Energy Conversion and Management 106 (2015) 1230–1241

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Energy Conversion and Management

journal homepage: www.elsevier .com/locate /enconman

Energetic and exergetic analysis of a steam turbine power plantin an existing phosphoric acid factory

http://dx.doi.org/10.1016/j.enconman.2015.10.0440196-8904/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: National Engineering School of Gabes, Omar IbnKhattab Street, 6029 Gabes, Tunisia.

E-mail address: hafdhi.fathia@gmail.com (F. Hafdhi).

Fathia Hafdhi a,⇑, Tahar Khir a, Ali Ben Yahyia b, Ammar Ben Brahim a

aApplied Thermodynamic Research Unit UR11ES80, National Engineering School of Gabes, Tunisiab Tunisian Chemical Group (TCG), B.P. 72, Gabes 6000, Tunisia

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 July 2015Accepted 15 October 2015Available online 11 November 2015

Keywords:Energy investigationExergy analysisSteam turbinePhosphoric acid factoryHeat recovery system

An energetic and exergetic analysis is conducted on a Steam Turbine Power Plant of an existingPhosphoric Acid Factory. The heat recovery systems used in the different parts of the plant are also con-sidered in the study. Mass, energy and exergy balances are established on the main compounds of theplant. A numerical code is established using EES software to perform the calculations required for thethermal and exergy plant analysis considering real variation ranges of the main operating parameterssuch as pressure, temperature and mass flow rate. The effects of theses parameters on the system perfor-mances are investigated.The main sources of irreversibility are the melters, followed by the heat exchangers, the steam turbine

generator and the pumps. The maximum energy efficiency is obtained for the blower followed by theheat exchangers, the deaerator and the steam turbine generator. The exergy efficiency obtained for theheat exchanger, the steam turbine generator, the deaerator and the blower are 88%, 74%, 72% and 66%respectively.The effects of High Pressure steam temperature and pressure on the steam turbine generator energy

and exergy efficiencies are investigated.� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The phosphate sector is one of the important key vectors for theTunisian economy balance, both in terms of employment and interms of the trade balance. The annual production of PhosphoricAcid is about 500,000 tonnes. Globally, the Tunisian phosphateindustry occupies the 5th place among the largest internationaloperators in this activity. Indeed, phosphate and its derivatives(phosphoric acid, DAP, TSP, DCP, etc.) are currently exported toabout fifty countries in the five continents. While the industry isvery important for the economy, it is considered among the largestenergy consumers.

Many national programs are conducted in Tunisia in thepurpose to improve the energy efficiency of the industrial sector,especially in the heavy industry factories such as the chemicalindustry field.

This study focuses on the analysis of a Steam Turbine PowerPlant with heat recovery systems used in Phosphoric Acid Factory.

Energy and exergy analysis may be constituted as a keymethodology for thermal system design and optimization. It isused to locate and determine the magnitude of irreversibility ratesoccurring in the streams and components of any energy system.

Many works were developed on energy and exergy optimiza-tion of industrial plants.

Vuckovic et al. [1] established an exergy analysis and exergoe-conomic investigation of a real industrial plant constituting a partof a rubber factory. This plant is used to supply production unites,services and working spaces by steam, compressed air and coolingand hot water. The exergy efficiency and the exergy destructionrate of the main components of the plant are determined. Thepossibilities of plant performance improvement are suggested.

Kaushik et al. [2] presented a review on energy and exergy anal-ysis of thermal power plant. A comparison between thermal powerplant stimulated by coal and gas was made. For coal based thermalpower plant, obtained results show that the highest energy loss islocated in the boiler. While for gas fired combined cycle thermalpower plant the maximum losses are located in the combustionchamber.

Ray et al. [3] developed an exergy analysis for proper operatingand maintenance decisions in a 500 MW steam power plant. Thestudy is conducted considering design and off-design conditions

Nomenclature

AC additional condenserBl blowercp specific heat at constant pressure (kJ/kg K)CU concentration phosphoric acid unitDe deaeratorDU distillation unit_E exergy (kW)h specific enthalpy (kJ/kg)HP high pressure steamLP low pressure steamMP medium pressure steam_m mass flow rate (kg/s)PAP unit of phosphoric acidPh. A phosphoric acidR gas constant (kJ/kmol K)SMM Sulfur melting and maintenanceSTGI Steam Turbine Generator ISTGII Steam Turbine Generator IIT temperature (�C)Tb turbineTC turbine condenser

Subscripts0 reference stateCT condenser of turbineD destructionDe deaeratorda dry aire energyex exergyGe gearha humid airin inletout outletPm pumpsw seawaterv water vaporval valve

Greek letterse specific exergy (kW/kg)g efficiencyx air humidity ratio

F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241 1231

for various values of superheat and reheats sprays. Obtain resultsconstitute guide procedures for exergy, economy and maintenancescheduling similar power plants.

Aljundi [4] performed an exergy and energy analysis of a steampower plant with a capacity of 396 MW. The effect of the referenceenvironment temperature variation on the exergy analysis of theconsider power plant has been studied. The results showed thatthe exergy efficiency of the power cycle was about 25%. The rateof exergy destruction and the exergy efficiency, of each compo-nent, changed with reference environment temperature. The mainconclusion indicates that the boiler is the major source of irre-versibility in the system. Indeed the exergy destruction in the boi-ler system is about 77% of the fuel exergy input. The exergydestruction in the turbine, condenser and all heaters and pumpsare respectively about 13%, 9% and 2%.

An exergy analysis for thermal power plant is conducted by Houet al. [5] using Aspen plus software. The effects of main operatingparameters such as combustion exes air coefficient, steam temper-ature and pressure and combustion temperature on the exergy effi-ciency are analyzed. The obtained results reveal that the boilerengenders the maximum irreversibility rates followed by the tur-bine. Furthermore the authors suggest that the increase of thecombustion temperature as well as the steam pressure and tem-perature leads to improvement of exergy efficiency.

