3D simulation of hydrogen distributions affected by a passive auto-catalytic recombiner in an oxygen-starved condition

In this paper, we simulated the change in hydrogen behavior according to a Passive Autocatalytic Recombiner (PAR) under oxygen depletion conditions and compared the results with the THAI project experimental results. In order to reduce calculation cost, we calculated the hydrogen-oxygen bonding reaction in the catalyst region through correlation equations of the hydrogen removal rate according to the PAR type, and we did not consider the shape of the catalyst with a relatively small size and the detailed hydrogen-oxygen bonding reaction. The hydrogen concentration at the PAR inlet and outlet, mixed gas temperature, flow rate at PAR inlet, pressure and total hydrogen removal rate were similar to the experimental results and analysis values in all cases and the reduction in the hydrogen removal rate agreed well under oxygen depletion conditions. However, after the hydrogen injection was stopped, errors of the hydrogen concentration and flow rate at the PAR inlet increased as the buoyancy decreased because the high temperature of the catalyst was not taken into account.     

Design and testing of a novel catalytic reactor to generate hydrogen

A catalytic reactor to generate hydrogen with a large conversion efficiency and a stable rate of generation is based on a π-shaped design that decreases the effect of hydrogen on the catalyst surface so as to increase the opportunities for contact between sodium borohydride (NaBH₄) and the catalyst. This novel design is tested in terms of the effect of its rate of volumetric flow, position of catalyst, angle of flow channel, ratio of areas of gas channel and flow channel, and ratio of widths of gas channel and flow channel, on the efficiency of chemical conversion and the stability of hydrogen generation. We compare this efficiency and stability with the corresponding properties of a conventional reactor. The results indicate that placing the catalyst at the back of the flow channel provided uninterrupted space for liquid and gas at the front end, thereby improving the sustainability of the sodium borohydride for the catalytic reaction. An increased angle of the flow channel improved the capability of bubbles to escape from the surface of the catalyst, which, when appropriately designed, increased the efficiency by 13.4%. The increased rate of volume flow of sodium borohydride resulted in a decreased duration of contact between sodium borohydride and the catalyst, thereby decreasing the conversion efficiency. When the rate of volume flow of sodium borohydride was 0.5–2.0 mL/min, the effect of ratios of area and widths of gas channel to flow channel on the overall conversion efficiency followed no significant pattern. A comprehensive comparison between a conventional reactor and this new gas-flow channel-based reactor showed that, when appropriately designed, the new reactors can increase the efficiency of chemical conversion from 69.7% to 90.2%, with a decreased amplitude of hydrogen generation from 250% to 42.9%.     

Kinetics of non-premixed hydrogen–oxygen catalytic recombination reaction at ambient temperature

This study proposes the use of the hydrogen–oxygen catalytic recombination reaction to safely eliminate the leaked hydrogen in a confined environment. Experiments on the hydrogen–oxygen reaction catalyzed by using Pt/C as a catalyst are conducted at ambient temperature in a small cylindrical vessel. The macroscopic kinetic process of the hydrogen–oxygen recombination reaction is investigated, and the effects of the reaction parameters, such as the initial hydrogen volume fraction and catalyst layer position, on the reaction temperature and hydrogen conversion are examined. The reaction temperature and temperature rise rate are shown to reach the maximum values when the initial hydrogen fraction is 70 vol%. When the initial hydrogen fraction is ≤ 67 vol%, the hydrogen conversion reaches 100%. After the initial hydrogen fraction is > 67 vol%, the hydrogen conversion decreases significantly, and the hydrogen conversion is only 53% for the initial hydrogen fraction is up to 80 vol%. Moreover, the position of the catalyst layer has a significant effect on the reaction rate and heat distribution inside the vessel. When the catalyst layer is near the bottom of the reaction vessel, the reaction rate is accelerated and the released heat accumulates at the bottom of the vessel. The influence law of the aforementioned factors can provide a technical reference for applications of the hydrogen–oxygen catalytic reaction.     

