boilers of thermal power plants

8/12/2019 Boilers of Thermal Power Plants

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Boilers Of Thermal Power

PlantsDebanjan Basak

CESC Ltd


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Points of Discussion

Thermodynamic Cycles Discussion on Sub and Supercritical

Boilers

Performance Indicators and Benchmarks

o a ower a on Constructional and design features of

Boilers

Boiler Auxiliaries

Losses and performance optimisation


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First Law of Thermodynamics

Energy cannot be created nor destroyed.

Therefore, the total energy of the universe

is a constant.

nergy can, owever, e conver e rom

one form to another or transferred from a

system to the surroundings or vice versa.


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Spontaneous Processes

Spontaneous processes

are those that can

proceed without any

outside intervention.

The gas in vessel B will

spontaneously effuse into

vesselA, but once the

gas is in both vessels, it

will not spontaneously


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Processes that are

spontaneous in one

direction are

nonspontaneous in

the reverse direction.


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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.


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Reversible Processes

In a reversible processthe system changes insuch a way that thesystem andsurroundings can beput back in their original

reversing the process.

Changes areinfinitesimally small in

a reversible process.


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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.


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Entropy

Entropy (S) is a term coined by Rudolph

Clausius in the 19th century.

Clausius was convinced of the significance

temperature at which it is delivered,

q

T


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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 u on the surroundin s theentropy is said to increase.

For the universe as a whole the entropy is

increasing!

Entropy is not conserved like energy!


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Entropy

Entropy can be thought of as a measure of

the randomness of a system.

It is related to the various modes of motion

.


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Entropy

Like total energy, E, and enthalpy, H,

entropy is a state function.

Therefore,

= final initial


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Second Law of

ThermodynamicsThe second law of thermodynamics:

The entropy of the universe does not

change for reversible processes and

Reversible (ideal):

Irreversible (real, spontaneous):


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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.

rotation about bonds.


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Each thermodynamic state has a specific number ofmicrostates, W, associated with it.

Entropy is

S = k lnW

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


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The number of microstates and,

therefore, the entropy tends to increase

with increases in

. Volume (gases).

The number of independently moving

molecules.


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Entropy and Physical States

Entropy increases with

the freedom of motion

of molecules.

Therefore,

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


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Solutions

Dissolution of a solid:

Ions have more entropy

(more states)

But,

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.)


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Entropy Changes

In general, entropy

increases when

Gases are formed from

qu s an so s.

Liquids or solutions are

formed from solids.

The number of gas

molecules increases.

The number of moles

increases.


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Third Law of Thermodynamics

The entropy of a pure crystall inesubstance at absolute zero is 0.


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Standard Entropies

Larger and more complex molecules have

greater entropies.


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Link S and H: Phase changes

A phase change is isothermal

(no change in T).

Entropysystem

For water:

Hfusion = 6 kJ/mol

Hvap = 41 kJ/mol

If we do this reversibly: Ssurr= Ssys


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Change in entropy

> 0

irreversible

Change in entropy

= 0

reversible

Change in entropy

< 0

impossible

process process process


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When a liquid evaporates its go

through a process where

the liquid heats up to the evaporation

temperature

the liquid evaporate at the vaporation

from fluid to gas

the vapor heats above the vaporation

temperature - superheating


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The heat transferred to a substance

when temperature changes is oftenreferred to as sensible heat.

The heat required for changing state as

evaporation is referred to as latent heat

of evaporation.


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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)where

H = enthalpy (kJ)

m = mass (kg)

h = specific enthalpy (kJ/kg)

Specific Enthalpy

Specific enthalpy is a property of the fluid and can be expressed

as:h = u + p v (2)

where

u = internal energy (kJ/kg)

p = absolute pressure (N/m2)

v = specific volume (m3/kg)


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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= mg

mg + mf

Quality of Steam

It is the representation of dryness fraction in

percentage: Quality of Steam = x X 100


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Steam Quality

Steam should be available at the point of

use:

At the correct temperature and pressure

Free from air and incondensable gases

CleanDry


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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

