Short description of gases
There is a whole range of various gas fuels, or the mixture of hydrocarbon, air and other admixtures. They differ both in their composition and in the energy liberated during the combustion. In the continuation of the text, we will discuss only three characteristic groups of gases related to the methanization project.
The first group of gases, the most widely spread in the world based on their use, is made up of natural gases. In some linguistic areas, they are called "marsh gases", because they are obtained from the earth from gas wells. The largest part in the structure of natural gases (most frequently above 90%) is made up by gas methane (CH4). To a lesser extent, depending on the site, the rest of the structure is made up by higher hydrocarbons, nitrogen, helium, and the practically insignificant share of other elements. Due to a high contribution of methane in the structure of natural gases, there are additional synonyms: natural gas is also called "methane", and the system of gas installation is called "methanisation". Their lower fuel power is 1.) at the temperature of 15°C and the atmospheric pressure (1 atm) about 9,5 kWh/m3 (there are slight differences between them in their fuel power, primarily related to the share of higher carbohydrates in the structure of gas). Their relative density is 0,55 (air = 1), which means they are lighter than air. At the atmospheric pressure, they are condensed at the temperature of -162°C, and their critical temperature is -83°C (the critical temperature is the temperature above which gas does not pass into a fluid state under no kind of pressure). This characteristic of natural gases is very important because of transportation. For this reason, the largest share of work consumption of natural gases is transported in pipelines in gassy state. Only a smaller part of world consumption of natural gas produced on exploitation fields of overseas countries, where it is technically almost impossible to build pipelines, is transported by ships in condensed state. Ships transporting natural gas in condensed state have especially made containers that can last even the lowest temperatures (-162°C), and they are popularly called "methaniers". 1 cubic metre of condensed natural gas yields 600 cubic metres of evaporated natural gas.
The second group of gases are oil gases. These gases also have a wide range of use, especially in areas where there is no constructed infrastructure, or a distribution pipeline network for the users' supply with natural gas. Their use is frequent in households where they are used either through bottles (10 or 35 kg) or through installed outdoor tanks. To a great extent, they consist of the gases of propane (C3H8) and butane (C4H10), which explains their synonym propane-butane. They are obtained in refineries from the processed raw oil, and from the separation from the natural gas excerpted from wells. Their fuel power mainly depends on shares of propane and butane, and the condensed oil gas distributed by the INA company has the declared lower fuel power of 12,8 kWh/kg and the relative density in evaporated state of 1,88, which means it is heavier than air. As opposed to natural gases, oil gases are condensed already at pressures of 1,7 to 7,5 atm at the temperature of 15°C, which makes them very suitable for storage in containers in condensed state, as well as for the transportation in trucks and wagon cisterns.
The third group of gases mentioned in the project comprises substitute gas. It is nothing but the mixture of evaporated propane-butane and air (about 53% of propane-butane and 47% of air). It is used in territories in which the arrival of natural gas is expected in the near future. The reason is of technical nature, because the transfer from one fuel to the other can only be performed without the stoppage of the supply of users, or without technical changes on the appliances. It is important to point out that substitute gas has more calories than natural gas (its lower fuel power is 15 kWh/cubic metre).
The above described gases belong to the organic chain of saturated carbohydrates which through combustion (connecting the substance and the oxygen) liberate chemically linked energy, and the product of the combustion is mainly carbon dioxide and water as it is represented in the example of the combustion of methane:
1 kg CH4 (methane) + 4 kg O2 (oxygen) = 2,75 kg CO2 (carbon dioxide) + 2,25 kg H2O (water) + 9,5 kWh
Practically, gases contain an insignificant share of harmful substances that can be shown in emissions of discharge gases during the combustion as polluters. The only harmful substance is carbon dioxide (known as "greenhouse gas"), but it is an unavoidable by-product of combustion.
1.) Lower fuel power is the quantity of heat as a result of a total combustion of the quantity of one unit of fuel, and the water steam as a product of the combustion goes out with smoke gases through the chimney as a condensate.