(A 300-µm coating developed by Dr. Werner Stamm (left) increases the service life of turbine blades, including those on the world’s largest gas turbine)
As every cook knows, a pinch of salt can transform a bland dish into a tasty one. But just how big that pinch should be is usually a question of experience, and sometimes it has to be mixed with other spices to get the right taste. The lesson isn’t lost on Dr. Werner Stamm—the star chef of materials research at Siemens Power Generation (PG) in Mülheim an der Ruhr, Germany. Stamm is always thinking up new "recipes" for which he’s never received any cooking awards, but instead 52 patents and the title "2006 Inventor of the Year." That’s because his recipes help make gas turbine blades more resistant to heat and corrosion.
The latest spice in Stamm’s kitchen is rhenium, a rare metal characterized by a very high melting point and high density. Adding one to 2 % of rhenium to a mixture of cobalt, nickel, chromium, aluminum, and yttrium (so-called MCrAlY coatings) imbues the complex mixture with extraordinary properties.
At high temperatures, the mixture forms a barrier of aluminum oxide on the MCrAlY surface that protects turbine blades from oxygen in a combustion gas. The rhenium improves the mechanical properties of the protective coating and simultaneously prevents the aluminum from diffusing into the base material. "The coating stops the base material from oxidizing," says Stamm. Without it, the nickel base alloy in the blade would only survive 4,000 hours of operation at maximum operating temperatures. With the coating, however, the alloy can hold out against the oxygen for more than 25,000 hours, longer than power plant operators demand as a minimum.
Stamm’s coating, which is only around 300 micrometers thick, also has another function—to serve as an adhesive agent for ceramic thermal insulation layers. Given a gas temperature of approximately 1,500 °C, this composite system of adhesive agent and ceramic—in conjunction with a special blade-cooling setup that blows air from narrow jets onto the blades—reduces the surface temperature on the metal in the first row of blades from 1,200 to around 950 °C. The newest thermal insulation coating systems can even accommodate ceramic surface temperatures of up to 1,350 °C.
Percentage Points Worth Fighting For. But Stamm and his coworkers still aren’t satisfied. That’s because as temperature increases, the efficiency of the system (the share of useful energy obtained from combustion) improves. And with raw material prices rising, power plant operators and designers are struggling to achieve gains of just tenths of a percent. This was the rationale behind development of the most modern—and with 340 MW of output also the largest—gas turbine in the world, which Siemens delivered to the E.ON plant in Irsching in 2007. Plans call for the giant powerhouse to be used in conjunction with a steam turbine beginning in 2011—a system that is set to break the 60-% efficiency mark (see World’s Largest Gas Turbine). "This moves us into a completely new realm of technology," says Dr. Johannes Teyssen, chief operating officer of E.ON AG in Düsseldorf. "And we fully expect the higher efficiency to result in lower power generation costs."
Additional efficiency could be gained by reducing air cooling in the turbine blades, as the air used here is carried through the turbine, thus lowering efficiency. Less cooling air would, however, raise the temperature in the first row of blades by over 100 °C—too much for the materials currently used. The gas turbine in Irsching already has an optimal cooling system—thanks to Werner Stamm’s MCrAlY protective coating. However, as Stamm points out, it won’t be possible to determine exactly how the turbine handles the strain until after it’s been operating normally for several years. "Labs and real machines are two different things," he says.
Heat-resistant and heat-insulating protective coatings like Stamm’s still offer huge untapped potential. If, for example, researchers are able to increase the surface temperatures of the ceramic material and reduce the formation of oxides on the MCrAlY layer, both efficiency and operating life could be significantly increased. And ultimately, the special ceramics are only an interim step on the road to full ceramics that require no cooling. But that’s a long way off, says Stamm, "Maybe in 15 years—but people were also saying that 15 years ago."
Siemens' acquisition of Westinghouse has brought new life to ceramic development, and engineers are now trying to increase temperatures—and thus efficiency—by utilizing oxide ceramics. Other companies in the sector are opting for a base material of silicon carbide, whose structure and properties resemble those of diamonds. Silicon carbide is a high-strength material that has one key disadvantage: It oxidizes when in contact with oxygen at high temperatures—and oxygen is something gas turbines have plenty of. Siemens researchers are therefore focusing on the development of oxide ceramics that have already reacted with oxygen. The material’s lower rigidity is not a drawback, as the most important thing is its actual useful expansion, which is greater than that of silicon carbide.
Still, ceramic blades need to be reinforced if they’re going to survive at least the 25,000 hours of operation customers demand of them. That’s because ceramics are brittle. Dr. Ulrich Bast of Siemens Corporate Technology in Munich, together with colleagues in Orlando, Florida, are therefore developing and testing fiber-reinforced ceramics . "The fibers provide a reserve for handling stress and keep the ceramic intact, even if it already has cracks in some places," says Bast. The combination of two brittle materials—a ceramic matrix and fiber—results in high tolerance to strain and damage. The oxidized fibers of aluminum oxide and silicon dioxide nevertheless remain the weakest link in the chain. Although they too no longer react with oxygen, they can only withstand temperatures up to 1,200 °C. Ceramic alone can handle up to 1,700 °C; when used in certain gas turbine components, it therefore requires no cooling. The fiber compound thus has to be protected from the extreme temperature of the heated gas by a thick ceramic insulation. Tests on a ring segment made of fiber-reinforced ceramic have already produced very promising results.
