Calorific value of wood. Woody biomass Ash content of birch firewood

Table 1 - Content of ash and ash elements in wood of various tree species

Woody plant	Ash,
														Sum
Pine	0,27	1111,8	274,0	53,4	4,08		5,59	1,148	0,648	0,141	0,778	0,610	0,191	1461,3
Spruce	0,35	1399,5	245,8	11,0	9,78	12,54	7,76	1,560	1,491	0,157	0,110	0,091	0,041	1689,8
Fir	0,46	1269,9	1001,9	16,9	16,96	6,85	6,16	1,363	2,228	0,237	0,180	0,098	0,049	2322,8
Larch	0,22	845,4	163,1		23,80	13,34	3,41	1,105	0,790	0,194	0,141	0,069	0,154	1057,4
Oak	0,31	929,7	738,3	14,4	7,88	3,87	1,29	2,074	0,987	0,524	0,103	0,082	0,024	1699,2
Elm	1,15	2282,2	2730,3	19,2	4,06	10,05	4,22	2,881	1,563	0,615	0,116	0,153	0,050	5055,4
Linden	0,52	1860,9	792,6	12,3	9,40	8,25	2,58	1,199	1,563	0,558	0,136	0,102	0,043	2689,6
Birch	0,45	1632,8	541,0	17,8	23,81	4,30	20,12	1,693	1,350	0,373	0,163	0,105	0,081	2243,6
Aspen	0,58	2100,7	781,4	12,4	5,70	9,19	12,99	1,352	1,854	0,215	0,069	0,143	0,469	2926,5
Poplar	1,63	4759,3	1812,0	18,1	8,19	17,18	15,25	1,411	1,737	0,469	0,469	0,273	0,498	6634,8
Alder black	0,50	1212,6	599,6	131,1	15,02	4,10	5,08	2,335	1,596	0,502	0,251	0,147	0,039	1972,4
Gray alder	0,43	1623,5	630,3	30,6	5,80	6,13	9,35	2,059	1,457	0,225	0,198	0,152	0,026	2309,8
Bird cherry	0,45	1878,0	555,6		4,56	11,49	4,67	1,599	1,287	0,347	0,264	0,124	0,105	2466,0

All tree species, based on the content of ash elements in their wood, are combined into two large clusters (Fig. 1). The first, headed by Scots pine, includes black alder, aspen and balsam poplar (Berlin), and the second includes all other species, led by spruce and bird cherry. A separate subcluster consists of light-loving species: silver birch and Siberian larch. The smooth elm stands apart from them. The greatest differences between clusters No. 1 (pine) and No. 2 (spruce) are noted in the content of Fe, Pb, Co and Cd (Fig. 2).

Figure 1 - Dendrogram of the similarity of tree species based on the ash composition of their wood, constructed using the Ward method using a matrix of normalized data

Figure 2 - Nature of difference woody plants belonging to different clusters, according to the ash composition of their wood

Conclusions.

1. Most of all the wood of all tree species contains calcium, which is the basis of the cell membrane. This is followed by potassium. There is an order of magnitude less iron, manganese, strontium and zinc in wood. Ni, Pb, Co and Cd close the rank series.

3. Tree species growing within the same floodplain biotope differ significantly from each other in the efficiency of their use nutrients. The most effective use of soil potential is Siberian larch, 1 kg of whose wood contains 7.4 times less ash than poplar wood, the most environmentally wasteful species.

4.The property of high consumption of minerals by a number of woody plants can be used in phytomelioration when creating plantings on technogenic or naturally polluted lands.

List of sources used

1. Adamenko, V.N. Chemical composition tree rings and the state of the natural environment / V.N. Adamenko, E.L. Zhuravleva, A.F. Chetverikov // Dokl. Academy of Sciences of the USSR. - 1982. - T. 265, No. 2. - P. 507-512.

2. Lyanguzova, I.V. Chemical composition of plants under atmospheric and soil pollution/ I.V. Lyanguzova, O.G. Devil // Forest ecosystems and atmospheric pollution. - L.: Nauka, 1990. P. 75-87.

3. Demakov, Yu.P. Variability of the content of ash elements in wood, bark and needles of Scots pine / Yu.P. Demakov, R.I. Vinokurova, V.I. Talantsev, S.M. Shvetsov // Forest ecosystems in a changing climate: biological productivity, monitoring and adaptation technologies: materials international conference with elements scientific school for youth [Electronic resource]. - Yoshkar-Ola: MarSTU, 2010. P. 32-37. http://csfm.marstu.net/publications.html

4. Demakov, Yu.P. Dynamics of the content of ash elements in the annual rings of old-growth pines growing in floodplain biotopes / Yu.P. Demakov, S.M. Shvetsov, V.I. Talantsev // Bulletin of MarSTU. Ser. "Forest. Ecology. Nature management". 2011. - No. 3. - P. 25-36.

5. Vinokurova, R.I. Specificity of the distribution of macroelements in the organs of woody plants of spruce-fir forests of the Republic of Mari El / R.I. Vinokurova, O.V. Lobanova // Bulletin of MarSTU. Ser. "Forest. Ecology. Nature management." - 2011. - No. 2. - P. 76-83.

6. Akhromeyko A.I. Physiological justification for creating sustainable forest plantations / A.I. Akhromeiko. – M.: Forest industry, 1965. – 312 p.

7. Remezov, N.P. Consumption and circulation of nitrogen and ash elements in the forests of the European part of the USSR / N.P. Remezov, L.N. Bykova, K.M. Smirnova.- M.: MSU, 1959. – 284 p.

