1.COAL – TYPES, WHY PULVERISATION?2.FIRING – DIRECT FIRING, INDIRECT FIRING3.PULVERISERS – TYPES, CLASSIFICATION4.BOWL MILLS – CONSTRUCTIONAL

FEATURES5.GRINDING PRINCIPLE 6. MILL REJECT SYSTEM7. OPERATIONAL PARAMETERS8. MAINTENANCE PRACTICES9. COMPARISION OF TUBE & BALL MILL

IT IS THE MOST EASILY AND ABUNDANTLY AVAILABLE, CHEAP AND TRANSPORTABLE, ENERGY

SOURCE

TYPES OF COALBASED ON PROXIMATE ANALYSIS OF COAL ON DRY BASIS:

1.ANTHRACITE

2.BITUMINOUS

3.SUB-BITUMINOUS

4.LIGNITE

PULVERISED FORM OF COAL• IT HAS MADE MODERN STEAM

GENERATING UNITS HIGHLY THERMAL EFFICIENT, RELIABLE AND SAFE.

• IT HAS MADE EFFICIENT USE OF LOW GRADE COALS.

• IT PROVIDES GOOD CONTROL OVER COAL AIR MIXTURE PROPORTION, FLAME STABILITY WITH MINIMUM LEVEL OF UNBURNT CARBON.

GRINDING PROCESSRAW COAL FROM COAL FEEDER

P.F. TO BOILER

P.F.+AIR

GRINDING ZONE

REJECTS

HOT AIR

FIRINGDIRECT FIRING:Coal is ground and is directly carried from the Pulveriser to the Furnace by use of Pressurised air. Pressurised Air from Primary Air Fans goes to the Pulverisers. Sufficient temperature is maintained for drying of coal and moving to the furnace without settling.

INDIRECT FIRING:Here coal is ground and is carried to an Intermediate Bunker. Based on the Unit load demand coal moves from the Intermediate Bunker to the Furnace. A suction fan helps in sucking the pulversised fuel to the furnace. This system is suitable, where the boiler load varies and the mills consume huge power. But, the disadvantage is that, the capital cost is high due to extra bunkers and fans.

FUNCTIONAL REQUIREMENTS OF PULVERISER

• GRINDING OF COAL • DRYING OF COAL• REJECTING UNDESIRABLE REJECTS

LIKE PYRITES & STONES• TRANSPORTATION OF PULVERISED

COAL

TYPES OF PULVERISERS• BASED ON PRINCIPLES OF PARTICLE

SIZE REDUCTION

- IMPACT- ATTRITION- CRUSHING

PULVERISERS USE ONE, TWO OR ALL THE THREE PRINCIPLES.

TYPES OF PULVERISERS Speed Low

10 to 20 RPMMedium

40 to 70 RPMHigh

900 to 1000 RPM

Type Ball tube mills(16 rpm in Vindhyachal)

Bowl Mill, MPS Mill, Ball and Race mill

Impact or Hammer Mill.

Beater Mill or Fan Mill.

TYPES OF MILL• Tube or ball mills: These are low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. If the diameter of the cylinder is greater than the length of the cylinder, the mill is called a ball mill. If the length of the cylinder is greater than the diameter of the cylinder, it is called a tube mill.

• Vertical spindle mill: These are medium-speed machines that include bowl mills, ring roll mills, and ring and ball mills. The bowl mills are further divided into deep bowl or shallow bowl mills.

• Impact mill: These are high-speed impact machines that use beater wheels to crush the coal.

Ball & Tube Mills

Tube mills: The tube mills are low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. If the length of the cylinder is greater than the diameter of the cylinder, it is called a tube mill.

TUBE MILLThese are slow speed Mills. The Mills are designated as BBD-4772Where,

B – Broyer (Name of inventor).B – Boulet (French word for Balls).D – Direct firing.47 – Diameter of shell (in Decimeters) i.e. 4.7m dia.72 – Length of shell (in Decimeters) i.e. 7.2 m length.

