Protection of wood from ignition. Normal wood burning

Many private houses are equipped with a stove. But it is not enough just to put any firewood into it and wait for maximum heat transfer. To properly heat your home, you need to have complete information about the burning temperature of firewood, as well as the material that will be used.

The efficiency will directly depend on the thermal conductivity of the material. Any owner of a private house with a stone oven knows about this nuance. The quality of combustion also depends on another indicator - the combustion temperature. By increasing the degrees, you can heat the water in the pipes much faster or brick walls thus protecting your home from severe frosts.

If you put a poplar in the furnace, you can observe a very high flame, but its temperature will not exceed 500 degrees, and this is not so much for heating the room. It is preferable to use ash, beech and hornbeam. They actively burn, but at the same time emit a temperature of 1000 degrees. This indicator is ideal for space heating.

Criteria for choosing the type of wood depending on the purpose

When choosing required material, you should be aware of a few nuances. For example, if you use ash or beech, you can raise the temperature to high levels, but if you use them for a bath or a stove, it is very expensive and unprofitable - firewood burns quickly. For this reason, people began to use a different wood - birch. The combustion of birch firewood is accompanied by obtaining 800 degrees.

Oak and larch are also often used. Their burning temperature ranges from 840 to 900 degrees. When there is a need to make an open fire, a fire, light logs in the barbecue in your summer cottage or private area, it is advisable to use pine. It is also often used for home heating by placing it in a stove. The combustion temperature of the material is about 610-630 degrees. But for this reason, you will have to use about half as much firewood as when using birch or oak.

Features of coniferous breed:

1. The combustion temperature is low.
2. When placed in a fire, it forms a large number of soot and smoke.

The appearance of smoke and soot is due to the large amount of resin contained in the wood. It settles on the walls of the chimney, and therefore it must be periodically cleaned after use. So conifers not so popular for the firebox - the cleaning process is very laborious. Such material is used only as a last resort, if there is no other option.

Also, when making a fire, it is necessary to pay attention to the moisture content of materials, since this percentage directly affects combustion. The wetter the wood, the worse it will burn. But it also creates a lot of smoke.

Popular experience shows that in order to obtain required heat to heat the house, it is necessary to use firewood from beech, oak, which is cut down in winter, mountain pines, birch and acacia.

Ash, resinous larch, maple, pine or oak, felled in summer, cause the strongest flames.

A little less heat forms fir, chestnut and cedar.

Poplar, alder, and aspen have the worst heat capacity.

From all this, we can conclude that those firewoods that are the most weighty and dense are best formed heat.

Factors affecting the combustion temperature of firewood

There are several factors that contribute to combustion:

1. The type of wood used for burning.
2. material moisture.
3. The volume of air entering the furnace.

These are the main indicators to look at. Special attention, since it is on them that the efficiency of burning wood, and the temperature that can rise during the combustion process, will depend.

Humidity level

The moisture content of the wood plays a key role in kindling, so this important point requires separate consideration. Any tree that has just been cut down has a certain moisture content. In most cases, this figure is 50%. But in some cases it increases to 65%. And this suggests that this type of material will dry for a very long time under the influence of high temperature before igniting.

Part of the heat will go only to remove excess moisture by evaporation. For this reason, the temperature will not reach maximum indicator. Heat transfer under this condition will decrease.

For maximum benefit, there are a few basic options to use:

1. Most suitable option- drying. To do this, the tree is cut into small pieces, and then folded into a dry place in a barn or shed. AT vivo the drying process will take approximately 1 year. And if the firewood is stored longer and lies for two summers, then their humidity will be 20%. This is already the best indicator.
2. The second option is less preferable - to burn what is, not paying attention to humidity. But in this situation, you will have to spend twice as much firewood to form the desired temperature. In addition, you should be prepared to clean the chimney from soot.

The better the wood dries, the higher the combustion temperature can be taught. And it depends on the release of heat. Heat will not work with wet wood.

Warming up process

Warming up is the heating of a separate area wooden material to a temperature sufficient to ignite the entire surface.

After that, the process will continue when coal is formed. When heated to 250-350 degrees, the selected material will begin to decompose into components. Then smoldering begins, but the flame does not appear yet. At this point, smoke formation can be observed. When the temperature continues to rise, the level of pyrolysis gases increases - a flash occurs. Firewood will burn completely.

