Fossil fuels are warming the planet because when they’re burned, they release heat-trapping gases like carbon dioxide (CO2). Carbon dioxide is found in the “flue gas” that billows out of power plants’ smokestacks, as part of a mix with nitrogen, water, and small amounts of soot and other chemicals.

But it’s possible to remove the climate-warming CO2 from flue gas before it’s vented, using a technology called carbon capture and storage (CCS).

Most capture systems use chemicals called “amines" to separate CO2 from the gas. The captured CO2 is then compressed, so it can be shipped off for use or to be stored underground. This process is quite energy-intensive. The flue gas must be pumped to different parts of the plant; the CO2 must be separated, cooled down, and pumped to its destination; and the amines need to be “recharged” at the end of the process so the whole operation can begin again. About five percent of the equipment’s total energy use goes to pumping various liquids and gases around the plant, while roughly one-third is used to compress the CO2, according to Howard Herzog, a Senior Research Engineer at the MIT Energy Initiative. The most energy-intensive part of the process is recharging the amines, he says.

Just how much energy this all takes depends on two things, says Herzog: how much CO2 the plant produces, and how concentrated that CO2 is in the flue gas.

Most fossil fuel plants in the United States burn either coal or natural gas. Coal plants outfitted with carbon capture technology use about 20 to 25 percent of the energy they produce to run CCS equipment, according to Herzog. Natural gas plants use about 15 percent of their energy to run the equipment.

Interestingly, although coal plants need to spend more total energy on CCS, their CCS equipment is a bit more energy-efficient. That’s because coal releases more CO2 than natural gas when burned, so the flue gas is richer in CO2, making it a little easier to separate out. “In the natural gas power plant, you have less CO2 to capture per unit of electricity, so that's why it doesn't need quite as much energy—even though, per molecule of CO2 captured, the natural gas power plant actually uses a little more energy,” said Herzog. “It all goes back to the concentration level.”

All this energy use is a big reason why coal and gas plants with carbon capture and storage are struggling to gain a foothold in the competitive energy market. Because some energy goes to run the capture equipment—energy that operators can’t then sell to the electric grid—the plant must charge more for the energy it does sell to make a profit. And on top of this, designing and installing the equipment is a large upfront cost, especially because few of these plants have been built and there is no standard design that can be mass-manufactured.