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.     

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.     

Solar enriched methane production by steam reforming process: Reactor design

A novel hybrid plant for a mixture of methane and hydrogen (enriched methane) production from a steam reforming reactor whose heat duty is supplied by a molten salt stream heated up by a concentrating solar power (CSP) plant developed by ENEA is here presented. By this way, a hydrogen stream, mixed with natural gas, is produced from solar energy by a consolidated production method as the steam reforming process and by a pre-commercial technology as molten salts parabolic mirrors solar plant. After the hydrogen production plant, the residual heat stored in molten salt stream is used to produce electricity and the plant is co-generative (hydrogen + electricity).The heat-exchanger-shaped reactor is dimensioned by a design tool developed in MatLab environment. A reactor 3.5 m long and with a diameter of 2″ is the most efficient in terms of methane conversion (14.8%) and catalyst efficiency (4.7 Nm³/h of hydrogen produced per kg_cat).     

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.     

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.     

Production of hydrogen from hydrogen sulfide assisted by dielectric barrier discharge

Hydrogen production by non-thermal plasma (NTP) assisted direct decomposition of hydrogen sulfide (H₂S) was carried out in a dielectric barrier discharge (DBD) reactor with stainless steel inner electrode and copper wire as the outer electrode. The specific advantage of the present process is the direct decomposition of H₂S in to H₂ and S and the novelty of the present study is the in-situ removal of sulfur that was achieved by operating DBD plasma reactor at ∼430 K. Optimization of various parameters like the gas residence time in the discharge, frequency, initial concentration of H₂S and temperature was done to achieve hydrogen production in an economically feasible manner. The typical results indicated that NTP is effective in dissociating H₂S into hydrogen and sulfur and it has been observed that by optimizing various parameters, it is possible to achieve H₂ production at 300 kJ/mol H₂ that corresponds to ∼3.1 eV/H₂, which is less than the energy demand during the steam methane reforming (354 kJ/mol H₂ or ∼3.7 eV/H₂).     

Thermodynamic analysis of hydrogen production via sorption-enhanced steam methane reforming in a new class of variable volume batch-membrane reactor

Combined reaction–separation processes are a widely explored method to produce hydrogen from endothermic steam reforming of hydrocarbon feedstock at a reduced reaction temperature and with fewer unit operation steps, both of which are key requirements for energy efficient, distributed hydrogen production. This work introduces a new class of variable volume batch reactors for production of hydrogen from catalytic steam reforming of methane that operates in a cycle similar to that of an internal combustion engine. It incorporates a CO₂ adsorbent and a selectively permeable hydrogen membrane for in situ removal of the two major products of the reversible steam methane reforming reaction. Thermodynamic analysis is employed to define an envelope of ideal reactor performance and to explore the tradeoff between thermal efficiency and hydrogen yield density with respect to critical operating parameters, including sorbent mass, steam to methane ratio and fraction of product gas recycled. Particular attention is paid to contrasting the variable volume batch-membrane reactor approach to a conventional fixed bed reaction–separation approach. The results indicates that the proposed reactor is a viable option for low temperature distributed production of hydrogen from methane, the primary component of natural gas feedstock, motivating a detailed study of reaction/adsorption kinetics and heat/mass transfer effects.     

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.     

Enhancing the ammonia to hydrogen (ATH) energy efficiency of alternating current arc discharge

In order to enhance the ammonia to hydrogen (ATH) energy efficiency, systematic study was carried out with atmospheric pressure alternating current arc discharge reactor (using a pair of stainless steel (SS) tube electrodes). Results showed that, using small-diameter SS tube electrodes with small discharge gaps forced more ammonia molecules to go through the effective plasma volume and obtained high electrodes temperature. Adopting low discharge frequency increased the discharge time, the effective plasma volume and the electrode temperature. These changes can enhance both the gas-phase plasma decomposition and the electrode-surface catalytic decomposition of ammonia. Insulating the reactor significantly increased the electrode temperature in small-diameter reactor, so as to enhance the electrode-surface catalytic decomposition of ammonia. In large-diameter reactors, however, the electrodes temperature increased less rapidly and more ammonia bypass of the effective plasma volume occurred. A 12.5 mol/kW h ATH energy efficiency was reached when ammonia was completely converted under the conditions of electrode diameter 3 mm, electrode gap 4 mm, discharge frequency 5 kHz, reactor diameter 8 mm, NH₃ flow rate 150 ml/min and input power 48 W. The ATH energy efficiency was further enhanced under similar conditions when incomplete ammonia conversion was allowed.     

