Performance characteristics of modified gas turbine cycles with steam ...

INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2012; 36:1346–1357 Published online 10 October 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1916

Performance characteristics of modified gas turbine cycles with steam injection after combustion exit Mahmoud Salem Ahmed1,*,†,{ and Hany Ahmed Mohamed2,} 1

Mechanical Engineering Department, Faculty of Industrial Education, Sohag University, Sohag, Egypt Mechanical Engineering Department, Faculty of Engineering, Assiut University, Assiut, Egypt

2

SUMMARY Gas turbine cycle technologies will play a major role in future power generation, and several well-justified concepts have been developed or are the subject of major feasibility studies. In the present work, gas turbine cycles are modified with steam injection between the combustion chamber exit and the gas turbine inlet. Heat recovery steam generators, utilizing the exhaust gases, provide these cycles with the injected steam at saturated vapor. The thermodynamic characteristics of the various cycles are considered in order to establish their relative importance to future power generation markets. The irreversibility of the different composing units of the cycles and the variation of gas properties due to steam injection as well as changes in the interrelation of component performance parameters are taken into account. The isentropic temperature ratio and maximum to minimum cycle temperature ratio are varied over some ranges that slightly exceed their practically acceptable bounds in order to comprehensively investigate their effects on cycle characteristics. The performance characteristics for various modified and regeneration cycles are presented at the same values of the operating parameters. The present modified cycles with steam injected cycles achieve an additional power output and higher efficiencies, resulting in a lower specific cost. At the chosen values of the operating parameters, the enhancement achieved in the overall efficiency for the simple, reheat (with steam injection at high and low pressures) and partial oxidation (with steam injection at high and low pressures) gas turbine cycles are of about 20–30%, 120–200%, 10–12%, 120–260% and 20%, respectively. The present modified cycles technique can be considered among the possible ways to improve the performance of gas turbine cycles-based power plants at feasible costs. This concept can be used for similar core engines. Copyright © 2011 John Wiley & Sons, Ltd. KEY WORDS gas turbines; partial oxidation cycle; reheat cycle Correspondence *Mahmoud Salem Ahmed, Mechanical Engineering Department, Faculty of Industrial Education, Sohag University, Sohag, Egypt. † E-mail: [email protected] { Assistant Prof. at Faculty of Industrial Education, Sohag Univ. } Prof. Dr. at Faculty of Eng, Assiut Univ. Received 2 March 2011; Revised 15 April 2011; Accepted 12 July 2011

1. INTRODUCTION Gas turbine simple cycle mode has long been used by utilities for limited peak power generation. In addition, industrial facilities use gas turbine units for on-site power generation, usually in combination with process heat production, such as hot water and process steam. The increase in process conditions (temperature and pressure) through advancements in materials and cooling methods is the major driver to enhance the performance of the gas turbine simple cycle. Ongoing development and near- term introduction of advanced gas turbines will improve the efficiency of the simple cycle operation more than 40%. The combination of the gas turbine cycle (Brayton cycle) with a medium 1346

or low-temperature bottoming cycle (like the Rankine cycle), known as the conventional combined cycle, is the most effective way to increase the thermal efficiency of a gas turbine cycle. Inexpensive and readily available media (like air and water), well-developed technologies (gas turbine, HRSG, steam turbine), short construction time and, in particular, the high overall efficiency, have led to wide acceptance of this scheme. Combined cycle plants are already achieving efficiencies well over 58%, with plant capacities in the range between 350MW and 500MW [1]. However, the development pace decelerated as the most readily available technical advances were exploited. Furthermore, in small-scale power generation (less than 50MW), it is generally more cost effective to install a less complex power plant, due to the Copyright © 2011 John Wiley & Sons, Ltd.

