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Ind. Eng. Chem. Res. 2004, 43, 1340-1348
Performance of Porous CaO Obtained from the Decomposition of Calcium-Enriched Bio-Oil as Sorbent for SO2 and H2S Removal Stratis V. Sotirchos*,†,‡ and Adam R. Smith‡ Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation of Research and Technology Hellas (ICEHT-FORTH), P.O. Box 1414, 265 00 Patras, Greece, and Department of Chemical Engineering, University of Rochester, Rochester, New York 14627
The performance of calcium oxide particles produced from the decomposition of calcium-enriched bio-oil (CEB), the product of the reaction of bio-oil and calcium hydroxide, as a sorbent for the in situ removal of SO2 and H2S from the flue gases of coal combustors and the coal gas produced in coal gasifiers, respectively, is investigated. Reactivity evolution experiments are carried out in a thermogravimetric analysis system using CaO samples prepared through decomposition of calcium-enriched bio-oil applied as a thin coating on a quartz pan or through calcination of two naturally occurring calcitic solids of high CaCO3 content. CEB is converted to CaO in one or two steps, that is, first conversion to CaCO3 in the presence of CO2 and then calcination. Calcination and sulfidation or calcination and sulfation are carried out both sequentially and simultaneously. Because of its high porosity, the CaO material that results from the decomposition of CEB is found to be capable of reaching much higher conversions during sulfation than calcium carbonate calcines, with ultimate conversions that can be almost 100%. This material exhibits higher reactivities also during sulfidation, but all calcined materials can reach conversions close to 100% in this reaction because of the absence of pore plugging phenomena. However, because of the much faster calcination of the CEB-derived CaCO3, there is minimal interference between its calcination and sulfidation in the simultaneous process, while this is not the case for calcium carbonate solids. 1. Introduction The use of calcium oxide materials derived from calcium carbonate or calcium hydroxide decomposition for in situ SOx capture in fossil fuel combustion units is plagued by pore plugging phenomena, which lead to incomplete conversion of CaO to CaSO4 and low calcium utilization, much lower than the 1:1 Ca:S ratio that is theoretically possible.1-4 The sulfidation of CaO (its conversion to CaS), a reaction encountered in the use of CaO for in situ capture of H2S in fossil fuel gasification units, is also accompanied by porosity reduction and pore plugging. Because of the smaller difference between the molar volumes of CaS and CaO than those of CaSO4 and CaO, it is possible to reach complete conversion of CaO to CaS before all pores are plugged with solid product.5 However, the diminishing porosity intensifies the intraparticle diffusional limitations of H2S in the pore space and causes a significant reduction in the overall reactivity as the reaction progresses. Incomplete conversion arises when sulfided CaO (partially or completely) is reacted in an oxygen-containing environment to convert CaS to CaSO4 and thus make possible the stable and environmentally safe disposal of the spent desulfurization sorbent.6,7 For complete conversion of CaO (through sulfation) or sulfided CaO (through oxidation) to CaSO4, it is necessary to employ CaO materials that possess higher porosity than the calcines that result from calcination * To whom correspondence should be addressed at the ICEHT-FORTH address. Tel.: +30-2610-965202. Fax: +302610-965223. E-mail:
[email protected]. † ICEHT-FORTH. ‡ University of Rochester.
