Why would anyone want to make their own charcoal? For one thing, good hardwood lump burns hotter and cleaner than briquettes and is much easier to light. You also know where it came from, what it contains and what was done to it en route. There are endeavors other than barbecue that require high quality natural charcoal: It is still the preferred fuel for forges and blacksmithing. Folks who make their own fireworks and black powder need specialty charcoals with specific burning properties such as that made from willow or grapevine. When grilling or even barbecuing in open pits with charcoal and wood, the quality of the charcoal is really not that critical. There is enough airflow to dilute impurities. Now that I have a Weber Smoky Mountain, though, charcoal quality, impurities and additives become very important. It is a great little cooker and will do everything folks say it will, BUT there is precious little airflow and the meat is bathed in smoke for hours. What you burn, you eat! I have read how the major manufacturers make briquettes. That leaves me either burning to coals, which is impractical for the small amount of coals needed by the WSM, or making my own lump, which is just a way to burn to coals and store them for use as needed. Being somewhat of a skinflint, I would rather utilize the resources at hand and make my own lump as opposed to buying it. My objective in this endeavor was to use existing technology to design a simple, cheap, reliable and efficient method for the small-scale production of charcoal for home use utilizing readily available materials and minimizing the release of pollutants.
How to make Charcoal:
Timing is important. Plan to start your burn on the hottest, muggiest day of the year with a comfort index of at least 105 and air quality just above the minimum to sustain life. These conditions won't affect the charcoal process at all but will ensure that the experience is memorable.
There are two basic methods of making charcoal: direct and indirect:
For my first tests, I decided to try the indirect method. There had been some posts on a pyrotechnics newsgroup describing a procedure for making small quantities of willow or grapevine charcoal in a cookie tin or five gallon bucket. For the furnace, I used a 55 gal oil drum with the top cut out and a 12" wide X 10" high hole cut in the lower side for maintaining the fire. I used two iron rods stuck through the sides about 8" from the bottom to support the retort. I also kept the top which had been cut out. After the fire was well established , the top was placed on the drum and supported by rods to help hold the heat in yet allow a good draft. The retort was a 16 gal. steel drum with lid and I cut about six 3/8" holes in the bottom with an acetylene torch. I burned it out well in the furnace to eliminate petroleum residues. These drums are used for lubricants such as transmission fluid and gear grease and are readily available.
After the retort was loaded with air dried hickory the top was sealed and the drum was placed in the furnace or burn barrel. Wood scraps and bark were placed under the retort and around the sides and lit with newspaper assisted by a little burnt motor oil to get things off to a fast start. There was right much smoke for the first hour, but as things heated up and the moisture was driven off, it burned so clean that all you could see were heat waves. With the vent holes located in the bottom of the retort, the vapors and gasses were discharged into the hottest part of the fire and burned.
I stopped the first test too soon and only had about 1/3 charcoal. The rest was charred chunks of wood. The second test burned for about 3 hours, until the gasses had just stopped burning around the holes in the bottom. Results: 56# of wood yielded 17 1/2# charcoal or 32% by wet weight. Assuming an EMC (equilibrium moisture content) of 12%, The yield exceeds 35% on a dry matter basis. This is very good as most direct burns result in 20 to 25% at the best. I got over 2 1/2 five gallon buckets of good lump and only one large (4"X6") chunk showed signs of incomplete conversion with some brown in the center.
I was going to run a series of trials to compare the indirect method with direct (bottom lit) and direct (top lit). After several burns using the retort, I decided that there were such obvious advantages to the indirect method that I abandoned studies of direct burns. The retort method is easy, reliable, and does not require the skill and attention of direct burns. The equipment and materials which I used are readily available worldwide. As the gasses and volatiles are discharged into a hot bed of coals, I believe that most of the pollutants are burned, adding to the furnace heat. I also suspect that yield and quality are better. From what I have read, 35% by dry weight is excellent; the resulting charcoal burns hot and clean; you can almost light it with a match.
The indirect method also appears to be more compatible with heat recovery and waste wood utilization systems. I live on a farm in Virginia and my wife operates a small sawmill. Disposing of slabs and wood waste is a serious problem. I can burn a lot of the hardwood slabs in my indoor masonry heater/cooker. We have not found an economical use for pine slabs (we can't give them away) and have started burning them in a field. This is obviously a wasteful and polluting practice. My ultimate goal is to build a small masonry furnace that would hold several 55 gallon drum retorts and recover heat for domestic space heating during the winter. Charcoal could be a marketable by-product. I would burn pine slabs and waste wood in the furnace and make charcoal from hardwoods in 55 gallon drums. This approach appears to be very energy efficient as the gasses released by destructive distillation are utilized.