Safety

Hydrogen as a new and ecologic energy source is tempting though it creates the challenge of ensuring the safe use of hydrogen for all future consumers. Making sure that a hydrogen vehicle can be simply and safely used by anyone while performing as expected requires that the car be light with built-in safety features. This is achieved by combining high pressure composite cylinders with strict test procedures. Composite cylinders of up to 150 L operated to a maximum of 700 bar are required for vehicle applications. Air Liquide has developed test benches to hydraulically cycle such cylinders at 1400 bar and up to 3500 bar for burst tests. These tests are performed under controlled temperature conditions at ambient and extreme temperatures in order to simulate cylinder aging. Components in gas service such as valves hoses and other pressure devices are tested up to 1400 bars with hydrogen to simulate actual usage conditions. Hydrogen is used as a testing gas instead of nitrogen which is commonly used for such tests because hydrogen interacts with materials (e.g. hydrogen embrittlement) and because hydrogen has a special thermodynamic behaviour ( pressure drop velocity heat exchange…)

Numerical simulations have been carried out for spontaneous ignition of pressurized hydrogen release directly into air. Results showed a possible mechanism for spontaneous ignition due to molecular diffusion. To accurately calculate the molecular transport of species momentum and energy in a multi-component gaseous mixture a mixture-averaged multi-component approach was employed in which thermal diffusion is accounted for. To reduce false numerical diffusion extremely fine meshes were used along with the ALE (Arbitrary Lagrangian-Eulerian) method. The ALE method was employed to track the moving contact surface with moving clustered grids. A detailed kinetic scheme with 21 elementary steps and 8 reactive chemical species was implemented for combustion chemistry. The scheme gives due consideration to third body reactions and reaction-rate pressure-dependant “fall-off” behavior. The autoignition of pressurized hydrogen release was previously observed in laboratory tests [2-3] and suspected as possible cause of some accidents. The present numerical study successfully captured this scenario. Autoignition was predicted to first take place at the tip region of the hydrogen-air contact surface due to mass and energy exchange between low temperature hydrogen and shock-heated air at the contact surface through molecular diffusion. The initial flame thickness is extremely thin due to the limiting molecular diffusion. The combustion region extends downward along the contact surface as it moves downstream. As the hydrogen jet developed downstream the front contact surface tends to be distorted by the developed flow of the air. Turbulence plays an important role in mixing at the region of the distorted contact surface. This is thought to be a major factor for the initial laminar flame to turn into a final stable turbulent flame.

In the present paper the results of experiments on study of high-speed deflagrations in flat layer of hydrogen-air mixtures unconfined from below are presented. The experiments were performed in two different rectangular channels: small-scale with mixture volume up to 0.4 m3 and large-scale with volume up to 5.5 m3. The main goal of the experiments was to examine the possibility of the layer geometries to maintain high-speed deflagration and detonation. With the aim to study a range of combustion regimes the experiments were performed varying degree of channel obstruction hydrogen concentration and thickness of the layer. Depending on the experimental conditions all major combustion regimes were observed: slow flame fast – ‘choked’ flame and steady-state detonation. It was found that minimum layer layer thickness in the range of 8 to 15 detonation cell widths is required for sustainable detonations.

The report deals with the investigation of non-stationary combustion of hydrogen-air mixtures extremely relevant to the issues of safety. Considered are the conditions of its formation and development in the tubes in the conic element and in the spherical 12-m diameter chamber. The report shows that at the formation of non-stationary combustion in the conic element in its top the pressure can develop exceeding 1000 atmospheres. It is also shown that in large closed volumes non-stationary combustion can develop from a small energy source in contrast to detonation for whose stimulation in large volumes significant power influences are required. Simultaneously in the volume a pressure can be formed by far exceeding the Chapman-Jouguet pressure in the front of stationary detonation.

Two key aspects of hydrogen safety are (1) incorporating data and analysis from research development and demonstration (RD&D) into the codes and standards development process; and (2) adopting and enforcing these codes and standards by state and local permitting officials. This paper describes work that the U.S. Department of Energy (DOE) is sponsoring to address these aspects of hydrogen safety. For the first DOE is working with the automobile and energy industries to identify and address high priority RD&D to establish a sound scientific basis for requirements that are incorporated in hydrogen codes and standards. The high priority RD&D needs are incorporated and tracked in an RD&D Roadmap adopted by the Codes and Standards Technical Team of the FreedomCAR and Fuel Partnership. DOE and its national laboratories conduct critical RD&D and work with key standards and model code development organizations to help incorporate RD&D results into the codes and standards process. To address the second aspect DOE has launched an initiative to facilitate the permitting process for hydrogen fueling stations (HFS). A key element of this initiative will be a Web-based information repository a toolkit that includes information fact sheets networking charts to encourage information exchange among code officials who have permitted or are in the process of permitting HFS templates to show whether a proposed station footprint conforms to requirements in the jurisdiction and a database of requirements incorporated in key codes and standards. The information repository will be augmented by workshops for code officials and station developers in jurisdictions that are likely to have HFS in the near future.

