Biodiesel antioxidants and their impact on the behavior of diesel engines: A comprehensive review

Climate change is a worldwide concern due to unfavorable consequences on human health and ecosystem quality. Accordingly, the world is looking for strategies to stop this destructive phenomenon. Biodiesel production is one of the promising strategies to decrease diesel consumption as one of the most important contributors to climate change. The carbon contained in biodiesel is derived from biogenic carbon dioxide, and therefore, it has a lower contribution to the atmospheric carbon pool and global warming. However, biodiesel suffers from poor stability due to the radical-mediated oxidative degradation of the fuel. The susceptibility of biodiesel towards oxidative degradation, attributed to the presence of points of unsaturation in the form of mono and polyunsaturated fatty acids, can be addressed by antioxidants. However, the presence of antioxidants in biodiesel might also change fuel behavior in diesel engines. Accordingly, the present study reviews and critically discusses biodiesel stability in the presence of antioxidants and, subsequently, the behavior of biodiesel doped with antioxidants in diesel engines. This review shows that antioxidants are highly effective in quenching the free radicals involved in oxidative chain reactions. Biodiesel treatment with synthetic antioxidants is a promising approach to increase biodiesel stability, but they may be toxic to humans and other organisms. Antioxidants extracted from plants and agri-food residues can help overcome this challenge and even lead to sustainable antioxidants production within circular bioeconomy frameworks. In addition to the stability problem, biodiesel suffers from higher nitrogen oxides emissions vs diesel. Antioxidants are also an effective solution to mitigate this emission by quenching free radicals. However, the quenching of radicals is accompanied by a setback in the oxidation process in the combustion engine, leading to higher smoke, unburned hydrocarbons, and carbon monoxide. Due to the impacts of these gases on human health, future studies should move towards the introduction of antioxidant compounds whose negative aspects are discounted.

Automated summary

Biodiesel production is a promising strategy to reduce diesel consumption and mitigate climate change, but it suffers from stability issues and higher nitrogen oxides emissions compared to diesel. Antioxidants can effectively improve biodiesel stability and mitigate nitrogen oxides emissions, but their use may have negative effects on human health and the environment. Extracting antioxidants from plants and residues could lead to sustainable production.