chapter 7 pulverisers plant

BOILER AND AUXILIARIES PULVERISER PLANT

CHAPTER 7

PULVERISER PLANT

7.1 INTRODUCTION

Coal can be burnt in a many ways such as hand firing, stoker firing, and pulveriser coal

because of many advantages.

1. STOKER FIRING SYSTEM

Stokers are equipment, which mechanically feed coal uniformly into a grate at the

bottom of the furnace and remove the ash residue from the bottom of the furnace.

2. PULVERISED FIRING SYSTEM

Here the coal is pulverised into a fine powder of size around 70 to 80 microns in a

pulveriser plant. This powdered coal is then sprayed into the furnace by suitable

methods. The coal burns when it floats inside the furnace. This is termed as suspension

firing. Burning coal in pulverised form makes combustion efficiency as high as 99%

compared to below 80% in stoker firing.

7.2 FUNDAMENTAL REQUIREMENTS OF A PULVERISER PLANT

Supply of coal in the pulverised form to the boiler furnace can be accomplished by

using different types of equipment and systems. The use of a particular type of

equipment is decided on the type of coal used, boiler requirements, user’s preference

etc. However any pulveriser plant must meet the following essential requirements:

(i) Row coal feeding

(ii) Drying

(iii) Grinding and circulating

(iv) Classifying

(v) Transporting

Raw coal Feeders

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A raw coal feeder is a device that supplies the pulveriser with an uninterrupted flow of

raw coal from the bunker to meet system requirements. The feeders have to regulate the

rate of coal flow corresponding to boiler load, calorific value of coal etc, as shown.

Fig. Pulveriser Plant

There are several types of coal feeders in use. The raw coal feeders are classified

according to the method by which the rte of coal flow is determined as:

(i) Volumetric feeder

(ii) Gravimetric feeder

(i) Volumetric Feeder

In this feeder keeping the volume of coal moved per rotation and bulk density of coal as

constants, by varying the speed of the feeder alone, the desired rate of coal flow is

achieved.

(ii) Gravimetric Feeder

In a volumetric feeder if there is any variation in bulk density of coal or the volume

moved per rotation, the actual mass rte of coal flow for a given speed will be varied.

The Gravimetric feeders are used to provide an accurate rate of coal flow.

Some of the raw coal feeders more commonly found are:

(i) Scrapper or Drag link feeder

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(ii) Belt feeder

(iii) Rotary volumetric feeder

(iv) Gravimetric feeder

7.3 PULVERISERS

To effect the coal particle size reduction, needed for pulverised coal firing, machines

known as pulverisers or mills are used to grind or comminute the coal. The pulverisers

are generally based on rock and mineral-ore grinding machinery. The coal mills uses

one, two or all three of the basic principles of particle size reduction namely impact,

attrition and crushing.

There are mainly five types of mill:

(i) Ball Mill

(ii) Bowl Mill

(iii) Ball and Race Mill

(iv) Hammer Mill

(v) Beater wheel Mill

The principal type of coal pulverisers are classified according to their operating speed

as Slow, Medium and High speed mills. Five major types of coal mills used are

tabulated below according to their speed.

Speed Category Speed Range Mill

Slow speedBelow 50 rpm (Normal

range 20-30 rpm)(i) Ball/Tube/Drum Mill

Medium speed 50-100 rpmBowl Mill

Ball and race mill

High speed Above 225 rpmImpact or Hammer mill

Beater wheel mill

Of the above medium speed bowl mills are more commonly deployed in Indian Power

Stations followed by Ball mills and Ball and Race mills. High speed mills are rarely

used and generally limited to pulverising lignite only.

7.3.1 BALL MILL

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This mill is also called as Tube mill or Drum mill.

In this mill the coal and hot air called primary air or hot flue gas can be admitted and

pulverised coal is taken out at both ends. Otherwise at one end the raw coal is admitted

with hot air/hot gas and the pulverised coal is taken out at he other end. Large capacity

mills that are from 50 T/hr to 100 T/hr adopt the first method.

Fig. Ball Mill Coal Pulveriser

When the drum is rotating the row coal from the

feeder falls on the screw conveyor of the

trunnion. The screw conveyor pushes the coal

inside the mill by its rotational movement and the

coal intermingles with the ball charge inside the

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mill. Pulverisation of coal is accomplished through continued cascading of the

mixtures, result from

a) Impact of the failing balls on the coal,

b) Attrition as particles slide over each other as well as over the liners and

c) Crushing as balls roll over each other and over the liners with coal particles between

them.

