Power plant engineering unit 2 notes by Varun Pratap Singh

Power Plant Engineering
Unit-2
NOTES
by
Varun Pratap Singh
Assistant Professor
Mechanical Engineering Department
College of Engineering Roorkee

Disclaimer
This document does not claim any originality and cannot be used as a substitute for prescribed
textbooks. The information presented here is merely a collection by the subject faculty members
for their respective teaching assignments. Various sources as mentioned at the end of the
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SYLLABUS
Unit-II
Steam power plant
Power plant boilers including critical and super critical boilers. Fluidized bed boilers, boilers
mountings and accessories.
General layout of steam power plant. Different systems such as fuel handling system,
pulverizes and coal burners, combustion system, draft, ash handling system, feed water
treatment and condenser and cooling system, turbine auxiliary systems such as governing, feed
heating, reheating, flange heating and gland leakage.
Operation and maintenance of steam power plant, heat balance and efficiency.

Steam Power Plant
INTRODUCTION
Steam is an important medium of producing mechanical energy. Steam has the advantage that,
it can be raised from water which is available in abundance it does not react much with the
materials of the equipment of power plant and is stable at the temperature required in the plant.
Steam is used to drive steam engines, steam turbines etc. Steam power station is most suitable
where coal is available in abundance. Thermal electrical power generation is one of the major
method. Out of total power developed in India about 60% is thermal. For a thermal power plant,
the range of pressure may vary from 10 kg/cm2 to super critical pressures and the range of
temperature may be from 250°C to 650°C.
The average all India Plant load factor (P.L.F.) of thermal power plants in 1987-88 has been
worked out to be 56.4% which is the highest P.L.F. recorded by thermal sector so far.
ESSENTIALS OF STEAM POWER PLANT EQUIPMENT
A steam power plant must have following equipment’s:
Figure: General layout of Steam Power Plant
1. A furnace to burn the fuel.
2. Steam generator or boiler containing water. Heat generated in the furnace is utilized to convert
water in steam.
3. Main power unit such as an engine or turbine to use the heat energy of steam and perform work.
4. Piping system to convey steam and water.

In addition to the above equipment the plant requires various auxiliaries and accessories depending
upon the availability of water, fuel and the service for which the plant is intended.
The flow sheet of a thermal power plant consists of the following four main circuits:
(i) Feed water and steam flow circuit
(ii) Coal and ash circuit
(iii) Air and gas circuit
(iv) Cooling water circuit.
A steam power plant using steam as working substance works basically on Rankine cycle.
Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser and
fed into the boiler again.
The different types of systems and components used in steam power plant are as follows:
(i) High pressure boiler
(ii) Prime mover
(iii) Condensers and cooling towers
(iv) Coal handling system
(v) Ash and dust handling system
(vi) Draught system
(vii) Feed water purification plant
(viii) Pumping system
(ix) Air preheater, economizer, super heater, feed heaters.
Above mentioned Figure shows a schematic arrangement of equipment of a steam power station. Coal
received in coal storage yard of power station is transferred in the furnace by coal handling unit. Heat
produced due to burning of coal is utilized in converting water contained in boiler drum into steam at
suitable pressure and temperature. The steam generated is passed through the superheated. Superheated
steam then flows through the turbine. After doing work in the turbine die pressure of steam is reduced.
Steam leaving the turbine passes through the condenser which maintain the low pressure of steam at the
exhaust of turbine.
Steam pressure in the condenser depends upon flow rate and temperature of cooling water and on
effectiveness of air removal equipment. Water circulating through the condenser may be taken from the
various sources such as river, lake or sea. If sufficient quantity of water is not available, the hot water
coming out of the condenser may be cooled in cooling towers and circulated again through the
condenser. Bled steam taken from the turbine at suitable extraction points is sent to low pressure and
high pressure water heaters.
Air taken from the atmosphere is first passed through the air pre-heater, where it is heated by flue gases.
The hot air then passes through the furnace. The flue gases after passing over boiler and superheated
tubes, flow through the dust collector and then through economiser, air pre-heater and finally they are
exhausted to the atmosphere through the chimney.
Steam condensing system consists of the following:
(i) Condenser
(ii) Cooling water
(iii) Cooling tower
(iv) Hot well

(v) Condenser cooling water pump
(vi) Condensate air extraction pump
(vii) Air extraction pump
(viii) Boiler feed pump
(ix) Make up water pump.
POWER STATION DESIGN PARAMETERS
Power station design requires wide experience. A satisfactory design consists of the
following steps :
(i) Selection of site
(ii) Estimation of capacity of power station.
(iii) Selection of turbines and their auxiliaries.
(iv) Selection of boilers, and their auxiliaries.
(v) Design of fuel handling system.
(vi) Selection of condensers.
(vii) Design of cooling system.
(viii) Design of piping system to carry steam and water.
(ix) Selection of electrical generator.
(x) Design and control of instruments.
(xi) Design of layout of power station. Quality of coal used in steam power station plays an
important role in the design of power plant. The various factors to be considered while
designing the boilers and coal handling units are as follows :
(a) Slagging and erosion properties of ash.
(b) Moisture in the coal. Excessive moisture creates additional problems particularly in case
of pulverized fuel power plants.
(c) Burning characteristic of coal.
(d) Corrosive nature of ash.
CHARACTERISTICS OF STEAM POWER PLANT
The desirable characteristic for a steam power plant are as follows :
(i) Higher efficiency.
(ii) Lower cost.
(iii) Ability to burn coal especially of high ash content, and inferior coals.
(iv) Reduced environmental impact in terms of air pollution.
(v) Reduced water requirement.
(vi) Higher reliability and availability.

COAL HANDLING
Coal delivery equipment is one of the major components of plant cost. The various steps
involved in coal handling are as follows:
(i) Coal delivery
(ii) Unloading
(iii) Preparation
(iv) Transfer
(v) Outdoor storage
(vi) Covered storage
(vii) In plant handling
(viii) Weighing and measuring
(ix) Feeding the coal into furnace.
(i) Coal Delivery. The coal from supply points is delivered by ships
or boats to power stations situated near to sea or river whereas coal
is supplied by rail or trucks to the power stations which are
situated away from sea or river. The transportation of coal by trucks
is used if the railway facilities are not available.
(ii) Unloading. The type of equipment to be used for unloading the
coal received at the power station depends on how coal is received
at the power station. If coal is delivered by trucks, there is no need
of unloading device as the trucks may dump the coal to the outdoor storage.
(iii) Preparation. When the coal delivered is in the form of big lumps and it is not of proper
size, the preparation (sizing) of coal can be achieved by crushers, breakers, sizers driers and
magnetic separators.
(iv) Transfer. After preparation coal is transferred to the dead storage by means of the
following systems:
1. Belt conveyors. 2. Screw conveyors. 3. Bucket elevators. 4. Grab bucket elevators.
5. Skip hoists. 6. Flight conveyor.
1. Belt conveyor. Belt conveyor is suitable for
the transfer of coal over long distances. It is used in
medium and large power plants. The initial cost of the
system is not high and power consumption is also low.
The inclination at which coal can be successfully
elevated by belt conveyor is about 20.
Average speed of belt conveyors varies between 200-
300 r.p.m. This conveyor is preferred than other
types.
Advantages of belt conveyor
1. Its operation is smooth and clean.
2. It requires less power as compared to other types of systems.
3. Large quantities of coal can be discharged quickly and continuously.
4. Material can be transported on moderate’s inclines.

2. Screw conveyor. It consists of an endless
helicoid screw fitted to a shaft, this system is
suitable, where coal is to be transferred over
shorter distance and space limitations exist. The
initial cost of the system is low. It suffers from the
drawbacks that the power consumption is high and
there is considerable wear of screw. Rotation of
screw varies between 75-125 r.p.m.
3. Bucket elevator. It consists of buckets fixed to a chain. The chain
moves over two wheels. The coal is carried by the buckets from
bottom and discharged at the top.
4. Grab bucket elevator. It lifts and transfers coal on a single rail
or track from one point to the other. The coal lifted by grab buckets
is transferred to overhead bunker or storage. This system requires
less power for operation and requires minimum maintenance. The
grab bucket conveyor can be used with crane or tower as shown in Fig.
Although the initial cost of this system is high but operating cost is less.
5. Skip hoist. It consists of a vertical or inclined Hostway a bucket or
a car guided by a frame and a cable for hoisting the bucket. The bucket
is held in upright position. It is simple and compact method of elevating
coal or ash. Fig. 4.7 shows a skip hoist.
6. Flight conveyor. It consists of one or two strands of chain to which steel scraper or flights
are attached’. it scraps the coal through a trough having identical shape This coal is discharged
in the bottom of trough. It is low in first cost but has large energy consumption. There is
considerable wear.
Flight conveyors possess the following advantages.
(i) They can be used to transfer coal as well as ash.
(ii) The speed of conveyor can be regulated easily.
(iii) They have a rugged construction.
(iv) They need little operational care.
Disadvantages. Various disadvantages of flight conveyors are as follows:
(i) There is more wear due to dragging action.
(ii) Power consumption is more.

The coal is stored by the following methods:
(i) Stocking the coal in heats. The coal is piled on the ground up to 10-12 m height. The pile
top should be given a slope in the direction in which the rain may be drained off.
The sealing of stored pile is desirable in order to avoid the oxidation of coal after packing an
air tight layer of coal. Asphalt, fine coal dust and bituminous coating are the materials
commonly used for this purpose.
(ii) Under water storage. The possibility of slow oxidation and
spontaneous combustion can be completely eliminated by storing
the coal under water.
(iii) Coal should be stored at a site located on solid ground, well
drained, free of standing water preferably on high ground not
subjected to flooding.
(vi) In Plant Handling. From the dead storage the coal is brought to
covered storage (Live storage) (bins or bunkers). A cylindrical
bunker shown in Fig. 4.9. In plant handling may include the
equipment such as belt conveyors, screw conveyors, bucket
elevators etc. to transfer the coal. Weigh lorries hoppers and
automatic scales are used to record the quantity of coal delivered to
the furnace.
(vii) Coal weighing methods. Weigh lorries, hoppers and automatic scales are used to weigh
the quantity coal. The commonly used methods to weigh the coal are as follows:
(i) Mechanical (ii) Pneumatic (iii) Electronic.
The Mechanical method works on a suitable lever system mounted on knife edges and bearings
connected to a resistance in the form of a spring of pendulum. The pneumatic weightier use a
pneumatic transmitter weight head and the corresponding air pressure determined by the load
applied. The electronic weighing machines make use of load cells that produce voltage signals
proportional to the load applied.
The important factor considered in selecting fuel handling systems are as follows:
(i) Plant flue rate
(ii) Plant location in respect to fuel shipping
(iii) Storage area available.
DEWATERING OF COAL
Excessive surface moisture of coal reduces and heating value of coal and creates handling
problems. The coal should therefore be dewatered to produce clean coal. Cleaning of coal has
the following advantages:
(i) Improved heating value.
(ii) Easier crushing and pulverising(iii) Improved boiler performance
(iv) Less ash to handle.
(v) Easier handling.
(vi) Reduced transportation cost.

