Exergy analysis of Rankine cycle power plantcomponentSaghi T.Khabbaz1& John Shobin1Halmstad University, [email protected]

se Abstract: considering the strategicimportance of hydrocarbon resources, accurate management of them, all the wayfrom the production to consumption process, has attracted a lot of attentionduring the past decades. For this purpose, several studies were performed onengine thermodynamic cycles. In this study, exergy analysis of a Rankine cyclefor Shahin Shahr refinery is discussed. As a matter of fact, exergy analysisutilized to find efficiency of the system helps to improvise the power plantperformance. The power plant consists of four turbines, four boilers,condenser, and deaerator. The primary objective of this paper is to analyze theexergy and energy destruction of different components in order to find whichcomponents are significant to find exergy efficiency. The second objective ofthis study is to find exergy efficiency of whole plant and more significantdestructive exergy component.Keywords: Exergy, Rankin cycleand efficiency1.

IntroductionInthe past many years, with rising energy prices and environmental damage, aswell as the limited resources of non-renewable energy sources, the use ofhigh-efficiency energy production and conversion systems has becomeincreasingly important to save on these valuable resources.Theexergy analysis combined with the first and second law of thermodynamics allowsthis makes comprehensive method for analyzing energy change systems, as well asrecognizing energy levels and undesirable thermodynamic processes in energysystems. The calculation is a useful tool for seeing the big difference betweenenergy reductions with internal irreversibility in a process. Exergy analysis isan appropriate method for measuring the function of the process components. Theexergy analysis of the different component in the realistic plant is thehighlight of this paper. In addition, that efficiency of the cyclic componentlike boiler, turbine etc also calculated. The results emphasize that exergyanalysis the efficiency optimization of individual components. The results suggest that; boileris a more significant component for exergy destruction.

In addition, thecalculations indicate that exergy losses occur at chimney and during thecombustion. Theexergy analysis of the cycle shows that the energy consumed in the condenser isnot thermodynamic in high quality, and the main share of exergy losses in theboiler is that of 52.7% of the total exergy input to the cycle.2. Materials and MethodsTheexergy and efficiency of each Rankin cycle components were carried out in thissection 2.

1.The cycle of power plant generation: In this product, we conduct quantitateanalysis of “Shahin Shahr” refinery. The plant is running according to thefollowing layout. the majorprocesses are:a) process1-2: the 700 ton/hour water in 2.7 barpressure with the temperature of 125°C is pumped and the pressure increases to62 bar in point 2.b) Process2-3: after exiting the boiler, the temperature reaches 400°C and the pressureis 45 bar. then the steam leads to steam transportation line with the pressureof 40 bar.c) Process3-4: in this step, the 165 ton/hour mas flow steam enters to turbines.

d) Process4-5: at point 5, the saturated steam with a quality of 90%, the temperature of50°C and 12.35 kPa goes into condensers.e) Process 5-6: the fluid goes out fromcondensers with 350 ton/hour mass flow, 12 kPa pressure and 60°C temperature. 3. Results 3.

1.Chemicalexergy for external gases from chimneyFirstof all, we need to determine the chemical exergy of thermos mechanical and thencalculate the chemical exergy of the gases from the fuels. Table 1 demonstrates thenatural gases which are used in the boiler. The relative humidity of air is60%. CH4 47.5% C2H6 8.

2% C3H8 8.6% C4H10 3.7% C5H12 0.8% H2 28% N2 3.2% TOTAL 100% Table 1: Themolar percentage of natural gases in boiler The balanced equation of fuel is definedbelow: (47.5%CH4 + 8.2% C2H6+ 8.

6% C3H8+3.7% C4H10+ 0.8 C5H12+ 28% H2+3.2 N2 ) + 2.933 ( O2+ 3.7 N2+ 0.

09 H2O) 1.085 CO2+ 2.323 H2O+ 11.27 N2+0.

878 O2 Thermomechanicalexergy of combustion gases at the chimney temperature is introduced below: = molar percentage of gases which areproduced by combustion= enthalpy of each gas in temperature ofT= entropy of each component of combustiongases Thechemical exergy of gases which are produced by combustion is calculated by nextformula. = ambient temperature= partial pressure of the components inlimited die state= partial pressure in ambient condition Thetotal exergy of external gases from the chimney is determined by sum of boththermomechanical exergy and chemical exergy for the released gases. 3.2.Chemicalexergy for natural gases Thechemical exergy of natural gases is achieved by the distribution percentage ofgases which is mentioned in Table 1. 3.3.Exergyof entrance airBecauseof entrance air does not heat before the air goes into the cycle, so thisexergy is equal zero.

3.4.Exergyof boiler Boilertransfer the fuel chemical exergy with the efficiency of 0.385 to the steam.

Most part of this transfer lost which are included exergy loss in exiting gasesfrom the chimney, exergy loss in combustion and transferring the heat. 3.5.Exergyloss in the chimneyThisexergy loss is obtained by the total exergy of thermomechanical exergy andchemical exergy which has been done in previous parts.Theproportional of exergy loss in combustion gases and exergy of entrance fuel iscalculated below: Therefore,the exergy loss from chimney is 3.6.

Exergyloss in combustion and transferring the heatAccordingto the first law of thermodynamic, the efficiency of the boiler is calculatedby next formula: = the produced steam flow by the boiler= the consumption fuel flow by the boiler= Low Heat Value for natural gas Thesecond law of thermodynamic efficiency formula is written below. = the exergy changes of steam in entranceand exiting from the boiler= the chemical exergy of fuel Hence,the proportion of total exergy loss in the boiler to the chemical exergy offuel is . So the irreversible exergy to the boileris 81421.08 kw and in the combustion step is 71622 kw. 3.6.1.

Turbine exergycalculationThe turbine exergy efficiency is 76%. The inlet exergy tothe turbine is: andthe outlet work of the turbine is so, the irreversible exergy of the turbine is defined bythe following formula. besides,the exergy loss in the turbine is 3.6.2. CondensersBy using the thermodynamically conditions, the inletexergy to the condenser is calculated by next formula.

theinlet exergy in condenser transfers to the cooling towel, besides this value isnot reversible to the cycle. Therefore, the irreversible amount of thecondenser is calculated by multiplying the mass flow and the inlet exergy of thecondenser. 3.6.3. Energyand exergy calculation of whole cycle The energy which is given to fluid is In addition, the energy which is given to the fluid by theboiler is represented by .So, the total energy is given to the fluid is calculatedby the sum of and .

the energy efficiency of electricity production by cycleis determined by the produced work and the total energy of the fluid. Respectively,the exergy efficiency is calculated by dividing the produced work to the exergyof the source. 3.

7.Exergyanalysis The exergy analysis of steam generator,turbine and condenser are shown in the below graph in which the percentage ofexergy destruction highest for condenser followed by steam generator andfinally turbine. 4. Conclusion · Exergyand efficiency analysis of Rankin cycle were performed.· Thepercentage of exergy destruction of condenser is higher than other components.· Theexergy destruction of turbine is lesser than any other component.· Theexergy destruction of chimney is · Thechemical exergy destruction of natural gases 5.

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