Waste cooking oil biodiesel alters combustion pathways to enhance volatile organic compound emissions and reduce intermediate/semi -volatile organic compounds in agricultural machinery

Biodiesel has attracted considerable attention in recent years due to its zero-carbon emission characteristics, especially in relation to non-road mobile sources that are challenging to electrify. However, limited attention has been paid to the impact of blending waste cooking oil biodiesel on emissions of volatile organic compounds (VOCs) and intermediate/semi-volatile organic compounds (I/SVOCs). Therefore, real-world measurement of organic matters from typical agricultural machines in China was conducted. Results indicated that while the addition of biodiesel effectively reduced I/SVOCs in PM—from 281 mg (kg fuel)−1 for D100–152 mg (kg fuel)−1 for B5 (46% reduction), and to 179 mg (kg fuel)−1 for B20 (36% reduction)—it simultaneously resulted in a notable increase in VOC emissions, from 140 mg (kg fuel)−1 for D100–217 mg (kg fuel)−1 for B5 (55% increase) and 254 mg (kg fuel)−1 for B20 (81% increase), mainly driven by elevated levels of oxygenated VOCs (OVOCs) and alkenes. This phenomenon was likely associated with alterations in combustion pathways induced by the high ester content and elevated viscosity characteristic of biodiesel. In addition, although biodiesel can serve as an effective alternative fuel for agricultural machinery in reducing CO₂ and PM emissions, the substantial increase in VOC emissions led to a significant rise in ozone formation potentials (OFP), which was 1.6 times and 1.8 times higher for B5 and B20 compared to D100. Given the current context of elevated O₃ concentrations in China and globally, the use of biodiesel derived from waste cooking oil should be carefully evaluated.