The invention relates to a
coal underground gasification method. The
coal underground gasification method comprises the following sequential steps of (1) establishing a gas inlet passage, a gas outlet passage and a gasification passage which enables the gas inlet passage to be communicated with the gas outlet passage; (2) injecting a gasification agent through the gas inlet passage, wherein the gasification agent comprises
water vapor and
oxygen-enriched gas, and an initial volume ratio between the
water vapor and the
oxygen-enriched gas ranges from 0.5 : 1 to 4 : 1; (3) igniting a
coal seam; (4) forming a new volume ratio by adjusting the ratio between the
water vapor and the
oxygen-enriched gas according to the characteristics of generated
coal gas. According to the coal underground gasification method disclosed by the invention, a little water vapor is enabled to be carried in raw
coal gas, the cost for recovering waste water in raw
coal gas is reduced, the situation that the water vapor and the coal seam are in full reaction so as to generate
hydrogen can be ensured, and thus the quality of coal gas can be increased; in addition, the temperature uniformity on a gasification working surface can be ensured due to reactions, i.e.
exothermic reaction of oxygen and the coal seam and endothermic reaction of water and the coal seam, which are carried out in the same time, and the phenomenon that the continuous gasification of the coal seam is obstructed as gasification coal ash is melted and attached to a gasification coal wall due to over-high local temperature can be prevented from happening.