Transition of deflagration to detonation in ethylene–hydrogen–air mixtures

The results of systematic experiments on deflagration-to-detonation transition (DDT) in homogeneous ethylene–hydrogen–air mixtures at normal pressure and temperature conditions are reported. Experiments are performed in a pulse-detonation tube of three different configurations with one open end. Hydrogen content and fuel-to-air equivalence ratio in the mixture are varied from 0 to 100% and from 0.5 to 3.5, respectively. The measured DDT run-up distance and time are shown to sharply decrease only at hydrogen content exceeding 70%vol. in the tube of all three configurations. The observed effect is explained by multidirectional influence of hydrogen addition on the mixture physicochemical properties relevant to the DDT phenomenon.     

Effect of solid obstacle distribution on flame acceleration and DDT in obstructed channels filled with hydrogen-air mixture

Ensuring hydrogen safety has become of great significance nowadays, whose leakage can possible result in the deflagration to detonation (DDT). The numerical study aims to explore the effect of solid obstacle distribution on the DDT in a homogeneous hydrogen-air mixture. Results show that the detonation initiation process can be classified into two types: i) local spherical detonation caused by the coupling of flame surface and high-pressure region; ii) detonation triggered by the interaction between the upper wall and multiple compression waves in front of the flame. Also, this study finds that the flame acceleration (FA) experiences two periodic “acceleration-deceleration” processes before the detonation initiation, and the initiation distance and time are the shortest when the obstacles are symmetrically distributed. Further, the higher the unilateral blockage ratio, the more unfavorable DDT occurs. The present results highlight the effect of different solid obstacle distribution patterns on the FA and DDT process.     

Assessment of ammonia as energy carrier in the use with reversible solid oxide cells

Ammonia represents one of the most promising potential solutions as energy vector and hydrogen carrier, having a higher potential to transport energy than hydrogen itself in a pressurized form. Furthermore, solid oxide fuel cells (SOFCs) can directly be fed with ammonia, thus allowing for immediate electrical power and heat generation. This paper deals with the analysis of the dynamic behavior of commercial SOFCs when fueled with ammonia. Several measurements at different temperatures have been performed and performances are compared with hydrogen and a stoichiometrically equivalent mixture of H₂ and N₂ (3:1 M ratio). Higher temperature led to smaller drops in voltage for both fuels, thus providing higher efficiencies. Ammonia resulted slightly more performant (48% at 760 °C) than hydrogen (45% at 760 °C), in short stack tests. Moreover, different ammonia-to-air ratios have been investigated and the stack area-specific resistance has been studied in detail by comparing numerical modeling predictions and experimental values.     

Numerical simulation of flame acceleration and deflagration-to-detonation transition in hydrogen-air mixtures with concentration gradients

The present study aims to test the capability of our newly developed density-based solver, ExplosionFoam, for flame acceleration (FA) and deflagration-to-detonation transition (DDT) in mixtures with concentration gradients which is of important safety concern. The solver is based on the open source computational fluid dynamics (CFD) platform OpenFOAM^® and uses the hydrogen-air single-step chemistry and the corresponding transport coefficients developed by the authors. Numerical simulations have been conducted for the experimental set up of Ettner et al. [7], which involves flame acceleration and DDT in both homogeneous hydrogen-air mixture as well as an inhomogeneous mixture with concentration gradients in an obstucted channel. The predictions demonstrate good quantitative agreement with the experimental measurements in flame tip position, speed and pressure profiles. Qualitatively, the numerical simulations have reproduced well the flame acceleration and DDT phenomena observed in the experiment. The results have revealed that in the computed cases, DDT is induced by the interaction of the precursor inert shock wave with the wall close to high hydrogen concentration rather than with the obstacle. Some vortex pairs appear ahead of the flame due to the interaction between the obstacles and the gas flow caused by combustion-induced expansion, but they soon disappear after the flame passes through them. Hydrogen cannot be completely consumed especially in the fuel rich region. This is of additional safety concern as the unburned hydrogen can be potentially re-ignited once more fresh air is available in an accidental scenario, resulting in subsequent explosions.     

