UNIT-I ==> pulverized coal, Ash Handling System

 Pulverized coal:

    Coal is pulverized (powdered) to increase its surface exposure thus permitting rapid combustion. The efficient use of coal depends greatly on the combustion process employed. For large-scale generation of energy, the efficient method of burning coal is confined still to pulverized coal combustion. The pulverized coal is obtained by grinding the raw coal in pulverising mills. The various pulverising mills used are as follows:

(i) Ball mill                                                                                     (ii) Hammer mill

(iii) Ball and race mill                                                                     (iv) Bowl mill.

The essential functions of pulverising mills are as follows: 

(i) Drying of the coal 

(ii) Grinding

(iii) Separation of particles of the desired size.

Proper drying of raw coal which may contain moisture is necessary for effective grinding.

 The coal pulverising mills reduce coal to powder form by three actions as follows:

(i) Impact             (ii) Attrition (abrasion)             (iii) Crushing.

Most of the mills use all the above-mentioned three actions in varying degrees. In impact-type mills, hammers break the coal into smaller pieces whereas in attrition type the coal pieces rub against each other or metal surfaces to disintegrate. In crushing type mills coal caught between metal rolling surfaces gets broken into pieces. The crushing mills use steel balls in a container. These balls act as crushing elements.

BALL MILL

A line diagram of ball mill using two classifiers is shown in Fig.  It consists of a slowly rotating drum which is partly filled with steel balls. Raw coal from feeders is supplied to the classifiers from where it moves to the drum by means of a screw conveyor.

As the drum rotates the coal gets pulverized due to the combined impact between coal and steel balls. Hot air is introduced into the drum. The powdered coal is picked up by the air and the coal air mixture enters the classifiers, where sharp changes in the direction of the mixture throw out the oversized coal particles. The over-sized particles are returned to the drum. The coal air mixture from the classifier moves to the exhauster fan and then it is supplied to the burners.

BALL AND RACE MILL

            In a ball and race mill, the coal passes between the rotating elements again and again until it has been pulverized to the desired degree of fineness. The coal is crushed between two moving surfaces namely balls and races. The upper stationary race and lower rotating race driven by a worm and gear hold the balls between them. The raw coal supplied falls on the inner side of the races. The moving balls and races catch coal between them to crush it to a powder. The necessary force needed for crushing is applied with the help of springs. The hot air supplied picks up the coal dust as it flows between the balls and races, and then enters the classifier. Where oversized coal particles are returned for further grinding, whereas the coal particles of the required size are discharged from the top of classifier. In this mill coal is pulverized by a combination of' crushing, impact and attrition between the grinding surfaces. The advantages of this mill are as follows:

(i) Lower capital cost                                                         (ii) Lower power consumption

(iii) Lower space required                                                    (iv) Lower weight.

However, in this mill, there is greater wear as compared to other pulverizes. The use of pulverized coal has now become the standard method of firing in the large boilers. The pulverized coal burns with some advantages that result in economic and flexible operation of steam boilers.

The preparation of pulverized fuel with an intermediate bunker is shown in Fig. The fuel moves to the automatic balance and then to the feeder and ball mill through which hot air is blown. It dries the pulverized coal and carries it from the mill to separator.

    The air fed to the ball mill is heated in the air heater. In the separator dust (fine pulverized coal) is separated from large coal particles which are returned to the ball mill for regrinding. The dust moves to the cyclone. Most of the dust (about 90%) from cyclone moves to bunker. The remaining dust is mixed with air and fed to the burner.

    Coal is generally ground in low speed ball tube mill. It is filled to 20-35% of its volume. Steel balls having diameters varying from 30-60 mm. The steel balls crush and ground the lumps of coal. The average speed of rotation of tube or drum is about 18-20 r.p.m.

       

Advantages

The advantages of using pulverized coal are as follows:

1. It becomes easy to burn wide variety of coal. Low-grade coal can be burnt easily.
2. Powdered coal has more heating surface area. They permits rapids and high rates of combustion.
3. Pulverized coal firing requires low percentage of excess air.
4. By using pulverized coal, rate of combustion can be adjusted easily to meet the varying load.
5. The system is free from clinker troubles.
6. It can utilize highly preheated air (of the order of 700°F) successfully which promotes rapid flame propagation.
7. As the fuel pulverising equipment is located outside the furnace, therefore it can be repaired without cooling the unit down.
8. High temperature can be produced in furnace.

