Sustainable energy production based on the combustion of woody biomass requires the recycling of the ash into the soil to close the nutrient cycles. The removal of the finest ash fraction separated in a second-stage de-dusting unit from the ash cycle provides a solution for eliminating volatile heavy metals. For this purpose the enrichment of these components in the finest fly ash should be maximized without incurring enrichment of the nutrients, such as Ca, Mg, K, and PO4 3–. A literature search about the enrichment of inorganic components in the fly ashes from grate-fired combustion of woody biomass revealed six sources containing enrichment factors, respectively, the concentration data required for the calculation of such factors. Extremely high variation of the enrichment factors for various elements was found. Most results from the analysis of the ashes from a district heating plant using forest residues as fuel were in the range of the published data. However, the enrichment of Cd and Zn in the finest fly ash even exceeded the published values. In view of the wide variation in the available enrichment factors, further investigations are required to optimize the enrichment of heavy metals in the second-stage fly ash. In these investigations more attention should be paid to the operation data of the combustion process, e.g., the combustion temperature and the off-gas oxygen content.
- Fly ash
- Grate-fired biomass combustion
- Heavy metals