Biochar production

Biochar is produced is via a thermal process called pyrolysis which is similar to combustion. While complete combustion occurs in the presence of oxygen which fully converts the organic material in the feedstock into CO₂, pyrolysis takes place in the absence of oxygen. This ensures that the carbon in the biomass is converted into energy-rich gases, liquids and biochar.

Slow pyrolysis is the process typically used for biochar production because it maximises the formation of the solid biochar fraction.

Slow pyrolysis uses comparitvely low biomass heating rates and long residence times of the biomass at the respective temperature (5 min to several hours). The resulting material is structurally different from the feedstock material. It consists of a very stable carbon framework which changes its chemical and phycial properties.

Pyrolysis units come in all different shapes and sizes. They either produce biochar continuously or in a batch process.

A low-cost way of biochar production is the Kon-Tiki which is a batch pyrolysis unit that can produce up to around 1000 L of biochar in one day. Here you can see a Kon-Tiki operated by Hans-Peter Schmidt and Paul Taylor:

In contrast, commercial units process several tonnes a day in a continuous process. For example, the pyrolysis unit of  converts up to 2,500 t of biomass into biochar per year and is suitable, e.g. to treat sewage sludge in waste water treatment facilities.

Pyrolysis destroys contaminants in the feedstock, such as PCBs, dioxins, furans, PAHs and even pharmaceuticals, e.g. hormones.

Furthermore, heavy metals present in the feedstocks are immobilised and hence rendered unavailable for plant uptake. Therefore, pyrolysis is the ideal waste management strategy, it converts potentially toxic waste material into a non-toxic and valuable resource.