Steam Power Plant.pdf 594h5m

STEAM POWER PLANT • •

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Power is the basic necessity for the economic development of a country. Development of heavy or large scale industries as well as medium scale and small scale industries, agriculture, transportation, etc., depend on electric power generation It is therefore necessary to utilise the present resources of energy with utmost care and with maximum efficiency. It is the duty of engineers and scientists to find ways and means to supply the required power at the cheapest rate.

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Renewable sources of energy are continuously produced in nature, and they will not get exhausted eventually in future. Examples: Hydel energy, solar energy, wind energy, tidal energy, geo-thermal energy and biomass. All the renewable energy sources have their common origin in sun. Non-renewable sources of energy will get exhausted eventually in future. Examples: Energy from fossil fuels.

Non-renewable sources of energy •

Conventional and Non-conventional sources of energy •

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The basic energy sources for generating electric power are thermal, hydel, and nuclear. These sources are known as "conventional sources of energy", as these sources are used for the last two hundred years for power generation. In the case of hydro-power, generation of power is dependent at the mercy of the nature. The other two resources, viz., thermal and nuclear are exhaustible.

Renewable and Non-renewable sources of energy •

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Renewable sources of energy is neither consumed nor converted into something else. Examples: hydel, solar, wind, tidal, geo-thermal. On the other hand, non-renewable sources of energy are exhaustible. Examples: fossil fuels (coal, petroleum products), natural gas, nuclear fuel.

At the present rate of consumption, unless new reserves are found, the existing reserves of coal and oil will last for another 150 and years respectively. Commercial and Non-commercial energy sources •

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The energy sources available can also be classified into three major types based on the yield of the net energy. They are: • • •

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The various energy sources are grouped under two main types. They are: Renewable energy sources and non-renewable energy sources.

Primary energy sources. Secondary energy sources. Supplementary energy sources.

Primary energy sources

CLASSIFICATION OF ENERGY SOURCES •

Energy sources like, coal, oil, gas, uranium and hydel power are known as commercial energy sources, because they are directly used to produce electricity. Energy sources like, wood, dung, waste etc. are known as non-commercial energy sources. These are mainly used as fuel for cooking and are also used in cottage industries (e.g., smithy). In sugar mills also non-commercial energy sources are utilised.

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The energy source which provides a net supply of energy is called primary energy source. Examples: coal, natural gas, uranium, oil etc. The energy to be expended to obtain these fuels is very much less than the energy that can be obtained from them by combustion or nuclear reaction.

Renewable sources of energy Secondary energy sources

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From the secondary energy sources, the yield of energy is less than the input. Examples: solar energy, wind energy, tidal energy, water energy, etc.

Supplementary energy sources •

If the net energy yield provided by the energy source is zero, it is called supplementary energy source. Example: thermal insulation.

LAYOUT OF STEAM POWER PLANT • •

Steam is an important medium for producing mechanical energy. Steam is used to drive steam engines and steam turbines.

Steam has the following advantages. 1. 2. 3.

Steam can be raised quickly from water which is available in plenty. It does not react much with materials of the equipment used in power plants. It is stable at temperatures required in the plant.

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This includes coal delivery, preparation, coal handling, boiler furnace, ash handling and ash storage. The coal from coal mines is delivered by ships, rail or by trucks to the power station. This coal is sized by crushers, breakers etc. The sized coal is then stored in coal storage (stock yard). From the stock yard, the coal is transferred to the boiler furnace by means of conveyors, elevators etc. The coal is burnt in the boiler furnace and ash is formed by burning of coal. Ash coming out of the furnace will be too hot, dusty and accompanied by some poisonous gases. The ash is transferred to ash storage. Usually, the ash is quenched to reduced temperature, corrosion and dust content. There are different methods employed for the disposal of ash. They are hydraulic system, water jetting, ash sluice ways, pneumatic system etc. In large power plants hydraulic system is used. In this system, ash falls from the furnace grate into high velocity water stream. It is then carried to the slumps.

Equipment of a Steam Power Plant A steam power plant must have the following equipment. 1. 2. 3.

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A furnace for burning the fuel. A steam generator or boiler for steam generation. A power unit like an engine or turbine to convert heat energy into mechanical energy. A generator to convert mechanical energy into electrical energy. Piping system to carry steam and water.

The working of a steam power plant can be explained in four circuits. 1. 2. 3. 4.

Fuel (coal) and ash circuit. Air and flue gas circuit. Feed water and steam flow circuit. Cooling water flow circuit.