Preparation of Ru–Cs catalyst and its application on hydrogen production by ammonia decomposition

Ammonia can be a hydrogen source for many applications including fuel cells. Using Ru or Cs–Ru as the catalyst, hydrogen is generated from ammonia by decomposition reaction. These catalysts are deposited on carbon powder by either chemical reduction or precipitation method in this study. Different carbon powder pre-treatment solutions and catalyst deposition conditions are evaluated. Nitric acid pre-treatment followed by precipitation at pH of 6 produces the highest catalyst loading from solution with given concentration of catalyst precursor. Hydrogen generation rate is measured at different catalyst compositions, ammonia inlet flow rates, decomposition temperatures, amount of catalyst packing, and ratio of Cs/Ru. The optimal condition for the ammonia decomposition reaction is Cs/Ru weight ratio at 3, ammonia inlet flow at 6 ml min⁻¹, reaction temperature at 400 °C. At this condition, the ammonia conversion rate reaches 90% and hydrogen generation rate reaches 29.8 mmol/min-g_cat.     

CFD simulation of CO2 removal from hydrogen rich stream in a microchannel

The main object of this research is to perform computational fluid dynamics simulation of CO₂ capturing from hydrogen-rich streams by aqueous DEA solution in a T-Junction microchannel contactor with 250 μm diameter and 5 mm length at dynamic conditions. To develop a comprehensive mathematical framework to simulate the flow hydrodynamics and mass transfer characteristics of system, the continuity and Navier-Stokes equations, two phase transport, and reaction rate model are coupled in COMSOL Multiphysics software. The developed model is solved and the effects of gas and liquid velocities as well as amine concentration on the CO₂ absorption rate, hydrogen purification fraction, and flow hydrodynamic are investigated. The absorption process consists of CO₂ diffusion from bubble bulk toward the bubble boundary, CO₂ solubility in the liquid boundary, diffusion from the boundary into the liquid bulk, and reaction with the amine molecules. The results show that when the gas and liquid streams are mixed in the junction point to form a bubble, the gas cross-section area becomes narrow, and the fluid velocity increases due to the applied force on the bubble by the liquid layers. It appears that increasing the DEA concentration in the inlet from 5% to 20% increases hydrogen purification fraction from 42.3 to 66.4%, and up to 96.7% hydrogen purity is achieved by 20% aqueous solution of DEA.     

Modeling and optimization of an industrial hydrogen unit in a crude oil refinery

The main goal of this research is the modeling and optimization of an industrial hydrogen unit in a domestic oil refinery at steady state condition. The considered process consists of steam methane reforming furnace, low and high temperature shift converters, CO₂ absorption column and methanation reactor. In the first step, the reactors are heterogeneously modeled based on the mass and energy balance equations considering heat and mass transfer resistances in the gas and catalyst phases. The CO₂ absorption column is simulated based on the equilibrium non-ideal approach. In the second step, a single objective optimization problem is formulated to maximize hydrogen production in the plant considering operating and economic constraints. The feed temperature, firebox temperature, and steam flow rate in the reformer, feed temperature in shift converters, lean amine flow rate in the absorption column, and feed temperature in the methanator are selected as decision variables. The calculated effectiveness factors and mass transfer coefficients prove that the methane reforming is inertia-particle mass transfer control, while shift and methanation reactions are surface reaction control. The simulation results show that applying the optimal condition on the system increases hydrogen production capacity from 85.93 to 105.5 mol s⁻¹.     

Simulation of methane steam reforming in a catalytic micro-reactor using a combined analytical approach and response surface methodology

In this study, a steady-state analytical model for heat and mass transfer in a 2D micro-reactor coated with a Nickel-based catalyst is developed to investigate microscale hydrogen production. Appropriate correlations for each species’ net rate of production or consumption, mass diffusivity, and the heat of reactions are developed using a detailed reaction mechanism of methane steam reforming. The energy and species conservation equations are then solved for the reactive mixture coupled with the wall energy equation. Finally, the response surface methodology (RSM) is employed to study the effects of channel height, inlet velocity and temperature, wall thickness and conductivity, and external heat flux on CH4 conversion. It is found that the inlet gas temperature, among different parameters, has the most influence on the overall performance of the microchannel hydrogen production. Also, the maximum necessary heat of reforming reaction increases by 84% and 26% if the CH₄ conversion changes from 50% to 60% and 60% to 70%, respectively. The developed analytical simulation can be a useful tool for designing experiments in micro-scale hydrogen production.     

