Analyze of gas turbine performances with the presence of the steam water in the combustion chamber

327

Analyze of gas turbine performances with the presence of the steam water in the combustion chamber

R. Kadi^1*, A. Bouam¹ and S. Aissani²

1 Nuclear Research Center of Birine, ‘CRNB’

P.O. Box 180, 17200 Aïn-Oussera, Djelfa, Algeria 2Faculty of Hydrocarbons and Chemistry, M’Hamed Bougara University of Boumerdes

Street of Independence, Boumerdes, Algeria

Abstract - The role of gas turbine in the nuclear and conventional power industries has received considerable attention over the past several decades. However, in order to improve the performances of gas turbine, different methods such as regeneration, intercooler intermediate, pre-heating and injection of the steam water have been used. This later method is the more beneficial to increase efficiency. The steam water injection at the upstream of the combustion chamber has been proposed.

The main purpose of the present work is to analysis the effect of the environmental conditions on the thermodynamic cycle processes of gas turbine, by using analytical relations. The quantity of water vapor carry through will be injected only when the ambient conditions become higher than standards. The influence of inlet temperature and pressure, ambient temperature and the combustion comber temperature over a wide operating range are evaluated. Design data of an industrial gas turbine are employed to show the variation of injected steam water. The results obtained of injection are in good agreement with the qualitative variation of these conditions in the case of normal operation without injection of the steam. For better illustration of the physical phenomena, the results are represented in the form of curves into both two and three dimensions.

Key words: Gas turbine - Simple cycles - Steam water injection - Energy balance - Specific power - Total thermal efficiency.

1. INTRODUCTION

Gas turbines are used by themselves in a very wide range of services, most notably for powering aircraft of all types but also in industrial plants for driving mechanicals equipment such as pumps, compressors and small electric generators in electrical utilities and for producing electric power for peak loads as well as for intermediate and some base-load duties There is also growing interest in using gas turbine in combined cycle plants [1, 2]. They have many advantages such as flexibility in supplying process.

They are also subject to fewer environmental restrictions. In spite of these advantages, their high sensitivity to the influence of air ambient temperature which varies considerably between day and night, summer and winter, makes that the thermal efficiency of exploitation is affected.

The use of gas turbines in combined cycle is one scheme to overcome their present low cycle efficiency. In other hand, an economic compromise between high capital and operating costs would have be found.

The cycle of gas turbine is very flexible cycle so that its parameters of performance, i.e.

output and specific net work, can be improved by adding the additional components to a simple cycle [3-14]. At the present, the combustion process in the combustion chamber is carried out in the presence of an additional quantity of the steam. This quantity is injected at the upstream of the combustion chamber.

This idea has grown out of the need to improve the simple Brayton cycle efficiency by utilizing the waste heat in the turbine exhaust gases to generate steam. The increase of the output and the exiting power of the turbine is a consequence of the additional mass crossing the channels inter-blades of turbine [1].

* [email protected] _ [email protected] __ [email protected]

2. BASIC GAS TURBINE CYCLE 2.1 General description

The ideal and basic cycle is called the Joule cycle is also known as the constant cycle because the heating and cooling processes are conducted at constant pressure. A simple layout is shown in figure 1.

Fig. 1: Illustrative diagram 2.2 Cycle proposed

Figure 2 presents the arrangement of a cycle injected by the steam water. Air is driven back downstream from the compressor with a pressure p2, combustion is carried out in the combustion chamber where fuel is injected in the presence of an additional quantity of the steam water whose physical properties are calculated for conditions of injection to the upstream of the combustion chamber [15, 16]. The flow is constituted by the mixture of combustion gases and the additional of steam water quantity, having a more raised temperature, crossing the channels interblending of the turbine led to an increase progressively with the power delivered by this machine.

Fig. 2: Schematic of an steam water injected circuit

3. EVALUATION OF THE CYCLE PERFORMANCE PARAMETERS In order to make the gas turbine insensitive with the variation in the ambient temperature, the steam water injection method before the combustion chamber has been proposed. This operation will be carried only when the inlet parameters of compressor exceed the standards conditions values of gas turbine. The calculation of new processes is obtained from an energy balance applied to an elementary volume of the combustion chamber presented in figure 3.

