Reforming of methane and carbon dioxide by DC water plasma at atmospheric pressure

An experimental plasma chemical reactor, equipped with a novel water plasma torch, was used for reforming methane and carbon dioxide mixture to produce synthesis gas (syngas). Water plasma is generated by the torch at atmospheric pressure, in the absence of carrier gases, water cooling system and special steam supply system. The influence of the ratio of CO₂ to CH₄ and total feed gas rate on syngas production, composition and energy conversion efficiency were investigated. Compared to other plasma technologies, the higher reaction performance was obtained by the novel water plasma process. The results show that, under optimum experimental conditions, the energy conversion efficiency reaches up to maximum value of 1.87 mmol/kJ and the highest energy efficiency of 74.63% is achieved, which is higher than that of other plasma processes. Furthermore, the obtained syngas with high mole ratio of H₂ to CO (close to 2) is suitable for the direct industrial application.     

Production of synthesis gas from propane using thermal water vapor plasma

In this work, an experimental plasma-chemical reactor equipped with a water vapor plasma torch was used for catalyst-free thermal plasma reforming of propane to produce a synthesis gas. Thermal arc discharge plasma (a mixture of water vapor and argon) was generated at atmospheric pressure.     

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.     

Modelling of thermal removal of tars in a high temperature stage fed by a plasma torch

The thermal degradation of tars in a chamber fed by a non-transferred plasma torch is theoretically examined in this study. The input of this reactor is a product gas coming from a gasification unit with a temperature of about 800 °C. According to literature, naphthalene and toluene are chosen as model compounds to represent the behaviour of their classes. According to this choice and to the data available in the literature, a reaction pathway for the thermal degradation of tars and its associated kinetics are proposed in this study. This mechanism is introduced in a CSTR model in order to check the influence of the operating parameters of the reactor on the degradation efficiency. These computations clearly show that a complete conversion of toluene (>99.9%) and an important conversion of naphthalene (96.7%) can be reached in the reactor, with concentration levels compatible with the further use of gas engines for electricity production. This theoretical study requires to be validated by comparison with experimental results.     

Continuous hydrogen production via the steam–iron reaction by chemical looping in a circulating fluidized-bed reactor

The steam–iron reaction was examined in a two-compartment fluidized-bed reactor at 800–900 °C and atmospheric pressure. In the fuel reactor compartment, freeze-granulated oxygen carrier particles consisting of Fe₃O₄ supported on inert MgAl₂O₄ were reduced to FeO with carbon monoxide or synthesis gas. The reduced particles were transferred to a steam reactor compartment, where they were oxidized back to Fe₃O₄ by steam, while at the same time producing H₂. The process was operated continuously and the particles were transferred between the reactor compartments in a cyclic manner. In total, 12 h of experiments were conducted of which 9 h involved H₂ generation. The reactivity of the oxygen carrier particles with carbon monoxide and synthesis gas was high, providing gas concentrations reasonably close to thermodynamic equilibrium, especially at lower fuel flows. The amount of H₂ produced in the steam reactor was found to correspond well with the amount of fuel oxidized in the fuel reactor, which suggests that all FeO that was formed were also re-oxidized. Despite reduction of the oxygen carrier to FeO, defluidization or stops in the solid circulation were not experienced. Used oxygen carrier particles exhibited decreased BET specific surface area, increased bulk density and decreased particle size compared to fresh. This indicates that the particles were subject to densification during operation, likely due to thermal sintering. However, stable operation, low attrition and absence of defluidization were still achieved, which suggest that the overall behaviour of the oxygen carrier particles were satisfactory.     

The heterogeneous condensation of mercury from a high velocity carrier gas

An analytical study of the condensation characteristics of mercury onto a cesium substrate in a nitrogen carrier gas was made. Good quantitative agreement was indicated between theory and experimental data [9] for heterogeneous condensation. A one-dimensional flow model was formulated, based on two-phase, three component gas dynamics, non-continuum growth relations and reaction heat term due to Cs-Hg amalgam formation. The results indicate that neglecting reaction heat, mass removal effects are dominant in the temperature range of this study. Substrate concentration was shown to have a direct effect on the magnitude of heterogeneous condensation. The effects of changing stagnation parameters on data presentation is considered.     

