The Fundamental Principle of our business: Pyrolysis of Wood

The Fundamental Principle of our business:
Pyrolysis of Wood

  1. General provisions

Charcoal is a macroporous high-carbon product obtained by sub-stoichiometric heating of wood beyond atmospheric ignition temperature. The structure and properties of charcoal are determined by the time/temperature regime of pyrolysis. Industrial charcoal is obtained at a final pyrolysis temperature of 450-550 °C.

Pyrolysis of wood (dry distillation of wood) – decomposition of wood when heated without access to air with the formation of gaseous and liquid products and a solid residue – charcoal.

The properties of charcoal and the rate of the process are affected by:

  1. Species and quality of wood
  2. The particle size of the raw materials
  3. The initial moisture content of the raw materials
  4. Temperature and rate of heating of wood during drying
  5. Speed and temperature of pyrolysis
  6. The amount of air available during the pyrolysis process

The key properties of charcoal according to GOST 7657-84:

roperty

Charcoal type

Test method

А

B

Unclassified, OKP 5571 0150

Top grade, OKP 24 5571 0132

Grade 1, OKP 24 5571 0132

Grade 1, OKP 24 5571 0143

Grade 2, OKP 24 5571 0144

Minimum Bulk density, g/cm3

0,37

0,37

Not standardized

GOST 7657–94, p. 4.6

Ash, max % by weight

2,5

3,0

2,5

3,0

4,0

GOST 7657–94, p. 4.7 & GOST 12596–67

Minimum fixed carbon, % by weight

90

78

88

77

67

GOST 7657–94, p. 4.8

Moisture, max % by weight

6

6

6

6

6

GOST 16399–70, part 2

Dust at point of loading, max % by weight

 

 

 

GOST 7657–94, p. 4.9

Particles < 25 mm

5

5

Not standardized

Particles < 12 mm

5

5

7

7

7

Smut, weight. %, <

None

2

None

2

2

GOST 7657–94, p. 4.10

Minimum density, grammes/dm3

210

210

Not standardized

GOST 7657–94, p. 4.11

  Classification of charcoal production systems (per  Y.D. Yudkevich)

2. Physical properties of wood

Timber is classified into hard and soft wood species. Softwoods include pine, spruce, fir, larch and others. Hardwoods include birch, oak, beech, hornbeam, maple, ash, poplar, lime, willow, fruit species and others. This classification is based upon the presence or absence of specific cellular characteristics which have no significant effect upon the pyrolysis process.

In particular, the classification «hardwood» is applied to all those species whose cellular composition includes «vessels»; long, open, tubular cells. This type of cell is absent in the softwood species. This is why e.g. balsa, a light and easily workable timber, is called a «hardwood» whereas e.g. larch, a tough and often dense timber, is a softwood.

In our activity we consider only those properties of wood that are relevant in the procurement, storage, drying and pyrolysis of wood:

  • Porosity
  • Specific or volumetric weight (density)
  • Humidity (moisture capacity)
  • Drying ability
  • Heat capacity

 

2.1. Wood porosity

Wood consists mainly of tubular cells and therefore has great porosity. Porosity affects the specific gravity, hygroscopicity, thermal conductivity and other physical properties of wood and can be calculated based on bulk density. The degree of porosity also affects the product of wood pyrolysis – charcoal, which has a significantly higher porosity (air/water capacity by weight) compared to the wood from which it is obtained.

 

2.2. Specific or volumetric weight

It is important to distinguish the specific gravity of the solid matter component of wood and that of wood as it is ordinarily presented, i.e. as a cellular body.

The specific gravity of woody solid matter, regardless of species, is on average about 1.55 kg/dm3.

The specific mass (density) of wood as we see and recognise it mainly depends on the porosity and moisture of the wood. Porosity changes according to the species and age of the tree, its growing conditions and other factors.

Due to the dependence of the volumetric weight of wood on moisture content, it is usually defined in 3 ways: for absolutely dry («bone-dry»), air-dry («seasoned») and freshly cut («green») wood. The average specific gravity of different species of wood in bone-dry* condition is given in table 1.

