Thermal power plants, also known as steam power plants, are the backbone of electricity generation. They play an important role in meeting the world’s energy demands by converting heat energy from fuel combustion into electricity.
How electricity is generated in thermal power plant?
The electricity generation process in a thermal power plant involves converting the chemical energy of fuel into heat energy, mechanical energy and finally electrical energy. The main stages of this process are as follows:
1. Combustion of Fuel: Fuel such as coal, natural gas, oil or biomass is burned in the boiler furnace, where combustion releases heat energy. This heat is used to generate steam for power production.
2. Steam Generation: The boiler transfers heat from fuel combustion to water, converting it into high-pressure, high-temperature steam. The steam may pass through a superheater to increase its temperature before entering the turbine.
3. Turbine Operation: The high-pressure, high-temperature steam expands through the turbine stages, transferring its energy to the turbine blades. This causes the turbine rotor and shaft to rotate, converting steam energy into mechanical energy.
4. Electricity Production: The rotating turbine shaft drives a generator, which converts mechanical energy into electrical energy through electromagnetic induction. The generated electricity passes through the electrical system and transformer before being supplied to the transmission grid.
After passing through the turbine, the exhaust steam enters the condenser, where it is cooled and converted back into water. The condensate is returned to the feedwater system and supplied to the boiler again, completing the steam-water cycle.
This continuous process enables a thermal power plant to convert the heat energy released from fuel into electrical energy for domestic, commercial and industrial use.
Important Terms Used In Thermal Power Plant Or Steam Power Plant
Saturation Temperature – It is the temperature for a corresponding saturation pressure at which a liquid boil into its vapor phase.
Wet Steam or Unsaturated Steam – It is a mixture which contain both water vapor and liquid water droplets. Most of the best boiler discharge steam containing 3% to 5% wetness. If the mass of steam contains 5 % water it’s said to be 5% wet & 95 % dry.
Dry Steam or Saturated Steam – Dry steam is steam that is at the temperature of saturation, but does not contain water particles in suspension.
Superheated Steam – It is an extremely high temperature vapour generated by heating the saturated steam or wet steam beyond the saturation point.
Condensate – A liquid phase produced by the condensation of steam.
Rankine Cycle in a Thermal Power Plant
The Rankine cycle is the fundamental thermodynamic cycle used in conventional steam power plants to convert heat energy into mechanical work and ultimately electrical energy. It explains how water and steam circulate through the boiler, steam turbine, condenser and feedwater pump during power generation.
The Rankine cycle consists of four main processes:
Pumping: The feedwater pump increases the pressure of condensate before it enters the boiler system.
Heat Addition: Heat is supplied in the boiler to convert water into high-pressure steam, which may be further heated in a superheater.
Expansion: Steam expands through the turbine, producing mechanical power to drive the generator.
Heat Rejection: Exhaust steam releases heat in the condenser and changes back into water, which is returned to the feedwater system.
These four processes form a continuous cycle that enables the plant to generate electricity. Actual power plants may incorporate superheating, reheating and regenerative feedwater heating to improve thermal efficiency.
Understanding the Rankine cycle helps explain the working principle of a thermal power plant, the functions of its major components and the factors that influence plant efficiency.
Thermal Power Plant Major Components and Their Functions
A thermal power plant consists of several major components that work together to convert heat energy into electrical energy. The type and arrangement of equipment depend on the fuel, heat source and plant technology. The following are the major components and their functions.
1. Fuel Supply System
Thermal power plants use different energy sources, including coal, natural gas, oil, biomass and nuclear fuel. Coal is a major fuel used in conventional steam power plants.
The fuel supply system delivers the required energy source to the power-generation system. For example, coal-fired plants use coal handling and pulverizing equipment, while gas-fired plants use fuel-gas supply and control systems.
The boiler is the heart of a conventional steam power plant, where heat released from fuel combustion converts water into high-pressure, high-temperature steam to drive the steam turbine. Boilers are classified according to their construction, circulation method and operating pressure.
Types of Boilers Used in Thermal Power Plants
1. Based on Construction
Water-Tube Boiler: Water flows inside the tubes, while hot gases pass outside. Commonly used in large thermal power plants.
Fire-Tube Boiler: Hot gases flow inside tubes surrounded by water. Generally used for small-capacity applications.
2. Based on Combustion Technology
Pulverized Coal (PC) Boiler: Pulverized coal burns in suspension inside the furnace. Widely used in large coal-fired power plants.
Fluidized Bed Combustion (FBC) Boiler: Fuel burns in a bed of particles fluidized by air.
