No Burn Scars From Char

Three months after a kiln burn at my place

At a recent meeting of my local fire safe council, someone warned against burning the soil when you’re making biochar. This is not a thing, I said, and I followed it up with this email.

Hi everyone,

I want to clarify a point that came up in the meeting for everyone's benefit. As the fire safe council for the region, we have to be informed about scientific facts. Biochar is new to many of us, so it's no surprise that not all of us have the full information.

There's not a problem with soil damage from making biochar. Burn piles cause burn scars; biochar piles do not.

A biochar pile is just a burn pile that's extinguished when the flames go out.

What's the difference to the soil? Temperature. Flames aren't hot enough or close enough to the ground to burn the soil. When you make biochar, you're putting out the fire when the flames go out, to obtain the charcoal. Flames keep most of the air from reaching the wood. If you don't put out the fire when the flames disappear, air has a chance to reach the solid residue (charcoal), and it burns to ash. Burning charcoal is what will burn your soil, not the flames.

It's helpful to consider that wood does not actually burn; it pyrolyzes, which has been defined as destructive distillation. The polymers in wood are broken apart by heat, releasing vapors that burn as flame and leaving behind relatively pure carbon with a different molecular structure (rings) that doesn't decompose. This different form of carbon is charcoal. (We use the term "biochar" for charcoal that we designate as a soil amendment and for other applications besides heating, but essentially biochar is charcoal.)

Kelpie Wilson has a slide in her presentation deck showing an aerial view of 50-year-old burn scars from incinerated slash piles in a logged area. The soil under the piles is still relatively sterile after 50 years, and from the air, the landscape appears polka-dotted. All the soil organic matter under those piles has been oxidized. Her next slide shows plants coming up through biochar, where the same type of material was extinguished with water and raked out when the flames disappeared.

Whether or not a kiln is used, the result is nearly the same; the differences are in the speed of production and the conversion efficiency from biomass to biochar. The kiln also makes the operation safer due to the heat shielding. (In both cases, the fire is lit on top, which drastically cuts down on smoke.) Kilns generate higher production temperatures than piles, which results in a superior biochar product. But quality and productivity are not important considerations when the purpose is fuel reduction.

Charcoal burns much hotter than the wood from which it is made. That is why for centuries it was used for blacksmithing. (Coal replaced it.)

I make biochar at my place up Seely Creek Road every wet season, and if you ask I will notify you so that you can come see it being done. Susan Nolan and I also occasionally present biochar science to the public for ISF. You can read about what I do at blackripple.com.

- Gray

Kelpie has a new book, The Biochar Handbook, with many of the photos that she uses in her slide deck. One of the cover images shows plants emerging from biochar in the woods.