Steam Turbine Design Project 2011

Engineering Faculty

THERMAL-FLUID SYSTEM DESIGN INGM 427 EES DESIGN AND OPTIMISATION ASSIGNMENT October 2011

SCHOOL FOR MECHANICAL ENGINEERING

Designed by:

AC Du Randt 21108293

SUMMARY This report consists of a Thermodynamic Rankine cycle design for a power station. The design has been done through programming to solve the calculations to gather solutions. After the necessary calculations have been done, the optimisation of the Rankine cycle can be done to achieve the optimum specific work output and efficiency. The Rankine cycle design consists of the following: • • • • • •

The Cycle has a maximum steam temperature, Tmax, which can be achieved through the boiler. The Cycle also has a minimum steam temperature, Tmin, which is determined by the vacuum pressure and where the power station is situated. There are two pumps for the cycle, the first, a condensate extraction pump and the second, a boiler feed pump. A total of four turbines are present in this cycle, a high pressure, intermediate pressure and two low pressure turbines. A condenser to change the phase of the steam to saturated water by using the cooling water or system plant. Feed water heaters to preheat the water before the boiler.

The design parameters include the following: • • • • • •

The maximum temperature is limited by the metallurgic characteristics of the metal used in the boiler. The minimum temperature is limited by the location of the power plant. The maximum pressure for the boiler is determined by the critical point of water. The pressures in the different turbines. The different pressures for the steam bleed for feed water heating. The pressures for the boiler feed pump.

The goal with this design is to design and optimise the Rankine cycle for optimum specific work output as well as the efficiency of the cycle. The cycle will be solved as it is encountered in the industry with losses and efficiencies of various elements. The report also includes the discussion between steam turbine driven pump as well as electrically driven pump, wet and dry cooling systems, coal analysis.

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DECLERATION I, Armand Charl du Randt (Identity Number: 890817 5149 083), hereby declare that the work contained in this dissertation is my own work. Some of the information contained in this dissertation has been gained from various journal articles; text books etc, and has been referenced accordingly. ________________ Initial & Name

______________ Date

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TABLE OF CONTENTS Summary.................................................................................................................................i Declaration...............................................................................................................................ii Table of contents....................................................................................................................iii List of Tables.........................................................................................................................iv List of Figures..........................................................................................................................v List of Symbols and Abbreviations.......................................................................................vi

1 INTRODUCTION...................................................................................1-1 1.1 BACKGROUND..........................................................................................................1-1 1.1.1 THERMODYNAMICS..........................................................................................1-1 1.1.2 MECHANICAL DESIGN.......................................................................................1-2 1.1.3 CONTROL AND INSTUMENTATION..................................................................1-2 1.2

PROBLEM STATEMENT...........................................................................................1-2

1.3

BOUNDARY VALUES AND ASSUMPTIONS FOR IDEAL AND ACTUAL CYCLE. .1-1

1.4

GOAL STATEMENT...................................................................................................1-1

2 TASK.......................................................................................................2-3 2.1 TASK DESCRIPTION.................................................................................................2-3 2.1.1 PRIMARY PROBLEM STATEMENT...................................................................2-3 2.1.2 OUTPUTS............................................................................................................2-3 2.1.3 METHOD OF WORK...........................................................................................2-3 2.1.3.1 CARNOT CYCLE.............................................................................................2-4 2.1.3.2 RANKINE CYCLE............................................................................................2-5 2.1.3.3 RANKINE CYCLE WITH SUPERHEATING.....................................................2-7 2.1.3.4 RANKINE CYCLE WITH REHEATING............................................................2-8 2.1.3.5 RANKINE CYCLE WITH FEED HEATING.......................................................2-9

3 3.1

PRACTICAL SITUATION........................................................................3-1 ACTUAL RANKINE CYCLE.......................................................................................3-1

3.2 OPTIMISATION..........................................................................................................3-2 3.2.1 BOILER FEED PRESSURE................................................................................3-2 3.2.2 TEMPERATURE..................................................................................................3-4 3.2.3 REHEAT CYCLE PRESSURE.............................................................................3-5 3.2.4 BLEED STEAM TAPPING POINTS.....................................................................3-7

4

ADDITIONAL..........................................................................................4-10

4.1

MATERIAL SELECTION..........................................................................................4-10

4.2

STEAM VS ELECTRICALLY DRIVEN FEEDPUMP................................................4-11

4.3

WET VS DRY COOLING SYSTEMS........................................................................4-12 4

5 CONCLUSION..........................................................................................5-1 CONCLUSION LIST OF TABLES TABLE 1: BOUNDARY VALUES FOR IDEAL CYCLE..................................................................................1-1 TABLE 2: BOUNDARY VALUES FOR ACTUAL CYCLE...............................................................................1-1 TABLE 3: CARNOT CYCLE VALUES..................................................................................................2-5 TABLE 4: RANKINE CYCLE COMPARISON..........................................................................................2-6 TABLE 5: RANKINE CYCLE WITH SUPERHEATING COMPARISON..................................................................2-7 TABLE 6: RANKINE CYCLE WITH FEED HEATING COMPARISON.................................................................2-9 TABLE 7: RANKINE CYCLE WITH FEED HEATING COMPARISON................................................................2-11 TABLE 8: OPTIMISATION CHANGES...................................................................................................3-9 TABLE 9: COMPARISON FOR IDEAL AND ACTUAL.................................................................................3-9 TABLE 10: PUMP DRIVERS COMPARISON.........................................................................................4-11 TABLE 11: COOLING SYSTEM COMPARISON......................................................................................4-12 Table 11: Cooling system comparison

