Fuel Burning Method TPP.ppt

Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Mechanical Engineering
Subject:- Energy Engineering (402047)
B.E.Mechanical
UNIT 1 :- Introduction and Thermal Power Plant
Purushottam W. Ingle
Assistant Professor

• B) Thermal Power Plant : General layout of modern thermal power
plant with different circuits, site selection criteria, classification of
coal, coal blending, coal beneficiation, selection of coal for thermal
power plant, slurry type fuels, pulverized fuel handling systems, fuel
burning methods, FBC systems, high pressure boilers, ash handling
system, Rankine cycle with reheat and regeneration (Numerical
Treatment), steam power plants with process heating (Numerical
Treatment)
9/4/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon

Fuel Burning Methods
• One of the most important factors in the economical working of a power
plant is the efficient combustion of fuel. Two commonly used methods for
burning the fuel are stoker Firing and pulverized fuel firing.
• Methods of fuel firing: The solid fuels are fired into the furnace by the
following methods:
• 1. Hand firing
• 2. Mechanical firing
1. Hand firing
• This is a simple method of firing coal into the furnace. It requires no capital
cost.
2. Mechanical (stoker) firing
• This is used in medium- and large-size power plants. Operating cost is
high.

Overfeed and underfeed fuel bed stoker
• Stoker is a power-operated fuel feeding mechanism. The working of
different types of stokers is based on the following two principles:
• 1. Overfeed principle
• Figure 3.9 shows the operating principle of an overfeed stoker. The
primary air enters the grate from the bottom. The air while moving through
the grate openings gets and picks up additional energy.
• The air then passes through a layer of incandescent coke where O2 reacts
to form CO2 and H2O vapor reacts with coke to form CO2, CO and free
H2. The gases leaving the incandescent coke consist of CO2, CO, H2, N2
and H2O and volatile matter of raw fuel. Then additional air known as
secondary air is supplied to burn the combustible gases. The combustion
of gases entering the combustor is not complete. The combustion of the
remaining combustible gases is completed in the combustion chamber. A
schematic diagram of an overfeed stoker is shown in Figure.

• The advantages and disadvantages of this stoker are as shown
below:
• Advantages
• (i) The clinkering difficulties are reduced even with coals having
high clinkering tendencies, by the spreading action.
• (ii) A wide variety of coal varying from lignite to semi-anthracite
as well as high ash coal can be burnt easily.
• (iii) The coking tendency of the coal is reduced before it reaches
the grate by the release of volatile gases, which burn in
suspension.

• (iv) It gives quick response to load change similar to pulverized fuel system
because there is only a small amount of fuel on the grate at any time and
most of the heat is released during burning of the coal in suspension.
• (v) The use of high-temperature preheated air is possible.
• (vi) This form of fixing provides thin and even fire bed and results in a high
rate of combustion (350 kg/m2 h). Therefore, it gives quick response of the
load change and with less sensitivity to the swelling characteristics of the
fuel.
• (vii) Its operation cost is considerably low, as 0.6 kW/h fi red.
• (viii) Due to fire bed equal pressure drop and proper air distribution,
combustion can be completed with minimum quantity of excess air.

• Disadvantages
• (i) It is always diffcult to operate spreader with varying sizes of a
coal and with varying moisture content.
• (ii)A natural result of suspension burning of fine fuel particles is
the entrainment of ash in the products of combustion. To avoid
the nuisance of fly ash, a dust collector is essential with this
stoker.
• (iii) Many fine unburnt carbon particles are also carried with the
exhaust gases, and it is necessary to trap these and return to
the furnace for burning. Otherwise, it would add as a loss to the
combustion system.

