Sulphur Oxidation States in Residues from a Small-Scale Circulating Fluidized Bed Combustor
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SULPHUR OXIDATION STATES IN RESIDUES FROM A SMALL-SCALE CIRCULATING FLUIDIZED BED COMBUSTOR E.E. BERRY,a R.T. HEMMINGS,b* B.J. CORNELIUSb* and E.J. ANTHONYc a. Matex Consultants Inc., Mississauga, Ontario, Canada L4Z 2G7 b. Ortech International, Sheridan Park, Mississauga, Ontario, Canada L5K 1B3 c. CANMET, Energy, Mines and Resources Canada, Ottawa, Canada Received 17 April, 1989; refereed ABSTRACT Significant concentrations (-6%, as CaS) of sulphides or other reduced-sulphur species in solid residues from a small-scale circulating fluidized bed (CFB) combustor have been reported in the literature. The presence of sulphides in similar quantities in residues from a utility-scale combustor would present significant difficulties with handling, disposal or utilization of the residues. This paper discusses the preliminary findings of an investigation of sulphur capture in a small-scale circulating fluidized bed combustion (CFBC) unit using a limestone bed and burning a high-S, high-Fe, Eastern Canadian coal. Data are presented on sulphur capture and chemical speciation for residue samples taken from a number of locations in the circulating bed during operation. The results are discussed in terms of probable mechanisms for the formation of sulphur compounds of reduced oxidation state in the bed and the combustion-gas cleaning system. INTRODUCTION The environmental problems associated with the combustion of low-grade or high-sulphur coals for thermal power generation are being widely investigated in North America. Of particular concern are emissions of sulphur oxides to the atmosphere and their contribution to "acidic rain." One approach to reducing acidic gas emissions is to burn such fuels in either a bubbling or circulating atmospheric pressure fluidized bed (AFB) combustor containing crushed limestone. During combustion, the products from oxidation of the sulphur in the fuel are absorbed into the calcined bed material and discharged as a solid residue, rather than with the combustion gases. Most of the sulphur captured by the limestone in AFB combustors is present in the discharged residues as anhydrous calcium sulphate (CaSO 4 ) [1-4]. However, significant accumulations of sulphide have been reported in residues from some small- and pilot-scale circulating fluidized bed combustors [5,6]. The presence of sulphide in residues from a full-scale, utility fluidized bed combustion (FBC) system would present considerable problems in terms of their utilization or disposal. This paper describes work in progress to examine the nature of sulphur in solid phases from a pilot-scale CFB combustor. The purpose of this study is to investigate the oxidation states of the major sulphur-containing species, and their location and mode of formation in the circulating fluid bed. EXPERIMENTAL DETAILS Combustion The work was conducted on materials obtained from a pilot-scale CFB combustor at the University of British Columbia. Full details of this installation have been reported by Grace et al. [7]. Figure 1 shows a schematic of the circul
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