Can-In-Can Kiln, Take 1

My capable collaborator for pyrolysis pursuits in the Osa Peninsula has been Jason (a.k.a. "Pollo Loco"), proprietor of one of Costa Rica's leading renewable energy companies. I'd been introduced to Jason the season before when, while blathering on with another local expat about the unalloyed goodness of biochar, I was told, "You've got to talk to Pollo." Jason was already into pyrolysis; his interest was more on biomass gasification for powering generators--perfectly compatible with my own biochar fixation.

Jason got us queued onto a nascent biochar project coming to the Osa that was funded by a philanthropic organization, with Costa Rica's tropical agricultural research institution and the quasi-governmental "clean production center" as primary participants. We inserted ourselves into the group's circle of communications. Their mission was to create a biochar pilot plant and investigate biochar application rates and plant response. As production scaled up, surplus biochar would be used for local habitat restoration work. Target biomass waste streams included bamboo, residue from African palm oil pressings (a major regional crop), and Melina mill waste (a fast-growing, plantation-grown utility wood species used in pallets, plywood, etc.).

The bamboo and palm oil operations were some distance away, but the Melina mill was just a stone's throw from Jason's shop. So we got a pick-up load (which they were only too happy to load into my truck; mill waste was choking the site, and they regularly had to haul the stuff off to dump it out in the forest--nasty business). First impression: This stuff is WET. So we sticker-stacked it to dry in the sun. Now we needed a kiln.

Designs for pyrolysis kilns are all over the map, from traditional charcoal pits to simple can-based affairs for the independent-minded backyard barbecue-er. If you google "making charcoal", you'll find heaps of links, including lots of YouTube videos. There are two basic approaches; direct, and indirect. Direct method kilns smolder-burn the feedstock by restricting airflow (producing lots of smoke). Indirect method kilns enclose the feedstock in a container and fire it from the outside, using the escaping combustible gases to feed the flames. I found an indirect design based on a 1-gal. can inside a 5-gal. can. I scaled up the design to incorporate a 22 gal. drum inside a 55 gal. drum. Jason made the necessary modification with his cutting torch, and a kiln was born.

We cut the Melina to fit the inner drum (known as the "retort") and stuffed it full. With some awkward gyrations we managed to center it open-face-down on the bottom of the 55 gal. drum. We filled the space between the drums with wood, piled more on top, and lit it. With holes cut around the lower rim of the outer drum, flames were drawn downward toward the incoming air. After a while, combustible gases escaping from the bottom rim of the inner drum fed the flames; it really roared.

After the pyrotechnics burned out and cooled, we overturned the whole affair to see what we got. There was a bit of char, but mostly some slightly blackened wood. This was not to be as simple as advertised on the YouTubes!