The lower calorific value of natural gas is mJ m3. Gas fuel

Every day, turning on the burner on the kitchen stove, few people think about how long ago gas production began. In our country, its development began in the twentieth century. Before this, it was simply found during the extraction of petroleum products. Calorific value The supply of natural gas is so great that today this raw material is simply irreplaceable, and its high-quality analogues have not yet been developed.

The calorific value table will help you choose fuel for heating your home

Features of fossil fuels

Natural gas is an important fossil fuel that occupies a leading position in the fuel and energy balances of many countries. In order to supply fuel to cities and various technical enterprises, they consume various flammable gases, since natural gas is considered dangerous.

Environmentalists believe that gas is the cleanest fuel; when burned, it emits much less toxic substances than firewood, coal, oil. This fuel is used daily by people and contains an additive such as an odorant; it is added in equipped installations in a ratio of 16 milligrams per 1 thousand cubic meters of gas.

An important component of the substance is methane (approximately 88-96%), the rest is other chemicals:

• butane;
• hydrogen sulfide;
• propane;
• nitrogen;
• oxygen.

In this video we will look at the role of coal:

The amount of methane in natural fuel directly depends on its deposit.

The described type of fuel consists of hydrocarbon and non-hydrocarbon components. Natural fossil fuels are primarily methane, which includes butane and propane. Apart from the hydrocarbon components, the described fossil fuel contains nitrogen, sulfur, helium and argon. Liquid vapors are also found, but only in gas and oil fields.

Types of deposits

There are several types of gas deposits. They are divided into the following types:

• gas;
• oil.

Their distinctive feature is the hydrocarbon content. Gas deposits contain approximately 85-90% of the present substance, oil fields contain no more than 50%. The remaining percentages are occupied by substances such as butane, propane and oil.

A huge disadvantage of oil production is its flushing from various kinds additives Sulfur is used as an impurity in technical enterprises.

Natural gas consumption

Butane is consumed as fuel at gas stations for cars, and organic matter, called “propane”, is used to refill lighters. Acetylene is a highly flammable substance and is used in welding and metal cutting.

Fossil fuels are used in everyday life:

• columns;
• gas stove;

This type of fuel is considered the most inexpensive and harmless; the only drawback is the release of carbon dioxide into the atmosphere when burned. Scientists all over the planet are looking for a replacement for thermal energy.

Calorific value

The calorific value of natural gas is the amount of heat generated when a unit of fuel is sufficiently burned. The amount of heat released during combustion is referred to one cubic meter taken under natural conditions.

The thermal capacity of natural gas is measured in the following indicators:

• kcal/nm 3 ;
• kcal/m3.

There is high and low calorific value:

1. High. Considers the heat of water vapor generated during fuel combustion.
2. Low. It does not take into account the heat contained in water vapor, since such vapors cannot be condensed, but leave with combustion products. Due to the accumulation of water vapor, it forms an amount of heat equal to 540 kcal/kg. In addition, when the condensate cools, heat comes out from 80 to one hundred kcal/kg. In general, due to the accumulation of water vapor, more than 600 kcal/kg is formed, this is the distinguishing feature between high and low heat output.

For the vast majority of gases consumed in the urban fuel distribution system, the difference is equivalent to 10%. In order to provide cities with gas, its calorific value must be more than 3500 kcal/nm 3 . This is explained by the fact that the supply is carried out through a pipeline over long distances. If the calorific value is low, then its supply increases.

If the calorific value of natural gas is less than 3500 kcal/nm 3, it is more often used in industry. It does not need to be transported over long distances, and combustion becomes much easier. Serious changes in the calorific value of gas require frequent adjustment and sometimes replacement large quantity standardized burners of household sensors, which leads to difficulties.

This situation leads to an increase in gas pipeline diameters, as well as increased costs for metal, network installation and operation. A big disadvantage of low-calorie fossil fuels is the huge content of carbon monoxide, which increases the level of threat during fuel operation and pipeline maintenance, in turn, as well as equipment.

The heat released during combustion, which does not exceed 3500 kcal/nm 3, is most often used in industrial production, where it is not necessary to transfer it over a long distance and easily form combustion.

PHYSICAL AND CHEMICAL PROPERTIES OF NATURAL GASES

U natural gases no color, smell, taste.

The main indicators of natural gases include: composition, calorific value, density, combustion and ignition temperature, explosive limits and explosion pressure.

