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Primary products of wood pyrolysis

Gaseous, liquid and solid products of wood pyrolysis consist of three main elements: carbon, hydrogen and oxygen. They may also contain a small amount of nitrogen-containing substances.

The composition of the gases formed during the wood pyrolysis almost does not depend on the tree species. During the pyrolysis of 1 m3 of wood it is formed 75-90 m3 of non-condensable gases. 

The average composition and temperature of released flue gases

                               1. The temperature                                               150-280 °C
                               2. Oxygen O2                                                              10-16 %
                               3. Sulphur dioxide SO2                                       (22-94)•10-6 %
                               4. Nitric oxide NO                                                  (0-40)•10-6 %
                               5. Methane CH4                                                        0-0,01 %
                               6. Carbon dioxide CO2                                                5-9 %

The liquid products. The condensate obtained during cooling of steam-gas mixture, formed in the process of wood pyrolysis, is called pyroligneous distillate. It has a density of 1.02—1.03 g/cm3 and contains a variety of organic substances, both soluble and insoluble in water. During carbonization of some species of the wood, in particular birch and aspen, it is formed a small layer of oils, floating in the surface of pyroligneous distillate and having a composition close to the tar. These oils are formed due to the decomposition of bark.

The composition of pyroligneous distillate depends on the type of wood, its moisture content and conditions of the process conducting. It contains: acids - formic, acetic, propionic, butyric, valeric, etc.; alcohols - methyl, propyl, allyl, etc.; ketones; aldehydes; methyl esters of acetic and other acids; phenols and many other compounds.

Pyroligneous distillate

The better wood is dried, the less the yield of pyroligneous distillate is, but respectively the concentration of useful components in it is more. More is pyroligneous distillate obtained from 1 m3 of a birch than from aspen, because birch has a high density.

The process of decomposition of dry and wet wood occurs in different ways. Dry wood with humidity less than 10%releases more heat per time unit during decomposition than freshly cut wood; exothermic reaction starts faster and goes more rapidly; he process is accelerated; the output of charcoal is reduced. During decomposition of wet wood the process is self-regulated: the temperature is reduced due to the large heat consumption for evaporation of moisture, the exothermic reaction is stretched out and the speed of carbonization is reduced, as a result the output of acids and charcoal increases slightly.

During artificial drying of wood the evaporation of moisture from the surface takes place, simultaneously, the movement of moisture comes from the more humid inner layers of the wood to the less humid outer layers. Both processes accelerate with increasing temperature, but the second one is slower, which leads to cracking of the wood and reduction of physico-mechanical properties of produced charcoal. To avoid this, it is necessary to limit the drying temperature and to apply the partially humidified heat carrier. An important factor in drying is the circulation of the heat carrier in the drying unit.

The value of the wood pieces has a significant influence on the yield of products, in particular acetic acid. From lumps it can be obtained 8% more acetic acid than from metre pieces of wood.

To speed up the process of pyrolysis, obtain a sufficiently concentrated pyroligneous distillate and reduce the cost of fuel for its processing, one should use a dry chock with a moisture content of 10-15%.

Solid products (charcoal). Under the same conditions of wood carbonization charcoal has almost the same composition. With increasing carbonization temperature, the output of charcoal is reduced, but at the same time its quality is increased (see table below); output of charcoal is slightly reduced when carbonization accelerates. 

Charcoal output depending on types of wood


Type of wood

Weight of absolutely

dry wood, kg/m3

Output*, kg






Oak, maple, ash




Larch, elm












Pine, aspen, lime-tree, spruce








Fuel briquette

1 300


* Charcoal output also depends on conditions of growth and age of wood.

Charcoal should not be overburned or unburned. Overburned charcoal is obtained under the action of free oxygen on the hot charcoal; it has a lot of cracks and is fragile. Unburned charcoal is obtained in case of insufficient temperature or long burning, it contains charred logs, has a brown color, lits a flame during burning.

Fragile charcoal with cracks

Cracks reduce the strength of the charcoal, so the most durable charcoal obtained from short pieces. The volume of well-calcined charcoal is about 60 % of the volume of the primary wood.

The ash content of charcoal ranges from 1 to 4 %, the large ash content of charcoal from wood usually does not exceed 1.5%. Charcoal just unloaded from the kiln does not contain moisture, but absorb it from the air up to 10-15%.

An important characteristic of charcoal is the content of nonvolatile carbon. The higher the temperature of charcoal burning is, the greater content of nonvolatile carbon is. Charcoal has a high porosity, which explains its adsorption properties.

The output of charcoal can be significantly increased, if the pyrolysis will be carried out with the catalysts. For this it is necessary to impregnate the wood with aqueous solutions. Especially the acid - peroxide catalysts (2.5% of sulfuric acid and 0.2 % of hydrogen peroxide of wood weight) and diamoni phosphates are the most effective. The catalyst can significantly reduce the temperature of the thermal decomposition of wood, several times to increase the speed of this process and increase 1.3—1.5 times the output of charcoal.


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