Pyrolysis of methane via thermal steam plasma for the production of hydrogen and carbon black

Methane pyrolysis for the production of hydrogen and solid carbon was studied in plasma reactor PlasGas equipped with a DC plasma torch with the arc stabilized by a water vortex. Steam plasma is produced by direct contact of electric arc discharge with water surrounding the arc column in a cylindrical torch chamber. The composition of the gas produced was compared with the results of the equilibrium calculations for different flow rates of input methane. We have found that for the net plasma power 52 kW the optimal flow rate of the input methane was between 200 slm and 300 slm, for which high methane conversions of 75% and 80% are achieved. For the flow rate of 500 slm, the methane conversion is only 60%; however, the output still consists of a mixture of hydrogen, methane and solid carbon, without other unwanted components. For the flow rate of 100 slm, the methane conversion is 88%. For 100 and 200 slm of input methane the energy excess for the reaction with respect to the calculated value is 16 kW and 4 kW. On the other hand, for 300 and 500 slm of input methane we have the energy lack of 10 kW and 38 kW. The solid carbon produced was composed of well-defined spherical particles of the size about 1 μm. Comparison with the steam and dry reforming of methane in the same system shows that the presence of oxygen increases the methane conversion, despite lower available energy produced.     

Characteristics of methane wet reforming driven by microwave plasma in liquid phase for hydrogen production

Discharge plasma reforming of methane to produce hydrogen has been a hotspot in recent years. At present, there is no report on liquid-phase discharge for methane reforming. In this paper, directly coupled liquid-phase microwave discharge plasma (LPMDP) is used for the first time to realize liquid-phase methane wet reforming to produce hydrogen. When methane gas is injected into the water in the reactor, plasma is generated in the water by microwave discharge. The type and relative intensity of active radicals produced during discharge are detected by emission spectroscopy. Methane gas is introduced into the reactor through two electrode structures. When the microwave power was 900 W, the optimal methane conversion rate reached 94.3%, and the highest concentration of hydrogen reached 74.0%. In addition, through the optimization of the electrode structure, while improving the stability of the plasma system, the higher yield of hydrogen and energy efficiency of hydrogen production were obtained, and the highest energy efficiency of hydrogen production was approximately 0.92 mmol/kJ. This investigation provides a new method for hydrogen production by liquid-phase plasma methane wet reforming.     

Hydrogen production via catalytic pulsed plasma conversion of methane: Effect of Ni–K2O/Al2O3 loading,applied voltage,and argon flow rate

Despite industrial application of methane as an energy source and raw material for chemical manufacturing, it is a potent heat absorber and a strong greenhouse gas. Evidently reduction of methane emission especially in the natural gas sector is essential. Methane to hydrogen conversion through non-thermal plasma technologies has received increasing attention. In this paper, catalytic methane conversion into hydrogen is experimentally studied via nano-second pulsed DBD plasma reactor. The effect of carrier gas flow, applied voltage, and commercial Ni–K₂O/Al₂O₃ catalyst loading on methane conversion, hydrogen production, hydrogen selectivity, discharge power, and energy efficiency are studied. The results showed that in the plasma alone system, the highest methane conversion and hydrogen production occurs at argon flow rate of 70 mL/min. Increase in the applied voltage increases the methane conversion and hydrogen production while it decreases the energy efficiency. Presence of 1 g Ni–K₂O/Al₂O₃ catalyst shifts the optimum voltage for methane conversion and hydrogen production to 8 kV, reduces the required power, and increases the energy efficiency of the process. Finally in the catalytic plasma mode the optimum process condition occurs at the argon flow rate of 70 mL/min, applied voltage of 8 kV, and catalyst loading of 6 g. Compared with the optimum condition in the absence of catalyst, presence of 6 g Ni–K₂O/Al₂O₃ catalyst increased the methane conversion, hydrogen production, hydrogen selectivity and energy efficiency by 15.7, 22.5, 7.1, and 40% respectively.     

Hydrogen production by radio frequency plasma stimulation in methane hydrate at atmospheric pressure

Methane hydrate, formed by injecting methane into 100 g of shaved ice at a pressure of 7 MPa and reactor temperature of 0 °C, was decomposed by applying 27.12 MHz radio frequency plasma in order to produce hydrogen. The process involved the stimulation of plasma in the methane hydrate with a variable input power at atmospheric pressure. It was observed that production of CH₄ is optimal at a slow rate of CH₄ release from the methane hydrate, as analyzed by in light of the steam methane reforming (SMR) and the methane cracking reaction (MCR) processes in accordance with the content of gas production. In comparison with the steam methane reforming (SMR), it was found that methane-cracking reaction (MCR) was dominant in conversion of CH₄ into hydrogen. An H₂ content of 55% in gas production was obtained from conversion of 40% of CH₄ at an input power of 150 W. The results clearly show that hydrogen can be directly produced from methane hydrate by the in-liquid plasma method.     

