What can cause black smoke during combustion in a boiler furnace?

Black smoke during combustion in a boiler furnace is primarily caused by insufficient combustion air. When there isn't enough air introduced into the combustion process, the fuel does not burn completely. This incomplete combustion results in the release of carbon particles or soot, which manifests as black smoke. Proper air-to-fuel ratios are crucial for reaching optimal combustion temperatures and ensuring a clean burn.

The other options involve factors that do not directly contribute to the formation of black smoke. For example, fly ash is typically a byproduct of combustion that can affect the efficiency of the system but is not responsible for the smoke color itself. Nitrogen in the air is generally inert and does not influence the coloration of smoke. Conversely, too much combustion air usually leads to more efficient burning of the fuel and results in clearer emissions, rather than black smoke. Understanding the importance of air supply in combustion helps prevent issues like black smoke and ensures a more efficient and cleaner combustion process.