Detonation propagation characteristics of H2/O2/AR in multi-angle bifurcation tubes

The propagation characteristics of the detonation wave in the bifurcated tube with the angular variation range of 30°–90° are simulated with 25% AR as dilution gas for H₂/O₂ mixture fuel at chemical equivalence ratio using the solver DCRFoam built on the OpenFOAM platform. The diffraction and reflection phenomena of detonation waves passing through bifurcation tubes with different angles are studied and analyzed. The results show that the distance from regular reflection to Mach reflection increases with the increase of the bifurcation angle so that after one reflection, the detonation forms three reflection forms with the angle of the different bifurcation tubes. After the first reflection, the detonation waves are more likely to induce the formation of transverse waves in the low-angle bifurcation tube. The lowest collision pressure after the detonation collides with the upper wall to form a secondary reflection occurs in the bifurcation tube between 50° and 60°.     

Influence of longitudinal obstacle spacing on the deflagration-to-detonation transition through a bank of obstacles in a hydrogen-air mixture

Flame acceleration and deflagration-to-detonation transition (DDT) in a channel containing an array of staggered cylindrical obstacles and a stoichiometric hydrogen-air mixture were studied by solving the fully-compressible reactive Navier-Stokes equations using a high-order numerical algorithm and adaptive mesh refinement. Four different longitudinal spacings (ls) of the neighboring obstacle rows (i.e., ls = 15.28, 19.1, 25.4, and 38.2 mm, corresponding to 1.2, 1.5, 2 and 3 times of obstacle diameter, respectively) were used to examine the effect of obstacle spacing on flame acceleration and DDT. The results show that the main mechanisms of flame acceleration and transition to detonation in all the cases studied are consistent. While the flame acceleration is caused by the growth of flame surface area in the initial stage, it is governed by shock-flame interactions in the later stage when shock waves are generated. The focusing of strong shocks at flame front is responsible for the initiation of detonation. It was found that the flame propagation speed and the DDT run-up distance and time are highly dependent on ls. Specifically, the flame acceleration declines as ls increases, since a larger ls leads to less disturbance of flow by obstacles per unit channel length. For detonation initiation, both the run-up distance and time increase with the increase of ls. It is interesting to note that the DDT distance and time increase significantly as ls increases from 19.1 mm to 25.4 mm. This is related to the slowdown of the increase rate of energy release over a period before DDT occurs under large ls condition, because every time the flame passes over an obstacle row the shock-flame interaction is delayed and numerous isolated pockets of unburned gas material are formed.     

Thermophysical properties of the liquid organic hydrogen carrier system based on diphenylmethane with the byproducts fluorene or perhydrofluorene

Fluorene (H0-F) and perhydrofluorene (H12-F) represent process-related byproducts formed by a dehydrocyclization step in the liquid organic hydrogen carrier (LOHC) system based on diphenylmethane (H0-DPM) and dicyclohexylmethane (H12-DPM). The influence of these byproducts on the liquid viscosity, surface tension, and liquid density of the DPM-based system was experimentally determined by studying three dehydrogenated binary mixtures with H0-F mole fractions of 0.05, 0.10, and 0.20 as well as one hydrogenated binary mixture with an H12-F mole fraction of 0.10 close to 0.1 MPa from (283–573) K. The densities increase with increasing share of H0-F or H12-F by around 1% per added byproduct mole fraction of 0.1. For the surface tension, an increase relative to the values of H0-DPM or H12-DPM by up to 6% is found. The addition of H0-F to H0-DPM or H12-F to H12-DPM yields a relative increase in viscosity by up to 9% at the lowest temperature studied.     

Effect of obstacles on the detonation diffraction and subsequent re-initiation

The DCRFoam solver (density-based compressible solver) built on the OpenFOAM platform is used to simulate the reflection and diffraction processes that occur when detonation waves collide with various objects. Static stoichiometric hydrogen–oxygen mixtures diluted with 70% Ar are used to form stable detonation waves with large cells, with initial conditions of 6.67 kPa pressure and 298 K temperature. The diameters of the cylindrical obstacle range from 6 mm to 22 mm, with x = 230 mm, x = 244 mm, and x = 257 mm being the chosen position. Cylindrical, square, triangular, and inverted triangular obstacles are used, and the quenched detonation re-initiation processes behind them are investigated. In the detonation diffraction process, four triple points exist at the same time due to the effect of cylindrical obstacles of smaller diameters. The re-initiation distance of the detonation wave increases with the increase of cylindrical obstacle diameter. Both the Mach reflection angle and the decoupled angle decrease as the diameter increases. When the location of the cylindrical obstacles is changed, the detonation wave dashes into the obstacles with its different front structures, it is easier to realize the detonation re-initiation when the weak incident shock at the front of a detonation wave strikes the obstacles, and the re-initiation distance decreases by 17.1% when compared with the longest re-initiation distance. The detonation re-initiation distance is shortest under the action of cylindrical obstacles, however the quenched detonation cannot be re-initiated when the inverted triangle and square obstacles are used. The suppression effects of inverted triangle and square obstacles on detonation waves are more evident.     

