Thermal Power Station Notes

THERMAL POWER STATION

Merits of Thermal Power plant:-

•   Fuel Used is Cheaper
•   Initial installation Cost is Less
•  Less space is required as compared to hydroelectric power plant
•  Cheaper  in production cost as compared to diesel power plant
•  Able to respond to rapidly changing load without much difficulty
•  Portion of steam out of the plant can be used  for various plants

Demerits Of Thermal Power Plant:-

•  High maintenance and operation cost
•  It pollutes the atmosphere due to production of large amount of ash and dust
•  Large amount of pure water is required, this leads to the installation of water       treatment equipments
•  Disposal of ash produced is quite difficult
•  Its running cost is more than hydro-electric plants

Choice of site for thermal power stations:-

•   The cost of the land should be reasonable
•  Abundant amount of water should be available near by the allocated site
•  Availability of cheap transportation facility
•  Preferably should be near the coal mines
•  Facility for ah disposal and storage coal
•  The choice of site should allow for the economical extensions consistent  with the estimated growth of load

Coal Handling System:-

Elements of  & Processing of Raw Coal in a Coal Handling System:-

(1) Fuel Delivery: Use of Rail, Truck or Boat

     

(2) Unloading: Use of any of the following most convenient equipment

                           (i) Bridge or Tower truck

                          (ii) Grab bucket

                          (iii) Car shakers

(3) Preparation: It involves the use of following equipments

                            (i) Crushers

                           (ii) Breakers

                           (iii) Sizer

                           (iv) Dryers

                           (v) Magnetic separators

Ash Handling System:-

Problems in Handling & Disposal of  Ash

•   It is hot when it comes out of the furnace
•   It is abrasive and wears out the container
•   It produces dust nuisance
•   Poisonous gases are produced
•   It has corrosive action
•   It forms clinkers by fusing together in lumps

Principal Requirements of Ash Handling system

•   The  system should be able to handle large clinkers, soot dust with minimum attention from the operator

•   The system should be able to handle the requisite amount of ash daily
•   The operation and maintenance cost should be kept to minimum
•   The system should be able to deal with hot and wet ashes, and operate with little noise, and keep the nuisance   within desirable limits

Fuel Burning Systems:-

• One of the chief factors in the economical working of a power plant is the efficient combustion of fuel
•  Two most commonly used methods for burning of coal are Stoker firing and Pulverized firing
•  The selection of one of the above method depends of the following factors

    (i) Characteristics of the coal available

    (ii) Capacity of boiler units

    (iii) Station load factor

    (iv) Load fluctuations

    (v) Reliability and efficiency of the various types of combustion

          equipments available

Methods of Burning Coal:-

• Stoker Firing: is a power operated fuel feeding mechanism and grate(metal framework keeping fuel in fireplace)
•   By the use of stokers, a cheaper grade of fuel can be used, a higher efficiency attained, greater flexibility of operation assured, better furnace conditions maintained and less smoke produced than is usual with hand firing

Stoker Firing: Advantages:-

•  Generally less building space is necessary
•  Can be used for small or large boiler units
•  Very reliable and maintenance charges are reasonably low
•  Cheaper grade of fuel is used
•  Reduction in auxiliary plant
•  Less smoke produced

Stoker Firing: Disadvantages:-

•  With very large unites the initial cost may be rather high than with pulverized fuel.
•  Construction is complicated.
•  Sudden variations in the steam demand can't be met to the same degree.
•  Banking and stand by losses are always presents.
•  Trouble due to sagging and clinkering of combustion chamber walls are experienced.

Classification of Stoker Firing:-

(1)  Overfeed stokers: the coal is fed into the grate above the point of air

admission, mainly two types:

       (i) Traveling grate stoker and

       (ii) Spreader stoker.

(2)  Underfed stokers: the coal is fed into the grate below the point of air

admissions, and suitable for non-clinkering, high volatile coals having coking properties and low ash content

Pulverized Fuel Firing:-

•  In this type firing system the coal is reduced to a fine powder with the help of grinding mill and then projected into the combustion chamber with the help of hot air current.
•  Efficiency depends upon the size of the powder.
•  Use in modern thermal power plant where available coal is cheap.

