In a commentary in the journal Joule, Rob McGinnis, founder and and CEO of Prometheus, a company that is developing technology to remove carbon dioxide from the air and turn it into fuels, discusses the technology advances that could lead to the potential price-competitiveness of renewable gasoline and jet with fossil fuels.

Recent breakthroughs in separations and catalysis, along with long-trend reductions in solar and wind electricity costs, have significantly increased the potential for cost-competitive renewable fuels from direct air capture (DAC) of CO₂. This is an important development because there is little time available to reduce CO₂ emissions sufficiently to avoid the worst effects of climate change. Transportation fuels contribute a significant portion of current CO₂ emissions, accounting for 23% of global greenhouse gas (GHG) emissions and up to 40% of GHGs in developed economies, offering significant opportunities for emissions reduction from the decarbonization of such fuels.

Electrification of the global vehicle fleet, which now totals over 1 billion cars and trucks, or conversion of vehicles to use novel fuels like hydrogen, cannot proceed quickly enough to address the climate crisis. Replacing fossil-fuel gasoline, diesel, and jet fuels would be able to proceed at a much faster pace, because it does not require the replacement or retrofit of the existing vehicle fleet. In order for this fuel replacement to occur, however, renewable fuels must offer the same or better performance than fossil fuels at the same or lower price.

—Rob McGinnis

McGinnis highlights three technical advances that he believes will make this possible.

1. Aqueous CO₂ electrolysis with base-metal catalysts. The conversion of CO₂ to fuels in these inexpensive water-based systems has shown high faradic efficiencies for reduction of CO₂ to C₂ fuel products such as ethanol. Previously, the reduction of CO₂ to CO for syngas for use in Fischer-Tropsch was considered promising; however, he noted, these processes are energy and capital intensive, and require massive scale to become practical.

2. The effective upgrading of alcohols to gasoline, diesel, and jet fuels. This upgrading requires an inexpensive catalysis step (oligomerization and dehydration), which is exothermic and compact, operating at moderate temperatures and pressures.

3. The separation of ethanol and other fuel products from water. A previous obstacle to the aqueous CO₂ electrolysis pathway has been the difficulty of separating ethanol and other alcohols from water. Separation of these fule products from water can now be achieved by nanotechnology-based separation, operated at room temperature and pressure.

These advances together allow for inexpensive systems that can make true gasoline, diesel, and jet fuels from atmospheric CO₂. These systems can run at room temperature and atmospheric pressure using only electricity rather than the mix of electricity and fossil fuels (such as methane for process heat) typically required by previous approaches. One practical advantage of non-thermal processes using only renewable electricity is that they can be turned on and off quickly to match intermittent supplies of renewable power. This is much better suited to the availability of renewable energy sources than thermal processes that must run 24/7 for stable operation.

—Rob McGinnis

McGinnis projects that by putting all of these advances together, it will be possible to offer renewable gasoline from direct air capture CO₂-to-fuels processes within the next two years that is price competitive with fossil gasoline. Once demonstrated, the main challenge will be in achieving speed to scale.

In order to replace all fossil gasoline in the United States, assuming 66% resource availability and a 60% conversion efficiency, an additional 1.4 TW of combined solar and wind capacity for the United States alone will be required.

—Rob McGinnis

Resources

• Rob McGinnis (2020) “CO₂-to-Fuels Renewable Gasoline and Jet Fuel Can Soon Be Price Competitive with Fossil Fuels,” Joule doi: 10.1016/j.joule.2020.01.002