Pulverized Fuel Burning System and its Components:

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 pulverizing mills.

Fig-1.21- Components of pulversied fuel burning

The various pulverizing mills used are as follows:

(i) Ball mill

(ii) Hammer mill
(iii) Ball and race mill
(iv) Bowl mill.

The essential functions of pulverizing 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 pulverizing mills reduce coal to powder form by three actions as follows:

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(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. 1.22. 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.

Fig-1.22-Ball Mill

As the drum rotates the coal gets pulverized due to the combined impact between coal and steel
balls. Hot air is introduced into the drum. The powdered coal is picked up by the air and the coal
air mixture enters the classifiers, where sharp changes in the direction of the mixture throw out
the oversized coal particles. The over-sized particles are returned to the drum. The coal air
mixture from the classifier moves to the exhauster fan and then it is supplied to the burners.

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BALL AND RACE MILL

Fig. 1.23 shows a ball and race mill. In this mill the coal passes between the rotating elements
again and again until it has been pulverized to desired degree of fineness. The coal is crushed
between two moving surfaces namely balls and races. The upper stationary race and lower
rotating race driven by a worm and gear hold the balls between them. The raw coal supplied falls
on the inner side of the races. The moving balls and races catch coal between them to crush it to
a powder. The necessary force needed for crushing is applied with the help of springs. The hot
air supplied picks up the coal dust as it flows between the balls and races, and then enters the
classifier. Where oversized coal particles are returned for further grinding, where as the coal
particles of required size are discharged from the top of classifier.

Fig-1.23-Ball and Race Mill

In this mill coal is pulverized by a combination of' crushing, impact and attrition between the grinding
surfaces. The advantages of this mill are as follows:
(i) Lower capital cost (ii) Lower power consumption
(iii) Lower space required (iv) Lower weight.

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However in this mill there is greater wear as compared to other pulverizes. The use of pulverized
coal has now become the standard method of firing in the large boilers. The pulverized coal
burns with some advantages that result in economic and flexible operation of steam boilers.
Preparation of pulverized fuel with an intermediate bunker is shown in Fig. 1.23. The fuel moves
to the automatic balance and then to the feeder and ball mill through which hot air is blown. It
dries the pulverized coal and carries it from the mill to separator.

The air fed to the ball mill is heated in the air heater. In the separator dust (fine pulverized coal)
is separated from large coal particles which are returned to the ball mill for regrinding. The dust
moves to the cyclone. Most of the dust (about 90%) from cyclone moves to bunker. The
remaining dust is mixed with air and fed to the burner.

IMPACT/HAMMER MILL:

In impact mill coal passes through coal feeder and pulverization takes place due to impact.

Fig-1.24-Impact mill

The coal in pulverizer remains in suspension during the entire pulverizing process. All the
grinding elements and the primary air fan are mounted on a single shaft as shown in figure. The
primary air fan induces flow of air through the pulverizer which carries the coal to the primary
stage of grinding where the coal is reduced to fine granular state by impact with a series
hammers and then into final stage where pulverization is completed by attririon. The final stage
of grinding consists of pegs carried on a rotating disc and travelling between stationary pegs.

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BOWL MILL:

This pulverizer consists of stationary rollers and a power driven bowl in which pulverization
takes place as the coal passes between the sides of the bowl and the rollers.

Fig-1.25-Bowl mill

Advantages:

The advantages of using pulverized coal are as follows:

1. It becomes easy to burn wide variety of coal. Low grade coal can be burnt easily.
2. 2. Powdered coal has more heating surface area. They permits rapids and high rates of
combustion.
3. Pulverized coal firing requires low percentage of excess air.
4. By using pulverized coal, rate of combustion can be adjusted easily to meet the varying load.
5. The system is free from clinker troubles.
6. It can utilize highly preheated air (of the order of 700°F) successfully which promotes rapid
flame propagation.
7. As the fuel pulverising equipment is located outside the furnace, therefore it can be repaired
without cooling the unit down.
8. High temperature can be produced in furnace.

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Disadvantages

1. It requires additional equipment to pulverize the coal. The initial and maintenance cost of the
equipment is high.
2. Pulverized coal firing produces fly ash (fine dust) which requires a separate fly ash removal
equipment.
3. The furnace for this type of firing has to be carefully designed to withstand for burning the
pulverized fuel because combustion takes place while the fuel is in suspension.
4. The flame temperatures are high and conventional types of refractory lined furnaces are
inadequate. It is desirable to provide water cooled walls for the safety of the furnaces.
5. There are more chances of explosion as coal burns like a gas.
6. Pulverized fuel fired furnaces designed to burn a particular type of coal cannot be used to any
other type of coal with same efficiency.
7. The size of coal is limited. The particle size of coal used in pulverized coal furnace is limited to
70 to 100 microns.

