Pore structure evolution of semi-coke under steam atmosphere in the context of deep underground coal gasification

Underground coal gasification (UCG) is one of the important ways for in-situ clean conversion of deep coal resources. Affected by groundwater influx, steam gasification becomes the dominant reaction for syngas production in the UCG process. This paper investigates the pressurized gas production characteristics of semi-coke under steam atmosphere range from atmospheric to 3 MPa. The gasified residual coke is characterized by N₂ adsorption-desorption, SEM, AFM, and Raman spectroscopy to analyze the evolution of pore structure and functional groups of semi-coke with changes in pressure and carbon conversion rate. The results show that the action of steam leads to the development of abundant microporous structures below 1 nm on the semi-coke surface. Under 3 MPa, the specific surface area of micropores increases from 17.64 m²/g to 435.46 m²/g, which is 4 times that under CO₂ atmosphere. Micropores serve as the main site for the initial reaction of steam gasification. Evolution of semi-coke structure based on functional groups is the result of competition between pressure-dominated chain scission and H radical-dominated polycondensation. The research results provide a theoretical basis for strengthening the hydrogen production process of deep underground coal gasification affected by groundwater.