Combustion behaviors and explosibility of suspended metal hydride TiH2 dust

In the production and storage processes of metal hydride material of TiH₂, there are at least three kinds of explosion hazards, for example, TiH₂ dust explosion, H₂ explosion and hybrid H₂/TiH₂ dust explosion. In this study, combustion behaviors of TiH₂ dust cloud under isobaric and isochoric conditions were studied using a visual dust combustion facility and a standard 20-L spherical explosion vessel bomb, respectively, and Ti dust and hybrid H₂/Ti dust were used as the reference materials. Experimental results showed that at equal dust concentrations, the flame propagation speed S_f, burning velocity S_L, maximum pressure rise P_ex and maximum rate of pressure rise (dP/dt)_ex of TiH₂ dust were all higher than those of Ti dust, while much smaller than those of hybrid H₂/Ti dust except the maximum pressure rise P_ex. The hydrogen state and content were the primary factors for the combustion differences of dust explosions. The values of explosion index K_st showed that the explosion risks of these samples increased as follows: Ti ˂ TiH₂ ˂ hybrid H₂/Ti dust.     

Ammonia suppression during decomposition of sodium amide by the addition of metal hydride

The decomposition of NaNH₂ has been reported, mainly decomposing into NaH, N₂ and H₂. Ammonia is also produced in addition to N₂ and H₂. To the best of our knowledge, very few scattered reports on the effect of alkali hydrides on NaNH₂ exist in literature. Thus, we choose NaNH₂–MH (M = Li, Na, K, Mg, Ca) system to be investigated in detail. Since NaNH₂–NaH is the simplest combination due to same cation, it was tested for the establishment of reaction mechanism using transmission electron microscopy (TEM). It is observed that the entire process follows NH₃ mediated reaction similar to LiNH₂–LiH system. Sodium amide first decompose into Na metal and NH₃, then generated NH₃ reacted with added NaH to form NaNH₂ and release H₂. This process continues until the consumption of NaH, thus suppresses NH₃ evolution to a great extent. The investigation has been extended further to the other metal hydrides and it is found that the addition of other metal hydride i.e. LiH, KH, MgH₂, and CaH₂ have also effectively suppressed the NH₃ evolution. The detailed reaction mechanism has been elucidated for all the amide hydride systems. It is observed that the decomposition takes place through an intermediate step of double-cation amide formation.     

Modelling and experimental results of heat transfer in a metal hydride store during hydrogen charge and discharge

A numerical model for the transient hydrogen charge/discharge rates and thermal behaviour of metal hydride stores was developed and verified against experiments using a cylindrical reactor filled with AB₅-type metal hydride. The model assumes local thermal equilibrium between the gas and solid phases, and incorporates the pressure and temperature-dependent hydrogen reaction rates, as well as heat transfer in the porous metal hydride bed. The model was verified through experimental data. The experiments were performed using a unit with hydrogen storage capacity of 130 Nl H₂; the store was submerged in an isothermal water bath. Experiments at different water bath temperatures and charge/discharge hydrogen pressures indicated a relation between charge/discharge time and these parameters. The reactor's ability to deliver a constant hydrogen flow at different water bath temperatures was experimentally investigated. During simulations it was found that the model applied is sensitive to perturbations of some of its parameters; activation energy of absorption, effective conductivity and heat of reaction were found to be the most important ones. The charge and discharge performances of the store are controlled by the reaction rate in the first half-part of the H absorption/desorption experiments and by a heat transfer in the second half-part of charge/discharge.     

Experimental study of a metal hydride vessel based on a finned spiral heat exchanger

The reaction time of hydrogen in metal hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure, and hydrogen tank volume) has been discussed and obtained results show that a good choice of these parameters is important.     

