A Guide to Making Biochar

Biochar has been touted as a silver bullet soil amendment and climate change solution that adds nutrients to soil while also sequestering carbon. Biochar is created by burning biomass in a high temperature, low oxygen environment, through a process known as pyrolysis. 

On Sunday, April 27, 2025 Wisconsin Farmers Union's South Central, Dane, and Kenosha-Jefferson-Racine-Walworth County Chapters co-hosted a special program and demonstration on biochar. The event was hosted at Seven Acre Dairy in Paoli, Wisconsin and a featured speaker from the Wisconsin Department of Natural Resources (DNR).

The DNR's Brian Zweifel discussed biochar and how it can be used as a soil amendment for farms or garden soil. The event also featured a demonstration of a Ring of Fire biochar kiln. 

The design for the kiln demonstrated at the event was inspired by Kelpie Wilson’s Ring of Fire Flame Cap Kiln design with modifications by Philip Rutter, Mark Hamman, and Alex Tanke. The group put together a guide for setting up a kiln, loading, and snuffing to help others looking to make their own biochar.

Step 1: Set Up Your Kiln

Obtain a Steel Culvert: The kiln demonstrated on April 27 was created using a 5ft diameter by 4ft tall aluminum plated steel culvert. So far, the aluminum plating on the kiln is still present on the inside and outside after many firings. Zinc plated (galvanized) steel is not recommended due to lead contaminants and potential health hazards. Don’t opt for anything taller than 4 feet.

Seal the Kiln: Four foot cut sections of steel were purchased from Metal Culverts Inc. Use vice grips to straighten any ends bent during transport. Next, seal both sides of the cracks very well. The group used Kraken Bond Fireplace Sealant and this was critical. Apply prior to first firing otherwise it may not adhere well to the dirty sections. Harden via firing the kiln; fire before rain washes the un-cured sealant away. The group used $10 worth of sealant per kiln.

Seal the lid with 20 feet of fireplace gasket rope.

Kiln Lid: The group used a 14 gauge steel lid that was 64 inches in diameter. Be sure the lid is at least four inches larger than the culvert diameter. Culverts are not exactly round and the lid won’t seal if you make it smaller. Seal the lid with 20 feet of fireplace gasket rope that’s ¾ of an inch in diameter. Fiberglass rope is cheap and more stretchy, making it much easier to use. Ceramic is more expensive but less irritating to the skin. Bend a small nail or two around the ends of the rope so that they do not fray. If using fiberglass, handle with gloves only. Use a six inch spring clamp for pinching the rope to seal. 

The total cost per kiln with snuffing systems is about $1,000 and it makes over $500 of biochar per firing. 

Other Considerations: 

• A shovel and heavy duty rake should always be on hand. 

• Be sure to have welding gloves. They will keep your hands safe when handling flaming logs. 

• You may want a P100 Respirator to minimize smoke inhalation. 

Step 2: Load and Light the Kiln

Kiln should be placed where the bottom can seal into the ground.

Weather conditions: Choose a day to make your biochar when the wind is less than 10 miles per hour (ideally less than 5 miles per hour). First, roll your kiln and lid to your site. Make sure you have water on hand and that your brush is already cut. Ideally the brush has rested for a bit to dry it out. Expect to need 3-4 large brush piles to fill a kiln. If the brush is dry, well prepped, and in high density, you should be able to run 1.5 kilns/person per day. One person can easily bundle, haul, and load one kiln.

Seal the kiln: Place your kiln in a place where the bottom can seal into the ground. It is harder to get a good seal in turf, but easy in a forest setting. If on turf, really chop up the dirt or till with a tiller finely before piling onto the bottom of the kiln to seal.

You'll need 3-4 large brush piles to fill a kiln.

Lighting the kiln: 

1. To light the kiln, start a small brush pile (about 5 feet in diameter) on fire on relatively even ground. Don't use long thick materials otherwise sealing the kiln to the ground can be an issue. If the brush pile is hard to start, the brush won’t make biochar in a timely and efficient way. Let your brush rest and burn on a day when the brush isn't too soggy.

