Steam Power Plant

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CHAPTER 2 STEAM POWER PLANT

LAYOUT OF STEAM POWER PLANT

The general layout of a steam power plant consists of mainly the following four circuits:

Coal and Ash Circuit

Air and Gas Circuit

Feed Water and Steam Circuit

Cooling Water Circuit Coal and Ash Circuit

Coal received at the coal storage is supplied to boiler furnace after necessary coal handling processes through fuel feeding devices.

After combustion of coal, the ash is collected from boiler furnace and removed to ash storage yard through ash handling equipment.

Air and Gas Circuit

Air is drawn from atmosphere by the forced or induced draught fan.

This air is sent through the air preheater where it is heated by the flue gases.

The heated air is passed on to the boiler furnace.

The high temperature flue gases formed in combustion chamber is used for transferring heat to feed water and steam in boiler tubes and superheater tubes.

These gases are then passed over the reheater section where it heats steam drawn from HP turbine.

Then it is passed through dust collector device to remove ash and finally passed over economizer and air preheater before being exhausted through the chimney.

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Feed Water and Steam Circuit

Steam generated in boiler tubes and superheated in superheater is fed to HP, IP and LP turbine to develop mechanical power.

After expansion in HP turbine, some steam is bled for feed water heating and remainder is passed through reheater for reheating steam and then to LP turbine.

Then this steam passes for further expansion in LP feed water heater and expansion in LP turbine.

The mechanical power generated by turbines is supplied to alternator where mechanical energy is converted to electrical energy.

Cooling Water Circuit The cooling water requirements for condenser are very large and usually taken from various sources like river, sea or lake. Components and Systems of Steam Power Plant:

1. Steam Generator : Boiler, Superheater, Reheater, Economiser, Air preheater 2. Coal handling system: wagon tippler, crusher house, coal mill 3. Ash and Dust handling system: Ash precipitator 4. Steam Turbines 5. Condensers, Cooling Towers 6. Water Treatment Plant 7. Forced and induced draught fans, boiler chimney 8. Generators

GENERAL FEATURES OF SELECTION OF SITE

1. Availability of Fuel (coal or oil):

Modern steam power plants require a huge quantity of fuel per annum. A thermal power plant of 400 MW requires 5000-6000 tons of coal per day. Therefore the plant should be as near as possible to coal fields or oil rigs.

Otherwise, transportation cost will be high; space requirement will increase for storage at plant site which might lead to losses in storage and hence require additional investments.

If it is not possible to locate plant near coal fields, then plant should be at least located near railway station and also provision for shunting coal wagons to the site.

2. Ash Disposal Facilities:

Ash disposal problem is serious; particularly in India as the coal here has large percentage of ash (20 to 40%).

The ash handling problem is more serious than coal handling as it comes out in hot condition and is highly corrosive. It also leads to atmospheric pollution.

In 400 MW power station, 10 hectares area is required per year for dumping ash upto 6.5 meters height.

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3. Space Requirement:

The average land requirement is 3 to 5 acres per MW capacity.

The cost of land adds up to the plant cost. So land should be available at cheap rates. 4. Nature of Land:

The selected site for power plant should have good load bearing capacity (10 kg/cm2) as it has to bear the dead load of the plant and forces transmitted to foundation due to machine operations.

5. Availability of Water:

Theoretically, there should be no loss of water in the boiler circuit but some make up water is required (6 to 10 tons/hr for 60 MW plant).

Cooling water required in condenser is also very high. (20 to 30,000 tons/hr for 60 MW plant)

Large quantity of water required for disposing ash if hydraulic system is used.

So power plant should be located near water source which will supply water throughout the year.

6. Transport Facilities:

It is necessary to have a railway line near the plant to bring in heavy machinery for installation and also for fuel.

7. Labour Availability:

Labour should be available at the proposed site at cheap rate. 8. Size of the Plant:

In small plants, expenses involved in electric transmission are more severe, so this becomes an important consideration for plant location.

For big plants however transportation costs of coal and fuel are a more important factor.

9. Future Extensions:

Plant should have scope of future extensions. HIGH PRESSURE BOILERS

For a given steam condition and boiler size, there is not much variation in efficiency between different types of boilers.

The widest scope for increasing efficiency and reducing costs is to increase the pressure and temperature of steam.

Modern high pressure boilers used for power generation have steam capacity of 30 to

650 tons/hr with pressure of up to 160 bar and 540 C. FEATURES OF HIGH PRESSURE BOILERS COMPARED TO CONVENTIONAL BOILERS

Method of Water Circulation: o In conventional boilers the water is circulated through tubes by natural circulation due

to density differences. o But in high pressure boilers, forced circulation is done by using a pump to increase the

rate of heat transfer.

Types of Tubing:

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o In conventional boilers, the flow takes place through a continuous tube leading to large pressure drop due to friction.

o This is reduced in modern high pressure boilers by having flow through parallel system of tubing.

Improved Methods of Heating: o As compared to conventional boilers in high pressure boilers the heat transfer is

increased by improving methods of heating. o Saving of latent heat of evaporation of water above critical pressure of steam. o Heating of water with superheated steam. o Increasing water velocity and gas velocity.

ADVANTAGES OF HIGH PRESSURE BOILERS

1. Tendency of scale formation is eliminated due to high velocity of water through tubes. 2. Light weight tubes can be used. Space required is less. Cost of foundation, time of erection and

cost are reduced due to less weight. 3. Due to forced circulation, there is more freedom in arrangement of furnace, tubes and boiler

components. 4. Danger of overheating is reduced and thermal stress problem is simplified as all parts are

uniformly heated. 5. Steam can be raised quickly to meet the variable load without the use of complicated control

devices. 6. Efficiency of plant increases by 40 to 42% as reduced quantity of steam is required for the same

power generation if high pressure steam is used. 7. The differential expansion is reduced due to uniform temperature and this reduces the

possibility of gas and air leakages. SUB CRITICAL BOILERS: Boilers which operate below the critical pressure of 221.2 bar are called sub critical boilers. The boilers in this category are La Mount, Loeffler and Velox boilers. LA MOUNT BOILER: These boilers have been built to generate 45 to 50 tons of superheated steam at a pressure of 120 bar

and at a temperature of 500 C. They are flexible, compact and have small size of separating drum. Working

o It is a modern high pressure water tube steam boiler working on a forced circulation. o The feed water from hot well is supplied to a storage and separating drum (boiler) through

economizer. o It is then drawn through circulation pump to evaporator tubes and the part of the vapour is

separated in the separator drum. The large quantity of feed water circulated by forced circulation through the water walls and drums equals to ten times the mass of steam evaporated. This prevents the tubes from being overheated.

o The steam separated in the boiler is further passed through the superheater and finally supplied to the prime mover.

o The disadvantage of this boiler is formation of bubbles on inner surface of the tube which reduces heat transfer and steam generation rate.

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LOEFFLER BOILER: These boilers have generating capacity of 100 tons/hr at a pressure of 140 bars. Loeffler boiler overcomes the major difficulty of deposition of salt and sediments on inner surface of water tubes in La Mount boiler, which leads to reduced heat transfer and danger of overheating of tubes. Working

1. It is a water tube boiler using a forced circulation. 2. Its main principle of working is to evaporate the feed water by means of superheated steam

from the superheater. 3. The high pressure feed pump draws water through the economizer and delivers it to

evaporating drum. 4. The steam circulating pump draws saturated steam from evaporating drum and passes it to

radiant and convective superheaters. 5. One third of this steam is supplied to prime mover and remaining two third passes through

water in evaporating drum in order to evaporate feed water.

