Safety

A key element in the safe operation of a modern gas distribution system is gas detection. The addition of hydrogen to natural gas will alter the characteristics of the fuel and therefore its impact on gas detection must be considered. It is important that gas detectors remain sufficiently sensitive to the presence of hydrogen and natural gas mixtures and that they do not lead to false readings. This paper presents analyses of work performed as part of the Office for Gas and Energy Markets (OFGEM) funded HyDeploy project on the response of various natural gas industry detectors to blended mixtures up to 20 volume percent (vol%) of hydrogen in natural gas. The scope of the detectors under test included survey instruments and personal monitors that are used in the gas industry. Four blend ratios were analysed (0 10 15 and 20 vol% hydrogen in natural gas). The laboratory testing undertaken investigated the following:

• Flammable response to blends in the ppm range (0-0.2 vol%);
• Flammable response to blends in the lower explosion limit range (0.2-5 vol%);
• Flammable response to blends in the volume percent range (5-100 vol%);
• Oxygen response to blends in the volume percent range (0-25 vol%); and
• Carbon monoxide response to blends in the ppm range (0-1000 ppm).

Catalytic and thermal conductivity based flammable detectors in the volume percent range are as expected affected in the form of a relative increase in output. All of the carbon monoxide detectors tested were found to be cross-sensitive to hydrogen in the blended mixtures tested at levels which have the potential to cause false alarms at current permissible leakage rates. In summary a number of measurement ranges across multiple detectors commonly used in the gas industry have been found to be cross-sensitive to hydrogen in natural gas blends to differing degrees. This required the HyDeploy project to select alternative instruments for the purpose of gas detection as part of the trial. This paper relates to the preparatory work to introduce hydrogen into the Keele closed network.

For safety issues related to the storage of hydrogen under high pressure it is necessary to determine how the gas is released in the case of failure. In particular there exist limited quantitative information on the near-field properties of the gas jets which are important for establishing proper decay laws in the far-field. This paper reports recent CFD results for air and helium obtained in the near-field of the highly under-expanded jets. The gas jets are released from a 30-bar tank with the same opening (orifice). The Reynolds number based on the diameter of the orifice and corresponding gas conditions at the exit was well beyond 106 . The 3D Compressible Multi-Component Navier-Stokes equations were solved directly without relying on the compressibility-corrected turbulence models. The numerical model was initially tested on a one-component (air-air) case where a few aerospace-driven data sets are available for validation. The shock geometry is characterized through the Mach disk position and diameter. These are compared to the results known from the literature and to the scaling laws developed based on the dimensional analysis. In the second two-component (helium-air) jet scenario the density field was validated and examined together with other fields in the attempt to suggest potential initial conditions for the forthcoming far-field simulations.

To advance hydrogen into the energy market it is necessary to consider risk assessment for scenarios that are complicated by accidental hydrogen release mixing with other combustible hydrocarbon fuels. The paper is aimed at examining the effect of mixing the hydrocarbon and inert gas into the hydrogen flame on the kinetic mechanisms the laminar burning velocity and the flame stability. The influences of hydrogen concentration on the flame burning velocity were determined for the hydrogen/propane (H2-C3H8) hydrogen/ethane (H2-C2H6) hydrogen/methane (H2-CH4) and hydrogen/carbon dioxide (H2-CO2) mixtures. Experimental tests were carried out to determine the lift-off blow-out and blowoff stability limits of H2 H2-C3H8 H2-C2H6 H2-CH4 and H2-CO2 jet flames in a 2 mm diameter burner. The kinetic mechanisms of hydrogen interacting with C3 C2 and C1 fuels is analysed using the kinetic mechanisms for hydrocarbon combustion.

Previous experiments demonstrated that the accidental release of high pressure hydrogen into air can lead to the possibility of spontaneous ignition. It is believed that this ignition is due to the heating of the mixing layer between hydrogen and air that is caused by the shock wave driven by the pressurized hydrogen during the release. Currently this problem is poorly understood and not amenable to direct numerical simulation. This is due to the presence of a wide range of scales between the sizes of the blast wave driven and the very thin mixing layer. The present study addresses this fundamental ignition problem and develops a solution framework in order to predict the ignition event for given hydrogen storage pressures and dimension of the release hole. In this problem only the mixing layer between the hydrogen and air is considered. This permits us to use much higher resolution than previous studies. This mixing layer at the jet head is advected as a Lagrangian fluid particle. The key physical processes in the problem are identified to be the mixing of the two gases at the mixing layer the initial heating by the shock wave and a cooling effect due to the expansion of the mixing layer. The results of the simulations indicate that for every storage pressure there exists a critical hole size below which ignition is prevented during the release process. Close inspection of the results indicate that this limit is due to the competition between the heating provided by the shock wave and the cooling due to expansion. Furthermore the results also indicate that the details of the mixing process do not play a significant role to leading order. The limiting ignition criteria were found to be well approximated by the Homogeneous Ignition Model of Cuenot and Poinsot supplemented by a heat loss term due to expansion. Therefore turbulent mixing occurring in reality is not likely to affect the ignition limits derived in the present study. Comparison with existing experiments showed very good agreement.

