Hydrogen Mole Fraction Research Articles

Mathematical approaches to modelling the mass transfer process in solid oxide fuel cell anode

In the recent literature, the influence of lean fuel, type of fuel, velocity profile and the flow direction on the performance operation of solid oxide cells (SOCs) is commonly investigated. SOCs generate electricity when they operate in fuel cell mode (SOFC – Solid Oxide Fuel Cell) or produce hydrogen in electrolysis mode (SOEC – Solid Oxide Electrolyzer Cell). They consist of several layers, including two porous electrodes separated by a gas-tight electrolyte. Typical anode porosity (fuel electrode in SOFC mode) varies from 20% to 50% (after sintering). The porosity value may limit the mass transport and the overall performance of SOC. This work investigates computationally and experimentally, the existing challenge such as diffusion mechanism in porous medium using the example of SOC electrode. A set of mass transport models (Fick's laws, modified Fick's law, Maxwell-Stefan Model, Dusty Gas Model) are described and discussed, highlighting their similarities, differences, advantages, and disadvantages as well as restrictions in usage. Finally, a detailed computational fluid dynamics (CFD) model of the diffusive transport in porous electrode was elaborated and validated. An experimental campaign was conducted using anode supported SOC, fabricated at the Institute of Power Engineering in Poland. Simulations were performed of the mass transport of pure hydrogen and pure water vapor or carbon dioxide (substitute of real product of electrochemical reaction in SOC for validation purposes) through porous anode. Tests were conducted in the wide flow range 10 [ml/min] up to 200 [ml/min] for both gases. The results demonstrate that hydrogen mole fractions reached similar values (0.5–0.6) at both outlets only for the maximum residence time or high porosity of anode. This implies that residence time and gas diffusion can be controlled by a combination of operating and microstructural parameters and can be estimated by numerical simulation using computational fluid dynamics. The main motivation and outcome of this work were to reduce the cost and time required for the optimization of the mass transport process through SOC's porous electrode.

Integrated electricity and gas system modelling with hydrogen injections and gas composition tracking

Despite the direct physical coupling between them, electricity and gas networks were traditionally modelled and operated independently. However, the heavy reliance on gas-fired generators to balance intermittent generation from renewable energy sources (RES), and the promising role of green hydrogen in decarbonising the natural gas system, have prompted a paradigm shift towards integrated electricity and gas system (IEGS) modelling. While many previous studies investigated the role of hydrogen in future low-carbon energy systems, a detailed assessment of hydrogen system integration into the electricity and gas transmission networks is still not addressed. Therefore, this paper presents a novel IEGS model with green hydrogen injections and gas composition tracking. The electricity system is modelled as a unit commitment model, formulated as a mixed-integer linear programming problem, and the gas system is modelled as a steady-state optimal gas flow (OGF). The developed model is demonstrated on two sets of case studies. The first case study validates the proposed OGF methodology on a small meshed gas test network, whereas the second case study demonstrates the applicability of the overall IEGS model with green hydrogen injection on the large-scale, real-world electricity and gas transmission networks of the state of Victoria (Australia). Results show that the proposed methodology can accurately capture the variations in gas flow direction while maintaining the hydrogen molar fraction within limits under hydrogen injections from multiple locations. Moreover, the amount of injected hydrogen not only depends on the level of RES curtailment, but also on local gas network constraints and local demand.

Improved kinetic model of hydrogen absorption and desorption in titanium with subsurface transport

We improve our previous kinetic model of hydrogen transport in Ti [Y. Hamamoto et al., Nucl. Mater. Energy 23 (2020) 100751]. The model becomes applicable to both the directional processes of absorption and desorption. The limitation in the covered range of hydrogen molar fraction, previously up to 1 mol-H/mol-Ti, is dissolved. The numerical calculations based on the model with a single set of kinetic parameters closely reproduce a series of experimental hydrogen absorption and desorption data for various temperatures, and thus verify the validity of the model.