of calcium carbonate solids (limestones) and Ca(OH)2. It has been found that the high temperature decomposition of organic calcium salts, such as calcium acetate and calcium magnesium acetate,8,9 in particle form or as sprays of aqueous solutions yields calcium oxide particles that are characterized by very high porosities, well above the value that is needed for 100% conversion before all pores are filled with solid product (CaSO4). The formation of high porosity is caused by the formation of bubbles by the decomposition gases in the molten mass of the organic salt. Experimental sulfation studies8,10-13 revealed that the CaO particles that are obtained from the decomposition of organic salts of calcium can reach much higher sulfation conversions than the CaO particles that are obtained from the decomposition of CaCO3 or other inorganic calcium solids (e.g., Ca(OH)2). One of the main drawbacks in the use of organic calcium salts as precursors for in situ formation of CaO sorbent particles in fossil fuel utilization reactors stems from the relatively high cost of the organic acids needed for the production of these materials. A relatively inexpensive source of organic acids for production of organic salts of calcium is provided by bio-oil, the product of the flash (fast) pyrolysis of biomass.14-17 The composition of the pyrolysis oil varies depending on the raw material, the treatment temperature, and the residence time in the reactor. In general, bio-oil consists of water, 10-20%, a mixture of carboxylic acids, several aldehydes and alcohols, and pyrolytic lignin. Formic and acetic acids are the main acids contained in bio-oil, with their total weight content being about 10%.18,19 By reacting bio-oil with calcium hydroxide, calcium-enriched bio-oil (CEB), a mixture of calcium hydroxide,
10.1021/ie034176w CCC: $27.50 © 2004 American Chemical Society Published on Web 02/20/2004
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organic calcium salts, and other organic compounds, is obtained. Studies in pilot scale units have shown20,21 that, as in the case of powders of pure organic calcium salts or sprays of aqueous solutions of carboxylic salts, the CaO particles that result from the decomposition of CEB possess very high porosity, a consequence of the formation of bubbles within the decomposing structure because of the release of volatile species from the decomposition or evaporation of the components of calcium-enriched bio-oil. The high porosity can lead during desulfurization to very high utilization of the calcium contained in the sorbent, well above the values observed for sorbents derived from CaCO3 or Ca(OH)2. In a previous study,22 we presented a comprehensive experimental investigation of the decomposition of calcium-enriched bio-oil at high temperatures and of the calcination of the CaCO3 material that is obtained when the decomposition is carried out in the presence of CO2. Decomposition and calcination experiments were conducted in a thermogravimetric analysis system, and the pore structure of dried, decomposed, or calcined samples was characterized using nitrogen adsorption-desorption, mercury intrusion-extrusion, and photomicrographic examination. The pore structure characterization results showed that the decomposition and calcination products of CEB possess very high porosity, higher than 80-90%. Because of its high porosity, the decomposition product of CEB in the presence of CO2 (CaCO3) showed much higher calcination rate than particles of limestones and calcites of similar characteristic size as the CEB layer. This led us to conclude that CEB may perform as a more efficient sorbent than limestones for SO2 capture in fossil fuel combustors. The performance of calcium-enriched bio-oil as a sorbent for SO2 and H2S removal under conditions at which calcination of CaCO3 is favored to occur is investigated in this study. The first step in the decomposition of the organic calcium salts is the formation of calcium carbonate23,24 which decomposes to CaO as the temperature increases and the equilibrium partial pressure of CO2 for CaCO3 decomposition becomes higher than the partial pressure of this species in the reactor. The decomposition of the Ca(OH)2 material that may be present in CEB in free form should also go through a step of CaCO3 formation if CO2 is present in the reactor,22 which is always the case in fossil fuel utilization units. In view of these facts, the CEB-derived samples that are employed in our sulfation and sulfidation experiments are prepared by first converting the CEB to CaCO3 by decomposing it in a CO2-containing environment, with the pressure of CO2 being higher than its equilibrium pressure in the calcium carbonate decomposition reaction. In order to examine the influence of the decomposition procedure on the performance of the obtained CaO particles, a few experiments are carried out using a one step decomposition process, that is, decomposition in an environment free of CO2. Since the presence of CO2 in the gas phase of combustors or gasifiers forces the calcination reaction to occur at temperatures above 650-700 °C at which the sulfation and sulfidation reactions occur at significant rates, one would expect the CaO produced in the calcination reaction to react with SO2 or H2S as it is formed. The calcination and sulfuration reactions are therefore expected to take place in a coal utilization unit simultaneously and not sequentially as they are usually studied experimentally. In order to investigate the
