comparative energy and exergy analysis of a thermal … is based on both first law of thermodynamics...

International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 618

(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

Abstract--- Energy and exergy concepts come from thermodynamics and are applicable to all fields of science

and engineering. Therefore, this paper intends to provide background for better understanding of these concepts

and their differences for energy conversion system.

The first law of thermodynamics used for analysis and optimization of energy systems. The use of energy as a

measure for understanding and improving the efficiencies of energy systems can be misleading and confusing. First

law analysis leads to false impressions about the energy conversion processes because it does not consider energy

degradations during the processes. The second law of thermodynamics describes the quality of energy and

degradations in the quality i.e., it compliments and enhances an energy balance by enabling calculation of both the

true thermodynamic value of an energy carriers, and the real thermodynamic inefficiencies and losses from the

processes or systems. The concept of exergy is extremely useful for this purpose. Exergy analysis, or the second law

analysis as it is called, is based on both the first and second laws, and exergy based methods, therefore , most be

adopted while designing or rehabilitating energy systems.

In this paper a 500 MW coal based thermal power plant of APGENCO, Warangal is chosen for case study

purpose. A detailed energy and exergy analysis is carried out for the each component of the plant. And the results

show that, according to energy analysis the condenser is main source of losses and according to exergy analysis the

boiler is the major source of losses where there are more irreversibilies. The use of supercritical technology is

strongly recommended to improve the performance of boiler, it not only improves the performance of boiler, but

also the performance of the turbine due to the enhanced steam parameters and higher pressure ratio across the

turbine.

Keywords--- Energy, Exergy, Exergy Efficiency and Exergy Destruction Ratio

I. INTRODUCTION

HE science of thermodynamics is built primarily on two fundamental natural laws, known as the first and the

second laws. The first law of thermodynamics (FLT) is simply an expression of the conservation of energy

principle. It asserts that energy is a thermodynamic property and that during an interaction, energy can change from

one form to another but the total amount of energy remains constant. The first law places no restriction on the

direction of a process, but satisfying the first law does not ensure that the process can actually occur. This

inadequacy of the first law to identify whether a process can take place is remedied by introducing another general

principle, the second law of thermodynamics. The second law of thermodynamics (SLT) asserts that energy has

quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. The high-

temperature thermal energy is degraded as it is transferred to a lower temperature body. The attempts to quantify the

quality or “work potential” of energy in the light of the second law of thermodynamics has resulted in the definition

of the properties entropy and exergy.

Efficiency is one of the most frequently used terms in thermodynamics, and it indicates how well an energy

conversion or process is accomplished. Efficiency is also one of the most frequently misused terms in

thermodynamics and is often a source of misunderstanding. This is because efficiency is often used without being

G. Buchi Babu, Assistant Professor of Department of Mechanical Engineering, Warangal Institute of Technology and Science (WITS), Guddeppad X Road, Oorugonda, Atmakur(M), Warangal – 506 342 (A.P), E-mail: [email protected].

Dr. K. Sridhar, Professor of Department of Mechanical Engineering, Kakatiya Institute of Technology and Science (KITS), Opposite

Yerragatu hillock, Hasanparthi, Warangal – 506 015 (A.P), E-mail: [email protected].

PAPER ID: MET15

Comparative Energy and Exergy Analysis

of a Thermal Power Plant

G. Buchi Babu and Dr. K. Sridhar

T

mailto:[email protected]

mailto:[email protected]


(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

properly defined first. Efficiency traditionally has been primarily defined based on the first law (i.e., energy). In

recent decades, exergy analysis has found increasingly widespread acceptance as a useful tool in the design,

assessment, optimization and improvement of energy systems. Determining exergy efficiencies for an overall system

and/or the individual components making up the system constitutes a major part of exergy analysis. A

comprehensive analysis of a thermodynamic system includes both energy and exergy analyses in order to obtain a

more complete picture of system behavior.

