Pulveriser System

Ways of burning coal in a boiler:

Manual firing Stoker firing Pulverised coal firing Cyclone furnace, etc.

The conventional fuel firing methods such as stoker firing were found to be unable to take the fluctuating loads on the plant due to limited capacity of combustion. Further, the conventional methods are unsuitable for large capacity (above 100 MW) plants and coals containing high percentage of ash due to the difficulties in removing large quantities of ash and interference of the formed ash in the combustion process.

The pulverised fuel systems are used extensively for large capacity plants and using low grade fuel as it gives high thermal efficiency and better control as per the load demand.

In a pulverised fuel firing system, the coal is reduced to a fine powder with the help of grinding mill and then projected into the combustion chamber with the help of hot air current. The amount of air required to complete the combustion (known as secondary air) is supplied separately through windbox to the combustion chamber.

The amount of air which is used to dry coal before pulverisation and carry the pulverised fuel to the combustion chamber is known as Primary Air. The amount of primary air may vary from 30% to almost the entire combustion air requirements as per the type of pulveriser used and load on it. The efficiency of the pulverised fuel firing system mostly depends upon the size of the powder. The fineness of the coal should be such as 70% of it would pass through a 200 mesh sieve and 98% through 50 mesh sieve

(200 mesh sieve – 75µm; Nominal sieve opening – 0.075 mm, nominal wire dia – 0.053 mm)

The success of the pulverised firing system lies in the fact that by breaking a given mass of coal into smaller pieces exposes more coal surface area for combustion.

For example:

Variation of surface area with particle size for one cubic inch volume is given:

Dia = 0.1 inch; surface area = 60 sq inch per cubic inch.

Dia = 0.01 inch; surface area = 600 sq inch per cubic inch.

Dia = 0.001 inch; surface area = 6000 sq inch per cubic inch.

After 0.001 inch dia, there is no substantial increase in the coal particle surface area.

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Advantages of Pulverised System of Firing:

Greater surface area of coal per unit mass of coal allows faster combustion as more coal surface is exposed to heat and oxygen. This reduces the excess air required to ensure complete combustion and the fan power also. High combustion efficiency.

Wide variety and low grade coal can be burnt more easily.

It gives fast response to load changes as rate of combustion can be controlled easily and immediately. Easily lighted and controlled.

The clinkering and slagging tendencies in this system is minimum.

This system works successfully with or in combination with gas and oil.

It is possible to use highly preheated secondary air (310 oC), which helps for rapid flame propagation.

Large amount of heat release is possible and with such rate of heat generation, each boiler of pulverised fuel fired system can generate as large as 2000 tonnes of steam per hour.

The boilers can be started from cold very rapidly and efficiently. This is highly important during emergency.

The furnace volume required is considerably less as the use of burners which produce turbulence in the furnace makes it possible to complete combustion with minimum travel of flame length.

Ability to design and build high unit capacity steam generators.

High availability.

Disadvantages of Pulverised System of Firing:

The capital cost of the pulverised system is considerably high as it requires many additional equipments. Its operation cost is also high compared with stoke firing.

This system produces fly-ash which requires special and costly fly-ash removal equipments as ESPs.

The flame temperatures are high and the conventional types of refractory lined furnaces are inadequate. It is always necessary to provide water-cooled walls for the safety of the furnace. The maintenance cost is also high as working temperature is high which causes rapid deterioration of the refractory surface of the furnace.

The possibilities of explosion are more as coal burns like a gas.

The storage of powdered coal requires special attention and high protection from fire hazards.

Skilled operators are required.

Special starting-up equipments are required.

The removal of slag formed from low fusion temperature ash requires special equipments and creates additional problems of its removal.

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System Requirements of a Pulveriser Plant

1. Raw coal feeding

2. Drying

3. Grinding and circulating

4. Classifying

5. Transporting

1. Raw coal feeding: This is an arrangement for feeding the raw coal from a bunker to the pulveriser at controlled rates. The rate of feeding depends on the boiler load, quality of coal, pulveriser condition, etc. A RC Feeder is used for carrying out this function.

2. Drying: Drying of coal is required for effective grinding of the coal in order to avoid sticking of wet coal to the surface of grinding elements. Further drying is also required to supply proper air-coal mixture at higher temperature to the burner for ignition without coking. In a pulverizer plant, coal is dried in the pulveriser either using hot air (called as Primary Air) or hot flue gas.

