farmblog towards actually sustainable farming in Ahualoa

It’s been seven years since I posted anything to this blog, but it’s a reasonable place to put things I’ll want to refer back to, so…

There are currently two biochar pits. Pit 1 is the original, and Pit 2 (built July 2015) is a larger pit which incorporates lessons learned from the first pit. I always fire both together.  When the pit is fired then opened up later, there is a gap at the top with air and a few chunks of wood that were not fully charred, which are removed, leaving a depth of char which is less than the depth of the pit.  Here are both pits, freshly opened, with the torrefied wood still in place:

New item by Ben Discoe / Google Photos

And Pit 2, emptied:

New item by Ben Discoe / Google Photos

In terms of carbon, it is difficult to say how much C is present in this partly-crushed, loose char. A solid block of charcoal, a web search indicates, might be 13 lbs/ft3. Another source, biochar-specific, says it could vary widely from 5 to 20 lbs/ft3. Using a value of 10, that gives 346 lbs of carbon, equivalent to 1270 lbs of CO2.  By a (similarly, very rough) approximation of the carbon footprint of air travel, at 1 lb CO2/mile, a flight from Hawaii to California (~2400 miles) would require two complete firings of the charpits to offset the flight emissions, or four complete firings for a round trip.  This analysis does not include the other (relatively small) carbon costs of the charpits, including per-burn costs (small amounts of gasoline for chainsaw and log-splitter) and amortized costs (footprint of the bricks used to build the pits).

In terms of time and money, looking around on this island, the approximate price of biochar is $150 per cubic yard.  (That equals $5.55 per ft3, so a 5-gallon bucket (0.67 ft3) would be $3.70.)  At that price, the pits produce $192 worth of char.  Firing the pits takes around 4.5 hours (can be anywhere from 4 to 5 depending on many factors).  There is also time spent unloading, as well as gathering, cutting, splitting, and moving the wood to the charpits; these hours are harder to estimate but we can imagine another 3 hours per amount of wood that goes into a firing.  That gives around $25/hour for the 7.5 hours of labor involved in each batch of char.

Those of you following the biochar-hawaii list know that i’ve stopped using my kiln, and am now focused on making biochar in a pit. This is both for reasons of scalability and wear; my 55-gal steel drum kiln/retort could only make ~23-gal of char, and the surrounding kiln blocks cracked from repeated heating.

Hence, a pit. Mine is lined with blocks for clean char and easy unloading. Continuously fed wood, pyrolysis occurs at the air-starved bottom of the pile, gradually the pit fills up, then i cover and let it cool for a day, before opening and scooping out the finished char:

That first small pit worked well, so i made it bigger and sure enough, it scales well:

That 82-gallon operation took 2.5 hours to do the burn, then 2.25 hours the next day to unload, crush, sort, sift, and load into buckets. That’s 82/4.75 = 17.25 gallons of char per hour of work. That’s not bad, given that i’m working with some cheap concrete blocks, a piece of old corrugated roofing, and a shovel. With more money and technology, like a continuous pyrolysis machine, you could certainly get vastly more char per hour of labor, but those machines start at $100,000. I’m feeling quite happy about my pit. The Biochar2010 album has all the pictures.

I gave a biochar talk to the Kona Coffee Grower’s Association on June 2. 10 minutes of that talk got uploaded to YouTube. I then addressed the Hawaii Tropical Fruit Growers on July 19, that time with a fancy presentation with charts and pictures. Next will probably be an evening talk in Waimea on August 8, and then a 1-day workshop on making and using biochar here at our farm, date TBA.