Properties and Environmental Considerations Related to AFBC Solid Residues
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PROPERTIES AND ENVIRONMENTAL CONSIDERATIONS RELATED TO AFBC SOLID RESIDUES E.E. BERRY* and E.J. ANTHONY** *PO Box 7261, Oakville, Ontario, Canada **E.J. Anthony, CANMET, Energy Mines and Resources Canada, Canada Received 2 February,
Ottawa, Ontario,
1987; communicated by G.J. McCarthy
ABSTRACT Atmospheric-pressure fluidized bed combustion (AFBC) produces solid residues that are different from the familiar pulverized coal ashes. When limestone beds are used to adsorb SOx, high-Ca residues, comprised largely of CaO and SO4 , are produced. Leachates from high-Ca AFBC residues are strongly alkaline (pH >11) and contain high levels of dissolved solids (TDS >3000 mg/L). If water is added during handling, hydration of CaO may cause a temperature rise and hydration of CaSO 4 may result in premature hardening of the residues. Trace elements and organic components may leach from disposal sites. This paper presents an overview of the nature of AFBC residues and the factors influencing their disposal. INTRODUCTION Atmospheric-pressure fluidized bed combustion (AFBC) is rapidly gaining acceptance in North America as a means to use "problem" fuels, such as lowgrade or high-sulphur coals for thermal power generation. In AFBC, coarsely crushed fuel is suspended in a combustion chamber with a bed-material, such as sand or limestone. The suspension is sustained by fluidizing the mass with air injected from below the bed. There are two general classifications of the AFBC process, illustrated schematically in Figure 1, namely: Bubbling or dense-phase AFBC in which coal is combusted in a mass of limestone or an inert materials that is fluidized at high solids density in such a way that it remains largely in place as a well defined bed. Circulating AFBC in which both the fuel and a bed of low solids density are suspended in a circulating gas stream. Although, in principal a number of solids could be used as the bed material, in practice two general types can be distinguished. These in turn produce two distinct classes of AFBC residues: Limestone or dolomite beds are used to absorb acidic combustion gases (sulfur oxides) when higher sulfur fuels are burned. The solid wastes from limestone-bed systems, termed high-calcium residues in this paper, are usually produced in two forms: a coarse bed-drain comprising spent limestone granules; and a fine baghouse or carry-over ash. When low-grade coals or coaly wastes are burned, a bed of sand or spent shale, is frequently used, in which little or no absorption of acidic gases occurs. The solid wastes from such systems are termed low-calcium residues in this paper. They are also produced in two forms, of differing particle sizes, both comprise largely calcined clay or shale. Recently considerable attention has been given to the nature, disposal and possible use of the solid residues from AFBC boilers [1-18]. As has been noted by previous authors [5], the properties of fluidized bed wastes is very Mat. Res. Soc. Symp. Proc. Vol. 86. n1987 Materials Research Society
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