Most of the wood is made up of organic compounds. Under the influence of high temperatures in a vacuum or a minimum flow of air, they decompose into solid, semi-liquid and gaseous components. This process is called dry distillation, tar coating, pyrolysis. It differs from burning in the higher preservation of decomposable components. Chemical reactions take place very quickly and without the formation of flame and smoke.
Distillation technology appeared in the XII century. Then, pine and other conifers were used as raw materials. Resin was obtained from them for the impregnation of the wooden parts of ships and ropes, and charcoal. In the XIX century, with the help of the destruction of the carbon-containing mass, acetic acid began to be obtained.
Essence of the process
The usual burning of wood with the participation of oxygen leads to ignition and evaporation of the released gases, the complete destruction of solid components, their transformation into smoke, soot, ash and ash. The flame temperature in this case reaches 1000 ° C. Pyrolysis is also a thermal degradation. Dry distillation is carried out in a confined space at a constant temperature of 250–450 ° C, the resulting gases and liquid emitted components are immediately removed and cooled. The process is accompanied by a large heat loss, but smoke and soot are not formed. The resulting residues can then be used for industrial purposes or the household.
Harvesting raw materials
As starting material, wood unsuitable for other needs is used, production wastes, including sawdust. It is customary to distinguish several groups of raw materials:
- hard-leaved: beech, birch, elm, oak, hornbeam, maple, ash;
- soft-leaved: linden, alder, aspen, poplar;
- conifers: larch, pine, cedar, fir, spruce.
There is a strict production regulation, according to which all timber coming to the processing plants is sawn, cut and collected in piles of a certain width and height in special warehouses. They are placed on flat areas, provide access to air and lighting.
Preparation for pyrolysis involves preliminary drying. This process can be difficult, especially when working with an array of aspen or poplar, which, with increased dampness, is prone to damage by fungus and the development of decay.
Drying is carried out naturally in the ventilated area of the warehouses. To speed up the process, the material is split into small pieces. Wood of an air-dry state is considered suitable for further processing: about 12-15%.
Sometimes quick drying is used: the array is crushed, placed in an oven or poured with dry hot air.
Equipment
Decomposition of wood is carried out in retort furnaces. Cases - cylindrical containers welded from metal. The thickness of their walls is about 15 mm. At the top there is a loading hole, inside there are grids for placing raw materials, a system for supplying coolants, removal and cooling of the emitted liquid products, gases and charcoal, at the bottom there is a discharge port for the resulting components.
Industrial equipment has various sizes. Most often they use large furnaces, the combustion chambers of which are about 2–2.5 m in diameter. The following heating systems are used:
- external: the metal walls of the retort are heated, starting the process of decomposition of wood;
- internal heat supply: the temperature is maintained by a mixture of the released gases; their efficiency is usually several times lower than that of external heaters.
Equipment is continuous, semi-continuous and batch. In the first case, all stages of the cycle occur simultaneously. When another batch of wood enters, finished coal is unloaded from the outlet. For semi-continuous devices, the process is streamlined. First, the first batch of raw materials undergoes full processing; after unloading, the next one arrives. The type of equipment affects the speed at which decomposition of wood occurs.
There are boilers similar to industrial ones, but more compact. They are intended for pyrolysis on a small scale.
The distillation result is influenced by the conditions in which the process occurs and the condition of the incoming material.
Distillation process
If the loaded batch of wood is not dry enough, after grinding it is dried in a closed chamber at a temperature of 130 ° C. This stage is the most energy-intensive, because it requires a mandatory external heat source. The evaporation of moisture is accompanied by the primary decay of some wood components.
Further heating to 155–200 ° С leads to the beginning of the release and evaporation of gaseous substances.
The direct decomposition of the entire mass of material occurs upon subsequent heating to 280–420 ° C. In this case, resins, acetic acid, carbonyl compounds are released and discharged. At the same time, charcoal is formed.
The final stage is calcination. The temperature inside the retort rises to 500 ° C and higher. Heavy resins, carbon compounds are removed from the wood residue. Then the products are cooled and discharged from the chamber.
The amount of material obtained, the speed of the process and energy costs depend on the type and size of pieces of wood, equipment systems. Fast pyrolysis using external heat sources leads to more coal and its high purity at relatively low energy costs.
Pyrolysis Products
The main components for which wood is distilled are coal and acetic acid.
Coal
The amount of solid residue obtained depends on the type of wood. For hardwood beech and birch, the yield is about 25% of the primary material. In conifers it is slightly higher. Softwood plants give the least amount of coal. Pyrolysis of sawdust allows you to get coal flour. Moreover, the yield of liquid residue is higher.
High-quality coal does not have cracks, brown or whitish deposits, burns without smoke. Defective product is obtained in violation of the pyrolysis technology: insufficient temperature, air penetration in the furnace.
Charcoal is an environmentally friendly and affordable type of fuel that is used for industrial and domestic stoves, home fireplaces. It emits a large amount of heat, during combustion it practically does not form by-products and odor, has a low cost. Coal is used in the metallurgical industry, agriculture, for the production of filters, plastics, dyes, glass, and medicines.
Condensate
Liquid pyrolysis products, or condensate, contain resinous compounds called liquids. Its amount reaches 50% of the total residue and depends on the species and wood moisture, type of pyrolysis. The composition of the fluid includes ketones, resins, aldehydes, alcohols, esters, water. As a result of multi-stage reactions, acetic acid is formed from it - a compound used in the chemical, textile, pharmaceutical, cosmetic, and food industries.
Formic and butyric acids, acetone, methyl and isopropyl alcohols, formaldehyde, resins are obtained from the slurry.
Gases
Gaseous compounds released as a result of distillation are formed in an amount of 20–25%. The composition of pyrolysis gases includes:
- CO: 40-50%;
- CO2: 28–38%;
- CH4: 8–20%;
- H2: 1–2%;
- carbon impurities: about 1%.
On average, about 70–80 m³ of gaseous compounds are released during the dry distillation of 1 m³ of wood.
Distillation of wood at home
Pyrolysis of wood or its waste can be done at home. In this case, only coal can be obtained.
As a retort, metal barrels of large volume are used.. Do not take containers from chemicals. A clean container with several small openings for exhausting gases is needed.
First, prepare the platform:
- A large sheet of iron is laid on the ground.
- They install several refractory bricks on the edge, between which firewood is laid out.
- Make a bonfire.
The barrel is filled with dried chopped wood, tightly closed. After that, it is installed on a platform with a bonfire. When the container becomes hot and oxidation begins, gas will flow from the holes. The process may take several hours.
When the gas flow stops, the barrel is left at the stake for 30 minutes. After cooling, remove the lid and remove the finished charcoal. It can be used for kindling baths, home stoves, fireplaces. After distillation of the sawdust, the obtained flour is added to the garden soil, used to process plant sections.