Boilers Of Thermal Power PlantsPlants

Debanjan Basak

CESC Ltd

Points of Discussion• Thermodynamic Cycles• Discussion on Sub and Supercritical

Boilers• Performance Indicators and Benchmarks

of a Power Stationof a Power Station• Constructional and design features of

Boilers• Boiler Auxiliaries• Losses and performance optimisation

First Law of Thermodynamics

• Energy cannot be created nor destroyed.

• Therefore, the total energy of the universe is a constant.

• Energy can, however, be converted from • Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.

Spontaneous Processes

• Spontaneous processes are those that can proceed without any outside intervention.

• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously

Spontaneous Processes

Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.the reverse direction.

Spontaneous Processes• Processes that are spontaneous at one

temperature may be nonspontaneous at other temperatures.

• Above 0°C it is spontaneous for ice to melt.• Below 0°C the reverse process is spontaneous.• Below 0°C the reverse process is spontaneous.

Reversible ProcessesIn a reversible process

the system changes in such a way that the system and surroundings can be put back in their original states by exactly states by exactly reversing the process.

Changes are infinitesimally small in a reversible process.

Irreversible Processes

• Irreversible processes cannot be undone by exactly reversing the change to the system.

• All Spontaneous processes are irreversible.

• All Real processes are irreversible.

Entropy

• Entropy (S) is a term coined by Rudolph Clausius in the 19th century.

• Clausius was convinced of the significance of the ratio of heat delivered and the of the ratio of heat delivered and the temperature at which it is delivered,

qT

Entropy is used to define the unavailable energy in a system.

Entropy defines the relative ability of one system to act on an other. As things move toward a lower energy level, where one is less able to act upon the surroundings, the less able to act upon the surroundings, the entropy is said to increase.

For the universe as a whole the entropy is increasing!

•Entropy is not conserved like energy!

Entropy

• Entropy can be thought of as a measure of the randomness of a system.

• It is related to the various modes of motion in molecules.in molecules.

Entropy

• Like total energy, E, and enthalpy, H, entropy is a state function.

• Therefore,

DS = S - SDS = Sfinal - Sinitial

Second Law of Thermodynamics

The second law of thermodynamics:The entropy of the universe does not change for reversible processes and

increases for spontaneous processes.increases for spontaneous processes.

Reversible (ideal):

Irreversible (real, spontaneous):

Entropy on the Molecular Scale

• Ludwig Boltzmann described the concept of entropy on the molecular level.

• Temperature is a measure of the average kinetic energy of the molecules in a sample.kinetic energy of the molecules in a sample.

Entropy on the Molecular Scale• Molecules exhibit several types of motion:

– Translational: Movement of the entire molecule from one place to another.

– Vibrational: Periodic motion of atoms within a molecule.

– Rotational: Rotation of the molecule on about an axis or – Rotational: Rotation of the molecule on about an axis or rotation about s bonds.

Entropy on the Molecular Scale• Boltzmann envisioned the motions of a sample of

molecules at a particular instant in time.– This would be akin to taking a snapshot of all the

molecules.

• He referred to this sampling as a microstate of the • He referred to this sampling as a microstate of the thermodynamic system.

Entropy on the Molecular Scale• Each thermodynamic state has a specific number of

microstates, W, associated with it.

• Entropy is

S = k lnW

where k is the Boltzmann constant, 1.38 ´ 10-23 J/K.where k is the Boltzmann constant, 1.38 ´ 10-23 J/K.

Entropy on the Molecular Scale

• The number of microstates and, therefore, the entropy tends to increase with increases in– Temperature.– Temperature.

– Volume (gases).

– The number of independently moving molecules.

Entropy and Physical States

• Entropy increases with the freedom of motion of molecules.

• Therefore,• Therefore,

S(g) > S(l) > S(s)

SolutionsDissolution of a solid:

Ions have more entropy (more states)

But,

Some water molecules Some water molecules have less entropy (they are grouped around ions).

Usually, there is an overall increase in S.(The exception is very highly charged ions that make a lot of water molecules align around them.)

Entropy Changes

• In general, entropy increases when– Gases are formed from

liquids and solids.liquids and solids.

– Liquids or solutions are formed from solids.

– The number of gas molecules increases.

– The number of moles increases.

Third Law of ThermodynamicsThe entropy of a pure crystalline substance at absolute zero is 0.

Standard Entropies

Larger and more complex molecules have greater entropies.

Link S and DH: Phase changes

A phase change is isothermal (no change in T).

Ent

ropy

syst

em

For water:

DHfusion = 6 kJ/molDHvap = 41 kJ/mol

If we do this reversibly: DSsurr = –DSsys

Change in entropy > 0

irreversible

Change in entropy = 0

reversible

Change in entropy < 0

impossibleprocess process process

When a liquid evaporates its go through a process where•the liquid heats up to the evaporation temperature

•the liquid evaporate at the vaporationtemperature by changing state temperature by changing state from fluid to gas

•the vapor heats above the vaporationtemperature - superheating

The heat transferred to a substance when temperature changes is often referred to as sensible heat.

The heat required for changing state as evaporation is referred to as latent heat of evaporation.