Optimization of Methane Reforming for High Efficiency and Stable Operation of SOFC Stacks

: Solid Oxide Fuel Cells (SOFCs) is an efficient and clean pathway to generate electricity. In addition to hydrogen, SOFCs also provide the possibility for the utilization of a variety of carbon containing fuels. However, the application of carbon-based fuels in SOFC remains to be further studied. In this study, an external reformer was used to create appropriate reforming syngas from methanol and avoid carbon deposition. The performance and the efficiency of SOFC were researched with methanol-reformed syngas as fuel. The thermodynamic calculation and experimental work of water vapor reforming reaction of methanol fuel were carried out to study the composition of reforming gas under different temperature and steam-carbon ratio conditions, where the experimental results and the computational results matched well. The mole fraction of hydrogen in reformed syngas was about 55% within the temperature of 700~800 ℃, nevertheless, this value reached 70% if the moisture was removed. Therefore, the reformed syngas should be applied to SOFCs before being condensed and dried. Based on the comparison between the experimental results and the calculated ones, the appropriate components were selected to prepare the reforming syngas, feeding into the anode of SOFC to test the performance and the efficiency. Optimal temperature and steam-carbon ratio conditions for methanol vapor reforming were obtained. Under these conditions, the power density of SOFC exceeded 0.4 W/cm2 and the efficiency reached 50% (LHV). The main conclusions confirmed the high efficiency and feasibility of the methanol-fueled SOFC power generation system.

Green hydrogen leaking accidentally from a motor vehicle in confined space: A study on the effectiveness of a ventilation system

Green hydrogen can play a considerable role in helping the world achieve net zero emissions by forming a bridge between the energy sector and transportation. Currently, it is, at the initial phase of development, due to the low demand for hydrogen (H2) fuel for transportation in general. However, an increase in the H2 fuel demand in the future will require high investments in the infrastructure sector. In this regard, parking hydrogen vehicles in residential garages pose a potential safety hazard because of the accidents that could arise from hydrogen leaks. The diffusion of hydrogen in a ventilated garage has been investigated using FLUENT software. This study provides some insight into the hydrogen extraction efficiency of a natural ventilation system. We studied the influence of important ventilation parameters (shape and aspect ratio R of opening ventilation) on hydrogen stratification. First, results show a remarkable efficiency in the case of the square shape. Second, when the aspect ratio R decreases, there is an augmentation in fresh air drawn and hydrogen evacuation which generates the decrease of hydrogen concentration inside the garage and increases the extraction efficiency. Highlights This article develops a hot issue in the context of scarce resources, namely the use of hydrogen as a more environmental-friendly source of energy. Eventual leakage of hydrogen in confined spaces frequented by motor vehicles poses a significant hazard. The flow structure and molar fraction of hydrogen is strongly influenced by ventilation parameters The square shape has the lower concentration values; it presents the highest extraction efficiency. The hydrogen extraction of the transverse rectangular shape (R = 0.5) is more efficient and it presents better results than the others do.

The role of oxygenated species in the growth of graphene, fullerenes and carbonaceous particles

The growth of carbonaceous materials was studied using a Kinetic Monte Carlo model that captures the growth and oxidation of six-member and partially-embedded five-member rings. A novel algorithm was used to resolve the migration of partially-embedded five-member rings. Circumcoronene molecules were grown at 1500 K and 1 atm in the presence of varying mole fractions of atomic and molecular oxygen and constant mole fractions of hydrogen and acetylene. Four regions of carbon growth associated with different carbonaceous products were identified. Graphene was formed in the presence of high mole fractions of atomic oxygen 10−4<XO≤10−2. Fullerenes were formed in the presence of low mole fractions of atomic oxygen and high mole fractions of molecular oxygen (XO ≤ 10−4 and 10−2 < XO2 ≤ 10−1). Low mole fractions of both atomic and molecular oxygen (XO ≤ 10−4 and XO2 ≤ 10−2) resulted in structures that became curved as time progressed. Small structures were found at the highest mole fractions of atomic oxygen (XO > 10−2). The production and consumption of partially-embedded five-member rings appear to explain the formation of the observed structures. The oxidation of partially-embedded five-member rings produces armchair sites that grow to form flat graphenic structures. Formation and subsequent embedding of partially-embedded five-member rings result in curved structures that resemble fullerenes.

