Charcoal (medical ( DAB ): Carbo pulveratur , Carbo medicinalis ) is a solid fuel and is produced when air-dry wood ( dried to 13% to 18% water) is heated to 275 ° C in the absence of air and without oxygen supply ( pyrolysis ). The temperature rises by itself to 350 ° C to 400 ° C (charring, similar to the coking of coal). The volatile components of the wood burn . In addition to gaseous decomposition products (see methanol ), about 35% charcoal is obtained as residue .
Using the same technology can also peat to peat carbon and generally herbal basic materials into biochar convert. Peat charcoal, which has properties similar to charcoal, was quite common in the 18th and early 19th centuries due to a lack of wood, but is rarely found today. Biochar is a potential as a means to compensate for the carbon dioxide - emissions in view of global warming attributed to their production has been intensified in recent years.
Since ancient times , metals, especially iron and steel , could only be made by burning charcoal. Hard coal could not be used for this because of various harmful ingredients. The great demand for charcoal led to a shortage of wood in the 16th century. At the beginning of the 18th century, the forests, especially in Great Britain, had been exploited to such an extent by the increasing consumption of charcoal (and by the other uses such as shipbuilding, lumber, pile foundations , canal construction etc.) that the shortage of charcoal in iron production became one serious national problem developed. The ironworks were therefore forced to look for other combustion materials. This raw material crisis is one of the triggers of the industrial revolution . The solution to the problem was the use of coke as fuel for the blast furnaces and the development of the coal-powered puddling process to produce wrought iron. With the expansion of the railways and traffic routes, the charcoal was more and more replaced by the hard coal. Electricity and gas also became more and more important as energy sources. During the First and Second World Wars , charcoal was in demand again by trade, industry and the army ( wood gasifier, etc.). Today, almost everywhere, charcoal is only used for barbecue purposes. Brazil is an exception , where large quantities of charcoal are produced for iron smelting , as the country has hardly any hard coal. Large eucalyptus plantations were created to meet the need for wood .
Even today, deforestation for charcoal is a serious problem in some countries, for example in Haiti , where it provides 60% of domestic energy. The same fate threatens Madagascar , where charcoal is also the most common fuel.
Properties and composition
Charcoal is a mixture of organic compounds with 81% to 90% carbon, 3% hydrogen, 6% oxygen, 1% nitrogen, 6% moisture and 1% to 2% ash and only insignificant amounts of sulfur .
The charcoal forms a loose, black product with a bulk density between 0.15 and 0.40 g cm −3 and a true density between 1.38 and 1.46 g cm −3 . The porosity of the charcoal varies depending on the type of wood as well as the speed and final temperature of the char between 72% and 85%, the inner surface is 50-100 m 2 / g, which is why it has a high adsorption capacity . The thermal conductivity is around 0.042 W / (mK) at 0 ° C and 0.073 W / (mK) at 200 ° C.
In relation to the wood feed, a volume loss of 35% to 45% and a mass loss of 65% to 75% occur during pyrolysis.
Charcoal is relatively easy to ignite (350 ° C to 400 ° C) and continues to burn without a flame because the flame-forming gases have already escaped during the charring. It burns at a higher temperature than wood. The combustion temperature of the charcoal is 800 ° C.
Depending on the quality, about 28–35 MJ of energy are released per kilogram of charcoal during combustion . According to another source, the calorific value is the equivalent of 31.6–32.9 MJ / kg, depending on the type of wood used.
The calorific value increases as the carbonization temperature increases. By increasing the pyrolysis end temperature from 400 ° C to 1200 ° C, the specific heat increases from 1.02 to 1.60 kJ / (kg · K).
Resin-free, non-juice rich wood are lackluster , highly porous char consisting of resinous , juice rich charcoal obtained wood, contains inside the cells , which formed from the constituents juice lustrous carbon . Charcoal is always easily friable , but only because of its structure; the carbon substance itself is hard and a good polish for metal. At ordinary temperature it is extremely stable and lies in the ground for centuries without changing; in the air it adsorbs certain gas components and vapors as well as substances suspended from liquids .
