Heat of combustion. Specific heat of combustion of fuel and combustible materials Heat of combustion of varnish

5. Categories of buildings according to explosion and fire hazard

5.1. A building belongs to category A if the total area of ​​category A premises in it exceeds 5% of the area of ​​all premises or 200 m 2.

It is allowed not to classify a building as category A if the total area of ​​category A premises in the building does not exceed 25% of the total area of ​​all premises located in it (but not more than 1000 m2), and these premises are equipped with automatic fire extinguishing installations.

5.2. A building belongs to category B if two conditions are simultaneously met:

a) the building does not belong to category A;

b) the total area of ​​premises of categories A and B exceeds 5% of the total area of ​​all premises or 200 m2.

It is allowed not to classify a building as category B if the total area of ​​premises of categories A and B in the building does not exceed 25% of the total area of ​​all premises located in it (but not more than 1000 m2), and these premises are equipped with automatic fire extinguishing installations.

b) the total area of ​​premises of categories A, B and B1-B3 exceeds 5% (10% if the building does not have premises of categories A and B) of the total area of ​​all premises.

It is allowed not to classify a building as categories B1-B3 if the total area of ​​premises of categories A, B and B1-C3 in the building does not exceed 25% of the total area of ​​all premises located in it (but not more than 3500 m2), and these premises are equipped with automatic fire extinguishing

5.4. A building belongs to category G if two conditions are simultaneously met:

b) the total area of ​​premises of categories A, B, B1-B3 and D exceeds 5% of the total area of ​​all premises.

It is allowed not to classify a building as category D if the total area of ​​premises of categories A, B, B1-C3 and D in the building does not exceed 25% of the total area of ​​​​all premises located in it (but not more than 5000 m2), and premises of categories A, B and B1-B3 are equipped with automatic fire extinguishing installations.

5.5. A building belongs to category B4 if it does not belong to categories A, B, B1-B3 or D.

5.6. A building belongs to category D if it does not belong to categories A, B, B1-B4, D.

Annex 1

Initial data for calculating the specific temporary fire load in premises

Table 1

Lower calorific value and density of THM, flammable liquid and gas liquid,

circulating in the premises of railway transport facilities

Name of substances and materials	Lower calorific value, MJ kg -1	Density,
Liquid flammable substances and materials
4. Butyl alcohol
5. Diesel fuel
6. Kerosene
8. Insulating impregnating varnish (BT-99, FL-98) (volatile content - 48%)
10. Industrial oil
11. Transformer oil
12. Turbine oil
13. Methyl alcohol
15. Solar oil
16. Toluene
17. White spirit
18. Enamel PF-115 (volatile content - 34%)
19. Ethyl alcohol
20. Glue (rubber)
Solid flammable substances and materials
21. Paper loosened
22. Paper (books, magazines)
23. Vinyl leather
24. Staple fiber
25. Construction felt
26. Pine wood ( W p = 20%)
27. Fiberboard (Fibreboard)
28. Chipboard (chipboard)
30. Carbolite products
31. Natural rubber
32. Synthetic rubber
33. Cable (power, lighting, control, automation)
34. Gray cardboard
35. Triacetate film
36. PVC linoleum
37. Flax loosened
38. Mipora (porous rubber)
39. Organic glass
40. Wiping material
41. Joiner's plate
42. Polyurethane foam
43. Polystyrene foam boards
44. Rubber
45. Fiberglass
46. ​​Cotton fabric (in bulk)
47. Wool fabric (in bulk)
48. Plywood
49. Rubber and polyvinyl chloride insulation of wires

Combustible material		Combustible material	Heat of combustion, MJ× kg -1
Paper loosened	13,4	Phenoplastics	11,3
Staple fiber	13,8	Cotton loosened	15,7
Wood in products	16,6	Amyl alcohol	39,0
Carbolite products	24,9	Acetone	20,0
Synthetic rubber	40,2	Benzene	40,9
Organic glass	25,1	Petrol	41,9
Polystyrene	39,0	Butyl alcohol	36,2
Polypropylene	45,6	Diesel fuel	43,0
Polyethylene	47,1	Kerosene	43,5
Rubber products	33,5	Fuel oil	39,8
Oil	41,9	Ethanol	27,2

The specific fire load q, MJ× m -2 is determined from the relationship, where S is the area where the fire load is located, m 2 (but not less than 10 m 2).

