Environ. Sci. Technol. 2003, 37, 3709-3715
Characterization of the Enhancement Effect of Na2CO3 on the Sulfur Capture Capacity of Limestones KARIN LAURSEN* Department of Chemical Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan ARNT A. KERN Bruker AXS GmbH, O ¨ stliche Rheinbru ¨ ckenstrasse 50, 76187 Karlsruhe, Germany JOHN R. GRACE AND C. JIM LIM Department of Chemical and Biological Engineering, University of British Columbia, 2216 Main Mall, Vancouver, Canada V6T 1Z4
It has been known for a long time that certain additives (e.g., NaCl, CaCl2, Na2CO3, Fe2O3) can increase the sulfur dioxide capture-capacity of limestones. In a recent study (1) we demonstrated that very small amounts of Na2CO3 can be very beneficial for producing sorbents of very high sorption capacities. This paper explores what contributes to these significant increases. Mercury porosimetry measurements of calcined limestone samples reveal a change in the pore-size from 0.04-0.2 µm in untreated samples to 2-10 µm in samples treated with Na2CO3sa pore-size more favorable for penetration of sulfur into the particles. The change in pore-size facilitates reaction with lime grains throughout the whole particle without rapid plugging of pores, avoiding premature change from a fast chemical reaction to a slow solid-state diffusion controlled process, as seen for untreated samples. Calcination in a thermogravimetric reactor showed that Na2CO3 increased the rate of calcination of CaCO3 to CaO, an effect which was slightly larger at 825 °C than at 900 °C. Peak broadening analysis of powder X-ray diffraction data of the raw, calcined, and sulfated samples revealed an unaffected calcite size (∼125-170 nm) but a significant increase in the crystallite size for lime (∼60-90 nm to ∼250-300 nm) and less for anhydrite (∼125-150 nm to ∼225-250 nm). The increase in the crystallite and pore-size of the treated limestones is attributed to an increase in ionic mobility in the crystal lattice due to formation of vacancies in the crystals when Ca is partly replaced by Na.
Introduction Sulfur dioxide emissions from combustion of sulfur-containing fuels such as petroleum-coke and coal causes serious environmental damage. The most commonly applied method of controlling these emissions is reaction with limestonederived sorbents in the form of lime or calcium hydroxide. In fluidized bed boilers, limestone (primarily calcite) is added directly to the combustion chamber, and the dense stone is * Corresponding author phone: +81-75-753-5599; fax: +81-75753-5909; e-mail:
[email protected]. 10.1021/es026090k CCC: $25.00 Published on Web 07/15/2003
2003 American Chemical Society
rapidly transformed into a highly porous calcined lime of large surface area. In the presence of oxygen, the lime reacts with SO2 forming CaSO4. Traditionally, the sulfation process has been visualized as taking place in two stages: (1) SO2/O2 diffusion through the pores of the lime particle and (2) chemical reaction of SO2 and O2 with CaO on the surfaces of individual grains (2). Due to the larger molar volume of CaSO4 compared to CaO, small pores in the lime are plugged, leading to particles with sulfated rims and unsulfated cores. This picture of the sulfation process is commonly referred to as the unreacted-core model (3-6). Similar behavior can apply also to sulfation of individual CaO grains within limestone particles (7-8). Once a CaSO4 coating has formed on the grain surfaces, ionic diffusion through the CaSO4 layer becomes rate-limiting (9). At a certain CaSO4 layer thickness, solid-state diffusion becomes rate-controlling. Eventually diffusion becomes so slow that full utilization of the CaO is not achieved, and the sulfation process is usually considered to have stopped (10), although there is recent evidence that sulfation continues at a very slow rate (11-12). Recent research on sulfation of limestones (13) reveals that some limestones do not follow the unreacted-core model. We divide sulfated particles into three groups: (a) unreactedcore particles have a highly sulfated rim (70-95% local utilization efficiency) and a slightly (0-5%) or unsulfated core; (b) uniformly sulfated particles exhibit a relatively homogeneous degree of sulfation (40-65%); and (c) network particles are only sulfated around the periphery and near fractures, with slightly or unsulfated cores of each “subgrain” separated by fractures (13). Due to the finite time of exposure and low calcium utilization (25-45% (14)), a large proportion of limestone sorbents pass through combustors without capturing sulfur. Several methods have been proposed to increase sorbent utilization, mostly focused on water or steam hydration (13, 15-17). Less attention has been focused on other techniques such as pretreatment with additives (1, 19-24). Salts have been used to change the physical properties of calcined limes in the lime industry for many years, but have found little use in fluidized bed combustors and other flue-gas De-SOx methods. Earlier work has indicated that the sulfur capture capacity of limestones under FBC conditions can be increased significantly through treatment with various additives including NaCl, CaCl2, Na2CO3, and Fe2O3 (19-24). When selecting an additive, it is important to choose compounds with no, or very limited, side-effects that can easily and costeffectively