Mathematical modeling of fluidized bed combustion — 2: combustion of gases

Abstract

Combustion of volatiles within a fluidized bed combustor is expected to influence the pollutant formation and emission characteristics as well as the combustion behavior of carbon. The difficulties involved in developing system models which include details of the release of volatiles from coal and their subsequent combustion are well recognized. In this paper, the behavior of combustible gas mixtures is considered as a sub-model of the more complex problem. The hydrodynamic description of the bubbling fluidized bed is based on the three-phase dispersion model, which accounts for the variation of the visible bubble flow with height above the distributor (the ability of this model to explain experimental data on gas-phase mixing and reaction in isothermal bubbling beds has already been established). The non-isothermality of the bed resulting from combustion reactions is taken into account through inclusion of an energy balance for the bubble phase. Finally, the effect of the variation in superficial gas velocity, on bubble properties and cross-flow, is included through an overall mass balance. Species (intermediates and products) and energy balances are solved numerically. Transient calculations show that the ignition first occurs close to the top of the bed; with time, the ignition front moves closer to the distributor. Steady-state parametric analyses show that the ignition front moves closer to the distributor with increase in bed temperature and particle size, and decrease in the excess gas velocity. Model calculations were also compared with experimental data reported in the literature on the axial variation in mole fractions of various reactant and product species. These comparisons show that under typical fluidized bed combustion conditions, methane and propane burn in a region of parametric sensitivity; thus, significant variations in calculated mole fractions result from comparatively minor variations in reaction rate parameters. Nevertheless, with activation energies well within acceptable limits established in independent kinetic studies, good agreement is obtained with experimental data for combustion of gases in fluidized beds.