Transient reaction and exergy analysis of catalytic partial oxidation of methane in a Swiss-roll reactor for hydrogen production

Catalytic partial oxidation of methane (CPOM) is a promising method for hydrogen production with autothermal reaction. To figure out the unsteady reaction characteristics of CPOM in a Swiss-roll reactor along with heat recirculation, a numerical method is employed to simulate the transient reaction dynamics, with emphasis on energy recovery using exergy analysis. Three different gas hourly space velocities (GHSVs) of 5000, 10,000 and 50,000 h⁻¹ with the condition of atomic O/C ratio of 1 are considered. The predictions indicate that increasing GHSV substantially shortens the transient period of chemical reactions; however, it also reduces the methane conversion, as results of more reactants sent into the reactor and shorter residence time of the reactants in the catalyst bed. Within the investigated range of GHSV, the methane conversion with energy recovery at the steady state is larger than 80%, much higher than the reaction without heat recovery. The selectivities of H₂ and CO in the product gas are always larger than 90%. The exergy recovery is in the range of 66–80%, implying that over two-third useful work contained in the product gas can be reused to preheat the reactants in the reactor, thereby enhancing the performance of CPOM.     

Effect of ruthenium addition on molybdenum catalysts for syngas production via catalytic partial oxidation of methane in a monolithic reactor

Hydrogen is mainly produced from hydrocarbon resources. Natural gas, mostly composed of methane, is widely used for hydrogen production. As a valuable feedstock for ‘Fischer–Tropsch’ (FT) process and ‘Gas to Liquids’ (GTL) technology, syngas production from catalytic partial oxidation of methane (CPOM) is gaining prominence especially owing to its more desirable H₂/CO ratio; relatively less energy consumption, and lower investment, compared to steam reforming processes (SMR), the leading technology.In the present study, effect of ruthenium (Ru) addition on molybdenum (Mo) catalysts for syngas production from methane (CH₄) via partial oxidation in a monolithic reactor was investigated. Mo based catalysts supported on Nickel (Ni) and Cobalt (Co) metal oxides and Ni-Co bimetallic oxides and their Ru added versions were developed, characterized, and tested for performance in a monolithic type reactor system. Catalyst activity was investigated in terms of H₂ and CO selectivity, CH₄ conversion; and CO₂ emission and it is concluded that addition of Ru over the structure led to increase in catalytic activity and reduction in carbon deposition over the catalyst surface.     

Hydrogen production through partial oxidation of methane in a new reactor configuration

Performance of the side feeding (SF) air injection in the process of partial oxidation of methane (POM) has been investigated by means of developing a one-dimensional steady-state non-isothermal model. A fixed bed reactor (FBR), a one-side feeding reactor (One-SF), and a membrane reactor (MR) has been compared for the conversion of methane, selectivity of hydrogen and reactor temperature. The results of the model revealed that the One-SF can operate within FBR and MR, and increasing the number of air injections of SF could achieve to the performance of the MR. The performance of the two to five-SF was studied according to the hydrogen selectivity, methane conversion, temperature profile and H₂/CH₄ ratio. It was observed that increasing the number of injections up to the three, increased the selectivity of hydrogen from 0.496 to 0.530 and decreased the outlet temperature from 1269 K to 1078 K. These results lead to creating of a process with controllable operating temperature and enhancing the selectivity of hydrogen. Consequently, decreasing the problems of high operating temperature in FBR and reduction of the process cost compared with MR.     