Water and Steam circuit •

• Coal and Ash circuit

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It consists of feed pump, economiser, boiler drum, superheater, turbine condenser etc. Feed water is pumped to the economiser from the hot well. This water is preheated by the flue gases in the economiser.

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This preheated water is then supplied to the boiler drum. Heat is transferred to the water by the burning of coal. Due to this, water is converted into steam.

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The steam raised in boiler is ed through a super heater. It is superheated by the flue gases. The superheated steam is then expanded in a turbine to do work. The turbine drives a generator to produce electric power. The expanded (exhaust) steam is then ed through the condenser. In the condenser, the steam is condensed into water and recirculated. A line diagram of water and steam circuit is shown separately in figure 1.3.

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This pre-heated air is supplied to the furnace to lid the combustion of fuel. Due to combustion of fuel, hot gases (flue gases) are formed.

The flue gases from the furnace over boiler tubes and superheater tubes. (In XJiler,wet steam is generated and in superheater the wet steam is superheated by the flue ;ashes.) Then the flue gases through economiser to heat the feed water. After that, it es through the air preheater to pre-heat the incoming air. It is then ed through a dust catching device (dust collector). Finally, it is exhausted to the atmosphere through chimney. A line diagram of air and flue gas circuit is shown separately in figure 104.

Cooling water circuit • • • •

This circuit includes a pump, condenser, cooling tower etc. The exhaust steam from the turbine is condensed in a condenser. In the condenser, cold water is circulated to condense the steam into water. The steam is condensed by losing its latent heat to the circulating cold water.

Air and Flue gas circuit •

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It consists of forced draught fan, air preheater, boiler furnace, superheaterer, economiser, dust collector, induced draught fan, chimney etc. Air is taken from the atmosphere. The action of a forced draught fan. It is ed through an air pre-heater. The air is preheated by the flue gases in the pre-heater.

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Thus the circulating water is heated. This hot water is then taken to a cooling tower. In cooling tower, the water is sprayed in the form of droplets through nozzles.

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The atmospheric air enters the cooling tower from the openings provided at the bottom of the tower. This air removes heat from water. Cooled water is collected in a pond (known as cooling pond). This cold water is again circulated through the pump, condenser and cooling tower. Thus the cycle is repeated again and again. Some amount of water may be lost during the circulation due to vaporisation etc. Hence, make up water is added to the pond by means of a pump. This water is obtained from a river or lake. A line diagram of cooling water circuit is shown in figure 1.5 separately.

Fuel transportation, handling and storage charges are more. FUEL HANDLING SYSTEM 8.

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Coal delivery equipment is one of the major components of plant cost. The various involved in coal handling are as follows:

Merits (Advantages) of a Thermal Power Plant 1.

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The unit capacity of a thermal power plant is more. The cost of unit decreases with the increase in unit capacity. Life of the plant is more (25-30 years) as compared to diesel plant (2-5 years). Repair and maintenance cost is low when compared with diesel plant. Initial cost of the plant is less than nuclear plants. Suitable for varying load conditions. No harmful radioactive wastes are produced as in the case of nuclear plant. Unskilled operators can operate the plant. The power generation does not depend on water storage. There are no transmission losses since they are located near load centres.

De-merits (Disadvantages) of thermal power plants 1. 2. 3. 4. 5. 6. 7.

Thermal plants are Jess efficient than diesel plants. Starting up the plant and bringing into service takes more time. Cooling water required is more. Space required is more. Storage required for the fuel is more. Ash handling is a big problem. Not economical in areas which are remote from coal fields.

i) Coal delivery •

The coal from supply points is delivered by ships or boats to power stations situated near to sea or river whereas coal is supplied by rail or trucks to the power stations which are situated away from sea or river. • The transportation of coal by trucks is used if railway facilities are not available. ii) Unloading •

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The type of equipment to be used for unloading the coal received at the postation depends on how coal is received at the power station. If coal is delivered trucks, there is no need of unloading device as the trucks may dump the coal to the outdoor storage. Coal is easily handled if the lift trucks with scoop are used. In case the coal is brought by railway wagons, ships or boats, the unloading may be done by car shakes, rotary car

dumpers, cranes, grab buckets and coal accelerators. • Rotary car dumpers although costly are quite efficient for unloading closed wagons. iii) Preparation •

When the coal delivered is in the form of big lumps and it is not of proper size, the preparation (sizing) of coal can be achieved by crushers, breakers, sizers, driers and magnetic separators. iv) Transfer •

After preparation coal is transferred to the dead storage by means of the following ~'SteI11S:

Advantages of belt conveyor 1. 2. 3. 4.

