The moisture content of woody biomass is a quantitative characteristic showing the content of moisture in the biomass. There are 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 - absolute humidity,%; m is the weight of the sample in the wet state, g; m0 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 Wp - relative, or working, humidity, 10
The conversion of absolute humidity into relative humidity and vice versa is carried out according to the formulas:
Ash is subdivided into internal, contained in the wood substance, and external, which got into the fuel during the harvesting, storage and transportation of biomass. Depending on the type of ash has a different fusibility when heated to high temperatures. Low-melting ash is called, having a temperature of the beginning of the liquid-melting state 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 inner ash of woody biomass is refractory, while the outer ash is fusible. The ash content in various parts of trees of various species is shown in Table. four.
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 on the normative method thermal calculation boiler units in the calculations of combustion devices, the ash content of stemwood of all species should be taken equal to 1% of the dry mass
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4. Distribution of ash in parts of a tree for various species
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Wood. This is justified if the ingress of mineral inclusions into the chopped stem wood is excluded.
Ash content of the bark. The ash content of the bark is greater than the ash content of the stem wood. One of the reasons for this is that the surface of the bark is constantly blown by atmospheric air during the growth of the tree and captures the mineral aerosols contained in it.
According to the observations carried out by TsNIIMOD for driftwood under the conditions of Arkhangelsk sawmills and woodworking enterprises, the ash content of barking waste was
In spruce 5.2, in pine 4.9% - The increase in the ash content of the bark in this case is explained by contamination of the bark during the rafting of whips along the rivers.
The ash content of the bark of various species per dry weight, according to A. I. Pomeransky, is: pine 3.2%, spruce 3.95, birch 2.7, alder 2.4%. According to NPO CKTI im. II 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 place of growth. According to V. M. Nikitin, the ash content of the leaves is 3.5%. Branches and branches 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. Pollution of branches and branches in 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 There are the following indicators: density of wood substance, density of absolutely dry wood, density of wet wood.
The density of a wood substance is the ratio of the mass of the material that forms the cell walls to the volume it occupies. The density of the 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 ro is the density of absolutely dry wood; then - the mass of the wood sample at No. p = 0; V0 - the volume of the wood sample at №р=0.
The density of wet wood is the ratio of the mass of a sample at a given moisture content to its volume at the same moisture content:
Р w = mw/Vw, (2.4)
Where mouth is the density of wood at humidity Wp; mw is the mass of the wood sample at moisture content Vw is the volume occupied by the wood sample at moisture content Wр.
Density of stem wood. The value of the density of stem wood depends on its species, humidity and swelling coefficient /Cf. All types of wood in relation to the coefficient of swelling KR are divided into two groups. The first group includes rocks whose 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, larch, the density of stem wood can be calculated using the following formulas:
Pw = 0.957 -------- ------- р12, W< 23%;
100-0.4WP" (2-5)
Loo-UR p12" No. p>23%
For all other species, the density of stem wood is calculated by the formulas:
0* = P-Sh.00-0.5GR L7R<23%; (2.6)
Ріг = °,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 prn standard moisture n in a completely 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 crust. 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 use formulas of the same structure as the formulas for calculating the density of stem wood, substituting in them the coefficients of volumetric swelling of the bark. The density of the bark will be calculated according to 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
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W P<23%; W*> 23%; Gr<23%; Гр>23%. |
P w - (100 - WP) p12 102.38 - 1.222 WP
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birch bark |
1.253(1_0.01WP)
(100-WP)pia 101.19 - 1.111WP
1.277(1 -0.01WP)
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 crust 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 process of decay, the density of rotten wood decreases and in the final stage it becomes much less than the density of healthy wood,
The dependence of the density of rotten wood on the stage of damage by 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: 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 is practically not studied. In fuel chips from crown elements, the predominant component in terms of volume is chips from twigs and branches, which are close in terms of density to stem wood. Therefore, when carrying out practical calculations, in the first approximation, it is possible to take the density of the elements of the crown equal to the density of the stem wood of the corresponding species.
