Determination of the heat of combustion of gas. Calorific value of various types of fuel: firewood, coal, pellets, briquettes

Every day, turning on the burner on the stove, few people think about how long ago they began to produce gas. In our country, its development was started in the twentieth century. Before that, it was simply found when extracting oil products. Calorific value natural gas is so great that today this raw material is simply irreplaceable, and its high-quality counterparts have not yet been developed.

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

Feature of fossil fuel

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

Ecologists believe that gas is the purest fuel; when burned, it releases much less toxic substances than wood, coal, and oil. This fuel is used daily by people and contains such an additive as an odorant, it is added at equipped installations in a ratio of 16 milligrams per 1,000 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 consider the role of coal:

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

The described type of fuel consists of hydrocarbon and non-hydrocarbon components. The natural fossil fuel is primarily methane, which includes butane and propane. In addition to the hydrocarbon components, nitrogen, sulfur, helium and argon are present in the described fossil fuel. Liquid vapors are also found, but only in gas and oil fields.

Deposit types

Several types of gas deposits are noted. They are divided into the following types:

• gas;
• oil.

Them hallmark is the hydrocarbon content. Gas deposits contain approximately 85-90% of the presented 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 generation is its flushing from various kinds of additives. Sulfur as an impurity is exploited at technical enterprises.

Natural gas consumption

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

Fossil fuel is used in everyday life:

• columns;
• gas stove;

This kind of fuel is considered the most budgetary and harmless, the only drawback is the emission of carbon dioxide during combustion into the atmosphere. 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 with sufficient burnout of a unit of fuel. 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 terms:

• kcal / nm 3;
• kcal / m 3.

There is a high and low calorific value:

1. High. Considers the heat of water vapor that occurs during the combustion of fuel.
2. Low. It does not take into account the heat contained in water vapor, since such vapors do not lend themselves to condensation, 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 from 80 to one hundred kcal / kg is released. In general, due to the accumulation of water vapor, more than 600 kcal / kg are formed, this is the distinguishing feature between high and low heat output.

For the vast majority of gases consumed in an urban fuel distribution system, the difference equates 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 the 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 for long distances, and it becomes much easier to carry out combustion. Serious changes in the calorific value of the gas need frequent adjustment and sometimes replacement a large number standardized burners of household sensors, which leads to difficulties.

This situation leads to an increase in the diameter of the gas pipeline, as well as an increase in the cost of metal, laying networks and operation. The big disadvantage of low-calorie fossil fuels is the huge content of carbon monoxide, in connection with this, the level of danger increases during the operation of the fuel and during the maintenance of the pipeline, in turn, as well as equipment.

The heat released during combustion, not exceeding 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.

5. THERMAL BALANCE OF COMBUSTION

Consider methods for calculating the heat balance of the 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 44, and the corresponding molecular weights would 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, 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 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 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 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.

The product of complete combustion of 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, halogens 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 .

Determine the molecular weight of methane in accordance with its chemical formula (CH 4):

М=1∙12+4∙1=16 g/mol.

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/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 substance - 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 of 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 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 (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) 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 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

The heat of combustion is determined by the chemical composition of the combustible substance. The chemical elements contained in the combustible substance are designated by the accepted symbols With , H , O , N , S, and ash and water are symbols BUT and W respectively.

Encyclopedic YouTube

• 1 / 5

  The heat of combustion can be related to the working mass of the combustible Q P (\displaystyle Q^(P)), that is, to a combustible substance in the form in which it enters the consumer; to dry matter Q C (\displaystyle Q^(C)); to the combustible mass of matter Q Γ (\displaystyle Q^(\Gamma )), that is, to a combustible substance that does not contain moisture and ash.

  Distinguish higher ( Q B (\displaystyle Q_(B))) and lower ( Q H (\displaystyle Q_(H))) heat of combustion.

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

  Net calorific value corresponds to the amount of heat that is released during complete combustion, without taking into account the heat of condensation of water vapor. The heat of condensation of water vapor is also called latent heat of vaporization (condensation).

  The lower and higher calorific value are related by the ratio: Q B = Q H + k (W + 9 H) (\displaystyle Q_(B)=Q_(H)+k(W+9H)),

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

  Calculation of heat of combustion

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

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

  Q H P = Q B P − 2514 ⋅ ((9 H P + W P) / 100) (\displaystyle Q_(H)^(P)=Q_(B)^(P)-2514\cdot ((9H^(P)+W^ (P))/100))(for solid)

  Q H P = Q B P − 600 ⋅ ((9 H P + W P) / 100) (\displaystyle Q_(H)^(P)=Q_(B)^(P)-600\cdot ((9H^(P)+W^ (P))/100))(for a liquid substance), where:

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

  H P (\displaystyle H^(P)) and W P (\displaystyle W^(P))- the content of hydrogen and water vapor in the working fuel,%;

  9 is a coefficient showing that when 1 kg of hydrogen is burned in combination with oxygen, 9 kg of water is formed.

