There is nothing I enjoy more than freshly baked bread from a cob oven. This seemingly anachronistic activity – burning wood to cook – is what defines us as humans. And reenacting this ancient routine is deeply grounding and satisfying. One of the things I enjoy most about it is that burning wood carries with it a certain honest because unlike fossil fuels, whose origin is anything but recognizable, when you burn a log there’s no hiding that what you are burning has come from a tree. But for those who assert that wood is a “dirty” fuel, let us consider what happens when you burn it.
Generally speaking, trees produce roughly the same amount of oxygen when growing as their wood consumes in rotting or burning. Burning wood produces carbon dioxide but the amount is no greater that has been absorbed by the tree when it was alive. Of course, there are other things going on.
In Heating Your Home with Wood(Soderstrom, 1978) wood burning is explained:
As the wood’s surface temperature approaches 212oF (100oC), the water in the wood begins to boil and evaporates as steam…Beyond 212oF to about 540oF, other gases and liquids are produced, [including] carbon dioxide, carbon monoxide, and the acetic and formic acids that are abundant in creosote…A new phase begins once the outside heat source has raised the wood’s surface above 540oF and continues to heat it toward 900oF. Now the reactions begin to produce their own heat and to generate gases, including methane (the basis of natural gas) and methanol (wood alcohol) as well as more acids, water vapor, and carbon dioxide. Tar droplets emerge and are borne upward by the gases. As these gases emerge from the wood, they mix with air and ignite at temperatures of about 1100oF. Once ignited, these gases burn at about 2000oF.
From this we see that much is going on when a piece of wood is burned. Smoke – a clear sign of incomplete combustion and something that we are all familiar with – is the result of hot combustible gases, tar, and carbon particles not mixing enough or at high enough temperatures to ignite. Too much smoke, as one could guess, contributes to poor air quality. But by following a few simple guidelines these issues can be minimized, thus allowing us to participate responsibly in this age-old ritual.
This list, though perhaps not exhaustive, provides the basis of a responsible wood burning ethic:
- Only burn as much as you need
- Make sure the wood is dry
- Be sure the fire has sufficient oxygen (air flow)
- Use the right size of wood (kindling should be small (a finger width at biggest) and only after you have a decent fire, burning of the gases of the wood (not paper), should you add incrementally larger pieces of wood).
- Be respectful of your neighbours (suggestion: invite them for pizza or bake them a loaf of bread)
An ethic of responsible wood use, combined with a strong ethic of planting trees, makes wood one of the most responsible fuel sources available to us today. Wood not only provides generous heat but the trees from which wood originates temper global climate, stabilize soil, sequester carbon and have the ability to provide local communities with economic durability. They are also, perhaps, our greatest chance at attaining a fuel democracy, especially in a society as addicted to fossil fuels as ours.