Biomass gasification coupled with producer gas cleaning,bottling and HTS catalyst treatment for H2-rich gas production

The aim of the present study is to demonstrate the production of hydrogen-rich fuel gas from J. curcas residue cake. A comprehensive experimental study for the production of hydrogen rich fuel gas from J. curcas residue cake via downdraft gasification followed by high temperature water gas shift catalytic treatment has been carried out. The gasification experiments are performed at different equivalence ratios and performance of the process is reported in terms of producer gas composition & its calorific value, gas production rate and cold gas efficiency. The producer gas is cleaned of tar and particulate matters by passing it through venturi scrubber followed by sand bed filter. The clean producer gas is then compressed at 0.6 MPa and bottled into a gas cylinder. The bottled producer gas and a simulated mixture of producer gas are then subjected to high temperature shift (HTS) catalytic treatment for hydrogen enriched gas production. The effect of three different operating parameters GHSV, steam to CO ratio and reactor temperature on the product gas composition and CO conversion is reported. From the experimental study it is found that, the presence of oxygen in the bottled producer gas has affected the catalyst activity. Moreover, higher concentration of oxygen concentration in the bottled producer gas leads to the instantaneous deactivation of the HTS catalyst.     

Experimental study of end-vented hydrogen deflagrations in a 40-foot container

Experiments were conducted in an enclosure with the same overall dimensions as a 40-foot ISO container to study the vented hydrogen-air deflagrations. This work focuses on the effects of hydrogen concentration, ignition location and obstacles on the overpressure and the structural response of the container wall. For center ignition, three overpressure peaks, which resulted from the vent opening, Helmholtz oscillation and acoustic oscillation, respectively, were recorded inside the container without obstacles. However, with the increase of hydrogen concentration, the third overpressure peak disappears when the obstacles are added in the container. Unlike center ignition, only two overpressure peaks were observed for back ignition. Due to the difference in reactivity of hydrogen-air mixture, the first overpressure peak is generated by the vent burst for low hydrogen concentration, or the venting of flame for high hydrogen concentration. The overpressure induced by the flame-acoustic interaction was not monitored with the increase of the hydrogen concentration and the installation of obstacles for back ignition. The overpressure for back ignition is more influenced by the obstacles than that for center ignition, when hydrogen concentration is larger than 12%. The displacement-time curves share similar trends with the pressure-time curves. The first peak displacement changes linearly with the corresponding first peak overpressure. However, the displacement caused by the second overpressure peak is significantly increased, especially for high hydrogen concentration and back ignition in the case with two obstacles.     

Pd-decorated CNT as sensitive material for applications in hydrogen isotopes sensing - Application as gas sensor

Applicability of multiwall carbon nanotubes (MWCNTs) decorated with palladium nanoparticles as sensitive layer in a resistive microsensor for identification of hydrogen isotopes, Deuterium (²H) and Protium (¹H), has been demonstrated. Palladium nanoparticles were anchored on the MWCNTs surface via a chemical process involving micellization, from a precursor chloride solution, in high ultrasonic density field. Pd-MWCNTs are quasi-aligned between the interdigitated gold electrodes of a SiO₂ substrate by drop casting and di-electrophoretic alignment in Tetrahydrofuran (THF) and Nafion solution. The morphostructural characterization of the sensitive material has been carried out through SEM, TEM and Raman spectroscopy and its gas sensing properties were evaluated using electrical measurements performed on a series of isotope concentrations (ranging from 0.1% up to 1%, and from 1% to 4%, value to which hydrogen becomes explosive) diluted in argon, to observe the evolution of the sensor sensibility. The two hydrogen isotopes have different behaviors related to the adsorption on the Pd-MWCNT, which is well observed in the resistance change. Therefore, the sensor based on Pd-MWCNTs could be a viable solution to be integrated in systems for hydrogen leakage detection.     