Coarse coals after combustion and uniform heat are a sign of good raw materials
Main criteria
The most important indicators for the furnace material are density, humidity and heat transfer. All of them are closely related and determine how efficient and useful wood burning is. It is worth considering each of them in more detail, given the different types of wood and how it is harvested.
Density
The first thing that a competent buyer pays attention to when ordering wood heating material is its density. The higher this indicator, the better the breed is.
All types of wood are divided into three main categories:
- low-density (soft);
- medium-density (moderately hard);
- high density (solid).
Each of them has a different density, and hence the specific heat of combustion of firewood. Hard varieties are considered the highest quality. They burn for a long time and give off more heat. In addition, they form a lot of coals that maintain heat in the firebox.
Due to their hardness, these firewoods are difficult to process, which is why some consumers prefer medium-density woods such as birch or ash. Their structure allows you to chop logs manually without much effort.
Humidity
The second indicator is humidity, that is, the percentage of water in the wood structure. The higher this value, the greater the density, while the resource used will release less heat for the same amount of effort.
The specific heat of combustion of dry birch firewood is characterized as more productive than wet firewood. It is worth noting such a feature of birch: it can be put into the firebox almost immediately after felling, because it is characterized by low humidity. To maximize the beneficial effect, it is better to prepare the material properly.
To improve the quality of wood by reducing the percentage of moisture content in it, the following approaches are used:
- Fresh firewood is left for a certain period under a canopy to dry. The number of days depends on the season and can vary from 80 to 310 days.
- Part of the firewood is dried indoors, which increases its calorific value.
- The best option is artificial drying. The calorific value is brought to the maximum level by bringing the percentage of moisture to zero, and the time for wood preparation is required to a minimum.
Heat dissipation
Such an indicator as the heat transfer of firewood, as it were, sums up the previous two characteristics. It is he who indicates how much heat the selected material can give under specific conditions.
The highest is the heat of combustion of firewood in hardwoods. Accordingly, the opposite is true for soft wood. Under equal conditions and natural shrinkage, the difference in readings can reach almost 100%. That is why, in order to save money, it makes sense to purchase high-quality firewood that is more expensive to purchase, since their production is more efficient.
Here it is worth mentioning such a property as the combustion temperature of firewood. It is the highest in hornbeam, beech and ash, more than 1000 degrees Celsius, while producing the maximum amount of heat at the level of 85-87%. Oak and larch are approaching them, while poplar and alder are the least productive with a yield of 39-47% at a temperature of around 500 degrees.
Wood species
The calorific value of firewood to the greatest extent depends on the type of wood. There are two main categories: coniferous and deciduous. High-quality furnace material belongs to the second group. It also has its own classification, since not all varieties are suitable for a particular purpose in terms of their density.
Conifers
Often the most affordable wood is needles. Its low cost is determined not only by the prevalence of firs and pines, but also by its properties. The fact is that the heat capacity of firewood of such a plan is low, and there are also a lot of other shortcomings.
The main disadvantage of conifers is the presence of a large amount of resin. When such firewood is heated, the resin begins to expand and boil, which results in the scattering of sparks and burning fragments over a long distance. Resin also leads to the formation of soot and burning, which clog the fireplace and chimney.
Deciduous
It is much more profitable to use hardwood. All varieties are divided into three categories, depending on their density. Soft breeds include:
- Linden;
- aspen;
- poplar;
- alder;
They quickly burn out and therefore have little value in terms of heating the house.
Medium density trees include:
- maple;
- Birch;
- larch;
- acacia;
- cherry.
The specific heat of combustion of birch firewood approaches the species that are classified as hard, in particular oak.
- hornbeam;
- nut;
- dogwood;
The calorific value of this type of firewood is maximum, but at the same time, wood processing is difficult due to its high density.
Oak is another popular type of fuel
The useful qualities of such rocks determine their higher cost, but this allows you to reduce the amount of material that will be needed to maintain a comfortable temperature in the house.
