SO2 Capture Characteristics of Three Limestones

Energy & Fuels 2006, 20, 1621-1628

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Simultaneous CO2/SO2 Capture Characteristics of Three Limestones in a Fluidized-Bed Reactor Ho-Jung Ryu,*,† John R. Grace,‡ and C. Jim Lim‡ Korea Institute of Energy Research, Daejeon, 305-343 Korea, and Department of Chemical and Biological Engineering, UniVersity of British Columbia, 2360 East Mall, VancouVer, BC, Canada V6T 1Z3 ReceiVed August 24, 2005. ReVised Manuscript ReceiVed April 16, 2006

Simultaneous CO2/SO2 capture characteristics of three limestones were investigated in a pilot scale fluidizedbed reactor. For each of these sorbents, the measured CO2 capture capacity decreased as the number of cycles increased and as the SO2 concentration increased. On the other hand, the SO2 capture increased with the number of cycles and the SO2 concentration. The total calcium utilization decreased as the number of cycles increased, but the effect of SO2 concentration on the total calcium utilization depended on the sulfation pattern of limestone. For one limestone (with unreacted-core-type sulfation), the total calcium utilization decreased with increasing SO2 concentration. However, for the other two limestones (with uniform-type sulfation), the total calcium utilization was almost independent of SO2 concentration for the range investigated. The results show that SO2 reduces the CO2 capture capacity of limestone and indicate that the sulfation patterns affect the CO2 capture capacity.

1. Introduction Carbon dioxide, the major greenhouse gas, is produced in large quantities from the combustion of fossil fuels, much of this related to electric power generation. Current technologies for CO2 sequestration include the disposal of CO2 in deep oceans, depleted oil and gas fields, and deep saline formations (aquifers) and the recovery of enhanced oil, gas, and coal-bed methane. However, these technologies are expensive. Capturing CO2 from flue-gas streams is an essential step for sequestrating CO2. Reducing the cost of CO2 capture will be a critical step in overall carbon management.1 Much attention has been recently directed toward costeffective and energy-efficient CO2 capture techniques for fossilfuel-fired power plants. There are many techniques available to capture CO2 emissions with different advantages and limitations. One of the more promising concepts involves absorption by dry regenerable sorbents.2 Limestone-based sorbents are very attractive as dry sorbents for CO2 capture because of their natural abundance and low cost. The possibility of using calcined limestone to remove CO2 from a gas stream was considered as early as the late 19th century.3 Recently, it has been suggested that calcium-based sorbents (limestone and dolomite) may be able to remove CO2 from flue gas and, by means of subsequent calcinations, produce * Corresponding author. Tel: +82-42-860-3794. Fax:+82-42-860-3134. E-mail: [email protected]. † Korea Institute of Energy Research. ‡ University of British Columbia. (1) Gray, M. L.; Champagne, K. J.; Soong, Y.; Kilmeyer, R. P.; Baltrus, J.; Maroto-Valer, M. M.; Andersen, J. M.; Ciocco, M. V.; Zandhuis, P. H. EnVironmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Centur, 1st ed.; Academic/Plenum: New York, 2002. (2) Hoffman, J. S.; Pennline, H. W. Investigation of CO2 Capture Using Regenrable Sorbents. 17th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, 2002. (3) Salvador, C.; Lu, D.; Anthony, E. J.; Abanades, J. C. Enhancement of CaO for CO2 capture in an FBC Environment. Chem. Eng. J. 2003, 96, 187-195.

a pure CO2 stream for sequestration. Figure 1 shows a basic concept for CO2 capture and regeneration with a limestonebased sorbent. The overall system consists of a carbonator and a calciner. In the carbonator, the fuel is burned with excess lime at temperatures of 600-750 °C under atmospheric pressure,4 where the carbonation reaction takes place to form CaCO3:

CaO + CO2 f CaCO3

(1)

The reverse calcination (regeneration) reaction takes place in the calciner at higher temperatures to regenerate the sorbent and produce concentrated CO2:

