Biochar Retort

Home-scale double-barrel top-lit-up-draft biochar retort stove!

“Biochar can be made cleanly and effectively on a home-scale, using a biochar retort.”

Biochar is a desirable material for anyone looking to improve their soil. Buried, it acts as a water retainer, nutrient trapper, and home for beneficial bacteria and plant roots, and it continues to support life and fertility for thousands of years! On a home-scale, biochar can be made from waste wood or mulch, using a biochar retort.

Benefits:

  • Produces biochar – a permanent soil amendment
  • Burns clean and hot
  • Can be used for heating or cooking
  • Can be fuelled with waste materials

Materials:

  • 44 gallon steel drum (empty)
  • 50 litre steel beer keg (empty)
  • 800x110mm corrugated steel sheet
  • 3 meters metal bracing

Materials needed for a burn:

  • 50 litres dry mulch (this will become biochar)
  • ~75 litres dry mulch (this will be burnt cleanly and become ash)
  • Some cardboard or newspaper for ignition

Jump to:

What is biochar?
What is a biochar retort?
Design
Construction
Testing
Applications
Other Uses
FAQ
Why does 50L of feed stock only yield 30L of biochar?
What else can I use to make a biochar retort?
What materials can I pyrolyse to make biochar?
My biochar retort produces a lot of smoke. What can I do?
Related Articles
Making Biochar using Mulch (video)
Mulching
How to Compost

What is biochar?

Biochar, made from dry mulch

Biochar, made from dry mulch

Biochar is a term given to wood that has been through a process of pyrolysis. It is charcoal without the soot, ash, and volatile organic compounds – it’s pretty much just carbon. It looks and feels like charcoal, but if you crush biochar in your hands, you shouldn’t have any oily stains on your hands – just black dust. ‘Pyrolysis’ might sound technical, but it just means the wood is cooked without oxygen. As it is cooked, wood gives off its substances as gases, and what remains is just ‘a wisp of the wood’ – biochar! The gas, known as syngas, is flammable, and most biochar stoves and retorts make use of this gas to perpetuate the pyrolysis. Also, the off gases can be trapped and stored, or redirected elsewhere for heating or cooking.

What is a biochar retort?

Home-scale double-barrel top-lit-up-draft biochar retort stove!

Home-scale double-barrel top-lit-up-draft biochar retort stove!

A biochar retort is a stove dedicated to the pyrolysis of wood and other materials to make biochar. While this is its prime function, a biochar retort can be modified or augmented to serve other functions such as heating or cooking. Retort designs vary greatly, from home-scale to commercial, with different methods for achieving pyrolysis.

Design

Biochar Retort Design

Biochar Retort Design

 

My biochar retort is a home-scale double-barrel top-lit up-draft rocket-stove retort (gasp!). Basically it is a large rocket stove with a beer keg inside it. It is capable of producing about 30 litres of biochar per burn, using only dry mulch – a common waste material.

Construction

My 44-gallon (200Litre) steel drum had a few litres of oil left in it, which I drained into a bucket and kept. The drum had a sealed lid, so I cut the top off it using an angle-grinder. This top became the lid, and I prepared a bracing using metal strapping to support the lid back on the barrel. I also prepared the lid to fit a chimney pipe, which was made from a scrap piece of corrugated sheet metal. Any kind of metal piping will do.

I vented and removed the valve from the beer keg, and cut a large opening in the top. A friend passed me the top of a keg he had cut open, which meant I had a nice lid to fit over my keg, but you can use any metal covering to prevent material falling out of the keg during a burn.

Testing

A simple method is to keep a fire going around the keg.

A simple method is to keep a fire going around the keg.

My first test was unforgettable. It was a windy day, when all the neighbours’ clothes were hung out to dry. I had no idea how much smoke would ensue. The burn took a long time to get properly underway, and all the while the paint on the outside of the barrel was boiling and burning, giving off a few credits worth of carbon pollution! I gritted my teeth and watched on. Eventually the rocket effect began, and the unit became too hot to stand within a metre of. The fire cleaned itself up, and no smoke could be seen for the rest of the burn. The sound was a phenomenal roar! Unfortunately there’s no decent way to remove paint and other polluting residues from a barrel. It’s best to roll the barrel through a very hot fire to combust as much residue as possible, and then to put the barrel to good use (make as much biochar as you can) to justify this initial environmental cost.

This is the hottest phase of a burn - when the fuel stock is fully combusting, and the gasses from inside the keg (the feed stock) are bellowing out the bottom, further fuelling the fire.

