The lowest heat of combustion of enamel paint. Specific heat of combustion of fuel and combustible materials

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 various types AI with different weighting factors W R equivalent dose is defined as the sum of equivalent doses for these types of AI:

(8.4)

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

The calorific value is understood as the heat of complete combustion of a unit mass of a substance. It takes into account heat losses associated with the dissociation of combustion products and the incompleteness of chemical combustion reactions. Calorific value is the maximum possible heat of combustion per unit mass of a substance.

Determine the calorific value of elements, their compounds and fuel mixtures. For elements, it is numerically equal to the heat of formation of the combustion product. The calorific value of mixtures is an additive quantity and can be found if the calorific value of the components of the mixture is known.

Combustion occurs not only due to the formation of oxides, therefore, in a broad sense, we can talk about the calorific value of elements and their compounds not only in oxygen, but also when interacting with fluorine, chlorine, nitrogen, boron, carbon, silicon, sulfur and phosphorus.

Calorific value is important characteristic. It allows you to evaluate and compare with others the maximum possible heat release of a particular redox reaction and determine in relation to it the completeness of the actual combustion processes. Knowledge of the calorific value is necessary when selecting fuel components and mixtures for various purposes and when assessing their completeness of combustion.

There are higher H in and lower H n calorific value. Higher calorific value, in contrast to lower calorific value, includes the heat of phase transformations (condensation, solidification) of combustion products when cooled to room temperature. Thus, the highest calorific value is the heat of complete combustion of a substance when the physical state of the combustion products is considered at room temperature, and the lowest calorific value is at the combustion temperature. The higher calorific value is determined by burning the substance in a calorimetric bomb or by calculation. It includes, in particular, the heat released during the condensation of water vapor, which at 298 K is equal to 44 kJ/mol. The lower calorific value is calculated without taking into account the heat of condensation of water vapor, for example, according to the formula

Where % H is the percentage of hydrogen in the fuel.

If calorific value values indicate the physical state of combustion products (solid, liquid or gaseous), then the “highest” and “lowest” subscripts are usually omitted.

Let us consider the calorific value of hydrocarbons and elements in oxygen per unit mass of the original fuel. The lower calorific value differs from the highest for paraffins by an average of 3220-3350 kJ/kg, for olefins and naphthenes - by 3140-3220 kJ/kg, for benzene - by 1590 kJ/kg. At experimental determination calorific value, it should be borne in mind that in a calorimetric bomb the substance burns at constant volume, and in real conditions - often at constant pressure. The correction for the difference in combustion conditions is for solid fuel from 2.1 to 12.6, for fuel oil - about 33.5, gasoline - 46.1 kJ/kg, and for gas reaches 210 kJ/m3. In practice, this correction is introduced only when determining the calorific value of the gas.

For paraffins, the calorific value decreases with increasing boiling point and increasing C/H ratio. For monocyclic alicyclic hydrocarbons this change is much less. In the benzene series, the calorific value increases when moving to higher homologues due to the side chain. Dinuclear aromatic hydrocarbons have a lower calorific value than the benzene series.

Only a few elements and their compounds have a calorific value that exceeds that of hydrocarbon fuels. These elements include hydrogen, boron, beryllium, lithium, their compounds and several organoelement compounds of boron and beryllium. The calorific value of elements such as sulfur, sodium, niobium, zirconium, calcium, vanadium, titanium, phosphorus, magnesium, silicon and aluminum lies in the range of 9210-32,240 kJ/kg. For the remaining elements of the periodic system, the calorific value does not exceed 8374 kJ/kg. Gross calorific value data various classes combustibles are given in table. 1.18.

Table 1.18

Gross calorific value of various combustibles in oxygen (per unit mass of fuel)

Substance
Substance

Carbon monoxide





iso-butane





n-Dodecane
n-Hexadecane

Acetylene
Cyclopentane
Cyclohexane


Ethylbenzene


Beryllium


Aluminum





Zirconium
Beryllium hydride

Psntaboran
Metadiborane
Ethyldiborane

For liquid hydrocarbons, methanol and ethanol, heating values are based on the liquid starting state.

