DOI : 10.17577/IJERTV5IS060478
- Open Access
- Total Downloads : 701
- Authors : Krishnakumar Dipak Pilankar, Dr. Rajesh Kale
- Paper ID : IJERTV5IS060478
- Volume & Issue : Volume 05, Issue 06 (June 2016)
- DOI : http://dx.doi.org/10.17577/IJERTV5IS060478
- Published (First Online): 18-06-2016
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License:
This work is licensed under a Creative Commons Attribution 4.0 International License
Energy and Exergy Analysis of Steam and Power Generation Plant
Krishnakumar Dipak Pilankar1
Post Graduate Scholar Dept. of Mechanical Engineering,
Rajiv Gandhi Institute of Technology, Mumbai, Maharashtra, India
Dr. Rajesh Kale2
Associate Professor
Dept. of Mechanical Engineering Rajiv Gandhi Institute of Technology, Mumbai, Maharashtra, India
Abstract This paper deals energy and exergy analysis of steam and power generation plant in a chemical and fertilizer industry. Conventional energy analysis is based on first law of thermodynamics and exergy analysis is based on second law of thermodynamics. The real energy loss in components cannot be justified by first law of thermodynamics alone, because it does not differentiate between quality and quantity of energy. First the main components of steam and power generation system selected. Selected components of the system are then analyzed separately and sites having largest energy and exergy losses are identified. By energy analysis, highest energy loss occurs in condensers where 47.16 MW is lost which represents 52.89% of total energy loss in plant. After condensers, energy loss in boilers is significant where 30.26 MW is lost which represents 34% of total energy loss. From exergy analysis highest exergy destruction occurred in two boilers where 238.6 MW exergy is destroyed, it represents 90.8% of total exergy destruction of plant. Exergy destruction in condenser is 4.426 MW which is only 1.78% of total exergy destruction. Total energy loss for plant is 89.17 MW while total exergy destruction for the plant is
-
MW. It is also seen that energy efficiencies of components are greater than exergy efficiencies. In power generation section, turbine 1 cycle is found to be more efficient than turbine 2 cycle. Energy and exergy efficiencies of turbine 1 cycle are found as 35.29% and 66.30% respectively and that of turbine 2 cycle are 32.07% and 64.33% respectively. For power generation cycles exergy efficiencies are greater than energy efficiencies.
KeywordsEnergy; Exergy; Exergy Destruction; Efficiency; Analysis
-
INTRODUCTION
Steam has been a popular mode of conveying energy since the industrial revolution. Steam is used for generating power and also used in process industries such as sugar, paper, fertilizer, refineries, petrochemicals, chemical, food, synthetic fiber and textiles. Major part of this steam production comes from fossil fuels like coal and natural gas. Energy conversion of chemical energy of fuel into steam takes place in boiler mostly. This steam is then utilized for electricity generation and for the processes. There is vast potential for saving by improving the efficiencies of steam generation system. Performance assessment of steam and power generation system is very essential for industry for proper utilization of available energy resources. By assessing performance of plant one can pin point areas or components where energy conversion is poor and where the improvement is required. This will help to improve energy efficiency, minimize operating expenses and increasing the profitability of industry.
Most commonly used method for thermodynamical performance assessment is based on first law of thermodynamics i.e. energy analysis [1]. Another method used is exergy analysis which is based on 2nd law of thermodynamics. There is increasing interest in combine utilization of both first law and second law thermodynamics. Exergetic analysis provides distinction between energy losses to environment and internal irreversibilities in the process [2]. Conventional method of energy analysis is based on first law of thermodynamics which concerned with conservation of energy principle. The First Law deals with the amounts of energy of various forms transferred between the system and its surroundings and with the changes in the energy stored in the system. It treats work and heat interactions as equivalent forms of energy in transit [3]. However first law sometime gives misleading results about performance of energy conversion device and optimization through first law has almost reached saturation level [4]. Also first law is concerned with quantity of energy and its transformation from one form to another. It does not account quality aspects of energy [5].
