Energy & Fuels 1989,3, 284-287
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Reaction between Calcium Carbonate and Sulfur Dioxide Claes Tullin* and Evert Ljungstrom Department of Inorganic Chemistry, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden Received November 2, 1988. Revised Manuscript Received January 18, 1989
It has been unclear if CaO or CaC03 is the reactive species in the desulfurization process in pressurized fluidized-bed combustion (PFBC), where the high partial pressure of C02should, in terms of elementary thermodynamics,inhibit the calcination of CaC03 In this investigation the simultaneous sulfation and recarbonation of calcined limestone and calcined CaC03 (pro analyse) was studied by means of thermogravimetric analysis at atmospheric pressure and 860 "C. When the samples were exposed to 0.3% SO2, 70% C02,and 3% 02,with the balance being N2,the recarbonation reaction initially dominates heavily over the sulfation until it becomes diffusion controlled. The CaC03 formed subsequently reacts with SO, (i.e. SO2and/or SO3)producing CaSO1. The sulfation rate of uncalcined CaC03 (pro analyse) was shown to be comparable with the sulfation rate of calcined material. Thus, it is concluded that the desulfurization in PFBC's is mainly caused by a direct reaction between CaC03 and SO,, regardless of precalcination.
Introduction The use of calcium-containing minerals, such as limestone (CaC03), as agents for desulfurization in fluidizedbed combustion of coal a t atmospheric pressure (AFBC), as well as at elevated pressures (PFBC), is a well-established technology. Due to the low specific surface area of CaC03, it is generally held that effective desulfurization passes from an initial heating and calcining stage, resulting in a CaO with a large surface area.'V2 CaC03(s) F? CaO(s) + C02(g) (1) The calcined limestone (CaO) subsequently reacts with
SO,, i.e. SO2 and/or SO3. CaO(s) + SOdg) + f/ZOdg)F? CaSOds) CaO(s) + S03(g)
* CaS04(s)
(2)
(3)
In the temperature range of interest in PFBC units
(750-950 "C), the CaS04 formed will always be stable under oxidizing conditions. From reaction 1it is clear that the CaC03 will be stable if the partial pressure of C02 exceeds the equilibrium pressure. A typical flue gas contains about 15% COz, where the calculated equilibrium pressure is 0.57 bar at 860 0C.3 The calcination should therefore be inhibited at total pressures exceeding 4 bar. At a normal operating pressure of 16 bar, with a partial pressure of C02 of about 2.4 bar, the calcination temperature would be approximately 966 "CS3A possible explanation for the successful desulfurization in PFBC might be due to differences in the C 0 2 partial pressures within the bed. The calcination is then assumed to take place near the air distributor where the C02pressure is low, and the completely or partly calcined particles are then sulfated or recarbonated in the upper regions of the bed.ll2 In bed samples extracted from PFBC test rigs, however, only traces of CaO have been found: suggesting that an al(1) Stantan, J. E.; Barker, S. N.; Wardell, R. V.; Ulerich, N. H.; Keairns, D. L.Proc. Znt. Conf. Fluid. Bed Combust. 1982,7th,1064-1075. ( 2 ) Jansaon, S. A.; OConnell, L. P.; Stantan,J. E. Proc. Znt. Conf. Fluid. Bed Combust. 1982, 7th, 1095-1100. (3)Baker, E. H. J. Chem. SOC.1962,464-470. (4) Ljunptrcm, E.; Lindqvist, 0. Proc. Znt. Conf. Fluid. Bed Combust. 1982,7th, 466-472.
