Coal ash composition as a function of feedstock composition

The extended capability of thermodynamic predictive programs now available admits realistic modelling of coal ash from combustion of a coal with known mineralogy. Previous investigations in which the equilibrium ash composition corresponding to the bulk mineral analysis of a coal was examined are extended here to evaluate trends which could occur as a result of local inhomogeneity. In order to obtain realistic modelling it is necessary to refer to experimental investigations of ash composition and regard the formation of complex minerals involving more than two oxides to be too slow to occur to any significant extent during pf combustion. Products from the markedly different mineral contents of two coals predicted using a database which includes a model of a silicate melt with potential immiscible Liquid formation are compared. The difference in the nature of oxide melt formation between the two examples is significant. Results are presented for predictions assuming that 2,4 and 8 x the baseline diagenetic pyrite and kaolinite content of a representative coal are locally available for equilibration, during reaction with air up to 5% excess. Adiabatic temperatures are predicted, and show maxima which occur at higher air levels and lower temperatures for increasing mineral content. The formation of oxide and sulphide melts is also examined. The range of existence of sulphide melts varies significantly with mineral content. The persistence of silica to higher fraction of air added is noted for 8 x diagenetic mineral, and a mullite phase is potentially formed at higher air addition for 8 x diagenetic mineral content than for baseline content. The chlorine-containing species variation with air addition also differs for differing amounts of mineral present. The formation of condensed phases on cooling is also examined. (C) 1999 Elsevier Science Ltd. All rights reserved.