UCG pilot tests in the United States and their contributions to modern UCG technologies

The industrialization of underground coal gasification (UCG), which is conducive to the transformation and development of the coal energy structure and the improvement in the low-carbon utilization level of coal resources, is an environmentally friendly energy development technology in line with the sustainable development strategy. The international energy crisis from the 1980s to the 1990s prompted the United States to vigorously develop UCG technologies. Accordingly, this country has successively carried out six series of UCG pilot tests, which contribute greatly to the progress in UCG technologies and provided the critical foundation for the formation of modern UCG technologies worldwide. This study systematically collects and summarizes over 40 years of UCG pilot test results in the United States and sorts the development paths of key technologies, with the purpose of providing inspiration and a reference for the ongoing industrialization process of UCG technologies in China. The results show that the United States has determined the coal seams and coal quality suitable for the applications of UCG technologies through UCG pilot tests and technical exploration. Moreover, this country has innovatively developed drilling-type UCG and relevant processes. Most especially, it has invented the controlled retractable injection point (CRIP) and parallel CRIP (P-CRIP) technologies, providing solutions to three technical challenges: gasification channel construction, gasification process control, and synthesis gas quality improvement. The United States has proposed the technical philosophy of the mutual fusion of adjacent cavities, which lays the feasible foundation for expanding the scale of UCG gasification from a single gasifier. The systematic collaborative R&D (research and development) schemes of science and technologies organized and implemented at the governmental level are the key to the longstanding innovation of UCG technologies in the United States. Nevertheless, there are still many technical challenges to be further solved in the UCG test series in the United States, such as the geological sealing performance of UCG gasifiers and its dynamic monitoring and evaluation, the dynamic monitoring and stability of gasification channels, technologies for guaranteeing the full gasification of coal seams, technologies for the safe and effective expansion of the gasifier width, the environmental monitoring and control of UCG gasifiers, and UCG technologies closely combined with carbon emission reduction. These challenges provide the focus for the development of UCG technologies in China.