FUEL BURNING FURNACES
Fuel is burnt in a confined space called furnace. The furnace provides supports and enclosure
for burning equipment. Solid fuels such as coal, coke, wood etc. are burnt by means of stokers
whereas burners are used to burn powdered (Pulverized) coal and liquid fuels. Solid fuels
require a grate in the furnace to hold the bed of fuel.
TYPES OF FURNACES
According to the method of firing fuel furnaces are classified into two categories :
(i) Grate fired furnaces (ii) Chamber fired furnaces. Grate fired furnaces. They are used to
burn solid fuels. They may have a stationary or a movable bed of fuel.
These furnaces are classified as under depending upon the method used to fire the fuel and
remove ash and slag.
(i) Hand fired (ii) Semi-mechanized (iii) Stocker fired.
Hand fired and semi-mechanized furnaces are designed with stationary fire grates and stoker
furnaces with traveling grates or stokers.
Chamber fired furnaces. They are used to burn pulverized fuel, liquid and gaseous fuels.
Furnace shape and size depends upon the following factors:
(i) Type of fuel to be burnt.
(ii) Type of firing to be used.
(iii) Amount of heat to be recovered.
(iv) Amount of steam to be produced and its conditions.
(v) Pressure and temperature desired.
(vi) Grate area required.
(vii) Ash fusion temperature.
(viii) Flame length.
(ix) Amount of excess air to be used.
To burn fuels completely, the burning equipment should fulfill the following conditions:
1. The flame temperature in the furnace should be high enough to ignite the incoming fuel and
air. Continuous and reliable ignition of fuel is desirable.
2. For complete combustion the fuel and air should be thoroughly mixed by it.
3. The fuel burning equipment should be capable to regulate the rate of fuel feed.
4. To complete the burning process the fuel should remain in the furnace for sufficient time.
5. The fuel and air supply should be regulated to achieve the optimum air fuel ratios.
6. Coal firing equipment should have means to hold and discharge the ash.
Following factors should be considered while selecting a suitable combustion equipment
for a particular type of fuel:
(i) Grate area required over which the fuel burns.
(ii) Mixing arrangement for air and fuel.
(iii) Amount of primary and secondary air required.
(iv) Arrangement to counter the effects of capping in fuel or of low ash fusion temperature.
(v) Dependability and easier operation.
(vi) Operating and maintenance cost.

METHOD OF FUEL FIRING
The solid fuels are fired into the furnace by the following methods:
1. Hand firing. 2. Mechanical firing
HAND FIRING
This is a simple method of firing coal into the furnace. It requires no capital investment. It is
used for smaller plants. This method of fuel firing is discontinuous process, and there is a limit
to the size of furnace which can be efficiently fired by this method. Adjustments are to be made
every time for the supply of air when fresh coal is fed into furnace.
Fig. Various Types of Hand Fired Grates.
Hand fired furnaces are simple in design and can burn the fuel successfully but they have
some disadvantages also mentioned below:
(i) The efficiency of a hand fired furnace is low.
(ii) Attending to furnace requires hard manual labour.
(iii) Study process of fuel feed is not maintained.
MECHANICAL FIRING (STOKERS)
Mechanical stokers are commonly used to feed solid fuels into the furnace in medium and
large size power plants. The various advantages of stoker firing are as follows :
(i) Large quantities of fuel can be fed into the furnace. Thus greater combustion capacity is
achieved.
(ii) Poorer grades of fuel can be burnt easily.
(iii) Stoker save labour of handling ash and are self-cleaning.
(iv) By using stokers better furnace conditions can be maintained by feeding coal at a uniform
rate.
(v) Stokers save coal and increase the efficiency of coal firing. The main disadvantages of
stokers are their more costs of operation and repairing resulting from high furnace
temperatures.

Principles of Stokers. The working of various types of stokers is based on the
following two principles:
1. Overfeed Principle. According to
this principle the primary air enters
the grate from the bottom. The air
while moving through the grate
openings gets heated up and air while
moving through the grate openings
gets heated up and the grate is
cooled.
The hot air that moves through a
layer of ash and picks up additional energy. The air then passes
through a layer of incandescent coke where oxygen reacts with coke to form-C02 and water
vapours accompanying the air react with incandescent coke to form CO2, CO and free H2. The
gases leaving the surface of fuel bed contain volatile matter of raw fuel and gases like CO2,
CO, H2, N2 and H2O. Then additional air known as secondary air is supplied to burn the
combustible gases. The combustion gases entering the boiler consist of N2, CO2, O2 and H2O
and also CO if the combustion is not complete.
2. Underfeed Principle. Fig. shows
underfeed principle. In underfeed
principle air entering through the holes in
the grate comes in contact with the raw
coal (green coal).
Then it passes through the incandescent
coke where reactions similar to overfeed
system take place. The gases produced then passes through a layer of ash. The secondary air is
supplied to burn the combustible gases. Underfeed principle is suitable for burning the semi-
bituminous and bituminous coals.
Types of Stokers. The various types of stokers are as follows:
Charging of fuel into the furnace is mechanized by means of stokers of various types. They are
installed above the fire doors underneath the bunkers which supply the fuel. The bunkers
receive the fuel from a conveyor.

AUTOMATIC BOILER CONTROL
By means of automatic combustion control it becomes easy to maintain a constant steam
pressure and uniform furnace draught and supply of air or fuel can be regulated to meet the
changes in steam demand. The boiler operation becomes more flexible and better efficiency of
combustion is achieved. This saves manual labour also.
Hagan system of automatic combustion control is shown in Fig. Master relay R1, is sensitive
to small venations in steam pressure and is connected to steam pressure gauge.
A fall in pressure operates the master relay R1 which in turn operates the servomotor coupled
to the vanes of the induced draught (LD) fan to open them slightly and simultaneously the
secondary air fan damper gets opened proportionately. By this readjustment of induced draught
takes place and stabilized conditions in the combustion chamber get changed. These changes
operate relay R2 to alter the position of forced draught fan servo-motor to adjust the position
of forced draught fan vanes so that stable conditions in combustion chamber are maintained.
This change causes more air to flow through passage which in turn operates relay R3. This
causes stoker motor to supply extra fuel into the furnace. In case of an increase of pressure of
steam the above process is reversed. Hand regulators are provided to servo motors and master
relay for manual control of system.

PULVERIZED COAL
Coal is pulverized (powdered) to increase its surface exposure thus permitting rapid
combustion. Efficient use of coal depends greatly on the combustion process employed.
For large scale generation of energy, the efficient method of burning coal is confined still to
pulverized coal combustion. The pulverized coal is obtained by grinding the raw coal in
pulverising mills. The various pulverising mills used are as follows:
(i) Ball mill (ii) Hammer mill
(iii) Ball and race mill (iv) Bowl mill.
The essential functions of pulverising mills are as follows:
(i) Drying of the coal (ii) Grinding (iii) Separation of particles of the desired size.
Proper drying of raw coal which may contain moisture is necessary for effective grinding.
The coal pulverising mills reduce coal to powder form by three actions as follows:
(i) Impact (ii) Attrition (abrasion) (iii) Crushing.
Most of the mills use all the above mentioned all the three actions in varying degrees. In impact
type mills hammers break the coal into smaller pieces whereas in attrition type the coal pieces
which rub against each other or metal surfaces to disintegrate. In crushing type mills coal
caught between metal rolling surfaces gets broken into pieces. The crushing mills use steel
balls in a container. These balls act as crushing elements.

BALL MILL
A line diagram of ball mill using two classifiers is shown in Fig. It consists of a slowly rotating
drum which is partly filled with steel balls. Raw coal from feeders is supplied to the classifiers
from where it moves to the drum by means of a screw conveyor.
BALL AND RACE MILL

PULVERISED COAL FIRING
Pulverised coal firing is done by two systems:
(i) Unit System or Direct System.
(ii) Bin or Central System.
Unit System. In this system (Fig. 4.25) the raw coal from the coal bunker drops on to the
feeder.
Hot air is passed through coal in the feeder to dry the coal. The coal is then transferred to the
pulverising mill where it is pulverised. Primary air is supplied to the mill, by the fan. The
mixture of pulverised coal and primary air then flows to burner where secondary air is added.
The unit system is so called from the fact that each burner or a burner group and pulveriser
constitute a unit.
Advantages
(i) The system is simple and cheaper than the central system.
(ii) There is direct control of combustion from the pulverising mill.
(iii) Coal transportation system is simple.
Bin or Central System. It is shown in Fig. Crushed coal from the raw coal bunker is
fed by gravity to a dryer where hot air is passed through the coal to dry it. The dryer may use
waste flue gases, preheated air or bleeder steam as drying agent. The dry coal is then transferred
to the pulverising mill. The pulverised coal obtained is transferred to the pulverised coal bunker
(bin). The transporting air is separated from the coal in the cyclone separator. The primary air
is mixed with the coal at the feeder and the mixture is supplied to the burner.

Advantages
l. The pulverising mill grinds the coal at a steady rate irrespective of boiler feed.
2. There is always some coal in reserve. Thus any occasional breakdown in the coal supply
will not affect the coal feed to the burner.
3. For a given boiler capacity pulverising mill of small capacity will be required as compared
to unit system.
Disadvantages
1. The initial cost of the system is high.
2. Coal transportation system is quite
complicated.
3. The system requires more space.
To a large extent the performance of
pulverised fuel system depends upon the
mill performance.
The pulverised mill should satisfy the
following requirements:
1. It should deliver the rated tonnage of
coal.
2. Pulverised coal produced by it should
be of satisfactory fineness over a wide
range of capacities.
3. It should be quiet in operation.
4. Its power consumption should be low.
5. Maintenance cost of the mill should be
low
Sidewise mentioned figure shows the
equipment’s for unit and central system of
pulverised coal handling plant.

