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LAYOUT of Thermal Power Station

• CW System• CT system• Equipment cooling water system• Water system• Coal Handling plant & MGR • Compressed Air system Switchyard• Boiler & Auxiliaries• Turbine & auxiliaries• Generator

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CW System

• Individual unitwise system

• Common header system

• Combination system

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Cooling Tower

• TYPE OF AIR & WATER FLOW

• Cross flow type

• Counter flow type

• DRAFT:

• Induced draft

• Forced draft

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ECW System

• DM water system

• Clarified water system

• Raw water system

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Boiler - Auxiliaries

• Coal Bunker• Coal Feeder• Mill & HFO system• PA Fan• Seal Air fan• Air PreHeater• Burner• FD Fan• Scanner Air Fan

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Boiler - Auxiliaries

• Wind Box

• ESP

• ID Fan

• Chimney

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Air & Draft System

• Natural, Induced, Forced & Balanced draft

• Air- Primary Air- Hot PA & Cold PA

Secondary Air

Seal Air

Scanner Air & Ignitor Air

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• Theoretical Air

• PA Fan- functions, Requirements

• FD Fan

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Windbox Assembly

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Windbox Arrangement (Typical)

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Fuel System

• HFO

• Ignitors

• Coal Mill

• Burners

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BOILER

• BOILER FUNDAMENTAL • Combustion • Thermal efficiency of a boiler• Water system- Circulation system ( Natural, controlled &

once through) Economiser, D/C, Water wall, Drum • SuperHeater • Types- Horzontal, Vertical & combined• Radiant Convective• Desuperheater• Reheater• FSSS

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•Heat loss from furnace surface.

•Unburned carbon losses.

•Incomplete combustion losses.

•Loss due to hot ash.

•Loss due to moisture in air.

•Loss due to moisture in fuel.

•Loss due to combustion generated moisture.

•Dry Exhaust Gas Losses

•~ 4.5%

Heat gained by superheater & reheater

40%

Heat gained by economizer & air preheater

12%

Fuel Energy100%

Heat gained by boiling water40%

Hot gas

Flue gas

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Steam Theory

• Within the boiler, fuel and air are forced into the furnace by the burner.

• There, it burns to produce heat.

• From there, the heat (flue gases) travel throughout the boiler.

• The water absorbs the heat, and eventually absorb enough to change into a gaseous state - steam.

• To the left is the basic theoretical design of a modern boiler.

• Boiler makers have developed various designs to squeeze the most energy out of fuel and to maximized its transfer to the water.

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Basic Boiler Configurations

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DPNL SH

Platen SHT

R

RHTR

LTSH

Economiser

APH ESP ID Fan

drum

Furnace

BCWpump

Bottom ash

stack

screentubes

Thermal Structure of A SG

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Boiler Furnace

• Structurally boiler furnace consists of the combustion space surrounded by water walls.

• The furnace is designed to perform two functions simultaneously, namely:

• Release of the chemical energy of fuel by combustion• The first task of combustion technology is • to burn the fuel efficiently and steadily,• to consume controlled excess air (as little as possible),• To generate a flame with controlled shape which will generate lowest

amount of pollutants.• Transfer of heat from the furnace to the working fluid inside the

water walls.• The important task of furnace heat removal is to produce a controlled

Furnace Exit Gas Temperature (FEGT).• FEGT is an important aspect of boiler safety.

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Five Requisites for Good Combustion

• Fuel• MATt Theory:

– M : Proper mixing of the reactants.

– A: Sufficient air.

– T: Conducive temperature

– T: Sufficient time Satisfactory density of reactant.

Fourth DF systems contains various auxiliaries to staisfy MATt theory as completely as possible.

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Essentials of Optimum Coal Combustion

Fuel Preparation: Distribute the fuel in a larger combustion volume.

Fuel feed quality and size shall be consistent. Fuel feed shall be measured and controlled as accurately as

possible. Fuel line fineness >75% passing a 200-mesh screen. Distribution to Burners : Distribute the total air required

into many streams, namely Primary air, Secondary air and tertiary air.

Primary airflow shall be accurately measured and controlled to ±3% accuracy.

Primary air to fuel ratio shall be accurately controlled when above minimum.

Fuel line minimum velocities shall be 12 m/sec. Fuel lines shall be balanced in fuel flow to within 10% of

average.

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Pulverized fuel

• Coal is ground to face powder size to ensure rapid combustion.

• For pulverized fuel, the processes of drying, devolatilization and char combustion are sequential.

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Drying

• Heat is convected and radiated to the particle surface and

• Conducted into the particle

• Drying time = time required for the particle to the vaporization point

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Volatile matter

• Volatile matter should be at least 20% to maintain flame stability.

• For low-volatile and/or high ash coal the flame speed is low and the air from the secondary nozzle should be mixed in more slowly to avoid instability.

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Ignition

• Low-rank coals represent ignition difficulties because of their high moisture content as-fired

• Low-volatile coals (anthracites and semi-anthracites) are much more difficult to ignite.