Studies of off-stoichiometric AB2 metal hydride alloy: Part 1. Structural characteristics

In Part 1 of this two-part series of papers, phase abundances, lattice parameters, crystallite sizes, and microstructures of three series of AB₂-based metal hydride alloys were studied. The base alloys with B/A stoichiometry of 2.0 in series 177, 190, and 193 are rich in C14, equal in C14/C15, and rich in C15 phases, respectively. In each series of alloys, the B/A stoichiometry varies from 1.8, 1.9, 2.0, 2.1, to 2.2. The effects of varying B/A stoichiometry to microstructures are the same for these three series of alloys. As the alloy formula changes from AB_1.8, AB_1.9, AB_2.0, AB_2.1, to AB_2.2, the following events occur: C14-to-C15 phase ratio decreases, both C14 and TiNi secondary phase lattice parameters and unit cell volume reduce; the a/c aspect ratio of C14 phase first decreases and then increases; abundances of non-Laves secondary phases decrease; and the Zr/Ti ratio in AB phase decreases. The C14/C15 ratio is closely related to the average electron density with a threshold that first decreases from 7.13 (AB_1.8) to 7.08 (AB_1.9) and to 7.06 (AB_2.0) and then increases to 7.08 (AB_2.1) and 7.09 (AB_2.2) as the stoichiometry increases. The distributions of B-site elements are not uniform with most of the V, Cr, Mn, Co residing in AB₂ phase and Sn in Zr₇Ni₁₀ phase.     

Improvement in the electrochemical properties of gas atomized AB2 metal hydride alloys by hydrogen annealing

The effects of high temperature hydrogen annealing were studied on powders made by gas atomization of both conventional vanadium-containing AB₂ metal hydride alloys and new vanadium-free AB₂ alloys designed for high power and low self-discharge applications. In both alloy systems, annealing in 950 °C hydrogen for 30 min was proven to be effective in improving the capacity, formation, high power, and low temperature performance in the nickel metal hydride battery compared to previous gas atomization trials where each property was reduced. The advantage in improving the cycle life by gas atomization was further extended by the hydrogen annealing process. Reduction in the surface oxide was confirmed by the use of Auger electron spectroscopy depth profiling and magnetic susceptibility. Metallic elements were reduced from the oxide state by hydrogen to react with the metallic nickel particulates originally embedded in the surface oxide in a high temperature environment and created a new surface free of oxygen.     

Effect of Ti and Al on the pulverization resistance of MgNi-based metal hydride electrodes evaluated by acoustic emission

The pulverization of amorphous MgNi, Mg_0.9Ti_0.1Ni and Mg_0.9Ti_0.1NiAl_0.05 materials prepared by mechanical alloying was evaluated by acoustic emission (AE) measurements during their electrochemical hydriding. It was confirmed that the pulverization of MgNi-based electrode occurs mostly at the end of the charge when the hydrogen evolution reaction is initiated. On the basis of the AE activity measurements, the positive influence of the Ti substitution and Al addition on the MgNi-based alloy pulverization resistance is demonstrated. This is mainly attributed to the lower porosity of these materials, limiting the accumulation of H₂ bubbles in the agglomerate pores. Combined with a better corrosion resistance, this characteristic produces an improvement of the cycle life of the MH electrodes. It was also shown that the AE energy distribution can be described by the Gutenberg–Richter relationship. The AE energy is higher for cracking related to the volume expansion than for the pulverization associated with the formation of H₂ bubbles in the agglomerate pores. Ti substitution and Al addition also induce a larger proportion of high-energy AE signals.     

Numerical investigation of hydrogen absorption in a stackable metal hydride reactor utilizing compartmentalization

In this paper, a three-dimensional model for hydrogen absorption in a metal alloy has been developed, validated against the experimental data in the literature, and then applied to a novel design for a hydrogen storage unit. The proposed design is similar to the fuel cell stack, but here the Membrane Electrode Assembly (MEA) has been replaced by a metal hydride (MH) reactor placed between the flow-field plates. These are stacked together to achieve the required amount of hydrogen storage. The flow-field plates have channels engraved on one side for hydrogen supply and on the other, for coolant/heating medium. It is known that the effectiveness of a hydrogen storage unit is directly related to its heat transfer area, and therefore, the choice of its geometry is very important. The larger the size, the more the resistance to heat transfer. Although, the internal tubular heat exchangers have proven to be effective in heat transfer, they pose severe challenges such as cooling/heating medium leakage due to tube erosion, stresses generated, etc. and they displace the active metal hydride from the tank. The present stacked MH reactor configuration helps to overcome these challenges by stacking small MH reactors together and there is no chance of the cooling/heating medium leaking into the metal hydride. Numerical simulations were performed to investigate the effect of coolant flow rate and percentage of flow-field plate rib area exposed to the MH reactor on temperature evolution and the amount of hydrogen stored. Further, a detailed study was carried out to understand the effect of compartmentalization of the MH reactor on temperature distribution. The results revealed that compartmentalization substantially helps to uniformly distribute the temperature in the metal bed, which is very important to maintain uniform utilization of the metal powder. Consequently, the uniform metal powder density for repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities.     

