The “Conservation” burn: An easy way to transform slash to biochar using the top-lit up-draft approach

Richard Freeman, Designed Ecosystems

This charring technique is a relatively easy and quick way to char a large amount of forest residue wood, also known as “slash.” It combines roughly sorting the wood by diameter, stacking and charring with a top-lit up-draft approach (TLUD).

With the TLUD approach, the heat radiating from the flame burns off the volatile gases released from heating the wood directly below the flame. This flame consumes most of the oxygen on the flame-front so that the top char does not burn to ash. As the flame moves downward it ignites the progressively smaller wood, which reduces to char more quickly due to the larger surface area to volume ratio.

For this char, I used the largest diameter wood for bottom rails. Then, perpendicular to the rails, I stacked the smallest diameter wood as parallel as feasible. Next I stacked the largest-diameter wood on top on the smaller wood.

Sometimes — but not this time — I will top off the pile with a layer of kindling-sized wood, perpendicular to the pile.

Next, I ignited the pile from the top using a propane-fueled weed torch.

Within ten minutes, all the top fuel was involved in the flame. The temperature on the flame front was 600-800°C (roughly 1100-1500° F.)

Within another ten minutes, the flame had nearly subsided, leaving only hot char with a coating of ash (mostly calcium carbonate).

When the flame fully subsided, I mixed and spread the hot char using a rake and pitchfork, briefly reigniting a small flame.

When the flame subsided, I doused the pile with water. Unfortunately, I did not take a photo of the remaining char, but this pile yielded about 2/3 cubic yard.

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