Highlights•Key problems and challenges regarding safety of hydrogen utilization are reviewed.•A total of 120 hydrogen incidents are examined.•Subsonic and under-expanded jet formation, and their modeling methods, are studied.•Several hydrogen self-ignition hypothetical theories are analyzed and summarized.•Deflagration-to-detonation transition and its interfering factor are described.AbstractThe development and application of hydrogen energy in power generation, automobiles, and energy storage industries are expected to effectively solve the problems of energy waste and pollution. However, because of the inherent characteristics of hydrogen, it is difficult to maintain high safety during production, transportation, storage, and utilization. Therefore, to ensure the safe and reliable utilization of hydrogen, its characteristics relevant to leakage and diffusion, ignition, and explosion must be analyzed. Through an analysis of literature, in combination with our practical survey analysis, this paper reviews the key issues concerning hydrogen safety, including hydrogen incident investigation, hydrogen leakage and diffusion, hydrogen ignition, and explosion. Access through your organization Check access to the full text by signing in through your organization. IntroductionHydrogen (H2) energy has been receiving increasing attention in recent years. The application of hydrogen energy combined with fuel cells in power generation, automobiles, and other industries will effectively solve the problems of traffic energy and pollution [[1], [2], [3]]. However, it is difficult to maintain safety in production, storage, transportation, and utilization because of the inherent characteristics of hydrogen [[4], [5], [6], [7]]. Hydrogen is characterized by low molar mass (which indicates low gas density), lack of odor and color (which increases difficulty of detection), and possibility of flaming (when air is mixed with a hydrogen volume fraction of only 4%). Hydrogen must be manipulated in safe and reliable circumstances that will restrict equipment contact with H2 and satisfy certain conditions to avoid leaks, ignition, and possible explosion [8,9]. A comparison of the physical properties of hydrogen and natural gas is presented in Table 1. The minimum ignition energy, ignition limit range, and energy density of hydrogen indicate that H2 can be ignited easily and will release a large amount of thermal energy. In addition, compared to natural gas, hydrogen is characterized by lower ignition temperature and faster burning velocity, which indicate that spontaneous ignition may occur if hydrogen encounters sparks or hot surfaces [10]. Therefore, ensuring safety is the highest priority for hydrogen utilization.H2 is the substance with the smallest relative molecular mass, and thus, compared to natural gas, it more easily leaks or permeates from high-pressure environments. When leakage occurs, three jet-flow statuses are possible: subsonic jet flow [11], critical jet flow, and supersonic jet flow [12,13]. Leaking H2 proceeds in the direction of the leak under the influence of the initial momentum and then floats upward by aerostatic buoyancy. In a confined space, a mixture of hydrogen and air converges at the top of the available volume. If a heat source exists, and the volume fraction is appropriate, hydrogen ignition will occur. However, the mixture gas will not ignite if it spreads into an unrestricted region where the volume fraction of the hydrogen is lower than 4%.Hydrogen has the widest flammable range, fastest flame propagation speed, and lowest ignition energy, which are not conducive to safe use. Hydrogen has the smallest molecular volume and easily diffuses into steel and other metals in large quantities, which results in material strength reduction and embrittlement; at the same time, hydrogen can easily enter pipe gaps to form local stresses and cause leakage. After hydrogen leakage occurs, a jet is produced and gradually transforms into a plume, which floats up quickly under the action of buoyancy and forms a hydrogen–air mixture. When the concentration of the mixture is in the combustible zone and encounters the smallest possible ignition energy, hydrogen ignition occurs. As the flame continuously accelerates, deflagration-to-detonation transition (DDT) occurs, often completing within a few seconds. Fig. 1 shows a flow chart from hydrogen leakage to occurrence of hazard. Normally, a minuscule leakage spreads rapidly without causing danger, but a large amount of leakage may produce jet flames and explosions.Hydrogen ignites and explodes when the burning condition is satisfied. Ignition and explosion produce higher temperature and pressure fields, which will cause great harm to surrounding people and property. In addition, compressed hydrogen spontaneously ignites without any apparent ignition source if it is suddenly released into air. This situation is usually accompanied by the focusing of shock waves, the release of charged particles, and even the local heating of surfaces. Therefore, it is necessary to evaluate explosion hazards and identify the mechanism of spontaneous ignition.Through an analysis of literature, in combination with our survey analysis, this paper provides a brief review of the key issues concerning hydrogen safety. We attribute different incidents to safety issues before, during, and after the incidents. With regard to safety issues before an incident, the focus is on hydrogen incident investigation; with regard to safety issues during an incident, the focus is on hydrogen leakage and diffusion; whereas with regard to safety issues after an incident, the focus is on hydrogen ignition and explosion. This review was conducted in the hope of providing some inspiration for the design and research of hydrogen safety management systems.Section snippetsHydrogen incident investigationMany types of incidents have occurred in hydrogen-related environments [14,15]. From an incident database supported by the U.S. Department of Energy, we analyzed 120 hydrogen incidents in 1999–2019 and their statistical data. All event data came from factories, governments, research institutes, and other institutions worldwide [16]. The results of the analysis are presented in Fig. 2. According to Fig. 2a, laboratories are the most accident-prone locations, accounting for 38.3% of incidents,Hydrogen leakage and diffusionThe initial manifestation of a safety accident in hydrogen utilization is, typically, leakage and diffusion. When high-pressure hydrogen leaks into ambient air, the leakage first occurs as a hydrogen jet. Depending on the pressure ratio between the source of the gas and the surrounding atmosphere, the leak jet will be in three flow states at the leak outlet: 1) as a subsonic jet, wherein the gas flow is fully expanded at the exit; 2) as a critical-state jet, wherein the exit velocity reachesHydrogen ignition and explosionThe combustion and explosion of hydrogen produce high temperature and high pressure, further escalating the hazard. Hydrogen requires very low ignition energy to be ignited in air, and its minimum ignition energy (MIE) is only 4% that of methane. Typically, when hydrogen is ignited with a low ignition energy, laminar combustion first occurs. Afterwards, because of the inherent instability of the flame, interaction of various pressure waves, etc., the flame undergoes turbulent combustion, whichConclusionThis article reviews several key issues regarding the safe utilization of hydrogen energy. First, through the analysis of 120 hydrogen safety incidents that occurred from 1999 to 2019, statistical data on the locations, facilities, possible causes, and losses are obtained. According to the statistical data, failures of pipes/valves/filters in the hydrogen system were notably the most common cause of hydrogen-related incident. Therefore, when performing fault diagnosis and regular leakDeclaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.AcknowledgmentThis research was funded by the Ministry of Science and Technology (MOST) of China under the contracts No. 2020YFF0305700 and No. 2019YFB1504902.