Study on absorption/desorption characteristics of a metal hydride tank for boil-off gas from liquid hydrogen

We have been performing research on the Totalized Hydrogen Energy Utilization System (THEUS) which has applications to commercial buildings and a planned added function of supplying energy to stations for hydrogen and electric vehicles. In that case we will utilize liquid hydrogen transported from a hydrogen station and all Boil-Off Gas (BOG) will be recovered in THEUS’s metal hydride tanks. It is known that BOG is chiefly composed of para-hydrogen, which has different thermo-physical properties from normal hydrogen. It has been reported that some metal hydride alloys work as a catalyst to accelerate the para-ortho conversion and the conversion proceeds relatively fast in the case of La–Ni₅. The conversion is considered to be an endothermic reaction. A misch metal (Mm)-Ni₅ metal hydride alloy, which contained La and Ni, was used in our THEUS metal hydride tank. To examine the effect of the para-ortho conversion on the THEUS operation, we investigated the absorption/desorption characteristics of the metal hydride tank with BOG. We confirmed that the effect of the heat of conversion was very small and BOG could be treated as normal hydrogen for practical application.     

Studies of off-stoichiometric AB2 metal hydride alloy: Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two-part series of papers, gaseous hydrogen storage and electrochemical properties of three series of alloys with different combinations of Cr/Mn/Co ratios are studied and compared to the structural properties reported in Part 1. As the B/A stoichiometry in each series of alloys increases from 1.8 to 2.2, systematic trends in certain storage properties are found: the hydrogen dissociation pressure and heat of hydride formation increases; the alloy with a AB_2.0 stoichiometry has the highest electrochemical full capacity; and slightly higher and lower B-contents increase the electrochemical high-rate-dischargeability and gaseous phase maximum storage capacity, respectively. Stoichiometric or slightly hyper-stoichiometric AB₂ alloys have lower PCT hysteresis which are expected to reduce pulverization during cycling. The full and high-rate discharge electrochemical capacities correlate well with the maximum and reversible gaseous hydrogen storages, respectively. Slight hyper-stoichiometry increases the high-rate dischargeability. Open circuit voltage, an important parameter in high-power application, is also found to be more relevant to the surface reaction than to the bulk hydride stability.     

Mathematical modeling,numerical simulation and experimental comparison of the desorption process in a metal hydride hydrogen storage system

A two-dimensional axisymmetric model is developed to study the hydrogen desorption reaction and its subsequent discharge in a metal hydride canister. Experimental tests are performed on an in-house fabricated setup. An extensive study on the effects of the metal properties and boundary conditions on discharging performance is carried out through non-destructive testing (NDT). Results show that the desorption process is more effective if the activation energy for desorption (E_d) and the reaction enthalpy (ΔH) decrease, and when the desorption rate coefficient (C_d) and the external convection heat transfer coefficient when the bottle is being heated (h) increase. Furthermore, porosity (ε) can be useful for the design of hydrogen storage systems, with a trade-off between charge/discharge time and storage capacity. Numerical and experimental results are compared achieving a good agreement. These results can be used to select metal hydride materials and also for the future evaluation of metal hydride degradation.     

In-situ study of the cracking of metal hydride electrodes by acoustic emission technique

Pulverisation phenomena occurring during the charge/discharge cycling of metal hydride materials were studied by acoustic emission coupled to electrochemical measurements. Two kinds of materials were studied: a commercial LaNi₅-based alloy and a ball-milled MgNi alloy. In both alloys, two populations of acoustic signals were detected during charging steps: P1, showing peak frequencies between 230 and 260 kHz, high energy and low rise time, and P2 with peak frequencies between 150 and 180 kHz, lower energy and longer rise time. Population P2 is related to the hydrogen evolution reaction whereas P1 is associated with pulverisation phenomena. No acoustic activity was detected during discharge. We also investigated pulverisation phenomena through cycles by monitoring the P1 population. It appears that pulverisation occurs mainly during the five first cycles for LaNi₅ with a maximum at the second cycle, while pulverisation takes place all along the cycling for MgNi, but at a decreasing rate. By comparing the P1 activities, it appears that the pulverization phenomenon is less intensive on the MgNi electrode than on the LaNi₅-based electrode.     

