Ash content of birch firewood. Ash composition of wood of various tree species in the floodplain biotope. The essence of the combustion process

Ash content in various constituent parts of the bark of various breeds: 5.2% for spruce, 4.9% for pine - The increase in the ash content of the bark in this case is explained by the contamination of the bark during the rafting of the whips along the rivers. The ash content in various constituent parts of the bark, according to V.M. Nikitin, is shown in table. 5. Ash content of bark of various breeds on dry weight, according to AI Pomeransky, is: pine 3.2%, spruce 3.95, 2.7, alder 2.4%.

According to NPO TsKTI im. II Pol - zunova, the ash content of the bark of various rocks varies from 0.5 to 8%. Ash content of crown elements. The ash content of the crown elements exceeds the ash content of wood and depends on the type of wood and the place of its growth. According to V.M. Nikitin, the ash content of the leaves is 3.5%.

Branches and twigs have an internal ash content of 0.3 to 0.7%. However, depending on the type of technological process, their ash content changes significantly due to their contamination with external mineral inclusions. Contamination of branches and twigs during harvesting, skidding and hauling is most intense in wet weather in spring and autumn.

Moisture and density are the main properties of wood.

Humidity Is the ratio of the mass of moisture in a given volume of wood to the mass of absolutely dry wood, expressed as a percentage. The moisture impregnating the cell membranes is called bound or hygroscopic, and the moisture that fills the cell cavities and intercellular spaces is called free or capillary.

When the wood dries, free moisture first evaporates from it, and then bound moisture. The state of wood, in which the cell walls contain the maximum amount of bound moisture, and only air is in the cell cavities, is called the limit of hygroscopicity. The corresponding humidity at room temperature (20 ° C) is 30% and does not depend on the breed.

There are the following levels of wood moisture content: wet - moisture content above 100%; freshly cut - humidity 50.100%; air - dry humidity 15.20%; dry - humidity 8.12%; absolutely dry - humidity about 0%.

This is the ratio at a certain humidity, kg, to its volume, m 3.

Increases with increasing humidity. For example, the density of beech wood with a moisture content of 12% is 670 kg / m3, and with a moisture content of 25% - 710 kg / m3. The density of late wood is 2.3 times higher than that of early wood; therefore, the better developed late wood, the higher its density (Table 2). The relative density of wood is the ratio of the mass of the sample in an absolutely dry state to the volume of the sample at the limit of hygroscopicity.

Table 1 - The content of ash and ash elements in the wood of various tree species

Woody plant	Ash,
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
Alder gray	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

According to the content of ash elements in their wood, all tree species are combined into two large clusters (Fig. 1). The first, headed by Scots pine, includes black alder, aspen and balsamic poplar (Berlin), and the second includes all other species, led by spruce and bird cherry. A separate subcluster is composed of light-loving species: drooping birch and Siberian larch. 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 by the ash composition of their wood, built by the Ward method using a matrix of normalized data

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

Conclusions.

1. Most of all calcium is contained in the wood of all tree species, which is the basis of the cell membrane. Potassium follows. There is an order of magnitude less iron, manganese, strontium and zinc in wood. Ni, Pb, Co, and Cd close the ranks.

(3) Wood species growing within the same floodplain biotope differ significantly from each other in the efficiency of their use of nutrients. The most efficient use of the soil potential is Siberian larch, in 1 kg of wood ash contains 7.4 times less than poplar wood - the most wasteful species in the ecological plan.

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

List of sources used

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

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

3. Demakov, Yu.P. Variability of the content of ash elements in wood, bark and needles of Scotch pine / Yu.P. Demakov, R.I. Vinokurov, V.I. Talantsev, S.M. Shvetsov // Forest ecosystems in a changing climate: biological productivity, monitoring and adaptation technologies: materials of an international conference with elements of a scientific school for youth [Electronic resource]. - Yoshkar-Ola: MarSTU, 2010.S. 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. - S. 25-36.

