Combined gases are subdivided into two groups: natural and artificial, and natural gases are divided into three groups:

• Gases extracted from pure gas fields;
• Gas mixtures of methane with petroleum and ethane, propane and butane fractions;
• Gases extracted from gas-condensate deposits.

In nature, pure gases, without impurities, almost do not meet. Impurities in natural gas are most often: hydrogen sulphide, nitrogen, carbon dioxide, water vapor.

Gases extracted from pure gas fields consist mainly of methane.

Natural gases have formed as a result of the decomposition of organic residues of plant and animal origin.

Natural gas, which is used in Bulgaria, is imported from Russia and is mainly extracted from the Tyumenko field in Russia. Since August 1992, the locality near the village of Aglen, Lovech district is included in the main gas pipeline. In 2004, is included the deposit at cape Galata, Varna district. Besides these, on the territory of Bulgaria there are small deposits of local importance - Balgarevo village, Devetaki village, Bardarski Geran village.

Gaseous fuel combustion is a process of quickly combining its combustible elements with oxygen. This process proceeds in certain quantitative ratios, which are expressed by the equations for the oxidation reaction.

Burning can begin if the air mixture is heated to a specific temperature called a flash point.

The starting burning process of the gas mixture is maintained independently if the amount of heat released by the combustion is sufficient to heat the new fuel to a flashpoint.


Methane 98.48 %
Ethane 0.28 %
Nitrogen 1.11 %
Propane 0.11 %
Carbon dioxide 0.02 %
Sulfur 25 mg/nm3
Density 0.677 kg/nm3
Calorific value 7 980 ccal/nm3

As you can see, the main component of natural gas is methane. That is why the basic properties of natural gas are almost the same as methane; no color, fiery and extremely explosive. It is lighter than air for this reason, if there is a gap in an enclosed space, it accumulates in the up.

If the gas in the mixture is insufficient or the supply air is too much, the heat released by the combustion is not reached and the combustion is stopped.

The smallest and largest volumes of gas content in the gas mixture, within which a flash or explosion may occur, are called the lower and upper limits of explosion.

Burning temperature
Explosive limits, volumetric% explosive mixture
365° C
1.50 %
8.50 %
510° C
4.00 %
7.50 %
595° C
5.00 %
15.00 %
470° C
2.10 %
9.30 %

In the earth's womb in the composition of gases, there is sometimes hydrogen sulphide - a substance without color and with a sharp, suffocating smell. Hydrogen sulphide is an active substance with a strong corrosion effect. Its removal from the natural gas composition takes place in the purification plants.

Before passing through the gas pipelines, natural gas is also subjected to drying to remove the accompanying quantities of water. Drying takes place in installations where the gas is treated with diethylene glycol.

Since natural gas is colorless and odorless in order to detect its leakage, it is odorized before it is fed into the distribution networks. After purification and drying, 16 g of ethyl mercaptan per 1000 m3 of gas are added.