Safety

The main objectives were to identify materials issues relating to the widespread use of hydrogen as a fuel.

MAIN FINDINGS

• Hydrogen is seen by many as the answer to the environmental problems of reliance on fossil fuels for energy needs. A great deal of effort is currently being invested in research into all areas of the hydrogen economy such as fuel cells hydrogen generation transportation and storage.
• Fuel cells have the potential to provide power for a very wide range of applications ranging from small portable electronics devices to large stationary electricity production and vehicles covering the whole range of road vehicles and possibly extending to rail marine and even aviation.
• The main obstacles to achieving a viable hydrogen economy are costs of producing hydrogen from renewable sources issues relating to transportation and storage due to the low energy density of hydrogen gas and the cost and reliability of fuel cells.
• The main material considerations relating to the use of hydrogen are hydrogen embrittlement material properties at cryogenic temperatures (due to use of liquid hydrogen) and permeability.
• A number of new materials are likely to come to prominence in a hydrogen economy; high performance composites are likely to be used extensively for high pressure hydrogen cylinders new materials or combinations of materials may be used for hydrogen pipelines and a range of new materials are currently being considered for hydrogen storage such as metal hydrides and carbon nanotubes.
• Due to the effect of hydrogen on materials it is important to test any materials in the environment in which they would be used. Depending on the type of test this could require the use of very specialist expensive equipment. 

A set of original experiments is presented covering the topic of hydrogen-air mixture ignition by a hot surface. The hot surface is a 30 mm long/10 mm diameter heated coil which temperature is controlled by IR techniques. The coil is placed into the flowing stream of hydrogen air mixtures. The variable parameters are the composition of the flammable atmosphere (4 to 75% H2 v/v) the flow speed (from 0.5 m/s to 30 m/s) and its temperature (from -110°C to ambient). The experimental techniques and results are presented and a tentative interpretation is proposed based on ignition theories and highspeed video recordings. It is shown that the ignition temperature (600°C) is insensitive to flowing conditions which is a very unexpected result.

To define a strategy of mitigation for containerized hydrogen systems (fuel cells for example) against explosion the main characteristics of flammable atmosphere (size concentration turbulence…) shall be well-known. This article presents an experimental study on accidental hydrogen releases and dispersion into an enclosure of 4 m3 (2 m x 2 m x 1 m). Different release points are studied: two circular releases of 1 and 3 mm and a system to create ring-shaped releases. The releases are operated with a pressure between 10 and 40 bars in order to be close to the process conditions. Different positions of the release inside the enclosure i.e. centred on the floor or along a wall are also studied. A specific effort is made to characterize the turbulence in the enclosure during the releases. The objectives of the experimental study are to understand and quantify the mechanisms of formation of the explosive atmosphere taking into account the geometry and position of the release point and the confinement. Those experimental data are analyzed and compared with existing models and could bring some new elements to improve them.

With the development of the hydrogen economy it requires a better understanding of the potential for fires and explosions associated with the unintended release of hydrogen within a partially confined space. In order to mitigate the hydrogen fire and explosion risks effectively accurate predictions of the hydrogen transport and mixing processes are crucial. It is well known that turbulence modelling is one of the key elements for a successful simulation of gas mixing and transport. GASFLOW-MPI is a scalable CFD software solution used to predict fluid dynamics conjugate heat and mass transfer chemical kinetics aerosol transportation and other related phenomena. In order to capture more turbulence information the Large Eddy Simulation (LES) model and LES/RANS hybrid model Detached Eddy Simulation (DES) have been implemented and validated in 3-D CFD code GASFLOW-MPI. The standard Smagorisky SGS model is utilized in LES turbulence model. And the k-epsilon based DES model is employed. This paper assesses the capability of algebraic k-epsilon DES and LES turbulence model to simulate the mixing and transport behavior of highly buoyant gases in a partially confined geometry. Simulation results agree well with the overall trend measured in experiments conducted in a reduced scale enclosure with idealized leaks which shows that all these four turbulent models are validated and suitable for the simulation of light gas behavior. Furthermore the numerical results also indicate that the LES and DES model could be used to analysis the turbulence behavior in the hydrogen safety problems.

