Firewood. Features of burning firewood of different types of wood. Wood biomass Ash content of firewood per working weight

Firewood - pieces of wood that are intended to be burned in stoves, fireplaces, furnaces or fires to obtain heat, heat and light.

Firewood firewood are mainly procured and supplied in sawn and chipped form. The moisture content should be as low as possible. The length of the logs is mainly 25 and 33 cm. Such firewood is sold in bulk storage meters or packaged and sold by weight.

Various firewoods are used for heating purposes. The priority characteristic by which one or another firewood is chosen for fireplaces and stoves is their calorific value, duration of burning and comfort during use (flame pattern, smell). For heating purposes, it is desirable that the heat release occurs more slowly, but for a longer time. All hardwood firewood is best suited for heating purposes.

For heating stoves and fireplaces, firewood is mainly used of such species as oak, ash, birch, hazel, yew, hawthorn.

Features of burning firewood of different types of wood:

Firewood from beech, birch, ash, hazel is difficult to melt, but they can burn damp, because they have little moisture, and firewood from all these tree species, except for beech, is easily split;

Alder and aspen burn without the formation of soot, moreover, they burn it out of the chimney;

Birch firewood is good for warmth, but when there is a lack of air in the firebox, it burns smoky and forms tar (birch tar), which settles on the walls of the pipe;

Stumps and roots give an intricate pattern of fire;

Juniper, cherry and apple branches give a pleasant aroma;

Pine firewood burns hotter than spruce because of the higher resin content. When burning resinous wood, a sharp rise in temperature, small cavities in the wood burst with a crash, in which resin accumulates, and sparks fly in all directions;

Oak firewood has the best heat transfer, their only drawback is that they split poorly, just like hornbeam firewood;

Firewood from pear and apple trees is easily split and burns well, emitting a pleasant smell;

Medium-hard wood is usually easy to chop;

Long smoldering coals produce cedar firewood;

Cherry and elm firewood smoke when burning;

Sycamore firewood is easily melted, but hard to chop;

Less suitable for burning softwood firewood, because they contribute to the formation of resinous deposits in the pipe and have a low calorific value. Pine and spruce firewood is easy to chop and melt, but they smoke and spark;

Poplar, alder, aspen, linden are also classified as softwood tree species. Firewood of these species burns well, poplar firewood sparks strongly and burns out very quickly;

Beech - wood of this breed is considered a classic firewood, since beech has a beautiful flame pattern and good heat development with almost complete absence of sparks. It should be added to all of the above - beech firewood has a very high calorific value. The smell of burning beech wood is also highly appreciated - therefore, beech wood is mainly used for smoking products. Beech firewood is versatile in use. Based on the above, the cost of beech firewood is high.

It is necessary to take into account the fact that the calorific value of firewood of different types of wood varies greatly. As a result, we obtain fluctuations in wood density and fluctuations in conversion factors cubic meter \u003d\u003e warehouse meter.

Below is a table with the average calorific value per firewood storage meter.

There will be no fire without firewood. As I said, in order for the fire to burn for a long time, you need to prepare for this. Prepare firewood. The bigger, the better. You don't need to overdo it, but you need to have a small margin just in case. After spending two or three nights in the forest, you will probably be able to more accurately determine the required supply of firewood for the night. Of course, you can calculate mathematically how much wood is needed to keep the fire going for a certain number of hours. Convert knots of one thickness or another to cubic meters. But in practice, this calculation will not always work. There are a lot of factors that cannot be calculated, and if you try, the spread will be quite large. Only personal practice gives more accurate results.

A strong wind increases the burning speed by 2-3 times. Wet, calm weather, on the other hand, slows down combustion. A bonfire can burn even during rain, only for this it is necessary to constantly maintain it. When it rains, you do not need to put thick logs in the fire, they take longer to flare up and the rain can simply extinguish them. Remember, thinner branches burn quickly, but they burn out quickly. They should be used to light the thicker branches.

Before talking about some of the species properties of wood during combustion, I want to remind you once again that if you are not forced to spend the night in the immediate vicinity of the fire, try to burn the fire no closer than 1-1.5 meters from the edge of your bed.

Most often we find the following tree species: spruce, pine, fir, larch, birch, aspen, alder, oak, bird cherry, willow. So, in order.

Spruce,like all resinous trees it burns hot, fast. If the wood is dry, the fire spreads over the surface quickly enough. If you do not have the opportunity to somehow divide the trunk of a small tree into relatively small equal parts, and you use the whole tree for a fire, be very careful. Fire, on wood can go beyond the border of the fireplace and cause a lot of trouble. In this case, clear enough space under the fireplace so that the fire cannot spread further. Spruce has the ability to "shoot". During combustion, the resin that is in the wood, under the influence of high temperatures, begins to boil, and not finding a way out, it explodes. A piece of burning wood, which is at the top, flies away from the fire. Probably many who burned a fire noticed this phenomenon. To protect yourself from such surprises, it is enough to put the logs upside down to you. Coals usually fly perpendicular to the trunk.

