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Experimental Determination of Dust Cloud Combustion Parameters of Alpha-AlH3 Powder in its Charged and Fully Discharged States for H2 Storage Applications
... Aluminum hydride (AlH 3 ), a solid-state hydrogen storage material, is considered to be the most candidate material for hydrogen storage with the advantages of relatively safe [2], high volumetric density (148 kg/m 3 ) [4], and low specific volumetric storage cost ($125/m 3 ) [5]. As an ideal solid hydrogen storage material, AlH 3 is utilized in fuel cells for mobile applications [6]. It is also used in propellants and explosives, increasing energy output and reducing combustion temperature [7,8]. ...
... The high dispersion of the AlH 3 particles is maintained by the release of hydrogen, accelerating the combustion rate [19]. Khalil et al. [6] revealed that the residual Al dust after hydrogen evolution had higher reactivity and higher explosion hazard compared to AlH 3 dust. The induction time of the AlH 3 dust explosion is shorter than that of the Al dust, the addition of AlH 3 dust can significantly shorten the induction time of the Al/AlH 3 /air mixture [20]. ...
... In his quantitative risk assessment (QRA) of on-board hydrogen storage in LD-FCV, Khalil (2013a;2013b;2013c;2015;2016a) postulated an accident scenario in which the hydride storage vessel ruptures as a result of an accidental vehicular collision leading to ex-vessel spewing (dispersal) of some of the enclosed hydride powder in ambient air. To estimate a realistic probability for potential dispersal of hydride dust in air, a subscale test rig has been designed and fabricated to mimic fast depressurization (blowdown) of a 15-ml stainless steel vessel containing 30 grams of NaAlH4 power (Khalil, 2010b;2011a;2011b). ...
The objective of this research is to examine the safety-related characteristics of candidate hydrogen storage materials being considered for use in light-duty fuel-cell vehicles (LD-FCV) under the U.S. Department of Energy (DOE) Hydrogen Program. This research aims to provide useful meaning to the general DOE safety target by establishing a link between the safety-related characteristics of candidate storage materials and satisfaction of DOE safety target. Accordingly, a science-based framework has been developed and consists of standardized materials tests (based on internationally accepted ASTM and United Nations testing protocols), novel risk mitigation strategies, and subscale system demonstration. The examined storage materials include NaAlH4, AlH3, 2LiBH4 + MgH2, 3Mg(NH2)2.8LiH, NH3BH3, and activated carbon (Maxsorb AX-21). The scope of safety tests covers conditions that the storage material may encounter during postulated accident scenarios such as dust cloud explosion, materials reactivity in air and other fluids, hot-surface contact, mechanical impact, and fast depressurization. The generated results uncovered potential fire and explosion risks under accidental conditions. The generated insights can be useful for assigning realistic probability values needed for quantifying risk scenarios, characterizing material’s hazard class, and supporting current and new hydrogen safety codes and standards. For risk mitigation, this study showed that powder compaction could be effective in suppressing pyrophoricity of hydride powders such as NaAlH4. Also, the study has experimentally demonstrated that adding (NH4)H2PO4 as a flame retardant to the hydride powder before compaction could suppress sensitivity of hydrides like NaAlH4 to ignite due to mechanical impact. The results also revealed that Maxsorb AX-21 to be a safer hydrogen storage medium compared to the examined hydrides which exhibited potential safety concerns under certain accident conditions.
This technical report on hydrogen safety (Task 37) of the Technical Collaboration Program (TCP), Internal Energy Agency (IEA) is comprised of five chapters as follows:
Chapter 1: Task goal and objectives
Chapter 2: Task member countries and organizational structure
Chapter 3: Task main achievements, deliverables, and safety knowledge dissemination
Chapter 4: Hydrogen safety and the path forward
Chapter 5: Task 37 key messages and recommendations
This technical report also has 3 appendices as follows:
Appendix A: Task 37 website
Appendix B: Hydrogen Safety Journal
Appendix C: Examples of Task 37 annual meetings
Task 37 provided the following key messages and recommendations for future hydrogen R&D activities:
1) Over its six-years duration, Task 37 provided key quantitative risk insights (both physics-based and probabilistic) to support the development of both new and revised hydrogen safety C&S (e.g., NFPA-2 and ISO standards).
