3rd Class Power Engineering
Section 3 - Module 7
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Boiler Draft and Flue Gas
Equipment
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This learning module is part of curriculum approved by
the ‘Standardization of Power Engineering Examinations
Committee’ and supports the following section of their
3rd Class Syllabus: Part B Section 3
Printing Date: October 2003
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Boiler Draft and Flue Gas Equipment
Section 3 – Module 7
Learning Outcome
When you complete this learning material you will be able to:
Explain boiler draft systems and fans and describe the equipment used to remove ash from flue
gas
Learning Objectives
You will specifically be able to complete the following tasks:
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1. Define and explain the applications and designs of natural, forced, induced and
balanced draft.
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2. Explain how draft is measured, monitored, and controlled in a large, balanced draft
boiler. Explain the position of control dampers.
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3. Describe typical draft fan designs, single and double inlet arrangements, and explain
methods used to control fan output.
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4. Explain the start-up and running checks that must be made on draft fans.
5. Describe typical windbox and air louver arrangements and distinguish between primary
and secondary air.
6. Describe the design and operation of flue gas particulate clean-up equipment, including
mechanical and electrostatic precipitators and baghouse filters.
7. Describe the design and operation of ash handling systems, including hydro and air
systems, bottom ash systems, and scraper conveyor systems.
8. Describe the designs and operation of SO2 recovery systems, including lime and wet gas
scrubbing.
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OBJECTIVE 1
Define and explain the applications and designs of natural, forced, induced and balanced
draft.
DRAFT
Draft is defined as the difference between atmospheric pressure and the static pressure of
combustion gases in a furnace, gas passage, flue or stack.
Draft Applications
Draft is classified into natural and mechanical draft. A stack of sufficient height to cause the
necessary pressure differential creates natural draft with the resulting air and flue gas flows.
Combustion gases are created when a fuel is burned in a boiler or a furnace. These combustion
gases are often called “flue gases” as they are dispelled via the flues, which are the passes or
ducts that connect the boiler with the stack.
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Mechanical draft is partially created by the use of mechanical fans. They may push the air and
combustion gases through the boiler, in which case they are called forced draft fans (F.D. fans).
They may also pull the air and gases through the boiler, in which case they are called induced
draft fans (I.D. fans). When furnace draft is maintained at atmospheric pressure (or just
below), by use of a combination of forced and induced draft fans, the draft is referred to as a
balanced draft system. Fig. 1 illustrates these four systems.
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Figure 1
Types of Draft Arrangements
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Natural Draft
Natural draft is the difference in pressure created solely by the stack, or chimney. In effect the
stack, boiler and the outer air constitute a large U-tube. The column of gases in the stack is one
vertical limb; the other limb is a column of cold air. The lower horizontal portion of the U-tube,
as shown in Fig. 2, represents the boiler with the interconnecting flues.
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Figure 2
Natural Draft
The draft will depend on:
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The temperature of the outside air, as colder more dense air will increase the draft
The temperatures of the hotter gases in the stack increase the draft
The stack height produces more draft
The outside air temperature cannot be controlled, therefore, only the gas temperatures in the
stack or the stack height are factors to be considered in the provision of draft. A higher stack
temperature means a greater loss of heat, and lower unit efficiency, while a high stack means
increased capital cost.
Forced Draft
One or more fans, creates mechanical draft. The fans are driven by a steam turbine or an
electric motor. A common type of the forced draft system is shown in Fig. 3, in which the fan
delivers air along an air duct to an enclosed furnace front. When using a forced draft system
the entire furnace casing is under a positive pressure. To prevent the escape of gases the
furnace and all furnace openings must be carefully sealed against outward leakage. The casing
must be strong enough to withstand the internal pressure.
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Figure 3
Forced Draft Boiler
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Induced Draft
Another method of producing mechanical draft is the induced draft system. This consists of a
fan installed in the flue gas duct between the boiler and the stack. This fan pulls the gases
through the boiler and pushes them up the stack. A pressure slightly lower than atmospheric is
created in the boiler. It is important that the boiler casing and openings are sealed to prevent air
leaking into the boiler, which would rapidly lower the capacity and efficiency of the fans. The
boiler casings must be made strong enough to withstand the external pressure of the
atmosphere.