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VELOX BOILER: These boilers have generating capacity of 100 tons/hr at a pressure of 85 bars. When the gas velocity exceeds the sound-velocity, the heat is transferred from the gas at a much higher rate than rates achieved with sub-sonic flow. The advantage of this theory is taken to effect the large heat transfer from a smaller surface area in this boiler. Working

1. Air is compressed to 2.5 bars with the help of a compressor run by gas turbine before supplying to the combustion chamber to get the supersonic velocity of the gases passing through the combustion chamber and gas tubes and high heat release rates (35 to 45 million kJ/m3).

2. The burned gases in the combustion chamber are passes through the annulus of the tubes. The heat is transferred from gases to water while passing through the annulus to generate the steam.

3. The mixture of water and steam thus formed then passes into a separator which enters with a spiral flow separating heavier water particles from steam.

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4. The separated steam is further passed to superheater and then supplied to the prime mover. The water removed from steam in the separator is again passed into the water tubes with the help of a pump.

5. The gases coming out from the annulus at the top are further passes over the superheater where its heat is used for superheating the steam and then used to run a gas turbine.

6. The power output of the gas turbine is used to run the air compressor. The extra power required to run the compressor is supplied with the help of electric motor.

7. The exhaust gases coming out from the gas turbine are passed through the economizer to utilize the remaining heat of the gases.

SUPER CRITICAL BOILERS: Usually a sub-critical boiler consists of three distinct sections as economizer, evaporator and superheater and in case of super-critical boiler, only economizer and superheater are required. These days almost all boilers above 300 MW capacity are super critical. Advantages of Super Critical boilers

1. The heat transfer rates are considerably large compared with sub critical boilers. 2. The pressure level is more stable due to less heat capacity of the generator and therefore gives

better response. 3. Higher thermal efficiency (40- 42%) of power station can be achieved with the use of super

critical steam.

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4. The problems of erosion and corrosion are minimized in super critical boilers as two phase mixture does not exist.

5. There is a great ease of operation and their comparative simplicity and flexibility make them adaptable to load fluctuations.

BENSON BOILER: These boilers have generating capacity of 150 tons/hr at a maximum pressure upto 500 bars. La Mount Boiler experiences formation and attachment of bubbles on the inner surfaces of the heating tubes which reduces the heat flow and steam generation as it offers high thermal resistance. Therefore, Benson suggested that if the boiler pressure was raised to critical pressure ie 225 bar, the steam and water would have the same density thus bubble formation can be easily eliminated. Working

This boiler does not use any drum.

The water is passed through the economizer into the radiant evaporator where majority of the water is converted into steam.

The remaining water is evaporated in the final evaporator absorbing the heat from hot gases by convection.

The saturated high pressure steam at 225 bars is further passed through the superheater and finally through the prime mover.

Major difficulty of salt deposition was experienced in the transformation zone when all remaining water converts into steam.

To avoid this, the final evaporator is normally flashed out after every 4000 working hours to remove the salt.

During starting, first circulating pumps are started and then the burners are started to avoid the overheating of evaporator and superheater tubes.

Advantages of Benson Boiler

It requires smaller floor area for erection.

As there are no drums, the total weight of Benson boiler is 20% less than other boilers.

Expansion joints are not required in these boilers instead the pipes are welded due to these erection of boiler is easy and quicker.

The benson boiler can be operated most economically by varying the temperature and pressure at partial loads and over loads.

The flow down losses are hardly 4% of natural circulation boilers of same capacity

Explosion hazards are not at all severe as it consists of only tubes of small diameter and has very little storage capacity compared to drum type boiler.

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COAL AND ASH HANDLING EQUIPMENTS In large power stations, it is not possible to handle large quantities of coal manually; therefore, some mechanical handling system must be introduced to the plant for easy and smooth operation and better control. The inplant coal handling system should be designed in such a way that the inplant transportation should be minimum. The following points should be kept in mind while designing the coal handling plant.

1. The handling method should be simple and sound and require minimum operations and transport.

2. There should not be double handling of coal in the plant. 3. The handling units should be centralized to facilitate inspection and maintenance. 4. The electrical motors should be used as prime mover as much as possible as they are reliable,

flexible and with high residual value. 5. The working parts should be enclosed to avoid abrasion and corrosion. 6. It should be able to deliver required quantity of coal during peak hours.

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Coal Delivery: The coal from supply point is delivered by ships, rails or even pipelines. Unloading: The unloading equipment used depends on how the coal is received at power station. If it is by ship or railway wagons, unloading may be done by car shakers, rotary car dumpers, cranes and buckets, lift trucks etc. Coal Preparation: The preparation of coal before feeding to the combustion chamber is necessary if un-sized coal is brought to the site. The coal preparation plant includes the following equipments: Crushers, Sizers, Dryers and Magnetic Separators.

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Transfer of Coal After preparation of coal, it is transferred from the site of preparation to storage by means of equipments like belt conveyors, screw conveyers, bucket elevators, grab bucket elevators etc. 1. Belt Conveyors

This is very suitable means of transporting large quantities of coal over large distances.

Belt conveyor consists of endless belt made of rubber, canvas or balota running over a pair of end drums or pulleys and supported by a series of rollers (known as idlers) provided at regular intervals.

The return idlers which support the empty belt are plain rollers and are spaced wide apart.

Maximum inclination of belt conveyors to horizontal is 20.

Average speed of the belt conveyor is 60 to 100 m/min.

Load carrying capacity varies from 50 to 100 tones/hr and can be easily transferred through 400 meters.

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Advantages of belt conveyors 1. The rate of coal transfer can be easily varied by just varying the speed of the belt. 2. The repair and maintenance costs are minimum. 3. The coal over the belt can be easily protected from wind and rain just by providing overhead

covers. 4. The power consumption to carry the coal is minimum compared with other conveyor belts.

Disadvantages of belt conveyors 1. It is not suitable for short distances and greater heights as its inclination is limited to 20.

2. Screw Conveyors

It consists of an endless helicoids screw fitted to a shaft.

The driving mechanism is connected to one end of the shaft and the other end of the shaft is supported in an enclosed ball bearing.

The screw while rotating in a trough transfers coal from one end to the other end.

The diameter of screw varies from 15 cm to 50 cm and its speed varies from 70 to 120 rpm as per the capacity required.

Advantages of screw conveyors

1. It requires minimum space and is cheap in the first cost. 2. It is most simple and compact. 3. It can be made dust tight.

Disadvantages of screw conveyors

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1. The power consumption per unit weight transferred is considerably high. 2. The length of feed hardly exceeds 30 metres due to torsional strain on the shaft. 3. The wear and tear is very high and therefore the life of the conveyor is considerably short

compared with belt conveyor. 3. Bucket Elevators

This conveyor is extensively used for vertical lifts.

It consists of buckets fixed to a chain which moves over two wheels.

The coal is carried by the buckets from the bottom and discharged at the top.

The maximum height of the elevator is limited to 30.5 m and maximum inclination to the horizontal is limited to 60.

The speed of the chain is 35 to 75 m/min for about 60 tonnes capacity per hour.

4. Grab Bucket Conveyor

A grab bucket conveyor lifts as well as transfers the coal from one point to another.

The grab conveyor can be used with crane or tower.

A 2 to 3 m3 bucket operating over a distance of 60 m transfers nearly 100 tonnes of coal per hour.

The initial cost of this machine is high but operation cost is less.

Its use for transferring the coal is justified only when other arrangements are not possible.

