Most small scale methods of producing biochar, such as the Kon-Tiki kiln, fire pits, traditional heaps, many barrel methods, rocket stoves, gasifiers, etc, can only function at high pyrolysis temperatures, and particularly in the case of the heap method, long process times. Biomass particles are exposed to temperatures of about 1000° C when the pyrolysis method used allows air to circulate in the bed of evolving char. The thermo-decomposition from these methods is much too aggressive to produce a biochar that is ideal to enhance soils. Instead, such char will remain a relatively inert bystander to the microbiochemical processes of soil fertility.
Research has shown that high temperature biochar that is composted with nutrient-rich organic matter develops an organic coating, and it is the organic coating that contributes to soil fertility, not the biochar itself. However, as that thin coating decomposes, the fertility effect should diminish, which is not ideal. If what we are looking for is a "coral reef" for decomposing organic matter, then perlite might provide a similar functionality at a lower cost.
As described on our Rationale page, a gentle pyrolysis in the range of 450-500° C that retains as much oxygen and hydrogen as possible in the evolving biochar will optimize its ability to participate in the soil microbiochemical ecosystem. Described here is a simple, affordable kiln we have developed that will produce biochar at low temperatures.
The kiln features an insulated firebrick enclosure designed to fit a 200 liter (55 gallon) dry-goods steel barrel with a clamp-on cover as a retort. The barrel is filled with split wood or other large particle biomass feedstock, covered, and heated from below with a separate wood fire until it reaches pyrolysis temperatures (over some 300° C). Once pyrolysis begins, the startup fire is allowed to die out. In our experience, pyrolysis temperature within the kiln self-regulates to about 470° C, +/- 10°.
As the biomass in the kiln reaches pyrolysis temperatures, free oxygen and carbon atoms are released in the decomposition process. As they recombine to form either CO or CO2 on their way out of the biomass, energy is released, heating the particle from within and others nearby in a uniformly distributed way. Since there is a limited amount of oxygen within biomass, the exothermic heat released is limited. But if the kiln is well insulated, and the feedstock is dry and of relatively uniform particle size, this heat alone is enough to reliably sustain low temperature pyrolysis in a small batch kiln.
A small hole in the center of barrel cover, about 8 mm in diameter, vents the evolving offgases to an "afterburner" positioned above the barrel. Depending on the moisture content of the feedstock, a small support flame may be needed to keep the burnable gases ignited. A generous supply of air to the afterburner is needed. A squirrel cage blower mounted so that it injects a tangential stream of air into the top of the afterburner is an optional enhancement, as well as a stainless steel mesh or firebrick plate positioned about 3/4 of the way up the afterburner barrel to deflect and heat the gas stream so it mixes well with the air and keeps the flame ignited. However, a simple afterburner without a blower or mesh works relatively well.
Thermocouple probes extend into the retort at top and bottom to monitor the temperature. To keep costs minimal, we purchased standard bimetal thermocouple wire, stripped the insulation about 1 cm and twisted the ends together. Small diameter metal tubes were placed through the brick enclosure, and corresponding holes drilled in the barrel retort to allow these tubes to be pushed into the barrel. The twisted end of the bimetal wire was then inserted through the protective metal tubing until it protruded into the barrel to monitor process temperature.
Biochar produced at high temperatures will eventually weather and decompose. It will oxidize and may aggregate with surrounding decomposed biomass to form a char with OH functional groups on its surfaces that enhances cation exchange capacity, the ability of a soil to retain and release nutrients to plant roots. But this process will likely take a very long time, centuries.
First, choose a barrel to use for the retort. A retort in a pyrolysis kiln encloses the feedstock in a heated, oxygen deprived environment. For this kiln, we used a 200 liter (55 gallon) "open head steel drum", which has a cover that can be opened and closed with a relatively tight seal using a steel clamp. One of the benefits of this type of barrel is that the seal between cover and barrel is sufficiently maintained during heating.
