9.10 Pyrolysis Unit

Slow pyrolysis has been found to optimise char production. The most popular reactors that are typically employed for slow pyrolysis are rotary kilns, drums, and screw augers. In comparison to an incinerator, the pyrolysis unit does not require an air supply but needs to create an air-free space.

The setup shown in the following picture was developed for biochar production in Dar es Salaam, Tanzania. It has three main components: a pyrolysis reactor, a heating system, and a heat-retaining brick kiln. A metal barrel (200 L), which is usually widely available, was used as a pyrolysis reactor. The barrel is placed horizontally on guiding metal rails inside the kiln. The kiln was made of burnt clay bricks. Two large burners heated the reactor.

In the initial stages of pyrolysis, a 2 cm vent vole in the barrel is aligned to a temporary and removable chimney pipe to emit the gases out of the kiln. Once the whitish-coloured smoke (water vapour) changes its colour to yellow/brown (flammable gases such as carbon dioxide, hydrogen, and methane), the barrel was rotated by 180° so that the vent hole in the barrel is aligned to direct the flammable pyrolysis gases to the LPG burners. This reduces the requirement of fuel for external heating and eliminates methane and carbon monoxide pollution emissions.

When the pyrolysis gases exiting the vent hole extinguish, the pyrolysis process has finished. The burners can be switched off, and the barrel can be removed from the kiln and left to cool outside the kiln. Another barrel can be loaded immediately to take advantage of the still-hot kiln.

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