CONSTRUCTION OF TUBE MILLCYLINDRICAL SHELL LINER

SHORT CONICAL LINER

LONG CONICAL LINER

SCREW CONVEYOR RIBBON

HOT AIR TUBE

CONNECTING ROD

INLET LINER WITH BOSS

INLET LINER

RIBBON

REMOVABLESHAFT

ANGLE

CHAIN&

CHAIN LINK

DETAILS FOR SCREW CONVEYOR ASSEMBLY

PRINCIPLES OF GRINDING

Raw Coal is fed into the Mill for Grinding. Grinding takes place by Impact and attrition.

MILL DETAILS• Design coal capacity- 83.15 Tons/Hr• Ball charged- 90 Tons• Speed – 15 to 16 rpm• Screw assebly- Right hand helix(DE side)

Left hand helix (NDE side)• Direction of rotation from NDE end- Clockwise.• Shell door opening- two numbers.• Shell liners- 600 nos.• weight of liners- 60.26 MT

Bowl MillsThese are medium speed Mills. The Mills are designated as XRP-803/ XRP-1003 Where,

X – 3 Phase, 50 Hz Supply (Y for 60 Hz supply)R – Raymond (Inventor of the mill),P – Pressurized,80/100 – Nominal dia. of the Bowl (inches)3 – Number of the Grinding Rolls.

BOWL MILL

Bowl mills are employed to pulverize the pre-crushed raw coal to the required fineness before it is admitted into the boiler furnace for combustion. The operating characteristics of bowl mills make it the best mill available for the direct firing systemof the coal. The best features of all the pulverisers have been incorporated in the design of the bowl mill.

DEEP BOWL MILL

ROLLER ZONE

SCRAPPER ZONE

GEARBOX

FUEL DISCHARGE ARRANGEMENT

SHALLOW BOWL MILL

Motor

Primary Air Inlet Duct

Journal spring assembly

Roller Journal assembly

Worm Gear

Mill Base and Gear Housing

Main Vertical Shaft

Classifier

Inner Cone

Inverted Cone

Scraper assembly

P F discharge arrangement

Mill Foundation

Construction features

ROLLER ZONEROLLER ZONE INCLUDES:

1. ROLLER JOURNAL ASSEMBLY (03 NOS.)2. INNER CONE3. INVERTED CONE4. CENTRAL FEED PIPE5. VANE WHEEL (AIR PORT ASSLY.)6. BULL RING SEGMENT (BRS)7. BRACKETS

SCRAPER ZONESCRAPER ZONE CONSISTS OF :

1. SIDE LINERS2. AIR CHAMBER3. GUIDE VANES4. SCRAPER5. INSULATION COVER PLATE6. REJECT OUTLET

FUEL DISCHARGE SYSTEMTHE FUEL DISCHARGE SYSTEM CONSIST OF :

1. CLASSIFIER2. VENTURI VANES3. VENTURI O-COLLAR4. MDV & MPO5. TRANSITION PIECE & PF PIPES

SEAL AIR SYSTEM

Seal air for the journal assembly is supplied through the ends of the trunnion shaft, through holes bored through the shaft and into the journal head. The mill area above the bowl is usually under suction, and air is pulled into this area. The air flows from the journal head through the small annular clearance between the upper journal housing and the journal head skirt. The journal housing rotates, and the journal skirt is stationary. The flow of seal air prevents the coal dust from going up to the area around the oil and possibly contaminating the oil supply.

MILL REJECT SYSTEM

MILL HAS BEEN DESIGNED TO REJECT TRAMP IRON, PYRITES & OTHER DENSER & DIFFICULT TO GRIND MATERIAL. WHEN THESE COME UNDER THE ROLLER, THE ROLLER LIFTS UP ALLOWING THESE TO PASS UNDER. THESE ARE FURTHER CARRIED OVER THE EDGE OF THE BOWL & DROP THROUGH THE MILL SIDE AIR INLET HOUSING. THEN, SCRAPPED BY SCRAPPERS THROUGH TRAMP IRON SPOUT TO PYRITE HOPPER, FROM WHERE THEY ARE PERIODICALLY REMOVED

MILL REJECT SYSTEM

GEAR BOX

THE MAIN PARTS OF GEAR BOX ARE :

1.WORM SHAFT2.WORM GEAR 3.VERTICAL SHAFT

WORM SHAFT & WORM GEAR

GEAR BOX

LUBRICATION•Lubrication of Gearbox components• Upper radial bearing by external Lubrication

system• Sump lubrication of Lower radial bearing and

thrust bearing• Worm gear and worm shaft bearings lubricated

by sump oil. The worm shaft bearings are splash lubricated.

