2000

Mikkanen, Pirita

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. In addition to the experimental studies, the particle formation was simulated with an Aerosol Behaviour in Combustion (ABC) model, which includes models for gas-to-particle conversion and for deposition. The results indicate that seed particle formation is involved in the fume particle formation. Particle growth occurs primarily in the furnace at temperatures greater than 800°C where the particles grow by simultaneous condensation and coagulation. Below the melting temperature of the particles, coalescence of the particles becomes slower than the particle collisions, due to increasing viscosity of the particles. Subsequently, the particles are not completely coalesced spheres but form agglomerated structures. The mean fume particle aerodynamic diameter measured at the furnace was 1.1 µm whereas it was almost 2 µm at the boiler exit. This growth is primarily due to agglomeration. The results showed two sources of coarse particles. Coarse particles formed in the furnace were efficiently depositing on the heat exchangers. The other source, which turned out to be significant as well, is the re-entrainment of particle aggregates from the heat exchangers.