220242en_ CCPP Concepts.pdf

8/14/2019 220242en_ CCPP Concepts.pdf

1/42


2/42


3/42


4/42


5/42


6/42


7/42


8/42


9/42


10/42


11/42


12/42


13/42


14/42


15/42


16/42


17/42


18/42


19/42


20/42


21/42


22/42


23/42


24/42


25/42


26/42

CCPP Combined Cycle Power PlantOperation Concepts

Power Plant Training Center

CSXA220242en_ Page 26

AB C D

Triple-Pressure Cycle (continued)

The energy flow diagram (Fig. 19) shows the slight improvement in the steam

turbine output over the dual-pressure cycle for the reasons stated above. The stacklosses are the same (same stack temperature) but there is a slight decrease in heatload in the condenser that corresponds to the gain in steam turbine output. For gasturbines with lower exhaust gas temperatures, more energy is available for IP steamproduction because the HP steam production is lower, making this concept moreattractive.

100 %

Energy in Fuel

37 .6%

21 .1%

0.5%

0.3%

8.2%0.3%

32 .0%

Losses in HRS GGas Turbine

Steam Turbine

Condenser

Stack

Losses

Q

Losses

CSXA401799. cd r

Figure 19: Energy Flow Diagram for the Triple Pressure Combined-Cycle Plant


27/42




AB C D

Reheat Cycles

Reducing ofMoisture Content

From our analysis, the moisture content in the steam turbine exhaust significantlylimits further improvements in the performance of the various cycles. By extendingthe idea of mild reheat, the full reheat cycle is derived in which this moisture contentis reduced (Fig. 22) and improvement in the performance is possible. The idea isthat (taking the triple-pressure cycle as an example) after the expansion of the HPsteam in the steam turbine to IP level, the steam returns to the HRSG and mixes withthe steam leaving the IP superheater. This steam is then heated to a temperaturesimilar or equal to the HP live-steam temperature before being admitted to thesteam turbine. In a dual-pressure reheat cycle there is no mixing on re-entering theHRSG, the cold reheat goes to an independent reheater section.

The reheater takes more exergy out of the HRSG and increases the steam turbineenthalpy drop resulting in a higher steam turbine output. Because the last part ofthe steam turbine expansion line is moved further to the right on theenthalpy/entropy diagram, there is a reduction in the moisture content (Fig 22).

Figure 20 is the flow diagram of the triple-pressure cycle with reheat. The steamturbine has separate HP and IP/LP casings (13, 14) in order to accommodate theextraction of the cold reheat steam. The high-pressure bypass (15), instead ofdumping steam into the condenser, dumps it into the cold reheat line to keep thereheat cooled (i.e., the line leaving the HP steam turbine).

(Continued next page)


28/42




AB C D

Reheat Cycles (continued)

GG

1 Comperssor 2 G as Turbine3 Dual HP Supperheater / Reheater 4 HP Evaporator 5 HP Economizer / IP Supperheater 6 IP Evaporator 7 Dual IP Economizer 8 LP Evaporator 9 Dual LP Economizer 10 HP Drum11 IP Drum12 LP Drum

13 HP Steam Turbine14 IP / LP Turbine15 HPSteam Bypass16 IP Steam Bypass17 LP Steam Bypass

P Feedwater Pump23 G as Turbine Cooler

18 Condenser 19 Condensate Pump20 Feedwater Tank / Dearator 21 HP Feedwater Pump22 I

7

8

HP

IP

14

19

22 21

1 2

3

4

5

6

9

13

18

15

10

11

12

16

LP

17

20

Natural Gas

CSXA401 818.cd r

Figure 20: Flow Diagram of a Triple Pressure Reheat Cycle



29/42


30/42




AB C D


Decrease of

Moisture Content

The decrease in the moisture content is illustrated on the enthalpy/Entropy diagram

(Fig. 22). The value is 10% compared to 16% in the triple-pressure example. Thepercentage of the stack losses has increased 0.4% (Fig. 23) because the stacktemperature is slightly higher 103 C (217 F). This occurs when the reheatersteals energy from the HP section of the HRSG, resulting in less HP steamproduction and therefore less feedwater heating in the HP economizers. Increases inIP and LP mass flows do not compensate for this and so the stack temperatureincreases. Combined with the additional steam turbine output, this explains thelower heat loads in the condenser.

