Seminar “Efficient pellet production.” Ash composition of wood of various tree species in a floodplain biotope Thermal characteristics of wood
Humidity
The moisture content of woody biomass is a quantitative characteristic showing the moisture content in the biomass. There are absolute and relative humidity biomass.
Absolute humidity is called the ratio of the mass of moisture to the mass of dry wood:
Where W a is absolute humidity, %; m is the mass of the sample in a wet state, g;
m 0 - mass of the same sample, dried to a constant value, g. Relative or operating humidity
The ratio of the mass of moisture to the mass of wet wood is called:
Where W p is relative, or operating, humidity, %
When calculating wood drying processes, absolute humidity is used.
In thermal calculations, only relative, or operating, humidity is used. Taking into account this established tradition, in the future we will use only relative humidity. There are two forms of moisture contained in woody biomass: bound (hygroscopic) and free. Bound moisture is located inside the cell walls and is held by physicochemical bonds; Removing this moisture involves additional energy costs and significantly affects most of the properties of the wood substance.
Free moisture is found in cell cavities and intercellular spaces. Free moisture is retained only by mechanical bonds, is removed much more easily and has less impact on, in the environment in which it is located, i.e. its temperature and relative humidity.
Moisture content of stem wood. Depending on the moisture content, stem wood is divided into wet, freshly cut, air-dry, room-dry and absolutely dry.
Wet wood is wood that has been in water for a long time, for example during rafting or sorting in a water basin. The moisture content of wet wood W p exceeds 50%.
Freshly cut wood is wood that has retained the moisture of the growing tree.
It depends on the type of wood and varies within the range W p =33...50%.
The average moisture content of freshly cut wood is, %, for spruce 48, for larch 45, for fir 50, for cedar pine 48, for Scots pine 47, for willow 46, for linden 38, for aspen 45, for alder 46, for poplar 48, for warty birch 44, for beech 39, for elm 44, for hornbeam 38, for oak 41, for maple 33. Air-dry is wood that has been aged for a long time outdoors
. While staying in the open air, the wood constantly dries out and its humidity gradually decreases to a stable value. Humidity of air-dried wood W p =13...17%.
Room-dry wood is wood that has been in a heated and ventilated room for a long time. Humidity of room-dry wood W p =7...11%.
Absolutely dry - wood dried at a temperature of t=103±2 °C to constant weight.
In a growing tree, the moisture content of the stem wood is unevenly distributed. It varies both along the radius and along the height of the trunk. The maximum moisture content of stem wood is limited by the total volume of cell cavities and intercellular spaces. When wood rots, its cells are destroyed, resulting in the formation of additional internal cavities
, the structure of rotten wood, as the decay process develops, becomes loose and porous, and the strength of the wood decreases sharply.
For these reasons, the moisture content of wood rot is not limited and can reach such high values that its combustion becomes ineffective.
The increased porosity of rotten wood makes it very hygroscopic; being in the open air, it quickly becomes moisturized. Ash content Ash content called the content in the fuel
Ash is divided into internal, contained in wood matter, and external, which got into the fuel during the procurement, storage and transportation of biomass. Depending on the type, ash has different fusibility when heated to high temperature
.
Low-melting ash is ash that has a temperature of the onset of the liquid-melting state below 1350°C.
Medium-melting ash has a temperature of the beginning of the liquid-melting state in the range of 1350-1450 °C.
For refractory ash, this temperature is above 1450 °C. The internal ash of woody biomass is refractory, and the external ash is low-melting. The ash content of the bark of various species varies from 0.5 to 8% and higher in case of severe contamination during harvesting or storage. Wood Density The density of wood substance is the ratio
mass of material
| , forming cell walls, to the volume it occupies. The density of wood substance is the same for all types of wood and is equal to 1.53 g/cm3. On the recommendation of the CMEA commission, all indicators | physical and mechanical properties | |
| wood values are determined at an absolute humidity of 12% and are converted to this humidity. | Density of different types of wood | |
| Breed | 660 | 630 |
| Density kg/m3 | 500 | 470 |
| At standard humidity | 435 | 410 |
| Absolutely dry | 375 | 350 |
| Larch | 800 | 760 |
| Pine | 800 | 760 |
| Cedar | 710 | 670 |
| Fir | 690 | 650 |
| Hornbeam | 690 | 650 |
| White acacia | 680 | 645 |
| Pear | 670 | 640 |
| Oak | 650 | 615 |
| Maple | 630 | 600 |
| Common ash | 520 | 490 |
| Beech | 495 | 470 |
| Elm | 495 | 470 |
| Birch | 455 | 430 |
Alder
Aspen
Linden Willow The bulk density of waste in the form of various shredded wood waste varies widely. For dry chips from 100 kg/m 3, up to 350 kg/m 3 and more for wet chips.
Thermal characteristics of wood
Woody biomass in the form in which it enters the furnaces of boiler units is called
working fuel.
The composition of woody biomass, i.e. the content of individual elements in it, is characterized by the following equation: C р +Н р +О р +N р +A р +W р =100%,
where C p, H p, O p, N p are the content of carbon, hydrogen, oxygen and nitrogen in the wood pulp, respectively, %; A p, W p - ash and moisture content in the fuel, respectively.
