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Hannover Messe - Power Plant Technology Forum 20115. 8. April 2011, Hannover, Deutschland

KA26 Combined Cycle Power Plant as Ideal Solution to Balance Load

Fluctuations

Dr. Michael Ladwig, Mark Stevens Alstom Power, Baden, Switzerland

Abstract As commonly accepted, combined cycle operation regimes change over the lifetime of a plant. Especially in the last few years these changes have been noticeably evident as a consequence of the increasing proportion of electricity generation production from renewables, such as wind turbines. Therefore, today's modern combined cycle power plants need to be flexible in operation. This flexibility is not only requested regarding turndown capability, but also regarding operation regimes. Plants may operate one year in more or less base-load mode, and the next year in cyclic mode with daily starts and stops.

Accurate modelling of thermal cycles is required to avoid premature fatigue failure of plant components for such requirements. Detailed temperature histories during transient events are required to determine maximum stress ranges for life assessment. These details are obtained from a dynamic simulation of the thermal process in the component of interest. The simulation of the HRSG, for example, is used to tune the operation concept and then to determine the temperature history for the four main types of cycles: hot start, warm start, cold start, and trip. Heat-up, steady state, and cool down phases of each cycle must be modelled. Thick walled components and those exposed to higher temperatures limit the life of the overall system.

From the output of detailed analyses, a comparison to code-allowable stresses can be made to determine the fatigue usage factor. Due to the relative high flexibility in start-up and operation of combined cycle power plants, these power plants are expected to be called upon more and more to deliver greater operational flexibility. Components therefore within the CCPP that are designed for base-load applications are likely to be less tolerant to cycling than many of these plants will actually experience.

This paper will show examples of the components of the KA26 combined cycle power plant, that demonstrate how Alstom achieved a fatigue-tolerant design to allow base-load and cycling plant operation.

Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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1 Introduction Power markets around the world are facing new challenges as the need to build new power generation plant to meet growing demand is at the same time having to take in to consideration public pressure and measures to address the global environmental issues and of the impact the conventional power plants are having on our planet. This challenge is further complicated in many of the so-called developed power markets by the trend-shift in structural set-up from one of regulated (closed) to de-regulated (open) power markets. The result of all these changes and developments is that many of the advanced gas-fired combined cycle power plants that where installed in the late 1990s and 2000s, which due to their relative high efficiencies were specified and designed based on base-load dispatch, are today being called on to operate under a wide ranging dispatch regime, including daily stop/starts and intermediate regimes, which was never foreseen at the outset.

Operational flexibility is now becoming more and more a buzzword in the gas-fired power industry, and OEMs and Operators alike have to re-define the way such power plants should be designed and the possible load regimes that could be expected today and in the future. The emergence and growth of renewable power, in particular wind-farms, also brings new challenges and issues for power companies and grid operators, who have to balance the power generation with the load demand. Although the emerging renewables and other low carbon technologies are expected to play an increasing role in the longer term, fossil energy supply from fuel gas, oil and coal is likely to remain for decades. The increasing installation of power generation systems using renewable energies and their dependency on ambient conditions (like wind power) calls for a balance with the reliable and rapidly available power resources covering periods of sudden supply shortage, peak demands or simply following the automated generation control over a wide range of relative load.

Alstoms modern combined cycle design concepts depict multi-purpose power plants, perfectly fitting plant operators needs for operation flexibility. It is understood that plants cannot be optimised for just one operation regime, as operators face different power market requirements and opportunities over the lifecycle of a power plant.

Alstom has established a Plant Integrator approach in the way it designs and builds modern power plants. Alstom looks for the overall plant optimisation, rather than focusing at the component level, thus leading to complementary well-aligned development of Alstoms in-house core components such as the gas turbine, steam turbine, HRSG, generators, and heat exchanger equipment.

Plant Integrator thinking is reflected in the continuous improvement of the KA26 product family, based on the Alstom advanced-class GT26 gas turbine, the Alstom steam turbine portfolio for combined cycles and Alstoms heat recovery steam generators (HRSG).

In this paper we shall look specifically at the design considerations taken by Alstom with regards its gas turbine GT26, steam turbines and HRSGs to ensure the KA26 products are able to deliver the very high operational flexibility needed in todays power markets.

2 The Alstom KA26 Combined Cycle Products The KA26 combined cycle power plants have been offered by Alstom since the mid 1990s, so for around 15 years now. These combined cycle power plants have at their core the Alstom GT26 gas turbine, which is now well known for having the unique sequential (2-stage) combustion technology in the advanced-class GT/CC market.

