Critical review of key variables affecting potential recycling applications of ash produced at large-scale biomass combustion plants

Abstract

Growing quantities of biomass ash are produced worldwide due to an increasing use of biomass for heat and electricity generation. Physical-chemical properties and elemental composition of ash at industrial biomass thermal power plants show significant variations. Experimental methods to characterize ashes are time-consuming and difficult to implement on a routine basis, posing severe limitations to the selection of most appropriate recycling options for these waste materials at the ash production stage. This review focused on relevant biomass ash properties that must be evaluated prior to environmental applications and recycling and on the identification of key process variables that determine such properties. A database characterizing 77 biomass ashes collected at large-scale industrial facilities was compiled. Ash type (bottom ash (BA) or fly ash (FA)) are distinguished by their pH and electrical conductivity ranges. Ash chemical composition was largely influenced by the type of biomass fuel burnt, and to a lesser extent by the technology and temperature of combustion. Ash from combustion of contaminated biomass, as well as the smallest fraction of FA particles (< 0.200 μm) show constraints for use in soils or construction materials due to high potentially toxic elements concentrations, soluble salts, and/or chloride contents posing both environmental and technical concerns. Both BA and FA generated by combustion of wood and biomass mixtures, regardless of the combustion technology, were found suitable for soil application as well as for incorporation in road or cementitious materials, provided that technical and risk assessment protocols to regulate their use are produced and standardized.