Fig. 3: Control volume of combustion chamber (Application of energy balance)

(

)

inj , s s f

a , 2 a

h m h m m

h m PCI m h m

× +

× +

=

× +

× +

×

&

&

&

&

&

&

(1) Since the vapour is injected just at the upstream of the combustor thus, the calculating parameters in the part of compression are unchanged. The flow of the fuel, in the case without injection, is given by:

const const

f PCI

m P

η

= ×

& (2)

a f

m f m

&

&

= ;

a s

m s m

&

&

= (3)

However, to maintain the manufactory combustor at constant temperature, with the presence of steam (which the parameters of injection are T_inj and P_inj), the fuel must be added even more.

This quantity of the fuel is given by:

( ) ( )

g , 3 CC

s , 2 s , 3 a

, 2 g ,

" 3

h PCI

h h s h f h

−

× η

−

× +

= − (4)

Thus

( ) ( )

(

)

−

× γ

γ

×

− + α

×

= −

1 1 1

1 1 1

1

C A D

C B C

s A (5)

with

⎪⎩

⎪⎨

⎧

−

× η

= γ

−

= β

−

= α

g , 3 CC

s , 2 s , 3

a , 2 g , 3

h PCI

h h

h h

and

( )

⎪⎪

⎪

⎩

⎪⎪

⎪

⎨

⎧

−

=

−

==ω η η

× η

=

s , 4 s , 3 1

g , 4 g , 3 1

mec C 1

mec gb

_ The 1

h h D

h h C

/ B

/ PCI A

As the vapour rate is very small comparing with the rate of air, pressure increase in the combustion chamber has been neglected while the steam is injected. The compressor of the engine determines the pressure in the combustion chamber.

3.1 Power provided by the turbine

The power developed by the turbine is given by:

(

) (

)

(

)

T m m h h m h h

P = & + & × − + & × − (6)

3.2 Available useful power The useful output power is:

( ) ( )

(

)

Disp _

Ut P P /

P = η − η (7)

4. ASSUMPTIONS

The characteristics of gas turbine are given in the following table 1. The calculations are taken in wide range of pressure ratio and ambient temperature.

) GN (

PCI 45119 kJ/kg ηT 88 %

Cf 1.02 ηmec 95 %

ηC 90 % ∆pG 1,25 %

pAmb 1.0132 bar ∆pCC 4 %

ε 7.3761 ∆pAdm 1%

5. RESULTS & INTERPRETATIONS

Assuming a gas turbine functioned in ranges of extreme temperatures (winter: 0 °C, summer:

50 °C) and of pressure ratio: (1<ε>10).

The performances of gas turbine are significantly influenced by the change in ambient conditions. So to keep those performances when the ambient temperature becomes higher than the reference conditions, the method of the steam water in the upstream of combustor is proposed.

5.1 Influence of inlet parameters on the injected vapour quantity

Wide rage of parameters values of the injected vapour (pressure and the temperature) have been taken in order to study their influence on the turbine performances. The figures (4, 5, 6 and 7) show the evolutions respectively, according to the ambient temperature, turbine power, net power, useful power and the necessary quantity of vapour injected in order to bring back the operation of this turbine to a standard temperature.

Fig. 4: Turbine power

Fig. 5: Useful output available

Figures 4 and 5 show that all curves of the performances (powers and net power) changes starting from T_iso. This latter is the beginning of improvement of the gas turbine performances.

Fig. 6: Quantity of steam injected

The first parts of curves are identical (in the case without injection). In the case of injection, the amount of injected vapour is reduced when the inlet parameters increased.

Fig. 7: Efficiency

At beginning of injection after the standard temperature, the efficiency will be kept constant as the ambient temperature changes. The figure 7 shows this variation. Also, we note that the injected parameters, such as temperature and pressure have not influence on the injection.

Analysis of the figures 4, 5 6 and 7 show that when we increase the injection steam temperature we have a reduction in the quantity injected of this one. Due to the temperature of mixture, the quantity of vapour injected decrease considerably.

5.2 Influence of operating ambient temperature

It is interesting to visualize the displacement of the point where the injection of the steam water starts, by varying the standard operating temperature (T_iso) of a given step. The profiles of turbine power, the necessary injected quantity of the vapour, available useful power and the total thermal efficiency are presented successively on the figures (8, 9, 10 and 11).

Fig. 8: Turbine power

Fig. 9: Quantity of steam injected

Figure 9 shows the displacement of the beginning point of an injected steam water which corresponds to T_iso.

