Ppe unit 1

UNIT-I
INTRODUCTIONTO POWER PLANTS AND
BOILERS
M.NAMBIRAJ
ASSISTANT PROFESSOR
DHANALAKSHMICOLLEGEOF ENGINEERING,
CHENNAI

• Layout of Steam , Hydro , Diesel , MHD,
Nuclear and Gas turbine Power Plants
• Combined Power cycles
• Comparison and selection
• Load duration Curves Steam
• Boilers and cycles
– High pressure and
– Super Critical Boilers
– Fluidized Bed Boilers
CONTENTS

UNIT-I
INTRODUCTION TO POWER PLANTS AND
BOILERS
• A power plant may be defined as a machine or assembly of
equipment that generates and delivers a flow of mechanical or
electrical energy.
• The main equipment for the generation of electric power is
generator. When coupling it to a prime mover runs the
generator, the electricity is generated.
The major power plants, which are discussed in this Chapter is,
1. Steam power plant
2. Nuclear power plant
3. Hydro electric power plant
4. Diesel power plant
5. Gas turbine power plant

The heat energy is converted into mechanical
energy by the steam turbine and that mechanical
energy is used for generating power with the
help of generator.
The layout of the steam power plant consists of
four main circuits:
1.Coal and ash circuit
2.Air and flue gas circuit
3.Water and steam circuit
4.Cooling water circuit

Advantages:
• Power production does not depends on nature of mercy .
• Initial investment is low.
• The time requirement for construction and
commissioning of thermal power plant require less
period of time.
Drawbacks:
• As compared to hydro power plant, life and efficiency
are less .
• Transportation of fuel is a major problem in this type of
power plant.
• It cannot be used as a peak load power plant .
• The coal (fuel) needed may be exhausted by gradual use.

HYDRO POWER PLANT
• Hydro electric power was initiated in India in
1987 near Darjeeling.
• In Hydro electric Power plants, the potential
energy of water is converted into kinetic
energy.
• The potential energy of water is used to run
water turbine to which the electric generator is
coupled.
• The arrangement of different components used
in hydraulic power plant is discussed below.

Components of Hydro electric power plant:
1.Water reservoir- To store the water during rainy
season.
2.Dam- To increase the height of water level and
thereby it increases the capacity of reservoir.
3.Spillway- It is acting as a safety valve for dam.
4.Trash rack- The water is taken from the dam or from
forebay is provided with trash rack.
5.Pressure tunnel- It carries water from the reservoir to
surge tank.
6.Forebay- It serves as a regulating reservoir. It stores
the water temporarily when the load on the plant
reduces.

7.Penstock- A pipeline fixed between the surge tank and
prime mover. It is commonly made of reinforced concrete
or steel.
8.Surge tank- The surge tank is introduced b/w the dam
and powerhouse to keep reducing sudden rise of pressure
(water hammer) in the penstock due to sudden backflow
of water.
9.Water turbine- It converts kinetic energy of water into
mechanical energy to produce electrical energy.
10.Draft tube- It is connected at the outlet of the water
turbine.
11.Tailrace- It is a waterway to lead the water discharged
from the turbine to the river.

Advantages:
• Water is the renewable source of energy. It is neither
consumed nor converted into something else
• Variable load does not affect the efficiency in the case of a
hydro plant
• Maintenance cost is low
• It requires less supervising staff for the operation of the
plant
Disadvantages:
• Hydro power projects are capital intensive with low of
rate of return
• Initial cost of the plant is high
• It takes considerably longer time for its installation
compared with thermal power plants

NUCLEAR POWER PLANT
• The heat produced due to fission of U and Pu
is used to heat water to generate steam which
is used for running turbo generator.
• 1kg of U can produce as much energy as
possible by burning 4500tonnes of high-grade
coal.
• The two materials most commonly used are
uranium-235 and plutonium-239.

Nuclear fission - This process takes place when
the nucleus of a heavy atom like uranium or
plutonium is split when struck by a neutron.