interaction of the calcination and sulfation or sulfidation of the CaCO3 material that results from the decomposition of CEB in a CO2-containing environment, experiments are carried out not only on the sequential but also on the simultaneous occurrence of the calcination and sulfuration reactions. 2. Experimental Procedures and Materials Reactivity evolution experiments (decomposition, calcination, sulfation, sulfidation) were performed in a thermogravimetric (TGA) system. The samples used in the experiments were placed on a quartz or gold pan, which had about 10 mm2 area. The pan was suspended using platinum wire from the sample arm of the microbalance in the thermogravimetric arrangement. The reacting gases entered from a side port at the top of the hangdown tube and flowed down toward the pan together with the inert gas stream used to purge the housing of the microbalance. A small amount of sample, enough to give 0.3-1 mg of CaO, was employed in the experiments. This was done because the calcination, sulfation, sulfidation reactions are relatively fast, and large samples may lead to bulk concentrations of gaseous reactants or products (e.g., CO2, SO2, and H2S) in the stream flowing around the sample that are much different than those in the feed mixture. The CEB samples were placed as a thin coating of the homogenized material on the bottom of the pan. They were decomposed following the procedure described in our previous study.22 This procedure entailed drying at room temperature, further drying at 100-110 °C, and heating to the reaction temperature under 70% CO2 in N2 (for conversion to CaCO3) or in N2 with traces of oxygen (for combustion or residual carbon). The CEB material used in our experiments was prepared by reacting wood-derived bio-oil with a Ca(OH)2 suspension at about 60 °C. It was provided to us by BTG (Biomass Technology Group) B.V. (Enschede, The Netherlands), and it was prepared using bio-oil produced through flash pyrolysis of wood using the rotating cone reactor technology that BTG B.V. has developed.15,16 The calcium contained in the CEB samples was about 13-14 wt %. Water was present at about 45 wt %, and it included water contained in bio-oil and water contained in the Ca(OH)2 suspension added to it for the preparation of CEB. The addition of Ca(OH)2 in the bio-oil and the ensuing formation of organic calcium led to a pH of about 11.5, whereas the pH of bio-oil was about 3. More information on the properties of the CEB used in the experiments is given in ref 22, where its behavior during decomposition and calcination is studied in detail. Some results on the variation of the weight of the sample for decomposition under 70% CO2 in N2 at 200 mL/min total flow rate during heating to 750 and 850 °C are given in Figure 1. Before starting the experiment, the CEB samples were heat treated at 110 °C until they showed insignificant weight change. It is seen in Figure 1 that initially the weight decreases up to 350-450 °C, then shows a small increase, and finally decreases above 600-700 °C. The total weight loss is about 24% of the initial (dry) weight of the sample after drying (baking) at 100-110 °C. The differences between the weight versus time curves reflect both the different temperature rise and the differences seen in the experiments among different samples. The comparison of the results of Figure 1 with those obtained for the decomposition of Ca(OH)2 samples in the presence and absence
1342 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 Table 1. Internal Surface Areas and Porosities of Calcined Materials Prepared from Calcium-enriched Bio-oil and the Calcium Carbonate Solids for Calcination at 750 and 850 °C from Mercury Porosimetry Data porosity internal surface (cm3/g) 2 area (m /g) (radius e 50 nm)
total porosity (cm3/g)
calcined material 750 °C 850 °C
750 °C
850 °C 750 °C 850 °C
calcium-enriched bio-oil
15
24
0.056
0.11
0.9
0.965
Greer limestone Iceland spar
48 84
34 65
0.125a 0.305 0.32
0.065a 0.29 0.325
0.34 0.35
0.33 0.34
a
These values are from N2 sorption data.
Figure 1. Variation of weight and temperature during decomposition of calcium-enriched bio-oil dried at 110 °C in 70% CO2.
