Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results

Explosion relief panels or doors are often used in industrial buildings to reduce damages caused by gas explosions. Decades of research have contributed to the understanding of the phenomena involved in gas explosions in order to establish an effective method to predict reliably the explosion overpressure. All the methods predict a monotonic increase of the overpressure with the concentration of the gas in the range from the lower explosion limit to the stoichiometric one. Nevertheless, in few cases, a non-monotonic behaviour of the maximum developed pressure as a function of hydrogen concentration was reported in the literature. The non-monotonic behaviour was also observed during experimental tests performed at the Scalbatraio laboratory at the University of Pisa, in a 25 m³ vented combustion test facility, with a vent area of 1,12 m². This paper presents the results obtained during the tests and investigates the possible explanations of the phenomena.     

Experimental studies on vented deflagrations in a low strength enclosure

This paper describes an experimental programme on vented hydrogen deflagrations, which formed part of the Hyindoor project, carried out for the EU Fuel Cells and Hydrogen Joint Undertaking. The purpose of this study was to investigate the validity of analytical models used to calculate overpressures following a low concentration hydrogen deflagration. Other aspects of safety were also investigated, such as lateral flame length resulting from explosion venting. The experimental programme included the investigation of vented hydrogen deflagrations from a 31 m³ enclosure with a maximum internal overpressure target of 10 kPa (100 mbar). The explosion relief was provided by lightly covered openings in the roof or sidewalls. Uniform and stratified initial hydrogen distributions were included in the test matrix and the location of the ignition source was also varied. The maximum hydrogen concentration used within the enclosure was 14% v/v. The hydrogen concentration profile within the enclosure was measured, as were the internal and external pressures. Infrared video images were obtained of the gases vented during the deflagrations. Findings show that the analytical models were generally conservative for overpressure predictions. Flame lengths were found to be far less than suggested by some guidance. Along with the findings, the methodology, test conditions and corresponding results are presented.     

Experimental study of vented hydrogen deflagration with ignition inside and outside the vented volume

Experiments were carried out inside a 25 m³ vented combustion test facility (CVE) with a fixed vent area sealed by a plastic sheet vent. Inside the CVE, a 0.64 m³ open vent box, called RED-CVE was placed. The vent of the RED-CVE was left open and three different vent area were tested. Two different mixing fans, one for each compartment, were used to establish homogeneous H₂ concentrations. This study examined H₂ concentrations in the range between 8.5% vol. to 12.5% vol. and three different ignition locations, (1) far vent ignition, (2) inside the RED-CVE box ignition and (3) near vent ignition (the vent refers to the CVE vent). Peak overpressures generated inside the test facility and the smaller compartment were measured. The results indicate that the near vent ignition generates negligible peak overpressures inside the test facility as compared to those originated by far vent ignition and ignition inside the RED-CVE box. The experiments with far vent ignition showed a pressure increase with increasing hydrogen concentration which reached a peak value at 11% vol. concentration and then decreased showing a non-monotonic behaviour. The overpressure measured inside the RED-CVE was higher when the ignition was outside the box whereas the flame entered the box through the small vent.     

CFD modeling and consequence analysis of an accidental hydrogen release in a large scale facility

In this study, the consequences of an accidental release of hydrogen within large scale, (>15,000 m³), facilities were modeled. To model the hydrogen release, an LES Navier–Stokes CFD solver, called fireFoam, was used to calculate the dispersion and mixing of hydrogen within a large scale facility. The performance of the CFD modeling technique was evaluated through a validation study using experimental results from a 1/6 scale hydrogen release from the literature and a grid sensitivity study. Using the model, a parametric study was performed varying release rates and enclosure sizes and examining the concentrations that develop. The hydrogen dispersion results were then used to calculate the corresponding pressure loads from hydrogen-air deflagrations in the facility.     