•Lubrication of Roller Journal assembly Bearings• The journal assembly is filled with oil and

internal circulation takes place as the roller assembly rotates around its axis.

LUBRICATION IN RJ ASSLY

LUBRICATION•Lubricant Used and Qty.

•Gear box - Servo mesh SP-320 – 2800 lit/mill

•Roller Journal assembly- Servo mesh SP 680 qty- 83 lit/RJ assly. - 250 lit/Mill

OIL REPLACEMENTParts to beLubricated

Lubricants Frequency ofOil check

Remarks

Mill Drive GearHousing

Servo mesh sp-320

03 months Change oil as per lab report

Roller Journals

Servo mesh sp-680

03 months Change oil as per lab report

Journal stop bolt

Servogem-3

01 month -

Worm shaft oil seal

Molikote33 01 month -

PARAMETERS WHICH DETERMINE THE PERFORMANCE OF MILL

• RAW COAL-HGI,TM,SIZE

• PF FINENESS

• R.C.FEEDER CALIBRATION

• PA FLOW

• MILL OPERATING PARAMETERS

• MILL INTERNALS CONDITION AND TUNNING

Sizes of raw coal Larger the size of raw coal fed to mill, amount of work per unit mass

is increased to get fine coal of small fineness. Hence the mill capacity varies inversely with the size of the raw

coal. Using smaller size raw coal feed to mill to increase the milling

capacity only shift the workload to the raw-coal precrusher.Normally

optimum coal size is selected so that total initial, operating and

maintenance cost of precrusher and mill can be kept minimum. Coal passing through 20/25 mm mesh is found suitable for Indian

conditions

FINENESS ANALYSIS

FINENESS

FINENESS DETERMINES

SPRING COMPRESSION DEFLECTOR BLADE POSITION WEAR OF GRINDING ELEMENTS WHICH IN TURNS EFFECTS -P.F. RECIRCULATION -UBC IN BA & FA -REJECTION -DP (BOWL DP)

COAL GRINDABILITYGrindability is defined as the ease with which the coal can be pulverized. This should not be confused with hardness. Coal of the same hardness may have a range of different grindability indices because of other constituents, such as moisture. A standard index has been developed based on use of the Hardgrove Grindability machine and is called the Hardgrove Grindability Index. Grindability is determined by the amount of new material that will pass through a 200 mesh sieve. A 50-g air-dried sample sized to greater than 16 mesh and less than 30 mesh is placed in the Hardgrove machine with eight 1-in. steel balls. A weighted race is placed on the balls, and the machine turned for 60 revolutions. The resultant coal size is then compared to an index and a value assigned from the index.

GRINDABILITY VS CAPACITY

The design rating on all Alstom RB pulverizers is based on a grindability index of 55 with 70% passing through a 200 mesh screen.

RC FEEDER CALLIBRATION

FEEDER RATEThe manufacturer’s recommendation is not to operate the mills below 40% of the design capacity without ignition support in the boiler. Below 40% design capacity, the air and fuel mixture can cause coal flame stability problems and boiler explosions. With ignition support the minimum feeder rate is 25% of the pulverizer capacity. At feed rates below 25% capacity, any momentary interruption of coal feed will allow the pulverizer to empty. This will cause a lossof boiler fire and possible boiler explosion.

PRIMARY AIR FLOWPrimary air provides the means to dry, classify, and transport the coal from the grinding zone of the mill through the classifier & PF pipes to the furnaceTwo constraints for the PA flow are:• Ability to maintain the minimum air velocity (15–16 m/sec) to transport the coal• To maintain the correct air & fuel ratio between 1.6-2.4

OPERATING PARAMETERSMILL AIR FLOW – MILL SHOULD BE OPERATED AT THE DESIGN AIRFLOW AT ALL LOADS. OPERATING AT HIGHER AIRFLOW WILL CAUSE EXCESS WEAR AND UPSET THE PERFORMANCE OF CLASSIFIER ETC.

