Calorific value of diesel fuel kJ kg. What to choose: gas or diesel fuel

specific heat of combustion - specific heat— Topics oil and gas industry Synonyms specific heat EN specific heat …

The amount of heat released during complete combustion fuel weighing 1 kg. The specific heat of combustion of fuel is determined empirically and is the most important characteristic fuel. See also: Fuel Financial Dictionary Finam ... Financial vocabulary

specific heat of combustion of peat by bomb- Higher calorific value of peat, taking into account the heat of formation and dissolution of sulfuric and nitric acids in water. [GOST 21123 85] Inadmissible, non-recommended calorific value of peat according to the bomb Topics peat General terms peat properties EN ... ... Technical Translator's Handbook

specific heat of combustion (fuel)- 3.1.19 specific calorific value (fuel): The total amount of energy released under regulated conditions of fuel combustion. A source …

Specific heat of combustion of peat according to the bomb- 122. Specific calorific value of peat by bomb Higher calorific value of peat taking into account the heat of formation and dissolution of sulfuric and nitric acids in water Source: GOST 21123 85: Peat. Terms and definitions original document ... Dictionary-reference book of terms of normative and technical documentation

specific heat of combustion of fuel- 35 calorific value of a fuel: The total amount of energy released under specified fuel combustion conditions. Source: GOST R 53905 2010: Energy saving. Terms and definitions original document ... Dictionary-reference book of terms of normative and technical documentation

This is the amount of heat released during the complete combustion of mass (for solid and liquid substances) or volumetric (for gaseous) units of matter. It is measured in joules or calories. The heat of combustion, referred to a unit mass or volume of fuel, ... ... Wikipedia

Modern Encyclopedia

Heat of combustion- (heat of combustion, calorific value), the amount of heat released during the complete combustion of fuel. There are specific heat of combustion, volumetric, etc. For example, the specific heat of combustion of coal is 28 34 MJ / kg, gasoline is about 44 MJ / kg; voluminous ... ... Illustrated Encyclopedic Dictionary

Specific heat of combustion of fuel- Specific calorific value of fuel: the total amount of energy released under specified conditions of fuel combustion...

Any fuel, when burned, releases heat (energy), quantified in joules or calories (4.3J = 1cal). In practice, to measure the amount of heat that is released during the combustion of fuel, calorimeters are used - complex devices laboratory use. The heat of combustion is also called the calorific value.

The amount of heat obtained from the combustion of fuel depends not only on its calorific value, but also on its mass.

To compare substances in terms of the amount of energy released during combustion, a more convenient value is specific heat combustion. It shows the amount of heat generated during the combustion of one kilogram (mass specific heat of combustion) or one liter, cubic meter (volume specific heat of combustion) of fuel.

The units of specific heat of combustion of fuel accepted in the SI system are kcal / kg, MJ / kg, kcal / m³, MJ / m³, as well as their derivatives.

The energy value of fuel is determined precisely by the value of its specific heat of combustion. The relationship between the amount of heat generated during the combustion of fuel, its mass and the specific heat of combustion is expressed by a simple formula:

Q = qm, where Q is the amount of heat in J, q is the specific heat of combustion in J/kg, m is the mass of the substance in kg.

For all types of fuel and most combustible substances, the values ​​of the specific heat of combustion have long been determined and tabulated, which are used by specialists when calculating the heat released during the combustion of fuel or other materials. In different tables, slight discrepancies are possible, obviously explained by slightly different measurement methods or different calorific value of the same type of combustible materials extracted from different deposits.

Of the solid fuels, coal has the highest energy intensity - 27 MJ / kg (anthracite - 28 MJ / kg). Charcoal has similar indicators (27 MJ / kg). Brown coal is much less calorific - 13 MJ/kg. In addition, it usually contains a lot of moisture (up to 60%), which, evaporating, reduces the value of the total calorific value.

Peat burns with a heat of 14-17 MJ/kg (depending on its condition - crumb, pressed, briquette). Firewood dried to 20% moisture emits from 8 to 15 MJ/kg. At the same time, the amount of energy received from aspen and from birch can almost double. Approximately the same indicators give pellets from different materials- from 14 to 18 MJ/kg.

