Note: This program first aired Saturday January 11, 2014.
If you are one of the 12 to 14 % of Mainers like me, who use wood as our primary or only heating source, you, like me, have been spending a great deal of time lately, attending to your wood stove. The recent ice storm and subsequent power outages, as well as the recent arctic cold have gotten me thinking about fire and our reliance on it. When the high for the day doesn’t get above zero, you know very concretely just how much you depend on that iron box of glowing embers in the middle of your house.
The reaction that causes fire is a combustion reaction, the rapid oxidation of a fuel (rapid being the key word here, oxidation can also happen slowly, like rust). In this case the fuel is wood; chemically wood is a carbohydrate, made of carbon and hydrogen and oxygen, and contains lots of other elements that aren’t part of the combustion reaction. When combustion happens, the bonds between the carbon and the hydrogen atoms break apart due to the input of heat, breaking bonds always takes or absorbs energy. In the course of the reaction, those atoms rearrange themselves into different compositions, and to do so they need to form new bonds. The formation of a bond, any bond, releases energy. Fire is hot, or exothermic, because the new bonds that form in the products release more energy than it took to break the old bonds in the reactants. The bonds in the reactants are in the wood. The bonds that form in the products are those in carbon dioxide, water, and other byproducts of fire. The difference in the bond energies of the players involved gives us a reaction with a temperature we can feel. This time of year, we only want exothermic reactions.
This is all well and good, but when I sit in front of the woodstove and watch the fire, I want to know, what is fire? What are flames? What is smoke? When you first light your fire, you need a spark, some initial heat to get the reaction going. What that initial heat is doing is vaporizing the wood. When wood is heated to approximately 300 F, volitaile organic gasses or VOCs are evolved, or released. Pure carbon, carbon dioxide and carbon monoxide can also be released at this time. Initially, these gasses and particles aren’t burning. They are what we call smoke. Smoke is primarily unburned VOCs and carbon particles. And the VOCs? Chemically, if you do the math and balance your equation, what you end up with as the formula for the VOCs is formeldahyde. Smoke, it turns out, is nasty stuff, and that nastiness is why the people who design, build and sell woodstoves put so much energy into clean burning technology. The next thing that should happen in your fire building progression is flame. Flame is the smoke, the VOC’s catching on fire, combusting. That is a good thing, it means the formeldahyde goes away, and leaves us in the best scenario, with carbon dioxide and water. When burning wood, complete combustion is not totally realistic, so unburned carbon, carbon monoxide, and even a few unburned VOCs can persist. It really depends on how much oxygen can reach the fire, and how hot the fire can burn. The hotter the fire, the more fully evolved the gasses, the easier they burn.
Additionally, when the VOCs are evaporated out of the wood, pure carbon, and ash are left behind. Once all the VOCs are gone, all you have left is char or charcoal. These are the glowing embers left behind when the fire burns down. They are still burning, but it is the pure carbon that is burning, so there is no gas left to evolve and make a flame.
Lastly, when the fire is out, all that are left are bits of unburned charcoal, and ash. Ash is everything in the wood that can’t burn, all the mineral components of the tree. There is still potential heat in the charcoal, but the ash is at the end of its role as part of wood, completely liberated from the biological structure through chemical emancipation. When you empty the ashes from your stove, you are closing the last of the great winter circles. The carbon, hydrogen and water have gone up the chimney. The minerals, the bones if you will, of the tree, you empty into a metal bucket and spread on the garden, the driveway, the icy path. You didn’t know you were running a tree crematorium did you? So scatter those ashes with respect and dignity, and gratitude for the trees of the future, the trees yet to come you are fertilizing.
References:
Nice level headed info from the “How Stuff Works” website: http://science.howstuffworks.com/environmental/energy/question43.htm
http://science.howstuffworks.com/environmental/earth/geophysics/fire.htm
Wow, there are a lot of great videos on science on Youtube. Here’s one: http://www.youtube.com/watch?v=1pfqIcSydgE
An amusing blog by Cecil Adams, a person who attempts to answer any and all questions: http://www.straightdope.com/columns/read/2425/what-exactly-is-fire
Bangor Daily article citing census data on number of homes in Maine with wood as primary heat: http://bangordailynews.com/2012/01/22/news/state/wood-heat-heats-up-as-homeowners-give-boot-to-oil/ (newer? numbers cited in a Portland Press Herald opinion piece put the number at 14%)
