Abstract
With the increasing depth of coalbed methane (CBM) exploitation, temperature becomes the main factor affecting the efficiency of CBM exploitation. The change of temperature has significant influence on the mineral composition and pore structure of coal. To study the effects of thermal treatment on mineral composition and pore structure of coal, X-ray diffraction (XRD) test, scanning electron microscopy (SEM) test, and mercury intrusion test were carried out for three groups of coal. The mineral composition and pore structure of coal specimens after thermal treatment (25, 50, 75, and 100°C) were analyzed. The results show that the main mineral compositions of three groups of coal specimens after different temperatures are basically unchanged, and the maximum diffracted intensity after different temperature treatments decreases first and then increases with the increasing temperature. The count of fissures decreases first and then increases with temperature, and the count of pores increases first and then decreases with the increasing temperature. The velocity of mercury injection in high pressure (100~400 MPa) of coal specimens increases first and then decreases with temperature. The porosity, pore area, median pore diameter, and average pore diameter increase with the increasing temperatures. The volume of microfracture decreases, then increases, and finally decreases. The volume of macropore and mesopore increases slowly, and that of transition pore decreases slowly with the increasing temperature. Meanwhile, the volume of micropore increases first and then decreases during the process of thermal treatment. The fractal dimension of pore size ranges from 2.6 to 2.9 and increases linearly with the increasing temperature.
Highlights
With the depletion of shallow coal resources, more and more coal mines are entering the deep mining stages
The coal lumps after thermal treatments were prepared for the X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion tests to analyze the variation of mineral composition, microstructure, and pore structure
Specimens is basically unchanged; namely, no chemical changes occur after the thermal treatment
Summary
With the depletion of shallow coal resources, more and more coal mines are entering the deep mining stages. It is urgent to study the evolution of mineral composition and microstructure of coal after different temperature treatments. To analyze the changes of physical and mechanical properties from a microscopic point of view, the X-ray diffraction (XRD) tests, mercury intrusion tests, and scanning electron microscopy (SEM) tests were carried out for three groups of coal after thermal treatment at different temperatures (25, 50, 75, and 100°C). The mineral composition and microstructure of coal specimens after thermal treatment were analyzed, and velocity of mercury injection, pore parameters, and distribution of pore size for three groups of coal were investigated based on the mercury intrusion tests. The result is significance for CBM exploitation and gas extraction
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