Much less than solid ones, they differ in the values ​​of the specific heat of combustion liquid types fuel. Thus, the specific heat of combustion of diesel fuel is 43 MJ / l, gasoline - 44 MJ / l, kerosene - 43.5 MJ / l, fuel oil - 40.6 MJ / l.

The specific heat of combustion of natural gas is 33.5 MJ/m³, propane - 45 MJ/m³. The most energy-intensive gaseous fuel is hydrogen gas (120 MJ/m³). It is very promising for use as a fuel, but has not yet been found. best options its storage and transportation.

Comparison of the energy intensity of different types of fuel

When comparing energy value the main types of solid, liquid and gaseous fuels, it can be established that one liter of gasoline or diesel fuel corresponds to 1.3 m³ of natural gas, one kilogram of coal - 0.8 m³ of gas, one kg of firewood - 0.4 m³ of gas.

The calorific value of fuel is the most important indicator of efficiency, but the breadth of its distribution in the areas of human activity depends on technical capabilities and economic indicators use.

Today, people are extremely dependent on fuel. Without it, heating of dwellings, cooking, operation of equipment and Vehicle. Most of the fuels used are hydrocarbons. To evaluate their effectiveness, the values ​​of the specific heat of combustion are used. Kerosene has a relatively impressive indicator. Due to this quality, it is used in rocket and aircraft engines.

Due to its properties, kerosene is used in rocket engines.

Properties, obtaining and application

The history of kerosene goes back more than 2 thousand years and begins when Arab scientists came up with a method for distilling oil into individual components. It was officially discovered in 1853, when Canadian physician Abraham Gesner developed and patented a method for extracting transparent flammable liquid bitumen and oil shale.

After drilling the first oil well in 1859, oil became the main raw material for kerosene. Due to its ubiquitous use in lamps, it was considered a staple of the petroleum refining industry for decades. Only the advent of electricity reduced its importance for lighting. Kerosene production has also fallen as the popularity of automobiles has risen.- this circumstance significantly increased the importance of gasoline as a petroleum product. However, today in many parts of the world, kerosene is used for heating and lighting, and modern jet fuel is the same product, but of a higher quality.

With the increase in the use of cars, the popularity of kerosene has fallen

Kerosene is a light transparent liquid, chemically a mixture of organic compounds. Its composition largely depends on the raw material, but, as a rule, it consists of a dozen different hydrocarbons, each of which contains from 10 to 16 carbon atoms. Kerosene is less volatile than gasoline. The comparative ignition temperature of kerosene and gasoline, at which they emit flammable vapors near the surface, is 38 and -40°C, respectively.

This property makes it possible to consider kerosene as a relatively safe fuel in terms of storage, use and transportation. Based on its boiling point (150 to 350°C), it is classified as one of the so-called middle distillates of crude oil.

Kerosene can be obtained straight-run, that is, physically separated from oil, by distillation, or by chemical decomposition of heavier fractions as a result of a cracking process.

Characteristics of kerosene as a fuel

Combustion is the process of rapid oxidation of substances with the release of heat. As a rule, oxygen contained in the air participates in the reaction. During the combustion of hydrocarbons, the following main combustion products are formed:

• carbon dioxide;
• water vapor;
• soot.

The amount of energy generated during the combustion of a fuel depends on its type, combustion conditions, mass or volume. Energy is measured in joules or calories. Specific (per unit of measurement of the amount of substance) calorific value is the energy obtained by burning a unit of fuel:

• molar (for example, J / mol);
• mass (for example, J / kg);
• volumetric (for example, kcal / l).

In most cases, to evaluate gaseous, liquid and solid fuels, they operate with an indicator of the mass heat of combustion, expressed in J / kg.

The value of the calorific value will depend on whether the processes occurring with water during combustion were taken into account. Evaporation of moisture is an energy-intensive process, and taking into account heat transfer during the condensation of these vapors can also affect the result.

The result of measurements made before the condensed steam returns energy to the system is called the lower calorific value, and the figure obtained after the vapors condense is called the higher calorific value. Hydrocarbon engines cannot use the additional energy of water vapor in the exhaust, so the net figure is relevant for engine manufacturers and is found more often in reference books.