To characterize fuel in thermal engineering calculations, the concepts of dry mass and combustible mass of fuel are used. Dry weight
In this case, the fuel is biomass dried to an absolutely dry state. Its composition is expressed by the equation
The indices of the signs of biomass components mean: p - the content of the component in the working mass, c - the content of the component in the dry mass, g - the content of the component in the combustible mass of fuel.
One of the remarkable features of stem wood is the amazing stability of its elemental composition of combustible mass. That's why The specific heat of combustion of different types of wood is practically the same.
The elemental composition of the combustible mass of stem wood is almost the same for all species. As a rule, the variation in the content of individual components of the combustible mass of stem wood is within the error of technical measurements. Based on this, during thermotechnical calculations, setting up combustion devices that burn stem wood, etc., it is possible to accept the following composition of stem wood for fuel without a large error mass: C g =51%, N g =6.1%, O g =42.3%, N g =0.6%.
Heat of combustion Biomass is the amount of heat released during the combustion of 1 kg of a substance. There are higher and lower calorific values.
Higher calorific value- this is the amount of heat released during the combustion of 1 kg of biomass with complete condensation of all water vapor formed during combustion, with the release of heat spent on their evaporation (the so-called latent heat of evaporation).
The highest calorific value Q in is determined by the formula of D. I. Mendeleev (kJ/kg):
Q in =340С р +1260Н р -109О р. Net calorific value
(NTS) - the amount of heat released during the combustion of 1 kg of biomass, excluding the heat spent on the evaporation of moisture formed during the combustion of this fuel. Its value is determined by the formula (kJ/kg):
Q р =340C р +1030H р -109О р -25W р.
The heat of combustion of stem wood depends only on two quantities: ash content and humidity. The lower heat of combustion of the combustible mass (dry, ash-free!) stem wood is almost constant and equal to 18.9 MJ/kg (4510 kcal/kg).
Types of wood waste
Depending on the production in which wood waste is generated, it can be divided into two types: logging waste and wood processing waste. Logging waste
- These are the separated parts of wood during the logging process. These include needles, leaves, non-lignified shoots, branches, twigs, tips, butts, peaks, trunk cuttings, bark, waste from the production of crushed pulpwood, etc. In his logging waste is poorly transportable; when used for energy, it is first crushed into chips.
Wood waste- This is waste generated in woodworking production. These include: slabs, slats, cuttings, short lengths, shavings, sawdust, production waste of industrial chips, wood dust, bark.
Based on the nature of biomass, wood waste can be divided into the following types: waste from crown elements; stem wood waste; bark waste; wood rot.
Depending on the shape and particle size, wood waste is usually divided into the following groups: lump wood waste and soft wood waste.
Lump wood waste- these are cut-offs, peaks, cutouts, slabs, laths, cuts, short lengths. Soft wood waste includes sawdust and shavings.
The most important characteristic shredded wood is its fractional composition.
Fractional composition is the quantitative ratio of particles of certain sizes in the total mass of crushed wood. The crushed wood fraction is the percentage of particles of a certain size in the total mass.
- — Shredded wood can be divided into the following types according to particle size: wood dust
- — , formed during sanding of wood, plywood and wood boards; the main part of the particles passes through a sieve with a hole of 0.5 mm;
- — sawdust, formed during longitudinal and transverse sawing of wood, they pass through a sieve with holes of 5...6 mm;
- wood chips
obtained by grinding wood and wood waste in chippers;
the main part of the chips passes through a sieve with 30 mm holes and remains on a sieve with 5...6 mm holes; wood material. The content of abrasive material in wood dust can reach up to 1% by weight.
Features of burning woody biomass
Important feature The advantage of woody biomass as fuel is the absence of sulfur and phosphorus in it. As you know, the main heat loss in any boiler unit is the loss of thermal energy with flue gases.
The magnitude of this loss is determined by the temperature of the exhaust gases. When burning fuels containing sulfur, this temperature is maintained at least 200...250 °C in order to avoid sulfuric acid corrosion of the tail heating surfaces.
When burning wood waste that does not contain sulfur, this temperature can be lowered to 100...120 °C, which will significantly increase the efficiency of boiler units. The moisture content of wood fuel can vary within very wide limits. In furniture and woodworking industries, the moisture content of some types of waste is 10...12%; in logging enterprises, the moisture content of the bulk of the waste is 45...55%; the moisture content of bark when debarking waste after rafting or sorting in water basins reaches 80%. Increasing the moisture content of wood fuel reduces the productivity and efficiency of boiler units. The yield of volatiles when burning wood fuel is very high - reaches 85%. This is also one of the features of woody biomass as a fuel and requires a large flame length in which the combustion of combustible components leaving the layer is carried out.
The product of coking woody biomass, charcoal, is highly reactive compared to fossil coals. High reactivity charcoal provides the ability to operate combustion devices at low values of the excess air coefficient, which has a positive effect on the efficiency of boiler plants when burning woody biomass in them. However, along with these positive properties in boiler and furnace equipment.
At a humidity of 10% and an ash content of 0.7%, the NCV will be 16.85 MJ/kg, and at a humidity of 50% only 8.2 MJ/kg. Thus, the fuel consumption of the boiler at the same power will change by more than 2 times when switching from dry fuel to wet fuel. Characteristic feature wood as a fuel has an insignificant internal ash content (does not exceed 1%). At the same time, external mineral inclusions in logging waste sometimes reach 20%. The ash formed during the combustion of pure wood is refractory, and its removal from the combustion zone of the furnace does not present any particular technical difficulty. Mineral inclusions in woody biomass are fusible.