Main components of Nuclear power plant

1. Fuel- The fuel which is used in the nuclear
reactors are uranium-235, plutonium-239,
uranium-233.
2. Nuclear Reactor- A nuclear reactor is a
device that permits a controlled fission chain
reaction. It consists of Reactor core, reflector,
shield etc. The various types of reactors used
in nuclear power plant is
- Pressurized-Water Reactor (PWR)
- Boiling Water Reactor (BWR)
- Heavy Water Reactor (HWR)

3. Steam generator- It is fed with feed water
and the feed water is converted into steam by
the heat of the hot coolant. The coolant is used
to transfer the heat from reactor core to the
steam generator.
4. Moderator- It is used to the slow down the
neutrons from high velocities without
capturing them.
5. Reflector- It is done by surrounding the
reactor core by a material called reflector
which will send the neutrons back in to core.

6. Control rods- It is used to control the nuclear
chain reaction and functions of the nuclear
reactor.
7. Coolant pump and feed pump- It is used to
maintain the flow of coolant and feed water in the
power plant. The ordinary water or heavy water is
a common coolant.
8. Shielding- The reactor is a source of intense
radioactivity. These radiations are very harmful,
shielding is provided to absorb the radioactive
rays. A thick concrete shielding and a pressure
vessel are provided to prevent the radiations
escaped to atmosphere.

Working principle of Nuclear Reactor

Advantages:
• Space requirement of a nuclear power plant
is less
• There is increased reliability of operation
• No ash handling
Drawbacks:
• It is not suitable for variable load condition
• It requires high initial cost
• It requires well trained personnel
• Maintenance cost of plant is high.

Diesel power plant
• Diesel engine power plant is suitable for small
and medium outputs.
• It is commonly used where fuel prices or
reliability of supply favour oil over coal, where
water supply is limited, where loads are
relatively small, and where electric line service
is unavailable or is available at too high rates.

Main components of Diesel power plant

Components of diesel power plant
1. Diesel engine- this is the main component of a
diesel power plant. The engines are classified as
2-stroke and 4-stroke engines. Engine is directly
coupled with generator for developing power.
2. Air filter and super charger- The air filter is
used to remove the dust from the air which is
taken by the engine.
The function of the supercharger is to increase
the pressure of the air supplied to the engine and
thereby the power of the engine is increased.

3. Engine starting system- This includes air
compressor and starting air tank. This is used to
start the engine in cold conditions by supplying
the air.
4. Fuel system- It includes the storage tank, fuel
transfer pump, strainers and heaters. The fuel is
supplied to the engine according to the load
variation.
5. Lubrication system- It includes oil pumps, oil
tanks, filters, coolers and pipes. It is used to
reduce the friction of moving parts and reduce
wear and tear of the engine.

6. Cooling system- The temperature of burning fuel
inside the combustion chamber is 1500oC to
2000oC. To maintain the temperature water is
circulated around the engine water jackets.
7. Governing system- It is used to regulate the
speed of the engine. This is done by varying fuel
supply according to the engine load.
8. Exhaust system- After the combustion process,
the burned gases are exhausted to the atmosphere
through silencer. The exhaust gas has high
temperature and so it is used to preheat the oil
and air.

Applications:
• Quite suitable for mobile power generation
• Used as peak load plants in combined with
thermal or hydro plants
• Used as stand by plants for emergency service
Advantages:
• Diesel power plants are cheaper
• Plant layout is simple
• Location of the plant is near the load centre
• It has no stand by losses

GAS TURBINE POWER PLANT
The gas turbine power plant has relatively low
cost and can be quickly put into commission.
It requires only a fraction of water used by
their steam turbine counter-parts.
It requires less space. This plant is of smaller
capacity and is mainly used for peak load
service.
The size of gas turbine plants used varies from
10MW to 50MW and the thermal efficiency of
about 22% to 25%.

Elements of Gas turbine plant
The simple gas turbine plant consists of
1.Compressor
2.Intercoolers
3.Regenerator
4.Combustion Chambers
5.Gas turbine
6.Reheating unit.

1. Compressor- In gas turbine plant, the axial and
centrifugal flow compressors are used. In most of
the gas turbine power plant, two compressors are
used.
1. Low pressure compressor
2. High pressure compressor
2. Intercooler- It is used to reduce the work of the
compressor and it is placed b/w HP and LP
compressor. The cooling of compressed air in
intercooler is generally done by water.
3. Regenerator- It is used to preheat the air which is
entering into the CC to reduce the fuel consumption
and to increase the efficiency.