Pine.Burns hotter and faster than ate. It breaks easily if the tree is no more than 5-10 cm in diameter. "Shoots". Thin dry twigs are well suited as second and third plan firewood to light a fire.

Fir... The main distinguishing feature is that it practically does not "shoot". Deadwood trunks with a diameter of 20-30 cm are very suitable for a "nodia", a fire for the whole night. Burns hot, evenly. Burning rate between spruce and pine.

Larch. This tree, unlike other resinous trees, sheds needles for the winter. The wood is denser and stronger. Burns for a long time, ate longer, evenly. Gives a lot of heat. If you find a piece of dry larch on the bank of a river, chances are that before this piece hit the bank, it lay in the water for some time. Such a tree will burn much longer than usual from the forest. A tree, being in water, without access to oxygen, becomes denser and stronger. Of course it all depends on the length of time in the water. After lying there for several decades, it turns into dust.

Properties of wood for the firebox

Firewood must be prepared carefully and in advance. Good wood should dry for at least a year. The minimum drying time depends on the month of stacking the woodpile (in days):

Another important indicator that characterizes the quality of firewood for a fireplace or stove is the density or hardness of the wood. Hardwood has the highest heat transfer, and softwood is the lowest. The density values \u200b\u200bof wood with a moisture content of 12% are shown in the table below:

Specific calorific value of various wood species.

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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 \u003d 4306-35W kcal / kg, W-humidity.
2. Bulk 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 bituminous coal
6. When dry wood is burned, 567 grams of water per kilogram of firewood are released.
7. The theoretical minimum air supply for combustion is 5.2m3 / kg_dry_wood. Normal air supply is about 3m3 / l_pine and 3_5m3 / l_birch.
8. In the chimney, the temperature of the inner walls of which is higher than 75 degrees, condensation does not form (with firewood up to 70% humidity).
9. The efficiency of the boiler / furnace TT without heat recovery cannot exceed 91% at a flue gas temperature of 200 degrees.
10. The heat exchanger of flue gas heat with steam condensation can return up to 30% or more of the heat of combustion of firewood, depending on their initial moisture content.
11. The difference between the expression obtained here for the specific calorific value of firewood and the literary dependence is primarily due to the use of different definitions of moisture
12. The volumetric calorific value of rotten wood with a dry density of 0.3 kg / l is 1.45 kW * h / l in 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 kWhliters of these firewood practically regardless of humidity. I will call the rule of four

Content
1. General Provisions.
2. Calorific value of absolutely dry wood.
3. Calorific value of damp 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 moisture
5. Bulk calorific value.
6. About wood moisture.
7. Smoke, charcoal, soot and ash
8. How much water vapor is formed during the combustion of wood
9. Latent warmth.
10. The amount of air required for burning wood
10.1. Flue gas quantity
11. Heat of flue gas
12. About the efficiency of the furnace
13. Total potential of heat recovery
14. Once again about the dependence of the calorific value of firewood on moisture
15. About the calorific value of rotten firewood
16. About the volumetric calorific value of any firewood.

Finished for now. I would be glad to see 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 further operate only with the moisture that is said for lumber. Those. the mass of water in the tree divided by the mass of dry residue, 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

The first concept. Of course, it is possible to talk about the calorific value of firewood in kilograms, but it is inconvenient, since the moisture content of the firewood is very different and, accordingly, the specific calorific value too. At the same time, we buy firewood in cubic meters, not in tons.
We buy coal in tons, so the calorific value for it 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. They write about 10 to 20 MJ / kg about firewood. Understanding. 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 element-wise composition of the wood.
Hence, I believe that the percentages are given in mass.
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-\u003e CO2 + 94 kcal / mol ~ 400 kJ / mol
H2 + 0.5O2-\u003e H2O + 240 kJ / mol

Now let's define the additional oxygen - which will give the heat of combustion.
495g C -\u003e 41.3 mol
442g O2-\u003e 13.8 mol
63g H2-\u003e 31.5 mol
For the combustion of carbon, 41.3 moles of oxygen are needed and for the combustion of hydrogen, 15.8 moles of oxygen.
Consider two limiting options. In the first, all the oxygen in the wood is bound with carbon, in the second with hydrogen
We consider:
1st option
Received heat (41.3-13.8) * 400 + 31.5 * 240 \u003d 11000 + 7560 \u003d 18.6 MJ / kg
2nd option
Received heat 41.3 * 400 + (31.5-13.8 * 2) * 240 \u003d 16520 + 936 \u003d 17.5 MJ / kg
Truth, along with all the chemistry, is somewhere in between.
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 + -0.5 MJ / kg ~ 5.0 + -0.1 kW * h / kg

