Role of thermodynamic effect on coal-gas interactions during underground pre- and post- mining coal seams in the environmental geology

Abstract

U nderground coal mining generally occurs in the terrifically thermodynamic environment of high geostress and geotherm, resulting in many coal-gas disasters, such as coal-gas outburst, coal spontaneous combustion and even gas explosion etc. (1-3). Geological statistics and relative researches show that when mining depth is over 1000 m, the geotherm will reach the range of 40 and 45°C with the commonly geothermal gradient of 30-50°C/km, and the geostress is 95-135 MPa at 3500-5000 m depth (4). In addition, the geotherm has a significant impact on geomechanics of coal-rock mass: the variation of 1°C within the coal-rock medium may trigger the insitu stress change of 0.4-0.5 MPa (5).Coal-gas disasters pose a difficult, persistent and costly problem for coal industries worldwide, usually causing huge economic losses, personal casualties, perilous land subsidence and massive environmental contamination (6-10). The fundamental understanding of gas migration and coal self-heating is essential to eliminate their dangers as mining hazards or develop the potential as an unconventional gas resource recovered (11)(12)(13)(14). The evolution of coal-gas interaction processes is a chain of physicochemical reactions in underground pre-and post-mining coal seams, which is labeled as "coupled processes" implying that one reaction process affects the initiation and progress of another (15)(16)(17). This reaction chain is linked together through dominant mechanisms, including compositional gas flow and diffusion, reaction kinetics, energy transport and coal deformation (1,2,15). The individual reaction process, in the absence of full consideration of cross couplings, forms the basis of well-known disciplines such as hydrology, chemistry, elasticity and heat transfer (1,15).Advances in our understanding of coal-gas interactions have provided some effective measures to retard or suppress underground coal-gas disasters and enhance potential resource utilization quality. Some typically mathematical models had been established to reveal coupled hydro-mechanical mechanism of coal seam gas flow, such as Palmer-Mansoori model (18), Shi-Durucan model (19), Zhang-Liu model (17) and Xia-Zhou model (20,21) etc., but failed to consider the influence of thermal effect on coal-gas interactions. However, in real coal mining environment, the thermodynamic effect may not only have a significant impact on the micro-and macro-structure evolution of coal-rock mass, but on gas ad/desorption, diffusion and migration behaviors. Thus, the thermodynamic effect should not be ignored in addressing the coal-gas interactions, which has become a kind of common recognition among many contemporary experts and scholars. Levy J et al. carried out an adsorption experiment of some coal sample, which showed that when the gas pressure of the coal sample was 5 MPa and the temperature was the range of 20 and 65°C, the adsorbed quantity of coal gas would be reduced by 0.12 m 3 /t as the temperature increased by 1°C (22). Yin GZ, et al. experimentally studied the influence...