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Coal Weighing Methods The cost of fuel used being major running cost of the plant, therefore it is necessary to weigh the coal at the unloading point inorder to have an idea of the total quantity of coal delivered at the site. The different weighing methods used as weigh bridge, belt scale, weigh lorry and automatic scale. Coal Crushing Crushing is required when handling unsized coal and may be done near the receiving area before the coal is stored in the yard. Plant burning pulverized coal generally specifies a coal to size larger than what cannot be handled by the pulverized, making crusher necessary. Most coal for stoker fired plants is brought in the required size, so crushing is not needed unless it becomes economically advantageous to buy larger lump coal. Several types of crushers are rotary breakers, single roll crusher etc.

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Rotary Type Crusher Single Roll Type Crusher Coal Burning Methods The two commonly methods used for burning coal are stoker firing and pulverized fuel firing. The stoker firing method is used for solid fuel and pulverized firing method is used for pulverized coal. The selection of firing method adopted for a particular power plant depends upon the following factors:

1. The characteristics of the available coal. 2. Capacity of the plant. 3. Load factor of the power plant. 4. Nature of load fluctuation and 5. Reliability and efficiency of the various combustion equipments available.

Solid Fuel Firing Hand Fired Stoker Feed Pulverized Fuel Fired Overfeed Stokers Underfeed Stokers Travelling Spreader Single Multi Grate type Retort Retort

Chain Bar Unit Central Grate Grate System System Hand Fired The hand firing system is the simplest of fuel firing but it cannot be used in modern power plants as it gives lower combustion efficiency, it does not respond quickly to fluctuating loads and the control of draught is difficult.

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Stoker Firing A stoker is a power operated fuel feeding mechanism and grate. Advantages of Stoker Firing:

1. This can be used for small or large boiler units. 2. A cheaper grade of fuel can be used with higher efficiency. 3. A greater flexibility of operations assured. 4. Less building space is required. 5. Very reliable, maintenance charges are low. 6. Capital investment as compared to pulverized fuel system is less. 7. Some reserve is gained by large amount of coal stored on the grate in case there is failure of

coal handling plant. Disadvantages of Stoker Firing:

1. Construction is complicated. 2. For large units, the initial cost may be higher. 3. There is always a certain amount of loss of coal in the form of riddling through the grates. 4. There is always possibility of slagging and clinkering of combustion chamber walls. 5. There is excessive wear of moving parts due to abrasive action of coal. 6. Banking and standby losses are always present.

Overfeed Stokers In overfeed stokers; the coal is fed into the grate above the point of air admission. It receives fresh coal on its top surface.

1. A layer of fresh or green coal. 2. A layer of coal losing moisture-drying zone. 3. A coking layer of coal losing its volatile matter- distillation zone. 4. A layer of incandescent coke, where the fixed carbon is consumed- combustion zone. 5. A layer of ash, which is progressively getting cooler.

From zone 4, due to combustion, heat transfer occurs upward and downward by conduction, while the air which flows from below will tend to carry away the heat upward by convection. The primary air passes through the incandescent coke layer at higher temperature (1200C) giving exothermic reaction C+O2 = CO2 and provides the heat release for continuing combustion process. It continues till all oxygen

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is consumed. Further CO2 is reduced to CO and H2 is separated from moisture. Both are endothermic reactions and bring down the temperature of bed and gas stream considerably. The stream passes upward through distillation zone and drying zone where volatile matter and moisture are picked up and finally emerges above the fuel bed. Thus, it contains N2, CO2, CO, H2, volatile matter and water vapour. To have complete combustion, sufficient fresh air is necessary. This air is fed at right angles to the up flowing gas stream, to create turbulence, so as to penetrate the gas mass and secure thorough mixing of air and gases. If black smoke appears at the chimney top, it indicates the inefficient combustion and energy wastage. Therefore, proper design of coal feeding apparatus and furnace is important. The overfeed stokers are used for large capacity boilers where coal is burnt with pulverization. These stokers are mainly of following two types:

a. Travelling Grate Stoker 1.) Chain Grate Type 2.) Bar Grate Type b. Spreader Stoker

a. Travelling Grate Stoker The travelling stoker may be chain grate type or bar grate type. These two differ only in the details of grate construction. The grate surface of a chain grate stoker is made of a series of cast iron links connected by pins to form an endless chain. The grate surface of a bar stoker is made of a series of cast iron sections mounted on carrier bars. The carrier bar rides on two endless type drive chains.

Construction and Working:

The chain grate consists of flexible endless chain which forms a support for the fuel bed.

The chain travels over two sprocket wheels. The sprocket on front side of furnace is connected to variable speed drive mechanism, while the other is at rear of the furnace.

The coal bed thickness is regulated either by adjusting the opening of fuel grate or by speed control of the stoker driving motor. The speed of the stoker is 15 to 50 cm/min.

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The combustion control automatically regulates the speed of the grate to maintain the required steam generation rate.

The ash containing a small amount of combustible material is carried over the rear end of the stoker and deposited in the ash pit.

The air is admitted from the underside of the grate which is divided into several compartments each connected to the air duct.

Coal with high ash content if used, will minimize the overheating of grate. As there is no agitation of fuel bed, non caking coals are suitable for chain grate stokers.

Advantages:

The chain grate stoker is simple in construction, low initial cost, low maintenance, self cleaning, controlling the speed of chain helps to control heat release rate.

Disadvantages:

It requires ignition arches.

There is always some loss of coal, clinker problems.

This cannot be used for high capacity boilers (200 tons/hrs or more) as the rate of burning is 200 to 300 kg/m2-hr when forced draught is used.

There is always some loss of coal in the form of fine particles carried with the ashes. b. Spreader Stoker

Construction and Working:

In this type of stoker, coal from the coal hopper, is fed into the path of a rotor by means of conveyor and is thrown into the furnace by means of the rotor and is burnt in suspension.

The air for combustion is supplied through the holes in the grate. The function of grate is only to support a bed of ash and move it out of the furnace.

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The secondary air is supplied through nozzles to create turbulence and supply oxygen for thorough combustion of coal.

Spreader stokers can burn any type of coal. Advantages:

There is less problem of clinker.

Use of high temperature preheated air is possible, in case of chain grate, it is limited to 180 C maximum.

The coking tendency of the coal is reduced before it reaches the grate by the release of volatile gases which burn in suspension.

It can be used for boiler capacities from 70 to 140 tonnes of steam per hour.

The fire bed gives equal pressure drop and proper air distribution so that combustion can be completed with minimum quantity of excess air.

Disadvantages:

It is always difficult to operate spreader with varying sizes of a coal and with varying moisture content.

A natural result of suspension burning of fine fuel particles is the entrainment of ash in the products of combustion. To avoid the nuisance of fly ash, a dust collector is almost a necessity with this stoker.

Many fine unburnt carbon particles are also carried with the exhaust gases and it is necessary to trap these and return to the furnace for burning. Otherwise it would add as a loss to the combustion system.

Underfeed Stokers

The underfeed principle is suitable for burning the semi bituminous and bituminous coals. The coal is fed from below the grate by a screw-conveyor or ram. Primary air passing through holes in the tuyers diffuses through spaces in the raw or green coal, picks up moisture and volatile matter through distillation zone. The gas stream passes next through the incandescent coke region; the volatile matter breaks up and burns with secondary air fed at the top. In overfeeding, burning the volatile matter will be somewhat cooler, therefore need longer time to ignite and burn. The underfeed stokers fall into two main groups

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a. Single Retort Stoker b. Multi- Retort Stoker

a. Single Retort Stoker

The fuel is placed in large hopper on the front of the furnace, and then it is further fed by reciprocating ram or screw conveyor into the bottom of the horizontal trough.

The air is supplied through the tuyeres provided along the upper edge of the grate.

The ash and clinkers are collected on the ash plate provided with dumping arrangement.

The coal feeding capacity of a single retort stoker varies from 100 to 2000 kg per hour.

