In thermal power station, Coal is a principal fuel and hence a careful thought is givento the design and layout of the coal handling plant. As the consumption of coal is very large,the design and layout of a coal handling plant should be simple but robust with a view toreduce the maintenance and running cost to the lowest possible figure consistent to reliability.

Mode of Coal Transportation :Coal is brought to the power station by three modes of transportation :

1. Roadways : Coal is carried in trucks and a truck can carry about 8-10 tons of coal. But dueto low capacity, low unloading rate and time consuming, this mode is not in much use forlarge thermal power stations.

2. Railways : coal is brought by railway wagons. One rack consists of 58 wagons. Eachwagon contains 58 MT of coal. Locos bring the wagons from the marshalling yard and placethem on wagon tippler. These wagons are then unloaded with the help of wagon tippler. Ifthese wagons are not unloaded in stipulated time period (generally 7 hrs.), demurrage chargesare lavied by railway department.

There are two types of wagon tipplers.

a) Side Wagon Tippler : Wagon is unloaded into a hopper which at the side of the railwaytrack. The max. angle of tilt is generally set between 140 to 150. The rate of unloading is 13wagons per hour. The time required for one cycle of operation of this wagon tippler is asbelow.

1. Weighing Wagon + coal before tippling - 15 sec.

2. Tippling of Wagon to hopper - 90 sec.

3. Pause - 5 sec.

4. Tippling of Wagon back to home position - 90 see.

5. Weighing Wagon after tippling - 15 sec.

The weighing machines are integral with tippler mechanism and are fitted with a ticketprinting recorder and totaliser.

b) Ring type (Rotary) Wagon Tippler : In rotary tipplers the wagon is fixed between thetwo large rings which are fastened to form a cage like structure. The cage is rotated anddischarged coal falls into the hopper right below the rail track. Angle of tippling is 140 - 160.The rate of unloading is 25 wagons per hour. The time required for one cycle of operation ofthis wagon tippler is 60 sec. only.

GENERAL WORKING AND DESCRIPTION OFCOAL HANDLING PLANT

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For easy and speedy movement of wagons, mechanized bettle chargers are providedbefore and after wagon tippler i.e. inhaul and outhaul bettle chargers.

These wagon tipplers are provided with photocell protection to avoid the entry of otherwagons when tippling cycle is in progress.

Track hopper system : This system is provided at Chandrapur thermal power station. BOBR(bottom opening wagons) wagons are unloaded in track hoppers. The holding capacity oftrack hoppers is 4500 MT.

MGR Railway system : This system is provided where coal mines are located near the powerstation. Railway wagons are used to transport the coal from coal mines to power station andunloaded wagons are returned to coal mines for refeeding. So this forms a ring type system.Wagons alongwith railway tracks being the MSEB property, this becomes the most economicalway of coal transportation having a very low maintenance time and cost as compared to theropeway system.

3. Ropeways : This mode of coal transportation is used where coal mines are located nearthe power stations. Coal is brought by hanging buckets/trolleys travelling on track ropes,which are pulled by a haulage rope with a driving mechanism. The payload of each bucketvaries from 1 to 3 tons. Automatic loading and unloading mechanisms are provided at loadingand unloading stations. Rate of unloading varies from 75 to 275 MT/Hr depending on the typeof ropeways used. This type of coal transportation is very economical compared to road or railtransportation and gives assured supply of coal, being the MSEB property. The only disadvantageof this system is long time for maintenance works.

There are mainly two types of ropeway systems used in power stations.

1. Mono Cable Ropeway :This ropeway operates on one single endless haulage rope. This continuously moving

rope serves the double purpose of supporting as well as hauling the ropeway bucket along theline. Between the stations the rope is supported on sheaves mounted on articulated beamequalizing the load on the sheaves. While travelling along the line the ropeway car runningfirmly attached to the main rope, their travel being entirely automatic requiring no attentionof operator. The capacity of each bucket is 1.0 T/hr. and the line capacity is 75 T/hr.

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b) Bi-cable bucket type ropeway :The essential characteristic is the use of two tensioned fixed track ropes on which the

carriages are run. Each carriage with its buckets suspended by means of hangers is locked tothe endless continuously moving haulage rope. One of the two track ropes (topside) carriesfull buckets, the second track rope on the bottom side of the line carries empty buckets. Thetrack ropes are supported on along the line at a convenient height above the ground bymeans of intermediate trestles, each trestles is provided with oscillating saddles with groovesfor carrying ropes and sheaves for hauling ropes. Capacity of each bucket is 1.8-2.5 T/hr andline capacity is 200-275 T/hr.

2. Bi-cable Ropeway: This is further divided in two types:

a) Tram Car type ropeway :In this system, two nos. of track ropes are provided at the top and bottom side. One of

the two track ropes (topside) carries full cars, the second track rope on the bottom side of theline carries empty cars. Each tram car body is fitted with steel axle at each end to receive twowheeled tramcar track assembly with a central bushing. Four tracks are fitted with each car sothat each is carried on 6 wheels. Since the tram cars turn completely upside down and downside up at the discharge and loading terminal respectively, no catches, latches or othermechanism is required to discharge or receive loads. Capacity of each bucket is 2.5 T/hr andline capacity is 200 T/hr.