An inter-comparison exercise on CFD model capabilities to simulate hydrogen deflagrations with pressure relief vents

The comparison between experimental data and simulation results of hydrogen explosions in a vented vessel is described in the paper. The validation exercise was performed in the frame of the European Commission co-funded Network of Excellence HySafe (Hydrogen Safety as an Energy Carrier) that has the objective to facilitate the safe introduction of hydrogen technologies. The mitigation effect of vents on the strength of hydrogen explosions is a relevant issue in hydrogen safety. Experiments on stoichiometric hydrogen deflagrations in a 0.95 m³ vessel with vents of different size (0.2 m² and 0.3 m²) have been selected in the available scientific literature in order to assess the accuracy of computational tools and models in reproducing experimental data in vented explosions. Five organizations with experience in numerical modelling of gas explosions have participated to the code benchmarking activities with four CFD codes (COM3D, REACFLOW, b0b and FLUENT) and one code based on a mathematical two-zone model (VEX). The numerical features of the different codes and the simulations results are described and compared with the experimental measurements. The agreement between simulations and experiments can be considered satisfactory for the maximum overpressure while correctly capturing some relevant parameters related to the dynamics of the phenomena such as the pressure rise rate and its maximum has been shown to be still an open issue.     

Hydrogen dispersion phenomenon in nominally closed spaces

The hydrogen dispersion phenomenon in an enclosure depends on the ratio of the gas buoyancy-induced momentum and diffusive motions. Random diffusive motions of individual gas particles become dominative when the release momentum is low, and a uniform hydrogen concentration appears in the enclosure instead of the gas cumulation below the ceiling. The expected hydrogen behavior could be projected by the Froude number, which value ~1 predicts a decline of buoyancy. This paper justifies this hypothesis by demonstrating full-scale experimental results of hydrogen dispersion within a confined space under six different release variations. During the experiments, hydrogen was released into the test room of 60 m³ volume in two methods: through a nozzle and through 21 points evenly distributed on the emission box cover (multi-point release). Each release method was tested with three volume flow rates (3.2 × 10⁻³ m³/s, 1.6 × 10⁻³ m³/s, 3.3 × 10⁻⁴ m³/s). The tests confirm the decrease of hydrogen buoyancy and its stratification tendencies when the Mach, Reynolds, and Froud number values decrease. Because the hydrogen dispersion phenomenon would impact fire and explosive hazards, the presented experimental results could help fire protection systems be in an enclosure designed, allowing their effectiveness optimization.     

Hydrogen production by a low-cost electrolyzer developed through the combination of alkaline water electrolysis and solar energy use

Electrolysis is a relatively simple process for obtaining hydrogen and can be combined with use of renewable energy sources, such as solar photovoltaic energy, for clean, sustainable gas production. This study designed a cylindrical electrolytic cell made of acrylic and 304 stainless steel electrodes to produce hydrogen. The electrolyte used was sodium hydroxide (NaOH 2–5 mol L^?1), and the direct current voltages applied were 2.0, 2.7, and 3.4 V. The maximum hydrogen production was achieved with 5.0 mol L^?1 NaOH and 3.4 V electric voltage. The system was connected to a photovoltaic panel of 20 W and exposed to solar radiation from 10 a.m. to 2 p.m. Approximately 2 L of hydrogen was produced within a period, and an average irradiance of 800.0 W m^?2 ± 60 W m^?2 was achieved. The system was stable throughout the tests.     

Modeling of vented deflagrations

A mathematical model of vented gas-phase deflagrations is presented. By introducing several empirical parameters, account is taken of initial turbulence in the gases, flame acceleration due to hydrodynamic instabilities prior to vent opening, and increased burning velocity due to turbulence generated by the venting process. Additionally, a mixture of burned and unburned gases is vented. Essential information needed to compute the pressure development during vented deflagrations (or in large closed vessels) is the rate of increase of flame area due to cell formation in the flame front prior to the vent opening.The model has been tested against methane/air mixtures at initial pressures of 45 psia in vessels up to 3.8 m³ in volume. Good agreement has been obtained.Further work is underway to gather data on vented deflagrations for gases such as propane, ethylene, and hydrogen (which represent a series of increasing burning velocities) and to investigate more fully the effect of initial turbulence and elevated pressures.     