The increase of capacity in an underfeed cannot be obtained simply by building larger single retort stoker. The size of retort for increasing the capacity is limited by virtue of inability of obtaining even air distribution from the sides of retorts.

b. Multi- Retort Stoker

It consists of a series of alternate retorts and tuyere boxes for supply of air.

Each retort is fitted with a reciprocating ram for feeding and pushers plates for the uniform distribution of coal.

The coal falling from the hopper is pushed forward during the inward stroke of the stoker ram.

Then the distributing rams (pushers) push the entire coal down the length of the stoker.

The ash formed is collected at the other end.

It is supplied with forced draught for maintaining sufficient air flow through the fuel bed.

The primary air is supplied to the fuel bed from main wind box situated below the stoker. The partly burnt coal moves on to the extension grate.

The quantity of air supplied is regulated by an air damper.

Means are provided for varying the air pressure under the different sections of the stoker in order to correct for irregular fuel bed conditions.

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The use of forced draught causes rapid combustion and it also becomes necessary to introduce overfire air when high volatile coals are used to prevent the smoke formation.

Advantages of underfeed stokers:

This gives higher thermal efficiency compared with chain grate stokers.

The part load efficiency is high with multiple retort stoker.

The combustion rate is considerably higher.

Sufficient amount of coal always remains on the grate so that the combustion is continued in the event of temporary breakdown of the coal supply system.

The grate is self cleaning.

Different varieties of coals can be used with this type of stoker.

Tuyeres, grate bars and retorts are not subjected to high temperature as they remain always in contact with fresh coal.

The use of force draught and relatively large quantity of fuel on the stoker make them responsive to rapid changes in load.

Underfeed stokers are suitable for non-clinkering, high volatile and low ash content coals.

Disadvantages of underfeed stokers:

The initial cost of the unit is high.

It requires large building space.

The clinker troubles are usually present.

Low grade fuels with high ash content cannot burn economically.

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Pulverized Fuel Firing In this system, the coal is reduced to a fine powder with the help of grinding mill and is then projected into combustion chamber with the help of hot air current. The amount of air required known as secondary air is supplied separately to combustion chamber to complete the combustion. The amount of air used to carry the coal and to dry it before entering into the combustion chamber is known as primary air. The efficiency of the system depends on the size of the powder. To burn pulverized coal successfully, following two conditions must be satisfied:

a. Large quantities of very fine particles of coal, that would pass a 200 mesh sieve must exist to ensure ready ignition because of their large surface to volume ratio.

b. Minimum quantity of coarser particles should be present since these particles cause slagging and reduce combustion efficiency.

Greater surface area per unit mass of coal allows faster combustion reactions, which reduces the excess air needed. This also reduces the dry exhaust loss through chimney and raises the steam generator efficiency. This system has ability to burn a wide variety of coals, also ease of burning alternatively with or in combustion with gas or oil. The ash resulting from combustion

Partly falls to the furnace bottom

The rest is carried in gas stream as fly ash to flue gas outlet or

Is deposited on the boiler heating surfaces. This fly ash is useful for making bricks etc. but investment is needed to remove fly ash before induced draught fan. There are less pressure losses; therefore less draught is needed for the system.

Pulverized fuel plants may be divided into following two systems:

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1. Unit System

Here each burner or burner group and the pulverize constitute a unit.

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2. Central System It employs a limited number of large capacity pulverizes at a central point to prepare coal for all the burners.

Unit System Central System 1. Layout is simple and permits easy operation. 1. Central preparation requires separate

Less space is required. Building and layout becomes complicated. 2. There is no complex transportation system. 2. Additional cost and complexity of coal 3. Without drying of coal, system works 3. Driers are usually necessary.

satisfactorily. 4. System is cheaper than central system. 4. The system is costly. 5. Flexibility is less than central system. 5. Flexibility is more. 6. Fans handle both air and coal, therefore 6. Fans handle only air, there is no problem of

excessive wear takes place. Excessive wear. 7. It allows direct control of combustion from 7. Fire hazard of quantities of stored pulverized

pulveriser. No storage of pulverized coal. Coal. Burners can be operated independent of Coal preparation plant.

8. If there is interruption of fuel supply, system 8. The large storage is protection against stops. interruption of fuel supply to the burners.

9. The mills operate at variable load, but not 9. The pulverising mill operates at constant load with best results. because of storage capacity between it and the burners.

10. Less power consumption of auxiliaries. 10. Power consumption of auxiliaries is high. Advantages of Pulverized Fuels:

1. Pulverizing exposes more coal surface area for combustion. 2. 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.

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3. Wide variety and low grade coal can be burnt more easily. 4. It gives fast response to load changes as rate of combustion can be controlled easily and

immediately. Automatic control applied to pulverized fuel fired boilers is effective in maintaining an almost constant steam pressure under wide load variations.

5. The system is perfectly free from clinker and slagging troubles. 6. This system works successfully with or in combination with gas and oil. 7. It is possible to use highly preheated secondary air (350 C) which helps for rapid flame

propagation. 8. The pulverizing system can be repaired without cooling the unit as the pulverizing equipment is

located outside the furnace. 9. Large amount of heat release is possible and with such rate of heat generation, each boiler of

pulverized fuel fired system can generate as large as 2000 tons of steam per hour. 10. The banking losses are low compared with stoker firing system. 11. The boiler can be started from cold very rapidly and efficiently. This is highly important during

emergency. 12. The external heating surfaces are free from corrosion and fouling as smokeless combustion is

possible. 13. There are no moving parts in the furnace subjected to high temperature, therefore the life of

system is more and the operation is trouble less. 14. Practically no ash handling problems. 15. 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.

Disadvantages of Pulverized Fuels: 1. The capital cost of the pulverized system is considerably high as it requires many additional

equipments. Its operation cost is also high compared with stoker firing. 2. This system produces fly-ash (fine dust) which requires special and costly fly-ash removal

equipments as electrostatic precipitators. 3. 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.

4. The possibilities of explosion are more as coal burns like a gas. 5. The storage of powdered coal requires special attention and high protection from fire hazards. 6. The fine grinding of fuel at all loads is not possible particularly in unit system. 7. The building space required is large particularly for central system. 8. The skilled operators are required. 9. Special starting up equipments are required. 10. Nuisance (high air pollution) is caused by the emission of very fine particles of grit and dirt as they

remain in suspended condition in air for a considerable long period. 11. The removal of liquid slag formed from low fusion temperature ash requires special equipments

and creates additional problems of its removal.

Pulverizer A pulverizer (mill) is the most important part of a pulverized coal system. In order to increase surface exposure, coal is pulverized. It promotes rapid combustion without using large quantities of excess air.

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Selection of Pulverizer Following points should be considered 1. Coal characteristics are important in selecting and sizing pulverizers. 2. Coal analysis determines the number of pulverizers required and their capacity. 3. Pulverizer capacity increases with coals grindability index and varies inversely with required

fineness. 4. The power required to pulverize 1 kg of coal also increases with fineness of the coal. 5. Variation in coal surface moisture also affect pulverizer and boiler operating characteristics.

Pulverizers (mills) are classified as follows: 1. Attrition Mills: a.) Bowl Mills b.) Ball and Race Mills 2. Impact Mills: a.) Ball Mills b.) Hammer Mills

Bowl Mill

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It consists of stationary rollers and power driven bowl. The raw coal is supplied through hopper.

The pulverization takes place in bowl as the coal passes between the sides of the bowl and the rollers.

The primary air induced draught fan draws a stream of heated air through the mill, and carries the pulverized coal into a stationary classifier.

