Electro Sustainable Aviation Fuel Powers First US Flight

Power-to-liquids technology seems almost like alchemy - converting gas and electricity into liquid fuel. However, the basic chemistry involved is well understood and is based on processes that have been known for more than a century. The difficulty has always been making it efficient and economical enough to compete with extracting crude oil from the ground.

The process starts off with electrolysis, during which water (H₂O) is split into hydrogen (H₂) and oxygen (O₂) by means of an electric current. This reaction is one that is known from GCSE and A-level Chemistry. If the electricity used comes from renewable sources such as wind, solar or hydroelectric power, the hydrogen produced is regarded as "green" since no fossil fuels have been burned in the process. The efficiency of the electrolyser is of great importance. Today's proton exchange membrane (PEM) electrolysers convert about 60 to 70 per cent of the electrical energy input into chemical energy which is stored in the hydrogen. Solid oxide electrolysers, which work at higher temperatures, can achieve greater efficiencies but are less developed in a commercial sense.

The carbon dioxide that has been captured then reacts with hydrogen through a process which usually has two stages. In the first stage, a reverse water-gas shift reaction converts the carbon dioxide and hydrogen into carbon monoxide (CO) and water; since the equilibrium of this reaction lies in favour of the production of CO at high temperatures, a large amount of thermal energy is needed. The carbon monoxide and the remaining hydrogen then go on to a Fischer-Tropsch reactor, where they combine on a catalyst (this is usually cobalt or iron based) to form hydrocarbon chains of different lengths. The Fischer-Tropsch process was developed in Germany in the 1920s, initially in order to produce synthetic fuel from coal gas; the chemistry is the same but the source of carbon is different.

Fischer-Tropsch synthesis yields a variety of hydrocarbons, including the lighter gases such as methane and ethane as well as heavy waxes. The kind of products obtained is determined by the catalyst, the temperature, and the pressure. In order to produce jet fuel it is necessary to have hydrocarbons with from eight to sixteen carbon atoms in each molecule, that is, in the C8 to C16 range. The lighter fractions can be pumped back into the reactor or employed as feedstock for naphtha. The more heavy fractions are cracked (that is, broken down into smaller molecules) by means of heat and catalysts, which is another common refinery process.

The synthetic kerosene produced is chemically very much like Jet A which is derived from petroleum. It has to comply with the ASTM D7566 specifications, these specifying strict limits on various properties such as flash point, freezing point, thermal stability, and energy density. It is currently allowed to be mixed with ordinary jet fuel in amounts up to 50 per cent, although the proportion that was used on the flight to Corpus Christi was not made public. The reason for the ASTM certification is that engines, seals, fuel pumps and storage tanks which are designed for Jet A function in the same way with the certified mixture and no modifications are needed.

When measured against battery-electric standards, the overall energy efficiency of the power-to-liquids process is fairly low. About 40 to 50 percent of the renewable electricity input is eventually stored as chemical energy in the liquid fuel. Compared with the 90 percent efficiency achieved when charging an electric vehicle battery, that seems wasteful, and it does. However, aircraft cannot operate on batteries (the energy density of jet fuel is some 50 times that of lithium-ion batteries on a mass basis), and for decades to come there is no feasible alternative to liquid hydrocarbons for long-haul aviation. Thus, power-to-liquids serves to fill that gap.

For A-level students studying either Chemistry or Physics, this process brings together the topics of electrolysis, chemical equilibria (specifically Le Chatelier's principle in the case of the reverse water-gas shift), catalysis, thermodynamics (using Hess's law for enthalpy calculations along the reaction chain), and energy efficiency calculations. It serves as a convincing illustration of how basic principles can be applied on an industrial scale.