Its operation is smooth and clean. It requires less power as compared to other types of systems. Large quantities of coal can be discharged quickly and continuously. Material can be transported on moderate inclines.

Screw conveyor • •

It consists of an endless helicoids screw fitted to a shaft (shown in figure). The screw while rotating in a trough transfers the coal from feeding end to the discharge end.

Belt conveyor • • • • • • •

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Figure 2.2 shows a belt conveyor. It consists of an endless belt moving over a pair of end drums (rollers). At some distance a ing roller is provided at the centre. The belt is made up of rubber or canvas. Belt conveyor is suitable for the transfer of coal over long distances. It is used in medium and large power plants. The initial cost of the system is not high and power consumption is also low.

The inclination at which coal can be successfully elevated by belt conveyor is about 200 Average speed preferred than other types.

This system is suitable, where coal is to be transferred over shorter distance and space limitations exist. The initial cost of the consumption is high and there is considerable wear of screw. Rotation of screw varies between 75-125 r.p.m. Bucket elevator • It consists of buckets fixed to a chain ( as shown figure). • The chain moves over two wheels. • The coal is carried by the buckets from bottom and discharged at the top.

Grab bucket elevator • It lifts and transfers coal on a single rail or track from one point to the other. • The coal lifted by grab buckets is transferred to overhead bunker or storage. • This system requires less power for operation and requires minimum maintenance. • The grab bucket conveyor can be used with crane or tower as shown in figure below. • Although the initial cost of this system is high but operating cost is less.

Storage of Coal • It is desirable that sufficient quantity of coal should be stored. • Storage of coal gives protection against the interruption of coal supplies when there is delay in transportation of coal or due to strikes in coal mines. • Also when the prices are low, the coal can be purchased and stored for future use. • The amount of coal to be stored depends on the availability of space for storage, transportation facilities, the amount of coal that will whether away and nearness to coal mines of the power station. • Usually coal required for one month operation of power plant is stored in case of power stations are situated at longer distance from the collieries whereas coal need for about 15 days is stored in case of power station situated near to collieries. • Storage of coal for longer periods is not advantageous because it blocks the capital and results in deterioration of the quality of coal. Pulverised Coal Storage in Bunker • Periodically a power plant may encounter the situation where coal must be stored for some time in a bunker, for instance during a plant shut down.

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The bunker, fires can occur in dormant pulverized coal from spontaneous heating within 6 day of loading. This time can be extended to 13 days when a blanket of CO2 is piped into the top of the bunker. The perfect sealing of the bunker from air leakage can extend the storage time as two months or more. The coal in the bunker can be stored as long as six months by expelling air from above the coal with the use of Co2 and then blanketting of all sources of air. A control system used for storing the pulverised fuel in bunker is shown in figure .

Pulverised Fuel Handling System Two methods are in general use to feed the pulverised fuel to the combustion chamber of the power plant. • • •

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First is ‘Unit System’ and second is ‘Central or Bin System’. In unit system, each burner of the plant is fired by one or more unit pulverisers connected to the burners, while in the central system, the fuel is pulverised in the central plant and then distributed to each furnace with the help of high pressure air current. Each type of fuel handling system consists of crushers, magnetic separators, driers, pulverising mills, storage bins, conveyors and feeders.

Ball and Race Mills

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The coal received by the plant from the mine may vary widely in sizes. It is necessary to make the coal of uniform size before ing the pulveriser for efficient grinding. The coal received from the mine is ed through a preliminary crusher to reduce the size to allowable limit (30 mm). The crushed coal is further ed over magnetic separator which removes pyrites and tramp iron.

PULVERISING MILLS: Ball Mill • A line diagram of ball mill using two classifiers is shown in figure. • 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 pulverised 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.

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In this mill the coal es between the rotating elements again and again until it has been pulverised to 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.

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Where oversized coal particles are returned for further grinding, whereas the coal particles of required size are discharged from the top of classifier.

PULVERISED COAL FIRING SYSTEM Pulverised coal firing is done by two systems: • •

Unit system or Direct system. Bin or Central system.

Unit System • In this system (figure below) the raw coal from the coal bunker drops on to the feeder.

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Hot air is ed through coal in the feeder to dry the coal. • The coal is then transferred to the pulverising mill where it is pulverised. • Primary air is supplied to the mill, by the fan. • The mixture of pulverised coal and primary air then flows to burner where secondary air is added. • The unit system is so called from the fact that each burner or a burner group and pulveriser constitute a unit. Advantages • 1. The system is simple and cheaper than the central system. • 2. There is direct control of combustion from the pulverising mill. • 3. Coal transportation system is simple. Central or Bin System • As shown in figure crushed coal from the raw coal bunker is fed by gravity to a dryer where hot air is ed through the coal to dry it. • The dryer may use waste flue gasses, preheated air or bleeder steam as drying agent.