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LIST OF FIGURES FIGURE 1: CARNOT CYCLE...........................................................................................................2-4 FIGURE 2: RANKINE CYCLE...........................................................................................................2-5 FIGURE 3: RANKINE CYCLE PUMP WORK (ZOOM)..............................................................................2-6 FIGURE 4: RANKINE CYCLE WITH SUPERHEATING...............................................................................2-7 FIGURE 5: RANKINE CYCLE WITH REHEATING....................................................................................2-8 FIGURE 6: RANKINE CYCLE WITH FEED HEATING...............................................................................2-10 FIGURE 7: HP FEEDWATER HEATER...............................................................................................2-10 FIGURE 8: DEAERATOR CONTACT FEEDWATER HEATER........................................................................2-11 FIGURE 9: LP FEEDWATER HEATER...............................................................................................2-11 FIGURE 10: ACTUAL RANKINE CYCLE..............................................................................................3-2 FIGURE 11: T-S DIAGRAM FOR STEAM.............................................................................................3-2 FIGURE 12: EFFICIENCY AND WORK OUTPUT FOR FEEDPUMP PRESSURE....................................................3-3 FIGURE 13: MAXIMUM TEMPERATURE...............................................................................................3-4 FIGURE 14: EFFICIENCY AND WORK OUTPUT FOR REHEAT CYCLE PRESSURE...............................................3-5 FIGURE 15: DRYNESS FRACTION FOR REHEAT CYCLE PRESSURE.............................................................3-6 FIGURE 16: PRESSURE FOR BLEED POINT FOR HP HEATER...................................................................3-7 FIGURE 17: PRESSURE BLEED POINT FOR DEAERATOR..........................................................................3-8 FIGURE 18: PRESSURE FOR BLEED POINT FOR LP HEATER...................................................................3-9 FIGURE 19: PRESSURE TAPPING POINT FOR TURBINE DRIVEN FEEDPUMP.................................................4-11 FIGURE 19: PRESSURE TAPPING POINT FOR TURBINE DRIVEN FEEDPUMP

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LIST OF SYMBOLS AND ABBREVIATIONS (NOMENCLATURE) EES LP IP HP HX isn opt tur p cond eff act SSC

Engineering Equation Solver Low Pressure Intermediate Pressure High Pressure Heat Exchanger Isentropic Optimum Turbine Pump condenser Efficiency Actual Specific Steam Consumption

Ta Tmax Pa Tx Px xx Δsx ΔP η rp Wc Wt qin qout

Atmospheric Temperature Maximum Temperature Atmospheric Pressure Temperature at specific point x Pressure at specific point x Quality at specific point x Entropy generated at specific point x Pressure loss at specific point Efficiency at specific point Pressure ratio Work needed for pumps Work delivered from turbines Heat input Heat rejected

K kPa o C

Kelvin Pressure Degrees Celsius

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EES Design Assignment 2011

INTRODUCTION

The Rankine cycle is a cycle that converts heat input into work output. The heat is supplied externally to a closed loop, with water as working fluid. The Rankine cycle is the most commonly used for power generation. The Rankine cycle is sometimes referred to as a practical Carnot cycle, because, when an efficient turbine is used, the T-S diagram begins to resemble the Carnot cycle. The main difference is that heat addition and rejection are isobaric in the Rankine cycle and isothermal in the theoretical Carnot cycle. A pump is used to pressurize the working fluid received from the condenser as a liquid instead of as a gas as it is in the Carnot cycle. All of the energy in pumping the working fluid through the complete cycle is lost, as is most of the energy of vaporization of the working fluid in the boiler. This energy is lost to the cycle because the condensation that can take place in the turbine is limited in order to minimize blade erosion; the vaporization energy is rejected from the cycle through the condenser. But pumping the working fluid through the cycle as a liquid requires a very small fraction of the energy needed to transport it as compared to compressing the working fluid as a gas in a compressor as in the Carnot cycle. The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure reaching super critical levels for the working fluid, the temperature range the cycle can operate over is quite small: turbine entry temperatures are typically 565°C (the creep limit of stainless steel) and condenser temperatures are around 30°C depending on the vacuum pressure in the condenser and the temperature of the surroundings. This gives a theoretical Carnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coalfired power station.

1.1 BACKGROUND In the designing process of the Rankine cycle, the background knowledge in other applications is also important and needs to be looked at for the design. The wide field of mechanical engineering can be divided into the following focus areas:

1.1.1 THERMODYNAMICS The fields included in this section are: • • • •

Fluid mechanics Thermodynamics Thermal Machines Heat Transfer

The design of a Rankine cycle starts with the working fluid. One needs to understand the basic principles of the characteristics of the working fluid as well as how it behaves in the cycle. The thermodynamics is the main design parameter and everything is based on the thermo dynamical behaviour of the working fluid. There are various characteristics of every fluid and in some instances, a few assumptions have to be made that has to be very accurate and based on the actual cycle characteristics. In the Feed heating Rankine cycle, the working fluid passes through feed heaters that preheat the fluid before the boiler, and therefore background knowledge is important for heat transfer.

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EES Design Assignment 2011

1.1.1 MECHANICAL DESIGN The fields included in this section are: • • • • • •

Material Science Strength of Materials Mechanical Design Structure analysis Engineering Graphic Design Computer methods of programming

To design and develop all the main parts for a Rankine cycle, it is important to understand all the principles of design and the design considerations for various parts. There are a few things to take into account when it comes to design. There are various materials and with that there are lots of ways to implement them. One of the main factors of a machine is its resistance to fail, and this can be prevented with the right designing methods and materials.

1.1.1 CONTROL AND INSTUMENTATION The fields included in this section are: • • •

Control Systems Measure and control Machine dynamics

On the plant, there will be a control system that will adjust the system and all the cycle parameters to achieve the desired work load. There will also be various safety measurement instruments that take measurements and can adjust the cycle or shut-down the cycle for safety reasons. Also if there is a need for maintenance on the machines or parts of it.