Flammability of materials

Flammability is directly affected by the percentage of moisture contained in the selected rock. An important role is played by the power of the heating source, as well as the cross-section of the wood and the speed of the air flow.

To make the flame flare up faster, it is desirable to use light wood, which has a large porosity. Wet wood will ignite very slowly, because it will dry out before an open fire forms.

Burning also depends on the shape of the tree - it is advisable to use a rectangle, since the circle will flare up much longer. To speed up the process, it is necessary to select a material with a small cross section and sharp edges. It is important to ensure that the area being heated is supplied with required amount oxygen.

The combustion temperature of firewood and flammability are also greatly influenced by the design of a home stove. It can be made from different materials and this directly affects the combustion temperature of the materials put inside. If the stove is massive, then the firewood in it will burn out almost completely, but this process will take a very long time. Great care must be taken when using. Failure to follow safety precautions may result in fire wood-burning sauna at high combustion temperatures.

The stove-potbelly stove, made of steel sheet, cools down quickly, while the heat is distributed over the surrounding space, but first it will pass from the combustion zone to the walls, and only then into the room.

combustion process

By observing the functioning of the furnace, one can think about why the supplied air does not affect the color of the resulting flame. Oxygen must have a chemical effect and impart soot bright color, which can even become white. But this phenomenon can be easily explained, because the particle size also affects the temperature. The smaller it is, the lower the temperature will be. Therefore, small hot particles form the same temperature as the gas that surrounds them. It should also be noted that each type of wood has a certain heat transfer. To find out these figures, you can study the table, which shows all the thermal conductivity indicators for each type of material.

Combustion temperature measurement

It is very difficult to measure the combustion temperature at home. An ordinary thermometer will not work here. Of course, “by eye” it will also not be possible to determine the correct combustion temperature of a certain material. To bring such studies, you need to purchase a special device called a pyrometer.

But you need to know that the high temperature of burning wood in the stove will not mean that they will release the required amount of heat. Therefore, you should also take care of high-quality equipment. AT good stoves it is possible to artificially reduce the supply of oxygen to the wood. Thus, it is possible to achieve an increase in the combustion temperature and a decrease in heat transfer.

Because the home combustion temperature different firewood is very difficult, expensive, and sometimes impossible to measure, then you can rely on official data. All indicators have long been calculated in the laboratory by specialists, by comparative analysis. In order to obtain the necessary results, the firewood was carefully dried before testing - it was brought into the optimal condition for experiments with open fire.

Thermal conductivity of materials:

The concept of "burning temperature of firewood" does not quite correctly reflect main characteristic. It is necessary to pay more attention to the ability to generate heat. The unit of measure for this parameter is calories. thermal energy, which heats 1 gram of ordinary water by 1 degree.

Heating capacity

In practice, a person should be interested in the heat output of the selected material. This is the temperature that can be reached when burning a certain type of firewood.

Firewood table:

• If the house is heated by a stove and during the burning process it smells of damp firewood, then you should immediately examine your equipment. It is possible that there is a leak somewhere.
• When burning, a large amount of acids is released, so the chimney should be built from reliable materials that can resist aggressive environments.
• If firewood with resin is used, the chimney must be thoroughly cleaned after use.
• To heat stones, for example, in a steam room, it is advisable to use firewood that burns weakly and emits a large amount of heat.
• To quickly heat the steam room, a material with a high combustion temperature is used. In this case, the air supply to the furnace must be increased.

Having studied the material, you can understand what temperature of burning firewood is needed for the most efficient heating of the room.

Wood fire and protection

Combustion is a process of thermal decomposition of wood, consisting of a fiery phase and smoldering, during which oxygen moves into the thickness of the wood.

Combustion can occur only when there is a sufficient supply of oxygen, and the heat of combustion itself is not dissipated, but is used to warm up new adjacent

areas of wood to the ignition temperature. Ignition temperature, i.e. the moment of flash of combustible gases for various breeds wood fluctuates within relatively small limits - from 250 to 300 °. Prolonged heating of wood at a temperature of 120-150 ° is accompanied by slow and gradual charring, with the formation of self-igniting coal in air, which is very dangerous for unprotected wooden elements.

