Performance improvement of a simple gas turbine cycle through integration of inlet air evaporative cooling and steam injection

Transcription

1 544 Journal o Scientiic & Industrial Research J SCI IND RES VOL 70 JULY 0 Vol 70, July 0, pp Perormance improvement o a simple gas turbine cycle through integration o inlet air evaporative cooling and steam injection Shyam Agarwal, S S Kachhwaha * and R S Mishra Department o Mechanical Engineering, Dhi Technological University, Bawana Road, Dhi 0 04, India School o Technology, Pandit Deendayal Petroleum University, Raisan, Gandhinagar , India Received 8 February 0; revised 8 May 0; accepted 6 May 0 Among many available retroitting technologies to improve power generation capacity and eiciency o simple cycle gas turbine, inlet air cooling (IAC) and steam injection gas turbine (STIG) are considered most eective ways to modiy an existing simple cycle unit In this study, a simple cycle generation unit is considered as base unit and STIG and IAC eatures are sequentially retroitted to the system To evaluate individual eects ater system modiications, a computer program has been devoped in EES (Engineering Equation Solver) sotware to stimulate perormance parameters Retroitting o simple cycle combined with IAC and STIG has been ound to boost power output rom 30 MW to 485 MW, while generation eiciency can be increased rom 99% to 334% Exergy destruction rate per MW o power output reduces or combustion chamber, compressor and HRSG, while increases or gas turbine or retroitted cycles Keywords: Exergy destruction, Gas turbine, Inlet air cooling (IAC), Retroitting, Steam injection gas turbine (STIG) Introduction Simple gas turbine power generation systems are widy used in Indian industries due to quick startup and shutdown capabilities Steam injection gas turbine (STIG) and inlet air cooling (IAC) by evaporation are the most common practices to enhance perormance o power generation Kumar et al devoped design methodology or parametric study and thermodynamic perormance evaluation o a gas turbine cogeneration system (CGTS) Wang & Chiou concluded that implementing both STIG and IAC eatures cause more than a 70% boost in power and 04% improvement in heat rate Bouam et al 3 studied combustion chamber steam injection or gas turbine perormance improvement during high ambient temperature operations Srinivas et al 4 concluded that steam injection decreases combustion chamber and gas reheater exergetic loss rom 385 to 374% compared to the case without steam injection in combustion chamber Minciuc et al 5 ocused on solutions o tri-generation plants based on gas turbine or internal combustion engine with absorption chilling machine Moran et al 6 devoped *Author or correspondence sskachhwaha@redimailcom design and economic methodology or gas turbine cogeneration system Nishida et al 7 analyzed perormance characteristics o two coniguration o regenerative steam-injection gas turbine (RSTIG) systems and concluded that thermal eiciencies o RSTIG systems are higher than those o regenerative, water injected and STIG systems IAC technology is simply to cool down inlet air entering compressor with a cooler Sinha & Bansode 8 studied eect o og cooling system (FCS) or IAC and showed improvement in turbine power and heat rate Chaker et al 9 have devoped ormulation or og droplet sizing analysis Salvi & Pierpaloli 0 have studied optimization o IAC systems or steam injected gas turbines and proposed technique o compression IAC through an ejection system supplied by exhaust heat o gas turbine Bassily studied perormance improvements o intercooled, reheat and recuperated gas turbine cycle using absorption inlet-cooling and evaporative atercooling A parametric study on eect o pressure ratio, ambient temperature and rative-humidity, turbine inlettemperature (TIT), and eectiveness o recuperated heat-exchanger on perormance o varieties o cycles is carried out Bhargava & Homji showed eects o inlet

