Pictures of the Future      Spring 2008

Testing Eternal Incarceration

Emissions from coal-fired power plants must become cleaner—which means removing their carbon dioxide content. The best place to store this greenhouse gas permanently is deep underground. That’s exactly what is happening at a test facility near Potsdam, Germany.

It’s raining in Ketzin. A drill tower rises up toward the dark clouds; a few gas tanks and a plain shack stand in a green meadow in the middle of the Havelland district, a half-hour west of Potsdam. Professor Frank Schilling from the research facility GeoForschungsZentrum Potsdam (GFZ) points down into a mud-filled hole from which a pipe as wide as a man protrudes. A tangle of cables can be seen inside it. "Here’s where we measure the spread of carbon dioxide underground," says Schilling, who is a mineralogist. At the other end of the meadow, a second hole plunges down, this one also filled with a mass of cables, and 100 m away there is a third hole. At the latter, pipes from a tank run into the damp soil. 700 m under Schilling’s feet, these pipes will pump up to four tons of carbon dioxide per hour into the sandstone at high pressure, thus  displacing salt water from pores in the rock.

The GFZ project near Ketzin, a town with a population of 4,000, is called CO₂SINK. For two years, beginning in the spring of 2008, it will inject 60,000 t of carbon dioxide underground for storage. That’s as much as the 150,000 residents of Potsdam will exhale during the same period, but it’s nothing compared to the more than 10 bn. t of this greenhouse gas that are blown into the atmosphere by the human race each year through power plant chimney stacks. And the problem will grow more acute, judging from the forecasts of the International Energy Agency, which indicate that fossil fuels will account for 85 % of the increase in power production over the next 20 years. The capacity of coal-fired power plants worldwide will then be 2,200 GW—about twice what it is today. The trend is already noticeable. China, for example, put 174 coal-fired power plants in the 500-MW class into operation in 2006 alone, which corresponds to the commissioning of one plant every two days (see Renewable Resources).

Underground Disposal. In view of these developments, CO₂SINK could, in spite of its modest scope, provide important answers to basic unresolved questions regarding CO₂ sequestration and therefore contribute significantly to environmental protection. If the measurements in Ketzin confirm the models, which predict that the gas can be securely confined underground in porous rock for thousands if not millions of years, the project would send an ­important signal worldwide. It would prove that CO₂ from coal-fired power plants, refineries, cement factories, and steel mills can be pumped into the earth and stored there. And if the gas isn’t emitted into the air, it can’t harm the climate.

Moreover, there is an abundance of room underground for carbon dioxide. The capacity for CO₂ sequestration in Germany alone is estimated at 30 bn. t. That’s enough for about 100 years at the current rate of CO₂ emissions from German coal-fired power plants—about 350 mill. t. The Intergovernmental Panel on Climate Change (IPCC) of the U.N., a Nobel Prize recipient that galvanized the political class and the media last year with its reports on climate change, estimates global sequestration capacity to be up to 900 bn. t in oil and gas deposits and at least 1,000, possibly even 10,000 bn. t in saline aquifers, which are sandstone deposits saturated with salt water, like those found in Ketzin. These potential sequestration sites around the world are also often found near large CO₂ producers, where liquefied CO₂ can be easily transported in pipes to storage depots. This is the case not only in Brandenburg, but also in the U.S. state of Illinois, where a prototype CO₂-free power plant is being tested in the Future-Gen project. The dream of a coal-fired power plant with a direct exhaust line into the subterranean rock could become a reality in many places around the world if policymakers quickly lay the groundwork and research efforts are intensified.

Studies show that CO₂ remains underground for an extremely long time. It will dissolve there in saline aquifers, much as it dissolves in mineral water when pumped by a CO₂ carbonator, and will then be retained in the pores of the sandstone. Over time, more and more of it will precipitate as a mineral compound and thus be kept out of the atmosphere forever. It is known that after thousands of years calcium carbonate is produced, as well as other carbonates such as magnesite and siderite. Verifying the underlying models and furnishing proof of whether and how CO₂ can be reliably sequestrated over the long term are among the central aims of the CO₂SINK project.

