Report.docx

ABSTRACT

In view of the present significance of the power crisis we endeavored to interpret this

Combined Cycle Power Plant. In this project we aimed to minimize the energy and exergy

losses. We have also briefed about the sophisticated and state of the art facilities such as Heat

Recovery Steam Generator in Combined Cycle Power Plant. We have chosen Octane and

Methane as fuels and plots have been drawn by varying input pressure and temperature

conditions. We have also embraced the performance of Combined Cycle Power Plant with

HRSG and without HRSG.

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CHAPTER 1

1.1 AIM

1.2 INTRODUCTION

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1.1 AIM:

To analyze the energy and exergy losses of combined cycle power plant with heat

recovery steam generator by Octane and Methane as fuels and to draw the variation of overall

efficiency for different temperature and pressure conditions.

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AIM

Octane as fuel

Energy& Exergy losses

calculation

Methane as fuel

1.2 INTRODUCTION:

Rapid degradation of fuel resources questions the survival of human being on the earth.

Electricity is the greatest form of energy which is been interpreted in the lives of human.

Electricity is the only form of energy which is easy to produce, easy to transport, easy to use and

easy to control.

This is the only energy which can be easy for generation, easy for transmission and easy for

distribution. Electricity consumption per capita is the living standard of people of country.

Thermal power plants generate more than 80% of total electricity production in the world.

In order to satisfy the growing energy needs, a new technology combined cycle has been evolved

as the big source of power. Combined Cycle power plants are gaining wider acceptance due to

more and more availability of natural gas, now a days, because of their higher overall thermal

efficiencies, peaking two-shifting capabilities, fast start-up capabilities and lesser cooling water

requirements. The principle of combined cycle power plant is that the exhaust of one Heat engine

is used as the heat source for another, thus extracting more useful energy from the heat,

increasing the overall efficiency. In Gas turbine we need approximately 12000C as heat required

to generate the power. In this temperature of exhaust gases is 5000C. That is high amount of heat

is wasted through exhaust gases. We take this disadvantage as advantage by supplying exhaust

gases heat as input to the steam turbine.

Heat Recovery Steam Generator is used to recover exhaust heat from the gas turbines and to

generate superheated steam which operates Rankine cycle.

Gas turbines can use a variety of liquid and gaseous fuels (Conventional or non conventional

type). Conventional fuels, which are used regularly at gas turbine installations, are various grades

of oils, ranging from light petroleum naphtha to residual fuels. In this project we chose Octane

and Methane of Paraffin family as fuels. We have analyzed the energy and exergy losses of

CCPP by varying pressure and temperature conditions.

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CHAPTER 2

2.1 RANKINE CYCLE

2.2 BRAYTON CYCLE

2.3 COMBINED CYCLE OF RANKINE CYCLE & BRAYTON CYCLE

2.4 HEAT RECOVERY STEAM GENERATOR

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2.1. RANKINE CYCLE:

Saturated or superheated steam enters the turbine at state 1, where it expands isentropically

to the exit pressure at state 2. The steam is then condensed at constant pressure and temperature

to a saturated liquid, state 3. The heat removed from

the steam in the condenser is typically transferred to the cooling water. The saturated liquid then

flows through the pump which increases the pressure to the boiler pressure (state 4), where the

water is first heated to the saturation temperature, boiled and typically superheated to state 1.

Fig 2.1Circuit layout of diagram of rankine cycle

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2.1.1. Steam power plant:

Power plants generate electrical power by using fuels like coal, oil or natural gas. A

simple power plant consists of a boiler, turbine, condenser and a pump. Fuel, burned in the boiler

and super heater, heats the water to generate steam. The steam is then heated to a superheated

state in the super heater. This steam is used to rotate the turbine which powers the generator.

Electrical energy is generated when the generator windings rotate in a strong magnetic field.

After the steam leaves the turbine it is cooled to its liquid state in the condenser. The liquid is

pressurized by the pump prior to going back to the boiler. Since the fluid undergoes a cyclic

process, there will be no net change in its internal energy over the cycle, and consequently the

net energy transferred to the unit mass of the fluid as heat during the cycle must equal the net

energy transfer as work from the fluid.

Fig 2.1.1 layout diagram of steam power plants

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An important application of thermodynamics is the analysis of power cycles through which the

energy absorbed as heat can be continuously converted into mechanical work. A thermodynamic

analysis of the heat engine cycles provides valuable information regarding the design of new

cycles or for improving the existing cycles.

2.2. BRAYTON CYCLE:-

fig 2.2 idealized Brayton cycle

Brayton cycle is the air standard cycle for the gas turbine power plant. Here air is first

compressed reversibly and adiabatically, heat is added to it reversibly at constant pressure, air

expands in the turbine reversibly and adiabatically, and heat is then rejected from the air

reversibly at constant pressure to bring it to the initial state. The Brayton cycle consists of :

TWO REVERSIBLE ISOBARS

TWO REVERSIBLE ADIABATICS

ŋ = 1 – ( Q1/Q2 ) (or) ŋbrayton = 1 – 1 / ( rp )^(γ-1/γ) (5)

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The efficiency of the Brayton cycle, therefore, depends upon either the compression ratio or the

pressure ratio. For the same compression ratio the Brayton cycle efficiency is equal to the Otto

cycle efficiency.

2.2.1. Gas turbine power plant:

A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It

has an upstream rotating compressor coupled to a downstream turbine, and a combustion

chamber in-between.

Energy is added to the gas stream in the combustor, where fuel is mixed

with air and ignited. In the high pressure environment of the combustor, combustion of the fuel

increases the temperature. The products of the combustion are forced into the turbine section.

