Ammonia suggested as a possible future “low carbon” jet fuel – but problematic

It aims to build lightweight reactors to “crack” the chemical to produce hydrogen to burn as fuel, a design it says could allow existing planes to be modified to store liquid ammonia rather than kerosene.

Hydrogen is currently seen as the only possible “clean” fuel for future long-haul aviation, but the difficulty of safely storing it in fuel tanks, either as a gas or highly cooled liquid, means aerospace manufacturers have argued that vastly different planes would be needed.

Small reactors could be retrofitted into passenger planes to allow the hydrogen to be obtained from ammonia, according to scientists on the UK’s state-funded Science and Technology Facilities Council, who have demonstrated that a mix of cracked ammonia can burn with similar properties as the kerosene normally used as jet fuel.

The new joint venture, as yet unnamed, will combine their findings with rocket engine technology from Reaction Engines, with seed funding from cleantech investor IP Group.

They believe the first sector likely to adopt their technology is shipping. Ammonia has already been seen as a cleaner fuel for the maritime sector, and could be a readily available fuel, as a product that is currently widely transported and stored globally.

However, most of the world’s ammonia is produced from fossil fuels in an energy-intensive process [the Haber Bosch process – very energy intensive – used to make fertiliser] that is responsible for 1-2% of global carbon emissions.

To be truly carbon-neutral, the new aircraft would have to run on “green ammonia”, produced from water and air using renewable energy.

Cracking the ammonia using the reactors on the plane produces hydrogen and nitrogen, and the emissions are water and nitrous oxides (NOx). NOx is an indirect greenhouse gas and can lead to the formation of health-damaging air pollutants such as particulate matter.

The cost of ammonia, or hydrogen, would far outstrip kerosene as a jet fuel, but the firms hope carbon taxes and legislation will alter the future economics.

Aviation and shipping currently account for 5% of worldwide CO2 emissions and their impact is expected to grow without significant technological or behavioural change.

The British government last year set up a jet zero council with the aim of decarbonising flight, with Boris Johnson suggesting that the UK could build an actual zero-emission transatlantic plane by 2050. [Which is, of course utter irresponsible Boris bollocks. AW comment]. 

The industry has signed up to a net zero pledge for 2050, which relies heavily on offsetting and sustainable fuels. Cracking ammonia onboard, if proved feasible, could give zero-carbon flight 20 years earlier, the new joint venture suggests, although large challenges would remain to decarbonise production of ammonia, reduce NOx, and tackle the effects of aircraft contrails that contribute to global warming. [The non-CO2 impacts probably at least double the global warming impact of aviation]. 

Bill David, STFC senior fellow and professor of energy materials chemistry at Oxford, said: “I am excited about the impact that our technology can have in enabling low-impact transitions in hard-to-abate energy sectors.

“Playing to the complementary strengths of ammonia and hydrogen, our cracker technology can rely on the global ammonia infrastructure to provide, at scale, blended ammonia-hydrogen fuels that mimic fossil fuel performance and offer affordable retrofitted energy solutions.”

David said that they were “on a journey” to show NOx emissions could be reduced with the right mix and temperatures. Ammonia itself is a large part of the AdBlue used to reduce NOx emissions from diesel combustion engines.

Robert Trezona, head of Cleantech, IP Group, said the combination of technologies was “a profound breakthrough” with “myriad applications”. He added: “This is a credible, amazing combination of science and engineering … it’s a possible thing.”

The firm will aim to raise tens of millions in funding from other investors next year to build larger scale demonstrations – initially very much on the ground, Trezona said: “This works – but we know we need to show hardware to get investment.”

https://www.theguardian.com/environment/2021/nov/05/british-firm-to-unveil-technology-for-zero-carbon-emission-flights-at-cop26

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Comment on use of ammonia by an jet engine expert

Whereas liquid hydrogen has to be stored and transported at a temperature of –253 °C, ammonia can be stored and transported at the much higher temperature of –33 °C. Large scale and well understood supply chains and storage facilities for ammonia already exist, and ammonia has a higher volumetric energy density than hydrogen..

Reaction Engines’ heat exchanger will use waste heat from a jet engine to power a compact and lightweight ammonia catalyst reactor, or ‘cracker’, developed by STFC. Each cracker/heat exchanger unit would be installed next to each engine pod on an aircraft wing. Installation would require changes to (replacement of) the aircraft’s wings and fuel tanks, but not to its fuselage or empennage. The engines themselves would need only small adjustments. One drawback, identified in modelling by Reaction Engines (based on an Airbus A320), would be that an airliner subjected to these minimum modifications would suffer a reduction in range to 2 000 km, but that this would still cover 90% of the most popular routes. This lost range would be restored if greater changes were made to the aircraft’s design.

Whereas liquid hydrogen has to be stored and transported at a temperature of –253 °C, ammonia can be stored and transported at the much higher temperature of –33 °C. Large scale and well understood supply chains and storage facilities for ammonia already exist, and ammonia has a higher volumetric energy density than hydrogen..

Reaction Engines’ heat exchanger will use waste heat from a jet engine to power a compact and lightweight ammonia catalyst reactor, or ‘cracker’, developed by STFC. Each cracker/heat exchanger unit would be installed next to each engine pod on an aircraft wing. Installation would require changes to (replacement of) the aircraft’s wings and fuel tanks, but not to its fuselage or empennage. The engines themselves would need only small adjustments. One drawback, identified in modelling by Reaction Engines (based on an Airbus A320), would be that an airliner subjected to these minimum modifications would suffer a reduction in range to 2 000 km, but that this would still cover 90% of the most popular routes. This lost range would be restored if greater changes were made to the aircraft’s design.

https://www.sciencedirect.com/science/article/pii/S1540748918306345

[There is a process to turn the NOx produced by burning ammonia (NH3) into nitrogen and water by  selective catalytic reduction (SCR).  Unclear if that could be used in jet engines, or if NOx would be produced.]