4. Combustion chambers- Hot air from regenerator
flows to the CC and the fuel like coal, natural gas
or kerosene are injected into the CC. After the fuel
injection, the combustion takes place.
5. Gas turbine- (1. Open cycle gas turbine)

MAGNETO HYDRO DYNAMICS (MHD)
GENERATOR

Contents
1. Introduction
2. Principle Of MHD Power Generation
3. Types of MHD SYSTEM
4. Open Cycle MHD System
5. Closed Cycle MHD System
6. Difference between Open Cycle and Closed
Cycle MHD System
7. Advantages OF MHD System
8. Disadvantages of MHD System
9. Applications
10. Conclusion

Introduction
Magneto Hydro Dynamic (MHD) system is a
non- conventional source of energy which is
based upon Faraday’s Law of Electromagnetic
Induction, which states that energy is generated
due to the movement of an electric conductor
inside a magnetic field.

Concept given by Michael Faraday in
1832 for the first time.

Principle Of MHD Power Generation
Faraday’s law of electromagnetic induction : When an
electric conductor moves across a magnetic field, an emf
is induced in it, which produces an electric current .

TYPES OF MHD SYSTEM
(1)Open cycle System
(2)Closed cycle System
(i) Seeded inert gas systems
(ii) Liquid metal systems

DIFFERENCE BETWEEN OPEN CYCLE
AND CLOSED CYCLE SYSTEM
Open Cycle System
 Working fluid after generating
electrical energy is discharged
to the atmosphere through a
stack .
 Operation of MHD generator is
done directly on combustion
products .
 Temperature requirement :
2300˚C to 2700˚C.
 More developed.
Closed Cycle System
 Working fluid is recycled to the
heat sources and thus is used
again.
 Helium or argon(with cesium
seeding) is used as the working
fluid.
 Temperature requirement : about
530˚C.
 Less developed.

ADVANTAGES OF MHD SYSTEM
Conversion efficiency of about 50% .
Less fuel consumption.
Large amount of pollution free power generated .
Ability to reach full power level as soon as started.
Plant size is considerably smaller than
conventional fossil fuel plants .
Less overall generation cost.
No moving parts, so more reliable .

DISADVANTAGES OF MHD SYSTEM
 Suffers from reverse flow (short circuits) of
electrons through the conducting fluids around
the ends of the magnetic field.
 Needs very large magnets and this is a
major expense.
 High friction and heat transfer losses.
 High operating temperature.
 Coal used as fuel poses problem of molten
ash which may short circuit the electrodes.
Hence, oil or natural gas are much better
fuels for MHDs. Restriction on use of fuel
makes the operation more expensive.

APPLICATIONS
Power generation in space craft.
Hypersonic wind tunnel experiments.
 Defense application.

SELECTION OF POWER PLANT
1. Availability of raw material.
2. Nature of land.
3. Cost of land.
4. Availability of water.
5. Transport facilities
6. Ash disposal facilities
7. Availability of labour
8. Size of plant
9. Load centre
10. Public Problems
11. Future Extension

CLASSIFICATION ON THE BASIS OF PRESSURE
Boilers are classified into two basic classes
• Low Pressure Boiler: Pressure is below 80bar.
• High Pressure Boiler: Pressure is above 80bar.
High Pressure Boilers are further classified in to Sub
Critical, Supercritical and Ultra Super Critical
• Sub-Critical (Operated around 19 Mpa):
Example: 170 bar, 540˚C, Efficiency: 38 %
• Super-Critical (Operated above 22.1 Mpa):
Example: 250 bar, 600/615˚C, Efficiency: 42 %
• Ultra Super-Critical:
Example: 300 bar, 615/630˚C, Efficiency: 44 %

MODERN HIGH PRESSURE BOILERS
A boiler which generates steam at pressure
greater than 80bar a temperature of about 500˚C,
producing more than 250 tones of steam/hr
called high pressure boilers.
By using high-pressure boilers, low grade
fuels can be burned easily.
High pressure boilers are water tube boilers
and uses pulverized coal firing. Examples of
these boilers are Lamont, Banson, Loeffler and
Velox boilers.

