Estimation of trace element release and accumulation in the sand bed during bubbling fluidised bed co-combustion of biomass, peat, and refuse-derived fuels

The experimental study and thermodynamic equilibrium calculations were performed to investigate the interactions of fuel-based lead, copper, zinc, and manganese with the bed material of the bubbling fluidised bed boiler (BFB) during the co-combustion of sawdust, peat, and refuse-derived fuel. Flue gas trace element measurements, a chemical analysis of process streams, and mass balance calculations over the boiler reveal that the bed material captures substantial amounts of Pb, Cu, Zn and Mn, but also that these metals are released from the bed when the fuel characteristics or the process parameters are changed. The study shows that the trace metal emissions of a BFB boiler are not necessarily directly related to metal concentrations of the fuel but are rather a result of a complex process combining the release of trace metals from the fuel, the interaction between the fuel and fuel ash particles within the bed material, and the release of trace metals from the bed. The EDS analysis of the bed material particles shows that the original sand particles are covered with a Ca- and K- rich layer. In combustion temperatures, the layer is assumably in a melt form and has an important role in the trace metal accumulation/release process. The thermodynamic multi-phase multi-component equilibrium calculation for zinc suggests that the release of zinc from the bed material is strongly increased with an increasing Cl-concentration of the fuel, due to the conversion of the bed-bound zinc to ZnCl₂.