Prediction of NO_x and unburned carbon in ash in highly staged pulverised coal furnace using overfire air

NOx emissions from power plants burning pulverised coal are subject to increasingly stringent regulations, and this has a consequential effect on the amount of unburned carbon (UBC) produced. The prediction of both is important, and a number of modelling techniques are available, but there could be difficulties especially under highly staged combustion. This is because these regions have low oxygen concentrations where char gasification reactions might take place in addition to the normal combustion. The present paper discusses the role of these reactions mainly within a staged tangentially T fired furnace, which uses overfire air for NOx reduction, but it also considered a wall fired case. Predictions for UBC and NOx in these furnaces, which are represented by a series of slices, were made with and without the gasification equations and are compared with test data from furnaces. These results indicate that the char gasification reactions play only a small role even under highly staged conditions and only for the larger UBC particles.