Melina Mill

Our feedstock of choice is forestry waste from the Melina mill, just a stone's throw from the shop. Melina (Gmelina arborea) is a fast-growing tropical hardwood native to Asia, related to Teak, that's widely used as a utility timber (pallets, plywood, construction lumber, millwork, even furniture making). Our local mill cranks out pallet pieces which get carted off and assembled elsewhere. The waste comprises "flitches" (semi-rounds from squaring up the logs), trim scrap (mostly in the 2cm by 5-10cm range), and sawdust. We're focused first on the trim scrap, because of its abundance and the fact that it's easy to handle and dries relatively quickly.

Melina is widely grown in vast monoculture plantations and harvested after only 10-12 years. In our area it is cut into roughly 2M logs, dragged out by oxen, and loaded onto flatbeds. There are several Melina mills on the Osa, at least two in the greater Puerto Jimenez area (where our shop is). Once cut, the stumps sprout multiple shoots, and seedlings also sprout from the newly sunlit soil. If plantation sites are not cleaned after harvest, a dense impenetrable Melina thicket will grow up through the trim slash with little prospect of commercial utility.

Can-In-Can Kiln, Takes 2-20

Moisture is the enemy. And waste biomass in Costa Rica's Osa Peninsula during the rainy season is nothing if not moist! To make matters worse, our feedstock of choice, Melina mill scrap, has a reputation in lumberman circles for persistent high moisture content. Moisture messes with you on several fronts: You have to drive it off before pyrolysis begins, so you consume more fuel wood (and time). We're using the same mill scrap for fuel wood, so the difficulty is compounded by smoky fires that are slow to get hot and require that much more wood to get the job done, resulting in that much more ash in the bottom of the fire chamber,

further confounding kiln performance. If that weren't enough, the volume of wood in the kiln retort (inner chamber that gets charred) is substantial enough so the outer portion (closest to the heat) has begun pyrolysis while the inner portion is still, quite literally, blowing off steam. The inevitable result is non-uniformity in the char, and frequently a high fraction that does not get charred at all.

We made a number of modifications to the kiln to improve its performance and accommodate the high-moisture fuel. These included more air intakes, a perforated kiln shelf to minimize ash blockage, corrugated roofing panels as thermal shields surrounding the kiln, and fitting the retort (inner drum) with a metal "diaper" to ease handling and minimize post-firing char burn. We also did our best to dry the scrap by sticker-stacking it and exposing it to the sun by day (running out in the middle of the night wearing nothing but our flip-flops to cover the stacks with plastic tarps if we get caught unawares by a tropical downpour!).

But the most important innovation we implemented was the afterburn. As described in my first can-can post, when pyrolysis begins, the fire really rips, promoting a positive feedback loop of heat-pyrolysis-combustion until (theoretically) all the pyrolysis gases have been driven off and you're left with nothing but char. Problem was, ours left us with a lot of wood, too. The inner portion of the load wasn't getting hot enough fast enough to "keep up" with the outer portion, because it had all that steam to blow off. So I implemented the afterburn:

After active pyrolysis winds down (no more roaring jet-engine-in-a-can), let it sit there being hot (driving off any remaining moisture) until the last embers in the bottom of the fire chamber have nearly burned out. Then re-load with fuel. This "second burning" takes a crack at the now-dry wood in the middle of the retort. You get a second coming of pyrolysis (jet-engine-in-a-can), though not quite as robust as the first. But when it's all done, you'll have nearly all char. (We never got 100% char, due to the fact that the "diaper" zone never sees the flame.)

We were ready to declare victory and proceed to a new kiln design when we were implored by the plant research group fraction in the project to make them some biochar for their field trials. A much larger kiln was under construction but was running behind schedule. Meanwhile, the plant research group had made commitments to graduate students from other institutions who were coming to Costa Rica for biochar thesis research. Plus, the growing season had begun and waiting longer could mess up an entire research season. So we made batch after batch of biochar using high-moisture-content Melina mill scrap in the inefficient can kiln.

How inefficient? An efficient kiln design should have a yield ratio of around 1:3; in other words you'll "sacrifice" (burn) about 1/3 of the wood volume as fuel for the 2/3 that gets turned to biochar. Given design limitations of the can-in-can kiln, at best it's probably a 1:1 proposition. With our high-moisture wood, we were running at 3:1 or worse, turning the yield ratio on its head. Not good. But they really needed biochar!

The English Kiln, Take 1

That's not "English" as in country of origin; we named our kiln after a charcoal-making design that Alex English had experimented with while attending an appropriate technology conference in India some nine years ago (see English Kiln). Unlike our can-can kiln (previous post), this is a "direct" burn design, i.e. the feedstock is partially combusted directly and reduced to char, rather than being cooked "indirectly" from without. The knock on direct burn designs is the smoke they produce--including potent greenhouse gases. Alex added a chimney and afterburner to his, intending to flare-off the smoke, ideally reducing it to mostly heat, water vapor, and CO2.

(CO2?! Isn't that bad? Only if the "C" comes from burning fossil fuel.

The fate of most all living biomass is to wind up as CO2 in the atmosphere; quickly if it is burned, or more slowly as it gets metabolized through the lower reaches of the food chain. Since we're making biochar, we'll be carbon negative--reversing global warming--even if we spill a bit of "C" along the way.)

Our experiences with the can-can kiln told us that the high moisture content of the wood was problematic. We also observed that the board-like form of the Melina scrap often led to broad surface contact between pieces, effectively reducing the useful surface area. In the interests of improving our chances for a decent burn the first time out, we cut the wood into smaller chunks to increase surface area and reduce contact between pieces.

Still concerned about moisture content, we decided to fill the kiln with our little wood chunks and build a small fire underneath to drive off moisture before formally igniting it from the top, as intended. We thought we were pretty clever as we watched a cloud of steam wafting out the top. When the cloud became denser, hotter, and tainted with a brownish color and creosote odor, we started feeling less clever. Wood just doesn't conduct heat very well, and things got a bit too toasty down in the bottom. We had unwittingly initiated pyrolysis in the load.

Of course, pyrolysis is what it's all about; but the design calls for initiating pyrolysis in the top and controlling intake air so the pyrolysis front will move slowly downward through the load. By initiating pyrolysis in the bottom with our external drying fire, we were faced with the prospect of the entire load going off at once. Sort of a green-biofuel version of the "China Syndrome". Not good.

We tried to suffocate the load it by collaring the intake vents, and put on the lid and chimney stack in an attempt to shunt smoke away from the building. But the damned thing had the bit in its mouth and was making a break for it. Dense smoke was pouring out with no sign of abating.

Reluctantly, we decided to abort the mission and overturn the load.

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!