Energy & Fuels 2000, 14, 973-979
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Influence of Periodically Changing CO2 Partial Pressure on Sulfur Capture and Free Lime Content of Residues in PFBC Kuanrong Qiu* and Britt-Marie Steenari Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Gothenburg, Sweden Received January 12, 2000. Revised Manuscript Received June 10, 2000
PFBC technology is still under development. Further research into the capture of SO2 by limestone during PFBC is needed. The present work investigates the influence of periodically changing partial pressure of CO2 on the sulfation of limestone and also the free lime content present in residual products. Experiments were conducted in a fixed-bed reactor under conditions typical of PFBC, that is, limestone particles were alternately exposed to the conditions of calcination and recarbonation as the reaction of sulfation proceeded. It was found that the degree of sulfation achieved was considerably higher with periodically changing partial pressure of CO2 than with a steady high partial pressure of CO2. This was true even when the average value of the pressure during its periodical change exceeded the calcination equilibrium pressure. These findings may partly explain the high conversion of limestone obtained in PFBC units. Furthermore, the free lime content proved remarkably different for various cycle times of changing CO2 partial pressure and/or various periods of a cycle time elapsing in the calcining region. This indicates that the content of free lime in solid residues from PFBC boilers can vary considerably with respect to overall operating conditions.
Introduction Pressurized fluidized bed combustion (PFBC) is one of the advanced fossil fuel technologies or so-called clean coal technologies and is still under development. The removal of SO2 during PFBC processes can be achieved by the addition of a sorbent, such as limestone or dolomite. The SO2 released from the combustion of coal reacts with the sorbent to form CaSO4. The reaction of limestone with SO2 under the conditions of atmospheric fluidized bed combustion (AFBC) has been the subject of extensive studies. However, a direct application of the results and observations obtained to the removal of SO2 by limestone during PFBC of coal is not possible. Under atmospheric conditions, where the partial pressure of CO2 is below 0.02 MPa and the temperature is above 800 °C, limestone calcines to its oxide prior to sulfation:
CaCO3 f CaO + CO2
(1)
The porous CaO thus formed subsequently reacts with SO2 under oxidizing conditions in the overall reaction:
CaO + SO2 + 1/2O2 f CaSO4
(2)
The porous structure of CaO developed during calcination is known to be favorable to the sulfation reaction. The calcination of limestone particles is normally considered to be inhibited in PFBC since the average * To whom correspondence should be addressed. Telephone: +46 31 7722866. Fax: +46 31 7722853. E-mail:
[email protected].
partial pressure of CO2 exceeds the calcination equilibrium pressure of limestone. In this sense, uncalcined limestone can still react with SO2. The reaction between them, however, occurs through a different path, i.e., via direct sulfation of CaCO3:
CaCO3 + SO2 + 1/2O2 f CaSO4 + CO2
(3)
Unlike AFBC with its optimum temperature of 850 °C for sulfur capture by calcined limestone, no optimum temperature for the removal of SO2 by uncalcined limestone under PFBC conditions was apparent. During direct sulfation of CaCO3, CO2 is released from the reaction interfaces of the limestone particles. The release of CO2 produces a different structure in the product layer than that formed from the sulfation of calcined limestone. Snow et al.1 suggested that the CaSO4 product layer formed during direct sulfation was more porous than the product layer formed from the sulfation of calcined limestone. Higher levels of conversion were found to be achieved via direct sulfation, as compared to the sulfation of calcined limestone.1-3 In all cases, the experiments were conducted using thermogravimetric analysis (TGA) with relatively small particle sizes, namely, from a few µm to several tens of µm. However, Ulerich et al.4 found that the sulfur (1) Snow, M. J. H.; Longwell, J. P.; Sarofim, A. F. Ind. Eng. Chem. Res. 1988, 27, 268-273. (2) Iisa, K.; Hupa, M. Proceedings of the 23rd Symposium on Combustion, Pittsburgh, PA, 1990; pp 943-948. (3) Hajaligol, M. R.; Longwell, J. P.; Sarofim, A. F. Ind. Eng. Chem. Res. 1988, 27, 2203-2210.
