Submicron Aerosol Formation During Combustion of Pulverized Coal

Abstract

A detailed investigation is conducted into the fundamental processes responsible for the formation of submicron aerosol during combustion of pulverized coal. To this end, both theoretical and experimental tools are developed. The first part of the work consists of a numerical simulation of the vaporization of ash and formation of aerosol. The work combines a model of a single, burning coal particle with one of the formation of aerosol by nucleation and the growth of aerosol by coagulation. A quasi-steady approach is used to model combustion and aerosol formation and this is shown to be valid a posteriori. Calculations are performed for vaporization of refractory oxides during combustion. The effect of thermophoresis on the transport of aerosol around a burning coal particle is found to be small. For refractory species like silica, nucleation typically occurs within 4-5 particle radii from the surface of the coal particle. Thus, in this case, nucleation is controlled by the combustion process. The vaporization rate is found to be weakly dependent on the presence of aerosol in the gas. The second part of this work is the development of an experimental system in which to study the aerosol formation processes. A new material is produced that is similar to coal in many respects, but is chemically simpler and more well-characterized. This makes it possible for the first time to study the fundamental aerosol formation processes without interference from the complex chemistry of coal ash. Experiments in a laminar drop-tube furnace confirm that this material burns in a manner similar to coal and that combustion produces an aerosol from vaporization of ash.