2. Once coals have formed, tip the kiln over the burning brush pile. Make sure that the top corrugation of the culvert points inward for snuffing purposes.

3. Seal the bottom edge of the kiln with a shovel and rake. The harder the ground is frozen, the more helpful it is to burn a larger starter pile to thaw the ground for easier sealing. Make sure there aren’t too many twigs in the sealing mud to make the seal better. Immediately after sealing, start loading the brush from the top.

4. Brush is best burned by bridging the brush across the top of the kiln. When the brush is turned to char it crumbles and falls into the kiln suffocating itself. Continue until full. Regularly add dirt to the base seal and make sure no tunnels have formed. Check the kiln about every 30 minutes, and more frequently at the beginning. It will take 3-6 hours to fill the kiln, depending on materials, load rate, and moisture content. A faster burn may mean you’ve reduced your char quality.

5. Let the flames get smaller before partially covering the kiln with the lid. Keep in mind that once you cover the kiln, pyrolysis pretty much stops because there is very little heat being generated. You can choose when to fully seal.

6. Optional step if you have a lot of thick, 2-4 inch diameter wood:

7. Prop the edges with some sticks or dig out small tunnels frequently around the kiln so air can flow through small gaps. You can hear the air rushing like a jet engine. 

8. Fill the kiln with all of your thick chunky wood very quickly. Note how full the kiln is once you are done loading. Ideally use thicker wood between 1-2ft length to reduce air space but keep good airflow. Wait for the volume inside the kiln to reduce to half then seal the bottom. All the thick wood should be char now and doesn’t need more oxygen. 

Step 3: Snuffing

Use a clamp to pull a good seal between the lid and the culvert.

To snuff out the kiln, cover it with the lid completely. Let rest for a little to let the smoke slow down. During this time, add more mud to the bottom seal. Place logs on the lid all around to put pressure down. Use your gasket rope and clamp to pull a good seal between the lid and the culvert in the concave section of the corrugation as seen in the photos. Tighten, twist the rope around itself and clamp to hold. There should be no smoke coming from the kiln after 3 or so hours of well sealed snuffing. If snuffing for longer than overnight, check all parts of the surface of the kiln for hot spots. 

Step 4: Quenching

After snuffing out the kiln, the final step is to quench the biochar. The amount of water required to quench and wet the biochar varies based on how long you snuff. Making the char moist is important for accidental burning safety and for reducing dust that gets into your eyes and lungs. Keep your char wet prior to spreading in the final location. 

• Expect to need about 130 gallons of water with no extended snuff time. No rope or fireplace sealant is required for an immediate snuff.

• Expect to need 70 gallons of water with an overnight 12 hour snuff.

• Expect to need no water to quench the biochar after 4 days snuffing.

• Expect to need 30 gallons to wet the char for dust reduction and ash washing.

Keep rakes and shovels on hand.

To quench the biochar, dump two-thirds of the expected water use on the top of the uncovered kiln. Tip over the kiln and roll it to the side, then tip it back up to get the rest of the biochar out. Use a shovel to turn over the pile and get all the chunks out. If they do not crumble in the hand, the quality is low and these should be discarded or burned in the next firing. These chunks are not as easily quenched and tend to be the culprit for smokers and burning charcoal in the pile. Make sure the char is wetted all throughout; wet any ashy or dry looking spots. The shovel also makes the biochar pieces smaller. The wet biochar will freeze so if you plan to move it while it is solidly frozen, you may need to use a small tiller to break it up.

One kiln makes about 2.7 cubic yards of biochar.

One kiln mostly filled makes about 2.7 cubic yards or about 600 lbs of dry biochar. Your kiln of biochar is equal to 1 ton of CO2 sequestered and worth about $500 before delivery in the current bulk biochar market.