Kinetics of coal pyrolysis and devolatilization

Abstract

An experimentally based, conceptual model of the devolatilization of a HV bituminous coal is outlined in this report. This model contends that the relative dominance of a process type-chemical kinetic, heat transport, mass transport -- varies with the extent of reaction for a given set of heating conditions and coal type and with experimental conditions for a given coal type and extent of reaction. The rate of devolatilization mass loss process is dominated initially by heat transfer processes, then coupled mass transfer and chemical kinetics, and finally by chemical processes alone. However, the chemical composition of the initial tars are determined primarily by the chemical characteristics of the parent coal. Chemically controlled gas phase reactions of the initial tars and coupled mass transfer and chemically controlled reactions of heavy tars determine the bulk of the light gas yields. For a HV bituminous coal this conceptual model serves to quantify the Two-Component Hypothesis'' of volatiles evolution. The model postulates that the overall rates of coal devolatilization should vary with coal type insofar as the characteristics of the parent coal determine the potential tar yield and the chemical characteristics of the initial tars. Experimental evidence indicates chemical characteristics and yields of primary'' tars vary significantly with coal type. Consequently, the conceptual model would indicate a shift from transport to chemical dominance of rate processes with variation in coal type. Using the conceptual model, United Technologies Research Center has been able to correlate initial mass loss with a heat transfer index for a wide range of conditions for high tar yielding coals. 33 refs., 30 figs., 6 tabs.