Torrefaction of densified biomass using flue gases in a fixed bed combustor

Abstract

Torrefaction is a low-temperature thermochemical treatment technology to enhance solid biomass fuel properties for energy applications. The available literature on torrefaction thermal pre-treatment presented the extent of improvement in fuel quality at various operating conditions however with non-practical industrial approach using energy intensive heating methods i.e. external heaters and with simulated gas composition. This work investigates the utilisation of real-time low-grade combustion waste heat and flue gases to produce torrefied biomass pellets at 225 °C and 250 °C in a fixed-bed reactor. The fuel quality, hydrophobicity, chemical functionality inorganics contents, and combustion kinetics of the flue gas torrefied (FGT) biomass fuel were thoroughly analysed and compared with the torrefied pellets produced under partially oxidative conditions (at 250 °C) using electrical heaters. The results suggest that flue gas torrefaction improves the heating value of the raw biomass by up to 11 %. The hydrophobicity (water resistivity) of the FGT biomass was also improved and comparable with the fuels derived from electrically heated partial oxidative torrefaction (POT). The presence of carbon dioxide in the flue gases did not adversely affect the quality of the FGT biomass fuel. In fact, FGT biomass attained relatively higher i.e. 92.1 % energy yield compared to 87.9 % of POT biomass. In addition, low concentrations of alkali and alkaline earth metals (AAEM) were observed in FGT biomass compared to POT biomass fuel. The outcome of this work suggests that torrefaction using combustion waste heat and flue gases is a viable and cost-effective thermal treatment method to upgrade biomass fuel quality.