Table 1

Bone-dry density of the most common wood species

 

Species

Density,  g/cm3

Density range

Average

Fir

0,31—0,50

0,38

Spruce

0,30—0,56

0,42

Pine

0,31—0,65

0,47

Larch

0,43-0,82

0,63

Aspen

0,32—0,61

0,47

Lime

0,33—0,62

0,47

Alder

0,33—0,64

0,49

Birch

0,42—0,79

0,60

Beech

0,45—0,79

0,64

Sycamore, maple

0,48—0,74

0,65

Oak

0,46—0,88

0,68

*Wood dried to constant weight at a temperature of 103 ± 2 ° C

The following relationship exists between porosity and bulk density of wood in an absolutely dry state:

С =  (1/х) yd

where:

С – wood density:

х — percentage porosity;

— density of timber as presented;

—density of solid woody material.

The volumetric weight of wood of the same species varies significantly depending on the growing conditions and age of the tree, resinousness and other conditions. So, for birch, it ranges from 0.51 to 0.74: for pine, from 0.31 to 0.71: for spruce, from 0.35 to 0.60 (for bone-dry wood). The specific gravity of wood taken from different parts of the same tree is not uniform: at the base of the branches it is largest, in the root greater than at the crown, and in the heartwood greater than in sapwood.

The solid biomass content of 1 m3 wood can thus be calculated. On average it is as follows: pine – 285kg, spruce — 253kg, fir – 250kg, larch – 385kg, aspen – 265kg, birch – 354 kg.

Further, a 1 m3  stack or pallet of cleft firewood has equivalent solid wood content to a single 680 kg block of wood.

2.3. Wood moisture

Water in wood is present in the cavities (lumena) of the cells and also impregnates the cell walls, where it is held by loose chemical bonding. Accordingly, free (or capillary) and bound (or colloidal) water are distinguished in wood. In general, bound water accounts for wood moisture content (MC) up to circa 28%: removing this moisture requires a relatively large input of energy. Above this MC, the water is not chemically attached to the wood structure and requires a lesser input of energy for elimination..

The MC transition betwee bound water and free water is called the fiber saturation point (FSP): it lies in the range of 23-30% of absolute humidity depending on the type of wood. In general, wood scientists would consider 27% absolute MC, or thereabouts, as being FSP in temperate species.

Moisture distribution is not uniform in different parts of the tree. Sapwood (the outer part of the stem) tends to a higher MC than heartwood. The basal trunk contains more moisture than the crown. A tree growing on marshy soil contains more moisture than a tree growing on dry soil. MC can be subject to seasonal variations.

For dry distillation (pyrolysis) of wood, the moisture content is of paramount importance. The higher the moisture content in the wood,  the greater will be the process heat requirement or the lower will be the yield of valuable products.

2.3.1. The difference between Absolute and Relative Humidity of wood.

Absolute humidity of wood is the ratio of the weight of moisture contained in wood to the mass of absolutely dry wood, expressed as a percentage.

Example: If a raw 300 g sample weighs 200 g after drying to constant mass, then its absolute moisture content (300-200) / 200 * 100% = 50%)

Relative humidity of wood is the ratio of the weight of the moisture contained in the wood to the weight of the raw wood, expressed as a percentage.

Example: If a raw 300 g sample weighs 200 g after drying to constant mass, then its relative humidity (300-200) / 300 * 100% = 33%

It is very important to define which expression of MC is in use. As a generality, wood producers, traders and processors will speak of MC as relative humidity: researchers and wood scientists prefer to use absolute humidity.

2.3.2. Degrees of Absolute Wood Moisture

 

Name

Absolute MC, %

Preparation conditions

Wet wood

More than 100 %

prolonged exposure to water

Freshly cut

50—100 %

 variable by season of harvesting

Air-dried

15—20 %

Prolonged period in exposure to warm, dry natural air

chamber-dried

6—10 %

Dried in artificially controlled environment

Absolutely dry

0 %

Dried by excess heat until stable mass is reached

2.4. Natural (atmospheric) drying of wood.

During natural drying, the wood loses moisture through contact with circulating natural atmospheric air of relative humidity < 100%,. The speed of natural drying is greater, the higher the temperature of the air, the lower its relative humidity and the faster its movement. Moisture release per surface area of wood is variable, according to the exposed dimension of the wood: most of all from a transverse section of wood (across the grain), less from a longitudinal split (along the grain) and even less from a surface covered with bark.

