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)
Standard of times 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 heart 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 decimetres. 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 builders-woodworkers and is alternative method for those cases when firewood is easier to weigh than to determine their 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 most fast way accounting 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 close in value 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. Under useful heat refers to the heat that can be taken away from the hearth without prejudice to 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 of the wood - renders big 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 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 calorific value of firewood for the same tree species 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. | |
Ash content in various constituent parts 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.
On the issues under consideration, I will write a summary here, and then something like paragraphs from which these summaries follow.
1. Specific calorific value of any wood 18 - 0.1465W, MJ / kg = 4306-35W kcal/kg, W-humidity.
2. Volumetric calorific value of birch (10-40%)
2.6kW*h/l
3. Volumetric calorific value of pine (10-40%) 2.1kW*h/l
4. Drying up to 40% and below is not so difficult. For round timber, it is even necessary if splitting is planned.
5. Ash does not burn. Soot and charcoal are close to coal
6. During the combustion of dry wood, 567 grams of water per kilogram of firewood is released.
7. Theoretical minimum air supply for combustion - 5.2m3/kg_dry_wood Normal air supply is about 3m3/l_pine and 3_5 m3/l_birch.
8. In the chimney, the temperature of the inner walls of which is above 75 degrees does not form condensate (with firewood up to 70% humidity).
9. The TT efficiency of the boiler/furnace without heat recovery cannot exceed 91% at a flue gas temperature of 200°C.
10. A flue gas heat exchanger with steam condensation can, in the limit, recover up to 30% or more of the combustion heat of firewood, depending on their initial humidity.
11. The difference between the expression obtained here for the specific calorific value of firewood and the literature dependence is primarily due to the use of different definitions of moisture
12. The volumetric calorific value of rotten firewood with a dry density of 0.3 kg/l is 1.45 kW*h/l over a wide range of humidity.
13. To determine the volumetric calorific value of various types of firewood, it is enough to measure the density of air-dry firewood of this type, multiply by 4 and get the calorific value in kWh liters of firewood data almost regardless of humidity. Call it the rule of four
Content
1. General Provisions.
2. Calorific value of absolutely dry wood.
3. Calorific value of wet wood.
3.1. Theoretical calculation of the heat of evaporation of water from wood.
3.2. Calculation of the heat of evaporation of water from wood
4. Dependence of wood density on humidity
5. Volumetric calorific value.
6. About the humidity of firewood.
7. Smoke, charcoal, soot and ash
8. How much water vapor is formed during the combustion of wood
9.Latent heat.
10. Amount of air required for burning wood
10.1. Flue gas quantity
11. Flue gas heat
12. About the efficiency of the furnace
13. Total heat recovery potential
14. Once again about the dependence of the calorific value of firewood on humidity
15. On the calorific value of rotten firewood
16. On the volumetric calorific value of any firewood.
Until finished. I will be glad to additions and constructive comments/suggestions.
1. General Provisions.
I’ll make a reservation right away that it turned out that I understand two different concepts by the moisture content of wood. I will continue to operate only with the moisture content that is mentioned for lumber. Those. the mass of water in the tree divided by the mass of dry matter, not the mass of water divided by the total mass.
Those. humidity 100% means that in a ton of firewood there are 500 kg of water and 500 kg of absolutely dry firewood
Concept one. Of course, it is possible to talk about the calorific value of firewood in kilograms, but it is inconvenient, since the moisture content of firewood varies greatly and, accordingly, the specific calorific value too. With all this, we buy firewood in cubic meters, not tons.
We buy coal in tons, so for it the calorific value is primarily interesting per kg.
We buy gas in cubic meters, so the calorific value of gas is interesting precisely per cubic meter.
Coal has a calorific value of about 25MJ/kg and gas about 40MJ/m3. About firewood they write from 10 to 20 MJ / kg. We understand. Below we will see that the volumetric calorific value, in contrast to the mass for firewood, does not change so much.
2. Calorific value of absolutely dry wood.
To begin with, let's determine the calorific value of completely dry firewood (0%) simply by the elemental composition of the wood.
Hence, I believe that the percentages given are massive.
1000 g of absolutely dry firewood contains:
495g C
442g O
63g H
Our final reactions. We omit the intermediate ones (their thermal effects, to one degree or another, sit in the final reaction):
С+O2->CO2+94 kcal/mol~400 kJ/mol
H2+0.5O2->H2O+240 kJ/mol
Now let's determine the additional oxygen - which will give the heat of combustion.
