17098_coalhandling

8/12/2019 17098_coalhandling

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Thermal ( Steam ) Power plants

mainly consists of 4 circuits

Coal and ash Circuit

Coalproduced in the miningsite is

transportedto power plant site

Coal handlingequipment for generation of

steam

The combustionof coal producesashwhich

is collected and removed to ash storageyard through ash handling equipments


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FD or IDfans are used for supply the air to

combustion chamber of the boilerthrough air-preheater

The air preheater is placed in the path of flue gasesto preheat the air

The flue gases produced by combustion of fuels inthe boiler furnaces after passing around boiler tubes

and super heatertubes Pass through a dust collectoror precipitator where

most of dust is removed before venting it of toatmosphere through chimney

Air and gas circuit


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Prime moverdevelops powerby utilizing steamgenerated in the boiler

Then condenseris used to condensethe steamcoming out of prime mover

A pumpis used to feedthe condensate tothe boiler

The condensate leaving the condenser is heated in

feed heatersthrough extracted steamfrom lowestpressure extractionpoint of the turbine

The feed watermay also be supplied from externalsource to compensateany lossof steam and water.

In the boiler shell and tubeswater circulationissetup due to density differenceof water betweenlowand high temperaturesections

A super heateris used to super heatthe wet steamfrom boiler drumand is then supplied to prime

movers

Feed water and steam circuit:


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In the condenser, quantity of cooling water

required to condense the steamis largeandis taken either from lake, river or sea

The cooling wateris taken from upper side

of the river and then passedthrough the

condenser

The hot wateris then discharged to lower

sideof the river

The system is known as open system Where wateris not availablein abundant

water from condenseris cooled either in

cooling pondor in cooling towerthe system

is known as closed system

Cooling water circuit


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Main Components of Thermal

Power Plants

1. Boiler

2. Super heater

3. Economizer

4. Air preheater5. Reheater

6. Steam turbine

7. Generator

8. Condensers

9. Cooling towers

10. Pumps

11.Coal mills

12.FD and ID Fans

13.ASH Precipitators

14.Water treatment plant


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Essentials of steam power plant


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Fuels used in thermal power plant

Combustion of fuel is accomplished by mixing

with air at elevated temperatures

Oxygen in the air chemically unites with Carbon,

Hydrogen of fuels and produce heat

In thermal power plants normally steam is

produced from water by using combustion heat offuels (Except in Gas turbines)

Various fuels were used in thermal power plants

Fossil fuels( Coal, Oil & Gas) Industrial waste gases

Synthetic fuels or SYNFUELS


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Coal Coal is the principal energysource for Indiabecause of its

large depositsand availability

Coal originatedfrom vegetable matter, which grew millionsof years ago

Trees and plantsfalling into water decayed and laterproduced peat bogs

Huge geologicalupheavals buried these bogs under layers

of silt Subterranean heat, soil pressureand movement of earth's

crust distilled off some of the bog's moisture and hardenedit to form coal

Basically classification of coal is based on Physical andchemicalcomposition

Peat

Lignite and brown coal

Bituminous coal

Anthracite


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Coal analysis Two types of coal analysis

Proximate Analysis

Gives: Behavior of coal when heated

FC + VM + M + A = 100% by mass

Ultimate Analysis

Gives: chemical elements along with ash and

moisture

C + H + 0 + N + S + M + A = 100% by mass


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Desirable properties of coal

Highcalorific value

Smallsulphur content ( les than 1%)

Good burningcharacteristics for complete

combustion

Highgrind-ability index

Highweather-ability

Grading of coal done on the basis

Heating value

Size

Ash content

Sulphur content


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Peat

Lowgrade coal

First stage coal formation

Contains 90% moisture

Smallamount of volatile matter Not suitablefor power plants

Used in domesticpurposes

Requires 1-2 months for drying in sunlight Peat(20% water, Dried) has CV of 16 MJ / kg


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Lignite (Brown coal)