Enthalpy of a system is defined as the mass of the system - m -multiplied by the specific enthalpy - h - of the system and can be expressed as:H = m h (1)whereH = enthalpy (kJ)m = mass (kg)h = specific enthalpy (kJ/kg)

Specific EnthalpySpecific enthalpy is a property of the fluid and can be expressed as:h = u + p v (2)whereu = internal energy (kJ/kg)p = absolute pressure (N/m2)v = specific volume (m3/kg)

Dryness Fraction of Saturated Steam (x or q)

It is a measure of quality of wet steam. It is the ratio of the mass of dry steam (mg) to the mass of total wet steam (mg+mf), where mf is the mass of water vapor.

X= mgmg + mf

Quality of SteamQuality of Steam

It is the representation of dryness fraction in percentage: Quality of Steam = x X 100

Steam Quality

Steam should be available at the point of use:•In the correct quantity•In the correct quantity•At the correct temperature and pressure•Free from air and incondensable gases•Clean•Dry

Advantages of Superheated Steam

• At a given pressure, its capacity to do the work will be comparatively higher.

• It improves the thermal efficiency of boilers and prime movers

• It is economical and prevents condensation in case of Steam turbines

Disadvantages of Superheated SteamDisadvantages of Superheated Steam

• Rise in Superheated temperature poses problems in lubrication

• Initial cost is more and depreciation is higher

Carnot Cycle

• Most efficient cycle operating between two heat sources

• Practically impossible• Difficulty in ending the • Difficulty in ending the

condensation process• High energy

consumption for pumping / compression

Rankine Cycle

• Practical Carnot cycle with much less efficiency

• Pump power is much less compared to less compared to turbine output (within 1%)

• Efficiency limited for lower steam inlet temperature

Understanding Basic CycleUnderstanding Basic Cycle

Rankine Cycle

• Process 1-2: Pump Work

• Process 2-3: Sensible and latent heat addition in the boiler at constant pressurepressure

• Process 3-4: Expansion in steam turbine

• Process 4-1: condensation of the steam in condenser

Rankine cycle with Reheat

• Average temp of heat addition increases with higher pressure

• Restricted for metallurgical limitsmetallurgical limits

• Reheating the expanded steam to improve efficiency

• Exit Dryness Fraction improved

Rankine cycle with Reheat and Regeneration

• Most commonly used in power plant

• Bled steam is utilised to exchange heat to exchange heat before being cooled at the condenser

Steam Condition Vs Design efficiencySteam Pr (Bar) Steam Temp

(0C)Reheat Steam Temp (0C)

Design Efficiency (%)

Size of set (MW)

41.4 462 27.5 30

89.1 510 30.5 60

103.4 566 33.7 100

103.4 538 538 35.7 120103.4 538 538 35.7 120

162 566 538 37.3 200

158.6 566 566 37.7 275

158.6 566 566 38.4 550

241.3 593 566 39.0 375

158.6 566 566 39.25 500

Heat Rate Improvement

Parameters at Turbine Inlet (bar/oC / oC)

% Improvement In Station Heat Rate

170 / 538 / 538 Base

170 / 538 / 565 0.5%

170 / 565 / 565 1.3%

246 / 538 / 538 1.6%

246 / 538 / 565 2.1%

246 / 565 / 565 3.0%

246 / 565 / 598 3.6%

306 / 598 / 598 5.0%

Steam cycle theory and constraints

• Higher the size of plant, lower is the capital cost per MW and higher is the plant efficiency

• The terminal steam condition tend to increase with the size of plantwith the size of plant

• Limitation in metallurgy is the constraints for higher terminal condition and hence efficiency

Sample Calculationof

Cycle efficiencies Cycle efficiencies under different condition

A cycle operating between

100 bar and 30 mbar

Heat addition-Sensible Heat• The sensible heat is mostly

added in the feed water heaters and the economisers

• The cycle operates between 100 bar (310.9610C saturation temp) and 30 mbar(24.10C temp) and 30 mbar(24.10C saturation temp)

• Sensible heat at A =101 KJ/Kg

• Sensible heat at B =1408 KJ/Kg

• Sensible heat added = 1307 KJ/Kg

Variation of sensible heat with Pressure

Absolute pressure

(bar)

Saturation Temperature

(°C )

Sensible Heat

( kj / kg )

50 263.9 1154.550 263.9 1154.5

100 311.0 1408.0

150 342.1 1611.0

200 365.7 1826.5

221.2 374.15 2107.4

Heat addition - Latent Heat

• The latent heat is mostly added in the water wall tubes of the boiler

• Latent heat diminishes with pressure and is zero at critical pressurepressure

• The latent heat is added from B to C at constant temp

• Entropy at C is 5.6198 kj/KgK• Entropy at B is 3.3605 kj/KgK• Latent heat added = 1319.7

KJ/Kg

Absolute pressure

(bar)

Saturation Temperatur

e(°C )

Latent Heat

( kj / kg )

50 263.9 1639.7

Variation of Latent heat with Pressure

50 263.9 1639.7

100 311.0 1319.7

150 342.1 1004.0

200 365.7 591.9

221.2 374.15 0

Heat addition - Super heat

• The Super heat is mostly added in the superheater tubes of the boiler arising out from the drum