Rise in Superheated temperature poses problems in lubrication

Initial cost is more and depreciation is higher


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Carnot Cycle

Most efficient cycleoperating betweentwo heat sources

Practically impossible

cu y n en ng econdensation process

High energyconsumption forpumping /compression


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Rankine Cycle

Practical Carnot cyclewith much lessefficiency

Pump power is much

turbine output (within1%)

Efficiency limited forlower steam inlet

temperature


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Rankine Cycle

Process 1-2: Pump Work

Process 2-3: Sensible

and latent heat addition in

the boiler at constant

Process 3-4: Expansion

in steam turbine

Process 4-1:

condensation of the

steam in condenser


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Rankine cycle with Reheat

Average temp of heataddition increaseswith higher pressure

Restricted for

Reheating theexpanded steam toimprove efficiency

Exit Dryness Fractionimproved


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Rankine cycle with Reheat and

Regeneration Most commonly used

in power plant

Bled steam is utilised

to exchange heat

before being cooled

at the condenser


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Steam Condition Vs Design efficiency

Steam 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

. .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


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Heat Rate Improvement

Parameters at Turbine Inlet(bar/oC / oC)

% Improvement In StationHeat 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%


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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 plant

Limitation in metallurgy is the constraints for

higher terminal condition and hence efficiency


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Heat addition-Sensible Heat

The sensible heat is mostlyadded in the feed water

heaters and the economisers

The cycle operates between

100 bar (310.9610C saturation

.saturation temp)

Sensible heat at A =101 KJ/Kg

Sensible heat at B =1408

KJ/Kg

Sensible heat added = 1307KJ/Kg


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Variation of sensible heat with Pressure

Absolute

pressure

(bar)

Saturation

Temperature

(C )Sensible

Heat

( kj / kg )

. .

100 311.0 1408.0

150 342.1 1611.0

200 365.7 1826.5

221.2 374.15 2107.4


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Heat addition - Latent Heat

The latent heat is mostly addedin the water wall tubes of theboiler

Latent heat diminishes withpressure and is zero at critical

pressure 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 = 1319.7KJ/Kg


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Absolute

pressure

(bar)

Saturation

Temperatur

e

(C )

Latent

Heat

( kj / kg )

50 263.9 1639.7

Variation of Latent heat with Pressure

100 311.0 1319.7

150 342.1 1004.0

200 365.7 591.9

221.2 374.15 0


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Heat addition - Super heat

The Super heat is mostlyadded in the superheatertubes of the boiler arisingout from the drum

from C to D at constantpressure

The amount of superheatcan be found by deducting

the total heat of Point Cfrom total heat of point D


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Variation of Superheat with Pressure

Absolute pressure

(bar)

Superheat required

( kj / kg )

50 800.9

100 821.5

150 885.4

200 1033.2


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Useful Heat

Useful heat : Total Heat Rejected Heat


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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


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Back pressure correction curve

Back Pressure in mb

Heat

cons

Optimum Back

pressure


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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 coolin of condensate. Flow to be o timised to et 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


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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 %


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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

Thermal Efficiency = 44.23 %

The Efficiency of basic Rankine

cycle can be improved with

superheat

The scope however is limited dueto materials to withstand high

temperature


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Ideal Efficiency of

Rankine Cycle with Reheat

The same 100 bar cycle with

reheat

At pressure 20 bar after

expansion in the turbine, the

566 0C

The steam expands to the

condenser pressure in IP/LP

turbine

The efficiency of this cycle is46.09%


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Ideal Efficiency of

Rankine Cycle with Reheat and regeneration

Sensible heat addition from

M to B

Latent heat and superheat

addition as before

.Kj/Kg

Heat rejected = 1192.2

Kj/Kg

Thermal Efficiency =

51.4%


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Increasing the maximum operating temperature can also increaseefficiency, as this takes steam to the superheated region, whichincreases the area and also enhances the quality of steam exiting theturbine.

The maximum temperature is limited by the metallurgicalquality of the pipes of boiler.


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Advantages of Reheat CycleIncreases 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.

