Determination of the calorific value and the composition of gases

Reliable determination of quality and calorific value of complex gas mixtures

Natural gas is a natural product and typically contains about 90 percent methane, as well as ethane, propane, butane and other hydrocarbons, plus nitrogen and carbon dioxide. Gas from a broad variety of countries is fed into the German natural gas network, increasingly also via LNG terminals, which regasify liquefied natural gas and feed it into the gas grid. In addition, the volume of gas fed into the grid from renewable sources such as biogas, biomethan and, to an increasing extend, hydrogen – which is generated as a temporary storage for surplus electricity according to the power-to-gas principle – is growing.

 The calorific value varies depending on the composition of the gas. This is decisive for the natural gas quality and also determines the gas price. In addition, the gas composition influences the combustion process. To ensure optimal combustion, the oxygen supply must be adjusted to the gas mixture. For industry and commerce, precise knowledge of the gas composition is crucial for operating combustion processes safely, efficiently, and with low emissions.

IR spectroscopy replaces complex gas chromatography

Therefore, regular measurements at distribution points in the gas network or at industrial consumers with critical processes provide information on the actual calorific value of the natural gas. The current standard method of measurement is gas chromatography (GC). This technique is comparatively expensive, quite slow and complex in operation, for example because carrier and calibration gases have to be stocked. Fraunhofer IPM is therefore focusing on alternative methods. Using infrared spectroscopy, we are developing sensors that can analyze gas mixtures and determine calorific values – without the extensive effort required to operate a process gas chromatograph.

IR spectrometer measures gas concentrations at sub-minute intervals – with very high accuracy

In cooperation with RMA Mess- und Regeltechnik GmbH & Co. KG in Rheinau, Fraunhofer IPM has developed an alternative measuring system for the analysis of complex gas mixtures that operates on the basis of infrared spectroscopy (IR). The spectrometer is able to measure the individual gas concentrations at sub-minute intervals and requires little maintenance as no calibration or carrier gases are needed. Hydrocarbons up to C₆, CO₂ and N₂ are quantitatively determined with very high accuracy – which is a challenge, since the different natural gas components appear in a very broad concentration dynamic: Methane has a share of about 90 percent, ethane and propane are in the 1 percent range - while the higher hydrocarbons such as pentanes are found at concentrations below 0.1 %. All components must be recorded with an accuracy down to the sub-per-mille range – at least for billing-relevant applications.