Torrefaction performance of Macadamia husk under a flue gas atmosphere for solid biofuel applications

Torrefaction of Macadamia husk (MAH) under a proposed exhaust gas atmosphere (O2 of 5%, CO2 of 15%, and balance N2 v/v) was conducted. It was performed in a batch torrefactor at temperature of 200, 250, and 300 °C and duration of 30 min. Influence of torrefaction temperature on the physicochemical properties, energy yield (EY), and energy mass co-benefit index (EMCI) was investigated. It was found that conventional torrefaction had a higher mass and energy yield than flue gas torrefaction. The flue gas remarkably enriches the calorific value of MAH due to the larger fixed carbon and lower volatile matter. The torrefied husks have contact angles between 118 and 133°, which are larger than 90°, representing hydrophobic fuel. The optimal flue gas torrefaction was suggested to operate at 200 °C and 30 min to reach the highest EMCI. Utilization of flue gas in torrefaction can be a choice to use waste heat energy efficiently and reduce the cost of air separation.