-RAJESH RANJAN

Pulveriser are employed to pulverize

the pre crushed raw coal to the

required fineness before it is admitted

into the boiler furnace for combustion

in Coal-fired Thermal Power Station.

I) RAW COAL FEEDING

II) DRYING

III) GRINDING AND CIRCULATING

IV) CLASSIFYING

V) TRANSPORTING

HIGH SPEED HORIZONTAL MILL

MEDIUM SPEED VERTICAL MILLS

BEATER MILLS

1000 RPM

LOW SPEED HORIZONTAL MILLS

BALL TUBE

MILLS 10 TO

20 RPM BOWL MILL

E TYPE MILL

MPS MILLS

30 TO 65 RPM

A Ball Tube Mills is designated as BBD 4772 where:

B - BOYERS (BODY)

B - BOULLETS (BALLS)

D - DIRECT (DIRECT FIRING)

47 - Diameter of the mill shell in decimeters.

72 - Length of the mill shell in decimeters.

BALL TUBE MILL NOMENCLATURE/ DESIGNATION

Ball Tube Mills used for Indirect Firing is designated as

BBI.

BALL TUBE MILLS ARE LOW SPEED HORIZONTAL PULVERISERS

USED IN COAL FIRED THERMAL POWER STATIONS FOR

PULVERISATION OF COAL.

IN 1989 BHEL COLLABORATED WITH STEIN INDUSTRIE, FRANCE

FOR MANUFACTURING OF THESE MILLS IN INDIA.

AT PRESENT 250 STEIN INDUSTRIE, DESIGN MILLS ARE IN

OPERATION THROUGH OUT INDIA.

HIGH AVAILABILITY

LOW MAINTENANCE

CONSTANT CAPACITY AND FINENESS

HARD AND ABRASIVE FUELS GROUND EFFICIENTLY

LARGE RESERVE CAPACITY

INCREASED FINENESS AT LOW LOADS.

LOW AIR TO COAL RATIO

PULVERIZES WIDE RANGE OF COALS

UNAFFECTED BY FOREIGN MATERIAL

NO REJECT HANDLING SYSTEM

ADVANTAGES OF TUBE MILLS

HIGH WEAR OF GRINDING MEDIA

HIGH POWER CONSUMPTION

LARGER IN SIZE REQUIRING MORE MILL BAY AREA

VERY NOISY REQUIRING SPECIAL INSULATION FOR NOISE

ATTENUATION

HIGH MOISTURE COALS PRODUCE A LARGE REDUCTION IN

MILL CAPACITY.

HIGH CAPITAL COST DUE TO THE SIZE AND WEIGHT.

HIGH CONSUMPTION OF GIRTH GEAR GREASE

MILL FIRE

DISADVANTAGES OF BALL TUBE MILL

CLASSIFIER ASSY.

SCREW CONVEYOR

MAIN REDUCER

RAW COAL CHUTE

MILL BODY

GRINDING MEDIA

GIRTH GEAR

AUXILIARY REDUCER

CLASSIFIER

MILL BODY

REFUSAL DUCT

GIRARTH GEAR

SOUND HOOD

CONVEYOR BODY

MAIN REDUCER

AUXILIARY REDUCER

HOT AIR BOX

SCREW CONVEYOR

MAIN MOTOR

CONNECTING DUCT

GIRTH GEAR GREASING

GIRARTH GEAR HOUSING

LOWER PART OF INTERNAL CONE

LINERS

INTERNAL CONE CLASSIFIER VANE ASSY

P F OUTLET BOX

REFUSAL BOX

The BBD mill consists of two perfectly symmetrical grinding circuits.

Raw coal is taken from the hopper by a gravimetric belt feeder. Coal falls into a mixing box where it is pre-dried by the hot by-pass air.

This coal then enters the mill by the trunion, whose screw conveyor pushes the coal inside the mill by its rotational movement.

The coal is then pulverized by the movement of the balls inside the rotating body. Grinding inside the mill takes place due to impact of alloy steel balls on liners of the Mill shell and due to friction and attrition.

The only grinding medium in this type of mill being the ball charge.

Contd…/-

Hot primary air enters the mill through the central tube of the

trunion. It completes coal drying and carries the pulverized coal out of the mill via the trunion annulus around the central tube, counter to the direction of raw coal feeding to the mill.

The Pulverized coal and primary air mixed with the by-pass air

from the mixing box enter the classifier installed above the mills, often at the feeder level.

Inevitably, the grinding medium suffers from wear related to the grinding phenomena, and this wear is all the greater if the coal contains a high percentage of abrasive ash or if a very fine grind is necessary.

The manufacturing cost of the balls, which are generally forged, is very low, also additional balls can be added without shutting down the mill, thanks to a mechanized system called ball feeding system.

This results in a mill with a very low maintenance cost and virtually 100% availability.

CONTROL

Unlike with other types of grinding, the flow of ground coal in the Tube mill is not controlled by adjusting the feeder speed but by adjusting the flow of primary air through the mill.

In the Tube mill the air to coal ratio remains steady whatever load in the mill.

At a given load, to increase the flow of pulverized coal at the mill outlet, it is sufficient to give an opening order to the primary air damper and instantaneously the flow of air, and thus of entrained pulverised coal, will increase.

Ball Tube Mill Performance basically depends on two factors:

Size reduction capacity of the coal,

Geometrical Characteristics of the mill.

This can be expressed as Q = f (HC, HR, FC, PB, DB, NB) where

Q = Flow of Pulverised coal (T/hr)

HC = Moisture content of coal (%)

HR = Hardgrone Grindability Index

FC = Coal fineness at mill outlet

PB = Weight of balls in the mill.

DB = Inside diameter of the mill.

nB = Rotational speed of mill.

For a given size of the tube mill DB and nB are fixed and hence the mill capacity/output depends mainly on HC, HR, FC and PB. As the mill clocks more number of operational hours, the shell liners do wear and this also plays a roll in reducing the capacity of the mill.