Ind. Eng. Chem. Res. 2004, 43, 5715-5720
5715
Cyclic Steam Reactivation of Spent Limestone K. Laursen* Department of Chemical Engineering, Kyoto University, Kyoto-Daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
W. Duo Pulp and Paper Research Institute of Canada, 3800 Wesbrook Mall, Vancouver, British Columbia, Canada V6S 2L9
J. R. Grace and C. J. Lim Department of Chemical and Biological Engineering, University of British Columbia, 2216 Main Mall, Vancouver, British Columbia, Canada V6T 1Z4
Steam reactivation experiments were conducted in a specially designed dual-environment fixedbed reactor to determine the reactivation effect of multiple cycles of sulfation and steam hydration at conditions simulating those experienced by sorbent particles in circulating fluidized-bed combustors (CFBCs) with steam reactivation in the return loop. Three spent sorbents from industrial CFBCs utilizing three different limestones for emission control were tested. Only two showed improved sulfur capture capacity after steam reactivation at 250 °C for a total of 180 min distributed over 1-18 cycles. These two spent sorbents responded very differently to cyclic sulfation and hydration; one showed significantly higher reactivation after the second cycle than after the first, whereas the other showed better performance after the first cycle. The increase in the reactivation effect after the second cycle in the one case is likely due to calcination of calcium carbonate formed in the outer reaches of the calcium core during storage of the sorbent. The cyclic experiments showed that, for the conditions investigated, one longterm hydration can be more effective than hydration over multiple short-term hydration cycles of the same overall duration. Introduction Reaction with limestone- or dolomite-derived sorbents is the most commonly applied method for controlling sulfur emissions during combustion of fossil fuels and other sulfur-containing fuels. In circulating fluidizedbed combustors (CFBCs), the limestone or dolomite is injected directly into the furnace and SO2 is captured in situ. The capture of sulfur by limestone can be described by a two-step global reaction scheme:
CaCO3 f CaO + CO2
(1)
CaO + SO2 + 1/2O2 f CaSO4
(2)
Upon entering the combustor, the relatively compact limestone is transformed into a porous calcium oxide (reaction 1); sulfation (reaction 2) then occurs at the outer surface and around the pores formed during calcination. Aided by its larger molar volume (∼46 cm3/ mol) than either CaCO3 (∼37 cm3/mol) or CaO (∼17 cm3/ mol), the CaSO4 product can seal the outer reaches of the pores, causing the center of the sorbent particles to remain essentially unsulfated. Calcium utilization in the sorbents is therefore usually significantly lower than 100%, with utilization efficiencies (moles of S adsorbed ÷ moles of Ca available) typically from 30 to 50%. The sulfation pattern leading to the formation of particles with a sulfated outer rim and an unsulfated interior is commonly described using the concept of the unreactedcore model.1 This behavior is not restricted to whole particles, but may apply also (in what is referred to as
the grain-model2,3 or grain-micrograin-model4) to sulfation of the individual CaO grains of limestone particles. Recent research4-6 reveals that many individual particles do not adhere to the simple core-rim structure. We have divided sulfated particles into three groups: (a) unreacted-core, (b) patchy/network, and (c) uniformly sulfated particles. Unreacted-core particles have a highly sulfated rim (70-95% local utilization efficiency) and an unsulfated or slightly (0-5%) sulfated core. Uniformly sulfated particles exhibit a relatively homogeneous degree of sulfation (40-65%). Patchy/ network particles consist of “subgrains”, separated by fractures or macropores, that are either fully sulfated or only sulfated around the periphery and along, or very close to, fractures and macropores.5-7 Because of the finite residence time of the particles and the low utilization caused by the pore plugging, much of the sorbent passes through the combustion process without capturing sulfur. Increasing the calcium utilization would decrease the amount of raw limestone required, reduce handling and disposal, and diminish CO2 (greenhouse gas) emissions because less limestone would need to be calcined. Several methods have been proposed to increase sorbent utilization, mostly focused on water or steam hydration.5-17 Reaction between the water or steam and CaSO4 of the product layer is not thermodynamically or kinetically favorable.12 Thus, the smaller water molecules are able to penetrate the product layer and to react with the CaO core:
CaO + H2O f Ca(OH)2
10.1021/ie030795t CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004
(3)
5716 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004