2) Dr. Khalil emphasizes the importance of expanding the current scope of hydrogen safety beyond the hydrogen-powered light-duty electric vehicles (LDEV) application and H2 refueling stations. To this end, Dr. Khalil recommends expanding the scope of H2 safety to other applications such as maritime, commercial aviation (hybrid-electric & all-electric aircraft), power-to-gas (P2G), heavy-duty vehicles, trains, and H2 transport in long road tunnels and other confined-spaces such as garages. Accordingly, additional R&D efforts are needed to ensure the safety of emerging hydrogen-based technologies and associated infrastructures.
3) Bulk storage of hydrogen (whether as compressed gas or liquified) would require investigation of novel tanks design, materials selection, more robust risk mitigation and control methods, and H2 leakage detection devices.
4) Comprehensive safety-related research efforts are needed to address materials-compatibility issues associated with hydrogen.
5) Improved understanding of safety issues is needed with respect to separation distances (aka, setback distances or safety distances), underground and above-ground hydrogen storage, leakage of hydrogen from transport pipelines, injecting hydrogen gas in existing natural gas networks, risks associated with blending hydrogen with natural gas for domestic heating, etc.
6) Hydrogen production, transport & distribution, supply chains safety risks, infrastructure physical security and vulnerability assessment, as well as safety codes & standards continue to be central issues for achieving the desired economies of scale and reliable adoption of hydrogen-based technologies.
7) There is a need for harmonizing hydrogen safety codes & standards to remove (or at least lower) unnecessary regulatory barriers and to establish common standards that ensure safety in each stage of the hydrogen value chain. Achieving this goal will accelerate the deployment of at-scale hydrogen-based technologies.
As a concluding remark on what Task 37 of the Hydrogen Technical Collaboration Program (H2 TCP) has accomplished over the past six years, Dr. Khalil emphasizes that: We must continue to seize all emerging opportunities to demonstrate, via science-based methods, the safety of hydrogen-based technologies.
The author of this technical report is Dr. Y. F. Khalil, the Operating Agent (OA) and Manager of the Hydrogen Safety Task 37 (which started in January 2015 and ended in December 2021) of the International Energy Agency (IEA) headquartered in Paris, France.
Dr. Yehia F. Khalil delivered an invited lecture at the UK Research and Innovation - Hydrogen Experts Meeting, Department of Engineering Science, University of Oxford, UK. The meeting was held during the period May 22 – 23, 2018.
In this lecture, Dr. Khalil presented his portfolio of R&D related to sustainable hydrogen production, life cycle assessment (LCA), and life cycle impact assessment (LCIA). He also provided an overview of the Task 37 (Hydrogen Safety) which he manages for the International Energy Agency (IEA), Hydrogen Technical Collaboration Program (H2 TCP).
In this program, Dr. Khalil manages a portfolio of hydrogen R&D conducted by a group of H2 safety experts from several countries around the world including UK, France, Norway, Denmark, U.S., Canada, Japan, and China.
This technical report highlights key technical achievement, results, deliverables, and hydrogen safety knowledge dissemination of Task 37 (Hydrogen Safety) managed by Dr. Yehia F. Khalil for the International Energy Agency (IEA), Hydrogen Technical Collaboration Program (H2 TCP).