Experimental and numerical study of detailed reaction mechanism optimization for syngas (H2 + CO) production by non-catalytic partial oxidation of methane in a flow reactor

The National Institute of Standards and Technology (NIST) detailed reaction mechanism of methane combustion was optimized based on a flow reactor experiment to obtain syngas (H₂ + CO). The experimental methane partial oxidation was conducted with pre-mixed gas in a flow reactor. Specifically, 0.2% methane and 0.1% oxygen were diluted with 99.7% argon, restraining the exothermic effect. The experiment was conducted from 1223 K to 1523 K under pressure. Through a comparison of the experimental results with calculated values, the NIST mechanism was selected as a starting point. Rate coefficients of O + OH = O₂ + H, CH₃ + O₂ = CH₃O + O, and C₂H₂ + O₂ = HCCO + OH were replaced with results from other studies. The replaced rate coefficient for CH₃ + O₂ = CH₃O + O was again optimized, within its reported uncertainty of 3.16, based on the experimental results of this study. The revised value of the rate coefficient for CH₃ + O₂ = CH₃O + O was k₃₇ = 7.92 × 10¹³ × e^{(−31400/RT)}. The optimized mechanism showed better performance in predicting the results of other studies, as well as this study. The optimization reduced the RMS error for the results of this study from 6.7 to 1.18.     

Catalytic partial oxidation of methane for the production of syngas using microreaction technology: A computational fluid dynamics study

Catalytic partial oxidation of methane in high temperature environments under extremely short contact time conditions has emerged as a very promising reaction pathway for the production of syngas. This paper addresses the issues related to the favorable operating conditions for the process. Computational fluid dynamics simulations were performed to gain insight into the underlying mechanism and the key factors affecting primary reaction products. Particular emphasis was given to the role of homogenous and heterogeneous reaction pathways in determining the distribution of reaction products. The effect of preheating temperature, pressure, feed composition, and reactor dimension was investigated in order to identify conditions that will maximize the yield of syngas. Comparisons were made between air-feed and oxygen-feed systems. The relative importance of homogeneous and heterogeneous reactions was assessed, and the reaction pathways responsible for the production of syngas were identified. It was shown that there is a strong interplay between gas-phase and surface chemistry due to the competitive oxidation reactions occurring simultaneously in the system. The contribution of homogeneous and heterogeneous reaction pathways is highly dependent on the operating conditions. Gas-phase chemistry is favored at high preheating temperatures, high pressures, and large reactors, whereas surface chemistry is favored at low preheating temperatures, low pressures, and small reactors, with a tendency to shift towards higher syngas yields. It is particularly beneficial to utilize air instead of oxygen as the oxidant, especially at industrially relevant pressures, thereby inhibiting or avoiding the onset of undesired gas-phase chemistry.     

Hydrogen production by partial oxidation of methane: Modeling and simulation

A one-dimensional non-isothermal model for oxygen permeable membrane reactor has been developed to simulate the partial oxidation of methane to produce hydrogen. The performance of two fixed bed reactors (FBRs) viz. one with pure O₂ in feed (FBR1), other with air in feed (FBR2), and a membrane reactor (MR) having air in non-reaction side have been studied at various feed conditions and inlet temperatures in order to investigate the effect of these parameters on conversion of methane and yield of hydrogen. The fixed bed reactor with pure O₂ in feed has been found to provide better performance as compared to fixed bed reactor with air and membrane reactor.     

Modeling and analysis of flow distribution in an A-type microchannel reactor

Flow distribution among microchannels is a fatal factor affecting the performance of laminated microchannel reactors for hydrogen production. Homogeneous flow strongly depends on the structural design of the microchannel reactor. The present work concentrates on improving the flow distribution in microchannel reactors for hydrogen production by optimization of the structural design. An innovative A-type microchannel reactor for hydrogen production with one inlet/two outlets was developed and analyzed. The equivalent electrical resistance network model was used to calculate the flow distribution in the microchannel reactor which was validated by computational fluid dynamics (CFD). The influences of structural parameters on flow distribution in the A-type were investigated quantitatively. The calculated results showed that longer microchannels with a higher aspect ratio and a small side length in the manifolds were beneficial for attaining uniform flow distribution in the A-type microchannel reactor. Furthermore, it was found that flow distributions among the microchannels in the A-type were much more uniform than those in the conventional Z-type microchannel reactor with one inlet/one outlet. Finally, an optimization strategy was proposed to optimize the manifold geometries to obtain a comparatively even flow distribution among microchannels.     

