Despite all the talk of moving to greener energy sources, coal will be with us for the foreseeable future. It’s just too cheap and plentiful. But if we’re really serious about cutting carbon dioxide emissions, coal plants everywhere will need to substantially reduce the billions of metric tons of CO2 they annually emit into the atmosphere. The big hope is that in the next few years the plants will begin capturing and storing a large portion of that CO2 deep underground, in the oceans, or in mineral form.
But the technology needed to capture carbon has a huge downside: It could nearly double the amount of water a plant uses for every kilowatt of electricity it delivers—easily erasing any gains from techniques aimed at conserving water.
"This technology was not developed in a water-constrained environment," says Jared Ciferno, technology manager for the existing plants program of the National Energy Technology Laboratory (NETL). "The bottom line is that [carbon] capture takes energy, and that translates to additional water use."
Just how much water is pretty shocking. By 2030, the addition of carbon-capture technology would boost water consumption in the U.S. electricity sector by 80 percent, or about 7500 megaliters per day, according to research at NETL, which is operated by the U.S. Department of Energy. For plants in water-stressed areas, that’s a deal breaker. "It is not likely that there is enough water supply available to any of our plants to allow for double the water use," says John Coggins, manager of resource planning at Salt River Project, a water and energy utility in Arizona.
The 80 percent figure assumes that the electricity generation lost to powering the carbon-capture system is made up for by adding more water-cooled coal-fired power. In other words, for a 550-megawatt plant to both capture its carbon and still deliver 550 MW of electricity, it would need to add more than 125 MW of additional generating capability to cover the energy used in capture. If you don’t make up for the lost generation, or make it up in some way that requires no water and emits no carbon—with a wind farm, say—the additional water consumption is more like 40 to 50 percent, according to NETL’s Ciferno.
That’s still a lot of water. For coal power plants, the state-of-the-art carbon-capture technology is known as amine-based wet scrubbing [see "Catching Carbon," above]. It’s basically the technology that puts the fizz in your Fanta. First, the plant’s flue gas is scrubbed of sulfurous nasties; what’s left is a mixture of nitrogen, water vapor, and CO2. An amine solution then reacts with the CO2, yielding a gas stream of mostly nitrogen, which goes out the smokestack, and a CO2-rich amine solution. The solution is heated to strip the CO2 from the amines. The CO2 is then cooled and compressed for storage, and the amines cycle back to pick up more CO2.