1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 2, February (2014), pp. 17-25
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IJMET
©IAEME
SENSITIVITY ANALYSIS OF HEAT RECOVERY STEAM GENERATOR
FOR A GE 6FA GAS TURBINE
S.Naga Kishorea,
Dr. T.V.Raob
a, b
Department of Mechanical Engineering, DBS Institute of Technology, Kavali-524201
ABSTRACT
The objective of the present study is to optimize the bottoming cycle for a 6 FA gas turbine.
The present work focuses on optimization of steam cycle alone by analyzing the sensitivity of steam
pressure and steam temperature and configuration of waste heat recovery boiler. A two pressure
configuration will have better opportunity for power generation and for applying service steam for
feed stack and drying etc. From cycle analysis at 4 kg/cm2 low pressure steam appear to be most
suitable for the thermodynamic cycle. Also from the analysis of various configurations a two
pressure steam cycle was chosen with integrated deaerator. Both low temperature and intermediate
pressure were used to extract maximum heat from the gas turbine exhaust. A fixed 2% blow down
steam was assumed as per the industrial practice. Two super heater sections were introduced with a
desuperheater to minimize the energy loss. The temperature of the high pressure steam was kept at
5000c and the pressure of steam has varied to calculate the variation of power and efficiency. And
also keeping the steam temperature at 4000C, 4500C. The power and efficiency at different pressures
ranging from 40 ata to 62 ata were calculated to get an operating point and to allow the steam turbine
designer to have suitable pressure and temperature variation to select the appropriate steam turbine at
bottom cycle.
Keywords: Combined Cycle (Cc), HRSG, Efficiency, NTU, Off Design, Effectiveness.
INTRODUCTION
The gas/steam combined cycle has already become a well-proven and important technology
for power generation due to its numerous advantages. The advantages include its high efficiency in
utilizing energy resources, low environmental emissions, short duration of construction, low initial
investment cost, low operation and maintenance cost, and flexibility in fuel selection, etc.[11]. These
features justify the fact that the combined cycle power plants are quite competitive in the power
market.
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
The heat recovery steam generator (HRSG) is the component of the bottoming steam cycle,
which absorbs energy of exhaust gas of the gas turbine and produces steam at subcritical pressures
suitable for the process or for further electricity generation by a steam turbine. Power plant engineers
can design their own HRSGs and the bottoming steam cycles at the initial stage. On the other hand,
gas turbine is not made in order and steam turbine is selected according to the condition of the steam
delivered from a HRSG. In this respect, the design of a HRSG is indispensable to the improvement
of the overall system efficiency and power output, and to the reduction of the main equipment cost
[5, 8, 9]. HRSGs are classified into single, dual, and triple pressure types depending on the number
of drums in the boiler. Dual pressure HRSGs have been widely used because they showed higher
efficiency than single pressure systems and lower investment cost than triple pressure HRSGs.
Murad A Rahim et al studied three types of HRSG and worked on the effects of HRSG design to the
net power generation and overall efficiency of the cycle was performed. Steam pressure, pinch point
temperature difference and approach temperature difference were taken as variables [1]. Meeta
Sharma and Onkar Singh presented a detailed mathematical modeling and analysis for segmented fin
in the HRSG for its various sub-components. The optimization was also done on the basis of
maximum heat recovery with minimum pressure drop for a given heat flow [2]. P.Ravindra Kumar
and V.Dhana Raju studied the operating characteristics of triple pressure reheat HRSGs which were
analyzed. The effects of the configuration of HP superheater and reheater on the thermal
performance and economics of the plant were investigated. The arrangement of IntermediateTemperature components such as intermediate pressure (IP) superheater, HP economizer, and low
pressure (LP) superheater were changed to check its effect on the performance of a steam turbine.
The off-design performance was also examined considering the operating ranges of the plant [3].
