Principle of Operation of Thermal Station

In the some Thermal Stations, feed water is pumped with the aid of boiler filling pump into the drum; the water from the boiler drum flows downward through the down comers by gravity to the common headers which supply each of the tubes of which the furnace wall is made up of. The water is allowed to rise in the tubes above the burner level and the burners are lit off in an arranged order from the central control room.  The feed water gains heat energy up to the saturation temperature and the wet steam moves to the drum. As the drum is designed to be half filled with water, the upper part is for the wet steam.

A device called cyclone separator (with the aid of centrifugal system) separates the wet from dry steam before the dry steam leaves for the primary superheater.  The steam leaves the primary superheater for the secondary superheater, at this point the pressure is about 12.5Mpa at 541⁰C.  Just before the steam reaches the turbine blades, it losses about 1⁰C and steam at 12.5Mpa, 540⁰C impinges on the HP turbine blade. The heat energy is converted to mechanical energy on the HP turbine and the turbine spins.  The weak steam from the HP turbine exhaust is piped back to a heating system called reheater.  This is designed to raise the temperature back to 540⁰C but at full load (220MW) the pressure will be 3.3Mpa.

The steam from the Reheater is then sent to the intermediate pressure turbine.   (IPTBN) As the three stages of the turbine are linked via one shaft connected together with the generator a speed of 3000rpm is achieved at the generator end.

At the     LP turbine exhaust, the steam has lost almost all the energy and the LP exhaust temperature is as low as 45⁰C at the point of condensation! The makes it easier for condensation to take place in the condenser.  The steam condenses and settles beneath the condenser.  This is called “hot well”.  The pressure is far lower than atmospheric pressure and hence a large pump is required to pump the water because it has to overcome the negative pressure, take it to zero pressure (atmospheric pressure).  This is done with the aid of condensate extraction pump (CEP). The water is taken via steam jet Air Ejector, in order to remove all dissolved 0₂, to the condensate polishing plant.  The Booster pump increases the pressure as the water passes through LP HTR 1----> LPHTR 2-------->LP HTR3 and then to the Dearator.

The Dearator removes any remaining 0₂ in the feed water and serves as the HTR 4. The boiler feed pump (BFP) takes the feed water and increases the pressure to the boiler pressure to the boiler pressure.  The water is taken through HP HTR 5------>HP HTR 6------>he Economiser and then to the boiler Drum and the cycle begins again.

705tons/hr of dry steam (Dryness fraction of about 0.9) is required for 220MW.

Note: The following article describes one of the ways incorporated by thermal Power Plant.The following article is a piece from a reoport of Lagos Thermal Station by P. B. Osofisan.