Hydrogen production by methanol steam reforming in an annular microchannel on a Cu-ZnO catalyst

Steam reforming of methanol into a hydrogen-containing gas under activated methanol conversion on annular microchannel walls was experimentally investigated. The methanol conversion was carried out on a copper-zinc catalyst deposited on the channel internal wall. The concentrations of the chemical conversion products in the output gas mixture were determined at different reactor temperatures and residence times. The channel wall temperature range within which the methanol steam reforming is intensified was also determined. It was shown that the CO content in the reaction products is determined by the CO₂ partial pressure rather than by the reactor temperature. A kinetic model of methanol steam reforming on a copper-zinc catalyst was developed.     

Experimental study of incomplete oxidation of methane in a ring channel

Chemical transformations at incomplete methane oxidation in the air medium were studied experimentally at reaction activation on the wall of an annular microchannel. Methane was oxidized incompletely on a rhodium catalyst deposited on an inner wall of the channel. Concentrations of the products of chemical transformations were measured in the outlet gas mixture for different reactor temperatures and stay times. We have determined the range of channel wall temperatures and stay times of the mixture corresponding to an increase in the portion of hydrogen and carbon dioxide indicating transition from predominant methane combustion to cascade chemical reactions with activation of steam and carbon dioxide methane conversions. It is shown that the kinetic model of chemical transformations of methane in the air medium depends significantly on the temperature of channel walls and stay time of the mixture. The effect of outer diffusion deceleration on the rate of chemical transformations at incomplete methane oxidation under the strained conditions is determined. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 05-08-65526).     

Modeling of methane steam reforming in a microchannel with a heat flow distributed in length

Numerical modeling of a flow of a high-temperature mixture of methane with water vapors in a two-dimensional plane microchannel with activation of chemical conversions on the channel wall has been performed. The modeling was performed within the framework of Navier-Stokes equations for a laminar flow of a multicomponent compressible gas. The influence of the external heat flux supplied to the gas mixture and its distribution along the channel length on the properties of the methane steam reforming have been investigated. It has been shown that not only the amount of heat supplied to the reaction zone but also the method of heat supply along the channel length are important. All the reactions with the residence time of the mixture on the order of tens of milliseconds terminate several centimeters downstream from the channel inlet, which makes it possible to optimize a compact reactor for obtaining a synthesis gas.     

Modeling of methane steam reforming in a microchannel subject to multicomponent diffusion

Catalytic methane steam reforming in a slot microchannel under external heat supply to the mixture reacting on walls is considered based on numerical simulation of a complete system of Navier-Stokes equations. Three ways of heat supply to channel walls are represented, namely, a uniform heat flux, a heat flux linearly decreasing in channel length, and a heat flux following the reaction rate profile of the main reaction. The thermophysical parameters of the mixture depend on its temperature and composition. Two diffusion models are considered, namely, models with equal and different diffusion coefficients for each mixture component. It is shown that consideration of multicomponent diffusion does not practically affect the concentration of the components and the methane reforming at the outlet. For the above-mentioned ways of heat supply, the methane reforming with a heat flux linearly decreasing in channel length is most significant.     

Hydrodynamics and heat and mass transfer at chemical conversions in slot reactors

In this paper, experimental and numerical investigations of the hydrodynamics and heat transfer in a disk slot heat exchanger-reactor for a radial flow of a gas mixture reacting on the channel walls are described. Data for the coefficients of heat transfer from the wall being heated to the gas flowing inside the reactor are presented. The temperature field of a catalytically active reactor plate at heat release on it has been investigated experimentally. Calculations of the flow and heat transfer in a slot reactor element for a catalytic reaction with heat release have been performed. Partial oxidation of methane in an oxygen medium with the formation of a hydrogen-containing synthesis gas in a two-dimensional microchannel has been investigated numerically. Data for the extent of the chemical conversion of methane versus the initial mixture consumption and reaction temperature are presented.     

Theoretical and experimental study of methane steam reforming reactions over nickel catalyst

In this work we perform DFT theoretical calculations of methane and steam interactions on Ni(1 1 1) surface. The calculations allow us to improve our understanding of the competition between these reactants by catalytic sites in methane steam reforming (MSR) process. For this purpose we compare theoretical results with kinetic measurements of MSR on a Ni(II)-Al(III) catalyst prepared from lamellar double hydroxides as precursor. This comparison shows that, for low H₂O/CH₄ ratios methane and water intermediate species adsorb on different catalytic sites. While CHO species adsorbs on top of Ni atom, OH one occupies preferentially a tri-coordinate surface site. On the other hand, for high H₂O/CH₄ ratios a competency between these species by Ni sites would establish, diminishing methane conversion. In addition competition between methane and steam for Ni sites would lead to a decrease in CO production. Nevertheless, intermediate species adsorbed on different active sites would produce CO₂, whatever the steam/methane ratio. Thus, it would be optimum steam concentration in hydrocarbon feed and active sites distribution on catalyst surface, which could maximize H₂ production and minimize CO selectivity. The theoretical findings agree with kinetic measurements, which show that maximum methane conversion depends on steam partial pressure in the feed; whereas always, selectivity to CO₂ increases and to CO diminishes.     