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TRACK ROPE (48/33 mm)

General Working of a Coal Handling Plant :As mentioned above, coal is brought to power station by either of three means of coal

transportation. This coal is first conveyed to primary crusher with the help of differentcombination of conveyor belts and its rate of feeding is controlled by Electro-magnetic vibratingfeeders. Conveyor belt before the crusher is provided with hanging magnets to separateferrous materials. Stones are picked up manually. In primary crusher, coal is first crushed to100 mm size. This coal is again conveyed to secondary/final crusher on belt system. Herevibrating screens are used to feed crushers, which bypasses coal of size more than 25 mm.In final crushers, coal is further crushed to required 25 mm size. This sized coal is then sendto bunkering belt and with the help of coal trippers. This sized coal is finally fed to coalbunkers. This cycle is called coal bunkering.

In case bunkers are full, then available coal is stored in stock yard with the help ofstacking belts /automatic stacker cum reclaimer. This cycle is called stacking.

In Koradi thermal power station, 2 nos. of ropeways are provided

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In emergency when coal is not available in plant by railways/ropeways, then thisstacked coal is diverted to the coal bunkers by reclaimimg conv. belts. This cycle is calledreclaiming. The coal stored in bunkers is further send to coal mill for pulverization andcombustion in boiler furnace.

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Equipments used in Coal Handling Plant :1. Conveyor Belt :

These are made up of cotton or synthetic fibers and rubber piles placed in alternatepositions normally vary from 4 to 6 ply. These are generally 900 to 1600 mm in width. Inselecting a belt, following factors are considered:1. Durability 2. Strength 3. Toughness 4. Elasticity 5. Lightness 6. Pliability

Belt Tensioning :1. Screw type : The horizontal and small conv. belts are fitted with a screw operated gear toadjust the belt tension and take up the slack belt. This gear is of robust construction anddesigned to protect from dust. It is fixed at an accessible place for adjustment and cleaning.

Rubber Scrappers : Rubber scrappers are provided at the head end of each conveyor toclean off damp coal dust and to prevent it from carrying on to the return rollers. Thesescrappers are always remain in contact with the belt with the help of spring arrangement sothat the belt is preserved and pulleys are kept clean which ensures straight running of the belt.

2. Automatic gravity take ups : Theses are provided in conv. belt system to maintain slackside tension, to permit length variation due to belt stretch / shrinkage, removal of startingjerks and extra length for vulcanizing. Belt tension is automatically and continuously maintainedby gravity take-ups.

It consists of 2 bend pulleys and a tensioning pulley to which balancing weights areprovided. This tensioning pulley is mounted on a travelling carriage, which is pulled by steelropes to which a counter weight is attached on sheaves. The length of take-up gear should notbe more than 1.5 % of the belt centre length.

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2. Idlers :Conveyor belt is rotating on head and tail pulleys placed at very large distance apart.

Belt can sag between these two pulleys because of its weight. In order to avoid this sagging,idlers are fixed at certain distance between these pulleys. Idlers consist of three rollers attachedto the brackets at an angle of 20-350 so that conv. belt can take shape like a arc of a circle,thus preventing objectionable sharp bends to the belt and carries maximum coal load withoutany spillage. It is shown in the figure as below.

In power station, following types of idlers are generally used.

1. Carrying Troughing Idlers :These idlers are provided for carrying and transporting the required coal load from

point of feeding to the unloading point. It consists of 3 rollers, which are fitted with bearings/Life sealed bearings. Profile makes an arc of a circle to avoid sharp bends to increase beltlife.(As shown in above figure)

2. Return Idlers :These are provided to give support to the belt from return side. As empty belt run over

these idlers, it consist of one plain roller for smaller belt width (upto 1400 mm approx.) andfor higher belt width, it may consists of 2 rollers.

3. Carrying Self-Aligning Idlers :These idlers are provided on carrying side of the conv. system. It consists of 3 roller

system mounted on a fulcrum which is free to oscillate in a pivot on a fixed frame. Wheneverbelt goes out of run, these idlers oscillate on either side, bringing the belt in center of axis ofthe conv. system. This avoids damaging of the belt.

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3. Pulleys :Conveyor pulleys are heavy cast iron construction having machine crowned faces, the

driving pulley being faced with ferodo or other similar friction material. The diameters ofpulleys are large enough to reduce belt stresses. The width of the pulley is more by 150 mm.The dia. of head, tail, snub and bend pulleys depend on the thickness of the belt and a usefulrule is as follow :

4. Return Self Aligning Idlers : These idlers also bring the return side belt to its centerposition if goes out of run.

5. Impact Idlers : These are provided at feeding points to increase the life of the belt andreduce spillage due to sagging below the side scals. The rollers of these idlers are fitted withrubber liners as shown in the following figure.

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4. Coal Feeders :There are two types of feeders used in coal handling plant.

a) Electro-magnetic feeders : These are of the vibrating or jiggling type and are placedimmediately below the coal receiving hopper. The feeders are designed to take coal from thehoppers and deliver it to their corresponding conveyors without spilling.

Vibrating feeders give the trays vibrations caused by the use of AC and DC together.Half wave rectified current is passed through the stator coils, forming the magnetic circuit tocreate a sequence of uninterrupted magnetic pulls on the armature which is connected to thevibrating bars through the centre clamp.