Often, when specifying the calorific value, they do not specify which of the quantities is meant, which can lead to confusion. Knowing that in the Russian Federation it is traditionally customary to indicate the lowest helps to navigate.

Net calorific value - important indicator

It should be noted that for some types of fuel, the division into net and gross energy does not make sense, since they do not form water during combustion. With regard to kerosene, this is irrelevant, since the content of hydrocarbons in it is high. With relatively low density (between 780 kg/m³ and 810 kg/m³) its calorific value is similar to that of diesel fuel and is:

• the lowest - 43.1 MJ / kg;
• the highest - 46.2 MJ / kg.

Comparison with other types of fuel

This indicator is very convenient for estimating the potential amount of heat contained in the fuel. For example, the calorific value of gasoline per unit mass is comparable to that of kerosene, but the former is much denser. As a consequence, in the same comparison, a liter of gasoline contains less energy.

The specific heat of combustion of oil as a mixture of hydrocarbons depends on its density, which is not constant for different fields (43-46 MJ/kg). Calculation methods allow with high precision determine this value if there is initial data on its composition.

The average indicators for some types of combustible liquids that make up oil look like this (in MJ / kg):

• diesel fuel - 42-44;
• gasoline - 43-45;
• kerosene - 43-44.

The calorie content of solid fuels, such as peat and coal, has a greater range. This is due to the fact that their composition can vary greatly both in terms of the content of non-combustible substances and the calorific value of hydrocarbons. For example, the calorific value of peat various types can fluctuate within 8-24 MJ/kg, and coal - 13-36 MJ/kg. Among common gases, hydrogen has a high calorific value - 120 MJ / kg. Next in terms of specific heat of combustion is methane (50 MJ/kg).

We can say that kerosene is a fuel that has stood the test of time precisely because of its relatively high energy intensity at a low price. Its use is not only economically justified, but in some cases there is no alternative.

5. THERMAL BALANCE OF COMBUSTION

Consider calculation methods heat balance combustion process of gaseous, liquid and solid fuels. The calculation is reduced to solving the following problems.

· Determination of heat of combustion (calorific value) of fuel.

· Determination of the theoretical combustion temperature.

5.1. HEAT OF BURNING

Chemical reactions are accompanied by the release or absorption of heat. When heat is released, the reaction is called exothermic, and when it is absorbed, it is called endothermic. All combustion reactions are exothermic, and combustion products are exothermic compounds.

Released (or absorbed) during the course chemical reaction heat is called the heat of reaction. In exothermic reactions it is positive, in endothermic reactions it is negative. The combustion reaction is always accompanied by the release of heat. Heat of combustion Q g(J / mol) is the amount of heat that is released during the complete combustion of one mole of a substance and the transformation of a combustible substance into products of complete combustion. The mole is the basic SI unit for the amount of a substance. One mole is such an amount of a substance that contains as many particles (atoms, molecules, etc.) as there are atoms in 12 g of the carbon-12 isotope. The mass of an amount of a substance equal to 1 mole (molecular or molar mass) numerically coincides with the relative molecular weight of a given substance.

For example, the relative molecular weight of oxygen (O 2) is 32, carbon dioxide(CO 2) is equal to 44, and the corresponding molecular weights will be equal to M =32 g/mol and M =44 g/mol. Thus, one mole of oxygen contains 32 grams of this substance, and one mole of CO 2 contains 44 grams of carbon dioxide.

In technical calculations, not the heat of combustion is often used Q g, and the calorific value of the fuel Q(J / kg or J / m 3). The calorific value of a substance is the amount of heat that is released during the complete combustion of 1 kg or 1 m 3 of a substance. For liquid and solids the calculation is carried out per 1 kg, and for gaseous - per 1 m 3.

Knowledge of the heat of combustion and the calorific value of the fuel is necessary to calculate the combustion or explosion temperature, explosion pressure, flame propagation speed, and other characteristics. The calorific value of the fuel is determined either experimentally or by calculation. In the experimental determination of the calorific value, a given mass of solid or liquid fuel is burned in a calorimetric bomb, and in the case of gaseous fuel, in a gas calorimeter. These devices measure the total heat Q 0 , released during the combustion of a sample of fuel weighing m. Calorific value Q g is found according to the formula

Relationship between heat of combustion and
fuel calorific value

To establish a relationship between the heat of combustion and the calorific value of a substance, it is necessary to write down the equation for the chemical reaction of combustion.

