Water adsorption characteristic and its impact on pore structure and methane adsorption of various rank coals

Coalbed methane not only is a new clean energy source, but also has potential damage to ecological environment. Water and methane coexist in coal reservoir; understanding the adsorption of water on coal and its impact on pore structure and methane adsorption of coal is vital to evaluate the reserves and productivity of coalbed methane. In the paper, water adsorption characteristics of various rank coals are firstly investigated by ten mathematical models. The modified Dent model provides a best fit, followed by GAB and Dent models. For GAB model, the primary site adsorption is more difficult to reach saturation, and the contribution rate of the secondary site adsorption is surprisingly high at P/P₀ approaching 0, which can be attributed to the possible overestimation of GAB monolayer adsorption capacity and secondary site adsorption. Besides, the low-rank coal sample YZG2 exhibits more prominent hysteresis than middle- to high-rank coals. The low-pressure hysteresis can be attributed to the water-water interactions over the primary site and the strengthened binding forces of water molecules in the water desorption process. In contrast, the high-pressure hysteresis largely depends on pore structure of coal such as ink-bottle pores, especially for the studied sample YZG2. Besides, pore analyses by low-temperature nitrogen adsorption method show that the pre-adsorbed water has remarkable influence on micropores smaller than 10 nm, and the micropores smaller than 4 nm almost disappear for water-equilibrated coals, which is closely related to the formed water clusters and capillary water in pore throats. This finding reveals that more methane gas can only be adsorbed in the larger pores of moist coal, and provides an explanation for water weakening methane adsorption capacity.