Because Ca(OH)2 has a larger molar volume (∼33 cm3/mol) than CaO, expansion of the core causes cracks in the sulfate shell. Upon re-injection of these particles into the fluidized bed, Ca(OH)2 dehydrates (reverse of reaction 3), causing further opening of the sulfated shell and exposing fresh unreacted CaO for further sulfation. Shearer et al.10 and Marquis12 showed decreasing reactivation with increasing temperature in the range of 100-300 °C. They explained the temperature dependence as being due to changes in the rate-limiting factor. On the basis of steam hydration experiments on fly ash in the temperature range of 100-600 °C, Julien et al.14 found maximum reactivation at 450 °C. Our recent research on sulfation and steam reactivation of limestones under conditions more relevant for industrial CFBCs5-7,17 has revealed that the effectiveness of steam hydration depends on the characteristics of the limestones; unreacted-core and patchy/network sulfated particles can be reactivated with steam, whereas uniformly sulfated particles cannot be appreciably reactivated under the same conditions. In the range of 250450 °C, increasing the steam temperature leads to decreasing reactivation,17 and for practical reasons, temperatures below 250 °C are unlikely to be of commercial interest. Spent sorbents from industrial CFBCs are much harder to reactivate than laboratory sulfated limestones; thus, lower temperatures and longer hydration times are needed if any reactivation of spent sorbents is to be achieved. This paper extends our work on identifying methods for increasing the sulfur capture capacity of limestones. We present the results of steam reactivation experiments on spent sorbents from commercial fluidized-bed combustors. Our previous work and most research reported in the literature have focused on a single period of hydration followed by resulfation. In this paper, we evaluate the effect of cycling (i.e., repeated sequences of hydration and sulfation), a treatment much closer to that experienced by sorbent particles in a CFBC with steam reactivation in the return loop. Samples Our tests involved three spent recirculating bed materials (denoted spent sorbents A-C) sampled during combustion of coals with three different limestones as SO2 sorbents from three different industrial circulating fluidized-bed boilers. The spent sorbents were immediately sealed after cooling, shipped in sealed vessels, and stored in a desiccator, except when screening. There was no further treatment of the materials before they were tested in our equipment, except where specified below. For spent sorbents A and C, the particle size range 212-355 µm was investigated, while somewhat smaller particles (150-212 µm) were tested for spent sorbent B. These ranges represent a major fraction of the ash in all three as-received samples. The calcium utilization of the size ranges tested was as follows: spent sorbent A, 41%; spent sorbent B, 27%; spent sorbent C, 46%. All three types of sulfated particles (i.e., unreacted-core, uniformly sulfated, and patchy/network) were present in each of the spent sorbents. Spent sorbent A contained approximately equal amounts of unreacted-core and uniformly sulfated particles, whereas B was rich in patchy/network sulfated particles, and C predominantly contained uniformly sulfated particles. In addition to the partially sulfated limestone particles, ash particles were also present. Scanning electron
Table 1. Chemical Composition of the Three Spent Sorbents spent sorbent A, spent sorbent B, spent sorbent C, 212-355 µm 150-212 µm 212-355 µm Ca (%) Mg (%) Si (%) Al (%) Fe (%) Na (%) K (%) Ti (%) Mn (ppm) V (ppm) Sr (ppm) Ba (ppm) Ni (ppm) Zn (ppm)
30.54 0.36 6.54 3.86 1.29 0.05 0.26 0.28 99 85 460 113 38 50
34.72 0.26 8.63 1.73 0.65 0.38 0.08 0.13 289 1524 281 145 1735 41
11.48 2.01 21.27 8.55 1.76 0.19 0.88 0.42 343 101 584 342 15 53
microscopy analyses showed that spent sorbent C had a significantly higher fraction of ash particles than A or B. This is also evident based on chemical analyses, which reveal a significantly higher content of Si and Al in spent sorbent C (Table 1). The ash particles in spent sorbents A and C were mainly aluminum silicates and iron-aluminum silicates, while B mainly contained quartz. Many ash particles, especially in spent sorbent B, were covered with a thin layer of calcium silicates, indicating reaction between the sorbents and the ash. Some limestone particles in spent sorbent A were enriched in iron in their outer reaches, indicating some reaction with the ash. No coatings or reaction zones were observed6 on limestone particles in sorbents B and C. Experimental Equipment and Methodology The specially designed dual-environment fixed-bed reactor consisted of three sections: a main body, a removable upper section, and a removable lower section. In total, the reactor was 2.0 m long and had an inside diameter of 37 mm. The sample holder at the center of the reactor consisted of a tube with a sintered quartz filter acting as the bed support. The reactor was located inside one of two furnaces, one on top of the other on a scissors-lifting table (Figure 1). The upper furnace (A) was held at the sulfation temperature (825 °C), while the lower furnace (B) was maintained at the hydration temperature (250 °C). When the scissors table was raised or lowered and the inlet gas composition was simultaneously varied with the aid of solenoid valves, samples could be switched quickly between hydration and sulfation conditions. Two thermocouples located immediately below and above the bed were used to control and record the gas temperature. During sulfation, gases were fed from the bottom of the reactor and heated as they passed upward along the outer part of the reactor and then downward through the central core toward the sample holder. Gas mixtures (air, N2, and SO2) were fed from gas cylinders and then passed through mass flow controllers to adjust their flows to give simulated flue gas concentrations of 2250 ppm of SO2 and 3% oxygen at a 1600 mL/min total gas flow. A nondispersive infrared gas analyzer continuously monitored the SO2 concentration of the outlet stream. The calcium utilization of the samples was based on weight gain and also on integration of the sulfur dioxide emission curves, with good agreement between the two methods. All results presented here are based on the former method.