3. Calorific value of damp 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 \u003d 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. we obtained for dry firewood 18 MJ / kg \u003d 4306 kcal / kg.
and 50W corresponds to 20.9 kJ / g of water. The heat of vaporization of water is 2.3 kJ / g. And here is a discrepancy. Therefore, the formula may not be applicable in a wide range of moisture parameters. At low humidity due to undefined A, at high (more than 20-30%) due to incorrect 50.
In the data on the direct calorific value, there is a contradiction from source to source and there is uncertainty about what is meant by humidity. I will not give 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 will use the dependencies

let's restrict ourselves to 20 degrees.
from here
3% -\u003e 5% (rel)
4% -\u003e 10% (rel)
6% -\u003e 24% (rel)
9% -\u003e 44% (rel)
12% -\u003e 63% (rel)
15% -\u003e 73% (rel)
20% -\u003e 85% (rel)
28% -\u003e 97% (rel)

How to get the heat of evaporation from this? rather simple.
mu (pair) \u003d mu0 + RT * ln (pi)
Accordingly, the difference in the chemical potentials of steam over wood and water is defined as delta (mu) \u003d RT * ln (pi / psat). pi is the partial pressure of steam above the tree, psat is the partial pressure of saturated vapors. Their ratio is the relative humidity of the air expressed as a fraction, let us denote it by H.
respectively
R \u003d 8.31 J / mol / K
T \u003d 293K
chemical potential difference is the difference in heat of vaporization expressed in J / mol. Let's write the expression in more digestible units in kJ / kg
delta (Qtest) \u003d (1000/18) * 8.31 * 293/1000 ln (H) \u003d 135ln (H) kJ / kg accurate to sign

3.2. Calculation of the heat of evaporation of water from wood
From here, our graphical data is converted into instantaneous values \u200b\u200bof the heat of vaporization of water:
3% -\u003e 2.71 MJ / kg
4% -\u003e 2.61 MJ / kg
6% -\u003e 2.49 MJ / kg
9% -\u003e 2.41 MJ / kg
12% -\u003e 2.36 MJ / kg
15% -\u003e 2.34 MJ / kg
20% -\u003e 2.32 MJ / kg
28% -\u003e 2.30 MJ / kg
Further 2.3 MJ / kg
Below 3%, we will consider 3MJ / kg.
Well. We have universal data applicable to any wood, assuming that the original picture is also applicable to any wood. It is very good. Now let's consider the process of moistening wood and the corresponding drop in calorific value
let us have 1kg of dry residue, moisture 0gr, calorific value 18MJ / kg
moistened to 3% - added water 30g. The mass increased by these 30 grams, and the heat during combustion decreased by the heat of vaporization of these 30 grams. Total we have (18MJ-30/1000 * 3MJ) / 1.03kg \u003d 17.4MJ / kg
then moistened by another 1%, the mass increased by another 1%, and the latent heat increased by 0.0271 MJ. Total 17.2 MJ / kg
And so on, we recalculate all the values. We get:
0% -\u003e 18.0 MJ / kg
3% -\u003e 17.4 MJ / kg
4% -\u003e 17.2 MJ / kg
6% -\u003e 16.8 MJ / kg
9% -\u003e 16.3 MJ / kg
12% -\u003e 15.8 MJ / kg
15% -\u003e 15.3 MJ / kg
20% -\u003e 14.6 MJ / kg
28% -\u003e 13.5 MJ / kg
30% -\u003e 13.3 MJ / kg
40% -\u003e 12.2 MJ / kg
70% -\u003e 9.6 MJ / kg
Hooray! This data, again, does not depend on the type of wood.
In this case, the dependence is perfectly described by the parabola:
Q \u003d 0.0007143 * W ^ 2 - 0.1702W + 17.82
or linearly 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 moisture
I will consider two breeds. Pine and birch

To begin with, I rummaged around and decided to stop at the following data on wood density

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 saw cut, since the humidity is not determined.
Hence the density of birch is 2.10E-05x2 + 2.29E-03x + 6.00E-01
pine 1.08E-05x2 + 2.53E-03x + 4.70E-01
here x is humidity.
I will simplify to a linear expression in the range of 0-40%
It turns out
pine ro \u003d 0.47 + 0.003W
birch ro \u003d 0.6 + 0.003W
It would be nice to collect statistics according to the data, since the pine is 0.47 mb. and about business, but birch is easier, and 0.57 somewhere.