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

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

  Q H P = 339 ⋅ C P + 1256 ⋅ H P − 109 ⋅ (O P − S L P) − 25.14 ⋅ (9 ⋅ H P + W P) (\displaystyle Q_(H)^(P)=339\cdot C^(P)+1256\ cdot H^(P)-109\cdot (O^(P)-S_(L)^(P))-25.14\cdot (9\cdot H^(P)+W^(P)))

  Q H P = 81 ⋅ C P + 246 ⋅ H P − 26 ⋅ (O P + S L P) − 6 ⋅ W P (\displaystyle Q_(H)^(P)=81\cdot C^(P)+246\cdot H^(P) -26\cdot (O^(P)+S_(L)^(P))-6\cdot W^(P)), where:

  C P (\displaystyle C_(P)), H P (\displaystyle H_(P)), O P (\displaystyle O_(P)), S L P (\displaystyle S_(L)^(P)), W P (\displaystyle W_(P))- the content of carbon, hydrogen, oxygen, volatile sulfur and moisture in the working mass of fuel in% (by mass).

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

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

  Substances and materials	Net calorific value Q H P (\displaystyle Q_(H)^(P)), MJ/kg
  Petrol	41,87
  Kerosene	43,54
  Paper: books, magazines	13,4
  Wood (bars W = 14%)	13,8
  Natural rubber	44,73
  Polyvinyl chloride linoleum	14,31
  Rubber	33,52
  Staple fiber	13,8
  Polyethylene	47,14
  Styrofoam	41,6
  Cotton loosened	15,7
  Plastic	41,87

  Gas fuel is divided into natural and artificial and is a mixture of combustible and non-combustible gases containing a certain amount of water vapor, and sometimes dust and tar. The amount of gas fuel is expressed in cubic meters under normal conditions (760 mm Hg and 0 ° C), and the composition is expressed as a percentage by volume. Under the composition of the fuel understand the composition of its dry gaseous part.

  natural gas fuel

  The most common gas fuel is natural gas, which has a high calorific value. The basis of natural gas is methane, the content of which is 76.7-98%. Other gaseous hydrocarbon compounds are part of natural gas from 0.1 to 4.5%.

  Liquefied gas is a product of oil refining - it consists mainly of a mixture of propane and butane.

  Natural gas (CNG, NG): methane CH4 more than 90%, ethane C2 H5 less than 4%, propane C3 H8 less than 1%

  Liquefied gas (LPG): propane C3 H8 more than 65%, butane C4 H10 less than 35%

  Combustible gases include: hydrogen H 2, methane CH 4, Other hydrocarbon compounds C m H n, hydrogen sulfide H 2 S and non-combustible gases, carbon dioxide CO2, oxygen O 2, nitrogen N 2 and a small amount of water vapor H 2 O. Indices m and P at C and H characterize compounds of various hydrocarbons, for example, for methane CH 4 t = 1 and n= 4, for ethane С 2 Н b t = 2 and n= b etc.

  Composition of dry gaseous fuel (in percent by volume):

  
  CO + H 2 + 2 C m H n + H 2 S + CO 2 + O 2 + N 2 = 100%.

  The non-combustible part of dry gaseous fuel - ballast - is nitrogen N and carbon dioxide CO 2 .

  The composition of the wet gaseous fuel is expressed as follows:

  CO + H 2 + Σ C m H n + H 2 S + CO 2 + O 2 + N 2 + H 2 O \u003d 100%.

  The heat of combustion, kJ / m (kcal / m 3), 1 m 3 of pure dry gas under normal conditions is determined as follows:

  Q n s \u003d 0.01,

  where Qco, Q n 2 , Q with m n n Q n 2 s. - heat of combustion of individual gases that make up the mixture, kJ / m 3 (kcal / m 3); CO, H 2, Cm H n , H 2 S - components that make up the gas mixture, % by volume.

  The heat of combustion of 1 m3 of dry natural gas under normal conditions for most domestic fields is 33.29 - 35.87 MJ / m3 (7946 - 8560 kcal / m3). Characteristics of gaseous fuel is given in table 1.

