The use of biomass materials in energy generation is increasing. Biomass can be derived from a variety of materials, such as virgin wood, treated wood and agricultural residues. Combustion of these fuels generates a residual ash which may be hazardous or non-hazardous based on its chemical composition.
Unlike municipal waste incinerator bottom ash (IBA) (http://www.wrcplc.co.uk/ps_ash-factory) there is no significant body of research providing insight into the geochemistry of biomass/wood waste ash. Knowledge of the metal compounds present is required to inform a hazardous property assessment, as required by the joint agencies Technical Guidance WM3 (http://bit.ly/1SQ2SL6), which includes assessment of hazardous property HP14 (ecotoxicity).
Some treated waste wood fuels may contain elevated levels of potentially toxic elements that are carried through to the ash, but not all metal compounds are ecotoxic. As an alternative to time consuming and costly geochemical modelling, WRc has developed extraction tests to identify concentrations of ecotoxic and non-ecotoxic portions of compounds present in the ash.
Thermodynamic modelling of data from selective extraction processes can provide the evidence to enable a non-hazardous classification of certain biomass ash streams (http://www.wrcplc.co.uk/ps-zinc-speciation). Non-ecotoxic compounds have been shown to be present in municipal waste ash and this extraction methodology is verifying previous geochemical modelling findings.