Analysis of inorganic compounds in submicronic and ultrafine fractions of wood combustion dust above the flame zone
Résumé
Dust-related problems such as deposit formation, corrosion and particulate emissions are of great relevance regarding wood combustion facilities. Investigations focusing on the physico-chemical composition and formation of fly ashes, especially fine particles PM2.5 (<2.5 μm), are thus of particular interest for the conception, design and operation of wood biomass boilers. Among the chemical constituents of wood-combustion dust, inorganic species may be largely released (Frandsen et al., 2007). The aim of this work was therefore to study the submicronic (PM0.1-1, corresponding to the accumulation mode) and ultrafine (PM0.1, encompassing the nucleation mode) fractions of dust emitted above the flame zone during wood combustion, especially the size distribution and chemical composition of inorganic species, particularly water-soluble ions. Combustion experiments were conducted on a laboratory-scale wood combustion tube reactor. The influence of the fuel size distribution and moisture on the fine particle formation and composition was studied for two reactor configurations: fluidized bed and grate-fired. For each combustion run, high temperature (480-540 °C) size-segregated fine particle samplings were carried out above the flame zone by cascade impaction with subsequent off-line chemical particle analyses by ion chromatography and ICP-AES. Potassium, sulphate and chloride account for more than 95% of the total mass of detected ions, mainly as K2SO4 and KCl. The potassium mass proportion in the PM0.1 fraction varies from 35% to 73%, depending on runs.