What is Terra Preta?

Biochar is widely associated with Terra Preta, the famously fertile and persistent anthropogenic "black soils" of Amazonia. I had understood Terra Preta soils to be the result of deliberate "slash and char" practices by native Amazonians, as opposed to the much more destructive "slash and burn" agriculture introduced by European settlers, which leaves soils degraded and depleted after only a few growing seasons.

As reported in Charles Mann's book 1491, an extraordinary account of civilizations in the New World prior to contact (everything they taught us in school was wrong!), native peoples lacked the steel tools that would have been required to slash much of anything. He even recounts an experiment wherein rainforest locals were hired to chop down trees using the relatively primitive tools that were known to have existed prior to contact. Slashing your way into burning a forest plot for a brief agricultural fling, without steel tools, just doesn't fit into anyone's energy budget, sustainable or otherwise!

When we look at the agricultural practices of the contemporary descendents of Terra Preta's creators, a different story emerges. An episode of the BBC documentary series Around the World in 80 Gardens tells of Terra Preta soils made by smolder-burning rotting wood and then mixing with ashes, shards of unfired clay pottery, spoiled food, and other organic wastes; as demonstrated in this six-minute video clip:

For a more extensive recounting of the Terra Preta phenomenon, check out the BBC special: The Secret of El Dorado. This more complex view of Terra Preta as a fertilizer that was deliberated produced from char, minerals, and organic debris is consistent with lab analysis of Terra Preta soil particles, which appear to be accretions of organo-mineral nutrients around a char core.

When you think about the way plants acquire nutrients from the soil, and the long timescale of human cultural evolution, it just makes sense that, once the benefits of char as a soil amendment were observed, it would be embellished and improved upon by astute gardeners whose very livelihoods depended on successful experimentation and innovation. The question now is, can we likewise improve on basic biochar by learning a lesson from this ancient agricultural wisdom; only do it on a much grander scale through the judicious application of technology?

Biochar Guru

I had the pleasure of spending some time in the company of Stephen Joseph, co-founder of the International Biochar Initiative, and one of biochar's true pioneers. Stephen was visiting Costa Rica as a consultant to our project, a slight detour from his home in New South Wales, Australia en route to Boulder, CO to attend the North American Biochar Conference

Stephen is a scientific genius, brilliant engineer, and visionary. Drawing on clues from multiple disciplines, ranging from surveys of "fired biomass" agricultural traditions to analyses utilizing state-of-the-art laboratory research tools, he has deduced how Terra Preta soils were created (not simply "biochar", as widely believed) and postulated plausible mechanisms by which they derive their extraordinary fertility and persistence. Next, he reverse-engineered their structure and came up with a recipe for their synthesis. He then went on to perform field trials with this synthetic Terra Preta, and has demonstrated extraordinary plant response, exceeding both biochar and conventional fertilizer treatments. This is an important accomplishment, coming at a time when biochar field research shows generally promising but often inconclusive results, and the mechanisms of the biochar/soil/plant interaction are still subject to speculation.

While Stephen's work is recent and needs to be corroborated by further study, it has important implications for commercialization potential, carbon balance analyses, and future research directions. Following Stephen's lead (a hasty white board sketch and much fast talking and hand waving), we are building a reactor to create synthetic Terra Preta from biochar, soil minerals, and other ingredients. This synthetic Terra Preta will be evaluated in field and greenhouse studies by CATIE (tropical agriculture research institution) along with simple biochar and conventional fertilizer treatments .

Osa Biochar Project Kiln

The kiln built for the Osa Biochar Project was designed by Nikolaus Foidl, through association

with the International Biochar Initiative (IBI). It is a moderate size retort kiln (4M3 capacity) with a removable lid and basket for loading/unloading by means of an overhead hoist. It's a clever design, with a central chimney that passes through the retort from the wood-fired primary combustion chamber below. Once pyrolysis is underway, secondary air inlets promote combustion around the sides of the retort as well, for more even heating.

