Combined Cycle Gas Turbine.ppt

Combined Cycle Gas Turbine

Combined Cycle Gas Turbine. CCGT Power Plant Abbas A M Al Fardan

Natural Gas Fueled Combution Turbine Combined Cycle Electricity Generator.flv


What is the CCGT? A combined cycle gas turbine power plant, frequently identified by CCGT shortcut, is essentially an electrical power plant in which a gas turbine and a steam turbine are used in combination to achieve greater efficiency than would be possible independently. The gas turbine drives an electrical generator. The gas turbine exhaust is then used to produce steam in a heat exchanger (steam generator) to supply a steam turbine whose output provides the means to generate more electricity. However the Steam Turbine is not necessarily, in that case the plant produce electricity and industrial steam which can be used for heating or


Basic Gas Turbine Information •Main Gas Turbine Manufactures: General Electrics, Simens Westinghouse & Alstom •Approximately Cost per MW – 0.7mln E •Efficiency approx 40% for gas turbine however in the CCGT plant the efficiency is 50-60% (even higher for cogenerated plant) •Low Green Gas Emission C02, NOx & SOx •Chepear comparing to other technology e.g. CCS •Lifetime 30-40 years


How it works?

220kV Tabert Substation

110kV Clahane Substation


CCGT Fuel Available in KSA Natural Gas. Resources available in KSA Synthetic Gas from coal. Resources not available in KSA

Fuel Oil. Resources available in KSA

Biogas from forestry, domestic and agricultural waste. Resources not available in KSA


CCGT Plants Conventional or Cogeneration Variable

CCGT

High Efficiency Cogeneration

Transmission Network

Lower Impact

Higher Impact

Power Losses

Less power losses

Higher Power Losses

Heat Market

Required

Not Required

Fuel consumption

-33%

+33%

CO2 Emission

-67%

+67%

Water Consumption

-30%

+30%

Capital Cost per kW delivered

630

1200


grid Grid Code Grid Code contains general conditions and rules for general application. The specification and conditions for each application are adjust individually. Those information are included in Grid Connection Offer & Agreement between developer and Transmission Operator TSO. •Client (Requires connection) and TSO must implement Grid Code specification during each stages of the project, for project above 10MW •TSO may be disconnected or terminated the Grid Connection Agreement if the Grid Code is not implemented by client. •The Implementation of the Grid Code may have significant impact on the cost of the Grid Connection


Grid Constraints •Capacity of the transmission lines • Small Infrastructures of the High Voltage Lines •Distance from Energy Load Centres (West Coast) • High Cost of Design and planning permission for Shallow Connection, significantly for OHL 220kV •Planning Restrictions regarding OHL Construction


Grid Connection Costs Variable

Cost

Gas & Steam Turbine Generator

210’000’000

2 bay 110kV/220kV Substation

4’420’000

220kV OHL

710’000/km (12km)

110kV OHL

320’000/km (15km)

Buried Cable 500MVA (optional)

2’150’000/km

Total Cost

227’740’000


Gas Turbine Basics • Gas Turbines – Types – How They Work – Applications – Components of Plant – Flow Paths – Operation

10


Gas Turbine Applications • Simple Cycle • Combined Cycle • Cogeneration

11


Types of Gas Turbine Plants • Simple Cycle – Operate When Demand is High – Peak Demand – Operate for Short / Variable Times – Designed for Quick Start-Up – Not designed to be Efficient but Reliable • Not Cost Effective to Build for Efficiency • Combined Cycle – Operate for Peak and Economic Dispatch – Designed for Quick Start-Up – Designed to Efficient, Cost-Effective Operation – Typically Has Ability to Operate in SC Mode

12


Principles of Operation • Open Cycle Also referred to as simple cycle)

 

The energy contained in a flowing ideal gas is the sum of enthalpy and kinetic energy. Pressurized gas can store or release energy. As it expands the pressure is converted to kinetic energy. Link to picture

13


Brayton Cycle – Gas Turbine Cycle

14


Thermodynamic Fundamentals •

Pressure Ratio & CT Components

15


Combustion or Gas Turbine

16


Principles of Operation Compressor • As air flows into the compressor, energy is transferred from its rotating blades to the air. Pressure and temperature of the air increase. • Most compressors operate in the range of 75% to 85% efficiency. Combustor • The purpose of the combustor is to increase the energy stored in the compressor exhaust by raising its temperature. Turbine • The turbine acts like the compressor in reverse with respect to energy transformation. • Most turbines operate in the range of 80% to 90% efficiency.

17


Principles of Operation Overall Energy Transformations (Thermal Efficiency) • Useful Work = Energy released in turbine minus energy absorbed by compressor. The compressor requires typically approximately 50% of the energy released by the turbine. • Overall Thermal Efficiency = Useful Work/Fuel Chemical Energy *100 Typical overall thermal efficiencies of a combustion turbine are 20% - 40%.

18


Gas Turbine Applications • Simple Cycle

COMBUSTOR

COMPRESSOR

INLET AIR

TURBINE

GENERATOR

EXHAUST GAS

19


Simple Cycle Power Plant Westinghouse 501D5 – 340 MW

20


Combined Cycle Power Plant

21

GT PRO 13.0 Drew Wozniak 12.54 p 90 T 30 %RH 944 m 4327 ft elev.

