Direct conversion of methane to acetylene or syngas at room temperature using non-equilibrium pulsed discharge

Non-catalytic direct conversion of methane to valuable products was studied using non-equilibrium pulsed discharge under the conditions of ambient temperature and atmospheric pressure. Acetylene was produced with 95% selectivity and 52% methane conversion. An addition of oxygen, carbon dioxide and steam contributed significantly to suppress the carbon deposition and produced carbon monoxide as well as acetylene. Methane conversion increased with an increase in the pulse frequency while the product selectivity remained almost constant. The selectivity depended on the composition of the feed gas. The effect of the partial pressure of oxygen was examined, and it was found that the pulsed discharge would be able to produce synthesis gas by partial oxidation of methane. Carbon monoxide selectivity of 79% with methane conversion of 76% was obtained under the conditions of CH₄/O₂=25/25 cm³ min⁻¹, gap distance of 10 mm and the frequency of 45 Hz.     

Reaction pathways of methane conversion in dielectric-barrier discharge

Conversion of methane to C₂, C₃, C₄ or higher hydrocarbons in a dielectric-barrier discharge was studied at atmospheric pressure. Non-equilibrium plasma was generated in the dielectric-barrier reactor. The effects of applied voltage on methane conversion, as well as selectivities and yields of products were studied. Methane conversion was increased with increasing the applied voltage. Ethane and propane were the main products in a dielectric-barrier discharge at atmospheric pressure. The reaction pathway of the methane conversion in the dielectric-barrier discharge was proposed. The proposed reaction pathways are important because they will give more insight into the application of methane coupling in a DBD at atmospheric pressure.     

Sensing and identification of carbon monoxide using carbon films fabricated by methane arc discharge decomposition technique

Carbonaceous materials have recently received attention in electronic applications and measurement systems. In this work, we demonstrate the electrical behavior of carbon films fabricated by methane arc discharge decomposition technique. The current-voltage (I-V) characteristics of carbon films are investigated in the presence and absence of gas. The experiment reveals that the current passing through the carbon films increases when the concentration of CO₂ gas is increased from 200 to 800 ppm. This phenomenon which is a result of conductance changes can be employed in sensing applications such as gas sensors.     

Hydrogen production from methane in a dielectric barrier discharge using oxide zinc and chromium as catalyst

The hydrogen fuel cell is a promising option as a future energy resource; however, the nature of the gas is such that the conversion process of other fuels to hydrogen on board is necessary. Among the raw fuel resources, methane could be the best candidate as it is plentiful. In this experiment, the possibility of producing hydrogen with less carbon formation from methane by a dielectric barrier discharge (DBD) was investigated. Without the addition of a catalyst, the formation of hydrogen reached between 30% and 35% at methane residence time of 0.22 min and supplied powers in the range of 60-130 W. The hydrogen selectivity increased at higher supplied power, but the process efficiency, defined as a ratio of the produced hydrogen to the supplied power, decreased slightly. In order to boost the hydrogen production with less carbon formation, a mixed oxide catalyst of zinc and chromium was added to the reactor. It was shown that the production of hydrogen was ca. 40% higher than the non-catalytic plasma process.     

Conversion of methane to higher hydrocarbons in pulsed DC barrier discharge at atmospheric pressure

Conversion of methane to C₂/C₃ or higher hydrocarbons in a pulsed DC barrier discharge at atmospheric pressure was studied. Non-equilibrium plasma was generated in the barrier discharge reactor. In this plasma, electrons which had sufficient energy collided with the molecules of methane, which were then activated and coupled to C₂/C₃ or higher hydrocarbons. The effect of the change of applied voltage, pulse frequency and methane flow rate on methane conversion, selectivities and yields of products was studied. Methane conversion to higher hydrocarbons was about 25% as the maximum. Ethane, propane and ethylene were produced as primary products, including a small amount of unidentified C₄ hydrocarbons. The selectivity and yield of ethane as a main product came to about 80% and 17% as the highest, respectively. The selectivities of ethane and ethylene were influenced not by the change of pulse frequency but by the change of applied voltage and methane flow rate. However, in case of propane, the selectivity was independent of those condition changes. The effect of the packing materials such as glass and A1₂O₃ bead on methane conversion was also considered, showing that A1₂O₃ played a role in enhancing the selectivity of ethane remarkably as a catalyst.     