The calorific value of some fuels was calculated on a computer. It is 24.75 kJ/kg for magnesium and 31.08 kJ/kg for aluminum (the state of the oxides is solid) and practically coincides with the data in Table. 1.18. The highest calorific value of paraffin C26H54, naphthalene C10H8, anthracene C14H10 and methenamine C6H12N4 are 47.00, 40.20, 39.80 and 29.80, respectively, and the lowest calorific value is 43.70, 39.00, 38.40 and 28.00 kJ/kg.

As an example, in relation to rocket fuels, we present the heats of combustion of various elements in oxygen and fluorine, per unit mass of combustion products. The heats of combustion are calculated for the state of combustion products at a temperature of 2700 K and are shown in Fig. 1.25 and in table. 1.19.

Puc. 1.25. Heat of combustion of elements in oxygen (1) and fluorine(2), calculated per kilogram of combustion products

As follows from the data presented, to obtain maximum combustion heat, the most preferred substances are those containing hydrogen, lithium and beryllium, and secondarily, boron, magnesium, aluminum and silicon. The advantage of hydrogen due to the low molecular weight of combustion products is obvious. It should be noted that beryllium has an advantage due to its high heat of combustion.

There is the possibility of the formation of mixed combustion products, in particular gaseous oxyfluorides of elements. Since the oxyfluorides of trivalent elements are usually stable, most oxyfluorides are not effective combustion products rocket fuels due to high molecular weight. The heat of combustion with the formation of COF2 (g) has an intermediate value between the heats of combustion of CO2 (g) and CF4 (g). The heat of combustion with the formation of SO2F2 (g) is greater than in the case of the formation of SO2 (g) or SF6; (G.). However, most rocket fuels contain highly reducing elements that prevent the formation of such substances.

The formation of aluminum oxyfluoride AlOF (g) releases less heat than the formation of oxide or fluoride, so it is not of interest. Boron oxyfluoride BOF (g) and its trimer (BOF)3 (g) are quite important components of the combustion products of rocket fuels. The heat of combustion to form BOF (g) is intermediate between the heats of combustion to form oxide and fluoride, but oxyfluoride is thermally more stable than either of these compounds.

Table 1.19

Heat of combustion of elements (in MJ/kg), per unit mass of combustion products ( T = 2700 K)


			oxyfluoride


Beryllium



Oxygen



Aluminum




Zirconium

When beryllium and boron nitrides are formed, enough large number heat, which makes it possible to classify them as important components of combustion products of rocket fuels.

In table Table 1.20 shows the highest calorific value of elements when they interact with various reagents, referred to a unit mass of combustion products. The calorific value of elements when interacting with chlorine, nitrogen (except for the formation of Be3N2 and BN), boron, carbon, silicon, sulfur and phosphorus is significantly less than the calorific value of elements when interacting with oxygen and fluorine. The wide variety of requirements for combustion processes and reagents (in terms of temperature, composition, state of combustion products, etc.) makes it advisable to use the data in Table. 1.20 in the practical development of fuel mixtures for one purpose or another.

Table 1.20

Higher calorific value of elements (in MJ/kg) when interacting with oxygen, fluorine, chlorine, nitrogen, per unit mass of combustion products

See also: Joulin S., Clavin R. Op. cit.

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 fuels differ in their 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 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 makes it possible to carry out preliminary calculations and determine the effectiveness of using a certain type of fuel.

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 it becomes brown and coal, 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 heat generating chemical element, therefore 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. Huge number ash has a negative impact on the operation of the boiler.

In conclusion

A significant influence on the combustion process of any type of fuel is exerted by volatile substances. 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 chemical composition There are three stages of solid fuel formation: peat, brown coal, 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 variants of solid, gaseous, liquid fuel, which will be the most effective and inexpensive to use in a certain situation.

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 exceeds 5% of the area of all premises or 200 m2.

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.