Alstom has focused on developing two Reference Plant products for the KA26:

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. 8. April 2011, Hannover, Deutschland Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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1. KA26-1 (1-on-1) single-shaft combined cycle power plant (Figure 1) 2. KA26-2 (2-on-1) multi-shaft combined cycle power plant (Figure 2), also developed

as Integrated Cycle Solution (ICS) utilizing once-through HRSG technology

The primary 50 Hz advanced-class CCPP markets have tended to lean to date more towards the 1-on-1 (KA26-1) CCPP configurations, but at the same time, there are markets where a 2-on-1 (KA26-2) CCPP configuration is preferred. Todate, Alstom has received orders for a total of 36 CCPP projects utilising the KA26-1 (1-on-1) platform and 8 CCPP projects utilising the KA26-2 (2-on-1) platform.

Figure 1 KA26-1 Single-Shaft CCPP

Figure 2 KA26-2 Multi-Shaft CCPP

The current CCPP performance of these two KA26 Reference Plant offerings from Alstom at ISO ambient conditions are:

KA26-1 KA26-2 KA26-2 ICS

Net Output 431 862 872

Net Efficiency 58.7% 58.7% 59.1%


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3 Key Component Design

3.1 GT26 Gas Turbine

The GT26 gas turbine (Figure 3), first introduced in the mid 1990s, is Alstoms advanced-class GT offering for the 50 Hz CCPP market. This GT26 is the only 50 Hz advanced-class gas turbine employing sequential (2-stage) combustion technology. This unique approach to the combustion process in the GT, applying the reheat principle, makes the GT26 particularly well suited for high CCPP base- and part-load performance. Combined with three (3) rows of variable inlet guide vanes at the inlet to the GT26 compressor, the GT26 is able to provide exceptionally high part-load efficiency in combined cycle operation, a factor that is becoming increasingly important as such large CCPPs are being called upon to operate more and more at loads other than just base-load, including even for up to ca. 8 hours per day at minimum parking loads during off-peak periods (such as during the night-time), which can see CCPPs operating at loads down to 50% or below. It is easy to see therefore why part-load performance is becoming a more and more important consideration for power companies planning to build new CCPPs. The unique sequential combustion of the GT26 is also a key factor enabling the KA26 products from Alstom to be parked at very low CCPP load, typically around 20%. This Low Load Operation Capability (LLOC) will be looked at in more detail later in this paper.

At the same time, in addition to being called on to operate at various loads, these advanced-class GTs are also being expected to cycle (stop/start), potentially on a daily basis, and this in-turn poses a greater duty and burden on these engines. It is a well accepted and recognised fact that high cycling shortens the inspection interval and expected lifetime of the GT, and all OEMs apply some form of start factor to their inspection interval formula to take this fact in to account. This is a factor that cannot be eliminated, but at the same time the OEMs can and are looking at ways to mitigate this impact and to still enable reasonable lifetime expectations from the components, especially in the hot gas path zone. Advancements in the special exotic alloy materials and thermal barrier coatings are helping significantly in this respect.

SEV Combustor

EV Combustor

3 rows Variable GuideVanes (VIGV)

Compressor

High Pressure Turbine (HPT)

Low Pressure Turbine (LPT)ExhaustDiffuser

Solid Welded Rotor

Figure 3 GT26 Gas Turbine


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Although Alstom recognises that cycling duty is a requirement and expectation of todays advanced-class GT-engines, the company is also looking at ways to assist the power companies to increase the options / choices they have with respect to the way the CCPP is operated, so as to provide power companies with greater flexibility and higher part-load performance so as to possibly avoid or mitigate the need for cycling, and the associated increased maintenance cost impact associated with such stop/start regimes.

3.2 STF15C & STF30C Steam Turbine Modules

The key features of the Alstom steam turbine portfolio, which make them ideally suited for high operational flexibility, are:

Inlet module portfolio for optimal fit to all steam turbine sizes and operation modes Shrink Ring design of HP Turbines Design of IP Turbines Welded Rotor design Operators can take advantage of very significant additional payments for frequency response support via the operational flexibility advantages derived from these key features, in particular:

Low start-up times Rapid loading and de-loading ramp rates

Start-up Times While the ramp rate of a steam turbine depends substantially on boiler capacities and the overall margins available in the overload valves or the throttle reserve available in the main control valves, the start-up times of an Alstom 660 MW steam turbine generator set (cold start in 240 minutes) compares favourably even with existing 210 MW machines in India (going up to 360 minutes). Figure 4 shows typical start-up times, to full load, of Alstoms steam turbines.