T 568 C

HP IP LP

1 2

0 b a r

3 0

b a r

5 b a

r

S a t u r a t io n L in e

0, 0 4 5 b a

r

(1-x ) = 10%

(1-x ) = Moisture Content

T

S(Entropy ) ( T e m p e r a t u r e

)

CSXA401 805.cd r

max

Figure 22: Temperature / Entropy Diagram

Showing the Effect of Full Reheat on the Steam Turbine Expansion Line

100 %

Energy in Fuel

37 .6%

21 .7%

0.5%

0.3%

8.6%0.3%

31 .0%

Losses in HRS GGas Turbine

Steam Turbine

Condenser

StackLosses

Q

Losses

CSXA401 800. cd r

Figure 23: Energy Flow Diagram for the Triple Pressure Reheat Combine-Cycle Plant(Continued next page)


31/42




AB C D


Energy /

TemperatureDiagram

The energy/temperature diagram for the triple pressure reheat cycle is shown in

Fig. 24. The energy taken out in the HP superheater/reheater and HP evaporator isapproximately 138 MW compared to 140 MW for the triple-pressure cycle. This ismainly due to the HP evaporator pressure level difference. However, the energy istransferred at a higher temperature level and because the combined mass flow ofthe HP superheater/reheater is higher than that of the HP/IP superheater in thetriple pressure cycle, this part of the diagram moves closer to the exhaust gastemperature. This results in an exergy gain and therefore a higher steam turbineoutput.

HP / IP / LPEconomi zer

IP Evaporatotor

LP Evaporator

E x h a u s t G a s

H P S u p e r h e a t e r / R e h e a t e r

HP Evaporator

Energy Transfer (MW)

T e m p e r a t u r e ( C )

0

100

200

300

400

500

600

700

50 100 1 50 200 2500

HP / IP Economi zer

HP Economi zer /IP Super heater

CSXA40179 8.cd r

Figure 24: Energy/Temperature Diagram for a Triple Pressure Reheat HRSG

Clearly, there is a performance benefit in a reheat cycle because of the greaterexpansion in the steam turbine and the improved exergy utilization in the HRSG.Still more is achieved if the live-steam pressure can be raised. State of the arttechnology can accomplish this task. For dual and triple-pressure cycles, the HPpart of the cycle makes the main contribution to the performance. Therefore, this isthe area in which major investments should be concentrated in order to improve thereheat cycle performance.



32/42


33/42




AB C D


1 Gas Turbine2 HP Supperheater / Reheater 34 HP Evaporator 5 Water Separator

HP Economizer

6 LP Supperheater 7 Recirculation Pump8 LP Evaporator 9 LP Drum

10 LP Economizer 11 HP Steam Turbine12 IP / LP Steam Turbine13 HP Feedwater Pump14 HP Steam Bypass15 LP Steam Bypass16 Condenser 17 Condensate Pump18 G T Cooler

78

1

2

3

4

5

6

9

13

15

10

11 12

16

14

LP

17

Natural Gas

G

G

CSXA401 82 0. cd r

Figure 25: Flow Diagram of a High Pressure Rehaet Cyclewith a HP Obe Through and a Drum Type LP Section

The cycle has no feedwater tank, so deaeration occurs in the condenser and arecirculation loop in the LP economizer of the HRSG raises the temperature of theincoming feedwater. The condensate pumps (17) must be designed to bring thecondensate to the LP drum, which also fulfils the function of LP feedwater pumps.The HP feedwater pumps (13) take suction from the LP drum.



