Biomass waste contains considerable amounts of ash-forming elements that adversely affect boiler operation and consequently reduce economic efficiency. Moreover

such waste may contain heavy metals

requiring thorough assessment prior to use. In this study

the mode of association of ash-forming matter in various categories of biomass waste was determined using a chemical fractionation method. The results obtained were subsequently verified through experimental testing in a bubbling fluidized bed pilot plant. Laboratory results showed that K and Cl exhibit similar modes of association across the different biomass types examined

whereas notable differences were identified for major ash-forming elements such as Na

Ca

S

and P. Based on the differing modes of occurrence of these elements

wheat straw presented the highest risk of hydroxide or carbonate release into the flue gas phase during combustion

followed by eucalyptus wood (EW). This indicates a greater risk of fouling and corrosion compared with the other biomasses studied. Analyses of ash samples from fluidized bed combustion experiments were consistent with the chemical fractionation results

confirming the reliability of the method as a useful tool for predicting the ash-related behavior of new fuels. In the final part of the study

heavy metals in biomass combustion tests were assessed. A significant content of certain commercially valuable metals was found in fly ash (FA) from different biomass wastes

indicating the potential for recovery and contributing to the promotion of a circular economy.