Numerical analysis of accidental hydrogen releases from high pressure storage at low temperatures

Evaluations of the performance of simplified engineering and CFD models are important to improve risk assessment tools e.g. to predict accurately releases from various types of hydrogen storages. These tools have to predict releases from a wide range of storage pressures (up to 80 MPa) and temperatures (down to 20 K), e.g. cryogenic compressed gas storage covers pressures up to 35 MPa and temperatures between 33 K and 338 K. Accurate calculations of high pressure releases require real gas EOS. This paper compares a number of EOS to predict hydrogen properties typical in different storage types. The vessel dynamics are modeled using a simplified engineering and a CFD model to evaluate the performance of various EOS to predict vessel pressures, temperatures mass flow rates and jet flame lengths. It is shown that the chosen EOS and the chosen specific heat capacity correlation are important to model accurately hydrogen releases at low temperatures.     

Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure

An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm × 13.8  mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.     

Failure pressure analysis of hydrogen storage pipeline under low temperature and high pressure

Low temperature and high pressure line pipes are widely used in hydrogen storage, air separation plant, liquefied natural gas (LNG) transportation etc. The material properties of pipes at low temperature are different from those at room temperature. If the medium in the pipe is corrosive, it will cause the pipe wall thickness to decrease. However, the failure pressure of the corroded hydrogen storage pipeline at extremely low temperature is lacking of adequate understanding. In this paper, we provided a novel failure pressure equation of the mild steel line pipe with corrosion defects at extremely low temperature. Firstly, a mechanical model of the line pipe with corrosion defects is established. And then, an analytical solution of the mechanical model is obtained based on elastic theory. Next, a failure pressure equation of the corroded hydrogen storage pipeline at extremely low temperature is developed. In the end, the accuracy of the failure pressure equation is verified by comparing with finite element method (FEM). The results suggest that the calculated value of the failure pressure equation is consistent with that of FEM. This paper provides a theoretical basis for the safety assessment of low temperature hydrogen storage pipeline. The new equation presented in this paper can provide useful guidance for the design of low temperature and high pressure pipelines.     

Modeling of ammonia combustion at low pressure

In the context of a hypothetic hydrogen based economy, ammonia has been suggested as a potential alternative to liquid fossil fuels for spark ignition (SI) engines. However, its use requires a detail understanding of its combustion characteristics, particularly the comprehension of the unavoidable thermal and fuel nitrogen oxides formation. This study presents the elaboration of an improved ammonia combustion mechanism validated for the flame structure prediction of ammonia, hydrogen, oxygen, argon flames investigated at several low pressures and for various conditions of equivalence ratio and of initial hydrogen content. The improved kinetic model is then reduced in order to be used in a complete SI engine numerical simulation. The comparison of the predictions of these mechanisms for the same ammonia flame structure is presented. The reduced mechanism contains 80 elementary reactions and 19 chemical species and allows a better understanding of the complete nitrogen oxides formation pathways.     

Fabrication of titanium bipolar plates for proton exchange membrane fuel cells by uniform pressure electromagnetic forming

Electromagnetic forming (EMF) is a type of noncontact and high-speed forming process that offers several advantages, such as high material formability, low springback, and low die cost. In this study, titanium bipolar plates (BPPs) for proton exchange membrane fuel cells are fabricated by EMF using a uniform pressure actuator that provided a uniform pressure distribution and high efficiency. A three-dimensional coupled electromagnetic-mechanical simulation model is established to optimize the dynamic forming process. Simulation results show that the channel depth is proportional to the impact velocity of the titanium workpiece. A forming window is established as a design guideline. Finally, a titanium BPP with sufficient channel depth (0.4 mm), high aspect ratio (0.67), low thinning rate (not exceeding 15.89%) and high corrosion resistance is successfully fabricated at a velocity of 286 m/s, thereby demonstrating that EMF is a feasible process for fabricating titanium BPPs.     

