High Efficiency Hearth

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Wood Burning Combustion

WOOD BURNING COMBUSTION

Stage one combustion

The first stage of combustion is the heating and evaporating stage. Initially, heat is brought into contact with a piece of wood in the presence of air. Heat causes several reactions.

First, it raises the temperature of an area on the wood surface to some depth into the wood. As the wood’s surface temperature approaches 212 degrees Fahrenheit, the water in the wood begins to boil, then evaporates. As long as the water remains in the wood, its boiling and evaporation robbing heat energy from the source, thereby keeping the wood cells from gaining more heat. Moisture must be driven off before combustion can begin, so wood with a high moisture content is hard to ignite.

Unlike moisture, volatile gases are combustible. They burn and release heat. As the wood surface temperature rises beyond 212° F to about 450° F, major gases abundant in creosote are produced: carbon dioxide, carbon monoxide and acetic and formic acids. However, the gases generated in the first stage of combustion do not ignite until the moisture evaporates and the kindling temperature is hot enough.

Stage two combustion

After moisture is driven from the wood and the heat raises the temperature of the wood above 540° F, the second stage of combustion takes place. This is the heat-producing stage. It occurs at two different temperature levels: primary and secondary combustion.

The process by which gases are released from wood and burned is called primary combustion. Primary combustion begins at about 540° F, continues toward 900° F and results in the release of a large amount of energy. Primary combustion also releases large amounts of unburned combustible gases, including methane and methanol as well as more acid, water vapor and carbon dioxides.

These gases, called secondary gases, contain up to 60 percent of the potential heat in the wood. Their combustion is important to achieve high overall combustion efficiency. The secondary gases are not burned near the wood because of lack of oxygen (oxygen is being consumed by primary combustion) or insufficient temperature.

The conditions needed to burn secondary gases are sufficient oxygen and temperatures of at least 1100° F. The air supply is critical. Too little air will not support combustion and too much will cool the temperature to a point where combustion cannot occur.

Remember that air is about 80 percent inert gas and, when introduced into a wood stove, is well below the 1100° F needed to sustain secondary combustion. The more air that mixes with the secondary gases, the greater the quantity of heat absorbed by the nitrogen, and the lower the temperature of the secondary gas-air mixture.

Secondary combustion can and does occur in wood burning stoves that are designed to meet or exceed the EPA’s requirements for clean air, but only if the stove is used with seasoned wood, operated in a manor consistent with it’s design and is connected to a properly functioning chimney system.

Many people do not realize that the chimney is the engine that drives the stove (or fireplace) and that if the chimney is not up to par (sized correctly, have adequate height, or does not hold enough heat) then the draft will be inadequate and the best stove in the world will be a disappointment at best, and possibly even a danger at worst. 

Stage three combustion

During wood burning, after the gases are driven from the wood, the carbon chains of cellulose and lignin molecules remain. Carbon, or charcoal, burns a long time with a low rate of heat output. Charcoal burning is important for two reasons. Additional energy is released, which is important to overall combustion efficiency.

Also, charcoal burns at a low rate of combustion, which means that a good charcoal bed will burn a long time, allowing a fire to last the night. The fire can be rekindled by adding wood and opening the draft to supply new oxygen.

Summary

Choosing the proper wood stove is important to wood burn more efficiently. However, proper operation of your stove is also critical. Understanding combustion principles and learning to manipulate the various conditions enables you to achieve maximum comfort and efficiency from your wood heating system.

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