5. Bulk calorific value.
Now we calculate the calorific value of the volume of the capacity 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 fresh sawing to 10.8 for absolutely dry ones. In a practically significant range of 10-40%, from about 9 to 10 MJ / l ~ 2.6 kW * h / l

For pine
humidity density specific heat capacity 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 fresh sawing to 8.5 for absolutely dry ones. In a practically significant range of 10-40%, from about 7 to 8 MJ / L ~ 2.1 kW * h / L

6. About humidity of firewood.
Earlier I mentioned the practically significant range of 10-40%. I want to clarify. From the previous reasoning, it becomes obvious that it is more expedient to burn dry firewood than raw ones, 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 for the same 20 degrees above 30% the equilibrium humidity of the air next to such a tree is 100% (rel.). What does it mean? AK the fact that the log behaves like a puddle, and dries in all weather conditions, can even dry in the rain. The drying speed is limited only by diffusion, which means that the length of the log is not chopped.
By the way, the drying speed of a 35 cm long log is approximately equivalent to the drying speed of a fifty-piece board, while due to cracks in the log, the drying speed increases additionally compared to the board, and laying in single-row pollenitsa even improves drying compared to the board. It seems that in a couple of months in the summer in a single-row pollenitsa outside, you can reach a humidity of 30% and less than half a meter of firewood. Chipped naturally dry even faster.
I am ready to discuss if there are results.

It is not difficult to imagine what kind of log it looks and feels. It does not contain cracks in the end, it is slightly damp to the touch. If it lies in the water, mold and fungi may appear. Joyfully running around if the warmth of all kinds of bugs. Of course, he injects, but reluctantly. I think above 50% somewhere is not pricked practically at all. The ax / cleaver comes in with a "squelch" and the whole effect

Dry wood already has cracks and humidity of less than 20%. Already relatively easy to inject and burns perfectly.

What is 10%? We 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. These firewood burn - just have time to toss, flare up perfectly, light and “ringing” to the touch. Also excellently planed on splinters.

7. Smoke, charcoal, soot and ash
The main products of wood burning 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 a 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 essence, as suggested
Charcoal also contains 1-3% miner. impurities, ch. arr. carbonates and oxides K, Na, Ca, Mg, Si, Al, Fe.
And here ash what is non-combustible metal oxides. By the way, ash is used in the world as an additive to cement, too, clinker, in fact, only received for delivery (without additional energy costs).

soot
Elemental composition,
Carbon, C 89 - 99
Hydrogen, N 0.3 - 0.5
Oxygen, O 0.1 - 10
Sulfur, S 0.1 - 1.1
Minerals 0.5
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 they not only burn, but also have a high calorific value - at the level of 25 MJ / kg. I think the formation of both coal and soot is primarily associated with insufficient temperature in the furnace / lack of oxygen.

8. How much water vapor is formed during the combustion of wood
1 kg of dry wood contains 63 grams of hydrogen or
Water from these 63 grams during combustion will result in a maximum of 63 * 18/2 (we spend two grams of hydrogen to obtain 18 grams of water) \u003d 567 grams / kg_dwood.
The total amount of water formed during the combustion of wood will thus be
0% -\u003e 567 g / kg
10% -\u003e 615 g / kg
20% -\u003e 673 g / kg
40% -\u003e 805 g / kg
70% -\u003e 1033 g / kg

9.Hidden heat.
An interesting question is, and if the moisture formed during the combustion of wood is condensed and the heat obtained is taken away, how much is there? Let's estimate.
0% -\u003e 567 g / kg -\u003e 1.3 MJ / kg -\u003e 7.2% of the heat of combustion of firewood
10% -\u003e 615 g / kg -\u003e 1.4 MJ / kg -\u003e 8.8% of the heat of combustion of firewood
20% -\u003e 673 g / kg -\u003e 1.5 MJ / kg -\u003e 10.6% of the calorific value of firewood
40% -\u003e 805 g / kg -\u003e 1.9 MJ / kg -\u003e 15.2% of the heat of combustion of firewood
70% -\u003e 1033 g / kg -\u003e 2.4 MJ / kg -\u003e 24.7% of the heat of combustion of firewood
Here it is, in theory, the limit of the additive that can be squeezed out from water condensation. Moreover, if you heat with non-damp wood, then the entire marginal effect is within 8-15%

10. The amount of air required for burning wood
The second potential heat source for improving the efficiency of the boiler / furnace TP is heat extraction from the flue gas.
We already have all the necessary data, so we will not go into the sources. First you need to calculate the theoretical minimum air supply for burning wood. Dry to begin with.
Turn to paragraph 2

1 kg of firewood:
495g C -\u003e 41.3 mol
442g O2-\u003e 13.8 mol
63g H2-\u003e 31.5 mol
For the combustion of carbon, 41.3 moles of oxygen are needed and for the combustion of hydrogen, 15.8 moles of oxygen. Moreover, 13.8 moles of oxygen are already present. Total oxygen demand for combustion is 43.3 mol / kg_wood. from here need for air 216 mol / kg_ firewood \u003d 5.2 m3 / kg_wood (oxygen is one-fifth).
For different wood moisture we have
0% -\u003e 5.2 m3 / kg -\u003e 2.4 m3 / l_pine! 3.1 m3 / l_, birch
10% -\u003e 4.7 m3 / kg -\u003e 2.4 m3 / l_pine! 3.0 m3 / l_, birch
20% -\u003e 4.3 m3 / kg -\u003e 2.3 m3 / l_pine! 2.9 m3 / l_, birch
40% -\u003e 3.7 m3 / kg -\u003e 2.2 m3 / l_ pine! 2.7 m3 / l_, birch
70% -\u003e 3.1 m3 / kg -\u003e 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 weakly dependent on their moisture content.