M. S. Ahmed and H. A. Mohamed

Thermodynamic cycle development

adverse effect of the economics of scale. Combined cycle plants in this power output range have usually higher specific investment costs and lower electrical efficiencies but, on the other hand, robust and reliable performance [2-4]. Thermodynamic cycle developments, such as recuperation, intercooling or aftercooling and cycle integration, such as Mixed Air Steam Turbines (MAST) [5] are among the possible ways to improve the performance of gas turbine-based power plants at feasible costs. MAST technologies can improve the performance of a simple-cycle gas turbine by the integration of the bottoming water/steam cycle into the gas turbine cycle in the form of water or steam injection. Such configuration has a higher electrical efficiency than the simple-cycle gas turbine and produces more electricity per unit fuel input. Well-known schemes of this technology are the steam injection gas turbines [5] and the humid air turbines [6]. The waste heat of the gas turbine is recovered and is utilized to produce steam, which is afterwards injected into the combustion chamber, or is used to humidify the compressed combustion air. Hence, the mass flow of the expanding flue gas is increased and, thereby, the fuel-to-electricity efficiency and the electrical power output are raised. This approach is favorable because water, unlike air, requires significantly less compressor work. Nevertheless, it should be mentioned that the expansion of steam inside the gas turbine to atmospheric pressure is less efficient than inside a steam turbine, where the steam leaves the steam turbine at much lower pressures and, thus, provides more power and higher efficiency. Therefore, a MAST technology will always have a lower efficiency than in combined cycle operation [7]. However, in MAST technology, steam is injected into the compressor discharge casing of the gas turbine as well as into the combustion chamber achieving a reduce in nitric oxide emissions NOx [8]. The changes in performance of a gas turbine resulting from the injection of steam or water into the combustion chamber are evaluated using explicit analytic relations [9,10]. Evaporative gas turbines are distinguished by humidifying the working fluid before combustion at temperatures below the boiling point of water, and the heat required for evaporation of water is partly taken out of the exhaust gas [11]. Three key questions in the developing humidified gas turbine cycles have been addressed in [12,14,15]. Two advanced power cycles with mixtures as the working fluid: the Kalina cycle, alternatively called the ammonia–water cycle, and the evaporative gas turbine cycle were considered by [13]. These cycles show that the mixture working fluids improve the performance regarding electrical efficiency, specific, power output, specific investment cost and cost of electricity compared with the conventional technology. The previous studies were carried out on simple gas turbine cycles with injected water or steam into the compressor, the combustion chamber, the turbine, or the line between the compressor exit and the combustion chamber inlet. All of them show that the output power increases as more ofsteam is injected into the gas turbine cycle and vice versa (due to the increased working fluid mass flow and

higher specific heat). No work is done and used the same techniques to improve the performance characteristics for the reheat and partial oxidation gas turbine cycles. In this paper, various modified gas turbine cycles, in which saturated steam is injected into the line between the combustion chamber exit and the gas turbine inlet, are suggested for the study. The saturated steam is obtained from the heat recovery of the exhaust gases. The amount of the heat recovery limits the water mass flow rate required for generating the saturated steam. The performance effects due to steam injection at low and high pressures into reheat and partial oxidation gas turbine (RHGT and POGT) cycles are analyzed as well as simple gas turbine (SGT) cycle. The values of performance parameters (power output, and overall, first and second law efficiencies) for the modified gas turbine cycles are compared with those obtained for the corresponding advanced cycles with heat regeneration. The irreversibility of the different composing units of the cycles and the variation of gas properties due to steam injection as well as changes in the interrelation of component performance parameters are taken into account. The isentropic temperature ratio and maximum to minimum cycle temperature ratio are varied over some ranges that slightly exceed their practically acceptable bounds in order to comprehensively investigate their effects on cycle characteristics. The enhancement in the values of performance parameters, power output and overall, first law and second law efficiencies, for gas turbine cycles are presented.

Int. J. Energy Res. 2012; 36:1346–1357 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

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2. ANALYSIS 2.1. Modified gas turbine cycles The modified gas turbine cycles suggested for the present study are schematically shown in Figures 1 to 3. The waste heat of the gas turbine is recovered and is utilized to produce saturated water vapor, which is afterwards injected into the cycle between combustion chamber exit and the turbine inlet. The design of modern HRSG includes two or more pressure levels and reheaters, which allows advanced recovery of the fuel gas thermal energy. The amount of saturated water vapor can be adjusted by using control valves in the line at the process steam outlet from the HRSG. Figure 4 shows schematic drawing for the modified SGT cycle which is the simple gas turbine cycle with steam injection before the turbine inlet. In modified RHGT and POGT cycles, steam can be injected at low pressure,

Figure 1. Schematic diagram of the modified SGT cycle.



Figure 2. Schematic diagrams of the modified RHGT and POGT cycles with steam injection at low pressure.

Figure 3. Schematic diagrams of the modified RHGT and POGT cycles with steam injection at high pressure.

