ROLE OF MOISTURE IN ADSORPTION AND DESORPTION OF GAS FROM COAL

The role of inherent moisture in coals of various ranks on sorption phenomena have been investigated using high and low pressure gas and water adsorption isotherms and surface area experiments. Inherent moisture is the main control on the adsorption capacity and contrary to earlier studies (including our own) is found to be the dominant control and mask the effects of decreasing pore size distribution that occurs with increasing rank or with maceral composition. Low rank coals, with inherent moisture contents reduced to that equivalent of higher rank coals, have consistently higher adsorptive capacities than the corresponding higher rank coals. Experimental results also show that the declining ratio of adsorptive capacities of carbon dioxide to methane found with increasing rank (i.e. normally about 15:1 in sub bituminous coals to less than 2:1 in low volatile and higher rank coals) is largely non existent if the inherent moisture content is markedly reduced. Our analyses and computations show that neither solution of gas in water, competition of adsorption sites by water nor blockage of surface area by water completely accounts for the dramatic increase in sorption capacity to gas with removal of inherent water nor the impact of inherent moisture on selective adsorption by various gases. Solution gas in inherent moisture is not normally considered but adsorption isotherms collected at equilibrium moisture content include solution gas although differentiation of solution from adsorbed gas is not possible during routine experiments.

For coals that have no free water, such as much of the producing fairway of the Horse Shoe Canyon coals in western Canada, moisture contents less than equilibrium moisture may exist which will have a dramatic effect on the adsorptive and differential adsorption capacity and hence carbon dioxide sequestration potential.