Task Determine the fire hazard category of the premises with an area of ​​S=84 m2.

The room contains: 12 tables made of wood chip material weighing 16 kg each; 4 stands made of wood chip material weighing 10 kg each; 12 benches made of chipboard, 12 kg each; 3 cotton curtains, 5 kg each; fiberglass board weighing 25 kg; linoleum weighing 70 kg.

Solution

1. The lower calorific value of the materials in the room is determined (Table 7.6):

Q =16.6 MJ/kg – for tables, benches and stands;

Q =15.7 MJ/kg – for curtains;

Q =33.5 MJ/kg – for linoleum;

Q =25.1 MJ/kg – for a fiberglass board.

2. Using formula 7.9, the total fire load in the room is determined

3. Specific fire load q is determined

Comparing the obtained values ​​of q = 112.5 with the data given in Table 7.4, we assign the premises to category B4 in terms of fire hazard.

RADIATION SAFETY

8.1. Basic concepts and definitions

Question What kind of radiation is called ionizing radiation?

Answer Ionizing radiation (hereinafter referred to as IR) is radiation whose interaction with a substance leads to the formation of ions of different signs in this substance. AI consists of charged (a and b particles, protons, fragments of fission nuclei) and uncharged particles (neutrons, neutrinos, photons).

Question What physical quantities characterize the interaction of AI with matter and with biological objects?

Answer The interaction of an AI with a substance is characterized by the absorbed dose.

Absorbed dose D is the main dosimetric quantity. It is equal to the ratio of the average energy dw transferred by ionizing radiation to a substance in an elementary volume to the mass dm of the substance in this volume:

The energy can be averaged over any given volume, in which case the average dose will be equal to the total energy delivered to the volume divided by the mass of that volume. In the SI system, the absorbed dose is measured in J/kg and has a special name gray (Gy). Non-systemic unit – rad, 1rad = 0.01 Gy. The dose increment per unit time is called dose rate:

To assess the radiation hazard of chronic human exposure, according to [8.2], special physical quantities are introduced - equivalent dose in an organ or tissue H T, R and effective dose E.

Equivalent dose H T,R – absorbed dose in an organ or tissue T, multiplied by the corresponding weighting factor for a given type of radiation W R:

Н T,R =W R × D T,R , (8.3)

where D T,R is the average absorbed dose in tissue or organ T;

W R – weighting factor for type R radiation.

When exposed to different types of irritants with different weighting factors W R, the equivalent dose is determined as the sum of equivalent doses for these types of irritants:

(8.4)

The values ​​of the weighting coefficients are given in table. 8.1 [8.1] .

First of all, let’s define the terms, since the question is not posed quite correctly.

, and you won’t find a list “cable type - value in MJ/m2”, it doesn’t exist and cannot exist. Specific fire load is calculated for indoors, in which different types and quantities of cable are laid, taking into account how much area they occupy. That is why the specific fire load dimension is Joules (Megajoules) per square meter.

The calculation of the specific fire load includes the quantities of different materials that create this fire load - in fact, everything that can burn. You write about the weight of one linear meter of cable, but in fact you need to take into account the mass flammable components in the cable, not the entire cable. It is the combustible mass that forms the fire load - mainly cable insulation.

There are no amendments to the wording of the third paragraph; it is correct.

All these terms, indicators and values ​​are used in the “Method for determining the categories of premises B1 - B4”, as described by the documents of the Ministry of Emergency Situations “On approval of the set of rules “Determination of categories of premises, buildings and external installations for explosion and fire hazards”, mandatory Appendix B. That the same approach is used in other regulatory documents, including departmental instructions.The following are excerpts from the document relevant to your question and our comments.

According to explosion and fire hazard, premises are divided into categories A, B, B1 - B4, D and D, and buildings - into categories A, B, C, D and D.

[Comment from the consultation section]: your question is about premises, we give a classification for them.

