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Don't Think 'Carbon Tax,' Think 'Carbon Deposit' - Technologue

Motor Trend logo Motor Trend 9/5/2014 Frank Markus

If you've consumed a carbonated beverage in a bottle or can, you've probably noticed the little note inscribed somewhere listing a ransom to be paid for the container's safe return. The dime deposit in my home state of Michigan is the nation's highest. You could call it a tax on littering or landfill abuse, but it works -- Michigan gets 97 percent of its containers back, which is triple the national rate. Might a similar approach be applied to the carbon content of vehicle fuels? As carbon capture and sequestration (CCS) technology inches toward viability, researchers are starting to ask that question.

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Here's how I see it working: New vehicles -- initially larger, more powerful ones that struggle to meet CAFE/CO2 regs -- will employ technology to capture CO2 from the exhaust stream, thereby earning hybrid-like CO2 ratings despite big-car gas mileage. Three typical approaches include absorption of CO2 into a liquid for later separation using waste engine heat; membrane separation, using elevated pressure to drive the gas through the semi-permeable membrane; and adsorption of CO2 onto a catalytic surface for later purging via increased pressure or decreased temperature. A fourth novel MIT approach uses a molybdenum metal oxide in an organic solvent to bind with dissolved CO2, capturing it in a replaceable cartridge. The CO2 in this solution can then be transformed into formate, a useful organic compound that doesn't need to be stored underground.

Research

Back in the car, CO2 captured by one of the first three methods would be pressurized and stored onboard, possibly in the fuel tank, separated by a bladder, or elsewhere. (Longer aluminum extrusions seeing increasing use in body structures could potentially work.) As you might know, a gallon of gas weighs about 6 pounds, but burning it produces 20 pounds of CO2, and capturing, pressurizing, and liquefying all of it would require three times the fuel tank volume and weigh down the car noticeably. Therefore many onboard CCS concepts envision capturing just 20-40 percent, which requires energy and increases complexity and fuel consumption. The CO2 would then be removed during refueling, probably via a fitting built onto a special fuel hose.

Building out this reclamation infrastructure won't happen overnight, so the vehicles would have to be capable of being refueled by conventional pumps, with the CCS hardware going offline until the storage tanks could be emptied. Cost estimates for that hardware are tough to come by, but a recent University of Michigan survey provides engineers with what sounds like an awfully optimistic target -- respondents expressed willingness to pay just $100 for 20 percent capture, losing at most 5 percent of their fuel economy and 10 percent of their trunk volume.

The per-gallon CO2 deposit would need to be chosen to help fund build-out of the reclamation infrastructure, and only a tiny percentage of those deposits would be refunded at first. Might saving a dime per gallon encourage you to buy a carbon-capture vehicle, presuming we figure out how to economically transport the stuff to one of America's 36 geologic sequestration basins, or use it to make things? More important, could this deposit concept realistically be sold as something other than a carbon tax?

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