Fate of bromine and chlorine in bubbling fluidized bed combustion - Formation of alkali halide aerosols

In solid substances used as fuels, bromine originates mainly from flame retarded plastics and textiles, as well as from wastewater sludge. It has been indicated that bromine behaves in a similar way as chlorine with respect to ash forming matter, aerosol formation and high temperature corrosion in biomass and waste fired boilers. In this work, a measurement campaign was carried out in a 20 kWth bubbling fluidized bed (BFB) bench scale reactor to characterize the fate of Cl and Br in BFB combustion conditions. Spruce bark was used as the base fuel - serving as a source of alkali metals (mainly K, little Na). The gases HCl and HBr were fed with the fluidization air at different proportions into the BFB unit. In total, six experiments were done. The fate of bromine and chlorine was evaluated by means of several measurements including: FTIR (flue gas measurement), a low-pressure impactor (size distribution and chemical composition of aerosol particles) and deposit probe sampling (deposit growth rate and chemical composition of the windward, 90° side and leeward deposits). It was shown that the addition of HBr or HCl greatly increased the release of alkali metals (K, Na) from the original fuel (spruce bark) during combustion. The promoting effect of HBr was more obvious than that of HCl. HBr was found more reactive towards alkali metals than HCl.