be added to the sorbent. Addition of chloridecontaining compounds could create unwanted side effects such as increasing superheater corrosion and toxic byproducts (25). High levels of sodium are also undesirable in an FBC environment due to its association with ash-softening leading to increasing superheater deposition and bed agglomeration (26) and to the formation of alkali-metal sulfates which enhance corrosion of heat-exchanger surfaces (25). However, if the amount of sodium is maintained at a low level (less than 1%) it should not increase agglomeration problems (27). Potential side effects of an additive will depend strongly on its fate during the calcination and sulfation processes, issues discussed in this paper. Another factor which has limited the interest in using additives to improve the utilization of the sorbents is the low cost of raw limestone. However, increasing waste disposal costs, the need to reduce CO2 emissions, and global moves to minimize the use of natural resources make it important to reconsider the possibility of salt addition. VOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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In recent work (1), we showed that addition of small amounts of Na2CO3 can significantly increase the sulfur capture capacity of limestones normally sulfating in an unreacted-core and patchy-network mode, while treatment has less effect on uniformly sulfating particles. For the particle size range 250-425 µm, the increase for the three sulfating modes were as follows: 20-24% to 45-47% for unreactedcore, 33% to 49% for patchy-network, and 44% to 52% for uniformly sulfating. Thus, the treatment more or less eliminated the strong influence of limestone type on the sulfur capture capacity. It is commonly accepted that the capture capacity of limestones is strongly dependent on particle size (4, 28-29). However, experiments on three sizeranges (37-72 µm, 250-425 µm, 1000-1180 µm) of the unreacted-core sulfating limestone revealed that the treatment significantly reduced the strong dependence on particle size of the untreated sample. The amount of Na2CO3 solution added had no effect on the increase in calcium utilization, with as little as 0.6 wt % of Na2CO3 being sufficient to significantly increase the capture capacity. The purpose of the research presented in this paper was to further characterize the Na2CO3 treated limestone by mercury porosimetry and power X-ray diffraction (XRD), to determine what causes the significant sulfur capture capacity enhancement of limestones treated with Na2CO3. Combined porosimetry and XRD can provide essential information on two very important parameters controlling the sulfur capture capacity: pore and crystallite size.
Experiments Calcination and sulfation experiments were carried out in two specially designed dual-environment reactors: a fixed bed and a thermogravimetric reactor (TGR). The reactors were located inside two furnaces one on top of the other, on a scissors-lifting table. The temperature of the upper furnace was held at high temperature (825-900oC), while the lower furnace was maintained at room temperature. The fixed bed quartz 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 tall and had an inside diameter of 37 mm. Thermocouples located directly below and above the bed controlled and continuously recorded the local gas temperatures. The sample holder, located at the center of the reactor, consisted of a tube with a sintered quartz filter acting as the bed support. The dualenvironment TGR utilized the same main quartz body as the fixed bed reactor, while the upper and lower adaptors were replaced by different upper and lower sections. The upper section consisted of a quartz adaptor below a steel chamber. The chamber housed a load cell (Transducer Technique, GSO10), while the quartz adaptor minimized stress on the joint between the steel chamber and the main reactor during heating. The load cell was connected by a very thin wire to a permeable wire-mesh basket suspended from above through a small aperture, across which there was a small positive pressure difference to prevent any gas leakage into the load cell chamber from the reactor. A thermocouple immediately below the basket monitored the temperature of the sample. Experiments were conducted on Strasburg Limestone. This limestone was found in our previous work to predominantly sulfate according to the unreacted-core model, the sulfation pattern which benefits most from reactivation and pretreatment (1, 13, 30). Samples of prescreened (250-425 µm and 1000-1180 µm) limestone were mixed with 15% of a 20%-by-weight aqueous Na2CO3 solution. The salt solution was poured over the samples, blended with a cake mixer for 1 min, and dried overnight at 110-120 °C. The samples were calcined at 825 °C or 900 °C in the fixed bed reactor or the TGR with a flow of 250 mL/min air and 50 mL/min N2. After 3710
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complete calcination the samples were loaded into the fixed bed reactor and sulfated at 825 °C (2250 ppm SO2, 1600 mL/ min total gas flow, and 3% oxygen). An NDIR analyzer continuously monitored the SO2 concentration of the outlet stream during sulfation. The calcium-utilizations of the samples were determined by weight gain and by integrating the sulfation curves.