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

reducedSize of the LP turbine blades is reduced because

sp. Steam volume is reduced by 8%


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Disadvantages of Reheat Cycle

Cost increases for additional pipes and

reheaters

Greater floor space required for longer

turbine

At light loads, steam passing through the

last blade rows are highly superheated if

same reheat is maintained


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Boiler: Definition as per IBR

Boilermeans any closed vessel

exceeding 22.75 litres (five gallons)

in ca acit which is used ex ressl

for generating steam under pressure

and includes any mounting or other

fitting attached to such vessel, which

is wholly or partly under pressurewhen steam is shut off:


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Classification of PF Boilers

Based on Operating

Pressure

Super-CriticalUltra-Super-

Critical

Sub-Critical

u - r ca : < Critical Pr.221.2 Bar

Super critical: > Critical

Pr. 221.2 Bar

Ultra-super critical >Pr > 300 Barand 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

EfficiencyIncrease

%

1.0

5660C/5660C

Super cri tical and ul tra supercritical conditions


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Super cri tical and ul tra supercritical conditions

Critical Conditions

Temperature -374.150C

Pressure-225.56kg/cm2

Ultra super critical condit ions

Temperature above 5930C

Pressure above 306kg/cm2

Improvement of thermal efficiency

Increasing the steam temperature ( 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 ( increases 0.1%

increase with increase of 10 bar pressure)


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Based on Types of Circulation

Natural Circulation Boiler


Assisted circulation Boiler

Once through Boiler


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Circulation in Boiler


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

and returns through riser tubes after being heated in thefurnace

The static head difference generated due to densitydifference of the steam and water mixture in the risertubes and water in the down comer is the driving force

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 thedensities 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


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Circulation Quantity VS Steam Produced

End Point

The circulation increases

with increase in Heat inputLosses due to friction from

high specific volume is

higher than the pressure

differential

Steam Produced

Total

Circulation

Quantity


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Circulation Ratio

Circulation ratio is the weight of water fed to thesteam generating circuits to the steam actuallygenerated

Kg. of water

Circulation ratio =

Kg. of Steam

Circulation ratio depends upon operatingpressure, available circulation head and flowresistance

For sub critical boilers, circulation ratio variesfrom 10-30


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Relationship

of density of

water-steam

withoperating

pressure


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Forced circulation (Once through)

No drum to separate change of state

Once through boiler can operate at anypressure below or above critical pressure

to the load and hence a minimum flow of25-30 % is needed always by recirculationpumps or by dumping

Spirally wound tube to average the heatinput per tube


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Once Through Boilers


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Based on Types of firing

Wall fired: Front / Opposed


Corner fired: Tangential

Down-shot fired : Single / Double


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Wall Firing (TGS Boiler)

The stability is imposed by a combination ofsecondary of air swirl and a flow reversal in theprimary air by an impeller

The refractory quarl though acts as a radiantheat 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 whichconsumes less fan power, intimate mixing and

better control


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Down shot Firing (BBGS Boiler)

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

Long particle residence time for completecombustion

The coal is fed downwards from the archalong with about 30-40 % combustion air

The secondary air and tertiary air isdistributed to form the flame characteristicsand shape


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Tangential Firing (SGS / BBGS#3 Boiler)

A turbulent zone is created in the center of thefurnace by the turbulent flames fired from thecorners towards the imaginary circle to which theflame path is tangent

of coal

The mixing of coal and air is obtained by theadmission 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 inBBGS #3)


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Our BoilersOur BoilersTitagarh Generating StationTitagarh Generating Station

