Smoke
and
dust
removal
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, clothings,
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.
The presence of dense smoke indicates poor furnace conditions and a loss in efficiency and
capacity of a boiler plant. A small amount of smoke leaving chimney shows good furnace
conditions whereas smokeless chimney does not necessarily mean a better efficiency in the
boiler room.
To avoid the atmospheric pollution the fly ash must be removed from the gaseous products
of combustion before they leaves the chimney.
The removal of dust and cinders from the flue gas is usually effected by commercial dust
collectors which are installed between the boiler outlet and chimney usually in the chimney
side of air preheater.
Classification of dust collectors
Dust collectors are Classified as;
• Mechanical dust collectors;
(a) Wet type (scrubbers).
• Spray type, packed type and impingement type.
(b) Dry type.
• Gravitational separators, cyclone separators,
• Electrical dust collectors;
•
Rod type and plate type.
Mechanical dust collectors
Mechanical dust collectors are sub-divided into wet and dry types.
In wet type collectors also known as scrubbers water sprays are
used to wash dust from the air. The basic principles of mechanical
dust collectors are shown in Fig. 4.38. As shown in Fig. 4.39(a) by
increasing the cross-sectional area of duct through which dust laden
gases are passing, the velocity of gases is reduced and causes
heavier dust particles to fall down. Changing the direction of flow
[Fig. 4.39(b)] of flue gases causes the heavier particles of settle out.
Sometime baffles are provided as shown in Fig. 4.39(c) to separate
the heavier particles.
Mechanical dust collectors
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, louvred and
baffle dust collectors.
A cyclone dust collector uses a downward flowing vortex for dust
laden gases along the inner walls. The clean gas leaves from an
inner upward flowing vortex. The dust particles fall to the bottom
due to centrifuging action.
Electrostatic Precipitators
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.
For fly ash scrubbers of large importance is the content of free
lime (CaO) in the ash. With a high concentration of CaO the ash
can be cemented and impair the operation of a scrubber.
The efficiency of operation of gas cleaning devices depends
largely on the physico-chemical properties of the collected ash
and of the entering waste gases.
FLY ASH SCRUBBER
Fig. 4.40 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.
Classification of Steam Power Plants
Steam Power Plants are also Classified as;
• Central stations; the electrical energy available
from these stations is meant for sale to the consumers
who wish to purchase it.
• Industrial/ captive power stations; this type of
power station is run by the manufacturing company
for its own use and its output is not available for
general sale.
Comparison between jet and
surface condenser
• Jet condenser; low manufacturing cost.
Low upkeeps, requires small floor space and
more auxiliary power required.
• surface condenser; high manufacturing
cost. high upkeeps, requires large floor space
and less auxiliary power required.
FLUIDISED BED COMBUSTION (FBC)
Burning of pulverised coal has some problems such as particle
size of coal used in pulverized firing is limited to 70-100
microns, the pulverised fuel fired furnances designed to burn a
particular can not 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
maincharacteristics. Fig. 4.41 shows basic principle of Fluidised
bed combustion (FBC) system.
FLUIDISED BED COMBUSTION (FBC)
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 disintegra-tion
and should have similar density as that of coal. Limestone, or
dolomite, fused alumina, sintered ash are commonly used as inert
materials.
FLUIDISED BED COMBUSTION (FBC)
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.
Soot blower
• The fuel used in thermal power plants causes soot and this is
deposited on the boiler tubes, economizer tubes, air pre heaters, etc.
• This drastically reduces the amount of heat transfer of the heat
exchangers. Soot blowers control the formation of soot and reduce
its corrosive effects.
• The types of soot blowers are fixed type, which may be further
classified into lane type and mass type depending upon the type of
spray and nozzle used.
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 superheater.
Feed Water Treatment
• Natural water supplies contain solid, liquid and gaseous
impurities.
• The various impurities present in the natural water (raw
water) may be in the following form:
(i) Dissolve salts such as carbonate, sulphates chlorides of
calcium, sodium and magnesium. Sometimes some iron,
aluminium or silica salts are also present.
(ii) Dissolve gases such as carbon dioxide, oxygen and SO2.
(iii) Mineral acids
(iv) Suspended matter such as alumina and silica may be
present as mud and salt.
Methods of Feed Water Treatment
• The impure water is chemically treated in different ways
depending upon the nature and concentration of
impurities. The different treatments adopted to remove
the various impurities:
(i) Mechanical Treatment
(ii) Thermal Treatment
(iii) Chemical Treatment
(iv) Demineralisation
(v) Blow down.