Regulagadda et al. [6] performed a thermodynamic analysis of asubcritical boiler–turbine generator for a 32 MW coal-fired powerplant. Energy and exergy equation governing the cycle are estab-lished. A parametric study is conducted for a range of operatingvariables. That permits to define the optimum parameters leadingto the best plant performances. The boiler and turbine engenderthe maximum exergy destruction rates in the power plant. Theidentification of the exergy losses in the different cycles has per-mitted to develop an environmental impact and sustainabilityanalysis.

In the purpose to analyze the opportunities to improve theenergy efficiency of existing lignite thermoelectric power plant of300 MW, Koroneos et al. [7] was performed a comparative studybetween the indicated plant and three proposed combinedheat and power systems CHP, working according to Rankine

cogeneration cycle and using the same fuel mass flow rate.Therefore, different arrangements of recovery and cogenerationsystems are adopted for the investigated CHP power plants. Themain comparison criteria are based on the power production.According to analytic study, the authors confirm that it is possibleto improve the efficiency of the existing power of about 8.5% byusing the most efficient prosed CHP system.

A comparison between nine coal-fired power plants in Turkey isconducted by Erdem et al. [8]. For each plant a calculation model isproposed and the mass, energy and exergy balances are estab-lished. That permits to determine the energy and exergy efficiencyas well the exergy destruction rate of each component. A compar-ison is then accomplished between the considered power plants.The obtained results may constitute helpful tools for further inves-tigations in the field of energetic and exergetic industrial powerplant analysis.

Ghannadzadeh et al. [9] developed a general methodology forexergy balance in chemical and thermal process integrated in theProSimPlus code. In order to fully automate exergy analysis, thewhole exergy balance of the system is presented under the formof single software. The essential elements for exergy analysis areprovided that can be applied for every process or utility system.

An energy and exergy investigation of a cement plant was car-ried out by Atmaca et al. [10]. In order to assess the performance ofthe whole factory and their components, the authors are applied amass, thermal as well as the exergy balances taking into accountthe variation of operating parameters. A set of performance criteriaare defined in the aim to conduct this analysis.

Molés et al. [11] conducted a thermodynamic analysis of a com-bined organic Rankine cycle and vapor compression cycle systemusing two different fluids with low Global Warming PotentialsGWP for each cycle. System performances are determined forranges of operating conditions variations. Results show that thecombined cycle COP varied between 0.30 and 1.10 while the com-puted electrical COP is varied between 15 and 110. Furthermore,for vapor compression system the selection of working fluid doesnot affect significantly the thermal and electrical efficiencies.Whereas, for ORC the working fluid has an important effect,especially on electrical efficiency.

1232 F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241

Hajabdollahi et al. [12] established a soft computing basedmulti-objective optimization of steam cycle power plant usingNon-dominated Sorting Genetic Algorithm (NSGA-II) and ArtificialNeural Network (ANN). The main cycle parameter at the inlet andoutlet of the different components are considered for the optimiza-tion design. The maximization of the thermal efficiency and theminimization of the total cost rate are taken as objective functionis chosen in the purpose to optimize the running conditions of thepower plant. Obtain results reveal an increase of the thermal effi-ciency of about 3.76% and a decrease of the total cost rate of about3.84%.

Keçebas� [13] carried out a thermal, exergo-economic and envi-ronmental investigation of an existing geothermal district heatingsystems installed in Afyon, Turkey. Based on data collected fromthe plant, authors conduct an analysis in order to evaluate theheating system performance, the energy and exergy efficiencies,the specific exergy index as well as the exergy destruction.Obtained results show an energy and exergy efficiencies of theoverall heating system of about 34.86% and 48.78%, respectively.Authors suggest that the main exergy destruction rates are dueto fluid reinjection, losses in heat exchangers and pipe lines, natu-ral direct discharge and the pump losses. Others advantages of thesystem are pointed out by authors such as positive effects on theenvironment and low investment costs.

Adibhatla et al. [14] carried out an exergetic analysis of a600 MWe coal fired thermal power plant at three operating loads(100%, 80% and 60%) and under different running conditions: con-stant pressure and at pure sliding pressure operation. The energy,and exergy efficiencies as well as the exergy destruction rates aredetermined for each component of cycles at the indicated condi-tions. The highest exergy destruction rate is obtained for the boilerfollowed by the turbine. At constant pressure, the exergy destruc-tion rate in the turbine decreases sensibly when the operating loadpasses from 100% to 80%. While for the operating loads off 80% and60% the exergy destruction rate is practically the same. For puresliding pressure operations, the exergy destruction rate in the tur-bine decreases significantly with the load condition.

According to an energy and exergy analysis, Edge et al. [15] areconducted a study on the uncertainty of parameter defining thepower plant performances. The study is performed on a lignitefired power plant cycle with load conditions varying from 100%to 40%. A sensitivity analysis is also carried out in the purpose todetermine the effect of the main parameter on the overall poweruncertainty efficiency. The performed study is based on real mea-surements of the operating parameters by appropriate sensorswith known accuracies, installed in the different components ofthe plant. According to obtained result, the accuracy in determin-ing the Low Heating Value of the used coal constitutes the maincause of uncertainty. That affects the performance criteriadetermination.

An exergetic and exergoeconomic analysis for solar thermalpower plant is developed by Elsafi [16]. Two steam power cyclesare studied, with and without reheating system. Exergy andeconomic balances are established for each component of thecycle. The obtained results show that the main sources of exergydestruction are the solar field followed by the condenser, the LPturbine and the HP turbine. From thermo-economic point ofview and based on the total cost rate, the most expensivecomponent is the solar field followed by the LP turbine, HPturbine and the condenser. Authors analyzed the effect of steamreheat degree at the inlet of the LP turbine on the systemperformances. They observe that an increase in reheat degreeof about 100 K leads to an increase of 9.1% in vapor fraction atthe turbine outlet and a decrease of 1.5% in energetic andexergetic efficiencies. Unfortunately an increase in electricitycost of about 2% is obtained.

An energy and exergy optimization of a drying plant is carriedout by Taner [17] in the purpose to define the optimum energyand exergy efficiencies. Real measurements are accomplished onthe drying plants taking into consideration the different accuraciesof the experimental devices. That permits to conduct an exergyanalysis and defined the optimum mass and thermal values lead-ing to the best system efficiencies. As result of the optimizationstudy, a significant improvement in the energy and exergy efficien-cies of about 41% and 43% respectively are obtained.