Components:
Steam separator drum
Water circulating pump
Distributing header
Evaporator
Convection super heater
Economiser
Air preheater
Combustion chamber

Advantages:
1. It is forced circulation boiler
2. High working pressure
Disadvantages:
1. The salt and sediment are deposited on
the inner surface of water.
2. Danger of overheating of tubes

• This is the first drumless boiler.
• The entire process takes place in a continuous
tube. This is also called once through boiler.
COMPONENTS:
1.Economiser- To preheat the water
2.Radiant evaporator-Partly converted into steam
3.Convection evaporator-Remaining water is
absorbed from the hot gases by convection.
4.Convection super heater-Saturated high-Pr
steam is converted into superheated steam.

COMPONENTS
Economiser-To preheat the water
Evaporator drum- It contains steam and water, 2/3 of
SH steam is used to heat the water in the drum and
evaporates it to saturated steam.
Mixing nozzles-It is used to distribute and mix the SH
steam throughout the water in the evaporator drum.
Steam circulating pump-To forces the steam from the
evaporator drum to the radiant super heater.
Radiant super heater-It is placed in the furnace. Used
for SH the saturated steam from the drum.
Convective super heater-Finally heated to the desired
temperature of 500˚C.

This boiler makes use of pressurized
combustion. This boiler can generate a pressure of
about 84kg/cm².
COMPONENTS:
1.Economiser
2.Axial flow compressor
3.Water circulating pump
4.Convection super heater

Fluidized Bed (FBC) Boiler
An Overview-
Fluidized bed combustion has emerged as a
viable alternative and has significant advantages over
conventional firing system.
It offers multiple benefits – compact boiler
design, fuel flexibility, higher combustion efficiency
and reduced emission of noxious pollutants such as
SOx and NOx. The fuels burnt in these boilers include
coal, washery rejects, rice husk, bagasse & other
agricultural wastes. The fluidized bed boilers have a
wide capacity range.

Mechanism of Fluidised Bed Combustion
When an evenly distributed air or gas is passed
upward through a finely divided bed of solid particles
such as sand supported on a fine mesh, the particles are
undisturbed at low velocity. As air velocity is gradually
increased, a stage is reached when the individual
particles are suspended in the air stream – the bed is
called “fluidized”.
With further increase in air velocity, there is
bubble formation, vigorous turbulence, rapid mixing
and formation of dense defined bed surface. The bed of
solid particles exhibits the properties of a boiling liquid
and assumes the appearance of a fluid – “bubbling
fluidized bed”.

At higher velocities, bubbles disappear, and
particles are blown out of the bed. Therefore, some
amounts of particles have to be recirculated to
maintain a stable system – “circulating fluidized
bed”. Fluidization depends largely on the particle
size and the air velocity.
If sand particles in a fluidized state is heated
to the ignition temperatures of coal, and coal is
injected continuously into the bed, the coal will
burn rapidly and bed attains a uniform temperature.
The fluidized bed combustion (FBC) takes place at
about 840OC to 950OC.

Since this temperature is much below the ash fusion
temperature, melting of ash and associated problems are
avoided.
The lower combustion temperature is achieved because
of high coefficient of heat transfer due to rapid mixing in the
fluidized bed and effective extraction of heat from the bed
through in-bed heat transfer tubes and walls of the bed. The gas
velocity is maintained between minimum fluidisation velocity
and particle entrainment velocity. This ensures stable operation
of the bed and avoids particle entrainment in the gas stream.
Combustion process requires the three “T”s that is Time,
Temperature and Turbulence. In FBC, turbulence is promoted
by fluidization. Improved mixing generates evenly distributed
heat at lower temperature. Residence time is many times
greater than conventional grate firing. Thus an FBC system
releases heat more efficiently at lower temperatures.

