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Wood pyrolysis (dry distillation of wood)

Currently hardwood (e.g. birch) is mostly used to carry out the process; softwood is rarely used (mostly during the complex processing of raw-material). During the process of birch wood pyrolysis (humidity is 10-15%), 24-25% of charcoal is obtained, 50-55% of liquid (condensate) and 22-23% of gaseous products. The bigger the size of peaces taken for pyrolysis is, the larger solid residue is. Though in the issue of uneven shrinkage of raw material and rapid discharge of volatile products there is a cracking of carbonizing material and 20% of fine coal with the size of peaces not exceeding 12 mm.  Obtained charcoal after the sorting by size of pieces is delivered directly to the customer or to the processing.

When the condensate is stored or processed the wood resin is settled (7-10%) and numerous transformations of its components run simultaneously. There is wide range of valuable products obtained from the resin. Settled condensate has 1,025-1,050 g/cm³ density and contains 6-9% mass of acetic acid and its homologs, 2,5-4,5% of methanol, 5-6% of different compounds (aldehydes, ketones, esters etc.), 4,5-14% of soluble wood resin and 67-81% of water. Acetic acid is obtained from the condensate by the way of extraction and it is processed into the foodstuff by the way of rectification and chemical purification.

Gaseous products (noncondensing gases) include dioxide (45-55% by volume) and carbon monoxide (28-32%), hydrogen (1-2%), methane (8-21%) and other hydrocarbons (1,5-3,0%). The composition of noncondensing gases depends on the final temperature of pyrolysis, speed and kind of heating (internal or external coolant circulation – usually fuel gases obtained by the burning of fuel and noncondensing gases); their combustion heat fluctuates from 3,05 to 15,2 MJ/m3 . The above listed factors and also the type, quality and humidity of wood determine the output of pyrolysis products.  When the temperature rises the outputs of wood resins and noncondencing gases increase but the outputs of charcoal, acetic acid and spirituous products decrease; the produced charcoal contains more carbon. The average output of the main products of wood pyrolysis is (per dry wood): acetic acid 5-7%, wood resin 10-14%, charcoal (per nonvolatile carbon) 23-24%.

charcoal firewood

Free radical reactions of thermal decomposition of hemicelluloses, cellulose and lignin (running correspondingly under 200-260, 240-350 and 250-400 °C) are the basis of wood pyrolysis; the correlation of rate constants under 320°C is 10:1:0,25. Kinetic characteristics of wood pyrolysis and its components found by different authors differ significantly. Decomposition reactions of wood, hemicelluloses, cellulose and lignin have the first order and the activation energies of these reactions are changed within the significant limits; for the mentioned components of wood correspondingly 70-80, 135-210 and 55-110 кJ/mole. The rate constant of wood pyrolysis is higher than of cellulose and for instance under 350°C for different species it is over the range (2,8-8,3)×10-3с-1. Wood pyrolysis is exothermal process under which a great number of heat is emitted (1150 kJ/kg).

The schematic flowsheet  of wood pyrolysis is: dividing of raw material into pieces, drying of divided wood, the process of pyrolysis in special charcoal kilns and retorts; cooling of the charcoal and its stabilization (to prevent the spontaneous combustion); vapor condensation of volatile products. The most long-term and power-consuming stage is the drying of wood from 45% humidity up to 15 % humidity.

The technique of wood pyrolysis is diverse but the most charcoal burning kilns and retorts used in the world practice became obsolete and don’t meet modern requirements. The most perfect technology of wood pyrolysis is one that works continuously. It is loaded with wood portions and the charcoal is uploaded periodically.

Wood pyrolysis


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