Mechanical equipment in Thermal
power station.
BOILER
SUPER HEATER
ECONOMISER
AIR PREHEATER
TURBINE
CONDENSER
Superheater
The superheater consists of a superheater header
and superheater elements. Steam from the main
steam pipe arrives at the saturated steam chamber
of the superheater header and is fed into the
superheater elements.
Superheated steam arrives back at the superheated
steam chamber of the superheater header and is fed
into the steam pipe to the cylinders. Superheated
steam is more expansive.
Advantages of superheated steam
• Capacity to do work is increased without increasing its
pressure.
• High temperature of super heated steam results in an increase
in thermal efficiency.
• Heat losses due to condensation of stem on cylinder walls are
avoided to a great extent.
• Does not produce corrosion effect on turbine.
Superheater
• It is a heating device.
• It is used to raise temp of steam at const pressure.
• It removes even last traces of moisture.
Classification of super heater
• Convection.
• Radiation.
• Combination of convection and radiation.
• Reheater
• The function of reheater is similar to the
superheater in that it serves to elevate the steam
temperature. Primary steam is supplied to the
high pressure turbine.
• After passing through the high pressure turbine,
the steam is returned to the steam generator for
reheating (in a reheater) after which it is sent to
the low pressure turbine. A second reheat cycle
may also be provided.
Condenser
• The use of a condenser in a power plant is to
improve the efficiency of the power plant by
decreasing the exhaust pressure of the steam below
atmosphere.
• Another advantage of the condenser is that the
steam condensed may be recovered to provide a
source of good pure feed water to the boiler and
reduce the water softening capacity to a
considerable extent. A condenser is one of the
essential components of a power plant.
• Functions of Condensers
• The main purposes of the condenser are to condense the
exhaust steam from the turbine for reuse in the cycle and to
maximize turbine efficiency by maintaining proper
vacuum.
• As the operating pressure of the condenser is lowered
(vacuum is increased), the enthalpy drop of the expanding
steam in the turbine will also increase. This will increase
the amount of available work from the turbine (electrical
output).
Cooling Tower
• The importance of the cooling tower is felt when
the cooling water from the condenser has to be
cooled.
• The cooling water after condensing the steam
becomes hot and it has to be cooled as it belongs
to a closed system. The Cooling towers do the job
of decreasing the temperature of the cooling water
after condensing the steam in the condenser.
Cooling Towers have one function :
• Remove heat from the water discharged from
the condenser so that the water can be
discharged to the river or re-circulated and
reused.
• A cooling tower extracts heat from water by
evaporation. In an evaporative cooling tower, a small
portion of the water being cooled is allowed to
evaporate into a moving air stream to provide
significant cooling to the rest of that water stream.
• Cooling Towers are commonly used to provide
lower than ambient water temperatures and are
more cost effective and energy efficient than
most other alternatives.
• The smallest cooling towers are structured for
only a few litres of water per minute while the
largest cooling towers may handle upwards of
thousands of litres per minute. The pipes are
obviously much larger to accommodate this
much water in the larger towers and can range up
to 12 inches in diameter.
Advantages of regenerative cycle
• Improve overall plant efficiency.
• Protect boiler corrosion.
• Avoid the thermal stresses due to cold water entering the
boiler .
• Increased the quantity of steam produced by boiler.
Function of economizer
To extract a part of heat from the fuel gas
coming out of the boiler.
• To use heat for heating feed water to the boiler.
• To increases the efficiency of boiler.
•
 The economizer is a feed water heater, deriving heat
from the flue gases. The justifiable cost of the
economizer depends on the total gain in efficiency. In
turn this depends on the flue gas temperature leaving
the boiler and the feed water inlet temperature.
Air Pre-heater
• The flue gases coming out of the economizer is used
to preheat the air before supplying it to the combustion
chamber. An increase in air temperature of 20 degrees
can be achieved by this method. The pre heated air is
used for combustion and also to dry the crushed coal
before pulverizing.
Advantages of mechanical handling
• Higher reliability.
• Less labour required.
• Operation is easy and smooth.
• Economical for large capacity plant.
• Losses in transport are minimised.
• Easily started.
Disadvantages of mechanical handling
• Need continuous maintenance and repair.
• Capital cost of plant is increased.
• Superheated steam then flows through the turbine. After
doing work in the turbine the pressure of steam is reduced.
Steam leaving the turbine passes through the condenser
which is maintained 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 superheater
tubes, flow through the dust collector and then through
economiser, air pre-heater and finally they are exhausted to
the atmosphere through the chimney.
