Pelletization of torrefied biomass with solid and liquid bio-additives

Abstract

Biomass commonly presents poor physical properties and low energy density that need to be enhanced. Torrefaction and densification are two of the most suitable techniques to this aim. In this work the pelletization of torrefied pine, blended with solid (raw pine and grape pomace) and liquid (glycerol) additives at low temperature (i.e., 40–80 °C) was carried out in a continuous bench-scale flat die pellet mill that simulates the features of a production-scale system. Among all the tested blends, four formulations fulfilled the requirements of the EN-Plus and ISO 17225-2 Industrial Pellets quality standards, which make them suitable fuels for both residential heating systems and power plant boilers. The produced pellets hold higher heating values between 18.8 and 21.7 MJ/kg, energy densities in the range 12.18–15.33 GJ/m3 and optimum physical properties (L = 20–25 mm, Ø = 6.1–6.2 mm, ρp = 1.1–1.2 g/cm3), in line with the reference pine pellets.A preliminary economic analysis was pursued proving that the improvements observed in formulations of torrefied pine, with 20 wt.% glycerol and with 10 wt.% grape pomace and 10 wt.% glycerol, may provide savings up to 20% in construction and transport costs.