SinS 2025: Renewable diesel fuels differs significantly in composition depending on source and treatment process: study 

Renewable fuels produced from biological sources such as hydrotreated vegetable oils (HVOs) – which are suitable for use in diesel engines – exhibit significant variability in chemical composition depending on feedstock origin and treatment processes, according to research presented at the Solutions in Science 2025 conference in Brighton. Mr Laval Nicolas, a doctoral candidate at the University of Southampton, reported findings from a multi-technique analytical study designed to characterise the compositional complexity of HVOs at molecular level.

Manufactured from non-crop biomass and waste vegetable oils, HVOs undergo catalytic hydrogenation in a two-stage hydrotreatment process to produce hydrocarbon-rich mixtures that can substitute for conventional fossil fuel-derived diesel. While the resulting fuels are often treated as interchangeable, Nicolas’s work demonstrates that they in fact differ substantially in their chemical profiles – with implications for performance, combustion, and storage.

Nicolas analysed more than 40 HVO samples using a combination of advanced analytical methods, including supercritical fluid chromatography (SFC), comprehensive two-dimensional gas chromatography (GC×GC) with mass spectrometry (MS), flame ionisation detection (FID) and proton nuclear magnetic resonance (¹H NMR) spectroscopy. His approach sought to disentangle the complex mixture of hydrocarbons and oxygenated species typically present in renewable diesel fuels.

Three case-study samples were discussed in detail. The first, HVO-1, was dominated by saturated hydrocarbons, including linear alkanes, isoalkanes and cycloalkanes, confirmed through GC×GC-MS as a mixture of C17 to C23 chain-length species. SFC-MS operating in negative ionisation mode also identified trace deprotonated fatty acids, indicating that some residual feedstock-derived components persist through processing.

The second sample, HVO-2, showed greater compositional diversity. Alongside a saturated fraction, an aromatic component of approximately 3% was detected. GC×GC-MS and SFC-MS jointly revealed the presence of fatty acid methyl esters (FAMEs), cyclic hydrocarbons and minor polar compounds, many of which would not have been captured by a single technique alone. This underscored the value of combining orthogonal separation and detection platforms.

HVO-3 presented a more limited profile in comparison, lacking some of the key constituents observed in the other samples. Nonetheless, targeted analysis revealed low-abundance species that were otherwise below the detection threshold in other instruments, reaffirming the sensitivity benefits of integrated workflows.

To support the chromatographic results, ¹H NMR spectroscopy was used to quantify the relative abundance of saturated and aromatic functional groups. Signals in the 0.5–2.5 ppm range confirmed the dominance of alkyl chains, while minor aromatic peaks near 7 ppm aligned with GC×GC-MS findings. Normalisation between platforms remains an ongoing challenge, but initial comparisons indicated good agreement in peak area distributions, particularly for saturated species.

An important observation across multiple samples was the presence of free fatty acids, monoacylglycerides and triacylglycerides – compounds that could have an impact on long-term fuel stability within storage infrastructure. In addition, compositional mapping showed wide variation in the degree of hydrocarbon isomerisation, with differences in the ratio of linear to branched alkanes potentially affecting cold-flow properties and ignition behaviour.

Nicolas noted that while many HVOs share a broadly similar production pathway, their molecular composition can be shown to not be uniform. His findings indicate that accurate characterisation requires the integration of multiple analytical approaches. Developing harmonised protocols for such analysis could help fuel producers and regulators better assess the quality and behaviour of renewable diesel products across different applications.

The study highlights the need for a more nuanced understanding of renewable diesel chemistry as the sector moves toward broader commercial deployment. Nicolas is continuing to refine his analytical pipeline, with the aim of establishing a standardised framework for compositional assessment and quality control in the emerging biofuel market.