The Azimuth Project
Experiments in biochar (Rev #3)

Biochar production from wood pellets

Method: Top lit updraft (TLUD) pyrolysis in a small selfmade stove. When the pyrolysis zone hits bottom and the big bright flame reduces to a small blue flame, then the stove is closed airtight with a lid and covered with a can and let cool down. Burn time ca. 35min. Batch size: 325g. Total pellet input: 2264g.

Resulting char:

  • Ashes: negligible (mostly from lighting the stove)

  • Pellet energy yield: 75%

  • Volume reduction: ca. 50% (from visual inspection, To do: measurement from photo)

  • Weight: 15%

  • Water absorption: 125% of char weight (at ca. 16°C)

"Energy yield": Alas my TLUD stove just gives nice light and is not useable for indoor heating. Modern pellet heating systems run at 85%-95% efficiency. According to the manufacturer, pellets give 5kWh/kg, being 18GJ/t. According to this wood has 18-22GJ/t and charcoal has 30GJ/t, i.e. 8.3kWh/kg. Alas I don’t have the lab to check this with my peculiar kind of charcoal. Energy yield computed as (5-0.158.3)/5=0.751

Volume reduction is roughly in accordance with estimates in en.wikipedia Charcoal and de.wikipedia Holzkohle.

According to en.wikipedia, weight reduction should be 25%. The lower number is to be expected,

  • from the characteristics of TLUD pyrolysis: "more of the bio-oil condensates are driven off" (loc. cit.).

  • as wood pellets are made from saw dust, the wood pores are more accesible and volatile components easier driven off.

The higher porosity is reflected in the huge water absorption of 125% of char weight:

  • Great benefit for use as soil additive

  • Probably better control of long term recalcitrance, as the short term decaying matter is burned out. (See discussion on blog here)

Other than standard BBQ charcoal, my pellet charcoal needs not to be cooked to gain its full water holding capacity: A few days in cold water suffice. (To do: determine water holding capacity of BBQ char.)


The charred pellets don’t fall apart. They could be used for a simplified and superior construction of a bokashi compost bucket producing non-acidic seepage fluid: Instead of a grate separating material from seepage, use wood pellet char. They provide enough space for the seepage. Without char, the fluid has pH <4 and needs to be diluted 1:100 - 1:500 before use in garden. Whith char it is pH 6.5. (Tests running, 2nd filling still gives pH 6.5. Applied at 1:2, my indoor palm tree is still alive.)


TLUD stove made of a steel thermo can: 8 holes drilled inside tube at top (difficult, 2 drills wasted). 2x4 holes at bottom cut out. Bottom made of rough clay, dried and burned in place (better heat insulation, optimized operation).

During operation: Light flame without smell and soot.

End of operation discernible by smell