Effect of fixed and removable gas-injection patterns on the expansion of reaction zones during underground coal gasification

The expansion pattern of the three reaction zones, including oxidation, reduction, and dry distillation zones in the underground coal gasification (UCG) process, is recognized to be different using various gas-injection patterns, on account of which the corresponding stability and syngas components are notably distinguished. The current work carried out UCG model tests in a removable gas-injection point (receded five times). During the stable gasification process, the temperature field map and changes in syngas composition were monitored to determine the area proportions among the three zones and their expansion rates. The differences in the expansion of the three zones between fixed and removable gas-injection points were then explained according to cavity growth. The results showed that the UCG process using a removable gas-injection point exhibited a better performance than that using a fixed point in some aspects, including gasification stability, syngas calorific value, area ratio of the reduction zone and oxygen diffusion velocity, etc., as follows. In the RGIP process, especially in the second retraction stage, the gasification was the most stable, and the syngas calorific value achieved the highest (7.83 MJ/m³). Meanwhile, the area ratio of the oxidation, reduction, and dry distillation zones was 1.00:1.58:1.00, and the corresponding expansion rates in forward and lateral directions were 0.074 and 0.008 m/h, respectively. Compared to gasification using a fixed point, removable gas-injection point method has led to cavity formation in each retraction stage, which can increase the oxygen diffusion velocity. It can thereby realize not only a stable gasification process but also a high thermal efficiency for the energetic reaction. In addition, that stable process can be lengthened by controlling the removable device. This explained why the expansion of the three zones using a removable gas-injection point was better mainly in gasification stability, efficiency, and syngas calorific value than that using a fixed one.