Natural gases from pure gas fields mainly consist of methane (82-98%) and other hydrocarbons.

Combustible gas contains flammable and non-flammable substances. Combustible gases include: hydrocarbons, hydrogen, hydrogen sulfide. Non-flammable gases include: carbon dioxide, oxygen, nitrogen and water vapor. Their composition is low and amounts to 0.1-0.3% C0 2 and 1-14% N 2. After extraction, the toxic gas hydrogen sulfide is removed from the gas, the content of which should not exceed 0.02 g/m3.

Heat of combustion is the amount of heat released during complete combustion of 1 m3 of gas. The heat of combustion is measured in kcal/m3, kJ/m3 of gas. The calorific value of dry natural gas is 8000-8500 kcal/m3.

The value calculated by the ratio of the mass of a substance to its volume is called the density of the substance. Density is measured in kg/m3. The density of natural gas completely depends on its composition and is in the range c = 0.73-0.85 kg/m3.

The most important feature of any combustible gas is its heat output, i.e. Maximum temperature achieved with complete combustion of gas, if required amount air for combustion exactly corresponds to the chemical formulas of combustion, and the initial temperature of the gas and air is zero.

The heat output of natural gases is about 2000 -2100 °C, methane - 2043 °C. The actual combustion temperature in furnaces is significantly lower than the heat output and depends on the combustion conditions.

The ignition temperature is the temperature of the air-fuel mixture at which the mixture ignites without an ignition source. For natural gas it is in the range of 645-700 °C.

All flammable gases are explosive and can ignite if exposed to an open flame or spark. Distinguish lower and upper concentration limit of flame propagation , i.e. the lower and upper concentration at which an explosion of the mixture is possible. The lower explosive limit of gases is 3÷6%, the upper 12÷16%.

Explosive limits.

A gas-air mixture containing the following amount of gas:

up to 5% - does not light;

from 5 to 15% - explodes;

more than 15% - burns when air is supplied.

The pressure during a natural gas explosion is 0.8-1.0 MPa.

All flammable gases can cause poisoning to the human body. The main toxic substances are: carbon monoxide (CO), hydrogen sulfide (H 2 S), ammonia (NH 3).

Natural gas has no odor. In order to detect a leak, the gas is odorized (that is, it is given a specific smell). Odorization is carried out by using ethyl mercaptan. Odorization is carried out at gas distribution stations (GDS). When 1% of natural gas enters the air, it begins to smell. Practice shows that the average rate of ethyl mercaptan for the odorization of natural gas that enters city networks should be 16 g per 1,000 m3 of gas.

Compared to solid and liquid fuel Natural gas wins in many ways:

Relative cheapness, which is explained more the easy way mining and transport;

No ash or release of solid particles into the atmosphere;

High calorific value;

No preparation of fuel for combustion is required;

The work of service workers is made easier and the sanitary and hygienic conditions of their work are improved;

The conditions for automating work processes are simplified.

Due to possible leaks through leaks in gas pipeline connections and fittings, the use of natural gas requires special care and caution. Penetration of more than 20% of the gas into a room can lead to suffocation, and if it is present in a closed volume, from 5 to 15% can cause an explosion of the gas-air mixture. Incomplete combustion produces toxic carbon monoxide CO, which even at low concentrations leads to poisoning of operating personnel.

According to their origin, natural gases are divided into two groups: dry and fatty.

Dry gases are gases of mineral origin and are found in areas associated with present or past volcanic activity. Dry gases consist almost exclusively of methane with an insignificant content of ballast components (nitrogen, carbon dioxide) and have a calorific value Qn = 7000÷9000 kcal/nm3.

Fat gases accompany oil fields and usually accumulate in the upper layers. By their origin, wet gases are close to oil and contain many easily condensable hydrocarbons. Calorific value of liquid gases Qn=8000-15000 kcal/nm3

To the benefits gaseous fuel should include ease of transportation and combustion, lack of ash moisture, significant simplicity of boiler equipment.

Along with natural gases artificial flammable gases obtained during processing are also used solid fuels, or as a result of the operation of industrial installations as waste gases. Artificial gases consist of flammable gases of incomplete combustion of fuel, ballast gases and water vapor and are divided into rich and poor, having an average calorific value of 4500 kcal/m3 and 1300 kcal/m3, respectively. Composition of gases: hydrogen, methane, other hydrocarbon compounds CmHn, hydrogen sulfide H 2 S, non-flammable gases, carbon dioxide, oxygen, nitrogen and a small amount of water vapor. Ballast – nitrogen and carbon dioxide.

