The heat of combustion is determined by the chemical composition of the combustible substance. Chemical elements contained in a combustible substance are indicated by accepted symbols WITH , N , O , N , S, and ash and water - symbols A and W respectively.
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The heat of combustion can be referred to the working mass of the combustible substance Q P (\ displaystyle Q ^ (P)), that is, to the combustible substance in the form in which it comes to the consumer; to dry matter Q C (\ displaystyle Q ^ (C)); to the combustible mass of the substance Q Γ (\ displaystyle Q ^ (\ Gamma)), that is, to a combustible substance that does not contain moisture and ash.
Distinguish between the highest ( 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 the substance, including the heat of condensation of water vapor when cooling the combustion products.
Net calorific value corresponds to the amount of heat that is released during complete combustion, excluding the heat of condensation of water vapor. The heat of condensation of water vapor is also called latent heat of vaporization (condensation).
The lowest and highest calorific values 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 is the amount of water in the combustible substance,% (by weight); H is the amount of hydrogen in the combustible substance,% (by weight).
Calculation of the calorific value
Thus, the gross 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 thermal engineering calculations, the gross calorific value is taken as 100%. Latent heat of combustion of gas is the heat that is released during the condensation of water vapor contained in the combustion products. In theory, it can reach 11%.
In practice, it is not possible to cool the combustion products to complete condensation, and therefore the concept of the lowest heat of combustion (QHp) was introduced, which is obtained by subtracting from the highest heat of combustion the heat of vaporization of water vapor, both contained in the substance and formed during its combustion. The vaporization of 1 kg of water vapor consumes 2514 kJ / kg (600 kcal / kg). The net calorific value is determined by the formulas (kJ / kg or kcal / kg):
QHP = QBP - 2514 ⋅ ((9 HP + WP) / 100) (\ displaystyle Q_ (H) ^ (P) = Q_ (B) ^ (P) -2514 \ cdot ((9H ^ (P) + W ^ (P)) / 100))(for solid)
QHP = QBP - 600 ⋅ ((9 HP + WP) / 100) (\ displaystyle Q_ (H) ^ (P) = Q_ (B) ^ (P) -600 \ cdot ((9H ^ (P) + W ^ (P)) / 100))(for liquid substance), where:
2514 - heat of vaporization at a temperature of 0 ° C and atmospheric pressure, kJ / kg;
H P (\ displaystyle H ^ (P)) and W P (\ displaystyle W ^ (P))- content of hydrogen and water vapor in working fuel,%;
9 is a coefficient showing that when 1 kg of hydrogen is burned in combination with oxygen, 9 kg of water are formed.
The heat of combustion is the most important characteristic fuel, as it determines the amount of heat obtained by burning 1 kg of solid or liquid fuel or 1 m³ 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 elementary composition, by a calculation method in accordance with the formula of D. I. Mendeleev, kJ / kg or kcal / kg:
QHP = 339 ⋅ CP + 1256 ⋅ HP - 109 ⋅ (OP - SLP) - 25.14 ⋅ (9 ⋅ HP + WP) (\ 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)))
QHP = 81 ⋅ CP + 246 ⋅ HP - 26 ⋅ (OP + SLP) - 6 ⋅ WP (\ 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))- 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, the calculation of the heat load to determine the category of a room for explosion and fire hazard) is usually carried out according to the lowest heat of combustion, in the USA, Great Britain, France - according to the highest. In the United Kingdom and the United States, prior to the introduction of the metric system, calorific value 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 Linoleum, polyvinyl chloride 14,31 Rubber 33,52 Staple fiber 13,8 Polyethylene 47,14 Expanded polystyrene 41,6 Loose cotton 15,7 Plastic 41,87 What is fuel?
It is one component or a mixture of substances that are capable of chemical transformations associated with the release of heat. Different types Fuels are distinguished by the quantitative content of an oxidizer in them, which is used to release thermal energy.
V broad sense fuel is an energy carrier, that is, a potential type of potential energy.
Classification
At present, the types of fuel are subdivided according to their state of aggregation into liquid, solid, and gaseous.
To solid natural species include stone and firewood, anthracite. Briquettes, coke, thermoanthracite are types of artificial solid fuel.
Substances that contain substances of organic origin are classified as liquids. Their main components are: oxygen, carbon, nitrogen, hydrogen, sulfur. Artificial liquid fuel will be a variety of resins, fuel oil.