Pulverized coal firing is done by two systems:

(i) Unit System or Direct System.

(ii) Bin or Central System.

Unit System. In this system (Fig. 1.26) 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
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. The unit
system is so called from the fact that each burner or a burner group and pulveriser constitute a unit.

Fig-1.26- Unit System

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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. 1.27. 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 pulverizing mill. The
pulverized coal obtained is transferred to the pulverized 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.

Fig-1.27-Central System
Advantages:

1. 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 effect 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.

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To a large extent the performance of pulverised fuel system depends upon the mill performance.
The pulverised mill should satisfy the following requirements:

 It should deliver the rated tonnage of coal.

 Pulverized coal produced by it should be of satisfactory fineness over a wide range of
capacities.
 It should be quiet in operation.
 Its power consumption should be low.
 Maintenance cost of the mill should be low.

CYCLONE FURNACE:

Cyclone-furnace firing, developed in the 1940s, represents the most significant step in coal firing
since the introduction of pulverized-coal firing in the 1920s. It is now widely used to burn poorer
grades of coal that contain a high ash content with a minimum of 6 percent to as high as 25
percent, and a high volatile matter, more than 15 percent, to obtain the necessary high rates of
combustion. A wide range of moisture is allowable with pre-drying. One limitation is that ash
should not contain a high sulfur content or a high Fe2O3; (CaO + MgO) ratio. Such a coal has a
tendency to form high ash-fusion temperature materials such as iron and iron sulfide in the slag,
which negates the main advantage of cyclone firing.

The main advantage is the removal of much of the ash, about 60 percent, ao molten slag that is
collected on the cyclone walls by centrifugal action and drained off the bottom to a slag-
disintegrating tank below. Thus only 40 percent ash leave, with the flue gases, compared with
about 80 percent for pulverized-coal firing. this materially reduces erosion and fouling of steam-
generator surfaces as well as the size of dust-removal precipitators or bag houses at steam-
generator exit. Other advantages are that only crushed coal is used and no pulverization
equipment is needed and that the boiler size is reduced.Cyclone-furnace firing uses a range of
coal sizes averaging 95 percent passing a 4-mesh screen.

The disadvantages are higher forced-draft fan pressures and therefore higher power
requirements, the inability to use the coals mentioned above, and the formation of relatively
more oxides of nitrogen, NO2 which are air pollutants, in the combustion process. The cyclone is
essentially a water-cooled horizontal cylinder (Fig.1.28) located outside the main boiler furnace,
in which the crushed coal is fed and fired with very high rates of heat release. Combustion of the
coal is completed before the resulting hot gases enter the boiler furnace.

The crushed coal is fed into the cyclone burner at left along with primary air, which is about 20
percent of combustion or secondary air. The primary air enters the burner tangentially, thus
imparting a centrifugal motion to the coal. The secondary air is also admitted tangentially at the
top of the cyclone at high speed, imparting further centrifugal motion. A small quantity of air,
called tertiary air, is admitted at the center.

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Fig-1.28-Cyclone Furnace

The whirling motion of air and coal results in large heat-release-rate volumetric densities,
between 450,000 and 800,000 Btu/(h.ft) (about 4700 to 8300 kW/m3), and high combustion
temperatures, more than 3000°F (1650°C). These high temperatures melt the ash into a liquid
slag that covers the surface of the cyclone and eventually drains through the slag-tap opening to a
slag tank at the bottom of the boiler furnace, where it is solidified and broken for removal. The
slag layer that forms on the walls of the cyclone provides insulation against too much heat loss
through the walls and contributes to the effi ciency of cyclone firing. The high temperatures also
explain the large production of NO, in the gaseous combustion products. These gases leave the
cyclone through the throat at right and enter the main boiler furnace. Thus combustion takes
place in the relatively small cyclone, and the main boiler furnace has the sole function of heat
transfer from the gases to the water-tube walls. Cyclone furnaces are also suitable for fuel-oil and
gaseous-fuel firing. Initial ignition is done by small retractable oil or gas burners in the
secondary air ports.

Like pulverized-coal systems, cyclone firing systems can be of the bin, or storage. or direct-
firing types, though the bin type is more widely used, especially for most bituminous coals, than
in the case of pulverized coal. The cyclone system uses either one-wall, or opposed-wall, firing,
the latter being preferred for large steam generators The size and number of cyclones per boiler
depend upon the boiler size and the desired load response because the usual load range for good
performance of any one cyclone is from 50 to 100 percent of its rated capacity. Cyclones vary in
size from 6 to 10 fv in diameter with heat inputs between 160 to 425 million Btu/h (about 47,000
to 125,Oa_ kW), respectively .

The cyclone component requiring the most maintenance is the burner, which is subjected to
erosion by the high velocity of the coal. Erosion is minimized by the us; of tungsten carbide and
other erosion-resistant materials for the burner liners, which are usually replaced once a year or
so.

