Ultrafine Particle Formation in Pulverized Coal, Biomass, and Waste Combustion: Understanding the Relationship with Flame Synthesis Process

The adverse effect of ultrafine particulate matter (PM) from the combustion of solid fuels calls for fundamental insights to guide effective control strategies. In this work, we investigated the ultrafine PM formation from burning several solid fuel samples, with special emphasis on the relationship with an emerging field of flame synthesis. Diluted fly ash was sampled and characterized in a bench-scale flat-flame burner at the early stage (similar to 40 ms) and in a 25 kW quasi-one-dimensional combustor at the burnout stage (similar to 1.5 s). Then, the population balance model framework was transplanted from the community of flame aerosol synthesis to simulate the particle size distributions (PSDs). From the experiments, unimodal submicron PSDs were revealed for the combustion of coal, rice husk, and sewage sludge under 40 ms of residence time and a postflame ambience of 1500 K and 20 vol % O-2. The compositional analysis reveals both the contributions from Na/K/P/S and from refractory species are important for ultrafine PM formation. This vaporization-nucleation mechanism highlights the similarity between solid fuel combustion and flame synthesis. However, the difference is raised by the medium and coarse mode PM that only appears in solid fuel combustion. The volatilized mineral and the ultrafine PM can be scavenged via coagulation, and the scavenged ratio takes similar to 10-60% of the total volatilized minerals for different coal samples. This ratio is inversely correlated with the volatilized minerals on a basis of ash input.