1.1 PROBLEM STATEMENT Formulate a Rankine cycle for calculating, optimising and graphic representation through programming in EES. For the closed cycle the system consists of the following components: • • • • • • •

The atmospheric conditions namely Tmin and Patm. There are two pumps with different work input and different pressures. One boiler with a temperature limit due to metallurgy namely Tmax. A High pressure turbine followed by a reheat to Tmax. After the reheat, an Intermediate pressure turbine followed by two low pressure turbines. From various point, steam is bled off for the feed water heaters. After there the turbines, the fluid enters the condenser where the heat gets rejected.

The feed water heater and boilers have heat transfer efficiencies as well as pressure losses. The two pumps have isentropic efficiencies. Depending on the power delivery, the drive mechanism has either a mechanical or isentropic efficiency. The maximum temperature that can be achieved is limited by the metallurgic limit of the material used.

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1.1 BOUNDARY VALUES AND ASSUMPTIONS FOR IDEAL AND ACTUAL CYCLE Boundary Value Patm Tatm Tmax Pcondenser Tcondenser

Value Description 90 kPa The atmospheric pressure 25 oC The atmospheric temperature 535 oC The maximum temperature of the steam in the boiler 5 kPa The vacuum pressure in the condenser 32.88 oC The temperature in the condenser Table 1: Boundary values for Ideal cycle

Boundary Value Patm Tatm Tmax Pcondenser Tcondenser

Value 90 kPa 25 oC 535 oC 8 kPa 41.49 oC

Description The atmospheric pressure The atmospheric temperature The maximum temperature of the steam in the boiler The vacuum pressure in the condenser The temperature in the condenser

Pressure losses ΔPboiler ΔPvalve

5.61 MPa 150 kPa

The head and pipe losses from the feedpump to the boiler The pressure loss over the turbine inlet valve

Efficiencies ηHPturbine ηLPturbine ηFPturbine ηpump

92% The efficiency of the HP turbine 88% The efficiency of the LP and IP turbine 85% The efficiency of the steam turbine driving the feedpump 90% The efficiency of the Feedpump Table 2: Boundary values for Actual cycle

The assumptions that were made are as follow: • • • • • • •

There is an isentropic efficiency for the two pumps. There is isentropic efficiency for the HP turbine. There is isentropic efficiency for the three turbines after the reheat. The three turbines can be seen as one turbine. There are pressure losses on various components of the cycle. The feed water heaters have an efficiency of 100%. There is no subcooling in the HP and LP heaters.

1.1 GOAL STATEMENT In the design of the Rankine cycle, all four turbines are mounted on a common shaft. The following is needed to achieve the optimum performance of the cycle: • • • • • •

The heat input for the cycle, qin [kJ/kg] The heat rejected for the cycle, quit [kJ/kg] Specific work output, wnett [kJ/kg] The efficiency of the Carnot and Rankine cycle, ηRankine en ηCarnot [%] Specific Steam Consumption, SSC [kgs/kWh] Dryness Fraction, x [%]

The cycles will be represented graphically on a T-Δs diagram. In order to achieve the optimum efficiency as well as the highest specific work output, graphs for various pressures and different points for steam bleeding will be shown. 1 .

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TASK

1.1 TASK DESCRIPTION 1.1.1PRIMARY PROBLEM STATEMENT The problem state that one has to formulate an advance Rankine cycle as encountered in the industry and then optimise the cycle for maximum efficiency possible for a certain specific work output. A report has to be compiled from the design as done in EES. The design will be built up from the simple Carnot cycle to the Rankine cycle with feed water heating. The advanced cycle will have pressure losses, efficiencies etc. and will be solved for various configurations to optimise the cycle. The effect on the cycle will be discussed for each configuration. There will also be a discussion about various configurations that influence the plant and Rankine cycle such as a steam driven feed water pump, wet and dry cooling systems and coal analysis.

1.1.2OUTPUTS The following calculations will be done for the different cycles: • • • • • •

The heat input for the cycle, qin [kJ/kg] The heat rejected for the cycle, quit [kJ/kg] Specific work output, wnett [kJ/kg] The efficiency of the Carnot and Rankine cycle, ηRankine en ηCarnot [%] Specific Steam Consumption, SSC [kgs/kWh] Dryness Fraction, x [%]

The following graphical representations will be shown: • The cycles T-Δs diagrams • Efficiency against pressure for the feed pump • Specific work output against pressure for the feed pump

1.1.1METHOD OF WORK The method used for the process for reaching the advance Rankine cycle with feed heating was to start with the simple Carnot cycle, then build one element at a time into the design in order to reach the advance cycle. The steps are shown in the following points to come.

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1.1.1.1 CARNOT CYCLE For a theoretical Carnot cycle, the boiler produces dry saturated steam at a pressure of 16 MPa. It then expands through a turbine and exhausts into a condenser at 5 kPa.

Carnot Cycle

400 350

1

2

4

3

300

T [°C]

250 200 150 100 50 0 -1.0

0.1

1.2

2.3

3.4

4.5

5.6

6.7

7.8

8.9

10.0

s [kJ/kg-K] Figure 1: Carnot Cycle For the Carnot cycle: ηCarnot= Tmax-TminTmax From this equation, it can be seen that the greater the difference between that of the heat source and that of the heat sink, the greater the efficiency of the cycle. For the limit to the maximum temperature, due to the strength of the material, only the minimum temperature can still play a part and that is limited by the atmosphere and the surroundings. With a few calculations, it can be seen that the heat sink temperature is more important than the maximum temperature. Even though the efficiency of the cycle is good, the Carnot cycle can never work in practice. One of the reasons for this is that the turbine will not last that long due to the dryness factor at the last stage and therefore the blades will not last long. The other and main reason for this is that from point 4, there is a need for a compressor to compress wet steam to saturated water. Even if this were possible, the size of the compressor will be over 50 % of that of the turbine. By letting the steam condense fully from point 3 to saturated water, the water can then be pumped and the cycle evolves into a simple Rankine cycle.

