Atmospheric pressure microwave plasma source for hydrogen production

Nowadays hydrogen is considered as a clean energy carrier and fuel of the future. That is why the interest in production and storage of hydrogen is still increasing. One of the promising technology is using microwave plasma for hydrogen production. In this study we propose two types of an atmospheric pressure microwave plasma source (MPS) for hydrogen production via methane conversion. The first one was a nozzleless waveguide-supplied coaxial-line-based. The second one was a nozzleless waveguide-supplied metal-cylinder-based. They can be operated with microwave frequency of 2.45 GHz and power up to a few kW with a high gas flow rates (up to several thousands l/h). We present experimental results concerning electrical properties of the MPS, plasma visualization, spectroscopic diagnostics and hydrogen production. The experiment was carried out with methane flow rate up to 12,000 l/h. An additional nitrogen or carbon dioxide swirl flow was used. The absorbed microwave power was up to 5000 W. Our experiments show that MPSs presented in this paper have a high potential for hydrogen production via hydrocarbon conversion.     

Application of atmospheric pressure microwave plasma source for hydrogen production from ethanol

In contrast to conventional technologies of hydrogen production like water electrolysis or coal gasification we propose a method based on the atmospheric pressure microwave plasma. In this paper we present results of the experimental investigations of the hydrogen production from ethanol in the atmospheric pressure plasma generated in waveguide-supplied cylindrical type nozzleless microwave (915 MHz and 2.45 GHz) plasma source (MPS). Argon, nitrogen and carbon dioxide were used as a working gas. All experimental tests were performed with the working gas flow rate Q ranged from 1500 to 3900 NL/h and absorbed microwave power P_A up to 6 kW. Ethanol was introduced into the plasma as vapours carried with the working gas. The process resulted in the ethanol conversion rate greater than 99%. The hydrogen production rate was up to 210 NL[H₂]/h and the energy efficiency was 77 NL[H₂] per kWh of absorbed microwave energy.     

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.     

Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles

This paper presents a process analysis of ZnO/Zn, Fe₃O₄/FeO and Fe₂O₃/Fe₃O₄ thermochemical cycles as potential high efficiency, large scale and environmentally attractive routes to produce hydrogen by concentrated solar energy. Mass and energy balances allowed estimation of the efficiency of solar thermal energy to hydrogen conversion for current process data, accounting for chemical conversion limitations. Then, the process was optimized by taking into account possible improvements in chemical conversion and heat recoveries. Coupling of the thermochemical process with a solar tower plant providing concentrated solar energy was considered to scale up the system. An economic assessment gave a hydrogen production cost of 7.98$ kg⁻¹ and 14.75$ kg⁻¹ of H₂ for, respectively a 55 MW_th and 11 MW_th solar tower plant operating 40 years.     

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.     

Plasma catalytic reforming of methane

Thermal plasma technology can be efficiently used in the production of hydrogen and hydrogen-rich gases from methane and a variety of fuels. This article describes progress in plasma reforming experiments and calculations of high temperature conversion of methane using heterogeneous processes. The thermal plasma is a highly energetic state of matter that is characterized by extremely high temperatures (several thousand degrees Celsius), and a high degree of dissociation and a substantial degree of ionization. The high temperatures accelerate the reactions involved in the reforming process. Hydrogen-rich gas (40% H₂, 17% CO₂ and 33% N₂, for partial oxidation/water shifting) can be efficiently made in compact plasma reformers. Experiments have been carried out in a small device (2–3 kW) and without the use of efficient heat regeneration. For partial oxidation/water shifting, it was determined that the specific energy consumption in the plasma reforming processes is 16 MJ/kg H₂ with high conversion efficiencies. Larger plasmatrons, better reactor thermal insulation, efficient heat regeneration and improved plasma catalysis could also play a major role in specific energy consumption reduction and increasing the methane conversion. A system has been demonstrated for hydrogen production with low CO content (1.5%) with power densities of 30 kW (H₂ HHV)/l of reactor, or 10 m³/h H₂ per liter of reactor. Power density should further increase with increased power and improved design.     