Minimization of hot spot in a microchannel reactor for steam reforming of methane with the stripe combustion catalyst layer

Hot spot formation is inevitable in a heat exchanger microchannel reactor used for steam reforming of methane because of the local imbalance between the generated and absorbed heat. A stripe configuration of the combustion catalyst layer was suggested to make the catalytic combustion rate uniform in order to minimize the hot spot near the inlet. The stripe configuration was optimized by response surface methodology with computational fluid dynamics. With the optimal catalyst layer, the hot spot was not observed near the inlet and the maximum temperature decreased by 130 K from that of the uniform catalyst layer without any conversion loss. The maximum relative particle diameters of the uniform and the optimal stripe catalyst layer were about 3.68 and 2.51, respectively, and the surface-averaged particle diameter of the optimal stripe catalyst layer was 7.64% less than that of the uniform stripe catalyst layer.     

A performance study of methanol steam reforming in an A-type microchannel reactor

The methanol steam reforming (MSR) performance in a microchannel reactor is directly related to the flow pattern design of the microchannel reactor. Hydrogen production improvements can be achieved by optimal design of the flow pattern. In this study, an A-type microchannel reactor with a flow pattern design of one inlet and two outlets was applied to conduct the MSR for hydrogen production. The MSR performance of the A-type microchannel reactor was investigated through numerical analysis by establishing a three-dimensional simulation model and compared with that of the conventional Z-type microchannel reactor. Experiments were also conducted to test the MSR performance and validate the accuracy of the simulation model. The results showed that compared with the conventional Z-type microchannel reactor, the species distributions in the A-type microchannel reactor were more homogeneous. In addition, compared with the Z-type microchannel reactor, the A-type microchannel reactor was shown to effectively increase the methanol conversion rate by up to 8% and decrease the pressure drop by about 20%, regardless of a slightly higher CO mole fraction. It was also noted that with various quantities of microchannels and microchannel cross sections, the A-type microchannel reactor was still more competitive in terms of a higher methanol conversion rate and a lower pressure drop.     

Hydrogen production over Ni/ceria-supported catalysts by partial oxidation of methane

The catalytic performance of Ni dispersed on ceria-doped supports, (Ce_0.88La_0.12) O_2-x, (Ce_0.91Gd_0.09) O_2-x, (Ce_0.71Gd_0.29) O_2-x, (Ce_0.56Zr_0.44) O_2-x and pure ceria, was tested for the catalytic partial oxidation of Methane (CPOX). The catalysts were characterized by Brunauer Emmett Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) and temperature programmed oxidation (TPO). Ni/ (Ce_0.56Zr_0.44) O_2-x showed higher Hydrogen production than the Ni/Gadolinium-doped catalysts, which may be due to its higher reducibility and surface area. By enhancing the support reducibility in Ni/doped-ceria catalysts, their catalytic activity is promoted because the availability of surface lattice oxygen is increased, which can participate in the formation of CO and H₂. It was also found that Ni/(Ce_0.56Zr_0.44) O_2-x showed higher catalytic performance after redox pretreatments. Similarly, a higher amount of H₂ or O₂ was consumed during hydrogenation and oxidation pretreatments, respectively. This may be correlated to re-dispersion of metallic particles and changes on the metal-support interface. In addition, it was observed that the ionic conductivity of Ni/(Ce_0.56Zr_0.44) O_2-x had an effect on the amount of carbon formed during the CPOX reaction at oxygen concentrations lower than the stoichiometric required, O/C ratios lower than 0.6. Its high oxygen mobility may have accelerated the surface oxidation reactions of carbon by reactive oxygen species, thus, inhibiting carbon growth on the catalyst surface.     