The quality aspect of energy is accounted by second law of thermodynamics. Second law provides necessary means to determine quality as well as degree of degradation of energy during process. Exergy is defined as maximum amount of work which can be obtained by a system or stream of matter or energy when it brought from specified initial state to the state of its environment, that is, the dead state. Exergy is a measure of the potential of the system or flow to cause change, as a consequence of not being completely in stable equilibrium relative to the reference environment. Unlike energy, exergy is not conserved during any real process; it is always destroyed in a process. Exergy destroyed is proportional to entropy generated due to irreversibilities [5].
Aim of this project is perform combine energy and exergy analysis on steam and power generation plant. Components having major energy loss and exergy destruction will be determined. Results of energy and exergy analysis will be then compared. At time of analysis, plant is operated at total steam load of 500 TPH and power generation of 23.8 MW.
Nomenclature
Nomenclature
Subscripts
Subscripts
P T
m h s E Q W
ex Ex Exd WT
Wshaft
P T
m h s E Q W
ex Ex Exd WT
Wshaft
pressure (bar) temperature (oC) mass flow rate (kg/s)
specific enthalpy (kj/kg) specific entropy (kj/kg K)
energy flow rate (MW) heat transfer rate (MW) work done (MW) specific exergy (kj/kg) exergy flow rate (MW) exergy destruction (MW) turbine work (MW)
actual power developed at turbine shaft (MW) energy efficiency or first law efficiency
exergy efficiency or second law efficiency
pressure (bar) temperature (oC) mass flow rate (kg/s)
specific enthalpy (kj/kg) specific entropy (kj/kg K)
energy flow rate (MW) heat transfer rate (MW) work done (MW) specific exergy (kj/kg) exergy flow rate (MW) exergy destruction (MW) turbine work (MW)
actual power developed at turbine shaft (MW) energy efficiency or first law efficiency
exergy efficiency or second law efficiency
s f 0
i
e w
s f 0
i
e w
steam fuel
dead or reference state inlet
exit
water
steam fuel
dead or reference state inlet
exit
water
Abbreviations TPH
TG DA HPH FP BM PRDS
CV
Abbreviations TPH
TG DA HPH FP BM PRDS
CV
tones per hour turbo generator deaerator
high pressure heater feed pump
boiler master
pressure reducing distribution system calorific value
tones per hour turbo generator deaerator
high pressure heater feed pump
boiler master
pressure reducing distribution system calorific value
PLANT DESCRIPTION AND WORKING
-
Plant Description
In this study the operating data of steam generation plant in chemicals and fertilizers industry is used. This steam generation plant also contains captive power plant. This plant falls in cogeneration category but steam is used separately for processes in other plants and in power generation. Normal steam production is 480 to 500 TPH of which around 150-170 TPH steam used in power generation section. The steam generation plant consist tree boilers each having capacity of 275 TPH. Two boilers are kept operating and third boiler is kept as standby. Steam produced in steam generation plant used in turbo generating unit, urea plants, and chemical group of plants and in feed pumps. Fuel used is natural gas. Power generation Section consists of two turbo-generators each of 15 MW capacities. Turbines are of extraction-condensing type. This captive power generation takes care of high risk load of all plants. Operating load conditions of boiler and turbine are shown in Table 1.