of CO2 suggested that the rise in the sample weight is most probably caused by the carbonation of CaO produced from the decomposition of Ca(OH)2. Ca(OH)2 must be present in free (unreacted) form in CEB since the acidic content of bio-oil (the content of bio-oil in acetic and formic acid (the major acids found in it) is about 10 wt %18,19) is well below the level that is required for the conversion of all Ca contained in it to carboxylic salts. Sulfation experiments were carried out using 0.35% SO2, 15% O2, and the balance N2 at 200 mL/min, and for sulfidation experiments, we used a steam of 0.7% H2S in N2 at 200 mL/min. The CEB samples were heated to the reaction temperature under 70% CO2 after they were first dried at 110 °C. They were calcined by sending through the reactor a mixture free of CO2, and were sulfated or sulfided by switching to the reactive mixture. For the simultaneous processes, calcination and sulfidation or sulfation were carried out at the same time by replacing the 70% CO2 mixture with the reactive mixture. When calcium-enriched bio-oil droplets are injected into a combustor, the decomposition of the components of CEB and the calcination of CaCO3 and Ca(OH)2 occur simultaneously. For this reason, in some experiments decomposition and calcination occurred simultaneously by heating the material in the presence of N2 (with small amounts of O2) or air only. The sulfation and sulfidation results of CEB-derived CaO were compared with those obtained for CaO particles derived from calcination of two calcium carbonate solids of very high CaCO3 content: a microcrystalline limestone distributed by Greer Limestone Co. (Greer limestone) and a calcite (Iceland spar) distributed by Wards Inc. in the form of large single crystals. The calcium carbonate content in Iceland spar is 99.2%, and in Greer limestone, 97.9%. Two particle size ranges (297-350 and 53-62 µm) were used in the sulfation and sulfidation experiments of these two solids. As in the case of CEB, calcination and sulfidation or sulfation were carried out sequentially or simultaneously. Mercury penetration and N2 adsorption were used to characterize the pore structure of the decomposed and calcined CEB and of the calcined calcium carbonate solids. The solids obtained from CEB were also characterized using N2 adsorption-desorption. The porosity of the calcium carbonate calcines was about 50%, while that of the CEB-derived CaO was estimated to be in
Figure 2. Weight change vs time for calcium-enriched bio-oil (CEB) decomposed and reacted at 750 °C in sequential or simultaneous mode.
excess of 80-90%. Table 1 gives the BET surface area, the total porosity, and the porosity of pores smaller than 100 nm of the CaO material that is obtained from CEB and the two calcium carbonate solids at two calcination temperatures (750 and 850 °C). More information on the pore structure properties of the calcines of Greer limestone and Iceland spar is given in ref 4, and on the pore structure characterization of CEB-derived CaO, in ref 22. 3. Sulfation Results Sulfation results obtained under various combinations of reaction conditions are shown in Figures 2-5. As mentioned in the preceding section, the reactive stream consisted of 3500 ppm SO2, 15% O2, and the balance N2, and the flow rate was set at 200 mL/min. All weight change data are presented in dimensionless form as 1 + ∆W/W0, where W0 is the weight of the calcium carbonate sample, that is, the final weight of the sample after decomposition in the presence of 70% CO2 in N2. The conversion versus time results obtained in the calcination experiments are also shown in some of the figures for comparison. The weight loss that was observed during calcination corresponded to the stoichiometry of the conversion of CaCO3 to CaO, and this observation was also made in the decomposition and calcination experiments.22 This was construed as an indication that very small quantities of compounds other than CaCO3 existed in the sample at the end of the heat
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Figure 3. Weight change vs time results for calcium-enriched bio-oil (CEB) decomposed and reacted at 850 °C in sequential or simultaneous mode.
Figure 4. Effects of temperature of decomposition and temperature of sulfation on the behavior of the weight change vs time curve for sulfation of CEB in simultaneous mode.
treatment under enough CO2 to prevent decomposition of CaCO3. Figures 2 and 3 present results obtained at 750 and 850 °C, respectively. Much larger conversions (corresponding to almost complete conversion) are reached at 750 °C. For complete conversion of CaO to CaSO4, it can easily be shown that 1 + ∆W/W0 should be 1.8 for the sequential sulfation curve and 1.36 for the simultaneous calcination-sulfation curve or sequential calcination-sulfation curve (composite curve). The composite curve is obtained by adding the weight changes obtained in independent (sequential) calcination and sulfation experiments at the same reaction times, which is equivalent to assuming that there is no interference of the progress of the two reactions in the simultaneous process. The deviations of the actual simultaneous curve from the composite curve give a direct indication of the extent of interaction of the two reactions. The sulfation results of Figures 2 and 3 clearly show that larger weight changes are obtained at 750 °C than at 850 °C. The material prepared at 750 °C reaches
Figure 5. Effect of weight of CEB sample on weight change vs time for calcium-enriched bio-oil (CEB) decomposed and reacted at 850 °C in sequential or simultaneous mode.