Underground Laboratory. One essential task of CO₂SINK is therefore to monitor the three-dimensional propagation of CO₂ in rock and draw conclusions applicable to commercial CO₂ sequestration at other locations. No other project anywhere is going to such great lengths to gather measurements in this respect:

? In the project’s two measuring pipes, which are 50 and 100 m away from the pipe carrying the gas, chains of electrodes measure electrical resistance in the rock. This array of electrodes is supplemented by electrodes at the surface. Concentrated salt water in the pores of the sandstone conducts the electrical current very well. When the water is displaced by CO₂ , conductivity decreases and resistance increases. Thanks to this geoelectric tomography, the gas can be monitored in great detail in three dimensions as it spreads.

? The project team is also carrying out experiments modeled on medical ultrasound. Here, intense sound waves are transmitted into the ground from the surface between the boreholes and reflected back. Since sound has a lower velocity in pores filled with CO₂ than in those filled with salt water, the spread of the gas can be monitored this way as well.

? Optical sensors measure temperature changes underground through the scattering of photons and thereby show the flow of CO₂ below the surface.  In the area of the reservoir around the bores there are narrow tubes with a semi-permeable membrane through which CO₂ can pass. High-purity argon forces the CO₂ upward through capillary tubes to the surface, where its concentration is measured.

Whatever the results of the measurements, one thing is certain, says Frank Schilling: "Practically nothing travels upward through the rock." The reason for this is the cap layer of ­gypsum and clay that lies like a bowl over the approximately nine-square-kilometer dome of sandstone and completely seals it. It served the same purpose over the past forty years, when power companies used a sandstone layer here at a depth of between 250 and 400 m to store natural gas. This repository was significantly larger than the planned CO₂ reservoir.

What would happen if the CO₂ managed to escape to the surface? Since the gas is heavier than air, critics fear that it could collect in pools where it would suffocate all life. But there’s no risk of this in Ketzin, says Schilling. Even if it were to escape, the CO₂ would be literally gone with the wind. We breathe it in small quantities all the time, and drink it in sparkling mineral water and soft drinks. Besides, the quantity of CO₂ stored in two years will merely be equal to the amount naturally generated in the same period by bacteria through degradation processes in the soil in the area above the CO₂ reservoir in Ketzin.

Ideal CO₂ reservoirs exist wherever gases or liquids have long accumulated underground. That basically means all petroleum and natural gas deposits, which have manifestly been sealed for millions of years. Some oil and gas producers already pump CO₂ back into such deposits in order to raise the yield through increased pressure. There are three industrial-scale showpiece projects in Canada, Algeria, and Norway. StatoilHydro of Norway, for instance, has the most experience here. Since 1996 it has pumped 10 mill. t of CO₂ down to a depth of 1,000 m beneath the North Sea. The CO₂ is an impurity that is extracted with the natural gas. But it would cost StatoilHydro dearly to vent it, as Norway levies a tax of $50 on each ton of CO₂.

Toward Affordable Sequestration. The IPCC report calculates the cost of CO₂ capture by low-CO₂ power plants and its transportation and sequestration to be 20 to 70 dollars per ton. That’s worth the price in Norway, but in countries without a CO₂ tax other market mechanisms must come into play. In Europe, the certificates in the emissions trading system provided for by the Kyoto Protocol currently cost less than $20—not enough to create an incentive. But in the event of a state subsidy or a CO₂ tax of 0.02 to 0.03 U.S. $/kWh, the technology would pay for itself, although the cost of electricity would increase by 20 %.

Siemens is helping to fund the CO₂SINK project and participating as an observer. "CO₂ sequestration won’t be one of our core areas of expertise," says Günther Haupt of Siemens’ Fossil Power Generation division. But since the construction of coal-fired power plants is an important part of Siemens’ business and depends on a solution to the CO₂ problem, the company will be involved. Siemens will also play an active role in cases where hardware does not yet exist, as in the Adecos project, which is developing an oxyfuel power plant with CO₂ removal with support from the German government. Here, Siemens is designing compressors for the CO₂ that will force it underground as a gas—but with the density of a liquid. These compressors have applications in multiple fields, since they also compress CO₂ from pre- and post-combustion processes (see CO₂ Separation and Compressors). "So far, CO₂ compressors of this kind haven’t been customized for large power plants," says Haupt.
                                                                                                                    Bernd Müller