UNIT-I
UNIT - I
Syllabus; Introduction to the sources of Energy -
resources and development of power in India. Steam Power Plant: plant
layout, working of different circuits, fuel and handling equipment, types of
coals and coal handling, choice of handling equipment, coal storage and
handling systems. Combustion: properties of coal – overfeed & underfeed
fuel beds, travelling grate strokers, spreader strokers, retort strokers,
pulverized fuel burning system & its components, combustion needs and
draught system, cyclone furnace, design and construction, dust collectors,
cooling towers and heat rejection, corrosion and feed water treatment.
Introduction to the sources of Energy:
Energy
can be defined as the capacity or ability to do work or produce a change. It
is a fundamental concept in physics and is often described as the "currency"
of the universe. Energy exists in various forms and can be converted from
one form to another.
Energy comes in different forms, including:
1. Kinetic Energy: This is the energy possessed by an object due to its motion. For example, a moving car or a flowing river has kinetic energy.
2. Potential Energy: This is the energy stored in an object or system based on its position or condition. Examples include gravitational potential energy (related to an object's height), elastic potential energy (stored in stretched or compressed materials), and chemical potential energy (stored in chemical bonds).
3. Thermal Energy: Also known as heat energy, it refers to the internal energy of a substance due to the motion of its particles. Thermal energy determines the temperature of an object and can be transferred between objects through processes like conduction, convection, and radiation.
4. Electrical Energy: This is the energy associated with the movement of electric charges. It powers electrical devices and is generated through various means such as generators, batteries, and solar cells.
5. Light Energy: Also referred to as radiant energy, it is the energy carried by electromagnetic waves, including visible light. Light energy allows us to see and is used in technologies such as lighting, lasers, and solar panels.
6. Chemical Energy: This is the energy stored within the bonds of chemical substances. When chemical reactions occur, the bonds are broken or formed, releasing or absorbing energy. Chemical energy is found in fuels, food, and batteries.
7. Nuclear Energy: Nuclear energy is released through nuclear reactions, specifically nuclear fission (splitting of atomic nuclei) or nuclear fusion (combining of atomic nuclei). It is the energy that powers the Sun and is harnessed in nuclear power plants.
These are just a few examples of the different forms of energy. The
conservation of energy principle states that energy cannot be created or
destroyed, only transferred or transformed from one form to another. This
principle underlies the study of energy and its various applications across
different scientific disciplines.
Sources of energy refer to the various natural resources or phenomena that can be harnessed to generate power and perform useful work. These sources provide the necessary fuel to produce electricity, heat, and mechanical energy, which are essential for our daily activities and industrial processes. Here's an introduction to some of the major sources of energy:
1. Fossil Fuels: Fossil fuels are derived from ancient organic matter, such as plants and animals, that underwent geological processes over millions of years. The three primary fossil fuels are coal, oil, and natural gas. These fuels are burned to release energy in the form of heat, which is then used to generate electricity or power various devices. Fossil fuels are non-renewable resources and contribute to environmental issues like air pollution and climate change.
2. Renewable Energy: Renewable energy sources are derived from naturally replenishing resources that do not deplete over time. They offer a sustainable and cleaner alternative to fossil fuels. Some common examples include:
a. Solar Energy: Solar power harnesses energy from the sun using photovoltaic (PV) cells or solar thermal collectors. PV cells convert sunlight directly into electricity, while solar thermal systems use sunlight to heat fluids or create steam, driving turbines to generate power.
b. Wind Energy: Wind turbines capture the kinetic energy of wind and convert it into electricity. As the wind blows, it rotates the turbine blades, which then spin a generator to produce electrical energy.
c. Hydropower: Hydropower utilizes the gravitational force of falling or flowing water to generate electricity. It typically involves constructing dams or utilizing the kinetic energy of rivers and tidal movements to turn turbines.
d. Biomass: Biomass energy is derived from organic materials such as plants, agricultural residues, and organic waste. These materials can be burned directly or converted into biogas or biofuels, which can then be used for heat or electricity generation.
e. Geothermal Energy: Geothermal power utilizes heat from the Earth's interior to generate electricity. This is done by tapping into hot water reservoirs or geothermal heat pumps that exploit the stable temperature of the ground for heating and cooling purposes.
3. Nuclear Energy: Nuclear power is generated through the process of nuclear fission, where the nucleus of an atom is split, releasing a significant amount of energy. This energy is used to heat water, produce steam, and drive turbines to generate electricity. Nuclear energy has the advantage of producing large amounts of electricity without direct carbon emissions, but it carries concerns related to safety, waste disposal, and the potential for nuclear proliferation.
4. Other Alternative Sources: Apart from the major sources mentioned above, there are several other alternative energy sources being explored and developed. These include tidal power, wave energy, hydrogen fuel cells, and various emerging technologies that aim to harness energy from unconventional sources.
