Alkali salt ash formation in four Finnish industrial recovery boilers

Combustion aerosol measurement methods were introduced and applied for exte nsive ash formation studies at four operating recovery boilers in Finland. Ash particle mass size distributions determined with a Berner-type low-pres sure impactor downstream the heat exchangers were clearly bimodal with the fine mode at about 2 mu m and the coarse mode above 3 mu m aerodynamic diam eter. According to SEM images, fine ash mode consists of individual, almost spherical 0.3-0.7 mu m alkali salt particles and of agglomerates with few primary particles of similar diameter and shape. The degree of fine mode pr imary particle sintering increased when increasing boiler heat load. Coarse mode includes large agglomerates with up to thousands of 0.3-0.7 mu m alka li 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 w ith an ion chromatography decreased from the upper furnace sampling point t o electrostatic precipitator inlet conditions, indicating sulfation of ash particles within the heat exchanger section. Chlorine in ash was bound as s odium chloride, no potassium chloride was detected with X-ray absorption fi ne structure spectroscopy. Furnace measurements showed that fine mode ash p articles are formed already in the furnace via vapor condensation. The exte nts 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 calc ulation on the recovery boiler. Coarse particles observed downstream the he at exchangers are proposed to form mainly via entrainment of large agglomer ates of fine ash particles deposited on the heat exchangers. The fine mode particle size was insensitive to the furnace conditions although the partic le concentration increased when the furnace gas temperature increased. This and the increase of Na/S molar ratio in the particles indicates that Na vo latilization increases with the increasing furnace temperature, whereas S r elease is less sensitive to the temperature increase.