The classifier which is located at the top of the pulverizer may be adjusted to charge the coal fineness while the pulverizer is in operation.

It returns the coarse particles of coal to the bowl for further grinding through the centre cone of it. The coal pulverized to the desired fineness is carried t the burner through the fan.

The leakage of coal from the mill casing is completely eliminated as the mill operates under negative pressure.

The power consumption of mill is about 5 kWH of electricity per tonne of coal. Ball and Race Mill

The coal is crushed between two moving surfaces: balls and races. The upper stationary race and lower rotating race driven by a worm and gear, hold the balls between them.

The grinding pressure is maintained by adjustable springs. The coal passes between the rotating elements repeatedly until it has been pulverized to desired degree of fineness.

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Hot air is supplied to the mill through the annular space surrounding the races by a forced draught fan.

The air picks up the coal dust and enters into the classifier. The coal particles of required size are taken to the burners and oversized particles slide down for further grinding in the mill.

Mill, feeder and fan require up to 14 kWh energy per tonne of coal pulverized. These mills have greater wear as compared to other mills.

It requires lower space, lower power consumption, lower weight and lower capital cost.

Ball Mill

It consists of a large cylinder partly filled with various sized steel balls.

The coal is fed into the cylinder and mixes with these balls.

The cylinder is rotated (100-200 rpm) and pulverization takes place because of the action between the balls and the coal.

The mill consists of coal feeder, pulverizer, classifier and exhauster like other mills.

The system is simple in operation, low in initial cost but its operating cost is high.

The grinding elements in this mill are not affected much, by metal scrap and other foreign material in the coal if any.

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It can be used for a wide range of fuels including anthracite and bituminous coal which are difficult to pulverize.

Impact or Hammer Mill

These mills have swinging hammers or bars, into the path of which is fed the coal to be pulverized.

All grinding elements and the primary air fan are mounted on a single shaft .

In the primary stage grinding, coal is reduced to fine granular state by impact of series of hammers.

The final stage grinding is completed by attrition, between stationary pegs and rotating pegs.

The rotating scoop shaped rejecter arms separate large particles and throw back into the grinding section.

The output of pulverizer is controlled by varying the coal feed and the flow of primary air either by hand or by automatic control. This pulverizer operates at high speed and requires minimum floor area.

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Pulverized Fuel Burners The pulverized coal burners should satisfy the following requirements

Thorough mixing of coal and primary air.

Proper turbulence and stable combustion of coal.

Control the flame shape and its travel in the furnace.

The mixture of coal and air should move away from the burner at a rate equal to flame travel to avoid the flash back with the burner.

Adequate protection against overheating, internal fires and excessive abrasion wear. Pulverized fuel burners are classified as: 1. Long Flame or U Flame or Streamlined burners. 2. Short Flame or turbulent burners. 3. Tangential burners 4. Cyclone burners

Ash Handling System A huge quantity of ash is produced in large power stations nearly 10 to 20% of the total quantity of coal burnt in a day. Therefore, it is to be removed from the furnace regularly and should be disposed off by scale or otherwise. The ash coming from the furnace is too hot, also it is dusty and irritating to handle and is accompanied by some poisonous gas. Therefore, it is to be quenched before handling. Quenching gives following advantages: 1. It reduces corrosion action of the ash. 2. It reduces the dust accompanying ash. 3. Ash forms clinkers by fusing in large lumps. These clinkers will disintegrate by quenching.

Modern ash handling systems are classified into four groups:

Mechanical Handling System

Hydraulic System

Pneumatic System

Steam Jet System

The general layout of ash handling and dust collection plant is below:

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The principle requirements of good ash handling system are as follows: 1. It should be able to handle large clinker, boiler refuse etc. with little attention of the workmen. 2. It should have high rate of handling. 3. It should be able to handle hot and wet ash effectively and with good speed. 4. The operating cost should be minimum. 5. The operation of the plant should be noiseless. 6. It should be able to operate under all variable load conditions. 7. It should be possible to minimize the corrosive or abrasive action of ashes and dust nuisance should

not exist. 8. In case of addition of units, it should need minimum changes in the original layout of plant.

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Hydraulic System:

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In this system, ash is carried with the help of flow of water with high velocity through a channel and dumped in the sump.

The system is either a low pressure system or high pressure system.

In low pressure system, ash directly falls into the troughs, provided below boilers and is carried by water to sumps.

In the sump, the ash and water are made to pass through a screen so that water is separated from ash and this water is reused. The ash is removed to the dumping yard.

The ash carrying capacity is about 50 tonnes/hr and distance covered is about 500 m.

In high pressure system, the hoppers below the boilers are fitted with water nozzles at the top and on the sides.

The function of top nozzles is to quench the ash while side nozzles provide driving force for the ash.

Trough carries water and ash. The water is again separated from ash and recirculated.

The capacity of this system is higher than low pressure system and distance covered is also more.

The hydraulic system is clean and healthy.

Working parts do not come in contact with ash. The system is dustless and closed.

It is suitable for large thermal power plants.

Pneumatic System:

This system can handle abrasive ash as well as fire dusty materials like fly ash and soot.

In this system, the exhauster which is provided at the discharge end creates a high velocity stream, picks up ash and dust from all discharge points and then carried in the conveyor pipe to the point of delivery.

Mobile crushing units are used to crush large ash particles to smaller size.

The separator working on the cyclone principle removes dust and ash which is collected in the ash hopper at the bottom, while clean air is discharged from the top.

The exhauster may be mechanical or may use steam jet or water jet for its operation.

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For mechanical exhauster, filter is essential; it consumes less power, hence preferred in a large station.

For small and medium plants, steam exhauster may be used.

The system is flexible. There is no spillage and rehandling.

Cost is less in comparison to other systems.

The system is totally closed, no chance of ash freezing or sticking in the storage bin and material is discharged freely by gravity.

Only it requires high maintenance and more noisy operation than any other system.

Mechanical Handling System:

This system is generally used for low capacity power plants using coal as fuel.

The hot ash from the boiler furnace is collected over the belt conveyor after cooling it through water seal.

This cooled ash is transported to an ash bunker and from ash bunker, it is removed to the dumping site through trucks.

Steam Jet System

In this system, steam at high velocity is passed in the direction of ash travel through a conveying pipe and dry solid materials of considerable size are carried along with it.

The ash is deposited in the ash hopper.

The system can remove the ash through a horizontal distance of 200 m and a vertical distance of 30 m.

It requires less space, less capital cost, no auxiliary drive required.

It is possible to place the equipment in any awkward position.

The capacity of the system is limited to 7 tonnes per hour, it requires continuous operation.

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Bag House Dust Collectors

Bag house collectors are favorable where coal has low sulphur content (less than 1%) and temperatures are under 300C.

In this type of design, flue gas is sent inside of the bags, then through the cloth into the house and then out.

A gentle reverse flow of air periodically cleans the bags with a minimum of bag flexing, avoiding excess fabric friction that increases the bag life.

A well designed and maintained bag house collector (particle size above 1) will collect 99.9% of dust and the efficiency is independent of the amount of dust in the flue gas.

However, the bag houses are more sensitive to condensation of objectionable gases than other types of collectors and require more maintenance.

For efficient function of bag filters it is necessary to maintain operating temperature above DPT (Dew Point Temperature) of the exhaust gases (approx. 150C).

The other factors affecting the performance are the type of boiler combustion, type of coal and boiler operation.

The different types of fabric Bag house filters are:

Open Pressure Type o In this a fan is located to the dust loaded side and it can operate with open sides as long as

protection is provided from weather. o It is constructed with corrugated steel or asbestos cement walls. o It may have open grating at the cell plate level and may not require hopper insulation.

