e-Fuels

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 H2, obtained via electrolysis from sustainable electricity sources, such as solar & wind.  E-fuels are a class of synthetic fuels that differ from biofuels, which are primarily derived 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 well-established and commercially proven process for producing conventional fuels, e-fuels, and chemicals.  It comprises a set of catalytic chemical reactions that convert synthesis gas (syngas), a mixture of CO and H₂.  Syngas can be produced from coal or biomass via gasification, and from natural gas via steam methane reforming (SMR).  The process occurs in the presence of metal catalysts, typically iron (lower cost) or cobalt (higher efficiency), at temperatures ranging from 150 to 300 °C and pressures from about 20 to 40 bar.

  

FT is a key 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 sustainable aviation fuel (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 (reaching 5x 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.