TPi July 2009

62 / 68

60 Tube Products International 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
Special 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