enhancing the dynamic performance of electricity …...sources into the feed-water pre-heating...

50 Int. J. Energy Technology and Policy, Vol. 3, Nos. 1/2, 2005

Copyright © 2005 Inderscience Enterprises Ltd.

Enhancing the dynamic performance of electricity production in steam power plants by the integration of transient waste heat sources into the feed-water pre-heating system

K. Roth and V. Scherer Department of Energy Plant Technology (LEAT), University of Bochum, 44801 Bochum, Germany E-mail: [email protected] E-mail: [email protected]

K. Behnke ALSTOM Power Support AG, 68309 Mannheim, Germany E-mail: [email protected]

Abstract: Since the deregulation of the energy market the enhancement of steam power plant’s dynamic performance has become more and more important. Nowadays, peak load is a profitable product at the energy stock markets if it can be provided reliably. This is in contrast to many power plants running constantly at full load. A very promising technique to improve the peak load capability of such power plants is to integrate transient external waste heat sources into the cycle. Feed-water pre-heaters are one favourable location to integrate the external heat into the water steam cycle. Since the addition of the external energy causes less steam consumption in the high-pressure and low-pressure pre-heaters, one or more steam extraction lines can be closed. By doing this the steam can be used for additional power generation in the steam turbine. The analysis of the occurring time-dependent processes are studied on a numerical basis, using the dynamic process simulation software APROS from VTT. With APROS, a comprehensive power plant model of a fossil-fuelled conventional steam power plant with an output of 393 MW was built, adapted and parameterised. In the simulations two different kinds of external heat sources have been integrated into the cycle: a fast-starting gas turbine and an industrial steam line. The simulation results show a fast steam turbine power increase of 32 MW within less than 14 minutes in both the cases. Adding the power output of the gas turbine, an additional 118 MW is provided accompanied by a 2% efficiency increase of the entire plant. This means less consumption of fossil fuels and consequently a significant reduction of CO2 and other pollutants.

Keywords: dynamic process simulation; waste heat; steam power plant.

Reference to this paper should be made as follows: Roth, K., Scherer, V. and Behnke, K. (2005) ‘Enhancing the dynamic performance of electricity production in steam power plants by the integration of transient waste heat sources into the feed-water pre-heating system’, Int. J. Energy Technology and Policy, Vol. 3, Nos. 1/2, pp.50–65.

Enhancing the dynamic performance of electricity production 51

Biographical notes: Kolja Roth graduated at the University of Bochum and is working as a PhD student at the Department of Energy Plant Technology. In his work he is particularly concerned with the simulation of unsteady power plant processes.

Professor Viktor Scherer is the head of the Institute of Energy Plant Technology in the Faculty of Mechanical Engineering at the Ruhr-University of Bochum. He holds a diploma and a doctoral degree in mechanical engineering from the University of Karlsruhe, Germany. From 1990 to 2000 he was a member of ABB-ALSTOM working on R&D in different fields of power plant layout. In 2000 he joined the University of Bochum.

Klaus Behnke joined ALSTOM Power (former ABB) in 1995 after studying mechanical engineering and following research engineering activities at the Technical University of Aachen. After three years of field experience as leading commissioning engineer for gas turbines and combined cycle power plants he worked for the R&D Steam Turbine Department and as Technical Project Manager for steam turbine projects. Since 2000 he joined the Technical Services Division in ALSTOM Power, Germany, where he led a fuel cell development project and several other R&D projects. He continued his career as the head of the thermo- and aerodynamical department. In addition to this position, he is currently the Deputy General Manager of the Technical Service Business in the ALSTOM Turbo-Systems Group.

1 Introduction and purpose

The enhancement of the dynamic performance of the steam power plant, i.e. the capability of supplying additional electrical peak power has become more and more important in the liberalised European energy market. Peak load is a profitable product at the energy stock markets if it can be provided reliably. Therefore, improved peak load capability is a substantial design characteristic of modern power plants. This is in contrast to many existing steam power plants, which are designed for base load operation.