Net Power 95959 kW LHV Heat Rate 7705 BTU/kWh 967.3 m

1X GE 6581B 149.2 p 684 T

Fogger 4.717 m

143.2 p 2072 T

2 X GT

73.85 %N2 13.53 %O2 3.233 %CO2+SO2 8.497 %H2O 0.8894 %Ar

12.93 p 1034 T 1934.6 M

33781 kW

12.39 p 68 T 948.7 m 30813 kW Natural gas 18.58 m LHV 369671 kBTU/h 77 T

122 T 292.6 M 122 T

850 p 950 T 248.6 M

96 T

1.694 p 120 T 222.1 M

17.19 p 220 T

292.6 M 29.65 M 29.58 M

0.1296 M

26.36 M

6.89 M V8

195.8 p 597 T

V4

183 p 375 T 70 M

879.8 p 954 T


Combined Cycle Plant Design

120 T

6.89 M FW

LPB

IPE2

IPB

HPE2

IPS1

HPE3

IPS2

HPB1

HPS3

268 T 1934.6 M

1031 T 1934.6 M 17.19 p 220 T 29.65 M 268

203.6 p 373 T 292.6 M 326

p[psia], T[F], M[kpph], Steam Properties: Thermoflow - STQUIK 1512 10-13-2004 23:27:31 file=C:\Tflow13\MYFILES\3P 0 70.gtp

203.6 p 924.2 p 383 T 472 T 36.75 M 251.1 M 419

481

534

199.7 p 910.5 p 460 T 523 T 36.75 M 251.1 M 538

568

195.8 p 910.5 p 500 T 533 T 36.75 M 248.6 M 569

879.8 p 954 T 248.6 M 897

1031 Natural gas 0M

22


Gas Turbine Components Compressor – Combustor - Turbine

23


Gas Turbine Components & Systems (cont’d) • Combustion System – Silo, Cannular, Annular – Water, Steam, DLN

• Turbine – Multiple Shaft, Single Shaft – Number of Stages – Material and Manufacturing Processes

 Exhaust

System

Simple Cycle Stack  Transition to HRSG 

 Generator

Open-Air cooled  TEWAC  Hydrogen Cooled 

 Starting

Systems

Diesel  Motor  Static 

24


Combustion Turbine Fuels • Conventional Fuels – Natural Gas – Liquid Fuel Oil

• Nonconventional Fuels – Crude Oil – Refinery Gas – Propane

• Synthetic Fuels – Chemical Process – Physical Process 25


GE Combustion Turbine Comparisons

26


Gas Turbine Types  

Advanced Heavy-Duty Units Advanced Aero derivative Units

27


Gas Turbine Major Sections • • • • • •

Air Inlet Compressor Combustion System Turbine Exhaust Support Systems

28


Gas Turbine Barrier Inlet Filter Systems

29


Gas Turbine Pulse Inlet Filter System

30


Inlet Guide Vanes

31


Inlet Guide Vanes

32


Gas Turbine Compressor Rotor Assembly

33


6B Gas Turbine

34


Gas Turbine Cut Away Side View

35


Gas Turbine Combustor Arrangement

36


Frame 5 GT

37


GE LM2500 Aero-derivative Gas Turbine

Compressor

Compressor Turbine Section

Power Turbine Section

38


FT4 Gas Turbine

39


FT4 Gas Turbine – Gas Generator Compressor)

40


FT4 Gas Turbine – Gas Generator (Compressor)

41


FT4 Gas Turbine – Free Turbine

42


FT4 Gas Turbine – Free Turbine Gas Path

43


FT4 Gas Generator Performance

44


FT4 Free Turbine Performance

45


Aero-derivative Versus Heavy Duty Combustion Turbines • Aero-derivatives – Higher Pressure Ratios and Firing Temperatures Result in Higher Power Output per Pound of Air Flow – Smaller Chilling/Cooling Systems Required – Compressor Inlet Temperature Has a Greater Impact on Output and Heat Rate – Benefits of Chilling/Cooling Systems are More Pronounced

46


Typical Simple Cycle CT Plant Components • Prime Mover (Combustion Turbine) • Fuel Supply & Preparation • Emissions Control Equipment • Generator • Electrical Switchgear • Generator Step Up Transformer • Starting System (Combustion Turbines) • Auxiliary Cooling • Fire Protection • Lubrication System 47


Typical Peaking Plant Components

Lube Oil System

Switchgear / MCC

GSU

Generator

Starting Engine

Fire Protection

48


Combining the Brayton and Rankine Cycles • Gas Turbine Exhaust used as the heat source for the Steam Turbine cycle • Utilizes the major efficiency loss from the Brayton cycle • Advantages: – – – – – –

Relatively short cycle to design, construct & commission Higher overall efficiency Good cycling capabilities Fast starting and loading Lower installed costs No issues with ash disposal or coal storage

• Disadvantages – High fuel costs – Uncertain long term fuel source – Output dependent on ambient temperature

49


How does a Combined Cycle Plant Work?

