Performance evaluation of steam injected gas turbine

Applied Thermal Engineering 102 (2016) 454–464

Contents lists available at ScienceDirect

Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Performance evaluation of steam injected gas turbine based power plant with inlet evaporative cooling Anoop Kumar Shukla a,⇑, Onkar Singh b a b

Harcourt Butler Technological Institute, Kanpur, U.P., India Madan Mohan Malaviya University of Technology Gorakhpur, U.P., India

h i g h l i g h t s Steam injection in the combustor increases expanding mass in the gas turbine. Gas turbine (GT) cycle with SI produces 7.2% more sp. work output than a simple cycle. GT cycle with IEC & SI produces 9.5% more sp. work output than a simple cycle. GT cycle with IEC, SI & FC produces 10.1% more sp. work output than a simple cycle.

a r t i c l e

i n f o

Article history: Received 6 October 2015 Accepted 21 March 2016 Available online 1 April 2016 Keywords: Steam injection Specific power Steam air ratio Thermal efficiency

a b s t r a c t Present paper deals with the study for performance evaluation of steam-injected gas turbine (STIG) based power plant with inlet evaporative cooling. It investigates the combined effect of inlet evaporative cooling (IEC), steam injection (SI) and film cooling (FC) on the power augmentation of simple gas turbine cycle. Thermodynamic modeling has been carried out and presented along with results showing the influence of inlet evaporative cooling on various performance parameters of STIG based power plant. Results show that there occurs increment of 3.2% in cycle thermal efficiency due to lowering of the compressor inlet temperature from 318 K to 282 K at 5% steam to air ratio (SAR). At 1850 K turbine inlet temperature and cycle pressure ratio of 24 there occurs increase in thermal efficiency of the GT cycle with IEC, SI and FC as compared to the simple GT cycle. Injection of steam in the combustion chamber enhances the specific expansion work in the gas turbine, which increases at rate of 2.95% for every increase in SAR by 2%. The study shows that gas turbine cycle configuration with inlet evaporative cooling (IEC), steam injection and film cooling is the best combination for obtaining more efficiency and power. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Global energy demand is predicted to increase continuously as evident from the International Energy Agency which projects a 50% increase in natural gas from 2015 to 2040 under international planned energy policies (compared to an overall energy growth of 1.1% per year) [1]. Strong economic growth in developing countries leads to increase in electricity prices, environmentalists concern to control anthropogenic global warming and gas prices having shown high volatility are compelling the power producers to find cost effective ways to increase energy efficiency and reduce generation costs from all prevalent power producing options, including simple gas turbines and combined cycle power plants. ⇑ Corresponding author. E-mail addresses: [email protected] rediffmail.com (O. Singh). http://dx.doi.org/10.1016/j.applthermaleng.2016.03.136 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.

(A.K.

Shukla),

onkpar@

The gas turbine cycle is extensively used in the topping cycle of gas/steam combined cycle power plants for power generation. The steam-injected gas turbine cycle is the modified arrangement of simple gas turbine cycle, wherein part of steam recovered in heat recovery steam generator (HRSG) is injected into the combustion chamber to increase power output and the efficiency of power generation. Amongst different options for gas turbine power augmentation the potential ones are namely inlet air cooling, combustion chamber injection systems and higher turbine inlet temperatures with suitable gas turbine blade cooling system [2]. Gas turbine blade cooling systems enable increasing the gas turbine inlet temperature (TIT) above the metallurgical temperature limits of turbine blades. Inlet air cooling (IAC) system decreases the temperature of air entering compressor, which eventually decreases the compressor work requirement and increased working fluid mass through the gas turbine resulting in improved thermodynamic performance of gas turbine. The injection of an