Fixing, bubbling and
fast fluidized beds
As the velocity of a
gas flowing through a
bed of particles
increases, a value is
reaches when the bed
fluidizes and bubbles
form as in a boiling
liquid. At higher
velocities the bubbles
disappear; and the
solids are rapidly
blown out of the bed
and must be recycled
to maintain a stable
system.principle of fluidization

Types of Fluidised Bed Combustion Boilers
There are two basic types of fluidised bed combustion boilers:
1. Pressurised Fluidised Bed Combustion System (PFBC).
2. Circulating (fast) Fluidised Bed Combustion system(CFBC)

Bubbling Bed Boilers
In the bubbling bed type boiler, a layer of solid
particles (mostly limestone, sand, ash and calcium
sulfate) is contained on a grid near the bottom of the boiler.
This layer is maintained in a turbulent state as low
velocity air is forced into the bed from a plenum chamber
beneath the grid.
Fuel is added to this bed and combustion takes place.
Normally, raw fuel in the bed does not exceed 2% of the
total bed inventory.
Velocity of the combustion air is kept at a minimum,
yet high enough to maintain turbulence in the bed. Velocity
is not high enough to carry significant quantities of solid
particles out of the furnace.

This turbulent mixing of air and fuel results in a residence time
of up to five seconds. The combination of turbulent mixing and
residence time permits bubbling bed boilers to operate at a furnace
temperature below 1650°F. At this temperature, the presence of
limestone mixed with fuel in the furnace achieves greater than 90%
sulfur removal.
Boiler efficiency is the percentage of total energy in the fuel
that is used to produce steam. Combustion efficiency is the percentage
of complete combustion of carbon in the fuel.
Incomplete combustion results in the formation of carbon
monoxide (CO) plus unburned carbon in the solid particles leaving the
furnace. In a typical bubbling bed fluidized boiler, combustion
efficiency can be as high as 92%. This is a good figure, but is lower
than that achieved by pulverized coal or cyclone-fired boilers. In
addition, some fuels that are very low in volatile matter cannot be
completely burned within the available residence time in bubbling
bed-type boilers.

Features of Bubbling bed boiler
•Fluidized bed boiler can operate at near atmospheric or
elevated pressure and have these essential features:
• Distribution plate through which air is blown for
fluidizing.
• Immersed steam-raising or water heating tubes which
extract heat directly from the bed.
• Tubes above the bed which extract heat from hot
combustion gas before it enters the flue duct.

1. Pressurized Fluidized Bed Combustion System (PFBC).
Pressurized Fluidized Bed Combustion (PFBC) is a variation of
fluid bed technology that is meant for large-scale coal burning
applications. In PFBC, the bed vessel is operated at pressure up to 16
atm ( 16 kg/cm2).
The off-gas from the fluidized bed combustor drives the gas
turbine. The steam turbine is driven by steam raised in tubes immersed
in the fluidized bed. The condensate from the steam turbine is pre-
heated using waste heat from gas turbine exhaust and is then taken as
feed water for steam generation.
The PFBC system can be used for cogeneration or combined
cycle power generation. By combining the gas and steam turbines in this
way, electricity is generated more efficiently than in conventional
system. The overall conversion efficiency is higher by 5% to 8%.
At elevated pressure, the potential reduction in boiler size is
considerable due to increased amount of combustion in pressurized
mode and high heat flux through in-bed tubes.

2. Circulating (fast) Fluidized Bed Combustion
system(CFBC)
The need to improve combustion efficiency (which also
increases overall boiler efficiency and reduces operating costs)
and the desire to burn a much wider range of fuels has led to the
development and application of the CFB boiler.
This CFBC technology utilizes the fluidized bed principle
in which crushed (6 –12 mm size) fuel and limestone are injected
into the furnace or combustor. The particles are suspended in a
stream of upwardly flowing air (60-70% of the total air), which
enters the bottom of the furnace through air distribution nozzles.
The fluidizing velocity in circulating beds ranges from 3.7
to 9 m/sec. The balance of combustion air is admitted above the
bottom of the furnace as secondary air.