Room category	Characteristics of substances and materials located (circulating) in the premises
A increased explosion and fire hazard	Combustible gases, flammable liquids with a flash point of not more than 28°C in such quantities that they can form explosive vapor-gas-air mixtures, upon ignition of which a calculated excess explosion pressure in the room develops exceeding 5 kPa, and (or) substances and materials capable of exploding and burn when interacting with water, atmospheric oxygen or with each other, in such quantities that the calculated excess pressure of the explosion in the room exceeds 5 kPa.
B explosion and fire hazard	Combustible dusts or fibers, flammable liquids with a flash point of more than 28°C, flammable liquids in such quantities that they can form explosive dust-air or steam-air mixtures, the ignition of which develops a calculated excess explosion pressure in the room exceeding 5 kPa.
B1 – B4 fire hazard	Flammable and low-flammable liquids, solid flammable and low-flammable substances and materials (including dust and fibers), substances and materials that can only burn when interacting with water, air oxygen or with each other, provided that the rooms in which they are located (apply) do not belong to category A or B.
G moderate fire hazard	Non-combustible substances and materials in a hot, incandescent or molten state, the processing of which is accompanied by the release of radiant heat, sparks and flames, and (or) flammable gases, liquids and solids that are burned or disposed of as fuel.
D reduced fire hazard	Non-flammable substances and materials in a cold state.

Classification of a room into category B1, B2, B3 or B4 is carried out depending on the quantity and method of placing the fire load in the specified room and its space-planning characteristics, as well as on the fire hazardous properties of the substances and materials that make up the fire load.

[Comment from the consultation section]: your case includes categories B1 – B4, fire hazard. Moreover, there is a high probability that your premises will be classified as category B4, but this must be supported by calculations.

Methods for determining categories of premises B1 - B4

Determination of categories of premises B1 - B4 is carried out by comparing the maximum value of the specific temporary fire load (hereinafter referred to as the fire load) in any of the areas with the value of the specific fire load given in the table:

Specific fire load and placement methods for categories B1 – B4

For a fire load that includes various combinations (mixtures) of flammable, combustible, low-flammable liquids, solid flammable and low-flammable substances and materials within a fire-hazardous area, the fire load Q (in MJ) is determined by the formula:

- quantity i th material fire load, kg;

- net calorific value i th material fire load, MJ/kg.

(in MJ/m2) is defined as the ratio of the calculated fire load to the occupied area:

Where S– fire load placement area, m2, not less than 10 m2.

Part 2. Application practice

To perform calculations, it is necessary to determine the mass in kg for each combustible material that will be located in the room. Strictly speaking, for this you need to know how much insulation and other combustible components are in each meter of cable of the corresponding type, and take the footage from your project. But conventional product specifications, at best, contain a linear weight in g/m or kg/km for the cable as a whole; it is formed by all elements, including non-flammable ones. Only packaging – reel or box – is excluded from the net value.

In optical cables that do not have armor or built-in supporting metal cables, one can agree with this and use the linear weight in the calculations as is, deliberately neglecting the mass of the quartz fiber, since it is small. Here, for example, are the linear weights for universal XGLO™ and LightSystem cables with a tight buffer, intended for indoor/external use (the article begins with the symbols 9GD(X)H......, such cables are in your list):

Number of fibers	Linear weight, kg/km
4	23
6	25
8	30
12	35
16	49
24	61
48	255
72	384

And this is a table for XGLO™ and LightSystem cables with a free buffer, also intended for indoor/external use (the article begins with the symbols 9GG(X)H......):

Number of fibers	Linear weight, kg/km
2	67
4	67
6	67
8	67
12	67
16	103
24	103
36	103
48	115
72	115
96	139
144	139

So, if a 25 m long section of ten cables of 24 fibers each is laid in a room, their total weight will be 15.25 kg for a cable with a tight buffer and 25.75 kg for a cable with a loose buffer. As you can see, the numbers may vary, and for large quantities of cable the difference can be quite significant.

In armored optical cables and twisted pair copper cables, a significant proportion of the linear weight is formed by the mass of metal, and then the spread of numbers and the difference between the linear weight and the content of flammable substances can be even greater. For example, the net weight of 1 km of twisted pair cable can vary from 21 kg to 76 kg depending on the category, manufacturer and the presence/absence of a screen and other structural elements. At the same time, a simple calculation shows that for category 5e with a core diameter of 0.511 mm, the minimum weight of copper in 1 km (8 conductors, copper density 8920 kg/m3) will be 14.6 kg, and for category 7A with a core diameter of 0.643 mm - not less than 23.2 kg. And this does not take into account the laying, which leads to the fact that in fact the length of the copper conductors will obviously be more than 1 km.