8. Rodin, L.E. Dynamics organic matter and biological cycle of ash elements and nitrogen in the main types of vegetation of the globe / L.E. Rodin, N.I. Bazilevich. – M.-L.: Nauka, 1965. -

9. Methodology for measuring the gross content of copper, cadmium, zinc, lead, nickel, manganese, cobalt, chromium using atomic absorption spectroscopy. – M.: FGU FCAO, 2007. – 20 p.

10. Methods of biogeochemical research of plants / Ed. A.I. Ermakova. – L.: Agropromizdat, 1987. – 450 p.

11. Afifi, A. Statistical analysis. Computer approach / A. Afifi, S. Eisen. - M.: Mir, 1982. - 488 p.

12. Factor, discriminant and cluster analysis / J. Kim, C. Muller, U. Klekka, etc. - M.: Finance and Statistics, 1989. - 215 p.

The calorific value of firewood depends on the type of tree and its humidity

We call firewood pieces of wood used in rapid oxidation reactions with atmospheric oxygen to produce light and heat. We simply light a fire on the ground after going on a picnic. Or in special devices- barbecues, hearths, boilers, stoves, takyrs or others.

There are various types of firewood, the amount of heat obtained from burning it, divided by mass (volume), is called specific heat combustion of heating oil. The calorific value of firewood depends on the type of tree and its moisture content. In addition, the completeness of combustion and the efficiency of combustion energy utilization depend on other factors. Different stoves, draft force, chimney design - everything affects the result.

The essence of the physical parameter

Energy is measured in “joules” - the amount of work done to move 1 meter when a force of 1 newton is applied in the direction of application. Or in “calories” - the amount of heat required to heat 1 g of water by 1 °C at a pressure of 760 mm Hg. An international calorie corresponds to 4.1868 Joules.

The specific heat capacity of a fuel is the amount of heat produced by complete combustion divided by the mass or volume of the fuel.

The value is not constant, since firewood can vary greatly, and this parameter also varies accordingly. In the laboratory, specific heat is measured by combustion in special devices. The result is true for a specific sample, but only for that sample.

The total specific heat of heating oil is measured with simultaneous cooling of combustion products and condensation of evaporated water - to take into account the ENTIRE amount of energy received.

In practice, working rather than specific heat of combustion is more often used, without taking into account all the energy received.

The essence of the combustion process

If you heat wood, then at 120–150 ˚C it becomes dark in color. This is a slow charring, turning into charcoal. Raising the temperature to 350–350 ˚С, we will see thermal decomposition, blackening with the release of white or brown smoke. When heated further, the released pyrolysis gases (CO and volatile hydrocarbons) will ignite, turning into flames. After burning for some time, the amount of volatile substances will decrease, and the coals will continue to burn, but without a flame. In practice, to ignite and maintain combustion, the wood must be heated to 450–650 ˚C.

Wood burning process

Further combustion temperature heating oil in the firebox ranges from approximately 500 ˚С (poplar) to 1000 and higher (ash, beech). This value greatly depends on the draft, the design of the furnace and many other factors.

Humidity dependent

The higher the humidity, the worse the combustion, the lower the efficiency of the stove, and the more difficult it is to ignite and maintain the fire. And the calorific value of firewood is lower.

Indicators of calorific value (the amount of heat released during complete combustion of 1 kg of firewood, depending on humidity)

Both the specific heat of heating oil and its utilization rate decrease. The reasons are as follows.

1. The water in the composition reduces the amount of fuel as such: at a humidity of 50%, the firewood contains half the water. And it won’t burn...
2. Part of the heating oil energy will be spent on heating and moisture evaporation.
3. Wet wood conducts heat better, which makes it difficult to warm up the part of the log being set on fire to the combustion temperature.

Freshly cut wood varies in moisture content depending on the time of felling, the type of tree, and the place of growth, but on average it contains about 50% water.

That's why they put it in woodpiles under a canopy. During storage, some of the moisture will evaporate. When humidity decreases from 50 to 20%, the specific heat of combustion of heating oil approximately doubles.

Density dependence

Oddly enough, the composition of trees of different species is similar: 35–46% cellulose, 20–28% lignin + esters, resins, and other substances. And the difference in the heat of combustion of heating oil is due to porosity, that is, how much space the voids occupy. Accordingly, the denser the tree, the greater the calorific value of firewood from it. High-quality fuel pellets obtained by drying and pressing wood waste have a density of 1.1 kg/dm 3, that is, higher than the density of water. In which they drown.

Economic features of various firewood

The shape matters: the smaller the logs, the easier they ignite and burn faster. It is clear that the length also depends on the design: it is impossible to place too long ones in a stove or fireplace, the ends protrude outward. Too short - extra labor when cutting or chopping. The combustion temperature of firewood depends on the amount of humidity, the type of wood, and the amount of air supplied. The temperature is lowest when burning firewood from poplar, highest when burning hardwood: ash, mountain maple, oak.

The importance of humidity was written above. Not only the heat transfer of fuel in the furnace, but also the labor costs for splitting or sawing greatly depend on it. It is easier to split and saw damp, freshly cut wood. However, it is too wet and viscous, which makes it hurt badly. The butt part is denser, and uprooted stumps and areas near knots have increased strength. There the layers of wood are intertwined, which makes it much stronger. Oak splits well in the longitudinal direction, which has been used by coopers since ancient times. Getting shingles, shingles, and splitting firewood has its secrets.

Spruce is a “shooting” species, which is why it is undesirable for use in fireplaces or fires. When heated, the internal “bubbles” with resin boil and throw burning particles quite far away, which is dangerous: it is easy to burn clothes near a fire. Or it may cause a fire near the fireplace. In a closed furnace firebox this does not matter. Birch produces a hot flame and is excellent firewood. But with poor draft, a lot of resinous substances are formed (before they used to make birch tar), and a lot of soot is deposited. Alder and aspen, on the contrary, produce little soot. Matches are mainly made from aspen.

