figure 10. The carbonation-reactivated ash sample shows a predominantly calcitic CaCO3 structure, similar to the one shown in Figure 6. The energy-dispersive spectroscopy was performed on samples reactivated by both carbonation and hydration and the results are summarized in Table 3. After reactivation, the Ca/S ratio in the vicinity of the surface is significantly higher for carbonation-reactivated ash samples than for the hydration-reactivated samples. The reactivation via carbonation is found to redistribute the available calcium within the sorbent particle more effectively than hydration. This corroborates well with the findings of reactivation of the laboratory-generated partially utilized sorbent. The reactivated ash samples are subjected to the second cycle of sulfation. The results of the second cycle
Reactivation of the partially utilized calcium-based sorbents via carbonation is demonstrated to be a promising process. Carbonation-reactivated sorbents and ashes show markedly higher sorbent utilization upon further sulfation. This novel reactivation process can favor the economics of dry sorbent technologies by enhancing the SO2 removal efficiency and sorbent utilization. Redistribution and increased exposure of unreacted/available calcium is suggested to be the main reason for heightened sorbent utilization. On the basis of the surface morphology and chemical composition analyses a “source and sink” mechanism for reactivation via carbonation is proposed. The postulated mechanism suggests that the presence of adequate amounts of water hydrates the unreacted CaO to Ca(OH)2 which provides by dissolution a “source” of Ca2+ ions for CaCO3 formation in the aqueous phase. The low solubility of CaCO3 leads to its precipitation and acts as a continuous “sink” for Ca2+ ions. Increased sulfur removal achieved from the carbonation-reactivated spent sorbents and ashes would reduce the costs of fresh sorbent needed and solid waste
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disposal compared to the processes without reactivation or with reactivation based on hydration, for the same extent of sulfur removal. However, a detailed economic analysis of the overall process is necessary before largescale applications can be considered. Acknowledgment The authors would like to acknowledge the contributions made by Mr. Himanshu Gupta and Mr. Raja Jadhav in performing analyses and Dr. A. GhoshDastidar of Union Carbide Corp. for his valuable suggestions and insights. Literature Cited (1) Couturier, M. F.; Marquis, D. L.; Steward, F. R.; Volmerange, Y. Reactivation of Partially-Sulfated Limestone Particles From a CFB Combustor by Hydration. Can. J. Chem. Eng. 1994, 72, 91. (2) Bobman, M. H.; Weber, G. F.; Keener, T. C. Additive Enhancement of Pressure-Hydrated Lime for Control of SO2/NOx Emissions. Presented at the Spring National Meeting of AIChE, Houston, TX, 1985. (3) Khan, T.; Kulvarlainen, R.; Lee, Y. Y. Improving Limestone Utilization in Circulating Fluidized Bed Combustors Through the Reactivation and Recycle of Partially Utilized Limestone in the Ash. Fluidized Bed Combustion; ASME: Miami, FL, 1995; Vol. 2. (4) Tsuchiai, H.; Ishizuka, T.; Ueno, T.; Hattori, H.; Kita, H. Highly Active Absorbent for SO2 Removal Prepared from Coal Fly Ash. Ind. Eng. Chem. Res. 1995, 34, 1404. (5) Al-Shawabkeh, A.; Matsuda, H.; Hasatani, M. Enhanced SO2 Abatement with Water-Hydrated Dolomitic Particles. AIChE J. 1997, 43 (1), 173. (6) Melia, M. T.; McKibben, R. S.; Pelsor, B. W. Utility Flue Gas Desulfurization Survey July 1982-March 1983; Project Summary, EPRI Contract No. RP982-32; EPRI: Palo Alto, CA, 1983.
(7) Palazzolo, M. A.; Brna, T. G.; Kelly, M. E. Current Status of Dry SO2 Control Systems. Proceedings of the Eighth EPA/EPRI Symposium on Flue Gas Desulfurization; EPA-600/9-84-017b (NTIS PB84-223049); EPA: San Diego, CA, 1984; Vol. 2. (8) Parson, E. L., Jr.; Hemenway, L. F.; Kragh, O. T.; Brna, T. G.; Ostop, R. L. SO2 Removal in Dry FGD. Proceedings of the Symposium on Flue Gas Desulfurization; EPA-600/9-81-019b (NTIS PB81-243164); EPA: San Diego, CA, 1981; Vol. 2. (9) Shearer, J. A.; Smith, G. W.; Myles, K. M.; Johnson, I. Hydration Enhanced Sulfation of Limestone and Dolomite in the Fluidized-Bed Combustion of Coal. J. Air. Pollut. Control Assoc. 1980, 30, 684. (10) Marquis, D. L. Reactivation of Spent CFB Limestone by Hydration. M.Sc. Dissertation, University of New Brunswick, Fredericton, NB, 1992. (11) Josewicz, W.; Rochelle, G. T. Fly Ash Recycle in Dry Scrubbing. Environ. Prog. 1986, 5 (4), 219. (12) Ghosh-Dastidard, A.; Mahuli, S. K.; Agnihotri, R.; Fan, L.S. Investigation of High-Reactivity Calcium Carbonate Sorbent for Enhanced SO2 Capture. Ind. Eng. Chem. Res. 1996, 35 (2), 598. (13) Wei, S.-H.; Mahuli, S. K.; Agnihotri, R.; Fan, L.-S. High Surface Area Calcium Carbonate: Pore Structural Properties and Sulfation Characteristics. Ind. Eng. Chem. Res. 1997, 36, 2141. (14) Gullett, B. K.; Bruce, K. R. Pore Distribution Changes of Calcium Based Sorbents Reacting with Sulfur Dioxide. AIChE J. 1987, 33, 1719. (15) Fan, L.-S.; Ghosh-Dastidar, A.; Mahuli, S. K. Chem. Sorbent and Methods of Making and Using Same. U.S. Patent No. 08/584,089, 1998. (16) Fan, L.-S.; Mahuli, S. K.; Agnihotri, R.; Chauk, S. Reactivation of Spent Sorbents and Ashes for Enhanced SO2 Capture. U.S. Patent (pending), 1998.
Received for review June 1, 1998 Revised manuscript received September 11, 1998 Accepted September 14, 1998 IE980368T