Mustafa Zeki Yilmazoglu and Ehsan Amirabedin carried out exergy analysis of a combined cycle
gas turbine (CCGT) power plant, in Ankara, in an exergy aspect. The exergy efficiency of each
component and overall plant was studied by calculating exergy destructions and sensitivity analysis
was performed by changing some critical parameters of the system [4]. C.Casarosa and Franco
investigated the thermodynamic analysis to design the operating parameters for the various
configurations of HRSG systems to minimize Exergy losses, taking into account only the
irreversibility due to the temperature difference between the hot and cold streams (pressure drop not
accounting). All the solutions lead to the zero pinch point and infinite heat transfer surface [7].
T.Srinivas projected thermodynamic modelling and optimization of a dual pressure reheating HRSG
in the CC with a deaerator. The variations in CC performance were plotted with the compressor
pressure ratio, gas turbine inlet temperature, HRSG HP, steam reheat pressure and deaerator
pressure. From this study of thermodynamic modelling, the optimized conditions to air compressor,
HP steam, LP steam, steam reheater and deaerator were developed [13].
In this study, sensitivity analysis of a heat recovery steam generator in a combined cycle gas turbine
(CCGT) power plant for GE 6FA Gas Turbine is performed. The power and efficiency at different
pressures ranging from 40 ata to 62 ata are calculated by varying temperatures of steam from 400oC
to 500oC for getting an operating point and for allowing the steam turbine designer to have suitable
pressure and temperature variation to select the appropriate steam turbine at bottom cycle.
PLANT DESCRIPTION
The heat recovery steam generator is a series of heat exchangers. It consists of three heat
exchangers (economizer, evaporator, superheater) for every pressure level. Economizers are used to
heat water close to saturation, evaporators to produce saturated steam and superheaters to produce
superheated steam. Every heat exchanger is a bundle tubes placed in-line or staggered arrangement
according to the manufacture. The flow of working fluid (water or steam) in the pipes is horizontal
flow and the flow of exhaust is vertical flow that's mean, each heat exchanger in the HRSG could be
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3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
considered a cross flow heat exchanger. The type of circulation fluid in the evaporator is a forced
circulation by circulated pump for each pressure level. Steam drums are used in HRSG to separate
the water from outlet steam from evaporator .The exhaust gases of the gas turbine consider are a
wake heat transfer coefficients comparison with heat transfer coefficients of working fluid in
evaporator and economizer therefore, finned tubes are used in HRSG. Fins can be sold or serrated
.The most important parameters of HRSGs are pinch point, approach point and gas side pressure
drop through the heat recovery system, which affect the effectiveness of heat exchange. Pinch point
is the difference between the gas temperature leaving the evaporator section of the system and the
saturation temperature corresponding to the steam pressure in that section. Approach point is the
difference between the saturation temperature of fluid and inlet temperature of fluid in evaporator.
The coils in the present configuration of HRSG are HP superheater 1, HP superheater 2, HP
Evaporator, IP Superheater, IP Evaoprator, LT Economizer, LP Evaporator is illustrated in Fig. 1.
Fig 1: Aschematic arrangement of combined cycle power plant Triple pressure HRSG
PERFORMANCE ANALYSIS
In the HRSG, (water or steam) and exhaust gas of the gas turbine flow inside and outside of
the tube respectively. Water or steam and exhaust gas exchange heat through a Cross-flow type Heat
Exchanger at each section such as Economizer, Evaporator, and Superheater. For the simplicity of
simulation procedure, each heat exchanger is assumed to be a Counter-flow type through the whole
HRSG because the inlets of the water and the exhaust gas locate in the opposite side. The heat
transferred at each module is calculated using the energy balance equation as described in Eq. (1).
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4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
----------(1)
The gas side and water/steam side temperatures at the inlet and outlet of each section of the
HRSG are determined using this energy balance equation Using the well Known effectiveness –NTU
method with overall heat transfer coefficient, Udesign, the area of HRSG together with other related
parameters is calculated based on design parameters shown in equation
---------------(2)
Q max is the maximum heat transfer rate that could possibly be delivered by the heat
exchanger [12].
The heat transfer area can be evaluated with the fin geometry data, and this area should match
the one determined by the effectiveness-NTU method as above. Through the iterative procedure, the
area is determined when the one equals to the other within a prescribed criterion.
