Methane steam reforming in an annular microchannel with Rh/Al2O3 catalyst

Regularities of methane conversion in the presence of water steam were obtained experimentally while activating chemical conversions on the inner convex wall of an annular microchannel. The steam methane reforming was done on the Rh/Al₂O₃ nanocatalyst with the heat applied through the microchannel gap from the outer wall. Concentrations of the products of chemical reactions in the outlet gas mixture are measured at different temperatures of the outer microchannel wall. The range of channel wall temperatures at which the ratio of hydrogen and carbon oxide in the outlet mixture grows substantially is determined. Data on the composition of methane conversion products for the ratio H₂O/CH₄ = 1.77 and the activation energy of methane steam reforming at reactor outer wall temperatures of up to 880°C are obtained. The effect of the radiation heat exchange and the external diffuse limitation on the rate of chemical conversions in methane steam reforming in an annular microchannel with external heat supply is determined.     

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.     

The AC monitoring of a mixed conducting Li-Ni-Co-O catalyst during temperature programmed reactions

The temporal analysis of effluent species with temperature linearly increasing with time for two reactions, carbon dioxide hydrogenation (250° to 600°C) and the oxidative coupling of methane (400° to 800°C) have been studied. A non-stoichiometric, mixed-oxide catalyst, Li_0.9Ni_0.5Co_0.5O_2-δ, containing both mobile ions and electrons at elevated temperatures was investigated. The exit gas stream was monitored by using the multiple-ion monitoring facility of a quadrupole mass spectrometer. The AC electrical characteristics of the catalyst pellet between 100 Hz and 1 MHz were simultaneously monitored in-situ. A clear correlation between catalytic behaviour and electrical properties was obtained. Frequency-dependent AC losses due to surface dipole effects were dominant during phases of strong chemisorption and reaction, whilst reversion to thermally activated behaviour occurred during times of reduced chemical activity. In the case of carbon dioxide hydrogenation, the catalyst becomes partially reduced as a result of the chemical reaction, as shown by the changes in conductance-temperature-frequency characteristics. The original oxidised state could be recovered by re-oxidation at 700°C for 1 hour in air. For the partial oxidation of methane, mobile lattice oxygen participates in the reaction. The temporary changes in the state of oxidation of the catalyst during this reaction are correlated with changes in the electrical characteristics of the material. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy     

Studies of catalytic process of complete oxidation of methane by SSITKA method

This paper presents the results obtained by means of the steady state isotopic transient kinetic analysis for complete methane oxidation over the Pd(PdO)Al₂O₃ catalyst. The average surface life-time and surface concentration of methane and carbon dioxide were determined. It was found out that on the palladium catalyst there are adsorbed small amounts of methane (which does not take part in the process of oxidation) only at the temperature corresponding to the starting point of methane oxidation. Additionally, in the steady state of methane oxidation on the palladium catalyst there are present two different kinds of carbon dioxide: short- and long-resided on the catalyst surface. The average surface life-time of both kinds of carbon dioxide decreases with temperature. The surface concentration of long-resided carbon dioxide increases with temperature whereas the small maximum at about 380 °C is noticed for the surface concentration of short-resided carbon dioxide.     

Kinetics of coal char gasification in a carbon dioxide medium

Solid fuel samples with different carbon contents are gasified by successively subjecting to pyrolysis in argon and oxidation in carbon dioxide at various temperatures to determine the rate of the chemical reactions and the activation energy required for simulating and optimizing the operation of gas generators. The samples were prepared from bituminous coal, lignite, and anthracite of the Kuznetsk and Kansk-Achinsk coal basins. The gasification of coal char samples in a carbon dioxide medium at 900–1200°C is analyzed by thermogravimetry. The temperature dependences of the weight change rate and gasification time of coal char samples are measured and used to calculate the preexponential factor and activation energy of the carbon oxidation reaction. It is found that, with increasing oxidizing medium temperature from 900 to 1200°C, the gasification time of the coal char samples obtained from anthracite and bituminous coal decrease 8- and 22-fold, respectively. A physicomathematical model of coal char gasification in a fixed bed, with the oxidizing gas diffusing through the ash layer formed, is proposed.     