Direct conversion from methane to methanol for high efficiency energy system with exergy regeneration

Direct synthesis of methanol from methane and water vapor mixtures has a high possibility to realize a highly sophisticated energy recycling system with exergy regeneration. Because, combined with a reforming process from methanol to hydrogen, exergy rate of low temperature thermal energy sources at about 100 °C can be enhanced to high quality chemical energy in hydrogen. For realizing this newly proposed energy system, direct synthesis of methanol from methane and water-vapor mixture has been successfully realized by non-equilibrium plasma chemical reactions under atmospheric pressure using a newly developed ultra-short pulsed barrier discharge in an extremely thin glass tube reactor. Various effects of reaction time, water-vapor concentration and discharge parameters on the conversion efficiency and reaction selectivity have been clarified. Methanol yield has reached the order of 1% at the water-vapor concentration of about 50%, and it has been proposed that the value can be largely enhanced by adding rare gas such as Kr or Ar to the source gas. Possible mechanisms for this effect have been also discussed.     

Experimental investigation on the partial oxidation of methane with the effect of shrunk structure

Hydrogen energy is an ideal clean energy to solve the expanding energy demand and environmental problems caused by fossil fuels. In order to produce hydrogen, a double-layer porous media burner with shrunk structure was designed to explore the partial oxidation (POX) of methane. And the combustion temperature, species concentration and reforming efficiency were studied under different shrunk parameters and operating conditions. The results indicated that the shrunk structure greatly influenced the flame position and temperature distribution. The flame moved to the downstream section with the decreasing of the inner shrunk diameter and the increasing of the shrunk height. When the diameter of the filled Al₂O₃ pellets was 8 mm, the hydrogen yield reached the highest value of 43.8%. With the increasing of equivalence ratio, the reforming efficiency increased first and then decreased, and the maximum value of 53.0% was reached at φ = 1.5. However, the reforming efficiency and axial temperature kept increasing when the inlet velocity increased from 10 to 18 cm/s. The corresponding results provided theoretical reference for the control of flame position and species production by the design of shrunk structure in porous media burner.     

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.     

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.     

基于太阳能的甲烷重整制氢技术研究进展

该文对近年来有关太阳能甲烷重整制氢技术的研究进行回顾。首先从甲烷重整反应制氢的工艺流程及其热力学与动力学特性角度,介绍甲烷重整反应的基本特点。其次,从高效、低成本催化剂体系开发以及新型反应器性能研究等两个方面重点回顾近年来关于太阳能甲烷重整技术的研究进展。最后,从高效催化剂开发研究、新型反应器设计以及制氢成本控制等方面,给出基于太阳能的甲烷重整制氢技术的未来发展方向。     

Production of hydrogen via methane reforming using atmospheric pressure microwave plasma

In this paper, results of hydrogen production via methane reforming in the atmospheric pressure microwave plasma are presented. A waveguide-based nozzleless cylinder-type microwave plasma source (MPS) was used to convert methane into hydrogen. Important advantages of the presented waveguide-based nozzleless cylinder-type MPS are: stable operation in various gases (including air) at high flow rates, no need for a cooling system, and impedance matching. The plasma generation was stabilized by an additional swirled nitrogen flow (50 or 100 l min⁻¹). The methane flow rate was up to 175 l min⁻¹. The absorbed microwave power could be changed from 3000 to 5000 W. The hydrogen production rate and the corresponding energy efficiency in the presented methane reforming by the waveguide-based nozzleless cylinder-type MPS were up to 255 g[H₂] h⁻¹ and 85 g[H₂] kWh⁻¹, respectively. These parameters are better than those typical of the conventional methods of hydrogen production (steam reforming of methane and water electrolysis).     

Fast start-up of microchannel fuel processor integrated with an igniter for hydrogen combustion

A Pt–Zr catalyst coated FeCrAlY mesh is introduced into the combustion outlet conduit of a newly designed microchannel reactor (MCR) as an igniter of hydrogen combustion to decrease the start-up time. The catalyst is coated using a wash-coating method. After installing the Pt–Zr/FeCrAlY mesh, the reactor is heated to its running temperature within 1 min with hydrogen combustion. Two plate-type heat-exchangers are introduced at the combustion outlet and reforming outlet conduits of the microchannel reactor in order to recover the heat of the combustion gas and reformed gas, respectively. Using these heat-exchangers, methane steam reforming is carried out with hydrogen combustion and the reforming capacity and energy efficiency are enhanced by up to 3.4 and 1.7 times, respectively. A compact fuel processor and fuel-cell system using this reactor concept is expected to show considerable advancement.     

