Spontaneous combustion of coal stockpiles - an unusual chemical reaction engineering problem

Abstract

Spontaneous combustion of stockpiled coal occurs because of reaction between atmospheric gases and coal, and causes pollution, as well as the potential loss of all or part of the stockpile. The stockpile may be viewed as a chemical reactor, and chemical engineering methods may be used to analyse and understand this problem. The analysis is complicated by the fact that coal stockpiles in general possess little or no symmetry. In addition, it has been shown that the flow in coal stockpiles is mainly due to natural convection, which makes the problem rather different to those traditionally analysed in the chemical reaction engineering literature. In order that a realistic model of this phenomenon be formulated, it is necessary to derive mass, momentum and energy balance equations. The various models that have been used to describe this phenomenon are discussed, starting with simple steady-state one-dimensional models, and culminating in a three-dimensional formulation. The simple one-dimensional model gives valuable insights into the behaviour of stockpiled coal. The solution of the more realistic model, using finite element techniques, is described. The models are compared in the light of the assumptions made in their derivation. Finally the practical implications of the work are discussed.