Grate-Fired Biomass Combustion Plants Using Forest Residues as Fuel: Enrichment Factors for Components in the Fly Ash

The aim of this work is to investigate the enrichment factors for various nutrients and heavy metals in the fly ash from grate-fired combustion plants using forest residues as fuels. Sustainable energy production requires recycling of the ash on the soil to close the nutrient cycles. The coarser bottom ash which is discharged from the boiler usually contains lower amounts of heavy metals compared to the fly ash which is separated from the off-gas. The discharge of the finest fly ash to landfill sites serves to remove the unwanted heavy metals from the cycle. For this purpose, the enrichment of these components in the finest fly ash should be maximized. At the same time the enrichment of the nutrients in the fine fly ash should be minimal. The enrichment of the components in the electrostatic precipitator fly ash of three biomass combustion plants was determined. The enrichment of the critical heavy metals Cd, Pb and Zn in the fly ash was higher than the values reported in one study but less than values calculated from the data presented in another study. Further investigations would be required to clarify this deviation. Thereby additional data on the furnace operation conditions e.g. combustion temperature should be included. The enrichment of most nutrients (Ca, Mg and PO₄ ^3-) in the fly ash was low. For K the enrichment in the fly ash was higher resulting in notable K losses to the fly ash.