Intermediatestage of coal development

Highamount of moisture30-40%

Brownin color

High heating value and carbon compared to

peat

Should be stored to avoid spontaneous

combustion

Used in pulverizedform

Can be air driedeasily

Suitable for local useinstead of transporting


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Bituminous coal Containing 46-86% of fixed carbonand 20-40% of VM

Ashcontent may vary 6-12 % High percentage of volatile matter CVof 32 MJ / Kg

Sub-Bituminousis having less moisture, ashthan

lignite, used in briquette or pulverizedform

Semi-Bituminouscoal high carbonand heating value,

contain less moisture, ash, sulphur, &VM, Has

tendencyto breakinto pieces


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Anthracite Coals Contains more than 86% fixed carbon&VM

around 8% Ignites slowlyhas high CV of 36 MJ / Kg

Low ash, zero CAKINGpower

Difficultto pulverizeAnthracite coal

C l h dli


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Coal handling


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Coal handling system

In Case of Rail DeliveryMost Commonly Used are :

Wagon Tippler Hopper

Hydraulic Gates Opening


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Hydraulic Gates Opening

Operation

The gates bellow the wagons are hydraulic controlled

By opening the gates the coal flows downwards due to gravitational force

and is moved to the bellow pit

Demerits

Due to the dust the Hydraulic will not work smoothly

Require a lot of maintenance


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Wagon Tippler Hopper


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Crushers and breakers used to crus the big size coal into small size

coal

Driers used to dry the coal if it contains lot of moisture it is also further

done in the pulverizes

Magnetic sprayers used to separate the iron parts that come along withthe coal from the mines these may damage the parts of pulverizes or

may have a bad effect on the boiler

Preparation


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Coal Crushers

Why?Reduce size from 200mm to 20mm

Types of crushers

1.Ring Type Coal Crusher

2.Hammer Mill Coal Crusher

3.Brad Ford Breaker


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Ring Type Coal Crusher


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Operation

The coal is fed at the top left corner

Is crushed in between the plane loose

rings and the adjustable plates.

The loose rings with the jaws used to take

the crushed coal down words

Adjustable plate and the screen determine

the maximum size of the discharged coal


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Hammer Mill Coal Crusher


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OPERATION

The coal is fed at the top

Is crushed by the action of rings that pivot

off center on a rotor or by swinging

hammers attached to it

Adjustable screen bars determine the

maximum size of the discharged coal


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Brad Ford Breaker


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Brad Ford Breaker

Bradford breaker which is used for largecapacity work

It consists of a large cylindermade up of

perforated steel platesto which liftingshelvesare attached on the inside

The cylinder rotating slowly at about 20

rpmreceives coal at one end The shelves liftthe coal up and then the

coal dropsdown by gravity


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Belt conveyor


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Belt conveyor


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Screw conveyor


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Bucket & Grab bucket elevator


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Skip Hoist and flight conveyor


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Stoker and Re-claimer

Stokerterms for storing the coal intothe stock yard

Re-claimermeans to take the coal

back from the stock yard

It moves on the rail trackalong withadjustable length of belt conveyor


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Combustion Equipments

for Lumped coal burning

1. Grate-fired furnaces

2. Chamber-type or flame

furnaces


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Grate


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Chamber type


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Combustion Equipment For

Burning Coal

Fuel bed furnaces (coarse particles)

Pulverized coal furnaces (fine particles)

Cyclone furnaces (crushed particles) Fluidized bed furnaces (crushed small

particles)


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Fuel bed furnaces (coarse particles)

There are two ways of feeding coal on to the

grate

1. Overfeeding

2. Underfeeding

O f d t k


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Overfeed stoker

Underfeed stoker


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Overfeeding


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Overfeeding

Receives fresh coal from top surface has

following distinct zones

Fresh or green coal

Coal losing moisture (Drying zone)

Coking layer (loosing of VM) Distillation zone

Incandescent coke ( Fixed carbon is

consumed) Combustion zoneAsh layerprogressively cooled

Operations


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Operations

Primary airgets warmed up as it flows through the ash

layer

As it passes throughthe incandescent coke

layer(1200 C) Carbon convertedto Carbon dioxide

releasing heat continues till oxygenis consumed, iflayer is thickCO2is converted to CO reducing layer

temperature

The stream while passing through distillation zone VM

is added

In distillation zone moistureis added

now stream contains N,CO2,CO,H2,VM&moisture


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For combustion of this stream

