EDF’s Bouchain combined-cycle plant in France entered the Guinness Book of Records as the most efficient gas-fired facility in the world. We discover the technology that made it happen

Operational flexibility and combined-cycle plant efficiency are typically viewed as countervailing forces – success in one comes at the expense of the other. Recent technological advances, however, are challenging that presumption.

At EDF’s 605 MW Bouchain combined-cycle plant in northern France, pairing GE’s 9HA.01 gas turbine with CMI’s heat recovery steam generator (HRSG) has enabled the plant to set a world record for efficiency – 62.22 per cent under ISO conditions – while having the operational flexibility to accommodate frequent starts and stops and achieve a sub-30-minute ramp to maximum power output.

The high efficiency is particularly notable in light of the fact that the Bouchain plant is replacing a two-unit coal-fired power station that had an average efficiency of 35 per cent, thereby providing more power with half the carbon emissions.

The 605 MW Bouchain plant
The 605 MW Bouchain plant
Credit: CMI Energy

Moreover, the plant has already demonstrated its operational flexibility: in response to a grid event in France, the plant successfully ramped up 94 MW in 10 seconds and 147 MW in 30 seconds. And the flexibility is critical to economic operation considering the variability of electricity prices in France, which can range from €24/MWh during low-demand periods to €200/MWh in peak winter demand periods.

Matching equipment

Modern gas turbines provide the energy to generate steam at higher temperatures and pressures than ever before. To optimize the value of advanced gas turbines in combined-cycle configurations, and to respond to power market trends, the HRSG must be able to efficiently handle the selected steam conditions.

At Bouchain, the plant design team originally intended to base the unit on a GE F-class turbine, the 9FB.05, with five rows of gas expansion. This gas turbine was an evolution of the 9FB.03 with three rows of gas expansion. Prior to detailed design, however, EDF and GE shifted to the 400 MW 9HA.01 model, establishing Bouchain as the reference plant for GE’s H-class turbine in 50 Hz power markets.

Upgrading to the 9H turbine meant that the reheater and superheater sections could potentially encounter steam operating temperatures above 600oC and pressures up to 200 bar. This presented challenges since Bouchain was based on a steam cycle delivering superheated steam at 585oC and 158 bar.

The steam conditions forced CMI to think carefully about materials selection in the reheat and superheat sections of the HRSG. Ferritic steel (SA 213 P91) is the conventional choice, and ASME code authorizes its use up to 650oC, but there are concerns about steam oxidation above 600oC.

And while P91 is an acceptable material choice at 100,000 operating hours and 650oC with respect to allowable stress, the maximum allowable stress is reduced significantly, thereby increasing component thickness and restricting the unit’s cycling ability.

CMI ultimately decided to replace the P91 with an austenitic steel (SA 213 S30432) in the first rows of the reheat and superheat sections. This ASME material, commercially marketed as Super 304H, confers high strength and steam oxidation resistance, and is also resistant to stress relaxation cracking.

The decision to go with Super 304H was not made lightly. While austenitic steels have a ten-year-plus track record in the fossil industry – where steam temperatures in ultra-supercritical boilers exceed 600oC – they’ve not yet penetrated the combined-cycle HRSG market. HRSG applications can provide similarly advanced steam cycles, but the likelihood of cyclic operation introduces an additional challenge.

CMI has been evaluating the use of austenitic stainless steels in HRSGs for many years, collaborating with a number of research organizations, universities and steel suppliers across Europe and Asia. They have examined a host of issues associated with application in an HRSG environment, including maximum allowable stress, steam oxidation resistance, weldability, conductivity, thermal expansion and cycle fatigue. With respect to thermal expansion, for example, investigators researched impacts on component clearances, expansion joints and cycle fatigue in metal welds.

The results of these investigations were successful enough to convince CMI to use Super 304H in combined-cycle designs in Turkey and Mexico, paired with advanced Siemens SGT 8000H and Mitsubishi M501J gas turbines in configurations operating at 600oC steam temperatures.

Increase in electricity sales possible through rapid-start capabilities, compared with a conventional plant
Increase in electricity sales possible through rapid-start capabilities, compared with a conventional plant
Credit: CMI Energy

“During the design phase for Bouchain, we were aware of the GE upgrade of the 9FB to the 9HA, so we had begun evaluating the potential impacts on boiler design,” said Pascal Fontaine, CMI vice president. “At 585oC operating steam temperature, we are at the limit for using tubes made of T91. We could have still used it, but after extensive discussions, GE requested us to shift to higher grade materials considering some future upgrades.”

Welding worries

One of the main concerns with the switch to austenitic steels involves the associated need to use dissimilar metal welds. Because ferritic carbon steels are all from the same material family, their thermal expansion properties are comparable, introducing fewer complications when welding components together. However, when austenitic steels are introduced to the mix, the thermal expansion properties are sufficiently different that larger stresses can be imparted to connecting joints, particularly in cyclic operation.

At Bouchain, austenitic steel is used in the first rows of the superheater/reheater – where steam temperatures are highest – but connections are needed to the rest of the boiler where ferritic steels are used. CMI had two primary options for where to transition from ferritic to austenitic steels: firstly, in the connections between the steam manifold and the header of the superheater/reheater tubes. These connectors are made using 163-mm diameter pipes. Secondly, in the superheater/reheater tubing itself. Diameters of these tubes are 31 or 38 mm.