5. Vinokurov, 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 substantiation of the creation of sustainable forest plantations / A.I. Akhromeyko. - M .: Lesnaya prom-st, 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. Smirnov, Moscow: Moscow State University, 1959, 284 p.

8. Rodin, L.E. Dynamics of organic matter and biological circulation 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. Technique for measuring the gross content of copper, cadmium, zinc, lead, nickel, manganese, cobalt, chromium by atomic absorption spectroscopy. - M .: FGU FTSAO, 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. Mueller, W. Kleck, et al. - M .: Finance and Statistics, 1989. - 215 p.

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

Main criteria

The most important indicators for the fuel material are: density, humidity and heat transfer. All of them are closely related to each other and determine how effective and useful the burning of wood is. It is worth considering each of them in more detail, taking into account 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 is.

All wood species are divided into three main categories:

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

Each of them has a different density, and hence the specific heat of combustion of firewood. Hard varieties are considered to be of the highest quality. They burn for a long time and generate more heat. In addition, they form a lot of coals, which maintain the heat in the furnace.

Due to its hardness, such wood is difficult to process, which is why some consumers prefer medium-dense wood, such as birch or ash. Their structure allows you to chop the logs by hand without much effort.

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 for 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.
Some 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 for preparing the wood is required a 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. Correspondingly, 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, which is more expensive to purchase, since their production is more efficient.

It is worth mentioning here such a property as the burning temperature of firewood. It is the largest 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 combustion 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 low, and there are also many other disadvantages.

The main disadvantage of conifers is the presence of a large amount of resins. When such wood 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 the processing of wood is difficult due to its high density.

Oak is another popular fuel

The useful qualities of such breeds determine their higher cost, but this allows you to reduce the amount of material that will be needed to maintain a comfortable temperature in the house.

Material selection

Even the highest qualities of wood can be negated if it is chosen incorrectly for a specific type of activity. For example, it practically does not matter what was used for a night fire when gathering with friends. A completely different matter is kindling a fireplace or stove in a 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 wood and the heat emanating from the fireplace are the highlight of the living room.

For long burning and the release of a large amount of heat, it is worth giving preference to oak, acacia, as well as birch and walnut. 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 out soot.

For a bath

To ensure a high temperature in the steam room of a bath, 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, of course, oak and birch logs will be the best choice. They are hard, give good heat in a small volume and also give off pleasant vapors. Linden and alder can also have an additional healing effect. You can only use well-dried materials, but not older than one and a half to two years.

For barbecue

When cooking on the grill and barbecue, the main point is not the burning of the wood itself, but the formation of coals. 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 hard logs to the firebox. In order for the smoke to have a special aroma, it is recommended to use fruit firewood for the barbecue. You can combine them with oak and acacia.

When using different types of wood, pay attention to the size of the chocks. For example, an oak tree will take longer to burn and smolder than an apple tree, so it makes sense to use thicker fruit logs.

Alternative combustion materials

The calorific value of certain types of firewood is quite large, 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. The use of pressed briquettes is optimal.

For the same kiln load, the 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. By pressing the waste, it is possible to create an incredibly dense combustion material that even the best types of wood cannot match. 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.

Humidity

The moisture content of woody biomass is a quantitative characteristic showing the moisture content of biomass. Distinguish between absolute and relative humidity of biomass.

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

Where W a - absolute humidity,%; m is the mass of the sample in a wet state, g; m 0 is the mass of the same sample, dried to a constant value, g.

Relative or working humidity the ratio of the mass of moisture to the mass of wet wood is called:

Where W p - relative, or working, humidity,%

When calculating wood drying processes, absolute moisture is used. In heat engineering calculations, only relative, or working, humidity is used. Taking into account this established tradition, in the future, we will only use relative humidity.

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

Free moisture is found in cell cavities and in intercellular spaces. Free moisture is retained only by mechanical bonds, it is removed much more easily and has less effect on the mechanical properties of wood.

