Combined-Cycle Power Plant Concepts meeting the demand for ... · The operating regime of Siemens...

Power-Gen Asia 2003 ID203 Rev.3

24.07.2003

Combined-Cycle Power Plant Concepts meeting the demand for

operational flexibility

Wolfgang Menapace,

Matthias Fränkle,

Dr. Bert Rukes

Siemens AG, Power Generation, Germany

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Abstract

Based on observations of the power industry in different regions, the suppliers of power plants

have to cope with different market and customer requirements. The prioritization of aspects

like low life cycle costs, proven design, performance and operational flexibility differs for

each market. The challenging task is to combine and optimize these different aspects to fulfill

the specific customers requirements.

This presentation illustrates how plant concepts are being developed for our customers under

the varying conditions of deregulated markets. Many plants which were built with a focus on

base-load operation will e.g. need to be operated with considerably more flexibility to

compete in changing power markets.

It will be demonstrated how the operation mode of a plant – base load vs. intermediate load –

influences the plant design and the economic evaluation. Due to the change in operating

requirements, more plants will be spending additional time off line. This increased down time

typically extends the time to restart the facility. Therefore it is important to provide a higher

level of operating flexibility to enhance the value of the new generation assets. The goal will

be to own a plant that has the efficiency of a traditional base-load combined cycle plant while

incorporating fast start-up and cycling capability and perceive potential market changes to

allow a highly competitive product over life time.

The paper will address results of a program within the authors’ company to develop an

integrated combined-cycle plant design that offers fast start-up and cycling flexibility without

adversely affecting efficiency or reliability. Examples of plant concepts are evaluated

regarding the economic benefits of its application depending on the market conditions.

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Introduction

Market environment of deregulation

Current forecasts show a progressing deregulation of the power market worldwide of about

40% until 2005 (Fíg.1).

source: internal data collection and analysis

Figure 1 Deregulated markets: Power production [TWh], Share of deregulation [%]

Besides the additional power demand the replacement of aging power plant units will become

the most important driver for the future development of the power plant market. The

liberalization and more restrictive environmental aspects will even accelerate this process.

In a deregulated business environment and given the unstable natural gas and electricity prices

Combined Cycle Power Plants are favored because of their relatively short pay-back periods,

advantageous emission values and therefore will dominate the market. However, modern

Clean Coal technologies (ultra supercritical steam plants, gasification, fluidized bed

combustion) will be part of the market to establish a well balanced energy mix, especially

where indigenous resources are cheaply available.

Demand for more flexible plants

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Figure 2 Market segmentation model

In a deregulated market the competitive position of a power plant must be defended

constantly. This effect is illustrated by the “market segmentation model” (Fig. 2). In this

model the individual units in the market are ranked by their variable operating costs. From the

load curve of the power supply system over the cumulative capacity of the units the projected

dispatch factor for each unit can be determined.

Due to the market entrance of more modern and cost effective plants (e.g. unit X in Fig.2) the

existing units will be forced to dispatch in a different operation mode after a certain time, i.e.

base load-designed plants are demanded to be operated in a non-base load regime to have

them in operation only when the revenues of electrical production are favorable or upon

dispatcher’s request.

This might force power companies to overcome this described scenario by stepping forward

in ranking with the help of modernization or the adaptation of a new operation mode in order

to ensure the competitiveness of a power plant in future.

The operating regime of Siemens PG’s 50Hz V94.3A gas turbines in commercial operation

reveals today’s requirement to operate V94.3A-class based combined cycle power plants in

intermediate or alternating operation. Power plants based on the predecessor model V94.2 are

already forced into daily cycling or peaking operation (Fig.3 ).

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Figure 3 Operating regimes of V94-engines

To help owners cope with such uncertainties, Siemens PG provides combined-cycle power

plants with enhanced operational flexibility. This enables power plant owners to continuously

operate their plants under the most economic conditions.

From a plant operating capability standpoint the change of the operational regime toward

more intermediate duty significantly compromises its economics, especially when the power

plant is designed as a base-load facility. The effect of “hidden” or unknown costs of cycling is

caused by increased forced outages, lost production, increased frequency of failures in major

components [1].

The major challenge for plant managers in a deregulated market is to respond quickly to

profitable dispatch opportunities. The majority of recently built combined-cycle plants do not

fully support these challenges and little progress has been made to improve this situation.

Several publications have already outlined existing industry practices on plant cycling [2].

They recommend obvious scope provisions such as auxiliary boilers, automated drains and

vents, etc. This paper will not elaborate on these.

