The carbonization process of activated carbon

The carbonization process is one of the important processes in the process of producing activated carbon by the gas activation method. This process is to heat the raw material in the air and reduce the non-carbon elements to produce the carbonaceous material suitable for the activation process. It is the pre-activation process,also the main preparation and foundation. In the production process of coal-based activated carbon, the carbonization process usually includes carbonization of materials and carbonization tail gas treatment.

The carbonization process is actually the dry distillation process of the material under low temperature conditions. In this process, the material is gradually heated and heated under a certain low temperature range and the condition of air isolation. The low-molecular substances in the material are first volatilized, and then the coal and coal tar pitch are decomposed and solidified. A series of materials will occur during the entire carbonization process Complex physical changes and chemical changes, of which physical changes are mainly dehydration, degassing and drying processes; chemical changes are mainly two types of reactions, thermal decomposition and thermal polycondensation.

During the thermal decomposition and thermal polycondensation reaction of the material, gas and coal tar are precipitated. The oxygen bonding group of the organic compounds in the material is destroyed, and the oxygen element is precipitated with gases such as Hz0, CO, CO: etc. At the same time, aromatic compounds and cross-linked High-strength carbon molecular structure solid; during the carbonization process, due to the discharge of non-carbon materials such as oxygen and hydrogen during the decomposition at high temperature, the carbon atoms after the loss of oxygen and hydrogen are recombined to form an order with a basic graphite microcrystalline structure The crystals consist of hexagonal carbon atom planes, and their arrangement is irregular, thus forming voids between crystallites. These voids are the initial pores of the carbonized material.
Therefore, the purpose of carbonization is to make the material form a secondary pore structure that is easily activated and to give the mechanical strength required to withstand activation. The requirement for the carbonization of materials is that the appearance of the carbonized material obtained through carbonization must meet certain specifications and shape requirements, the internal structure must have a certain initial pore structure, and at the same time have a high mechanical strength

The carbonization process can generally be divided into the following stages.
(1) The temperature in the drying stage is below 120 ° C, and the external moisture and internal moisture are released from the raw coal. At this time, the appearance of the raw coal is unchanged.
(2) In the pyrolysis stage, the raw coal starts to decompose and release pyrolysis water to form gaseous products (such as CQ, C02, H2S, etc.). Different coal types have different pyrolysis temperatures, and coal with low metamorphism starts to heat. The solution temperature is also low. Northeast peat is about 100-1600, lignite is about 200-3000C, bituminous coal is about 300-4000C, and anthracite is about 300-450C. Because the molecular structure and generation conditions of coal are quite different, the above pyrolysis temperature is just a relative reference value between different coal types.
(3) The temperature in the carbonization stage is 300-600 degrees Celsius, mainly polycondensation and decomposition reactions, the raw coal largely precipitates volatile matter, and almost all the tar and gas products precipitated in the carbonization process are produced in this stage. Cohesive bituminous coal gradually softens and melts at this stage to form a colloidal body with three phases of gas, liquid, and solid, and then turns into semi-coke through the processes of flow, polycondensation, and solidification; non-adhesive forms needle-like semi-coke or lump Shaped half-focus

The final temperature and rate of carbonization are the main operating conditions controlled by the carbonization process. For different coal types, the tar formation process ends at around 550 ° C. A lot of laboratory research and industrial production experience have shown that 600 ℃ is the best final carbonization temperature. If the temperature is too low, the carbonization product cannot form sufficient mechanical strength. If the temperature is too high, the graphite crystallite structure in the carbonization product will be promoted. Ordering, reducing the gap between crystallites, affecting the activation pore formation process. The carbonization heating rate has a great influence on the yield of carbonized products.
The high heating rate can make the material precipitate more tar and coal gas and reduce the yield of carbonized material. When the heating rate is reduced, the material is heated for a long time in the low temperature region, and the pyrolysis reaction has a strong selectivity. The initial pyrolysis breaks the weaker bonds in the material molecule, and parallel and sequential thermal polycondensation reactions occur, forming a The structure with high thermal stability, thereby reducing the yield of volatiles of the thermal decomposition products at high temperature stage, and obtaining a higher yield of solid carbonized products (ie carbonized materials).

The quality of carbonized materials in the carbonization process is mainly evaluated by volatile matter, coke index, water capacity and strength. The volatility of qualified carbonized materials is 7% -18%, the characteristic index of coke yesterday is 1-3, the water capacity is 15% -25%, and the strength of the ball disc is 90%.
Because the measurement of the above indicators requires a certain amount of time, and during the commissioning of the production site, it is often necessary to quickly adjust the process parameters according to the quality of the carbonized material, so the quality of the carbonized material can also be roughly evaluated through the senses. Qualified carbonized materials should have a smooth, crack-free surface, high strength, and consistent color of the material.

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