5. THERMAL BALANCE OF COMBUSTION
Consider calculation methods heat balance 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) 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 basic unit the amount of matter in the SI system. 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 the given substance.
For example, the relative molecular weight of oxygen (O 2) is 32, carbon dioxide(CO 2) is equal to 44, and the corresponding molecular weights will be equal to 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 solids the calculation is carried out per 1 kg, and for gaseous - 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 of 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, halides 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 .
Let's define molecular weight 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 at 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 values of the heat of combustion and calorific value have been measured with high accuracy and are given in the relevant reference literature. Here is a table of calorific values 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 gaseous fuels
|
Substance |
Acetylene |
|||||
|
Q |
||||||
combustible 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 liquid fuels
|
Substance |
Methyl alcohol |
Fuel oil, oil |
|||||
|
Q |
|||||||
Table 5.3
Calorific value of solid fuels
|
Substance |
wood fresh |
wood dry |
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, it is necessary to know the elemental composition of the fuel ( equivalent formula 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 loss 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 exothermic reactions combustion 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
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
|
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 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. Determine the amount of firewood needed to heat 50 liters of water from 10° C to 100° C:
Thus, about 22 kg of firewood is required to heat water. |
kg.Classification of combustible gases
For the gas supply of 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 extracted from wells of pure gas fields or oil fields along with oil. The gases of oil fields are called associated gases.
The gases of pure gas fields mainly consist of methane with a small content of heavy hydrocarbons. They are characterized by the constancy of 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 on special gas plants- or obtained as a by-product from the combustion of coal in metallurgical plants, as well as in oil refineries.
Gases produced from hard coal, in our country for urban gas supply are used in very limited quantities, and specific gravity they are decreasing all the time. At the same time, the production and consumption of liquefied hydrocarbon gases obtained from associated petroleum gases at gas-gasoline plants and at oil refineries during oil refining. Liquid hydrocarbon gases used for urban gas supply consist mainly of propane and butane.
Composition of gases
The type of gas and its composition largely predetermine the scope of the gas, the scheme and diameters gas network, Constructive decisions gas burners and individual units of gas pipelines.
Gas consumption depends on the calorific value, and hence the diameters of gas pipelines and the conditions for gas combustion. When using gas in industrial installations, the combustion temperature and the speed of flame propagation and the constancy of the composition are very significant. gas fuel The composition of gases, as well as physiochemical properties they 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 the gaseous fuel includes: hydrogen (H 2) - a gas without color, taste and smell, its lower calorific value is 2579 kcal / nm 3 \ methane (CH 4) - a colorless, tasteless and odorless gas, is the main combustible part of natural gases, its lower calorific value is 8555 kcal / nm 3; carbon monoxide (CO) - a colorless, tasteless and odorless gas, obtained from the incomplete combustion of any fuel, very toxic, lower calorific value 3018 kcal / nm 3; heavy-hydrocarbons (C p N 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 called ballast. Natural gases are characterized by high calorific value and complete absence of carbon monoxide. At the same time, a number of fields, mainly gas and oil, contain a very toxic (and 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 is 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 in percentage terms.However, given that thousands and even millions of cubic meters of gas, then the total amount of impurities reaches a significant value.Many impurities fall out in gas pipelines, which ultimately leads to a decrease in their throughput, and sometimes to a complete cessation of gas flow.Therefore, the presence of impurities in gas must be taken into account both in the design of gas pipelines , as well as 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 during gas transportation through pipelines. Resin is a product of thermal decomposition of fuel and accompanies many artificial gases. In the presence of dust in the gas, the resin contributes to the formation of tar-mud plugs and blockages in 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 vapors is contained in almost all natural and artificial gases. It enters natural gases in the gas field itself due to the contacts of gases with the water surface, and artificial gases are saturated with water during the production process. The presence of moisture in gas in significant quantities is undesirable, since it reduces the calorific value of the gas. In addition, it has a high heat capacity of vaporization , moisture during gas combustion carries away a significant amount of heat together with combustion products into the atmosphere.A large moisture content in gas is also undesirable because, condensing when the gas is cooled in the "burden of its movement through pipes, it can create water plugs in the gas pipeline (in lower points) to be deleted. This requires the installation of special condensate collectors and pumping them out.
Sulfur compounds, as already noted, include hydrogen sulfide, as well as carbon disulfide, mercaptan, etc. These compounds not only adversely affect human health, but also cause significant corrosion of pipes.
Other harmful impurities include ammonia and cyanide compounds, which are found mainly in coal gases. The presence of ammonia and cyanide compounds leads to increased corrosion of 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 a ballast that reduces the calorific value, which leads to an increase in the diameter of gas pipelines and a decrease in the economic efficiency of using gaseous fuel.
The composition of gases used for urban 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 | |
| Shebelinskoye .............................. | 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 oil and gas 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 | - | ||
| Ashy....... | 23,5 | 9,3 | 3,5 | ____ | 0,2 | 4,5 | 1,132 | - | ||
| Bold.......... ............................. . | 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 amount of fuel is called the calorific value (Q) or, as it is sometimes called, 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 as 1 m 3, taken under normal conditions.
