Dispersive Diffusion of Gas in Coal: An Experimental Investigation into the Fundamentals of Gas Sorption and Diffusion in Coal

Abstract

Gas diffusion in coals plays an important role in enhanced coalbed methane production, but a consistent picture of diffusion behaviour in coals is yet to emerge. There is no agreement on the most appropriate model, pressure dependence of diffusion coefficients, or the physical explanation to why CH4 diffusion is slower than CCh. The aim of the work towards this Thesis is to determine sorption kinetics more accurately, model them more systematically over a wider range of experimental conditions and resolve discrepancies present in the literature to explain (i) why uptake of gas in coal takes a considerable amount of time and (ii) why C02 sorbs faster than CH4. A manometric sorption system was constructed that was optimized for uptake rate measurements. This was used to study the effects of coal-type, particle size, gas-type and mixture, and temperature on sorption kinetics. A gravimetric apparatus was also utilised to measure isotherms for different gases at different fractions of the same coaL Further work combined manometric uptake rate measurements with Small Angle X-Ray Scattering (SAXS) along with an analysis of previously obtained sorption-induced coal swelling kinetic data.