To reduce carbon dioxide emissions in energy production, more emphasis will be placed on hydro and nuclear power plants. Here is a glimpse of the operating characteristics of these power plants of the future.
Emission-Free Coal-Fired Power Plants
Laboratories, utilities and engineers all over the world are working on concepts for CO2-free coal-fired power plants. The keys to clean energy are the use of pure oxygen for combustion or gasification and the liquefaction and storage of product carbon dioxide.
Germany is regarded as a global leader in environmental protection. In terms of greenhouse gas emissions, for instance, the country has lowered its output of carbon dioxide by 19 percent since 1990 and will attain a 21 percent reduction by 2012. This trend is in the right direction, but the lion’s share of this abatement has been due to the closure of industrial and power plants in the new German sates. The conservation group World Wide Fund for Nature (WWF) holds that Germany has fallen behind in some aspects of climate protection.
One WWF study showed that five of the ten highest-emitting power plants in Europe stand on German soil. Experts do not find this result surprising. All the plants in question are fueled with brown coal (lignite), and “Lignite has very high-CO2 energy to begin with,” says Stefan Lechtenböhmer of the Wuppertal Institute for Climate, Environment and Energy.
“Global climate change is the biggest challenge for environmental policy today,” maintains Lars G. Josefsson, president of Vattenfall (Stockholm, Sweden). He sees this as the main theme in his country’s environmental efforts. Business considerations play a role too, of course. The Swedish concern has taken over lignite mining and power generation in the new German states and would like to continue using this cheap fuel— a goal is shared by RWE AG (Essen, Germany), which is in a similar business position in the state of North Rhine-Westphalia. Brown coal accounts for a large share of electrical power output in Germany, some 27 percent.
In years to come, part of the problem may vanish into thin air – literally. Germany must replace generating capacity of around 40-gigawatt by 2020, as older fossil-fuel plants reach the end of their lives while energy demand rises. Since a larger plant supplies about one gigawatt, it will take at least 40 of them just to modernize the installed base.
CCGT power plants of the future
What technology and what fuel will utilities adopt? With regard to CO2 emissions, the case seems clear: The use of “combined-cycle gas turbine” (CCGT) plants can yield a saving of 48 percent relative to the coal-fired plants replaced in the coming modernization campaign.
A CCGT plant burns natural gas in a gas turbine and then, from the residual heat, extracts enough energy to drive steam turbines and generate additional power. Such plants today have efficiencies of 58 percent, far better than those of plants burning bituminous coal (48 percent) and lignite (43 percent). Higher efficiency also means lower CO2 emissions.
What is more, cost arguments favor CCGT technology, since the investment cost of 400 euro ($510) per kilowatt of capacity is substantially less than the 700 euro ($893) per kilowatt for coal-fired plants.
In view of these advantages, the European Union (EU) projects that the fuel mix will shift toward natural gas while lignite consumption stagnates, with bituminous coal regaining ground after a low around 2010. Rising natural gas prices, however, cast doubt on this forecast. The price of natural gas has more than doubled since September 11, 2001. Although there is no consensus as to whether crude oil and natural gas prices are linked, present knowledge still suggests that the world’s reserves of natural gas will last only a few decades longer than those of petroleum.
In contrast, there is enough bituminous coal and lignite to last at least 300 years, and with these there is no danger of terrorist attack on pipelines. The EU study, “World Energy, Technology and Climate Policy Outlook” therefore projects stable oil prices out to 2030. A reassessment is also taking place in the U.S., where many gas-fired plants have been built in recent years. Domestic natural gas is no longer sufficient to satisfy the rising demand for energy, and imports will be needed in the future. Coal, on the other hand, is plentiful, and so the U.S. government’s Clean Coal Power Initiative calls for the construction of two billion dollars’ worth of new coal-fired plants over ten years.
The more power plants are fueled with coal, the more pressing the CO2 problem will become. If the entire world’s estimated reserves of coal – 5 trillion tons – were burned, the amount of carbon dioxide released into the atmosphere would be 17 times as much as the total for the past 150 years. This is the outcome that engineers seek to prevent through new concepts.
Lignite-fired plants using “optimized plant technology” (BOA) are already in service. The Niederaussem facility in North Rhine-Westphalia – the most modern in Germany with a capacity of 965-MW and an efficiency of 43 percent – employs this principle. BOA facilities emit 30 percent less CO2 than older plants chiefly by virtue of two practices: drying the feed lignite to boost efficiency and increasing the steam temperature at the turbine inlet.
