PARTICULATE FORMATION IN FUEL OIL COMBUSTION

In an attempt to develop a simple screening method to predict potential air pollution by particulate emissions from residual fuel oil burners, a comprehensive and detailed study of the combustion of single droplets of a wide range of oils suspended on a fine thermocouple was made, and compared with particulate emissions from a spray burner fired with identical fuels. Single droplet combustion parameters such as the pre-ignition time (t_i), ignition temperature (T_i), flame time (t_f), coke combustion time (t_i) and temperature (T_c) were derived from temperature-time traces of the droplet and particle combustion. The following relationships were established between the combustion parameters and the original droplet diameter (d_o):

where A, T_i K_i, K_f and K_c are constants dependent upon the fuel. The Conradson Carbon Residue of each fuel correlated with both K_c and particulate emissions. Other relationships between single droplet parameters, fuel composition and particulate emissions were evaluated. A strong relationship was found between T_c and the naturally occuring vanadium content of fuels. Addition of vanadium to a poor fuel increased T_c dramatically and improved both the single particle combustion and the particulate emissions level. A significant correlation between the effect of a calcium additive on T_c in the droplet experiments and its effect on particulate emissions was found. The action of additives cannot be explained simply an catalysis of coke combustion since a catalyst would not normally be effective at the high temperatures of the coke burn-out; all other results indicate diffusion control which is expected under these conditions. It is possible that metals have a catalytic effect earlier in the combustion process during the formation of the coke particle, modifying the structure of the coke and hence affecting its subsequent combustion.