An innovative and comprehensive approach for the consequence analysis of liquid hydrogen vessel explosions

Abstract Hydrogen is one of the most suitable solutions to replace hydrocarbons in the future. Hydrogen consumption is expected to grow in the next years. Hydrogen liquefaction is one of the processes that allows for increase of hydrogen density and it is suggested when a large amount of substance must be stored or transported. Despite being a clean fuel, its chemical and physical properties often arise concerns about the safety of the hydrogen technologies. A potentially critical scenario for the liquid hydrogen (LH2) tanks is the catastrophic rupture causing a consequent boiling liquid expanding vapour explosion (BLEVE), with consequent overpressure, fragments projection and eventually a fireball. In this work, all the BLEVE consequence typologies are evaluated through theoretical and analytical models. These models are validated with the experimental results provided by the BMW care manufacturer safety tests conducted during the 1990's. After the validation, the most suitable methods are selected to perform a blind prediction study of the forthcoming LH2 BLEVE experiments of the Safe Hydrogen fuel handling and Use for Efficient Implementation (SH2IFT) project. The models drawbacks together with the uncertainties and the knowledge gap in LH2 physical explosions are highlighted. Finally, future works on the modelling activity of the LH2 BLEVE are suggested

Cryogenic Hydrogen Storage Tanks Exposed to Fires: a CFD study

Hydrogen is one of the most suitable candidates in replacing heavy hydrocarbons. Liquefaction of fuels is one of the most effective processes to increase their low density. This is critical especially in large-scale or mobile applications such as in the maritime or aeronautical fields. A potential loss of integrity of the cryogenic storage equipment might lead to severe consequences due to the properties of these substances (e.g. high flammability). For this reason, this critical event must be avoided. The aim of this study is to analyse the behaviour of the cryogenic vessel and its lading when it is exposed to a fire and understand how to prevent a catastrophic rupture of the tank during this accident scenario. A two-dimensional computational fluid dynamic (CFD) analysis is carried out on a cryogenic liquid hydrogen (LH2) vessel to investigate its thermal response when engulfed in a fire. The model accounts for the evaporation and condensation of the substance and can predict the tank pressurization rate and temperature distribution. It is assumed that the vessel is completely engulfed in the fire (worst-case scenario). The CFD model is validated with the outcomes of a small-scale fire test of an LH2 tank. Critical indications on the dynamic response of the cryogenic tank involved in a worst-case accident scenario are provided. Tank pressurisation and temperature distributions of the case study can be exploited to provide conservative estimations of the time to failure (TTF) of the vessel. These outcomes represent useful information to support the emergency response to this type of accident scenario and can aid the selection of appropriate and effective safety barriers to prevent the complete destruction of the tank

A Machine Learning Approach to Predict the Materials' Susceptibility to Hydrogen Embrittlement

Hydrogen is widely considered a promising energy carrier capable of mitigating human environmental impact. Nevertheless, safety aspects represent one of the major bottlenecks for the widespread utilization of hydrogen technologies. Industrial equipment operating in hydrogen environments is prone to hydrogen-induced damages, which may manifest through a reduction of mechanical properties, fracture toughness, and fatigue performance. They may cause component failures at stress levels significantly below the design level, therefore determining loss of containment. The occurrence of hydrogen embrittlement (HE) relies on the synergy of several factors, such as hydrogen concentration, operating conditions, level of internal and applied stress, microstructure and chemical composition of the material. However, the interlinked dependence of these factors makes a direct and clear evaluation challenging, subsequently creating serious difficulties in planning inspection and maintenance activities. In this study, a comprehensive review of the experimental data of tensile tests carried out in hydrogen was performed and analyzed through an advanced machine learning approach. This study can provide critical insights into the susceptibility to hydrogen embrittlement for several materials operating under different environmental conditions. In particular, the Embrittlement Index was estimated and used as determining parameter to predict the likelihood of component failures. The model demonstrated accurate and reliable predicting capabilities. The outcome of this study can increase the understanding of hydrogen-induced material damages and facilitate decision-making processes in planning the inspection and maintenance of hydrogen technologies

