The 'Flat-Tiki'

It's hard to know where to start. There may be no beginning and there may be no ending. I've been researching biochar for 6 years off and on. Sometimes I feel that I want to help save the planet and other times it all seems too hard. It depends on what I am reading and how bad things seem to be getting regarding the imminent climate chaos/change/disruption and what is being done by other people to address it. I know I can always do more with less but how that benefits the greater good is difficult to say. One thing is certain - I am an appropriate tech-head and proud of it.

My initial interest in biochar was stimulated by the late Geoff Moxham who showed me a top-lit updraft (TLUD) stove in 2009. It completely bamboozled me. How was it possible that such a clean burning flame could be produced from such a small amount of biomass aka virtually any organic material known. I decided that I would design and build a biomass-biochar stove for my main project for the Diploma of Permaculture at the previously running course at the Permaforest Trust. This was co-managed by Tim Winton best known for his Pattern Dynamics (TM) systems ecology approach to permaculture and general understanding of living systems.

My final design for the TLUD stove retrofitted a stainless steel birko and innovated with its use of insulation in a third concentric chamber. Otherwise it was yet another proof of concept that many others had achieved with their modified designs. Probably my favourite TLUD design is the '1G Toucan' developed by Dr Hugh McLaughlin.

I wanted to make more biochar and in order to do this I scaled up my TLUD design for an oil barrel called the 'Don't worry, be happy' biochar kiln. The problem with the design was observation and probably safety too given the pressurised nature of the combustion chamber. I never built it and waited for a few years for something better to come along. Things needed to be simplified and cheaper. Then came along Charmaster Dolph's 'Moxham' tubular kiln. I couldn't believe how simple yet how effective the design seemed to be. The output was clean, efficient and could produced quite a lot of char for the effort required (a recent engineering report produced by 'Black is Green' is available at This pioneered a new breed of biochar kiln known as the 'top-fed open draught' (TFOD) kiln. Then came along Dr Paul Taylor's modification of the Japanese 'Moki' kiln. He scaled up the size and gave it a bottom with legs, drain, handles and heat shield at the Ithaka Institute known as the 'Kon-Tiki' kiln named after the Kon-Tiki expedition in 1947 which was a  journey by raft across the Pacific Ocean from South America to the Polynesian islands, led by Norwegian explorer and writer Thor Heyerdahl.

The 'Kon-Tiki' is an advanced design. It is virtually clean-burning, can be quenched from below and scaled up to about 1000l capacity (or larger) and can pump out large amounts of biochar for a good day's work. The latest 'Kon-Tiki' kilns also use A-Frames to allow convenient cone emptying. I built a 1.2m 'Kon-Tiki' with legs, drain, handles and heat shield using youtube specs and modified it to Paul's sheet design for a 1.2m diameter. It seemed to work perfectly the first time. But then I thought what if I want to produce smaller batches of biochar in less time? I downloaded Kelpie Wilson's 'Pyramid' kiln design ( and had a couple built with rim edge folding for safety. It worked efficiently but couldn't really take larger feedstock (which she has addressed via a design of a larger 'Pyramid' kiln used to process forestry waste). At the time however this design was not available and I wanted to do more with less. That's when I started researching Hawaiian Luau pits and then came across a hybrid pit kiln designed by Hans-Peter Schmidt also working at the Ithaka Institute. I was impressed with the design for application in less industrialised countries with less access to advanced metal fabrication technologies.

At this point I thought to myself why couldn't I design something that tried to take the best of all these designs and make it flatpackable, mobile and light with a batch volume suitable for large gardens and small farms? Not only that, I wanted to design something that could use state of the art tech in the West (Oz) with the flexibility to be built using traditional fabricating technology without a roller. Enter the 'Flat-Tiki'.