• The Super heat is added • The Super heat is added from C to D at constant pressure

• The amount of superheat can be found by deducting the total heat of Point C from total heat of point D

Variation of Superheat with Pressure

Absolute pressure(bar)

Superheat required( kj / kg )

50 800.9

100 821.5

150 885.4

200 1033.2

Thermal Efficiency of Cycle

Useful Heat

Thermal Efficiency =

Total HeatTotal Heat

Useful Heat

Useful heat : Total Heat – Rejected Heat

Effect of Back Pressure

Improvement of back pressure induces

certain losses too:

• Increase in the CW pumping power

• Higher Leaving loss

• Reduced condensate temperature

• Increased wetness of the steam

Back pressure correction curve

Back Pressure in mb

Heat cons

Optimum Back pressure

Causes for departure of back pressure

• CW inlet temperature different from designBalance between increase T/A output to extra pumping power required

• CW quantity flowing through the condenser is incorrectLow across temperature requires closing of the valves otherwise will result in under cooling of condensate. Flow to be optimised to get desired acrossin under cooling of condensate. Flow to be optimised to get desired across

• Fouled tube plateIf the CW across rise is independent of increase of flow then it is assumed

that the tube plates are fouled with debris

• Dirty tubesCondenser back pressure is independent of increase of flow

• Air ingress into the system under vacuumIncrease of TTD. More air ejection improves the vacuum. Helium leak testing may be employed

Calculation of Ideal EfficiencyBasic Rankine Cycle between 100bar and 30 mbar

• Total Heat supplied: 2626.7.3 kj/Kg

• Total Heat rejected, [T X (S2-S1))]: 1917.2 kj/Kg

• Useful heat : Total Heat – Rejected heat

• Thermal Efficiency = 27.01 %

• The Highest possible efficiency for a basic Rankine cycle with steam at 100 bar (abs) and dry saturated condition and back pressure at 30mbar is 27.01 %

Ideal Efficiency of Rankine Cycle with superheat

• Total Heat supplied: 3438.3 kj/Kg

• Total Heat rejected, [T X (S2-S1))]:1917.2 kj/Kg

• Useful heat : Total Heat –Rejected heatRejected heat

• Thermal Efficiency = 44.23 %

• The Efficiency of basic Rankine cycle can be improved with superheat

• The scope however is limited due to materials to withstand high temperature

Ideal Efficiency of Rankine Cycle with Reheat

• The same 100 bar cycle with reheat

• At pressure 20 bar after expansion in the turbine, the steam is heated in the boiler to steam is heated in the boiler to 566 0C

• The steam expands to the condenser pressure in IP/LP turbine

• The efficiency of this cycle is 46.09%

Ideal Efficiency of Rankine Cycle with Reheat and regeneration

• Sensible heat addition from M to B

• Latent heat and superheat addition as before

• Total heat supplied 2453.5 • Total heat supplied 2453.5 Kj/Kg

• Heat rejected = 1192.2 Kj/Kg

• Thermal Efficiency = 51.4%

Changes in cycle efficiency• The ideal efficiency of the cycle changes with

superheating, reheating and feed heating as under• Basic efficiency: 27.01 %• With Superheat: 44.23%• With Reheat: 46.09%• With reheat and feed heating 51.4%• With reheat and feed heating 51.4%• A combination of reheating and feed heating will give

higher ideal cycle efficiency• In practice, due to losses in turbine and other parts the

actual efficiency is much less than the ideal cycle efficiency stated above

In the figure the dark green area is the area decreased and light green area is the area increased when the boiler pressure of the Rankine cycle is increased. Area increased in the cycle is clearly more than the area decreased in the previous cycle so Rankine efficiency is increased. But this leads to decrease of quality of steam that comes out of turbine. This quality should not be less than 85%, which limits to maximum pressure of the power plant.

Increasing the maximum operating temperature can also increase efficiency, as this takes steam to the superheated region, which increases the area and also enhances the quality of steam exiting the turbine.

The maximum temperature is limited by the metallurgical quality of the pipes of boiler.

Advantages of Reheat Cycle•Increases the dryness fraction of steam•Reduces fuel consumption by 4 to 5%•Reduces steam flow with corresponding reductions in boiler, turbine and feed heating equipments capacity.•Reduces pumping power•Reduces pumping power•Reduction in exhaust blade erosion of turbine•Reduction in steam volume and heat to the condenser is reduced by 7 to 8%.•Condenser size and cooling water flow also reduced•Size of the LP turbine blades is reduced because sp. Steam volume is reduced by 8%

Disadvantages of Reheat Cycle

•Cost increases for additional pipes and reheaters•Greater floor space required for longer turbine•Complex operation and control increases•Complex operation and control increases•At light loads, steam passing through the last blade rows are highly superheated if same reheat is maintained

Boiler: Definition as per IBR

Boiler means any closed vessel exceeding 22.75 litres (five gallons) in capacity which is used expressly in capacity which is used expressly for generating steam under pressure and includes any mounting or other fitting attached to such vessel, which is wholly or partly under pressure when steam is shut off:

Classification of PF Boilers

Based on Operating Pressure

• Sub-Critical: < Critical Pr.