The dry coal is then transferred to the pulverising mill. The pulverised coal obtained is transferred to the pulverised coal bunker (bin). The transporting air is separated from the coal in the cyclone separator. The primary air is mixed with the coal at the feeder and the mixture is supplied to the burner.

Advantages • I. The pulverising mill grinds the coal at a steady rate irrespective of boiler feed. • 2. There is always some coal in reserve. Thus any occasional breakdown in the coal supply will not affect the coal feed to the burner. • 3. For a given boiler capacity pulverising mill of small capacity will be required as compared to unit system. Disadvantages • 1. The initial cost of the system is high. • 2. Coal transportation system is quite complicated. • 3. The system requires more space. COMBUSTION EQUIPMENT FOR BURNING COAL • Since the source of heat is the combustion of a fuel, a working unit must have whatever equipment is necessary to receive the fuel and air, proportioned to each other and to the boiler steam demand, mix, ignite, and perform any other special combustion duties, such as distillation of volatile from coal prior to ignition. • Fluid fuels are handled by burners; solid lump fuels, by stokers. Classification of Combustion system

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The hand firing system is the simplest method of fuel firing but it cannot be used in modern power plants as it gives lower combustion efficiency, it does not respond quickly to fluctuation loads.

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Combustion equipment for steam boiler •

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Classification of mechanical stokers • In small boilers, the grate is stationary and coal is fed manually by shovels. • But for more uniform operating condition, higher burning rate and greater efficiency, moving grates or stokers are employed. Stokers may be of the following types: 1. 2. 3. 4. 5.

Travelling grate stoker Chain grate stoker Spreader stoker Vibrating grate stoker Underfeed stoker.

Travelling grate stokers • The grate surface is made up of a series of cast-iron bars ed together by links to form an endless belt running over two sets of sprocket wheels with a surface as wide as needed (figure 2.12). • A coal gate at the rear of the coal hopper regulates the depth of the fuel bed.

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The gate can be raised or lowered as needed. Simultaneous adjustment of grate speed, fuel bed thickness, and air flow controls the burning rate so that nothing but ash remains on the grate by the time it reaches the furnace rear. The ash falls into the ash pit as the grate turns on the rear sprocket to make the return trip. As the raw or green coal on the grate enters the furnace, the surface coal gets ignited from heat of the furnace flame and from radiant heat rays reflected by the ignition arch. The fuel bed becomes thinner toward the furnace rear as the combustible matter burns off. Under grate air pressures are varied by dampers from front to rear of the stoker to it gradually reduced quantity of primary air fed by the FD fan. The secondary air aids in mixing the gases and supplies oxygen to complete combustion.

Chain grate stokers • A travelling type chain grate is shown in figure 2. 13.

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The chain grate stoker consists of an endless chain which forms a for the fuel bed.

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The chain travels over two sprocket wheels one at the front and other at the rear of furnace as shown in figure. • The front sprocket is connected to a variable speed drive mechanism. • The coal is fed by gravity from a hopper located in front of the stoker. • The depth of the fuel on the grate is regulated by hand adjusted gate as shown in figure. • The speed of the grate varies at the rate at which the coal is fed to the furnace. • 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 as shown in figure. Advantages • 1. It is simple in construction and its initial cost is low. • 2. It is more reliable in service therefore maintenance charges are low. • 3. It is self-cleaning stoker. • 4. The heat release rates can be controlled j ust by controlling the speed of chain. • 5. It gives high heat release rates per unit volume of the furnace. Disadvantages • 1. The amount of coal carried on the grate is small as the increase in grate size creates additional problems. • This cannot be used for high capacity boilers (200 tons/hr. or more). • 2. The temperature of preheated air is limited to 180°C. • 3. The clinker troubles are very common.

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Feeder is a rotating drum fitted with blades. • Feeders can be reciprocating rams, endless belts, spiral worm etc. • From the feeder the coal drops on the spreader distributor which spread the coal over the furnace. • The spreader system should distribute the coal evenly over the entire grate area. • The spreader speed depends on the size of coal. Advantages • The various advantages of spreader stoker are as follows: • I. Its operation cost is low. • 2. A wide variety of coal can be burnt easily by this stoker. • 3. A thin fuel bed on the grate is helpful in meeting the fluctuating loads. Method of Feeding Coal to Combustion Chamber Overfeed supply of coal • In case of overfeed stoker the coal is fed on to the grate above the point of air ission as shown in figure

Spreader Stoker • A spreader stoker is shown in figure . • In this stoker the coal from the hopper is fed on to a feeder which measures the coal in accordance to the requirements.

