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Steam power plant


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Power Plant Engineering Topics

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Steam power plant

  1. 1. Unit – 2 Steam Power Plant
  2. 2. Systems and Components  High Pressure Boiler  Prime Mover  Condensers and cooling towers  Coal handling systems  Ash and dust handling system  Draught system  Feed water purification plant  Pumping system  Air preheater, economizer, super heater and feed heaters
  3. 3. Power Station Design ❖ Selection of site ❖ Estimation of capacity of power station ❖ Selection of turbines and their auxiliaries ❖ Selection of boilers and their auxiliaries ❖ Design of fuel handling system ❖ Selection of condensers ❖ Design of cooling system ❖ Design of piping system to carry steam and water ❖ Design and control of Instruments
  4. 4. Characteristcs ❖ Higher efficiency ❖ Lower cost ❖ Ability to burn coal especially of high ash content and inferior coals ❖ Reduced environmental impact in terms of air pollution ❖ Reduced water requirement ❖ Higher reliability and availability
  5. 5. Coal Handling 1. Coal Delivery: Coal from supply points is delivered to the power stations by ships, boats, rails or trucks 2. Unloading: cranes, grab buckets and coal accelerators etc 3. Preparation: If coal is in the form of big lumps or in improper size, then sizing (preparation) of coal is done using crushers, breakers and magnetic separators. 4. Transfer: After preparation coal is transferred to the dead storage by means of following systems: ❖ Belt Conveyer ❖ Screw Conveyers ❖ Bucket Elevators ❖ Grab Bucket Elevators etc
  6. 6. Coal Handling ❖ Belt Conveyer: It consists of an endless belt moving over a pair of rollers. The belt is made up of canvas or rubber. Belt conveyor is suitable for the transfer of coal over long distances. Initial cost of the system is not high and power consumption is too low. The inclination at which coal can be successfully elevated by belt conveyer is about 20. Average speed varies between 200-300 rpm. ❖ Advantages: ❖ 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 moderates inclines.
  7. 7. Coal Handling ❖ Screw Conveyer: It consists of an endless helicoid screw fitted to a shaft. The screw while rotating in a trough transfers the coal from feeding end to the discharge end. This system is suitable where coal is to be transferred over shorter distance (or space limitations) ❖ Although initial cost is too low but there is considerable amount of wear in screw.
  8. 8. Coal Handling ❖ Bucket Elevator: It consists of buckets fixed to a chain. The chain moves over two wheels. The coal is carried by the buckets from bottom and discharged at top. ❖ Grab Bucket Elevator: It lifts and transfers coal from one point to the other. The coal lifted by grab buckets is transferred to overhead storage.
  9. 9. Coal Handling 5. Storage of Coal: It gives protection against the interruption of coal supplies when there is delay in transportation 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, transportation facilities, nearness to coal mines etc. However, storage of coal for longer periods is not advantageous because it blocks the capital and results in deterioration of quality of coal. ❖ Tendency to whether (to combine with oxygen of air) : coal loses some of its heating value and ignition quality.
  10. 10. Coal Handling 6. In Plant Handling: It includes transfer of coal from dead storage to the furnace using equipments like belt conveyers, screw conveyers etc. Even lorries are used to record the quantity of coal delivered to the furnace. 7. Supply to the furnace.
  11. 11. De-watering of coal  Excessive moisture of coal reduces heating value of coal and creates handling problems. The coal should therefore be dewatered to produce clean coal. Cleaning of coal has the following advantages: (i) Improved heating value. (ii) Easier crushing and pulverising (iii) Improved boiler performance (iv) Less ash to handle. (v) Easier handling.
  12. 12. Method of fuel firing : Hand firing  This is a simple method of firing coal into the furnace. It requires no capital investment. It is used for smaller plants.  Adjustments are to be made every time for the supply of air when fresh coal is fed into furnace.  Hand fired grates: used to support the fuel bed and admit air for combustion.  While burning coal the total area of air openings varies from 30 to 50% of the total grate area.  The grate area required for an installation depends upon various factors such as its heating surface, the rating at which it is to be operated and the type of fuel burnt by it.  The width of air openings varies from 3 to 12 mm.  The construction of the grate should be such that it is kept uniformly cool by incoming air. It should allow ash to pass freely.  Hand fired grates are made up of cast iron.
