Adsorption Characteristics and Mechanisms of Coal–Microorganisms in the Process of Biogenic Methane Production from Highly Volatile Bituminous Coal

To investigate the adsorption characteristics and mechanisms of coal–microorganisms in the process of biogenic methane production from coal, simulation experiments of biogas production were conducted under suitable environmental conditions (initial pH = 7 and constant temperature at 35 °C) using highly volatile bituminous coal from China’s Yima coalfield. The microbial adsorption characteristics, organic liquid products, coal wettability, and adsorption heat were measured using scanning election microscopy, gas chromatography–mass spectrometry, contact angle measurement, and micro-calorimetry, respectively, to reveal the adsorption characteristics of the coal and its interaction mechanisms with microorganisms. The results show the following: (1) The microbial adsorption capacity changed as fermentation time varied and the maximum values of optical density (at 600 nm) and RNA were 0.122 and 1772.73 μg, respectively, and occurred at 9 d. (2) The contact angle of the coal surface was <90° during the experiments, and the coal wettability reached its maximum on day 9 of heat production. (3) There was correspondence between the exothermic process and production of intermediate liquid products, and the content of small organic molecules was more and the heat output was higher. (4) The generation of gas products differed remarkably as adsorption capacity and adsorption heat during the reactions varied. The maximum methane production and adsorption heat values were, respectively, 1.849 mL/g (at 19 d) and 306.031 J/g (at 9 d). The adsorption heat showed a variable trend (first decreasing, then increasing, decreasing again, and finally increasing), but all phases resulted from an exothermic reaction. This research provides a reference for understanding the biogas production pathway, the degradation mechanism, and the improvement of biogas-producing efficiency as microorganisms degrade coal, and enhances knowledge about the potential concentrations of target products. The study further enriches the theory of biogenic gas production from coal.