almost complete conversion for large reaction times both in the simultaneous and the sequential process. On the other hand, the conversions reached by the material that was prepared at 850 °C (Figure 3) are only about 50% for both processes. Figure 4 presents some results on the effects of the maximum temperature used during decomposition, which was the same as the temperature of calcination, and of the temperature of sulfation on the behavior of the weight change versus time curve for sulfation of CEB in sequential mode (calcination followed by sulfation). It is seen that that the porous material that is obtained through decomposition of CEB at 850 °C does not reach complete conversion even when it is sulfated at 750 °C. This points to the conclusion that the temperature that the material sees during its decomposition has a very strong influence on the performance of the resulting CaO as a sorbent for SO2 removal. In our study of the decomposition and calcination of CEB,22 the differences between the calcination curves of CaCO3 produced through CEB decomposition and those of the two calcitic solids (Greer limestone and Iceland spar) were found to be considerably larger at 750 °C. Since calcination is a very fast reaction and its progress is mainly controlled by the transport rate of CO2 through the converted part of the solid (CaO layer), it was argued that the differences between the diffusion coefficient of CO2 through the pore structure of the CaO material produced from the decomposition of CEB and its diffusion coefficient through the CaO layers produced from the decomposition of the two naturally occurring calcium carbonate solids are much larger at 750 °C. This should also be true for the differences of the diffusion coefficients of SO2 in the pore space of the calcines of the three materials, and therefore, the above explanation is consistent with the sulfation behavior at 750 °C of CaO samples prepared at 750 and 850 °C (Figure 4). A possible reason for the negative influence of temperature on the diffusion coefficient of gases in the pore space of the calcined CEB is that exposure to high temperatures increases the rate of sintering, reducing the small pore porosity and the surface area of the sorbent material. As it is seen from the results of Table 1, the CaO material prepared from CEB at 750 °C has 2 times larger pore space of small pores, as determined
1344 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004
Figure 6. Effects of temperature on the weight change vs time curve for sulfation of CEB in sequential mode. The material was brought to the reaction temperature by heating it under nitrogen.
from the N2 sorption data, than the sample prepared at 850 °C. The latter has larger surface area, but the complete N2 adsorption-desorption curves22 suggest that this difference is due to very small pores, which should be plugged with solid product at very low conversion levels. It should be noted that it is rather precarious to reach conclusions on the differences in the pore size distributions of the porous solids obtained from CEB decomposition on the basis of the mercury penetration data since these solids are fragile and it is possible that their structure may be altered at the high pressures applied in that measurement method. Several runs were carried out at the reaction conditions of Figures 2-4 to ascertain that the obtained results were reproducible. It was found that while there were some differences among the corresponding weight change versus time curves of different experiments, the main trends were consistent with those shown in Figures 2-4. Weight versus time curves for simultaneous and sequential sulfation obtained from another set of experiments conducted at the conditions of Figure 3 are presented in Figure 5, and they are compared with those shown in Figure 3. It is seen that the two sets of results are comparable to each other. Significant differences are encountered mainly at small reaction times. Because of the very high reaction rates during the initial stages of the sulfation process, the weight change curve is very sensitive to the diffusional limitations that SO2 encounters in the pore structure. These limitations are strongly influenced by the amount of the sample used and the way in which it is coated on the bottom of the pan. The results were also validated using a second batch of CEB. Figure 6 shows two sulfation curves obtained using sorbents prepared by decomposition and calcination of CEB during heating under N2, that is, in one step. It is seen that the results are similar to those shown in Figure 4. The conversion reached by the solid prepared at 750 °C is again much higher than that of the sample prepared at 850 °C. It should be noted that the conversions in Figure 6 are higher than those in Figure 4 at both temperatures, but experiments at other conditions indicated that this was more a reflection of
Figure 7. Comparison of the behavior of calcium-enriched biooil (CEB) with those of 53-62 µm particles of Greer limestone and Iceland spar during sulfation at 750 °C in sequential or simultaneous mode.
