Natural gas mainly consists of methane (> 80%), ethane, propane, butane, nitrogen and carbon dioxide. In the composition of components, the individual natural gas types considerably differ sometimes. For the gas application, however, the individual components are of less importance, since in practice the most important parameters such as the calorific value, methane number and Wobbe index are used. Measuring instruments in the natural gas industry therefore measure these parameters, which ultimately can be derived from the gas composition. IR spectroscopy plays an important role in this case, since this technique allows rapid gas analysis. Although the alternative gas chromatography (GC) has much better selectivity than the IR technique, the response time is much greater. Furthermore, the GC technique is extremely expensive and very complex in terms of equipment and also maintenance-intensive.
The following figure shows the absorption bands of an L-gas mixture.
At high pressure and elevated temperature from incompletely degraded and sedimented marine biomass, natural gas is produced in geological sediments under air closure. In conventional natural gas deposits, the natural gas is trapped in porous reservoir rock under an impermeable overburden, so it can freely flow into a downhole. This conventional type of deposit has so far dominated natural gas production because of its high yield and economic viability. In recent years, however, natural gas from unconventional deposits has also been increasingly promoted (Tight gas, shale gas, coal sewage gas, methane hydrates).
Natural gas has a particularly high attractiveness among fossil primary energy sources because of its flexibility, efficiency and cleanliness. Compared to other fossil primary energy sources, natural gas produces less local pollutant emissions and the lowest CO2 emission per unit of energy, since the mass ratio of hydrogen to carbon atoms in natural gas is relatively high. The high flexibility of natural gas allows a wide field of application.
Measurement tasks in the gas industry
The primary measurement task in the gas industry is energy measurement. In addition to volumetric measurement, gas-sensing measurements play an important role here, and it also performs various secondary measurement tasks for the natural gas industry, gas transport and gas utilization. In the following the essential measurement tasks in the gas industry are explained.
Energy: Gas trading is based on energy billing since the energy content of the gas is the decisive value for the consumer. For example, in Germany the thermal energy bill is regulated in a standard of the German Association of Gas and Water (Deutsche Vereinigung des Gas- und Wasserfachs DVGW). Therefore the energy determination is the basic task for the gas measurement. The energy E is calculated as the product of the gas volume V with the volumetric calorific value Hs. In this process, the volume and calorific value must relate to the same gas state (pressure p, temperature T).
In practice, the volume V is measured under operating conditions (p, T) of the pipeline, whereas the calorific value Hs,0 refers to standard conditions (p0,T0). Here, the index 0 denotes the standard state, p0 = 1013.25 mbar and T0 = 273.15 K. Therefore, for the energy calculation, the operating volume V must first be converted to standard volume V0. Natural gas is not an ideal gas; its real gas behavior is taken into account by the compressibility coefficient K during the conversion:
In addition to the state (p, T), the compressibility coefficient K depends decisively on the gas characteristics, i.e. on the gas composition and its physical parameters. Standardized state equations for the calculation of the compressibility are used in the gas economy, for example according to ISO 12213. As input data, these equations require either the molar gas composition or characteristic gas parameters, such as calorific value, standard density and various gas components. Therefore the determination of the energy requires not only the volume measurement, but also an analysis of the gas structure.
Gas characteristics: Natural gas in pipeline quality consists essentially of hydrocarbons (typ.75 – 100mol%), as well as the inert gases nitrogen (typ. 0 – 20mol%) and carbon dioxide (typ. 0 – 5mol%). The hydrocarbon content consists essentially of methane CH4 as well as the series of alkanes (sum formula CnH2n+2, with order n), representatives are ethane C2H6, propane C3H8, butane C4H10, pentane C5H12, hexane C6H14, etc., as well as its isomers. The concentration of the alkanes typically decreases with increasing order n. Other components such as helium, oxygen, sulfur components and water vapor occur only in traces in transport networks, typically in the concentration range from a few 1ppm to a few 100ppm (ppm= parts per million). By feeding biogas and hydrogen from regenerative energy sources, the new gas components oxygen and hydrogen can occur in significant concentrations. The physical parameters of the gas vary with the gas composition. The gas properties play an important role in the energy billing, on the one hand directly as the calorific value of the gas and on the second hand indirectly as compressibility in the volume determination. In addition, there are numerous other gas parameters which play an important role in gas production, transport, trade and, in particular, in the usage of gas. Due to their importance to the gas industry, the gas characteristics are defined in various national and international standards. In Germany, gases in public supply must comply with the specifications of the DVGW work sheet G260. The DVGW worksheet G262 applies to the injection of gases from regenerative sources. At European level, there is the industry agreement EASEE-gas with concerted business practices for H-gas in cross-border gas trading. Currently, CEN working groups are involved in standardization for H-gas (mandate M400) and biogas (mandate M475). On the international level, there is no standard for the gas structure, but different standards, for the measurement of gas characteristics and for the calculation of gas parameters.
A number of other gas parameters and their significance are presented below:
Wobbe-Index: A relevant parameter of the gas composition is the Wobbe index Ws,0, which is calculated from the calorific value Hs,o divided by the square root of the relative density d (GL. 10.3.3, see also chapter 2.6). The Wobbe index is a measure of the thermal performance of a gas burner, i.e. two different gases with the same Wobbe index deliver the same burner power with the same settings. So, the Wobbe index is significant for the majority of gas applications, especially in heating. The fluctuation bandwidth of the Wobbe index is therefore locally limited in the gas networks to avoid problems with gas usage. As a result of the above-described trends in gas industry, the limitation of the fluctuation band widths will become more difficult in the future and will require increased use of measuring, control and regulation technology for gas conditioning and process control.
Methane number: The fuel-tightness of the fuel is important in the combustion of gas in engines. For high efficiency, high compression is applied to gas engines, which, however, entails the risk of wear-promoting knocking due to early auto-ignition of the fuel. The knock resistance is described by the so-called methane number MZ, which corresponds approximately to the octane number of the gasoline (see also chapter 2.6). The methane number is scaled by the methane content of a binary methane/hydrogen mixture. MZ=0 is the methane number of pure hydrogen and MZ=100 for pure methane, the intermediate values are defined by the methane content of the corresponding binary methane/hydrogen mixture. The methane number of a natural gas mixture is obtained by comparison with the binary methane / hydrogen mixture of the same knock strength. Inert gases, such as nitrogen and carbon dioxide, increase the knock-resistance; hydrocarbons such as ethane, propane, butane, etc., reduce them. Current engine control systems usually operate with knock sensors which detect the occurrence of early ignition and take appropriate measures, such as ignition timing shift, mixture adjustment or power reduction. By means of a fast and continuous measurement of the methane number of the fuel, the engine operation could be proactively optimized before knocking occurs at all.
List of references:
Kaster, J.: Energiemessung und andere Messaufgaben in der Gaswirtschaft. Kap. 15.1 aus Gasmesstechnik in Theorie und Praxis (Wiegleb, G.) Springer-Vieweg Verlag 2016 S. 971-944
Schley, P.: Brennwertverfolgung in Gasnetzen (SmartSim). Kap. 15.4 aus Gasmesstechnik in Theorie und Praxis (Wiegleb, G.) Springer-Vieweg Verlag 2016 S. 1019-1033