At the same time, it is impossible to supply air less than the obtained value - there will be incomplete burnout of the fuel, the formation of 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 flue gases, and large losses in the pipe.

The coefficient of excess (gamma) of air is introduced as the ratio of the actual air supply to the theoretical minimum (5m3 / kg). The magnitude of the coefficient of excess can be different and is usually from 1 to 1.5.

10.1. Flue gas amount
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 from the furnace is greater than at the inlet and is calculated in terms of room temperature
0% -\u003e 5.9 m3 / kg, of which water vapor 0.76 m3 / kg
10% -\u003e 5.5 m3 / kg, of which water vapor 0.89 m3 / kg including evaporated 0.13
20% -\u003e 5.2 m3 / kg, of which water vapor 1.02 m3 / kg including evaporated 0.26
40% -\u003e 4.8 m3 / kg, of which water vapor 1.3 m3 / kg
70% -\u003e 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 the chimney needs to be maintained so that there is never condensation in it. (by the way, I don’t have any condensation in the pipe).
To do this, we find the temperature corresponding to the relative humidity of the flue gas for 70% of firewood. It is possible according to the schedule above. We are looking for 1.68 / 4.4 \u003d 0.38.
And here it is not 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 hot, there will be no condensation in it.

For excess factors greater than unity, the flue gas quantity should be considered as the calculated flue gas quantity (5.2 m3 / kg at 20%) plus (gamma-1) multiplied by the theoretically required air quantity (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 \u003d 6.1m3 / kg

11. Flue gas heat
We will restrict ourselves to the case in which the flue gas temperature is 200 degrees. I took one of the values \u200b\u200bfrom the link http://celsius-service.ru/?page_id\u003d766
And we will look for an excess of flue gas heat in comparison with room temperature - the potential for heat recovery. Let's take an excess air factor of 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 \u003d 1.097.
at zero, 1.295 and 1.042.
Please note that the density is connected according to the law of an ideal gas: 0.748 \u003d 1.295 * 273/473. And the heat capacity is almost constant. Since we operate with fluxes 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. Total heat capacity of our standard smoke cube is 1.29 kJ / m3 / K

0% -\u003e 6.9 m3 / kg -\u003e 1.6 MJ / kg -\u003e 8.9% of the heat of combustion of firewood
10% -\u003e 6.4 m3 / kg -\u003e 1.5 MJ / kg -\u003e 9.3% of the heat of combustion of firewood
20% -\u003e 6.1 m3 / kg -\u003e 1.4 MJ / kg -\u003e 9.7% of the heat of combustion of firewood
40% -\u003e 5.5 m3 / kg -\u003e 1.3 MJ / kg -\u003e 10.5% of the calorific value of firewood
70% -\u003e 5.0 m3 / kg -\u003e 1.2 MJ / kg -\u003e 12.1% of the calorific value of firewood

In addition to that, we will try to substantiate the difference between the literary calorific value of firewood 4400-50W and those obtained above 4306-35W. Justify the difference in the coefficient.
Suppose that the authors of the formula consider heat to heat additional steam to be the same losses as latent heat and wood shrinkage. Between 10 and 20% we have allocated an additional pair of 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 \u003d 30.4 KJ / kg_wood. One percent moisture is ten times less than 3 kJ / kg /% or 0.7 kcal / kg /%. Received not 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 waste. Losses through the walls are also unconditional (if not considered good). Latent heat - losses? Not. Latent heat from evaporated moisture sits in the reduced calorific value of firewood. In chemically formed water is a combustion product, and not a loss of power (it does not evaporate, but immediately forms in the form of steam).
In total, the limiting efficiency of the boiler / furnace is determined by the heat recovery potential (excluding condensation) written just above. And it is about 90% and no more than 91. To increase the 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 formation of soot should be expected - smoky and not 100% burning of wood -\u003e decrease in efficiency.

13. Total potential of heat recovery.
From the data presented above, it is quite simple to calculate for the case of cooling from flue gas 200 to 20 and moisture condensation. For the simplicity of all moisture.