When wood is kept in air, moisture is exchanged between the air and the wood substance. If the moisture content of the wood substance is very high, then during this exchange the wood dries out. If its moisture content is low, then the woody substance is moistened. With a long stay of wood in the air, stable temperature and relative humidity of the air, the moisture content of the wood also becomes stable; this is achieved when the water vapor pressure of the surrounding air is equal to the water vapor pressure at the surface of the wood. The value of stable moisture content of wood, kept for a long time at a certain temperature and air humidity, is the same for all tree species. Stable humidity is called equilibrium, and it is completely determined by the parameters of the air in which it is located, that is, its temperature and relative humidity.

Stem moisture. Depending on the moisture content, stem wood is divided into wet, freshly cut, air-dry, room-dry and absolutely dry.

Wet refers to wood that has been in water for a long time, for example, when rafting or sorting in a water basin. The moisture content of wet wood W p exceeds 50%.

Freshly cut is the wood that has retained the moisture of the growing tree. It depends on the type of wood and varies within W p = 33 ... 50%.

The average moisture content of freshly cut wood is,%, in spruce 48, in larch 45, in fir 50, in cedar pine 48, in common pine 47, in willow 46, in linden 38, in aspen 45, in alder 46, in poplar 48, warty birch 44, beech 39, elm 44, hornbeam 38, oak 41, maple 33.

Air-dry is wood that has been cured for a long time in the open air. During the stay in the open air, the wood constantly dries out and its moisture content gradually decreases to a stable value. Humidity of air-dry wood W p = 13 ... 17%.

Room-dry wood is wood that is kept 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 decays, its cells are destroyed, as a result of which additional internal cavities are formed, the structure of rotten wood becomes loose, porous as the decay process develops, and the strength of the wood decreases sharply.

For these reasons, the moisture content of wood rot is not limited and can reach such high values that its combustion becomes ineffective. The increased porosity of rotten wood makes it very hygroscopic, being outdoors, it quickly moisturizes.

Ash content

Ash content the content of mineral substances in the fuel remaining after the complete combustion of the entire combustible mass is called. 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 the wood substance, and external, which got into the fuel during harvesting, storage and transportation of biomass. Depending on the type of ash, it has different fusibility when heated to a high temperature. Low-melting ash is called if the temperature of the beginning of the liquid-melting state is 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 inner ash of woody biomass is refractory, and the outer ash is low-melting.

The ash content of the bark of various breeds varies from 0.5 to 8% and higher with severe contamination during harvesting or storage.

Density of wood

The density of a woody substance is the ratio of the mass of the material that forms the cell walls to the volume it occupies. The density of the woody substance is the same for all types of wood and is equal to 1.53 g / cm 3. On the recommendation of the CMEA Commission, all indicators of the physical and mechanical properties of wood are determined at an absolute moisture content of 12% and are recalculated for this moisture content.

Density of different types of wood

Breed	Density kg / m 3
Breed	At standard humidity	Completely dry
Larch	660	630
Pine	500	470
Cedar	435	410
Fir	375	350
Hornbeam	800	760
White acacia	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 crushed wood waste varies widely. For dry chips from 100 kg / m 3, up to 350 kg / m 3 and more for wet chips.

Thermal characteristics of wood

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

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

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

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

The indices at the signs of the 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 the fuel.

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

The elementary composition of the combustible mass of stem wood is practically 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. mass: C g = 51%, H 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. Distinguish between higher and lower heat of combustion.

Higher calorific value- this is the amount of heat released during the combustion of 1 kg of biomass during the complete condensation of all water vapors formed during combustion, with the release of heat spent on their evaporation (the so-called latent heat of vaporization). The highest heat of combustion Q in is determined by the formula of D. I. Mendeleev (kJ / kg):
Q in = 340C p + 1260H p -109O p.

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 p = 340C p + 1030H p -109O p -25W p.

