A carbon-negative aviation fuel being developed by wind-power pioneer Henrik Stiesdal, produced from agricultural waste and green hydrogen, will reduce greenhouse-gas emissions significantly more than initially thought, Recharge has learned.
The fuel, which is on track to be commercialised by 2025, promises to be the most climate-friendly option to clean up the aviation industry, which is often described as the hardest sector to decarbonise.
As Recharge has previously reported, the end product of Stiesdal’s SkyClean process will be a fuel that is chemically identical to the A-1 jet fuel currently used by airlines but produced in a process that actually removes carbon from the atmosphere. So, the more SkyClean fuel is burned, the more CO2 is removed from the air.
Other green aviation fuels being developed — biofuels and synthetic “e-fuels” made from green hydrogen combined with captured CO2 — are merely carbon-neutral, neither adding to nor reducing overall CO2 levels.
SkyClean, however, is based on a pyrolysis process that uses agricultural waste as feedstock and delivers both fuel and biochar.
In simple terms, a pyrolysis oven is used to heat agricultural waste in the absence of oxygen, converting the biomass into three components: biooil, syngas and biochar. The biooil and syngas are then combined with green hydrogen (produced by using renewable energy to split water molecules into hydrogen and oxygen) to create biomethanol, which is then synthesised into aviation fuel.
About half of the carbon content of the agricultural waste is not converted into biooil and syngas but is delivered as biochar, a carbon-based solid similar to charcoal. Biochar does not decompose or rot when spread on the ground; it is stable in nature for millennia. Since the plants that comprise the feedstock for the SkyClean process took all their carbon content from the atmosphere in the form of CO2, and since about half of the carbon is sequestered as biochar — leaving only half to become converted into fuel — the process is carbon negative.
Due to biochar’s ability to retain water and nutrients, spreading it on the ground can improve soil quality by reducing nutrient-leaching, acidity, and the amount of irrigation and fertilisers required. It can also reduce the emissions of two very potent greenhouse gases — nitrous oxide and methane.
Recent research by the Technical University of Denmark (DTU) has shown greater climate benefits when using different types of agricultural waste.
Initial calculations had revolved around the use of straw — the abundant remains of cereal plants after grain has been removed.
“By applying the process to other types of agriculture waste than straw, we get a significantly larger reduction in agricultural emissions because we prevent the emission of methane and nitrous oxide, which are very potent greenhouse gases,” Stiesdal tells Recharge.
He explains that animal manure and farm slurry — a noxious combination of manure, water and other organic waste — is often left to decompose on farmland, resulting in large amounts of methane and nitrous oxide being released. By using these materials in the SkyClean pyrolysis process, those greenhouse gases would not be emitted.
“So, the end result is that the net effect on greenhouse gas emissions is somewhat higher than what we expected,” Stiesdal says.
SkyClean could also reduce emissions from biogas produced via anaerobic digestion by using the “fibre fractions” — the organic material left at the end of the process — as an aviation-fuel feedstock. This leftover waste, which is usually spread onto farmland, also emits methane and nitrous oxide as it decomposes.
The inventor explains that his country, Denmark, aims to reduce annual greenhouse-gas emissions by about 17 million tonnes CO2-equivalent per year by 2030 to meet its climate targets.
“It turns out that the SkyClean process, if applied throughout Danish agriculture, using only waste products — so, in other words, not using products that could have other uses — we can remove between eight and 12 million of those 17 million tonnes with this single technology.”
This could be achieved, he says, with 100 SkyClean plants across Denmark, each of which would cost about €35m ($39.5m) and represent 10-40MW of feed-in power, depending on the feedstock. And it would produce 130% of the country’s annual aviation-fuel needs.
