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Module 8Pulverized coal firing
BasicPower Plant
Training
Printed in Canada
©1995, 1996
British Columbia Institute of Technology
These materials were developed by the Learning Resources Unit at British ColumbiaInstitute of Technology (BCIT) in Canada.
All rights reserved. No part of the text or graphics may be reproduced in any formor by any electronic or mechanical means (including photocopying, recording, orinformation storage and retrieval) without written permission from BCIT LearningResources Unit.
British Columbia Institute of Technology
Learning Resources Unit
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Canada
MODULE 8 PULVERIZED COAL FIRINGBritish Columbia Institute of Technology—Basic Power Plant Training
MODULE 8 CONTENTS Pulverized coal firing
UNIT 1
Coal characteristics, preparation, and transportation
UNIT 2
Reclaim, handling, and storage systems for coal fuel
UNIT 3
Pulverized coal combustion
UNIT 4
Pulverized coal firing
UNIT 5
Boiler control systems
UNIT 6
Furnace explosions
UNIT 2 COAL HANDLING
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UNIT 2 COAL HANDLING
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Unit 2 Reclaim, handling,and storage systemsfor coal fuel
This unit discusses the common methods of handling, storing, andprocessing coal for use in pulverized coal-fired boilers.
UNIT 2 COAL HANDLING
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UNIT 2 COAL HANDLING
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UNIT 2 CONTENTS Reclaim, handling, andstorage systems for coal fuel
TOPIC 1
Reclaim systems .................................................................................... 5
TOPIC 2
Conveyors............................................................................................... 9
TOPIC 3
Outdoor storage .................................................................................. 18
TOPIC 4
In-plant, enclosed storage ................................................................. 21
TOPIC 5
Weighing .............................................................................................. 28
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Figure 1Automatic reclaim system
Track hopper
Beltscale
Coal sampler
Reclaim hoppers and feeder
Collecting belt
Live pile
Permanent storage
Stackerboom
Reclaimbelts
Crusherhouse
Beltscale
In-plant silos
Surge bin
Sampler
Conveyors
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TOPIC 1 Reclaim systems
Coal handling systems are usually flexible in design for twomain reasons:
• to by-pass breakers or crushers when necessary
• so loads may be divided, sending part to storage and thebalance to the boiler.
Most reclaim systems are arranged so coal can be stacked out byconveyors, and spread and compacted by bulldozers or tractorcarryall teams. It is reclaimed by the same equipment into areclaim hopper, then passes through the preparation system andinto the bunkers.
Automaticreclaim system
In a modern, automatic reclaim system, the coal is stacked out instorage piles, directly over one or more reclaim hoppers (seeFigure 1 opposite). This coal is not moved away or compactedunless there is a surplus over and above boiler requirements. Thispile is a large, immediately available supply, eliminating the needfor large bunkers within the mill. By using silos with 4 to 5 hourscapacity, instead of bunkers with 24 to 30 hours capacity, savingsin mill space and materials requirements are realized. In this typeof system, the equipment starts automatically as need arises andstops when the silo is filled. If necessary, bulldozers are used toservice the reclaim hoppers, and usually at least one hopper isequipped with a crusher for frozen coal.
Coal breakers The breaker is a large cylinder (size is dependent on capacity)made up of perforated steel plates on which are mounted rows oflifting shelves. The cylinder rotates at 12 to 20 rpm inside a steelcasing with a hopper bottom which collects the coal that passesthrough the screen plates (see Figure 2 on the next page).
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British Columbia Institute of Technology—Basic Power Plant Training
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Figure 2Cylindrical coal breaker
Receiving ring
Rawcoalfeed
Removable sections
Dischargering
Oversizeand trashdischarge
Oversizeand trashdischarge
14 ft (4.3 m)
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Figure 3Hammer mill
Coal passes into the breaker feed end, and the fines areimmediately screened out. Larger lumps are lifted by the shelvesand dropped down to the screen. These lumps of coal are brokendown by gravity impact. The harder lumps are gradually fedtoward the rear end of the breaker, where they are reduced by ahammer mill (Figure 3). Uncrushed stone and tramp iron areremoved by hand. Some breakers are center-fed, and others are thereject-type, where tramp metal, wood, and stone are rejected to arefuse conveyor or hopper.