An experimental study of syn-gas production via microwave plasma reforming of methane,iso-octane and gasoline

A newly developed microwave plasma system for fuel reforming was tested for three different hydrocarbon fuels. The microwave plasma system was powered by a low cost commercial magnetron and power supply. The microwave power was delivered to the nozzle from the magnetron via a coaxial cable, which offers tremendous flexibility for system design and applications. A non-premixed configuration was achieved by delivering a separate stream of fuel to the plasma plume, which is composed of diluted oxygen only. The feasibility of syn-gas production capability of the microwave plasma system was demonstrated and the reforming characteristics of methane, iso-octane and gasoline were compared. The effects of input power, injected fuel amount, total flow rate and O/C ratio were evaluated. The production rates of both hydrogen and carbon monoxide were proportional to the input power and the inverse of the total flow rate. As a result, the maximum efficiency of 3.12% was obtained with iso-octane for power consumption of 28.8 W, O/C ratio of 1, and 0.1 g/min of fuel supply. Liquid fuels produced more syn-gas and showed better efficiency than methane for the same input powers and O/C ratios.     

Hydrogen production from (cane-molasses) stillage by citrobacter freundii and its use in improving methanogenesis

Citrobacter freundii is a facultatively anaerobic fermentative organism capable of H₂ production from sugars and lactate. A locally isolated strain of C. freundii was shown to be capable of H₂ production on an equimolar basis (i.e. 1:1.2 mol/mol) from glucose. It was also found capable of H₂ production from cane-molasses stillage (having BOD₅ of 54 kg m⁻³) at a rate of 1.2 m³ (m³waste) ¹ day⁻¹. This was tested on various scales, ranging from 0.06 to 100 1 and was found to be consistent over these variations in scale. Simultaneously, the BOD₅ value was reduced at a rate of 5.4 kg (m³waste)⁻¹ day⁻¹.

One of the uses of the evolved H₂ was as feed for an anaerobic digestor containing (untreated) cane-molasses stillage as substrate. With this arrangement, the methanogenesis was enhanced by way of increased production of methane (i.e. rise in volumetric methane content of the evolved gas), while added H₂ vanished totally from the digestor. The question of whether microbially produced H₂ can be used more profitably in microbial utilization of H₂ for energy production is considered in detail.     

Synthesis and characterization of a highly stable amorphous silicon hydride as the product of a catalytic helium–hydrogen plasma reaction

A novel highly stable surface coating SiH(1/p) which comprised high-binding-energy hydride ions was synthesized by a microwave plasma reaction of a mixture of silane, hydrogen, and helium wherein it is proposed that He⁺ served as a catalyst with atomic hydrogen to form the highly stable hydride ions. Novel silicon hydride was identified by time of flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). The ToF-SIMS identified the coatings as hydride by the large SiH⁺ peak in the positive spectrum and the dominant H⁻ in the negative spectrum. XPS identified the H content of the SiH coatings as hydride ions, H⁻(1/4), H⁻(1/9), and H⁻(1/11) corresponding to peaks at 11, 43, and 55 eV, respectively. The silicon hydride surface was remarkably stable to air as shown by XPS. The highly stable amorphous silicon hydride coating may advance the production of integrated circuits and microdevices by resisting the oxygen passivation of the surface and possibly altering the dielectric constant and band gap to increase device performance.

The plasma which formed SiH(1/p) showed a number of extraordinary features. Novel emission lines with energies of q·13.6 eV where q=1,2,3,4,6,7,8,9, or 11 were previously observed by extreme ultraviolet spectroscopy recorded on microwave discharges of helium with 2% hydrogen (Int. J. Hydrogen Energy 27 (3) 301–322). These lines matched H(1/p), fractional Rydberg states of atomic hydrogen where p is an integer, formed by a resonant nonradiative energy transfer to He⁺ acting as a catalyst. The average hydrogen atom temperature of the helium–hydrogen plasma was measured to be 180–210 eV versus ≈3 eV for pure hydrogen. Using water bath calorimetry, excess power was observed from the helium–hydrogen plasma compared to control krypton plasma. For example, for an input of 8.1 W, the total plasma power of the helium–hydrogen plasma measured by water bath calorimetry was 30.0 W corresponding to 21.9 W of excess power in 3 cm³. The excess power density and energy balance were high, 7.3 W/cm³ and −2.9×10⁴ kJ/molH₂, respectively. This catalytic plasma reaction may represent a new hydrogen energy source and a new field of hydrogen chemistry.     

