Fundamentals of char production
Charcoal production is basically the transformation of virgin (often wet) biomass into charcoal by heating in the absence of air. Charcoal production (or carbonisation) Is one of various pyrolysis processes. Essential for all these processes is a confined surrounding with little or no air present and a heat source to reach the necessary temperature.
The first step in the carbonisation process is drying of the biomass which starts at low temperature (ca. 100 °C). This is followed by thermal breakdown of the biomass at increased temperature starting at 200-250 °C up to the final carbonisation temperature. During this dissociation the structure of the biomass changes, large molecules are broken into smaller ones and evolved in the form of gas. The gas mixture contains CO, CO2, H2O but also larger hydrocarbons and tars. In traditional charcoal processes these hydrocarbons and tars cause the black smoke, typical smell and are hazardous to both environment and health.
Chimney filled with tarry sludge after production and hazardous vapours coming off a traditional kiln
In the course of the process the carbon content of the remaining solid residue slowly increases (carbonisation) as preferentially non-carbon components are emitted through the gases. Due to the high temperature chemical bonds within the complex biomass molecules are broken. The weakest chemical bonds i.e. O-O, C-O and N-C bonds are broken more easily than C-C bonds, leading to a carbon rich solid residue, the char. At increasing temperatures the carbon content of the char get higher as more and more non-carbon components are released.
The charcoal production process is similar to the extremely slow natural process of coal formation from plants in the soil.
Restaurant char quality and difficult picking of quality char from a pit
The energy needed for heating the biomass can be derived from combustion of part of the biomass or by adding heat from an external source. In the latter case the process can be controlled better. Once the carbonisation process has started the produced gases (off-gases) contain sufficient caloric value to supply at least part of the necessary heat for the carbonisation.
Key issue for an efficient carbonisation process and good quality end products is the combination of the heating rate, final temperature and residence time.
In the figure the principle of the CharcoTec process is depicted. For heat generation during start-up a wood stove is used in which wood or other biomass is combusted. The hot flue gases from the wood stove enter the CharcoTec furnace and heat the dried biomass in the carbonisation unit. During heating of the biomass off-gases are produced containing lighter and heavier hydrocarbons. This gas stream is directed back to the wood stove and combusted. Hereby emissions to the atmosphere of these hydrocarbons are prevented and the caloric value of the gases is used for the carbonisation process. After start-up the caloric value of the off-gases is sufficient for maintaining the process temperature at the desired level. Utilising the caloric value of the off-gases increases the overall process efficiency.
The hot flue gases passes to the second reactor containing the container with wet biomass (drying unit). The water vapour from the drying unit and the flue gases leave the process through the chimney.