Evaporation from wood proceeds at a lower rate than evaporation from an open source of water. The more cell walls the water molecules encounter when passing from the inside of a piece of wood to the outside surface, the slower this process. On a path parallel to the axis of the trunk, the cell walls are fewer than on a path perpendicular to the axis of the trunk. Therefore, through a cross-section of wood, more water vapor is released per unit of drying time from a cross-sectional than from a lateral surface of the wood.

The relative humidity and air temperature have a very large effect on the speed of drying viz:

  •  It has been shown that under conditions, for example, of the Urals, the most intensive drying of wood is observed in the period April — July;
  • during the first year post-felling, coniferous industrial firewood does not dry as completely as firewood cleared of bark;
  • In the autumn months, due to the easier absorption of moisture by firewood without bark, the humidity of both types of firewood differs only slightly;
  • Coniferous firewood not de-barked dries slightly during the first year, the bark is shed during the second year and the wood becomes air-dry;
  • Birch dries slower than conifers.  De-barked birch firewood reaches its maximum dryness only in the second year of storage, and birch remaining in the bark dries out less.
  • Split firewood, bark on, dries faster than de-barked roundwood.
  • The average moisture content in freshly cut birch is 56.2% (from 54% to 59%), in pine — 49.9% (from 46% to 53%), in spruce — 50.2% (from 44% to 55%) ) (percentage of relative humidity), in oak – 55%. (from 52% to 58%)
  • With natural drying, no matter how long it lasts, the wood will never attain a completely dry state. Accordingly, the temperature and relative humidity of the outdoor air will only achieve equilibrium humidity.
  • In natural drying for 1.5 years after felling, the relative humidity of air-dried firewood reaches approximately 20%.
  • When lying (in woodpiles in a forest or in a warehouse), firewood gradually loses moisture. The speed of this natural drying depends on the type of wood, its initial moisture content, method of cutting, relative humidity, temperature and air velocity.

How to dry firewood 1 m long, harvested in January, at storage under a canopy is shown in table 2.

Drying a piece of wood, due to its low thermal conductivity, is non-uniform. The outer layers of wood dry much faster than the inner ones. With fast drying of thick and damp logs, the outer layers are dry, while the inner layers are just starting to lose moisture. As a result, wood shrinkage is anisotropic (differential by dimension); this causes the formation of cracks in the wood, extending from the outer surface inward along the radius. Cracks in the wood may also occur as a result of faster drying of the core compared to sapwood.

2.5. Heat capacity

The heat capacity of absolutely dry wood of all species is 0.324 kcal / kg, and resinous substances – 0.5 kcal / kg.

When wood is heated by 1 ° (within 0 ° – 100 °), it expands by 0.040 mm/metre in the transverse direction and by 0.005 mm/metre in the longitudinal direction.

Wood is a poor conductor of heat. Its thermal conductivity depends on the conditional density of the wood (variable by species), the direction of the heat flux relative to the axis of the wood fiber, temperature and humidity. The coefficient of thermal conductivity of dry wood ranges from 0.1-0.4 W / m². An increase in the density of dry wood, i.e., an increase in the proportion occupied by wood substance per unit gross volume, leads to an increase in thermal conductivity. This is due to the fact that the wood substance has a 20 times greater thermal conductivity than air. The thermal conductivity of wood along the fibers is 3 times greater than across the fibers. In the radial and tangential directions, the thermal conductivity of  wood may differ slightly because the zones of late wood of annual layers are elongated in the tangential direction. Late wood, especially coniferous, is denser and therefore more thermally conductive. Moistening of wood, i.e. replacing the air contained in it with water, which has 23 times greater thermal conductivity, leads to an increase in the thermal conductivity of wood. An increase in the temperature of wet wood leads to an even greater increase in thermal conductivity. 