Fabrication of titanium bipolar plates by rubber forming and performance of single cell using TiN-coated titanium bipolar plates

In this study, a rubber forming method is used to fabricate titanium bipolar plates for proton exchange membrane fuel cells. A titanium blank with a thickness of 0.1 mm is compressed using a stamping mold equipped with a 200-ton hydraulic press to fabricate bipolar plates. A forming experiment is carried out by changing different parameters such as the punch velocity, punch pressure, rubber thickness, rubber hardness, and draft angle of the channel. The optimum forming conditions are found to be a rubber thickness of 10 mm, rubber hardness equivalent to that of Shore A 20, punch velocity of 30 mm s⁻¹, punch pressure of 55 MPa, and punch draft angle of 30°. The fabricated titanium bipolar plate is coated with a TiN layer. A single cell with a TiN-coated titanium bipolar plate is examined and compared to those with uncoated titanium and graphite bipolar plates. The initial performances (in terms of current densities) of the single cells with the uncoated titanium, TiN-coated titanium, and graphite bipolar plates are 396, 799, and 1160 mA cm⁻², respectively, at a cell voltage of 0.6 V.     

Determination of lifetime probabilities of carbon fibre composite plates and pressure vessels for hydrogen storage

It is shown that an analogy can be made between the failure of unidirectional carbon fibre reinforced epoxy plates and filament wound carbon fibre composite pressure vessels and that their strengths and failure probabilities can be determined. Fibres in filament wound composite structures are placed on geodesic paths around the mandrel, which becomes the liner; so that when the structure is pressurised the fibres are only subjected to tensile forces, as in a unidirectional composite. Multiscale modelling reveals that composite failure is controlled by fibre breakage and that clustering of fibre breaks determines ultimate reliability of the structure. Time dependent relaxation of the matrix leads to delayed failure of the elastic fibres. A statistical study, using the stochastic properties of the fibres, determines the range of lifetimes which will be obtained in a given class of pressure vessel, leading to an evaluation of failure probabilities as a function of internal pressures. In this way the definition of safety factors, based on an understanding of the physical processes governing damage accumulation, becomes possible.     

Interaction of ZrMo2 with hydrogen at high pressure

The interaction of hydrogen with ZrMo₂ intermetallic compound at pressure up to 2500 bars has been studied. In ZrMo₂–H₂ system desorption isotherms were measured and thermodynamic parameters of hydride phase decomposition were calculated. The structure of ZrMo₂D_4.0 deuteride has been investigated by X-ray and neutron diffraction methods. It was revealed that ordering of deuterium in the cubic lattice of ZrMo₂ led to the formation of superstructure with tetragonal lattice. The occupancy of the positions of deuterium and metal atoms was determined.     

Sorption enhanced reaction process for direct production of fuel-cell grade hydrogen by low temperature catalytic steam-methane reforming

New experimental data are reported to demonstrate that a sorption enhanced reaction (SER) concept can be used to directly produce fuel-cell grade H₂ (<20 ppm CO) by carrying out the catalytic, endothermic, steam-methane reforming (SMR) reaction (CH₄ + 2H₂O ↔ CO₂ + 4H₂) in presence of a CO₂ selective chemisorbent such as K₂CO₃ promoted hydrotalcite at reaction temperatures of 520 and 550 °C, which are substantially lower than the conventional SMR reaction temperatures of 700-800 °C. The H₂ productivity of the sorption enhanced reactor can be large, and the conversion of CH₄ to H₂ can be very high circumventing the thermodynamic limitations of the SMR reaction due to the application of the Le Chetalier's principle in the SER concept. Mathematical simulations of a cyclic two-step SER concept showed that the H₂ productivity of the process (moles of essentially pure H₂ produced per kg of catalyst-chemisorbent admixture in the reactor per cycle) is much higher at a reaction temperature of 590 °C than that at 550 or 520 °C. On the other hand, the conversion of feed CH₄ to high purity H₂ product is relatively high (>99+%) at all three temperatures. The conversion is much higher than that in a conventional catalyst-alone reactor at these temperatures, and it increases only moderately (<1%) as the reaction temperature is increased from 520 to 590 °C. These results are caused by complex interactions of four phenomena. They are (a) favorable thermodynamic equilibrium of the highly endothermic SMR reaction at the higher reaction temperature, (b) faster kinetics of SMR reaction at higher temperatures, (c) favorable removal of CO₂ from the reaction zone at lower temperatures, and (d) higher cyclic working capacity for CO₂ chemisorption at higher temperature.     