MILL OUTLET TEMPERATURE – TOO HIGH A OUTLET TEMPERATURE MAY LEAD TO A PULVERISER FIRE & TOO LOW OUTLET TEMPERATURE MAY NOT DRY THE COAL SUFFICIENTLY.

PULVERISED FUEL FINENESS – LOW FINENESS RESULT IN UNBURNT CARBON & HIGH FINENESS RESULT IN INCREASE IN WEAR RATE & REDUCTION OF MILL CAPACITY.

MILL CAPACITY – THE CAPACITY OF MILL DEPENDS ON THE HARD GROOVE GRINDABILITY INDEX, THE MOISTURE CONTENT OF COAL & THE PULVERISED FUEL FINENESS OUTPUT REQUIRED.

OPERATING PARAMETERS• MAINTAIN Mill Outlet Temperature

BETWEEN 70-80°C.

• REDUCE THE MILL LOAD IF Mill Outlet temperature DROPS BELOW 60°C

• MAINTAIN PF FINENESS between 65-75% THRU 200 MESH AT A MILL LOAD OF DESIGNED MILL CAPACITY

OPERATING PARAMETERSCONT….

• MAINTAIN BOWL DP ≤ 300 mm WC

• REDUCE THE SPRING COMPRESSION BY 5% FOR EVERY 10 POINT INCREASE IN HGI

• KEEP THE UBC IN FA & BA AS PER THE REQUIRED VALUES, BY TUNING BOILER OPERATING PARAMETERS TOO

• MAINTAIN VARIBLE AIR FLOW TO IMPROVE COMBUSTION, REDUCE WEAR AND MAINTAIN P.F. FINENESS

MILL CAPACITY

• MILL CAPACITY WHEN HGI

• MILL CAPACITY P.F.FINENESS

• MILL CAPACITY RC TM

MILL CAPACITY

CONT…

• MILL CAPACITY RC SIZE

• MILL CAPACITY MOT

• MILL CAPACITY BOWL DP

MILL INTERNALS & MAINTENANCE PRACTICES

- The grinding elements are the most wear prone elements. - The Grinding elements consist of 3 Nos. Grinding Rolls and One Set

Bull Ring Segments.- A life chart based on the running hours is maintained on monthly

basis.- Based on the running hours of the elements and their wear

measurement, the Grinding Elements are replaced.- Presently, the Grinding Rolls replaced are of High chrome Carbide

Insert Rolls and the Bull Ring Segments are of High chrome Materials.- The Grinding Rolls are replaced after 6000 hours and at the same

time the Bull ring segments are also replaced.- The Liners are another wear prone components to protect the Mill

Body.- The liners are of Ni-Hard material and are replaced when they get

damaged due tp erosion.- Apart from this, the ceramic tiled items like Inner cone, Ventury vanes,

Mill discharge Valves, Multiport Outlets, Orifices are replaced after 10000-15000 operating hours.

DAMAGED BRSRJ ASSEMBLY

WEAR MEASUREMENT OF GRINDING ROLL

CLASSIFIER CONE WITH CERAMIC INSTALLED

CLASSIFIER SETTING

• Setting of Classifier is adjusting the opening of the classifier.

• There are 2 positions at the extreme. – 0 : Means all classifier vanes are fully Open– 10 : means all classifier vanes are closed.

• Based on the fineness analysis, the classifier is set to achieve the desired fineness level.

CLASSIFIER SETTING

For optimum mill operation, the classifier pointers should be set between 0 and 3. If the coal is too fine when the setting is on point 1, the spring pressure on the rolls may be too great. If the coal is too coarse when the setting is on 3, the springpressure on the rolls may not be enough.

INVERTED CONE POSITIONING

If the inverted cone is raised to a point that the clearance between the inverted cone and inner cone is greater than 4 in., coarse coal will be carried out of the mill and not returned to the bowl for grinding.

SPRING COMPRESSIONOne indication that the spring pressure is too high is a rumbling noise at low loads. If the spring pressure is too low, the rumbling noise can occur at high loads.