Disadvantages

1. It requires additional equipment to pulverize the coal. The initial and maintenance cost of the equipment is high.
2. Pulverized coal firing produces fly ash (fine dust) which requires a separate fly ash removal equipment.
3. The furnace for this type of firing has to be carefully designed to withstand for burning the pulverized fuel because combustion takes place while the fuel is in suspension.
4. The flame temperatures are high and conventional types of refractory lined furnaces are inadequate.
5. It is desirable to provide water-cooled walls for the safety of the furnaces.
6. There are more chances of explosion as coal burns like a gas.
7. Pulverized fuel-fired furnaces designed to burn a particular type of coal cannot be used to any other type of coal with the same efficiency.
8. The size of coal is limited. The particle size of coal used in pulverized coal furnaces is limited to 70 to 100 microns.

SHAFT MILL

Fuel pulverization with a shaft mill. The fuel from bunker is moved to feeder via automatic balance. Then from duct fuel goes to mill where it is crushed by beaters secured on the spindle of the mill rotor. The pulverised fuel is dried up and then blown into the shaft by hot air. Secondary air is delivered into the furnace through holes to burn the fuel completely.

ASH HANDELING SYSTEM:

            A 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 delivering to the fill from where it can be disposed off.

            Ash handling refers to the method of collection, conveying, interim storage and load out of various types of ash residue left over from solid fuel combustion processes.

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 trouble, the equipment should be able to handle them smoothly.
4. The equipment used should remove the ash from the furnace, load it to the conveying system 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.

    Ash handling systems may employ different forms of pneumatic ash conveying or mechanical ash conveyors. A typical ash handling system may employ vacuum pneumatic ash collection with ash conveying from several ash pick-up stations and resulting in delivery to an ash storage silo for interim holding prior to load out for disposal or reuse. Pressurized pneumatic ash conveying may also be employed.

 Mechanical Ash handling system:

In this system ash cooled by a water seal falls on the belt conveyor and is carried out continuously to the bunker.

Hydraulic System:

 In this system, ash from the furnace grate falls into a system of water possessing high velocity and is carried to the sumps. It is generally used in large power plants. Hydraulic system is of two types namely low-pressure hydraulic system used for continuous removal of ash and high-pressure system which is used for intermittent ash disposal.

Pneumatic system:  

In this system ash from the boiler furnace outlet falls into a crusher where larger ash particles are crushed to small sizes. The ash is then carried by a high-velocity air or steam to the point of delivery. Air leaving the ash separator is passed through filter to remove dust etc. so that the exhauster handles clean air which will protect the blades of the exhausted.

Boiler Draught

            Boiler draught is defined as the small difference between the pressure of outside cold atmospheric air and that of gases within a furnace or chimney. The draught is necessary to force air through the fuel grate to help in proper combustion of fuel and to remove the products of combustion

or

The difference of pressure to maintaining the constant flow of air and discharging the gases through the chimney to the atmosphere is known as draught. Draught can be achieved by the use of a chimney, fan, steam or air jet or a combination of these...

Draught is achieved by a small pressure difference which causes the flow of air or gas to take place.

The draught is one of the most essential systems of the thermal power plant which support the required quantity of air for combustion and removes the burnt products from the system. To move the air through the fuel bed and to produce a flow of hot gases through the boiler economiser, preheater and chimney require a difference of pressure.

 When the draught is produced with the help of chimney only, it is known as Natural Draught and when the draught is produced by any other means except chimney it is known as Artificial Draught.

Purpose of Boiler Draught

• To provide an adequate supply of air for fuel combustion.
• For throw out the exhaust gases of combustion from the combustion chamber.
• To discharge these gases to the atmosphere through the chimney.

 Types of Boiler Draught

In general, the draughts may be classified into the following two types,

1. Natural Draught
2. Artificial Draught

1. Natural Draught

The natural draught system employs a tall chimney as shown in the figure. The chimney is a vertical tubular masonry structure or reinforced concrete. It is formed for enclosing a column of flue gases to produce the draught.

Advantages of Natural Draught

• It does not require any external power for producing the draught.
• The capital investment is less. The maintenance cost is low as there is no mechanical part.
• Chimney keeps the flue gases at a high place in the atmosphere which prevents contamination of the atmosphere.
• It has a long life.

Disadvantages of Natural Draught

• The maximum pressure available for producing natural draught by the chimney is hardly 10 to 20 mm of water under the normal atmospheric and flue gas temperatures.
• The available draught reduces with increases in outside air temperature and for generating enough draught, the exhaust gases have to be discharged at relatively high temperatures resulting in the loss of overall plant efficiency. Thus maximum utilization of Heat is not possible.