A.K. Shukla, O. Singh / Applied Thermal Engineering 102 (2016) 454–464

455

Nomenclature c cp f h _ m s v w A C E L M P T Q_ l Q_ s Nu Pr Re St

blade chord specific heat at constant pressure (kJ/kg K) ratio of mass flow rate of fuel to mass flow rate of air enthalpy (kJ/kg) mass flow rate blade spacing specific volume (m3/kg) work done area (m2) velocity (m/s) evaporative cooler effectiveness latent heat of evaporation (kJ/kg) molar mass pressure (N/m2) temperature (K) latent cooling load sensible cooling load Nusselt number Prandtl number Reynolds number Stanton number

Acronyms GT gas turbine CPR cycle pressure ratio CIT compressor intake temperature SAR steam to air ratio HRSG heat recovery steam generator STIG steam injected gas turbine TIT turbine inlet temperature LHV lower heating calorific value IAC inlet air cooling

additional mass of humidified air or steam directly into the combustion chamber improves the performance of gas turbine cycles. A brief literature review shows that Rice [3] proposed a steam injected gas turbine cycle. In this the steam produced from the HRSG is injected into the combustion chamber in which fuel energy is received by both steam from the HRSG and air from the compressor. The expansion of these combustion products inside the gas turbine boosts the power output of the turbine. Larson and Williams [4] provided back of the envelope method for performance calculations of STIG cycle, also analyzed the economic significance of steam-injection technology for cogeneration and combined cycle applications. Borat [5] has shown that the use of steam injection increases the thermal efficiency and net power output of the gas turbine in the range between 20% and 40%. Cheng [30] proposed and patented steam injected free turbine type gas turbine, reported 62% increased horsepower and 40% increased efficiency using a 501KF gas turbine. Paepe and Dick [29] analyzed the thermodynamic possibilities of steam injected gas turbines along with turbine blade cooling. They found that a basic steaminjected gas turbine with steam blade cooling had an efficiency of 49% at CPR of 30 and TIT of 1523 K, and this efficiency increased to 52% at TIT of 1673 K. Wang and Chiou [23] found that with both steam injection and inlet air cooling, the power output and efficiency were 85.56 MW and 37.24% respectively for a STIG cycle using all the injected steam. Jonsson and Yan [27] reviewed the literature on wet and humid gas turbine power cycles and classified the humidified gas turbines into three different categories, namely gas turbines with an injection of water that evaporates completely, gas turbines with injection

IEC SI FC

inlet evaporative cooling steam injection film cooling

Greek symbols coolant flow discharge angle u relative humidity k ratio of cooled blade surface area to hot gas flow area (Ab/Ag) e film cooling effectiveness g efficiency x humidity ratio

a

Subscripts a air b blade e exit g gas f fuel i inlet s steam vap vapor amb ambient com compressor evc evaporative cooling cl coolant gt gas turbine cc combustion chamber aw adiabatic pt polytropic DB dry bulb WB wet bulb

of steam and gas turbine cycles with injection of water in a humidification tower with a recirculation water loop. The humidified gas turbine output in the Smart-AHAT system can be increased by approximately 23% due to the added mass flow injected in the form of steam [32]. Despite increased turbine output, the fuel flow to the combustor is not significantly increased since the steam injected into the combustor is already superheated. Athari et al. [24] compared exergoeconomic analyses of gas turbine steam injection cycles with and without fogging inlet cooling. They reported 0.5% and 0.4% increment in the energy and exergy efficiency for steam injection cycle with fogging inlet cooling as compared to simple steam injection cycle. Paweł Ziółkowski et al. [25] described the possibilities of modification for existing gas turbine in the PGE Gorzow power plant into the Cheng cycle. It reported 42.5% electrical efficiency of modified PGE Gorzow power plant into the Cheng cycle as compared to 41.21% of classical combined cycle power plant. Hagen [31] used thermal diluent to enhance the performance of thermodynamic cycles that include a heat recovery system and preferably recovers and recycle thermal diluent from expanded energetic fluid to improve cycle thermodynamic efficiency and reduce energy conversion costs. Agarwal et al. [26] analyzed the performance of simple gas turbine cycle through integration of steam injection and inlet air evaporative cooling. Retrofitting of simple gas turbine cycle with integration of steam injection and inlet air cooling boosted the power output from 30 MW to 48.25 MW and generation efficiency can be raised from 29.9% to 33.4%. Sanjay and Prasad [6] studied and compared the thermodynamic performance of a gas turbine power plant for different