Material selection
Even the highest quality wood can be reduced to nothing if it is not properly selected for a particular type of activity. For example, it practically does not matter what was used for a night fire when gathering with friends. A completely different matter is kindling a fireplace or stove in a bathhouse.
for fireplace
Heating your home can be a problem if you load the wrong wood into the stove. This is especially dangerous when using a fireplace, as a sparkling log can even lead to a fire.
The unobtrusive burning of firewood and the heat emanating from the fireplace are the highlight of the living room
For long burning and the release of a large amount of heat, it is worth giving preference to oak, acacia, as well as birch and walnut. To clean the chimney from time to time, you can burn aspen and alder. The density of these rocks is small, but they have the ability to burn out soot.
For a bath
To ensure a high temperature in the steam room of the bath, maximum heat transfer of firewood is necessary. In addition, you can improve the conditions of rest if you use such breeds that saturate the room with a pleasant smell, without releasing harmful substances and resins.
Read also about in addition to this article.
For heating the steam room, oak and birch logs will, of course, be the best choice. They are solid, give good heat with a small volume, and also emit pleasant fumes. Linden and alder can also have an additional healing effect. You can use only well-dried materials, but not older than one and a half to two years.
BBQ
When cooking on the grill and barbecue, the main point is not the burning of firewood itself, but the formation of coals. That is why it does not make sense to use thin loose branches. They can only be taken to light a fire, and then add large hard logs to the firebox. In order for the smoke to have a special aroma, it is recommended to use fruit firewood for the barbecue. You can combine them with oak and acacia.
When using different types of wood, pay attention to the size of the chocks. For example, an oak tree will take longer to burn and smolder than an apple tree, so it makes sense to take thicker fruit logs.
Alternative fuel materials
The calorific value of firewood of certain species is quite high, but far from the maximum possible. In order to save money and space for storing furnace material, more and more attention is now being paid to alternative options. It is optimal to use pressed briquettes.
Pressed wood generates much more heat for the same kiln load. This effect is possible by increasing the density of the material. In addition, there is a much lower percentage of humidity. Another plus is minimal ash formation.
Briquettes and pellets are made from sawdust and wood chips. By pressing the waste, it is possible to create an incredibly dense furnace material, which even the best types of wood cannot be compared with. With a higher cost per cubic meter of briquettes, the resulting savings can be quite significant.
It is necessary to prepare and purchase furnace materials on the basis of a thorough analysis of their properties. Only high-quality firewood can provide you with the necessary heat without harming either your health or the heating structure itself.
The content of ash in various components of the bark of various species Spruce 5.2, pine 4.9% - The increase in the ash content of the bark in this case is due to contamination of the bark during the rafting of whips along the rivers. The ash content in various constituent parts of the bark, according to V. M. Nikitin, is shown in Table. 5. The ash content of the bark of various species on a dry basis, according to A. I. Pomeransky, is: pine 3.2%, spruce 3.95, 2.7, alder 2.4%.
According to NPO CKTI im. II 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 place of growth. According to V. M. Nikitin, the ash content of the leaves is 3.5%.
Branches and branches have an internal ash content of 0.3 to 0.7%. However, depending on the type of technological process, their ash content changes significantly due to contamination with external mineral inclusions. Pollution of branches and branches in the process of harvesting, skidding and hauling is most intense in wet weather in spring and autumn.
Humidity and density are the main properties of wood.
Humidity- this is the ratio of the mass of moisture in a given volume of wood to the mass of absolutely dry wood, expressed as a percentage. Moisture that impregnates cell membranes is called bound or hygroscopic, and moisture that fills cell cavities and intercellular spaces is called free or capillary.
When wood dries, free moisture first evaporates from it, and then bound moisture. The state of wood, in which the cell membranes contain the maximum amount of bound moisture, and only air is in the cell cavities, is called the hygroscopic limit. The corresponding humidity at room temperature (20 ° C) is 30% and does not depend on the breed.
The following levels of wood moisture content are distinguished: wet - humidity above 100%; freshly cut - humidity 50. 100%; air-dry humidity 15.20%; dry - humidity 8.12%; absolutely dry - humidity is about 0%.