During the first half of the cycle, the armature is flexed towards stator coil. And duringother half wave rectified cycle, armature is pulled back with the help of the springs. This toand fro motion in the gap between armature and the stator coil causes vibrations in feeder.

b) Vibrating Screens : These are of double deck type. The upper deck is trash screen, whichallows large size coal to the crusher. The lower deck is a sizing screen, which allows the coalto bypass the crusher. The screens are mechanically vibrated by an eccentric drive.

5. Coal Crushers:There are two types of crushers

a) Primary Crushers : The primary crushers are either hammer type or single roll crushers.They are designed to crush the coal from 450 mm to 100 mm size. Coal lumps bigger than450 mm size causes serious trouble in the crushers very often.

b) Secondary Crushers : The secondary crushers, which are either hammer type or ringtype crusher. These crushers further crushes coal to the size of 25 mm size.

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! Head Pulley - 5 * Belt Ply! Tail Pulley 4 * Belt Ply! -Snub Pulley - 3 * Belt Ply! Tripper Pulley -5 * Belt Ply

Snub Pulley : These pulleys are used to relieve the adjacent return idler and increase the arcof contact of the main pulley for effective gripping of the belt.

Magnetic Separators :These are provided to get rid of foreign material (i.e. tramp iron) which finds its way

into the coal. The points requiring attention for magnetic separation to be efficient are depthof coal on belt and speed of the belt.

There are two types of magnetic separators used:a) Suspended Magnets : These magnetic separators are fixed on conveyor delivering coalto the crushers and are operated manually by travelling winches.

b) Rotating Magnets : These are also fixed on driving top end of the conveyor belt beforecrushers. Small size belt is rotated across the running conv. belt with a separate drivingmechanism. Material attracted to the portion under magnet is automatically thrown in thedischarge chute.

6. Trippers :Belt conveyors passing over the top of overhead coal bunkers are fitted with travelling

trippers having chutes on one or both sides of the conveyor. These trippers are power propelledand travels on rails. It has been provided with clamping device to prevent it from running away.

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c) Magnetic Pulleys : The head pulleys of the conveyors prior to crushers are arranged asmagnetic pulleys. On long belts, tramp iron gradually takes up a lower position near the beltand so come under the influence of the pulley to a greater extent. These magnetic pulleysautomatically discharge the extracted tramp iron through tramp iron chutes.

Non magnetic materials like stones, shells etc. are removed manually from the running belt.

Protections provided in coal handling Plant :1. Pull chord switch : This protection does not work automatically but is to be operatedmanually by the operator when he senses some severe disaster. This pull chord can be operatedfrom any position along the length of the conveyor belt.

2. Belt sway switch : These are mounted on the conveyors and protect the belt f r o mexcessive running out and getting edge worn / damaged.

3. Zero speed switch : When the speed of the conveyor drops below predetermined speed,it operates and trips the system to save it from congestion at the transfer points. It is usuallyfixed nearer to the tail pulley.

Interlocks provided in Coal Handling Plant :If one of the belts trips for any reason, all earlier belts will trip on auto along with the

associated vibrating feeders provided at input points.

Operating Sequence of Coal Handling Plant:There are three types of operating sequences:

1. Direct to the bunker : Coal received from different modes of coal transportation, istransferred to the crusher with conv. system where coal is crushed to 25 mm size. It is thentransferred to the bunkers through tripper trolley as per the boiler unit requirement. Feedingrate is controlled by Electro-magnetic feeders at feeding points. This cycle is called BUNKERING.

2. Direct to stack : In case bunkers are full and coal by railways / ropeways is available,then coal is first brought to the crusher house, then it is either crushed or bypassed and thendiverted to the coal stock yard with the help of stacking conv. belts. This stacked coal can beused when coal supply is not available by any means of coal transportation. This cycle iscalled STACKING.

3. Stacking to Bunkering : In case bunkers are empty and wagon / ropeway coal is notavailable, then coal is first brought from stack-yard. It is then send to the crusher and thereafterto the bunkers with the help of reclaiming conv. belts. This cycle is called RECLAIMIMG.

Automatic stacker cum reclaimers are used for stacking and reclaiming purpose, if available.

General Problems faced in Coal Handling Plant :1. Design Problems : Cal. Value and Ash %

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Coal received in power station is having cal. Value much less and ash percentage more thanthe rated values recommended by manufacturer. Hence the systems in coal handling plant getoverloaded resulting in low bunkering.

2. Rainy Season Problems : Chute choke ups, Coal yard -Slurry FormationTransfer chutes gets choked up due to wet or muddy coal. Slurry formed in coal yard maycause problems with electro-magnetic feeders at input points, frequent choke-ups at transferchutes etc.

3. Other Misc. Problems: Snapping of belts /ropes : Conv. belts and ropeway ropes get damaged or broken becauseof jerks and overloading problems due to various reasons. Repairing and replacement ofthese belts and ropes require more time for maint.

Derailment of coal wagons : De-railment of wagons result in obstacle in unloading ofbalance wagons in line. This results in lower bunkering and may attract demurrage chargesfrom railway department.

Oversized coal/Muddy Coal : Oversized / muddy coal may cause damage to the beltsystem, frequent choke-ups of transfer chutes and damages to the crusher rings.

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There are three types of Boilers :1) Fire tube Boiler.2) Water tube Boiler.3) Forced once through or Mono tube steam Generator.

Fire tube Boiler is Locomotive engine boiler. Forced once through steam generator is used as a marine boiler. In power station practice we are associated with water tube boiler.

