Volatilization of the heavy metals during circulating fluidized bed combustion of forest residue

The environmental impact of the heavy metals contained in the combustion product ash depends on the speciation of the heavy metals and the size distributions of the heavy metals in the ash. Therefore, the behavior of cadmium, lead, copper, and zinc was studied experimentally during circulating fluidized bed combustion (CFBC) of Swedish forest residue. The size distributions and concentrations of the heavy metals in the fly ash particles and in the gas phase were determined by low-pressure impactors and filters upstream of the convective back pass at 830 °C. Downstream of the convective back pass at T = 150 °C, the size distributions were determined. The fly ash from CFBC was found to contain two separate particle classes. Fine particles (D_p < 0.5 μm) consisted mainly of KCl, and coarse particles (D_p > 0.5 μm) contained as major elements Ca and Si. Major fraction of all the studied heavy metals were found in the coarse fly ash particles at location 1 at 830 °C; 7−26% of Pb, 24−27% of Cu, 1−8% of Cd, and less than 1% of Zn were found in the gas phase. The gas-to-particle conversion route for Cd, Pb, and Cu was found by chemical surface reaction, probably with silicates. None of the studied heavy metals were enriched in the fine particles at the inlet of the electrostatic precipitator.