The adsorption of gases and especially water vapor causes the charcoal to increase in weight after it has cooled down in the absence of air. The increase in weight of fresh charcoal when lying in the air is 4–5% for oak and birch charcoal, 5–8% for spruce, beech and alder, 8–9% for pine, willow and poplar , and fir charcoal in 24 hours 16%.
Quality criteria for charcoal are lumpiness , water content, ash content and content of volatile components. Fresh, dry charcoal discharged from the charring tends to self-ignite. It is therefore necessary to age under controlled conditions, avoiding heat build-up in the first 48 hours after pyrolysis. Good charcoal is glossy black and has the same structure as wood, badly charred coals have a brown, brownish-red color, these are referred to as red coals , fires , foxes and their charring temperature is too low, below 300 ° C. This was also created on purpose, for the production of black powder or in the metallurgical process.
Charcoal is produced by heating wood in the absence of air, resulting in charcoal, wood vinegar , wood gas , wood tar . Different phases of the pyrolysis process are distinguished depending on the temperature .
In the initial phase, temperatures of up to 220 ° C mainly lead to heating and drying of the material, with hydrogen and traces of carbon dioxide , acetic acid and formic acid being released . When heated up to 150 ° C wood gives off only hygroscopic water; then acidic vapors develop; if the temperature rises above 150 ° C, it decomposes. Up to around 280 ° C, the release of these substances increases in a pyrolytic decomposition phase , which, like the initial phase, is endothermic . From 280 ° C, a strong exothermic reaction occurs, in which around 880 kJ / kg of wood are released as energy and heat the process to over 500 ° C. From 300 ° C onwards, increasingly dense yellow or yellow-brown vapor and gases develop . The flammable gases (see also under wood gas ), especially carbon monoxide , methane , formaldehyde , acetic and formic acid as well as methanol and hydrogen, which burn and emerge with smoke development . When the escaping products cool down, wood tar and wood vinegar are obtained. The structure of the wood changes above 400 ° C from the fibrillar structure of the wood to the crystalline structure of graphite . In the last endothermic process, the flue gases are split into flammable carbon monoxide and hydrogen as they pass through layers that have already been carbonized ; the charcoal remains as a residue.
- Red coal ( roasted coal ) is produced at carbonization temperatures between 270 ° C and 350 ° C, is brown-black and has almost the same calorific value as the black coal produced above 350 ° C, with a yield that is half as much . This is therefore often produced for metallurgical purposes and, because of certain properties, for gunpowder production.
- Black coal is produced at temperatures above 350 ° C. As the carbonization temperature rises, the tightness and conductivity of the coal for heat and electricity increases; but at the same time the flammability of the coal and its tendency to attract moisture decrease .
- Coal produced below 270 ° C is solid, not burned out, red-brown (redwood).
The tightness and quantitative yield of the coal is determined by the charring speed; a lower speed results in a denser coal and a higher yield. The yield of coal decreases with increasing temperatures. At the same time, the coal is constantly becoming richer in carbon and ash and correspondingly poorer in hydrogen and oxygen.
Both red and black charcoal clearly show the structure of the wood macroscopically, while the type of wood from which the charcoal was produced can be seen microscopically.
Both hardwood and coniferous wood are processed into coal, the former mainly when the liquid distillation products, acetic acid and wood spirit → wood vinegar , are the main product, the latter when the main emphasis is placed on the extraction of coal, tar and turpentine oil .
Charring in piles and piles
The oldest method of producing charcoal (coal distillery) is the ancient kiln operation ( charcoal burning ), in which the wood was erected in almost hemispherical or conical piles (kilns) in large logs regularly (standing or lying) around three in the middle Stakes ( Quandel ) are put on and covered with a blanket.
In the past, especially in southern Germany , Russia and Sweden , the wood was charred in heaps or lying plants. The charring of the stratified wood was gradual from one end of the elongated pile to the other. The charred pieces were gradually pulled out .
Milk coal is differentiated as follows:
- Lump, coarse, reading or drawing coal, the densest or largest pieces, still in the form of the logs used.
- Forged or medium coals, dense but only fist-sized pieces.
- Small charcoals made of branch wood .