A hot ignition point ( In the range of 1000-

1300oC

Done by providing a fire brick lined arch which stores

up the heat and remains at high temperature Sufficient fresh air

Secondary air or over-fire air

Turbulence

Providing secondary air at right angles to up-flowing

gas stream emerging out of fuel bed


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Results

Carbon is in CO, CO2 both are color less

Carbon with hydrogen in VM which will be

crackedto form free carbonat high

temperaturesuspended in gas stream

If Burneris not designed properlyor

operatedproperly leadsto unburnt carbon

particles this appears as black smokeonchimney top

U d f di


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Underfeeding


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Underfeeding

In underfeeding coal is fed from below

Primary air passing through holes diffusesthrough spaces in the raw green coal picking upmoisture

In distillation zoneto stream VM is added

In incandescent zoneVM breaks readily burnswith secondary air present aboveit where it is

fed from top VM burning is somewhat cooler need longer

time to ignite and burn


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Mechanical Stokers

Overfeeding

1. Traveling grate stoker

2. Chain grate stoker

3. Spreader stoker

4. Vibrating grate stoker

Underfeed stoker

1. Single retort

2. Multiretort

Traveling grate stoker


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Grate surface is made up of Cast Iron barsjoined together by links to form endlessbelt

Belt wound around two sprockets A coal gate regulates the depth of fuel bed

Simultaneous adjustment of Fuel bed

thickness, primary air flow controls theburning rate so that at the end of its rearash only remains


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Advantages & Disadvantages

Simple and Initial cost is low

Reliable in service and maintenance is low

Gives high rate of heat release per volume of the

furnace Limited coal can be carried on grate

Clinker problems are common

Ignition arches are requiredThere is always some loss in the form of

particles


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Spreader stoker


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Spreader stoker

Coal from hopperis fed by rotating feeder Feedernormally will have Blades fittedon the

drum

Fine particlesburn in suspension

Speed of the feeder varieswith the steam outputof boiler

Grate is made up of CI bars, Links underneaththe grate are connected to a lever

Fuels used may be Bituminous, lignite, woodwaste, baggase

Coal size used in 6-36 cm


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Advantagesand Disadvantages

A wide variety of fuels can be used

Clinker formation is reduced

High temperature preheated air can be used

Quick response to load variation Gives equal pressure drop and proper air

distribution

Operation cost is low

Difficult to operate variable sized coal particles

Fly ash and entrapped carbon particles


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Underfeed stokers


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Multiple retort


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Pulverised coal


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Why pulverisers?

Size reduction

(mm to micron)

Other things:

Drying (primary

airapprox 300

degree)

Feeding


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Types of Pulverisers

Ball and tube mill (low speed < 75 rpm)

Ball and tube race mill (medium speed 75-

225 rpm)

Bowl mill (medium speed 75-225 rpm)

Impact or hammer mill (> 225 rpm)

.depending upon the nature of coal

Pulverised coal firing


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Pulverised coal firingPulverised coal firing is done by two system :

(i) Unit System or Direct System. (ii) Bin or Central System.

Unit System. In this system the raw coal fromthe coal bunker drops on to the

feeder.

Hot air is passed through coal in

the feeder to dry the coal.

The coal is then transferred tothe pulverising mill where it is

pulverised.

Primary air is supplied to the

mill, by the fan.

The mixture of pulverised coal

and primary air then flows toburner where secondary air is

added.

The unit system is so called from

the fact that each burner or a

burner group and pulveriser

constitute a unit.


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Unit system

Advantages

(i) The system is simple and cheaper than the central system.

(ii) There is direct control of combustion from the pulverising mill.

(iii) Coal transportation system is simple

Bin or Central System.

C h d l f th l b k i f d b it t d h h t i i d


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Crushed coal from the raw coal bunker is fed by gravity to a dryer where hot air is passed

through the coal to dry it.

The dryer may use waste flue gases, preheated air or bleeder steam as drying agent.