Study of partial oxidation reforming of methane to syngas over self-sustained electrochemical promotion catalyst

A self-sustained electrochemical promotion (SSEP) catalyst is synthesized for partial oxidation reforming (POXR) of CH₄ to produce syngas (H₂ and CO) at a relatively low temperature ranging from 350 to 650 °C. The SSEP catalyst is comprised of 4 components: microscopic Ni/Cu/CeO₂ anode, La_0.9Sr_0.1MnO₃ cathode, copper as electron conductor, and yttria-stabilized-zirconia as oxygen ion conductor, which form microscopic electrochemical cells to enable the self-sustained electrochemical promotion for the POXR process. The SSEP catalyst exhibited much better catalytic performance in POXR of CH₄ than a Ni–Cu–CeO₂ catalyst and a commercial Pt–CeO₂ catalyst. The CH₄ conversion over the SSEP catalyst is 29.4% at 350 °C and reaches 100% at 550 °C and the maximum selectivity to H₂ is on the level of 90% at 450–650 °C under a GHSV of 42,000 h⁻¹. The mechanism of the SSEP is discussed.     

Entropy generation from hydrogen production of catalytic partial oxidation of methane with excess enthalpy recovery

Catalytic partial oxidation of methane (CPOM) is an important route for producing hydrogen and it is featured by autothermal reaction. To recognize the reaction characteristics of CPOM, H₂ production and entropy generation from CPOM in Swiss-roll reactors are studied numerically. The considered parameters affecting the performance of CPOM include the excess enthalpy recovery, gas hourly space velocity (GHSV), number of turns and atomic O/C ratio. The impact of chemical reactions, heat transfer and friction on entropy generation is also analyzed. The results indicate that preheating reactants through waste heat recovery as well as increasing GHSV or number of turns is conducive to enhancing H₂ yield, whereas the maximum H₂ yield develops at O/C = 1.2. A higher H₂ yield is always accompanied by a higher value of entropy generation, and chemical reactions are the main source of entropy generation, especially from steam methane reforming. In contrast, viscous dissipation almost plays no part on entropy generation, compared to heat transfer and chemical reactions. From the analysis of entropy generation, detailed mechanisms of H₂ production from CPOM can be figured out.     

A novel tubular oxygen-permeable membrane reactor for partial oxidation of CH4 in coke oven gas to syngas

Dense BaCo_0.7Fe_0.2Nb_0.1O_3−δ (BCFNO) membrane tubes were prepared by slip casting and readily brazed to 310S stainless steel supports using a silver-based alloy. A novel tubular membrane reactor was constructed by placing a cylindrical Ni-based monolithic catalyst coaxially around the tubular membrane and a conventional Ni-based catalyst-bed apart from the membrane tube. The novel membrane reactor was successfully applied to partial oxidation of CH₄ in coke oven gas (COG). At 850 °C, 94% of CH₄ conversion, 93% of H₂ and as high as 11.3 cm³ cm⁻² min⁻¹ of oxygen permeation flux were obtained. The experimental H₂ and CO selectivity and CH₄ conversion were close to the thermodynamically predicated ones. There was a good match in the coefficient of thermal expansion (CTE) among BCFNO membrane, Ag-based alloy and 310S metal support. Long-term operation test results indicate that the novel tubular BCFNO membrane reactor exhibited not only high activity but also good stability for the partial oxidation of CH₄ in COG to syngas.     

Optimization of a counter-flow microchannel reactor using hydrogen assisted catalytic combustion for steam reforming of methane

The objective of this study is to optimize a microchannel reactor using hydrogen assisted catalytic combustion for steam reforming of methane. Hydrogen assisted catalytic combustion does not require preheating because the catalytic combustion of hydrogen occurs at room temperature. After start-up by hydrogen catalytic combustion, fuels of hydrogen and methane were changed to methane. The geometric configuration of the counter-flow reactor was optimized by the simulation model under steady state condition. The hydrogen flow rate in the counter-flow reactor was also optimized by transient simulations using the response surface methodology. As a result, the counter-flow reactor showed extremely short start-up time because of the optimized configuration and the optimized hydrogen flow rate. Hot spots were avoided because of the hydrogen shut-off after start-up. The operating characteristics of the counter-flow reactor were compared with those of the co-flow reactor.     

