Quick and easy biochar in a 200 litre (44 gallon or 55 US gallon) drum

The bush here provides an abundance of sticks!

This is a cheap, quick, easy way to make bulk biochar for soil improvement.
For a clear description of the burning method, see my page: http://bruceteakle.blogspot.com.au/p/blog-page.html

Recently we made our first few drums-full of charcoal purely for the garden. For years we have been making charcoal for cooking and blacksmithing, and using the siftings for the garden and orchard. Unfortunately I haven’t tried to make any sort of controlled experiment with our charcoal use in soil, but Erika and I both feel confident in it as a soil improver, lightening the heavy yellow clay we have in a small part of our garden, and hopefully holding water and nutrients as the evidence indicates.

We had a pile of waste sticks and branches, piled and dried after some tree cutting a few months ago. The material was too thin and light for fuel charcoal, but I was keen to try making biochar with it. A couple of Danish backpacker friends who were keen to play with fire did a some very successful burns in 200litre drums.

The bin is lit lying down, building a strong fire before standing it up with a shovel

Most of the time is spent feeding the fire with sticks

The method is exactly the same as our normal method of making fuel charcoal. The thin sticks mean that the wood chars more quickly and the whole burn takes less time. The work is also more intense as the pieces each have less wood in them and the branches need to be broken and fed in. Burning a full drum of charcoal took our 2 friends about 2 ½ hours, from lighting the fire to tipping the drum over to snuff the fire. They could probably have done 2 drums in a similar time, with enough sticks.

Using the biochar in the garden

Once cold (it was ready the next day), we trolleyed the drums up the hill to the garden, and opened full drums of charcoal. 

The pieces were smaller than usual, and less dense: most bigger pieces were easily crushed in hand. We didn’t sift it (which is usually the dirtiest part of making fuel charcoal) and dumped the bin contents straight into trenches in a new section of garden. 

To charge the biochar with water and nutrients, we flooded the charcoal-filled trenches with septic tank effluent (it’s our own microbes…). Then the trenches were given a quick dig over, backfilled with topsoil, mulched with lawn clippings and planted.

A good method

I think the 200litre drum, open top method is a really good way to make biochar from light material:

• It’s quick: in 2 to 3 hours you can make nearly 200 litres (and probably nearly 400 litres with 2 drums in similar time) of biochar, probably about 30kg per drum (the material was very low density, so the charcoal is also low density).
• It’s cheap: the kiln for making this charcoal is very cheap, simple and easily obtained: a standard 200 litre drum with the lid chiselled off, holes punched under the rim, and 2 steel skewers. If you oil the drum after the burn, it will last for years.
• It’s dry: the method doesn’t need water, so doesn’t waste precious Australian rural water and can be done away from a water supply. The resulting biochar/charcoal is dry, so it is easily transported (this drum-full had to be trolleyed up a steep, rough hill after it had cooled).
• I am confident in the quality of the charcoal, relative to other methods (more on this below).

Less effective methods?

I see a lot of biochar making methods on the net which strike me as difficult and unproductive. Most involve a double drum system, in which a small drum, preloaded with wood, is cooked inside a larger vessel. This clearly works, but is sadly unproductive: sticks are wild, crooked things that don’t easily pack, and which shrink a lot on charring. This leaves many of these double drum methods producing 2 or 3 buckets of biochar after hours of work, using equipment which is relatively complex and time-consuming to construct.

I don’t see the benefts of using double drums, compared to using a single container with fire and charcoal within. There is probably a higher proportion of the wood in the inner drum becoming charcoal, but this is (at least) balanced by the loss of wood burnt in the outer drum. Most importantly, this method is highly inefficient in time: kilograms of charcoal per hour of work; when most of us have abundant sticks to burn.

Other approaches are more similar to our drum method: sometimes called “Kontiki”, “flame curtain”, “open top”. Wood is burnt in an open-top vessel, or pit in the ground, very similar to burning in a drum. However most of these methods extinguish the fire using water. This is demanding: you need to be able to get 100s of litres of water to the fire; and the resulting charcoal is wet, heavy and much harder to transport. Also, I suspect you will often leach out valuable ash minerals from the charcoal, when draining the vessel after the fire is cold.

To me these methods are looking for a lid to extinguish the fire, to avoid needing water and to produce a dry, easily transported charcoal.

Biochar quality

I’m confident that charcoal made this way is as good as any other DIY method available for use as a soil additive - as biochar. I don’t have empirical evidence from experiments: the fertile soils of our gardens and orchards might have been just as good without charcoal, but we get the idea that the charcoal has been helpful.

Claims about what characteristics are important in biochar are diverse and deserve sceptical consideration. Some sources consider that it is important for biochar to include wood-derived tars and oils (from the smoking wood), to feed micro-organisms which will take up residence in the pores. Others are concerned about the toxicity of these same chemicals, and advocate charcoal of a high purity with minimal condensates.

Charcoal making is always a balance between over-cooking and burning some of your charcoal to ash; and under-cooking and making smokey charcoal. Slower methods in better-insulated kilns are likely to produce more consistent batches, as they give larger pieces of wood time to heat through and boil out the smokey content. These methods can take days per burn. Fast, open methods (like I use, or Kontiki) are less consistent, and always over-burn some charcoal to ash, as well as leaving some pieces of undercooked wood.

My conclusion so far is that claims that particular methods of making biochar are correct, or that particular types of charcoal are “biochar” and other types aren’t, are not supported by evidence. Biochar is simply charcoal being used as a soil additive. There is a huge diversity of types of biochar, made from a wide range of biological matter, using a range of cooking methods, and having various effects on plants.

I recommend reading some of the science-based literature on biochar, such as Biochar in Horticulture by NSW Department of Primary Industries.

Room for improvement

I’m sure this method could be improved:

§  A bigger vessel makes more charcoal in less time with less work. There is less cutting and handling, as bigger pieces can be put in whole. The fire is hotter and heats the wood quicker and cooks it more consistently. A bigger vessel would either need to be still able to be tipped over, or would need a method to seal the lid reliably (probably with soil, like my big bin).

§  Some closable air vents would make starting easier. I used to weld in threaded pipe pieces to charcoal drums, but I stopped because it makes more work, makes the method less accessible to people without metalworking skills and equipment, and it works pretty well without them.  

Making charcoal in a pit

Charcoal can be made in a pit in the ground. This is perhaps the oldest and easiest way to make some charcoal.

When I first learnt how to make charcoal for blacksmithing, from my (some time later to be…) Father in Law Adrian Hobba, it was in a pit in the ground. The pit was around 0.5m deep, something under a metre wide, and around 2m long. We made charcoal in the pit very much like we do now in drums.

We brought wood to the pit, lit a fire in the bottom, and fed and spread the fire until it filled the bottom of the pit. We would keep adding wood on top and letting it burn, until the pit was full of coals. When finished, the pit was covered with overlapping pieces of old corrugated iron placed across the pit, and then sealed with sandy soil, placed wherever smoke squeezed out.

At first Adrian’s pit was just a hole in the earth. Later we lined the walls with brick to stop the sides falling in and to help keep soil out of the charcoal. I’ve also made pits and lined them with old corrugated iron to keep the charcoal clean.

The pit made all of our charcoal until I became dissatisfied with the need to handle and transport several times more weight of wood than we produced in charcoal. Surely it would be more easy and efficient to take a steel container to the wood supply than to carry the wood to our pit! That’s when I started to try various steel bins and drums.

If you want to make charcoal from a large supply of wood in one area, a pit could be a good way to go. It’s cheap, not that hard to dig, and won’t rust or burn out.