Figure 4. Characteristics performance of the modified SGT cycle.

before the low pressure turbine inlet as in Figure 2 and at high pressure, before the high pressure turbine as in Figure 3.

exhaust ducting pressure drops ΔPcc, ΔPcc2 and ΔPex, and mass fraction of the air in the first combustor m.

2.2. Controlling parameters

2.3. Performance characteristics

The controlling parameters for the above cycles are compressors and turbines isentropic efficiencies, c , t , isentropic temperature ratio X, maximum to minimum cycle temperature ratio θ , fuel/air ratio f, heat exchanger effectiveness X, ratios of pressure drops to initial pressures in the first and second combustors, heat exchanger and

The performance characteristics of the cycles are the dimensionless heat supplied to the cycle q (heat supplied/ Cp.T1), the dimensionless network output from the cycle wnet (network output/ Cp.T1), steam injection/air ratio s, first law efficiency I and second law efficiency II. In calculating the second law efficiency, the available energy

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associated with the heat input into the combustion chambers (rather than its chemical value based on composition and amount of fuel introduced into the cycle) has been utilized as an approximation to its exact value. The performance of gas turbine cycles is affected by various factors such as fuel type, inlet air properties, mass flow rate, temperature and pressure of the gases exhaust, losses through the various processes, leakage, cycle maintenance and component efficiencies. In the present analysis, a unit mass of air (entering at atmospheric conditions) is used as the basis for the

h Cps ¼

i

18:015

- The polytropic efficiency of turbines and compressors, T and C, is 0.9. - The effectiveness of the HRSG and H.E., x, is 0.8. - The fuel air ratio is constant and equals 0.02. - The isentropic temperature ratio, X, for two stage compressions corresponds to the high to low pressures ratio for one stage compression. - P ¼ P ¼ P ¼ 101:325 kPa 1 5 6 pffiffiffiffiffiffiffiffiffiffi Pc ¼ Pa ¼ P1 P2 - P ¼ P ð1 ΔP Þ 3 2 cc1 - P ¼ P ¼ P ð1 ΔP Þ b e a cc2 - P ¼ P =ð1 ΔP Þ 4 1 ex - ΔP ¼ ΔP ¼ ΔP ¼ 0:02 cc1 cc2 ex - P ¼P ¼P 7

T 400 T 400 2 0:04 Cpf ¼ 1:01 þ 0:32 1400 1400

32:24 þ 0:001923ðT 273:15Þ þ 1:055:105 ðT 273:15Þ2 4:187:109 ðT 273:15Þ3

calculations. The actual operations of the cycle units (e.g. compressors, turbines, combustion chambers, heat exchanger and exhaust ducts) are resembled by assumed efficiencies, effectiveness and pressure drop values to account for the irreversibilities of each unit. The properties of the working medium in each respective line are utilized by their specific heats and isentropic exponent values. The isentropic temperature ratio, effectiveness values as well as the values of the operating parameters (maximum to minimum cycle temperature ratio and fraction of air entering primary combustion chamber) are varied over some ranges that slightly exceed their practically acceptable bounds in order to comprehensively investigate their effects on cycle characteristics. The irreversibilities in compressors, turbines, HRSG and water pump are accounted for by the thermal efficiency or effectiveness of each unit beside the pressure drop ratio through some units. The assumptions for the above cycles shown in Figures 1 to 3 are:

3

- T ¼ saturatedtemperatureatP 8 8 - water specific heat, Cpw, is 4.18kJ/kg.K - The constant pressure specific heats for products from the combustion chambers assuming octane fuel, Cpf, and steam, Cps, are calculated at different temperature, T, from the equations presented in [10] as follow:

8

¼ Pe whensteamisinjectedatcyclehighpressure

- P ¼P ¼P b 7 8 ¼ Pe whensteamisinjectedatcyclelowpressure - T ¼ T ¼ 0:9T ¼ 298:15K 1 6 5 - T ¼T 3 b Int. J. Energy Res. 2012; 36:1346–1357 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

