Stages of Wood Combustion

Stage 1: The Evaporation of Moisture

Anyone who has ever tried to burn wet or unseasoned wood will find this stage very frustrating. During the moisture evaporation stage, instead of producing heat, heat is absorbed. Water trapped inside of the wood is turned into steam and removed from the wood. Once this process is complete, it leads to Stage 2.

Stage 2: The Vaporization of Hydrocarbon Compounds

Stage 2 we still are not into the heat producing phase (maybe a tiny bit) but we are close! At this point, we are over five hundred degrees and temperatures are on the rise. The chemical structure of the wood starts to break down and the process of pyrolysis begins. Pyrolysis “liberates organic gases and leaves carbon rich charcoal”. This process also creates a mixture of hydrocarbons in the form of liquid tar droplets and combustibles gases, and digging in is very complex. At this point we have hydrocarbon vapors, carbon monoxide, methane, water vapor, carbon dioxide, and a nice mix of other vapors. This is a turning point as the temperatures continue to rise.

Stage 3: Gas Vapor Ignition and Combustion

Now that we have all of these gases being produced, all they need now is the minimum threshold temperature for combustion to occur. Side note, we are assuming all of he components of the fire triangle are present during this process. The NFI states that between 540 degrees and 1,225 degrees is where we finally have combustion! Carbon is the first to react with oxygen, producing potentially deadly carbon monoxide. Interestingly enough, more than half of the heat produced from the fire at this points is from burning gaseous hydrocarbons and carbon monoxide. For combustion to continue the temperature should remain typically over 1,100 degrees but can reach 2,000 degrees! Ironically enough, at this stage of combustion, water is produced. Hydrogen and oxygen molecules combine and large amounts of water vapor is found in the flue gases.

Stage 4: Char Burning

The first three processes have left carbon in the charcoal as the only remaining combustible material. a temperature above 950 degrees is needed to burn this carbon but it can burn with little or no flame at all. You will notice the sound and heat at the tail end of a fire. It is actually carbon burning in the charcoal!

In summary, the burning process is complex and different logs are in different stages during a fire. Care must be used to ensure that your fireplace is operating safely and carbon monoxide detectors should be in every room. Also don’t forget fire extinguishers. Happy burning!