34/42




AB C D


Heat Balance The heat balance is shown in Figure 1-46. As expected, performance is further

improved. The gross output is 2.4 MW above that of the triple-pressure reheat cycleand efficiency increases from 59.3% to 59.8% - a considerable improvement. Partof this benefit comes because a once-through HRSG is designed with an approachpoint temperature of zero but the same pinch-point (12 K (22 F)) is used as in theother examples. However, due to the increased pressure of the HP system, powerconsumed in the feedwater pumps is higher, slightly reducing the gain in the netpower output. The example is offered with a fuel that has no sulphur content so thefeedwater temperature has been reduced to optimize the energy in the cold end ofthe HRSG. In such a cycle, the feedwater temperature affects the HRSG surface butnot the performance. This is valid as long as there is sufficient energy available inthe exhaust gas to preheat the incoming condensate. Gross efficiencies

approaching 60% are the current state of the art for combined-cycle power plantswith this concept under the given ISO conditions.

P bar T CM kg/sX rel. Humidity

Total Power ( Gross) = 282 .9 MWGross Efficency (LHV) = 59 .8%

Natural Gas

P 1.013T 80M 386 .7

T 4 5M 79.1

M 8.0 M 0

P 7.4T 1 67

P 7.0

T 3 22M9. 5

T 647M 386 .7

P 1.103T 1 5X 60

473 MW

17 8 MW

P 42.8T 568M 61. 6

P 40.0T 371M 61. 6

P 38.0T 565M 61. 6 P 6 .6

T 3 20M 9.5104. 8 MW

P 0.04 5T 31M 71.1

M 0

M 0

P 153. 6T 565M 61. 6

P 160T 568

M 61. 6G

G

CSXA401 82 1. cd r

Figure 26: Heat Balance for a dUal Pressure Reheat Cycle with Once Through HRSG


35/42




AB C D

Cycles with Supplementary Firing

Purpose Supplementary firing increases the plant output by installing duct burners in theHRSG inlet duct. This adds energy to the cycle by increasing the exhaust gastemperature often at the expense of efficiency. Supplementary firing is appropriatefor HRSGs because there is usually sufficient oxygen content in the exhaust gas toact as combustion air. In an open cycle gas turbine, with a single stage ofcombustion, only 30-50% of the oxygen contained in the air is used for combustion.

Earlier combined-cycle installations generally had supplementary firing. This is notthe case today due to progress in the development of the gas turbine. As gas-turbine inlet temperatures and hence exhaust gas temperatures increase, theimportance of supplementary firing diminishes for two reasons. First, because thetemperature window between the gas turbine exhaust and the duct burner exhaustdecreases so the added benefit of supplementary firing decreases. Second,optimum values can be given to the water/steam cycle parameters with the gasturbine alone. Since current levels of efficiency are set by reheat cycles, theefficiencies achieved in cycles with supplementary firing lead to a high cost ofelectricity.

Nevertheless, increased operating and fuel flexibility of the combined-cycle withsupplementary firing may be an advantage in special cases, particularly ininstallations used for cogeneration of heat and power where this arrangement

makes it possible to control the electrical and thermal outputs separately.(Continued next page)


36/42




AB C D

Cycles with Supplementary Firing (continued)

Energy /

TemperatureDiagram

Figure 27 shows energy/temperature diagrams for a single-pressure HRSG with

constant live-steam conditions and inlet exhaust gas temperatures of 647

C,(1,197 F), 750 C, (1,382 F) and 1,000 C, (1,832 F), the latter two, aftersupplementary firing. At 647 C the temperatures of gas and water in theeconomizer are convergent, with the minimum difference in temperature on theevaporator end, typical for an HRSG without supplementary firing. At 1,000 C(1,832 F), on the other hand, the minimum difference in temperature is at the inletto the economizer on the waterside. This pattern corresponds more to that of aconventional steam generator.

