(163c) Kinetics of Coal Drying in Bubbling Fluidized Beds

Some coals used in U.S. coal-fired power plants have unusually high moisture levels. High fuel moisture leads to low power plant thermal efficiency, increased stack emissions of pollutants and maintenance and operational problems. One solution is to dry the coal before burning it, and if this is done using power plant waste heat, it can result in significant efficiency improvements.

This paper will describe laboratory experiments to determine the kinetics of coal drying in a bubbling fluidized bed and a simple theoretical model of coal drying. The experiments were performed with coal which had been crushed to minus 1/4? and fluidized with air heated to temperatures ranging up to 160°F. In-bed electrical heaters, used to simulate an in-bed tube bundle, provided additional thermal energy for drying. The experiments determined the effects of superficial air velocity, drying temperature and inlet air humidity level on rate of drying.

A theoretical model of the drying process was developed in which the air and coal particles are assumed to be at the same temperature and the air?water vapor mixture leaving the bed at the free surface is in equilibrium with the local values of particle moisture. This model is in good agreement with laboratory data, showing that for this application, the drying rates do not depend on fluidized bed bubble behavior or on particle-gas contact, but are controlled by heat and mass transfer processes within the particle pore structure.