Hydrogen Mole Fraction Research Articles

Effects of flow and operation parameters on methanol steam reforming in tube reactor heated by simulated waste heat

In order to utilize waste heat that is widely available in many industries, effects of flow and operation parameters on micro-tube reactor performance for hydrogen production through methanol steam reforming were studied by simulation. Results showed that outlet parameters are not significantly affected by the flow arrangements in both cocurrent and countercurrent flow conditions of reactant/heating media. However, the choice of aligned or crossing reactor tubes would affect methanol conversion and CO molar fraction. Methanol conversion rate and hydrogen molar fraction were higher for aligned arrangement than that of the crossing arrangement at different inlet velocities except for the inlet velocity of 0.1 m/s condition. They were also higher for aligned arrangement at different water/methanol molar ratios. CO molar fraction at reactor outlet was also examined for different tube arrangements. The results from this study can be used to optimize the design parameters for waste heat recovery coupled with methanol steam reforming reactors for hydrogen production.

Boltzmann Analysis of Cryogenic $\text{He}$ –$\text{H}_{\text{2}}$ Gas Mixtures as Dielectric Media for High-Temperature Superconducting Power Devices

Analysis of the dielectric properties of gas mixtures in the context of their application as cryogens for high-temperature superconducting power applications is presented. Boltzmann analysis was conducted for pure gaseous helium (He) and its mixtures consisting of small mole fractions of gaseous hydrogen (H 2 ) to establish a theoretical foundation for the observed enhancement in dielectric strength at cryogenic temperatures. Boltzmann analysis showed that the electron energy distribution function shifts to the lower energy region, the reduced ionization coefficient (α/N), and the reduced effective ionization coefficient ((α - η)/N) decreases, whereas the reduced attachment coefficient (η/N) and the reduced critical electric field ((E/N)cr) increases as the mole fractions of H 2 increase in the He-H 2 mixture. The results of the analysis predict a persisting enhancement of dielectric strength by adding larger amounts of H 2 beyond the reported 4 mol% in the He-H 2 mixtures. The analysis also provides a basis for identifying a suitable gas mixture for gaseous cryogen cooled superconducting device with a required power rating.

Innovative hydrogen recombiner concept for severe accident management in nuclear power plants

Excursion of catalyst surface temperature is an important safety concern in deployment of a passive catalytic recombiner device in the nuclear reactor containment. In order to minimize the temperature on the catalyst surface, heat transfer from the surface has to be enhanced. In the present paper, a conceptual design of an innovative PAR is presented. This recombiner uses pool boiling of water to enhance heat transfer from the catalyst surface and limit its temperature. Numerical studies are presented to demonstrate the efficacy of this design. An in-house numerical code called HDS that couples CFD techniques with models for reaction kinetics and water pool boiling heat transfer is used in the analysis. It is shown that unlike conventional recombiner designs, the present design limits the surface temperature even at very high hydrogen mole fractions. The recombiner ensures passive and safe operation for hydrogen mole fraction as high as 16% and for the duration in which high hydrogen concentration is expected. The recombiner can be installed in areas such as the fuelling machine vault and pump room, where high hydrogen concentration is expected.

Effects of fuel composition and exhaust gas recirculation on ignition delay characteristics of n-heptane/hydrogen blended fuel

The autoignition characteristics of n-heptane/hydrogen were investigated numerically. The effects of the temperature, fuel compositions and Exhaust gas recirculation (EGR) on the autoignition characteristics were evaluated. The numerical study on the ignition delay time was performed using the CHEMKIN-PRO software to calculate ignition delay time and predict the chemical species in the combustion process. The results revealed that ignition delay time decreased with an increase in the hydrogen fraction in the mixture in low temperature regime, but the opposite behavior observed in high temperature regime. The oxidation reaction of n-heptane in low temperature regime is limited with increasing mole fraction of hydrogen in blended fuel. Thus, cool flame which is characteristics of n-heptane is weakened. As consumption rate of n-heptane and temperature are decreased, total reactivity is decreased. The ignition delay time sharply decrease with EGR due to intermediate species in the exhaust gas.

