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The fact that biodiesel is prone to oxidative degradation is a major problem that threatens fuel stability and results in increased production of hazardous emissions such as nitrogen oxides (NOX) and particulate matter (PM) in diesel engines. This review evaluates critically the effectiveness of antioxidant additives as a solution to this problem. Through a combination of experimental data on lots of engine tests, the current study proves that certain synthetic antioxidants, e.g., tert-butyl hydroquinone (TBHQ), are especially useful. In the reviewed engine literature, the TBHQ formulations have been reported to reduce NOX by as much as 27.9% and unburned hydrocarbon by as much as 71.4% in selected blends of biodiesel, although there are other studies that indicated lower or no benefits. These values represent the best‑case outcomes within individual experiments rather than generalized averages and are influenced by engine type, operating conditions, and fuel formulation. Antioxidants are also found to substantially stabilize fuels, and some formulations have raised the temperature of oxidation onset more than 20 °C. This review concludes that the strategic choice of the type of antioxidant and concentration of the chosen antioxidant is the most important, and the synthetic antioxidants are usually better choices than the natural ones. In addition, the current review extends previous studies by providing a quantitative, comparative analysis of a broad set of antioxidants and biodiesel feedstocks and by relating antioxidant chemistry to emission outcomes. These observations give a very important roadmap in developing next-generation and low-emission biofuel blends. Due to the large differences between research in terms of engine configurations, test regimes, and the content of the fuel, the percentage reductions reported cannot be seen as definite universal decreases but simply as a range.
Fayad et al. (Sat,) studied this question.