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DUST COLLECTORS:

A dust collector is a system used to enhance the quality of air released from industrial and
commercial processes by collecting dust and other impurities from air or gas. Designed to handle high-
volume dust loads, a dust collector system consists of a blower, dust filter, a filter-cleaning system,
and a dust receptacle or dust removal system. It is distinguished from air cleaners, which use
disposable filters to remove dust.

Five main types of industrial dust collectors are:

 Inertial separators
 Fabric filters
 Wet scrubbers
 Unit collectors
 Electrostatic precipitators

Inertial separators separate dust from gas streams using a combination of forces, such as
centrifugal, gravitational, and inertial. These forces move the dust to an area where the forces
exerted by the gas stream are minimal. The separated dust is moved by gravity into a hopper,
where it is temporarily stored.
The three primary types of inertial separators are:
 Settling chambers
 Baffle chambers
 Centrifugal collectors
Neither settling chambers nor baffle chambers are commonly used in the minerals processing
industry. However, their principles of operation are often incorporated into the design of more
efficient dust collectors.

Fabric filters Commonly known as baghouses, fabric collectors use filtration to separate dust
particulates from dusty gases. They are one of the most efficient and cost effective types of dust
collectors available and can achieve a collection efficiency of more than 99% for very fine
particulates.

Fig-1.29- Baghouse

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Dust-laden gases enter the baghouse and pass through fabric bags that act as filters. The bags can
be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope shape.

WET SCRUBBERS:

Dust collectors that use liquid are known as wet scrubbers. In these systems, the scrubbing liquid
(usually water) comes into contact with a gas stream containing dust particles. Greater contact of
the gas and liquid streams yields higher dust removal efficiency.

There is a large variety of wet scrubbers; however, all have one of three basic configurations:

1. Gas-humidification - The gas-humidification process agglomerates fine particles, increasing

the bulk, making collection easier.
2. Gas-liquid contact - This is one of the most important factors affecting collection efficiency.
The particle and droplet come into contact by four primary mechanisms:
a) Inertial impaction - When water droplets placed in the path of a dust-laden gas stream, the
stream separates and flows around them. Due to inertia, the larger dust particles will
continue on in a straight path, hit the droplets, and become encapsulated.
b) Interception - Finer particles moving within a gas stream do not hit droplets directly but brush
against and adhere to them.
c) Diffusion - When liquid droplets are scattered among dust particles, the particles are
deposited on the droplet surfaces by Brownian movement, or diffusion. This is the
principal mechanism in the collection of submicrometre dust particles.
d) Condensation nucleation - If a gas passing through a scrubber is cooled below the dewpoint,
condensation of moisture occurs on the dust particles. This increase in particle size makes
collection easier.

3. Gas-liquid separation - Regardless of the contact mechanism used, as much liquid and dust as
possible must be removed. Once contact is made, dust particulates and water droplets combine to
form agglomerates. As the agglomerates grow larger, they settle into a collector.

The "cleaned" gases are normally passed through a mist eliminator (demister pads) to remove
water droplets from the gas stream. The dirty water from the scrubber system is either cleaned
and discharged or recycled to the scrubber. Dust is removed from the scrubber in a clarification
unit or a drag chain tank. In both systems solid material settles on the bottom of the tank. A drag
chain system removes the sludge and deposits in into a dumpster or stockpile.

UNIT COLLECTORS:

Unlike central collectors, unit collectors control contamination at its source. They are small and self-
contained, consisting of a fan and some form of dust collector. They are suitable for isolated, portable,
or frequently moved dust-producing operations, such as bins and silos or remote belt- conveyor
transfer points. Advantages of unit collectors include small space requirements, the

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return of collected dust to main material flow, and low initial cost. However, their dust-holding
and storage capacities, servicing facilities, and maintenance periods have been sacrificed.

A number of designs are available, with capacities ranging from 200 to 2,000 ft³/min (90 to
900 L/s). There are two main types of unit collectors:
 Fabric collectors, with manual shaking or pulse-jet cleaning - normally used for fine dust
 Cyclone collectors - normally used for coarse dust
Fabric collectors are frequently used in minerals processing operations because they provide high
collection efficiency and uninterrupted exhaust airflow between cleaning cycles. Cyclone
collectors are used when coarser dust is generated, as in woodworking, metal grinding, or
machining.
The following points should be considered when selecting a unit collector:
 Cleaning efficiency must comply with all applicable regulations.
 The unit maintains its rated capacity while accumulating large amounts of dust between
cleanings.
 Simple cleaning operations do not increase the surrounding dust concentration.
 Has the ability to operate unattended for extended periods of time (for example, 8 hours).
 Automatic discharge or sufficient dust storage space to hold at least one week's
accumulation.
 If renewable filters are used, they should not have to be replaced more than once a
month. Durable.
 Quiet.
Use of unit collectors may not be appropriate if the dust-producing operations are located in an area
where central exhaust systems would be practical. Dust removal and servicing requirements are
expensive for many unit collectors and are more likely to be neglected than those for a single, large
collector.

