Standard process of wood decomposition under the influence of heating in the oxygen-free gaseous atmosphere (pyrolysis) is carried out in the closed tank (retort) heated through the walls.

Retort contains branch pipe to take formed steam-gases into the condensation device, and it also contains a device to separate liquid from gas. The wood is placed into the retort, the loading hole is closed and the devise is heated by regulating the temperature using the pyrometer placed into the recreation space. The heating is held up to 400-500°C.

According to the external features and check measurements the process can be divided into the next stages:

1. Drying – only humidity is emitted, practically 0-150°C.
2. Dry distillation (pyrolysis). There is a range of organic products in the distillate and the gas is emitted. Usually this stage is referred to the 150-350°C temperature intervals.  So-called exothermic period is also typical for this stage. This period is noticed under 278-285°C when the process goes very actively emitting reaction heat significantly.
3. Tempering stage. It is characterized by the separation of small amount of resins (1,5-2%)  and significant amount of noncondensable gases from the coal. The beginning of this stage is 350-400°C and the end is set by the production conditions and usually doesn’t exceed 400-550 °C.

The first component of wood that starts to decay under the temperature lower 150^o C is xylan but usually its decay goes under 250-260 oC producing furfural, ascetic acid and gases. The decay of lignin starts under the approximate temperature of 200^oC. This process in consequence of hetero- and homolytic dissociation of chemical bonds between and inside the base units of lignin leads to the formation of low-molecular volatile compounds and complete rearrangement of the initial structure of lignin. When the temperature is above 300 ^oC the cellulose starts to decay.

The wood is a complicated complex of organic compounds, notable for high molecular weight; therefore its process of decay is extremely complicated and it is difficult to describe and to quantify. Undoubtedly in the course of the process the range of consecutive and parallel reactions, characterized by the bonds breakage in the initial complex takes place.

As a result new matters are formed, many of which are unstable under these conditions and react easily with each other. In the devices of batch operation the stages of pyrolysis process go sequentially, whereas there is at the same time the process of drying in the upper part, the heating of the wood up to the temperature of exothermic reaction is lower, wood decay and tempering of charcoal is in the middle part, cooling of charcoal before the unloading is in the bottom of continuous working device. Therefore during the process of batch operating device the compound of vapour-gas mixture changes eventually, whereas it stays almost invariable during continuous process.

Accumulated experience of production and researches conducted in the laboratory conditions let to estimate the range of relations determining the influence of different factors on the pyrolysis process and also connection between chemical composition of wood and products of its thermal decay. Different factors influence the process of pyrolysis but raw and operating ones are in the first place (see below).

Products of pyrolysis

The next primary products are produced as a result of the process:

·

• charcoal, about 30%
• wood dry-distillating gas, about 20%
• distillate (condensate), about 50%

Condensate or damp wood vinegar obtained during the process of usual dry distillation using condenser makes 15-20% solution of organic matters in the water. Specific weight is 1,025-1,035.

The condensate acidity fluctuates from 7 to 12%.

The condensate includes about 380 individual chemical agents.

The main groups are:

• acid, fatty acids of homologous series of acetic acid (formic, propynole, butyric and valeric acids) with 80% dominance of the acetic acid.
• alcohol – mainly methanol with admixture of allyl alcohol.
• ether – methyl acetate, ethyl acetate etc.
• aldehyde – formaldehyde, acetic aldehyd etc.
• ketone – acetone, methylethylketone.

Quantitatively in the average numbers one can assume the content of the acid part in the condensate – 7-12%, wood-alcohol (ether, aldehyde and ketone groups) – 3-5%, soluble resins – 5-7%.

Charcoal is a porous, fragile matter of the shining black color shot with bluish in the fracture keeping the structure of the initial wood.

True specific weight is 1,4; specific weight of the lump coal made of spruce is 0,25, pine – 0,27, birch – 0,3-0,4.

Charcoal burned under 400 oC of the final temperature of all species has almost the same content of the organic mass: C – 82,5%; H2 — 4%; O+H – 13,5%. Carbon content in the charcoal depending on the final temperature of the burn-off fluctuates within considerable limits reaching 99,7% from the organic mass under very high temperature (1600 °C). The content of ashes in the charcoal is 2,5-3,5, it mostly contains СаО, K2O, Na2O and small amount of phosphorus that is very significant. Thermal value of the charcoal is 7000-8000 cal/kg.

Noncondensable gases.

Wood dry-distillating gas is combustible though it contains up 50% of CO₂. Thermal value of the cumulative gas emitted during the process of pyrolysis is 800-1200 cal/m?. This rate can be changed sharply during the process. When the temperature rises dry-distillating gas enriches itself with combustible components.

Up-to-date charcoal burning installations oriented to produce only charcoal when environmentally friendly devices produce wood (pyrolysis) gas and condensate recycling. Such charcoal burning kiln becomes energy-independent. Produced heat is enough to warm the new portion of raw material, the process goes continuously and more efficient.