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.     

Syngas production by integrating CO2 partial gasification of pine sawdust and methane pyrolysis over the gasification residue

The aim of this work was to study syngas production by integrating CO₂ partial gasification (for CO production) of pine sawdust (PS) and methane pyrolysis (for H₂ production) over the gasification residue. Effect of the gasification conditions (including CO₂ flow rate, reaction temperature, mass ratio of PS:Ni and reaction time) was investigated on properties of the gasification residue. Besides CO-rich gas released from the gasification process with CO₂ conversion up to about 92%, the gasification residue could serve as robust catalyst for H₂ production by methane pyrolysis. Thanks to the nickel crystallites formed with high reduction degree and high dispersion on the surface after the gasification process, the gasification residue was competent for high and stable methane conversion (about 91%) at 850 °C. In addition to the flexible syngas output (in theory, with an arbitrary ratio of H₂/CO), valuable filamentous carbons can be achieved by regulating the process parameters.     

Review of methane catalytic cracking for hydrogen production

Methane catalytic cracking is a process by which carbon monoxide-free hydrogen can be produced. Despite the fact that hydrogen produced from methane cracking is a pure form of hydrogen, methane cracking is not used on an industrial scale for producing hydrogen since it is not economically competitive with other hydrogen production processes. However, pure hydrogen demand is increasing annually either in amount or in number of applications that require carbon monoxide-free hydrogen. Currently, hydrogen is produced primarily via catalytic steam reforming, partial oxidation, and auto-thermal reforming of natural gas. Although these processes are mature technologies, CO is formed as a by-product, and in order to eliminate it from the hydrogen stream, complicated and costly separation processes are required. To improve the methane catalytic cracking economics, extensive research to improve different process parameters is required. Using a highly active and stable catalyst, optimizing the operating conditions, and developing suitable reactors are among the different areas that need to be addressed in methane cracking. In this paper, catalysts that can be used for methane cracking, and their deactivation and regeneration are discussed. Also, methane catalytic cracking kinetics including carbon filament formation, the reaction mechanisms, and the models available in the literature for predicting reaction rates are presented. Finally, the application of fluidized beds for methane catalytic cracking is discussed.     

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).     

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

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

Hydrogen production by methane cracking using Ni-supported catalysts in a fluidized bed

Nickel, supported on porous alumina (γAl₂O₃), non-porous alumina (αAl₂O₃), and porous silica, was used to catalyze methane cracking in a fluidized bed reactor for hydrogen production. The effects of temperature, PCH₄${\text{P}}_{{\text{CH}}_4}$, and particle diameter, and their interactions, on methane conversion were studied with each catalyst. Temperature was the dominant parameter affecting the hydrogen production rate for all catalysts and particle diameter had the strongest effect on the total amount of carbon deposited. Maximum methane conversion as a function of support type followed the order Ni/SiO₂ > Ni/αAl₂O₃ > Ni/γAl₂O₃. Nonetheless, better fluidization quality was obtained with Ni/γAl₂O₃. Methane conversion was increased by increasing temperature and particle size from 108 to 275 μm due to better fluidization achieved with 275 μm particles. Increasing the flow rate and methane partial pressure (PCH₄${\text{P}}_{{\text{CH}}_4}$) caused a drop in methane conversion. Tests were also run in a fixed bed reactor, and at constant weight hourly space velocity (WHSV), higher conversion was achieved in the fixed bed, but at the same time faster deactivation occurred since a higher methane conversion led to increase in carbon filament and encapsulating carbon formation rates. A critical problem with the fixed bed was the pressure build-up inside the reactor due to carbon accumulation. Finally, a series of cracking/regeneration cycle experiments were carried out in the fluidized bed reactor. The regeneration was performed through product carbon gasification in air. Ni/αAl₂O₃ and Ni/γAl₂O₃ activity decreased significantly with the first regeneration, which is attributed to Ni sintering during exothermic regeneration/carbon oxidation. However, Ni/SiO₂ was thermally stable over at least three cracking/regeneration cycles, but mechanical attrition was observed.     