The diversification of energy sources is crucial for reducing dependence on fossil fuels, mitigating climate change, and ensuring a sustainable energy future. Governments, industries, and individuals are increasingly focusing on adopting and promoting cleaner and more renewable forms of energy to meet the growing global energy demand.
Power:
Power is the rate of doing work, which equals energy per time.
The units of power are watts, joules per second, and horsepower,
where ;
1 Watt
= 1
joule per second
1 Kilowatt = 1,000 Watts
1 Megawatt = 1,000 kilowatts
= 1 horsepower
Classification of power plants
The power plants cycle generally divided into the following groups
1. Vapour Power Cycle
(Carnot cycle, Rankine cycle, Regenerative cycle, Reheat cycle, Binary vapour cycle)
2. Gas Power Cycles
(Otto cycle, Diesel cycle, Dual combustion cycle, Gas turbine
cycle.)
Steam Power plant
The above 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 super heater.
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 the boiler and
super heater tubes, flow through the dust collector and then through
the economizer, air pre-heater and finally, they are exhausted to the
atmosphere through the chimney.
Steam is generated in a boiler, expanded in the prime mover and
condensed in the condenser and fed into the boiler again.
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A furnace to burn the fuel.
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Steam generator or boiler containing water. Heat generated in the
furnace is utilized to convert water in steam.
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Main power unit such as an engine or turbine to use the heat energy
of steam and perform work.
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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:
a.
Feed water and steam flow circuit
b.
Coal and ash circuit
c.
Air and gas circuit
d.
Cooling water circuit.
Feed water and steam flow circuit:
The
steam generated in the boiler passes through super heater and is
supplied to the steam turbine. Work is done by the expansion of steam in
the turbine and the pressure of steam is reduced. The expanded steam
then passes to the condenser, where it is condensed. The condensate leaving the condenser is
first heated in a l.p. water heater by using the steam taken from the
low pressure extraction point of the turbine. Again steam taken from the
high pressure extraction point of the turbine is used for heating the
feed water in the H.P water heater. The hot feed water is passing
through the economizer, where it is further heated by means of flue
gases. The feed water which is sufficiently heated by the feed water
heaters and economizer is then fed into the boiler.
Coal from the storage yard is transferred to the boiler furnace by means of coal handling equipment like belt conveyor, bucket elevator, etc., ash resulting from the combustion of coal in the boiler furnace collects at the back of the boiler and is removed to the ash storage yard through the ash handling equipment.
Ash Disposal:
Indian coal contains 30% to 40% ash. A power plant of 100MW 20 to 25
tonnes of hot ash per hour. Hence sufficient space near the power plant
is essential to dispose such large quantities of ash.
Air is taken from the atmosphere to the air preheater. Air is heated in
the air preheater by the heat of flue gas which is passing to the
chimney. The hot air is supplied to the furnace of the boiler. The flue
gases after combustion in the furnace, pass around the boiler tubes. The
flue gases then passes through a dust collector, economizer and
pre-heater before being exhausted to the atmosphere through the chimney.
By this method the heat of the flue gases which would have been wasted
otherwise is used effectively. Thus the overall efficiency of the plant
is improved.
Cooling water circuit:
Abundant quantity of water is required for condensing the steam in the
condenser. Water circulating through the condenser may be taken from
various sources such as river or lake, provided adequate water supply is
available from the river or lake throughout the year. If adequate
quantity of water is not available at the plant site, the hot water from
the condenser is cooled in the cooling tower or cooling ponds and
circulated again.
The different types of systems and components used in steam power plant
are as follows:
(1)
High pressure boiler
(2)
Prime mover
(3)
Condensers and cooling towers
(4)
Coal handling system
(5)
Ash and dust handling system
(6)
Draught system
(7)
Feed water purification plant
(8)
Pumping system
(9)
Air preheater, economizer, super heater, feed heaters.
Advantages of thermal power plants
1) Initial cost is low compared with hydro-plant.
2) The power plant can be located near load center, so the transmission
losses are considerably reduced.
3) The generation of power is not dependent on nature’s mercy like
hydro plants.
4) The construction and commissioning of a thermal plant requires less
period of time than a hydro plant.
Disadvantages of thermal power plants
1) It pollutes the atmosphere due to production of large amount of
smoke and fumes.
2) It is costlier in running cost as compared to hydroelectric
plants.
3) The heated water that comes from thermal power plant has an adverse
effect on the lives in the water and disturbs the ecology.
4) Maintenance cost is high.
CHARACTERISTICS OF STEAM POWER PLANT
The desirable characteristic of a steam
power plant is 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. Coal is easily handled if the lift trucks with scoop are
used. In case the coal is brought by railway wagons, ships or boats, the
unloading may be done by car shakes, rotary car dumpers, cranes, grab buckets
and coal accelerators. Rotary car dumpers although costly are quite efficient
for unloading closed wagons.
(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. Fig. shows a belt conveyor. It consists of an endless
belt. Moving over a pair of end drums (rollers). At some distance, a supporting
roller is provided at the center. The belt is made, up of rubber or canvas. A 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. The average speed of belt
conveyors varies between 200-300 r.p.m. This conveyor is preferred over 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 inclines.