Numerical parametric study on the burner arrangement design for hydrogen production in a steam methane reformer

A numerical parametric study on the burner arrangement design for a steam methane reforming reactor was conducted under three burner configurations: a single co-axial inside burner (SCIB), a single top burner (STB), and multiple top burners (MTB). The burner combustion gas inlet angle was changed to 25°, 45°, and 90° with respect to the horizontal plane. Moreover, the number of reformer tubes inside the reactor was changed to four, six, and eight tubes arrangements. Furthermore, the models were numerically evaluated for temperature distribution inside the combustion space and hydrogen mole fraction in the reactor by incorporating the Xu and Forment model using a user-defined function (UDF). The results show that the STB configuration is preferable over the SCIB and MTB configuration due to the uniform temperature distribution inside the combustion space and efficient heating from the combustion gas. Furthermore, the standard deviation of the temperature inside the combustion space was found to be lower for the STB arrangement. Moreover, the STB configuration reported a maximum temperature uniformity index of 0.92 for the eight-tubes reformer arrangement. The value of the average wall heat transfer coefficient and wall heat flux at the reformer tube surface for STB configuration was 396.6 W/(m2K) and 21.7 W/m2, respectively. The velocity inlet angle of the burner combustion gas also plays an important role in the methane steam reforming reaction. A burner combustion gas inlet angle of 45° was observed to have better hydrogen conversion than the 25° and 90° inlet angles due to the proper mixing, leading to more effective heat transfer to the reformer tubes.

Investigation of syngas exergy value and hydrogen concentration in syngas from biomass gasification in a bubbling fluidized bed gasifier by using machine learning

In this study, an artificial neural network (ANN) model as a machine learning method has been employed to investigate the exergy value of syngas, where the hydrogen content in syngas reached maximum in bubbling fluidized bed gasifier which is developed in Aspen Plus® and validated from experimental data in literature. Levenberg-Marquardt algorithm has been used to train ANN model, where oxygen, hydrogen and carbon contents of sixteen different biomass, gasification temperature, steam and fuel flow rates were selected as input parameters of the model. Moreover, four different biomass samples, which hadn't been used in training and testing, have been used to create second validation. The hydrogen mole fraction of syngas was also evaluated at the different steam to fuel ratio and gasification temperature and the exergy value of syngas at the point where the hydrogen content in syngas reached maximum were estimated with low relative error value.

Source Term Estimation under the SBLOCA-Induced Severe Accident Condition in the SMART

The SMART is a system-integrated modular reactor in which a nuclear steam supply system with a thermal power of 365 MW is contained inside of the reactor vessel. Although the probability is very low, the reactor core can be damaged during a small break loss-of-coolant accident when both the passive safety injection system and the passive residual heat removal system are completely unavailable. In this work, a total of five cases were analyzed considering the reactor vessel condition and the availability of the radioactivity removal tanks and the ancillary containment spray system as containment condition variables using MELCOR code. It was estimated that there is no containment failure based on pressure, hydrogen mole fraction, and ablation depth, so that the release fractions of the 12 classes of fission products in MELCOR were evaluated considering design leak only for all cases. The overall source term of the case in which the integrity of the reactor vessel is maintained by the early initiation of the cavity flooding system was similar to that of the reactor vessel failure case. While the release fraction of cesium to the environment was analyzed to increase when there is no water in the radioactivity removal tanks, the fraction is small enough at which the radioactivity of the released cesium-137 remains well below 100 TBq, a regulatory limit. Moreover, it was found that the source term can be cut in half if the ancillary containment spray system is available. The results of this study verify the safety performance of the SMART under the small break loss-of-coolant severe accident condition with respect to the source term of interest.