456


means of blade cooling. Open and closed loop cooling scheme using air and steam as coolants have been discussed. The closedloop cooling scheme involved only the internal convection cooling method while the open-loop cooling schemes involve film cooling and transpiration cooling method along with the internal convection. Kumar and Singh [7] performed the comparative study of transpiration-cooled gas turbine cycle performance considering the radiation effects for different coolants. Lucia [8] studied different methods for inlet air cooling in a cogeneration plant operated at two different locations in Italy and recognized the benefits and limitations for implementation of IAC systems. Bhargava et al. [9] have analyzed a wide range of combined cycle power plants using evaporative cooling and inlet fogging as a means of inlet air cooling. Bhargava et al. [2] also studied the effect of droplet dimension on inlet fogging and presented thermodynamic considerations of inlet and overspray fogging in IAC systems. Rahim [10] carried out a performance analysis of a combined cycle gas turbine power plant with four methods of IAC i.e. fogging, evaporative cooling, absorption cooling and electrical chiller cooling systems. Mahto and Pal [11] performed thermodynamic and thermo-economic analysis of simple combined cycle power plant, integrated with compressor inlet air cooling by fogging and gas turbine blade cooling by means of bleeding of compressed air. They proposed a simple gas turbine combined cycle configuration with triple pressure HRSG for obtaining the highest specific work and efficiency for the given operating range of gas turbine inlet temperature and cycle pressure ratio. Mohapatra and Sanjay [12] compared two different inlet air cooling techniques (evaporative cooing and vapor compression cooling) integrated with the cooled gas turbine. It showed that the vapor compression inlet air cooling improves the plant specific work by 18.4% compared to 10.48% for evaporative cooling. Carmona [13] evaluated the impact of evaporative cooling on an existing simple cycle, namely General Electric Frame 9E performance in Lagos n Nigeria. The study proposed the prospective benefits associated with evaporative cooling in consequence to power plant operators whose power generation assets are situated in coastal areas. Barigozzi et al. [14] investigated a techno-economical parametric analysis of an IAC system applied to combined cycle power plant with aero-derivative gas turbine. Their investigation reported that the best techno-economic performance is achieved for sites with high ambient temperature with low relative humidity, typical of desert areas. While there are numerous studies are available for power enhancement techniques in gas turbine cycles few were found

studying steam injection, inlet air cooling and gas turbine blade cooling. This study investigates the following: Analysis of integrated effect of the inlet evaporative cooling, steam injection into the combustion chamber and film blade cooling on gas turbine cycle. The effect of various parameters i.e. relative humidity, temperature of ambient air, turbine inlet temperature and cycle pressure ratio on the performance parameters of a film cooled gas turbine cycle with evaporative inlet air cooling and steam injection in the combustor. 2. System description & thermodynamic modeling A schematic of the steam-injected gas turbine with evaporative cooling at compressor inlet is shown in Fig. 1. Atmospheric air is drawn into the compressor via evaporative cooler which cools the ambient air before entering into the compressor. The compressed air goes to combustion chamber and burns with fuel. Some amount of steam generated from the heat recovery steam generator (HRSG) is injected into the combustion chamber for increasing the power at a given turbine inlet temperature and some part is used as a coolant in gas turbine for blade cooling to sustain the increased turbine inlet temperature. In this study the air admitted in the compressor is assumed to behave as an ideal gas and natural gas whose composition is given in Table 1 is considered as fuel. The pressure loss in the combustion chamber and coolant injection in turbine blade is taken as given in Table 1. Mathematical modeling of components of steam injected gas turbine based power plant is carried out on the basis of mass and energy balance. 2.1. Compressor In the present study an axial flow compressor is used in steam injected gas turbine based power plant. Here polytropic efficiency is considered to take care of various thermodynamic losses occurring in it. The relation between temperature and pressure of air at any section of the compressor is given as in Eq. (1) [7].