Combustion-torch ignition: Fluorescence imaging of OH concentration

The temporal and spatial development of the OH concentration during the ignition of a lean methaneair mixture (φ = 0.65) by a combustion torch has been studied. In the experiment the combustion torch was formed by a jet of high temperature combustion product gases that exit a thin-plate circular orifice connecting a small cylindrical prechamber with the main combustion chamber. This starting jet was driven by a spark-ignited flame which propagates through the prechamber. By decreasing the diameter of the prechamber orifice the initial gas velocity of the combustion torch was systematically increased. With this variation in velocity the flow field of the combustion torch, determined from high-speed schlieren videography, was altered significantly. At the lowest velocity a laminar vortex-ring structure was formed. As the velocity was increased the combustion-torch flow field develops the features of a highly turbulent jet. The fluid physics of the combustion torch has a significant influence on its chemical structure and the development of the subsequent ignition process in the main combustion chamber. Unique observations of the chemical structure of the combustion-torch ignition process were obtained by quantitative imaging of the OH concentration using laser-induced fluorescence.     

A novel configuration for hydrogen production from coupling of methanol and benzene synthesis in a hydrogen-permselective membrane reactor

Coupling energy intensive endothermic reaction systems with suitable exothermic reactions improve the thermal efficiency of processes and reduce the size of the reactors. One type of reactor suitable for such a type of coupling is the heat-exchanger reactor. In this work, a distributed mathematical model for thermally coupled membrane reactor that is composed of three sides is developed for methanol and benzene synthesis. Methanol synthesis takes place in the exothermic side and supplies the necessary heat for the endothermic dehydrogenation of cyclohexane reaction. Selective permeation of hydrogen through the Pd/Ag membrane is achieved by co-current flow of sweep gas through the permeation side. A steady-state heterogeneous model of the two fixed beds predicts the performance of this novel configuration. The co-current mode is investigated and the simulation results are compared with corresponding predictions for an industrial methanol fixed-bed reactor operated at the same feed conditions. The results show that although methanol productivity is the same as conventional methanol reactor, but benzene is also produced as an additional valuable product in a favorable manner, and auto-thermal conditions are achieved within the both reactors and also pure hydrogen is produced in permeation side. This novel configuration can increase the rate of methanol synthesis reaction and shift the thermodynamics equilibrium. The performance of the reactor is numerically investigated for various key operating variables such as inlet temperatures, molar flow rates of exothermic and endothermic streams, membrane thickness and sweep gas flow rate. The reactor performance is analyzed based on methanol yield, cyclohexane conversion and hydrogen recovery yield. The results suggest that coupling of these reactions in the presence of membrane could be feasible and beneficial. Experimental proof-of-concept is needed to establish the validity and safe operation of the novel reactor.     

Experimental validation of local thermal equilibrium in a MW plasma torch for hydrogen production

In this paper, experimental and numerical studies are made to investigate local thermal equilibrium in a microwave plasma torch at atmospheric pressure for hydrogen and carbon black production from methane dissociation. The microwave induced plasma can be operated up to 2 kW power at 2.45 GHz frequency. Methane is dissociated in argon, air or nitrogen plasma and optical emission spectroscopy is used to characterize the plasma. C₂, CN and OH ro-vibrational bands are used for rotational and vibrational temperature estimation while stark broadening of H-line is used for electron temperature calculation. Temperatures are determined at varying operating parameters of microwave power, axial gas flow rate, and methane flow rate. The rotational (heavy particle), vibrational, and electron temperatures are found to be equal to 5000 ± 500 K. The plasma is thus at local thermodynamic equilibrium.     

