How Does Wood Burn in a Stove

With an understanding of these basic characteristics of heat, we now turn to the phenomenon of combustion and observe what happens when we ignite a log of wood. It is important to understand combustion because the many types of woodstoves are made to take advan­tage of the various processes which occur in an actively burning fire.

Vaporization of Water in the Wood

The first stage in the combustion of wood is the vaporization of moisture (water). Moisture content usually represents the ratio of the weight of contained water to the weight of dry wood. "Dry wood" is con­sidered absolutely dry when all water that is not chemi­cally bonded to other elements in the wood is removed. This can be done by drying wood in an oven at the boiling point of water (100 degrees C or 212 degrees F). A freshly felled cypress tree in a swamp in South Caro­lina may contain 200 pounds of water for each 100 pounds of dry wood. It is thus described as having 200 percent water. More frequently the water content of sap-wood in freshly felled timber amounts to about 100 per­cent, with the heartwood containing much less. Wood allowed to dry ("season") in the open air will generally dry until the moisture content equals that of the atmo­sphere. Brought indoors, the wood may dry further until it reaches 6 or 7 percent water. Even well-seasoned wood will contain as much as 20 percent water.¹

As the wood becomes hotter, it loses more and more of its moisture and begins to give off a complex mixture of gases and tar-forming vapors called volatiles, which immediately burst into flame. The ignition temperature of these gases can be anywhere between 800 and 1200 degrees F, but is usually around 1100 degrees F. It has been estimated by scientists that about 40 to 60 percent of the heat-value of wood comes from the burning of these gases.

Charcoal Stage

While the gases continue to flame, the wood goes through complex chemical changes until, as the flames subside, the wood turns into charcoal. These red-hot coals burn with hardly any flame because the volatiles have been mostly burned up. Charcoal, however, needs large quantities of air in order to keep burning, and the coals are so hot that the mass of air is nearly always heated enough to help burn the carbon monoxide gas produced by the coals. This gas turns into carbon diox-ide, the same gas we exhale from our lungs and drink in carbonated beverages.

The charcoal stage of the burning cycle lasts longer in hardwoods like oak and hickory than when relatively soft woods such as white pine and spruce are burned. Soft woods, that is, woods of lesser density, will pro­duce longer flames and burn more fiercely in the begin­ning stages of the fire. It has been discovered, however, that the flaming stage, even in hardwood fires, can be prolonged by igniting the fire with a more intense heat-source and by allowing the fire access to much more oxygen. The charcoal stage will then be correspondingly briefer.

Modern Stoves

Designers and users of wood-burning stoves must be aware of these processes if they are to obtain maximum heat from the fuelwood. For example, it is known that in an open fireplace, most of the gases given off in the first stage of combustion are cooled by too much oxy­gen; therefore, they do not ignite completely and are lost up the chimney. In the older-model box stoves, the flame-path to the flue is so direct that much of the gas passes up the flue before it can mix with enough oxygen to burn more vigorously. These gases must be heated enough to ignite and to be maintained in flame both by the surrounding flames and an adequate source of secondary (preferably preheated) air.

1. See Donald George Coleman, Encyclopedia Britannica, 1967, s. v. "Wood."

Modern stoves deal with this problem in a variety of ways (to be dis­cussed in chapter 2) and with varying degrees of success (Fig. 5).

Fig. 5. Most gases are burned in a modern, airtight stove, leav­ing a light smoke, or no smoke, coming from chimney.

Charcoals likewise need high temperatures to continue burning. A contiguous cold surface or open space can absorb enough radiation from a charcoal particle so that combustion ceases. Therefore, a properly designed stove will maintain a high enough temperature in the combus­tion chamber to keep the charcoals burning and to mix the incoming air with the combustion gases.²

2. See Bernard Lewis and Guenther von Elbe, Combustion, Flames, and Explosions of Gases (New York: Academic Press, 1951).

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