It is known that the source of energy used in industry, transport, agriculture, and 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 during the complete combustion of 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, construct 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.

The tables present the mass specific heat of combustion of fuel (liquid, solid and gaseous) and some other combustible materials. The following fuels were considered: coal, firewood, coke, peat, kerosene, oil, alcohol, gasoline, natural gas, etc.

List of tables:

During the exothermic reaction of fuel oxidation, its chemical energy is converted into thermal energy with the release of a certain amount of heat. The resulting thermal energy is usually called the heat of combustion of the fuel. It depends on its chemical composition, humidity and is the main one. The heat of combustion of fuel per 1 kg of mass or 1 m 3 of volume forms the mass or volumetric specific heat of combustion.

The specific heat of combustion of a fuel is the amount of heat released during the complete combustion of a unit mass or volume of solid, liquid or gaseous fuel. In the International System of Units, this value is measured in J/kg or J/m 3.

The specific heat of combustion of a fuel can be determined experimentally or calculated analytically. Experimental methods for determining calorific value are based on practical measurement of the amount of heat released when a fuel burns, for example in a calorimeter with a thermostat and a combustion bomb. For fuel with a known chemical composition, the specific heat of combustion can be determined using the periodic formula.

There are higher and lower specific heats of combustion. The higher calorific value is equal to the maximum amount of heat released during complete combustion of the fuel, taking into account the heat expended on the evaporation of moisture contained in the fuel. The lowest heat of combustion is less than the highest value by the amount of heat of condensation, which is formed from the moisture of the fuel and hydrogen of the organic mass, which turns into water during combustion.

To determine fuel quality indicators, as well as in thermal calculations usually use lower specific heat of combustion, which is the most important thermal and performance characteristic of the fuel and is shown in the tables below.

Specific heat of combustion of solid fuels (coal, firewood, peat, coke)

The table presents the values ​​of the specific heat of combustion of dry solid fuel in the dimension MJ/kg. Fuel in the table is arranged by name in alphabetical order.

Of the solid fuels considered, coking coal has the highest calorific value - its specific heat of combustion is 36.3 MJ/kg (or in SI units 36.3·10 6 J/kg). In addition, high calorific value is characteristic of hard coal, anthracite, charcoal and brown coal.

Fuels with low energy efficiency include wood, firewood, gunpowder, milling peat, and oil shale. For example, the specific heat of combustion of firewood is 8.4...12.5, and that of gunpowder is only 3.8 MJ/kg.

Specific heat of combustion of liquid fuels (alcohol, gasoline, kerosene, oil)

A table is given of the specific heat of combustion of liquid fuel and some other organic liquids. It should be noted that fuels such as gasoline, diesel fuel and oil have high heat release during combustion.

The specific heat of combustion of alcohol and acetone is significantly lower than traditional motor fuels. In addition, liquid rocket fuel has a relatively low calorific value and, with complete combustion of 1 kg of these hydrocarbons, an amount of heat will be released equal to 9.2 and 13.3 MJ, respectively.

Specific heat of combustion of gaseous fuels and combustible gases

A table is presented of the specific heat of combustion of gaseous fuel and some other combustible gases in the dimension MJ/kg. Of the gases considered, it has the highest mass specific heat of combustion. The complete combustion of one kilogram of this gas will release 119.83 MJ of heat. Also, fuel such as natural gas has a high calorific value - the specific heat of combustion of natural gas is 41...49 MJ/kg (for pure gas it is 50 MJ/kg).

Specific heat of combustion of some combustible materials

A table is provided of the specific heat of combustion of some combustible materials (wood, paper, plastic, straw, rubber, etc.). Materials with high heat release during combustion should be noted. Such materials include: rubber of various types, expanded polystyrene (foam), polypropylene and polyethylene.

Sources:

1. GOST 147-2013 Solid mineral fuel. Determination of the higher calorific value and calculation of the lower calorific value.
2. GOST 21261-91 Petroleum products. Method for determining the higher calorific value and calculating the lower calorific value.
3. GOST 22667-82 Natural flammable gases. Calculation method for determining the calorific value, relative density and Wobbe number.
4. GOST 31369-2008 Natural gas. Calculation of calorific value, density, relative density and Wobbe number based on component composition.
5. Zemsky G. T. Flammable properties of inorganic and organic materials: reference book M.: VNIIPO, 2016 - 970 p.

