Synthetic hydrocarbon fuels made using captured CO2 are likely to be worse for the climate than conventional oil products, because the renewable energy used for their manufacture would be better used reducing emissions.
This post looks at synthetic hydrocarbon fuels made from combining hydrogen and CO2 – either fossil CO2 captured from an industrial process or CO2 captured directly from the air. (It excludes fuels made from biomass or other biogenic feedstock, which raise different issues.) These fuels (usually called e-fuels) are very similar to conventional oil products such as diesel, so they can be used in a very similar way.
However in practice e-fuels offer few if any climate benefits over using conventional oil products. Indeed they are worse for the climate, because their manufacture requires vast amounts of renewable electricity that would be better used elsewhere to reduce emissions.
Burning e-fuels has similar emissions to burning conventional oil products – by design they are very similar products. If e-fuels are made from fossil carbon captured from an industrial process they end up putting that fossil carbon into the atmosphere when they are burnt. This is not compatible with a net-zero economy[i].
In practice using e- fuels is even worse than this simple comparison implies. Their manufacture uses huge amounts of renewable electricity[ii]. All this energy is used in just recreating a product already found in nature. The renewable electricity could be put to much better use reducing emissions elsewhere – renewable electricity will be scarce for the foreseeable future because of the huge scale-up needed to displace existing fossil fuel use.
This makes e-fuels worse for the climate than conventional oil products: they have similar emissions, but making e-fuels diverts large amount renewable electricity from reducing emissions elsewhere.
Instead of using fossil CO2 that has been captured from an industrial process, some proponents of e-fuels make their case based on use of CO2 captured directly from the air (Direct Air Capture – DAC)[iii],[iv]. However, again this makes no sense.
Making e-fuels with CO2 captured from the air is roughly carbon neutral, but so is burning conventional fuels and removing CO2 from the atmosphere using DAC with permanent storage (usually referred to as DACCS). Capturing a tonne from the air then emitting a tonne from e-fuels is simply equivalent to emitting a tonne from conventional oil products, then capturing that tonne and permanently storing it (see Figure below). It achieves this same result – approximate carbon neutrality – using either e-fuels or conventional hydrocarbon fuels. In both cases it’s the DAC that makes the cycle carbon neutral. But, again, much greater amounts of renewable electricity are required in the case of e-fuels, with no compensating climate benefits over conventional oil products. It would be better to use the additional renewables elsewhere, including additional DAC if necessary, which although expensive would at least take more carbon out of the atmosphere.
Much better is DACCS without any use of hydrocarbon fuels. This is carbon negative, with net removal of CO2 from the atmosphere – likely to be a crucial long term need.
Figure: Comparison of various uses of Direct Air Capture
Furthermore, DAC is itself very expensive and energy intensive, requiring yet more renewable electricity. It likely to remain so due to the low concentration of CO2 in the air – about a factor of 100 less than in a typical exhaust gas at a factory or power plant. This is likely to make any approach using DAC difficult and expensive over at least the next two or three decades.
A comparison with enhanced oil recovery (EOR) illustrates the problems with e-fuels. A tonne of CO2 captured from the air can be used to make e-fuels, or to produce conventional oil by injecting the captured CO2 into an oil reservoir to recover more of the oil. These approaches make similar products, and therefore have similar emissions, but the EOR route uses much less renewable energy. The energy can instead by used for reducing emissions or additional DAC. (To be clear, I am not advocating EOR, simply suggesting in had advantages over e-fuels.)
E-fuels are simply not a good way to reach net-zero. We need to focus on real solutions, including using renewable electricity wisely.
Adam Whitmore – 18th May 2021
[i] A recent article in Nature Climate Change made this point, but did not go on to make the comparison with continued burning of fossil fuels.
[ii] The huge demand for renewable electricity will remain even if technology improves. This is because manufacture of e-fuels is intrinsically energy intensive, as CO2 is such a stable molecule. Conventional oil refining also uses energy, but much less than making e-fuels.