The variation of gas properties due to steam injection and changes in the interrelation of component performance parameters are taken into account. Some of the thermodynamics relations are used to obtain the specific heat at constant pressure, the specific heat ratio and various temperatures of mixture at different parts in the aforementioned cycles. The specific heat ratio of different mixtures is estimated from the mass fractions of the mixture components and the universal gas constant. Average specific heats at constant pressure through turbines expansion and HRSG are considered for calculating the turbine work and the recovered heat, depending on inlet and exit temperatures, Cpi-e. Applications of the conventional thermodynamic laws and relationships to the different parts of the cycle and with some manipulations formulated for the performance characteristics of the aforementioned cycles in the equations are given below: The modified POGT cycle with steam injection at high pressure s¼

x Cp 45 ð1 þ f ÞðT4 T5 Þ hfg þ Cpw ðT8 T7 Þ x Cp45 ðT4 T5 Þ

(1)

wC ¼ ½ðTc T1 Þ þ mðT2 Tc Þ=T1 wP ¼ sv6 ðP7 P6 Þ=Cp T1

(2) (3) (4)

wT ¼ ½ðm þ f þ sÞCpea ðTe Ta Þ þð1 þ f þ sÞCpb4 ðTb T4 Þ=Cp T1

wnet ¼ wT wC wp

(5)

q ¼ ½ð1 þ f þ sÞCpb ðTb T1 Þ ðm þ f þ sÞCpa ðTa T1 Þ ð1 mÞCp ðTc T1 Þ=Cp T1 þ ½ðm þ f ÞCp3 ðT3 T1 Þ (6) mCp ðT2 T1 ÞÞ=Cp T1

½

a ¼ q ð1 þ f þ sÞCpb ln

Tb Ta ðm þ f þ sÞCpa ln T1 T1

Tc ð1 mÞCp ln =Cp T1 T3 T2 =Cp ðm þ f ÞCp3 ln mCp ln T1 T1

I ¼

wnet q

and

II ¼

wnet a

(7)

(8)

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The modified POGT cycle with steam injection at low pressure s¼

x Cp45 ð1 þ f ÞðT4 T5 Þ hfg þ Cpw ðT8 T7 Þ x Cp45 ðT4 T5 Þ

(9)

wC ¼ ½ðTc T1 Þ þ mðT2 Tc Þ=T1

(10)

wP ¼ sv6 ðP7 P6 Þ=Cp T1

(11)

wT ¼ ½ðm þ f ÞCp3a ðT3 Ta Þ

(12)

þð1 þ f þ sÞCpe4 ðTe T4 Þ=Cp T1 wnet ¼ wT wC wp

(13)

q ¼ ½ð1 þ f ÞCpb ðTb T1 Þ ðm þ f ÞCpa ðTa T1 Þ ð1 mÞCp ðTc T1 ÞÞ=Cp T1 þ ½ðm þ f ÞCp3 ðT3 T1 Þ mCp ðT2 T1 ÞÞ=Cp T1 (14) Tb Ta Tc =Cp a ¼ q ð1 þ f ÞCpb ln ðm þ f ÞCpa ln ð1 mÞCp ln T1 T1 T1 T3 T2 (15) =Cp ðm þ f ÞCp3 ln mCp ln T1 T1

(15) I ¼

wnet q

and II ¼

wnet a

a

(16)

The modified RHGT cycles with steam injection The equations of the performance characteristics for the modified RHGT cycles with steam injection at high and low cycle pressures are obtained by substituting m=1.0 into Equations (1–16). The modified SGT cycle with steam injection The equations of the performance characteristics for the modified SGT cycle with steam injection are obtained by substituting into Equations (1–8) with m=1.0 and Ta =Tb =T4. The SGT, RHGT and POGT normal cycles The equations of the performance characteristics for the SGT, RHGT and POGT normal cycles are obtained by substituting into the equations of these cycles with s=0. The SGT, RHGT and POGT regeneration cycles The equations of the performance characteristics for the SGT, RHGT and POGT regeneration cycles are the same equations of the normal cycles excepting into q and a equations, T2 will be replaced by Tx.

3. RESULTS AND DISCUSSION The characteristics of the modified SGT cycle in Figure 4 show the dependence of wnet, I and II on isentropic temperature ratio X and maximum cycle temperature θ at the 1350