The flammability of wood is related to its volumetric weight, moisture, power external source heating, sectional shape of the wooden element, speed air flow(thrust), the position of the element in the heat flow (horizontal, vertical), etc. Crucial for the combustion process has the calorific value of the material. Dry and light wood ignites faster than dense wood (oak, etc.). Wet wood is more difficult to ignite, since before ignition it is necessary to expend an additional amount of heat to evaporate water. A retarding factor is also the increased thermal conductivity of wet wood; the ignited surface layer of it is rather cooled. Round and massive elements burn worse than with a rectangular profile and with a small cross section, with sharp ribs and a relatively developed side surface. The unplaned surface of the elements, like loose wood, ignites faster than a smooth one.

Good results gives impregnation of wood in hot and cold baths. For such impregnation, ammophos is used - a white crystalline powder, which is ammonium salts of phosphoric acid, ammonium sulphate (technical), diammonium sphate (technical), which do not cause corrosion of steel.

To obtain a solution that has both flame retardant and antiseptic properties, sodium fluoride is added to the composition.

More simple but less effective tool fire protection of wooden elements is their surface impregnation by immersion for 2-3 hours in an aqueous solution of salts (phosphate, ammonium sulphate, etc.) or surface treatment two or three times (with a spray gun or brush) with water fire retardant solutions the same composition. In this case, the solution penetrates to a depth of 1 - 1.5 mm.

Finally, one more and also a simple means is the painting of the surfaces of wooden elements with special fire-retardant silicate and other paints or coating flame retardant(superphosphate, etc.).

All flame retardant paints and coatings partially delay ignition. At high temperatures wood under the cover of paint or coating is subjected to dry distillation, with the release of decomposition products - combustible gases that go outside, followed by buckling and rupture of the cover. In this case, the combustion of gas jets occurs at a considerable distance from the surface of the wood with a reduced heating effect of the flame and a slow speed and decay of the wood. The fire-retardant effect of painting and coating is also explained by the heat-insulating effect of their cover, which, in some paints, can significantly increase in volume under the action of high temperatures, forming foam or bubbles that delay the start of dry distillation of wood.

Semi-finished products and building products

Semi-finished products and building products are made from coniferous and hardwood with humidity not higher than 12% for a clean floor and 15% for other details.

Depending on the type of processing, this group of wood materials includes: planed bars, planed and tongue-and-groove boards for flooring clean floors, parquet, plywood, profile materials- plinths, fillets, railings, platbands, etc.

Sheet pile boards, unlike the usual edged board have a tongue (notch) on one side of the edge, and on the other, a ridge that enters the tongue of the adjacent board. Tongue and tongue, with which the boards are tightly adjusted, may have different shape- rectangular, triangular, trapezoidal and segmental. Groove boards are used for flooring, partitioning and other works.

Profile materials - skirting boards and fillets - are used to close the corners between the wall and the floor, handrails - for the device stair railings and platbands - for sheathing window and door openings.

Parquet is produced in the form of parquet boards, type-setting and piece parquet with a wood moisture content of 8±2%. Parquet boards They consist of two layers: the upper one is a front covering made of parquet planks 6-8 mm thick and the lower one is in the form of a slatted base 18-19 mm thick. Parquet planks are made from high quality wood: oak, beech, ash, pine, larch, maple, elm and some other species.

The basis for parquet boards is wood of various types of wood, including cedar, pine, spruce, fir, as well as birch, alder, aspen, etc. treated with antiseptics. Planks of the front layer of parquet boards are glued to the base with waterproof synthetic adhesives. Parquet boards have edges on one side - a groove, and on the other - a comb for a tight connection when laying boards in a parquet floor. They have the following advantages over block parquet: less precious wood consumption, more durable parquet lathing, more mechanized production and faster flooring.

Type-setting parquet is a set of parquet planks (13 hardwoods: oak, beech, etc., glued face to paper in a certain order). After laying the typesetting parquet on the prepared floor base, the paper together with the adhesive is removed and the parquet floor is properly finished.

Strip parquet consists of planks of hardwood of a certain size and shape.

Plywood - sheets obtained by gluing three or more thin layers wood veneer with a mutually perpendicular arrangement of wood fibers. Plywood veneer is produced on special lathes by cutting a layer of wood in the form of a continuous wide tape, followed by cutting it into cut sheets. Plywood can be ordinary (glued), decorative and bakelized. Depending on the type of adhesive used, plywood of the FSF brand is distinguished, which has increased water resistance - glued with waterproof phenol-formaldehyde adhesives; medium water resistance grades FK and FBK - glued with carbamide or albumin-casein adhesives; limited water resistance brand FB - glued with protein adhesives. To obtain glued plywood, pine, spruce, fir, alder, oak, birch and beech wood is widely used.