Thus, the composition of dry gaseous fuel can be represented as the following mixture of elements:

CO + H 2 + ∑CmHn + H 2 S + CO 2 + O 2 + N 2 =100%.

The composition of wet gaseous fuel is expressed as follows:

CO + H 2 + ∑CmHn + H 2 S + CO 2 + O 2 + N 2 + H 2 O = 100%.

Heat of combustion dry gaseous fuel kJ/m3 (kcal/m3) per 1 m3 of gas at normal conditions defined as follows:

Qn= 0.01,

Where Qi is the heat of combustion of the corresponding gas.

The calorific value of gaseous fuel is given in Table 3.

Blast gas formed during the smelting of cast iron in blast furnaces. Its yield and chemical composition depend on the properties of the charge and fuel, the operating mode of the furnace, methods of process intensification and other factors. The gas output ranges from 1500-2500 m 3 per ton of cast iron. The share of non-combustible components (N 2 and CO 2) in blast furnace gas is about 70%, which determines its low thermal performance (the lower calorific value of gas is 3-5 MJ/m 3).

When burning blast furnace gas, the maximum temperature of the combustion products (without taking into account heat losses and heat consumption for the dissociation of CO 2 and H 2 O) is 400-1500 0 C. If the gas and air are heated before combustion, the temperature of the combustion products can be significantly increased.

Ferroalloy gas is formed during the smelting of ferroalloys in ore reduction furnaces. Gas exhausted from closed furnaces can be used as fuel SERs (secondary energetic resources). In open furnaces, due to the free access of air, the gas burns at the top. The yield and composition of ferroalloy gas depends on the grade of smelted

alloy, charge composition, furnace operating mode, its power, etc. Gas composition: 50-90% CO, 2-8% H2, 0.3-1% CH4, O2<1%, 2-5% CO 2 , остальное N 2 . Максимальная температура продуктов сгорания равна 2080 ^0 C. Запылённость газа составляет 30-40 г/м^3 .

Converter gas formed during steel smelting in oxygen converters. The gas consists mainly of carbon monoxide, its yield and composition vary significantly during smelting. After purification, the gas composition is approximately as follows: 70-80% CO; 15-20% CO 2 ; 0.5-0.8% O 2; 3-12% N 2. The heat of combustion of gas is 8.4-9.2 MJ/m 3. The maximum combustion temperature reaches 2000 0 C.

Coke gas formed during coking of coal mixture. In ferrous metallurgy it is used after the extraction of chemical products. The composition of coke oven gas depends on the properties of the coal charge and coking conditions. The volume fractions of components in the gas are within the following limits,%: 52-62H 2 ; 0.3-0.6 O 2; 23.5-26.5 CH 4; 5.5-7.7 CO; 1.8-2.6 CO 2 . The heat of combustion is 17-17.6 MJ/m^3, the maximum temperature of combustion products is 2070 0 C.

Substances of organic origin include fuels that, when burned, release a certain amount of thermal energy. Heat production must be characterized by high efficiency and the absence of side effects, in particular, substances harmful to human health and the environment.

For ease of loading into the firebox, wood material is cut into individual elements up to 30 cm long. To increase the efficiency of their use, the firewood must be as dry as possible and the combustion process must be relatively slow. In many respects, wood from hardwoods such as oak and birch, hazel and ash, and hawthorn are suitable for heating premises. Due to the high resin content, increased burning rate and low calorific value, coniferous trees are significantly inferior in this regard.

It should be understood that the value of the calorific value is affected by the density of wood.

It is a natural material of plant origin, extracted from sedimentary rock.

This type of solid fuel contains carbon and other chemical elements. There is a division of material into types depending on its age. Brown coal is considered the youngest, followed by hard coal, and anthracite is older than all other types. The age of a combustible substance also determines its moisture content, which is more present in young material.

During the combustion of coal, environmental pollution occurs, and slag is formed on the boiler grates, which to a certain extent creates an obstacle to normal combustion. The presence of sulfur in the material is also an unfavorable factor for the atmosphere, since in the air space this element is converted into sulfuric acid.

However, consumers should not fear for their health. Manufacturers of this material, taking care of private customers, strive to reduce the sulfur content in it. The heating value of coal can vary even within the same type. The difference depends on the characteristics of the subspecies and its mineral content, as well as the geography of production. As a solid fuel, not only pure coal is found, but also low-enriched coal slag, pressed into briquettes.