Kinetic modeling study of toluene pyrolysis at low pressure

A detailed kinetic model, consisting of 137 species and 530 reactions, was developed to simulate toluene pyrolysis at low pressure within the temperature range from 1270 to 1870 K. The mole fraction profiles predicted for pyrolysis species up to phenanthrene were in good agreement with the experiment. The decomposition pathways of toluene and the growth pathways to polycyclic aromatic hydrocarbons (PAHs) were discussed from reaction flux analysis. Toluene decomposes through the reaction sequence C₆H₅CH₃ → C₆H₅CH₂ → C₇H₆ → c-C₅H₅ → C₃H₃, which also has a predominant contribution to the production of acetylene. Furthermore, sensitivity analysis showed that the primary decomposition reactions of toluene, C₆ H₅CH₃ = C₆H₅CH₂ + H and C₆H₅CH₃ = C₆H₅ + CH₃, have great influences on the formation of small molecules, such as phenyl radical, benzyl radical, C2- and C3-species, which are critical to the formation of PAHs in the pyrolysis of toluene.     

Experimental studies on wind influence on hydrogen release from low pressure pipelines

At the DIMNP (Department of Mechanical, Nuclear and Production Engineering) laboratories of University of Pisa (Italy) a pilot plant called HPBT (Hydrogen Pipe Break Test) was built in cooperation with the Italian Fire Brigade Department. The apparatus consists of a 12 m³ tank connected with a 50 m long pipe. At the far end of the pipeline a couple of flanges have been used to house a disc with a hole of the defined diameter. The plant has been used to carry out experiments of hydrogen release. During the experimental activity, data have been acquired about the gas concentration and the length of release as function of internal pressure and release hole diameter. The information obtained by the experimental activity will be the basis for the development of a new specific normative framework arranged to prevent fire and applied to hydrogen. This study is focused on hydrogen concentration as function of wind velocity and direction. Experimental data have been compared with theoretical and computer models (such as CFD simulations).     

Raman spectroscopy study of molecular hydrogen solubility in water at high pressure

We have measured the Raman spectra of gaseous molecular hydrogen dissolved in liquid water at room temperature and as a function of pressure. Vibrational spectra of molecular hydrogen have been clearly detected. Band intensities and profiles have been carefully measured using, for calibration purposes, the water OH stretching band. From the measured intensities of the Raman band, we have obtained the behavior of hydrogen concentration in the liquid water, as a function of the gas partial pressure. The observed behavior is presented and compared to Henry’s law predictions. Additionally, we present a detailed analysis of the spectral band features from which important information on the interaction of hydrogen with water molecules could be derived.     

Absorption of deuterium in titanium plates induced by electric discharges

In this work, the absorption of deuterium in titanium plates induced by electric discharges is studied. The objective was to measure the amount of deuterium that is absorbed in the titanium structure under the influence of an electric discharge. The ionization and the free radicals produced by the electric field act as a promoter to the absorption mechanism. Thus, the absorption can be enhanced by the use of an electric discharge. The results indicated that there was a rapid desorption of deuterium at the beginning of the discharge, followed by an additional absorption to higher levels than those before the discharge. The additional absorption of deuterium was about 20% of the initial absorption. When the titanium was completely saturated with the gas, no additional absorption occurred through the electric discharges. As a result of the absorption conditions of deuterium in the titanium structure, anomalous emission of neutrons was recorded as tracks in a CR39 type plastic solid-state nuclear-track detector.     

Enhanced gaseous hydrogen solubility in ferritic and martensitic steels at low temperatures

Metals that are exposed to high pressure hydrogen gas may undergo detrimental failure by embrittlement. Understanding the mechanisms and driving forces of hydrogen absorption on the surface of metals is crucial for avoiding hydrogen embrittlement. In this study, the effect of stress-enhanced gaseous hydrogen uptake in bulk metals is investigated in detail. For that purpose, a generalized form of Sievert's law is derived from thermodynamic potentials considering the effect of microstructural trapping sites and multiaxial stresses. This new equation is parametrized and verified using experimental data for carbon steels, which were charged under gaseous hydrogen atmosphere at pressures up to 1000 bar. The role of microstructural trapping sites on the parameter identification is critically discussed. Finally, the parametrized equation is applied to calculate the stress-enhanced hydrogen solubility of thin-walled pipelines and thick-walled pressure vessels during service.     

