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Kon-Tiki - the democratization of biochar production

By Hans-Peter Schmidt and Paul Taylor

A simple but ingenious invention finally allows each farmer and gardener, everywhere in the world, to produce for themselves a sufficient quantity of high quality biochar. With reasonable investment and some know-how of the charmaker’s craft, farmers can produce in one afternoon a cubic meter of high quality biochar. This democratization of biochar production will be a key strategy to closing the agricultural production loop for small farmers.

Please find the citable PDF-version of the article here

In the next few decades, industrially produced biochar may become one of the key raw materials for the bio-based economy. Since the construction, electronics, paper making, waste water treatment, textile, 3D printing and other industries will all be competing for this biochar (see 55 uses of biochar), commercially-produced biochar will remain an expensive input for farmers to purchase.  Small farmers may find that weighing the cost of farm labor against the cost of commercial biochar comes out in favor of  making their own from accumulated farm, garden and household residues. This allows farmers to complete the resource loop on their own farms where biochar can enter the local use cascades (Schmidt, 2012; Shackley, 2014) and become the basis for the humus enrichment of soil.


Fig. 1. Biochar production in the Swiss Alps.

People of many ancient and preindustrial civilizations produced charcoal and biochar in such quantities that it even became one of the causes for the deforestation of whole regions and countries (Carlowitz, 2013; Fang and Xie, 1994; Willcox, 1974). The charcoal they produced was mainly used to melt ore, to work iron, to produce lime, to fire ceramics or simply for cooking and heating, however, a considerable part of the finer charcoal fraction was used in combination with organic wastes to improve soil fertility (Criscuoli et al., 2014; Glaser and Birk, 2012). If the ancient peoples were able to produce such large quantities of wood and biochar, how can it be that in this age of high technology, we have not been able to successfully  produce local, cost-effective biochar to be used on farm? How is it that the millions of euros and dollars spent on major biochar research projects have failed to develop a reliable and affordable pyrolysis system to give farms and communities access to biochar made from the residues they generate? How did our ancestors manage to produce, without chainsaws, steel, conveyor belts and electric motors, such substantial amounts of biochar that an average one-fifth of the humus content of the soils of the world is composed of biochar (Kluepfel et al., 2014; Rodionov et al., 2010; Schmidt and Noack, 2000)? Although the bulk of this biochar stems from natural causes, mostly forest and steppe fires, these fires were also the result, to some degree, of human agency (Gammage, 2012; Gerlach et al., 2012; Rodionov et al., 2010). There is no doubt, however, that within settlement areas where the proportion of biochar exceeds by a wide margin that found in other soils, the biochar was produced as a side effect of village fire management practices, and perhaps even deliberately produced and added to soil (Gerlach et al., 2012, 2006).


Learning from fire

Anyone who has ever tried to clear a hundred square meters of wild growing forest to make it plowable, even when chainsaw and backhoe are available, will see very quickly that fire helps.  For 25,000 years of human history, fire was the most ubiquitous and important means that every culture, people and clan had to carve out a place to live in nature. Only through fire was humankind able to develop the intellectual and physical advantages he had over the other animals to access resources and adapt the environment to his benefit (see also Richard Wrangham’s excellent book: “Catching Fire: How Cooking Made Us Human”).
Most folks who deal daily with fire, cook every meal, forge every tool and nail, burn lime, fire clay, warm themselves, and maintain their pastures and forests, learn how to light a fire that provides warmth without enveloping every house in the village with acrid smoke.  Contrary to what one intuitively believes, a smokeless fire is lit from above and not from below.


The analogy of the match

Although it seems counterintuitive  (see here the exciting field of intuitive physics, wood actually does not burn. Instead it is the gas emitted by heating the wood that burns. Only when the wood is finally charred under the flame of the woodgas, can oxygen penetrate the then porous structure of the newly charred wood and glow the carbon to ash.

Striking a match on the rough surface of the matchbox ignites a flaming chemical reaction of the sulfurous tip that generates enough heat to make the wood emit highly combustible gases. The flame ignites the gases so released from the wood and the process continues under the heat from the burning pyrolysis gases, causing further outgassing and burning of gas. But underneath the flame of the woodgas the wood itself does not burn but carbonizes, because the gas flame consumes all the oxygen, creating a pyrolysis zone where the flame protects the match from oxidation. As we know, the match burns with a clean flame until someone blows out the flame after which it will smoke. The smoke is just the last unburned and condensing residual wood vapors, released before the match cools sufficiently to stop outgassing them.


Fig. 2. The example of a match shows how the flame excludes oxygen allowing pyrolysis to take place  (image: Thomas Reed)

A smokeless fire can be made to work just like a match.  You light it from above, so that the fire in the uppermost layer heats the next lower layer, which consequently begins to outgas. The gas rises through the flame above, where it is burned. In contrast, when you light a fire from below, the heat will cause the wood layer above to outgas. Much of the ascending gas will escape the flame and condense in the cooler air. This is what we see as smoke.  Instead of burning completely, the bottom lit fire sends smoke out the chimney or into the house, or into the clothes, eyes and noses of those seated around the campfire.

If you layer a wood pile loosely, with enough small branches in the upper layer, and light it at the top, nearly all the resulting wood gas will pass through the overlying flame front and burn so there is only a clean, smoke free combustion gas. Radiant heat from the flame chars the wood beneath layer by layer. Air is drafted in from the sides of the pile, but is updrafted into the flame and consumed in combustion. Under the nearly oxygen-free fire front the char is mostly preserved. As the pyrolysis reduces the wood chunks to smaller pieces that pass down through the loose pile, fresh layers of wood are continually exposed to off-gassing heat below the fire front. By observing the flame and the onset of ash build up on the outer layers of the charred wood you can determine the right moment to quench with water or smother with dirt, and instead of producing ash alone, you may retain close to a fifth of the wood as charcoal, while utilizing your smokeless fire to cook or to warm yourself.


Fig. 3. These burn piles in the Oregon woods illustrate the difference between lighting a fire on the top (left) or lighting it near the bottom (right). Images: Kelpie Wilson.


From the basic principle of smokeless fire

The fundamental principle of the smokeless fire was the starting point for our design of the Kon-Tiki, an open-topped conical kiln for making biochar.  We chose the name Kon-Tiki in memory of Thor Heyerdahl, who asserted in the ‘40s of the last century that the inhabitants of South America were able to cross the Pacific to Polynesia in handmade boats. The experts virulently attacked Heyerdahl's theory until he finally silenced them by building such a boat with only the tools and materials of the South American natives, and crossing half the Pacific from Lima to Polynesia. He named his boat Kon-Tiki after the South American god of sun and fire.


Fig. 4. With this raft, built only with native South American materials and tools, Thor Heyerdahl crossed half the Pacific Ocean in 1946. He named it after the fire god, Kon-Tiki.

