Bioenergy production from invasive plants in the Lower Mekong Basin using hydrothermal liquefaction and anaerobic digestion

Invasive plants in the Lower Mekong Basin, although ecologically challenging, offer significant potential for bioenergy. This study assessed mimosa (Mimosa pigra), giant salvinia (Salvinia molesta), and water lettuce (Pistia stratiotes), collected from Nakhon Phanom province, Thailand, as feedstocks for hydrothermal liquefaction (HTL) and anaerobic digestion (AD). The biomass underwent HTL treatment at 160–200 °C and was subsequently evaluated for biomethane potential (BMP). The HTL of mimosa at 200 °C for 3 h (M-200-3) yielded 16.2 wt% bio-oil (BO) and 6.98 wt% hydrochar (HC). The BO from M-200-3 possessed a higher heating value (HHV) of 31.4 MJ/kg and energy recovery efficiency (ERE) of 26.6%. Giant salvinia BO also performed effectively, with an HHV of 26.7 MJ/kg and an ERE of 23.2% at 4 h. In contrast, water lettuce showed limited HTL effectiveness, with BO yields below 10 wt%. Characterization through GC–MS revealed that phenolic compounds dominated the BO composition, while FTIR confirmed the presence of ketones, aldehydes, carboxylic acids, and aromatic rings. TGA analysis demonstrated that BO thermal degradation occurred between 261 and 314 °C, with low-boiling-point components enhancing BO performance. Additionally, SEM images revealed the formation of carbon spheres in HC at 200 °C. Energy production calculations indicated mimosa could generate 49,418 kWh of BO and 19,273 kWh of HC per hectare of feedstock (kWh/ha) under maximum annual biomass conditions. BO and HC from giant salvinia could produce 5,023 and 986 kWh/ha, respectively. Notably, water lettuce excelled in anaerobic digestion, achieving the highest BMP of 0.238 ± 0.022 NL CH4/gVS and yielding 5,517 kWh/ha. These findings establish that invasive plant species can serve as viable bioenergy feedstocks, with mimosa optimal for HTL processing and water lettuce demonstrating superior performance in biogas production.