Super-Critical Ultra-Super-Critical

Sub-Critical

• Sub-Critical: < Critical Pr. 221.2 Bar

• Super critical: > Critical Pr. 221.2 Bar

• Ultra-super critical > Pr > 300 Bar

and Temp > 1100 0 F or 593 0C 4

THERMAL EFFICIENCY IMPROVEMENT

169 246 310

STEAM PRESSURE (kg/cm2)

Base

%

1.8

0.8

0.8

1.0

0.8

5380C/5380C

5380C/5660C

5660C/5660C

5660C/5930C

6000C/6000C

Eff

icie

ncy

Incr

ease

%

1.0

5660C/5660C

Super critical and ultra supercritical conditions

Critical Conditions

•Temperature -374.150C

•Pressure-225.56kg/cm2

Ultra super critical conditions

•Temperature above 5930C

•Pressure above 306kg/cm2

Improvement of thermal efficiency•Increasing the steam temperature (ή increases 0.31% •Increasing the steam temperature (ή increases 0.31% every 100C of increase of main steam temperature & 0.24% every 100C of increase of reheat steam temperature )

•Increasing in the steam pressure (ή increases 0.1% increase with increase of 10 bar pressure)

Based on Types of Circulation

• Natural Circulation Boiler

Classification of PF Boilers

• Assisted circulation Boiler

• Once through Boiler

Circulation in Boiler

Natural Circulation• The water flows from the drum vide down comer pipes

and returns through riser tubes after being heated in the furnace

• The static head difference generated due to density difference of the steam and water mixture in the riser tubes and water in the down comer is the driving force for the circulation. This is called ‘Thermo-Siphon’for the circulation. This is called ‘Thermo-Siphon’

• The steam and water mixture is separated in the boiler drum

• As the pressure rises, the difference between the densities tend to decrease and Natural circulation head cannot overcome the frictional resistance

• Higher the heat input, higher should be the flow rate through the tubes to avoid overheating

Circulation Quantity VS Steam Produced

End Point

The circulation increases with increase in Heat input

Losses due to friction from high specific volume is higher than the pressure differential

Steam Produced

Total

Circulation

Quantity

differential

Circulation Ratio

• Circulation ratio is the weight of water fed to the steam generating circuits to the steam actually generated

Kg. of waterCirculation ratio =

Kg. of SteamKg. of Steam

• Circulation ratio depends upon operating pressure, available circulation head and flow resistance

• For sub critical boilers, circulation ratio varies from 10-30

Relationship of density of water-steam water-steam with operating pressure

Assisted Circulation

• As the pressure goes high, the density difference between the steam and water decreases and therefore, additional assistance of pumps are needed to establish circulation

• The pumps are located at the bottom of the down comers

• The pumps are located at the bottom of the down comers

• The tube dimensions for assisted circulation boilers are less and have orifices to establish uniform temperature distribution

• They are restricted below critical pressure to near about 190 bar

Forced circulation (Once through)

• No drum to separate change of state• Once through boiler can operate at any

pressure below or above critical pressure• Flow in once through boiler is proportional • Flow in once through boiler is proportional

to the load and hence a minimum flow of 25-30 % is needed always by recirculation pumps or by dumping

• Spirally wound tube to average the heat input per tube

Once Through Boilers

Based on Types of firing

• Wall fired: Front / Opposed

Classification of PF Boilers

• Corner fired: Tangential

• Down-shot fired : Single / Double

Wall Firing (TGS Boiler)

• The stability is imposed by a combination of secondary of air swirl and a flow reversal in the primary air by an impeller

• The refractory quarl though acts as a radiant heat source but its major role is aerodynamic flow stabiliserheat source but its major role is aerodynamic flow stabiliser

• 80 % combustion air through secondary air and 20 % through primary air

• Modern design incorporate axial swirl which consumes less fan power, intimate mixing and better control

Down shot Firing (BBGS Boiler)

• Adopted for burning of low volatile coal < 16 % (Anthracite)

• Long particle residence time for complete combustioncombustion

• The coal is fed downwards from the arch along with about 30-40 % combustion air

• The secondary air and tertiary air is distributed to form the flame characteristics and shape

Tangential Firing (SGS / BBGS#3 Boiler)

• A turbulent zone is created in the center of the furnace by the turbulent flames fired from the corners towards the imaginary circle to which the flame path is tangent

• Simple in construction and can burn a wide variety • Simple in construction and can burn a wide variety of coal

• The mixing of coal and air is obtained by the admission of coal and air in alternate layers

• There can be provisions for tilting of the burners for super heater temperature control (not in SGS, available in BBGS #3)

Our BoilersOur BoilersTitagarh Generating StationTitagarh Generating Station

Designed for Coal with• Calorific Value – 4500• Ash + Moisture – 35.5%• Volatile Matter – 25%• Fixed Carbon – 39.5%

Southern Generating StationSouthern Generating Station

Designed for Coal with• Calorific Value – 3800• Ash + Moisture – 44%• Volatile Matter – 17%• Fixed Carbon – 39%

