Natural gases are mixtures of saturated hydrocarbons with different boiling points. The composition ranges from methane as the main component up to pentanes. In addition, natural gases often contain higher unsaturated hydrocarbon compounds with a highly complex phase behaviour. To avoid an explicit definition of these compounds, they are generically summed up under the term C6+.
Beside these hydrocarbon compounds, nitrogen and carbon dioxide are typical impurities. Hydrogen sulphide and carbonyl sulphide as well as mercaptans are further components that are frequently found in different concentrations. Natural gases containing hydrogen sulphide are generally called sour gases, gases without this component are defined as sweet gases. In addition, natural gas resources that contain mercury in varying concentrations exist worldwide.
In the reservoir, natural gas is found with chloride-containing water, the so-called reservoir water. Therefore, when natural gas is extracted, it is saturated with water corresponding to the partial pressure.
Natural gas specification
Natural gas injected into the pipeline network of the public gas supply is standardized. In Germany, this standardization ist set forth by the guidelines of the DVGW*1-regulations. In this, combustion characteristics and the guide values for the impurities are defined. In the near future, the specifications of the European Richtlinie EASEE-gas may apply.
The defined dew points of water and of the higher hydrocarbons are one of the criterions. At the usual injection pressure of approx. 70 bar(a), the H2O-dew point must be -8°C or lower, and in the pressure region from 1 to 70 bar(a) the hydrocarbons –dew point must be -2°C at least.
However, guide values for mercury in natural gas are neither taken into account in the DVGW-set of rules nor in the specification. According to a WEG*2-agreement, the mercury concentration may not exceed 28 μg/m3(Vn).
The adjustment of natural gas compositions, which widely vary, primarily three industrial processes have proven themselves:
The absorption process with triethylene glycol as a drying agent for the adjustment of the H2O-dew point.
The adsorption process, for the simultaneous adjustment of the H2O- and the hydrocarbons-dew point. Zeolites (molecular sieves) or silica gel are used as basis for the adsorbent
The condensation process is the process that competes with the adsorption process. In the LTS- (low temperature separation) or the CFP-design (cold fractionation plant), mercury is simultaneously extracted.
*1 Deutsche Vereinigung des Gas- und Wasserfaches e.V.
*2 Wirtschaftsverband Erdöl-/Erdgasgewinnung e.V.