In technical calculations, normal conditions are understood as the state of the gas at a temperature equal to 0 ° C, and, at a pressure of 760 mmHg Art. The volume of gas under these conditions is denoted nm 3(normal cubic meter).
For industrial gas measurements in accordance with GOST 2923-45, the temperature of 20 ° C and pressure of 760 are taken as normal conditions mmHg Art. The volume of gas referred to these conditions, in contrast to nm 3 we 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.
There are higher (Q in) and lower (Q n) calorific value. The gross calorific value takes into account the heat of condensation of water vapor formed during the combustion of fuel. The net calorific value does not take into account the heat contained in the water vapor of the combustion products, since water vapor does not condense, but is carried away with the combustion products.
The concepts Q in and Q n apply only to those gases, during the combustion of which water vapor is released (these concepts do not apply to carbon monoxide, which does not give water vapor during combustion).
When water vapor condenses, heat is released equal to 539 kcal/kg. In addition, when the condensate is cooled to 0°C (or 20°C), heat is released, respectively, in the amount of 100 or 80 kcal/kg.
In total, due to the condensation of water vapor, heat is released more than 600 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 values of the calorific value 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 the conditions of cities gas is supplied through pipes over considerable distances. With a low calorific value, it is required to supply a large amount. This inevitably leads to an increase in the diameters of gas pipelines and, as a result, to an increase in metal investments and funds for the construction of gas networks, and, subsequently, to an increase in operating costs. A significant disadvantage of low-calorie gases is that in most cases they contain a significant amount of carbon monoxide, which increases the danger when using gas, as well as when servicing networks and installations.
Gas with calorific value 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 of gas. Fluctuations, as we have already established, are allowed no more than 10%. A greater change in the calorific value of the gas requires a new adjustment, and sometimes a change of a large number of unified burners for household appliances, which is associated with significant difficulties.
The tables present the mass specific heat of combustion of fuel (liquid, solid and gaseous) and some other combustible materials. Fuels such as: coal, firewood, coke, peat, kerosene, oil, alcohol, gasoline, natural gas, etc. are considered.
List of tables:
In an exothermic fuel oxidation reaction, its chemical energy is converted into thermal energy with the release of a certain amount of heat. The resulting thermal energy is called the heat of combustion of the fuel. It depends on its chemical composition, humidity and is the main one. The calorific value of fuel, referred to 1 kg of mass or 1 m 3 of volume, forms the mass or volumetric specific calorific value.
The specific heat of combustion of 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 the calorific value are based on the practical measurement of the amount of heat released during the combustion of fuel, for example, in a calorimeter with a thermostat and a combustion bomb. For a fuel with a known chemical composition, the specific heat of combustion can be determined from Mendeleev's formula.
There are higher and lower specific heats of combustion. The gross calorific value is equal to the maximum amount of heat released during complete combustion of the fuel, taking into account the heat spent on the evaporation of the moisture contained in the fuel. The lower calorific value is less than the higher value by the value of the heat of condensation, which is formed from the moisture of the fuel and the hydrogen of the organic mass, which turns into water during combustion.
To determine fuel quality indicators, as well as in heat engineering calculations usually use the lowest specific heat of combustion, which is the most important thermal and operational characteristic of the fuel and is given in the tables below.
Specific heat of combustion of solid fuel (coal, firewood, peat, coke)
The table shows the values of the specific heat of combustion of dry solid fuel in the unit of MJ/kg. The fuel in the table is arranged by name in alphabetical order.
Of the considered solid fuels, coking coal has the highest calorific value - its specific heat of combustion is 36.3 MJ/kg (or 36.3·10 6 J/kg in SI units). In addition, high calorific value is characteristic of coal, anthracite, charcoal and brown coal.
Fuels with low energy efficiency include wood, firewood, gunpowder, freztorf, oil shale. For example, the specific heat of combustion of firewood is 8.4 ... 12.5, and gunpowder - only 3.8 MJ / kg.
| Fuel | |
|---|---|
| Anthracite | 26,8…34,8 |
| Wood pellets (pillets) | 18,5 |
| Firewood dry | 8,4…11 |
| Dry birch firewood | 12,5 |
| gas coke | 26,9 |
| blast-furnace coke | 30,4 |
| semi-coke | 27,3 |
| Powder | 3,8 |
| Slate | 4,6…9 |
| Oil shale | 5,9…15 |
| Solid propellant | 4,2…10,5 |
| Peat | 16,3 |
| fibrous peat | 21,8 |
| Milling 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 |
| freztorf | 8,1 |
| Slag | 27,5 |
Specific heat of combustion of liquid fuel (alcohol, gasoline, kerosene, oil)
The table of specific heat of combustion of liquid fuel and some other organic liquids is given. It should be noted that fuels such as gasoline, diesel fuel and oil are characterized by high heat release during combustion.
