Occurrence of water within different rank coals: a review

Abstract

ABSTRACT Water (H2O) within coal seams is capable of competing the occurrence space with methane (CH4), thereby, weakening its adsorption, diffusion, and flow in coal reservoirs. H2O coexisted in coals will reduce CH4 adsorption capacity and diffusion coefficient by up to 62.37% and 80%, respectively. Based on existing literature and data, this paper mainly reviews the occurrence of H2O within various rank coals and bridges the current research gaps. Results show that H2O content in coals varies between 0.5% and 25%, showing an L-shaped decreasing trend with coal rank. This trend is associated with the variation of oxygen-containing functional groups, pore specific surface area and pore volume, and clay minerals of coals. The modified Brunauer-Emmet-Teller (BET) model is applicable to describe the adsorption equilibrium of H2O on coals with multiple correlation coefficient (R 2) above 0.9997. The separation of water vapor adsorption and desorption hysteresis loop on coals into low relative pressure (P/P S) below 0.3 and high relative pressure above 0.3 can effectively distinguish the influence of coal matrix swelling effect, ink-bottle shaped pore effect and capillary condensation on H2O occurrence. Considering that traditional methods cannot well determine H2O in the closed pores, SANS and SAXS are predictive to estimate the occurrence space of H2O within coals. Moreover, the low-temperature plasma ashing can remove the organic matter from coals and maintain mineral structure, which is expected to determine the content distribution and spatial distribution characteristics of H2O in inorganic minerals and organic matters of coals. Finally, the accurate measurement of CH4 adsorption and desorption on moist coals via volumetric method should be focused on. Specifically, the terahertz waves with humidity monitor sensitivity being 62 ppm are recommended to detect H2O content in bulk phase for CH4 adsorption and desorption system. Thus, incorporating terahertz waves into volumetric method is helpful to generate reliable data regarding CH4 adsorption and desorption on moist coals.