100

0

200

300

400

500

600

700

800

900

1000

1100

10 20 30 40 50 60 70 80 90 1000

Energy Transfer (%)

T e m p e r a t u r e

( C )

100

0

200

300

400

500

600

700

800

10 20 30 40 50 60 70 80 90 1000

Energy Transfer (%)


( C )

100

0

200

300

400

500

600

700

10 20 30 40 50 60 70 80 90 1000

Energy Transfer (%)


( C )

Figure B

Figure C

Figure A

CSXA401 806.cd r

Figure 27: Energy/Temperature Diagramfor 647 C (A), 750 C (B) and 1000 C (C) Exhaust Gas Temperature Entering the HRSG

A temperature of 750 C (1,382 F) supplementary firing gives the best exergeticutilization of the exhaust gas with a constant difference in temperature along theentire economizer. The single pressure cycle here is at an optimum with the exhaustgas cooled down to a temperature close to the feedwater temperature. There is noexergy or energy available for additional pressure levels.



37/42




AB C D


Figure 28 shows how relative power output and efficiency depend on the

temperature after supplementary firing for a single and a dual-pressure cycle. Thereference point at 647 C (1,197 F) is the single-pressure cycle. The reference pointat 647 C (1,197 F) is the single-pressure cycle without supplementary firing. Above750 C (1,382 F) there is no longer any performance benefit in the dual-pressurecycle. Calculations assume the use of natural gas fuel. When burning oil, the curvepaths for the single-pressure system are not significantly changed, but there is lessdifference between the single and dual-pressure systems. Triple-pressure and triple-pressure reheat cycles follow the same pattern, starting out with a larger differencewithout supplementary firing and ending at the same point at 750 C (1382 F).

90

95

100

10 5

110

115

120

125

130

65 0 700 7 50 800 85 0 900 9 50 1000

Ex haust Gas Temperature (C)

Relati ve Gross Effi cien cy

Relati ve Gross Output

Dual P ressure

Single P ressure

R e l a t i v e

G r o s s

P l a n t

E f f i c i e n c y

a n d r e l a t i v e

G r o s s

O u t p u t

( % )

CSXA401 807. cd r

Figure 28: Effect of Temperature after Supplementary Firingon Power Output and Efficiency Relative to that of a Single Pressure Cycle

For installations with supplementary firing that are frequently operated at partloads, it could make economic sense to select a cycle with more pressure levels oreven reheat. At part loads or when the supplementary firing is switched off, theexhaust gas temperature at the outlet of the HP section rises and an LP sectionenables this exhaust gas energy to be used.



38/42




AB C D


Heat Balance Figure 29 shows the heat balance of a typical combined-cycle plant with

supplementary firing to 750

C, (1,382 F) with natural gas and a feedwatertemperature of 60 C (140 F). The basic arrangement for this installation is thesame as that for the single-pressure system in Figure 4.

P bar T CM kg/sX rel. Humidity

Gross Power Output = 303 .5 MWGross Efficiency (LHV) = 57 .9%

Natural Gas

T 60P 0.2

P 1.013T 71M 387.7

P 113.1T 31 8

17 8 MW

P 1.013T 1 5X 60%

T 647M 386 .7

P 10 5T 565M 96.5

M 0P 0.04 5

T 31M 91.3

125 .5 MW

P 0.3M 5.2

P 100. 8T 565M 96.5

M 96.5

52 4 MWT 7 50

51.0 MW

CSXA401 810. cd r

G

G

Figure 29: Heat Balance for a Single Pressure Cycle with Supplementary Firing

Supplementary firing increases the steam turbine output by 24.3 MW compared tothe cycle without supplementary firing. The efficiency rises slightly from 57.7% to57.9% because the increased steam production also results in increased mass flowsthrough the economizers removing more energy from the exhaust gas, thuslowering the stack temperature. Generally, for cycles with more pressure levelssupplementary firing has a negative effect on the efficiency because withoutsupplementary firing these cycle already make maximum use of the exhaust gasenergy.