2 AGARWAL et al : PERFORMANCE IMPROVEMENT OF A SIMPLE GAS TURBINE CYCLE 545 ogging on a large number o commercially available gas turbines This study presents perormance improvement o a simple cycle generation unit taken as a base unit and STIG and IAC eatures are sequentially retroitted to the system Experimental Section System Description Simple cycle gas turbine system integrated with IAC and STIG eatures (Fig ) comprises a base unit that includes compressor, combustor, gas turbine and a generator An HRSG was installed at downstream exit o turbine (state point 5) to recover heat rom exhaust gases Fraction o superheated steam generated rom HRSG is used or STIG (state point 9) and remaining super heated steam is used or process application An FCS is installed to cool ambient air (state point ) FCS uses very ine og droplets o high pressure water injected through special atomizing nozzles located at discrete points across inlet duct at high pressure to create cooling eect Amount o og is to be monitored based on dry and wet bulb ambient conditions to achieve required cooling A typical FCS consists o a high pressure pump skid connected or eeding to an array o maniolds located at a suitable place across compressor inlet duct Maniolds have a requisite number o og nozzles 6, which inject very ine droplets o water into inlet air Discharge through each nozzle is around 3 ml/s and produces 3 billion droplets per second Fine og evaporates very ast, thus dropping inlet air temperature Moding and Computer Simulation Formulations Assumptions considered or present study are as ollows: i) Molar raction (N = 07898, O = 00989, CO = and H O = 0) is assumed o mole o dry air; ii) Heat loss rom combustion t:chamber is % o lower heating value o u (All other components operate without heat loss); iii) Fog cooling system has been maintained or 00% saturation o ambient air at wet bulb temperature o air; iv) pressure o water injected rom nozzle into evaporative cooling chamber has been assumed 38 bar and converts into og (ine droplets), absorbs latent heat o air through adiabatic mixing; and v) Combustion chamber has been maintained at constant temperature A computer program has been devoped in Engineering equations solver (EES) to ormulate and simulate retroitting techniques over simple gas turbine with a set o steady-state governing equations including mass, energy, entropy and exergy balances using control volume analysis sequentially or compressor, combustor, gas turbine and HRSG Results o program were validated with available data 6 Ater successul validation, EES program has been devoped or analysis o IAC, STIG and integrated technologies retroitted with simple gas turbine For complete combustion o natural gas (methane) with steam injection in combustion chamber, chemical equation takes ollowing orm: 4 [,N,O,CO? λ CH + X N + X O + X CO + X HO HO,HO X',OO + X',COCO + X',HOHO () Mole raction o N, X Mole raction o O, X,N,O Mole raction o CO, X ] X,N = () +λ+ω =,CO Mole raction o H O, X X,O λ +λ+ω (3) X,CO+λ = (4) +λ+ω,ho = X,HO + λ+ω +λ+ω (5) where ω is steam injection ratio deined as ratio o mass o steam injected to mass o air supplied ω = m & &, = m & s m& g s m a m& s ω = m&, ] +ω [ ] [ ',N +λ+ω X N + ω, ω = ω ( + λ ), ω ω = (6) λ where ω is ratio o mass o steam injected to mass o combustion gases ormed and ω is ratio o mass o steam injected to mass o u supplied Maximum amount o permitted STIG is 0% o mass low rate o inlet air Heat transer between exhaust gases and condensate water has been taken place in water heat recovery boiler