When wood with a significant content is burned, sintered slag is formed, the removal of which from the high-temperature zone of the combustion device is difficult and requires special fireboxes to ensure efficient operation of the firebox. technical solutions . Sintered slag, formed during the combustion of high-ash wood biomass, has a chemical affinity with brick, and at high temperatures in the combustion device sinteres with the surface brickwork
furnace walls, which makes slag removal difficult.
Heat output usually called maximum combustion temperature
, developed during complete combustion of fuel without excess air, i.e. under conditions when all the heat released during combustion is completely spent on heating the resulting combustion products.
The influence of woody biomass moisture content on the efficiency of boiler plants is extremely significant. When burning absolutely dry woody biomass with low ash content, the operating efficiency of boiler units, both in terms of their productivity and efficiency, approaches the operating efficiency of liquid fuel boilers and, in some cases, exceeds the operating efficiency of boiler units using certain types of coal.
An increase in the humidity of woody biomass inevitably causes a decrease in the efficiency of boiler plants. You should know this and constantly develop and carry out measures to prevent atmospheric precipitation, soil water, etc. from getting into wood fuel.
The ash content of woody biomass makes it difficult to burn.
The presence of mineral inclusions in woody biomass is due to the use of insufficiently advanced technological processes for wood harvesting and its primary processing. It is necessary to give preference to such technological processes in which the contamination of wood waste with mineral inclusions can be minimized.
The fractional composition of crushed wood should be optimal for this type of combustion device. Deviations in particle size from the optimal, both upward and downward, reduce the efficiency of combustion devices. Chips used to chop wood into fuel chips should not produce large deviations in particle size in the direction of increasing them. However, the presence of a large number of too small particles is also undesirable.
To ensure efficient combustion of wood waste, it is necessary that the design of boiler units meet the characteristics of this type of fuel. Firewood
- pieces of wood that are intended to be burned in stoves, fireplaces, furnaces or fires to produce heat, heat and light. Fireplace wood
mainly prepared and supplied in sawn and chipped form. The moisture content should be as low as possible. The length of the logs is mainly 25 and 33 cm. Such firewood is sold in bulk storage meters or packaged and sold by weight. For heating purposes they are used various firewood . The priority characteristic by which certain firewood for fireplaces and stoves is selected is its, burning duration and comfort during use (flame pattern, smell). For heating purposes, it is desirable that the heat release occurs more slowly, but over a longer period of time. For heating purposes, all firewood from hardwood.
To fire stoves and fireplaces, they mainly use wood from such species as oak, ash, birch, hazel, yew, and hawthorn.
Features of wood burning different breeds wood:
Firewood made from beech, birch, ash, and hazel is difficult to melt, but they can burn damp because they have little moisture, and firewood from all these tree species, except beech, splits easily;
Alder and aspen burn without producing soot, moreover, they burn it out of the chimney;
Birch firewood is good for heat, but if there is not enough air in the firebox, it burns smoky and forms tar (birch resin), which settles on the walls of the pipe;
Stumps and roots provide intricate patterns of fire;
Branches of juniper, cherry and apple give a pleasant aroma;
Pine firewood burns hotter than spruce firewood due to its higher resin content. When tarred wood burns, a sharp increase in temperature causes small cavities in the wood to burst with a bang, in which resin accumulates, and sparks fly in all directions;
Oak firewood has the best heat transfer; its only drawback is that it splits poorly, just like hornbeam firewood;
Firewood from pear and apple trees splits easily and burns well, emitting a pleasant smell;
Firewood made from medium-hard species is generally easy to split;
Long-smoldering coals provide cedar firewood;
Cherry and elm wood smokes when burned;
Plane wood burns easily, but is difficult to split;
Less suitable for burning wood coniferous species, because they contribute to the formation of tar deposits in the pipe and have a low heating value. Pine and spruce firewood are easy to split and melt, but they smoke and spark;
Tree species with soft wood also include poplar, alder, aspen, and linden. Firewood of these species burns well, poplar firewood sparks strongly and burns out very quickly;
Beech - firewood of this species is considered classic fireplace wood, since beech has a beautiful flame pattern and good heat development with an almost complete absence of sparks. To all of the above, it should be added that beech firewood has a very high rate calorific value. The smell of burning beech wood is also highly rated - that’s why beech wood is mainly used for smoking food. Beech firewood is universal in use. Based on the above, the cost of beech firewood is high.
It is necessary to take into account the fact that the calorific value of firewood of different types of wood varies greatly. As a result, we get fluctuations in wood density and fluctuations in conversion factors cubic meter => storage meter
Below is a table with average calorific values per meter of firewood.
| Firewood (natural drying) | Calorific value kWh/kg | Calorific value mega Joule/kg | Calorific value MWh/ storage meter |
Bulk density in kg/dm³ |
Density kg/ storage meter |
| Hornbeam firewood | 4,2 | 15 | 2,1 | 0,72 | 495 |
| Beech firewood | 4,2 | 15 | 2,0 | 0,69 | 480 |
| Ash firewood | 4,2 | 15 | 2,0 | 0,69 | 480 |
| Oak firewood | 4,2 | 15 | 2,0 | 0,67 | 470 |
| Birch firewood | 4,2 | 15 | 1,9 | 0,65 | 450 |
| Larch firewood | 4,3 | 15,5 | 1,8 | 0,59 | 420 |
| Pine firewood | 4,3 | 15,5 | 1,6 | 0,52 | 360 |
| Spruce firewood | 4,3 | 15,5 | 1,4 | 0,47 | 330 |
1 storage meter of dry wood from deciduous trees replaces about 200 to 210 liters of liquid fuel or 200 to 210 m³ of natural gas.
