Exploring the Torrefaction Potential of Acacia mangium Leaves for Biocoal Production: An Oxidative Thermogravimetric Analysis Study

Acacia mangium is a flowering tree species native to Papua New Guinea and Australia. It is widely used in agroforestry, forestry, and reforestation due to its fast-growing and adaptable nature. Due to its semi-evergreen or semi-deciduous nature, A. mangium has a high leaf litter rate of 8.8–10.5 tonnes/hectare/annum. However, its high leaf litter presents potential biomass for conversion into biocoal and other solid fuels through torrefaction. The torrefaction of A. mangium leaves (AML) as previously reported yielded good biocoal properties. However, using ultra-pure nitrogen gas for torrefaction is expensive, impractical, and unsustainable, particularly in industrial biocoal production. Therefore, this study examines the potential of oxidative (in air) torrefaction of AML through thermogravimetric analysis from 200–300 °C (ΔT = 25 °C) at 20 °C/min for 30 minutes. The results revealed that mass losses (M_L) increased from 15.63 to 62.98%, whereas mass yields (M_Y) decreased from 84.37 to 37.02%. The higher heating value (HHV) increased from 21.31 to 25.73 MJ/kg, and energy density (D_E) from 1.01 to 1.22, whereas energy yield (E_Y) decreased from 85.17 to 45.12%. The variations in M_L and M_Y may be due to the impact of temperature on the thermo-chemical degradation of AML’s lignocellulosic components. In contrast, the high HHV and D_E values may be due to carbon deposition and oxygen depletion. In comparison, the oxidative process yielded higher HHV (25.73 MJ/kg) and D_E (1.22) at 300 °C, when compared to the non-oxidative process (HHV = 24.15 MJ/kg; D_E = 1.14) reported in the literature for AML. Overall, the study findings reveal that AML torrefaction is a practical approach for valorising streams of agroforestry wastes into biocoal—a solid biofuel with fuel characteristics comparable to lignite and sub-bituminous coals.