The induced draft fan which is required to provide the same volume of air, is larger than a
forced draft fan, due to the following:
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The I.D. fan must move a larger mass because the flue gases consist of the mass of fuel
as well as the mass of air (one kg of fuel that uses 15 kg of air, for complete
combustion, produces 16 kg of gases)
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The I.D. fan must be able to handle any air leakage into the boiler setting
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The I.D. fan must deal with a greater volume of gases since the temperature of the flue
gas is higher than the air moved by an F.D. fan
Balanced Draft
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An I.D. fan is not commonly used as the only mechanical means to provide draft. A
combination of forced draft and induced draft provides the more common balanced draft
system. A typical balanced draft system is shown in Fig. 1. In this system the furnace pressure
is maintained at, or slightly below atmospheric pressure.
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In a balanced draft system, sealing and strength of the furnace casing are important but not to
the same extent as in the forced and induced draft systems.
Stacks seldom provide sufficient draft to meet the requirements of modern boiler units.
For example, a 60 m stack provides a theoretical natural draft of 1 cm water gauge at the
bottom of the stack. The resistance to air and gas flow in modern units with superheaters,
economizers and air heaters, may require 50 cm water gauge, or more. These higher draft
systems require the use of fans and simple draft calculations cannot be made. In these cases the
requirements to move air or gases are expressed in terms of fan kilowatt. Fan requirements are
calculated and margins of safety are added to cover all conditions encountered during
operation. Various fans may be able to fulfill certain boiler requirements but, from fan
characteristic and capacity curves, the fan that most economically can do the job, is usually
chosen.
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OBJECTIVE 2
Explain how draft is measured, monitored, and controlled in a large, balanced draft boiler.
Explain the position of control dampers.
DRAFT
Draft is measured in centimeters of water gauge by an instrument called a draft gauge. Ten cm
of water gauge is approximately equal to a pressure of 1 kPa. Referring to Fig. 4, the normal
places of measurement are in the:
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•
•
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Wind box
Furnace
Economizer inlet
Air heater inlet
Stack
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Figure 4
Steam Generator with Balanced Draft Arrangement
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Instruments
The instruments used to monitor the air and flue gas system in a boiler consist of the types that
measure flow, temperature and pressure. Flow measurement is obtained by measuring the
pressure drop across a section of ductwork or across the fan. This pressure drop is calibrated to
indicate the airflow to the boiler. Temperature measurement is accomplished by use of
conventional thermometers and thermocouples.
Manometer
The measurement of draft involves the measurement of very small pressures in which a normal
pressure gauge cannot be used. A simple gauge for measuring draft consists of a glass U-tube
containing water. This type of gauge, as shown in Fig. 5 is known as a manometer. In the
illustration, the furnace pressure is less than atmospheric. The atmospheric pressure pushes the
water so that the level in the U-tube leg connected to the furnace is higher than the level in the
leg open to atmosphere. The difference in the levels is approximately 1.5 cm. The furnace
pressure or draft is -1.5 cm wg.
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Figure 5
Manometer
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Diaphragm Draft Gauge
A type of gauge that has superseded the U-tube for measuring boiler draft is the diaphragm
gauge, illustrated in Fig. 6.
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Figure 6
Diaphragm Draft Gauge
(Courtesy of Bailey Meter Co. Ltd)
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The diaphragm, which is connected by a push rod, range spring, and linkage to a pointer, is
open to the atmosphere at the top and subjected to the draft, being measured at the bottom.
Changes in the draft will cause the diaphragm to move and produce a proportional movement
of the pointer, which indicates the value of the draft on a scale calibrated in cm of water. Draft
indicators are often combined in one casing and displayed on the boiler control panel, as shown
in Fig. 7.
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Figure 7
Typical Draft Gauge
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Balanced Draft Control
The schematic arrangement as illustrated in Fig. 8, shows the location of the equipment that is
used to control the draft in the boiler.
The FD (forced draft) fan that controls the volume of combustion air to the boiler is equipped
with dampers at the inlet of the fan. The damper drive unit controls the dampers. The
combustion air then passes to the air heater where the hot flue gases exiting from the boiler,
preheat it. The volume of combustion air entering the boiler is controlled through the use of
primary and secondary air louvers, which are located at the burners.