Our goal was quite similar, although not nearly as adventurous. We wanted to show how our ancestors were able to produce with simple means and without high technology, large quantities of biochar. Additionally, we sought a simple, inexpensive, easily adaptable technology for a Terra Preta project in Nepal, where the mountain farmers cannot possibly engage with an unaffordable, high-tech pyrolysis machine. If earlier peoples in South America, Australia, Scandinavia, Palestine, China, and actually almost everywhere, were able to produce and apply such quantities of biochar that their soils were partially blackened throughout, this must be achievable today in even the poorest tropical countries. We also hoped to develop a technology that would allow farmers and gardeners in rich countries to convert their own residues into biochar as an alternative to buying it from industrial manufacturers.


Fig. 5. Typical profile section of a soil west of Cologne. Among the superficial unconsolidated sediments is a black soil horizon containing deep black soil pits up to 2 m deep with high proportions of charcoal. (from: Gerlach et al 2012)

In our thinking, we began with the smokeless fire of the ancestors and combined this with the observations of archaeologists, namely that black soil deposits are often found in soil profiles as clearly demarcated cone pits with a upper diameter of about 2 m and a depth of 1.5 (Eckmeier et al., 2008; Gerlach et al., 2006). First, we suspected that these soil cones were simply rubbish pits, which, when they were filled, were burned from the top down, only to be replenished. In some instances, this will have been the case. But what if these man-sized pits were used as open pits for pyrolysis? We had to exercise now some experimental archaeology.


Open Earth Kiln

If you take care to build a strong initial bed of flaming embers at the bottom of the hole in the ground, gradually, layer by layer you can add combustible material such as wood, food scraps, bones, leaves and straw while maintaining a smoke-consuming fire front. The burning pyrolysis gas consumes most of the oxygen drawn into the pit by the flame and therefore protects the pyrolysis zone, while the earthen walls keep air out from the sides and below. The fire itself is so effective at excluding air that the underlying layers outgas and char instead of burning to ash. After a few hours, by the steady piling and outgassing of fresh biomass, one or more cubic meters of biochar accumulate that can then be quenched by water or by a 5-10 cm thick layer of soil, sand or manure.


Fig. 6. Production of biochar in a 90 cm deep pit with a stone rim (Images: Ithaka-Institute).

Inspired by Josiah Hunt, we tested the production of biochar in an open earth kiln. And it worked just great. In the first attempt, we made a few hundred liters of excellent looking biochar in a conically excavated hole in the ground. This success was reason enough to construct a more precise theory about the system and to consider how it could be implemented with better technology and control.


With the fire, not against the fire

An investigation amid the global biochar community showed rapidly that we were not alone on the road of this development. At her most valuable Backyard Biochar website Kelpie Wilson had already presented examples of the Japanese Moki-Kiln, the Australian Moxham Kiln, Kelpie Wilson's own Pyramid Kiln and new cone and pyramid designs by Michael Wittman, Gary Gilmore and others. With the exception of the Moxham all these kilns are comparatively small and more suitable for gardeners and hobbyists, but the principle is clear: produce biochar using the fire and not by suppressing it. We also took as inspiration the form of fire containers that were used throughout the Orient for the offering of religious sacrifices. Under the name of Agni Hotra, the Vedic fire ritual, they are still widely used today in India. The size of the Agni Hotra bowls is generally small, but for temple rituals, there were larger fire bowls made of copper. The dynamics of smokeless flames over the fire pits, dancing to the heavens, clearly showed that we were on the right track with the physics of fire.



Fig. 7. The Vedic Agni Hotra ritual (image: agnikultur.

Based on these principles, which may mark a U-turn of the modern direction of pyrolysis for farm scale biochar production, we were now close to developing an optimized Kon-Tiki kiln for the production of high-quality biochar in large quantities and at very low cost. The first principle of Kon-Tiki art is this: Use the pyrolysis gases as cover gas and thus create with the fire the air exclusion for pyrolysis.


Kon-Tiki Cone Kiln

Although the biochar quality from the first experiments with an excavated earth kiln looked pretty good, it was too inhomogeneous for standardized products. The open combustion of the pyrolysis gases was fairly clean, but not always stable, especially in gusts of wind, and we were not able to completely prevent the emergence of smoke. We had to get one step further to study the operating principles more precisely and to optimize the different parameters of the system. At this stage we designed and built the first 750 liter aboveground Kon-Tiki made out of steel.

 Figure 8. The first Kon-Tiki had a diameter of 150 cm, a height of 90 cm and a capacity of 850 liters. It was built by Markus Koller.

With an upper diameter of 1.50 m, a height of 0.90 m and a wall inclination of 63°, a steep cone shape was chosen so that the resulting biochar was well compacted and would make a consistent fire front at the surface for a reliable barrier to oxygen. Unlike the earthen walls in the earth kiln, the steel walls reflect the pyrolysis and combustion heat back into the kiln, resulting in a more uniform temperature distribution and thus ensuring more homogeneous charring conditions and resulting biochar quality. More importantly, the decisive criterion for the success of the new steel shape was the difference in combustion dynamics with the change from a sunken to an aboveground form. We found that the combustion air that is drawn down onto the burning surface is preheated as it rises along the hot outer wall of the kiln. Pre-heating the combustion air significantly reduces the cooling of the unburned gases, generating more stable combustion dynamics and greatly reducing smoke production.


Fig. 9. Air is drawn in over the hot outer wall of the kiln and swirls above the fuel bed creating a vortex that ensures good mixing of pyrolysis and combustion air, resulting in very low emissions of the Kon-Tiki kiln.

Once the kiln reaches its working temperature of 650°-700°C, hardly any smoke is visible. The combustion air rolls in over the metal edge of the outer wall and into the kiln. But at the same time, the burning gases must escape upwards and so,  similar to a clockwork, a counter-rotating vortex is established in the center of the kiln (see Fig. 4). Thanks to the establishment of this horizontal vortex, the air supply to the fire zone is stabilized. The wood gas, which is heavier than air, is kept in the vortex until it is completely burned. Thus, the second fundamental principle of Kon-Tiki craft is the development of a horizontal gas-air vortex, which provides a stable, smokeless combustion regime.


Optimize combustion by providing a rim-shield

To further optimize combustion dynamics of the Kon-Tiki, we added a thin metal rim-shield. This provides additional preheating of combustion air that rises between the inclined kiln walls and the steel outer screen. Since the screen extends almost ten cm above the edge of the kiln, it prevents cold combustion air from being drawn directly into the kiln and protects the combustion dynamics from any disturbing gusts of wind. Indeed, the denser cold air that slides from the outside of the screen onto the preheated air stream from the space between the kiln and screen is extraordinarily stabilizing and prevents smoke or even fire from breaking out laterally from the Kon-Tiki. Another advantage of the screen is that the kiln wall is not cooled by the external air, or even gusts of wind, thereby improving the power of the kiln walls to reflect heat back to the interior of the pyrolysis zone. It also protects the workers from possible burns as the screen never heats to more than 60°C.


 Fig. 10. Video showing the efficiency of the rim shield.

Drying and pyrolysis

As we had observed with the open earth kiln, the fire front at the surface quickly dries the biomass after it is laid down on the blaze. The massive heat released during pyrolysis is thus used as drying energy and wet biomass with a water content of over 50% can be carbonized. Once a high-energy fuel bed forms at the bottom of the Kon-Tiki, you can even pyrolyze freshly cut wood, leaves or cattle dung. The Kon-Tiki thus works both as a dryer and a pyrolyzer. Unlike most closed pyrolysis systems, this in itself is a major advantage.