Aluminum hydride (AlH3) is a covalently bonded trihydride with a high gravimetric (10.1 wt%) and volumetric (148 kg·m−3) hydrogen capacity. AlH3 decomposes to Al and H2 rapidly at relatively low temperatures, indicating good hydrogen desorption kinetics at ambient temperature. Therefore, AlH3 is one of the most prospective candidates for high-capacity hydrogen storage materials. Firstly, this review briefly summarizes the basic chemical and physical characteristics of AlH3. Then, its synthesis, dehydrogenation thermodynamics and kinetics, regeneration and methods for improving reversibility of hydriding are described with the aim of applying this material for hydrogen storage. In accordance with the fact that AlH3 is generally formed by reacting Al with H2 at extremely high hydrogen pressure, the high-pressure study of this hydride is discussed in detail. Finally, the advantages, weaknesses, critical technical challenges and outlook of this field are discussed. 铝氢化物(AlH3)是一种三共价键的氢化物, 具有较高的质量(10.1 wt%)和体积(148 kg H2•m-3)储氢容量。在相对温和的温度下, AlH3可快速分解为铝和氢气, 具有良好的放氢动力学。因此, AlH3是最有前景的高容量储氢材料之一。本文首先简要概述了AlH3的基本化学和物理特性。然后介绍了该材料的合成、脱氢热力学和动力学、再生循环以及提高吸放氢可逆性的方法。因为由Al与H2反应生成AlH3需要极高的氢气压力, 本文详细讨论了AlH3的高压研究。最后, 讨论了AlH3研究领域中的优势、劣势、关键技术挑战和发展前景。
α-AlH3 is one of the most promising hydrogen storage materials due to its high gravimetric hydrogen capacity and low dehydriding temperature. In present work, a convenient and cost-efficient solid-state mechanochemical reaction is proposed to obtain α-AlH3 nano-composite. With the addition of TiF3, α-AlH3 nano-composite was formed in a short period by milling of LiH and AlCl3. Based on XRD and NMR results, the average grain size of the α-AlH3 in the nano-composite was 45 nm. The reaction pathway as well as the synergistic effect of TiF3 on the solid state reaction between LiH and AlCl3 were confirmed. In the α-AlH3/LiCl nano-composite, TiF3 reduced the temperature of dehydriding reaction and improved dehydrogenation rate of α-AlH3. Within the temperature range between 80 and 160 °C, dehydrogenation of the as-milled α-AlH3 nano-composite showed fast kinetics. At 160 °C, a maximum hydrogen desorption of 9.92 wt% was obtained within 750 s, very close to the theoretical hydrogen capacity of α-AlH3.
Dr. Khalil, Operating Agent for the Hydrogen Safety Task of the International Energy Agency (IEA), discussed a framework comprised of standardized safety tests to assess the safe use of metal hydrides, chemical hydrides, and adsorbents as solid-state hydrogen storage media for on-board light-duty fuel cell vehicles. He also presented two baselines design for on-board hydrogen storage systems. The first design is for on-board reversible hydrogen storage and the second design is for off-board regenerable hydrogen storage.
Solid-state hydrogen storage materials such as complex metal hydrides, chemical hydrides, and adsorbents are promising alternatives to the use of compressed hydrogen gas or liquefied hydrogen for on-board vehicular applications. However, acceptance by the general public to the use of hydrogen, in any of its on-board storage forms, as an energy carrier in the transportation sector requires assurance that this application is safe and poses no additional risks above the current risk acceptance criteria associated with use of gasoline/diesel in the transportation sector.
This research discusses the results of an experimental program covering materials reactivity tests, dust cloud combustion characterization tests, mechanical impact sensitivity tests, and material/hot surface contact tests. These tests were performed to identify safety-critical characteristics of selected solid-state hydrogen storage materials. Based upon the results of those tests, risk mitigation methods were proposed to eliminate or mitigate the identified risks. The effectiveness of the proposed risk mitigation methods was also examined. The insights gained from the experimental program could be useful to ongoing research efforts aimed at identifying the best hydrogen storage materials from a safety standpoint as well as supporting current and future risk-informed hydrogen safety standards such as NFPA-2, ISO, and IEC.
