Use of biomass for energy production is more sustainable than the use of fossil fuels provided the biomass is a waste from another industry or grown sustainably. Solid biomass combustion accounts for ~90% of bioenergy supply, and the resultant combustion ash must be properly managed to promote a circular materials economy. This study aims to address four important issues for a better understanding and management of biomass ash. Firstly, an up-to-date estimate of global biomass ash production is made. It shows an ash generating rate of ~170 megatonnes per year (Mt/yr), which are dominated by municipal solid waste (MSW) ash and wood ash. Future ash production could increase to ~1000 Mt/yr with greater use of agricultural residues and wastes.
Secondly, a phylogenetic analysis is conducted for virgin biomass ash categorisation. The results show plant taxonomy strongly influences the ash chemical composition. Biomass feedstocks that are burnt commercially can usefully be categorised as hardwood, softwood, grass crop residues, and non-grass crop residues. The most abundant elements in hardwood ashes are calcium > potassium > phosphorous, whereas in the softwood ashes they are calcium > silicon > potassium. The most abundant elements in eudicot straw ashes are potassium > calcium > phosphorous > chlorine, and in grass straw ashes they are silicon > potassium > calcium. Differences in major element chemistry between the feedstock categories are visualised using a ternary plot of the normalised CaO, K2O and SiO2 contents. Other properties depend principally on whether the feedstock is herbaceous or woody. Herbaceous feedstocks produce significantly more ash (typically 5-9%) than woody feedstocks (typically 1-2%) and their ash has a significantly lower initial deformation (melting) temperature than ash from woody feedstocks, and thus has greater potential to form slag and foul the furnace.
Thirdly, ash properties of five laboratory wood ashes (one softwood ash and four hardwood ashes), three industrial wood ashes, two rice husk ashes and one straw ash are characterised to compare with the established ash categorisation system. The results
confirm biomass type and ash generation procedure have significant influence on ash major chemical composition and minerology. Ash from Alnus spp., Crataegus spp. and Salix spp. (three hardwoods) falls within the hardwood ash region in K2O-SiO2-CaO ternary diagram and Cedrus spp. (softwood) ash situates at the edge of softwood ash region. Fraxinus spp. (hardwood) ash falls out of the hardwood ash range, reflecting high K potential in Asterids derived biomass ash. Three industrial wood ashes fall within the softwood ash region and two rice husk ashes of high silica content agree with reports. Aarhus ash is the straw fly ash and has very high K content (nominal oxide K2O > 45%) in forms of sylvite, arcanite and kalicinite. All biomass ashes are alkaline in nature (water leachate pH at liquid to solid ratio of 100 ranging from ~9-12). Biomass ash beneficial reuse as a soil improver should consider trace element leaching behaviour as well as concentration limits as some trace elements of potential concern (e.g. Cr and Mo) in ash may exist in easily solubilised forms. Potassium in most ashes is recyclable (>90% extractable) by water leaching. Industrial wood ash contains more problematic trace elements than laboratory wood ash and thus controlling trace elements entrainment during biomass processing and ash production/initial storage may be helpful.
Fourthly, the effect of persistent organic pollutants (POPs) content on potential reuse options for biomass combustion ash is elucidated. Concentrations of three classes of POPs (polycyclic aromatic hydrocarbons, PAHs; polychlorinated biphenyls, PCBs; and
polychlorinated dibenzodioxins/furans, PCDD/Fs) in biomass bottom/total ash and fly ash from important biomass fuel sources (agricultural residues, wood, waste wood, paper sludge, sewage sludge and MSW) are collated and 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 PCBs + PCCD/Fs content to be estimated conservatively as 1.25 x PCCD/Fs 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 ash must be treated to destroy their PCDD/Fs contents before disposal.
Overall, biomass ash amounts are set to increase with increasing biomass use as biofuels for the global transition of energy production in a more sustainable way. Virgin biomass ash properties are influenced by plant taxonomy and thus biomass evolution-based categorisation can guide ash management usefully. Biomass ash management should consider the leaching behaviour as well as concentration limits of trace toxic elements and meanwhile consider the POPs content in ash (especially for waste biomass fly ash).
