Pyrolysis and Oxidation of Methane in a RF Plasma Reactor

The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH₄ mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H₂, C₂ and C₃ hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C₂ and C₃ formation dramatically decreased, and syngas (H₂ and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH₄ mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH₃, CH₂, CH, H, O and O (¹D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.     

Semiconductor Photocatalysts for Non-oxidative Coupling,Dry Reforming and Steam Reforming of Methane

Methane is one of the promising alternatives of petroleum, which should be used for not only a fuel but also a resource for hydrogen and more useful chemicals as with the petroleum. However, the selective methane conversion to them is still difficult in contrast to the combustion. Three types of photocatalytic reactions for methane conversion, i.e., the photocatalytic non-oxidative coupling of methane (2CH₄ → C₂H₆ + H₂), the photocatalytic dry reforming of methane (CH₄ + CO₂ → 2CO + 2H₂) and the photocatalytic steam reforming of methane (CH₄ + 2H₂O → 4H₂ + CO₂), can take place around room temperature or at a mild condition such as 473 K using photoenergy and semiconductor photocatalyst. In the present short review, the details of each photocatalytic reaction and the design concept of the semiconductor photocatalysts for each photocatalytic methane conversion were summarized and discussed.     

Ethylene pyrolysis and oxidation: A kinetic modeling study

Ethylene oxidation and pyrolysis was modeled using a comprehensive kinetic reaction mechanism. This mechanism is an updated version of one developed earlier. It includes the most recent findings concerning the kinetics of the reactions involved in the oxidation of ethylene. The proposed mechanism was tested against ethylene oxidation experimental data (molecular species concentration profiles) obtained in jet stirred reactors (1–10 atm, 880–1253 K), ignition delay times measured in shock tubes (0.2–12 atm, 1058–2200 K) and ethylene pyrolysis data in shock tube (2–6 atm, 1700–2200 K). The general prediction of concentration profiles of minor species formed during ethylene oxidation is improved in the present model by using more accurate kinetic data for several reactions (principally: HO₂ + HO₂ → H₂O₂ + O₂, C₂H₄ + OH → C₂H₃ + H₂O, C₂H₂ + OH → Products, C₂H₃ → C₂H₂ + H).     

Development of a detailed pyrolysis mechanism for C1–C4 hydrocarbons under a wide range of temperature and pressure

A model of core mechanism of hydrocarbon pyrolysis with good predictive ability is crucial to the development of active cooling technology for advanced aeroengines. In this work, a detailed core kinetic model of pyrolysis of C₁–C₄ hydrocarbon fuels is developed through the combination of a series of potential energy surfaces and validated against a series of experimental results. The kinetic model contains 103 species and 1290 reactions, and most of the kinetic and thermochemical parameters are compiled from recent highly accurate quantum chemical calculations without modification. The pressure-dependent rate constants are considered for the dissociation/association reactions, isomerization reactions, and chemically activated reactions. Simulation results for various alkanes (methane, ethane, propane, n-butane, isobutane), alkenes (ethylene, propene, 1-butene, 2-butene, isobutene, allene, 1,3-butadiene), and alkynes (acetylene, propyne, vinylacetylene) indicate that the major product distributions at various temperatures (800-2300 K) and pressures (0.8-10 atm) can be predicted well by the developed core kinetic model. Thus, the developed pyrolysis mechanism for C₁–C₄ hydrocarbons can be used as a cornerstone to develop the pyrolysis mechanisms of larger hydrocarbon fuels and thus support the development of thermal management in advanced aeroengines.     