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5717
Figure 2. Sulfur emission during resulfation of spent sorbent A subjected to no hydration, 1, 6, or 18 cycles of hydration (250 °C, 0.83 atm water vapor partial pressure, 90 min total time) and sulfation (825 °C, 2250 ppm SO2 partial pressure, 180 min total time). Table 2. Results of Cyclic Hydration and Sulfation of the Spent Sorbents
Figure 1. Schematic showing the experimental setup of equipment used for cyclic steam reactivation and sulfation experiments (not to scale).
During hydration, steam was fed to the top of the quartz reactor and water was maintained as a gas until it left the bottom of the reactor. The steam was generated by pumping water into an evaporator connected to the top of the reactor by a heated transport line. Hydration was conducted at 250 °C with a steam partial pressure of 0.83 atm. This hydration temperature was chosen because previous work had shown that 250 °C is likely to be close to the optimum hydration temperature, taking commercial interest into account. Following hydration, the steam feed was interrupted and the lifting table lowered, subjecting the sample to dehydration and quick heating to the sulfation temperature (825 °C). Resulfation was initiated as soon as the temperature of the sample reached 825 °C. Hydration followed by resulfation was repeated from 1 to 19 times for each experimental run. The overall (cumulative) sulfation and hydration times were fixed at 90 and 180 min, respectively, with each sulfation period varying from 5 to 90 min and each hydration from 10 to 180 min. Results and Discussion To evaluate the effect of cycling time, spent sorbent A was subjected to 1, 6, and 18 cycles of hydration and sulfation. For comparison, a sample was also sulfated without hydration (Table 2 and Figure 2). For these experiments, the hydration temperature was set at 250 °C and the total hydration and sulfation times were maintained at 180 and 90 min, respectively. Steam reactivation led to a 10-15% higher final Ca utilization compared with resulfation without hydration. The reactivation effect was slightly higher when the sample was subjected to several shorter-term hydrations rather than a single long-term hydration (Table 2). However, there appeared to be no difference in the final total utilization for 6 and 18 hydration/sulfation cycles. Our previous experiments6 revealed that short-term hydration (15 min) was not sufficient to fully reactivate any
spent sorbent
conditions
A A A A B B B C C C
no hydration 1 cycle 6 cycles 18 cycles no hydration 1 cycle 6 cycles no hydration 1 cycle 6 cycles
hydration time per cycle total
sulfation time per cycle total
180 30 10
180 180 180
90 15 5
180 30
180 180
90 15
180 30
180 180
90 15
90 90 90 90 90 90 90 90 90 90
Ca utilization (%) 45 55 60 58 35 50 47 49 50 52
of these spent sorbents, whereas significantly greater reactivation was obtained for 180 min of hydration. However, the results presented here indicate that shortterm hydration can be effective if the ash is subjected to sulfation in between. Our previous work on these three sorbents and the corresponding pure limestones showed that the benefit of steam reactivation for the range of conditions studied is very dependent on which limestone is used. To determine if the limestone type also affects cyclic hydration and sulfation, single-cycle and 6-cycle experiments were conducted under the same conditions on two sorbents with different dominant sulfation patterns: spent sorbent B, which contains particles predominantly sulfating according to the patchy/network pattern, and spent sorbent C, in which the particles are predominantly homogeneously sulfated (Table 2 and Figure 3). Our previous work6 has shown that spent sorbent B could be reactivated, with a 15-16% increase in Ca utilization of steam-hydrated samples compared to nonhydrated samples. However, there appeared to be no additional effect of varying the number of cycles for this sorbent. Resulfation of spent sorbent C led to a 7-10% increase in utilization. However, neither single, long-term hydration nor short-term, cyclic hydration was able to reactivate this sorbent. The two sorbents that could be reactivated with steam behaved rather differently when exposed to cycling. Whereas cycling appeared to slightly improve the capture capacity of spent sorbent A (primarily unreacted-core sulfated particles), it had a negligible effect on spent sorbent B (primarily patchy/network sulfated particles). When the sulfur emission curves for the two
5718 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004
Figure 4. Sulfur emission during two cycles of resulfation (825 °C, 2250 ppm SO2 partial pressure, 30 min total time) for spent sorbent A with and without high-temperature pretreatment (stagnant air, 5 min; 300 mL/min air, 30 min) prior to steam hydration (250 °C, 0.83 atm H2O partial pressure, 60 min total time).
