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Could Carbon Capture Improve CAFE?

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A recent paper shows even a modest on-board carbon-capture system could produce significant corporate average fuel economy improvements for any automaker.

Could Big Oil play a pivotal role in slashing automotive carbon-dioxide emissions? Two giants of the oil industry, Saudi Aramco and ExxonMobil, are studying ways to strip the CO2 from the exhaust gases pumping out of every internal-combustion engine. And while the effort is very much in the research phase, the possibilities are intriguing.

They call it CCS, for carbon capture and storage. You already may have heard of efforts along these lines from coal-fired electric generating plants. There, the idea is to capture CO2 from the smokestacks and inject it into wells deep underground. For automotive applications, the idea is to follow a similar approach but use an on-board system for cars, trucks and buses.

Researchers at the University of Michigan Transportation Research Institute recently published a paper titled: An Overview of CAFE Credits and Incorporation of the Benefits of On-board Carbon Capture. The paper shows even a modest on-board carbon capture system could produce significant CAFE improvements for any automaker.

The authors show how a vehicle rated at 20 mpg (11.8 L/100 km), when fitted with an on-board system that captured 20% of the CO2 in the exhaust stream, could be rated at 37 mpg (6.4 L/100 km) from an emissions standpoint.

Remember, CAFE is not just about fuel consumption anymore. Starting in 2017, the EPA is adding regulations that stipulate reductions in greenhouse-gas (GHG) emissions as well.

Up to now, the only way to reduce GHG or CO2 has been to improve the fuel economy of the vehicle. That’s why automakers are investing so heavily in new powertrains and lightweight materials. But that is an expensive strategy that is starting to drive up the cost of cars significantly. The on-board capture approach, on the other hand, would unlink the MPG-GHG connection. But while the cost savings could be huge, it wouldn’t come easy.

Carbon dioxide is formed when two oxygen atoms join to a carbon atom. Bonding them together adds weight and volume. So the gas actually takes up more room and weighs more than the gasoline it once was a part of. Amazingly, burning one gallon (3.8 L) of gasoline weighing 6 lbs. (2.7 kg) generates almost 20 lbs. (9 kg) of CO2.  Capturing 100% of the CO2 would require storage in a reservoir three times larger than the gasoline tank. That’s why the researchers are proposing to capture only 20% of the CO2 in the exhaust stream, though their research model also shows the benefits of capturing 40%.

Interestingly, one of the proposed ideas is to capture CO2 and inject it into the gas tank as the gasoline is consumed. A membrane inside the tank would keep the CO2 separate from the gasoline.

But then what do you do with the CO2? It would have to be off-boarded every time a motorist stops to refuel. Could gas stations handle the off-boarding? And then what would they do with the CO2? Could motorists be paid for their CO2, which is used in food and beverage processing as well as in industrial applications? Clearly, there are plenty of questions to be answered.

There is a different, pre-combustion approach, too. Gasoline could be converted to a syngas and then shifted to CO2 and hydrogen. The CO2 then would be stripped out, and the engine would run on hydrogen. But a post-combustion system is considered the easiest to retrofit to a vehicle.

What I like about this research is that it’s looking at the GHG challenge from a completely different standpoint. Obviously Big Oil is interested, because it could extend the use of fossil fuels deep into this century.

But for automakers, it could provide a new way to reduce CO2 emissions much more cost-effectively than the way they’re doing it now.

John McElroy is editorial director of Blue Sky Productions and producer of “Autoline” for WTVS-Channel 56, Detroit, and “Autoline Daily,” the online video newscast.

 

 

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