The ID (induced draft) fan is used to remove the hot flue gases from the boiler. It is equipped
with dampers at the inlet of the fan. A damper drive unit controls the opening of these
dampers. The ID fan discharges into the stack and then to the atmosphere. Both of the damper
drives are adjusted to maintain a slightly negative pressure in the firebox of the boiler.
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Figure 8
Draft Equipment
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OBJECTIVE 3
Describe typical draft fan designs, single and double inlet arrangements, and explain
methods used to control fan output.
DRAFT FANS
Most fans are of the centrifugal type. These employ blades mounted on an impeller, rotating
within a spiral or volute housing. The fan gives sufficient energy to the air or gas to initiate
motion and overcome all friction. The blades or rotor do the actual work, while the housing
collects and directs the air or gas discharged by the impeller. Fig. 9 illustrates a type of fan that
is equipped with backward curved blades and vane-controlled inlet, suitable for forced draft
(FD) service.
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Figure 9
Forced Draft Fan
Fig. 10 illustrates a type of fan that is equipped with radial tip blades and double inlet, suitable
for induced draft (ID) service.
Figure 10
Induced Draft Fan
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Fan Components
Fans are made up of many parts. Some typical fan components are shown in Fig. 11.
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Figure 11
Fan Terminology
1.
2.
3.
4.
Blade
Shroud
Hub
Shaft
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5. Back Plate
6. Intermediate Shroud
7. Inductor Vane
8. Shroud Stiffener (Inner)
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9. Housing
10. Shroud Stiffener (Outer)
11. Inlet Cone
Induced and forced draft fans are most commonly divided according to the six types of blades
used in these fans, as illustrated in Fig. 12. The “airfoil” fan is primarily used for moving clean
air or non-corrosive gases. It is expensive, but is also the most efficient design available and is
sometimes used as an F.D. fan. A fan with backward curved blades is most commonly used for
F.D. service. It is used for moving clean air or gases, and runs at a high speed. Induced draft
fans normally use straight radial, radial tip or forward curved blades, which run at lower speeds
and are better suited to move dust laden and hot flue gases.
Figure 12
Types of Fan Blades
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The fan inlet may either be single or double, as shown in Figs. 13 and 14. The double inlet
design balances the pressure on both sides of the fan and, therefore, offers a relatively thrust
free operation. The single inlet fan, inherently, creates a thrust load on the bearings because the
pressure is unbalanced across the backplate. Inductor vanes, on the backside of the backplate
on a single inlet fan, are often incorporated to help minimize this thrust.
Figure 13
Single Inlet Fan
Fan Output Control
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Figure 14
Double Inlet Fan
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Fan output is usually controlled by one of the following methods:
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Inlet damper control
Outlet damper control
Variable speed control
Inlet Damper Control
Inlet damper control may be achieved by using inlet vanes, as fitted to the FD fan in Fig. 9.
The multi-bladed ID Fan, as shown in Fig.15, achieves this same control through the use of the
double inlet and ouver-damper arrangement. This fan is ideal for the handling of hot air, or
gases.
One advantage of inlet vanes is that the air entering the fan receives a spin in the direction of
wheel rotation. This results in a lower power requirement, especially at low loads and,
therefore, a more efficient operation at partial loads.
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Figure 15
ID Fan
Outlet Damper Control
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Outlet dampers throttle the airflow from the fan. The resistance produced by the damper,
causes the fan to operate at a higher pressure than needed. This requires more power to run at
partial loads than should be necessary and a less efficient operation is the result. Of the
controls used, outlet dampers have the lowest first cost and are simple to operate, but require
the largest power input to the fan.
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Variable Speed Control
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Variable speed is another way to control fan output. Common methods include variable speed
motors, magnetic couplings, fluid drive units and steam turbines. The latter is frequently used
in power plants to achieve a more efficient heat balance.
Variable speed control is the most efficient method used to control fan output as far as power
consumption is concerned, but original installation costs are usually the highest. It is often
used in combination with damper control.
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OBJECTIVE 4
Explain the start-up and running checks that must be made on draft fans.
FAN STARTUP CHECKS
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Ensure all debris, tools, rags, etc., are removed from the immediate fan and driver area.