3 546 J SCI IND RES VOL 70 JULY 0 where superheated steam is generated as ( h h ) = m ( h ) m exh 6 7 w sup h cond, where m exh and m w are mass low rate o exhaust gases o turbine and condensate water; h 6, h 7, h sup and h cond are enthalpies o exhaust gases at state 6 and 7, super-heated steam and condensate water Also, TPP = Tsat + TPP and TAP = Tsat TAP, where T pp, T sat and T AP are pinch point temperature, saturation temperature o water and approach point temperature, respectivy TPP is pinchpoint dierence and TAP is approach point dierence at saturation temperature Temperature o air ater og cooling can be obtained rom an energy balance on dry air, water spray and air-born water vapour beore and ater the system Assuming adiabatic mixing, energy gained by sprayed water is balanced by energy lost by dry air, and original air-born mixture, ater cooling such that (h h ) = m (h h ) +ωm (h h ), where mw v w a a a a v v m w and h are mass low rate and enthalpy o cooling w water, m a is mass low rate o dry air, ( ha ha ) is enthalpy change o dry air, ( hv hv ) is enthalpy change o water vapour during cooling Humidity ratio ( ω ) can 0 6 P v be speciied as ω = P P, where P v is partial pressure o water vapour and P is total atmospheric pressure From conservation o mass, amount o water evaporated is equal to the mass o water vapour at point minus water vapour originally in air at point as m w = ( ω ω ) m a, where ω is humidity ratio o air ater cooling Partial pressure o water vapour (P v ) can be ound rom respective rative humidity (RH) (φ ) as P = φp, where P is saturation pressure o water v sat sat vapour or corresponding temperature Pressure loss in adiabatic mixing is neglected Enthalpy, entropy, and exergy can be determined at each state point using mass and energy balances Perormance Parameters Perormance parameters required or thermodynamic analysis o simple cycle and retroitted systems include thermal eiciency, which is ratio o net work output ( W & ) to total heat input ( net Q & ) o a u v given as by W& net η Th = Q& Ration or Q & = m& CV u Q & (kw) is given, where m is mass low rate o u (kg/s) and CV u is caloriic value o u (kj/kg) Generation eiciency o a thermal system is the ratio o ectrical power output (W & ) to the total heat input o u ( Q & W ) given as η Gen = & Q& Ration between W & and W & net is given byw & = ε W& net, where ε is eectiveness o ectrical generation system Heat rate is the ratio o heat produced by u ( Q & ) to ectrical power output ( W & ) o thermal system and given as Q& HR = W& Heat rate is reciprocal o generation eiciency Speciic u consumption o a thermal system is the ratio o mass o u to net work output It is reciprocal o speciic net work (W spec ) and given as SFC m& W& = net First Law eiciency ( ) is the ratio o all useul energy extracted rom system to the energy o u input, ( W& + Q& ) Pro and given as η = I Q&, where (process Q & = m& ω h h, heat rate) is given as ( ) ( ) where pro m& is mass low rate (kg/s) o combustion products P and h 6 and h 7 are enthalpies (kj/kg) at states 6 and 7 respectivy Second Law eiciency ( ) is the amount o exergy associated with u and given as ( W& + ) pro η = II, where is exergy o process heat and is exergy o u input E pro = E PH,Pro + E, CH, Pro where E PH, Pro and E CH, Proare physical and chemical exergy o process heat, respectivy Similarly, E EPH, + CH, = E, where EPH, and ECH, are physical and chemical exergy o u, respectivy Exergetic eiciency o component (ε ) is the ratio o exergy rate recovered rom component ( E & ) to R exergy rate supplied to component ( E & ) and given as P S 6 7

4 AGARWAL et al : PERFORMANCE IMPROVEMENT OF A SIMPLE GAS TURBINE CYCLE 547 Table Comparison o various perormance parameters o simple gas turbine cycle and retroitted cycles Perormance parameters Simple gas Simple gas Simple gas Simple gas turbine cycle turbine cycle turbine cycle turbine cycle with og with STIG with og cooling cooling & STIG First law eiciency ( η I ), % Second law eiciency ( η II ), % Power generation eiciency ( η Gen ), % Thermal eiciency ( η Th ), % Fu-air ratio (λ) Steam injection ratio (ω), per kg o 0 0 mass o air Heat rate (HR), kw/kwh Speciic net work ( W SpecidicNet ), kj/kg o u Speciic u consumption (SFC), kg/kwh Work-heat ratio (WH ratio ), kj/kj Power-to-heat ratio (PH ratio ), kw/kj/s Speciic work ISO ( W SpecidicI SO ), kw-s/kg o air Turbine work ( W Tur ), MW Compressor work ( W Comp ), MW Net power output ( W net ), MW Electric work done ( W ), MW pro Process heat( Q ), MW R R D ε = = = S R + D R + Exergy D destruction rate ( E & DR D ) is given as = DR D, tot Results and Discussion In present study, ollowing three conigurations with retroitting have been studied in comparison to simple gas turbine cycle: i) Simple gas turbine cycle with IAC; ii) Simple gas turbine cycle with STIG; and iii) Simple gas turbine cycle with both IAC and STIG Initial conditions or system analysis were as ollows: Ambient air temperature at state, 985 K; Ambient air pressure at state, 03 kpa; Ambient air RH at state, 60%; Spray water temperature at state, 985 K; Spray water pressure at state, 3800 kpa; Air inlet pressure to compressor (P ), 03 kpa; Air inlet temperature to compressor (T ), 985 K; RH o inlet air to compressor at, 00%; Pressure ratio o compressor (r p ), 0:; Isentropic eiciency o compressor (η ), 086%; Isentropic eiciency o Turbine (η ), SC ST 086%; Lower heating value o u (LHV), 8036 kj/ kmol; Mass low rate o air ( m& a ),84 kg/s; Turbine inlet temperature (TIT), (T 4 ), 50 K; Injection pressure o u (methane) (P ), 00 kpa; Injection temperature o u (methane) (T ), 985 K; Pressure drop in combustion chamber,( p) combustion chamber, 5%; Exhaust pressure o combustion products ater HRSG (P 7 ), 03 bar; Exhaust temperature o combustion products ater HRSG (T 7 ), 4035 K; Pressure o steam generation (P 9 ), 000 kpa; Pressure o condensate water at inlet o