However, any cylindrical steel container could be used, as long as it can be easily opened for loading and unloading, and closed with a relatively tight seal. The objective is to prevent pyrolysis gases from leaking out the side of the cover, and air getting into the retort. A steel sheet placed over an open top steel container might also be used as a cover, but note that some thicknesses may warp when heated, even when weighted down.
Another detail to keep in mind when choosing a container for the retort is that ia finished batch will need to be lifted out of its insulated enclosure. A single man will be able to lift a 200 liter barrel with biochar out of the enclosure. A significantly larger retort may require a crane or winch to remove and empty.
Another important consideration is to locate the insulated enclosure for the retort in an open area outside, away from anything that could catch on fire. Another reason it should be outside is because unburned pyrolysis gases, particularly carbon monoxide and carbon dioxide, are very hazardous if they accumulate in a closed space. They can easily be fatal to humans or animals, particularly as loss of consciousness can occur before one has time to react.
As mentioned, we placed 2 ~8 mm diameter tubes through both walls, 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 6 mm hole positioned to allow this smaller tube into the barrel. To accurately locate these holes in the barrel, make a long centerpunch from a metal rod that fits snugly in the 8 mm tube. Place the barrel in position on the stand in the enclosure, and then use the long centerpunch you have made to mark where the thermocouple holes should be drilled in the barrel.
Note that the bottom 8 mm tube should be located very close to the front opening of the enclosure. In this kiln, it would be better if it was moved one outer brick to the left. This positioning will make it easier to reach in through the opening to align the 6 mm tube carrying the thermocouple wire with the hole in the barrel and push it in before a new batch is loaded in the kiln. The top thermocouple can be easily accessed from above.
The small diameter tube is meant to protect the thermocouple wire in the space between the inner wall and the barrel where flames and heat run up the sides from the fire at the bottom. If your thermocouple already has a protective layer, then the inner tube might not be necessaary on your kiln.
The gas emitted from a pyrolysis kiln operating at temperatures from about 400° C and up resembles a heavy grey smoke. This gas is full of burnable carbon monoxide (CO), methane (CH4), hydrogen (H2), and vaporized tars. They should be flared to eliminate harmful emissions. Hence we have fabricated a simple burner for this kiln made of a smaller barrel and a stovepipe.
The burner shown in these photos was made using a used closed head barrel that contained motor oil. We cut the top (that contained the threaded taps) out of it, turned it over, and cut a hole in the center matching the diameter of our stovepipe, and then welded a short section of that in place. Then we drilled a number of large diameter holes with a hole saw around the periphery as air inlets. An additional 2 meter section of stovepipe fit onto the welded short section completed our initial burner.
While the long stovepipe provides plenty of draft that draws air into the burner, one drawback to this design is its instability. Adding 3 or 4 angled legs bolted to the barrel that signficantly widen the base would make this burner safer. On a windy day, it could blow over. (Let it fall if this happens! It's much too hot to touch.) In use, we found it better to set the burner on firebricks to provide more air flow and access to start the flare. Metal legs could be positioned to also create a gap between the burner and retort cover of about 8 to 10 cm. Supporting legs on the burner would be a significant improvement to our intial design, providing both stability and air flow, as well as easier access to start and maintain the flare.
Several methods can be used to help maintain a pyrolysis gas flame. One would be to purchase a stainless steel mesh, for instance chicken fencing is available in 304 stainless, form it into a mass about the diameter of the burner, and affix it to the top of the burner with a few small bolts, or a threaded rod through the whole burner. The hot wire mesh will tend to continually relight the flame. You can also fix a piece of firebrick in the middle of the burner for the same purpose.
To help maintain the flame, we also found it helpful to position 2 firebricks on either side of the 8 mm pyrolysis gas exit hole in the retort cover. These bricks will become hot and help to keep the flame lit as the gas flows past them.
Alternative designs for the burner might be 2 small barrels cut open top and bottom, joined together end to end and appropriately stablized, or a single large barrel with ample flame maintenance apparatus within it. Unless you are an experienced welder, the stovepipe can be difficult to weld in place because of the thinness of the steel.