Characteristics of heat transfer and temperature rise of hydrogen during rapid hydrogen filling at high pressure

An experiment has been done to measure the rise in temperature of a gas during filling a tank at high pressure. The experimental condition is that filling gases are nitrogen and hydrogen at a pressure of 5 to 35 MPa and at a filling mass of G=45 to 324 g/min for hydrogen. The temperatures are measured either horizontally or vertically at five positions in the tank. It is found that heat loss transferred from compressed gas to the tank wall has a significant effect on the rise in the filled gas temperature. The heat transfer coefficient is estimated after the end of filling and is about α_h=270 W/(m²K) for the hydrogen at 35 MPa. A theoretical procedure is proposed to calculate the temperature increase of the gas on a basis of assumption that the gas temperature in the tank is uniform at any time, and the heat transfer coefficient is given. The calculation shows that the temperature is in reasonable agreement with the measured temperatures by assuming α_h=500 W/(m²K) during the filling of hydrogen at 35 MPa, although the estimated heat loss after the end of filling becomes larger than the actual one. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(1): 13–27, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20140     

Effect of some elements on oxygen reduction and hydrogen evolution at lead-acid battery negative plates

The effects which the impurities As, Bi, Cd, Co, Cr, Ni, Sb, Se, Sn and Te have on the processes of hydrogen evolution and oxygen reduction on the negative plates of lead-acid batteries have been tested. This was done by measuring the gas pressure in sealed vessels containing cycled, wet, negative plates. The addition of As, Co, Ni, Sb and Te to the electrolyte used during the cycling increases the hydrogen ion discharge rate, whereas Cr, Bi and Sn have the opposite effect. In the case of oxygen, As, Bi, Ni, Sb, Se, Sn, and Te promote its reduction while Co and Cr suppress it. The effect of impurity concentration has also been considered.     

Characterization of defects in titanium created by hydrogen charging

In the present work positron annihilation spectroscopy was employed for investigation of defects created in titanium by hydrogen loading. Pure titanium samples were firstly annealed to remove dislocations introduced by cutting and polishing. Subsequently the samples were loaded with hydrogen up to various hydrogen concentrations. Ti samples with different microstructures were compared: (i) conventional coarse grained sample, (ii) ultra fine grained material with microstructure refined by severe plastic deformation. Hydrogen gas loading of coarse grained and ultra fine grained samples was performed at hydrogen gas pressure of 103 bar and temperature of 150 °C. This resulted in formation of δ-TiH_x phase in Ti matrix. The hydrogen content absorbed in the samples was determined by thermogravimetric analysis. The phase composition of hydrogen-loaded samples was characterized by X-ray diffraction. Hydrogen loading introduced vacancies which agglomerated in the sample into small vacancy clusters. In addition to vacancies, dislocations were created by α-Ti → δ-TiH_x phase transition. Differential thermal analysis revealed that hydrogen is trapped at several kinds of traps characterized by different binding energies. The release of hydrogen from these traps precedes the decomposition of the δ-TiH_x phase.     

Accumulation of hydrogen in titanium exposed to neutron irradiation

Hydrogen saturation of titanium-based materials exposed to irradiation with resonance neutrons with an energy of 0.1 MeV is considered. Radioactive scandium ₂₂Sc⁴⁶, gamma-quanta with energy of 889 and 1120 keV, and hydrogen form during nuclear reactions in titanium. The intensity of the gamma radiation depends on the concentration of hydrogen in titanium pre-saturated with hydrogen. The gamma field likely effects the excitation of the hydrogen subsystem of titanium. Irradiated materials in the presence of gamma radiation are controlled by measuring the thermo-emf. Hydrogenation of titanium exposed to neutron irradiation increases by 10–12%, which changes the thermo-emf by 20%. The temperature of components required to obtain the most hydrogen-saturated titanium corresponds to room temperature. Using this method, the hydrogen saturation time of material decreases and its amount of hydrogen increases. The effective conductivity energy is 0.17/0.5 mV K for unirradiated titanium and 0.122/0.5 mV K for irradiated titanium, change of 30%. The effect of gamma radiation must be considered when producing neutron shields based on titanium borides. Intermetallic compounds used for the accumulation and transportation of hydrogen and exposed to irradiation lose titanium atoms, negating the composition stoichiometry. The quality of commercial titanium saturated with hydrogen under these conditions improves.