From raw material powder to solid fuel pellet: A state-of-the-art review of biomass densification

Densification is the most effective forming technology, the production of which, pellets are the most intensive energy content of biomass solid fuels. Increasing the overall utilization efficiency of bioenergy, extending available types of wastes and residues for pelleting, and improving the pellet quality comprehensively are necessary. This paper reviewed feedstocks used for pelleting, binding mechanism of biomass pellets, and evaluation systems of pellet qualities. To extend the supply amounts and improve the qualities of raw biomass, various feedstocks and formulation recipes were classified based on their regions, sources, and species. And applicable objects and conditions of several different pretreatment technologies for feedstocks were compared. Physical properties and biochemical analyses are also concluded to evaluate the flowability, compactibility, and potential characteristics as solid fuel. For a better understanding of energy consumption and conversion during the densification process, pelleting devices and binding mechanism are elaborated. Associations and differences between single pelleting press in lab-scale and industrial pelleting in pilot-scale are also discussed. Suggestions are proposed for the optimization design of pelleting devices and processes for different feedstock densification. Lastly, relationships between operating parameter factors of pelleting process and pellet quality are presented. This review could provide theoretical analyses for powder compression, new insights for waste to energy, and guidance for achieving global peak carbon and environmental strategic objectives. [Display omitted] •Biomass feedstocks and recipes used for pelleting are reviewed.•Pelleting process is explained from theoretical and experimental aspects.•Durability and water sorption of biomass pellets are discussed.•Suggestions are given for optimizing pelleting process and mechanical device design.•New methods and concepts in industrial pelleting are presented.