Ultrafine Particles Formed by Heating Droplets of Simulated Ash Containing Metals

Abstract

Coal and biomass combustion and waste incineration produce inorganic aerosol, whose ultrafine fraction is hypothesized to be generated mainly by metal vaporization and subsequent nucleation and growth in a high-temperature atmosphere. To investigate this mechanism, we investigated nanoparticle inception and early growth processes at high temperature by spraying monodisperse droplets of simulated ash containing metals in the centerline of a laboratory ethylene/air laminar premixed flame at stoichiometric conditions. Particles were collected by thermophoresis on mica substrates for atomic force microscopy dimensional and morphological analysis. Chemical nature, volume fraction, and mean diameter of particles were evaluated by in situ UV-Vis broadband light absorption and laser light-scattering measurements. We also measured size distribution of material sampled from the flame without dilution using differential mobility analysis and scanning electron microscopy. Energy dispersive X-ray spectroscopy provided further information on the chemical composition of particles deposited on substrates. Size distributions showed that the flame-generated aerosol have a high number concentration, are mostly smaller than 10 nm, and persist far downstream of the droplet injection point. Preliminary results showed that the coagulation rate of those nanoparticles is much slower than their collision rate, which implies that their lifetime may be long and may represent a severe problem for environmental pollution.