1. What Is Biochar?
Biochar is a stable form of carbon,
chemically equivalent to charcoal, that can
be used as a soil amendment to increase
moisture, nutrient retention, and habitat for
beneficial microorganisms, as well as
sequestering carbon in soils.
2. Biochar can be produced
using several methods.
One method suitable for
small-scale agriculture is the
two-barrel nested retort.
3. Building a two-barrel nested retort is quite simple. It takes
two barrels, one larger and one smaller. We used 55- and 30-
gallon steel drums.
4. Be sure to purchase a steel lid for the 55 gal drum, as this
becomes an important part of the retort.
5. The retort also
requires a
chimney.
Chimney pipe in 6-
or 8-inch diameter
works well; it's
available in most
hardware stores.
Avoid roof
flashing, as it will
melt in the intense
heat.
6. Adequate airflow from the bottom of the large barrel is
essential to a good burning process. To ensure this, air holes
are required around the bottom edge.
7. An exhaust hole must be cut into the steel lid to
accommodate the chimney.
8. We found it helpful to remove the bottom from a metal
coffee can and fix it over the exhaust hole using steel
brackets. This helps to stabilize the bottom of the
chimney, which remains removable.
9. Our primary feedstock to date has been dead and downed
hardwood branches, which are abundant on the ranch.
Hardwoods generally contain fewer tars and other chemicals
that might not be desirable in soil amendments.
10. Secondarily, we are using some fibrous wastes from our
horticulture projects, such as the shells of under-sized
birdhouse gourds.
11. Feedstocks for charring must also be dry. Green or wet
materials will not work in this retort.
12. Choose a level
area in the open,
with no trees or
electrical lines
directly above.
Keep a good
distance between
the retort and
flammable
buildings, hay
bales, and
anything else that
could easily catch
fire. We took the
added precaution
of constructing a
brick-enclosed
sand pad.
13. Fill the smaller bucket with the feedstock. Leave space
between the materials, and fill to the top.
Limbs larger than 4” in diameter do not char completely, so
twigs and small branches are best. Pieces between 6’'-18” in
length pack more easily into the retort, and make it easier to
transfer the finished char afterwards.
17. …and then invert it. The goal is to invert the barrels, leaving
the rim of the smaller barrel pressed cleanly against the
bottom of the large barrel with little or no gap.
19. Fill the gap between the barrels with the same size of fuel
wood that was used to fill the smaller barrel. Pack in as
much fuel as possible, stuffing very small twigs into gaps.
20. Pack the top of
the gap and the
charge barrel
with dry grass,
straw or similar
material. Avoid
using starter
fluids, diesel, or
other petroleum-
based fuels. Start
the fire using
matches or a
lighter. A small
hand-held
propane torch is
an easy way to
get the fire going
quickly.
21. Once the fire is successfully burning, place the lid on the
outer barrel. Be certain to wear fire-proof gloves and eye
protection.
Then place the chimney on the lid, and secure it so it doesn’t
blow or fall over. We use a few rocks or concrete blocks.
22. If all is working properly, air will be drawn through the
bottom air vents and a clean, very hot fire will begin to
develop.
23. This is a good time to wet or re-wet any grass or weeds
that might be growing near the retort, to reduce fire
hazard.
24. As the fire nears
the bottom of the
retort, it appears to
burn even faster
and hotter. This is
due to flammable
gases that are
cooked out of the
feedstock and leak
out of the open
bottom of the inner
barrel. These gases
provide the final
fuel necessary to
ensure that a good
biochar product
results.
25. When you judge
it safe, remove
the chimney and
the lid. Be
cautious of
leftover coals
and extinguish
them with water
or let the retort
sit a bit longer.
26. When safe, invert the retort
and remove the outer
barrel. There should be
very little ash remaining in
the outer barrel.
The charge barrel,
however, will be full of
finished biochar.
27. Biochar contains measurable but small amounts of plant nutrients.
It should not be viewed as a fertilizer, but as a soil amendment -
one with the potential to make added fertilizers more stable and
efficient.
28. Added directly to the soil, fresh biochar can act like a sponge and
temporarily rob plants of nutrients. To prevent this from
happening, we incorporate biochar into our compost first, where
it can absorb both water and nutrients. It is then spread on the soil
on the same schedule as our compost applications.
