CarbonZero Experimental Biochar Kiln
Introduction
We've constructed a simple closed retort kiln using an insulated firebrick enclosure designed for a 200 liter dry-goods steel barrel with a clamp-on cover as a retort. The barrel is filled with split wood or other biomass feedstock, covered, and heated from below with a separate wood fire until it reaches pyrolysis temperatures (over about 320 C). If the moisture content of the wood is low, very little if any additional heat is needed once pyrolysis begins, as the reaction gives off heat.
The advantages to the approach are several. One is that we can control to a significant degree the temperature of the pyrolysis reaction because of the closed retort. You cannot do that in a system where combustion and pyrolysis are combined in the same vessel. Another is that the firebrick enclosure retains and refracts a significant amount of heat, enabling the use of much less wood to drive the reaction. Ideally, several batches per day are run so that the batches that follow the first need even less fuel, as the thermal mass of the firebrick retains significant heat.
A small hole in the center of barrel cover, about 12 mm in diameter, vents the evolving offgases to an afterburner positioned above the barrel. Depending again on the moisture content of the feedstock, a small support flame may be needed to keep the gases ignited. A generous supply of air to the afterburner is key. A squirrel cage blower mounted so that it injects a tangential stream of air into the top of the afterburner would be ideal, as well as a stainless steel mesh or firebrick plate positioned about 3/4 of the way up the afterburner barrel to deflect the gas stream so it mixes well with the air and keep it ignited. However, a simple afterburner without the blower works relatively well.
Thermocouple probes extend into the retort at top and bottom to monitor the temperature. We purchased standard bimetal thermocouple wire, stripped the insulation about 1 cm and twist the ends together, and fed it into protective metal tubing that penetrates through a small holes drilled in the side of the barrel. There are cheap electrical meters available that have settings to measure temperature from a thermocouple, so you may also need to purchase an appropriate plug. Polarity is important when wiring the plug.
An image and description of the type of barrel recommended for the retort is here.
Operation
Operation of the kiln is relatively simple. Insert the barrel / retort into the fire brick enclosure. Fill it with dry wood or other dry biomass. Insert the temperature probes into the barrel so you can monitor the progress of the reaction. Cover it and place the afterburner on top, ready to go. Place a few sticks of wood under the retort and set them alight.
It can take as little as 15 minutes to bring temperatures up above 300 C to initiate pyrolysis if the feedstock is bone dry, and as much as 2 hours or more if it is somewhat moist. At some point, the heat given off by the thermal reaction occurring inside the retort will be enough to sustain the process, and you can let the flames die out underneath the barrel. Again, this is highly dependent on the moisture content of the feedstock. Use the temperature probes to guide you. If you see the temperature rapidly falling below 320 C or so, add more wood to the fire under the barrel. If you see the temperature holds above 320 C, likely in a range between 360 and 440 C, then the reaction has become self-sustaining and you don't need more wood.
Once temperatures in the retort approach the pyrolysis zone, a significant amount of smoke will begin to emanate from the hole in the center of the cover. Light the afterburner, as indicated below, and keep the flame maintained. When the afterburner flame goes out on it's own and very little smoke remains, the batch is done.
Carefully remove the barrel if you are going to immediately run another batch. Keep your nose and face away in case the char lights on fire if you remove the cover. 2 people and some sturdy heat resistant gloves may be necessary. A bit of engineering resourcefulness and some handles that catch to the outside of the barrel on some latch or hook welded to the outside would be better. And either quench and/or cover the barrel so the char doesn't burst into flames.
A word of warning. It is possible to get burned doing this if you are not careful, so think things through carefully and make sure you are prepared for each step before proceeding to make biochar in this, or in fact any way.
Construction
Construction began with the inner firebrick wall. If I had to do this again, I'd pour a level concrete pad to start with. It makes the walls much easier to construct.
The inner wall at full height. I used high temperature silicone to hold it together, and then mortared the gaps using a fire cement. The opening left in the bottom is to allow wood to be placed under the retort to heat it.