Thermal–fluid behavior of the hydriding and dehydriding processes in a metal hydride hydrogen storage canister

A study of the hydrogen absorption and desorption processes using LaNi₅ metal hydride is presented for investigation on the influences of expansion volume and heat convection. The hydrogen storage canister comprises a cylindrical metal bed and a void of expansion volume atop the metal. The expansion volume is considered as a domain of pure hydrogen gas. The gas motion in the metal hydride bed is treated as porous medium flow. Concepts of mass and energy conservation are incorporated in the model to depict the thermally coupled hydrogen absorption and desorption reactions. Simulation results show the expansion volume reduces the reaction rates by increasing thermal resistance to the heat transfer from the outside cooling/heating bath. The assumption usually adopted in simulating heat transfer in a metal hydride tank that heat convection in the reaction bed may be ignored is not valid when expansion volume is used because heat convection dominates the heat transfer through the expansion volume as well as the metal bed. The details of the thermal flow pattern are demonstrated. It is found that, due to the action of thermal buoyancy, circulations are likely to happen in the expansion volume. The hydrogen gas accordingly, instead of going directly between the inlet/outlet and the metal bed, tends to move with the circulation along the boundary of the expansion volume.     

Comparisons of the hydrogen-rich syngas compositions from wet rice husk slurry steam reforming reactions using different catalysts

Rice husk slurry is pumped into a packed reactor and the products from the steam reforming reactions using different catalysts are studied. The steam/biomass weight ratio of such a system is between 3.47 and 5.25. The solids, liquid and gaseous products are a mass fraction of 2.8-4.1%, a mass fraction of 92.4-93.0% and a mass fraction of 3.5-4.7%, respectively. The hydrogen concentration in the gaseous product is approximate a volume fraction of 41% using the Al₂O₃ catalyst of a CuO mass fraction of 13%, a volume fraction of 38% using the Al₂O₃ catalyst of a Ni mass fraction of 13%, a volume fraction of 31% using the Al₂O₃ catalyst of a ZnO mass fraction of 13%, and a volume fraction of 20% using the Al₂O₃ catalyst at the reactor temperature of 800 °C. In the reactor temperature range studied (350-800 °C), the hydrogen concentration in the product stream increases monotonically with the increasing of the reactor temperature and the steam/carbon molar ratio. The value of dry gas LHV is between 9.4 MJ m⁻³ and 12 MJ m⁻³ at the reaction temperature of 600-800 °C. Considering the simple catalyst used in current study, the syngas of a hydrogen volume fraction of approximate 40% is obtained by pumping the biomass slurry to carry out the catalytic steam reforming reaction.     

Hydrogen production by aqueous phase catalytic reforming of glycerine

Hydrogen is believed to be the one of the main energy carriers in the near future. In this research glycerine, which is produced in large quantities as a by-product of biodiesel process, was converted to hydrogen aiming to contribute to clean energy initiative. Conversion of glycerol to hydrogen was achieved via aqueous-phase reforming (APR) with Pt/Al₂O₃ catalyst. The experiments were carried out in an autoclave reactor and a continuous fixed-bed reactor. The effects of reaction temperature (160-280 °C), feed flow rate (0.05-0.5 mL/dak) and feed concentration (5-85 wt-% glycerine) on product distribution were investigated. Optimum temperature for hydrogen production with APR was determined as 230 °C. Maximum gas production rate was found at the feed flow rates around 0.1 mL/min. It was also found that hydrogen concentration in the gas product increased with decreasing glycerol concentration in the feed.     

