Coal Handling Plant Maintenance and Operation Philosophy

GENERAL WORKING AND DESCRIPTION OF COAL HANDLING PLANT In thermal power station, Coal is a principal fuel and hence a careful thought is given to 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 to reduce the maintenance and running cost to the lowest possible figure f igure consistent to reliability reliabili ty.. 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 due to low capacity, low unloading rate and time consuming, this mode is not in much use for large thermal power stations. 2. Railways : coal is brought by railway wagons. One rack consists of 58 wagons. Each wagon contains 58 MT of coal. Locos bring the wagons from the marshalling yard and place them on wagon tippler. These wagons are then unloaded with the help of wagon tippler. If  these wagons are not unloaded in stipulated time period (generally 7 hrs.), demurrage charges are 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 railway track. The max. angle of tilt is generally set between 140 to 150°. The rate of unloading is 13 wagons per hour. The time required for one cycle of operation of this wagon tippler is as below. 1. Weighing Wagon + coal before tippling

-

15 sec.

2. Tippling of Wagon to hopper

-

90 sec.

3. Pause

-

5 sec.

4. Tippl ppling ing of Wagon gon bac back to home pos positio ition n

-

90 see. ee.

5. Weighing Wagon after tippling

-

15 sec.

The weighing machines are integral with tippler mechanism and are fit ted with a ticket printing recorder and totaliser. totaliser. b) Ring type (Rotary) Wagon Tippler : In rotary tipplers the wagon is fixed between the two large rings which are fastened to form a cage like structure. The cage is rotated and discharged coal falls into the hopper hoppe r right below the rail track. Angle of tippling is 140 - 160°. The rate of unloading is 25 wagons per hour. hour. The time required requi red for one cycle of operation of  this wagon tippler is 60 sec. only. 84

For easy and speedy movement of wagons, mechanized bettle chargers are provided before and after wagon tippler i.e. inhaul and outhaul bettle chargers. These wagon tipplers are provided with photocell protection to avoid the entry of other wagons 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 of  track hoppers is 4500 MT. MGR Railway system : This system is provided where coal mines are located near the power station. Railway wagons are used to transport the coal from coal mines to power station and unloaded wagons are returned to coal mines for refeeding. So this forms a ring type system. Wagons alongwith railway tracks being the MSEB propert y, this becomes the most economical way of coal transportation having a very low maintenance time and cost as compared to the ropeway system. 3. Ropeways : This mode of coal transportation is used where coal mines are located near the 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 bucket varies from 1 to 3 tons. Automatic loading and unloading mechanisms are provided at loading and unloading stations. Rate of unloading varies from 75 to 275 MT/Hr depending on the type of ropeways used. This type of coal transportation is very economical compared to road or rail transportation and gives assured supply of coal, being the MSEB property. The only disadvantage of 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 supporti ng as well as hauling the ropeway bucket along t he line. Between the stations the rope is supported on sheaves mounted on articulated beam equalizing the load on the sheaves. While travelling along the line the ropeway car running firmly attached to the main rope, their travel being entirely automatic requiring no attention of operator. operator. The capacity of each bucket is 1.0 T/hr. T/hr. and the line capacity is 75 T/hr. T/hr.

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For easy and speedy movement of wagons, mechanized bettle chargers are provided before and after wagon tippler i.e. inhaul and outhaul bettle chargers. These wagon tipplers are provided with photocell protection to avoid the entry of other wagons 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 of  track hoppers is 4500 MT. MGR Railway system : This system is provided where coal mines are located near the power station. Railway wagons are used to transport the coal from coal mines to power station and unloaded wagons are returned to coal mines for refeeding. So this forms a ring type system. Wagons alongwith railway tracks being the MSEB propert y, this becomes the most economical way of coal transportation having a very low maintenance time and cost as compared to the ropeway system. 3. Ropeways : This mode of coal transportation is used where coal mines are located near the 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 bucket varies from 1 to 3 tons. Automatic loading and unloading mechanisms are provided at loading and unloading stations. Rate of unloading varies from 75 to 275 MT/Hr depending on the type of ropeways used. This type of coal transportation is very economical compared to road or rail transportation and gives assured supply of coal, being the MSEB property. The only disadvantage of 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 supporti ng as well as hauling the ropeway bucket along t he line. Between the stations the rope is supported on sheaves mounted on articulated beam equalizing the load on the sheaves. While travelling along the line the ropeway car running firmly attached to the main rope, their travel being entirely automatic requiring no attention of operator. operator. The capacity of each bucket is 1.0 T/hr. T/hr. and the line capacity is 75 T/hr. T/hr.