The findings of this research both experimental and probabilistic modeling is part of this Principal Investigator (PI) project funded by the U.S. Department of Energy.
Project title: Quantifying and Addressing the DOE Material Reactivity Requirements with Analysis and Testing of Hydrogen Storage Materials and Systems.
The objectives of this experimental investigation are two-fold:
1) Identify potential adverse reactivity effects associated with using reversible hydrides and activated carbon for storing hydrogen on-board vehicles.
2) Propose risk mitigation methods to prevent those adverse reactivity effects from occurring or at least mitigate their consequences should they occur.
The insights gained from this experimental investigation are used to support:
1) Quantitative risk analysis by reducing the epistemic uncertainties of modeled phenomenological events.
2) Development of additional risk mitigation strategies to address unforeseen events associated with use of these materials in vehicular applications.
Corresponding author: Dr. Y.F. Khalil, United Technologies Research Center (UTRC), USA
Also, Professor of Chemical & Environmental Engineering, School of Engineering and Applied Science (SEAS)
email: yehia.khalil@yale.edu
Office: M8, Teaching Laboratory: M288, Mason Laboratory
9 Hillhouse Avenue
Yale University, New Haven, CT 06520, USA
The key technical target of this project is EH&S, having a focus on the safety sub-target with some consideration for toxicity. The technical target for safety is specified generally as “Meets or exceeds applicable standards.” For metal hydride, chemical hydride and adsorbent materials and systems, however, no such standards exist today. Furthermore, standards currently under development will be high-level in scope, primarily focused on systems and will not provide adequate guidance for evaluating and selecting viable candidate materials. As part of this effort, trade-offs will be evaluated between residual risks after mitigation and the two technical barriers:
(A) System Weight and Volume
(E) Charging/Discharging Rates
Experimental analyses of laboratory-type composite solid rocket propellants and hybrid
solid fuels based on �-AlH3 (alane) are presented. Chemical and physical properties of this
peculiar energetic ingredient are discussed, ballistic properties experimentally evaluated
under a variety of configurations, and flame structures compared with the corresponding
aluminized propellants. The obtained results overall disclose a competitive performance of
alane-based propellants with respect to the aluminized formulations and encourage its use in
both solid and hybrid rocket propulsion for space exploration.
The possibility of using hydrogen as a reliable energy carrier for both stationary and mobile applications has gained renewed interest in recent years due to improvements in high temperature fuel cells and a reduction in hydrogen production costs. However, a number of challenges remain and new media are needed that are capable of safely storing hydrogen with high gravimetric and volumetric densities. Metal hydrides and complex metal hydrides offer some hope of overcoming these challenges; however, many of the high capacity “reversible” hydrides exhibit a large endothermic decomposition enthalpy making it difficult to release the hydrogen at low temperatures. On the other hand, the metastable hydrides are characterized by a low reaction enthalpy and a decomposition reaction that is thermodynamically favorable under ambient conditions. The rapid, low temperature hydrogen evolution rates that can be achieved with these materials offer much promise for mobile PEM fuel cell applications. However, a critical challenge exists to develop new methods to regenerate these hydrides directly from the reactants and hydrogen gas. This spotlight paper presents an overview of some of the metastable metal hydrides for hydrogen storage and a few new approaches being investigated to address the key challenges associated with these materials.
Phase stability of nano-structures and possible low energy surfaces of AlH3 polymorphs from thin film geometry have been investigated using all electron density-functional total energy calculations. The calculated structural data for α-, α′-, β-, and γ-AlH3 modifications are in very good agreement with experimental values. The electronic structures based on parameterized HCTH functionals reveal that all polymorphs are insulators with calculated band-gap varying between 2.53 and 4.41eV. From our theoretical simulation we have found that the (010) in α-, (100) in α′-, (101) in β-, and (101) in γ-AlH3 surfaces are the most stable surface in the corresponding polymorphs. We have predicted that the critical size for the AlH3 nano-cluster is less than 1nm. As opposite to complex hydrides we have investigated so far, the calculated formation energy as a function of particle size reveal that the nano particles of AlH3 are relatively stable than the corresponding decomposed phases.