Numerical Investigation of PEM Water Electrolysis Performance for Different Oxygen Evolution Electrocatalysts

AbstractOne of the hydrogen production methods, namely the proton exchange membrane water electrolyzer (PEMWE), shows good potential and agreement with renewable energy sources in the storage and conversion of energy. In this study, the effects of different anode catalyst materials on the performance of PEMWE are investigated. In line with this aim the temperature, membrane thickness, current collector length and molar fraction distribution of species in gas channels are investigated to observe and compare the effects of different anode catalyst. The cell performance is simulated with a two dimensional numerical model based on Comsol Multiphysics Software. The performance of the cell and hydrogen production are increased with the change of temperature from 303 K to 353 K. Different thickness of the membrane is investigated and the results bring out that use of thinner membrane is more important for Pt‐Ir anode catalyst than Pt anode catalyst. Additionally, Pt‐Ir anode catalyst is decreased cell voltage and provided four times higher current density. Maximum value of hydrogen molar fraction in the cathode gas channel is increased 60%.

Utilization of Crude Glycerin for Synthetic Gas Production and Potential Electricity Generation

The principal goal of this research was to explore a new avenue of utilizing crude glycerin in potential electricity generation via gasification process integrated with Stirling engine generator. Glycerin (C3H8O3) is a byproduct of vegetable oil or animal fat transesterification process for biodiesel production. An externally heated fixed bed gasification unit was installed to achieve thermal decomposition of glycerin. Glycerin was gasified in the externally heated gasifier. The effects of bed temperature (650, 700, and 750°C) and water to glycerin ratio (0%, 15%, and 30%) on the products yield (gas, liquid, and particulates), gas composition and heating value were investigated. The gas production rate reached 53.4% of the product weight during gasification of crude glycerin. Glycerin gasification produces gas with a relatively medium heating value of 13.14 MJ/m3. The maximum hydrogen mole fraction of 43.6% was obtained under the bed temperature of 750°C. A scenario of integrating crude glycerin gasification system and Stirling engine generators was created for potential electricity generation. An economic analysis model was developed to determine the cost of electricity generation. Using a 5 Mg/day fluidized bed gasifier integrated with four Stirling engine generators, electricity could be potentially generated with the cost of $0.142/kWh.

Numerical study of methane steam reforming and methane combustion over the segmented and continuously coated layers of catalysts in a plate reactor

Four separate 2D steady state numerical models are developed for a catalytic plate reactor (CPR), designed with the four different configurations between segmented and continuously coated layers of combustion and reforming catalysts for hydrogen production by combustion assisted methane steam reforming (MSR). MSR is simulated on one side of a plate by implementing experimentally validated surface microkinetic model for nickel/alumina catalyst. Required heat to an endothermic MSR is provided by simulating catalytic methane combustion (CMC) on an opposed-side of the plate by implementing reduced surface microkinetic model for platinum/alumina catalyst. Four different combinations of coating configurations between reforming and combustion catalysts are studied in terms of reaction heat flux and reactor plate temperature distributions as well as in terms of methane and hydrogen mole fraction distributions. These combinations are: (1) continuous combustion-catalyst and continuous reforming-catalyst (conventional CPR design), (2) continuous combustion-catalyst and segmented reforming-catalyst, (3) segmented combustion-catalyst and continuous reforming-catalyst, and (4) segmented combustion-catalyst and segmented reforming-catalyst. For the same reforming-side gas hourly space velocity, the study has shown that the CPR designed with the segmented catalysts requires 66% less combustion-catalyst to achieve similar methane conversion and hydrogen yield in MSR compared to the conventional CPR design. The study has also shown that maximum reactor plate temperature, thermal hot spots and axial thermal-gradients are reduced significantly in the CPR designed with the segmented catalysts than the CPR designed with the conventional continuous catalysts configuration.