  13. 13. Method of fuel firing : Hand firing  The main characteristic of a grate fired furnaces are the heat liberation per unit of grate area and per unit of volume of furnace.  These two characteristics depend on the following factors : (i) Grade of fuel (ii) Design of furnace (iii) Method of combustion.
  14. 14. Method of fuel firing : Hand firing  The grate divides the combustion chamber into the furnace space in which the fuel is fired and an ash pit through which the necessary air required for combustion is supplied.  The grate is arranged horizontally and supports a stationary bed of burning fuel. The fuel is charged by hand through the fire door.  Disadvantages: (i) The efficiency of a hand fired furnace is low. (ii) Attending to furnace requires hard manual labour. (iii) Study process of fuel feed is not maintained.
  15. 15. Mechanical Firing (Stokers)  Mechanical stokers are commonly used to feed solid fuels into the furnace in medium and large size power plants. The various advantages of stoker firing are as follows: (i) Large quantities of fuel can be fed into the furnace. Thus greater combustion capacity is achieved. (ii) Stoker save labour of handling ash and are self-cleaning. (iii) By using stokers better furnace conditions can be maintained by feeding coal at a uniform rate. (iv) Stokers save coal and increase the efficiency of coal firing. The main disadvantages of stokers are their more costs of operation and repairing resulting from high furnace temperatures.
  16. 16. Chain grate stoker  The chain travels over two sprocket wheels, one at the front and one at the rear of furnace. The traveling chain receives coal at its front end through a hopper and carries it into the furnace.  The speed of grate (chain) can be adjusted to suit the firing condition.  The air required for combustion enters through the air inlets situated below the grate.  The stokers are suitable for low ratings because the fuel must be burnt before it reaches the rear of the furnace.
  17. 17. Spreader Stoker  In this stoker, the coal from the hopper is fed on to a feeder which measures the coal in accordance to the requirements.  Feeder is a rotating drum fitted with blades.  From the feeder the coal drops on to spreader distributor which spread the coal over the furnace. The spreader system should distribute the coal evenly over the entire grate area.
  18. 18. Multi pushers stoker  The coal falling from the hopper is pushed forward during the inward stroke of stoker ram. The distributing rams (pushers) then slowly move the entire coal bed down the length of stoker.  The slope of stroke helps in moving the fuel bed downwards.  The primary air enters the fuel bed from main wind box situated below the stoker. Partly burnt coal moves on to the extension grate.  The air entering from the main wind box into the extension grate wind box is regulated by an air damper.  As sufficient amount of coal always remains on the grate, this stoker can be used under large boilers (upto 500,000 lb/hr capacity) to obtain high rates of combustion
  19. 19. Automatic Boiler Control
  20. 20. Pulverized Coal  Coal is pulverized (powdered) to increase its surface exposure thus permitting rapid combustion. Efficient use of coal depends greatly on the combustion process employed.  The pulverized coal is obtained by grinding the raw coal in pulverizing mills. The various pulverizing mills used are as follows: (i) Ball mill (ii) Hammer mill (iii) Ball and race mill (iv) Bowl mill.  The essential functions of pulverizing mills are as follows: (i) Drying of the coal (ii) Grinding (iii) Separation of particles of the desired size.  Proper drying of raw coal which may contain moisture is necessary for effective grinding.  The coal pulverizing mills reduce coal to powder form by three actions as follows: (i) Impact (ii) Attrition (abrasion) (iii) Crushing.
  21. 21. Tube & Ball Mill
  22. 22. Pulverized Coal Firing
  23. 23. Pulverized Coal Burners  Different burners are used to burn different quality of coal.  Them main difference between various burners lies in the rapidity of air-coal mixing i.e., turbulence.  A pulverised coal burner should satisfy following requirements:  It should mix the coal and primary air thoroughly and it should create sufficient turbulence.  It should deliver air to the furnace in right proportions and should maintain stable ignition of mixture.  It should also control flame shape and travel in the furnace.  The flame shape is controlled by secondary air vanes. If it is supplied in too much quantity; may cool the mixture and prevent its heating to ignition temperature.
  24. 24. Pulverized Coal Burners  Long flame burner (U-flame Burner): In this burner air and coal mixture travels a considerable distance thus providing sufficient time for complete combustion.  Short flame burner (Turbulent Burner): In this burner, flame enters the furnace horizontally and secondary air diverts the flame path.
  25. 25. Pulverized Coal Burners  Tangential Burner: In this system one burner is fitted to corner of the furnace. The inclination of the burner is so made that the flame produced are tangential to an imaginary circle at the centre.  Cyclone Burner: this burner uses an external source to create a cyclone inside the chamber for adequate flow of flame.