Parameter 1 .

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Cycle

qin [kJ/kg]

wnett [kJ/kg]

qout [kJ/kg]

η [%]

SSC [kg/kWh]

Dryne ss Fractio n

P [MW]

Carnot Cycle

930.7

471.7

459

50.6 8

7.63

0.60

118

Table 3: Carnot Cycle Values

1.1.1.2 RANKINE CYCLE For a simple Rankine cycle, a pump would now replace the compressor of the Carnot cycle. The pump increases the pressure of the saturated water to 16 MPa. The boiler needs to heat from point 5 to 1 as well as from point 1 to 2, same as in the Carnot cycle.

Rankine Cycle

450 400

2

1

350

T [°C]

300 250 200 150 100 50

5 3

4

0 -1.0

0.1

1.2

2.3

3.4

4.5

5.6

6.7

7.8

8.9

10.0

s [kJ/kg-K] Figure 2: Rankine Cycle In the figure shown, the working fluid condenses fully to saturated water. The pump takes the water from 5 kPa and pressurise it to 16MPa. It also shows that the work done by the pump is much less than the compressor in the Carnot cycle.

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Rankine Cycle 34.38

33.59

T [°C]

5

32.81

4

32.03

0.5

0.5

0.5

0.5

0.5

0.5

s [kJ/kg-K] Figure 3: Rankine Cycle Pump Work (zoom) Even though the pump work is less, the heat input is greater and therefore the efficiency is less than for the Carnot cycle, but the specific work output is higher.

Parameter Cycle

qin [kJ/kg]

wnett [kJ/kg]

qout [kJ/kg]

Carnot Cycle

930.7

471.7

459

Rankine Cycle

2426

967

1459

η [%]

SSC [kg/kWh]

Dryne ss Fractio n

P [MW]

7.63

0.60

118

3.72

0.60

242

50.6 8 39.8 5

Table 4: Rankine Cycle Comparison Also, much more heat is rejected to the condenser which corresponds to the lower cycle efficiency. The smaller pump work, as well as the greater turbine work output, gives a much smaller SSC. The first step to improving this simple Rankine cycle is to superheat the steam.

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EES Design Assignment 2011

1.1.1.3 RANKINE CYCLE WITH SUPERHEATING For the Rankine cycle with superheating, the steam is heated into the superheated region. The steam still gets pressurised through the feed pump into the boiler. The boiler superheats the steam from where it expands through the turbine into the condenser.

Rankine Cycle with Superheating

550

3

500 450 400

T [°C]

2

1

350 300 250 200 150 100 50 0 -1.0

6

4

5

0.1

1.2

2.3

3.4

4.5

5.6

6.7

7.8

8.9

10.0

s [kJ/kg-K] Figure 4: Rankine Cycle with Superheating It can be seen that the addition of superheating to the Rankine cycle, it greatly increases the work done for the cycle. This is because the mean temperature at which the heat is added is greater than the cycles previously considered. Even though there is an increase in the work output and the efficiency, on the downside is that the boiler needs to be bigger to superheat the steam as well as the condenser. However, the improvement of the dryness fraction is of importance both for the reduced erosion of the LP turbine blades as well as the wet stage efficiency. The next step to improve the cycle is to add a reheat cycle before the second stage turbines.

Parameter Cycle

qin [kJ/kg]

wnett [kJ/kg]

qout [kJ/kg]

Carnot Cycle

930.7

471.7

459

Rankine Cycle

2426

967

1459

Rankine Cycle with

3243

1421

1822

η [%] 50.6 8 39.8 5 43.8 2

SSC [kg/kWh]

Dryne ss Fractio n

P [MW]

7.63

0.60

118

3.72

0.60

242

2.53

0.75

355

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EES Design Assignment 2011

Superheating Table 5: Rankine cycle with superheating comparison

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1.1.1.4 RANKINE CYCLE WITH REHEATING From the superheated cycle, the reheat cycle improves the cycle even further. For the reheat cycle, the steam expands partially through a HP turbine to a lower pressure and lower temperature. At this lower temperature, the steam gets heated once more to the maximum temperature. In this Rankine cycle, the feed pump pressurises the water to a pressure of 16 MPa into the boiler. The boiler superheats the steam to a maximum temperature of 535°C and it partially expands through the HP turbine. Then it enters the boiler for the second time to be reheated to the maximum temperature of 535°C and it then expands through the IP and two LP turbine into the condenser. In the condenser it condenses to saturated water.

Rankine Cycle with Reheating

550

3

5

500 450 400 1

T [°C]

350

16 MPa

2

300

4

250

3.4 MPa

200 150 100 50 0 -1.0

8

6

7

0.1

1.2

2.3

3.4

4.5

5.6

6.7

7.8

8.9

10.0

s [kJ/kg-K] Figure 5: Rankine Cycle with Reheating There is a relative slight gain in efficiency with the reheating element in the boiler. The main advantage and real purpose of the reheating is that on the final stage of the turbine, the dryness fraction is greater and therefore there is less wet steam in the final stage. The major gain in thermal efficiency due to reheating is achieved therefore as a result of reduction in wetness of the steam passing through the latter stages, thereby improving the individual stage efficiencies. There is a particular pressure at which it is most economical to reheat the steam. That this is so may be seen by considering that, if the steam is reheated early in the expansion, the additional quantity of the heat will be supplied will be small with a consequently small gain. If the reheating is done at a fairly low pressure, then although a large amount of heat is supplied, the steam will still have a high degree of superheat at the entry to the condenser with the result of a large amount of the heat supplied in the process will be thrown to waste.

