Effects of congestion and confining walls on turbulent deflagrations in a hydrogen storage facility-part 2: Numerical study

Safety studies for hydrogen retail stations involve identification of possible accidental scenarios, modelling of consequences and measures to mitigate associated hazards with it. Accidental release of hydrogen during its handling and storage can lead to formation of ignitable mixture in a very short time. Ignition of such a mixture can lead to generation of overpressure affecting structure and people. Understanding of the possible overpressures generated is critical in designing the system safe from explosion hazards. In the present study, the worst-case scenario where high-pressure hydrogen storage cylinders are enveloped by a premixed hydrogen-air cloud is numerically simulated. The computational domain mimics the setup for premixed hydrogen cloud in a mock hydrogen cylinder storage congestion environment experimentally studied by Shirvill et al. [1]. Large Eddy Simulations (LES) are performed using OpenFOAM CFD toolbox solver. The Flame Surface Wrinkling Model in LES context is used for modelling deflagrations [2]. Numerical simulation results are compared against experiments. Simulations are able to predict experimental flame arrival and overpressure reasonably well. The effects of ignition location, congestion and confining walls on the turbulent deflagrations in particular on explosion overpressure are discussed. It was concluded that explosion overpressure increases with increase in confinement.     

Safety studies on high-pressure hydrogen vehicle refuelling stations: Releases into a simulated high-pressure dispensing area

If the general public is to use hydrogen as a vehicle fuel, customers must be able to handle hydrogen with the same degree of confidence, and with comparable risk, as conventional liquid and gaseous fuels. The hazards associated with jet releases from leaks in a vehicle-refuelling environment must be considered if hydrogen is stored and used as a high-pressure gas since a jet release in a confined or congested area can create an explosion hazard. As there was insufficient knowledge of the explosion hazards, a study was initiated to gain a better understanding of the potential explosion hazard consequences associated with high-pressure leaks from hydrogen vehicle refuelling systems. This paper describes the experiments with a dummy vehicle and dispenser units to represent refuelling station congestion. Experiments with ignition of premixed 5.4 m × 6.0 m × 2.5 m hydrogen–air clouds and hydrogen jet releases up to 40 MPa (400 bar) pressure are described. The results are discussed in terms of the conditions leading to the greatest overpressures and overall conclusions are made from these.     

Experimental investigation and theoretical modeling of dehydriding process in high-pressure metal hydride hydrogen storage systems

This study explores the endothermic dehydriding (desorption) reaction that takes place in a high-pressure metal hydride (HPMH) hydrogen storage system when hydrogen gas is released to the fuel cell. The reaction is sustained by circulating warm fluid through a heat exchanger embedded in the HPMH powder. A systematic approach to modeling the dehydriding process is presented, which is validated against experimental data using two drastically different heat exchangers, one using a modular tube-fin design and the other a simpler coiled-tube design. Experiments were performed inside a 101.6-mm (4-in) diameter pressure vessel to investigate the influences of hydrogen release rate, heat exchanger fluid flow rate and fluid temperature on the dehydriding process for the HPMH Ti_1.1CrMn. It is shown the dehydriding reaction rate can be accelerated by increasing the fluid temperature and/or the rate of pressure drop. HPMH particles located in warmer locations close to heat exchanger surfaces both began and finished dehydriding earlier than particles farther away. 2-D and 3-D models were created in Fluent to assess the dehydriding performances of the modular tube-fin heat exchanger and coiled-tube heat exchanger, respectively. The models are shown to be quite accurate at predicting the spatial and temporal variations of metal hydride temperature during the dehydriding reaction.     

Simulating vented hydrogen deflagrations: Improved modelling in the CFD tool FLACS-hydrogen

This paper describes validation of the computational fluid dynamics tool FLACS-Hydrogen. The validation study focuses on concentration and pressure data from vented deflagration experiments performed in 20-foot shipping containers as part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA), funded by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU). The paper presents results for tests involving inhomogeneous hydrogen-air clouds generated from realistic releases performed during the HySEA project. For both experiments and simulations, the peak overpressures obtained for the stratified mixtures are higher than those measured for lean homogeneous mixtures with the same amount of hydrogen. Using an in-house version of FLACS-Hydrogen with the numerical solver Flacs3 and improved physics models results in significantly improved predictions of the peak overpressures, compared to the predictions by the standard Flacs2 solver. The paper includes suggestions for further improvements to the model system.     

