An Introduction to Different Types of Fans in Thermal Plants
Combustion is pivotal for heating in steam boilers. Air is crucial for combustion and plays an important role in fuel burning and must be supplied with precision, in different ratios, depending on the type of fuel used. Steam boiler systems in Thermal Power plants use different types of centrifugal fans. It provides air for combustion, fuel to the burners, circulates the gasses for enhanced heat transfer, and releases exhaust gasses. Depending on the boiler size and airflow requirement, various types of fans are used with varying capabilities. Draught fans are essential in thermal power plants by providing tremendous support to the various process activities related to power generation.
Two types of fans used in a boiler power plant are– Draft Fans and Other Process Fans.
- Draft Fans:
Draft fans are important for thermal power plants as it maintains the flow of gasses through the steam boiler system. Draft fans are classified further as Forced Draft fans and Induced Draft Fans.
Forced Draft fans in the power plant are centrifugal fans, utilizing radial airflow blading. It provides the necessary air for fuel combustion by pushing air through the combustion air supply system to the furnace. Located at the inlet of the boiler to push high-pressure fresh air into the combustion chamber, it mixes with the fuel to produce positive pressure.
Compared to Induced Draft fans, Forced Draft fans are more efficient and easy to maintain with clean operating conditions. Backward curved and airfoil centrifugal fans are the most common centrifugal fans used as FD fans.
Induced Draft Fans exhausts all flue gasses from the boiler through the dust collector, passing it to the chimney and into the open atmosphere. Therefore, ID fans are located at the outlet of the steam boiler system. Induced Draft fan creates a negative pressure to discharge the gasses after combustion from the furnace.
As Induced Draft fans can capably handle hot flue gasses, it faces more corrosion and erosion issues. Radial fans and backward inclined blade centrifugal fans are the most common types of Induced Draft fans used in a boiler power plant.
Other Process Fans:
Other Process fans used in a boiler power plant are,
- Primary Air Fan
- Secondary Air Fan
- Flue Gas Recirculation Fan
- Primary Air Fan:
Primary air fans or PA fans are high-pressure fans that supply the air for transportation of coal in the boiler power plants directly from the pulverizer to the furnace. PA fans supply positive pressure upstream of the coal pulverizer.
In power plants, Primary Air fans provide air in conveying the pulverized coal to the furnace area. PA fans produce pressure upstream of the pulverizer for pushing the coal and air mixture through the pulverizer into the furnace.
The most common centrifugal fan used as a PA fan in a boiler power plant is Backward Curved Centrifugal Fan.
- Secondary Air Fan:
The secondary fans or SA fans play a vital role in power plants. The secondary fan helps with the complete combustion of the fuel in the furnace. SA fans increase the airflow as required, to improve efficiency and avoid fuel wastage. Coal-fired thermal plants account for the highest share of electricity generation compared to other methods like hydroelectric, wind, gas, or solar-based plants. Furthermore, SA fans can run continuously. Considering the transportation expenses and high costs of coal mining, it is not ideal to do inefficient burning of the coal in the furnace. Due to all the mentioned factors, secondary fans play a key role in maintaining the efficiency of the thermal power plant.
- Flue Gas Recirculation Fan:
Flue gas recirculation fan reduces the flue gas pollution content in the exhaust gasses from thermal combustion processes. Flue gas recirculation fans are used in a power plant boiler for controlling steam temperature and furnace heat absorption.
Flue gas recirculation fans extract gas from the economizer outlet and the inlet of the pre-heater. It later discharges the gas either to the bottom of the furnace to control the steam temperature or to different locations in the furnace.
Backward curved centrifugal fans are the most common type of gas recirculation fans used in a boiler power plant.
Role of Primary Air Fan and Secondary Air Fan in Thermal Plants:
Air supplied in thermal plants boilers for combustion reacts with the fuel as per its stoichiometric ratio. However, some percentage of air provided for combustion remains unreacted.
It usually occurs due to the fuel quantity fed at once and variation in the fuel particles size. Therefore, a certain amount of extra air requires to be provided to compensate for the air that went unreacted.
Conclusion:
Fans of various types and ratings are used as process fans in power plants. Draught fans are typically heavy-duty centrifugal fans that are an essential and irreplaceable component of a thermal power plant.
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- Published in Boiler