New insights into biomass combustion ash categorisation: a phylogenetic analysis

Combustion of biomass within power stations is a more sustainable way to generate electricity than the use of fossil fuels provided the feedstock is sustainably grown. The physical and chemical properties of biomass combustion ash from different feedstocks were statistically analysed using 168 published database records. Plant taxonomy of the feedstock has a strong influence on the major element composition of the ash, and 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 > chlorine > phosphorous, and in grass straw ashes they are calcium > potassium > calcium > chlorine. Differences in major element chemistry between the feedstock categories are visualised using a ternary plot of the normalised calcium oxide, dipotassium oxide, silicon dioxide 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. These findings allow for the impact on ash characteristics to be include in commercial and operational decisions about power station feedstock.