Bubbling Fluidized Bed Combustion (BFBC) Boiler: Bed particles remain in a bubbling state due to the upward flow of air.
Circulating Fluidized Bed Combustion (CFBC) Boiler: Bed particles circulate through the furnace and are separated and returned for continued combustion.
3. Based on Water Circulation
Natural Circulation Boiler: Water circulates through the boiler tubes due to density differences between water and the steam-water mixture.
Forced Circulation Boiler: A pump drives water through the boiler circuit.
Once-Through Boiler: Water passes through the heating circuits and becomes steam without recirculation through a steam drum.
4. Based on Operating Pressure
Subcritical Boiler: Operates below water’s critical pressure of approximately 22.064 MPa.
Supercritical Boiler: Operates above the critical pressure, without a distinct liquid-to-vapour phase transition.
Ultra-Supercritical Boiler: Operates at higher steam pressures and temperatures than conventional supercritical systems.
Note: Boilers are classified in different ways based on their construction, combustion technology, water circulation and operating pressure. Therefore, a single boiler can belong to several categories at the same time. For example, a boiler can be water-tube in construction, use pulverized coal as fuel, and operate at subcritical pressure.
The steam turbine converts the energy of high-pressure, high-temperature steam into mechanical rotational energy. As steam expands through stationary and moving blades, it causes the turbine rotor to rotate.
Large power plants may have high-pressure (HP), intermediate-pressure (IP) and low-pressure (LP) turbine sections. The turbine shaft is coupled to the generator to produce electricity.
The generator converts the mechanical energy supplied by the turbine into electrical energy through electromagnetic induction. When the turbine rotates the generator rotor within a magnetic field, an electrical voltage is induced in the stator windings.
The generated electricity is then transferred through the plant’s electrical system and transformer to the transmission grid.
The Demineralization (DM) plant removes dissolved mineral salts and ions from water to produce high-purity water. This water is used for boiler makeup and other applications requiring controlled water quality.
Proper water treatment helps prevent scale formation, corrosion and deposits on boiler tubes and other equipment. Depending on the plant design, additional treatment systems may be used to maintain the required water chemistry.
The Ash Handling Plant collects and transports ash generated during the combustion of solid fuels, particularly coal. Bottom ash is collected from the furnace bottom, while fly ash is carried by flue gases and captured by particulate-control equipment.
The collected ash is transported to designated storage, disposal or utilization facilities. Ash handling arrangements vary according to boiler design and the type of fuel used.
The condenser system receives exhaust steam from the turbine and removes its heat, converting the steam back into water. The recovered condensate is returned to the feedwater system for reuse.
The condenser also helps maintain low pressure at the turbine exhaust, supporting efficient steam expansion. Cooling water carries away the rejected heat and may circulate through a cooling tower or another cooling arrangement. The condenser system generally includes the condenser, condensate extraction pumps, circulating-water pumps and associated piping and auxiliary equipment.
The cooling tower removes heat from circulating cooling water by transferring it to the atmosphere. The cooled water is recirculated to the condenser in plants using a cooling-tower system.
In fuel-fired power plants, this system supplies combustion air and carries flue gases through heat-recovery and emission-control equipment before releasing them into the atmosphere through the chimney. It may include FD fans, PA fans, ID fans, an air preheater, an electrostatic precipitator (ESP), a flue gas desulfurization (FGD) system, and a chimney.
The chimney discharges treated flue gases into the atmosphere at an appropriate height to support dispersion. Emission-control equipment reduces pollutants before the gases are released, depending on the plant configuration.
10. Electrical Transformer and Switchyard
The generator transformer increases the generator output voltage to the required transmission level. The switchyard contains equipment for switching, isolation, protection and connection to the electrical grid.
11. Instrumentation and Control System
The instrumentation and control system monitors parameters such as pressure, temperature, flow, level and vibration. It regulates plant equipment and supports safe, stable and reliable operation.
12. Pumps and Auxiliary Equipment
Pumps, fans, motors and other auxiliary equipment support the circulation of water, movement of air and gases, fuel supply and operation of the main power-generation systems.
Heat Recovery Technologies for Improving Power Plant Efficiency
A significant amount of energy in a thermal power plant is lost through flue gases, hot boiler blowdown water and condenser cooling systems. Heat recovery technologies capture some of this energy and reuse it for heating feedwater, combustion air or other process streams, helping improve overall plant efficiency and reduce fuel consumption.