Designed for Coal wi th

Calori fic Value 4500

Ash + Moisture 35.5%

Volatile Matter 25%

Fixed Carbon 39.5%

Southern Generating StationSouthern Generating Station

Designed for Coal wi th

Calori fic Value 3800

Ash + Moisture 44%

Volatile Matter 17%

Fixed Carbon 39%

BBGS Generating StationBBGS Generating Station

Designed for Coal wi th Calori fic Value 3850

Ash + Moisture 50%

Volatile Matter 15%

Fixed Carbon 32%

Front wall firinFront wall firin Down shot firinDown shot firin


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Heat Transfer Zones


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Heat Transfer Zones

The Furnace: High temperature gases ofcombustion is used for heating water and steam

with low to medium superheat

gases is used to heat steam with medium to high

superheat

Heat Recovery zone: Comparatively cool gasesexchange heat to feed water to saturation temperature or

with low superheat


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Types of Boiling

Sub-cooled water heating: Initial stage of heating, Water in contact with thetube evaporates-bulk fluid is below the saturation temp

Sub-cooled Nucleate Boiling: Formation and collapsing of bubbles due totransfer of latent heat

Nucleate boiling: Bulk of the liquid reaches to saturation temperature, bubbles, .

this stage (Water velocity 1.5-3 mps)

DNB (Departure from Nucleate boiling): Even higher heat flux will result incollapsing 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


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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)

Flux D-E: Unstable Film BoilingE-F: Stable Film Boiling

ES

A

B


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Furnace- Duty

Furnace to have suitable surface area to

reduce the temperature of the furnace gas

to a level acceptable to super heater

Adequate water circulation in the furnace

tubes to prevent overheating

To avoid flame impingement in the

opposite wall tubes


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Furnace- Duty

Width sufficient to accommodate all

burners at an acceptable pitching

Overall dimension to ensure optimum

To reduce the furnace temperature below

ash softening temp to avoid slagging


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Coal Vs Oil Fired Furnace

Average oil droplet burnout time is half to

that of coal

Coal particle require higher residence time

Sticky ash hinders wall tube heat

absorption hence higher surface area


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Furnace- Performance & Control

Operation procedures

Firing Pattern

Soot blowing

Excess airOther methods

Gas recirculation (GR) as in BBGS

Tilting burners for Corner fired boiler


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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 mainstays

Advanta e: Easy maintenance

Older design

Membrane wall tube: Tubes joined with fins to form a fullywelded structure, the membrane wall

Advantage: Minimum ingress of tramp air

The outer casing requires only heat shielding


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Super heaters and Re heaters


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Super heaters and Re heaters

Convective: The heat transfer is through convection and Heat absorption rate increaseswith the boiler output

Radiant: Radiant su er heaters receives heat throu h radiation onl

With increase load in the boiler, the heat absorption in the furnace surfacesis 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


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RE,SCALEARBITRARY

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

STEAMTEMPERAT

STEAM TO IP TURBINE STEAM TO HP TURBINE


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PRIMARY

REHEATER

VERTICAL

PIMARY

SUPERHEATER

VERTICAL

PRIMARY

FINAL

REHEATER

FINAL

SUPER

HEATER

PLATEN

SUPERHEAER

STEAM

FROM

COMBUSTION

GASES

STEAMFROM DRUM

TO DRUM

FEED WATER

TO DRUM

FEED WATER

REHEATER

ECONOMISER

SUPERHEATER

ECONOMISER

FURNACE

AIR HEATER

GAS TO STACK

REHEATER PRIMARY

SUPERHEATER

AIR

COAL

TURBINE

BLOCK DIAGRAM SHOWING BOILER

ELEMENTS AND FLOWPATHS


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OXIDE

STEAM

WATERFOULING

GAS

FILM

BULK GAS

TEMPERATUE

TUBE

WALL

COMPOSITE TEMPERATURE DROP FROM GAS TO STEAM / WATER

THROUGH A BOILER TUBE WALL


98/150

GAS

GAS

1025*C

568*C

1025*C

930*C

568*C

930*C

STEAM

COUNTER FLOW PARALLEL FLOW

492*C

492*C

Tin=447.4*c Tin=442.0*c

SUPER HEATER GAS AND STEAM TEMPERATURE


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General

Arrangement of

a 210 MW

-

boiler


100/150

Typical section

of a Double

,250 MW Boiler

at BBGS,

CESC

Super heater temperature is affected by


101/150

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


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Super heater temperature control

Direct Attemperation / De super heaters Excess Air

Furnace division

Gas recirculation

Adjustment of burner tilt

Type pf burners


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104/150

Steam Separation and purity

Boiler operating below critical pressure needdrum to separate saturated steam from a

mixture of steam-water discharged by the boiler

tubes

Drum also serves as vessel for chemicaltreatment 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


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RWATER


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0.30

0.25

0.20

0.15

ATIO=

SILICACONTE

NTOFSTEAM

SILICACONTE

NTOFBOILER

0 1000 2000 3000 4000

.