It is a mixture of various gases: ethylene, methane, propane, butane. In addition to them, the gaseous fuel contains carbon dioxide and carbon monoxide s, hydrogen sulfide, nitrogen, water vapor, oxygen.
Fuel indicators
The main indicator of combustion. The formula for determining the calorific value is considered in thermochemistry. allocate " equivalent fuel", Which means the calorific value of 1 kilogram of anthracite.
Domestic heating oil is intended for combustion in low-power heating devices that are located in residential premises, heat generators used in agriculture for drying feed, canning.
Specific heat of combustion of fuel is such a value that demonstrates the amount of heat that is formed during complete combustion of fuel with a volume of 1 m 3 or a mass of one kilogram.
To measure this value, use J / kg, J / m 3, calorie / m 3. Calorimetry is used to determine the calorific value.
When increasing specific heat fuel combustion, the specific fuel consumption decreases, and the coefficient useful action remains unchanged.
The heat of combustion of substances is the amount of energy released during the oxidation of a solid, liquid, 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 highest and lowest heat of combustion is associated with the state of aggregation of water in the substances obtained after fuel combustion.
The highest calorific value is the amount of heat released during the 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.
Latent heat of condensation is the value of the energy of condensation of water vapor.
Mathematical relationship
The highest and lowest calorific values are related by the following relationship:
Q B = Q H + k (W + 9H)
where W is the amount by weight (in%) of water in a combustible substance;
H is the amount of hydrogen (% by mass) in the combustible substance;
k is a factor of 6 kcal / kg
Calculation Methods
The highest and lowest heat of combustion is determined by two main methods: calculated 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 in this case is completely absorbed by the water. Having an idea of the mass of water, it is possible to determine by the change in its temperature, the value of its heat of combustion.
This technique is considered simple and effective, it only assumes the possession of information about the data of technical analysis.
In the calculation method, the highest and lowest heat of combustion is 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 reference fuel.
The heat of combustion of gas allows preliminary calculations to be made, to reveal the efficiency of using a certain type of fuel.
Features of origin
In order to understand how much heat is released during the combustion of a certain fuel, it is necessary to have an idea of its origin.
There is in nature different variants solid fuels, which differ in composition and properties.
Its formation is carried out through several stages. First, peat is formed, then brown and bituminous coal is obtained, then anthracite is formed. The main sources of solid fuel formation are leaves, wood, and pine needles. Dying off, parts of plants when exposed to air 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, there is additional ballast in the fuel. The part that was formed from organic matter: hydrogen, carbon, nitrogen, oxygen. In addition to these chemical elements, it contains ballast: moisture, ash.
Furnace technology assumes the release of working, dry, as well as combustible mass of burned fuel. The working mass is called the fuel in its original form, supplied to the consumer. Dry mass is a composition in which there is no water.
Composition
The most valuable components are carbon and hydrogen.
These elements are found in any type of fuel. In peat and wood, the percentage of carbon reaches 58 percent, in bituminous and brown coal - 80 percent, 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 in any fuel. By binding 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. Oxygen, which is part of the fuel, acts as ballast.
It is not heat generating chemical element, therefore, negatively affects the value of the heat of combustion. Combustion of nitrogen contained in free or bound form in combustion products, it is considered harmful impurities, therefore, its amount is clearly limited.
Sulfur is included in the fuel in the form of sulfates, sulfides, and also as sulfurous gases. When hydrated, sulfur oxides form sulfuric acid, which destroys boiler equipment, negatively affects vegetation and living organisms.
That is why sulfur is that chemical element, the presence of which in natural fuels is extremely undesirable. If it gets inside the working room, sulfur compounds cause significant poisoning of the maintenance personnel.
There are three types of ash, depending on its origin:
- primary;
- secondary;
- tertiary.
The primary species is formed from minerals that are found in plants. Secondary ash is formed as a result of the ingress of plant residues by sand and earth during formation.
Tertiary ash is included in the composition of the fuel during extraction, storage, and transportation. With a significant deposition of ash, there is a decrease in heat transfer on the heating surface of the boiler unit, reducing the amount of heat transfer to water from gases. Great amount ash negatively affects the operation of the boiler.
Finally
A significant effect on the combustion process of any type of fuel is exerted by volatiles... The larger their output, the larger the volume of the flame front will be. For example, coal, peat, easily ignite, the process is accompanied by insignificant heat losses. The coke, which remains after the removal of volatile impurities, contains only mineral and carbon compounds. Depending on the characteristics of the fuel, the amount of heat varies significantly.
