NMR-based investigation of pore structure evolution in coal during underground gasification

XUE Yi; TENG Teng; WANG Linchao; GAO Feng; LIU Guannan; GAO Yanan

doi:10.13225/j.cnki.jccs.QH25.0179

Abstract：Against the backdrop of the global energy transition and the"dual carbon"goals,underground coal gasification(UCG)is widely regarded as a strategic pathway for the in-situ,efficient conversion and clean utilization of deep,hard-to-mine coal resources.During the UCG process,the coal structure continuously evolves under high temperatures and multi-stage chemical reactions,such as drying,pyrolysis,gasification,and oxidation.These structural changes,particularly in pore morphology,exert a significant influence on gasifying agent flow behavior and syngas production rates.To elucidate the response mechanism of coal pore structures under thermal conditions,the effects of different atmospheres(nitrogen and air)and heating temperatures(200 ℃,300 ℃,and 400 ℃)on porosity,pore size distribution,and pore connectivity were systematically investigated using low-field nuclear magnetic resonance(NMR)technology.The results indicate that thermal treatment promotes pore expansion and improved connectivity,as evidenced by the overall enhancement of trans-verse relaxation time spectra,with medium and large pores exhibiting the most pronounced increases.At the same temper-ature,the effect of thermal treatment in an air atmosphere is more significant than that in a nitrogen atmosphere.Thermal treatment under air notably increases the coal sample's porosity,with the growth rate surpassing that under nitrogen;at 400 ℃,porosity increased by 90.04％in air compared to 65.24％in nitrogen.Moreover,as the temperature rises,the pore structure undergoes marked transformation:the proportion of micropores decreases,while that of mesopores and macro-pores increases.Thermal effects also enhance pore connectivity,thereby improving the permeability of the coal matrix.At 400 ℃ in air,the effective porosity reaches 19.17％,accounting for 75.56％of the total porosity,which is higher than the 15.56％and 70.53％observed under nitrogen,respectively.Furthermore,the fractal dimensions of medium and large pores show a decreasing trend with increasing temperature,indicating that thermal treatment renders the pore structure more reg-ular,with simultaneous improvements in connectivity and permeability.During thermal treatment in an air atmosphere,the coal samples experience multiple stages including drying,gasification,and combustion,resulting in more severe internal structural damage.