The combustion takes place at 840-900oC, and the fine
particles (<450 microns) are elutriated out of the furnace with
flue gas velocity of 4-6 m/s. The particles are then collected by
the solids separators and circulated back into the furnace. Solid
recycle is about 50 to 100 kg per kg of fuel burnt.
There are no steam generation tubes immersed in the bed.
The circulating bed is designed to move a lot more solids out of
the furnace area and to achieve most of the heat transfer outside
the combustion zone - convection section, water walls, and at the
exit of the riser. Some circulating bed units even have external
heat exchanges.
The particles circulation provides efficient heat transfer to
the furnace walls and longer residence time for carbon and
limestone utilization. Similar to Pulverized Coal (PC) firing, the
controlling parameters in the CFB combustion process are
temperature, residence time and turbulence.

For large units, the taller furnace characteristics of CFBC boiler
offers better space utilization, greater fuel particle and sorbent residence
time for efficient combustion and SO2 capture, and easier application of
staged combustion techniques for NOx control than AFBC generators.
CFBC boilers are said to achieve better calcium to sulphur utilization – 1.5
to 1 vs. 3.2 to 1 for the AFBC boilers, although the furnace temperatures
are almost the same.
CFBC boilers are generally claimed to be more economical than
AFBC boilers for industrial application requiring more than 75 – 100 T/hr
of steam
CFBC requires huge mechanical cyclones to capture and recycle
the large amount of bed material, which requires a tall boiler.
A CFBC could be good choice if the following conditions are met.
1. Capacity of boiler is large to medium
2.Sulphur emission and NOx control is important
3.The boiler is required to fire low-grade fuel or fuel with highly
fluctuating fuel quality.

Circulating bed boiler (At a Glance)-
At high fluidizing gas velocities in which a fast
recycling bed of fine material is superimposed on a
bubbling bed of larger particles.
The combustion temperature is controlled by rate of
recycling of fine material.
Hot fine material is separated from the flue gas by a
cyclone and is partially cooled in a separate low velocity
fluidized bed heat exchanger, where the heat is given up to
the steam.
The cooler fine material is then recycled to the
dense bed.

Advantages of Fluidized Bed Combustion Boilers
1. High Efficiency
FBC boilers can burn fuel with a combustion efficiency of over 95%
irrespective of ash content. FBC boilers can operate with overall efficiency
of 84% (plus or minus 2%).
2. Reduction in Boiler Size
High heat transfer rate over a small heat transfer area immersed in the bed
result in overall size reduction of the boiler.
3. Fuel Flexibility
FBC boilers can be operated efficiently with a variety of fuels. Even fuels
like flotation slimes, washer rejects, agro waste can be burnt efficiently.
These can be fed either independently or in combination with coal into the
same furnace.
4. Ability to Burn Low Grade Fuel
FBC boilers would give the rated output even with inferior quality fuel. The
boilers can fire coals with ash content as high as 62% and having calorific
value as low as 2,500 kcal/kg. Even carbon content of only 1% by weight
can sustain the fluidised bed combustion.

5. Ability to Burn Fines
Coal containing fines below 6 mm can be burnt efficiently in FBC boiler,
which is very difficult to achieve in conventional firing system.
6. Pollution Control
SO2 formation can be greatly minimised by addition of limestone or dolomite
for high sulphur coals. 3% limestone is required for every 1% sulphur in the
coal feed. Low combustion temperature eliminates NOx formation.
7. Low Corrosion and Erosion
The corrosion and erosion effects are less due to lower combustion
temperature, softness of ash and low particle velocity (of the order of 1
m/sec).
8. Easier Ash Removal – No Clinker Formation
Since the temperature of the furnace is in the range of 750 – 900o C in FBC
boilers, even coal of low ash fusion temperature can be burnt without clinker
formation. Ash removal is easier as the ash flows like liquid from the
combustion chamber. Hence less manpower is required for ash handling.