CaCO3 f CaO + CO2

(2)

Fresh “makeup” sorbent is continuously fed to the system to compensate for the decay in sorbent capacity. The concentrated CO2 produced by the calciner can be sequestered or used for some other purpose. For the process to be continuous, the sorbent must be regenerated after the carbonation reaction. Several previous studies,3,5-9 summarized in Table 1, have investigated the multicycle performance of carbonation/regeneration. This work (4) Abanades, J. C.; Alvarez, D. Conversion Limits in the Reaction of CO2 with Lime. Energy Fuels 2003, 17, 308-315. (5) Curran, G. P.; Fink, C. E.; Gorin, E. CO2 Acceptor Gasification Process-Studies of Acceptor Properties. AdV. Chem. Ser. 1967, 69, 141165. (6) Barker, R. The Reversibility of the Reaction CaCO3 ) CaO + CO2. Appl. Chem. Biotechnol. 1973, 23, 733-742. (7) Silaban, A.; Narcida, M.; Harrison, D. P. Characteristics of the Reversible Reaction between CO2 and Calcined Dolomite. Chem. Eng. Commun. 1996, 146, 149-162. (8) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. A Twin Fluid-Bed Reactor for Removal of CO2 from Combustion Processes. Trans. IchemE 1999, 77A, 62-68. (9) Aihara, M.; Nagai, T.; Matsushita, J.; Negishi, Y.; Ohya, H. Development of Porous Solid Reactant for Thermal-Energy Storage and Temperature Upgrade Using Carbonation/Decarbonation Reaction. Appl. Energy 2001, 69, 225-238.

10.1021/ef050277q CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006

1622 Energy & Fuels, Vol. 20, No. 4, 2006

Ryu et al.

Figure 1. Simplified schematic of CO2 capture and regeneration process (adapted from Salvador et al.3).

has indicated that the recarbonation is far from being reversible in practice. Instead, the maximum carbonation capacity decreases rapidly with cycling as a result of the loss of suitable pore volume in the sorbent during successive calcination steps.4 However, most of these investigations were carried out using thermogravimetric analyzers and fixed-bed reactors.6-9 Little previous work has been reported on multicycle operation under fluidized-bed conditions.3,5 Moreover, previous studies did not consider the effect of SO2 on the reversibility of the carbonation/calcination reaction. Limestones (and to a lesser extent dolomite) are commonly used as sorbents for SO2 in fluidized-bed combustors, on the basis of the overall reaction

1 CaO + SO2 + O2 f CaSO4 2

(3)

This reaction is almost irreversible. Although fossil fuels inevitably contain at least some sulfur so that reaction 3 is bound to occur, to some extent at least, in parallel with reaction 1 under combustion conditions, previous carbonation studies have been predominantly carried out in pure CO2 or in gas mixtures of CO2 and N2 (or air). Therefore, experimental work is needed to investigate the effect of SO2 concentrations on cyclic simultaneous CO2/SO2 capture for typical flue gas conditions. This study examines the effects of the number of cycles and the SO2 concentrations on simultaneous CO2/SO2 capture characteristics. Experiments were carried out in a small fluidized-bed reactor for three limestones. SO2 concentrations were varied, and multicycle runs were carried out to determine the CO2 capture capacity, SO2 capture capacity, and total calcium utilization for three limestones over time.

Figure 2. Schematic of bubbling fluidized-bed reactor. (T and P designate thermocouples and pressure transducers, respectively.)

