Preliminary evaluation of biofuel production potentials for Southern Africa's encroacher and invasive bush biomass

Abstract

The biofuel production potentials for encroacher and invasive bush biomass species found in Southern Africa were assessed using different valorization routes. Theoretical models were employed to calculate the biofuel yields. The gasification-catalytic route produced highest ethanol yields (450–488 L/t) while the lowest values were from enzymatic/acid hydrolysis-to-fermentation route. Blue gum gave the highest ethanol yields. Biodiesel and naphtha yields produced through Fischer-Tropsch synthesis were highest for blue gum (196 L/t) and lowest for Acacia raficiens (176 L/t). The highest biogas and biomethane potential of 458 L/kg.VS and 229 L/kg.VS respectively were obtained from black wattle while the respective lower values (270 L/kg.VS and 132 L/kg.VS) were recorded for blue gum. Senegalia mellifera gave the highest torrefied biofuel energy and mass yields at 0.92 and 0.97 respectively while black wattle had the lowest mass and energy yields at 0.75 and 0.83 respectively. From an energy yield basis, the acid hydrolysis-fermentation route yielded an average of 3.69 GJ/t of biomass while the highest yields came from the gasification-catalytic conversion route which was 9.7 GJ/t. The average energy yield variations across biomass species ranged 5.11–6.19 GJ/t which is around 30 % of the raw biomass' calorific value. These early results provide insights towards the best pairing of appropriate biomass species and energy conversion route. Further evaluations of these biomass-valorization technology pairing to unpack process efficiencies, cost and kinetics are required using real process experiments instead of using theoretical models. These additional tests should include sustainability assessment to guide future commercialization decisions.