Solar thermal cracking of methane in a particle-flow reactor for the co-production of hydrogen and carbon

An experimental investigation on the thermal decomposition of CH₄ into C and H₂ was carried out using a 5 kW particle-flow solar chemical reactor tested in a solar furnace in the 1300–1600 K range. The reactor features a continuous flow of CH₄ laden with μm-sized carbon black particles, confined to a cavity receiver and directly exposed to concentrated solar irradiation of up to 1720 suns. The reactor performance was examined for varying operational parameters, namely the solar power input, seed particle volume fraction, gas volume flow rate, and CH₄ molar concentration. Methane conversion and hydrogen yield exceeding 95% were obtained at residence times of less than 2.0 s. A solar-to-chemical energy conversion efficiency of 16% was experimentally reached, and a maximum value of 31% was numerically predicted for a pure methane flow. SEM images revealed the formation filamentous agglomerations on the surface of the seed particles, reducing their active specific surface area.     

Rotating gliding arc assisted methane decomposition in nitrogen for hydrogen production

Hydrogen production from methane decomposition via an atmospheric pressure rotating gliding arc (RGA) discharge reactor co-driven by a magnetic field and tangential flow is investigated. The motion and V–I characteristics of the RGA are studied with a high-speed camera and oscilloscope. Optical emission spectroscopy (OES) is used to characterize RGA plasmas in N₂ and CH₄ + N_2, and the RGA plasma is shown to occur as a warm plasma. For the CH₄ + N₂ plasma, CN, C₂, and CH spectral lines are observed. The vibrational and rotational temperatures are 0.56–0.86 eV and 1325–1986 K, respectively. The effects of load resistance, the CH₄/N₂ ratio, and the feed flow rate on the performance of methane decomposition are investigated. The maximum CH₄ conversion rate and H₂ selectivity are 91.8% and 80.7% when the CH₄/N₂ ratio is 0.1 and 0.05, respectively, at a flow rate of 6 L/min. The possible reaction mechanisms of the methane decomposition process are discussed. This study is expected to establish a basis for the further industrial applications of H₂ production.     

Variations of methane conversion process with the geometrical effect in rotating gliding arc reactor

Controllability of the kinetic path of methane conversion in plasma driven oxidation reaction is investigated. Different geometries in a rotating gliding arc reactor are adopted to control reaction paths in methane oxidation reaction. Diverging and converging type reactor product different reaction environments in view point of both the reaction time and the degree of thermal activation. In the diverging reactor, the partial oxidation process is dominant with high methane conversion because the diverging section facilitates to elongate the arc length and decrease the flow velocity. Thus, the convective and radiative heat transfer from the arc column to the reactants could be enhanced. The role of plasma in the diverging reactor is mainly igniting and sustaining the partial oxidation, which is rather different from that in the converging reactor where the plasma plays as a heat source for thermal pyrolysis of methane with the help of focusing thermal energy of the arc.     

Plasma reforming for hydrogen production: Pathways,reactors and storage

Hydrogen is an energy carrier with a very high energy density (>119 MJ/kg). Pure hydrogen is barely available; thus, it requires extraction from its compounds. Steam reforming and water electrolysis are commercially viable technologies for hydrogen production from water, alcohols, methane, and other hydrocarbons; however, both processes are energy-intensive. Current study aims at understanding the methane and ethanol-water mixture pathway to generate hydrogen molecules. The various intermediate species (like CH_X, CH₂O, CH₃CHO) are generated before decomposing methane/ethanol into hydrogen radicals, which later combine to form hydrogen molecules. The study further discusses the various operating parameters involved in plasma reforming reactors. All the reactors work on the same principle, generating plasma to excite electrons for collision. The dielectric barrier discharge reactor can be operated with or without a catalyst; however, feed flow rate and discharge power are the most influencing parameters. In a pulsed plasma reactor, feed flow rate, electrode velocity, and gap are the main factors that can raise methane conversion (40–60%). While the gliding arc plasma reactor can generate up to 50% hydrogen yield at optimized values of oxygen/carbon ratio and residence time, the hydrogen yield in the microwave plasma reactor is affected by flow rate and feed concentration. Therefore, all the reactors have the potential to generate hydrogen at lower energy demand.     

