Role of alkali chloride on formation of ultrafine particulate matter during combustion of typical food waste

Particulate matter (PM) emission from municipal solid waste (MSW) incineration poses a threat to both the environment and human health. The current understanding of PM from MSW incineration is limited to its emission characteristics, while the formation mechanism in flame is far from clear. In this work, a flat-flame burner and a thermophoretic sampling system were used to investigate the formation and evolution of nanoparticles during the combustion of typical food waste. The diameter, morphology, and chemical composition of particles sampled at different burning stages and positions were analyzed. We found that the formation of nanoparticles is governed by the sooting process and the gas-to-solid conversion of mineral vapors in the early burning stage, while in the late stage, cubic nanoparticles are observed on the surface of the fuel pellet and are carried into flame by the gas flow, leading to a remarkable increase in the size of nanoparticles sampled at different heights. Based on the element mapping, the EDS spectra, and a time scale analysis, we prove that the main component of these cubic particles is KCl. Moreover, adding NaCl with the mass ratio 1.3% similar to 5.1% into the fuel pellet can promote the formation of cubic KCl particles and increase the particle size at all sampling points, while further increasing the NaCl mass ratio to 8.8% leads to the formation of low-temperature KCl/NaCl eutectic and reduces the size of nanoparticles. The gas-carrying of alkali chloride particles reported in the current work helps to form a more complete picture of PM formation from the combustion of solid fuels.