In Figure 1 typical electricity prices at the European Energy Exchange (EEX) in Leipzig are shown for a period of one month in 2002.1 Significant fluctuations of the price are visible. For a given day at noon the demand of electricity is high and consequently the prices are high, too. During the night it is not unusual to have prices falling below the cost of production.

Therefore it is necessary to enhance the power plants dynamic performance to meet the requirements of such deregulated markets. A very promising technique of improving the peak load capability is to integrate transient external waste heat for feed-water heating into the cycle (Roth, Scherer and Behnke, 2002; Roth et al., 2003; Scherer, Roth and Behnke, 2003). A schematic process diagram is shown in Figure 2. Less steam is consumed by the high-pressure and low-pressure pre-heaters on account of this process. One or more steam extraction lines can be closed and the steam can be used for very fast and dynamic additional power generation in the steam turbine. The efficiency of the power plant is improved simultaneously.

Note that closing steam extraction without replacing the feed-water pre-heating by external heat is an already well-known technique of providing peak load capability.

52 K. Roth, V. Scherer and K. Behnke

However, this additional electricity can be provided only temporarily and because of it the power plant efficiency decreases.

Since the integration of the transient heat sources leads to substantial dynamic energy shifts within the power plant’s water–steam cycle, one objective of the developed automation concept is to keep the boiler inlet feed-water temperature constant to avoid any effects on the complex boiler control. The description and analysis of these shifts, together with a cycle evaluation using an unsteady process simulation tool, is the subject of study of the current paper.

Figure 1 Electricity prices at the Leipzig European Energy Exchange (EEX) in 20021

Figure 2 Schematic drawing of the steam power plant with external heat source


2 Analysis of available heat sources

A wide range of waste heat sources is available in the energy market. However, for the purposes of supplying heat to a feed-water pre-heater this range is limited. The limiting parameters are operational and temporal availability, quantity of waste heat and temperature level. An overview of the possible heat sources is given in Table 1.

Table 1 Overview of waste heat sources

Heat Source Temperature Range

Gas turbines

Heavy duty gas turbine

Aero-derivative gas turbine

500 °C

400 °C

Fuel cells

PEM

PAFC

SOFC

MCFC

80 °C

200 °C

650 °C

1000 °C

Processes

Food sector

Cooling

Drying

Sugar fabrication

Textile industry

Washing, drying

Paper industry

Bleaching, drying

Construction industry

Drying of bricks and lime

Baking lime

Petrochemical industry

Thermal cracking of mineral oil

Catalytic cracking and reforming

Miscellaneous

Alcohol production from biomass

Drinking water conditioning

Coal gasification

Steam reforming of methane

140–180 °C

120–230 °C

600 °C

60–150 °C

140–185°C

75–300 °C

900 °C

400–500 °C

490–740 °C

180 °C

90–150 °C

700–800 °C

750–850 °C

Thermal solar energy

Solar collector

75–390 °C

One potential external heat source could be the exhaust gases of fast-starting gas turbines. With this system configuration heat, as well as electrical energy, is provided, leading to a significant increase up to 35% of the total power output. This kind of heat source was


chosen as an example for the calculation of the dynamic cycle behaviour in the current study. The results for the steady cycle calculations were presented by Kotschenreuther (Kotschenreuther and Miemann, 2000).

An alternative option is the use of fuel cells. The waste heat of PEM, PAFC, SOFC or MCFC (Bergmann and Uttich, 2001) fuel cells could be used. In general fuel cells require large financial investments but have high efficiency and short response times. One fuel cell that could come into consideration is the PAFC, because the waste heat is available at a temperature level favourable for feed-water heating.

Other concepts could be based on utilising waste heat from chemical engineering processes such as cooling, drying or hydro-cracking. In all chemical plants, and many other production facilities, steam lines are available at different temperature and pressure levels and are potential heat sources. Such a steam line at 43 bars and 290 °C was chosen as another example for dynamic simulation.

