Powering aeroplanes with renewable fuels is crucial for making flying less climate-damaging, but it will get aviation nowhere near climate neutrality, environmental NGOs, industry representatives and researchers agreed at a conference in Berlin. They said that making synthetic fuels with renewable power – so-called power-to-liquid – is a top priority for the rapidly growing sector and requires immediate government action to get the technology off the ground to reach industrial scale so it can have a real impact soon. But experts also warned that planes’ CO2 output is only part of the problem, because their “non-CO2 effects” – such as condensation trails, particles and other greenhouse gases emitted at high altitudes – contribute even more to the climate crisis. This is why Germany’s environment agency (UBA) has proposed a host of measures to make flying more environmentally friendly. However, it also suggested in a new study that flying longer distances will never be climate-neutral (Clean Energy Wire).

That being said, conference participants were in broad agreement that synthetic fuels made with renewables are key for significantly reducing aviation’s direct CO2 emissions. 

Building up an infrastructure for renewable fuels requires international cooperation and treaties.

Aviation Carbon emissions have doubled since 1990

Aviation is already a major and growing emitter. In Europe its emissions have doubled since 1990, and globally they could, without action, double or treble by 2050. Such emissions growth needs to be reversed and brought to zero by 2050 if we are to meet the goals of the Paris Agreement. Otherwise growth in aviation emissions could rapidly consume the limited carbon budget to remain within the 1.5 and 2°C targets of that Agreement.

Aviation however is at risk of having its emissions locked in due to the growth in passenger numbers and aircraft fleet. While uncertainties exist, we do know that the sector will have a substantial fuel demand well into the 2030s, 2040s and beyond, the period when our economy needs to increasingly decarbonise. 

Further operational improvements do not achieve decarbonisation

The expected technology and operations improvements will not mitigate the expected fuel demand and emissions growth from aviation. Generating incremental efficiency improvements from current aircraft designs is becoming ever more costly and difficult. Further operational improvements remain possible but do not achieve decarbonisation and require the right policies to be in place. To significantly reduce the expected fossil fuel demand and ultimately eliminate it from the sector would require further measures according to the Transport & Environment study of october 2018.

Alternative fuels to close the gap

To succeed in putting aviation on a pathway to decarbonisation, new types of alternative fuels need to be brought forward. Synthetic aviation fuels (Gas To Liquid, biokerosene and Power To Liquid) can contribute to less air pollution around airports due to ultra-fine dust, soot and benzene. Biokerosene and PTL kerosene (if produced with sustainable electricity) can also lead to lower greenhouse gas emissions.

Currently, all synthetic kerosene’s are only very poorly available, mainly because it is significantly more expensive than conventional kerosene. Production is not getting under way and too few economies of scale can be achieved. In addition, PTL kerosene is only at the beginning of its technical development. The amount of biokerosene is also limited by the availability of sufficient biomass and by competitive demand for this biomass from other sectors. EU legislation and self-regulation ensure that no food is used for biokerosene, and aim to avoid indirect damage to food production, biodiversity and vulnerable nature. Large areas are needed for both renewable electricity production and biomass production. This leads to difficult balancing processes with other claims on these surfaces

Power to Liquid kerosene (Electro fuels) will be needed to close the gap. Electro fuels are produced through combining hydrogen with carbon from CO2. With the hydrogen produced using additional renewable electricity and with the correct source of CO2 (ideally air capture), such fuels can be close to near zero emissions and carbon circular. 

Again however strict safeguards are needed to ensure synthetic kerosene would be produced only from zero emission electricity. If produced at scale, electro fuels are likely to cost between three and six times more than untaxed ,100 per tonne in 2050, electro fuel uptake will increase ticket prices by 59%, resulting in a 28% reduction in projected passenger demand compared to a business-as-usual scenario. However, compared to the ticket price with an equivalent CO2   tonne, the ticket price increase would only be 23%. 

Introducing a progressively more stringent low carbon fuel standard (GHG target) on aviation fuel suppliers will leave all operators flying within or from Europe needing to purchase such fuels. These rising fuel costs will increase operating costs which will inevitably be passed onto consumers, causing a fall in demand for jet fuel compared to forecasts and reducing the volume of alternative fuels that will be required to replace kerosene.

