The Azimuth Project
Experiments in biochar (Rev #2)

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. (Tests running.)

Summa summarum, TLUD produced wood pellet char is superior to standard char for agricultural use.