Generation 50plus. E.ON plans to begin building a new generation of coal-fired steam power plants in 2014 that will achieve an efficiency of above 50 %. Several preliminary projects are now under way for "Generation 50plus," with Siemens working on the development of components for such a plant. At the Scholven power generation center near Gelsenkirchen, Germany, for example, the COMTES700 project is testing materials for use in boilers, pipes and turbines that will be exposed to a steam temperature of 700 °C. This high temperature will enable the new plants to make the leap in efficiency from today’s maximum 46 % to 50 %. But higher temperatures alone won’t be enough, according to Dr. Ernst-Wilhelm Pfitzinger, project manager for the 700-degree turbine in Mülheim. Pfitzinger says achieving the final percentage point will depend on finding a favorable location with good cooling conditions—like the Baltic Sea. In a study known as NRWPP700, several partners, including Siemens, are already designing a demonstration plant whose components will withstand steam temperatures of 720 °C.
While 720 °C might sound almost refreshing compared to the hellish temperatures in a gas turbine, the demands placed on high and medium-pressure turbines are nevertheless enormous. In addition to the heat, there’s also the stress of 250 bar of pressure; in E.ON’s 50plus plant, that will likely increase to 350 bar. By comparison, a normal gas turbine is subjected to a pressure of only 25 bar or so.
Engineers building the steam turbine at Siemens PG in Mülheim can call upon the material expertise of their colleagues from gas turbine development, but the processing of the materials is extremely difficult. Whereas housings, blades and shafts in a gas turbine have a filigree design and are formed from thin plates and sheets, the forged shafts of a large steam turbine can be up to a meter thick, and individual components can weigh more than 20 t. What’s more, after being processed, all components may not deviate from pre-calculated shapes by more than a few hundredths of a millimeter. Welded seams 20 cm wide require the use of completely new welding techniques and, above all, new testing methods such as X-ray procedures, and ultrasound cannot penetrate far enough into the metal. The processing of alloys into thick-walled forged and cast components also necessitates a complicated recalculation of material data that takes into account the hot steam atmosphere. Such efforts increase costs—and the new alloy is also five times more expensive than high-quality turbine steel. Designers therefore only want to use a nickel-based alloy for those components such as rotor cores, blades, and internal housings that are truly subjected to high stresses. "This requires not only new processing techniques for compounds of various metals but also new cooling concepts," says Pfitzinger. "However, it should be possible to go above 720 °C."
World’s Largest Turbine Blade. Jörn Bettentrup doesn’t have to worry about too much heat. A development project manager at Siemens PG, Bettentrup designs new running blades for the final stage of low-pressure steam turbines, which are generally used together with high and medium-pressure turbines. Steam in the three turbines gradually expands and then slackens in the end, cooling down to 30 °C at a pressure of 45 mbar. The expansion sharply increases the volume of flow, however, which means the last blade wheel has to be the largest. The biggest blade wheel made by Siemens for final-stage operation has a flow surface of 12.5 m². "The trend is toward even larger areas," says Bettentrup, which is why he and his team are looking to build a steam turbine with the world’s largest blade wheel area—16 m². The turbine is also to be used at the E.ON plant in Irsching. Even the jet engines in an Airbus A380 don’t come close to this.
There’s a simple reason why the giant wheel is so attractive to customers: A 16-m² turbine can replace two 8-m² turbines, which saves a lot of money in terms of room, bearings and piping. It presents a major challenge for developers, however, as associated centrifugal forces put huge stresses on the blades. At 3,000 revolutions per minute, several hundred tons of force act on the blade roots and the grooves that join it with the rotor. Conventional blade steel is not strong enough to withstand this, so engineers need a very rigid material that’s light, thereby reducing the centrifugal force. They’ve now decided on titanium, an expensive metal with a matte finish that’s also popular among jewelers. Titanium weighs around half as much as normal turbine steel, is somewhat stronger, and displays good erosion-resistance properties. Titanium’s ability to damp oscillations is, however, slightly lower than that of steel, which is why titanium blades are equipped with special coupling and support elements. The structure of this blade system is extremely complex.
Most manufacturers now offer titanium blades for the final stages of their low-pressure turbines—but none have dared to build one as big as Siemens plans to produce. Tests and experiments designed to overcome technical hurdles still need to be carried out before the design is approved. But all operating parameters have already been tested for around two years using a small model turbine. The blade development team’s job is now to employ the material in an optimal design at a favorable cost, as production of titanium blades is more complicated—and therefore more expensive—than the process for conventional steel blades. Additional costs are generated by the high and increasingly volatile price of raw materials. Despite this, Bettentrup’s calculations show that, "It will definitely pay off for our customers."
No comments:
Post a Comment