Comparison of Stefan model with two- phase model of coal drying

Abstract The Stefan model has frequently been proposed to describe highly non-isothermal rapid drying of porous media. In particular it has been used to analyze the drying of Western coals and lignites which contain up to 40% water, almost all of which is unbound. The Stefan model may be appropriate for freezing processes but may break down when describing processes where much steam is produced within the moist porous material and flows outward because of an internally generated pressure build up. Recently, the first author developed a mechanistic moisture dynamics model called the two-phase model to remedy this deficiency and, thus, to rationally extend the drying theory. The objectives of this paper are: (1) to compare the Stefan and two-phase drying models by analyzing lignite block pyrolysis and subbituminous coal drying experiments; and (2) to determine the range of heating rates for which the Stefan model is adequate. This paper is, to our knowledge, the first to quantitatively assess the adequacy of the Stefan model to describe coal drying.