The mechanics of combustion in overfeed stoker is described below • The pressurised air coming from F.D. fan enters under the bottom of the grate. • The air ing through the grate is heated by absorbing the heat from the ash and grate itself, whereas the ash and grate are cooled.

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The hot air then es through a bed of incandescent coke. As the hot air es through incandescent coke, the 02 reacts with C to form CO2' The rate of carbonoxidation depends entirely on the rate of air supply. The water vapour carried with air also reacts with C in incandescent zone and forms CO, CO2 and H2. Part of CO2 formed reacts with C ing through incandescent zone and converts into CO. The gases leaving the incandescent region of fuel bed consist of N2' CO2' CO, H2 and H2O· The raw coal is continuously supplied on the surface of the bed. Here it loses its volatile matter by distillation. The heat required for the distillation of coal is given by incandescent coke below the fresh fuel. The ignition zone lies directly below the raw fuel undergoing distillation. The gases leaving the upper surface of the fuel bed contain combustible volatile matter formed from the raw fuel, N2, CO2' CO, H2 and H2O. Additional secondary air is supplied at a very high speed to create turbulence which is required for complete combustion of unburned gases. The combustion of the remaining combustible gases is completed in the combustion chamber. The burned gases entering the boiler contain Nz, COz' 0z and HzO and some CO if the burning is incomplete.

place and part of the broken volatile matter reacts with the oxygen of air. ASH HANDLING SYSTEM • Boilers burning pulverized coal have dry bottom furnaces. • The large ash particles are collected under the furnace in a water-filled ash hopper. • Fly ash is collected in dust collectors with either an electrostatic precipitator (ESP) or a baghouse. • Ash must be collected and transported from various points of the plants. • Pyrites, which are the rejects from the pulverizers, are disposed of with the bottom ash system. Three major factors should be considered for ash disposal systems. 1. 2. 3. •

Plant site. Fuel source. Environmental regulation. The sluice conveyor system is the most widely used for bottom ash handling, while the hydraulic vacuum conveyor is the most frequently used for fly system.

Sluice Conveyor System

Hydraulic Vacuum Conveyor

Under-feed stoker • In this type of stokers, the fuel and air move in the same direction.

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The reactions which take place in the incandescent zone of under-feed stoker are exactly the same as in the incandescent zone of over-feed stoker except some breaking of the molecular structure of the volatile matter takes

ASH HANDLING EQUIPMENT

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Mechanical means are required for the disposal of ash. The handling equipment should perform the following functions: 1.

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Capital investment, operating and maintenance charges of the equipment should be low. It should be able to handle large quantities of ash. Clinkers, soot, dust etc. create troubles. The equipment should be able to handle them smoothly. 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. The equipment should be corrosion and wear resistant.

CLASSIFICATION OF ASH HANDLING SYSTEM 1. Hydraulic system. 2. Pneumatic system. 3. Mechanical system. The commonly used ash discharge equipment is as follows: 1. 2.

Rail road cars. Motor truck. Etc.

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 intermittent ash disposal, second water jetting system. • Figure shows hydraulic system.

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In this method water at sufficient pressure is used to take away the ash to sump. Where water and ash are separated. The ash is then transferred to the dump site in wagons, rail cars or trucks. The loading of ash may be through a belt conveyor, grab buckets. If there is an ash basement with ash hopper the ash can fall, directly in ash car or conveying system.

Water Jetting System

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Water jetting of ash is shown in figure. In this method a low pressure jet of water coming out of the quenching nozzle is used to cool the ash. The ash falls into a trough and is then removed.

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 ed through filter to remove dust etc. so that the exhauster handles clean air which will protect the blades of the exhauster.

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Mechanical System

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

The purpose of draught is as follows: • To supply required amount of air to the furnace for the combustion of fuel. • The amount of fuel that can be burnt per square foot of grate area depends upon the quantity of air circulated through fuel bed. • To remove the gaseous products of combustion. CLASSIFICATION OF DRAUGHT

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Figure shows a mechanical ash handling system. In this system ash cooled by water seal falls on the belt conveyer and is carried out continuously to the bunker. The ash is then removed to the dumping site from the ash banker with the help of trucks.

