Study of the dry methane reforming process using a rotating gliding arc reactor

Dry methane reforming (DMR) via rotating gliding arc (RGA) discharge, co-driven by a magnetic field and tangential flow, was investigated in this study. Optical emission spectroscopy (OES) was used to characterize the major active species (energetic electrons, radicals, ions, atoms and excited molecules) in the DMR chemical process. The influence of the operational conditions (applied voltage and CH₄/CO₂ ratio) on the basic spectroscopic parameters (electron excitation temperature, electron density and rotational temperature) was determined by spectroscopic methods. The rotational and electron excitation temperatures were approximately 1100–1200 K and 1.1–1.7 eV, respectively, indicating the non-thermal equilibrium characteristics of the RGA discharge. The electron density was approximately 5–20 × 10²¹ m⁻³ by fitting the line shape of H_α at 656 nm. The conversions of the reactants (CH₄ and CO₂) and the selectivities of the products (H₂, CO and C₂ hydrocarbon) were analyzed using a gas chromatograph (GC) under different energy inputs or feed gas proportions. The structure and morphology of carbon black produced during the chemical process was characterized by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy, indicating the properties of electrical conductivity and high absorption capacity that can be useful for potential application.     

Characteristics of methane reforming using gliding arc reactor

A gliding arc reformer was proposed to produce hydrogen-rich gas. The reformer has two reactors including a plasma reactor and a catalyst reactor.     

Experimental study and kinetic modeling of methane decomposition in a rotating arc plasma reactor with different cross-sectional areas

Methane decomposition into hydrogen and carbon is analyzed in a plasma reactor, with a rotating arc and different cross-sectional areas for the passing gas. This novel setup helps the arc discharge to sweep a larger fraction of the reactant which could cause a better interaction of methane molecules with plasma phase causing higher conversions. The effects of angular velocity of arc discharge, feed flow rate, and cross-sectional area for the passing gas were investigated on the reactor performance. Methane conversion increased significantly by changing the arc mode from stationary to rotating. Increasing the cross-sectional area for the passing gas causes conversion drop for stationary arc whereas a slight increase in conversion is observed for rotating arc mode. Hydrogen production rate of 100 ml/min with an energy yield of 26.8 g/kWh achieved at a methane flow rate of 150 ml/min. The residence time is estimated to be 0.2–3.9 s in the range of the present study, which is a much longer period compared to the plasma process time. Therefore, it is suggested that the mass transfer rate between the gas and plasma phase is the controlling factor for methane conversion. In this respect, an apparent reaction rate constant is derived by considering methane conversion as that fraction of gas, which is exposed to the active area of the plasma arc column.     

Oxidative reforming of n-heptane in gliding arc plasma reformer for hydrogen production

The exploration of novel technologies to reduce the air pollution and greenhouse gas emissions has been of great interest. Gliding arc plasma reformer at atmospheric pressure has been developed for converting n-heptane to hydrogen. The system has been evaluated by H₂ yield and energy yield via continuous n-heptane oxidative reforming at room temperature. Effects of some process parameters (discharge gap, input power, residence time, and O/C) have been studied on the reaction performance. The maximum H₂ yield and energy yield are 50.1% and 94.5 L (kW h)⁻¹. To investigate the role of inert gas (N₂, Ar) in the plasma oxidative reforming system, the performance of C₇H₁₆/air, C₇H₁₆/N₂/O₂/Ar and C₇H₁₆/O₂/Ar have been investigated. The results show that N₂ (B³Π_g) and Ar¹ can accelerate the formation of active oxygen species (such as O⁺, O (¹D) and O). The presence of active oxygen species promotes the progress of the oxidative reforming reaction. What's more, N₂ (B³Π_g) is also conducive to the direct conversion of n-heptane. The reaction mechanism of hydrogen production from gliding arc plasma oxidative reforming of n-heptane was proposed based on the analysis of the OES and GC–MS.     