Transient numerical modeling and model predictive control of an industrial-scale steam methane reforming reactor

A steam methane reforming reactor is a key equipment in hydrogen production, and numerical analysis and process control can provide a critical insight into its reforming mechanisms and flexible operation in real engineering applications. The present paper firstly studies the transport phenomena in an industrial-scale steam methane reforming reactor by transient numerical simulations. Wall effect and local non thermal equilibrium is considered in the simulations. A temperature profile of the tube outer wall is given by user defined functions integrated into the ANSYS FLUENT software. Dynamic simulations show that the species distribution is closely related to the temperature distribution which makes the temperature of the reactor tube wall an important factor for the hydrogen production of the reformer and the thermal conductivity of the catalyst network is crucial in the heat transfer in the reactor. Besides, there exists a delay of the reformer's hydrogen production when the temperature profile of the tube wall changes. Among inlet temperature, inlet mass flow rate and inlet steam-to-carbon (S/C) ratio, the mass flow rate is the most influencing factor for the hydrogen production. The dynamic matrix control (DMC) scheme is subsequently designed to manipulate the mole fraction of hydrogen of the outlet to the target value by setting the temperature profile trajectory of the reforming tube with time. The proportional-integral control strategy is also studied for comparison. The closed-loop simulation results show that the proposed DMC control strategy can reduce the overshoot and have a small change of the input variable. In addition, the disturbances of feed disturbance can also be well rejected to assure the tracking performance, indicating the superiority of the DMC controller. All the results give insight to the theoretical analysis and controller design of a steam methane reformer and demonstrate the potential of the CFD modeling in study the transport mechanism and the idea of combining CFD modeling with controller design for the real application.

Development of severe accident mitigation technology and analysis for SMART

Severe accident mitigation technology was developed and evaluated for a small integral reactor of SMART. The containment pressure and hydrogen behavior were analyzed using MELCOR computer code (Sandia National Laboratory, 2018) in the SBO (Station Black Out) sequence, which was selected from PSA (Probabilistic Safety Assessment) results for the SMART. The severe accident mitigation technology to improve the SMART safety include reactor vessel depressurization using the ADS (Automatic Depressurization System) to prevent DCH (Direct Containment Heating) in case of a reactor vessel failure, a reactor cavity flooding using the CFS (Cavity Flooding System) with the IRWST (In-containment Refueling Water Storage Tank) for the IVR-ERVC (In-Vessel corium Retention through External Reactor Vessel Cooling) to prevent the reactor vessel failure, and a hydrogen control system of PARs (Passive Autocatalytic Recombiners) to remove hazards from hydrogen combustion considering the amount of hydrogen to be generated by 100% fuel cladding oxidation. The MELCOR results showed that the containment pressures during the SBO sequence was below the design pressure, which meant that the containment integrity is maintained during a severe accident in the SMART. The hydrogen mole fraction of the containment was much lower than the hydrogen safety criteria of 10 vol%, which meant that the possibility of the hydrogen burn in the containment was negligible. For this reason, it could be concluded that the hydrogen was controlled by the hydrogen moving path and the hydrogen control system of PARs in the SBO sequence of the SMART.

A theoretical and experimental study on hydrodynamics, heat exchange and diffusion during methane pyrolysis in a layer of molten tin

A theoretical and experimental study was performed on the heat exchange, hydrodynamics and diffusion during the transition of methane through a layer of tin melt with the aim of intensifying the pyrolysis process in a reactor filled with metal melt. Analysis of the experiment data shows that, as the methane supply rate increases from 25 to 250 ml/min, the duration of the contact between the gas bubbles and tin melt shortens, which leads to a decrease in the mole fraction of hydrogen in the resulting gas from 12% to 4.4% (for a reactor 10 cm in height). Using a cascade of mesh diaphragms is suggested to intensify the pyrolysis process for high methane supply rates. A unique float-type structure may be placed inside the reactor to solve the problem of continuous removal of the hard carbon deposits generated during methane pyrolysis and to control the metal melt level.

