Steam plant improvement methods and regeneration

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 regeneration increases efficiency

              1
  +--------+------>+---------------+   +-----+
  | Boiler |       |    Turbine    |===| Gen |
  |  + S/H |       +---------------+   +-----+
  +--------+          | m kg (2)  | (1-m) kg (3)
      ^ 7             v           v
      |         +-----------+ +-----------+
  +--------+  6 |  Open FWH | | Condenser |
  | Pump 2 |<---| (mixing)  | +-----------+
  +--------+    +-----------+       | 4
                      ^ 5           |
                 +--------+         |
                 | Pump 1 |<--------+
                 +--------+

Regeneration means heating the feed water, before it enters the boiler, with steam bled (extracted) from the turbine at one or more intermediate stages.

In the simple Rankine cycle the cold feed water (about 40 °C) enters the boiler and is heated by the hot flue gases over a large temperature range. This low-temperature part of the heat addition lowers the mean temperature of heat addition and so lowers efficiency.

In the regenerative cycle a fraction $m$ kg of steam per kg is bled from the turbine at state 2 and used to heat the feed water in a feed water heater (FWH). The remaining $(1-m)$ kg expands to the condenser.

Why efficiency increases:

• The feed water enters the boiler hotter (state 7 instead of 5), so the heat supplied in the boiler falls from $(h_1 - h_5)$ to $(h_1 - h_7)$, and all of it is added at a higher mean temperature. In the limit (infinite heaters) the cycle approaches the Carnot efficiency.
• The latent heat of the bled steam is not rejected in the condenser; it is returned to the feed water. Heat rejected per kg of boiler steam is only $(1-m)(h_3 - h_4)$.
• Work output per kg falls slightly (bled steam does not expand fully), but heat supplied falls more, so the ratio $W/Q$ rises (typically by 10-15% with several heaters).
• Other benefits: less thermal stress in the boiler, smaller condenser and LP blade height (less steam flow at the exhaust), deaeration in an open heater.

Efficiency (one open heater):

$$\eta = \frac{(h_1 - h_2) + (1-m)(h_2 - h_3) - W_p}{h_1 - h_7}, \qquad m = \frac{h_6 - h_5}{h_2 - h_5}$$

  T
  ^                    1
  |                   /|
  |      f----------g  |
  |     /              |
  |    7               |
  |    6 . . . . . . . 2  bleed (m kg)
  |   /                |
  |  5                 |
  |  4-----------------3  (1-m) kg
  +-------------------------> s
  4-5 pump 1, 5 and 2 mix in FWH -> 6,
  6-7 pump 2, 7-f-g-1 boiler,
  1-2-3 turbine, 3-4 condenser