I asked a proper chemist about my table of synthetic fuels (synfuels). They said something to the effect that they were in two minds about the use of carbon in fuels, and that they thought it was a shame to put so much effort into synthesising molecules, only to burn them. On the other hand, they were perfectly happy with synthesised molecules being used as raw materials for chemical engineering.
I have encountered the expression of similar ideas before, and I think it partly results from a well-established paradigm that considers chemical engineering somewhat apart from energy engineering. The fact of the matter is that molecules are being restructured all the time, everywhere in the vast and sprawling petroleum- and Natural Gas-based energy system, as well as in Big Chemistry.
These days, there are very few things that happen at oil refineries that don’t involve altering molecules; including the use of combustion and gasification to deal with waste disposal, and provide on-site energy. Synthesis is part of the bedrock and fabric of fuel production – it’s not a step too alien.
My reply was basically to say that I understood the chemist’s reticence about maintaining the use of carbon in the new fuels of the Energy Transition. It would make a lot of sense to jump straight to a Hydrogen-and-Renewable-Power Economy. But, I said, as there are a billion Internal Combustion Engines (ICE) on the road around the world, and this is going to be that case for at least the next couple of decades, we need to continue to provide liquid fuels, and that this can be most easily done using carbon-based molecules, as they naturally have higher boiling points.
As for the implication that there is a high cost and high inefficiency in synthesising molecules for use as road/rail/ship/plane fuel, that ain’t necessarily so. Like all things, it will depend on concentrating effort in improving processes and equipment. Task forces. Investment. Focus.
Basically, as we are stuck with needing to provide liquid carbon-based drive for the global fleet for decades to come, and yet we need to undergo an Energy Transition to much lower net emissions fuels, we have two main choices for an approach :-
1. Decomposition
Decomposition of biomass can be done in a range of biological and thermochemical ways, some of which result in complex hydrocarbon/carbohydrate molecules; and others of which produce simple compounds (usually gases) that would need synthesis to make them into appropriate liquid fuels.
Where biomass can be decomposed directly to liquid fuels, there are often problems arising from contaminants and unwanted by-products. This sometimes gives a poor “atom economy”, and will lead to continuing criticism about waste disposal – essentially rejecting molecules and atoms – which is innately inefficient.
The current petroleum production and refining system has high levels of rejected molecules, such as carbon dioxide and sulfur. Managing this carries a high burden. Do we really want to reproduce that ? Where’s the optimisation ?
2. Synthesis
By taking biomass and water and industrial waste gases and using thermochemistry to break them all down into basic, foundational molecules, and then using renewable electricity to synthesise them into usuable fuels, stands a chance of being highly efficient in the use of molecules. Starting chemistry with smaller and neater molecules, and choosing which ones we use, means a higher possible eventual atom economy.
Sure, it would require a certain amount of solar power and wind power, but this wouldn’t be inefficiency in the traditional sense. There are plenty more sunshine where the last rays came from, and no waste is created by using their energy with less than 100% efficiency. And the wind keeps on blowing, even though we might use up a lot of wind power for chemistry, without creating slag heaps, or needing to bury carbon dioxide.
With synthesis, energy is chemistry, and chemistry is energy. But then, that’s the way it’s becoming anyway. Virtually every atom that goes into a petroleum refinery has to be processed before it’s fit for purpose. We are getting to the point where crude petroleum is no longer the best option for input feedstocks for liquid fuels.
In addition, synthesis allows us to put carbon dioxide to good use. At the moment, this would be unavoidable carbon dioxide created by industrial processes, and gathered for use in synthetic fuels.