The manufacture of plywood consists mainly of the following technological operations: steaming wooden logs in hot water and peeling them to obtain veneer, cutting the veneer into sheets of a given format, drying and smearing with glue, laying layers of veneer and pressing on hot presses into plywood, leveling the edges of plywood by cutting, drying and storing. Plywood is produced in dimensions of 725X1230 mm with a thickness of 1.5 to 12 mm. Plywood, obtained on the basis of synthetic adhesives, is sufficiently waterproof, durable and widely used for cladding exterior walls, roofing works, manufacturing of load-bearing and enclosing structures ( plywood FSF); for device internal partitions in factory housing construction and indoor wall cladding (FB plywood), etc.

Ignition of wood is possible only when its outer layers are heated to active pyrolysis temperatures (see Fig. 95), including radiant heating (see Fig. 164), when a combustible mixture of pyrolysis products (volatile) and air becomes capable of igniting from external ignition source (fire, spark, burner, etc.). If there is no external ignition source, then ignition becomes possible in the self-ignition mode, when some part of the wood, overheating, not only emits volatiles, but chars. At the same time, active charcoal can begin to interact with air (smolder) with spontaneous combustion and, in the end, due to its high temperature, ignites the combustible mixture above the surface of the wood. Thus, self-ignition of wood occurs due to the smoldering of the resulting charcoal. And smoldering charcoal, as everyone knows, occurs primarily on the villi of wood in the form of coals (Fig. 95). Therefore, the protection of wood from spontaneous combustion (for example, on the shelf of a bathhouse, where there are no sources of ignition, but there are high temperatures) should first of all imply protection against ignition of the wood villi.

Wood always has villi: structural irregularities and uneven processing. Structural irregularities are a consequence of the capillary-porous structure of wood. When cut, some of the fibers are torn off, and some are cut right along the cells. Therefore, on the surface of the wood there are always elevations, grooves, depressions and deep channels, when visible to the eye and when not. But the structure of wood is always visible, it is always visible that different areas absorb paint and water differently. Irregularities in processing are the result of poor-quality mechanical processing of wood (sawing, planing, grinding, etc.). All these irregularities in everyday life are called barbs. According to GOST 7016-82, all irregularities are clearly classified (risks, kinematic waviness, fracture irregularities, elastic recovery irregularities in annual layers, pressing irregularities, etc.) and are called wood roughness. Roughness is measured in accordance with GOST 15612-85, taking into account the presence of individual torn off fibers (hairiness) and fiber bundles (mossiness) by the size of the heights of irregularities above the surface.

To reduce the roughness, the wood is planed, ground, and then fired with a short but powerful action of a gas burner. Burrs burn out without igniting the wood, because it does not have time to warm up to the temperatures of active pyrolysis. Possible sooty deposits formed during firing are removed by wiping with hard felt. Burrs on the surface of the wood, of course, remain, but very small.

To make wood even more inert to fire, it is impregnated with water salts followed by drying. It is clear that if all the pores in the wood (and in the villi too) are clogged with non-combustible salt, then the wood becomes more heat-consuming (it is more difficult to warm up) and more heat-conducting (heat is better removed from the ember starting to ignite). A lot of salt must be introduced into the surface layer, at least 20 kg per 1 m² of wood. Strengthening the effect will be achieved by choosing crystalline hydrates as salts (borax, sodium carbonate - household (crystal) soda, copper or iron vitriol it. which, when heated, decompose with the release of water, which evaporates and thereby cools the wood that is ready to flare. It is better if the salt decomposes with the absorption of heat and the release of gases that blow air away from the wood or break chains. chemical reactions ignition of pyrolysis products. It is even better if the decomposing salt also produces low-melting oxides and closes all the pores of the wood with the melt. So there can be a lot of impregnating compounds and the principles of their work.

If the work is responsible, done to order, then the impregnating composition should be chosen industrial (even if made from production waste), but certified in accordance with GOST 16363-76 (see section 5.7.16), providing the customer with a formal certificate. The trouble, however, is that it is now dangerous to believe in certificates in our country, and you can only rely on the authority of the company (if the products are not fake). Therefore, for your own needs, you can purchase the salts themselves at the chemical base, best of all, ammonium phosphate and / or ammonium sulfate. The fire-retardant amount of these salts will be 20-80 kg per 1 m³ of wood (SNiP I-A. 12-55). These salts can be dissolved in solution liquid glass(sodium or potassium), as well as with antiseptic salts such as sodium fluoride, zinc chloride, blue vitriol etc.