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EES Design Assignment 2011

Parameter Cycle

qin [kJ/kg]

wnett [kJ/kg]

qout [kJ/kg]

Carnot Cycle

930.7

471.7

459

Rankine Cycle

2426

967

1459

3243

1421

1822

3813

1733

2080

Rankine Cycle with Superheating Rankine Cycle with Reheating

SSC [kg/kWh]

Dryne ss Fractio n

P [MW]

7.63

0.60

118

3.72

0.60

242

43.8 2

2.53

0.75

355

45.4 6

2.08

0.86

433

η [%] 50.6 8 39.8 5

Table 6: Rankine Cycle with feed heating comparison

1.1.1.5

RANKINE CYCLE WITH FEED HEATING

The Rankine cycle with feed heating is the same basic cycle as in the reheat cycle. The boiler produces superheated steam with quality 16 MPa and 535°C. The steam expands through a HP turbine to a pressure of 3.4 MPa from where the steam is reheated. The steam then expands through a IP turbine to 1 MPa. In the IP turbine, steam is bled off at 2 MPa to a HP heater to preheat feedwater before it enters the boiler. This HP heater is a closed heat exchanger. The steam then expands through two LP turbines from 1 MPa to 5 kPa in the condenser. In the LP turbine, bled steam is taken at 600 kPa to a deaerator. The deaerator is a contact heat exchanger and the steam is fed directly into the feedwater. The distillate from the HP heater also feeds into the deaerator. Also from a pressure of 150 kPa in the LP turbine, bled steam is taken to a LP feed heater, which is the same as the HP heaters, only at a lower pressure and another stage. The distillate of the LP heater exhausts into the condenser.

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EES Design Assignment 2011

Rankine Cycle with Feed Heating

550

3

5

m1

500

m2

450

12

m3

400

T [°C]

350

1

2

300

4 16

250 14

200

13 9,10

150

17

100 50 0 -1.0

15

20

18 19

7,8

6

21

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

s [kJ/kg-K] Figure 6: Rankine Cycle with feed heating For this feedwater heating cycle, two feed pumping stages are now necessary because of the intersection of the deaerator. A condenser extraction pump (point 7 & 8) takes the saturated water from the condenser and pressurise it from 5 kPa to 600 kPa and discharges it into the deaerator. The second and main feed pump takes suction in the deaerator and pressurise it from 600 kPa to 16 MPa for an ideal cycle, and discharge it into the boiler after passing through the HP heater. For the calculation of the mass flows for the bleed steam, an energy balance is made for every heater. The energy balances shown is with respect to the T-s diagram above. h(m1. h h12)+mt.h10= m1.h15+(mt. h14) 15 2 14 0

Figure 7: HP feedwater heater

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h(m2. h h16)+m1.h8 + mt-m1-m2.h20=(mt. h9) 25 1 0 9 6

Figure 8: Deaerator contact feedwater heater h(m3. h h18)+mt-m1-m2.h8= mt-m1-m2.h20+(m3. h21) 28 8 1 1 20

Figure 9: LP feedwater heater It is important to note that the efficiency of the regenerative feed heating cycle may be considerably improved by increasing the number of stages of the feed heating, but it is limited for practical reasons in the industry. The main reason for this improvement is that less heat is thrown to the condenser without doing any work. On the practical side, there are a few additional improvements, such as due to feed heating, the mass flow in the final stages of the turbine is lower and therefore a smaller turbine can be used. Also, due to less mass flow in the condenser, a smaller area is needed for heat transfer so the condenser can be smaller.

Parameter SSC [kg/kWh]

Dryne ss Fractio n

P [MW]

7.63

0.60

118

3.72

0.60

242

43.8 2

2.53

0.75

355

2080

45.4 6

2.08

0.86

433

1541

49.6 2

2.37

0.86

379

Cycle

qin [kJ/kg]

wnett [kJ/kg]

qout [kJ/kg]

Carnot Cycle

930.7

471.7

459

Rankine Cycle

2426

967

1459

3243

1421

1822

3813

1733

3058

1517

Rankine Cycle with Superheating Rankine Cycle with Reheating Rankine Cycle with Feed Heating

η [%] 50.6 8 39.8 5

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Table 7: Rankine cycle with feed heating comparison

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PRACTICAL SITUATION

2.1 ACTUAL RANKINE CYCLE For the ideal Rankine cycle, all the calculations have been done and compared to other cycles with added elements. Now, the Rankine with feed heating cycle have to be done as it is in reality. For a practical cycle in the industry, every part of the cycle has losses and efficiencies. The following will be added to the ideal cycle to make it an actual cycle: • • • •

Pressure losses over the boiler, for pipe and head loss. Pressure losses over the valves at the inlet of the turbines. Isentropic efficiencies for the HP, IP and LP turbines. Isentropic efficiencies for the extraction and boiler feed pump.

The optimisation is done so that the balance between cycle efficiency and specific work output can be found. For optimisation, the following will be looked at: • The working pressure of the boiler, or delivery pressure of the main feed pump. • The effect of the maximum temperature of the steam. • The reheat cycle pressure, or inlet pressure for the IP turbine. • The bleed steam tapping point for the HP heater. • The bleed steam tapping point for the deaerator which will also be the delivery pressure of the extraction pump as well as the inlet pressure for the main feed pump. • The bleed steam tapping point for the LP heater. Even though there are a lot of configurations for the optimum cycle, there are a few parameters that cannot be changed that are set to a value that is determined by other factors. These parameters are the following: • • • •

The maximum temperature of the boiler is limited to the metallurgic limit of the material used. Therefore, due to practical reasons, the maximum temperature is fixed at the temperature before the metal excursions takes place. The minimum temperature in the condenser is also a set value, because of the maximum vacuum pressure in the condenser as well as the atmospheric temperature and the cooling water temperature. The maximum pressure of the boiler cannot exceed the critical pressure of water. The dryness fraction on the final stages cannot exceed a certain amount because of erosion on the turbine blades.