Hydrogen as a future energy carrier is receiving a significant amount of attention in Japan. From the viewpoint of safety risk evaluation is required in order to increase the number of hydrogen refuelling stations (HRSs) implemented in Japan. Collecting data about accidents in the past will provide a hint to understand the trend in the possibility of accidents occurrence by identifying its operation time However in new technology; accident rate estimation can have a high degree of uncertainty due to absence of major accident direct data in the late operational period. The uncertainty in the estimation is proportional to the data unavailability which increases over long operation period due to decrease in number of stations. In this paper a suitable time correlation model is adopted in the estimation to reflect lack (due to the limited operation period of HRS) or abundance of accident data which is not well supported by conventional approaches. The model adopted in this paper shows that the uncertainty in the estimation increases when the operation time is long owing to the decreasing data.

Methane and hydrogen-enriched (25 vol% and 50 vol% H₂ -enriched CH₄) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re) showing the dependence on Re and H₂ concentrations. The effects of equivalence ratio (Φ) Re and H₂ enrichment on flame structure were examined employing OH-PLIF measurement. For CH₄/air cases the flame was stabilized with an M shape; while for H₂ -enriched cases the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H₂ enrichment occurring due to the increase in flame speed higher resistance of the flame to the strain rate and change in the inner recirculation zone. Flow fields of CH₄/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events led by two mechanisms (combustion-induced vortex breakdown CIVB and boundary-layer flashback BLF) were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH4 flames with M shape whereas the BLF occurs for all H₂ -enriched flames with П shape.

The UK Government signed legislation on 27th June 2019 committing the UK to a legally binding target of Net Zero emissions by 2050. Climate change is one of the most significant technical economic social and business challenges facing the world today.

The H21 NIC Phase 1 project delivered an optimally designed experimentation and testing programme supported by the HSE Science Division and DNV GL with the aim to collect quantifiable evidence to support that the UK distribution network of 2032 will be comparably as safe operating on 100% hydrogen as it currently is on

natural gas. This innovative project begins to fill critical safety evidence gaps surrounding the conversion of the UK gas network to 100% hydrogen. This will facilitate progression towards H21 Phase 2 Operational Safety Demonstrations and the H21 Phase 3 Live Trials to promote customer acceptability and ultimately aid progress towards a government policy decision on heat.

DNV GL and HSE Science Division were engaged to undertake the experimentation testing and QRA update programme of work. DNV GL and HSE Science Division also peer reviewed each other’s programme of work at various stages throughout the project undertaking a challenge and review of the experimental data and results to provide confidence in the conclusions.

A strategic set of tests was designed to cover the range of assets represented across the Great Britain gas distribution networks. The assets used in the testing were mostly recovered from the distribution network as part of the ongoing Iron Mains Risk Reduction Replacement Programme. Controlled testing against a well-defined master testing plan with both natural gas and 100% hydrogen was then undertaken to provide the quantitative evidence to forecast any change to background leakage levels in a 100% hydrogen network.

Key Findings from Phase 1a:

• Of the 215 assets tested 41 of them were found to leak 19 of them provided sufficient data to be able to compare hydrogen and methane leak rates.
• The tests showed that assets that were gas tight on methane were also gas tight on hydrogen. Assets that leaked on hydrogen also leaked
• on methane including repaired assets.
• The ratio of the hydrogen to methane volumetric leak rates varied between 1.1 and 2.2 which is largely consistent with the bounding values expected for laminar and turbulent (or inertial) flow which gave ratios of 1.2 and 2.8 respectively.
• None of the PE assets leaked; cast ductile and spun iron leaked to a similar degree (around 26-29% of all iron assets leaked) and the proportion of leaking steel assets was slightly less (14%).
• Four types of joint were responsible for most of the leaks on joints: screwed lead yarn bolted gland and hook bolts.
• All of the repairs that sealed methane leaks also were effective when tested with hydrogen.