Hydrogen generation system using sodium borohydride for operation of a 400 W-scale polymer electrolyte fuel cell stack

Sodium borohydride (NaBH₄) in the presence of sodium hydroxide as a stabilizer is a hydrogen generation source with high hydrogen storage efficiency and stability. It generates hydrogen by self-hydrolysis in aqueous solution. In this work, a Co–B catalyst is prepared on a porous nickel foam support and a system is assembled that can uniformly supply hydrogen at >6.5 L min⁻¹ for 120 min for driving 400-W polymer electrolyte membrane fuel cells (PEMFCs). For optimization of the system, several experimental conditions were changed and their effect investigated. If the concentration of NaBH₄ in aqueous solution is increased, the hydrogen generation rate increases, but a high concentration of NaBH₄ causes the hydrogen generation rate to decrease because of increased solution viscosity. The hydrogen generation rate is also enhanced when the flow rate of the solution is increased. An integrated system is used to supply hydrogen to a PEMFCs stack, and about 465 W power is produced at a constant loading of 30 A.     

Large capacity hydrogen production by microwave discharge plasma in liquid fuels ethanol

In situ hydrogen production technologies have attracted attentions because of hydrogen storage and transportation safety issues. Discharge plasma technology for hydrogen production is of fast response, large capacity, small scale and portability, which is suitable for automobiles and ships. In this paper, a method for producing hydrogen by microwave discharge in ethanol solution was introduced. A microwave discharge reactor of direct standing wave coupling (MDRSWC) was designed, which was suitable for on-board hydrogen production. The characteristics of large capacity hydrogen production by applying MDRSWC in liquid ethanol were investigated. Depending on the experimental conditions of ethanol concentration and microwave power, the flow rate of hydrogen production was achieved ranging from 28.93 to 72.48 g/h. In addition to main hydrogen and carbon dioxide, a small amount of methane and acetylene as by-products were detected. By optimizing the experimental conditions, the experimental results showed that the flow rate of hydrogen, the percentage concentration of hydrogen and the energy yield of hydrogen production were 72.48 g/h, 58.1% and 48.32 g/kWh respectively. This work could provide a potentially effective hydrogen production method for on-board hydrogen utilization device.     

A mini-type hydrogen generator from aluminum for proton exchange membrane fuel cells

A safe and simple hydrogen generator, which produced hydrogen by chemical reaction of aluminum and sodium hydroxide solution, was proposed for proton exchange membrane fuel cells. The effects of concentration, dropping rate and initial temperature of sodium hydroxide solution on hydrogen generation rate were investigated. The results showed that about 38 ml min⁻¹ of hydrogen generation rate was obtained with 25 wt.% concentration and 0.01 ml s⁻¹ dropping rate of sodium hydroxide solution. The cell fueled by hydrogen from the generator exhibited performance improvement at low current densities, which was mainly due to the humidified hydrogen reduced the protonic resistivity of the proton exchange membrane. The hydrogen generator could stably operate a single cell under 500 mA for nearly 5 h with about 77% hydrogen utilization ratio.     

Electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 hydrogen storage alloy

The electrochemical properties of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 hydrogen storage alloy have been studied through chronopotentiometric, chronoamperometric and cyclic voltammogram measurements. The maximum capacity value obtained was 265 mAh g⁻¹ at rate C/6 and the capacity decrease was recorded by 1.5% after 30 cycles. The values of the hydrogen diffusion coefficient D_H obtained through cyclic volammogram and chronoamperometric techniques were, respectively, 7.01 × 10⁻⁸ cm² s⁻¹ and 4.23 × 10⁻¹¹ cm² s⁻¹.     

Nano-Cu catalyze hydrogen production from formaldehyde solution at room temperature

In this paper, high efficient production of CO-free hydrogen from formaldehyde (HCHO) aqueous solution catalyzed by various nano-metal catalysts was reported. It was found that nano-metal catalyst could catalyze formaldehyde into hydrogen and formic acid under room temperature and atmospheric pressure. Among Pt, Au, Ni, and Cu nano-metal particles, nano-Cu catalyst exhibited the highest activity and the long-term stability. The temperature seems influence the reaction significantly. For example, when the temperature was increased from 0 to 60 °C, the rate of hydrogen production increases from 2.34 to 140 mL min⁻¹ g⁻¹ catalyst over nano-Cu catalyst. Hydrogen was produced via the formic acid intermediate. When NaOH concentration was high, Cannizzaro reaction occurred, which resulted in the retardation of hydrogen generation at high concentration of NaOH and HCHO.     