3. Charcoal raw materials

 3.1. Firewood is solid wood.  Offcuts and lumpwood waste of logging and woodworking.

Wood should be prepared for drying and pyrolysis. The ratio of the length of the pieces to the maximum diameter is 1/3, i.e. if the length of the wood is 300 mm. its diameter should not exceed 100 mm. In the process of drying and pyrolysis of wood, the basic principle applies: the shorter the piece, the faster and more uniformly the drying and pyrolysis process takes place. Keep in mind that rot in wood prevents coal formation and affects the cooling and stabilization of charcoal.

  • Various kinds of fuel briquettes (pini-kay, ruff, nestro).

  Fuel briquettes have the following advantages compared to firewood:

  • High density — increases the duration of burning
  • Homogeneous fraction – uniform heat distribution during combustion
  • High mechanical strength – minimum dust loss when physically damaged
  • Low humidity (5-8%)
  • discontinuous wood structure
  • Low ash

These properties allow the assertion that charcoal produced from fuel briquettes has numerous advantages compared to charcoal made from solid wood.

The most common presentation is charcoal made from Pini-kay type fuel briquettes. This type of charcoal briquette is frequently available from Asia. 

3.3. Nut-shells: coconut, walnut and many others.

The shells of various nuts are a good raw material for pyrolysis, as they tend to have a high specific density.

Coconut shells are a globally important raw material for charcoal production, especially in developing countries such as Indonesia, Malaysia, India. Coconut shell charcoal is used in many industries due to its advantages and properties. Moreover, coconut charcoal can be ground and used to produce granular activated carbon. Due to its pleasant smell, coconut charcoal is recognized as one of the best types of fuel for cooking.

3.4. Agricultural plant waste.

Small-fragment and low-density plant waste can be subject to processing into BioChar, also known as Terra Preta. This material is gaining new momentum every year as a soil conditioning & improvement agent. Entrepreneurs in the agricultural industry have begun to pay more attention to restoring soil composition, and they are also looking for safe solutions to the problem of soil depletion. Biochar is a proven option in these situations.

Biochar is a quality fertilizer which:

  • improves the composition of infertile stony, volcanic or sandy soils;
  • neutralizes soils with a high level of acidity;
  • prevents pests;
  • prevents the development of purulent processes;
  • provides accelerated growth and development of plants, as the soil warms up more quickly and uniformly;
  • absorbs and fixes residues of chemicals (pesticides, herbicides and other toxins) from the soil;
  • contributes to the functioning in the soil of micro-organisms that have a positive effect on productivity (mycorrhizal associations);
  • enhances soil porosity, thereby providing plant roots with access to oxygen and air circulation;
  • is an antiparasitic and antibacterial agent;
  • preserves and maintains the saturation of soils with necessary trace elements and nutrients, and eliminates the problem of their leaching.

4. A brief review of thermal methods for processing wood raw materials.

«Pitch production» is the process of producing solid, meltable pine resin by thermal decomposition of pine sap. The appearance of pitch in Russia dates back to the 12th century. For a long time, pine resin was one of the main items of Russian export. It was essential to wooden ship-building By now, in the absence of any commercial drivers, this process is extinct.

„Tar digestion» is the process of obtaining tar during thermal decomposition of birch bark. This is also a very old Russian craft. Birch tar, like pine resin, was exported in large volumes abroad. Tar production has survived to this day, albeit in much lesser volume.

«Wood gasification» is the process of producing gaseous fuels from wood. In Russia, gasification of wood in direct process gas generators appeared in the middle of the 19th century. to provide combustible generator gas for furnaces in the iron and steel industry and in glass production. Large gas generating stations, such as Izhevskaya, equipped with gas generators with a capacity of 30 thousand m3 of processed wood per year, worked until the end of the 1950s.

«Carbonization» is one of the typical forest chemical industries of the past. Even in ancient times, charcoal was used for smelting metals and as fuel. The only carbonization product is charcoal. The volume of charcoal production in Russia was very significant, the maximum production – about 1 million tons per year – was achieved before the First World War. The overwhelming majority of coal was used in metallurgy, for the smelting of pig iron. However, charcoal in the iron and steel industry was replaced by cheaper coal-coke and its production decreased sharply. Currently, charcoal is practically not used in this industry. With the general development of technology, the charcoal process itself also changed: pit and heap charcoal was gradually replaced by charcoal in various kinds of furnaces, at first periodic and then semi-continuous.

