I've spent a considerable time both researching and experimenting to find a good way to make biochar. My main conclusion is that there are many many bad ways shown on the internet to make biochar and the main problems are:
I concluded that I wanted a fast method, that could burn green freshly cut branches from hedge trimming and scrub clearning in the garden. It needed to be simple and not produce excessive smoke. It also needed to be able to deal with relatively large quantities at once. I wasn't concerned about getting the maximum possible weight of biochar from the waste material burnt, instead I sacrificed a small amount of yield for ease of operation and clean burning.
The counter-intuitive method I settled on basically consists of filling a drum with close packed branches and twigs. The drum has an open top and bottom and is supported a few inches off the ground by three sturdy blocks of wood. The drum itself has large air holes cut into it two thirds of the way up the side - mine are triangular, but only because that was easiest to cut with the jig saw at the time.
Once the drum is stacked to the top dry newspaper and kindling is placed across the TOP of the drum and lit.
This dry kindling must spread across the whole top and be sufficient quantity to get the fire established across the width of the drum quickly - this is important to prevent smoke while you make biochar
I haven't seen anything quite like this anywhere on the internet and I can vouch for its effectiveness - I've done about 10 loads in the past few months and will use it to make biochar through the winter too.
The newspaper and kindling is lit and flame spreads rapidly across the drum. The heat from the fire cooks the wood beneath which pyrolyses and gives off smoke. This smoke rises through the very hottest part of the fire where it breaks down into very simple 'clean' syngas (mostly hydrogen and carbon monoxide). Once this gas rises above the hottest part of the fire it combusts with oxygen from the surrounding air above the top of the drum.
Within ten minutes the flames will have descended all the way to the bottom of the drum and within around 20 minutes of being lit the flames will die down substantially. At this point the wood has essentially finished charring and you are ready to put the fire out and collect your fresh homemade biochar.
To do this I tend to douse the whole thing with some water from a hosepipe then, wearing a big heavy fireproof glove, I drag the drum away from the remaining pile of biochar. Spray a bit more water to cool the whole area down then simply shovel the char into a big container of water to stop it burning further.
At this point you could spread your wet biochar onto the soil, however I prefer to spend a few minutes breaking up the larger bits first and then compost it with my normal garden waste. Then when you spread the compost on the garden the biochar is already mixed in with it and has had a chance to absorb nutrients and allow fungi and bacteria to get established - a real bonus for developing healthy soil.
Currently I don't have anyway to measure my yields so I can't say for certain what mass of carbon I'm getting out per load. Unfortunately adding water to the char messes with any yield calculations based on mass. My gut feeling is that it is a 'worthwhile' amount and it certainly gets rid of garden waste quickly (and spectacularly!!!).
To get through lots of waste I would like to have a second one made up exactly the same way - then you could be loading one while the other burns out and get through twice as much in the time.
When loading try to put smaller diameter sticks at the bottom and larger at the top - the higher they are in the barrel the more time they get to char completely
After you first make biochar for your own garden I'm sure you will see both many more ways to produce it, as well as many possible fuels to use. I'd love to hear about any successful (or even unsuccessful!) experiments.
We are looking for many more personal stories of your successes (and failures!) making biochar. Whether you are making 100 tonnes a week or a have an occasional burn in your backyard, let us know.
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