The potential for using aluminum hydride, AlH3, for vehicular hydrogen storage is explored. It is shown that particle-size control and doping of AlH3 with small levels of alkali-metal hydrides (e.g. LiH) results in accelerated desorption rates. For AlH3 20 mol % LiH, 100 °C desorption kinetics are nearly high enough to supply vehicles. It is highly likely that 2010 gravimetric and volumetric vehicular system targets (6 wt % H2 and 0.045 kg/L) can be met with onboard AlH3. However, a new, low-cost method of off-board regeneration of spent Al back to AlH3 is needed.
The aim of this paper is to present a risk analysis method that can be applied to factories where combustible dust is handled, in the form of raw materials, products or by-products, and therefore at risk to explosion. The work was carried out on site: a consistent number of companies that deal with the surface finishing of objects in aluminium through grinding were examined. The aluminium powder produced as a by-product is generally captured by suction plants and then subjected to dry or wet type abatement. In order to provide a rational approach to the risk assessment and frequency estimation, each company was divided into the so-called fields of study; and four risk assessment topics were identified for each field. A brief review of the methods that are available for the consequence magnitude estimation, regarding both the pressure wave and the launching of missiles, is also provided.
Multinuclear and multidimensional solid state NMR techniques including magic-angle-spinning (MAS) and multiple-quantum (MQ) MAS experiments have been used to characterize various AlH_3 samples. At least three distinct polymorphic AlH_3 phases have been prepared by desolvating the alane etherate product from its organometallic synthesis. MAS-NMR spectra for the ^(1)H and ^(27)Al nuclei have been obtained on a variety of AlH_3 samples that include the β- and γ-phases as well as the α-phase. ^(27)Al MAS NMR was found to respond with high sensitivity for showing differences in spatial arrangements of AlH_6 octahedra in the three polymorphs studied. Based on the characteristic NMR signatures determined, phase transition of the γ-AlH_3 to the α-AlH_3 was studied at room and high temperatures. Direct decomposition of the γ-AlH_3 to aluminum metal at room temperature was also unambiguously confirmed by NMR studies.
A reactive force field, REAXFF, for aluminum hydride has been developed based on density functional theory (DFT) derived data. REAXFF(AlH(3)) is used to study the dynamics governing hydrogen desorption in AlH(3). During the abstraction process of surface molecular hydrogen charge transfer is found to be well described by REAXFF(AlH(3)). Results on heat of desorption versus cluster size show that there is a strong dependence of the heat of desorption on the particle size, which implies that nanostructuring enhances desorption process. In the gas phase, it was observed that small alane clusters agglomerated into a bigger cluster. After agglomeration molecular hydrogen was desorbed from the structure. This thermodynamically driven spontaneous agglomeration followed by desorption of molecular hydrogen provides a mechanism on how mobile alane clusters can facilitate the mass transport of aluminum atoms during the thermal decomposition of NaAlH(4).
Solid-state hydrogen storage materials such as complex metal hydrides, chemical hydrides, and adsorbents are promising alternatives to the use of compressed hydrogen gas or liquefied hydrogen for on-board vehicular applications. However, acceptance by the general public to the use of hydrogen, in any of its on-board storage forms, as an energy carrier in the transportation sector requires assurance that this application is safe and poses no additional risks above the current risk acceptance criteria associated with use of gasoline/diesel in the transportation sector.
