Pulverised Coal Combustion Under Transient Cloud Conditions in a Drop Tube Furnace

Abstract

A low volatile coal was burned with oxygen-enriched air in a drop tube furnace at 1223 and 1523 K, either as single particles or as small batches added as a pulse. The particle temperatures and burnout times at the two furnace temperatures were recorded for each test. The ignition / combustion processes were recorded by pyrometer and video camera. The pulsed batch burnout times were ten to twenty times longer than the single particle times, indicating that cloud combustion was taking place. A mathematical model of the transient behaviour of the batch combustion process was developed, based on plug flow. Effects arising from volatile combustion were apparent in the visualisations but were ignored in the modelling. Burning was predominantly under diffusion control especially at high oxygen concentrations. The effect of particle segregation due to feeder performance and aerodynamic drag was simulated. From the dispersion of the particles, values of the local cloud combustion number G during burnout were determined. The presence of cloud combustion during most of the burning process was confirmed. In a drop tube furnace the mass transfer process supplying oxygen to the interior of the particle cloud is convection brought about by the slip between the particles and the gas. A new simplified cloud combustion number Gi, which combines convection and reaction kinetics was developed for this situation. Gi can then be related to the effectiveness factor for combustion rate in the cloud. The new approach predicts that serious oxygen depletion will arise during the bumout of the cloud, as is observed in practice. The cloud number Gi was successful in predicting the cloud bumout times from the single particle values.