9. Less Excess Air –
Higher CO2 in Flue Gas The CO2 in the flue gases will be of the order of 14 –
15% at full load. Hence, the FBC boiler can operate at low excess air - only 20
– 25%.
10. Simple Operation, Quick Start-Up
High turbulence of the bed facilitates quick start up and shut down. Full
automation of start up and operation using reliable equipment is possible.
11. Fast Response to Load Fluctuations
Inherent high thermal storage characteristics can easily absorb fluctuation in
fuel feed rates. Response to changing load is comparable to that of oil fired
boilers.
12. No Slagging in the Furnace-No Soot Blowing
In FBC boilers, volatilisation of alkali components in ash does not take place
and the ash is non sticky. This means that there is no slagging or soot blowing.
13 Provisions of Automatic Coal and Ash Handling System
Automatic systems for coal and ash handling can be incorporated, making the
plant easy to operate comparable to oil or gas fired installation.

14 Provision of Automatic Ignition System
Control systems using micro-processors and automatic ignition
equipment give excellent control with minimum manual supervision.
15 High Reliability
The absence of moving parts in the combustion zone results in a high
degree of reliability and low maintenance costs.
16 Reduced Maintenance
Routine overhauls are infrequent and high efficiency is maintained for
long periods.
17 Quick Responses to Changing Demand
A fluidized bed combustor can respond to changing heat demands more
easily than stoker fired systems. This makes it very suitable for
applications such as thermal fluid heaters, which require rapid responses.
18 High Efficiency of Power Generation
By operating the fluidized bed at elevated pressure, it can be used to
generate hot pressurized gases to power a gas turbine. This can be
combined with a conventional steam turbine to improve the
efficiency of electricity generation and give a potential fuel savings
of at least 4%.

General Arrangements of FBC Boiler
FBC boilers comprise of following systems:
i) Fuel feeding system
ii) Air Distributor
iii) Bed & In-bed heat transfer surface
iv) Ash handling system
Many of these are common to all types of FBC boilers
1. Fuel Feeding system
For feeding fuel, sorbents like limestone or dolomite, usually two
methods are followed: under bed pneumatic feeding and over-bed
feeding.
Under Bed Pneumatic Feeding
If the fuel is coal, it is crushed to 1-6 mm size and pneumatically
transported from feed hopper to the combustor through a feed pipe
piercing the distributor. Based on the capacity of the boiler, the number
of feed points is increased, as it is necessary to distribute the fuel into the
bed uniformly.

Over-Bed Feeding
The crushed coal, 6-10 mm size is conveyed from coal bunker to a
spreader by a screw conveyor. The spreader distributes the coal over the surface
of the bed uniformly. This type of fuel feeding system accepts over size fuel also
and eliminates transport lines, when compared to under-bed feeding system.
2. Air Distributor
The purpose of the distributor is to introduce the fluidizing air evenly
through the bed cross section thereby keeping the solid particles in constant
motion, and preventing the formation of defluidization zones within the bed.
The distributor, which forms the furnace floor, is normally constructed
from metal plate with a number of perforations in a definite geometric pattern.
The perforations may be located in simple nozzles or nozzles with bubble caps,
which serve to prevent solid particles from flowing back into the space below
the distributor.
The distributor plate is protected from high temperature of the furnace by:
i) Refractory Lining
ii) A Static Layer of the Bed Material or
iii) Water Cooled Tubes.

3. Bed & In-Bed Heat Transfer Surface:
a) Bed
The bed material can be sand, ash, crushed refractory or limestone,
with an average size of about 1 mm. Depending on the bed height these are of
two types: shallow bed and deep bed.
At the same fluidizing velocity, the two ends fluidise differently, thus
affecting the heat transfer to an immersed heat transfer surfaces. A shallow bed
offers a lower bed resistance and hence a lower pressure drop and lower fan
power consumption. In the case of deep bed, the pressure drop is more and this
increases the effective gas velocity and also the fan power.
b) In-Bed Heat Transfer Surface
In a fluidized in-bed heat transfer process, it is necessary to transfer
heat between the bed material and an immersed surface, which could be that of
a tube bundle, or a coil. The heat exchanger orientation can be horizontal,
vertical or inclined. From a pressure drop point of view, a horizontal bundle in a
shallow bed is more attractive than a vertical bundle in a deep bed. Also, the
heat transfer in the bed depends on number of parameters like (i) bed pressure
(ii) bed temperature (iii) superficial gas velocity (iv) particle size (v) Heat
exchanger design and (vi) gas distributor plate design.

Ppe unit 1

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Ppe unit 1