*Wood Burning 101*
20 September 2009
This article’s main goal is to describe the stages of combustion for burning wood. This is the foundation of knowledge required for more advanced discussions regarding fireplaces and wood stoves, particularly the modern, high-efficiency appliances available on the market today. The bulk of this information comes from my training with the Hearth Education Foundation (www.hpbef.org)... primarily the manual for Woodburning Appliance Certification. Many parts are verbatim, since the concise and precise wording they provide is difficult to improve upon.
As wood burns, there are four stages through which the wood passes on its journey from log to ash. These four stages of wood combustion are: 1) Moisture Evaporation, 2) Vaporization of Hydrocarbon Compounds, 3) Gas Vapor Ignition and Combustion, and 4) Char Burning. These stages occur in sequence for each tiny segment of the wood fuel, even though different segments of the fuel may be going through these stages beginning at different times and progressing with different rates.
Before wood fuel can burn, the embedded water must be removed through evaporation so as to leave the remaining wood dry enough for combustion. In order for water to turn from a liquid to a gas, the water molecules must absorb heat and rise in temperature. This stage of combustion absorbs heat from the fire, and does not give off heat to the system.
VAPORIZATION OF HYDROCARBON COMPOUNDS:
As temperatures rise, the chemical structure of the wood molecules begins to break down, and hydrocarbons begin to vaporize. This chemical process is known as Pyrolysis. Pyrolysis produces a complex mixture of hydrocarbons in the form of a fog of tar droplets and other combustible gasses. As this stage begins, heat is still being absorbed in order to drive this chemical process. As the temperatures increase, these combustible compounds begin to also combust, and we move toward the next stage of combustion.
GAS VAPOR IGNITION AND COMBUSTION:
The temperature required for combustion of the combustible gasses is higher than that for the solid wood. These threshold temperatures vary, but occur between about 540 degrees and 1225 degrees, provided that adequate oxygen is available. More than half of the available heat in wood comes from the burning of gaseous hydrocarbons and carbon monoxide. As the wood burns, carbon reacts with oxygen to first form carbon monoxide, which is itself a combustible gas. If sufficient oxygen and heat are present, carbon monoxide with react with the oxygen to form carbon dioxide. Complete combustion of wood produces almost exclusively 1) water vapor, 2) carbon dioxide, 3) heat, and 4) noncombustible ashes. The less complete the combustion is, the more carbon monoxide, combustible hydrocarbons, and other gasses are left unburned. Thorough combustion thus depends on combustible gasses remaining in the combustion zone long enough to reach ignition temperatures. As these gasses burn, internal firebox temperatures can reach up to 2000 degrees.
Charred wood must come into direct contact with oxygen before it will burn. This occurs only after moisture vaporization, Pyrolysis, and the release of gasses have subsided. Basically the carbon in the charcoal is the only remaining combustible material. Charcoal burns with little or no flame, producing temperatures in excess of 1100 degrees.
Obtaining the most complete combustion possible requires the stages of combustion to proceed as smoothly as possible. Each stage of combustion requires the necessary supply of fuel, heat, and oxygen. If a stove or fireplace is not operating properly, it is because one of these elements is being negatively affected. Now that you have the foundation of combustion basics under your belt, you are more prepared to move forward with the next article: Woodburning 102.
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