1 best of : Stacked DAC: A Direct Air Capture CO2-to-Gasoline Scheme that Works?

Twice now I’ve been dazzled by schemes for scrubbing CO₂ out of the atmosphere and turning it into fuel. In July 2008 it was NaOH scrubbers and exotic catalysts; in March 2012, it was ionic amine fluids and a formic-acid fuel cell. Both were tricky to describe, energy intense, and costly; years later, neither is yet ready for prime time. Then this June my newsfeed lit up with tales of a refreshingly simple new direct air capture (DAC) CO₂-to-fuel scheme that’s already running in a pilot plant.

Canadian energy company Carbon Engineering—founded by Harvard professor David Keith with venture capital funding from Bill Gates—has been scrubbing CO₂ for three years now in Squamish, British Columbia, for conversion to liquid fuels. An obvious secret to CE’s success is its reliance on existing industrial technology. The scrubbing operation is essentially a power-station cooling tower grafted onto a pulp/paper mill, augmented with tweaks from the water-treatment and gold-extraction industries. The fuel conversion involves universally understood Fischer-Tropsch gas-to-liquid tech.

Instead of condensing steam, air in this cooling tower flows past an alkaline solution of potassium hydroxide and water. Mildly acidic CO₂ gloms onto the basic potassium hydroxide, forming potassium carbonate. Calcium is then used to grab the carbon from the potassium in a pellet reactor borrowed from the water-treatment biz. Finally, pure CO₂ is liberated the way gold is separated from ore in a gold-ore roasting calciner. A paper mill’s caustic recovery loop keeps the potassium and calcium in continual reuse.

The process above is expected to consume between 1.5 and 2.4 megawatt-hours of energy per metric ton of CO₂ depending on plant configuration, with the lion’s share heating the roaster. Energy is a major driver of the operation’s cost, which, when scaled up for larger commercial production, is estimated to cost $232 per metric ton in the near term, falling to as low as $94 per metric ton in five to 10 years—a significant savings relative to chief rival, Zurich-based Climeworks, which is currently converting CO₂ via solid-oxide fuel cell at a cost of $600 per metric ton.

Carbon Engineering’s cost projections, published in the energy journal Joule, are based on scaling up the Squamish plant from its current 1-metric-ton-per-day scrubbing rate. Oh, and if you’re wondering why CE doesn’t just tap the CO₂-rich stream from a coal or natural-gas smokestack, it’s because that carbon originates underground—fuel produced from it wouldn’t qualify with regulators as carbon neutral. That’s bureaucracy for you.

One indication that CE’s pricing is in the ballpark: It aligns nicely with the $94 to $185 per metric ton that companies are currently paying for offset credits to comply with the California Low Carbon Fuel Standard. That standard will likely assign a carbon-intensity figure of about 30 to CE’s fuel—down from 53 for California corn ethanol and 100 for crude-based gasoline. Selling this fuel is what will eventually pay back Bill Gates’ investment.

Of course, carbon harvested from the sky has no energy value until hydrogen atoms are attached via Fischer-Tropsch, and that hydrogen accounts for about 70 percent of the cost of the resulting fuel. Keith expects Carbon Engineering to generate hydrogen using electrolyzers that are similar to proton-exchange-membrane fuel cells, powered by solar or wind electricity. He projects that PEM costs will drop from $1,000 per kilowatt to less than $500 in five years when his commercial plant comes online. He’s designing that plant for an annual scrub of at least 300,000 metric tons of CO₂—enough to produce 2,000 barrels of gasoline per day. His projected gasoline cost target is within 50 percent of today’s crude-oil refined product.

Here’s why we should all be rooting for projects like this to work: As liquid fuels approach carbon neutrality, we can burn them virtually guilt free. This will be a boon for difficult-to-electrify air and sea transportation, and it could allow us to continue driving our beloved combustion engine vehicles forever.

Stacked DAC: A Direct Air Capture CO2-to-Gasoline Scheme that Works? – Technologue

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