Experiments on hydrogen production from methanol steam reforming in fluidized bed reactor

A novel approach for the hydrogen production which integrated methanol steam reforming and fluidized bed reactor (FBR) was proposed. The reaction was carried out over Cu/ZnO/Al₂O₃ catalysts. The critical fluidized velocities under different catalyst particle sizes and masses were obtained. The influences of the operating parameters, including that of H₂O-to-CH₃OH molar ratio, feed flow rate, reaction temperature, and catalyst mass on the performance of methanol steam reforming were investigated in FBR to obtain the optimum experimental conditions. More uniform temperature distribution, larger surface volume ratio and longer contacting time can be achieved in FBR than that in fixed bed reactor. The results indicate that the methanol conversion rate in FBR can be as high as 91.95% while the reaction temperatures is 330 °C, steam-to-carbon molar ratio is 1.3, and feed flow rate is 540 ml/h under the present experiments, which is much higher than that in the fixed bed.     

Parametric study of the decomposition of methane using a NiCu/Al2O3 catalyst in a fluidized bed reactor

CO₂-free production of hydrogen via catalytic decomposition of methane (CDM) was studied in a fluidized bed reactor (FBR) using a NiCu/Al₂O₃ catalyst. A parametric study of the effects of some process variables, including catalyst particle size, reaction temperature, space velocity and the ratio of gas flow velocity to the minimum fluidization velocity (u_o/u_mf), was undertaken. A mean particle size of 150 μm allows optimization of results in terms of hydrogen production without agglomeration problems. The operating conditions strongly affect the catalyst performance: hydrogen production was enhanced by increasing operating temperature and lowering space velocity. However, increases in operating temperature, space velocity and the ratio u_o/u_mf provoked increases in the catalyst deactivation rate. At 700 °C, carbon was deposited as carbon nanofibers, while higher temperatures promoted the formation of encapsulating carbon, which led to rapid catalyst deactivation.     

A numerical study on methanol steam reforming reactor utilizing engine exhaust heat for hydrogen generation

Internal combustion engines are used in most vehicles around the world to power the transport sector. Efficiency improvement, emission reduction, and utilization of alternative fuels are the main aspects of current IC engine research. Hydrogen-enhanced combustion proved to be one of the efficient ways to achieve such goals. But the problem lies in the storage of hydrogen for the transportation sector, and on-board fuel reforming is a promising option for solving this issue. It deals with transforming a suitable liquid fuel (methanol) into an H₂-rich gas using a catalytic conversion process. For sustaining the reforming reaction, the required heat energy is taken from engine exhaust waste heat, this process is known as thermochemical recuperation. Number of studies on the reformers utilized for on-board hydrogen generation using engine exhaust heat are limited in the literature. The present study investigates the performance of a reactor that uses the exhaust gas heat energy for sustaining the reforming reaction. A numerical analysis was performed over a selected reactor where exhaust gases were flowing at one side, while on the other, the reforming reaction was taking place with the help of heat provided by high-temperature exhaust gases. A packed bed-type reactor was chosen for the current study and a parametric study was conducted where the effects of various operating parameters on both reacting and heating sides on the reactor's performance were investigated. It was found that temperature was the most influential inlet parameter among others. Steam/Carbon ratio and flow configuration had a negligible effect on the hydrogen yield as well as methanol conversion. Reactant inlet velocity increment revealed a significant drop in methanol conversion as it reduces the residence time for reforming reaction in the catalyst zone.     

Phase change in the cathode side of a proton exchange membrane fuel cell

A three-dimensional steady state two-phase non-isothermal model which highly couples the water and thermal management has been developed to numerically investigate the spatial distribution of the interfacial mass transfer phase-change rate in the cathode side of a proton exchange membrane fuel cell (PEMFC). A non-equilibrium evaporation-condensation phase change rate was incorporated in the model which allowed supersaturation and undersaturation take place. The most significant effects of phase-change rate on liquid saturation and temperature distributions are highlighted. A parametric study was also carried out to investigate the effects of operating conditions; namely as the channel inlet humidity, cell operating temperature, and inlet mass flow rate on the phase-change rate. It was also found that liquid phase assumption for produced water in the cathode catalyst layer (CL) changed the local distribution of phase-change rate. The maximum evaporation rate zone (above the channel near the CL) coincided with the maximum temperature zone and resulted in lowering the liquid saturation level. Furthermore, reduction of the channel inlet humidity and an increase of the operation temperature and inlet mass flow rate increased the evaporation rate and allowed for dehydration process of the gas diffusion layer (GDL) to take place faster.     