There, the high velocity and volume of the gas flow is directed through a nozzle over the

turbine's blades, spinning the turbine which powers the compressor and, for some turbines,

drives their mechanical output. The energy given up to the turbine comes from the reduction in

the temperature and pressure of the exhaust gas.

Energy can be extracted in the form of shaft power, compressed air or thrust or any combination

of these and used to power aircraft, trains, ships, generators, or even tanks.

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http://en.wikipedia.org/wiki/Tank

http://en.wikipedia.org/wiki/Electrical_generator

http://en.wikipedia.org/wiki/Ship

http://en.wikipedia.org/wiki/Train

http://en.wikipedia.org/wiki/Aircraft

http://en.wikipedia.org/wiki/Nozzle

http://en.wikipedia.org/wiki/Volume

http://en.wikipedia.org/wiki/Velocity

http://en.wikipedia.org/wiki/Temperature

http://en.wikipedia.org/wiki/Ignition_system

http://en.wikipedia.org/wiki/Earth's_atmosphere

http://en.wikipedia.org/wiki/Combustion_chamber

http://en.wikipedia.org/wiki/Combustion_chamber

http://en.wikipedia.org/wiki/Turbine

http://en.wikipedia.org/wiki/Gas_compressor

http://en.wikipedia.org/wiki/Internal_combustion_engine

fig 2.2.1 Layout diagram for Gas turbine power plant

2.3. COMBINED CYCLE POWER PLANT:

Combined Cycle power plants are those which have both gas and Steam turbines

supplying power to the network. Combined cycle power plants employ more than one

thermodynamic cycle – Rankine (steam) and Brayton (gas). In a combined cycle power plant, a

gas turbine generator generates electricity and the waste heat is used to make steam to generate

additional electricity through a steam turbine, which enhances the efficiency of electricity

generation. Additionally, combined cycles are characterized by flexibility, quick part-load

starting, suitability for both base-load and cyclic operation, and a high efficiency over a wide

range of loads.

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Fig 2.3.Layout diagram for combined cycle power plant

Combined Cycle Power Plant assets need to be flexible to meet rapidly fluctuating demand

levels. As well, they need to remain reliable and demonstrate that every effort has been made to

minimize environmental impacts and maximize efficiency. Ensuring flexible, reliable operation

with minimum forced outages, implementing innovative strategies that reduce emissions and

dealing with volatile power markets while achieving the lowest operating costs possible are the

new industry reality.

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2.4 HRSG (HEAT RECOVERY STEAM GENERATOR):

Fig 2.4.Combined cycle power plant with HRSG

The HRSG is designed to extract maximum recoverable heat from the exhaust gas of

the gas turbine. For this purpose the exhaust gas flow from the gas turbine is arranged in a

direction counter to the water/ steam circuit of HRSG. The exhaust gas from the gas turbine

enters HP ,IP & LP section of the Boiler .All the three section include the Secondary and primary

superheaters evaporators & Economizers., & finally through CPH module before exhausted to

the atmosphere by the stack.

The steam drum placed above the evaporators serves as a balancing vessel for water

and steam. It receives feedwater from the Economizer and maintains positive water supply to the

evaporator modules. Drum receives the mixture of steam and water from the evaporator modules

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by the heat transfer. After separating water from the steam / water mixture at drum, the saturated

steam is supplied to the main steam line through Superheaters.

2.4.1 Scope of HRSG:

The heat recovery steam generator (HRSG) comes in numerous shapes, designs, configurations,

arrangements, etc. The HRSG here we are discussing is water tube (as opposed to a fired tube)

type heat recovery unit. This refers to the process fluid that is the steam or water being on inside

of the tube with the products of combustion being on the outside of the tube. The products of

combustion are normally at or close to atmospheric pressure, the shell side is generally not

considered to be a pressure vessel.

Fig 2.5.HRSG

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2.4.2. Evaporator section:

Evaporation of pre-treated feed water (w) by motive steam(m) bled from the turbine is

often used where the boiler make-up water requirement is not large. Both evaporated water (v)

and condensed steam (c) exiting the evaporator are fed back as make-up in to the plant.

2.4.3. Super heater section:

The super heater section of the HRSG is used to dry the saturated vapor being separated

in the steam drum. In some units it may be heated to little above the saturation point where in

other units may be significant temperature for additional energy storage. The super heater section

is normally located in the hotter gas steam, in front of the evaporator.

2.4.4. Economizer section:

The economizer section, sometimes called a pre heated or preheat coil, is used to preheat

the feed water being introduced to the system to replace the steam(vapor)being removed from the

steam via the super heater or a stream outlet and the water loss through the blow down. It is

normally located in the colder gas downstream of the evaporator. Since the evaporator inlet and

outlet temperatures are both close to the saturation temperature for the system pressure, the

amount of heat that may be removed from the flue gas is limited due to approach to the

evaporator, known as pinch point which is discussed later, where as the economizer inlet

temperature is low allowing the flue gas temperature to be taken lower.

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CHAPTER 3

3.1 EXERGY

3.2 OCTANE (C8H18) AS FUEL

3.3 METHANE (CH4) AS FUEL

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3.1 EXERGY

The First law of Thermodynamics makes only quantitative analysis of system. It doesn’t

discriminate quality wise. It is the Second law which discriminate the quality of energy. The

exergy of a system is the maximum work obtainable as the system comes to equilibrium with the

surroundings (i.e. dead state). The higher the value of exergy, more is the work obtainable from

the system. This exergy is a composite property depending on the state of system and

surroundings. When the system is in thermal equilibrium with its surroundings is said to be at

dead state having zero exergy.

We know that energy can be conserved. But what is not conserved is exergy. Once the exergy is

wasted it can never be recovered. When we use electricity we are not destroying any energy, we

are converting it to less useful form that is from high exergy value to lower exergy value.