10.1021/ef0000072 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/31/2000
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capture capacity of large limestone particles, also tested by means of TGA, was lower under uncalcined conditions than under calcined conditions. Also, Illerup et al.5 reported that by using a pressurized fixed-bed reactor to perform tests on limestone sulfation, a dramatic drop was observed in the degree of sulfation when the partial pressure of CO2 exceeded the equilibrium pressure. Furthermore, studies on PFBC at Exxon6 showed better sulfur capture by precalcined limestone, whereas Stantan et al.7 reported that precalcination gave no improvement upon sorbent utilization. Some authors7,8 have proposed that there exist zones in PFBC where the concentration of CO2 is low, e.g., near a distributor, enabling the occurrence of calcination of limestone. However, Tullin and Ljungstro¨m9 as well as Iisa et al.10 demonstrated a fairly rapid recarbonation reaction between CaO and CO2, compared to the reaction of CaO with SO2. The CaO so produced in a zone with low CO2 concentration may recarbonate to CaCO3 before reacting with SO2. Sulfur capture in PFBC was thus believed to proceed mainly via direct sulfation of CaCO3. The free lime content in sulfurous ashes is one of the key factors to be taken into account when disposing of the ashes. This is due to the free lime leading to some of the problematic behavior patterns found in the landfilled ashes, such as exothermic reactions with water and destructive expansion. According to certain investigations into ashes from PFBC boilers, the free lime contents are generally low (e.g., 1% or less).11,12 However, there is some disagreement in the literature regarding this aspect. For instance, data from a PFBC, reported by Hoy et al.,13 showed that, typically, more than 50% of the unsulfated limestone was calcined despite the exit partial pressure of CO2 exceeding the equilibrium pressure of limestone calcination. It is clear that there is no general agreement in the literature about the effects of pressurized conditions on the sulfur capture process, though many investigators proposed that favorable desulfurization is possible in PFBC. Therefore, further research is needed into the reaction and mechanisms of sulfation pertinent to PFBC processes. Moreover, despite the fact that CO2 partial pressure varies with space and time in the fluidized bed combustor and thus the particles experience alternating gas compositions, no previous work has yet been reported on the effect of periodically changing CO2 partial pressure on the sulfation of limestone and the free lime content of PFBC ashes. (4) Ulerich, N. H.; Newby, R. A.; Keairns, D. L. Thermochim. Acta 1980, 36, 1-6. (5) Illerup, J. B.; Dam-Johansen, K.; Lunde´n, K. Chem. Eng. Sci. 1993, 48, 2151-2157. (6) Hoke, R. C.; Bertrand, R. R.; Nutkis, M. S.; Kinzler D. D.; Ruth, L. A. Studies of the Pressurized Fluidized-bed Coal Combustion Process; Technical Report EPA-600/7-77-107, 1977. (7) Stantan, J. E.; Barker, S. N.; Wardell, R. V.; Ulerich, N. H.; Keairns, D. L. Proc. Int. Conf. Fluid. Bed Combust. 1982, 7, 10641075. (8) Jansson, S. A.; O’Connell, L. P.; Stantan, J. E. Proc. Int. Conf. Fluid. Bed Combust. 1982, 7, 1095-1100. (9) Tullin, C.; Ljungstro¨m, E. Energy Fuels 1989, 3, 284-287. (10) Iisa, K.; Tullin, C.; Hupa, M. Proc. Int. Conf. Fluid. Bed Combust. 1991, 11, 83-90. (11) Anthony, E. J.; Iribarne, E. J.; Iribarne, J. P. Can. J. Chem. Eng. 1997, 75, 1115-1121. (12) Bland, A. E.; Brown, T. H. Proc. Int. Conf. Fluid. Bed Combust. 1997, 14, 683-692. (13) Hoy, H. R.; Roberts, A. G.; Stantan, J. E.; Wilkons, D. M. Pressurized Fluidized Bed Combustion; 1986, DOE/MC/22190-2356.
