The quality of natural gas is subject to strong fluctuations. Not only different natural gas deposits are responsible for this, but increasingly also the feed-in of hydrogen from power-to-gas-plants. For fuel gas analysis, Fraunhofer IPM has developed an optical measuring system, which now recognizes the hydrogen content in the gas in addition to the hydrocarbons contained.
The spectrometer for the infrared active components of the natural gas and the thermal conductivity sensor for hydrogen measurement are installed on a natural gas bypass. (Source: Fh.-IPM)
With the composition of the gas, the calorific value also fluctuates. For consumers, the gas composition is an important factor, because the price of the gas is determined on the basis of the volume supplied and the calorific value. In industry, there is another aspect: the gas composition influences the combustion process. If, for example, the proportion of hydrogen in the gas mixture increases, the calorific value decreases. If the gas flow is not adjusted, for example in an oven, its temperature decreases. Such fluctuations can lead to quality problems, for example in the case of glass melting. Today, natural gas quality is usually determined using gas chromatographs at distribution points in the gas network or directly at large consumers.
The new fuel gas measuring system EcoSpectro, which Fraunhofer IPM developed on behalf of the company RMA Mess- und Regeltechnik, determines the main components of natural gas spectroscopically: An infrared spectrometer detects the percentages of methane, ethane, propane, butane and longer hydrocarbon chains. Compared to gas chromatography, spectroscopy has several advantages: Gas chromatographs are expensive, comparatively slow and laborious to operate. The IR-spectrometer, on the other hand, measures at subminute intervals, works without recalibration over a period of several months and does not require rinsing gases. The spectra are analyzed using chemometric methods. In this way, gas concentrations of more than 70 percent can be determined down to the 100 ppm range.
For the detection of regeneratively produced gases, the IR spectrometer was equipped with an additional sensor for the determination of the hydrogen content as part of the project “Gas Efficiency”. Since hydrogen cannot be detected with infrared light due to its molecular structure, the spectrometer was supplemented by a thermal conductivity sensor. The thermal conductivity of hydrogen is one factor seven greater than the thermal conductivity of all other fuel gases including air, so the sensor is very sensitive to hydrogen admixtures. The modular sensor system was tested in the Fraunhofer IPM gas laboratory and then in a field test on a natural gas bypass of the Fraunhofer Institute for Solar Energy Systems ISE. There, IR spectrometers and thermal conductivity sensors carried out a total of about 125,000 measurements. Over a period of several months, excluding hydrogen feed, the results were very close to the daily calorific value provided by the natural gas supplier.
However, the measured values revealed fluctuations in gas quality in the minute range. Consumers who need consistent gas quality could thus use the measuring system for fast process control. At the time of hydrogen feed, the modified hydrogen content could be determined with accuracy in the 100 ppm range. The system is therefore particularly suitable as a fast-reacting measuring system forpower-to-gassystems or consumers behind a hydrogen feed-in point. (Source: Fh.-IPM)
Links: Project “Gas Efficiency – Fast Measurement Technology for the Efficient Use of Renewably Generated Gases”, Fraunhofer Institute for Physical Measurement Techniques IPM, Freiburg, Germany • RMA Mess- und Regeltechnik GmbH & Co KG, Rheinau, Germany
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