How different is diesel from biodiesel, really?

Australian researchers have found that particulate emissions from a diesel exhaust are heavily influenced by the fuel’s composition

According to the European Commission, biofuels now represent around 5 per cent of all transport fuel used in member states. Biodiesel – made from plant or vegetable oils – has been attracting particular attention because of the ease with which it can be blended into traditional petrodiesel. But a new paper in the latest issue of Carbon [DOI: 10.1016/j.carbon.2016.03.061] has shown that the blending process alters much more than the final fuel’s chemical composition.

The particles produced on combustion of diesel fuel are primarily composed of black carbon (soot). Due to health concerns, since the 1970s most cars have used dedicated diesel particulate filters (DPFs) to catch these particles before they reach the vehicle’s exhaust. Because the reliability of DPFs depends on how soot oxidises on their surface, the size and shape of soot particles can have a direct impact on the overall engine performance.

This analysis, led by researchers from the Queensland University of Technology has shown that blended diesels (bio + petro) produce dramatically different particles to petrodiesel, even under the same engine conditions. To examine this effect, exhaust particles from petrodiesel, alongside biodiesel blends based on microalgae, cotton seed and waste cooking oil were investigated with high-resolution transmission electron microscopy.

On average, biodiesels were found to produce smaller primary soot particles than petrodiesel, which then led to more compact and spherical agglomerates. Particles from cotton seed and waste cooking oil were found to be smaller than those from microalgae fuel, and all were smaller than those of petrodiesel. In addition, the primary particle size was shown to decrease as biodiesel content increased. The authors attributed these morphological difference to the presence of oxygen, which seemed to result in a greater structural disorder in the carbon layers, reducing the particle’s overall size.

Microstructural changes like these will have an effect on the surface area of the final agglomerate, and therefore play role in their reactivity in the presence of catalysts, and their toxicity to humans. Given the growing popularity of biodiesel, the team conclude that, “This highlights the need for further investigation of the interaction of particles produced from combustion of oxygen-rich fuels with both living and non-living surfaces”.

This story originally appeared here on 18 May 2016

Original paper (£): N. Savic, M.M. Rahman, B. Miljevic, H. Saathoff, K.H. Naumann, T. Leisner, J. Riches, B. Gupta, N. Motta, Z.D. Ristovski, “Influence of biodiesel fuel composition on the morphology and microstructure of particles emitted from diesel engines”, Carbon 104 (2016) 178–189. DOI: 10.1016/j.carbon.2016.03.061