原位DRIFTS研究CH4部分氧化和CO2重整的耦合

8%Ru-5?/γ-Al2O3催化剂对于甲烷的低温活化具有较好的催化活性,在500℃下甲烷、二氧化碳和氧气的耦合反应中,吸热反应二氧化碳重整和放热反应甲烷部分氧化进行耦合强化,使得耦合反应中的甲烷转化率为38.8%。用原位漫反射傅里叶红外光谱法对钌系催化剂耦合甲烷部分氧化和二氧化碳重整反应体系机理进行研究。CO在8%Ru-5?/γ-Al2O3上吸附,表明CO在催化剂表面上波数为2 167 cm-1(2 118 cm-1)和2031 cm-1(2 034 cm-1)处形成孪生态Ru(CO)2和Ce(CO)2吸附物种,而且高温下CO吸附物种很容易从催化剂表面脱附出来。原位漫反射红外实验结果表明甲烷部分氧化反应时催化剂表面上有吸附物种碳酸根、甲酰基(甲酸盐)和一氧化碳的形成,其中表面的甲酰基和甲酸盐物种是甲烷部分氧化反应的主要活性中间物,这些中间活性中间体由甲烷吸附态CHx和催化剂表面的氧吸附态结合而形成的,随后这种中间物种再分解为CO产物;甲烷和二氧化碳重整反应时没有新的吸附物种产生,由此提出重整反应的机理是吸附态的甲烷和二氧化碳在催化剂活性中心上进行活化解离而生成合成气;甲烷、二氧化碳和氧气耦合反应过程中出现新的羟基物种(桥式羟基Ru-(OH)2),耦合反应机理复杂可能是由部分氧化和重整两类反应机理的复合,其中桥式羟基Ru-(OH)2参与了反应的进行。     

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.     

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.     

Investigation of products of thermal methane conversion in hydrocarbon mixtures by mass spectrometry

The influence of ethylene/acetylene admixture on catalytic methane pyrolysis is investigated. Mass spectrometric analysis of the products of reactions in synthesis of carbon structures by thermal chemical gas-phase deposition is carried out for different temperatures, gas mixture compositions, and catalyst concentrations. It is shown that in the presence of ethylene and acetylene, there occurs partial methane decomposition at lower temperatures.     

Modeling the process of chemical regeneration of air in airtight habitable facilities

Physical experiments and mathematical modeling are used to study the kinetics of the reactions of carbon dioxide and water with potassium superoxide accompanied by oxygen release at various values of the temperature and humidity of the breathing gas mixture. The kinetics of the chemisorption is demonstrated to be limited by the rate of air regeneration in an airtight habitable facility. Experimental and analytical approaches are applied to determine the kinetic coefficients of the chemical reactions using the experimental data and a mathematical model of chemisorption kinetics. To perform the above chemical reactions, an original-design chemisorption reactor was developed, which contains plates with potassium superoxide nanocrystalline fixed on the fibers and pore surface of a fibrous polymer matrix. A mathematical model of chemical air regeneration is developed to calculate the guaranteed values of the parameters of the reactor and the protective effect time of the chemisorbent during which, at a given load, the reactor provides the appropriate concentrations of oxygen and carbon dioxide in the breathing gas mixture in an airtight habitable.     

Development of anodes for direct electrocatalytic oxidation of methane in solid oxide fuel cells

Gold-doped nickel/zirconia and nickel/ceria cermet anodes incorporating different levels of gold have been prepared and studied as potential anodes for the direct electrocatalytic oxidation of methane in solid oxide fuel cells. The methane conversion activity and selectivity towards synthesis gas products of these anodes have been determined over a range of SOFC operating temperatures and methane/oxidant ratios, and compared with the corresponding undoped nickel cermet. The amount of carbon deposition has been determined using post-reaction temperature programmed oxidation. The influence of gold loading on the methane conversion activity, product selectivity and amount of carbon deposition has been determined. The addition of small amounts of gold to nickel cermets results in a dramatically increased tolerance to carbon deposition and a reduction in the activity of the anode and the selectivity towards partial oxidation compared to the corresponding undoped nickel cermet. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

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.     