On the same section of 25 m of, say, 120 twisted pair cables, the total mass of the cables can be from 63 kg to 228 kg depending on their type, while the copper in them can be from 43.8 kg and higher for category 5e and from 69.6 kg and above for category 7A.

The difference is large even for the quantities that we took, meaning not the largest telecommunications room, into which the cable is routed through a suspended tray or route under the raised floor. For armored and other specific cables with metal structural elements, the difference will be much greater, but at the same time they can be found mainly on the street, and not indoors.

If you take the calculation strictly, then for each type of cable you need to have a complete breakdown of the flammable and non-flammable components included in it and their weight content per unit length. In addition, the lower heating value in MJ/kg must be known for each combustible component. For polymers widely used in telecommunications, various sources give the following net calorific value values:

• Polyethylene – from 46 to 48 MJ/kg
• Polyvinyl chloride (PVC) – from 14 to 21 MJ/kg
• Polytetrafluoroethylene (fluoroplastic) – from 4 to 8 MJ/kg

Depending on what input data you use, the output may vary. Here are 2 examples of calculations for the already mentioned room with 120 twisted pair cables:

Example 1.

• 120 cables twisted pair category 5e
• Linear cable weight 23 kg/km

Total cable weight (excluding non-combustible components)

G i= 120 · 25 m · 23 · 10 -3 kg/m = 69 kg

Q= 69 kg · 18 MJ/kg = 1242 MJ

S tray= 25 m · 0.3 m = 7.5 m 2

g= 1242 / 10 = 124.2 MJ/m 2

The specific fire load refers to the range from 1 to 180 MJ/m 2, despite the fact that we have not subtracted the weight content of copper in the cable. If it had been subtracted, then the premises would have been classified as category B4.

Example 2.

• 120 twisted pair cables category 6/6A
• Conductor gauge 23 AWG
• PVC sheath, lower calorific value 18 MJ/kg
• Linear cable weight 45 kg/km
• Tray length 25 m, width 300 mm

Total cable weight excluding non-combustible components

G i= 120 · 25 m · 45 · 10 -3 kg/m = 135 kg

Q= 135 kg · 18 MJ/kg = 2430 MJ

S tray= 25 m · 0.3 m = 7.5 m 2

In accordance with the calculation methodology, it is necessary to use an area of ​​at least 10 m 2 in the calculations.

g= 2430 / 10 = 243 MJ/m2

The specific fire load exceeded 180 MJ/m2 and fell into the range corresponding to the higher room category B3. But if we subtracted the weight of the copper, the calculation would be different.

23 AWG conductor gauge corresponds to a diameter of 0.574 mm. The cable has 8 copper conductors, therefore, each kilometer of cable contains at least 18.46 kg of copper.

G i= 120 · 25 m · (45 – 18.46) · 10 -3 kg/m = 79.62 kg of combustible components

Q= 79.62 kg 18 MJ/kg = 1433.16 MJ

g= 1433.16 / 10 = 143.3 MJ/m2

In this case, we get room category B4. As you can see, the component component can influence the calculations quite significantly.

Accurate data on weight content and lower calorific value can only be obtained from the manufacturer of a specific product. Otherwise, you will have to personally “gut” each specific type of cable, measure the mass of each element on high-precision scales, and establish all the chemical compositions (which in itself can be a very non-trivial task, even if you have a well-equipped chemical laboratory). And after all this, make an accurate calculation. For category 6/6A cable, in our calculation, for example, the weight and material of the separator partition were not taken into account. If it is made of polyethylene, you need to take into account that its lower calorific value is higher than that of PVC.

Chemical and physical reference books provide values ​​for the lower calorific value for pure substances and indicative values ​​for the most popular building materials. But manufacturers can use mixtures of substances, additives, and vary the weight content of components. For accurate calculations, data from a specific manufacturer is needed for each type of product. They are usually not publicly available, but they should be provided upon request; this is not classified information.

