Kinetic contribution of CO2/O2 additive in methane conversion activated by non-equilibrium plasmas

A temperature-controlled and pressure-controlled coaxial dielectric barrier discharge(DBD) reactor was developed to decouple the thermal and kinetic effects of radio frequency(RF) discharge on methane conversion,and further to compare the kinetic behaviors of the mechanistically similar reactions of methane conversion with O_2 and CO_2 additives. A kinetic mechanism for RF plasma assisted methane conversion was assembled. The formation of products in the RF plasma reactor was measured with Gas Chromatography(GC–TCD) and the data were used to validate the kinetic model. The experimental and computational results showed the different kinetic roles of carbon dioxide and oxygen additives in methane conversion, due to the different dissociation and ionization energy of the two additive gases, as well as the thus produced electron energy distribution function(EEDF). Fuel oxidation by plasma generated O, O(~1 D), O_2(a~1Δg), O_2(b~1Σ_g~+) and O+in partial oxidation of methane was observed essential for methane consumption, which resulted in an increase in methane conversion rate,compared to pure methane pyrolysis and dry reforming of methane with CO_2 additive. It was also found that dry reforming of methane with CO_2 was by far the easier to produce the syngas as well as C_2 hydrocarbon species,due to the weak oxidation ability of CO_2 and also the significant deposition of the electron energy on CH_4 dissociation in a dry reforming discharge mixture. This kinetic study produced comparative data to demonstrate the contribution of CO_2/O_2 additive in non-equilibrium plasma assisted methane conversion.     

Mathematical Modeling of Methane Air Conversion on a Structured Porous Metal Catalyst

A one-dimensional two-phase mathematical model of an ideal plug-flow reactor for methane air conversion on a Ni–MgO monolith catalyst on porous nickel was proposed. The model describes the methane air conversion as the result of three simultaneous reaction stages: methane deep oxidation, methane steam reforming, and reverse shift reaction. The effect of the external gas–solid mass transfer is taken into account in two variants: (i) independent diffusion and (ii) multicomponent diffusion for all mixture components. The results of modeling were used to analyze the experimental data (obtained in our previous work) on the dependence of the temperature of the front layer of the catalyst on the pressure and excess air coefficient. The best agreement between the calculation and experiment was obtained under conditions of complete external diffusion control of the exothermic stage for oxygen and the transition (between the kinetic and external diffusion) region of the endothermic stage, the kinetic effect of the endothermic stage being further limited by internal pore-diffusion resistance of the rate of this stage for methane.     

Reduction kinetics of a fluidizable nickel–alumina oxygen carrier for chemical‐looping combustion

This study investigates the kinetic modelling of oxygen carrier reduction by methane in the combustion reactor of the chemical‐looping combustion process. The species being reduced and the stability of the fluidizable oxygen carrier sample over repeated reduction–oxidation cycles are established using the temperature programmed reduction and oxidation and the pulse chemisorption characterization methods. The power‐law relation, the nucleation and nuclei growth model and the shrinking‐core models are considered to interpret the experimental kinetics of the oxygen carrier reduction. The obtained results show that the nucleation and nuclei growth model best describes the experimental data providing parameters with adequate statistical fitting indicators.     

覆盖层氧气消耗通量模型及甲烷氧化能力预测

填埋场覆盖层生物气扩散规律和甲烷氧化能力的评估是甲烷减排研究的重要组成部分。以数值模拟方法分析了氧气在覆盖层中的扩散规律,得到了指数方程形式的氧气扩散模型(R²范围0.8941～0.9975);通过检测有机碳和甲烷浓度变化进一步考察了模拟覆盖层不同深度的甲烷氧化能力,证实了在0.05～0.25 m范围内甲烷氧化活性最高;以Fick定律和轴向扩散模型推导了模拟覆盖层中氧气消耗通量模型,该模型计算得到的氧气消耗通量与覆盖层中微生物甲烷氧化经验方程相比无显著差异;结合以上模型推演出覆盖层甲烷消耗通量模型,与实际检测值相比,预测结果理想(R²=0.9983)。该成果可为揭示填埋场覆盖层生物气扩散规律、强化甲烷氧化能力以及预测甲烷排放提供新的思路和理论依据。     

Fuel-rich methane combustion: Role of the Pt dispersion and oxygen mobility in a fluorite-like complex oxide support

For catalysts comprised of Pt supported onto dispersed complex fluorite-like oxides (ceria doped by Pr, Gd, Sm, or CeO₂–ZrO₂ doped by La, Gd or Pr), the effects of the oxygen mobility in supports and Pt dispersion on the performance in methane selective oxidation into syngas at short contact times were elucidated using combination of kinetic and spectroscopic methods. While in general any simple universal relation between the oxygen mobility, Pt dispersion and the rate of methane transformation into syngas was not found, for some series, a good correlation was observed agreeing with the bifunctional scheme of the methane selective oxidation into syngas.     