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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 the line carries empty cars. Each tram car body is fitted with steel axle at each end to receive two wheeled tramcar track assembly with a central bushing. Four tracks are fitted with each car so that each is carried on 6 wheels. Since the tram cars turn completely upside down and down side up at the discharge and loading terminal respectively, no catches, latches or other mechanism is required to discharge or receive loads. Capacity of each bucket is 2.5 T/hr and line capacity is 200 T/hr. T/hr.

b) Bi-cable bucket type ropeway : The essential characteristic is the use of two t ensioned fixed track ropes on which the carriages are run. Each carriage with its bucket s suspended by means of hangers is i s locked to the endless continuously moving haulage rope. One of the two track ropes (topside) carries full buckets, the second track rope on the bottom side of the line carries empty buckets. The track ropes are supported on along the line at a convenient height above the ground by means of intermediate trestles, each trestles is provided with oscillating saddles with grooves for carrying ropes and sheaves for hauling ropes. Capacity of each bucket is 1.8-2.5 T/hr and line capacity is 200-275 T/hr. T/hr. TRACK ROPE (48/33 mm)

86

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

General Working of a Coal Handling Plant : As mentioned above, coal is brought to power station by e ither of three means of coal transportation. This coal is first conveyed to primary crusher with the help of different combination of conveyor belts and its rate of feeding is controlled by Electro-magnetic vibrating feeders. Conveyor belt before the crusher is provided with hanging magnets to separate ferrous materials. Stones are picked up manually. In primary crusher, coal is first crushed to 100 mm size. This coal is again conveyed to secondary/final crusher on belt system. Here vibrating 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 send to bunkering belt and with the help of coal trippers. This sized coal is finally fed to coal bunkers. This cycle is called coal bunkering. In case bunkers are full, then available coal is stored in stock yard with the help of  stacking belts /automatic stacker cum reclaimer. This cycle is called stacking.

87

In emergency when coal is not available in plant by railways/ropeways, then this stacked coal is diverted to the coal bunkers by reclaimimg conv. belts. This cycle is called reclaiming. The coal stored in bunkers is further send to coal mill for pulverization and combustion in boiler furnace.

Equipments used in Coal Handling Plant : 1. Conveyor Belt : These are made up of cotton or synthetic fibers and rubber piles placed in alternate positions normally vary from 4 to 6 ply. These are generally 900 to 1600 mm in width. In selecting 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 to adjust the belt tension and take up the slack belt. This gear is of robust construction and designed to protect from dust. It is fixed at an accessible place for adjustment and cleaning. 88

2. Automatic gravity take ups : Theses are provided in conv. belt system to maintain slack side tension, to permit length variation due to belt stretch / shrinkage, removal of starting  jerks and extra length for vulcanizing. Belt tension is automatically and continuously maintained by gravity take-ups. It consists of 2 bend pulleys and a tensioning pulley to which balancing weights are provided. This tensioning pulley is mounted on a travelling carriage, which is pulled by steel ropes to which a counter weight is att ached on sheaves. The length of take-up gear should not be more than 1.5 % of the belt centre length.

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

89

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 wei ght. In order to avoid this sagging, idlers are fixed at certain distance between these pulleys. Idlers consist of three rollers attached to the brackets at an angle of 20-35 0 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 without any 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. I t 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 belt life.(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.) and for 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. Whenever belt goes out of run, these idlers oscillate on either side, bringing the belt in center of axis of  the conv. system. This avoids damaging of the belt. 90

4. Return Self Aligning Idlers : These idlers also bring the return side belt to its center position if goes out of run. 5. Impact Idlers : These are provided at feeding points to increase the life of the belt and reduce spillage due to sagging below the side scals. The rollers of these idlers are fitted with rubber liners as shown in the following figure.

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 of  pulleys 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 pulle ys depend on the thickness of the belt and a useful rule is as follow : 91

! ! ! !

