6Particle Formation in Combustion

Combustion processes emit large quantities of particles to the atmosphere. Particles fonned in combustion systems fall roughly into two categories. The first category, re-ferred to as ash, comprises particles derived from noncombustible constituents (primarily mineral inclusions) in the fuel and from atoms other than carbon and hydrogen (so-called heteroatoms) in the organic structure of the fuel. The second category consists of car-bonaceous particles that are fonned by pyrolysis of the fuel molecules. Particles produced by combustion sources are generally complex chemical mix-tures that often are not easily characterized in tenns of composition. The particle sizes vary widely, and the composition may be a strong function of particle size. This chapter has a twofold objective-to present some typical data on the size and chemical compo-sition of particulate emissions from combustion processes and to discuss the fundamental mechanisms by which the particles are fonned. 6.1 ASH Ash is derived from noncombustible material introduced in the combustor along with the fuel and from inorganic constituents in the fuel itself. The ash produced in coa