All alternative aviation fuel options have unique opportunities and limitations and despite best endeavours at developing and rolling out these fuels, a scenario may arise where the reliance is predominantly on hydrocarbon fuels if the alternatives cannot be manufactured and safely deployed at the scale needed, says a report by the Royal Society. The study, with input from nearly 20 UK scientists and experts, looked at four alternatives to traditional kerosene-based aviation fuels – biofuels, hydrogen, ammonia and synthetic e-fuels – examining their feedstock availability, lifecycle analysis, cost implications, technology readiness, safety concerns, non-CO2 impacts and operational considerations. Although the focus of the study was on sustainable fuels to meet the net zero emissions target for UK aviation, the report points out that feedstock availability and accessibility are an international challenge and not unique to the UK.
“Aviation is a key sector within the UK economy and although it dipped during the pandemic, it’s now growing again, and it’s important we find a solution to how we’re going to fly in the future and meet our climate targets,” commented Graham Hutchings, Regius Professor of Chemistry at Cardiff University and chair of the study’s working group. “We’ve been looking at the alternatives to current jet fuel and focused on four of these. We’ve looked at the resources to make them, the associated costs associated and whether they can be used in existing airframes or if they need new airframes and new infrastructure.
“The conclusion that we’ve come to in considering these fuels is that there is no clear winner. Each has its advantages and disadvantages, and some of the science exists but some of it doesn’t, and a lot of research still needs to be done. The one clear thing is that we need cleaner fuels for the future.”
It is evident there is no single, simple answer to decarbonising aviation and the solution is likely to be a portfolio, says the ‘Net zero aviation fuels’ report, and industry should exercise caution in choosing one alternative fuel solution over another because of the need for international acceptance. “The pathways to decarbonisation are different in different parts of the world and there is a need to encourage the best solution for each region rather than a one solution fits all, noting that long-distance travel will require compatible solutions at each end.”
The study looked at the equivalent resources that would be required for each of the four fuel options to replace fossil jet fuel consumed in the UK. Electric-powered aircraft were not covered as it was considered battery technologies would be unlikely to have been developed to give the energy density required for most commercial flights in the timescale available to reach net zero by 2050, although hybrid systems utilising batteries to support one of the other options “might be a potential solution”.
Aviation energy demand is predicted to be strong over the period to 2050, with projections ranging from flat demand compared to increases of 40-50% by 2050. Based on these trends, the UK annual demand for aviation fuel, which in 2019 was 12.3 million tonnes according to the government, is expected to grow up to 17 million tonnes (Mt) by 2050. The report looks at how each of the four alternative fuel options could meet the 2019 demand and the resources that would be needed.
For example, the total amount of land needed to produce the required 12.3 Mt from the use of a crop such as rapeseed is 68% of the total agricultural land in the UK. If taken together with other energy crops such as miscanthus and rapid-growth wood, the amount of land needed to replace all the UK’s aviation fuel is over 50% of available UK agricultural land.
Most sustainable aviation fuel used today has been made from waste cooking oil and fats and the UK produces around 250 million litres of used cooking oil (UCO) a day, with much of it used to feed livestock and manufacture soap, make-up, clothes, rubber and detergent. If 100-200 million litres of UCO were diverted to jet fuel production, a conservative estimate of 50% conversion efficiency would produce 50-100 million litres of jet fuel, which is only 0.3 to 0.6% of total UK jet fuel use per year.
The UK is highly reliant on importing feedstocks for its renewable fuel needs, with over 423 million litres of UCO sourced from China alone in 2021. The report points out that some imported cooking oil is not waste but virgin oil or oil of secondary use and that unless importation is properly controlled and regulated, this could lead to encouraging suppliers to clear more land and/or cut food production in order to expand oil production.
Agricultural residues, mainly straw from cereal production, are another potential source and estimates on the availability of this feedstock for jet fuel production range from 2 to 5 Mt/year, although this implies competition with current use and preventing it from being returned to the soil. In this case, soil carbon and nutrient content would be depleted, leading to increased use of fertilisers and therefore increased GHG emissions. Being spread over a wide area of arable land, collection and transportation to the refinery also incurs a penalty in terms of GHG emissions and the energy balance.