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 idl er and increase the arc of contact of the main pulley for effective gripping of the belt.

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 placed immediately below the coal receiving hopper. The feeders are designed to take coal from the hoppers 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 st ator coils, forming the magnetic ci rcuit to create a sequence of uninterrupted magnetic pulls on the armature which is connected to the vibrating bars through the centre clamp. During the first half of the cycle, the armature is flexed towards stator coil. And during other half wave rectified cycle, armature is pulled back with the help of the springs. This to and 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, which allows large size coal to the crusher. The lower deck is a sizing screen, which allows the coal to 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 than 450 mm size causes serious trouble in the crushers very often. b) Secondary Crushers : The secondary crushers, which are either hammer type or ring type crusher. These crushers further crushes coal to the size of 25 mm size.

92

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 propelled and travels on rails. It has been provided with clamping device to prevent it from running away.

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 depth of 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 coal to the crushers and are operated manually by travelling winches. b) Rotating Magnets : These are also fixed on driving top end of the conveyor belt before crushers. Small size belt is rotated across the running conv. belt with a separate driving mechanism. Material attracted to the portion under magnet is automatically thrown in the discharge chute.

93

c) Magnetic Pulleys : The head pulleys of the conveyors prior to crushers are arranged as magnetic pulleys. On long belts, tramp iron gradually takes up a lower position near the belt and so come under the influence of the pulley to a greater extent. These magnetic pulleys automatically 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 operated manually by the operator when he senses some severe disaster. This pull chord can be operated from any position along the length of the conveyor belt. 2. Belt sway switch : These are mounted on the conveyors and protect the belt excessive running out and getting edge worn / damaged.

f r o m

3. Zero speed switch : When the speed of the conveyor drops below predet ermined speed, it operates and trips the system to save it from congestion at the transfer points. It is usually fixed 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, is transferred to the crusher with conv. system where coal is crushed to 25 mm size. It is then transferred to the bunkers through tripper trolley as per the boiler unit requirement. Feeding rate 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 then coal is first brought to the crusher house, then it i s diverted to the coal stock yard with the help of stacking used when coal supply is not available by any means called STACKING.

by railways / ropeways is available, either crushed or bypassed and then conv. belts. This stacked coal can be of coal transportation. This cycle is

3. Stacking to Bunkering : In case bunkers are empty and wagon / ropeway coal is not available, then coal is first brought from stack-yard. It is then send to the crusher and thereafter to the bunkers with the help of reclaiming conv. belts. This cycle is called RECLAIMIMG. Automatic stacker cum reclaimers are used for stacking and recl aiming purpose, if available. General Problems faced in Coal Handling Plant : 1. Design Problems : Cal. Value and Ash % 94

Coal received in power station is having cal. Value much less and ash percentage more than the rated values recommended by manufacturer. Hence the systems in coal handling plant get overloaded resulting in low bunkering. 2. Rainy Season Problems : Chute choke ups, Coal yard -Slurry Formation Transfer chutes gets choked up due to wet or muddy coal. Slurry formed in coal yard may cause problems with electro-magnetic feeders at input po ints, frequent choke-ups at transfer chutes etc. 3. Other Misc. Problems: • Snapping of belts /ropes : Conv. belts and ropeway ropes get damaged or broken because of jerks and overloading problems due to various reasons. Repairing and replacement of  these belts and ropes require more time for maint. • Derailment of coal wagons : De-railment of wagons result in obstacle in unloading of  balance wagons in line. This results in lower bunkering and may attract demurrage charges from railway department. • Oversized coal/Muddy Coal : Oversized / muddy coal may cause damage to the belt system, frequent choke-ups of transfer chutes and damages to the crusher rings. ✦✦✦