PULVERISED COAL BURNERS
Burners are used to burn the pulverised coal. The main difference between the various burners
lies in the rapidity of air-coal mixing i.e., turbulence. For bituminous coals the turbulent type
of burner is used whereas for low volatile coals the burners with long flame should be used. A
pulverised coal burner should satisfy the following requirements:
(i) It should mix the coal and primary air thoroughly and should bring this mixture before it
enters the furnace in contact with additional air known as secondary air to create sufficient
turbulence.
(ii) It should deliver and air to the furnace in right proportions and should maintain stable
ignition of coal air mixture and control flame shape and travel in the furnace. The flame shape
is controlled by the secondary air vanes and other control adjustments incorporated into the
burner. Secondary air if supplied in too much quantity may cool the mixture and prevent its
heating to ignition temperature.
(iii) Coal air mixture should move away from the burner at a rate equal to flame front travel in
order to avoid flash back into the burner.

The various types of burners are as follows:
1. Long Flame Burner (U-Flame Burner). In this
burner air and coal mixture travels a considerable
distance thus providing sufficient time for complete
combustion
2. Short Flame Burner (Turbulent Burner). The
burner is fitted in the furnace will and the flame enters
the furnace horizontally.
3. Tangential Burner. In this system one burner is fitted
attach corner of the furnace. The inclination of the burner is
so made that the flame produced are tangential to an
imaginary circle at the centre.
4. Cyclone Burner. This burner uses
crushed coal intend of pulverised coal. Its
advantages are as follows:
(i) It saves the cost of pulverisation because of a crusher needs less power
than a pulveriser.
(ii) Problem of fly ash is reduced. Ash produced is in the molten form and
due to inclination of furnace it flows to an appropriate disposal system.

WATER WALLS
Larger central station type boilers have water cooled furnaces. The combustion space of a
furnace is shielded wholly or partially by small diameter tubes placed side by side. Water from
the boiler is made to circulate through these tubes which connect lower and upper headers of
boiler.
The provision of water walls is advantageous due to following reasons: (1) These walls provide
a protection to the furnace against high temperatures. (2) They avoid the erosion of the
refractory material and insulation. (3) The evaporation capacity of the boiler is increased.
The tubes are attached with the refractory materials on the inside or partially embedded into it.
Above mentioned figure shows the various water walls arrangement.

ASH DISPOSAL
A large quantity of ash is, produced in steam power plants using coal. Ash produced in about
10 to 20% of the total coal burnt in the furnace. Handling of ash is a problem because ash
coming out of the furnace is too hot, it is dusty and irritating to handle and is accompanied by
some poisonous gases.
It is desirable to quench the ash before handling due to following reasons:
1. Quenching reduces the temperature of ash.
2. It reduces the corrosive action of ash.
3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will disintegrate.
4. Quenching reduces the dust accompanying the ash.
Handling of ash includes its removal from the furnace, loading on the conveyors and delivered
to the fill from where it can be disposed off.
General Layout of Ash Handling and Dust Collection System
(i) Hydraulic system
(ii) pneumatic system
(iii) Mechanical system.
The commonly used ash discharge equipment is as follows:
(i) Rail road cars
(ii) Motor truck
(iii) Barge

(i) Hydraulic System. In this system, ash
from the furnace grate falls into a system of
water possessing high velocity and is carried
to the sumps. It is generally used in large
power plants. Hydraulic system is of two
types namely low pressure hydraulic system
used for continuous removal of ash and high
pressure system which is used for
intermittent ash disposal.
(ii) Water Jetting. Water jetting of ash is shown in Fig..
In this method a low pressure jet of water coming out of
the quenching nozzle is used to cool the ash. The ash falls
into a trough and is then removed.
(iii) Ash Sluice Ways and Ash Sump System. This
system shown diagrammatically in Fig.
used high pressure (H.P.) pump to supply high pressure
(H.P.) water-jets which carry ash from the furnace bottom
through ash sluices (channels) constructed in basement floor to ash sump fitted with screen.
The screen divides the ash sump into compartments for coarse and fine ash. The fine ash passes
through the screen and moves into the dust sump (D.S.). Dust slurry pump (D.S. pump) carries
the dust through dust pump (D.P), suction pipe and dust delivery (D.D.) pipe to the disposal
site. Overhead crane having grab bucket is used to remove coarse ash. A.F.N represents ash
feeding nozzle and S.B.N. represents sub way booster nozzle and D.A. means draining apron.
(iv) Pneumatic system. In this system ash from the boiler furnace
outlet falls into a crusher where larger ash particles are crushed to small
sizes. The ash is then carried by a high velocity air or steam to the point
of delivery. Air leaving the ash separator is passed through filter to
remove dust etc. so that the exhauster handles clean air which will
protect the blades of the exhauster.

(v) Mechanical ash handling system. In this
system ash cooled by water seal falls on the belt
conveyor and is carried out continuously to the
bunker. The ash is then removed to the dumping site
from the ash bunker with the help of trucks.

Efficient Combustion of Coal
The factors which affect the efficient combustion of coal are as follows:
1. Type of coal. The important factors which are considered for the selection of coal are as
follows:
(i) Sizing (ii) Caking (iii) Swelling properties (iv) Ash fusion temperature.
The characteristics which control the selection of coal for a particular combustion equipment
are as follows:
(i) Size of coal (ii) Ultimate and proximate analysis (iii) Resistance of degradation
(iv) Grind ability (v) Caking characteristics (vi) Slagging characteristics
(vii) Deterioration during storage (viii) Corrosive characteristics (ix) Ash Content.
The average ash content in Indian coal is about 20%. It is therefore desirable to design the
furnace in such a way as to burn the coal of high ash content. The high ash content in coal
has the following disadvantages:
(i) It reduces thermal efficiency of the boiler as loss of heat through unburnt carbon, excessive
clinker formation and heat in ashes is considerably high.
(ii) There is difficulty of hot ash disposal.
(iii) It increases size of plant.
(iv) It increases transportation cost of fuel per unit of heat produced.
(v) It makes the control difficult due to irregular combustion. High as content fuels can be used
more economically in pulverised form. Pulverised fuel burning increases the thermal efficiency
as high as 90% and controls can be simplified by just adjusting the position of burners in
pulverised fuel boilers. The recent steam power plants in India are generally designed to use
the pulverised coal.
2. Type of Combustion equipment. It includes the following:
(i) Type of furnace
(ii) Method of coal firing such as:
(a) Hand firing
(b) Stoker firing
(c) Pulverised fuel firing.
(iii) Method of air supply to the furnace. It is necessary to provide adequate quantity of
secondary air with sufficient turbulence.
iv) Type of burners used.
(v) Mixing arrangement of fuel and air.
The flames over the bed are due to the burning of volatile gases, lower the volatile content in
the coal, shorter will be the flame. If the volatiles burn up intensely high temperature is
generated over the furnace bed and helps to burn the carbon completely and vice versa.
For complete burning of volatiles and prevent unburnt carbon going with ash adequate quantity
of secondary air with sufficient turbulence should be provided.

SMOKE AND DUST REMOVAL
In coal fed furnaces the products of combustion contain particles of solid matter floating in
suspension. This may be smoke or dust. The production of smoke indicates that combustion
conditions are faulty and amount of smoke produced can be reduced by improving the furnace
design. In spreader stokers and pulverised coal fired furnaces the coal is burnt in suspension
and due to this dust in the form of fly ash is produced.
Dust particles are mainly ash particles called fly ash intermixed with some quantity of carbon
ash material called cinders. Gas borne particles larger than 1µ in diameter are called dust and
when such particles become greater in size than 100p they are called cinders. Smoke is
produced due to the incomplete combustion of fuels; smoke particles are less than 10p in size.
The disposal smoke to the atmosphere is not desirable due to the following reasons:
1. A smoky atmosphere is less healthful than smoke free air.
2. Smoke is produced due to incomplete combustion of coal. This will create a big economic
loss due to loss of heating value of coal.
3. In a smoky atmosphere lower standards of cleanliness are prevalent. Buildings, clothing’s,
furniture etc. becomes dirty due to smoke. Smoke corrodes the metals and darkens the paints.
To avoid smoke nuisance, the coal should be completely burnt in the furnace.
TYPES OF DUST COLLECTORS
The various types of dust collectors are as follows:
1. Mechanical dust collectors.
2. Electrical dust collectors.
Mechanical dust collectors. Mechanical dust collectors are sub-divided into wet and dry
types. In wet type collectors also known as scrubber’s water sprays are used to wash dust from
the air.
The basic principles of mechanical dust collectors are shown in Fig. By increasing the cross-
sectional area of duct through which dust laden gases are passing, the velocity gases are reduced
and causes heavier dust particles to fall down. Changing the direction of flow of flue gases
causes the heavier particles of settle out. Sometime baffles are provided as
shown in Fig. to separate the heavier particles. Mechanical dust collectors may be wet type or
dry type.
Wet type dust collectors called scrubbers make use of water sprays to wash the dust from
flue gases. Dry type dust collectors include gravitational, cyclone, louvered and baffle dust
collectors.

Electrostatic Precipitators (ESP)
An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulate collection device
that removes particles from a flowing gas (such as air) using the force of an induced
electrostatic charge.
It has two sets of electrodes, insulated from each other that maintain an electrostatic field
between them at high voltage. The flue gases are made to pass between these two sets of
electrodes. The electric field ionises the dust particle; that pass through it attracting them to
the electrode of opposite charge. The other electrode is maintained at a negative potential of
30,000 to 60,000 volts. The dust particles are removed from the collecting electrode by rapping
the electrode periodically. The electrostatic precipitator is costly but has low maintenance cost
and is frequently employed with pulverised coal fired power stations for its effectiveness on
very fine ash particles and is superior to that of any other type.
The principal characteristics of an ash collector is the degree of collection.
η = Degree of collection
where
Gl = Quantity of ash entering an ash collector per unit time (kg/s)
G2 = Quantity of uncollected ash passing through the collector per unit time (kg/s)
Cl = Concentration of ash in the gases at the inlet to the ash collector (kg/m3
)
C2 = Ash concentration at the exist (kg/m3
).

 the basic idea of an ESP:
 Charging
 Collecting.
 Removing
 Every particle either has or can be given a charge—positive or negative.
 It imparts a negative charge to all the particles in a gas stream in ESP.
 Then a grounded plate having a positive charge is set up.
 The negatively charged particle would migrate to the grounded collection plate and
be captured.
 The particles would quickly collect on the plate, creating a dust layer. The dust layer
would accumulate until we removed it.
 The structural design and operation of the discharge electrodes (rigid-frame, wires or
plate) and collection electrodes.
 tubular type ESP
 plate type ESP
 The method of charging
 single-stage ESP
 two-stage ESP
 The temperature of operation
 cold-side ESP
 hot-side ESP
 The method of particle removal from collection surfaces
 wet ESP
 Dry ESP
FLY ASH SCRUBBER
Below Mentioned Figure shows a fly wash centrifugal scrubber. It is similar to a mechanical
ash collector but has a flowing water film on its inner walls. Due to this film, the collected ash
is removed more rapidly from the apparatus to the bin and there is less possibility for secondary.
Capture of collected dust particles by the gas flow. The degree of ash collection in scrubbers
varies from 0.82 to 0.90. The dust laden
gas enters through the inlet pipe.
Cinder Catcher. Cinder catcher is used to
remove dust and cinders from the gas. In this
catcher the dust laden gas is made to strike a
series of vertical baffles that change its direction
and reduce its velocity. The separated dust and
cinders fall to the hopper for removal. Cinder
catchers are ordinarily used with stoker firing.

