COAGULATION OF SOOT TO SMOKE IN HYDROCARBON FLAMES

The soot cloud in a luminous diffusion flame contains carbon monoxide and other unburnt gases, and both soot and gases oxidize when additional oxygen diffuses into the flame. According to the literature, the relative oxidation rates satisfy d log [soot] d log [CO]{slanted equal to or greater-than} 0·5 × 10-6 D where D is the diameter of the soot particles in centimetres. Particles with D {reversed tilde equals} 1 × 10-6 cm (a size often observed in flames) should burn up with the gases. In order to survive the oxidation of the gases and give smoky flames, the particles must have considerably larger diameters. Plots of log [soot] versus log [CO], as both oxidized, were made for flames containing various mole fractions of soot. Mole fraction was defined as atoms of carbon precipitated as soot per gas molecule in the soot cloud. At sufficiently large mole fractions, the oxidation rate of soot decreased relative to the oxidation rate of CO until, eventually, soot survived to give smoky flames. This was taken as evidence for coagulation to larger particles the greater the mole fraction. Increasing the pressure increased the mole fraction of soot precipitated, and also increased the coagulation of a given mole fraction. A simple calculation suggests: 1. (1) When the log/log plots just failed to give evidence for coagulation, soot particles did not exceed 2·2 × 10-6 cm diameter. 2. (2) When the flames first became smoky, the particles could have grown as large as 6 × 10-6 cm diameter. © 1969.