95

FUEL FIRING 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 flow section or economiser, furnace, flue gas path, air pre-heaters and other auxiliaries of the boilers such as forced draught and induced draught fans, primary air fans, secondary air or sealing fans, coal pulverising mills, coal feeders, burners, ash removal and disposal arrangement such as Electrostatic precipitator, ash extractor, pressure conveyor, etc. In all methods of fuel firing, some basic principles are incorporated to get the most efficient combustion. These are – The Three ‘T’s i.e. Temperature, Turbulence and Time. Secondary air also place very important role in combustion of fuel. Often, the turbulence is provided 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 temperature must be maintained. For coal, ignition temp. is in the range 4000 C to 4250 C. A small rise in temperature 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 combustion as also complete combustion is possible wit h minimum excess air. (More than optimum excess air will increase the flue-gas loss through chimney). Consider oxy-acetylene flame without and 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 furnace design and type of firing. COAL FIRING : Stoker Firing : Modern high capacity boilers do not use stoker f iring. Even then, many stoker 96

fired boilers are in service even today and it is instructive to study the combustion process. The coal size is ¾” – 1". Primary air is admitted under the fuel bed. Ignition of coal and volatiles is due to temperature maintained by combustion of fuel and reflection of heat from arches. 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 fi xed carbon left behind starts burning at about 1/3 rd 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 the combustion of CO to CO2. Bottom layer ignites 2/3 rd down the grate. Combustion in a stoker fired can be controlled by changing the grate speed and adjusting air flow through the fuel bed. Uniform thickness of fuel bed is very important for proper and complete combustion. An Ignition plane is formed in the fuel bed, which may get disturbed due 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 now most essential to take advantage of high combustion rate and to reduce unburnt fuel to minimum. For high volatiles coals, a short flame is suitable as less time is needed for complet e combustion. 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 complete the combustion, which is a gradual process as volatile content is less. M ore residence time is therefore 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 tangential firing can be used if turbulence can be produced in the furnace. In this case combustion is not completed in an individual flame, but is completed in the FIRE BALL which is a turbulent mass of fuel, air and gases and volatiles. See figure (Short flame and tangential firing arrangement.)

97

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

DATA R. C. F. C. M. L. F. O.

= = =

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

Warm-up Guns

Heavy oil Guns

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

1. Capacity

✦✦✦

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BOILER AIR AND FUEL GAS SYSTEM

FUEL GAS PATH 10 0

Fig. 2

✦✦✦

10 1

BOILER WATER CIRCUIT / STEAM PATH

✦✦✦

10 2

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 & it’s working pressure was 6 to 10 kg/cm 2 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 1844 2) Water tube boilers developed in the year 1873. Water tube Boilers are used in Thermal Bower stations. These are sub divided according to water circulation 1) 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 of  circulation takes place. 2) Forced circulation : As operating pressure of the boiler approaches to the critical pressure, additional pumps are required to install in down comers, because at this pressure there is no appreciable density difference between water and steam to have a natural 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 pressure boiler”. 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 240 kg/cm2, it is called as, “Super critical”. All modern Boilers are top slung from steel structures. From the beams a series of  slings take up the boiler loads. Approximately suspended weight of one 210 MW boiler is 3640 metric tones. Height of Boiler is about 64 meters and Boiler drum is at a height of 52 meters from the ground. Boiler design consideration : Following factors are taken into consideration for designing the 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.

10 3

Period of constructions : In India the Boiler is being constructed in three years i.e. 36 months. 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 mm The 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. Three safety valves are provided. Drum vents, chemical dosing live. Emergency blow down line are provided. Inside the drum there is a position called separating chamber through which steam enters from riser tubes and goes through primary separators called turbo separators. Turbo separators have spinning blades, moisture is separated here and the steam further goes through secondary separator and finally through drying screens. The drying screens are located in the upper part of the drum. Water level is maintained 254 mm below the geometrical centerline of the drum, upper part is left to occupy the generated steam. 2)

Down comers : Made of SA106 Gr. C material There 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 323.9x24.4 mm joined to platen water wall header. Platen water wall header are not provided to every boiler. 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 is about 7 feet wide and there are 83 such panels. After getting heated water goes through these tubes by natural circulation to the drum. The latest design of furnace walls are fully cooled 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. 10 4