In practice, it is convenient to immediately saw and split freshly cut firewood. Then stack it under the awnings, making woodpiles so that air passes through, drying the fuel and increasing heat transfer. Chopping wood is a labor-intensive task, so when buying, pay attention to this. In addition, they will bring you stacked or bulk firewood.

In the second case, heating oil is placed in a “loose” body, and the client pays partly for the air. In addition, liquid or gaseous fuel used for heating has an advantage: it is easy to automate the supply. Firewood requires a lot of manual work. All this should be taken into account when choosing a stove or boiler for your home.

Video: How to choose firewood for the firebox

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

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

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

To fire stoves and fireplaces, they mainly use wood from such species as oak, ash, birch, hazel, yew, and hawthorn.

Features of burning firewood of different types of wood:

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

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

Birch firewood is good for heat, but if there is not enough air in the firebox, it burns smoky and forms tar (birch resin), which settles on the walls of the pipe;

Stumps and roots provide intricate patterns of fire;

Juniper, cherry and apple branches give a pleasant aroma;

Pine firewood burns hotter than spruce firewood due to its higher resin content. When tarred wood burns, a sharp increase in temperature causes small cavities in the wood to burst with a bang, in which resin accumulates, and sparks fly in all directions;

Oak firewood has the best heat transfer; its only drawback is that it splits poorly, just like hornbeam firewood;

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

Firewood made from medium-hard species is generally easy to split;

Long-smoldering coals provide cedar firewood;

Cherry and elm wood smokes when burned;

Plane wood burns easily, but is difficult to split;

Coniferous wood is less suitable for heating because it contributes to the formation of resinous deposits in the pipe and has a low calorific value. Pine and spruce firewood are easy to split and melt, but they smoke and spark;

Tree species with soft wood also include poplar, alder, aspen, and linden. Firewood of these species burns well, poplar firewood sparks strongly and burns out very quickly;

Beech - firewood of this species is considered classic fireplace wood, since beech has beautiful picture flames and good development of heat with almost complete absence of sparks. To all of the above, it should be added that beech firewood has a very high calorific value. The smell of burning beech wood is also highly rated, which is why beech wood is mainly used for smoking food. Beech firewood is universal in use. Based on the above, the cost of beech firewood is high.

It is necessary to take into account the fact that the calorific value of firewood of different types of wood varies greatly. As a result, we get fluctuations in wood density and fluctuations in conversion factors cubic meter => storage meter

Below is a table with average calorific values ​​per meter of firewood.

Firewood (natural drying)	Calorific value kWh/kg	Calorific value mega Joule/kg	Calorific value MWh/ storage meter	Bulk density in kg/dm³	Density kg/ storage meter
Hornbeam firewood	4,2	15	2,1	0,72	495
Beech firewood	4,2	15	2,0	0,69	480
Ash firewood	4,2	15	2,0	0,69	480
Oak firewood	4,2	15	2,0	0,67	470
Birch firewood	4,2	15	1,9	0,65	450
Larch firewood	4,3	15,5	1,8	0,59	420
Pine firewood	4,3	15,5	1,6	0,52	360
Spruce firewood	4,3	15,5	1,4	0,47	330

1 dry wood storage gauge deciduous trees replaces about 200 to 210 liters of liquid fuel or 200 to 210 m³ of natural gas.

Tips for choosing wood for a fire.

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

Strong wind increases the burning rate by 2-3 times. Humid, calm weather, on the contrary, slows down combustion. A fire can burn even during rain, but for this it is necessary to constantly maintain it. When it rains, you shouldn’t put thick logs on the fire; they take longer to burn and the rain can simply put them out. Don't forget, thinner branches flare up quickly, but also burn out quickly. They should be used to light thicker branches.

Before I talk about some of the properties of wood during combustion, I would like to remind you once again that if you are not forced by the need to spend the night in close proximity to a fire, try to burn a fire no closer than 1-1.5 meters from the edge of your bed.

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

Spruce, Like all resinous tree species, it burns hot and quickly. If the wood is dry, the fire spreads across the surface quite quickly. If you do not have the ability to somehow divide the trunk of a small tree into relatively small equal parts, and you use the entire tree for a fire, be very careful. Fire on wood can go beyond the boundaries of the fire pit and cause a lot of trouble. In this case, clear enough space for the fire pit so that the fire cannot spread further. Spruce has the ability to “shoot”. During combustion, the resin contained in the wood begins to boil under the influence of high temperatures and, finding no way out, explodes. The piece of burning wood that is at the top flies away from the fire. Probably many who burned a fire noticed this phenomenon. To protect yourself from such surprises, just place the logs with the end facing you. The coals usually fly perpendicular to the trunk.

Pine. Burns hotter and faster than spruce. It breaks easily if the tree is no more than 5-10 cm thick in diameter. "Shoots." Thin dry branches are well suited as second and third firewood for starting a fire.

Fir. Home distinctive feature is that it practically does not “shoot”. Dead wood trunks with a diameter of 20-30 cm are very well suited for “nodya”, a fire for the whole night. Burns hot and evenly. Burning rate between spruce and pine.

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

Properties of wood for burning

Wood suitable for burning is divided into the following main categories:

Softwood	Hardwood Soft breeds	Hardwood Hard rocks
Pine, spruce, thuja and others	Linden, aspen, poplar and others	Oak, birch, hornbeam and others
They are characterized by a high content of resin, which does not burn completely and clogs the chimney and internal parts of the firebox with its residues. When using such fuel, the formation of soot on the glass of the fireplace, if any, is inevitable. This type of fuel is characterized by longer drying of firewood.	Due to their low density, firewood from such species burns quickly, does not form coals, and has a low specific calorific value.	Firewood made from such wood species ensures a stable operating temperature in the firebox and high specific calorific value.