ENERGYin – ENERGYout = 0

EXERGYin – EXERGYout = EXERGY destroyed

ENERGY OF ISOLATED SYSTEM NEVER CHANGE,

ENTROPY OF ISOLATED SYSTEM NEVER DECREASE,

EXERGY OF ISOLATED SYSTEM NEVER INCREASE.

Below table shows the different grades of energy

High grade energy Low grade energy

Electrical Energy Heat or Thermal Energy

Mechanical work Heat derived from nuclear fission

Water power Heat derived from fossil fuels

Wind power

Tidal power

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3.2 OCTANE (C8H18) AS FUEL:

Octane is a hydrocarbon of Paraffin family and an alkane with the chemical formula C8H18.

Octane has many structural isomers that differ by the amount and location of branching in the

carbon chain. Because of low-molecular weight hydrocarbons, octane and its isomers are very

flammable. Octane and its isomers are components of gasoline (petrol). Octane became well

known in American popular culture in the mid- and late-sixties, when gasoline companies

boasted of "high octane" levels in their gasoline advertisements. We have chosen Octane as a one

fuel to compare overall efficiency of the system.

The combustion reaction when Octane burned,

C8H18 + 12.5O2 8CO2 + 9H2O

(Reactants) (Products)

The properties of Octane are,

Calorific value of Octane: 44.43MJ/kg

Specific heat ratio of Octane: 1.33

Specific heat of Octane: 1.148 KJ/kg

Ratio of chemical exergy to enthalpy formation (Ѱ): 1.0725

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http://en.wikipedia.org/wiki/Gasoline

http://en.wikipedia.org/wiki/Gasoline

http://en.wikipedia.org/wiki/Structural_isomer

http://en.wikipedia.org/wiki/Chemical_formula

http://en.wikipedia.org/wiki/Alkane

3.2 METHANE (CH4) AS FUEL:

Generally, Methane gas is an ideal fuel for use in gas turbines because of its clean burning,

availability at a lower cost and practically free from solid residue. The Methane gas has high

calorific value relatively. Moreover Methane has little inherent sulfur content and is

environmentally acceptable because the resulting SO2 emissions are inherently low. This is the

cleanest of all fossil fuels. It is free from ash and mixes well with air to undergo complete

combustion producing very little smoke. It has high hydrogen content and produces as

considerable amount of water vapor when burned.

The combustion reaction when methane burned,

CH4 + 2O2 CO2 + 2H2O

(Reactants) (Products)

The properties of Methane are,

Calorific value of Methane: 55.50MJ/kg

Specific heat ratio of Methane: 1.32

Specific heat of Methane: 2.24 KJ/kg

Ratio of chemical exergy to enthalpy formation (Ѱ): 1.0977

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CHAPTER 4

4.1 CALCULATION PROCEDURE OF COMBINED CYCLE POWER PLANT WITHOUT HRSG

4.2 CALCULATIONS FOR COMBINED CYCLE POWER PLANT WITH HRSG BY OCTANE AS FUEL

4.3 CALCULATIONS FOR COMBINED CYCLE POWER PLANT WITH HRSG BY METHANE AS FUEL

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4.1. Calculation procedure of combined cycle power plant without HRSG:

INPUT PARAMETERS:

Gas turbine:

Pressure ratio of gas turbine : 7.5

Air inlet temperature : 15 0C

Maximum cycle temperature : 750 0C

Steam turbine:

Gas leaves the steam generator : 100 0C

Steam supplied to the turbine : 50 bar, 600 0C

Condenser pressure : 0.1 bar

Total power output of the plant : 200 MW

Calorific value of the fuel : 43.3 MJ/kg

Take for combustion gases Cpg= 1.11 kJ/kg K and γ= 1.33

For air Cpa= 1.005 kJ/kg K and γ= 1.4

Fig 4.1 Layout diagram for combined cycle power plant without HRSG

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Solution:

For the gas turbine cycle:

T ₂T ₁ = ( P₂

P₁)( γ−1

γ) = (7.5)(0.4/1.4)

T2 = 2880×(7.5)2/7 = 512.165 K

T ₃T ₄

= ( P₃P₄

)( γ−1

γ) = (7.5)(0.33/1.33)

T4 = 1023/(7.5)(0.33/1.33) = 620.52 K

For the steam cycle, from Mollier chart,

ha = 3670, hb = 2305, hc = hd = 192 kJ/kg

(Wnet)GT = ma Cpg (T3-T4) - ma Cpa (T2-T1)

= ma {1.11× (1023-620.52) - 1.005× (512.65 - 288)}

= 221.47 ma

(WT)ST = ms(ha - hb) = ms(3670 - 2305) = 1365 ms

1365 ms +221.47 ma = 200×103 -------- (1)

Now, ma Cpg (T5-T6) = ms (ha-hc)

ma 1.11 (1023-373) = ms (3670-192)

msma

= 715/3478 = 0.2056 ------------- (2)

Substituting in Equation (1),

1365 ×0.2056 ma + 221.47 ma = 2,00,000

ma = (2,00,000/500.26) = 398.34 kg/s

ms = 0.2056×398.34 = 81.9 kg/s

(Wnet)GT = 221.47 ma = 221.47×398.34 = 88.22 MW

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(WT)ST = 1365 ms = 1365×81.9 = 111.78 MW

Wnet = (Wnet)GT + (WT)ST

= 88.22 + 111.78 = 200 MW

Q1 = ma Cpg (T3 - T2 - T5 - T4)

= 398.34×1.11(1023 - 512.165 + 1023 - 620.52)