Analytical Methods SEM analyses were carried out to determine the morphology and relative porosity of calcined and sulfated limestone samples. The sulfation morphologies were evaluated based on sulfur X-ray mapping (“dot-mapping”) of epoxy embedded, cross-sectioned, and polished particles. Pore-sizes of calcined samples were measured by a Micromeritics Instruments PoreSizer 9320. This instrument provides information on pore-size, volume, and distribution as well as pore surface area for pores in the diameter range 0.003-360 µm. Powder X-ray diffraction data were collected on a Siemens D5005 Bragg-Brentano diffractometer using CuKR radiation operating at 40 kV and 40 mA. Peak width changes were used to estimate the average crystallite size of the main phases (i.e., calcite (CaCO3) in raw limestones, lime (CaO) in calcined samples, and anhydrite (CaSO4) in sulfated samples). The crystallite size was estimated based on single-peak broadening (31) using the TOPAS R program (Bruker DiffracPlus, Version 2.0) and the fundamental parameters approach (FPA) (32). Because the instrument itself contributes to peak broadening, diffraction patterns of two standards with minimum broadening (LaB6 NIST SRM660a and R-A12O3 NIST SRM676) were analyzed to account for the instrument contributions. To monitor peak shifts due to sample displacement or misalignment, 10% of a Si-standard (NIST SRM 640c) was added to the powdered samples. For single-peak refinements a 2θ-range around the main peak(s) (calcite: [104] 28.2-30.5°; lime: [200] 36.8-38.2°; anhydrite: [200] and [020] 24.7-26.3°) was measured with a step-size of 0.01° and counting times of 10 s per step. Due to moisture absorption of the highly hydroscopic calcined samples causing transformation of lime (CaO) to portlandite (Ca(OH)2) even during analysis very few of the calcined limestones were suitable for determination of crystallite size.
Results TGR Experiments. To further characterize the effect of alkali treatment during calcination, Na2CO3-treated and untreated samples (1000-1180 µm) were calcined in the TGR at 825 and 900 °C (Figure 1 and Table 1) and sulfated in the fixed bed reactor. As seen by the weight loss curves registered during calcination, transformation of a 2 g sample at 900 °C was completed within less than 10 min for a Na2CO3-treated and 13 min for an untreated sample, whereas at 825 °C the process required 45 and 55 min, respectively. The calcination process is, as expected, strongly affected by temperature, but the reduction in time needed for the completion of the transformation is also influenced by the Na2CO3 treatment. The time required for the calcination process for both treated and untreated samples is much longer than the time reported for calcination of individual particles in TGA experiments (33, 34). However, similar times have been reported previously from experiments in a bench-scale reactor (35). The time required for calcination of individual particles in a TGA and a full-scale fluidized bed will of course be significantly less than determined here for a relatively dense column of particles in a wire-mesh basket where both the external heat and mass transfer will be lower. However, it is expected that the calcination rate of individual particles in an FBC would also be increased by the Na2CO3 treatment.
FIGURE 1. Weight loss during calcinations at 825 °C and 900 °C of untreated Strasburg Limestone and samples treated with a 20% Na2CO3 solution.
TABLE 1. Results of Sulfation of Untreated Strasburg Limestone (1000-1180 µm) and Samples Treated with a 20% Na2CO3 Solution Calcined in the TGR and Sulfated in Fixed Bed Reactora
TGA-ThL-C6 TGA-ThL-C7 TGA-ThL-C8 TGA-ThL-C9 TGA-ThL-C2 TGA-ThL-C15 TGA-ThL-C5 TGA-ThL-C16 a
Na2CO3 solution (wt %)
calc temp ( °C)
calc time (min)
Ca-util (%) mass gain
Ca-util (%) integration
porosity (%)
pore diameter average (µm)
0 0 15 15 0 0 15 15
825 825 825 825 900 900 900 900
60 60 60 60 15 60 15 60
7 11 46 47 9 11 36 31
11 12 49 51 12 13 41 35
40 42 37 51
0.07 0.08 0.73 1.04
Ca-utilizations are based on mass gain and integration of sulfur emission curves.
