What is the difference between primary and secondary air?

In a combustion process, particularly in biomass boilers, primary and secondary air serve distinct roles in facilitating efficient and complete combustion. Primary air is supplied directly to the fuel bed or grate. Its main purpose is to support the initial combustion of the fuel. In stoker-fired boilers, primary air is forced up through the grate, providing oxygen for the fuel to ignite and devolatilize, releasing volatile gases. In fluidized bed boilers, the primary air fluidizes the fuel bed, creating a turbulent mixing environment that promotes combustion. Secondary air, on the other hand, is introduced above the fuel bed or grate. Its primary function is to provide additional oxygen to burn the volatile gases and remaining unburned carbon particles that are released during the initial combustion stage. This ensures more complete combustion and reduces emissions of carbon monoxide and unburned hydrocarbons. The amount and distribution of secondary air are carefully controlled to optimize combustion efficiency and minimize the formation of pollutants. Often, adjusting the ratio of primary and secondary air allows tuning the combustion process. Primary air focuses on initial ignition and fuel bed conditions, while secondary air ensures complete combustion of the released gases.