Depending on the chemical composition there are three stages of solid fuel formation: peat, lignite, coal.
Natural wood is used in small boiler plants. They mainly use chips, sawdust, slabs, bark, the firewood itself is used in small quantities. Depending on the type of wood, the amount of heat released varies significantly.
As the heat of combustion decreases, firewood acquires certain advantages: quick flammability, minimal ash content, and absence of traces of sulfur.
Reliable information about the composition of natural or synthetic fuels, their 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, which will become the most effective and inexpensive to use in a particular situation.
PHYSICAL AND CHEMICAL PROPERTIES OF NATURAL GASES
Have natural gases there is no color, smell, taste.
The main indicators of natural gases include: composition, heat of combustion, density, combustion and ignition temperatures, 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 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, toxic gas, hydrogen sulfide, 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. The density is measured in kg / m3. The density of natural gas completely depends on its composition and is in the range of c = 0.73-0.85 kg / m3.
The most important feature any combustible gas is the heat output, i.e. Maximum temperature achieved with complete combustion of the gas, if the required amount of air for combustion exactly matches the chemical formulas of combustion, and the initial temperature of the gas and air is zero.
The heating capacity of natural gases is about 2000 -2100 ° C, methane - 2043 ° C. The actual combustion temperature in furnaces is significantly lower than the heating capacity and depends on the combustion conditions.
Ignition temperature is the temperature of the air-fuel mixture at which the mixture ignites without a source of ignition. For natural gas, it is in the range of 645-700 ° C.
All flammable gases are explosive, capable of being ignited by an open fire or spark. Distinguish lower and upper concentration limit of flame propagation , i.e. lower and upper concentration at which an explosion of the mixture is possible. The lower limit of explosiveness of gases is 3 ÷ 6%, the upper one is 12 ÷ 16%.
Explosive limits.
Air-gas 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 explosion pressure of natural gas is 0.8-1.0 MPa.
All combustible gases can cause poisoning of the human body. The main toxic substances are: carbon monoxide (CO), hydrogen sulfide (H 2 S), ammonia (NH 3).
Natural gas is odorless. In order to determine the leak, the gas is odorized (i.e., give it a specific odor). Odorization is carried out using ethyl mercaptan. Odorization is carried out at gas distribution stations (GDS). When 1% of natural gas gets into the air, its smell begins to be felt. Practice shows that average rate ethyl mercaptan for odorizing natural gas supplied to city networks should be 16 g per 1,000 m3 of gas.
Compared to solid and liquid fuels, natural gas is superior in many respects:
Relative cheapness, which is explained by more in an easy way mining and transport;
Lack of ash and removal of solid particles into the atmosphere;
High calorific value;
Fuel preparation for combustion is not required;
Facilitates the work of service workers and improves the sanitary and hygienic conditions of his work;
The conditions for automating work processes are facilitated.
Due to possible leaks through leaks in gas pipeline connections and valve connections, the use of natural gas requires special care and attention. Penetration of more than 20% of the gas into the room can lead to asphyxiation, 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 greasy.
Dry The gases are classified as gases of mineral origin and are found in areas associated with current or past volcanic activity. Dry gases consist almost exclusively of methane alone with negligible 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. Fat gases are similar in origin to oil and contain many easily condensable hydrocarbons. Calorific value liquid gases Qн = 8000-15000 kcal / nm3
The advantages of gaseous fuels include ease of transportation and combustion, lack of ash moisture, significant simplicity of boiler equipment.
As well as natural gases artificial combustible gases obtained during processing are also used 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 kcam3, respectively. Composition of gases: hydrogen, methane, other hydrocarbon compounds CmHn, hydrogen sulfide H 2 S, incombustible gases, carbon dioxide, oxygen, nitrogen and a small amount of water vapor. Ballast is 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 the 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 under normal conditions is determined as follows:
Qн = 0.01,
Where Qi is the heat of combustion of the corresponding gas.
The calorific value of gaseous fuel is shown in Table 3.
Blast furnace gas formed during the smelting of pig 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 intensifying the process, and other factors. Gas output ranges from 1500-2500 m 3 per ton of pig 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 lowest calorific value of gas 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 is formed during the smelting of ferroalloys in ore-reduction furnaces. The gas discharged from closed furnaces can be used as fuel RER (secondary energy resources). In open ovens due to free access air gas burns out on the top. The yield and composition of ferroalloy gas depends on the grade of the smelted
alloy, composition of the charge, operating mode of the furnace, 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 smelting. After cleaning, 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 С.
