Formation of Metal Oxide Aerosols for Conditions of High Supersaturation

Abstract

High concentrations of submicron particles of inorganic materials are found in the off-gases of various high-temperature processing units such as smelters and coal combustion systems. Metal oxide aerosols can form in combustion systems when hot gases that contain metal vapor species mix with oxygen-bearing gases and when these mixed gases are cooled. Experiments were conducted in an evaporation-oxidation furnace simulating the geometric configuration of a diffusion flame and using zinc as the test substance. The average metal oxide particle size and particle size distribution were measured for various experimental conditions. It can be shown that the conditions under which most metal oxides are formed in combustion gases correspond to extremely high degrees of supersaturation, with respect to the stable solid or liquid oxide phase. For these conditions, the homogeneous nucleation mechanism, utilizing the Becker-Doring theory, does not apply. An alternative approach is the “chemical nucleation” process, starting with an essentially irreversible chemical reaction in the gas phase to form a monomer of the oxide molecule and proceeding through steps of condensation and coalescence that are controlled by the diffusion rate and collision frequencies of the reacting species and the growing oxide particles. The fluid dynamics of the gases in the system are important in the kinetics of the process because they influence the concentrations of precursor species and particles and the lengths of diffusion paths. A chemical nucleation model is proposed to describe quantitatively the formation of zinc oxide particles as a function of process parameters such as partial pressure of metal vapor, oxidation temperature, and characteristic parameters of the gas flow system.