FIGURE 2. Porosimetry curves for untreated Strasburg Limestone and samples treated with a 20% Na2CO3 solution calcined at 825 °C and 900 °C. Whether an increase in calcination rate is beneficial for the sulfation process and the final capture-capacity of the sorbent is unclear. In utility boilers where limestone is added without precalcination, the sulfation and calcination processes within limestone particles occur more or less simultaneously and in competition. Mulligan et al. (16) reported that for TGA experiments at temperatures higher than 850 °C direct sulfation gave similar sulfation efficiencies to those determined for samples which were fully calcined prior to exposure to SO2-rich flue gas. These results are in agreement with our previous results on direct sulfation at 825 °C (1). However, Mulligan et al. (6) also reported that at 800 °C the
efficiencies were lower for direct sulfation, a result they attributed to incomplete calcination. These results indicate that the effect of any increase in calcination rate is likely to be beneficial to the capture capacity. Mercury porosimetry measurements of samples calcined at 900 °C for 15 and 60 min clearly reveal a significant influence of Na2CO3 on the pore-size distribution (Figure 2). Pores in the untreated samples are predominantly 0.04-0.2 µm, while in the treated samples they range from 2 to 10 µm with very few smaller than 1 µm. The pore-size distribution for the treated samples remains unchanged with increasing calcination time, while there is a slight shift toward larger VOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Sulfur X-ray mappings of cross-sectioned sulfated Strasburg Limestone particles (1000-1180 µm) (magnification: 40×) (a) sample without treatment and (b) sample treated with a 20% Na2CO3 solution (magnification: 40×).
FIGURE 4. SEM micrographs of epoxy embedded and cross-sectioned sulfated Strasburg Limestone particles (1000-1180 µm) treated with a 20% Na2CO3 solution (magnification: 500×) (a) backscattered electron (BSE) micrograph and (b) sulfur X-ray mapping of the same area as illustrated in (a). pores in the untreated sample calcined for 60 min compared to the shorter term calcined sample. This suggests that the untreated sample is slowly sintering, eliminating the small pores and creating larger ones. Analysis of a fully sintered (at 1300 °C) sample of this limestone had a primary pore-size in the range of several microns (30). Scanning Electron Microscopy (SEM) analyses of the sulfated limestones with and without Na2CO3 treatment revealed that the treatment changes the sulfation morphology from predominantly unreacted-core to uniformly sulfated in all treated samples (Figure 3), thereby almost eliminating the effects of particle size in the range 37 µm to 1180 µm. More detailed pictures of treated samples reveal that the sulfation morphology is uniform on a particle scale, but on a smaller scale sub-areas remain unreacted-core or patchynetwork (Figure 4). These sub-areas are apparently almost impermeable to SO2, explaining why the capture capacity, though improved significantly, remains incomplete. Previous work has shown that pores smaller than 0.07 µm close rapidly during sulfation due to the larger molar volume of CaSO4 (4). A significant part of the pores in the untreated samples are smaller than 0.07 µm, and these pores would plug-up rapidly forming the observed unreacted-core sulfation pattern. The major increase in the pore-size of the treated samples prevented pore plugging at the particle scale, but pore-plugging was still observed at a smaller scale. The high porosity of the fully sulfated treated particles reveals that the treatment, though improving the capture capacity significantly (especially for larger particles), may not be optimal. Powder X-ray Diffraction Analysis. Broadening of diffraction peaks of crystalline materials may be used to estimate crystallite size changes. In general, a decrease in crystallite size leads to an overall broadening of the peaks. Note that 3712
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the crystallite size may not be equivalent to the grain size but may be smaller. Further, the crystallite size determined by XRD is not a “true” crystallite size, but a value referred to as “domain size” which is equal to or less than the crystallite size. Thus, the crystallite sizes determined by XRD cannot be seen as absolute values but merely as an indication of the general size. Diffractograms of the [104] calcite peak in the dried limestone (with and without Na2CO3 treatment) indicate negligible changes in the average crystallite size at this stage of the treatment (Figure 5a). However, both the width change of the [200] lime peak in the calcined samples (Figure 5b) and the combined [200] and [020] anhydrite peaks in the sulfated samples (Figure 5c) indicate a significant increase in crystallite size due to Na2CO3 treatment. Based on single-peak width, the crystallite size in the raw limestones varied between 125 and 170 nm, and there was no apparent effect of the alkali treatment (Table 2). Crystallite size determinations revealed that the salt treatment enhanced crystal growth during both calcination and sulfation, leading to an increase in the average lime crystal size from ∼60-90 nm to ∼250-300 nm, while the increase in the sulfated samples was less apparent (∼125-150 nm to 225-250 nm). The TGR calcined samples indicate a slight increase in crystallite size with increasing temperature from 825 to 900 °C. Comparing the crystallite size of the lime with the size of the unreacted-core sub-areas in the samples treated with Na2CO3 (Figure 4) reveals that these unreacted-core subareas are not all individual lime crystals, but often conglomerates of several crystals.