 Artificial or Mechanical Draught

It has been seen that the draught produced by the chimney is affected by the atmospheric conditions. It has no flexibility, poor efficiency and tall chimney are required. In most of the modern power plants, the draught applied must be freedom of atmospheric condition, and It should have more flexibility (control) to bear the fluctuation loads on the plant.

Advantages of Artificial or Mechanical Draught

• It is more economical and its control is easy.
• The desired value of draught can be produced by mechanical means which cannot produced by means of natural draught.
• It increases the rate of combustion by which low-grade fuel can also be used.
• It reduces the smoke level and increases the heat transfer coefficient the flue gases side thus increases the thermal efficiency of the boiler.
• In mechanical draught, It saves the energy and the heat of flue gases can be best utilized by it.
• In this way, it reduces fuel consumption and makes boiler operation cheaper.
• It reduces the height of chimney which now is only controlled by the requirement of pollution norms.

Disadvantages of Artificial or Mechanical Draught

• The initial cost of mechanical draught system is high.
• Running cost is also high due to the requirement of electricity but that is easily compensated by the savings in fuel consumption
• Maintenance cost is also at a higher rate.
• Noise level of boiler is also high due to noisy fan/blower etc.

1. Steam Jet Draught

It is a very simple and easy method of producing artificial draught without the need for an electric motor. It may be forced or induced depending on where the steam jet is installed. Steam under pressure is available in the boiler.

When a small position of steam is passed through a jet or nozzle, pressure energy converts to kinetic energy and steam comes out with a high velocity. This high-velocity steam carries, along with it, a large mass of air or flue gases and makes it flow through the boiler. Thus steam jet can be used to produce draught and it is a simple and cheap method.

Actually the steam jet is directed towards a fix direction and carries all its energy in kinetic form. It creates some vacuum in it’s surrounding and attracts the air of flue gases either by carrying along with it. Thus it has the capacity to make the flow of the flue gases either by carrying or including towards chimney. It depends on the position of the steam jet.

 Types of Steam Jet Draught

The following are the main two types of steam jet draught:

1. Induced steam jet draught.
2. Forced steam jet draught.

1. Induced Steam Jet Draught

The jet of steam is turned into a smoke box or chimney. The kinetic head of the steam is high but static head is low i.e., it produces a partial vacuum which brings the air through the grate, ash pit, flues and then to motor box and chimney.

This type of induced steam jet draught arrangement is used in locomotive boilers. Here the steam jet is absorbing the exhaust gases through boiler so it is Induced Steam Jet Draught.

2. Forced Steam Jet Draught

Steam from the boiler after having been throttled to a gauge pressure of 1.5 to 2 bar is supplied to the jet or nozzle installed in the ash pit.

The steam rising out of nozzles with a great velocity drags air by the fuel bed, furnace, flue passage and then to the chimney. Here the steam jet is pushing or forcing the air and flue gases to flow through boiler hence it is forced steam jet draught.

2. Mechanical or Fan Draught

The draught, produced by means of a fan or blower, is known as a mechanical draught or fan draught. The fan used is, generally, of centrifugal type and is driven by an electric motor.

In an induced fan draught, a centrifugal fan is placed in the path of the flue gases before they enter the chimney. It draws the flue gases from the furnace and forces them up through the chimney. The action of this type of draught is similar to that of the natural draught.

In case of forced fan draught, the fan is placed before the grate, and the air is forced into the grate through the closed ash pit.

Types of Mechanical or Fan Draught

The following are the three types of mechanical or fan draught:

1. Induced draught.
2. Forced draught.
3. Balanced draught.

1. Induced draught

In induced draught, the blower is placed near the base of the chimney instead of near the grate. The air is absorbed in the system by decreasing the pressure through the system below the atmosphere. The induced draught fan sucks the burned gases from the furnace and the pressure inside the furnace is reduced below atmosphere and includes the atmospheric air to flow through the furnace.

2. Forced Mechanical Draught: 

3. Balanced Draught

It is always better to use a combination of forced draught and induced draught instead of forced or induced draught alone. If the forced draught is applied alone, the furnace cannot be opened for firing or inspection because high-pressure air inside the furnace will quickly try to blow out and there is every possibility of blowing out the fire completely and the furnace stops. If the induced draught is used alone, then also furnace cannot be opened either for firing inspection because the cold air will try to rush into the furnace as the pressure inside the furnace is under atmospheric pressure. This reduces the effective draught and dilutes the combustion.