The process of thermal processing of wood, in which liquid products are produced alongside charcoal, was called in the Russian literature «dry distillation of wood». Now, this term is considered obsolete and replaced by wood pyrolysis, and the name «dry distillation» is supplanted by «pyrolysis production».

5. Wood pyrolysis

  1. 1. The main stages of pyrolysis. The process usually involves four stages, which can be divided into exothermic and endothermic reactions. An exothermic reaction is a chemical reaction accompanied by the release of heat, and an endothermic reaction requires input of heat.

The composition and properties of pyrolysis products are influenced by the species and quality of the wood, particle size of the raw material and its initial moisture, heating rate, residence time of the raw material at a particular temperature, final heating temperature, gas flow rate through the wood layer, and other factors.

5.1.1. Wood drying: temperature not higher than 150 ° С; an endothermic process, characterized by heat absorption; the composition of the wood is almost unchanged.

5.1.2. The initial stage of breakdown of wood: temperature from 150 to 270–275 ° С; endothermic process; decomposition of hemicelluloses and individual fragments of lignin begins; low molecular weight products are formed (water, carbon oxides, methanol, acetic acid, etc.).

5.1.3. Stage of pyrolysis proper: temperature from 270–275 ° С to 450 ° С; exothermic process; intense decomposition of cellulose and lignin occurs with the formation of the bulk of the products of disassociation and the formation of the structure of the char residue.

5.1.4. Calcination of charcoal: temperature lies in the range 450–550 ° С; the residual functional groups are cleaved from the carbon skeleton; endothermic and exothermic reactions run in parallel, the overall balance of the stage is endothermic.

5.1.5. Charcoal cooling and stabilization. The hot charcoal is discharged from the charcoal kiln and absorbs oxygen from the air whilst hot, as a result spontaneous combustion can occur. Chars burned at low temperatures and containing up to 30% volatile substances have the greatest propensity for spontaneous combustion; the auto-ignition temperature of such chars, in excess oxygen, is below 150 ° C. Charcoal with a low content of volatile substances can spontaneously ignite at temperatures above 250 ° C. Unloaded char at 30-90 ° C may adsorb 0.5-2% atmospheric oxygen by weight of char; at the same time low molecular weight products, primarily water (0.3-1.5%), are released from the char.

5.2. Effects of process conditions on charcoal yield and properties

The composition and yield of the final products largely depends on the residence time, in the heating zone, of the vapor-gas mixture formed during primary decomposition. If we consider the general trends of thermal decomposition, it can be argued that a relatively low temperature (200–400 ° C) and a longer process duration contribute to the formation of solid char product, whilst at temperatures above 600 ° C an increased proportion of gases are formed. Resins will prevail at a more moderate temperature (400–600 ° C), with a high heating rate and short residence time.

In the process of pyrolysis, a large range of gas-phase products are evolved. These include carbon monoxide and dioxide, gaseous saturated and unsaturated hydrocarbons, hydrogen, water (not only wood moisture, but also the product of the chemical decomposition of its components), acids (formic, acetic, etc.), methanol, ketones, and ethers. All of these substances are removed as a gas-vapor mixture.

To begin the process of cellulose decomposition, the temperature must be about 270–280 ° С. At temperature above this threshold the process in dry wood becomes self-driven, with the release of heat and an increase in temperature. But if the core of the wood pieces contain moisture, then vapors are formed that reduce the temperature in the outer layers. At a temperature of about 300 ° C, lignin also decomposes. The process of thermolysis is complicated by the fact that the thermal decomposition products from the deep layers pass through the outer, more heated layers. This additional heat causes them to undergo further transformations.

Because wood has low thermal conductivity, heat is distributed slowly through it. A constant heat input is needed to maintain the process until the exothermic stage is reached.

Figure 1 shows how, when heat is supplied through the wall of the retort or trolley loaded with wood, the temperature rises inside the trolley. Here, the stages of wood drying, separation of chemically bound moisture and methoxyls, rapid exothermic decomposition of wood, and calcination of char with the evolution of mainly gaseous substances are clearly shown. The exothermic heat evolved is 1000–1150 kJ. This heat is sufficient that in the absence of heat loss, the pyrolysis process of air-dry wood will proceed without additional heat input.

