e-Fuels
Using the same infrastructure as fossil equivalents
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Electrofuels (e-fuels) (a.k.a. power-to-liquids, power-to-gas, or power-fuels) are fuels (in gas or liquid form) synthetically produced w/o petroleum or biomass, made by synthesizing CO or CO2, captured from gases emitted by industrial processes or directly from the air (CO2 is about 0.04% of the atmosphere), and hydrogen, obtained from sustainable electricity sources, such as solar & wind. E-fuels are a class of synthetic fuels: which differentiate from biofuels (primarily produced from biomass) [Engie, 2022].
Decarbonization
By drastically reducing the harmful emissions associated w/ combustion engines, e-fuels play a key role in decarbonization strategies. Thru the creation of a circular carbon cycle, their carbon footprint is a lot lower than oil-based fuels: as the process uses CO2 in production and releases around the same amount of CO2 when burned, they can be considered carbon neutral.
Examples of liquid e-fuels are e-methanol, e-ethanol, e-gasoline, e-kerosene, and e-diesel; of gaseous e-fuels are e-methane & e-ammonia. E-fuels have the advantage of using the same infrastructure as their fossil equivalents (petrol, diesel, kerosene, methanol, natural gas), putting them in competition with biofuels, which offer the same advantage [Engie, 2022].
Fischer-Tropsch
Fischer-Tropsch (FT) is a high successful, commercially available method for producing legacy fuels, e-fuels or chemicals. It is a suite of complex chemical reactions that converts a mixture of CO & H2 (syngas), produced from coal & biomass thru gasification, and from NG thru SMR, in into liquid hydrocarbons, which occur in the presence of metal catalysts (iron, cheaper, or cobalt, more efficient) at temperatures ranging from 150 to 300 °C, and pressures of one to several tens of atmospheres (typically 20–40 bar).
FT is an important reaction in coal & biomass liquefaction processes. It was use in Germany, during the WWII, and in South Africa in the 1950s to produce fuels (synthetic diesel & petrol fuels) for transport, and today is one of the main ASTM-approved methods for producing SAF.
Production routes
The primary targets are methanol & diesel [RG, 2022]. E-diesel (C12H24: C10H20 to C15H28) is produced via FT process w/ an efficiency of 69%. An alternative to the FT process is methanol-to-diesel. Figure 1 shows different process steps for e-fuels production. Figure 2 shows the e-fuels production routes. Figure 3 shows the energy efficiency among different technologies.
Cost & efficiency
While e-fuels can be very low-carbon if made from renewable electricity, they can’t be low-cost at the same time (they reach five times the price of oil products). The e-fuels production process is inherently inefficient, converting at best half of the energy in the electricity into liquid or gaseous fuels [ICCT, 2020].
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Applications
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In addition to being used to H2 transport & storage, e-fuels can be used from heavy-duty long-haul transport decarbonization to green chemistry [Engie, 2022]. They are also a way of recycling & recovering CO2, from which most e-fuels are made.
Figure 1: Process steps for the production of e-fuels
Figure 2: E-fuels production routes
Figure 3: Energy efficiency of different techs
Fossil fuels provide a large amount of energy for a small amount of fuel, but they add CO2 to the atmosphere. E-fuels can be dropped straight into the existing engines of cars, aircraft, commercial, and agricultural vehicles, allowing them to run sustainably in exactly the same way and same performance as they do on fossil fuels, w/o engine modifications [Zero, 2023].
Examples [CC, 2022]
Some e-fuel examples are:
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e-methane (CH4)
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e-hydrogen (H2)
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e-methanol (CH3OH)
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e-DME/ e-OME
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e-gasoline & e-diesel
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e-kerosene (or "e-jet": highly refined kerosene)
Advantages & challenges
Although biofuels are cheaper than e-fuels, they face availability limitations aggravated by competing demand in the bio-economy and sustainability constraints with respect to land use [EP, 2023]. Challenges to scaling up e-fuels include i) high production costs, ii) low energy efficiency, and iii) air pollutant emissions: despite the reduction in CO2 emissions, using e-fuels does not necessarily reduce the emissions of other GHG and gases responsible for air pollution, such as CO, ammonia & NOx [CC, 2022].
Long life
Recently, Germany, w/ its Italian & Polish allies, obtained an agreement authorizing sales beyond 2035 for combustion engine vehicles that use synthetic fuels. The EU must now establish a provision to classify vehicles running on e-fuels as carbon neutral. Berlin hopes that this legislative process will be completed by Fall/2024 [Le Monde, 2023].