Spontaneous ignition of pressurized releases of hydrogen and natural gas into air

This paper demonstrates the "spontaneous ignition'' (autoignition/inflammation and sustained diffusive combustion) from sudden compressed hydrogen releases that is not well documented in the present literature, for which little fundamental explanation, discussion or research foundation exists, and which is apparently not encompassed in recent formulations of safety codes and standards for piping, storage, and use of high pressure compressed gas systems handling hydrogen. Accidental or intended, rapid failure of a pressure boundary separating sufficiently compressed hydrogen from air can result in multi-dimensional transient flows involving shock formation, reflection, and interactions such that reactant mixtures are rapidly formed and achieve chemical ignition, inflammation, and transition to turbulent jet diffusive combustion, fed by the continuing discharge of hydrogen. Both experiments and simple transient shock theory along with chemical kinetic ignition calculations are used to support interpretation of observations and qualitatively identify controlling gas properties and geometrical parameters. Although the phenomenon is demonstrated for pressurized hydrogen burst disk failures with different internal flow geometries, similar phenomena apparently do not necessarily occur for sudden boundary failures of storage vessel or transmission piping into open air that have no downstream obstruction. However, subsequent reflection of the resulting transient shock from surrounding surfaces through mixing layers of hydrogen and air may have the potential for producing ignition and continuing combustion. Much more experimental and computational work is required to quantitatively determine the envelope of parameter combinations that mitigate or enhance spontaneous ignition characteristics of compressed hydrogen as a result of sudden release, particularly if hydrogen is to become a major energy carrier interfaced with consumer use. Similar considerations for compressed methane, for mixtures of light hydrocarbons and methane (simulating natural gas), and for larger carbon number hydrocarbons show similar autoignition phenomena may occur with highly compressed methane or natural gas, but are unlikely with higher carbon number cases, unless the compressed source and/or surrounding air is sufficiently pre-heated above ambient temperature. Spontaneous ignition of compressed hydrocarbon gases is also generally less likely, given the much lower turbulent blow-off velocity of hydrocarbons in comparison to that for hydrogen.