One-step torrefaction and densification of woody and herbaceous biomass feedstocks

Abstract

The investigation evaluates the potential of one-step torrefaction and pelleting of various woody and herbaceous feedstocks for the production of high-quality solid biofuels. The feedstocks were first subjected to a comprehensive analysis of chemical, thermal, physical and fuel properties. The samples were size reduced and densified at different temperatures (20–250 °C) and pressure (31–184 MPa). The resulting pellets were characterized for durability, real and bulk density, and lower heating value (LHV) and the results were then compared against quality standards for woody and non-woody pellets according to ISO 17225. The results showed clear responses to temperature and pressure in the ranges 20–80 °C and 31–92 MPa. Some of the feedstock (Jerusalem artichoke stalks, rock rose, scotch broom and vine sprouts) produced pellets of physical properties comparable or superior to pine wood, a feedstock that was used as a reference due to its potential to produce high quality densified fuels. Others (triticale and cynara) produced less satisfactory results and/or required very severe conditions to produce pellets of characteristics comparable to pine wood. However, the presence of specific components in the alternative feedstocks (mainly ash, Cl, K, N) resulted in pellets not complying with ISO quality standards.