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011

Figure 4 - Typical Alstom ST Start-up Times

Shrink-Ring design of the HP Turbine The unique shrink ring design of the Alstom HP turbine was introduced in the 1960s. It eliminates bolt flanges on the inner casing and results in a radially symmetrical structure with best thermo-elasticity. Therefore, the shrink ring design prevents casing distortions almost

as Ideal Solution to Balance Load Fluctuations 5. 8. April 2011, Hannover, Deutschland Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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completely, which is of increasing importance with the increasing steam temperatures of modern water-steam cycles. The benefits are long-term stable clearances and sustained efficiencies combined with long-term reliability and operational flexibility. Slots maintain the steam-extraction for the top heater in the inner casing and a simple and robust shrunk-on extraction chamber located between two shrink rings. The axial position of these slots is determined through the targeted final feed water temperature. Due to the double-shell design the outer-casing is exposed to the exhaust steam only, which allows relatively small flanges at the outer-casing. A pre-heating of the casings prior to a start-up is not required. Generally, Alstoms reheat turbines are designed in such a way that the casings do not limit thermal transients. The assembly of the HP turbine with its shrink rings is shown in Figure 5.

Figure 5 - HP Turbine Assembly showing Shrink Rings Design features of the IP Turbine The lower pressure level of the intermediate-pressure turbine allows a horizontal split of the inner and outer casing with conventional flanges. This standard double-shell design would however lead to large casing distortions with the increasing steam temperatures of modern water-steam cycles. Alstom has made intensive investigations to reduce the possible thermal casing distortion in particular in the inlet sections where the inlet scrolls cause additional asymmetries. The inner shell geometry of the IP modules in conventional double-shell design has been optimised to counterbalance the asymmetric mass distribution caused by the flange and the inlet scrolls. The inlet section with the inlet scrolls and some first stages build the inner-casing and supports the blade-carriers for the further steam path sections. The pressure drop over the blade-carriers is comparably small and therefore, their flanges can be kept small as well. The result is an overall more rotation-symmetrical design, which reduce thermal distortions almost completely. The benefits are stable clearances and sustained efficiencies. Thanks to the inlet scrolls with the integrated radial first stationary blade row and the welded rotor design a secondary steam cooling with its negative impact on the efficiency is not required, not even for the highest reheat temperatures of today. Welded Rotor Design The welded rotor design is a key feature of Alstom steam turbines since the 1930s. It allows the material selection of each rotor section to match the respective temperature of the steam path. Therefore, welded rotors can be easily adapted to suit the increasing steam temperatures of modern water-steam cycles. Because of the cavity formed by the two


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welded rotor sections, the stress levels due to temperature differences are lower. This design allows a faster start-up and/or lower life consumption rates compared to mono-block rotors and suits well the water-steam cycle parameters of today and for the future. Further, the small forgings (Figure 6) are easier to obtain on the market and to test than large forgings for mono-block rotors.

Figure 6 - Alstom Rotor Forgings prior to Welding Alstom has by far the most experience in welding of similar and dissimilar rotor materials. It is notable that Alstoms welded rotors have never suffered rupture or similar failures since their introduction back in the 1930s.

3.3 Heat Recovery Steam Generator (HRSG)

Advanced large combined cycles continue to drive toward increased thermal efficiency. Correspondingly, the water/stream cycle, including the HRSG, has also had to evolve to capture more energy from the gas turbine exhaust. The HRSG has become larger while accommodating higher steam pressures and temperatures accepting increased gas turbine exhaust mass flow and temperature. Alstom HRSGs (ref. Figure 7) for such applications are typically triple pressure with reheat producing over 160 MW without supplementary firing. Figure 7 - Alstom 3PRH Drum-Type HRSG Plant power output can be further increased with addition of (supplementary) firing in either the HRSG inlet-duct or inter-stage between HRSG tube banks. The range of operating modes for todays plants add further demands for HRSG capability: Grid frequency control participation, required in some countries (even when operating at

nominal load), may mean transient grid frequency excursions leading to temporarily increased turbine outlet temperatures. Pressure parts and structural supports must be designed with consideration for creep at higher temperatures to endure these excursions.

Unlimited operation at a range of GT loads from 100% down to 10% requires a highly flexible means of high-pressure superheater and reheater temperature control. Alstom


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HRSGs employ dual-stage desuperheating to maintain steam temperatures within metal design limits and avoid over-spraying to a saturated steam condition.

