Kinetics of coal swelling in gases: Influence of gas pressure, gas type and coal type

• Different gases swell coal at different rates.
• For a given gas different coals swell at very different rates.
• With increasing pressure slowly-swelling coals swell faster.
• With increasing pressure rapidly-swelling coals swell more slowly.

The increasing interest in enhanced coal bed methane (ECBM) production and reporting of greenhouse gas emissions from mines has spotlighted gas diffusion and gas-induced swelling in coals. Although it is generally agreed that carbon dioxide (CO2) diffuses faster than methane (CH4) in coal and that diffusion rates decline as the pressure is increased, there is no general agreement on the physical mechanism responsible. In this paper kinetic data for sorption-induced swelling for five Australian bituminous coals have been analysed. Gases investigated included CO2 and CH4, Xe and ethane. It was found that swelling rates are influenced by gas type. CH4 swelled coal more slowly than CO2 at the same pressures. Other swelling kinetic data showed that, of the gases investigated, xenon was the slowest, followed by ethane. CO2 swelling was found to be the fastest overall. The swelling rate was found to depend on maceral composition, with the slower-swelling coals being vitrinite-rich. However, the swelling rate of different coals varied differently with increasing pressure: the slowest swelling coal showed an increase in swelling rates with an increase in system pressure, whereas the fastest swelling coals showed a decrease in the swelling rates at high pressures.