Disadvantage of steam power plant
• Maintenance and operating cost are high.
• Long time required for erection and putting into action .
• Large quantity of water is required.
• Great difficulty experienced in coal handling .
• Efficiency decreases rapidly below about 75 percent load.
9-1
The Simple Ideal Rankine Cycle
© The McGraw-Hill Companies, Inc.,1998
How can We Increase the Efficiency of the
Rankine cycle?
• Rankine cycle efficiency can be increased by increasing
average temperature at which heat is transferred to the
working fluid in the boiler or decreasing the average
temperature at which heat is rejected from the working fluid
in the condenser. That is, the average fluid temperature
should be as high as possible during heat addition and as low
as possible during heat rejection.
The three ways by which efficiency of the Rankine cycle can
be increased are :
(a) Lowering the condenser pressure.
(b) Superheating the steam to high temperatures.
(c) Increasing the boiler pressure.
• The thermal efficiency of the Rankine cycle
can be increased by increasing the average
temperature at which heat is added to the
working fluid and/or by decreasing the
average temperature at which heat is
rejected to the cooling medium. The average
temperature during heat rejection can be
decreased by lowering the turbine exit
pressure.
• Consequently, the condenser pressure of most
vapor power plants is well below the
atmospheric
pressure.
The
average
temperature during heat addition can be
increased by raising the boiler pressure or by
superheating the fluid to high temperatures.
There is a limit to the degree of superheating,
however, since the fluid temperature is not
allowed to exceed a metallurgically safe value.
• Superheating has the added advantage of
decreasing the moisture content of the steam at
the turbine exit. Lowering the exhaust pressure
or raising the boiler pressure, however,
increases the moisture content. To take
advantage of the improved efficiencies at
higher boiler pressures and lower condenser
pressures, steam is usually reheated after
expanding partially in the high-pressure
turbine.
• This is done by extracting the steam after partial
extraction in the high-pressure turbine, sending it back to
the boiler where it is reheated at constant pressure, and
returning it to the low-pressure turbine for complete
expansion to the condenser pressure.
• The average temperature during the reheat
process, and thus the thermal efficiency of
the cycle, can be increased by increasing the
number of expansion and reheat stages. As
the number of stages is increased, the
expansion and reheat processes approach an
isothermal
process
at
maximum
temperature. Reheating also decreases the
moisture content at the turbine exit.
• Another way of increasing the thermal
efficiency of the Rankine cycle is by
regeneration. During a regeneration process,
liquid water (feed water) leaving the pump is
heated by some steam bled off the turbine at
some intermediate pressure in devices called
feed water heaters.
• The two streams are mixed in open feed
water heaters, and the mixture leaves as
a saturated liquid at the heater
pressure. In closed feed water heaters,
heat is transferred from the steam to
the feed water without mixing.
• The production of more than one useful form
of energy (such as process heat and electric
power) from the same energy source is called
cogeneration. Cogeneration plants produce
electric power while meeting the process heat
requirements of certain industrial processes.
This way, more of the energy transferred to
the fluid in the boiler is utilized for a useful
purpose. The faction of energy that is used for
either process heat or power generation is
called the utilization factor of the cogeneration
plant.
• The overall thermal efficiency of a power plant
can be increased by using binary cycles or
combined cycles. A binary cycle is composed of
two separate cycles, one at high temperatures
(topping cycle) and the other at relatively low
temperatures.
• The most common combined cycle is the gassteam combined cycle where a gas-turbine
cycle operates at the high-temperature range
and a steam-turbine cycle at the lowtemperature range. Steam is heated by the
high-temperature exhaust gases leaving the
gas turbine. Combined cycles have a higher
thermal efficiency than the steam- or gasturbine cycles operating alone.
Turbine performance can be expressed by the following factors :
(i) The steam flow process through the unit-expansion line or
condition curve.
(ii) The steam flow rate through the unit.
(iii) Thermal efficiency.
(iv) Losses such as exhaust, mechanical, generator, radiation etc.
The steam temperature may try to fluctuate because of the following reasons :
(i) Variation in heat produced due to varying amounts of fuel burnt according to
changing loads.
(ii) Fluctuation in quantity of excess air.
(iii) Variation in moisture content and temperature of air entering the furnace.
(iv) Variation in temperature of feed water.
(v) The varying condition of cleanliness of heat absorbing surface.
The efficiency of steam turbines can be increased:
(i) By using super heated steam.
(ii) Use of bled steam reduces the heat rejected to the condenser and this increases
the turbine efficiency.