Reduction of hydrogen peroxide production at anode of proton exchange membrane fuel cell under open-circuit conditions using ruthenium–carbon catalyst

This study examines the effect of hydrogen peroxide (H₂O₂) on the open-circuit voltage (OCV) of a proton exchange membrane fuel cell (PEMFC) and the reduction of H₂O₂ in the membrane using a ruthenium/carbon catalyst (Ru/C) at the anode. Each cathode and anode potential of the PEMFC in the presence of H₂O₂ is examined by constructing a half-cell using 1.0 M H₂SO₄ solution as an electrolyte and Ag/AgCl as the reference electrode. H₂O₂ is added to the H₂SO₄ solution and the half-cell potential is measured at each H₂O₂ concentration. The cathode potential is affected by the H₂O₂ concentration while the anode potential remains stable. A Ru catalyst is used to reduce the level of H₂O₂ formation through O₂ cross-over at the interface of a membrane and the anode. The Ru catalyst is known to produce less H₂O₂ through oxygen reduction at the anode of PEMFC than a Pt catalyst. A Ru/C layer is placed between the Nafion^® 112 membrane and anode catalyst layer and the cell voltage under open-circuit condition is measured. A single cell is constructed to compare the OCV of the Pt/C only anode with that of the Ru/C-layered anode. The level of hydrogen cross-over and the OCV are determined after operation at a current density of 1 A cm⁻² for 10 h and stabilization at open-circuit for 1 h to obtain an equilibrium state in the cell. Although there is an increase in the OCV of the cell with the Ru/C layer at the anode, excessive addition of Ru/C has an adverse effect on cell performance.     

Out of the cell performance of reforming catalysts for direct molten carbonate fuel cells (DMCFC)

Nickel catalysts supported on commercial MgO and LiAlO₂ have been tested and compared in the methane steam reforming reaction at temperatures ranging from 798 to 923 K and GHSV from 5.4 × 10⁶s⁻¹ to 5.4 × 10⁸s⁻¹. Initial molar ratio of the reactants, R = P_H2O/P_CH4 was 2.54. Results of the influence of the catalyst reduction temperature have been reported. Preliminary results of the reaction kinetic analysis, adopting first order pseudohomogeneous model, are given. The suitability of using these catalysts into the molten carbonate fuel cells (MCFC), to generate hydrogen directly by the methane steam reforming, is discussed.     

Kinetics of the NCO radical reacting with atoms and selected molecules

Rate constants for the reaction of isocyanate radicals (NCO) in its electronic ground state ( ²Π) with oxygen atoms were determined at 2.5 Torr total pressure in the temperature range 302–757 K. Excimer laser photolysis (ELP) of chlorine isocyanate (ClNCO) produced NCO radicals detected by laser-induced fluorescence (LIF). The reaction NCO + O exhibits a negative temperature dependence, described by the two-parameter equation: k_NCO+O(T) = (4.3_−2.2^+3.2) × 10⁻⁸ × T^{−1.14_−0.12+0.08} cm³ molecule⁻¹ s⁻¹. Measurements at 298 K and total pressures of 2.5 and 9.9 Torr, respectively, indicated a slight pressure dependence. For the reaction of NCO radicals with hydrogen atoms, the rate constant k_NCO+H = (2.2 ± 1.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹was obtained at 298 K and a total pressure of 2.6 Torr for the first time by a direct measurement. From a single measurement k = (3.8 ± 1.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ was determined at 548 K and 2.4 Torr total pressure. In addition, rate constants for the reactions of NCO radicals with molecular oxygen (O₂), carbon dioxide (CO₂), molecular hydrogen (H₂), and carbon monoxide (CO), which is a dissociation product of CO₂ in a microwave discharge, were measured at two different temperatures. At room temperature these reactions were slow and at the detection limit of the ELP/LIF technique. However, at elevated temperatures at least the rate constants of the reactions NCO + O₂ and NCO + H₂ become significantly larger and, therefore, should be taken into account, when modeling combustion processes under certain conditions.     

