CHEMICAL KINETICS OF SOOT PARTICLE GROWTH

The light given off by a candle flame and the black smoke from wood­ burning fireplaces, diesel engines, and industrial furnaces have a common origin: tiny carbonaceous soot particles. The light of a candle comes from incandescent soot particles that ultimately burn up in the air at the top of the flame. (But a cold object placed in the flame will quickly become covered with black soot.) In the other cases, however, the soot may be emitted into the atmosphere as a pollutant. In spite of generations of effort, soot emission from many practical combustion systems remains a serious problem, and the viability of the fuel-efficient passenger car diesel engine may rest on our ability to make significant reductions in soot emissions. The morphology of soot particles can be best observed with electron micrographs such as that shown in Figure 1. We see 10 to 30 nm spherical units attached in necklace-like chains, with each sphere containing the order of 105 carbon atoms and 104-105 hydrogen atoms, putting the spheres somewhat above molecular in size. The structure in Figure 1 is nearly the same for soot generated on a bunsen burner and in a diesel engine. A great deal of work has been reported that correlates soot formation with fuel type, flame temperature, or flow fields (1), yielding an extensive and very useful body of engineering data, but these data have provided little insight into the detailed chemistry of soot formation. Soot formation involves some interesting and unusual chemistry. First, soot formation represents growth from species the size of fuel molecules, with perhaps 1 to 10 carbon atoms, to particles with hundreds of thousands of carbon atoms. Such growth takes place at flame temperatures, generally between 1000 K