Effects of fuel oxidation on the emission characteristics and formation mechanism of aldehydes from biodiesel combustion: Experimental and simulation analysis

In this study, the emission characteristics and formation mechanisms of aldehydes from biodiesel combustion, both before and after oxidation, are comprehensively investigated through Bunsen burner experiments coupled with Chemkin and Gaussian software simulations. The results reveal that the aldehyde concentration initially decreases, then increases, and eventually decreases again as the oxidation time of the original biodiesel increases. During the first 0–6 h of biodiesel oxidation, its overall saturation increases, while the aldehyde generation potential decreases. After 8 h, oxidation products with low bond dissociation energy (BDE) and multiple double bonds readily dissociate into aldehyde intermediates, such as CH 3 O, CH 2 CHO, and C 4 H 6 . This causes an increase in the concentrations of formaldehyde and acetaldehyde by 120.2 % and 188.3 %, respectively. Between 10 and 12 h, the oxidation products undergo transformation, significantly reducing aldehyde emissions. As the overall carbon chain length of biodiesel shortens, combustion accelerates, and the aldehyde peaks rise more rapidly, appearing earlier. The lowest BDE values, both before and after biodiesel oxidation, are concentrated near the ester group and C=C bond. The oxidation product methyl alpha-eleostearate exhibits the lowest BDE of 315.67 kJ/mol. The formation of oxidation products of biodiesel like methyl alpha-eleostearate leads to incomplete combustion, with aldehyde emissions increasing by 9.5–183.2 %. This is due to the electron-withdrawing, electronegative, and resonance effects of the double bonds, which lower the energy required for H-abstraction and β-decomposition reactions during aldehyde formation.