Modeling the devolatilization behavior of high volatile bituminous coals

Abstract

A model for the devolatilization of high volatile bituminous coals developed by analogy with a single-stage, equilibrium flash distillation is evaluated against measured rates and yields of tar and noncondensible gases, and the size characteristics of tar. As seen in the comparison with data, the model accurately correlates transient weight loss for heating rates from 440 to 3300 K/s for temperatures to 1300 K; the ultimate distribution of the volatile products for pressures to 0.5 MPa; and the molecular weight distributions of tars prepared in atmospheric pyrolysis at temperatures to 1200 K. The tendency to form lighter tar at elevated pressures is also explained. The theory predicts that yields are insensitive to heating rate at moderate temperatures, in accord with wire-grid experiments, but conditions where tar yields are substantially enhanced by more rapid heating are also identified. Hence, the influences of all of the important operating conditions on the devolatilization of high volatile bituminous coals can be understood in terms of only three mechanisms: (1) the disintegration of the coal macromolecule into primary fragments which are widely distributed in size: (2) the partitioning of the primary fragments into volatile and condensed species according to a phase equilibrium for continuous mixtures; and (3) competitive char formation in the condensed phase with the simultaneous evolution of noncondensible gases.