3. Grinding and Circulating: The most important system in the pulveriser plant is the pulveriser where coal is grinded to the desired fineness. Pulveriser is also called as coal mill. The pulveriser should have the ability to grind the coal and reject foreign matter that enters with the feed. Also, sufficient circulation of coal within the pulveriser is to be effected to dry the coal, remove it from the grinding zone and deliver it to the classifier.

4. Classifying: For achieving the desired effects of highest combustion efficiency, coal of only desired fineness should be permitted to go to the burners and the oversized particles should be returned to the pulveriser for further grinding. A classifier in-built in to the system does this function. The separation/classifying process must be accurate enough to be effective over a wide load range otherwise the fine coal also will be returned to the pulveriser for further grinding resulting in the production of super-fines.

5. Transporting: The system must have the arrangement to convey/transport the coal powder from the pulveriser to the boiler furnace without any fall out of the coal particles in the pipelines. This is achieved by using hot air or hot flue gas under pressure or suction created by fans.

A typical Pulveriser Plant is as shown in the Picture below:

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1. Raw Coal Feeders

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 are made to deliver the required quantity by either increasing or decreasing the speed of the feeder. The feeders have to regulate the rate of coal flow corresponding to boiler load, calorific value of coal, etc.

Types of raw coal feeders:

a. Volumetric feeder and b. Gravimetric feeder

a. Volumetric feeder:

The rate of coal flow through the feeder is derived as:

R = Q x B x N

Where, R = Rate of coal flow in Kg/sQ = Volume of coal delivered by the feeder per rotation in m3/RotationB = Bulk density of coal in kg/m3

N = Speed of feeder rotation per second

Desired flow rate is achieved by varying the speed of the feeder.

b. Gravimetric feeder:

In volumetric feeders, if there is any variation in bulk density of coal or the volume moved per rotation, the actual mass rate of coal flow for a given speed will be varied, leading to errors in computing the rate of flow for various purposes such as excess air control, performance calculations, etc.

The gravimetric feeders are used to provide an accurate rate of coal flow as follows:

R = M x N

Where, R = Rate of coal flow in kg/sM = Mass of coal on unit length of feeder belt in kg/mN = Speed of feeder belt in m/s

1.1 Raw Coal Feeder Construction

a) Rotary Volumetric Feeder:

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In this type of feeder, a spider wheel is keyed to the centre of a feed roll shaft. This shaft extends through a cylindrical core, which forms the base of the feed roll. The core is made in two halves, which are bolted to opposite sides of the feeder body, and the spider wheel is placed in between these two halves. Number of plates are bolted to the spider wheel along its periphery, thereby making number of pockets. When the feed roll shaft rotates, the plates bolted to the spider revolve around the split stationary cylindrical core. This feeder is connected at the top to the raw coal bunker and at the bottom to the pulveriser through coal pipes. When the feeder runs, the coal is received by the pockets formed by the plates and emptied into the pulveriser at a rate, which depends on the speed of the feeder. A hinged levelling gate held in place by spring pressure limits the amount of coal entering each pocket but allows passage of foreign material, which might otherwise cause damage to the feeder.

Positively Infinite Variable (PIV) speed drive is employed for driving this feeder, which is connected to a speed reducer gearbox. A chain connects the sprocket on gearbox unit and a sprocket on feed roll shaft mounted on a clutch assembly.

Clamps are provided for disconnecting the links from the clutches so that the feed roll shaft may be revolved backward by the hand wheel for removal of obstructions in the feeder.

Limitations:

The rotary volumetric feeder is not the accurate method of controlling the required quantity of coal supply to the pulverizers because;

The bulk density of coal may vary and The coal may fail to spread across the pockets in the spider wheel assembly

b) Gravimetric Feeder:

The gravimetric feeder resembles a belt feeder and provides a precise weighing system to measure the mass of coal per unit length of the belt which is multiplied by the speed of belt to determine the rate of coal flow. The weighing can be done either by mechanical scales or electronic strain gauges. The electronic gravimetric feeders are more precise.

A typical electronic weighing gravimetric feeder is shown in the Fig below.

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An endless synthetic rubber belt moves over three pulleys inside the feeder body. The fabricated feeder body is having the inlet chute connected to the bunker at the top at one end and outlet opening at the bottom of the other end. The pulley near the outlet opening is known as head pulley and is connected to a variable speed drive and transmits the motion to the belt. The pulley at the inlet side, called as take-up pulley can be adjusted for belt tension and tracking with the help of take up screws. Another tension pulley is placed in between to keep the belt in constant tension.