A further concept could be based on the use of solar energy. This renewable generated heat energy leads to a maximum increase of the generator’s power output of about 4%. However, because heat availability is dependent on weather this concept is only an option if efficient means of heat storage can be provided (Tamme, Lang and Steinmann, 2003).

3 Software selection

The time-dependent processes are studied on a numerical basis, using an unsteady process-simulation software. Such software tools solve the one-dimensional unsteady conservation equations for mass, energy and momentum (Kotschenreuther and Miemann, 2000):

• mass balance:

0A A vt z

∂ ∂+ =

∂ ∂ρ ρ

(1)

• momentum balance: 2A v A v Ap S

t z z∂ ρ ∂ ρ ∂

+ + =∂ ∂ ∂ m (2)

• energy balance:

eA h A vh St z

∂ ∂+ =

∂ ∂ρ ρ

(3)

In order to get an overview about the commercially available programmes used for unsteady thermodynamic process simulation, a detailed market analysis of the existing software packages was carried out. For this reason different kinds of information sources such as the internet, the expertise of the project participants involved and the software tool database of TU Delft have been used.2 These sources led to a pre-selection of 15 software packages as shown in Table 2.

After a first evaluation several software tools had to be excluded, mainly due to the lack of pre-designed power plant modules.


Four software packages have been selected for detailed comparison. These are the APROS from VTT, MMS from nHance Technologies, HYSYS from Hyprotech and ProTRAX from TRAX. The suppliers provided non-commercial licences. This allowed an evaluation using several test cases and receiving a first impression of the programme’s look and feel.

Table 2 Evaluation matrix, status July 2001

Evaluation for Process Simulation Tools (Date 01.07.01) 2

0 si

m

Asp

en D

yn.

HYS

YS

Mas

sbal

Mat

lab

MS1

Pro

Trax

Sin

da/F

luin

t

Ari

ane

Aut

odyn

amic

s

CA

E

Vis

sim

MM

S

AP

RO

S

gP

RO

MS

Specification:

Power plant modules provided? × √ √ √ × × √ √ √ √ 160 √ 20

• Coal burner × ? √ √ × × √ × √ × √ √ ?

• HP turbine × √ √ √ × × √ × √ × √ √ ?

• Boiler × ? √ √ × × √ √ √ × √ √ ?

• Feed-water pre-heater × √ √ √ × × √ √ √ × √ √ √

• Pumps × √ √ √ × × √ √ √ × √ √ √

• Pipes × √ √ √ × × √ √ √ × √ √ ?

• Valves × √ √ √ × × √ √ √ × √ √ √

Are modules modifiable? × √ √ ? √ × √ × √ √ √ √ √

Can new modules be generated? √ √ √ √ √ √ √ √ √ √ √ √ √

Simulation of faults? ? ? √ ? × × √ × ? × √ √ ?

Programming language

C++

SID

OPS

Fort

ran,

C++

, VB

Fort

ran,

C++

, VB

?

C, J

AV

A

C++

C, F

ortr

an

C, F

ortr

an

Fort

ran

C, C

++

Fort

ran

AC

SL

C++

, For

tran

?

Source Code modifiable? × × × ? × × √ √ √ × √ × ?

Information about equations? ( √ ) ( √ ) √ ? ( √ ) ( √ ) √ √ √ ( √ ) √ × ?

Parameters by experimental data? √ √ ? ? √ × × √ √ √ √ × ?

Steady state independent from dynamics? × √ √ √ √ √ × √ × √ × × ?

Software updates ? ? √ ? ? ? ? ? ? √ √ √ ?

Base power plant available? × (X) √ ( √ ) × × √ × √ × × × ×


Table 2 Evaluation matrix, status July 2001 (Continued)

Evaluation for Process Simulation Tools (Date 01.07.01) 2

0 si

m

Asp

en D

yn.