In addition to the main components, coal contains various non-flammable ash-forming additives, “rock”. Ash pollutes the environment and is sintered into slag on the grate, which makes it difficult to burn coal. In addition, the presence of rock reduces the specific heat of combustion of coal. Depending on the type and mining conditions, the amount of minerals varies greatly; the ash content of hard coal is about 15% (10–20%).
Another harmful component of coal is sulfur. During the combustion of sulfur, oxides are formed, which are converted into sulfuric acid in the atmosphere. The sulfur content in the coal that we supply to customers through a network of our representatives is about 0.5%, this is a very low value, which means that the ecology of your home will be preserved.
The main indicator of any fuel is specific heat of combustion. For coal this figure is:

These figures refer to coal concentrate. Actual figures may vary significantly. So, for ordinary hard coal, which can be bought at coal warehouses, the value indicated is 5000-5500 kcal/kg. We use 5300 kcal/kg in our calculations.
The density of coal is from 1 to 1.7 (hard coal - 1.3–1.4) g/cm 3, depending on the type and content of mineral substances. In technology, “bulk density” is also used; it is about 800-1,000 kg/m3.

Types and grades of coal

Coal is classified according to many parameters (geography of extraction, chemical composition), but from a “domestic” point of view, when buying coal for use in furnaces, it is enough to understand the labeling and the possibility of use in ThermoRobot.

According to the degree of coalification, three types of coal are distinguished: brown, stone And anthracite. The following coal designation system is used: Variety = (brand) + (size).

In addition to the main grades given in the table, intermediate grades of coal are also distinguished: DG (long-flame gas), GZh (gas fatty), KZh (coke fatty), PA (semi-anthracite), brown coals are also divided into groups.
Coking grades of coal (G, coke, Zh, K, OS) are practically not used in thermal power engineering, since they are a scarce raw material for the coke-chemical industry.
According to the size class (size of pieces, fractions), graded coal is divided into:

In addition to graded coal, there are combined fractions and screenings available for sale (PK, KO, OM, MS, SSh, MSSh, OMSSh). The size of coal is determined based on the smaller value of the finest fraction and the larger value of the largest fraction indicated in the name of the coal grade.
For example, the OM fraction (M - 13–25, O - 25-50) is 13–50 mm.

In addition to the above-mentioned types of coal, you can find coal briquettes on sale, which are pressed from low-enriched coal slurry.

How coal burns

Coal consists of two flammable components: volatiles And solid (coke) residue.

During the first stage of combustion, volatile substances are released; When there is an excess of oxygen, they burn quickly, producing a long flame but little heat.

After this, the coke residue burns out; the intensity of its combustion and ignition temperature depend on the degree of coalification, that is, on the type of coal (brown, hard, anthracite).
The higher the degree of carbonization (the highest is for anthracite), the higher the ignition temperature and heat of combustion, but the lower the combustion intensity.

Coal grades D, G

Due to the high content of volatile substances, such coal flares up quickly and burns out quickly. Coal of these grades is available and suitable for almost all types of boilers, however, for complete combustion, this coal must be supplied in small portions so that the released volatile substances have time to completely combine with oxygen in the air. Complete combustion of coal is characterized by a yellow flame and clear flue gases; incomplete combustion of volatile substances produces a purple flame and black smoke.
To effectively burn such coal, the process must be constantly monitored; this operating mode is implemented in the Termorobot automatic boiler room.

Coal grade A

It is more difficult to light, but it burns for a long time and produces much more heat. Coal can be loaded in large batches, since they burn mainly coke residue and there is no mass release of volatile substances. The blowing mode is very important, since if there is a lack of air, combustion occurs slowly, it may stop, or, on the contrary, an excessive increase in temperature, leading to heat loss and burnout of the boiler.

Cost calculations for 1 kW*hour:

• Diesel fuel. Specific heat of combustion of diesel fuel is 43 mJ/kg; or, taking into account a density of 35 mJ/liter; Taking into account the efficiency of a diesel fuel boiler (89%), we find that when burning 1 liter, 31 mJ of energy is generated, or in more conventional units 8.6 kWh.
  • The cost of 1 liter of diesel fuel is 20 rubles.
  • The cost of 1 kWh of diesel fuel combustion energy is 2.33 rubles.

• Propane-butane mixture SPBT(Liquefied petroleum gas LPG). The specific heat of combustion of LPG is 45.2 mJ/kg or, taking into account the density of 27 mJ/liter, taking into account the efficiency of a gas boiler of 95%, we obtain that when burning 1 liter, 25.65 mJ of energy is generated, or in more conventional units - 7.125 kW* h.
  • The cost of 1 liter of LPG is 11.8 rubles.
  • The cost of 1 kWh of energy is 1.66 rubles.