= 400.19 MW

Ƞthermal = WnetQin =

200400.19

= 50%

Power developed by Gas Turbine 88.22 MW

Power developed by Steam Turbine 111.78 MW

Mass flow rate of steam 81.9 kg/s

Mass flow rate of air 398.34 kg/s

Overall efficiency 50%

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4.2. Calculation for combined cycle power plant with HRSG by Octane as fuel:

INPUT PARAMETERS

Inlet condition of air to the compressor : 1 bar, 25 0C

Pressure ratio of compressor : 8

Maximum gas temperature at inlet to the gas turbine : 900 0C

Pressure drop in the combustion chamber : 3%

Efficiency of the compressor : 0.88

Efficiency of the gas turbine : 0.88

Calorific value of liquid octane (C8H18) used as fuel : 44.43 MJ/kg

Specific heat of air : 1.006 kJ/kg K

Specific heat of gas : 1.148 kJ/kg K

Specific heat ratio of gas : 1.333

Specific heat ratio of air : 1.4

Condition of steam at inlet to the steam turbine : 40 bar, 425 0C


Feed water temperature to the HRSG : 170.4 0C

Efficiency of the steam turbine : 0.82

Pressure drop of gas in the HRSG : 5 kpa

Steam flow rate : 29.235 kg/s

Assume Ѱ= (∆G0)/(∆H0)=1.0401+0.1728(hc)

Where (h/c) is the mass ratio of hydrogen to carbon in the fuel.

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Fig 4.2 Layout diagram for combined cycle power plant with HRSG

Solution:

Gas turbine plant:

P1= 1 bar, P2= 8 bar, T1= 298 K, Ƞc= 0.88

T ₂T ₁ = ( P₂

P₁)( γ−1

γȠc) = 80.4/(1.4×0.88) = 1.965

T2= 298×1.965= 586K = 313 0C

Combustor:

Pressure loss = 0.03×8 = 0.24 bar

P3 = 8 - 0.24 = 7.76 bar

Let the flow rate of combustion gas be 1 kg/s and that of fuel f kg/s.

The flow of air = (1 - f) kg/s

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Therefore,

f×CV = 1.Cpg (T3 - T1) - (1 - f) Cpa(T2 - T1)

f×44,430 = 1×1.148 (900 - 25) - (1 - f)×1.006 (313 - 25)

= 1004.5 - 289.7 (1 - f)

f =714.8/44140.7 = 0.0162 kg/s

Air fuel ratio = (1−f )

f= 0.9838/0.0162= 60.73

Now, C8H18 + 12.5O2 = 8CO2 + 9H2O

Air fuel ratio for stoichiometric combustion = (12.5×32)/ (0.232×114) = 15.12

Excess air = (60.73-15.12)/15.12= 3.02 or 302%

Gas turbine:

P4= 1+0.05= 1.05 bar, T3= 1173 K

T ₃T ₄

= ( P₃P₄

)(

(γ−1) Ƞtγ

) = (7.76/1.05)(0.333×0.88)/1.333

= 1.5530

T4 = 11731.553 = 755 K = 482 0C

This is the turbine exhaust gas temperature.

HRSG:

Let the pinch point temperature difference (T5 - Tf) be 30 0C,

Tf = (Tsat) 40 bar = 250.40C

T5 = 250.4+30 = 2800C

Now, from steam tables, ha = 3272 kJ/kg and hf = 1087 kJ/kg.

According to energy balance equation we have,

mg cpg (T4 - T5) = ms (ha – hf)

1×1.148(482 – 80) = ms (3272 – 1087)

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ms = 0.106 kg/s

The total heat transfer in the HRSG yields the stack temperature T6,

That is for 170.40C, he = 721 kJ/kg

1.148 (482 – T6) = 0.106(3272 – 721)

T6 = 2470C

Power output:

ha = 3272 kJ/kg

sa = 3.853 kJ/kg

sb1 = sf + xbs (sfg) = 0.4226 + xbs (8.052)

xbs = 0.7986

hb1 = hf + xbs (hfg) = 121.46 + 0.7986 (2432.9) = 2064.37 kJ/kg

By neglecting pump work, output of steam turbine is

WST = ms (ha – hb1) ȠT = 29.235(3272 – 2064)×0.82 = 28950 kW

Mass flow rate of gas in gas turbine

ma = 29.2350.106

= 275.8 kg/s

Air flow rate entering the compressor ma = (1- f) 275.8 = 0.9838×275.8 = 271.3 kg/s

Power output from the gas turbine WGT = 275.18×1.148 (900 – 482) – 271.3×1.006 (313-25)

= 53,744 kW

Total power output = WST + WGT = 82,607 Kw

Fuel mass flow rate, mf = 0.01662 × 275.8 = 4.466 kg/s

Overall efficiency of combined plant Ƞo = 82607

4.466 × 44430 = 41.63%

Efficiency of steam plant ȠST = h a−hbh a−he

= 38.8%

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Efficiency of GT plant ȠGT = 53744

4.466 × 44430 = 27.09%

“Lost heat “coefficient in the exhaust stack XL = 275.8× 1.148(247−25)

4.466 × 44430 = 35.4%

Overall efficiency can also be calculated as Ƞo = ȠGT + ȠST - ȠGT ȠST - ȠST XL

= 0.271+0.388-0.271×0.388-0.388×0.354

= 0.417 = 41.7%

Thus, it is close to the value of 41.63% which is obtained earlier.

