Coke Formation in the Combustion of Isolated Heavy Oil Droplets

The combustion and coking of isolated, free and suspended droplets of a residual oil have been studied. The combustion history of the free and suspended droplets was found to be characterized by qualitatively similar phenomena. The coke particle was seen to form within the final 9 percent of the droplet burning period and the mass of the coke particle was about 3 percent of the mass of the initial residual oil droplet. Changing the temperature history of a droplet by varying the initial droplet size between 260 and 570 microns or by diluting the residual oil with a more volatile No. 2 fuel oil did not affect the mass fraction of the residual oil in the initial droplet which was converted to coke. The contribution to the coke particle from both the asphaltene and maltenc (residual oil from which the asphaltenes have been removed) components of the oil was found to be significant, and therefore fuel asphaltene content alone will not provide a satisfactory basis for determining the relative coking tendency of different oils. The results indicate that in the combustion of heavy fuel oils coke particles will be formed and that particulate emissions may be best reduced through enahanced particle burnout (through techniques such as improved atomization and oil-water emulsions), and not through an attempted reduction in the formation of coke.