FLUIDISED BED COMBUSTION (FBC)
Burning of pulverised coal has some problems such as particle size of coal used in pulverised
firing is limited to 70-100 microns, the pulverised fuel fired furnaces designed to burn a
particular cannot be used other type of coal with same efficiency, the generation of high temp.
about (1650 C) in the furnace creates number of problems like slag formation on super heater,
evaporation of alkali metals in ash and its deposition on heat transfer surfaces, formation of
SO2 and NOX in large amount. Fluidised Bed combustion system can burn any fuel including
low grade coals (even containing 70% ash), oil, gas or municipal waste. Improved
desulphurisation and low NOX emission are its main characteristics. Below mentioned figure
shows basic principle of Fluidised bed combustion (FBC) system. The fuel and inert material
dolomite are fed on a distribution plate and air is supplied from the bottom of distribution plate.
The air is supplied at high velocity so that solid feed material remains in suspension
condition during burning. The heat produced is used to heat water flowing through the tube
and convert water into steam: During burning SO2 formed is absorbed by the dolomite and
thus prevents its escape with the exhaust gases. The molten slag is tapped from the top surface
of the bed. The bed temperature is nearly 800-900’C which is ideal for sulphur retention
addition of limestone or dolomite to the bed brings down SO2 emission level to about 15% of
that in conventional firing methods.
Principles of Fluidized Bed Combustion Operation:
https://www.youtube.com/watch?v=pd9zCb-exkU
A fluidized bed is composed of fuel (coal, coke, biomass, etc.,) and bed material (ash, sand,
and/or sorbent) contained within an atmospheric or pressurized vessel. The bed becomes
fluidized when air or other gas flows upward at a velocity sufficient to expand the bed. The
process is illustrated in figure. At low fluidizing velocities (0.9 to 3 m/s). relatively high solids
densities are maintained in the bed and only a small fraction of the solids are entrained from the
bed. A fluidized bed that is operated in this velocity range is referred to as a bubbling fluidized
bed (BFB). A schematic of a typical BFB combustor is illustrated in figure.

The amount of NOX is produced is also reduced because of low temperature of bed and low
excess air as compared to pulverised fuel firing.
The inert material should be resistant to heat and disintegration and should have similar density
as that of coal. Limestone, or dolomite, fused alumina, sintered ash are commonly used as inert
materials.
Various advantages of FBC system are as follows:
(i) FBC system can use any type of low grade fuel including municipal wastes and therefore
is a cheaper method of power generation.
(ii) It is easier to control the amount of SO2 and NOX, formed during burning. Low emission
of SO2 and NOX. will help in controlling the undesirable effects of SO2 and NOX. during
combustion. SO2 emission is nearly 15% of that in conventional firing methods.
(iii) There is a saving of about 10% in operating cost and 15% in the capital cost of the power
plant.
(iv) The size of coal used has pronounced effect on the operation and performance of FBC
system. The particle size preferred is 6 to 13 mm but even 50 mm size coal can also be used
in this system.

TYPES OF FBC SYSTEMS
FBC systems are of following types :
(i) Atmospheric FBC system :
a. Bubbling fluidized bed combustors(Overbid feed)
b. Circulating fluidized (Underfeed)
In this system the pressure inside the bed is
atmospheric.
Below mentioned figure shows commercial
circulation FBC system. The solid fuel is made
to enter the furnace from the side of walls. The
Low Velocity (LV), Medium Velocity (MV) and
High Velocity (HV) air is supplied at different
points along the sloping surface of the
distribution ash is collected from the ash port.
The burning is efficient because of high lateral
turbulence.
(ii) Pressurised FBC system. In this system pressurised air is used for fluidisation and
combustion. This system: the following advantages: (a) High burning rates. (b) Improved
desulphurisation and low NO, emission. (c) Considerable reduction in cost.
Atmospheric Fluidized Bed Combustion (AFBC)
Bubbling fluidized bed combustor
A typical BFB arrangement is illustrated schematically in figure. Fuel and sorbent are
introduced either above or below the fluidized bed. (Overbid feed is illustrated.) The bed
consisting of about 97% limestone or inert material and 3% burning fuel, is suspended by hot
primary air entering the bottom of the combustion chamber. The bed temperature is controlled
by heat transfer tubes immersed in the bed and by varying the quantity of coal in the bed. As
the coal particle size decreases, as a result of either combustion or attrition, the particles are
elutriated from the bed and carried out the combustor. A portion of the particles elutriated from
the bed are collected by a cyclone (or multiline) collector down-stream of the convection pass
and returned to the bed to improve combustion efficiency.

Secondary air can be added above the bed to improve combustion efficiency and to achieve
staged combustion, thus lowering NOx emissions. Most of the early BFBs used tubular air
heaters to minimize air leakage that could occur as a result of relatively high primary air
pressures required to suspend the bed. Recent designs have included regenerative type air
heaters.
Circulating fluidized bed combustor
https://www.youtube.com/watch?v=Vjda91aVbYI
https://www.youtube.com/watch?v=4MQVJ6qbRuE
A typical CFB arrangement is illustrated schematically in figure. In a CFB, primary air is
introduced into the lower portion of the combustor, where the heavy bed material is fluidized
and retained. The upper portion of the combustor contains the less dense material that is
entrained from the bed. Secondary air typically is introduced at higher levels in the combustor
to ensure complete combustion and to reduce NOx emissions.
The combustion gas generated in the combustor flows upward with a considerable portion
of the solids inventory entrained. These entrained solids are separated from the combustion gas
in26 hot cyclone-type dust collectors or in mechanical particle separators, and are continuously
returned to the combustion chamber by a recycle loop.
The combustion chamber of a CFB unit for utility applications generally consists of
membrane-type welded water walls to provide most of the evaporative boiler surface. The
lower third of the combustor is refractory lined to protect the water walls from erosion in the
high velocity dense bed region. Several CFB design offer external heat exchangers, which are
unfired dense BFB units that extract heat from the solids collected by the dust collectors before
it is returned to the combustor. The external heat exchangers are used to provide additional
evaporative heat transfer surface as well as superheat and reheat surface, depending on the
manufacturer’s design.
The flue gas, after removal
of more than 99% of the
entrained solids in the
cyclone or particle
separator, exists the
cyclone or separator to a
convection pass. The
convection pass designs
are similar to those used
with unconventional coal-
fuelled units, and contain
economizer, superheat,
and reheat surface as
required by the
application.

Pressurized Fluidized Bed Combustion:
Figure: PFBC turbocharged arrangement
The PFBC unit is classified as either turbocharged or combined cycle units. In turbocharged
arrangements (figure) combustion gas from the PEBC boiler is cooled to approximately 394
C and is used to drive a gas turbine. The gas turbine drives an air compressor, and there is little,
if any, net gas turbine output. Electricity is produced by a turbine generator driven by steam
generated in the PFBC boiler.
In the combined cycle arrangement (figure) 815C to 871C combustion gas from the PFBC
boiler is used to drive the gas turbine. About 20% of the net plant electrical output is provided
by the gas turbine. With this arrangement, thermal efficiency 2 to 3 percentage points higher
than with the turbocharged cycle are feasible.
Figure: PFBC combined cycle rearrangement

DRAUGHT (OR DRAFT) SYSTEM OR CHIMNEY
Chimney:-
The most common method to achieve this difference in pressure, the draught, is to provide a
chimney. Chimney is a tall hollow structure, which creates the required draught due to
difference in pressure from the ground level to some altitude in the atmosphere.
• Chimneys are made of steel, bricks or concrete.
• Brick and concrete chimneys are generally used as they have a longer life.
• The average life of concrete chimneys is about 50 years.
• The life of steel chimneys is about 25 years, which depends upon the maintenance and care
taken to prevent corrosion.
• Chimneys ere provided with lightning conductor to protect from thunder lightning and
aircraft warning light as they are at higher altitudes.
Draught (or Draft) system: -
• Draught systems are essential for flue gas propagation.
• Flue gas propagation is the process of movement of the hot gases from the combustion
chamber through boiler pipes, economizer, air pre-heater and finally to the chimney.
• The function of draught is to supply required quantity of air for combustion, propagate the
flues and remove the flues from the system
• A difference in pressure is required to move the air through the fuel bed to produce a flow of
hot gases i.e., propagation of the flue gases through the boiler, economizer, pre heater and to
the chimney by overcoming the pressure losses in the system.
• This difference in pressure required maintaining a constant flow of air, through the boiler
systems and finally to discharge the hot flues to the atmosphere through chimney is termed the
draught
Types of Draught systems
1. Natural Draught
2. Mechanical Draught
a) Forced Draught
b) Induced Draught
c) Balanced Draught

1. Natural Draught:
• The natural drought is produced by chimney or
stack.
• It is caused by the density difference between
atmospheric air and hot gas in the stack.
• For a chimney of height ‘H’ meter. The pressure
difference is given by
Advantages of Natural Draught
1) No external power is required to run the system.
2) It requires small capital investment.
3) Maintenance costs are minimum.
4) The exhausts are discharged at a high altitude and levels.
5) The system has a long life.
Limitations:-
1) The maximum pressure created by natural draught is very low (20mm of water).
2) For sufficient draught, the flue gases should be discharged at a higher temperature,
which reduces the plant efficiency.
3) Economizer and pre heater cannot be used to recover heat from the flue gases.
4) The system will have poor combustion efficiency, since the velocity of air is low.
5) It cannot produce higher draughts under peak loads, hence not flexible.