1. Economizer
An economizer recovers heat from boiler flue gases to preheat feedwater before it enters the boiler. This reduces the heat required to convert water into steam and improves boiler efficiency.
2. Air Preheater (APH)
An air preheater transfers heat from outgoing flue gases to incoming combustion air. Preheated air improves combustion conditions and reduces heat loss through the flue gas system.
3. Low-Temperature Economizer
A low-temperature economizer recovers additional heat from flue gases downstream of conventional heat recovery equipment. The recovered heat can be used to preheat condensate or other suitable process streams, depending on plant design and operating conditions.
4. Boiler Blowdown Heat Recovery
Hot boiler blowdown water contains useful thermal energy. A flash vessel and heat exchanger can recover some of this energy for makeup water heating or other suitable applications, reducing energy losses.
5. Condenser Heat Recovery
The condenser rejects heat from exhaust steam after it leaves the turbine. Where a suitable heating demand exists, some of this low-grade heat can be recovered for industrial process heating or other applications using appropriate heat exchangers or heat pumps.
Note: Economizers and air preheaters are conventional heat recovery systems in thermal power plants. Additional systems, such as low-temperature economizers and boiler blowdown heat recovery, can further improve energy utilization. Their actual benefits depend on plant design, available heat sources, operating conditions and auxiliary power consumption.
Coal Based Thermal Power Plant Layout
How does a coal based thermal power plant work?
The coal based thermal power plant or steam power plant comprises of four main working circuits:
Coal and ash circuit
Air & gas circuit
Feed water and steam flow circuit
Cooling water circuit
Coal and Ash Circuit
Coal Handling: Coal is transported to the coal handling plant, processed, and fed into the furnace through pulverizers and fuel feeding devices.
Air is taken from atmosphere through the action of forced draught (FD) fan and preheated using air preheater. Flue gases are produced when coal is combusted in the boiler. The flue gases after passing around boiler tubes and superheater tubes in the furnace pass through an economizer and finally through the air preheater before being exhausted to the atmosphere via electrostatic precipitator (ESP) & chimney.
In the water and steam circuit condensate leaving the condenser is passes through the deaerator and water heaters before going the boiler through economizer.
In the boiler drum and tubes, water circulates due to the difference between the density of water in the lower temperature and the higher temperature section of the boiler. The wet steam produced in the drum is further heated in the superheater to form superheated steam. This high-pressure, high-temperature steam drives the turbine, which in turn rotates the generator to produce electricity.
After expanding in turbine, steam is condensed in the condenser. The condensate leaving the condenser is circulated to boiler. A part of steam and water is lost while passing through different components and this compensated by supplying additional feed water. This feed water is purified & softened before going the boiler, to avoid the scaling of the tubes of the boiler. The purification and softening of water are done in water treatment plant.
The cooling water supply to the condenser helps in maintaining a low pressure in it. The cooled water extract heats from turbine exhaust steam in the condenser and become hot. Hot water gets cooled after passing through cooling tower and again circulated to condenser through circulating water pump.
A part of water is lost in the cooling tower and this compensated by supplying the water from a natural source such as river, lake or sea.
Major Air Pollution Problems in Thermal Power Plants and Their Solutions
Thermal power plants produce flue gases during fuel combustion. These gases may contain pollutants such as sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which contribute to air pollution and environmental problems. Specialized emission control systems are used to reduce these pollutants before the flue gases are released into the atmosphere.
The performance of a thermal power plant is evaluated using several operating parameters that indicate its efficiency, reliability and energy consumption. Regular monitoring of these parameters helps identify performance losses and improve overall plant operation.
The important performance parameters include:
Heat Rate: The amount of heat energy required to generate one unit of electrical energy, generally expressed in kcal/kWh. A lower heat rate generally indicates better thermal efficiency.
Boiler Efficiency: Indicates how effectively the boiler converts the energy available in fuel into useful heat for steam generation.
Plant Load Factor (PLF): The ratio of actual electrical energy generated during a period to the maximum possible generation at rated capacity over the same period.
Plant Availability: Indicates the proportion of time a generating unit is available for operation.
Auxiliary Power Consumption: The electrical energy consumed by plant equipment such as pumps, fans, coal mills and material-handling systems.
Monitoring these parameters helps identify operational inefficiencies, reduce auxiliary power consumption, improve equipment performance and maintain reliable electricity generation.
