Thermal Power Station

Thermal Power Station

1. Basic Principle We used coals as fuel for the generation of heat energy. As the water in the Boiler evaporated due to the intense heat, it becomes high-pressurized steams.  And the steams are passing through a conduit (there is a turbine at the other end of the tunnel), it forces its way through the Turbine, thus rotating the Turbine. (As the steams are high-pressurized, the Turbine will rotate very fast.)  The Turbine is connected to a Generator via a coupler. As the Turbine is rotating (from the force of the steams), electrical energy is being produced.  After the steams have passed through the turbine, it enters a Condenser. The Condenser has got a cooling agent (namely seawater) and the steam will go through the cooling agent via a pipe. The steam thus changes back to its liquid form and returns to the Boiler.  And the whole process repeats.

2. There are four main circuits in any thermal power plant and these are Coal & Ash Circuit  – this circuit deals mainly with feeding the boiler with coal for combustion purposes and taking care of the ash that is generated during the combustion process and includes equipment and paraphernalia that is used to handle the transfer and storage of coal and ash. Air & Gas Circuit  – we know that air is one of the main components of the fire triangle and hence necessary for combustion. Since lots of coal is burnt inside the boiler it needs a sufficient quantity of air which is supplied using either forced draught or induced draught fans. The exhaust gases from the combustion are in turn used to heat the ingoing air through a heat exchanger before being let off in the atmosphere. The equipment which handles all these processes fall under this circuit.

3. Feed Water & Steam Circuit  – this section deals with supplying of steam generated from the boiler to the turbines and to handle the outgoing steam from the turbine by cooling it to form water in the condenser so that it can be reused in the boiler plus making good any losses due to evaporation etc. Cooling Water Circuit  – this part of the thermal power plant deals with handling of the cooling water required in the system. Since the amount of water required to cool the outgoing steam from the boiler is substantial, it is either taken from a nearby water source such as a river, or it is done through evaporation if the quantity of cooling water available is limited.  

4. Components of Coal Fired Thermal Power Station   i )Fuel preparation system: the raw feed coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder   ii)Dryers:   they are used in order to remove the excess moisture from coal mainly wetted during transport

5. iii)Magnetic separators : coal which is brought may contain iron particles. These iron particles may result in wear and tear. The iron particles may include bolts, nuts wire fish plates etc. so these are unwanted and so are removed with the help of magnetic separators

6. Purpose of fuel storage is two  Fuel storage is insurance from failure of normal operating supplies to arrive. Storage permits some choice of the date of purchase, allowing the purchaser to take advantage of seasonal market conditions. Storage of coal is primarily a matter of protection against the coal strikes, failure of the transportation system & general coal shortages.

7. There are two types of storage: Live Storage(boiler room storage):  storage from which coal may be withdrawn to supply combustion equipment with little or no remanding is live storage. This storage consists of about 24 to 30 hrs. of coal requirements Dead storage - stored for future use. Mainly it is for longer period of time, and it is also mandatory to keep a backup of fuel for specified amount of days depending on the reputation of the company and its connectivity

8. Boiler and auxiliaries A Boiler or steam generator essentially is a container into which water can be fed and steam can be taken out at desired pressure, temperature and flow. This calls for application of heat on the container. For that the boiler should have a facility to burn a fuel and release the heat. The functions of a boiler thus can be stated as:- To convert chemical energy of the fuel into heat energy To transfer this heat energy to water for evaporation as well to steam for superheating.

9. Economiser : Economiser :  Flue gases coming out of the boiler carry lot of heat.Function of economiser   is to recover some of the heat from the heat carried away in the flue gases up the chimney and utilize for heating the feed water to the boiler .It is placed in the passage of flue gases in between the exit from the boiler and the entry to the chimney.The use of economiser   results in saving in coal consumption , increase in steaming rate and high boiler efficiency  

10. 1) Fuel economy: – used to save fuel and increase overall efficiency of boiler plant. 2) Reducing size of boiler: – as the feed water is preheated in the economiser and enter boiler tube at elevated temperature. The heat transfer area required for evaporation reduced considerably.

11. Air Preheater The heat carried out with the flue gases coming out of economiser are further utilized for preheating the air before supplying to the combustion chamber. It is a necessary equipment for supply of hot air for drying the coal in pulverized fuel systems to facilitate grinding and satisfactory combustion of fuel in the furnace

12. Superheater : Superheater :  Most of the modern boliers are having superheater and reheater arrangement.  Superheater is a component of a steam-generating unit in which steam, after it has left the boiler drum, is heated above its saturation temperature .

13. Steam turbines Steam turbines have been used predominantly as prime mover in all thermal power stations. The steam turbines are mainly divided into two groups: - Impulse turbine Impulse-reaction turbine  The steam at high temperature (536 ‘c to 540 ‘c) and pressure (140 to 170 kg/cm2 )

14. Condenser The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycle increases. The functions of a condenser are:- 1) To provide lowest economic heat rejection temperature for steam. 2) To convert exhaust steam to water for reserve thus saving on feed water requirement. 3)  To introduce make up water .

15. Condensers are classified  as ( i ) Jet condensers or contact condensers (ii) Surface condensers In   jet condensers  the steam to be condensed mixes with the cooling water and  the temperature of the condensate and the cooling water is same when leaving the condenser ; In  surface condensers   there is no direct contact between the steam to be condensed and the circulating cooling water. There is a wall interposed between them through heat must be convectively transferred

16. Boiler feed pump Boiler feed pump is a multi stage pump provided for pumping feed water to economiser . BFP is the biggest auxiliary equipment after Boiler and Turbine. It consumes about 4 to 5 % of total electricity generation.

17. Cooling tower The cooling tower is a semi-enclosed device for evaporative cooling of water by contact with air. The hot water coming out from the condenser is fed to the tower on the top and allowed to tickle in form of thin sheets or drops. The air flows from bottom of the tower or perpendicular to the direction of water flow and then exhausts to the atmosphere after effective cooling.

18. Ash handling system The disposal of ash from a large capacity power station is of same importance as ash is produced in large quantities. Ash handling is a major problem. i )   Manual handling : While barrows are used for this. The ash is collected directly through the ash outlet door from the boiler into the container from manually.

19. ii)   Mechanical handling : Mechanical equipment is used for ash disposal, mainly bucket elevator, belt conveyer. Ash generated is 20% in the form of bottom ash and next 80% through flue gases, so called Fly ash and collected in ESP. iii)   Electrostatic precipitator : From air preheater this flue gases (mixed with ash) goes to ESP. The precipitator has plate banks (A-F) which are insulated from each other between which the flue gases are made to pass. The dust particles are ionized and attracted by charged electrodes. The electrodes are maintained at 60KV.Hammering is done to the plates so that fly ash comes down and collect at the bottom. The fly ash is dry form is used in cement manufacture

20. Generator Generator or Alternator is the electrical end of a turbo-generator set. It is generally known as the piece of equipment that converts the mechanical energy of turbine into electricity. The generation of electricity is based on the principle of electromagnetic induction.

21. Advantages of coal based thermal Power Plant They can respond to rapidly changing loads without difficulty A portion of the steam generated can be used as a process steam in different industries Steam engines and turbines can work under 25 % of overload continuously Fuel used is cheaper Cheaper in production cost

22. Disadvantages of coal based thermal Power Plant Maintenance and operating costs are high Long time required for erection and putting into  action A large quantity of water is required Great difficulty experienced in coal handling Presence of troubles due to smoke and heat in the plant Unavailability of good quality coal Maximum of  heat  energy lost Problem of ash removing