When choosing fuel for a fireplace or stove, the moisture content of the wood is of great importance. The calorific value of firewood largely depends on humidity. It is generally accepted that the best way Firewood with a moisture content of no more than 25% is suitable for burning. Indicators of calorific value (the amount of heat released during complete combustion of 1 kg of firewood, depending on humidity) are indicated in the table below:

Firewood for burning must be prepared carefully and in advance. Good firewood must dry for at least a year. The minimum drying time depends on the month the woodpile was laid (in days):

Another important indicator that characterizes the quality of firewood for heating a fireplace or stove is the density or hardness of the wood. Hard deciduous wood has the greatest heat transfer, while softwood has the least. The density of wood at a moisture content of 12% is shown in the table below:

Specific calorific value of wood of various species.

Humidity

The moisture content of woody biomass is a quantitative characteristic showing the moisture content in the biomass. A distinction is made between absolute and relative humidity of biomass.

Absolute humidity is called the ratio of the mass of moisture to the mass of dry wood:

Where W a is absolute humidity, %; m is the mass of the sample in a wet state, g;

m 0 - mass of the same sample, dried to a constant value, g. Relative or operating humidity

The ratio of the mass of moisture to the mass of wet wood is called:

Where W p is relative, or operating, humidity, %

There are two forms of moisture contained in woody biomass: bound (hygroscopic) and free. Bound moisture is located inside the cell walls and is held by physicochemical bonds; Removing this moisture involves additional energy costs and significantly affects most of the properties of the wood substance.

Free moisture is found in cell cavities and intercellular spaces.

Free moisture is retained only by mechanical bonds, is removed much more easily and has less impact on the mechanical properties of wood. When wood is exposed to air, moisture is exchanged between the air and the wood substance. If the moisture content of the wood substance is very high, this exchange causes the wood to dry out. If its humidity is low, the wood substance is moistened. At long stay wood in air, stable temperature and relative humidity

air humidity of wood also becomes stable; this is achieved when the water vapor pressure of the surrounding air becomes equal to the water vapor pressure at the surface of the wood. The amount of stable moisture content in wood kept for a long time at a certain temperature and air humidity is the same for all tree species.

Steady humidity is called equilibrium, and it is completely determined by the parameters of the air in which it is located, i.e., its temperature and relative humidity. Moisture content of stem wood. Depending on the moisture content, stem wood is divided into wet, freshly cut, air-dry, room-dry and absolutely dry.

Wet wood is called

long time

located in water, for example during rafting or sorting in a water basin. The moisture content of wet wood W p exceeds 50%. Freshly cut wood is wood that has retained the moisture of the growing tree. It depends on the type of wood and varies within the range W p =33...50%.

Room-dry wood is wood that has been in a heated and ventilated room for a long time. Humidity of room-dry wood W p =7...11%.

Absolutely dry - wood dried at a temperature of t=103±2 °C to constant weight.

In a growing tree, the moisture content of the stem wood is unevenly distributed. It varies both along the radius and along the height of the trunk.

The maximum moisture content of stem wood is limited by the total volume of cell cavities and intercellular spaces. When wood rots, its cells are destroyed, resulting in the formation of additional internal cavities; the structure of rotten wood, as the decay process progresses, becomes loose and porous, and the strength of the wood is sharply reduced.

By stated reasons The moisture content of wood rot is not limited and can reach as high as high values, at which its combustion will become ineffective.

The increased porosity of rotten wood makes it very hygroscopic; being in the open air, it quickly becomes moisturized.

Ash content Ash content

refers to the content of mineral substances in the fuel that remain after complete combustion of the entire combustible mass. Ash is an undesirable part of the fuel, as it reduces the content of combustible elements and complicates the operation of combustion devices.

Ash is divided into internal, contained in wood matter, and external, which got into the fuel during the procurement, storage and transportation of biomass.

Depending on the type, ash has different fusibility when heated to high temperatures.

Low-melting ash is ash that has a temperature of the onset of the liquid-melting state below 1350°C.

Medium-melting ash has a temperature of the beginning of the liquid-melting state in the range of 1350-1450 °C.

For refractory ash, this temperature is above 1450 °C.

The internal ash of woody biomass is refractory, and the external ash is low-melting.	The ash content of the bark of various species varies from 0.5 to 8% and higher in case of severe contamination during harvesting or storage.
	Wood Density	The density of woody matter is the ratio of the mass of the material forming the cell walls to the volume it occupies. The density of wood substance is the same for all types of wood and is equal to 1.53 g/cm3. According to the recommendation of the CMEA commission, all indicators of the physical and mechanical properties of wood are determined at an absolute humidity of 12% and are converted to this humidity.
Larch	660	630
Pine	500	470
Density of different types of wood	435	410
Fir	375	350
Breed	800	760
Density kg/m3	800	760
Pear	710	670
Oak	690	650
Maple	690	650
Common ash	680	645
Beech	670	640
Elm	650	615
Birch	630	600
Alder	520	490
Aspen	495	470
Linden	495	470
Willow	455	430

The bulk density of waste in the form of various shredded wood wastes varies widely. For dry chips from 100 kg/m 3, up to 350 kg/m 3 and more for wet chips.

Thermal characteristics of wood

Woody biomass in the form in which it enters the furnaces of boiler units is called working fuel. The composition of woody biomass, i.e. the content in it individual elements, is characterized by the following equation:
C р +Н р +О р +N р +A р +W р =100%,
where C p, H p, O p, N p are the content of carbon, hydrogen, oxygen and nitrogen in the wood pulp, respectively, %; A p, W p - ash and moisture content in the fuel, respectively.