0.05

0.00

STEAM DRUM PRESSURE IN , psi

DISTRIBUTION

EFFECT OF PRESSURE ON SILICA DISTRIBUTION RATIO


107/150

Performance Indicators and

Benchmarkin

B h ki Obj i


108/150

Benchmarking-Objectives

Benchmarking is

a continuous formal process of measuring,

understanding, and adapting

more e ec ve prac ces rom es - n-c assorganizations that lead to superior

performance.

Benchmarking is essential to

provide the best service to our customers.

B h ki B fit


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Benchmarking-Benefits

Improve our performance and organization

Learn about industry leaders and competitors

Determine what world-class performance is

Achieve breakthrough results

Improve customer satisfaction

Become the best in the business

St f b h ki


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Steps of benchmarking

What to benchmark

With whom to benchmark

Identification of potential improvement areas

based on benchmarkin .

Adoption of best practices for improvement

Monitor effectiveness of new practice

Modify practice as per requirement

Standardise practice

Key Benefits from Benchmarking at


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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

Ammonia dosing system at ESP

Boiler Insulation survey

Destaging of Condensate Extraction Pump

Installation of SS-304 chutes at CHP

K P f I di t


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Key Performance Indicators

Cost of Generation Plant Load Factor (PLF)

Plant Availability Factor (PAF)

Loss In Production

Specific Coal Consumption

Specific Oil Consumption

Auxiliary Power Consumption

Environmental Emissions

No of Accidents Implementation of Quality and SHE systems

K M it i


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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


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Technology

What is Supercritical Pressure ?

Critical point in water vapour cycle is a

thermod namic state where there is no clear

114

distinction between liquid and gaseous stateof water.

Water reaches to this state at a critical

pressure above 22.1 MPa and 374 oC.



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

clear distinction between liquid and

.

Water reaches to this state at a

crit ical pressure above 22.1 MPa and

374 oC.

R ki C l S b iti l U it


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Rankine Cycle Subcritical Unit

1 - 2 > CEP work

2 - 3 > LP Heating

3 - 4 > BFP work

4 - 5 > HP Heating

5 6 > Eco, WW

uper ea ng

7 8 > HPT Work8 9 > Reheating

9 10 > IPT Work

1011 > LPT Work

11 1 > Condensing

R ki C l S iti l U it


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Rankine Cycle Supercritical Unit

1 - 2 > CEP work

2 2s > Regeneration

2s - 3 > Boiler Superheating

>

4 5 > Reheating5 6 > IPT & LPT Expansion

6 1 > Condenser Heat rejection

VARIATION OF LATENT HEAT

WITH PRESSURE


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WITH PRESSURE

AbsoluteAbsolute

PressurePressure

(Bar)(Bar)

SaturationSaturation

TemperatureTemperature

((ooC)C)

LatentLatent

HeatHeat

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

5050 264264 16401640

150150200200

221221

342342366366

374374

10041004592592

00

D t f N l t B il i


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Departure from Nucleate Boiling

Nucleate boiling is a type of boiling that takes place when the surface temp is hotterthan 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.

