Evaluating the potential of woody and nonwoody invasive alien plant species for bio-oil and biogas production.

Abstract

Biomass energy, converted into biofuels such as biogas, bio-oil, and biochar, offers clean alternatives for energy and transport. However, in South Africa, securing sustainable feedstock without impacting food security, water, and land remains a challenge. This study seeks to bridge this gap by assessing the potential of woody invasive alien plant species (IAPs) (Lantana Camara, Chromoleana Odorata, Solanum Mauritianum) and nonwoody IAPs (Acacia Mearnsii, Eucalyptus Grandis, Pinus Patula). The study comprised four experiments, beginning with the analysis of physical and chemical characteristics of IAPs, including proximate, structural composition, and higher heating value (HHV), to assess the suitability of IAPs for bio-oil and biogas production. The results revealed that woody invasive species had significantly higher levels of cellulose, hemicellulose, and lignin, measured at 47.5%, 22.6%, and 25.5%, respectively, compared to nonwoody IAPs, which exhibited lower concentrations of 23.6%, 15.9%, and 9.8% (p < 0.005). This section contributes towards the strategic selection of high-performing substrates, making bio-oil and biogas production cost-effective. Experiment two applied multi-criteria decision making (MCDM) methods to determine the most suitable IAPs for bio-oil and biogas production based on physical and chemical properties. Results showed that woody IAPs, particularly E. grandis, ranked highest for bio-oil with a relative closeness of 0.774, whereas nonwoody IAPs, particularly L. camara ranked highest for biogas with a relative closeness of 0.874. These results offer practical guidance for selecting effective feedstocks in renewable energy applications. Experiment three confirmed the MCDM results, showing that woody IAPs produced higher bio-oil yields (22.93 to 27.55%) than nonwoody IAPs (18.77 to 20.45%). However, the high content of oxygenated compounds in bio-oil reduced fuel quality, indicating the need for upgrading methods such as hydrodeoxygenation to enhance combustion efficiency. Experiment 4, which focused on biogas production using biochemical methane potential (BMP) tests, revealed that nonwoody IAPs produced significantly higher average methane yields (p < 0.005) at 162.62 Nml/g VS compared to woody species (117.69 Nml/g VS). This confirmed the suitability of nonwoody IAPs for biogas production as indicated by MCDM findings. This study supports energy efficiency and environmental sustainability in South Africa by repurposing IAPs as biofuel sources. However, the need for high capital investment underscores the importance of further research into the economic viability of large-scale production.