Here we begin with the outer wall using ordinary construction brick. The thin steel plate allows us to extend the opening through the outer wall. The inner wall is now mortared with fire cement. Ordinary mortar is used for the outer wall.
A close up view of the opening. That's again high temperature silicone sealing the plate to the inner wall.
Here's a view down into the top of the enclosure that shows the triangular steel frame used to support the barrel about 20cm from the base, to provide space for the fire that will heat the retort. The pipe work was meant to redirect the offgases under the retort to be burnt, but this didn't work well. It turned out that the amount of heat available was MUCH more than needed. (The frame and barrel were both severely distorted from the heat and pressure that built up in an out of control feedback loop that developed when we tried this approach. The interior of the barrel went over 1000 C, well over our target range of about 400 - 450 C to produce low temperature biochar.) Since the equipment to regulate the gas flow would be expensive and complex, we dropped the idea to try and redirect the gases under the retort. It's too easy to cause a feedback loop that drives ever more gas out of the char as the temperature increases if you have no means to control the gas flow under the barrel, in this configuration at least.
Here the outer wall is at full height. The gap between the outer and inner wall is about 10 cm at the most narrow point. If a top plate is added to cover the gap, it should allow for liberal expansion and contraction.
Outer wall at full height.
We added perlite as an insulation layer between the inner and outer walls. It may be just as good to use one layer of regular sized fire bricks instead of this 2 layer approach. The pipework to recirculate the syngas to be burned under the reactor turned out to be a bad idea. When I tried it, temperatures in the retort became WAY too hot, over 1100 C. My temperature probe melted and the barrel blew up in a slow motion explosion that created quite a fireball. Lesson learned - if the barrel is within refractive brick or plate, by the time syngas is available, you don't need it - the reaction will give off enough heat to be self-sustaining.
Here's a photo of one of the first firings. You can see the thermocouple wire and meter in the foreground.
I placed 2 ~8 cm diameter brass tubes in the outer wall before I mortared the gaps, positioned so they would intersect with points 10 cm above the bottom and 10 cm below the top of the barrel. The thermocouple wire is pushed into a smaller diameter tube, and then this tube is pushed through the larger 8 cm tube in the outer wall and into a 6mm hole positioned "just right" in the barrel.
If I had to do this again I would place the bottom 8 mm tube very close to the front opening so it is easier to align the 6 mm tube carrying the thermocouple wire with the hole in the barrel and push it in.
The small diameter tube protects the thermocouple wire in the space between the inner wall and the barrel where flames lick up the sides from the fire at the bottom. This jury rigged system might certainly be improved somehow, but its cheap and it works.
Here's a view of through the opening at the wood fire under the barrel. If the material in the barrel is bone dry, it takes surprisingly little time and wood to bring the retort up to the pyrolysis zone, in as little as 15 minutes, and then the heat given off by the pyrolysis reaction proceeding in the barrel, and retained by the firebrick enclosure, is enough to sustain the reaction. You can let the fire go out under the barrel.
However, if the feedstock is wet, it can take several hours of heating from the bottom before the reaction begins. In this case, it may be best to maintain a small fire underneath to drive off the moisture first, which is necessarily a slow process, and then kick the fire up after some time to initiate pyrolysis.
The "Afterburner"
The advantage to the approach outlined above to making biochar in a closed retort heated from without is that you can fairly closely control the temperature at which it is produced. Low temperature biochar leaves many hydrogen atoms behind in the char. This may be advantageous because they remain available to become the docking points where OH and COOH functional groups can form, which is the basis of how cation exchange capacity functions in both biochar and humic substances and, it is believed, one of the main drivers behind the fertility increases seen with biochar.
The higher the temperature, the more hydrogen is driven out of the char, the lower the eventual cation exchange capacity potential.
However, the consequence of producing biochar in this way is that the unburned off-gases emitted from the retort during pyrolysis need to be dealt with in some way. As proponents of biochar, we need to manage the off-gases responsibly. We also have a further motivation beyond proper stewardship of the environment. Breathing this thick, acrid smoke is unhealthy and uncomfortable.
This image shows our kiln with the afterburner removed for a moment.
