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).     

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.     

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.     

Production of hydrogen and light hydrocarbons as a potential gaseous fuel from microwave-heated pyrolysis of waste automotive engine oil

Waste automotive engine oil was pyrolyzed in a continuous stirred bed reactor using microwave energy as the heat source; the yield and characteristics of the incondensable gaseous products are discussed. The recovered gases (41 wt% yield) were found to contain substantial concentrations of light aliphatic hydrocarbons (up to 86 vol.%) that could potentially be used as a chemical feedstock or a fuel source to power the process, or to be reformed to produce hydrogen for use as a second-generation fuel. Examination of the composition of the gases also showed the formation of H₂ (up to 19 vol.%) and CO that could also be used as a valuable syngas (with a H₂ + CO content of up to 35 vol.%). The high yield of gaseous hydrocarbons can be attributed to the unique heating mode and chemical environment present during microwave-heated pyrolysis. The use of a microwave-heated bed of particulate-carbon showed advantages in transforming waste oil into valuable gases. Hence an environmentally unfriendly waste material can be transformed into a useful resource and serves as an alternative source of hydrogen or hydrocarbon energy. The recovery of valuable gases shows advantage over traditional destructive approaches and suggests excellent potential for recycling problematic waste oil.     

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.     

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.     

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.     

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

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

The role of hydrogen in microwave plasma valorization of producer gas

Plasma methods are given significant attention in the context of conditioning the producer gas derived from biomass gasification. The goal of this work is to present the impact of hydrogen on the other producer gas compounds during microwave plasma valorization. These compounds include main producer gas components (CO, CO₂, CH₄, N₂) and minor impurities (tar compounds, H₂S and NH₃). The results prove a beneficial impact of hydrogen addition on the conversion of CH₄ and toluene, increasing it from ca. 68%–95% and ca. 97%–100%, respectively. Additionally, the presence of hydrogen changes the distribution of the products, inhibiting soot and aromatics production and promoting C₂ compounds. In the case of CO₂, the conversion increases from ca. 18%–63% when compared to nitrogen plasma, with CO being the resulting product. The presence of hydrogen inhibits H₂S conversion and does not affect CO and NH₃     

Hydrogen-rich syngas production through coal and charcoal gasification using microwave steam and air plasma torch

In the present study, microwave plasma gasification of two kinds of coal and one kind of charcoal was performed with various O₂/fuel ratios of 0–0.544. Plasma-forming gases used under 5 kW microwave plasma power were steam and air. The changes in the syngas composition and gasification efficiency in relation to the location of the coal supply to the reactor were also compared. As the O₂/fuel ratio was increased, the H₂ and CH₄ contents in the syngas decreased, and CO and CO₂ increased. When steam plasma was used to gasify the fuel with the O₂/fuel ratio being zero, it was possible to produce syngas with a high content of hydrogen in excess of 60% with an H₂/CO ratio greater than 3. Depending on the O₂/fuel ratio, the composition of the syngas varied widely, and the H₂/CO ratio necessary for using the syngas to produce synthetic fuel could be adjusted by changing the O₂/fuel ratio alone. Carbon conversion increased as the O₂/fuel ratio was increased, and cold gas efficiency was maximized when the O₂/fuel ratio was 0.272. Charcoal with high carbon and fixed carbon content had a lower carbon conversion and cold gas efficiency than the coals used in this study.     

Hot hydrogen atoms in a water-vapor microwave plasma source

A study of the hydrogen Balmer line shape in a water-vapor, microwave slot-antenna excited plasma source operated at 2.45 GHz is reported. The emission profiles of the H_α and H_β lines are well fitted by Gaussian profiles. Excited hydrogen atoms are detected in the remote plasma zone of the source up to 30 cm distance from the exciting antennas. The measured Doppler temperature corresponding to the H_β line broadening is about three times higher than the rotational temperature of the hydrogen molecular Fulcher-α band. It has been found clear evidence for the existence of a local source of excited “hot” hydrogen atoms in the “microwave field free” remote plasma zone. The measured Doppler broadening of the O(777.4 nm) triplet line indicates that “hot” oxygen atoms, with an energy around 0.3 eV, are also created in this source. Exothermic electron–ion and ion–ion recombination processes as well as DC distributed potentials existing in inhomogeneous remote plasma are possible local sources of “hot” atoms in the far remote plasma zone.     

Analysis and modelling of microwave plasma hydrogen production utilizing water vapor and tungsten electrodes

In this study, hydrogen production via microwave plasma is investigated, analyzed and simulated in a novel way for practical applications. The water vapor when in proximity of a tungsten electrode is modeled for the generation of hydrogen gas. A numerical simulation study is performed using plasma and electromagnetic wave COMSOL modules to analyze the plasmolysis of water vapor within a vacuum concealed reaction vessel entailing a tungsten electrode. A kinetic model is therefore developed to represent the reaction mechanisms and interactions between the species within the plasmolysis reactor. The dynamic results of electron density, electron temperature, plasma rate, and species interactions are extracted through the kinetic model. Within the time domain of 10⁻¹⁶ to 10⁻¹⁴ s, the hydrogen concentration is found to increase to 4.5815 $\times$ 10⁻¹¹ mol/m³ with a corresponding decrease in water vapor concentration of 1.782 $\times$ 10⁻⁸ mol/m³, respectively. The dynamic variations in the concentrations of other dissociated species are investigated across the geometry of the reaction domain studied, and it is therefore concluded that the tip of the electrode entails the highest species concentrations.     