  Example. Determine the net calorific value of natural gas (under normal conditions) of the following composition:

  H 2 S = 1%; CH 4 = 76.7%; C 2 H 6 = 4.5%; C 3 H 8 = 1.7%; C 4 H 10 = 0.8%; C 5 H 12 = 0.6%.

  Substituting into formula (26) the characteristics of gases from Table 1, we obtain:

  Q ns \u003d 0.01 \u003d 33981 kJ / m 3 or

  Q ns \u003d 0.01 (5585.1 + 8555 76.7 + 15 226 4.5 + 21 795 1.7 + 28 338 0.8 + 34 890 0.6) \u003d 8109 kcal / m 3.

  Table 1. Characteristics of gaseous fuel

  Gas	Designation	Heat of combustion Q n s
  		KJ/m3	kcal/m3
  Hydrogen	H,	10820	2579
  carbon monoxide	SO	12640	3018
  hydrogen sulfide	H 2 S	23450	5585
  Methane	CH 4	35850	8555
  Ethane	C 2 H 6	63 850	15226
  Propane	C 3 H 8	91300	21795
  Butane	C 4 H 10	118700	22338
  Pentane	C 5 H 12	146200	34890
  Ethylene	C 2 H 4	59200	14107
  Propylene	C 3 H 6	85980	20541
  Butylene	C 4 H 8	113 400	27111
  Benzene	C 6 H 6	140400	33528

  Boilers of the DE type consume from 71 to 75 m3 of natural gas to produce one ton of steam. The cost of gas in Russia in September 2008 is 2.44 rubles per cubic meter. Consequently, a ton of steam will cost 71 × 2.44 = 173 rubles 24 kopecks. The real cost of a ton of steam at factories is for DE boilers at least 189 rubles per ton of steam.

  Boilers of the DKVR type consume from 103 to 118 m3 of natural gas to produce one ton of steam. The minimum estimated cost of a ton of steam for these boilers is 103 × 2.44 = 251 rubles 32 kopecks. The real cost of steam for plants is at least 290 rubles per ton.

  How to calculate the maximum consumption of natural gas for a steam boiler DE-25? This is technical specifications boiler. 1840 cubes per hour. But you can also calculate. 25 tons (25 thousand kg) must be multiplied by the difference between the enthalpies of steam and water (666.9-105) and all this divided by the boiler efficiency of 92.8% and the heat of combustion of gas. 8300. and all

  Artificial gas fuel

  Artificial combustible gases are local fuels, since they have a much lower calorific value. Their main combustible elements are carbon monoxide CO and hydrogen H2. These gases are used within the limits of the production where they are obtained as fuel for technological and power plants.

  All natural and artificial combustible gases are explosive, capable of igniting on an open flame or spark. There are lower and upper explosive limits of gas, i.e. the highest and lowest percentage concentrations in the air. The lower explosive limit of natural gases ranges from 3% to 6%, while the upper limit ranges from 12% to 16%. All combustible gases can cause poisoning of the human body. The main toxic substances of combustible gases are: carbon monoxide CO, hydrogen sulfide H2S, ammonia NH3.

  Natural combustible gases, as well as artificial ones, are colorless (invisible), odorless, which makes them dangerous when they penetrate into the interior of the boiler room through leaks in gas pipeline fittings. To avoid poisoning, combustible gases should be treated with an odorant - a substance with an unpleasant odor.

  Obtaining carbon monoxide CO in industry by gasification of solid fuel

  For industrial purposes, carbon monoxide is obtained by gasification of solid fuel, i.e., its transformation into gaseous fuel. So you can get carbon monoxide from any solid fuel - fossil coal, peat, firewood, etc.

  The process of gasification of solid fuel is shown in a laboratory experiment (Fig. 1). Having filled the refractory tube with pieces of charcoal, we will heat it up strongly and let oxygen pass from the gasometer. Let the gases coming out of the tube pass through a lime water washer and then set it on fire. Lime water becomes cloudy, the gas burns with a bluish flame. This indicates the presence of CO2 dioxide and carbon monoxide CO in the reaction products.

  The formation of these substances can be explained by the fact that when oxygen comes into contact with hot coal, the latter is first oxidized into carbon dioxide: C + O 2 \u003d CO 2

  Then, passing through hot coal, carbon dioxide is partially reduced by it to carbon monoxide: CO 2 + C \u003d 2CO

  Rice. 1. Obtaining carbon monoxide (laboratory experience).

  Under industrial conditions, gasification of solid fuels is carried out in furnaces called gas generators.

  The resulting mixture of gases is called producer gas.

  The gas generator device is shown in the figure. It is a steel cylinder with a height of about 5 m and a diameter of approximately 3.5 m, lined inside with refractory bricks. From above, the gas generator is loaded with fuel; From below, air or water vapor is supplied by a fan through the grate.

