Flammability Limits Of Hydrogen Research Articles

CMOS-compatible plasmonic hydrogen sensors with a detection limit of 40 ppm

Sensing of leakage at an early stage is crucial for the safe utilization of hydrogen. Optical hydrogen sensors eliminate the potential hazard of ignition caused by electrical sparks but achieve a detection limit far higher than their electrical counterparts so far. To essentially improve the performance of optical hydrogen sensors in terms of detection limit, we demonstrate in this work a plasmonic hydrogen sensor based on aluminum-palladium (Al-Pd) hybrid nanorods. Arranged into high-density regular arrays, the hybrid nanorods are capable of sensing hydrogen at a concentration down to 40 ppm, i.e., one thousandth of the lower flammability limit of hydrogen in air. Different sensing behaviors are found for two sensor configurations, where Pd-Al nanorods provide larger spectral shift and Al-Pd ones exhibit shorter response time. In addition, the plasmonic hydrogen sensors here utilize exclusively CMOS-compatible materials, holding the potential for real-world, large-scale applications.

Critical concentration of helium making hydrogen–oxygen with various fractions nonflammable

Abstract The objective of this study is to determine the critical concentrations of helium making hydrogen–oxygen nonflammable under various volume fraction ratios of hydrogen and oxygen. Tests were conducted for three volume fraction ratios of hydrogen and oxygen 2:1, 1:1 and 1:2, at the initial pressures of 0.1, 0.2, 0.3 and 0.4 MPa and the initial temperatures of 21, 40, 60, 75 and 90 °C. The flammability limits of mixtures with the lower volume fraction ratios between hydrogen and oxygen are much different from the lower flammability limits of hydrogen in oxygen or air. Compared with the lower flammability limits of hydrogen in oxygen or air, the critical concentrations of helium determined in this study are significant for process safety of the mixtures with various volume fraction ratios between hydrogen and oxygen. For the mixtures at 21 °C, the critical concentrations of helium making hydrogen–oxygen nonflammable increase with the volume fraction ratios of hydrogen and oxygen in the range from 0.5 to 2. The variation of the critical concentrations of helium with the initial pressure depends on the initial temperature. For the mixtures at the initial temperature of 60 °C and the initial pressure of 0.3 MPa, the critical concentration of helium reaches to its limit of 95% in the case of volume fraction ratio of hydrogen and oxygen 2:1. The limit values of the critical concentration of helium are 93% for 1:1 volume fraction ratio of hydrogen and oxygen and 89% for 1:2 volume fraction ratio of hydrogen and oxygen ratio, respectively.

Effects of temperature and time of exposure on the flammability limits of HYDROGEN–AIR mixtures

The flammability limits of hydrogen in air were determined for upward flame propagation at elevated temperatures up to 350°C and atmospheric pressure in a conventional stainless steel (316) test tube apparatus. The effect of the extent to which the residence time, i.e. time interval between the time of acquiring the desirable temperature by the hydrogen–air mixture and initiation of spark ignition, and, consequently, the existence of preignition reactions that may influence the value of the flammability limits was also investigated. The flammability limits of hydrogen in air were widened with an increase in the initial temperature up to 200°C. In this initial temperature range the limits were not affected by the length of the residence time. However, at initial temperatures exceeding 200°C the flammability limits, especially, the rich limits narrowed with an increase in the temperature and were very significantly affected by the length of the residence time before spark ignition. This means that the time of exposure of the mixture to elevated temperatures before initiation of ignition should be taken into consideration when determining the flammability limits and should be reported together with the obtained values of the limits. It was suggested that the substantial drop in the value of the rich limit and increase in the value of the lean limit with the increase in the residence time was caused by the relatively low temperature catalytic reactions on the stainless steel surface of the flame tube. A simple method for calculating the hydrogen conversion catalytically to water was proposed. The results of calculations are in fair agreement with the experimental evidence.