Boiler consists of the following parts :Drum, down-comers, feeder tubes, headers, riser tubes, top water headers, connecting

tubes from headers to drum cage, primary super heater, secondary super heater, forced flowsection or economiser, furnace, flue gas path, air pre-heaters and other auxiliaries of theboilers such as forced draught and induced draught fans, primary air fans, secondary air orsealing fans, coal pulverising mills, coal feeders, burners, ash removal and disposal arrangementsuch as Electrostatic precipitator, ash extractor, pressure conveyor, etc.

In all methods of fuel firing, some basic principles are incorporated to get the mostefficient combustion. These are The Three Ts i.e. Temperature, Turbulence and Time.Secondary air also place very important role in combustion of fuel. Often, the turbulence isprovided by admitting the secondary air in a special manner.

Let us consider each requirement :1. Temperature :

The fuel must reach ignition temp. to ignite and for a stable flame this temperaturemust be maintained. For coal, ignition temp. is in the range 4000 C to 4250 C. A small rise intemperature can double or treble the rate of combustion and conversely a drop in temperature,slows down the process. Even flame may be lost. Thus correct temp. is very essential.

2. Turbulence :Combustion is after all a reaction between fuel particle and oxygen. Turbulance helps

each fuel particle to quickly contact the necessary oxygen molecules so that rapid combustionas also complete combustion is possible with minimum excess air. (More than optimum excessair will increase the flue-gas loss through chimney). Consider oxy-acetylene flame withoutand with air.

3. Time :Depending upon fuel particle size, some time is needed for complete combustion. This

time is reduced by turbulence and rise in temp. The necessary time is provided by furnacedesign and type of firing.

COAL FIRING :Stoker Firing : Modern high capacity boilers do not use stoker firing. Even then, many stoker

FUEL FIRING

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fired boilers are in service even today and it is instructive to study the combustion process.The coal size is 1". Primary air is admitted under the fuel bed. Ignition of coal andvolatiles is due to temperature maintained by combustion of fuel and reflection of heat fromarches. The coal bed may be considered in three distinct layers.

1. Top Layer :Here, as coal is heated-up, and volatiles are given up. They burn in secondary air

which is admitted above fuel bed and causes some turbulence. The coke and fixed carbon leftbehind starts burning at about 1/3rd length of stoker.This layer is DISTILLATION ZONE.

2. Middle Layer :Carbon/coke is burnt to CO2 at about half way down the stoker. Reduction zone.

3. Bottom Layer :Volatiles given-up pass through fuel bed passing through coke. As oxygen is limited

reduction of CO2 to CO occurs in middle layer. The secondary air above fuel bed completes thecombustion of CO to CO2. Bottom layer ignites 2/3rd down the grate.

Combustion in a stoker fired can be controlled by changing the grate speed and adjustingair flow through the fuel bed. Uniform thickness of fuel bed is very important for proper andcomplete combustion. An Ignition plane is formed in the fuel bed, which may get disturbeddue to uneven fuel bed and resulting uneven primary air flow.

P. F. FIRING :In P. F. Firing, surface area of fuel particles is greatly increased and this speeds up

release of volatiles and combustion. Turbulence by burner design or in the furnace is nowmost essential to take advantage of high combustion rate and to reduce unburnt fuel tominimum. For high volatiles coals, a short flame is suitable as less time is needed for completecombustion. The low volatile coals, however, need a long flame to enable complete combustion.

There are two basic types of P. F. Firing :Vertical or Down-shot Firing :

For low volatile coals. Secondary air is admitted in stages down the flame to completethe combustion, which is a gradual process as volatile content is less. More residence time istherefore necessary for each particle of coal to burn completely. This is LONG FLAME FIRING.

Horizontal Firing :Bituminous coals with high volatiles can be burnt with turbulent short flame burners

on front or rear or both walls of furnace. Burner design ensures turbulence. Long flame tangentialfiring can be used if turbulence can be produced in the furnace. In this case combustion is notcompleted in an individual flame, but is completed in the FIRE BALL which is a turbulent massof fuel, air and gases and volatiles.

See figure (Short flame and tangential firing arrangement.)

Oil Burners : Atomisation and Air entry design create in increase in surface area and turbulance.

Atomisation may be :

1. Mechanical OR Pressure atomisation :

High pressure oil escaping through a nozzle gets atomised into a fine spray. A swirl is

obtained by tangential slots just before the nozzle. Oil viscosity around 80-120 seconds (Redwood

no.1) is necessary, so temperature must be raised suitably. Oil flow is proportional to square of

oil pressure. Due to this turn down ratio is very small. Effective atomisation is not possible

below 14-16 Kg/cm2.

2. Steam atomisation :

The oil passing through nozzles is intersected by steam at slightly higher pressure.

Some heating occurs in burner also. Satisfactory atomisation is possible down to 5 kg/cm2 oil

pressure. Turn down ratio 10:1.

3. Air Atomisation :

Same as steam atomisation. When atomising steak is not available, air can be used in

the same burners.

4. Spinning Cup Burner :

For very small installations.

Oil Burner Installation :

The essential features are (a) Air register (b) shape of burner throat (c) Diffuser and its

location with respect to throat and burner tube. The air register controls secondary air flow and

gives it a swirl to enable air to penetrate the flame. The burner throat is convergent-divergent.

The convergent shape directs air towards the flame so that good mixing occurs. The divergent

part allows development of oil spray cone and maintains close contact between oil spray and

air- (necessary for good combustion).