Fig. 10: Available useful power

Fig. 11: Efficiency

Figure 11 shows, when the ambient temperature is inferior to the standard temperature (T_amb<T_iso) the efficiency decrease by increasing the ambient temperature. This is the consequence of volumic mass of air in the compressor.

5.3 Influence of the combustion chamber temperature

In order to represent the influence of the combustion temperature on the quantity of injected vapour; wide range of values is changed. The results are mentioned in figures 12 and 13.

We show that the injected quantity reduces when the combustion temperature increases. The efficiency depends also of this variation. The work of the compressor would also decreases.

6. CONCLUSION

This study was analysis the influence of the environmental conditions on gas turbine performances with the presence of steam vapour at the upstream of the combustor.

Fig. 12: Quantity of steam injected

Fig. 13: Efficiency

A detailed research has been made to modify the real cycle of gas turbine, to improve the output and to keep the efficiency constant with injection of the steam water. The quantity of water vapour carry through will be injected only when the ambient conditions become higher than standards. The heat energy coming from exhaust overheated the vapour injected. A device with feed water circuit, economizer and evaporator is proposed to obtain the necessary desired performances.

The results obtained prove that the available useful power and the total thermal efficiency of gas turbine have been kept constant, as in operating conditions (ISO conditions), when quantity of steam water is injected proportionally with change of ambient temperature. In south Algerian, the gas turbines are used in the industry of hydrocarbons work in hard climatic conditions. The ambient temperature varies considerably during the year can often reach 50 °C in summer reducing the efficiency of 28 %. By this study we want to make the gas turbines insensitive with the variation of the ambient temperature of an injection device of the steam water driven by the free energy of exhaust fumes.

Furthermore, the model developed in this work for the modified simple cycle can envisage the performances of a relatively large number of gas turbines similar to the type G E MS5002. At the end, we concluded that the results obtained by this model are questionable and must be re- evaluated using experimental data.

NOMENCLATURE

Cf Flow coefficient, (-) R Specific constant of gas, (J/kg.K) C ,p C_v Specific heats at constant pressure

and volume, (J/kg.K)

t Temperature in, (°C) Cp Average specific heats at constant

pressure, (J/kg.K)

T Temperature in, (K)

f (with steam water injection ),

carb a

"

"

m m

f = & & , (%)

T4

side of the turbine, (K)

GN Natural gas, (-) T_CC Temperature on the outlet side of the combustion chamber (indicated

T3), (K)

h Specific enthalpie, (J/kg) s Flow steam to flow air ratio,

a s/m m

s=& & , (%)

0

hf Standard specific enthalpie, (J/kg) w Specific work, (J/kg) k Polytropic coefficient of the fluid,

(-)

γ Isentropic coefficient, γ=c_p/c_v,(-)

m& Mass rate of flow, (kg/s) λ Excess coefficient of air, (-)

n i Fraction of an element in a gas mixture, (-)

∆p Drop of pressure, (bar) p

P

Pressure, (bar)

Power, (W) ^C

η Isentropic efficiency of the compressor, (%)

PC Absorber power by the

compressor, (W) ^CC

η Combustion efficiency, (%) PCI Lower calorific value of the fuel,

(kJ/kg) ^G

η Electric generator efficiency, (%)

Disp _

PUt Useful output available of the

thermodynamic cycle, (W) ^mec

η Mechanical efficiency, (%)

PT Power produced by the turbine, (W) η_T Isentropic out put of turbine, (%) ε Compressor pressure ratio,

1 2/p

=p

ε , (-)

gb _

ηTh Thermal efficiency of the cycle, (%)

Subscripts and superscripts 1, 2, 3, 4 Positions of the cycle presented by

the various elements of the gas turbine

const Quantity related to the manufacturer

a Quantity related to the air Gen Quantity related to combustion gases steam generator

Adm Quantity related to the admission g Quantity related to the combustion gases

Amb Quantity related to the ambient inj Quantity related to the parameters of injection

C Quantity related to the compressor iso Quantity related to the standard conditions

CC Quantity related to the combustion chamber

s Quantity related to the steam water f Quantity related to the fuel T Quantity related to the turbine

Acknowledgments - The authors are grateful for the support of the director of nuclear research center, Mr. Kerris Abd Moumen and Mr. Semine Mohamed, director of nuclear technologies division. We would like also Mr. Anis Bousbia Salah (Fabio Moretti, Francesco D’Auria Università di Pisa) for his expertise and generous help in generating this work.