The pictures and descriptions below demonstrated how the kiln is loaded with feedstock, closed with a simple flat steel plate, and how our afterburner is constructed and used. It can be a little difficult to ignite and maintain the flame if the feedstock is relatively wet, but persistence and a little experience will eventually pay off. One key seems to be to ensure an adequate supply of air into the afterburner.
Here's an image showing the retort loaded with wood, ready to be covered.
The retort is then covered with a steel plate. A plate about 8 mm thick is best to prevent it from warping. If it warps, off-gases will leak from the sides. A much better alternative, I found, is to use a dry goods or open head barrel with a clamp-on lid. The time spent trying to find one of those is well worth it.
A 12 mm hole is then drilled in the center of the lid (not shown in the image above) to allow the off-gases to escape.
Fire bricks positioned to elevate the afterburner, an attempt to allow more air to circulate inside. Be careful not to use ordinary bricks where it can get hot. They can and do explode.
The afterburner placed on top of the fire bricks. The bottom (as orientated in the photo - it was actually the top) is cut out. I drilled the holes using a sheet metal step drill. A short section of stove pipe is welded to the top, with matching hole cut out, and extended with a 2 meter section.
The long stove pipe is to give the afterburner plenty of draft.
The afterburner in action. It makes an impressive, audible roar - very satisfying.
This photo was taken immediately after the one above. There is no visible smoke.
If the afterburner starts to smoke, it probably indicates that it's not getting enough air, or that the air and off-gases are not mixing enough.
During this burn, I used the glowing end of the sticks of wood you see in the photo, inserted under the afterburner, to help maintain the flame. That worked surprisingly well. To start the afterburner, I used some fire starting cubes. A small propane torch might come in handy for this purpose.
As you can see in the above photo, the afterburner gets red hot at the top and may soon burn out. I will probably replace this with a 200 liter barrel sized afterburner and force feed air into it, tangentially and near the top so it swirls inside, with a squirrel cage blower.
I knocked the afterburner off the retort by mistake as it was getting dark, trying to position it to provide more air intake. The feedstock inside the retort was bone dry, and I had 2 fire bricks positioned side by side next to the outlet hole, so the flame remained lit. Lots of energy here, even when producing low temperature biochar.
A Better Afterburner - And a Final Note of Caution
The afterburner shown above could be replaced with a larger one of a different design that utilizes a tangentially orientated air stream injected near the top of the afterburner with a blower, so it swirls inside and thereby mixes the off-gases and air. Either a stainless steel mesh or fire brick plate would be located about 3/4 of the way up inside the afterburner to serve as a deflection point that helps disperse the stream of gas coming from the retort and a heat sink that keeps the flame lit.
A better, safer design would also ensure that the afterburner does not fall over, from a gust of wind for instance. It gets very hot, and even with heavy welders gloves, it's too hot to handle. If it is not secured and it starts to fall over, let it fall. Don't try to catch it!
The flare that this small reactor produces packs an impressive amount of energy and heat. Be careful when firing the reactor, especially when the raw syngas starts evolving and you need to light and manage the flare. Plan ahead for the materials you will need to start and maintain the flare. I found that a long stick of wood worked well. Start the end of the stick on fire under the kiln, and then place the glowing ember end near the gas outlet. Have fire starter cubes ready, and a pair of long metal tongs, to start the flare, or use a propane torch. Use only fire-brick on top of the cover to support the afterburner - common brick can and will explode from the heat.
Rather than placing the flare on top of firebrick supports, a better solution would be to construct a metal frame that allows more airflow into the afterburner from underneath, one that would secure the afterburner to the brick enclosure. Remember that it can be a little difficult to light and/or maintain the flare in the beginning because the raw syngas still contains significant moisture. If it doesn't light, proceed carefully to try again every few minutes until it does.
Commercial Scale Biochar Production
There are much more efficient ways of making biochar in quantity when sufficient investment capital, and of course feedstock, is available. If you are interested in a solution to produce agricultural quality biochar in larger quantities, we have a full range of possibilities on offer on this page.