Photoacoustic monitoring of gases using a novel laser source tunable around 2.5 μm

We present the first spectroscopic measurements using a tunable solid state Cr²+:ZnSe laser emitting at wavelengths between 2.2 μm and 2.8 μm. Photoacoustic measurements on various gases such as methane, carbon monoxide, carbon dioxide, water vapour, nitrous oxide, and ambient air were carried out. In this paper, we present measurements on methane, nitrous oxide, and ambient air. The deduced detection limits are in the low ppm or sub-ppm range, e.g., 0.2 ppm for carbon dioxide, 0.8 ppm for methane and 2.7 ppm for carbon monoxide.     

SiC/SiO2 coating for improving the oxidation resistive property of carbon nanofiber

The SiC/SiO₂ deposition was performed to improve the oxidation resistive properties of carbon nanofiber (CNF) from electrospinning at elevated temperatures through sol-gel process. The stabilized polyacrylonitrile (PAN) fibers were coated with SiO₂ followed by heat treatment up to 1000 and 1400 °C in an inert argon atmosphere. The chemical compositions of the CNFs surface heat-treated were characterized as C, Si and O existing as SiC and SiO₂ compounds on the surface. The uniform and continuous coating improved the oxidation resistance of the carbon nanofibers. The residual weight of the composite was 70-80% and mixture of SiC, SiO₂ and some residual carbon after exposure to air at 1000 °C.     

Experimental and modeling study on the pyrolysis and oxidation of iso-octane

High pressure iso-octane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, iso-octane represents the branched paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on iso-octane was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on iso-octane oxidation and pyrolysis were obtained for temperatures from 835 to 1757 K, pressures from 21 to 65 atm, reactions times from 1.11 to 3.66 ms, and equivalence ratios from 0.52 to 1.68, and ∞. Iso-octane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at conditions studied. This observation prompted an experimental and modeling study of iso-octane pyrolysis using an iso-octane sub-model taken from a recently published n-decane/iso-octane/toluene surrogate model. The revised iso-octane sub-model showed improvements in predicting intermediate species profiles from pyrolytic experiments and oxidation experiments. The modifications to the iso-octane sub-model also contributed to better agreement in predicting the formation of carbon monoxide and carbon dioxide when compared to the recently published 1st Generation Surrogate model and a recently published iso-octane oxidation model. Model improvements were also seen in predicting species profiles from flow reactor oxidation experiments and ignition delay times at temperatures above 1000 K at both 10 and 50 atm.     

Emission of carbon oxides during the combustion of lean methane-air premixed mixtures

A one-dimensional problem of propagation of a laminar flame front through a uniform methane-air mixture was solved using the GRI-Mech 3.0 reaction mechanism. An analysis of the composition of the combustion products behind the flame front at a pressure of 10 atm, an initial mixture temperature of 600 K, and two values of the air-to-fuel equivalence ratio (α = 1.8 and 2.5) was performed. It was demonstrated that, at short residence times, the carbon oxide emission increases as the mixture is made leaner, with the opposite tendency being observed at long residence times. Numerical calculations of the characteristics of turbulent flow and combustion in two axisymmetric homogeneous-combustion model chambers with relatively long residence times were performed within the framework of a bulk (quasi-laminar) combustion model. In calculations, the methane-air mixture composition and the wall temperature of one of the chambers were varied. The case of cooling air inflow through the chamber wall was considered. It was demonstrated that, over a wide range of parameters in the combustion chamber and on its wall, the CO emission monotonically decreases as the degree of mixture leaning grows, but it increases when the chamber wall is cooled and when cooling air is blown through the wall.     