Keçebas� et al. [18] carried out a conventional and advancedexergy analysis on a real geothermal power system of 6.35 MWand located in Denizli, Turkey. The studied system is constitutedby two-level and binary power plant using water for cooling. Realoperating parameter values are used to accomplish this investiga-tion. The investigated analysis method has permitted to identifythe system components that should be improved in the purposeto enhance the exergy efficiency. The proposed modifications leadto an increase in exergy efficiency of about 18.26%.

An energy and exergy analysis of subsea power system workingaccording to Rankine cycle is investigated by Yuan et al. [19]. Theworking fluids selected to perform the system analysis areWater–steam, CO2, C9H2O, C10H22 and C12H26. Simulation resultsshow that the Water–steam gives the best energy and exergy effi-ciencies as well as high turbine power output. However this sys-tem requires higher supply energy. While for CO2 system highenergy and exergy efficiencies are obtained with low energy input,but lower turbine power output is produced. Moreover the boilerand condenser engender the most important exergy destructionrate.

Peng et al. [20] carried out an exergy investigation on solarhybrid coal fired power plant of 330 MW. Solar system is used toheating feed water at temperature below 300 �C in the purposeto substitute the steam extraction from steam turbine. That per-mits to improve the net electrical power generated by the steamturbine. A thermal and economic comparison study is also estab-lished between solar-only and solar-hybrid coal-fired powerplants. According to the analysis results lower irreversibility ratesare achieved in the solar feed water heater and the steam turbine.An enhancement in exergy efficiency and solar energy conversionare obtained. Also the hybrid coal-fired power plant seems to beeconomically beneficial than the solar-only thermal power plant.

Three methods are considered by Memon [21] to conduct ananalytic study on a combine cycle power plant: exergoeconomic,thermo-environmental and statistical. The energy, exergy and eco-nomic balances are established. The influence of operating param-eters on the energy and exergy efficiencies, generated power andCO2 emission are analyzed. These performance criteria are corre-lated according to operating parameters in order to define the opti-mum values. Exergoeconomic analysis is also developed taking themaximization of exergy efficiency and the minimization of totalcost as objective functions. Analytic results show that the optimumoperating conditions giving best cycle performances, suitable eco-nomic balance and minimum gas release are obtained for a GasTurbine Inlet temperature of 1500 K.

Exergy analysis of a combined reheat regenerative steam tur-bine is performed by Gogoi [22]. The system consists of a basedpower cycle and a Water-LiBr vapor absorption refrigeration sys-tem VARS. Exergetic efficiency and exergy destruction rate in thedifferent elements of the system are determined as well as thewhole system energy utilization factor. The influence of the mainoperating parameters on the system performance is examined.Obtained results indicate that the optimum performances areachieved for a boiler pressure of 150 bars. Also the optimum tem-perature values in the each component of VARS are determined.The maximum Exergy destruction rate for the power cycle islocated in the cooling tower followed by the gas stream in the

F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241 1233

boiler outlet and the boiler. While for the VARS the generator rep-resents the most important irreversibility followed by the absor-ber, condenser and the evaporator.

An optimization procedure of the multi-step regenerative irre-versible Brayton cycle is conducted by Ahmadi et al. [23]. Energeticanalysis coupled with NSGA II algorithm is used in order to opti-mize the thermal efficiency and the normalized power output ofthe Brayton cycle. Furthermore the decision-making approachesTOPSIS, LINMAP and Fuzzy are used to compare the obtain resultswith the real one. Precision analysis is also applied to determinethe deviation from ideal state.

Sadatsakkak et al. [24] performed thermo-economic optimiza-tion of irreversible regenerative closed Brayton cycle. Ecologicalobjective function is considered as optimization criteria. Anequivalent system is defined based on thermodynamic first andsecond laws. Using NSGA-II method developed multi objectiveevolutionary approaches are applied to accomplish the analysis.Thermo-economic criterion, ecological function and power outputare taken into consideration, in the same time, for the optimizationprocedure.

Energy and exergy analysis study of a combined cycle powerplant is carried out by Ganjehkaviri et al. [25]. The combine cycleis with dual pressure heat recovery steam generator HRSG. Energy,exergy and economic balances are defined for the different cyclecomponents. Optimization is conducted taking into account thequality of steam turbine outlet. The energy and exergy efficienciesof the different elements are determined for three values of outletvapor quality. The optimum operating conditions are obtained foroutlet vapor quality of 88%. From economic point of view resultsshow that the improvement of the steam cycle quality leads to adecrease of the cycle total cost.

Manesh et al. [26] developed an exergoeconomic and exergoen-vironmental analysis on the coupling of a gas fired steam powerplant with a total site utility system. The main purpose of the studyis to analyze the incorporation of a steam power plant as an energysupply source for a site utility system. An appropriate method isused to optimize the integration of a steam power plant and a siteutility effect on the whole plant performances. The obtain resultsshow that this proposed design is a beneficial way leading to anenhancement of energy and exergy efficiencies as well as goodenvironmental impacts. Moreover this integration leads to adecrease of the total annualized cost of the whole system com-pared with initial base design.

Taillon et al. [27] proposed new graphs for thermal power plantexergy efficiency determination. These graphs permit to determinethe efficiency ranks compared with the normally obtained valuesfor the industrial systems.

In the present study an energetic and exergetic analysis isconducted on a thermal power plant installed in an existingindustrial chemical factory. The operating mode of the factoryand the power supply streams are presented. Mass, thermaland exergy balances are established on the main compounds. Acode is established in the purpose to perform all calculationsrequired for the exergetic analysis. The effects of the operatingparameter variations on the power plant performance areanalyzed.

2. System description

The Phosphoric Acid Plant (PAP) is one of the important indus-trial factories owned by the Tunisian Chemical Group (GCT) andinstalled in the industrial area of Gabes (South East – Tunisia).The plant is intended for the production of Phosphoric Acid. Themean daily production is about 1500 t/day. For the operating ofthe different units, the PAP plant consumes a meaningful electricalpower generated by two steam turbine power plants. Moreover a

connection with the Tunisian Society of Electricity and Gas (STEG)is established in the purpose to ensure a continuous plantoperation.