Ignition of hydrogen-enriched methane by heated air

This study investigates experimentally and computationally the effects of hydrogen addition on ignition in nonpremixed, counterflowing methane vs. heated air jets for ambient pressures between 0.2 and 8.0 atm, hydrogen concentrations in the range 0–60% by volume, and pressure-weighted strain rates of 150, 300, and 350 s⁻¹. The effect of flow strain rate was further investigated computationally for strain rates between 100 and 10,000 s⁻¹. Hydrogen addition was found to significantly improve methane ignition through a mechanism of increased radical production and weakening of kinetic inhibition by diffusive separation of branching and termination reactions. Three ignition regimes were identified, depending on the H₂ concentration: 1) hydrogen-assisted methane ignition, 2) transition, and 3) hydrogen-dominated ignition. Both experiments and modeling indicated two-stage ignition within the first two regimes, with the first stage controlled by radical runaway, and the second stage involving thermal feedback. The controlling chemistry within the three ignition regimes was investigated using the Computational Singular Perturbation (CSP) method applied to conditions within an ignition kernel identified similar to previous studies on counterflow ignition. Chemical heat release was shown to be indispensable at ignition in the first two regimes, but negligible within the third, kinetically dominated regime, except at high pressures. Similarly, transport effects were found to be significant in regimes 1) and 2), but the ignition temperatures were largely insensitive to strain within the third regime. Methane addition to the H₂/N₂/air system was found to inhibit ignition at low and moderate pressures, while facilitating it at pressures greater than 5 atm, primarily because of the interaction with the HO₂/H₂O₂ chemistry which is dominant in these regimes. A CSP-derived skeletal mechanism was found to represent, within a 3% deviation, the ignition temperatures and species concentrations calculated using the full mechanism.     

Effect of microwave plasma torch on the pyrolysis fuel oil in the presence of methane and ethane to increase hydrogen production

Pyrolysis fuel oil (PFO) processing by microwave plasma torch was developed for the production of hydrogen. The PFO cracking process was performed at atmospheric pressure in the absence of catalyst and effect of plasma gas on the production rate of hydrogen and light hydrocarbons (C₂–C₄) was evaluated. In the first step, effect of the applied power and the working gas flow rate was investigated. In the second step, the applied power and working gas rate were set to 650 W and 4000 sccm, respectively, which were provided by combining methane or ethane as 0%, 2.5%, 7.5%, and 20% with argon. By increasing the percentage of the existing methane in argon, production rate of the sum of the light hydrocarbons was increased and that of hydrogen was reduced, but it was more than the case when argon was applied alone. By increasing ethane percentage, hydrogen production and light hydrocarbon rate were increased. The best conditions of the plasma gas for producing hydrogen at the power of 650 W were obtained as 5CC PFO feed, 2500 sccm (80%) argon, and 500 sccm (20%). The hydrogen production rate in optimized conditions was 2343.16SCCM with selectivity of 84.41%. Sum of the obtained hydrocarbons in this test was 434.25 sccm. Another parameter in the present study was the feed volume processed by plasma. In this case, 5 cc, 3 cc, and 1 cc of the feed were tested when the plasma gas was 3000 sccm argon with the power of 650 W. The results showed that, by increasing the feed, the products were increased. In the processing of 5 cc feed with plasma, 896.41 sccm hydrogen and 61 sccm light hydrocarbon were produced.     

Thermodynamic possibilities and constraints for pure hydrogen production by iron based chemical looping process at lower temperatures

Iron offers the possibility of transformation of a syngas or gaseous hydrocarbons into hydrogen by a cycling process of iron oxide reduction (e.g. by hydrocarbons) and release of hydrogen by steam oxidation. From the thermodynamic and chemical equilibrium point of view, the reduction of magnetite by hydrogen, CO, CH₄ and a model syngas (mixtures CO + H₂ or H₂ + CO + CO₂) and oxidation of iron by steam has been studied. Attention was concentrated not only on convenient conditions for reduction of Fe₃O₄ to iron at temperatures 400–800 K but also on the possible formation of undesired soot, Fe₃C and iron carbonate as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of magnetite at low temperatures requires a relatively high H₂/H₂O ratio, increasing with decreasing temperature. Reduction of iron oxide by CO is complicated by soot and Fe₃C formation. At lower temperatures and higher CO₂ concentrations in the reducing gas, the possibility of FeCO₃ formation must be taken into account. The purity of the hydrogen produced depends on the amount of soot, Fe₃C and FeCO₃ in the iron after the reduction step. Magnetite reduction is the more difficult stage in the looping process. Pressurized conditions during the reduction step will enhance formation of soot and carbon containing iron compounds.