Effect of the position and the area of the vent on the hydrogen dispersion in a naturally ventilated cubiod space with one vent on the side wall

The design of ventilation system has implications for the safety of life and property, and the development of regulations and standards in the space with the hydrogen storage equipment. The impact of both the position and the area of a single vent on the dispersion of hydrogen in a cuboid space (with dimensions L x W x H = 2.90 × 0.74 × 1.22 m) is investigated with Computational Fluid Dynamics (CFD) in this study. Nine positions of the vent were compared for the leakage taking place at the floor to understand the gas dispersion. It was shown a cloud of 1% mole fraction has been formed near the ceiling of the space in less than 40 s for different positions of the vent, which can activate hydrogen sensors. The models show that the hydrogen is removed more effectively when the vent is closer to the leakage position in the horizontal direction. The study demonstrates that the vent height of 1.00 m is safer for the particular scenario considered. The area of the vent has little effect on the hydrogen concentration for all vent positions when the area of the vent is less than 0.045 m² and the height of the vent is less than 0.61 m.     

Small scale experiments and Fe model validation of structural response during hydrogen vented deflagrations

University of Pisa (UNIPI) conducted a series of vented deflagration tests at B. Guerrini Laboratory. The tests were part of the experimental campaign performed by UNIPI for the European HySEA project (Hydrogen Safety for Energy Applications). Experiments included homogeneous hydrogen-air mixture contained in an about 1 m³ enclosure, called SSE (Small Scale Enclosure). The mixture concentration was variable between 10% and 18% vol. During the deflagrations, structural response was investigated by measuring the displacement of a test plate. The collected data were used to validate the FE model developed by IMPETUS Afea. In this paper experimental facility, displacement measurement system and FE model are briefly described, then comparison between experimental data and simulation results is discussed.     

Hydrogen production by steam methane reforming in a membrane reactor equipped with a Pd composite membrane deposited on a porous stainless steel

With the aim of producing hydrogen at low cost and with a high conversion efficiency, steam methane reforming (SMR) was carried out under moderate operating conditions in a Pd-based composite membrane reactor packed with a commercial Ru/Al₂O₃ catalyst. A Pd-based composite membrane with a thickness of 4–5 μm was prepared on a tubular stainless steel support (diameter of 12.7 mm, length of 450 mm) using electroless plating (ELP). The Pd-based composite membrane had a hydrogen permeance of 2.4 × 10^?3 mol m^?1 s^?1 Pa^?0.5 and an H₂/N₂ selectivity of 618 at a temperature of 823 K and a pressure difference of 10.1 kPa. The SMR test was conducted at 823 K with a steam-to-carbon ratio of 3.0 and gas hourly space velocity of 1000 h^?1; increasing the pressure difference resulted in enhanced methane conversion, which reached 82% at a pressure difference of 912 kPa. To propose a guideline for membrane design, a process simulation was conducted for conversion enhancement as a function of pressure difference using Aspen HYSYS^®. A stability test for SMR was conducted for ～120 h; the methane conversion, hydrogen production rate, and gas composition were monitored. During the SMR test, the carbon monoxide concentration in the total reformed stream was <1%, indicating that a series of water gas shift reactors was not needed in our membrane reactor system.     

Hydrogen jet fires in a passively ventilated enclosure

This paper describes a combined experimental, analytical and numerical modelling investigation into hydrogen jet fires in a passively ventilated enclosure. The work was funded by the EU Fuel Cells and Hydrogen Joint Undertaking project Hyindoor. It is relevant to situations where hydrogen is stored or used indoors. In such situations passive ventilation can be used to prevent the formation of a flammable atmosphere following a release of hydrogen. Whilst a significant amount of work has been reported on unignited releases in passively ventilated enclosures and on outdoor hydrogen jet fires, very little is known about the behaviour of hydrogen jet fires in passively ventilated enclosures. This paper considers the effects of passive ventilation openings on the behaviour of hydrogen jet fires. A series of hydrogen jet fire experiments were carried out using a 31 m³ passively ventilated enclosure. The test programme included subsonic and chocked flow releases with varying hydrogen release rates and vent configurations. In most of the tests the hydrogen release rate was sufficiently low and the vent area sufficiently large to lead to a well-ventilated jet fire. In a limited number of tests the vent area was reduced, allowing under-ventilated conditions to be investigated. The behaviour of a jet fire in a passively ventilated enclosure depends on the hydrogen release rate, the vent area and the thermal properties of the enclosure. An analytical model was used to quantify the relative importance of the hydrogen release rate and vent area, whilst the influence of the thermal properties of the enclosure were investigated using a CFD model. Overall, the results indicate that passive ventilation openings that are sufficiently large to safely ventilate an unignited release will tend to be large enough to prevent a jet fire from becoming under-ventilated.     