The schematic diagram flow of the thermal power plant of thePAP factory is shown in Fig. 1. This power plant is mainly consti-tuted by two steam turbine cycles STGI and STGII used to provideabout 14 MW as total net electrical power required for the differ-ent units. The required High Pressure steam (HP) mass flow rateat about 40 bars and 410 �C is provided by the Evaporator BoilerPre-superheater Superheater group (EBPS).

The first steam turbine cycle is with extraction andcondensation, while the second one is with back pressureturbine. The net electrical power generated by each cycle isabout 7 MW.

Furthermore, to supply the other different units, a quantity ofHP steam is expanded through expansion valve to obtain MediumPressure steam (MP) at 12 bars and 280 �C and Low Pressure steam(LP) at 6 bars and 230 �C.

In the steam turbine cycle, The HP steam is expanded throughthe steam turbine STG I to reach the Low pressure level at point4 where an appropriate quantity is extracted. The remained LPsteam amount is expanded through the last stage of the turbineto reach 0.09 bar (point 5). The condensation occurs in the seawa-ter condenser (TC). The obtained liquid is transferred to the storagetank (CT).

Furthermore sufficient quantities of process water are injectedin points 10 and 13 for steam desuperheating. That permits todecrease the temperatures of MP and LP streams until 190 and165 �C respectively.

In the second cycle, about 82 t/h of HP steam is expandedthrough the steam turbine STGII. An extraction may be performedoptionally at medium pressure MP (point 15) according to workingcondition requirements. The remained stream is expanded to reachthe LP level.

The Turbo-blower is supplied by about 12 t/h of HP steam andused to provide the compressed air flow rate required for the sulfurcombustion process. From the Turbo blower the steam is con-densed in CTb and then transferred to the same storage tank CT.A Boiler MP is also used as additional energy source when animportant quantity of MP steam is required.

The MP and LP streams are used to supply the units (AC, CTb,CU, De, DU, SMM and TC).

The condensate streams issued from all the indicated units aretransferred to the storage tank (CT) and then the deaerator (De). Awater treatment is then performed in the purpose to supply theboilers.

Temperatures, pressures and mass flow rates are measured inthe inlet and outlet of each component by appropriate sensors.

The ranges of the operating parameters are indicated on Table 1.

3. System analysis

Energy and exergy analysis of power plant are conducted con-sidering the following assumptions [28]:

– All process are assumed as steady-state and steady flow.– The kinetic, potential and chemical exergy are neglected.– The dead state was considered as P0 = 1.013 bar andT0 = 293.15 K.

– No chemical reaction is occurred in the different processes.

3.1. Mass and energy balances

Energy analysis is based on the first law of thermodynamics,which is related to the conservation of energy [29].

25

10

26 28P1 P2

2 1

5

8

P4

4

6

7

3

27

P3

20

P6

22

17

Y

P5

P8

15

36

Boiler MP

35

30

16

14

29

HP Steam

LP Steam MP Steam

EBPS

TC AC

Y

STG II

De

P7

31

32 34 37

44

45

24

a

g

Sea water

DU

21

38

STG I

CT

11

12

33

9

13

23

19 18

SMM

c/e

d/f

Coupling with PAP

CTb CU

24’

25’

ST

Y

39 40

41 43

P9

b

h

To PAP 3

Tb

WTb

Bl

42

EBPS: Evaporator_Boiler_Pre-superheater_Superheater, STGI/II: Steam Turbine Generator I/II, Tb: Turbine, SMM: Sulfur Melting and

Maintenance, CTb:Turbo-blower Condenser, DU: Distillation Unit, CU: Concentration Unit, TC: Turbine Condenser, CT: Condensate Tank,

De: Deaerator, AC: Additional Condenser, Bl: Blower, ST: Storage Tank

Fig. 1. Schematic flow diagram of steam turbine power plant.

Table 1Operating parameter ranges.

Operating parameters Range/value

HP steam temperature 385–400 �CHP steam pressure 37–39 barMP steam temperature 187–190 �CMP steam pressure 11.5–12 barLP steam temperature 165 �CLP steam pressure 5.7–6 barHP steam mass flow rate 179 t/hMP steam mass flow rate 18 t/hLP steam mass flow rate 135 t/hSeawater temperature 15–32 �CSeawater salinity 0.039 kg/kgTotal Net power generated 14 MW

1234 F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241

For an open system the mass and energy balances are given byX_min ¼

X_mout ð1Þ

_Q � _W ¼X

_mouthout �X

_minhin ð2Þ

3.2. Energy and exergy analysis

The exergy of a system is the maximum work obtainable asthe system comes to equilibrium with the surroundings. Exergyanalysis predicts the thermodynamic performance of an energysystem and the efficiency of the system components by accuratelyquantifying the entropy generation of the components [30]. Itpermits to identify the causes, locations and magnitudes ofinefficiencies process [31].

For an open system and taking into account the indicatedassumptions, the exergetic balance can be expressed as flow:

_Xheat � _W ¼X

_mouteout �X

_minein þ _ED ð3Þ

where the exergy transferred by heat is given by : _Xheat

¼X

1� T0

T

� �_Q ð4Þ

and the specific exergy is showed as : ei¼ ðhi � h0Þ � T0ðsi � s0Þ ð5ÞThen the governing equations including energy and exergy bal-

ances for each component of the cycle showed in Fig. 1 areexpressed as follows:

(a) HP steam production

The HP steam used in thermal power plant of phosphoric acidfactory is provided from the Evaporator Boiler Preheater andSuperheater unit (EBPS). A connection with HP steam network ofother factory is provided.

