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Progress of Sulfation in Highly Sulfated Particles of Lime Juan C. Abanades,*,† Edward J. Anthony,*,‡ Francisco Garcı´a-Labiano,† and Lufei Jia‡ Department of Energy and Environment, Instituto de Carboquı´mica (CSIC), Miguel Luesma Casta´ n 4, 50015 Zaragoza, Spain, and CETC, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada
Sulfation of limestone-derived particles over periods of many hours may contribute significantly to limestone’s utilization in fluidized-bed combustion (FBC). Long-term sulfation may also cause agglomeration. The progress of sulfation in highly sulfated particles of a calcined limestone is investigated. Conversion curves lasting 14-60 h have been obtained in a thermobalance at different SO2 concentrations with particle sizes and temperatures typical of FBC boilers. Also, tests in a tube furnace under very strong sulfation conditions have been performed to elucidate the mechanism responsible for the residual activity of the sorbents and their final fate as fully sulfated solids. Scanning electron microscopy analysis of the highly sulfated samples shows cracks in the external surface of all of the particles, irrespective of their sulfation pattern during their fast sulfation period. Based on this observation, a shrinking-core model is used to fit the thermogravimetric analysis curves with an adjustable cracking constant. This simple model gives a satisfactory fit up to conversions close to the full sulfation of the particle. However, in samples allowed to agglomerate, the onset of agglomeration must produce a drastic change in the effective particle size; this explains deviations of the model from results obtained with real ashes in very long sulfation tests. Introduction One of the main advantages of fluidized-bed combustors (FBCs) is their ability to effectively retain sulfur with the addition of a low-cost sorbent such as limestone. Despite substantial research on the sulfation characteristics of limestone,1-5 it is still difficult to predict the behavior of these solids as sulfur sorbents in FBCs from laboratory tests and reaction models. It is generally accepted that the higher molar volume of CaSO4 with respect to CaO and CaCO3 causes pore plugging and therefore prevents full utilization of the sorbent. Pore plugging can take place in the external layer of the sorbent particle or in the external surfaces of small or large grains in the interior of the sorbent particle, leading to very different sulfation patterns. As shown by Laursen et al.,6 these patterns can all be present in different proportions for a given limestone. Despite the different possible sulfation patterns, the sorbent always reaches a so-called maximum conversion, Xmax, which corresponds to the plugging described above. This maximum conversion is reached in times typically shorter than 3 h at normal sulfation conditions in a FBC. The sulfation reaction is very slow for most practical applications after this fast sulfation period, although it has recently been demonstrated7 that the residual activity can be responsible for several percentage points of conversion for some fine particles used in circulating FBCs (CFBCs). The fate of the sorbent particles under time scales of days to months is of interest in understanding the formation of deposits in CFBC boilers.1,8-11 The exist* To whom correspondence should be addressed. † CSIC. Tel.: +34 976 733977. Fax: +34 976 733318. E-mail:
[email protected]. ‡ CETC. Tel.: 613 9962868. Fax: 613 9929335. E-mail:
[email protected].