39/42




AB C D

Summary of Cycle Performance

To give an overview of all the cycles discussed in this section, the performance datafor the examples in this section are summarized in Table 1. The auxiliary powerconsumption for each case ant the resulting net output and efficiency are alsogiven, which are important for comparing the cycles in real terms.

S i n g

l e P r e s s u r e

D u a

l P r e s s u r e

T r i p l e P r e s s u r e

T r i p l e P r e s s u r e

R e h e a

t

D u a

l P r e s s u r e

R e h e a

t

S i n g

l e P r e s s u r e

S u p p

l e m e n

t a r y

F i r i n g

Gas Turbine Fuel Input (LHV) MW 473 473 473 473 473 473

Duct Burner Fuel Input (LHV) MW 0 0 0 0 0 51

Total Fuel Input (LHV) MW 473 473 473 473 473 524

Gas Turbine Output MW 178 178 178 178 178 178

Steam Turbine Output MW 94.8 99.0 99.7 102.5 104.9 125.5

Gross Output MW 272.8 277 277.7 280.5 282.9 303.5

Gross Efficiency (LHV) % 57.5 58.6 58.7 59.3 59.8 57.9

Auxiliary Consumption MW 4.1 4.5 4.5 4.6 5.2 5.0

Net Output MW 268.7 272.5 273.2 275.9 277.7 298.5

Net Efficiency (LHV) % 56.8 57.6 57.8 58.3 58.7 57.0

Net Heat Rate (LHV) kJ/kWh 6.337 6.249 6.233 6.172 6.132 6.320

Net Heat Rate (LHV) Btu/kWh 6.006 5.923 5.908 5.850 5.812 5.990

Table 1: Performance Comparison for Different Cycle Concepts(Natural Gas Fuel with Low Sulphur Content)

The above examples illustrate how the net efficiency can rise from 56.8% to 58.7%and the output from 268.7 MW to 277.7 MW by changing the concept of thewater/steam cycle. Adding supplementary firing can increase this output further, butfor cycles with several pressure levels at the expense of the efficiency.


40/42




AB C D

Exercises Combined Cycle Power Plant ConceptsHRSG Energy / Temperature Diagram

Exercise Task Draw an energy/temperature diagram for your combined cycle power plants HRSGand do the following: Define (and calculate) the thermal energy exchanged in each boiler section. Color each specific heat exchanger section in your cross-sectional boiler

drawing. Compare and discuss the thermal energy calculations with your colored heat

exchanger sections. Discuss the effect a gas turbine exhaust gas temperature drop of 50 C (GT

remains at base load) would have on your boiler performance.

Exercise Aids HRSG process data table HRSG cross section drawing with tubing details Guarantee Heat balance

Group,Time Frame,Presentation

Group work with max. 4 trainees per group 30 minutes Group presentation in front of class


41/42




AB C D

Energy Flow Diagram

Exercise Task Calculate and draw an energy flow diagram for your combined cycle plant

including the energy flow internally in the gas turbine (energy consumption ofcompressor) to understand: Energy input GT internal energy use Power output (GT; ST) Energy losses

Exercise Aids Process flow sheet Water/Steam Cycle Guarantee Heat balance

Group,Time Frame,Presentation

Group work with max. 4 trainees per group 20 minutes Group presentation in front of class


42/42

CCPP Combined Cycle Power PlantOperation Concepts AB C D

SummaryObjectives Review

Carefully review the objectives stated on page 6 of this lesson together with the TestQuestions for Verification of Learning Progress. Note that this lesson is not completewithout the system specific and overall plant Operation & MaintenanceDocumentation.

220242en_ CCPP Concepts.pdf

Documents

Transcript of 220242en_ CCPP Concepts.pdf