495g C ->41.3 mol
442g O2->13.8 mol
63g H2->31.5 mol
The combustion of carbon requires 41.3 moles of oxygen and the combustion of hydrogen requires 15.8 moles of oxygen.
Let's consider two extreme options. In the first, all the oxygen available in the wood bound to carbon, in the second, to hydrogen.
We believe:
1st option
Received heat (41.3-13.8)*400+31.5*240=11000+7560=18.6 MJ/kg
2nd option
Received heat 41.3*400+(31.5-13.8*2)*240=16520+936=17.5 MJ/kg
The truth, along with all the chemistry, is somewhere in the middle.
The amount of carbon dioxide and water vapor released during complete combustion is the same in both cases.
Those. calorific value of any absolutely dry firewood (even aspen, even oak) 18+-0.5MJ/kg~5.0+-0.1kW*h/kg
3. Calorific value of wet wood.
Now we are looking for data for the calorific value depending on the humidity.
To calculate the specific calorific value depending on humidity, it is proposed to use the formula Q=A-50W, where A varies from 4600 to 3870 http://tehnopost.kiev.ua/ru/drova/13-teplotvornost-drevesiny-drova.html
or take 4400 in accordance with GOST 3000-45 http://www.pechkaru.ru/Svojstva drevesin.html
Let's figure it out. obtained by us for dry firewood 18 MJ / kg = 4306 kcal / kg.
and 50W corresponds to 20.9 kJ/g of water. The heat of evaporation of water is 2.3 kJ/g. And here is the inconsistency. Therefore, in a wide range of humidity parameters, the formula may not be applicable. At low humidity due to undefined A, at high humidity (more than 20-30%) due to incorrect 50.
In the data on the direct calorific value, there are contradictions from source to source and there is an ambiguity of what is meant by humidity. I will not provide links. Therefore, we simply calculate the heat of evaporation of water depending on humidity.
3.1. Theoretical calculation of the heat of evaporation of water from wood.
To do this, we use the dependencies
Let's limit ourselves to 20 degrees.
from here
3% -> 5%(rel)
4% -> 10%(rel)
6% -> 24%(rel)
9% -> 44%(rel)
12% -> 63%(rel)
15% -> 73%(rel)
20% -> 85%(rel)
28% -> 97%(rel)
How to get the heat of vaporization from this? but quite simple.
mu(pair)=mu0+RT*ln(pi)
Accordingly, the difference in the chemical potentials of steam over wood and water is defined as delta(mu)=RT*ln(pi/pus). pi - partial pressure of steam above the tree, pnas - partial pressure of saturated vapors. Their ratio is the relative humidity of the air expressed as a fraction, let's denote it H.
respectively
R=8.31 J/mol/K
T=293K
the chemical potential difference is the difference in the heat of vaporization expressed in J/mol. We write the expression in more digestible units in kJ / kg
delta(Qsp)=(1000/18)*8.31*293/1000 ln(H)=135ln(H) kJ/kg up to sign
3.2. Calculation of the heat of evaporation of water from wood
From here, our graphical data is processed into instantaneous values of the heat of evaporation of water:
3% -> 2.71MJ/kg
4% -> 2.61MJ/kg
6% -> 2.49MJ/kg
9% -> 2.41MJ/kg
12% -> 2.36MJ/kg
15% -> 2.34MJ/kg
20% -> 2.32MJ/kg
28% -> 2.30MJ/kg
Further 2.3MJ/kg
Below 3% we will consider 3MJ/kg.
Well. We have universal data applicable to any wood, assuming that the original image is also applicable to any wood. It is very good. Now consider the process of moistening wood and the corresponding drop in calorific value
let us have 1kg of dry residue, humidity 0g, calorific value 18MJ / kg
moistened to 3% - added water 30g. The mass increased by these 30 grams, and the heat of combustion decreased by the heat of evaporation of these 30 grams. Total we have (18MJ-30/1000*3MJ)/1.03kg=17.4MJ/kg
further moistened by another 1%, the mass increased by another 1%, and the latent heat increased by 0.0271 MJ. Total 17.2MJ/kg
And so on we recalculate all the values. We get:
0% -> 18.0 MJ/kg
3% -> 17.4MJ/kg
4% -> 17.2MJ/kg
6% -> 16.8MJ/kg
9% -> 16.3MJ/kg
12% -> 15.8MJ/kg
15% -> 15.3MJ/kg
20% -> 14.6MJ/kg
28% -> 13.5 MJ/kg
30%-> 13.3 MJ/kg
40%-> 12.2MJ/kg
70%-> 9.6MJ/kg
Hooray! These data again do not depend on the type of wood.