assumed values of the other operating parameters. For a constant maximum cycle temperature θ=5, the greatest effect of X is drastically noticed in the behavior of I, II and wnet; however, the maximum work output reached at about X=2.1. Then, the large variation of X results in a very small effect on I and II. Hence, X=2.1 (at compression ratio of 13.42) can be considered as the optimum choice of X for the specific values of other parameters with respect to the lack of work output. While at X=2.1, the variation of θ results in a small effect on I and II and linear increase of wnet. Thus, highest value of θ is recommended for optimum performance. It is observed that the modified SGT cycle at X=2.1 and θ=5 achieves an enhancement in both I and wnet with respect to that obtained from the normal SGT cycle of about 18% as shown in Figure 5. The performance characteristics of the modified POGT cycle with different steam injection pressures is shown in Figure 6 versus X at θ=5 and m=0.2, versus θ at X=2.1 and m=0.2 and versus m at X=2.1 and θ=5 at the assumed values of the other operating parameters. The general trends of variations of ZI, ZII and wnet are increased with increasing X or m values. However, higher values of θ result in higher values of wnet, almost very small effect on I and lower values of II. This means that increasing θ decreases the available energy associated with the heat input into combustion chambers. Hence, for optimum performance, higher values of X and m (m=1 means that POGT cycle become RHGT cycle) are recommended while θ value needs to compromise between the values of wnet and II. It is observed that the modified POGT cycle with steam injection at low pressure gives higher values of I and II and lower values of wnet compared to that obtained with steam injection HP. The effects of the controlling parameters X, θ and m on the enhancement of I and wnet due to steam injection into the POGT cycle with respect to the normal POGT cycle are given in Figure 7. The steam injection at LP at the chosen ranges of the controlling parameters achieves a bigger enhancement in I at higher value of X and lower values of θ and m, while there is no enhancement in wnet along the variation of these parameters. While for the injection at HP, bigger enhancement in I and wnet are obtained at lower values of m and X and higher values of θ. From Figures 6 and 7, the injection at LP is almost usefulness for future power generation markets where there is no enhancement in the wnet, which means no enhancement in the specific fuel consumption for constant fuel air ratio. Thus, injection at HP is recommended for the use at low m and X and high θ when the POGT cycle is available. The performance characteristics of the modified RHGT cycle at different steam injection pressures is shown in Figure 8 versus X at θ=5 and versus θ at X=2.1 with the assumed values of the other operating parameters. The general trends of variations of I and wnet are the increasing of their values with increasing X or θ. While increasing X and decreasing θ result an increase in II except at θ less than 3.5, II is degraded for HP injection. It is observed that the RHGT cycle with steam injection at low pressure Int. J. Energy Res. 2012; 36:1346–1357 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er



Figure 5. Enhancement in Ι and wnet resulted from the modified SGT cycle.

0.6

4

HP LP

0.4

3

wnet

LP

HP HP

0.2

2 LP

0 0.1

0.4

m

0.7

1

1

Figure 6. Characteristics performance of the modified POGT cycles for high, HP, and low, LP, steam injection pressures.

gives higher values of I and II and lower values of wnet compared to that obtained with HP steam injection except at θ less than 3.5 II has lower values. The high value of wnet means low specific fuel consumption for constant fuel air ratio leading to lower cost of the power output. Thus steam injection at HP in the RHGT cycle can be recommended for future power generation markets. On the other hand, for the optimum performance with steam injection at HP, a higher value of X is recommended while θ value needs to compromise between the values of wnet and II. The effects of the controlling parameters X and θ on the enhancement of I and wnet due to steam injection into the RHGT cycle with respect to the normal RHGT cycle are given in Figure 9. The steam injection at LP at the chosen ranges of the controlling parameters results in high and very small enhancement values in I and wnet, respectively with changes along X and θ variations. While for the injection at HP, increasing θ and decreasing X lead to increase and decrease in the enhancement in wnet and I respectively at the assumed values of the other controlling parameters.

From Figures 9 and 10, HP steam injection in the RHGT cycle at X =2.1 and θ=5 achieves enhancement in wnet of about 120% with reduction in I of about 2%. Discussions of the performance characteristics of SGT, RHGT and POGT regeneration cycles are established in [1–4]. The values of performance characteristics in the present work may deviate from the previous study [4] due to change in heat exchanger positions and gas properties values. According to the aforementioned characteristics equations, the regeneration cycles result in the same values of wnet obtained from corresponding normal cycles leading to nil enhancement in wnet. Comparisons between the effect of different values of X and θ on performance characteristics of SGT, RHGT and POGT regeneration cycles at the chosen values of the other controlling parameters are shown in Figures 10 to 13. The general trends of variations for all cycles are as follows:


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•

higher values of θ and X result in higher values of I except for the RHGT cycle, the values begin to decline beyond X =2.1.



1.5

0.25

0.15

0.05

LP

1

HP

wnet 0.5

Enhance (wnet)

Enhance ( )

HP

LP -0.05 0.1

0.4

0.7

1

0

m Figure 7. Enhancement in Ι and wnet resulted from the modified POGT cycles for high, HP, and low, LP, steam injection pressures.

