Investigation on the Gas Emission Law of Water-Containing Coal across the Rank Range

Water commonly occurs in coal reservoirs, and it can block the gas flow channels. This has a significant influence on methane transportation within coal. To reveal the gas emission law of water-containing coal across the rank range, three typical coal samples with different coal ranks covering lignite to anthracite were selected in this work. The initial velocity of gas emission (ΔP) under the effect of moisture was measured, and the combination of scanning electron microscopy and mercury injection method was adopted to study the pores and fracture characteristics within coal. Distribution features of oxygen-containing groups in coal were explored by X-ray photoelectron spectroscopy. The microscopic influence mechanism of the water content on ΔP in coal was also comprehensively elucidated. The experimental results show that the moisture content has an obvious inhibitory effect on the ΔP of coal, but the degree of influence on different coal rank samples was different. As the pore space of anthracite (sample XJ) is developed with numerous gas transportation channels, the ΔP has less changes at the lower moisture content (<4.36%). When the moisture content is >4.36%, a large number of water molecules will band together to form water clusters, hindering the gas release, thus greatly reducing the ΔP. However, the change of lignite (sample SL) shows an inverse trend to that of anthracite. Its ΔP is sensitive to the moisture content due to the small number of pores and low porosity. In addition, a great number of oxygen-containing groups in lignite can also provide good surface hydrophilicity for water molecules, and even a small amount of the moisture content (<3.21%) can block most of the pore and facture channels within coal, leading to the remarkable decrease in ΔP. For bituminous coal (sample ML), the distribution of pores and oxygen-containing groups is the most uniform, and the ΔP decreases linearly with the increase in the moisture content.