Optimal design of parallel channel patterns in a micro methanol steam reformer

This study describes the performance of micro methanol steam reformers with channel widths optimized using the simplified conjugate gradient method (SCGM), which uses a minimum objective function of the H₂ mass fraction standard deviation in channels. A three-dimensional numerical model and optimal simplified conjugate gradient algorithm were built to predict and search for the effects of channel widths and flow rate on the performance of chemical reactions. Furthermore, this simulation model was compared to; and corresponded well with existing experimental data. Distributions of velocity, temperature, and gas concentrations (CH₃OH, CO, H₂, and CO₂) were predicted, and the methanol conversion ratio was also evaluated. The mole fraction of CO contained in the reformed gas, which is essential to preventing poisoning of the catalyst layers of fuel cells, is also investigated. In the optimization search process, the governing equations use the continuity, momentum, heat transfer, and species equations to evaluate the performance of the steam reformer. The results show that channel width optimization can not only increase the methanol conversion ratio and hydrogen production rate but also decrease the concentration of carbon monoxide. The velocity and mixture gas density distributions in channels are discussed and plotted at various locations for an inlet liquid flow rate of 0.3 cc min⁻¹. Full development is not obtained in the downstream channel flow, the velocity in channel is increased from 1.28 m s⁻¹ to 2.36 m s⁻¹ at location Y = 1 mm–32 mm, respectively. This can be attributed to a continuous increase in the lightweight H₂ species as a result of chemical reactions in the channels.     

Hydrogen production from the pyrolysis–gasification of waste tyres with a nickel/cerium catalyst

Hydrogen production from the pyrolysis-gasification of waste tyres has been investigated with a Ni/CeO₂/Al₂O₃ catalyst using a two-stage fixed bed reaction system. The conditions of catalyst preparation such as Ni and CeO₂ content and calcination temperature were investigated in relation to product yield and composition. The fresh and reacted catalysts were analysed using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The results showed that there were small changes in the gas and hydrogen yield by adding 5 wt.% of CeO₂ into the Ni/CeO₂/Al₂O₃ catalyst. The gas yield related to the mass of waste tyre and the amount of reacted water were increased with the increase of CeO₂ content from 5 to 15 wt.%. However, with the further increase of CeO₂ content to 30 wt.%, the gas yield related to the mass of tyre and the amount of reacted water was reduced. Increasing the Ni content of the catalyst showed a positive influence on the gas yield and hydrogen production. The investigation of the calcination temperature of the Ni/CeO₂/Al₂O₃ catalyst showed that the oil yield related to the mass of tyre and reacted water decreased from 28.4 to 23.4 wt.% for the catalyst calcined at 500 °C, and decreased from 24.2 to 17.7 wt.% for the catalyst calcined at 750 °C. When the Ni content of the catalyst was increased from 5 to 20 wt.%. there were only small changes in total gas and hydrogen production from the pyrolysis-gasification of waste tyre. Lower coke deposition on the reacted catalyst was obtained for the Ni/CeO₂/Al₂O₃ catalyst prepared at the calcination temperature of 750 °C compared with the 500 °C calcination temperature.     