Synthetic fuels are the only technically viable solution

Using Electro fuels to meet the expected remaining fuel demand for aviation in 2050 would require renewable electricity equivalent to some 28% of Europe’s total electricity generation in 2050 and 95% of the electricity currently generated using renewables in Europe. It is also important to keep in mind that other sectors will need additional renewable electricity to decarbonise, for example for green hydrogen to be used in industry. However with today’s technology synthetic fuels are the only technically viable solution that would allow aviation to exist in a world that avoids catastrophic climate change.

Production could increase if policy support were available

CO2-based synthetic fuels are of increasing interest as a potential strategy to reduce petroleum consumption as well as greenhouse gas (GHG) emissions from the transportation sector. Production of CO2-based synthetic fuels has been very limited to date, but production could increase if policy support were available. In particular, the European Commission’s proposal for a recast Renewable Energy Directive (RED II) includes CO2-based synthetic fuels as an eligible pathway to meeting the 2030 target for renewable energy in transport. 

Technically possible to produce sizeable quantities of synthetic kerosene

The entire synthetic, carbon neutral kerosene production chain has been demonstrated on various occasions and via various routes. Most process steps remain to be scaled and some to be improved, but it is most likely that the next decade will be the start of mass-scale synthetic fuel production. It is technically possible to produce sizeable quantities of synthetic kerosene, yet it is unclear whether this is also (at least somewhat) economical. The uncertainties about the oil price and electricity price are so large and their values so critical, that a deviation in either can make synthetic kerosene cheaper than fossil kerosene or push its price to a point where it is very uneconomical. 

Price parity between fossil and synthetic kerosene can be achieved

However parity could be reached: With a moderate CO2 price, a slightly higher oil price, an electricity price which is in line (if not conservative) with wind power projections (of 2020), and the sale of a byproduct (oxygen), synthetic kerosene would cost as much as fossil kerosene 

Synthetic kerosene compares favourably to its alternatives and that there is reason to believe price parity between fossil and synthetic kerosene can be achieved in the year 2030. This means that there is a strong incentive to start up synthetic kerosene production!

Synthetic kerosene production will not stand on its own. Instead it will be able to play a significant role in the energy system, not only by providing a renewable fuel for aviation, but also by its ability to balance large variations in the supply and demand of electricity. 

First phase

A good first phase could be to operate the individual process steps, modularly and at a small scale. If this phase is successful, there are two main paths ahead. One would be to directly construct a full-scale synthetic kerosene production plant; the other would take an intermediate phase of constructing a small-scale plant before doing the same. Both come down to integrating the process and scaling it.

World’s first power-to-liquids production plant.

The world’s first power-to-liquids production plant opened in Dresden in November 2018. The new rig uses Power to Liquid technology to transform water and CO2 to high-purity synthetic fuels (petrol, diesel, kerosene) with the aid of renewable electricity.

Soon to be followed by the plant at the Karlsruhe Institue of Technology 

For the first time, they combined all four requisite chemical process steps in their compact pilot plant into a single continuous process. The research partners KIT, Climeworks, Sunfire, Ineratec achieved thereby the maximum utilization of carbon dioxide along with a particularly high level of energy efficiency. Especially as the material and energy cycles are recycled internally. 

The existing pilot plant can produce around ten liters of fuel per day. A plant capable of 200 liters per day will soon be developed as part of the second phase of the Kopernikus P2X project. After that, a semi-industrial pilot plant will be built with a production capacity of 1500 to 2000 liters per day. This would theoretically make it possible to achieve efficiency levels of around 60 percent. This means that 60 percent of the green electricity that is used could be stored as chemical energy in fuel.

Just 4 steps to fuel

In the first step, the plant extracts carbon dioxide from the surrounding air in a cyclic process. The Direct Air Capture technology from Climeworks, a spin-off from ETH Zurich, uses a specially treated filter material for this purpose. The filters, through which the air flows, absorb carbon dioxide molecules like a sponge. Under a vacuum and at 95 degrees Celsius, the adsorbed carbon dioxide separates from the surface again and is pumped out.

In the second step, carbon dioxide and water vapor are simultaneously split by electrolysis. Developed by the technology company Sunfire, this so-called co-electrolysis process produces hydrogen and carbon monoxide in a one-step process. In other words, they produce a synthetic gas that forms the basis for a variety of processes used in the chemical industry. With its high level of efficiency on an industrial scale, co-electrolysis can chemically bind 80 percent of all of the green electricity used in the synthetic gas.