Fig. 11. Once a strong ember bed brings the pyrolysis temperature up to 700 ° C, fresh wood can be added. The Kon-Tiki functions as both a feedstock dryer and a pyrolyzer.


Ignition and First Layer

In the first experiment of firing a deep Kon-Tiki we feared that we had made it too deep, because deep down in the Kon-Tiki steel container, the oxygen is used up very quickly. In fact, it was impossible at first, even with a strong igniter, to start a fire. After several attempts and considerations we found a highly effective ignition technique that we marvel at  anew every time we use it.

Start by building an open stacked square chimney of dry wood in the middle of the kiln and about three-quarters of the kiln height. This airy wood chimney is ignited at the top with some tinder. Once the top two rows of the fire are burning well, it creates a train that pulls air down the sidewalls of the kiln and back up through the middle of the wooden chimney. After about ten minutes, burning wood from the top of the chimney falls down the chimney and ignites the base. After another five minutes the entire burning “chimney” can be collapsed and spread evenly on the bottom of the kiln.


Fig. 12. igniting the wood chimney in the middle of the Kon-Tiki.

Another five to ten minutes later, a sufficiently hot bed of embers has been formed and the surface layer begins to be covered with white ash. This is the moment to add the first regular layer of biomass. Cover the zone of glowing coals evenly but not too thickly. Once this new biomass layer also becomes coated with white ash, this is the sign that the feedstock has solidly reached pyrolysis temperature and exothermic pyrolysis will continue even in the absence of flaming combustion. It is time now to add the next layer of biomass. This will maintain a powerful flame front above the pyrolyzing material to consume down-convecting oxygen while combusting the smoke, thus protecting the char. This process is repeated for all the subsequent layers every five to ten minutes until quenching. Consequently, working with the Kon-Tiki requires the constant presence of a person to add fresh biomass. If you wait too long, the char starts to oxidize, which reduces yield and increases the ash content of the biochar. Take care not to lay on too much, too fast as this will weaken the flame, reducing its ability to capture the fumes and allowing smoke to escape.


 Fig. 13. When the top layer of biomass begins to coat with ash, it is the right time to add the next fuel layer. The biomass becomes completely charred then in the lower layers.


Firing Duration

Compared to an automated installation, the disadvantage of the Kon-Tiki kiln is that it must be hand fed during the entire period of operation. Depending on the type, lumpiness and water content of the feedstock, it takes two to eight hours to produce roughly 1 cubic meter of biochar in the latest version of the Kon-Tiki kiln with side angles of 70° . If one uses dry wood chips, it only takes about two hours; undried prunings take four to five hours; green wood with logs, branches and leaves takes up to eight hours. Again, depending on the biomass, one person can operate two to four kilns in parallel. On a working day, a person can thus produce with two to four kilns between 1 and 1.5 tons of biochar, which corresponds approximately to the daily capacity (in 24-hour continuous operation) of a medium sized industrial pyrolysis plant.


Fig. 14. Mounting the rim shield to optimize combustion dynamics.

Another significant advantage of the Kon-Tiki is that the biomass does not need to be homogenized, chopped or even pelletized, but may simply be layered as coarse pieces up to 120 cm long. However, the charring time is considerably longer than with dry, small-sized biomasses. When using fresh twigs and branches, the capacity of the Kon-Tiki corresponds approximately to the amount of biomass that accumulates in eight hours of landscape maintenance or while cutting firewood. Instead of tossing the branches and brush unsuitable for firewood on a big pile that very slowly rots, or is burned to mostly ash in a smoky fire, they can be charred in the Kon-Tiki.



The Kon-Tiki should only be filled to a maximum of 10 cm below the top edge, otherwise the stable gas-air vortex will be disrupted and the charring of the upper layers will be uneven. As the Kon-Tiki becomes full, make sure the last two to three layers consist of only easily charred material such as thin branches or prunings, since larger pieces added in the final stages will either remain incompletely charred or will require too much time to burn, resulting in excessive ash production.

Quenching can take place either from the top or the bottom. We developed a method to quench from the bottom that works like this: About 20 minutes before the last layer is pyrolyzed, the water tap at the bottom of the Kon-Tiki is opened. Water flows slowly in from the bottom of the kiln. When the water meets the hot coals, it evaporates. The heated 600-700°C water vapor rises through the char bed, and not only makes for a slow quench, but partially activates the biochar at the same time. The hot steam serves to expel and react with condensates from the pores of the biochar. The biochar is thus cleaned, increasing the pore volume and the inner surfaces of the biochar. In this way, partially activated biochar is produced. The only specific surface area measurement taken so far was done for a Kon-Tiki biochar quenched from the top. This top-quenched biochar had a specific surface area of  289 m2 per gram. We hypothesize that vapor activation as described above will result in consistently higher surface areas. The fire in the uppermost layer of the Kon-Tiki is not snuffed by the steam, because the hot top layer of glowing coals, about 20 cm thick, floats on the rising water. Once you notice that the last coal layer begins to float, spray it with water from above to complete the quenching.


Fig. 15. Quenching from the top with water.

Alternatively, you can also completely douse the kiln from above, however, it would be to the detriment of the partial steam activation compared to the watering from below. The pore volume and the specific surface of the biochar would be smaller when doused from above. If you want to avoid wetting the char, so you can later use it, for example, as fuel charcoal, you can close the kiln either with an airtight lid or simply with a thick layer of dirt to snuff it out and allow it to completely cool. (Take care: this takes a long time and could lead to loss of char or fire if the lid distorts or the dirt leaks air.) The resulting “dry quenched” biochar is however much richer in condensates and also pollutants such as PAHs. For fuel charcoal, this may be good, since the condensates and pollutants burn well, but for biochar used as animal feed, certainly not. Our initial tests of using nutrient and mineral enriched water like liquid manure or liquid digestate for quenching are very promising for the production of carbon fertilizers or other enhanced biochars. However, this is a new field that needs further research.


Fig. 16. Crystal clear quench water is the best sign of clean pyrolysis.

The quench water can be left for a few hours or even days in the kiln. It drains out easily through the water tap at the bottom. The quench water looks clean and transparent, but it is soapy and has a very high pH. While the high pH is due to the approximately 10% ash which results from the open fire pyrolysis process, the soap is formed by the reaction of the ash with pyrolysis oils, which are expelled from the pores during quenching of the char. This soapy quench water is apparently excellent for pouring on fruit and vegetable plants. It discourages snails and fungus and generally acts as a tonic to the plants. The latter statement is based on personal observations of only two dozen plant species so far; systematic scientific investigations are still pending.



Biochar quenched with water generally fulfills all the requirements for the premium quality of the European biochar certificate (EBC). The open fire pyrolysis principle guarantees that the vast majority of the pyrolysis gas is expelled from the biochar and burned, not stuck on the biochar surfaces and pores in the form of toxic condensates (Bucheli et al., 2015). The biochar is additionally cleaned and partially activated when slow quenched with water from the bottom. Please find here an EBC analysis of a vine root biochar made with a Kon-Tiki.