Figure 8. Comparison of the behavior of calcium-enriched biooil (CEB) with those of 53-62 µm particles of Greer limestone and Iceland spar during sulfation at 850 °C in sequential or simultaneous mode.
the different batch of CEB and not of the method of preparation of the porous sorbent. Figures 7 and 8 compare the calcination and sulfation behavior of the porous sorbent obtained through CEB decomposition (Figures 2 and 3), in sequential or simultaneous mode, with the behavior of porous sorbents obtained from 53-62 µm particles of the two calcium carbonate solids. The results clearly show that even at 850 °C the porous material that is obtained from the decomposition of CEB exhibits much higher sulfation conversion. It must be noted that, under the conditions prevailing in a combustor, the porous particles that result from the decomposition of CEB are exposed to the high temperature environment only for a very short time. Therefore, the changes that their pore structure undergoes because of sintering effects will be of much lower extent than those experienced by the pore structure of the porous material formed in the thermo-
Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1345
Figure 9. Weight change vs time for calcium-enriched bio-oil (CEB) decomposed and sulfided at 750 °C in sequential or simultaneous mode.
Figure 11. Weight change vs time results for Greer limestone 297-350 µm particles decomposed and reacted at 850 °C in sequential or simultaneous mode.
Figure 12. Weight change vs time results for Greer limestone 53-62 µm particles decomposed and reacted at 750 °C in sequential or simultaneous mode. Figure 10. Weight change vs time for calcium-enriched bio-oil (CEB) decomposed and sulfided at 850 °C in sequential or simultaneous mode.
gravimetric analysis system if the temperatures experienced by the particles in two systems are similar. In the latter, the porous material was exposed to the high temperature environment for about 30 min, on the average, from the moment it reached the reaction temperature until the SO2-containing mixture was sent through the reactor. 5. Sulfidation Results Sulfidation experiments were carried out using the same procedure as in the case of the sulfation experiments, the only difference being that the reactive mixture sent through the reactor was a mixture of H2S (7000 ppm) and N2 instead of SO2, O2, and N2. As in the case of sulfation, both sequential and simultaneous calcination reaction experiments were carried out. The obtained results are presented in Figures 9-13. The weight change is defined in the same way as in the case of the sulfation experiments. For complete conversion
of CaO to CaS, 1+∆W/W0 should be 1.16 for the sequential sulfidation curve and 0.72 for the simultaneous calcination-sulfidation curve or sequential calcination-sulfidation curve (composite curve). Figures 9 and 10 present results for sulfidation carried out, in simultaneous or sequential mode, at 750 and 850 °C, respectively. In both figures, one may easily verify that the sulfidation process reaches complete conversion in both cases (simultaneous and sequential mode). It takes about 9-10 min for complete conversion to take place. Since the solid product of the sulfidation reaction (CaS) is less bulky than that of the sulfation reaction (CaSO4), it is possible to achieve complete sulfidation without plugging completely the pore structure even if the calcium carbonate from which the porous CaO is obtained is nonporous, and no particle size expansion takes place during calcination. The comparison of the results for 750 and 850 °C indicates that sulfidation takes place faster at the lower temperature. This behavior is consistent with the observations made on the effects of temperature in the calcination and sulfation results in our previous study22 and the preceding section, respectively, which led us to
1346 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004
Figure 13. Effect of the presence of oxygen in the gas mixture during decomposition on the weight change vs time curve for calcium-enriched bio-oil (CEB) sulfided sequentially at 850 °C.