Kinetic modeling of plasma methane conversion in a dielectric barrier discharge

Methane conversion by plasma offers a promising route to produce higher value-added products. As plasma reaction is a relatively complex process, kinetic modeling is necessary to obtain a general pattern of the complex interaction on the basis of chemical reaction and products. In this paper, we present a method to obtain the kinetic rate coefficient (k) from the experimental data. Although plasma reaction was classified as chemically complex interaction, the reactions showed a certain pattern of the mechanism. In pure methane injection, the decomposition of methane by plasma could initiate coupling reactions and produce C₂H₆, C₃H₈, and C₄H₁₀. Dehydrogenation of C₂H₆ into C₂H₄ and then to C₂H₂ could be clearly seen by the higher value of the reaction rate constant of C₂H_n + 2 to C₂H_n − 2. Using the rate constant values (k) obtained by this method, the pathways of the methane conversion by a dielectric barrier discharge can be drawn.     

Partial oxidation of methane using a microscale non-equilibrium plasma reactor

A flow-type, microscale, non-equilibrium plasma reactor was developed for partial oxidation of methane without a catalyst. A wide range of oxygen and methane mixtures was directly processed without dilution or explosion at ambient temperature because the microscale plasma reactor removes excess heat generated by partial oxidation, thereby maintaining a reaction field at temperatures near room temperature. Consequently, the least reactive methane was excited by high-energy electrons, whereas successive destruction of reactive oxygenates was minimized simultaneously within the extremely confined environment. A highly reactive and quenching environment is thereby obtained within a single reactor: these are paradoxical conditions in conventional thermochemical processes. A major product among liquid oxygenates was methanol, whose selectivity reached 34% at 30% of methane conversion. Selectivity of oxygenates such as methanol and formaldehyde depends strongly on the fragmentation pattern of methane dissociation by electron impact. Maximum selectivity of oxygenates, which is estimated from numerical simulation of a filamentary microdischarge, reaches 60% when the applied electric field corresponds to the breakdown field of methane (80 Td, 1 Td = 10⁻¹⁷ V cm²). The discharge current increases markedly with an applied electric field, but the selectivity of oxygenates decreases as the field strength increases.     

Novel oxidation reaction at ambient temperature and atmospheric pressure with electric discharge and oxide surface

We investigated a novel oxidation reaction with surface-oxygen and lattice-oxygen induced using a non-equilibrium electric discharge at ambient temperature. We employed MgO, ZrO₂, and TiO₂ for this novel reaction. Methane was oxidized easily and converted into H₂, CO, and CO₂ by the surface-oxygen and lattice-oxygen of oxide with activation of discharge at ambient temperature without gas-phase oxygen. The oxide itself was stable after the reaction. Among these oxides, the tetragonal phase and amorphous phase of ZrO₂ showed remarkably high activity for methane oxidation. Consequently, up to 8% of surface and lattice oxygen of the oxide was consumed by methane oxidation induced by electric discharge. The non-equilibrium electric discharge activated both the surface-oxygen and the lattice-oxygen of the oxides and methane molecules in the gas phase. After these reactions, the oxide surface vacant sites were recovered partially through steam post-treatment. Hydrogen formed simultaneously with steam decomposition. Other reactions were also studied by changing the reaction gas: methane into carbon monoxide, carbon monoxide with oxygen, and carbon monoxide with steam. Furthermore, the correlation of reactivity between the feed gas and surface oxygen was studied. Emission spectra under a CH₄ atmosphere with electric discharge showed complex peaks caused by carbon monoxide formation at 280-500 nm at 0-4 min, suggesting that surface oxygen on oxides was probably consumed within 4 min from the start of the reaction.     

脉冲放电NO脱除过程模拟

建立了数学模型,把所考虑的NO/N₂/O₂/H₂O脉冲放电体系中的主要化学反应,归结为各物种浓度的刚性常微分方程组的初值问题来求解,计算方法选用Rosenbrock法,得到了物种浓度随停留时间的演化规律。结果表明：NO还原和氧化在其脱除过程中同时共存,当NO,N₂,O₂,H₂O和CO₂的体积浓度分别为200×10^-6,80%, 5％,6％和9％时,NO氧化所占的比例比NO还原的大很多;脉冲频率增大导致NO还原率和氧化率均增大;H₂O浓度增大导致HNO₃浓度增大,表明NO氧化所占的比例随H₂O浓度增大而增大。     

