For quite a while I have wanted to make charcoal. It is useful for the barbi, it can be used in a burner to keep the frost out of the green house and it can be dug into the soil to make Terra Preta. In the mean time my pile of branches has grown into a fire hazard. If one of my pyromaniac boys lite it, it will wipe out at least three peach trees and probably damage a nearby green house. My first efforts were pretty unsuccessful. I tried making a bonfire with some of the branches, feeding in new wood for a while and then when it had burnt down, shoveling soil over the pile and adding more soil where ever a wisp of smoke appeared. Usually when I opened the pile the next morning, there were still glowing coals inside, showing that air was still reaching the center and there were large areas of white ash with very little charcoal. It also involved shoveling a lot of soil. Yesterday, I tried something new.
I had a 45gallon (200l) steel drum available so a fire was started in the bottom of it. Branches were added from the pile, often sticking up well above the drum and as the bottoms burnt off, the tops were picked up from around the drum and stuffed back in. Branches were added until the drum was about a third filled with burnt material and then only twigs were tossed in from around the drum. They flared up, broke and died down pretty quickly. At this point there was very little flame. The drum was then gently tipped over on its side and then upended so that the open side was face down in the dirt. Virtually all the contents were still inside. A bit of dirt was kicked around the outside of the drum to exclude air and the drum was left over night
Next morning the drum was tipped back on its side and there was about a third of a drum of cool, extinguished charcoal of all sizes from the largest branches to the smallest twigs and leaves.We broke open some pieces to see what was inside. They were charcoal right into the middle. Even very fine material such as leaves crumbled in out hands, carbonized but not consumed. So what is happening here.
Wood consists of mainly hydrogen and carbon. When you pyrolyze (heat without oxygen) wood at about 500 degrees C, you drive off volatile material which is much like crude oil with all the various fractions found in crude oil and you are left with carbon (charcoal) and some mineral material which if you burnt off the carbon would be the ash. The trick to making charcoal is to somehow burn the volatile fraction to create the heat to pyrolyze the wood without burning off the charcoal. The 45 gallon drum seems to fit the bill.
As you burn wood, the volatiles come off and make flame and of course some of the charcoal burns too. As you add more and more wood in the drum, a layer of charcoal starts to build up on the bottom of the drum but the oxygen coming down into the drum is used up by combining with the volatiles which are being driven off by the heat. There are no air holes in the bottom of the drum. You see the burning of the volatiles as flame. Very little oxygen gets to the charcoal and if the charcoal on the top is being burnt to some extent, it is excluding oxygen from deeper charcoal. In fact, twice now, when I tipped the barrel back over next morning, there was a layer of unburnt kindling that I had used to start the fire. A sure sign that Oxygen was not getting down to the bottom.
The experiment started with the drum sitting on its base in the vertical position. The burning was poor. The drum was then tilted by putting a piece of fire wood under one edge of the rim. That worked better. The drum was off-vertical by 20 or 30 degrees and what happened is that the hot gasses rose along the upper surface, drawing air down the lower surface. The burning worked really well once the drum was tilted.
In about the third batch it was noticed that if the branches were pushed down into the growing charcoal layer, the bottom of the branches didn't char. It is better to just let new branches to rest on the top of the building charcoal layer.
Just a warning. Charcoal is very easy to light. If an ember from the fire drops on the finished charcoal from a previous batch, it will light up. There will not be any flame and the only sign of burning, besides the heat, will be a white patch on the charcoal pile. Let this go for half an hour and the whole pile will be fully alight.
Happy charcoal making.