While both options are feasible, CMI preferred the second option because of the smaller diameters. GE requested to go with the first option, however, locating the dissimilar metal weld in the larger-diameter connections where Incoloy transition sections are needed between the P91 connectors and the 347H stainless steel connectors. Incoloy has thermal expansion properties just in between those of the P91 carbon steel and the 347H stainless steel header, enabling it to function as an effective transition in the dissimilar metal weld on larger diameter piping.

Accessibility for monitoring was a key factor in the decision to locate the transition between the steam manifold and the header. Inspections will be easier to perform over the life of the plants as the welds expand and contract during cyclic operation.

“We prefer to locate the dissimilar metal welds in the tubes because their smaller diameters will equate to less thermal expansion and lower stresses on the welds. Both options have been validated at our Welding Expertise Centre in Belgium,” says Fontaine. “Option 2 is used by CMI on our advanced HRSG references such as the Topolobampo II project in Mexico with the M501J turbine from Mitsubishi, and the Hamitabat project in Turkey with the SGT5 8000 H from Siemens.”

Drum-type boiler

Conventional HRSG design typically favours once-through designs over drum-type designs for higher-pressure applications. This is because the steam becomes denser as the pressure rises, rendering it more water-like, and making it more difficult to separate steam and water in a drum-type boiler. Once-through boilers avoid this issue because water simply enters at one end and exits as superheated steam on the other end.

The nominal upper bound on pressure for a drum-type boiler is around 180 bar. In two-phase steam-water conditions, there is a potential flow stability problem because some tubes will circulate more fluid than others for the same pressure drop. This issue can be magnified in combined-cycle power plants as the pressure is sliding according to load. This is the big challenge when comparing once-through boilers with drum-type boilers. With drum-type boilers, the thermodynamics remain in a portion of the phase diagram where flow stability occurs naturally.

Bouchain features operating pressure at 158 bar, which is still well within the acceptable pressure limits for drum-type boilers.

“CMI is anticipating much higher pressures in even more advanced steam cycles, perhaps exceeding 200 bar, which will likely dictate a shift to once-through boiler designs,” says Fontaine. “Horizontal once-through technology has the same pressure limits as the drum-type technology, but for vertical once-through boilers, there is basically no limit to what we can do. We could do 200 bar all the way up to the supercritical point if desired.At some point, however, other challenges would intervene, such as the need for thicker headers, which would reduce operational flexibility.”

CMI has been testing advanced once-through concepts anticipating such an evolution. For the next model enhancement to the GE 9H turbine, the 9HA.02, steam cycles could reach 200 bar. CMI has developed and tested steam cycles for advanced HRSGs to accommodate such conditions.

Rapid start

Power markets are changing in many countries and conventional grid-tied generation sources are increasingly subject to daily cycling.

Fast-start capabilities are integrated into the Bouchain power plant design. GE’s Rapid Start technology enables the plant to reach maximum power output in less than 30 minutes from a hot standstill. The H-class gas turbine accommodates fast start by starting up in simple-cycle mode, with the fuel heating provided by the startup gas heater.

Independently, the steam turbine valves control steam flow and temperature. The distributed control system then defines the optimal path to load all of the components and reach maximum output in the shortest possible time with the fewest possible emissions.

For the HRSG, rapid start is achieved primarily through dual attemperation – both intermediate and final – to reduce the steam temperature during rapid transients. Most of the desuperheating control is provided by the intermediate attemperator, but the addition of the second attemperator enables fine-tuning of steam temperature during rapid load changes, such as at startup.

The HRSG’s air purging functionality also contributes to fast starts. The boiler can be purged when the gas turbine is stopped instead of during startup, so the boiler is ready to go when the next loading cycle occurs. This function is referred to as the ‘purge credit.’

HRSG configuration

The physical layout of the HRSG at Bouchain is fairly similar to those used at combined-cycle plants based on earlier gas turbine designs. A total of 18 pre-fabricated modules are arranged in a three wide by six deep configuration, comprising 14,142 tubes and a heat transfer surface of 405,428 square metres.

To accommodate the design gas flow of 750 kg/s, CMI either had to increase the HRSG footprint from three to four modules or increase the length of the tubes. “Adding a fourth module would have significantly increased the costs of the HRSG, so we opted to increase the tube length,” explains Fontaine. “The size and weight of the individual modules increased, of course, but that drawback was less penalizing than the cost of a fourth module.”

To sustain the high efficiency of the HRSG, duct firing is not included at Bouchain. Available space has been reserved to accommodate a selective catalytic reduction system if future emission reductions require tighter NOx control. The HRSG is also equipped with an acoustic shroud around the inlet duct to control noise levels.

As scientists and engineers continue pushing the boundaries of gas turbine and heat transfer technology, ensuring a compatible fit between the gas turbine and the heat recovery steam generator will be paramount. The importance of this compatibility is further reinforced by the evolution of power markets that are pushing asset owners to provide greater operational flexibility.

Bouchain is in the vanguard of plants charting the path to this future.

About the author: John Evans is Senior Technical Director at Krishnan & Associates, a specialized energy industry marketing consulting firm servicing OEMs and technology providers. More information can be obtained at www.krishnaninc.com. John can be reached at john@Krishnaninc.com