Determine the lower calorific value of natural gas. Gaseous fuel
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 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.
Released (or absorbed) during flow 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 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. Calorific value fuel is determined either experimentally or by calculation methods. 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/m 3 at 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 |
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Q |
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A flammable substance is a 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 |
Fuel oil, oil |
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|
Q |
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Table 5.3
Calorific value of solid fuels
|
Substance |
The tree is fresh |
Dry wood |
Brown coal |
Dry peat |
Anthracite, coke |
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|
Q |
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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 in it the following elements:
Oxygen (O);
Hydrogen (H);
Carbon (C);
Sulfur (S);
Ashes (A);
Water (W).
Fuel combustion products always contain water vapor, 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:
Thus, about 22 kg of firewood is required to heat water. |
kg.The tables show the mass specific heat combustion of fuel (liquid, solid and gaseous) and some other combustible materials. The following fuels were considered: coal, firewood, coke, peat, kerosene, oil, alcohol, gasoline, natural gas, etc.
List of tables:
During the exothermic reaction of fuel oxidation, its chemical energy is converted into thermal energy with the release of a certain amount of heat. The resulting thermal energy is usually called the heat of combustion of fuel. It depends on its chemical composition, humidity and is the main one. The heat of combustion of fuel per 1 kg of mass or 1 m 3 of volume forms the mass or volumetric specific heat of combustion.
The specific heat of combustion of a fuel is the amount of heat released during the complete combustion of a unit mass or volume of solid, liquid or gaseous fuel. In the International System of Units, this value is measured in J/kg or J/m 3.
The specific heat of combustion of a fuel can be determined experimentally or calculated analytically. Experimental methods for determining calorific value are based on practical measurement of the amount of heat released when a fuel burns, for example in a calorimeter with a thermostat and a combustion bomb. For fuel with known chemical composition The specific heat of combustion can be determined using the Mendeleev formula.
There are higher and lower specific heats of combustion. The higher calorific value is maximum number the heat released during complete combustion of the fuel, taking into account the heat expended on the evaporation of moisture contained in the fuel. Net calorific value less than value higher by the amount of heat of condensation, which is formed from the moisture of the fuel and hydrogen of the organic mass, which turns into water during combustion.
To determine fuel quality indicators, as well as in thermal calculations usually use lower specific heat of combustion, which is the most important thermal and performance characteristics fuel and is shown in the tables below.
Specific heat of combustion of solid fuels (coal, firewood, peat, coke)
The table presents the values of the specific heat of combustion of dry solid fuel in the dimension MJ/kg. Fuel in the table is arranged by name in alphabetical order.
Of the solid fuels considered, coking coal has the highest calorific value - its specific heat of combustion is 36.3 MJ/kg (or in SI units 36.3·10 6 J/kg). In addition, high calorific value is characteristic of hard coal, anthracite, charcoal and brown coal.
Fuels with low energy efficiency include wood, firewood, gunpowder, milling peat, and oil shale. For example, the specific heat of combustion of firewood is 8.4...12.5, and that of gunpowder is only 3.8 MJ/kg.
| Fuel | |
|---|---|
| Anthracite | 26,8…34,8 |
| Wood pellets (pellets) | 18,5 |
| Dry firewood | 8,4…11 |
| Dry birch firewood | 12,5 |
| Gas coke | 26,9 |
| Blast coke | 30,4 |
| Semi-coke | 27,3 |
| Powder | 3,8 |
| Slate | 4,6…9 |
| Oil shale | 5,9…15 |
| Solid rocket fuel | 4,2…10,5 |
| Peat | 16,3 |
| Fibrous peat | 21,8 |
| Milled peat | 8,1…10,5 |
| Peat crumb | 10,8 |
| Brown coal | 13…25 |
| Brown coal (briquettes) | 20,2 |
| Brown coal (dust) | 25 |
| Donetsk coal | 19,7…24 |
| Charcoal | 31,5…34,4 |
| Coal | 27 |
| Coking coal | 36,3 |
| Kuznetsk coal | 22,8…25,1 |
| Chelyabinsk coal | 12,8 |
| Ekibastuz coal | 16,7 |
| Frestorf | 8,1 |
| Slag | 27,5 |
Specific heat of combustion of liquid fuels (alcohol, gasoline, kerosene, oil)
A table is given of the specific heat of combustion of liquid fuel and some other organic liquids. It should be noted that fuels such as gasoline, diesel fuel and oil.