In the third step, long chain hydrocarbon molecules are formed out of the synthetic gas using the Fischer-Tropsch process, the raw products for fuels. Ineratec, a spin-off from KIT, is supplying a micro-structured reactor that offers a large surface area in a very small space in order to reliably dissipate heat from the process and use it for other steps in the process. In this way, the process can easily be controlled, is able to cope well with alternating loads and is expandable in a modular way.

Finally, the fourth step optimizes fuel quality and the yield. KIT has integrated this sub-process, known as hydrocracking, into the process chain. In a hydrogen atmosphere, the long hydrocarbon chains are partially broken down in the presence of a zeolite-platinum catalyst. They thereby change the product range towards more usable fuels such as gasoline, kerosene and diesel.

On-site energy generation

This process offers enormous potential, especially in view of its modular character. Due to the low risk associated with scalability, the threshold for implementation is significantly lower than that of a large-scale central chemical plant. The process can be installed in a decentralized manner and can therefore be used where solar, wind or water power is readily available.

Targeting natural Hydrogen

The quest for sustainable energy supply at low environmental and economical cost is a major driver in the current context of the energy transition toward a low carbon society. Hydrogen (H2) is a carbon-free fuel par excellence because its oxidation (i.e. combustion, fuel cells) emits only water. However, to this point, H2 has only been regarded as an energy vector and not as a credible large-scale alternative to hydrocarbons since most production methods (e.g. methane steam reforming) currently in use only postpone CO2 emissions. 

Its massive production through water hydrolysis using renewable energy is also very challenging, as it requires both pure water resources and local storage infrastructures. 

The discoveries of hundreds of natural H2 seepages, generally connected with circulation of hydrothermal fluids through ultramafic rocks, both under the seafloors and on the continents, remove these obstacles but raise important questions regarding the energy potential that these sources can represent.

Exploration of substantial sources of natural hydrogen gas will yield low-cost hydrogen and helium. 

To explore for this new natural resource, Natural Hydrogen Energy LLC (NH2E), was formed in the United States. an international Team of geologists and chemists with extensive knowledge of natural hydrogen working togehther with experts in drilling and gas production.

After several years of intensive research, NH2E selected and obtained mineral leases over promising geological sites. The first drilling in the USA was successfully completed by NH2E in 2019. The results show that extraction of hydrogen and helium gases is possible.

Capturing large quantities of CO2 at the source will yield low-cost CO2

Four companies in the port of Rotterdam are participating in the plans to capture and store the CO2 that normally goes up in their processes. Shell, ExxonMobil, Air Liquide and Air Products are committed to the plans. A central pipeline transports the greenhouse gas to an empty gas field under the North Sea.

The storage of CO2 is the industry’s most important plan to substantially reduce emissions in the short term. “We start with about two megatons a year,” says project leader Wim van Lieshout of Porthos, as the project is called. “That’s equivalent to the emissions of a large coal-fired power station.”

ExxonMobil’s director Erik van Beek explains that it is primarily about the CO2 that is released during the production of hydrogen. Hydrogen is a feedstock at the refinery. “Of course we are also innovating, but CO2 capture and storage is something you can do relatively quickly.

Many international climate reports mention CO2 capture and storage as necessary to reduce emissions. And in the Dutch Climate Accord, too, it is a measure that the industry wants to take in order to ultimately achieve seven of the intended fourteen megatons of CO2 reduction. Once all permits and subsidy applications have been completed, the first CO2 can be stored in 2023.

The technique for capturing and storing CO2 is not new and is already being used in Norway and Canada, among other countries. Oil and gas companies pump CO2 into empty fields to get the last remnants out. Porthos may well be the first large-scale CO2 storage project designed to reduce emissions. The project is also being watched with interest in Europe.

Part of the captured CO2 can also be used for the production of Synthetic Kerosine. 

If low-cost CO2 and low-cost hydrogen would be available in large quantities, production of large quantities of low-cost Synthetic kerosene is feasible.

Sources: Transport & Environment Study, Published: October 2018,
CO2-Based Synthetic Fuel: Assessment of Potential European Capacity and Environmental Performance Report by Adam Christensen, Ph.D. Chelsea Petrenko, Ph.D, november 2017, Quintel Intelligence BV, The Chemical Engineer, Innovation Origins, Kopernikus Projekt, Natural Hydrogen Energy LLC, Shell, Clean Energy Wire.