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As a part of zero emission ambitions within the maritime, aviation and elsewhere, liquid hydrogen (LH 2) will increasingly be used as an energy carrier and fuel in the coming years. It is therefore important to understand safety aspects of LH 2 , including gas dispersion and explosion properties. In the process of building the world's first ferry using LH 2 as fuel the Norwegian Public Roads Administration commissioned large-scale LH 2 release tests to study dispersion and explosion properties relevant for the design and approval processes. Experiments with LH 2 releases both indoor and outdoor were performed. In this article these experiments are described and compared to simulations with the CFD-model FLACS. The aim of the modelling is both to better understand the experiments performed, and to demonstrate that important phenomena and physics related to LH 2 safety assessments for approval processes can be simulated and predicted with confidence. [ABSTRACT FROM AUTHOR]

As a part of zero emission ambitions within the maritime, and possibly within aviation, liquid hydrogen (LH2) will increasingly be used as fuel in the coming years. In that respect it is important to understand safety aspects of LH2, including gas dispersion and explosion properties. The Norwegian Public Roads Administration commissioned large-scale LH2 dispersion and explosion experiments both indoor and outdoor when developing their first LH2-fuelled ferry. In this article these experiments are simulated using a CFD-model with the aim to validate a modelling approach for gas dispersion from LH2-releases. [ABSTRACT FROM AUTHOR]

Heavy-duty vehicles (HDVs) and trains using hydrogen as fuel have significantly larger tanks than smaller fuel cell vehicles (FCVs). The requirement for depressurization time in case of fire may often be the same, thus there may be a need to dimension the thermal pressure relief devices (TPRDs) for significantly higher release rates than used in FCVs. This CFD-study assesses to what extent higher TPRD release rates than typically used for FCVs may be applied without compromising safety. The study has been performed in collaboration with partners in the MoZEES research centre. [ABSTRACT FROM AUTHOR]

Abstract: Computational fluid dynamics (CFD) tools are increasingly employed for quantifying incident consequences in quantitative risk analysis (QRA) calculations in the process industry. However, these tools must be validated against representative experimental data, involving combined release and ignition scenarios, in order to have a real predictive capability. Forschungszentrum Karlsruhe (FZK) has recently carried out experiments involving vertically upwards hydrogen releases with different release rates and velocities impinging on a plate in two different geometrical configurations. The dispersed cloud was subsequently ignited and resulting explosion overpressures recorded. Blind CFD simulations were carried out prior to the experiments to predict the results. The simulated gas concentrations are found to correlate reasonably well with observations. The overpressures subsequent to ignition obtained in the blind predictions could not be compared directly as the ignition points chosen in the experiments were somewhat different from those used in the blind simulations, but the pressure levels were similar. Simulations carried out subsequently with the same ignition position as those in the experiments compared reasonably well with the observations. This agreement points to the ability of the CFD tool FLACS to model such complex scenarios even with hydrogen as a fuel. Nevertheless, the experimental set-up can be considered to be small-scale. Future large-scale data of this type will be valuable to confirm ability to predict large-scale accident scenarios. [Copyright &y& Elsevier]

Abstract: When introducing hydrogen-fuelled vehicles, an evaluation of the potential change in risk level should be performed. It is widely accepted that outdoor accidental releases of hydrogen from single vehicles will disperse quickly, and not lead to any significant explosion hazard. The situation may be different for more confined situations such as parking garages, workshops, or tunnels. Experiments and computer modelling are both important for understanding the situation better. This article reports a simulation study to examine what, if any, is the explosion risk associated with hydrogen vehicles in tunnels. Its aim was to further our understanding of the phenomena surrounding hydrogen releases and combustion inside road tunnels, and furthermore to demonstrate how a risk assessment methodology developed for the offshore industry could be applied to the current task. This work is contributing to the EU Sixth Framework (Network of Excellence) project HySafe, aiding the overall understanding that is also being collected from previous studies, new experiments and other modelling activities. Releases from hydrogen cars (containing 700bar gas tanks releasing either upwards or downwards or liquid hydrogen tanks releasing only upwards) and buses (containing 350bar gas tanks releasing upwards) for two different tunnel layouts and a range of longitudinal ventilation conditions have been studied. The largest release modelled was 20kg H2 from four cylinders in a bus (via one vent) in 50s, with an initial release rate around 1000g/s. Comparisons with natural gas (CNG) fuelled vehicles have also been performed. The study suggests that for hydrogen vehicles a typical worst-case risk assessment approach assuming the full gas inventory being mixed homogeneously at stoichiometry could lead to severe explosion loads. However, a more extensive study with more realistic release scenarios reduced the predicted hazard significantly. The flammable gas cloud sizes were still large for some of the scenarios, but if the actual reactivity of the predicted clouds is taken into account, moderate worst-case explosion pressures are predicted. As a final step of the risk assessment approach, a probabilistic QRA study is performed in which probabilities are assigned to different scenarios, time dependent ignition modelling is applied, and equivalent stoichiometric gas clouds are used to translate reactivity of dispersed non-homogeneous clouds. The probabilistic risk assessment study is based on over 200 dispersion and explosion CFD calculations using the commercially available tool FLACS. The risk assessment suggested a maximum likely pressure level of 0.1–0.3barg at the pressure sensors that were used in the study. Somewhat higher pressures are seen elsewhere due to reflections (e.g. under the vehicles). Several other interesting observations were found in the study. For example, the study suggests that for hydrogen releases the level of longitudinal tunnel ventilation has only a marginal impact on the predicted risk, since the momentum of the releases and buoyancy of hydrogen dominates the mixing and dilution processes. [Copyright &y& Elsevier]

Abstract: One of the main benefits sought by including hydrogen in the alternative fuels mix is emissions reduction – eventually by 100%. However, in the near term, there is a very significant cost differential between fossil fuels and hydrogen. Hythane (a blend of hydrogen and natural gas) can act as a viable next step on the path to an ultimate hydrogen economy as a fuel blend consisting of 8–30% hydrogen in methane can reduce emissions while not requiring significant changes in existing infrastructure. This work seeks to evaluate whether hythane may be safer than both hydrogen and methane under certain conditions. This is due to the fact hythane combines the positive safety properties of hydrogen (strong buoyancy, high diffusivity) and methane (much lower flame speeds and narrower flammability limits as compared to hydrogen). For this purpose, several different mixture compositions (e.g. 8%, 20% and 30% hydrogen) are considered. The evaluation of (a) dispersion characteristics (which are more positive than for methane), (b) combustion characteristics (which are closer to methane than hydrogen), and (c) Combined dispersion + explosion risk is performed. This risk is expected to be comparable to that of pure methane, possibly lower in some situations, and definitely lower than for pure hydrogen. The work is performed using the CFD software FLACS that has been well-validated for safety studies of both natural gas/methane and hydrogen systems. The first part of the work will involve validating the flame speeds and flammability limits predicted by FLACS against values available in literature. The next part of the work involves validating the overpressures predicted by the CFD tool for combustion of premixed mixtures of methane and hydrogen with air against available experimental data. In the end, practical systems such as vehicular tunnels, garages, etc. is used to demonstrate positive safety benefits of hythane with comparisons to similar simulations for both hydrogen and methane. [Copyright &y& Elsevier]