To know clearly the reaction process of underground coal gasification was the theoretical base to set up the stable control technique. In comparison with the moving bed gasification,an expression of the underground coal gasification was a fixed seam,but the reaction area was in moving,the reaction process was limited by the h eat spalling and thermal fragmentation of the two side seams along the gasified channel. The essence was a percolation channel reaction with different size lump coal,sea m parting,roof falling rock mixed. With the analysis,the precipitation process of the underground coal gasification could explain gas precipitation process in the radial coal wall along the gasification channel,an axial gasification process along the gasification channel,secondary combustion of the combustible components in the aerobic area of the gasification channel and a water-gas exchanging reaction process in the air flow channel. As for a single reaction period,with the coal consumption,gasification channel expanded,secondary combustion enhanced and gas quality reduced,the stable period would be terminated. With an application of a control technique,such as the retreated gasification at an injection point,the parameters adjustment could be applied to control the fluctuation scope of single periodic gas components and could realize the conne ction of multi production periods in order to realize the stable production of the underground coal gasification.

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