The impeller or Diffuser protects the flame from secondary air. It also helps swirling as

part of air passes through the openings in the diffuser. Air through the diffuser, should meet

the oil spray at about 90.

Slag on diffuser, damaged or distorted diffuser will give unsatisfactory flame and poor

combustion.

So far we have seen the importance of turbulence. The temperature necessary to ignite

any fuel must be provided to establish a flame first. Then, if design is satisfactory and operating

conditions, parameters are right, the combustion will be self-sustaining and a good flame will

be established.

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IGNITION TEMPERATURES OF SOME FUELS :

Bituminous Coal 4080 C

Semi Bituminous Coal 4660 C

Anthracite 4960 C

Acetylene 4820 C

Ethylene(C2H6) 5380 C

Hydrogen 6100 C

Methane 6500 C

CO 6540 C

The starting point is generally a H.S.D. or L.D.O. igniter. Oil is atomised by air(pressurised). A high voltage spark provides the ignition energy to establish the igniter flame(or PILOT TORCH).

Vertical flames of pilot torches are provided across the LDO/FO main guns to providethe ignition energy to the latter.

The oil guns have sufficient ignition energy to establish coal flames at adjacent burners.

D A T A

R. C. F. = 7.4 to 43.6 t/h. 2.7 to 16 rpm.C. M. L. = 33.87 t/h for 55 H.G. 70% through 200 meshF. O. = HV grade of IS 1593, C.V. = 10270 K.Cal/Kg.

Igniters Warm-up Heavy oilGuns Guns

1. Capacity 0.5 million 10% MCR 10% MCR

K.Cal/hr /Elevation /Elevation.

2. Turndown Nil 2.5:1 4:1

3. Firing Rate 50 Kg/hr. 1350 Kg/hr 1320 Kg/hr.

4. Oil Pr. 12-14 Kg/cm2 4.36 kg/cm2 8.5 kg/cm2

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BOILER AIR AND FUEL GAS SYSTEM

FUEL GAS PATH

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Fig. 2

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BOILER WATER CIRCUIT / STEAM PATH

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BOILER CONSTRUCTION

HISTORY OF BOILERS:Boiler means any closed vessel exceeding 22.75 liters in capacity used for steam

generation under pressure. The first Boiler was developed in 1725 & its working pressure was6 to 10 kg/cm2 and was called Wagon Boiler.

TYPES OF BOILERS: There are two types of Boilers :1) Fire tube boilers (Carnish & Lauchashire blrs.) developed in the year 18442) Water tube boilers developed in the year 1873.

Water tube Boilers are used in Thermal Bower stations. These are sub divided according towater circulation1) Natural circulation : Drum to down comers to ring main header to water

wall tubes & back to drum. Due to difference in density of water and steam this type ofcirculation takes place.

2) Forced circulation : As operating pressure of the boiler approaches to the criticalpressure, additional pumps are required to install in down comers, because at thispressure there is no appreciable density difference between water and steam to have anatural circulation of water.

According to working pressure the Boiler, Boilers are classified as:1) Drum type sub critical pressure boiler: When working pressure of the boiler is between

130 kg/cm2 and 180 kg/cm2, the boiler is called as, Drum type sub critical pressureboiler.

2) Critical pressure Boilers : When boiler working pressure is 221.2 kg/cm2, it is termed as,Critical pressure Boilers.

3) Super critical or drum less once through boilers: When boiler working pressure is 240kg/cm2, it is called as, Super critical.

All modern Boilers are top slung from steel structures. From the beams a series ofslings take up the boiler loads. Approximately suspended weight of one 210 MW boiler is 3640metric tones. Height of Boiler is about 64 meters and Boiler drum is at a height of 52 metersfrom the ground.

Boiler design consideration : Following factors are taken into consideration for designingthe modern boiler.1) Lowest capital cost, ease of construction, simplicity, safety, good working condition,

ease of maintenance.2) Efficient operation, effective baffling for heat transfer, well insulated casings, ability to

deliver pure steam with effective drum internals to generate steam of fuall capacity.3) Availability of auxiliaries.

Period of constructions : In India the Boiler is being constructed in three years i.e. 36months.

The main parts of Boilers are :

1) Boiler drum

2) Down comers

3) Water walls

4) Furnace

5) Platen superheater

6) Reheater

7) Final superheater

8) Primary superheater

9) Economizer

10) Burners

11) Ignitors

1) Boiler drum : size : Length : 15.7 meters, ID: 1976 mm, Thickness 132 mmThe drum is made of special carbon steel plates of SA299 A grade A-1 by fusion

welding (submerged arc welding). Two gauge glasses are provided for level indication. Threesafety valves are provided. Drum vents, chemical dosing live. Emergency blow down line areprovided.

Inside the drum there is a position called separating chamber through which steamenters from riser tubes and goes through primary separators called turbo separators. Turboseparators have spinning blades, moisture is separated here and the steam further goesthrough secondary separator and finally through drying screens. The drying screens are locatedin the upper part of the drum. Water level is maintained 254 mm below the geometricalcenterline of the drum, upper part is left to occupy the generated steam.

2) Down comers : Made of SA106 Gr. C materialThere are 6 down comers from boiler drum of size 406x32 mm and are joined to ring

main header to provide water to water wall tubes. There are two down comers of size 323.9x24.4mm joined to platen water wall header. Platen water wall header are not provided to everyboiler.