- Quandel coal, small leaky pieces from near the Quandel.
- Coal , - Coal extinguish or clear , small pieces or dust.
- Fires, incompletely charred pieces from the edge or bottom of the kiln.
Charring in ovens
In a very similar way to that in piles or piles, the charring takes place in round or angular brick-built pitch furnaces , which allow easier, more complete extraction of the by-products (tar, wood vinegar, gases which are usually lost during kiln operation), but a lower yield and less good yield Deliver coal. In these single-chamber stoves, air also comes into the wood to be charred, and part of it generates the necessary temperature through its combustion. This coal is known as furnace coal .
However, the charring process is better under control if the wood is charred in vessels that are heated from the outside, i.e. without access to air. In two-chamber furnaces , retorts , tubes or cylinders, this is sometimes done with heated air, with furnace gases from the blast furnace, with superheated steam or with the use of forced air. This charcoal is called retort charcoal , at 500 ° C a hard charcoal is formed.
Charcoal used to make black powder
Careful carbonization of this kind is particularly necessary for the extraction of coal for black powder manufacture. Large iron cylinders are used for this, which are filled outside the oven, closed with a lid and pushed into the oven. A large movable lid closes the space in which the cylinder is located. The gases evolving from the wood are fed into the furnace. The temperature 300-400 ° C is determined using a pyrometer . Red coal for hunting powder is made with superheated steam. Buckthorn , poplar, or alder charcoal work best here.
In traditional Japanese fireworks, many companies also produce their own charcoal. Here are woods of pine and Paulownien also Hanfstengel charred in Erdmeilern which are mostly built into the slope. The technique for producing the hemp charcoal is described in
Results of charring with various types of wood
According to the actual volume, the average coal yield is 47.6%.
If one compares the apparent volume (without subtracting the gaps) of the wood with that of the coal, the wood types provide the following volume percentages of coal:
- Oak wood 71.8 to 74.3% vol
- Red beech wood 73% vol
- Birch wood 68.5% vol
- Hornbeam 57.3% vol
- Pine wood 63.6% vol
The following percentages by weight of the charcoal are obtained with different types of wood (dried at 150 ° C and charred at 300 ° C):
- made of oak 46%
- made of spruce wood 40.75%
- Elm 34.7%
- Hornbeam 34.6%
- Birch 34.17%
- Buckthorn 33.6%
- Ash 33.3%
- Linden tree 31.85%
- Poplar 31.1%
- Horse chestnut 30.0%
Adsorption property of charcoal
Charcoal was and remained indispensable as an adsorbent, filter and clarifier in many areas. Charcoal, the finely structured surface of which binds many undesirable organic substances, can also be used as activated charcoal for filtering and cleaning various substances. For example in the production of vodka , as a charcoal tablet for diarrhea or for filters of gas masks . In general, charcoal produced at a low temperature adsorbs the most. Carbon adsorbs oxygen and is oxidized in the process. This reacts, for example, with hydrogen sulfide to form sulfuric acid and water, with ammonia to form ammonium nitrate and with ammonium hydrogen sulfide to form ammonium sulfate .
Also rot products are vigorously destroyed. Meat surrounded by charcoal takes a long time to decompose, without any signs of putrefaction. Coal also adsorbs odors. Evil-smelling, putrid water can be cleaned by freshly annealed charcoal and alcohol from fusel oils are released.
But the charcoal has no effect on the microscopic organisms (bacteria, etc.) contained in the water, and when the water is filtered through charcoal they pass through the filter; the water becomes odorless, but not cleared of the disease-transmitting organisms. Coal can hold back some large, non-polar , organic substances in water, e.g. B. chlorinated hydrocarbons , plant treatment products or drugs. But heavy metal ions (e.g. from lead), nitrate and calcium / magnesium (lime) cannot be filtered out using this process.
Charcoal also adsorbs
- Colorants, in particular the nitrogen-containing charcoal ( bone charcoal in the first row) has a strong decolorizing effect .
- Salts are adsorbed by the charcoal, and this is in large part the value of the charcoal for sugar manufacture.