Qiu and Steenari
The present work evaluated the influence of CO2 partial pressures at which limestone calcines or fails to calcine on the rate of sulfation. Moreover, the effect of periodically changing CO2 partial pressure was studied. The conditions used either promoted or prevented the calcination of the sorbent. The study aimed to investigate the effect of the changing conditions on the degree of sulfation and also on the free lime content of residual products. Thus information was provided on the possible content of free lime in sulfurous ashes from PFBC. Experimental Section Experimental Setup. Experiments were performed in a fixed-bed reactor which consisted of quartz (Figure 1). The reactor was 700 mm in length, with a sintered quartz filter (diameter 19 mm) placed in the middle of the reactor to support the sample particles. The quartz reactor was mounted in an electrically heated tubular oven. There were two gas inlets in the reactor. This design enabled the introduction of CO2 with a periodically changing partial pressure while keeping constant the concentrations of SO2 and O2 in the reactant gas. O2, SO2 (0.5% SO2-N2 gas mixture), and some of the CO2 were let into the reactor from the lower inlet. In the meantime, N2 and the remaining CO2 were alternatingly introduced into the reactor from the upper inlet through two programmable threeway magnetic valves. The valves alternated between N2 and CO2, thus producing a periodically changing partial pressure of CO2 in the reactor while making constant the total flow through the reactor and the concentrations of SO2 and O2. Gas flow rates and gas compositions were regulated by mass flow controllers (Brooks 5850E). The concentrations of SO2, O2, and CO2 in the outlet gas from the reactor were measured using a URAS 10E gas analyzer. X-ray powder diffraction was applied to identify the crystalline compounds in the solid products from the fixed-bed reactor and was performed using a powder diffractometer (Siemens, D5000). Sorbent. A Swedish limestone, Ignaberga, was selected as a sorbent for this research. This limestone has been studied in previous investigations.14 The chemical composition of this limestone is given in Table 1. The sorbent was sieved into a particle size of 0.355-0.5 mm. Experimental Procedure. Experiments were conducted under three reactant gas conditions: (a) constant low CO2 partial pressure where the calcination of limestone occurs; (b) constant high CO2 partial pressures where limestone is unlikely to calcine on thermodynamic grounds; and (c) periodically changing partial pressure of CO2 between the low (0.012 MPa) and high (0.076 MPa) CO2 partial pressures. One cycle consisted of a period with the low CO2 partial pressure and a period with the high CO2 partial pressure. The experimental conditions used in this study are listed in Table 2. In all cases, isothermal conditions in the quartz reactor were first established and the desired reactant gas conditions were then achieved. For each run, 0.5 g of the limestone particles was rapidly loaded from a sample feeder (Figure 1) onto the sintered quartz filter in the reactor. The evolution of SO2 concentration in the outlet gas was recorded at intervals of two seconds. Figure 2 shows two typical curves of an SO2 concentration profile in the outlet gas. Curve (a) represents the profile of SO2 concentration under the conditions of a constant partial pressure of CO2. Curve (b) shows the experimental result under conditions where the partial pressure of CO2 was suddenly lowered during the sulfation reaction. (14) Mattisson, T. Ph.D. Thesis, University of Go¨teborg, Go¨teborg, Sweden, 1998.
S Capture and Lime Content of Residues in PFBC
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Figure 1. Schematic diagram of the experimental setup. Table 1: Chemical Composition of the Limestone Used Ignaberga limestone
chemical composition (wt %)
CaCO3 SiO2 Al Fe Mg K Na Ba + Sr
91.0 5.0 0.4 0.2 0.5 0.2
