In testing hydrogen-fueled engines, the purity of the hydrogen fuel is important. Hydrogen may become contaminated with nitrogen (N2), argon (Ar), or oxygen (O2), thereby making the hydrogen unusable for engine testing at Stennis Space Center (SSC). Therefore, for rocket engine testing, the quality of hydrogen from the fuel tank or feed line is tested before use. If there are contaminants found within the hydrogen, it is mandatory for the tank to be emptied; this results in testing delay. Therefore, NASA has specific interest in measuring concentrations of N2, Ar, and O2 in hydrogen gas. The approach currently used for testing the purity of hydrogen is conducted by collecting a gas sample from the hydrogen supply line or storage tank and then sending it to an analytical laboratory for evaluation. This procedure is time consuming and can, if the sampling is not carefully conducted, inadvertently introduce contamination, which would provide a misguided reading for the entire tank. Therefore, establishing a non-intrusive, on-line, near-real-time monitor that has a simultaneous multi-species impurity detection capability for hydrogen would eliminate these issues.

Therefore, a diagnostic technique for verifying hydrogen quality for engine testing and providing timely results was developed. An analytical system based on LIBS (Laser Induced Breakdown Spectroscopy) was developed by Mississippi State University for SSC to measure the concentrations of multiple impurity species in hydrogen. LIBS is a laser-based diagnostic technique used for measuring the concentration of various elements in a test media. A high-energy, pulsed laser beam is focused on the test medium. The intense laser energy heats the material in the focal volume and creates a small region of incandescent plasma. The light emitted by this plasma is collected and spectroscopically analyzed. LIBS provides a spatially and temporally resolved measurement. Gated detection with an intensified charge-coupled-device (CCD) detector discriminates against background emission and improves the detection limit. With properly selected atomic lines, several species can be simultaneously monitored in a spectral region. In the high-temperature plasma produced by the laser discharge, any molecular species that were originally present will dissociate, so the sample is in elemental form. The plasma consists of excited neutral atoms, ions, and electrons. The intensity of the emission lines in the spectrum is analyzed to deduce the total elemental concentrations in the sample.

This analytical technique is capable of measuring various impurities (molecular and atomic) found within hydrogen tanks. Additionally, the technique has the ability to provide good sensitivity to accurately measure simultaneous concentration of multiple species and is capable of detecting low levels of impurities in the hydrogen tanks and feed lines. The developed system is compact, sturdy, user-friendly, delivers results in near real time, and is easy to implement in the field. This makes the system highly adaptable and applicable to many vendors who require the use of pure hydrogen. The integrated system could provide NASA with a timely evaluation process for determining the quality of hydrogen fuel before rocket engine testing. Additionally, a sensor system that uses LIBS could be implemented to monitor the gas composition for other applications, like in large manufacturing plants in order to provide supplemental data, which could optimize the efficiency of the processing plant and control the processes involved in the manufacturing process. This technology could also be modified for other applications such as toxic continuous emission monitoring (CEM) and for quality control in pharmaceutical, chemical, and food processing industries.

This work was done by Jagdish Singh and Perry Norton of Mississippi State and Mississippi Ethanol LLC, for Stennis Space Center. For more information, contact Perry Norton (662) 324-7852, or Mississippi Ethanol LLC, 205 Research Blvd, Winona, MS 38967. Refer to SSC-00397.

NASA Tech Briefs Magazine

This article first appeared in the November, 2015 issue of NASA Tech Briefs Magazine.

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