Numerical study of hydrogen explosions in a refuelling environment and in a model storage room

Numerical simulations have been carried out for large scale hydrogen explosions in a refuelling environment and in a model storage room. For the first scenario, a high pressure hydrogen jet released in a congested refuelling environment was ignited and the subsequent explosion analysed. The computational domain mimics the experimental set up for a vertical downwards release in a vehicle refuelling environment experimentally tested by Shirvill et al. [6]. For completeness of the analysis, an analytical model has also been developed to provide the transient pressure conditions at nozzle exit. The numerical study is based on the traditional computational fluid dynamics (CFD) techniques solving Reynolds averaged Navier-Stokes equations. The Pseudo diameter approach is used to bypass the shock-laden flow structure in the immediate vicinity of the nozzle. For combustion, the Turbulent Flame Closure (TFC) model is used while the shear stress transport (SST) model is used for turbulence. In the second scenario, premixed hydrogen-air clouds with different hydrogen concentrations from 15% to 60% in volume were ignited in a model storage room. Analysis was carried out to derive the dependence of overpressure on hydrogen concentrations for safety considerations.     

Experimental study on hydrogen explosions in a full-scale hydrogen filling station model

In order for fuel cell vehicles to develop a widespread role in society, it is essential that hydrogen refuelling stations become established. For this to happen, there is a need to demonstrate the safety of the refuelling stations. The work described in this paper was carried out to provide experimental information on hydrogen outflow, dispersion and explosion behaviour. In the first phase, homogeneous hydrogen–air mixtures of a known concentration were introduced into an explosion chamber and the resulting flame speed and overpressures were measured. Hydrogen concentration was the dominant factor influencing the flame speed and overpressure. Secondly, high-pressure hydrogen releases were initiated in a storage room to study the accumulation of hydrogen. For a steady release with a constant driving pressure, the hydrogen concentration varied as the inlet airflow changed, depending on the ventilation area of the room, the external wind conditions and also the buoyancy induced flows generated by the accumulating hydrogen. Having obtained this basic data, the realistic dispersion and explosion experiments were executed at full-scale in the hydrogen station model. High-pressure hydrogen was released from 0.8 to 8.0 mm nozzle at the dispenser position and inside the storage room in the full-scale model of the refuelling station. Also the hydrogen releases were ignited to study the overpressures that can be generated by such releases. The results showed that overpressures that were generated following releases at the dispenser location had a clear correlation with the time of ignition, distance from ignition point.     

Viable storage of hydrogen in materials with off-board recharging using high-temperature electrolysis

Hydrogen storage in materials (hydrides) is challenging for vehicular applications, due to the large amounts of heat that must be removed in a short period of time during the filling process. It is proposed that rehydrogenation of the hydride instead is performed outside the vehicle, and linked to hydrogen production by water electrolysis at the filling station. The waste heat removed from the hydride during filling can be used to heat the electrolysis cell, increasing its efficiency. This solution can speedup the filling process, increase the efficiency of the hydrogen production, increase safety during filling, eliminate the need for massive transportation of hydrogen, and open the arena for many new hydrogen storage materials.     

Combined effect of ignition position and equivalence ratio on the characteristics of premixed hydrogen/air deflagrations