Coal feed
Broken coal
Trampmetal
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Either before or after the coal goes through the breaker, it passesover a magnetic pulley or separator which removes the trampiron, dropping it into a tramp iron collector (Figure 4). After thecoal passes through the breaker, it is delivered to the bunkers, tostorage, or to the hammer mill crushers.
Figure 4Magnetic pulley separator
Tramp iron
Magneticpulley
Conveyor belt
Non-magneticmaterial (coal)
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Conveyors
Conveyors are the most common mechanical means of moving coalfrom the unloading points to the storage pile, or to the preparationand firing equipment.
TOPIC 2
The belt conveyor is commonly used to transport coal in amounts upto 800 tons per hour. It consists of a belt made up of layers ofrubberized fabric, and is manufactured in widths capable of carryinga certain tonnage at speeds up to 550 feet per minute.
Figure 5 shows the basic components of a belt conveyor system. Thebelt ends are spliced together, creating an endless loop which travelscontinuously between a head and tail pulley.
Belt conveyors
Figure 5Belt conveyor components
Troughing idlers (front view)
Drive motorTroughing idlers
Tailpulley
Counterweight
Returning idlers
Take-up pulley
Headpulley
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Figure 6Coal reclaim conveyor system
The belt is supported on the top or carrying side by troughing rollers,and on the bottom by straight or return idlers. It is normally drivenby a motor connected to the head pulley shaft through a speedreducer. Proper load carrying tension is maintained by adjustingscrew take-ups, or by gravity take-up counterweights.
A coal reclaiming conveyor system is shown in Figure 6. Coal isfed from a receiving hopper to the main conveyor by feeders, at arate determined by the operator. In locations where frozen coalmay be encountered, a lump crusher is installed between thereceiving hopper and feeder. This reduces frozen lumps to a sizethat can be handled by the conveyor system. The coal can then befed into a breaker to be sized as required.
Load cells
Sampler
In-line magnet
Surge bin
Crusher house
Feeder Reclaim belts
Reclaim hopper and feeder
Control lineLoad cells
Feeder
Conveyor belts
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Safety standards for conveyor operation are published in theAmerican Standards Association manual, Section B.20.1 under“Safety Standards for Conveyors and Related Equipment.”However, when operating any type of conveyor it is important toobserve the following basic safety precautions:
• Properly adjust the emergency trip wires around the conveyor.This helps protect personnel and equipment in case of trouble,yet avoids nuisance trips. The tension on the trip wire shouldbe enough to keep the wire taut and trip the conveyor with anadditional firm tug.
• Be aware of the nip points on the conveyor and avoid them.Keep long hair and loose clothing tucked in so it won’t getcaught in a nip point and pull you in.
• Stay clear of any moving parts and always shut the conveyordown and lock it out before working on it.
• Enclose the conveyor and any transfer points in dust-tightenclosures with large access doors to keep dust to a minimumin windy areas. Regularly sweep out these enclosures to reducethe risk of fire.
• Install and maintain belt cleaners at the head end of the belt tominimize housekeeping problems. Belt cleaners may also beneeded on the inside of the belt downstream of the feed hopper.
• Thoroughly lubricate the belt rollers and adjust the belt tensionand tracking to avoid misalignment and fraying.
Belt conveyorsafety ..........
