A mathematical model to describe the seam-cavity growth during underground coal gasification(UCG) is developed. The effect of coal gasification or coal combustion on lateral growth of coal wall is mainly considered in this model. It is based on some basic assumptions and balances of heat and mass transfer in gasification zone of cavity,and considers the effect of dry distillation of seams. The gasifing zone of coal wall is assumpted to be a thin,highly permeable char layer and an ash layer seperating inert ash rubble of the main cavity from coal wall. The model can explain lateral growth mechanism of coal wall in entire gasification process,and determine the growth velocity v (x,t) of coal wall.
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