Effect of system pressure on char combustion in Fluidized Beds. Model predictions

Although Pressurized Fluidized Bed Combustion is already at the demonstration stage, fundamental studies on the effect of system pressure on the many aspects of the process are still under way. In this paper the effect of system pressure on fluidized bed char combustion is considered from a theoretical standpoint. Char combustion in bubbling fluidized bed is controlled by the oxygen transfer from bubbles to emulsion phase and from this latter to char particle surface, oxygen diffusion in the pore network and char intrinsic reactivity. According to previous indications this study confirms that the actual influence of system pressure on char combustion can only be deduced by means of a comprehensive mathematical model which accounts for the controlling steps altogether. The extention of a model previously developed by the authors to a pressure higher than atmospheric is finalized to this objective. This model is based on a char particle population balance and considers particle combustion, attrition and elutriation under simple bed fluid dynamics. With reference to a pre-pilot combustor, from which experimental data at atmospheric pressure were available, bed carbon loading, specific carbon combustion rate, characteristics resistances of the combustion mechanism and combustion efficiency have been calculated at a pressure of 12 bar and compared with results at 1 bar. In model evaluations, the excess air has been considered the main parametric variable. Different assumptions concerning bed fluid dynamics as well as combustion kinetics have been tested. Model results clearly indicate that bed carbon loading increases with pressure even though char combustion rate is faster due to higher oxygen partial pressure.