Tips for choosing wood for a fire.
There will be no fire without wood. As I already said, in order for the fire to burn for a long time, you need to prepare for this. Prepare firewood. The bigger, the better. There is no need to overdo it, but you should have a small supply just in case. After spending two or three nights in the forest, you will probably be able to more accurately determine required stock firewood for the night. Of course, you can mathematically calculate how much wood is needed to keep a fire going for a certain number of hours. Convert knots of one thickness or another into cubic meters. But in practice, such a calculation will not always work. There are a lot of factors that cannot be calculated, and if you try, the scatter will be quite large. Only personal practice gives more accurate results.
Strong wind increases the burning rate by 2-3 times. Humid, calm weather, on the contrary, slows down combustion. A fire can burn even during rain, but for this it is necessary to constantly maintain it. When it rains, you shouldn’t put thick logs on the fire; they take longer to burn and the rain can simply put them out. Don't forget, thinner branches flare up quickly, but also burn out quickly. They should be used to light thicker branches.
Before I talk about some of the properties of wood during combustion, I would like to remind you once again that if you are not forced by the need to spend the night in close proximity to a fire, try to burn a fire no closer than 1-1.5 meters from the edge of your bed.
Most often we come across the following tree species: spruce, pine, fir, larch, birch, aspen, alder, oak, bird cherry, willow. So, in order.
Spruce, Like all resinous tree species, it burns hot and quickly. If the wood is dry, the fire spreads across the surface quite quickly. If you don't have any way to split the trunk small tree into relatively small equal parts, and you are using the entire tree for the fire, be very careful. Fire on wood can go beyond the boundaries of the fire pit and cause a lot of trouble. In this case, clear enough space for the fire pit so that the fire cannot spread further. Spruce has the ability to “shoot”. During combustion, the resin contained in the wood begins to boil under the influence of high temperatures and, finding no way out, explodes. The piece of burning wood that is at the top flies away from the fire. Probably many who burned a fire noticed this phenomenon. To protect yourself from such surprises, just place the logs with the end facing you. The coals usually fly perpendicular to the trunk.
Pine. Burns hotter and faster than spruce. It breaks easily if the tree is no more than 5-10 cm thick in diameter. "Shoots." Thin dry branches are well suited as second and third firewood for starting a fire.
Absolutely dry. Home distinctive feature is that it practically does not “shoot”. Dead wood trunks with a diameter of 20-30 cm are very well suited for “nodya”, a fire for the whole night. Burns hot and evenly. Burning rate between spruce and pine.
Larch. This tree, unlike other resinous trees, sheds its needles in the winter. The wood is denser and stronger. Burns for a long time, longer than spruce, evenly. Gives off a lot of heat. If you find a piece of dry larch on the bank of a river, there is a chance that before this piece hit the bank, it lay in the water for some time. Such a tree will burn much longer than usual from the forest. A tree, being in water, without oxygen, becomes denser and stronger. Of course, it all depends on the length of time spent in the water. After lying there for several decades, it turns into dust.
Properties of wood for burning
Wood suitable for burning is divided into the following main categories:
Softwood |
Hardwood Soft breeds |
Hardwood Hard rocks |
| Pine, spruce, thuja and others | Linden, aspen, poplar and others | Oak, birch, hornbeam and others |
| They are characterized by a high content of resin, which does not burn completely and clogs the chimney and internal parts of the firebox with its residues. When using such fuel, the formation of soot on the glass of the fireplace, if any, is inevitable. This type of fuel is characterized by longer drying of firewood. |
Due to their low density, firewood from such species burns quickly, does not form coals, and has a low specific calorific value. | Firewood made from such wood species ensures a stable operating temperature in the firebox and high specific calorific value. |
When choosing fuel for a fireplace or stove, the moisture content of the wood is of great importance. The calorific value of firewood largely depends on humidity. It is generally accepted that the best way Firewood with a moisture content of no more than 25% is suitable for burning. Indicators of calorific value (the amount of heat released during complete combustion of 1 kg of firewood, depending on humidity) are indicated in the table below:
Firewood for burning must be prepared carefully and in advance. Good firewood should dry for at least a year. The minimum drying time depends on the month the woodpile was laid (in days):
Another important indicator that characterizes the quality of firewood for heating a fireplace or stove is the density or hardness of the wood. Hard deciduous wood has the greatest heat transfer, while softwood has the least. The density of wood at a moisture content of 12% is shown in the table below:
Specific calorific value of wood of various species.
The calorific value of a wood substance of any species and any density in an absolutely dry state is determined by the number 4370 kcal/kg. It is also believed that the degree of rottenness of wood has virtually no effect on the calorific value.