Fragments Generated During Liquid Hydrogen Tank Explosions

Liquid hydrogen (LH2) may be employed to transport large quantities of pure hydrogen or be stored onboard of ships, airplanes and trains fuelled by hydrogen, thanks to its high density compared to gaseous compressed hydrogen. LH2 is a cryogenic fluid with an extremely low boiling point (-253°C at atmospheric pressure) that must be stored in double-walled vacuum insulated tanks to limit the boil-off formation. There is limited knowledge on the consequences of LH2 tanks catastrophic rupture. In fact, the yield of the consequences of an LH2 tank explosion (pressure wave, fragments and fireball) depend on many parameters such as tank dimension, filling degree, and tank internal conditions (temperature and pressure) prior the rupture. Only two accidents provoked by the rupture of an LH2 tank occurred in the past and a couple of experimental campaigns focussed on this type of accident scenario were carried out for LH2. The aim of this study is to analyse one of the LH2 tank explosion consequences namely the fragments. The longest horizontal and vertical ranges of the fragments thrown away from the blast wave are estimated together with the spatial distribution around the tank. Theoretical models are adopted in this work and validated with the experimental results. The proposed models can aid the risk analysis of LH2 storage technologies and provide critical insights to plan a prevention and mitigation strategy and improve the safety of hydrogen applications

Modelling of Fireballs Generated After the Catastrophic Rupture of Hydrogen Tanks

The interest towards hydrogen skyrocketed in the last years. Thanks to its potential as an energy carrier, hydrogen will be soon handled in public and densely populated areas. Therefore, accurate models are necessary to predict the consequences of unwanted scenarios. These new models should be employed in the consequence analysis, a phase of risk assessment, and thus aid the selection, implementation, and optimization of effective risk-reducing measures. This will increase safety of hydrogen technologies and therefore favour their deployment on a larger scale. Hydrogen is known to be an extremely flammable gas with a low radiation flame compared to hydrocarbons. However, luminous fireballs were generated after the rupture of both compressed gaseous and liquid hydrogen tanks in many experiments. Moreover, it was demonstrated that conventional empirical correlations, initially developed for hydrocarbon fuels, underestimate both dimension and duration of hydrogen fireballs recorded during small-scale tests (Ustolin and Paltrinieri, 2020). The aim of this study is to obtain an analysis of hydrogen fireballs to provide new critical insights for consequence analysis. A comparison among different correlations is conducted when predicting fireball characteristics during the simulation of past experiments where both gaseous and liquid hydrogen tanks were intentionally destroyed. All the models employed in this study are compared with the experimental results for validation purposes. Specific models designed for hydrogen can support the design of hydrogen systems and increasing their safety and promote their future distribution

Lessons learned from HIAD 2.0. Inspection and maintenance to avoid hydrogen-induced material failures

Hydrogen has the potential to make countries energetically self-sufficient and independent in the long term. Nevertheless, its extreme combustion properties and its capability of permeating and embrittling most metallic materials produce significant safety concerns. The Hydrogen Incidents and Accidents Database 2.0 (HIAD 2.0) is a public repository that collects data on hydrogen-related undesired events mainly occurred in chemical and process industry. This study conducts an analysis of the HIAD 2.0 database, mining information systematically through a computer science approach known as Business Analytics. Moreover, several hydrogen-induced material failures are investigated to understand their root causes. As a result, a deficiency in planning effective inspection and maintenance activities is highlighted as the common cause of the most severe accidents. The lessons learned from HIAD 2.0 could help to promote a safety culture, to improve the abnormal and normal events management and to stimulate a widespread rollout of hydrogen technologies

