Combustion-generated NOx and coke in heavy residual fuel oil combustion

This paper deals with the numerical modelling of heavy residual fuel oil combustion. Some results of combustion-generated nitric oxide and coke are presented. The emissions of both species are greatly affected by a gas temperature and thus the modelling of soot concentration, which affects radiation heat transfer is of vital importance. The calculated results of three combustion cases, the thermal inputs of which are 1.8, 3.5 and 36 MW, respectively, are compared with the measured ones. A simplified model of the formation and reduction of nitric oxide, in which hydrogen cyanide is an intermediate phase between the phases of fuel nitrogen and nitric oxide, is used. The amount of coke generated is described with the so-called residual carbon rate coefficient xi, which has in actual practice a non-linear relationship to the heavy residues and chemical structure of the fuel. In this study it is assumed that a fixed portion of vaporized fuel oil remains as coke, i.e. xi has a constant value. The prediction of the absorptivity of soot is based on the idea that soot is formed in the zones where fuel vapor is burning.