To characterize fuel in thermal engineering calculations, the concepts of dry mass and combustible mass of fuel are used.

Dry weight In this case, the fuel is biomass dried to an absolutely dry state. Its composition is expressed by the equation
C s +H s +O s +N s +A s =100%.

Combustible mass fuel is biomass from which moisture and ash have been removed. Its composition is determined by the equation
C g + N g + O g + N r = 100%.

The indices of the signs of biomass components mean: p - the content of the component in the working mass, c - the content of the component in the dry mass, g - the content of the component in the combustible mass of fuel.

One of the remarkable features of stem wood is the amazing stability of its elemental composition of combustible mass. That's why The specific heat of combustion of different types of wood is practically the same.

The elemental composition of the combustible mass of stem wood is almost the same for all species. As a rule, the variation in the content of individual components of the combustible mass of stem wood is within the error of technical measurements. Based on this, during thermotechnical calculations, setting up combustion devices that burn stem wood, etc., it is possible to accept the following composition of stem wood for fuel without a large error mass: C g =51%, N g =6.1%, O g =42.3%, N g =0.6%.

Heat of combustion Biomass is the amount of heat released during the combustion of 1 kg of a substance. There are higher and lower calorific values.

Higher calorific value- this is the amount of heat released during the combustion of 1 kg of biomass with the complete condensation of all water vapor formed during combustion, with the release of heat spent on their evaporation (the so-called latent heat of evaporation).
Q in =340С р +1260Н р -109О р.

Net calorific value(NTS) - the amount of heat released during the combustion of 1 kg of biomass, excluding the heat spent on the evaporation of moisture formed during the combustion of this fuel. Its value is determined by the formula (kJ/kg):
Q р =340C р +1030H р -109О р -25W р.

The heat of combustion of stem wood depends only on two quantities: ash content and humidity. The lower heat of combustion of the combustible mass (dry, ash-free!) stem wood is almost constant and equal to 18.9 MJ/kg (4510 kcal/kg).

Types of wood waste

Depending on the production in which wood waste is generated, it can be divided into two types: logging waste and wood processing waste.

Logging waste- These are the separated parts of wood during the logging process. These include needles, leaves, non-lignified shoots, branches, twigs, tips, butts, peaks, trunk cuttings, bark, waste from the production of crushed pulpwood, etc.

In its natural form, logging waste is poorly transportable; when used for energy, it is first crushed into chips.

Wood waste- This is waste generated in woodworking production. These include: slabs, slats, cuttings, short lengths, shavings, sawdust, production waste of industrial chips, wood dust, bark.

Based on the nature of biomass, wood waste can be divided into the following types: waste from crown elements; stem wood waste; bark waste; wood rot.

Depending on the shape and particle size, wood waste is usually divided into the following groups: lump wood waste and soft wood waste.

Lump wood waste- these are cutouts, visors, cutouts, slabs, laths, cuts, short lengths. Soft wood waste includes sawdust and shavings.

The most important characteristic of crushed wood is its fractional composition.

Fractional composition is the quantitative ratio of particles of certain sizes in the total mass of crushed wood. The crushed wood fraction is the percentage of particles of a certain size in the total mass.

• — Shredded wood can be divided into the following types according to particle size: wood dust , formed when sanding wood, plywood and wood boards
• — ; the main part of the particles passes through a sieve with a hole of 0.5 mm;
• — wood chips obtained by grinding wood and wood waste in chippers;
• the main part of the chips passes through a sieve with 30 mm holes and remains on a sieve with 5...6 mm holes;

— large chips, the particle size of which is more than 30 mm.

Let us separately note the features of wood dust. Wood dust generated during sanding of wood, plywood, particle boards and fibreboards cannot be stored either in buffer warehouses of boiler houses or in off-season storage warehouses for small wood fuels due to its high windage and explosion hazard. When burning wood dust in combustion devices, it is necessary to ensure compliance with all rules for the combustion of pulverized fuel, preventing the occurrence of flashes and explosions inside combustion devices and in the gas paths of steam and hot water boilers. Wood sanding dust is a mixture of wood particles averaging 250 microns in size with abrasive powder separated from the sanding paper during the sanding process of wood material. Content

abrasive material

in wood dust can reach up to 1% by weight. Features of burning woody biomass

Important feature

The product of coking woody biomass, charcoal, is highly reactive compared to fossil coals.

The high reactivity of charcoal makes it possible to operate combustion devices at low values ​​of the excess air coefficient, which has a positive effect on the efficiency of boiler plants when burning woody biomass in them. However, along with these positive properties wood has features that negatively affect the operation of boilers. Such features, in particular, include the ability to absorb moisture, i.e., an increase in humidity in the aquatic environment. With increasing humidity, the lower calorific value quickly decreases and increases fuel consumption, combustion becomes difficult, which requires special

constructive solutions in boiler and furnace equipment. At a humidity of 10% and an ash content of 0.7%, the NCV will be 16.85 MJ/kg, and at a humidity of 50% only 8.2 MJ/kg. Thus, the fuel consumption of the boiler at the same power will change by more than 2 times when switching from dry fuel to wet fuel. Characteristic feature wood as a fuel has an insignificant internal ash content (does not exceed 1%).

At the same time, external mineral inclusions in logging waste sometimes reach 20%. The ash formed during the combustion of pure wood is refractory, and its removal from the combustion zone of the furnace does not present any particular technical difficulty. Mineral inclusions in woody biomass are fusible.