  

  Oxygen in the air reacts with the carbon of the fuel, forming carbon dioxide, which, rising up through a layer of hot fuel, is reduced by carbon to carbon monoxide.

  If only air is blown into the generator, then a gas is obtained, which in its composition contains carbon monoxide and nitrogen of the air (as well as a certain amount of CO 2 and other impurities). This generator gas is called air gas.

  If water vapor is blown into the generator with hot coal, then carbon monoxide and hydrogen are formed as a result of the reaction: C + H 2 O \u003d CO + H 2

  This mixture of gases is called water gas. Water gas has a higher calorific value than air gas, since its composition, along with carbon monoxide, also includes a second combustible gas - hydrogen. Water gas (synthesis gas), one of the products of gasification of fuels. Water gas consists mainly of CO (40%) and H2 (50%). Water gas is a fuel (calorific value 10,500 kJ/m3, or 2730 kcal/mg) and at the same time raw material for the synthesis of methanol. Water gas, however, cannot be obtained for a long time, since the reaction of its formation is endothermic (with the absorption of heat), and therefore the fuel in the generator cools down. In order to keep the coal hot, the injection of water vapor into the generator is alternated with the injection of air, the oxygen of which, as is known, reacts with the fuel to release heat.

  AT recent times steam-oxygen blast began to be widely used for fuel gasification. Simultaneous blowing of water vapor and oxygen through the fuel layer makes it possible to carry out the process continuously, significantly increase the generator productivity and obtain gas with a high content of hydrogen and carbon monoxide.

  Modern gas generators are powerful devices of continuous action.

  So that when fuel is supplied to the gas generator, combustible and toxic gases do not penetrate into the atmosphere, the loading drum is made double. While fuel enters one compartment of the drum, fuel is poured out of the other compartment into the generator; when the drum rotates, these processes are repeated, while the generator remains isolated from the atmosphere all the time. Uniform distribution fuel in the generator is carried out using a cone, which can be installed at different heights. When it is lowered, the coal lies closer to the center of the generator; when the cone is raised, the coal is thrown closer to the walls of the generator.

  Removal of ash from the gas generator is mechanized. The cone-shaped grate is slowly rotated by an electric motor. In this case, the ash is displaced to the walls of the generator and is thrown into the ash box with special devices, from where it is periodically removed.

  The first gas lamps were lit in St. Petersburg on Aptekarsky Island in 1819. The gas that was used was obtained by gasification hard coal. It was called light gas.

  
  

  The great Russian scientist D. I. Mendeleev (1834-1907) was the first to express the idea that the gasification of coal can be carried out directly underground, without lifting it out. The tsarist government did not appreciate Mendeleev's proposal.

  The idea of ​​underground gasification was warmly supported by V. I. Lenin. He called it "one of the great triumphs of technology." Underground gasification was carried out for the first time by the Soviet state. Already before the Great Patriotic War, underground generators were operating in the Donetsk and Moscow region coal basins in the Soviet Union.

  Figure 3 gives an idea of ​​one of the methods of underground gasification. Two wells are laid in the coal seam, which are connected at the bottom with a channel. Coal is set on fire in such a channel near one of the wells and blast is supplied there. Combustion products, moving along the channel, interact with hot coal, resulting in the formation of combustible gas, as in a conventional generator. The gas comes to the surface through the second well.

  Generator gas is widely used for heating industrial furnaces - metallurgical, coke and as a fuel in cars (Fig. 4).

  
  

  Rice. 3. Scheme of underground gasification of coal.

  A number of organic products, such as liquid fuels, are synthesized from hydrogen and carbon monoxide of water gas. Synthetic liquid fuel - fuel (mainly gasoline), obtained by synthesis from carbon monoxide and hydrogen at 150-170 degrees Celsius and a pressure of 0.7 - 20 MN / m2 (200 kgf / cm2), in the presence of a catalyst (nickel, iron, cobalt ). The first production of synthetic liquid fuels was organized in Germany during the 2nd World War due to the shortage of oil. Synthetic liquid fuels have not received wide distribution due to their high cost. Water gas is used to produce hydrogen. To do this, water gas in a mixture with water vapor is heated in the presence of a catalyst and as a result, hydrogen is obtained in addition to that already present in water gas: CO + H 2 O \u003d CO 2 + H 2

  PHYSICAL AND CHEMICAL PROPERTIES OF NATURAL GASES

  Natural gases have no color, smell or taste.

