How to Make Biochar with a Dome Kiln

Mar 10, 2015

This post has been written by Mike Thomas, a great bloke who was a student on our 2013 Permaculture Design Course. Enjoy! Last summer at the ‘Plumplot’ (our farm in Margate, southern Tasmania) we built a dome kiln with the aim of making biochar. 1 I’ll start by explaining biochar and pyrolysis, explain the Dome Kiln and then expand on our experience. Biochar: Is charcoal formed for the purpose of:

  1. Increasing soil fertility by housing bacteria and binding nutrients.
  2. Returning captured carbon almost permanently to the soil
(charcoal is about 50% carbon whereas ash is about 5% carbon) Historically biochar was used to enhance the fertility of the Amazon basin where most soils are only able to grow crops for up to three years and heavy rains leech the ground of nutrient.  Boosted with biochar  and the right bacteria those lousy soils were able to support civilisations.  It is well worth watching the BBC documentary ‘The secret of El Dorado’ which explains this further . Current scientific studies show mixed results with regard to biochar in different soil types; some showing a 100% improvement and others showing a depletion in fertility. Pyrolysis: Biochar is made by a process called pyrolysis. This is the chemical process which occurs in material which contains carbon (such as wood) when it is heated with minimal oxygen. Pyrolysis can therefore happen in a heated barrel with the lid on and a couple of holes drilled in the side. It takes a lot of wood to keep the barrel hot; In my experience was an inefficient design. Commercial operators have much larger furnaces working on a similar principle. A cone kiln is a design which originates in Japan. Cones can be made from a sheet of metal or dug into a hole in the ground.  Layers of wood are burnt sequentially such that the layer above ‘steals’ the oxygen from the layer below.  When the cone is full,  the kiln is doused with water and may be covered to prevent oxygen entry. It will remain hot and voila! the contents undergo pyrolysis to form char. Dome Kiln: Our kiln is a pit style brick lined cone kiln made in a dome shape. Advantages of our design are:
  • 2 metre diameter allows huge volume compared with most metal cones for similar burn time
  • Dome shape has a greater volume to lid ratio than a cone
  • Easier to reach the centre from the edge
  • Brick lined pit prevents edge breakdown and mud formation
  • Brick and crusher dust has good heat capacity
  • 1 metre crusher dust edge minimises fire risk

2Kiln construction detail showing support bricks and use of crusher dust

For us the dome kiln solved three problems:
  1. Use of excess wood not suitable for the fireplace
  2. Leaky dam = messy mudpit with no function
  3. Poor soil may be improved
Dome kiln materials:
  • Leaky dam
  • Salvaged bricks
  • Crusher dust (4 trailer loads)
  • Clay as mortar
  • Cement for final brick layer.
  • Flat metal for lid joined and cut to shape

3Kiln site pre construction showing failed dam. See spades for scale in top right hand corner

Substrate
  • 3 ute loads of wood from Cass our friendly and generous neighbour
  • Big pile of olive and apple prunings
Position considerations
  • Easy access
  • Near water for fire safety and dousing
  • Fire retardant vegetation nearby (deciduous trees)
  • Smoke moves away from neighbours in prevailing winds
  • Flat or downhill to garden
Drying the kiln Initially we had a problem as the most rain in about 30 years fell two days before our Dome Kiln ‘first burn’ party. With three helpers and a few buckets it didn’t take long to empty it out.

4Blooper!… lots of water in our kiln!

5Severin and bucket after emptying the kiln

6Kiln drying with a hot fire from prunings

The burn We laid the first layer of wood on pruning coals and when it was burning well carefully placed the next layer.  When this layer was burning we laid the next and so on.

7Wood layers at half full

8Lengths of wood that spanned the diameter of the dome made it easier to lie each layer flat

After 4 hours when the kiln was full we covered the burning wood in horse manure (to exclude oxygen and start building soil) and hosed it down. Finally we placed the metal lid on top.

9Amy shovels the horse manure to cover the char. See lid in background

10Lid in place – Loads of smoky steam escaping!

11Two days later… Biochar!!!

The kiln held about two cubed metres of char. I set up a flat metal ‘anvil’ where the char could be crushed easily by walking on it in our helper Severin’s big boots. He crushed and moved it to the garden in two half days.

12Amy and a sample of the final product

*Acknowledgements and thanks for building this dome kiln go to… Severin, Freya,Kati, Petra, Anna, Amy Lau, Marcus Higgs,  Finn Fagan, Cass Rea]]>

your thoughts:

3 Comments

  1. Rafaele Joudry

    Its great to see this example of how to make biochar with natural materials and a minimum of engineering and metalwork. Thanks for sharing!

    Reply
  2. Ian

    Thanks guys, very interesting. Question. For what area of garden will this amount of biochar serve – about one and a half cubic meters by visual estimate? How long did you leave it to burn?

    Reply
    • Hannah Moloney

      Hi Ian, This isn’t our kiln so I can’t give you a clear answer (sorry) – it belongs to some friends who have a small acreage. However I can say vaguely that it would be a lot as a little goes a long way!

      Reply

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