Functions and Characteristics of Blowers in Boiler Systems of Thermal Power Plants

In thermal power plants, the blower system is a key component for the safe and efficient operation of the boiler. Its core function is to provide power for the combustion process and maintain gas circulation within the furnace. Its main components include forced draft fans and induced draft fans, as well as pulverized coal fans, flue gas recirculation fans, desulfurization fans, primary and secondary air fans, and cooling tower fans.

Primary air's function is to directly transport fuel (such as pulverized coal) into the furnace. It is usually supplied by a forced draft fan, but can also be handled independently by a dedicated primary air fan. Its airflow carries pulverized coal into the combustion zone, forming the basis for ignition and initial combustion.

Secondary air supplements the primary air, ensuring complete combustion of pulverized coal within the furnace. Because primary air primarily undertakes transport tasks and has a limited oxygen content, it cannot achieve complete combustion alone. Therefore, secondary air is introduced around the burner and injected into the furnace synchronously with the primary air to improve oxygen supply and mixing efficiency. In primary and secondary air systems, the respective air pressure and volume can be flexibly adjusted via valves to match combustion conditions and achieve efficient combustion. In some designs, the exhaust gas containing fine powder discharged from the coal mill is reintroduced into the furnace for combustion via a pulverized coal exhaust fan, which is equivalent to tertiary air; however, if the coal mill is directly connected to the primary air duct, the tertiary air step can be eliminated.

The primary air blower (also known as a forced draft fan) primarily provides sufficient air to the boiler as a secondary air source, ensuring the oxygen required for pulverized coal combustion; in systems without a separate primary air blower, it also serves as a primary air conveyor. The induced draft fan is responsible for extracting the high-temperature flue gas (typically 150-250℃) from the furnace after combustion, along with coal dust. Therefore, its structure and materials need to be reinforced with enhanced wear resistance and high-temperature resistance, and its technical requirements are higher than those of the primary air blower.

Structurally, primary and induced draft fans are classified as centrifugal or axial flow types. Small and medium-sized units mostly use centrifugal fans, while large units use both centrifugal and axial fans. However, ultra-large units with a single unit capacity exceeding 600,000 kilowatts generally use axial fans. Centrifugal fans have a relatively simple structure, high efficiency, low noise, and low manufacturing cost under rated load, and are therefore widely used; however, their large size poses certain challenges to manufacturing and transportation when used in high-flow applications.