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Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore–fissure structure analysis, mechanical property test, and permeability experiments under different stress loading–unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore–fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading–unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading–unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

To explore the occurrence and distribution characteristics of fine-grained pyrites in coal and the effect of pyrite particle size on flotation efficiency, coal samples from Guizhou province and Shanxi province, China, were selected for pyrite morphology observation and sulfur content test before and after flotation desulfurization experiments with different coal particle sizes. Experimental results showed that the fine-grained fine pyrites in coal have various occurrence forms and complex connections with the coal matrix. The fragmentation process can change the distribution of pyrite content in coal. Flotation desulfurization experiments showed that the sum of pyrite content in the cleaned coal and middlings gradually became significantly higher in coal particles with size 15–37 μm compared with particle sizes 37–44 and 44–75 μm. The complex occurrence morphology and crystal structure of fine-grained pyrite make it difficult to be removed from the coal matrix by ore grinding during flotation. Fine-grained pyrite mainly occurs in the form of framboïdal pyrite, disseminated pyrite, and monomer pyrite with a size of 0.69–33.94 μm in the middlings and cleaned coal. Therefore, 37 μm is considered as the critical dimension for ore grinding to improve the effective flotation desulphurization efficiency in this study, and some more effective methods should be used to increase the desulfurization efficiency of fine-grained pyrite.

As one of the crucial factors contributing to coal spontaneous combustion, the oxidation of pyrite is a complex process involving multiple reactions, particularly in the presence of oxidants (Fe3+ and O2) and bacteria. However, experimental results based on mineral-pyrite are not entirely applicable to coal-pyrite due to their differences in formation environments and compositions. This study selected two types of coal-pyrite and one type of mineral-pyrite as research to conduct oxidation experiments with the participation of oxidant (Fe3+) and bacteria (Acidithiobacillus ferrooxidans), respectively, to obtain the following conclusions. Under natural conditions, the chemical oxidation rate of pyrite is slow, but the addition of oxidant Fe3+ and bacteria can significantly accelerate the oxidation rate. The promotion effect of oxidant Fe3+ on the oxidation reaction is stronger than that of bacteria. Under the same conditions, the oxidation rate of coal-pyrite samples is slightly higher than that of mineral-pyrite, due to the relatively higher impurities content, poorer crystal structure, and humic acid in the coal seams. Additionally, different compositions of coal-pyrite samples can lead to various oxidation degrees under different conditions. Therefore, the oxidation process and mechanism of pyrite in coal seams are complex and affected by many factors, which need further study to prevent coal spontaneous combustion accurately and effectively.

CO2-ECBM is a method of enhanced coalbed methane extraction followed by cutting greenhouse gas emissions and new energy development. In order to reveal the characteristics of gas flow in porous media and the pore structure response characteristics of coal rocks, the experiments were carried out to simulate the process of CO2 displacement of N2 at a buried depth of 900 m, including monitoring the changes in gas permeability and strain of coal samples along with a comparison of the pore structure of low-temperature liquid nitrogen adsorption on coal samples both before and after displacement were both done. The findings of the experiment are listed below. The N2 permeability of the LiuZhuang sample ranges from 0.0008mD to 0.0014mD, whereas the permeability of QiDong is around 0.0003mD. With an increase in gas injection duration and an expansion of the coal matrix for N2 adsorption, the permeability steadily decreases. The efficient stress compression of the coal pore fracture structure during sample preparation and testing avoids the visible fracture region, which results in poor permeability. The displacement stages of CO2 can be divided into three phases. Free nitrogen flows from the end of the position and the permeability diminishes during the phase of free nitrogen. When CO2 is introduced into the penetration stage, the permeability tends to rise, however when there is no penetration, the permeability test values are frequently low. During the CO2 steady displacement phase, gas permeability gradually declines. Axial and radial strains are progressively raised during the initial stage of the CO2 injection whereas they are gradually reduced during the initial stage of the N2 injection. While CO2 is continuously supplied through the coal body stage, there are modest axial and radial strain changes. The axial and radial stresses are stabilized by the CO2 displacement. The overall pore volume of the coal significantly rises following the displacement. The increase part of the pore volume is primarily focused on the pore of absorption and filling (aperture < 10nm), whereas the decreased part is mainly concentrated in the diffusion pore of the Fick type and the permeability part (aperture > 50nm). The increased in pore volume ratio surface area is centered mostly in the fill pore region (aperture 10 nm) and is four times greater than it was before the displacement. The CO2 injection exerts an expansion impact on the adsorptionfilled and diffusion pores during the CO2-ECBM process, whereas the compression effect on the percolation pores results in a reduction in permeability.