Demonstrated safety in the production distribution and use of hydrogen will be critical to successful implementation of a hydrogen infrastructure. Recognizing the importance of this issue the U.S. Department of Energy has established the Hydrogen Safety Program to ensure safe operations of its hydrogen research and development program as well as to identify and address needs for new knowledge and technologies in the future hydrogen economy. Activities in the Safety Program range across the entire safety spectrum including: R&D devoted to investigation of hydrogen behaviour physical characteristics materials compatibility and risk analysis; inspection and investigation into the safety procedures and practices of all hydrogen projects supported by DOE funds; development of critical technologies for safe hydrogen systems such as sensors and design techniques; and safety training and education for emergency responders code inspectors and the general public. Throughout its activities the Safety Program encourages the open sharing of information to enable widespread benefit from any lessons learned or new information developed.



This paper provides detailed descriptions of the various activities of the DOE Hydrogen Safety Program and includes some example impacts already achieved from its implementation.

The largest known experiment on hydrogen-air deflagration in the open atmosphere has been analysed by means of the large eddy simulation (LES). The combustion model is based on the progress variable equation to simulate a premixed flame front propagation and the gradient method to decouple the physical combustion rate from numerical peculiarities. The hydrodynamic instability has been partially resolved by LES and unresolved effects have been modelled by Yakhot's turbulent premixed combustion model. The main contributor to high flame propagation velocity is the additional turbulence generated by the flame front itself. It has been modelled based on the maximum flame wrinkling factor predicted by Karlovitz et al. theory and the transitional distance reported by Gostintsev with colleagues. Simulations are in a good agreement with experimental data on flame propagation dynamics flame shape and outgoing pressure wave peaks and structure. The model is built from the first principles and no adjustable parameters were applied to get agreement with the experiment.

If the hydrogen economy is to progress more hydrogen fuelling stations are required. In the short term in the absence of a hydrogen distribution network these fuelling stations will have to be supplied by liquid hydrogen (LH2) road tanker. Such a development will increase the number of tanker offloading operations significantly and these may need to be performed in close proximity to the general public.<br/>Several research projects have been undertaken already at HSL with the aim of identifying and addressing hazards relating to the storage and transport of bulk LH2 that are associated with hydrogen refuelling stations located in urban environments.<br/>The first phase of the research was to produce a position paper on the hazards of LH2 (Pritchard and Rattigan 2009). This was published as an HSE research report RR769 in 2010.  <br/>The second phase developed an experimental and modelling strategy for issues associated with LH2 spills and was published as an internal report HSL XS/10/06. The subsequent experimental work is a direct implementation of that strategy. LH2 was first investigated experimentally (Royle and Willoughby 2012 HSL XS/11/70) as large-scale spills of LH2 at a rate of 60 litres per minute. Measurements were made on unignited releases which included the concentration of hydrogen in air thermal gradients in the concrete substrate liquid pool formation and temperatures within the pool. Computational modelling on the un-ignited spills was also performed (Batt and Webber 2012 HSL MSU/12/01).<br/>The experimental work on ignited releases of LH2 detailed in this report is a direct continuation of the work performed by Royle and Willoughby.<br/>The aim of this work was to determine the hazards and severity of a realistic ignited spill of LH2 focussing on; flammability limits of an LH2 vapour cloud flame speeds through an LH2 vapour cloud and subsequent radiative heat and overpressures after ignition. The results of the experimentation will inform the wider hydrogen community and contribute to the development of more robust modelling tools. The results will also help to update and develop guidance for codes and standards.

Hydrogen is an alternative to conventional heavy marine fuel oil following the initial strategy of the International Maritime Organization (IMO) for reducing greenhouse gas emissions. Although hydrogen energy has many advantages (zero-emission high efficiency and low noise) it has considerable fire and explosion risks due to its thermal and chemical characteristics (wide flammable concentration range and low ignition energy). Thus safety is a key concern related to the use of hydrogen. Whereas most previous studies focused on the terrestrial environment we aim to analyze the effects of the ship’s motion on hydrogen dispersion (using commercial FLUENT code) in an enclosed area. When compared to the steady state our results revealed that hydrogen reached specific sensors in 63% and 52% less time depending on vessel motion type and direction. Since ships carry and use a large amount of hydrogen as a power source the risk of hydrogen leakage from collision or damage necessitates studying the correspondence between leakage diffusion and motion characteristics of the ship to position the sensor correctly.

Flame acceleration (FA) and explosion of hydrogen/air mixtures remain key issues for severe accident management in nuclear power plants. Empirical criteria were developed in the early 2000s by Dorofeev and colleagues providing effective tools to discern possible FA or DDT (Deflagration-to-Detonation Transition) scenarios. A large experimental database composed mainly of middle-scale experiments in obstacle-laden ducts at atmospheric pressure condition has been used to validate these criteria. However during a severe accident the high release rate of steam and non-condensable gases into the containment can result in pressure increase up to 5 bar abs. In the present work the influence of the unburnt gas initial pressure on flame propagation mechanisms was experimentally investigated. Premixed hydrogen/air mixtures with hydrogen concentration close to 11% and 15% were considered. From the literature we know that these flames are supposed to accelerate up to Chapman-Jouguet deflagration velocity in long obstacle-laden tubes at initial atmospheric conditions. Varying the pressure in the fresh gas in the range 0.6–4 bar no effects on the flame acceleration phase were observed. However as the initial pressure was increased we observed a decrease in the flame velocity close to the end of the tube. The pressure increase due to the combustion reaction was found to be proportional to the initial pressure according to adiabatic isochoric complete combustion.