Table I. Characteristics of the Materials Used CaC03 (pro analyse) chem compn, w t % CaC03 %COB
99
K2O
Yxhult limestone 83.1 1.0 1.3 0.8
2.7
A1203
Si compounds particle size, pm specific surface area of calcined material: m2/g
10-9P 2.4
11.1 90-125b 6.3
The limits enclose 90% of the particle mass. *Sieved material. 'The samples were calcined in a tube furnace under a flow of Nz. The samples were introduced into the cool furnace, heated to 830 OC during 2 h, and held a t 830-850 "C for 1.5 h.
ternate route for desulfurization could be the direct reaction of CaC03 with SO,."' CaC03(s) + SO&)
+ '/202(g) * CaSOds) + COdg)
CaC03(s) + S03(g)
* CaS04(s)+ C02(g)
(4)
(5)
Assuming that the main pathway responsible for the sulfur capture is the well-established reaction between CaO and SO, (reactions 2 and b), the relative rates of sulfation and recarbonation, i.e. the reverse of reaction 1, are of interest. As part of ongoing research examining the mechanisms of desulfurization in PFBC's, this investigation was undertaken for the purpose to study those relative rates of sulfation and recarbonation.
Experimental Section The experiments were performed with calcium carbonate (pro analyse, Merck No. 2066),and Yxhult limestone. The characteristics of the materials are given in Table I. (5) Dennis, J. S.; Hayhurst, A. N. Zmt. Chem. Eng. Symp. Ser. Inst. Chem. Eng. 1984,No. 87,61-68. (6) Tarradellas, J.; Bonnetain, L. Bull. Soc. Chim. Fr. 1973, 6, 1903-1908.
(7) Bulewicz, E. M.; Kandefer, S.; Jurys, C. J. Imt. Energy 1986,
188-195.
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Figure 1. SchematicTGA curve obtained when the material was calcined (A; 100% NZ),simultaneouslysulfated and recarbonated (B, 0.3% SO,, 70% COP, 3% 02,balance NZ),and finally recalcined (C; 100% N2).
Thermogravimetricanalyses (TGA) were carried out by using a Mettler TA1 apparatus at atmospheric pressure with various gas compositions. SO2 (2% in N2),COP,air (21% O2in N2),and N2 gas supplies were controlled by means of calibrated flow rotameters and set to achieve the desired concentrationswith a total flow of 15 L/h. A sample weight of 150 f 1.5 mg was used throughout the experiments, and the samples were mounted in a hemispherical-shaped A1203 crucible (width 17.0 mm, height 6.3 mm). The samples were heated to 860 OC at a rate of 25 OC/min and held at that temperature for 1h under N2 to ensure complete calcination. In one set of the experiments the calcined samples were either recarbonated in an atmosphereof C02(70%) and O2(3%)or sulfated in an atmosphere of SO2(0.3%)and O2 (3%) with N2 as balance. In another set of experiments the calcined sampleswere Simultaneouslysulfated and recarbonated in an atmosphere of 70% C02, 0.3% SO2, 3% 02,and N2 as balance. The total molar content of CaO was calculated from the initial weight loss during calcination (stage A in Figure 1). When the reaction products were either CaC03or CaS04,the fraction of CaO converted could be calculated directly from the increase in weight. In those experiments where CaC03 and CaS04 were formed simultaneously,as in Figure 1,the fraction of CaC03was calculated from the weight decrease obtained when the atmosphere was changed back to Nz (recalcination, stage C) after 4-180 min of reaction (stage B). If the weight increase caused by CaC03 formation in stage B was known,the fraction of CaS04formed could be determined. The direct sulfation of CaCOS,i.e. the sulfation of uncalcined material, was also studied. In order to prevent calcination, the CaCOS(pro analyse) sample was heated to 860 OC in an atmosphere of 70% C02and 3% 02 with N2as balance. After 1h at 860 OC, confirmingthat no weight decreaeeoccurred, SO2 (0.3%) was introduced. The fractional conversion of CaO to CaC03 and CaS04 is designated as Xcncoaand XCgo,,respectively, where moles of CaC03formed x 100% X ~ a = ~ total ~ s moles of Ca in sample moles of CaS04 formed x w 4 x 100% total moles of Ca in sample and the residual CaO content, Xho, is given by % Xcno = (100 - Xcnco~- XC~SOJ
