Our First Batch of Biochar

There's much to catch up on: how our practices, theory, and philosophy continue to deepen; lessons we've learned from our second food forest area; what our chickens have taught us; new ways we've learned to prepare and eat onsite crops; and processes we've designed to fill a vacancy opening up later this year.  But for this little blog entry, I'd like to focus on our first batch of biochar.

Biochar is an organic, charcoal-like soil amendment that has a surprising capacity to hold water, soil microbes, and nutrients and prevent those nutrients from leaching from the soil.  We made a small batch (it'll probably crush down to 2-3 gallons) in a cone-shaped pit.  This approach required little time and effort, but it did require constant attention and careful timing to expel and burn the volatile compounds from the wood and pyrolize the cellulose without burning the resulting carbon.  We started burning a small amount of wood at the bottom of the cone, and continually added fresh wood at the right rate to keep the surface burning, while denying oxygen to the charred layers below, just in time to prevent the carbon from burning.

Once we had enough char for our first trial, we filled the pit twice with water (making huge billows of steam), which cooled our biochar and prevented it from slowly smoldering into a pile of white ash.

What Went Well

Extent of burning and pyrolysis - We did have a little white ash at the bottom of the pit (overburned), and a few of the larger sticks at the top were still brown in the center (underburned), but the rest of the batch was solid black as obsidian and brittle throughout - perfect.

Arrangement of sticks in parallel - Usually when you're building a wood fire, you want to criss-cross the wood for ventilation.  But because we wanted to minimize ventilation, we laid the sticks in parallel.  I don't have a control group to compare against, but I suspect that helped to halt combustion at lower levels.

Synchronic layering - The instructions we found suggested using discrete layers, in sort of an iterative but diachronic pattern of add, pause, add, pause ...  But instead, we continuously added wood at a deliberate rate, keeping an eye on the size of the flames and watching for white ash.  Again, I don't have a control, but I suspect this approach accelerated the process and gave us more even results.

What to Do Different Next Time

Cut more sticks ahead of time - While the fire burned, I spent much of my time quickly cutting sticks to length.  Next time, I'll cut sticks to length ahead of time so I can pay more attention to the fire.

Next Steps

We'll crush our biochar to smaller bits, adjust the pH of the ash we unintentionally produced (using some vinegar we accidentally made while brewing kombucha), and use various methods to load it up with microbes and nutrients.  One method we'll use is to scatter biochar in our chicken coop, where it will not only reduce odor, but also absorb ammonia and nitrifying bacteria.  Another method we may try is to mix it with compost and flour from mesquite pods that are no longer suitable for human food.