Once the chambers are loaded and sealed, the retort is heated by burning wood in the central firebox. The wood that we burn here is generally waste wood that would otherwise end up in landfill, or being burnt for no use. Pallets, offcuts and chips as well as wood that has served its purpose such as old fencing, provides us with enough energy to heat the retort to temperatures of around 300°C, depending on the wood species and moisture that we are converting.
High temperatures are thermally destructive to the wood in the chambers. The structures start to break down, expelling moisture, oils and flammable gases such as hydrogen and methane. The process of pyrolysis, where the wood is heated in an oxygen limited environment, means that it is never in contact with a direct flame, and the gases do not catch alight in the chamber itself. Think of it like cooking, rather than burning. In our retort, the volatile gases are piped back into the firebox, where they are burnt to further fuel the process until the temperatures in the drums reach upwards of 500°C.
The self-sustaining process is kept at high temperature until the gases are completely burnt off, indicating that the wood has fully undergone the conversion process, and is now around 90% carbon.