Steam plant improvement methods and reheating

Methods of improving performance of a steam power plant

The efficiency of a Rankine (steam) cycle is $\eta = 1 - \dfrac{T_{\text{low}}}{T_{m,\text{in}}}$ roughly, so it improves by raising the mean temperature of heat addition $T_{m}$ or lowering the temperature of heat rejection. Common methods:

1. Increasing boiler (steam) pressure - raises the saturation temperature at which heat is added.
2. Superheating the steam (higher turbine inlet temperature) - raises mean temperature of heat addition and dryness at exhaust.
3. Reducing condenser pressure (better vacuum) - lowers heat rejection temperature.
4. Reheating - steam partly expanded in HP turbine is reheated in the boiler and expanded in LP turbine.
5. Regenerative feed water heating - steam bled from the turbine heats the feed water in feed water heaters.
6. Using economiser and air preheater - recover heat from flue gases.
7. Supercritical / binary vapour and combined cycles - use very high pressure, or a topping cycle (gas turbine) whose waste heat runs the steam cycle.
8. Reducing losses: better turbine blading, insulation, reduced throttling.

How reheating increases efficiency

               1
  +----------+----->+------+     +------+
  |  Boiler  |      |  HP  |=====|  LP  |===> Generator
  |  + S/H   |  2   | turb |     | turb |
  | Reheater |<-----+------+  +->+------+
  |          |------------3---+     | 4
  +----------+                      v
       ^ 6                    +-----------+
       |      +------+   5    | Condenser |
       +------| Pump |<-------+-----------+
              +------+

In a reheat cycle the steam leaving the boiler (state 1) expands in the HP turbine only up to an intermediate pressure (state 2). It is taken back to the reheater in the boiler and heated at constant pressure to nearly the original temperature (state 3). It then expands in the LP turbine to condenser pressure (state 4).

How reheating increases efficiency and output:

• Higher mean temperature of heat addition: heat added in reheating (2-3) is at a high temperature. If the reheat pressure is chosen properly (about 20-25% of boiler pressure), the average temperature at which heat is supplied rises, so $\eta$ rises by about 4-5%.
• More work per kg of steam: the expansion 3-4 lies to the right of the original expansion line, so the specific work output increases. The steam consumption (kg/kWh) falls and the plant output for the same boiler size increases.
• Drier exhaust steam: without reheat, expansion from high pressure ends in very wet steam (dryness below 0.85). Reheat gives exhaust dryness about 0.9 or more, which reduces blade erosion and friction loss, so the turbine internal efficiency improves.
• It makes very high boiler pressures usable, which is itself a means of raising efficiency.

Cycle efficiency with reheat:

$$\eta = \frac{(h_1 - h_2) + (h_3 - h_4) - W_p}{(h_1 - h_6) + (h_3 - h_2)}$$

Limitations: extra cost of reheater, piping and controls; useful only for large units (above about 100 MW) and high pressures.

  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