An experimental and kinetic modeling study of glycerol pyrolysis

Pyrolysis of glycerol, a by-product of the biodiesel industry, is an important potential source of hydrogen. The obtained high calorific value gas can be used either as a fuel for combined heat and power (CHP) generation or as a transportation fuel (for example hydrogen to be used in fuel cells). Optimal process conditions can improve glycerol pyrolysis by increasing gas yield and hydrogen concentration. A detailed kinetic mechanism of glycerol pyrolysis, which involves 137 species and more than 4500 reactions, was drastically simplified and reduced to a new skeletal kinetic scheme of 44 species, involved in 452 reactions. An experimental campaign with a batch pyrolysis reactor was properly designed to further validate the original and the skeletal mechanisms. The comparisons between model predictions and experimental data strongly suggest the presence of a catalytic process promoting steam reforming of methane. High pyrolysis temperatures (750–800°C) improve process performances and non-condensable gas yields of 70%w can be achieved. Hydrogen mole fraction in pyrolysis gas is about 44–48%v. The skeletal mechanism developed can be easily used in Computational Fluid Dynamic software, reducing the simulation time.

Atmospheric molecular hydrogen (H2) at the Shangdianzi regional background station in China

Atmospheric molecular hydrogen (H2) mole fractions have been continuously measured at the Shangdianzi regional station in China. In this study, we present the atmospheric H2 time series from January 2015 to April 2016, and investigate the diurnal and seasonal cycles, and the impact of meteorological factors on the observed values. Atmospheric H2 mole fractions at Shangdianzi vary from a minimum of 381 ppb (parts per billion, 10−9 dry air mole fraction) to a maximum of 1535 ppb, with a median of 510 ppb and a mean (± standard deviation) of 555 ± 113 ppb during the observation period. The results indicate that H2 mole fractions at Shangdianzi are frequently influenced by local sources and sinks. Regionally representative conditions account for 44.7% of the total records with a mean mole fraction of 488 ± 20 ppb. The highest regionally representative H2 mole fraction is observed in July, while the lowest is observed in October. Peak-to-trough amplitude in the seasonal cycle is 63 ± 3 ppb. H2 mole fractions show nighttime depletion in all seasons, with the lowest values in the morning (7:00–10:00 local time). The H2 mole fractions are also influenced by local surface wind direction at Shangdianzi. Winds from NW-NNW-N-NNE-NE-ENE-E directions are always associated with negative contribution to atmospheric H2 loading, whereas winds from SSW-SW-WSW-W directions generally enhance the H2 values. The results of trajectory clustering analysis demonstrate that air masses from a southerly direction induce high H2 mole fractions. Conversely, mean H2 mole fractions are low when air masses are from the north, northwest, and east directions.

Effects of various gasification parameters and operating conditions on syngas and hydrogen production

This paper investigates the effect of various gasification parameters and operating conditions on syngas composition. The gasification parameters include type of coal, oxygen and steam flow rates to the gasifier. The gasification process takes place in an entrained flow gasifier. Two gasifier models are developed with the Aspen Plus; one is based on the kinetics of the gasification obtained from experimental results reported in the literature, the second model is based on the Gibbs free energy. It was found that the kinetics based model had errors of 0.3%, 1.1%, and 2.7% by comparison, the Gibbs free energy based model exhibited an error of 4.1%, 6.5%, and 5.1% in the mole fraction of carbon monoxide, hydrogen, and carbon dioxide in the syngas respectively. Although the Gibbs free energy minimization approach based model produced slightly larger errors than the kinetics based model, the latter was chosen since the Gibbs model is more compatible with any coal type and with any combination of operating and input parameters. The latter model is used to find the effects of various gasification parameters and operating conditions on the resulting syngas composition. It is shown that the syngas composition is highly dependent on the flow rates of oxygen (gasification oxidant) and steam (gasification agent). The behavior of the syngas composition and hydrogen produced for three different types of coal is presented. Finally, the conditions leading to the maximum values of specific species in the syngas flow rates are presented.