Results and Discussion The rates of sulfation and recarbonation of the calcined CaC03 powder and the calcined limestone are shown in Figures 2 and 3. The rate of recarbonation, i.e. the reverse of reaction 1,was observed in both cases to be much faster than the rate of sulfation (reactions 2 and 3) during the initial stage of the reaction. Although CaO is reported to have a greater reactivity toward SO2than C02: the greater
30
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Figure 2. Sulfation and recarbonation of calcined CaC03(pro analyse) at 860 OC: (I) sulfation, 0.3% SO2,3% O,, balance N2; (11)recarbonation, 70% C02,3% 02,balance Nz. P = 1 atm. 100- X %
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Figure 3. Sulfation and recarbonation of calcined Yxhult limestone at 860 OC: (I)sulfation, 0.3% SOz, 3% 0 2 , balance N,; (11)recarbonation, 70% C02, 3% O,, balance N2. P = 1 atm. partial pressures of C02 allow the recarbonation reaction to dominate. It should be noted that initial reaction rates observed by TGA methods are often controlled by mass transport resistance between the bulk gas and the sample and are therefore not suitable for determination of rate constants. Since CaC03 has a larger molar volume than CaO, conversion to CaC03 through the recarbonation reaction results in a blockage of the pore structure within the reacting CaO particle. Thus, the rate of recarbonation becomes rapidly controlled by the mass transport of C02 through the formed CaC03 layer.*12 Similarly the sulfation reaction of CaO also becomes controlled by diffusion through the product layer after an initial surface rea~ti0n.l~The conversion of CaO to CaS04 and CaCOSwas greater for the CaC03 powder, due probably to differences in particle size, porosity, and chemical composition (Table I). If the sulfation reaction proceeds via CaO, then it must compete with the recarbonation reaction for free CaO, since sufficiently high partial pressures of C02are present in the larger part of the PFBC bed. Figures 4 and 5 illustrate what would be expected to happen to a particle that is allowed to calcine completely in those regions of the bed where the C02 pressure is low, after which it is transported to a zone of high CO, pressure. The sulfation and recarbonation reactions seem initially to occur inde(8)Clark, L. M. R e d . Trau. Chim. Pays-Baa 1949,68,969-982. (9) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1983, 29, 1, 79-86. (10) Hattori, T.; Mohri, J. Yogyo Kyokaishi 1981, 89, 10, 568-571. (11) Oakeeon, W. G.; Cutler, I. B. J. Am. Ceram. SOC.1979,62,11-12, 556-558. (12) Barker, R. J. Appl. Chem. Biotechnol. 1973,23, 733-742. (13) Hartman, M.; Coughlin, R. W. AZChE J. 1976,22,490-498.
Tullin and Ljungstrom
286 Energy & Fuels, Vol. 3, No. 3, 1989
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Figure 4. Simultaneous sulfation and recarbonation of calcined CaC03 (pro analyse) at 860 "C under 0.3% SO2, 70% COz, 3% 0 2 , and Nz as balance: ( 0 )Xcdo4;(EI) XcacOs,(m) CC.0. P = 1
atm.
1007
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Figure 6. Sulfation (0.3% SOz,3% 02,and N2as balance) for 2 h followed by recarbonation (70% COz, 3% Oz,and N2 as balance) for 1 h. P = 1 atm.
X%
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l o80o ]
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Figure 5. Simultaneous sulfation and recarbonation of calcined Yxhult limestone at 860 "C under 0.3% SOz, 70% COz,3% Oz, and Nzas balance: ( 0 )Xcas~(;(EI) XC.C~; (H) Xc.0. P = 1 atm.