2. Screw conveyor. It consists of an endless helicoid screw fitted to a
shaft. The screw while rotating in a trough transfers the coal from feeding end
to the discharge end. 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 hoistway 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. shows a skip hoist.
6. Flight conveyor. It consists of one or two strands of chain to which
steel scraper or flights are attached’. which scrap 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.
Skip hoist and bucket elevators lift the coal vertically while Belts and flight
conveyors move the coal horizontally or on inclines.
(v) Storage of coal. It is desirable that sufficient quantity of coal
should be stored. Storage of coal gives protection against the interruption of
coal supplies when there is delay in transportation of coal or due to strikes
in coal mines. Also when the prices are low, the coal can be purchased and
stored for future use. The amount of coal to be stored depends on the
availability of space for storage, transportation facilities, the amount of
coal that will whether away and nearness to coal mines of the power station.
Usually coal required for one month operation of power plant is stored in case
of power stations situated at longer distance from the collieries whereas coal
need for about 15 days is stored in case power station situated near to
collieries. Storage of coal for longer periods is not advantageous because it
blocks the capital and results in deterioration of the quality of coal.
The coal received at the power station is stored in dead storage in the
form of piles laid directly on the ground. The coal stored has the tendency to
whether (to combine with oxygen of air) and during this process coal loss some
of its heating value and ignition quality. Due to low oxidation the coal may
ignite spontaneously. This is avoided by storing coal in the form of piles which
consist of thick and compact layers of coal so that air cannot pass through the
coal piles. This will minimize the reaction between coal and oxygen. The other
alternative is to allow the air to pass through layers of coal so that air may
remove the heat of reaction and avoid burning. In case the coal is to be stored
for longer periods the outer surface of piles may be sealed with asphalt or
fine coal.
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. 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. 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.
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 caping in fuel or of low ash fusion temperature.
(v) Dependability and easier
operation.
(vi) Operating and maintenance
cost.
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.
Underfeed Principle. Fig. shows underfeed principle. In underfeed principle
air entering through the holes in the rate
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.
(i)
Chain Grate Stoker. Chain
grate stoker and traveling grate stoker differ only in grate construction. A
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 furnace. The traveling chain
receives coal at its front end through a hopper and carries it into the
furnace. The ash is tipped from the rear end of chain. The speed of grate
(chain) can be adjusted to suit the firing condition. The air required for
combustion enters through the air inlets situated below the grate. Stokers are
used for burning non-coking free burning high volatile high ash coals. Although
initial cost of this stoker is high but operation and maintenance cost is low.
1. 2. The
travelling grate stoker
The traveling grate stoker also uses
an endless chain but differs in that it carries small grate bars which actually
support the fuel fed. It is used to burn lignite, very small sizes of
anthracites coke breeze etc. The stokers are suitable for low ratings because
the fuel must be burnt before it reaches the rear of the furnace. With forced
draught, rate of combustion is nearly 30 to 50 lb of coal per square foot of grate
area per hour, for bituminous 20 to 35 pounds per square foot per hour for
anthracite.
Spreader Stoker. A spreader stoker is shown in Fig. In this stoker the
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.
Feeders can be reciprocating rams, endless belts, spiral worms etc. From the
feeder the coal drops on to spreader distributor which spread the coal over the
furnace. The spreader system should distribute the coal evenly over the entire
grate area. The spreader speed depends on the size of coal.
Advantages
The various advantages of spreader
stoker are as follows :
1. Its operation cost is low.
2. A wide variety of coal can be burnt
easily by this stoker.
3. A thin fuel bed on the grate is
helpful in meeting the fluctuating loads.
4. Ash under the fire is cooled by the
incoming air and this minimizes clinkering.
5. The fuel burns rapidly and there is
little coking with coking fuels.
Disadvantages
1. The spreader does not work
satisfactorily with varying size of coal.
2. In this stoker the coal burns in
suspension and due to this fly ash is discharged with flue gases which requires
an efficient dust collecting equipment.
Multi-retort Stoker. A multi-retort stoker is shown in Fig. The coal
falling from the hopper is pushed forward during the inward stroke of stoker
ram. The distributing rams (pushers) then slowly move the entire coal bed down
the length of stoker. The length of stroke of pushers can be varied as desired.
The slope of stroke helps in moving the fuel bed and this fuel bed movement
keeps it slightly agitated to break up clinker formation. The primary air
enters the fuel bed from main wind box situated below the stoker. Partly burnt
coal moves on to the extension grate. A thinner fuel bed on the extension grate
requires lower air pressure under it. The air entering from the main wind box
into the extension grate wind box is regulated by an air damper.
As sufficient amount of coal always remains on the grate, this stoker
can be used under large boilers (upto 500,000 lb per hr capacity) to obtain
high rates of combustion. Due to thick fuel bed the air supplied from the main
wind box should be at higher pressure.
