3rd Class Power Engineering

Section 3 - Module 7

Boiler Draft and Flue Gas Equipment

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:

1. Define and explain the applications and designs of natural, forced, induced and

balanced draft.

2. Explain how draft is measured, monitored, and controlled in a large, balanced draft boiler. Explain the position of control dampers.

3. Describe typical draft fan designs, single and double inlet arrangements, and explain

methods used to control fan output.

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. 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.

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.

Figure 2 Natural Draft

The draft will depend on:

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:

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)

The I.D. fan must be able to handle any air leakage into the boiler setting

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 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. 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:

Wind box Furnace Economizer inlet Air heater inlet Stack

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.

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.

Figure 6

Diaphragm Draft Gauge (Courtesy of Bailey Meter Co. Ltd)

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.

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.

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.

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.

Figure 11 Fan Terminology

1. Blade 5. Back Plate 9. Housing 2. Shroud 6. Intermediate Shroud 10. Shroud Stiffener (Outer) 3. Hub 7. Inductor Vane 11. Inlet Cone 4. Shaft 8. Shroud Stiffener (Inner)

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 Figure 14 Single Inlet Fan Double Inlet Fan

Fan Output Control Fan output is usually controlled by one of the following methods:

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 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. Variable Speed Control 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

Ensure all debris, tools, rags, etc., are removed from the immediate fan and driver area.

Give fan and driver a visual check; look for loose nuts, plates or bolts, disconnected rods, etc.

Ensure that the fan inlet screen is clean and clear of frost, rags or debris.

Check the dampers and damper drive connections for tightness.

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 manufacturers recommended oil.

Open cooling water to the water- cooled bearings.

Generally look for any condition which might impair the proper operation of the unit.

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.

Start fan, listen for unusual noises, feel bearings and fan casing for excessive vibration, check bearing lubrication, check for fan leaks, especially around seals.

Fan Operating Checks

Look for signs of the leakage of oil, water, air and hot gases.

Listen for unusual noises.

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Note unusual smells, such as those caused by hot bearings, leakage of gases, motor shorts, etc.

Check oil levels in bearings.

Check all indicators for the fan and driver, such as bearing temperatures, fan suction and discharge pressures, manometers, air or gas flows, etc.

Feel all bearings for normal temperature and vibration.

Check rotation of bearing oil rings.

Generally look for any unusual condition; a change normally indicates problems. 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. 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. 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. 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

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. 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 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: Force (newtons) = mass (kg) x acceleration (m/s2) F = m x a

Acceleration = 2change of velocity m/s m

time it takes to change s s= =

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) (b)

Figure 20 Particle Precipitators

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

Figure 22 Electrostatic Collector

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.

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:

High gas temperatures reduce the ability of the dust particles to become charged

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 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. 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. With todays technology fabrics such as polyamide or acrylic fibers may operate safely at 290C and may even accept gas up to 315C 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. 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. 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

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. 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:

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 abrasion-resistant 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 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.

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.

Figure 28 Submerged Scraper Conveyor

(Alstom Power) Bottom Ash Removal System 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:

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.

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Figure 29 Wet Type Bottom Ash Hopper

(Alstom Power)

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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:

Wet scrubbers Dry scrubbers

Wet Scrubbers 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. 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. 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.

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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.

Figure 31 Power Plant Lime Scrubbers

(Courtesy McGraw-Hill Education)

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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.

Figure 32 Wet Tray Tower Scrubber

(Courtesy of Babcock and Wilcox)

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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:

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.

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 121C to 177C 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.

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Figure 34

Pneumatic Horizontal Nozzle Dry Scrubber System (Courtesy of Babcock and Wilcox)

Vertical flow types, as shown in Fig. 35, are also used.

Figure 35 Pneumatic Vertical Nozzle Dry Scrubber

(Courtesy of Babcock and Wilcox) Reagents

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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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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

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. 6. With the aid of a simple sketch, explain the principal of operation of a cyclone precipitator. 7. What are the advantages of the wet type of bottom ash removal system? 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.

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This material copyright Power Engineering Training Systems, a division of PanGlobal Training Systems, 2003. All rights are reserved. No part of this material may be reproduced in whole or in part without the prior written permission of the copyright holder. Address all inquiries to: PanGlobal Training Systems 1301 16 Ave. NW, Calgary, AB Canada T2M 0L4 Attention: Chief Operating Officer www.powerengineering.ca

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Draft ApplicationsNatural DraftFigure 2Natural Draft

Forced DraftFigure 7Typical Draft GaugeFigure 8

DRAFT FANSFigure 9Figure 10Induced Draft Fan

Fan TerminologyFigure 12Types of Fan Blades

Fan output is usually controlled by one of the following metInlet damper control may be achieved by using inlet vanes, a

WINDBOX AND AIR LOUVERSElectrostatic CollectorFigure 24Flyash Removal SystemFigure 29Wet Type Bottom Ash Hopper