Electrostatic precipitators use electrostatic forces to separate dust particles from exhaust gases.
A number of high-voltage, direct-current discharge electrodes are placed between grounded
collecting electrodes. The contaminated gases flow through the passage formed by the discharge and
collecting electrodes. Electrostatic precipitators operate on the same principle as home "Ionic" air
purifiers.

The airborne particles receive a negative charge as they pass through the ionized field between
the electrodes. These charged particles are then attracted to a grounded or positively charged electrode
and adhere to it.

The collected material on the electrodes is removed by rapping or vibrating the collecting
electrodes either continuously or at a predetermined interval. Cleaning a precipitator can usually
be done without interrupting the airflow.
The four main components of all electrostatic precipitators are:
 Power supply unit, to provide high-voltage DC power
 Ionizing section, to impart a charge to particulates in the gas stream A means of removing the
collected particulates
 A housing to enclose the precipitator zone

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The following factors affect the efficiency of electrostatic precipitators:
 Larger collection-surface areas and lower gas-flow rates increase efficiency because of
the increased time available for electrical activity to treat the dust particles.
 An increase in the dust-particle migration velocity to the collecting electrodes increases
efficiency. The migration velocity can be increased by:
o Decreasing the gas viscosity
o Increasing the gas temperature
o Increasing the voltage field

COOLING TOWERS AND HEAT REJECTION:

Cooling towers are a very important part of many chemical plants. The primary task of a cooling tower
is to reject heat into the atmosphere.

They represent a relatively inexpensive anddependable means of removing low-grade heat from
cooling water. The make-up water sourceis used to replenish water lost to evaporation. Hot water
from heat exchangers is sent to thecooling tower. The water exits the cooling tower and is sent
back to the exchangers or to other units for further cooling. Typical closed loop cooling tower
system is shown in Figure 1.30.

Fig-1.30-Cooling system

The amount of heat that can be rejected from the water to the air is directly tied to the relative
humidity of the air. Air with a lower relative humidity has a greater ability to absorb water
through evaporation than air with a higher relative humidity, simply because there is less water
in the air. As an example, consider cooling towers in two different locations– one in Atlanta,
Georgia, and another in Albuquerque, New Mexico. The ambient air temperature at these two
locations may be similar, but the relative humidity in Albuquerque on average will be much
lower than that of Atlanta‘s. Therefore, the cooling tower in Albuquerque wil be able to extract
more process or building heat and will run at a cooler temperature because the dry desert air has
a greater capacity to absorb the warm water.

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Cooling towers can be split into two distinct categories: open circuit (direct contact) and closed
circuit (indirect) systems.In open circuit systems the recirculating water returns to the tower
after gathering heat and is distributed across the tower where the water is in direct contact with
the atmosphere as it recirculates across the tower structure. Closed circuit systems differ in that
the return fluid (often water, or sometimes water mixed with glycol) circulates through the
tower structure in a coil, while cooling tower water recirculates only in the tower structure itself
(see Figure 1). In this case, the return fluid is not exposed directly to the air.

 The cooling towers types: -

1. Natural Draft cooling tower

2. Mechanical Draught

Natural Draft Towers

Natural draft towers are designed to move air up through the structure naturally without the use
of fans. They use the natural law of differing densities between the ambient air and warm air in
the tower. The warm air will rise within the chimney structure because of its lower density
drawing cool ambient air in the bottom portion. Often times these towers are very tall to induce
adequate air flow, and have a unique shape giving them the name ―hyperbolic‖ towers

Mechanical draught Towers — Uses power-driven fan motors to force or draw air
through the tower.

 Induced draught — A mechanical draft tower with a fan at the discharge (at the top)
which pulls air up through the tower. The fan induces hot moist air out the discharge.
This produces low entering and high exiting air velocities, reducing the possibility of
recirculation in which discharged air flows back into the air intake. This fan/fin
arrangement is also known as draw-through.

 Forced draught — A mechanical draft tower with a blower type fan at the intake.
The fan forces air into the tower, creating high entering and low exiting air velocities.
The low exiting velocity is much more susceptible to recirculation. With the fan on the
air intake, the fan is more susceptible to complications due to freezing conditions.

Another disadvantage is that a forced draft design typically requires more motor horsepower than
an equivalent induced draft design. The benefit of the forced draft design is its ability to work
with high static pressure. Such setups can be installed in more-confined spaces and even in some
indoor situations. This fan/fill geometry is also known as blow-through.

 Fan assisted natural draught — A hybrid type that appears like a natural draft
setup, though airflow is assisted by a fan.

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