THERMAL POWER ENGINEERING

FLUIDIZED BED COMBUSTION
FLUIDIZATION is a method of mixing fuel and air in a specific
proportion, for obtaining combustion.
A FLUIDIZED BED may be defined as the bed of solid particles behaving
as a fluid.
It operates on the principal that when an
evenly distributed air is passed upward
through a finely divided bed of solid
particles at low velocity, the particles
remain undisturbed, but if the velocity of air
flow is steadily increased, a stage is reached
when the individual particles are
suspended in the air stream.

If the air velocity is further increased, the bed becomes highly turbulent
and rapid mixing of particles occur which appear like formation of
bubbles in a boiling liquid and the process of combustion as a result is
known as FLUIDIZED BED COMBUSTION.
The velocity of air, causing fluidization depends on a number of
parameters-
1. Size of fuel particles.
2. Density of air fuel mixture.
Hence, these parameters are given due consideration, while
manipulating with air flow velocity for desired rate of combustion.
A fluidized furnace has an enclosed space with a base having openings
to admit air.

Crushed coal, ash and crushed dolomite or limestone is mixed in the
bed furnace and high velocity combustion air is then passed through
the bed, entering from the furnace bottom.
With the steady increase in the velocity of air, a stage will be reached
when the pressure drop across the bed becomes equal to the weight
per unit cross-section of the bed, and this particular critical velocity is
called the minimum fluidizing velocity.
With further increase in velocity of air, the bed will begin to expand and
allow passage of additional air, in the form of bubbles. When the air
velocity becomes 3 to 5 times the critical velocity, the bed resembles to
that of a violently boiling liquid.
A pictorial representation of fluidized bed combustion is given in the
figure below:
The evaporator tubes of boiler are directly immersed in the fluidized
bed and the tubes, being in direct contact with the burning coal
particles, produce very high heat transfer rates. Because of this, the unit

size is reduced to a great extent, and also produces combustion with
very high efficiency.
TYPES
Fluidized Bed combustion can be in 2
variants
1. VERTICAL TYPE FBC
These are generally used in smaller plant, and has the capacity to
produce steam of up to 6 tons per hour only.
Their vertical shape reduces the overall dimension of the steam boiler,
and is extremely efficient in plants, where space provision is limited.
2. HORIZONTAL TYPE FBC
There are almost 10 times in
capacity when compared to
vertical type fluidized bed
combustion.
They can produce as much as 60
tons of steam per hour, and are
placed horizontally with respect to
the boiler tubes.
The high capacity of the horizontal type Fluidized boilers coupled with
their high efficiency, makes them an extremely desirable choice for the
coal fired thermal power generating station.

ADVANTAGES
FBC is being used exhaustively these days in all major power stations all
over the globe, owing to numerous advantages that it offers over the
other pre-dominant methods of combustion.
Few of those are:
1. High thermal efficiency.
2. Easy ash removal system, to be transferred for made cement.
3. Short commissioning and erection period.
4. Fully automated and thus ensures safe operation, even at extreme
temperatures.
5. Efficient operation at temperatures down to 150° C (i.e. well below the
ash fusion temperature).
6. Reduced coal crushing etc. (pulverized coal is not a necessity here).
7. The system can respond rapidly to changes in load demand, due to
quick establishment of thermal equilibrium between air and fuel
particles in the bed.
8. The operation of fluidized bed furnace at lower temperature helps in
reducing air pollution. The low temperature operation also reduces
the formation of nitrogen oxides. By adding either dolomite (a
calcium-magnesium carbonate) or lime stone (calcium carbonate) to
the furnace the discharge of Sulphur oxides to the atmosphere can
also be reduced if desired.

PULVERIZED COAL FIRING SYSTEM
[ADVANTAGES AND DISADVANTAGES]
In the pulverized fuel firing system, the coal is reduced to a fine powder
with the help of grinding mills and projected into combustion chamber
with the help of hot air current. The amount of air required to complete
the combustion is supplied separately to the combustion chamber. The
resulting turbulence in the combustion chamber helps in the uniform
mixing of fuel (coal) and air and through combustion. The amount of air
which is used to carry the coal and to dry it before entering into
combustion chamber is known as “primary air” and the amount of air
which is supplied separately for complete the combustion is known as
“secondary air”. The amount of primary air may vary from 10% to almost
the entire combustion air requirement as per the type of pulverize and
load on it.