Increased tube length, higher pressures and temperatures, frequent cycling, and different operation modes all work to fatigue HRSG pressure parts. Simple extension of existing technology from smaller HRSGs, operating under less demanding conditions, typically results in reduced availability. Alstoms HRSG design with Single Row Harps and Stepped Component Thickness was specifically developed to be more flexible to meet the demands of advanced large combined cycles.

4 Design Methodology for Alstom Combined Cycle Products Figure 8 shows a typical annual load profile that is being witnessed on a large percentage of the Alstom KA26 (50 Hz) and KA24 (60 Hz) fleet. It shows that the load profile for the KA26 plants is varying greatly, including a large number of stop/starts as well as parking at minimum load. Figure 9 is a 1 week snap-shot of a typical load regime being seen on the KA26 fleet in Europe, which also shows very clearly the different load points the plant is being called to operate at. Base-load is by no means the rule! The Combined Cycle Power Plants are being called upon very often by the grids to provide frequency support and being requested by the grid companies to run at reduced load so as to have reserve capacity for fast response (load ramp-up) to support grid frequency disturbance restorations. Fast ramping from part-load and quick start-up from shutdown therefore is being seen more and more a factor for consideration in the design of the combined cycle power plants.

Figure 8 Typical KA26 Annual Load Profile (Europe)

1 Year 1 Year

1 Week1 Week

Figure 9 - Typical KA26 Weekly Load Profile (Europe)


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4.1 Plant Base- and Part-Load Efficiency

Continuously increasing plant efficiency levels, always striving for a better use of natural resources, is a clear trend in the power industry and Alstom is certainly one of the leading global players in this field. Fluctuating market power demand, offering an operation reserve, a frequency response reserve or low load parking during periods of low demand, all means that CCPPs will spend more and more time at part loads. Part-load efficiency nowadays is therefore becoming a significant criterion in new CCPP projects.

In contrast to other OEMs / GT suppliers, Alstom has focused its GT/CCPP development to ensure high overall plant efficiency from base-load down to very low part-loads. With its sequential combustion concept combined and multiple variable inlet guide vanes the GT26 offers a very high turn-down ratio at low NOx emissions and high exhaust temperatures, thereby permitting todate higher part-load efficiency levels than can be achieved with single combustor technology.

General Possibilities of Reducing Load

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KA26 LLOC: Reduced GT load at low

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Period of low power demand or reduced power tariff

* Typical for single combustion engines

4.2 KA26 Turn-Down Ratio and Low Load Operation Capability

Utilising the competitive turndown ratio of the GT26, plant operators have a wide field of possible operation points. In principle a turndown to around 30% GT load is possible, subject to emission limit constraints, whilst still being able to provide primary and secondary frequency response. Where the outlook on future NOx and CO emission legislation makes catalysts installation/use (or their retrofit-ability at a later date) advisable, Alstom can provide proven and reliable solutions.

Figure 10 The KA26 LLOC Concept

Alstom can even offer an option reducing the steady-state power output to around 20% relative plant load or lower for the sole purpose of parking the CCPP fully on-line instead of shutting down. The KA26 Low Load Operation Capability (LLOC) concept permits the fuel consumption during operation at minimum parking load to be reduced to a minimum. Todate, power companies have had only two options during off-peak periods, to park at a minimum-load point of approximately 50% CCPP load or shut-down. Figure 10 shows the three options for the GT26/KA26 plant, namely normal operation at the high part-load point (around 50% CCPP load), complete shut-down plus the ability to park at a much lower load point (around 20% CCPP load). The KA26 LLOC works by having only the first-stage environmental (EV) burners of the GT26 in operation, the second-stage SEV-burners are switched off, thereby offering a feature that is special to the KA26. In the LLOC operation


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mode, the KA26 plant is parked and the GT26 still produces sufficient exhaust energy to permit with the water/steam cycle to remain in full operation at the very low load point, and importantly the plant is still able to meet close to base-load NOx and CO emission limits.

Figure 11 shows the significant difference in CCPP part-load heat consumption of the LLOC concept compared with the normal operating range.

It is re-emphasised that the LLOC is a specific operating point for the pure purpose of providing full on-line reserve power, and when in this mode the CCPP is parked.

Figure 11 Typical KA26-1 Heat Consumption vs. CCPP Load Graph

Some of the salient customer benefits that can be brought by this LLOC feature are:

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1. provides a true on-line reserve capacity due to fully functional water/steam cycle; 2. affords shorter re-loading time to full / high load compared to a full plant shut-down; 3. avoids risk of potential (uncertain) start-up failure at critical stage; 4. reduces the cumulative NOx / CO emissions compared to parking at the higher more

traditional part-load points; 5. project specific solutions combining the LLOC with district heating / cogeneration can

be worked-out, hence improving the fuel utilization at times of low power demand. 6. reduces fuel consumption during off-peak parking periods, thereby saving fuel costs

during the low or negative spark spread periods.