In this case, the dependence is perfectly described by a parabola:
Q=0.0007143*W^2 - 0.1702W + 17.82
or linear on the interval 0-40
Q \u003d 18 - 0.1465W, MJ / kg or in kcal / kg Q \u003d 4306-35W (not 50 at all) We will deal with the difference separately.
4. Dependence of wood density on humidity
I will consider two breeds. Pine and birch
To begin with, I rummaged and decided to stop at the following data on the density of wood
Knowing the density values, we can determine the volumetric weight of the dry residue and water depending on the humidity, we do not take into account the fresh cut, since the humidity is not determined.
Hence the birch density is 2.10E-05x2 + 2.29E-03x + 6.00E-01
pine 1.08E-05x2 + 2.53E-03x + 4.70E-01
where x is the humidity.
I will simplify to a linear expression in the range of 0-40%
It turns out
pine ro=0.47+0.003W
birch ro=0.6+0.003W
It would be nice to collect statistics on the data, since pine is 0.47 m.b. and about the case, but birch is lighter, and 0.57 somewhere.
5. Volumetric calorific value.
Now let's calculate the calorific value unit of volume of the ability of pine and birch
for birch
0 0,6 18 10,8
15 0,64 15,31541 9,801862
25 0,67 13,91944 9,326025
75 0,89 9,273572 8,253479
For birch, it can be seen that the volumetric calorific value varies from 8 MJ / l for freshly cut to 10.8 for absolutely dry. In a practically significant range of 10-40% from about 9 to 10 MJ / l ~ 2.6 kWh / l
For pine
humidity density specific heat volumetric heat capacity
0 0,47 18 8,46
15 0,51 15,31541 7,810859
25 0,54 13,91944 7,516497
75 0,72 9,273572 6,676972
For birch, it can be seen that the volumetric calorific value varies from 6.5 MJ / l for freshly cut to 8.5 for absolutely dry. In a practically significant range of 10-40% from about 7 to 8 MJ / l ~ 2.1 kWh / l
6. About the humidity of firewood.
Earlier, I mentioned a practically significant interval of 10-40%. I want to explain. From the discussions carried out earlier, it becomes obvious that it is more expedient to burn dry firewood than raw firewood, and it is simply easier to burn them, it is easier to carry them to the firebox. It remains to understand what dry means.
If we turn to the picture above, we will see that at the same 20 degrees over 30%, the equilibrium air humidity next to such a tree is 100% (rel.). What does it mean? AK the fact that the log behaves like a puddle, and dries under any weather conditions, it can even dry in the rain. The drying speed is limited only by diffusion, which means the length of the log if not chopped.
By the way, the drying speed of a log 35 cm long is approximately equivalent to the drying speed of a fifty-fifty board, while due to cracks in the log, its drying speed additionally increases compared to a board, and laying in single-row logs still improves drying compared to a board. It seems that in a couple of months in the summer in a single-row pollen on the street you can reach a humidity of 30% or less than half a meter of firewood. Chipped naturally dry even faster.
Ready to discuss if there are results.
It is not difficult to imagine what kind of log this is in appearance and touch. It does not contain cracks in the end, to the touch is slightly damp. If it lies haphazardly in the water, mold and fungi may appear. Joyfully run in if the heat is all kinds of bugs. It pricks of course, but reluctantly. I think above 50% somewhere it doesn’t prick practically at all. The ax/cleaver enters with a "slurp" and the whole effect
Air-dry wood, already has cracks and humidity less than 20%. It is already relatively easy to prick and burns perfectly.
What is 10%? Let's look at the picture. This is not necessarily chamber drying. This can be drying in a sauna or simply in a heated room during the season. This firewood burns - just have time to throw it up, it flares up perfectly, light and "ringing" to the touch. They are also superbly planed into splinters.
7. Smoke, charcoal, soot and ash
The main combustion products of wood are carbon dioxide and water vapor. Which, along with nitrogen, are the main components of the flue gas.
