Potential reuse options for biomass combustion ash as affected by the persistent organic pollutants (POPs) content

Burning biomass as fuel results in ash containing persistent organic pollutants (POPs). Concentrations of three classes of POPs (polycyclic aromatic hydrocarbon, PAH; polychlorinated biphenyl, PCB; and polychlorinated dibenzodioxin/furan, PCDD/Fs) in biomass ash samples are collated from the literature. Data for bottom/total ash and fly ash from important biomass fuel sources (agricultural residues, wood, waste wood, paper sludge, sewage sludge and municipal solid wastes) are compared to proposed limits for reuse as fertiliser, controlled use in soil, or disposal without treatment. Ash POPs content is related to feedstock composition and ash fraction. PAHs, PCBs and PCCD/Fs are significantly more concentrated in fly ash compared to the corresponding bottom/total ash for each biomass type. Data availability for PCBs is lower than other POPs, however a strong correlation between PCBs and PCCD/Fs allows PCB+PCCD/F content to be estimated conservatively as 1.25x PCCD/F content. Typically, bottom/total ash from virgin biomass (e.g. wood and agricultural residues) is compliant with use as fertiliser whereas waste sourced bottom/total ash (e.g. waste wood, municipal solid waste) is more suitable for controlled use in construction. Higher POPs contents in fly ash restrict its use and occasionally PCDD/F contents must be destroyed before disposal.