4.3 Cycling Capability

In addition to the availability of the LLOC concept, Alstom KA26 plants offer competitive solutions for daily start and stop (DSS) operation. An overall assessment of process parameters of the topping (GT) and bottoming (water/steam) cycles has led to advanced start-up concepts.

Alstoms GT26 with its sequential combustion in combination with the multiple and variable GT inlet guide vanes offers two unique characteristics.

Combined Cycle Power Plant Load [%]

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The more or less constant exhaust gas temperature over a wide operation range (30-100% GT load) reduces thermal transients during load changes as far as possible.

Due to the second sequential combustor system, the GT26 exhaust volume flow and exhaust temperature can be adapted to the actual start-up conditions of the underlying water/steam cycle.

Amply dimensioned steam de-superheaters and steam-bypass systems (a tradition in Alstom plants) also plays an important role in the KA26 design for cycling capability.

The Alstom HRSG pressure parts design is well prepared for any transient operation, which is addressed in section 5 of this Paper. Alstom developed ways to fulfil the EN12952 restrictions regarding the protection of drum magnetite layers, which allows Alstom to offer EN12952 compliant solutions with HP-drums rather than once-through evaporation systems.

Further improvement of the Alstom combined cycle steam turbine portfolio will allow Alstom to offer fast start-up options from hot and warm turbine conditions (e.g. after overnight shutdown or weekend shutdown). The same design principles contribute to a reduced expenditure of steam turbine fatigue lifetime.

It is worth noticing that Alstom plants can realise all of these plant characteristics without additional auxiliary systems such as: auxiliary steam boilers condensate polishers (for neither cycles employing drum-type nor for once-through HP

evaporation concepts)

Reduced capital expenditure (first, operational and maintenance) is the positive outcome for plant operators.

Alstom is able to offer HRSG with either fully drum-type or partial (i.e. only high pressure) once-through evaporation technology. Once-through evaporators are currently set in operation in two plants in Germany and UK, where high efficiency 850 MW blocks are installed and commissioned. Here the selected high operation pressures had driven the decision for once-through technology. For the base fleet with single-shaft 400 MW blocks, however, Alstom does not currently see a necessity for once-through evaporation systems: For 1-on-1 CCPP applications the impact of temperature on efficiency exceeds the

influence of HP system pressure. Actually the pressure / efficiency curve is relatively flat with a cost / efficiency optimum around 140 bar (depending on alloy material cost).

Drum-type applications provide a more robust chemical/steam purity behaviour characteristic, since drum blow-down is always possible. Condensate polishing is neither required for commissioning nor for commercial operation.

Alstom drum design and operation concepts allow for fast-start-up procedures and daily start-stop operation regimes.

The start-up and part load behaviour of a drum-assisted evaporator is easier to control than the one of a once-through evaporator with separation bottle.

Drum-type HRSGs achieve at least the same part-load efficiency as those equipped with once-through technology.

Alstom HRSG design employing HP-drums even allows for additional supplementary firing sufficient for most markets needs, as and when required.

The HP once-through evaporation system, at the selected moderate pressure levels, has no considerable first or secondary cost advantage over the drum-type design.

Once-through evaporators can be subject to dry-out at elevated steam qualities, which leads to reduced inner heat transfer coefficient and requires additional heat exchange surface.


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4.4 Grid Frequency Support

With its competitive turndown ratio outlined above, Alstom KA26 plants offer a unique frequency response load range. Alstom HRSGs in KA26 plants are ready to accept high GT exhaust temperatures, which allows maintaining power output characteristics according UK National Grid Code requirements, one of the most demanding grid codes worldwide.

Steam turbine assistance in frequency primary response is another optional feature developed for KA26 plants. It allows the water/steam cycle to contribute to the active grid frequency support of the plant. As an additional feature, the steam turbine may also be used to limit frequency excursions during partial load rejection in small island grids.

4.5 Power Augmentation

Air inlet cooling (AIC) by water injection is a well-established measure for gas turbine power augmentation. By cooling down the inlet air, the GT air mass flow is increased and the compressor work reduced. While fogging saturates the intake air, high fogging injects additional water, which evaporates primarily in the compressor. Alstom started back in 2000 its in-house GT inlet cooling development program for evaporative coolers, fogging systems and high fogging systems, with the goal to achieve the optimum performance and availability with a safe operation of the respective systems together with the gas turbine. Theses systems have been validated and today are offered by Alstom as standard options for the Alstom KA26 product offerings. These GT inlet cooling options permit more than 20% power boost. Figure 12 shows the locations in the air intake for the GT inlet cooling options.