Fig. 17. Beautiful open pore structure of the Kon-Tiki biochar. Image: Michael Hayes

The pyrolysis temperature in the Kon-Tiki is 650-700 °C with brief temperature peaks close to the flames going up to 750°-800°C. In this temperature range, the biomass, including its lignin, becomes completely charred. The result is a high-temperature biochar of high quality, which is particularly suitable for animal feed, as a litter additive, for manure treatment, for composting, for drinking water filtration, wastewater treatment and generally to bind toxins and volatile nutrients. The Kon-Tiki biochar is less useful for direct application to soil, since it might adsorb labile soil nutrients and bind plant-signaling chemicals. Be sure to enhance biochar from the Kon-Tiki with nutrients before using it as a soil conditioner biochar.


Gaps and Uncertainties

The development of the Kon-Tiki systems is still in it’s  early stages and needs more systematic research. For the moment, the quality assertions are based on only one complete EBC analysis plus some additional academic lab data of Kon-Tiki biochars. Initial emission testing indicates that clean combustion is feasible, although a systematic investigation of the influence of feedstock composition, degree of humidity, particle size and stronger winds has still to be done. Yield was measured at rates between 15 and 20 percent on a dry matter basis which correspond to other high temperature pyrolysis chars, however, the influence of feedstock composition, humidity, particle size and the art of the char maker’s craft will all impact yield.

We are currently investigating differences in specific surface area and volatile organic carbon content of biochars produced at different heights in the kiln. Biochar at the bottom of the kiln stays much longer in the pyrolysis zone than biochar from feedstock added towards the end of the firing. At the bottom of the kiln biochar cools slowly and vapor activation is much shorter and at lower temperatures than in the middle or the upper parts of the kiln which result in different activation levels and biochar characteristics. Systematic research of these variances in function of the charring position inside the kiln is needed.
These uncertainties are important and we are looking forward to initiate and do more advanced research about these questions. However, these uncertainties are congruent with most of the scientific uncertainties of more technical pyrolysis systems.



From the first attempt, the Kon-Tiki deep cone kiln worked better than we would have envisioned before we began. Nevertheless, everything was not as easy as it seems in retrospect. We spent many hours in the myriad attempts to optimize the shape and dimensions of the cone until the thermodynamic puzzles began to crack. What made it light work for us, however, was the magnificent pleasure it was to work directly with the fire. Each experiment lasted at least six, but usually eight to ten hours that we passed surrounded by the forest, on the terraces of the ancestors, facing the snow covered mountains. Sometimes we invited friends; often the children were there, who have long since become fire and biochar experts. It was the best summer of research. We filmed, photographed, carried out countless measurements and learned new measurement techniques.

Friendships have been strengthened by the fire while sharing the meals cooked on the Kon-Tiki. Just like our ancestors, we have experimented with the forces of primordial elements and discovered the awe of nature in a new way. In the age of high technology, we have put this hubris for a moment behind us. Like Thor Heyerdahl on his raft in the sea, we were shaking some foundations of the scientific and technical imagination. Just as the miracle of aging wine is based on the proper dose of air, that enemy of winemakers, so the quality of biochar, which could ultimately increase the fertility of our soils, is based on the proper dose of air and fire.

The first video that we released in July 2014 on YouTube (see fig. 8 above) has raced like the wind to reach many people around the world. It was clear from the beginning that we would make the design available as open source. Nevertheless, we have waited for the actual publication of this article on the Kon-Tiki craft, at the end of autumn 2014, to gather more knowledge about the principles of operation, to optimize the design and especially to gain more certainty about the quality of the biochar, the mass balances and emissions. However, based on just the preliminary videos and design documents we have made available, Kon-Tiki kilns have already been built in Australia, Ireland, Canada, California, England, Hungary, Switzerland and South Africa. More Kon-Tikis are under construction in Nepal, India, Indonesia, Hawaii, Germany and Malawi. In all the places where the Kon-Tiki produces biochar, enthusiasm is huge and to date no complications have arisen. On this basis, we expect that 2015 will usher in hundreds of Kon-Tikis all over the world pushing forward the democratization of biochar production.


Fig. 18. Frank Strie was the first outside of Switzerland to build a Kon-Tiki. In Tasmania, he built five of his Kon-Tiki "Tas" kilns, gaining valuable experience in the process and promoting the technology. Frank Strie was the first to test the char activating bottom quenching method.

Originally designed for agriculture in developing countries, it is more and more apparent that the farmers of Europe, Australia and America will also seize the chance to make their biochar themselves and use the Kon-Tiki to optimize their agricultural material cycles.

What we call Kon-Tiki is not a finite form but the technical realization of the open fire kiln principal. And this has many inventors and will result in many varying designs. All are invited to participate in this movement to reappropriate the craft of fire and biochar making.


Next development steps

With numerous partners in various countries, we are currently working on optimizing the geometry and thermodynamics of the Kon-Tiki. For a North American university, we have just developed a research Kon-Tiki, with which all parameters can be monitored and combustion can be measured and controlled by metering the air intake.  A giant Kon-Tiki was built to char large root wads with minimal size reduction for a composting facility. We also develop smaller sized Kon-Tikis for small gardeners who can use it to char their green residues and organic waste. The next technical development step will be the integration of heat recovery. One Kon-Tiki load produces more than 1 MWh heat, enough to heat a poorly insulated farmhouse for two weeks. These and other developments such as the automation of char removal will be the subject of future articles in tBJ.


Fig. 17: A master of the Kon-Tiki craft will learn to pyrolyze with fire, and like in this picture, with no visible emissions. The cigare of the char master makes more smoke than the Kon-Tiki, which produces 1 MWh of heat in an afternoon.

The genius of the Kon-Tiki is in the elegance of the simple form and the avoidance of expensive moving parts and controls. Thus, the Kon-Tiki is robust and inexpensive. However, larger scale commercial and industrial biochar production require elaborate automation to reduce labor. This becomes far more of the cost than the basic reactor vessel and here the automated, continuously operated plants may remain unsurpassed. But for small and medium-sized farmers, landscapers, small winemakers (using their grape prunings) and gardeners who occasionally want to create their own high-quality biochar, there is no alternative that will be more efficient, less expensive or as supremely beautiful.


Build or buy and contribute

(August 1st 2016): We have started collaborations with manufacturers in several parts of the world to bring the Kon-Tiki to market. If you prefer to build it yourself or to start the production in your region, we are willing to share the design and building instructions in exchange for a modest donation to help us building the knowledge and technology for a more carbon intelligent world.

As the Kon-Tiki kilns quickly caught fire around the world, we started to share pictures and stories from Kon-Tiki craftsmen and craftswomen from the Amazon to Zambia, from Switzerland to Nepal, and from Tasmania to California on our world map of Kon-Tiki that contains already more than 50 countries. Please participate and send us your Kon-Tiki photos and experiences to be included in the world map of Kon-Tiki.