Thermodynamic analysis of carbon dioxide reforming of methane to syngas with statistical methods

The present theoretical work deals with thermodynamic analysis based on Gibbs energy minimization method combined with a statistical approach: response surface methodology. By combining these two methodologies, the conditions of optimal operation were obtained. This study brings a novel methodology for thermodynamic analyses, which are normally based only on the Gibbs energy minimization method. Thermodynamic analysis of carbon dioxide reforming of methane has been investigated by method of Gibbs free energy minimization for production of desirable products at a wide range of temperature (600–1300 K), pressure (1–20 bar) and carbon dioxide to methane ratio (0.5–3). Moreover, the surface response methodology was used to determine the cubic polynomial models for mole fraction of hydrogen, carbon monoxide and ratio of H2/CO based on temperature, pressure and carbon dioxide to methane ratio. Numerical optimization was employed for determining the maximum production of hydrogen while keeping the production of carbon dioxide, methane and carbon monoxide to a minimum; in addition, the optimum ratio of hydrogen to carbon monoxide (1–3) occurs at 1 bar, 1011.99 K and carbon dioxide to methane ratio of 0.5.

HYDROGEN PRODUCTION FROM PHENOL STEAM REFORMING OVER Ni-Co/ZrO2 CATALYST: EFFECT OF CATALYST DILUTION

This study looked into the hydrogen production from phenol steam reforming over Zirconia (ZrO2)-supported nickel-cobalt catalysts diluted with silicon carbide (SiC). The objective of this study is to obtain the effect of catalyst dilution on hydrogen production and the phenol conversion in various SiC dilutions. The catalysts were prepared by impregnation method and their performance tests were carried out in a micro fixed bed reactor at atmospheric pressure and 800 °C temperature, feed flow rate 0.36 mL/min, weight of catalyst 0.2 g, and dilution range of 0.05 to 0.35 g (1:0 to 1:1.75). The results showed that the catalyst dilution does not affect much on the catalyst activity toward phenol conversion. However, it does improve the conversion of phenol with the presence of SiC. The maximum conversion was at 0.3 g (1:1.5) SiC dilution, which was of 98.9 % and 0.6 mole fraction of hydrogen.

Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor

Semicoke powders with particle size less than 6 mm are by-products during the pyrolysis of coal. Direct combustion of semicoke powders is difficult due to the low volatile content. Supercritical water gasification might provide an efficient conversion method for semicoke powders. In order to determine the optimum conditions of gasification of semicoke with the supercritical water fluidized bed reactor, the influences of the main operating parameters including temperature (540–660 °C), feedstock concentration (10–30 wt%), flow rate of preheated water (40–80 g/min) and alkali catalysts (K2CO3, KOH, Na2CO3 and NaOH) were systematically investigated in this study. The results showed that semicoke-water slurry of 30 wt% was continuously transported into the reactor and stably gasified without plugging problems. Hydrogen yield of 85.90 mol/kg was obtained with the hydrogen molar fraction of 61.02%. In particular, carbon gasification efficiency of more than 95% was obtained under the conditions of 660 °C, 60 g/min flow rate of preheated water and 10 wt% feedstock concentration with 5 wt% K2CO3.

Role of gas-phase and surface chemistry in methane reforming using a [formula omitted] oxygen transport membrane

This study investigates the performance of a La0.9Ca0.1FeO3−δ (LCF) membrane for applications including oxyfuel combustion, CO2 reuse and syngas production under a reactive environment using methane (CH4)–argon (Ar)–carbon dioxide (CO2) mixtures in the sweep stream. Through experimental measurements, we show that an LCF membrane can act as a catalyst for methane pyrolysis driven by the presence of iron at the B-site of the perovskite structure. Using methane–argon mixtures, the oxygen flux JO2 increases by one order of magnitude compared to the case with no methane. Measurements in the range of 900–950 °C suggest that the aforementioned enhancement is related to surface reactions consuming oxygen ions at the membrane surface. However, JO2 levels off when the inlet CH4 mole fraction reaches approximately 5%. The same applies for methane conversion. Addition of CO2 into the sweep stream has a significant impact on JO2 rise and syngas production. As more CO2 is added, surface carbon monoxide (CO) and hydrogen (H2) mole fractions increase by an order of magnitude. The high yield of syngas at the membrane surface results in a higher JO2 as more reactants participate in heterogeneous reactions with oxygen ions; methane conversion reaches approximately 100% showing that dry reforming (in the gas-phase) plays an important role. This oxygen flux enhancement is accompanied by significant reuse of CO2. The proposed technology can have a major impact on the CO2 recycle and the effort to mitigate its accumulation in the atmosphere.