Microwave-assisted catalytic decomposition of methane over activated carbon for -free hydrogen production

The aim of this work was to combine microwave heating with the use of low-cost granular activated carbon as a catalyst for the production of CO₂-free hydrogen by methane decomposition in a fixed bed quartz-tube flow reactor. In order to compare the results achieved, conventional heating was also applied to the catalytic decomposition reaction of methane over the activated carbon. It was found that methane conversions were higher under microwave conditions than with conventional heating when the temperature measured was lower than or equal to . However, when the temperature was increased, the difference between the conversions under microwave and conventional heating was reduced. The influence of volumetric hourly space velocity (VHSV) on the conversion tests using both microwave and conventional heating was also studied. In general, there is a substantial initial conversion, which declines sharply during the first stages of the reaction but tends to stabilise with time. An increase in the VHSV has a negative effect on CH₄ conversion, and even more so in the case of microwave heating. Nevertheless, the conversions obtained in the microwave device at the beginning of the experiments are, in general, better than the conversions reported in other works which also use a carbonaceous-based catalyst. Additionally, the formation of carbon nanofibres in one of the microwave experiments is also reported.     

Ni doped carbons for hydrogen production by catalytic methane decomposition

Ni doped carbons were prepared from raw coal and direct coal liquefaction residue (CLR) by KOH activation with addition of Ni(NO₃)₂, and used for catalytic methane decomposition (CMD) to produce hydrogen. The catalytic activity of the Ni doped carbon for CMD was compared with those of metal catalysts (Ni/SiO₂ and Ni/Al₂O₃), coal- and CLR-based carbons, and Ni-carbon catalysts prepared by traditional impregnation and precipitation methods. The results show that the Ni doped carbon has higher and more stable activity than the metal and carbon catalysts at 850 °C. The preparation method for Ni doped carbons can make full use of the reducibility of the carbon composition and simplify the traditional synthesis process. The Ni content and the morphology of carbon deposits produced during CMD have a great effect on the catalytic activity of the Ni doped carbon.     

Experimental study of hydrogen production from reforming of methane and ammonia assisted by Laval nozzle arc discharge

The production of hydrogen from hydrogen compounds for fuel cell or internal combustion engine applications is a potential method for responding to the energy crisis and environmental problems. In this work carbon dioxide reforming of methane and decomposition of ammonia using a Laval nozzle arc discharge (LNAD) reactor has been exploited at atmospheric pressure without external heating or catalysts. CH₄ (or NH₃) conversion and H₂ selectivity were observed to be negatively correlated with the concentration of CH₄ (or NH₃) and the flux of CO₂ (N₂) and positively correlated with voltage and the Laval nozzle throat radius. Power consumption increased with the concentration of methane at the same CO₂ flow rate, and the conversion of methane gradually increased with the content of water vapor in the gas mixture. A high conversion rate and fair H₂ selectivity were achieved, 51% and 37.5%, respectively, when the methane and carbon dioxide flow rates were 4 L/min and 14 L/min, respectively, and the minimum distance between the two electrodes was 2.5 mm. The LNAD reactor used in this study exhibited a good conversion rate and low energy consumption, which should be suitable for the industrial scale-up of the system.     

High temperature iron-based catalysts for hydrogen and nanostructured carbon production by methane decomposition

The production of hydrogen and filamentous carbon by means of methane decomposition was investigated in a fixed-bed reactor using iron-based catalysts. The effect of the textural promoter and the addition of Mo as a dopant affects the catalysts performance substantially: iron catalyst prepared with Al₂O₃ showed slightly higher catalytic performance as compared to those prepared with MgO; Mo addition was found to improve the catalytic performance of the catalyst prepared with MgO, whereas in the catalyst prepared with Al₂O₃ displayed similar or slightly poorer results. Additionally, the influence of the catalyst reduction temperature, the reaction temperature and the space velocity on the hydrogen yield was thoroughly investigated. The study reveals that iron catalysts allow achieving high methane conversions at operating temperatures higher than 800 °C, yielding simultaneously carbon nanofilaments with interesting properties. Thus, at 900 °C reaction temperature and 1 l g⁻¹_cat h⁻¹ space velocity, ca. 93 vol% hydrogen concentration was obtained, which corresponds to a methane conversion of 87%. Additionally, it was found that at temperatures higher than 700 °C, carbon co-product is deposited mainly as multi walled carbon nanotubes. The textural and structural properties of the carbonaceous structures obtained are also presented.     