The term heat output was proposed at one time by D.I. Mendeleev as a characteristic of the fuel, reflecting its quality from the point of view of its ability to be used for high-temperature processes. The higher the heat output of the fuel, the higher the quality of the thermal energy released during its combustion, the higher the operating efficiency of steam and hot water boilers. Heat output represents the limit to which the actual temperature in the furnace approaches as the combustion process improves.

The heat output of wood fuel depends on its moisture content and ash content. The heat output of absolutely dry wood (2022 °C) is only 5% lower than the heat output of liquid fuel.

When the wood moisture content is 70%, the heat output decreases by more than 2 times (939 °C). Therefore, a humidity of 55-60% is the practical limit for using wood for fuel purposes.

The influence of the ash content of wood on its heat performance is much weaker than the influence of humidity on this factor.

The influence of woody biomass moisture content on the efficiency of boiler plants is extremely significant. When burning absolutely dry woody biomass with low ash content, the operating efficiency of boiler units, both in terms of their productivity and efficiency, approaches the operating efficiency of liquid fuel boilers and, in some cases, exceeds the operating efficiency of boiler units using certain types of coal.

An increase in the humidity of woody biomass inevitably causes a decrease in the efficiency of boiler plants. You should know this and constantly develop and carry out measures to prevent atmospheric precipitation, soil water, etc. from getting into wood fuel. The ash content of woody biomass makes it difficult to burn. The presence of mineral inclusions in woody biomass is due to the use of insufficiently advanced technological processes for wood harvesting and its primary processing. Preference should be given to these

The fractional composition of crushed wood should be optimal for this type of combustion device. Deviations in particle size from the optimal, both upward and downward, reduce the efficiency of combustion devices. Chips used to chop wood into fuel chips should not produce large deviations in particle size in the direction of increasing them. However, the presence of a large number of too small particles is also undesirable.

To ensure efficient combustion of wood waste, it is necessary that the design of boiler units meet the characteristics of this type of fuel.

I will write a summary here on the issues under consideration, 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%) 2.6 kW*h/l
3. Volumetric calorific value of pine (10-40%) 2.1 kW*h/l
4. Drying 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. When dry wood burns, 567 grams of water per kilogram of firewood is released.
7. The theoretical minimum air supply for combustion is 5.2 m3/kg_dry_firewood. Normal air supply is about 3m3/l_pine and 3_5 m3/l_birch.
8. In a chimney whose internal wall temperature is above 75 degrees, condensation does not form (with firewood up to 70% humidity).
9. The efficiency of the boiler/furnace heater without heat recovery cannot exceed 91% at a temperature flue gases 200 degrees
10. A flue gas heat exchanger with steam condensation can, in the limit, return up to 30% or more of the heat of combustion of firewood, depending on its 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 humidity
12. The volumetric calorific value of rotten firewood 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-dried firewood of this type, multiply by 4 and obtain the calorific value in kWh liters of this firewood almost regardless of humidity. I'll 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 moisture content of firewood.
7. Smoke, charcoal, soot and ash
8. How much water vapor is produced when wood burns?
9.Latent heat.
10. The 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. About the calorific value of rotten firewood
16. About the volumetric calorific value of any firewood.

Finished for now. I will be glad to add additions and constructive comments/suggestions.

1. General Provisions.
Let me make a reservation right away that it turned out that by wood moisture content I mean two different concepts. I will further operate only with the moisture content that is discussed for lumber. Those. the mass of water in the tree divided by the mass of the dry residue, and not the mass of water divided by the total mass.

Those. 100% humidity means that a ton of firewood contains 500 kg of water and 500 kg of absolutely dry firewood

Concept one. It is of course 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. At the same time, we buy firewood by the cubic meter, not by the ton.
We buy coal in tons, so its calorific value is primarily interesting per kg.
We buy gas by the cubic meter, so the calorific value of the gas is interesting per cubic meter.
Coal has a calorific value of about 25 MJ/kg, and gas about 40 MJ/m3. About firewood they write from 10 to 20 MJ/kg. Let's figure it out. Below we will see that the volumetric calorific value, unlike the mass value for firewood, does not change that much.

2. Calorific value of absolutely dry wood.
To begin with, we will determine the calorific value of completely dry firewood (0%) simply by the elemental composition of the wood.
Hence, I believe that the percentages are given on a mass basis.
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, are present 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 provide 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 present in the firewood is associated with carbon, in the second with hydrogen
We count:
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+-0.5 MJ/kg~5.0+-0.1 kW*h/kg

3. Calorific value of wet wood.
Now we are looking for data for calorific value depending on 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. we obtained 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 there is a discrepancy. Therefore, the formula may not be applicable in a wide range of humidity parameters. At low humidity levels due to uncertain A, at high humidity levels (more than 20-30%) due to incorrect 50.
In the data on direct calorific value, there are contradictions from source to source and there is uncertainty about 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 will use 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 can we obtain 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 determined as delta(mu)=RT*ln(pi/psat). pi is the partial pressure of vapor above the tree, psat is the partial pressure of saturated vapor. Their ratio is the relative air humidity 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 evaporation expressed in J/mol. Let's write the expression in more digestible units in kJ/kg
delta(Qsp)=(1000/18)*8.31*293/1000 ln(H)=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 processed into instantaneous values ​​of the heat of evaporation of water:
3% -> 2.71 MJ/kg
4% -> 2.61MJ/kg
6% -> 2.49 MJ/kg
9% -> 2.41 MJ/kg
12% -> 2.36 MJ/kg
15% -> 2.34 MJ/kg
20% -> 2.32MJ/kg
28% -> 2.30MJ/kg
Next 2.3 MJ/kg
Below 3% we will consider 3MJ/kg.
Well. We have universal data applicable to any wood, considering that the original picture is also applicable to any wood. This is very good. Now let’s consider the process of wood moistening and the corresponding drop in calorific value
let us have 1 kg of dry residue, humidity 0g, calorific value 18 MJ/kg
moistened to 3% - added 30g of water. The mass increased by these 30 grams, and the heat of combustion decreased by the heat of evaporation of these 30 grams. Our total is (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 17.2 MJ/kg
And so on, we recalculate all the values. We get:
0% -> 18.0 MJ/kg
3% -> 17.4 MJ/kg
4% -> 17.2 MJ/kg
6% -> 16.8 MJ/kg
9% -> 16.3 MJ/kg
12% -> 15.8 MJ/kg
15% -> 15.3 MJ/kg
20% -> 14.6 MJ/kg
28% -> 13.5 MJ/kg
30%-> 13.3MJ/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 linearly in the interval 0-40
Q = 18 - 0.1465W, MJ/kg or kcal/kg Q=4306-35W (not 50 at all) We will deal with the difference separately later.

