B.B. Argent

Coal ash composition as a function of feedstock composition

Abstract 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× 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× diagenetic mineral, and a mullite phase is potentially formed at higher air addition for 8× 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.

Prediction of the distribution of alkali and trace elements between the condensed and gaseous phases generated during clinical waste incineration

Abstract The mobilisation of alkali and trace elements present in clinical waste can lead to accelerated deterioration of the plant and to environmental damage. The damage can be caused by transfer of low levels of trace elements, which are difficult to monitor, and a model of the underlying processes which predicts the degree of mobilisation of each element from waste of specified characteristics is thus desirable. The Equilibrium module of the FACT suite of computer programs has been used to make predictions for alkali and trace element mobilisation from a typical waste composition with variation in the S/Cl ratio which influences the volatilisation/condensation processes. Although thermodynamic data for some of the potential melts are incomplete, predictions made using the various oxide melt models, matte, salt and solid solution models available in FACT are combined to allow meaningful comment on Pb, Cu, Zn, Ni and Cr distributions. Separate consideration is given to mobilisation in primary (pyrolysis) and secondary combustion (oxidation) chambers. Comparisons are made with published data from municipal waste incineration plants. An interesting feature of the predictions for condensation during cooling of the waste gases is that if solution of Pb, Zn and Cu chlorides is permitted in alkali chloride or Pb sulphate into sulphate melts then Pb and Cu are predicted to be largely removed from the gas stream into these melts.

Sintering of the APC Residue from Municipal Waste Incinerators

Air pollution control (APC) residues from municipal waste incineration contain considerable amounts of water-soluble heavy metal compounds which can cause environmental problems. Therefore, a satisfactory and efficient detoxification technology is required. A thermal process via sintering has been developed that is capable of forming two major fractions of sintered products — that is, a small fraction in which the volatile heavy metals are concentrated for recycling or disposal and a major solid particle fraction that is more stable. This result has been obtained by heating the APC ash up to 850 °C with a high efficiency processing system. This novel process has been demonstrated in a large experimental facility that includes a 100 kW regenerative burner. The approach is based on the application of a regenerative heating concept to achieve an economic and efficient process. Thus, the hazardous material which is causing a problem when landfilled can be converted to a better quality product either for utilization by the construction industry or for disposal at lower cost.