This is the hottest phase of a burn – when the fuel stock is fully combusting, and the gasses from inside the keg (the feed stock) are bellowing out the bottom, further fuelling the fire. Notice there is no smoke.

The first burn managed to turn most of the feedstock in the keg to biochar, but wasn’t hot enough to penetrate to the centre of the keg. It took me a number of tests to find the best way to achieve complete pyrolysis, whilst maintaining a clean burn of the fuel stock. My best results were achieved by using dry mulch as the feed stock and as the fuel stock. I started a fire atop the fuel stock, which then burnt down around the outside of the keg. I also had success using dismembered pallets (single-use pine) as fuel stock, though these required energy to chop up.

Applications

A wheelbarrow-load of biochar, spread over and mixed with mulch. The mulch will eventually break down, but the biochar will stay in the soil for thousands of years!

A wheelbarrow-load of biochar, spread over and mixed with mulch. The mulch will eventually break down, but the biochar will stay in the soil for thousands of years!

In an agricultural setting, biochar can be tilled into the soil with machinery, but on a home-scale, it’s not so easy. Manually digging trenches, and involving biochar into your soil profile is a lot of work, and may not seem worthwhile to a short-term enthusiast (though in the big picture, the efforts will yield returns for 1000s of years to come!). A simple strategy is to mix biochar into your compost, so that it is inoculated with beneficial microbes, nutrients and water, then mix your compost into the top few inches of your soil. If you’re mulching and encouraging earthworms in your soil, the worms will take the finer particles of biochar and distribute them throughout your soil. The larger particles will be left closer to the surface, where they can hold onto moisture and nutrients and support shallow rooted plants. This strategy takes time, but the least amount of effort. Also, if you’re planting a tree, you can spend a bit of extra time mixing biochar into the soil below where the tree roots will grow.

Agricultural scientists have established that soil productivity will be improved with applications of biochar up to approximately 8% by volume, with some success reported at 13%. On a home-scale, it might be easiest to elect a portion of land to be the fore-runner in biochar application, with other plots trailing behind. Once productivity deteriorates in the lead plot, you might consider the others to be at an optimal saturation. If you want to achieve 8% biochar in the top 30cm of your soil, you will need a layer of biochar 2.5cm thick (which should then be mixed in).

Other Uses

Fuel

Biochar burns hot and clean, much the same as charcoal. This might be handy for those who are trying to achieve extremely high temperatures (i.e. for glass blowing or melting/shaping metal), or as a combustion fuel for a steam engine.

Water Filtration

Common water filters use a form of carbon similar to biochar, called ‘activated carbon’, which is clean and extremely porous. Biochar can be just as clean and just as porous, and so it can mechanically filter water. Spent biochar can then be added to compost.

Hydroponics

Biochar can be used as a grow-medium for hydroponic systems. It’s porosity resembles perlite, a common grow-medium. Perlite is a non-renewable resource (it is mined), whereas biochar can be made.

Insulation

Due to it’s porosity, biochar acts as a good insulator. It conducts heat poorly, won’t melt, and so long as it is not exposed to oxygen, it won’t burn. I’ve used it as a lightweight insulation inside a tin-can cooking stove.

Frequently Asked Questions

Why does 50L of feed stock only yield 30L of biochar?

There is some shrinkage involved in pyrolysis, as almost all the substances of the feedstock are gassed off. It doesn’t mean that any feed stock has disappeared nor burnt. However, if your inner barrel/keg allows a flow of oxygen into it, some of  your feed stock will combust and turn to ash.

Wood and other feed stocks shrink during pyrolysis, leaving around 60% of the original volume as biochar

Wood and other feed stocks shrink during pyrolysis, leaving around 60% of the original volume as biochar

What else can I use to make a biochar retort?

Retorts have been made using all kinds of materials – everything from concrete to clay, oil barrels to large industrial pyrolysis ovens. Having an understanding of the processes involved in the use of any retort, will help you see the differences between designs. Some are more efficient but less portable, others are effective but have a short lifespan (a follow-up use for your retort should be part of your plan). Just remember, temperatures inside a retort could exceed 700 degrees!

 What materials can I pyrolyse to make biochar?

 Any kind of biomass can be pyrolysed to make biochar; wood, leaves, straw, sawdust, corn husks, rice hulls and coffee grinds are all examples. Different materials yield biochar with different qualities, which can be important for large-scale agriculture. Mulch is a great choice because it is made up of different sized particles, leaves, twigs and wood chips, which will yield a varied biochar. Mulch is also a common waste product and can usually be got by the truck load for free! The most important factor is how dry the material is (the drier the better!), as water interferes with fire, and can cause smoke and incomplete pyrolysis.

