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Thermal Plant
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04/19/23 1
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
04/19/23 2
CW System
• Individual unitwise system
• Common header system
• Combination system
04/19/23 3
Cooling Tower
• TYPE OF AIR & WATER FLOW
• Cross flow type
• Counter flow type
• DRAFT:
• Induced draft
• Forced draft
04/19/23 4
ECW System
• DM water system
• Clarified water system
• Raw water system
04/19/23 5
Boiler - Auxiliaries
• Coal Bunker• Coal Feeder• Mill & HFO system• PA Fan• Seal Air fan• Air PreHeater• Burner• FD Fan• Scanner Air Fan
04/19/23 6
Boiler - Auxiliaries
• Wind Box
• ESP
• ID Fan
• Chimney
04/19/23 7
Air & Draft System
• Natural, Induced, Forced & Balanced draft
• Air- Primary Air- Hot PA & Cold PA
Secondary Air
Seal Air
Scanner Air & Ignitor Air
04/19/23 8
• Theoretical Air
• PA Fan- functions, Requirements
• FD Fan
04/19/23 9
Windbox Assembly
04/19/23 10
Windbox Arrangement (Typical)
04/19/23 11
Fuel System
• HFO
• Ignitors
• Coal Mill
• Burners
04/19/23 12
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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
04/19/23 15
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.
04/19/23 16
Basic Boiler Configurations
04/19/23 17
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.
04/19/23 21
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.
04/19/23 22
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.
04/19/23 23
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.
04/19/23 24
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
04/19/23 25
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.
04/19/23 26
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.
04/19/23 27
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
04/19/23 28
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 29
Simple Burner
Fuel
Air
Burning Velocity
Flow velocity
04/19/23 30
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
04/19/23 31
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.
04/19/23 32
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
04/19/23 33
• 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…
04/19/23 34
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.
04/19/23 36
04/19/23 37
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)
04/19/23 38
Slagging and Fouling
• Slagging – the formation of fused or melted deposits
• Fouling – the formation of bonded or lightly-sintered deposits at lower temperatures
04/19/23 39
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
04/19/23 40
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
04/19/23 41
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.
04/19/23 42
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
04/19/23 43
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
04/19/23 44
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)
04/19/23 45
Types of Circulation
04/19/23 46
Natural Circulation
Controlled Circulation
04/19/23 47
Controlled Circulation (Vs) Once Thru’
CC OT
04/19/23 48
Volume
Head to be developed
Temperature
Density
Fan Selection
04/19/23 49
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
04/19/23 50
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
04/19/23 51
Furnace Wall
04/19/23 52
Superheater System
04/19/23 53
04/19/23 54
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
04/19/23 55
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.
04/19/23 56
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.
04/19/23 57
Attemperators
04/19/23 58
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.
04/19/23 59
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 !
04/19/23 60
Regenerative Trisector Airheater
04/19/23 61
Regenerative Bisector Airheater
04/19/23 62
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
04/19/23 63
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
04/19/23 64
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)
04/19/23 65
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.
04/19/23 66
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
04/19/23 67
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
04/19/23 68
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.
04/19/23 69
• 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
04/19/23 70
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
04/19/23 71
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.)