Effect of high compression ratio on improving thermal efficiency and NOx formation in jet plume controlled direct-injection near-zero emission hydrogen engines

The Plume Ignition and Combustion Concept (PCC) developed by the authors significantly reduced nitrogen oxide (NOx) emissions in a direct-injection hydrogen engine under high-load operation. With PCC, a rich fuel plume is ignited immediately after completion of injection in the latter half of the compression stroke to reduce NOx formation. Simultaneously, high thermal efficiency was also achieved by mitigating cooling losses through optimization of the jet configuration in the combustion chamber. This basic combustion concept was applied to burn lean mixture in combination with the optimized hydrogen jet configuration and the application of supercharging to recover the power output decline due to the use of a diluted mixture. As a result, a near-zero-emission-level engine has been achieved that simultaneously provides high thermal efficiency, high power output and low NOx emissions at a single-digit ppm level [1]. In this study, a high compression ratio was applied to improve thermal efficiency further by taking advantage of the characteristics of hydrogen fuel, especially its diluted mixture with a high anti-knock property. As a result, NOx emissions at a single-digit ppm level and gross indicated thermal efficiency of 52.5% were achieved while suppressing knocking at a compression ratio of 20:1 by optimizing the excess air ratio and injection timing, and increasing power output by supercharging.     

Supercritical water gasification mechanism of polymer-containing oily sludge

In the offshore petroleum industry, polymer-containing oily sludge (PCOS) hinders oil extraction and causes tremendous hazards to the marine ecological environment. In this paper, an effective pretreatment method is proposed to break the adhesive structure of PCOS, and the experiments of supercritical water gasification are carried out under the influencing factors including residence time (5–30 min) and temperature (400–750 °C) in batch reactors. The increase of time and temperature all show great promoting effects on gas production. Polycyclic aromatic hydrocarbons, including naphthalene and phenanthrene, are considered as the main obstacles for a complete gasification. Carbon gasification efficiency (CE) reaches maximum of 95.82% at 750 °C, 23 MPa for 30 min, while naphthalene makes up 70% of the organic compounds in residual liquid products. The highest hydrogen yield of 19.79 (mol H₂/kg of PCOS) is observed in 750 °C for 25 min. A simplified reaction pathway is presented to describe the gaseous products (H₂, CO, CO₂, CH₄). Two intermediates are defined for describing the reaction process bases on the exhaustive study on organic matters in residual liquid products. The results show that the calculated data and the experimental data have a high degree of fit and tar formation reaction is finished within 10 min.     

A new approach for improving the efficiency of the indirectly coupled photovoltaic-electrolyzer system

Hydrogen as an energy storage medium is considered an effective solution to the problem of the intermittent availability of renewable and innovative energies. This study focused on improving the efficiency of an indirectly coupled photovoltaic-electrolyzer (PVE) system using an improved maximum power point tracking (IMPPT) technique. The IMPPT is based on a neural current estimator and a variable step size (VSS) technique. The neural network-based current estimator is employed to estimate the short circuit current (Isc) quickly and directly without the need for periodic disconnection of the PV system, which in turn, reduces the losses resulting from the measurement process. The VSS technique is used to achieve an optimal adjustment of the duty cycle. The accurate electrical model of the PVE system was simulated in MATLAB/Simulink environment. The system performance was evaluated for one day using actual radiation data of Zonguldak City on June 19, 2021. Simulation results show that using the proposed IMPPT system, the PVE system achieves higher performance compared to using both P&O-based MPPT and directly coupled mode. Assuming an ideal electrolyzer, the efficiencies of the PVE system were about 91% using the IMPPT, 67% using the P&O-based MPPT, and 45% using the directly coupled mode. This demonstrates that the proposed system exhibits superior performance and increases hydrogen production efficiency compared to directly coupled PVE systems that need fine-tuning of system parameters.     

Producing hydrogen from the fermentation of cheese whey and glycerol as cosubstrates in an anaerobic fluidized bed reactor

This study aimed to evaluate the effect of the organic loading rate (OLR) (60, 90, and 120 g Chemical Oxygen Demand (COD). L^?1. d^?1) on hydrogen production from cheese whey and glycerol fermentation as cosubstrates (50% cheese whey and 50% glycerol on a COD basis) in a thermophilic fluidized bed reactor (55 °C). The increase in the OLR to 90 gCOD.L^?1. d^?1 favored the hydrogen production rate (HPR) (3.9 L H₂. L^?1. d^?1) and hydrogen yield (HY) (1.7 mmol H₂. gCOD^?1_app) concomitant with the production of butyric and acetic acids. Employing 16S rRNA gene sequencing, the highest hydrogen production was related to the detection of Thermoanaerobacterium (34.9%), Pseudomonas (14.5%), and Clostridium (4.7%). Conversely, at 120 gCOD.L^?1. d^?1, HPR and HY decreased to 2.5 L H₂. L^?1. d^?1 and 0.8 mmol H₂. gCOD^?1_app, respectively, due to lactic acid production that was related to the genera Thermoanaerobacterium (50.91%) and Tumebacillus (23.56%). Cofermentation favored hydrogen production at higher OLRs than cheese whey single fermentation.     

