Steam plant components and performance improvement

Main components of a steam turbine power plant

A steam (thermal) power plant works on the Rankine cycle. Its main parts are:

                superheated steam (1)
  +---------------+ -------------> +---------+   +-----+
  | Boiler        |                | Steam   |===| Gen |
  | (furnace,     |                | turbine |   +-----+
  |  economiser,  |                +---------+
  |  superheater) |                     | exhaust (2)
  +---------------+                     v
     ^   ^   |                    +-----------+
 fuel| air|  v flue gas           | Condenser |<--cooling
     |   |  chimney               +-----------+   water
     |                                  | condensate (3)
     |  feed water (4)  +-----------+   |
     +------------------| Feed pump |<--+
                        +-----------+

• Boiler (steam generator): burns coal/oil/gas and converts feed water into high-pressure steam. Includes furnace, water walls, drum, superheater, reheater, economiser (heats feed water by flue gas) and air preheater.
• Steam turbine: expands steam and converts its heat energy into shaft work; drives the alternator (generator).
• Condenser: condenses exhaust steam under vacuum using cooling water; the low back pressure increases turbine work. Cooling water is cooled in a cooling tower or taken from a river.
• Feed pump (with condensate pump): returns the condensate to boiler pressure, usually through feed water heaters (regeneration).
• Fuel and ash handling, draught system (FD/ID fans), chimney, ESP/dust collector.
• Water treatment plant for make-up water.

Ways of increasing the performance

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.

Typical effect: raising boiler pressure from 40 to 160 bar, superheat to about 540 °C, one reheat and 6-7 regenerative heaters together raise a plant's efficiency from about 25% to about 38-40%.