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Figure 4 1S.V94.3A Combined Cycle Power Plant

This paper introduces details on the Siemens PG´s development of a straightforward

combined-cycle plant based on the single-shaft arrangement with improved start-up time

while not sacrificing base load efficiency.

Plant Design Considerations for Cycling/Fast Start Capability Operational flexibility in this regard means the power plant capability of performing increased

numbers of start-up and shut down events, high efficient part load operation and rapid load

following as well as special tasks like house load and grid support operation.

The major challenge to increase the cycling capabilities of a highly efficient combined cycle

power plant without sacrificing base load performance basically comes down to the question

on how thermal stresses in the HRSG and the steam turbine might be reduced.

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Starting point: Configuration and Definitions

Starting point of the analysis was the current design of the 50Hz single-shaft. The Siemens

PG single-shaft concept has been introduced to the market in 1994 (King´s Lynn, UK) and

accumulated a big pool of operational data for different load regimes which allowed to start

the analysis on real operational data.

The single-shaft concept offers some inherent advantages with regard to cycling requirements

due to the lower system complexity of the water-/steam cycle. (Fig. 5).

Since most of the systems subroutines are put into operation by the unit coordination program

during an automated start-up a critical path analysis offered the best opportunities to focus on

the limiting components.

Figure 5 water-/steam cycle of a single-shaft

In the next step the team had to agree on an operating regime. The established differentiation

between hot, warm and cold starts and the corresponding downtime assumption was found not

to be precise enough to base a thorough analysis on it. In addition to that a definition for

“Start-up time” based on unmistakable milestones was required.

Due to the fact, that it is hard to predict a future load regime for today’s base-loaded

combined-cycle plants, a “worst case” scenario was selected. Such a load regime might apply

to a merchant plant which needs to operate and survive with the opportunities and the risks of

a short-term market without the security of a long-term power purchase agreement that

guarantees the revenue stream.

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The attached table summarizes the assumptions of the analysis:

Definition of start-up time From ignition of gas turbine to gas turbine at

full load and all steam-bypasses closed

Definition of shut-down time From initiation of shutdown from base load

to flame-off signal of gas turbine

Duration of an overnight standstill 8-16h

Duration of a weekend standstill 16-64h

Duration of a prolonged standstill >64h

Number of starts after overnight standstill 200 starts/a

Number of starts after weekend standstill 50 starts/a

Number of starts after extended standstill 2 starts/a

With regard to start-up, the plant components restrict each other in terms of maximum

allowable temperature gradients or necessary heat soaking time. This is an effect of

interdependent systems and cannot be neglected. However, under the umbrella of the

Reference Plant program, experts from various disciplines including gas turbine, steam

turbine, plant engineering, HRSG engineering and including vendors, were brought together

to integrate component and system design to enable the following:

• Fast GT ramp-up:

The gas turbine (GT) to be ramped up in combined-cycle applications close to the load

ramps as they are in simple-cycle applications, almost unrestricted by downstream

components

• ST life –time consumption:

The ability to quantify the effect of start-ups on the steam turbine maintenance schedule,

enabling the power plant manager to set a business strategy considering the ST and

HRSG, harmonized with GT maintenance

The result of the development was successful as shown in Figure 6 for a plant start-up after an

overnight shutdown. The diagram compares the relative power output of a “Typical Plant”

and a single-shaft with improved cycling capability. Both configurations consider triple-

pressure reheat bottoming cycles:

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Figure 6 Load curves after 8h downtime

As per definition the preparation for start-up is not included in the start-up time which starts

with ignition of the gas turbine (A). At rated speed the generator is synchronized and

connected to the grid and the GT is loaded to minimum output (B). The gas turbine output and

hence the exhaust gas temperature is increased further with the target to reach a GT load to

operate completely in the low NOx premix mode. In parallel the steam lines and steam turbine

valves are warmed-up (C). The steam turbine is rolled on, synchronized and loaded (D) while

the GT is continuously ramped up, the steam bypass valves are closing (E) and the steam

turbine accepts all generated steam. The gas turbine load is increased to rated power and the

plant reaches approximately 97% of its rated power (F).

This optimized start-up scheme makes the plant start-up approx. twice as fast as its

predecessor after overnight, weekend and prolonged shutdowns. One key for this

improvement is Siemens’ patented Benson Once-Through Steam Generator (Benson-OTSG).

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Benson Once -Through Steam Generator (OTSG)

The Benson technology has proven its capability and reliability throughout the past 70 years.

This technology is well known throughout the industry in both sub- and supercritical designs

in conventional steam power plants.

