Natural Resources – Carbon-Dioxide Sequestration
Deep-Sixing Carbon Dioxide
Due to higher emissions taxes, the removal of carbon dioxide from the exhaust gases produced by power plants could become not only environmentally but also economically desirable in 15 to 20 years. Pilot projects are already under way.
On this natural-gas facility in the North Sea, Statoil pumps one million tons of CO2 a year into the earth’s crust
Even when oil finally runs out, there’ll be enough coal left to meet our energy needs for several centuries. But if the estimated 5,000 billion tons of carbon resources were burned the same way as today, the amount of carbon dioxide (CO2) emitted would be 17 times greater than the total for the past 150 years. Merely increasing the efficiency of power plants will therefore not be enough to prevent a dramatic greenhouse effect. One possible solution is to separate CO2 from exhaust gases and store it underground. This would allow us to continue to use fossil fuels to generate power, and give us time to develop a sustainable hydrogen-based economy.
But the separation and storage of CO2—the so-called sequestration—is expensive, costing between 30 and 42 € per ton of CO2. Nevertheless, as with other new technologies, these costs will fall significantly in the future. Furthermore, emitting CO2 will become also expensive, either through emission-certificate trading (see Emission Certificates) or because of taxes. "Due to these two trends, CO2 sequestration will probably become profitable in 15 to 20 years," says Frank Haffner of the strategy field for Energy at Siemens Corporate Technology. To ensure that the technology is ready by then, the EU initiated the CASTOR, ENCAP and CO2 SINK research projects in early 2004, and is providing around 15 mill. € of funding for each.
Detergents for CO2. Scrubbing carbon dioxide out of gas mixtures is a long-established process, and industrial companies sometimes do this to exhaust gases from power plants. Such steps have not been motivated by environmental considerations, but to re-use the gas in oil or food production. To extract CO2, flue gas is channeled through a solvent such as ethanolamine. After the amine binds the CO2, the cleaned exhaust gas can be released into the atmosphere. Then, the "detergent" has to be heated so that the CO2 is again released—a process that consumes a lot of energy. Flue-gas scrubbing and CO2 liquefaction reduces a coal-fired power plant’s efficiency by 11 to 14 percentage points, so operators have to use more fuel to generate the same amount of power.
One advantage of this approach is that the changes needed for power plants are minimal. However, a chemical plant is required to deal with huge amounts of exhaust gas, as the coal is burned with the help of air. But normal air consists of only 20 % oxygen, while most of the rest is "useless" nitrogen. One aim of the CASTOR project is to develop a large CO2-scrubbing research facility that is also cost-effective.
Meanwhile, the ENCAP project is following a different approach, involving combustion with pure oxygen, which produces only CO2 and water vapor as exhaust gases and no nitrogen. The water vapor can then be easily removed by means of condensation, with only CO2 left over. Chemical scrubbing is thus no longer necessary. But a gas—oxygen from the atmosphere—also has to be extracted when using this approach. The standard method of cooling air until it liquefies also consumes a great deal of energy and is expensive. Using this method therefore reduces a power plant’s efficiency by seven to 11 percentage points. The ENCAP project involves evaluating new types of membranes that can effectively extract oxygen without requiring large amounts of energy. This process will require a new turbine, which is currently under development.
CO2-Free Combustion. A third, elegant approach involves extracting CO2 before combustion takes place. This requires that power plants be equipped with Integrated Gasification Combined Cycle (IGCC, see Clean Future) technology. In an IGCC system, fuel reacts with oxygen-enriched air or with pure oxygen and water vapor. The result is synthesis gas, a mixture of carbon monoxide and hydrogen. But instead of pumping this gas directly into a combustion chamber, an additional, intermediate step could be employed to transform the IGCC system into a CO2-free power plant. Here, the synthesis gas is combined with water vapor to create CO2 and hydrogen. Because the resulting CO2 is highly concentrated, it can easily be extracted like any other concentrated gas. The leftover hydrogen is then combusted, producing only water vapor as emissions. This approach also reduces power-plant efficiency by seven to eleven percentage points.
As part of the ENCAP project, Siemens and partners like Alstom Power are developing burners and combustion chambers that can process hydrogen-rich gases. "Completely new technologies are needed because of the gases’ high flammability and rapid flame propagation," says Günter Haupt from Siemens Power Generation. Changes would also have to be made to the gas turbines. A burner prototype is due to be tested at the German Aerospace Center in Cologne in late 2005.
This technology has also been endorsed by the U.S. government and is being supported by the country’s coal industry and power suppliers. As part of the FutureGen initiative, the world’s first CO2-free IGCC power plant is due to be set up over the next ten years at a projected cost of one billion dollars. The facility will use coal to produce electricity as well as hydrogen. The resulting CO2 will be sequestered.
Carola Hanisch
Locking Carbon Dioxide Safely Underground
Underground carbon dioxide is nothing unusual. It is produced during the metamorphosis of rocks. CO2 can also often be found in natural-gas fields such as Statoil’s Sleipner field, where it is unavoidably extracted with the gas. Because of Norway’s high CO2 tax, Statoil pumps one million tons of CO2 per year into the Utsira Formation, a salt water imbued sandstone layer under the North Sea. This strategy saves the company some $50 million per year. According to Statoil’s Dr. Tore Torp, the Utsira Formation "could hold an estimated 600 bill. t of CO2—enough to accomodate the combined CO2 emissions of all European power plants for 600 years at current emission rates." In the U.S., CO2 is used to extract the remaining petroleum from nearly depleted oil fields, and most of the gas then stays underground. In one such program, CO2 is transported more than 300 km from a coal gasification power plant in North Dakota to the Weyburn oil field in Canada. At the Sleipner and Weyburn fields, researchers are studying how the gas behaves and spreads and how it reacts with water and rock.
In early 2004, the EU launched the CO2 SINK project (see image below), which is being coordinated by the Geoforschungszentrum (GFZ) Potsdam. Its goal is to pump CO2 from a biomass gasification plant into a layer of porous sandstone below an unused natural-gas reservoir near Ketzin, where it will be monitored. For the first time ever, researchers plan to use drill holes to directly observe how the CO2 behaves underground. Unlike the Sleipner and Weyburn fields, Ketzin is located near a major city: Berlin. This will also allow the researchers to gain experience with respect to approval processes and public acceptance of such projects.
Storing CO2 underground poses two risks: leakage and acidification of groundwater. Although CO2 is non-toxic and is contained, for example, in mineral water and beer, this odorless gas is heavier than air and can suffocate people and animals in depressions where there is no wind. In 1986, large amounts of CO2 erupted from the Nyos crater lake in Cameroon, killing over 1,500 people and all animals in a 14-km radius. Although this situation was very different from that of the sandstone layer near Berlin, such deposits must be closely monitored.
