Steam and Gas Turbine Power Plants: Types and Cycles
Steam Turbines
Types of Steam Turbines
Based on Outlet Vapor Pressure
Backpressure Turbines
In backpressure turbines, the steam is extracted at a pressure above atmospheric after being used. This steam can then be utilized for industrial processes. These turbines can be further classified into:
- Simple or Single Stage: These turbines have a single stage.
- Composite Single or Multistage: These turbines consist of two or more stages.
Based on Steam Flow Direction
Axial Turbines
In axial turbines, the steam flows in the same direction as the axis of rotation of the impeller. These are the most common type of steam turbines.
Radial Turbines
In radial turbines, the steam flow does not follow the same direction as the axis of rotation. They are less frequent than axial turbines.
Condensing Turbines
In condensing turbines, the steam exhaust is below atmospheric pressure due to condensation in a condenser. The condenser is a heat exchanger where circulating water condenses the steam, causing a vacuum and reducing the outlet pressure below atmospheric. These turbines are commonly used in large power plants, including coal-fired, combined cycle gas turbine, and nuclear power stations. The exhaust gases from gas turbines, which have a high energy content, can be used to generate steam in recovery boilers, further increasing efficiency. Combined cycle plants can achieve fuel efficiency exceeding 55%.
Backpressure Steam Turbine Cycle
In this cycle, the exhaust steam from the turbine is used in industrial processes. High-pressure steam from a boiler expands in the turbine, generating mechanical energy to drive a generator or compressor. This cycle can achieve efficiencies of 85-90%. A significant advantage of this cycle is its ability to utilize various fuels, including solid, liquid, and gaseous fuels, making it suitable for industries with residual fuels or waste heat.
Gas Turbine Plants
Gas Turbine Plant with Heat Recovery
In gas turbine systems, fuel is burned in a combustion chamber, generating mechanical energy. While their conversion efficiency is lower than reciprocating engines, gas turbines offer easy heat recovery from the exhaust gases. These gases, at temperatures around 500°C, can be used to produce steam in a recovery boiler.
Types of Gas Turbine Plants with Heat Recovery
Simple Cycle
This is the classic combined heat and power (CHP) plant, suitable for substantial steam requirements (>10t/h). The design of the heat recovery system is crucial for its economic viability. An afterburner or bypass valve can be used to adjust steam production.
Combined Cycle
A combined cycle optimizes the simple cycle by utilizing some of the generated steam in a second backpressure turbine, improving heat recovery. This configuration enhances efficiency by incorporating a high-pressure boiler and a backpressure steam pipe. Regulation of the plant is achieved through post-combustion and bypass valves.
Absorption Cycle
The absorption cooling process operates on similar principles as mechanical refrigeration, involving:
- Evaporation of the refrigerant at low temperature, extracting heat for cooling.
- Compression of the refrigerant vapor to a suitable condensing temperature.
- Condensation of the refrigerant by cooling it to a temperature higher than the fluid being cooled.
However, in absorption cooling, the compression stage is replaced by a refrigerant recirculation pump, significantly reducing mechanical energy input. This requires the evaporated refrigerant to be absorbed by a liquid with high affinity for it (e.g., water and ammonia). The solution is then pumped to the required condensing pressure and separated in a distillation column. The condensed gas is then ready to produce the desired cooling effect.