LABORATORY STUDIES OF SUBMICRON PARTICLE FORMATION IN PULVERIZED COAL COMBUSTION

A laboratory furnace was constructed to study the mechanism of submicron particle formation in pulverized coal boilers. Simulation was achieved by matching characteristic kinetic times in laboratory and field units. This was done by matching volumetric heat extraction rates. Reynolds number and geometric scaling were not deemed essential for the simulation.;The influence of macroscopic combustion parameters on the characteristics of fine particle emissions was investigated. Total submicron particle volume concentration was found to depend on burner type and to increase with temperature for each burner type tested. The particle volume concentration was found to increase with exhaust NO concentration.;The fine particle composition was found to differ from that of larger residual ash particles which generally reflected the composition of the bulk ash. It was also found that combustion conditions (overall fuel-air equivalence ratio and wall temperature) influenced the composition of the smallest ash particles.;Species enrichment data are generally supportive of the vaporization-condensation mechanism of submicron particle formation. Measured dependence of particle volume-mean diameter is consistent with theoretical predictions based on free-molecular Brownian coagulation of particles formed by nucleation of vaporized ash. Strong dependence of submicron particle volume concentration on char particle temperatures in the hot zone further suggests vaporization of ash to be the principal source of material from which fine particles are formed.;Combustion-generated submicron aerosols from the laboratory furnace were characterized by size and composition distribution measurements using an electrical mobility analyzer and a low-pressure impactor. Elemental analysis of size-classified mass samples was done by alpha particle-induced X-ray emission spectrum analysis.;A semi-quantitative combustion model was formulated to calculate char particle temperature-time histories in the laboratory furnace. An Arrhenius vaporization rate expression was fitted to the laboratory measurements of total submicron particle volume concentration using the particle temperature histories calculated from the combustion model. Estimates of kinetic parameters were compared with those of other investigators. The model calculations suggest that the mass-specific rate of ash vaporization is strongly dependent on parent coal particle size. This is due to higher burning temperatures of smaller particles together with a high apparent activation energy for ash vaporization.;Recommendations for further work are given.