2. Experimental Section The multicycle tests were carried out in a bubbling fluidizedbed reactor. A schematic of the reactor is shown in Figure 2. The major components consist of a preheater, the fluidization column, a hot gas filter, a gas cooler, and a gas-sampling unit. The preheater has a height of 0.71 m and an internal diameter of 0.1 m. The lower zone of the reactor above the preheater is 0.66 m high with an internal diameter of 0.1 m, whereas the upper zone is 0.37 m high with an internal diameter of 0.15 m. A conical section of height 0.14 m connects the two parts, so that the total height of the reactor is 1.17 m. A perforated gas-distributor plate, with 34 drilled 1.2 mm diameter holes on a hexagonal grid, separates the preheater and reactor. Reactant gas was fed to the lower part of the preheater. Three electrical heaters, heating different sections, could be controlled individually. Thermocouple measurements of temperature

and pressure transducer data were recorded by a data acquisition system. The exit stream from the fluidized-bed reactor was sampled at the outlet of the freeboard zone. The CO2, SO2, and O2 concentrations were determined using an on-line gas analyzer (Horiba PG-250) and recorded by a data acquisition system. Further details of the reactor system are available elsewhere.10,11 The three limestones tested were Strassburg from the U. S., Luscar from Canada, and Danyang from Korea. The compositions (10) Constantineau, J. P. Fluidized Bed Roasting of Zinc Sulfide Concentrate: Factors Affecting the Particle Size Distribution. Ph.D. dissertation, University of British Columbia, Vancouver, Canada, 2004. (11) Johnsen, K.; Ryu, H. J.; Grace, J. R.; Lim, C. J. Sorption-Enhanced Steam Reforming of Methane in a Fluidized Bed Reactor with Dolomite as CO2-Acceptor. Chem. Eng. Sci. 2006, 1195-1202.

0.65-1.675 completionb 0 (Air) 850 until completionb 0.15 (air + CO2) BFB Salvador et al.3

700

TGA Aihara et al.9

a BFB, bubbling fluidized bed; TGA, thermogravimetrical analyzer; PTGA, pressurized TGA; NA, not available; P b CO2, CO2 partial pressure in input gas. Completion: Each step was stopped when the solid mass or CO2 concentration was deemed to be essentially constant.

14

10 10 (spherical pellet)

char gasifier TGA PTGA packed-bed reactor Curran et al.5 Barker6 Silaban et al.7 Shimizu et al.8

750

1h

750

0 (N2)

1h

alkoxide, CaCO3 (reagent), CaCO3+CaTiO3 Havelock limestone, Cadomin limestone

70 26 5 4 >1 0.002-0.02 < 0.038 0.42-0.59 S. Dakota limestone CaCO3 (reagent) dolomite (National Lime Co.) Chichibu limestone NA until completionb 20 min NA 4 (regenerator outlet gas) 0 (N2) 0 (N2) 1 (pure CO2) 1060 866 750 950 NA until completionb 20 min NA

1.28 (gasifier outlet gas) 1.0 (pure CO2) 0.15 (N2 balance) 0.05 0.15 (N2 balance) 0.2

reaction time PCO2 (atm) reactor type authors

816 866 550 600

PCO2 (atm)

regeneration (calcination) carbonation (CO2 capture)

temp. (°C) temp. (°C)

Table 1. Summary of Reaction Conditions for Previous Studies on the Cyclic CO2 Capture/Regeneration Processa

particle size (mm) particles reaction time

number of cycles

CO2/SO2 Capture Characteristics of Three Limestones

Energy & Fuels, Vol. 20, No. 4, 2006 1623 Table 2. Origin and Properties of the Three Limestones limestone properties/ composition

Strassburg (U. S. A.)

Luscar (Canada)

Danyang (Korea)

MgO (%) CaO (%) Na2O (%) SiO2 (%) Al2O3 (%) Fe2O3 (%) K2O (%) TiO2 (%) P2O5 (%) MnO (%) Cr2O3 (%) Ba (ppm) Ni (ppm) Sr (ppm) Zr (ppm) Y (ppm) Nb (ppm) Sc (ppm) particle size range (µm) bulk density (kg/m3) sulfation patterns

0.59 55.46 0.01 0.7 0.2 0.05 0.08 0.01 0.01 0.01 0.001 80 6 284 5 5 5 1 355-600 1362 unreacted core

2.26 52.39 0.01 1.7 0.28 0.04 0.14 0.01 0.01 0.01 0.001 10 9 263 5 5 10 1 355-600 1194 uniform