3) Water Walls : Made of SA 210 Gr. A1 material, 63.5x6.3 mm, 76.1mm.The water wall tubes forms membrane panels. The each membrane panel is of 22

tubes joined by fins welding and having length of 60 to 70 feet each and width of panel isabout 7 feet wide and there are 83 such panels. After getting heated water goes throughthese tubes by natural circulation to the drum. The latest design of furnace walls are fullycooled on all sides by bare tubes. Refractory covere on blocked tube walls are being abandoned.

4) Furnace Size : 13.868-m width, 10.592-m depth, and 5494m3 volume.

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The tall rectangular radiant type furnace has now become a feature of the moderndesign of pulverised fuel boiler. The height of modern boiler is increased to lower gas temperatureand reduce slagging. The furnace is of two passes. The 1st pass comprises of main furnace,enclosed by four walls of membrane panels 7feet wide & 60 to 70 feet in lengths. The firingequipment such as burners, oil guns, igniters are mounted in the first pass of the furnace,here combustion of fuel takes place and hence this the most hot zone of the boiler and iscalled as firing zone. The maximum heat transfer takes place in furnace only. Temperature ofthe firing zone is about 1200 to 1400 0C, where the heat is generated due to conversion ofchemical energy of the fuel. This type of furnace is called water-cooled furnace, as the membranepanels are made of tubes through which water is circulating (water wall tubes).

Over the water wall tubes from out side skin welding is done with M.S. sheet and glasswool lagging of about 100 to 150 mm thick is placed under G.I. sheets to reduce the radiationlosses from furnace. The out side temp is about 45 to 500C if effective insulation is done.The height of membrane panel is 60 to 70 feet to avoid joints in firing zone. i.e. A,B,C,Delevations of the boilers.

The extended furnace is called second pass where primary superheater and economizer,A.H. is installed.

5) Superheaters : The Superheater material should be suitable for the transient highmetal temp. During the start up condition superheater receives relatively high heat input &there is low steam flow through it. steam is superheated in the super heaters.i) Primary superheater or low temperature superheater (LTSH) : From drum steam

comes to LTSH this is in two stages called lower bunch & upper bunch. There are 134assemblies in each bunch at 102-mm pitch. The material used are SA209T, SA210 Gr. A,SA 213 T11. The size of tubes are 44.5x4.5 mm & temperature range is 4500C to 4800C.Soot blowing steam is taken from LTSH outlet before attemperation.

ii) Platen superheater : It is situated in furnace vertically. Its headers are in pent house.There are 29 assemblies at pitch of 457 mm. The pitch is more in comparison to othersto avoid choking or fouling. From LTSH the steam comes to platen superheater afterattemperation. The material used is alloy steel as SA 213 T11, SA 213 T22. SA 213 to347 H and it stands pto 5800C. The size of tubes are 51x7.1 mm & 51x8.6mm.

iii) Final superheater : Its headers are in pent house header no 13 & 14. it is situatedvertically behind reheater. It is having 119 assembly at a pitch of 114 mm and size oftubes are 51x7.6 mm the materials are SA213 T22 alloy steel and stands up to 5800C(alloy steel)

iv) Reheater : The materials SA213 T11 alloy steel & stands upto 5500C. The reheater tubesize is 54x3.6 mm and are placed behind the hotter section of superheater. This is ingeneral gives adequate protection. Temperature control of superheater is achieved byburner tilt mechanism and this mechanism also controls the temperature of reheat steam.If reheaters are located close to furnace can receive too much heat for initial steam flowcausing an excessive rise in reheat steam temp. The steam, which is coming from HP

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turbine, is heated up in R.H. to its normal temp. of 5400C and used in IP turbine. Reheateris in two parts called front and rear. In front R.H. there are 59 assemblies at a pitch of229 mm and at rear there are 89 assemblies at a pitch of 152 mm.

v) Economizer : It is placed between LTSH and Air Heater in second pass of the furnace forutilization of heat of flue gas for heating feed water, other wise the heat which is receivedby the economiser may go waste if it is not utilised in this way. The feed water after HPheaters passes through economizer and is heated by flue gas. After passing through theeconomiser feed water reaches to boiler drum. Economiser is in two bunches calledlower bunch & upper bunch. There are 270 assemblies at a pitch of 102 mm, the materialused are carbon steel of SA 210 Gr. A1 stands up to 4500C, size of the tubes are 44.5x4.5mm.

6) Windbox : The wind box is situated at 11 m level of Boiler it is in two parts one is on LHSand other is on RHS of Boiler. There are thirteen compartments in it on each corner out ofwhich 3 for oil burners, 6 for coal mills, 4 for auxiliary air. These compartments are connectedto burner tilt mechanism which is operated +/- 300 as per requirement according to finaltemperature of steam. The secondary air after air preheater comes to wind box and is given tofurnace along with fuel for complete combustion of fuel as per requirement.

7) Burners : Coal is used as a primary fuel and oil as secondary fuel during start up of Boilerand for flame stability at low loads & during other transient operating conditions. Burner is toatomise fuel, penetrate & mix with proper proportions for complete combustion. The burnersare situated at 3 elevations called AB,CD,EF. At every elevation there are four burners. FO/LSHS can be fired at all three elevations but LDO can be taken at AB elevation only for start upof Boiler. For every burner whether LDO/FO there is one igniter to ignite the burner. Nowigniters are being changed form HSD/LDO to HEA (High energy arc igniters, purely electrical)

8) Soot Blowers : About 78 soot blowers are provided at different zones to remove theaccumulated soot on boiler tubes for effective heat transfer.

