1. We want a clean burn — no excess greenhouse gases or we are defeating one of the main advantages of biochar.
2. We want clean feedstocks — no toxic materials because this is going into the garden and we will eat it, eventually.
3. We want to stack our functions — can we harvest heat? power? dinner? hot chocolate?
4. We would prefer to use biomass that would otherwise be burned carelessly or allowed to decay to methane and other greenhouse gases.
5. We want this to be done safely, cheaply, and in a fair way for all the stakeholders.
Old methods of making charcoal — smoky kilns, woodpiles covered with dirt, rusty drums — are out.
The new method is to get the feedstock to 450°C or hotter in the absence of oxygen — to “bake” the feedstock — in an enclosure that does not generate a lot of smoke or add to greenhouse gases just to make the biochar.
One of the best ways to make biochar is to use a fire you are already burning to begin with. With a simple stove “insert” you can turn any wood heater or cooking stove into a biochar kiln. Here’s how:
Measure the firebox and door of your stove. The insert must be smaller than both of these, and it should be easy to put in and take out. Sheet metal stovepipe sections are one way to go, but they tend to be thin-walled and will wear out fairly quickly (wood gas is caustic and the “wood vinegar” produced by pyrolysis corrodes steel). A metal fabrication shop or tin smith can make a thicker-gauge steel cylinder to the size you specify. At least one of the ends should be removable.
As we evolved our insert designs here, we found that an eyebolt on one end (to make it easier to grasp and remove from the stove when hot) and a wingnut on the other end (to make it easy to open, remove biochar and re-fill with fresh biomass) worked well. The two bolts were connected by a threaded rod that ran inside the cylinder, from end to end.
The cylinder should have small holes to vent the hot gases that are produced by pyrolysis. This is important! Once the biomass reaches 400 degrees or more, these holes will show blue gas jets that will add to the heat of the stove while the biomass bakes. Once the biomass is fully charred, the jets will subside. This will let you know it is time to remove the insert from the stove.
Many types of biomass can go into the insert to bake into biochar: wood scraps, chips, pellets, bamboo, manure, algae, poultry litter, and various grasses. Each of these will have different baking times and end products. Once the biochar is done, it should cool in a safe place (it is highly flammable when hot). And, as I detail in my book, it should be steeped in compost and/or compost tea before going to the garden. This priming process assures the best results for healthy plants.