Premixed hydrogen/air deflagrations were performed in a 100 mm × 100 mm × 1000 mm square duct closed at one end and opened at the opposite end under ambient conditions, concerning with the combined effect of ignition position IP and equivalence ratio ?. A wide range of ? ranging from 0.4 to 5.0, as well as multiple IPs varying from 0 mm to 900 mm off the closed end of the duct were employed. It is indicated that IP and ? exerted a great impact on the flame structure, and the corresponding pressure built-up. Except for IP₀, the flame can propagate in two directions, i.e., leftward and rightward. A regime diagram for tulip flames formation on the left flame front (LFF) was given in a plane of ? vs. IP. In certain cases (e.g. the combinations of ? = 0.6 and IP₅₀₀ or IP₇₀₀), distorted tulip flames were also observed on the right flame front (RFF). Furthermore, the combinations of IP and ? gave rise to various patterns of pressure profiles. The pressure profiles for ignition initiated at the right half part of the duct showed a weak dependence on equivalence ratio, and showed no dependence on ignition position. However, the pressure profiles for ignition initiated at the left half part of the duct were heavily dependent on the combination of IP and ?. More specifically, in the leanest (? = 0.4) and the richest (? = 4.0–5.0) cases, intensive periodical oscillations were the prime feature of the pressure profiles. With the moderate equivalence ratios (? = 0.8–3.0), periodical pressure oscillations were only observed for IP₉₀₀. The maximum pressure peaks P_max were reached at ? = 1.25 rather than at the highest reactivity ? = 1.75 irrespective of ignition position. The ignition positions that produced the worst conditions were different, implying a complex influence of the combination of IP and ?.     

Experimental study of hydrogen storage with reaction heat recovery using metal hydride in a totalized hydrogen energy utilization system

Experimental results for hydrogen storage tanks with metal hydrides used for load leveling of electricity in commercial buildings are described. Variability in electricity demand due to air conditioning of commercial buildings necessitates installation of on-site energy storage. Here, we propose a totalized hydrogen energy utilization system (THEUS) as an on-site energy storage system, present feasibility test results for this system with a metal hydride tank, and discuss the energy efficiency of the system. This system uses a water electrolyzer to store electricity energy via hydrogen at night and uses fuel cells to generate power during the day. The system also utilizes the cold heat of reaction heat during the hydrogen desorption process for air conditioning. The storage tank has a shell-like structure and tube heat exchangers and contains 50 kg of metal hydride. Experimental conditions were specifically designed to regulate the pressure and temperature range. Absorption and desorption of 5,400 NL of hydrogen was successfully attained when the absorption rate was 10 NL/min and desorption rate was 6.9 NL/min. A 24-h cycle experiment emulating hydrogen generation at night and power generation during the day revealed that the system achieved a ratio of recovered thermal energy to the entire reaction heat of the hydrogen storage system of 43.2% without heat loss.     

Interferents for hydrogen storage on a graphene sheet decorated with nickel: A DFT study

In the present work, the decoration of a graphene sheet with nickel is considered as a hydrogen storage material by means of density functional theory calculations. A number of factors relevant for the handling and operation of this material were analyzed. This includes the interaction with potentially interfering chemicals, hydride formation and the hydrogen storage capacity. The present results show that unless the access of oxygen to the surface is restricted, its strong bond to the decorated systems will preclude the practical use for hydrogen storage. In the best case, the energy required to replace an adsorbed oxygen molecule by hydrogen is of the order of 1.7 eV, something that indicates the severe problem that the presence of oxygen represents for this type of systems.     

Theoretical study of the synthesis,characterization and hydrogen storage properties of a high-density hydrogen storage material: (CH3NH3)BH4