Plasma dry reforming of methane in an atmospheric pressure AC gliding arc discharge: Co-generation of syngas and carbon nanomaterials

An alternating-current (AC) gliding arc reactor has been developed offering a new route for the co-generation of syngas and value-added carbon nanomaterials by plasma dry reforming of methane. Different carbon nanostructures including spherical carbon nanoparticles, multi-wall carbon nanotubes and amorphous carbon have been obtained as by-products of syngas generation in the plasma system. Optical emission spectra of the discharge demonstrate the formation of different reactive species (Al, CO, CH, C₂, H_α, H_β and O) in the plasma dry reforming reaction. The effect of different operating parameters (feed flow rate, input power and CH₄/CO₂ molar ratio) on the performance of the plasma process has been evaluated in terms of the conversion of feed gas, product selectivity and energy conversion efficiency. It is interesting to note that gliding arc plasma can be used to generate much cleaner gas products of which syngas is the main one. The results also show that the energy efficiency of dry reforming using gliding arc plasma is an order of magnitude higher than that for processing using dielectric barrier or corona discharges. Both of these can be attributed to the higher electron density in the order of 10²³ m⁻³ generated in the gliding arc plasma.     

Microwave-assisted methane decomposition over pyrolysis residue of sewage sludge for hydrogen production

The microwave-assisted methane decomposition over a pyrolysis residue of sewage sludge (PRSS), which acted as a microwave receptor and a low-cost catalyst without further activation, was investigated in a multimode microwave reactor. For comparison purpose, methane conversion (MC) over an activated carbon (AC) was also studied. The results indicate that PRSS is a better microwave receptor than AC. Under the same microwave power (MWP), MC over PRSS is markedly higher than that over AC, due to the remarkably higher Microwave heating (MWH) performance of PRSS. MWH of PRSS and AC is heavily influenced by atmosphere. Under the same MWP, the stable temperatures of the catalysts in hydrogen, nitrogen and methane atmosphere follow the sequence: T_nitrogen > T_hydrogen > T_methane. On the other hand, it was observed that nitrogen showed different effect on MC over PRSS and AC under MWH. Specifically, under the microwave-assisted methane decomposition reactions, the effect of nitrogen on MC over PRSS is not obvious, but it has remarkable effect on MC over AC. Additionally, a large number of molten beads were formed on the surface of the used PRSS by microwave irradiation. The composition and formation mechanism of the molten beads were also reported.     

Life cycle assessment of hydrogen production by methane decomposition using carbonaceous catalysts

Methane decomposition to yield hydrogen and carbon (CH₄ ? 2H₂ + C) is one of the cleanest alternatives, free of CO₂ emissions, for producing hydrogen from fossil fuels. This reaction can be catalyzed by metals, although they suffer a fast deactivation process, or by carbonaceous materials, which present the advantage of producing the catalyst from the carbon obtained in the reaction. In this work, the environmental performance of methane decomposition catalyzed by carbonaceous catalysts has been evaluated through Life Cycle Assessment tools, comparing it to other decomposition processes and steam methane reforming coupled to carbon capture systems. The results obtained showed that the decomposition using the autogenerated carbonaceous as catalyst is the best option when reaction conversions higher than 65% are attained. These were confirmed by 2015 and 2030 forecastings. Moreover, its environmental performance is highly increased when the produced carbon is used in other commercial applications. Thus, for a methane conversion of 70%, the application of 50% of the produced carbon would lead to a virtually zero-emissions process.     