Discussion Sulfur dioxide emissions from fluidized bed boilers are dependent on many factors, including operating temperature,
FIGURE 5. Powder X-ray diffractograms of single peaks of untreated Strasburg Limestone and samples treated with a 20% Na2CO3 solution. (a) [104] calcite peak in dried limestone; (b) [200] lime peak in samples calcined at 825 °C and 900 °C; and (c) [200] and [020] anhydrite peaks in sulfated limestone; calcined at 825 °C and 900 °C and sulfated at 825 °C. fuel type and composition, sorbent type and amount, and air-fuel feed system. The sulfur capture of the added sorbent is controlled mainly by the particle size of the sorbent, its overall reactivity, and its physical properties such as porosity, pore-size, and specific surface area. The capture capacity of individual particles is determined by a balance between the pore-size and surface area of the particle. The optimum poresize for a calcined limestone sulfur sorbent should provide sufficient surface area for sulfation to proceed without leading to rapid plugging of pores and a premature change from a
relatively fast chemical reaction to a very slow solid-state diffusion-controlled process. The sodium treatment led to a pore-size distribution more favorable for penetration of sulfur into the particles and reaction with lime grains throughout the whole particle without plugging the pores in the outer reaches, as seen for the untreated samples. However, the increase in the crystallites led to formation of larger grains with unsulfated cores, preventing the transformation from becoming complete. The experiments revealed that the treatment increased the rate of the calcination process and led to formation of larger crystals of the lime and anhydrite (presumably also the grain size) as well as higher porosity. An increase in the reaction rate of the calcination and the structural changes of the resulting lime and anhydrite may result from enhanced ionic mobility in the crystal lattice, as suggested by Gasner and Setesak (21), or increased ionic mobility and diffusion due to formation of a thin layer of eutectic melt, as suggested by Shearer et al. (22). This leaves one unanswered question: did the change involve formation of a melt, or was it solely due to solid-state reaction. Comparisons between the overall pore-size distribution of limestone particles calcined with Na2CO3 at 850-900 °C and particles sintered at 1300 °C (30) reveal some distinct similarities, indicating that sintering may be contributing to the changes in the pore-size distribution in the treated samples. Sintering is a term describing densification of a polycrystalline material, with or without a liquid phase to aid the transport of matter (36). Diffusion rates in melts are much faster than in solids, so the presence of a small amount of melt usually accelerates sintering significantly. Moreover, sintering may commence at a much lower temperature than in the absence of a melt (36). Our TGR calcination experiments showed that calcination was indeed somewhat faster in the presence of Na2CO3, but the rate increase can hardly be described as “significant”. SEM sulfur X-ray mappings of fully sulfated samples treated with Na2CO3 reveal formation of sub-areas with limited permeability for sulfur dioxide. Because these sub-areas are significantly larger than the determined crystallite size of the lime, each sub-area represents more than one crystal. Single crystals are expected to be impermeable. However, low permeability of a conglomerate of crystals may arise either from densely packed crystals or from an impermeable coating layer (e.g., a thin layer of melt). The finding that some sub-areas appear to be sulfating in a patchy-network mode suggests that the subareas consist of densely packed crystals without any significant melt coating. Since the calcination rate for the treated samples was not significantly improved, the experiments support the hypothesis that a melt was probably not involved in the process and that the changes in the reaction rate of calcination of the Na2CO3-treated samples and the morphology of the calcined and the sulfated particles were due to increased ionic mobility within the crystal lattice. When a mineral is doped with an “impurity”, this impurity may either be incorporated into the crystal lattice of the host mineral or a new phase (either crystalline or amorphous/ melt) may be created. When an impurity atom of a different valence (an