Изменение температуры в процессе пиролиза

Temperature change during pyrolysis:

1 – temperature outside the retort (trolley); 2 – temperature inside the retort (trolley);

I = heating period; II = drying period; III = 1st stage of pyrolysis;

IV = exothermic phase; V = char calcination phase.

The rate of heating of solid wood by an external heat source is determined by the rate of heat transfer from the heat carrier medium through the wall of the container (retort or trolley) via heat conduction, and inside the trolley by natural thermal convection from the wall into the wood. Overall, the slowest of the processes is the determining factor – the thermal conductivity through the wall of the trolley. With internal heating by the circulating heat carrier the heat input is faster: in this case, the rate of heating is determined by the heat transfer from the gas stream to the wood and the internal thermal conductivity of the wood.

Char calcined to 500 ° C is not pure carbon. If calcination is continued, hydrocarbons will also form at 600 ° С (methane, ethane, ethylene, etc.). At 700–950 ° С the principal gas evolved is hydrogen. The table Figure 4 shows how the content of non-volatile carbon in coal increases and its overall mass yield decreases, with increasing temperature and the removal of volatiles.

Table 4

Dependence of the yield of coal and volatile components on carbonization temperature

End  temperature, oC

Volatile, output %

Charcoal characterisitcs

Output, % by mass of original wood

Carbon content of char, % by mass

370

9,0

40,1

75,5

380

10,0

39,1

77,5

390

10,5

38,6

78,5

400

11,0

38,1

79,5

410

11,5

37,6

80,6

420

12,0

37,1

81,7

430

12,5

36,6

82,8

440

13,0

36,1

83,9

450

13,5

35,6

85,1

470

14,0

35,1

86,3

500

14,5

34,6

87,6

530

15,5

33,6

90,2

800

22,0

27,1

96,0

Carbon content (% by mass) achieved at different process temperatures:

Picture. 2

зависимость содержания углерода от температуры

I general, char calcinated at 450–550 ° С is considered to be suitable for most uses,. Under high-speed pyrolysis, the yield of char is 30–50% less than with a longer residence time in the heated zone. Oxidative pyrolysis (in the presence of metered amounts of oxygen) also reduces the yield of coal. In industrial production, the yield of char is often noticeably less than stated above. The most common cause of this lost productivity is the ingress of atmospheric oxygen into the apparatus, as a result of which part of the char burns out. The specific parameters of the regime and kiln overloading can also impact negatively.

6. Technical, physical and chemical properties of charcoal

and its handling requirements

In good charcoal, the wood structure is preserved. In cross-sections of char, especially from coniferous species, annual rings should be clearly visible.

Good charcoal should be strong, shiny black, have few radial cracks and make a ringing sound when tapped. It should burn without smell or smoke.

Charcoal is hygroscopic, it readily absorbs moisture from the air. It absorbs especially quickly during rain, or if stored in low, humid and damp places without flooring. Therefore, charcoal should be stored indoors or under a canopy on flooring or pallets, in an elevated dry area.

The main qualities of charcoal include strength, which reduces losses during handling and transportation. The wood species has significant influence on strength. For example, birch charcoal is the most durable, pine and aspen are less durable; the most durable is stemwood of large trees and the least durable is knotty branchwood.

The chemical properties of charcoal

 

Element

Presence,  %age by mass

1

Carbon

72-95% (average 85%)

2

Oxygen

5-15%

3

Phosphorus

0,016-0,037%

4

Hydrogen

4 – 4,8%

5

Volatiles

3-20%

6

Ash

< 3%

7

Moisture

от 4%-15%

8

Calorific value

7000 – 8100 Kcal/kg (29 — 34 MJ/kg)

The Physical properties of charcoal

 

Property

Nominal value

1

Solid density

260-380 kg/m³

2

Bulk density

130-150 kg/m³ (average 143 kg/m³)

3

Specific surface area

160-400 m²/kg

4

Mass

Average 210 g/litre. Range 100 – 500 kg/m³

5

Ratio of pore volume to bulk volume of the piece (porosity)

72-80 %

6

Average specific heat

0,69-1,21 KJ/kg/K at 24 and 560°С

7

Thermal conductivity

0,058 W/(m/К)

8

Electrical resistivity

0,8·108 – 0,5·102 ohm/cm

In the CIS countries, the standard GOST 7657-84   applies to  manufacture of charcoal.