Analysis of AP1000 severe accident induced by SBO using MAAP5

The total loss of AC power is an essential initiating event of PWR which can lead to a severe accident like Fukushima in Japan. For such improbable accidents, three cases of severe accidents of AP1000 induced by the station blackout (SBO) were simulated based on MAAP5 code. This particular analysis was conducted at 100% core power with different approaches. For the case with auto depressurization system (ADS), the primary system pressure appeared to uphold a low level for a longer duration and the external reactor vessel cooling (ERVC) system had sufficient capacity to eliminate the heat from the molten pool with a ratio of ex-vessel heat flux to critical heat flux (CHF) at about 0.49. For the case without ADS, the reactor pressure vessel (RPV) rupture will occur due to the creep effect inducing a sudden shock to the containment, and the peak pressure in the containment is still under the level at which the inelastic deformation may take place. The hydrogen production of the case without ADS in the core is 83% more than that of having ADS, while the hydrogen mole fraction in the upper containment is lower before the RPV failure. Overall, our findings are meaningful and significant for comprehending the detailed process of AP1000 severe accident induced by SBO, which is the basic standard for the development of the severe accident management guidelines (SAMG).

Isomerization of n‐pentane over platinum promoted tungstated zirconia supported on mesoporous SBA‐15 prepared by supercritical impregnation

AbstractTungstated zirconia was supported on mesoporous SBA‐15 by impregnation in supercritical ethanol. The resultant catalyst promoted with 1 wt% of Pt shows 100% selectivity of iso‐pentane and two times higher conversion in n‐pentane isomerization than the one prepared by conventional impregnation. Supercritical impregnation exerts no destruction of the porous silica and offers better dispersion of tungstated zirconia than conventional impregnation. Optimized loading of WO3/ZrO2/SBA‐15 is 20/40/40 wt%, which infers to 2.4 W‐atoms/nm2 and 8.9 Zr‐atoms/nm2 dispersed on the SBA‐15 support. Moreover, introducing a proper mole fraction of hydrogen in the reactants markedly improves the catalytic activity in n‐pentane isomerization. The role of hydrogen was not only served as the reactants, it also acted to regenerate Brønsted acid sites during n‐pentane isomerization. However, overdose of hydrogen leaded to a leveling‐off in the rate of isomerization and enhanced hydrogenolysis reaction. The in situ IR spectroscopic studies suggested that Brønsted acid sites were the active centers for the isomerization. A reaction mechanism was, therefore, proposed that heterolytic fission of hydrogen and dehydrogenation of n‐pentane took place over Pt. The secondary carbocation formed over Brønsted acid sites was rearranged to a tertiary carbocation by methyl migration, followed by reacting with a hydride to generate iso‐pentane.

On the reasons of spatial heterogeneity of the plasma chemical composition in ablation controlled discharges

The reasons of spatial heterogeneity of the plasma chemical composition in ablation controlled discharges are analyzed. The “locking” of atoms of chemical elements with a low ionization potential in the vicinity of a negatively charged wall due to the backflow of a significant fraction of ionized atoms onto the wall is considered as one of the effective mechanisms of plasma demixing and change of its chemical composition in comparison with the initial stoichiometry of the ablating substance. The proposed approach is consistent with the results of spectroscopic studies of a capillary discharge with an ablating wall (polymethylmethacrylate), during which a significant decrease in the carbon mole fraction with a simultaneous increase in the hydrogen mole fraction in the discharge core was revealed. According to the obtained results, the physical processes leading to the spatial separation of chemical elements occur exactly in the capillary spatial domain. The observed demixing of the plasma jet is manly a consequence of the processes that take place inside the capillary.

Hydrogen effects on ignition delay time of methyl butanoate in a rapid compression machine

SummaryThe effect of hydrogen addition on the ignition delay time of methyl butanoate was studied using a rapid compression machine experiment and ANSYS CHEMKIN‐PRO 19.0 numerical analysis. The ignition delay was measured under engine‐relevant conditions by varying equivalence ratios (0.5 and 1.0), compression pressures (15 and 30 bar), compression temperatures (860‐1060 K), and hydrogen mole fractions in the methyl butanoate/hydrogen mixture (0%, 25%, 50%, and 75%). The kinetic mechanism was modified by adding the methyl butanoate mechanism to the NUIG Aramco 3.0 mechanism and modifying the rate parameters of the H‐atom abstraction reactions of methyl butanoate. The numerical results obtained using the modified mechanism were in good agreement with the experimental results. The methyl butanoate/hydrogen mixture did not show a negative temperature coefficient characteristic. With hydrogen addition, the ignition delay increased at low temperatures but decreased at high temperatures. The results of the chemical and dilution effect analysis using the imaginary inert H2 showed that the chemical effects differ depending on the temperature. Most added hydrogen molecules participate in the chain propagation reactions to produce H radicals and consume OH by the reaction H2 + OH → H + H2O. This, in turn, changes the reactions of other species, such as HO2 and H2O2. The results of the reaction path analysis for the related species showed that the addition of hydrogen reduced radical reactions at low temperatures, while increasing them at high temperatures.