HYS

YS

Mas

sbal

Mat

lab

MS1

Pro

Trax

Sin

da/F

luin

t

Ari

ane

Aut

odyn

amic

s

CA

E

Vis

sim

MM

S

AP

RO

S

gP

RO

MS

Specification:

Platform (Windows98/ NT – UNIX) W W W √ √ W √ U W √ √ √

Manual available? √ √ √ √ √ √ √ √ √ √ √ √

Demo version available? √ × √ ( √ ) √ √ √ × √ √ √ ×

Training available? √ √ √ √ √ √ √ √ √ √ √ √

Supplier location NL US Can Can US B US US US US US Fi GB

Training location NL US EU EU D B US US US US US Fi GB

Software price

$800

$1,6

00 p

a

$1,5

00 +

$50

0 pa

$500

pa

Exi

sten

t

€2,7

50

$20,

000

$6,1

20

€4,6

00

$25,

000

$300

$10,

000

€15,

136

+ €1

934

pa

£500

Training price

? ?

May

be

free

? ? ?

Free

?

$18,

000

?

$995

€4,2

00

£573

This evaluation yielded the result that APROS from VTT was the most suitable product for the purposes of the current project. The most important criterium was the modelling profoundness of the different power plant modules. APROS is one of the very few software tools that provide a 6-equation model for calculating the conservation equations for water and steam separately. The further advantages are the various interfaces providing several alternatives for connecting external software to the simulation program. A long list of reference users working with APROS especially in Europe is another advantage. The number of available power plant components is very extensive and nearly every plant type, from fluidised bed boilers up to an entire coal gasification combined cycle including automation and electrical systems can be modelled (Miettinen and Hänninen, 1992; Hänninen, 1994a, 1994b;VTT Report, 1998).

4 Reference plant

In order to examine the feasibility of integrating external waste heat into the cycle a power plant model based on an existing reference plant was defined in cooperation with the project partner ALSTOM Power Support.


This reference plant is a typical fossil-fuelled conventional steam power plant with an output of 393 MW and seven feed-water preheating stages. Using APROS a comprehensive model of this power plant was built, adapted and parameterised. The parameterising is especially a very time-consuming step. A large amount of lay-out data such as geometries, isometries, geodetic elevations, thermodynamic data, valve characteristics and automation concepts have to be integrated into the model.

A more detailed APROS view of the plant’s high-pressure pre-heater section including the bypass is illustrated in Figure 3. Automation and measuring points are not shown.

Figure 3 High-pressure pre-heaters modelled in APROS

The power plant has been subdivided into more than 500 single components. For each component the one-dimensional unsteady differential equations for the conservation of mass, momentum and energy are solved. Heat transfer, heat capacity of solid walls and two-phase flow phenomena are taken into account. On the basis of this model several different kinds of plant configurations with or without an external heat source have been simulated. Careful calibration of the model has been carried out to meet the steady-state design and guarantee values.

5 Integration of heat source and automation concept

The integration of a fast starting gas turbine with an electrical output of 86 MW was chosen as the first example for this paper. The concept is illustrated in Figure 4.

The amount of heat in the gas turbine’s exhaust is sufficient to fully substitute the high-pressure pre-heater numbers 6 and 7, so their steam extraction lines can be closed and the steam can be used for additional power generation in the turbine. The exhaust gases leaving heat exchanger HE1 are still at a temperature level to pre-heat a part of the


condensate in the low-pressure pre-heating section (HE2). The main part of the condensate is pre-heated in the pre-heater numbers 1–4 by extraction steam conventionally.

Figure 4 Integration of the gas turbine into the cycle

The concept with an industrial steam line that represents the second example for an integration of external heat into the cycle is shown in Figure 5. It is assumed that the total amount of steam needed in the heat exchangers (HE1, HE2) can be taken from the steam line. The condensate from HE1, together with additional live steam, pre-heats the water in HE2.