ence of localized low-gas-velocity regions in these CFBC systems permits the formation of deposits of sorbent, which can achieve quantitative conversion and agglomeration. This has led to severe problems in a number of boilers firing high sulfur coal or petroleum coke. The objective of this work is to understand better the slow sulfation process of the sorbent in these long time scales, which are usually neglected in studies to assess the sorbent performance in CFBCs. Experimental Section The limestone chosen for most studies in this work is Yucatan, which is currently used by the two 100 MWe NISCO CFBC boilers.12 This limestone contains 98% CaCO3 and has excellent sulfation characteristics. For comparison purposes, some tests have been conducted with limestone Omyacarb “la Blanca” (from Spain), used in a previous work7 on residual activities. Sulfation of the Yucatan limestone was done in a Setaram thermogravimetric analyzer (TGA) at atmospheric pressure and in a tube furnace. The reacting gas mixture in the TGA (20 dm3/h) contained SO2, O2, and N2 and was controlled by mass flow controllers. The reacting gas was introduced at the top of the reaction tube. In addition, N2 flowed through the microbalance head to keep the electronic parts free from corrosive gas. Four particle size fractions have been investigated: 0.070.1, 0.1-0.25, 0.25-0.4, and 0.4-0.63 mm. The particle size fraction of 0.1-0.25 mm was selected as a reference, together with the following set of sulfation conditions: T ) 850 °C; 1000 vppm of SO2 in air. To gain some insight into the mechanism of sulfation in highly sulfated particles, samples of both Yucatan and “la Blanca” limestones with particle sizes between 0.35 and 0.6 mm were prepared for observations by scanning electron microscopy (SEM). The sorbent
10.1021/ie020868t CCC: $25.00 © 2003 American Chemical Society Published on Web 04/03/2003
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samples were sulfated at 850 °C for 3.5 h with 1 vol % SO2 and 3 vol % O2 in N2 in a tube furnace. Most of the samples were prepared for observation by embedding the powder in resin, curing, and polishing, as is usual for elemental mapping purposes. However, because of problems in the interpretation of these observations (see below), some photographs were also taken with the powders glued onto a sticky surface. The instrument used was a Hitachi 4500 SEM with an energy-dispersive spectrometric analyzer, except for the photographs of glued powder, obtained by a Hitachi 570. These observations indicated three regions whose composition was basically pure CaSO4, CaO, or mixtures of CaSO4 + CaO. Most photographs were taken with backscattered electrons (BSE) and the rest with secondary electrons (SE). BSE images distinguish between areas of CaO, which appeared darker, and those of CaSO4, which appeared brighter; thus, an approximate mapping becomes apparent from observation of the photographs. SE gives a better feeling of three-dimensionality but a lack of detail. Finally, some sulfation tests were conducted in a temperature-controlled oven with Yucatan limestone to investigate the formation of agglomerates.13 All of these samples agglomerated under sulfation conditions (1 vol % of SO2 at temperatures between 850 and 950 °C) in 28 days. It has been shown10 that there is a typical penetration depth for sulfation of ∼4-5 mm through a loose-packed powdered bed. The analysis of the sulfated layer at the top of the crucibles showed the material to be highly sulfated with conversions between 70 and 75%, while the interior of the crucibles had conversions of 45-55% or below. This high conversion of the outer layer of particles in such tests is comparable with deposits found in utility-scale CFB boilers, which can contain 72-75% CaSO4 or more. The implications of this penetration layer will be discussed later.
Figure 1. Effect of the particle size on the conversion curves of Yucatan limestone: 850 °C; 1000 vppm of SO2 (lines are model predictions).
Results and Discussion The focus of this work is on the residual reactivity observed when the particles, and/or the grains forming the particles, have developed a product layer rich in CaSO4, which prevents the inner regions of CaO from reacting rapidly. In line with an earlier study7 on this subject, it is assumed here that after 3 h of reaction the progress of the sulfation reaction corresponds only to the “residual” period. The observations of Figures 1-3 confirm the validity of this assumption for Yucatan limestone. Figure 1 shows a clear dependence of the residual activity with the particle size. Figure 2 shows that higher temperatures tend to increase residual activities, with a sharp increase between 800 and 850 °C, in contrast with a much lower change between 850 and 900 °C. Figure 3 shows the effect of the SO2 concentration, including measurements obtained in the tube furnace, which generally follows the trend of the TGA results, despite the large difference in the SO2 concentration in the experiments. Small differences between the results from the TGA and the tube furnace are due to the latter not performing as a differential reactor during the fast sulfation period. The SEM results obtained confirm that particles of Yucatan limestone display two clearly different sulfation patterns (see Figure 4a): about half of the particles are sulfated throughout their mass (uniform sulfation), while in the other half, network sulfation patterns are clearly shown. By contrast, the particles of “la Blanca”
Figure 2. Effect of the temperature on the conversion curves of Yucatan limestone: dp ) 0.1-0.25 mm; 1000 vppm of SO2 (lines are model predictions).