Figure 3. Sulfur emission during resulfation of spent sorbents B (a) and C (b), subjected to no hydration, 1 or 6 cycles of hydration (250 °C, 0.83 atm H2O partial pressure, 90 min total time) and sulfation (825 °C, 2250 ppm SO2 partial pressure, 180 min total time). Table 3. Effect of High-Temperature Pretreatment on the Final Ca Utilization of Spent Sorbent A Hydrated over 2 Cycles at 250 °C and 30 min per Cycle pretreatment agent
pretreatment time (min)
Ca utilization (%)
no pretreatment 825 °C, stagnant air 825 °C, stagnant air 825 °C, 300 mL/min air 825 °C, steam
5 120 30 5
51.0 54.3 53.7 55.9 55.9
sorbents are compared, it becomes apparent that the differences are related to the first cycle (Figures 2 and 3a). For spent sorbent A, the sulfur emission curves for the two cyclic experiments show that the second hydration led to a higher degree of reactivation than the first, a feature not seen for spent sorbent B. These observations raise the question of whether the improved reactivation in the second cycle is due to the longer total hydration time involved in two cycles or perhaps to heating and cooling. Our previous work on sulfation and steam reactivation on spent sorbents A and B, as well as the pure two limestones, did not reveal that longer hydration times were needed to reactivate unreactedcore sulfating particles than patchy/network sulfating particles, indicating that the heating and cooling of the spent sorbent prior to hydration may account for the improved reactivation after the second cycle.5,6 To test this hypothesis, experiments were carried out in which “as received” spent sorbent A was first subjected to hightemperature treatment (hereafter referred to as pretreatment) in the presence of various agents including air, N2, and steam. Table 3 shows the effect of various pretreatments on the final calcium utilization of a series of 2-cycle experiments with a total hydration time of 60 min (30 min per cycle) and total sulfation time of 30 min (15 min per cycle). It is clear that pretreatment
Figure 5. XRD diffractograms of “as-received” and high-temperature-treated (air, 300 mL/min) spent sorbent A. The high temperature led to calcination of the CaCO3 present in the “asreceived” sorbent (A ) anhydrite, L ) lime, C ) calcite, and Q ) quartz).
increases the sulfur capture capacity. Also, in contrast to samples hydrated without pretreatment, the first hydration cycle becomes much more effective for these pretreated samples (Figure 4). A possible explanation for the effect of pretreatment could be the presence of calcium carbonate formed by recarbonization of CaO in the spent sorbent during storage, a reaction which could occur when the ash is exposed to air. If part of the unutilized CaO in the sorbent particles is converted to CaCO3 during storage, this part will not react with the steam, but needs to be recalcined to CaO, a process which would occur during the high-temperature pretreatment. An X-ray diffractogram of “as-received” spent sorbent A shows a minor calcite peak (Figure 5), whereas, as expected, this peak is absent in a sample heated to 825 °C, indicating that the effect of this type of pretreatment is, at least to some extent, related to calcination of CaCO3. The minor calcite peak reveals that only a small amount of calcite is present in the “as-received” spent sorbent. Even a small amount of calcite may have an effect, especially if the calcite, as would be expected, is predominantly present in the outer reaches of the unreacted CaO core
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5719 Table 4. Results of Pretreatment (30 min, 825 °C) and Cyclic Hydration and Sulfation of Spent Sorbent A
cycles
pretreatment
1 1 6 1 1 19
none 30 min, 0.83 atm steam 30 min, 0.83 atm steam none 30 min, 0.83 atm steam 30 min, 0.83 atm steam