Partial oxidation of methane to synthesis gas on a Rh/TiO2 catalyst in a fast flow porous membrane reactor

The partial oxidation of methane to synthesis gas has been studied over a 3% Rh/TiO₂ catalyst in a fixed bed and a novel membrane reactor under autothermal conditions using O₂ as oxidant. The membrane reactor allows the partial oxidation reaction to be performed without premixing the reactants reducing the risk of explosion even at low methane/oxygen ratios. The membrane reactor can operate autothermally and at millisecond residence time. Methane conversions of up to 65% with CO and H₂ selectivities of 90 and 82% respectively have been achieved. The low methane oxygen ratio and the high flow rates are the key factors to attain autothermal behavior. The most sensitive factor to attain high conversion and selectivities appears to be short contact time but high temperature. A kinetic model was used to interpret the experimental results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Some kinetic aspects of unsteady-state partial oxidation reactions. Dynamic processes on metal oxide surfaces

It is shown that steady-state kinetic data do not allow the discrimination between the redox and associated mechanisms of the partial oxidation reactions. A discrimination between these mechanisms was performed using transient experiments. The obtained rate expressions are in agreement with experimental kinetic data for catalytic partial oxidation of o-xylene.

An influence of the conjugate oxidation of a catalyst surface on dynamics and kinetics of the heterogeneous catalytic oxidative reactions is considered. Computing simulation of methane oxidative coupling of methane reaction at lowered temperature and elevated pressure has been performed. It showed that the reaction order with respect to oxygen exceeding unity is consistent with the chain branching mechanism of the reaction in the presence of TiSi₂ and TiB₂ and showed the important role of the branching chain cycles in the low-temperature OCM reaction at elevated pressure.     

Kinetics of the photocatalytic total oxidation of different alkanes and alkenes on TiO2 powder

Although photocatalyzed total oxidation reaction of hydrocarbon species has been discussed in the literature, most of these studies were performed to obtain an appropriate reaction mechanism. Studies on the kinetics of this type of reaction are rare. Using titanium dioxide as the photo catalyst, the kinetics of the total oxidation of methane, ethane, ethene, as well as propene, have been investigated using a continuous‐stirred tank reactor. In the experiments, the hydrocarbon concentrations, the oxygen concentration, and the irradiation intensity were varied. The results obtained are evaluated on the basis of a kinetic model to derive rate equations which can be used for reactor design.     

The reaction of NiO/NiAl2O4 particles with alternating methane and oxygen

The reduction and oxidation behaviour of oxygen carrier particles of NiO and NiAl₂O₄ has been investigated in a fluidized bed reactor as well as a thermogravimetric analyzer (TGA). The particles showed high reactivity and gas yield to CO₂ with methane in the temperature interval 750–950°C. In the fluidized bed the yield to CO₂ was between 90 and 99% using bed masses corresponding to 16–57 kg/MW_fuel. Complementary experiments in a TGA at 750 and 950°C showed a clear reaction of the NiAl₂O₄ with CH₄ at the higher temperature. There was methane released from the reactor at high degrees of solid oxidation, which is likely associated with the lack of Ni‐sites on the particles which can reform the methane. There was some carbon formation during the reduction, although the amount was minor when the gas yield to carbon dioxide and degree of oxidation of the solid was high. A simple reactor model using kinetic data from a previous study predicted the gas yield during the reduction in the fluidized bed experiments with reasonable accuracy. The oxygen carrier system investigated in this work shows high promise for use in a real CLC system, provided that the particle manufacturing process can be scaled up with reasonable cost.     

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

The partial oxidation of methane to synthesis gas has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoichiometric feed ratio the dominant pathways are the direct formation of CO and H₂ followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.     

Kinetics of catalytic oxidation of methane: Application of initial rate technique for mechanism determination

A kinetic investigation on the oxidation of methane using a palladium catalyst was conducted. Conversions of methane were small and limited to about 10% to facilitate the use of initial rate approach. Plots of initial rate against total pressure and varying feed composition were used to interpret the data qualitatively. Regression coefficients from linearized rate equations and criteria of best fit were used to arrive at a plausible mechanism. The experimental results indicated that the reaction rate is controlled by surface reaction in which the adsorbed oxygen and adsorbed methane react to produce adsorbed carbon dioxide and adsorbed water.     

Impact of oxygen transport properties on polypropylene thermal oxidation,part 1: Effect of oxygen solubility

A general kinetic model is proposed to describe the polypropylene thermal oxidation of thin polypropylene films in a wide range of temperatures (from 60 to 200°C) and oxygen partial pressures (from 0.02 to 5 MPa) using a single set of parameters. This model was calibrated with several series of experimental data including analyses of primary (hydroperoxides) and secondary oxidation products (carbonyl species), and subsequent changes in macromolecular properties (average molecular masses). It predicts the experimental data previously published in the literature in terms of induction times and maximal oxidation rates. The variability of the oxygen solubility coefficient allows to explain the scattering of induction times and oxidation rates among the whole iPP family, but also the dependence of this latter quantity with oxygen partial pressure. This variability is presumably due to iPP polymorphism in the temperature range where oxygen permeability is commonly measured. It is concluded that the kinetic model can be used to study the direct effect of iPP morphology on its thermal oxidation kinetics (chemistry of oxidation). © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41441.     