The Ithaka Institute has funded the entire development of the Kon-Tiki from our own resources, which has brought us to the limit of our financial possibilities. Since we provide the design as open source to farmers everywhere, we can draw no financial gain from this work. If, you, dear readers, would like to see further democratization of biochar production around the world, please support our work with a donation to the Ithaka Institute, so that we can continue our research for the development of the Kon-Tiki as well as other exciting projects which we hope to be able to share with you in the future.

Please download here the citable PDF version of the article

PS.: In the meantime we published some scientific articles about the biochar quality and emissions of Kon-Tiki kilns as well as about the application of Kon-Tiki biochar to enhance crop growth. These articles were published open source an can be downloaded for free (Fourfold Increase in Pumpkin Yield in Response to Low-Dosage Root Zone Application of Urine-Enhanced Biochar to a Fertile Tropical Soiland Emissions and Char Quality of Flame-Curtain "Kon Tiki" Kilns for Farmer-Scale Charcoal/Biochar Production).


Please download the list of references here


  • Jenkins Macedo, United States
    02.12.2014 00:02

    Pyrolysis system should be locally resourceful and accessible!

    This pyrolysis systems looks okay. However, to generalize that this system would now allow gardeners and farmers all over the world to be able to produce high quality biochar using this approach is somehow misleading as a matter of fact that those who most need the biochar technology are rural smallholders. Most smallholders might not be able to pay for the metal sheet used in this system. The most commonly used slow pyrolysis systems at zero cost would always be the traditional earth mound approach. I just completed a field research in Laos where I initially assumed that the metal drum approach would be great just to be faced with complications like the cost of drums (200 gallons for the retort and 100 gallon internal system, the kiln), excessive waste of burning additional biomass for the biomass in the kiln to turn to biochar, labor, time, and constant monitoring. After receiving feedbacks from smallholders associated with the metal drum pyrolysis systems that you see in Youtube videos, we switched to the modified traditional earth-mound approach, which cost us zero dollars, plus we didn't have to burn additional feedstock, time wise spend and we produce biochar from rice husk, which is one of the most common feedstock in most developing countries especially in Southeast Asian countries. I am wondering how would the quality of rice husk biochar turn out to be with this pyrolysis system as it seems to be less effective for rice husk feedstock, but better with wood. Here is a link to some of our work: Overall, the system looks great, how feasible it would be is another issue that needs to be considered.

  • Erich J. Knight, United States
    08.12.2014 00:54

    Double, Double No toil or Trouble,

    A million years of Playing with Fire has finally put together an Oz Astrophysicist & a Swiss Viticulturist. We get char activating bottom quenching, steam cleaning, in a simple, robust Democratic design. Inspired by the Vedic fire ritual, Agni Hotra, the cauldron that Frank Strie built on a pivot, should be used in a "Hairy Potter" movie. The Bard of Biochar says; Double, Double No toil or Trouble, Limited Fire burns and Char Cauldrons Bubble!

  • Hans-Peter Schmidt, Switzerland
    08.12.2014 08:05

    Kon-Tiki for poor rural regions

    Hi Jenkins, You are right that the full scale metal Kon-Tiki would be too expensive for poor rural regions. In that case we also propose to stick to soil pit open fire kilns that can be done with no material costs. Here in Nepal we use a mixture of both techniques using a cone metal sheeld of 50 cm height one third in soil two third above which significantly improves combustion cleaness. This kiln wall could alternativly also be made of clay or stones. The main "Kon-Tiki principle" keeps the same. Thanks for your very valuable comment, Hans-Peter

  • ,
    08.12.2014 11:12


    This is an outstanding article. Thank you for being so clear and desciptive. I'd love to see more farmers read and incorporate this.

  • Albert Bates, United States
    09.12.2014 16:11

    Cone Pit Method

    Like you, I also experimented with the earth pit method, but with the modification, following from the ideas of Josiah Hunt, Michael Wittman, Kelpie Wilson and others, of creating a cone shape in the earth that approximated the shape of metal cone designs. I agree that in some places fabricating metal kilns will be unaffordable, but they provide some advantages that may justify costs and might be constructed on a neighborhood or village-scale pattern. The advantages of metal rims (even if added to earthen kilns), bottom-up quench and tilting hinges for unloading are significant. If we have come up with so many innovations in such a short time, I can barely imagine what else may be discovered.

  • Laura Jones, Australia
    25.01.2015 03:35


    How amazing is this?? Fantastic work Paul and Hans-Peter

  • BenK,
    19.03.2015 18:18

    Shape of Rim Shield

    Here's my question; the purpose of the rim shield is to preheat the combustion air and feed the top of the flame with a minimum of turbulence. I would think it would be reasonable to use a concentric conic section that starts somewhere above the bottom of the kiln and goes a bit above the rim, rather than a cylinder. It would use less steel and more effectively heat the air. However, I presume, it would also be more trouble to fabricate because of the shape. Am I right? Has it been tested? Does it somehow limit the airflow too much?

  • Paul Taylor,
    20.03.2015 08:46

    Shape of rim shield

    Hello Benjamin: Thanks of your thoughtful input. A deep cone Kon Tiki is made out of 2 standard sheets of steel. The maximum volume Kon-Tiki that can be made out of two 2.4 x 1.2 m standard sheets of steel is about 1.65m diam (with a volume just over 1.1 m3). If that instead was the outside conical rim shield, then the cone kiln inside it would have to be smaller say 1.5m diameter to have a 75mm gap all around.

    The maximum cylinder that can be made out of the same two sheets is 1.53m diam, not quite enough for a 1.5m cone, but good for a 1.38m cone. So you see the conical section rim shield would take almost the same metal sheets, although there would be some corners left outside the circular cuts for the conic section. Generally a welded conic section is more expensive to fabricate than a cylinder, and for ease of welding a thickness of at least 1.5mm is desirable. Therefore while it might be desirable for the kiln itself, using conic sections for rim shields and for hoods seems to be diminishing returns. The rim shield only needs to be 0.8 or 0.6mm thick. I have made them out of inexpensive galvanized sheet used for ducting, or from wide, thin metal flashing. A thin metal shield can be curved without a machine shop, screwed together and supported off the rim of the much stiffer cone kiln.

    While I have made conic sections out of this thin galvanized material, it does require cutting large circles, whereas a cylindrical rim shield can just be rolled out of standard sheets. For me it is convenient to remove the rim shield to empty the kiln - even necessary with a light rim shield in order not to damage it. A cylindrical shield can just be lifted off, whereas a conical shield would need to be disassembled. See my youtube video: Kon-Tiki comes to Australia (

    The rim shield serves 5 purposes:
    1. For safety and to improve the work environment around the kiln – the shield stays cool so its easier to work closer to the kiln
    2. To insulate the kiln by reflecting heat back to the inner kiln wall, which is important when carbonizing more difficult feedstock that take more heat for longer, such as bigger limb wood, wetter wood, or smaller moister chips etc.
    3. To help maintain a homogenous pyrolysis temperature in the kiln.  Without the rim shield wind blows would cool down parts of the inner kiln wall creating temperature differences inside the kiln which would cause condensation of pyrolytic gases and thus inconsistent biochar quality. The cylindrical shield, particularly if shiny, around a conical kiln tends to diffuse or reflect heat radiating from higher hotter level of the kiln down towards the cooling lower portion.
    4. To refine and stabilize the rim vortex, particularly in a breezy situation.
    5. Preheat the combustion air.