hydration time
sulfation time
180 180 30 90 90 5
90 90 15 90 90 5
Ca per per utilization cycle total cycle total (%) 180 180 180 90 90 95
90 90 90 90 90 90
53.3 64.9 60.9 50.2 60.9 50.7
because this may block the pores and prevent steam from penetrating into the interior. Appreciable CO2 levels and reduced temperatures can exist in the return leg of CFBCs, thermodynamically favoring carbonation of free CaO. However, given the brief residence times there, carbonation is not expected to play a significant role. Thus, in terms of cyclic hydration in utility CFBCs, pretreatment is not an issue. However, when testing or determining the reuse potential of spent sorbents that have had time to recarbonize, carbonization may be important. Without this carbonation, hydration and reactivation results may be unduly pessimistic. Another transformation process that can be expected to take place readily in spent sorbents exposed to the atmosphere outside the combustor is hydration of CaO (lime) to Ca(OH)2 (portlandite). However, in this case, because of the sealing and storage in a desiccator as outlined above, it appears that this reaction did not take place because no portlandite was detected in the “asreceived” spent sorbent. If portlandite were to have been formed in the sorbent during storage, this would be expected to react in a way similar to steam hydration, and this could also lead to a pessimistic determination of the effect of steam reactivation because the background sample (i.e., sample resulfated without steam hydration) would already have been partially or fully hydrated. Because pretreatment appears to play a major role in the reactivation effect of cyclic hydration of spent sorbent A, more experiments were conducted with samples pretreated prior to long-term hydration (Table 4 and Figure 6). By pretreatment of samples prior to single-cycle experiments, it was easier to directly compare the effect of the cycling. Single-cycle hydration for 180 min with pretreatment led to a 14% increase in utilization, compared to a sample hydrated for the same period without pretreatment. However, a 6-cycle experiment with the same total hydration time resulted in a lower final Ca utilization than a corresponding singlecycle experiment with pretreatment. Similar effects of single-cycle experiments were found for samples hydrated for 90 min, but when the hydration was spread over a 19-cycle series, the total effect of hydration was even lower than that for the single-cycle experiment without pretreatment. These cyclic tests show that if the sorbents are pretreated, a single long hydration period is more effective than multiple short periods of hydration. The explanation of why one long-term hydration period is as good as, and in some cases even better than, several short-term hydration cycles is probably related to the amount of Ca(OH)2 formed. During shortterm hydration, only part of CaO is hydrated, and not enough Ca(OH)2 is formed to create sufficient force to completely break the CaSO4 product layer on all particles, only enough to create a limited number of cracks.
Figure 6. Sulfur emission during single and multiple (6 and 19) cycles of resulfation (825 °C, 2250 ppm SO2 partial pressure, 90 min total time) for spent sorbent A with and without hightemperature pretreatment prior to steam hydration. (a) 1 cycle without pretreatment and 1 and 6 cycles with pretreatment (30 min, 0.83 atm H2O partial pressure). Hydration conditions for all samples: 250 °C, 0.83 atm H2O partial pressure, 180 min total time. (b) 1 cycle without pretreatment and 1 and 19 cycles with pretreatment (30 min, 0.83 atm steam H2O partial pressure). Hydration conditions for all samples: 250 °C, 0.83 atm H2O partial pressure, 90 min total time.