Promotion of Non-thermal Plasma on Catalytic Reduction of NOx by C3H8 Over Co/BEA Catalyst at Low Temperature

The effects of non-thermal plasma on selective catalytic reduction of NO_x by C₃H₈ (C₃H₈-SCR) over Co/BEA catalyst were investigated over a wide range of reaction temperatures (473–773 K). The significant synergistic effect between non-thermal plasma and catalytic reduction by C₃H₈ was exhibited at low temperatures from 473 to 673 K. The synergetic effect diminished with increasing temperature. The NO_x removal efficiency of non-thermal plasma facilitated C₃H₈-SCR hybrid system increased significantly with the increase in NO₂/NO ratio from 0.13 to 1.06 when the specific input energy increased from 0 to 136 J L^?1. The oxidation performance of NO to NO₂ was significantly enhanced by C₃H₈ in the plasma reactor. Results of CO₂/CO ratio and CO₂ selectivity suggested that adding non-thermal plasma improved CO₂ selectivity of C₃H₈-SCR. 200 ppm SO₂ slightly inhibited NO_x conversion of the non-thermal plasma facilitated C₃H₈-SCR hybrid system at below 673 K, whereas it exhibited no obvious effect at over 673 K. Non-thermal plasma was more selective toward NO oxidation than SO₂ oxidation in the presence of C₃H₈. The non-thermal plasma facilitated C₃H₈-SCR hybrid system could be used stably in durability tests with several hundreds ppm of SO₂.     

Thermal decomposition of methane: Autocatalysis

The origin of autocatalysis in the pyrolysis of methane has been investigated by kinetic modeling. A mechanism is presented that provides good agreement with experimental data at 1038 K and 433 torr into the autocatalytic region. The main causes of autocatalysis are secondary initiation by hydrocarbon products larger than C₂H₆ and chain radical methylation sequences.     

Pyrolysis of methyl chloride,a pathway in the chlorine-catalyzed polymerization of methane

The reaction of CH₄ + Cl₂ produces predominantly CH₃Cl + HCl, which above 1200 K goes to olefins, aromatics, and HCl. Results obtained in laboratory experiments and detailed modeling of the chlorine-catalyzed polymerization of methane at 1260 and 1310 K are presented. The reaction can be separated into two stages, the chlorination of methane and pyrolysis of methylchloride. The pyrolysis of CH₃Cl formed C₂H₄ and C₂H₂ in increasing yields as the degree of conversion decreased and the excess of methane increased. Changes of temperature, pressure, or additions of HCl had little effect. In the absence of CH₄ C₂H₄ and C₂H₂ are formed by the recombination of ?H₃ and ?H₂Cl radicals. With added CH₄ recombination of ?H₃ forms C₂H₆, which dehydrogenates to C₂H₄ + H₂. C₂H₄ in turn dehydrogenates to C₂H₂ + H₂. While HCl, C, CH₄, and H₂ are the ultimate stable products, C₂H₄, C₂H₂, and C₆H₆ are produced as intermediates and appear to approach stationary concentrations in the system. Their secondary reactions can be described by radical reactions, which can lead to soot formation. ?H₃ - initiated polymerization of ethylene is negligible relative to the ?₂H₃ formation through H abstraction by Cl. The fastest reaction of ?₂H₃ is its decomposition to C₂H₂. About 20% of the consumption of C₂H₂ can be accounted for by the addition of ?₂H₃ to it with formation of the butadienyl radical. The addition of the latter to C₂H₂ is slow relative to its decomposition to vinylacetylene. Successive H abstraction by Cl from C₄H₄ leading to diacetylene has rates compatible with the experimental values. About 10% of ?₄H₅ abstracts H from HCl and forms butadiene. Successive additions of ?₂H₃ to butadiene and the products of addition can account for the formation of benzene, styrene, naphthalene, and higher polyaromatics. The following rate parameters have been derived on the basis of the experimentally measured reaction rates, the estimated frequency factors, and the currently available heat of formation of the ?₂H₃ radical (69 kcal/mol):      