The heat of combustion of stem wood depends only on two quantities: ash content and moisture content. The lowest heat of combustion of the combustible mass (dry ashless!) Of stem wood is practically 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, they can be divided into two types: logging waste and woodworking waste.

Waste logging- These are the parts of the tree that are separated during the logging process. These include needles, leaves, non-lignified shoots, branches, twigs, tops, otklevki, canopies, cut-outs of the trunk, bark, waste from the production of chipped pulpwood, etc.

In its natural form, logging waste is not easily transportable; when used for energy, it is preliminarily crushed into chips.

Waste woodworking- this is waste generated in the woodworking industry. These include: slabs, slats, cuttings, short cutter, shavings, sawdust, waste from the production of technological chips, wood dust, bark.

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

Depending on the shape and size of the particles, wood waste is usually classified into the following groups: lump wood waste and soft wood waste.

Lump wood waste- these are otklevki, visors, cutouts, slabs, rake, cutoffs, shorts. Soft wood waste includes sawdust and shavings.

The most important characteristic of shredded wood is its fractional composition. Fractional composition is the quantitative ratio of particles of certain sizes in the total mass of crushed wood. The fraction of crushed wood is the percentage of particles of a certain size in the total mass.

Shredded wood by particle size can be divided into the following types:

— wood dust generated when sanding wood, plywood and wood-based panels; the main part of the particles passes through a sieve with an opening of 0.5 mm;
— sawdust formed during longitudinal and transverse sawing of wood, they pass through a sieve with holes of 5 ... 6 mm;
— chips obtained by chopping wood and wood waste in chippers; the main part of the chips passes through a sieve with holes of 30 mm and remains on a sieve with holes of 5 ... 6 mm;
- large chips, the particle size of which is more than 30 mm.

Separately, we note the features of wood dust. Wood dust generated during the grinding of wood, plywood, chipboard and fibreboard is not subject to storage, both in buffer warehouses of boiler houses and in warehouses for off-season storage of small wood fuel due to its high windage and explosiveness. When burning wood dust in furnaces, it must be ensured that all the rules for burning pulverized fuels are met, preventing the occurrence of outbreaks and explosions inside furnaces and in the gas paths of steam and hot water boilers.

Wood sanding dust is a mixture of wood particles with an average size of 250 microns with abrasive powder, which is detached from the sandpaper during the sanding process of the wood material. The content of abrasive material in wood dust can be up to 1% by weight.

Features of combustion of woody biomass

An important feature of woody biomass as a fuel is the absence of sulfur and phosphorus in it. As you know, the main loss of heat in any boiler unit is the loss of heat energy with flue gases. The amount of this loss is determined by the temperature of the flue gases. This temperature during the combustion of fuels containing sulfur, in order to avoid sulfuric acid corrosion of the tail heating surfaces, is maintained at least 200 ... 250 ° C. When burning wood waste that does not contain sulfur, this temperature can be lowered to 100 ... 120 ° C, which will significantly increase the efficiency of boiler units.

The moisture content of the fuel wood can vary over a very wide range. In furniture and woodworking industries, the moisture content of some types of waste is 10 ... 12%, in logging enterprises, the moisture content of the main part of the waste is 45 ... 55%, the moisture content of the bark when debarking waste after rafting or sorting in water basins reaches 80%. An increase in the moisture content of the wood fuel reduces the productivity and efficiency of the boilers. The release of volatiles when burning wood fuel is very high - up to 85%. This is also one of the features of woody biomass as a fuel and requires a large length of the torch, in which combustion of the combustible components emerging from the layer is carried out.

Woody biomass coking product - 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 ratio, which has a positive effect on the efficiency of boiler plants when burning wood biomass in them.

However, along with these positive properties, wood has features that negatively affect the operation of boilers. These features, in particular, include the ability to absorb moisture, that is, an increase in moisture in the aquatic environment. With an increase in humidity, the lower heat of combustion rapidly decreases, fuel consumption increases, combustion becomes more difficult, which requires the adoption of special design solutions in boiler and furnace equipment. With a moisture content of 10% and an ash content of 0.7%, the NTS will be 16.85 MJ / kg, and with a moisture content 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.