A mechanistic investigation of a calcium-based oxygen carrier for chemical looping combustion

Chemical looping combustion (CLC) has been suggested as an energy-efficient method for the capture of carbon dioxide from combustion. It is indirect combustion by the use of an oxygen carrier, which can be used for CO₂ capture in power-generating processes. The possibility of CLC using a calcium-based oxygen carrier is investigated in this paper. In the air reactor air is supplied to oxidize CaS to CaSO₄, where oxygen is transferred from air to the oxygen carrier; the reduction of CaSO₄ to CaS takes place in the fuel reactor. The exit gas from the fuel reactor is CO₂ and H₂O. After condensation of water, almost pure CO₂ could be obtained. The thermodynamic and kinetic problem of the reduction reactions of CaSO₄ with CO and H₂ and the oxidization reactions of CaS with O₂ is discussed in the paper to investigate the technique possibility. To prevent SO₂ release from the process of chemical looping combustion using a calcium-based oxygen carrier, thermochemical CaSO₄ reduction and CaS oxidation are discussed. Thermal simulation experiments are carried out using a thermogravimetric analyzer (TGA). The properties of the products are characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray diffractometry (XRD), and the optimal reaction parameters are evaluated. The effects of reaction temperature, reductive gas mixture, and oxygen partial pressure on the composition of flue gas are discussed. The suitable temperature of the air reactor is between 1050 and 1150 °C and the optimal temperature of the fuel reactor between 900 and 950 °C.     

Heat, mass and momentum transfer in coating formation by plasma spraying

The plasma jets produced by d.c. spray torches exhibit unusual properties: high flow velocities (up to 2 500 m · s ⁻¹ ), high temperatures (up to 14 000 K), steep temperature and velocity radial gradients (up to 10 ⁷ K · m ⁻¹ and 5.10 ⁵ s ⁻¹ ) and low gas density ( 1/30 to 1/50 that of the cold gas). They are laminar in their core and turbulent in their fringes. When they exit the torch nozzle, the resulting vortices coalesce inducing an engulfment process of the ambient gas with large scale eddies entraining bubbles of cold gas. The latter do not mix instantaneously with the plasma due to the high density difference. Mixing occurs after the heating of the cold inclusions. In addition, the plasma jets are continuously fluctuating in length and position because of the continuous movement of the arc root on the anode wall at frequencies ranging between 3 and 20 kHz. This results in a sort of piston flow. In plasma spraying, the solid particles are injected in the plasma jet through an injector set downstream or upstream of the nozzle exit. In this injector, particles collide between themselves and the injector wall. Therefore, they have trajectory and velocity distributions at the injector exit. It results in a dispersion of their trajectories within the jet. The flow rate of the powder carrier gas has to be adjusted to give the particles about the same momentum as that of the plasma jet at the injection point. The large difference between particle and flow velocity can induce convective movements within the molten droplets resulting in a continuous renewing of the liquid material at the particle surface. For metal or alloy particles sprayed in air this internal movement brings about a high oxidation rate enhanced by the presence of atomic oxygen in the jet. Particles impact on the part to be covered at velocities between 150 and 300 m · s ⁻¹ . The liquid material spreads out from the point of impact and forms a lamella called “splat”. The flattening time is below a few μ s and splat solidification generally starts before the flattening process is completed. The next particle that impacts a few tens of μ s later, flattens on already solidified particles. The piling up of a few splats forms a pass in less than one millisecond, then, the next pass is deposited a few seconds later. The thickness of a pass varies between 3 and 60 μ m. The flow and heat phenomena during the impact and solidification processes control the microstructure and thermo-mechanical properties of coatings. The build-up of a coating in plasma spraying is a multiscale problem with time scales ranging between microseconds and seconds and length scales ranging between a few micrometers and a few hundred micrometers or more. Therefore, models and experiments deal with either the formation of splats or the piling of layers. This paper will review what is our present knowledge of the modeling and measurement of the transient phenomena involved in the various subsystems of the plasma spray process: jet formation, particle injection, particle heating and acceleration and coating formation.     

