Proper building of a fire key to eliminating particulates
With spring on the way, and outdoor burning on the horizon, I hereby implore all fire keepers to be mindful of some valuable principles. A fire only begins to burn clean when all of the gases are drawn into the flame path. Gases are pyrolized (released) from the wood at around 150 C. But the flash-point (ignition into flame) only occurs above 300 C. In between these temperatures the fire is hot enough to temporarily volatilize the cellulose and lignin materials, but when the gasses do not reach flash-point they condense into tar droplets. The extent to which this is happening is the extent to which the combustion process is smouldering and polluting the atmosphere with particulate matter. For people with compromized respiratory systems (like myself and my son) this is a dangerous situation.
Big sprawling slash piles DO NOT burn clean because the gases (being warmer than ambient air) want to wander straight upwards. The combustion process can be improved by building a tall narrow fire, which evokes a strong vertical draft up the middle that effectively draws air/smoke into the flame path from the perimeter. This horizontal air intake dynamic can be enhanced by stacking a few dozen bricks to make a fire ring about 2 foot tall and 2 or more feet in diameter. The air comes in through the small spaces between the bricks. Efficiency can be improved even further by cutting the ends off a 55 gallon drum and setting it on a brick tripod a couple of inches off the ground. Once you get the fire ramped up with dry wood, even the dampest slash will burn clean.
Yes, this takes some time setting up, and yes it requires tending/feeding every 15 minutes or so. But everything burns way faster, and at least you won’t be smoking your neighbours out. At home I keep a third of a barrel set up like this for cooking and gathering round. And when I want to burn off slash and yard cuttings I stack another third on top. Works like a charm.
For those who don’t see the point of transcending faulty burning habits, please consider what it might feel like to suffer from asthma. When I breathe in particulate matter from inefficient wood-fire my airway tightens and I feel like I am having to breathe through a pillow.
I love wood-fire. I build masonry heaters and cob ovens for a living. And I dare say that if we want to keep making fire then we had all best remember and remind each other how to do it well.