What, I thought, could be the design compromises required? I initially went with 3mm 300 grade mild steel on the 'Kon-Tiki' 1.2m but this was heavy and rusted easily. My offsider from the Philippines, Julio, suggested I go with 350 grade weathering (Corten) steel at the 1.6mm guage. Great, I thought, light and weather resistant. I then whipped up a design for a standard 1200 x 2400 sheet using a similar approach to the early 'Pyramid' kiln design but went hexagonal instead of pyramidal which would bridge the geometric gap between a rolled truncated cone and pyramid (assuming that a truncated cone is more efficient and a pyramid is less efficient). Furthermore, a hexagonal structure is probably the most stable one mimicking the Carbon 6 molecule not to mention other hexagonal patterns found in nature such as bee hives and basalt columns.  I also took the hybrid approach which would allow a small pit to be dug at the bottom therefore extending the volume of the hexagonal cone and would allow construction without welding.

I used laser cutting for the pieces, hand holes, logo and screw holes. I used galvanised wing nuts and side flaps on three of the sections in order to create easy assembly from flat-packable pieces. The volume is estimated to be similar to the 'Kon-Tiki' 1.2m kiln. I then tested the 'Flat-Tiki' with a tubular heat shield built for the 'Kon-Tiki' 1.2m and realised that this improved the performance of the burn so I went all the way and built a hexagonal heat shield to go around the hexagonal kiln, using a similar approach with airflow going up the side of the kiln from the bottom of the heat shield which could then feed flames at the rim of the kiln in hope of torroidal convection loops. The heat shield also extends 300mm vertically from the rim of the kiln as recommended by Dr Paul Taylor and can be shifted up and down. Julio cleverly alternated hinges (2 per join) in the heat shield which allows it to concertina and flat pack. From my prototyping it appears that flame turbulence is greater with only half-formed torroidal convection loops occurring. This was an improvement over the 'Pyramid' but not as pronounced as the 'Kon-Tiki' 1.2m.

This all says to me that I have hit the design middle ground for efficiency between the 'Kon-Tiki' and 'Pyramid' designs, as hypothesised.

The weight of the 'Flat-Tiki' kiln is about 21 kg and the 'Flat-Tiki' Heat Shield is about 30kg, placing the whole unit at 51kg. I am happy with that as I wanted to get below 50kg. Close enough for now.  The kiln and heat shield can be easily moved once in place, can be moved around from place to place (mobile) and can be operated by one person if need be. This could be the world's first flat-packable hexagonal hybrid 'Flat-Tiki' biochar kiln!

Other innovations include hand holes that double as observation holes from the side, a cooking plate off the same sheet (a standard 1200x2400) and a chapati plate.  Given that the side flaps on the kiln needed to be rolled, if rolling isn't available then the flaps can be omitted from the design. In this scenario the kiln pieces would need to be welded together - not ideal but still manageable by one person and could be transported in a stackable fashion similar to that used for the 'Kon-Tiki' cones - just not flat-packable for convenience and ease of transport.

Now that its' fire ban season, prototyping is off the table until May 1 2016. I want to get emissions testing done for the design at the University of Adelaide late next year using vineyard waste and start to sell the kiln in Oz. I would like to donate 10% of the cost of production to the Nepali Climate Farming Fund. If successful then I hope to assist fabricators to produce them in other countries too...

The designs for the 'Flat-Tiki' kiln and 'Flat-Tiki' heat shields, licenced under an International Creative Commons licence, will be available on request after emissions testing has been performed in May 2016. Read on for some photos of my kiln research...

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Comments: 3
  • #1

    Kelpie Wilson (Saturday, 13 May 2017 22:28)

    Nice job on the hex kiln! You capture well the design tradeoffs between cone and pyramid shapes. I appreciate the thought you have put into this. I am including on of your pictures in my latest slideshow of flame kilns.

  • #2

    Trevor Reddicliffe (Thursday, 24 May 2018 00:51)


    please send me details of your designs when finalized

    Best Regards

    Trevor Reddicliff

  • #3

    Tom Seaney (Wednesday, 09 February 2022 08:26)

    How did your design do in emissions testing? I am interested in fabricating the kiln if final plans are available.
    Thanks for your efforts in biochar production!