Biochar Production in Kon-Tiki Australia

After exploring and making deep cone kilns, cylindrical kilns and pyramid kilns in Switzerland, Canada, and California it seemed they presented the fastest way to get into biochar making on the small property. I was keen to set them up on my  own property in Australia.

The goals were: 

  • Make a Kon-Tiki and a variety of related flame-shield kilns and compare them for effectiveness, clean burn, ease of use and economy.
  • Operate them on a variety of biomass on my property including tree fall, limb wood, greenwaste, palm fronds, as well as saw mill off-cuts and chips.
  • Explore their viability for small scale production.
  • See if they could make biochar organo-mineral complexes by carbonizing mixtures of chips, manure and minerals.
  • Do emissions tests and compare with other small-property batch production methods like TLUDs and retorts.

These goals were largely accomplished between February and May 2015, and are reported in the next several blogs. Videos are published on The Biochar Revolution YouTube channel.

In this blog I discuss the design, construction, and operating of a Kon-Tiki kiln to produce (7.5cu meters or over 1 ton (oven dried basis) of biochar.


From an Australia 1.2m x 2.4m mild steel sheet, the maximum sized Kon-Tiki with a depth of 90cm (any deeper becomes inconvenient to use) is 1.65m diameter with an angle of 64.5 °.Volume is 1.1 m3

An extra partial sheet is required for the base. Base can be flat instead of domes or coned.

  • Kiln has handles and  3 sturdy legs so that cone can be tipped over two legs. 
  • Drain is a 50mm pipe nipple welded to center of base.

Trailering the Kon-Tiki

The load includes a smaller cone inside the large one, rim-shield,
cone-extensions & large flues all to be used eventually in emissions studies


  • A fire permit was obtained, but the fire marshal found it not needed for an enclosed burn.  Your area may be different. 
  • The marshal was so interested he stayed to build and light the fire to commission the Kon-Tiki! 
  • Both the large cone and a smaller 1.2m cone in the background were made from 2mm mild steel and required an angle iron stiffening ring welded to the top.
  • The rim shield is a 1.8m corrugated stainless steel ring – very light. It was lifted over the cone and sat on bricks.
  • Fuel & kindling were staked in the new Kon-Tiki and top lit.

The biochar was quenched by water spray from above. A pleasing start.


  • The tilt cradle was designed around triangular frames, made from 2 in (5cm) square section steel tube. 
  • The tilt axis was provided by welding a sturdy steel tube through the cone.
  • This tube also allows safely inserting a thermocouple into the middle of the kiln.
  • The horizontal bar on the frame is to support the rim shield at its desired height. 
  • Rim shield (not shown) is installed around the cone for firing and lifted off prior to tilting


  • 6 tons of wood flooring offcuts
  • 4 cu m of hardwood chips in bulk bags
  • Moved under solar roofed barn


The deep cone is stacked to near the rim with bushy fuel to allow top lighting in free air.


  • The fuel is lit from the top.
  • Some smoke escapes the fire initially.

Here, without the rim shield, can be seen:

  • The cradle made from scrap 5cm (2in) square section tube.
  • The 3cm diameter tube welded through the middle of the cone, sitting on the triangle frame
  • This tube is the tilting axis and also allows a thermocouple to be inserted into the middle of the kiln
  • Another lower tube allows a second TC to be inserted near the kiln bottom.
  • The rimshield will sit in angle iron cross bars on each side of the cradle. They are slanted relative to the base so the rimshield will be horizontal and rise about 10 cm above the cone rim.
  • Biochar from the previous run has been emptied onto a tarp to sun dry.

A bed of coals and fire in the bottom of the kiln

  • Here we have added some woodchips and blocks to test wood chips as a feedstock.
  • Wood chips tend to smother the flame. If moist they burn well only in an intense fire.
  • Woodchips, moist greenwaste, and complexes with manures clay and minerals were easier and cleaner to fire by mixing with the dry wood offcuts or dry limb wood.


  • Each batch of wood blocks was weighed in 60L bins on a scale.
  • Material is added at a rate that keeps the flames high and minimizes smoke.
  • Here too many blocks were added to one side of the kiln, snuffing the flame, but smoke and steam are rolled into the flame by the rim vortex.
  • Intensive flame coverage in the cone ensures the biochar is shielded from oxygen, which is consumed in the flame along with the smoke.


Pictured is a long thermocouple probe inserted into biochar after misting, which cooled top surface at end of day's run

Temperatures were measured by inserting long thermocouple probes into tubes 15cm from bottom, 41 cm from top of cone, and also directly into biochar at top. Typical: Low ~ 300oC, Middle ~ 650oC, Flaming Layer ~ 730oC Every layer in the kiln experiences these temperatures starting at ~730oC, soon dropping to ~650oC, then soaking at lower temperatures as the layers accumulate.


Quench water from 1000L bulk liquid tank (optionally with nutrients) is:

  • Pumped into the kiln through the bottom drain.
  • Soaked overnight.
  • Drained back into the smoke water/nutrient tank next morning.


  • The kiln is easily tilted and propped for emptying.
  • Biochar is raked out on tarps and sundried for at least 2 to 4 days, covered at nights.
  • Too much moisture in the biochar can clog the mill when grinding, and too little causes dust.
  • Moisture content of sundried biochar ranged from 34 to 63% wb.


  • After sun drying the biochar is passed through a hammer mill.
  • Biochar is volumed and weighed in buckets or bins before and after milling.
  • The milled biochar, raked out on the tarpaulin, may be sprayed with phosphoric acid or organic acid to adjust pH and further sun-dried
  • Moisture content of sun-dried biochar is determined by oven drying samples:
  • Wood blocks = 15%, wb
  • Biochar = 15-63%, wb (sundried)


  • Samples of all biochars were oven dried and weighed to compute moisture content.
  • pH was measured and buffered with phosphoric acid.


  • Lumpy biochar in the background.
  • Fluffy milled biochar in the foreground.
  • All biochar is volumed and weighed in bins.
  • Then stored in 1000 L bulk bags or bins.

Sending off for farm field trials

Biochar made in Kon-Tiki in 10 runs in 1 month:

  • 14500L, 5400kg of wood (oven dried basis)
  • 7500L, 1150kg of biochar (oven dried basis)
  • Volume Yield: 52%
  • Mass Yield:  21% (oven dried basis)
  • Biochars included straight, sea mineral amended, gypsum amended, and clay-mineral-manure complexes.


Unit Limit of reporting Plain Wood Biochar Sea Mineral Biochar Gypsum Biochar Enhanced Biochar BOMC
EC dS/m 0.01 1.2 1.5 2.7 3.0
pH(CaCl2) pH units 0.04 6.9 7.6 7.1 7.1
Ammonium-N mg/L 0.03 0.87 0.19 0.19 11
Nitrate-N mg/L 0.02 0.30 <0.20 0.10 0.30
Total Nitrogen % 0.02 0.14 0.16 0.12 1.4
Total Carbon % 0.2 81 81 56 43

Plain Wood Biochar: Biochar from wood blocks. Sea Mineral Biochar: Biochar from wood blocks quenched with Sea Minerals water. Gypsum Biochar: Biochar from wood blocks, wood chips and gypsum (64%, 23%, 13%). BOMC: Biochar-Organo-Mineral Complex from wood chips, manure, clay, minerals. See next blog. All biochar quenched also with smoke water. All biochar buffered with phosphoric acid to target near neutral.


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