Wood pyrolysis is the first stage of wood combustion. Flames on burning wood are formed not by combustion of the carbon in the wood, but due to gases, which are the volatile products of the pyrolysis.This stage represents the decomposition of wood by heating to 450-500°C without air access. In the pyrolysis process many different substances are produced. The following substances have the largest concentration in the gaseous products: methyl alcohol (by the way, this is why methanol`s old name is "wood alcohol"), acetone, acetic acid, benzene etc. Non-volatile products of incomplete pyrolysis include liquid and paste-like resin (including tar). In the result of the wood pyrolysis process, there can be obtained almost pure carbon - charcoal, which in the form of impurities contains some oxides of potassium, sodium, calcium, magnesium and iron.
Nowadays deciduous wood (e.g. birch) is more often used for the implementation of the pyrolysis process, less often (in cases of complex processing of raw materials) it is used wood of coniferous breeds. Depending on the size of wood pieces taken for pyrolysis, the size of the solid residues also varies. Nevertheless, one should take into account that, as a result of uneven shrinkage of raw materials and the active emission of volatile products, cracking of the material occurs and it can be produced up to 20% of fine charcoal, particle size of which does not exceed 12 mm. Upon completion of the pyrolysis process, the obtained charcoal is sorted by the size of pieces and then sent directly to the consumer or for next processing.
Basic technological scheme of wood pyrolysis is as follows:
The longest and most energy-consuming process among all of them is the wood drying.
As for equipment, the most improved technology of wood pyrolysis involves the use of continuously operating equipment, such as silo dryers, charcoal kins and vertical steel retorts. In the last option the wood is loaded on top by portions, and the charcoal is unloaded from the bottom.
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