Off-design performance analysis of the HRSG [6] is carried out using the design parameters
described as above. The procedure to determine the thermodynamic properties at each section (T, p ,
h ) is repeated until the parameters converge to the assumed value. For illustration, the NTU at offdesign operation is given by Eq. (3).
----------------(3)
Then, we can determine effectiveness as a function of NTU and actual heat transfer rate, as in
Eq. (4). Also we can determine thermodynamic properties such as temperature, pressure, and
enthalpy at each heat exchanger during off-design operation.
-----------------(4)
Off-design performance of gas turbine is estimated with the gas turbine performance curves.
Steam turbine is assumed to operate on the sliding pressure mode during off-design operation.
Performance of each section of steam turbine is analyzed using the well-known
Spencer/Cotton/Cannon correlations [10]
Design conditions such as atmospheric conditions and cycle parameters are shown in Table 1.
At this working condition, the exhaust gas of gas turbine is 687,747 kg/hr in flow rate and 610°C in
temperature, which is suitable for the conventional steam turbine. The total heat transfer area of heat
exchangers is found 21971.1263 m2. Mass flow rate, absolute pressure and temperature of various
heat exchangers are shown in Table 2.
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5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
Table 1:
Gas Turbine Model
Ambient air Dry Bulb temperature, oC
Capacity factor, %
Fuel Type
Fuel Flow Rate, Kg/S
Turbine Inlet Temperature, oC
Turbine Exhaust Temperature, oC
Pressure Ratio
Condenser Pressure, bar
Based Pinch Point temperature Difference, oC
Based Approach temperature, oC
Coil Description
HP Superheater 1
HP Superheater 2
HP Evaporator
HT Economizer
IP Superheater
IP Evaporator
LT Economizer
IP Economizer
LP Evaporator
Outlet
Pressure
(Kg/cm2
abs)
62.0
62.0
66.4
66.5
4.0
7.2
67.5
7.2
3.2
Table 2:
Gas inlet Gas Outlet
GE Frame 6F A
25
85
Natural gas
191
1206
610
0.05
40
10
(oC)
(oC)
Process
Inlet
(oC)
610.0
553.6
509.9
320.6
267.3
264.5
236.2
236.2
212.8
553.6
509.9
320.6
267.3
264.5
236.2
211.9
213.6
176.2
283.3
281.0
281.0
164.4
165.2
165.2
134.4
134.4
134.7
Process
Outlet
(oC)
Process
Flow
(Kg/h)
450.6
397.8
281.0
261.7
235.0
165.2
164.2
165.2
134.7
104,819
91,576
1,982,880
93,483
12,711
318,984
93,408
12,970
387,660
RESULTS AND DISCUSSIONS
The review of results are shown below that the inlet and out let temperatures of exhaust gas –
steam for the different coils in the given HRSG configuration. The coils in the configuration are HP
superheater 1, HP superheater 2, HP Evaporator, IP Superheater, IP Evaoprator, LT Economizer, LP
Evaporator.
It is observed that the variation of maximum power with pressures at different steam
temperatures as shown in figures . The mass of steamflow is increased by increasing the pressure of
steam. And the power and efficiency are also increased.
It is observed that power and efficiency are increased linearly with respect to pressure at a
temperature of 400oC as shown in the Graph1. The maximum power is developed at 58 ata and the
maximum efficiency is obtained at 60 ata when steam at a temperature of 425oC as shown in
Graph 2. Hence it is required optimization.
It is understood that the optimum point is obtained at a pressure of 58ata which is giving
maximum power and efficiency as shown in Graph 3 at a tempeature of 450oC. The maximum power
and the maximum efficiency are obtained at 60 ata when steam at a temperature of 475oC as shown
in Graph 4. Graph 5 shows the variations of power and efficiency with pressure at a temperature of
500oC. Here the maximum power and efficiency are obtained again at a pressure of 58ata. Graph 6
and 7 show the variations of steam flow of high pressure and low pressure superheaters with respect
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6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
to pressure. From the results, it was found that the most suitable pressure for the configuration is
58ata for the temperature ranges between 450oC to 500oC.