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Flame stability

• Fuel-air mixture must have a composition within the flammability limits

• Stability of the flame is determined by the concentration of gas phase volatiles and air

• The flame speed depends on the volatile matter and ash levels of the coal

• The maximum flame speed occurs at mixtures which are sub-stoichiometric

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Role of burners

• Burner Governs:• Fuel Ignition• Aerodynamics of Fuel air mixture• Generation of combustion conditions.

• The performance of the burner determines whether combustion equipment will operate reliably & economically.

• Types of burners:• Swirl types• Direct or Parallel-flow types

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Simple Burner

Fuel

Air

Burning Velocity

Flow velocity

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Stability & Flammability Limits

• Burning Velocity > Flow Velocity : Flash Back Limit• Burning Velocity < flow Velocity : Blow Off Limit• Burning Velocity = Flow Velocity : Stable Flame.

Air Flow rate

Fue

l Flo

w r

ate

Rich Mixture

Lean Mixture

Blow off Flash Back Stable Flame

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Burning Velocity & Residence Time• Quality of Fuel & Fuel Chemistry.• Air-fuel ratio• Turbulence level

• Time to be spent by fuel particle in the furnace before it burns completely.

• Residence time is inversely proportional to burning velocity.• Fuel particle is continuously moving.• The distance traveled by the fuel particle should be much larger

than furnace height.• Swirl motion will ensure the required residence time.• Internally generated swirl : Swirl Burners.• Externally generated swirl: Direct Burners.

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Coal particle size distribution

• Maximum amount of fine particles

- 79-75 mass % through 200-mesh

• Minimum amount of coarse particles

-less than 0.1-0.5% on 50-mesh

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• If 1-2% remain on 50 mesh screen, most of this very coarse coal will not burn and will end up in the bottom ash. It also frequently causes slagging around the burners.

• If 200 mesh fineness is poor, we may get high combustibles in the fly ash.

Coal particle size distribution… contd…

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Role of burners

• Burner Governs:• Fuel Ignition• Aerodynamics of Fuel air mixture• Generation of combustion conditions.

• The performance of the burner determines whether combustion equipment will operate reliably & economically.

• Types of burners:• Swirl types• Direct or Parallel-flow types

04/19/23 35Optimization of Furnace Parameters.

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Ash content

• Ash properties of coal are broadly rank-dependant

• A higher proportion of mineral matter of low-rank coals may have higher levels of some or all of the basic elements associated with fouling or slagging (iron, calcium, magnesium, sodium, potassium)

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Slagging and Fouling

• Slagging – the formation of fused or melted deposits

• Fouling – the formation of bonded or lightly-sintered deposits at lower temperatures

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Slagging and Fouling … contd …

• Influencing factors:

(a) Temperature:

--- The temperature history of the flame partly determines the condition of the ash as it approaches the boiler surfaces

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Slagging and Fouling …contd…

• Thus, fine grinding may be a good idea

• Also, particles less than 30 m will tend to follow the flow around the tubes rather than impinging on their surfaces.

• But, fine particles are more liable to stick together

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Slagging and Fouling …contd…

• (b) heat flux

The heat flux to the boiler surface, its temperature, and the thermal

conductivity

Of the deposit upon it, together determine the thickness that can be reached before

the outer surface melts or fuses.

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Slagging and Fouling …contd…

(c) Reducing conditions

-- gaseous atmospheres containing high levels of CO lower the melting temperature of the ash to some 200oC below its melting temperature under oxidizing conditions

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Water circulation--Drum Type Boiler

Natural Circulation Boiler

Circulation thru water walls by

thermo-siphon effect

Controlled Circulation Boiler

Thermo-siphon effect

supplemented

by pumps

Supercritical Boilers

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Boiler Water Circulating Systems

Choice of Circulating system depends on

Operating Pressure

The density difference between water and steam

provides the driving force for the Circulating

fluid

Higher pressures units warrant pumps to ensure

circulation (Natural circulation not adequate)

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Types of Circulation

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Natural Circulation

Controlled Circulation

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Controlled Circulation (Vs) Once Thru’

CC OT

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Volume

Head to be developed

Temperature

Density

Fan Selection

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Adoption of variable frequency drives for ID fan

Use of double-stage Axial reaction PA

fans

Micro-processor based integrated

operating system (IOS) controls for esp

to reduce corona power consumption.

Direct ignition of pulverised coal to

reduce oil consumption in coal-fired

boilers.

Energy Saving Measures

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Super heaters

• Super heater heats the high-pressure steam from its saturation temperature to a higher specified temperature.

• Super heaters are often divided into more than one stage.• The enthalpy rise of steam in a given section should not exceed

– 250 – 420 kJ/kg for High pressure. > 17 MPa– < 280 kJ/kg for medium pressure. 7 Mpa – 17 MPa– < 170 kJ/kg for low pressure. < 7 MPa

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Furnace Wall

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Superheater System

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Platen Superheater

• Platen Superheater : Flat panels of tubes located in the upper part of the furnace, where the gas temperature is high.

• The tubes of the platen SH receive very high radiation as well as a heavy dust burden.