Operation of the kiln is relatively simple. Insert the barrel retort into the center of the fire brick enclosure, lining up the thermocouple holes with the tubes through the enclosure. Insert the temperature probes into the retort so you can monitor the progress of the reaction. Fill it with dry, cut and split wood that is a relatively uniform thickness so each piece will be heated to the core uniformly. A batch kiln like this will only work with relatively large feedstock pieces that allow hot gases to circulate between them.
The next step is to light a fire under the retort to dry to wood completely and initiate pyrolysis.
If the biomass in the barrel is bone dry, it takes surprisingly little time, and fuel wood, to bring the retort up to pyrolysis temperature. It can take as little as 15 minutes. Once you see that pyrolysis temperatures have stablized above 300° C and are rapidly climbing toward 400° C, the fire under the barrel can be allowed to die out, and the opening covered to prevent a draft of cold air entering the enclosure.
If the feedstock is wet, however, it can take several hours of heating from the bottom before the reaction begins. Wood will absorb moisture from the air, so generally speaking it may take an hour or more drive that moisture out of the wood. A clear indicator that signficant moisture remains in the wood is if the temperature does not rise above 100° C. In this case, it may be best to save some wood and maintain a relatively gentle fire underneath the kiln. Drying biomass is inherently a slow process, so a hot fire won't help much. Once the temperature begins to rise steadily above 100° C, the feedstock is dry enough to bring it up to pyrolysis. Build the intensity of the fire up with more wood until the temperature is rising steadily past 300° C towards 400° C. Then you can stop adding wood to the fire, and cover the entrance to the enclosure to prevent a draft around the retort.
As the gas emitted from the retort changes color from white, mostly water vapor, to yellow, you can begin to try to light the burner, but you may find it difficult as there may still be substantial water vapor present. Yellowish smoke should be emitted at temperatures somewhere in the range of about 120-300° C particle temperature. The pieces of wood in the kiln will be at a variety of temperatures at they heat past 100° C. The pieces at the top will heat more quickly than those toward the bottom. The outer layers will heat faster than the inner layers. So during this phase, there is mix of condensible and flammable gases plus water vapor. It won't burn that well, but if you keep trying every 5 minutes or so, at a certain point you will hear a vvvummp! and there will be a fire roaring in the burner.
To light the burner, light a fire starting cube, place it on the retort cover, and push it toward the gas exit hole. Put some heavy gloves on to protect your hands from being burned. Place some small pieces of wood on the starting cube and get those burning under the burner next to the gas exit. You will need a pair of tongs for this stage. Keep feeding that small fire next to the gas exit. If there is wind, stack a few firebricks around the burner to block it. Do not use ordinary bricks around or under the burner, or on the retort cover, because they can and will explode if heated. The shrapnel can penetrate tissue anywhere on your body (how do I know that?).
Take a long stick and put the end of it in the fire you've built. Get the end glowing and keep feeding into the flame. When the gas lights on fire, keep the glowing end of this stick next to the gas exit to maintain the flame.
If the gas coming out of the retort changes to a grey color, it is definitely burnable at this point.
If you have room under the burner, you can position 2 firebricks on edge near the gas exit, in a V shape. The gas exit should be in the middle of the V. Once the flare is lit and stable, you can carefully move these firebricks to be parallel, on either side of the hole, with a 1 cm gap between them. They will quickly become hot and help maintain the flare, replacing the glowing ember.
Prioritize your safety while operating a do-it-yourself pyrolysis kiln. Here are some tips to help.
Pyrolysis will tend to begin at the top of the retort, because heat rises, and spread down through the batch. As the pyrolysis front is moving downward through the batch in general, for each piece of wood it starts at the outer layer and penetrates inward.
As mentioned before, when pyrolysis temperatures are first achieved and burnable gases begin to be emitted, only a portion of the wood is at that temperature. But some of the wood, particularly towards the core of the pieces, may still be drying. So at first the flare will need support to remain lit. But over time, the flare will become increasingly stable, and particularly if the gas contacts some firebrick or stainless wire mesh on its way into the afterburner, it probably will remain burning on its own.