conclude that the diffusion coefficient of gases through the pore space of the CaO material obtained from the decomposition of CEB is lower at the higher temperature, possibly because of the occurrence of sintering. Past studies5 showed that the temperature of the reaction has the opposite effect on the sulfidation of CaO sorbents obtained from the two calcium carbonate solids that we employed as test cases in the sulfation experiments (Greer limestone and Iceland spar). However, the characterization of the pore structure of these solids did not show that the temperature had any significant effects on their pore structure properties. It should be pointed out that the lower diffusion coefficient in CEBderived CaO at 850 °C does not lead to lower calcination rates (compare Figures 9 and 10) because one of the key driving forces for calcination, the difference between the equilibrium concentration of CO2 at the reaction interface and its concentration in the bulk phase, increases dramatically with increasing temperature. The results presented in past studies of the literature on the sulfidation of limestone-derived CaO refer to sulfidation using completely calcined samples, that is, to reaction in sequential mode. This approach was also adopted in the indirect limestone sulfidation experiments that were carried out in our laboratory in the past.5 Since these studies have been carried out in a different thermogravimetric analysis system from that used in this study, it was decided to carry out detailed studies of the behavior of Greer limestone and Iceland spar in indirect sulfidation experiments, both in simultaneous and sequential mode. The sequential calcination and sulfidation results that were obtained from our experiments were found to be, in general, in good agreement with those obtained in past studies.5 Results for 297-350 µm Greer limestone particles at 850 °C are shown in Figure 11 and for small particles (53-62 µm) of the same solid at 750 °C in Figure 12. It is seen from the results for the large particles that even though the temperature is high (850 °C), the sulfidation reaction reaches only about 60% conversion within the time that is required for the sulfidation of CaO obtained from CEB decomposition to be completed. Much smaller conversions were ob-
served for these particles at the low temperature. The small particles of Greer limestone of Figure 12 reach almost complete conversion when they are reacted in sequential mode despite the lower reaction temperature (750 vs 850 °C in Figure 11). Faster overall reaction rates were observed for these particles at the larger temperature. There are very large differences between the composite simultaneous curve and the actual simultaneous curve in Figure 12, and thus, it is not possible to say whether the sulfidation reaction has reached high conversion levels at 600 s in that case. Even though the total weight change is very close to the value expected for complete conversion, this weight difference may reflect partial calcination and partial sulfidation and not complete calcination and complete sulfidation. An observation that points toward the latter of these explanations is that the weight does attain a minimum, which would signify completion of the calcination reaction, but decreases monotonically. The differences between the composite and actual simultaneous sulfidation curve are very small in the case of the large particles at 850 °C, and this suggests limited interference of the two processes at this temperature. Negligible interference between the calcination and sulfidation reactions is also suggested by the results for the sulfidation of the CEBderived CaO material at 850 °C (see Figure 11). This, however, does not appear to be the case when the CEBderived material is reacted at 750 °C. As in the case of the limestone particles, there appears to be significant interference between the two reactions at this temperature. However, in the case of the CEB-derived material, the weight of the material goes through a minimum and then reaches an almost constant value that is very close to that obtained for complete conversion. These are strong indications that both the calcination and the sulfidation reactions have reached complete conversion. The main reason for the stronger interference of the calcination and sulfidation reactions at the lower temperature is the slowing of the calcination reaction. The rate of the calcination reaction becomes comparable to that of the sulfidation reaction, and this provides sufficient time to the latter to modify the pore structure of the solid and create a product layer of significant thickness before the calcination reaction is completed. The interference of the two processes is stronger in the case of Greer limestone because the calcination of this solid is much slower than that of CEB-derived CaO, especially at 750 °C (see ref 22 and the calcination curves shown in the figures that present sulfation and sulfidation results in the present study). Among the most important factors that influence the rate of calcination is the extent of the intraparticle diffusional limitations in the pore space of the decomposed solid (CaO). The calcination of Iceland spar proceeds much slower that that of Greer limestone, and thus, this solid was found to exhibit much lower sulfidation rates and smaller conversions at a certain reaction time than Greer limestone, in agreement with past studies.5 Moreover, the weight variation in the simultaneous process indicated very strong interaction of calcination and sulfidation for this solid even at 850 °C. The material that was produced from the decomposition of CEB in an environment free of oxygen and CO2 had dark brown color, an indication of residual carbon in it. The introduction of oxygen in the reactor after the completion of the decomposition caused a small weight
Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1347
decrease, and this was construed as an indication of the presence of residual carbon in the decomposed sample (CaO). Figure 13 examines the effects of the presence of oxygen in the gas mixture sent through the reactor during decomposition on the sulfidation behavior of the resulting CaO particles. The differences between the two curves are rather small, and the same observation was made for reaction at other conditions. This indicates that the presence of residual carbon in CaO samples prepared by decomposing CEB in the absence of oxidizing species (CO2 or O2) does not affect significantly their behavior in sulfidation or sulfation experiments. The sulfidation results that we have presented were obtained using a 1% H2S in nitrogen mixture that was diluted with N2 to a stream of 7000 ppm H2S. Efthimiadis and Sotirchos5 examined the effects of concentration on the sulfidation process of calcium carbonate solids by carrying out experiments with different concentrations of H2S in the gas phase. Their results indicated that the rate of weight change was proportional to the concentration of H2S, and since this is the case for the rates of the diffusion processes (diffusion through the pore space and diffusion through the CaS layer), they concluded that the sulfidation reaction was of first order with respect to the H2S concentration. We also investigated the concentration dependence of the sulfidation rates by carrying out experiments at H2S concentrations different from 3500 ppm. The obtained results were in agreement with those of Efthimiadis and Sotirchos for the calcium carbonate solids and indicated that the sulfidation of CaO produced from CEB decomposition proceeds with rate proportional to the concentration of H2S as well.