An experimental investigation of reverse vortex flow plasma reforming of methane

This paper focuses on the reforming of methane into hydrogen rich gas by means of gliding arc plasma stabilized in a reverse vortex flow. Parametric tests utilizing a 42 mm diameter reactor investigated the effects of electrode gap distance, reaction chamber exit diameter, steam input, methane input (fuel to oxygen ratio), and power input. Over the range of conditions tested, reactor performance was most sensitive to methane input. Decreasing the diameter of the reaction chamber exit impeded the performance of the reformer. A set of factorial tests determined the optimal operating conditions of the system to be at flow rates of 2 slpm nitrogen, 0.56 slpm oxygen, 1.25 slpm methane, an electrode gap distance of 34.5 mm, an outlet diameter of 12.65 mm, and a power input of 260 W. At these conditions the system yielded 83.3% hydrogen selectivity, 79.8% methane conversion and efficiency of 43.5%. Physical operating boundaries of the system defined by soot production and arc extinction were identified.     

Experimental study on partial oxidation of methane to produce hydrogen using low‐temperature plasma in AC Glidarc discharge

A reformer using low‐temperature plasma was designed and developed for hydrogen production. The reformer has three electrodes and uses AC gliding arc discharge. A reference condition, which is the highest hydrogen production, has a O₂/C ratio of 0.45, input flow rate of 4.9 l min^?1 and power supply of 1 kW. And the methane conversion rate, the high hydrogen selectivity and the reformer efficiency were 69.2, 77.8 and 35.2%, respectively. To investigate reforming characteristics, parametric studies were achieved for the gas components ratio, a gas flow rate, a reactor temperature, an input electric power and catalyst addition effect. The results are as follows: The gas components ratio was an important factor, which had maximum value. When the gas flow rate, the reactor temperature and the electric power were increased, the methane conversion rate and the hydrogen concentration also increased. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

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.     

Production of hydrogen by partial oxidation with thermal plasma

The purpose of this paper is to investigate the characteristics and optimum operating conditions of the plasmatron-assisted CH₄ reforming reaction for the hydrogen-rich gas production. In order to increase the hydrogen production and the methane conversion rate, parametric screening study was conducted at various CH₄ flow ratio and steam flow ratio and with and without adding catalyst in the reactor. High-temperature plasma flame was made with air and arc discharge, and the air flow rate and the input power were set to 5.1  L/min and 6.4 kW, respectively.When the steam flow ratio was 30.2%, the hydrogen production was maximized and the optimal methane conversion rate was 99.7%. Under these optimal conditions, the following syngas concentrations were determined: H₂, 50.4%; CO, 5.7%; CO₂, 13.8%; and C₂H₂, 1.1%. H₂/CO ratio was 9.7 and the hydrogen yield was 93.7%.     

Modelling the hydrodynamics and kinetics of methane decomposition in catalytic liquid metal bubble reactors for hydrogen production

Bubble reactors using molten metal alloys (e.g, nickel-bismuth and copper-bismuth) with strong catalytic activity for methane decomposition are an emerging technology to lower the carbon intensity of hydrogen production. Methane decomposition occurs non-catalytically inside the bubbles and catalytically at the gas-liquid interface. The reactor performance is therefore affected by the hydrodynamics of bubble flow in molten metal, which determines the evolution of the bubble size distribution and of the gas holdup along the reactor height. A reactor model is first developed to rigorously account for the coupling of hydrodynamics with catalytic and non-catalytic reaction kinetics. The model is then validated with previously reported experimental data on methane decomposition at several temperatures in bubble columns containing a molten nickel-bismuth alloy. Next, the model is applied to optimize the design of multitubular catalytic bubble reactors at industrial scales. This involves minimizing the total liquid metal volume for various tube diameters, melt temperatures, and percent methane conversions at a specified hydrogen production rate. For example, an optimized reactor consisting of 891 tubes, each measuring 0.10 m in diameter and 2.11 m in height, filled with molten Ni_0·27Bi_0.73 at 1050 °C and fed with pure methane at 17.8 bar, may produce 10,000 Nm³.h^?1 of hydrogen with a methane conversion of 80% and a pressure drop of 1.6 bar. The tubes could be heated in a fired heater by burning either a fraction of the produced hydrogen, which would prevent CO₂ generation, or other less expensive fuels.     