REFERENCES

[1] P.J. Potter, ‘Power Plant Theory and Design’, Second Edition of Steam Power Plant, John Wiley & Sons, New York, Chichester, Brisbane, Toronto, 1976.

[2] M.M. El-Wakil, ‘Power Plant Technology’, International Student Edition 1^st Printing, 1985.

[3] R. Gicquel, ‘Prise en Main - Exemple des Turbines à Gaz’, Logiciel Thermoptim Vers. JAVA 1.38 Avril 2001.

[4] M.J. Moore, ‘NOx Emission Control in Gas Turbines for Combined Cycle Gas Turbine Plant’, Proc. Instn.

Mech. Engrs., Vol 211, Part-A Imeche 1997.

[5] K Mathioudakis, ‘Evaluation of Steam and Water Injection Effects on Gas Turbine Operation using Explicit Analytical Relations’, Instn. Mech. Engrs., Vol. 216, Part A: J Power and Energy2002.

[6] H. Haselbacher, ‘Performance of Water/Steam Injected Gas Turbine Power Plants Consisting of Standard Gas Turbines and Turbo Expanders’, Int. J. Energy Technology and Policy, Vol. 3, N°1/2, 2005.

[7] D.Y. Cheng & A.L.C. Nelson, ‘The Chronological Development of the Change Cycle Steam Injected Gas Turbine During the Past 25 Years’, Proceedings of ASME Turbo Expo 2002, Amsterdam, the Netherlands, June 3-6, 2002.

[8] M. Milancej, ‘Advanced Gas Turbine Cycles: Thermodynamic Study on the Concept of Intercooled Compression Process’, Diploma Thesis, Institut für Thermodynamik und Energie wandlung, Technische Universität Wien, Vienna, July 2005.

[9] D. Zhao, Y. Ohno, T. Furuhata, H. Yamashita, N. Arai and Y. Hisazumi, ‘Combustion Technology in a Novel Gas Turbine System with Steam Injection and Two-Stage Combustion’, Journal of Chemical Engineering of Japan, Vol. 34, N°9, pp. 1159 - 1164, 2001.

[10] Y. Ohno, D. Zhao, T. Furuhata, H. Yamashita, N. Arai and Y. Hisazumi, ‘Combustion Characteristics and NOx Formation of a Gas Turbine System with Steam Injection and Two-Stage Combustion’, Journal of Chemical Engineering of Japan, 2001.

[11] D.L. Daggett, ‘Water Misting and Injection of Commercial Aircraft Engines to Reduce Airport NOx’, National Aeronautics and Space Administration Glenn Research Center NASA/CR—2004-212957.

[12] K. Brun and R. Kurz, ‘Gas Turbine Life Limiting Effects of Inlet and Interstage Water Injected’, Proceedings of the Thirty Fourth Turbomachinery Symposium, 2005.

[13] R.C. Hendricks, ‘Water Injected Turbomachinery’, National Aeronautics and Space Administration, Glenn Research Center, March 2005.

[14] V. Ganapathy, B. Heil and J. Rentz, ‘Heat Recovery Steam Generator for Change Cycle Application’, The American Society of Mechanical Engineers, Reprinted From 1988, Industrial Power Conference.

[15] International Association for the Properties of Water and Steam, ‘Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam’, Erlangen, Germany, September, 1997.

[16] H.D. Goldammer, ‘Computes Thermophysical Properties of Water / Steam’, Based on Rational Formulation for the Free Energy F = U - T*S (Helmholtz-Function), Program Written and Developed by H.D. Goldammer, B.T.W.B. Last, Update 30. 04. 1984; Schwaebisch Gmuend, Germany, Postbox 1303.

[17] J.H. Keenan, J. Chao and J. Kaye, ‘Gas Tables Thermodynamic Properties of Air Products of Combustion and Component Gases Compressible Flow Functions’, Second Edition, New York, Chichester, Brisbane, Toronto, Singapore, 1979.

[18] R. Bidard et J. Bonnin, ‘Energétique et Turbomachines’, Editions Eyrolles, Saint-Germain, Paris, 1979.

[19] R. Kling, ‘Thermodynamique Générale et Applications’, Editons Technip, Paris, 1980.

[20] S. Jebaraj and S. Iniyan, ‘A review of Energy Models’, Renewable and Sustainable Energy Reviews, Vol.

10, N°4, pp. 281 – 311, 2006.