Enhancement of simultaneous hydrogen production and methanol synthesis in thermally coupled double-membrane reactor

Coupling the methanol synthesis with the dehydrogenation of cyclohexane to benzene in a co-current flow, catalytic fixed-bed double-membrane reactor configuration in order to simultaneous pure hydrogen and methanol production was considered theoretically. The thermally coupled double-membrane reactor (TCDMR) consists of two Pd/Ag membranes, one for separation of pure hydrogen from endothermic side and another one for permeation of hydrogen from feed synthesis gas side (inner tube) into exothermic side. A steady-state heterogeneous model is developed to analyze the operation of the coupled methanol synthesis. The proposed model has been used to compare the performance of a TCDMR with conventional reactor (CR) and thermally coupled membrane reactor (TCMR) at identical process conditions. This comparison shows that TCDMR in addition to possessing advantages of a TCMR has a more favorable profile of temperature and increased productivity compared with other reactors. The influence of some operating variables is investigated on hydrogen and methanol yields. The results suggest that utilizing of this reactor could be feasible and beneficial. Experimental proof of concept is needed to establish the validity and safe operation of the recuperative reactor.     

Modeling a counter-current moving bed for fuel and steam reactors in the TRCL process

A mathematical model for the moving bed is developed to simulate the fuel and steam reactor in the TRCL (Three-Reactor Chemical-Looping) process. An ideal plug flow of the solid and gas is assumed in modeling the fuel and steam reactor in the TRCL process. The model considered the mass, heat balances, equilibrium, physical properties, such as the heat capacity and viscosity, and kinetics. From this model, the temperature, gas conversion and solid conversion profiles can be predicted for fuel and steam reactors. The oxygen carrier inventory (the mass of the oxygen carrier) in the fuel and steam reactor was calculated with variation of the solid inlet temperature, solid conversion, Fe₂O₃ content and steam feed rate. The temperature of the oxygen carrier to the reactor was the most sensitive parameter for determining the required inventory of the oxygen carrier. An increase in the solid inlet temperature was predicted to decrease the required inventory of the oxygen carrier. In the steam reactor, a solid inlet temperature increase over 1150 K will cause an increase in the inventory of the oxygen carrier due to the equilibrium conversion. An excessively low or high active material content will require a larger inventory of the oxygen carrier in the fuel reactor. In this study, approximately 20 wt.% of the Fe₂O₃ content was suitable for reducing the inventory of the oxygen carrier while achieving a solid conversion of 0.9 in the fuel reactor.     

Three-dimensional numerical modeling on high pressure membrane reactors for high temperature water-gas shift reaction

This study presents a three-dimensional numerical model that simulates the H₂ production from coal-derived syngas via a water-gas shift reaction in membrane reactors. The reactor was operated at a temperature of 900 °C, the typical syngas temperature at gasifier exit. The effects of membrane permeance, syngas composition, reactant residence time, sweep gas flow rate and steam-to-carbon (S/C) ratio on reactor performance were examined. Using CO conversion and H₂ recovery to characterize the reactor performance, it was found that the reactor performance can be enhanced using higher sweep gas flow rate, membrane permeance and S/C ratio. However, CO conversion and H₂ recovery limiting values were found when these parameters were further increased. The numerical results also indicated that the reactor performance degraded with increasing CO₂ content in the syngas composition.     

Intense heat simulation studies on window of high density liquid metal spallation target module for accelerator driven systems

The design of the spallation target system for accelerator driven systems requires a detailed understanding of the thermal-hydraulic issues involved as intense heat is deposited both in the window as well as in the target. Removal of heat from the window is a big thermal-hydraulic challenge. A lead–bismuth eutectic (LBE) experimental flow loop is currently being designed (1:1 size of an actual target both in terms of geometry and flow rate) to study thermal-hydraulics of the target system. It is proposed to simulate the proton beam heating of the window with a plasma heat source. In this paper, flow simulation studies have been performed for different target geometries and flow configurations with different window materials, which are proposed for the above said experimental loop. Optimal values of window geometry and flow configurations including the thermal loads the window experiences have been arrived at along with the required parameters for the plasma torch. The plasma torch has been tested and found to be suitable for simulation of proton beam heating in LBE loop to be set up.     

