Heat transfer through coals and other naturally occurring carbonaceous rocks

Abstract

The understanding of heat transfer through solid fossil fuels is essential to the phenomenology of pyrolysis, gasification and combustion of these fuels. While coals have thermal conductivities of 0.1 to 0.5 W/mK at 300/sup 0/K, heat transfer measurements are complicated by the changes found in these fuels caused by the heating processes. Such complications are clearly shown when one looks at thermal conductivity differences between virgin and heat-treated materials. Coals, upon heating, undergo a variety of chemical and physical modifications. Initially, these materials lose low molecular weight gases; additional heating removes moisture. Such pyrolytic processes result not only in a significant mass decrease (as much as 50 percent for low-rank coals), but a marked alteration in the internal structure of the material. In virgin coals, mass transfer is dominated by a system of pores. Drying these materials typically alters the flow mechanisms and consequently the permeability. Heat transfer becomes dominated by the convective transport of products generated within the specimen during the heating process. Studies are described that explore the concurrent and counter-current heat and mass transfer problems through semiporous materials such as coals and other model specimens