Component
Parameter
Component number
1
2
Boiler
Boiler steam load (TPH)
250
250
Steam temperature (oC)
510
510
Steam pressure (bar)
103.198
102.267
Mass flow rate of fuel (kg/s)
4.186
4.184
Feed water inlet temperature oC
193.5
188
Turbine
Power generated (MW)
11.5
12.3
Live steam flow (TPH)
73
76
Component
Parameter
Component number
1
2
Boiler
Boiler steam load (TPH)
250
250
Steam temperature (oC)
510
510
Steam pressure (bar)
103.198
102.267
Mass flow rate of fuel (kg/s)
4.186
4.184
Feed water inlet temperature oC
193.5
188
Turbine
Power generated (MW)
11.5
12.3
Live steam flow (TPH)
73
76
Table 1 Operating data of boiler and turbine
-
Working
Fig. 1 shows the schematic diagram of plant. In Fig. 1, letter a used with numbers to indicate that those streams are connected to deaerator 1, feed pump 1, HP heater 1, boiler 1, turbo generator 1 and condenser 1. Letter b with numbers shows that those streams are connected to component number 2 e.g. deaerator 2, boiler 2 etc. Condensates from urea plants are stored in tank and are supplied to deaerators. TG return condensates lines are also connected to deaerators dome. HP heater condensate lines are connected to deaerator storage tanks. There are two nos of deaerators. Deaerator steam for heating water is fed to steam dome from 2 Ata PRDS, TG extraction-2 steam, feed pump exhaust and flash steam from boilers. From deaerators feed water passes to the high pressure heater through the boiler feed pumps. There are two number of HP heaters of shell and tube type. There are four feed pumps, two electric motor driven and two turbines driven. Normally two turbine driven feed pumps are running. Steam for these pumps comes from boiler master BM. Extraction 1 from TG sets is used for heating the feed water. From HP heaters feed water passes to the individual boiler via feed water control station. From boiler steam goes to boiler master where steam distributed according to requirement. Around 30% of the steam from the boiler master goes to two turbo generators. As explained earlier, there are two extractions in each turbine. Extraction 1 is utilized in HP heater while extraction 2 goes to deaerator. Remaining steam from turbine is then exhausted into condenser where it gets condensed. Condensate then goes to the deaerator.
Fig. 1. Schematic diagram of the steam and power generation plant
-
-
METHODOLOGY AND DEFINITIONS
Mass, energy, and exergy balances for any control volume at steady state with negligible potential and kinetic energy changes can be expressed, by
mi = me (1)
The expression for energy efficiency () and exergy efficiency () for the component are obtained from following definitions
= energy in products . (7)
total energy input
Q W = m h m h
(2)
= exergy in products
(8)
e e i i
total exergy input
Ex,heat W = meexe miexi + Exd (3)
Where Exd is exergy destroyed and Ex,heat is net exergy transfer by heat at temperature T
Ex,heat = ( 1- To / T)Q (4) Specific exergy is given by
This equation establishes a relationship between the desired result (for instance, the heating of feed water, or the power in a turbine) and the input (the amount of energy or exergy spent to obtain the result). In some systems there is no universal agreement as to what are an input and an output. Therefore their efficiency must be defined by the expression
Energy or Exergy Out
ex = h ho To (s-so) (5)
Efficiency =
Energy or exergy in (9)
Total exergy flow at any point is given by
Ex = m× ex = m × [h ho To (s-so)] (6)
For steady state operation and considering each selected component in Fig. 1 as control volume, energy balance, energy efficiency, exergy balance and exergy efficiency can be defined.
Operating data of the plant is collected form computer operated control room of the plant. Thermodynamic properties at various points indicated in Fig. 1 are shown in Table 2. Note that points with letter a i.e. 1a, 2a etc. indicates that those streams are connected to number 1 components e.g. boiler 1, HP heater 1, deaerator 1 etc. Points with letter b i.e. 1b, 2b. etc. indicates that those streams are connected only to number 2 components, e.g. boiler no.2.