Figure 12 GT Inlet cooling option locations

The fogging and high fogging systems inject small water droplets (Dv90

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Figure 13 gives an indication of the amount of GT power gain that can be obtained by means of the various GT inlet-cooling options. The greatest gain is to be had by means of combining evap. cooler or fogging with high fogging.

Figure 13 Indication of GT26 power gain possibility with AIC

The main customer benefits of this optional system as offered by Alstom on its KA26 products are:

Up to 20% power boost in the GT by applying high fogging together with evap cooler or fogging

Short lead and installation time with potential for high return on investment Alstom offers complete turnkey inlet air cooling systems Alstom will guarantee performance of the plant operating with the Alstom GT air inlet

cooling systems No lifetime penalties to the GT thermal block when the Alstom GT air inlet cooling

systems are supplied and installed by Alstom

These aspects are discussed in some length in reference [1].

4.6 Evolution KA26 Reference Plants

The KA26 product portfolio is subject to continuous improvement driven by experience gained from executed projects as well as new market request.

With the on-going customer expectation for increasing plant efficiencies and operational flexibility, Alstom will moderately improve the water/steam cycle systems. Live steam and reheat steam temperatures will be moderately increased compared to todays single-shaft plants.


5 The HRSG Todays modern Heat Recovery Steam Generators (HRSGs) must be designed to endure a wide range of steady state and transient conditions, at times outside of the specified design envelope of a combined cycle power plant to provide reliable long-term service under a variety of operating modes. In modern plants with high capacity bypass systems, operation of the gas turbine and HRSG can essentially be decoupled from the steam turbine allowing significant flexibility, but the HRSG must be capable of enduring the complete range of gas


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turbine exhaust gas temperature, flow transients, and resulting water/steam temperatures. HRSG design must conform to the prevailing boiler codes, such as the EN12952 Water Tube Boiler Code or ASME Section I. These codes principally control the wall thickness of pressure parts (tubes, headers, etc.) so that the stress levels in these components during steady operation are sufficiently low that they provide long-term service, usually in excess of 200,000 hours, with consideration for operation at high temperatures. Additionally, the HRSG must be able to endure the variety of start-ups, load changes and low load operation that might occur over the planned operating life of the plant. To accomplish this, the transients must be well understood and appropriate fatigue calculations performed to ensure that the required lifetime would be met. Some of these fatigue calculations are defined in the EN12952 Water Tube Boiler Code. The challenge that faces the HRSG designer is that in some cases the requirements for long-term operation are counter to those for cycling service. For example, for a long service life at high temperature, additional wall thickness can be added to the component to decrease the operating stress and therefore decrease the rate of creep damage accumulation. However, a thicker wall component will have larger thermal stresses during temperature transients, which can make the component more vulnerable to fatigue damage during such transients. The designer, therefore, must take advantage of materials with superior high temperature strength, such as grade 91 or grade 92 (x10CrWMoVNb9-2), and develop pressure part arrangements that minimize wall thickness of key parts, and also minimize differences in wall thickness between adjacent parts to minimize stress intensification at critical joints.

Figure 14 Alstom HRSG Single Row Harp Design

Alstom has a long tradition of employing such technology, with over ten years experience in application of advanced alloys and single row harp assemblies. Diameter and wall thickness of the gas-touched headers is minimised and tube bends are eliminated with such assemblies. Also, using the stepped component thickness approach minimizes thickness differences between tubes and manifolds. In this case, all tubes are 1.5 (38 mm) or 1.75 (45 mm) diameter, single row harp headers are 3.5 (89 mm) or 4.5 (114 mm) diameter, connecting links are 3.5 (89 mm) or 4.5 (114 mm) diameter, and manifolds range from 8

Single Row Harps

Stepped Component Thickness


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(203 mm) to 20 (508 mm) diameter. Wall thickness is related to pressure, material, and temperature but otherwise increase proportionally with diameter. By stepping diameters, wall thicknesses are also more gradually stepped and the abrupt change in thickness from tubes to manifolds is avoided. These geometric factors influence the transient stress range of key pressure parts and weld joints. When selected in this manner, as shown in Figure 14, thermal stresses are minimised, the stress range from transient operation is minimized, and life maximized. This is particularly important for the high temperature superheater and reheater components that experience the most rapid heating from the gas turbine, under-go rapid temperature changes due to steam flow, and which must also endure condensate development during scenarios such as purges. This, therefore, requires that many operating scenarios be evaluated to ensure that overall thermal growth and temperature differentials both within and between parts do not result in thermal stresses that are prohibitively high. These aspects are discussed in some length in reference [2].

