<<why does flame impengement produce incomplete combustion in a natural gas furnace?
If gas burns completely, the end combustion products are carbon dioxide and water. But to get there a lengthy chain of chemical reactions have to occur. If the conditions for that chain aren't all present, the process stops at that point and the result is other, intermediate products of combustion, which can include alcohols called aldehydes, soot and carbon monoxide, among many others.
Flame impingement can cool the combustion gasses and restrict air from getting to portions of the flame, and that can stop the combustion process.