Figure 5 Integration of the steam line into the cycle


A control strategy has been developed that allows the addition of external heat into the preheating cycle without affecting the feed-water temperature at the boiler inlet and therefore without affecting the complex boiler control. This is illustrated in Figure 6.

Figure 6 Bypass control concept

Note that before the feed-water is switched into the external heat exchangers, a small amount of feed-water flows backwards from downstream of PH7 through HE1 to the main feed-water pump (MFP) entrance. This guaranties pre-heating of the bypass piping and HE1 heat exchanger before the gas turbine starts and minimises thermal stresses within the materials. After the main control valve (MCV) starts letting the feed-water into the bypass and the flue gases from the gas turbine, respectively, the steam flows into the heat exchanger HE1 and the water within the bypass pipes gets heated. The feed-water temperature at the HE1 exit is measured by a temperature sensor (TC1). The PI controller opens the main control valve and the feed-water is directed into the bypass successively in order to keep the given temperature at a set point. Another temperature sensor TC2 can be found in the feed-water line downstream of heat exchanger PH7. This temperature measurement is used to control the steam extraction valves. The control concept in the low-pressure pre-heating section works similarly.

6 Simulation results

6.1 Integration of a gas turbine

The simplified characteristics of the exhaust temperature and exhaust mass flow of the gas turbine representing the boundary conditions that affect the cycle are illustrated in Figure 7.

As can be seen in Figure 8, the feed-water temperature decreases immediately after the start of the gas turbine. This is because of the low exhaust gas temperature after the start of the gas turbine, which leads to feed-water cooling instead of heating. Then the increasing exhaust temperature causes a rise of the feed-water temperature, which is controlled by directing more and more mass flow through HE1 as shown in Figure 9.


Figure 7 Exhaust mass flow and exhaust temperature

Figure 8 Temperature characteristics

Correspondingly, the mass flow through the high pressure pre-heater numbers 6 and 7 is decreasing. Since the reduced feed-water mass flow needs less steam the controller closes down the extraction steam valves V1 and V2. The processes appearing in the low-pressure pre-heating section are very similar.

The parallel mass flows flowing through the pre-heater numbers 6 and 7 and the HE1 are mixed in front of the boiler. As shown in Figure 8 the boiler inlet temperature varies due to the feed-water shifting process. The maximum deviation from the original value is about three degrees over a period of a few minutes and therefore sufficient in real applications.


Figure 9 Mass flow characteristics

Closing the steam turbine extraction lines and starting the gas turbine leads to a fast increase of the steam power plant’s power output as shown in Figure 10. The combination of the integration of an external heat source into the cycle and closing the extraction steam lines leads to an additional and lasting rise in the steam power plant output from 393 MW up to 425 MW. Together with the gas turbine’s power output of 86 MW the total power increase is about 118 MW in 14 minutes. Associated with this, the power plant’s efficiency rises by 2%.

Figure 10 Power characteristics


6.2 Integration of a steam line

The steam consumption of the heat exchangers number 1 and 2 can be seen in Figure 11, which is controlled by two valves opening linearly. The steam flowing into HE1 condenses and gets conducted in the liquid state into HE2.

Figure 11 Steam mass flows into the heat exchangers

Figure 12 shows the temperature characteristics of the feed-water in the high-pressure pre-heater section. Compared to the example with the gas turbine (Figure 8), the characteristics are much smoother. This is caused by the steadily increasing steam mass flow with constant temperature and the nearly linearly increasing feed-water mass flow through the bypass as shown in Figure 13. The maximum deviation of the mixture water temperature in front of the boiler lies under 2 ºC.

Figure 12 Temperature characteristics


Figure 13 Mass flow characteristics

Figure 14 Power characteristics

These processes lead to a fast increase in the power output of the steam turbine, which rises from 393 to 425 MW in 12 minutes (see Figure 14). Further simulations have shown that even a faster operation is possible, assuming reduced valve operation times.

