The last few months have seen advances in the production of ‘electrofuels’, or liquid fuels made from green hydrogen and captured CO2 that can replace products made from fossil oil.

· Infinium, a Californian start-up raised $69m from Amazon and NextEra Energy to build manufacturing facilities that will make about 160 million litres a year of near-zero carbon liquid fuels.

· Prometheus Fuels, also in California, raised money from Maersk.

· Norway’s Nordic Electrofuels recently signed an agreement to take waste CO2 from a metals processor before building a 10 million litres a year plant.

· In the last few days Zero Petroleum, a UK start-up, provided the fuel for the first ever flight using 100% renewable aviation gasoline anywhere in the world.

I look briefly below at how low the prices of hydrogen and carbon dioxide will have to be for electrofuels to be competitive with oil.

The level of interest today in electrofuels (‘efuels’, ‘synthetic fuels’) remains low. There is, for example, absolutely no commentary in UK media or assessment in public policy documents. Even though these fuels can decarbonise difficult sectors, particularly aviation, understanding of the potential is very limited.

Many confuse efuels with conventional biofuels, particularly in the context of aviation. This is unfortunate; electrofuels can be made in unlimited quantities without needing agricultural land. They do not result in major greenhouse gas emissions, which biofuels generally do.

The reason for the lack of interest seems to be that electrofuels are assumed to be extremely expensive and unlikely ever to be competitive with liquid fossil fuels. This is a reasonable concern; at today’s costs electrofuels will be far more expensive to make than using fossil below. This will not always be the case.

In the table below, I write down the cost of the hydrogen and the CO2 that is necessary to make a tonne of C11H24, which I have used the representative hydrocarbon in aviation fuel.[1] I have not included any calculations of the cost to process the source chemicals into this hydrocarbon but this number will probably not exceed $200 per tonne of C11H24 and will not change the basic conclusion. (The details of the equations I have used are in the appendix to this note).

How the costs of H2 and CO2 affect the prospective price of aviation electrofuel

The calculations are described in the Appendix below.

What do these numbers show? In the left hand column, I suggest a price of $1.00 of hydrogen and $100 a tonne of CO2. These input price result in a total cost for a tonne of C11H24 of $746. On the right, costs of $2.50 and $250 result in an input cost per tonne of product of $1,865.

Are these numbers high or low? The current price of a tonne of aviation fuel is about $750 in the US. So in order for electrofuels to be competitive, the price of hydrogen needs to be about $1.00 per kilogramme and CO2 about $100 a tonne.

These are very demanding targets but - for example - the US government’s Earthshot programme is pushing for $1.00/kg hydrogen and $100/tonne CO2 by 2030. Direct capture of CO2 will eventually cost, according to leading industry player Carbon Engineering less than $100 a tonne at scale. H2 cost may well fall to $1.00 a kilogramme within five years in the sunniest locations where PV electricity is already very cheap.

Would a carbon tax on fossil fuels make much difference? Combusting a tonne of C11H24 will result in production of about 3.1 tonnes of CO2. A $100 a tonne carbon tax would therefore add about $310 to the cost of aviation fuel, taking it to approximately $1060, or about 5% below the electrofuels route at an H2 price of $1.50 and a CO2 price of £150.

In summary, at today’s costs for hydrogen and carbon dioxide, electrofuels would probably be at least five times the price of oil products, even when made in large quantities. But prices will fall as green hydrogen comes down in price and direct air capture of CO2 increases in scale. Even if electrofuels look expensive today, the logic of developing them is strong since they will work in existing applications such as aviation engines. Alternatives, such as pure hydrogen or ammonia require huge investment of time and money in designing, building and testing new aircraft and redesigned engines. Electrofuels may become the dominant source of energy for flying, even with today’s costs.

Appendix

A liquid fuel, such as aviation kerosene, consists of a mixture of hydrocarbons. A hydrocarbon is a molecule composed entirely of hydrogen and carbon atoms.

It is possible to manufacture hydrocarbons in chemical processes that result in no net emissions to the atmosphere after combustion. Put simply, all we need do is generate some hydrogen in an electrolyser, collect some CO2 from the atmosphere, turn it into carbon monoxide and then react the carbon and the H2 to make a hydrocarbon(s). The technology is well understood. The Fischer Tropsch process using this approach and is over 100 years old.

Making synthetic aviation fuel from zero carbon sources

Aviation fuel is a complex mixture of chemical compounds. Kerosene is the most important group of hydrocarbons in the mix. To make kerosene from hydrogen and captured CO2 requires two reactions.

1, The CO2 needs to be turned into carbon monoxide (CO). This is done using what is called the reverse water gas shift reaction.

CO2 + H2 -> CO + H20

This reaction requires hydrogen to be added. Not all the hydrogen that is added is captured in each run through the reactor but eventually it will be reacted with incoming carbon dioxide. The molecular weight of CO2 is 44 and that of H2 is 2. So for every tonne of CO2 converted to CO, 2/46ths of H2 will be needed.

CO has a molecular weight of 28. So a tonne of CO will require 44/28ths tonnes of CO2 as input and 44/28 * 2/46 tonnes of hydrogen.

2, The CO needs to be reacted with hydrogen to make a kerosene-like molecules. I have chosen C11H24 as the representative hydrocarbon. (The Fischer Tropsch reaction that turns CO and H2 into hydrocarbons produces many different molecules). The reaction is

23H2 + 11CO -> C11H24 + 11H2O

The total molecular weight of inputs is 354, including 46 for the hydrogen required. The weight of the useful output (C11H24) is 156. So to make a tonne of aviation fuel requires 46/156 tonnes of hydrogen and 110/156 tonnes of carbon monoxide.

3, Total inputs. I calculate that to make a tonne of C11H24, the refinery will require 3.10 tonnes of CO2 and 0.44 tonnes of hydrogen. (The extra 2.54 tonnes is lost as water, H20).

[1] Aviation kerosene includes a wide variety of hydrocarbons and other molecules. I have used C11H24 as a typical ingredient.