Figure 8. Characteristics performance of the modified RHGT cycles for high, HP, and low, LP, steam injection pressures.

Figure 9. Enhancement in Ι and wnet resulted from the modified RHGT cycles for high, HP, and low, LP, steam injection pressures.

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Figure 10. Effect of X and θ on the ZI for SGT, RHGT and POGT regeneration cycles.

•

• •

higher values of X result in higher values of II except for the RHGT cycle, the values begin to decline beyond X =2.1 while higher values of θ result in higher and lower values for RHGT and POGT cycles, respectively. However, for the SGT cycle, higher values of θ result in an increase of II to peak, value then begin to decline beyond θ = 3.5. higher values of θ and X result in higher values of wnet except for the SGT cycle, the values begin to decline beyond X =2.1. higher values of θ and X result in higher values of required heat additions q except for the SGT cycle, the values begin to decline beyond X=1.8, and the for POGT cycle, the q values almost remain constant along the variations of X.

According to the characteristics performance in Figures 10 to 13, the preferences for uses can be chosen as follows:

• •

When low power is required, the SGT regeneration cycle can be recommended for use at X=2.1 where it has the highest values of I and II, and wnet is reduced beyond X=2.1. When higher power is required, the RHGT regeneration cycle can be recommended for use at X=2.1 where higher compression values beyond X=2.1 have unappreciable effect in wnet and slowdown in I and II values.

Comparisons between the effect of X and θ on the performance characteristics of the SGT, (LP and HP injections) RHGT and (LP and HP injections) POGT modified cycles at the chosen values of the other controlling

parameters are shown in Figures 14 to 17. The figures show that higher values of X and θ result in higher values of I, wnet, the heat required q, and steam injection/air ratio s for all the modified cycles except for higher X values result lower values in steam injection/air ratio s. However, higher values in II are resulted at higher X and lower θ values except for SGT and LP injection RHGT modified cycles II begin to decline at θ = 3.5. The general trends of variations for all cycles are as follow:

• • • •

Descending values of I and II resulted from the SGT, (LP and HP injections) RHGT and (LP and HP injections) POGT modified cycles respectively except for: -RHGT modified cycle, the HP injection has higher values in I and II than LP injection for θ less than 3.2 and less than 3.5 respectively. -SGT modified cycle has lower values in II than the HP injection of the RHGT cycle at θ less than 3.3 and lower values than LP injection at θ less than 3.1. Ascending values of wnet, q and s resulted from the SGT, LP injections of POGT and RHGT, and HP injections of POGT and RHGT modified cycles, respectively, but the SGT modified cycle has higher values than the LP injection POGT cycle for X less than 2.7 and θ more than 3.7.

According to the characteristics performance in Figures 14 to 18, the preferences for uses can be chosen as follows:

•

When low power is required, the SGT modified cycle can be recommended for the use where it has the highest values of I.

Figure 11. Effect of X and θ on the ZII for SGT, RHGT and POGT regeneration cycles. Int. J. Energy Res. 2012; 36:1346–1357 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

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Figure 12. Effect of X and θ on the wnet for SGT, RHGT and POGT regeneration cycles.

Figure 13. Effect of X and θ on the q for SGT, RHGT and POGT regeneration cycles.

Figure 14. Effect of X and θ on the ZΙ for SGT, (LP and HP injections) RHGT and (LP and HP injections) POGT modified cycles.

•

When higher power is required, the HP injection RHGT cycle can be recommended for the use at X=2.1 and lower value of θ where higher compression values beyond X=2.1 have unappreciable effect in wnet and II values tend to decline with increasing θ values.

The figures given in the previous sections show that the regeneration cycles result in higher values of I and II compared to that result from the corresponding modified cycles. This result attributes to the higher values of the heat input to the modified cycles compared to that input for the corresponding regeneration cycles. This means that when the same fuel is introduced for these cycles, the burner efficiency for the modified cycles is higher than that for the corresponding regeneration cycles for the same fuel air ratio. On the other hand, values of wnet resulted from the modified cycles are higher than those resulted from the regeneration cycles as shown from the figures presented in the previous sections. Hence, the overall thermal efficiency for the modified cycles is higher than that for the 1354