- Bitter substances, glycosides , carbohydrates, especially alkaloids , are also adsorbed.
When lying in the air for a long time, charcoal loses its adsorptive capacity, but regains it through burning out ; The substances absorbed from liquids can also be withdrawn from the coal (revitalization) so that it can be used again after it has burned out.
Charcoal is used to generate intense heat, especially wherever smoke and flames must be avoided, for example in the forge, when glowing flat steel, in chemical processes, etc. Today, charcoal is mainly used for leisure purposes, e.g. B. for grilling DIN 1860 (51749), technically produced. Charcoal was before development of the coking of coal into coke, the necessary fuel for metal smelting as well as the sophistication of the blacksmith. Since it reduces metal oxides , contains little ash and practically no sulfur, it is ideal for extracting metals from the ores, but it is far too expensive and burns too quickly. Silver and copper salts are reduced by using carbon → soldering . Charcoal can also be used to harden steel ; the workpiece is placed in glowing charcoal for several hours and then quenched with water. The carbon then penetrates the outer layers of the steel. Furthermore, carbon disulfide , sodium cyanide → Castner-Kellner process , ferrosilicon can be obtained from charcoal .
- for the production of black powder
- for defusing brandy
- for clarifying and decolorizing liquids
- for filtering water
- for preserving putrefactive substances.
- for disinfecting
- For cleaning hydrogen- rancid fats and dull (damp) grain.
- as tooth powder
- as a polishing agent for stone, wood and metals.
- for filling aspirators for use in rooms with harmful substances.
- for processing in cigarette filters
- as coloring ( E 153) and flavoring (smoking agent ) in the food industry
- as charcoal in art, linden and willow charcoal are used for drawing, as black paint
- as an adsorbent for diarrhea and poisoning, for purulent ulcers.
- In semiconductor technology , retort charcoal was used in galvanic batteries and charcoal pencils in carbon arc lamps
- Fine charcoal dust is processed into charcoal briquettes
As a soil conditioner, charcoal loosens the soil and also works through its adsorption capacity for ammonia and carbon dioxide . Ornamental plants with rotting roots can be cured if they are placed in soil mixed with coal. Large wounds on sap plants heal easily if they are sprinkled with powdered charcoal, and such plants, tubers and seeds can be packed well in charcoal for longer transport. Recently, charcoal is also being discussed as a rediscovered soil additive in horticulture and agriculture under the name of biochar ( also biochar ) or terra preta .
Production and Market
The profession of charcoal maker, a worker who made charcoal in kilns, has practically died out in Europe. The decline is primarily due to increased environmental standards (in the course of the EU's eastward expansion also in Eastern Europe ) and the lack of cheaply available wood as a raw material.
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Most of the charcoal produced worldwide comes from what are known as emerging and developing countries . The total amount is estimated to be at least 43 million tons, but due to the data uncertainty this amount is assumed to be the minimum amount. Most of the charcoal used in European countries is imported.
Internationally, charcoal production is often criticized for having negative environmental impacts. On the one hand, emissions arise from both the production process and the use of charcoal. a. the release of CO 2 . On the other hand, the production of charcoal is often related to illegal clearing of forests. To make matters worse, the charcoal marketed in Europe is not subject to any obligation to provide evidence from the country of origin. While sustainability seals such as PEFC or FSC prove that the wood used comes from sustainable forestry, they say nothing about the geographical origin of the wood. In studies from 2017 and 2018, the WWF found that the majority of the charcoal tested in Germany contained tropical wood and that largely no correct information was noted on the product packaging. Only charcoal imported from Namibia was highlighted positively, as it is potentially produced from excess wood from the bushes . In 2019, Stiftung Warentest also carried out an analysis of charcoal marketed in Germany and also found that only charcoal from Namibia can be traced back to sustainability.
Almost all of the charcoal offered in Switzerland is also imported. The WWF Switzerland introduced in 2018 the grill coals available in Switzerland to the test. It was found that many of the products are incorrectly declared. Tropical wood species could be detected in almost half of the products tested. In 2018, charcoal in Switzerland was mainly imported from Poland. In 2019, tropical wood was found in only two Obi products .
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