Saturday, July 31, 2010

Big Kiln Burn

Our large kiln, while not particularly grand by commercial standards, is plenty big enough. At 2M tall and 1.6M diameter, the kiln basket has a volume of around 4M3, a capacity to produce about 300 kg of biochar from a load of scrap wood. The hood portion has an integral water jacket and internal gutter for condensing and collecting wood vinegar (insecticide and plant growth stimulant). The kiln base serves as the primary fire chamber. A large gantry crane is required to lift of the lid and remove the kiln basket for loading and unloading.

When the kiln was originally fabricated the prior year by a shop near San Jose, a series of miscues and lack of qualified engineering oversight resulted in a number of defects and deficiencies. These were noted last August when Stephen Joseph was on hand to commission the kiln. Stepehen determined that further operation would rapidly result in degradation and eventual mechanical failure, so he left the Costa Rican engineers with a fix-it list. They prepared some engineering drawings, but a breakdown in communications between the NGO administering the project, the organization charged with building the unit, and their engineers and machine shops resulted in a stalemate. Six months later, the kiln was still in the same sorry state. When funding was approved for the 2010 season, we (Biocombustibles de Costa Rica--BCR) were given the nod as mechanical contractors, with on-site engineering oversight by the kiln's original designer, Nik Foidl of Austria. That's when we settled on the big TLUD to prime the kiln (see prior post). 

When we took delivery of the kiln, a number of additional materials and mechanical faults were noted, and more still when Nik finally arrived. There was a flurry of work, with lots of improvisation to substitute for materials we were unable to track down, or specialty machine shop work that would have resulted in excessive delays. Throughout this period we experienced exceptionally high rainfall. Since the kiln and TLUD were out in the open, the rains significantly limited our testing time. In addition to abundant rains, here in the humid tropics the equilibrium moisture content percentage of seasoned wood hangs in the mid- to high-20's. Driving out all the water significantly increases the amount of time and fuel needed to prime and operate the kiln. It also adds so much water vapor to the exhaust stream that the combustible gases of pyrolysis can be impossible to ignite for much of the firing cycle, resulting in unacceptably high emissions. A drying oven--a converted shipping container powered by the waste exhaust from the kiln--had been designed into the facility, but this would be one of our last mechanical tasks once we were under cover. 

So there we were, out in the mud, dodging rainstorms, cobbling together hardware and complaining about wet wood. We did one run with high moisture content wood, quite memorable for the quantity of smoke produced that day! After that, we used the exhaust from my diesel truck to pre-dry the sawdust, and a half-load of sawdust in the TLUD to pre-dry the kiln load--incredibly inefficient, but better than smothering the planet with smoke.

Finally, we had a load of dry sawdust in the TLUD, and a reasonably dry load of wood in the kiln basket. Ready to fire! The TLUD was connected to the kiln through three 10cm automotive steel flex tubes, wrapped with insulation, and connected to injection ports in the kiln base. Though we'd already run and tested the TLUD, optimizing heat transfer to the kiln proved tricky. At one point the bonnet of the TLUD got so hot that the automotive flex tubes failed, resulting in an all-hands-on-deck fire drill with welding gloves and baling wire. We eventually figured out that the TLUD temperature was less important than the delta T between TLUD and injection port, and maximum air mass transfer was key. Finally, the kiln load crept up to 300C and could sustain pyrolysis without the TLUD. We switched off the blower, sat back, and watched the show. Click here for an annotated online slide show of the big burn.  

Next up, building a roof structure to house the kiln and TLUD, integrating the drying oven, building a new much larger BMC reactor to replace last years mechanical disaster, and a specialty cross-draft pyrolyzer for African oil palm waste (pending approval of funding).

My TLUD's Bigger 'n Yours

The kiln load (Gmelina mill scrap) must get up to around 280C before it will produce enough combustible gas to "kick over" into pyrolysis mode, then gases will burn in the kiln's fire chamber and generate sufficient heat to sustain pyrolysis until the entire load has been carbonized. Our kiln was designed to have combustible gas from a different source injected into the fire chamber to "prime" the kiln until it could produce enough combustible gas to sustain the reaction on its own.

We opted for a biomass gasifier for this purpose. Nik Foidl designed a beast for us––a TLUD (top-lit updraft) sawdust gasifier a meter in diameter, capable of generating 200-350kW. That's a lot of hot! A blower injects air into a space in the base of the unit, and the perforated floor of the basket allows air to migrate upward through the sawdust. Squirt a bit of kerosene onto the surface of the sawdust and ignite to get it started. Then put down the hood and turn on the blower.