If you have built the kiln as instructed with an insulating layer, used pieces of wood with a relatively uniform thickness, and closed off the fire opening in the enclosure once pyrolysis begins, the reaction will spread uniformly through the entire batch until it is all pyrolyzed, and then, rather suddenly, the flare will go out. You won't be able to light it again if you try, because there is not enough burnable gas left to support it. That said, if the flare goes out and you see a heavy grey smokey gas pouring out, the batch isn't finished yet. Relight the flare. With a bit of experience, you will know when the batch is finished.
When the batch is finished, the simplest thing to do is push a firebrick over the gas exit hole, open the fire opening to allow air to circulate past the retort, and leave the batch to cool for some hours. Make sure the the temperature inside the kiln is well below 350° C before opening the cover and pulling the retort out of the enclosure, or the char will immediately burst into flames. If you can let it cool overnight, you will avoid any risk of being burned.
However, if you want to quench it with water, prepare in advance a spray nozzle on a metal pipe or tube that is long enough to span the top of the kiln to reach the gas exit hole and spray inside of it while still allowing steam to escape. Devise a way to fix the metal tube in place so it will not move. The nozzle should inject a rather fine spray into the hole rather than a blast of water. You don't want to fill the retort with much water, or you won't be able to lift it out once it is cool! Nearly all of the water sprayed into the retort should flash to hot steam and carry the heat out of the hole without condensing.
When you are ready to quench the batch, fix the metal tube in place connected to a hose and then GET OUT OF THE WAY before turning the water on. Why? Because super heated steam can cause severe burns, and it will escape the retort under pressure. Experiment with your quenching setup from many meters away until you know how it behaves. Keep in mind that water flashing to steam will expand significantly, so turn the water on slowly, let spray for a some minutes. Turn it off, and then check the temperature. It probably will take much longer than you think to quench a batch.
Char will auto-ignite without a spark or flame at temperatures above 350° C, so the strategy is to quench with a light spray until the temperature stabilizes well below that point (without allowing much water to collect in the retort), and then you can safely open the cover, pull the retort out, and transfer the char to another closed metal container to finish cooling. A second hand 200 liter barrel with a steel sheet cover is sufficent.
If you are quenching to do multiple batches per day, you will need a way to get the hot retort out of the enclosure without burning yourself, such as a stationary crane. Consider fabricating several retorts. Use a stationary crane to pull a finished batch out hot, and set it aside to cool (or quench). Preload the retorts with wood outside of the enclosure and use the crane to immediately place the next batch in the enclosure.
Note that biochar can spontaneously ignite when it is freshly made and is exposed to air. Do not put fresh batches in buildings, vehicles, or near anything that can catch fire. Dump trucks have been burned to the ground transporting fresh char.
So you've let your first batch cool overnight. You come back to your kiln first thing in the morning, open the cover, and there it is, low temperature biochar, perfect to form OH functional groups and fully particpate in the biochemical processes of soil fertility. Now what?
Here is a simple formula.
See our Biochar Preparation page for more information how to prepare biochar before adding it to your soil.
It has been demonstrated in a number of trials that adding biochar to soil immediately after it is made sometimes does not enhance the growth of plants and may even retard growth. The main reason is that certain types of biochar can rapidly adsorb nitrogen and other cations from the soil solution, creating a short term nutrient deficit for plants, particularly if the soil lacks sufficient nitrogen. Other reasons could be that the size of the char particles is too large, the char has a low cation exchange capacity ( either because it is high temperature or unoxidized ), or it is coated with tar residues. Some components of the tars that remain on freshly made biochar surfaces and within pores tend to repress plant growth.
The recommendation to compost biochar allows it become hydrophillic as bacteria consume residual tars, adsorb cations such as nitrogen, calcium and magnesium and also anions such as phosphorus, and aggregate with other soil particles. Composting the char will also accelerate the complete oxidation of biochar surfaces, which will increase the cation exchange capacity while charging the char with mineralized nutrients made available from the nutrient-rich biomass being composted with the char.