The belt has kerfs at the ends and projection on the inside to track the belt correctly over the pulleys to minimise the spillage of coal.

Three precisely made stainless steel rollers are mounted in feeder body exactly at the same level. The two end rollers are called weigh span rollers and their centre to centre distance is weigh span. The centre roller known as weighing roller is pivoted on to the feeder body on either side and two load cells are connected in between the weighing roller and top of the body on either side. These load cells are strain gauge type. The load cell weighs the coal and gives a milli volt output corresponding to the load. The speed of the belt is measured on the head pulley by means of a high-resolution optical encoder in the form of frequency. The output of load cell and optical encoder are transmitted to the feeder electronics for processing and correcting the speed to meet the demand.

After weighed coal is delivered to the outlet a few pieces may continue to stick on the belt due to wetness, etc. To clean the belt from these sticking pieces and to ensure greater accuracy, a dead weight type belt scrapper is provided immediately below the delivery point of the belt. The belt scrapper is provided immediately below the delivery point of the belt. The scrapper is made of soft rubber so that it will not damage the belt.

A clean out conveyor assembly is placed below the belt to clear the area below the belt from the spill over coal of the belt as any accumulation of spill over below belt may lead to fire and damage to the belt. The cleanout conveyor assembly has a drive shaft and a take up shaft and two chains meshing with sprockets over the shafts are running between these two shafts.

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2. Pulverisers

Pulverisers are used to grind the coal to the required fineness needed for pulverised coal firing. The basic principles of particle size reduction could be one or all of the following actions:

Impact

Attrition

Crushing

Pulveriser design requirements:

Optimum fineness for design coals over the entire pulverizer operating range

Rapid response to load changes

Stable and safe operation over the entire load range

Continuous service over long operating periods

Acceptable maintenance requirements, particularly grinding elements, over the pulverizer life

Ability to handle variations in coal properties

Ease of maintenance (minimum no. of moving parts and adequate access)

Minimum building volume

Types of Pulverisers:

The pulverisers are classified based on their operating speed.

1. Slow speed Mills

Below 50 rpm (normal range 20-35 rpm); Mills – Ball or Tube or Drum mills

2. Medium speed Mills

50 to 100 rpm; Mills – Bowl mills/Ball and race mills

3. High speed Mills

Above 225 rpm; Mills – Impact or Hammer mills/Beater wheel mills

Of the above, medium speed 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 are generally limited to pulverising lignite only.

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Ball Mill

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

Construction: The mill consists of-

a)Single

compartment mill drumIt consists of a large heavy rolled plate shell having dished ends or heads, with trunnions. The shell and heads are lined inside with armoured steel plates of thickness about 40 mm. These liners are sufficiently hard to last for several years.

b) Anti-friction bearingsThe entire mill drum is supported and rotates on two anti-friction bearings by means of trunnions.

c) Mill driveAt one end of the shell a gear wheel is embedded on the shell. This gear wheel is meshing with a pinion rotated by a motor through a gear box. With this arrangement, the entire shell can be rotated at a speed of 20 to 25 rpm.

d) Ball chargeThe inside of the shell is filled to a little less than half with forged steel or cast alloy balls varying from 40 to 60 mm in diameter.

e) Fuel inlet and discharge elbowsIn this mil the coal and hot air called primary air or hot flue gas can be admitted and pulverised coal is taken out at both ends.

f) Lubricating equipment for mill bearingGrease or drip oil from a lubricating oil system lubricates the meshing point of gear wheel and pinion.

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g) Foundation frames for drive and bearingsThe foundation frames are fabricated from thick steel sections. The whole foundation is levelled gear box and countershaft is bolted to the foundation frame and frame is grouted.

When the drum is rotating the raw 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 mill. Pulverisation of coal is accomplished through continued cascading of the mixtures, results from:

a) Impact of the falling 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.

Classifier of Ball Mill:

The purpose of classifier is to control the range of particle sizes in the pulverized fuel which leave the mill. All particles above a pre-determined maximum size are recycled through the mill for further grinding while smaller particles pass the burners.

Classifiers are divided into two basic groups, rotary and static, both of which make use of a similar principle the resistance of a particle to a change of direction of speed. The greater the mass of a particle, the greater is its resistance to such change.