Charcoal is prone to spontaneous combustion. The spontaneous combustion of charcoal is the result of its auto-oxidation, which develops quickly, with a rapid increase in temperature under the influence of paramagnetic centers in the char. This is a chain branched process that has certain critical parameters. If these parameters (concentration of PMC, temperature, concentration of O2 and geometric dimensions of the mass of coal) are not exceeded upon contact of coal with air, then coal will not ignite. It is possible to prevent self-ignition by carrying out controlled stabilization.

7. Auto-ignition of charcoal and stabilization. Security measures during storage and transportation of charcoal

Charcoal has paramagnetic properties due to the presence of stabilizing macroradicals (paramagnetic centers – PMC). The presence of macroradicals determines the high reactivity of charcoal with respect to oxygen.

The hot charcoal discharged from the charcoal kiln absorbs oxygen from the air, while it is gaining heat. As a result, spontaneous combustion can occur. Coals burned at low temperatures and containing up to 30% volatile substances have the greatest ability to spontaneous combustion; the autoignition temperature of such coals is below 150 ° C. Charcoal with a low content of volatile substances can spontaneously ignite at temperatures above 250 ° C.

Cooled charcoal is also prone to autoignition. At ordinary temperature, charcoal can adsorb various substances from liquid solutions, as well as various gases, including inerts. In general, the easier the gas is liquefied the better it is absorbed by charcoal. When heated, charcoal will release adsorbed substances; this frees its capacity for adsorption. To further increase the adsorption capacity of charcoal, it may be deliberately «activated». However, charcoal which is well calcined also has effective activity: approximately 30% that of DAK (activated carbon).

Given the above, it is strictly unacceptable to freeze moist charcoal: it will fragment into small pieces. Charcoal should not be stored in the open, or in closed but moist rooms. When a small amount of moisture is absorbed, self-heating occurs due to surface activity. In the case of the slightest draught, this process could result in self-ignition.

If charcoal receives a small amount of air during the cooling process, or is intensively cooled by powerful air pressure, then it is less prone to spontaneous combustion. A simple example: in the wind, a bonfire flares up, and the flame goes out. It depends on the intensity of heat transfer. Thus, charcoal blown in a thin layer of air will not ignite, but will cool. The scientist Burmistrov, from Nizhny Novgorod, studied this phenomenon and made a quencher – on a mesh conveyor, a thin layer of hot coal moved and was purged with a strong air stream. This was an inclined conveyor with transverse scrapers, dressed in a casing. The pipe casing was approximately one and a half meters in diameter. Air entered at the upper end and was sucked from below, passing over a layer of charcoal.

Based on this, it can be concluded that the stabilization of hot charcoal can be carried out by controlled cooling with atmospheric air in a thin layer: on a conveyor: or by mixing it or cooling in a well-ventilated area. The optimal conditions for this process are: the temperature of the coal at the time of unloading from the oven or trolley <200 ° C, the thickness of the charcoal layer is 60-100 mm, the cooling time is 15-20 minutes, the temperature of the coal after “stabilization-cooling” is 70–80 ° C. Under these conditions, charcoal absorbs oxygen from the air, without heating up. It stabilizes and loses its ability to ignite spontaneously. At a low ambient temperature, the coal cools too quickly and does not have time to stabilize, therefore, in winter, the casing of the conveyor for cooling must be thermally insulated and the site must be closed. It is worth noting that after charcoal has cooled to ambient temperature it continues to stabilize, which process can take more than 10 hours.

 

8. Uses for Charcoal

As a fuel for fireplaces, barbecues and other similar devices

Unlike conventional fuels (for example, firewood), charcoal does not form smoke or an open flame when correctly ignited. It only gives the necessary temperature – heat. For the preparation of various dishes you do not need to wait for the wood to burn out – after all, charcoal is already a finished fuel. Charcoal is perfect for grilling, barbecue, etc in particular.