Cooperative activation effect on H2O adsorption in MnO–Co catalyzed steam methane reforming

Steam methane reforming is a very important chemical process in hydrogen production and solid oxide fuel cells (SOFCs). Cobalt (Co) is an important catalyst for dry and steam methane reforming. However, previous studies have confirmed that metal Co surfaces only have weak adsorption activity for H₂O, which is evidently unfavorable for steam reforming. In this work we used first-principles simulations to study the activity of MnO–Co catalysts for the adsorption of H₂O. Compared with the Co (111) surface and pristine Co clusters, the MnO–Co catalytic layer has a stronger adsorption capability for H₂O because of the introduction of the MnO substrate, which is crucial for improving the steam reforming reaction and inhibiting carbon disposition in SOFCs. The cooperation mechanism between MnO and Co is discussed based on the analysis of electronic structures. The conclusions from this work are universal for other metal-oxide composite catalyst layers.     

Three-dimensional numerical simulation of the characteristics of a ramjet power plant with a continuous-detonation combustor in supersonic flight

Multi-variant three-dimensional numerical simulations demonstrate the feasibility of the continuous- detonation process in an annular combustor of a ramjet power plant operating on hydrogen as fuel and air as oxidant in conditions of flight at a Mach number of M ₀ = 5.0 and an altitude of 20 km. Conceptual schemes of an axisymmetric power plant, 400 mm in external diameter and 1.3 to 1.5 m in length, with a supersonic intake, divergent annular combustor, and outlet nozzle with a frusto-conical central body are proposed. Calculations of the characteristics of the internal and external flows, with consideration given to the finite rate of turbulent-molecular mixing of the fuel mixture components with each other and with the combustion products, as well as the finite rate of chemical reactions and the viscous interaction of the flow with the bounding surfaces, have shown that, in these flight conditions, the engine of such a power plant has the following performance characteristics: the thrust, 10.7 kN; specific thrust, 0.89 (kN s)/kg; specific impulse, 1210 s; and specific fuel?consumption 0.303 kg/(N h). In this case, the combustor can operate with one detonation wave traveling in the annular channel at an average velocity of 1695 m/s, which corresponds to a detonation wave rotation frequency of 1350 Hz. It is shown that, an operating combustor has regions with subsonic flow of detonation products, but the flow is supersonic throughout its outlet section.     

Ethanol utilization in solid oxide fuel cells: A thermodynamic approach

A thermodynamic analysis of electrical power generation in ethanol fueled solid oxide fuel cells (SOFCs) was made in the temperature range between 800–1200 K at atmospheric total pressure. A SOFC was considered being fed with the thermodynamic equilibrium products of ethanol a) steam reforming b) CO₂ reforming and c) partial oxidation. In each case, ethanol, steam, carbon oxides, methane and hydrogen were considered coexisting in the equilibrium mixture produced by different ethanol to oxidant (H₂O, CO₂ and O₂) initial ratios. The boundary conditions for carbonization were also examined. The theoretically derived values of the molar fractions of the species in equilibrium were used for the evaluation of the thermodynamic value of the electromotive force (emf) established in the cell under the equilibrium conditions. Finally, the maximum electrical power obtainable in SOFC was calculated in each case under consideration. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Scaling relations for the length of coaxial oxy-flames with and without swirl

An extensive experimental study is carried out to analyze scaling laws for the length of methane oxy-flames stabilized on a coaxial injector. The central methane fuel stream is diluted with N₂, CO₂ or He. The annular air stream is enriched with oxygen and can be impregnated with swirl. Former studies have shown that the stoichiometric mixing length of relatively short flames is controlled by the mixing process taking place in the vicinity of the injector outlet. This property has been used to derive scaling laws at large values of the stoichiometric mixture fraction. It is shown here that the same relation can be extended to methane oxy-flames characterized by small values of the stoichiometric mixture fraction. Flame lengths are determined with OH* chemiluminescence measurements over more than 1000 combinations of momentum ratio, annular swirl level and composition of the inner and outer streams of the coaxial injector. It is found that the lengths of all the flames investigated without swirl collapse on a single line, whose coefficients correspond to within 15% of flame lengths obtained for fuel and oxidizer streams at much larger stoichiometric mixture fractions. This relation is then extended to the case of swirling flames by including the contribution of the tangential velocity in the flow entrainment rate and is found to well reproduce the mixing degree of the two co-axial streams as long as the flow does not exhibit a vortex breakdown bubble. At higher swirl levels, when the flow features a central recirculation region, the flame length is found to also directly depend on the oxygen enrichment in the oxidizer stream.     