An important feature of Alstom combined cycle plant (such as the KA26) is the Once-Through Cooler (OTC), which is a once-through steam generator acting as an intercooler for the gas turbine compressor airflow. The OTC provides steam flow to the HRSG superheater early during a cold start-up. This helps to control the temperature rise, and consequently fatigue damage in the superheater. This is relevant to this discussion as superheater manifolds are one of the two controlling components for HRSG life (the other is the high pressure drum). Referring to Figure 15, it can be seen that the OTC introduces steam to the low temperature superheater at a relatively low flow rate approximately 5 minutes before boiling and steam generation occurs in the high-pressure evaporator. Consequently, the high-pressure superheater components are more gradually heated during the cold start-up and thermal gradients in thicker pressure parts are reduced. The resulting thermal stresses and fatigue damage are also reduced.

Figure 15 Steam Flow to HP Superheater Outlet Manifold (Alstom)

While the superheater and reheater components can take advantage of high strength alloys, for some components, such as the high-pressure drum, high-strength alloys may not be as effective. High-pressure drums for HRSGs represent a particular design challenge when working to the EN12952 Water Tube Boiler Code because of the requirements for fatigue analysis, but more particularly because of an additional rule that limits the stress range to avoid magnetite cracking on the surface of the drum. The Code basically permits a fixed stress range beyond which magnetite cracking will occur, irrespective of the strength of the base material. Therefore, if a high strength material is used, for example an alloy such as WB36, then the drum shell thickness will be decreased but the absolute stress level will be higher. In fact, the stress level during normal operation can approach that permitted as a limit to avoid magnetite cracking. This means that little, if any, stress range remains to allow for the thermal stresses that occur during start-up. Therefore, in many cases for high strength materials, the drum may have to be thicker than that determined by simply applying Code allowable stresses to ensure that the combination of start-up thermal stresses and normal


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operating pressure stresses do not exceed the magnetite-cracking threshold. The transient experienced by the high-pressure steam drum during cold start-up is also quite complex, because it is highly non-linear and is governed by a combination of fundamental thermo-hydraulics during the early part of the start-up and by plant design during the latter part of the start-up. More specifically, during a start-up from cold (defined as a condition at which the drum fluid is close to ambient temperature) then when the plant is started the evaporator tubes will progressively warm until the fluid reaches boiling temperature. When this occurs, circulation will start to be established and the fluid in the drum will rapidly warm from its sub-cooled condition to boiling. Thus a rapid temperature transient results, during which the fluid temperature in the drum can change by approximately 80 to 100K in a matter of a few minutes. As the start-up progresses and pressure builds in the drum, then some control can be provided for the rate of temperature (or pressure) rise by modulation of the by-pass system. However, at low pressures when steam density is low, the by-pass system may not be sufficiently large to permit control. Therefore, it is quite common that the high-pressure drum must endure a cold start-up transient that is highly non-linear and which must be modelled with high-fidelity dynamic simulation to predict the temperature changes that occur.

The two graphs in Figure 16 show a linear transient approximation and non-linear transient calculated from a dynamic simulation of fluid temperature and pressure in the HP drum during a cold start-up for comparison. In the case of the linear approximation, a straight line is drawn from the starting point, at ambient temperature and pressure to the steady state operating temperature and pressure. The resulting stress, determined from a finite element model of the component of interest, in this case the HP drum, follows a similar linear progression.

Linear Transient Non-Linear Transient

Figure 16 - Graphs showing Linear & Non-Linear Transients

In the case of the non-linear graph, temperature and pressure over time are determined from a dynamic simulation of processes within the HRSG. Separation of fluid in the HP-drum from the gas path and the phase change lead to a highly non-linear start-up event. Temperature and pressure gradients are steep in the initial phase, flat in the intermediate phase, and steep in the final phase. The resulting stress plot of the HP drum is also highly non-linear. It can be seen that during the initial phase the inside of the drum shell experiences compressive stress. This is a result of the relatively rapid heating of the inside surface of the drum and the time it takes for the drum plate to heat through its thickness. Then as pressure builds, stress in the drum shell increases and changes to tensile. At the end of the cold start, steady state is achieved and the stress level is the same as for the linear case. However the stress range experienced by the drum shell is approximately 35% higher when the non-linear transient is considered. Therefore it can be seen that the simple linear approximation is not an adequate means of predicting life of HRSG components as it under-estimates transient stresses and consequently over-estimates fatigue-life. Through consideration of the non-linear transient and resulting stresses, Alstom has developed a unique high-pressure drum.