Frequently Asked Questions
1. What are the main causes of boiler tube failure in a thermal power plant?
Boiler tube failure can occur due to overheating, corrosion, erosion, creep, fatigue and other damage mechanisms. Identifying the actual root cause is essential to prevent repeated tube failures and unplanned boiler shutdowns. For more details, read our guide: Boiler Tube Failure: Causes and Mechanisms
2. Why can’t we simply increase boiler steam temperature to improve thermal power plant efficiency?
Increasing steam temperature can improve the thermal efficiency of a power plant, but it is limited by boiler tube material strength, allowable metal temperature, creep resistance and turbine operating limits. Excessive steam temperature can damage components and reduce equipment life. Therefore, any increase must remain within the plant’s design and operating limits. For more details, read our guide: Why Can’t We Simply Increase Boiler Steam Temperature to Improve Efficiency?
3. Why can a coal stockpile catch fire in a thermal power plant without any external flame?
Coal can react slowly with oxygen and generate heat inside a stockpile. If this heat cannot escape fast enough, the coal temperature gradually rises and may eventually lead to spontaneous combustion. Large stockpiles, poor temperature monitoring and unsuitable storage conditions can increase the risk. Regular temperature monitoring and proper coal yard management are essential to prevent stockpile fires. For more details, read our guide: Spontaneous Combustion in Coal Stockpiles
4. Why do some thermal power plants use natural draft cooling towers while others use induced draft cooling towers?
Natural draft cooling towers use the density difference between hot, moist air inside the tower and cooler outside air to create airflow without large fans. Induced draft cooling towers use mechanical fans to draw air through the tower, allowing better control of airflow. The choice depends on plant design, cooling capacity, space availability, auxiliary power consumption and operating requirements. For more details, read our guides: Natural Draft Cooling Tower Induced Draft Cooling Tower
5. Why can’t every thermal power plant use an ultra-supercritical boiler to generate electricity more efficiently?
Ultra-supercritical boilers operate at higher steam temperatures and pressures than subcritical and supercritical boilers, which can improve thermal efficiency and reduce coal consumption per unit of electricity generated. However, higher operating conditions require advanced materials, more demanding design requirements and greater investment. The choice depends on plant capacity, project cost, technology availability and operating requirements. For more details, read our guide: Subcritical vs Supercritical vs Ultra-Supercritical Boiler: Complete Comparison
6. Why does a thermal power plant need an auxiliary boiler if its main boiler can generate steam?
An auxiliary boiler provides steam for essential plant services during startup, shutdown and other operating conditions when the main boiler cannot supply the required steam. Depending on the plant design, this steam may be used for fuel oil heating, equipment warming and other auxiliary requirements. It helps support plant startup and maintain essential services when the main boiler is unavailable. For more details, read our guide: Auxiliary Boiler in Thermal Power Plant
7. Why do thermal power plants use the Rankine cycle?
Thermal power plants use the Rankine cycle because it provides a practical way to convert heat energy into mechanical work using steam and a turbine. Water can be condensed and pumped back to the boiler, allowing it to circulate through a closed cycle. This makes continuous power generation possible with established and reliable equipment. For more details, read our guide: Why Thermal Power Plant Uses Rankine Cycle
8. What happens if the coal handling plant fails to supply coal to the boiler in a thermal power plant?
If the coal handling plant fails to maintain the required coal supply, boiler bunkers may gradually empty, affecting continuous coal feeding to the furnace. If the shortage continues, the generating unit may need to reduce load or shut down, depending on bunker stock, coal consumption and the duration of the interruption. Reliable conveyor operation, adequate coal stock and effective equipment maintenance are essential for uninterrupted power generation. For more details, read our guide: Coal Handling Plant
9. Why does condenser vacuum play an important role in thermal power plant efficiency?
Condenser vacuum helps the steam turbine exhaust steam at a lower pressure, allowing the steam to expand further and produce more work. If condenser vacuum deteriorates, turbine exhaust pressure increases, which can reduce turbine output and overall plant efficiency. Cooling water temperature, condenser tube cleanliness, air leakage and cooling system performance can all influence condenser vacuum. For more details, read our guide: Efficiency of a Thermal Power Plant
10. How much can a cooling tower reduce water temperature in a thermal power plant?
In a thermal power plant, a cooling tower typically reduces the circulating cooling water temperature by around 8–15°C, depending on ambient wet-bulb temperature, cooling tower design, heat load and operating conditions. For example, water entering the cooling tower at 42°C might leave at 30–34°C under suitable conditions. The actual temperature reduction is called the cooling tower range and is calculated as the difference between the hot-water inlet and cold-water outlet temperatures. For more details, read our guide: Steam Condenser in Power Plant