Siemens PG also successfully applied the Benson evaporator principle in a 390 MW

combined-cycle application at Cottam Development Center, Nottinghamshire (United

Kingdom). This state-of-the-art single-shaft facility has been in commercial operation since

1999. During this period the Benson-OTSG (Figure 7) has shown its direct applicability for

fast-start up and cycling operation, even though the Cottam plant was not explicitly designed

for this duty.

Figure 7 Benson-OTSG at Cottam Development Center (triple-pressure reheat steam cycle)

The HP-/IP-evaporator stages are characterized by two bundles of vertical tubes, which are

arranged in series in the horizontal gas-path (Fig. 8). All tubes within an evaporator bundle

are connected in parallel, creating natural circulation characteristics. The low mass flux

design allows the fluid mass flow to self-adjust to the heat input in each tube row, meaning

tubes with higher heat absorption will see an increased fluid mass flow. This design ensures

static and dynamic flow stability throughout a wide load range. In addition it results in lower

pressure losses across the evaporator compared to other once-through concepts.

The Benson-OTSG is capable of handling high temperature transients during a fast start-up

due to the elimination of thick-walled steam drums. The HP drum, in particular, impedes fast

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start-up of conventional HRSG’s by dictating long soaking periods and slow maximum

allowable temperature ramp rates. The Benson-OTSG eliminates these restrictions by

replacing the drum with a small separator vessel. The separator performs the function of

water/steam separation during start-up and shutdown. At steady state operation, including low

loads, the steam flow passes through the separator as part of the interconnecting piping toward

the superheater. The steam at the evaporator outlet is slightly superheated; consequently no

separation occurs.

Figure 8 Once-Through Steam Generator Section

In contrast with a drum-type HRSG, the Benson OTSG is able to control the main steam

temperature by modulating the feedwater massflow. This enables the system to compensate

the effects of changing ambient conditions or GT loads within a certain range without placing

the steam attemperators in operation. Attemperators are incorporated as interstage, as well as

final-stage desuperheater for the high pressure and reheat steam systems. This arrangement

ensures compliance with the steam temperature limits, dictated by the steam turbine during

plant start-up.

Simply stated, the Benson OTSG retains all the positive features offered by the traditional

drum-type HRSG, while providing enhanced cycling capability over the drum-type HRSGs.

Steam Turbine

The plant concept is based on a 2 casing 3 pressure reheat HE-type Steam Turbine (ST). The

ST is started power-output oriented, which means that it is loaded fast with reduced and

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nearly constant steam temperatures until the bypass valves are closed and the GT reaches base

load. Special care must be taken to control the steam temperature in order to meet the optimal

steam temperature for fast loading of the ST.

The H-type steam turbine is designed for full-arc admission without the use of a control stage.

This design greatly reduces blade stresses during the start-up process. The combination of

full-arc admission with variable-pressure operation (which has become a standard for HRSGs

in the industry) provides high efficiency, maximum operating flexibility and minimized

maintenance.

In order to provide further flexibility, the Siemens patented Turbine Stress Controller (TSC) is

supplied. The TSC consists of a stress evaluation system coupled with the ST start-up

program incorporating an on-line life cycle counter. It calculates the stress in all critical ST

“thick wall” components (including the valves, HP casing and rotor and the IP rotor)

depending on three different ramp rates, i.e. normal, accelerated and fast. The function of the

TSC is to control the start-up ramp rates so as to minimize material fatigue and at the same

time to calculate the cumulative fatigue of the monitored turbine parts.

The TSC assigns an appropriate number of equivalent starts for each normal, accelerated and

fast start. With this knowledge of the effect of each type of start on the maintenance schedule

of the steam turbine, the owner can make prudent business decisions regarding whether or not

the plant should be used to satisfy transient dispatch requirements.

Steam/Water Cycle and Operational Chemistry

Despite the requirements for an improved cycling capability, power plant owners still expect

base-load plant efficiency levels. This is achieved by retaining the triple-pressure reheat

steam/water cycle (Figure 9).

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Figure 9 Triple pressure reheat steam cycle with Benson-OTSG

The cycle design has been thoroughly analyzed and as previously mentioned was first

introduced at the Cottam Development Center. The Benson-OTSG is a once-through steam

generator for the HP and IP part, while the LP section retains a drum-type cycle. This HRSG

configuration requires a combination of pH-control with ammonia and oxygenated treatment.

The oxygenated treatment creates and maintains a protective layer of magnetite and hematite

inside the tubes.

As a further enhancement to rapidly achieve proper steam quality and to support fast start-up,

the cycle incorporates a condensate polisher. The polisher provides high quality feedwater to

the Benson-OTSG thus avoiding deposits in the evaporator tubes and thereby decreasing the

evaporator pressure losses. The condensate is deaerated in the condenser hotwell by vacuum,

while the polisher removes the ammonia, which must then be added to the cycle for proper

pH-adjustment.