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In the rotary classifiers, ‘whizzer’ separators are situated so that the coal/air mixture from the mill is directed through them. Centrifugal force directs the coarser particles to the outside of the separator covering, where the air/gas velocity is at its minimum, and the particles then return to the mill table under the influence of gravity.

The static type separators use centrifugal force for classification. Some mills, employ adjustable vanes in the heat of the mill to produce a cyclonic effect which results in the necessary centrifugal force. Variation of the vane angle varies the intensity of the swirl or vortex within the classifier cone; the more intense the swirl the finer the pulverised fuel produced. But intensification of the swirl beyond an economic limit will reduce the throughput of the mill by creating restrictions at the mill outlet.

Advantages of Ball Mills:

High output possible up to 50-100 tonnes per hour No maintenance over long periods (apart from routine on-load recharging of mill balls) High availability Because of high availability, no standby capacity is required

No mill rejects Reserve of fuel within mill makes output more stable On a pressure mill installation the pressure is low and the air/fuel ratio is low; this keeps the primary

air power requirements to a minimum

Disadvantages of Ball Mills:

High power consumption Some problems with control of coal level within the mill Virtually constant power consumption at all loads; low load operation is therefore not economical With high moisture content fuels a high primary air temperature is required because of the low

air/fuel ratio Unplanned stops leave the mill full of coal which, under unfavourable conditions, can ignite. This coal

has to be quenched and even dug out otherwise the mill cannot be restarted

Bowl Mill

Bowl mill is a vertical spindle medium speed mill. In a bowl mill coal is pulverised between a disc called bowl rotated by the drive assembly and rollers kept above the disc loaded by springs or pneumatic or hydraulic loading devices. Many variations of bowl mill are available with certain changes in the construction.

Coal is fed to the centre of the pulveriser onto a revolving bowl. Centrifugal force causes the coal to travel towards the perimeter of the bowl. The coal

passes between the bull ring/bowl and the grinding rolls, which impart the pressure necessary for grinding.

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The partially ground coal continuous outward and over the edges of the bowl. Heated air enters the mill side housing below the bowl and is directed upward around the bowl outside diameter.

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Advantages of Bowl Mills

Low power consumption Reliability and minimum maintenance Wide capacity and ability to handle wide range of coals Quite and vibration less operation, inherent flexibility of the system

Disadvantages of Bowl Mills

Provision of sealing air supply necessary Periodic maintenance is required Tightness requirements of casing and inspection parts Pyrites and tramp iron must be removed during operation Outage required for replacement of worn-out bull ring/bowl segments and rollers

Constructional Details of Bowl Mill:

a) Mill drive and bowl assembly

The vertical shaft is supported by a thrust bearing and two radial bearings. The bowl is assembled at the top of the shaft and it rotates along with the scrapers. Wear resistant NIHARD alloy steel segments are lined on the top surface of the bowl. These are called as bull ring segments.

b) Sealing system

The mill works under pressure, to prevent the entry of dust laden air to the gearbox a sealing arrangement is provided.

c) Mill side liner assembly

It consists of thick cerwool insulation to preserve heat of hot air inside the mill so as to prevent excessive conducting of heat from mill side to mill base. It also has bottom liners to provide an enclosure for the scrapers to scrap the pyrites & rejects from this zone into the pyrite hopper.

d) Separator and Journal assembly

This houses the journal loading system namely the spring assembly. The spring is precompressed to a certain load for giving the grinding pressure. Due to increased coal feed, the coal bed rises and this further compresses the spring giving the required

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grinding pressure. This assembly also has classifier. The classifier vanes position is adjustable to get required fineness of the coal going out of the mill.

e) Grinding roll assembly

The mill has three grinding roll assemblies. In each assembly a grinding role is suspended on a journal shaft which is fixed to the journal head. Tapered roller bearings are provided inside the housings. Oil is filled in the housings to lubricate the bearings. The rollers are centrifugally casted with the outer surfaces made of NIHARD.

f) Venturi multiport outlet and mill discharge valve assembly

The venture is provided with a venture vane which eliminates the spin which is getting introduced as the coal air mixture is entering into the classifier. The venture vane is provided with ceramic tiles for wear resistance. The mill discharge valve is flap type. The flap is away from the coal system in the fully opened position.

Principle of Operation of Bowl Mill

Coal is fed to the centre of the pulveriser onto a revolving bowl. Centrifugal force causes the coal to travel towards the perimeter of the bowl. The coal passes between the bull ring and the grinding rolls, which impart the pressure necessary for grinding. The partially ground coal continuous outward and over the edge of the bowl.