From EBPS unit

Energy supply : _QEBPS ¼ _m1ðh1 � h45Þ ð6Þ

Exergy flux : _EEBPS ¼ _m1e1 ð7ÞCoupling with PAP III

Energy supply : _Qcp ¼ _m2h2 ð8Þ

Exergy flux : _Ecp ¼ _m2e2 ð9Þ(b) Steam Turbine Generator I (STGI) [4,8]

Energy balance : _WSTGI ¼ _m3ðh3 � h4Þ þ ð _m3 � _m4Þðh4 � h5Þð10Þ

F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241 1235

Exergy balance : _WSTGI ¼ _m3ðe3�e4Þþð _m3� _m4Þðe4�e5Þ� _ED;STGI

ð11Þ

Exergydestruction : _ED;STGI¼ _m3ðe3�e4Þþð _m3� _m4Þðe4�e5Þ� _WSTGI

ð12Þ

Energy efficiency : ge;STGI ¼_WSTGI

_m3h3 � _m4h4 � _m5h5ð13Þ

Exergy efficiency : gex;STGI ¼_WSTGI

_m3ðe3 � e4Þ þ ð _m3 � _m4Þðe4 � e5Þð14Þ

(c) Steam Turbine Generator II (STGII) [4,8]Energy balance : _WSTGII ¼ _m14ðh14 � h15Þ þ ð _m14 � _m15Þðh15 � h16Þ

ð15Þ

Exergy balance : _WSTGII ¼ _m14ðe14�e15Þþð _m14� _m15Þðe15�e16Þ� _ED;STGII

ð16Þ

Exergy destruction : _ED;STGII ¼ _m14ðe14�e15Þþð _m14� _m15Þðe15�e16Þ� _WSTGII

ð17Þ

Energy efficiency : ge;STGII ¼_WSTGII

_m14h14 � _m15h15 � _m16h16ð18Þ

Exergy efficiency : gex;STGII¼_WSTGII

_m14ðe14�e15Þþð _m14� _m15Þðe15�e16Þð19Þ

(d) Turbine Condenser (TC) [4,8]The energy balance of the turbine condenser gives the heat

transferred to the cooling fluid and the exergy balance gives theexergy destruction as:

Energy balance : 0¼ _m5ðh5 � h29Þ þ _m32ðh32 � h33Þ � Energy loss

ð20Þ

Exergy balance : 0 ¼ _m5ðe5 � e29Þ þ _m32ðe32 � e33Þ � _ED;TC ð21Þ

Exergy destruction : _ED;TC ¼ _m5ðe5 � e29Þ þ _m32ðe32 � e33Þ ð22Þ

Energy efficiency : ge;TC ¼ _m32ðh33 � h32Þ_m5ðh5 � h29Þ ð23Þ

Exergy efficiency : gex;TC ¼ _m32ðe33 � e32Þ_m5ðe5 � e29Þ ð24Þ

CondenserSeawater Outlet

Seawater Intlet

Condensate

35 36

26

7

6

b

a

WSTGear

Air Outlet

Air Intlet

BlowerSteam Turbine

HP Steam

Wbl

Fig. 2. Turbo-blower unit.

(e) Turbo-blowerThe turbo-blower is used to supply the drying tower by ambient

air. This group is constituted by steam turbine, Gear and blower aspresented in Fig. 2. The steam turbine converts the thermal energyto mechanical torque used to drive the gear and then the blowershaft. The steam at the turbine outlet is transferred into the seawa-ter condenser. Energy and exergy balance in each component ofthe turbo-blower can be expressed as follows.

Steam Turbinek

Energy balance : _WST ¼ _m6ðh6 � h7Þ ð25Þ

Exergy balance : _WTb ¼ _m6ðe6 � e7Þ � _ED;Tb ð26Þ

Exergy destruction : _ED;Tb ¼ _m6ðe6 � e7Þ � _WTb ð27Þ

Energy efficiency : ge;Tb ¼_WTb

_m6ðh6 � h7Þ ð28Þ

Exergy efficiency : gex;Tb ¼_WTb

_m6ðe6 � e7Þ ð29Þ

Blower

Energy balance : _WBl ¼ _WTbgGe ¼ _mairðhb � haÞ ð30Þ

Exergy balance : _WBl ¼ _mairðea � ebÞ � _ED;Bl ð31Þ

Exergy destruction : _ED;Bl ¼ _mairðea � ebÞ � _WBl ð32ÞThe humid air at the blower inlet is a mixture of dry air and

water vapor; the total exergy is expressed as [32]:

eha ¼ ðcpda þxcpvÞT0TT0

� 1� lnTT0

� �þ ð1þ ~xÞ RdaT0 ln

PP0

� �

þ RdaT0 ð1þ ~xÞ ln 1þ ~x0

1þ ~xþ ~x ln

~x~x0

� �ð33Þ

with ~x ¼ 1:608x.Through the Turbo-Blower, the humidity ratio is considered as

constant. Consequently the above equation becomes:

eha ¼ðcpdaþxcpvÞT0TT0

�1� lnTT0

� �þð1þ ~xÞ RdaT0 ln

PP0

� �ð34Þ

(f) Condenser of Turbo-blower [4,8]Energy balance : 0¼ _m7ðh7 � h26Þ þ _m35ðh35 � h36Þ � Energy loss

ð35Þ

Exergy balance : 0¼ _m7ðe7 � e26Þ þ _m35ðe35 � e36Þ � _ED;CTb ð36Þ

Exergy destruction : _ED;CTb ¼ _m7ðe7 � e26Þ þ _m35ðe35 � e36Þ ð37Þ

Energy efficiency : ge;CTb ¼ _m35ðh36 � h35Þ_m7ðh7 � h26Þ ð38Þ

Exergy efficiency : gex;CTb ¼ _m35ðe36 � e35Þ_m7ðe7 � e26Þ ð39Þ

(g) For expansion valveEnergy balance : _m8h8 ¼ _m9h9 ð40Þ

Exergy destruction : _ED;val ¼ _m8ðe8 � e9Þ¼ _m8½ðh8 � h9Þ � T0ðs8 � s9Þ�

ð41Þ

(h) Deaerator [12,17]The condensate streams from the different units are transferred

into the deaerator where oxygen O2 is extracted by LP steam

1236 F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241

injection. Obtained treated water will be used to feed boiler.The energy and exergy balances are established according to thefollowing equations:

Energy balance : 0¼ _m20h20þ _m23h23þ _m30h30þ _m41h41� _m42h42� _m43h43

ð42Þ

Exergy balance : _ED;De ¼ _m20e20þ _m23e23þ _m30e30þ _m41e41� _m42e42� _m43e43ð43Þ

Energy efficiency : ge;De ¼_m42h42 þ _m43h43

ð _m20h20 þ _m23h23 þ _m30h30 þ _m41h41Þð44Þ

Exergy efficiency : gex;De ¼_m42e42 þ _m43e43

ð _m20e20 þ _m23e23 þ _m30e30 þ _m41e41Þð45Þ

In the other hand the amount of LP steam used to feed thedesalination unit (stream 23, Fig. 1) is considered, for the presentstudy, as useful energy. An extended exergetic analysis of thedesalination unit will be performed in future investigations.

