The process of wood pyrolysis by its appearance and control measurements can be divided into the following stages:
1. Wood drying – wood gives off only moisture, almost 0-150°C.
2. Essentially, dry distillation (wood pyrolysis). At this stage in the distillate appears a number of organic products and exudes the gas. Usually this phase relates to the temperature interval of 150-350°C. This stage is also characterized by a so-called exothermic period, observed at temperature of 275-285°C, when the process goes very actively, with an apparent release of the reaction heat.
3. Stage of calcination. It is characterized by the separation of a small amount of resin (1,5-2%) and of a significant amount of non-condensable gases from the coal, produced on the previous stage,. This stage begins at the temperature of 350-400°C, and final temperature is set according to the production conditions and usually does not exceed 400-550°C.
The first component of wood that begins to disintegrate at a temperature slightly below 150°C, is xylan, but mostly its decomposition occur at a temperature of 250-260°C along with the formation of furfural, acetic acid and gases. The decomposition of lignin starts at a temperature approximately 200°C, this process leads to the formation of low molecular weight volatile compounds and to a complete restructuring of the primary structure of lignin. It happens due to hetero - and homolytic dissociation of chemical bonds between lignin structural units and inside of them. At the temperature of above 300°C cellulose begins to decompose.
Wood is a very complicated system of organic compounds, that differs from the others by its high molecular weight, and therefore the process of its disintegration is extremely complex and difficult to describe and, especially, to estimate. There is no doubt that there is a number of consecutive and parallel reactions, that are characterized by the break of ties that exist in the initial complex during the process.
As a result the new substances are formed, and many of them are unstable in these conditions, and easily react with each other. In the units of periodic action stages of the pyrolysis process occur consistently in time, whereas in continuously operating units in the top zone there is a process of drying, below – the process of heating wood to a temperature of the exothermic reaction, in the middle zone — the decomposition of wood and calcination of coal, in the lower part – the process of cooling the coal before unloading. And all of these processes occur at the same time. So, when someone works with periodically operating equipment the composition of the gas mixture varies over time, whereas in a continuous process it remains virtually unchanged in time.
Accumulated industrial experience and held in the laboratory studies gave the possibility to identify several relationships that determine the influence of various factors on the pyrolysis process and the relationship between the chemical composition of wood and products of its thermal decomposition. The pyrolysis process is influenced by various factors, but the most important among them are raw and modal factors, which are considered below.
The products of pyrolysis
As a result of the process following primary products are formed:
• charcoal is about 30%
• wood dry distillation gas – about 20%
• distillate (slush) – about 50% of the weight of air-dried wood.
The slush or raw wood vinegar is 15-20% solution of organic substances in water, produced by conventional dry distillation with using a condenser. Its specific weight is 1,025-1,035.
The acidity of the slurry is in the range of from 7 to 12%.
In the composition of the slurry it is found for about 380 individual chemicals.
Quantitatively, it is possible to take the average content of acidic parts in the slush — 7-12 %, wood alcohol (ether, aldehyde and ketone group) - 3-5%, the soluble resin - 5-7%.
Charcoal is a porous, shiny black fragile body with a bluish tint in the break, preserving the structure of the original wood to a certain extent. The true specific weight is 1.4; specific weight of lump charcoal from spruce is 0.25, charcoal from pine – 0.27, charcoal from birch - 0,3-0,4. Burned charcoal from all wood species has approximately the same composition of organic matter: C — 82,5%; H2 — 4%; O+H - 13,5%. The carbon content in charcoal, depending on the final temperature of burning, varies significantly reaching 99.7% of organic mass at a very high temperature (1600°C). Ash content in charcoal is 2.5-3.5%. There are dominated CaO, K2O, Na2O and is very little phosphorus and sulphur in its composition, which is very typical. The calorific value of charcoal is 7000 - 8000 cal/kg.
Wood dry distillation gas is combustible, although it contains to 50% of CO2. The calorific value of the total gas produced during pyrolysis is 800-1200 cal/m3. This figure changes dramatically in the process - dry distillation gas is gradually enriched by combustible components with increasing temperature.
The modern charcoal burning installations are focused on getting only charcoal, herewith the eco-friendly equipment provides utilization of wood (pyrolysis) gas and slush. Such a charcoal kiln is non-volatile. The resulting heat is sufficient to heat a new portion of raw materials; the process is continuous and effective.
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