Coke oven gas formed during coking of a 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 the coking conditions. Volume fractions of components in 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.
5.THERMAL BALANCE OF COMBUSTION
Let us 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 the heat of combustion (calorific value) of the fuel.
· Determination of the 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 absorbed, it is called endothermic. All combustion reactions are exothermic, and combustion products are exothermic.
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. By the 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 conversion of a combustible substance into products of complete combustion. The mole is the basic SI unit of the amount of matter. One mole is the amount of a substance in which there are 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 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 are 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 often not the heat of combustion that is 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 temperature of combustion or explosion, pressure during explosion, flame propagation speed and other characteristics. The calorific value of the fuel is determined either experimentally or by calculation. In the experimental determination of the calorific value, a given mass of solid or liquid fuel is burned in a calorimetric bomb, and in the case of gaseous fuel, in a gas calorimeter. With the help of these devices, the total heat is measured Q 0, released during the combustion of a fuel sample with a mass m... Calorific value Q g is found by the formula
The relationship between the heat of combustion and
calorific value of fuelTo establish a relationship between the heat of combustion and the calorific value of a substance, it is necessary to write down the equation of 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 free form.
Combustible substance - gas
As an example, let us calculate the calorific value of CH 4 methane, for which the heat of combustion is Q g=882.6 .
We 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 us find the volume of 1 kg of methane, knowing its density ρ = 0.717 kg / m 3 under normal conditions:
.· Let's determine the calorific value of 1 m 3 of methane:
The calorific value of any combustible gases is determined in a similar way. For many common substances, calorific values and calorific values have been measured with high precision and are listed in the relevant reference literature. Here is a table of the calorific value of some gaseous substances (Table 5.1). The magnitude Q in this table is given in MJ / m 3 and in kcal / m 3, since often 1 kcal = 4.1868 kJ is used as a unit of heat.
Table 5.1
Calorific value of gaseous fuels
Substance
Acetylene
Q
Flammable 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.
We 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.
Determine the calorific value of 1 kg of ethyl alcohol:
The calorific value of any liquid and solid fuels is determined in a similar way. Table 5.2 and 5.3 show the calorific value Q(MJ / kg and kcal / kg) for some liquid and solid substances.
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
Peat dry
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 of the following elements in it:
Oxygen (O);
Hydrogen (H);
Carbon (C);
Sulfur (S);
Ash (A);
Water (W).
The products of combustion of fuels always contain water vapor, which is 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 temperatures 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 formula of Mendeleev:
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 reactions of combustion of carbon, hydrogen and sulfur (with a plus sign). Oxygen, which is part of the fuel, partially replaces the oxygen in the air, therefore 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 the calculated and experimental data on the calorific value of different fuels (wood, peat, coal, oil) showed that the calculation by the Mendeleev formula (5.1) gives an error not exceeding 10%.
Net calorific value Q n(MJ / m 3) dry combustible gases with sufficient accuracy can be calculated 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 [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.
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 insignificant. The main combustible components are hydrogen H 2 and carbon monoxide CO. In coke oven gas, for example, the H 2 content 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 is ~ 30% and H2 is ~ 15%, while the lower calorific value of the generator gas is 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.
Let's consider examples of calculating the calorific value of substances according to Mendeleev's formula.
Let's determine the calorific value of coal, the elemental composition of which is given in table. 5.4.
Table 5.4
Elemental composition of coal
· Substitute given in table. 5.4 data in Mendeleev's 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.
Determine the amount of firewood required to heat 50 liters of water from 10 ° C to 100 ° C, if heating consumes 5% of the heat released during combustion, 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 according to Mendeleev's formula (5.1):
Q n= 0.339 ∙ 43 + 1.025 ∙ 7–0.1085 ∙ 41–0.025 ∙ 7 = 17.12 MJ / kg.
Determine the amount of heat spent on heating water when burning 1 kg of firewood (taking into account that it takes 5% of the heat (a = 0.05) released during combustion to heat it):
Q 2 = a Q n= 0.05 17.12 = 0.86 MJ / kg.
Determine the amount of firewood required to heat 50 liters of water from 10 ° C to 100 ° C:
kg.Thus, it takes about 22 kg of wood to heat the water.