The fate of methane in a claus plant reaction furnace

Experimental kinetic data are reported for key side reactions occurring in the front end [i. e. the reaction furnace (RF) and the waste heat boiler (WHB)] of modified Claus plants used for sulfur recovery from the sour gases evolved in the treatment of natural gas. An extensive experimental study was conducted in a high temperature tubular reactor system for two important homogenous gas‐phase reactions. Firstly, experiments were carried out to study the oxidation of hydrogen sulfide and methane mixtures in the presence of oxygen. Secondly, the reaction between methane and sulfur dioxide was investigated experimentally. These results showed that methane was much less competitive for oxygen than hydrogen sulfide. Hence, in a partially oxidizing environment of a RF, data showed that methane reacted significantly with other major sulfur containing species, as secondary reactions, to form COS and especially CS². This is highly problematic from an environmental point of view.     

Simulation of growth of methane-utilising bacteria in batch culture

A kinetic model for batch growth of a methane-utilising bacterium is presented, using data from previous experimental work on the specific rates of growth, respiration and methane consumption of the bacterium. A two-substrate model with respect to methane and oxygen was employed to represent the specific rate of methane consumption. Employing the various coefficients assessed previously, the bacterial growth was simulated in relation to changing partial pressures and mass-transfer coefficients for both gases. The simulated results are discussed in light of the experimental data published by other workers on both pure and mixed populations of methane-utilising bacteria. The optimal partial pressures of oxygen and methane derived from the model were found to play a decisive role in connection with inflammability.     

Reduction kinetics of SrFeO3−δ/CaO∙MnO nanocomposite as effective oxygen carrier for chemical looping partial oxidation of methane

Chemical looping reforming of methane is a novel and effective approach to convert methane to syngas, in which oxygen transfer is achieved by a redox material. Although lots of efforts have been made to develop high-performance redox materials, a few studies have focused on the redox kinetics. In this work, the kinetics of SrFeO_3−δ–CaO∙MnO nanocomposite reduction by methane was investigated both on a thermo-gravimetric analyzer and in a packed-bed microreactor. During the methane reduction, combustion occurs before the partial oxidation and there exists a transition between them. The weight loss due to combustion increases, but the transition region becomes less inconspicuous as the reduction temperature increased. The weight loss associated with the partial oxidation is much larger than that with combustion. The rate of weight loss related to the partial oxidation is well fitted by the Avrami–Erofeyev equation with n = 3 (A3 model) with an activation energy of 59.8 kJ∙mol^‒1. The rate law for the partial oxidation includes a solid conversion term whose expression is given by the A3 model and a methane pressure-dependent term represented by a power law. The partial oxidation is half order with respect to methane pressure. The proposed rate law could well predict the reduction kinetics; thus, it may be used to design and/or analyze a chemical looping reforming reactor.     

催化汽油氧化脱硫的动力学模型研究

通过脱硫实验及动力学模拟,开展催化汽油氧化脱硫的动力学模型研究.结果表明,汽油脱硫率随着催化剂用量增加、氧化温度提高、氧气分压增大、氧化时间、氧化水油体积比增大均持续提高,而汽油收率逐渐降低.依据反应动力学和萃取相平衡原理,确定了脱硫率和汽油收率模型,并确定了有关模型参数.建立的脱硫率和汽油收率模型在显著性水平α=0.01时均是显著的,具有较高的模拟计算精度.     

The effects of kinetics, hydrodynamics and feed conditions on methane coupling using fluidized bed reactor

A previously developed model describing bubbling fluidized bed reactors is used in this investigation to study the effect of various important hydrodynamic, operating and design parameters on the performance of a large scale fluidized bed reactor used in oxidative coupling of methane. Three kinetic schemes obtained from the literature have been used in this study. The model predicted fairly well the experimental results reported recently under different reaction conditions. The simulation results revealed that increasing the ratio of methane to oxygen in the feed leads to lower methane conversion but higher C₂ selectivity. As the ratio is decreased the system loses its fixed-point stability to a periodic stability. Higher methane conversion and product selectivity are obtained upon decreasing the feed flow rate and particle diameter.     

Scientific basis for process and catalyst design in the selective oxidation of methane to formaldehyde

The mechanism and kinetics of the gas-phase selective oxidation of methane to formaldehyde (MPO) are revised in the general context of catalytic oxidations. In agreement with ab initio calculations of the energy barrier for the activation of methane on transition metal oxide complexes, a formal Langmuir-Hinshelwood kinetic model is proposed which accounts for the "steady-state" conditions and activity-selectivity pattern of MPO catalysts, providing an original support to process design and catalyst development.