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Give fan and driver a visual check; look for loose nuts, plates or bolts, disconnected
rods, etc.
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Ensure that the fan inlet screen is clean and clear of frost, rags or debris.
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Check the dampers and damper drive connections for tightness.
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Check the oil in the fan and driver bearings. If the level is low, fill it to the correct level
with recommended oil. If the oil is dirty or polluted with water, change it to the fan
manufacturer’s recommended oil.
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Open cooling water to the water- cooled bearings.
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Generally look for any condition which might impair the proper operation of the unit.
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If the drive unit is a steam turbine or engine, open drains and slightly open the steam
supply and exhaust valves to warm up the unit. Open the steam exhaust valve wide,
after warm up.
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Start fan, listen for unusual noises, feel bearings and fan casing for excessive vibration,
check bearing lubrication, check for fan leaks, especially around seals.
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Fan Operating Checks
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Look for signs of the leakage of oil, water, air and hot gases.
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Listen for unusual noises.
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Note unusual smells, such as those caused by hot bearings, leakage of gases, motor
shorts, etc.
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Check oil levels in bearings.
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Check all indicators for the fan and driver, such as bearing temperatures, fan suction
and discharge pressures, manometers, air or gas flows, etc.
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Feel all bearings for normal temperature and vibration.
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Check rotation of bearing oil rings.
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Generally look for any unusual condition; a change normally indicates problems.
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During the first hour of operation make these checks frequently. Most problems occur shortly
after start up. When the fan has been operating for some time, these running checks can be
made less frequently, but they should be made on a regular basis.
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OBJECTIVE 5
Describe typical windbox and air louver arrangements and distinguish between primary
and secondary air.
WINDBOX AND AIR LOUVERS
The windbox is an extension of the air ducts and serves as a distributing chamber for the air. It
is a large chamber located where the burners enter the boiler. Air pressures are stabilized in
this chamber so that each burner receives equal amounts of air.
Louvers are installed in the windbox to direct the air to the base of the flame. In this case, they
are called primary air louvers as they supply the air for the initial combustion of the fuel. Air is
also directed to the surrounding area of the flame to cause complete combustion. This type is
referred to as secondary air louvers.
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For gaseous and liquid fuels and for powdered coal, the primary air enters the furnace mixed
with the fuel. This primary air passes through the fuel bed, when burning solid fuels.
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Secondary air helps to burn the volatile matter expelled from the fuel, during the primary or
preliminary stage of combustion. In the case of solid fuel, it is generally admitted above the
fuel bed.
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Figs. 16 to 19 illustrates various windbox and louver designs, as well as the locations where
primary and secondary air enters a furnace.
Figure 16
Burner Admission of Primary and Secondary Air
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Figure 17
Burner for Pulverized Coal, Oil and Natural Gas Firing
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Figure 18
Tangential Firing
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Figure 19
Solid Bed Fuel
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OBJECTIVE 6
Describe the design and operation of flue gas particulate clean-up equipment, including
mechanical and electrostatic precipitators and baghouse filters.
SOLID POLLUTANTS
Solid pollutants are dust particles known as ash, consisting mainly of the non-combustible parts
of the fuels the power plants use.
Ash is present in boilers that use coal as its fuel. Oil firing produces some ash during
soot-blowing and is practically absent with natural gas burning. With stoker firing methods,
most of the ash can be removed from the furnace, quenched with water and sent to a suitable
landfill for disposal. With modern pulverized coal firing, the high turbulence between the coal
and the combustion air stream causes most of the ash to be carried over with the stack effluents.
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Ash and soot cannot be prevented from entering the flue gas stream. They can be prevented
from entering the atmosphere by the methods outlined below.
Mechanical Precipitators
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The theory of particle precipitation is based on the fact that, when a moving particle changes its
velocity, a force is generated as a consequence. This law is represented as:
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Force (newtons) = mass (kg) x acceleration (m/s2)
F
=
Acceleration =
m
x
a
change of velocity
m/s m
=
= 2
time it takes to change
s
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Velocity is a vector quantity in that it has both magnitude and direction. A force on a moving
mass is generated whenever its velocity vector changes and the change can be either in
magnitude or direction, or both. This principle is used in the piece of equipment shown in Fig.
