Review of key characteristics of UCG from a multi-scale perspective

Underground Coal Gasification (UCG) is a clean and efficient technology for coal exploitation and utilization with broad application prospects. From a multi-scale perspective, the research systematically explores the "five-multi" characteristics of UCG technology: multiphase, multicomponent, multi-physical field coupled, multiscale, and multi-flow mode, along with their progress. At the molecular (nanoscale) scale, reaction pathways and mechanisms during coal pyrolysis and gasification are revealed through experimental and molecular dynamics simulation methods, clarifying the impacts of gasification agent type, coal rank, and ash content. At the pore (micrometer to centimeter) scale, the effects of high temperature and gasification reactions on coal pore structure and physical property evolution are investigated. During coal heating, the matrix softens, fractures develop, pores interconnect, volatiles and tar release, and strength decreases. At the laboratory (meter-level) scale, physical simulation experiments recreate key phenomena during gasification, such as temperature and pressure field changes, gasification cavity structure, as well as syngas generation and flow, revealing the influence of factors like operating pressure, gasification agent injection method, and coal rank. At the minefield (hundred to kilometer) scale, numerical simulation methods are employed to optimize UCG schemes, revealing the stability of the cavity, rock deformation, characteristic field development, cavity development, and syngas generation and flow patterns. Despite its promise, Underground Coal Gasification (UCG) technology encounters several challenges, including intricate multiphase and multicomponent reaction and flow mechanisms, ambiguous patterns of multi-physical field coupling, and the interplay of multiscale effects and multi-flow modes. To advance this technology, future research endeavors should emphasize enhancing fundamental research across various scales. This includes investigating the production and migration processes of multiphase and multicomponent fluids in extreme conditions, developing methods for cross-scale correlation, refining multi-physical field coupling mechanisms, accounting for multi-flow-mode influences and comprehensively considering the synergistic impact of the full cycle of efficient production during the gasification period and pollution prevention and control during the shut-in period. By addressing these areas, the industrial application and sustainable development of UCG technology can be facilitated.