0% -\u003e 2.9 MJ / kg-\u003e 16% of the heat of combustion of firewood
10% -\u003e 3.0 MJ / kg -\u003e 18.6% of the heat of combustion of firewood
20% -\u003e 3.0 MJ / kg -\u003e 20.6% of the heat of combustion of firewood
40% -\u003e 3.2 MJ / kg -\u003e 26.3% of the calorific value of firewood
70% -\u003e 3.6 MJ / kg -\u003e 37.4% of the heat of combustion of firewood
It should be noted that the values \u200b\u200bare quite noticeable. 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 the engineering calculation. In the indicated effect, about half is condensation, the rest is the heat capacity of the flue gas.

14. Once again on the dependence of the calorific value of firewood on moisture
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 heat to heat additional steam to be the same losses as latent heat and wood shrinkage. Between 10 and 20% we have allocated an additional pair of 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 \u003d 30.4 KJ / kg_wood. One percent moisture is ten times less than 3 kJ / kg /% or 0.7 kcal / kg /%. Received not 15. Still inconsistency.

Let's assume one more 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 relative.
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 item 1).
Based on these definitions, we construct the dependence of the absolute humidity on the relative
0% (rel) -\u003e 0% (abs)
10% (rel) -\u003e 9.1% (abs)
20% (rel) -\u003e 16.7% (abs)
40% (rel) -\u003e 28.6% (abs)
70% (rel) -\u003e 41.2% (abs)
100% (rel) -\u003e 50% (abs)
Separately, we again consider the interval 10-40. It can approximate the obtained dependence of the straight line W \u003d 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. Finally, we got a result much closer to the literature data.

15. About the calorific value of rotten firewood
In the case of stoking a fire in nature, including on barbecue, I probably like many people prefer to heat with dry wood. These firewood are rather rotten dry branches. They burn well, quite hot, but for the formation of a certain amount of coal it takes about twice as much as a normal dry-birch birch. But where can I get this dry birch in the forest? Therefore, I’m drowning with what is and what does not harm the forest. The same firewood is excellent for heating the stove / boiler in the house.
What is this dry forest? This is the same wood in which the process of decay usually took place, incl. directly at 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 at the root under certain conditions.
I'm talking about such firewood

You can also use rotten tree trunks if they are dry. It is very difficult to burn raw rotting wood, so we will not consider it for now.

I have never been able to measure the density of such firewood. But subjectively, this density is somewhere in one and a half times lower than ordinary pine (with wide tolerances). Based on this postulate, we calculate the volumetric heat capacity depending on the humidity, while the fire is usually dry wood from deciduous species, the density of which was initially higher than that of pine. Those. consider the case when a rotten log has a dry matter density of half that of the original wood.
Since for birch and pine the linear formulas for the density dependence have coincided (up to the density of absolutely dry firewood), then for rotten wood we also use this formula:
ro \u003d 0.3 + 0.003W. This is a very rough estimate, but no one seems to have done much research on the issue raised here. Mb Canadians have information, but they also have their own forest, with their own properties.
0% (0.30 kg / l) -\u003e 18.0 MJ / kg -\u003e 5.4 MJ / l \u003d 1.5 kW * h / l
10% (0.33 kg / l) -\u003e 16.1 MJ / kg -\u003e 5.3 MJ / l \u003d 1.5 kW * h / l
20% (0.36 kg / l) -\u003e 14.6 MJ / kg -\u003e 5.3 MJ / l \u003d 1.5 kW * h / l
40% (0.42 kg / l) -\u003e 12.2 MJ / kg -\u003e 5.1 MJ / l \u003d 1.4 kW * h / l
70% (0.51 kg / l) -\u003e 9.6 MJ / kg -\u003e 4.9 MJ / l \u003d 1.4 kW * h / l
Which is no longer particularly surprising the bulk calorific value of rotten firewood again weakly depends on humidity and is about 1.45 kW * h / l.

16. About the volumetric calorific value of any firewood.
In general, the rocks considered, including rot, can be combined under one formula for the 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 -\u003e 2.7 kW * h / l
0.53 kg / l -\u003e 2.1 kW * h / l
0.36 kg / l -\u003e 1.5 kW * h / l
Those. volumetric calorific value of air-dried firewood, regardless of breed, is approximately Q \u003d 4 * density (in kg / l), kW * h / l

Those. to understand what your specific firewood will give (various fruit, rotten, coniferous, etc.) You can determine the density of conditionally air-dry firewood once - by weighing and determining the volume. Multiply by 4 and apply the resulting value for almost any wood moisture.
I would make a similar measurement by making a short log (within 10 cm) close to a cylinder or rectangular parallelepiped (board). The goal is not to bother measuring volume and air dry quickly enough. I recall that along the fibers, drying is 6.5 times faster than across. And this 10cm dry little air in the summer in a week.

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Large coals after combustion and uniform heat are a sign of good raw materials.

Main criteria

The most important indicators for the combustion material are: density, humidity and heat transfer. All of them are closely related to each other and determine how effective and useful burning wood is. It is worth considering each of them in more detail, given the different types of wood and methods of harvesting it.