The difference in the price of 1 kW of heat obtained from the combustion of diesel and LPG was 29%. The given figures show that of the listed heat sources, liquefied gas is the more economical. To get a more accurate calculation, you need to put current energy prices.

Features of the use of liquefied gas and diesel fuel

DIESEL FUEL. There are several varieties that differ in sulfur content. But for the boiler this is not very important. But the division into winter and summer diesel fuel is important. The standard establishes three main grades of diesel fuel. The most common is summer (L), the range of its application is from O°C and above. Winter diesel fuel (3) is used at subzero air temperatures (up to -30°C). At lower temperatures, arctic (A) diesel fuel should be used. A distinctive feature of diesel fuel is its cloud point. In fact, this is the temperature at which the paraffins contained in diesel fuel begin to crystallize. It really becomes cloudy, and with a further decrease in temperature it becomes like jelly or congealed fatty soup. The smallest crystals of paraffin clog the pores of fuel filters and safety nets, settle in pipeline channels and paralyze work. For summer fuel the cloud point is -5°C, and for winter fuel it is -25°C. An important indicator, which must be indicated in the passport for diesel fuel, is the maximum filterability temperature. Cloudy diesel fuel can be used up to the filterability temperature, and then the filter becomes clogged and the fuel supply stops. Winter diesel fuel does not differ from summer diesel fuel either in color or smell. So it turns out that only God (and the gas station attendant) knows what is actually flooded. Some craftsmen mix summer diesel fuel with BGS (gas gasoline) and other stuff, achieving a decrease in the filterability temperature, which risks either pump failure or simply an explosion due to the fact that this hellish stuff’s flash point decreases. Also, instead of diesel, light heating oil can be supplied; it does not differ in appearance, but it contains more impurities, and those that are not present in diesel at all. Which is fraught with contamination of fuel equipment and expensive cleaning. From the above, we can conclude that if you purchase diesel at a low price, from private individuals or unverified organizations, you may end up in need of repairs, or the heating system may be defrosted. The price of diesel fuel, delivered to your home, fluctuates by a ruble from the prices at gas stations, both down and up depending on the remoteness of your cottage and the amount of fuel transported, anything cheaper should alert you unless you are an extreme sports enthusiast. , and are not afraid to spend the night in a cooling house in 30 degree frost.

LIQUEFIED GAS. Just like diesel fuel, there are several grades of SPBT, differing in the composition of the mixture of propane and butane. Winter mixture, summer and arctic. The winter mixture consists of 65% propane, 30% butane and 5% gas impurities. The summer mixture consists of 45% propane, 50% butane, 5% gas impurities. Arctic mixture - 95% propane and 5% impurities. A mixture of 95% butane and 5% impurities can be supplied, this mixture is called household. A very small amount of a sulfurous substance, an odorant, is added to each mixture in order to create a “gas smell.” From the point of view of combustion and the effect on the equipment, the composition of the mixture has practically no effect. Butane, although much cheaper, is slightly better for heating than propane - it has more calories, but it has a very big drawback that makes it difficult to use in Russian conditions - butane stops evaporating and remains liquid at zero degrees. If you have an imported tank with a low neck or vertical (the depth of the evaporation surface is less than 1.5 meters) or is located in a plastic sarcophagus that worsens heat transfer, then during prolonged frosts the tank may stop evaporating butane, not only due to frost, but also from -due to insufficient heat transfer (during evaporation, the gas cools itself). At temperatures below 3 degrees Celsius, imported containers made for the conditions of Germany, the Czech Republic, Italy, Poland, with intense evaporation, stop producing gas after all the propane has evaporated, and only butane remains.

Now let’s compare the consumer properties of LPG and diesel fuel

Using LPG is 29% cheaper than diesel fuel. The quality of LPG does not affect its consumer properties when using AvtonomGaz tanks; moreover, the higher the butane content in the mixture, the better the gas equipment works. Low-quality diesel fuel can lead to serious damage to heating equipment. Using liquefied gas will eliminate the smell of diesel fuel in your home. Liquefied gas contains less toxic sulfur compounds and, as a result, there is no air pollution in your garden. You can run not only a boiler, but also a gas stove, as well as a gas fireplace and a gas electric generator from liquefied gas.

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.

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 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 clearly 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.