In the figure below, the efficiency of the turbines were added as well as the pressure losses over the valves. The boiler feed pump normally delivers a pressure of 22 MPa into the boiler where the pressure has dropped to 18 MPa due to the head and pipe loss. The pressure drops even further through pipe and valve loss to 16 MPa for the HP turbine. The steam expands through the turbine to a pressure of 3.5 MPa before the entry to the IP turbine where there is another valve loss to a pressure of 3.4 MPa. The steam expands through the IP and LP turbines to 5 kPa in the condenser where the steam condenses to saturated water. The condenser extraction pump takes suction from 5 kPa and delivers 600 kPa which is the deaerator pressure and the suction pressure of the boiler feed pump. The points where the steam is bled from is normally 2 MPa for the HP heater, 600 kPa for the deaerator and 150 kPa for the LP heater. For this cycle, a steam drive feed pump is added to the cycle where steam is bled off from 2 Mpa and expands into the condenser.

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EES Design Assignment 2011

Actual Rankine Cycle 550 3

m1

5

500

m2

450

m3

400 12 ,22

350

1

m4

4

2

T [°C]

300 250

16 14

200

13 9,10

150

17 20

100 50

15 18

19

7,8

6 ,23

21

0 -1

0

1

2

3

4

5

6

7

8

9

s [kJ/kg-K] Figure 10: Actual Rankine Cycle

1.1 OPTIMISATION 1.1.1BOILER FEED PRESSURE Steam

500 450 400

21.5 MPa

Critical Pressure Point

18 MPa

350

T [°C]

300 250 200 150 100 50 0 -1.0

0.1

1.2

2.3

3.4

4.5

5.6

6.7

7.8

8.9

10.0

s [kJ/kg-K]

Figure 11: T-s diagram for Steam For water as working fluid, the critical point is the point where no change of state can take place when the pressure is sufficient or if heat is added. At the critical point the water and steam can't be distinguished, and there is no point referring to water or steam. For states above the critical point the steam is supercritical. Supercritical is not the same as superheated - which is saturated steam at lower pressures and temperatures heated above the saturation temperature. 1 .

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EES Design Assignment 2011

For a supercritical steam boiler, the operating pressure is so high (22.06 MPa) that actual boiling ceases to occur, the boiler has no liquid water - steam separation. There is no generation of steam bubbles within the water, because the pressure is above the critical pressure at which steam bubbles can form. It passes below the critical point as it does work in a high pressure turbine and enters the condenser. For this design, a normal boiler will be used with only superheated capability. The figure below, for the actual cycle, show that the greater the pressure of the feedpump, the greater is the specific work output and the more improved the efficiency. However, the boiler pressure is limited by the critical pressure of steam and therefore cannot exceed that pressure. The boiler pressure is lower than the delivery pressure of the feedpump due to the pipe and head losses by a 18/22 factor.

Feedpump Pressure

1280

0.44

1276

0.436

1271

0.432

1267

0.428

0.424 17

ηRANKINE wnetto 19

21

23

25

w netto [kJ/kg]

η RANKINE

0.444

1262

1258 27

Pfeedpump [MPa] Figure 12: Efficiency and work output for feedpump pressure Therefore the boiler pressure selected will be close to the critical pressure for the better performance of the cycle. However the dryness fraction decreases and this will lead to more wet steam in the final stages of the turbine. For the ideal cycle, the dryness fraction is too small for the final stage of the turbine to prevent erosion on the blades, but for the actual cycle, the steam is still dry enough in the final stages of the turbine. The next step is to find the optimum outlet pressure of the HP turbine which will also be the reheat cycle pressure.

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EES Design Assignment 2011

1.1.2TEMPERATURE For the maximum temperature that the boiler can deliver, the efficiency as well as the specific work output improves as the temperature increases. However there is a limit to this maximum temperature, because of the creep rupture strength of the tubes in the boiler. For a temperature of 540 °C, the temperature in the boiler is around 1600 °C and therefore the limit to the temperature is in the boiler and not in the HP turbine.

T emperature

0.448

1400

1380 0.444

0.44

1340

ηRANKINE wnetto

0.436

1320

w netto [kJ/kg]

η RANKINE

1360

1300 0.432 1280

0.428 500

510

520

530

540

550

560

1260 570

Tmax [C] Figure 13: Maximum temperature

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EES Design Assignment 2011

1.1.3REHEAT CYCLE PRESSURE For the pressure at which the reheat should take place, there are a few parameters to consider. One of the parameters that will change is the efficiency; because of the heat input will change as the pressure changes. It’s the same with the specific work output that will change with the pressure change. Another parameter that is controlled by the reheat pressure is the dryness fraction in the final stage of the turbine or inlet into the condenser; because the expansion through the IP and LP turbines are depended of the reheat pressure.

Reheat Cycle Pressure

1370

0.438

ηRANKINE 1350 wnetto

0.436

1330

0.434

1310

0.432

1290

0.43 2

3

4

5

6

w netto [kJ/kg]

η RANKINE

0.44

1270 7

Preheat [MPa] Figure 14: Efficiency and work output for reheat cycle pressure For this figure shown, the efficiency improves the greater the pressure, but it flattens out. However, the specific work output improves as the pressure lowers, and keeps improving. The reason for this is that the HP turbine delivers more work and the steam gets reheated to the same temperature and then expands through the IP and LP turbines. The reason for the efficiency decreasing at lower pressure is that the temperature drops through the turbine, so the further the pressure drops through expanding in the turbine, the more heat input is needed and therefore the efficiency is less. Therefore a balance between the efficiency and the specific work output is needed for the optimum performance of the cycle.