Liquid hydrogen production via hydrogen sulfide methane reformation

Hydrogen sulfide (H₂S) methane (CH₄) reformation (H₂SMR) (2H₂S + CH₄ = CS₂ + 4H₂) is a potentially viable process for the removal of H₂S from sour natural gas resources or other methane containing gases. Unlike steam methane reformation that generates carbon dioxide as a by-product, H₂SMR produces carbon disulfide (CS₂), a liquid under ambient temperature and pressure—a commodity chemical that is also a feedstock for the synthesis of sulfuric acid. Pinch point analyses for H₂SMR were conducted to determine the reaction conditions necessary for no carbon lay down to occur. Calculations showed that to prevent solid carbon formation, low inlet CH₄ to H₂S ratios are needed. In this paper, we analyze H₂SMR with either a cryogenic process or a membrane separation operation for production of either liquid or gaseous hydrogen. Of the three H₂SMR hydrogen production flowsheets analyzed, direct liquid hydrogen generation has higher first and second law efficiencies of exceeding 80% and 50%, respectively.     

Membrane bubble column reactor model for the production of hydrogen by methane pyrolysis

A membrane reactor model is developed to describe, model, and design molten metal methane pyrolysis bubble column reactors. It is utilized to demonstrate that a membrane reactor allows conversions in excess of the equilibrium conversion implied by the feed and operating conditions. Ultra-high conversion eliminates the need to separate product hydrogen from unreacted methane, thereby eliminating the need to recycle un-reacted methane and reducing the total equipment sizes and energy costs. Furthermore, it is shown that the hydrogen can be completely removed through the membrane reactor walls before the gas bubbles breakthrough the molten metal layer into the reactor headspace. The equations also apply to non-membrane reactors, and are therefore useful for future general conceptual design studies. The general applicability is demonstrated by comparison of the model predictions to published experimental data on methane pyrolysis in a non-membrane bubble column reactor.     

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.     

Microwave-assisted catalytic decomposition of methane over activated carbon for -free hydrogen production

The aim of this work was to combine microwave heating with the use of low-cost granular activated carbon as a catalyst for the production of CO₂-free hydrogen by methane decomposition in a fixed bed quartz-tube flow reactor. In order to compare the results achieved, conventional heating was also applied to the catalytic decomposition reaction of methane over the activated carbon. It was found that methane conversions were higher under microwave conditions than with conventional heating when the temperature measured was lower than or equal to . However, when the temperature was increased, the difference between the conversions under microwave and conventional heating was reduced. The influence of volumetric hourly space velocity (VHSV) on the conversion tests using both microwave and conventional heating was also studied. In general, there is a substantial initial conversion, which declines sharply during the first stages of the reaction but tends to stabilise with time. An increase in the VHSV has a negative effect on CH₄ conversion, and even more so in the case of microwave heating. Nevertheless, the conversions obtained in the microwave device at the beginning of the experiments are, in general, better than the conversions reported in other works which also use a carbonaceous-based catalyst. Additionally, the formation of carbon nanofibres in one of the microwave experiments is also reported.     

Numerical modeling of methane pyrolysis in a bubble column of molten catalysts for clean hydrogen production

Methane pyrolysis using molten catalysts in a bubble column reactor (BCR) has recently been proposed to produce hydrogen with separable carbon particles as byproducts. In this study, a numerical model of the BCR of molten catalysts for methane pyrolysis was developed and validated using experimental data. Based on a non-isothermal 1-D simplification, continuous liquid and discrete bubble phases were considered by incorporating submodels for bubble behaviors, catalytic and homogeneous reactions, heat/mass transfer, and a submerged orifice for methane supply. The initial bubble diameter was predicted using the correlation derived from measurements. When applied to experiments with Ni₍₂₇₎Bi₍₇₃₎ and mixtures of KCl–MnCl₂, the model accurately reproduced the methane conversion at different temperatures and column heights. Furthermore, detailed information on the key phenomena was acquired, including the profiles of the bubble diameter, rise velocity, reaction rates, temperature, and gas composition. A sensitivity analysis confirmed that the uncertainties regarding the physical properties of molten catalysts had a negligible impact. A comparison of the performances of Ni₍₂₇₎Bi₍₇₃₎ and KCl₍₅₀₎MnCl₂₍₅₀₎ under the same reaction conditions revealed a favorable influence of the catalyst density on methane conversion because of the increased pressure. The proposed model would be useful in reactor optimization and scale-up with high hydrogen productivity.     

Technoeconomic analysis of hydrogen production via hydrogen sulfide methane reformation

Feasibility analysis of methane reforming by hydrogen sulfide for hydrogen production from technical and economical viewpoints was made. An improved Hydrogen Sulfide Methane Reformation (H₂SMR) process flowsheet was proposed in order to compare its production costs with those of Steam Methane Reformation (SMR) conventional process. Major findings were: high production of hydrogen, a partial self-sustainability process since some of the hydrogen produced could be used as an energy source, no greenhouse gases generated, common sizes of main equipment for a typical H₂ production and the possibility of eliminating Claus plants. Aspen Plus^® V8.4 simulation software was used. Results showed H₂SMR is a more economical source of H₂ production than SMR conventional process, with an estimated cost of 1.41 $/kg.     

