Steam Power Plant and Rankine Cycles

The Rankine cycle or Rankine Vapor Cycle is the process widely used by power plants such as coal-fired power plants or nuclear reactors. In this mechanism, a fuel is used to produce heat within a boiler, converting water into steam which then expands through a turbine producing useful work.
The steam-electric power station is a power station in which the electric generator is steam driven.

1. STEAM POWER PLANT

2. 1 2 3 4

3. STEAM POWER PLANT 1 2 3 4

4. RANKINE CYCLE

5. HOW TO INCREASE EFFICIENCY OF RANKINE CYCLE

6. Lowering the condenser pressure (Lower T low,avg ) Steam in saturated mixture during condensation Increase in net work output Additional heat input requirements (2 to 2’), but small compared to W net Condenser operating pressure is limited by the temperature of the cooling medium Side effect: Lowering the condenser pressure increases the moisture content of the steam at the final stages of the turbine (4´). Could cause air leakage to condenser and moisture content of the steam in the turbine due to low pressure

7. Superheating the steam to high temperatures (Increases T high,avg ) Steam is superheated at constant pressure (3 to 3’) which increase the net work output . Decreases the moisture content of the steam at the turbine exit. Steam superheated temperature is limited by metallurgical considerations and material limitation (may create damage to turbine blades) Possible solution: Expand the steam in the turbine in two stages, and reheat it in between. (Decrease in Steam remains same as before)

8. Superheating the steam to high temperatures with Reheat

9. Increasing the boiler pressure (Increases T high, avg ) P boiler increase which will automatically increase the boiling temperature Increases the net work output, but at the same time increases the moisture content in the turbine High moisture content (at 4’ than 4) causes: erosion in turbine blades and Viscous effect i.e. resistance to flow

10. Ideal Regenerative Rankine Cycle A feed water heater is basically a heat exchanger where heat is transferred from the steam to the feed water . An open (or direct-contact ) feed water heater is basically a mixing chamber, where the steam extracted from the turbine mixes with the feed water leaving the pump. Ideally, the mixture leaves the heater as a saturated liquid at the heater pressure.

11. Ideal Regenerative Rankine Cycle This improves the average heat-addition and thus the cycle efficiency. In steam power plants, steam is extracted from the turbine at various points to heat the feed water to boiler and thus improves cycle efficiency . The device where the feed water is heated by regeneration is called a regenerator, or a feed water heater (FWH).

12. Ideal Regenerative Rankine Cycle with Open FWH 1-2 pump-I. 2-3 Mixing of Feed water 6-3 Steam from turbine. 3-4 pump II. 4-5 heat addition in boiler. 5-6 Turbine Work out (‘y’ portion of steam to FWH). 5-7 Turbine Work out (1-y portion of steam). 7-1 heat rejection in condenser.

13. Ideal Regenerative Rankine Cycle with Open FWH y x h 6 + (1-y) x h 7 = 1 x h 3 y x h 6 (1-y) x h 7 1 x h 3

14. Closed Feed-Water Heaters Another type of feed water heater frequently used in steam power plants is the closed feed water heater , in which heat is transferred from the extracted steam to the feed water without any mixing taking place . The two streams now can be at different pressures, since they do not mix.

15. Closed Feed-Water Heaters m 1-m

16. Cogeneration  In production of electricity some energy is rejected as waste heat, but in cogeneration this thermal energy is put to good use . Concept is to utilize waste heat from the power plant Heat is extracted from at least two heat sources the first one being the flow rate of waste heat obtained from the exhaust gases from the power plant. Heat from the condensing steam taken from the low pressure turbine extraction points The resulting low-temperature waste heat extracted is then used for water or space heating .

17. Cogeneration