(i) Pumps

For each pump used in the different units of the steam turbinepower plant, the energy and exergy balances can be expressed as[4,8]:

Energy Balance : _WPm ¼ _mPmðhout � hinÞ ð46Þ

Exergy Balance : _ED;Pm ¼ _Ein � _Eout þ _WPm ð47Þ

Energy efficiency : ge;Pm ¼_Ein � _Eout

_WPm

ð48Þ

Exergy efficiency : gex;Pm ¼_Ein � _Eout

_WPm

ð49Þ

(j) Sulfur melting and maintenance unit (SMM)The melting of the Solid Sulfur is occurred in the melter using

MP steam as shown in Fig. 3. Obtained liquid is transferred by grav-ity to the pit. After filtration the sulfur liquid is stored and main-tained at suitable temperature in an appropriate tank equippedby a heating coil.

Melter

Energy balance : 0¼ _m18ðh18 � h180 Þ þ _meðhe � he0 Þ � Energy loss

ð50Þ

Exergy balance : 0 ¼ _m18ðe18 � e180 Þ þ _mgðeg � eg0 Þ � _ED;Ml ð51ÞStorage tank

Energy balance : ð0¼ _m19ðh19�h190 Þþ _mhðhg00 �hhÞþ _WP�Energy loss

ð52Þ

Exergy balance : 0 ¼ _m15ðe15 � e150 Þ þ _mdðec00 � edÞ þ _WP � _ED;Stsu

ð53Þ(k) Phosphoric acid Concentration Unit (CU)This unit is used to obtain phosphoric acid with 54% P2O5 from

the phosphoric acid with 28% P2O5 by evaporation. The main com-ponents of this unit are: boiler, condenser, heat exchanger and bas-ket filter.

The acid loop (Boiler, Filter and GBHE) is initially filled by phos-phoric acid 54% P2O5. Then an adequate quantity of phosphoric

acid 28% P2O5 is injected in the circuit through the filter in orderto prevent the solid crusts. After that the filtered acid is evacuatedby circulation pump to the Graphitic Block Heat Exchanger (GBHE)where it is heated by LP steam. Both streams, LP steam and phos-phoric acid, circulate in cross flow through the GBHE as indicatedin Fig. 4. Then the heated acid is evacuated to the boiler operatingunder vacuum in the order to reduce the acid evaporation temper-ature. The vacuum in the boiler is created by seawater jet permit-ting the condensation of steam and gas from the boiler by directcontact. A concentrate phosphoric acid 54% P2O5 is so obtained inthe boiler that leaves gravitationally by overflow to be stored inappropriate tanks.

In the concentration unit the energy supply is only used in theGBHE. For this study the exegetic analysis is exclusively performedon the vapor streams. Consequently the energy and exergy bal-ances are established for the graphitic block heat exchanger asfollows:

Energy balance : 0 ¼ _m24ðh24 � h25Þ þ _mcðhc � hdÞ � Energy loss

ð54Þ

Exergy balance : 0¼ _m24ðe24 � e25Þ � _mcðec � edÞ � _ED;GBHE ð55Þ

Energy efficiency : ge;GBHE ¼_mcðhd � hcÞ

_m24ðh24 � h25Þ ð56Þ

Exergy efficiency : gex;GBHE ¼_mcðed � ecÞ

_m24ðe24 � e25Þ ð57Þ

4. Results and discussion

The thermal power plant was analyzed for real operating condi-tions during whole year. The main operating parameters are theturbine supply flow rate, HP steam temperature and pressure. Inthe other hand the seawater temperature varies sensibly for thedifferent seasons in the local region. That may affect the perfor-mances of power plant components supplied by seawater. Henceall the indicated parameters will be taken into consideration forthe following analytic study.

A numerical code is established using EES software to performthe calculations required for the thermal and exergy plant analysis.The values of the different operating parameters for real workingconditions are indicated in Table 2.

In the other hand, the exergy losses due to drains or leaks aredetermined according to mass balances established on the relevantcomponents. These exergy loss rates are indicated in the bottom ofTable 2.

The Exergy destruction rates of the different power plant com-ponents, determined for real operating conditions are presented inFig. 5. The minimum irreversibility rates are obtained for the con-densers (0.5 MW), the deaerators (0.4 MW) and the blower(1.5 MW) followed by the pumps and steam turbines. The heatexchangers present an irreversibility rate of about 5 MW. The max-imum of irreversibility rates is obtained for the melters due to theimportant heat losses during open melting process.

The energy and exergy efficiencies are shown in Fig. 6. The max-imum energy efficiency is obtained for the blower (98%) followedby the heat exchangers (97%), the turbine (93%), the deaerators Iand II (94–92%), and Steam Turbine Generators II and I (87–74%).The minimum energy efficiencies are obtained for the melters(30%) and the condensers (57–68%). The exergy efficiency obtainedfor the heat exchanger, the STGII, the deaerator II and the blower,are 88%, 74%, 72% and 66% respectively. The minimum exergy effi-ciencies are obtained for the melters (28%) and the condensers(24–27%).

18’

19

18

19’

g'’

h

g’ g

Filte

r

Storage Tank

MP Steam

Steam condensate

Solid Sulfur

Melter Pit Liquid sulfur

BP Steam

Steam condensate

Fig. 3. Sulfur melting and maintenance unit.