• Mechanism of HT : High Radiation & Low convection• Thermal Structure:

– No. of platens– No. of tubes in a platen– Dia of a tube– Length of a tube

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Reheater

• The pressure drop inside reheater tubes has an important adverse effect on the efficiency of turbine.

• Pressure drop through the reheater should be kept as low as possible.

• The tube diameter : 42 – 60mm.• The design is similar to convective superheaters.

• Overall Heat Transfer Coefficient : 90 – 110 W/m2 K.

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Thermal Structure of Economizer

• Out side diameter : 25 – 38 mm.• Tube thinckness: 3 – 5 mm• Transverse spacing : 2.5 – 3.0• Longitudinal spacing : 1.5 – 2.0• The water flow velocity : 600 – 800 kg/m2 s• The waterside resistance should not exceed 5 – 8 %.

Of drum pressure.• Flue gas velocity : 7 – 13 m/s.

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Attemperators

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Various Energy Losses in A SG

• Heat loss from furnace surface.• Unburned carbon losses.• Incomplete combustion losses.• Loss due to hot ash.• Loss due to moisture in air.• Loss due to moisture in fuel.• Loss due to combustion generated moisture.• Dry Exhaust Gas Losses.

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Furnace Exit Gas Temperature

• The temperature of products of combustion at the exit of the furnace is called FEGT.

• Defines the ratio of furnace heat absorption to outside heat absorption.

• High FEGT – Compact furnace & Large secondary section

• FEGT < Ash Deformation Temperature.• Generally FEGT = Ash Softening Temperature – 100.

FEGT is a Right Measure of Steam Generator Health !A Healthy SG will Always Perform Better !!!

A SG Always Healthy will Perform Better !

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Regenerative Trisector Airheater

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Regenerative Bisector Airheater

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REGENERATIVE AIR HEATER

• The heating surfaces are alternately heated up by the flue gas passing through and cooled by the air passing over it

• The heat is absorbed by the heat transfer matrix from the flue gases and then released to the air

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SALIENT COMPONENTS OF AIRHEATER

1. Modular Rotor2. Rotor housing and connecting plates3. Heating elements4. Sealing system5. Support bearing and guide bearing6. Lubricating oil pumps and circuits.7. Drive mechanism including auxiliary drive8. Access doors9. Observation port and light10. Cleaning and washing devices 11. Rotor stoppage alarm12. Deluge system • Elements handling arrangement• Fire sensing device

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HEATING ELEMENTS

• Air heater is provided with multi-layers of heating elements.

• The material for the cold end element is selected based on the Sulphur content expected in the starting and stabilizing fuel oils.

• The cold end elements are basketed for easy removal and re placement from the sides.

• Hot end elements are removable from the top of the gas ducts. Provision is made to accommodated the future addition of elements at hot end. (If planned)

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SEALING SYSTEM

• The rotor is divided into equal sectors each forming a separate air or gas passage through the rotor.

• Fixed leaf type metal seals are attached radially and axially to the rotor structure between each sector.

• Sector shaped unrestrained radial sealing plates provide the sealing surfaces that divide the rotor into air and gas passages.

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Gas temperature entering

airheater

Gas quantity entering airheater

Air quantity leaving airheater

Desired pressure drop on air/gas

side

Gas temperature leaving airheater

Airheater Selection

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Airheater Performance

• Gas temperature entry Actual Vs Design• Air entry temp Actual Vs Design• Coal Moisture Actual Vs Design• Gas side minimum pressure drop to ensure

at min gas flow condition for stable load. • Condition of element thickness• Condition of seals

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Performance Indicators

• Air-in-Leakage

• Gas Side Efficiency

• X - ratio

• Flue gas temperature drop

• Air side temperature rise

• Gas & Air side pressure drops

The indices are affected by changes in entering air or gas temperatures, their flow quantities and coal moisture.

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• Air ingress can be quantified by the increase in oxygen % in flue gas; The temperature drop of the flue gas from air heater outlet to ID fan discharge also provides an indication of the same.

Oxygen % at various locations in boiler

0

2

4

6

8

10

Furn Outlet AH Inlet AH Outlet ID outlet

O2

%

210 MW 210 MW 500 MW 210 MW

Boiler Air Ingress

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Boiler Exit Gas Temperature

Ideal flue gas temperature at stack outlet should be just above the dew point to avoid corrosion; Higher gas temperatures reduce efficiency; Possible causes of temperature deviations are

• Dirty heat transfer surfaces• High Excess air• Excessive casing air ingress• Fouled/corroded/eroded Air heater baskets • Change in ambient temp or AH leakage levels• Non - representative measurement

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OFF – Design/Optimum Conditions

Parameter Deviation Effect on Heat

Rate Excess Air (O2) per % 7.4 Kcal/kWh Exit Gas Temp per oC 1.2 Kcal/kWh Unburnt Carbon per % 5-15 Kcal/kWh Coal moisture per % 2-3 Kcal/kWh

Boiler Efficiency per % 25 Kcal/kWh

Effect of Boiler side Parameters (Approx.)