Prospect for recycling critical elements in combustion residues of coal, lignite, and biomass feedstocks

The combustion residues derived from coal, lignite, biomass, and spent wash could be recycled to extract critical elements required for energy transition. To recycle these elements from the combustion residues, it is necessary to understand their chemical mode of occurrence in the ash. This study presents the content of critical elements and their chemical mode of occurrence in coal, lignite, biomass, and incinerator ash. Lignite ash, rich in Ca and Si, offers Sc (31 mg/kg) and Nd (212 mg/kg), while coal ash, dominated by Si and Al, contains Ga (44.8 mg/kg). Biomass and spent wash ash, characterized by K, Ca, and S, present substantial potential for potash and Sc. Lignite ash primarily contains rare earth elements (REEs) in metal oxide-bound fractions, whereas in coal ash, the REEs are associated with the hard mullite or quartz phase. Biomass and incinerator ashes have significant water-soluble potash, and the Sc is associated with metal oxides. Green acids can extract critical elements from lignite, biomass, and incinerator ashes, but extracting from coal ash requires harsh conditions. Future research should concentrate on green extraction processes considering the chemical patterns of occurrence of critical elements.