THE ENERGY INDUSTRY TIMES - JANUARY 2017 high-pressure steel alloy that is used more widely in coal fired boilers. Values for parameters such as creep and fatigue and the maximum stress remaining after 200 000 hours of operation are known up to 650°C design conditions. But even though the material is potentially still useable above this temperature, Fontaine says the ‘Maximum Allowable Stress’ is reduced and thus component thickness increases, which has an impact on cycling ability. In anticipation of going to higher steam conditions following the upgrade of the 9FB.05 to the 9HA.01, it was decided that Super304H would be the best choice of material for the superheater and reheater tubes. “The designation of the gas turbine was changed around the time of project execution,” said Fontaine. “The turbine had been operated at higher gas flow, while keeping the steam temperature at 585°C but GE saw the potential for even higher steam temperature. So when we designed the boiler, we had in mind the upgrade of the 9FB to the 9HA. “Other projects using Mitsubishi 701J and Siemens 8000H machines already have steam cycles at 600°C. For example, the Hamitabat project we are building in Turkey also has an operating steam temperature of 600°C for both high pressure superheater and reheater.” Super304H, which is specified Bouchain is on the boil Since breaking the 60 per cent combined cycle electrical efficiency barrier at the Irsching 4 power plant (later renamed Kraftwerk Ulrich Hartmann) in Germany in 2011, the major gas turbine manufacturers have been pushing the performance envelope of their large machines, with the latest milestone being 62.22 per cent power plant efficiency achieved in June 2016 at the 605 MW Bouchain power plant located in north Calais, northeast France. While much of the increase in plant efficiency can be attributed to the move to H-class and J-class gas turbine technology, developments have also been required in the boiler or heat recovery steam generator (HRSG) that sits behind the turbine. Not only do these boilers have to handle the higher gas turbine exhaust temperatures and flows, they also have to be flexible – working in tandem with the gas turbine to cycle and ramp quickly to meet the demands of today’s power markets. Bouchain comprises a GE 9HA.01 gas turbine with a power output of 400 MW. Exhaust gas from the turbine at a temperature of 650°C is fed to an HRSG designed and supplied by CMI, based in Belgium and the US. The HRSG is a triple-pressure, drumtype boiler, designed to deliver superheated high pressure steam at 585°C and 158 bara. Following the award of the contract in March 2012, CMI had to come up with several innovative features to enable the HRSG to exploit the technical abilities of the GE 9HA.01 gas turbine at the heart of the facility. Among the key innovations were the use of materials in the superheater and reheater sections that allow the HRSG to potentially handle steam temperatures in excess of 600°C and pressures approaching 200 bar. The Bouchain combined cycle plant is the first commercial reference of the GE 9HA.01 (an upgraded version of the 9FB.05). It represents a huge investment that saw GE tie-up with the French utility EDF for the development and commercialisation of the turbine. GE also worked with CMI on developing the boiler to utilise the exhaust heat from the turbine. Development of the new HRSG began in 2013, around the time when GE was in the process of changing the design of its H-class gas turbine. Pascal Fontaine, Vice President Marketing & Licence Manager, CMI Energy said: “We were just finishing two projects in France that were based on the GE Frame 9FB.03. GE wanted to change the design of this gas turbine and developed the 9FB.05. This was quite a different machine, with two additional rows in the turbine section. We worked with GE to develop the boiler behind this new generation of gas turbines to take advantage of their ability to enable higher steam temperature and pressure in the boiler.There was a lot of discussion on the maximum steam temperature and pressure, and GE fixed the steam cycle at 585°C in the superheater and reheater.” Such high steam temperature prompted significant debate over the most suitable materials needed in the superheater and reheater. The decision was taken to use austenitic stainless steel (Super304H) tubes in the superheater/reheater sections instead of the ferritic alloy (carbon) steel (SA335 P91) that is more commonly used. Fontaine noted: “Even though we are at the limit for P91 and have a few years experience of using austenitic stainless steel at such a temperature (585°C), the market is still questioning its use.” ASME codes authorise carbon steel P91 up to 650°C at design conditions. It has been on the market for many years and is used extensively worldwide. However there is a concern about steam oxidation above about 605°C. Fontaine said: “We have to be very careful with the heat treatment and welding procedures – a lot of problems can occur, especially with small piping. There has been a lot of debate, and even today ASME is considering the maximum allowable stress for this material.” He notes that there is similar discussion around P92, another Special Project Supplement The demands placed on the heat recovery steam generators used in the current crop of advanced combined cycle power plants is calling for innovations in boiler technology. Junior Isles Record-breaking performance: the Bouchain combined cycle power plant in France
Energy Industry Times January 2017
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