Heat flow inside a catalyst particle for steam methane reforming: CFD-modeling and analytical solution

Numerical investigation of a steam methane reforming process was performed from point of view to understand the heat flows inside a catalyst particle. To verify the numerical results, a new method based on the thermal conductivity equation has been developed to determine the temperature distribution inside the catalyst particle. The CFD-model was realized via ANSYS Fluent. To model the steam methane reforming process, the industrial Ni-based catalyst with a spherical particle was chosen. The temperature contours inside the catalyst particle and hydrogen mole fraction in the reaction space was calculated both numerically and analytically. The results show the irregularity in the distribution of the temperature field inside the catalyst. In the direction of flow, a minimum catalyst temperature occurs. In this case, the temperature decrease inside the catalyst occurs unevenly. Also, the temperature change on the catalyst surface as a function of flow time was analyzed.

Study of silicon nitride deposition in III-N MOVPE reactors

Si3N4 deposition from silane and ammonia in a number of III-N MOVPE reactors of various sizes was studied in a wide range of reactor conditions. It was revealed that Si3N4 deposition rate depends on temperature, pressure, carrier gas type, and ammonia concentration. Deposition rate rises with temperature up to 1050–1100 °C in a manner typical for temperature-activated processes, but under any studied conditions, including the 800–850 °C range, it is strictly linear with SiH4 concentration indicating the absence of high-order parasitic reactions at high temperature and surface passivation at low temperature. For a low-volume reactor the higher the pressure is, the faster the deposition is both for N2 and H2 carrier gases. For large reactors the dependence is non-monotonic. At temperature above 900 °C using H2 as a carrier gas results in a higher Si3N4 deposition rate than when using N2. If nitrogen is used as a carrier gas, deposition rate gradually rises with ammonia concentration. If hydrogen is used, deposition rate rapidly rises with ammonia concentration and then gradually falls. If hydrogen-nitrogen mixture is used as a carrier gas, deposition rate changes in a linear manner with the mole fraction of hydrogen in the carrier gas.

A novel integrated structure for hydrogen purification using the cryogenic method

A cryogenic-based module is developed for the separation of the hydrogen from a feed gas mixture including nitrogen and linear branched or cyclic C1−C6 hydrocarbons. The feed gas mixture is initially separated in a network of heat exchangers and phase separators to produce hydrogen-rich and hydrogen-lean streams. The hydrogen-enrich stream is extracted from the cryogenic separation heat exchangers and the hydrogen-lean stream is processed through the cryogenic sections to remove non-hydrogen components and produce aromatic, low pressure, and high-pressure fuel streams. A Joule-Bryton propane refrigeration cycle and an auxiliary nitrogen refrigeration stream supply the required cryogenic streams. The process is energy and exergy analyzed and results show that the more than 88.4% of exergy destruction occurred in heat exchangers and the valves have the least role in exergy destruction. Moreover, sensitivity analysis shows that when the mole fraction of hydrogen in feed gas increases by 60.0%, the specific energy decreases by 38.0%, the required power increases by 68.6%, the exergy efficiency reduces by 58.3%, and the required heat increases by 60.0%. As well as, when the pressure of stream 23 increases by 10%, the reduction of exergy efficiency is 9.2%, the promotion of specific energy is 3.0%, the reduction of hydrogen recovery rate is 0.08%, and the increase of nitrogen refrigeration capacity is 10.60%. Therefore, a comprehensive view is needed to change the operational parameters to achieve the desired results. The proposed module may be combined with many other gas upgrading systems.

The role of co 2 in improving sonic hydrogen production

The role of co ₂ in improving sonic hydrogen production