Table 2 Thermodynamic properties of the points of steam and power generation plant
Point
Mass flow rate
m (kg/s)
Pressure P (bar)
Temperature T (oC)
Specific enthalpy h (kj/Kg)
Specific entropy
s (kj/kg K)
Specific exergy
ex (kj/kg)
Energy flow E (MW)
Exergy flow Ex (MW)
1a
48.71
5.08
63.0
264.11
0.868
7.492
12.865
0.365
2a
8.89
5.62
64.0
268.34
0.881
7.949
2.385
0.071
3a
11.39
10.66
182.7
776.68
2.169
125.961
8.845
1.435
4a
0.53
1.91
200.0
2871.23
7.531
595.795
1.520
0.315
5a
1.67
2.02
211.0
2892.82
7.549
611.860
4.821
1.020
6a
0
–
–
–
–
–
–
–
7a
3.78
1.92
230.0
2931.38
7.651
619.643
11.074
2.341
8a
74.93
4.15
115.0
482.73
1.473
42.816
36.171
3.208
9a
74.93
127.52 116.0
495.80
1.474
55.823
37.150
4.183
10a
74.93
127.52
197.0
843.84
2.285
157.870
63.229
11.829
11a
70.00
120.21
192.0
821.44
2.239
149.445
57.501
10.461
12a
69.44
103.20
510.0
3397.00
6.614
1399.418
235.903
97.182
13a
3.78
99.10
500.0
3376.17
6.605
1381.481
12.754
5.219
14a
21.11
97.12
496.0
3368.32
6.603
1374.118
71.109
29.009
15a
11.39
15.23
313.0
3066.48
6.962
963.585
34.924
10.974
16a
9.72
0.18
61.0
2415.15
7.378
186.017
23.481
1.808
17a
9.72
0.18
59.0
246.97
0.819
5.428
2.401
0.053
1b
42.40
5.08
63.0
264.11
0.868
7.492
11.198
0.318
2b
8.89
5.62
65.0
272.52
0.893
8.378
2.422
0.074
3b
8.33
10.59
171.1
723.97
2.052
108.656
6.033
0.905
4b
0.49
1.93
199.0
2869.10
7.521
596.863
1.406
0.292
5b
1.67
1.99
210.0
2890.96
7.554
608.746
4.818
1.015
6b
0
–
–
–
–
–
–
–
7b
3.07
1.91
227.0
2927.37
7.641
618.668
8.985
1.899
8b
64.85
4.15
114.0
478.49
1.462
41.890
31.029
2.717
9b
64.85
128.50
116.0
495.87
1.473
55.919
32.156
3.626
10b
64.85
128.50
186.0
795.51
2.181
141.130
51.587
9.152
11b
69.31
117.81
188.0
803.74
2.202
143.147
55.704
9.921
12b
69.44
102.27
510.0
3398.10
6.619
1398.926
235.979
97.148
13b
3.07
99.00
500.0
3376.29
6.605
1381.427
10.363
4.240
14b
20.28
98.10
496.0
3367.09
6.597
1374.666
68.277
27.875
15b
8.33
15.13
308.0
3055.67
6.946
957.515
25.464
7.979
16b
11.94
0.18
63.0
2438.80
7.450
187.986
29.130
2.245
17b
11.94
0.18
61.0
255.34
0.844
6.187
3.050
0.074
-
E
NERGY AND EXERGY BALANCE EQUATIONS AND
Boiler efficiency = energy gain by steam
energy supplied by fuel
(10)
A. Boiler
EFFICIENCY FORMULAE FOR COMPONENTS
Or
Boiler is main component of steam generation. Boiler energy balance, exergy balance and efficiencies can be obtained by different ways but in an industry, a main criterion
ms×(hshw)
=
=
boiler mf×CV
(11)
is fuel to steam conversion. So in this study steam to fuel efficiencies are considered. Boiler energy efficiency is given as
Where hs and hw are enthalpies of steam an feed water resp.
and CV is the calorific value of fuel (Natural gas).
ms×(p2p1)
(12)
D. Deaerator
boiler =
mf×CV
Energy balance is given as
In equation (12), if we write points 11 and 12 with a i.e.11a and 12a we will get energy efficiency formula for boiler 1. If we write with b i.e. 11b and 12b we will get energy efficiency formula for boiler 2. Same way other components
m1p+m2p+m3p+m4h4+m5p+m6p+m7h7 = m8h8+Eloss (24) Energy efficiency
formulas can be derived.
