North of Los Angeles, near Bakersfield, California, is a pilot plant full of rocket technology. Rudi Beichel, the space pioneer with German roots who helped the U.S. to reach the moon, worked there on the development of rocket engines for a long time. He was nearly 80 years old - an age at which most of his colleagues had retired - when he accepted a new challenge and set out to develop a fossilfuel power plant that generates electricity with practically zero emissions.
In 1993, six years before his death at 86, Beichel established the Clean Energy Systems (CES) company. Today the company’s work is bearing fruit. CES has developed a combustion chamber that can burn an extremely wide variety of fuels for a 50-megawatt (MW) test power plant. What makes this plant special is the fact that it emits no carbon dioxide (CO2) or other exhaust gases into the atmosphere. It is one of the first zero-emission plants in the world - and the largest of its kind. The company’s innovative technology has piqued the interest of Siemens. “We worked on similar ideas in the 1990s,” says Frank Bevc, Director of Technology Policy and Research Programs at Siemens Energy in Orlando, Florida. “We were impressed by how Clean Energy Systems has implemented its ideas.”
The central innovation from CES is its “direct oxyfuel process.” Whereas natural gas requires little pretreatment, coal, coke, and biomass must first be converted into a gas and then cleansed of sulfur or ammonia compounds. The resulting gas is then fed into a combustion chamber where pure oxygen rather than air is used for combustion. The advantage of this is that the nitrogen that consti tutes three quarters of the air does not have to be passed through the combustion process, and only oxygen, hydrogen, and hydrocarbons such as methane are burned in the combustion chamber. The flue gas produced by this process is composed mainly of carbon dioxide and water vapor.
Pilot plants built by power producers Vattenfall and E.ON in the Lusatia region of eastern Germany and in Ratcliff, UK, respectively, have also recently begun burning coal with oxygen, but in these cases the flue gas is recirculated into the combustion process to increase the level of CO2 and to control the temperature (see article “Coal’s Cleaner Outlook”, Pictures of the Future 1/2008). CES, on the other hand, uses water for cooling, as well as higher pressure, which in turn results in higher efficiency for electricity generation. In the CES plant, a heat exchanger is used to cool the hot flue gas after it has passed through the turbine. The water vapor condenses out of the flue gas as it cools, leaving behind the CO2, which can then be drawn off. In this way, more than 99 percent of the carbon dioxide can be prevented from entering the atmosphere.
CES’s 50 MW plant is too small to generate electricity commercially, according to Keith Pronske, President and CEO of CES. “But the plant is already industrially attractive to anyone who has natural gas available as a fuel and needs carbon dioxide for the extraction of gas or oil from the ground,” says Pronske. He points out that liquefied carbon dioxide from such a plant can be pumped into oil-bearing layers of rock to increase pressure and extract oil from old wells.
What is it about CES’s technology that intrigues Siemens? “The company’s innovative combustion chamber is an excellent complement to our turbine expertise,” says Bevc. Working closely with CES, and with financial support from the U.S. Department of Energy, in 2006 Engineers from Siemens Energy in Florida began development of a 200 MW power plant based on combustion with oxygen. Siemens is contributing an innovative gas turbine design to the project.
The gas turbine must be able to withstand a hot and moist environment that is normally the domain of steam turbines. The dense gas stream has a pressure of 15 bars, a temperature of roughly 1,200 degrees Celsius, and is comprised of 80 percent water vapor and 20 percent CO2.
A vintage Siemens SGT 900 gas turbine has been specially adapted for such conditions, and the efforts of its developers are paying off in the form of high efficiency. Because the temperature of the stream entering the turbine is very high for such a moist, high-pressure environment, the plant’s efficiency is over 40 percent with natural gas and over 30 per cent with gasified coal. These are modest numbers compared to the efficiency of a modern coal-fired power plant, which without carbon dioxide separation, is over 40 percent. However, Siemens hopes to exceed these values with its next generation of turbines, which are scheduled to be introduced in 2015. The new turbines should have an efficiency of roughly 50 percent for natural gas and 40 percent for coal.
Carbon Dioxide Laundry. This isn’t the only approach to the separation of carbon dioxide that Siemens is pursuing. In addition to the oxyfuel method, the company is pressing forward with development of so-called IGCC (integrated gasification combined cycle) plants. These installations use entrained flow bed gasification and scrubbing processes to separate greenhouse gases from fuel gas prior to combustion (pre-combustion carbon capture). IGCC technology is now so mature that it can be deployed on an industrial scale. Siemens is also currently working to develop an efficient and environmentally-friendly post-combustion carbon capture process based on amino acid salts, which can even be retrofitted to meet the requirements of existing fossil-fueled power plants (see article “Scrubbing Agent is a Winner”).
“Despite our internal development work, we are always on the lookout for partners such as Clean Energy Systems that can help us to further advance our CO2 separation technologies,” says Robert Shannon of Siemens Energy in Florida. “We’re also interested in experimental, potentially revolutionary research approaches.”
Siemens found one such development at the Massachusetts Institute of Technology (MIT), which has been chosen by Siemens as a Center for Knowledge Interchange (CKI). CKIs are special universities with which the company has signed close framework and research contracts. Chemical Engineering Professor T. Alan Hatton and Howard Herzog, an MIT specialist in carbon dioxide sequestration, told Siemens about a method by which CO2 can be removed from a flue gas stream at a potentially low energy cost, which makes the technique extremely economical. A cooperation project on the topic commenced in 2008.
The basic idea behind this partnership can be summed up as follows: Most separation methods remove carbon dioxide from flue gas by using special scrubbing liquids, which are later heated. The process is effective, but it is also very energy-intensive. Hatton’s idea is to pass the flue gas through special salts rather than scrubbing agents. Unlike known scrubbing agents, the salts have a melting point of less than 100 degrees Celsius. They absorb CO2 in the liquid state and release it again when they are induced by an electromagnetic field to change to a semicrystalline solid state.
“This could reduce the energy consumption associated with carbon dioxide separation by 50 or even 75 percent,” says Hatton’s research partner, Dr. Thomas Hammer of Siemens Corporate Technology (CT) in Erlangen, Germany. “However,” he adds, “with this brand new method, we can’t expect a commercial application for at least ten years.” The quantities with which the MIT and Siemens researchers are working in the laboratory are modest at the moment. “No more than a thimblefull,” says Hatton.
CO2 Goes Underground. If carbon dioxide separation is successful, the gas will still need to be disposed of permanently. CES, for example, has already found one way to do this. The fact that it could be easily reconfigured to suit the company’s needs is not the only reason that CES purchased the Bakersfield power plant. The plant is also strategically located over rock strata that can hold billions of tons of trapped CO2. That’s enough to store centuries worth of the CO2 produced each year by the planned 200 MW power plant. Another option is to sell the separated CO2 - for example, to the operators of depleted oil fields in the surrounding area, who would pump the CO2 deep below the surface to increase oil extraction rates.
Hubertus Breuer
A Clean Energy Systems pilot plant near Bakersfield, California burns fossil fuels without emitting carbon dioxide to the atmosphere. Siemens has developed a gas turbine suitable for use with this technology.
Siemens is working with experts at MIT on methods for scrubbing CO2 out of plower plant flue gas.