Impact of coal ranks on dynamic gas flow: An experimental investigation

To investigate the dynamic process of adsorbing-gas flow in different rank coals, the CO2 flow rate and corresponding CO2 permeability were measured at decreasing pressures under a constant 5 MPa confining stress condition using 46 coal samples from Chinese coal-bearing basins (Qinshui, Huoxi and Ordos) and a coal-bearing zone (Sanjiang-Mulinghe). Two parameters (η: change ratio of permeability and, △S: total loss of flow rate) were proposed to quantitatively evaluate the dynamic process of gas flow. Furthermore, the impact of coal ranks on the dynamic gas flow was observed. Results show that: (1) dynamic gas flow occurs in coals and associates with the coal rank; (2) △S is positively related to η and decreases with increasing coal rank from high volatile bituminous to semi-anthracite coal but increases in anthracite coal; (3) in different rank coals, η and △S are comprehensively affected by the maceral composition and initial permeability of the coal, of which the maceral composition primarily affects the η and △S changes in bituminous and semi-anthracite coals. The initial permeability dominates η and △S changes as coal rank varies from semi-anthracite to anthracite coal. For the same rank, coals (especially some bituminous and anthracite coals) with lower and higher initial permeabilities have smaller η and △S values.