corresponding regeneration cycle achieving lower running cost of the generated power by using the modified cycles. However, for market uses, the assessment of the power generation cycles is mainly evaluated by the running cost of the net output power. Thus, the modified cycles are recommended for market uses instead of the corresponding regeneration cycles. Although these modified cycles have much lower absolute performance than the combined cycle technology, the variations in selection criteria, which tend to apply to smaller scale systems, create niche markets to very specific requirements. In practical, the development of the existing power generation stations is usually made by replacing the current cycle with a modern one or by modifying it. Thus, the enhancements in wnet resulted from the modified cycles with respect to that obtained for the corresponding normal or regeneration cycles are given in Figure 19 against X or θ at the assumed values of the other controlling parameters. This figure shows that the enhancements in wnet for the SGT, HP and LP injections of RHGT, and HP and LP injections of POGT modified cycles of about 20–30%, Int. J. Energy Res. 2012; 36:1346–1357 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er



Figure 15. Effect of X and θ on the ZΙΙ for SGT, (LP and HP injections) RHGT and (LP and HP injections) POGT modified cycles.

Figure 16. Effect of X and θ on the wnet for SGT, (LP and HP injections) RHGT and (LP and HP injections) POGT modified cycles.

Figure 17. Effect of X and θ on the q for SGT, (LP and HP injections) RHGT and (LP and HP injections) POGT modified cycles.

Figure 18. Effect of X and θ on the s for SGT, (LP and HP injections) RHGT and (LP and HP injections) POGT modified cycles. Int. J. Energy Res. 2012; 36:1346–1357 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

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Figure 19. Enhancement in wnet resulted from SGT, HP and LP injections of RHGT, and HP and LP injections of POGT modified cycles.

120–200%, 10–12%, 120–260% and 20% respectively. These enhancements which are the ratio between the values of wnet for the modified and normal or regeneration cycles mean the same value of the enhancement in the overall thermal efficiency. Since modern gas turbines employ burner temperatures not too far below the optimum temperature [9], it must be concluded that in the future, increasing burner exit temperature is not a way to increase thermal efficiencies as it was in the past. Increasing pressure ratio yields a moderate improvement potential, and true improvements in thermal efficiencies are only possible with alternate gas turbine cycle configurations.

requirements. Also, the present modification technique can be considered among the possible ways to improve the performance of gas turbine cycles-based power plants at feasible costs. This concept can be used for similar core engines.

NOMENCLATURE Alphabetic symbols a CPf

4. CONCLUSIONS The present modified cycles with steam injected cycles achieve an additional power output and higher efficiencies, resulting in a lower specific cost. At the chosen values of the operating parameters, the enhancement achieved in the overall efficiency for the simple, reheat (with steam injection at high and low pressures) and partial oxidation (with steam injection at high and low pressures) gas turbine cycles are of about 20–30%, 120–200%, 10–12%, 120–260% and 20% respectively. The present research leads to the following recommendation for using of the various modified cycles:

• •

When low power is required, the SGT modified cycle can be recommended for the use where it has the highest values of I. When higher power is required, the HP injection RHGT cycle can be recommended for the use at X=2.1 and lower value of θ where higher compression values beyond X=2.1 have not appreciable effect in wnet and II values tend to decline with increasing θ values.

CP CPs CPw f m P q s T w X

= dimensionless available energy input to cycle = fuel air mixture specific heat at const. pressure = air specific heat at const. pressure = steam specific heat at const. pressure = water specific heat at const. pressure = fuel/air mass ratio = mass fraction of the air = pressure = heat supplied (dimensionless) = steam to air mass ratio = temperature = work (dimensionless) = isentropic temp. ratio = T2s/T1

Greek Symbols D h θ

= ratio of pressure drop to entering pressure to cycle unit = efficiency = max. /min. cycle temp. ratio, T3/T1

Subscripts

Although the present modified cycles have much lower absolute performance than the combined cycle technology, the variations in selection criteria, which tend to apply to smaller scale systems, create niche markets to very specific

(a, b, c, e, 1, 2,. . .etc.) c cc1 cc2

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= = = =

state point in the cycle figures compressor first combustion chamber second combustion chamber


I II net s t x

= first law = second law = net work = isentropic state = turbine = exchanger

Abbreviations HP LP HRSG LHV LPC LPT POGT RHGT SGT

= high pressure = low pressure = heat recovery steam generator = lower heating value = low pressure compressor = low pressure turbine = partial oxidation gas turbine cycle = reheat gas turbine cycle = simple gas turbine cycle

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