In-water gas combustion in linear and annular gas bubbles

A new pulsed-cyclic method of in-water gas combustion was developed with separate feed of fuel gas and oxygen with the focus on development of new technologies for heat generators and submerged propellers. The results of calorimetric and hydrodynamic measurements are presented. In-water combustion of acetylene, hydrogen, and propane was tested with the operation frequency of 2–2.5 Hz and with a linear injector. The combustion dynamics of combustion of stoichiometric mixture with propane (C₃H₈+5O₂) was studied for a bubble near a solid wall; the produced gas bubble continues expansion and oscillations (for the case of linear and annular bubbles). It was demonstrated that gas combustion in annular bubbles produces two same-magnitude pulses of force acting on the wall. The first pulse is produced due to expansion of combustion products, and the second pulse is produced due to axial cumulative processes after bubble collapse. This process shapes an annular vortex which facilitates high-speed convective processes between combustion products and liquid; and this convection produces small-size bubbles.     

Effects of initial water content on steam reforming of aliphatic hydrocarbons with nonthermal plasma

The effects of initial water content on steam reforming of aliphatic hydrocarbons such as methane, propane, and neopentane with nonthermal plasma were analyzed in terms of substrate conversion, carbon recovery, and product selectivity. It was found that water addition increased CO₂ yield despite a decrease in substrate conversion. The number of carbon atoms in the substrate hydrocarbon affected the additive effect due to the insufficient supply of oxygen atoms from water. Plausible reaction pathways for the conversion of substrate to CO_x are proposed to explain the relative concentrations of CO_x species; these pathways involved different precursors for CO and CO₂.     

Experimental Investigation of Transient Forced Convection of Liquid Methane in a Channel at High Heat Flux Conditions

Transient heat transfer of liquid methane under forced convection in a 1.8 mm × 1.8 mm asymmetrically heated square channel was investigated. This study is aimed at understanding the heat transfer behavior of cryogenic propellant in cooling channels of a regeneratively cooled rocket engine at the start-up condition. To simulate high heat load conditions representative of regeneratively cooled rocket engines, a high heat flux test facility with cryogenic liquid handing capabilities was developed at the Center for Space Exploration Technology Research. The time history of inlet and outlet fluid temperatures and test section channel wall temperatures were measured at high heat flux conditions (from 1.19 to 3.80 MW/m²) and a Reynolds number (Re) range of 1.88 × 10⁵ to 3.45 × 10⁵. The measured wall temperature data point toward possible film boiling within the test section during certain tests, particularly with higher heat fluxes and lower Reynolds number conditions that resulted in higher wall temperatures. The transient average Nusselt numbers (Nu_L) of the channel obtained from the experimental measurements are lower than those calculated from the Sieder–Tate correlation (Nu_O); however, the ratio (Nu_L/Nu_O) increases with the increase in Reynolds number. The ratio is around 0.25 at the lower end of Re and then increases to 0.7 at the maximum Re studied in the present investigation.     

A novel DME steam-reforming catalyst designed with fact database on-demand

Novel catalysts for dimethyl ether (DME) steam reforming (SR) were designed based on catalysis database on-demand. A catalyst library consisting of precious metals loaded on various metal oxides was tested for DME SR and its elemental reactions of DME hydrolysis and MeOH SR. Platinum loaded on alumina, Pt/Al₂O₃, shows high activity for DME SR as reported previously. The drawback of the catalyst was also confirmed; the formation of methane leading to the reduction of hydrogen formation. From the fact database for DME hydrolysis and MeOH SR built up with high-throughput experimentation tools, the high activity of Pt/Al₂O₃ for DME SR is owing to its high activity on DME hydrolysis because its activity on MeOH steam reforming is not remarkable. Based on these facts, novel catalysts were designed and achieved by physical mixing of Pt/Al₂O₃ which reveals high activity on DME hydrolysis with an active catalyst on MeOH steam reforming. By mixing of Pt/Al₂O₃ with Pd/Al₂O₃, methane formation was suppressed without loss of hydrogen production activity.     