4. Dependence of wood density on humidity
I will consider two breeds. Pine and birch

To begin with, I rummaged around and decided to settle on 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 fresh sawn wood, 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
here x is 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 per unit volume 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 wood to 10.8 for completely dry wood. In a practically significant range of 10-40% from approximately 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 freshly cut wood to 8.5 for completely dry wood. In a practically significant range of 10-40% from approximately 7 to 8 MJ/l ~ 2.1 kW*h/l

6. About the moisture content of firewood.
Earlier I mentioned the practically significant interval of 10-40%. I want to clarify. From the previous considerations, it becomes obvious that it is more advisable to burn dry wood than wet wood, and it is simply easier to burn it and easier to carry it to the firebox. It remains to understand what dry means.
If we look at the picture above, we will see that at the same 20 degrees above 30%, the equilibrium air humidity next to such a tree is 100% (rel.). What does it mean? AK is that the log behaves like a puddle, and dries in any weather conditions, it can even dry in the rain. The drying rate is limited only by diffusion, which means the length of the log if it is 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, and due to the cracks in the log, its drying speed additionally increases compared to a board, and laying it in single-row half logs further 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 for a half-meter of firewood. Chipped ones naturally dry even faster.
Ready to discuss if there are results.

It is not difficult to imagine what kind of log this looks and feels like. It does not contain cracks at the end, and feels slightly damp to the touch. If it lies haphazardly in the water, mold and fungi may appear. All sorts of bugs run happily if it’s warm. Of course he injects himself, but reluctantly. I think above 50% there is almost no pricking at all. The ax/cleaver enters with a “squelch” and the whole effect

Air-dried wood already has cracks and moisture content is less than 20%. It already pricks relatively easily and burns well.

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 in, it flares up perfectly, it is light and “ringing” to the touch. They are also excellently planed into splinters.

7. Smoke, charcoal, soot and ash
The main products of wood burning are carbon dioxide and water vapor. Which, together with nitrogen, are the main components of flue gas.
In addition, unburned residues remain. This is soot (in the form of flakes in the chimney, 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% mineral. impurities, ch. arr. carbonates and oxides of K, Na, Ca, Mg, Si, Al, Fe.
And here it is ash What are Non-flammable 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 slightly different soots - but technical soots. But I think the difference is small.

Both charcoal and soot are close to coal in composition, which means that 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 due to insufficient temperature in the firebox/lack of oxygen.

8. How much water vapor is produced when wood burns?
1 kg of dry firewood contains 63 grams of hydrogen or
When burned, these 63 grams of water will yield a maximum of 63*18/2 (we spend two grams of hydrogen to produce 18 grams of water) = 567 grams/kg_wood.
The total amount of water generated during the combustion of wood will thus be
0% ->567 g/kg
10%->615 g/kg
20%->673 g/kg
40%->805 g/kg
70%->1033 g/kg

9.Latent heat.
An interesting question is: if the moisture formed during the combustion of wood is condensed and the resulting heat is taken away, how much of it is there? We'll evaluate it.
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 calorific value of firewood
40%->805 g/kg->1.9MJ/kg->15.2% of the calorific value of firewood
70%->1033 g/kg->2.4MJ/kg->24.7% of the heat of combustion of wood
This is the theoretical limit of the additive that can be squeezed out from water condensation. Moreover, if you do not heat with raw 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 increasing the efficiency of a TT boiler/furnace is heat extraction from the flue gas.
We already have all the necessary data, so we won’t go into the sources. First you need to calculate the theoretical minimum air supply for burning wood. To start with dry ones.
Let's look at 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, there are already 13.8 moles of oxygen. 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 wood moisture contents we have
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 depends slightly on its humidity.

In this case, it is impossible to supply air less than the obtained value - there will be incomplete combustion of fuel, the formation carbon monoxide, soot and coal. It is also not advisable to supply much more, since this will result in incomplete combustion of oxygen, a decrease in the maximum temperature of the flue gases, and large losses into the chimney.

Enter the excess air coefficient (gamma) as the ratio of the actual air supply to the theoretical minimum (5 m3/kg). The value of the excess coefficient can vary and is usually 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 chemical water and all the moisture in the wood.
Thus, the amount of flue gas at the exit from the furnace is greater than at the entrance and is calculated 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. First, we can determine what temperature the chimney needs to be maintained so that there is never condensation in it. (by the way, I have no condensate in the pipe at all).
To do this, we will find the temperature corresponding to the relative humidity of the flue gas for 70% of the firewood. It is possible according to the schedule above. We are looking for 1.68/4.4=0.38.
But it can’t 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% humidity we have 5.2+0.2*4.3=6.1m3/kg

11. Flue gas heat
Let us limit ourselves to the case in which the flue gas temperature is 200 degrees. 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- heat recovery potential. Let us assume an excess air coefficient 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=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 recalculated by 20 degrees, we determine the density at a given temperature - 1.207. and Cp we take the average, about 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% of the calorific value of firewood
10%->6.4 m3/kg->1.5MJ/kg->9.3% of the calorific value of firewood
20%->6.1 m3/kg->1.4MJ/kg->9.7% of the 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% heat of combustion of wood

In addition, we will 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.
Let us assume 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 allocated between 10 and 20% additional steam of 0.13 m3/kg_wood. Without bothering with finding 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/%. We didn't get 15. Still an inconsistency. I don’t see any more reasons yet.

