Fly ash particle formation in the recovery boilers has been studied experimentally in five industrial scale recovery boilers operating in Finland. The formation and growth mechanisms of the particles were studied by measuring particle characteristics including number, mass and composition size distributions in the gas phase at the recovery furnace exit, at superheater area and at boiler exit. As part of the thesis work, measurement methods were developed for recovery boiler conditions. Electron microscopy was utilised to study the particle morphology and in the case of the coarse particle study, to determine the compositions of various particle types. During this study, a method for the direct determination of chemical compounds in the particles was applied along with the traditional chemical analyses methods. In addition to the experimental studies, the particle formation was simulated with an Aerosol Behaviour in Combustion (ABC) model, which includes models for gas-toparticle conversion and for deposition. The particle size distribution was found to be trimodal at the furnace and bimodal at the boiler exit. Unlike in other boilers burning solid or sludge fuels, most of the particle mass appeared in the particle size fraction smaller than 5 μm, i.e. in fume particles. The average size of the fume particles remained constant at almost 2 μm in aerodynamic mass mean diameter in the convective sections of all five boilers. However, the mass concentration increased while the boiler heat load increased. The coarse particle concentration was dependent on the boiler operation and on sootblowing. Carryover particles were detected only in the furnace. The results indicate that seed particle formation is involved in the fume particle formation. In the measured case, the seed forming elements Fe and Mn, showed clearly bimodal composition size distribution suggesting partial vaporisation of these elements. Particle growth occurs primarily in the furnace at temperatures greater than 800°C where the alkali hydroxide vapours react with SO2 or CO2 and form condensed species and simultaneously the particles grow by
Combustion aerosol measurement methods were introduced and applied for extensive ash formation studies at four operating recovery boilers in Finland. Ash particle mass size distributions determined with a Berner-type low-pressure impactor downstream the heat exchangers were clearly bimodal with the fine mode at about 2 μm and the coarse mode above 3 μm aerodynamic diameter. According to SEM images, fine ash mode consists of individual, almost spherical 0.3−0.7 μm alkali salt particles and of agglomerates with few primary particles of similar diameter and shape. The degree of fine mode primary particle sintering increased when increasing boiler heat load. Coarse mode includes large agglomerates with up to thousands of 0.3−0.7 μm alkali salt primary particles and spherical silica particles. Ash particle main component was sodium sulfate as determined with X-ray diffraction. Sodium-to-sulfur molar ratio of ash particles calculated on the analyses results with an ion chromatography decreased from the upper furnace sampling point to electrostatic precipitator inlet conditions, indicating sulfation of ash particles within the heat exchanger section. Chlorine in ash was bound as sodium chloride, no potassium chloride was detected with X-ray absorption fine structure spectroscopy. Furnace measurements showed that fine mode ash particles are formed already in the furnace via vapor condensation. The extents of release of 12% for Na, 24% for S, and 48% for Cl were determined on the basis of ion concentrations in fine particles and the mass balance calculation on the recovery boiler. Coarse particles observed downstream the heat exchangers are proposed to form mainly via entrainment of large agglomerates of fine ash particles deposited on the heat exchangers. The fine mode particle size was insensitive to the furnace conditions although the particle concentration increased when the furnace gas temperature increased. This and the increase of Na/S molar ratio in the particles indicates that Na volatilization increases with the increasing furnace temperature, whereas S release is less sensitive to the temperature increase.
