Method of oxygen-enriched two-stage underground coal gasification

Two-stage underground coal gasification was studied to improve the caloric value of the syngas and to extend gas production times. A model test using the oxygen-enriched two-stage coal gasification method was carried out. The composition of the gas produced, the time ratio of the two stages, and the role of the temperature field were analysed. The results show that oxygen-enriched two-stage gasification shortens the time of the first stage and prolongs the time of the second stage. Feed oxygen concentrations of 30%, 35%, 40%, 45%, 60%, or 80% gave time ratios (first stage to second stage) of 1:0.12, 1:0.21, 1:0.51, 1:0.64, 1:0.90, and 1:4.0 respectively. Cooling rates of the temperature field after steam injection decreased with time from about 19.1–27.4 °C/min to 2.3–6.8 °C/min. But this rate increased with increasing oxygen concentrations in the first stage. The caloric value of the syngas improves with increased oxygen concentration in the first stage. Injection of 80% oxygen-enriched air gave gas with the highest caloric value and also gave the longest production time. The caloric value of the gas obtained from the oxygen-enriched two-stage gasification method lies in the range from 5.31 MJ/Nm3 to 10.54 MJ/Nm3.