20 (a). The particle laden gas stream enters at the left with a certain velocity, through the
relatively narrow duct. As it enters into (A), the diverging section, its forward velocity is
reduced in order to fill up the larger section of duct (A). A forward force will act on the
particles, pushing them toward the hopper.
At point (B), an upward turn is made. At this point, a downward force will be exerted on the
dust particles, pulling them towards the hopper. At point (C), the duct converges and the
stream must now accelerate, since it has to pass through a narrower space. As the velocity
increases, the force will pull the particles backwards.
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Gas particles experience these forces, as well. But the masses of CO2, N2 and O2 molecules are
negligible by comparison to the dust particles. Since the forces generated in sections A, B, and
C are also dependant on the moving masses according to the formula F = m x a, then the forces
acting on the gas molecules will also be negligible, by comparison.
Another variation of a mechanical precipitator is the one shown in Fig.20 (b). Here, the sharp
turn, at B, is caused by baffles rather than changing direction of the whole duct. This has the
advantage of saving space and the ducting is neater and more streamlined.
(a)
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(b)
Figure 20
Particle Precipitators
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Cyclone Precipitator
These principles are further employed in the combination spray and cyclone precipitator, as
shown in Fig. 21. The gas stream enters with high velocity from the narrow section (A) to the
larger section (B). It enters in such a way as to create a whirling motion.
A spray of water turns the individual dust particles into heavy mud particles. They are not
likely to reach the top where the exit is. They hit the walls and the water spray washes them
down into the basin, where they settle as sludge. The sludge can be removed as required. This
is an extremely effective way of dust removal from the combustion gases.
However it has a serious drawback in cases where the combustion gases contain sulphur
dioxide (SO2), from the combustion of sulphur. SO2 in water forms weak sulphuric acid, which
is very corrosive to iron based metals. It is also an irritant to human skin and is a soil pollutant.
Unless a safe and proper method is available for the disposal of the acidic water, this spray type
cannot be used. Dry cyclones are used extensively, but their effectiveness is somewhat limited
because the very minute dust particles will still escape, since the forces that will separate them
decrease as the size of the mass of the particles decreases.
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The scrubbing water may be treated with lime or dolomite slurry. Both of these compounds
exhibit an alkali chemical behavior (high pH >7). The acidic (low pH <7) scrubbing water is
thus neutralized to about a pH of 7 and sulphur is retained in solid form in a non-polluting
waste compound.
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Figure 21
Cyclone Precipitator
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Electrostatic Precipitators
Dust laden flue gas from the boiler enters the precipitator that contains a charging electrode.
The dust particles receive a charge of electricity and become electrified. The closer the dust
particles come to the charging electrode, the greater is the effect. Referring to Fig. 22, for good
precipitation of dust, the distance between the collector wall and the electrode must be kept to a
minimum. As the particles continue to flow in the precipitator, they tend to attach themselves
to the fixed walls. They get compacted into heavier particles and in turn fall, into the hopper.
The build up of particles on the walls of the enclosure may become thick enough to reach the
charging electrode before the buildup is heavy enough to fall into the hopper. In such a case
the precipitator is “shorted”. When this happens the charging electrode starts conducting to the
wall and loses part of its ability to charge the flying particles.
This can cause the passages to become clogged. To prevent this, a device called a “rapper” is
employed with small, hammer balls continuously tapping the walls. This causes the compacted
particles to fall into the hopper.
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Figure 22
Electrostatic Collector
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Fig. 23 shows an electrostatic precipitator. Since the distance between the electrode and
surfaces must be small, several collecting surfaces are employed instead of just the walls of an
enclosure. These are arranged as curtains, alternating with curtains of electrodes. The rappers
that are used are sophisticated mechanisms instead of simple hammer balls. Nevertheless, the
principle is the same as that illustrated in Fig. 22.
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Figure 23
Electrostatic Precipitator
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Electrostatic precipitators are efficient devices for dust removal, but because the distance
between electrodes and collecting plates are small, one or more cyclone precipitators should
precede them. They are used as final polishers of the flue gas. According to some
manufacturers, they can be as much as 90% efficient. In order to maintain this high efficiency,
attention should be paid to the following:
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High gas temperatures reduce the ability of the dust particles to become charged
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Idle or defective rappers will cause internal build-up, resulting in the shorting out of the
electrodes
Finally, there are some particles that cannot be easily charged. These particles may still pass
through the chimney and to the atmosphere.