Having impregnated with an aqueous solution of salts and dried, the wood can be coated with fire-retardant paint, which should not be deeply absorbed into the wood, but create a non-combustible film on the surface that covers the unevenness of the wood. These paints include silicate, oil with the obligatory addition of effective flame retardants, vinyl chloride, organosilicon, etc. The amount of paint should be at least 0.5-0.8 kg per 1 m² of wood surface. From improvised means, as a paint, you can use a solution of liquid glass (“office” glue for paper) with the addition of a fine filler (lithopone, chalk, titanium oxide) so that the powder clogs the pores and remains on the surface in the form of a layer glued together with silicate (or other varnish ) particles.

On top of the paint (or instead of it), you can apply a fire-retardant coating (coating) like plaster, but containing specific components: fibrous fillers, gas-forming substances, water-releasing crystalline hydrates, low-melting oxides. The cheapest samples include the well-known superphosphate coating SFO (dispersion of superphosphate in water), lime-clay salt coating IGSO (a mixture of lime dough - slaked lime with clay and table salt). Intumescent coatings are more advanced, for example, VPD for wood (analogue of VPM-2 for metal). As a coating, you can use ordinary lime-alabaster, lime-cement and cement-sand plasters, which should fit snugly against the wood surface so that all the uneven surfaces of the wood are smeared and have reliable thermal contact with the plaster. Such coatings and plasters prevent the ignition of wood, at least from a flame. short circuit equipment power supply wires during operation circuit breakers or 3 minutes exposure to flame blowtorch, although intumescent coatings can provide fire resistance even at the level of EI45 and can withstand the action of electric and gas welding.

In ordinary baths, reliable fire protection of wood in the area of ​​\u200b\u200bthe furnace unit is rare. Often wooden wall upholstered with a sheet of metal on asbestos. The fire resistance of such protection is low due to the high thermal conductivity of asbestos. To increase the effectiveness of this standard protection, laying the first layer of asbestos in wet form on a silicate-clay mortar, tightly adhering to all uneven surfaces of the wood.

All these methods of protection can make it difficult for the wood to ignite spontaneously, but with prolonged exposure to fire, the wood can still flare up, since the pyrolysis of wood cannot be prevented by any means. The combustion of wood can be hindered by restricting the access of air to the surface of the wood (with the appearance of smoke), limiting the transfer of heat from the flame zone to the wood, as well as impregnating the wood very large quantity salts and flame retardants (up to 200 kg per 1 m³ of wood). Moreover, the task is precisely that the smoke (the appearance of which cannot be prevented) does not degenerate into a flame.

Fundamentally, the combustion of TGM is similar to the combustion of gases and liquids and is a homogeneous, diffusion process the transformation of combustible substances into combustion products with the release of heat and light. Combustion is based on a redox reaction.

In the combustion of liquids and TGM there is an additional similarity: the need to prepare the substance for combustion (evaporation, melting, decomposition) and the release of combustible vapors; ignition occurs when the concentration of combustible vapors and gases of NKPRP is reached.

Let us consider the occurrence of TGM combustion by the example wood, which is one of the most widely used solid combustible building materials. The following stages of ignition and combustion of wood can be distinguished:

1) heating of a wet substance (wood temperature - up to 50 0 С);

2) wood drying (removal of physically bound water) - temperature up to 120-150 0 C. The first two stages are the longest and take about 55% of the total ignition duration. It must be added that at these stages the destruction of the substance does not yet occur;

3) removal of intracapillary and chemically bound water - temperature 150-180 0 C. At the same stage, the decomposition of the least stable components of wood (luminic acids) occurs. Mostly non-combustible gases and vapors are emitted - CO 2 and H 2 O, but there is a relatively small amount of combustible gases and vapors, such as carbon monoxide CO.

In order to substantiate its appearance, we recall that two stages of carbon combustion are distinguished. At the first stage, carbon is oxidized to carbon monoxide: C + 0.5O 2 = CO. Therefore, in combustion products there is always a toxic and flammable gas - CO ( carbon monoxide). Due to the fact that the decomposition products contain a certain amount of combustible gases and vapors, at this stage there is a possibility of spontaneous combustion of wood.

