Thermal plant advantages, superheating and reheating

Advantages of thermal (steam) power plant

• Fuel (coal, oil, gas) is readily available and easy to transport, and the initial cost is lower than that of hydro and nuclear plants.
• Can be located near the load centre, so transmission cost and losses are small.
• Needs less space than a hydro plant of the same capacity.
• Construction time is short compared to hydro plants.
• Output does not depend on rainfall; it can run as a base-load plant all year.
• Unit sizes up to about 1000 MW are possible; the steam can also be used for process heating (cogeneration).
• Overload capacity is good.

Superheating

Superheating is heating dry saturated steam at constant boiler pressure, in the superheater, to a temperature above its saturation temperature (process g-1).

  T
  ^                   1
  |                  /|    superheat g-1
  |      f---------g  |
  |     /  boiling    |
  |    /              |
  |   3               |
  |   4---------------2
  +-----------------------> s
  3-4 pump, 4-f-g-1 boiler (heating,
  boiling, superheating), 1-2 turbine,
  2-3 condenser (4 and 3 almost coincide)

Benefits:

• Heat is added at a higher temperature, so the mean temperature of heat addition and cycle efficiency increase.
• Work per kg of steam increases (area of the T-s diagram grows), so steam consumption falls.
• Exhaust steam (state 2) is drier, reducing erosion and friction loss in the last turbine stages.
• Superheated steam has no moisture, so losses in pipes are lower.

Limit: maximum temperature is fixed by metallurgy of tubes and blades (about 540-600 °C).

Reheating

In reheating, steam that has partially expanded in the HP turbine is returned to the boiler and reheated at constant (intermediate) pressure to about the original temperature, then expanded in the LP turbine.

  T
  ^                  1          3
  |                 /|         /|
  |                / |        / |
  |     f--------g'  |       /  |
  |    /             2------'   |
  |   /                         |
  |  6                          |
  |  5--------------------------4
  +--------------------------------> s
  5-6 pump, 6-f-g'-1 boiler (heat,
  boil, superheat), 1-2 HP turbine,
  2-3 reheater, 3-4 LP turbine,
  4-5 condenser

Benefits:

• Higher mean temperature of heat addition (reheat at 20-25% of boiler pressure), so efficiency increases by about 4-5%.
• Work output per kg increases, steam rate falls.
• Exhaust dryness increases (above about 0.88), reducing blade erosion; allows use of high boiler pressure.

Drawbacks: higher cost and more complex piping and control; economic only for large units.