Hydrogen production via catalytic steam reforming of the aqueous fraction of bio-oil using nickel-based coprecipitated catalysts

Hydrogen production was studied in the catalytic steam reforming of a synthetic and a real aqueous fraction of bio-oil. Ni/Al coprecipitated catalysts with varying nickel content (23, 28 and 33 relative atomic %) were prepared by an increasing pH technique and tested during 2 h under different experimental conditions in a small bench scale fixed bed setup. The 28% Ni catalyst yielded a more stable performance over time (steam-to-carbon molar ratio, S/C = 5.58) at 650 °C and a catalyst weight/organic flow rate (W/m_org) ratio of 1.7 g catalyst min/g organic. Using the synthetic aqueous fraction as feed, almost complete overall carbon conversion to gas and hydrogen yields close to equilibrium could be obtained with the 28% Ni catalyst throughout. Up to 63% of overall carbon conversion to gas and an overall hydrogen yield of 0.09 g/g organic could be achieved when using the real aqueous fraction of bio-oil, but the catalyst performance showed a decay with time after 20 min of reaction due to severe coke deposition. Increasing the W/m_org ratio up to 5 g catalyst min/g organic yielded a more stable catalyst performance throughout, but overall carbon conversion to gas did not surpass 83% and the overall hydrogen yield was only ca. 77% of the thermodynamic equilibrium. Increasing reaction temperatures (600–800 °C) up to 750 °C enhanced the overall carbon conversion to gas and the overall yield to hydrogen. However, at 800 °C the catalyst performance was slightly worse, as a result of an increase in thermal cracking reactions leading to an increased formation of carbon deposits.     

The ReSER-COG process for hydrogen production on a Ni–CaO/Al2O3 complex catalyst

The reactive sorption-enhanced reforming process of simulated coke oven gas (ReSER-COG) was investigated in a laboratory-scale fixed-bed reactor with Ni–CaO/Al₂O₃ complex catalyst. Simulated coke oven gases that are free of or contain C2+ hydrocarbons (C₂H₄, C₂H₆, C₃H₆, C₃H₈) have been studied as feed materials of the ReSER process for hydrogen production. The effects of temperature, steam to methane molar ratio (S/CH₄) and carbon space velocity on the characteristics of ReSER-COG were studied. The results showed that the hydrogen concentration reaches up to 95.8% at a reaction temperature of 600 °C and a S/CH₄ of 5.8 under normal atmospheric pressure conditions. This reaction temperature was approximately 200 °C lower than that of the coke oven gas steam reforming (COGSR) processes used for the hydrogen production. The amount of H₂ generated by ReSER-COG was approximately 4.4 times more than that produced by the pressure-swing adsorption (PSA) method per unit volume of COG. The reaction temperature was 50 °C lower when simulated COG with C2+ was used, as opposed to when COG without C2+ was used. The complex catalyst has a better resistance of coking during the ReSER-COG process when C2+ gas is present.     

质子交换膜燃料电池阳极内流动与传质特性模拟

针对直流道质子交换膜燃料电池阳极，建立二维稳态的数学模型研究流道和电极内的流动和传质特性。模型采用通用Darcy定律来描述多孔介质与非多孔介质区域的流体流动，可以模拟沿流动方向上的物质变化情况，并探讨进口速度、进口氢气质量分数和催化层厚度对质量传输的影响。结果表明：增大进口速度、增加进口氢气质量分数、降低催化层厚度有利于氢气的质量传递。     

Numerical and experimental study on hydrogen production via dimethyl ether steam reforming

This work presents the characteristics of catalytic dimethyl ether (DME)/steam reforming based on a Cu–Zn/γ-Al₂O₃ catalyst for hydrogen production. A kinetic model for a reformer that operates at low temperature (200 °C–500 °C) is simulated using COMSOL 5.2 software. Experimental verification is performed to examine the critical parameters for the reforming process. During the experiment, superior Cu–Zn/γ-Al₂O₃catalysts are manufactured using the sol-gel method, and ceramic honeycombs coated with this catalyst (1.77 g on each honeycomb, five honeycombs in the reactor) are utilized as catalyst bed in the reformer to enhance performance. The steam, DME mass ratio is stabilized at 3:1 using a mass flow controller (MFC) and a generator. The hydrogen production rate can be significantly affected depending on the reactant's mass flow rate and temperature. And the maximum hydrogen yield can reach 90% at 400 °C. Maximum 8% error for the hydrogen yield is achieved between modeling and experimental results. These experiments can be further explored for directly feeding hydrogen to proton exchange membrane fuel cell (PEMFC) under the load variations.