ELECTROSTATIC PRECIPITATOR (ESP) • For power generation steam power plant mainly depend on coal and other fossil fuels to produce electricity. • A natural result from the burning of fossil fuels, particularly coal, is the emission of fly ash. • Ash is mineral matter present in the fuel. • Two emission control devices for flyash are the traditional fabric filters and the more recent electrostatic precipitators. • The fabric filters are large baghouse filters having a high maintenance cost (the cloth bags have a life of 18 to 36 months, but can be temporarily cleaned by shaking or back flushing with air). DRAUGHT • Draught is defined as the difference between absolute gas pressure at any point in a gas flow age and the ambient (same elevation) atmospheric pressure. • Draught is plus if P atm

P gas.

If only chimney is used to produce the draught, it is called natural draught. • Artificial draught If the draught is produced by steam jet or fan it is known as artificial draught. • Steam jet draught It employs steam to produce the draught. • Mechanical draught It employs fan or blowers to produce the draught. • Induced draught The flue is drawn (sucked) through the system by a fan or steam jet. • Forced draught The air is forced into the system by a blower or steam jet. Natural Draught • Natural draught system employs a tall chimney as shown in figure below. • The chimney is a vertical tubular masonry structure or re-inforced concrete. • It is constructed for enclosing a column of exhaust gases to produce the draught. • It discharges the gases high enough to prevent air pollution. • The draught is produced by this tall chimney due to the temperature difference of hot gases in the chimney and cold external air outside the chimney.

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Due to this pressure difference (p), the atmospheric air flows through the furnace grate and the flue gases flow through the chimney. • The pressure difference can be increased by increasing the height of the chimney or reducing the density of hot gases. Merits of Natural Draught • 1. No external power is required for creating the draught. • 2. Air pollution is prevented since the flue gases are discharged at a higher level. • 3. Maintenance cost is practically nil since there are no mechanical parts. • 4. It has longer life. • 5. Capital cost is less than that of an artificial draught. Demerits of natural draught • 1. Maximum pressure available for producing draught by the chimney is less. • 2. Flue gases have to be discharged at higher temperature since draught increases with the increase in temperature of flue gases. • 3. Heat cannot be extracted from the flue gases for economiser, superheater air pre-heater, etc. since the effective draught will be reduced if the temperature the flue gases is decreased. • 4. Overall efficiency of the plant is decreased since the flue gases are discharged higher temperatures. Artificial Draught • It has been seen that the draught produced by chimney is affected by the atmospheric conditions. • It has no flexibility, poor efficiency and tall chimney is required. • In most of the modern power plants, the draught used 'must be independent of atmospheric condition, and it must have

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greater flexibility (control) to take the fluctuating loads on the plant. Today's large steam power plants requiring 20 thousand tons of steam per hour would be impossible to run without the aid of draft fans. A chimney of any reasonable height would be incapable of developing enough draft to move the tremendous volume of air and gases (400 x 103 m3 to 800 x 103 m3 per minute). The further advantage of fans is to reduce the height of the chimney needed. The draught required in actual power plant is sufficiently high (300 mm of water) and to meet high draught requirements, some other system must be used, known as artifical draught. The artificial draught is produced by a fan and it is known a fan (mechanical) draught. Mechanical draught is preferred for central power stations.

Forced Draught • In a forced draught system, a blower is installed near the base of the boiler and air is forced to through the furnace, flues, economiser, air-preheater and to the stack.

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This draught system is known as positive draught system or forced draught system because the pressure and air is forced to flow through the system. A stack or chimney is also used in this system as shown in figure but its function is to discharge gases high in the atmosphere to prevent the contamination. It is not much significant for producing draught therefore height of the chimney may not be very much.

Induced Draught

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In this system, the blower is located near the base of the chimney instead of near the grate. The air is sucked in the system by reducing the pressure through the system below atmosphere. The induced draught fan sucks the burned gases from the furnace and the pressure inside the furnace is reduced below atmosphere and induces the atmospheric air to flow through the furnace. The action of the induced draught is similar to the action of the chimney. The draught produced is independent of the temperature of the hot gases therefore the gases may be discharged as cold as possible after recovering as much heat as possible in air-preheater and economiser. This draught is used generally when economiser and air-preheater are incorporated in the system. The fan should be located at such a place that the temperature of the gas handled by the fan is lowest. The chimney is also used in this system and its function is similar as mentioned in forced draught but total draught produced in induced draught system is the sum of the draughts produced by the fan and chimeny.

Balanced Draught • It is always preferable to use a combination of forced draught and induced draught instead of forced or induced draught alone. • If the forced draught is used alone, then the furnace cannot be opened either for firing or inspection because the high pressure air inside the furnace will try to blowout suddenly and there is every chance of blowing out the fire completely and furnace stops. • If the induced draught is used alone, then also furnace cannot be opened either for firing or inspection because the cold air

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will try to rush into the furnace as the pressure inside the furnace is below atmospheric pressure. This reduces the effective draught and dilutes the combustion.