Types of sootblowers :a) Wall Soot blowers : These are situated on the furnace and are 56 in numbers. These aredriven by electric motors. Super heated steam is blown through them to clean the designatedarea of the water wall.

b) L.R.S.B. : Long Retractable Soot Blowers are 20 in numbers. These are used to clean S.H.and R.H. and are located in 2nd pass of the furnace.

C) Two soot blowers are located on Air Heater to clean the baskets of A.H. Steam from P. R. D.S. is taken for this purpose.

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1.0 Description :The best features of all the pulverisers have been incorporated in the design of the

Bowl Mill. The bowl mill consists essentially of a reduction gear box, mill side and liner assemblyprimary air and mill reject chamber, revolving bowl and scraper, separator body with separatorbody liner assembly, grinding rolls and journal assembly, pressure spring assembly, classifier,multiport outlet assembly, central feed pipe and separating inner cone.

Motor is coupled directly to Worm shaft of the reduction gear which rotates the bowl ata reduced speed and transmits the total power required for pulverising the Coal.

2.0 Operation :

2.1 Grinding :Coal of @1 inch size is fed by the R.C. feeder through central feed pipe into the

revolving bowl of the bowl mill. Centrifugal force feeds the coal uniformly between the bullring and independently rotating spring loaded rolls to travel through the outer periphery ofthe bowl.

The springs, which load the rolls, impart the pressure necessary for grinding. Thepartially pulverized coal continues to move up over the edge of the bowl due to centrifugalforce.

2.2 Transporting the fuel to the furnace :Mixture of hot and cold primary air enters the mill side housing below the bowl and is

directed upwards around the bowl and around the separator body liners, which carry pulverizedcoal upwards into the deflector openings at the top of the inner cone. Then it comes outthrough the ventury and the multiport outlet assembly. As air passes upward around thebowl, it picks up the partially pulverized coal. The heavier particles strike the separator bodyliners and are returned to the bowl immediately for further grinding. The lighter particles arecarried up through the deflector openings.

The deflector blades in the opening impart the spinning action to the material with thedegree of spin set by the angle of opening of the blades, determining the size of the pulverzedcoal. Any oversized coal particle is returned down the inside of the inner cone to the bowl foradditional grinding, when pulverized to the desired extent the coal leaves the mill and entersthe pulverised fuel pipes and finally enters into the furnace through coal burners which areconnected at four corners of the boiler.

Orifice plates are installed in the coal piping leaving the pulverizer discharge valves tocompensate for unequal resistance to flow due to different lengths/bends of pipings to theburners.

BOWL MILL ( PULVERIZER )

2.3 Reject Removal :Any tramp iron or dense foreign material in the raw coal feeder, which is difficult to

grind, if carried over to the top of the bowl is dropped out through the air stream to the lowerpart of the mill side housing. Pivoted scrapers, attached to the bowl hub sweep the tramp ironor other foreign material around to the tramp iron snout through normally open by pyritehopper gate. The mill rejects can be intermittently taken out from the pyrite hopper, first byclosing the inner gate and opening the outer gate of the hopper.

2.4 Temperature Control :The Bowl mill can be isolated completely for maintenance work by closing the Hot air

shut off gates, cold air shut off gates, pulverizer discharge valves and seal air valve.Hot air control damper and cold air control damper, regulate the temperature of the air

entering the pulverizer by proportioning the air flow from the hot air and cold air supply ductso that mill outlet temperature is maintained between 75 to 850C irrespective of coal feed rateand moisture content of coal.

3.0 Mill air flow :Mill should be operated at the design airflow at all loads. Operating at higher airflow

will cause excess wear and fineness will be decreased. If mill is operated at lower airflow, itmay result into coal rejects & excess fineness.

4.0 Sealing arrangement :Since it is a pressurised mill, there is a possibility of entering coal dust into bearing /

gear box housing & damaging the bearings, worm & worm gear. To avoid this, sealingarrangement is provided. Sealing arrangement comprises seal air fans and filter. Alternativearrangement is made for getting seal air from service air compressors in the absence of sealair fans.

5.0 Bearings & Lubrication :Figure indicates the general arrangement of Bowl mill and showing the number of

bearings and their respective locations.Radial and thrust bearings of the worm shaft, Radial and thrust bearings of the vertical

shaft are lubricated by the same oil, which is filled in the reduction gear box housing andserves the purpose of lubricant for the main drive worm, worm gear, where as the upper andlower bearings roller journals are lubricated by means of self-contained circulation system.

The deflector regulators, journal stop bolts and spring adjusting bolts, bushings andbearings of spring stud and mill rotor geared couplings are grease lubricated.

The horizontal worm shaft bearings are fully oiled from the bath of oil in the gearcasing. The pumping action of worn shaft thrust bearing circulates oil through it, the radialbearing oil circulation is provided by action of worm gearing.

The vertical shaft lower thrust radial bearings are immersed in oil and are completelylubricated. The pumping action of this lower bearing assembly circulates oil through it.

