Study On The Combustion Cavity Growth And Stability Of The Roof During Underground Coal Gasification

Underground coal gasification is a new coal mining technology, having manyadvantages, likes safety, less investment, quick effect, less pollution etc. During UCGthe gasification face is closed and the mechanical properties of the rock are affectedby high temperature so that roof stability for underground coal gasification processresearch is less now. Therefore researching the surrounding rock’s temperature fieldand the roof damage is vital significance. The paper studies the above questions byusing theoretical analysis, physical test and numerical simulation and thecorresponding results are obtained.(1) Before burning through roof and floor, the section of gasification channel isapproximated into a round shape, got the stress distribution of surrounding rock underthe in-situ stress, temperature and gasification pressure. Research suggests thatgasification pressure of surrounding rock control effect is unapparent, ground stressand thermal stress in the underground gasification process plays a main role to theextension of plastic zone.(2)Through analyzing the internal stress of clamped beam distribution influencedby temperature. Research suggests that the compression thermal stress caused bytemperature reduce the roof tensile stress and increase the compressive stress at thesame time. The limit span of roof during UCG depends on the rock ultimate tensilestrength decrease and the thermal stress changes.(3) With gasification, the combustion cavity roof temperature presents differentcharacteristics, including the stage of temperature field formation for the first time,the stage of temperature field disturbance after roof caving, the stage of temperaturefield stationary after roof stability. Combustion cavity roof separation, and the rockbreakage will affect rock layer temperature value and temperature gradient, but littleinfluence on temperature extended range.(4) Numerical analysis the regular of strata behaviors during UCG, researchsuggests that the existence of thermal stress changes the state of vertical stressdistribution, making the stress concentration is more obvious. And because of rockproperties is affected by temperature, vertical stress contour of surrounding rock isindented. Gasification combustion cavity displacement of roof and floor is muchbigger than the conventional mining and displacement value includes both curved roofsubsidence and the rock deformation thermal expansion values.(5) The plastic zone of surrounding rock expanded form is different. The plastic zone develops along up the shoulder of the gob ends during conventional mining.Nevertheless the extension of plastic zone distribution is related to the temperature. Inthe area of low temperature plastic zone expanded in line with conventional miningand plastic zone is larger than conventional mining during UCG.