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EES Design Assignment 2011

Another factor that plays a roll on the reheat pressure is the dryness fraction. If the dryness fraction is too low, the steam is too wet in the final stages in the turbine and then erosion occurs. If the dryness fraction is too high, more heat is rejected to the condenser and therefore the efficiency drops. So the higher the pressure, the lower the dryness fraction and therefore the efficiency are higher. 0.44

0.98

0.439 0.97

x6 ηRANKINE 0.438

0.96

0.437

η RANKINE

x6

0.95

0.436

0.94

0.435

0.93

0.434

0.92

0.433

0.91

0.432

0.9 2

3

4

5

6

0.431

Preheat [MPa] Figure 15: Dryness fraction for reheat cycle pressure The optimum performance for the cycle will be decided with the balance in the pressure for the efficiency and the specific work output. The next step of the optimisation is to decide where the tapping points for the bleed steam will be and what effect it will have on the cycle.

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EES Design Assignment 2011

1.1.4BLEED STEAM TAPPING POINTS For the feed heating cycle, there are three points where the steam can be bled from. The first is in the IP turbine cycle where the steam is bled for the HP heater. For the feed heating cycle, the HP heater is there to preheat the water after the feedpump and before the boiler. In the graph shown, the efficiency improves as the bleeding pressure increases, up to a point and then it decreases again. The specific work output decreases as the pressure rises, because there is less mass flow to do work in the turbine as the bleeding pressure increases. If the bleeding pressure is less, it is possible for the turbines to do more work before the tapping point. Therefore it will be best to find the right balance for the tapping point depending on the interest in efficiency or in specific work output. For this practical cycle, a value in between will be picked for the best efficiency and output.

0.44

Pressure for HP heater tapping point

1380

ηRANKINE wnetto 1360

0.439

1340

1320 0.437 1300 0.436 1280 0.435

0.434 1

wnetto [kJ/kg]

η RANKINE

0.438

1260

1.5

2

2.5

3

1240 3.5

Pm,dot,1 [MPa] Figure 16: Pressure for bleed point for HP heater

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EES Design Assignment 2011

The deaerator pressure is the delivery pressure of the condensate extraction pump and the inlet pressure of the main feedpump. Therefore the deaerator pressure is the same pressure from which the steam for the contact heater is bled. The steam is bled from the LP turbine cycle for the contact heater. In the graph shown, there are a specific point where the efficiency and specific work output is a maximum at a certain pressure. As the pressure increases, the efficiency as well as the work output is decreased. The reason for this is that the turbines do less work and the extraction pump must do more to deliver the sufficient pressure. Therefore the tapping point pressure will be selected on the point for maximum efficiency and specific work output.

Pressure for deaerator tapping point

0.444

ηRANKINE wnetto

0.442

1330

1320 0.438 1315 0.436 1310 0.434

wnetto [kJ/kg]

1325

0.44

η RANKINE

1335

1305

0.432

1300

0.43 0.5

1

1.5

2

2.5

1295 3

Pm,dot,2 [MPa] Figure 17: Pressure bleed point for deaerator

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EES Design Assignment 2011

The LP heater takes steam from the LP turbine cycle and heats up the water from the outlet of the extraction pump and heats it up to a certain point. For obvious reasons, if the pressure is too low, there can be no work done and therefore the efficiency also lowers. The graph shows that there is a certain point where the values are a maximum. Therefore the pressure will be selected for the maximum efficiency and specific work output.

Pressure for LP heater tapping point

0.44

1340

ηRANKINE

0.439

wnetto

1335

1330

0.437 0.436

1325

0.435

1320

wnetto [kJ/kg]

η RANKINE

0.438

0.434 1315

0.433 0.432 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1310 0.4

Pm,dot,3 [MPa] Figure 18: Pressure for bleed point for LP heater In the table below, the values for the optimisation changes are shown. Parameter Value 26 MPa Pfeedpump 4 MPa Preheat 8 kPa Pcondenser 1.75 MPa Pm_dot_1 0.66 MPa Pm_dot_2 0.1 MPa Pm_dot_3 0.66 MPa Pdeaerator 1 MPa PSFP Table 8: Optimisation changes

Cycle Rankine Cycle with Feed Heating (Ideal) Rankine Cycle with Feed Heating

qin [kJ/kg]

wnett [kJ/kg]

qout [kJ/kg]

η [%]

SSC [kg/kWh]

Dryne ss Fracti on

P [MW]

