Steam Power Plants


 

 

It is the power plant which is used to generate electricity by the use steam turbine. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser. The greatest variation in the design of steam-electric power plants is due to the different fuel sources.

Almost all coal, nuclear, geothermal, solar thermal electric power plants, waste incineration plants as well as many natural gas power plants are steam-electric. Natural gas is frequently combusted in gas turbines as well as boilers. The waste heat from a gas turbine can be used to raise steam, in a combined cycle plant that improves overall efficiency.

 

Working Mechanism

Steam Power Plant Working

  • The pulverised coal is fed into the boiler where the pulverised coal is burnt into the furnace
  • Due to heat from the furnace, the water present in the boiler drum changes to the high pressure steam.
  • From the boiler this high pressure steam is passed to the super heater where it is again heated up to its dryness.
  • After that this super heated steam strikes the turbine blades with a high speed and the turbine blades starts rotating to at high speed. Here the stored potential energy of the steam is gets converted into mechanical energy.
  • A generator is coupled with the turbine rotor. As the turbine rotates, the generator also rotate with same speed and mechanical energy of the turbine gets converted into electrical energy.
  • Steam after hitting the turbines blades lost its most of the energy and leaves the turbine with low pressure steam.
  • This low pressure steam enters into the condenser. Cold water circulates in the condenser from the cooling tower. Here the low pressure wet steam is converted into water.
  • After that condensed water with the feed water passed to the economiser where it gets heated up by the economiser. And finally the feed water enters into the boiler by a feed water pump to repeat the cycle.
  • The burnt flue gases from the furnace passes through the super heater, economiser and air pre-heater. This heat of the flue gases is used to heat the steam in the super heater to its dryness, to heat feed water in the economiser before entering into the boiler and to heat air form the atmosphere in the air pre-heater before it enters into the furnace.
  • The ash from the furnace is transported to ash handling plant and finally to the ash storage.

Advantages 

  • Low initial cost.
  • Less land area is required as compared with the hydro power plant.
  • Economical power generation cost. Coal is used as fuel and the cost of coal is cheaper than petrol and diesel fuel. 
  • Easy maintenance cost.
  • Can be installed in any area where water sources and transportation facility are easily available.

Components

The major components of this power plant are boiler, steam turbine, condenser and feed water heater and superheater.

Boiler The boiler transfers energy to the water by the chemical reaction of burning some type of fuel. The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum. Once the water enters the steam drum it goes down the downcomers to the lower inlet waterwall headers. From the inlet headers the water rises through the waterwalls and is eventually turned into steam due to the heat being generated by the burners located on the front and rear waterwalls (typically).

As the water is turned into steam/vapor in the waterwalls, the steam/vapor once again enters the steam drum. The steam/vapor is passed through a series of steam and water separators and then dryers inside the steam drum. The steam separators and dryers remove the water droplets from the steam and the cycle through the waterwalls is repeated. This process is known as natural circulation.

Steam turbine

Steam Turbines

A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft.

Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible expansion process.

Condenser

Steam power plants utilize a surface condenser cooled by water circulating through tubes. The steam which was used to turn the turbine is exhausted into the condenser. The steam is therefore condensed as it comes in contact with the cool tubes full of circulating water. This condensed steam is withdrawn from the bottom of the surface condenser. The condensed steam is now water, commonly referred to as condensate water. It is attached to the cooling tower.
Feed water heater In the case of a conventional steam-electric power plant utilizing a drum boiler, the surface condenser removes the latent heat of vaporization from the steam as it changes states from vapor to liquid. The heat content (btu) in the steam is referred to as Enthalpy. The condensate pump then pumps the condensate water through a feedwater heater. The feedwater heating equipment then raises the temperature of the water by utilizing extraction steam from various stages of the turbine.

Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system.This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.

Superheater As the steam is conditioned by the drying equipment inside the drum, it is piped from the upper drum area into an elaborate set up of tubing in different areas of the boiler. The areas known as superheater and reheater. The steam vapor picks up energy and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves of the high pressure turbine.