To assist in improving the efficiencies of power plants, their thermodynamic characteristics and performances

are usually investigated. Power plants are normally examined using energy analysis but, as pointed out previously, a

better understanding is attained when a more complete thermodynamic view is taken, which uses the second law of

thermodynamics in conjunction with energy analysis via exergy methods. Although exergy analysis can be generally

applied to energy and other systems, it appears to be a more powerful tool than energy analysis for power cycles

because of the fact that it helps determine the true magnitudes of losses and their causes and locations, and improve

the overall system and its components.

The objective of this paper is to analyze the results of energy analysis and exergy analysis performed on a 500

MW coal based thermal power plant of APGENCO, Warangal, Andhra Pradesh. Components of energy loss and

exergy destruction are identified. And also effect of varying parameters on energy analysis and exergy analysis are

investigated.

II. ENERGY VS EXERGY

The traditional method of assessing the energy disposition of an operation involving the physical or chemical

processing of materials and products with accompanying transfer and/or transformation of energy is by the

completion of an energy balance. This balance is apparently based on the first law of thermodynamics. In this

balance, information on the system is employed to attempt to reduce heat losses or enhance heat recovery. However,

from such a balance no information is available on the degradation of energy, occurring in the process and to

quantify the usefulness or quality of the heat content in various streams leaving the process as products, wastes, or

coolants.

The exergy method of analysis overcomes the limitations of the first law of thermodynamics. The concept of

exergy is based on both first law of thermodynamics and second law of thermodynamics. Exergy analysis can

clearly indicate the locations of energy degradation in a process that may lead to improved operation or technology.

It can also quantify the quality of heat in a reject stream. So, the main aim of exergy analysis is to identify the causes

and to calculate the true magnitudes of exergy losses. Table 1 presents a general comparison of both energy and

exergy.

Table 1: Comparison of Energy and Exergy

ENERGY EXERGY

is dependent on the parameters of matter or

energy flow only, and independent of the

environment parameters.

is dependent both on the parameters of matter or

energy flow and on the environment parameters.

has the values different from zero.

is equal to zero (in dead state by equilibrium

with the environment).

is governed by the FLT for all the processes. is governed by the FLT for reversible processes

only (in irreversible processes it is destroyed

partly or completely).

is limited by the SLT for all processes. is not limited for reversible processes due to the

SLT.

is motion or ability to produce motion. is work or ability to produce work.

is always conserved in a process, so can

neither be destroyed or produced.

is always conserved in a reversible process, but is

always consumed in an irreversible process.

is a measure of quantity only. is a measure of quantity and quality.


(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

III. PLANT DESCRIPTION

3.1. Configuration of Power Plant

The power plant of 500 MW chosen for case study was erected in 2010 as KTPP stage I of APGENCO. This

power plant is coal based and works in subcritical conditions of steam.

The important conditions of steam are given below:

Main steam (MS): 176.3 bar and 540 0C.

Reheat steam: 540 0C.

Feed water temperature at economizer inlet: 253.7 0C.

Condenser pressure: 9 kPa.

Feed pump pressure: 196.2 bar.

Superheated steam first expands in high pressure turbine (HP turbine). HP turbine exhaust as cold reheat (CRH)

is reheated to MS temperature and exits the reheater as hot reheat (HRH). This HRH expands in an intermediate

pressure turbine (IP turbine) followed by a low pressure turbine (LP turbine). The LP turbine exhaust is condensed

in condenser and this condensate is heated in three LP heaters. Feed water from the exit of LP heaters is fed to the

HP heater with the help of boiler feed pump and enters into the boiler circuit. The schematic diagram of the plant is

shown in figure 1.

Figure 1: Power Plant Scheme

In the boiler circuit, pulverized coal is burnt with air, the resulting combustion gases are cooled to heat the

working fluid. Gases exit the airpreheater (APH) to chimney. Reheater (RH) is arranged in between two stages of

superheater (SH), viz., panal SH and platen SH, as per the flue gas path. Working fluid path in boiler circuit is as

follows:

Economizer,

Evaporator,

Low Temperature Super Heater (LTSH),

Panel SH, and

Platen SH.