Underfeed Principle
• Figure shows underfeed principle. In this case, air entering
through the holes in the grate comes in contact with the volatile
matter in raw coal is given off of by distillation.
• Then it passes through the incandescent coke where reactions
similar to overfeed stoker take place. The gases produced then
pass through a layer of ash.
• The secondary air is supplied to burn the combustible gases.
The advantages and disadvantages of the underfeed stokers.
Figure shows a schematic diagram of an overfeed stoker

• Advantages
• (i) This gives higher thermal efficiency compared with chain grate stokers.
• (ii) The part load efficiency is high with multiple retort stoker.
• (iii) The combustion rate is considerably higher.
• (iv) Sufficient amount of coal always remains on the grate so that the
combustion is continued in the event of temporary breakdown of the coal
supply system.
• (v) The grate is self-cleaning.
• (vi) Different varieties of coals can be used with this type of stoker.
• (vii) Tuyeres, grate bars and retorts are not subjected to high temperature;
they remain always in contact with fresh coal.
• (viii) The use of forced draft and relatively large quantities of fuel on the
stoker make them responsive to rapid changes in load.

• The coal is continuously agitated by the plunger and pusher
plates and due to this, the fuel bed remains porous and free
from clinkers.
• (x) Smokeless operation is possible even at very light load.
• (xi) It can be used with all refractory furnaces because of non-
exposure of stoker mechanism to the furnace. Under the
existing conditions in the furnace of this type, it would not be
possible to use other types.
• (xii) Underfeed stokers are suitable for non-clinkering high
volatile and low ash content coals.

• Disadvantages
• (i) The initial cost of the unit is high.
• (ii) It requires large building space.
• (iii) The clinker troubles are usually present.
• (iv) Low-grade fuels with high ash content cannot burn
economically. The anthracite coals
• with relative low ash fusion temperatures are not suited to the
underfeed stoker.

Chain Grate Stoker
• Chain grate stoker consists of an endless chain, which forms a
support for the fuel bed. The chain travels over two sprocket
wheels one at the front and one at the rear of the furnace.
• The chain receives coal at its front through a hopper and carries
it into the furnace. The ash is carried over the rear end of the
stoker and deposited in the ash pit.
• The air enters through the air inlets situated below the grate.
Initial cost is high and operations and maintenance cost are low.

Travelling Chain grate stoker
• It differs from chain grate stoker only in grate construction. It carries small
grate bars, which actually support the fuel bed.
• The stokers are suitable for low ratings because the fuel must be burnt
before it reaches the rear of the furnace.
• The travelling stoker may be chain grate type or bar grate type. These two
differ only in the details of grate construction. The grate surface of a bar
grate stoker is made of a series of cast iron sections mounted on carrier
bars.
• The carrier bar rides on two endless-type drive chains. A travelling-type
chain grate is shown in Figure 3.14. The chain grate stoker consists of an
endless chain that forms a support for the fuel bed. The chain travels over
two sprocket wheels: one at the front and other at the rear of furnace as
shown in fi gure. The front sprocket is connected to a variable speed drive
mechanism.

• The coal is fed by gravity from a hopper located in front of the
stoker. The depth of the fuel on the grate is regulated by a
hand-adjusted gate as shown in figure.
• The speed of the grate varies at the rate at which the coal is fed
to the furnace. The combustion control automatically regulates
the speed of the grate to maintain the required steam
generation rate.
• The ash containing a small amount of combustible material is
carried over the rear end of the stoker and deposited in the ash
pit as shown in Figure 3.14.

• The air required for combustion is supplied through the air inlets
situated below the grate. The secondary air is supplied through
the openings provided in the furnace well above the grate.
• The combination of primary air and overfire air supplied provide
turbulence required for rapid combustion.
• The primary air is brought in from the sides and then forced
through the upper grate. The air-ducts under the stoker are
divided into sections. The air supply to different parts of the
stoker is regulated to meet the changing demand through these
sections.

• The air openings in the grate depend on the kind of coal burned and vary
from 20 to 40 per cent of the total grate area.
• Air dampers are provided to control air supply to the various zones. The
operator controls the rate of burning in different zones to minimize the
coke carried over into the ash pit, or coal supplied to the grate is regulated
by varying either the depth of coal on the grate with the help of grate valve
or the rate of grate travel. These grates are suitable for low rating of fuel
because the fuel must be burnt before it reaches the rear end of the
furnace.
• The rate of burning with this stoker is 200–300 kg/m2/h when forced
drought is used. Any type of fuel except caking bituminous coal can be
used with chain grate stoker because of the formation of large percentage
of fi ne particles resulting in increased carbon loss.

