Chemical composition and heating capacity of common fuels

First, the chemical composition of the fuel

1. Chemical composition of solid and liquid fuels

The physical state of solid fuels and liquid fuels differs in the structure of chemical molecules, but their chemical elements are the same, consisting of carbon ( C ), hydrogen ( H ), nitrogen ( N ), oxygen ( O ), sulfur ( S ). Made up of other elements. In addition, both fuels contain water and ash.

Carbon (C) is the main component of the solid and liquid fuels, such as heavy carbonaceous down about 85%, 90% or anthracite.

When oxygen is sufficient, carbon burns to produce carbon dioxide ( CO ₂ ). When oxygen is insufficient, it burns to form carbon monoxide ( CO ), which releases heat. The reaction is as follows:

C+O ₂ =CO ₂ +96 , 600 kcal

C+0.5O ₂ =CO+28,400 kcal

As can be seen from the above, each 12 kg Pure carbon and 32 kg Oxygen interaction produces CO ₂ , so 1 kg The heat released by pure combustion to generate CO ₂ is

96.600 kcal ——————— = 8,050 kcal 12

When 12 kg of carbon and 16 kg of oxygen form CO , the heat released by CO combustion per kg of carbon can be determined.

28.400 kcal ——————— = 2,370 kcal 12

Hydrogen ( H ) is the second major component of solid fuels and liquid fuels. It exists in the fuel in two forms, one in the form of a combustible state, such as a hydrocarbon, which emits a large amount of heat when burned. On the other hand, it has been oxidized with fuel to form a stable oxide such as water, which does not exotherm when burned.

The combustion of hydrogen is as follows:

H ₂ +0.5O ₂ =H ₂ O _water +68,380 kcal

In another reaction, when hydrogen is burned, the generated water is converted into water vapor, and the reaction is as follows:

H ₂ +0.5O ₂ =H ₂ O _vapor +57,70 kcal

The difference between 68,380 kcal and 57,700 kcal is good. 18 kg The heat required for water to become steam.

The molecular weight of hydrogen is equal to 2.016 , so one kilogram The heat released by hydrogen combustion produces water

68.380
———— = 33,919 kcal / kg 2.016

The heat released by the combustion of 1 kg of hydrogen to produce water vapor is: [next]
57.700
———— = 28,621 kcal / kg 2.016

Nitrogen ( N ) does not participate in combustion and does not exotherm. It is an inert substance in fuel.

Oxygen ( O ) exists as a compound in solid and liquid fuels and therefore does not support combustion. It is a harmful component. The presence of it reduces the heating capacity of the fuel, so it is desirable that the oxygen content of the fuel is as small as possible.

Sulfur (S) is harmful composition, is present in the fuel in three states :( 1) organic sulfur; (2) pyrite; (3) sulfate. The first two types can be burned and exothermic, and they are treated as free sulfur in the calculation, which is called volatile sulfur. The last type of sulfur cannot be burned and it is present in the fuel in the form of various sulfates. The chemical reaction of sulfur combustion is:

FeS+1.5O ₂ =FeO+SO ₂ +111,400 kcal

S+O ₂ =SO ₂ +70,000 kcal

According to the above reaction formula, the calorific value per gram of sulfur in the pyrite can be determined as:

111.400
————— = 3,500 kcal 32

combustion one kilogram The heat released by sulfur in organic matter is:

28.400 kcal —————— = 2,370 kcal 12

When sulfur is burned, the released SO ₂ will corrode the equipment and pollute the air, so it is a harmful component and generally does not allow the sulfur content in the fuel to exceed 0.5% .

Moisture ( W ) H ₂ O itself does not exotherm, but also absorbs a large amount of heat into steam, reducing the amount of fuel used, so moisture is a harmful component of fuel.

Ash ( A ) solid and liquid fuels contain many mineral impurities such as SiO ₂ , Al ₂ O ₃ , CaO , FeO, etc., and after combustion, the fuel remains as a solid, which is called ash. For smelting, it is the most harmful component. The liquid fuel ash content is very low, generally below 0.3% . There are many ash in solid fuels, and the fluctuation range is large, which is unfavorable for smelting production. Therefore, the ash content must be considered when selecting fuel.

Representation of solid fuel and liquid fuel components:

    First, the representation of the ingredient: that is, the percentage of the weight of the solid fuel and liquid fuel actually used:

C ^with +H ^with +O ^with +N ^with +S ^with +A ^with +W ^with =100%

Second, the dry component means: regardless of the moisture in the fuel, expressed as a percentage of each component of the dry material: [next]

                                  C ^dry + H ^dry + O ^dry + N ^dry + S ^dry + A ^dry = 100%

Third, it is expressed only as solid and liquid fuel combustible components, regardless of moisture and ash:

C ^combustion + H ^combustion + O ^combustion + N ^combustion + S ^combustion = 100%

Fourth, expressed as organic components: C , H , O , N The four elemental components are commonly referred to as organic substances of fuel, and are expressed as:

C ^machine + H ^machine + O ^machine + N ^machine = 100%

2. Chemical composition of gaseous fuel

Gaseous fuels are mechanical mixtures of various simple compounds. The most important ones are: carbon monoxide, hydrogen, methane ( CH ₄ ), ethylene ( C ₂ H ₄ ), heavy hydrocarbons ( C _m H _n ), hydrogen sulfide ( H ₂ S ). They are all flammable compounds. In addition, there are incombustible components such as N ₂ , CO ₂ , SO ₂ , and H ₂ O _vapor .

When a combustible component in a gaseous fuel is burned, its reaction formula is as follows:

CO+0.5O ₂ =CO ₂ +68, 220 kcal

H ₂ +0.5O ₂ =H ₂ O _vapor +57,810 kcal

CH ₄ +2O ₂ =CO ₂ +2H ₂ O _vapor +191,902 kcal

C ₂ H ₄ +3O ₂ =CO ₂ +2H ₂ O _vapor +320,402 kcal

H ₂ S+1.5O ₂ =H ₂ O _vapor +SO ₂ +123,960 kcal

According to the Aphrodite law, in the standard case (the temperature is 0 °C , the pressure is 760 mm When the mercury column), the volume occupied by the molecule in any given gaseous substance are equal (i.e., 22.4 Nm ^3), but slightly different from the component small.

68.220
1 standard meter ³ CO is released when burning —————— = 3,046 kcal 22.4

57.810
1 standard meter ³ H ₂ is released when burning —————— = 2,580 kcal 22.4

191.902
1 standard meter ³ CH ₄ is released during combustion

—————— = 8,567 kcal