A characteristic feature of wood as a fuel is a low content of internal ash (less than 1%). At the same time, external mineral inclusions in logging waste sometimes reach 20%. The ash formed during the combustion of pure wood is refractory, and its removal from the combustion zone of the furnace does not present any particular technical difficulty. Mineral inclusions in woody biomass are fusible. When wood with a significant content is burned, caked slag is formed, the removal of which from the high-temperature zone of the combustion device is difficult and requires special technical solutions to ensure the effective operation of the furnace. The sintered slag, formed during the combustion of high-ash woody biomass, has a chemical affinity with brick, and at high temperatures in the furnace it is sintered with the surface of the brickwork of the furnace walls, which makes it difficult to remove the ash.

Heat output usually called the maximum combustion temperature, developed with complete combustion of fuel without excess air, that is, under conditions when all the heat released during combustion is completely consumed to heat the resulting combustion products.

The term "heat output" was proposed at one time by D.I. The higher the heat output of the fuel, the higher the quality of the thermal energy released during its combustion, the higher the efficiency of the steam and hot water boilers. The heating capacity is the limit to which the actual temperature in the furnace approaches as the combustion process improves.

The heating capacity of wood fuel depends on its moisture content and ash content. The heating capacity of absolutely dry wood (2022 ° С) is only 5% lower than the heating capacity of liquid fuel. With a wood moisture content of 70%, the heat output decreases by more than 2 times (939 ° C). Therefore, the moisture content of 55-60% is a practical limit for the use of wood for fuel purposes.

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

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

An increase in the moisture content of woody biomass inevitably leads to a decrease in the efficiency of boiler plants. You should be aware of this and constantly develop and carry out measures to prevent the ingress of atmospheric precipitation, soil water, etc. into the 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 perfect technological processes of timber harvesting and its primary processing. It is necessary to give preference to such technological processes in which the contamination of wood waste with mineral inclusions can be minimized.

The fractional composition of the crushed wood should be optimal for this type of combustion device. Deviations in particle size from the optimum, both upward and downward, reduce the efficiency of combustion devices. Chippers used to chop wood into fuel chips should not show large deviations in particle size towards their increase. 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 the boiler units meets the characteristics of this type of fuel.

Firewood is the oldest and most traditional source of heat energy, which is a renewable fuel. By definition, firewood is a piece of wood commensurate with the hearth that is used to light and maintain a fire in it. By its quality, firewood is the most unstable fuel in the world.

However, the weight percentages of any wood mass are roughly the same. It includes - up to 60% cellulose, up to 30% lignin, 7 ... 8% associated hydrocarbons. The rest (1 ... 3%) -

State standard for firewood

On the territory of Russia
GOST 3243-88 Firewood. Technical conditions
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The standard from the times of the Soviet Union defines:

Range of firewood by size
Allowable amount of rotten wood
Range of firewood by calorific value
Methodology for accounting for the amount of firewood
Transportation and storage requirements
wood fuel

Of all the GOST information, the most valuable are methods for measuring wood stacks and coefficients for converting values from a fold measure to a dense one (from a storage meter to a cubic meter). In addition, there is still some interest in a fad on limiting sound and sapwood rot (no more than 65% of the end area), as well as a ban on external rottenness. It’s just difficult to imagine such rotten wood in our space age of the pursuit of quality.