Reactor scale modeling of multi-walled carbon nanotube growth

As the mechanisms of carbon nanotube (CNT) growth becomes known, it becomes important to understand how to implement this knowledge into reactor scale models to optimize CNT growth. In past work, we have reported fundamental mechanisms and competing deposition regimes that dictate single wall carbon nanotube growth. In this study, we will further explore the growth of carbon nanotubes with multiple walls. A tube flow chemical vapor deposition reactor is simulated using the commercial software package COMSOL, and considered the growth of single- and multi-walled carbon nanotubes. It was found that the limiting reaction processes for multi-walled carbon nanotubes change at different temperatures than the single walled carbon nanotubes and it was shown that the reactions directly governing CNT growth are a limiting process over certain parameters. This work shows that the optimum conditions for CNT growth are dependent on temperature, chemical concentration, and the number of nanotube walls. Optimal reactor conditions have been identified as defined by (1) a critical inlet methane concentration that results in hydrogen abstraction limited versus hydrocarbon adsorption limited reaction kinetic regime, and (2) activation energy of reaction for a given reactor temperature and inlet methane concentration. Successful optimization of a CNT growth processes requires taking all of those variables into account.     

Thermodynamic properties of surface carbon on ruthenium

The thermodynamic properties of surface carbon on alumina-supported ruthenium have been determined from measurements of the Boudouard equilibrium in the temperature range from 525 to 675 K. The results indicate that the enthalpy of surface-carbon formation on Ru is positive relative to graphite and insensitive to the fractional coverage with carbon. Measurements of the chemical reactivity of surface carbon towards hydrogen indicate that the high-enthalpy carbon forms methane at relatively low temperatures (275 K).     

A wire microcalorimetric study of catalytic ignition of methane–air mixtures over palladium oxide

Catalytic ignition and heat release of methane oxidation over a Pd wire covered with a 1–2 μm PdO surface layer were investigated by wire microcalorimetry over the temperature range of 600–770 K and pressure range of 0.5–4 atm. Ignition temperatures and heat release rates for different methane concentrations (1–4 vol.% in dry air) were determined, showing that the ignition temperatures decrease with increasing the methane concentration and increasing ambient pressure. At total pressure of 1 atm and 2% methane concentration, the global activation energy for the catalytic reaction is 21.5 ± 0.9 kcal/mol and 14.3 ± 0.2 kcal/mol in the temperature ranges of 600–670 K and 670–770 K, respectively. The reaction order for methane is 0.9 ± 0.1 over the temperature range of 630–770 K.     

Propylene deep oxidation on the Pt(111) surface: temperature programmed studies over extended coverage ranges

Propylene oxidation was studied on the Pt(111) surface over a wide range of reaction stoichiometries using temperature programmed methods. Reaction of propylene with excess oxygen results in complete oxidation to water and carbon dioxide, with oxydehydrogenation to form water beginning at 290 K. The initiation of skeletal oxidation occurs after water formation begins, except for the highest propylene coverages. A stable dehydrogenated intermediate with a C₃H₅ stoichiometry is formed in the 300 K temperature range during oxidation. Reaction of propylene with substoichiometric amounts of oxygen results in incomplete oxidation with hydrocarbon decomposition dominating after depletion of surface oxygen. Increasing oxygen coverage results in more complete oxidation. Oxidation processes result in water, carbon dioxide, and carbon monoxide, while decomposition results in hydrogen, propylene, and propane desorption with some surface carbon remaining. Propylene-d₆ and selectively labeled propylene-3,3,3-d₃ (CH₂CHCD₃) experiments indicated initial water formation results from oxydehydrogenation of one of the olefinic hydrogens. At the highest propylene and oxygen coverages studied, we observed small amounts of partial oxidation which indicate that the vinyl hydrogen is removed initially, resulting in the formation of an adsorbed H₂CCCH₃ intermediate. The partial oxidation products observed are acetone desorbing at 200 K and acetic acid at 320 K. Removal of the first skeletal carbon begins at 320 K, except for the highest propylene coverages. Preadsorption of molecular oxygen limits adsorption of propylene and preadsorption of propylene limits molecular oxygen adsorption at 110 K. Similar oxidation mechanisms are observed following initial adsorption of both molecular and atomic oxygen, which is expected since molecular oxygen dissociates and/or desorbs well below oxidation temperatures.