dT ¼ T

! Rcom dP gpt;com cp;com P

ð1Þ

where gpt,com is the polytropic efficiency of the compressor and cp,com and Rcom are the specific heat at constant pressure and the gas constant respectively. Rcom is expressed by:

Fig. 1. Schematic of steam injected gas turbine with evaporative inlet air cooling.

457


V_ amb;a cp;amb ðT DB;amb T com;i Þ Q_ s ¼

Table 1 Input data for analysis [15–18,20,21,23,28].

v wair

Parameter

Symbol

Unit

Compressor

(a) Polytropic efficiency (gpt,com) = 92 (b) Mechanical efficiency (gm,com) = 98.5

% %

Gas turbine

(a) Turbine blade temperature (Tb) = 1123 (b) Exhaust pressure = 0.109 (c) Exhaust hood temperature loss = 5 (d) Polytropic efficiency (gpt,gt) = 92

K MPa K %

Evaporative cooling

(a) Evaporative cooler effectiveness (E) = 90

%

Combustion chamber

(a) Lower heating calorific value of fuel (LHV) = 44,769 (b) Pressure loss (Dp) = 3% of entry pressure (c) Combustion efficiency (gcc) = 99.5 (d) Fuel composition CH4 = 90%, C2H6 = 4.5%, CO2 = 4%, and N2 = 1.5% by weight (e) Air composition O2 = 21%, N2 = 79% by volume (f) Steam injection pressure = 3 (g) Steam injection temperature = 610

kJ/kg

v wair ¼ ð0:287 þ xamb;DB 0:462Þ

kJ/kg K kJ/kg

Film cooling

(a) Film cooling efficiency (gc) = 70 (b) Adiabatic wall effectiveness (eaw) = 40 (c) Prandtl number (Prg) = 0.7 (d) Reynolds number (Reg) = 1 106 (e) Lamda (k) = 10

% %

T Patm

ð11Þ

where xamb;DB is the specific humidity of ambient air at its dry bulb temperature and is calculated on the basis of saturation pressure (Psat) of ambient air, relative humidity (u), molar mass of water vapor (Mv) and molar mass of air (Ma) as given in Eq. (12):

xamb;DB ¼

% MPa K

(a) Specific heat at constant pressure cp = f(T) R (b) Enthalpy h = cpdT

Rcom ¼ cp;com cv ;com

where V_ amb;a , T DB;amb , and T com;i are the volume flow rate of ambient air, dry bulb temperature of the ambient air, and compressor inlet air temperature respectively. The specific volume of wetted air per kg of dry ambient air (v wair ) is taken from Dawoud et al. [19] as given in Eq. (11):

% % %

Gas properties

ð10Þ

Psat Mv Ma ðPatm P Þ sat u

ð12Þ

If the compressor intake air temperature is less than or equal to the dew point temperature of the ambient air then the latent cooling load Q_ l will be given as:

V_ amb;a ½xamb;DB ðcp;v T DB;amb þ LÞ xc;i ðcp;v T com;i þ LÞ Q_ l ¼

v wair

ðxamb;DB xc;i Þ cp;w T com;i

ð2Þ

ð13Þ

where cp,v, L, xc,i and cp,w are specific heat of water vapor at constant pressure, latent heat of evaporation of water at 273.15 K, specific humidity at compressor inlet temperature, and specific heat of liquid water at constant pressure respectively.