Pulverized Fuel Firing Advantages:-

•  Any grade of coal can be used since coal is powder before use.
•  Rate of feed of the fuel can be regulated properly resulting in fuel economy
•  Higher boiler efficiency
•  Greater capacity to meet peak loads
•  Practically no ash handling troubles
•  No moving parts in the furnace subjected to high temperatures
•  No standby losses due to banked fires
•  Practically free from sagging and clinkering troubles
•  The external heating surfaces are free from corrosions
•  This system works successfully with or in combination with gas or oil

Pulverized Fuel Firing Disadvantages:-

•  High capital cost
•  Lot of fly ash in the exhaust which makes the removing of fine dust uneconomical
•  The possibilities of explosion are more as coal burns like gas
•  Maintenance of furnace brick work is costly
•  Special equipment is needed to start this system 
•  The skilled operaters are required

Super Heaters:-

• Used to raise the temperature of the vaporized portion of the working substance
•  Classification of super heaters on the basis of heat transference
• Radiant super heaters
• Convection super heaters
•  Classification of super heaters on the basis of their position
• Over deck super heaters
• Inter super heaters
• Interbank super heaters
•  Primary super heater super heat the saturated steam just produced in the boiler
•  Intermediate super heaters or the re-heaters worked-off steam returned from the turbine is super heated

Economizer:-

•  Economizer or the feed water heater is used to heat up the feed water by utilizing the heat of the waste flue gases before they are discharged to the chimney

 Advantages of Economizer:

•   Saves expenditure of fuel
•   Increase the overall efficiency of the boiler plant
•   Makes the heat surface of the boiler more effective
•   Increases  steam capacity of the boiler
•   Increases the life of boiler

 Shortcomings of Economizer:

•  Extra expenses in the cost of installation
•  Extra expenses in the cost of maintenance
•  Extra floor place is required

Air Pre-heater:

• The flue gases coming out of the economizer is used to preheat the air before supplying it to the combustion chamber. An increase in air temperature of 20 degrees can be achieved by this method. The pre heated air is used for combustion and also to dry the crushed coal before pulverizing.

Types of Air Heaters:-

• Tubular Air Heater:

The flue gas flows outside the tubes in which the air flows heating it. To increase the time of contact horizontal baffles are provided.

• Plate Type Air Heater:

It consists of rectangular flat plates spaced 1.5 to 2 cm apart leaving alternate air and gas passages. This is not used extensively as it involves high maintenance.

 Draft Systems & Cooling Towers:-

• The importance of the cooling tower is felt when the cooling water from the condenser has to be cooled. The cooling water after condensing the steam becomes hot and it has to be cooled as it belongs to a closed system. The Cooling towers do the job of decreasing the temperature of the cooling water after condensing the steam in the condenser.

Classification of cooling towers:-

     (i) Hyperbolic cooling tower

     (ii) Atmospheric cooling tower

     (iii) Natural draft cooling tower

     (iv) Forced draught cooling

     (v)  Induced draft cooling tower

Draft System: - It supplies required air for the combustion and to bring it into intimate

contact with fuel.

 Feed Water Treatment:-

Adverse Effect of Presence of Impurities in Water:-

    (i) Scale formation                       (ii) Corrosion

    (iii) Foaming & priming                 (iv) Embrittlement

Methods of Feed Water Treatment:-

   (1) Mechanical

         (i) Sedimentation                     (ii) coagulation

         (iii) Filtration                             (iv) Interiror painting

  

 (2) Thermal

          (i) Distillation by evaporation

          (ii) Deaeration

   

(3) Chemical

          (i) Lime soda treatment

          (ii) Ion exchange process

          (iii) Demineralization

Steam Turbines:-

Ø  Steam generated in the boiler is expanded in the turbine which is coupled to the alternator

 Classification of Steam Turbines:-

    (1) Impulse Turbines

          (a) Simple Impulse

          (b) Compound Impulse

                 (i) Pressure Compounded

                 (ii) Velocity Compounded

                 (iii) Pressure-Velocity Compounded

          (c) Combined impulse

  

    (2) Reaction Turbine

         (a) Axial flow

         (b) Radial and Axial flow

        

Boilers:-

 Boiler or steam generator is used to convert water into steam at desired temperature and pressure

Boiler Components: Fuel burning system, Furnace  and boiler surface

 Classification of Boilers:-

    (1)  Water Tube Boilers

          (i) Straight or bent tube

          (ii) Longitudinal or cross-drum

          (iii) Sectional or box heater

          (iv) Cross or parallel baffles

          (v) Horizontal, Vertical or inclined tubes

    (2) High Pressure Boilers

         (i) Lamont

         (ii)  Loeffler

         (iii) Benson

         (iv) Velox