Exergy flux and irreversibilities:

Given, Ѱ = −∆Gₒ−∆ Hₒ

= 1.0401+0.1729(hc)


For Octane (C8H18), Ѱ = 1.0401+0.1729(18 ×1)(8 ×12)

= 1.0725

−∆ Hₒ = mf×(CV)0 = 4.466×44,430 = 198,424 kW

−∆ Gₒ = −∆ Hₒ×Ѱ = 1.0725×198424 = 212810kW

Tₒ ∆Sₒ = ∆ Gₒ −∆ Hₒ = 212810 – 198424 = 14386 kW

Exergy destruction in various components

Compressor: Rate of irreversibility given as Ic = ma T0 (s2 – s1)

Where s2 – s1 = cpa ln (T ₂T ₁

) - Raln ( P ₂P ₁

)

= 1.006 ln (586298 )–

0.4 ×1.0061.4

ln 8

= 0.0818 kJ/kg K

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Ic = 271.3×298×0.0818 = 6613 kW

Combustor: Rate of irreversibility is given by Icomb = T0 [(Sp) 3 – (SR) 2]

i.e., Icomb = T0 [{mgcpg ln (T ₃T ₁

)- mgRg ln ( P ₃P ₁

)} - { mgcpg ln (T ₂T ₁

)– maRa ln ( P ₂P ₁

)}] + T0 ∆S0

= 80,947 – 6672 + 14,386 = 88661 kW

Gas Turbine: Rate of energy dissipation is given by IGT = mg T0 (s4 – s3)

Where, s4-s3 = cpa ln (T ₄T ₃

) - Rg ln ( P ₄P ₃

)

= -0.5053+0.5740 = 0.0687 kJ/kg K

IGT = 275.8× 298(0.0687) = 5646 kW

HRSG: Rate of work lost in HRSG is

IHRSG = T0 [ms (sa – se) + (s6 – s4)]

Where sa – se = 6.853 – 2.046 = 4.807 kJ/kg K

s6 – s4 = cpa ln (T ₆T ₄

) - Rg ln ( P ₆P ₄

)

= -0.428+0.014 = -0.414 kJ/kg K

IHRSG = 298×29.235×4.807 – 275.8×298×0.414

= 41,789 – 34,026 = 7853 kW

Steam Turbine: Rate of energy dissipation is IST = ms (sb – sa)T0

sa = 60853kJ/kg K

sb = sf + xb (sfg) = 0.4226+0.89×8.052

= 7.589 kJ/kg K

IST = 29.235 (7.589-6.85)×298 = 6412 kW

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Exhaust losses: Rate of exergy losses due to exhaust flue gases is

Iexh = ∫T ₆

Tₒ

[1−(TₒT )]dQ = mg cpg [(T6 – T0) – T0 ln (T ₆

T ₁)]

= 275.8×1.148 [(247-25) – 298 ln (520298 )] = 17,760 kW

Exergy balance:

Exergy input (kW) Power output(kW) Exergy losses(kW)

−∆ Gₒ= 212,810 WGT = 53,744 Compressor : 6,613

W ST = 28,950 Combustor : 88,661

Gas turbine : 5,646

HRSG : 7,853

Steam turbine : 6,412

Exhaust gases : 7,760

Input = 212,810 kW Total output = 82,607 kW Total losses = 1,32,945 kW

Exergy output + exergy destruction = 82,607 + 1, 32,945 = 2, 15,552 kW

This is 1.3% greater than the exergy input.

Exergetic (or) second law efficiency (or) measure of perfectness of the system

ȠII= Minimumexergy ¿ perform task ¿Actual exergy available ¿

perform thetask ¿ = 82,607212810

=

38.8%

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4.3. Calculation for combined cycle power plant with HRSG by Methane as fuel:

INPUT PARAMETERS

Inlet condition of air to the compressor : 1 bar, 25 0C

Pressure ratio of compressor : 8

Maximum gas temperature at inlet to the gas turbine : 900 0C

Pressure drop in the combustion chamber : 3%

Efficiency of the compressor : 0.88

Efficiency of the gas turbine : 0.88

Calorific value of liquid methane (CH4) used as fuel : 55.5 MJ/kg

Specific heat of air : 1.006 kJ/kg K

Specific heat of gas : 2.24 kJ/kg K

Specific heat ratio of gas : 1.32

Specific heat ratio of air : 1.4

Condition of steam at inlet to the steam turbine : 40 bar, 425 0C


Feed water temperature to the HRSG : 170.4 0C

Efficiency of the steam turbine : 0.82

Pressure drop of gas in the HRSG : 5 kpa

Steam flow rate : 29.235 kg/s

Assume Ѱ= (∆G0)/(∆H0)=1.0401+0.1728(hc)


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Solution:

Gas turbine plant:

P1= 1 bar, P2= 8 bar, T1= 298 K, Ƞc= 0.88

T ₂T ₁ = ( P₂

P₁)( γ−1

γȠc) = 80.4/(1.4×0.88) = 1.965

T2= 298×1.965= 586K = 313 0C

Combustor:

Pressure loss = 0.03×8 = 0.24 bar

P3 = 8 - 0.24 = 7.76 bar

Let the flow rate of combustion gas be 1 kg/s and that of fuel f kg/s.

The flow of air = (1 - f) kg/s

Therefore,

f×CV = 1.Cpg (T3 - T1) - (1 - f) Cpa(T2 - T1)

f×55,500 = 1×2.24 (900 - 25) - (1 - f)×1.006 (313 - 25)

= 1960 - 289.7 (1 - f)

f = 0.03025 kg/s

Air fuel ratio = (1−f )

f= 0.96975/0.03025= 32.05

Now, CH4 + 2O2 = CO2 + 2H2O

Air fuel ratio for stoichiometric combustion = (2×32)/ (0.232×16) = 17.24

Excess air = (32.05 - 17.24)/17.24= 0.85 or 85%

Gas turbine:

P4= 1+0.05= 1.05 bar, T3= 1173 K

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T ₃T ₄

= ( P₃P₄

)(

(γ−1) Ƞtγ

) = (7.76/1.05)(0.333×0.88)/1.333

= 1.5530

T4 = 11731.553 = 755 K = 482 0C

This is the turbine exhaust gas temperature.