where

cp;com ¼ cp;a xa;i cp;v ap

ð3Þ

2.3. Evaporative cooling

cv ;com ¼ cv ;a xa;i cv ;v ap

ð4Þ

Evaporative cooling is based on the basic principle that as the water evaporates, the latent heat of vaporization is absorbed from the water body and the surrounding air leading to cooling of the air entering compressor. Dry bulb temperature of the air leaving the evaporative cooler is calculated as:

cp,a and cv,a are the specific heat of air at constant pressure and specific heat of air at constant volume respectively, both in kJ/kg K. xa,i is the specific humidity of air at the intake section of the compressor inlet. The values of cp,a and cv,a are given as a function of mean temperature in Kelvin across the compressor [18]: 4

cp;a ¼ 1:048 3:837 10

5:491 1010 T 3m þ

T m þ 9:453 107 T 2m 7:929 1014 T 4m

cv ;a ¼ cp;a 0:287

ð5Þ ð6Þ

cp,vap and cv,vap are the specific heat of water vapor at constant pressure and the specific heat of water vapor at constant volume respectively, and are given as a function of mean temperature across the compressor [18]: 4

cp;v ap ¼ 1:8778 5:112 10

T m þ 1:9157

1:367 109 T 3m þ 3:723 1014 T 4m cv ;v ap ¼ cp;v ap 0:4614

106 T 2m ð7Þ ð8Þ

Mass and energy balance across the compressor yields, the compressor work as given by:

_ com ¼ m _ com;e hcom;e m _ com;i hcom;i W

ð9Þ

2.2. Estimation of cooling load For cooling the air entering compressor, the sensible cooling load Qs is obtained on the basis of relative humidity and dry-bulb temperature of the ambient air using equation:

E¼

T DB;i T DB;o T DB;i T WB;o

ð14Þ

where E is evaporative cooler effectiveness, TDB,i & TDB,o are dry bulb temperatures of ambient air at the inlet and exit of the evaporative cooler and TWB,o is wet bulb temperature at the exit of the evaporative cooler. Assuming adiabatic evaporation and the temperature of the liquid water being evaporated is equal to Ta,o, the specific humidity ratio of air at the exit of evaporative cooler is given by:

xa;o ¼

cp;a ðT DB T a;o Þ þ xamb;DB ðcp;v T DB þ L cp;w T a;o Þ cp;v T a;o þ L cp;w T a;o

ð15Þ

_ w;ev c required for evaporative The mass flow rate of water m cooling is given by:

_ w;ev c ¼ m

V_ amb;a

v wair

½xa;o xDB

ð16Þ

2.4. Combustion chamber Heat addition to combustion chamber occurs due to combustion of fuel (natural gas). The compressed air from the compressor enters the combustion chamber for burning the fuel and additional quantity of the steam is added. Assuming an adiabatic steady-flow combustion process, with a given combustion efficiency, Lower Heating calorific Value (LHV) of fuel and combustion chamber exit

458


temperature, the simple energy & mass balance gives the fuel mass flow rate. The governing equations are given by:

The average Nusselt number of flat plates is considered as given by El-Masri and Pourkey [22].

_ f þm _ s;cc ¼ m _g _ com;e þ m m

ð17Þ

1=3 Nug ¼ 0:037 Re0:8 g Pr g

_ a ð1 þ xcom;e Þ _ com;e ¼ m m

ð18Þ

_ f LHV gcc þ m _ s;cc hs;cc _ a ha;com;e þ xcom;e hx;com;e þ m m _ a ð1 þ xcom;e Þ þ m _ f þ ms;cc Þ hcc;e ¼ ðm

ð19Þ

Fuel–air ratio (f) can be given as:

_ f ð1 þ xcc;e þ SARÞcpg T cc;e SAR hs;cc ðha;com;e þ xcom;e hx;com;e Þ m f¼ ¼ _a LHV gcc cpg T cc;e m ð20Þ The steam–air ratio (SAR) is given by:

_ s;cc m SAR ¼ _a m

Gas turbine blade cooling helps to keep the blade material temperature within safe limits so as to get higher creep life, lower oxidation rates, and lower thermal stresses. Here the gas turbine blades are considered to have been cooled by film cooling using steam as coolant in which coolant is injected at one or more discrete locations along the surface exposed to a high temperature gas to protect the blade surface in the immediate region of injection and also in the downstream region. Fig. 2 depicts the film cooled blade cooling model considered in the analysis in which steam is used as coolant for blade cooling. A flat plate surface model is used to model, the film cooling, in which the coolant requirement for gas turbine blade can be given as [33]:

_ cl cpg ðT gt;i T b Þ eaw T gt;i ðT cl;i þ gc ðT b T cl;i ÞÞ m ¼ k St g _g cp;cl gc ðT b T cl;i Þ m

ð22Þ where k is the ratio of internal cooled blade surface area to external hot gas flow area (Ab/Ag) and gc is the cooling efficiency.

ð23Þ

where a is the coolant flow discharge angle, c is the blade chord and s is the blade spacing. Adiabatic film cooling effectiveness (eaw) is given as:

eaw ¼

T g T aw T g T cl;e

ð24Þ

Average Stanton number (Stg) is expressed as a function of the average Nusselt number (Nug), Reynolds number (Reg) and Prandtl number (Prg) as

Stg ¼

Nug Reg Prg

Expansion in gas turbine is modeled considering polytropic efficiency as given by:

gpt ðc1 c Þ dT dP ¼ T P

ð25Þ

ð27Þ

Turbine work is obtained through mass and energy balance across the control volume of the gas turbine as in Eq. (27):

ð28Þ

The specific work output, cycle efficiency and specific fuel consumption are mathematically considered as described ahead:

wnet ¼ wgt wcom

2.5. Cooled gas turbine blade

k ¼ 2c=s cos a

2.6. Gas turbine

_ gt ¼ m _ gt;i ðhgt;i hgt;e Þ þ m _ cl ðhcl;i hcl;e Þ W ð21Þ

ð26Þ

w

ð29Þ

gth ¼ _ net mf LCV

ð30Þ

_f 3600m wnet

ð31Þ

sfc ¼

Based on thermodynamic modeling an exhaustive computer program is written in C++ for carrying out thermodynamic analysis of STIG based power plant and to obtain the results. The computer program is supported by fundamental thermodynamic relations including real gas behavior, pressure losses, and thermodynamic properties of steam & air. 3. Results & discussion Based on the thermodynamic modeling the results have been obtained and presented here for studying the effect of turbine inlet temperature, compressor intake temperature, pressure ratio and ratio of mass of steam injected in combustion chamber to mass of air entering into combustion chamber, relative humidity and ambient air conditions on the performance of the gas turbine cycle. The input parameters considered are given in Table 1. Fig. 3 depicts the variation of compression work for various cycle pressure ratios with different CIT at TIT of 1650 K and 5% SAR. It shows that the requirement of compression work decreases with decreasing CIT for individual cycle pressure ratio. The variation is almost linear and a minimum compression work is reached at 282 K. Fig. 4 shows the effect of the CIT on the thermal efficiency of the steam injected gas turbine cycle. Results show that there is an increase in cycle thermal efficiency by 3.2% for the decrease in CIT from 318 K to 282 K on taking the steam to air ratio as 5% in the combustion chamber. This is due to reduction in the

Fig. 2. Film cooling model for turbine model.


459

Fig. 3. Variation of compressor work versus compressor intake air temperature, at a TIT of 1650 K.

Fig. 4. Variation of thermal efficiency versus compressor intake air temperature, at a TIT of 1650 K.

