Renewable biofuel production from biomass: a review for biomass pelletization, characterization, and thermal conversion techniques

The exhaustion of fossil fuel resources has instigated a necessity to find new alternatives like biofuels, for heat and power generation. Biofuels are usually generated from the thermal conversion of densified biomass material. Densification systems convert biomass into pellets, and consist of three phases: pre-pelletization, pelletization, and post- pelletization. This article provides an overview of available biomass densification techniques. A detailed discussion has been provided to emphasize the effect of raw material properties on the pellet’s durability and bulk density. A quality parameter (Q) has been proposed to evaluate the quality of pellets considering the factors involved in pellet characterization. Particularly, accounting for pellet compression rather than tensile forces were found to be better when quantifying the pellet quality parameter Q. A discussion regarding the binding mechanisms, types of binders used, and their effect on the pellet’s durability is provided, in addition to the pelletization process by itself with the main parameters that affect its operation. The post-pelletization processes were presented, focusing on three thermal conversion techniques: gasification, pyrolysis, and combustion. A comparison between these techniques has been provided, in addition to recommendations regarding pros and cons of each one. Finally, the environmental footprints of densification systems have been reviewed.