Effects of lithium addition in AB2 metal hydride alloy by solid-state diffusion

In this study, microstructures and electrochemical properties of a Zr-based AB₂-type metal hydride alloy (Ti_12.1Zr_21.3V_10.5Cr_7.5Mn_8.1Co_8.0Ni_31.7Al_0.8) doped with 6 wt% LiH by solid-state diffusion at different temperatures were investigated and compared with those of the base alloy sintered at the same conditions. Structural study by x-ray diffraction analysis exhibited a lattice expansion in the main C14 phase of the Li-doped alloys with a relatively low sintering temperature, indicating that Li incorporated into the C14 Laves phase but was evaporated at higher sintering temperatures. Addition of Li deteriorated both the electrochemical capacity and activation easiness; however, it improved the high-rate dischargeability (HRD). The benefit of Li addition on HRD can be associated with the increase in amount of metallic nickel clusters embedded in the surface oxide (estimated by the saturated magnetic susceptibility) and was a strong function of sintering temperature. As the sintering temperature increased, HRD increased due to the increase in amount of surface catalytic metallic nickel.     

Inhibition of hydrogen production reactions in the wet dust removal system using CeCl3 solutions

For treatment of aluminum dust, a wet dust removal system has been used worldwide. During treatment, aluminum dust is inhaled into a water tank of the dust collector. As hydrogen production reactions are likely to take place in the water tank, there exists a great risk of fire or explosion accidents associated with the wet dust removal system. Based on field research and laboratory experiments, Hydrogen Inhibition Method (HIM) by using CeCl₃ solutions was proved capable of inhibiting reactions between aluminum dust and water. When the concentration of CeCl₃ solutions reached 6.02 g/L, there was basically no hydrogen gas produced. SEM, EDS and XPS characterizations were used to assess the aluminum particles before and after being reacted with water or CeCl₃ solutions, respectively. Shrinking core model was utilized to identify the corresponding chemical reaction kinetics. Additionally, a physicochemical mechanism was established to explain these phenomena.     

Role of heat pipes in improving the hydrogen charging rate in a metal hydride storage tank

Metal hydrides can store hydrogen at high volumetric efficiencies. As the process of charging hydrogen into a metal powder to form its hydride is exothermic, the heat released must be removed quickly to maintain a rapid charging rate. An effective heat removal method is to incorporate a heat exchanger such as a heat pipe within the metal hydride bed. In this paper, we describe a two-dimensional numerical study to predict the transient heat and mass transfer in a cylindrical metal hydride tank embedded with one or more heat pipes. Results from a parametric study of hydrogen storage efficiency are presented as a function of storage tank size, water jacket temperature and its convective heat transfer coefficient, and heat pipe radius and its convective heat transfer coefficient. The effect of enhancing the thermal conductivity of the metal hydride by adding aluminum foam is also investigated. The study reveals that the cooling water jacket temperature and the heat pipe's heat transfer coefficient are most influential in determining the heat removal rate. The addition of aluminum foam reduces the filling time as expected. For larger tanks, more than one heat pipe is necessary for rapid charging. It was found that using more heat pipes of smaller radii is better than using fewer heat pipes with larger radii. The optimal distribution of multiple heat pipes was also determined and it is shown that their relative position within the tank scales with the tank size.     

The structure, hydrogen storage, and electrochemical properties of Fe-doped C14-predominating AB2 metal hydride alloys

A series of Fe-substituting cobalt C14-predoninating AB₂ alloys with the general formula Ti₁₂Zr_21.5V₁₀Cr_7.5Mn_8.1Fe_xCo_8−xNi_32.2Sn_0.3Al_0.4 (x = 0-5) were studied for the impacts of Fe to structure, gaseous, and electrochemical hydrogen storage properties. All alloys exhibit hyper-stoichiometric C14 main phase due to the formation of A-rich non-Laves secondary phases and the loss of Zr and Ti in the melt. Lattice parameters together with the unit cell volume increases and then decreases with increasing Fe-content which indicates the existence of anti-site defects. The amount of TiNi secondary phase increases with the increase of Fe-content up to 4% and shows a detrimental effect to the high-rate dischargeability of the alloys. Most of the gaseous storage characteristics remain unchanged with the addition of Fe. In the electrochemical properties, Fe-addition in the AB₂ alloys facilitates activation, increases the total electrochemical capacity and effective surface reaction area, decreases the half-cell high-rate dischargeability and bulk hydrogen diffusion, and deteriorates both −10 and −40 °C low-temperature performance. Fe-substituting Co in AB₂ alloys as negative electrode of nickel metal hydride battery can reduce the raw material cost with the trade-off being mainly in the low-temperature performance.     