This research discusses the results of an experimental program covering materials reactivity tests, dust cloud combustion characterization tests, mechanical impact sensitivity tests, and material/hot surface contact tests. These tests were performed to identify safety-critical characteristics of selected solid-state hydrogen storage materials. Based upon the results of those tests, risk mitigation methods were proposed to eliminate or mitigate the identified risks. The effectiveness of the proposed risk mitigation methods was also examined. The insights gained from the experimental program could be useful to ongoing research efforts aimed at identifying the best hydrogen storage materials from a safety standpoint as well as supporting current and future risk-informed hydrogen safety standards such as NFPA-2, ISO, and IEC.
The surface energy values of (100), (110), (111) surfaces in Ni, Al and ordered alloys NiAl and Ni3Al have been calculated within the framework of methodology based upon the electron density functional method. The results of calculations are in good agreement with the known for pure metals experimental data and in case of Ni3Al alloy. There is also a good agreement with the results obtained by calculation using the embedded atom method. The investigation has been carried out in this work to show that the surface energy sigma of alloys can not be interpreted as an averaged concentration sigma of pure metals. For NiAl, the obtained results reveal considerable distinctions in anisotropy of sigma in comparison to the anisotropy of surface energy in pure metals.
Aluminum hydride is a covalent, binary hydride that has been known for more than 60 years and is an attractive medium for on-board automotive hydrogen storage, since it contains 10.1 % by wt. hydrogen with a density of 1.48 g/ml. There are at least 7 non-solvated AlH3 phases, namely α, α', β, γ, δ, ε and ζ. The properties of α-AlH3, obtained from the Dow Chemical Co. in 1980, have been previously reported. Here we present a description of the thermodynamic and kinetic properties of freshly prepared α, β and γ phases of AlH3. In all cases the decomposition kinetics are appreciable below 100°C and all will meet the DOE 2010 gravimetric and volumetric vehicular system targets (6 wt% H2 and 0.045 kg/L). However, further research will be required to develop an efficient and economical process to regenerate AlH3 from the spent Al powder.
This paper aims to develop quantitative insights based on measured deflagration parameters of hybrid mixtures of activated carbon (AC) dust and hydrogen (H-2) gas in air. The generated experimental evidence is used to reject the claim of the null hypothesis (H-0) that severity of deflagrations of H-2/air mixtures always bound the severity of deflagrations of heterogenous combustible mixtures of AC dust/H-2/air containing the same H-2 concentrations as in the H-2/air binaries. The core insights of this investigation show that the maximum deflagration pressure rise (Delta P-MAX) and maximum rate of pressure rise ((dP/dt)(MAX)) of this hybrid mixture are greater than those corresponding to deflagrations of H-2/air mixtures for all the dust and H-2 concentrations being examined. The deflagration severity indices (K-st and ES) of the hybrid mixture containing 29 M01% H-2 are found to be greater than those of the H-2/air mixture containing 29 mol% H-2. Also, the minimum explosible concentration (MEC) of the hybrid mixture is lower than that of the AC dust in air only. The insights gained should lead to better realization of the severity of a postulated safety-significant accident scenario associated with on-board cryadsorption H-2 storage systems for fuel-cell (FC) powered light-duty vehicles. The identified insights could also be relevant to other industrial processes where combustible dusts are generated in the vicinity of solvent vapors. Moreover, these insights should be useful for supporting quantitative risk assessment (QRA) of on-board H-2 storage systems, designing improved safety measures for cryoadsorption H-2 storage tanks, and guiding H-2 safety standards and transportation regulations.
This study discusses results of an experimental program to determine dust cloud combustion characteristics of 2LiBH4 + MgH2 binary system in air. The determined parameters of hydrided and partially-dehydrided states of this system include: maximum deflagration pressure rise (PMAX), maximum rate of pressure rise (dP/dt)MAX, minimum ignition temperature (TC), minimum explosible concentration (MEC), minimum ignition energy (MIE), and explosion severity index (KSt). Impact of dust particle size on the measured parameters is evaluated for the partially-dehydrided state. For dust of same mean particle size, results show the hydrided state to be more explosible in air compared to its partially-dehydrided state. Moreover, MIE of the partially-dehydrided mixture is identified in the test with lowest ignition delay time (IDT) and highest dust cloud concentration (DC). Taguchi's mixed-levels design of experiments (DoE) methodology is employed to calculate dust's MIE response surface as a function of DC and IDT. The one-at-a-time effect and interaction effect between DC and IDT on dust MIE are determined. The core insights of this contribution are useful for quantifying risks in mobile and stationary H2 storage applications, informing H2 safety standards, and augmenting property databases of H2 storage materials.
