Wood, sawdust, roots, kernels and shells are carbonized by heating them in a carbonization plant for thermal decomposition. During pyrolysis, a series of complex chemical reactions are produced, resulting in many new products and changes in wood materials. The carbonization process of activated carbon can be divided into four stages according to the characteristics of temperature change and product formation.
1. drying stage
The temperature of this stage is from 20 to 150 degrees centigrade, the pyrolysis rate is very slow, mainly because the moisture content in wood is evaporated by the heat supplied by the external supply, and the chemical composition of the wood material has almost no change.
2. pre carbonization stage
The temperature of this stage is 50 - 275 degrees centigrade, and the thermal decomposition reaction of wood materials is obvious. The chemical composition of wood materials begins to change, in which the unstable components, such as the decomposition of hemicellulose, produce carbon dioxide, carbon monoxide and a small amount of acetic acid.
The above two stages need external heat supply to ensure the rise of pyrolysis temperature, so it is also called endothermic decomposition stage.
3. phase of carbonization of wood activated carbon
The temperature of this stage is 75 - 400 degrees Celsius, in this stage, the wood material thermal decomposition rapidly, produce a large number of decomposition products. The generated liquid products contain a large amount of acetic acid, methanol and wood tar. The content of carbon dioxide in the generated gas products decreases gradually, while methane and ethylene and other combustible gases increase gradually. At this stage, a large amount of reaction heat is released, so it is also called exothermic reaction stage.
4. calcination stage
The temperature rises from 450 to 500 degrees Celsius. This stage depends on the calcination of the charcoal with external heat supply, and the volatile substances in the charcoal are discharged to improve the fixed carbon content of the charcoal. At this point, the production of liquid products is very small.
It should be pointed out that, in fact, the limits of the four stages are difficult to be clearly divided. The thermal conductivity of wood materials is small because of the heat of each part of the wood activated carbon carbonization equipment, and the thermal conductivity of the wood materials is small. Therefore, the location of the wood materials in the equipment is different, even the internal and external of the large lumber may also be in different pyrolysis stages.
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