New research shows how effective injecting carbon dioxide (CO2) into the volcanic rock basalt could be for fighting carbon emissions.
Burning fossil fuels produces carbon dioxide. As the gas rises and the atmosphere traps it, it retains heat as part of a process called the greenhouse effect. The increased temperatures associated with the greenhouse effect can cause melting ice caps, higher sea levels, and a loss of natural habitat for plant and animal species.
Environmental scientists trying to mitigate the effects of CO2 have experimented with injecting it deep underground, trapping it. These trials have mainly taken place in sandstone aquifers, however, the CO2 primarily remains present as a bubble that can return to the surface if is there are fracture in the capping formation.
A different approach using basalt flows as injection sites—chiefly at the CarbFix site in Iceland and in Washington state—has yielded dramatic results. Metals in basalt have the ability to transform CO2 into a solid inert mineral in a matter of months.
While the new method holds promise, the underground injections can be imprecise, difficult to track and measure. The new research sheds light on what happens underground after injecting CO2 into basalt.
“In a field site, you inject the carbon dioxide in, and it’s a very open system,” says Daniel Giammar, professor of environmental engineering in the School of Engineering & Applied Science at Washington University in St. Louis. “You can’t get a good constraint in terms of a capacity estimate. You know you made some carbonate from the CO2, but you don’t really know how much. In the lab, we have well-defined boundaries.”
To obtain a clearer, quantifiable look at carbon trapping rates in basalt, Giammar collected samples of the rock from Washington state, where researchers previously injected a thousand tons of CO2 gas deep underground into a basalt flow. He placed the rocks in small reactors that resemble slow cookers to simulate underground conditions, and then injected CO2 to test the variables involved in the carbonization process.
“We reacted it at similar pressure and temperature conditions to what they had in the field, except we do all of ours in a small sealed vessel,” Giammar says. “So we know how much carbon dioxide went in and we know exactly where all of it went. We can look at the entire rock afterwards and see how much carbonate was formed in that rock.”
The lab kept the basalt in the pressurizers and followed up, using 3D imaging to analyze their pore spaces at six weeks, 20 weeks, and 40 weeks. They watched moment to moment the CO2 turned into mineral, saw the exact voids within the basalt it filled, and observed the precise spots in the rock where the carbonization process began.
Once they collected and analyzed all of the data, Giammar and his team predicted that one cubic meter of basalt could convert 47 kilograms of CO2 into mineral. Researchers can use this estimate as a baseline to scale up, quantifying how much CO2 entire areas of basalt flow can effectively convert.
“People have done surveys of available basalt flows,” Giammar says. “This data will help us determine which ones could actually be receptive to having CO2 injected into them, and then also help us to determine capacity. It’s big. It’s years and years worth of US CO2 emissions.”
Giammar’s lab is currently sharing its results with colleagues at the University of Michigan, who will assist in developing a computational model to further help researchers to look for a solid fix for CO2 abatement. The researchers have also been invited to take part in the second phase of the US Department of Energy’s Carbon Storage Assurance Facility Enterprise, or CarbonSAFE, which investigates new technologies for CO2 abatement.
The researchers report their findings in the journal Environmental Science & Technology Letters.
The US Department of Energy funded the research. Additional collaborators are from the Pacific Northwest National Laboratory and Washington University.