Appendix 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

Types of calorific value

The heat of combustion can be related to the working mass of the combustible substance, that is, to the combustible substance in the form in which it reaches the consumer; to the dry mass of the substance; to a combustible mass of a substance, that is, to a combustible substance that does not contain moisture and ash.

There are higher () and lower () calorific values.

Under higher calorific value understand the amount of heat that is released during complete combustion of a substance, including the heat of condensation of water vapor when cooling the combustion products.

Lower heating value corresponds to the amount of heat that is released during complete combustion, without taking into account the heat of condensation of water vapor. The heat of condensation of water vapor is also called latent heat of combustion.

The lower and higher calorific values are related by the relation: ,

where k is a coefficient equal to 25 kJ/kg (6 kcal/kg); W is the amount of water in the flammable substance, % (by mass); H is the amount of hydrogen in a combustible substance, % (by mass).

Calculation of calorific value

Thus, the higher calorific value is the amount of heat released during complete combustion of a unit mass or volume (for gas) of a combustible substance and cooling of the combustion products to the dew point temperature. In thermal engineering calculations, the higher calorific value is taken as 100%. The latent heat of combustion of a gas is the heat that is released during the condensation of water vapor contained in the combustion products. Theoretically, it can reach 11%.

In practice, it is not possible to cool the combustion products until complete condensation, and therefore the concept of lower calorific value (QHp) has been introduced, which is obtained by subtracting from the higher calorific value the heat of vaporization of water vapor both contained in the substance and those formed during its combustion. The vaporization of 1 kg of water vapor requires 2514 kJ/kg (600 kcal/kg). The lower calorific value is determined by the formulas (kJ/kg or kcal/kg):

(for solid matter)

(for a liquid substance), where:

2514 - heat of vaporization at a temperature of 0 °C and atmospheric pressure, kJ/kg;

I is the content of hydrogen and water vapor in the working fuel, %;

9 is a coefficient showing that the combustion of 1 kg of hydrogen in combination with oxygen produces 9 kg of water.

Heat of combustion is the most important characteristic of a fuel, as it determines the amount of heat obtained by burning 1 kg of solid or liquid fuel or 1 m³ of gaseous fuel in kJ/kg (kcal/kg). 1 kcal = 4.1868 or 4.19 kJ.

The lower calorific value is determined experimentally for each substance and is a reference value. It can also be determined for solid and liquid materials, with a known elemental composition, by calculation in accordance with the formula of D.I. Mendeleev, kJ/kg or kcal/kg:

Content of carbon, hydrogen, oxygen, volatile sulfur and moisture in the working mass of fuel in% (by weight).

For comparative calculations, the so-called conventional fuel is used, which has a specific heat of combustion equal to 29308 kJ/kg (7000 kcal/kg).

In Russia, thermal calculations (for example, calculating the heat load to determine the category of a room for explosion and fire hazard) are usually carried out using the lowest calorific value, in the USA, Great Britain, and France - according to the highest. In the UK and US, before the introduction of the metric system, specific heat of combustion was measured in British thermal units (BTU) per pound (lb) (1Btu/lb = 2.326 kJ/kg).

The highest values of heating value of natural gases from various sources

These data were obtained from the International Energy Agency.

Algeria: 42,000 kJ/m³
Bangladesh: 36,000 kJ/m³
Canada: 38,200 kJ/m³
Indonesia: 40,600 kJ/m³
Netherlands: 33,320 kJ/m³
Norway: 39,877 kJ/m³
Russia: 38,231 kJ/m³
Saudi Arabia: 38,000 kJ/m³
UK: 39,710 kJ/m³
United States: 38,416 kJ/m³
Uzbekistan: 37,889 kJ/m³
Belarus: 33,000 kJ/m³

The required amount of fuel to operate a 100 W light bulb for a year (876 kWh)

(The fuel quantities listed below are based on 100% heat-to-electricity conversion efficiency. Since most power generating plants and distribution systems achieve efficiencies of about 30% - 35%, the actual amount of fuel used to power a 100 W light bulb is will be approximately three times the specified amount).