“Preliminary results of hydrogen production from water vapor decomposition using DBD plasma in a PMCR reactor”

The water decomposition is considered one of the most attractive chemical processes for the production of hydrogen. The present work describes the preliminary results obtained in the experimental study of the water vapor dissociation into hydrogen and oxygen species using Dielectric-Barrier Discharge (DBD) plasma in a plate micro-channel reactor (PMCR). The water vapor molecules are injected without using carrier gas into the PMCR reactor at pressure of 100 kPa and temperature of 573 K. The applied high voltage of the plasma was within range of 14–18 kV and different steam flow rates have been analyzed within range of 100–200 ml/h. The product gases have been separated in ice trap which it was connected directly to the PMCR reactor to prevent the recombination of hydrogen and oxygen species. The concentration of the outlet species has been measured in a gas phase chromatography (GC) instrument. The PMCR reactor heating temperature effect on the water vapor decomposition has been analyzed. It was found that the water vapor is dissociated into their constituent molecular elements of hydrogen and oxygen gas using plasma. The maximum obtained mole fraction, hydrogen flow rate and conversion rate were 2.3%, 9.42 g/h, 42.51% respectively, at steam temperature of 573 K, pressure 100 kPa, PMCR heating temperature 403 K, steam flow rate of 200 ml/h and the plasma discharge high voltage of 18 kV. It was observed that the amount of evolved hydrogen concentration increased with the increase of the PMCR reactor heating temperature. Also, the thermal efficiencies versus the heat supplied have been calculated and the maximum obtained efficiency was 49.32%. Consequently, the evolved hydrogen flow rate appears to depend mainly on the plasma voltage, PMCR reactor heating temperature and the separating temperature of outlet hydrogen and oxygen species. The steam dissociation experiment will be extended to separate hydrogen and oxygen species elements at high temperature conditions.     

A novel reactor for large-area epitaxial solar cell materials

A novel vertical stagnation flow organometallic vapor phase epitaxy reactor was designed and fabricated for the growth of GaAs and AlGaAs for solar cell applications. The reactor had an inverted configuration to eliminate recirculation problems. The susceptor and gas inlet nozzle were closely spaced (about 1 cm) in order to achieve improvements in deposition efficiency, layer uniformity and abruptness of interfaces. A specially designed water-cooled inlet nozzle was used to maintain the nozzle surface at relatively low temperatures under all operating conditions. A computer model was formulated to study the various thermal processes in this reactor. The model used rigorous thermal boundary conditions which included thermal radiation effects. Simulated and experimental nozzle temperatures were compared for different susceptor temperatures, susceptor-nozzle distances, gas flow rates and reactor pressures. The maximum nozzle temperature was about 100 °C, which is sufficiently low to prevent premature decomposition of the reactants on its surface.     

Hydrogen production system combined with a catalytic reactor and a plasma membrane reactor from ammonia

Ammonia is a 1promising raw material for hydrogen production because it may solve several problems related to hydrogen transport and storage. Hydrogen can be effectively produced from ammonia via catalytic thermal decomposition; however, the resulting residual ammonia negatively influences the fuel cells. Therefore, a high-purity hydrogen production system comprising a catalytic decomposition reactor and a plasma membrane reactor (PMR) has been developed in this work. Most of the ammonia is converted to hydrogen and nitrogen by the catalytic reactor. After the product gas containing unreacted ammonia is introduced to the PMR, unreacted ammonia is decomposed and hydrogen is separated in the PMR. Based on these processes, hydrogen with a purity of 99.99% is obtained at the output of the PMR. Optimal operation conditions maximizing the hydrogen production flow rate were investigated. The gap length of the PMR and the gas differential pressure and applied voltage of the plasma influence the flow rate. A pure hydrogen flow rate of ∼120 L/h was achieved using the current operating conditions. The maximum energy efficiency of the developed hydrogen production system is 28.5%.     

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.     

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.