Stability of acetylene/methane and acetylene/hydrogen/methane gas mixtures at elevated temperatures and pressures

Hydrogen-enriched natural gas (HENG) containing a mixture of acetylene, hydrogen, and methane is produced from natural gas feedstock in our plasma dissociation process. Storage of this HENG fuel at pressures up to 4000 psig is required for rapid vehicle refueling. Little information on the stability of acetylene mixtures at elevated pressures is presently available; therefore we have performed stability testing on gas mixtures that simulate our HENG fuel. This report describes the stability testing of binary gas mixtures of acetylene and methane containing up to 10%(v) acetylene, and a ternary gas mixture of 4%(v) acetylene, 20%(v) hydrogen, and 76%(v) methane, at pressures up to 3600 psig and temperatures up to 200 °C. The mixtures tested were found to be stable to rapid spontaneous decomposition at all test conditions; however, some degree of hydrogenation of acetylene to ethylene may have occurred in an intermediate mixture of acetylene and hydrogen while preparing the highest pressure ternary test mixture.     

Direct measurement of formaldehyde and methanol emissions from gasohol engine via pulsed discharge helium ionization detector

Unregulated formaldehyde (HCHO), acetaldehyde (CH₃CHO) and methanol (CH₃OH) emissions from M10 blend (methanol/gasoline = 1/9 in volume) fuelled spark ignition (SI) engine were directly detected by a gas chromatography (GC) with a Gs-OxyPLOT capillary column and pulsed discharge helium ionization detector (PDHID). Chromatograms show that HCHO and CH₃OH are well separated and linearly responded on PDHID within the range of 0.8-800 μg/L. The method is proved suitable and reliable for measuring unregulated pollutants from the engine. Experimental results show that compared with gasoline, HCHO emission from M10 is greatly larger while CH₃CHO is smaller. HCHO emission increases while CH₃OH emission decreases with engine speed and tailpipe temperature. The three-way catalytic converter (TWC) can remove over 85% of CH₃OH and CH₃CHO emissions when it is lightened off. However, an unfamiliar phenomenon of minus HCHO conversion efficiency happens when engine speed is over 3500 r/min, which infers that TWC may enhance HCHO emission rather than weaken it at high engine speeds.     

Cation effects in the oxidative dehydrogenation of methane on sulphates

The oxidative coupling of methane has been studied on Mg, Ca, Sr and Ba sulphate. The conversions and selectivities are found to be dependent on the nature of the cation and to be approximately correlated with their electronegativities, thus implying that the C-H bond scission in CH₄, at least with these catalysts, is heterolytic, is rate determining, and further, that the formation of C₂₊ hydrocarbons is dependent, among other factors, on the concentration of methyl radicals.     

Reaction rate constant of methane clathrate formation

Particle size distribution measurements were performed during the growth stage of methane hydrate formation in a semi-batch stirred tank crystallizer. Experiments were carried out at temperatures between 275.1 and 279.2 K and pressures ranging from 3873 to 5593 kPa. The reaction rate constant of methane hydrate formation was determined using the model of Bergeron and Servio (AIChE J 2008;54:2964). The experimental reaction rate constant was found to increase with temperature, following an Arrhenius-type relationship, from 8.3 × 10⁻⁸ m/s to 6.15 × 10⁻⁷ m/s over the 4° range investigated, resulting in an activation energy of 323 kJ/mol. An increase in pressure of approximately 600 kPa did not have any effect on the reaction rate constant. Population balances, based on the measured critical nuclei diameter and that predicted by homogeneous nucleation theory, were also used for comparison purposes. The initial number of hydrate particles was calculated using the mole fraction of methane in the bulk liquid phase and compared to that predicted by an energy balance.     

The low-temperature oxidative coupling of methane over zirconium oxide

Zirconium oxide is shown to be capable of catalyzing the conversion of methane to ethane at temperatures as low as 530 °C. The lowest temperature at which ethane is produced is found to be dependent upon the method employed for the preparation of the catalyst. The presence of surface hydroxyl groups appears to be necessary for the production of ethane at these low temperatures.     

Catalytic oxidative coupling of methane over hydroxyapatite modified with lead

Hydroxyapatite modified with lead catalyzes the oxidative dehydrogenation of methane with high selectivity to C₂ compounds at reaction temperatures as low as 700 °C. The activity is stabilized after reduction in the surface area of the catalyst during the reaction.     

