Calculation of heat of combustion of gas. Natural gas and its calorific value for domestic use

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

natural gas fuel

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

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

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

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

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

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

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

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

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

Q n s \u003d 0.01,

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

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

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

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

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

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

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

Table 1. Characteristics of gaseous fuel

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

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

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

Substances of organic origin include fuel, which, when burned, releases a certain amount of thermal energy. Heat generation should be characterized by high efficiency and the absence of side effects, in particular, substances harmful to human health and the environment.

For ease of loading into the furnace, wood material is cut into individual elements up to 30 cm long. To increase the efficiency of their use, firewood should be as dry as possible, and the combustion process should be relatively slow. In many respects, firewood from such hardwoods as oak and birch, hazel and ash, hawthorn is suitable for space heating. Due to the high resin content, increased burning rate and low calorific value, conifers are significantly inferior in this regard.

It should be understood that the density of wood affects the value of the calorific value.

It is a natural material of plant origin, extracted from sedimentary rock.

This type of solid fuel contains carbon and other chemical elements. There is a division of material into types depending on its age. Brown coal is considered the youngest, followed by hard coal, and anthracite is the oldest of all other types. The age of the combustible substance also determines its moisture content, which is more present in the young material.

During the combustion of coal, the environment is polluted, and slag is formed on the grate of the boiler, which, to a certain extent, creates an obstacle to normal combustion. The presence of sulfur in the material is also an unfavorable factor for the atmosphere, since this element is converted into sulfuric acid in the air space.

However, consumers should not be afraid for their health. Manufacturers of this material, taking care of private customers, seek to reduce the sulfur content in it. The calorific value of coal can differ even within the same type. The difference depends on the characteristics of the subspecies and the content of minerals in it, as well as the geography of production. As a solid fuel, not only pure coal is found, but also low-enriched coal slag pressed into briquettes.

Pellets (fuel pellets) is a solid fuel created industrially from wood and plant waste: shavings, bark, cardboard, straw.

The raw material crushed to the state of dust is dried and poured into the granulator, from where it already comes out in the form of granules of a certain shape. To add viscosity to the mass, a vegetable polymer, lignin, is used. The complexity of the production process and high demand form the cost of pellets. The material is used in specially equipped boilers.

The types of fuel are determined depending on what material they are processed from:

• round timber of trees of any species;
• straw;
• peat;
• sunflower husk.

Among the advantages that fuel pellets have, it is worth noting the following qualities:

• environmental friendliness;
• inability to deform and resistance to fungus;
• ease of storage even outdoors;
• uniformity and duration of burning;
• relatively low cost;
• the possibility of using for various heating devices;
• suitable pellet size for automatic loading into a specially equipped boiler.

Briquettes

Briquettes are called solid fuel, in many respects similar to pellets. For their manufacture, identical materials are used: wood chips, shavings, peat, husks and straw. During the production process, the raw material is crushed and formed into briquettes by compression. This material also belongs to environmentally friendly fuel. It is convenient to store it even outdoors. Smooth, uniform and slow burning of this fuel can be observed both in fireplaces and stoves, and in heating boilers.

The varieties of environmentally friendly solid fuels discussed above are a good alternative to generating heat. Compared to fossil sources of thermal energy, which adversely affect the environment during combustion and are, moreover, non-renewable, alternative fuels have clear advantages and relatively low cost, which is important for certain categories of consumers.

At the same time, the fire hazard of such fuels is much higher. Therefore, some precautions must be taken regarding their storage and the use of fire resistant wall materials.

Liquid and gaseous fuels

As for liquid and gaseous combustible substances, the situation is as follows.

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

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

Feature of fossil fuel

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

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

An important component of the substance is methane (approximately 88-96%), the rest is other chemicals:

• butane;
• hydrogen sulfide;
• propane;
• nitrogen;
• oxygen.

In this video, we will consider the role of coal:

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

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

Deposit types

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

• gas;
• oil.

Their distinguishing feature is the hydrocarbon content. Gas deposits contain approximately 85-90% of the presented substance, oil fields contain no more than 50%. The remaining percentages are occupied by substances such as butane, propane and oil.

A huge disadvantage of oil generation is its flushing from various kinds of additives. Sulfur as an impurity is exploited at technical enterprises.

Natural gas consumption

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

Fossil fuel is used in everyday life:

• columns;
• gas stove;

This kind of fuel is considered the most budgetary and harmless, the only drawback is the emission of carbon dioxide during combustion into the atmosphere. Scientists all over the planet are looking for a replacement for thermal energy.

Calorific value

The calorific value of natural gas is the amount of heat generated with sufficient burnout of a unit of fuel. The amount of heat released during combustion is referred to one cubic meter, taken under natural conditions.

The thermal capacity of natural gas is measured in the following terms:

• kcal / nm 3;
• kcal / m 3.

There is a high and low calorific value:

1. High. Considers the heat of water vapor that occurs during the combustion of fuel.
2. Low. It does not take into account the heat contained in water vapor, since such vapors do not lend themselves to condensation, but leave with combustion products. Due to the accumulation of water vapor, it forms an amount of heat equal to 540 kcal / kg. In addition, when the condensate cools, heat from 80 to one hundred kcal / kg is released. In general, due to the accumulation of water vapor, more than 600 kcal / kg are formed, this is the distinguishing feature between high and low heat output.

For the vast majority of gases consumed in an urban fuel distribution system, the difference equates to 10%. In order to provide cities with gas, its calorific value must be more than 3500 kcal/Nm 3 . This is explained by the fact that the supply is carried out through the pipeline over long distances. If the calorific value is low, then its supply increases.

If the calorific value of natural gas is less than 3500 kcal / Nm 3, it is more often used in industry. It does not need to be transported for long distances, and it becomes much easier to carry out combustion. Serious changes in the calorific value of the gas 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 the diameter of the gas pipeline, as well as an increase in the cost of metal, laying networks and operation. The big disadvantage of low-calorie fossil fuels is the huge content of carbon monoxide, in connection with this, the level of danger increases during the operation of the fuel and during the maintenance of the pipeline, in turn, as well as equipment.

The heat released during combustion, not exceeding 3500 kcal/nm 3 , is most often used in industrial production, where it is not necessary to transfer it over a long distance and easily form combustion.

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 caloric 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 the water vapor does not condense, but is carried away with the combustion products.

The concepts of 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 consequence, 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 in a large number of unified burners for household appliances, which is associated with significant difficulties.

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