The efficiency of the pulverized fuel firing system mostly depends on
the size of the powder. The fineness of the coal should be such as 70%
of it should pass through 200 mesh sieve and 98% through 50 mesh
sieve..
ADVANTAGES
 The main advantage of pulverized firing system lies in the fact that
by breaking a given mass of coal into smaller pieces exposes more
surface area for combustion.
 Greater surface area of coal per unit mass of the coal allows faster
combustion as more coal surface is exposed to heat and oxygen.
This reduces the excess air required to ensure complete
combustion and the required fan power also
 Wide variety and low grade coal can be burnt more easily when
the coal is pulverized
 Pulverized coal gives faster response to load changes as the rate
of combustion can be controlled easily and immediately.
Automatic control applied to pulverized coal fired boilers is
effective in maintaining an almost constant steam pressure under
wide load variations
 This system is free from clinker and slagging troubles
 This system works successfully with or in combination with the gas
and oil
 It is possible to use highly pre-heated secondary air (350oC) which
helps in rapid flame propagation
 The pulverized system can be repaired easily without cooling the
system as the pulverizing equipment is located outside the furnace
 Large amount of heat release is possible in this system compared
to stoke firing system
 The banking losses are low compared to stoke firing system
 The boiler can be started from cold very rapidly and efficiently.
This is highly important when grid stability is of the important
concern
 The external heating surface is free from corrosion and fouling as
smokeless combustion is possible

 There are no moving parts in the furnace or boiler subjected to
high temperature. Therefore, the life of the pulverized fuel firing
system is more and operation is trouble-less
 Practically no ash handling problem in this type of firing system
 The furnace volume required is considerably less as the use of the
burners which produce turbulence in the furnace makes it possible
to complete combustion with minimum travel.
DISADVANTAGES
 The capital cost of the pulverized coal firing system is considerably
high as it requires many additional auxiliary equipment. Its
operation cost is also high compared to stoke firing system
 The system produces fly ash (fine dust) which requires special and
costly fly-ash removal equipment’s as electrostatic precipitators
 The flame temperatures are high and the conventional types of
refractory lined furnaces are not inadequate. It is always necessary
to provide water cooled walls for the safety of the furnace. The
maintenance cost is also high as working temperature is high
which causes rapid deterioration of the refractory surface of the
furnace
 The possibility of the explosion is more as coal burn like gas
 The storage of powdered coal requires special attention and high
protection from the fire hazards

COMBUSTION EQUIPMENT FOR BURNING COAL
PULVERISED COAL BURNERS
Burners are devices that allow uniform mixing of fuel with air hence lead to
efficient and complete combustion. The burner receives the fuel along with
the primary air in a central passage, while the secondary air is supplied
around the passage. A good design of the burner is essential to achieve
complete combustion of the fuel. Thus, a good burner should meet a number
of design requirements.

LONG-FLAME OR U - FLAME OR STREAM LINED BURNER
The arrangement of a long flame, U-shaped burner is schematically shown
Fig. The burner is placed such that it produces a long, u-shaped flame. The
burner injects a mixture of primary air and fuel vertically downwards in thin
streams practically with no turbulence and produces a long flame.
Secondary hot air is sup plied at right angles to the flam e which provides
necessary turbulence e and mixing for proper and rapid combustion. A
tertiary air is supplied around the burner for better mixing of the fuel with
air. In this burner, due to long flame travel, high volatile coals can be burnt
easily. Velocity of the air fuel mixture at the burner tip is around of 25 m/sec.
SHORT FLAME OR TURBULENT BURNER
The schematic arrangement of a short-flame or turbulent burner is
illustrated in fig. These burners are generally built into the furnace walls, so
that the flame is projected horizontally into the furnace. Primary air and the
fuel mixture is combined with secondary air at the burner periphery, before
the entry into the furnace as shown in figure. This burner gives out a
turbulent mixture which burns rapidly and combustion is completed within a
short distance. Therefore, the combustion rate is high. The velocity of mixture
at the burner tip is about 50 m/sec. In such burners, the bituminous coal can
be burnt easily. Modern high capacity power plants use such burners.
TANGENTIAL BURNERS