Picture courtesy of Nooter/Eriksen

50


Combined Cycle Heat Balance

51


Combined Cycles Today • Plant Efficiency ~ 58-60 percent – Biggest losses are mechanical input to the compressor and heat in the exhaust

• Steam Turbine output – Typically 50% of the gas turbine output – More with duct-firing

• Net Plant Output (Using Frame size gas turbines) – up to 750 MW for 3 on 1 configuration – Up to 520 MW for 2 on 1 configuration

• • • • •

Construction time about 24 months Engineering time 80k to 130k labor hours Engineering duration about 12 months Capital Cost ($900-$1100/kW) Two (2) versus Three (3) Pressure Designs – Larger capacity units utilize the additional drums to gain efficiency at the expense of higher capital costs

52


Combined Cycle Efficiency • • •

•

•

Simple cycle efficiency (max ~ 44%*) Combined cycle efficiency (max ~58-60%*) Correlating Efficiency to Heat Rate (British Units) = 3412/(Heat Rate) --> 3412/ = Heat Rate* – Simple cycle – 3412/.44 = 7,757 Btu/Kwh* – Combined cycle – 3412/.58 = 5,884 Btu/Kwh* Correlating Efficiency to Heat Rate (SI Units) = 3600/(Heat Rate) --> 3600/ = Heat Rate* – Simple cycle – 3600/.44 = 8,182 KJ/Kwh* – Combined cycle – 3600/.58 = 6,207 KJ/Kwh* Practical Values – HHV basis, net output basis – Simple cycle 7FA (new and clean) 10,860 Btu/Kwh (11,457 KJ/Kwh) – Combined cycle 2x1 7FA (new and clean) 6,218 Btu/Kwh (6,560 KJ/Kwh)

*Gross LHV basis

53


Gas Turbine Generator Performance Factors that Influence Performance – Fuel Type, Composition, and Heating Value – Load (Base, Peak, or Part) – Compressor Inlet Temperature – Atmospheric Pressure – Inlet Pressure Drop • Varies significantly with types of air cleaning/cooling – Exhaust Pressure Drop • Affected by addition of HRSG, SCR, CO catalysts – Steam or Water Injection Rate • Used for either power augmentation or NOx control – Relative Humidity

54


Altitude Correction

55


Humidity Correction

56


Cogeneration Plant • A Cogeneration Plant – Power generation facility that also provides thermal energy (steam) to a thermal host. • Typical thermal hosts – paper mills, – chemical plants, – refineries, etc… – potentially any user that uses large quantities of steam on a continuous basis. • Good applications for combined cycle plants – Require both steam and electrical power

57


Major Combined Cycle Plant Equipment • • • • •

Combustion Turbine (CT/CTG) Steam Generator (Boiler/HRSG) Steam Turbine (ST/STG) Heat Rejection Equipment Air Quality Control System (AQCS) Equipment • Electrical Equipment

58


Heat Recovery Steam Generator (HRSG)

59

Combined Cycle Gas Turbine Steam Turbine

GE D11

60


Primary to Secondary to End-Use Energy

P r im a r y E n e rg y

Losses

Losses

T r a n s fo r m a tio n T r a n s p o r ta tio n D is tr ib u tio n

U tiliz a tio n D e v ic e o r S y s te m

S e c o n d a ry E n e rg y

F in a l U s e fu l E n e rg y


Outline • • • •

Electricity Basics Electricity from Fossil Fuels Co-generation and Tri-generation Economics


Electricity Basics • Electricity can be either direct current (DC) or alternating current (AC) • In AC current, the voltage and current fluctuate up and down 60 times per second in North America and 50 times per second in the rest of the world • The power (W) in a DC current is equal to current (amps) x voltage (volts): P=VI • The power in an AC current is equal to the product of the root mean square (RMS) of the fluctuating current and voltage if the current and voltage are exactly in phase (exactly tracking each other): P=Vrms x Irms • The standard electricity distribution system consists of 3 wires with the current in each wire offset by 1/3 of a cycle from the others, as shown in the next figure

Combined Cycle Gas Turbine Three-phase AC Current


Two Pole Synchronous Generator

Source: EWEA


• Electricity demand continuously varies, and power utilities have to match this variation as closely as they can by varying their power production. The following distinctions are made: • Base_load power plants: these are plants that run steadily at full load, with output equal to the typical minimum electricity demand during the year. Plants (such as coal or nuclear) that cost a lot to build but are cheap to operate (having low fuel costs) are good choices • Peaking powerp lants: these are plants that can go from an off state to full power within an hour or so, and which can be scheduled based on anticipated variation in demand (natural gas turbines or diesel engines would be a common choice) • Spinning reserve: these are plants that are on but running at part load – this permits them to rapidly (within a minute) vary their output, but at the cost of lower efficiency (and so requires greater fuel use in the case of fossil fuel power plants).


Electricity from Fossil Fuels • Pulverized coal • Integrated Gasification/Combined Cycle (IGCC) • Natural gas turbines and combined cycle • Diesel and natural gas reciprocating engines • Fuel cells


Technical issues related to electricity from fossil fuels • • • • •

Full load efficiency Part-load efficiency Rates of increase of output Impact of temperature on output Auxiliary energy use


Generation of electricity from a conventional, pulverized-coal power plant G e n e ra to r

s te a m

H ig h - P r e s s u r e B o ile r

e le c tr ic ity o u t

fo s s il fu e l in

S te a m T u r b in e

a ir ( O 2 ) CO

to c o o lin g t o w e r o r c o ld riv e r w a te r

2

w a te r c o n d e n s a te CO

2

a n d /o r c o g e n e r a tio n

u p th e s ta c k s e q u e s te r e d C O out

2

C ondenser P um p

Source: Hoffert et al (2002, Science 298, 981-987)

c o o lin g w a t e r r e t u r n f lo w


The upper limit to the possible efficiency of a power plant is given by the Carnot efficiency: η = (Tin-Tout)/Tin So, the hotter the steam supplied to the steam turbine, the greater the efficiency. Hotter steam requires greater pressure, which requires stronger steel and thicker walls. so there is a practical limit to the achievable Carnot efficiency (and actual efficiencies are even lower)


Coal power plant operating temperatures and efficiencies • Typical: 590ºC, 35% efficiency • Best today: > 600ºC, 42-44% efficiency • Projected by 2020: 720ºC, 48-50% efficiency


