Research review of underground coal gasification: principles, challenges and prospects

• Comprehensive review of UCG principles, challenges, and global trials. • Deep coal UCG offers sustainable energy and CO 2 storage potential. • Novel metal extraction via chemical leaching-gasification-surface recovery. • UCG enables hydrogen production and heavy oil recovery synergy. • Identifies barriers and solutions for UCG industrialization success. As a primary fossil fuel, coal combustion generates escalating CO 2 emissions, intensifying the global climate change crisis. Thus, curbing CO 2 emissions from coal combustion is a pressing imperative for climate mitigation. Underground coal gasification (UCG), a clean, efficient, and low-carbon technology for coal resource exploitation, enables in-situ conversion of coal to syngas. This approach holds substantial value for CO 2 sequestration, resource efficiency enhancement, and low-carbon energy advancement. This review systematically examines the fundamental principles and mechanisms of underground coal gasification. It emphasizes critical theoretical elements, including gasification processes and thermodynamic reaction mechanisms under complex geological settings, coal seam cavity expansion and evolution patterns, and surrounding rock thermal deformation mechanisms. Additionally, it traces UCG’s developmental trajectory and categorizes it into two primary methods: shaft-based and borehole-based gasification. Building upon this foundation, it evaluates UCG’s core technical hurdles, such as reaction control and process stability, environmental impact management, and geological-resource limitations affecting economic feasibility and sustainability. An analysis of prominent domestic and international UCG experimental initiatives reveals that, despite notable trial and pilot successes, UCG industrialization persists as a major obstacle, particularly concerning medium-deep coal gasification, operational stability, environmental safeguards, and cost management. Finally, this review explores UCG’s expansive potential in medium-deep coal resource exploitation, strategic metal recovery, hydrogen generation, and CO 2 storage. Nevertheless, realizing these opportunities necessitates advanced gasification process optimization, monitoring technology refinements, and bolstered environmental risk evaluation. In summary, UCG furnishes a pivotal route toward low-carbon energy production, optimized coal resource use, and worldwide energy shifts, poised to contribute meaningfully to carbon abatement and energy resilience.