However, if you have to wait a long time for such information, and you need to do the calculation now, you can perform approximate calculations, setting the maximum values ​​- i.e. take the worst case scenario. The designer chooses the maximum possible value of the lower calorific value, the maximum weight content of combustible substances, deliberately making a big mistake, not in his favor. In some cases, because of this, the premises will fall into a more dangerous category, as we first did in Example 2. It is absolutely impossible to “err” in the other direction, deliberately making the calculations more optimistic. In case of any doubt, the interpretation should always be in the direction of additional security measures.

Chemical reactions are accompanied by the absorption or release of energy, in particular heat. reactions accompanied by the absorption of heat, as well as the compounds formed during this process, are called endothermic . In endothermic reactions, heating of the reacting substances is necessary not only for the occurrence of the reaction, but also during the entire time of their occurrence. Without external heating, the endothermic reaction stops.

reactions accompanied by the release of heat, as well as the compounds formed during this process, are called exothermic . All combustion reactions are exothermic. Due to the release of heat, they, having arisen at one point, are able to spread to the entire mass of reacting substances.

The amount of heat released during complete combustion of a substance and related to one mole, unit of mass (kg, g) or volume (m 3) of a combustible substance is called heat of combustion. The heat of combustion can be calculated from tabular data using Hess's law. Russian chemist G.G. Hess in 1840 discovered a law that is a special case of the law of conservation of energy. Hess's law is as follows: the thermal effect of a chemical transformation does not depend on the path along which the reaction occurs, but depends only on the initial and final states of the system, provided that the temperature and pressure (or volume) at the beginning and end of the reaction are the same.

Let's consider this using the example of calculating the heat of combustion of methane. Methane can be produced from 1 mole of carbon and 2 moles of hydrogen. When methane is burned, it produces 2 moles of water and 1 mole of carbon dioxide.

C + 2H 2 = CH 4 + 74.8 kJ (Q 1).

CH 4 + 2O 2 = CO 2 + 2H 2 O + Q horizon.

The same products are formed by the combustion of hydrogen and carbon. During these reactions, the total amount of heat released is 963.5 kJ.

2H 2 + O 2 = 2H 2 O + 570.6 kJ

C + O 2 = CO 2 + 392.9 kJ.

Since the initial and final products are the same in both cases, their total thermal effects must be equal according to Hess's law, i.e.

Q 1 + Q mountains = Q,

Q mountains = Q - Q 1. (1.11)

Therefore, the heat of combustion of methane will be equal to

Q mountains = 963.5 - 74.8 = 888.7 kJ/mol.

Thus, the heat of combustion of a chemical compound (or their mixture) is equal to the difference between the sum of the heats of formation of combustion products and the heat of formation of the burned chemical compound (or substances that make up the combustible mixture). Therefore, to determine the heat of combustion of chemical compounds, it is necessary to know the heat of their formation and the heat of formation of the products obtained after combustion.

Below are the heats of formation of some chemical compounds:

Aluminum oxide Al 2 O 3 ………		Methane CH 4 …………………………
Iron oxide Fe 2 O 3 …………		Ethane C 2 H 6 ……………………
Carbon monoxide CO………….		Acetylene C 2 H 2 ………………
Carbon dioxide CO2………		Benzene C 6 H 6 …………………
Water H 2 O ………………………….		Ethylene C 2 H 4 …………………
Water vapor H 2 O ……………		Toluene C 6 H 5 CH 3 …………….

Example 1.5 .Determine the combustion temperature of ethane if the heat of its formationQ 1 = 88.4 kJ. Let's write the combustion equation for ethane.

C 2 H 6 + 3.5O 2 = 2 CO 2 + 3 H 2 O + Qmountains.

For determiningQmountainsit is necessary to know the heat of formation of combustion products. the heat of formation of carbon dioxide is 396.9 kJ, and that of water is 286.6 kJ. Hence,Qwill be equal

Q = 2 × 396,9 + 3 × 286.6 = 1653.6 kJ,

and the heat of combustion of ethane

Qmountains= Q - Q 1 = 1653.6 - 88.4 = 1565.2 kJ.

