The Pyrolysis Property of a Pulsed Plasma of Methane

This study examined the discharge property of methane in a pulsed plasma using single and sequential pulse modes. A pulsed corona discharge occurred, followed by a pulsed spark discharge. An equivalent single channel model (ESC model) for the pulsed plasma of methane was established. The maximum temperature of methane in the discharge channel was estimated. The pyrolysis property of the channel methane was estimated from Senkin simulation and single channel discharge experiment.     

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.     

Chemical Kinetics of Methane Pyrolysis in Microwave Plasma at Atmospheric Pressure

Results of chemical kinetics modeling in methane subjected to the microwave plasma at atmospheric pressure are presented in this paper. The reaction mechanism is based on the methane oxidation model without reactions involving nitrogen and oxygen. For the numerical calculations 0D and 1D models were created. 0D model uses Calorimetric Bomb Reactor whereas 1D model is constructed either as Plug Flow Reactor or as a chain of Plug Flow Reactor and Calorimetric Bomb Reactor. Both models explain experimental results and show the most important reactions responsible for the methane conversion and production of H₂, C₂H₂, C₂H₄ and C₂H₆ detected in the experiment. Main conclusion is that the chemical reactions in our experiment proceed by a thermal process and the products can be defined by considering thermodynamic equilibrium. Temperature characterizing the methane pyrolysis is 1,500–2,000 K, but plasma temperature is in the range of 4,000–5,700 K, which means that methane pyrolysis process is occurring outside the plasma region in the swirl gas flowing around the plasma.     

Plasma Thermal Conversion of Methane to Acetylene

This paper describes a re-examination of a known process for the direct plasma thermal conversion of methane to acetylene. Conversion efficiencies (% methane converted) approached 100% and acetylene yields in the 90–95% range with 2–4% solid carbon production were demonstrated. Specificity for acetylene was higher than in prior work. Improvements in conversion efficiency, yield, and specificity were due primarily to improved injector design and reactant mixing, and minimization of temperature gradients and cold boundary layers. At the 60-kilowatt scale cooling by wall heat transfer appears to be sufficient to quench the product stream and prevent further reaction of acetylene resulting in the formation of heavier hydrocarbon products or solid carbon. Significantly increasing the quenching rate by aerodynamic expansion of the products through a converging–diverging nozzle led to a reduction in the yield of ethylene but had little effect on the yield of other hydrocarbon products. While greater product selectivity for acetylene has been demonstrated, the specific energy consumption per unit mass of acetylene produced was not improved upon. A kinetic model that includes the reaction mechanisms resulting in the formation of acetylene and heavier hydrocarbons, through benzene, is described.     

Catalytic Pyrolysis of Methane

Methane decomposition over metal oxides/SiO₂ surface was investigated. At 1400 K obtained product distribution of this decomposition varied with metal oxide used. The effectiveness of these catalysts has been discussed in terms of activity and C₂ selectivity. ThO₂/SiO₂ was found to be the most effective catalyst for the catalytic decomposition of methane. Positive catalytic effect of ThO₂/SiO₂ on the pyrolysis has also been confirmed at 1073 K. At low reaction conversions, ethane and ethylene are found as major products. Yields of ethylene and other unsaturated products are sensitively inhibited by NO impurities in the methane. A reaction mechanism has been proposed to account observed experimental results.     

Quenching Experiment Study on Thermal Plasma Pyrolysis Process of Coal Tar

Quenching is a key approach to obtain high acetylene yield in the process of coal tar pyrolysis to produce acetylene in a thermal plasma reactor due to the thermodynamic characteristics of acetylene. Experiments of coal tar pyrolysis were carried out in a lab-scale H₂/Ar plasma reactor under various quenching conditions. Meanwhile, thermodynamic analysis was performed to assist the optimization of quenching temperature and the maximization of acetylene yield. As quenching media in the experiments, hydrogen, argon, methane, and water were used separately to study the influence of quenching process on acetylene yield and specific energy requirement. The experimental results indicate that the acetylene concentration in quenched product gas was significantly affected by quenching operation, and the acetylene yield was significantly affected by quenching medium flow rate. The acetylene yields of 24.6, 17.8, 44.9 and 23.6 wt% can be reached by using hydrogen, argon, methane, and water as quenching media, respectively. The specific energy requirement analysis indicates that process energy efficiency can be improved by a suitable quench operation.     

On the Autocatalysis of Methane Pyrolysis

Methane pyrolysis has been performed in a recycled flow system in the temperature range of 1103 to 1220 K to investigate the time profile of product distribution. Hydrogen, ethylene, and benzene are found to be the major products before the soot formation. Similar to the literature reports of studies in conventional flow systems, the rate of methane conversion is slow at the beginning of the decomposition and becomes fast after an incubation period. This increase in the decomposition rate after the incubation period is generally called autocatalysis. The proposed reason for this autocatalysis has been the catalytic effect of the secondary products, i.e., soots and carbon deposits. The present study showed that autocatalysis started before the accumulation of these products, and that decomposition of propylene and other minor but reactive products might be attributed to this autocatalysis effect.     