limestone appear most frequently with thin sulfated coatings consisting of pure or nearly pure CaSO4, while the core consists of nonsulfated CaO (see Figure 4b). Moreover, the transition from the sulfated coating to the nonsulfated core is always sharp rather than gradual (unreacted core). The observation of broken coatings in both the photographs and S maps (see Figure 5) raised an interpretation problem. Three possibilities can be considered: (1) all the cracks and fissures were present in the original particle, (2) they are caused by preparation of the sample (setting of resin and polishing), or (3) they are the consequence of the sulfation process itself. The first possibility is excluded because the strong sulfation conditions would not allow the existence of the fresh and accessible CaO regions as observed in Figure 5. To investigate the case of the second possibility, the photographs of Figure 6 were obtained from sulfated limestone particles glued onto a sticky surface. Figure 6a shows that, before any sample preparation using resin, particles of Yucatan limestone have a large
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Figure 3. Effect of the SO2 concentration on the conversion curves of Yucatan limestone: 850 °C; dp ) 0.1-0.25 mm. Large points: tube furnace results at 10 000 vppm. Solid dots: ICP elemental S/Ca analysis. Squares: Eschka for S (lines are model predictions).
Figure 5. Example of a sulfated particle from limestone Yucatan with cracks in the external sulfated layer (a) and its associated S map (b).
Figure 4. S maps of a highly sulfated sample (3.5 h, 850 °C, 10 000 vppm of SO2) of Yucatan (a) and “la Blanca” (b) limestones.
abundance of cracks, leading to partial disintegration of the particle. These cracks were also clear in the external layers of limestone “la Blanca” (Figure 6b), despite the large difference in sulfation patterns between the two limestones as described above. Therefore, there is only one possibility left, which is that the increase in volume associated with the sulfation of the
external surface layer must produce some kind of tension and disruptions that may force cracks to develop. The interpretation of the conversion curves with a sulfation model for the residual activity period needs to consider the above observations. As mentioned above, Laursen et al.6 showed that it is common for natural limestones to show more than one sulfation pattern. Yucatan limestone used in this work is an example of this behavior. The fact that the particles within a small sample of limestone can be so heterogeneous is obviously a very undesirable thing to consider when modeling the system, and to our knowledge, it has not yet been incorporated in existing models of individual particles, let alone at the full-size reactor level.1-5 As a consequence, it would go well beyond the scope of this work to follow this rigorous approach to interpret the limited amount of experimental information available. Instead, we follow a more simple approach, relying on the observed external cracking of all of the particles irrespective of their sulfation pattern during the fast reaction period. Figure 1 shows that the slopes of the conversion curves for the period of residual activity are inversely proportional to particle size. This is consistent with a shrinking-core model with reaction control at the interface between the core and the product layer.14 In view of the general appearance of cracks in the highly sulfated particles as observed by SEM, the first mechanism to be considered in the modeling reaction at such
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Figure 7. Effect of the particle size on conversion predicted by eq 1 at very long time scales and at 850 °C and 1000 vppm of SO2. Sulfation levels measured in agglomeration tests8 are represented by the hatched area in the graph.
τ)
FCadp 0.2 2CSO k ′ 2 ap
and kap′ ) 0.0105 exp[-65 (kJ/mol)/RT] (2)
Figure 6. External view of a sulfated particle of Yucatan (a) with SEM (BSE) and a general overview (SE) of cracking in the external layers of limestone “la Blanca” (b).