The following dosing regime is used:

• Oxygenated Treatment (low ammonia, low oxygen): for the condensate system

• All Volatile Treatment (high ammonia, low oxygen): for the feedwater to the LP-system

• Oxygenated Treatment (low ammonia, high oxygen): for HP- and IP once-through system

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The combined oxygen/ammonia treatment with the integrated condensate polisher of the plant

concept has already contributed to the high reliability of Cottam Development Center (Figure

10) and reduced the time to meet steam purity requirements to minutes [4].

Figure 10: Cottam Development Center

BOP Considerations for Fast Start-up/Shutdown and Cycling operation

Let’s reflect now the impact of additional measures that support combined-cycle power plants

in coping with the requirements of fast start-up/shutdown and cycling operation mode. As

most of the readers that are associated with the power industry may already know, these

measures can be acknowledged as industry standards and are also part of drum-type HRSG

cycles. Noteworthy measures that are also applied are:

• Component redundancies

• Auxiliary boiler

• Vacuum pumps

• Stack damper

• Automated drain and vent valves

• Optimized steam piping warm-up concept

• High level of system and plant automation

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Benefits A plant designed for increased cycling capability offers operational benefits that will

positively affect the life-cycle costs, due to:

• Increased flexibility

• Fuel savings

Since this plant can be operated in a fast-start, intermediate-load or a base-load mode, there is

a high degree of flexibility for the owner. The ability to rapidly reach dispatchable load allows

the owner to participate in more revenue generation opportunities and allows additional profit

depending on it’s specific contractual conditions.

An advantage that can be quantified are the fuel savings. Due to accelerated start-up, the

improvements significantly reduce start-up fuel. For the economic calculation on NPV-basis a

set of assumptions were made with regard to fuel prices, electrical revenues and interest rates.

Furthermore a load regime and a fixed relation between expected and unexpected starts has

been fixed. This results in savings of approx. € 6 million NPV over 20 years for a once-

through design (Figure 11).

Figure 11 Fuel savings on NPV basis

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Conclusions

The single-shaft concept with enhanced cycling capabilities represents an approach for the

market that targets the needs of combined cycle power plants in intermediate duty with

frequent start-up and shut-down cycles, without sacrificing base-load efficiency.

Special detail in system integration using expertise and experience of a full turnkey contractor

is key to the success of the plant design. Controlled steam temperatures and an advanced ST

start-up mode are considered to mitigate ST induced start-up restrictions.

Its improvement in operating flexibility helps power plant owners to be prepared for a

deregulated market and maintain their revenue stream over the uncertainty of the dispatch

requirements.

References

[1] “The real cost of cycling powerplants: What you don’t know will hurt you” by S A

Lefton, P M Besuner, G P Grimsrud, Aptech Engineering, POWER, Nov/December

2002

[2] “Design of High Load Cycling Operation for Combined Cycle Power Plants” by

Andrew Chrusciel, Justin Zachary, Sam Keith: Bechtel Power Corp., Published for

PowerGen International 2001, Las Vegas, NV

[3] “Successful Combined Cycle Debut for Novel Horizontal-Flow Vertical-Tube Benson-

HRSG” by Joachim Franke, Uwe Lenk, Robert Taud: Siemens Power Generation,

Erlangen, Germany; Friedrich Klauke: Babcock Borsig Power, Oberhausen, Germany.

Published in Modern Power Systems, July 2000

[4] “Chemical Operation Experience With The CCPP Cottam” by M. Rziha, B. Senger:

Siemens Power Generation, Erlangen, Germany; S. Merry, A. Greig: Powergen Cottam,

Retford, UK. Published for ESAA-Power Station Chemistry 2002, Rockhampton

Queensland

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[5] “Combined Cycle Steam Turbine Operation” by Leo Bize, Harry Martin: Siemens

Westinghouse Power Corporation, Orlando FL; Norbert Henkel, Edwin Gobrecht:

Siemens Power Generation, Muelheim, Germany. Published for 2001 International Joint

Power Conference, June 4-7, New Orleans, LO. JPGC2001/PWR-19127

[6] “An Integrated Combined-Cycle Plant Design that Provides Fast Start and Cycling

Capability at Base-Load Efficiency” by Raymond Baumgartner, Christian Engelbert,

Mathias Groepner, Michael McManus, Erich Schmid: Siemens Westinghouse Power

Corporation, Orlando FL. Published for Electric Power 2003 Conference, March 4-6,

Houston, Texas, USA

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