Efficient hydrogen production strategy for full-process underground coal gasification: Synergistic effect of microwave modification and mobile gas injection

Underground coal gasification (UCG) technology converts coal into secondary fuel (high calorific value syngas) in the high-temperature environment. UCG for hydrogen production is a low-cost and coal-based hydrogen production route that aligns with China's national condition. However, the dense pore structure and poor thermal conductivity in deep coal are common problems in UCG, leading to the worse hydrogen-rich syngas production efficiency. In this work, the global optimization strategy including microwave modification and mobile gas injection is proposed to adjust the temperature distribution and enhance the gasification reaction intensity in UCG. As microwave modification time increases from 0 to 4.5 min, the specific surface area of coal samples first rises from 4.64 m 2 /g to 7.82 m 2 /g, then drops to 4.96 m 2 /g, and the heat release first increases from 23537 J/g to 23816 J/g, then decreases to 23076 J/g. When the microwave time increases from 0 to 1 min, the total production of syngas increases; while when the microwave time further increases to 4.5 min, the syngas production performance decreases. When the mobile gas injection method is used for reaction zone reconstruction, the moving speed should be determined based on the cavity expansion. For the conditions in this work, the moving speed at 0.4 mm/min is beneficial to improve the gas production and optimize the expansion of the high-temperature zone. After the global optimization strategy, the gasification performance is greatly improved, with the maximum H 2 content enhancing to 36.06% and the effective gas production time increasing by 35.43%.