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 degrees 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 degrees C. In this initial temperature range the limits were not affected by the length of the residence time. However, at initial temperatures exceeding 200 degrees 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. (C) 1998 International Association for Hydrogen Energy.