Mechanical Draught:
There are two types of mechanical draught systems, depending upon the type of fan
used for creating the draught effect.
If a forced draught fan is used it is termed as a forced draught system, and if an induced
draught fan is used it is termed
induced draught system.
(a) Forced Draught
In this system, a blower is
provided before the furnace.
The blower forces the air
through the furnace,
economizer, air preheater and
finally to the stack.
This system is termed a positive
or forced draught system, since
the pressure throughout the
system is above atmospheric,
and the flues are force driven.
The function of chimney in this
arrangement is only to discharge the exhaust at high altitudes.
The chimney has got nothing to do with draught creation and hence its height need not
be too much, but a higher altitude is desirable to discharge the flues to minimize
atmospheric pollution.
b) Induced Draught
In this system, a blower is installed before the
chimney which sucks air into the system and
creates a low pressure condition below
atmospheric pressure.
• This causes the air to be induced into· the
furnace through the entrance ports and hot
gases flow through the boiler, economizer,
preheater, blower and· then finally to the
chimney.
• The action of induced draught is similar to
the action of natural draught chimney, but the
draught produced it independent of the
temperature of hot gases.
• Hence, maximum heat can be recovered in
the air pre heater and economizer, and
comparatively cooler gases can be discharged
to the atmosphere.

c) Balanced Draught
 Balanced draught is a combination of both forced draught and induced draught.
 In this system, both forced draught and induced draught fans are used, thus eliminating
the difficulties of forced draught and induced draught systems.
 The forced draught fan provided at the entry to the furnace supplies the air through the
fuel bed/grate, while the induced draught fan sucks in the hot flues from the furnace
and discharges them at the chimney.
 Forced draught supplies sufficient air for combustion and induced draught prevents
blow off flames when the doors are opened
Comparison between Forced Draught and Induced Draught Systems:
1) The induced draught handles a higher volume of gases at high temperature, therefore the
size of fan required and power to drive it are larger as compared to· the forced draught system.
`2) Water cooled bearings are required in induced draught system since the hot gases come in
contact with the fan.
3) There are chances of air leakage in the forced draught system, since the pressure inside the
furnace is above atmospheric. In the induced draught, the pressure is below atmospheric
(suction), chances of leakage.
4) In the induced draught system, air flow is more uniform through the grate and furnace, as
compared to the forced draught system.
5) In an induced draught system, cold air may rush into the furnace while fuel charging
This cold air rush will reduce the heat transfer efficiency.
6) The fan blade wear is more in induced draught system as the blades come in contact with
hot gas.
Advantages of Mechanical Draught over Natural Draught
1) In a mechanical draught system, the rate of combustion is high since high draught is available.
2) The rate of air flow, hence the combustion can be controlled by changing the draught pressures
through the fan operations.
3) The operation of the mechanical draught system does not depend on the environmental temperature.
However, the natural draught is highly dependent on the environmental temperature.
4) Low grade fuels can be easily burnt in mechanical draught system since a higher level of draught is
available in a mechanical draught system.
5) In mechanical draughts, maximum heat can be recovered and hence the overall efficiency is higher.
6) The chimney height need not be as high as that of natural draught as its function is only to discharge
the flues.

STEAM GENERATOR (BOILERS)
https://www.youtube.com/watch?v=dVBoZ4PfZmE
https://www.youtube.com/watch?v=nL-J5tT1E1k
Boiler is an apparatus to produce steam. Thermal energy released by combustion of fuel is
transferred to water, which vaporizes and gets converted into steam at the desired temperature
and pressure.
The steam produced is used for:
(i) Producing mechanical work by expanding it in steam engine or steam turbine.
(ii) Heating the residential and industrial buildings
(iii) Performing certain processes in the sugar mills, chemical and textile industries.
Boiler is a closed vessel in which water is converted into steam by the application of heat.
Usually boilers are coal or oil fired. A boiler should fulfill the following requirements
(i) Safety. The boiler should be safe under operating conditions.
(ii) Accessibility. The various parts of the boiler should be accessible for repair and
maintenance.
(iii) Capacity. The boiler should be capable of supplying steam according to the requirements.
(iv) Efficiency. To permit efficient operation, the boiler should be able to absorb a maximum
amount of heat produced due to burning of fuel in the furnace.
(v) It should be simple in construction and its maintenance cost should be low.
(vi) Its initial cost should be low.
(vii) The boiler should have no joints exposed to flames.
(viii) The boiler should be capable of quick starting and loading.
The performance of a boiler may be measured in terms of its evaporative capacity also called
power of a boiler. It is defined as the amount of water evaporated or steam produced in kg per
hour. It may also be expressed in kg per kg of fuel burnt or kg/hr/m2 of heating surface.
TYPES OF BOILERS
The boilers can be classified according to the following criteria.
According to flow of water and hot gases.
1. Water tube. 2. Fire tube.
Fire tube boilers are classified as follows.
l. External furnace:
(i) Horizontal return tubular (ii) Short fire box (iii) Compact.
2. Internal furnace:
(i) Horizontal tubular
(a) Short firebox (b) Locomotive (c) Compact (d) Scotch.
(ii) Vertical tubular.
(a) Straight vertical shell, vertical tube
(b) Cochran (vertical shell) horizontal tube.
Various advantages of fire tube boilers are as follows.
(i) Low cost

(ii) Fluctuations of steam demand can be met easily
(iii) It is compact in size
Water tube boilers are classified as follows.
1. Horizontal straight tube boilers
(a) Longitudinal drum (b) Cross-drum.
2. Bent tube boilers
(a) Two drum (b) Three drum
(c) Low head three drum (d) Four drum.
3. Cyclone fired boilers
Various advantages of water tube boilers are as follows.
(i) High pressure of the order of 140 kg/cm2 can be obtained.
(ii) Heating surface is large. Therefore, steam can be generated easily.
(iii) Large heating surface can be obtained by use of large number of tubes.
(iv) Because of high movement of water in the tubes the rate of heat transfer becomes large
resulting into a greater efficiency.
According to position of furnace.
(i) Internally fired (ii) Externally fired
In internally fired boilers the grate combustion chamber are enclosed within the boiler shell
whereas in case of extremely fired boilers and furnace and grate are separated from the boiler
shell.
According to the position of principle axis.
(i) Vertical (ii) Horizontal (iii) Inclined.
According to application.
(i) Stationary (ii) Mobile, (Marine, Locomotive).
According to the circulating water.
(i) Natural circulation (ii) Forced circulation.
According to steam pressure.
(i) Low pressure (ii) Medium pressure (iii) Higher pressure.

BOILER ACCESSORIES
• A boiler requires many accessories for continuous trouble free steam generation.
• Some accessories are needed to increase the efficiency of the boiler.
• High economy in power generation can be achieved by utilizing the heat energy to the
maximum extent. Some of the essential boiler accessories as follows,
1. Super heater
2. Re heater
3. Economizer
4. Air pre heater
Other essential accessories include:
1. De super heater
2. Soot blower
3. Cooling Towers
MERITS AND DEMERITS OF WATER TUBE BOILERS OVER FIRE TUBE
BOILERS
Merits
1. Generation of steam is much quicker due to small ratio of water content to steam content.
This also helps in reaching the steaming temperature in short time.
2. Its evaporative capacity is considerably larger and the steam pressure range is also high-200
bar.
3. Heating surfaces are more effective as the hot gases travel at right angles to the direction of
water flow.
4. The combustion efficiency is higher because complete combustion of fuel is possible as the
combustion space is much larger.
5. The thermal stresses in the boiler parts are less as different parts of the boiler remain at
uniform temperature due to quick circulation of water.
6. The boiler can be easily transported and erected as its different parts can be separated.
7. Damage due to the bursting of water tube is less serious. Therefore, water tube boilers are
sometimes called safety boilers.
8. All parts of the water tube boilers are easily accessible for cleaning, inspecting and repairing.
9. The water tube boiler's furnace area can be easily altered to meet the fuel requirements.
Demerits:
1. It is less suitable for impure and sedimentary water, as a small deposit of scale may cause
the overheating and bursting of tube. Therefore, use of pure feed water is essential.
2. They require careful attention. The maintenance costs are higher.
3. Failure in feed water supply even for short period is liable to make the boiler over-heated.

REQUIREMENTS OF A GOOD BOILER
A good boiler must possess the following qualities:
1. The boiler should be capable to generate steam at the required pressure and quantity as
quickly as possible with minimum fuel consumption.
2. The initial cost, installation cost and the maintenance cost should be as low as possible.
3. The boiler should be light in weight, and should occupy small floor area.
4. The boiler must be able to meet the fluctuating demands without pressure fluctuations.
5. All the parts of the boiler should be easily approachable for cleaning and inspection.
6. The boiler should have a minimum of joints to avoid leaks which may occur due to expansion
and contraction.
7. The boiler should be erected at site within a reasonable time and with minimum labour.
8. The water and flue gas velocities should be high for high heat transfer rates with minimum
pressure drop through the system.
9. There should be no deposition of mud and foreign materials on the inside surface and soot
deposition on the outer surface of the heat transferring parts.
10. The boiler should conform to the safety regulations as laid down in the Boiler Act.

HIGH PRESSURE BOILERS
In all modern power plants, high pressure boilers (> 100 bar) are universally used as they offer
the following advantages. In order to obtain efficient operation and high capacity, forced
circulation of water through boiler tubes is found helpful. Some special types of boilers
operating at super critical pressures and using forced circulations are described in this topic.
I. The efficiency and the capacity of the plant can be increased as reduced quantity of steam is
required for the same power generation if high pressure steam is used.
2. The forced circulation of water through boiler tubes provides freedom in the arrangement of
furnace and water walls, in addition to the reduction in the heat exchange area.
3. The tendency of scale formation is reduced due to high velocity of water.
4. The danger of overheating is reduced as all the parts are uniformly heated.
5. The differential expansion is reduced due to uniform temperature and this reduces the
possibility of gas and air leakages.
LA MONT BOILER
https://www.youtube.com/watch?v=dHtKeSOeeck&list=PLfxgbb0UqKThziq6s4VFDz_P54EL6BL7R&in
dex=5
A forced circulation boiler was first introduced in 1925 by La Mont. The arrangement of water
circulation and different components are shown in below mentioned figure.
The feed water from hot well is supplied to a storage and separating drum (boiler) through the
economizer. Most of the sensible heat is supplied to the feed water passing through the
economizer. A pump circulates the water at a rate 8 to 10 times the mass of steam evaporated.
This water is circulated through the evaporator tubes and the part of the vapour is separated in
the separator drum. The large quantity of water circulated (10 times that of evaporation)
prevents the tubes from being overheated.
The centrifugal pump delivers the water to the headers at a pressure of 2.5 bar above the drum
pressure. The distribution headers distribute the water through the nozzle into the evaporator.
The steam separated in the boiler is further passed through the super-heater.
Secure a uniform flow of feed water through each of the parallel boiler circuits a choke is fitted
entrance to each circuit.