In total six bleeds are taken from the turbines. The bleed pressures and corresponding destinations are given:

0.29 bar – LPH1

1.439 bar – LPH2

2.797 bar – LPH3

7.69 bar – Deaerator (DA)

19.78 bar – HPH5

45.60 bar – HPH6

The sixth bleed is extracted from HP turbine exhaust, the bleeds to LP heaters are extracted from LP turbine and

the remaining bleeds are extracted from IP turbine.


(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

3.2. Fuel

The fuel used in the boiler is coal. The ultimate analysis is given in the table 2.

Table 2: Coal Ultimate Analysis [10]

Element % by weight

Carbon 35.28

Hydrogen 2.54

Oxygen 7.05

Nitrogen 0.68

Sulphur 0.45

Moisture 12

Ash 42

Lower heating value(kJ/kg) 13384.50

Exergy (kJ/kg) 15493.35 [4]

IV. THERMODYNAMIC ANALYSIS

A detailed energy analysis and exergy analysis is carried out for all major components of the power plant.

4.1. Assumptions

The following assumptions have been made during the analysis is

Kinetic and Potential energies are neglected,

Various components operate at study state conditions,

Isentropic efficiency of pump is 80%,

Generator efficiency is 98%,

Excess air is 20%, and

The reference environment used is environment of Beahr [4]: 1 atm, 25 0C

Table 3: Reference Environment Composition

Element Mole fraction (%)

N2 75.65

O2 20.30

H2O 3.12

Ar 0.90

CO2 0.03

4.2. Calculations

The energy flow is given by:

En = mΔH

where m is mass flow and H is the enthalpy.

The physical exergy of a material stream at a given state is given by:

Exph = H- H0-T0 (S-S0)

where H is the enthalpy and S is the entropy at the state; H0 and S0 are the values at the environment state.

The chemical exergy of the component gases of the environment on the molar basis is given by:

Exch =mRT ln (P0/P00)

where P00 is the partial pressure of the component and R is universal gas constant.

The chemical exergy of an ideal gas mixture on the molar basis is is given by:

Exch,mix =m[∑i Xiech i + RT0∑i Xi ln Xi]

where Exch is the standard chemical exergy of the component with mole fraction X. Standard chemical


(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

exergy is based on standard values of the environment temperature and pressure. The exergy transfer with work

interaction is associated with work transfer rate or shaft power. Because exergy is defined as the maximum work

potential, it is equivalent to exergy in every respect. The exergy transfer rate (Ex) connected with the heat transfer

rate Q is given by:

Ex = ∫A ( 1-T0/T ) Q dA

Where A is the heat transfer area and T is the temperature at which the heat transfer occurs.

Finally, The total exergy is given by:

Ex = Exph + Exch

The energy or first law efficiency η I of a system or system component is defined as the ratio of energy output to

the energy input of system or system component, i.e.

η I = (Desired output energy / Input energy supplied)

The exergy or second law efficiency is defined as

η II = (Desired output / Maximum possible output)

i.e., η II = (Exergy of product / Exergy of fuel)

The exergy destruction ratio is given by:

yd = (Exergy destruction / Total exergy of fuel)

The exergy loss ratio is given by:

yl = (Exergy loss / Total exergy of fuel)

The energy loss ratio is given by:

Enl = (Energy loss / Energy of fuel)

Boiler

Inlet air molar composition and combustion gas composition are given in table 4 and table 5.

Table 4: Inlet Air Composition

Table 5: Combustion Gas Composition


(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

Table 6: Exergy and Energy Flows Across the Boiler

Turbine

Generator efficiency is assumed as 98%, with generator output as 500 MW. This makes the turbine output

as 510.204 MW = W.