Density

The first thing that a competent buyer pays attention to when ordering a heating material made of wood is its density. The higher this indicator, the better the breed.

All wood species are divided into three main categories:

• low-density (soft);
• medium dense (moderately hard);
• high density (hard).

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 support the heat in the furnace.

Due to its hardness, such wood is difficult to process, so some consumers prefer medium-dense wood, such as birch or ash. Their structure makes it easy to chop the logs by hand.

Humidity

The second indicator is moisture, that is, the percentage of water in the wood structure. The higher this value, the greater the density, while the resource used will generate less heat with the same effort.

The specific heat of combustion of dry birch firewood is characterized as more productive than wet. It is worth noting this feature of birch: it can be put into the firebox almost immediately after felling, because it has a low moisture content. It is best to prepare the material properly to maximize the beneficial effect.

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 out. The number of days depends on the season and can range from 80 to 310 days.
• Part of the wood 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 required to prepare the wood is minimum.

Heat dissipation

An indicator such 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, subject to specific conditions.

The heat of combustion of firewood in hard rocks is the highest. Accordingly, the opposite is the case with softwood. 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.

It is worth mentioning such a property as the burning temperature of firewood. It is greatest in hornbeam, beech and ash, more than 1000 degrees Celsius, while the maximum amount of heat is produced at the level of 85-87%. Oak and larch are approaching them, and poplar and alder have the lowest indicators with a production of 39-47% at a temperature of about 500 degrees.

Wood species

The calorific value of firewood depends to the greatest extent 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

Needles are often the most readily available wood. Its low cost is due not only to the prevalence of spruce and pine trees, but also to its properties. The fact is that the heat capacity of firewood of such a plan is not high, and there are also many other disadvantages.

The main disadvantage of conifers is the presence of a large number of resins. When such firewood is heated, the resin begins to expand and boil, which as a result leads to the spread of sparks and burning fragments over a long distance. Also, resin leads to the formation of soot and burning, which clog the fireplace and chimney.

Deciduous

It is much more profitable to use hardwoods. All varieties are divided into three categories, depending on their density. Soft breeds include:

• linden;
• aspen;
• poplar;
• alder;

They burn out quickly and therefore have little value in terms of heating the house.

Medium dense trees include:

• maple;
• birch;
• larch;
• acacia;
• cherry.

The specific heat of combustion of birch firewood is close to the species that are classified as solid, in particular oak.

• hornbeam;
• nut;
• dogwood;

The calorific value of this type of firewood is maximum, but at the same time processing of wood is difficult due to its high density.

Oak is another popular fuel

The useful qualities of such breeds make them more expensive, but this reduces the amount of material that will be needed to maintain a comfortable temperature in the house.

Material selection

Even the highest qualities of wood can be nullified if it is chosen incorrectly taking into account a specific type of activity. For example, it doesn’t matter what was used for the night bonfire for gatherings with friends. A completely different matter is kindling a fireplace or stove in a bath.

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, since a sparkling log can even lead to a fire.

The unobtrusive burning of firewood and the heat coming from the fireplace is 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. Aspen and alder can be burned from time to time to clean the chimney. The density of these rocks is low, but they have the property of burning off soot.

For the bath

To ensure a high temperature in the steam room, the maximum heat transfer of firewood is required. In addition, you can improve the resting conditions if you use rocks 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, the best choice will, of course, be oak and birch logs. They are hard, give good heat in a small volume and also give off pleasant vapors. Linden and alder can also provide an additional healing effect. You can use only well-dried materials, but not older than one and a half to two years.

For barbecue

When cooking on the barbecue and barbecue, the main point is not the burning of firewood itself, but the formation of coal. This is why it doesn't make sense to use thin, loose branches. They can be taken only to light a fire, and then add large solid 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 will need more time for burning and decay than an apple tree, so it makes sense to take thicker fruit logs.

Alternative combustion materials

The calorific value of firewood of certain species is large enough, but far from the maximum possible. In order to save money and space for storing fuel material, more and more attention is now being paid to alternative options. Optimal is the use of pressed briquettes.

With the same furnace load, pressed wood generates much more heat. This effect is possible by increasing the density of the material. In addition, there is a much lower percentage of humidity here. Another plus is the minimum ash formation.

Briquettes and pellets are made from sawdust and wood chips. Due to the pressing of the waste, it is possible to create an incredibly dense furnace material that even the best types of wood cannot compare with. With a higher cost per cubic meter of briquettes, the resulting savings can be quite significant.

It is necessary to prepare and purchase heating materials based on 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.

Wood is a rather complex material in terms of its chemical composition.

Why are we interested in the chemical composition? But 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 wood species.