The dry coal is then transferred to the pulverising mill.

The pulverised coal obtained is transferred to the pulverised coal bunker (bin) The transporting air is separated from the coal in the cyclone separator.

The primary air is mixed with the coal at the feeder and the mixture is supplied to the

burner.

Advantages

l. The pulverising mill grinds the coal at a steady rate irrespective of boiler feed.2. There is always some coal in reserve. Thus any occasional breakdown in the coal supply will

not effect the coal feed to the burner.

3. For a given boiler capacity pulverising mill of small capacity will be required as compared to

unit system.

Disadvantages

1. The initial cost of the system is high.2. Coal transportation system is quite complicated.

3. The system requires more space.

Bi t l t


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Bin or central system

Pulverised fuel firing system


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Pulverised fuel firing system


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Pulverized coal handling system


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g y

B ll d t b ill


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Ball and tube mill


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Ball and tube mill


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Ball and tube mill

B ll d t b ill


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Ball and tube mill

B ll d t b ill


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Ball and tube mill Mills can be designed and manufactured for very large

capacity. Also each mill can supply coal to two sets ofburner. Boiler can be designed with lesser number ofmills.

Higher power consumption per tons of coal and highstarting torque makes drive motor large.

Suitable for base load stations only where load variation isless.

Response to load change is very fast. During operationpresence of relatively large quantity of pulverized coal ingrinding zone acts as a storage reservoir. In case of

sudden increase in fuel demand fuel flow can beincreased almost instantaneously.

Start up and shutdown time is longer.

Product fineness is very good.


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Bowl Mill


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Bowl Mill

Bowl mill


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Bowl mill

B l ill


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Bowl mill

Replacement of grinding elements takes lesser time. Capacity and product fineness deteriorate with wearingout of grinding elements. Rate of erosion of bowl andrings are very high.

Frequent shutdown of mills are required for adjustment of

loading springs which becomes loose for wearing out ofroller and bowl.

Ingress of non crushable material may damage millinternals.

Reject rate is very high. Elaborate mill reject system

becomes essential for larger units. Disposal of mill rejectis another problem to be tackled by power station.


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Impact/Hammer Mill


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Impact/Hammer Mill

/


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Impact/Hammer Mill

A simple, compact and low cost mill.

Mill can be designed for very high temperature. Thisfacility makes this mill suitable for use with high moisture

lignite fuel.

Wear of grinding elements reduces mill capacity.

Product fineness is low.

Ball and Race mill


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Ball and race mill


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Ball and race mill

Capacity and product fineness does not deteriorateappreciably with wearing out of grinding elementsbecause of self compensating mechanism.

Hydro -pneumatic loading system maintains constantloading pressure. But it is costly and needs maintenance.

Response to load change is slow compared to tube mills. Start up and shut down time is less.

Ingress of non crushable material may damage millinternals.

Chances of grinding zone fire are almost nil.

B ll d ill


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Ball and race mill

B ll d ill


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Ball and race mill

P l i ti


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Pulverization

Advantages

Low excess air requirement

Less fan power

Ability to use highly preheated air reducing exhaust losses

Higher boiler efficiency Ability to bum a wide variety of coals

Fast response to load changes

Ease of burning alternately with, or in combination with gas and oil

Ability to release large amounts of heat enabling it to generateabout 2000 t/h of steam or more in one boiler

Ability to use fly ash for making bricks etc. Less pressure losses and draught need.

Di d t


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Disadvantages

1. Added investment in coal preparation unit2. Added power needed for pulverizing coal

3. Large volume of furnaces needed to permit desired heat

release and to withstand high gas temperature

Ash handling system


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Ash handling systemA large quantity of ash is, produced in steam power plants using coal. Ash produced in about 10

to 20% of the total coal burnt in the furnace. Handling of ash is a problem because ash coming

out of the furnace is too hot, it is dusty and irritating to handle and is accompanied by some

poisonous gases.