Renewable hydrogen from ethanol by a miniaturized nonthermal arc plasma-catalytic reforming system

Ethanol–water mixtures were reformed directly into H₂-rich gas without extra heat source with conversion rates of 69.8% and 88.0% by nonthermal arc plasma and plasma-catalytic reactors, respectively. The plasma reactor consists of a Laval nozzle electrode and a central electrode. The ethanol, water and air mixtures were mixed by a spray nozzle, and then introduced into the Laval nozzle. In terms of energy efficiency, the optimal reforming condition was determined to be O/C ∼ 0.5 and S/C ∼ 1.0 with an ethanol input rate of ∼0.10 g s⁻¹. Furthermore, it is also found that applying Ni/γ-Al₂O₃ catalyst just at the downstream of the discharge region contributed to a better conversion extent and a higher hydrogen production rate, while the power consumption increased slightly, thus the specific energy required for hydrogen production reduced from 68.5 to 40.1 kJ mol⁻¹ at O/C = 0.44, S/C = 1.28 and inlet ethanol = 0.10 g s⁻¹. This reforming technology has promising prospects not only for low-cost hydrogen generation and efficiency improvement for inner combustion engine, but also for many other potential chemical applications, such as nanophase material preparation and solar fuel cell manufacturing.     

H2 production by low pressure methane steam reforming in a Pd–Ag membrane reactor over a Ni-based catalyst: Experimental and modeling

Nowadays, there is a growing interest towards pure hydrogen production for proton exchange membrane fuel cell applications. Methane steam reforming reaction is one of the most important industrial chemical processes for hydrogen production. This reaction is usually carried out in fixed bed reactors at 30–40 bar and at temperatures above 850 °C. In this work, a dense Pd–Ag membrane reactor packed with a Ni-based catalyst was used to carry out the methane steam reforming reaction between 400 and 500 °C and at relatively low pressure (1.0–3.0 bar) with the aim of obtaining higher methane conversion and hydrogen yield than a fixed bed reactor, operated at the same conditions. Furthermore, the Pd–Ag membrane reactor is able to produce a pure, or at least, a CO and CO₂ free hydrogen stream. A 50% methane conversion was experimentally achieved in the membrane reactor at 450 °C and 3.0 bar whereas, at the same conditions, the fixed bed reactor reached a 6% methane conversion. Moreover, 70% of high-purity hydrogen on total hydrogen produced was collected with the sweep-gas in the permeate stream of the membrane reactor. From a modeling point of view, the mathematical model realized for the simulation of both the membrane and fixed bed reactors was satisfactorily validated with the experimental results obtained in this work.     

Low-carbon hydrogen production via electron beam plasma methane pyrolysis: Techno-economic analysis and carbon footprint assessment

Electron beam plasma methane pyrolysis is a hydrogen production pathway from natural gas without direct CO₂ emissions. In this work, two concepts for a technical implementation of the electron beam plasma pyrolysis in a large-scale hydrogen production plant are presented and evaluated in regards of efficiency, economics and carbon footprint. The potential of this technology is identified by an assessment of the results with the benchmark technologies steam methane reforming, steam methane reforming with carbon capture and storage as well as water electrolysis. The techno-economic analysis shows levelized costs of hydrogen for the plasma pyrolysis between 2.55 €/kg H₂ and 5.00 €/kg H₂ under the current economic framework. Projections for future price developments reveal a significant reduction potential for the hydrogen production costs, which support the profitability of plasma pyrolysis under certain scenarios. In particular, water electrolysis as direct competitor with renewable electricity as energy supply shows a considerably higher specific energy consumption leading to economic advantages of plasma pyrolysis for cost-intensive energy sources and a high degree of utilization. Finally, the carbon footprint assessment indicates the high potential for a reduction of life cycle emissions by electron beam plasma methane pyrolysis (1.9 kg CO₂ eq./kg H₂ – 6.4 kg CO₂ eq./kg H₂, depending on the electricity source) compared to state-of-the-art hydrogen production technology (10.8 kg CO₂ eq./kg H₂).     

Plasma steam methane reforming (PSMR) using a microwave torch for commercial-scale distributed hydrogen production

Although large-scale hydrogen production through conventional steam methane reforming (SMR) is available at an affordable cost, there is a shortage of hydrogen pipeline infrastructure between production plants and fueling stations in most places where hydrogen is needed. Due to the difficulties of transporting and storing hydrogen, onsite hydrogen production plants are desirable. Microwave plasma torch-based methods are among the most promising approaches to achieving this goal.The plasma steam methane reforming (PSMR) method discussed here has many benefits, including a high energy yield, a small carbon footprint, real-time fueling because of the short start-up time (<10 min), and the absence of expensive metal-based catalysts. Methane reforming and water gas shift reaction (WGSR) co-occur in the method advanced without a separate WGSR to achieve a high H₂ yield.This study examines an experimental investigation of commercial-scale hydrogen production through PSMR utilizing a microwave torch system. The optimum results obtained showed that the hydrogen production rate was 2247 [g(H₂)/h], and energy yield was 70 [g(H₂)/kWh] of the absorbed microwave power. An assessment of the results indicated a similar trend to that of simulated data (ASPEN Plus). The experimental results presented in this paper demonstrate the potential of a catalyst-free PSMR for commercial-scale hydrogen production.