3058

1517

1541

49.6 2

2.37

0.86

379

3042

1336

1706

2.694

0.930 3

334

43.9 2

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EES Design Assignment 2011

(Actual) Table 9: Comparison for Ideal and Actual

2

ADDITIONAL

2.1 MATERIAL SELECTION The material selection for a boiler is very important for the performance of the boiler in terms of the heat transfer to the working fluid, the resistance against corrosion and resistance against abrasive wear. The typical challenges for the selections of materials for a boiler are the high temperature and pressure at which the boiler operates as well as the aggressive media in the boiler. The material used for the tubes in the boiler is mainly stainless steel. The difference between normal stainless steel and these used for the boiler tubes is that the material is more stable in the austenite region. The reason for this stability is the increased content CrNi and CrNiMo in the steel over the standard 18/8 CrNi and 18/8/2 CrNiMo steels, and more especially by additions of nitrogen, which is particularly effective in promoting the austenite stability. Typical properties of austenitic heat resistance materials include: • High creep rupture strength above 550 °C • Resistance to high-temperature corrosion and oxidation • Excellent processing characteristics Creep rupture strength is one of the most important properties of materials used for tubes working under pressure. The improvement in creep rupture strength can be attributed to metallurgical changes related to specific alloying. The stainless steels alloys with the highest creep rupture strength are the “supercritical” grades. The high creep rupture strength in the high-nickel steels is due to precipitates in the matrix. Despite differences in the mechanical and physical properties of the materials, due to their chemical composition, there are certain similar characteristics that are attributable to their metallurgical face-centred cubic lattice structure. Besides creep rupture strength, ductility is an important factor governing the suitability of a material for a given application. Under long-term stress both stabilized and non-stabilized steels display lower values of reduction in area after fracture. This is due to the hardening effect of special carbide precipitates and metallic phases. However, high-nickel steels, as well as nickel base alloys, also shows very good ductility values after long-term exposure to service stresses across the range of high-temperature applications.

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EES Design Assignment 2011

1.1 STEAM VS ELECTRICALLY DRIVEN FEEDPUMP For a practical Rankine cycle, there are two pumps providing pressure for the system. The first pump is the condenser extraction pump and is much smaller than the main boiler feedpump. The extraction pump takes suction from the condenser and delivers the water in the deaerator with a pressure that will be the intake to the main feedpump. The extraction pump is electrically driven, which is very practical because it is quite small compared to the main feedpump. For the main feedpump which takes suction from the deaerator and delivers pressure to the boiler, is quite large and takes a lot of power from the system to pump the condensate. The difference between the steam turbine driven and the electrically driven pumps is their advantages and disadvantages as well as the electrical units sent out. The table below shows a few characteristics of the two different drive systems: (the figures shown is for a 600 MW unit with 700 MW available before the HP turbine) Steam turbine driven Advantage Disadvantage Better efficiency for Expensive turbine the whole cycle No auxiliary power Difficult to use on needed different loads

Electrically driven Advantage Disadvantage Very easy on start-up Expensive motor Easy to control over different loads

Needs auxiliary power (20 MW)

Efficiency Generator reading Units Generated Units sent-out

85% Efficiency 75% 591.6 MW Generator reading 616 MW 573.9 MW Units Generated 597.5 MW 571 MW Units sent-out 563 MW Table 10: Pump drivers comparison For a steam driven feedpump, the tapping point for the bleed steam is not that important as is was for the HP and LP heaters. The amount of work needed by the pump is equal to the amount of work that the turbine will deliver. So for a higher pressure selected, the mass flow of the steam through the turbine will be less. In the figure shown, the efficiency and the specific work output of the cycle increases as the tapping pressure decreases. 0.4394

Steam turbine driven feedpump tapping point pressure ηRANKINE

1337 1337

wnetto

0.4394

1336 1336 1336 0.4393 1336 0.4392

1336

wnetto [kJ/kg]

η RANKINE

0.4393

1336 0.4392 1336 0.4391 0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

1336 2

PSFP [MPa] 1 .

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Figure 19: Pressure tapping point for turbine driven feedpump

1.2 WET VS DRY COOLING SYSTEMS In the power generating industry, there are always factors that cannot be changed and forms part of the location of the power station. These factors are the coal, water, ambient temperature. One of the most important components of the power generating cycle is the condenser, which condense the steam back to water to be pumped. For the condenser to condense the steam, a cooling system is needed to cool down the water used to condense the steam. There are three ways of doing so, the first is a wet cooling system, the second a dry cooling system using fans and the third also a dry cooling system, but a dry cooling tower is used. The wet cooling tower is full of tubes that circulate the water through the cooling tower and condenser. The cooling water is cooled by sprinkler system in the tower, with a large dam at the bottom of the tower. The disadvantage of this system is that the location of the power station needs to be close to water, because a lot of water is needed and it evaporates. The two dry systems are basically the same. The first using fans for the convection heat transfer between the tubes and air, whereas the second uses a tower that is full of tubes, but the tower is shaped so that there is natural draft of air through the tower which provides the convection heat transfer. The disadvantages of the first system are that it uses a lot of auxiliary power and the second is that it relies on the ambient temperature. In the table below, the three systems are compared. Wet cooling system

Dry cooling system Fan draft system Tower draft system Not very expensive Very expensive construction

Efficiency of the cycle is higher Specific work output is Maintenance needs to be Very little maintenance greater done on the fans The water used for this Uses a lot of auxiliary power No auxiliary power needed system is high Table 11: Cooling system comparison

2

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CONCLUSION

The generating of power starts with a simple principal of energy cannot be created or destroyed. Therefore when it comes to a coal fired power station, the energy that is in the coal as it comes from the mine, is converted into heat energy and used to heat water which becomes steam. The steam is then used in turbines that put out work and drives the generator and delivers electricity which is then fed into the grid. The cycle used for delivering work is a Rankine cycle that has been optimised for better efficiency and work output. It all starts with a simple Carnot cycle and from there it is developed into a Rankine cycle. The normal Rankine cycle is then developed into a cycle that takes the steam into the superheated region, with a reheat cycle and steam is bled off for the feed water heating. These added components are there to improve the cycle and to make it more feasible for practical cases. After the basic cycle has been formed, the optimisation of the cycle starts. This is done to get the best possible performance out of the turbines as well as what is needed and certain compromises that have to be made. Components that can be optimised are the feedpump delivery pressure, feed heater tapping points and reheat pressure. The optimisation can be limited by certain parameters that cannot be changed and has to for part of the cycle. The power station as a whole has a major impact on the environment and the location where it is based. The pollution coming from a power station and the effect of the cooling water that evaporates into the sky can also have an effect on the weather. Nevertheless, power is needed everywhere and therefore it must be generated, but the challenge is to make it as clean and efficient as possible.

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