Kinetic behavior of subbituminous coal drying; effects of confining pressure

Abstract

A model is developed which explains the drying behavior of a subbituminous coal. This model, a variation of the moving-boundary approach, has the vapor-liquid interface in the macropores receding from the exterior surface of the coal as drying progresses. On the liquid side of the interface, liquid water exists in all three pore systems - macropores, transitional pores, and micropores. On the vapor side of the interface, in the transitional and micropore systems branching off the macropores, the moisture essentially is in equilibrium with the water vapor in the macropore system. Water diffuses from the liquid interface in the macropores through these pores to the exterior surface of the coal. Experiments with a Washington State subbituminous coal are described which test this model. The predictions of the model match the behavior of experimental drying curves. The effective diffusivities obtained were within the range expected for the constricted pore systems which coal possesses. Higher initial stresses apparently decrease the total porosity and increase the severity of pore constrictions. Disparities between model prediction and experimental behavior, which occurred at a late drying time, result from the assumed water adsorption isotherm