E-Fuels as Reduced Carbon Emission Options

Abstract

Synthetic “E-fuels,” or electro-fuels, have been introduced as a potential reduced carbon emissions energy source for power generation and vehicle propulsion applications. Environmental change is needed, with the transportation sector alone producing approximately a quarter of global greenhouse gas (GHG) emissions. Hydrogen, produced from electrolysis to avoid GHG production, is used along with CO2 or nitrogen to generate these electro-fuels, through the Fischer–Tropsch process. Direct air capture (DAC) of atmospheric carbon dioxide or biomass combustion effluents can provide sources for these gases to be combined with hydrogen to generate synthetic methanol, methane, or ammonia—the three most widely discussed E-fuels. In addition, “ER-fuels,” or electrically reformed fuels, are a similar option to E-fuels, where refinery fuel gases, such as ethane or propane, are reformed before synthesis to produce the final fuel. E-fuels, on the other hand, are generated from hydrogen and carbon either captured or produced, not from existing fuels. Redox couples, such as vanadium, can additionally be used as zero emission fuels; they are “electrically rechargeable” in that, through electrolysis, the reducing agent is produced, and then using a fuel cell (FC), the reverse occurs, and electrons plus the oxidizing agent are produced. These synthetic fuels are approximately carbon neutral when the hydrogen is sourced from renewable powered electrolysis, mainly solar and wind energy, as the amount of carbon dioxide consumed is roughly equivalent to the amount produced upon their combustion or FC energy conversion. Combustion in gas turbines or reciprocating piston-cylinder engines and FC electricity generation are the two main uses for extracting energy from E-fuels. Fuel cells are shown to have higher efficiency, but combustion provides fewer infrastructure changes and easier implementation. Both options provide a method for controlling carbon emissions using E-fuels as opportunities for energy storage.