BENSON BOILER
https://www.youtube.com/watch?v=wIStYEIXJ0c&list=PLfxgbb0UqKThziq6s4VFDz_P54EL6BL7R&ind
ex=2
The main difficulty experienced in the La Mont boiler is the formation and attachment of
bubbles on the inner surfaces of the heating tubes. The attached bubbles reduce the heat flow
and steam generation as it offers higher thermal resistance compared to water film
1. Benson in 1922 argued that if the boiler pressure was raised to critical pressure (225 atm.),
the steam and water would have the same density and therefore the danger of bubble formation
can be completely
2. Natural circulation boilers require expansion joints but these are not required for Benson as
the pipes are welded. The erection of Benson boiler is easier and quicker as all the parts are
welded at site and workshop job of tube expansion is altogether avoided.
3. The transport of Benson boiler parts is easy as no drums are required and majority of the
parts are carried to the site without pre-assembly.
4. The Benson boiler can be erected in a comparatively smaller floor area. The space problem
does not control the size of Benson boiler used.
5. The furnace walls of the boiler can be more efficiently protected by using small diameter
and close pitched tubes.
6. The superheated in the Benson boiler is an integral part of forced circulation system,
therefore no special starting arrangement for superheated is required.
7. The Benson boiler can be started very quickly because of welded joints.
8. The Benson boiler can be operated most
economically by varying the temperature and pressure
at partial loads and overloads. The desired temperature
can also be maintained constant at any pressure.
9. Sudden fall of demand creates circulation problems
due to bubble formation in the natural
circulation boiler which never occurs in Benson boiler.
This feature of insensitiveness to load fluctuations
makes it more suitable for grid power station as it has
better adaptive capacity to meet sudden load
fluctuations.
10. The blow-down losses of Benson boiler are hardly
4% of natural circulation boilers of same
capacity.
11. Explosion hazards are not at all severe as it consists
of only tubes of small diameter and has very little
storage capacity compared to drum type boiler.
During starting, the water is passed through the economiser, evaporator, superheated and back
to the feed line via starting valve A. During starting the valve B is closed. As the steam
generation starts and it becomes superheated, the valve A is closed and the valve B is opened.
During starting, first circulating pumps are started and then the burners are started to avoid the
overheating of evaporator and super heater tubes.

LOEFFLER BOILER
https://www.youtube.com/watch?v=d0Bqaehp4q0&list=PLfxgbb0UqKTjprUBrxoIBJ2l1VvJSMTX7
The major difficulty experienced in Benson boiler is the deposition of salt and sediment on the
inner surfaces of the water tubes. The deposition reduced the heat transfer and ultimately the
generating capacity. This further increased the danger of overheating the tubes due to salt
deposition as it has high thermal resistance.
The difficulty was solved in Loffler boiler by preventing the flow of water into the boiler tubes.
Most of the steam is generated outside from the feed water using part of the superheated steam
coming out from the boiler.
The pressure feed pump draws the water through the economiser and delivers it into the
evaporator drum as shown in the figure. About 65% of the steam coming out of superheater is
passed through the evaporator drum in order to evaporate the feed water coming from
economiser.
The steam circulating pump draws the saturated steam from the evaporator drum and is passed
through the radiant superheater and then connective superheater. About 35% of the steam
coming out from the superheater is supplied to the H.P. steam turbine. The steam coming out
from H.P. turbine is passed through reheater before supplying to L.P. turbine as shown in the
figure. The amount of steam generated in the evaporator drum is equal to the steam tapped
(65%) from the superheater. The nozzles which distribute the superheated steam through the
water into the evaporator drum are of special design to avoid priming and noise.

SCHMIDT-HARTMANN BOILER
https://www.youtube.com/watch?v=Joylu3FOma4
The operation of the boiler is similar to an electric transformer. Two pressures are used to effect
an interchange of energy. In the primary circuit, the steam at 100 bar is produced from distilled
water. This steam is passed through a submerged heating coil which is located in an evaporator
drum as shown in the figure. The high pressure steam in this coil possesses sufficient thermal
potential and steam at 60 bar with a heat transfer rate of 2.5 kW/m2-°C is generated in the
evaporator drum.
The steam produced in the evaporator drums from impure water is further passed through 'the
superheater and then supplied to the prime-mover. The high pressure condensate formed in the
submerged heating coil is circulated through a low pressure feed heater on its way to raise the
feed water temperature to its saturation temperature. Therefore, only latent heat is supplied in
the evaporator drum. Natural circulation is used in the primary circuit and this is sufficient to
effect the desired rate of heat transfer and to overcome the thermo-siphon head of about 2 m to
10 m. In normal circumstances, the replenishment of distilled water in the primary circuit is
not required as every care is taken in design and construction to prevent leakage. But as a
safeguard against leakage, a pressure gauge and safety valve are fitted in the circuit.
Advantages
1. There is rare chance of overheating or burning the highly heated components of the primary
circuit as there is no danger of salt deposition as well as there is no chance of interruption to
the circulation either by rust or any other material. The highly heated parts run very safe
throughout the life of the boiler.
2. The salt deposited in the evaporator drum due to the circulation of impure water can be easily
brushed off just by removing the submerged coil from the drum or by blowing off the water.
3. The wide fluctuations of load are easily taken by this boiler without undue priming or
abnormal increase
in the primary
pressure due to high
thermal and water
capacity of the
boiler.
4. The absence of
water risers in the
drum, and moderate
temperature
difference across
the heating coil
allow evaporation to
proceed without
priming.

VELOX-BOILER
https://www.youtube.com/watch?v=xmdJMEKo5jA&list=PLfxgbb0UqKTjprUBrxoIBJ2l1VvJSMTX7&in
dex=9
Now, it is known fact that when the gas velocity exceeds the sound-velocity, the heat is
transferred from the gas at a much higher rate than rates achieved with sub-sonic flow. The
advantages of this theory are taken to effect the large heat transfer from a smaller surface area
in this boiler.
Air is compressed to 2.5 bar with a help of a compressor run by gas turbine before supplying
to the combustion chamber to get the supersonic velocity of the gases passing through the
combustion chamber and gas tubes and high heat release rates (40 MW/m3). The burned gases
in the combustion chamber are passed through the annulus of the tubes as shown in figure. The
heat is transferred from gases to water while passing through the annulus to generate the steam.
The mixture of water
and steam thus formed
then passes into a
separator which is so
designed that the
mixture enters with a
spiral flow. The
centrifugal force thus
produced causes the
heavier water particles
to be thrown outward
on the walls. This
effect separates the
steam from water. The
separated steam is
further passed to
superheater and then
supplied to the prime-
mover. The water
removed from steam in
the separator is again passed into the water tubes with the help of a pump.
Advantages
1. Very high combustion rates are possible as 40 MJ/m3 of combustion chamber volume.
2. Low excess air is required as the pressurised air is used and the problem of draught is
simplified.
3. It is very compact generating unit and has greater flexibility.
4. It can be quickly started even though the separator has a storage capacity of about 10% of
the maximum hourly output.

RAMSON’S ONCE THROUGH BOILER
• The boiler consists of inclined evaporator coil arranged in spiral.
• Forty such coils are paralleled around the furnace.
• Steam generated in evaporator flows into headers and then convection superheater.
• The superheated steam is utilized for power generation.
Advantages:
1. Heat transfer rate is large.
2. High thermal efficiency.
3. Problem of corrosion and erosion are minimized.
4. Adaptable to load fluctuations.
Disadvantages:
It is costly due to increased requirement for steel for heat transfer surface, pump and feed
water piping.

Generation of steam using forced circulation, high and
supercritical pressure
High-pressure boilers use the forced circulation of water which ensures the positive
circulation of water and increased evaporative capacity. They require less heat of vaporization.
They are compact and thus require less floor space. Due to the high velocity of water, the
tendency of scale formation is minimized.
High pressure boilers can be further classified
into
1. Natural circulation
2. Forced circulation and
3. Once through boilers.
Natural circulation (steam drum
boilers)
Natural circulation boilers use a steam drum. The
steam drum level is maintained by the boiler feed
pump. Water is fed into the steam drum and
travels down to the bottom mud drum where it is
fed into the boiler tubes. Heat is applied and the
hot water rises to the top where it accumulates
and enters the drum again. A drum separates the
steam and the water where the water goes down
to the bottom mud drum again. The steam is then
fed to the superheater for further heating. The steam drum has a fixed saturation point, which
means a molecule of water can make many passes through the evaporation tubes before turning
into steam for further heating.
• A typical pattern of natural circulation boiler is as shown in figure.
• Here the water is circulated purely by density difference with most of the heat from the fuel
flame is being radiated to the water walls directly.
• The steam pressure of such boiler is
limited to 180bar, with water steam being
separated in boiler drum.

Advantages
 Easier construction and cheaper to build, no spiral walls is
required.
 Less water consumption.
 More tolerant to feed water impurities.
 High reliability.
 Constant heat transfer areas.
 High partial load range.
Disadvantages
 The drum is part of the high-pressure components and
limits the operating flexibility due to high
thermal stresses.
 Only one evaporation end point, the drum.
 High circulation ratio, which leads to a big evaporator area.
 More tube failures because of larger diameter tubes.
 Sensitive to load variations and cannot be used in supercritical Rankine cycle designs.
Forced circulation (once-through boilers)
Forced circulation or once-through boilers do not make use of a steam drum. Water enters the
boiler from the boiler feed pump. The water level is controlled by the firing rate through the
evaporation and circulation rate. Water travels through the boiler tubes and evaporates fully
(only applies after start-up). During start up the boiler uses separating vessels to separate the
steam and water mixture. Fully evaporated steam travels to the superheater for further heating.
In these boilers water is circulated by using additional pump. These boilers often use orifice,
which control which control flow circulation.
• Orifice is located at bottom of tubes that ensure even distribution of flow through water wall
tubes.
• These boilers can produce steam pressure up to 200bar.
Advantages
 Does not have a high-pressure drum, thus more operating flexibility and
lower stress operation.
 High overall efficiency, even at part loads.
 Shorter start-up time.
 Suitable for all coal grades.
 An equal distribution of the water in the tubes.
 Ideal for sliding pressure operation, thus more control over load changes.
 Produces less 𝐂𝐎𝟐, because of an increase in overall efficiency.
Disadvantages
 Necessary for higher grade material as the evaporator forms part of the
first stage superheater.
 Difficult construction because of spiral tubes.
 Feed pumps needed for forced circulation.
 Recirculating pump needed during the start-up phase.

