Determining the minimum ignition energy of toluene vapor containing hydrogen towards a risk assessment for liquid organic hydride storage in hydrogen refueling stations

Abstract

Hydrogen refueling stations associated with on-site hydrogen production systems based on the organic chemical hydride method (HRS-OCH) will process both hydrogen (H2) and toluene generated by the dehydrogenation of methylcyclohexane. In such systems, toluene containing varying levels of dissolved H2 will be stored in on-site tanks, such that H2 could be released into the tank headspace to form flammable toluene/H2/air mixtures. As a means of examining the associated safety hazards, the present work experimentally determined the minimum ignition energy (MIE) values for toluene/H2/air mixtures with varying proportions, using a cylindrical explosion vessel. The MIE for toluene containing 1 vol% H2 was found to be 0.319 mJ (the ignition probability is greater than 3.3 %), and thus similar to the values for pure toluene/air mixtures. H2 proportions above 2 vol% decreased the MIE by an order of magnitude, such that the value approached that for pure H2/air mixtures (0.021 mJ). This low MIE would allow these mixtures to be readily ignited by a static electrical discharge from toluene after accumulating a charge through flow processes. To reduce the ignition risk to an acceptable level, the concentration of H2 in the gas phase of HRS-OCH storage tanks should therefore be less than 1 vol%. Consequently, safe operation of HRS-OCH facilities requires the installation of H2 stripping processes, such as the use of membrane separation units.