Fig. 5. Variation of specific work output versus compressor intake air temperature, at a TIT of 1650 K.

temperature of air at the inlet of the compressor, which decreases compression work and increases thermal efficiency of the cycle. Fig. 5 depicts the effect of CIT on the specific work output of the cycle at different pressure ratio for TIT of 1650 K and 5% SAR. It is evident that as the temperature of the air at inlet to the compressor increases the specific work output of the cycle decreases for given pressure ratio and fixed TIT. This is attributed to increment of compression work with increasing compressor inlet temperature while the expansion work from turbine remains same for given TIT and cycle pressure ratio. Due to the increased mass flow while lowering the CIT from 318 K to 282 K, specific work output for the given cycle is increased by 4.2%.

Figs. 6–8 show the effect of variation of TIT on cycle thermal efficiency, specific work output and the specific fuel consumption respectively at 5% SAR and 288 K CIT. Fig. 6 shows that the thermal efficiency increases as TIT increases which is because the increase in specific work output dominates the rising fuel requirement due to combined effect of steam injection and increased TIT. The effect of variation of turbine inlet temperature on the specific work output at different cycle pressure ratio is given in Fig. 7. It is observed that maximum specific work output is obtained at TIT of 1850 K, CPR of 27 and 5% SAR. Steam injection results in increased work output from the gas turbine due to increase in specific heat of expanding fluid and mass expanding through it. Fig. 8 depicts the effect of

460


Fig. 6. Variation of cycle thermal efficiency versus turbine inlet temperature at CIT of 288 K.

Fig. 7. Variation of specific work output with different turbine inlet temperature at CIT of 288 K.

Fig. 8. Variation of specific fuel consumption with different turbine inlet temperature at CIT of 288 K.

TIT on the sfc at different CPR and 5% SAR limit. At any TIT value the specific fuel consumption increases due to increase in fuel requirement due to steam injection in the combustion chamber. Fig. 9 shows the effect of variation of mass of steam injection in the combustion chamber on the GT cycle thermal efficiency. GT cycle thermal efficiency increases with the increase of the SAR % in the combustion chamber, this is because of increase in the expansion work in gas turbine is more despite than the additional fuel burning due to injection of steam in the combustion chamber. At TIT of 1750 K and the compressor intake temperature of 288 K maximum thermal efficiency is obtained for cycle pressure ratio of 27 and 11% SAR. Fig. 10 depicts the variation of specific work output with different fraction of steam injection in the combustion chamber for various cycle pressure ratio at 288 K CIT. It is seen that as the amount of steam injected in the combustion chamber is increased the specific work output of the cycle is increased due to increase in

expanding mass because of steam injection and thus larger expansion work is obtained without any additional compressor work. There is 2.59% average increase in the specific work output for every increase of SAR by 2%. Figs. 11 and 12 show the effect of ratio of mass of coolant injected in gas turbine for film cooling to mass of gas flowing at turbine inlet for different TIT on cycle performance. Fig. 11 exhibits that the specific work output of the cycle increases slightly with increase in the ratio of mass of coolant to mass of gas for fixed TIT. There occurs a gain of 4.764 kJ/kg in specific work output upon increasing the ratio of mass of coolant to gas by 0.002. This increment occurs because of increase in expansion work due to addition of mass of coolant in the gas turbine. Fig. 12 depicts the variation of cycle thermal efficiency with injection of the mass of coolant in the gas turbine blade. It shows that the increase of mass of coolant injected in the gas turbine for blade cooling offers increase in thermal efficiency of the cycle.


Fig. 9. Variation of cycle thermal efficiency versus percentage steam air ratio at CIT of 288 K.

Fig. 10. Variation of specific work versus percentage steam air ratio at CIT of 288 K.

Fig. 11. Variation of specific work output to ratio of mass of steam coolant to gas at different TIT.

Fig. 12. Variation of cycle thermal efficiency to ratio of mass of steam coolant to gas at different TIT.

461

462


Fig. 13. Variation of specific work output with cycle pressure ratio for different combination of GT cycle.

Fig. 14. Variation of thermal efficiency with cycle pressure ratio for different combination of GT cycle.

Fig. 15. Variation of thermal efficiency with turbine inlet temperature for different combination of GT cycle.