Product complete burning carbon is carbon dioxide:

C + O 2 → CO 2.

The product of complete combustion of hydrogen is water:

2H 2 + O 2 → 2H 2 O.

The product of complete combustion of sulfur is sulfur dioxide:

S + O 2 → SO 2.

At the same time, nitrogen, halides and other non-combustible elements are released in a free form.

combustible gas

As an example, we will calculate the calorific value of methane CH 4, for which the heat of combustion is equal to Q g=882.6 .

Let's define molecular weight methane in accordance with its chemical formula(CH 4):

М=1∙12+4∙1=16 g/mol.

Let's define calorific value 1 kg of methane:

Let's find the volume of 1 kg of methane, knowing its density ρ=0.717 kg/m 3 under normal conditions:

.

Determine the calorific value of 1 m 3 of methane:

The calorific value of any combustible gases is determined similarly. For many common substances, the calorific values ​​and calorific values ​​have been measured with high accuracy and are given in the relevant reference literature. Let's give a table of values ​​for the calorific value of some gaseous substances (Table 5.1). Value Q in this table it is given in MJ / m 3 and in kcal / m 3, since 1 kcal = 4.1868 kJ is often used as a unit of heat.

Table 5.1

Calorific value of gaseous fuels

combustible liquid or solid

As an example, we will calculate the calorific value of ethyl alcohol C 2 H 5 OH, for which the heat of combustion Q g= 1373.3 kJ/mol.

Determine the molecular weight of ethyl alcohol in accordance with its chemical formula (C 2 H 5 OH):

М = 2∙12 + 5∙1 + 1∙16 + 1∙1 = 46 g/mol.

Determine the calorific value of 1 kg of ethyl alcohol:

The calorific value of any liquid and solid combustibles is determined similarly. In table. 5.2 and 5.3 show the calorific values Q(MJ/kg and kcal/kg) for some liquid and solid substances.

Table 5.2

Calorific value liquid fuels

Table 5.3

Calorific value of solid fuels

Mendeleev's formula

If the calorific value of the fuel is unknown, then it can be calculated using the empirical formula proposed by D.I. Mendeleev. To do this, you need to know the elemental composition of the fuel (the equivalent formula of the fuel), that is, the percentage of the following elements in it:

Oxygen (O);

Hydrogen (H);

Carbon (C);

Sulfur (S);

Ashes (A);

Water (W).

The combustion products of fuels always contain water vapor, formed both due to the presence of moisture in the fuel, and during the combustion of hydrogen. Waste products of combustion leave the industrial plant at a temperature above the dew point temperature. Therefore, the heat that is released during the condensation of water vapor cannot be usefully used and should not be taken into account in thermal calculations.

The net calorific value is usually used for the calculation. Q n fuel, which takes into account heat loss with water vapor. For solid and liquid fuels, the value Q n(MJ / kg) is approximately determined by the Mendeleev formula:

Q n=0.339+1.025+0.1085 – 0.1085 – 0.025, (5.1)

where the percentage (mass %) content of the corresponding elements in the fuel composition is indicated in parentheses.

This formula takes into account the heat of exothermic combustion reactions of carbon, hydrogen and sulfur (with a plus sign). Oxygen, which is part of the fuel, partially replaces the oxygen in the air, so the corresponding term in formula (5.1) is taken with a minus sign. When moisture evaporates, heat is consumed, so the corresponding term containing W is also taken with a minus sign.

Comparison of calculated and experimental data on the calorific value of different fuels (wood, peat, coal, oil) showed that the calculation according to the Mendeleev formula (5.1) gives an error not exceeding 10%.

Net calorific value Q n(MJ / m 3) dry combustible gases can be calculated with sufficient accuracy as the sum of the products of the calorific value of individual components and their percentage in 1 m 3 of gaseous fuel.

Q n= 0.108[Н 2 ] + 0.126[СО] + 0.358[CH 4 ] + 0.5[С 2 Н 2 ] + 0.234[Н 2 S ]…, (5.2)

where the percentage (vol.%) content of the corresponding gases in the mixture is indicated in parentheses.