• The advantages and disadvantages of chain grate stoker are
listed below:
• Advantages
• (i) It is simple in construction and its initial cost is low.
• (ii) It is more reliable in service, therefore maintenance charges
are low.
• (iii) It gives high heat release rates per unit volume of the
furnace.
• (iv) The heat release rates can be controlled just by controlling
the speed of chain.
• (v) It is self-cleaning stoker.

• Disadvantages
• (i) The amount of coal carried on the grate is small as the
increase in grate size creates additional
• problems. This cannot be used for high-capacity boilers 200 t/h
or more.
• (ii) The temperature of preheated air is limited to 180°C.
• (iii) The clinker troubles are very common.
• (iv) Ignition arches are required.
• (v) There is always some loss of coal in the form of fine
particles carried with the ashes.

Spreader Stoker
• Figure 3.15 shows a spreader stoker. This is an overfeed-type stoker. The
coal burns on this stoker remains partly in suspension and partly on the
grate as shown in Figure 3.15.
• In this stoker, coal from the hopper is fed on to a feeder, which measures
the coal in accordance to the requirements. Feeder is a rotating drum fitted
with blades. From the feeder the end drops on to spreader or distributor,
which spreads the coal over the furnace. The spreader stoker distributes
the coal evenly over the entire grate area. The spreader speed depends
on the size of coal.
• The spreader stoker consists of variable feeding device, a mechanism for
throwing the coal uniformly on the grate and suitable openings for
admitting the air. The coal feeding and distributing mechanism is located in
the front wall above the grate. A portion of coal supplied containing fine
particles of coal burns in suspension and remaining falls to the grate. The
air supplied by FD fan enters the furnace through the openings provided in
the grate.

• A portion of this air is used to burn the fuel on the grate and the
remaining air is utilized to burn the volatile matter and fi ne
particles in suspension. Overfire or secondary air is supplied
through the nozzle.
• The secondary air creates high turbulence and completes the
combustion of volatile matter and fi ne particles of the coal.
• The unburnt coal and ash are deposited on the grate, which
should be removed periodically.

• The feeder is a slow-speed rotating drum on which a large number of
small blades is mounted. It supplies coal to the spreaders in a continuous
stream. The speed of the feeder can be adjusted as per the load on the
plant.
• The feeders are operated with variable speed drive to control the
combustion as per requirement. The feeders may be reciprocating ram,
endless belt or spiral worm. The spreader consists of a rapidly rotating
shaft carrying blades. These blades are twisted to provide uniform
distribution of the coal over the grate.
• The fast rotating blades hit the coal particles coming from the feeder and
throw it into the furnace. The distribution of coal over the grate depends on
the rotating speed of the spreader and on the size of the coal.
• Variations in performance of the spreader due to the changes in coal size
and moisture content can be done by means of an external hand
adjustment of the mechanism.

• The selection of the size of the coal used in spreader stoker
should be in between 6 and 36 cm. Stationary grates are used
up to 10 MW capacity plant while moving grates are used in the
range of 10–30 MW capacity plant.
• The spreader stoker has wide applications with respect to the
fuels used as well as to the boiler sizes. A wide variety and poor
quality coal can be burnt efficiently with this type of stoker.
• This type of stoker can be used for boiler capacities from 80 to
150 tons of steam per hour. The heat release rate of 40 × 106
kJ/m2 h is possible with stationary grate and 80 × 106 kJ/m2 h
is possible with travelling grate.

• Advantages
• (i) A wide variety of coal varying from lignite to semi-anthracite as well as
high ash coal can be burnt easily.
• (ii) The clinkering problems are reduced even with coals that have high
clinkering tendencies due to the spreading action.
• (iii) The high-temperature preheated air can be used.
• (iv) The coking tendency of the coal is reduced before it reaches the grate
by the release of volatile gases, which burn in suspension.
• (v) This system provides thin and even fi re bed and results in high rate of
combustion (350 kg/m2 h) giving quick response to the load change and
with less sensitivity to the swelling characteristics of the fuel.