SPRING COMPRESSION

ROLLER JOURNAL & SPRING ASSLY

REPLACEMENT OF RJ ASSLY

JOURNAL OPENING COVER

REPLACEMENT OF RJ ASSLY

COOLER CLEANING

• Drain oil from gear housing & drain water from cooler tubes. • Remove cover head of cooler• Use wire brush of standard Length to remove the scale formed in tubes. • Pressurize the tube bundle with water. • Repeat the process till clean water will come.

FOREIGN MATERIALS IN MILLS

These materials do not pulverise readily and therefore should be kept out of the raw fuel supply. When they get into the system, they collect and may catch fire. Each time the mill is opened for any reason, remove any such debris from air inlet, inner cone, bowl or other areas.

SCRAPER REPLACEMENT

A scraper is attached to the bowl hub skirt and pushes debris to the reject chute. A rigid guard acts as a shield for the scraper pivot arm. Bottom edges are most susceptible to wear and should be replaced after wear. Maintaining proper support (pins and bushings) helps to minimize wear.

SCRAPER ASSEMBLY

VENTURI OUTLETThe outlet venturi is located at the top of the mill just below the mill discharge valves

The venturi distributes the pulverized coal into the fuel lines as the coal exits the mill. The newdesign outlet venturi distributes the coal and air mixture more evenly and with less turbulence tothe fuel outlets. The less turbulent flow reduces the wear on the venturi components and thedischarge valve bodies.

MDV & MPO

The discharge valves prevent the boiler gas from returning to the mills. The flapper type valveuses a disc that is removed from the coal stream when the valve is open. Removing the discduring operation eliminates disc wear and maintains a positive barrier between the mill andboiler when the valve is closed.

MILL CLEARANCES

• Vertical gap between body liner segment and rotating vane segment of air port assembly. - 5 to10 mm• Gap between inverted cone and inner cone. -100 mm• Gap between center feed pipe lower and inner cone. -115 mm• Vertical gap between scraper and insulation cover plate. -16 to19mm• Vertical gap between skirt and insulation cover plate. -19 to21mm• Vertical gap between scraper guard and insulation cover plate. -10 to16mm• Radial gap between scraper and mill side bottom liner. - 25 to 30mm

• Radial clearance between Air Seal housing & Seal runner - 1 + 0.0 - 0.05

BEST PRACTICES IN MAINTENANCE OF BOWL MILLS

• Both sides Ceramic Tiled Inner Cones have been used to improve Mill fineness for better combustion.

• Spring loaded Mechanical face seals are being used for higher life of Mill Gearbox bearings and seals.

• High Chrome plated Roller Journal shaft sleeves are being used for better life of shaft seals and increased life of Roller Journal bearings.

• Ceramic Tiled Orifices are being used for higher life of Orifices and even distribution of Fuels in Pulverised fuel pipes.

BEST PRACTICES IN MAINTENANCE OF BOWL MILLS

• Ceramic Mill Discharge Valves and Multiple Port Outlets are being used for Higher life.

• Inside of Mill Body has been lined with ceramic Tiles for preventing any erosion.

• Rotating vane Wheels have been installed to improve the fineness and even distribution of Flow in all pipes to improve Boiler efficiency by reducing unburnt carbon.

BOWL MILL PERFORMANCE OPTIMISATION TENCHNIQUE

1. TO AIM AT THE DESIGN- MILL OUTPUT- LOW RATE OF MILL REJECTS, ESPECIALLY

COAL IN IT TO BE LESS THAN 10%- PULVERISED COAL FINENESS AT MILL

OUTLET, WITH COMBUSTIBLES IN FLYASH AND BOTTOM ASH IN LIMITS.

- OPTIMUM AUXILIARY POWER CONSUMPTION.

BOWL MILL PERFORMANCE OPTIMISATION TECHNIQUE

• THE PROCEDURE FOR MILL OPTIMISATION IS A SERIES OF TESTS CARRIED OUT TO ACHIEVE THE DESIRED RESULTS, LIKE-

- CLEAN AIR FLOW TEST AND HOT AIR FLOW TEST, CALIBRATION OF AIR FLOW INSTRUMENTS.