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By carefully selecting shell thickness, diameter, materials, nozzles, and joint construction the Alstom high-pressure drum can accommodate operating pressure levels used in todays advanced CCPP and fully comply with pressure part codes, meeting client life requirements.

Due to the highly non-linear transients experienced by the HRSG, it is essential that accurate dynamic models are developed to predict transient temperatures and pressures. Detailed stress analysis must then be performed based on temperature and pressure predictions from these models. The results are far from the conventional simplified approach employed in some Boiler Codes, based on assumptions of simple linear transients. Also the need for an integrated approach is highlighted, where key characteristics of the gas turbine, HRSG, and balance of plant are included in the dynamic model. The brief discussion here as highlighted the challenges facing the HRSG designer and some of the approaches to address the issues while respecting the requirements defined in boiler/pressure vessel codes. Other considerations also enter for a cycling plant, such as turndown performance and the related issues of de-superheater performance and economizer steaming; however, these are usually addressed with more conventional boiler technology. By proper evaluation of actual conditions and design of HRSG components, reliable operation can be achieved.

6 Summary Combined-cycle power plants for base-load duty in all but a few markets globally is today a very unlikely scenario. Operational flexibility is becoming more and more important as owners have to address increasing competition within the power industry as well as dynamic market forces. New challenges are being introduced, in particular with the growth in the renewables market, and the gas-fired power plants are being required to operate under all possible dispatch modes, from base-load right through to daily stop/starts.

Alstom, as one of the leading designers and builders of large turnkey gas-fired combined cycle power plants, is focused on developing the KA26 products to have the in-built features plus options along-with the design considerations to allow power plant owners the ability to dispatch these plants in as wide a range of conditions and factors as possible. Operational flexibility (i.e. grid frequency support, fast start-up and/or ramp-up, high base- and part-load efficiency, along-with high fuel handling capability to deal with fuel gas composition variations) are indeed a key features required in todays combined cycle power plants, as well as at the same time delivering the high performance and reliability needed.

The core components and overall plant design have to take in to consideration the higher operational burden being demanded of todays combined cycle power plants, which in turn offers exciting and new challenges for the plant integration companies like Alstom, to come up with the combined cycle power plants needed for tomorrows markets. Having all of the core technologies in-house puts Alstom in a strong position to address these needs and challenges. The Low Load Operation Capability is a good example of how Alstom can take and use inherent features of a component and implement it in the overall plant design so as to enable this special mode of operation. Designing the plant so as to be able to deliver high plant efficiency not just at base-load but also over a very wide load range is another area where having the total know-how in-house, at the plant and component level, allows the power plants to be optimised and tailored to meet the individual customers needs on a case-by-case basis.

Environmental legislations combined with public pressure will continue to challenge companies like Alstom to find solutions to produce energy cleaner and more efficiently. A good power mix of technologies has been and will remain important, and the combined-cycle power plant will continue to have a part to play in this arena, but we can and must accept that such plants will be called on to operate in ways that ten years ago would never have been thought likely.


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References [1] PowerGen Europe 2004 Paper Fogging and High Fogging: Alstoms Experience and

Customer Benefits by Stefan Lecheler (Universitt der Bunderswehr Mnchen), Stefan Florjancic and Giovanni Cataldi (Alstom (Switzerland) Ltd.)

[2] PowerGen International, Las-Vegas Paper Fast Start-up and Design For Cycling of Large HRSGs by Wesley Bauver, Ian Perrin, and Thomas Mastronarde ALSTOM, Heat Recovery & Plants, Windsor , CT, USA (POWER-GEN International, 9-11 December 2003 Las Vegas, Nevada)


1 Introduction2 The Alstom KA26 Combined Cycle Products3 Key Component Design3.1 GT26 Gas TurbineSTF15C & STF30C Steam Turbine Modules3.3 Heat Recovery Steam Generator (HRSG)

Design Methodology for Alstom Combined Cycle Products4.1 Plant Base- and Part-Load Efficiency4.2 KA26 Turn-Down Ratio and Low Load Operation CapabilityCycling Capability 4.4 Grid Frequency Support4.5 Power AugmentationEvolution KA26 Reference Plants

5 The HRSG6 SummaryReferences

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