Experimental and theoretical studies were conducted to investigate the pyrophoricity and water-reactivity risks associated with employing sodium alanate (NaAlH4) complex metal hydride in on-board vehicular hydrogen (H2) storage systems. The ignition and explosivity of NaAlH4 upon exposure to oxidizers in air or water were attributed to the spontaneous formation of stable hydroperoxyl intermediates on the NaAlH4 surface and/or H2 production, as well as the large driving force for NaAlH4 conversion to favorable hydroxide products predicted by atomic and thermodynamic modeling. The major products from NaAlH4 exposure to air: NaAl(OH)4, gibbsite and bayerite Al(OH)3, and Na2CO3 observed by XRD, were identified to be formed by surface-controlled reactions. The reactivity risks were significantly minimized, without compromising de-/re-hydrogenation cyclability, by compacting NaAlH4 powder into wafers to reduce the available surface area. These core findings are of significance to risk mitigation and H2 safety code and standard development for the safe use of NaAlH4 for on-board H2 storage in light-duty vehicles.
Using 20L spherical explosion vessel, the explosion characteristics of nano-aluminum powders with different sizes were investigated. Compared with micro-scale aluminum powders explosion, nano-powders explosion processes and mechanisms were analyzed based on the auxiliary analysis of scanning electron microscopy (SEM) and X-Ray diffraction (XRD). The results show that the maximum explosion pressure and maximum rate of pressure rise mainly depended on the dust concentrations. With the increasing of dust concentration, the maximum explosion pressure increases gradually to the maximum when the dust concentrations below 1000g/m3 and then the maximum explosion pressure decreased especially for the dust concentrations higher about 1250g/m3. At the same time, the trends of maximum rate of pressure rise performed the similar rules with the dust concentrations. For the selected nano-powders, particle size change seems no obvious explosion differences. However, for micro-sized aluminum powders, explosion characteristic presents decreased change rules with the particle size increase. At the same time, the lower explosion concentration limits of aluminum powders explosion were measured and presented. Research result may have important implications for nano-sized aluminum powders utilization and safety operation.
The thermal decomposition of alane was investigated by application of synchrotron X-ray diffraction (SR-XRD) and thermal desorption spectroscopy (TDS). Two polymorphs were studied, α- and γ-AlH3. Activation energies, anisotropic volume expansions, and phase transformation paths were found. In addition, the crystal structure data, including structure of hydrogen sublattice, and small charge transfer from the aluminium towards the hydrogen sites were observed during a high-resolution SR-XRD study of α-AlH3.
X-Ray and neutron powder diffraction data for aluminum hydride, A1H3, and aluminum deuteride, A1D3, were used to solve this structure. Both compounds crystallize in the trigonal space group R3c with six molecules in a hexagonal unit cell of dimensions a = 4.449 Å and c = 11.804 Å for the hydride and a = 4.431 Å and c = 11.774 Å for the deuteride. Least-squares refinement of this three-parameter problem using seven unique A1D3 neutron powder diffraction peaks gave a final R1 of 0.026. The refined parameters gave R1 = 0.040 for 29 A1H3 X-ray powder diffraction peaks. Columns of A1 atoms and spirals of H atoms are parallel with the c axis and are packed so that the A1 has octahedral coordination symmetry. The final A1⋯H distance of 1.72 Å, the participation of each A1 in six bridges, and the equivalence of all A1⋯H distances suggest that 3c-2e bonding occurs. The closest A1-A1 distance is 3.24 Å. The alternating planes of A1 and H atoms, perpendicular to the c axis, result in a stable structure which is a three-dimensional network of A1⋯-H⋯A1 bridges and is consistent with the observed high density of the crystal.