The flame front migrates downward, toward its oxygen source, producing a mess of smoke (mix of combustible gases). A pair of air ports in the hood of the TLUD introduce more air, causing the smoke to burst into flames, and the hot exhaust gases are injected into the kiln. The TLUD is powered by a single large blower regulated by butterfly valves. The primary butterfly controls the total air introduced into the system. A "Y" and pair of butterfly valves control the relative amount of primary air, which is injected into the base and blows through the sawdust; and secondary air, which is injected into the hood to ignite the gases.

Hot, hot hot!

Know of a bigger TLUD? We'd love to hear about it!

Monday, June 7, 2010

Nod to Nik

Nikolaus Foidl, designer of the Costa Rica Biochar Project's large kiln, is a brilliant scientist & engineer with wide-ranging intellectual appetites and an unquenchable thirst for knowledge. He marked his birthday with us here on the Osa Peninsula. We celebrated with a BioChar-B-Que. Happy Birthday, Nik!

Sunday, May 30, 2010

The Big Chill, and Fire!

After making biochar in our kiln, we allow the retort drum to cool to ambient temperature, for ease of handling and to avoid the risk of the hot char self-igniting. Curiously, when we take the lid of the drum, the char cools abruptly. What's up with that?! We kicked it around and concluded that it's probably due to rapid evaporation of volatile compounds. On completion of pyrolysis the atmosphere in the retort is saturated with residual volatiles. When the lid is removed, these diffuse into the air, triggering evaporation from the manifold char surfaces as the system equilibrates to the new atmosphere. The coolness we're experiencing is the latent energy of evaporation (in the same way that rubbing alcohol feels cool on the skin, and then is gone).

Far more consequential is spontaneous combustion! We had just bagged some biochar (day-old and auto-chilled as above) and chucked it into the back of a station wagon for transport. Then we resumed work re-loading the kiln. Fortunately, one of our group noticed the smoke and we quickly removed the burning bags and quenched them with water.  If you google "spontaneous combustion" (bypass the apocryphal tales of "Human Spontaneous Combustion"), you find sites insisting that spontaneous combustion of a bag of charcoal is the stuff of urban myths (I can assure you, it is NOT). The best explanation I found was here: Crushing the char before stuffing it in the bag exposes heaps of surface area to fresh air; stir in the heat of friction from crushing, add a lick of water from humidity condensing on the char surfaces to catalyze oxidation reactions, and Voilá! Instant fire. Take care with handling fresh biochar! 

Dr. TLUD and the Coffee Caper

A couple weeks ago I took a break from my kiln work on the Osa to visit a very different sort of biochar undertaking in the coffee country of Costa Rica's Santos region: a women owned and run stove building workshop (APORTES/ Givers) for use by the families of coffee pickers.

The project originated when Arturo Seguro of Sol ColibrĂ­ coffees of Costa Rica, on a marketing mission in the Seattle area, happened upon an environmental fair where Art Donnelly of SeaChar (Seattle IBI chapter) was manning a table demonstrating a TLUD biochar-producing cook stove. Arturo saw in the stove a solution to a problem: Coffee pickers are mostly seasonal migrants from Nicaragua and Panama who live in tin-roof shacks, generally without electricity or running water, cooking on smoky open-hearth fires—a notorious source of upper respiratory health problems and driver of habitat destruction from overharvesting wood. This is a big problem, impacting the lives of 10's of thousands of agricultural workers. 

After a couple of design iterations, and the capable guidance of Paul Anderson (aka Dr. TLUD), they had devised a stove based on a 5-gallon pail that would be efficient, smoke-free, and produce biochar in the bargain. Art and Paul were on hand to help Arturo and crew kick-off the workshop. The stoves will sell for around $30. I brought sample back to the Osa with me; judging from the reaction of my Costa Rican colleagues, the TLUD stove may enjoy a much wider reception. Although most area farm families have propane cook stoves, they still do a lot of cooking on wood as well, and the advantages of a high-efficiency smoke-free stove are obvious!

(Unfortunately, the trip was marred by a failed wheel bearing on my truck during our descent back to the coast. Ay caray!)

Saturday, May 15, 2010

Rocket Retort Rocks!

