An integrated full, pilot and laboratory scale study of the effect of coal quality on NOx and unburnt carbon formation

PowerGen and EPRI have co-funded a two year research project to investigate the effect of coal quality and coal blends on NOx formation and unburnt carbon in low-NOx combustion systems. The objective of the programme was to develop a better understanding of these effects in relation to combustion staging and coal blending in order to derive more reliable predictive methods. Eight bituminous coals covering a wide range of properties and geographical source were included in the study. The comprehensive work programme comprised full-scale plant trials on a 500 MW{sub e} tangentially fired boiler fitted with LNCFS backed up by smaller scale testing at the Power Technology Center using a 1MW (3.4 MBtu/h) Combustion Test Facility and a laboratory Drop Tube Furnace. Coal and char samples pertinent to all experimental scales have also been examined at Nottingham University using a recent development of an automated image analysis technique for coal petrology and char characterization. The overall results of the programme are summarized in this paper. The value of rig testing for the prediction of NOx emissions from individual coals in full-scale plant has been successfully demonstrated, through calibration with plant data. As expected, NOx emissions do not correlate simply with eithermore » nitrogen content or with fuel ratio based on laboratory determined volatile matter; a stronger correlation is found with a function based on both coal volatile matter and nitrogen content. It has been shown that the sensitivity of NOx emissions to coal quality is reduced as the degree of combustion staging increases. All coals tend towards a NOx minimum of 150-200vpm under deep air staging. The level of unburnt carbon, an often ignored disadvantage of low NOx combustion systems, has been shown to be highly dependent on coal quality, but is much less sensitive to combustion staging.« less