Burning Wood

As the wood stove continues its comeback, many who had never given the matter much thought are now becoming interested in how to burn wood efficiently.

Stan Parkerson and Kent Severns point out in the Fall 1979 issue of The Northern Engineer (a quarterly publication of the Geophysical Institute) that, surprisingly, our earliest fuel is one of the most complicated.

Wood burns in three stages. First, the water which it contains must be evaporated. Because certain kinds of wood may contain as much as 230 gallons of water per cord (a cord is the equivalent of a four-foot high stack of 4'x8' plywood), it is easy to understand the importance of air-drying stove wood for a year or more before it is used. Otherwise, a large percentage of the energy that could be used for heat goes into turning all that water into steam. A further detrimental effect of burning unseasoned wood is that it results in a higher rate of creosote buildup in the chimney. This is not because green wood contains more creosote-forming materials, but that it produces a cooler fire. This permits the excess moisture, along with the dissolved volatile gases and tars, to condense more readily on the inner walls of the chimney. The mixture then runs back down the chimney until the moisture is evaporated again, leaving behind solid, flammable creosote which creates a chimney fire hazard.

The second burning stage is when the volatile gases are distilled out of the wood and ignited. This stage produces approximately half the total energy available in wood.

The third and final phase of combustion is the charcoal stage containing the remaining half of the heat energy in wood. This stage is clean-burning and the most efficient. Wood, therefore, burns as two types of fuels--gas and solid. Each type has different burning characteristics, making it difficult to maintain controlled and efficient combustion in even well-designed stoves.

Highly efficient airtight stoves are a must for anyone with a serious intent to heat with wood. Open fireplaces or barrel stoves are nice for effect, but they are washouts when it comes to fuel efficiency. Unless one has an unlimited supply of wood on hand, they are not a good solution to the heating problem.

Stove efficiency is a combination of two factors--heat transfer and combustion efficiency. Most airtight stoves are reasonably effective in transferring heat, so if the overall efficiency is to be improved, it is necessary to get more complete combustion of the volatile gases.

There are a number of ways in which airtight stoves currently on the market achieve this. One is to preheat the air before it enters the firebox by circulating it around the outside walls first. Another is to provide an additional oxygen supply for secondary combustion at a point higher in the system than the primary combustion chamber.

Other approaches, such as the use of baffle systems to control air flow over and around the coals, all perform virtually the same function--that of promoting secondary combustion of the hot gases. In a way, this is similar to the function performed by that of an afterburner in a jet aircraft. The principal difference is that with a jet engine additional fuel must be added to the hot exhaust. With a good stove, all that need be added is oxygen. This is one reason why good ventilation is so important to houses using this type of heating.