Combustion Fume Structure and Dynamics

During pulverized coal combustion, a fume of submicron particles is formed from the mineral matter in the parent coal. Studies of the variation in chemical composition with particle size have revealed that much of the submicron fume is formed from volatilized coal ash [l, 2,3]. The formation and evolution of the ash fume is governed by homogeneous nucleation, condensation, and coagulation. Vapors of refractory species nucleate relatively early in the combustion process. Coagulation of those fine particles results in a size distribution that is approximately log normal. More volatile species remain in the gas phase until after the nucleation has taken place. Condensation on the surfaces of both the fume and the larger residual ash particles results in the enrichment of the fine particles with volatile, and frequently toxic trace species. The resultant concentration of heavy metals in the size interval between 0.1 and 1 pm may allow disproportionate amounts of these species to escape collection, even by the best of gas cleaning systems. Flagan and Friedlander [l] first modeled the evolution of the ash particle size distribution in pulverized coal combustion beginning with the hypothesis that the fine particles resulted from homogeneous nucleation and grew primarily by coagulation. They predicted much more distinct peaks in the submicron size range than had been observed at that time. Improved instrumentation has verified those predictions, and shown that the situation can be even more complex than their simple model indicated. In some cases multiple peaks are seen in the size distribution of the submicron fume particles [4]. This could occur relatively late in the cooling of the combustion products when a second vapor becomes sufficiently supersaturated to undergo homogeneous nucleation in spite of the large numbers of fume particles produced in the initial nucleation burst. If heavy metals are responsible for this additional nucleation event or if it occurs before the heavy metals condense, further enrichment of the fine particles with heavy metals could result. A comprehensive theoretical treatment of the aerosol dynamics of pyrogenous fumes requires a number of extensions of the classical descriptions. Rigorous descriptions of the coagulation of dense, spherical particles are available [5, 6, 71, but fume particles are rarely spherical. The materials involved tend to be refractory, so high temperatures are required to achieve complete coalescence. Flame temperatures may be hot enough to melt some materials, so coalescence is not always achieved. Even with systems that can melt the particles in the primary reaction zone, coagulation during the cooling' or quench process can form agglomerates. To predict the dynamics of the fumes produced when coalescence is rate limiting, the structure and dynamics of the resulting aggregates must be understood. Pyrogenous fumes, including soot, coal ash, and synthetic fumes (TiO2, Si02, etc.) exhibit a common structure, namely agglomerates of approximately equiaxed particles (spherules)