There are concepts of volumetric calorific value and mass calorific value. The volumetric calorific value of firewood is a rather unstable value, depending on the density of the wood and, therefore, on the type of wood. After all, each rock has its own density; moreover, the same rock from different areas can differ in density.
It is most convenient to determine the calorific value of firewood by mass calorific value depending on humidity. If the humidity (W) of the samples is known, then their calorific value (Q) can be determined with a certain degree of error using a simple formula:
Q(kcal/kg) = 4370 – 50 * W
Based on moisture content, wood can be divided into three categories:
- room-dry wood, humidity from 7% to 20%;
- air-dried wood, humidity from 20% to 50%;
- driftwood, humidity from 50% to 70%;
Table 1. Volumetric calorific value of firewood depending on humidity.
| , forming cell walls, to the volume it occupies. The density of wood substance is the same for all types of wood and is equal to 1.53 g/cm3. On the recommendation of the CMEA commission, all indicators | Calorific value, kcal/dm3, at humidity, % | Calorific value, kW h/m 3, at humidity, % | ||||
|---|---|---|---|---|---|---|
| 12% | 25% | 50% | 12% | 25% | 50% | |
| Fir | 3240 | 2527 | 1110 | 3758 | 2932 | 1287 |
| Breed | 2640 | 2059 | 904 | 3062 | 2389 | 1049 |
| Maple | 2600 | 2028 | 891 | 3016 | 2352 | 1033 |
| At standard humidity | 2280 | 1778 | 781 | 2645 | 2063 | 906 |
| Density kg/m3 | 2080 | 1622 | 712 | 2413 | 1882 | 826 |
| Beech | 1880 | 1466 | 644 | 2181 | 1701 | 747 |
| Spruce | 1800 | 1404 | 617 | 2088 | 1629 | 715 |
| Absolutely dry | 1640 | 1279 | 562 | 1902 | 1484 | 652 |
| Poplar | 1600 | 1248 | 548 | 1856 | 1448 | 636 |
Table 2. Estimated mass calorific value of firewood depending on humidity.
| Humidity degree, % | Calorific value, kcal/kg | Calorific value, kW h/kg |
|---|---|---|
| 7 | 4020 | 4.6632 |
| 8 | 3970 | 4.6052 |
| 9 | 3920 | 4.5472 |
| 10 | 3870 | 4.4892 |
| 11 | 3820 | 4.4312 |
| 12 | 3770 | 4.3732 |
| 13 | 3720 | 4.3152 |
| 14 | 3670 | 4.2572 |
| 15 | 3620 | 4.1992 |
| 16 | 3570 | 4.1412 |
| 17 | 3520 | 4.0832 |
| 18 | 3470 | 4.0252 |
| 19 | 3420 | 3.9672 |
| 20 | 3370 | 3.9092 |
| 21 | 3320 | 3.8512 |
| 22 | 3270 | 3.7932 |
| 23 | 3220 | 3.7352 |
| 24 | 3170 | 3.6772 |
| 25 | 3120 | 3.6192 |
| 26 | 3070 | 3.5612 |
| 27 | 3020 | 3.5032 |
| 28 | 2970 | 3.4452 |
| 29 | 2920 | 3.3872 |
| 30 | 2870 | 3.3292 |
| 31 | 2820 | 3.2712 |
| 32 | 2770 | 3.2132 |
| 33 | 2720 | 3.1552 |
| 34 | 2670 | 3.0972 |
| 35 | 2620 | 3.0392 |
| 36 | 2570 | 2.9812 |
| 37 | 2520 | 2.9232 |
| 38 | 2470 | 2.8652 |
| 39 | 2420 | 2.8072 |
| 40 | 2370 | 2.7492 |
| 41 | 2320 | 2.6912 |
| 42 | 2270 | 2.6332 |
| 43 | 2220 | 2.5752 |
| 44 | 2170 | 2.5172 |
| 45 | 2120 | 2.4592 |
| 46 | 2070 | 2.4012 |
| 47 | 2020 | 2.3432 |
| 48 | 1970 | 2.2852 |
| 49 | 1920 | 2.2272 |
| 50 | 1870 | 2.1692 |
| 51 | 1820 | 2.1112 |
| 52 | 1770 | 2.0532 |
| 53 | 1720 | 1.9952 |
| 54 | 1670 | 1.9372 |
| 55 | 1620 | 1.8792 |
| 56 | 1570 | 1.8212 |
| 57 | 1520 | 1.7632 |
| 58 | 1470 | 1.7052 |
| 59 | 1420 | 1.6472 |
| 60 | 1370 | 1.5892 |
| 61 | 1320 | 1.5312 |
| 62 | 1270 | 1.4732 |
| 63 | 1220 | 1.4152 |
| 64 | 1170 | 1.3572 |
| 65 | 1120 | 1.2992 |
| 66 | 1070 | 1.2412 |
| 67 | 1020 | 1.1832 |
| 68 | 970 | 1.1252 |
| 69 | 920 | 1.0672 |
| 70 | 870 | 1.0092 |
The moisture content of woody biomass is a quantitative characteristic showing the moisture content in the biomass. A distinction is made between absolute and relative humidity of biomass.
Absolute humidity is the ratio of the mass of moisture to the mass of dry wood:
Wa= t~t° 100,
Where Noa is absolute humidity, %; t is the mass of the sample in a wet state, g; t0 is the mass of the same sample dried to a constant value, g.