5 548 J SCI IND RES VOL 70 JULY 0 Air Fu Steam-injection ω? 9 3 Combustion products Combustion chamber 4 Turbine Compressor Fogged & cooled air Fog cooling system 5 6 Water Remaining superheatedsteam P Heat recovery (-?) ω Ambient air steam generator 8 Condensate water water G Fig Simple cycle gas turbine with og cooling and STIG 7 Exhaust gases HRSG (P 8 ), 000 kpa; Temperature o condensate water at inlet to HRSG (T 8 ), 985 K; Pressure drop in HRSG on the gas side, ( p) HRSG, 5%; Amount o steam injected (ω ), 0% o mass low rate o air; Temperature o superheated steam STIG (T 9 ), 7535 K; Approach point ( T AP ), K; and Pinch point ( T pp ),0 K In the calculation, steady state operation is assumed without considering turbine blade cooling Perormance analysis o these retroitted gas turbine system is done by preparing a computer program in EES validated with available data 6 Temperature, pressure and gas concentration in each component are calculated by taking into consideration o compositions and proportions o gases and consequently, various perormance parameters and exergy loss in these systems are estimated Net power output and power generation eiciency or simple cycle are 30 MW and 993% respectivy (Table ) Attachment o evaporative cooler with simple cycle improves perormance parameters (system eiciencies, heat rate and speciic power output etc) Gas turbine inlet air ogging is a commonly used method o cooling intake air, where demineralized water is converted into og droplets by means o special atomizing nozzles operating at 38 bar Evaporation o small size (5-0 µ) droplets in - intake duct cools - air and consequently increases - moist air mass low rate to improve power perormance This technique allows close to 00% evaporation eectiveness in terms o attaining saturation conditions and wet bulb temperature at compressor inlet Thus variation in ambient temperature inluences exit air temperature o compressor, entry and exit temperature o turbine, mass low rate, speciic work, speciic u consumption and power When the ambient temperature drops, net power supplied by the machine increases Thereore, it is useul in many cases to cool compressor inlet air to obtain a greater production o ectric power associated with reduction in compressor work Using evaporative cooling, available air (5 C and 60% RH) can be cooled up to 95 C Impact o evaporative cooling will be higher in dry summer season when dry bulb temperature is higher and RH is lower Comparison o simple cycle gas turbine with and without og cooling shows (Table ) that net power output increases by 3% and various eiciencies increase by 08% while heat rate decreases by 06% Comparison o simple cycle gas turbine with and without STIG shows (Table ) that net power output and thermal eiciency increase by 74% and 35% respectivy, while heat rate decreases by 0% In the process o recovering energy rom exhaust gases via HRSG, temperature at outlet o stack (state point 7 in Fig ) is usually kept above 7 C (dew point temperature o acid) in order to prevent condensation o SO and NO, which ultimaty hydrolyzed into sulphuric acid (H SO 4 ) and nitric acid (HNO 3 ) and inally cause scale and corrosion to air preheater o HRSG Pinch point dierence and approach point dierence or present analysis are taken as 0 K and K respectivy Under these conditions, maximum low rate o generated superheated steam at 753 K and 0 bar is 5 kg/s I all generated steam is injected into combustor (STIG only), maximum injection ratio (m steam /m air ) is 06 Thereore, there is a wide range o STIG available to optimize power cycle Calculated power output or injection ratio (0) shows that eect o STIG is quite substantial Net power output is increased