A unique feature of this kiln design is a water jacket for condensing pyrolignous acids (wood vinegar) and lower temperature pyrolysis gases. Water is circulated through the jacket while temperatures climb through the 150-280C range, and the condensate is routed to an exit spout with a dip tube immersed in water to further condense the smoke. These "smoke chemicals" have been demonstrated to stimulate seed germination and promote plant growth, and are now considered the main actors in fire-response vegetation such as chaparral. (Back in the day when I was studying this stuff, heat of the fire was held to be the trigger that stimulated germination and sprouting.)

Once temperatures in the load have reached 300C, the reaction became self-sustaining (no more need to stoke the firebox with wood), and the chimney outlet holds a strong flame. A few hours later, pyrolysis burns out, and the load needs to cool before exposing to air. The char is then crushed and screened. First production from the new kiln is being used in plant growth field trials conducted by CATIE, the Costa Rican agricultural research institution.

The kiln will eventually be set up at the site of the Sustainable Agricultural Center at La Palma, where diverse feedstocks can be processed into biochar for local farmers. It may be fitted with additional apparatus to utilize waste heat for crop drying and pre-drying on-deck kiln loads. A specific need that has already been identified is powering a drying kiln for timber bamboo grown by the local "Amigos de Bamboo" agricultural cooperative--a key step in promoting commercialization of locally cultured bamboo as an alternative to harvesting rainforest trees for construction.

How Dry, and Why?

Dryness matters, especially when you're working with wood waste, as we are. As discussed in a previous post, excessive moisture content (MC) in the feedstock hurts you on several fronts:

• It takes more time and fuel to reach pyrolysis temperatures;
• More fuel means more ash in the fire chamber that can interfere with thermal flows;
• In a retort kiln pyrolysis of the load will be uneven, compromising the quality and yield of the char;
• In a direct-burn kiln, the result is excessive polluting smoke and difficulty flaring-off waste gases.

A word about Moisture Content (MC) of wood and Relative Humidity (RH) of air. Both are percentages of water, which only serves to mislead, since they have about as much in common as apples and Frisbees. MC refers to the amount of water in the wood as a percentage of dry weight. Depending upon the wood species, freshly cut saturated wood will have an MC in the 30's. Relative Humidity is the percentage of the maximum potential water content of air at a given temperature. Here's the tricky bit: the maximum potential water content of air varies with temperature. A lot.

Under most terrestrial conditions, the equilibrium moisture content of wood (EMC) will be in the teens or drier. Wood waste that's allowed to air dry will approach the EMC as a function of surface/volume and how it's cut (end grain dries faster than other surfaces). Just age it a while, and you're good to go. But in the humid tropics, where temperatures are high and the relative humidity hangs in the 90's, typical EMCs will be in the 20's. Residents of the humid tropics are all too familiar with high EMC's; perfectly clean T-shirts get all moldy-smelling, not because they're dirty, but because the material is damp enough to support fungal growth (yuck!). EMC's in the 20's are high enough to complicate pyrolysis. Simple air drying is not dry enough.

The trick is to elevate the temperature and move air over the surface. Heating air from 30 to 60C takes the RH of the air from 100% to 25%. A given parcel of air that's been heated contains the same amount of water, but now its capacity to do the "work" of drying is much greater. Going from 30 to 60C is a piece of cake with a solar drying kiln. Making a drying kiln that's cheap and simple to use with unwieldy mounds of biomass is the design challenge. Stay tuned...

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!

The Pyrolysis Arts

Just as "biochar" sounds like something you don't want to have happen to your steak; "pyrolysis" sounds like something you don't want to catch. The connotations are unfortunate. Because "pyrolysis" of biomass is how "biochar" is made.

Pyrolysis is defined as thermal degradation in a restricted-oxygen atmosphere. When you pyrolyze biomass, you wind up with a mix of combustible gases, volatiles, and solid residue. The nature and relative proportion of these co-products depends on the pyrolysis environment and the type of biomass you started out with.