1.03 52.94 0.01 1.55 0.9 0.29 0.32 0.05 0.01 0.03 0.001 17 12 185 7 5 5 1 355-600 1317 uniform

of these limestones are provided in Table 2, together with their particle size ranges, bulk densities, and sulfation patterns. Prior to the start of each experiment, the limestone was sieved to ensure that all particles were initially between 355 and 600 µm in size. The static bed height was 0.2 m in all cases, and the experiments were carried out batchwise for the solids; that is, no particles were added during the run. The fluidized-bed reactor operated with a total inlet gas flow of 1.08 Nm3/h in all cases, corresponding to superficial gas velocities between 0.13 m/s at 700 °C and 0.15 m/s at 850 °C. Limestone was calcined in the air as the bed temperature was increased from room temperature to 850 °C. An on-line gas analyzer was used to determine complete calcination, corresponding to the CO2 dropping to a negligible level in the exit stream. Once the limestone was fully calcined, the

Figure 3. CO2 concentration vs time during CO2 capture tests.


Ryu et al. Table 3. Summary of Reaction Conditions

steps

temp. (°C)

initial calcination CO2 capture CO2/SO2 2000 ppm capture CO2/SO2 4000 ppm capture calcination a

A.C.a

f 850 700 700 700 700 f 850

input gas concentration air CO2 16%, O2 5%, N2 balance CO2 16%, O2 5%, SO2 2000 ppm, N2 balance CO2 16%, O2 5%, SO2 4000 ppm, N2 balance air

flow rate Nm3/h

1.1 1.1 Nm3/h 1.1 Nm3/h 1.1 Nm3/h 1.1 Nm3/h

static bed height 0.2 m 0.2 m 0.2 m 0.2 m 0.2 m

A.C.: ambient conditions.

Figure 5. CO2 capture capacity as a function of the number of cycles. (For details of literature results, see Table 1.)

Figure 4. CO2 concentration as a function of the number of cycles during regeneration tests.

temperature was lowered to 700 °C and the lime was exposed to a simulated flue gas for a simultaneous CO2/SO2 capture test. The simulated flue gas contained 16% CO2 and 5% O2 for all sorption tests. The balance was either nitrogen alone (for CO2 capture tests) or 2000 or 4000 ppm SO2 and the rest N2 for the simultaneous CO2/SO2 capture tests. The inlet concentrations were measured by bypassing the reactor. A rapid increase in the exit CO2 concentration marked the end of each cycle. Once CO2 or CO2/SO2 capture was deemed to be complete, the bed temperature was increased again to 850 °C to regenerate/calcine the limestone in the air. For each set of operating conditions, 10 cycles of CO2 or CO2/SO2 capture regeneration were carried out. The experimental conditions are summarized in Table 3.

3. Results and Discussion Typical traces of CO2 concentration versus time for the Strassburg and Danyang limestones during the CO2 capture tests (0 ppm SO2) for 10 cycles appear in Figure 3. The outlet CO2 concentrations were only 2-5% for an initial period, similar to the CO2 partial pressure corresponding to thermodynamic equilibrium, that is, 0.035 atm, for this temperature and pressure.12 After some time, the CO2 concentration increased quickly and then increased very slowly because of the depletion of CaO in the limestone. The abrupt increase in CO2 concentra-

Figure 6. CO2 concentration vs time during the simultaneous capture of CO2 and SO2.

tion occurred when the CO2 capture capacity of the CaO was nearly exhausted. It is clear from Figure 3 that the duration of (12) Barker, E. H. The Calcium Oxide-Carbon Dioxide System in the Pressure Range 1-300 Atmospheres. J. Chem. Soc. 1962, 464-470.