Ph. A 54% P2O5Ph. A 54% P2O5

Gas & Steam

Boiler

24 25

c

d

GB

HE

Ph. A 28% P2O5

Basket filter

Pump

Condensate LP Steam To sewers

Dire

ct

Con

dens

er

Seawater

Fig. 4. Diagram flow of Phosphoric Acid concentration unit.

F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241 1237

The variation of the power generated by the steam turbine gen-erator STGI according to HP steam mass flow rate is presented inFig. 7 for different value of condensate rate. The extraction massflow rate in STGI (see point 4, Fig. 1) is defined according to oper-ating conditions required for the desired production rates. Thataffects the generated net power of the turbine. In fact, the gener-ated power increases gradually with HP steam mass flow rate.For HP steam mass flow rate less than 35 t/h, the generated poweris relatively low and is not significantly affected by the condensaterate especially for 8, 12 and 20 t/h. While for HP steam flow rateabove 40 t/h the condensate rate affects sensibly the generatedpower. Indeed, in this range, increasing the condensate rate leadsto the enhancement of the net generated power. A maximum netpower of about 6 MW is obtained for 20 t/h of condensate massflow rate.

For the back pressure steam turbine STGII, the variation of thegenerated power according to HP steam mass flow rate is pre-sented in Fig. 8. The generated power increases linearly to achieveabout 7 MW for a mass flow rate of about 82 t/h.

The total power generated by the two Steam turbines is widelysufficient for the power plant requirements. The generated powerover than the plant supplies is transferred to the national electric-ity network.

The variation of the exergetic efficiency of steam turbine STGIaccording to HP steam flow rate is presented in Fig. 9 for differentvalues of condensate flow rate. The exergy efficiency increaseswith _mHP to reach maximum values of about 49%, 51%, 52% and54% for condensation flow rates of 8, 12, 18 and 20 t/h respectively.The optimum _mHP values leading to the indicated maximum exergyefficiencies are respectively 55, 46, 52 and 54 t/h.

For the back pressure steam turbine STGII, the variation of theexergetic efficiency power according to HP steam mass flow rateis presented in Fig. 10. The exergetic efficiency increases sensiblywithmHP to reach a maximum value of about 75.5% for a mass flowrate of about 73 t/h. That can be considered as an optimum valuefor the STGII supply.

Fig. 11 illustrates the variations of steam turbine generatorenergetic and exergetic efficiencies according to HP steam temper-ature. The efficiencies increase linearly with THP. For a rise of 30 �C,the energy efficiency increases of about 2% for STGI and 8.8% forSTGII. The exergy efficiency is also enhanced of about 1.61% forSTGI and 8.3% for STGII.

The effects of HP steam pressure on steam turbine generatorenergetic and exergetic efficiencies are presented in Fig. 12. Forthe explored High pressure range, the energy efficiency increasesof about 2.1% for STGI and 8.9% for STGII. While an improvementin exergy efficiency of about 2.4% and 7.5% are obtained for STGIand STGII respectively.

According to above results, one can see that for the variationranges of THP and PHP, practically the same increase of the energyefficiency is obtained for the two steam turbine generators. Whilethe exergy efficiencies of STGI and STGII are slightly affected by thevariations of HP steam temperature and pressure that agrees withthe difference in exergy destruction rates between the two steamturbine generators obtained for studied case as showns in Fig. 5.

As result of exergy efficiency increase, the irreversibility ratesthrough the steam turbine generators decrease with THP as shownin Fig. 13. The irreversibility rate of STGII is more affected by THPincrease. These results can be explained by the fact that irre-versibility decrease with temperature rise.

Table 2Thermodynamic and exergetic data of different streams for real operating conditions.

Stream Mass flow rate (kg/s) Temperature (�C) Pressure (bar) Enthalpy (kJ/kg) Entropy (kJ/kg K) Exergy (kW)

No. Material

1 Steam 34.17 397 38 3210 6.786 40,6992 Steam 15.55 389 39.65 3188 6.734 18,4263 Steam 18.05 386 37.5 3185 6.754 21,2264 Steam 12.5 220 5.64 2895 7.086 90315 Steam 5.55 43 0.09 2338 7.422 720.076 Steam 3.33 392 37.7 3199 6.772 39467 Steam 3.33 50 0.12 2542 7.937 601.98 Steam 0.84 386 37.5 3185 6.754 979.79 Steam 0.84 350 5.8 3166 7.562 763.110 Water 1.39 27 6 113.7 0.395 0.733411 Steam 3.34 392 37.7 3199 6.772 394612 Steam 3.34 360 12 3186 7.579 310413 Water 1.66 27 12.5 114.3 0.395 1.96614 Steam 22.77 392 37.7 3199 6.772 26,96715 Steam 0 0 0 0 0 016 Steam 22.77 250 5.7 2958 7.206 18,53617 Steam N.O. N.O. N.O. N.O. N.O. N.O.18 Steam 3.88 190 12 2790 6.534 392019 Steam 3.34 165 5.7 2773 6.281 3015.3520 Water 7.22 99 1 417.4 1.302 168.821 Steam 1.39 165 5.7 2773 6.281 165422 Water 8.34 54 3 226.1 0.7543 48.7223 Water 5 49 2.8 205.2 0.6901 34.1524a Steam 6.95 120 1.1 2715 7.42 3542240 Steam 7.78 121 1.2 2716 7.383 404125a Water 6.95 97 0.92 406.2 1.272 219.1250 Water 7.78 96 0.9 402 1.261 239.126 Water 3.33 40 0.08 167.4 0.5719 4.90627 Steam N. O. N. O. N. O. N. O. N. O. N. O.28 Water N. O. N. O. N. O. N. O. N. O. N. O.29 Water 5.55 41 0.08 171.6 0.5852 9.3530 Steam 1.95 165 5.7 2773 6.819 144731 Water 521.41 27 1.013 113.2 0.3951 14,16232 Water 341.91 29 2.7 121.7 0.4228 171,10033 Water 341.91 33 1.5 138.3 0.4777 171,10034 Water 341.91 33 1.5 138.3 0.4777 171,10035 Water 151.01 29 2.7 121.7 0.4228 42,52636 Water 151.01 37 1.5 155 0.5318 159,78237 Water 28.49 29 2.7 121.7 0.4228 802438 Water 21.37 50 1.5 209.3 0.703 91.6439 Water 38.33 97 1.5 406.3 1.272 121240 Water 6.95 99 4 415 1.295 23341 Water 31.38 99 4 415 1.295 105242 Steam 0.55 104 1.1 2683 7.336 27843 Water 45.55 104 1.2 435.7 1.350 117844 Water N. O. N. O. N. O. N. O. N. O. N. O.45 Water 37.5 104 75 443.5 1.351 1697a Air 66.92 27 1.013 67.26 5.842 5.27b Air 66.92 61 1.331 245.9 6.331 1048ca Phosphoric Acid L.I 478.5 73 3.2 100 0.311 3482da Phosphoric Acid L.I 478.5 82 0.3 119.3 0.366 4849ea Phosphoric Acid L.II 739 72 3.3 97.88 0.304 5165fa Phosphoric Acid L.II 739 81 0.4 117.1 0.359 7232g Sulfur Solid 5.59 27 1.013 1.346 0.001 108,064h Sulfur Liquid 5.59 155 1.013 179.19 0.0014 29,983