These burners are built into the furnace walls at the corners. They inject the
air-fuel mixture tangentially to an imaginary circle in the center of furnace.
As the flames intercept, it leads to a swirling action. This produces sufficient
turbulence in the furnace for complete combustion. Hence in such burners,
there is no need to produce high turbulence within the burners. Tangential
burners give fast and high heat release rates.
CYCLONE BURNER
This burner burns the coal particles in suspension, thus avoiding fly-ash
problems, which is common in other types of burners. This burner uses
crushed coal (about 5 to 6 mm size) instead of pulverized coal. This burner
can easily burn low grade coal with high ash and moisture content. Also, this
can burn biofuels such as rice husk. The cyclone burner consists of a
horizontal cylinder of about 3 m diameter and about 4 m length. The cylinder
wall is water cooled, while the inside surface is lined, with chrome ore. The
horizontal axis of the burner is slightly inclined towards the boiler. The coal
used in cyclone burner is crushed to about 6 mm size. Coal and primary air
(about 25% of the combustion or secondary air) are admitted tangentially
into the cylinder so as to produce a strong centrifugal motion and turbulence
to the coal particles. The primary air and fuel mixture flows centrifugally
along the cylinder walls towards the furnace. From the top of the burner, the
secondary air is also admitted tangentially, at a high velocity (about 100 m/s).
The high velocity secondary air causes further increase in the centrifugal
motion, leading to a highly turbulent whirling motion of the coal air mixture.
Tertiary air (about 5 to 10% of the secondary air) is admitted, axially at the
center as shown in fig, so as to move the turbulent coal-air mixture towards
the furnace.

PRINCIPLES
NATURAL CIRCULATION
Boilers are designed with Economizer,
Evaporator and superheated
depending on the Design parameters.
HEAT INPUT STEAM DRUM
DOWNCOMER TO FURNACE TUBES
FIG 1. FLOW DUE TO DENSITY
DIFFERENCE Economizers add sensible
heat to water. The economizer water
outlet temperature will be closer to
saturation temperature. The water is
forced through the economizer by the
boiler feed pumps. Super heaters add heat to steam. That is the heat is added
to steam leaving the Boiler steam drum / Boiler shell. The steam passes
through the superheated tubes by virtue of the boiler operating pressure.
Evaporators may be multi tubular shell, Water wall tubes, Boiler bank tubes
or Bed coils as in FBC boiler. In evaporators, the latent heat is added. The
addition of heat is done at boiling temperature. The Flow of water through
the evaporator is not by the pump but by the fact called thermos siphon. The
density of the water, saturated or subcooled is higher as compared the water
steam mixture in the heated evaporator tubes. The circulation is absent once
the boiler firing is stopped.

FORCED CIRCULATION
A forced circulation boiler is a
boiler where a pump is used to
circulate water inside the boiler.
This differs from a natural
circulation boiler which relies on
current density to circulate water
inside the boiler. In some forced
circulation boilers, the water is
circulated twenty times the rate of
evaporation. In water tube boilers,
the way the water is recirculated
inside the boiler before becoming
steam can be described as either natural circulation or forced circulation. The
forced circulation boiler begins the same as a natural circulation boiler, at
the feed water pump. The secondary pump takes the feed water going to
the boiler and raises the pressure of the water going in. In a natural
circulation boilers, the circulation of water is dependent on the differential
pressures caused by the change of density in the water as it is heated. That
is to say that as the water is heated and starts turning to steam, the density
decreases sending the hottest water and steam to the top of the furnace
tubes. In contrast to the natural circulation boiler, the forced circulation
boiler uses a water circulation pump to force that flow instead of waiting for
the differential to form. Because of this, the generation tubes of a forced
circulation boiler are able to be oriented in whatever way is required by space
constraints.

NATURAL DRAFT
Natural draft uses the stack or chimney affect. Flue gases are hotter and less
dense than surrounding air around the stack opening. As flue gases rise
through the stack, a natural convection current is formed creating a pressure
gradient through the stack, ducting and furnace. This causes combustion
gases to be sucked into the stack and combustion air to be drawn into the
furnace. Natural draft boilers are not as efficient as mechanized draft boilers.
Natural draft furnaces operate below atmospheric pressure.