Figure 7. CO2 concentrations as a function of the number of cycles during regeneration after the simultaneous capture of CO2 and SO2.

the effective sorption period decreased as the number of cycles increased, because of the loss of calcium capture capacity. Silaban and Harrison13 reported that the loss of capacity was associated with a change in structural properties of the sorbent. It was claimed that reduced porosity left the interior of the particles inaccessible to CO2. Abanades and Alvarez4 reported that the maximum carbonation capacity decreases rapidly with multiple cycles as a result of the loss of suitable pore volume in lime-based sorbents during successive calcination steps. As a consequence, the CO2 capture capacity for the limestones decreased with the number of cycles. The CO2 capture capacity was determined from the difference in the CO2 concentrations between the inlet and exit integrated over time before the carbonation was completed and the initial amount of limestone, that is

Energy & Fuels, Vol. 20, No. 4, 2006 1625

Figure 8. SO2 concentration vs time during the simultaneous capture of CO2 and SO2 at two SO2 concentrations.

agreement (average difference of 2.1%). In this study, we used an average of these two values as the CO2 capture capacity. The effects of CO2 capture-regeneration cycling on the CO2 capture capacities of the three limestones in the fluidized bed are shown in Figure 5. For the sake of comparison, data from previous studies5-9 (see Table 1) are also presented. The solid line represents an empirical correlation

xCO2,N ) f N+1 + b

(5)

After the CO2 capture tests, all sorbents were subjected to regeneration. Figure 4 illustrates typical regeneration (calcination) curves for the Strassburg and Danyang limestones. The CO2 concentration increased with time, before dropping abruptly to 0%. The duration of the regeneration and the total amount of CO2 released decreased with an increase in the number of cycles, as reported in previous work. Figure 4 can also be used to determine the CO2 capture capacity by integration of the CO2 released with time. The CO2 capture capacity can be determined either from the CO2 response curves during CO2 capture or from the regeneration tests, and these two values showed excellent

proposed for the decay of the CO2 capture capacity by Abanades,14 based on TGA and fixed-bed data of several studies. Here, xCO2,N is the relative CO2 capture capacity after the Nth cycle, whereas f and b are fitted empirical constants, f ) 0.782 and b ) 0.174. The CO2 capture capacities for all three of our limestones decreased with the number of cycles, and in general, all three followed the same trend, although there were small differences in the capacities, particularly during the first few cycles. However, the results of this study show a somewhat slower decay of the CO2 capture capacity with increasing cycle number than in previous work and in the empirical curve corresponding to eq 5. For simultaneous CO2/SO2 capture, some portions of CaO absorb CO2, while others absorb SO2, so that the CO2 capture capacity decreases with increasing SO2 concentration. Figure 6 shows CO2 concentration versus time curves for Strassburg limestone for simultaneous CO2/SO2 capture at different SO2 concentrations. The results show that the duration of the simultaneous CO2/SO2 capture reaction decreased as the number

(13) Silaban, A.; Harrison, P. High-Temperature Capture of Carbon Dioxide: Characteristics of the Reversible Reaction Between CaO (s) and CO2 (g). Chem. Eng. Commun. 1995, 137, 177-190.

(14) Abanades, J. C. The Maximum Capture Efficiency of CO2 Using a Carbonation/Calcination Cycle of CaO/CaCO3. Chem. Eng. J. 2002, 90, 303-306.

CO2 capture capacity )

moles of CO2 absorbed (4) moles of Ca in the sorbent


Ryu et al.

Figure 9. Comparison of CO2 capture capacity, SO2 capture capacity, and total calcium utilization for three limestones.