Pellets (fuel granules) are solid fuels created industrially from wood and plant waste: shavings, bark, cardboard, straw.

The raw material, crushed to dust, is dried and poured into a granulator, from where it comes out in the form of granules of a certain shape. To add viscosity to the mass, a plant polymer, lignin, is used. The complexity of the production process and high demand determine the cost of pellets. The material is used in specially equipped boilers.

Types of fuel are determined depending on the material from which they are processed:

• round timber of trees of any species;
• straw;
• peat;
• sunflower husk.

Among the advantages that fuel pellets have, it is worth noting the following qualities:

• environmental friendliness;
• inability to deform and resistance to fungus;
• easy storage even outdoors;
• uniformity and duration of combustion;
• relatively low cost;
• Possibility of use for various heating devices;
• suitable granule size for automatic loading into a specially equipped boiler.

Briquettes

Briquettes are solid fuels that are in many ways similar to pellets. For their manufacture, identical materials are used: wood chips, shavings, peat, husks and straw. During the production process, raw materials are crushed and formed into briquettes by compression. This material is also an environmentally friendly fuel. It is convenient to store even outdoors. Smooth, uniform and slow combustion of this fuel can be observed both in fireplaces and stoves, and in heating boilers.

The types of environmentally friendly solid fuel discussed above are a good alternative for generating heat. Compared to fossil sources of thermal energy, which have a negative impact on the environment when burned and are, moreover, non-renewable, alternative fuels have clear advantages and a relatively low cost, which is important for certain categories of consumers.

At the same time, the fire hazard of such fuels is much higher. Therefore, it is necessary to take some safety measures regarding their storage and the use of fire-resistant materials for walls.

Liquid and gaseous fuels

As for liquid and gaseous flammable substances, the situation here is as follows.

The amount of heat released during complete combustion of a unit amount of fuel is called calorific value (Q) or, as is sometimes said, calorific value, or calorific value, which is one of the main characteristics of fuel.

The calorific value of gases is usually referred to as 1 m 3, taken under normal conditions.

In technical calculations, normal conditions mean the state of the gas at a temperature of 0°C and, at a pressure of 760 mmHg Art. The volume of gas under these conditions is denoted nm 3(normal cubic meter).

For industrial gas measurements according to GOST 2923-45, temperature 20°C and Pressure 760 are taken as normal conditions mmHg Art. The volume of gas assigned to these conditions, as opposed to nm 3 we'll call m 3 (cubic meter).

Calorific value of gases (Q)) expressed in kcal/nm e or in kcal/m3.

For liquefied gases, the calorific value is referred to as 1 kg.

There are higher (Qc) and lower (Qn) calorific values. Gross calorific value takes into account the heat of condensation of water vapor generated during fuel combustion. The lower calorific value does not take into account the heat contained in the water vapor of the combustion products, since the water vapor does not condense, but is carried away with the combustion products.

The concepts Q in and Q n refer only to those gases whose combustion releases water vapor (these concepts do not apply to carbon monoxide, which does not produce water vapor upon combustion).

When water vapor condenses, heat is released equal to 539 kcal/kg. In addition, when the condensate is cooled to 0°C (or 20°C), heat is released in the amount of 100 or 80, respectively. kcal/kg.

In total, more than 600 heat is released due to the condensation of water vapor. kcal/kg, which is the difference between the higher and lower calorific value of the gas. For most gases used in urban gas supply, this difference is 8-10%.

The calorific values ​​of some gases are given in table. 3.

For urban gas supply, gases are currently used that, as a rule, have a calorific value of at least 3500 kcal/nm 3 . This is explained by the fact that in urban areas gas is supplied through pipes over considerable distances. When the calorific value is low, a large quantity must be supplied. This inevitably leads to an increase in the diameters of gas pipelines and, as a consequence, to an increase in metal investments and funds for the construction of gas networks, and subsequently to an increase in operating costs. A significant disadvantage of low-calorie gases is that in most cases they contain a significant amount of carbon monoxide, which increases the danger when using gas, as well as when servicing networks and installations.

Gas calorific value less than 3500 kcal/nm 3 most often used in industry, where it is not necessary to transport it over long distances and it is easier to organize combustion. For urban gas supply, it is desirable to have a constant calorific value of gas. Fluctuations, as we have already established, are allowed no more than 10%. A larger change in the calorific value of gas requires new adjustments and sometimes replacement of a large number of standardized burners of household appliances, which is associated with significant difficulties.

