Underground coal gasification field tests in China: History and prospects

To achieve peak carbon dioxide emissions and carbon neutrality in China, a low-carbon and efficient coal energy structure is essential for the sustainable energy development of China. Underground coal gasification (UCG) is an important strategy for transforming the coal energy structure in China. Based on the review of previous literature, this study analyzes the exploration history of the UCG field tests in China and delivers the prospects of UCG industrialization in China. The analytical results show that the development and field tests of UCG technologies in China have experienced five stages, namely the initial attempt, silence/recovery, extensive explorations, making technological breakthroughs, and tests and promotion, laying a solid scientific and technological foundation for a new round of explorations into the UCG industrialization in China. Three achievements have contributed substantially to the development of UCG technology in China as milestones: (1) the success of the first UCG pilot test of mines in Mazhuang, Xuzhou; (2) the invention of the long channel - large cross-section-two stage (LLTS) UCG process; and (3) the success of the UCG field test in wells in Ulanqab, Inner Mongolia. Among them, the LLTS-UCG process is the only internationally recognized innovative achievement in the field of UCG in China. This process breaks through the international limitation on the length of gas flow channels of the time, thus increasing the amount of coal to be gasified by a single furnace and greatly improving the gas production capacity of a single gasifier. Moreover, this process yields high-quality synthetic gas by increasing the single-gasifier daily gas production using large cross-sections and increasing the coal-to-gas conversion rate through two-stage gas injection. The LLTS-UCG process has opened commercial UCG production in China. With simple control methods of the process, the basic philosophy and processes of the process can be used as a reference for the development of UCG technologies for middle-deep coal seams. This study delivers the prospects of UCG industrialization from the aspects of national demand, technological maturity, resource potential, and safeguard measures. Accordingly, some UCG development priorities and suggestions for constructing an industrial policy system are given. The UCG industrialization in China will be concentrated in shallow coal seams in the short term, it is necessary to further explore the regulating mechanism of the UCG process with low damage to the environment and a high coal-to-gas conversion rate and to establish a UCG technology system for complex geological conditions, such as tectonic coal seams and largely thin coal seams. In the long run, it is expected to achieve the UCG industrialization of middle-deep coal seams, for which the key is to develop the collaborative and efficient co-production technology of UCG – CBM (coal bed methane)-carbon capture, utilization, and storage (CCUS) under multi-field coupling conditions. Moreover, it is recommended that the government should formulate a feasible UCG industry policy system, aiming to provide incentive mechanisms and regulatory guarantees for promoting UCG industrialization. The results of this study help boost the confidence to explore UCG industrialization in China. Furthermore, and provide some references for further innovating UCG technologies suitable for the geological conditions of China.