    The last may very well be the least important, because the Kon-Tiki has nice mixing of air and pyrogas. As long as there is enough pyrogas being generated, the flaming above the carbonizing biomass in the kiln will be strong, which aside from the clean combustion benefit is important to protect the forming biochar from oxidation. Preheating air is often a trade off, because the heat is coming from the kiln, and for difficult feedstock heat is needed in the kiln for a good flow of pyrogas.
    Our work has indicated that the open space between the cone and the bottom of the rim shield is important for good air entry inside the shield. Remember the air entrained into the rim vortex can be entrained from inside the shield or outside it, and this competition will lead to more air coming from outside the shield if the flow inside the shield is too restricted, which reduces its vortex stabilizing benefit.

  • Sven Norén,
    08.02.2016 17:05

    Octogonal shape of kiln

    I am new to this and have just ordered my first Kon-Tiki kiln from a local workshop. I got some ideas and will write three separate comments. For the first try, I start with 2 sheets 2.0 times 1.0 meter. The standard swedish sheet. Cut it into eight fractions with a trapezoidal shape. Height 1.0 meter. With 0.7 meter on top and 0.3 meter at the base. With these 8 pieces we can form a kiln. Not exactly round but cheaper to manufacture than a completely round kiln. A first try.

  • Sven Norén,
    08.02.2016 17:13

    Reflecting umbrella over the kiln

    I think of making a reflecting roof over the kiln, to reflect the heat and the light back to the ground. I think it would be fairly easy to make a big cone from cardboard covered with aluminum foil. Say 4 meters above the ground with a big hole in the center to let the air pass. If the concave part is turned downwards heat will be reflected to the kiln. If the convex part is turned down then the area around the kiln will be lit up and constitute a cosy place to sit in the evening, drinking coffee or wine and chatting. I will name it "Café Kon-Tiki"

  • Sven Norén,
    08.02.2016 17:22

    Use of the Kon-TIki kiln to pyrolyze faeces

    Imagine you have a toilet of the UDDT type - Urine Diversion and Dehydration Toilet, where the faeces are dried and kept in textile sacks. The faeces needs to be posttreated to get rid of dangerous stuff. Instead of composting the faeces, it may be a better idea to turn it into biochar with the Kon-Tiki kiln. If you want to use the biochar as fertilizer one problem is that the nutrients will be washed away with the water that is used to quench the fire. I guess that also goes for the potassium in the wood. It will end up in the water. Or?

  • Rasmus,
    20.05.2016 00:06

    suggestion: protect surface with temporary earth-based material

    I was wondering what the long-term experience is in terms of corrosion resistance of the inner surface of the kiln. The harsh pyrolysis environment may over time lead to flaking off or embrittlement of the steel. To prevent this and improve longevity of the kiln, the inner surface could be coated (smeared) with a thin lining of fresh clay, for example. The metal surface may then have to be modified in a way (e.g. spikes) that would prevent this clay/terracotta lining from falling off when the kiln is tilted for emptying.

  • Hans-Peter,
    20.05.2016 09:02

    clay coating

    as the potentially aggressive pyrolitic gases, liquids and acids are rather completely burned in the Kon-Tiki process, the steel has a quite long life expectancy. In the still young history of the Kon-Tiki (2 years), I have not seen yet any severe damage of the kiln material. Coating the steel with clay would be rather difficult because of the unavoidable cracks. You could make a Kon-Tiki completely with clay (either in the soil or above) but no need then all the trouble with welding an iron form. But I agree that it would look lovely.

  • Peter Hirst,
    14.06.2016 03:27

    Hood or stack for the Kon Tiki?

    I have been asked twice in recent days about the possibility of capturing and using process heat from the Kon Tiki. That raises an interesting topic about the nature of the draft above the flame front that I believe can be looked into profitably The concern expressed in both inquiries is that any kind of hood or collector might interfere with either the rim vortex or the draft itself, the latter of which is actually the engine that drives all open or flame cap kilns. There is a configuration for such a stack that is well known to blacksmiths that has been demonstrated with smaller pyramid kilns not only not to interfere with the important flows but to enhance them tremendously. Contrary to the instincts of many HVAC experts and experimenters, tis configuration is decidedly NOT a hood or collector that feeds into a narrower stack well above the fire. It is instead a straight open stack of even cross section with its opening very close to the flame front. The diameter or cross section of the stack is about the same as the base of the kiln or fire pit. This, careful observers will note, is about the same size as the flame from such a kiln is naturally tapered to by the draft of a strongly burning open fire or kiln. the reason that a high ,wide collecting hood does not work but this narrow, lower one does is a function of the volume and temperature of the gasses at its lowes tor entrance level. With a high, wide collecting hood, by the time the flames and exhaust from the fire have reached it, they have induced a tremendous amount of cool ambient air along with them. The result is that at the entrance to the narrower stack at the top of the hood, there is collected a large volume of fairly cool air. With just the stack, of the right dimension, place closer to the flame front, a much smaller volume of very much hotter gasses are at the entrance of the stack, and the gap between the flame and the ambient atmosphere is very much smaller. This gap may be adjusted to get just the right mix of burning gasses and fresh ari in that gap, and the result is instead of the choking of a hood and pipe, the roar of a balanced gas combustion. If this sounds familiar, it should : it is essentially a rocket stove. You can demonstrate the principle casually with a length of stove pipe and any open fire. Hold the pipe, say 6 to 8" diameter, 4 to 6 feet in length , at a high angle with its lower end over some part of the open fire. Experiment with height of the opening above above the flame front, and you will find that sweet spot where the flame roars into the pipe. Note also that when the stack is sized correctly, the entire draft of the fire is drawn strongly into it: none escapes up the sides of the stack. For this reason, the draft may be carried off at an angle - sometimes a surprisingly low angle - without interfering with the draft at all, and delivered elsewhere, perhaps to serve another heat process. This configuration has been shown to vastly improve the performance of the pyramid kiln, and I have every expectation that it would also improve the Kon Tiki. In fact, one result seen in the pyramid could be very important. It so improves the draft in the pyramid that it can be filled to and even above its rim without losing effectiveness of combustion. This could significantly increase the capacity of the Kon Tiki, obviating the need to stop adding to it with so much volume left, as reported above.