decomposition and sulfation had a very strong influence on the sulfation behavior of the CEB-derived sorbent. Almost complete conversion of CaO to CaSO4 could be achieved for samples prepared and reacted at 750 °C, but the conversions reached by samples prepared at 850 °C were much lower at both reaction temperatures. However, even in the latter case, the observed conversions of CaO to CaSO4 were much higher than those measured for the calcines of the two calcium carbonate solids, which even for the small particles (53-62 µm) of the most reactive solid (Greer limestone) were below 20-40%. Higher treatment or reaction temperatures lead to incomplete conversion of CaO to CaSO4 for the CEB-derived sorbent. The main conclusion from the findings of our experimental studies is that the CaO material that results from the decomposition of calcium-enriched bio-oil has the potential to be a much more efficient SOx removal sorbent than calcined limestones. This is mainly due to the high porosity of this material, which allows the pore structure to remain open even at the point of complete conversion of CaO to CaSO4. It must be noted that the material formed from the CEB droplets in a combustor could be characterized by greater porosity than the material used as sorbent in our studies. The latter was prepared under much milder conditions than those encountered by the CEB droplets in the combustor but also was kept at the reaction temperature for a relatively large period of time (about 30 min) before it was subjected to sulfation or sulfidation. Therefore, the performance of CEB-derived sorbents as agents for the in situ removal of SOx or H2S in coal utilization units may be better than that seen in the experimental results of this study.
6. Summary and Conclusions An experimental investigation was carried out of the performance of the calcium oxide particles that are produced from the decomposition of calcium-enriched bio-oil (CEB), the product of the reaction of bio-oil with calcium hydroxide, as sorbent material for the in situ removal of SO2 and H2S from the flue gases of coal combustors and the coal gas produced in coal gasifiers, respectively. Sulfation and sulfidation experiments were carried out in a thermogravimetric analysis system, operating at atmospheric pressure, using samples of calcium-enriched bio-oil applied as thin coatings on the bottom of a quartz pan. After drying, these samples were decomposed to CaO under conditions of slow heating, and the resulting CaO was reacted with SO2 or H2S. In most experiments, the conversion of CEB to CaO was achieved in two steps: first converting the material to CaCO3 by heating it in the presence of CO2 and then calcining it by switching to a CO2-free mixture. Since in an actual application calcination and sulfidation or calcination and sulfation take place simultaneously, reactivity experiments were carried out by conducting these reactions both sequentially and simultaneously. The performance of the CEB-derived CaO sorbents was investigated using as reference those of CaO materials, in particle form, obtained from the decomposition of two naturally occurring calcitic solids of very high content in CaCO3 (over 97%), a microcrystalline calcitic limestone and a high purity calcite existing in the form of large single crystals. The results from the sulfation experiments (in simultaneous or sequential model with calcination) showed that the maximum temperature experienced during
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Received for review October 10, 2003 Revised manuscript received December 23, 2003 Accepted January 21, 2004 IE034176W
