Fire Physics

The chemistry of wood burning is a complicated subject in itself, but in general the hydrocarbon molecules of the wood fibers combine with oxygen from the available combustion air. The hydrogen atoms combine with oxygen to produce water vapor. The carbon atoms combine with the oxygen to form carbon monoxide and/or eventually carbon dioxide. See other entries for description of the carbon reactions.

In any case, all three of these chemical reactions are exothermic and so give off more energy (heat and light) than they use. This is why burning hydrocarbon fuels (wood, coal, oil, gas, coke, peat, etc) gives off heat.

Wood contains about 6000 usable Btu/pound of latent heat. (See "Amount of heat in wood" #311). This energy can only be liberated at an interface between wood and air, i.e. at the surface of the wood.

If ten pounds of wood is a single log (about 6" diameter and 20" long), its surface area is about 3 square feet. If the same wood is split in tiny pieces, it could have twenty times the surface area. The second situation would burn (oxidize) at potentially twenty times the rate, and therefore could produce twenty times the heat output (for one twentieth as long). It would not actually burn twenty times as fast because oxygen molecules would not always be where they needed to be at the right time, but fifteen times as fast is realistic.

There are some secondary considerations. Because fifteen times the oxygen is necessary, then the fire-induced draft will have fifteen times the velocity. This rapid draft shortens the time that oxygen molecules are near the complex hydrocarbon molecules.

Some of the volatile gases released from the wood may not have enough time to completely oxidize, so partially oxidized hydrocarbon gases can escape the fire vicinity. In general, these gases are called creosote.

The term creosote is somewhat generic. It refers to about 160 complex hydrocarbon chemicals which are gaseous at elevated temperatures and solid at room temperature. If air supply is restricted, as in an air-tight stove, the problem is exascerbated, and huge amounts of creosote can be released which represents much pollution and wasted fuel energy.

There is a simplification concept which says that three T's are necessary for adequate combustion. Time, Temperature, and Turbulence.

Assuming adequate air supply, if there is high enough Temperature (over 1100°F) for all the necessary reactions to occur; adequate Time to allow those reaction sequences to occur; and enough Turbulence to ensure oxygen availability when and where necessary; then proper and complete combustion will occur. Virtually no wasted fuel and no pollution will be sent out the chimney.

Air-tight stoves usually fulfill these requirement, but they do not allow enough oxygen to burn completely. Therefore, they later have to attempt secondary combustion with either a catalytic combustor or by introducing secondary combustion air.