The Effect of Moisture on the Structural Stability of a Coal Cavity

The drying of coal and the associated thermal and moisture-induced stresses are examined with a numerical model to estimate the rate of surface regression in underground coal gasification. The model includes internal convection, diffusion, conduction and flow of liquid, vapor and gas. The structural failure is modeled by three different mechanisms based upon a strength reduction due to heating, drying, or crack formation. Using properties and boundary conditions appropriate to Western United States coal, the model predicts a regression rate which is in qualitative agreement with measured results. Using the model, it is possible to examine different thermal and failure modes and to gain an understanding of some of the mechanisms which may control the surface regression of coal during gasification. These results may be applicable to the formation of rubble during gasification and to internal cavity collapse.