大气压非平衡等离子体甲烷干法重整零维数值模拟

大气压非平衡等离子体由于其独特的非平衡特性,可为甲烷和二氧化碳稳定温室气体分子活化和重整提供非热平衡和活化环境.本文采用了零维等离子体化学反应动力学模型,考虑了详细的CH₄/CO₂等离子体化学反应集,重点研究了反应气体CH₄/CO₂摩尔分数(5%—95%)对大气压非平衡等离子体甲烷干法重整制合成气和重要含氧化合物的影响.首先,给出了进料气体不同体积比时电子密度和温度随时间的演化规律,结果表明初始甲烷摩尔分数的提高有利于获得较高的电子密度和电子温度.随后,讨论了主要自由基和离子数密度在不同的甲烷摩尔分数下随着时间的变化规律,并给出了反应气体的转化率、合成气体和重要含氧化合物的选择性.此外,还明确了合成气和含氧化合物主要生成和损耗的化学反应路径,发现甲基和羟基是合成含氧化合物的关键中间体.最后,归纳总结给出了主要等离子体粒子之间的总体等离子体化学反应流程图.     

Experimental study of heat transfer of dielectric liquid perfluorohexane at flowboiling in a microchannel heat exchanger

Presented are results of an experimental study of local heat transfer characteristics in boiling of the dielectric liquid perfluorohexane under forced convection in a horizontal microchannel heat exchanger. The experiments with a copper microchannel heat exchanger comprising 21 channels with sections of 335 × 930 μm were conducted with a mass velocity of 250 to 1000 kg/m²s and a heat flux through the outer wall of the heat exchanger of 3 to 60 W/cm². The dependence of the local heat transfer coefficient on the heat flux density on the inner wall of the microchannels was established, as well as the critical heat flux. The experimental data are compared with calculations based on known models of heat transfer.     

煤与天然气适度转化的甲醇-动力联产系统

本文开拓性地提出了一个新型煤和天然气综合利用、甲醇和电力联产的能源动力系统。新系统采用了燃料化学能适度转化的方法,煤先和纯氧与蒸汽发生部分气化反应,适度转化成合成气和半焦,碳转化率约为50%;半焦在天然气/水蒸气重整反应器中燃烧,为重整反应提供反应热;将甲醇生产系统与联合循环发电系统有机整合,来自煤和天然气的合成气先用于甲醇生产,未反应合成气用作发电燃料。研究表明在相同产出条件下,多功能系统比分产系统少消耗10%左右的化石燃料。本文工作为煤和天然气综合高效利用提供了新途径。     

Internal reforming in intermediate temperature solid oxide fuel cells running on natural gas

Methane steam reforming has been studied over a range of nickel/ceria-gadolinia cermet anodes, over the temperature range 500–700 °C appropriate for intermediate temperature ceriagadolinia based SOFCs. The influence of operating temperature and methane/steam ratio on the reforming characteristics, methane conversion and product selectivity, and the carbon deposition on the anode during reforming, has been determined for each anode. Nickel/ceria powders made from gas atomised intermetallic precursors have been studied as potential anode materials for intermediate temperature ceria-gadolinia based SOFCs running on natural gas. The powders have been characterised structurally and evaluated for their methane reforming characteristics and their resistance to carbon deposition during internal reforming, especially at low steam/methane ratios, over the relevant operating temperature ranges of ceriagadolinia based SOFCs. Their performance has been compared to conventionally prepared anodes and anodes generated from cast alloys, with very favourable results. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Thermalhydraulic Characteristics in Annular Pebble Beds with Axial and Radial Gas Flows

The purpose of this article is to determine the heat transfer and hydraulic drag in thin annular pebble beds with axial gas flow and investigate the flow distribution along annular pebble beds with radial flow. The experimental investigations showed that in thin annular pebble beds heat transfer values, equal to those for large (unlimited) pebble beds, could be achieved. The observed distributions of the radial flow in annular pebble beds demonstrated that regulation of flow distribution is possible by changing the permeability of the inner wall (outlet header) of the annular channel.     

催化剂薄层内甲烷水蒸气重整反应强化管内对流换热的数值模拟

本文以竖直圆管内壁催化剂薄层内发生甲烷水蒸气重整反应强化对流换热作为研究对象,对其进行了数值模拟．结果发现,催化剂薄层内的吸热化学反应可以有效地强化对流换热,降低流体和壁面温度,从而对壁面起到保护作用;极限热流密度的大小与流体的入口温度有关,存在最佳入口温度使极限热流密度最大．