The tall rectangular radiant type furnace has now become a feature of the modern design of pulverised fuel boiler. The height of modern boiler is increased to lower gas temperature and reduce slagging. The furnace is of two passes. The 1 st pass comprises of main furnace, enclosed by four walls of membrane panels 7feet wide & 60 to 70 feet in lengths. The firing equipment 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 is called as firing zone. The maximum heat transfer takes place in furnace onl y. Temperature of  the firing zone is about 1200 to 1400 0C, where the heat is generated due to conversion of  chemical energy of the fuel. This type of furnace is called water-cooled furnace, as the membrane panels are made of tubes through which water is circulating (water wall tubes). Over the water wall tubes from out si de skin welding is done with M.S. sheet and glass wool lagging of about 100 to 150 mm thick is placed under G .I. sheets to reduce the radiation losses from furnace. The out side temp is about 45 to 50 0C 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,D elevations 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 high metal 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 134 assemblies 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 450 0C to 4800C. Soot blowing steam is taken from LTSH outlet before attemperation. ii)

Platen superheater : It is situated in furnace vertically. It’s headers are in pent house. There are 29 assemblies at pitch of 457 mm. The pitch is more in comparison to others to avoid choking or fouling. From LTSH the steam comes to platen superheater after attemperation. The material used is alloy steel as SA 213 T11, SA 213 T22. SA 213 to 347 H and it stands pto 580 0C. 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 situated vertically behind reheater. It is having 119 assembly at a pitch of 114 mm and size of  tubes are 51x7.6 mm the materials are SA213 T22 alloy steel and stands up to 580 0C (alloy steel)

iv)

Reheater : The materials SA213 T11 alloy steel & stands upto 550 0C. The reheater tube size is 54x3.6 mm and are placed behind the hotter section of superheater. This is in general gives adequate protection. Temperature control of superheater is achieved by burner 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 initi al steam flow causing an excessive rise in reheat steam temp. The steam, which is coming from HP 10 5

turbine, is heated up in R.H. to its normal temp. of 540 0C and used in IP turbine. Reheater is in two parts called front and rear. In front R.H. there are 59 assemblies at a pitch of  229 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 for utilization of heat of flue gas for heating feed water, other wise the heat which is received by the economiser may go waste if it is not utilised in this way. The feed water after HP heaters passes through economizer and is heated by flue gas. After passing through the economiser feed water reaches to boiler drum. Economiser is in two bunches called lower bunch & upper bunch. There are 270 assemblies at a pitch of 102 mm, the material used are carbon steel of SA 210 Gr. A1 stands up to 450 0C, size of the tubes are 44.5x4.5 mm.

6) Windbox : The wind box is situated at 11 m level of Boiler it is in two parts one is on LHS and other is on RHS of Boiler. There are thirteen compartments in it on each corner out of  which 3 for oil burners, 6 for coal mills, 4 for auxiliary air. These compartments are connected to burner tilt mechanism which is operated +/- 30 0 as per requirement according to final temperature of steam. The secondary air after air preheater comes to wind box and is given to furnace 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 Boiler and for flame stability at low loads & during other transient operating conditions. Burner is to atomise fuel, penetrate & mix with proper proportions for complete combustion. The burners are 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 up of Boiler. For every burner whether LDO/FO there is one igniter to ignite the burner. Now igniters 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 the accumulated soot on boiler tubes for effective heat transfer. Types of sootblowers : a) Wall Soot blowers : These are situated on the furnace and are 56 i n numbers. These are driven by electric motors. Super heated steam is blown through them to clean the designated area 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 2 nd 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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BOWL MILL ( PULVERIZER ) 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 assembly primary air and mill reject chamber, revolving bowl and scraper, separator body with separator body liner assembly, grinding rolls and journal assembly, pressure spring assembl y, 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 at a 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 bull ring and independently rotating spring loaded rolls to travel through the outer periphery of  the bowl. The springs, which load the rolls, impart the pressure necessary for grinding. The partially pulverized coal continues to move up over the edge of the bowl due to centrifugal force. 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 pulverized coal upwards into the deflector openings at the top of the inner cone. Then it comes out through the ventury and the multiport outlet assembly. As air passes upward around the bowl, it picks up the partially pulverized coal. The heavier particles stri ke the separator body liners and are returned to the bowl immediately for furthe r grinding. The lighter particles are carried up through the deflector openings. The deflector blades in the opening impart the spinning action to the materi al with the degree of spin set by the angle of opening of the blades, determining the size of the pulverzed coal. Any oversized coal particle is returned down the inside of the inner cone t o the bowl for additional grinding, when pulverized to the desi red extent the coal leaves the mill and enters the pulverised fuel pipes and finally enters into the furnace through coal burners which are connected at four corners of the boiler. Orifice plates are installed in the coal piping leaving the pulverizer discharge valves to compensate for unequal resistance to flow due to different lengths/bends of pipings to the burners.