Larger pieces of coal are broken by impact and the fine grinding is done by attrition and

crushing as the balls rolls and slide within the charge.

The hot primary air/gas introduced into the mill through the central tubes of the

trunnions completes the coal drying and carries the pulverised coal out of the mill via

the trunnions annulus around the central tube, counter to the raw coal feeding the mill.

Any bigger size particles are pushed back into the drum by the movement of the screw

conveyor for further grinding. The pulverised coal and the air enter the classifier

installed above the mill.

When the pulverised coal and air mixture enters the classifier, the vanes impart swirling

to the mixture as it travels towards the outlet. The swirling depends on the position of

vanes and separates the bigger coal particles by centrifugal force. These bigger particles

fall back by gravity to the trunnion inlet where they mix with the raw coal and re

pulverised.

In these mills operating under pressure, there is a special joint in the trunnion between

fixed part and the rotating part to prevent coal dust leaking out on to the atmosphere.

The joint consists of a lip of synthetic material rubbing on a smooth part. Counter

pressure provided by a seal air fan prevents any leakage to the outside.

The major advantages of this mill are

1) High availability

2) Low maintenance

3) Constant capacity and fineness

4) Hard and abrasive fuels grind efficiency

5) Large reserve capacity

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6) Increased fineness at low loads

7) Fast response over wide load range

8) Pulverise wide range of coals

9) Unaffected by foreign material

This mill having following disadvantages also which influence the decision of choosing

this mill for a given application

1) High power consumption. KW per unit of coal pulverised is very high particularly

at part load.

2) Larger in size requiring more mill bay area.

3) Chance of mill fire when the mill is idle.

4) Very noisy requiring special insulation for noise attenuation.

5) High moisture coals produced a large reduction in mill capacity.

6) High capital cost due to the size and weight.

7.3.2 BOWL MILL

Bowl mill is a vertical spindle medium speed mill. In a bowl mill the coal is pulverised

between discs called bowl rotates by the drive assembly and rollers kept above the disc

loaded by spring or pneumatic or hydraulic loading devices.

Some of the typical bowl mills are

Lopulco Mill

Berz mill

Raymond mill (Deep and shallow bowls)

HP mill (developed version of Raymond mill)

Coal from the raw coal feeder is fed at the centre of the bowl through a raw coal inlet

chute inserted at the centre of separator body top. Due to centrifugal force the coal

moves towards the periphery. The three rolls exert the required grinding pressure

through the springs. The primary air supplied to the mill side moves up through the

vanes around the bowl. By the deflector liners the air is directed towards the centre of

the mill which causes the re-circulation of the coal through the grinding area. The air

moving upwards picks the fine coal and enters the classifier through the vanes. The

vanes introduced spin and as a result course particles get separated from the stream and

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return through the annulus between the centres feed pipe and multiport assembly at the

outlet of the classifier.

Fig. Bowl Mill

Any hard foreign material entering the mill with coal cannot be pulverised. Such

particles when move on the bowl lift the roller journal assembly compressing the spring

and increase the gap between roller and the bowl. Once such particles clear the bowl the

spring brings back the roller journal assembly to the initial position. The pyrites fall

down from the bowl into the mill side assembly where they are swiped by the scrappers

and discharged into the pyrites hopper through the tramp iron spout. Periodically the

rejects are removed from the pyrite hopper by first closing the inlet gate and then

opening outlet gate.

The major advantages of this mill are

a) Low power consumption.

b) Reliability

c) Minimum maintenance.

d) Wide capacity.

e) Quiet and vibration less operation.

f) Ability to handle wide range of coals.

The demerits of this mill are

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a) Need for removal of mill rejects.

b) Foreign materials in coal mill cause damage to rollers.

c) Outage required for replacement of worn-out bull ring segments and rollers.

7.3.3 BALL AND RACE MILL

The construction and working of this mill is more or less similar to the bowl mill. The

major difference is instead of bowl and roller this mill has a lower grinding ring

connected to the drive assembly and a stationary upper grinding ring applied pressure

from pneumatic loading cylinder containing pressurised inert gas. Hollow steel balls are

carried between these two rings.