Yesterday was the christening of my new kiln, the Rocket Retort, a culmination of many months of research, design, contemplation; and a recent spate of hard work. Like so many others bit by the biochar bug, I wanted to create a kiln for my own use. I also recognized the potential that a practical, high-performance, "personal" biochar kiln could have in leveraging distributed production among home gardeners and other small stakeholders, and perhaps ultimately, subsistence farmers worldwide. My prior experience of small biochar kilns, gleaned from YouTube profiles and my own backyard pyrotechnics, had been of barely-contained conflagrations that produced an uncertain sort of biochar. My Rocket Retort design was informed by my work as hardware development manager for a philanthropic-funded biochar project in Costa Rica, involving a much larger kiln designed by Nikolaus Foidl and guided by Stephen Joseph, two of biochar's leading lights. Design criteria for my personal kiln include:

  • Low cost materials
  • Basic shop tools only
  • Low emissions
  • Efficient biomass conversion
  • Controlled firing profile
  • Recycle pyrolysis gases
  • Collect wood vinegar

The 55 gallon drum--durable, affordable, widely available, easily handled--is at the heart of the design. A removable-lid drum stuffed with wood mill scrap serves as the retort. To prime the kiln, I had been considering scaling up one of the newer innovative biomass stove designs, but felt stymied by the challenge of refueling and controlling output. On a suggestion from stover-friend Charlie Sellers, I looked into the Rocket Stove (, a versatile design that addressed my emissions, fuel feed, and control concerns. The rest of the hardware fell into place after a bit of "outside the drum" thinking: Create fire chamber and insulating jackets (two total) by cutting ends off drums, slitting open, and welding inserts cut from a third drum. The tricky bit was opening the slit drums evenly to maintain the roundness of the now-larger cylinders. The nesting Russian doll cylinders rest on staircase ledges in the modified rocket stove base. Each cylinder is topped by a shallow cone-shaped lid with a central exhaust vent made by cutting a sliver wedge out of a sheet metal disc and welding the cut edges together. The lids are secured by bolts welded to the inside rim of the cylinders.

The other design consideration was collecting wood vinegar (natural pesticide and plant growth stimulant) and recycling pyrolysis gases. A two-inch steel pipe was threaded onto the bung hole on the lid of the retort drum, exiting holes cut into the shallow cone lids, and elbowing down toward the stove's fuel feed opening. A "T" fitting and valves enable directing evolved gases toward either a condenser pipe leading away from the kiln to collect wood vinegar, or directly into the fuel chamber to fire the kiln. The fuel feed opening is divided horizontally by a stainless plate, with the lower portion intended for intake air. Being able to block the throat of the upper portion of the feed chamber enables greater range of control and can improve combustion efficiency by limiting excess air.

We were thrilled with our first firing! The rocket stove enables ramping up temperatures gradually, which could be a big advantage when working with high moisture content feedstocks. The cross-over from distillation to pyrolysis was fairly tender. Directing all of the gasses into the stove's fuel chamber resulted at first in an over-temperature condition, which was alleviated by diverting pyrolysis gasses out the vinegar condenser pipe--at one point flames were shooting out two meters (very dramatic!)--stimulating conversation on the various uses to which these surplus combustible gasses could be put.

For future firings the kiln will be fitted with thermocouples and a multi-station digital thermometer so we can approach pyrolysis temperatures a bit more gingerly, with the goal of achieving a longer soak at the lower end of the pyrolysis range to retain more organic compounds in the carbon matrix for a more plant-effective biochar. Separately, I'm  working on a design for a rotisserie-style reactor for making biochar mineral complex (BMC)--a step up from garden variety biochar. Wood biochar, clay, chicken litter, and mineral nutrients (rock phosphate, calcium, etc.) will be blended and loaded into a 55 gallon drum mounted laterally over the rocket stove for tumble-heating at sub-pyrolysis temperatures, to create a substance resembling aged terra preta (based on the pioneering work of Stephen Joseph). 

It is worth noting that labor was not among my design considerations. Although labor cost is crucial in commercial economic analysis, home gardeners are known to lavish lots of time on their gardens, heedless of return on their labors. Likewise, backyard biocharers generally do it for the benefit of their garden and for sport (the thrill of the burn). As for the ultimate target audience, subsistence farmers, the low-value of their labor is one of the snares of the poverty trap. Producing biochar, and improving the productivity of their agriculture, might just help them pick the lock.

For a captioned slideshow, go to: Rockin' Rocket Retort.  We'll get a YouTube together soon.