Single-step conversion of wet Nannochloropsis gaditana to biodiesel under subcritical methanol conditions

•One-step transesterification of wet algae under subcritical methanol condition.•Fatty acid methyl esters analyzed with GC–MS, 1H NMR, and TGA.•Optimization of the one-step esterification under subcritical methanol.•No need for drying the algae and a conventional catalyst. A single-step process for a direct conversion of wet algal biomass containing about 80% of water into biodiesel under subcritical methanol conditions is developed and presented in this work. This process may have energetic advantages with high free fatty acid (FFA) and water contents as the feedstock. At elevated temperatures, water hydrolysis and FFA esterification can simultaneously occur in a subcritical state. The influences of process parameters including reaction temperature (150–225°C), reaction time (15–120min) and methanol to wet biomass (vol./wt.) ratio (3:1–8:1) were investigated. The optimal conditions for this process are reported as: methanol to wet biomass (vol./wt.) ratio of around 6:1, temperature and time of about 225°C, and reaction time of 90min. The fatty acid methyl esters (FAMEs) were analyzed with GC–MS, FT-IR and thermogravimetric (TGA). This process does not require a catalyst and any drying pre-treatment steps. Thus, the process is cost-saving and environmental friendly for renewable biodiesel production.