Factors Influencing Ultrafine Particulate Matter (PM_(0.1)) Formation under Pulverized Coal Combustion and Oxyfiring Conditions

摘要

This paper explores the early processes of coal ultrafine ash (D < 100 run) formation under both conventional air-blown and oxyfiring conditions. An innovative flow reactor and a high-resolution differential mobility analysis technique have been coupled to measure the particle size distribution functions (PSDFs) in a size range extending down to 1 nm. Information on the formed fly ash chemical nature has been obtained by ultraviolet-visible (UV-vis) light absorption and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDXS). Five coals of different rank, covering a broad range of ash compositions, have been tested under three oxygen concentration levels. A multimodal behavior of coal ultrafine ash PSDFs has always been observed. The first mode at 1-5 nm has been attributed to carbonaceous particles based on the UV-vis light absorption measurements and the results obtained burning a carbon black powder under the same operating conditions. The volume fractions of larger mode particles have been correlated to coal components. SEM-EDXS analyses have mostly supported the correlation indications. Results suggest that the particle size modes derive from size-selective nucleation of refractory oxides and metal nanoparticles and their subsequent growth. The oxygen concentration influences the size of nucleating particles and the preferential vaporization of some compounds with respect to others through both char-burning temperatures and the local reducing properties of the gas environment. Nevertheless, an enhanced oxygen concentration promotes ultrafine particle formation.