Question to a psychologist 

Heat of combustion of fuel. Thermal machines. ICE. Specific heat of combustion of fuel

It is known that the source of energy that is used in industry, transport, agriculture, in everyday life, is fuel. These are coal, oil, peat, firewood, natural gas etc. When fuel burns, energy is released. Let's try to find 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 the atoms, that is, work must be done, and therefore energy must be spent. 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 join. For example, the carbon atoms contained in the fuel combine 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 - the combination of a carbon atom and two oxygen atoms into a carbon dioxide molecule

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

    A physical quantity showing how much heat is released during the complete combustion of 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

From this table it can be seen that the specific heat of combustion, for example, of gasoline is 4.6 10 7 J / kg.

This means that the complete combustion of gasoline weighing 1 kg releases 4.6 10 7 J of energy.

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 fuel combustion calculated?

Exercise 9

  1. What amount of heat is released during the complete combustion of 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. When dry wood was completely burned, 50,000 kJ of energy was released. What mass of wood burned?

Exercise

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

An important thermal characteristic of fuel is its specific heat of combustion.

Specific heat of combustion of fuel

A distinction is made between specific higher and lower calorific values. The specific heat of combustion of working fuel, taking into account the additional heat that is released during the condensation of water vapor found in combustion products, is called highest specific heat of combustion of working fuel. This additional amount of heat can be determined by multiplying the mass of water vapor generated from the evaporation of fuel moisture /100 and from the combustion of hydrogen 9 /100 , to the latent heat of condensation of water vapor equal to approximately 2500 kJ/kg.

Specific lower heating value of fuel – the amount of heat that is released in ordinary practical conditions, i.e. when water vapor does not condense, but is released into the atmosphere.

Thus, the relationship between the highest and lowest specific heat of combustion can be expressed by the equation - = =25(9 ).

64. Conditional fuel.

Fuel is any substance that, upon combustion (oxidation), releases a significant amount of heat per unit mass or volume and is available for mass use.

Natural and derivative organic compounds in solid, liquid and gaseous states are used as fuel.

Any organic fuel consists of carbon, hydrogen, oxygen, nitrogen, volatile sulfur, and solid and liquid fuels consist of ash (mineral residues) and moisture.

An important thermal characteristic of fuel is its specific heat of combustion.

Specific heat of combustion of fuel is the amount of heat that is released during the complete combustion of a unit amount of fuel.

The lower the specific heat of combustion of the fuel, the more it is consumed in the boiler unit. For comparison various types fuels based on their thermal effect, the concept of conventional fuel was introduced, the specific heat of combustion of which was taken = 29.3 MJ/kg.

The ratio of Q Н Р of a given fuel to Q specific fuel is called the equivalent E. Then the conversion of the consumption of natural fuel V N into standard fuel V UT is carried out according to the formula:

Conditional fuel- a unit of accounting for organic fuel adopted in calculations, that is, oil and its derivatives, natural and specially obtained from the distillation of shale and coal, gas, peat - which is used to calculate the beneficial effect of various types of fuel in their total accounting.

In the USSR and Russia per unit standard fuel(ce) the calorific value of 1 kg of coal was taken = 29.3 MJ or 7000 kcal. International Energy Agency ( I.E.A.) took the unit of oil equivalent, usually denoted by the abbreviation TOE(English) . Tonne of oil equivalent). One ton of oil equivalent equals 41.868 GJ or 11.63 MWh. The unit also used is the barrel of oil equivalent ( BOE).

65. Excess air coefficient.

The number showing how many times the actual air flow is greater than the theoretically required amount of air is called excess air coefficient, i.e. actual air flow L (in kg/kg) or V (m 3 / m 3) is equal to its theoretically required amount L o or V o > multiplied by the excess air coefficient a

V= aV 0 .

Everyone knows that in our lives huge role fuel use plays a role. Fuel is used in almost any industry modern industry. Fuel derived from oil is especially often used: gasoline, kerosene, diesel fuel and others. Combustible gases (methane and others) are also used.

Where does fuel energy come from?

It is known that molecules are made up of atoms. In order to divide any molecule (for example, a water molecule) into its constituent atoms, it is necessary to expend energy (to overcome the forces of attraction of atoms). Experiments show that when atoms combine into a molecule (this is what happens when fuel is burned), energy, on the contrary, is released.

As is known, there is also nuclear fuel, but we won't talk about it here.

When fuel burns, energy is released. Most often this is thermal energy. Experiments show that the amount of energy released is directly proportional to the amount of fuel burned.

Specific heat of combustion

To calculate this energy, a physical quantity called the specific heat of combustion of fuel is used. The specific heat of combustion of a fuel shows how much energy is released during the combustion of a unit mass of fuel.