Steam turbine tests are made for the following:
(i) Power
(ii) Valve setting
(iii) Speed regulation
(iv) Over speed trip setting
(v) Running balance.
Thermal efficiency of steam turbine depends on the following
factors:
(i) Steam pressure and temperature at throttle valve of turbine.
(ii) Exhaust steam pressure and temperature.
(iii) Number of bleedings.
Lubricating oil should be changed or cleaned after 4 to 6 months.
Steam turbine specifications consist of the following:
(i) Turbine rating. It includes :
(a) Turbine kilowatts
(b) Generator kilovolt amperes
(c) Generator Voltage
(d) Phases
(e) Frequency
(f) Power factor
(g) Excitor characteristics.
(ii) Steam conditions. It includes the following:
(a) Initial steam pressure, and Temperature
(b) Reheat pressure and temperature
(c) Exhaust pressure.
(iii) Steam extraction arrangement such as automatic or nonautomatic extraction.
(iv) Accessories such as stop and throttle valve, tachometer etc.
(v) Governing arrangement.
Selection of plant site
• The selection of plant site for thermal power plant
compared with hydro-power plant is more difficult as it
involves number of factors to be considered for its
economic justification.
• A few important factors to be considered for the selection
of thermal power plants.
Selection of plant site
• AVAILABILITY OF COAL.
• Huge quantity of coal is required for
large thermal plants.
• ASH DISPOSAL FACILITIES.
• SPACE REQUIREMENT.
• NATURE OF LAND.
• AVAILABILITY OF WATER.
Selection of plant site
• TRANSPORT FACILITYIES.
• AVAILABILITY OF LABOUR.
• PUBLIC PROBLEMS.
• SIZE OF THE PLANT.
ABOUT ELECTROSTATIC
PRECIPITATOR
Nowadays, the environment protection has
become a crucial problem and the authorities
are requested to set increasingly more stringent
limits, one of which is the emissions from the
industrial plants of solid particulate and other
gaseous pollutants.
ABOUT ELECTROSTATIC PRECIPITATOR
What is ESP
Electrostatic precipitator (ESP) is a widely
used device in so many different domains
to remove the pollutant particulates,
especially in industrial plants.
HOW ESP WORKS
Main process of ESP
Generally,
the
processes
of
electrostatic precipitator are known as
three main stages: particle charging,
transport and collection.
Schematic of wire-plate ESP
Schematic of wire-plate electrostatic
precipitator
Mechanism of ESP
Mechanism of electrostatic precipitator
PROCESS OF Particle charging
Particle charging is the first and
foremost beginning in processes.
As the voltage applied on precipitator
reach threshold value, the space inside
divided into ionization region and drift
region.
The electric field magnitude around the
negative electrode is so strong that the
electrons escape from molecule.
Under the influence of electric field, the positive
ions move towards the corona, while the
negative ions and electrons towards the
collecting plates.
Particle transport
In the moving way, under the influence of
electric field, negative ions cohere and charge the
particles, make the particles be forced towards
collecting-plate.
Particle collection
As soon as the particles reach the plate,
they will be neutralized and packed by
the succeeded ones subsequently. The
continuous process happens, as a result,
particles are collected on the collecting
plate.
Main pollutants from a power
system
• Non –toxic dust
• Sulphurous anhydride
• Carbon monoxide
• Nitrogen dioxide
• Soot (fly ash)
• Hydrogen sulphide
• Pollution can be define as the contamination of soil, air
and water with undesirable amount of material and heat.
• Acid rain; the rain which contain acid as its
constituents, brings all the acid down from high
above the environment.
• Contaminant; it is the another name of
pollution. It is undesirable substances which
may be physical, chemical or biological.
•
• Pollutant; these are undesirable substances
present in the environment these can be NO2,
SO2, CO2,smoke,salt, bacteria.
Bad effects of thermal pollution
• Lot of heat is injected into biosphere from thermal power
plant, through exhaust gases and waste water. The major
problem is the effect of discharge of large quantity of heated
wasted water into natural water basins. Hot water raises the
temperature and disturbs the natural ecological balance
Advantages of combined operation of plants
• Greater reliability of supply to the consumers.
• Avoid complete shut down.
• The overall cost of energy per unit of an
interconnected system is less.
• There is a more effective use of transmission line
facilities.
• Less capital investment required.
• Less expenses on supervision, operation and
maintenance.
• Due to limited generating capacity diesel power stations is
not suitable for base load plants.
• Nuclear power stations is not suitable for peak load
plants.
• Incremental rate curve shows that as output power
increases, cost of plant also increases.