A review of biomass ash properties towards treatment and recycling

The generation of biomass ash (BA) is expected to increase in the future because biomass is generally recognized as carbon neutral fuel. Since BA is known, in many cases, to contain hazardous concentrations of trace elements, this will simultaneously produce more potentially hazardous ash. Soon this will need to be handled amidst stricter waste policies and a societal evolution towards a circular economy. In many cases, to allow recycling of BA, trace elements need to be removed for the protection of health and the environment. A better understanding of trace element origins in BA, and knowledge of which trace elements are most critical to advantageous recycling schemes are also needed. In this work available BA data were reviewed and processed for study by multivariate statistical analyses. This allowed for reorganization of the complex nature of BA data into simpler forms for interpretation. An established connection between peat fuels and As was thoroughly reinforced. Wood co-fired with peat would produce BA most advantageous for any recycling, while other biomass for forest recycling and wood, bark and wood waste split between forest recycling and needing treatment or disposal. Some trace elements were still an obstacle to the recycling schemes presented, therefore current state of the art ash treatments for targeting individual trace elements or for total treatment of ash were discussed. Additionally, treatment methods for ash were reviewed because their options are varied, and the goal of utilization will be achieved through matching treatment methods and recycling targets. In the future, the method of using legislative limit values and multivariate analyses to determine BA recycling routes could be replicated for other national limit values and other wastes.