Biodiesel via supercritical ethanolysis within a global analysis “feedstocks-conversion-engine” for a sustainable fuel alternative

The challenges in reducing the world's dependence on crude oil and the greenhouse gas accumulation in the atmosphere, while simultaneously improving engine performance through better fuel efficiency and reduced exhaust emissions, have led to the emergence of new fuels, with formulations blending petrodiesel, biodiesel, bioethanol and water in various proportions. In parallel, the sustainability of the new biofuel industries also requires to maintain a high level of biodiversity while playing on techno-diversity, using a variety of resources that do not compete with edible crops (nor by using arable land for energy crops or food crops for energy production) and flexible conversion technologies satisfying the eco-design, eco-energy and eco-materials criteria. In addition, it would be relevant to consider blending ethyl biodiesel, instead of methyl biodiesel, with petrodiesel, particularly if the fuel formulation is completed with bioethanol (or even water). The supercritical ethanolysis of lipid resources to produce ethyl biodiesel is a simple but efficient route that should have the potential to satisfy the sustainability criteria if analyzed holistically. Therefore, this review focuses specifically on the production of ethyl biodiesel via triglyceride supercritical ethanolysis within a global analysis “feedstocks-conversion-engine”. The scientific and technical bottlenecks requiring further development are highlighted by emphasizing (i) the kinetic and thermodynamic aspects (experiments and modeling) required for the process simulation, the results of which aim at securing the life cycle assessment, first at the process level and then at the fuel level; (ii) the proposals to improve the supercritical process performance in terms of eco-material and eco-energy; (iii) the impacts of ethyl vs. methyl biodiesel fuels and of biodiesel–ethanol–petrodiesel blends (with or without water) on the diesel engine emissions and performance; (iv) the technological flexibility of the supercritical process allowing its conversion toward production of other key products. Finally, built on the state-of-the art review, a new R&D direction combining supercritical ethanolysis of lipids with the addition of CO₂, glycerol recovery, and cogeneration, according to the biorefinery concept, is proposed and discussed.