Underground coal fires are difficult to extinguish. As an alternative they might be used for heat production. Hence, their controlling factors have to be investigated. Experimental work on in-situ coal gasification and geological surveys on underground coal fires are used to develop a model to understand the combustion process and subsidence behavior. Coal fires are divided in five thermal zones with different thermo-mechanical characteristics. The three zones in and above the seam are used in a convection model. In experiments temperatures, ranging from surface temperature to 1200°C, are applied to claystones, shales and sandstones. Changing mineralogy, thermal expansion, shrinkage, bulk moduli and permeability of grain aggregates are established under defined in-situ T,P-conditions. The results are implemented in finite difference- and creep models to calculate subsidence. Volume changes are converted to permeability fields for a convection model. In this model, seam-, rubble- and overburden thicknesses, their depths and fault densities, are other input parameters. The model calculates the temperature-, convection- and oxygen distribution in the sub-surface. For shallow coal fires (15 m), the overburden, faults and collapsed rubble zone are used for oxygen supply and thermal convection. Deep coal fires (40 m) use the rubble zone and faults for the process.


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