I. Drying Stage. The temperature in this stage is 120–150 degrees Celsius. The pyrolysis rate is very slow, primarily involving the evaporation of moisture contained in the wood relying on externally supplied heat. The chemical composition of the woody material undergoes almost no change.

II. Pre-Carbonization Stage. The temperature in this stage is 150–275 degrees Celsius. The pyrolysis reaction of the woody material becomes more apparent, and the chemical composition of the woody material begins to change. Among these, unstable components such as hemicellulose decompose, generating substances such as carbon dioxide, carbon monoxide, and small amounts of acetic acid.

III. Carbonization Stage. The temperature range in this stage is 275–400 degrees Celsius. During this stage, the woody material undergoes rapid thermal decomposition, generating a large quantity of decomposition products. The liquid products produced contain significant amounts of acetic acid, alcohols, and wood tar. In the gases generated, the carbon dioxide content gradually decreases, while combustible gases such as methane and ethylene gradually increase. This stage releases a large amount of reaction heat, hence it is also called the exothermic reaction stage.

IV. Calcination Stage. The temperature rises to 450–500 degrees Celsius. This stage relies on externally supplied heat to calcine the charcoal, expelling residual volatile substances within the charcoal and improving the fixed carbon content of the charcoal. At this point, very little liquid product is generated.

V. Refining Stage. The temperature in this stage is 500–1,200 degrees Celsius. This stage is the critical one.