Literature review of the catalytic pyrolysis of methane for hydrogen and carbon production

This review highlights recent developments and future perspectives in CO_x-free hydrogen production through methane pyrolysis. We give detailed discussions on thermal and catalytic methane cracking into hydrogen and carbon. Various types of solid and liquid catalysts were reviewed in terms of hydrogen selectivity, methane conversion, and deactivation. Some pilot scale technology was discussed; however, large-scale industrialisation is impeded by rapid solid catalyst deactivation, low-priced carbon (by-product) of molten catalysts, harsh conditions for reactor materials, and performance of stable molten catalysts. For catalytic methane cracking in molten catalysts (salt or metal), substantial advances in catalyst development, product separation, and reactor design are still required to commercialise methane pyrolysis for hydrogen production. To provide guidance to future works in this area, the review is specifically focused on (i) design of catalysts (ii) recent developments of molten salt-based methane cracking, (iii) reactor design and process design.     

Hydrogen production by direct injection of ethanol microdroplets into nitrogen microwave plasma flame

Liquid ethanol introduced as microdroplets into the tip of microwave nitrogen plasma, operating at 2.45 GHz under atmospheric pressure, has been investigated. Injection of ethanol outside the region of plasma generation eliminated a problem of soot formation at that region, which was responsible for short reactor lifetime. Using liquid ethanol allows to save energy needed for vaporization. Hydrogen, carbon monoxide and solid carbon were the main outlet products. Other products detected with gas chromatography were CH₄, C₂H₄ and C₂H₂. The best results concerning hydrogen production were as follows: concentration in the outlet gas up to 28%, production rate up to 1043 L/h, energy yield up to 209 L per kWh of microwave power, and were obtained for liquid C₂H₅OH flow rate of 3.7 L/h. A numerical 0D model was used to determine contributions of chemical reactions in formation of measured gaseous products. Simplified model involving only radical reactions without any ions and electrons predicts final concentrations of main compounds quite well for microwave power up to 4 kW.     

Hydrogen and syngas production from glycerol through microwave plasma gasification

Glycerol which is a byproduct of biodiesel production is considered as a potential feedstock for syngas production with the increase of biodiesel demand. In this study, the characteristics of glycerol gasification under a microwave plasma torch with varying oxygen and steam supply conditions were investigated. The experimental results demonstrated that the gasification efficiency and syngas heating value increased with the supplied microwave power while the increase of oxygen and steam led to a lower gasification performance. In order to achieve high carbon conversion and cold gas efficiency in the microwave plasma gasification of glycerol, the O₂/fuel ratio should be maintained at 0–0.4. It was revealed that the fuel droplet size and the mixing effect and retention time inside the plasma flames are critical factors that influence the product gas yield and gasification efficiency. This study verified that syngas with a high content of H₂ and CO could be effectively produced from glycerol through microwave plasma gasification.     

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.     

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).     

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

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

Thermodynamic and thermoeconomic assessment of hydrogen production employing an efficient multigeneration system based on rich fuel combustion

The recent development of distributed multigeneration energy systems is changing the focus of producing different energy vectors from large centralized plants to local energy systems. A novel multigeneration system is designed in the present work to supply domestic energy demands of power, hydrogen and heating. The proposed system mainly consists of a supercritical CO₂ cycle, a gas turbine equipped with a rich-fueled combustion chamber, a membrane for hydrogen separation and a water-gas shift reactor. Feeding the combustion chamber with a rich fuel mixture leads to the availability of a significant hydrogen amount in the products, which can be separated and stored. Thermodynamic analysis revealed that the highest irreversibility belongs to the combustion chamber, which is responsible for almost half of total exergy destruction. The cost of the produced hydrogen is estimated to be 2.2–6.8 $/kg for a natural gas price of 9.51 $/GJ and equivalence ratios of 2.9–1.65. The overall energy and exergy efficiencies, hydrogen production rate, total system cost rate, and cost of produced electricity are found to be 75.1%, 58.9%, 40.6 kg/h, 222 $/h and 51 $/MWh, respectively, assuming an equivalence ratio of 2.     

Evaluation of process conditions for methane pyrolysis applying the triple thermal plasma system

Among various thermal plasma sources for CH₄ pyrolysis, singular direct current (DC) thermal plasma has limited capacity, erosion, and efficiency. However, the triple thermal plasma system has been applied for nanomaterial synthesis and exhibited results overcoming these limitations. This study used the triple thermal plasma system to investigate the most suitable conditions for CH₄ pyrolysis. CH₄ conversion rate and selectivity of H₂ and C₂H₂ were analyzed by varying the CH₄ flow rate and quenching conditions at a fixed power supply of approximately 30 kW, and the specific energy requirement (SER) per 1 kg H₂ was compared with that of previous works. The maximum conversion rate was 97% at 50 L/min of CH₄, which is approximately 7% higher than earlier studies under conditions with similar process enthalpy. In addition, the conversion of CH₄ to C₂H₂ and further to heavier hydrocarbons proceeded one order of magnitude faster than the reaction time expected by the gas-phase reaction. This result is attributed to the easy penetration of CH₄ into the core region with the highest temperature and the strong interaction between the processing gas and graphite surface due to the arrangement of the torches in the triple plasma system. C₂H₂ selectivity was relatively high, while it was less affected by the increase in the quenching gas than generally expected. This finding was attributed to the naturally fast quenching rate without quenching gas due to the structure expanding from the first to the second graphite. While quenching can enhance selectivity by stabilizing the radicals as intermediates such as H₂ or C₂H₂, it depressed the following reaction with dehydrogenation. Thus, the quenching conditions must be optimized. Finally, we demonstrated that the triple thermal plasma enhanced CH₄ pyrolysis regarding H₂ production efficiency.