Tailored hydrotalcite-based Mg-Ni-Al catalyst for hydrogen production via methane decomposition: Effect of nickel concentration and spinel-like structures

Thrive for the CO_x-free hydrogen production via methane decomposition has gained much interest owing to its feasibility and environmental friendliness. Herein, ahydrotalcite based Nickel catalyst was synthesized via co-precipitation method by varying the amount of Nickel concentration and tested for methane decomposition reaction in a fixed bed reactor. In addition, the effect of calcination temperature in the development of the spinel-like structure of as-developed catalyst was comprehensively discussed. It was found that the hydrotalcite based Nickel catalyst prepared at 40% Nickel concentration has the highest performance of above 80% conversion for 7 h of methane decomposition which was owing to its effective diffusion of carbon particles and its spinel-like structure, evidently from the XRD and FESEM analysis. The profound performance monitored here was attributed to the formation of carbon nanofibers (CNFs) on the surface of the catalyst which levitates the active Ni^ospecies on its tips, results in more available active sites for the chemisorptions of the methane molecules. Nevertheless, the excessive of Nickel concentration leads to the detrimental methane decomposition performance, hencepromotes the formation of large particle size and successive development of bulk NiO phases during the reduction process, consequently abnegate the overall methane decomposition reaction. The aforementionedfindingsshow that the spinel-like structure is the key factor in the growth of long uniform CNFs and elevation of active sites on the fibre tips.     

Semi-continuous hydrogen production from catalytic methane decomposition using a fluidized-bed reactor

Non-oxidative, catalytic decomposition of hydrocarbons is an alternative, one-step process to produce pure hydrogen with no production of carbon oxides or higher hydrocarbons. Carbon produced from the decomposition reaction, in the form of potentially valuable carbon nanotubes, remains anchored to the active catalyst sites in a fixed bed. To facilitate periodical removal of this carbon from the reactor and to make hydrogen production continuous, a fluidized-bed reactor was envisioned. The hypothesis that the tumbling and inter-particle collisions of bed material would efficiently separate nanotubes anchored to the active catalyst sites of the bed particles was tested and shown to be invalid. However, a switching mode reaction system for the semi-continuous production of hydrogen and carbon nanotubes by periodic removal and replenishment of the catalytic bed material has been successfully demonstrated.     

Novel micro-channel methane reformer assisted combustion reaction for hydrogen production

A metal catalyst-containing, 80 ml, micro-channel reactor (MCR) with a section dedicated to combustion reaction was investigated for the potential application of on-board methane steam reforming (MSR) to hydrogen production. The metal catalyst was introduced into the MCR as a shape of a thin plate that was diffusion-bonded with the other micro-channel plates. The combustion reaction was performed on the other side of the MCR for direct provision of the necessary heat for the endothermic MSR and for miniaturizing the system volume. In the MCR, both the methane conversion and the hydrogen production rate are extremely high compared with those of the equilibrium under atmospheric pressure. The required heat of reaction is successfully provided by the combustion of either hydrogen or the methane mixture on the other side of the MCR without the need for any heating cartridges. This novel micro-channel reformer is suitable for application as a compact fuel processor due to its production of hydrogen-rich syn-gas, small volume, simple catalyst loading and use of an active and easily stackable catalyst.     

Plasma assisted dry methane reforming using gliding arc gas discharge: Effect of feed gases proportion

The influence of feed gases proportion on the performance of gliding arc discharge (GAD) plasma assisted methane reforming with carbon dioxide process (DMR) is investigated, especially focused on the reagents conversion, the main by-product distribution and the energy consumption. An additional analysis of the theoretical conditions of equilibrium and its comparison with the experimental results are also included. The GAD assisted DMR process is effective in converting CH₄/CO₂ into synthesis gas with C₂H₂ and C₂H₄ detected as the main hydrocarbon compounds. Strong dehydrogenation in the GAD assisted DMR process is suggested with no obvious C₂H₆ observed in the effluent and serious coke formation and deposition detected in the relative high CH₄/CO₂ feed ration and supply voltage condition. Compared to other plasma systems reported before, the GAD reactor used in current work shows lower energy consumption for methane conversion but higher energy consumption for C₂H₂ formation. The increase of CH₄/CO₂ molar ratio in the feed gases favors the reagents conversion and consequently promotes the formation of each by-product. Meanwhile, the by-product selectivities, the value of H₂/CO ratio in the effluent, the coke formation, and the special energy consumption for reagents conversion and synthesis gas formation are shifted. It is clearly demonstrated that under some certain circumstances, the GAD plasma reactor assisted reaction can be totally out of thermodynamic equilibrium.     

