Operating the Kon-Tiki

The flame curtain principle

Conventional pyrolysis separates two processes that nature runs together: the thermal decomposition of biomass (carbonisation) and the combustion of the gases released during that decomposition. Industrial reactors seal the biomass in an oxygen-free chamber, extract the pyrolysis gases, burn them separately, and transfer the heat back to the reactor. This works, but it requires airtight construction, gas and heat handling - engineering that costs money and is prone to failure.

The Kon-Tiki does the opposite. It runs both processes in the same open vessel, using the combustion of pyrolysis gases as radiative heat source and as the mechanism that protects the char from burning. As biomass is heated in the kiln, it releases volatile gases - hydrogen, methane, carbon monoxide, tars. These gases rise and ignite at the surface where they meet oxygen. The resulting flame layer - the flame curtain - forms a relatively continuous blanket of fire across the top of the kiln. Because the flame consumes all available oxygen at the surface, the zone below it is effectively anoxic. The char bed sits in this oxygen-depleted zone, carbonising without combusting.

The flame curtain is self-regulating. As long as fresh biomass is heated, it generates pyrolysis gas; as long as pyrolysis gas is burning, it excludes oxygen from the char. The system maintains itself without valves, sensors, or operator intervention - only the operator's judgment of when to add the next layer.

Why the cone

The conical geometry solves two problems simultaneously.

First, the sloped outer walls act as a chimney. Air heated by contact with the hot steel rises along the outside of the kiln, drawing fresh combustion air over the rim and into the flame zone. The preheated air is more reactive, enabling more complete combustion of pyrolysis gases at the flame curtain. The rim shield - a vertical extension above the kiln edge - stabilises this airflow and prevents lateral wind from disrupting the flame curtain.

Second, the cone narrows toward the bottom. The weight of each new layer compresses the char below it, and because the lower layers occupy progressively less volume, the char bed consolidates naturally. The exothermal pyrolysis process continues until the charing reactions are completed. 

Loading: why top-down matters

The Kon-Tiki is loaded in thin layers, each added only after the previous layer has visibly ashed over — the white ash on the surface signals that carbonisation is largely complete and the layer is ready to be buried under fresh feedstock. This top-down loading is the key to both quality and emissions. Each new layer of biomass immediately releases pyrolysis gases that feed the flame curtain, while the layer below transitions from active pyrolysis to stable char. The char never re-encounters oxygen because it is continuously buried.

If too much biomass is added at once, the flame curtain collapses: the volume of pyrolysis gas exceeds what the flame can combust, unburned gases escape as smoke, and oxygen may reach the char bed. Smoke is always a sign that the operator has overloaded the kiln. The remedy is to stop loading and let the flame re-establish before adding the next layer.

Quenching

When the kiln is full, the biochar must be cooled before the flame curtain dies. The Kon-Tiki has a water inlet at the bottom of the cone. Water rises slowly from below, cooling the char from the bottom up while the flame curtain continues to combust pyrolysis gases from the still-hot upper layers. This bottom-up quench avoids the smoke burst that would result from dousing the fire from above, and the rising water vapor drives off pyrolysis gases between and inside the chared particles. Only when the water level approaches the surface is a final top spray applied to extinguish remaining embers.

The quench water - alkaline (pH ~11), slightly soapy, rich in dissolved potash - is itself a useful by-product: it can be used as a foliar spray, a mild organic pest deterrent, or returned to the compost.

Biochar quality

Correctly operated, a Kon-Tiki produces biochar that meets EBC certification thresholds. Typical pyrolysis temperatures of 620–750°C yield H/Corg ratios of 0.15–0.20, well below the 0.7 limit — indicating a high degree of carbonisation and long-term stability. PAH levels are consistently low when the flame curtain is maintained throughout the burn, as demonstrated by Cornelissen et al. (2016).

The quality depends entirely on the operator. A well-run Kon-Tiki matches industrial biochar; a poorly run one produces smoke and under-carbonised material. This is why the Global Artisan C-Sink Standard requires operator training and direct monitoring of the production process before producers can certify carbon sinks.

Download: Kon-Tiki Operating Instructions (PDF)