Combustible gas classification
For gas supply to cities and industrial enterprises, various combustible gases are used, differing in origin, chemical composition and physical properties.
By origin, combustible gases are divided into natural, or natural, and artificial, produced from solid and liquid fuels.
Natural gases are produced from wells of purely gas fields or oil fields along with oil. Gases from oil fields are called associated gases.
Gases from pure gas fields are mainly methane with a small content of heavy hydrocarbons. They are characterized by a constant composition and calorific value.
Associated gases, along with methane, contain a significant amount of heavy hydrocarbons (propane and butane). The composition and calorific value of these gases vary widely.
Artificial gases are produced at special gas plants - or are obtained as a by-product when coal is burned at metallurgical plants, as well as at oil refineries.
Gases produced from coal are used in our country for urban gas supply in very limited quantities, and their proportion is constantly decreasing. At the same time, the production and consumption of liquefied hydrocarbon gases, obtained from associated petroleum gases at gas-petrol plants and at oil refineries during oil refining, is growing. Liquefied petroleum gases used for urban gas supply are composed primarily of propane and butane.
Gas composition
The type of gas and its composition largely determine the field of gas application, the scheme and diameters of the gas network, the design solutions of gas burners and individual gas pipeline units.
Gas consumption depends on the calorific value, and hence the diameters of gas pipelines and the conditions for gas combustion. When gas is used in industrial installations, the combustion temperature and flame propagation speed and the constancy of the gas fuel composition are very important.The composition of gases, as well as their physicochemical properties, primarily depend on the type and method of obtaining gases.
Combustible gases are mechanical mixtures of various gases.<как горючих, так и негорючих.
The combustible part of gaseous fuel includes: hydrogen (H 2) -gas without color, taste or smell, its net calorific value is 2579 kcal / nm 3 \ methane (CH 4) is a colorless, tasteless and odorless gas that is the main combustible part of natural gases, its net calorific value is 8555 kcal / nm 3; carbon monoxide (CO) is a gas without color, taste or smell, it turns out due to incomplete combustion of any fuel, it is very poisonous, net calorific value 3018 kcal / nm 3; heavy-hydrocarbons (C p H t), By this name<и формулой обозначается целый ряд углеводородов (этан - С2Н 6 , пропан - С 3 Нв, бутан- С4Н 10 и др.), низшая теплотворная способность этих газов колеблется от 15226 до 34890 kcal / nm *.
The non-combustible part of the gaseous fuel includes: carbon dioxide (CO 2), oxygen (O 2) and nitrogen (N 2).
The non-combustible part of gases is usually called ballast. Natural gases are characterized by a high heating value and a complete absence of carbon monoxide. At the same time (a number of fields, mainly gas-oil fields, contain a very poisonous (and corrosively corrosive gas - hydrogen sulfide (H 2 S). Most artificial coal gases contain a significant amount of highly toxic gas - carbon monoxide (CO). The presence of oxide in the gas) carbon and other toxic substances are highly undesirable, since they complicate the production of operational work and increase the danger when using gas.In addition to the main components, the composition of gases includes various impurities, the specific value of which is negligible. even millions of cubic meters of gas, the total amount of impurities reaches a significant value. , and during operation.
The amount and composition of impurities depend on the method of production or extraction of gas and the degree of its purification. The most harmful impurities are dust, tar, naphthalene, moisture and sulfur compounds.
Dust appears in gas during production (extraction) or when transporting gas through pipelines. Tar is a product of thermal decomposition of fuel and is associated with many artificial gases. In the presence of dust in the gas, the resin contributes to the formation of tar-mud plugs and blockages of gas pipelines.
Naphthalene is commonly found in artificial coal gases. At low temperatures, naphthalene precipitates in pipes and, together with other solid and liquid impurities, reduces the flow area of gas pipelines.
Moisture in the form of vapor is found in almost all natural and artificial gases. It enters natural gases in the gas field itself as a result of gas contacts with the water surface, and artificial gases are saturated with water during the production process. The presence of moisture in the gas in significant quantities is undesirable, since it lowers the calorific value of the gas. , moisture during gas combustion carries away a significant amount of heat along with combustion products into the atmosphere. points) to be deleted. This requires the installation of special condensate traps and their evacuation.
Sulfur compounds, as already noted, include hydrogen sulphide, as well as carbon disulphide, mercaptan, etc. These compounds are not only harmful to human health, but also cause significant corrosion of pipes.