Controlled Circulation System
Beyond 175 kg/cm2 of pressure, circulation is to be assisted with mechanical pumps, to
overcome frictional losses. To regulate the flow through various tubes, orifice plates are used.
This system is applicable in the high sub-critical regions (say 200 kg /cm2).
Combined Circulation System
Beyond the critical pressure, phase transformation is absent, and hence a once through system
is adopted. However, it has been found that even at supercritical pressures, it is advantageous
to recirculate the water through the furnace tubes at low loads. This protects the furnace tubes
and simplifies the start-up procedure. A typical operating pressure for such a system is 260
Kg/cm2
.

Once through boilers
• Figure shows the flow diagram of
once through boilers.
• These boilers operate about
critical pressure i.e. above 221bar
• As density of water and steam is
same above critical pressure, there
will be no recirculation.
• In these boilers water enters
bottom of the tubes and completely
transforms into steam as it pass
through tubes and reaches the top.
• Thus, these boilers does not
require steam drum and hence
referred to as drumless boilers.

What is Super-critical
The critical line for water is 22.06 MPa and 374°C. From this point and above the water doesn’t
enter the two-stage phase when turned into steam. Figure 1 below illustrates the difference
between sub-critical and supercritical with pressure plotted on the X-axis, enthalpy on Y1 -
axis and temperature on the Y2 - axis. The left-hand side of the critical line show the transition
phase of water to
steam under sub-
critical conditions
and the right-hand
side shows the
transition phase of
water under
supercritical
conditions. It’s clear
that there is no two
phase stage under
supercritical
conditions. (Ultra-
Super Critical
Pressure Coal Fired
Boiler).
Figure: Difference between sub-critical and supercritical
illustrated on a pressure, enthalpy and temperature graph (Ultra
Super Critical Pressure Coal Fired Boiler)
Metallurgical conditions of
material improved over the
years and higher pressures and
temperatures become possible.
In figure 2 below the
improvement in the estimated
gross plant efficiency can be
seen as the pressure and
temperature, improved to make
supercritical and ultra-
supercritical conditions
possible.
Figure: Difference between sub-critical, supercritical and ultra-
super critical illustrated on a graph with pressure against gross
cycle efficiency (Ultra Super Critical Pressure Coal Fired Boiler)

Difference between sub- and super-critical Rankine cycles
To compare sub- and supercritical Rankine cycles only the once-through boiler design was
used. Figure shows different pressure lines and an example of a Rankine cycle plotted on the
T-s diagram. The lines running through the top part of the T-s diagram (6-1) represent the
temperature of the steam input generated in the boiler. The line at the bottom (4-5) represents
the heat rejection part of the steam. The greater the area between the top and bottom line the
more energy is available for the turbine to perform net mechanical work.
The line running at the bottom (4-5) is determined by external factors such as atmospheric
temperature, thus it’s not always sustainable to lower this line even more. The only other way
to increase the efficiency is by increasing the temperature of the steam input (top line 6-1).
Because the temperature is limited by the metallurgical properties of the material, and with
new material development it is now possible to go to higher temperatures during superheat and
reheat. To balance that with the wetness at the last turbine stages, higher boiler pressure is
required. The critical point for water is 22.06 MPa (indicated in figure 14). The lines running
above this point can be classified as super-critical and the line below this point as sub-critical.
It is clear that the supercritical Rankine cycle operated at much higher temperatures and
pressures. Subcritical Rankine cycles are used in drum boilers as well as in once-through,
cycles where supercritical Rankine cycles can only be used by once-through boilers.
Parameters to define subcritical, supercritical and ultra-supercritical-
Table: – Parameters to define supercritical
Sr.
No.
Technology Pressure
( MPa)
Temperature
(ºC)
Efficiency
(%)
1. Subcritical up to 22.1 540- 565 36-37
2. Supercritical 24.2-27.2 565-593 40-42
3. Ultra supercritical >27.2 > 593 48-55

FACTORS DISTINGUISHING SUPERCRITICAL FROM
SUBCRITICAL BOILERS (Only for M. Tech)
1) Capital cost:
Most sources indicate that a supercritical boiler involves an equipment capital cost increase of
2%. Though this has come down in recent years because of improved equipment designs and
increased experience.
These increased in cost is due to the associated system such as boiler, steam turbines, pumps,
feed water heater and piping. However, these cost increases are offset by overall cost savings
in balance of plant equipment such as coal handling, emission control and heat rejection which
results in increased cycle efficiency.
2) Efficiency:
Assuming similar plant configurations, conventional supercritical steam conditions are
expected to provide an efficiency improvement of 2% over subcritical steam conditions.
Current supercritical designs that employ sliding pressure technology have significant better
part load efficiencies than subcritical units. At 75% load efficiency reduction in case of
supercritical boiler is 2% whereas in the case of subcritical it is about 4% reduction.
3) Reliability and availability:
Currently literature indicates that the reliability and availability of new supercritical unit is
expected to be equivalent to subcritical units. Improvements in materials design and
experiences contribute to this assumption. Studies of the units prior to 1986 indicates that the
first and second generation of supercritical units have underperformed their subcritical
counterparts in terms of equivalent availability factor(EAF) and equivalent forced outage
rate(EFOR). Though, the second generation units have seen some improvement than the first
generation in this regard.
In 1985 Electric Power Research institute (EPRI) presented a report that showed unavailability
associated with pressure parts of supercritical plant decreased before finally levelling off at less
than 500 hours per year after 10 years of service. In contrast the unavailability associated with
pressure part in subcritical boilers after 10 years of service was equal to that of super critical
units but was increasing.
Critical reliability issues in the boiler have been tube leakages and water wall tubes cracks.
Water chemistry is a major contributor to tube leakages as the deposition of corrosion products
is a root cause of failure. The water wall cracking has been resolved through the use of better
materials.

DIFFERENCE BETWEEN SUBCRITICAL AND SUPERCRITICAL
TECHNOLOGY (Only for M. Tech)
Table: Comparison of subcritical and supercritical
Comparison of Sub critical and Supercritical TPPS
Input Parameters Unit Subcritical TPP Supercritical TPP
Steam Pressure MPa 16.7 24.1
Steam temperature degree celsius 538/538 565/593
Plant configuration Units x rating 4 x 500 3 x 660
Plant capacity MW 2000 1980
EPC cost per MW Rs million 40 46
Auxiliary consumption % 7.0 5.5
Station heat rate kCal per kWh 2235 2100
O & M cost Rs million per MW 1.46 1.34
Land requirement Acres per MW 0.80 0.65
Source: L & T Power
Table: Parameters differentiating subcritical and supercritical
Parameters Differentiating Sub critical and Supercritical Technology
Sr.
No
Description Supercritical Subcritical
01. Circulation Ratio 1 Once-thru=1
Assisted Circulation=3-4
Natural circulation= 7-8
02. Feed water Flow
Control
Water to Fuel Ratio
(7:1)
-OHDR(22-35 O
C)
-Load Demand
Three Element Control
-Feed Water Flow
-MS Flow
-Drum Level
03. Latent Heat Addition Nil Heat addition more
04. Specific Enthalpy Low More
05. Sp. Coal Consumption Low ( approx. 4% less
than subcritical)
High
06. Air Flow & Dry Flu
Gas Loss
Low High
07. Coal and Ash handling Low High
08. Pollution Low High
09. Auxiliary Power
Consumption
Low (6%) More (7-8%)
10. Overall Efficiency High
(40-42%)
Low
(36-37%)
11. Total Heating Surface
Required
Low
(84439m2
)
High
(71582m2
)
12. Tube Diameter Low High
13. Material Requirement Low High

( Tonnes) 7502 MT 9200 MT
14. Start Up Time Less More
15. Blow Down Loss Nil More
16. Water Consumption Less More
17. Cost Of Generation Less More
DESIGN FEATURES OF SUPER CRITICAL BOILERS (Only for M. Tech)
1. Sliding Pressure Operation
Sliding Pressure implies the variable pressure required at the turbine inlet based on load
& steam flow rate. Again the sliding pressure can be classified as pure sliding pressure,
modified sliding pressure etc.The basic nature of a simple, rotating turbine is to require less
pressure as load and flow rate are reduced, and if the main steam pressure is limited to only
that required for each load, this mode is referred to as pure sliding pressure.
However, when we speak generally of "sliding pressure" we often mean “modified sliding
pressure”. This mode has a limited amount of pressure throttling to provide a modest amount
of fast-response load reserve. The modified sliding pressure operation combines the advantages
of constant-pressure operation with those of the sliding pressure mode. The ability to activate
the storage capacity of the boiler by opening the throttle valves is combined with the advantages
of low lifetime consumption of the plant and high part load efficiency.
Figure - Water wall arrangements
The design for sliding pressure requires certain drastic adaptations of the steam generator design.
In sliding-pressure operation, because the steam generator operates under both super-critical
and sub-critical conditions as load is varied, the furnace must be designed to accommodate
both single- and two-phase fluid flow. Because the two pressure regimes and the wide variation
in fluid specific volume make continual forced re-circulation rather impractical, it is
appropriate to use a once-through design, in which flow rate through the furnace is directly
proportional to load. Steam flow rate and velocity through the furnace tubes are critical for
cooling the tubes, and with flow proportional to load, low-load operation presents a challenge

to proper furnace tube cooling. In this contest the boiler can be of spiral water wall
design or vertical tube water wall with rifled tubing.
Further, in sliding-pressure mode at low load, the fluid is subcritical, posing specific challenges
to heat transfer and tube cooling. Both departure from nucleate boiling (DNB) and steam dry-
out carry the potential for elevated tube metal temperatures. These conditions are mitigated or
avoided, in part, by providing sufficient steam mass flow density at subcritical, once-through,
low loads. Designing for proper steam cooling effect at low loads produces very high steam
mass flow density and pressure drop at full load in a once-through design. Therefore, specifying
minimum once-through load should be done with careful consideration of its consequences at
full load. Below the minimum design once-through flow rate, recirculation pumps are usually
used to protect the furnace.
2. Spiral Water wall Tubes
Among the heat-absorbing surfaces, the furnace walls are exposed to the highest heat flux. This
is because of the intense radiant heat from the fireball.
Currently, two design variants are used for once-through units: the spiral furnace tube
arrangement and the vertical tube arrangement. Design choice is governed by furnace size and
customer preference – both variants have advantages, depending on project drivers.
Spiral Configuration
1) Benefits result from averaging of lateral heat absorption variation (each tube forms a part of each furnace
wall).
2) Simplified inlet header arrangement.
3) Large number of operating units
4) Use of smooth bore tubing throughout entire furnace wall system
5) No individual tube orifices.
For any given furnace size, the spiral wall unit – in which the tube is “wrapped” around the
unit – has fewer tubes than the vertical wall unit. Refer Figure-4
Figure - Spiral water wall tubes
Vertical Configuration