Integrated Gasification Combined Cycle (IGCC) • This is an alternative advanced coal power plant concept • Rather than burning pulverized solid coal, the coal is heated to 1000ºC or so at high pressure in (ideally) pure oxygen • This turns the coal into a gas that is then used in a gas turbine, with heat in the turbine exhaust used to make steam that is then used in a steam turbine • Efficiencies of ~ 50% are expected, but are much lower at present


Generation of electricity with natural gas • • • •

Simple-cycle power generation Combined-cycle power generation Simple-cycle cogeneration Combined-cycle cogeneration


Simple-cycle turbine • Has a compressor, combustor, and turbine proper • Because hot gases rather than steam are produced, it is not restricted in temperature by the rapid increase in steam pressure with temperature • Thus, the operating temperature is around 1200ºC


Simple-cycle gas turbine and electric generator EXHAUST FUEL C O M B U STO R

SHAFT

E L E C T R I C IT Y G EN ERATO R

CO M PRESSO R

T U R B IN E

IN T A K E A IR

Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)


Efficiency of generating electricity using natural gas • One might expect a high efficiency from the gas turbine, due to the high input temperature (and the resulting looser Carnot limit) • However, about half the output from the turbine has to be used to compress the air that is fed into it • Thus, the overall efficiency is only about 35% in modern gas turbines


Turbine efficiency vs turbine size (power)


Efficiency and cost of a simple-cycle gas turbine with and without water injection


Due to the afore-mentioned high operating temperature of the gas turbine, the temperature of the exhaust gases is sufficiently hot that it can be used to either: Make steam and generate more electricity in a steam turbine (this gives combined cycle power generation). Or: provide steam for some industrial process that can use the heat, or to supply steam for district heating (this gives simple cycle cogeneration)


Combined-cycle power generation using natural gas C O O L IN G T O W E R

C O ND EN S ER

EX H AU ST

E L E C T R I C IT Y W ATER P U M P

S T E A M T U R B IN E

STEA M

FU EL H EAT RE CO V ERY STEA M G E NE RATO R

CO M BUSTO R

SH A FT

E L E C T R I C IT Y G EN ER ATO R

C O M PR ES SO R

T U R B IN E

IN T A K E A IR



Simple-cycle cogeneration EXHAUST

W ATER PU M P PR O CESS STEA M FUEL H EAT RECO VERY STEAM G ENERATO R

C O M B U STO R

SHAFT

E L E C T R I C IT Y G EN ERATO R

CO M PRESSO R

T U R B IN E

IN T A K E A IR



The energy can be cascaded even further, as follows: • Gas turbine → steam turbine → useful heat as steam from the steam turbine (combined cycle cogeneration), or • Gas turbine → steam turbine → steam → hot water (also combined cycle cogeneration), or • Gas turbine → steam → hot water


Combined-cycle cogeneration C O O L IN G T O W E R PR O C ES S S TEA M EX H A U ST

C O N D EN S ER E L E C T R I C IT Y S T E A M T U R B IN E

W ATER P U M P ST EA M

FU EL H EAT R E C O V ER Y ST EA M G E N E R ATO R

C O M B U STO R

SH A FT

E L E C T R I C IT Y G EN E R ATO R

C O M PR ES SO R

T U R B IN E

IN T A K E A IR



Cogeneration system with production of steam and hot water E L E C T R IC IT Y

FU E L

G A S T U R B IN E

G E N E R ATO R S T E A M

E X H A U S T G A S

H E AT R E C O V E R Y S T E A M G E N E R ATO R

H E AT E X C H A N G E R

E X H A U S T G A S

H O T W AT E R

Source: Malik (1997, M. Eng Thesis, U of Toronto)


• State-of-the-art natural gas combined-cycle (NGCC) systems have electricity generation efficiencies of 55-60%, compared to a typical efficiency of 35% for single-cycle turbines • However, NGCC systems are economical only in sizes of 25-30 MW or greater, so for smaller applications, only the less efficient simple-cycle systems are used • Thus, a number of techniques are being developed to boost the electrical efficiency of simple gas turbines to 42-43%, with one technique maybe reaching 54-57%


In cogeneration applications, the overall efficiency (counting both electricity and useful heat) depends on how much of the waste heat can be put to use. However, overall efficiencies of 90% or better have been achieved


Reciprocating engines • These have pistons that go back and forth (reciprocate) • Normally they use diesel fuel – so these are the diesel generators normally used for backup or emergency purposes • However, they can also be fuelled with natural gas, with efficiencies as high as 45%


Fuel cells • These are electrochemical devices – they generate electricity through chemical reactions at two metal plates – an anode and a cathode • Thus, they are not limited to the Carnot efficiency • Operating temperatures range from 120ºC to 1000ºC, depending on the type of fuel cell • All fuel cells require a hydrogen-rich gas as input, which can be made by processing natural gas or (in the case of hightemperature fuel cells) coal inside the fuel cells


Fuel cells (continued) • Electricity generation efficiencies using natural gas of 40-50% are possible, and 90% overall efficiency can be obtained if there is a use for waste heat • In the high-T fuel cells, the exhaust is hot enough that it can be used to make steam that can be used in a steam turbine to make more electricity • An electrical efficiency of 70% should be possible in this way – about twice that of a typical coal-fired.


F u e l (H 2 )

A ir (M o s tly N 2 + O 2) D C P ow er

Cross section of a single fuel cell.

