Tuesday, May 10, 2016

A New Angle on Carbon Capture

In my last couple of posts I looked at the difficulty of meeting ambitious targets for cutting greenhouse gas emissions (GHG) without help from the lower-emitting portions of our current energy mix. Last week ExxonMobil announced that it is pursuing a new pathway for capturing carbon from power plant exhaust. That could help revive another important strategy for large-scale emissions reduction from our existing energy sources.

Carbon capture and sequestration (CCS) has fallen out of favor, lately, mainly due to the high cost and technical challenges of the early prototypes for large-scale implementation of the technology. Not only are the initial investment costs of today's CCS hardware still very high, but it is also inherently expensive to operate. That's because of the high energy consumption of the process, resulting in a "parasitic" load on the host power plant that reduces its net output by up to 20%, making the remaining output much more expensive. That creates a large deterrent in any market that doesn't provide either direct subsidies for carbon removal, or a high carbon tax or price for traded emissions offsets.

Another reason that CCS has received less attention recently is that the costs of renewable energy technologies like wind and solar power have kept falling. To some they now look cheap enough, especially with further cost improvements extrapolated, to enable us to reach our emissions goals mainly through wider deployment of solar modules and wind turbines.

Even if that were technically feasible, like most other energy industry experts I have met I am convinced that the deep emissions cuts desired for mid-century will require implementing or retro-fitting CCS onto the fleet of coal and gas-fired power plants that will likely still be in service decades from now. CCS underpins several of the emissions stabilization wedges pioneered by Princeton engineering professor Rob Socolow and his colleagues ten years ago.

What makes the approach that ExxonMobil and FuelCell Energy, Inc. have described so attractive is that, instead of being a drain on power generation, capturing CO2 via fuel cells would actually add significantly to a facility's reliable power output. It would increase revenue, rather than curtailing it.

The clever bit, and its potential advantage over current carbon-capture technology, is that CO2 capture in a carbonate fuel cell occurs as a byproduct of the power generation step. That means that it doesn't require a big, expensive, power-hungry process unit, the only function of which is to strip CO2 from flue gas and concentrate it for subsequent shipment and storage.

These fuel cells would still require natural gas for fuel, and they would produce CO2 emissions in the process of generating electricity, though at a lower rate than the coal or gas-fired plant with which they would be partnered. However, both their direct emissions and the CO2 extracted from the power plant exhaust would come out in a highly purified form suitable for geological sequestration and stay out of the atmosphere.

That brings up an important advantage of this approach over various schemes to capture CO2 directly from the atmosphere. Although the article on the Exxon/Fuel Cell Energy development in MIT Technology Review  described the CO2 concentration in power plant flue gas (5%-15%) as "low", that is still hundreds of times higher than its concentration in air.

400 parts per million of CO2 in the atmosphere may be worrying from a climate perspective, but it is still just 0.04% of air that remains mostly nitrogen and oxygen. And the lower the concentration, the harder--and normally more expensive--it is to extract. (Green plants can do this trick cheaply thanks to billions of years of evolution combined with cost-free sunlight.)

The press release makes it very clear that this new carbon-capture technology has so far only been demonstrated in the lab. Scaling it up will require additional work, and success is uncertain. Many other promising innovations, including a host of cellulosic biofuel technologies, have failed to scale. However, its potential applications are compelling enough to justify a lot of patience and persistence. I wish them luck.

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