In addition, unburned residues remain. This is soot (in the form of flakes in the pipe, and actually what we call smoke), charcoal and ash. Their composition is as follows:
charcoal:
http://www.xumuk.ru/encyklopedia/1490.html
composition: 80-92% C, 4.0-4.8% H, 5-15% O - the same stone in fact, as suggested
Charcoal also contains 1-3% miner. impurities, ch. arr. carbonates and oxides of K, Na, Ca, Mg, Si, Al, Fe.
And here is ash what is non-combustible metal oxides. By the way, ash is used in the world as an additive to cement, also clinker, in fact, only received for delivery (without additional energy costs).
soot
elemental composition,
Carbon, C 89 - 99
Hydrogen, H 0.3 - 0.5
Oxygen, O 0.1 - 10
Sulfur, S0.1 - 1.1
Minerals0.5
True, these are not the same soots - but technical soots. But I think the difference is small.
Both charcoal and soot are close to coal in composition, which means that not only do they burn, but they also have a high calorific value - at the level of 25 MJ / kg. I think the formation of both coal and soot is primarily due to insufficient temperature in the furnace / lack of oxygen.
8. How much water vapor is formed during the combustion of wood
1 kg of dry firewood contains 63 grams of hydrogen or
From these 63 grams of water, when burned, a maximum of 63 * 18 / 2 will be obtained (we spend two grams of hydrogen to obtain 18 grams of water) \u003d 567 grams/kg_firewood.
The total amount of water formed during the combustion of wood in this way will be
0% ->567 g/kg
10%->615 g/kg
20%->673g/kg
40%->805 g/kg
70%->1033 g/kg
9.Latent heat.
An interesting question is, and if the moisture formed during the combustion of wood is condensed and the resulting heat is taken away, how much is there? Let's estimate.
0% ->567 g/kg->1.3MJ/kg->7.2% of the calorific value of firewood
10%->615 g/kg->1.4MJ/kg->8.8% of the calorific value of firewood
20%->673 g/kg->1.5MJ/kg->10.6% of the combustion heat of firewood
40%->805 g/kg->1.9 MJ/kg->15.2% of the calorific value of firewood
70%->1033 g/kg->2.4MJ/kg->24.7% of the calorific value of firewood
Here it is theoretically the limit of the additive that can be squeezed out from the condensation of water. Moreover, if you still heat with non-damp firewood, then the entire marginal effect is within 8-15%
10. Amount of air required for burning wood
The second potential source of heat to improve the efficiency of the HT boiler/furnace is heat extraction from the flue gas.
We already have all the necessary data, so we will not go into sources. First you need to calculate the theoretical minimum air supply for burning wood. To start dry.
Let's turn to paragraph 2
1 kg of firewood:
495g C ->41.3 mol
442g O2->13.8 mol
63g H2->31.5 mol
The combustion of carbon requires 41.3 moles of oxygen and the combustion of hydrogen requires 15.8 moles of oxygen. Moreover, 13.8 mol of oxygen is already there. The total oxygen requirement for combustion is 43.3 mol/kg_wood. from here air requirement 216 mol/kg_wood= 5.2 m3/kg_wood(oxygen - one fifth).
For different humidity we have wood
0%->5.2 m3/kg->2.4 m3/l_pine! 3.1 m3/l_, birch
10%->4.7 m3/kg->2.4 m3/l_pine! 3.0 m3/l_, birch
20%->4.3 m3/kg->2.3 m3/l_pine! 2.9 m3/l_, birch
40%->3.7 m3/kg->2.2 m3/l_pine! 2.7 m3/l_, birch
70%->3.1 m3/kg->2.1 m3/l_pine! 2.5 m3/l_, birch
As in the case of calorific value, we see that the required air supply per liter of firewood is slightly dependent on their moisture content.
In this case, it is impossible to supply air less than the obtained value - there will be incomplete burnout of the fuel, the formation carbon monoxide, soot and coal. It is also impractical to supply much more, since, at the same time, incomplete combustion of oxygen, a decrease in the limiting temperature of the flue gases, and large losses in the pipe.
The excess air coefficient (gamma) is entered as the ratio of the actual air supply to the theoretical minimum (5m3/kg). The value of the excess coefficient can be different and usually ranges from 1 to 1.5.
10.1. Flue gas quantity
At the same time, we burned 43.3 mol of oxygen, but released 41.3 mol of CO2, 31.5 mol of chemical water and all the moisture content of the wood.