The aim of the study is to identify and quantify the additional risks related to hydrogen explosions during the operation of a hydrogen-driven car. In a first attempt the accidents or failures of a simple one-tank hydrogen storage system have been studied as a main source of risk. Three types of initiators are taken into account: crash accidents fire accidents without crash (no other cars are involved) and hydrogen leakages in normal situation with following ignition. The consequences of hydrogen ignition and/or explosion depend strongly on environmental conditions (geometry wind etc.) therefore the different configurations of operational and environmental conditions are specified.<br/>Then Event Tree/Fault Tree methods are applied for the risk assessment.<br/>The results of quantification permit to draw conclusions about the overall added risk of hydrogen technology as well as about the main contributors to the risk. Results of this work will eventually contribute to the on-going pre-normative research in the field of hydrogen safety.

Spontaneous ignition of compressed hydrogen release through a length of tube with different internal geometries is numerically investigated using our previously developed model. Four types of internal geometries are considered: local contraction local enlargement abrupt contraction and abrupt enlargement. The presence of internal geometries was found to significantly increase the propensity to spontaneous ignition. Shock reflections from the surfaces of the internal geometries and the subsequent shock interactions further increase the temperature of the combustible mixture at the contact region. The presence of the internal geometry stimulates turbulence enhanced mixing between the shock-heated air and the escaping hydrogen resulting in the formation of more flammable mixture. It was also found that forward-facing vertical planes are more likely to cause spontaneous ignition by producing the highest heating to the flammable mixture than backward-facing vertical planes.

There are currently projects and demonstration programs aiming at introducing Hydrogen powered Fuel Cell (HFC) vehicles into the market. Regione Toscana has been cofounder of the project “H2 Filiera Idrogeno” whose goal is to achieve a clean and sustainable mobility through HFC vehicle studies covering their production storage and use. Among the goals of the project was the substitution of the electric propulsion system with a hydrogen fuel cells propulsion system. This work presents a brief overview of the necessary modifications of the electric propulsion version of a Piaggio Porter to host a H2 fuel cell and experimental studies of realistic H2 releases from the vehicle. The scenarios covered H2 unintended releases underneath the vehicle when at rest and focused on three types of releases diffusive major and minor that might reach the interior of the vehicle and potentially pose a direct risk to the passengers.

HELION a subsidiary of AREVA in charge of the business unit Hydrogen and energy storage is deploying for the first time in a French public building a hydrogen-based energy storage system the Greenergy Box™. The 50 kWe system is coupled with a photovoltaic farm to ensure up to 45% electrical autonomy and power backup to the building. The safety system and siting measures of the complete hydrogen chain are described. The paper also highlights the work accomplished with Fire Authorities and Public to gain the acceptance of the project and allow the deployment of four other hydrogen-based green buildings.

An increase in the number of hydrogen-fuelled applications in the marketplace will require a better understanding of the potential for fires and explosion associated with the unintended release of hydrogen within a structure. Predicting the temporally evolving hydrogen concentration in a structure with unknown release rates leak sizes and leak locations is a challenging task. A simple analytical model was developed to predict the natural and forced mixing and dispersion of a buoyant gas released in a partially enclosed compartment with vents at multiple levels. The model is based on determining the instantaneous compartment over-pressure that drives the flow through the vents and assumes that the helium released under the automobile mixes fully with the surrounding air. Model predictions were compared with data from a series of experiments conducted to measure the volume fraction of a buoyant gas (at 8 different locations) released under an automobile placed in the center of a full-scale garage (6.8 m × 5.4 m × 2.4 m). Helium was used as a surrogate gas for safety concerns. The rate of helium released under an automobile was scaled to represent 5 kg of hydrogen released over 4 h. CFD simulations were also performed to confirm the observed physical phenomena. Analytical model predictions for helium volume fraction compared favourably with measured experimental data for natural and forced ventilation. Parametric studies are presented to understand the effect of release rates vent size and location on the predicted volume fraction in the garage. Results demonstrate the applicability of the model to effectively and rapidly reduce the flammable concentration of hydrogen in a compartment through forced ventilation.

Hydrogen transportation systems require very high pressure hydrogen storage containers to enable sufficient vehicle range for practical use. Current proposed designs have pressures up to 70 MPa with leakage due to damage or deterioration at such high pressures a great safety concern. Accurate models are needed to predict the flammability envelopes around such leaks which rapidly vary with time. This paper compares CFD predictions of jet flows for low pressure jets with predictions using the integral turbulent buoyant jet model. The results show that the CFD model predicts less entrainment and that the turbulent Schmidt number should be smaller with 0.55 giving better results. Then CFD predictions for very high pressure flows are compared with analytical models for choked flows that generate underexpanded jets into the ambient to evaluate the effects of the model assumptions and the effects of real exit geometries. Real gas effects are shown to accelerate the blowdown process and that real flow effects in the CFD model slow the flow rate and increase the exit temperature.