As the energy crisis becomes worse hydrogen as a clean energy source is more and more widely used in industrial production and people’s daily life. However there are hidden dangers in hydrogen storage and transportation because of its flammable and explosive features. Gas detection is the key to solving this problem. High quality sensors with more practical and commercial value must be able to accurately detect target gases in the environment. Emerging porous metal-organic framework (MOF) materials can effectively improve the selectivity of sensors as a result of high surface area and coordinated pore structure. The application of MOFs for surface modification to improve the selectivity and sensitivity of metal oxides sensors to hydrogen has been widely investigated. However the influence of MOF modified film thickness on the selectivity of hydrogen sensors is seldom studied. Moreover the mechanism of the selectivity improvement of the sensors with MOF modified film is still unclear. In this paper we prepared nano-sized ZnO particles by a homogeneous precipitation method. ZnO nanoparticle (NP) gas sensors were prepared by screen printing technology. Then a dense ZIF-8 film was grown on the surface of the gas sensor by hydrothermal synthesis. The morphology the composition of the elements and the characters of the product were analyzed by X-ray diffraction analysis (XRD) transmission electron microscope (TEM) scanning electron microscope (SEM) energy dispersive spectrometer (EDS) Brunauer-Emmett-Teller (BET) and differential scanning calorimetry (DSC). It is found that the ZIF-8 film grown for 4 h cannot form a dense core-shell structure. The thickness of ZIF-8 reaches 130 nm at 20 h. Through the detection and analysis of hydrogen (1000 ppm) ethanol (100 ppm) and acetone (50 ppm) from 150 °C to 290 °C it is found that the response of the ZnO@ZIF-8 sensors to hydrogen has been significantly improved while the response to ethanol and acetone was decreased. By comparing the change of the response coefficient when the thickness of ZIF-8 is 130 nm the gas sensor has a significantly improved selectivity to hydrogen at 230 °C. The continuous increase of the thickness tends to inhibit selectivity. The mechanism of selectivity improvement of the sensors with different thickness of the ZIF-8 films is discussed.

After nearly 25 years of service some of the wires of the tendons of “La Arena” bridge (Spain) started to exhibit the effects of environmental degradation processes. “La Arena” is cable-stayed bridge with 6 towers and a reference span between towers of about 100 meters. After a maintenance inspection of the bridge evidences of corrosion were detected in some of the galvanized wires of the cables. A more in-deep analysis of these wires revealed that many of them exhibited loss of section due to the corrosion process. In order to clarify the causes of this degradation event and to suggest some remedial actions an experimental program was designed. This program consisted of tensile and fatigue tests on some strand samples of the bridge together with a fractographic analysis of the fracture surfaces of the wires its galvanized layer thickness and some hydrogen measurements (hydrogen embrittlement could be another effect of the environmental degradation process).Once the type and extension of the flaws in the wires was characterized a structural integrity assessment of the strands was performed with the aim of quantifying the margins until failure and establishing some maintenance recommendations.

In situ electrochemical microcantilever bending tests were conducted in this study to investigate the role of grain boundaries (GBs) in hydrogen embrittlement (HE) of Alloy 725. Specimens were prepared under three different heat treatment conditions and denoted as solution-annealed (SA) aged (AG) and over-aged (OA) samples. For single-crystal beams in an H-containing environment all three heat-treated samples exhibited crack formation and propagation; however crack propagation was more severe in the OA sample. The anodic extraction of H presented similar results as those under the H-free condition indicating the reversibility of the H effect under the tested conditions. Bi-crystal micro-cantilevers bent under H-free and H-charged conditions revealed the significant role of the GB in the HE of the beams. The results indicated that the GB in the SA sample facilitated dislocation dissipation whereas for the OA sample it caused the retardation of crack propagation. For the AG sample testing in an H-containing environment led to the formation of a sharp severe crack along the GB path.