Study on CRIP process of underground coal gasification coupled with high-power microwave heating

The Controlled Retraction Injection Point (CRIP) process of underground coal gasification can mine in situ medium-deep coal resources efficiently and transform rich coal resources in China into strategic and scarce gas resources. In order to replace the injections of oxygen and igniting agent and improve the gasification reactivity of coal seam, the CRIP process of underground coal gasification coupled with high-power microwave heating was proposed in this paper based on technical characteristics of underground coal gasification and microwave heating. Gasification agents and microwave generated were injected into the underground coal seam through a continuous oil pipe and released in the target gasification area. Theoretical and experimental research were conducted to grope for solutions to microwave generation, transmission, release, coal seam heating, and industrial development model. The results show that the CRIP process of underground coal gasification coupled with high-power microwave heating is technically feasible. High-power magnetrons can supply a large amount of heat to meet the industrial production scale. Microwave can be transmitted remotely to underground coal seam through the annular space between the inner tube outer wall and the outer tube inner wall coated with copper in the double-layer continuous oil pipe. The higher microwave frequency is, the faster microwave power decays. If 2450 MHz and 915 MHz microwaves are transmitted a distance of 500 m in the double-layer continuous oil pipe, their power decays to 15.6% and 32.1% respectively. Lower frequencies should be selected to transmit microwaves in long distance with low loss in medium-deep underground coal gasification. A microwave energy release device produced by cutting gaps on the outer wall surface at the front end of the double-layer continuous oil pipe according to certain rules can leak out the microwave and radiate the coal seam. Raw coal can absorb microwave energy well and be heated during the drying, pyrolysis, and combustion stages. When the sample temperature rises to 587 ℃, the coke is ignited in the air atmosphere. And a maximum temperature of 1080 ℃ is observed. An integrated industrial model of wind-solar power, energy storage, and underground microwave gasification of coal is able to fully utilize green energies and achieve clean and efficient in-situ mining of medium-deep coal seams. The conclusions in the present study provide the theoretical basis and technical support for microwave assisted underground coal gasification.