High fuel prices and the increasingly complex nature of power markets are forcing combined cycle generators into mid-load operation, which requires high efficiency and availability combined with rapid starting capability to be economically viable. How are such plants meeting this challenge?

Eric Jeffs

Flexibility of operation was a property of the earliest combined cycles, many of which are still in service in Europe, and is in contrast to today’s much larger plants with high steam pressures. These modern combined cycles were built primarily for base-load duty but are being forced into mid-load duty – primarily by the presence of high natural gas prices – and are having to perform a large number of starts per year.

In countries such as the UK, the majority of new capacity additions in recent years have been gas fired combined cycles, designed to replace the aging fleet of baseload coal and oil fired units. However, the market is increasingly looking towards combined cycles to supply peak demand.

In a thermal power system based on steam plants, as in several European countries, the flexible systems of the past were the large oil fired steam sets with their low auxiliary loads and a liquid fuel, which could run up from cold in an hour. This provides a benchmark performance for the modern combined cycle, which after an overnight shutdown must be able to come up to full load in a similar order of time.

The early European combined cycles with moderate steam pressures and vertically configured heat recovery boilers were able to do this. But a large number of modern combined cycles, particularly in North America, are unable to achieve the rapid starting and loading required for mid-load operation. The common factor is in the use of horizontal, natural circulation heat recovery boilers with high steam pressures and high thermal inertia.

Benson design

The significant announcement at the end of last year was by Siemens on the new SGT5-8000H advanced gas turbine. Of the few details then released was the fact that it was not steam-cooled, and was designed specifically for flexible operation in a combined cycle. Furthermore, the combined cycle will only be supplied with a Benson (once through) heat recovery boiler.

The Benson heat recovery boiler was first installed at Cottam in the UK in 1998 in a combined cycle with the current model SGT5-4000F. That unit has now run for seven years and has operated for some periods on base load and at other times on a mid-load regime with on average 150 starts per year. Cottam was built originally as a Siemens Development Centre. The boiler has Benson HP and IP sections and a drum type LP and was designed for a maximum pressure of 160 bars, in anticipation of the later installation of a larger gas turbine. Following postponement of this development, ownership transferred to E.ON (UK), which runs it commercially as part of its generation portfolio. The boiler can only run at 125 bars with the existing unit.

Figure 1. Cottam, near Nottingham, UK, was the first combined cycle to be supplied with a Benson heat recovery boiler, behind a Siemens SGT5-4000F, and has operated for seven years with about 150 starts/year
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Two German orders to Siemens last year for SCC5-4000F combined cycle blocks at Hamm-Uentrop (2) and Hardecke (1) are being supplied with Benson heat recovery boilers, but with a Benson HP section at 125 bars, and drum type IP and LP stages. Despite the political hostility of the former SDP/Green coalition, Germany still receives 33 per cent of its electricity supply from nuclear plants, which run baseload, and has about 14 000 MW of renewable capacity in wind farms which are intermittent in output. Therefore Benson boilers for these two contracts provide a flexible system which can carry spinning reserve to bring on quickly when the wind drops to cover the loss of output from the wind farms, and at times of coal fired and nuclear plant trips.

Because of its original license from Mark Benson in 1924, Siemens has had a long interest in the technology and could see a need for an adaptation of the principle to a heat recovery boiler. Cottam has provided the proof of the design in commercial operation, and in the winter of 2004-05 the license was offered to the world’s boiler-makers. Siemens could see that there was a need for rapid starting capability, which the Benson boiler can achieve but the horizontal natural circulation boilers cannot. Following the takeover of the Westinghouse business in 1998, it saw at first hand the problems of the Westinghouse combined cycles based on W501F with tri-pressure reheat boilers operating at pressures over 120 bars.

Fast start

Siemens has since developed control packages for a warm start, after an eight hour overnight shut-down, to achieve better temperature soaking of pipes and casings, and control of the gas turbine so that it can load up quickly. As soon as enough steam is available the steam turbine starts to roll. The gas turbine does not have to wait at part load with higher emissions for the steam turbine to catch up, and since the efficiency is higher during the start-up period there is a consequent saving of fuel.

