Study on porosity and permeability changes in coal freezing by liquid nitrogen based on the chemical structure

Abstract

The porosity and permeability of coal can be effectively increased by freezing and cracking with liquid nitrogen. In order to study the changing characteristics of coal's functional groups and coal's chemical structure on porosity and permeability and its microscopic change mechanism, industrial analysis, elemental analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman) and carbon-13 nuclear magnetic resonance spectroscopy (¹³C-NMR) were used to analyze the functional groups and chemical structure of coal, and CT scanning and numerical simulation software were used to study the changes in pore cracks before and after freezing the coal body and to conduct quantitative analysis. Results show that with an increase in coal metamorphism, the chemical structure parameters of coal, such as the content of aromatic carbon, order degree, and bridge carbon ratio, increase. The change rate of the porosity and permeability of low-rank coal is higher than that of high-rank coal. The higher the aromatic carbon content of the coal sample, the smaller the porosity change rate and permeability change rate after liquid nitrogen freezing. The higher the oxidation degree (oxygen-grafted carbon, oxygen-grafted aromatic carbon, and carbonyl–carboxyl carbon) of the coal sample, the larger the rate of change in the porosity and permeability after liquid nitrogen freezing. The results of this study establish the relationship between macroscopic (porosity and permeability) and microscopic (chemical structure and functional groups) effects and provide ideas and references for further improving the permeability of coal freezing using liquid nitrogen.