The heat of combustion is experimentally determined in a bomb calorimeter and a gas calorimeter. There are higher and lower calorific values. Higher calorific value Q in is the amount of heat released during the complete combustion of 1 kg or 1 m 3 of a combustible substance, provided that the hydrogen contained in it burns to form liquid water. Lower calorific value Qn is the amount of heat released during the complete combustion of 1 kg or 1 m 3 of a combustible substance, provided that hydrogen is burned until water vapor is formed and the moisture of the combustible substance is evaporated.

The higher and lower heats of combustion of solid and liquid combustible substances can be determined using the formulas of D.I. Mendeleev:

where Q in, Q n - higher and lower calorific values, kJ/kg; W – content of carbon, hydrogen, oxygen, combustible sulfur and moisture in the combustible substance, %.

Example 1.6. Determine the lowest combustion temperature of sulfur fuel oil consisting of 82.5% C, 10.65% H, 3.1%Sand 0.5% O; A (ash) = 0.25%,W = 3%. Using the equation of D.I. Mendeleev (1.13), we obtain

=38622.7 kJ/kg

The lower calorific value of 1 m3 of dry gases can be determined by the equation

The lower calorific value of some flammable gases and liquids, obtained experimentally, is given below:

Hydrocarbons: methane………………………..
ethane …………………………
propane………………………
methyl………………….
ethyl…………………………
propyl………………………

The lower calorific value of some combustible materials, calculated from their elemental composition, has the following values:

Gasoline……………………		Synthetic rubber
Paper ……………………		Kerosene………………
Wood		Organic glass..
air-dry………..		Rubber ………………..
in building structures...		Peat ( W = 20 %) …….

There is a lower limit of calorific value, below which substances become incapable of combustion in the air atmosphere.

Experiments show that substances are non-flammable if they are not explosive and if their lower calorific value in air does not exceed 2100 kJ/kg. Consequently, the heat of combustion can serve as an approximate estimate of the flammability of substances. However, it should be noted that the flammability of solids and materials largely depends on their condition. Thus, a sheet of paper, easily ignited by the flame of a match, when applied to the smooth surface of a metal plate or concrete wall, becomes difficult to combust. Consequently, the flammability of substances also depends on the rate of heat removal from the combustion zone.

In practice, during the combustion process, especially in fires, the heat of combustion indicated in the tables is not completely released, since combustion is accompanied by underburning. It is known that petroleum products, as well as benzene, toluene, acetylene, i.e. substances rich

carbon, burn in fires with the formation of a significant amount of soot. Soot (carbon) can burn and produce heat. If it is formed during combustion, then, consequently, the combustible substance emits less heat than the amount indicated in the tables. For substances rich in carbon, the underburning coefficient h is 0.8 - 0.9. Consequently, in fires when burning 1 kg of rubber, not 33520 kJ can be released, but only 33520´0.8 = 26816 kJ.

Fire size is usually characterized by the area of ​​the fire. The amount of heat released per unit area of ​​fire per unit time is called heat of fire Q p

QP= Qnυ mh ,

Where υ m– mass burnout rate, kg/(m 2 ×s).

The specific heat of fire during internal fires characterizes the thermal load on the structures of buildings and structures and is used to calculate the fire temperature.

1.6. Combustion temperature

The heat released in the combustion zone is perceived by the combustion products, so they heat up to a high temperature. The temperature to which combustion products are heated during combustion is called combustion temperature . There are calorimetric, theoretical and actual combustion temperatures. The actual combustion temperature for fire conditions is called fire temperature.

The calorimetric combustion temperature is understood as the temperature to which the products of complete combustion are heated under the following conditions:

1) all the heat released during combustion is spent on heating the combustion products (heat loss is zero);

2) the initial temperatures of air and flammable substances are 0 0 C;

3) the amount of air is equal to the theoretically required (a = 1);

4) complete combustion occurs.

The calorimetric combustion temperature depends only on the composition of the combustible substance and does not depend on its quantity.

Theoretical temperature, in contrast to calorimetric temperature, characterizes combustion taking into account the endothermic process of dissociation of combustion products at high temperature

2СО 2 2СО + О 2 - 566.5 kJ.

2H 2 O2H 2 + O 2 - 478.5 kJ.