= m8h8
(25)
Exergy efficiency of boiler is given by
DA m1p+ m2p + m3p + m4h4 + m5p + m6p+m7h7
Exergy efficiency = exergy gain by steam
exergy supplied by fuel
(13)
Exergy balance of deaerator is given as m1ex1+m2ex2+m3ex3+m4ex4+m5ex5+m6ex6+m7ex7=m8ex8+Exd (26)
boiler
= ms×(ex12ex11) mf×exf
(14)
Exergy efficiency
exf is the exergy of a fuel can be calculated by using equation
DA =
m e
8 x8
= exf
LHVf
(15)
m1ex1+ m2ex2+m3ex3+m4ex4+ m5ex5+m6ex6+m7ex7
(27)
term is the ratio of chemical exergy of the fuel to the LHV(lower heating value) or of fuel. The value of is taken as 1.06 for natural gas. Calorific value (CV) or LHV of natural gas (fuel) is taken as 46500 kj/kg.
B. HP heater
Feed water flow through HP heater, feed pump and deaerator is not known but it can be easily found by HP heater energy balance.
Energy supplied by steam = energy gain by feed water (16) m15 × (p5 p) = m10 × (p0 h9) (17)
Exergy balance of HP heater is given as
m15 × (ex15 ex3) = m10 × (ex10- ex9) + Exd (18) Exergy efficiency of HP heater
E. Turbine
Energy input (Ein) to the turbine derived as
Ein = m14p4 (28)
Energy out (Eout) from turbine given as
Eout = m15p5 +m6p+m16p6 (29) Turbine work done (WT)
WT = Ein -Eout = m14p4 – m15p5 -m6p-m16p6 (30) Actual Power/work develop by turbine shaft (Wshaft)
g
g
Wshaft = Generator power × -1 earbox x -1generator (31)
Where gearbox =0.984 and generator =0.9803 are gearbox and generator efficiencies respectively.
Energy or first law efficiency of turbine is given as
C. Feed pump
HPH
= m10× ( ex10 ex9) m15 × ( ex15 ex3)
(19)
turbine=
p>Wshaft (32)
WT
Energy balance of feed pump is derived as
m13 (p3 h7) = m9 (h9 h8) + Eloss (20)
Exergy input to turbine is derived as
Exin = m14ex14 (33)
Energy efficiency of feed pump
Exergy out from turbine
FP
m9 × ( h9 h8)
= m13 × ( p3 h7)
(21)
Exout = m15ex15 + m6ex6 +m16ex16 (34)
Exergy destruction in turbine
Exergy balance of feed pump is derived as
m13 × (ex13 ex7) = m9 × (ex9 ex8) + Exd (22) Exergy efficiency
Exd = Exin Exout – Wshaft (35) Exergy or second law efficiency of turbine
FP
= m9 ( ex9 ex8) m13 ( ex13 ex7)
(23)
turbine = Wshaft
ExinExout
(36)
-
Condenser
In condenser energy is rejected to environment (cooling water) external to the plant. This heat rejection is necessary for power cycle to complete. Efficiency term for condenser is not used. For condenser amount of heat rejected and exergy destruction are considered, which are of more importance.
Heat/energy rejected in condenser is given by
Qrej = m17× (p6 p7) (37)
Exergy destruction in condenser is given as
Exd = m17 × (ex16 ex17) (38)
-
Turbine power cycle
Energy efficiency / thermal efficiency of turbine cycle is given as
efficiencies or energy efficiencies greater than 85% except two feed pumps. However energy analysis can be sometime misleading because it does not consider quality of the energy. Energy loss can be large in quantity but it becomes insignificant when quality of energy is poor.