Design and performance of asymmetric supported membranes for oxygen and hydrogen separation

Producing syngas and hydrogen from biofuels is a promising technology in the modern energy. In this work results of authors’ research aimed at design of supported membranes for oxygen and hydrogen separation are reviewed. Nanocomposites were deposited as thin layers on Ni–Al foam substrates. Oxygen separation membranes were tested in CH₄ selective oxidation/oxi-dry reforming. The hydrogen separation membranes were tested in C₂H₅OH steam reforming. High oxygen/hydrogen fluxes were demonstrated. For oxygen separation membranes syngas yield and methane conversion increase with temperature and contact time. For reactor with hydrogen separation membrane a good performance in ethanol steam reforming was obtained. Hydrogen permeation increases with ethanol inlet concentration, then a slight decrease is observed. The results of tests demonstrated the oxygen/hydrogen permeability promising for the practical application, high catalytic performance and a good thermochemical stability.     

Performance evaluation of a solarized gas turbine system integrated to a high temperature electrolyzer for hydrogen production

In this paper, the performance of a solar gas turbine (SGT) system integrated to a high temperature electrolyzer (HTE) to generate hybrid electrical power and hydrogen fuel is analyzed. The idea behind this design is to mitigate the losses in the electrical power transmission and use the enthalpy of exhaust gases released from the gas turbine (GT) to make steam for the HTE. In this context, a GT system is coupled with a solar tower including heliostat solar field and central receiver to generate electrical power. To make steam for the HTE, a flameless boiler is integrated to the SGT system applying the SGT extremely high temperature exhaust gases as the oxidizer. The results indicate that by increasing the solar receiver outlet temperature from 800 K to 1300 K, the solar share increases from 22.1% to 42.38% and the overall fuel consumption of the plant reduces from 7 kg/s to 2.7 kg/s. Furthermore, flameless mode is achievable in the boiler while the turbine inlet temperature (TIT) is maintained at the temperatures higher than 1314 K. Using constant amounts of the SGT electrical power, the HTE voltage decreases by enhancing the HTE steam temperature which result in the augmentation of the overall hydrogen production. To increase the HTE steam temperature from 950 K to 1350 K, the rate of fuel consumption in the flameless boiler increases from 0.1 m/s to 0.8 m/s; however, since the HTE hydrogen production increases from 4.24 mol/s to 16 mol/s it can be interpreted that the higher steam temperatures would be affordable. The presented hybrid system in this paper can be employed to perform more thermochemical analyses to achieve insightful understanding of the hybrid electrical power-hydrogen production systems.     

Hydrogen binding and dissociation in MgScHn clusters (n ≤ 20)

Transition metal doped magnesium hydride solids are a leading candidate for hydrogen storage materials. In this investigation, MgScH_n clusters (n = 1–20) are theoretically studied using density functional theory and Møller-Plesset perturbation theory. It is determined that hydrogen binds successively to the MgSc diatomic metal center up to MgScH₁₃ when the cluster becomes saturated at 15.9% hydrogen by mass. In contrast to earlier predictions, we shown that for MgScH₁₄ and larger clusters molecular hydrogen dissociates from the core cluster structure. A local minimum is observed on the potential energy surface for larger clusters where dissociated hydrogen interacts with a negatively charged hydride of the core cluster in a dipole-induced dipole intermolecular force, providing insight into the dissociation pathway in bulk magnesium hydride materials doped with transition metals. Analysis of the frontier orbitals and natural bonding analysis of these clusters support this logical dissociation pathway.     

Transition to detonation in a hydrogen-air mixtures due to shock wave focusing in the 90 - deg corner

This work presents experimental observation of the ignition modes due to shock wave focusing in a 90 - deg corner in a mixture of 20%–55% H₂ in air with the main purpose of recognizing critical conditions for transition to detonation. The results showed three ignition modes, first ‘weak’ ignition followed by deflagration with ignition delay time higher than ∼1 μs, second ‘strong’ with instantaneous transition to detonation, and third with deflagrative ignition and delayed transition to detonation. The transition was observed only when specific shock wave velocity was reached. The transition velocity for stoichiometric mixture was approx. 715 m/s corresponding to M = 1.75 and 71% of speed of sound in products. For leaner or richer mixtures, the transition velocity increased approaching the speed of sound in products at approx. 18% and 58% H₂.