BBGS Generating StationBBGS Generating Station

Designed for Coal with• Calorific Value – 3850• Ash + Moisture – 50%• Volatile Matter – 15%• Fixed Carbon – 32%

Front wall firingFront wall firing Corner firingCorner firing Down shot firingDown shot firingFront wall firingFront wall firing Corner firingCorner firing Down shot firingDown shot firing

Heat Transfer Zones

Heat Transfer Zones

• The Furnace: High temperature gases of combustion is used for heating water and steam with low to medium superheat

• The Convection Zone: Medium temperature • The Convection Zone: Medium temperature gases is used to heat steam with medium to high superheat

• Heat Recovery zone: Comparatively cool gases exchange heat to feed water to saturation temperature or with low superheat

Boiler

Showing Heat transfer areas

Types of Boiling• Sub-cooled water heating: Initial stage of heating, Water in contact with the

tube evaporates-bulk fluid is below the saturation temp

• Sub-cooled Nucleate Boiling: Formation and collapsing of bubbles due to transfer of latent heat

• Nucleate boiling: Bulk of the liquid reaches to saturation temperature, bubbles will not collapse, fluid flow along with the bubbles. Sub critical boiler operates in will not collapse, fluid flow along with the bubbles. Sub critical boiler operates in this stage (Water velocity 1.5-3 mps)

• DNB (Departure from Nucleate boiling): Even higher heat flux will result in collapsing of bubble to form a layer of superheated steam on the tube face. Breakdown of mode of heat transfer-leads to ‘burn out’ of the tube to overheating.

• Film Boiling: Complete film of steam is formed at the solid liquid interface, results in reduction in heat transfer, High velocities of steam is required

Types of Boiling

Log Heat

A-B: Water Heating

B-S: Sub cooled Nucleate Boiling

S-C: Nucleate Boiling

C-D: Onset of Film Boiling

F

C

D

Critical Heat Flux or DNB

Log (Tsurface – Tbulk)

Log Heat Flux D-E: Unstable Film Boiling

E-F: Stable Film BoilingE

S

A

B

Furnace- Duty

• Furnace to have suitable surface area to reduce the temperature of the furnace gas to a level acceptable to super heater requirementsrequirements

• Adequate water circulation in the furnace tubes to prevent overheating

• To avoid flame impingement in the opposite wall tubes

Furnace- Duty

• Width sufficient to accommodate all burners at an acceptable pitching

• Overall dimension to ensure optimum absorption and total combustionabsorption and total combustion

• To reduce the furnace temperature below ash softening temp to avoid slagging

Coal Vs Oil Fired Furnace

• Average oil droplet burnout time is half to that of coal

• Coal particle require higher residence time – higher flow path– higher flow path

• Sticky ash hinders wall tube heat absorption hence higher surface area

Furnace- Performance & Control

Operation procedures

• Firing Pattern

• Soot blowing

• Excess air• Excess air

Other methods

• Gas recirculation (GR) as in BBGS

• Tilting burners for Corner fired boiler

Furnace Construction

Basically two types• Tangent wall tube: Tubes are arranged tangentially and the

skin casing is used to seal. The skin casing is supported from main stays

Advantage: Advantage: • Easy maintenance• Older design

• Membrane wall tube: Tubes joined with fins to form a fully welded structure, the membrane wall

Advantage: • Minimum ingress of tramp air• The outer casing requires only heat shielding

Super heaters and Re heaters

Convective: • The heat transfer is through convection and Heat absorption rate increases

with the boiler output

Radiant:• Radiant super heaters receives heat through radiation only• Radiant super heaters receives heat through radiation only

• With increase load in the boiler, the heat absorption in the furnace surfaces is increased at a lesser rate hence, the radiant superheat decrease with load

Combination • Fairly flat superheat curve with wide range of load

• Type of material, tube diameter, positioning in the furnace, gas temperature zone, superheating surface etc. are important factors for designing a super heater

ST

EA

M T

EM

PE

RA

TU

RE

, SC

ALE

AR

BIT

RA

RY

20 40 60 80 100

STEAM OUTPUT PERCENTAGE

A SUBSTANTIALLY UNIFORM FINALSTEAM TEMPERATURE OVER A RANGE OF OUTPUT CAN BE ATTAINED BY A SERIES OF ARRANGEMENT OF RADIANT AND CONVECTION SUPERHEATER COMPONENT