Aluminum hydride (AlH3) and its associated compounds make up a fascinating class of materials that have motivated considerable scientific and technological research over the past 50 years. Due primarily to its high energy density, AlH3 has become a promising hydrogen and energy storage material that has been used (or proposed for use) as a rocket fuel, explosive, reducing agent and as a hydrogen source for portable fuel cells. This review covers the past, present and future research on aluminum hydride and includes the latest research developments on the synthesis of α-AlH3 and the other polymorphs (e.g., microcrystallization reaction, batch and continuous methods), crystallographic structures, thermodynamics and kinetics (e.g., as a function of crystallite size, catalysts and surface coatings), high-pressure hydrogenation experiments and possible regeneration routes.
An experimental investigation was carried out on the influences of dust concentration, particle size distribution and humidity on aluminum dust explosion. Tests were mainly conducted thanks to a 20 L explosion sphere. The effect of humidity was studied by storing the aluminum particles at constant relative humidity until the sorption equilibrium or by introducing water vapour in the explosion vessel. The tested particles sizes ranged from a volume median diameter of 7 to 42 μm and the dust concentrations were up to 3000 g m−3.Among other results, the strong influence of the particle size was pointed out, especially when the Sauter mean diameter is considered. These results stressed the predominance of the specific surface area on the mass median particle diameter.The effect of water on aluminum dust explosion was decoupled: on the one hand, when water adsorption occurs, hydrogen generation leads to an increase of the explosion severity; on the other hand, when the explosion of dried aluminum powder occurs in a humid atmosphere, the inhibiting effect of humidity is put forward.A model based on mass and heat balances, assuming a shrinking core model with chemical reaction limitation, leads to a satisfactory representation of the pressure evolution during the dust explosion.
The thermal decomposition of alane was investigated by application of synchrotron X-ray diffraction (SR-XRD) and thermal desorption spectroscopy (TDS). Two polymorphs were studied, alpha- and gamma-AlH3. Activation energies, anisotropic volume expansions, and phase transformation paths were found. In addition, the crystal structure data, including structure of hydrogen sublattice, and small charge transfer from the aluminium towards the hydrogen sites were observed during a high-resolution SR-XRD study of alpha-AlH3. (C) 2006 Elsevier B.V. All rights reserved.
Metal powders or dusts can represent significant dust explosion hazards in industry, due to their relatively low ignition energy and high explosivity. The hazard is well known in industries that produce or use aluminum powders, but is sometimes not recognized by facilities that produce aluminum dust as a byproduct of bulk aluminum processing. As demonstrated by the 2003 dust explosion at aluminum wheel manufacturer Hayes Lemmerz, facilities that process bulk metals are at risk due to dust generated during machining and finishing operations [U.S. Chemical Safety and Hazard Investigation Board, Investigation Report, Aluminum Dust Explosion Hayes Lemmerz International, Inc., Huntington, Indiana, Report No. 2004-01-I-IN, September 2005]. Previous studies have shown that aluminum dust explosions are more difficult to suppress with flame retardants or inerting agents than dust explosions fueled by other materials such as coal [A.G. Dastidar, P.R. Amyotte, J. Going, K. Chatrathi, Flammability limits of dust-minimum inerting concentrations, Proc. Saf. Progr., 18-1 (1999) 56-63]. In this paper, an inerting method is discussed to reduce the dust explosion hazard of residue created in an aluminum buffing operation as the residue is generated. This technique reduces the dust explosion hazard throughout the buffing process and within the dust collector systems making the process inherently safer. Dust explosion testing results are presented for process dusts produced during trials with varying amounts of flame retardant additives.
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