Relative or working humidity is the ratio of the mass of moisture to the mass of wet wood:
Where Wр - relative, or working, humidity, 10
Conversion of absolute humidity to relative humidity and vice versa is carried out using the formulas:
Ash is divided into internal, contained in wood matter, and external, which got into the fuel during the procurement, storage and transportation of biomass. Depending on the type, ash has different fusibility when heated to high temperatures. Low-melting ash is an ash that has a temperature at which the melting point begins below 1350°. Medium-melting ash has a temperature of the beginning of the liquid-melting state in the range of 1350-1450 °C. For refractory ash, this temperature is above 1450 °C.
The internal ash of woody biomass is refractory, and the external ash is low-melting. The ash content in various parts of trees of various species is shown in table. 4.
Ash content of stem wood. The content of internal ash of stem wood varies from 0.2 to 1.17%. Based on this, in accordance with the recommendations for the standard method of thermal calculation of boiler units in the calculations of combustion devices, the ash content of stem wood of all species should be taken equal to 1% of the dry mass
|
4. Distribution of ash in parts of wood for different species
|
Wood. This is legal if mineral inclusions are excluded from the crushed stem wood.
Ash content of bark. The ash content of the bark is higher than the ash content of the stem wood. One of the reasons for this is that the surface of the bark is blown with atmospheric air all the time the tree is growing and traps the mineral aerosols it contains.
According to observations carried out by TsNIIMOD for driftwood in the conditions of Arkhangelsk sawmills and woodworking enterprises, the ash content of debarking waste was
For spruce 5.2, for pine 4.9% - The increase in ash content of the bark in this case is explained by contamination of the bark during rafting of the logs along the rivers.
The ash content of the bark of various species on a dry weight basis, according to A.I. Pomeransky, is: pine 3.2%, spruce 3.95, birch 2.7, alder 2.4%. According to NPO TsKTI im. I. I. Pol-Zunova, the ash content of the bark of various rocks varies from 0.5 to 8%.
Ash content of crown elements. The ash content of crown elements exceeds the ash content of wood and depends on the type of wood and its location. According to V. M. Nikitin, the ash content of the leaves is 3.5%. Branches and twigs have an internal ash content of 0.3 to 0.7%. However, depending on the type of technological process of wood harvesting, their ash content changes significantly due to contamination with external mineral inclusions. Contamination of branches and twigs during the process of harvesting, skidding and hauling is most intense in wet weather in spring and autumn.
Density. The density of a material is characterized by the ratio of its mass to volume. When studying this property in relation to woody biomass, the following indicators are distinguished: density of wood substance, density of absolutely dry wood, density of wet wood.
The density of woody matter is the ratio of the mass of the material forming the cell walls to the volume it occupies. The density of wood substance is the same for all types of wood and is equal to 1.53 g/cm3.
The density of absolutely dry wood is the ratio of the mass of this wood to the volume it occupies:
P0 = m0/V0, (2.3)
Where po is the density of absolutely dry wood; then is the mass of the wood sample at Nop=0; V0 is the volume of the wood sample at Nop=0.
The density of wet wood is the ratio of the mass of a sample at a given humidity to its volume at the same humidity:
P w = mw/Vw, (2.4)
Where is the density of wood at humidity Wp; mw is the mass of the wood sample at humidity Vw is the volume occupied by the wood sample at humidity Wр.
Stem wood density. The density of stem wood depends on its species, humidity and swelling coefficient /Avg. All types of wood in relation to the swelling coefficient of the KR are divided into two groups. The first group includes species with a swelling coefficient /Ср = 0.6 (white acacia, birch, beech, hornbeam, larch). The second group includes all other breeds in which /<р=0,5.
For the first group, for white acacia, birch, beech, hornbeam, and larch, the density of stem wood can be calculated using the following formulas:
Pw = 0.957--------------- p12, W< 23%;
100-0.4WP" (2-5)
Loo-UR р12" №р>23%
For all other species, the density of stem wood is calculated using the formulas:
0* = P-Sh.00-0.5GR L7R<23%; (2.6)
Pig = °.823 100f°lpp Ri. її">"23%,
Where pig is the density at standard humidity, i.e. at an absolute humidity of 12%.
The density value at standard humidity is determined for various types of wood according to table. 6.
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6. Density of stem wood of various species at standard humidity and in an absolutely dry state
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Bark density. The density of the crust has been studied much less. There are only fragmentary data that give a rather mixed picture of this property of the bark. In this work we will focus on the data of M. N. Simonov and N. L. Leontiev. To calculate the density of the bark, we will accept formulas of the same structure as the formulas for calculating the density of stem wood, substituting into them the coefficients of volumetric swelling of the bark. We will calculate the density of the bark using the following formulas: pine bark
(100-THR)P13 ^p<230/
103.56- 1.332GR "" (2.7)
1.231(1-0.011GR)" ^>23%-"
Spruce bark Pw
|
W P<23%; W*> 23%; Gr<23%; Гр>23%. |
Р w - (100 - WP) р12 102.38 - 1.222 WP
|
Birch bark |
1.253(1_0.01WP)
(100-WP)pia 101.19 - 1.111WP
1.277(1 -0.01 WP)
The density of the bast is much higher than the density of the crust. This is evidenced by the data of A.B. Bolshakov (Sverd - NIIPdrev) on the density of parts of the bark in an absolutely dry state (Table 8).