An energy-efficient plasma methane pyrolysis process for high yields of carbon black and hydrogen

A novel thermal plasma process was developed, which enables economically viable commercial-scale hydrogen and carbon black production. Key aspects of this process are detailed in this work. Selectivity and yield of both solid, high-value carbon and gaseous hydrogen are given particular attention. For the first time, technical viability is demonstrated through lab scale reactor data which indicate methane feedstock conversions of >99%, hydrogen selectivity of >95%, solid recovery of >90%, and the ability to produce carbon particles of varying crystallinity having the potential to replace traditional furnace carbon black. The energy intensity of this process was established based on real-time operation data from the first commercial plant utilizing this process. In its current stage, this technology uses around 25 kWh per kg of H2 produced, much less than water electrolysis which requires approximately 60 kWh per kg of H2 produced. This energy intensity is expected to be reduced to 18–20 kWh per kg of hydrogen with improved heat recovery and energy optimization.     

Kinetic and autocatalytic effects during the hydrogen production by methane decomposition over carbonaceous catalysts

The reaction kinetics of methane decomposition to yield hydrogen and carbon has been investigated comparing different types of carbonaceous catalysts: two ordered mesoporous carbons (CMK-3 and CMK-5) and two commercial carbon blacks (CB-bp and CB-v). The evolution of the reaction rate along the time has been analyzed concluding that it is governed by different and opposite events: reduction of active sites by carbon deposition, autocatalytic effects of the carbon deposits and pore blockage and diffusional constraints. A relatively simple kinetic model has been developed that fits quite well the experimental reaction rate curves in spite of the complexity of the involved phenomena.     

Effect of hydrogen additive on methane decomposition to hydrogen and carbon over activated carbon catalyst

The effect of H₂ addition on CH₄ decomposition over activated carbon (AC) catalyst was investigated. The results show that the addition of H₂ to CH₄ changes both methane conversion over AC and the properties of carbon deposits produced from methane decomposition. The initial methane conversion declines from 6.6% to 3.3% with the increasing H₂ flowrate from 0 to 25 mL/min, while the methane conversion in steady stage increases first and then decreases with the flowrate of H₂, and when the H₂ flowrate is 10 mL/min, i.e. quarter flowrate of methane, the methane conversion over AC in steady stage is four times more than that without hydrogen addition. It seems that the activity and stability of catalyst are improved by the introduction of H₂ to CH₄ and the catalyst deactivation is restrained. Filamentous carbon is obtained when H₂ is introduced into CH₄ reaction gas compared with the agglomerate carbon without H₂ addition. The formation of filamentous carbon on the surface of AC and slower decrease rate of surface area and pores volume may cause the stable activity of AC during methane decomposition.     

Effects of reaction conditions on hydrogen production and carbon nanofiber properties generated by methane decomposition in a fixed bed reactor using a NiCuAl catalyst

In this paper, the results obtained in the catalytic decomposition of methane in a fixed bed reactor using a NiCuAl catalyst prepared by the fusion method are presented. The influences of reaction temperature and space velocity on hydrogen concentration in the outlet gases, as well as on the properties of the carbon produced, have been investigated. Reaction temperature and the space velocity both increase the reaction rate of methane decomposition, but also cause an increase in the rate of catalyst deactivation. Under the operating conditions used, the carbon product is mainly deposited as nanofibers with textural properties highly correlated with the degree of crystallinity.