A numerical model for char combustion in packed bed reactors.

A numerical simulation of the combustion of centimeter-sized char particles in an overfeed fuel bed is presented. One-dimensional mass, species and energy balances for the reading system are solved by finite volume discretization methods to predict operating curves and temperature and concentration profiles for char combustion under a variety of conditions. The simulation is unique in that it employs separate gas and solid phase energy balances with finite-rate CO kinetics and a particle number balance to account for the shrinkage of the char particles as they descend through the bed. All the major energy transport processes, such as particle-to-fluid heat and mass transfer, conduction, convection and radiation, are modelled. The simulation was run for beds of 1-3 cm particles ranging from 30-60 cm in height. The range of air flow rates studied ranged from 0.036 to 0.87 m/s. Comparisons between predicted gas and temperature profiles and similar profiles obtained from the literature generally showed good agreement, indicating that the simulation was reliable enough for use as a design tool over the range of conditions that were studied. The simulation proved to be quite sensitive to choices of the heat and mass transfer coefficients and also to the CO$\sb2$ reduction rate expression, indicating that there is a need for more research on large-particle kinetics and high-temperature packed bed energy transport processes