pendently of each other, and the sharp decrease in recarbonation rate indicates that the surface of the particles is almost instantaneously covered by CaC03 product. If the diffusion rate of SO2through the CaC03 layer is comparable with that of C02,then the contribution via reaction 2 or 3 to the overall conversion of CaO to CaS04 ought to be negligible under partial pressures of C02exceeding the equilibrium pressure. Since the C02 pressure exceeds the equilibrium pressure, no CaO could be formed from the recalcination of CaC03, and therefore, the decrease of CaC03 content is caused by direct sulfation (reaction 4 or 5 ) . Thus, it is evident that the sulfation reaction may proceed via the direct reaction of SO, with CaC03. In view of these observations, the sulfur capture by a calcined particle in a PFBC probably procedes from an initial recarbonation of the available surface, producing a CaC03 layer. This reaction will heavily dominate over the sulfation to the point where the recarbonation becomes controlled by diffusion through the product layer. The formed CaC03 subsequently reacts with SO,, resulting in an outer layer of CaS04 and C02 gas. As a result of this, the layer of CaC03 will decrease in thickness, and further recarbonation of internal CaO may occur. It is conceivable that SO, could, in addition to C02, diffuse through the CaC03 layer and react with the internal CaO, although in view of the bulk concentration differences and the consumption of SO, at the surface (sulfation of CaC03),this mechanism is believed to be of minor importance. If a noncalcined material (CaC03) is subjected to sulfation, the released C02 molecules (reaction 4 or 5) must diffuse through the outer CaS04 layer to the bulk gas. If this process is slow compared with the sulfation rate (reaction 4 or 5), the intraparticle pressures of C02 will be
0
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Figure 7. Sulfation of CaCOS (pro analyse) at 860 "C. The calcination status of the samples, and the sulfation conditions were as follows: (I) calcined, 0.3% SO2, 3% Oz,70% COz (from ; calcined, 0.3% SOz, 3% O2 (from Figure 2); Figure 4 ( 0 ) ) (11) (111) calcined and recarbonated (70% C02, 3% 02)for 3 h to Xcac0 = 93.5%, 0.3% SO2, 3% 02, 70% CO,; (IV) uncalcined, 0.3% bo2,3% 02,70% COP In all of the experiments Nzwas used as balance. P = 1 atm.
high. In order to study the diffusion of C02 through the CaS04layer, a sample of CaC03 (pro analyse) was calcined, sulfated for 2 h, and finally recarbonated for 1h (Figure 6). The recarbonation reaction is still fast and seems to be only marginally affected by the CaS04 layer present. Thus, the COz diffusion rate through the CaS04layer was assumed to be fast, implying that the intraparticle pressure of C02 ought to be commensurate with the bulk pressure. Furthermore, thermodynamic calculations based on the free energy changes in reactions 4 and 5 indicate that the equilibrium pressure of C02 is of the order of lo6 bar at 860 "C; psq = 0.003 bar and pq = 0.04 bar. Thus, the rate of reaction appears to be independent of the C02 levels of interest. If the desulfurization is chiefly due to the direct reaction of SO, with CaC03, the relative sulfation rate of a noncalcined material under high partial pressures of COz should be investigated. To maintain a stable CaCO3?ample and prevent the calcination reaction, heating in an environment of C02 above the equilibrium pressure is necessary. The Yxhult limestone was, however, found to be unstable under these conditions, and therefore, it was omitted from further investigations. The CaC03 powder, however, remained uncalcined when heated under 70% C02 to 860 "C. Furthermore, a sample of CaC03 (pro analyse) was calcined and recarbonated for 3 h in order to compare the desulfurization activity of the uncalcined CaC03 powder with that of the CaC03 formed during recarbonation. The results are shown in Figure 7, where the
Energy & Fuels 1989, 3, 287-291 sulfation curves from Figures 2 and 4 are included for comparison. The simultaneously recarbonated and sulfated material (curve I) shows a high desulfurization activity. This may be explained as the CaC03 layer formed during recarbonation is built up by small crystallites, thus resulting in a relatively large specific surface area. The calcined and recarbonated material (curve 111) was subsequently expected to be more reactive than the uncalcined material (curve IV). The difference between the two curves is however small, possibly due to crystallite growth occurring during the recarbonation reaction. Perhaps the most surprising result in Figure 7, however, is the sulfation rate of the uncalcined material (curve IV), which clearly is commensurate with the sulfation rate of calcined material in absence of C02 (curve 11). This may be an effect of the small particle size. Since air was used as a source of oxygen in the experiments traces of C02 were always present. In order to investigate if low C02 concentrations had a catalyzing effect on the sulfation rate, one sulfation experiment was
287
undertaken where the C02was removed by passing the air through a bed of Ascarite. No significant effect on the sulfur capture was found.