aliovalent impurity) is incorporated in the host mineral, defects in the form of vacancies form in the crystal lattice (36). If the Ca2+ is replaced by the lower charge Na1+ ion, charge balance may be maintained by the formation of point-defects in the form of either anion vacancies or interstitial cations. Such point-defects in a crystal mediate solid-state diffusion and formation of larger crystals, as observed in the Na2CO3-treated calcined samples. Before an additive, such as Na2CO3, is introduced into a fluidized bed combustor, it is important to evaluate all potential side-effects of the foreign element including such issues as corrosion, deposition, agglomeration, and enviVOL. 37, NO. 16, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Crystallite Size of Raw, Calcined, and Sulfated Strasburg Limestonea sample
reactor
particle size (µm)
Na2CO3 solution (wt %)
calc temp (°C)
calc time (min)
Ca-util. (%) mass gain
ThL-Na-1 ThL-Na-11 ThL-Na-2 ThL-Na-3 ThL-Na-12 ThL-Na-13 ThL-Na-5 ThL-Na-6 ThL-Na-7 ThL-Na-14 TGA-ThL-C6 TGA-ThL-C7 TGA-ThL-C8 TGA-ThL-C9 TGA-ThL-C2 TGA-ThL-C3 TGA-ThL-C4 TGA-ThL-C5 TGA-ThL-C15 TGA-ThL-C16
FB FB FB FB FB FB FB FB FB FB TGR TGR TGR TGR TGR TGR TGR TGR TGR TGR
250-425 250-425 250-425 250-425 250-425 250-425 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180 1000-1180
0 0 3 15 15 200 0 3 15 200 0 0 15 15 0 0 15 15 0 15
825 825 825 825 825 825 825 825 825 825 825 825 825 825 900 900 900 900 900 900
90 90 90 90 90 90 90 90 90 90 60 60 60 60 60 60 60 60 15 15
21 24 47 45 45 45 10 44 45 40 7 nab 46 47 9 nab nab 36 11 31
crystallite size (nm) calcite lime anhydrite 96
71 81
124 134 48 149 118 170 95
transf transf transf transf 30 62 240 249 108 71 298 301 129 transf
187 156 285 267 286 290 144 350 276 307 nab 258 225 125 nab nab 252 143 267
a The crystallite sizes were determined based on single-peak refinements of the [104] calcite peak in the raw limestone, the [200] lime peak in calcined samples, and the combined [200] and [020] anhydrite peaks in the sulfated samples. b Not available: na.
ronmental emissions. The larger crystallite size of anhydrite in the Na2CO3-treated samples compared with the untreated samples indicates that the salt remained associated with the solids throughout the reaction period without vaporizing. This is important as it suggests that Na2CO3 treatment should neither enhance deposition on downstream heat exchanger surfaces nor agglomeration within the bed. However, pilot plant or utility boiler test runs are needed to establish whether these operational problems may increase. Addition of the Na2CO3 created a very porous sorbent, which may be prone to attrition. Attrition could be of further benefit if it causes shredding of fully sulfated parts from a partly sulfated particle, a process that could expose fresh unreacted CaO. However, if attrition leads to generation of small CaO particles that are only slightly utilized, these small particles are likely to be entrained and removed from the reactor before they are fully utilized. It has been shown that limestone addition for removal of sulfur dioxide emission can affect NOx and N2O emissions (e.g. refs 37 and 38). Whether Na2CO3 addition will affect the levels of emission NOx or other pollutants is uncertain but should be investigated. Three environmental factors clearly support the use of this additive: (i) decreasing CO2 emission from the calcination process; (ii) generating less solid waste for disposal; and (iii) less treatment of waste prior to disposal. As environmental issues become more important and costs associated with environmentally unfriendly practices escalate (e.g., due to CO2 emission taxes), increasing the sulfur capture capacity of limestones is an option that needs to be considered.
Acknowledgments The experiments were carried out at the Pulp and Paper Centre, University of British Columbia, Canada, while the analyses were conducted at the Institute of Environmental Science and Engineering, Nanyang Technological University, Singapore. The limestones were generously supplied by Alstom Power. Assistance from P. Mehrani with porosimetry measurements is gratefully acknowledged. The authors also acknowledge discussions with H. Andrus, J. Chui, and W. Duo. 3714
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Received for review August 26, 2002. Revised manuscript received April 24, 2003. Accepted May 12, 2003. ES026090K
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