Bag Houses
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With certain coals, some particles cannot be easily charged to allow them to be effectively
removed by the electrostatic precipitators, as shown in Fig. 22 and 23. In many cases, the
answer could be the use of fabric filters arranged in bag houses. This arrangement uses several
filter bags, in parallel, and the principle is the same as that of the domestic vacuum cleaner.
The fabric of the bags is large enough to allow the flue gas molecules to pass through but small
enough to catch the dust particles.
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Fig. 24 illustrates an example of a bag house. Due to its size, cost and installation, a fabric bag
cannot be discarded after it gets clogged. A rapper system similar to that of the electrostatic
precipitator is used to shake the bags free from the compacted particles. Jets of air used in the
same way as a backwash principle may substitute or be used in combination with the rapper
(shaker) system.
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With today’s technology fabrics such as polyamide or acrylic fibers may operate safely at
290°C and may even accept gas up to 315°C for short periods.
Figure 24
Bag House Filters
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OBJECTIVE 7
Describe the design and operation of ash handling systems, including hydro and air
systems, bottom ash systems, and scraper conveyor systems.
ASH HANDLING SYSTEMS
The type of ash handling system used at a plant depends upon the method of firing and the size
of the plant. A small plant firing with stokers may be arranged so that trucks can be driven into
the basement and the ash from the ash pit dumped directly into them, Medium sized plants may
use a pneumatic system that removes the ash from the pit and transports it to an outside storage
bin, or silo. From the silo it is removed by truck. Large pulverized coal-fired plants use much
more elaborate systems and these may be hydraulic, pneumatic or a mixture of both.
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A typical ash handling system for a large plant, as shown in Fig. 25, uses both hydraulic and
pneumatic methods. Ash from the dry bottom type furnace is quenched and collected in a
water-filled ash hopper. Any sticky slag that is discharged from the furnace with the dry ash
will disintegrate when quenched. The ash is periodically removed from the hopper through a
hydraulically operated gate and forced, by means of a jet pump or hydraulic ejector through a
pipeline to a fill area. As the ash leaves the hopper, it passes through a clinker grinder, which
reduces the size of large pieces of clinker and slag, to permit passage through the pipeline.
When the ash-water mixture reaches the fill area, the water is drained off to a clarifying basin
before entering any stream, river or lake.
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In the system illustrated in Fig. 25, the flyash is collected from the economizer section, the dust
collectors and from the bottom of the stack. The flyash is conveyed from the economizer to
intermediate storage bins by means of a blower operated air pressure system. The flyash is
then collected from the intermediate bins, the dust collectors and stack by means of a vacuum
system. A mechanical blower or pump produces the vacuum. The ash is separated from the
conveying air in a primary mechanical separator and then in a secondary cyclone separator.
Then before the air is discharged to atmosphere by the vacuum pump, it passes through an air
washer and removes any remaining flyash, which is discharged with the water to a clarifying
pond.
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Figure 25
Ash Handling System
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The fly ash is carried from the bottom of the primary and secondary separators by another
blower-operated air pressure system, which carries it to storage silos. Trucks are then used to
haul away the flyash from the storage silos.
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The boiler, in Fig. 25, is a dry bottom type, but a similar ash handling system could be used for
a wet bottom or slag tap type. In this type, the molten slag drops into a water- filled hopper.
The water is agitated by jets to aid in the disintegration of the slag, which also passes through a
clinker grinder, before entering the ash-handling pipeline.
Three types of ash handling systems are:
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•
•
Hydro, or water powered system
Pneumatic, or air-powered system
Mechanical system
Hydro System
Hydro systems often use jet pumps to move a mixture of water and ash to the disposal area, via
a closed pipeline. This is the most common method for handling bottom ash. Fig. 26 is a
schematic arrangement of a type of jet pump, used for handling abrasive solids.
The jet pump is a simple device with no moving parts. It has three main components: body,
nozzle and diffuser. To minimize the effects of abrasion, the jet pump is made of abrasionresistant alloys. The piping, downstream of the jet is also designed for abrasion resistance,
especially the elbow.