6 AGARWAL et al : PERFORMANCE IMPROVEMENT OF A SIMPLE GAS TURBINE CYCLE 549 Net increase in work Cycles Fig Net work output or retroitting cycles in comparison to simple cycle 40 Generation eiciency, % Fig 3 Comparison o generation eiciency or dierent retroitted cycles

7 550 J SCI IND RES VOL 70 JULY 0 Table Comparison o exergy destruction in components or simple gas turbine cycle and retroitted cycles[*exergy destruction rate (%) is the ratio o exergy destruction rate within a component to total exergy destruction rate o the system] E È E È

8 AGARWAL et al : PERFORMANCE IMPROVEMENT OF A SIMPLE GAS TURBINE CYCLE 55 Eiciency, % Steam injection ratio (% o mass low rate o air) Process heat, MW Fig 4 Eect o steam injection ratio on irst-law eiciency, generation eiciency and process-heat or simple cycle integrated with og cooling and STIG to 383 MW Proound eect rom STIG alone is because the required pressure o injected steam is obtained rom pump Since pumping work is to 3 orders o magnitude smaller than that o compressor, net power output produced by steam is, thus, higher than that o air per unit mass low rate Besides, this speciic heat o superheated steam is almost double the value o air and thereore, enthalpy o steam is higher than that o air at a certain temperature Thereore, STIG method is a very eective way to boost net power output and to increase overall eiciency o gas turbine Simple gas turbine cycle with STIG (steam injection ratio 0) signiicantly improves system eiciencies Comparison o simple cycle gas turbine integrated with FC (og cooling) and STIG shows that net power output increases by 305% and thermal eiciency increases by 354%, while heat rate decreases by 04% Comparison o simple cycle gas turbine with and without FC (Table ) shows that exergetic eiciency gets also improved by 0% However u-air ratio increased by % As compared to this, simple cycle gas turbine with and without STIG shows that exergetic eiciency gets improved by 58%, however u-air ratio increased by 44% Integration o simple cycle with STIG and evaporative (og) cooling urther improves system perormance in terms o exergetic eiciency, which improved by 569% and uair ratio increased by 54% There is net increase in work output or dierent retroitted cycles in comparison to simple cycle (Fig ) Maximum increase in work output obtained is 85 MW or simple cycle combined with STIG (injection ratio 0) and og cooling Maximum generation eiciency achieved is 3766% or integrated og cooling and STIG retroitted cycle with injection ratio 0 (Fig 3) Thus combination o ogging and STIG with simple cycle gas turbine cycle is a good approach to enhance perormance o system on the basis o irst and second laws Beneit o adding STIG eature can be estimated rom Fig 4, which shows eect o STIG on generation eiciency, irst law eiciency and process heat or ixed inlet air conditions as air gets saturated up to 00% RH due to og cooling First law eiciency alls with increasing amount o steam injection ratio, may be because slope o decreasing process heat is steeper than slope o increasing generation eiciency or reduction in process heat takes place with aster rate In present case, maximum amount o injection steam is limited by available energy recovered rom HRSG Maximum injection ratio taken as 0 is still bow the allowable injection limit (prescribed by manuacturer) or available industrial turbines Exergy destruction rate (MW) represents waste o available energy While examining rative exergy destruction or all components, combustor has largest exergy destruction and shows major location o thermodynamic ineiciency because o large irreversibility arising rom combustion reaction and heat transer (Table ) Steam injection will increase exergy destruction due to mixing o high temperature superheated steam (753 C) and compressed air (at 5948 C) in combustor Exergy-losses at position 7 (Fig ) are considered as exergy loss through stack Since part o exhaust heat is recovered in HRSG, exhaust exergy out o stack can be reduced substantially ater retroitting Exergy losses through stack will not only waste available exergy but also dump thermal pollution to living environment For a retroitted cycle with og cooling and STIG, exergetic eiciencies are as ollows: compressor, 9; turbine, 93; combustor, 68; and HRSG, 75% Although exergy destruction rate o combustor is highest, exergy eiciency o combustor is higher than that o HRSG Thereore, a greater improvement margin exists or HRSG as compared to combustor Exergy destruction rate o each system component except compressor increases due to increasing mass low rate o air and steam mixture Exergy destruction rate increases with increasing STIG quantity in combustion chamber, turbine and HRSG (Fig 5) Exergy destruction in combustion chamber is highest among all system components Increasing steam injection amount reduces stack-losses