The specific heat of combustion of alcohol and acetone is significantly lower than traditional motor fuels. In addition, liquid rocket fuel has a relatively low calorific value and, with complete combustion of 1 kg of these hydrocarbons, an amount of heat will be released equal to 9.2 and 13.3 MJ, respectively.
| Fuel | Specific heat of combustion, MJ/kg |
|---|---|
| Acetone | 31,4 |
| Gasoline A-72 (GOST 2084-67) | 44,2 |
| Aviation gasoline B-70 (GOST 1012-72) | 44,1 |
| Gasoline AI-93 (GOST 2084-67) | 43,6 |
| Benzene | 40,6 |
| Winter diesel fuel (GOST 305-73) | 43,6 |
| Summer diesel fuel (GOST 305-73) | 43,4 |
| Liquid rocket fuel (kerosene + liquid oxygen) | 9,2 |
| Aviation kerosene | 42,9 |
| Kerosene for lighting (GOST 4753-68) | 43,7 |
| Xylene | 43,2 |
| High sulfur fuel oil | 39 |
| Low sulfur fuel oil | 40,5 |
| Low-sulfur fuel oil | 41,7 |
| Sulphurous fuel oil | 39,6 |
| Methyl alcohol (methanol) | 21,1 |
| n-Butyl alcohol | 36,8 |
| Oil | 43,5…46 |
| Methane oil | 21,5 |
| Toluene | 40,9 |
| White spirit (GOST 313452) | 44 |
| Ethylene glycol | 13,3 |
| Ethyl alcohol (ethanol) | 30,6 |
Specific heat of combustion of gaseous fuels and combustible gases
A table is presented of the specific heat of combustion of gaseous fuel and some other combustible gases in the dimension MJ/kg. Of the gases considered, it has the highest mass specific heat of combustion. The complete combustion of one kilogram of this gas will release 119.83 MJ of heat. Also, fuel such as natural gas has a high calorific value - the specific heat of combustion of natural gas is 41...49 MJ/kg (for pure gas it is 50 MJ/kg).
| Fuel | Specific heat of combustion, MJ/kg |
|---|---|
| 1-Butene | 45,3 |
| Ammonia | 18,6 |
| Acetylene | 48,3 |
| Hydrogen | 119,83 |
| Hydrogen, mixture with methane (50% H 2 and 50% CH 4 by weight) | 85 |
| Hydrogen, mixture with methane and carbon monoxide (33-33-33% by weight) | 60 |
| Hydrogen, mixture with carbon monoxide (50% H 2 50% CO 2 by weight) | 65 |
| Blast furnace gas | 3 |
| Coke Oven Gas | 38,5 |
| Liquefied hydrocarbon gas LPG (propane-butane) | 43,8 |
| Isobutane | 45,6 |
| Methane | 50 |
| n-Butane | 45,7 |
| n-Hexane | 45,1 |
| n-Pentane | 45,4 |
| Associated gas | 40,6…43 |
| Natural gas | 41…49 |
| Propadiene | 46,3 |
| Propane | 46,3 |
| Propylene | 45,8 |
| Propylene, mixture with hydrogen and carbon monoxide (90%-9%-1% by weight) | 52 |
| Ethane | 47,5 |
| Ethylene | 47,2 |
Specific heat of combustion of some combustible materials
A table is provided of the specific heat of combustion of some combustible materials (wood, paper, plastic, straw, rubber, etc.). Materials with high heat release during combustion should be noted. These materials include: rubber various types, expanded polystyrene (foam), polypropylene and polyethylene.
| Fuel | Specific heat of combustion, MJ/kg |
|---|---|
| Paper | 17,6 |
| Leatherette | 21,5 |
| Wood (bars with 14% moisture content) | 13,8 |
| Wood in stacks | 16,6 |
| Oak wood | 19,9 |
| Spruce wood | 20,3 |
| Wood green | 6,3 |
| Pine wood | 20,9 |
| Capron | 31,1 |
| Carbolite products | 26,9 |
| Cardboard | 16,5 |
| Styrene butadiene rubber SKS-30AR | 43,9 |
| Natural rubber | 44,8 |
| Synthetic rubber | 40,2 |
| Rubber SKS | 43,9 |
| Chloroprene rubber | 28 |
| Polyvinyl chloride linoleum | 14,3 |
| Double-layer polyvinyl chloride linoleum | 17,9 |
| Polyvinyl chloride linoleum on a felt basis | 16,6 |
| Warm-based polyvinyl chloride linoleum | 17,6 |
| Fabric-based polyvinyl chloride linoleum | 20,3 |
| Rubber linoleum (Relin) | 27,2 |
| Paraffin paraffin | 11,2 |
| Polystyrene foam PVC-1 | 19,5 |
| Foam plastic FS-7 | 24,4 |
| Foam plastic FF | 31,4 |
| Expanded polystyrene PSB-S | 41,6 |
| Polyurethane foam | 24,3 |
| Fiberboard | 20,9 |
| Polyvinyl chloride (PVC) | 20,7 |
| Polycarbonate | 31 |
| Polypropylene | 45,7 |
| Polystyrene | 39 |
| High pressure polyethylene | 47 |
| Low-pressure polyethylene | 46,7 |
| Rubber | 33,5 |
| Ruberoid | 29,5 |
| Channel soot | 28,3 |
| Hay | 16,7 |
| Straw | 17 |
| Organic glass (plexiglass) | 27,7 |
| Textolite | 20,9 |
| Tol | 16 |
| TNT | 15 |
| Cotton | 17,5 |
| Cellulose | 16,4 |
| Wool and wool fibers | 23,1 |
Sources:
- GOST 147-2013 Solid mineral fuel. Determination of the higher calorific value and calculation of the lower calorific value.