• (vi) It gives quick response to load change similar to pulverized
fuel system due to small amount of fuel on the grate at any time
and most of heat is released during burning of the coal in
suspension.
• (vii) The fi re bed gives equal pressure drop and proper air
distribution so that combustion can be completed with minimum
quantity of excess air.
• (viii) Its operation cost is considerably low.

• Disadvantages
• (i) It is difficult to operate the system with varying sizes of coal
and with varying moisture content.
• (ii) Due to suspension, burning of fine fuel particles is the
entrainment of ash in the products of combustion; a dust
collector is necessary with this stoker.
• (iii) Many fine unburnt carbon particles are also carried with the
exhaust gases resulting in a loss to the combustion system.

Retort Stoker
• In underfeed stokers, the fuel is fed from underneath the fire and moves
gradually upwards. The primary air is supplied from below where
combustion takes place. The fuel releases the volatile matter as it passes
through the initial fuel bed from bottom.
• The released volatile matter mixes with fresh air and enters into the
combustion zone. The entire combustion process is highly efficient and
gives high rates of heat release. Bituminous and semi-bituminous coals
with small ash content and fusing temperature above 1300°C (caking or
non-caking) can be burnt very effectively using these stokers.
• The underfeed stokers can be divided into the following two main types:
• 1. Single retort stoker
• 2. Multi-retort stoker

• 1. Single retort stoker
• Figure 3.16 shows the arrangement of a single retort stoker.
The fuel is placed on a large hopper on the front of the furnace.
• Then it is further fed by reciprocating ram or screw conveyor
into the bottom of the horizontal trough. The air is supplied
through the tuyeres provided along the upper edge of the grate.
The ash and clinkers are collected on the ash plate provided
with dumping arrangement. The coal-feeding capacity of a
single retort stoker varies from 100 to 2000 kg/h. Due to the
inability of obtaining even air distribution from the sides of
retorts, multi-retort stokers are generally used for increasing the
burning capacity of the stoker.

• 2. Multi-retort stoker
• The multi-retort stoker is shown in Figure 3.17. It consists of a
series of alternate retorts and tuyere boxes for supply of air.
• Each retort is fitted with a reciprocating ram for feeding and
pusher plates for the uniform distribution of coal. The coal falling
from the hopper is pushed forward during the inward stroke
ram.
• Then the distributing rams (pushers) push the entire coal down
the length of the stoker. The ash formed is collected at the other
end as shown in figure. The number of retorts may vary from 2
to 20 with local burning capacity ranging from 300 to 2000
kg/h/retort.

• Underfeed stokers use forced draft for maintaining sufficient air
flow through the fuel bed. The primary air is supplied from main
wind box to the fuel bed situated below the stoker. The partly
burnt coal moves on to the extension grate. The low-pressure
air enters into the extension grate and wind from main wind box
is supplied to the thinner fuel bed on the extension grate. The
quantity supplied can be regulated by an air damper. The air
pressure in the main wind box under the stoker is also varied to
meet variable load. Provision is also made for varying the air
pressure under the different sections of the stoker in order to
correct for irregular fuel-bed conditions. Forced draft causes
rapid combustion.

• It is also necessary to introduce ‘overfire air’
• when high volatile coals are used to prevent the smoke
formation. Combustion control is introduced into the stoker drive
by either varying the ram stoke or changing the rate of
reciprocation. On–off control is used on motor-driven stokers.

• Provision is also made for varying the air pressure under the
different sections of the stoker in order to correct for irregular
fuel-bed conditions.
• Forced draft causes rapid combustion. It is also necessary to
introduce ‘overfire air’ when high volatile coals are used to
prevent the smoke formation. Combustion control is introduced
into the stoker drive by either varying the ram stoke or changing
the rate of reciprocation. On–off control is used on motor-driven
stokers.