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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 lower part of the mill side housing. Pivoted scrapers, attached to the bowl hub sweep t he tramp iron or other foreign material around to the tramp iron snout through normally open by pyrite hopper gate. The mill rejects can be intermittently taken out from the pyrite hopper, first by  closing 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 t he air entering the pulverizer by proportioni ng the air flow from the hot air and cold air supply duct so that mill outlet temperature is maintained between 75 to 850C irrespective of coal feed rate and 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, it may 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, sealing arrangement is provided. Sealing arrangement comprises seal air fans and filter. Alternative arrangement is made for getting seal air from service air compressors in the absence of seal air 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 and serves the purpose of lubricant for the main drive worm, worm gear, where as the upper and lower bearings roller journals are lubricated by means of self-contained circulation system. The deflector regulators, journal stop bolts and spring adjusting bolts, bushings and bearings 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 gear casing. The pumping action of worn shaft thrust bearing circulates oil through it, the radial bearing oil circulation is provided by action of worm gearing. The vertical shaft lower thrust radial bearings are immersed in oil and are completely lubricated. 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 oil pump busing through suitably drilled hole. As the shaft rotates, spiral grooves in the pump hub force the oil into a cavity below the vertical shaft. From the cavity the oil rises into a hole drilled in the shaft to a upper radial bearing. The oil then returns to the gear housing through return 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 the bowl 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 through the oil return holes back to the reservoir. The upper journal housing is provided with a duel ti p type seal to prevent oil wastage. The journals are kept clean by clean seal air brought through fl exible tubing to holes provided in the trunion shaft and journal head for this purpose. This inward moving air prevents dust from 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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11 0

COAL FEEDER  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 Boiler Pressure and load. Coal feeders are of two types. 1. DRAG – CHAIN TYPE 2. ROTARY TYPE. 100 lb rotary feeder means 100 lb coal i n one revolution. The figure attached with this indicate the features of Raw coal feeder (Volumetric). Following are main parts of Raw Coal Feeder. 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. :  While doing the maintenance works Raw Coal Feeder, P.A. Fan air must be cut off before opening any inspection doors, hot & cold air gates / dampers also must be closed. Note

2) SPEED CONTROL : Raw coal feeder that are generally in use are rotary type or drag link chain t ype. There are volumetric feeders i.e. constant volume of coal per revolution of the feeder. Coal feeding can be changed by varying the speed of the feeder. For achieving this a constant speed motor is 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 mill can 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 motor is coupled with the feeder through a PIV Gearbox. Variable speed between N1 to N2 is achieved by means of varying the distance between the two wheels W1 and W2 as shown. W1 CHAIN W2

N2

N1

Fig. 1 : PIV Drive Principle 11 1

In case of drag link chain type feeder the coal bed thickness can be varied and thus additional flexibility permits the use of less speed variation. Shear pins are provided in the coal feeders so that this pi n will fail and coal feeder will stop with an alarm in case of obstruction b y any foreign material coming along with coal and protect 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 flux density within the coupling, slip between the couplings input and output shafts is increased and 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 Coil varies directly in proportion to the load requirement. Thus in case, the load requirement increases, the required coil voltage to maintain the set speed increases. This in turn increases the coil current. This current value ind icates the loading factor, which in case if goes out of the limit, the unit gives a visual indication of “OVERLOAD FAULT” and parallely operates one potentially free contact (NO to NC) for the annunciation system. Under normal running conditions visual display shows “OVERLOAD HEALTHY” condition on the front panel and Potential Free contact remains NO. The setting potentiometer is located inside the Panel which gives 0 to 100% range corresponding to 0 to 6 amps current. Once the fault has occurred the fault is latched by means of internal logic. The visual indication on the front panel and the potential free changeover contact changeover their states for fault indications. The system doesn’ t trip automatically on occurrence of this fault. If the fault is acknowledged and serviced, the Reset pushbutton on the panel will Reset the card i.e. fault condition. If the fault is still present the Reset pushbutton will momentarily reset the system, but as soon as the Pushbutton is released the fault condition will reappear. 11 2

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