To overcome both the difficulties mentioned above either using forced draught or induced draught alone, a balanced draught is always preferred. The balanced draught is a combination of forced and induced draught. The forced draught overcomes the resistance of the fuel bed therefore sufficient air is supplied to the fuel bed for proper and complete combustion. The induced draught fan removes the gases from the furnace maintaining the pressure in the furnace just below atmosphere. Also the pressure inside the furnace is near atmospheric therefore there is no danger of blowout or there is no danger of inrushing the air into the furnace when the doors are opened for inspection.

COMPARISON OF FORCED AND INDUCED DRAUGHT

MERITS AND DEMERITS OF MECHANICAL DRAUGHT OVER NATURAL DRAUGHT

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Thus air is forced through the fuel bed, furnace and ed through the chimney. Merits of steam jet draught •

1. This system is very simple and low in cost. • 2. Low grade fuels can be used. • 3. Space required is less. Demerits of steam jet draught • • STEAM JET DRAUGHT • The artificial draught produced by the steam jet is known as steam jet draught. • They may be induced or forced depending upon the location of the steam jet. Induced steam jet draught

1. It can be operated only when the steam is raised. 2. The draught produced is very low.

CONDENSER • A condenser is a device in which the steam is condensed by cooling it with water. • The condensed steam is known as condensate. • The following are the advantages of installing a condenser in a steam power plant. 1. More work is done by the given amount of steam than could be obtained without a condenser. Thus, the efficiency of the power plant is increased. 2. Steam consumption is reduced for the given output. 3. The condensate is recovered for the boiler feed water. • The steam power plants using condenser are shown in figure shows that the cooling water used in condenser is not recirculated again and again but discharged to the downstream side of the river.

Forced steam jet draught • In this system, steam es through a throttle valve from the boiler. • Then the steam es through a nozzle which is projecting into a diff pipe. • The steam comes out of the nozzle with high velocity and draws air along with it. • The kinetic energy of the mixture of steam and air is converted into pressure energy when it es through the diff pipe.

• Whereas figure shows that the cooling water is re-circulated again and again by ing through the cooling tower.

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• The essential elements of a steam condensing plant is given below:

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1. A closed vessel in which the steam is condensed.

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2. A pump to deliver condensed steam to the hot well from the condenser. 3. A dry air-pump to remove air and other noncondensable gases. 4. A feed pump to deliver water to the boiler from hot well. 5. Another pump for circulating cooling water. 6. An arrangement for re-cooling the circulating water from the condenser such as cooling tower or spray pond. CLASSIFICATION OF CONDENSERS

In counter flow jet condensers, the steam and cooling water flow in opposite directions. In low level jet condensers, the condensate is pumped by means of a condensate pump into the hot well. In high level jet condensers, the condensate falls to the hot well by the barometric leg provided in the condenser. In ejector condensers, a number of convergent nozzles are used. In down flow surface condensers, the condensed steam flows down from the condenser. In central flow surface condensers, the condensed steam moves towards the centre of condenser tubes. In single surface condensers, the cooling water flows in the condenser tubes only once. In multi surface condensers, the cooling water flows in the condenser tubes number of times.

Jet Condensers • In a jet condenser, the steam to be condensed and the cooling water come in direct and the temperature of the condensate is the same as that of the cooling water leaving the condenser. • For jet condensers the recovery of the condensate for reuse as boiler feed water is not possible. • Depending upon the arrangement of the removal of condensate, the jet condensers • are sub-divided into the following categories: • 1. Low level counter flow jet condenser. • 2. High level (or) barometric jet condenser. • 3. Ejector condenser. Low level jet condenser •

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In jet condensers, there is direct between the cooling water and the steam which is to be condensed. In surface condensers, there is no direct between the cooling water and the steam which is to be condensed. In parallel flow jet condensers, the flow of steam and cooling water are in the same direction.

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A low level counter flow jet condenser is shown in figure 2.35. In this condenser, the cooling water enters at the top and sprayed through jets. The steam enters at the bottom and mixes with the fine spray of cooling water. The condensate is removed by a separate pump. The air is removed by an air pump separately from the top.

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There is a pressure drop at the throat of the nozzle. The reduction in pressure draws exhaust steam into the nozzle through a nonreturn valve. Steam is mixed with water and condensed. In the converging cones, pressure energy is partly converted into kinetic energy. In diverging cones, the kinetic energy is partly converted into pressure energy. The pressure obtained is higher than atmospheric pressure and this forces the condensate to the hot weII.