  • Hans-Peter Schmidt,
    14.06.2016 13:29

    stack or chimney

    Dear Peter, thank you very much for your thoughts and propositions concerning the heat recovery. The principle you described seems correct for most Kon-Tiki type kilns. However, larger deep cone kilns have the tendency to have the flames at around ⅔ of the flame curtain diameter and not in the middle. The small diameter middle stack would certainly modify this dynamic though this may not be a disadvantage. A clear advantage of your stack is the better mixing of the gases and the combustion air resulting in even lower emissions. If the stack is further equipped with ribbons, we might even see vertical vortexes inside the stack (like small fire tornados). How do you propose to recover the heat then, do you want to install heat exchanger inside the stack? We built distillation units to fit above the Kon-Tiki - they have a diameter of about ⅔ of the flame curtain and are about 20 cm above the kiln. The flames built up below the distillation cylinder and rise at it's rim. With this technique we recover 25% of the potential heat without the need of heat exchanger. The most important detail of your proposition is to not to use chimneys but stacks which will certainly become standard soon. In Europe we are building closed Kon-Tiki systems with pre-heated combustion air drawing in between the rim shield and the kiln which is very efficient as you keep a lot more heat in the pyrolysis zone. This, however, could be combined with a stack. Good things to try out. Thanks, Hans-Peter

  • Peter Hirst, United States
    14.06.2016 16:08

    Kon TIki Stack, redux

    Hans-Peter Thank you for your thoughtful response; it has prompted some clarifications. First, on the heat recovery: that could be accomplished in a number of ways, and I am in discussions about them at the moment. For an application that can use the entire heat stream, and provide adequate stacking to keep up with it, it could be ducted directly to flow-through drying ovens or heat exchangers. Obviously the stacks on the discharge side of such heat sinks would have to be sized to ssure adequate draft with the reduced temperatures, Another option is to tap just a portiion of the flow at or near the top of the stack, again with adequate blowers or stacks. Heat exchangers can also be located in the stack itself: coiled monotube, flame tube or water tube. As for the flame structure of the larger Kon Tiki's, let me make one particular clarification. I have seen a couple of pictures of the phenomenon of the flames being concentrated other than at the center. But I note two things, at least in the pictures I have seen. First that the flame annulus formed by this is itself tapered toward the top, as one would expect, and that the diameter of the flame annulus is about the same as that of the proposed stack. In relation to the flame front, the stack is not narrow at all: that is the mistake many professionals make, thinking that such a heat source may be collected and concentrated into a smaller column. It is why many hooded devices, particulalrly early forges, do not work. IN one experiment in Sonoma, the stack was about 80 percent of the section of the top of the kiln below it, and the sweet spot seems to be a little less than that, perhaps around 2/3. So I have reason the believe the larger Kon Tiki would positively benefit from the stack. It seems likely that the flameless section in the middle of the larger unit is due directly to the oxygen being consumed entirely by the outer ring , and that the flame front would benefit from being drawn closer to that center and the added turbulence created by the higher velocity induced by the stack. I would very much like to this experiment conducted with Kon Tiki kilns of increasing diameter.

  • Rasmus,
    28.06.2016 19:46

    journal: "Hardware X"

    There is a new journal called "Hardware X" that is specifically geared towards the open source hardware movement. This might be a good venue to aggregate some specifications as well as implements (examples: "standard" kiln version and how to build it, flat-pack version, mobile version (as a bicycle trailer), flame curtain, various other attachments (for hose, etc.)

  • Curtis Rock,
    21.04.2017 20:49

    Cone Calculations

    I have been trying to find information on kon-tiki kiln wall thickness. I found a cone calculator at I put in thickness of .1", minor diameter of 16", major diameter of 60", and height of 36", and it returned a wall angle of 31.5 degrees, which gives a 58.5 degree angle when the cone is inverted. I am trying to find a source for basic cone fabrication in North Texas.

  • Hans-Peter Schmidt,
    22.04.2017 09:25

    steel thickness

    Hi Curtis, For the 1 m3 kiln, we usually use 3 mm thick steel sheets. 2.5 mm will also do the job and we also did some with 2mm in Nepal but 3 mm seems best. The cone angle should be between 60 and 70°. Good luck, Hans-Peter

  • Jehan,
    01.02.2018 15:07

    Large Scale Biochar Making

    Hi, I would like to hear any thoughts on my idea of making biochar on a large scale. I have many hectares of invader trees to clear. Using a bulldozer will be the most cost effective. So I was thinking: the bulldozer could pile the trees by simply pushing them in more or less round piles of lets say 8 meter diameter and 3 to 4 meters high; compacting the piles as much as possible. When the piled trees have dried, we build a kiln around it using lets say 4m corrugated sheet metal end to end standing upright with some supports. This will form a 4 meter high kiln roughly 8 meter in diameter. We light the fire from the top inside the kiln and let it burn to charcoal then put it out with water. The idea is not to have the most efficient kiln or best quality biochar, but to deal with the large amount of biomass we have without having to use expensive machinery like chippers or having labor intensive processes. Any ideas or advice will be appreciated.

  • Hans-Peter,
    01.02.2018 23:26

    Large Scale, yes

    Your idea will work very nicely. Kelpie Wilson does it similar in large scale forest projects. Have a look to her web site: And keep tBJ informed how it works out for you.

  • Jehan,
    02.02.2018 07:30

    Large Scale Biochar Making

    Hi, I would like to hear any thoughts on my idea of making biochar on a large scale. I have many hectares of invader trees to clear. Using a bulldozer will be the most cost effective. So I was thinking: the bulldozer could pile the trees by simply pushing them in more or less round piles of lets say 8 meter diameter and 3 to 4 meters high; compacting the piles as much as possible. When the piled trees have dried, we build a kiln around it using lets say 4m corrugated sheet metal end to end standing upright with some supports. This will form a 4 meter high kiln roughly 8 meter in diameter. We light the fire from the top inside the kiln and let it burn to charcoal then put it out with water. The idea is not to have the most efficient kiln or best quality biochar, but to deal with the large amount of biomass we have without having to use expensive machinery like chippers or having labor intensive processes. Any ideas or advice will be appreciated.

  • Michael Shafer,
    03.04.2018 12:00

    Anyone can make one and thoughts on quenching

    Hans-Pieter, As always, kudos. Two quick thoughts, the first inspired by an earlier comment, the second by our experience at Warm Heart with quenching in the field. First, there is no reason for anyone not to build and use a flame cap oven. Anywhere in the developing world the basic materials can be purchased for almost nothing at recycling centers (junkyards) and every market village I know has someone with the tools to put them together. Second, in the countryside of the developing world, water is a scarce commodity because there simply isn't any or because it is so hard to carry it to the field. Snuffing the oven, however, requires no water or very little. The simplest way to do this is to make a fitted top to the burner that can be sealed with dirt. If it is possible to bring any water to the field at all, dumping even a single bucket in before applying the top and dirt will greatly speed "quenching." A second alternative is to use a homemade "fire blanket." We use medium weight canvas that we pre-soak in water. Spread across the top of the oven or (better) laid over the top, this too works (but watch out for hot spots! Remember, the "everyman" (and more often than not "everywoman") we are talking about are the poor, small farmers of the developing world's rural fringe. By making biochar from their crop wastes they not only sequester CO2 and reduce eCO2 emissions, but they avert PM2.5 emissions reducing a critical risk to their health and can improve their crop yields through their own efforts - essential since no one else is going to help!

  • Frank Strie, Terra-Preta Developments, Tasmania Australia,
    14.04.2018 13:15

    Effective Quenching ...