Density of rotten wood. The density of rotten wood in the initial stage of decay usually does not decrease, and in some cases even increases. With the further development of the decay process, the density of rotten wood decreases and in the final stage becomes significantly less than the density of healthy wood,
The dependence of the density of rotten wood on the stage of its damage to rot is given in table. 9.
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9. Density of wood rot depending on the stage of its damage |
RC(YuO-IGR) 106- 1.46WP
The pis value of rotten wood is equal to: aspen rot pi5 = 280 kg/m3, pine rot pS5=260 kg/m3, birch rot p15 = 300 kg/m3.
Density of tree crown elements. The density of crown elements has practically not been studied. In fuel chips from crown elements, the predominant component in terms of volume is chips from twigs and branches, which are close in density to stem wood. Therefore, when carrying out practical calculations, as a first approximation, the density of the crown elements can be assumed to be equal to the density of the stem wood of the corresponding species.
Firewood is the most ancient and traditional source of thermal energy, which is a renewable type of fuel. By definition, firewood is pieces of wood commensurate with the hearth, used to start and maintain a fire in it. In terms of quality, firewood is the most unstable fuel in the world.
However, the weight percentage composition of any wood mass is approximately the same. It includes up to 60% cellulose, up to 30% lignin, 7...8% associated hydrocarbons. The rest (1...3%) -
State standard for firewood
Operates on the territory of Russia
GOST 3243-88 Firewood. Specifications
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The standard from the times of the Soviet Union defines:
- Firewood assortment by size
- Permissible amount of rotten wood
- Firewood range by calorific value
- Methodology for calculating the amount of firewood
- Transportation and storage requirements
wood fuel
Of all the GOST information, the most valuable is the methods for measuring wood stacks and the coefficients for converting values from a folded measure to a dense one (from a folding meter to a cubic meter). In addition, the point of limiting heart and sapwood rot (no more than 65% of the end area), as well as the ban on external rottenness, is of some interest. It’s just hard to imagine such rotten firewood in our cosmic age of the pursuit of quality.
Regarding calorific value,
then GOST 3243-88 divides all firewood into three groups:
Firewood accounting
To take into account any material value, the most important thing is the ways and methods of calculating its quantity. The amount of firewood can be taken into account either in tons and kilograms, or in folded and cubic meters and decimeters. Accordingly - in mass or volumetric units of measurement
- Accounting for firewood in mass units of measurement
(in tons and kilograms)
This method of accounting for wood fuel is used extremely rarely due to its bulkiness and clumsiness. It was borrowed from woodworkers and is an alternative method for cases where it is easier to weigh firewood rather than determine its volume. So, for example, sometimes during wholesale deliveries of wood fuel it can be easier to weigh loaded wagons and timber trucks, rather than determine the volume of shapeless wood “caps” rising on them.Advantages
- ease of information processing for further calculation of the total calorific value of fuel during thermal engineering calculations. Because the calorific value of a weight measure of firewood is calculated according to and is practically unchanged for any type of wood, regardless of its geographical location and degree. Thus, when accounting for firewood in mass units, the net weight of the combustible material is taken into account minus the weight of moisture, the amount of which is determined by a moisture meterFlaws
accounting of firewood in mass units of measurement
- the method is absolutely unacceptable for measuring and accounting for lots of firewood in field logging conditions, when the required special equipment (scales and moisture meter) may not be at hand
- the result of measuring humidity soon becomes irrelevant, the firewood quickly becomes damp or dries out in the air - Accounting for firewood in volumetric units of measurement
(in folded and cubic meters and decimeters)
This method of accounting for wood fuel has become the most widely used as the simplest and fastest way to account for wood fuel mass. Therefore, firewood accounting is carried out everywhere in volumetric units of measurement - fold meters and cubic meters (fold and dense measures)Advantages
accounting of firewood in volumetric units of measurement
- extreme simplicity in measuring wood stacks with a linear meter
- the measurement result is easily controlled, remains unchanged for a long time and does not raise doubts
- the methodology for measuring wood lots and the coefficients for converting values from a folded measure to a dense one are standardized and set out inFlaws
accounting of firewood in mass units of measurement
- the price for the simplicity of accounting for firewood in volumetric units is the complication of further thermotechnical calculations for calculating the total calorific value of wood fuel (you need to take into account the type of tree, where it grows, the degree of rottenness of the firewood, etc.)
Calorific value of firewood
The calorific value of firewood
it is also the heat of combustion of wood,
it is also the calorific value of firewood
How does the calorific value of firewood differ from the calorific value of wood?
The calorific value of wood and the calorific value of firewood are related and similar values, identified in everyday life with the concepts of “theory” and “practice”. In theory, we study the calorific value of wood, but in practice, we deal with the calorific value of firewood. At the same time, real wood logs can have a much wider range of deviations from the norm than laboratory samples.
For example, real firewood has bark, which is not wood in the literal sense of the word and, nevertheless, occupies volume, participates in the process of burning wood and has its own calorific value. Often, the calorific value of bark differs significantly from the calorific value of the wood itself. In addition, real firewood may have different wood densities depending on the wood, have a large percentage, etc.