Advantages and disadvantages of stoker firing over pulverized firing
• Advantages
• (i) There is no necessity of coal preparation plant as the coal obtained from
mines can be directly used. Sometimes it is necessary to size (crush) the
coal in order to suit the furnace conditions.
• (ii) This can be used for medium capacity plant more economically.
• (iii) It is free from danger of explosion.
• (iv) The building space requirement compared with pulverized system is
less.
• (v) The capital investment as compared to pulverized system is less.
• (vi) The maintenance and operating costs are less.
• (vii) The auxiliary plant required is considerably reduced.

• (vii) The auxiliary plant required is considerably reduced.
• (viii) It also works for few hours in the event of coal-handling
plant failure as large amounts of coal are stored on the grate.
• (ix) The dust collection problems are less severe compared with
pulverized system as most of the ash is removed from the
grate.
• (x) The stoker firing systems are more reliable.

• Disadvantages
• (i) The loss of coal is more through riddling.
• (ii) There is heavy wear and tear of moving parts due to abrasive action of
coal.
• (iii) The troubles of clinkering of combustion chamber walls are very
common.
• (iv) The sudden variations of load cannot be met to the same degree of
effciency as in the
• case of pulverized fuel fi ring.
• (v) The furnaces need fire arches, which increase the construction cost
and create troubles during operation.
• (vi) Standby losses are considerably more.

PREPARATION AND BURNING OF PULVERIZED COAL
• Coal is pulverized (powdered) to increase its surface exposure thereby
permitting rapid combustion. Coal is reduced to a fine powder in grinding
mill or pulverizes.
• There are two methods used to feed pulverized fuel to the furnace.
1.Unit or Direct System
• Figure 3.18 shows the schematic diagram of a unit system. In the unit
system, a separate pulverizing unit is provided for each furnace and the
fuel is fired directly into the furnace without being stored.
• The raw coal from overhand coal bunker falls into a feeder and there it is
dried by supplying hot air. The coal is then transferred to the pulverizing
mill where it is pulverized. Primary air is supplied to the mill by the fan. The
mixture of pulverized coal and primary air then flows to burner where
secondary air is added.

• Advantages
• (i) System is simple and cheaper.
• (ii) Direct control of combustion from pulverizing mill is possible.
• (iii) Coal transportation system is simple.
• Disadvantages
• (i) Mill operates at variable load.
• (ii) System is less flexible.
• (iii) No reserve.
• (iv) Fan handles air + coal mixture.

Bin or Central system
• Figure 3.19 shows
the schematic
diagram of a bin
system. In central
system, the coal is
ground in a central
grinding plant and
stored in bins or
bunkers. From the
bunkers it is
distributed in
various burners
through separate
feeders in
accordance with
load demands.

• Coal from the raw coal bunker is fed by gravity to a dryer where
it is dried by the hot air mill.
• The pulverized coal from the mill is transferred to pulverized
coal bunker.
• The air from the coal is separated in the cyclone separator and
returned to the mill.
• The primary air is mixed at the feeder and the mixture is
supplied to the burner.

ASH HANDLING SYSTEM
• Coal that is available in nature has some percentage of ash.
When coal is burnt, about 10–20 percent of it is converted into
ash.
• Considering the large coal-burning capacity of modern power
plant, the amount of ash that is produced annually accounts to
thousands of tonnes per annum. Thus, it is necessary to have
modern ash-handling systems.
• An ash-handling system should perform the following
operations in sequence as shown in Figure 3.33.

• 1. Mechanical handling system
• Figure shows a mechanical dust-handling system. This system
is used to handle limited amount of ash especially in small
plants. Hot ash from the boiler furnace is made to fall through a
water seal over the belt conveyor.
• The cooled ash is carried away by the belt to the dumping site
or to an overhead bunker. The ash is disposed o from the
bunker by means of trucks. The system can deliver ash up to 3
t/h with a speed of 30cm/min for a period ranging from 5 to 10
years. Power consumption using this system is low.

• 2. Pneumatic
system
• Figure 3.36 shows a
typical pneumatic
ash-handling
system. In this
system, air is used
as a medium to
carry ash over long
distances at the rate
of 5–30 t/h. The
system consists of
ash crushers,
separators, hopper

• The ash collected in the ash hopper is crushed by using ash
crushers. This ash is carried to the primary and secondary
cyclone separators using air. Heavier ash settles down due to
centrifugal action and is collected in the ash hopper.
• Air leaving the secondary separator is further cleaned by
passing through filter. Clean air is exhausted to the atmosphere
using exhaust fan. This system is prone to rapid wear and tear
resulting in failure of conveying pipes; it is noisy in operation.