Predictive calculation of the effective thermal conductivity in a metal hydride packed bed

The effective thermal conductivity of a metal hydride packed bed was calculated by considering the influence of expansion during hydrogen absorption and contraction during hydrogen desorption. The porosity was calculated using an experimental formula developed by direct observation, which was used in combination with other referenced methods. However, none of the methods could express the reported experimental value response to pressure change using only the experimental porosity formula. The area contact model was modified so that the porosity and the contact area changed with expansion and contraction. The contact area change was calculated by assuming a simple geometrical deformation caused the difference between the particle expansion and the packed bed expansion. The calculation results of the improved area contact model with the deformed factor and the shape factor were in good agreement with the reported experimental data. This calculation method of the effective thermal conductivity with the influence of expansion and contraction is expected to be useful for designing of heat transfer enhancement of a hydrogen storage tank.     

In situ formation of Ti hydride and its catalytic effect in doped NaAlH4 prepared by milling NaH/Al with metallic Ti powder

X-ray diffraction (XRD) analysis revealed that nanocrystalline Ti hydride(s) was in situ formed during mechanical milling 1:1 NaH/Al mixture with metallic Ti powder. The composition of the formed Ti hydrides varies slightly upon changing the milling atmosphere from inert Ar to reactive H₂. Directly doping sodium alanate with commercial Ti hydride was found to result in similar dehydriding kinetics, hydrogen capacity, and cycling stability to those of the samples doped with metallic Ti. Moreover, according to the XRD results, the Ti hydride(s) remains stable in the de-/hydrogenation cycles. On the basis of these results, a discussion regarding the nature of catalytically active species was given.     

Study on the influence of Mg content on the risk of hydrogen production from waste alloy dust in wet dust collector

We use a proprietary automatic Al–Mg alloy–water reaction test apparatus to compare the hydrogen evolution profiles of Al-xMg (x = 10%,20%) with different particle sizes, characterize the waste Al-xMg alloy dust particles before and after reaction through SEM, EDS, and XRD, and present a three-stage four-step hydrogen evolution model of Al-xMg (x ≤ 35%) alloy dust particles. It is discovered that the reaction of the Al–Mg alloy in water is a hydrogen evolution–adsorption–slow diffusion process. The particular β-Al₃Mg₂ in Al-xMg (x ≤ 35%) will adsorb the resulting hydrogen to form MgH⁺ and adhere to the surface of the particles. As the Mg content in the alloy increases, the hydrogen evolution reduces. The entire process lasts around 5–6 h, with maximum hydrogen conversion rate of 54% (Al–10%Mg, d (50) = 12 μm, α = 0.544). Our hydrogen evolution model provides very useful theoretical references for avoiding hydrogen explosion in Al–Mg alloy manufacturing facilities.     

Numerical simulation of the hydrogen storage with reaction heat recovery using metal hydride in the totalized hydrogen energy utilization system

Numerical simulation of a hydrogen storage tank of a Totalized Hydrogen Energy Utilization System (THEUS) for application to commercial buildings was done to verify the practicality of THEUS. THEUS consists of a fuel cell, water electrolyzer, hydrogen storage tank and their auxiliary machinery. The hydrogen storage tanks with metal hydrides for load leveling have been previously experimentally investigated as an important element of THEUS. A hydrogen storage tank with 50 kg AB5 type metal hydride was assembled to investigate the hydrogen-absorbing/desorbing process, which is exothermic/endothermic process. The goal of this tank is to recover the cold heat of the endothermic process for air conditioning, and thus improve the efficiency of THEUS. To verify the practical effectiveness of this improved system, we developed a numerical simulation code of hydrogen storage tank with metal hydride. The code was validated by comparing its results with experimental results. In this code the specific heat value of the upper and lower flanges of the hydrogen storage tank was adjusted to be equal to the thermal capacity of the entire tank. The simulation results reproduce well the experimental results.