Recycling Combustion: Creating Fuel From Carbon Dioxide
[Aug. 4, 2020: Motor Trend]
I admit that writing about this month's subject makes me feel a bit like Charlie Brown running toward Lucy and her football. This particular football, as a general concept, is just so alluring: Take a problem (excessive atmospheric CO2) and turn it directly into a solution that lets us keep driving the vehicles we love—via carbon-neutral gasoline, diesel, and jet fuel. This latest scheme solves many of the problems that have yanked the ball on my previous runs, giving me the courage to barrel toward it one more time.
My three prior columns explaining ways of converting atmospheric CO2 into liquid fuels ran in July 2008, June 2012, and Oct 2018. The early ideas started by scrubbing CO2 from the air with an absorbent compound such as sodium hydroxide in water, isolating the CO2, cracking it into carbon monoxide, and then hydrogenating it via various catalytic reactions such as Fischer-Tropsch. Capital and energy input costs doomed these ideas.
Ionic salts and formic-acid fuel cells running in reverse helped bring the projected energy input cost down to around $5-$8/gallon when I wrote my second column. Then Carbon Engineering (with backing from Bill Gates) leveraged pulp/paper mill tech to drop the CO2-scrubbing cost to about the price established for carbon-offset credits by the California Low Carbon Fuel Standard. Combined with anticipated drops in the cost of the proton-exchange membranes that were to provide the clean hydrogen, Carbon Engineering reckoned the per-gallon fuel price would fall to within 50 percent of higher 2018 pump prices.
Now Prometheus Fuels is aiming three new silver bullets at this problem:
Aqueous CO2 electrolysis using base-metal catalysts to turn CO2 directly into ethanol;
Separating the ethanol from the water without an energy-intensive distilling process; and
Upgrading the ethanol to gasoline, diesel, or jet fuels using exothermic catalytic reactions, which give off heat rather than absorbing it.
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The CO2 collection step still requires a large air-to-water contact area like a cooling tower. But rather than trying to chemically isolate pure CO2, the electrolysis step can happen with just 2 percent CO2 in water.
The electrolysis step employs sheets of graphene covered in carbon nanospikes doped with copper nanoparticles. This anode's intense folds and spikes help reduce the CO2 to CO and then grab hydrogen from the water to form ethanol at a 63 percent Faradaic efficiency (the number of electrons that end up in the ethanol) with 84 percent of the CO2 becoming ethanol.
Ethanol then gets separated from the water via carbon nanotube membranes that are highly selective for alcohols while rejecting water (I have friends like that). Minimal energy is consumed in this step.
Upgrading from ethanol to complex hydrocarbons involves novel but inexpensive zeolite catalysts involving indium and vanadium (InV-ZSM-5). The reaction takes place at atmospheric pressure without added hydrogen, and the process can tolerate water in the intake stream. It occurs at elevated temperature, but the overall net reaction produces its own heat.
The resulting CO2-based crude stock contains no sulfur, benzene, heavy metals, or other impurities that petrochemical refiners must contend with. A third of the resulting liquid hydrocarbon mix includes chains of five carbons or more, and the calculated research and motor octane numbers are 105.7 and 90.6, respectively. The entire process can be operated with (clean) electrical input only and can be turned on and off quickly to match intermittent renewable energy supplies.
Los Angeles just contracted a long-term purchase of solar energy at 1.997 cents/kWh. Presuming 60 percent electrochemical efficiency, the 36.3 kWh of energy in a gallon of Prometheus gasoline may require 61 kWh of input energy, or $1.22 worth of sunshine. This price doesn't include the CO2 scrubbing or final refining and other costs, but neither does it presume carbon offset credits that may apply.
Water usage in the CO2 scrubbing could be where Lucy yanks my football again, but BMW i Ventures just invested $12.5 million in June, and Prometheus has announced plans to start selling its gasoline in California at the end of 2020. Might I finally make contact with my football and split the uprights?.... Read More