Recent years have seen the introduction of a new generation of large gas turbines (eg, the GE HA, the Siemens H and the MHI J) able to deliver combined cycle power plant efficiencies well beyond what used to be seen as the major milestone of 60%, while at the same time providing enhanced operational flexibility.

In order to get the most out of these new machines in combined cycle mode, new demands have been made of heat recovery steam generators, in terms of steam temperatures and pressures, output, fast start-up and resilience under cycling.

Currently, the world record for combined cycle fuel efficiency, 62.22% (ISO conditions), is held by the Bouchain plant in northern France, jointly developed by GE and EDF, which was inaugurated in June 2016 (see Modern Power Systems, July 2016, p 12).

With an installed capacity of 605 MW this plant employs a GE 9HA.01 gas turbine coupled to a CMI heat recovery steam generator that incorporates a range of advanced design features designed to fully match the capabilities of the HA technology. As well as being capable of achieving unprecedented efficiency, the plant is also able to perform frequent starts and stops, and the power train (gas turbine, CMI HRSG, and steam turbine) can reach maximum power in less than 30 minutes (from “hot standstill”).

Using stainless steel

Among innovations incorporated in the Bouchain HRSG is the use of austenitic stainless steel (Super304H) tubes in the superheater and reheater sections, rather than ferritic alloy (carbon) steel.

At Bouchain, the steam temperature at the outlet of both the HP superheater and reheater is around 585°C, about 20°C higher than for conventional HRSGs in an F class application. CMI and GE decided to employ austenitic stainless steel even though P91 would have been possible but at the expense of thicker tubes.

The ASME code authorises carbon steel SA335 P91 up to 650°C at design conditions, however there is a concern about steam oxidation above about 605°C.

Looking ahead to future combined cycle technology upgrades, with steam temperatures surpassing 600°C, austenitic stainless steel can be seen as a prudent materials choice.

Austenitic stainless steel tubes are well established and widely used in fossil fired ultrasupercritical boilers (where steam temperatures exceed 600°C) but this experience is not directly transferrable to combined cycle HRSG applications because of the much higher levels of cycling anticipated in combined cycle plants.

However CMI has been investigating the use of austenitic stainless steels in HRSGs for a number of years, in collaboration with universities (in Belgium and Germany), research organisations (CRM, BWI and Laborelec in Belgium, TNO in the Netherlands) and steel suppliers (Vallourec/Mannesmann- Tubacex,andNipponSteel&SumitomoMetal Corp (NSSMC)). Issues addressed include: maximum allowable stress; conductivity and thermal expansion; steam oxidation resistance; stress relaxation cracking; material availability; impacts on design; weldability; cycle fatigue; and location of dissimilar (also called “black & white”) welds.

Dissimilar welds are those joining the austenitic stainless steel components to the ferritic alloy steel used elsewhere in the HRSG. The question arises as to whether these joints should be in the superheater/ reheater tubes themselves or in the connectors, ie the pipework linking the steam manifold to the header of the superheater/ reheater tubes (see p22, diagrams upper left).

Although CMI’s initial preference was for the dissimilar welds to be located in the tubes (building on long term operating experience with ultrasupercritical plants), it was decided, in the case of the Bouchain heat recovery steam generators, to place the dissimilar welds in the connectors, with Incoloy sections between. Particular attention will cerainly be paid to monitoring these dissimilar welds over the boiler operating life, particularly in view of the expected cyclical operating regime.

Dealing with the challenges posed by dissimilar welds in HRSGs of the Bouchain type was one of the reasons why CMI established a welding expertise centre at its Seraing site in Belgium.

The Super304H selected for the Bouchain HRSG superheater and reheater tubes includes Nb for stabilisation and special heat treatment. It has high strength and good resistance to steam oxidation and stress relaxation cracking.

TP347H stainless steel is employed in the connectors and for the header of the superheater/reheater tubes, again with Nb as a stabilising element. TP347H stainless steel also has good steam oxidation resistance but has a courser grained structure and is considered potentially more susceptible to stress relaxation cracking than a material with high fine grain (HFG) microstructure, such as the Super304 used for tubing in the Bouchain HRSG. Such a microstructure can only be obtained for thin wall components such as tubes.

Nickel-based high-temperature “superalloy”, Incoloy 617, which costs about ten times more than P91, is used for the transition pipework between the P91 and TP347H. It has a thermal expansion coefficient about midway between that of P91 and that of TP347H and serves a buttering role in the dissimilar welds.

As well as steam temperatures being higher than for typical F class combined cycle plants, HP steam pressures are also greater, 154 bar compared with 125 bar typically. At this pressure, and up to about 180 bar, a drum type natural circulation HRSG with horizontal exhaust gas flow, as at Bouchain, is still applicable, while at higher pressures a vertical flow configuration would be required, based on a once-through boiler without drum.

A drum type HRSG with fast start capabilities can be regarded as one of the enabling technologies for achieving what GE calls Rapid Response operational characteristics for the new combined cycle plant. Other enablers are double attemperation (intermediate, between the superheater and reheater sections, and final, for fine tuning), to reduce the steam temperature during rapid transients, and cascading steam bypass.

The HRSG is designed to be able to cope with daily start-ups and cyclic operation over a 30 year life. It is equipped with a stress monitoring system, which calculates equivalent operating hours amassed by the HRSG, adjusting for fatigue arising from cycling and operating in Rapid Response mode.

600°C steam cycles

With the Bouchain HRSG reference, CMI sees itself as well placed to provide HRSGs for the growing number of H and J class combined cycle plants where operating steam temperatures over 600°C are envisaged and where stainless tubing is therefore required. Recent examples are the HRSGs that CMI is developing for the Hamitabat combined cycle plant in Turkey (Siemens H) and the Topolobampo II combined cycle plant in Mexico (MHI J).