Steam Cycle in Coal-Fired Power Plants (PLTU)
A coal-fired power plant (PLTU) is a type of power plant that uses high-pressure steam to drive a turbine, which is connected to a generator to produce electricity. In the process of generating electricity, the steam cycle plays a critical role. To understand how a coal-fired power plant works, we need to examine the steam cycle, which typically refers to the Rankine Cycle.
Understanding the Rankine Cycle
The Rankine Cycle is a thermodynamic cycle that describes the conversion of heat energy into mechanical energy, which is then converted into electrical energy by a generator. This cycle involves several key stages: heating water to turn it into steam, expanding the steam in a turbine, condensing the steam back into water, and pumping the water back into the boiler.
In general, the steam cycle in a coal-fired power plant can be divided into several main stages:
1. Water Heating (Process in the Boiler)
The first stage in the steam cycle is the heating of water, which begins in the boiler. The water, which has been treated to remove dissolved gases like oxygen, is pumped into the boiler. Inside the boiler, the water is heated by the combustion of fossil fuels such as coal, oil, or natural gas.
Boilers in coal-fired power plants are usually of the water tube boiler type, where water flows through tubes that are heated from the outside. The heating process takes place in several steps:
- Preheating (Economizer): The water is first preheated by the hot exhaust gases from the fuel combustion, reducing the amount of fuel needed in the boiler.
- Main Heating (Boiler): The preheated water enters the boiler drum, where it partially turns into saturated steam.
- Superheating: The saturated steam from the boiler drum is further heated in the superheater to achieve higher temperatures, producing superheated steam that is used to drive the turbine.
At this point, the steam reaches extremely high temperatures (typically above 500°C) and pressures (usually between 150-200 bar), which are necessary for producing mechanical energy in the turbine.
2. Steam Expansion in the Turbine
After being heated to superheated steam, the steam is directed to the steam turbine. The turbine is divided into several stages, typically high-pressure turbine, intermediate-pressure turbine, and low-pressure turbine.
Expansion in the High-Pressure Turbine: The superheated steam first enters the high-pressure turbine, where its thermal energy is converted into mechanical energy. Here, the steam undergoes a reduction in both pressure and temperature.
Reheating: After exiting the high-pressure turbine, the steam is sent back to the boiler for reheating. The purpose of reheating is to restore the steam's temperature without increasing its pressure, thereby enhancing the efficiency of the expansion process in the intermediate and low-pressure turbines.
Expansion in the Intermediate and Low-Pressure Turbines: The reheated steam then flows into the intermediate-pressure and low-pressure turbines, where further expansion takes place, generating additional mechanical energy that is also converted into electrical energy.
In the turbines, the thermal energy of the steam is converted into kinetic energy, which spins the turbine shaft. The turbine shaft is connected to a generator, which converts the mechanical energy into electrical energy.
3. Steam Condensation (Process in the Condenser)
After the steam leaves the low-pressure turbine, it still contains residual energy. The steam, having lost most of its energy, is then condensed in a condenser, a device that converts the steam back into water by cooling it.
The condensation process involves several steps:
- Cooling with Water: The condenser is typically connected to a cooling system that uses water from an external source, such as a river or the sea. The cooling water flows through tubes in the condenser, absorbing heat from the steam, causing the steam to turn back into water.
- Vacuum Operation: The condenser operates under a vacuum to enhance the efficiency of the condensation process. At low pressure, the boiling point of water decreases, allowing the steam to condense at a lower temperature, making the process faster and more efficient.
At the end of this stage, the steam has fully condensed into water (condensate), which is collected in a condensate tank.
4. Pumping Water Back to the Boiler
The condensed water, stored in the condensate tank, is then pumped back to the boiler to begin the cycle again. Before the water returns to the boiler, several additional processes are carried out to improve efficiency:
- Feedwater Heating: The condensate water is typically heated before entering the boiler through a feedwater heater. The heat for this process comes from residual steam exiting from various stages of the turbine, increasing the temperature of the water before it enters the boiler. This process reduces fuel consumption since the water is already preheated when it reaches the boiler.
After this preheating step, the water is ready to return to the boiler, initiating a new steam cycle.
5. Control and Regulation of the Cycle
The steam cycle in a coal-fired power plant is not only mechanically driven but also involves complex control systems. These systems are designed to ensure that each process, from water heating to steam pressure and temperature regulation, runs optimally.
Some key parameters that are controlled in this cycle include:
- Steam Pressure and Temperature: These must be maintained at optimal levels to ensure the efficiency of the cycle. Too high or too low pressures can lead to damage to the turbine or reduced efficiency.
- Steam and Water Flow: The flow of steam and water must be regulated to prevent any surplus or deficit at each stage, which can reduce the plant’s performance.
- Water Quality: The water used in the cycle must be free of contaminants and dissolved gases, as they can cause corrosion or operational disturbances.
6. Efficiency of the Rankine Cycle and Technological Advancements
The thermodynamic efficiency of the Rankine Cycle in coal-fired power plants ranges from 30-45%, depending on the type of fuel used, the technology of the plant, and the maintenance and operation of the unit. One way to improve efficiency is by implementing a Regenerative Rankine Cycle. In this cycle, leftover steam from the turbine is used to heat the feedwater, reducing the amount of fuel needed to heat the water in the boiler.
Modern technology also integrates the Rankine Cycle with other cycles, such as the Brayton Cycle (used in gas power plants), to form a combined cycle power plant. In a combined cycle, hot gases from fuel combustion are used to drive a gas turbine (Brayton Cycle), and steam from the Rankine Cycle is used to recover the remaining heat from the combustion process, achieving a total efficiency of over 60%.
Conclusion
The steam cycle in coal-fired power plants, based on the Rankine Cycle, is a thermodynamic process involving water heating, steam generation, steam expansion in turbines, condensation, and the return of water to the boiler. Each stage in this cycle is crucial to ensuring high efficiency in electricity generation. With technological advancements and improved control systems, the steam cycle in coal-fired power plants continues to improve in terms of efficiency and operational effectiveness.
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