4) Heating dry material and thermal decomposition (pyrolysis) of wood:

start of pyrolysis (temperature 180-250 0 C). Wood at this temperature turns mainly into coal (60-70%). In general, few vapors and gases are released, most of them are non-combustible - carbon dioxide CO 2, water vapor H 2 O, as well as a small amount of carbon monoxide CO, methane CH 4, etc. With increasing temperature, the amount of combustible gases and vapors increases. By the end of this stage, the HPVA is ready for ignition from the ignition source. So, the ignition temperature pine wood 255 0 C, oak - 238 0 C. Note that with the grinding of a substance, its ignition temperature decreases (for example, the ignition temperature of pine sawdust is 196 0 C) in the absence of IZ, vapor ignition will not occur, and only with further heating, at higher temperatures (370-400 0 C), they will self-ignite;

· intensive decomposition of wood (temperature 280-400 0 С). At this stage, cellulose is converted mainly into gaseous combustible products and the main amount of combustible gases is released - about 40% of their total amount. In addition to the listed gases, hydrogen H 2 and ethylene C 2 H 4 are released. In addition to them, pairs of alcohols, aldehydes, ethers, ketones, etc. can be noted. In general, there are more than 350 types of products of thermal decomposition and combustion of wood.

We emphasize the fact that during the decomposition of wood, two ways are possible: a) at temperatures of 180-250 0 C, it turns mainly into coal; b) at temperatures of 280-400 0 C, predominantly volatile products are released. This is of great importance in the fire protection of wood. Knowing the factors that affect the burning rate allows you to control it.

5) Termination of the release of volatile compounds and the beginning of combustion of the carbonaceous residue - charcoal (temperature 500-600 0 C). The carbon residue is formed in the previous stages, but its combustion is hindered by the fact that atmospheric oxygen does not penetrate to it, since it burns out in the zone of flame reactions. At temperatures above 500 0 C, the release of "volatiles" practically stops and oxygen gains access to the surface of the carbonaceous residue (coal). From this moment, there is a simultaneous heterogeneous combustion(smoldering) coal and homogeneous combustion decomposition products that continue to exit through cracks from the underlying layers of wood. The thickness of the coal varies within 2.5 cm. When all the layers of wood are converted into coal, the release of gaseous decomposition products stops, and only the combustion of coal continues.

The thermal decomposition of coal, peat and a number of other materials proceeds similarly to wood. However, each case has its own characteristics. So, in peat, the total amount of volatile substances is less and their release begins at more low temperatures than wood (see Fig. 5.6). Coal consists of more heat-resistant components than wood, so its decomposition proceeds at higher temperatures and less intensively.

Rice. 5.6. Dependence of the relative yield of volatile pyrolysis products of solids on temperature 1 – wood; 2 - peat; 3- coal

It is known that wood is construction material has numerous advantages. However, it is combustible and flammable. To reduce the combustibility of wood, numerous methods (means) of fire protection are used.

The fire hazard of wood is determined by the laws of its thermal decomposition under the action of external heat fluxes, which begins at a temperature of 110˚С. Further heating is accompanied by the removal of free and bound moisture from the wood. This process is completed at a temperature of 180˚С, after which the decomposition of the least heat-resistant components begins with the release of CO 2 and H 2 O. At a temperature of ~ 250˚С, wood pyrolysis occurs with the release of gaseous products: CO, CH 2, H 2, CO 2, H 2 O. The escaping gas mixture is combustible and can be ignited from an ignition source. At higher temperatures, the process of thermal decomposition of wood is accelerated. The main mass of combustible gases, containing up to 25% hydrogen and up to 40% combustible hydrocarbons, is released in the temperature range from 350 to 450˚С.

One of the important factors that determine the fire hazard of wood is its ability to ignite and spread combustion when heated in air.

Burning wood occurs in the form of fiery combustion and smoldering. Under fire conditions, the main amount of heat is released during the period of flame combustion (up to 60%) and ~40% - during the smoldering period.

Indicators fire hazard some types of wood are shown in table 4.

Table 4 - Fire hazard indicators of various types of wood

The temperature indicators of the fire hazard of wood - the ignition and self-ignition temperatures - are determined by the laws of its thermal decomposition. The values ​​of these indicators for different breeds wood, as can be seen from Table 2, are in a fairly narrow temperature range.