an interface is a purely physical mechanism, whereby the external product layer is cracking in these highly converted particles as a consequence of sulfation. There are different rate expressions for a cracking-core model with these characteristics, as obtained and analyzed by Park and Levenspiel.15 However, these authors also showed that in many practical cases it is possible to fit the plots of conversion versus time with the rate expression of the shrinking-core model if the experimental evidence does not justify the inclusion of additional parameters, as is the case here. Therefore, the TGA results obtained here with Yucatan have been fitted (solid lines in Figures 1-3) to the following equation:
t - t0 ) (1 - X0)1/3 - (1 - X)1/3 τ with
(1)
The low apparent order of reaction, 0.2, is consistent with the fact that the residual activities plotted in Figure 3 are almost independent of the SO2 concentration, in agreement with earlier results using the same sorbent.10 The reaction order with respect to the oxygen has not been investigated in this work and has been considered zero. As can be seen with the solid lines of Figures 1 and 3, the quality of the fit is satisfactory for the curves obtained for different particle sizes and SO2 concentrations, respectively. The quality is less satisfactory for the curves at different temperatures (Figure 2). This is not surprising considering that a number of very complex mechanisms associated with the physical cracking process itself have been lumped into a single cracking constant, kap′. These mechanisms must interact closely with the sulfation process, which is ultimately responsible for creating the necessary tensions to open new cracks. As mentioned above, the reaction rates shown in Figures 1-3 are higher than those obtained with the “la Blanca” limestone.7 The fitting of the cracking model of eq 1 to all of the conversion curves for “la Blanca” limestone leads to a quality of fit similar to the one obtained using an effective diffusion coefficient for the external product layer. The similarity in fit quality of the two different models is because the shrinking-core model generates, in the interval of conversions investigated in both cases, almost identical results in terms of (t - t0)/τ. However, we believe that the new cracking mechanism described in this work is more adequate because it is supported by the observations obtained by SEM. Finally, in Figure 7 a gross extrapolation of the modeling predictions is made to the time scales longer than 2 weeks, which are typically used in agglomeration studies.8-11 For small particles (0.15 and 0.3 mm), the model tends to overpredict conversions; i.e., at 4 days, the predicted conversion is very high. The experimental
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conversion level at 1000 vppm for 0.1-0.25 mm particles seems to level off at conversions higher than 80% (Figure 3), while the predicted values go to full conversion in just under 100 h. This can be attributed to two causes: to increased difficulty for cracks to open as the core of the particle becomes smaller and/or to the prevalence of small grains of CaO left behind in the CaSO4 product layer. In both cases, a product layer of CaSO4 without cracks surrounds the CaO, and only a very slow diffusion process may take place. The diffusion of reactants through this pure CaSO4 product layer16,17 would be much slower than that through the cracks of a particle. At time scales over 2 weeks, eqs 1 and 2 appear to overpredict the conversion considerably compared with the oven test results, obtained with samples with particle sizes in the range of 0-1.4 mm and a mean particle size of about 0.3 mm. However, it was clear from the results in the oven test that the agglomeration process itself decreases the sulfation levels, presumably because of mass-transfer effects produced in the beds of material in the crucibles.8 It is also evident that the concept of particle size will not apply to a mass of particles once they have started to agglomerate, as is the case in many of these very long sulfation tests. Clearly, the present modeling approach could be used to predict the onset of agglomeration but not the subsequent increase in sulfation levels of the agglomerated particles. Conclusions The final sulfation of particles of calcined limestones is relevant to understanding the formation of agglomerates in ash deposits found in FBC boilers. Long thermogravimetric tests and tests in a tube furnace with Yucatan limestone revealed a period of strong residual activity that brings a sorbent of particle size between 0.1 and 0.25 mm to conversions higher than 80% in just under 3 days of reaction at 850 °C and 1000 vppm of SO2. SEM observation of the interior of the sulfated particles of Yucatan limestone shows a complex mixture of sulfation patterns, in contrast with the simple coreshell structure of “la Blanca” limestone used as a reference. Despite these differences, both limestones display extensive cracking of the highly sulfated external layers of the particles. A cracking model, in its simplest version as a shrinking-core model with an apparent cracking constant, fits the measured conversions for both limestones with reasonable agreement. The model accounts for the strong effect of the particle size on the conversion curves in the final period of residual activity. It is argued that this is the result of the gross simplifications of including in a single constant the intrinsically complex mechanism of crack opening. The extrapolation of the model to time scales relevant for agglomeration studies predicts the onset of agglomeration for fine particles much earlier than expected. This is because the concept of the particle size does not apply to a mass of particles once they have started to agglomerate, as is the case in many of the very long sulfation tests carried out. It is concluded that the present model could be used to predict the onset of agglomeration but not the subsequent increase in sulfation levels of the agglomerated particles. Acknowledgment The authors acknowledge the SEM studies carried out by Professor J. V. Iribarne and Dr. A. P. Iribarne of the University of Toronto.
