Sequestering carbon poses more potential problems

The notion that we can effectively bury the harmful byproducts of energy production (be they radioactive or climate-changing) is on shaky ground (literally and figuratively). Coal producers are desperately trying to convince regulators and the public that they should be considered "clean" if they can demonstrate that they can first capture their carbon emissions and then sequester that carbon underground.

Scientists have discovered that there are more problems associated with carbon capture and sequestration than originally assumed. (GW)

Carbon Capture Remains Elusive

Despite subsidies and new projects, carbon dioxide sequestration is still a long way off.

By Phil McKenna
Technology Review
October 5, 2009

On October 1, a coal-fired plant in West Virginia operated by American Electric Power (AEP) became the first power station in the U.S. to pump a portion of its carbon dioxide emissions underground. At the same time, the U.S. Department of Energy is funneling billions of stimulus dollars into carbon capture and sequestration. And FutureGen, a government-backed project to build the first zero-emissions coal-fueled plant, looks set to rise from the ashes.

At first blush, it seems carbon capture and sequestration (CCS) is on its way to making clean coal a reality. However, no commercial-scale CCS operation is near completion in the U.S., and until a market price is set on carbon dioxide, experts say things aren't likely to change.

"Until there is a market, the technology won't take off," says Howard Herzog, principal research engineer with the MIT Energy Initiative. "It's amazing that there are as many projects going on that there are today; they are all research and development projects that are funded with subsidies."

The American Recovery and Reinvestment Act of 2009 provided $3.4 billion in federal funding for CCS projects, including $1 billion for FutureGen and more than $1 billion for other commercial-scale operations. Yet even with this money, significant hurdles remain.

"There are an array of technical challenges that have to be overcome," says Tom Williams, a spokesperson for utility company Duke Energy, which recently invested $17 million in carbon-capture research at a coal gasification power plant in Edwardsport, IN, and is currently seeking federal funding to further develop capture and sequestration technology at the plant. "Permitting challenges, sequestration challenges, geological challenges, [and] efficiency challenges all have to be figured out," Williams says.

One of the geological challenges faced by Duke Energy and others investigating in CCS is ensuring that the pressure inside reservoirs deep beneath the surface of the earth doesn't climb too high as carbon dioxide is injected. "There are only certain safe levels that you can raise the pressure to before you get into issues of seismicity," Herzog says.

Ernest Majer, a seismologist at Lawrence Berkeley National Laboratory, briefed members of the U.S. Senate in September on these potential hazards. He says that pumping pressurized, liquid carbon dioxide underground has the potential to cause minor earthquakes, although with proper site selection and injection rates, this shouldn't be an issue. "If you inject great volumes into an active fault, then yes, you are going to have problems, but we've been injecting wastewater from municipalities for years without a problem," he says. "You just have to engineer it properly."

In particular, this means implementing reliable monitoring systems to track the movement of carbon dioxide deep underground. Sensors used in oil and gas fields are well developed for this purpose, though less-expensive monitoring systems would make carbon dioxide sequestration for coal plants more cost-competitive.

"Every time you place a sensor thousands of feet down, it requires drilling a well bore that, depending on depth and diameter, can cost between 5 [million] and 10 million dollars," says Ken Humphreys, of the FutureGen Alliance. Humphreys says less-expensive systems such as acoustic sensors that monitor the movement of carbon dioxide from the surface are currently under development.

As engineers develop new technologies for carbon capture and sequestration, technical setbacks may be inevitable. AEP, the utility company that began pumping 2 percent of its carbon dioxide emissions underground on October 1, had hoped to begin sequestration earlier, but the project was delayed when sensors showed higher-than-anticipated moisture content in the carbon dioxide. If the liquefied gas contains too much water, carbonic acid can form, corroding the steel pipes used to transport it underground.

To bring the water content down to a safe level, AEP said it would have to further cool the carbon dioxide to remove water through precipitation before pumping it underground. Additional testing, however, revealed that the moisture content had been misread and was actually within safe levels.

"There are definitely teething pains in getting it up and running," says Gary Spitznogle of AEP. "It's just the nature of a new process. Not everything works right in the first iteration."

The cap-and-trade legislation now making its way through Congress could help hasten solutions to many of the technical issues that CCS still faces. But one of the biggest remaining questions is whether sufficient reservoirs exist to store all of the carbon dioxide that may be captured.

The best-studied storage deposits are former oil and gas reservoirs capped by layers of nonporous rock that kept the petrochemicals locked deep underground for millions of years. Yet of an estimated 3,947 gigatonnes of carbon dioxide storage capacity under the U.S., only 1 percent consists of depleted natural gas and oil reservoirs. The vast majority of capacity--3,630 gigatonnes--consists of deep saline formations that have received less scrutiny.

"We're at the place where there is no problem doing millions of tonnes a year, but to solve the climate problem we need to do billons of tonnes or gigatonnes a year, and at that scale, storage becomes a real issue," Herzog says.

Majer, of Lawrence Berkeley National Laboratory, says small-scale tests such as AEP's pilot project will go a long way toward determining the viability of storage in saline aquifers. "We don't know all the answers yet, but we pretty much know how to get the answers," he says. "And who knows, the answer may still be, it ain't gonna work."

Copyright Technology Review 2009.