Table 7: Exergy and Energy Flows Across the Turbine

Stream

No. Stream

Pressure,

bar

Temperature, 0C

Mass flow rate.,

kg/s

Exergy flow,

MW

Energy flow,

MW

1 MS 176.3 540 418.731 625.596 1377.159

2 CRH 45.6 338.3 377.994 426.594 1112.996

3 HRH 43.54 540 377.994 530.024 1296.125

4 Exhaust 0.09 43.76 283.329 38.089 654.693

5 LPH1 0.29 68.32 12.602 3.713 29.119

6 LPH2 1.439 139.91 21.776 12.055 57.662

7 LPH3 2.797 202.37 12.107 8.266 33.498

8 DA 7.67 311.36 24.93 22.710 74.210

9 HPH5 19.78 429.62 23.024 26.773 73.877

10 HPH6 45.60 338.30 40.737 45.975 119.95

Condenser


(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

Table 8: Exergy and Energy Flows Across the Condenser

4.3. Results and Discussions

After a detailed analysis of the power plant, the Exergy balance sheet and Energy balance sheet are given in table

9 and table 10 respectively.

Table 9: Exergy Balance Sheet

Table 10: Energy Balance Sheet

The plant exergy efficiency is 35.41% and energy efficiency is 38.14%. There is not much difference in

exergy and energy efficiencies.

According energy analysis, across the condenser 15282.5 kg/s water is circulated and its temperature is rising

from 32.22 0C to 42.22

0C. Thus, the condenser is major source of energy loss.

According exergy analysis, about one third the exergy entering is lost in the combustion process and half in

the boiler. Thus, the boiler is major source of exergy loss.

V. CONCLUSIONS

The exergy techniques presented in this paper provide a powerful and systematic tool for identifying the

location, magnitude and source of real thermodynamic losses in an energy system. It is always recommended to go

for an exergy analysis after an energy analysis.

More than half of the exergy entering the plant is lost in boiler. Turbine and condenser have relatively very low

losses compared to the boiler.

The reasons for the high destruction in the boiler are:

Combustion process is highly irreversible.

Steam is operated in subcritical conditions, therefore temperature difference between steam and the

combustion gas is high, which increases the irreversibility due to heat transfer.

Stream

No.

Pressure,

bar

Temperature, 0C

Mass flow rate.,

kg/s

Exergy flow,

MW

Energy flow,

MW

1 0.09 43.76 283.329 38.088 654.686

2 0.09 43.76 46.125 0.555 8.416

3 0.09 43.76 329.107 0.751 25.853

4 2.00 33.00 15282.5 7.008 464.644

5 2.00 43.00 15282.5 31.179 1100.424


(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

Effect of excess air and inlet air temperature on boiler exergy efficiency is shown in figure 2.

Figure 2: Effect of Excess Air and Inlet Air Temperature on Boiler Exergy Efficiency

Some suggestions regarding the boiler are:

Excess air may be reduced to improve efficiency.

Heat transfer is much dependent on materiel properties, therefore material improvements may be considered

to improve heat transfer performance.

Working fluid operating in supercritical conditions is recommended as it decreases the temperature

difference between steam and the combustion gas, and enhances the heat transfer.

Here are some conclusions for the overall plant:

Use of supercritical technology is strongly recommended as it not only improves the boiler performance, but

also the performance of the turbine due to the enhanced steam parameters and high pressure ratio across the

turbine.

It is very important to identify which irreversibilities can be avoided and which cannot be.

e.g, although combustion process is the major source of loss, it cannot be avoided. Also, it is important to

identify whether the components are sensitive to the changes wish to be made.

Use of cogeneration is always recommended wherever feasible.

REFERENCES

[1] N Arai, H Taniguchi, K Mouri and T Nakahara., Exergy Analysis on Combustion and Eenergy Conversion

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[2] Bejan A., Tsatsaronis G., Moran M., Thermal Design and Optimization, Wiley ( 1996 ).

[3] Cornelissen R L., Thermodynamics and sustainable development: The use of exergy analysis and reduction of

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[5] Dincer I and Cengel Y A., Energy, Entropy and Exergy Concepts and Their Roles in Thermal Engineering,

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(ICCOMIM - 2012), 11-13 July, 2012

ISBN 978-93-82338-04-8 | © 2012 Bonfring

[12] Wall G (1997), Exergy – A uesful Concept within Resource Accounting, Institute of Theoretical Physics,

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comparative energy and exergy analysis of a thermal … is based on both first law of thermodynamics...

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