Influences of removable gas injection methods on the temperature field and gas composition of the underground coal gasification process

In this paper, the effects of changing the injection position of a gasification agent on temperature fields and gas compositions are investigated through an experimental model of the underground gasification of coaxial coal. A 3D model of the temperature field is constructed based on the experimental temperature parameters using kriging interpolation; the 900 C isothermal surface is used as the reference plane to define the oxidation zone, and its plane and volume expansion rate are analyzed. The underground coal gasification experiment is divided into five stages according to the injection location. The average expansion rates of the oxidation zone in each stage are 2.04 cm/h, 1.93 cm/h, 3.44 cm/h, 3.68 cm/h, and 3 cm/h along the X-direction and 0.69 cm/h, 0.09 cm/h, 0.56 cm/h, 1.69 cm/h, and 1.01 cm/h along the Z-direction; the expansion rate along the X-direction is greater than that along the Z-direction. The expansion volumes of the oxidation zone after the five stages increase from 0.0097 m3 to 0.0168 m(3), 0.0321 m(3), 0.0521 m(3) and 0.0657 m(3). An analysis of the expanded volume of the oxidation zone compared with the volume of the cavity shows that both features are similar in shape and that the final volume of the cavity is smaller than the final expanded volume of the oxidation zone; the volume of the cavity is approximately 77.3 % of the expanded volume of the oxidation zone. The formation of a cavity requires the temperature to reach the conditions for the oxidation reaction of the coal and the rupture and flaking of the ash layer that accumulates after the reaction by the actions of certain stresses, such as thermal. The composition of the product gas and the strength of the chemical reactions involved are evaluated based on stoichiometric conservation. A short period after a shift in the injection position of the gasifier greatly impacts the chemical equilibrium of the UCG process with a large change in the strength of the chemical reaction, followed by a relatively small change when the gasification process stabilizes.