FORCED DRAFT
A forced draft furnace uses a fan or blower to force combustion air through
the system. Control is accomplished by regulating the fan speed or damper
operation. This type of furnace is operated slightly above atmospheric
pressure. Forced draft furnace must be airtight to prevent leakage of flue
gases into surrounding area.

INDUCED DRAFT
An induced draft fan draws the gases through the flue ducting and the
combustion air into the furnace making high stacks unnecessary. Control is
accomplished by regulating the fan speed or damper operation. An induced
draft furnace is operated slightly below atmospheric pressure. Induced draft
is used with solid fuel burning or stokers, because the furnace is not airtight.

BALANCED DRAFT
Furnaces equipped with both an FD (Forced Draft) and ID (Induced Draft)
fans are called balanced draft systems; see Figure 1. In balanced draft
systems, the forced and induced draft fans work together to move
combustion air and flue gases through the furnace. The FD fan is used to
regulate the combustion airflow and the ID fan is used to regulate furnace
pressure. Balanced draft furnaces are typically operated slightly below
atmospheric pressure.

INTRODUCTION AND CLASSIFICATION OF STEAM TURBINES
INTRODUCTION
De Laval, Parsons and Curtis developed the concept for the steam turbine in
the 1880s. Modern steam turbines use essentially the same concept but
many detailed improvements have been made in the intervening years
mainly to improve turbine efficiency.
Steam turbines are used in all of our major coal fired power stations to drive
the generators or alternators, which produce electricity. The turbines
themselves are driven by steam generated in ‘Boilers ‘or ‘Steam Generators
‘as they are sometimes called.
Energy in the steam after it leaves the boiler is converted into rotational
energy as it passes through the turbine. The turbine normally consists of
several stages with each stage consisting of a stationary blade (or nozzle)
and a rotating blade. Stationary blades convert the potential energy of the
steam (temperature and pressure) into kinetic energy (velocity) and direct
the flow onto the rotating blades.

CLASSIFICATION
EXTRACTION STEAM TURBINE
The extraction turbine contains two
outlets as shown in figure 1. The first
outlet extracts the steam with
intermediate pressure for the feeding of
the heating process while the second
outlet extracts the remaining steam with
low-pressure steam for the condensation.
The extraction of heat from the first outlet
can be stopped to generate more output.
Steam control valves at this outlet make
this steam very flexible and allow
adjusting the output as per demand. The
steam from the second outlet goes to the condensation chamber where
cooling water brings the temperature of the steam down. The condensed
water then goes back to the boiler for the regeneration of the electricity of
power, therefore, it is also known as the regenerative steam turbine. The
scheme of extraction turbine with cogeneration system is shown in figure 1.
This turbine has following benefits and disadvantages.
Advantages:
 This type of steam turbine can be used to generate a high amount of
electricity.
 It is a flexible turbine with the ability to regulate output as per changing
need.
Disadvantages:
 It is costly turbine with lots of auxiliary components
 Heat rejection in the condensation process reduces the overall
efficiency of the system.
 It is usually used on industrial level and requires complex configuration

BACK PROCESS STEAM TURBINE
The non-condensing steam turbine uses
high-pressure steam for the rotation of
blades. This steam then leaves the turbine at
the atmospheric pressure or lower pressure.
The pressure of outlet steam depends on in
the load, therefore, this turbine is also known
as the back-pressure steam turbine. This low-
pressure steam uses for processing and no
steam is used for condensation. The
schematic diagram of the back-process
steam turbine with cogeneration system is
shown in figure 2. There are lots of benefits of this steam turbine but at the
same time it has few disadvantages which are listed below.
Advantages:
 The configuration of this steam turbine is very simple
 It is relatively inexpensive as compared to extraction steam turbine
 It requires very less or no cooling water
 Its efficiency is higher as it does not reject heat in the condensation
process
Disadvantages:
 The biggest disadvantage of this type of steam turbine is that it is
highly inflexible. The output of this turbine can’t be regulated as it does
not allow changing the pressure and temperature of steam in the
turbine, therefore, it works best with the constant load.
 The thermal load of this turbine defines the flow of steam mass which
makes it difficult to change the output value. Other methods to
regulate output reduce the efficiency of the overall system.

THERMAL POWER ENGINEERING

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