Analysis of a Hydrogen Pump Driven Refrigeration Cycle

Hydrogen oxidation and reduction are fast reactions in polymer-electrolyte cells with modest amounts of noble metal catalyst. Hydrogen can be pumped and compressed in a cell that oxidizes hydrogen at the anode at low pressure and reduces protons at high pressure at the cathode. A typical hydrogen cell resembles a polymer-electrolyte fuel cell or electrolyzer with gas-diffusion electrodes containing platinum or a platinum alloy catalyst surrounding a polymer-electrolyte membrane like Nafion. Hydrogen pumps can be more efficient and durable than mechanical compressors. Conventional polymer-electrolyte membranes like Nafion need to be moist in order to have high ionic conductivity. This water is a nuisance in most applications; however one exception is a refrigeration cycle. The thermodynamics of a refrigeration cycle with water as the working fluid driven by an electrochemical hydrogen pump is studied in this work. In particular, an expression for the work done by the reactor is derived that includes terms associated with the partial pressure changes of hydrogen and water and irreversible losses.The coefficient-of-performance (COP) of the hydrogen-pump system approaches the Carnot limit when the mole fraction of hydrogen entering the reactor is small (< 0.01). Unfortunately, the amount of water crossing the membrane under these conditions far exceeds what can be supplied by osmotic drag. The calculated COP at a hydrogen mole fraction of 0.1 is approximately half of the Carnot limit, a promising result, when the hot and cold reservoirs are at 40oC and 20oC. Unfortunately, replacing a mechanical compressor with a hydrogen pump does not address the main issue with using water as refrigerant which is low total gas pressure. Water has low vapor pressures at typical hot and cold reservoir temperatures. Therefore, the electrochemical reactor would need to operate under vacuum and be designed to simultaneously provide low pressure drop and high mass transport rates.

CFD modeling and control of a steam methane reforming reactor

This work initially focuses on developing a computational fluid dynamics (CFD) model of an industrial-scale steam methane reforming reactor (reforming tube) used to produce hydrogen. Subsequently, we design and evaluate three different feedback control schemes to drive the area-weighted average hydrogen mole fraction measured at the reforming tube outlet (x¯H2outlet) to a desired set-point value (x¯H2set) under the influence of a tube-side feed disturbance. Specifically, a CFD model of an industrial-scale reforming tube is developed in ANSYS Fluent with realistic geometry characteristics to simulate the transport and chemical reaction phenomena with approximate representation of the catalyst packing. Then, to realize the real-time regulation of the hydrogen production, the manipulated input and controlled output are chosen to be the outer reforming tube wall temperature profile and x¯H2outlet respectively. On the problem of feedback control, a proportional (P) control scheme, a proportional-integral (PI) control scheme and a control scheme combining dynamic optimization and integral feedback control to generate the outer reforming tube wall temperature profile based on x¯H2set are designed and integrated into real-time CFD simulation of the reforming tube to track x¯H2set. The CFD simulation results demonstrated that feedback control schemes can drive the value of x¯H2outlet toward x¯H2set in the presence of a tube-side feed disturbance and can significantly improve the process dynamics compared to the dynamics under open-loop control.

Investigation on supersonic combustion of hydrogen with variation of combustor inlet conditions

