Densification of agricultural wastes and forest residues: A review on influential parameters and treatments

Abstract

Biomass densification is an effective process to overcome specific biomass application limitations such as low density, nonuniform particle size and shape, and cost of transportation. Lignocellulosic materials (e.g. agricultural wastes and forest residues) are the main precursors used for pelletization. The quality of fuel pellets is determined based on their mechanical strength, hydrophobicity, heating value, and density. These properties are influential in handling, transportation, storage, and fuel applications of this product. There are several parameters affecting the quality of fuel pellets: precursor chemical structure, pelletization operating conditions, precursor pre-treatments, and pellet posttreatments. Formation of a strong binding structure in biomass pellet depends on the internal structure of precursors (e.g. lignin, cellulose, hemicellulose, extractives, moisture), particle size range of precursor, additives (e.g. binders, lubricants, plasticizers, and moisture), and pelletization operating conditions. Pre-treatments such as steam explosion and torrefaction can be used to facilitate the pelletization process or improve some precursor properties such as energy content or hydrophobicity. Post-treatments such as coating and torrefaction are applied to biomass pellets to improve their hydrophobicity or heating value. This chapter provides an overview of the parameters affecting the quality of biomass fuel pellets, biomass pre-treatments, and pellet post-treatments, as well as safety aspects related to transportation and storage of fuel pellets.