HRSG:

Let the pinch point temperature difference (T5 - Tf) be 30 0C,

Tf = (Tsat) 40 bar = 250.40C

T5 = 250.4+30 = 2800C

Now, from steam tables, ha = 3272 kJ/kg and hf = 1087 kJ/kg.

According to energy balance equation we have,

mg cpg (T4 - T5) = ms (ha – hf)

1×2.24(482 – 280) = ms (3272 – 1087)

ms = 0.2179 kg/s

The total heat transfer in the HRSG yields the stack temperature T6,

That is for 170.40C, he = 721 kJ/kg

2.24 (482 – T6) = 0.2179(3272 – 721)

T6 = 234.7 0C

Power output:

ha = 3272 kJ/kg

sa = 3.853 kJ/kg

sb1 = sf + xbs (sfg) = 0.4226 + xbs (8.052)

xbs = 0.7986

hb1 = hf + xbs (hfg) = 121.46 + 0.7986 (2432.9) = 2064.37 kJ/kg

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By neglecting pump work, output of steam turbine is

WST = ms (ha – hb1) ȠT = 29.235(3272 – 2064)×0.82 = 28950 kW

Mass flow rate of gas in gas turbine

mg = 29.2350.2179

= 134.17 kg/s

Air flow rate entering the compressor ma = (1- f) 134.17 = 0.969×134.16 = 130.10 kg/s

Power output from the gas turbine WGT = 134.17×2.24 (900 – 482) – 130.10×1.006 (313-25)

= 84,749.59 kW

Total power output = WST + WGT = 1, 13,699.74 kW

Fuel mass flow rate, mf = 0.03025 × 134.17 = 4.0583 kg/s

Overall efficiency of combined plant Ƞo = 1,13,699.74

4.0583 ×55,500 = 50.47%

Efficiency of steam plant ȠST = h a−hbh a−he

= 38.8%

Efficiency of GT plant ȠGT = 84,749.59

4.0583 ×55,500 = 37.62%

“Lost heat “coefficient in the exhaust stack XL = 134.17 ×2.24 (233−25)

4.0583 × 55,500 = 29.27%

Overall efficiency can also be calculated as Ƞo = ȠGT + ȠST - ȠGT ȠST - ȠST XL

= 0.3762+0.388-0.3762×0.388-0.388×0.2927

= 0.5048 = 50.48%

Thus, it is close to the value of 50.47% which is obtained earlier.

Exergy flux and irreversibilities:

Given, Ѱ = −∆Gₒ−∆ Hₒ

= 1.0401+0.1729(hc)

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For Octane (C8H18), Ѱ = 1.0401+0.1729(4 × 1)(1 ×12)

= 1.0977

−∆ Hₒ = mf×(CV)0 = 4.0583×55,500 = 225,235.62 kW

−∆ Gₒ = −∆ Hₒ×Ѱ = 225235.62 ×1.0977 = 247,241.17 kW

Tₒ ∆Sₒ = ∆ Gₒ −∆ Hₒ = 247241.17 – 225235.62 = 22,005.52 kW

Exergy destruction in various components

Compressor: Rate of irreversibility given as Ic = ma T1 (s2 – s1)

Where s2 – s1 = cpa ln (T ₂T ₁

) - Raln ( P ₂P ₁

)

= 1.006 ln (586298 )–

0.4 ×1.0061.4

ln 8

= 0.0818 kJ/kg K

Ic = 130.10×298×0.0818 = 3240.52 kW

Combustor: Rate of irreversibility is given by Icomb = T0 [(Sp) 3 – (SR) 2]

i.e., Icomb = T0 [{mgcpg ln (T ₃T ₁

)- mgRg ln ( P ₃P ₁

)} - { mgcpg ln (T ₂T ₁

)– maRa ln ( P ₂P ₁

)}] + T0 ∆S0

= 1, 59,985.18 – 95,858.61 + 22,005.52 = 96,997.49 kW

Gas Turbine: Rate of energy dissipation is given by IGT = mg T0 (s4 – s3)

Where, s4-s3 = cpa ln (T ₄T ₃

) - Rg ln ( P ₄P ₃

)

= -0.5053+0.5740 = 0.0687 kJ/kg K

IGT = 134.17× 298(0.0687) = 15,264.24 kW

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HRSG: Rate of work lost in HRSG is

IHRSG = T0 [ms (sa – se) + (s6 – s4)]

Where sa – se = 6.853 – 2.046 = 4.807 kJ/kg K

s6 – s4 = cpa ln (T ₆T ₄

) - Rg ln ( P ₆P ₄

)

= -0.8776 + 0.01400 = -0.8626 kJ/kg K

IHRSG = 298×29.235×4.807 – 134.17×298×0.8626

= 7392.52 kW

Steam Turbine: Rate of energy dissipation is IST = ms (sb – sa)T0

……………… sa = 60853kJ/kg K

sb = sf + xb (sfg) = 0.4226+0.89×8.052

= 7.589 kJ/kg K

IST = 29.235 (7.589-6.85)×298 = 6412 kW

Exhaust losses: Rate of exergy losses due to exhaust flue gases is

Iexh = ∫T ₆

Tₒ

[1−(TₒT )]dQ = mg cpg [(T6 – T0) – T0 ln (T ₆

T ₁)]

= 134.17 ×2.24 [(233 - 25) – 298 ln (507298 )] = 16,524.48 kW

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Exergy balance:

Exergy input (kW) Power output (kW) Exergy losses (kW)

−∆ Gₒ= 247,241.17 W GT = 84,749.59 Compressor : 3240.52

WST = 28,950 Combustor : 86,132.08

Gas turbine : 15,264.24

HRSG : 7392.52

Steam turbine : 6,412

Exhaust gases : 16,524

Input = 247,241.17 kW Total output = 113,699.74 kW Total losses = 134,965.84 Kw

Exergy output + exergy destruction = 113,699.74 + 134,965.84 = 248,665.58 kW

This is 0.57% greater than the exergy input.

