Environmental performance of different end-of-life alternatives of wood fly ash by a consequential perspective

Abstract

The increasing demand for bioenergy has intensified the pressure to utilise forest residues. Consequently, wood ash production has increased, which has prompted the need to find environmentally friendly alternatives for its end-of-life processes. However, studies assessing the environmental impacts of wood ash valorisation have assumed a static market while disregarding the potential environmental consequences and economic causalities of producing new products from wood ash. Therefore, this study assesses the outcomes of changing the conventional end-of-life of wood fly ash from its disposal in sanitary landfills to valorisation alternatives through its incorporation in construction materials (mortar and concrete) or soil amelioration. First, a consequential life cycle assessment was applied to identify the most environmentally sustainable choices for two types of wood fly ash (namely, those derived grate furnaces and fluidised bed furnaces). The characterisation factors used in this study are those that have been suggested for conducting a Product Environmental Footprint (PEF). The results show that mortar production is the best alternative for fly ashes generated in grate furnaces. Meanwhile, concrete and mortar production are the best alternatives for fly ashes generated in fluidised bed furnaces, concrete production has a slight advantage (smaller than 1%) over mortar production. Valorisation is less beneficial for soil amelioration than the incorporation in construction materials, and the impacts are higher than those of ash landfilling in some impact categories (human toxicity, freshwater eutrophication and ecotoxicity) due to the emission of trace elements into soil. A sensitivity analysis was carried out by changing the market trend of the conventional materials displaced from stable to strongly declining. This analysis showed that market trends could affect the net environmental performance of the investigated alternatives and modify their rankings.