Using tiny glass beads, the researchers simulated the way liquified carbon dioxide would spread through salty water in the pores of deep rock formations. Image: Michael Szulczewski, of the Juanes Research Group, MIT |
A
new study by researchers at Massachusetts Institute of Technology (MIT) shows
that there is enough capacity in deep saline aquifers in the United States
to store at least a century’s worth of carbon dioxide emissions from the
nation’s coal-fired power plants. Though questions remain about the economics
of systems to capture and store such gases, this study addresses a major issue
that has overshadowed such proposals.
The
MIT team’s analysis—led by Ruben Juanes, the ARCO Associate Professor in Energy
Studies in the Department of Civil and Environmental Engineering, and part of
the doctoral thesis work of graduate students Christopher MacMinn PhD ’12 and Michael
Szulczewski—is published in the Proceedings of the National Academy of
Sciences.
Coal-burning
power plants account for about 40% of worldwide carbon emissions, so climate
change “will not be addressed unless we address carbon dioxide emissions from
coal plants,” Juanes says. “We should do many different things” such as
developing new, cleaner alternatives, he says, “but one thing that’s not going
away is coal,” because it’s such a cheap and widely available source of power.
Efforts
to curb greenhouse gases have largely focused on the search for practical,
economical sources of clean energy, such as wind or solar power. But human
emissions are now so vast that many analysts think it’s unlikely that these
technologies alone can solve the problem. Some have proposed systems for
capturing emissions—mostly carbon dioxide from the burning of fossil fuels—then
compressing and storing the waste in deep geological formations. This approach
is known as carbon capture and storage, or CCS.
One
of the most promising places to store the gas is in deep saline aquifers: Those
more than half a mile below the surface, far below the freshwater sources used
for human consumption and agriculture. But estimates of the capacity of such
formations in the United
States have ranged from enough to store just
a few years’ worth of coal-plant emissions up to many thousands of years’
worth.
The
reason for the huge disparity in estimates is twofold. First, because deep
saline aquifers have no commercial value, there has been little exploration to
determine their extent. Second, the fluid dynamics of how concentrated,
liquefied carbon dioxide would spread through such formations is very complex
and hard to model. Most analyses have simply estimated the overall volume of
the formations, without considering the dynamics of how the carbon dioxide
would infiltrate them.
The
MIT team modeled how the carbon dioxide would percolate through the rock,
accounting not only for the ultimate capacity of the formations but the rate of
injection that could be sustained over time. “The key is capturing the
essential physics of the problem,” Szulczewski says, “but simplifying it enough
so it could be applied to the entire country.” That meant looking at the
details of trapping mechanisms in the porous rock at a scale of microns, then
applying that understanding to formations that span hundreds of miles.
“We
started with the full complicated set of equations for the fluid flow, and then
simplified it,” MacMinn says. Other estimates have tended to oversimplify the
problem, “missing some of the nuances of the physics,” he says. While this
analysis focused on the United
States, MacMinn says similar storage
capacities likely exist around the world.
Howard
Herzog, a senior research engineer with the MIT Energy Initiative and a
co-author of the PNAS paper, says this study “demonstrates that the rate
of injection of carbon dioxide into a reservoir is a critical parameter in
making storage estimates.”
When
liquefied carbon dioxide is dissolved in salty water, the resulting fluid is
denser than either of the constituents, so it naturally sinks. It’s a slow
process, but “once the carbon dioxide is dissolved, you’ve won the game,”
Juanes says, because the dense, heavy mixture would almost certainly never
escape back to the atmosphere.
While
this study did not address the cost of CCS systems, many analysts have
concluded that they could add 15% to 30% to the cost of coal-generated
electricity, and would not be viable unless a carbon tax or a limit on carbon
emissions were put in place.
While
uncertainties remain, “I really think CCS has a role to play,” Juanes says. “It’s not an ultimate salvation, it’s a bridge, but it may be essential because
it can really address the emissions from coal and natural gas.”