高压脉冲放电等离子体处理工业废水实验研究

采用针-板式高压脉冲放电等离子体反应器处理工业废水.考察了脉冲电压峰值、电极间距、氧气通入量等因素对废水CODCr去除率的影响.在脉冲电压峰值为35 kV,电极间距为15 mm,氧气鼓人量为150 mL/min的实验条件下,废水被放电处理150 min,CODCr的去除率可达81.2%.研究表明,高压脉冲放电等离子体技术处理工业废水具有良好的应用前景.     

Upgrading biomass pyrolysis gas by conversion of methane at high temperature: Experiments and modelling

Biomass gasification at temperatures below 1273 K produces gas which contains methane and too much tar for Fischer-Tropsch synthesis. The aim of this study is to investigate methane conversion at high temperature. Experimental tests were performed between 1273 and 1773 K, with a mixture of gas representative of wood pyrolysis at 1100 K (main components only: CO, CO₂, CH₄, H₂, H₂O). Two different kinetic schemes were used to predict the gas composition, and PAH molecules formation. For a residence time of 2 s in the reactor, the gas must be heated to at least 1650 K to reach a methane conversion rate of 90%. A parametric study was performed at 1453 K, by varying the initial methane, steam and hydrogen contents, so as to find out which components are the most influent on methane conversion and soot production.     

Effects of non-gray thermal radiation on the heating of a methane laminar flow at high temperature

The effects of the non-gray thermal radiation on the heating of a methane/argon laminar flow at high temperature are investigated. The preheating zone of a tubular reactor is studied, where the thermal decomposition of methane does not occur. The laminar flow is simulated with the commercial Fluent code in an axisymmetric geometry. The discrete ordinates method is applied to the numerical simulation of radiative heat transfer. The non-gray gas radiative model used is the absorption distribution function (ADF) using high temperature methane radiative properties which were recently published. Several thermal entrance regions of tubular reactors are compared and the influence of methane participation in radiative heat transfer is studied. The results show that the temperature field is significantly influenced by radiation due to methane absorption. Furthermore, the average flow temperature increases when the wall temperature, the tube diameter or the methane mole fraction increases. Due to intense absorption bands of methane, it is shown that the influence of methane non-gray thermal radiation should be included in simulations of such reactors.     

Growth of highly oriented graphite films at room temperature by pulsed laser deposition using carbon-sulfur targets

Carbon-sulfur films were grown by pulsed laser deposition at room temperature using different graphite-sulfur mixtures as targets. The structure of the films was characterized by X-ray diffraction and transmission electron microscopy. The composition and the chemical bonds were analyzed by Rutherford-backscattering spectroscopy, X-ray photoelectron spectroscopy and energy dispersive X-ray analysis. The films were composed of amorphous carbon with sp²-, sp³- and S-C-C-S bonds and textured graphite on the top of the film. The thin graphite layer on top of the carbon-sulfur films is highly oriented, comparable to highly oriented pyrolytic graphite, and free of sulfur in the graphite lattice. The lateral size of the oriented graphite grains in the films was up to 8 μm. Magnetic measurements reveal that the films prepared under the conditions of our study show neither magnetic ordering nor superconductivity in the studied temperature range T > 2 K.     

Reaction mechanism of partial oxidation of methane to synthesis gas over supported ni catalysts

A mechanistic study on the partial oxidation of methane to synthesis gas (H₂ and CO) was conducted with supported nickel catalysts. To investigate the reaction mechanism, pulse experiments, O₂-TPD, and a comparison of the moles of reactants and products were carried out. From the O₂-TPD experiment, it was observed that the active catalyst in the synthesis gas production desorbed oxygen at a lower temperature. In the pulse experiment, the temperature of the top of the catalyst bed increased with the pulses, whereas the temperature of the bottom decreased. This suggests that there are two kinds of reactions, that is, the total oxidation of methane (exothermic) at the top and reforming reactions (endothermic) at the bottom. From the comparison of the moles of reactants and products, it was found that the moles of CO₂, CH₄ and H₂O decreased as the moles of H₂ and CO increased. The results support the mechanism that synthesis gas is produced through a two-step reaction mechanism: the total oxidation of methane to CO₂ and H₂O takes place first, followed by the reforming reaction of the produced CO₂ and H₂O with residual CH₄ to form synthesis gas. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.