What is the function of excess air in boiler combustion control, and what specific consequence arises from operating with insufficient excess air?

The function of excess air in boiler combustion control is to ensure complete combustion of the fuel. Complete combustion means that all the carbon, hydrogen, and other combustible elements in the fuel are fully oxidized to form carbon dioxide (CO2), water (H2O), and other stable oxides. Excess air provides enough oxygen to overcome imperfect mixing between the fuel and air, ensuring that fuel molecules can readily find oxygen atoms to react with. Operating with insufficient excess air leads to incomplete combustion. Incomplete combustion results in the formation of undesirable products such as carbon monoxide (CO), unburned hydrocarbons (HC), and soot (unburned carbon particles). Carbon monoxide is a toxic gas and represents a loss of energy. Unburned hydrocarbons also represent a loss of energy and can contribute to air pollution. Soot can foul heat transfer surfaces, reducing boiler efficiency and potentially leading to increased maintenance costs. Furthermore, insufficient excess air can lead to increased slagging and fouling on boiler tubes due to the reducing atmosphere created by incomplete combustion. This reduces heat transfer and can accelerate tube corrosion. Therefore, maintaining adequate excess air is crucial for efficient and clean boiler operation.