As for the calorific value,
then GOST 3243-88 divides all firewood into three groups:

Firewood accounting

For accounting for any material value, the most important thing is the ways and methods of calculating its quantity. The amount of firewood can be taken into account, either in tons and kilograms, or in fold and cubic meters and decimeters. Accordingly - in mass or volume units

Accounting for firewood in mass units
(in tons and kilograms)
This method of accounting for wood fuel is used extremely rarely due to its bulkiness and sluggishness. It is borrowed from wood builders and is an alternative method for those cases where it is easier to weigh the wood than to determine its volume. So, for example, sometimes with wholesale deliveries of wood fuel, it is easier to weigh wagons and timber trucks shipped "on top" than to determine the volume of shapeless wood "hats" towering on them.
Advantages

- simplicity of information processing for further calculation of the total calorific value of fuel in heat engineering calculations. Because the calorific value of the weight measure of firewood is calculated by and is practically unchanged for any type of wood, regardless of its geographical location and degree. Thus, when accounting for firewood in mass units, the net weight of combustible material is taken into account minus the weight of moisture, the amount of which is determined by the moisture meter
disadvantages
accounting of firewood in mass units
- the method is absolutely unacceptable for measuring and accounting for lots of firewood in the field of logging, when the required special equipment (scales and moisture meter) may not be at hand
- the result of measuring humidity soon becomes irrelevant, firewood quickly damp or dry out in the air
Accounting for firewood in volumetric units
(in fold and cubic meters and decimeters)
This method of accounting for wood fuel has become the most widespread, as the simplest and fastest way to account for wood fuel mass. Therefore, the accounting of firewood is carried out everywhere in volumetric units of measurement - stock meters and cubic meters (fold and dense measures)
Advantages
accounting of firewood in volumetric units
- extreme simplicity in the execution of measurements of wood stacks with a linear meter
- the measurement result is easily controlled, remains unchanged for a long time and is beyond doubt
- the method for measuring wood lots and the coefficients for converting values from a stock measure to a dense one are standardized and set out in
disadvantages
accounting of firewood in mass units
- the payment for the simplicity of accounting for firewood in volumetric units becomes the complication of further heat engineering calculations for calculating the total calorific value of wood fuel (you need to take into account the type of wood, its place of growth, the degree of rottenness of firewood, etc.)

Calorific value of firewood

Calorific value of firewood,
she is the heat of combustion of firewood,
she is the calorific value of firewood

How does the calorific value of firewood differ from the calorific value of wood?

The calorific value of wood and the calorific value of firewood are related and close in value, identified in everyday life with the concepts of “theory” and “practice”. In theory, we study the calorific value of wood, but in practice, we deal with the calorific value of firewood. At the same time, real wood blocks can have a much wider range of deviations from the norm than laboratory samples.

For example, real firewood has bark, which is not wood in the literal sense of the word and, nevertheless, occupies volume, participates in the process of burning wood and has its own calorific value. Often, the calorific value of the bark differs significantly from the calorific value of the wood itself. In addition, real firewood may have different wood density, depending on, have a large percentage, etc.

Thus, for real firewood - the calorific value indicators are generalized and slightly underestimated, since for real firewood - all negative factors that reducetheir calorific value. This explains the downward difference in value between the theoretical-calculated values of the calorific value of wood and the practically-applied values of the calorific value of firewood.

In other words, theory and practice are different things.

The calorific value of firewood is the amount of useful heat generated during its combustion. Useful heat means heat that can be removed from the hearth without affecting the combustion process. The calorific value of wood is the most important indicator of the quality of wood fuel. The calorific value of firewood can vary widely and depends, first of all, on two factors - the wood itself and it.

The calorific value of wood depends on the amount of combustible woody substance present in a unit of mass or volume of wood. (for more details about the calorific value of wood in the article -)
The moisture content of wood depends on the amount of water and other moisture present in a unit of mass or volume of wood. (for more details about wood moisture in the article -)

Firewood volumetric calorific value table

Calorific value gradation by
(with wood moisture 20%)

