Better use of bioenergy: A critical review of co-pelletizing for biofuel manufacturing

Better and more efficient use of biomass energy is seen as a favourable option for reducing carbon emissions. Biomass pellet fuel is one of the most common and important ways of utilising biomass energy. The use of diverse biomass resources with different properties for pelletizing is a current research hotspot. This paper provides an innovative overview of co-pelletization technology for biomass pellet fuels. The chemical composition of various types of biomass is classified and the influence of different biomass matrix feedstocks and pelletizing operating parameters on pellet quality and pelletizing process is summarized. The research results indicate that the natural chemical fraction of biomass has a significant influence on the pelletizing process and fuel quality. Furthermore, co-pelletizing can be divided into biomass-biomass co-pelletizing and biomass-non-biomass co-pelletizing. Common co-pelleting combinations include: woody and herbaceous crop co-pelleting; woody or herbaceous crops with high starch and oil content (e.g. microalgae, peanut shells, etc.); mixed pelleting of different parts of the same crop (e.g. bark and leaves of the same quality); co-pelleting of biomass with solid waste (e.g. municipal sludge, waste paper, etc.). Crop types such as oil and sugar used for co-pelletizing with herbaceous and woody crops help to reduce energy consumption in the preparation of biomass pellet fuel and improve the quality of the pellets. Co-pelletization of non-biomass-based materials (domestic production waste, inorganic additives) with biomass also facilitates the pelletizing process and improves the quality of pellet fuel. It is obvious that mixing different raw materials for co-pelletizing is considered a promising way for upgrading biomass pellet and that economical, environmentally friendly and sustainable raw materials are prerequisites for co-pelletizing. Moreover, co-pelletizing seems to be a strong option for enhancing the competitiveness of biomass pellet fuel production at scale in the future.