What type of air typically provides additional oxygen for the combustion process?

Primary air is crucial in the combustion process as it directly supports the burning of the fuel. It is the air that is mixed with the fuel before it enters the combustion area, ensuring there is enough oxygen available for the efficient combustion of the fuel. This primary air can also influence the flame characteristics, combustion efficiency, and overall performance of the boiler.

In contrast, exhaust air consists of the gaseous byproducts formed during combustion, which typically contains carbon dioxide and water vapor but lacks enough oxygen for continued combustion. Forced air refers to the technique of blowing air into the system to help with combustion, but it does not specifically denote the air that is mixed with fuel prior to combustion. Draft air, on the other hand, refers to the air that enters into the system due to the pressure differences created by the exhaust gases, and while it plays a role in the combustion process, it is not the primary source of oxygen needed for the combustion itself. Hence, primary air is uniquely positioned as the type that directly provides additional oxygen for the combustion process.