Inspired by both alkaline metal borohydrides and organic-inorganic hybrid perovskite, we predict a pair of complex structures of (CH₃NH₃)BH₄ with tremendous high hydrogen capacity (21.27 wt.%). Through comparison and analysis of the electronic structures of alkali metal atoms, CH₃NH₃, NH₄, and NH₃BH₃ molecule, it is concluded that similar spatial and electronic structures show the feasibility of synthesizing (CH₃NH₃)BH₄ by a substitution reaction. Firstly, theoretical structures (S₁ and S₂ in P₁) with stable configurations have been reconstructed by cation substitution followed by a series of restrictive structural optimizations, and both the lattice parameters and the position coordinate information of (CH₃NH₃)BH₄ are obtained. Ignoring the relatively mobile hydrogen, the structural symmetries of S₁ and S₂ are I4mm and P4/nmm, respectively. X-ray diffraction characterizations of S₁ and S₂ are consistent with the experimental results. Secondly, the calculated elastic constants of (CH₃NH₃)BH₄ (S₁ and S₂) with P₁ symmetry indicate that angles α, β and γ oscillate at right angles due to the influence of the cation orientation. The calculated spatial dependence of bulk (B), Young's (E), and shear (G) modulus obviously show that the two P₁ phases all have strong elastic anisotropy. Thirdly, the calculated electronic properties show that the protonic amine-H, hydridic borane-H, and neutral methane-H are widely distributed in (CH₃NH₃)BH₄, which allow for weaving in a planar dihydrogen bonding network, which in turn influences the dehydrogenation reaction. Last and most important, we propose the following dehydrogenation process of (CH₃NH₃)BH₄ via the intermediate compounds: 2(CH₃NH₃)BH₄ → CH₃NH₂BH₂NHCH₃BH₃+3H₂. For each dehydrogenation step, the free energy change is negative, which means (CH₃NH₃)BH₄ can decompose spontaneously, similar to ammonium borohydride, which is strongly related to the planar dihydrogen bonding network.     

Effects of turbulence on nonpremixed ignition of hydrogen in heated counterflow

Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet. In contrast to pseudo-turbulent flows, where turbulence was generated by only a perforated plate on the fuel side, resulting in little effect on ignition in a hydrogen system, fully turbulent flows with perforated plates on both sides of the flow were found to produce noticeable effects. The difference was attributed to the fact that in fully turbulent flows, a significantly larger range of turbulent eddies extend to smaller scales than in pseudo-turbulent flows. At atmospheric pressure, the lowest turbulence intensity studied had ignition temperatures notably lower than laminar ones, while further increases in turbulence intensity resulted in rising ignition temperatures. As a result, optimal conditions for nonpremixed hydrogen ignition exist in weakly turbulent flows where the ignition temperature is lower than can be obtained in other laminar or turbulent flows at the same pressure. Similar trends were seen for all fuel concentrations and at all pressures in the second ignition limit (below 3-4 atm). At higher pressures, turbulent flows caused the ignition temperatures to continue to follow the second limit resulting in ignition temperatures higher than the laminar values. The extension of the second limit ends at the highest pressures (7 to 8 atm) where evidence of third limit behavior appears. Three mechanisms were noted to explain the experimental results. First, turbulent eddies similar in size to the ignition kernel can promote discrete mixing of otherwise isolated pockets of gas. Second, this mixing can promote HO₂ chain branching pathways, which can account for the enhanced ignition noted in the second limit where reaction is governed by crossover temperature chemistry. Third, turbulence limits the excursion times available for reaction, inordinately affecting the slower HO₂ reactions. This is responsible for the increasing ignition temperature with turbulence intensity and pressure.     

A study on the hydrogen storage capacity of Ni-plated porous carbon nanofibers

In this study, we prepared highly porous carbon-nanofiber-supported nickel nanoparticles as a promising material for hydrogen storage. The porous carbons were activated at 1050 °C, and the nickel nanoparticles were loaded by an electroless metal-plating method. The textural properties of the porous carbon nanofibers were analyzed using N₂/77 K adsorption isotherms. The hydrogen storage capacity of the carbons was evaluated at 298 K and 100 bar. It was found that the amount of hydrogen stored was enhanced by increasing nickel content, showing 2.2 wt.% in the PCNF-Ni-40 sample (5.1 wt.% and 6.4% of nickel content and dispersion rate, respectively) owing to the effects of the spill-over of hydrogen molecules onto the metal–carbon interfaces. This result clearly indicates that the presence of highly dispersed nickel particles can enhance high-capacity hydrogen storage.     