UNIT 2 COAL HANDLING
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British Columbia Institute of Technology—Basic Power Plant Training
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Figure 7Flight conveyor details
A
A
B
B
C
C
Transferconveyorchute
Top trough
Bottom trough
Drive chain
Boiler feedchutes
Return conveyorfeed chute
Section CC
Fuel drops into theboiler feed chutesand excess fuel iscarried to the fuelreturn conveyor chute
Section BB
Fuel carried in onedirection in the toptrough and in theopposite directionin the bottom trough
Section AA
Fuel drops fromtop trough intobottom trough
Top trough
Bottom trough
Flight counterswitch
Timer
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This type of conveyor is equipped with strong, upright flightscarried between two chains. To reduce friction, the bottom andsides of metal flight conveyor troughs are often lined with Teflon.Teflon and similar plastics have a very slippery surface. Sheets ofthese materials are usually bolted or screwed to the surface ofconveyor troughs.
Flight conveyors normally have two troughs (Figure 7 opposite).One trough is on the top, with the flights traveling in onedirection, and another is directly below it, where the flights maketheir return pass. Fuel is often fed to the top trough from a transferconveyor chute. At some point, usually near an end, the toptrough has holes in the bottom. These holes allow the fuel to dropdown into the bottom trough. When the fuel drops into the bottomtrough, the flights carry it in the opposite direction. More holesalong the bottom of the lower trough allow the fuel to drop intochutes supplying the boiler feeders.
Each boiler feed chute is equipped with a gate for closing thechute off when the boiler is out of service. Enough fuel normallyfalls into the feed chutes to keep them full. The excess is draggedover the top of the opening and along the trough to the next chute.When all the chutes are full, any fuel remaining in the lowertrough is usually dropped into a transfer chute supplying a fuelreturn conveyor. This excess fuel is returned to storage, or back tothe beginning of the system.
Flight conveyors
UNIT 2 COAL HANDLING
MODULE 8 PULVERIZED COAL FIRING
British Columbia Institute of Technology—Basic Power Plant Training
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Figure 8Bucket elevators
Fuel in Fuel in Fuel in
Fuelout
Fuelout
Fuelout
Continuous Mechanical Centrifugal Spaced loading
Continuous loading
Idlersprocket
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A bucket elevator is a special type of conveyor used for verticallifts. The buckets are carried on a single chain, or between twochains (Figure 8 opposite). This type of conveyor moves the fuel intraveling compartments. It is protected by a shear pin torquelimiting device and by bucket counters and timers.
Bucket elevators can be continuously loaded by a regulated feedstream. The feed stream can also be unregulated, just flowingfreely into the lower end of the elevator. In this case, the elevatoracts as a hopper. The buckets then scoop up loads of material asthey change direction at the bottom. This arrangement takes careof any spillage occurring at the top end where the buckets dump.
The bucket elevator can discharge at the top end by flinging thefuel out into an outlet hopper. This is known as centrifugaldischarge. The speed of the elevator must be fast enough to flingthe material out into the hopper. This can result in dust problemsif the elevator is not well-enclosed.
Another discharge system uses idler sprockets to carry the bucketchain around the head sprocket more than 180°. This causes thebuckets to tilt over further, completely dumping their load. Theidler sprockets prevent the buckets from contacting the lower lipof the discharge hopper located under the dumping buckets.
Bucket elevators
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British Columbia Institute of Technology—Basic Power Plant Training
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Figure 9␣ Redler conveyor
Fuel in
Fuel out
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Shuttle conveyor
Redler conveyor A special type of flight conveyor known as a Redler conveyorhas the flights of the lower pass completely enclosed in a close-fitting tube (Figure 9 opposite). By completely enclosing theflights, the conveyor can be adjusted from horizontal tovertical, or any angle in between. This allows great flexibilityin design, and allows this type of conveyor to be used inlocations with little available space. This type of conveyor isnot open to routine inspection. If there is a problem in theenclosed tube, it is more difficult to detect and repair.
When the coal is delivered to the bunkers, it is distributed over thelength of each bunker by a device called a tripper or shuttleconveyor. This may be motor-driven, cable-operated, or driven byconveyor pulley through a gear reduction arrangement. Theshuttle conveyor travels over the bunker on rails, and unloads thecoal from the conveyor belt over the side of the belt into thebunker below.