Dry wood of all species is a flammable (B3) highly combustible (G4) material with a high smoke-generating ability (D3). According to the toxicity of combustion products, wood belongs to the group of highly hazardous materials (T3). The linear velocity of flame propagation over the surface is 1-10 mm/s. This speed significantly depends on a number of factors: the type of wood, its moisture content, the magnitude of the incident heat flow, orientation of the burning surface. The smoldering rate is also not a constant value - for various types of wood it varies between 0.6 - 1.0 mm / min.

In construction, wood-based finishing materials are widely used: chipboard, fibreboard, wood panels, slats, plywood. All of these materials are combustible. Modified panels, slats, plywood. All of these materials are combustible. Modification of wood with polymers, as a rule, increases its fire hazard.

Table 5 shows the flammability characteristics of some wood-based building materials.

Table 5 - Flammability wood materials

Spread of flame over the surface of wood

Experimental studies of flame propagation over the surface of wood materials using different test methods have shown that not only the conditions of external thermal exposure, but also the type of wood affects the characteristics of flame propagation.

The influence of the type of wood can be traced to some extent when considering the values ​​of the so-called flame spread index (FRI).

IRP according to GOST 12.1.044-89 is a complex indicator, because when it is calculated, in addition to the flame propagation speed by separate sections surface of the sample and the limiting propagation distance, also uses data on the maximum temperature of the outgoing flue gases and time to reach it. Materials with FRI≤20 are referred to as slowly spreading flames, with FRI˃20 as fast spreading flames. All types of wood belong to the last group of materials. Their index is over 55.

Table 4 shows the IRP values ​​of untreated wood samples with a thickness of 19-25 mm.

Although most types of wood belong to the 3rd most dangerous class in terms of their ability to spread flames over the surface of ceiling structures in case of fire, some softwood samples, as follows from Table 6, have lower RFI values ​​and belong to class 2.

Table 6 - RFI value and fire spread class

An increase in the heat flux to the wood surface causes a significant increase in the speed of flame propagation. Termination of the process is possible if the heat flux from its own flame becomes less critical for this material.

Tests of wood-based finishing building materials under conditions simulating the development of a real fire showed fairly high flame propagation rates over them (Table 7).

Table 7 - Flame propagation speed over wood-based claddings

Smoke-generating ability and toxicity of wood combustion products

Emission of smoke from toxic gases is the dominant fire hazard. It manifests itself in the toxic and irritating effect of combustion products, as well as in the deterioration of visibility in a smoky environment. The deterioration in visibility makes it difficult to evacuate people from the danger zone, which, in turn, increases the risk of poisoning by combustion products. The situation in case of fire is further complicated by the fact that flue gases quickly spread in space and penetrate into rooms far from the source of the fire. The concentration of the emitted smoke and its nature depend on the structural features and chemical composition of the combustible material, combustion conditions.

More than 200 compounds, products of incomplete combustion, were found in the flue gases generated during the combustion of wood. The maximum value of the optical density during combustion of each type of wood depends in a complex way on the density of the external heat flux. Smoke generation coefficient during decomposition and smoldering combustion of wood different types depends on the density of the external heat flux (Figure 14).

1 - spruce; 2 - Moscow pine; 3 - thongkaribe pine; 4 - Ilyim elm; 5 - acacia keolai; 6 - chestnut; 7 - acacia; 8- eucalyptus bachdan.

Figure 14 - Characteristics of smoke generation.

A similar extreme character of the curves for the dependence of the toxicity index of wood combustion products on the density of the external heat flux (Figure 15). In the mode of smoldering combustion of spruce wood, the CO output is 70–240 times higher than the CO output during flame combustion.

In the smoldering mode in the temperature range of 450-550 ˚С, all types of wood manifest themselves as highly hazardous in terms of the toxicity of combustion products and belong to the T3 group. With an increase in the intensity of thermal exposure to 60-65 kW / m 2 (which corresponds to a temperature of 700-750 ˚С), according to the toxicity of combustion products, wood of various types passes into the group of moderately hazardous materials T2.

1- linden; 2 - birch; 3 - Ilyim elm; 4 - oak; 5 - aspen; 6 - pine; 7 - spruce.

Figure 15 - Toxicity of combustion products from the temperature of thermal exposure.

When burning wood, a fairly intense smoke generation occurs. The greatest amount of smoke is emitted during the combustion of wood materials in the smoldering mode (table 8).

Table 8 - Smoke-generating ability of wood materials during tests in the smoldering mode

4 Fire safety measures in the construction of wooden buildings