E e le c tr o n flo w N e g a tiv e io n s or P o s itiv e io n s

F uel d is tr ib u tio n p la te

Several such cells would be placed next to each other to form a fuel cell stack. O x id iz e d F u e l (H 2 O )

N itr o g e n


United Technologies Company 200-kW phosphoric acid fuel cell that uses natural gas as a fuel. 1=fuel processor, 2=cell stack, 3=power conditioner, 4=electronics and controls

Source: www.utcfuelcells.com


Solid Oxide Fuel Cell / Gas Turbine System Fuel

Air o

o

25 C

25 C

FC

AC

o

236 C

o

847 C

SOFC = Solid Oxide Fuel cell AC,FC = Air & Fuel compressor CB = Catalytic burner GT = Gas turbine HRSG = Heat recovery steam generator HE = Heat exchanger

GT-2 o

1079 C GT-1

o

301 C

HE-1

o

738 C o

448 C

o

526 C HE-2

SOFC

o

985 C CB

M 468 C

Turbine Exhaust

o

Pump

o

440 C HRSG o

509 C

o

1290 C

o

HE-3

224 C o

25 C

To heat load From heat load


Electrical efficiency vs. load


Figure 3.11b Relative electrical efficiency vs. load


Summarizing the preceding slides and other information,

• Natural gas combined-cycle has the highest full-load efficiency (55-60%) and holds its efficiency well at part load • Reciprocating engines have intermediate full-load efficiencies (40-45%) and load their efficiencies well at part load • Gas turbines and micro-turbines have low full-load efficiencies (typically 25-35%, but ranging from 16% to 43%) and experience a substantial drop at part load • Fuel cells using natural gas have intermediate full-load efficiency (40-45%) but this efficiency increases at part load


Capital Costs Today • Pulverized coal power plant with state-ofthe-art pollution controls: $1200-1400/kW • Natural gas combined cycle: $400-600/kW in mature markets, $600-900/kW in most developing countries • Reciprocating engines: $600-1200/kW • Fuel cells: $3000-5000/kW


Cogeneration


Cogeneration is the simultaneous production of electricity and useful heat – basically, take the waste heat from electricity generation and put it to some useful purpose. Two possible uses are to feed the heat into a district heating system, and to supply it to an industrial process


Figure 3.12 Proportion of electricity produced decentrally (overwhelmingly as cogeneration)


Technical issues • Impact of withdrawing useful heat on the production of electricity • Ratio of electricity to heat production • Temperature at which heat is supplied • Electrical, thermal and overall efficiencies • Marginal efficiency of electricity generation


Four efficiencies for cogeneration:

• The electrical efficiency – the amount of electricity produced divided by the fuel use (later I’ll need to call this the direct electrical efficiency) • The thermal efficiency – the amount of useful heat provided divided __by the fuel use • The overall efficiency – the sum of the of two • The effective or marginal efficiency of electricity generation – explained later


Impact of withdrawing heat • In simple-cycle cogeneration, capturing some of the heat in the hot gas exhaust does not reduce the production of electricity, but the electrical production is already low • In cogeneration with steam turbines, the withdrawal of steam from the turbine at a higher temperature than would otherwise be the case reduces the electricity production • The higher the temperature at which we want to take heat, the more that electricity production is reduced


Example of the tradeoff between production of useful heat and loss of electricity production using steam turbine cogeneration

Source: Bolland and Undrum (1999, Greenhouse Gas Control Technologies, 125-130, Elsevier Science, New York)


Thus, to maximize the electricity production, we want to be able to make use of heat at the lowest possible temperature. If the heat is to be provided to buildings, that means having well insulated buildings that can be kept warm with radiators that are not very hot


The alternative to cogeneration is the separate production of heat and electricity. The effective efficiency in generating electricity is the amount of electrical energy produced divided by the extra fuel used to produce electricity along with heat compared to the amount of fuel that would be used in producing heat alone. The extra amount of fuel required in turn depends on the efficiency with which we would have otherwise have produced heat with a boiler or furnace.


For example, suppose that we have a cogeneration system with an electrical efficiency of 25% and an overall efficiency of 80%. Then, the thermal efficiency is 80%-25%=55% - we get 55 units of useful heat from the 100 units of fuel. If the alternative for heating is a furnace at 80% efficiency, we would have required 68.75 units of fuel to produce the 55 units of heat. Thus, the extra fuel use in cogeneration is 100-68.75=31.25 units, and the effective electricity generation efficiency is 25/31.25=80%. I call this the marginal efficiency, because it is based on looking at things on the margin (this is a concept from economics).


With a little algebra, it can be shown that the marginal efficiency is given by nmarginal = nel/(1-nth/nb) where nel and nth are the electrical and thermal efficiencies of the cogeneration system, and nb is the efficiency of the boiler or furnace that would otherwise be used for heating


Marginal efficiency of electricity generation in cogeneration (ηel = efficiency of the alternative, central power plant for electricity generation)


Key points • For a given thermal efficiency, the effective electrical efficiency is higher the higher the direct electrical efficiency • However, very high effective electrical efficiencies can be achieved even with low direct electrical efficiencies if the thermal efficiency is high – that is, if we can make use of most of the waste heat • To get a high thermal efficiency requires being able to make use of lowtemperature heat (at 50-60ºC), as well as making use of higher temperature heat


Electricity:heat ratio

• Because the marginal electricity generation efficiency in cogeneration is generally much higher than the efficiency of a dedicated central powerplant, there is a substantial reduction in the amount of fuel used to generate electricity when cogeneration is used • Thus, we would like to displace as much inefficient central electricity generation as possible when cogeneration is used to supply a given heating requirement • This in turn requires that the electricity-to-heat production ratio in cogeneration be as large as possible • (Remember – none of the gains that we’ve talked about occur if we can’t use the waste heat produced by cogeneration)