Exergetic (or) Second law efficiency (or) Measure of perfectness of the system

ȠII= Minimumexergy ¿ perform task ¿Actual exergy available ¿

perform thetask ¿ = 113,699.74247,241.17

= 45.98%

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CHAPTER 5

5. RESULTS AND DISCUSSIONS

5.1.RESULTS FOR COMBINED CYCLE POWER PLANT (Without HRSG)

5.2 RESULTS FOR COMBINED CYCLE POWER PLANT (With HRSG)

USING OCTANE AS FUEL

5.3. RESULTS FOR COMBINED CYCLE POWER PLANT (With HRSG)

USING METHANE AS FUEL

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5.1.RESULTS FOR COMBINED CYCLE POWER PLANT (Without HRSG)

5.1.1. Varying the inlet temperature of compressor

PARAMETERS

150C 250C 350C 450C

Mass flow rate of steam (kg/s) 81.89 83.19 84.53 85.9

Mass flow rate of air (kg/s) 398.32 404.62 411.13 417.85

Total heat input (MW) 403.82 402.21 400.56 398.86

Work done by gas turbine (kW) 88.22 86.45 84.52 82.73

Work done by steam turbine (kW) 111.77 113.55 115.38 117.25

Total power out (kW) 199.99 200 200 199.98

Thermal efficiency (%) 49.52 49.72 49.93 50.13

5.1.2. Varying the pressure ratios

PARAMETERS

6 bar 7.5 bar 9 bar 10.5 bar




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Total power out (kW) 200 200 199.98 200


5.1.3. Varying the inlet temperatures of Gas turbine:

PARAMETERS

6000C 7500C 9000C 10500C






Total power out (kW) 200 200 199.99 199.99


5.1.4. Varying the steam generation in the boiler:

PARAMETERS

40 bar 50 bar 60 bar 70 bar






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Total power out (kW) 200 199.99 200 199.99


5.1.5. Varying the steam temperatures:

PARAMETERS

5000C 6000C 7000C 8000C






Total power out (kW) 200 199.99 199.99 200


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5.2 RESULTS FOR COMBINED CYCLE POWER PLANT (With HRSG)

USING OCTANE AS FUEL:

5.2.1. Varying the inlet temperatures of compressor

PARAMETERS

15 0C 250C 350C 450C


Mass flow rate of gas (kg/s) 275.8 275.8 275.8 275.8

Mass flow rate of fuel (kg/s) 4.578 4.467 4.33 4.19

Work done by gas turbine (kW) 56,494.84 53,734.51 51,239.04 48,464.91

Work done by steam turbine (kW) 28,863 28,863 28,863 28,863

Total power out (kW) 85,357.84 82,597.51 80,102.04 77,327.91

Overall efficiency (%) 41.51 41.63 41.61 41.96

5.2.2. Varying the pressure ratios.

PARAMETERS



Mass flow rate of gas (kg/s) 275.8 273.2 406 487.2

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Total power out (kW) 82,416.31 71,479.12 1,04,442.98 1,15,593.43


5.2.3. Varying the inlet temperature of Gas turbine

PARAMETERS

9000C 10000C 11000C 12000C


Mass flow rate of gas (kg/s) 275.8 208.8 168 140.5







PARAMETERS





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Work done by steam turbine (kW) 28,261.15 28,986.25 29,456.45 29,813.65




PARAMETERS

425 0C 525 0C 625 0C 725 0C





Work done by steam turbine (kW) 28,998.09 32,271.08 35,769.90 39,528.40

Total power out (kW) 82,741.37 91,604.45 1,01,237.42 1,12,548.05


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5.3. RESULTS FOR COMBINED CYCLE POWER PLANT (With HRSG)

USING METHANE AS FUEL:

5.3.1. Varying the inlet temperatures of compressor

PARAMETERS

15 0C 25 0C 35 0C 45 0C






Total power out (kW) 1,15,040.55 1,13,699.74 1,12,466.71 1,11,182.18


5.3.2. Varying the pressure ratios

PARAMETERS




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Work done by gas turbine (kW) 84,749.59 1,07,237.38 1,31,836.50 1,59,144.79




5.3.3. Varying the inlet temperatures of Gas turbine:

PARAMETERS

900 0C 1000 0C 1100 0C 1200 0C






Total power out (kW) 1,13,699.74 1,03,288.93 95,732.48 90,599.54



PARAMETERS




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Work done by steam turbine (kW) 25,171.33 28,950 26,369.37 27,328.28




PARAMETERS

425 0C 525 0C 625 0C 725 0C




Work done by gas turbine (kW) 84,749.59 1,03,061.68 1,11,919.26 1,22,112.5

Work done by steam turbine (kW) 28,950 29,965.89 33,082.32 36,678.23



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Discussion for the above cases:

The above tables represent the performance calculations of without & with HRSG in combined cycle power plant by Octane and Methane as fuels. The overall efficiency varied by varying the parameters like inlet temperature of compressor, gas turbine inlet temperature, pressure ratio.

5.4 Plots for the above results:

Graph 5.1.Inlet temperature vs Thermal efficiency

The above graph represents the variations in efficiency of CC plant with respect to inlet temperatures of compressor. It clearly conveys the efficiency will be improved with HRSG.

