Thermal Behavior of Biomass Ashes with the Influence of Additives: A Critical Review

Alessandro Blasi

Abstract

The gasification of biomass is undoubtedly one of the most promising processes for sustainable energy and heat production. The formation of low-melting ashes can compromise the proper functioning of plants. To address potential risks of ash fusion and agglomerate formation in high-temperature zones that may occur in downdraft reactor configurations, a detailed literature review on the use of additives to counteract the risk of ash formation and deposition has been carried out. The reduction of ashes through chemical reaction proves to be the most effective methodology. Additives are grouped based on their reactive compound content, such as Al-based additives, Al-silicates, sulfur-based additives, calcium-based additives, and phosphorus-based additives. Additives with strong chemical adsorption and reaction capabilities can minimize sintering, deposition, and slagging of ashes during biomass gasification processes. The actual effectiveness of chemical reaction mechanisms is closely related to interactions among active components K-Al-Si, K-Ca-Si, and K-Ca-P. Additives' abilities to mitigate ash-related problems are strongly influenced by various parameters, such as mass/molar ratios between reactive components in additives and problematic elements in biomass ashes, as well as the reaction atmosphere and gasification technology. The bibliographic monitoring also highlighted the current trend towards using waste materials as additives, primarily based on carbonates, oxides, silicates, and aluminates.