(There isn't yet universally accepted terminology in this field, so here's my take: The generic solid residue of pyrolysis can be referred to as "char". When this char is burned as fuel, as it has been for millennia, call it "charcoal". When we apply it as a carbon-sequestering soil amendment, call it "biochar". OK?)

The biomass, or "feedstock", is typically crop residue or agroforestry waste; but just about anything that was once living or derived from living material can be pyrolyzed, including sorted municipal waste and de-wetted sewage. As for the pyrolysis environment, that's all over the map--maximum temperature reached, heating rate, dwell time at different levels, presence of small amounts of oxygen, suffusing with certain gases, size of the biomass particle, etc. Larger scale engineered plants operate more like refineries, and the pyrolysis environment can be manipulated to maximize the yield of particular products; bio-oil for processing into biodiesel, combustible gases for resale or direct power production, and other commercially valuable products. Alchemy comes to mind. Given the huge range of inputs and variables and possible co-products, referring to the "Pyrolysis Arts" seems well justified.

Our interest is the solid stuff, the lowest common denominator. In the most basic char-making kilns, the gases and volatile compounds produced are consumed in the process, with the excess either released to the atmosphere or burned. Since many of these compounds can be potent greenhouse gases, atmospheric release is a no-no; if nothing else, we'll want to flare them off. Better still would be to capture some of the heat and use it (for drying more biomass, for example).

Many future posts will be devoted to experimentation with and optimization of char-making kilns.

Birth of an obsession

It was April 30, 2008. I know because it's stamped in my passport. Diane and I were driving back to Costa Rica's Osa Peninsula from the cool coffee highlands of Boquete, Panama where we'd gone on a "visa vacation" (non-residents are required to leave the country every 90 days). We took advantage of the hotel's high-speed internet for downloading and were plowing through a pile of Science Friday podcasts on the drive back, when Ira Flato's guest spoke of something he called "biochar", which he described as fertilizer made from pulverized charcoal. Whoa! Hang on a sec! We both had MS degrees in Botany. We know what fertilizer is; charcoal is NOT. Who was this crackpot, and how'd he get to be on this highly respected, nationally syndicated science program?!

Curious, I googled "biochar". What I learned blew me away. As a soil amendment, biochar is not only a potential source of plant nutrients, but it also improves water retention, reduces acidity, and changes soil surface properties to improve nutrient availability. It persists in the soil, so you only need to add it once. It reduces reliance on petrochemical fertilizers, improves groundwater quality, and reduces emissions of greenhouse gases common with traditional agricultural. As if that weren't enough, a co-product of making biochar in specially designed processing plants can be biofuel or electric power. And it's made from agricultural waste or timber slash, solving a waste disposal problem. Best of all, biochar persists in the soil for hundreds to thousands of years, sequestering CO2 and reversing the effects of global warming.

Is this possible? A potent and permanent soil amendment that improves environmental quality, is a non-fossil fuel energy source, solves waste disposal problems, and reverses global warming? Why didn't I know about this? Why doesn't everyone know about this?!

It's not even new. Turns out, there are areas in the Amazon basin know as Terra Preta (dark soil) where native peoples appear to have been practicing "char agriculture" for millennia. It was described in the soil science literature back in the '60s, but for some reason languished in relative obscurity until very recently. And while charcoal making is among the most ancient of the industrial arts, even modern "high tech" methods of simultaneously producing both char and power date back to WWII. Yet, despite its huge range of benefits and technical feasibility, nobody I talked to had even heard of biochar, and hardly anyone was actually practicing it.

Biochar embraces a constellation of disciplines that mirrors my personal interests, experiences, education, and aspirations; a "perfect storm" of environmental activism, plant science, technology development, sustainable agriculture; a chance to contribute, learn and grow. Plus, burning stuff is fun! Unbound by work or other hard commitments, and already dedicated to the principles of sustainable living, I decided to up the ante by making the advancement of biochar my personal mission.