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Ƞth

Inlet Temp

Inlet Temp vs Ƞth

Graph 5.2.Pressure ratio vs Thermal efficiency

The above graph represents variation of thermal efficiency with respect to pressure ratio of gas turbine. It is found that high efficiency can be obtained by with HRSG.

Graph 5.3.Maximum temperature vs Thermal efficiency

The above graph represents the variations in efficiency of CC plant with respect to maximum temperature of gas turbine. It clearly conveys the efficiency will be improved with HRSG.

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Pressure Ratio vs Ƞth

Ƞth

Pressure Ratio

Max Temp vs Ƞth

Ƞth

Max Temp

Graph 5.4.Steam turbine pressure vs Thermal efficiency

The above graph represents the variations in efficiency of CC plant with respect to pressure of boiler of steam turbine. It clearly conveys the efficiency will be improved with HRSG.

Graph 5.5.Steam turbine Temperature vs Thermal efficiency

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Steam Turbine Pressure vs ȠthȠth

Max Pressure

Steam Turbine Temp vs Ƞth

Ƞth

Max Temp

The above graph shows the variation of thermal efficiency with respect to maximum temperature reached in steam turbine. From above plots it is observed that combined cycle power plant gives high efficiency with Heat Recovery Steam Generator exceeding 50%.

5.5 RESULTS (OCTANE VS METHANE):

By taking Octane and Methane as fuels efficiency of Combined Cycle Power Plant with HRSG has been evaluated. The results plotted below.

Graph 5.6.Inlet temperature vs Thermal efficiency

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Inlet Temp vs Ƞo

Ƞo

Inlet Temp

Graph 5.7.Pressure ratio vs Thermal efficiency

Graph 5.8.Maximum temperature vs Thermal efficiency

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Pressure Ratio vs Ƞo

Ƞo

Pressure Ratio

Max Temp vs ȠoȠo

Max Temp

Graph 5.9.Maximum pressure in steam turbine vs Thermal efficiency

Graph 5.10.Maximum temperature in steam turbine vs Thermal efficiency

The above plots show that Methane shows immortal properties than that of Octane. Methane gave better results because of higher calorific value and specific heat. Fuel burning rate directly affects the power developed by Gas turbine. By varying various input parameters Methane gave better results.

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Ƞo

Steam Turbine Max Pressure vs Ƞo

Max Pressure

Steam Turbine Max Temp vs Ƞo

Ƞo

Max Temp

Below figure briefs the energy and exergy variations of CCPP with Octane and Methane as fuels.

However there are more losses with Methane as fuel, they can be neglected when compare to the efficiency. Second law efficiency is the measure of perfectness of the system. It shows the quality of the system. From above we can found that Methane fuel possesses high exergetic efficiency. Variation of fuel directly affects the power generated by Gas turbine.

PARAMETERS OCTANE METHANE

Power Developed By Gas Turbine 53,744 kW 84,749.59 kW

Power Developed By Steam Turbine 28,863 kW 28,950.15 kW

Total Power Developed 82,607 kW 1,13,699.74 kW

Efficiency Of Gas Turbine 27.09% 37.62%

Efficiency Of Steam Turbine 38.80% 38.81%

Overall Efficiency 41.63% 50.47%

Chemical Exergy 212,810 kW 247,241.17 kW

Change in Enthalpy Of Formation 198.424 kW 225,235.65 kW

Rate Of Exergy Loss In Combustion 14,386 kW 22,005.52 kW

EXERGY LOSSES:(kW)

1. Compressor 6,613 3,240.52

2. Combustor 88,661 86,132.08

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3. Gas Turbine 5,646 15,264.24

4. HRSG 7,856 7,392.52

5. Steam Turbine 6,412 6,412

6. Exhaust Losses 17,760 16,524.48

TOTAL LOSSES 1,32,945 kW 1,34,965.84 kW

SECOND LAW EFFICIENCY 38.80% 46.00

CONCLUSION:

To conclude, combined cycle power plant with HRSG can meet growing energy needs. Selection of best fuel can certainly improves the performance of the Combined Cycle Power Plant.

From the above results it is observed that

1. Combined Cycle power plant with Heat Recovery Steam Generator gives more efficiency.

2. Methane gas is the best fuel to improve the overall efficiency because of its high calorific value.

3. Change of fuel directly affects the work done by Gas turbine.

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REFERENCES:

1. Power plant engineering by P.K.Nag.

2. Engineering Thermodynamics by Cengel.

3. T. Srinivas1*, B. V.Reddy2, A. V. S. S. K. S. Gupta3:Parametric Simulation of Combined Cycle Power Pant by pp. 29-36, 2011 Int. J. of Thermodynamics ISSN 1301-9724 / e-ISSN 2146-1511

4. Thermal engineering by Rajput.

5. Manuel valdes, Jose L. Rapun : Optimization of heat recovery steam generators for combined cycle gas turbine power plants, Applied Thermal Engineering 21 (2001) 1149-1159.

6. Gas turbine combined cycle, http: // www.gepower.com

7. B.V. Reddy, G. Ramkiran, K. Ashok Kumar, P.K. Nag, Second law analysis of a waste heat recovery steam generator, International Journal of Heat and Mass Transfer 45 (2002) 1807–1814.

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http://www.gepower.com/



8. Xiang, W., Chen, Y. (2007). Performance improvement of combined cycle power plant based on the optimization of the bottom cycle and heat recuperation. Journal of Thermal Science, 16(1), 84-89.

9. Engineering thermodynamics by P K Nag

10. Ravi kumar ,N.,rama Krishna, k., sita rama raju, A .V., Exergy analysis of gas turbine power plant with alternative configuration of regenerator proceedings on CD, 2nd international exergy energy environment symposium(IEEES2), kos,Greece, 2005,VI-13

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