Natural gas
What natural gas from all sources has in common is that it is composed of methane, usually 75…95%. The rest of the mix typically consists of higher alkanes, like ethane and propane and fractions of nitrogen and carbon dioxide. The exact composition depends on the source of the gas, so when a grid is supplied with a variety of gases the composition will change. Furthermore, other recent trends contribute to the fluctuations in composition.
Trends in gas compositions
Natural gas is very suitable to facilitate in the increasing use of renewable energies.
Biogas produced from renewable energy sources in biogas plants can, after proper treatment , be fed into the grid. However, biogas composition will depend on the feedstock, which is not always constant in time.
Another important trend is power to gas or P2G; here electricity, produced from renewable sources like solar or wind, is used to produce a gas as an energy carrier. This can be hydrogen produced with electrolysis, or synthetic methane by combining carbon dioxide and hydrogen produced from electrolysis.
A major factor in renewable energy is the mismatch between supply and demand. As you can imagine solar energy is only being produced during daytime. Transferring electrical energy into chemical energy by producing combustible gases and feeding this in the national grid can help to balance this mismatch by utilizing the large buffer capacity of the available gas networks. Recent research, by for instance Kiwa, has shown that the current gas grid in the Netherlands can handle several tens of percent of hydrogen with limited modifications.
All these factors are leading to increasing changes to the gas composition in the network. Composition and quality are strongly correlated; increasing amounts of inert gases, like nitrogen or carbon dioxide, reduce the amount of energy produced when burned, also known as the calorific value.
The presence of hydrogen in natural gas can change flame characteristics, such as temperature and flame speed.
Measuring the composition
With changing chemical compositions it becomes increasingly important to measure calorific value and components. With only a single point of entry, one measurement sufficed to analyse the composition in the downstream network. In the present day grid, networks are more intertwined and have multiple points where gases are blended. At every point of entry, it is necessary to measure the composition, not only for quality control but also for fiscal purposes. In this way, the suppliers can make sure consumers receive the quality they need and are charged for the heating value of the gas rather than the volume they receive.
The current standard for determining gas quality is gas chromatography; this method is very accurate but also slow and expensive. Alternative methods like calorimetry are similarly expensive and have a large footprint, making it hard to implement in small-scale applications.
All these future trends lead to a need for measurement technology that can be used in-line and in small-scale applications. This requires sensors that are compact, cost-effective and preferably measure composition.