Thus, the amount of flue gas at the outlet of the furnace is greater than at the inlet and is in terms of room temperature
0% ->5.9 m3/kg, of which water vapor 0.76 m3/kg
10%->5.5 m3/kg, of which water vapor 0.89 m3/kg including evaporated 0.13
20%->5.2 m3/kg, of which water vapor 1.02 m3/kg including evaporated 0.26
40%->4.8 m3/kg, of which water vapor 1.3 m3/kg
70%->4.4 m3/kg, of which water vapor 1.69 m3/kg
Why do we need all this?
But why. To begin with, we can determine what temperature it is necessary to maintain the chimney so that there will never be condensate in it. (By the way, I have no condensation in the pipe at all).
To do this, we find the temperature corresponding to the relative humidity of the flue gas for 70% of the firewood. You can see the chart above. We are looking for 1.68 / 4.4 \u003d 0.38.
And here is and cannot be on schedule! There's a mistake
We take this data http://www.fptl.ru/spravo4nik/davlenie-vodyanogo-para.html and get a temperature of 75 degrees. Those. if the chimney is hotter, there will be no condensation in it.
For excess factors greater than one, the amount of flue gas should be calculated as the calculated amount of flue gas (5.2 m3/kg at 20%) plus (gamma-1) times the theoretically required amount of air (4.3 m3/kg at 20%). .
For example, for an excess of 1.2 and 20% moisture, we have 5.2 + 0.2 * 4.3 = 6.1 m3 / kg
11. Flue gas heat
We restrict ourselves to the case in which the temperature of the flue gas is 200 deg. I took one of the values from the link http://celsius-service.ru/?page_id=766
And we will look for the excess heat of the flue gas compared to room temperature - the potential for heat recovery. Let's take the coefficient of excess air 1.2. Flue gas data from here: http://thermalinfo.ru/publ/gazy/gazovye_smesi/teploprovodnosti_i_svojstva_dymovykh_gazov/28-1-0-33
Density at 200 degrees 0.748, Cp=1.097.
at zero 1.295 and 1.042.
Please note that the density is related according to the ideal gas law: 0.748=1.295*273/473. And the heat capacity is practically constant. Since we operate with flows converted to 20 degrees, we will determine the density at a given temperature - 1.207. and Cp we take the average, somewhere around 1.07. The total heat capacity of our standard smoke cube is 1.29 kJ/m3/K
0% ->6.9 m3/kg->1.6MJ/kg->8.9% calorific value of firewood
10%->6.4 m3/kg->1.5MJ/kg->9.3% calorific value of firewood
20%->6.1 m3/kg->1.4MJ/kg->9.7% calorific value of firewood
40%->5.5 m3/kg->1.3MJ/kg->10.5% of the calorific value of firewood
70%->5.0 m3/kg->1.2MJ/kg->12.1% calorific value of firewood
In addition to that, let's try to justify the difference between the literary calorific value of firewood 4400-50W and the 4306-35W obtained above. Justify the difference in the coefficient.
Suppose that the authors of the formula consider the heat for heating additional steam to be the same losses as latent heat and wood shrinkage. We have between 10 and 20% allocated additional steam 0.13m3/kg_wood. Without bothering with the search for the value of the heat capacity of water vapor (they still do not differ much), we get additional losses for heating additional water 0.13 * 1.3 * 180 = 30.4 KJ / kg_wood. One percent moisture is ten times less than 3 kJ/kg/% or 0.7 kcal/kg/%. Got no 15. Still inconsistency. I don't see any more reasons.
12. About the efficiency of the furnace
There is a desire to understand what lies in the so-called. boiler efficiency. Flue gas heat is definitely a loss. Losses through the walls are also unconditional (if they are not considered useful). Latent heat - loss? No. The latent heat from the evaporated moisture sits in our reduced calorific value of firewood. In chemically formed water is a product of combustion, and not a loss of power (it does not evaporate, but immediately forms in the form of steam).
The total limiting efficiency of the boiler / furnace is determined by the heat recovery potential (excluding condensation) written a little higher. And is about 90% and not more than 91. For increase efficiency it is necessary to reduce the temperature of the flue gas at the outlet of the furnace, for example, by reducing the intensity of combustion, but at the same time, more extensive soot formation should be expected - smoky and not 100% combustion of wood-> reduction in efficiency.