The main chemical binders in wood are lignin and cellulose. They form cells - a kind of container, inside of 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 variations in the chemical composition of different wood species determine the differences in the heating value of different wood species. The calorific value is measured in kilocalories - that is, the amount of heat obtained by burning one kilogram of a tree of a particular breed is calculated. There are no fundamental differences between the calorific values \u200b\u200bof different types of wood. And for household purposes, it is enough to know the average values.

Differences between rocks in calorific value look minimal. It should be noted 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 higher. However, on the market, firewood is supplied by volume, not by weight, so there will simply be more of them in one cubic meter of firewood harvested from hardwood.

Harmful impurities in wood

During the chemical combustion reaction, the wood does not burn completely. After combustion, ash remains - that is, not the burnt part of the wood, but moisture evaporates from the wood during combustion.

Ash affects the quality of combustion and the calorific value of firewood less. 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, when burning such wood, the lion's share of the energy released with the flame can simply go to evaporate the wood moisture itself, without doing any useful work.

The moisture present in the wood drastically reduces the calorific value of any firewood. Burning wood 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 wood does not burn out completely; when burning such firewood, a hanged amount of smoke is released, which pollutes both the chimney and the furnace space.

What is wood moisture content, what does it affect?

A physical quantity that describes the relative amount of water contained in wood is called moisture. The moisture content of the wood is measured in percent.

When measuring, two types of humidity can be taken into account:

• Absolute humidity is the amount of moisture that is currently in the wood relative to a fully dried wood. Such measurements are usually carried out for construction purposes.
• Relative humidity is the amount of moisture that is currently in the wood relative to its own weight. These 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 in an indicator of 150%.

Analyzing this formula, it can be established that firewood harvested from coniferous wood with a relative humidity of 12 percent when burning 1 kilogram will emit 3940 kilocalories, and firewood harvested from deciduous trees with a comparable humidity will emit 3852 kilocalories.

To understand what a relative humidity of 12 percent is - let us explain that such humidity is acquired by firewood, which is dried for a long time 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 the derivative of the density and the calorific value.

Experimentally, information was obtained on the specific calorific value of certain types of wood. Data is given for the same moisture content of 12 percent. Based on the results of the experiment, the following was compiled table:

Using the data from this table, you can easily compare the calorific value of different types of wood.

What kind of firewood can be used in Russia

Traditionally, the most favorite type of firewood for burning in brick ovens in Russia is birch. Although in essence birch is a weed, the seeds of which easily catch on any soil, it is extremely widely used in everyday life. An unpretentious and fast-growing tree faithfully served our ancestors for many centuries.

Birch firewood has a relatively good calorific value and burns rather slowly, evenly, without over-heating the stove. In addition, even the soot obtained from the combustion of birch firewood goes into business - it includes tar, which is used both for domestic and medicinal purposes.

In addition to birch, aspen, poplar and linden are used as firewood from hardwood. Their quality compared to birch, of course not very, but in the absence of others it is quite possible to use such firewood. In addition, linden wood, when burned, emits a special aroma that is considered beneficial.

Firewood from aspen gives a high flame. They can be used at the final stage of the combustion to burn off the soot formed when burning other wood.

Alder also burns quite evenly, and after combustion it leaves a small amount of ash and soot. But again, by the sum of all, 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 prepare food, especially baked goods.

Firewood harvested from fruit trees is quite rare. Such firewood, and especially maple, burns very quickly and the flame when burning reaches a very high temperature, which can adversely affect the condition of the stove. In addition, you just need to heat the air and water in the bath, and not to melt the metal in it. When using such firewood, they must be mixed with firewood with low calorific value.

Coniferous wood is rarely used. Firstly, such wood is very often used for construction purposes, and secondly, the presence of a large amount of resin in coniferous trees contaminates furnaces and chimneys. It makes sense to heat the stove with coniferous wood only after prolonged drying.

How to harvest firewood

Firewood harvesting usually begins in late autumn or early winter, before a permanent snow cover is established. The felled trunks are left on the plots for primary drying. After some time, usually in winter or early spring, firewood is removed from 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 technological development, firewood can be prepared all year round. Enterprising people can bring you already sawn and chopped firewood any day for a reasonable fee.

How to cut and chop wood

Saw the delivered log into pieces suitable for the size of your firebox. After that, 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 chopped into logs so that the cross-section of the resulting log is about 80 square cm. Such firewood will burn for a rather long time in a bath stove and generate more heat. Smaller logs are used for kindling.

Chopped logs are stacked in a woodpile. It is intended not only for accumulating fuel, but also for drying firewood. A good woodpile will be located in an open area, 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 with wet soil.

Drying of wood to an acceptable moisture content takes about a year. In addition, wood dries much faster in logs than in logs. Chopped wood reaches an acceptable moisture content already in three months of summer. When the wood is dried for a year, the woodpile will get 15 percent moisture, which is ideal for combustion.

Calorific value of firewood: video