This is the ratio at a certain humidity, kg, to its volume, m 3.
Increases with increasing humidity. For example, the density of beech wood at a moisture content of 12% is 670 kg/m3, and at a moisture content of 25% it is 710 kg/m3. The density of late wood is 2.3 times higher than that of early wood; therefore, the better developed late wood, the higher its density (Table 2). The conditional density of wood is the ratio of the mass of the sample in an absolutely dry state to the volume of the sample at the limit of hygroscopicity.
The calorific value of firewood depends on the type of trees and their moisture content.
We call firewood pieces of wood used in reactions of rapid oxidation with atmospheric oxygen to produce light and heat. We kindle a fire just on the ground, leaving for a picnic. Or in special devices - barbecues, hearths, boilers, stoves, takyrs or others.
Firewood is diverse, the amount of heat obtained from their combustion, divided by mass (volume), is called the specific heat of combustion of heating oil. The calorific value of firewood depends on the type of trees and their moisture content. In addition, the completeness of combustion and the coefficient of utilization of combustion energy also depend on other factors. Different furnaces, traction force, chimney device - everything affects the result.
Essence of a physical parameter
Energy is measured in "joules" - the amount of work to move 1 meter when a force of 1 newton is applied in the direction of application. Or in "calories" - the amount of heat needed to heat 1 g of water by 1 ° C at a pressure of 760 mm Hg. The international calorie corresponds to 4.1868 Joules.
The specific heat capacity of a fuel is the amount of heat produced by complete combustion divided by the mass or volume of the fuel.
The value is not constant, since firewood can vary greatly, respectively, this parameter also varies. In the laboratory, specific heat is measured by combustion in special devices. The result is true for a particular sample, but only for it.
The total specific heat of the heating oil is measured with the simultaneous cooling of the combustion products and the condensation of the evaporated water - to account for ALL the amount of energy received.
In practice, the working rather than the specific heat of combustion is used more often, without taking into account all the energy received.
The essence of the combustion process
If you heat the wood, then at 120-150 ˚С it becomes dark in color. This is a slow charring, turning into charcoal. Bringing the temperature to 350–350 ˚С, we will see thermal decomposition, blackening with the release of white or brown smoke. Heating further, the released pyrolysis gases (CO and volatile hydrocarbons) will ignite, turning into flames. After burning for some time, the amount of volatile substances will decrease, and the coals will continue to burn, but without a flame. In practice, to ignite and maintain combustion, the wood must be heated to 450-650 ˚С.
firewood burning process
In the future, the combustion temperature of heating oil in the furnace ranges from approximately 500 ˚С (poplar) to 1000 and higher (ash, beech). This value is highly dependent on draft, furnace design and many other factors.
Humidity dependence
The higher the humidity, the worse the combustion, the lower the efficiency of the stove, the more difficult it is to light and maintain the fire. And less calorific value of firewood.
Calorific value indicators (the amount of heat released during the complete combustion of 1 kg of firewood, depending on humidity)
The specific heat of heating oil and the coefficient of its use also decrease. The reasons are as follows.
- Water in the composition reduces the amount of fuel as such: at a moisture content of 50% in the wood, water is half. And it won't burn...
- Part of the energy of heating oil will be spent on heating and evaporation of moisture.
- Wet wood conducts heat better, which makes it difficult to warm up the ignited part of the log to the ignition temperature.
Freshly cut wood varies in moisture depending on the time of felling, wood species, place of growth, but on average it contains about 50% water.
Therefore, they put it in woodpile under a canopy. During storage, some of the moisture will evaporate. With a decrease in humidity from 50 to 20%, the specific heat of combustion of heating oil approximately doubles.
Density dependence
Oddly enough, the composition of trees of different species is similar: 35–46% cellulose, 20–28% lignin + esters, resins, and other substances. And the difference in the heat of combustion of heating oil is due to porosity, that is, how much space is occupied by voids. Accordingly, the denser the tree, the greater the calorific value of firewood from it. High-quality fuel pellets obtained by drying and pressing wood waste have a density of 1.1 kg / dm 3, that is, higher than the density of water. in which they drown.
