EMISSION OF UNBURNED COKE FROM COMBUSTION OF RESIDUAL FUEL-OIL IN WALL-FIRED ELECTRIC UTILITY BOILERS

A model is proposed for the calculation of unburned-coke emission from the combustion of residual oil in electric utility boilers. The calcu lation begins with the spray droplet-size distribution, and includes q uantitative descriptions of the fraction of fuel converted to coke, th e size of the residues formed, their reaction with oxygen, fragmentati on near complete burnout, separation from flue gas, and re-injection i nto the furnace. The gas and particles were assumed to be at a fixed t emperature, but allowances were made for distributions of residence ti me and oxygen concentration. The standard deviation of the oxygen-conc entration distribution and the characteristic mixing time in the post- flame region were adjusted to fit measurements of unburned carbon emis sion from 400 MW (electric) power boilers. Reasonable values of all pa rameters reproduced the measurements of unburned carbon under normal o perating conditions, near 0.5 mol% excess oxygen, oil viscosity of 16 mm2 s-1, and 10 wt % asphaltenes in the oil. The model did not reprodu ce the variation of unburned coke with the asphaltenes content of the oil, thereby demonstrating the need for a direct measurement of coke y ield under flame conditions. Influences of asphaltenes on atomisation behaviour and coke structure may also be important. The limitations on the accuracy of the calculation arise from lack of knowledge of condi tions in the furnace and from incomplete understanding of the processe s of coke formation, combustion and separation from flue gas.