Open Pressure Type Closed Pressure Type

Closed Pressure Type o It consists of a closed air-tight structure with fan located to the dust loaded side. o The floor of the unit is closed and structure walls and hopper are insulated. o It is used for gases having high DPT.

Closed Suction Type

o In this the fan is located to the outlet of baghouse clean gas side. o The floor, walls and hopper are insulated.

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o It is used for gases having DPT between 75C to 85C. o Blower maintenance is cheaper as it is located to clean side of the gas.

Closed Suction Type

Advantages of the Baghouse Filters

Bag houses are not sensitive to flyash resistivity therefore efficiency remains constant.

They have high collection efficiency about 99.9%.

They are less costly than ESP.

Baghouses easily comply with capacity requirements. Disadvantages of the Baghouse Filters

Bag house fabrics are very sensitive to fluctuations in gas temperatures.

Continued operation at or below acid DPT lead to corrosion of metal parts and reduces the bag life. Electrostatic Precipitators (ESP)

It is extensively used in removal of fly ash from electric utility boiler emissions.

It is designed to operate at any desired efficiency for use as a primary collector or as a supplementary unit to a cyclone collector. This is required to meet air pollution knobs or strict air quality codes.

The dust laden gas is passed between positively charged and negatively charged conductors and it becomes ionized as sufficiently large voltage (30,000- 60,000 V) is applied between the conductors.

When gas passes through electrodes, both negative and positive ions are formed, positive ions are more as high as 80%.

In the collecting unit, there are number of vertical plates, alternately negatively charged and earthed.

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This produces high intensity electrostatic field existing between the plates. This electrostatic field

exerts a force on positively charged dust particles and drives them towards the grounded plates.

The deposited dust particles are removed from the plates by giving shaking motion to the plates by external means.

The dust is collected in the dust hoppers.

Advantages:

It can effectively remove very small particles like smoke, mist and fly ash.

It has easy operation.

The draught loss is quite less.

This is most effective for high dust loaded gas (as high as 100 grams/m3).

The maintenance charges are minimum as compared to other separators.

The dust collected is in dry form and can be removed either dry or wet depending on its application.

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Disadvantages:

It is necessary to convert low voltage AC to high voltage DC, which increases capital cost of the equipment.

Running charges are also higher.

The space required is more as compared to wet system.

Necessary to protect the entire collector from sparking.

The collector efficiency depends on gas velocity. If gas velocity exceeds that for which the plant is designed, the collector efficiency decreases.

Boiler Feed Water Treatment LAYOUT OF BOILER FEED WATER TREATMENT PLANT

Necessity of Feed Water Treatment 1. Natural water contains solid, liquid and gaseous impurities and therefore cannot be used in boilers. 2. It is also necessary to reduce the corrosive nature and quantity of dissolved and suspended solids in

feed water with the beginning of high pressure, critical and supercritical boilers. 3. Once through the boilers, the problem of removing dissolved solids is important as it passes through

turbine and condenser as well. 4. The effects of these solids are the corrosion and erosion of boiler tubes, turbine blades and

condenser tubes. 5. These solids also block the boiler tubes eventually by scale formation which reduces the rate of heat

transfer and thus resulting in tube failure due to overheating. 6. The external water is used as a make up (up to 1- 5%)to the feed water system in order to

counterbalance the loss of working medium throughout the cycle from blow-down, leaks etc.

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Therefore, it is absolutely necessary to have a separate water-softening plant to polish the water taken from outside source to remove any contamination caused by corrosion in make up water.

7. The most probable reason of condensate contamination is leakage since the system is not sufficiently tight under normal load conditions.

8. The overall objective of the water softening plant is to maintain the operation at the best possible levels of availability, economy and efficiency.

9. Also to attain chemical control of the water and steam system to prevent the corrosion in the boiler and steam turbine, to prevent the scale and deposit formations on heating surfaces and to maintain high level purity of the steam.

Different Impurities in Water The impurities present in the feed water are classified as given below:

a. Undissolved and suspended solid materials. b. Dissolved salts and minerals. c. Dissolved gases. d. Other materials (as oil, acid) either in mixed and unmixed forms. Effects of Impurities in Water 1. Scale Formation

The calcium and magnesium salts are the major source of scale formation because of their low solubility. The scale forming salts are those that become less soluble as water temperature increases.

A thin film of water next to the boiler heating surface remains always hotter than the main body of water. Therefore, less soluble salts deposit directly on the heating surface earlier.

The major trouble of scale forming arises due to the calcium sulphate and the formed scale is further hardened by the presence of silica in water. Calcium and magnesium bicarbonates are broken down by moderate heating (100C) into relatively insoluble monocarbonate and CO2 as given off as per the following chemical reaction

Ca(HCO3)2 + Heat CaCO3(soft sludge) +CO2 +H2O Mg(HCO3)2 + Heat MgCO3 (soft sludge) + CO2 + H2O

The scale formation takes place mainly in feed water piping and boiler tubes. Its effect on piping system is to choke the flow by reducing the flow area and increases the pressure required to maintain the water delivery. It also reduces the heat transfer from the hot gases to water. It also leads to overheating of tubes.

The formation of scale is prevented either by removing or reducing the contents of calcium, magnesium or silica before feed water reaches the boiler or by internal treatment ie adding chemicals in feed water which causes dissolved solids to form a non sticky soft sludge.

2. Corrosion

The corrosion is the eating away process of boiler metal. It causes deterioration and failure of the equipments. The corrosion of boilers, economizer, feed water heaters and piping is caused by an acid or presence of dissolved oxygen and CO2 in the boiler feed water.

Oxygen generally enters a closed system through make up condenser leakage and condensate pump packings.

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The CO2 comes out of bicarbonates on heating and it combines with the water to form weak acid known as carbonic acids. The acid slowly reacts with iron and other metals to form their bicarbonates.

The corrosion can be minimized by adding alkali salts to neutralize acids in water. The effect of CO2 is neutralized by the addition of ammonia or neutralizing amines in water.

The effect of oxygen is reduced only by removing the oxygen from water by use of mechanical deaeration followed by scavenger chemicals.

The corrosion of metal surfaces can be prevented by applying protective coating of amines to the internal surfaces of boilers and economizers.

3. Priming, Foaming and Carryover

The priming is a violent discharge of water with steam from the boiler. In priming, the water level in the boiler undergoes rapid and great changes and there are violent discharges of bursting bubbles.

The priming is caused due to improper boiler design, improper method of firing, overloading, sudden load changing or a combination of these factors.

The priming effect is reduced by installing steam purifiers, lowering water level in the boiler drum and maintaining constant load on boiler.

The foaming is the formation of small and stable bubbles throughout the boiler water. The high percentage of dissolved solids, excessive alkalinity and presence of oil in water are responsible for foaming.

When the concentration of solids in water increases and it also becomes contaminated with oil, numerous small sized steam bubbles are formed which are stable and do not burst easily. These bubbles form thick layer on the surface producing violent foaming.

The foaming contaminates the steam with appreciable amounts of boiler water which contains the corrosive salts.

Boiler water solids are also carried over in the moisture mixed with steam even when there is no indication of either priming or foaming. This is known as Carryover.

The carryover is caused due to improper boiler design, high water level, overloading and fluctuating loads on boiler.

Steam washers and mechanical separators in the boiler drums effectively control carryover within reasonable and tolerable limits.

Proper water treatment including the right amount of blowdown is the key for maintaining these limits.

4. Caustic Embrittlement

The caustic embrittlement is the weakening of boiler steel as a result of inner crystalline cracks.

This is caused by long exposure of boiler steel to a combination of stress and highly alkaline water.

