Thermal Power Plants

At present 54.09% or 93918.38 MW (Data Source CEA, as on 31/03/2011) of total electricity production in India is from Coal Based Thermal Power Station. A coal based thermal power plant converts the chemical energy of the coal into electrical energy. This is achieved by raising the steam in the boilers, expanding it through the turbine and coupling the turbines to the generators which converts mechanical energy into electrical energy.

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Coal-based-power-Station

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Introductory overview

In a coal based power plant coal is transported from coal mines to the power plant by railway in wagons or in a merry-go-round system. Coal is unloaded from the wagons to a moving underground conveyor belt. This coal from the mines is of no uniform size. So it is taken to the Crusher house and crushed to a size of 20mm. From the crusher house the coal is either stored in dead storage( generally 40 days coal supply) which serves as coal supply in case of coal supply bottleneck or to the live storage(8 hours coal supply) in the raw coal bunker in the boiler house. Raw coal from the raw coal bunker is supplied to the Coal Mills by a Raw Coal Feeder. The Coal Mills or pulverizer pulverizes the coal to 200 mesh size. The powdered coal from the coal mills is carried to the boiler in coal pipes by high pressure hot air. The pulverized coal air mixture is burnt in the boiler in the combustion zone.
Generally in modern boilers tangential firing system is used i.e. the coal nozzles/ guns form tangent to a circle. The temperature in fire ball is of the order of 1300 deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to steam in the boiler and steam is separated from water in the boiler Drum. The saturated steam from the boiler drum is taken to the Low Temperature Superheater, Platen Superheater and Final Superheater respectively for superheating. The superheated steam from the final superheater is taken to the High Pressure Steam Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating this steam is taken to the Intermediate Pressure Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high pressure steam is converted to electrical energy in the Generator.

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Diagram of a typical coal-fired thermal power station

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Coal-Based-Thermal-Power-Plant1

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Principal

Coal based thermal power plant works on the principal of Modified Rankine Cycle.

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Modified-Rankine-Cycle

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Components of Coal Fired Thermal Power Station:

  • Coal Preparation

                        i)Fuel preparation system: In coal-fired power stations, 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, so that coal will undergo complete combustion during combustion process.

** pulverizer is a mechanical device for the grinding of many different types of materials. For example, they are used to pulverize coal for combustion in the steam-generating furnaces of fossil fuel power plants.
Types of Pulverisers: Ball and Tube mills; Ring and Ball mills; MPS; Ball mill; Demolition.
                       ii)Dryers:  they are used in order to remove the excess moisture from coal mainly wetted during transport. As the                                                      presence of moisture will result in fall in efficiency due to incomplete combustion and also result in CO emission.
                        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.
The coal we finally get after these above process are transferred to the storage site.
Purpose of fuel storage is 2 –
  •  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.

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There are 2 types of storage:

  1. 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 of the plant and is usually a covered storage in the plant near the boiler furnace. The live storage can be provided with bunkers & coal bins. Bunkers are enough capacity to store the requisite of coal. From bunkers coal is transferred to the boiler grates.
  2. 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.There are many forms of storage some of which are –
    1. Stacking the coal in heaps over available open ground areas.
    2. As in (I). But placed under cover or alternatively in bunkers.
    3. Allocating special areas & surrounding these with high reinforced concerted retaking walls.
  • 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:-

  1. To convert chemical energy of the fuel into heat energy
  2. To transfer this heat energy to water for evaporation as well to steam for superheating.

The basic components of Boiler are: -

  1. Furnace and Burners
  2. Steam and Superheating

a. Low temperature superheater

b. Platen superheater

c. Final superheater

  • Economiser

It is located below the LPSH in the boiler and above pre heater. It is there to improve the efficiency of boiler by extracting heat from flue gases to heat water and send it to boiler drum.

Advantages of Economiser include

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.

  • 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

  •  Reheater

Power plant furnaces may have a reheater section containing tubes heated by hot flue gases outside the tubes. Exhaust steam from the high pressure turbine is rerouted to go inside the reheater tubes to pickup more energy to go drive intermediate or lower pressure turbines.

  • Steam turbines

Steam turbines have been used predominantly as prime mover in all thermal power stations. The steam turbines are mainly divided into 2 groups: -

  1. Impulse turbine
  2. Impulse-reaction turbine

The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, 2 low pressure turbines, and the generator. The steam at high temperature (536 ‘c to 540 ‘c) and pressure (140 to 170 kg/cm2) is expanded in the turbine.

  • 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.

We normally use surface condenser although there is one direct contact condenser as well. In direct contact type exhaust steam is mixed with directly with D.M cooling water.

  • 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.

  • 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.

The cooling towers are of 4 types: -

1. Natural Draft cooling tower

2. Forced Draft cooling tower

3. Induced Draft cooling tower

4. Balanced Draft cooling tower

  • Fan or draught system

In a boiler it is essential to supply a controlled amount of air to the furnace for effective combustion of fuel and to evacuate hot gases formed in the furnace through the various heat transfer area of the boiler. This can be done by using a chimney or mechanical device such as fans which acts as pump.

i) Natural draught 

When the required flow of air and flue gas through a boiler can be obtained by the stack (chimney) alone, the system is called natural draught. When the gas within the stack is hot, its specific weight will be less than the cool air outside; therefore the unit pressure at the base of stack resulting from weight of the column of hot gas within the stack will be less than the column of extreme cool air. The difference in the pressure will cause a flow of gas through opening in base of stack. Also the chimney is form of nozzle, so the pressure at top is very small and gases flow from high pressure to low pressure at the top.

 

ii) Mechanized draught

There are 3 types of mechanized draught systems

1)                  Forced draught system

2)                  Induced draught system

3)                  Balanced draught system

Forced draught: – In this system a fan called Forced draught fan is installed at the inlet of the boiler. This fan forces the atmospheric air through the boiler furnace and pushes out the hot gases from the furnace through superheater, reheater, economiser and air heater to stacks.

Induced draught: – Here a fan called ID fan is provided at the outlet of boiler, that is, just before the chimney. This fan sucks hot gases from the furnace through the superheaters, economiser, reheater and discharges gas into the chimney. This results in the furnace pressure lower than atmosphere and affects the flow of air from outside to the furnace.

Balanced draught:-In this system both FD fan and ID fan are provided. The FD fan is utilized to draw control quantity of air from atmosphere and force the same into furnace. The ID fan sucks the product of combustion from furnace and discharges into chimney. The point where draught is zero is called balancing point.

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  • 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.

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.

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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.

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  • 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.

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 in comparison with that of diesel power stations

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

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