  The main indicators of natural gases include: composition, heat of combustion, 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 combustible and non-combustible substances. Combustible gases include: hydrocarbons, hydrogen, hydrogen sulfide. Non-flammables include: carbon dioxide, oxygen, nitrogen and water vapor. Their composition is low and amounts to 0.1-0.3% CO 2 and 1-14% N 2 . After extraction, toxic hydrogen sulfide gas is extracted from the gas, the content of which should not exceed 0.02 g/m3.

  The calorific value is the amount of heat released during the 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/m 3 .

  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 depends entirely on its composition and is within c = 0.73-0.85 kg/m3.

  The most important feature of any combustible gas is the heat output, i.e. the maximum temperature reached with complete combustion of the gas, if required amount combustion air exactly matches the chemical formulas of combustion, and the initial temperature of the gas and air is zero.

  The heat capacity of natural gases is about 2000 -2100 °C, methane - 2043 °C. The actual combustion temperature in furnaces is much 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 combustible gases are explosive, capable of igniting with an open flame or spark. Distinguish lower and upper concentration limit of flame propagation , i.e. the lower and upper concentrations at which an explosion of the mixture is possible. The lower explosive limit of gases is 3÷6%, the upper limit is 12÷16%.

  Explosive limits.

  Gas-air mixture containing the amount of gas:

  up to 5% - does not burn;

  from 5 to 15% - explodes;

  more than 15% - burns when air is supplied.

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

  All combustible gases are capable of causing poisoning of the human body. The main toxic substances are: carbon monoxide (CO), hydrogen sulfide (H 2 S), ammonia (NH 3).

  Natural gas has no smell. In order to determine the leak, the gas is odorized (i.e., they give it a specific smell). Carrying out odorization is carried out by using ethyl mercaptan. Carry out odorization at gas distribution stations (GDS). When 1% of natural gas enters the air, its smell begins to be felt. Practice shows that the average rate of ethyl mercaptan for the odorization of natural gas supplied to city networks should be 16 g per 1,000 m3 of gas.

  Compared to solid and liquid fuels, natural gas wins in many ways:

  Relative cheapness, which is explained by an easier way of extraction and transport;

  No ash and removal of solid particles into the atmosphere;

  High heat of combustion;

  No preparation of fuel for combustion is required;

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

  Facilitates the automation of work processes.

  Due to possible leaks through leaks in gas pipeline connections and fittings, the use of natural gas requires special care and caution. The penetration of more than 20% of the gas into the room can lead to suffocation, and if it is present in a closed volume from 5 to 15%, it 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 the 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 alone with an insignificant content of ballast components (nitrogen, carbon dioxide) and have a calorific value Qн=7000÷9000 kcal/nm3.

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

  The advantages of gaseous fuels include the ease of transportation and combustion, the absence of moisture ash, and the significant simplicity of boiler equipment.

  As well as natural gases artificial combustible gases are also used, obtained during the processing of solid fuels, or as a result of the operation of industrial plants as waste gases. Artificial gases consist of combustible 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 kkam3, respectively. Composition of gases: hydrogen, methane, other hydrocarbon compounds CmHn, hydrogen sulfide H 2 S, non-combustible 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 \u003d 100%.

  The composition of the wet gaseous fuel is expressed as follows:

  CO + H 2 + ∑CmHn + H 2 S + CO 2 + O 2 + N 2 + H 2 O \u003d 100%.

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

  Qn \u003d 0.01,

  Where Qi is the calorific value of the corresponding gas.

  The heat of combustion of gaseous fuel is given in table 3.

  Blast furnace gas formed during iron smelting 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 intensifying the process, and other factors. The gas output ranges from 1500-2500 m 3 per ton of pig iron. The proportion of non-combustible components (N 2 and CO 2) in blast-furnace gas is about 70%, which causes its low thermal performance ( lower heat gas combustion is 3-5 MJ / m 3).

  When burning blast-furnace gas, the maximum temperature of the combustion products (excluding 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 formed during the smelting of ferroalloys in ore reduction furnaces. The exhaust gas from closed furnaces can be used as fuel SER (secondary energy resources). In open furnaces, due to the free access of air, the gas burns on the top. The yield and composition of ferroalloy gas depends on the grade of the smelted

  alloy, charge composition, furnace operation mode, its power, etc. Gas composition: 50-90% CO, 2-8% H 2 , 0.3-1% CH 4 , O 2<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 change significantly during melting. After purification, the composition of the gas 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 the gas is 8.4-9.2 MJ/m 3 . The maximum combustion temperature reaches 2000 0 C.

  coke oven gas formed during the coking of coal charge. 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. 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 the combustion products is 2070 0 С.