The carbon atoms combine with oxygen atoms to eventually create carbon dioxide. Pairs of hydrogen atoms combine with oxygen to form water vapor. These are the desired end products of burning. Fuels such as natural gas, propane, or methane, start out with rather simple hydrocarbon molecules, so the process described above is extremely efficient and clean-burning, as long as sufficient oxygen is provided. Fuels such as heating oil or coal have moderately complex hydrocarbon molecules.
Wood tends to have even more complex molecules to start with.
For these fuels with more complex molecules, it is very important for there to be appropriate conditions for the intermediate-sized molecules created in the first stage, to be further broken down into individual carbon and hydrogen atoms, for the burning process to complete itself. Temperature, Turbulence and Time. High enough temperature must be present to break down the complex molecules to release the individual carbon and hydrogen atoms. Turbulence is necessary to make sure that the oxygen in the air gets a chance to spread around well enough so each carbon atom and hydrogen atom gets a chance to be near some oxygen atoms. Unfortunately, this often is not the case when burning wood.