CaO exposed via such cracks would readily sulfate upon reexposure to SO2, and the particle would remain partly unsulfated. However, during one long-term hydration period, a major part of CaO in unreacted-core and patchy/network sulfated particles is expected to be hydrated, creating sufficient force to completely break the product layers. Overall, the cyclic tests confirm that the duration of hydration in each cycle and pretreatment are important factors for reactivating spent sorbents. Conclusions Experimental results show that, to maximize the reactivation of spent sorbents, it is more beneficial to subject a spent sorbent to a smaller number of relatively long-term hydration periods than to many short-term hydration periods having the same cumulative hydration time. These findings are important for the design and operation of steam reactivation for the return loop of CFBCs. The results further show that when the reactivation potential of a spent sorbent from a largescale combustor is evaluated, it is very important that the spent sorbent not be subjected to any conditions that may alter its chemistry after removal from the combustor. Acknowledgment The samples included in this study were generously provided by Power Plant Laboratories, Alstom, Windsor,
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CT. The authors acknowledge useful discussions with H. Andrus and J. Chiu. K.L. and W.D. express their gratitude to John Grace for his introduction, neverending support, and high commitment to this interesting topic. Literature Cited (1) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; John Wiley & Sons: New York, 1999. (2) Hartman, M.; Coughlin, R. W. Reaction of Sulfur Dioxide with Limestone and the Grain Model. Ind. Eng. Prod. Res. Dev. 1974, 13, 248-256. (3) Mulligan, T.; Pommeroy, M.; Bannard, J. E. J. The Mechanism of the Sulphation of Limestone by Sulphur Dioxide in the Presence of Oxygen. Inst. Energy 1989, 3, 40-47. (4) Dam-Johansen, K.; Hansen, P. F. B.; Østergaard, K. HighTemperature Reaction between Sulphur Dioxide and Limestones III. A Grain-Micrograin Model and Its Verification. Chem. Eng. Sci. 1991, 46, 847-853. (5) Laursen, K.; Duo, W.; Grace, J. R.; Lim, C. J. Sulfation and Reactivation Characteristics of Nine Limestones. Fuel. 2000, 79 (2), 153-164. (6) Laursen, K.; Duo, W.; Grace, J. R.; Lim, C. J. Characterization of Steam Reactivation Mechanisms in Limestones and Spent Calcium Sorbents. Fuel 2001, 80 (9), 1293-1306. (7) Laursen, K.; Duo, W.; Lim, C. J.; Grace, J. R. Sulfation and Steam Reactivation Characteristics of Limestones of Worldwide Origin. In Advanced in Environmental Materials. Vol. II. Environmentally Preferred Materials; White, T., Stegemann, J., Eds.; Materials Research Society: Singapore, 2001; pp 67-76, ISBN 981-04-4992-7. (8) Johnson, I.; Moulton, D. S.; Nunes, F. F.; Swift, W. M.; Teats, F. G.; Jonke, A. A. Regeneration of Sulfated Limestone from FBCs. Annual Report 1979, ANL/CEN/FE-79-13. (9) Newby, R. A.; Bachovchin, D. M.; Peterson, C. H.; Rohatgi, N. D.; Ulerich, N. H.; Keairns, D. L. FBC Sulfur Removal-Do We Know Enough? Proceedings of the 6th International Conference on Fluid-Bed Combustion, Atlanta, GA, Apr 9-11, 1980.
(10) Shearer, J. A.; Smith, G. W.; Moulton, D. S.; Smyk, E. B.; Myles, K. M.; Swift, W. M.; Johnson, I. Hydration Process for Reactivating Spent Limestone and Dolomite for Reuse in Fluidized-Bed Coal Combustion. Proceedings of the 6th International Conference on Fluidized Bed Combustion, Atlanta, GA, Apr 9-11, 1980; pp 1015-1027. (11) Hamer, C. A. Methods to Improve Limestone Utilization in Fluidized Beds; Division Report ERL/MSL 84-77; CANMET, Energy, Mines and Resources Canada: 1984. (12) Marquis, D. L. Reactivation of Spent CFB Limestone by Hydration. M.A.Sc. Thesis, University of New Brunswick, Fredericton, New Brunswick, Canada, 1992. (13) Couturier, M. F.; Marquis, D. L.; Steward, F. R.; Volmerange, Y. Reactivation of Partially-Sulphated Limestone Particles from a CFB Combustor by Hydration. Can. J. Chem. Eng. 1994, 72, 91-97. (14) Julien, S.; Brereton, C. H. M.; Lim, C. J.; Grace, J. R.; Chiu, J. H.; Skowyra, R. S. Spent Sorbent Reactivation Using Steam. Proceedings of the 13th International Fluidized Bed Combustion Conference; ASME: New York, 1995; Vol. 2, pp 841-850. (15) Khan, T.; Kuivalainen, R.; Lee, Y. Y. Improving Limestone Utilization in Circulation Fluidized Bed Combustors through the Reactivation and Recycle of Partially Utilized Limestone in the Ash. 13th International Fluidized Bed Combustion Conference; ASME: New York, 1995; Vol. 2, pp 831-840. (16) Couturier, M. F.; Volmerange, Y.; Steward, F. Hydration of Partially Sulfated Lime with Water. Proceedings of the 15th International Conference on Fluidized Bed Combustion, Savannah, GA, May 16-19, 1999. (17) Laursen, K.; Mehrani, P.; Grace, J. R.; Lim, J. Steam Reactivation of Partially Utilized Limestone Sorbents. Environ. Eng. Sci. 2003, 20, 11-20.
Received for review October 28, 2003 Revised manuscript received May 5, 2004 Accepted May 6, 2004 IE030795T