Modelling of Accident Scenarios from Liquid Hydrogen Transport and Use

Hydrogen is one of the most suitable candidates to replace hydrocarbons and reduce the environmental pollution and CO2 emissions. Hydrogen is valuable energy carrier, potentially clean and renewable thanks to its peculiar properties. However, hydrogen has a few characteristics, such as high flammability and low density that must be taken into account when stored or handled, especially in relation to the associated safety. For this reason, this PhD study aims to increase the knowledge on safety of hydrogen technologies. Hydrogen safety is a broad topic which involves several disciplines. This PhD focusses on the modelling of atypical accident scenarios of liquid hydrogen (LH2) technologies by adopting a multidisciplinary approach. This type of accident scenarios is called atypical because they have low probability to happen but high consequences. A few times, the neglection of these scenarios by conventional risk assessment techniques led to major accidents. For this reason, the atypical accident scenario cannot be omitted during a risk assessment and must be further analysed. Firstly, through a comprehensive literature review, this PhD study investigates the causes of loss of integrity (LOI) and loss of containment (LOC) of hydrogen equipment since the atypical accident scenarios always occurred after these critical events. The consequences of an LH2 release are then analysed. The focus is placed on the boiling liquid expanding vapour explosion (BLEVE) and the rapid phase transition (RPT) explosions for liquid hydrogen technologies because a significant dearth of knowledge is still present. Secondly, the possibility for the BLEVE to occur after the catastrophic rupture of an LH2 vessel is theoretically assessed by gathering information on previous accident and applying accepted thermodynamic theories for this event. The consequences of a potential BLEVE for LH2 (pressure wave, missiles and fireball) are evaluated. Unique experimental series on LH2 bursting tank scenario and fire tests are simulated. Different approaches are employed for the BLEVE event: analytical models, empirical correlations and CFD analysis. Finally, the time to failure of an LH2 tank exposed to a fire is estimated with a thermal node model. Thirdly, the RPT event is analysed from a more theoretical approach since no records of LH2 RPT are found in literature. The knowledge gained for other substances such as liquefied natural gas (LNG) and liquid nitrogen (LIN) is applied to LH2. The consequences of a hypothetical LH2 RPT are evaluated by means of an analytical model and compared to the LNG RPT aftermath. The main contributions of this PhD study are the following: • investigation on the causes of LOI of hydrogen technology; • identification of the LH2 release consequences; • understanding of the BLEVE feasibility for LH2 storage systems; • determination of the LH2 BLEVE consequences; • estimation of the time to failure of LH2 tanks exposed to a fire; • analysis of the theories and mechanisms of RPT explosions; • determination of the LH2 RPT consequences. This PhD study provides relevant safety indications on the causes of LOI of hydrogen technologies as well as on the BLEVE and RPT phenomena for LH2 technologies. The knowledge gap in these topics is highlighted and partially fulfilled. The limitations of existing models for the simulation of these explosions are emphasised. The results of this thesis serve as a starting point for future studies

Hydrogen Fireball Consequence Analysis

A fireball may occur after the catastrophic rupture of a tank containing a flammable substance such as a fuel, if an ignition source is present. The fireball is identified by the combustion of the flammable cloud created after the fuel release and composed by the mixture of the latter and air. In particular, the fuel concentration is higher at the center of the fireball compared with the external layers where the ignition takes place. After its formation, the fireball tends to rise vertically due to the buoyancy of the hot gases involved in the combustion. Moreover, the fireball emits its energy mainly through radiant heat. Hence, the fireball formation may be one of the consequences of both a liquid and a compressed gaseous hydrogen tank explosion. For instance, the fireball is a consequence of a boiling liquid expansion vapor explosion (BLEVE). A BLEVE may occur after the catastrophic rupture of a tank containing a liquid at a temperature higher than its boiling point at atmospheric pressure. The explosion is characterized by the rapid expansion of the liquid and vapor phases due to the depressurization of the vessel. The aim of this study is to model a liquid hydrogen (LH2) fireball generated subsequently the BLEVE phenomenon. Different empirical correlations were selected to estimate the fireball dimensions and duration. Moreover, the fireball radiation was estimated by means of a theoretical model. As case study, the fireball generated from the explosion of the LH2 tank with a volume of 1 m3 , which will be tested during the safe hydrogen fuel handling and use for efficient implementation (SH2IFT) project, was simulated. The results achieved from the fireball numerical models can be employed to estimate the safety distance from an LH2 tank and propose appropriate safety barriers. Furthermore, these outputs can aid the writing of critical safety guidelines for hydrogen technologies. Finally, the outcome of this study will be validated with the experimental results during the SH2IFT project