It is designated Latin letter q. In the SI system, the unit of measurement for this quantity is J/kg. Note that each fuel has its own specific heat of combustion. This value has been measured for almost all types of fuel and is determined from tables when solving problems.

For example, the specific heat of combustion of gasoline is 46,000,000 J/kg, kerosene is the same, and ethyl alcohol is 27,000,000 J/kg. It is easy to understand that the energy released during the combustion of fuel is equal to the product of the mass of this fuel and the specific heat of combustion of the fuel:

Let's look at examples

Let's look at an example. 10 grams of ethyl alcohol burned in an alcohol lamp in 10 minutes. Find the power of the alcohol lamp.

Solution. Let's find the amount of heat released during the combustion of alcohol:

Q = q*m; Q = 27,000,000 J/kg * 10 g = 27,000,000 J/kg * 0.01 kg = 270,000 J.

Let's find the power of the alcohol lamp:

N = Q / t = 270,000 J / 10 min = 270,000 J / 600 s = 450 W.

Let's look at a more complex example. An aluminum pan of mass m1 filled with water of mass m2 was heated using a kerosene stove from temperature t1 to temperature t2 (00C< t1 < t2

Solution.

Let's find the amount of heat received by aluminum:

Q1 = c1 * m1 * (t1 t2);

Let's find the amount of heat received by water:

Q2 = c2 * m2 * (t1 t2);

Let's find the amount of heat received by a pan of water:

Let's find the amount of heat given off by burned gasoline:

Q4 = Q3 / k * 100 = (Q1 + Q2) / k * 100 =

(c1 * m1 * (t1 t2) + c2 * m2 * (t1 t2)) / k * 100;

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.

To the solid natural look include stone and firewood, anthracite. Briquettes, coke, thermoanthracite are types of artificial solid fuel.

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

It is a mixture of various gases: ethylene, methane, propane, butane. In addition to them, gaseous fuel 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 state of aggregation 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 is different options 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 brown mass, then a brown gas is obtained.

At high blood 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 the part that is formed from organic matter: 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 the chemical element whose presence in natural fuel is highly 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.

In conclusion

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 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 appearing real opportunity identifying those main variants of solid, gaseous, liquid fuel, which will be the most effective and inexpensive to use in a certain situation.

In this lesson we will learn how to calculate the amount of heat that fuel releases during combustion. In addition, we will consider the characteristics of the fuel - the specific heat of combustion.

Since our whole life is based on movement, and movement is mostly based on the combustion of fuel, studying this topic is very important for understanding the topic “Thermal Phenomena”.

After studying the issues related to the amount of heat and specific heat capacity, let's move on to consider amount of heat released when burning fuel.

Definition

Fuel- a substance that in some processes (combustion, nuclear reactions) produces heat. Is a source of energy.

Fuel happens solid, liquid and gaseous(Fig. 1).

Rice. 1. Types of fuel

  • TO hard types fuels include coal and peat.
  • Liquid fuels include oil, gasoline and other petroleum products.
  • Gaseous fuels include natural gas.
  • Separately, we can highlight the very common lately nuclear fuel.

Combustion of fuel is chemical process, which is oxidative. During combustion, carbon atoms combine with oxygen atoms to form molecules. As a result of this, energy is released, which a person uses for his own purposes (Fig. 2).

Rice. 2. Formation of carbon dioxide

To characterize the fuel, the following characteristic is used: calorific value. Calorific value shows how much heat is released during fuel combustion (Fig. 3). In physics, calorific value corresponds to the concept specific heat of combustion of a substance.

Rice. 3. Specific heat of combustion

Definition

Specific heat of combustion - physical quantity, which characterizes the fuel, is numerically equal to the amount of heat that is released during complete combustion of the fuel.

The specific heat of combustion is usually denoted by the letter . Units of measurement:

There is no unit of measurement, since fuel combustion occurs at an almost constant temperature.

The specific heat of combustion is determined experimentally using sophisticated instruments. However, there are special tables for solving problems. Below we present the values ​​of the specific heat of combustion for some types of fuel.

Substance

Table 4. Specific heat of combustion of some substances

From the given values ​​it is clear that during combustion a huge amount of heat is released, so the units of measurement (megajoules) and (gigajoules) are used.

To calculate the amount of heat released during fuel combustion, the following formula is used:

Here: - mass of fuel (kg), - specific heat of combustion of fuel ().

In conclusion, we note that most The fuel used by humanity is stored using solar energy. Coal, oil, gas - all this was formed on Earth due to the influence of the Sun (Fig. 4).

Rice. 4. Fuel formation

In the next lesson we will talk about the law of conservation and transformation of energy in mechanical and thermal processes.

Listliterature

  1. Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
  2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
  3. Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.
  1. Internet portal “festival.1september.ru” ()
  2. Internet portal “school.xvatit.com” ()
  3. Internet portal “stringer46.narod.ru” ()

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