As heat-source for hookah

Charcoal provides the necessary intensity, duration and cleanliness of burning for a pleasing shisha experience.

In industry

Charcoal is used in non-ferrous metallurgy (for example, to produce aluminum, boron, etc.); in the production of pure silicon, which is used to produce semiconductors; in the chemical industry; as fireplace fuel (abroad), etc. In metallurgy, for example, it it used as a reducing agent (charcoal has a high carbon content). In the production of glass, crystal, paints, electrodes, plastics. In the production of charcoal, liquid by-products are formed in the form of wood tar (tar), from which turpentine, food grade acetic acid, rosin, methyl alcohol, alcohol solvents, etc. can be obtained.

As a feed supplement in animal husbandry

Charcoal is known to be beneficial in the digestive system of mammals.

In construction

As an insulating material during construction, since charcoal is very hygroscopic and absorbs odors well.

As anti-corrosion powders and lubricants

Charcoal finds some application in instrumentation and printing industry, where it is used for grinding and polishing parts and forms. The most suitable for these purposes is char from broadleaved species of wood, obtained according to a special technological regime. In mechanical engineering, solid lubricant is required, mainly graphite. Charcoal, due to its low ash content and pollution, can also be used to produce this lubricant. To do this, coal is mixed with sedimentary resin, calcined at a temperature of 1400-1500 ° C, and then treated with sour potassium manganese, sulfuric acid or tannin.

In the production of gunpowder

In gunpowder production, char from alder or buckthorn wood with a carbon content of 72-80% is preferred. Gunpowder prepared using char from other wood species is more difficult to ignite, so the use of other types of char is not commonplace. The amount of char and the carbon content in the char affect the rate of combustion of gunpowder. With an increase in the proportion of char the burning rate of gunpowder decreases, and with an increase in the carbon content of the char, it increases. In the composition of gunpowder, charcoal content ranges from 12 to 20%. So, hunting gunpowder contains coal 14-16%, cord gunpowder – 12%, sulfurless – 20%, etc.

In the manufacture of carbon products

Electrical carbon products are made from pure carbon materials, such as petroleum and pitch coke, carbon black, graphite, charcoal, etc., by mixing with coal tar or pitch. These products are used in many sectors of industry. They are used in the electrical equipment of various types: in electric motors, for thermal purposes, in vacuum technology, etc. This includes all types of carbon resistors, switching contacts, brushes, products for technology, communications and many other items.

As a filler for plastics

Charcoal can be used as a filler in plastics. Plastics of this type, where the powder is carbonaceous material, include, for example, some types of faolite, special-purpose molding materials, etc. In these plastics, char can replace expensive and scarce graphite. Charcoal, as already noted, is a low ash material, with few impurities. It is stable in chemically aggressive environments and is quite heat resistant. Raw char has a high electrical resistivity. When calcining char, its electrical conductivity increases rapidly, therefore, by calcining char at different temperatures and using some additives, it is possible to obtain products with desired dielectric properties. Calcined charcoal acquires significant adsorption activity, which enhances its main service function as a filler – adsorption hardening of the product. The basis of this function is a change in the mechanical properties of liquids near solid surfaces. The molecules of the dissolved resin near the surface of a solid char particle are oriented under the action of adsorption-attracting forces. The fluid around the particle acquires an ordered structure, and its mechanical properties change greatly. Upon solidification, this structure is preserved and the physicomechanical properties of the solid formed are improved. By-products of charcoal production, i.e. fines and dust, may not always find effective use. Coal fines differ from commercial char, having a somewhat higher ash content, but this fact can only affect the acid resistance the material, without affecting other properties. Resins such as phenol-formaldehyde, furfuraldehyde and others can serve as a binding agent for charcoal plastics. Wood-tar (pitch) can also be included in the press material composition, the presence of which in small quantities improves the ductility of products.

As a raw material for the production of activated carbon

Active carbons are porous carbon bodies that, when in contact with a gaseous or liquid medium, provide a significant surface area for the sorption process to occur.

 

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