It is desirable to quench the ash before handling due to following reasons:

1. Quenching reduces the temperature of ash.

2. It reduces the corrosive action of ash.3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will disintegrate.

4. Quenching reduces the dust accompanying the ash.

Handling of ash includes its removal from the furnace, loading on the conveyors and delivered

to the fill from where it can be disposed off.


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ASH HANDLING EQUIPMENT

Mechanical means are required for the disposal of ash. The handling equipment should

perform the following functions:

(1) Capital investment, operating and maintenance charges of the equipment should be low.

(2) It should be able to handle large quantities of ash.

(3) Clinkers, soot, dust etc. create troubles, the equipment should be able to handle them

smoothly.

(4) The equipment used should remove the ash from the furnace, load it to the conveyingsystem

to deliver the ash to a dumping site or storage and finally it should have means to dispose of

the stored ash.

(5) The equipment should be corrosion and wear resistant.


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Hydraulic Ash Handling SystemL V l it S t


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Low Velocity System

Hydraulic Ash Handling SystemHi h V l it


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High Velocity

System


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Pneumatic Ash Handling System


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Pneumatic Ash Handling System


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Dust collection system


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The size of dust particles is designated in microns (1 = 0.001 mm). Dust particles are mainly

ash particles called fly ash intermixed with some quantity of carbon ash material called cinders.

Gas borne particles larger than 1 in diameter are called dust and when such particles become

greater in size than 100 they are called cinders. Smoke is produced due to the incompletecombustion of fuels, smoke particles are less than 10 in size.

The disposal smoke to the atmosphere is not desirable due to the following reasons :

1. A smoky atmosphere is less healthful than smoke free air.

2. Smoke is produced due to incomplete combustion of coal. This will create a big economic

loss due to loss of heating value of coal.

3. In a smoky atmosphere lower standards of cleanliness are prevalent. Buildings, clothings,

furniture etc. becomes dirty due to smoke. Smoke corrodes the metals and darkens the paints.To avoid smoke, the coal should be completely burnt in the furnace.

Types of dust collectorsThe various types of dust collectors are as follows :


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yp

1. Mechanical dust collectors.

2. Electrical dust collectors.

Mechanical dust collectors. Mechanical dust collectors are sub-divided into wet and dry types.

In wet type collectors also known as scrubbers water sprays are used to wash dust from the air.The basic principles of mechanical dust collectors are shown in Fig. As shown in Fig. by

increasing the cross-sectional area of duct through which dust laden gases are passing, the

velocity of gases is reduced and causes heavier dust particles to fall down. Changing the direction

of flow of flue gases causes the heavier particles of settle out. Sometime baffles are provided as

to separate the heavier particles. Mechanical dust collectors may be wet type or dry type. Wet

type dust collectors called scrub scrubbers make use of water sprays to wash the dust from fluegases. Dry type dust collectors include gravitational, cyclone, and baffle dust collectors.

Electrostatic Precipitators. It has two sets of electrodes, insulated from each other that maintain

an electrostatic field between them at high voltage. The flue gases are made to pass between these


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two sets of electrodes. The electric field ionises the dust particle; that pass through it attracting

them to the electrode of opposite charge. The other electrode is maintained at a negative potential

of 30,000 to 60,000 volts. The dust particles are removed from the collecting electrode by

rapping the electrode periodically. The electrostatic precipitator is costly but has low maintenancecost and is frequently employed with pulverised coal fired power stations for its effectiveness on

very fine ash particles and is superior to that of any other type.

ESP


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ESP

4.13.1 FLY ASH SCRUBBER

It is similar to a mechanical ash collector but has a flowing water film on its inner walls. Due to

thi fil th ll t d h i d idl f th t t th bi Th d f


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this film, the collected ash is removed more rapidly from the apparatus to the bin. The degree of

ash collection in scrubbers varies from 0.82 to 0.90. The dust laden gas enters through the inlet

pipe.

Cinder Catcher. Cinder catcher is used to remove dust and cinders from the gas. In this catcher

the dust laden gas is made to strike a series of vertical baffles that change its direction and

reduce its velocity.

The separated dust and cinders fall to the

Hopper for removal. Cinder catchers

are ordinarily used with stoker firing.

17098_coalhandling

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