Thus, for real firewood, the calorific value indicators are generalized and slightly underestimated, since for real firewood, all the negative factors that reducetheir calorific value. This explains the smaller difference in magnitude between the theoretically calculated values of the calorific value of wood and the practically applied values of the calorific value of firewood.
In other words, theory and practice are different things.
The calorific value of firewood is the amount of useful heat generated during its combustion. Useful heat means heat that can be removed from the fireplace without harming the combustion process. The calorific value of firewood is the most important indicator of the quality of wood fuel. The calorific value of firewood can vary widely and depends, first of all, on two factors - the wood itself and its .
- The calorific value of wood depends on the amount of combustible wood substance present per unit mass or volume of wood. (more details about the calorific value of wood in the article -)
- Wood moisture content depends on the amount of water and other moisture present per unit mass or volume of wood. (more details about wood moisture content in the article -)
Firewood volumetric calorific value table
Calorific value gradation according to
(at wood moisture content 20%)
| Wood species | specific calorific value of wood (kcal/dm 3) |
|
| Birch | 1389...2240 |
First group birch, beech, ash, hornbeam, elm, elm, maple, oak, larch |
| beech | 1258...2133 | |
| ash | 1403...2194 | |
| hornbeam | 1654...2148 | |
| elm | not found (analogue - elm) |
|
| elm | 1282...2341 | |
| maple | 1503...2277 | |
| oak | 1538...2429 | |
| larch | 1084...2207 | |
| pine | 1282...2130 |
Second group pine, alder |
| alder | 1122...1744 | |
| spruce | 1068...1974 |
Third group spruce, cedar, fir, aspen, linden, poplar, willow |
| cedar | 1312...2237 | |
| fir |
not found |
|
| aspen | 1002...1729 | |
| Linden | 1046...1775 | |
| poplar | 839...1370 | |
| willow | 1128...1840 | |
Calorific value of rotten wood
It is absolutely true that rot deteriorates the quality of firewood and reduces its calorific value. But how much the calorific value of rotten firewood decreases is a question. Soviet GOST 2140-81 defines the methodology for measuring the size of rot, limits the amount of rot in a log and the number of rotten logs in a batch (no more than 65% of the end area and no more than 20% of the total mass, respectively). But, at the same time, the standards do not indicate in any way a change in the calorific value of the firewood itself.
It's obvious that within the limits of GOST requirements There is no significant change in the overall calorific value of the wood mass due to rot, therefore, individual rotten logs can be safely neglected.
If there is more rot than is acceptable according to the standard, then it is advisable to take into account the calorific value of such firewood in units of measurement. Because when wood rots, processes occur that destroy the substance and disrupt its cellular structure. At the same time, accordingly, the wood decreases, which primarily affects its weight and practically does not affect its volume. Thus, mass units of calorific value will be more objective for taking into account the calorific value of very rotten firewood.
By definition, the mass (weight) calorific value of firewood is practically independent of its volume, type of wood and degree of rottenness. And, only wood moisture has a great influence on the mass (weight) calorific value of firewood
The calorific value of a weight measure of rotten and rotten firewood is almost equal to the calorific value of a weight measure of ordinary firewood and depends only on the moisture content of the wood itself. Because only the weight of water displaces the weight of combustible wood substances from the weight measure of firewood, plus heat loss due to evaporation of water and heating of water vapor. Which is exactly what we need.
Calorific value of firewood from different regions
Volumetric The calorific value of firewood for the same type of tree growing in different regions may differ due to changes in wood density depending on the water saturation of the soil in the growing area. Moreover, these do not necessarily have to be different regions or regions of the country. Even within a small area (10...100 km) of logging, the calorific value of firewood for the same type of wood can change with a difference of 2...5% due to changes in wood. This is explained by the fact that in arid areas (in conditions of lack of moisture) a smaller and denser cellular structure of wood grows and forms than in swampy land rich in water. Thus, the total amount of combustible substance per unit volume will be higher for firewood harvested in drier areas, even for the same logging area. Of course, the difference is not that big, about 2...5%. However, for large-scale firewood collections this can have a real economic effect.
The mass calorific value for firewood from the same type of wood growing in different regions will not vary at all, since the calorific value does not depend on the density of the wood, but depends only on its moisture content
Ash | Ash content of firewood
Ash is a mineral substance that is contained in firewood and remains in the solid residue after complete combustion of the wood mass. The ash content of firewood is the degree of its mineralization. The ash content of firewood is measured as a percentage of the total mass of wood fuel and indicates the quantitative content of mineral substances in it.
Distinguish between internal and external ash
| Internal ash | External ash |
| Internal ash is mineral substances that are contained directly in | External ash is mineral substances that entered the firewood from the outside (for example, during harvesting, transportation or storage) |
| Internal ash is a refractory mass (above 1450 °C), which is easily removed from the high-temperature fuel combustion zone | External ash is a low-melting mass (less than 1350°C), which is sintered into slag, which adheres to the lining of the combustion chamber of the heating unit. As a consequence of such sintering and sticking, external ash is poorly removed from the high-temperature fuel combustion zone |
| The content of internal ash of wood matter ranges from 0.2 to 2.16% of the total wood mass | The external ash content can reach 20% of the total wood mass |
| Ash is an undesirable part of the fuel, which reduces its combustible component and complicates the operation of heating units | |