The fast start package has been applied to several SGT5-4000F combined cycles in Europe. It is an engineered solution for plants with horizontal, natural circulation boilers running at high steam pressures. But there are other combined cycles with vertical boilers that are naturally fast starting. The vertical boilers have intrinsically lower inertia because the tubes are arranged horizontally across the gas path and are supported in tube plates suspended from the top of the frame. The tubes are also of narrower wall thickness because, unlike in the horizontal designs, the tubes do not hang from a drum or header under their own weight.

Figure 2. Lake Road, Connecticut, USA: this plant consists of three blocks of Alstom GT24 with dual pressure once through boilers. It is a mid-load merchant plant dispatched daily from Boston
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Belgium, with more than 50 per cent nuclear supply has also installed combined cycles which have been designed for load following duty. Gent Ringvaart, Baudour, Vilvoorde, Brugge Herdersbrug, Drogenbos, Seraing, and Angleur all have vertical heat recovery boilers supplied by CMI, and operate in mid-load duty. These are conventional drum type boilers with assisted circulation and two pressures, except for Gent Ringvaart and Baudour which, being based on the GE Frame 9FA, have tri-pressure reheat steam cycles with vertical, natural circulation boilers.

Gent Ringvaart went into operation in November 1997 and ran initially on baseload, but in recent years has run a mixed operation with baseload at the winter peak and at other times mid-load with, on weekdays, a run down to 50 per cent load overnight and a 36 hour shutdown at weekends.

Perhaps the most flexible combined cycles in current service are the GT24 blocks in North America of which 42 are in service, including ten in Mexico and one in Canada. Of the other 31 in the United States the ten at sites in New England are dispatched by National Grid (US), out of Boston.

These plants all have a standard design of power train that was developed by ABB in consultation with American National Power, specifically for the merchant plant market which depends on high efficiency and rapid starting capability. In addition to a 2-pressure Benson type boiler with HP output at 160 bars, 560°C, and a drum type LP running at 7 bars, 320°C, the other notable feature is the two speed steam turbine which has the HP cylinder running at 8900 r/min and an LP at 3600 r/min, which offers about one percentage point net gain in efficiency by optimizing the HP blade design. Total plant efficiency is 57.5 per cent, which is compatible with what is achieved with the larger GT26 blocks in the 50 Hz market with conventional drum type boilers operating at lower pressures.

With the once-through boiler, there is no HP drum and a much smaller water volume, which gives it a low thermal inertia. On a warm start after an overnight shutdown the GT24 block can come up to full load in just under an hour from start. These plants operate mainly on a mid-load regime with daily start and stop in accordance with dispatch.

There are two projects in North America with vertical drum type boilers. EDF International’s Rio Bravo site 60 km south of the US border in northern Mexico has three 570 MW blocks with two SGT6-5000F gas turbines and a 210 MW steam turbine. Vertical boilers were specified because the area has a high air conditioning load in summer and a lower more variable demand for the rest of the year. CMI supplied six tri-pressure reheat boilers with an HP output at 133 bars, 566°C.

The other combined cycle is in Canada at Brighton Beach, near Windsor, Ontario. Ontario Power Generation wanted flexible operation and specified vertical boilers, this time a natural circulation design with supplementary firing, from Austrian Energy. The plant has two Frame 7FA gas turbines and a 200 MW steam turbine, an HP output at 136 bars, 567°C when fully fired.

Engineering solutions

The flexibility of a combined cycle ultimately depends on the type of boiler and its thermal inertia. High steam pressures contribute to higher efficiency. For many combined cycles running today which were built as base load units at a time of low fuel prices, there is an engineered solution that will improve start-up performance.

Particularly in Europe the older combined cycles with vertical boilers have a low thermal inertia, which lends itself to load following and frequency control. But there is also a growing experience with once-through boilers, particularly in the United States, which are similarly capable.

In the future, with a growing component of wind energy and the return of nuclear power for base load, the combined cycle will be plant for mid-load and frequency control for which flexibility and high efficiency will be at a premium.