Component
Energy loss (MW)
Percentage energy loss
Energy efficiency (%)
Boiler 1
15.827
17.75
91.87
Boiler2
14.437
16.19
92.58
FP 1
0.701
0.79
58.30
FP2
0.526
0.59
61.86
DA 1
5.246
5.88
87.33
DA 2
3.559
3.99
89.79
Turbine 1
0.783
0.88
93.84
Turbine 2
0.932
1.05
93.19
Condenser 1
21.080
23.64
–
Condenser 2
26.080
29.25
–
Total
89.170
100.00
–
Component
Energy loss (MW)
Percentage energy loss
Energy efficiency (%)
Boiler 1
15.827
17.75
91.87
Boiler2
14.437
16.19
92.58
FP 1
0.701
0.79
58.30
FP2
0.526
0.59
61.86
DA 1
5.246
5.88
87.33
DA 2
3.559
3.99
89.79
Turbine 1
0.783
0.88
93.84
Turbine 2
0.932
1.05
93.19
Condenser 1
21.080
23.64
–
Condenser 2
26.080
29.25
–
Total
89.170
100.00
–
Table 3 Energy analysis results
cycle
= power developed at shaft heat supplied or net energy input to cycle
(39)
Heat supplied or net energy input to the cycle is given by
Qin = WT + Qrej (40)
Or
Qin= m14p4 – m15p5 – m6p- m17p7 (41)
Energy efficiency of turbine cycle
Results of exergy analysis are summarized in Table 4. It is found that maximum exergy destruction occurs in two boilers. In boilers around 238.6 MW exergy destroyed which is 90.8% of total exergy destruction. This shows that exergy destruction in two boilers is dominant over all components. Main reason for this exergy destruction is combustion process which is
cycle
= Wshaft Qin
(42)
highly irreversible and heat transfer through finite temperature difference across heat exchanging components in boiler. Next to boiler major source of exergy destruction is turbine. In two
Exergy efficiency of turbine cycle is given as
Power developed at turbine shaft
cycle= net exergy input to cycle
cycle = Wshaft
Exi
Exi is net exergy input to the turbine cycle, it derived as
(43)
(44)
turbines around 3.5% exergy destroyed. In condensers exergy destroyed is only 1.5% of total exergy destruction. This is because in condenser energy is ejected to environment at low temperature and pressure i.e. quality of energy is poor. According to energy analysis energy losses in two condensers are significant as they are about 53% of total energy lost in plant. However, exergy analysis showed that only 1.5% exergy destroyed in condensers. Real loss occurs in two boilers. First law analysis tells us that scope for improvement
Exi = m14ex14 – m15ex15 – m6ex6 +m17ex17 (45)
-
-
RESULTS AND DISCUSSIONS
Energy and exergy analysis is performed on the components of plant using above relations. All the calculations were done using Microsoft Excel software.
Results of energy analysis of the steam and power generation plant are summarized in Table 3. Note that HP heaters are not included. To find out feed water flow, it is assumed there is no energy loss in HP heater to surrounding. Total energy lost for the plant is 89.17 MW. Energy analysis also reveals that energy loss in two condensers is much higher than any other components although only 30% steam is used in power cycles. Energy loss in two condensers is 47.16 MW which is 52.89% of total energy loss in the plant. Two boilers are the second major contributors to energy loss. Energy loss in two boilers is 30.26 MW which is about 34% of total energy loss. In deaerators 9.87 MW of energy is lost. For condenser efficiency is not defined as it reject heat to environment. Most of the components have first law
exists in condenser however second law analysis showed that scope for improvement is more in boiler rather than in condenser.
Component
Exergy destruction (MW)
Percentage exergy destruction
Exergy efficiency (%)
Boiler 1
119.563
45.51
42.06
Boiler2
119.071
45.32
42.28
HPH 1
1.889
0.72
80.19
HPH 2
1.545
0.59
78.15
FP 1
1.903
0.72
33.86
FP2
1.492
0.57
36.29
DA 1
2.338
0.89
57.84
DA 2
1.787
0.68
60.32
Turbine 1
4.305
1.64
73.47
Turbine 2
4.899
1.86
72.24
Condenser 1
1.756
0.67
–
Condenser 2
2.171
0.83
–
Total
262.719
100.00
–
Component
Exergy destruction (MW)
Percentage exergy destruction
Exergy efficiency (%)
Boiler 1
119.563
45.51
42.06
Boiler2
119.071
45.32
42.28
HPH 1
1.889
0.72
80.19
HPH 2
1.545
0.59
78.15
FP 1
1.903
0.72
33.86
FP2
1.492
0.57
36.29
DA 1
2.338
0.89
57.84
DA 2
1.787
0.68
60.32
Turbine 1
4.305
1.64
73.47
Turbine 2
4.899
1.86
72.24
Condenser 1
1.756
0.67
–
Condenser 2
2.171
0.83
–
Total
262.719
100.00
–
Table 4 Exergy analysis results
Fig 2 shows graphical representation of energy loss and exergy destruction of components. Fig 3 shows comparison between energy and exergy efficiencies of components. It is found that exergy efficiencies all components are less compared energy efficiencies. Boilers have much less exergy efficiency than energy efficiency. It was assumed that there is no energy loss in two HP heaters so their energy efficiencies will be 100%. However their exergy efficiencies are less than 100%, with HP heater 1 having higher exergy efficiency than HP heater 2.
