This was a post from Kelpie Wilson, one of the world's foremost biochar experts. I subscribe to her blog (you can too!). She wanted to make sure that people are making good burn piles so they get lots of carbon in their biochar and not so much smoke or ash.
John S
PDX OR
New Biochar Burn Pile Guidelines
Make biochar, not smoke
Kelpie Wilson
Nov 18
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Every molecule of carbon that is retained in biochar is a molecule that does not form unhealthy particulates in the air or carbon dioxide to add to the overload in the atmosphere. Everywhere I look around here in the autumn, people are burning their accumulated piles of yard debris in just about the most polluting way possible.
I put together this biochar burn pile guideline with the hope that people will use it to improve their practices. We are burning anyway, so why not do it better?
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Here’s what usually happens: people clear around their homes and chuck the brush, fallen branches in the yard, old lumber, and raked up leaves all into one pile. Nervously, they wait for the moment when enough rain has fallen so it is safe to torch the mess and get rid of the flammable liability.
Typically, folks light the pile near the bottom, with their understanding that “heat rises,” so you need to put the fire under the pile to get it going. There are two problems with this. First, we need to understand that heat does not rise. Hot air rises due to the buoyancy effect (hot air expands and is lighter than the surrounding cold air), but the heat itself is only transferred by radiation and conduction.
The reason that this is important is because when you heat wood from below, the material starts to emit flammable gasses like hydrogen, methane and carbon monoxide. As these gasses escape the pile, they cool and condense into particulates. In other words, smoke. If, on the other hand, there is a flame already going on top of the pile, that flame will burn most of the gasses before they can condense into smoke. The emissions from the flame are mostly just carbon dioxide, an unavoidable product of combustion that is invisible and odorless and not a respiratory hazard.
Let’s light our piles closer to the top so the flame burns up the smoke!
The second problem with a smokey pile that is lit near the bottom, is that without a strong flame on top, you lose a lot of your heat. The pile, especially if it is poorly built, will sit there and slowly smolder all day long, filling the neighborhood with smoke. Flame is hot and smoke is not. So the burn pile takes a lot longer to complete. People get bored standing around in the smoke waiting for it to finish so they go into the house to do the dishes and that’s when a breeze picks up and carries an ember into the dry grass and the fire department gets a call.
A hot, flaming burn pile is a safer burn pile because you are vigilant, standing by with a hose. It won’t take all day and you can be done in a couple of hours before you get too tired of it all. If you put out the fire at the end with water, you also get biochar for your trouble.
However, you won’t achieve this goal with a sloppy, jumbled up pile full of dirt and leaves that cut off the air and make smoke. You need to build your pile strategically. First, separate out the big logs. Do you really need to burn them? They could be firewood or they could be cut to short lengths and left in contact with the ground to rot and provide habitat for forest fungi and creatures. Or use them in a hugelkultur garden bed to hold moisture.
Then make a dense pile of the smaller stuff. It needs to be dense so that the flame can progress downward and light each layer in turn. Leave the leaves out of it! Leaves are wonderful compost for your garden, so if you need to rake them up, make piles and let them molder into microbe-rich leaf mold. Or use them as winter mulch on your garden beds.
Finally, lay the bigger logs or boards around the inner pile in a teepee formation. This helps concentrate the heat for a good, fast burn and keeps the pile from falling apart as it burns down.
A rick is another option that is especially good if you have a lot of old lumber. We made this pile of densely stacked boards that I thought would not burn well because it was too dense. But I made sure that there was at least one inch of space between each board and it burned, quick and clean, making a beautiful pile of char that we put out with water at the end. I have a big magnet for picking up the nails. Easy peasy!
I will be sharing the guideline at the top at a workshop this week at the South Slough National Estuarine Research Reserve - Biochar as a Tool for Forest Management. We will give a hands-on demonstration of several biochar techniques, including the Ring of Fire Biochar Kiln, a Burn Boss air curtain burner with Valley Environmental, and the Biochar Burn Pile.
It’s exciting for me to be back at the South Slough Reserve because their mission is ecological restoration. They will have an opportunity to use the biochar they make in various ecological restoration projects.
In my opinion, we have a solid set of techniques for making biochar on site in forests and other natural areas. What we most need now is more experimental work on applications to demonstrate the benefits of using biochar to repair and restore our injured ecosystems.
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Food forest in a suburban location. Grows fruit, vegetables, herbs, and mushrooms. Forages for food and medicine. Teaches people how to grow food. Shares plants and knowledge with students at schools.
