Do-It-Yourself Low Temperature Biochar Kiln
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.