The whole charring process can generally be divided into the following three stages.
- Drying stage
From the start of ignition to the temperature of the furnace rising to 160 ° C, the moisture in the ingredient is mainly evaporating by the external heating and the heat generated by the external combustion. The chemical composition of materials has hardly changed.
- Carbonization start
Continue external heating, so that the temperature inside the furnace rises to between 160 and 280 °C. At this time, the biomass material undergoes a thermal decomposition reaction, and its composition begins to change. Among them, the unstable composition, such as hemicellulose, decomposes to form CO2, CO and a small amount of acetic acid.
- Overall carbonization stage
At this stage, temperature rises to 400-600 ° C. Biomass materials undergoes thermal decomposition abruptly, and produce a large amount of liquid products such as acetic acid, methanol, and wood tar. In addition, flammable gases such as methane, ethylene, etc are generated, and they flow through the duct to the combustion zone, and then be ignited. When the combustible gas is ignited, the external fuel continues to be burned until all the pores on the combustion tube are ignited. At this time, the addition of external fuel is stopped. Subsequent carbonization only needs to burn the recovered gas until the carbonization is completed.