Wind-power pioneer Henrik Stiesdal has begun work on what may be the most ingenious of his decades’ worth of inventions — carbon-negative aviation fuel.

Plenty of carbon-neutral aviation fuels are in the pipeline, including biofuels and synthetic fuels made using green hydrogen, but this would appear to be the first that will actually result in CO2 being extracted from the atmosphere — so the more fuel produced, the more carbon will be removed.

The Danish inventor’s eponymous company, Stiesdal Fuel Technologies A/S, is targeting a pilot plant in the next four years that will enable the production of 60.000 tons of carbon-negative jet fuel per year — enough to supply 6-7% of the Danish aviation sector’s requirements.

And it may well prove to be cheaper than the highly polluting fossil jet fuel used today. He believes he will able to sell the carbon-negative jet fuel for “roughly equivalent” to standard Jet A-1 fuel produced when the price of crude oil is $50 per barrel. And this could be reduced further with income from a carbon premium. (The average price of a barrel of crude was just under $58 per barrel when this article was published.)

With the global aviation industry emitting twice as many greenhouse gases as the UK each year, it is a solution that could play a major role in the fight against climate change.

How it works

Ironically, the route to this decarbonisation is carbonisation — literally converting agricultural waste into solid carbon, or biochar — a substance similar to charcoal.

This is achieved through a process known as pyrolysis — heating in the absence of oxygen — which converts plant waste into three components: biooil, syngas and char. A small fraction of this biooil or syngas would provide the required 500-600°C heat for the pyrolysis oven.

The remaining biooil and syngas would then be combined with additional hydrogen to produce methanol, which can then be further converted to jet fuel through well-established chemical processes. (To remain carbon-negative, this conversion would have to be powered by renewable energy, with the added hydrogen being green — ie, produced by electrolysis powered with renewable energy.)

About half of the carbon that the plant absorbed as it grew is released back into the atmosphere when the green jet fuel is burned, but the other half is stored for millennia as biochar, a product that can be used to enrich agricultural soil.

Stiesdal’s process — which he calls SkyClean — therefore has three potential revenue streams: the sale of both carbon-negative aviation fuel and biochar, as well as carbon credits. Revenues from biochar and carbon credits help to reduce the cost of jet-fuel production to a comparable level to existing Jet A-1 production.

Due to biochar’s ability to retain water and nutrients, it can improve soil quality by reducing nutrient-leaching, acidity, and the amount of irrigation and fertilisers required. It can also reduce the natural emissions from soil of two very potent greenhouse gases — nitrous oxide and methane.

The SkyClean process is very different to the normal carbon-neutral biofuel process, in which biomass is fermented to produce ethanol. This leaves residues known as “fibre fractions” that are either left to rot or mixed into animal feed — both of which result in all the biomass’ carbon returning to the atmosphere.

“We have had big interest lately from the Danish agricultural sector because pyrolysis of waste products from agriculture will solve many things,” Stiesdal tells Recharge. “My main target when I started was to solve the aviation fuel issue. But when you carbonise waste products from agriculture, you get a lot of benefits. With the SkyClean concept, Denmark could reduce the climate impact from agriculture by 50% so that's really very, very substantial. It would probably be the same rates for other countries' agriculture sectors. And that is because we reduce the emissions from agricultural products that would otherwise just decompose and release CO2.”

"And at the same time, the country’s total consumption of jet fuel could be covered.”

According to Stiesdal, there is enough “sustainable waste biomass” — that which is not used for food, clothing or any other value-creating purpose — to yield about a third of the world’s current energy consumption in the transport sector, or several times the world’s consumption for aviation purposes. This leaves room for considerable growth in aviation without running out of biological feedstock. The associated carbon sequestration would correspond to about 10% of global CO2 emissions, he says.

And the process could also be tweaked to enable plastics or old tyres to be used as a feedstock instead of agricultural waste.

The pilot project

“We hope to get a project started with actual equipment within the course of the next year or so,” Stiesdal tells Recharge.

“And the hope is that we can build a pilot plant that would lead to a real-life demo here in Denmark. Such a project would have a time horizon of about four years, out of which we expect two-and-a-half years to go with the pilot and completing all key research on the fertilising effect [of the biochar] and non-toxicity [to remove harmful tars] and what have you in the final design of the pyrolysis oven, and then one-and-a-half years to get a full blown demo plant in place. After about a year of testing, the technology could be made commercially available somewhere around 2025.”

Pyrolysis is not a new idea — pyrolysis ovens have been tried and tested for many years in various university laboratories around the world, and a handful of companies are offering such equipment for sale — but the technology has yet to be commercialised.

“If you go out there you will find no end of beautiful pilot pyrolysis ovens,” Stiesdal tells Recharge. “We have these fantastic smart oven principles that can produce clean sequestered carbon and nice gases. But what is actually preventing this from being commercialised? Who's picking up the ball? Who's been driving it? The answer? Nobody. So they have done research projects with external parties and they have developed beautiful smart processes and mechanisms, but nobody is picking up the ball. I would like to pick up the ball.

“The need is there to take solutions that exist and fold them into products that can be commercialized and industrialized because industrialization is what brings down prices.”

Partners in the project include the Technical University of Denmark (DTU), Aarhus University, and Siemens Gamesa, which Stiesdal says is interested in supplying the green hydrogen that needs to be added to the biooil/syngas mix to create jet fuel. The Danish Agriculture and Food Council, which has set a vision to become climate neutral by 2050, is also strongly supporting the project.

Stiesdal Fuel Technologies does not plan to convert the methanol, syngas and hydrogen into aviation fuel at the pilot project, but rather to use external companies that have already perfected the conversion process.

“It seems obvious to me that we are unlikely to be able to outperform existing refineries and conversion units,” says Stiesdal. “Consequently, we will concentrate on the upstream part of the process and leave the downstream part to the incumbent players.”

The production of carbon-negative jet fuel is something that Big Oil should be involved in, Stiesdal tells Recharge, but he says now is not the time to get the fossil-fuel companies interested in SkyClean.

“I think this technology is relevant, doable and necessary. But does it sound a little bit fantastic? Could be. Quite frankly, you have this old guy sitting in Denmark, he cooks up ideas all the time and let's talk when he has something that he can show us that really works.

“So, I would like to be a bit further on before I approach the people who can really put volumes into this. That's how it has to be. You don't cut any ice on ideas. You cut ice once you can show tangible equipment that delivers tangible products.”

The genius of Henrik Stiesdal

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.