Losses

Stream Mass flow rate (kg/s) Exergy losses (MW)

HP steam 1.39 1654.65MP steam 1.11 940.1LP steam 1.39 14.97

a Two heat exchangers, two melters.

1238 F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241

Fig. 14 depicts the variation of the condenser exergyefficiency according to seawater temperature. It can be seenthat increasing the seawater temperature from 12 to 24 �C leadsto an increase of the exergy efficiency of about 4 times for theturbo blower condenser and 14 times for the turbine condenser.For Tsw above 25 �C the exergy efficiency increases slightly with

Tsw to reach maximum values of about 35% and 45% for theturbine condenser and the turbo-blower condenser respectively.Although the indicated rise of the exergetic efficiency theobtained values are very low especially in cold seasons whenthe seawater temperature is less than 15 �C. These results agreewith Aljundi investigations [4] on energy and exergy analysis of

Fig. 5. Exergy destruction rates of thermal power plant components.

Fig. 6. Energetic and exergetic efficiency of main components.

20 25 30 35 40 45 50 55 60 65 701

1,52

2,53

3,54

4,55

5,56

6,57

HP (t/h)

Pow

er (

MW

)

18 t/h

8 t/h12 t/h

20 t/h

m

Fig. 7. Variation of STGI net power with HP steam flow rate.

40 45 50 55 60 65 70 75 80 853

4

5

6

7

8

mHP (t/h)

Pow

er (

MW

)

Fig. 8. Variation of STGII net power with HP steam flow rate.

20 25 30 35 40 45 50 55 60 65 700,2

0,3

0,4

0,5

0,6

mHP (t/h)

η ST

GI

20 t/h 18 t/h 12 t/h8 t/h

Fig. 9. Variation of exergetic efficiency of steam turbine STG I according to HPsteam flow rate.

40 45 50 55 60 65 70 75 80 850,64

0,68

0,72

0,76

0,8

mHP (t/h)

η ST

GII

Fig. 10. Variation of exergy efficiency of STGII according to HP steam flow rate.

F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241 1239

a steam power plant. In fact the authors obtained the samevalues of condenser exergy efficiency in similar operating condi-tions. Indeed if the Tsw increases the temperature difference

between the two streams decreases, therefore the irreversibilityrate due to temperature gradient IDT decreases. That improvessignificantly the exergetic efficiency.

375 380 385 390 395 400 405 410 4150,55

0,6

0,65

0,7

0,75

0,8

0,85

0,9

0,95

THP (°C)

η Tur

bine

ηe,STG I

ηe,STG II

ηex,STG II

ηex,STG I

Fig. 11. Energetic and exergetic efficiency variations according to THP.

36 37 38 39 40 41 420,6

0,65

0,7

0,75

0,8

0,85

0,9

0,95

PHP (°C)

η Tur

bine

ηe,STG II

ηex,STG II

ηe,STG I

ηex,STG I

Fig. 12. Energetic and exergetic efficiency variations according to PHP.

375 380 385 390 395 400 405 410 4151500

2000

2500

3000

3500

4000

4500

THP (°C)

IrTu

rbin

e (k

W)

IrSTGI

IrSTGII

Fig. 13. Influence of HP steam temperature variation on steam turbineirreversibility.

10 15 20 25 30 350

0,1

0,2

0,3

0,4

0,5

Tsw (°C)

η Con

dens

er

ηcdTb

ηcdT

Fig. 14. Variation of exergy efficiency of condensers according to seawatertemperature.

1240 F. Hafdhi et al. / Energy Conversion and Management 106 (2015) 1230–1241

5. Conclusion

An Energetic and Exergetic Analysis is conducted on a SteamTurbine Power Plant used in existing Phosphoric Acid Factory.The heat recovery systems used in the different parts of the plantare also considered in the analysis. Mass, thermal and exergy

balances are established on the main compounds of the factory.The effects of the key operating parameters such as seawatertemperature, and mass flow rate on the cycle performances areinvestigated. The obtained results can be presented as follows.

For the explored ranges of HP steam temperature and pressure,the same increase of the energy efficiency is obtained for the twosteam turbine generators. While the exergy efficiencies of STGIand STGII are slightly affected by THP and PHP variations. Aboutsteam mass flow rate effect, obtained results show that for STGIand considering condensation mass flow rates of 8, 12, 18 and20 t/h, the optimum HP steam folw rate values leading to themaximum exergy effeciencies are respectively, 55, 46, 52 and54 t/h. While for STGII a maximum exergetic efficiency of about75.5% is obtained for _mHP of 73 t/h.

The seawater temperature affects significantly the exergy effi-ciency of the condensers. That should by taking into considerationfor the operating condition in cold seasons.

The obtain results constitute helpful tools to analyze the realperformances of industrial plants and permit to better undertakethe future perfections that can be carried out on the differentstreams in order to improve the efficiency and reduce the energeticlosses.

Acknowledgments

This work is conducted with cooperation of Tunisian ChemicalGroup. The authors would like to thank the collaborators for theirsupport and the interest given to this work.

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