Experimental and modeling study on auto-ignition characteristics of methane/hydrogen blends under engine relevant pressure

Auto-ignition characteristics of methane/hydrogen mixtures with hydrogen mole fraction varying from 0 to 100% were experimentally studied using a shock tube. Test pressure is kept 1.8 MPa and temperatures behind reflected shock waves are in the range of 900–1750 K and equivalence ratios from 0.5 to 2.0. Three ignition regimes are identified according to hydrogen fraction. They are, methane chemistry dominating ignition (XH₂≤40%)$\left(X_{{\text{H}}_2}\le 40\mathrm{\%}\right)$, combined chemistry of methane and hydrogen dominating ignition (XH₂=60%)$\left(X_{{\text{H}}_2}=60\mathrm{\%}\right)$, and hydrogen chemistry dominating ignition (XH₂≥80%)$\left(X_{{\text{H}}_2}\ge 80\mathrm{\%}\right)$. Simulated ignition delays using four models including USC Mech 2.0, GRI Mech 3.0, UBC Mech 2.1 and NUI Galway Mech were compared to the experimental data. Results show that USC Mech 2.0 gives the best prediction on ignition delays and it was selected to conduct sensitivity analysis for three typical methane/hydrogen mixtures at different temperatures. The results suggest that at high temperature, ignition delay mainly is governed by chain branching reaction H + O₂ ⇔ OH + O, and thus increasing equivalence ratio inhibits ignition of methane/hydrogen mixtures. At middle-low temperature, contribution of equivalence ratio on ignition delay of methane/hydrogen mixtures is mainly due to chemistries of HO₂ and H₂O₂ radicals.     

A study on crystal structure and chemical state of TiCrVMn hydrogen storage alloys during hydrogen absorption-desorption cycling

The evolution of crystal structure and chemical state of the V-based hydrogen storage alloy (Ti_0.32Cr_0.46V_0.22)₉₆Mn₄ during hydrogen absorption/desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Reasons for the degradation in cycling capacity of the alloy are presented and discussed. One reason is the continuous reduction of the V-based cell volume during cycling, which cannot hold further hydrogen atoms. The decrease in cycling capacity can also be attributed to the oxidation of Ti, V, and Cr elements during cycling.     

Experimental investigation of hydrogen storage in single walled carbon nanotubes functionalized with borane

Single walled carbon nanotubes (SWCNTs) dispersed in 2-propanol are deposited on the alumina substrate using drop cast method. The deposited SWCNTs are characterized using the techniques SEM, EDS and FTIR. Then the SWCNTs are functionalized with BH₃ using LiBH₄ as the precursor. FTIR, XPS and CHNS techniques are used to confirm the functionalization. The functional groups are identified from FTIR studies. The various elements present in the functionalized SWCNTs are identified from XPS and CHNS studies. The functionalized samples are hydrogenated and the hydrogen storage capacity of these samples is estimated using CHNS studies.     

First-principles study of hydrogen storage in metal functionalized [4,4]paracyclophane

Storing hydrogen for commercial purpose with high gravimetric density is a major task. Li and Sc are functionalized over delocalized π electrons of [4,4]paracyclophane to explore reversible hydrogen storing capacity. Electronic structure calculations are performed with Minnesota 06 hybrid functional and 6-311G(d,p) basis set. [4,4]paracyclophane binds strongly to Sc showing Dewar coordination. Sc functionalized [4,4]paracyclophane complex has a capacity of holding 10 H₂ molecules while Li functionalized complex holds 8 H₂ molecules with hydrogen weight percentage of 11.8% and 13.7% respectively. Conceptual DFT parameters namely hardness and electrophilicity confirm the high stability of the complexes. Atom Density Matrix Propagation simulations at various temperatures and their desorption pattern indicate reversibility of adsorbed hydrogens. The study confirms the potential of Sc functionalized [4,4]paracyclophanes as a hydrogen storage material.     

Effects of mechanical milling on hydrogen storage properties of alloy

The hydrogen storage alloy of Ti_0.32Cr_0.43V_0.25 was prepared by arc melting and high energy ball milling. Effects of ball milling were studied for various time periods (30–300 min) at 200 rpm. The hydrogen storage capacity of the alloy decreased with the increase in milling time. The reasons for the drop in the hydrogen storage capacity are twofold: surface contamination of the alloy powder and the microstructural changes. The latter includes the increase in lattice strain, the decrease in crystallite size and the consequent increase in subgrain boundaries. Despite the microstructural changes, the BCC phase of the alloy was maintained and its lattice constant remained nearly the same.