Capturing CO2 with seawater

A study explores using ocean water and limestone to strip carbon from fossil fuel emissions. The byproduct might even help strengthen coral reefs.

Many minds are clamoring to find a way to economically burn fossil fuels without the carbon consequences.

Last spring I discussed how cleantech start-up Calera was looking to sequester carbon dioxide via seawater and then make cement with the resulting calcium carbonate. A researcher in California has a different take on the scheme: throw calcium bicarbonate Ca(HCO3)2* into the ocean—where it might help support coral reefs.

Greg Rau of the Carbon Management Program at the Lawrence Livermore National Laboratory stripped 97 percent of CO2 from natural gas with a seawater and limestone gas scrubber.

The process treats the CO2 with water, producing carbonic acid, which then reacts with limestone. Dissolved calcium bicarbonate results. The hard and white CaCO3 is a key building block for living marine structures, such as coral skeletons and the protective shells of shellfish, as well as algae and zooplankton at the base of marine food chain.

The experiments, published recently in ACS's Environmental Science & Technology, mimic the natural weathering of limestone in the ocean, but they do so at an accelerated pace.

The ocean naturally absorbs some of the CO2 in the atmosphere. But with rising atmospheric CO2 levels, the ocean have been absorbing more. As the gas dissolves into the seawater, the reaction produces hydrogen ions, leading to the ocean becoming more acidic. The higher pH levels result in less calcium carbonate available to marine life.

While Rau's work took place in a lab, he hopes natural gas power plants near coastlines, some of which already use seawater for cooling, might one day catch their CO2 and then release the CaCO3 into the ocean.

Rau, who is also a senior scientist at UC Santa Cruz Institute of Marine Sciences, says in a statement:

This approach not only mitigates CO2, but also potentially treats the effects of ocean acidification. Further research at larger scales and in more realistic settings is needed to prove these dual benefits.

This method allows a power plant to continue burning fossil fuel but eliminate at least some of the carbon dioxide that is emitted, and in a way that in some locations should be less expensive and more environmentally friendly than other carbon dioxide sequestration methods.

An article published this week in Nature discusses how carbon sequestration remains uneconomical (though one company in China at least seems to be pulling it off).


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