ST

EA

M T

EM

PE

RA

TU

RE

, SC

ALE

AR

BIT

RA

RY

PRIMARY

REHEATER

VERTICAL

PIMARY SUPERHEATER VERTICAL

PRIMARY

FINAL

REHEATER

FINAL

SUPER

HEATERPLATEN SUPERHEAER

STEAM TO IP TURBINE STEAM TO HP TURBINE

STEAM FROM HP

COMBUSTION GASES

STEAM FROM DRUM

TO DRUM

FEED WATER

TO DRUM

FEED WATER

REHEATER

ECONOMISER

SUPERHEATER ECONOMISER

FURNACE

AIR HEATER

GAS TO STACK

REHEATER PRIMARY

SUPERHEATER

AIR

COAL

HP TURBINE

BLOCK DIAGRAM SHOWING BOILER ELEMENTS AND FLOWPATHS

OXIDE

STEAM WATER FILMFOULING

GAS FILM

BULK GAS TEMPERATUE

TUBE WALL

FILMFOULING

COMPOSITE TEMPERATURE DROP FROM GAS TO STEAM / WATER THROUGH A BOILER TUBE WALL

GAS

STEAM

GAS

1025*C

568*C

1025*C

930*C

492*C568*C

930*C

STEAM

STEAM

COUNTER FLOW PARALLEL FLOW

492*C492*C

568*C

Tin=447.4*c Tin=442.0*c

SUPER HEATER GAS AND STEAM TEMPERATURE

General Arrangement of a 210 MW radiant Re-heat radiant Re-heat boiler

Typical section of a Double Down shot, Down shot, 250 MW Boiler at BBGS, CESC

Super heater temperature is affected by

• Load

• Excess Air

• Feed Water temperature

• Heating surface cleanliness

• Burner operation

• Burner tilt

• Coal burnt

Super heater temperature control

• Direct Attemperation / De super heaters

• Excess Air

• Furnace division

• Gas recirculation

• Adjustment of burner tilt

• Type pf burners

Steam Separation and purity

• Boiler operating below critical pressure need drum to separate saturated steam from a mixture of steam-water discharged by the boiler tubes

• Drum also serves as vessel for chemical treatment of water and storage of water

• The drum sizing is done primarily to house the separation equipment and should accommodate the changes in water level with variation of load

0.30

0.25

0.20

0.15

0.10

DIS

TR

IBU

TIO

N

RA

TIO

=S

ILIC

A C

ON

TE

NT

OF

ST

EA

M

SIL

ICA

CO

NT

EN

T O

F B

OIL

ER

WA

TE

R

0 1000 2000 3000 4000

0.10

0.05

0.00

STEAM DRUM PRESSURE IN , psi

DIS

TR

IBU

TIO

N

RA

TIO

=

EFFECT OF PRESSURE ON SILICA DISTRIBUTION RATIO

Performance Indicators and Benchmarking Benchmarking

Benchmarking-Objectives

• Benchmarking is

• a continuous formal process of measuring, understanding, and adapting

• more effective practices from best-in-class • more effective practices from best-in-class organizations that lead to superior performance.

• Benchmarking is essential to

• provide the best service to our customers.

Benchmarking-Benefits

• Improve our performance and organization

• Learn about industry leaders and competitors

• Determine what world-class performance is

• Accelerate and manage change • Accelerate and manage change

• Achieve breakthrough results

• Improve customer satisfaction

• Become the best in the business

Steps of benchmarking

• What to benchmark

• With whom to benchmark

• Identification of potential improvement areas based on benchmarking.based on benchmarking.

• Adoption of best practices for improvement

• Monitor effectiveness of new practice

• Modify practice as per requirement

• Standardise practice

Key Benefits from Benchmarking at CESC Ltd

• Reduction in Annual overhaul time • High pressure jet cleaning of boiler tubes• Operating at zero pressure differential of

feed control stationfeed control station• Ammonia dosing system at ESP• Boiler Insulation survey• Destaging of Condensate Extraction Pump• Installation of SS-304 chutes at CHP

Key Performance Indicators

• Cost of Generation• Plant Load Factor (PLF)• Plant Availability Factor (PAF)• Loss In Production • Heat Rate• Heat Rate• Specific Coal Consumption• Specific Oil Consumption• Auxiliary Power Consumption• Environmental Emissions• No of Accidents• Implementation of Quality and SHE systems

Key Monitoring • PF Sample analysis• PA and PF flow distribution• Performance of Boiler feed pumps• Performance of Fans• Insulation survey of boiler casings• Thermographic assessment of valves• Reject analysis from pulverisers• Helium leak test of condensers• Energy consumption of major axillaries• Physical inspection of fly ash• Measurement of boiler and air heater efficiency• Measurement of turbine efficiency• Fuel sampling and analysis from coal feeders

Introduction to Supercritical Technology

What is Supercritical Pressure ?

Critical point in water vapour cycle is athermodynamic state where there is no clear

114

thermodynamic state where there is no cleardistinction between liquid and gaseous stateof water.Water reaches to this state at a critical

pressure above 22.1 MPa and 374 oC.

What is Supercritical Pressure ?

• Critical point in water vapour cycle is a thermodynamic state where there is no clear distinction between liquid and gaseous state of water.gaseous state of water.

• Water reaches to this state at a critical pressure above 22.1 MPa and 374 oC.

