Steam drying of coal. Part 2. Modeling the operation of a fluidized bed drying unit

Drying of coal in steam offers, besides reduction of fire and explosion hazards, improved thermal efficiency. In addition, steam-dried coals appear to be less prone to spontaneous combustion. In a previous paper, a mathematical model was developed for the drying of a single porous particle in steam. In this paper, the single particle model has been integrated with a two-phase hydrodynamic model to simulate the continuous drying of coal in a bed fluidized with superheated steam. The model incorporates: (i) variation of particle temperature and moisture content with residence time within the bed; (ii) the effect of the initial condensation during the heating period of coal fed to the fluidized bed; and (iii) variation in the superficial gas flow-arising from the vaporization of moisture from the wet coal or the condensation of inlet steam-for various operating conditions. Model studies show that, for a specified reactor size, system performance is most sensitive to the steam-tube duty and the initial moisture content of the wet coal. The conceptual problems in defining an inversion temperature, above which vaporization rate in steam is greater than that in air, for the continuous drying of porous particles are discussed. It is shown that the inversion temperature is a consequence of departure from thermal equilibrium between the gas and solid phases. Model studies show that the inversion temperature depends on design and other operating parameters. (C) 2000 Elsevier Science Ltd. All rights reserved.