“The UK’s agricultural waste can only provide a small fraction of the demand for jet fuel, its status as waste is debatable and its removal from current usage will have negative impacts on life-cycle analysis for energy accounting and GHG emissions,” says the report.
Forest residues are another potential source of feedstock but the UK is one of least forested countries in Europe. Estimates of feedstock availability in the UK for jet fuel production range from 0.8 to 2 Mt/year but using the high estimate would only produce around 0.2 Mt/year of fuel, which is 1.7% of the total amount of fuel required.
Using municipal solid waste as a feedstock to produce sustainable jet fuel is the favoured pathway of four out of eight current SAF projects ongoing in the UK. The total renewable content of MSW in the UK is estimated to be about 40 Mt/year, of which 12 Mt/year could be used for bioenergy production, although there is competition for this waste. However, using the estimated 12 Mt/year for jet fuel production could yield 1.2 Mt/year of fuel, that is 10% of the total amount of fuel required.
To work out resource requirements for hydrogen as a fuel for aviation, the UK’s 2019 jet fuel use of 12.3 Mt was converted to 145 TWh of energy. Electrolysis is said to be between 50 and 70% energy efficient for hydrogen production, with the potential of improved efficiencies to 76% by 2050. The study estimated the electricity required to provide the 2019 jet fuel consumption would therefore be between 207 TWh (at 70% efficiency) and 290 TWh (at 50% efficiency), which is 68-95% of the UK’s current electricity production, but to be sustainable, only renewable electricity should be considered. The use of green hydrogen for aviation to replace current fossil jet fuels requires around 2.4 to 3.4 times the total current renewable electricity in the UK. This route requires increases in wind and solar power generation, notes the report, adding electrolysis also requires deionised water. To produce 1 tonne of H2 requires 9 tonnes of water, so to supply the 3.8 Mt of H2 required to replace fossil jet fuel, 34.2 Mt of ionised water is required.
The power-to-liquid e-fuels route requires significantly more energy than for hydrogen production. The process when done sustainably using renewable electricity would require 5 to 8 times the UK’s 2020 renewable electricity capacity without biofuels. This route requires considerable energy input that outweighs the energy produced by the fuel itself. It takes between 140 and 198 gigajoules to make 1 tonne of synthetic e-fuel, which is 3.2 to 4.6 times the energy content of the end fuel, when compared to jet fuel at the same energy density.
Although few have shown interest in ammonia as a fuel for aviation – it is though being considered for maritime use – it does have significantly higher volumetric energy density than both liquid and high-pressure hydrogen. While ammonia has six times less energy by mass than hydrogen, the system storage mass for hydrogen is at least a factor of two more than for ammonia. To supply the quantity of energy (145 TWh) needed to match the 2019 UK jet fuel consumption, the mass of ammonia needed was calculated at 30.2 Mt. Overall, green ammonia production requires 5-10% more energy than the equivalent hydrogen production. Similar to green synthetic fuels, ammonia as a jet fuel requires a major increase in the annual UK sustainable (wind and solar) electricity production.
“Accessing the required amount of electricity will be a challenge, particularly as other energy uses will also require large amounts of renewable electricity,” says the report. “The production of ammonia and e-fuels requires more energy than hydrogen. However, this is partly offset by reductions in the energy needed to store these fuels.”
It cautions that while green ammonia and hydrogen show promise as potential fuels, they come with significant safety concerns that will need to be addressed. “While both are widely used in industry, existing standards on handling of these fuels would need to be updated to suit the civil aviation context,” it adds.
“Considerations will have to be made on handling multiple technologies in the airport and on aircraft. Staff and crew will need specialised training on handling alternative fuels and the public will need to be informed about the relevant safety concerns within the airport and aircraft.”
The report concludes: “The selected solutions need to be globally accepted and each of the options considered in a holistic manner, both to provide the best solution now and for the coming years. The options available now offer some carbon savings but are not ideal. Further research and development will be needed to produce better alternative fuels, including accessing sustainable feedstocks, and the development of the efficient production, storage and use of green hydrogen, ammonia and e-fuels.
“Some of the solutions will require substantial redesign of airframes and support infrastructure. R&D is also needed to understand and mitigate the non-CO2 climate impacts of all the fuel options.”
Photo: Heathrow Airport
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