Fluidization analysis for catalytic decomposition of methane over carbon blacks for solar hydrogen production

A series of bed collapse tests were conducted for determining the dense fluidization flow rate of a gas-solid mixture in a micro-channel fluidized bed reactor, and a separate simulation was created for calculating the reactor conversion and temperature of the catalytic methane pyrolysis. The minimum fluidization and minimum bubbling flow rates were determined to be 3.04 and 8.07 sccm for a 2 × 4 mm² reactor channel with an average voidage of 0.57; 6.21 and 15.9 sccm for a 4 × 6 mm² channel with an average voidage of 0.42, respectively. By building a correlation between these critical velocities and the cross-sectional area of the fluidized bed reactor channel, the dense fluidization flow rate at the micro-/mini-channel level with an internal diameter range from 0.3 to 1 mm is predicted between 1.47 to 4.21 sccm. In the simulation, an internal diameter of 0.6 mm, a 10-kW solar input rate, and an initial gas flow rate from 0.08 to 0.23 sccm that expands to 1.5–4.3 ccm at the reaction temperature, are considered as the optimal conditions to maintain a reasonable conversion of methane pyrolysis and to keep the mixed fluid in the dense fluidization within the laminar flow range. The conversion of 79% under these conditions was calculated numerically and found to be promising compared to literature reports. An additional force analysis on a single carbon black particle is shown with different reactor orientations to validate the experimental data and simulation results.     

Experimental investigation of hydrogen production by CH4–CO2 reforming using rotating gliding arc discharge plasma

Hydrogen produced from CH₄–CO₂ reforming by an optimized rotating gliding arc discharge plasma reactor is investigated in this study. The effect of CH₄/CO₂ ratio (mole ratio), total input flow rate, discharge gap, voltage, and discharge frequency are analyzed. The results show that H₂ yield increases with the increase of CH₄/CO₂ ratio. Arc can be stretched effectively by increasing total input flow rate, then the discharge region is enlarged. Increasing discharge gap can enlarge the discharge region, but the reaction of the gas mixture would be suppressed if the discharge region was excessively large. The discharge region decreases with the increased discharge frequency to a certain degree. Based on the experimental results, the optimal experimental condition is concluded as applied voltage 60 V, discharge frequency 20 kHz, and minimum discharge gap 3 mm. It is anticipated that the results would serve as a good guideline to the application of hydrogen production from hydrocarbon fuels by plasma reforming onboard.     

Dehydrogenation characteristics of lean methane in a thermal reverse-flow reactor

In order to study the dehydrogenation reaction mechanism of ultra-low concentration methane in a thermal reverse-flow reactor, the effects of the cyclic period (120s–240s), the lean methane volume flow (90 Nm³/h to 180 Nm³/h), and the methane concentration (0.2 vol% to 0.8 vol%) on the dehydrogenation performance were studied systematically by using a thermal reverse-flow experimental system. When the methane concentration is 0.2 vol%, the reactor can achieve self-heat maintaining operation. With the increase in the methane concentration, the width of the high-temperature zone, the exhaust gas temperature, the methane conversion rate, and the maximum temperature of the heat-accumulator bed increase. With the increase in the lean methane volume flow, the width of the high-temperature zone, the distance between the center of the high-temperature zone and the center of the reactor, the maximum temperature, the exhaust gas temperature, and the methane conversion rate increase. With the increase in the cyclic period, the exhaust gas temperature and the deviation of the high-temperature zone increase, but the methane conversion rate and the maximum temperature decrease slightly.     

Effect of additive gases on methane conversion using gliding arc discharge

Methane conversion using gliding arc plasma has been studied. The process was conducted at atmospheric pressure. Four kinds of additive gases—helium, argon, nitrogen, and CO₂—were used to investigate their effects on methane conversion, as well as product selectivity, and discharged power. Methane conversion was increased with the increasing concentration of helium, argon, and nitrogen in the feed gas but decreased when CO₂ concentration increased. Qualitatively, hydrogen and acetylene were the major gas products. No liquid product was produced.     

Catalytic DBD plasma reactor for low temperature partial oxidation of methane: Maximization of synthesis gas and minimization of CO2

In this study, room temperature synthesis gas production from partial oxidation of methane through a dielectric barrier discharge plasma reactor was investigated experimentally. In this case, operating conditions including applied voltage and Ar flow rate were evaluated to obtain the best operating conditions for the purpose of enhancing CH₄ conversion and selective synthesis gas production. In addition, plasma alone system was compared with the catalytic system (NiO–CaO/Al₂O₃), revealing great improvement of synthesis gas selectivity through utilization of the catalyst. The maximum CH₄ conversion of 99.9% with the synthesis gas module of 1.84 and energy efficiency of 3.2% was gained in the catalytic system where the applied voltage was 10 kV. Besides, synthesis gas module of almost 2, which is in favor of commercial applications including methanol production, was achievable at the applied voltage of 8 kV in the catalytic system. Finally, the obtained results were compared with the similar previous studies.