This increment in the efficiency occurs because of the additional expansion work obtained in the turbine due to injection of mass of coolant. Figs. 13–15 show the comparison of the performance parameters for the different combination of the GT cycle (i.e. simple gas turbine cycle, GT cycle with inlet evaporative cooling (IEC), GT cycle with steam injection (SI), GT cycle with IEC & SI and GT cycle with IEC, SI & FC with varying pressure ratio). Fig. 13 depicts the specific work output is more for the GT cycle intake air cooling

with film cooling & steam injection combination. It shows that there is sharp increase in the specific work output in the GT cycle with steam injection as compared to the simple GT cycle and the simple GT cycle with IEC at particular CPR. TIT of 1750 K and cycle pressure ratio of 24 yields increment of 7.2% in the specific work output for combination of the GT cycle with steam injection (SI), 9.5% increment for the combination GT cycle with IEC & SI and 10.1% increment for the GT cycle with IEC, SI & FC as compare to the GT cycle with IEC.


Fig. 14 depicts the variation of thermal efficiency with CPR for various combination of the gas turbine cycle. At the TIT of 1750 K and cycle pressure ratio 24 there occurs increment of 2.75% in thermal efficiency for the combination of GT cycle with IEC, 11% increment for the combination GT cycle SI, 14.1% increment for the combination GT cycle with IEC & SI and 15.79% increment for the combination GT cycle with IEC, SI & FC as compare to the simple GT cycle. Fig. 15 gives the variation of thermal efficiency of the different combinations of gas turbine cycle with varying TIT at cycle pressure ratio 24. For all gas turbine cycle configurations the thermal efficiency is seen to increase with increasing TIT in the considered range of TIT and cycle pressure ratio. Cycle configuration integrated with IEC, SI & FC produces more specific work output. At 1850 K TIT there occurs an increment of 2.77% in thermal efficiency for the combination of the GT cycle with IEC, 11.57% increment for the combination of the GT cycle with SI, 14.93% increment for the combination of the GT cycle with IEC & SI and 16.35% increment for the GT cycle with integration of IEC, SI & FC as compared to the simple GT cycle. 4. Conclusions On the basis of thermodynamic analysis of gas turbine cycle configuration integrated with IEC, SI & FC the following conclusions have been drawn. (i) Decreasing the temperature of air at the compressor inlet improves specific work output and thermal efficiency of the gas turbine integrated with IEC, SI & FC. There is 3.2% increase in cycle thermal efficiency by lowering the CIT from 318 K to 282 K at 5% SAR in the combustion chamber. (ii) Injection of steam in the combustion chamber increases expanding mass in the gas turbine, which augments the specific power output of the cycle. There is average increase of 2.59% in the specific work output for increasing SAR limit by every 2%. (iii) Steam-film cooling helps to increase TIT level for GT cycle configuration for the same blade material temperature. There occurs a gain of 4.764 kJ/kg in specific work output upon increasing the ratio of mass of coolant to gas by 0.002. (iv) At 1750 K turbine inlet temperature and cycle pressure ratio of 24 there is 7.2% increment in the specific work output for the combination of GT cycle with steam injection (SI), 9.5% increment for combination GT cycle with IEC & SI and 10.1% increment for GT cycle with IEC, SI & FC as compared to the GT cycle with IEC. (v) Gas turbine cycle configuration with inlet evaporative cooling, steam injection & film cooling is better combination for obtaining more efficiency and power. At 1850 K TIT & CPR of 24 there is 16.35% proportional increase in thermal efficiency GT cycle with IEC, SI & FC as compared to the simple GT cycle which is greater than all other combination compared in this analysis.

Acknowledgements The authors thankfully acknowledge the support received from TEQIP-II at Harcourt Butler Technological Institute, Kanpur for this work. References [1] World Energy Outlook 2015 10 November www.worldenergyoutlook.org/energyclimate/>.

2015