TPi July 2009

Nickel alloys for advanced ultra-supercritical boiler service By Gaylord Smith, Brian Baker and Lewis Shoemaker, Special Metals R&D team, Huntington, West Virginia, USA

S pecial Metals Corporation, part of Precision Castparts Corporation, has been developing and manufacturing nickel alloys for critical applications for over 100 years. With manufacturing facilities in the USA and UK, SMC is one of the largest global nickel alloy producers, and the range of products manufactured include forged and rolled bar, sheet, strip and plate, tube and extruded shaped sections. Introduction Coal-fired electric utilities worldwide are facing increasing demand for additional electrical capacity, while at the same time facing mandates to reduce plant emissions, principally CO 2 , through improved efficiency or carbon capture technology. To meet these future goals will necessitate the employment of established and specifically developed austenitic nickel-base alloys that are capable of meeting these stringent demands. The challenge is to provide boiler alloys that provide creep strength at very high steam temperatures, 700ºC (1,290ºF) or higher, and steam pressures as high as 350 bar [35MPa (5.1ksi)], while at the same time providing coal-ash and steam oxidation corrosion resistance. Meeting this challenge has brought nickel- base metallurgists and aerospace superalloys into the effort. That meeting this challenge is critical due to the fact that over 40% of the electrical energy produced

worldwide is produced from coal, a percentage that is expected to remain high in the coming years, as depicted in Figure 1 . Historically, supercritical steam boilers have typically delivered steam to the turbine at temperatures up to 566°C (1,050°F) and pressures up to 238 bar [24.1MPa (3,500psi)] with fuel efficiency between 42 and 43% (LHV basis). Notable efforts to improve efficiency began in the late 1950s in the United States, to raise the conditions to 621°C/310 bar [1,150°F/31MPa (4.5ksi)] and in 1962 to 649°C/340 bar [1,200°F/ 34.5MPa (5ksi)]. Material issues and other problems ultimately resulted in these plants scaling back their operating conditions. For the next three decades, the best state- of-the-art supercritical boiler stood at 593°C/310 bar [1,100°F/31.5MPa (4.57ksi)]. During this period, coal- ash corrosion was minimised by the development of Incoclad ® 671/800HT ® that provided long term service (20+ years) in boilers fuelled with a variety of high- sulphur, low-grade coals. However, strength issues prohibited raising the steam temperature and pressure. By the mid-to-late 1990s, utilities, their plant fabrica- tors and certain far-sighted governments decided to make the necessary developments to achieve a 700°C (1,290°F) advanced ultra-supercritical technology for coal-fired power plants. Two notable initiatives were inaugurated in Europe: AD700 and MARKO. Then, in 2002, US DOE Vision 21, ‘Material Development and Qualification Project’ was started. The performance target of the European projects was established as 400- 1,000MW, 700°C/350 bar [1,290°F/35.5MPa (5.1ksi)] and the goal for the DOE project became a more aggressive 760°C/ 380 bar [1,400°F/38.6MPa (5.6ksi)]. While raising steam temperature is more efficient than raising steam pressure, unfortunately raising steam temperature disqualifies even the most advanced fer- ritic tube steels at temperatures much above 620°C (1,150°F) due to their lack of strength and coal ash

Figure 1 : Worldwide electricity generation by type of fuel, 2005-2030

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Tube Products International July 2009

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