of cycles increased, consistent with Figure 3. A comparison of Figure 3a and Figure 6 indicates that, for a given cycle number, the abrupt increase in CO2 concentration occurred earlier at a higher SO2 concentration, and the duration of appreciable CO2/ SO2 capture decreased with increasing SO2 concentration. Figure 7 shows outlet CO2 concentration curves for Strassburg limestone during regeneration periods after simultaneous CO2/SO2 capture at different SO2 concentrations. The total release of CO2 decreased with successive cycles, consistent with Figure 4. A comparison of Figure 4a and Figure 7 indicates that, for any given number of cycles, the total amount of released CO2 during regeneration decreased with increasing SO2 concentration. Figure 8 shows typical SO2 concentration versus time curves for Strassburg limestone during simultaneous CO2/SO2 capture tests at different SO2 concentrations. At 2000 ppm SO2, the input SO2 was absorbed entirely and not detected in the outlet gas until the fourth cycle. During the fifth cycle, SO2 was detected after 100 minutes. After this cycle, SO2 was detected earlier in each successive cycle. At 4000 ppm SO2, the SO2 concentration versus time curves show similar trends to those for 2000 ppm SO2, but SO2 was detected earlier, from the fourth cycle onward. For both SO2 concentrations, the slopes of the SO2 concentration versus time curves increased with the number of cycles, presumably because of an accumulation of SO2 on the sorbent, because the SO2 capture reaction, eq 3, is irreversible during the regeneration intervals.

SO2 capture capacity was determined from the difference in the SO2 concentrations between the inlet and exit gases, the total reaction time, and the initial mass of limestone present:

cumulative SO2 capture capacity ) cumulative moles of absorbed SO2 (6) moles of Ca in the sorbent Figure 9 compares the CO2 capture capacities, SO2 capture capacities, and total calcium utilizations of the three limestones for different SO2 concentrations. The total calcium unitization is the ratio of absorbed moles of CO2 and SO2 to the number of moles of Ca in the sorbent, that is

total calcium utilization ) cumulative moles of absorbed CO2 and SO2 (7) moles of Ca in the sorbent For all three limestones, the CO2 capture capacity decreased with increasing number of cycles and SO2 concentration. Increased exposure to SO2 clearly reduced the CO2 capture. At the same time, the SO2 capture increased with the number of cycles because absorbed SO2 during simultaneous CO2/SO2 capture was not desorbed during the regeneration. The cumulative SO2 capture capacity also increased with the SO2 concentration. The results indicate that, even though it is present in a


Energy & Fuels, Vol. 20, No. 4, 2006 1627

Figure 10. Illustration of two different sulfation patterns (modified from Laursen et al.15).

much smaller concentration (2000 or 4000 ppm vs 16% CO2), the SO2 capture reaction eventually shuts down the CO2 capture reaction for simultaneous CO2/SO2 capture conditions. The CO2 capture capacities are almost the same for all three limestones at 0 ppm SO2 concentration, as shown in Figures 5 and 9a. When SO2 was present, the CO2 capture capacities differed for the three limestones. Danyang limestone gave the best CO2 capture capacities, with Luscar limestone the next best, and Strassburg limestone the worst, for the same SO2 concentrations. However, at the same SO2 concentration (2000 ppm), Luscar limestone showed better cumulative SO2 capture capacities than Danyang and Strassburg limestones. The total calcium utilization decreased with cycling for all three limestones. However, the effect of SO2 concentration on the total calcium utilization depended on the limestone. For Strassburg limestone, the total calcium utilization decreased with increasing SO2 concentration, whereas the total utilization for the Luscar and Danyang limestones was almost independent of the SO2 concentration. The results can be considered in the light of different sulfation patterns of the three sorbents. Laursen et al.15 reported that sorbent particles may sulfate in three different ways, that is, unreacted core, network, and uniform, depending on how the sulfur is distributed through the sorbent particles. These sulfation trends depend on the morphology (i.e., porosity, grain size, fracture size, and configuration) of calcined limestone. The Strassburg limestone sulfated in an unreacted core manner, whereas Luscar limestone showed uniform sulfation. These two types of limestone reacted quite differently when subjected to sulfation as shown in Figure 10. For the unreacted-core-type sulfation, the reaction between the CaO and SO2 takes place mainly in the outer layers of the limestone particle, and a dense, nearly nonporous CaSO4 layer or shell forms on the outside. For the uniform-type sulfation, on the other hand, CaSO4 forms in the outer layers of individual CaO grains throughout the sorbent particles.15,16 At the same SO2 concentrations, the unreacted-core-type limestone (Strassburg) forms thicker CaSO4 layers than the uniform-type limestone (Luscar), and it is more difficult for CO2 to penetrate the product layer of the sulfated limestone. This is consistent with the Luscar limestone showing better CO2 and SO2 capture capacity than the Strassburg limestone. (15) Laursen, K.; Duo, W.; Grace, J. R.; Lim, J. Sulfation and Reactivation Characteristics of Nine Limestones. Fuel 2000, 79, 153-163. (16) Laursen, K.; Duo, W.; Grace, J. R.; Lim, J. Characterization of Steam Reactivation Mechanisms in Limestones and Spent Calcium Sorbents. Fuel 2001, 80, 1293-1306.

