Halide aerosols in circulating fluidised bed co-combustion. Role of coal bound kaolin

An experimental campaign was carried out with a circulating fluidised bed (CFB) pilot scale combustor to study the role of coal bound kaolin in the fate of solid recovered fuel (SRF) originated halide aerosols. A combustion experiment was carried out with SRF-Spruce Bark mixture as a reference. High kaolinite coal and paper pigment kaolin, one at a time, were mixed with the SRF-Bark in increasing proportions until dp <1 mu m fine particles were absent as measured from 780 degrees C combustion gases by means of a dilution probe and low pressure impactor (LPI). This fine particle mode was absent after mixing sufficiently either coal or kaolin with SRF-Bark and only traces of water soluble alkali metal salts were found in the CFB fly ash. These conditions were achieved when kaolin was mixed with the SRF-Bark for 52 times on a molar basis compared to the Na + K initially found in the aerosols. This proportioning was found to be the same for the additive kaolin and coal bound kaolinite. Na and K in the fly ash seem to be bound chemically to the kaolin as alkali aluminosilicates rather than in water soluble alkali sulphates. This is indicated by their solubility behaviour.