Rankine Cycle Subcritical Unit

1 - 2 > CEP work2 - 3 > LP Heating3 - 4 > BFP work4 - 5 > HP Heating5 – 6 > Eco, WW6 – 7 > Superheating6 – 7 > Superheating7 – 8 > HPT Work8 – 9 > Reheating9 – 10 > IPT Work10–11 > LPT Work11 – 1 > Condensing

Rankine Cycle Supercritical Unit

1 - 2 > CEP work2 – 2s > Regeneration2s - 3 > Boiler Superheating3 – 4 > HPT expansion3 – 4 > HPT expansion4 – 5 > Reheating5 – 6 > IPT & LPT Expansion6 – 1 > Condenser Heat rejection

VARIATION OF LATENT HEAT WITH PRESSURE

Absolute Absolute PressurePressure

(Bar)(Bar)

Saturation Saturation TemperatureTemperature

((ooC)C)

Latent Latent HeatHeat

(K J/Kg.)(K J/Kg.)

5050150150

264264342342

1640164010041004150150

200200221221

342342366366374374

10041004592592

00

Departure from Nucleate Boiling

• Nucleate boiling is a type of boiling that takes place when the surface temp is hotter than the saturated fluid temp by a certain amount but where heat flux is below the critical heat flux. Nucleate boiling occurs when the surface temperature is higher than the saturation temperature by between 40C to 300C.

DE

NS

ITY

WATER

PRESSURE(ksc)

DE

NS

ITY

STEAM

175 224

No Religious Attitude

Supercritical Boiler Water Wall Rifle Tube And Smooth Tube

Natural Circulation Vs. Once Through System

From CRH Line

LTRH

LTSH4430C

FRH

Platen Heater

Mixer Header

FSH

To HP Turbine To IP

Turbine

Separator

3260C

4230C

4730C

4620C5340C

5260C

5710C

5690C

3240C

From FRS Line

Boiler Recirculation Pump

Economizer Phase 1

Economizer Phase 2

Bottom RingHeader

2830C

2800C

NRV

Feed water control

• In Drum type Boiler Feed water flowcontrol by Three element controller–1.Drum level–2.Ms flow–3.Feed water flow.

• Drum less Boiler Feed water control by–1.Load demand–2.Water/Fuel ratio(7:1)–3.OHD(Over heat degree)

Difference of Subcritical(500MW) and Supercritical(660MW)Supercritical(660MW)

COMPARISION OF SUPER CRITICAL & SUB CRITICAL

DESCRIPTION SUPERCRITICAL

(660MW)

SUB-CRITICAL

(500MW)

Circulation Ratio 1 Once-thru=1

Assisted Circulation=3-4

Natural circulation= 7-8

Feed Water Flow Control -Water to Fuel Ratio

(7:1)

Three Element Control

-Feed Water Flow(7:1)

-OHDR(22-35 OC)

-Load Demand

-Feed Water Flow

-MS Flow

-Drum Level

Latent Heat Addition Nil Heat addition more

Sp. Enthalpy Low More

Sp. Coal consumption Low High

Air flow, Dry flu gas loss Low High

Continue..

DESCRIPTION SUPERCRITICAL

(660MW)

SUB-CRITICAL

(500MW)

Coal & Ash handling Low High

Pollution Low HighPollution Low High

Aux. Power Consumption

Low More

Overall Efficiency High(40-42%)

Low(36-37%)

Total heating surface area Reqd

Low(84439m2)

High(71582m2)

Tube diameter Low High

Continue..

DESCRIPTION SUPERCRITICAL

(660MW)

SUB-CRITICAL

(500MW)

Material / Infrastructure

(Tonnage)

Low

7502 MT

High

9200 MT

Start up Time Less MoreStart up Time Less More

Blow down loss Nil More

Water Consumption Less More

Advanced Supercritical Tube Materials(300 bar/6000c/6200c)

129

Material Comparison

DescriptionDescription 660 MW660 MW 500 MW500 MW

Structural SteelStructural Steel Alloy SteelAlloy Steel Carbon SteelCarbon Steel

Water wallWater wall T22T22 Carbon SteelCarbon Steel

SH CoilSH Coil T23, T91T23, T91 T11, T22T11, T22

RH CoilRH CoilT91,Super T91,Super 304 H304 H

T22, T22, T91,T11T91,T11

LTSHLTSH T12T12 T11T11

EconomizerEconomizer SA106SA106--CC Carbon SteelCarbon Steel

Welding Joints (Pressure Parts)Welding Joints (Pressure Parts) 42,000 Nos42,000 Nos 24,000 Nos24,000 Nos

Advantages of SC Technology

I ) Higher cycle efficiency meansPrimarily– less fuel consumption– less per MW infrastructure investments– less emission

131

– less emission– less auxiliary power consumption– less water consumptionII ) Operational flexibility– Better temp. control and load change flexibility– Shorter start-up time– More suitable for widely variable pressure operation

ECONOMY

Higher Efficiency (η%)

•Less fuel input.•Low capacity fuel handling system.•Low capacity ash handling system.•Less Emissions.

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•Less Emissions.

Ø Approximate improvement in Cycle Efficiency

Pressure increase : 0.005 % per barTemp increase : 0.011 % per deg K

Challenges of supercritical technology

• Water chemistry is more stringent in super critical once through boiler.

• Metallurgical Challenges• More complex in erection due to spiral water wall.• More feed pump power is required due to more friction • More feed pump power is required due to more friction

losses in spiral water wall.• Maintenance of tube leakage is difficult due to complex

design of water wall.• Ash sticking tendency is more in spiral water wall in

comparison of vertical wall.

