Effect of thermal steam on the physicochemical structure and methane adsorption/desorption characteristics of lignite and anthracite

Thermal steam technology for coalbed methane extraction is an intensive research focus. This paper aims to explore the mechanism of thermal steam enhanced coal seam permeability. Computed tomography, Fourier transform infrared spectroscopy, and high-pressure methane adsorption/desorption were selected for characterization to explore the effects of steam at 100 and 200 °C on the physicochemical structure and methane adsorption/desorption properties of lignite and anthracite. The results show that the total porosity of lignite and anthracite increased with temperature by 1.79% and 1.38% at 200 °C, respectively. Meanwhile, the 3 D fractal dimension and absolute permeability of lignite and anthracite reach maximum values of 2.25 and 2.19, 75.05 mD, and 31.04 mD at 200 °C, respectively. Moreover, the equivalent diameter distribution of pore throats shifts toward values greater than 300 μm due to the generation of well-connected pores and fractures after thermal steam treatment. It can reduce the hydroxyl groups and the sulfur content in coal, lower the average chain length, decrease coal aliphaticity, increase aromaticity, and thus lead to an increase in coal rank. As the temperature increases, the methane adsorption and desorption characteristic parameters a and 1n values of lignite and anthracite increase. The adsorption characteristic parameters of lignite are smaller than of anthracite, indicating that lignite has weaker methane adsorption than anthracite. This study provides a theoretical basis for the application of thermal steam to improve the extraction efficiency of coalbed methane.