Economic features of various firewood
The shape matters: the smaller the logs, the easier they light up and burn faster. It is clear that the length also depends on the design: too long cannot be placed in a stove or fireplace, the ends protrude outward. Too short - extra work when cutting or cutting. The combustion temperature of firewood depends on the size of the humidity, the type of wood, the amount of air supplied. The temperature is lowest when burning poplar firewood, higher when burning hardwoods: ash, mountain maple, oak.
About the value of humidity was written above. Not only the heat transfer of the fuel in the furnace, but also the labor costs for splitting or sawing strongly depend on it. Moist, freshly cut wood is easier to prick and saw. However, too wet viscous, from this it pricks badly. The butt part is denser, and uprooted stumps, areas near the knots have an increased strength. There, the layers of wood are intertwined, which makes it much stronger. Oak splits well in the longitudinal direction, which has been used by coopers since ancient times. Obtaining shingles, shingles, chopping firewood has its own secrets.
Spruce is a "shooting" breed, therefore it is undesirable for use in fireplaces or bonfires. When heated, the internal "bubbles" with resin boil up and throw burning particles quite far, which is dangerous: it is easy to burn clothes near a fire. Or it can cause a fire near the fireplace. In a closed furnace furnace, this does not matter. Birch gives a hot flame, it is excellent firewood. But with poor traction, it produces a lot of resinous substances (they used to make birch tar), a lot of soot is deposited. Alder and aspen, on the other hand, produce little soot. It is from aspen that matches are mainly made.
In practice, it is convenient to immediately saw and split freshly cut firewood. Then fold under the sheds, making the woodpile so that the air passes through, drying the fuel and increasing heat transfer. Chopping firewood is a time-consuming task, so when buying, pay attention to this. And they will bring you stacked or bulk firewood.
In the second case, heating oil is placed in the body "loose" and the customer pays partly for the air. In addition, the liquid or gaseous fuel used for heating has a plus: it is easy to automate the supply. Firewood requires a lot of manual work. All this should be taken into account when choosing a stove or boiler for a home.
Video: How to choose firewood for a firebox
Firewood is the most ancient and traditional source of thermal energy, which belongs to a renewable type of fuel. By definition, firewood is pieces of wood that are proportionate to the hearth and are used to build 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
On the territory of Russia operates
GOST 3243-88 Firewood. Specifications
Download (downloads: 1689)
The standard of the times of the Soviet Union defines:
- Assortment of firewood by size
- Permissible amount of rotten wood
- Assortment of firewood by calorific value
- The method of accounting for the amount of firewood
- Requirements for transportation and storage
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 folding measure to a dense measure (from a warehouse meter to a cubic meter). In addition, there is still some interest in the fad on limiting heartwood and sap rot (no more than 65% of the butt area), as well as a ban on external rot. It's just hard to imagine such rotten firewood in our space age of the pursuit of quality.
As regards the calorific value,
then GOST 3243-88 divides all firewood into three groups:
Firewood accounting
To account for any material value, the most important thing is the ways and methods of counting its quantity. The amount of firewood can be taken into account, either in tons and kilograms, or in storage and cubic meters and decimeters. Accordingly - in mass or volume units
- Accounting for firewood in mass units
(in tons and kilograms)
This method of accounting for wood fuel is used extremely rarely due to its bulkiness and sluggishness. It is borrowed from woodworking builders and is an alternative method for those cases where it is easier to weigh the firewood than to determine its volume. So, for example, sometimes in the case of wholesale deliveries of wood fuel, it is easier to weigh wagons and timber trucks shipped “on top” than to determine the volume of shapeless wood “caps” towering on them.Advantages
- ease of processing information for further calculation of the total calorific value of the fuel in heat 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 taking into account firewood in mass units, the net weight of combustible material is taken into account minus the weight of moisture, the amount of which is determined by a moisture meterFlaws
accounting for firewood in mass units
- the method is absolutely unacceptable for measuring and accounting for batches of firewood in the field of logging, 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 up in the air - Accounting for firewood in volumetric units of measurement
(in folding and cubic meters and decimeters)
This method of accounting for wood fuel is the most widely used, as the simplest and fastest way to account for wood fuel mass. Therefore, accounting for firewood is everywhere carried out in volumetric units of measurement - warehouse meters and cubic meters (fold and dense measures)Advantages
accounting for firewood in volume units
- extreme simplicity in the execution of measurements of 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 batches and the coefficients for converting values from a folding measure to a dense measure are standardized and set out inFlaws
accounting for firewood in mass units
- the price for the ease of accounting for firewood in volume units is the complication of further heat engineering calculations for calculating the total calorific value of wood fuel (you need to take into account the type of wood, its place of growth, the degree of rottenness of firewood, etc.)
