Recession of a coal face exposed to a high temperature

Abstract

A one-dimensional, transient model is developed to describe drying, pyrolysis, endothermic gasification and spalling(thermomechanical failure) of a wet coal face exposed to a high temperature. Such a situation occurs, for example, at the roof of an underground coal gasification (UCG) cavity. Emphasis is placed on thermochemistry, and rock mechanics are simplified by use of two parameters, a failure length and temperature, which measure the strength of the coal. Drying of the coal and convection of the evaporated water are modeled as a Stefan problem, and reaction of this water vapor with carbon at the free surface is described by an asymptotic solution in the limit of a large activation energy for this reaction. Thus, evaporation and gasification effects become analytic boundary conditions on the numerical solution to the transient heat penetration in the dry coal. Pyrolysis is treated numerically as the release of a single component in the dry coal according to one-step Arrhenius kinetics. Both surface gasification and spalling are shown to be of importance for typical UCG conditions, and their relative interaction can provide an explanation for UCG field test observations. The model, in particular the perturbation solution developed for the surface recession rate due to gasification, has applications in pyrolyzing ablative and related systems.