Turbines | Energy Conservation in Plants
Combustion Gas Turbine
Combustion gas turbines (CGT) can be used to drive generators, compressors and pumps. They are suitable for utility plants either for base or peak loads. Energy in exhaust gases of gas turbines can be recovered in Heat Recovery Steam Generator (HRSG) to raise steam for either power generation (combined cycle) or process heating (cogeneration) purposes or sometimes both.
- CGT has several advantages as follows:
- Wide power range from fraction of Megawatts up to hundreds of Megawatts
- High power to weight ratio
- Fuel flexibility where it can use gaseous as well as liquid fuels
- No need for cooling water
- Suitable for combined cycle and cogeneration
- Minimum environmental impact due to combustion at very lean conditions
- Short delivery and quick to install
However, it also has few disadvantages as follows:
- Relatively low thermal efficiency when used as simple cycle
- Not suitable for solid fuels
- Power output and efficiency are sensitive to site conditions (ambient temperature and humidity)
Gas turbine can be classified according to their application or design. Regarding the application gas turbine can be either, micro gas turbine (small power range), aero-derivative (gas turbine originally designed for air craft propulsion or derived from unit designed for air jets, industrial gas turbine and heavy duty gas turbine.
Regarding the design, CGTs are single shaft and two shaft turbines (eliminates the need of a gear box; suitable for driving compressors and pumps).
Gas turbine performance (output power and heat rate (efficiency)) is affected by the ambient conditions as well as installation. Gas turbine performance is usually characterized at ISO conditions. Inlet air chilling of the air going to the CGT compressor can enhance the power output of the machine due to the increase in density and consequently the mass flowrate to the constant volume CGT machine besides the enhancement of the machine heat rate. (heat input per power output). This important fact will be discussed later in this book.
- Ambient pressure equal sea level pressure
- Ambient temperature at 15°C
- Relative humidity 60%
- Inlet pressure loss 100 mm H2O
- Exit pressure loss 200 mm H2O
There are three basic types of steam turbines:
1. Condensing turbines, in which all the steam that enters the throttle expands to the high-vacuum condensing pressure.
2. Back-pressure turbines, which operate with an exhausts pressure equal or greater than, atmospheric pressure to meet the process steam requirements. And,
3. Extraction turbines, in which steam is extracted at one or more intermediate stages, often at comparatively high pressures, either to meet process load or to heat boiler feed water. Extraction can be applied to condensing or back-pressure turbines.