This alone, however, will not be enough to abate greenhouse gas emissions significantly. The only way for coal-fired plants to achieve this goal is to release no carbon dioxide into the atmosphere in the first place. All the industrialized countries are therefore working on CO2 capture, and numerous research projects on this topic have been started in recent years.
Three Options for Removing Carbon Dioxide
1. Post Combustion
This method has the advantage that it can be retrofitted to older power plants. Many plants already have carbon dioxides scrubbers, usually not for environmental reasons but to secure CO2 for use in oilfields or in the food processing industry.
Flue gas from the power plant is passed through a solvent such as ethanolamine. The spent scrub liquor is then heated to release CO2, which is liquefied by compression. The result, however, is a loss of efficiency, as much as 14 percentage points, so that more coal must be burned in order to get the needed output. In the EU’s CASTOR project, engineers are designing a higher-efficiency pilot installation that can handle the off-gas streams from a large power plant.
2. Pre combustion
The greatest obstacle to carbon dioxide capture is the large amount of nitrogen in the combustion air. This nitrogen must be transported through the power plant but plays no part in the combustion process. The IGCC (integrated gasification combined cycle) process works with a much lower level of nitrogen.
Pulverized coal and pure oxygen are reacted to yield a synthesis gas made up primarily of carbon monoxide, hydrogen, carbon dioxide and water vapor. The admission of further steam converts most of the carbon monoxide to carbon dioxide and hydrogen. CO2 is now easily captured, while the hydrogen is used to fire a gas turbine.
Engineering firms such as Siemens AG (Munich, Germany) and Alstom (Levallois-Perret Cedex, France) are developing the IGCC technology, and projects like ENCAP, funded by the EU, and the German COORTEC initiative – aim to create power plant designs using gas turbines capable of withstanding the hot hydrogen flame.
Many gasification systems are already used in refineries because they can handle not only coal but also heavy oil and even asphalt. The technology has not yet gained acceptance in power plants, and work toward improving the overall reliability is still under way.
3. Oxyfuel
The Oxyfuel process, in which coal is burned with oxygen, is a middle way. Because the combustion temperature with pure oxygen would shoot up to 4,892-deg F (2,700-deg C), while today’s steam turbines can handle only 1202-deg F (650-deg C), a portion of the off-gas is recycled and used to dilute the oxygen in order to control the temperature. The nitrogen burden is eliminated at the outset, so the carbon dioxide concentration in the flue gas increases to some 73 percent, making it easier to capture, liquefy and ship the CO2. The Oxyfuel concept is under study by 20 partners in the ENCAP and COORETEC projects.
Oxyfuel on the Test Stand
The Oxyfuel concept has recently received a boost. At the Schwarze Pumpe site in the Lausitz (easternmost Germany), Vattenfall will build a 30-MW pilot plant, slated for startup in 2008. This facility, costing 40 million euro ($51 million), will demonstrate that the Oxyfuel process will work in the context of power plant operations with all their complexity.
If everything goes as planned, the next decade will see an Oxyfuel demonstration plant with an output of 250- to 600-MW, and a 1,000-MW commercial facility offering competitive generation costs will go into service in 2020.
In the Oxycoal project, companies such as RWE, E.ON (Duesseldorf, Germany) and Linde (Wiesbaden, Germany) are already working – under the leadership of the Rhine-Westphalia Technical University (RWTCH) in Aachen – to realize the concept of an advanced coat-dust-fired power plant. Oxyfuel and IGCC plant concepts offer new market possibilities. After all, know-how in the generation and handling of gases will be crucial when coal is not burned with air but is either combusted with pure oxygen or converted with it to synthesis gas.
“Separating air is like distilling liquor, only at minus 356-deg F (minus 180-deg C),” says Dr. Harald Ranke, Research and Development Manager in Linde’s Engineering Division. After the air is cooled to this lower temperature, controlled heating and cooling cause oxygen and nitrogen to condense out of it in different parts of the system. Depending on the purity target, noble gases and valuable trace gases such as argon, xenon and krypton can also be produced in the same way.
Future IGCC and Oxyfuel power plants will pose enormous new challenges to the technology of air separators. Today’s largest installations deliver 5,000 tons of oxygen per day, enough for an Oxyfuel plant rated at some 300-MW. A typical gigawatt Oxyfuel plant would therefore need three of the largest air separating systems.
The cryogenic process in an IGCC or Oxyfuel power plant would cost from seven to eleven percentage points of efficiency, and so the integration of air separation into power plant operations must be optimized.