Performance of turbine power generation cycles is summarized in Table 5.
50.00
40.00
Loss %
Loss %
30.00
20.00
10.00
0.00
% Energy loss % Exergy destruction
Fig. 2. Percent energy loss and percent exergy destruction in components
100.00
Efficiency (%)
Efficiency (%)
80.00
60.00
40.00
20.00
0.00
Energy efficiency Exergy efficiency
Parameter
TG 1 cycle
TG 2 cycle
Energy loss (MW)
Total
21.86
27.01
In condenser
21.08
26.08
In turbine
0.78
0.93
Exergy destruction (MW)
Total
6.06
7.07
In condenser
1.76
2.17
In turbine
4.30
4.90
Energy or thermal efficiency (%)
35.29
32.07
Exergy efficiency (%)
66.30
64.33
Power generated (MW)
11.50
12.30
Parameter
TG 1 cycle
TG 2 cycle
Energy loss (MW)
Total
21.86
27.01
In condenser
21.08
26.08
In turbine
0.78
0.93
Exergy destruction (MW)
Total
6.06
7.07
In condenser
1.76
2.17
In turbine
4.30
4.90
Energy or thermal efficiency (%)
35.29
32.07
Exergy efficiency (%)
66.30
64.33
Power generated (MW)
11.50
12.30
Fig. 3. Comparison between energy and exergy efficiencies Table 5 Analysis results of power generation cycles
It is clear from Table 5 that exergy efficiencies of both turbine cycles are greater than thermal/energy efficiencies. The reason for low thermal efficiencies is large quantity of heat rejection in condenser to environment. Exergy efficiencies are higher because exergy destruction in condenser is very less compared to energy rejected. Analysis also showed that turbine 1 cycle is more efficient due comparatively less energy and exergy loss.
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CONCLUSIONS
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This paper represents an energy and exergy analysis performed on steam and power generation plant. The analysis performed when total steam load of the plant was 500 TPH and power generation of 23.8 MW from two turbo generators. Highest energy loss was found in two condensers where
47.16 MW energy loss which was around 53% of total energy loss. Next to condensers it was two boilers where major energy loss occurred. Energy loss in boilers was 30.26 MW which represents 34% of total loss. But the results obtained from exergy analysis were different from energy analysis. Exergy analysis showed that energy loss in condensers is insignificant due to its low quality as this energy is lost at low pressure and temperature. Exergy analysis proved that major losses are occurring in boiler rather than in condenser. In two boilers exergy destruction was 238.6 MW which represents 90.8% of total exergy destruction in plant. After boilers it was two turbines where 9.2 MW exergy destroyed which represents 3.5% of total destruction. Exergy destruction in two condensers was very less. Less than 5% exergy is destroyed in deaerators, HP heaters and feed pumps.
Exergy efficiencies of all components were less than energy efficiencies. However exergy efficiencies of two turbine cycles were higher than energy or thermal efficiencies.
Exergy efficiencies of two boilers were considerably less than energy efficiencies on account large exergy destruction. Exergy destruction in boilers is mainly due to highly irreversible combustion process and heat transfer through finite temperature difference. Analysis also showed that in power generation, turbine 1 cycle (TG 1 cycle) is more efficient than turbine 2 cycle (TG 2 cycle). Both the energy and exergy efficiencies of TG 1 cycle were greater than TG 2 cycle.
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