Detailed measurements in and modelling of an industry-type pulverised-coal flame

Abstract

Measurements have been performed in a large-scale laboratory furnace fired by an industry-type pulverised-coal (PF) swirl burner. New data are reported for gas phase species concentration, temperature and char burnout for two swirl numbers, 1.03 and 1.45, other operating conditions being fixed. These data are distinguished by the emphasis they give to the crucial near burner field. They complement previously obtained data for a lower firing rate. The characteristic particle residence time in the burner internal recirculation zone emerges as the most important quantity influencing combustion efficiency. Too excessive, as well as too little, swirl can result in insufficient residence times. A parallel mathematical modelling exercise is summarised. Comparisons of the predictions with the data reveal that the ignition distance is significantly underpredicted, a model deficiency that was not previously recognised due to the lack of near field measurements. Varying the parameters of the existing physical modelling within physically acceptable bounds produced no improvement. This and the good prediction of a gas flame leads to the conclusions that: the timescale of the secondary cracking of the primary products of pyrolysis, a process not hitherto modelled, is significant, and/or that the pyrolysis kinetic rates established in relatively low heating rate and/or final temperatures experiments are not appropriate in combustors. Novel benchtop scale experiments will have to be devised to produce data of the kind required.