How it works - Synthetic fuels
Synthetic fuels are nothing particularly new, and we're familiar with GTL (Gas to Liquid) fuels that are made from natural gas.
The Fischer-Tropsch process that's used for this can employ all kinds of hydrocarbon raw materials, to create designer fuels like kerosene, petrol and diesel, all of them significantly purer and lower in emissions than equivalent fuels derived from crude oil. It can also be used to synthesise the base materials for advanced synthetic lubricating oils. There are other variations, where bulk methanol is first produced by this process, and then the ExxonMobil MTG (methanol to gasoline) process used to create the actual fuels. Most GTL fuels are blended with normal petrol and diesel, or used in low-pollution areas by city utility vehicles and public transport But the new interpretation of synthetic fuel is being taken to mean products not derived from unsustainable crude oil and natural gas, but fuels constructed from the fundamental petroleum hydrocarbon building blocks of hydrogen, oxygen, and carbon. This essentially means fuel made from just atmospheric air and carbon dioxide, along with green hydrogen. How is this possible? Well, internal combustion engines and many other combustion processes involve breakdown of a mixture of very complex hydrocarbons.
The products of combustion contain a number of simple compounds – carbon dioxide, carbon monoxide, and water, along with some pollutants – as we know. The magic of the synthetic fuel process is that, with the help of green hydrogen, and suitably cleaned up, those combustion products and air can be re-combined to create the same types of straight chain aliphatic hydrocarbons as found in the original fossil fuels, but without the nasty stuff. The results are sustainably sourced zero carbon synthetic fuels that are purer than the natural product, and which burn more cleanly in internal combustion engines.
The Fischer-Tropsch process itself was used in wartime Germany for the production of oils and fuels when the fuel resources available were just coal. It requires carbon monoxide to be produced from around half the carbon dioxide input, which is then mixed with hydrogen and air to produce Syngas to feed the process. Zero carbon synthesis means the hydrogen must be created using green energy, usually by electrolysis of water using green electricity. Also involved are catalysts, usually in various forms of cobalt, nickel, or iron, that help determine the chemical reactions and the end products. Along with the processing conditions, primarily temperature and pressure, the end product can be designed to produce the particular hydrocarbon compounds required.
Similar synthesis of fuels are possible using other zero carbon raw materials, such as biomass derived from the pyrolysis of waste vegetation matter, and waste vegetable oils, but the green hydrogen route appears to offer the greatest possibilities for bulk production. We can speculate that promotion of these synthetic fuels will target specific transport situations where battery charging and electric power are impractical, like city transport, off-road equipment, and vehicles used in unpopulated areas, like deserts and polar regions. They also might be used for leisure pursuits, like track racing, rallying, and perhaps for marine use, when the charging practicalities are challenging for pure electric power. There will inevitably be some availability of synfuels for use in our existing fossil fuel cars, and the costs may be low enough to attract those stubbornly resistant to electric transport, and for use in historic cars – but only when, and where, internal combustion engines remain legal.
But such fuels should realistically be seen as the dying gasps of a condemned species, the combustion engined car, sad though that may seem for many of us. If we should need a clinching argument for accepting this, there's no prospect of changing the laws of thermodynamics to make combustion engines any more efficient, or anywhere near comparable with zero carbon electric power. Over 50% of the energy content of any green synthetic fuel will still be lost as heat, to an atmosphere that's warming rapidly enough. For all the teething problems of road transport electrification, it's all too easily forgotten that emissions-free, and 80 to 90% efficient, electric cars largely eliminate the irrecoverable waste heat discharged to the atmosphere by existing internal combustion engine transport. Whatever clever green fuel they may burn, the vast majority of them are seemingly heading for the end of the road, like it or not.