Wood species	specific calorific value of firewood (kcal / dm 3)
Birch	1389...2240	First group according to GOST 3243-88: birch, beech, ash, hornbeam, elm, elm, maple, oak, larch
beech	1258...2133
ash	1403...2194
hornbeam	1654...2148
elm	not found (analogue - elm)
elm	1282...2341
maple	1503...2277
oak	1538...2429
larch	1084...2207
Pine	1282...2130	Second group according to GOST 3243-88: pine, alder
alder	1122...1744	Second group according to GOST 3243-88: pine, alder
spruce	1068...1974	Third group according to GOST 3243-88: spruce, cedar, fir, aspen, linden, poplar, willow
cedar	1312...2237
fir	not found (analogue - spruce)
aspen	1002...1729
Linden	1046...1775
poplar	839...1370
willow	1128...1840

The calorific value of rotten wood

It is absolutely true that rot degrades the quality of firewood and reduces its calorific value. But how much the calorific value of rotten firewood decreases is a question. Soviet GOST 2140-81 and determine the method for measuring the size of rot, limit the amount of rot in a log and the amount of rotten logs in a batch (no more than 65% of the end area and no more than 20% of the total mass, respectively). But, at the same time, the standards do not in any way indicate a change in the calorific value of the firewood itself.

It's obvious that within the requirements of GOSTs there is no significant change in the total calorific value of the wood mass due to rot, therefore, individual rotten logs can be safely neglected.

If there is more rot than is permissible according to the standard, then it is advisable to take into account the calorific value of such firewood in units of measurement. Because, when wood decays, processes occur that destroy the substance and disrupt its cellular structure. At the same time, accordingly, wood decreases, which primarily affects its weight and practically does not affect its volume. Thus, mass calorific value units will be more objective for accounting for the calorific value of very rotten wood.

By definition, the mass (weight) calorific value of firewood is practically independent of its volume, wood species and degree of rottenness. And, only the moisture content of the wood - has a great influence on the mass (weight) calorific value of firewood

The calorific value of the weight measure of rotten and rotten wood is practically equal to the calorific value of the weight measure of ordinary firewood and depends only on the moisture content of the wood itself. Because only the weight of water displaces the weight of the combustible wood substance from the weight measure of firewood, plus the loss of heat due to evaporation of water and heating of water vapor. Which is exactly what we need.

Calorific value of firewood from different regions

Volumetric the calorific value of firewood for the same species of wood growing in different regions may differ due to changes in the density of wood depending on the water saturation of the soil in the growing area. Moreover, it does not necessarily have to be different regions or regions of the country. Even within a small area (10 ... 100 km) of logging, the calorific value of firewood for the same wood species can change with a difference of 2 ... 5% due to the change in wood. This is due to the fact that in arid areas (under conditions of a lack of moisture) a finer and denser cellular structure of wood grows and forms than in water-rich swampy land. Thus, the total amount of combustible substance per unit volume will be higher for firewood harvested in drier areas, even for the same logging area. Of course, the difference is not that great, about 2 ... 5%. Nevertheless, with large logs, this can give a real economic effect.

The mass calorific value for firewood from the same wood species growing in different regions will absolutely not differ at all, since the calorific value does not depend on the density of the wood, but depends only on its moisture content

Ash | Ash content of firewood

Ash is a mineral that is contained in wood and which remains in the solid residue after the wood mass has completely burned out. The ash content of firewood is the degree of its mineralization. The ash content of wood is measured as a percentage of the total mass of wood fuel and shows the quantitative content of mineral substances in it.

Distinguish between internal and external ash

Internal ash	External ash
Internal ash is a mineral that is contained directly in	External ash is mineral substances that have entered the firewood from the outside (for example, during harvesting, transportation or storage)
Internal ash is a refractory mass (above 1450 ° C), which is easily removed from the high-temperature zone of fuel combustion	External ash is a low-melting mass (less than 1350 ° C), which is sintered into slag, which sticks to the lining of the combustion chamber of the heating unit. As a result of such sintering and sticking, external ash is poorly removed from the high-temperature zone of fuel combustion
The internal ash content of the woody substance is in the range from 0.2 to 2.16% of the total wood mass	External ash content can be up to 20% of the total wood mass
Ash is an unwanted part of the fuel, which reduces its combustible content and complicates the operation of heating units