In a parallel flow type of this condenser, the cooling water and steam to be condensed move in the same direction. [i.e., from top to bottom].

High level jet condenser • A high level jet condenser is shown in figure 2.36. This is similar to a low level condenser, except that the condenser shell is placed at a height of I0.36 m [barometric height] above the hot well. • The column of water in the tail pipe forces the condensate into the hot well by gravity. • Hence condensate extraction pump is not required.

Merits and Demerits of jet condensers Merits 1. Intimate mixing of steam and cooling water. 2. Quantity of cooling water required is less. 3. Simple equipment and cost is low. 4. Less space is required. 5. Cooling water pump is not needed in low level jet condenser. Condensate extraction pump is not required for high level and ejector condensers. Demerits 1. Condensate is wasted. 2. The cooling water should be clean and free from harmful impurities.

Ejector condenser • An ejector condenser is shown in figure 2.37. In this condenser cooling water under a head of5 to 6 m enters at the top of the condenser. • It is ed through a series of convergent nozzles.

3. In low level jet condensers, the engine may be flooded, if condensate extraction pump fails. Surface Condenser • In surface condensers there is no direct between the steam and cooling

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water and the condensate can be re-used in the boiler. In such a condenser even impure water can be used for cooling purpose whereas the cooling water must be pure in jet condensers. Although the capital cost and the space needed is more in surface condensers but it is justified by the saving in running cost and increase in efficiency of plant achieved by using this condenser. Depending upon the position of condensate extraction pump, flow of condensate and arrangement of tubes the surface condensers may be classified as follows: 1. Down flow condenser 2. Central flow condenser 3. Evaporative condenser

Down flow condenser

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Steam enters at the top and flows downward. The water flowing through the tubes in one direction lower half comes out in the opposite direction in the upper half In this type of condenser, the cooling water and exhaust steam do not come in direct with each other as in case of jet condensers. This is generally used where large quantities of inferior water are available and better quantity of feed water to the boiler must be used most economically. It consists of cast iron air-tight cylindrical shell closed at each end as shown in figure. A number of water tubes are fixed in the tube plates which are located between each cover head and shell.

The exhaust steam from the prime mover enters at the top of the condenser and surrounds the condenser tubes through which cooling water is circulated under force. The steam gets condensed as it comes in with cold surface of the tubes. The cooling water flows in one direction through the first set of the tubes situated in the lower half of condenser and returns in the opposite direction through the second set of the condeser is discharged into the river or pond. The condensed steam is taken out from the condenser by a separate extraction pump and air is removed by an air pump.

Central flow condenser • Figure 2.40 shows a central flow condenser. In this condenser the steam ages are all around the periphery of the shell. • Air is pumped away from the centre of the condenser. • The condensate moves radially towards the centre of tube next. • Some of the exhaust steam while moving towards the centre meets the undercooled condensate and pre-heats it thus reducing undercooling.

Evaporative condenser • In this condenser steam to be condensed in ed through a series of tubes and the cooling water falls over these tubes in the form of spray. A steam of air flows over the tubes to increase evaporation of cooling water which further increases the condensation of steam. • These condensers are more preferable where acute shortage of cooling water exists. • The arrangement of the condenser is shown in figure. • Water is sprayed through the nozzles over the pipe carrying exhaust steam and forms a thin film over it. • The air is drawn over the surface of the coil with the help of induced fan as shown in figure. • The air ing over the coil carries the water from the surface of condenser coil in the form of vapour. • The latent heat required for the evaporation of water vapour is taken from the water film formed on the condenser coil and drops the temperature of the water film and this helps for heat transfer from the steam to the water. • This mode of heat transfer reduces the cooling water requirement of the condenser to 10% of the requirement of surface condensers. • The water particles carried with air due to high velocity of air are removed with the help of eliminator as shown in the figure. • The make-up water (water vapour and water particles carried with air) is supplied from outside source.

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The quantity of water sprayed over the condenser coil should be just sufficient to keep the condenser coil thoroughly wetted.

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The water flow rate higher than this will only increase the power requirement of water pump without materially increasing the condenser capacity. This type of condenser works better in dry weather (low WBT) compared with wet weather as the water vapour carrying capacity of dry air is higher than wet air at the same temperature. The arrangement of this type of condenser is simple and cheap in first cost. It does not require large quantity of water therefore needs a small capacity cooling water pump. The vacuum maintained in this condenser is not as high as in surface condensers therefore the work done per kg of steam is less with this condenser compared with surface condenser. These condensers are generally preferred for small power plants and where there is acute shortage of cooling water.