5. THERMAL BALANCE OF COMBUSTION

Let's consider methods for calculating the heat balance of the combustion process of gaseous, liquid and solid fuels. The calculation comes down to solving the following problems.

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

· Determination of theoretical combustion temperature.

5.1. HEAT OF COMBUSTION

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

The heat released (or absorbed) during a chemical reaction 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 of quantity of a substance. One mole is the amount of substance that contains the same number of 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 mass of this substance.

For example, the relative molecular weight of oxygen (O 2) is 32, carbon dioxide (CO 2) is 44, and the corresponding molecular weights will be 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, it is not the heat of combustion that is most 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 released during complete combustion of 1 kg or 1 m 3 of a substance. For liquid and solid substances, the calculation is carried out per 1 kg, and for gaseous substances - per 1 m 3.

Knowledge of the heat of combustion and 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. When experimentally determining 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 instruments measure the total heat Q 0 released during combustion of a sample of fuel weighing m. Calorific value Q g is found by the formula

The relationship between the heat of combustion and
calorific value of fuel

To establish a connection 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.

The product of complete combustion of carbon is carbon dioxide:

C+O2 →CO2.

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.

In this case, nitrogen, halogens and other non-combustible elements are released in free form.

Combustible substance - gas

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

· Let's determine the molecular weight of methane in accordance with its chemical formula (CH 4):

M=1∙12+4∙1=16 g/mol.

· Let's determine the calorific value of 1 kg of methane:

· Let's find the volume of 1 kg of methane, knowing its density ρ=0.717 kg/m3 under normal conditions:

.

· Let's determine the calorific value of 1 m 3 of methane:

The calorific value of any combustible gases is determined similarly. For many common substances, heat of combustion and calorific values ​​have been measured with high accuracy and are given in the relevant reference literature. Here is a table of the calorific values ​​of some gaseous substances (Table 5.1). Magnitude Q in this table 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

Substance			Acetylene
Q

Combustible substance - liquid or solid

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

· Let's determine the molecular weight of ethyl alcohol in accordance with its chemical formula (C 2 H 5 OH):

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

Let's 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 liquids and solids.

Table 5.2

Calorific value of liquid fuels

Substance		Methyl alcohol	Ethanol			Fuel oil, oil
Q

Table 5.3

Calorific value of solid fuels

Substance		The tree is fresh	Dry wood	Brown coal	Dry peat	Anthracite, coke
Q

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 (equivalent fuel formula), that is, the percentage content of the following elements in it:

Oxygen (O);

Hydrogen (H);

Carbon (C);

Sulfur (S);

Ashes (A);

Water (W).

The products of fuel combustion always contain water vapor, which is formed both due to the presence of moisture in the fuel and during the combustion of hydrogen. Waste combustion products leave an industrial plant at a temperature above the dew point. 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 calculation Q n fuel, which takes into account heat losses 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 (wt.%) 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 included in the fuel partially replaces 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.

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

Net calorific value Q n(MJ/m3) of 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 content in 1 m3 of gaseous fuel.

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

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

On average, the calorific value of natural gas is approximately 53.6 MJ/m 3 . In artificially produced combustible gases, the content of methane CH4 is insignificant. The main flammable components are hydrogen H2 and carbon monoxide CO. In coke oven gas, for example, the H2 content reaches (55 ÷ 60)%, and the lower calorific value of such gas reaches 17.6 MJ/m3. The generator gas contains CO ~ 30% and H 2 ~ 15%, while the lower calorific value of the generator gas is Q n= (5.2÷6.5) MJ/m3. The content of CO and H 2 in blast furnace gas is lower; magnitude Q n= (4.0÷4.2) MJ/m 3.

Let's look at 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 those given in the table. 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 consumed for heating, and the heat capacity of water With=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

	· Let's find the calorific value of firewood using the Mendeleev formula (5.1): Q n=0.339∙43+1.025∙7–0.1085∙41–0.025∙7= 17.12 MJ/kg. · Let's determine the amount of heat spent on heating water when burning 1 kg of firewood (taking into account the fact that 5% of the heat (a = 0.05) released during combustion is spent on heating it): Q 2 =a Q n=0.05·17.12=0.86 MJ/kg. · Let's determine the amount of firewood required to heat 50 liters of water from 10° C to 100° C: kg. Thus, about 22 kg of firewood is required to heat water.