The vertical water wall design uses internal ribbing in the tubes to improve heat transfer. The
vertical wall option is suitable for larger units where lower perimeter-to-furnace plan area ratios
result in higher fluid flow per tube. Refer Figure-5. The vertically oriented tubes are self-
supporting within the wall, allowing a simpler support system. The relatively simple vertical
wall furnace has certain significant advantages like:
1) Simpler wind box openings.
2) Simpler furnace water wall support system.
3) Elimination of intermediate furnace wall transition header.
4) Less costly to construct.
5) Easier to identify and repair tube leaks.
6) Lower water wall system pressure drop thereby reducing required feed pump power.
Figure - Furnace wall configuration
The success of the low mass flow vertical tube variable pressure design depends on the
capability of the tube internal geometry to promote cooling of the tube when exposed to high
heat flux. Two rib design, single and multi-lead, have been applied in the past. The single lead
design promotes the significant turbulence at the wall and is excellent in preventing departure
from nucleate boiling (DBN) but it also produce a high pressure drop due to friction and must
be produced by machining, which is expensive.
Figure- Rifle tube and plane tube

Start up and low load re-circulation system
One of the critical parameters of a once-through system is the proper selection of minimum
acceptable once-through flow in the evaporator tubes. Figure-6 shows the water wall flow as
a function of load.
From the figure below, it can be seen that the water wall flow decreases proportionately with
the load. Below a particular load, the water wall flow is kept constant in order to ensure
flow, high enough to cool the tubes. This load is typically 30 - 40% of BMCR and below this
load; the boiler will operate under the low load re-circulation system
Graph - Low load recirculation system
.
At low loads where the water flow is to be kept constant, a water-steam separator and a drain
water return system are required. The water separator consists of one or more vertical vessels
with tangential inlets. The separator is in a wet condition when operating under the low load
circulation range. In the once-through mode, the separator runs dry.
3. Steam Separator
Generally, a steam separator and a separator drain tank were installed to separate the steam and
the water at the furnace outlet during a low-load recirculation operation. This design is different
from that of a conventional NC boiler, for which a steam drum is installed to separate the water
from the steam under all operating loads. The steam drum is designed to have sufficient water
storage capacity, and usually contains complicated internal parts, such as steam cyclones,
scrubbers, internal feed pipes, and baffles. Because of the complex internals, steam drums
require a large amount of maintenance work during outage periods. However, the steam
separator design of a Benson boiler is simple in configuration and has no internal, therefore
significantly less maintenance work is required
4. Boiler Start –up systems
The start-up system in super critical boilers is used to protect super-heaters from water carry-
over by separating water from steam and re-circulating it through the evaporator surfaces
during start-up, low load operation and shutdown of the boiler. The required water flow rate

through the evaporator tubes is therefore maintained greater than the evaporation rate to protect
them against overheating.
A schematic of the boiler water-steam and start-up system arrangement is shown in Fig. 5. The
start-up system equipment consists of two steam water separators, a water collection tank, a boiler
circulating pump and the associated piping and control valves to return the fluid from the water
collection tank to the economizer inlet.
During start-up, the unit is operated much like a drum boiler where water is recirculated to
maintain a minimum flow through the furnace equivalent to 30% of full load flow. The system
is similar to a pumped circulation drum boiler with the steam water separators and the water
collection tank functioning like the steam drum. The water flowing through the furnace is a
combination of water from the water collection tank and boiler feedwater. The boiler feed pump
controls the total flow through the furnace so the minimum required mass flow is maintained.
Steam generated through the furnace circuits is separated from the water in the vertical
separator, routed to the super heater and then to either the steam turbine or the turbines bypass
system. The water from the vertical separator is returned to the water collection tank and then
to the circulating pump. The 381 valve, located at the discharge of the circulating pump,
controls the flow proportionally to tank level to maintain the water inventory in the collection
tank. Water is also recirculated from the pump discharge to the collecting tank to assure that
the minimum flow required through the pump is maintained.
Above the minimum boiler load (or Benson load) the unit switches to once-through operation.
The circulating pump is taken out of service but is kept pressurized. A small flow of feedwater
from the economizer outlet is routed to the circulating pump inlet and back to the separator to
maintain the components in the ready state for use during shutdown.
From the figure below we can understand the complete system of the start up of the supercritical
boiler. If we can see the start up thoroughly we will find the starting procedure of a supercritical
boiler is similar to the subcritical boiler.

Figure No. 5- Boiler Start-Up circuit
5. Main and reheat steam temperature control
Steam Reheat System
In utility boilers, it is important to achieve best possible heat rate to reduce the fuel cost and
hence the operators try to maintain superheat and reheat steam temperatures at rated value to
the extent possible. In once through boilers, SH steam temperature is maintained by means of
coordinated feed water flow and spray attemperation. There are many methods to control RH
steam temperature: like burner tilt, gas recirculation (GR), divided back pass dampers (gas
biasing), excess air and steam bypass. Spray, though envisaged as an emergency control, is not
preferred as a means of RH steam temperature control in constant pressure operation as it
affects plant heat rate. However, in case of once through boilers which are generally operated
in sliding pressure mode, quantum of RH spray is expected to be lower. In this case RH spray

attemperation is preferred as it will result in simpler design and operation of the boiler and also
less maintenance as systems like burner tilt, GR fans; divided back pass dampers are
eliminated.
Steam Temperature Control Method
Superheat and reheat steam temperatures should not be allowed to increase beyond the rated
value as it will result in metallurgical problems in superheater and reheater tubes and also
turbine components. On the other hand, steam temperature lower than rated value will result in
higher cycle heat rate. Typically a temperature reduction of 10 deg C in large capacity power
plant will result in about 0.3 % increase in plant heat rate. Hence it is essential to maintain the
superheat and reheat temperatures within a narrow range around the rated values.
As many operating variables change the steam temperature both superheater and reheat to get
the best performance from the turbine as well to prevent any overheating suitable means of
steam temperature control are required. The various means available are-
a) Burner Tilting
b) Gas Recirculation
c) Divided back pass dampers
d) Excess Air
e) Reheat Spray
a) Burner Tilting
Figure- Burner tilting

Tilting burners are provided in corner or tangential fired boilers. The burners can be tilted up
or down in unison in all the four corners to move the fire ball inside the furnace either upward
or downward to change the furnace absorption. When RH temperature is lower than the rated
value, burners are tilted up to reduce the furnace absorption and increase the furnace outlet
temperature. As more heat is now available for RH pick up, RH temperature can be maintained.
When RH temperature is more than the rated value, the burners are tilted down.
b) Gas recirculation:
Flue gas at economizer outlet or ID fan outlet is drawn and reintroduced into the furnace by a
Gas Recirculation (GR) fan. Tight shut off dampers are positioned both upstream and
downstream of the fan. Refer Figure for a typical arrangement of gas recirculation. As the
quantity of re-circulated gas is changed, the quantity of heat absorbed in the furnace and the
heat at furnace outlet are changed. When RH outlet temperature is lower than rated value, GR
quantity is increased to increase the heat available for RH pick up. In this case power consumed
by the GR fan is additional loss and will increase the net plant heat rate.
The major disadvantage in this method is, the gas recirculation fan to handle high temperature
dust ladle. Any failure of the fan will result in outage of the boiler. As Indian coal has a high
percentage of abrasive ash, this method of control is not generally preferred in Indian Power
Station.
Figure: Gas recirculation

c) Divided back pass dampers:
Figure- Divided back pass dampers
The divided back pass arrangement is used in wall fired boilers with fixed burners. In wall fired
boilers, the convective back pass is divided into two gas passes. On one side, Low Temperature
Reheat (LTRH) section is located and on the other side Low Temperature Superheat (LTSH)
section is located. These two sections are divided by steam cooled wall or a baffle plate. A
common economiser heat transfer section is located across both the LTRH and LTSH sections
outlet. The gas mass flow through LTRH side can be increased or decreased (gas biasing) by
the multi louver dampers positioned at the outlet of each pass (generally at the outlet of
economizer section in lower gas temperature region). Refer Figure for a typical arrangement
of dived back pass with control damper. By opening the dampers on LTRH side, the heat
transfer in LTRH section which is predominantly convective is increased due to the increase
in gas mass flow thereby increasing the RH steam temperature. In this type of control, draft
loss through the dampers will increase the power consumed by induced draft fans.
d) Excess air:
Excess air by itself is not used as a means of RH steam temperature control as an increase in
excess air will increase the stack loss and reduces the boiler efficiency. Typically 0.3 to 0.4 %
of boiler efficiency will be lost for every 10 % increase in excess air. In some cases especially
when the control load is very low, in addition to burner tilt or gas biasing, excess air is also to
be increased to achieve the RH steam temperature.
e) Reheat spray:
The reheat spray is done in a reheat de-superheater located in the cold reheat piping at the inlet
of low temperature reheat (LTRH) section or in between stages in a two stage reheater. Due to
the lower operating pressure for reheat cycle, RH spray is normally taken from boiler feed

Power plant engineering unit 2 notes by Varun Pratap Singh

Power plant engineering unit 2 notes by Varun Pratap Singh

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Power plant engineering unit 2 notes by Varun Pratap Singh