- OPTIMISATION OF JOURNAL SPRING COMPRESSION LOAD.

- OPTIMISATION OF BOWL DP VALUE, KEEPING % REJECTS UNDER CONTROL

- CLASSIFIER VANE SETTINGTO MAINTAIN PULVERISED FUEL FINENESS.

- HEIGHT OF THE VENTURI COLLAR TO MAINTAIN THE DESIRED PULVERISER FUEL FINENESS AND DP ACROSS THE BOWL.

- RING-ROLL CLEARANCE TO REDUCE HIGH BED FORMATIONON THE BOWL AND TO REDUCE REJECTION RATE.

- INVERTED CONE GAP TO MAINTAIN THE PULVERISED FUEL FINENESS.

- SPRING SEAT GAP TO ENSURE EQUAL LOAD ON BOWL BY SPRING ASSEMBLIES.

BOWL MILL PERFORMANCE OPTIMISATION TECHNIQUE

MILL FIRE

The usual causes of pulveriser fires are:i)Excessive Mill Outlet temperaturesii)Foreign material such as paper, bags, belt etc.iii)Excessive accumulation of pyrites or coal at the mill bottom or air inlet to the mill.iv)Improper or unusual operation

INDICATIONS:The two most common indications of a fire in a milling system are:

a)A high or rapidly increasing mill outlet temperature without other cause.

b) Paint peeling from the mill and/ or piping.

Fire extinguishing procedure

If a mill system fire does occur, perform the following:

a) Regardless of the fire location DO NOT shut down the mill and NEVER open any mill inspection doors until all evidence of the fire has disappeared. b) SHUT THE HOT AIR SHUT OF GATE.

c) Place the feeder on manual control and continue feeding coal at a high rate without overloading the mill.

d) Cool the mill by admitting steam or water.

• FIRES UNDER THE BOWL USUALLY DO NOT CONTRIBUTE TO ABOVE BOWL FIRES OR EXPLOSIONS UNLESS SOMETHING IS ALLOWED TO VIOLENTLY DISPERSE THEM, SUCH AS A SUDDEN SLUG OF WATER. THE MOST EFFECTIVE WAY OF TREATING THEM IS TO CONTINUE RUNNING THE MILL WITH COLD AIR UNTIL THE FIRE GOES OUT. THE MILL AIR INLET SHOULD BE CLEARED AT EVERY OPPORTUNITY.

Fire extinguishing procedure

Advantages & DisadvantagesBall & Tube Mills Bowl Mills

Mill output is controlled by Variation of Primary Air Flow

Mill output is varied by variation of Coal flow into the Mill

Air –Fuel ratio is less thereby erosion in the down stream of Mill is low

Air-Fuel ratio is high enough to cause increased erosion in the downstream of the Mill

The Mill responds to faster load variation and is quite helpful in the ABT regime for UI charges

The response to load variation is slow

There is no Mill reject and hence, there is no Mill reject handling system

The Mill rejects require a Mill reject handling system

Wearing elements (Grinding Balls) are refilled during running condition of Mill and hence, less downtime

For replacement and setting of clearances of wear elements (Grinding Rolls & BRS) mill shutdown is required

Mill fineness is higher and hence, the combustion in Boiler is better

Mill fineness is comparatively lower

Large reserve capacity Less reserve capacity

Advantages & DisadvantagesBall & Tube Mills Bowl Mills

No Mill purging is required Mill purging is required on regular basis depending on coal quality

Requires open gear lubricants, attention is required on handling of grease

Does not require a grease handling system

Requires longer cooling time to start work Requires very short cooling time

Mills require minimum spares if maintained properly and foreign material ingress is controlled

Mills require major spares due to high wear & tear

Mill availability is marginally higher as compared to Bowl mills

Mill availability is lower as compared to Ball mills

Cost on Consumption of wearing elements is high

Cost on Consumption of wearing elements is low

Cost on power consumption is very high

Cost on consumption of power is very much lower.

Maintenance is difficult and cumbersome Easy maintainability

Can grind a wide range of coal Difficult to grind varieties of coal

OPERATE AND MAINTAIN YOUR

EQUIPMENT SAFELY