Comparative Study of Methane Activation Process by Different Plasma Sources

In this paper, we compare the characteristics of methane activation by diverse plasma sources. The test conditions of reactant flow rate and composition are fixed for each plasma source to eliminate any possible misleading effects from varying test conditions. Among the diverse characteristics of each plasma source, we focus on the electron energy and degree of thermal activation in evaluating the cost-effectiveness of methane decomposition. The reaction is evaluated based on the selectivity of specific products, including H₂, C₂H₆, and C₂H₂. Among the tested plasma sources, those that provide a somewhat thermal environment have a rather high degree of warmness, resulting in higher methane conversion and lower operational costs. As the non-thermal characteristics of the plasma sources become stronger, the selectivity of C₂H₆ increases. This reflects C₂H₆ formation from the direct collision of CH₄ with high-energy electrons. On the other hand, as the degree of warmness increases, the selectivity of H₂ and C₂H₂ increase. The results give an insight into possible tools for process control or selectivity control by varying the degree of warmness in the plasma source. The process optimization and cost reduction of methane activation should be based on this concept of selectivity control.     

An experimental and modeling study of ethanol oxidation kinetics in an atmospheric pressure flow reactor

Experimental profiles of stable species concentrations and temperature are reported for the flow reactor oxidation of ethanol at atmospheric pressure, initial temperatures near 1100 K and equivalence ratios of 0.61–1.24. Acetaldehyde, ethene, and methane appear in roughly equal concentrations as major intermediate species under these conditions. A detailed chemical mechanism is validated by comparison with the experimental species profiles. The importance of including all three isomeric forms of the C₂H₅O radical in such a mechanism is demonstrated. The primary source of ethene in ethanol oxidation is verified to be the decomposition of the C₂H₄OH radical. The agreement between the model and experiment at 1100 K is optimized when the branching ratio of the reactions of C₂H₅OH with OH and H is defined by (30% C₂H₄OH + 50% CH₃CHOH + 20% CH₃CH₂O) + XH. As in methanol oxidation, HO₂ chemistry is very important, while the H + O₂ chain branching reaction plays only a minor role until late in fuel decay, even at temperatures above 1100 K.     

Ethylene Epoxidation in an AC Dielectric Barrier Discharge Jet System

In this work, ethylene epoxidation was investigated in a dielectric barrier discharge jet (DBDJ) with a separate ethylene/oxygen feed under oxygen lean conditions. The ethylene (C₂H₄) stream was directly injected behind the plasma zone in order to reduce all undesired reactions, including C₂H₄ cracking and further reactions, while the oxygen (O₂) balanced with argon was fed through the plasma zone totally to maximize the formation of active oxygen species. The effects of various operating parameters, such as total feed flow rate, O₂/C₂H₄ feed molar ratio, applied voltage, input frequency, and C₂H₄ feed position on the ethylene epoxidation activity, were investigated to determine the optimum operating conditions for this new DBDJ system. The highest ethylene oxide (EO) selectivity (55.2 %) and yield (27.6 %), as well as the lowest power consumption (3.3 × 10^?21 and 6.0 × 10^?21 Ws/molecule C₂H₄ converted and EO produced, respectively) were obtained at a total feed flow rate of 1,625 cm³/min (corresponding to a residence time of 0.022 s), an O₂/C₂H₄ feed molar ratio of 0.25:1, an applied voltage of 9 kV, an input frequency of 300 Hz, and a C₂H₄ feed position of 3 mm behind the plasma zone. The superior activity of the ethylene epoxidation in the DBDJ system resulted from a small reaction volume as well as a separate ethylene/oxygen feed.     

Kinetics of ethane oxidation

Ethane oxidation in jet-stirred reactor has recently been investigated at high temperature (800–1200 K) in the pressure range 1–10 atm and molecular species (H₂, CO, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) concentration profiles were obtained by probe sampling and GC analysis. Ethane oxidation was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of C₁? C₄ hydrocarbons. The proposed mechanism is able to reproduce experimental data obtained in our high-pressure jet stirred reactor and ignition delay times measured in shock tube in the pressure range 1–13 atm, for temperatures extending from 800 to 2000 K and equivalence ratios of 0.1 to 2. It is also able to reproduce atoms concentrations (H,O) measured in shock tube at ≈2 atm. The same detailed kinetic mechanism can also be used to model the oxidation of methane, ethylene, propyne, and allene in similar conditions.     