Heated air enters the mill side housing below the bowl and is directed upward around the bowl outside diameter and separator body annulus, bu the vane wheel. It continues upward and in to the deflector

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openings in the classifier at the top of the inner cone, then out through the venture and multiple port outlet assembly. As the air passes upward around the bowl, it picks up the partially pulverized coal. The heavier particles lose their momentum and are returned to the bowl for further grinding. The lighter particles are carried up through the deflector openings.

The deflector blades in the openings cause the coal-air mixture to spin within the inner cone. The angle of the blades determines the velocity of the spin and the resulting fineness of the finished product. Any pulverized coal is returned through the inside of the inner cone to the bowl for further grinding. Coal that is pulverized to the desired fineness leaves the pulveriser and enters the fuel piping system.

Ball and Race Mill

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Factors affecting Mill Performance

The performance of the mill plant is affected by a number of factors mainly associated with the properties of coal being ground.

Grindability index of coal

When determining pulverizer size, the most important physical coal characteristic to consider is grindability. 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 this number easier it is to pulverise. Normal HGI values of Indian bituminous coals vary from 45 to 60. Lignite has HGI upto 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.

Grindability is not strictly a matter of hardness. Some materials, fibrous in nature, are not hard but are very difficult to grind. Sticky or plastic materials can also defy grinding.

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.

Moisture content

The total moisture content of the raw coal is made of inherent and free/surface moisture. In any coal milling system drying of coal is adopted by using hot flue gas or hot air. This drying removes entire free and part of inherent moisture. Insufficient drying produces agglomeration of fines in the pulverising zone and is difficult to remove the fines efficiently. This limits the output capacity of the mills when high moisture coal is used. The problem is more in ball mill due to availability of more surface area for wet coal to stick.

Size of raw coal

Larger the size of raw coal fed to the mill amount to work per unit mass is increased to get fine coal of same fineness. Hence mill capacity varies inversely with the 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 property of the material etc. mill output will reduce as

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wear increases and this is because of loss of contact surfaces. This necessitates provision of spare capacity in each mill as well standby mills.

Pulverised Fuel Handling Systems

Two methods are in general use to feed the pulverised fuel to the combustion chamber:

1. Unit System or Direct Firing System

In unit system, each burner of the plant is fired by one or more unit pulverisers connected to the burners.

2. Central or Bin System or Indirect Firing System

In the central system, the fuel is pulverised in the central plant and then distributed in each furnace with the help of high pressure air current.

Each type of fuel handling system consists of crushers, magnetic separators, driers, mills, conveyours and feeders.

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Advantages of Unit System

It is simple in layout and cheaper than central system The coal transportation system is simple It allows direct control of combustion from the pulveriser Maintenance cost is less Coal which requires drying for satisfactory function of the central system is generally supplied without

drying in the unit system It affords better control of fuel feed into the furnace

Disadvantages of Unit System

The mill operates at variable load as per the load on the power plant, which results in poor performance of the pulverising mill (more power consumption per ton of coal at part load)

The total capacity of all mills must be higher than for the central system with the load factors common in practice

The degree of flexibility is less than the central system In the event of the failure of the auxiliaries of one of the burners, the burner has to put off, as there is

no reserve capacity. Strict maintenance planning is required as the generation of the unit directly depends upon the mill

performance

Advantages of Central System

The central system is flexible and changes can be made to accommodate quick changes in demand. There is always supply of fuel available in reserve in the boiler bunkers

There is a great degree of flexibility as the quantity of fuel and air can be separately controlled The pulverizer always runs at its rated load irrespective of the load on the plant Burners can be operated independently of the operation of coal preparation The pulverizer can be shutdown when sufficient reserve capacity has been achieved. The same can be

used during peak load periods The fan handles only air therefore there is no problem of excessive wear of fan blades Offers good control over fineness of coal

Disadvantages of Central System

Occupies large space Power consumption of auxiliaries is high. Therefore, overall power consumption per ton of coal

handled is higher than the unit system There is possibility of fire hazard due to the stored pulverised coal The coal transportation system becomes more complex Driers are essential The o&m charges are higher

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Direct Firing System – Classification

1. Pressurised System

a) Cold PA System

b) Hot PA System

c) Flue Gas System

2. Suction System

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a) With Air Drying

b) With Flue Gas Drying

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