According to the research of solid fuel combustion theory and solution dynamics of biomass, the biomass carbonization process can be divided into the following stages:
The first is drying stage. The biomass materials absorb heat in the carbonization furnace. The water evaporates and volatilizes, while the chemical compositions inside the biomass almost have no change.
Follow by the volatile pyrolysis stage. The biomass continues absorbing heat until the temperature reaches about 200°C. Inside the materials, molecular bond has breaking and rearrangement and the organic matter volatilize gradually. The internal materials begins thermal decomposition. Under hypoxia conditions, a bit of the volatile gaseous fuel has combustion, which can provide heating for materials gradually.
The last one stage is comprehensive carbonization stage. This stage’s temperature is 300-500°C, in which the materials are rapidly pyrolysis and generate wood tar, acetic acid and other liquid products as well as methane, ethylene and other combustible gas. Along with the separating precipitated, finally we get the solid product, which is consists of carbon and ash.
Biomass pyrolysis carbonization is a complex process and has many reaction. It has the following characteristics:
1. Smaller heating rate, typically 30°C / min or less. Experimental studies have shown that with respect to the rapid heating manner, the slow heating manner can improve the yield of carbon by 5.6%.
2. Lower pyrolysis temperature. The temperature lower than 500°C is beneficial to the production and quality guarantee of the carbon.
3. Longer gas retention time. According to the different raw materials, the time is often various between 15min and several days.