Oil is supplied to the vertical shaft upper bearings by a screw pump bolted to the

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vertical shaft bottom end. Oil from the gear housing enters on an annular chamber in the oilpump busing through suitably drilled hole. As the shaft rotates, spiral grooves in the pumphub force the oil into a cavity below the vertical shaft. From the cavity the oil rises into a holedrilled in the shaft to a upper radial bearing. The oil then returns to the gear housing throughreturn oil gauge glass.

6.0 Oil Coolers :Tube type coolers are installed in the gear housing reducing the oil temperature when

the mill is in operation. Mechanical face seal arrangement through the space between thebowl hub skirt and the mill bottom casing prevents dust from blowing into gear casing.

The roller journals are filled with the lubricating oil upto the top seals.The pumping action of the roller bearings circulates oil from the reservoir in the journal

housing to the annular chamber between the bearings, then into the shaft bore and throughthe oil return holes back to the reservoir.

The upper journal housing is provided with a duel tip type seal to prevent oil wastage.The journals are kept clean by clean seal air brought through flexible tubing to holes providedin the trunion shaft and journal head for this purpose. This inward moving air prevents dustfrom getting into the bearing.

7.0 Mill specification :

1. Total number per boiler 6

2. Type Pressurised

3. Size XRP 803

4. Capacity of each mill 39 t/hr

5. Design coal grindability 55 HGI scale

6. Maximum moisture content 12 %

7. Fineness through 200 mesh screen 70%

MOTOR

Power rating 340 KW

Voltage 6.6 KV 3 phase

Frequency 50 C.P.S.

R.P.M. 980

Rated current 41.7 Amps.

No load current 20 amps.

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1) DESCRIPTION :Coal Feeding to any coal mill is regulated by means of a coal feeder. By changing

speed, a coal feeder controls the feeding to a coal mill and thus ultimately controls the BoilerPressure and load. Coal feeders are of two types.

1. DRAG CHAIN TYPE2. ROTARY TYPE.

100 lb rotary feeder means 100 lb coal in one revolution. The figure attached with thisindicate the features of Raw coal feeder (Volumetric). Following are main parts of Raw CoalFeeder.1. Raw coal feeder body with seal air provision.2. Inspection doors, Isolating gate, Rod gate.3. Spring loaded flap gate.4. Protective shear pin (key)5. No coal Flow device.6. R.P.M. indicator.7. Air cut-off solenoid in case of PIV gear box.8. Seal air to shaft glands.Note : While doing the maintenance works Raw Coal Feeder, P.A. Fan air must be cut off beforeopening any inspection doors, hot & cold air gates / dampers also must be closed.

2) SPEED CONTROL :Raw coal feeder that are generally in use are rotary type or drag link chain type. There

are volumetric feeders i.e. constant volume of coal per revolution of the feeder. Coal feedingcan be changed by varying the speed of the feeder. For achieving this a constant speed motoris coupled with PIV (Positive infinitely variable drive) gear box or Eddy Current Coupling.Feeder speed is varied as per the required mill loading. Accordingly loading of the coal millcan be done depending upon the speed of Raw Coal Feeder.

Equipments used for speed control of raw coal feeder are as follows :1. P.I.V. (positive infinitely variable drive.)2. Eddy current couplings (popularly known as Dynodrives)

2.1 P.I.V. Drive (positive infinitely variable drive) : The constant speed induction motoris coupled with the feeder through a PIV Gearbox. Variable speed between N1 to N2 is achievedby means of varying the distance between the two wheels W1 and W2 as shown.

COAL FEEDER

W 1

W 2

CHAIN

N 2 N 1 Fig. 1 : PIV Drive Principle

In case of drag link chain type feeder the coal bed thickness can be varied and thusadditional flexibility permits the use of less speed variation.

Shear pins are provided in the coal feeders so that this pin will fail and coal feeder willstop with an alarm in case of obstruction by any foreign material coming along with coal andprotect the equipment from possible damages.

2.2 Eddy current couplings :An eddy current coupling connects an AC motor driven fixed-speed input shaft to a

variable speed output shaft through a magnetic flux coupling. By reducing the level of fluxdensity within the coupling, slip between the couplings input and output shafts is increasedand speed is reduced. Slip is wasted energy in the form of heat.

A typical Eddy current coupling is shown in the fig.

Operating principle of Eddy Current coupling :The current signal of the Eddy Current coil is obtained through the Current sensing

Device. This value is always compared with the set value. The current in the Eddy Current Coilvaries directly in proportion to the load requirement. Thus in case, the load requirementincreases, the required coil voltage to maintain the set speed increases. This in turn increasesthe coil current. This current value indicates the loading factor, which in case if goes out of thelimit, the unit gives a visual indication of OVERLOAD FAULT and parallely operates onepotentially free contact (NO to NC) for the annunciation system. Under normal runningconditions visual display shows OVERLOAD HEALTHY condition on the front panel and PotentialFree contact remains NO.

The setting potentiometer is located inside the Panel which gives 0 to 100% rangecorresponding to 0 to 6 amps current. Once the fault has occurred the fault is latched bymeans of internal logic. The visual indication on the front panel and the potential free changeovercontact changeover their states for fault indications. The system doesnt trip automatically onoccurrence of this fault. If the fault is acknowledged and serviced, the Reset pushbutton onthe panel will Reset the card i.e. fault condition. If the fault is still present the Reset pushbuttonwill momentarily reset the system, but as soon as the Pushbutton is released the fault conditionwill reappear.

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