Membrane Separators for High Temperatures
The solution may lie in so-called membrane separators. In such a unit, a ceramic membrane separates oxygen from the air; the process may serve energy in comparison with a distillation-type air separator. The partial pressure of oxygen is the sole driving force causing the gas to migrate through the dense ceramic to the other, oxygen-poor side.
This sounds simple, but in reality it is a major challenge. Separation takes place at temperatures as high as 1652-deg F (900-deg C). The ceramic must be able to withstand mechanical stresses and chemical attacks at these temperatures in continuous service, and this goal has not yet been attained.
Several companies, chiefly American, are hard at work on this technology but have not achieved a breakthrough that would lead to a commercial product. Linde is also working on membrane separators in the context of a project for the German Federal Ministry for Education and Research. The resulting know-how will find use in the power plants being designed in the Oxycoal project, among others.
CO2 belongs underground
The decisive factor for the efficiency of an air separator, no matter of what type, will be that it is integrated as neatly as possible into the power plant.
In a future CO2-free power plant, carbon dioxide capture is only half the story. Simply blowing it off into the air would mean no benefit in climate protection; this is especially true of the Oxyfuel process. The greenhouse gas, therefore, has to be permanently confined (sequestered). The most obvious choice is to pump the carbon dioxide into oil or gas fields, where it will raise the pressure in the formation and thus boost yield.
In countries that impose high CO2 taxes or where emission rights are traded, this is a way to save a lot of money. Statoil (Stavanger, Norway), for example, injects a million tons of CO2, unavoidably produced from natural gas fields, back beneath the sea floor every year, thus saving eight-figure sums. Statoil estimates that the Utsira rock formation in the North Sea could hold 600 billion tons of CO2, as much as would be emitted over the next 600 years by all the power plants now operating in Europe.
Another option is to inject the CO2 into mined-out coal seams or salt domes. Ketzin, near Berlin, features the CO2-Sink project, coordinated by the Potsdam Georesearch Center. Carbon dioxide will be injected into a salt dome there on a test basis.
This effect would ideally be linked to Vattenfall’s Oxyfuel pilot plant, but that facility will not enter service until mid-2008. Because it is near a large city, CO2-Sink is likely to raise many questions about the permit process and public acceptance.
At present, the public knows almost nothing about CO2 sequestration, so special effort needs to be put into convincing people that carbon dioxide can be safely confined underground for centuries to come. Geologists predict that the gas will gradually combine with potassium salts present in the formation, forming harmless limestone and thus being rendered innocuous.
A Question of Economics
Just as important to the success of CO2 capture as the surmounting of technical obstacles, however, is whether the process can be carried out economically. At present it costs between 30 and 40 euro ($38 and $51) to capture one ton of CO2; 6 to 15 euros ($7 to $19) to transport it 100-km in high pressure pipelines; and another 10 to 25 euros ($12 to $32) to store it underground.
These expenses would increase the cost of electricity by 0.015 to 0.06 euros ($0.019 to $0.07) per kilowatt-hour. Vattenfall is seeking to lower long-term costs dramatically with the Oxyfuel process, projecting around 20 euros ($25) per ton of CO2.
At the same time, prices in the emissions market are on the rise. The cost of emission rights per ton of CO2 – 20 to 30 euros ($25 to $38) in the European market – is “more than was first anticipated for this point in time,” says Karoline Rogge of the Fraunhofer Institute for Systems and Innovation Research at Karlsruhe, which advises the German government on the granting of certificates. Frank Haffner, a Siemens energy strategist, is optimistic: “The two opposing trends should make CO2 capture economical in 15 to 20 years.”
Americans are also looking to this trend; their FutureGen Initiative is set to build the world’s first CO2-free IGCC power plant within the coming decade. The budget for the project is a billion dollars.
CASTOR
This EU-funded project concerning CO2 capture and storage in Europe began in February 2004 and is to run for three years. Participants include numerous energy utilities, industrial companies and research institutes.
ENCAP
The goals of the ENCAP project include developing “pre-combustion” technologies with which carbon dioxide emissions from power plants can be greatly reduced. Scheduled to run until early 2009, the project claims 33 industrial partners as well as many universities and well-known research institutes as its participants. The total budget is 22 million euros ($28 million).
COORETEC
The COORETEC project (CO2 Reduction TECnologies in fossil-fuel power plants) was conceived jointly by businesses, scientific interests and the German Federal Ministry of Economics and Labor (BMWA). The aim is to develop technologies needed for highly efficient, profitable coal- and gas-fired power plants emitting virtually no carbon dioxide. The project will create the world’s first realistic “road map” for power plant development.
Pumps & Systems, December 2006