Typical Pore Characteristics of Nontectonic and Tectonic Low-Rank Lignite in Mining Areas and Their Influence on Gas Desorption

Abstract

Abstract Pore characteristics determine the control mechanism of gas adsorption and desorption in the coal bodies. The fundamental investigation into microscopic pore structures and adsorption–desorption characteristics of long-flame coal represents necessary support for the safe mining of low-rank coal and the exploration and development of low-rank coalbed methane. The research aims to fully reflect the differences in pore and gas desorption characteristics between nontectonic and tectonic low-rank long-flame coal. To this end, experiments on pore characteristics and pore size distribution of nontectonic and tectonic low-rank long-flame coal were conducted, as well as comparative cross-experiments on gas desorption laws under different adsorption equilibrium pressures, different particle sizes, and diverse types of damage. The trend of mercury increment changes in coal bodies with various degrees of structure was analyzed; characteristic parameters of coal bodies including pore size, pore volume, and specific surface area (SSA) were explored; and differences in the gas desorption speed and amount of coal bodies with varying degrees of structure were compared. Additionally, the main factors influencing the gas desorption in the main pores of the coal body were quantitatively analyzed. The results show that compared with nontectonic coal, the porosity of tectonic coal decreases and the SSA increases. The pore diameters are mainly large pores and visible pores, which are over half of the pore volume, followed by micropores and small pores, exceeding one-third of the pore volume; the increment of mercury injection and pore volume shows six special turning points, and the overall trend in each interval intuitively presents a W-shaped change with three peaks; the cumulative gas desorption amount and initial gas desorption amount of nontectonic and tectonic low-rank caking coal decrease gradually as the adsorption equilibrium pressure drops and the coal sample particle size enlarges. The difference factor of the cumulative gas desorption amount of nontectonic and tectonic low-rank caking coal affected by the adsorption equilibrium pressure ranges from 2.44 to 2.93, with an average of 2.69, and the difference factor of the initial gas desorption amount ranges from 2.50 to 2.73, with an average of 2.60; the difference factors of the cumulative gas desorption amount of nontectonic and tectonic low-rank caking coal affected by the coal sample particle size ranges from 1.24 to 1.59, with an average of 1.41, and the difference factors of the initial gas desorption amount affected by the coal sample particle size ranges from 1.40 to 1.74, with an average of 1.56. The difference in the degree of gas desorption influencing factors of nontectonic and tectonic low-rank caking coal is that the adsorption equilibrium pressure is greater than the coal sample particle size. Research findings provide important technical support and theoretical foundation for the rational selection of gas extraction and prevention and control measures during the mining of low-grade long-flame coal in nontectonic and tectonic areas, in terms of proper division of the mining areas.