585 divacancy-defective germanene as a hydrogen separation membrane: A DFT study

As sustainable and clean energy, hydrogen is the most attractive and promising energy source in the future. Membrane separation is attractive due to its high hydrogen separation performance and low energy consumption. Van-der-Waals-corrected density functional theory (DFT) calculations are performed to investigate the hydrogen separation performance of 585 divacancy-defective germanene (585 germanene). It is found that the 585 germanene presents a surmountable energy barrier (0.34 eV) for hydrogen molecule passing through the membrane, and that membrane exhibits extremely high selectivity for H₂ molecules over CO, CO₂, N₂, CH₄ and H₂S molecules in a wide range of temperatures. Meanwhile, the hydrogen permeance of 585 germanene can reach 1.94 × 10^?7 mol s^?1 m^?2 Pa^?1 at the low limit temperature of methane reforming (at 450 K), which is higher than the industrially acceptable gas permeance. With high selectivity and permeance, the 585 germanene is a promising candidate for hydrogen separation.     

Microbial electrolysis cell powered by an aluminum-air battery for hydrogen generation,in-situ coagulant production and wastewater treatment

Microbial electrolysis cells (MECs) are an efficient technology for generating hydrogen gas from organic matters, but an additional voltage is needed to overcome the thermodynamic barrier of the reaction. A combined system of MEC and the aluminum-air battery (Al-air battery) was designed for hydrogen generation, coagulant production and operated in an energy self-sufficient mode. The Al-air battery (28 mL) produced a voltage ranged from 0.58 V to 0.80 V, which powered an MEC (28 mL) to produce hydrogen. The hydrogen production rate reached 0.19 ± 0.01 m³ H₂/m³/d and 14.5 ± 0.9 mmol H₂/g COD. The total COD removal rate was 85 ± 1%, of which MEC obtained 75 ± 1% COD removal and 10 ± 1% COD removal was achieved by in-situ coagulating process. The microorganisms removal of MEC effluent was 97 ± 2% through ex-situ coagulating process. These results showed that the Al-air battery-MEC system can be operated in energy self-sufficient mode and recovered energy from wastewater with high quality effluent.     

Optimal interval of periodic polarity reversal under automated control for maximizing hydrogen production in microbial electrolysis cells

Microbial electrolysis cells (MECs) are a promising approach for producing hydrogen gas from low-grade substrates with low energy consumption. However, pH increase in a cathode due to proton reduction and thus the need for buffering this pH increase remains a challenge for MEC operation. In this study, a previously reported operational strategy for pH buffer - periodic polarity reversal (PPR) was further studied by developing and applying an automatically control system. The effect of PPR interval on the hydrogen production was investigated and the optimal PPR interval was determined. With an optimal PPR interval of 40 min, the MEC had a significantly low pH increase rate of 0.0085 min^?1 in its cathodes, and this resulted in the highest current density of 1.58 ± 0.02 A m^?2, Coulombic efficiency of 130.3 ± 1.8%, hydrogen production rate of 1.65 ± 0.01 m³ H₂ m^?3d^?1, overall hydrogen recovery of 75.9 ± 0.4%, and energy efficiency relative to the substrate input of 140.8 ± 1.4%. Further analysis suggested that this optimal value of PPR interval was affected by both reaction time and hydrogen supply. When the PPR interval increased from 10 min to 40 min, a longer reaction time helped produce more protons and thus generated a stronger buffer capacity. Beyond 40 min, the mass transfer of the dissolved hydrogen gas could become a limiting factor, leading to a weaker buffer capacity with a longer PPR interval. Those findings have provided an effective pH control strategy with a convenient control system for maximizing hydrogen production in MECs.     

