Several people have inquired as to my method for making charcoal. I'll explain here what I have come up with; it has served me well for a while now.
What Charcoal Is
Uses For Charcoal
I tried various other methods and contraptions. I tried heating wood in a sealed container, trying to pipe the gas off to a burner. This failed. I tried building one fire under another. This failed.
Finally I realized that since only heat was necessary, the best way to heat the wood is from the wood fire itself. The key is not to waste the heat, but to retain it so that it can "cook" the wood. We can do this in an insulated furnace. We admit a small amount of air at the bottom to keep combustion going, and in a nice insulated furnace, the steam and volatiles will escape, and we will be left with a nice glowing bed of charcoal.
We want to burn off the volatiles for two reasons:
My first version was a 20 gallon galvanized trash can, inside a 30-gallon trashcan, with packed earth filling the space between the two, on the bottom and all around. This provides about two inches of packed earth insulation. Then, around the bottom, I drove eight peices of 1/2" copper pipe through both cans from the inside, evenly spaced. That was it. No particular reason to use copper; I just had some on hand, and 1/2" seemed like a good place to start.
I built a fire inside, and slowly began to figure out how much wood I could put in this thing without putting out the fire. This is key; I'm pretty sure most of the heat is generated from burning off the volatiles present in the wood, not from burning the charcoal.
To see this, think of what happens if you heat a log up. The temperature of the outside of the log will rise, and first the steam and then the volatiles will be given off. The temperature is lower inside the log. Let's say the water is driven out at 212F (thereabouts; it's not exactly free water), and other components start around 400 F degrees. We don't need to actually burn the charcoal, in fact we would prefer not too, if we can just heat the log up to 1100 F degrees. Of course what actually happens is that the log shrinks, checks, splits, and crumbles into chunks when it is heated, exposing the hot char (not quite charcoal) to the air, which begins combustion around 720 F degrees.
This is OK, as we need enough heat coming from below since the volatiles mostly combust at the top surface of the can. I was stacking wood around the top to take advantage of this heat somewhat; but this wood will start to smoke, and the smoke will not be hot enough to combust. Perhaps things would be more efficient if air was introduced lower down in the can, at the top of the charcoal bed, so the volatiles would combust closer to the wood.
Once the fire was going, I found that larger logs (all cut to stove length) worked better than small, as there was more air space between them, allowing the heat from the bottom of the fire to rise and keep everything hot enough to combust the gases (smoke) being released by the wood.
I kept loading and burning down until the furnace was about half full of charcoal, and then shovelled it all out into another garbage can, which I covered to smother the coals.
A word on galvanized cans: galvanization is the process of coating the steel cans with zinc. Zinc burns off at about 900F, producing white smoke (which is zinc oxide). Avoid this smoke; it can causes metal fume fever, which makes you feel like you have the flu. This smoke smells slightly sweet to me, so I generally know when it is about, if there isn't enough to see. The dose seems to correlate with the severity of the symptoms: if you only breath a little, you will only feel slightly weird. I've gotten touches of metal fume fever, so I stay back from the area if I know I'm heating galvanized things beyond the temperature at which the zinc will combust. I've read lots of remarks about metal fume fever on the internet, mostly claiming it's a horrible poison; this simply isn't true at the levels of exposure here. As we all know, or should know, the dose makes the poison.
After making five or so batches of charcoal, the inner garbage can in the furnace burnt through. So I used thicker gauge metal to weld up a sort of octagonal barrel to use for the inside, and it is holding up well. I also enlarged the tuyeres to 3/4" iron pipe, which cut down on the flame-outs.
What I will probably do is work out a better way of constructing the cans and the tuyeres all together. I'd be very interested to hear how you improve your furnace, and I'd be glad to add a FAQ section based on any feedback.
Keep the fire going, adding wood judiciously, and you will see the bed of charcoal grow. As the bed of charcoal grows, the source of oxygen for the combustion of the smoke is from the air above the furnace, and the main function of the air coming through the charcoal bed below is to keep enough hot gases rising through the charge, so the smoke is hot enough to ignite and burn off when it reaches the oxygen above the furnace. If you stack in too much wood, it will cool the rising gases below the ignition point, and you will have a huge cloud of smoke fill up the area (neighborhood, in my case!) in short order. Flame out! To remedy this, the best I have been able to do is point a blower (like a hair dryer) into the tuyeres to get the fire hot enough so the smoke coming out ignites again.
When the barrel starts to get full of charcoal, I shovel it out into a metal trash can, and then quickly put on the lid to put the fire out. It will be cool in an hour or so. Typically it is the end of the day for me, and I let it cool overnight.
The best charcoal for the forge is good sized lumps of hardwood. Small bits tend to blow around and shower you with sparks. A simple briquetting device might help make use of the low-quality charcoal.
I understand that some artists like to make their own charcoal. A friend of mine is interested in making some charcoal for distillation of spirits.