Soot is generally caused by incomplete combustion. It is hard find soot on a natural gas burner with a clean blue flame. All carbon is oxidized. With wood the chains of carbon is very long and the combustion is slower cooler and longer many newer wood burning stoves have a reburn where super heated fresh air is introduced to oxidize and ignite some of the gasified wood that would be soot. Imagine soot as tiny particles of charcoal. Also remember combustible means it’s flash point is above 100c.