My biochar retort produces a lot of smoke. What can I do?

It has been my goal over the course of this project to eliminate smoke altogether from a burn. I have achieved this only a handful of times, and cringed every other time as I watched smoke rocketing out the top of the chimney. The dynamics of fire is a fascinating subject, and is complicated when it is contained inside a rocket stove or biochar retort. I have made many small adjustments to my unit – more air-intake holes at the bottom, sides and at the base of the chimney. I have discovered that more air isn’t always better, but is better than too little, and what would be best is to have all of these openings adjustable during a burn. The most important factor seems to be the dryness of the stock used – it should be as dry as possible!

Related Articles

Making Biochar using Mulch (video)

See how I’ve made enough biochar for the whole backyard!

Mulching

“Make your soil as healthy as a forest floor, by mulching!”

How to Compost

“Turn waste into fertiliser and save the planet, by composting!”

11 comments on “Biochar Retort

  1. Biochar is the real tool of the people. Anyone can make biochar and only nature can turn biochar into the holy grail called terra pretta.

    I have a massed a lot of different ways to make biochar along with simple testing methods on my website biocharproject.org its free and open source.

    If everyone becomes self sustaining with their own biochar making then we are one step closer to freedom.

    Awesome post keep spreading the word.

    Charmaster Dolph Cooke
    Australia.

    • Hi Dolph,

      Thank you for stopping by. Here in Perth we have sandy depleted soils. It’s my hope biochar, along with other amendments, can turn Perth sand into sustainable soil. Great to see you’re spreading the word.

  2. Just watched your video on the retort, very cool. I learned today that wood ash can make your soil go alkaline, do you need to filter out the white stuff?

    • Hi Benjamin, yes, wood ash is a common soil additive, and it will temporarily make your soil more alkaline. This will only be a problem if your soil is already very alkaline and you are trying to adjust it back toward neutral, or if you are growing acid-loving plants such as strawberries or blueberries. Otherwise, what’s most important to remember when considering pH of soil, is that life will balance it out. Encourage worms and other decomposers into your soil (you may have to modify it slightly to begin with), keep it moist and covered, and soon enough the pH will adjust to neutral/slightly alkaline. I hope this helps.

  3. would this work at -20C with snow around it. If so, what would be a simple way to capture some heat for my barn?Could copper tubing be installed around it or a boiler on top of it? Great article and drawing. Thank you so much. Fay in Farnham Quebec Canada

    • Hi Fay, so sorry for the delay, I’m sure you’ve found an answer to your question by now! I had a similar thought about freezing cold air rushing into the bottom of the retort, and that pre-warming the air might be a good idea, until a friend of mine told me it’s better to have cold air, because the greater density means more oxygen for the fire. I still wonder if humidity might cause problems, and if your wood is not bone dry you will get some smoke. I did create a lot of smoke during my experimentation, which is horrible, but I plan for the biochar to be in continuous use for the next few thousand years, so hopefully the earth will forgive me by then. Good luck with your experiments, and let me know how it goes.

  4. hi,your sketch of the workings are a little hard to work out.a keg with a hole cut it the top filled with wood material,then lid replaced and soround fire ignighted,how is the moisture from the wood in the keg able to escape? cheers.

    • Hi Mike, thanks for your question.
      The keg is placed upside down in the barrel, and all steam and smoke are forced to escape out the bottom, straight into the fire, which then goes on to cook the keg. If the mulch is too moist, you’ll get a lot of smoke, so make sure you dry your stock in the sun as much as possible before a burn. Check out the video I made showing a burn and how I set it up. There’s a link at the bottom of the article.

      Good luck with your project, and let me know how it turns out!

  5. Thanks for info.I have built a ss thermete that we take camping,it has a firebox elevated ubove the ground with a conicle chimnee surrounded by a 50lt water tank,heats very fast with very little smoke,this fire roars.was thinking of a larger version with an internal fuel pod.I think the key to the efficiency is the cone shaped flue that increases the velocity of the burn.just thought I would share.cheers.

    • Hi David, there’s a basic diagram of my retort in the article, which shows the locations of air intakes and is roughly to scale. None of my holes were measured or calculated, and so I cant tell you what is ideal. What I would do differently if I made another unit, is make sure all the intakes are adjustable during a burn. As it was I had to make adjustments between burns and watch mistakes literally go up in smoke. With adjustable air intakes, you might even find its best to have the intakes a certain way at the beginning of a burn, and another way to help finish it off. Good luck with your design, send me a photo, and let me know how it goes!

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