Multiple-Experimental Investigation on the Physicochemical Structures Alternation During Coal Biogasification

The aim of microbially-enhanced coalbed methane (MECBM) generation is to replicate the natural process of microbial methane production in coal seams and to increase coalbed methane production. However, a systematic evaluation of the response of coal structure during biogasification is still lacking. Here, alternations of the pore and molecular structures were investigated during the bioconversion of coal to methane using proximate/ultimate analysis, scanning electron microscope, low-field nuclear magnetic resonance (LNMR), low-pressure nitrogen adsorption (LPNA), micro-Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. The results showed that MECBM is a process of carbon enrichment, nitrogen/sulfur fixation and dehydrogenation/deoxygenation. Moreover, microbial modify coal pore structure by destroying aliphatic side chains, dissociating small molecular clusters and reducing the degree of aromaticity. These changes in the coal pore structure result in the "loose" of the coal reservoir spatial structure. We found that micropores (<10 nm) became transition pores (10-100 nm) and mesopores (100-1000 nm) after microbial degradation, which then led to the increase of the average pore width and the reduction of the specific surface area. This process benefits the desorption and transportation of coalbed methane. In addition, the surface fractal dimension decreased, and the volume fractal dimension increased according to the fitting date of LPNA. The fractal dimension of transition pores and mesopores increased of most coal after biotreatment based on NMR. These results led us to draw the hypothesis that the organic compounds formed by biodegradation may adhere to the pores, thus blocking the pores, making the structure of transition pores and mesopores complex, as demonstrated by the SEM images. This phenomenon is thought to be improved by continuous degradation or water movement.