The specific heat of combustion of alcohol and acetone is significantly lower than traditional motor fuels. In addition, liquid propellant has a relatively low calorific value and, with the complete combustion of 1 kg of these hydrocarbons, an amount of heat equal to 9.2 and 13.3 MJ, respectively, will be released.
| 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 propellant (kerosene + liquid oxygen) | 9,2 |
| Aviation kerosene | 42,9 |
| Lighting kerosene (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 |
| Oil methane | 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 fuel and combustible gases
A table of the specific heat of combustion of gaseous fuel and some other combustible gases in the dimension of MJ/kg is presented. Of the considered gases, the largest mass specific heat of combustion differs. With the complete combustion of one kilogram of this gas, 119.83 MJ of heat will be released. Also, a 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 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 mass) | 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 mass) | 65 |
| Blast Furnace Gas | 3 |
| coke oven gas | 38,5 |
| LPG liquefied hydrocarbon gas (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 |
| Propadien | 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 given of the specific heat of combustion of some combustible materials (, wood, paper, plastic, straw, rubber, etc.). It should be noted materials with high heat release during combustion. Such materials include: various types of rubber, expanded polystyrene (polystyrene), polypropylene and polyethylene.
| Fuel | Specific heat of combustion, MJ/kg |
|---|---|
| Paper | 17,6 |
| Leatherette | 21,5 |
| Wood (bars with a moisture content of 14%) | 13,8 |
| Wood in stacks | 16,6 |
| Oak wood | 19,9 |
| Spruce wood | 20,3 |
| wood green | 6,3 |
| Pine wood | 20,9 |
| Kapron | 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 SCS | 43,9 |
| Chloroprene rubber | 28 |
| Polyvinyl chloride linoleum | 14,3 |
| Two-layer polyvinyl chloride linoleum | 17,9 |
| Linoleum polyvinylchloride on a felt basis | 16,6 |
| Linoleum polyvinyl chloride on a warm basis | 17,6 |
| Linoleum polyvinylchloride on a fabric basis | 20,3 |
| Linoleum rubber (relin) | 27,2 |
| Paraffin solid | 11,2 |
| Polyfoam PVC-1 | 19,5 |
| Polyfoam FS-7 | 24,4 |
| Polyfoam FF | 31,4 |
| Expanded polystyrene PSB-S | 41,6 |
| polyurethane foam | 24,3 |
| fibreboard | 20,9 |
| Polyvinyl chloride (PVC) | 20,7 |
| Polycarbonate | 31 |
| Polypropylene | 45,7 |
| Polystyrene | 39 |
| High density polyethylene | 47 |
| Low-pressure polyethylene | 46,7 |
| Rubber | 33,5 |
| Ruberoid | 29,5 |
| Soot channel | 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 gross calorific value and calculating the net calorific value.
- GOST 22667-82 Combustible natural gases. Calculation method for determining the calorific value, relative density and Wobbe number.
- 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.
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The amount of heat released during the complete combustion of a unit amount of fuel is called the calorific value (Q) or, as it is sometimes called, 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 as 1 m 3, taken under normal conditions.
In technical calculations, normal conditions are understood as the state of the gas at a temperature equal to 0 ° C, and, at a pressure of 760 mmHg Art. The volume of gas under these conditions is denoted nm 3(normal cubic meter).
For industrial gas measurements in accordance with GOST 2923-45, the temperature of 20 ° C and pressure of 760 are taken as normal conditions mmHg Art. The volume of gas referred to these conditions, in contrast to nm 3 we 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.
There are higher (Q in) and lower (Q n) calorific value. The gross calorific value takes into account the heat of condensation of water vapor formed during the combustion of fuel. The net calorific value does not take into account the heat contained in the water vapor of the combustion products, since water vapor does not condense, but is carried away with the combustion products.
The concepts Q in and Q n apply only to those gases, during the combustion of which water vapor is released (these concepts do not apply to carbon monoxide, which does not give water vapor during combustion).
When water vapor condenses, heat is released equal to 539 kcal/kg. In addition, when the condensate is cooled to 0°C (or 20°C), heat is released, respectively, in the amount of 100 or 80 kcal/kg.
In total, due to the condensation of water vapor, heat is released more than 600 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 values of the calorific value 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 the conditions of cities gas is supplied through pipes over considerable distances. With a low calorific value, it is required to supply a large amount. This inevitably leads to an increase in the diameters of gas pipelines and, as a result, to an increase in metal investments and funds for the construction of gas networks, and, subsequently, to an increase in operating costs. A significant disadvantage of low-calorie gases is that in most cases they contain a significant amount of carbon monoxide, which increases the danger when using gas, as well as when servicing networks and installations.
Gas with calorific value 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 of gas. Fluctuations, as we have already established, are allowed no more than 10%. A greater change in the calorific value of the gas requires a new adjustment, and sometimes a change of a large number of unified burners for household appliances, which is associated with significant difficulties.
