THE ENERGY INDUSTRY TIMES - JANUARY 2017 600°C HRSG • (1) Tube to Tube • (2) Connectors is the same limit in pressure for the horizontal once-through under the Benson patent,” explained Fontaine. “Indeed, we have to rely on gravity and pressure to perform the separation of steam and water in the vertical tubes” “This is why we noticed a shift in the market to vertical-type once-through boilers. There’s no limit; we can do 200 bar or even up to the supercritical point if we wish but I don’t think this will happen as it would present other challenges. It would require thicker headers, thicker tubes, which would reduce operational flexibility.” Reduced operational plant flexibility is sometimes argued as a draw- back of drum-type boiler versus the once-through design but Fontaine does not see this as being the case at Bouchain. “The plant potentially will be shifting everyday, as is required of many combined cycle plants today. It will start in the morning, stop in the evening and we will keep the boiler warm overnight. “Under hot conditions Bouchain can make a straight start-up without limitations on the gas turbine. The limitation generally does not come from the HRSG; it’s more from the Special Project Supplement Connectors Headers This is explained by thermodynamic principles that dictate how water changes phase according to pressure and temperature. Fontaine explained: “As steam goes to the superheater, we have to go through an S-curve in a typical phase diagram. This means there is potentially a problem with flow stability because there could be several points of operation since some tubes will circulate more than others in a large heat exchanger. This could result in significant flow instability. This is even more critical for combined cycle power plants, as you go through a range of gas turbine loads during operation and pressure sliding. Flow stability is solved naturally in a drum-type boiler because we remain on the increasing part of the S-curve under biphasic conditions.” Solving this flow stability issue, while minimising the flow pressure drop, is one development that CMI has worked on. Regarding the oncethrough steam generator, he points out that these developments are on vertical type boilers, as opposed to horizontal type boilers. “The horizontal design is valid for a range up to about 180 bar – whether it’s once through or drum type – there steam turbine side where we have to heat up the large components of the machine.” Notably, GE’s trademarked ‘Rapid Start’ is also implemented here. GE says the plant is capable of reaching maximum combined cycle power output in less than 30 minutes from a hot standstill. This fast start capability is made possible by features such as a double intermediate and final attemperator, which according to Fontaine is not so common for GE. “Bouchain is a breakthrough for GE. In the past they have preferred an intermediate attemperator-superheater only. Siemens has used a double attemperator – both intermediate and final – for some time. “A final attemperator located between the boiler and the steam cycle is used for fine-tuning of the steam temperature control during rapid load changes such as start-up. It is a way of fine-tuning at the outlet of the boiler, even though the gas turbine is loading. Most of the control is done with the intermediate attemperator and the fine-tuning is performed by the final attemperator.” Another feature of the boiler at Bouchain that also allows rapid start, is what Fontaine describes as ‘purge credit’. This means the HRSG is purged when the gas turbine is stopped instead of during start up, so the boiler is ready to start during the next loading cycle. When Bouchain started in June 2016, it set a new world record for combined cycle plant efficiency under ISO conditions. Commenting on the achievement, Fontaine said: “It is an achievement we are proud of because it’s a CMI boiler, which we have developed with GE, behind the 9HA.01 gas turbine.” Since then, however, GE’s purchase of Alstom’s power business and Doosan’s HRSG segment may mean there will be fewer opportunities for CMI to undertake projects with GE. “The market trend is to keep the complete power train, including the boiler – within the group. So, the market for us, as an independent boiler supplier, of these big sized projects is a bit more complicated,” said Fontaine. Nevertheless, he remains optimistic, noting that Bouchain is a flagship project for both CMI and the industry. “It is the first new generation of HRSGs in commercial operation for the advanced H-class and J-class machines.” In the meantime, CMI says it will continue developing its HRSGs to accommodate bigger sizes of gas turbines for steam cycles with higher temperatures and pressures. In particular, further developments in the vertical once-through steam generator can be expected. “At the end of the day it will be up to the designer, who is establishing the heat balance. The heat balance is the key. Even behind a big gas turbine, it is the developer’s decision as to whether to go for a conventional steam cycle or not. But the trend, in order to increase efficiency, is take advantage of the potential to increase steam temperature and pressure,” said Fontaine. He notes that if power output increases in future gas turbine models, it is possible we may see steam pressure of 200 bar and will have to go to once-through. “We developed such cycles with GE for the 9HA.02. The Bouchain unit is rated at 400 MW, providing 605 MW in combined cycle. The 9HA.02 would be a much bigger machine and the steam cycle behind it could be up to 200 bar.” Having already developed and tested this advanced boiler design at its test facility, CMI believes it is ready, whichever way the market goes. Fontaine concluded: “We already have a new design to propose to the market; the large vertical oncethrough HRSG is already there. For CMI, it will be the next big step.” Possible locations for dissimilar welds: The configuration adopted for Bouchain is shown in (2) on the right of the lower diagram (dissimilar welds located within connectors) The Hamitabat combined cycle plant featuring two Siemens GT 8000H gas turbines and CMI boiler is due for completion in 2017 Boiler technical parameters and guaranteed performance Type Three-pressure, drum-type Superheated steam Flow (kg/s) 90.2 Temperature (°C) 584.3 Pressure (bar) 158.4 Reheated steam Flow (kg/s) 104.4 Temperature (°C) 582.5 Pressure (bar) 27.8 LP steam Flow (kg/s) 15.5 Temperature (°C) 315.1 Pressure (bar) 4.7 ©CMI May 2013 600°C - Dissimilar Weld Location 3 2 1 Steam Manifold SHT / RHT Tubes 600°C HRSG ©CMI May 2013 600°C - Dissimilar Weld Location P91 (Connector) P91 (Header) T91 (Tubes) S304H (Transition) S304H (Tubes) P91 (Connector) Incoloy (Transition) 347H (Connector) 347H (Header) S304H (Tubes)
Energy Industry Times January 2017
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