Fuel cells for airborne usage: Energy storage comparison

The global drone market is growing every year. The number of applications is increasing: from search and rescue, security, surveillance to science and research and unmanned cargo systems. A limiting factor for drone exploitation is that for the energy storage, normally, a battery is used and this solution affects flight time. A possible solution could be the utilization of fuel cells. This paper focuses on the utilization of fuel cells power as an alternative solution for drone propulsion. The aim of the study is to determine when it is more appropriate, in terms of mass, to use a battery or a hybrid (fuel cell \ufe battery) system to power drones. To compare the different systems, a numerical simulation model has been developed in order to choose the best power system once the drone operation profile has been defined. The model allows comparing different type of fuels and battery systems. The data to tune the model have been taken from commercial products, today already available. The simulation model considers a light-weight open-air cathode PEM (Polymer Exchange Membrane) fuel cell. The stack power output is chosen according to the mission profile and rages from 200 W to 1000 W. The presented results show that, for the considered drone segment, multi-rotor drones with weight of 7 kg at take-off, lithium batteries are still the best choice for time flight shorter than about 1 h. A hybrid system, appears to be interesting for longer flights. For example, it has been calculated that a hybrid quadcopter drone with a mass of 7 kg, considering a flight profile that requires 1089 Wh can be powered with a 4.4 kg hybrid system composed by a 500 W and 1.4 kg PEM fuel cell system, 1.9 kg hydrogen composite pressure vessel and a 0.8 kg lithium battery. The same amount of energy can be stored in a lithium battery with a weight of about 6.6 kg. These means a weight saving of more than 30%. The hybrid system, in term of weight, is even more convenient for flight profiles that require more energy

An Extensive Review of Liquid Hydrogen in Transportation with Focus on the Maritime Sector

The European Green Deal aims to transform the EU into a modern, resource-efficient, and competitive economy. The REPowerEU plan launched in May 2022 as part of the Green Deal reveals the willingness of several countries to become energy independent and tackle the climate crisis. Therefore, the decarbonization of different sectors such as maritime shipping is crucial and may be achieved through sustainable energy. Hydrogen is potentially clean and renewable and might be chosen as fuel to power ships and boats. Hydrogen technologies (e.g., fuel cells for propulsion) have already been implemented on board ships in the last 20 years, mainly during demonstration projects. Pressurized tanks filled with gaseous hydrogen were installed on most of these vessels. However, this type of storage would require enormous volumes for large long-range ships with high energy demands. One of the best options is to store this fuel in the cryogenic liquid phase. This paper initially introduces the hydrogen color codes and the carbon footprints of the different production techniques to effectively estimate the environmental impact when employing hydrogen technologies in any application. Afterward, a review of the implementation of liquid hydrogen (LH2) in the transportation sector including aerospace and aviation industries, automotive, and railways is provided. Then, the focus is placed on the maritime sector. The aim is to highlight the challenges for the adoption of LH2 technologies on board ships. Different aspects were investigated in this study, from LH2 bunkering, onboard utilization, regulations, codes and standards, and safety. Finally, this study offers a broad overview of the bottlenecks that might hamper the adoption of LH2 technologies in the maritime sector and discusses potential solutions