  26. 26. Ash Disposal  A large quantity of ash is produced in steam power plant using coal. Ash produced is about 10 to 20% of the total coal burnt in the furnace.  Handling of ash is a problem because ash coming out of the furnace is too hot, it is dusty and irritating to handle and it contains some poisonous gases.  It is desirable to quench the ash before handling due to following reasons:  Quenching reduces the temperature of ash  It reduces the corrosive action of ash  It reduces the dust accompanying the ash  It includes : removal from the furnace, loading on the conveyors and delivered to the place where it can be disposed off
  27. 27. Ash Handling System
  28. 28. Requirements  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
  29. 29. Ash Handing Systems  Hydraulic Systems: ash from the furnace grate falls into a system of water possessing high velocity and is carried to the sumps.  Hydraulic system is of two types namely low pressure hydraulic system used for continuous removal of ash and high pressure system which is used for intermittent ash disposal.  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.
  30. 30. Ash Handing Systems  Water Jetting: 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.
  31. 31. Ash Handing Systems  Pneumatic System: In this system, ash from the boiler furnace outlet falls into a crusher where larger ash particles are crushed to small sizes.  The ash is then carried by a high velocity air or steam to the point of delivery.  Air leaving the ash separator is passed through filter to remove dust, so that the exhauster handles clean air which will protect the blades of the exhauster.
  32. 32. Ash Handing Systems  Mechanical System: In this system ash cooled by water seal falls on the belt conveyor and is carried out continuously to the bunker.  The ash is then removed to the dumping site from the ash bunker with the help of trucks.
  33. 33. Smoke and Dust Removal  The products of combustion contain particles of solid matter floating in suspension. This may be smoke or dust.  The production of smoke indicates that combustion conditions are faulty and amount of smoke produced can be reduced by improving the furnace design.  Smoke is produced due to the incomplete combustion of fuels, smoke particles are less than 10µ in size.  To avoid the atmospheric pollution the fly ash must be removed from the gaseous products of combustion before they leaves the chimney.  The removal of dust and cinders from the flue gas is usually effected by commercial dust collectors which are installed between the boiler outlet and chimney usually in the chimney side of air pre heater.
  34. 34. Dust Collectors  Mechanical Dust Collectors: by increasing the cross-sectional area of duct through which dust laden gases are passing, the velocity of gases is reduced and causes heavier dust particles to fall down.  Changing the direction of flow of flue gases causes the heavier particles of settle out.  Sometime baffles are provided, to separate the heavier particles.  Mechanical dust collectors may be wet type or dry type. Wet type dust collectors called scrubbers make use of water sprays to wash the dust from flue gases.
  35. 35. Dust Collectors  Electrostatic Precipitators: It has two sets of electrodes, insulated from each other that maintain an electrostatic field between them at high voltage. The flue gases are made to pass between these two sets of electrodes.  The electric field ionizes the dust particle; that pass through it attracting them to the electrode of opposite charge.  The dust particles are removed from the collecting electrode by rapping the electrode periodically.  The electrostatic precipitator is costly but has low maintenance cost and is frequently employed with pulverized coal fired power stations for its effectiveness on very fine ash particles and is superior to that of any other type.
  36. 36. Dust Collectors  The principal characteristics of an ash collector is the degree of collection
  37. 37. Fluidised Bed Combustion  Burning of pulverized coal has some problems such as particle size of coal used in pulverised firing is limited to 70-100 microns.  The generation of high temp. about (1650 C) in the furnace creates number of problems like slag formation on super heater, evaporation of alkali metals in ash and its deposition on heat transfer surfaces, formation of SO2 and NOX in large amount.  Fluidized Bed combustion system can burn any fuel including low grade coals (even containing 70% ash), oil, gas or municipal waste.  Improved de-sulphurization and low NOX emission are its main characteristics.
  38. 38. Fluidised Bed Combustion  The fuel and inert material dolomite are fed on a distribution plate and air is supplied from the bottom of distribution plate.  The air is supplied at high velocity so that solid feed material remains in suspension condition during burning.  During burning SO2 formed is absorbed by the dolomite and thus prevents its escape with the exhaust gases.  Addition of limestone or dolomite to the bed brings down SO2 emission level to about 15% of that in conventional firing methods  The amount of NOX is produced is also reduced because of low temperature of bed and low excess air as compared to pulverised fuel firing.