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Figure 26
Jet Pump For Ash Removal
(Alstom Power)
Pneumatic System
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A pneumatic system moves the ash in a stream of air, or flue gas. The air is moved either by
upstream pressure or a downstream vacuum. This is the most common way to move flyash.
Fig. 27 shows a vacuum-to-pressure, dry pneumatic, flyash system. It uses a mechanical
exhauster to create a vacuum on the upstream system, from the precipitators to the surge
transfer tank and bag filters. A mechanical blower is used for the pressurized part of the
system, from the transfer tank to the flyash silo.
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Figure 27
Flyash Removal System
(Alstom Power)
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Mechanical System
The oldest method of ash removal is the mechanical method. This involves the use of solids
handling equipment, such as scraper conveyors, bucket elevators, and conveyor belts. The
most common mechanical arrangement used is the submerged scraper conveyor for bottom ash
removal. Fig. 28 illustrates this type of arrangement. The scraper operates below the water
level of the bottom ash pit. A belt conveyor is used to dewater the bottom ash mixture and
transport it to the clinker grinder.
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Figure 28
Submerged Scraper Conveyor
(Alstom Power)
Bottom Ash Removal System
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Bottom ash hoppers may be of the dry or wet, type. In the dry type the bottom ash pit is dry
and lined with insulation material. The ash is crushed and removed dry. Dry bottom hoppers
are normally not used on boilers above 180 MT/h, of steam production. In a wet type of
system, the bottom ash hopper is filled with water. This type of system has the following
advantages:
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It cools the ash and helps to break it up into smaller pieces
It keeps the slag submerged so that large masses do not fuse together
The water also helps in the removal of the ash
Fig. 29 is a side view of an intermittent-removal ash hopper. Note the water level, clinker
grinder, and jet pump. There are also jetting nozzles to remove slag from the sides of the
hopper. Each hopper is usually drained only periodically (approximately every 8 hours),
during normal operation.
PE3-3-7-27
Figure 29
Wet Type Bottom Ash Hopper
(Alstom Power)
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PE3-3-7-28
OBJECTIVE 8
Describe the designs and operation of SO2 recovery systems, including lime and wet gas
scrubbing.
SULPHUR DIOXIDE RECOVERY SYSTEMS
Recovery systems, for the removal of SO2 can be classified into two major types:
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Wet scrubbers
Dry scrubbers
Wet Scrubbers
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In the flue gas scrubbing system as shown in Fig. 30, a solution of lime/limestone is pumped to
the venturi scrubber unit. The venturi creates a negative pressure atmosphere, which in turn,
causes the boiler flue gas to enter the scrubber and be in contact with the absorbent slurry. This
absorbent slurry absorbs the sulphur dioxide, fly ash and lesser amounts of oxygen, from the
flue gas. This solution of spent absorbent and gases then passes to the bottom of the spray
tower. Here, the remainder of the flue gases separate from the absorbent and to rise up the
spray tower.
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An ID fan is a part of the system and it causes the remainder of the flue gas to be drawn up the
spray tower. A solution of the absorbent slurry, from the holding tank, is pumped to the spray
tower where it is sprayed into the uprising gases to further remove any remaining traces of
sulphur dioxide. A spray of water is also added to the top of the spray tower to aid in the
cooling of the hot flue gases. The flue gases are then discharged to the stack and out to
atmosphere.
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The spent absorbent drains to the scrubber effluent holding tank where the dissolved sulphur
compounds and fly ash are precipitated. Fresh lime slurry is added to regenerate the spent
absorbent. The slurry contains from 5 to 15 percent suspended solids, consisting of fresh
additive, absorption products and flyash. To regulate the accumulation of solids, a bleed
stream from the scrubber effluent holding tank, is pumped to the solid/liquid separation section
of the system. Here, the liquids are removed from this solution and recycled back to the
scrubber effluent holding tank. The solids are removed and disposed of.
PE3-3-7-29
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Figure 30
Flue Gas System Process Flow Diagram
(Alstom Power)
Fig. 31 shows an overview of the layout of a scrubbing system that is used in a. power plant. In
this type, flue gas is scrubbed in the absorber sections, which contain a series of sprays. The gas
comes in contact with lime or limestone slurry. Usually about 90% of the SO2 is removed.