Use of Solid-Phase Microextraction to Monitor Gases Resulting from Thermal Plasma Pyrolysis

This paper proposes use of solid-phase microextraction to monitor the gases resulting from thermal plasma pyrolysis, instead of the traditional process using an air-tight syringe. The results confirm that thermal plasma technology can be used to eliminate carbon tetrachloride, because dioxins and furans were not detected. Some chlorinated compounds, for example tetrachloroethylene, hexachloroethane, 1,1,2,3,4,4 hexachloro-1,3-butadiene, 1,2,3,4,5,5-hexachloro-1,3-cyclopentadiene, and octachlorocyclopentadiene were detected, however. The results also confirm that this technique is more efficient than injection by means of an air-tight syringe for monitoring gases formed by the pyrolysis of carbon tetrachloride and enables detection of all viable sample constituents.Revised: 18 December 2003 and 12 February 2004     

Direct Non-oxidative Methane Conversion by Non-thermal Plasma: Experimental Study

The direct non-oxidative conversion of methane to higher hydrocarbons in non-thermal plasma, namely dielectric barrier discharge and corona discharge, has been investigated experimentally at atmospheric pressure. In dielectric barrier discharge, the methane is mainly converted to ethane and propane with small amounts of unsaturated and higher hydrocarbons. While in corona discharge, methane was activated mainly to acetylene with small amount of other higher hydrocarbons. Decreasing the gas flow or increasing power input will improve the methane conversion and product yields. It is found that the methane conversion and main product yield against the input specific energy were special functions in both dielectric barrier discharge and corona discharge, independent of the reactor size, and whether fixing flow rate or power input and changing the power input or flow rate. The corona discharge is a promising alternative method for methane conversion to produce acetylene and hydrogen at low temperature.     

Methane Pyrolysis Stimulated by Admixture of Atomic Hydrogen: 1. An Experimental Study

A method for enhancing the hydrocarbon pyrolysis process by introducing atomic hydrogen into the reaction medium from an arcjet plasma source was considered. It was shown that hydrogen atoms could effectively be introduced by mixing under low pressure. The atomic hydrogen–stimulated methane pyrolysis process was experimentally studied in a continuous stirred reactor with a plasma plume. When hydrogen atoms were present in the plasma jet, the amount of the valuable product increased by a factor of two.     

Experimental Study of Liquid Hydrocarbons Pyrolysis to Acetylene in H2/Ar Plasma

Liquid hydrocarbons including n-hexane, cyclohexane and toluene are pyrolyzed in H₂/Ar plasma to investigate the effects of feedstock properties and key operating conditions (e.g., the feedstock specific input power and residence time) on the reaction performance. The experiments verify that the non-aromatic hydrocarbons show better chemical reactivity than partially aromatic substances. Meanwhile, the straight-chain alkanes and cycloalkanes have better yields of ethylene during the pyrolysis. The results also demonstrate that the pyrolysis reactions are almost completed within the first 0.8?ms in Ar/H₂ plasma independent of the feed substances (i.e., liquid hydrocarbons), where the increased feedstock specific input power enhances the reactant conversions and correspondingly raises the yields of acetylene. At a feedstock specific input power of 4.7?×?10⁴?kJ/kg, the n-hexane conversion is over 90?% and the yield of acetylene reaches 70?%. In addition, when using n-hexane as the feedstock, very little coke is formed during the course of reaction. Comprehensive comparisons of the current experiments with the data reported in the literature are made to point out the key influencing factors, i.e., the effective mass ratio of C/H (R _C/H) in the gaseous phase and the quench temperature. Both two factors would need to be enhanced in order to get a better performance. Finally, the improvements on the specific energy requirement of this process are discussed.     

Mathematical Modeling of the Plasma-Chemical Pyrolysis of Methane

A mathematical model was developed for the plasma-chemical pyrolysis of methane, which includes the latest data on the mechanism and kinetics of chemical processes of hydrocarbon pyrolysis and mixing of methane jets with hydrogen heated in an arc plasma torch. The results of calculations on methane conversion and the synthesis of acetylene and its homologues satisfactorily agree with experimental data over a wide range of parameters of the process. It was shown that the methane conversion is initiated via interaction with atomic hydrogen, acetylene is produced through the dissociation of intermediate products involving radicals, and the consumption of acetylene is due to the synthesis of its homologues involving vinylidenecarbene and methylenecarbene in the ground and excited states.     