In practice, the dissociation of combustion products must be taken into account only at temperatures above 1700 0 C. During diffusion combustion of substances in fire conditions, the actual combustion temperatures do not reach such values, therefore, to assess fire conditions, only the calorimetric combustion temperature and the fire temperature are used. There is a distinction between internal and external fire temperatures. The internal fire temperature is the average temperature of the smoke in the room where the fire occurs. External fire temperature – flame temperature.

When calculating the calorimetric combustion temperature and the internal fire temperature, it is assumed that the lower heat of combustion Qn of a combustible substance is equal to the energy qg required to heat the combustion products from 0 0 C to the calorimetric combustion temperature

, - heat capacity of the components of combustion products (heat capacity of CO 2 is taken for a mixture of CO 2 and SO 2), kJ/(m 3 ? K).

In fact, not all the heat released during combustion under fire conditions is spent on heating the combustion products. Most of it is spent on heating structures, preparing flammable substances for combustion, heating excess air, etc. Therefore, the temperature of an internal fire is significantly lower than the calorimetric temperature. The combustion temperature calculation method assumes that the entire volume of combustion products is heated to the same temperature. In reality, the temperature at different points of the combustion center is not the same. The highest temperature is in the region of space where the combustion reaction occurs, i.e. in the combustion (flame) zone. The temperature is significantly lower in places where there are flammable vapors and gases released from the burning substance and combustion products mixed with excess air.

In order to judge the nature of temperature changes during a fire depending on various combustion conditions, the concept of average volumetric fire temperature was introduced, which is understood as the average value of the temperatures measured by thermometers at various points of the internal fire. This temperature is determined from experience.

What is fuel?

This is one component or a mixture of substances that are capable of chemical transformations associated with the release of heat. Different types of fuel differ in the quantitative content of oxidizer, which is used to release thermal energy.

In a broad sense, fuel is an energy carrier, that is, a potential type of potential energy.

Classification

Currently, fuel types are divided according to their state of aggregation into liquid, solid, and gaseous.

Natural hard materials include stone, firewood and anthracite. Briquettes, coke, thermoanthracite are types of artificial solid fuel.

Liquids include substances containing substances of organic origin. Their main components are: oxygen, carbon, nitrogen, hydrogen, sulfur. Artificial liquid fuel will be a variety of resins and fuel oil.

Gaseous fuel is a mixture of various gases: ethylene, methane, propane, butane. In addition to them, the composition contains carbon dioxide and carbon monoxide, hydrogen sulfide, nitrogen, water vapor, and oxygen.

Fuel indicators

The main indicator of combustion. The formula for determining the calorific value is considered in thermochemistry. emit “standard fuel”, which implies the calorific value of 1 kilogram of anthracite.

Household heating oil is intended for combustion in heating devices of low power, which are located in residential premises, heat generators used in agriculture for drying feed, canning.

The specific heat of combustion of a fuel is a value that demonstrates the amount of heat that is generated during the complete combustion of fuel with a volume of 1 m 3 or a mass of one kilogram.

To measure this value, J/kg, J/m3, calorie/m3 are used. To determine the heat of combustion, the calorimetry method is used.

With an increase in the specific heat of combustion of fuel, the specific fuel consumption decreases, and the efficiency remains unchanged.

The heat of combustion of substances is the amount of energy released during the oxidation of a solid, liquid, or gaseous substance.

It is determined by the chemical composition, as well as the state of aggregation of the combustible substance.

Features of combustion products

The higher and lower calorific values ​​are related to the state of aggregation of water in the substances obtained after combustion of fuel.

The higher calorific value is the amount of heat released during complete combustion of a substance. This value also includes the heat of condensation of water vapor.

The lowest working heat of combustion is the value that corresponds to the release of heat during combustion without taking into account the heat of condensation of water vapor.

The latent heat of condensation is the amount of energy of condensation of water vapor.

Mathematical relationship

The higher and lower calorific values ​​are related by the following relationship:

QB = QH + k(W + 9H)

where W is the amount by weight (in %) of water in a flammable substance;

H is the amount of hydrogen (% by mass) in the combustible substance;

k - coefficient equal to 6 kcal/kg

Methods for performing calculations

The higher and lower calorific values ​​are determined by two main methods: calculation and experimental.

Calorimeters are used to carry out experimental calculations. First, a sample of fuel is burned in it. The heat that will be released is completely absorbed by the water. Having an idea of ​​the mass of water, you can determine by the change in its temperature the value of its heat of combustion.