The present work numerically investigated the effect of variation in the inlet Mach number and stagnation temperature on the mixing of fuel with the oxidizer and the subsequent stabilization of a flame in a combustor at supersonic conditions. Dimensions of the studied combustor were taken from literature. It had a 10° wedge located at the top wall of the combustor. The combustor was modeled and analyzed using ANSYS FLUENT software. Three-dimensional, compressible, reacting flow calculations with a detailed chemistry model were performed. Turbulence was modeled using SST k-ω model. Necessary grid refinement was done to capture the incident oblique shock formed at the 10° wedge. Hydrogen was injected through the fuel inlet port. The computations were performed for Mach numbers of 2.0, 2.5 and 3.0 at the combustor inlet for a combustion inlet stagnation temperature of 1500 K. Later, the combustor inlet Mach number was kept constant at 2.5 and the combustor inlet stagnation temperature was varied as follows: 1500 K, 1700 K, and 1900 K. The results indicated that as the combustor inlet Mach number increased, the location of incidence of the oblique shock shifted to the downstream of the fuel inlet and it resulted in the better mixing of the fuel with cross flow stream of air and led to better degree of combustion of hydrogen. The contours of mole fraction of OH radical and hydrogen also corroborated the improvement in the mixing of fuel with the cross flow air and the subsequent flame stabilization at higher Mach numbers. The flow pattern, mixing of fuel with air and flame stabilization was not affected significantly till 1700 K whereas for 1900 K, combustion of hydrogen was more uniform.

Experimental and theoretical analysis of cellular instability in lean H2-CH4-air flames at elevated pressures

Cellular instability in spherical propagating hydrogen-methane-air flames was studied experimentally in a constant volume chamber at an equivalence ratio of 0.8 and mixture temperature of 350 K. The mole fraction of hydrogen in the binary fuel was varied from 0 to 1.0 for mixture pressures up to 0.50 MPa. Cellular instability started earlier with an increase in the hydrogen mole fraction and mixture pressure. Self-acceleration of some of the propagating cellular flames was recorded and the acceleration increased with hydrogen mole fraction and mixture pressure. The unstretched laminar burning velocity was obtained from experiments and 1-D simulations of the outwardly propagating flames. Asymptotic theories gave a satisfactory qualitative prediction of the trends in the Markstein length, and the critical flame size for the onset of cellular instability. It was concluded that the Markstein length changed to a negative value at elevated pressure due to increased sensitivity of the burning velocity to thermo-diffusive effects.

High-purity hydrogen production from ash-free coal by catalytic steam gasification integrated with dry-sorption CO2 capture

This study is aimed at the improvement of process conditions for catalytic steam gasification of ash-free coal (AFC) integrated with CO2 capture. A novel modified catalytic hydrogen production reaction integrated gasification (M-HyPr-RING) process was proposed for high-purity H2 production from steam gasification of AFC. AFC was prepared from lignite coal by solvent extraction with maximum yields of 67% on a dry ash free basis at 400°C using hydrotreated heavy aromatic hydrocarbons with a coal-to-solvent ratio of approximately 1:10.The gasification temperature and sorbent-to-carbon (CaO/C) ratio were maintained constant for all experiments at 675°C and 2, respectively, as determined by our previous findings. The steam gasification experiments were conducted on AFC in a fixed-bed reactor in the presence of different catalyst loadings (5, 10 and 20wt% of potassium carbonate) with and without sorbent. A maximum hydrogen molar fraction of 85% (dry basis with nitrogen) was observed with catalyst loading of 20% and CaO/C of 2 after 5min of steam gasification under mild process conditions.

Numerical Investigation of the Effects of Nonuniform Premixing on Shock-Induced Combustion

A parametric numerical study was performed to study effects of the nonuniform premixing of a hydrogen–air mixture on the structures of oblique detonation and shock-induced combustion formed on the surface of a hypersonic spherical projectile. Axisymmetric two-dimensional Navier–Stokes equations were solved with a detailed chemical kinetic mechanism involving nine species. A smooth-front oblique detonation formed on the hypersonic projectile at a Mach number of 6.46 under a uniformly stoichiometric condition was taken as the completely premixed case. For nonuniform cases, hydrogen mole fraction was distributed at the inlet boundary based on the Gaussian function. As the nonuniformity was increased, the following occurred: 1) deformed oblique detonation, 2) oscillating shock-induced combustions with nonuniform corrugated structures, 3) steady shock-induced combustions with (and without) the minimum induction length located outside the centerline, and 4) nose-confined combustion. Analyses on distributions of induction lengths combined with zero-dimensional additional simulations demonstrated that a reactivity gradient became strongly influential on the nonuniformly premixed structures as the nonuniformity was increased, and finally the postshock flowfield became the most determinant factor.