This would be affordable because the SkyClean process has four potential revenue streams — the carbon-negative aviation fuel, as well as biochar, waste heat and carbon credits. The three additional products effectively subsidise the cost of the aviation fuel, potentially enabling it to reach price parity with A-1 jet fuel if the operator of a SkyClean plant receives the equivalent of €65 per tonne of sequestered CO2, according to Stiesdal’s calculations. This could come in the shape of carbon credits, an added tax on each flight paid by passengers or airlines, or through government subsidies from general taxation. And this could be done incrementally, with the proportion of SkyClean fuel in jet A-1 gradually increasing.
If the EU’s carbon price — currently around €25 per tonne — rises in the coming years as expected, it could soon be more profitable for airlines to buy jet A-1 based on the SkyClean process, rather than on conventional fossil oil (although international standards only currently allow up to a 50% biofuel/A-1 blend on commercial flights). This fossil fuel was responsible for 915 million tonnes of CO2 emissions globally in 2019, and passenger numbers are expected to increase by 70% by 2050.
SkyClean’s environmental credentials have been further improved by the recent invention by DTU researchers Jesper Ahrenfeldt and Ulrik Birk Henriksen of a pyrolysis process that delivers tar-free biochar. Tars are a potentially harmful by-product of the biomass pyrolysis process that may cause cancer and other health problems. The risk of creating biochar containing tars had been Stiesdal’s biggest concern since he first came up with the idea for SkyClean around ten years ago. Through the cooperation with DTU this concern has now been alleviated.
Stiesdal Fuel Technologies is now planning to build a 2MW SkyClean pilot plant at DTU, which would be “semi-commercial” as it would be sold to a private company once testing is completed. “There are many clients that are interested [in buying the pilot plant] here in Denmark,” Stiesdal explains.
After that, the company plans to start work on the first 20MW commercial pilot plant, which would be fully operational before 2025.
“We are hoping for some public funding, but I have to say that if we can't get the public funding, then we are hopeful that we can get some private funding,” says Stiesdal, pointing to the considerable interest in the technology from the agricultural sector, airlines, aircraft builders and airports, both inside and outside Denmark.
“I think it would be fair that we get some financial contribution from the public because I think that this is very much to society’s benefit,” he adds.
“My own hope is that we could get this fully implemented across Denmark by 2030. That basically means a plan whereby agriculture waste would go into this process that would otherwise be sprayed on agricultural land, ploughed down after harvest or burned in incinerators. On top of that there's an additional, large amount of waste that we could also use — waste from gardens and parks, waste from forestry, and washed-up seaweed on the beaches, which is a nuisance in the summer and therefore collected.
“SkyClean is really a no-brainer,” he concludes, “because it solves both the agriculture challenge and the aviation challenge.”
Henrik Stiesdal has been responsible for some of the most important inventions in the wind power industry over a 40-year career, building his first turbine by hand on his parents’ farm in 1978.
His turbine design — comprising upwind rotors, automatic yawing and two-speed generators — was later sold to Vestas, then a manufacturer of farm wagons and truck cranes, helping the company become the world’s leading turbine maker.
Stiesdal was responsible for the world’s first offshore wind farm, Vindeby, in Denmark in 1991, and the marinisation of wind turbines to enable them to survive at sea. Later, as chief technology officer (CTO) at OEM Bonus Energy, he designed the first one-piece turbine blade and then the first variable-speed turbine. Then as CTO of Siemens Wind Power, which purchased Bonus in 2004, he was in charge of the direct-drive technology that eliminated the then-unreliable gearbox that had become the Achilles heel of the wind sector.
He retired from Siemens in 2014, and has since formed his own eponymous innovation company Stiesdal A/S, which aims to provide cost-effective climate-fighting solutions to the energy and transport sectors.
The company is split into four subsidiaries: Stiesdal Offshore Technologies, which has developed the low-cost TetraSpar floating turbine foundation; Stiesdal Storage Technologies, which is developing a hot-rock thermal energy storage technology called GridScale that can enable 24-hour wind and solar power; Stiesdal Fuel Technologies, which is developing SkyClean, the carbon-negative jet fuel; and Stiesdal PtX Technologies, which is developing a low-cost electrolyser called HydroGen.