This technique is considered simple and effective; it only requires knowledge of technical analysis data.

In the calculation method, the higher and lower calorific values ​​are calculated using the Mendeleev formula.

Q p H = 339C p +1030H p -109(O p -S p) - 25 W p (kJ/kg)

It takes into account the content of carbon, oxygen, hydrogen, water vapor, sulfur in the working composition (in percent). The amount of heat during combustion is determined taking into account the equivalent fuel.

The heat of combustion of gas allows preliminary calculations to be made and the effectiveness of using a certain type of fuel to be determined.

Features of origin

In order to understand how much heat is released when a certain fuel is burned, it is necessary to have an idea of ​​its origin.

In nature, there are different versions of solid fuels, which differ in composition and properties.

Its formation occurs through several stages. First, peat is formed, then brown and hard coal are formed, then anthracite is formed. The main sources of solid fuel formation are leaves, wood, and pine needles. When parts of plants die and are exposed to air, they are destroyed by fungi and form peat. Its accumulation turns into a brown mass, then brown gas is obtained.

At high pressure and temperature, brown gas turns into coal, then the fuel accumulates in the form of anthracite.

In addition to organic matter, the fuel contains additional ballast. Organic is considered to be that part that is formed from organic substances: hydrogen, carbon, nitrogen, oxygen. In addition to these chemical elements, it contains ballast: moisture, ash.

Combustion technology involves the separation of the working, dry, and combustible mass of burned fuel. The working mass is the fuel in its original form supplied to the consumer. Dry mass is a composition in which there is no water.

Compound

The most valuable components are carbon and hydrogen.

These elements are contained in any type of fuel. In peat and wood, the percentage of carbon reaches 58 percent, in hard and brown coal - 80%, and in anthracite it reaches 95 percent by weight. Depending on this indicator, the amount of heat released during fuel combustion changes. Hydrogen is the second most important element of any fuel. When it binds with oxygen, it forms moisture, which significantly reduces the thermal value of any fuel.

Its percentage ranges from 3.8 in oil shale to 11 in fuel oil. The oxygen contained in the fuel acts as ballast.

It is not a heat-generating chemical element, therefore it negatively affects the value of its heat of combustion. The combustion of nitrogen, contained in free or bound form in combustion products, is considered harmful impurities, therefore its quantity is strictly limited.

Sulfur is included in fuel in the form of sulfates, sulfides, and also as sulfur dioxide gases. When hydrated, sulfur oxides form sulfuric acid, which destroys boiler equipment and negatively affects vegetation and living organisms.

That is why sulfur is a chemical element whose presence in natural fuel is extremely undesirable. If sulfur compounds get inside the work area, they cause significant poisoning of operating personnel.

There are three types of ash depending on its origin:

• primary;
• secondary;
• tertiary

The primary species is formed from minerals found in plants. Secondary ash is formed as a result of plant residues entering sand and soil during formation.

Tertiary ash appears in the composition of fuel during extraction, storage, and transportation. With significant ash deposition, a decrease in heat transfer on the heating surface of the boiler unit occurs, reducing the amount of heat transfer to water from gases. A huge amount of ash negatively affects the operation of the boiler.

Finally

Volatile substances have a significant influence on the combustion process of any type of fuel. The greater their output, the larger the volume of the flame front will be. For example, coal and peat ignite easily, the process is accompanied by minor heat losses. The coke that remains after removing volatile impurities contains only mineral and carbon compounds. Depending on the characteristics of the fuel, the amount of heat changes significantly.

Depending on the chemical composition, there are three stages of solid fuel formation: peat, lignite, and coal.

Natural wood is used in small boiler installations. They mainly use wood chips, sawdust, slabs, bark, and the firewood itself is used in small quantities. Depending on the type of wood, the amount of heat generated varies significantly.

As the heat of combustion decreases, firewood acquires certain advantages: rapid flammability, minimal ash content, and the absence of traces of sulfur.

Reliable information about the composition of natural or synthetic fuel, its calorific value, is an excellent way to carry out thermochemical calculations.

Currently, there is a real opportunity to identify those main options for solid, gaseous, liquid fuels that will be the most effective and inexpensive to use in a certain situation.