Calorific value of firewood
calorific value of firewood
she is the heat of combustion of firewood,
she is the calorific value of firewood
How is the calorific value of firewood different from the calorific value of wood?
The calorific value of wood and the calorific value of firewood are related and similar quantities, identified in everyday life with the concepts of "theory" and "practice". In theory, we study the calorific value of wood, but in practice we are dealing 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 truest sense of the word, and yet it occupies volume, participates in the process of burning firewood and has its own calorific value. Often, the calorific value of the bark is significantly different from the calorific value of the wood itself. In addition, real firewood can be, have different wood density depending on, have a large percentage, etc.
Thus, for real firewood, the calorific value indicators are generalized and slightly underestimated, since for real firewood, all negative factors that reducetheir calorific value. This explains the difference in the smaller side 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 two different things.
The calorific value of firewood is the amount of useful heat generated during their combustion. Useful heat refers to the heat that can be taken away from the hearth without compromising 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 in a unit mass or volume of wood. (more details about the calorific value of wood in the article -)
- The moisture content of wood depends on the amount of water and other moisture present in a unit of mass or volume of wood. (more details about wood moisture in the article -)
Table of volumetric calorific value of firewood
Gradation of calorific value according to
(at wood moisture content 20%)
| wood species | specific calorific value of firewood (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
Absolutely true is the statement that rot worsens the quality of firewood and reduces their calorific value. But how much the calorific value of rotten firewood decreases is a question. Soviet GOST 2140-81 and determine the methodology for measuring the size of rot, limit the amount of rot in a log and the number of rotten logs in a batch (no more than 65% of the butt area and no more than 20% of the total mass, respectively). But, at the same time, the standards do not indicate a change in the calorific value of the firewood themselves.
It's obvious that within the requirements of GOSTs there is no significant change in the total calorific value of the wood mass due to rot, therefore, individual rotten logs can be safely neglected.
If there is more rot than is permissible 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, 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 their volume, wood species and degree of rottenness. And, only the moisture content of wood - 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 substance from the weight measure of firewood, plus heat loss for water evaporation 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 species of wood growing in different regions may differ due to changes in the density of wood depending on the water saturation of the soil in the growing area. Moreover, it does not 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 wood species can vary with a difference of 2...5% due to changes in wood. This is explained by the fact that in a dry area (under conditions of lack of moisture) a finer and denser cellular structure of wood grows and forms than in water-rich marshy land. 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 so great, about 2...5%. However, with large firewood harvesting, 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 differ 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 which remains in the solid residue after the complete combustion of the wood mass. The ash content of firewood is the degree of their 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
| Inner ash | outer ash |
| Inner ash is a mineral substance that is found directly in | External ash is mineral substances that have entered the firewood from 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, sticking to the lining of the combustion chamber of the heating unit. As a result of such sintering and sticking, external ash is poorly removed from the high-temperature fuel combustion zone. |
| The internal ash content of the wood substance is in the range from 0.2 to 2.16% of the total wood mass | The content of external ash can reach 20% of the total wood mass |
| Ash is an undesirable part of the fuel, which reduces its combustible component and makes it difficult to operate heating units. | |
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