Electricity : heat output ratio in cogeneration


Figure 3.17 Dependence of overall savings through cogeneration on the electricity:heat ratio and on the central powerplant efficiency, assuming a 90% overall efficiency for cogeneration and 90% efficiency for the alternative heating system


Cost of Electricity


Issues related to the cost of electricity: • Capital cost, interest rate, lifespan • Fuel cost (impact of depends on efficiency) • Fixed and variable operation & maintenance costs • Baseload vs peaking costs • Transmission line costs and transmission losses • Amount of backup capacity


Capital cost of natural gas combined cycle cogeneration plants


Amortization of capital cost: CRF x Ccap / (8760 x CF) units: $/kWh where CRF = i /(1-(1+i)-N) is the cost recovery factor _i = interest rate _N = financing time period Ccap = capital cost ($/kW) 8760 is the number of hours in a year CF= capacity factor (annual average output as a fraction of capacity)


Fuel contribution to the final cost: Cfuel ($/GJ) x 0.0036 (GJ/kWh) / efficiency The cost of electricity from less efficient power plants will be more sensitive to the cost of fuel than the cost of electricity from efficient power plants, but more efficient power plants will tend to have greater capital cost


Typical overnight capital costs and best efficiencies • Pulverized coal: $1200-1400/kW,η= 0.45-0.48 • IGCC: $1400-2600/kW today, η= 0.41-0.55 $1150-1400/kW hoped for, future • NGCC: $400-600/kW, η = 0.55-0.60 • Reciprocating engine: $600-1200/kW,η=0.40-0.46 • Micro-turbine: $1800-2600/kW, η= 0.23-0.27 • Fuel cells: $3000-5000/kW, η= 0.35-0.45 $1000-1500/kW hoped for, future • NGCC/FC hybrid: $2000-3000/kW, η= 0.70-0.80


Cost of electricity from coal and natural gas


Cost of heat from boilers, electricity with or without cogeneration, and heat from cogeneration


Cost of electricity from central coal (at $2/GJ) and from natural gas (at $10/GJ)


Water requirements • Most thermal power plants use water to cool the condenser of a steam turbine and for other, minor, purposes • There are two approaches: a once-through cooling system a recirculating system in a cooling tower • Water use by power generation represents the largest or second largest use of water in most countries (with irrigation sometimes being a larger use)


• In once-through systems, the water is returned to the source (but at a warmer temperature). Large volumes of water are needed – not available in arid regions • In a recirculating systems, water that has removed heat from the condenser is sprayed through a cooling tower, where it is cooled by evaporation, then returns to the condenser • This consumes water, but the amount that is withdrawn from the water source (lakes, rivers or groundwater) is smaller than in oncethrough systems


Typical water requirements • Steam turbines (as in coal power plants) Once through: 80-190 liters withdrawn per kWh of __generated electricity, ~ 1 liter / kWh consumed Recirculating: 1-3 liters/kWh withdrawn 1-2 liters/kWh consumed • Natural gas combined cycle Once through: 30 liters/kWh withdrawn ~ 0.4 liters/kWh consumed Recirculating: 0.9 liters/kWh withdrawn 0.7 liters/kWh consumed


Bottom line: • More efficient power plants, such as natural gas combined cycle power plants, use less water per kWh of generated electricity than less efficient power plants • The water requirements can be a constraining factor in arid regions • It is possible to use air rather than water to cool the condenser, but then the efficiency drops


Section 3.1 – Steam Turbine Fundamentals

Overview • Hero Reaction Turbine – 120 B.C. • First Practical Turbine – 1884, C. Parsons • First Power Plant – 7.5 kw – 1890 • Reaction, Impulse and Velocity-Compounded • Reheat Steam – 1930’s • Last 100 years Turbine is the key element in generating electricity • Turbines run Generators, Pumps, Fans, etc. • Today up to 1,500 MW 126


Steam Turbine Fundamentals Overview

127


Steam Turbine Fundamentals Fundamentals

Energy Transfer Coal, Natural Gas, Nuclear, Biofuel, Waste Fuel 128


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Reaction Turbines Newton’s third law of motion – For every action there is an equal and opposite reaction.

Narrowing Steam Path

Narrowing Steam Path

129


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Impulse Turbines Steam / Gas Flow Fixed Vanes

Moving Blades

130



Reaction – Impulse Comparison

131


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Velocity-Compounded Turbine Velocity compounding is a form of staging which by dividing the work load over several stages results in improved efficiency and a smaller diameter for the blade wheels due to a reduction in Ideal blade speed per stage.

Inlet Pressure

1 P= V

Inlet Velocity

132


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Turbine Components - Blades Impulse

Reaction 133


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Turbine Diaphragms

Diaphragms contain the fixed blades

134


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Steam Turbine Casing

135


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Turbine Rotor

136



Turbine Shaft and Casing Seals

137


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Turbine Types

Straight HP Tandem HP Tandem LP

138


Steam Turbine Fundamentals Section 3.1 – Steam Turbine Fundamentals Turbine – Multiple Sets

139


Steam Turbine Fundamentals Section 3.2 – Steam Turbine Design Overview Classification by; • Type – Reaction or Impulse • Steam Temperature and Pressure • Configuration – Compound, Tandem Compound, Cross Compound • Reheat • Output – MW • Structural Elements

140


Steam Turbine Fundamentals Section 3.2 – Steam Turbine Design Turbine Design - Basics

141


Steam Turbine Fundamentals Section 3.2 – Steam Turbine Design Materials • Blades • Stainless Steel – 403 & 422 (+Cr) • 17-4 PH steel (+ Ti) • Super Alloys • Rotor • High “Chrome – Moley” Steel – Cr-Mo-V • Low “Ni Chrome Steel – Ni-Cr-Mo-V

142

Combined Cycle Gas Turbine 143

A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Sir Charles Parsons in 1884. Steam Turbine may also be define as a device which converts heat energy of to the steam to the mechanical energy which finally converted into electrical energy.


Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 90% of all electricity generation in the United States, is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.


The modern steam turbine was invented in 1884 by Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW (10 hp) of electricity. The Parsons turbine also turned out to be easy to scale up. Parsons had the satisfaction of seeing his invention adopted for all major world power stations, and the size of generators had increased from his first 7.5 kW set up to units of 500MW capacity.


Steam turbines are made in a variety of sizes ranging from small <0.75 kW units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 1,500 MW turbines used to generate electricity. There are several classifications for modern steam turbines.


WORK IN A TURBINE VISUALIZED

147


Further the steam turbine is based upon Rankine cycle • An ideal Rankine cycle operates between pressures of 30 kPa and 6 MPa. The temperature of the steam at the inlet of the turbine is 550°C. Find the net work for the cycle and the thermal efficiency. – Wnet=Wturbine-Wpump OR Qin-Qout – Thermal efficiency hth=Wnet/Qin – Net work done is converted into power output of turbine.


Ideal Rankine Cycle This cycle follows the idea of the Carnot cycle but can be practically implemented. 1-2 isentropic pump 3-4 isentropic turbine

2-3 constant pressure heat addition 4-1 constant pressure heat rejection


CLASSIFICATION OF STEAM TURBINE Classification of steam turbines may be done as following: 1.According to action of steam (a) Impulse turbine (b) Reaction turbine (c) Combination of both 2. According to direction of flow: (a) Axial flow turbine (b) Radial flow turbine 3. According to number of stages (a) Single stage turbine (b) Multi stage turbine


(4). According to number of cylinders (a) Single cylinder turbine (b) Double cylinder turbine (c) Three cylinder turbine (5) According to steam pressure at inlet of Turbine: (a) Low pressure turbine (b) Medium pressure turbine. (c) High pressure turbine (d) Super critical pressure turbine.


Description of common types of Turbines. The common types of steam turbine are 1. Impulse Turbine. 2. Reaction Turbine. The main difference between these two turbines lies in the way of expanding the steam while it moves through them. In the impulse turbine, the steam expands in the nozzles and it's pressure does not alter as it moves over the blades. In the reaction turbine the steam expanded continuously as it passes over the blades and thus there is gradual fall in the pressure during expansion below the atmospheric pressure.


PRESSURE-VELOCITY DIAGRAM FOR A TURBINE NOZZLE PRESSURE ENTRANCE HIGH THERMAL ENERGY HIGH PRESSURE LOW VELOCITY STEAM INLET

EXIT LOW THERMAL ENERGY LOW PRESSURE HIGH VELOCITY STEAM EXHAUST

VELOCITY


Simple impulse Turbine. It the impulse turbine, the steam expanded within the nozzle and there is no change in the steam pressure as it passes over the blades ROTOR

NOZZLE

STEAM CHEST

155



PRESSURE-VELOCITY DIAGRAM FOR A MOVING IMPULSE BLADE DIRECTION OF SPIN

REPRESENTS MOVING IMPULSE BLADES

PRESSURE

VELOCITY TURBINE SHAFT ENTRANCE HIGH VELOCITY STEAM INLET

EXIT LOW VELOCITY STEAM EXHAUST


Reaction Turbine In this type of turbine, there is a gradual pressure drop and takes place continuously over the fixed and moving blades. The rotation of the shaft and drum, which carrying the blades is the result of both impulse and reactive force in the steam. The reaction turbine consist of a row of stationary blades and the following row of moving blades. The fixed blades act as a nozzle which are attached inside the cylinder and the moving blades are fixed with the rotor as shown in the figure.


When the steam expands over the blades there is gradual increase in volume and decrease in pressure. But the velocity decreases in the moving blades and increases in fixed blades with change of direction. Because of the pressure drops in each stage, the number of stages required in a reaction turbine is much greater than in a impulse turbine of same capacity. It also concluded that as the volume of steam increases at lower pressures therefore the diameter of the turbine must increase after each group of blade rings.


REACTION TURBINE PRINCIPLE ROTOR

STEAM CHEST

160



PRESSURE-VELOCITY DIAGRAM FOR A MOVING REACTION BLADE DIRECTION OF SPIN

REPRESENTS MOVING REACTION BLADES

PRESSURE TURBINE SHAFT ENTRANCE HIGH PRESSURE HIGH VELOCITY STEAM INLET

EXIT LOW PRESSURE LOW VELOCITY STEAM EXHAUST

VELOCITY

162



.

Compounding in Steam Turbine.

The compounding is the way of reducing the wheel or rotor speed of the turbine to optimum value. It may be defined as the process of arranging the expansion of steam or the utilization of kinetic energy or both in several rings. There are several methods of reducing the speed of rotor to lower value. All these methods utilize a multiple system of rotors in series keyed on a common shaft, and the seam pressure or jet velocity is absorbed in stages as the steam flows over the blades.