13. Total heat recovery potential.
From the data presented above, it is quite simple to consider for the case of cooling from flue gas 200 to 20 and moisture condensation. For ease of all moisture.
0% ->2.9MJ/kg->16% of the calorific value of firewood
10%->3.0MJ/kg->18.6% of the calorific value of firewood
20%->3.0MJ/kg->20.6% of the calorific value of firewood
40%->3.2MJ/kg->26.3% of the calorific value of firewood
70%->3.6MJ/kg->37.4% of the calorific value of firewood
It should be noted that the values are quite significant. Those. there is a potential for heat recovery, while the magnitude of the effects in absolute terms in MJ/kg weakly depends on humidity, which possibly simplifies engineering calculations. About half of the indicated effect is due to condensation, the rest is due to the heat capacity of the flue gas.
14. Once again about the dependence of the calorific value of firewood on humidity
Let's try to justify the difference between the literary calorific value of firewood 4400-50W and those obtained above 4306-35W in the coefficient before W.
Suppose that the authors of the formula consider the heat for heating additional steam to be the same losses as latent heat and wood shrinkage. We have between 10 and 20% allocated additional steam 0.13m3/kg_wood. Without bothering with the search for the value of the heat capacity of water vapor (they still do not differ much), we get additional losses for heating additional water 0.13 * 1.3 * 180 = 30.4 KJ / kg_wood. One percent moisture is ten times less than 3 kJ/kg/% or 0.7 kcal/kg/%. Got no 15. Still inconsistency.
Let's take another option. Consisting in the fact that the authors of the well-known formula operated on the so-called absolute moisture content of wood, while here we operated on the relative one.
In absolute terms, W is taken as the ratio of the mass of water to the total mass of firewood, and in the relative ratio of the mass of water to the mass of dry residue (see paragraph 1).
Based on these definitions, we construct the dependence of the absolute humidity on the relative
0%(rel)->0%(abs)
10%(rel)->9.1%(abs)
20%(rel)->16.7%(abs)
40%(rel)->28.6%(abs)
70%(rel)->41.2%(abs)
100%(rel)->50%(abs)
Separately, consider again the interval 10-40. It is possible to approximate the obtained dependence by a straight line W= 1.55 Wabs - 4.78.
We substitute this expression into the formula for the previously obtained calorific value and we have a new linear expression for the specific calorific value of firewood
4306-35W \u003d 4306-35 * (1.55 Wabs - 4.78) \u003d 4473-54W. We finally got a result much closer to the literature data.
15. On the calorific value of rotten firewood
In the case of burning a fire in nature, including barbecues, I probably, like many others, prefer to heat with dry wood. These firewood are rather rotten dry branches. They burn well, quite hot, but to form a certain amount of coals, it takes about twice as much as normal dry birch. But where can I get this dry birch in the forest? Therefore, I drown with what I have and with what does not harm the forest. The same firewood is perfectly applicable for heating the stove / boiler in the house.
What is this dryer? This is the same wood in which the decay process usually took place, incl. directly on the root, as a result, the density of the dry residue has greatly decreased, a loose structure has appeared. This loose structure is more vapor permeable than ordinary wood, so the branch dried right on the vine under certain conditions.
I'm talking about these woods.
You can also use rotten tree trunks if they are dry. Raw rotten wood is very difficult to burn, so we will not consider it for now.
I have never measured the density of such firewood. But subjectively, this density is about one and a half times lower than ordinary pine (with wide tolerances). Based on this postulate, we calculate the volumetric heat capacity depending on humidity, while I usually heat with dry wood from hardwoods, the density of which was initially higher than that of pines. Those. Let us consider the case when a rotten log has a dry residue density that is half that of the original wood.
Since for birch and pine the linear formulas for the dependence of density coincided with us (up to the density of absolutely dry firewood), we will also use this formula for rotted wood:
ro=0.3+0.003W. This is a very rough estimate, but no one seems to have done much research into the issue raised here. M.b. Canadians have information, but they also have their own forest, with their own properties.