• 3. Hydraulic system
• In this system, water is used as the medium to carry ash at high velocity
through channels. Depending on water pressure or velocity, the system is
subdivided as low-pressure system and high-pressure system.
• (i) Low-pressure system
• In this system, ash falling from the boiler furnace is carried away by water
moving at a low velocity of 3–5m/s into sumps. In the sump, ash and water
are separated by passing over a screen.
• Water is pumped back to the trough and used again. The ash collected is
removed using carriages. This system is continuous and can handle about
40–50 t/h of ash through a distance about 500 m. Figure shows a low-
pressure system.

• (ii) High-pressure system
• In high-pressure system, the hoppers below the boilers are
fitted with water nozzles at the top and on the sides. Ash is
quenched by top nozzle spray and is carried away by the side
nozzle spray.
• The quenched ash flows along with water at high velocity and
gets collected in the sump. Water is separated by ash by
passing it over screen and is re-circulated. Figure shows a high-
pressure system.

• Advantages of hydraulic system
• (i) It is clean, dustless and totally enclosed.
• (ii) Its ash-carrying capacity is considerably large, therefore it is more
suitable for large thermal power plants.
• (iii) It can discharge ash to a considerable distance (1000m) from the
power plant.
• (iv) It can also be used to handle a stream of molten ash.
• (v) The unhealthy aspects of ordinary ash basement work are eliminated.
• (vi) The whole system is clean and healthy.
• (vii) The important feature of the system is the absence of working parts in
contact with the ash.

• 4. Steam jet system
• In this system, high-pressure jet of steam is used as a medium
to carry away the ash from boiler hopper. This system can
remove ash through a horizontal distance of 200 m and a
vertical distance of 30m.
• This system is useful particularly when there is limited space so
that no other handling plants can be installed. As pipes carry
ash that is highly abrasive, it is necessary to use nickel alloy as
lining to prevent wear and tear.
• This system does not require any other auxiliary drives except
steam generated by the boiler itself. For each tonne of ash
removal, about 110 kg of steam is required.

• Advantages of wet system
• (i) Transportation of ash by pipelines eliminates noise,
dust and traffic problems.
• (ii) Use of manned equipment is eliminated.
• (iii) The system is unaffected by transportation strikes.

• Disadvantages of wet system
• (i) Large quantities of leachate under a positive pressure head in pond
pose a constant treat to ground water quality. This is prevented by surface
preparation and artificial lining, which are very costly.
• (ii) The transport water is normally recycled. This requires additional
pipelines, pumping equipment, treatment facilities and substantial
operating and maintenance costs.
• (iii) A larger area is required. Area of wet system may be twice that of the
dry system.
• (iv) Water requirements are very large.
• (v) Scaling and cementation within pipeline, particularly when the slurry
contains calcium, magnesium and sulphate ashes, may render this system
unsuitable in certain cases. (vi) It is not flexible to relocate the other
discharge site.

• Advantages of dry system
• (i) Leachate quantities are significantly reduced. Liners to disposal area
can be eliminated by fixation of ash. Ash piles can be designed to provide
drainage at different levels.
• (ii) Water and power requirements are considerably less.
• (iii) Compacted ash is a structural material that can be sold.
• (iv) Required storage volume and area are reduced considerably. The
density of compacted ash is 1400 kg/m3 against 900 kg/m3 of loose ash.
• (v) The ash disposal site has wider choice of land after closure.
• (vi) This system offers greater flexibility in operation as ash is transported
by vehicles to different sites.

• Disadvantages of dry system
• (i) Use of trucks makes this system totally dependent.
• (ii) It presents increased visual impact along transportation
route.
• (iii) Wetting of ash containing calcium and magnesium forms
lumps, which may stick to conveying belt.

Fuel Burning Method TPP.ppt

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