水城褐煤热解的气体产物析出特征及甲烷的生成反应类型研究

利用傅里叶红外光谱仪研究了煤中主要官能团的分布,利用热重-质谱联用（TG/MS）在10℃/min的条件下研究了水城褐煤的热解行为,获得了煤热解主要挥发分气体（H₂、CH₄、H₂O、CO、CO₂）生成的速率曲线。采用分峰拟合的方法将甲烷的生成速率曲线分解为五个峰,通过化学动力学分析,结合煤的结构特性、热解特性及其他挥发分气体的生成特征,认为甲烷的生成主要由一个脱吸附过程和四个化学反应组成。     

Additive effect of hydrocarbons on OH radical production in H2 oxidation

Mixtures of hydrocarbons (methane, allene, propyne, propene, and propane)–H₂–O₂ highly diluted with argon were heated to a temperature ranging from 1200 to 1900 K behind reflected shock waves, and the additive effects of methane, allene, propyne, propene, and propane on OH radical production in H₂ oxidation were studied by observing time‐resolved UV‐absorption (306.7 nm). It was found that, in H₂ oxidation below 1500 K, the addition of these hydrocarbons prolonged the delay time of the onset of the rapid OH radical production. An analysis using reported kinetic modeling of C₁–C₄ oxidation gave valuable information for reactions between hydrocarbons and H, O atoms and OH radicals. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 50–55, 2005     

Thermal decomposition of 1,2 butadiene

The thermal decomposition of 1,2 butadiene has been studied behind reflected shock waves over the temperature and total pressure ranges of 1300–2000 K and 0.20–0.55 atm using mixtures of 3% and 4.3% 1,2 butadiene in Ne. The major products of the pyrolysis are C₂H₂, C₄H₂, C₂H₄, CH₄ and C₆H₆. Toluene was observed as a minor product in a narrow temperature range of 1500–1700 K. In order to model successfully the product profiles which were obtained by time-of-flight mass spectrometry, it was necessary to include the isomerization reaction of 1,2 to 1,3 butadiene. A reaction mechanism consisting of 74 reaction steps and 28 species was formulated to model the time and temperature dependence of major products obtained during the course of decomposition. The importance of C₃H₃ in the formation of benzene is demonstrated.     

Kinetic Modeling of Secondary Methane Formation and 1‐Olefin Hydrogenation in Fischer–Tropsch Synthesis over a Cobalt Catalyst

A detailed kinetic model of Fischer–Tropsch synthesis (FTS) product formation, including secondary methane formation and 1‐olefin hydrogenation, has been developed. Methane formation in FTS over the cobalt‐based catalyst is well known to be higher‐than‐expected compared to other n‐paraffin products under typical reaction conditions. A novel model proposes secondary methane formation on a different type of active site, which is not active in forming C₂₊ products, to explain this anomalous methane behavior. In addition, a model of secondary 1‐olefin hydrogenation has also been developed. Secondary 1‐olefin hydrogenation is related to secondary methane formation with both reactions happening on the same type of active sites. The model parameters were estimated from experimental data obtained with Co/Re/γ‐Al₂O₃ catalyst in a slurry‐phase stirred tank reactor over a range of conditions (T = 478, 493, and 503 K, P = 1.5 and 2.5 MPa, H₂/CO feed ratio = 1.4 and 2.1, and X _CO = 16–62%). The proposed model including secondary methane formation and 1‐olefin hydrogenation is shown to provide an improved quantitative and qualitative prediction of experimentally observed behavior compared to the detailed model with only primary reactions.     