Conclusions The results show that the desulfurization in PFBC units is mainly caused by a direct reaction between SO, and CaC03. If precalcined material is used it will initially recarbonate rather than sulfate due to the high partial pressure of C02in PFBC units. Furthermore, the sulfation rates of uncalcined and calcined CaC03 (pro analyse) are comparable. Acknowledgment. We express our gratitude to the National Energy Administration for financial support and to David Cooper, Sohbat Ghardashkhani, Anders Lyngfelt, and Professor Oliver Lindqvist for helpful discussions as well as technical assistance. Registry No. SOz, 7446-09-5;COz, 124-38-9;CaO, 1305-78-8; CaC03, 471-34-1; CaS04,7778-18-9.
Effect of Carbon Monoxide on Olefin Hydrogenation and Isomerization on a Reduced Fused Magnetite Catalyst David K. Matsumoto and Charles N. Satterfield* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received November 23, 1988. Revised Manuscript Received February 2, 1989 Some of the effects of high syngas conversion on the secondary reactions of olefins formed by Fischer-Tropsch synthesis on a reduced fused magnetite catalyst were simulated. Studies were of olefins in the presence of hydrogen and low concentrations of CO, or in hydrogen alone, at 232 "C and total pressures of 0.30-0.79 MPa. No significant incorporation of olefins into growing chains was observed.
Introduction 1-Olefins are a major primary product of the FischerTropsch synthesis on iron catalysts. They can undergo secondary reactions, the two dominant ones being hydrogenation to form paraffins and double-bond migration to form 2-olefins (for olefins of four carbon atoms or more). Generally 3-olefins are not observed. Olefins may also become incorporated into higher molecular weight products, but the extent to which this occurs on iron is the subject of some debate. In recent papers we have reported how the secondary reactions of olefins formed on a fused magnetite catalyst during Fischer-Tropsch synthesis can be modified by treatment with dibenzothiophene.1*2 Notably, after the reduced catalyst was contacted with dibenzothiophene, olefin selectivities became much more sensitive to syngas conversion as compared to the untreated catalyst. In another study3selected 1-olefinswere added to syngas during Fischel-Tropsch synthesis. The secondary reactions were enhanced at high CO conversions (low CO partial pres(1)Stenger, H. G., Jr.; Sattefield, C. N. Znd. Eng. Chem. Process Des. Deu. 1985,24, 415. (2) Matsumoto, D. K.; Satterfield, C. N. Energy Fuels 1987, I , 203. (3) Hanlon, R. T.; Satterfield, C. N. Energy and Fuels 1988,2, 196.
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Table I. Analysis of Organics in 2% Olefin in H, Mixtures olefin ethene propene 1-butene 2-butene (I
mol % of organics 99.5 99.6 99.7 55.6 (trans) 42.0 (cis)
minor components (mol % of component) 1-butene (1.3), butane (0.9), unidentified propane (0.4) butane (O.l), trans-2-butene (0.1) 1-butene (1.3), butane (0.9),unidentified impurity (0.2)"
Passed through reactor unconverted.
sure). This enhancement was attributed to a decrease in competitive adsorption by CO. This and other studies suggested that the olefin reaction network in the Fischer-Tropsch synthesis may be markedly affected by CO conversion, especially very high conversions. High conversion effects have received little attention in the past, probably because it is difficult to isolate its effect with studies in a plug-flow (fixed-bed) reactor. A continuous-flow stirred tank reactor (CSTR), as used here, is particularly well suited to study effects of this nature. The purpose of the present studies was to simulate some representative Fischer-Tropsch reaction conditions on iron ranging from very high conversions (high H2/C0 ratios) to complete conversion of carbon monoxide (reactions of olefins with no CO present). 0 1989 American Chemical Society