Nomenclature CSO2 ) SO2 concentration in the bulk gas (mol/m3) dp ) particle diameter (m) kap′ ) apparent cracking constant [(mol/m3)0.8‚m/s] R ) gas constant (J/mol‚K) T ) temperature (K) t ) time (s) t0 ) time required to reach X0 (s) X ) calcium conversion during sulfation X0 ) sulfation conversion after 3 h of reaction τ ) time required to reach the limit sulfation conversion (s) FCa ) molar density of Ca in the particles (mol/m3)
Literature Cited (1) Anthony, E. J.; Granatstein, D. L. Sulfation Phenomena in Fluidised Bed Combustion Systems. Prog. Energy Combust. Sci. 2001, 27, 215. (2) Mattison, T.; Lyngfelt, A. A Sulfur Capture Model for Circulating Fluidized-Bed Boilers. Chem. Eng. Sci. 1998, 53, 1163. (3) Ada´nez, J.; Gaya´n, P.; Garcı´a-Labiano, F. Comparison of Mechanistical Models for the Sulfation Reaction in a Broad Range of Particle Sizes of Sorbents. Ind. Eng. Chem. Res. 1996, 35, 2190. (4) Dennis, J. S.; Hayhurst, A. N. A Simplified Analytical Model for the Rate of Reaction of SO2 with Limestone Particles. Chem. Eng. Sci. 1986, 41, 25. (5) Dennis, J. S.; Hayhurst, A. N. Mechanism of the Sulphation of Calcined Limestone Particles in Combustion Gases. Chem. Eng. Sci. 1990, 45, 1175. (6) Laursen, K.; Duo, W.; Grace, J. R.; Lim, J. Sulfation and Reactivation Characteristics of Nine Limestones. Fuel 2000, 79, 153. (7) Abanades, J. C.; de Diego, L. F.; Garcı´a-Labiano, F.; Ada´nez, J. Residual Activity of Sorbent Particles with a Long Residence Time in a CFBC. AIChE J. 2000, 46, 1888. (8) Anthony, E. J.; Jia, L. Agglomeration and Strength Development of Deposits in CFBC Boilers firing High Sulfur Fuels. Fuel 2000, 79, 1933. (9) Anthony, E. J.; Iribarne, A. P.; Iribarne, J. V. A New Mechanism for FBC Agglomeration and Fouling in 100% Firing of Petroleum Coke. J. Energy Resour. Technol. 1997, 119, 55. (10) Anthony, E. J.; Talbot, R. E.; Jia, L.; Granatstein, D. L. Agglomeration and Fouling in Three Industrial Petroleum-Coke Fired CFBC Boilers due to Carbonation and Sulfation. Energy Fuels 2000, 14, 1021. (11) Anthony, E. J.; Jia, L. Fouling in a 160 MWe FBC Boiler Firing Coal and Petroleum Coke. Fuel 2001, 80, 1009. (12) Zierold, D. M.; Voyles, R. W. NISCO Cogeneration Facility. In Proceedings of the 12th International Conference on Fluidized Bed Combustion; Rubow, L., Commonwealth, G., Eds.; ASME Press: New York, 1993; Vol. 1, p 501. (13) Abanades, J. C.; Anthony, E. J.; Garcı´a-Labiano, F.; Jia, L. The Sulfation Reaction of Limestone Particles Over a Time Scale of Weeks. In Proceedings of the 16th International Conference on Fluidized Bed Combustion; Geiling, D. W., Ed.; ASME Press: New York, 2001; FBC01-0125. (14) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; John Wiley: New York, 1999. (15) Park, J. Y.; Levenspiel, O. The Crackling Core Model for the Reaction of Solid Particles. Chem. Eng. Sci. 1975, 30, 1207. (16) Borgwardt, R. H.; Bruce, K. R.; Blake, J. An Investigation of Product Layer Diffusivity for CaO Sulphation. Ind. Eng. Chem. Res. 1987, 26, 1993. (17) Hsia, C.; Pierre, G. R.; Raghunathan, K.; Fan, L. S. Diffusion Through CaSO4 Formed During the Reaction of CaO with SO2 and O2. AIChE J. 1993, 39, 698.
Received for review November 1, 2002 Revised manuscript received February 28, 2003 Accepted February 28, 2003 IE020868T
