Hydrogen and C2–C6 Alkane Sensing in Complex Fuel Gas Mixtures with Fiber-Enhanced Raman Spectroscopy

Abstract

Power-to-gas is a heavily discussed option to store surplus electricity from renewable sources. Part of the generated hydrogen could be fed into the gas grid and lead to fluctuations in the composition of the fuel gas. Consequently, both operators of transmission networks and end users would need to frequently monitor the gas to ensure safety as well as optimal and stable operation. Currently, gas chromatography-based analysis methods are the state of the art. However, these methods have several downsides for time-resolved and distributed application and Raman gas spectroscopy is favorable for future point-of-use monitoring. Here, we demonstrate that fiber-enhanced Raman gas spectroscopy (FERS) enables the simultaneous detection of all relevant gases, from major (methane, CH 4 ; hydrogen, H 2 ) to minor (C2−C6 alkanes) fuel gas components. The characteristic peaks of H 2 (585 cm −1 ), CH 4 (2917 cm −1 ), isopentane (765 cm −1 ), i-butane (798 cm −1 ), n-butane (830 cm −1 ), n-pentane (840 cm −1 ), propane (869 cm −1 ), ethane (993 cm −1 ), and nhexane (1038 cm −1 ) are well resolved in the broadband spectra acquired with a compact spectrometer. The fiber enhancement achieved in a hollow-core antiresonant fiber enables highly sensitive measurements with limits of detection between 90 and 180 ppm for different hydrocarbons. Both methane and hydrogen were quantified with high accuracy with average relative errors of 1.1% for CH 4 and 1.5% for H 2 over a wide concentration range. These results show that FERS is ideally suited for comprehensive fuel gas analysis in a future, where regenerative sources lead to fluctuations in the composition of gas.

Analysis of CH4, C2H6, C2H4, C2H2, H2, CO, and H2S by forward Raman scattering with a hollow‐core anti‐resonant fiber

“…The detection capability of the Raman system was validated by the detection experiment of gas samples with critical concentrations. Experimental results showed that this system can be used in fields of trace gas detection in ppm level; this better LODs compared with the recent results [18,24] on fuel gas mixtures (hydrocarbons, air constituents, as well as H 2 S) from HC-ARFs might be the contribution of collection of forward Raman scattering and the digital spatial filtering of an imaging spectrograph. The spectral resolution and detection range of our homemade imaging spectrograph allowed the simultaneous detection of multiple gases.…”

“…At present, several signal-enhancing devices have been proved to be effective: passively locked cavities, [1][2][3][4] where the work of Ohara et al showed a limit of detection (LOD) of 100 ppm for methane with 1-s exposure time and 40-W intra-cavity power, [2] and the work of Frosch et al showed a signal enhancement of 6 orders of magnitude [3] as well as LODs between 150 and 350 ppm in the process monitoring of biogas production [4] ; actively locked cavities, [5][6][7] where Hippler's work arrived a LOD of 1 mbar in 1 bar of total pressure (1000 ppm) with a diode laser of 10 mW and integration time of 30 s [5] ; multipass cells, [8][9][10] where the work of Velez et al recorded a LOD of methane below 1 ppm with a multi-mode diode laser of 6 W and exposure time of 5 s [10] ; functionalized waveguides, where the work of Holmstrom et al showed an enhancement over 9 orders of magnitude with a strong dependence on analytes [11] ; and hollowcore fibers (HCFs). [12][13][14][15][16][17][18][19][20][21][22][23][24][25] Among these, HCF is an excellent component, which can be used not only as gas sample cells with long optical path but also as effective collectors of Raman radiation.…”

“…Along with the development of microstructure fiber technology, many types of HCFs had been applied for fiber-enhanced Raman spectroscopy (FERS) of gaseous samples, such as metal-coated capillaries (MCCs), [13,14] hollow-core photonic bandgap fibers (HC-PBFs), [15][16][17][18][19] kagome HCFs, [20,21] and hollow-core anti-resonant fibers (HC-ARFs). [22][23][24][25] The advantage of MCCs is that the gas exchange rate is fast because of the large core diameter, but its enhancement of Raman scattering is not as good as other types of fiber. [12] In the previous studies, the best LOD of FERS system based on HC-PBF was reported as 0.2 ppm for methane (CH 4 ) with the excitation laser power of 2 W and the gas pressure of 20 bar, [15] and the best LOD of FERS system based on HC-ARF was reported as 3 ppm for CH 4 , 49 ppm for hydrogen (H 2 ) with the excitation laser power of 1.3 W and the gas pressure of 4 bar.…”

Comment on Hydrogen and C2–C6 Alkane Sensing in Complex Fuel Gas Mixtures with Fiber-Enhanced Raman Spectroscopy

“…Calculations of Raman spectra of polyatomic molecules are rather complicated and always require verification with experimental data. According to the data of Knebl et al, 12 among the given characteristic peaks of ethane (993 cm −1 ), propane (869 cm −1 ), n-butane (830 cm −1 ), isobutane (798 cm −1 ), n-pentane (840 cm −1 ), isopentane (765 cm −1 ), and nhexane (1038 cm −1 ), only the peaks of n-butane and n-pentane are overlapped. Based on this, it can be concluded that the concentrations can be estimated by using the peak or integral intensities of the indicated bands without taking into account the overlap with the spectra of other components.…”

“…In addition, these tables contain data for methane, carbon dioxide, and nitrogen (at 1 atm). Note that the peak at 1038 cm −1 chosen by Knebl et al 12 for the estimates of n-hexane concentration is not the best choice. According to Figure 1, the n-hexane peak is located at 1040 cm −1 and it is overlapped by more intense bands of n-pentane and isopentane with a maxima at 1038 cm −1 as well as the rotational S 3 line of hydrogen (1035 cm −1 ).…”