Figure 11. BSE images (a) and corresponding sulfur X-ray mappings (b) for sulfated Danyang limestone. (Dark spots represent areas richer in sulfur.)

To determine the sulfation pattern of Danyang limestone, the sulfur distribution through the sorbent particle was checked by X-ray mapping after full sulfation. The sorbent was sulfated in a thermogravimetric reactor, used in previous studies of limestone sulfation and reactivation.16,17 The Danyang limestone was dried overnight at 125 °C and sieved. The 355-600 µm size fraction of limestone was then calcined in the air as the temperature was increased from room temperature to 900 °C. The initial solid mass was 0.85 g, and the air flow rate was 1600 mL/min. An on-line CO2 analyzer determined the completion of calcination, corresponding to the CO2 dropping to a negligible level in the exit stream and the solid mass reaching a steady level. After the limestone was fully calcined, the temperature was lowered to 850 °C, and the lime was exposed to gas containing 3% O2, 2900 ppm SO2, and the balance N2 at a total flow rate of 1600 mL/min. The sulfation conditions were (17) Duo, W.; Laursen, L.; Lim, J.; Grace, J. R. Crystallization and Fracture: Formation of Product Layers in Sulfation of Calcined Limestone. Powder Technol. 2000, 111, 154-167.


Ryu et al.

maintained for 10 h. An on-line SO2 analyzer and balance were used to determine the completion of sulfation. After the limestone was fully sulfated, backscattering electron (BSE) imaging and X-ray mapping of the fully sulfated limestone (i.e., the sample was sulfated until the outlet SO2 concentration had fully stabilized) were performed. Figure 11a shows a BSE image of sulfated limestone, and Figure 11b shows the corresponding sulfur X-ray mapping. The dark spots represent areas rich in sulfur. As shown in Figure 11b, Danyang limestone, like Luscar, is primarily composed of uniform sulfated particles, whose product layers can be more easily penetrated as shown in Figure 10. This is consistent with the Luscar and Danyang limestone showing better capture and more similar behavior than Strassburg limestone, as shown in Figure 9.

decreased more quickly when SO2 was present and at higher SO2 concentrations. The SO2 capture capacity increased with an increasing number of cycles and increasing SO2 concentration because of the accumulation of SO2 in the sorbent. The total (CO2 and SO2) calcium utilization decreased with cycling, but the effect of SO2 concentrations on the total calcium utilization differed for the three limestones. For Strassburg limestone, which follows unreacted-core-type sulfation, the total calcium utilization decreased with increasing SO2 concentration. However, for Luscar and Danyang limestones, both of which follow uniform sulfation patterns, the total calcium utilization was almost independent of the SO2 concentration. SO2 in the flue gas reduces CO2 capture capacity. The sulfation patterns also appear to affect CO2 capture capacity.

4. Conclusions

Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean government (MOEHRD, Basic Research Promotion Fund No. M01-2004-00010116-0) and Natural Sciences and Engineering Research Council of Canada (NSERC). The authors thank Dr. E. J. Anthony for supplying the Luscar limestone.

Experiments were performed on three limestones in a fluidized-bed reactor. In all three cases, the CO2 capture capacity decreased as the number of cycles increased. The decay in CO2 capture capacity limits the usefulness of limestone for CO2 capture-regeneration processes. The CO2 capture capacity

EF050277Q

SO2 Capture Characteristics of Three Limestones

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