- GOST 21261-91 Petroleum products. Method for determining the higher calorific value and calculating the lower calorific value.
- GOST 22667-82 Natural flammable gases. Calculation method determination of combustion heat, relative density and Wobbe numbers.
- GOST 31369-2008 Natural gas. Calculation of calorific value, density, relative density and Wobbe number based on component composition.
- Zemsky G. T. Flammable properties of inorganic and organic materials: reference book M.: VNIIPO, 2016 - 970 p.
The heat of combustion is determined by the chemical composition of the combustible substance. The chemical elements contained in a flammable substance are indicated by accepted symbols WITH , N , ABOUT , N , S, and ash and water are symbols A And W respectively.
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The heat of combustion can be related to the working mass of the combustible substance Q P (\displaystyle Q^(P)), that is, to the flammable substance in the form in which it reaches the consumer; to the dry weight of the substance Q C (\displaystyle Q^(C)); to a flammable mass of substance Q Γ (\displaystyle Q^(\Gamma )), that is, to a flammable substance that does not contain moisture and ash.
There are 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 complete combustion of a 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, 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 values are related by the relation: 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 flammable substance, % (by mass); H is the amount of hydrogen in a combustible substance, % (by mass).
Calculation of calorific value
Thus, the higher calorific value is the amount of heat released during complete combustion of a unit mass or volume (for gas) of a combustible substance and cooling of the combustion products to the dew point temperature. In thermal engineering calculations, the higher calorific value is taken as 100%. The latent heat of combustion of a gas 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 combustion products until complete condensation, and therefore the concept of lower calorific value (QHp) has been introduced, which is obtained by subtracting from the higher calorific value the heat of vaporization of water vapor both contained in the substance and those formed during its combustion. The vaporization of 1 kg of water vapor requires 2514 kJ/kg (600 kcal/kg). The lower 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 matter)
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))- content of hydrogen and water vapor in the working fuel, %;
9 is a coefficient showing that the combustion of 1 kg of hydrogen in combination with oxygen produces 9 kg of water.
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 of gaseous fuel in kJ/kg (kcal/kg). 1 kcal = 4.1868 or 4.19 kJ.
The lower 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))- content of carbon, hydrogen, oxygen, volatile sulfur and moisture in the working mass of fuel in% (by weight).
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, calculation of the heat load to determine the category of a room in terms of explosion and fire hazard) are usually carried out using the lowest calorific value, in the USA, Great Britain, and France - according to the highest. In the UK and US, before the introduction of the metric system, specific heat of 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 (blocks W = 14%) 13,8 Natural rubber 44,73 Polyvinyl chloride linoleum 14,31 Rubber 33,52 Staple fiber 13,8 Polyethylene 47,14 Expanded polystyrene 41,6 Cotton loosened 15,7 Plastic 41,87 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
The advantages of gaseous fuel include ease of transportation and combustion, absence of ash and moisture, and significant simplicity of boiler equipment.
Along with natural gases Artificial flammable gases obtained during the processing of solid fuels, or as a result of the operation of industrial plants as waste gases, are also used. 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 under normal conditions is determined 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 ( lower heat gas combustion 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.
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. The calorific value of natural gas is so high 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 releases much less toxic substances than firewood, coal, and 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 distinguishing feature is their 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 of various additives. Sulfur is used as an impurity in technical enterprises.
Natural gas consumption
Butane is used as fuel in car gas stations, and an organic substance 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:
- High. Considers the heat of water vapor generated during fuel combustion.
- Low. It does not take into account the heat contained in water vapor, since such vapors do not condense, 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 of a large number of 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.