Hydrogen production by catalytic partial oxidation of methane and propane on Ni and Pt catalysts

In recent years the catalytic partial oxidation has been taken into consideration as a suitable process for hydrogen production, because of its exothermic nature which makes the process less energy and capital cost intensive with respect to steam reforming. In this paper the behaviour of three different catalyst typologies, two based on Ni–_Al2O3${\mathrm{Al}}_2{\mathrm{O}}_3$ (different in active phase composition) and one constituted by Pt supported on _CeO2${\mathrm{CeO}}_2$, is studied for partial oxidation of propane (as representative of liquefied petroleum gas). For comparison the same catalysts have been tested also in methane partial oxidation.     

Hydrogen production by non-catalytic partial oxidation of coal in supercritical water: The study on reaction kinetics

Supercritical water gasification (SCWG) has attracted great attention for efficient and clean coal conversion recently. A novel kinetic model of non-catalytic partial oxidation of coal in supercritical water (SCW) that describes formation and consumption of gas products (H₂, CO, CH₄ and CO₂) is reported in this paper. The model comprises 7 reactions, and the reaction rate constants are obtained by fitting the experimental data. Activation energy analysis indicates that steam reforming of fixed carbon (FC) is the rate-determining step for the complete gasification of coal. Once CH₄ is produced by pyrolysis of coal, steam reforming of CH₄ will be the rate-determining step for directional hydrogen production.     

Modeling electrochemical partial oxidation of methane for cogeneration of electricity and syngas in solid-oxide fuel cells

This paper uses computational models to evaluate strategies for scaling electrochemical partial oxidation (EPOX) processes from the laboratory scale to practical application. In addition to producing electrical energy alone, solid-oxide fuel cells (SOFC) can be operated with hydrocarbon fuel streams to produce synthesis gas (H₂ and CO) as well. SOFC systems are usually operated to consume most of the fuel and produce electricity. However, by operating with a hydrocarbon fuel at relatively high flow rates, the exhaust-gas composition can be predominantly syngas. In this case the steam (and CO₂), produced from electrochemical and thermal reactions, reacts to reform the hydrocarbon fuel within the catalytic anode support structure. A practical limitation of electrochemical partial oxidation operation is the fact that carbon tends to deposit on Ni-based anode catalysts. The present paper explores the use of barrier layers to prevent carbon deposits. The results show that a tubular cell can be designed to deliver syngas and electricity using methane as the primary fuel.     

Modeling study on the production of hydrogen/syngas via partial oxidation using a homogeneous charge compression ignition engine fueled with natural gas

The production of hydrogen and syngas from natural gas using a homogeneous charge compression ignition reforming engine is investigated numerically. The simulation tool used was CHEMKIN 3.7, using the GRI-3 natural gas combustion mechanism. This simulation was conducted on the changes in hydrogen and syngas concentration according to the variations of equivalence ratio, intake temperature, oxygen enrichment, engine speed, initial pressure, and fuel additives with partial oxidation combustion. The simulation results indicate that the hydrogen/syngas yields are strongly dependent on the equivalence ratio with maxima occurring at an optimal equivalence ratio varying with engine speed. The hydrogen/syngas yields increase with increasing intake temperature and oxygen contents in air. The hydrogen/syngas yields also increase with increasing initial pressure, especially at lower temperatures, yet high temperature can suppress the pressure effect. Furthermore, it was found that the hydrogen/syngas yields increase when using fuel additives, especially hydrogen peroxide. Through the parametric screening studies, optimum operating conditions for natural gas partial oxidation reforming are recommended at 3.0 equivalence ratio, 530 K intake temperature, 0.3 oxygen enrichment, 500 rpm engine speed, 1 atm initial pressure, and 7.5% hydrogen peroxide.