After passing through a mist eliminator, the flue gas goes to the stack. There are also
subsystems within this design for reagent or limestone slurry preparation, waste slurry
dewatering, and solids disposal. The disposal of products is often accomplished by mixing the
dewatered sludge with flyash and sent to a landfill.
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Figure 31
Power Plant Lime Scrubbers
(Courtesy McGraw-Hill Education)
PE3-3-7-30
Another type of wet scrubber, shown in Fig. 32, is a simple loop absorber tray tower. Flue gas
enters at the bottom and flows into the absorber tower quench section. A lime slurry solution is
injected into the gas stream through the use of spray nozzles. This is the initial stage of SO2
absorption.
The flue gas then passes through a perforated tray where further removal of the SO2 takes place
due to the violent action, taking place. Any liquid droplets entrained in the upward flowing
flue gas are removed through the use of moisture separators. The clean flue gas then passes to
the atmosphere. The sludge, from the bottom of the unit, is then disposed of.
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Figure 32
Wet Tray Tower Scrubber
(Courtesy of Babcock and Wilcox)
PE3-3-7-31
Dry Scrubbers
Dry scrubbing is the main alternative to wet scrubbing for SO2 control on utility boilers. The
advantages of dry scrubbing over wet scrubbing are:
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Simplicity of construction
Dry waste products
Fewer unit operations
Less costly construction materials
In the dry scrubbing process, the heat of the flue gas is used to dry finely atomized slurry of
alkaline reactants. As the slurry dries, a majority of the SO2, in the flue gas, reacts with the
reagent. The reacted material, now dry powder is removed along with the flyash in the
precipitators, or bag-house. Fig. 33 shows a utility size dry scrubber installation, coupled with
a baghouse. Unlike a wet scrubber installation, the dry scrubber is installed before the dust
collector.
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Figure 33
Dry Scrubber Location
(Courtesy of Babcock and Wilcox)
The unit shown in Fig. 34 is a horizontal scrubber with air nozzles, for atomization. Flue gas
leaves the air heater at a temperature of 121°C to 177°C and then passes through a finely
atomized spray of alkaline slurry. The atomized droplets absorb SO2 while the hot flue gases
dry the slurry into a powder. The dried products are removed from the hopper at the bottom of
the scrubber or pass to the precipitator for removal. This design is common with industrial or
utility boilers.
PE3-3-7-32
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Figure 34
Pneumatic Horizontal Nozzle Dry Scrubber System
(Courtesy of Babcock and Wilcox)
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Vertical flow types, as shown in Fig. 35, are also used.
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Figure 35
Pneumatic Vertical Nozzle Dry Scrubber
(Courtesy of Babcock and Wilcox)
Reagents
PE3-3-7-33
Lime is the most common reagent used in dry scrubbers. Limestone is the preferred reagent
with wet scrubbers. Lime is much more expensive than limestone. Therefore, the operating
cost for a dry scrubber is higher than for a wet scrubber. The dry scrubbers are often used
where reagent cost is not the main factor, such as in plants using lower sulphur coals and in
smaller sized plants.
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PE3-3-7-34
Assignment
1. What factors affects the creation of draft in a boiler?
2. With the aid of a single line sketch, explain the following:
a) How balanced draft is created and controlled in a boiler.
b) What pressure is normally maintained in a boiler that operates under a balanced draft
system?
3. List the advantages and disadvantages of the following types of fan speed control:
a) Inlet damper
b) Outlet damper
c) Variable speed
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4. Briefly discuss the difference between primary and secondary air lovers for a boiler.
5. List the steps that you feel should be followed in the starting of a draft fan.
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6. With the aid of a simple sketch, explain the principal of operation of a cyclone precipitator.
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7. What are the advantages of the wet type of bottom ash removal system?
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8. With the aid of a simple sketch, explain the principal of operation of a lime wet scrubbing
unit used for the removal of sulphur dioxide from flue gas.
9. List the advantages that dry scrubbers, used for sulphur dioxide removal, have over the wet
type scrubbers.
PE3-3-7-35
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