0% (0.30 kg/l) ->18.0MJ/kg ->5.4MJ/l=1.5kW*h/l
10% (0.33 kg/l) ->16.1MJ/kg->5.3MJ/l=1.5kW*h/l
20% (0.36 kg/l) ->14.6MJ/kg->5.3MJ/l=1.5kW*h/l
40% (0.42 kg/l) ->12.2MJ/kg->5.1MJ/l=1.4kW*h/l
70% (0.51 kg/l) ->9.6MJ/kg->4.9MJ/l=1.4kW*h/l
What is no longer surprising The volumetric calorific value of rotten firewood is again weakly dependent on humidity and is about 1.45 kWh/l.
16. On the volumetric calorific value of any firewood.
In general, the considered breeds, including rotten, can be combined under one formula for calorific value. In order to get not quite an academic formula, but applicable in practice, instead of absolutely dry wood, we write for 20%:
Density Calorific value
0.66 kg/l -> 2.7 kW*h/l
0.53 kg/l -> 2.1 kW*h/l
0.36 kg/l -> 1.5 kW*h/l
Those. the volumetric calorific value of air-dry firewood, regardless of the species, is approximately Q=4*density(in kg/l), kW*h/l
Those. to understand what your specific firewood will give (various fruit, rotten, coniferous, etc.) You can once determine the density of conditionally air-dry firewood - by weighing and determining the volume. Multiply by 4 and apply the resulting value to almost any moisture content of firewood.
I would carry out a similar measurement by making a short log (within 10 cm) close to a cylinder or a rectangular parallelepiped (board). The goal is not to bother with measuring the volume and dry quickly enough in the air. I remind you that drying along the fibers is 6.5 times faster than across. And this 10 cm field will dry in the air in the summer in a week.
_____________________________________________________________________________
The pictures posted here are located on other resources. In order to preserve the information content and in pursuance of clause 6.8 of the Forum Rules, I am attaching them as attachments. If these attachments violate someone's rights, please inform - then they will be deleted.
Attachments:
Comments
Wood is a rather complex material in terms of its chemical composition.
Why are we interested in chemistry? Why, combustion (including burning wood in a stove) is a chemical reaction of wood materials with oxygen from the surrounding air. The calorific value of firewood depends on the chemical composition of a particular type of wood.
The main binding chemical materials in wood are lignin and cellulose. They form cells - a kind of container, inside which there is moisture and air. The wood also contains resin, proteins, tannins and other chemical ingredients.
The chemical composition of the vast majority of wood species is almost the same. Small fluctuations in the chemical composition of different species and determine the differences in the calorific value of different types of wood. Calorific value is measured in kilocalories - that is, the amount of heat obtained by burning one kilogram of a tree of a particular species is calculated. There are no fundamental differences between the calorific values of different types of wood. And for domestic purposes, it is enough to know the average values.
Differences between rocks in calorific value appear to be minimal. It is worth noting that, based on the table, it may seem that it is more profitable to buy firewood harvested from coniferous wood, because their calorific value is greater. However, on the market, firewood is supplied by volume, not by mass, so there will simply be more of it in one cubic meter of firewood harvested from hardwood.
Harmful impurities in wood
During chemical reaction burning wood does not burn completely. After combustion, ash remains - that is, the unburned part of the wood, and during the combustion process, moisture evaporates from the wood.
Ash has less effect on the quality of combustion and the calorific value of firewood. Its amount in any wood is the same and is about 1 percent.
But the moisture in the wood can cause a lot of problems when burning them. So, immediately after felling, wood can contain up to 50 percent moisture. Accordingly, during the combustion of such firewood, the lion's share of the energy released with the flame can be spent simply on the evaporation of the wood moisture itself, without doing any useful work.
The moisture present in wood dramatically reduces the calorific value of any firewood. Burning firewood not only does not fulfill its function, but also becomes unable to maintain the required temperature during combustion. At the same time, the organic matter in the firewood does not burn out completely; when such firewood burns, a suspended amount of smoke is released, which pollutes both the chimney and the furnace space.
What is the moisture content of wood, what does it affect?
The physical quantity that describes the relative amount of water contained in wood is called moisture content. The moisture content of the wood is measured as a percentage.
When measuring, two types of humidity can be taken into account:
- Absolute moisture content is the amount of moisture that is contained in the wood at the current moment in relation to a completely dried tree. Such measurements are usually carried out for construction purposes.
- Relative humidity is the amount of moisture that wood currently contains relative to its own weight. Such calculations are made for wood used as fuel.
So, if it is written that wood has a relative humidity of 60%, then its absolute humidity will be expressed as 150%.