The average calorific value of natural gas is approximately 53.6 MJ/m 3 . In artificially produced combustible gases, the content of CH 4 methane is negligible. The main combustible components are hydrogen H 2 and carbon monoxide CO. In coke oven gas, for example, the content of H 2 reaches (55 ÷ 60)%, and the net calorific value of such gas reaches 17.6 MJ/m 3 . In the generator gas, the content of CO ~ 30% and H 2 ~ 15%, while the net calorific value of the generator gas Q n= (5.2÷6.5) MJ/m 3 . In blast-furnace gas, the content of CO and H 2 is less; magnitude Q n= (4.0÷4.2) MJ/m 3 .

Consider examples of calculating the calorific value of substances using the Mendeleev formula.

Let us determine the calorific value of coal, the elemental composition of which is given in Table. 5.4.

Table 5.4

Elemental composition of coal

Let's substitute given in tab. 5.4 data in the Mendeleev formula (5.1) (nitrogen N and ash A are not included in this formula, since they are inert substances and do not participate in the combustion reaction):

Q n=0.339∙37.2+1.025∙2.6+0.1085∙0.6–0.1085∙12–0.025∙40=13.04 MJ/kg.

Let us determine the amount of firewood required to heat 50 liters of water from 10 ° C to 100 ° C, if 5% of the heat released during combustion is spent on heating, and the heat capacity of water With\u003d 1 kcal / (kg ∙ deg) or 4.1868 kJ / (kg ∙ deg). The elemental composition of firewood is given in Table. 5.5:

Table 5.5

Elemental composition of firewood

It is known that the source of energy used in industry, transport, agriculture, in everyday life, is the fuel. These are coal, oil, peat, firewood, natural gas and others. During the combustion of fuel, energy is released. Let's try to figure out how energy is released in this case.

Let us recall the structure of the water molecule (Fig. 16, a). It consists of one oxygen atom and two hydrogen atoms. If a water molecule is divided into atoms, then it is necessary to overcome the forces of attraction between atoms, i.e., to do work, and therefore to expend energy. Conversely, if atoms combine to form a molecule, energy is released.

The use of fuel is based precisely on the phenomenon of energy release when atoms combine. For example, the carbon atoms contained in the fuel are combined with two oxygen atoms during combustion (Fig. 16, b). In this case, a molecule of carbon monoxide - carbon dioxide - is formed and energy is released.

Rice. 16. Structure of molecules:
a - water; b - connection of a carbon atom and two oxygen atoms into a carbon dioxide molecule

When designing engines, an engineer needs to know exactly how much heat the fuel being burned can release. To do this, it is necessary to determine experimentally how much heat will be released during the complete combustion of the same mass of fuel of different types.

The physical quantity showing how much heat is released during the complete combustion of a fuel weighing 1 kg is called the specific heat of combustion of the fuel.

The specific heat of combustion is denoted by the letter q. The unit of specific heat of combustion is 1 J/kg.

The specific heat of combustion is determined experimentally using rather complex instruments.

The results of the experimental data are shown in Table 2.

table 2

This table shows that the specific heat of combustion, for example, of gasoline is 4.6 10 7 J / kg.

This means that with the complete combustion of gasoline weighing 1 kg, 4.6 10 7 J of energy is released.

The total amount of heat Q released during the combustion of m kg of fuel is calculated by the formula

Questions

1. What is the specific heat of combustion of fuel?
2. In what units is the specific heat of combustion of fuel measured?
3. What does the expression “specific heat of combustion of fuel equal to 1.4 10 7 J / kg” mean? How is the amount of heat released during the combustion of fuel calculated?

Exercise 9

1. How much heat is released during complete combustion charcoal weighing 15 kg; alcohol weighing 200 g?
2. How much heat will be released during the complete combustion of oil, the mass of which is 2.5 tons; kerosene, the volume of which is 2 liters, and the density is 800 kg / m 3?
3. With the complete combustion of dry firewood, 50,000 kJ of energy were released. How much firewood burned?

Exercise

Using Table 2, build a bar chart for the specific heat of combustion of firewood, alcohol, oil, hydrogen, choosing the scale as follows: the width of the rectangle is 1 cell, the height of 2 mm corresponds to 10 J.