The same chemical reactions that allow water to carve out caves in limestone could be used to capture carbon dioxide from smokestacks, say researchers at Stanford and the University of California at Santa Cruz.
The process—which uses seawater and crushed limestone to capture carbon dioxide—would be simpler than conventional carbon capture and storage (CCS) technologies, and potentially cheaper and more practical. The researchers have demonstrated the idea in laboratory tests, but not yet at an actual power plant.
Conventional CCS is a complex process that involves first isolating carbon dioxide from other exhaust gases, then compressing it and shipping it to an underground storage site. As a result, the technologies are expensive and haven’t been demonstrated at a large scale (see “What Carbon Capture Can’t Do” and “Grasping for Ways to Capture Carbon Dioxide on the Cheap”).
The new approach captures and stores the carbon dioxide in a single step. “The basic concept is extremely simple,” says Ken Caldeira, a professor of Environmental Earth System Sciences at Stanford University. He says it involves speeding up a natural process. When carbon dioxide in air mixes with water, it makes the water slightly more acidic. If this water comes into contact with limestone, the limestone reacts with carbon dioxide to form calcium bicarbonate—a common material that’s a constituent of hard water.
Crushing the rock and exposing it to the relatively concentrated levels of carbon dioxide found in exhaust gases makes it possible to capture and store 70 to 80 percent of the carbon dioxide emitted from a power plant, based on lab experiments, says Greg Rau, a senior researcher with the Institute of Marine Sciences at UC Santa Cruz. The product of the reaction, water containing dissolved calcium bicarbonate, would be pumped into the ocean. Even if all coastal power plants deployed the technology they would only modestly increase the amount of calcium bicarbonate already in the ocean as a result of natural processes, he says.
The main challenge with the approach is that it would require large amounts of water and limestone. The system for capturing the carbon dioxide would be about the size of a big-box store like Walmart. Coastal power plants already pump large amounts of seawater for cooling, so they’d be the best candidates to use the approach.
Although the researchers calculate that the process could be cheaper than other carbon capture and storage processes, they need to confirm this by demonstrating a small version of the system at a power plant. The demonstration would also be needed to measure the environmental effects of pumping calcium bicarbonate into the ocean.
Rau says there would be one environmental benefit in the area close to a power plant. The calcium bicarbonate could make the water slightly less acidic, counteracting the ocean acidification that’s been happening as the result of elevated carbon dioxide levels in the atmosphere.