Different methods of compounding are: 1.Velocity Compounding 2.Pressure Compounding 3.Pressure Velocity Compounding. These are explained in detail as given below:


Velocity Compounding: There are a number of moving blades separated by rings of fixed blades. All the moving blades are keyed on a common shaft. When the steam passed through the nozzles where it is expanded to condenser pressure, it's Velocity becomes very high. This high velocity steam then passes through a series of moving and fixed blades When the steam passes over the moving blades it's velocity decreases. The function of the fixed blades is to re-direct the steam flow without altering it's velocity to the following next row moving blades where a work is done on them and steam leaves the turbine with a low velocity as shown in diagram.


VELOCITY COMPOUNDED TURBINE

167



Pressure Compounding: These are the rings of moving blades which are keyed on a same shaft in series, are separated by the rings of fixed nozzles. The steam at boiler pressure enters the first set of nozzles and expanded partially. The kinetic energy of the steam thus obtained is absorbed by moving blades. The steam is then expanded partially in second set of nozzles where it's pressure again falls and the velocity increase the kinetic energy so obtained is absorbed by second ring of moving blades. This process repeats again and again and at last, steam leaves the turbine at low velocity and pressure. During entire process, the pressure decrease continuously but the velocity fluctuate as shown in diagram.


PRESSURE COMPOUNDED TURBINE

170



Pressure velocity compounding This method of compounding is the combination of two previously discussed methods. The total drop in steam pressure is divided into stages and the velocity obtained in each stage is also compounded. The rings of nozzles are fixed at the beginning of each stage and pressure remains constant during each stage as shown in figure. The turbine employing this method of compounding may be said to combine many of the advantages of both pressure and velocity staging By allowing a bigger pressure drop in each stage, less number stages are necessary and hence a shorter turbine will be obtained for a given pressure drop.


PRESSURE-VELOCITY COMPOUNDED IMPULSE TURBINE CURTIS STAGE NOZZLE, MOVING BLADE, FIXED BLADE, AND MOVING BLADE NOZZLE MOVING BLADE

PRESSURE VELOCITY

FIXED BLADE

MOVING BLADE

RATEAU STAGE – NOZZLE & MOVING BLADE NOZZLE MOVING BLADE

173



Steam supply and exhaust conditions These types include condensing, non-condensing, reheat, extraction and induction. Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser. Non-condensing or back pressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are available.


Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.


Casing or shaft arrangements These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications.


Two-flow rotors A two-flow turbine rotor. The steam enters in the middle of the shaft, and exits at each end, balancing the axial force. The moving steam imparts both a tangential and axial thrust on the turbine shaft, but the axial thrust in a simple turbine is unopposed. To maintain the correct rotor position and balancing, this force must be counteracted by an opposing force. Either thrust bearings can be used for the shaft bearings, or the rotor can be designed so that the steam enters in the middle of the shaft and exits at both ends. The blades in each half face opposite ways, so that the axial forces negate each other but the tangential forces act together. This design of rotor is called two-flow or double-exhaust.

178



Principle of operation and design An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine No steam turbine is truly isentropic, however, with typical isentropic efficiencies ranging from 20–90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or buckets as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.


Turbine efficiency Schematic diagram outlining the difference between an impulse and a reaction turbine To maximize turbine efficiency the steam is expanded, doing work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.


Impulse turbines An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which is converted into shaft rotation by the bucket-like shaped rotor blades, as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure (atmospheric pressure, or more usually, the condenser vacuum). Due to this high ratio of expansion of steam, the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the carry over velocity or leaving loss.


Reaction turbines In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.


Operation and maintenance When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also, a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10–15 RPM (0.17– 0.25 Hz) to slowly warm the turbine.


Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade breaking away from the rotor at high velocity and being ejected directly through the casing. To minimize risk it is essential that the turbine be very well balanced and turned with dry steam that is, superheated steam with a minimal liquid water content. If water gets into the steam and is blasted onto the blades (moisture carry over), rapid impingement and erosion of the blades can occur leading to imbalance and catastrophic failure. Also, water entering the blades will result in the destruction of the thrust bearing for the turbine shaft.


To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine. Modern designs are sufficiently refined that problems with turbines are rare and maintenance requirements are relatively small. The steam turbine operates on basic principles of thermodynamics using the part of the Rankine cycle. Superheated vapor (or dry saturated vapor, depending on application) enters the turbine, after it having exited the boiler, at high temperature and high pressure. The high heat/pressure steam is converted into kinetic energy using a nozzle. Once the steam has exited the nozzle it is moving at high velocity and is sent to the blades of the turbine.


A force is created on the blades due to the pressure of the vapor on the blades causing them to move. A generator or other such device can be placed on the shaft, and the energy that was in the vapor can now be stored and used. The gas exits the turbine as a saturated vapor (or liquidvapor mix depending on application) at a lower temperature and pressure than it entered with and is sent to the condenser to be cooled


Isentropic turbine efficiency To measure how well a turbine is performing we can look at its isentropic efficiency. This compares the actual performance of the turbine with the performance that would be achieved by an ideal, isentropic, turbine. When calculating this efficiency, heat lost to the surroundings is assumed to be zero. The starting pressure and temperature is the same for both the actual and the ideal turbines, but at turbine exit the energy content ('specific enthalpy') for the actual turbine is greater than that for the ideal turbine because of irreversibility in the actual turbine.


The isentropic efficiency is found by dividing the actual work by the ideal work. where •h1 is the specific enthalpy at state one •h2 is the specific enthalpy at state two for the actual turbine •h2s is the specific enthalpy at state two for the isentropic turbine

Combined Cycle Gas Turbine.ppt

Recommend Documents