Gas formation mechanism during deep coal underground gasification and influencing factors

Deep underground coal gasification (UCG), as a highly promising in-situ conversion technology, provides an important approach for the low-carbon transformation of energy by converting underground coal into hydrogen-rich gases such as H₂, CH₄, etc. With the annual increase in the depth of coal mining in our country, the efficient development of deep coal resources has become the key to ensuring energy security. However, deep UCG is confronted with challenges such as high gasification pressure, high in-situ stress and complex hydrogeological conditions, and it is urgent to conduct in-depth exploration of its gas production mechanism and stability control factors. This paper comprehensively analyzes the gas production mechanism and influencing factors of deep UCG from three aspects: the underground gasification gas production reaction of deep coal, the influencing factors of stable gas production in gasification, and the development, evolution and constraint mechanism of the combustion cavity. It also looks forward to the future research directions. Research shows that the gas production process of deep UCG is characterized by non-steady-state, non-uniformity and reverse heat and mass transfer. The gas production reaction is jointly driven by the thermal spalling of coal, the pyrolysis of coal, the reduction of semi-coke and homogeneous reactions. High-pressure conditions promote the formation of CH₄ in the product while inhibiting the production of H₂. The stable gas production process is mainly influenced by the properties of coal, the characteristics of coal seam occurrence, gasification agents, the evolution of combustion cavities and groundwater. The formation of combustion cavities and the intrusion of groundwater are important factors affecting deep UCG. The moisture generated by dry distillation and oxidation reactions during the gasification process, as well as the groundwater seeping in, can alter the structural characteristics of coal, affecting the continuous and stable operation of underground gasifiers. The dynamic evolution of the combustion cavity space leads to complex and variable heat transfer, mass transfer and momentum transfer within the cavity, and deteriorates the stability of gas production. The expansion of the combustion cavity is significantly influenced by the combustion rate of the coal body, the flow rate of oxygen, and the spillage of the coal body. The retreat of the injection point of the gasification agent can constrain the evolution process of the combustion cavity. The coupling effect of multiple physical fields significantly affects the kinetics of the gasification reaction, the characteristics of cavity expansion, and ultimately determines the composition and output of the syngas. In the future, in view of the characteristics of high pressure and high ground stress in deep areas, it is still necessary to systematically study the complex dynamic characteristics of typical multiphase reactions under the influence of high reactive gas pressure in deep areas, deeply reveal the influencing factors and control mechanisms of stable gas production in deep coal gasification, so as to improve the basic theory of underground coal gasification in deep areas and provide theoretical guidance for the operation and stable gas production of deep UCG projects.