The caustic embrittlement takes place under following conditions: a. When the boiler water contains free hydroxide alkalinity and some silica, the feed water is high in

sodium bicarbonate which breaks down into sodium carbonate in the boiler and partially hydrolizes as shown below Na2CO3 + HOH CO2 + 2NaOH

b. Slow leakage of boiler water through a joint or seam.

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c. Boiler metal is highly stressed at the point of leakage. This may be caused by faulty riveting, misalignment and expansion.

Prevention of embrittlement consists of reducing the causticity or adding inhibiting agents to the feed water.

The most practical method of preventing caustic embrittlement is to regulate the chemical composition of the boiler water.

The method to prevent embrittlement is to eliminate free NaOH using phosphates such as sulphate liquor, quebracho tannin and sodium nitrate.

Different Methods of Water Treatment

The basic objective of the water treatment system is to remove the suspended solids, dissolved solids and dissolved gases from the water before supplying the water to the boiler.

If the dissolved solids in the water are removed in the boiler itself by a chemical treatment then the method is known as Internal Treatment

If they are removed from the water before supplying to the boiler then it is known as External Treatment.

Internal Boiler Water Treatment

The aim of this treatment is to adjust boiler water chemically to prevent scale formation, corrosion, steam contamination and embrittlement.

The amount and type of treating chemicals used depend on the plant operating conditions and feed water analysis.

An internal treatment is accomplished by adding chemicals to the boiler water either to precipitate the impurities so that they can be removed in the form of sludge or to convert them into salts which will stay in water and do not harm.

The common internal treatments used are discussed below: 1. Sodium Carbonate (Soda Ash) Treatment:

The added sodium carbonate reacts with sulphates of calcium and magnesium in boiler water to form calcium and magnesium carbonates as given by the following equation: CaSO4 + Na2CO3 CaCO3 (insoluble sludge) + Na2SO4

The sodium carbonate at high pressure and temperature conditions react with water to form free caustic soda Na2CO3 + 2H2O + Heat 2NaOH + H2O + CO2

The free caustic soda formed reacts with the soluble magnesium salts to form desirable insoluble magnesium hydroxide sludge MgSO4 + 2NaOH Mg(OH)2 (sludge) + Na2SO4

This system of cleaning destroys sulphate hardness. This treatment is suitable for low pressure boilers.

The main disadvantage of this system is, it forms CO2 which goes with steam and dissolves in the condensate to form carbonic acid.

2. Phosphate Treatment:

For high pressure and temperature conditions (ie above 10 bar), the required degree of alkaline stability is obtained with treatment by phosphate compounds.

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The common phosphates which are used are Trisodium phosphate (Na3PO4.12H2O) highly alkaline, Disodium phosphate (Na2HPO4.12H2O) moderately alkaline and mono-sodium phosphate (NaH2PO4) slightly acidic.

The phosphate precipitate will be either tricalcium phosphate or hydroxyapatite [(Ca10(OH)2(PO4)6] if hydrate content of boiler water is high enough. The hydroxyapatite is more preferable form of precipitate as it is less sticky than tricalcium phosphate.

The phosphate treatment should be given directly to the boiler drum with a chemical pump.

Because if introduced into the suction or the boiler feed pump, most phosphates will react with the impurities of water and cause deposits in pumps, piping feed water regulators and valves.

The phosphate treatment should not be used to boiler water which is very hard because the sludge produced is heavy and tends to aggravate carryover conditions in the boiler.

The solution to this problem is to reduce the hardness with external treatment before the use of phosphate treatment.

3. Colloidal Treatment:

To remove the sludge formed effectively from the boiler, proper colloidal materials are added to the boiler water which will prevent the sludge from sticking to each other or to the boiler drum surface so that it can be more easily removed by blow down.

The colloidal materials have excellent absorbing and coagulating properties which readily absorb formed inorganic sludge onto its surface.

The common colloidal materials used are tannins, lignins, starch and seaweed derivatives. Other organic colloids used are sodium mannuronate and sodium alginate which react with calcium and magnesium salts to form a floc that entangles precipitates.

The ethylene diaminetetra acetic acid (EDTA) and other chemicals have chelating powder, so calcium, magnesium and other common metals are tied up in the water. This action prevents the formation of either scale or sludge in boilers, heat exchangers and piping and is effective over the normal range of alkalinity encountered in boiler plant operation.

4. Use of Volatile Amines:

Alkalinity of the feed water is one of the important parameters to control corrosion of boilers because in alkaline solution, the cathodic as well as anodic reaction rates slow down due to less number of hydrogen ions.

Ammonia, cyclohexylamine and morpholine are the main volatile amines used for the purpose.

Morpholine is better volatile alkaline chemical for corrosion control compared with other amines because it increase pH of steam as well as condensate and neutralizes carbonic acid and other corrosion causing acid components.

5. Blowdown Systems:

Draining off some of the boiling water carrying excessive concentration solids and replacing it with fresh water keeps the solid concentration within safe limits. This process is known as blowing down and discharged water is called blowdown.

The blowdown may be either intermittent or continuous.

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External Water Treatment Systems

When the make-up water quantity is large and contains considerable suspended and dissolved solid material, then the external treatment of water becomes essential.

The suspended solid material is generally removed by mechanical means.

The dissolved gases are generally removed by thermal treatment and dissolved solids are removed with the use of chemical treatment.

a. Mechanical Means (Removal of Suspended Solids)

The suspended material is generally removed with the use of the following mechanical means: i. Sedimentation

The water is allowed to remain stand still in big tanks or to flow at a very low velocity.

The solid matter settles down due to gravity and it is removed either periodically or continuously. The clean water is taken from the surface of the settling chambers.

The settling of solid material is accelerated many times by adding a coagulant like aluminium sulphate or sodium aluminate.

The reaction between these salts and alkalinity in the water forms a floc which makes small particles adhere to each other, forming larger particles that settle out more easily.

To remove organic impurities, the usual practice is to use oxidizing agents line chlorine or potassium permanganate and then passed through granular activated carbon filters.

ii. Filtration

Water is filtered by passing it through a fine strainer or other porous media to remove suspended

solids mechanically. The degree of filtration depends on the fineness of filtration media.

The suspended matter adheres to the filter materials leaving the water clear as it drains from the bottom.

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It is necessary to wash the beds periodically to remove the dirt collected in the voids of the filter material.

The different types of the filters are pressure filters, gravity filters, tubular filters, horizontal filters, compartmented type and cartridge type and many others.

b. Thermal Treatment (Removal of Dissolved Gases)

The water generally contains oxygen, carbon dioxide, air, H2S and other gases in dissolved condition. It is necessary to remove these gases before supplying the water to the boiler as they are fully responsible for corrosion.

The removal of gases is accomplished by heating the water to 105-110 C with subsequent agitation during heating. The simple heating removes the dissolved gases from water because the absorbing capacity of water is reduced at higher temperatures.

The different types of deaerators are steam deaerators, forced draft degasifiers, pressure aerators, coke tray aerators and wood slat aerators.

In tray type deaerator, water which is to be deaerated is first passed through the vent condenser where it is preheated by the gases and air is liberated from the water and part of steam carried with the gases.

Then it is passed through a spray distributor over an entire width of heating tray in a form of uniform shower.

The water from the heating trays falls over the air separating trays and then it passes into storage space of the deaerator. The released air and part of the steam is vented passing over the condenser.

The heated gases give their heat to the make up water and the steam carried with gases is also condensed.

The steam enters through the nozzle in the side of the shell filling the entire space between the shell and tray compartment.

It then flows downward through perforations in the top plate meeting the water sprayed upward through the distributor.