YWATER

PRESSURE(ksc)

DENS

IT

STEAM

175 224

No Religious Attitude


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Supercritical Boiler Water Wall


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p

Rifle Tube And Smooth Tube

Natural Circulation Vs. Once


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Through System

Mixer Header

To HP

TurbineTo IP

Turbine

5340C

5710C

5690C


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From CRH Line

LTRH

LTSH

4430C

FRH

Platen

Heater

FSH

Separator

3260C

4230C

4730C

4620C

534 C5260C

3240C

From FRS Line

Boiler

Recirculation Pump

Economizer

Phase 1Economizer

Phase 2

Bottom Ring

Header

2830C

2800C

NRV

Feed water control


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Feed water control

In Drum type Boiler Feed water flowcontrol by Three element controller1.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)


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Difference ofSubcritical(500MW) and

Su ercritical 660MW

COMPARISION OF SUPER CRITICAL & SUB CRITICAL


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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

-OHDR(22-35 OC)-Load Demand

-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..


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DESCRIPTION SUPERCRITICA

L(660MW)

SUB-CRITICAL

(500MW)

Coal & Ash handling Low High

Aux. Power

Consumption

Low More

Overall Efficiency High

(40-42%)

Low

(36-37%)

Total heating surfacearea Reqd Low(84439m2)High

(71582m2)

Tube diameter Low High

Continue..


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DESCRIPTION SUPERCRITIC

AL(660MW)

SUB-

CRITICAL(500MW)

Material / Infrastructure

(Tonnage)

Low

7502 MT

High

9200 MT

Blow down loss Nil More

Water Consumption Less More

Advanced Supercritical Tube Materials


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p

(300 bar/6000c/6200c)

129

Material Comparison


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DescriptionDescription 660 MW660 MW 500 MW500 MWStructural 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,SuperT91,Super304 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


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Advantages of SC Technology

I ) Higher cycle efficiency meansPrimarily less fuel consumption

less per MW infrastructure investments

131

less auxiliary power consumption less water consumption

II ) Operational f lexibil ity

Better temp. control and load change flexibility

Shorter start-up time

More suitable for widely variable pressure operation

ECONOMY


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ECONOMY

Higher Efficiency (%)

Less fuel input.

Low capacity fuel handling system.

Low capacity ash handling system.

132

.

Approximate improvement in Cycle

Efficiency

Pressure increase : 0.005 % per bar

Temp increase : 0.011 % per deg K

Challenges of supercritical technology


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g p gy

Water chemistry is more stringent in super critical oncethrough boiler.

Metallurgical Challenges

More complex in erection due to spiral water wall.

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 incomparison of vertical wall.


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CombustionCombustion

Combustion Basics


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Fuel

Combustion Stoichiometry

Air/Fuel Ratio

Air Pollutants from Combustion

5/8/2013 135Aerosol & Particulate Research Laboratory

Fuel


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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



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Combust ion Stoichiometry


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

22222 )78.3( NOHCONOHC mn

222224

78.32

)78.3(4

Nm

nOHm

nCONOm

nHC mn

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


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q Air -Fuel (AF) ratio

AF = 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

qAir -Fuel molal rat io

AFmole = 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?


Air-Fuel Ratio


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q Rich mixture

- more fuel than necessary(AF) mixture < (AF)stoich

q Lean mixture

- more air than necessary

(AF) mixture > (AF)stoich

Most combustion systems operate under lean conditions.

Why is this advantageous?


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Formation of NOx and CO in Combustion


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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


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How do you explain the trends of the exhaust HCs, CO,

and NOx as a funct ion 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


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Facilitators of Combustion

Time Temperature

Turbulence

Improper Combustion


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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


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Main types of combustion

Flame combustion Cyclone Combustion

Fluidised Bed combustion

Flame Combustion


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Flame Combustion

Burning of pulverized coal or coal dust in asuspended state inside the furnace.

Fine particles of coal are easily moved by 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)


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Fluidized Bed Combustion

(FBC)


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(FBC)

Solid fuel ground to particle size of 16mm isplaced on a grate.

It is blown from beneath with an airflow at suchspeed that the fuel particles are lifted above the

The speed of the gas-air flow within the bed ishigher than above it

The finer and partially burnt particles rise to theupper portion of the bed where the flow velocity

decreases and are burnt completely.

Boiler Auxillaries


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o e u a es

Fans Blowers

Feed Pumps & Circulation Pumps

Airheaters Dampers and gates

Soot Blowers

boilers of thermal power plants

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