The Azimuth Project
Experiments in biochar

Written in 2011.

Biochar production from wood pellets

Method and results

Method: Top lit updraft (TLUD) pyrolysis in a small and simple 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%

  • Weight: 15%

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

  • Charred pellets don’t fall apart

"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 - 0.15 * 8.3) / 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.)

An application

The char pellets 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 and the seepage can be used 1:2 as fertilizer. In my test, after a 2nd bucket filling and adding urine (no smell) the pH goes down to 6 at the end. The char can then be used as “instant Terra preta”, e.g. to improve spent soil. (Addendum summer 2018: A big “instant Terra Preta” flower pot with 80%vol biochar and high density planting is still doing well.)

The fossil fool’s cost in Euro per ton

Computing the cost of char per metric ton relative to heating oil, where 1kg pellets = 0.5l oil. German prices as of 5.Feb.2011, incl. 19% VAT. Oil price includes <10% “eco”tax, precise value irrelevant due to the large price fluctuations (cf. e.g. this chart).

  • Wood pellets: 241€/t for a 3-12t order.
  • Heating oil: 0.78€/l

1t char requires 1t / 0.15 = 6.67t pellets.

Pellet cost: 6.67t * 241€/t = 1607€

Energy yield as heating oil equivalent: 0.75 * 6670kg * 0.5l/kg = 2500l oil

Energy yield in oil price: 2500l * 0.78€/l = 1950€

Result: 1607€ - 1950€ = -343€/t (incl. VAT). In U.S.$ (VAT substracted, 1.36€/$) -378$/t


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). The inner bottom air inlet is slightly above the outer bottom holes. The inner tube is wrapped in aluminum foil. This thermal insulation is essential for reliable operation and needs replacement after some time.

The construction is a paraphrase of a plan I found on the Internet (link lost). There are many other plans offered, but this one is the simplest and seems optimized for char production.

The stove is actually not trivial to light. As primary starter I use a few pellets soaked in resin and start these with a little birch bark. One can learn a lot about fire from the stove. When the gas/smoke gets not hot enough to burn, then the stove enters an irreversible state of smoke production. (Once I used it to smoke and desinfect the pantry of an abandoned farm house - the mouse there did not survive.)

During operation: Light flame without smell and soot.

(comment nad: It seems that by looking at this chart the burning temperature is important in assessing the type and severity of air pollution. – quick reply Martin: Thanks for this! Need to run, look later. Yes, temp is important 1 for char characteristics 2 complete combustion of soot, for this also size of combustion chamber is crucial. I’m currently exploring this with a bigger oven. Will come back soon. )

End of operation: Flame retreats inside and gets blue. Smell and soot.

Suggested projects

Home heat and electricity generation with IC engine

The most energy efficient application would be stationary home electricity and heat generation (efficiency up to 90%). As shown above this would still be competitive with fossil fuels even if biochar is left behind.

The cheapest and easiest motor for this would be an internal combustion engine. The technology is already commercially available in Germany ( and in use at progressive farms. But it needs daily maintenance and does not produce much char. UPDATE 2013: A small German engineering company has a better machine, which is now being scaled down for home use.

The Gasifier Experimenters Kit made in U.S.A. has a variant optimized for biochar production. The GEK project is an experiment in collaborative science and open source engineering.

Using micro gas turbines

It would be a highly interesting experiment to power a micro gas turbine with wood gas. Typical microturbine efficiencies are 25 to 35%. An electric heater could be used to start pyrolysis and regulate optimal temperature. To clean the gas a combination of cyclonic separation and electrostatic precipitation could be used.

Woodgas hybrid upgrade pack for electric vehicles

Gas turbines have already been tried and tested in vehicles, e.g. the M1 Abrams battle tank.

One 15kg bag of wood pellets contains 75kWh energy. Using a 30% effective microturbine and only 75% pellet energy to keep the biochar, this would result in 75kWh * 0.3 * 0.75 = 16.9 kWh.

The Mitsubishi i MiEV 4-passenger vehicle comes with batteries of 16kWh or 20kWh. So, the carbon negative wood gas hybrid car is a totally serious idea. It seems feasible to design a small gasifier-microturbine pack that can be placed on one passenger seat.

category: experiments, carbon