Sustainable biodiesel production and properties enhancement from locally sourced Roselle (Hibiscus sabdariffa L.) as an alternative biofuel feedstock in Thailand

This study investigates the potential of biodiesel production from roselle seed oil (Hibiscus sabdariffa L.) as a sustainable energy source. The oil extraction, performed via Soxhlet extraction with hexane, yielded a higher oil content of 21.10 ± 0.53 % w/w, compared to the cold pressing method (8.94 ± 0.10 % w/w). Notably, the cloud point and pour point were both below 0 °C, effectively reducing wax formation in cold conditions. Chemical composition analysis revealed that roselle seed oil contains a high level of unsaturated fatty acids (79.65 %), predominantly cis-oleic acid (C_18:1) and cis-linoleic acid (C_18:2). Structural confirmations were obtained through ¹H NMR, ¹³C NMR and FT-IR analyses, validating the fatty acid structure and functional groups in the oil. In terms of biodiesel production, the reaction rate constant for roselle seed oil was determined to be 6.27 × 10⁻¹ h⁻¹, comparable to other vegetable oils. Blending the biodiesel with B7 diesel at 20 % ratio significantly enhanced fuel properties: kinematic viscosity decreased from 4.50 ± 0.05 cSt to 3.85 ± 0.03 cSt, density decreased from 882 ± 4 kg/m³ to 864 ± 4 kg/m³, and acid value dropped from 0.43 ± 0.021 mg KOH/g oil to 0.28 ± 0.011 mg KOH/g oil, benefiting engine longevity. Additionally, the heating value increased from 9410 ± 67 kcal/kg to 9864 ± 73 kcal/kg, improving combustion characteristics. The findings indicate that roselle seed oil can serve as an effective raw material for biodiesel production, supporting the development of renewable energy sources in Thailand and contributing to the nation's energy security.