7 Conclusions

The integration of external heat into the steam cycle is a promising concept for increasing the competitiveness of fossil-fuelled power plants in a liberalised market as well as to reduce the CO2 emissions. The results of the integration of a gas turbine and a steam line into the pre-heating section were provided as examples of external heat sources.


The simulations show a fast steam turbine power increase from 393 to 425 MW in less than 14 minutes. Adding the power output of the gas turbine, an additional 118 MW is provided. It is interesting to note that the dynamics of the water steam cycle are not the limiting factors for the load gradient of the power plant but the time-dependent temperature rise of the gas turbine exhaust gases with respect to the valve operation time when integrating a steam line. In the first example of using a gas turbine, the increase in power is accompanied by a 2% efficiency increase in the entire plant. This means less consumption of fossil fuels and consequently a significant reduction of CO2 of about 220,000 tons per year.

Acknowledgements

This research was carried out within the Arbeitsgemeinschaft Turbomaschinen 2 (AG Turbo 2). The authors acknowledge the support from the project partners BMWi Germany (Ministry of Economy) and ALSTOM Power Support Mannheim.

References

Bergmann, H. and Uttich, R. (2001) ‘Brennstoffzellen bei RWE’, VGB PowerTech, Nr. 2, pp.S35–S39.

Hänninen, M. (1994a) ‘Summary on the APROS calculations of three LOCA experiments in the LOFT test facility’, Report, VTT Technical Research Centre of Finland, Finland.

Hänninen, M. (1994b) ‘Summary of the validation results of the APROS thermal hydraulic models’, Report, VTT Technical Research Centre of Finland, Finland.

Kotschenreuther, H. and Miemann, L. (2000) ‘Leistungs- und Wirkungsgradverbesserungen von kohlebefeuerten Anlagen durch kostengünstige Repowering Konzepte’, VGB-Kongress Kraftwerke 2000, Düsseldorf.

Miettinen, J. and Hänninen, M. (1992) ‘APROS five equation thermohydraulics’, Report, VTT Technical Research Centre of Finland, Finland.

Roth, K., Scherer, V. and Behnke, K. (2002) ‘Wirtschaftliche Nutzung von stark transienten Wärmequellen im Wasser-Dampf-Kreislauf’, 8. Statusseminar der AG Turbo, Köln, Germany.

Roth, K., Scherer, V., Reinig, G., Gebhardt, B. and Behnke, K. (2003) ‘Steigerung der Leistungsdynamik von Dampf-kraftwerken durch Einbindung externer transienter Wärmequellen in die Speisewasservorwärmung’, VDI Fachtagung Fortschrittliche Energiewandlung und -anwendung, Stuttgart.

Scherer, V., Roth, K. and Behnke, K. (2003) ‘Enhancing the dynamic performance of electricity production in steam power plants: integration of transient waste heat sources into the water steam cycle’, 7th International Conference on Energy for a Clean Environment, Lissabon.

Tamme, R., Lang, D. and Steinmann, W-D. (2003) ‘Advanced thermal energy storage technology for parabolic trough’, International Solar Energy Conference, Hawaii.

VTT Report (1998) ‘Accuracy of APROS’ fast steam tables for real time applications’, VTT Report, VTT Technical Research Centre of Finland, Finland.

Notes

1 http:/www.eex.de, European Energy Exchange EEX. 2 http://www.interduct.tudelft.nl/PItools/index.html: Software tools for process integration,

TU Delft, 2000.


Nomenclature

Symbols

A area in m2

h enthalpy in kJ/kg

p pressure in N/m2

Se source term of energy equation in kJ/m

Sm source term of momentum equation in N/m

v flow velocity in m/s

ρ density in kg/m3

Subscripts

e energy

m momentum

enhancing the dynamic performance of electricity …...sources into the feed-water pre-heating...

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