Gas production performance of underground coal gasification with continuously moving injection: Effect of direction and speed

•The UCG behavior with different moving directions was studied.•Effects of injection moving speed and oxygen flow rate were analyzed.•Potential application of different methods for moving injection was discussed. Underground coal gasification (UCG) is an in-situ gasification technology where the controlled retreating injection point (CRIP) method is usually adopted to optimize the cavity evolution and gas production stability. However, the reported studies only focus on the gasification behavior of the moving injection backward under fixed time intervals. In this work, the gas production performance and reaction region evolution were studied for UCG with continuously moving injection for different directions and different speeds. For moving backward injection, when a uniform moving speed is used, gas production is not satisfactory with a heating value of 1 MJ/m3. In order to optimize the gas production, the injection should move backward for a fixed distance at regular time intervals rather than continuously, and the distance should be long enough for a new distribution of combustion zone, gasification zone and pyrolysis zone. For moving forward injection, when the moving speed and oxygen flow rate match well, the maximum heating value is larger than 11 MJ/m3, and the gas production is stable. The moving forward method is recommended for low ash coal to avoid “ash cover pipe” problem, and the injection pipe should be manufactured with high temperature resistant material. The moving backward method is recommended for low volatile coal to avoid “gas product leakage” problem, and the retreatment distance should be specially designed to avoid re-ignition.