Physical properties of aerosol particles measured from a bubbling fluidized bed boiler

Increased use of biomass and waste fuels, and the consequent corrosion problem have led to an increased need to study and monitor the combustion processes. This study presents an extensive physical characterization of aerosol particles measured from a bubbling fluidized bed boiler with different fuel mixtures and optional ferric sulfate feeding. The fuel mixtures included bark, sludge, peat and solid recovered fuel. Previously, the characterization of the particles analyzed from a fluidized bed reactor has mainly focused on chemical off-line analysis of collected impactor samples, large coarse mode particles or laboratory-scale reactors. In this study, the focus is in the particle size range from 3 to 500 nm, where mobility size distributions, effective density, morphology and electric net charge of particles were measured and analyzed. In the boiler, the particle size distribution in the measurement range was unimodal. Gas phase species formed a second smaller particle mode in the dilution. The number concentration of the smaller mode, peaking around 20 nm, was mostly dominating but variations were seen with respect to measurement location, fuel mixture and additive feeding. The effective density of these particles was approximately 1.4 g/cm3. The larger mode, peaking around 80 nm, was found to be more stable and the effective density of these particles decreased as a function of particle size, being 3-4 g/cm3 at the maximum. The results of this work suggest that the cores of these particles already exist in the boiler and partly consist of heavier lead and zinc compounds. The ferric sulfate feeding decreased the number and mass concentration of the smaller mode particles, which are formed in the sampling and dilution processes mainly from the gas phase alkali chlorides. These condensable species were also linked to the negative net charge of particles. This study deepens the understanding of the combustion process and the sampling of aerosol particles with an aspect of on-line monitoring.