Anthony King
A study from the University of Georgia, US, suggests a non-edible oilseed crop could generate jet fuels and reduce carbon emissions by 68% per litre (GCB Bioenergy, doi: 10.1111/gcbb.12888).
The crop, Ethiopian mustard (Brassica carinata), produces oil-rich seeds. A $15m project funded by the US Department of Agriculture looked at its potential to grow in the South of the US. Previously, on a small scale, oil from this plant was refined to produce jet fuel and tested in commercial flights.
‘We can crush seeds from carinata and refine the oil to produce jet fuel and other commercially valuable products,’ says Puneet Dwivedi, a sustainability scientist at the University of Georgia. ‘We desperately need to reduce the carbon footprint of the aviation sector.’ What comes out depends on the conversion technology, but can be a mixture of jet fuel, naphtha, biodiesel and pelletised animal feed.
Carinata crop trials were run in Georgia, Florida, Alabama, Mississippi and North Carolina. Dwivedi stresses this oil crop fits well with local crop systems, which in Georgia would typically be a rotation of cotton and peanuts. ‘I’m sure farmers will be interested in growing carinata, because carinata can grow only in the winter months, and in the South, we don’t grow any crops in winter. A farmer would not be losing anything,’ he says. The crop would need to be sown and harvested between November and March.
He says the crop would provide valuable soil cover during winter, reducing soil erosion and moisture loss, and also notes that its flowers are very attractive to pollinators such as bees. His calculations suggest carinata should provide farmers with higher profits than canola. Also, as a non-food crop, he believes that the plant itself could be improved by CRISPR-Cas9 gene editing technology without controversy.
‘Double-cropping systems have possible economic and environmental benefits,’ comments Ric Hoefnagels at Utrecht University in the Netherlands: ‘[However], a single-issue [life cycle analysis], such as the carbon footprint method used in this paper, cannot identify possible risks of burden-shifting. These include, for example, other potential environmental impacts that result from the additional fertiliser inputs.’
As noted in a recent economic feasibility study for carinata, biofuels from new feedstocks face significant challenges to achieve the scale necessary to compete with fossil fuels, and require substantial investment in refining capacity, ‘which inherently involves risks’ (GCB Bioenergy, doi: 10.1111/gcbb.12873). This study looked at the price variability of jet fuel and suggested a carinata biorefinery could be supported by long-term contracts or price guarantees. Without subsidies, Dwivedi’s study indicated that jet fuel from carinata would cost $0.80 to $1.28/L, compared with $0.50 for conventional aviation fuel.
In 2019, 101bn L of aviation fuel was used in the US, around 18% of global consumption. Around 1.4m ha of land in the south-eastern US were previously shown to be suitable for carinata production, and Dwivedi estimates carinata oil could replace up to 4% of overall aviation fuel consumed in the US. Hoefnagels says by his calculations this figure is closer to 1.5%.
Carinata seeds include 44% oil and 56% meal, notes Hoefnagels, with the meal rich in proteins and suitable for animal feed. However, he adds that the methods used in the paper shifts the environmental burden largely to carinata meal, in a way that is inconsistent with European Union and US rules for liquid biofuels. Both the EU and US methods would slightly reduce the carbon emission savings from the jet fuel from carinata.
Oil from the mustard plant could be transformed into aviation fuel using the hydro-processed esters and fatty acid (HEFA) process, wherein triglycerides in vegetable oil are hydrogenated to saturate the double bonds and fatty acids are released. The crude-oil-to-fuel ratio was about 72%, and co-products were propane and naphtha at around 9% and 6%, respectively.
HEFA is the most mature technology, notes Hoefnagels. ‘One of the main challenges of this pathway is that the upscaling potential of HEFA is constrained by the availability and cost of sustainable feedstock,’ he explains. The supply of waste fats and oils is relatively limited, and many vegetable oils do not comply with sustainability measures. ‘Alternative non-food oilseed crops, such as carinata (and camelina), could indeed be suitable candidates for upscaling supply markets,’ he adds.
The missing piece of the puzzle, according to Dwivedi, is a lack of local infrastructure for crushing the seed and processing the oil. Current research focuses on modelling economic and environmental feasibility across Georgia, Alabama and Florida.
The development is timely. Ahead of COP26 in Glasgow, the Chief Executive of London’s Heathrow airport said the UK should push scale up of sustainable aviation fuel (SAF) by bringing in rules for its use, a price support mechanism and loan guarantees.
But there is no silver bullet for replacing fossil fuels in aviation. ‘Climate targets in aviation can only be achieved if multiple SAF production pathways with various sustainable feedstocks, including waste and residues, crops and electro-fuels are developed,’ Hoefnagels says.
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