Pyrolysis of propane for CVI of pyrocarbon: Part III: Experimental and modeling study of the formation of pyrocarbon

Carbon/carbon composites synthesis involves the deposition of a matrix of pyrocarbon produced by the pyrolysis of a gaseous hydrocarbon in a preform made of carbon fibers. This work describes an experimental and modeling study of the formation of pyrocarbon obtained by the pyrolysis of propane. The pyrolysis of propane is carried out in a perfectly stirred reactor at low pressure (2.6 kPa) in a wide range of temperature (1173–1298 K) with a residence time of 1 s. During the pyrolysis, the pyrocarbon is quantified by weighing and 29 other products of propane pyrolysis are also analysed by Gas-Chromatography (GC). In order to reproduce the experimental deposit of pyrocarbon but also the gas phase species, an original way of modeling the deposition of pyrocarbon, which contains a homogeneous model completed with lumped heterogeneous reactions, is proposed. This model tries to target which species gives what pyrocarbon although what really happens at the carbon fibers surface remains unknown. Two kinds of reactions of deposition are discussed; those involving small gaseous unsaturated species such as C₂H₂ and those involving large species (≥C₆). The results of modeling seem to show, in agreement with the literature, that the pyrocarbon deposition could be quantitatively explained by the deposition of small unsaturated species.     

Thermal reactions of acetonitrile at high temperatures. Pyrolysis behind reflected shocks

The thermal decomposition of acetonitrile was studied behind reflected shocks in a single pulse shock tube over the temperature range 1350–1950 K at overall densities of approximately 3 × 10^?5 mol/cc. Methane and hydrogen cyanide are the major reaction products. They are formed by an attack of H and CH₃ radicals on acetonitrile. The initiation step of the pyrolysis is the self dissociation of acetonitrile: for which the following rate constant was obtained: k₁ = 6.17 × 10¹⁵exp(?96.6 × 10³/RT)sec^?1. Where R is given in units of cal/K mol. Additional reaction products which appear in the pyrolysis are: C₂H₂, C₂H₄, CH₂?CHCN, CH?CHCN, C₂H₅CN, C₂N₂, and C₄H₂. Acetylene is formed from methane pyrolysis and becomes a major reaction product at high temperatures. Acrilonitrile and cyanoacetylene are secondary products originating from the CH₂CN radical. Rate parameters for the formation of the reaction products are given.     

Methane conversion under cold plasma over Pd-containing ionic liquids immobilized on γ-Al2O3

Pd-containing ionic liquid (IL) l-hexyl-3-methylimidazolium tetrafluoroborate (C₆MIMBF₄) immobilized on γ-Al₂O₃ (Pd-IL/γ-Al₂O₃) was prepared and characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis. The influences of C₆MIMBF₄ loading and Pd on methane conversion to C₂ hydrocarbons under cold plasma were investigated. FTIR and SEM analyses indicated that C₆MIMBF₄ had been successfully immobilized on γ-Al₂O₃ and the C₆MIMBF₄ showed excellent stability under cold plasma. The results of BET and methane conversion showed that with the increase in immobilization amount of C₆MIMBF₄ onto γ-Al₂O₃, the specific surface area and pore volume of IL/γ-Al₂O₃ decreased, while the selectivity and yield of C₂ hydrocarbons increased. The selectivity of C₂ hydrocarbons was 94.6% when the loading of C₆MIMBF₄ was 40%, and the percentage of C₂H₄ in C₂ hydrocarbons was as high as 64% when using Pd-IL/γ-Al₂O₃ as a catalyst with no conventional thermal reduction treatment. Optical emission spectra (OES) from the cold plasma reactor during methane conversion were also studied. The results indicated that the intensity of the C₂, CH, H, and C active species from methane and hydrogen decomposition increased when IL/γ-Al₂O₃ or Pd-IL/γ-Al₂O₃ was introduced into the plasma system. Based on the analyses of the gas product and OES spectra, it can be concluded that the surface catalyzed reactions between plasma and ionic liquid were very important for the reduction of Pd²⁺ and the formation of C₂H₄