Combustion Combustion BasicsBasicsBasicsBasics

Combustion Basics

• Fuel

• Combustion Stoichiometry

• Air/Fuel Ratio

• Equivalence Ratio• Equivalence Ratio

• Air Pollutants from Combustion

5/8/2013 135Aerosol & Particulate Research Laboratory

Fuel

q Gaseous Fuels• Natural gas

• Refinery gas

q Liquid Fuels• Kerosene

• Gasoline, diesel

• Alcohol (Ethanol)

• Oil

q Solid Fuels• Coal (Anthracite, bituminous, subbituminous, lignite)

• Wood

5/8/2013 136Aerosol & Particulate Research Laboratory

Combustion Stoichiometry

q Combustion in Oxygen

OHCOOHC mn 222 +®+

OHm

nCOOm

nHC mn 222 24+®÷

øö

çèæ ++

OHCOOCH 2224 22 +®+

OHCOOHC 22266 365.7 +®+

5/8/2013 137Aerosol & Particulate Research Laboratory

Combustion Stoichiometry

q Combustion in Air (O2 = 21%, N2 = 79%)

22222 )78.3( NOHCONOHC mn ++®++

22222 478.3

2)78.3(

4N

mnOH

mnCONO

mnHC mn ÷

øö

çèæ +++®+÷

øö

çèæ ++

222224 56.72)78.3(2 NOHCONOCH ++®++

2222266 35.2836)78.3(5.7 NOHCONOHC ++®++

1. What if the fuel contains O, S, Cl or other elements?2 Is it better to use O2 or air?

Air-Fuel Ratio

q Air-Fuel (AF) ratioAF = m Air / m Fuel

Where: m air = mass of air in the feed mixture

m fuel = mass of fuel in the feed mixture

Fuel-Air ratio: FA = m Fuel /m Air = 1/AF

q Air-Fuel molal ratioAFmole = nAir / nFuel

Where: nair = moles of air in the feed mixture

nfuel = moles of fuel in the feed mixture

What is the Air-Fuel ratio for stoichiometric combustion of methane and benzene, respectively?

5/8/2013 139Aerosol & Particulate Research Laboratory

Air-Fuel Ratio

q Rich mixture- more fuel than necessary

(AF) mixture < (AF)stoich

q Lean mixture- more air than necessary

(AF) > (AF)(AF) mixture > (AF)stoich

Most combustion systems operate under lean conditions. Why is this advantageous?

Equivalence Ratio

Equivalence ratio: shows the deviation of an actual mixture from stoichiometric conditions.

actual

stoich

stoich

actual

AFAF

FAFA

)()(

)()(

==factualstoich

The combustion of methane has an equivalence ratio Φ=0.8 in a certain condition. What is the percent of excess air (EA) used in the combustion?

How does temperature change as Φ increases?

Formation of NOx and CO in Combustion

q Thermal NOx- Oxidation of atmospheric N2 at high temperatures

- Formation of thermal NOx is favorable at higher temperature

NOON 222 «+

2221 NOONO «+

q Fuel NOx- Oxidation of nitrogen compounds contained in the fuel

q Formation of CO- Incomplete Combustion

- Dissociation of CO2 at high temperature

221

2 OCOCO +«

Air Pollutants from Combustion

How do you explain the trends of the exhaust HCs, CO, and NOx as a function of air-fuel ratio?How do you minimize NOx and CO emission?

Source: Seinfeld, J. Atmospheric Chemistry and Physics of Air Pollution.

Facilitators of Combustion

• Time

• Temperature

• Turbulence

Improper Combustion

Excess Combustion• Explosion

• Tube burn out

• Refractory damages

Incomplete Combustion• Waste of fuel

• Fall in steam parameters

• Fall in thermal efficiency

• Fall in thermal efficiency

• Generation of pollutants

• Slagging

• Generation of pollutants

• High FGET

• Explosion

Main types of combustion

• Flame combustion

• Cyclone Combustion

• Fluidised Bed combustion

•Flame Combustion

• Burning of pulverized coal or coal dust in a suspended state inside the furnace.

• Fine particles of coal are easily moved by the flow of air and combustion products through the flow of air and combustion products through the section of the furnace

• Combustion takes place in a short time of the presence of particles in the furnace ( 1 to 2 secs)

Cyclone combustion

• Fuel particles go through intensive turbulent motion

• The coal particles burn off more quickly• Permits the combustion of coarse coal • Permits the combustion of coarse coal

dust and even crushed coal• Develops a higher temperature with the

result that slag are removed in the molten state.(slagging-type furnace)

Fluidized Bed Combustion (FBC)

• Solid fuel ground to particle size of 1–6mm is placed on a grate.

• It is blown from beneath with an airflow at such speed that the fuel particles are lifted above the grategrate

• The speed of the gas-air flow within the bed is higher than above it

• The finer and partially burnt particles rise to the upper portion of the bed where the flow velocity decreases and are burnt completely.

Boiler Auxillaries

• Fans

• Blowers

• Feed Pumps & Circulation Pumps

• Airheaters• Airheaters

• Dampers and gates

• Soot Blowers