    Hello all, Considering that the key issue of the production methodology is the intended first use of the resulting black, stable carbon sponge, be that as Biochar (to foster life above or below the ground, as Feedchar for Livestock, animals and people, be that as Filtrationchar of either gaseous or liquid substances, for clean water or polluted water or to capture nutrients or treat effluent in holding basins, as Charcoal for cooking or BBQs, The earlier we can create the desired characteristics via liquid quenching or dry snuffing the better the intended results. This is why I thought in August 2014 when I first thought about our original design of the prototype model of the KON-TIKI-TAS kilns, how to optimise the uptake / coating of desired substances at this early, immediate stage. By injecting / flood quenching the ultra-dry , possibly still hot char in the kiln, the optimum coating of the Char's cell walls can be achieved. So, if you like to produce a designerchar with particular effect and desired characteristic, you may think of what the quenching process should / could contain. ... The limit is in our imagination what we intend to achieve in the short, medium and long term... Our own, FRANK'S CHAR branded products are made with these considerations. Nutrient Enriched Biochar for example is flood quenched with liquid pig manure (slurry) mostly the urine of pigs captured in a holding lagoon. This process activates the char at this fresh, ultra dry and still hot stage so that plants will grow their feeder roots to seek it. Our Asparagus ferns in our home garden patch are between 3.5m and 4.5 meters tall, just to use one example. You can visit FRANK'S CHAR Tasmania on facebook and / or look for

  • Ken Reiman,
    11.09.2018 00:26

    Quench water

    I would like to ask what the pH of the quench water generally is as I am concerned about it's long term effects on the soil where it is discharged, as I haven't seen this addressed anywhere. Thankyou for your consideration.

  • Hans-Peter,
    11.09.2018 06:01

    pH of quench water

    The pH of the Kon-Tiki quench water is usually around 11 and due to minerals from partial ashing. The quench water can be used for foliar treatments (plant fortifiant, good against snails and fungi). The pH buffer capacity is low and there is no risk when you spread it over a large enough surface. However, I would not empty the quench water always on the same spot but rather recover it and use it as foliar spray. We often ferment the quench water (adding 3% sugar and about 1/3 (vol) fresh leaves). After ten days, the pH of the quench water will thus reach 4 and can be used as bio-active fertilizer in drip irrigation or to nutrient enhance biochar.

  • Frank Strie, Terra-Preta Developments, Tasmania Australia,
    16.10.2018 15:35

    What to do with the quench water / the quench liquids

    Spot on HP, I noticed how instantly the earthworms wiggle out of the ground when the soapy, soft quench water/ quench liquid runs over the ground. I have used the high pH - Biochar quenching water and liquid pig manure around our Hazelnut bushes and Fruit Trees, Apples, Pears, Plums, Peaches and Apricots, in fact today I did just that! It is a constant matter of observation and learning. Keep up the great work my friend Frank

  • Ian Gordon,
    22.02.2019 03:30

    Cone or Pyramid

    I'm seeing recommendations being made for pyramid kilns (easier to fabricate and fill) but I'm wondering whether the pyrolysis would be as efficient. What concerns me are those corners, How would that affect airflow? I'm also seeing octagonal kilns.

  • Schmidt, Hans-Peter,
    22.02.2019 21:32

    cone versus pyramid

    The advantage of the cone forme is a more homogenous inflow of combustion air and easier manufacturing of the rim shield. But we also build lots of pyramid kilns that work just fine. If you want to manufacture it yourself and the pyramid form is easier to handle than the cone, just make what is easier for you as it is more important to build your Kon-Tiki yourself than to have it perfectly done by computers. Good Luck, Peter

  • Kelly Pope,
    26.03.2019 14:04

    material longevity

    Does anyone have tips, experience, or advice on material longevity of kon-tiki kilns? I currently have a small conical pit in my backyard that I can make about 7 dry gallons of biochar in (5 when crushed), but I want to commission a unionized sheet metal worker to fabricate a kon-tiki for a bike cart. However, I don’t want to make mistakes in this process and commission something with a material that will degrade within a few burns (as I seem to notice happening with many TLUD designs). It is easy to see rust forming on some kon-tikis in pictures and videos, but if I commission a mobile teaching kiln, the ideal would be that it would have some longevity. Do people recommend a treatment for the metal after their biochar has cooled & been removed after soaking? Similar to a cast iron being treated? Any advice?

  • Schmidt, Hans-Peter,
    28.03.2019 08:18


    Hi Kelly, We usually built the Kon-Tiki with 3 mm mild steel. The very first Kon-Tiki from 2015 was since then always outside without a cover and was used for many biochar making seasons: the steel is still intact and it is perfectly useable. During winter, when we do not use it for several months, I keep some biochar in the kiln which avoids iron corroding inside the kiln. Kindly, Hans-Peter

  • Eck Volkmann,
    11.08.2020 20:45

    salinity of quenching water

    Dear Ithacas, thank you for sharing your great innovation - I hope to return in some way or other by participating in your enthusiasm! Where I am, water is very scarce with the exception of rather saline, groundwater close to ocean shore, too brackish for drinking, yet certainly below 4% NaCl concentration. From domestic compost I learned, that some seeds are extremely sensitive to salinity, even only conductivity of water from just kitchen waste. So I was wondering, whether I would spoil my bio-char, if I would quench with saline water. I have no water-chemistry data, except knowing that from what we pump into swimming pools around here is always very alkaline - > 8.5. Any of you, who could say something about this in terms of soil-chemistry/biology if brack-water-quenched bio-char would be applied to sandy, tropical soils on corral-rock? Hoping you're still passing by here every now and then to see if there are new developments or questions... its been 6 years now - wow!!

  • Hans-Peter Schmidt,
    12.08.2020 01:04


    Hi Eck, I will not be able to provide a complete and satisfying reply but I will try to help with what I know. Biochar was shown to reduce salt stress in crops cultivated in saline soils (see here:, so the biochar would certainly help agronomy in your region. Biochar is also used to filter saline water (e.g. in aquariums). However, your question is different. There is some evidence that NaCl-treatment of biochar has an activating effect. We tested sea minerals for quenching, which worked well (not at least because of the > 50 different minerals contained in sea water. What I do no know is what happens with the salt of the quenching water when it is adsorbed by the biochar and this biochar is applied to soil. I think that the risk is very low, But without any experimental data, I do not know it- Best, Hans-Peter

  • Frank Strie,
    24.08.2021 07:05

    Saline water quench options

    That sounds like a very useful consideration Hans-Peter and Eck, It may be hard to get sufficient seawater or even brackish water / estuarine water in a land locked country like Switzerland and Austria, however down her on our Island under Down Under it is no problem at all to get such quench liquids, the Tamar estuary is just down hill from our house say 300 metres away. The ocean just 35 km North from here. So I will discuss with our local researchers if there is an interest in scientific trials that we are willing to support in kind. I like to think that a seawater flood quenched char could work as an effective surface application to act as a growth retardant, non toxic (black herbicide mulch) ... It would require to rinse the mild steel kiln (now made with 5mm plate) with fresh water to avoid additional corrosion. So far none of our original prototype KON-TIKI-TAS kilns from Aug. 2014 has rusted through. for updates visit and Facebook and FRANK'S CHAR Tasmania. By the way, our 2021 Spring is here!

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