The present analysis is allowed for wide range of steam turbines and it can be selected for the
purpose of without depending on a single steam tubine with its specific steam pressure and
temperature. The system designer can be selected a turbine which could give maximum power or
efficiency through the analysis.
Pr. Vs Power & Efficiency
38
36.5
36
36
35.5
35
34.5
32
34
30
Efficiency
Power
34
power
efficiency
33.5
33
28
32.5
26
32
40
45
56
58
60
62
Pr(ata)
Graph 1. Variation of power and efficiency with pressure at temperature of 4000C
35
34.5
34
33.5
33
32.5
32
31.5
31
30.5
30
35
34.5
34
33.5
33
32.5
32
31.5
31
30.5
30
35
40
45
50
55
60
Efficiency
Power
Pr. Vs Power, Efficiency
Power
Efficiency
65
Pr (ata)
Graph 2. Variation of power and efficiency with pressure at temperature at 4250C
Pr. Vs Power & Efficiency
36
35
35
34
34
33
33
32
32
31
31
30
30
29
29
28
28
27
efficiency
37
36
Power
37
Power
Efficiency
27
40
45
56
58
60
62
Pr(ata)
Graph 3. Variation of power and efficiency with pressure at temperature at 4500C
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7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
35
34.5
34
33.5
33
32.5
32
31.5
31
30.5
30
35
34.5
34
33.5
33
32.5
32
31.5
31
30.5
30
35
40
45
50
55
60
Efficiency
Power
Pr. Vs Power, Efficiency
Power
Efficiency
65
Pr (ata)
Graph 4. Variation of power and efficiency with pressure at temperature at 4750C
Pr. Vs Power & Efficiency
36
35
35
34
34
33
33
32
32
31
31
30
30
29
29
28
28
27
27
40
45
56
58
60
Efficincy
37
36
power
37
62
Pr(ata)
Power
Efficiency
Graph 5. Variation of power and efficiency with pressure at temperature at 5000C
Ms(HP) tons/hr
Pr. Vs Ms (hp) tons/hr
110
100
90
80
70
60
50
HP
50
55
60
65
Pr(ata)
Graph 6. Variation of steam flow of high pressure superheater steam
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8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 17-25, © IAEME
Pr. Vs Ms(IP)
Ms(IP) tons/hr
11.5
11.1
10.7
10.3
9.9
9.5
0
20
40
60
80
Pr(ata)
Graph 7. Variation of steam flow of low pressure superheater steam
CONCLUSIONS
1) From the sensitivity analysis it was found that most suitable pressure appears to be at 58 ata
and steam temperature between 450oCand 500oc will be adequate to give for 100MW power
output and system efficiency around 35% which is on the higher side for that rating. Avoiding
reheating the system cycle configuration was kept crucial.
2) The present analysis allows a wide range of steam turbine that can be selected for the purpose
with out depending on a single steam turbine with its specific steam pressure and temperature.
Through the analysis the system designer select a turbine which could give maximum power
or efficiency.
3) The detailed mechanical design also carried out which provides a reasonable size for the waste
recovery boiler. Two pressure steam not only gives higher power using injection in steam
turbine but also if required provide service steam for drying and process requirement.
4) BHEL manufactures 6 FA gas turbine under license, 6 FA was selected because it was locally
manufactured is having higher efficiency. This is mostly suitable for IGCC plant. It can also to
be noted that the work carried out at so much work for burning low Btu gas such as coal gas
generated in fluidized bed using air whose calorific value is limited to be about 1000 kcal/kg.
Low Btu gas increases the pressure drop in combustion chamber and requires modification for
combustion stability and also air fuel ratio to maintain same entry turbine temperature as
desired, inspite of 7 to 8 times low calorific value has been introduced in the combustion
chamber. This requires modification of gas feeding system, blade cooling concept as well as
fuel gas controlling system.
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