Among other harmful impurities, ammonia and cyanide compounds should be noted, which are found mainly in coal gases. The presence of ammonia and cyanide compounds leads to increased corrosion of the pipe metal.
The presence of carbon dioxide and nitrogen in combustible gases is also undesirable. These gases do not participate in the combustion process, being ballast that reduces the calorific value, which leads to an increase in the diameter of gas pipelines and to a decrease in the economic efficiency of using gaseous fuel.
The composition of gases used for city gas supply must meet the requirements of GOST 6542-50 (Table 1).
Table 1
The average values of the composition of natural gases of the most famous fields in the country are presented in table. 2.
From gas fields (dry)
Western Ukraine. ... ... 81,2 7,5 4,5 3,7 2,5 - . 0,1 0,5 0,735 Shebelinskoe ............................... 92,9 4,5 0,8 0,6 0,6 ____ . 0,1 0,5 0,603 Stavropol region. ... 98,6 0,4 0,14 0,06 - 0,1 0,7 0,561 Krasnodar region. ... 92,9 0,5 - 0,5 _ 0,01 0,09 0,595 Saratov ............................... 93,4 2,1 0,8 0,4 0,3 Traces 0,3 2,7 0,576 Gazli, Bukhara region 96,7 0,35 0,4" 0,1 0,45 0,575 From gas and oil fields (associated) Romashkino ............................... 18,5 6,2 4,7 0,1 11,5 1,07 7,4 4,6 ____ Traces 1,112 __ . Tuymazy ............................... 18,4 6,8 4,6 ____ 0,1 7,1 1,062 - Ash ....... 23,5 9,3 3,5 ____ 0,2 4,5 1,132 - Fat .......... ............................. 2,5 . ___ . 1,5 0,721 - Syzran oil ............................... 31,9 23,9 - 5,9 2,7 0,8 1,7 1,6 31,5 0,932 - Ishimbay ............................... 42,4 20,5 7,2 3,1 2,8 1,040 _ Andijan. ............................... 66,5 16,6 9,4 3,1 3,1 0,03 0,2 4,17 0,801 ; Calorific value of gases
The amount of heat released during the complete combustion of a unit of the amount of fuel is called the calorific value (Q) or, as they sometimes say, the calorific value, or calorific value, which is one of the main characteristics of the fuel.
The calorific value of gases is usually referred to 1 m 3, taken under normal conditions.
In technical calculations, normal conditions mean the state of the gas at a temperature equal to 0 ° C, and, at a pressure of 760 mmHg Art. The gas volume under these conditions is denoted nm 3(normal cubic meter).
For industrial gas measurements in accordance with GOST 2923-45, the normal conditions are taken as a temperature of 20 ° C and a Pressure of 760 mmHg Art. The volume of gas attributed to these conditions, in contrast to nm 3 will call m 3 (cubic meter).
Calorific value of gases (Q)) expressed in kcal / nm e or in kcal / m 3.
For liquefied gases, the calorific value is referred to 1 kg.
Distinguish between higher (Q in) and lower (Q n) calorific value. The gross calorific value takes into account the heat of condensation of water vapor generated during fuel combustion. The net calorific value does not take into account the heat contained in the water vapor of the combustion products, since water chests do not condense, but are carried away with the combustion products.
The concepts of Q in and Q n refer only to those gases, the combustion of which emits water vapor (these concepts do not apply to carbon monoxide, which does not produce water vapor during combustion).
During condensation of water vapor, heat is released, equal to 539 kcal / kg. In addition, when the condensate is cooled to 0 ° C (. Or 20 ° C), respectively, heat is released in the amount of 100 or 80 kcal / kg.
In total, more than 600 heat is released due to condensation of water vapor. kcal / kg, which is the difference between the gross and net 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, which, as a rule, have a calorific value of at least 3500 kcal / nm 3. This is explained by the fact that in urban conditions gas is supplied through pipes over considerable distances. If the calorific value is low, a large amount must be fed. 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 in the following: and to an increase in operating costs. A significant disadvantage of low-calorific 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 with a heating value of less than 3500 kcal / nm 3 most often used in industry, where it is not required to transport it over long distances and it is easier to organize incineration. For urban gas supply, it is desirable to have a constant calorific value. Fluctuations, as we have already established, are allowed no more than 10%. A large change in the calorific value of a gas requires a new adjustment, and sometimes a change in a large number of standardized burners for household appliances, which is associated with significant difficulties.