The bottom ring receiving the raw coal at the centre rotates and in turn rotates the ball.

In this process the trapped in between the grinding element gets pulverised and moves

towards the outer edge of the lower grinding ring. The coal air mixture is carried up to

the classifier, where the coarser particles are returned for further grinding and the finer

once go into the mill outlet pipes for the distribution to the burner.

Fig. Ball and Race Mill

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7.3.4 HAMMER MILL

It consists of thick casing with sufficient stiffeners. The inside of the casing is fully

lined with armoured steel plate. The mill shaft supported on two bearings run through

the centre of the casing. Discs, which carry the arms and hammer tips, are mounted on

to the shaft. The hammer tips are made from manganese steel to take up greater wear

during impact and shock. The shaft is deep drilled for the circulation of cooling water

for cooling the shaft, which comes in contact with hot air or hot gas as well as the inner

race of both bearings. The motor shaft and mill shaft are directly coupled, so the mill

runs at motor speed.

Fig. Hammer Mill

The hot air/hot gas supplied dries the coal in the mill and transports the coal to the

classifier. The coarser particles are returned by classifier for further grinding.

7.3.5 BEATER WHEEL MILL

The construction and working of a beater wheel mill is similar is many ways to a radial

fan. The mill consists of a spiral housing fully lined inside with lateral and

circumferential armours. The beater wheel mounted on an overhung shaft revolves

within the housing. Sets of heavy beater plates are fitted to the beater wheel supported

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by webs. The beater wheel shaft is supported on one side by two self-aligning roller

bearings, which are lubricated by an oil pump. The mill shaft is coupled to the motor by

a PULVIS coupling. During operation the beater wheel rotates at a speed of 420 rpm.

The beater wheel mill functioning like a fan creates vacuum in its suction side and

sucks the drying fluid, mostly the hot flue gas from the furnace upper section through a

duct. The fuel is also fed into this duct by the raw coal feeder. Grinding takes place by

the impact of the coal against the beater plates. Further grinding effects due to the

impact when the material is discharged by the wheel at high velocity over the armoured

liners of the housing.

These mills are excellent dryers and so they are mainly used for lignite and peat only.

Factors Affecting Mill Performance

The performance of the mill plant especially the pulveriser output is affected by a

number of factors mainly associated with the properties of coal being ground. Important

of these factors are

Grind ability index of coal

It is a measure of the ease with which coal can be pulverised. One of the methods of

measuring grindability index of coal is Hard Grove Grindability Index Number (HGI).

Higher the number easier the coal pulverisation. Normal HGI values of Indian

bituminous coals vary from 45 to 60. Lignite has high HGI up to 120. Thus a mill

designed to handle a coal having a particular HGI will have a greater output with

grinding coal more than that value and conversely a reduced output if low HGI coals

are handled. Mill output increases with high grindability index coal.

Fineness of milled product

Normally for bituminous coals to give optimum combustion efficiency a fineness of

70% through 200 mesh will be desired. The mill and classifier will be designed to

produce this fineness. Any increase in fineness will waste mill power. Reducing the

fineness can increase mill output but it is permissible only if the coarser fuel provide

satisfactory combustion.

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Moisture content

The total moisture content of the raw coal is made of inherent and free or surface

moisture. In any coal milling system drying of coal is adopted by using hot flue gas or

hot air. Insufficient drying produces agglomeration of fines in the pulverising zone and

difficulty to remove the fines efficiently and as quickly as they are produced. This

limits the output capacity of the mill when high moisture coal is used.

Size of raw coal

Larger the size of raw coal fed to the mill amount of work per unit mass is increased to

get fine coal of same fineness. Hence the mill capacity varies inversely with size of raw

coal. Generally the mills will be supplied by a uniform size of raw coal prepared in the

coal crushers.

Mill wear

Due to the grinding action and abrasive nature of coal, mill and exhauster parts wear

which depends on the period of service, type of coal, wearing properly of the material

etc. mill output will reduce as wear increases and this is because of loss of contact

surfaces. This aspect will be taken care of during mill design as such the mill can

deliver near full capacity till considerable wear takes place but after that the output

rapidly falls calling for replacement of worn out parts.

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chapter 7 pulverisers plant

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