Experimental Studies and Simulation of Methane Pyrolysis and Oxidation in Reflected Shock Waves Accompanied by Soot Formation

Kinetics and Catalysis - An experimental and computational study of methane pyrolysis and oxidation and formation of soot particles in reflected shock waves has been performed using a previously...     

低温催化裂解烷烃法制备碳纳米管

低温催化裂解烷烃法制备碳纳米管陈萍，王培峰，林国栋，张鸿斌，蔡启瑞（厦门大学化学系固体表面物理化学国家重点实验室，厦门，３６１００５）关键词碳纳米管，催化裂解，甲烷碳纳米管的制备与研究是国际上新材料领域的探索热点［１］．由于具有纳米级的管径，碳纳米管...     

Pyrolysis of Methane on Resistive MgO/SiC Catalyst

Dynamics of methane pyrolysis on the MgO/SiC catalyst was examined at different temperatures of the resistive catalyst. The conversion of methane on the MgO/SiC catalyst at 1200°C passes through a minimum (29%) by 60th minute, with the selectivity with respect to acetylene steadily increasing during the whole experiment. The scanning microscopy with EDAX analysis demonstrated that the full carbonization of the MgO/ SiC sample at 1200°C also occurs after 60 min of the experiment. It was found that a carbon coating of layered structure is formed on the catalyst surface in the course of the experiment, with C₂ hydrocarbons still present among the pyrolysis products. It was shown that carbon deposits formed on the surface of the MgO/SiC catalyst are catalytically active in the process of acetylene formation.     

Synthesis of Ultrafine Carbon Black by Pyrolysis of Polymers Using a Direct Current Thermal Plasma Process

A direct current thermal plasma system was developed and applied to synthesize ultrafine carbon black by using PS (polystyrene) and HDPE (high density polyethylene) as carbon sources. The precursors were pyrolyzed at different temperatures and the pyrolysis products were employed to investigate the actual synthesis of carbon black through the plasma jet. Spherical carbon nanoparticles with a high degree of turbostratic structure were obtained, showing a fingerprint graphite structure with a large surface area and ultrafine particle size. The carbon black was characterized by transmission and scanning electron microscopy, powder X-ray diffraction, Raman spectroscopy, and nitrogen physisorption. Particle size distribution and dispersion stability in solvents were also investigated.     

An Experimental Study of Plasma Cracking of Methane Using DBDs Aimed at Hydrogen Production

Plasma Chemistry and Plasma Processing - We report the results of an experimental campaign about the production of hydrogen from methane cracking using a non-thermal plasma. Experiments have been...     

Methane Conversion Using Dielectric Barrier Discharge：Comparison with Thermal Process and Catalyst Effects

The direct conversion of methane using a dielectric barrier discharge has been experimentally studied. Experiments with different values of flow rates and discharge voltages have been performed to investigate the effects on the conversion and reaction products both qualitatively and quantitatively. Experimental results indicate that the maximum conversion of methane has been 80% at an input flow rate of 5 ml/min and a discharge voltage of 4 kV. Experimental results also show that the optimum condition has occurred at a high discharge voltage and higher input flow rate. In terms of product distribution, a higher flow rate or shorter residence time can increase the selectivity for higher hydrocarbons. No hydrocarbon product was detected using the thermal method, except hydrogen and carbon. Increasing selectivity for ethane was found when Pt and Ru catalysts presented in the plasma reaction. Hydrogenation of acetylene in the catalyst surface could have been the reason for this phenomenon as the selectivity for acetylene in the products was decreasing.     

催化裂解CH4制备不同形貌的碳纳米管

通过甲烷于较低温度(500～700℃)下在镍催化剂上催化裂解制备了各种形貌的碳纳米管.透射电镜测试结果表明,碳纳米管的外径和内径明显地受催化剂的大小和形貌的影响.本文考察了催化剂前驱体的种类、反应温度和原料气流速对镍催化剂和碳纳米管形貌的影响.     

Steam Plasma Methane Reforming for Hydrogen Production

Reactions of methane with water and CO₂ in thermal plasma generated in a special plasma torch with a water-stabilized arc were investigated. Steam plasma with very high enthalpy and low mass flow rate was produced in a dc arc discharge which was in direct contact with water vortex surrounding the arc column. Composition of produced gas, energy balance of the process and its efficiency were determined from measured data. The output H₂/CO ratio could be adjusted by a choice of feed rates of input reactants in the range 1.1–3.4. Depending on experimental conditions the conversion of methane was up to 99.5%, the selectivity of H₂ was up to 99.9%, and minimum energy needed for production of 1 mol of hydrogen was 158 kJ/mol. Effect of conditions on process characteristics was studied. Comparison of measured data with results of theoretical computations confirmed that the reforming process produces gas with composition which is close to the one obtained from the thermodynamic equilibrium calculations. Relations between process enthalpy, composition of produced syngas and process characteristics were determined both theoretically and experimentally.