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 harmful). Latent heat - loss? No. The latent heat from evaporated moisture sits in the reduced calorific value of firewood. Chemically formed water is a combustion product, and not a loss of power (it does not evaporate but is immediately formed in the form of steam).
In total, the maximum efficiency of the boiler/furnace is determined by the heat recovery potential (without taking into account 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 exit from the furnace, for example, by reducing the combustion intensity, but at the same time one should expect more extensive formation of soot - it is smoky and not 100% burning of wood -> a decrease in efficiency.

13. Total heat recovery potential.
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 simplicity 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 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 perhaps simplifies the engineering calculation. In the indicated effect, about half 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 the 4306-35W obtained above in the coefficient before W.
Let us assume 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 allocated between 10 and 20% additional steam of 0.13 m3/kg_wood. Without bothering with finding 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/%. We didn't get 15. Still an inconsistency.

Let's assume one more option. The point is that the authors of the well-known formula operated with the so-called absolute humidity of wood, while here we operated with relative humidity.
In absolute terms, W is taken to be the ratio of the mass of water to the total mass of firewood, and in relative terms, the ratio of the mass of water to the mass of dry residue (see paragraph 1).
Based on these definitions, we will construct the dependence of absolute humidity on 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)
Let's look separately at the interval 10-40 again. It is possible to approximate the obtained dependence of the 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=4306-35*(1.55 Wabs - 4.78)=4473-54W. We finally obtained a result much closer to the literature data.

15. About the calorific value of rotten firewood
When starting a fire outdoors, including at barbecues, I, probably like many people, prefer to burn it with dry wood. This firewood consists of rather rotten dry branches. They burn well, quite hot, but to form a certain amount of coal it takes approximately twice as much as normal air-dry birch. But where can I get this dry birch in the forest? That’s why I drown with what I have and with what doesn’t harm the forest. The same firewood is perfect for heating a stove/boiler in the house.
What is this dry wood? This is the same wood in which the process of rotting usually took place, incl. directly on the root, as a result, the density of the dry residue greatly decreased and a loose structure appeared. This loose structure is more vapor-permeable than ordinary wood, so the branch dried right on the root under certain conditions.
I'm talking about this kind of firewood

You can also use rotten tree trunks if they are dry. It is very difficult to burn damp rotten wood, 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 Scots pine(with wide tolerances). Based on this postulate, we calculate the volumetric heat capacity depending on humidity, while I usually burn dry wood from deciduous trees, the density of which was initially higher than pine. 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 (up to the density of absolutely dry firewood), then for rotten wood we will also use this formula:
ro=0.3+0.003W. This is a very rough estimate, but no one seems to have really researched the issue raised here. M.b. The Canadians have information, but they also have their own forest, with its 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
Which is no longer particularly surprising, the volumetric 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 rotten wood, can be combined under one formula for calorific value. In order to get a formula that is not entirely academic, 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-dried firewood, regardless of the species, is approximately Q=4*density (in kg/l), kW*h/l

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

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Comments

1. Serious work, Alexander!
   However, there are also questions:
   

   I will further operate only with the moisture content that is discussed for lumber. Those. the mass of water in the tree divided by the mass of the dry residue, and not the mass of water divided by the total mass.

   building materials...
   Or is the definition the same?
   

   1. Specific calorific value of any wood is 4306-35W kcal/kg, W-humidity.

   
   
   
   

   1. Andrey-AA said:

      Interesting movie. You're talking about combustion, and humidity is for building materials...
      We probably need to determine the moisture content of the firewood! Or is the definition the same?

      This is exactly the definition. All tables that exist on wood, “feelings” and comparisons with numbers are based on precisely these relative percentages. About absolute humidity (natural % (mass)), everything that I could dig up relates to the near-war period, and there is no talk of any real values ​​here. Further, as I understand it, moisture meters for wood measure precisely these relative percentages, which are discussed in the article.

      Andrey-AA said:

      There are tables in which at 80% it will be 413 kcal/kg.
      And this really doesn’t fit with your formula...
      Same as with this one: 4473-54W.
      At small percentages - more or less.

      At what 80%? If absolute (although I can hardly imagine how it is possible to wet a tree like that), then
      for 4 kg of water 1 kg of dry residue, respectively, the calorific value will be roughly 0.25 * 18-0.75 * 2.3 = 2.8 MJ/kg => 679 kcal/kg
      A further decrease may be due, for example, to the measurement technique.
      In general, there is confusion in the tabular data, which as a result causes distrust in all the data. That is why I sat for a day and studied the question.

      1. 1. Andrey-AA said:

            Don't know. I have attached the table.

            The authors of the table confused relative and absolute percentages. We are talking about 80% absolute 4 kg of water per 5 kg of firewood
            Then they use the term net calorific value. I forgot what this is. I'll take a look later.

            1. mfcn said:

               The authors of the table confused relative and absolute percentages.

               It seems to me that for firewood, 50% water and 50% completely dry wood counts as 50% relative humidity.
               And you took it as for building materials and called the same proportion 100 percent relative humidity.
               I hinted at this a little higher...