Analyzing this formula, it can be established that firewood harvested from coniferous wood with a relative humidity index of 12 percent will release 3940 kilocalories when burning 1 kilogram, and firewood harvested from hardwood with comparable humidity will already release 3852 kilocalories.
To understand what constitutes a relative humidity of 12 percent - let's explain that firewood acquires such humidity, which long time dry outside.
Density of wood and its effect on calorific value
To estimate the calorific value, you need to use a slightly different characteristic, namely the specific calorific value, which is a value derived from density and calorific value.
Experimentally, information was obtained on the specific calorific value of certain types of wood. Information given for the same indicator humidity at 12 percent. Based on the results of the experiment, the following table:
Using the data from this table, you can easily compare the calorific value of different types of wood.
What firewood can be used in Russia
Traditionally, the most favorite type of firewood for burning in brick ovens in Russia is birch. Although, in fact, birch is a weed, the seeds of which easily cling to any soil, it is extremely widely used in everyday life. An unpretentious and fast-growing tree has faithfully served our ancestors for many centuries.
Birch firewood has a relatively good calorific value and burns quite slowly, evenly, without overheating the stove. In addition, even the soot obtained by burning birch firewood is used - it includes tar, which is used both for domestic and medicinal purposes.
In addition to birch, aspen, poplar and linden wood is used as firewood from hardwoods. Their quality compared to birch, of course, is not very good, but in the absence of others, it is quite possible to use such firewood. In addition, linden firewood emits a special aroma when burned, which is considered beneficial.
Aspen firewood gives a high flame. They can be used at the final stage of the firebox to burn off the soot formed by burning other firewood.
Alder also burns quite evenly, and after combustion it leaves a small amount of ash and soot. But again, in terms of the sum of all the quality, alder firewood cannot compete with birch firewood. But on the other hand - when used not in a bath, but for cooking - alder firewood is very good. Their even burning helps to cook food efficiently, especially pastries.
Firewood made from fruit trees are quite rare. Such firewood, and especially maple, burns very quickly and the flame reaches a very high temperature during combustion, which can adversely affect the condition of the stove. In addition, you just need to heat air and water in the bath, and not melt the metal in it. When using such firewood, it must be mixed with firewood with a low calorific value.
Softwood firewood is rarely used. Firstly, such wood is very often used for construction purposes, and secondly, the presence a large number resin in coniferous trees contaminates furnaces and chimneys. It makes sense to heat the stove with coniferous wood only after a long drying time.
How to prepare firewood
Firewood harvesting usually begins in late autumn or early winter, before permanent snow cover is established. Felled trunks are left on the plots for primary drying. After some time, usually in winter or early spring, firewood is taken out of the forest. This is due to the fact that during this period no agricultural work is carried out and the frozen ground allows you to load more weight on the vehicle.
But this is the traditional order. Now, due to the high level of development of technology, firewood can be harvested all year round. Entrepreneurial people can bring you already sawn and chopped firewood any day for a reasonable fee.
How to saw and chop wood
Saw the brought log into pieces that fit the size of your firebox. After the resulting decks are split into logs. Decks with a cross section of more than 200 centimeters are pricked with a cleaver, the rest with an ordinary ax.
The decks are pricked into logs so that the cross section of the resulting log is about 80 sq.cm. Such firewood will burn for quite a long time in sauna stove and release more heat. Smaller logs are used for kindling.
Chopped logs are stacked in a woodpile. It is intended not only for the accumulation of fuel, but also for drying firewood. A good woodpile will be located in an open space, blown by the wind, but under a canopy that protects the firewood from precipitation.
The bottom row of woodpile logs is laid on logs - long poles that prevent firewood from contacting wet soil.
Drying firewood to an acceptable moisture content takes about a year. In addition, wood in logs dries much faster than in logs. Chopped firewood reaches an acceptable moisture content already in three months of summer. When dried for a year, firewood in a woodpile will receive a moisture content of 15 percent, which is ideal for combustion.
Calorific value of firewood: video
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 heart 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. | |
How to understand: will the kitten be fluffy?
What kind of light alcohol can be drunk for pregnant women: the consequences of drinking
Why do the legs swell in the ankles and ankles of the feet in pregnant women: causes and methods of treatment
The wedding of Prince Harry and Meghan Markle: scandalous and secret details of the marriage (photo) The future marriage of Prince Harry year NTV
How to close white plums for the winter