Modelling of heat and mass transport phenomena and chemical reaction in underground coal gasification

A two-dimensional axi-symmetric computational fluid dynamic model of an underground coal gasification cavity partially filled with an ash bed has been developed. The model is used to simulate the combined effects of heat and mass transport and chemical reaction during the gasification process. Simulations have revealed that when bottom injection of the oxidant is applied, the flow in the void space above the ash bed is dominated by a single buoyant force due to temperature gradients established by combustion. Optimum oxygen injection rates can be found which maximise the production of chemical energy in the product gas. When the oxidant is injected into the cavity from the top, most of the valuable gasification products are oxidized leading to a product gas with a high temperature and a low calorific value. The simulations eludicate the important transport and reaction processes occurring in the underground cavity and the results are in qualitative agreement with observations from field trials of underground coal gasification.