9 55 J SCI IND RES VOL 70 JULY 0 Exergy destruction rate (MW) Components Fig 5 Comparison o exergy destruction rate o system components or simple and retroitted cycle integrated with og cooling and STIG 5 Exergy destruction rate (MW) MW o power output 05 0 Components Fig 6 Comparison o exergy destruction rate (MW) per MW o output or system components as large amount o heat o exhaust gas has been utilized to convert into superheated steam (at state point 7 in Fig ) Exergy destruction in combustion chamber increases with increasing amount o STIG due to increased amount o mass low rate o air and steam mixture Due to signiicant increase in power output, rate o exergy destruction per MW o power output reduces (Fig 6) or combustion chamber, compressor, HRSG and stack gases, while increases or gas turbine due to increase in mass low rate (mass low rate o air rom compressor plus mass low rate o injected-steam with lower exergy) Conclusions In this study, an existing simple cycle gas turbine was considered as basic system and has been converted into modiied retroitted system with either IAC or /and STIG eatures Steam needed in STIG eature is generated rom energy recovered rom system s own exhaust gases Under average local weather conditions

10 AGARWAL et al : PERFORMANCE IMPROVEMENT OF A SIMPLE GAS TURBINE CYCLE 553 (5 C and 60% RH), beneit o adding STIG eature can substantially improve power output rom 30 MW to 395 MW and power generation eiciency rom 993% to 334% Maximum power that can be reached by the system with both IAC and STIG eatures is 485 MW or steam injection pressure ratio at 0 Although steam injection will increase total exergy losses, exergy loss per MW output is smaller than that o simple cycle It also reveals that degree o energy wasting and thermal pollution can be reduced through retroitting Reerences Kumar A, Kachhwaha S S & Mishra R S, Thermodynamics analysis o a regenerative gas turbine cogeneration plant, J Sci Ind Res, 69 (00) 5-3 Wang F J & Chiou J S, Integration o steam injection and inlet air cooling or a gas turbine generation system, Exergy Convers Mgmt, 45 (004) Bouam A, Aissani S & Kadi R, Combustion chamber steam injection or gas turbine perormance improvement during high ambient temperature operation, J Engg Gas Turbines & Power, 30 (008) Srinivas T, Gupta A V S S K S & Reddy B V, Sensitivity analysis o STIG based combined cycle with dual pressure HRSG, Int J Therm Sci, 47 (007) Minciuc E, LeCorre O, Athanasovici V, Tazerout M & Bitir I, Thermodynamic analysis o tri-generation with absorption chilling machine, Appl Therm Engg 3 (003) Moran M J, Thermal System Design And Optimization (John Wiley & Sons, New York) 996, Nishida K, Takagi T & Kinoshita S, Regenerative steam-injection gas turbine systems, Appl Energy 8 (005) Sinha R & Bansode S, A thermodynamic analysis or gas turbine power optimization by og cooling system, in 0th Nat & 9th Int ISHMT-ASME Heat and Mass Transer Con, edited by N Iyer Khannan (Research Publishing Services) 00 9 Chaker M, Meher-Homji C B & Mee I I I T, Inlet ogging o gas turbine engines-partii: og droplet sizing analysis, nozzle types, measurement, and testing, J Engg Gas Turbines & Power, 6 (004) Salvi D & Pierpaoli P, Optimization o inlet air cooling systems or steam injected gas turbines, Int J Therm Sci, 4 (00) 85-8 Bassily A M, Perormance improvements o the intercooled reheat recuperated gas turbine cycle using absorption inlet-cooling and evaporation ater-cooling, Appl Exergy, 77 (004) 49-7 Bhargava R & Meher-Homji C B, Parametric analysis o existing gas turbines with inlet evaporative and overspray ogging, J Engg Gas Turbines & Power, 7 (005) 45