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c. Chemical Treatment (Removal of Dissolved Solids) The external water softeners are mainly divided into precipitation type and ion-exchange type.

i. Hot Lime[Ca(OH)2] Soda(Na2CO3) Process

This process uses lime (calcium hydroxide) and soda ash (sodium carbonate) which react with all types of calcium and magnesium salts and precipitate them at boiling point (100 C) of the water.

The lime precipitates the carbonate hardness as given below: Ca(HCO3) + Ca(OH)2 2CaCO3 + H2O Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 +2H2O

The sulphate hardness of calcium and magnesium is removed by soda ash. The reactions are as below: CaSO4 + Na2CO3 CaCO3 + Na2SO4 MgSO4 + Na2CO3 MgCO3 + Na2SO4 MgCO3 + Ca(OH)2 Mg(OH)2 + CaCO3

The magnesium carbonate (MgCO3) formed reacts with lime and forms magnesium hydroxide sludge.

The chloride hardness of calcium and magnesium is also removed by soda and lime respectively as below: CaCl2 +Na2CO3 CaCO3 + 2NaCl MgCl2 + Ca(OH)2 Mg(OH)2 + CaCl2

The disadvantage is the softened water must be filtered before use to avoid carrying the precipitate with water. The precipitate which does not settle in reaction tank is removed by filtration.

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In a hot process lime soda softener with filtration, the raw make-up water enters through a float controlled regulating valve and then passes over vent condenser and then sprayed into the reaction chamber.

Water is heated by spraying it into the upper steam space. Inlet flow actuates a proportioning device to control the amount of lime and soda fed to the heating and mixing zone.

The above chemical reactions take place almost instantly. The sludge collecting cone receives the precipitates at the bottom and discharges to the sewer periodically. After this, the softened water leaves the settling tank.

ii. Ion Exchange of Zeolite Process

Impurities that dissolve in water dissociate to form positively and negatively charged particles known as ions and these impurities are known as electrolytes.

The ion exchange materials such as sodium zeolite have the ability to exchange one ion for other, hold it temporarily in chemical combination and give it up to a strong regenerative solution.

Sodium is much like an ordinary pressure filter, holds a bed of active zeolite supported by layers of graded gravel lying over a water distribution and collection system.

The raw water is supplied to the zeolite tank at the top. The water sprayed at the top of the shell flows downward through the zeolite bed and the hardness of the water is removed by ion exchange.

Typical sodium zeolite reactions are:

Ca(HCO)2 + Na2Z CaZ + 2NaHCO3

CaSO4 + Na2Z CaZ + Na2SO4

CaCl2 + Na2Z CaZ + 2NaCl

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Zeolite reacts in similar way with magnesium salts.

Once the bed is saturated water is used to remove the dirt that might be collected on the top of the zeolite bed.

Then the sodium chloride salt solution of predetermined strength and amount is injected with the help of hydraulic rejector at the top of the vessel thus reactivating the bed.

Environmental Aspects of Steam Power Plant

The influence of Thermal Power Plant on the surrounding is determined by a. Contamination of air by flue gases b. Warming up of atmosphere due to heat rejected c. Contamination of air

The main emissions from coal combustion at thermal power plants are carbon dioxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx), chlorofluorocarbons (CFCs), and air- borne inorganic particles such as fly ash, soot, and other trace gas species.

Carbon dioxide, methane, and chlorofluorocarbons are greenhouse gases. These emissions are considered to be responsible for heating up the atmosphere, producing a harmful global environment.

Oxides of nitrogen and sulphur play an important role in atmospheric chemistry and are largely responsible for atmospheric acidity.

Particulates and black carbon (soot) are of concern, in addition to possible lung tissue irritation resulting from inhalation of soot particles and various organic chemicals that are known carcinogens.

1. Sulphur Dioxide (SO2)

Sulphur present in coal after combustion releases SO2 as flue gases and gets converted to sulphuric acid with the atmosphere.

This sulphuric acid falls down as acid rain and contaminates fresh water sources, damages plants and buildings.

Acid rain also reduces ground fertility and crop yield.

Sulphuric acid causes respiratory tract problems.

2. Carbon Dioxide (CO2)

Accumulation of CO2 in atmosphere leads to green house effects.

Due to which climate changes takes places which can transform fertilize land into barren.

It may also lead to melting of ice at the poles.

3. Carbon Monoxide (CO)

Once carbon monoxide has been breathed in by humans, it replaces the oxygen in the blood, thus killing off cells and starving vital organs of oxygen.

In small extent, it is also responsible for acid rain.

4. Oxides of Nitrogen (NOx)

It helps form acid rain which hampers plant growth.

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It also contributes to global warming.

NOx combines with other pollutants to form toxic chemicals.

Small levels of NOx can cause nausea, irritated eyes and/or nose, respiratory problems etc.

5. Ash and Dust

The fine particulate matter emitted when coal is burned has the potential to harm human health.

These small particles are breathed into the body, damaging lung or helping to trigger lung cancer.

The smallest particles can work their way directly into the blood stream.

Control of Pollutants: 1. Control of SO2

Iron Sulfide (FeS2) pyrite in coal is the source of sulphur.

Desulphurization of coal is done by chemical treatment, magnetic separation and froth floatation methods

Cleaning of flue gases by different methods ie wet scrubbing, catalytic conversion etc.

Avoid using fuel high with sulphur content and using tall chimney for exhausting the flue gases.

2. Control of NOx

By adjusting combustion condition ie low combustion temperature and use of low nitrogen fuel.

Reduction of residence time of combustion products.

3. Control of Particulate Material

Treating these materials by use of electrostatic precipitator and various filters can reduce the particulate material to release into the atmosphere.

http://www.epa.gov/air/particlepollution/basic.htmlhttp://www.epa.gov/air/particlepollution/health.html

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CHAPTER 2 STEAM POWER PLANT QUESTIONS FROM PREVIOUS PAPERS

1. What is need of boiler Feed water treatment? Explain in brief steps of boiler feed water treatment process. (4)

2. The use of regenerative feed water heating increases the capital cost but reduces the operating cost of steam power plant. Explain. (4)

3. What is pulverizing of coal? Mention their advantages. (4) 4. Explain construction and working of sub critical boiler. (6) 5. Explain principle of ESP (electrostatic precipitator). (4) 6. Explain multi retort stoker system. (4) 7. What are the environmental aspects of steam power plant? (4) 8. Explain coal handling system in power plant. (4) 9. Describe Hydraulic ash handling system. (6) 10. Describe any one high pressure boiler. State its critical stage. (4) 11. What is pulverizing of coal? What are the advantages of it over other coal firing systems? (4) 12. Draw general layout of steam power plant with labeled components. (4) 13. State any four advantages of pulverized coal system. (4) 14. Draw layout of feed water treatment plant and explain. (4) 15. With the help of neat sketch, explain the principle of underfeed strokers. (4) 16. Why feed water treatment is necessary? Explain scale formation in boiler? (4) 17. What are the various steps involved in coal handling system? Sum12(4) 18. Draw schematic diagram of a screw conveyor. State four advantages of screw conveyors.

Sum2012(4) 19. Explain pneumatic ash handling system with neat sketch. State its advantages. Win12(4),

Sum12 (8) 20. Explain Lamont Boiler with sketch. State its critical stage. Sum12 (8) 21. Name four impurities present in feed water. State the procedure to remove any one of them

- Sum12(4) 22. State the advantages of electrostatic precipitator. Sum12(4) 23. Write factors to be considered for site selection of steam power plant. Win12(4) 24. State necessity of feed water treatment used for boiler. Explain any one method feed water

treatment. Win12(8) 25. State advantages of pulverized coal system-sketch elements of pulverized coal system and label

it. Win12(8)