Characteristic of a multi-pass membrane separator for hydrogen separation through self-supported PdAg membranes

Dense PdAg membranes have shown immense potential to achieve high hydrogen purity required for proton exchange membrane (PEM) fuel cell. However, high hydrogen recovery and flux at lower transmembrane partial pressure is still a concern. In current study self-supported dense PdAg membranes were used to study the hydrogen recovery in a multi-pass membrane separator. Performance of a single and four collective membranes are tested in a single (without baffle) and multi-pass (with longitudinal baffles) membrane separator. Further, array of membrane configurations were tested experimentally by using longitudinal baffles and placing membranes at different locations. The hydrogen recovery for each configuration was measured experimentally. Experiments were performed using binary gas mixture 50H₂:50N₂ (v/v) at 3 bar pressure, 673 K temperature and gas-hourly space velocity (GHSV) 43 h^?1. The best assembly was further tested with typical methanol reformate gas composition by using simulated gas mixture of 50H₂:30N₂:18CO₂:2CO (v/v) at same operating condition. Numerical simulations were performed by using commercial software ANSYS 14.5 to understand the flow dynamics inside the separator with and without baffle. The results demonstrate that a multi-pass membrane separator enables to control hydrogen partial pressure radially along the length of reactor. This resulted in 33% enhancement in hydrogen recovery with multi-pass in comparison to single pass membrane separator.     

Hydrogen production rates with closely-spaced felt anodes and cathodes compared to brush anodes in two-chamber microbial electrolysis cells

Flat anodes placed close to the cathode or membrane to reduce distances between electrodes in microbial electrolysis cells (MECs) could be used to develop compact reactors, in contrast to microbial fuel cells (MFCs) where electrodes cannot be too close due to oxygen crossover from the cathode to the anode that reduces performance. Graphite fiber brush anodes are often used in MECs due to their proven performance in MFCs. However, brush anodes have not been directly compared to flat anodes in MECs, which are completely anaerobic, and therefore oxygen crossover is not a factor for felt or brush anodes. MEC performance was compared using flat felt or brush anodes in two-chamber, cubic type MECs operated in fed-batch mode, using acetate in a 50 mM phosphate buffer. Despite placement of felt anodes next to the membrane, MECs with felt anodes had a lower hydrogen gas production rate of 0.32 ± 0.02 m³-H₂/m³-d than brush anodes (0.38 ± 0.02 m³-H₂/m³-d). The main reason for the reduced performance was substrate-limited mass transfer to the felt anodes. To reduce mass transfer limitations, the felt anode electrolyte was stirred, which increased the hydrogen gas production rate to 0.41 ± 0.04 m³-H₂/m³-d. These results demonstrate brush electrodes can improve performance of bioelectrochemical reactors even under fully anaerobic conditions.     

Hydrogen effect on 2.25Cr–1Mo–0.25V bainitic steel under aging heat treatment

In the present work, the mechanical properties and hydrogen interaction with microstructures obtained after thermal treatment of a 2.25Cr–1Mo–0.25V bainitic steel were investigated. The samples were heat-treated under two different conditions simulating shop-repairs and manufacturing procedures of reactors. The microstructures after each heat treatment showed MC, M₂C, and M₇C₃ carbides, characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The hydrogen diffusivity values after the first permeation for both conditions were 4.2 × 10^?11 m²s^?1 and 8.6 × 10^?11 m² s^?1, and the solubility was 7.4 molHm^?3 and 5.3 molHm^?3, showing the influence trapping in the behavior of permeation curves. The TDS results indicate that a greater hydrogen trapping capability can be achieved with one of the studied conditions. Tensile testing of the hydrogenated and heat-treated samples, showed that the number of heat treatment cycles has a strong influence on the loss of ductility, which is mainly due to carbide growth and coalescence.     

Hydrogen adsorption on graphene sheets and nonporous graphitized thermal carbon black at low surface coverage

Behavior of hydrogen adsorption on nonporous carbon based materials was comparatively studied for selection of an efficient carrier for catalytic metals. Graphene sheets (GS) and graphitized thermal carbon black, which respectively has a specific surface area about 220 m²/g and 36 m²/g, were selected for adsorption equilibrium testes within temperature–pressure range from 77 K–87 K and 0–1 kPa. Henry law constants were employed to calculate the second virial constants and the limit isosteric heat of adsorption. The Weeks, Chandler and Andersen (WCA) perturbing scheme and the fundamental measure theory (FMT) were used to determine the interaction energy between solid atoms and hydrogen molecules. Adsorption potential well was determined by linear interpolation based on the Boltzmann distribution approximation. It shows that the potential well between hydrogen molecules and the GS, BP280 is respectively about 33.55 K and 31.97 K, suggesting that the bonding energy between the GS and hydrogen molecules is larger than that on carbon black.