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.     

Influence of process parameters on direct solar-thermal hydrogen and graphite production via methane pyrolysis

Current hydrogen and carbon production technologies emit massive amounts of CO₂ that threaten Earth's climate stability. Here, a new solar-thermal methane pyrolysis process involving flow through a fibrous carbon medium to produce hydrogen gas and high-value graphitic carbon product is presented and experimentally quantified. A 10 kW_e solar simulator is used to instigate the methane decomposition reaction with direct irradiation in a custom solar reactor. From localized solar heating of fibrous medium, the process reaches steady-state thermal and chemical operation from room temperature within the first minute of irradiation. Additionally, no measurable carbon deposition occurs outside the fibrous medium, leaving the graphitic product in a form readily extractable from the solar reactor. Parametric variations of methane inlet flow rate (10–2000 sccm), solar power (0.92–2.49 kW) and peak flux (1.3–3.5 MW/m²), operating pressure (1.33–40 kPa), and medium thickness (0.36–9.6 mm) are presented, with methane conversion varying from 22% to 96%.     

Modeling of fluidized bed membrane reactors for hydrogen production from steam methane reforming with Aspen Plus

Hydrogen production via steam methane reforming with in situ hydrogen separation in fluidized bed membrane reactors was simulated with Aspen Plus. The fluidized bed membrane reactor was divided into several successive steam methane sub-reformers and membrane sub-separators. The Gibbs minimum free energy sub-model in Aspen Plus was employed to simulate the steam methane reforming process in the sub-reformers. A FORTRAN sub-routine was integrated into Aspen Plus to simulate hydrogen permeation through membranes in the sub-separator based on Sieverts' law. Model predictions show satisfactory agreement with experimental data in the literature. The influences of reactor pressure, temperature, steam-to-carbon ratio, and permeate side hydrogen partial pressure on reactor performances were investigated with the model. Extracting hydrogen in situ is shown to shift the equilibrium of steam methane reactions forward, removing the thermodynamic bottleneck, and improving hydrogen yield while neutralizing, or even reversing, the adverse effect of pressure.     

Simulation of chlorine-mediated autothermal methane pyrolysis for hydrogen production

Given the present need to access a large-scale supply of hydrogen in the short-term, methane pyrolysis for hydrogen production could be utilized as a transition technology, since existing natural gas infrastructures can be exploited. Here, a novel chlorine-mediated pyrolysis process is presented that overcomes the considerable challenges posed by the input of external energy in the direct methane pyrolysis. By operating at a methane to chlorine inlet ratio of roughly 1.5 to 1, the heat released by the exothermic chlorination reaction can be leveraged to generate hydrogen by pyrolysis in addition to the hydrogen chloride. A downstream hydrochloric acid electrolysis enables the chlorine to be recycled and produces further hydrogen resulting in an overall hydrogen yield of 99%. An approximate cost calculation highlights the main costs of the process and reveals the high outlays for the electrolysis unit.     

A novel rotary reactor configuration for simultaneous production of hydrogen and carbon nanofibers

A novel reactor configuration, a rotary bed reactor (RBR), was used to study at large scale production the Catalytic Decomposition of Methane (CDM) into hydrogen and carbon nanofibers using a nickel–copper catalyst. The results were compared to those obtained in a fluidized bed reactor (FBR) under the same operating conditions. Tests carried out in the RBR provided higher hydrogen yields and more sustainable catalyst performance in comparison to the FBR. Additionally, the effect of the rotation speed and reaction temperature on the performance in the RBR of the nickel–copper catalyst was studied. The textural and structural properties of the carbon nanofibers produced were also studied by means of N₂ adsorption, SEM and XRD, and compared to those obtained in the FBR set-up under the same operating conditions.     

Hydrogen production using methane: Techno-economics of decarbonizing fuels and chemicals

In the near-to-medium future, hydrogen production will continue to rely on reforming of widely available and relatively low-cost fossil resources. A techno-economic framework is described that compares the current best practice steam methane reforming (SMR) with potential pathways for low-CO₂ hydrogen production; (i) Electrolysis coupled to sustainable renewable electricity sources; (ii) Reforming of hydrocarbons coupled with carbon capture and sequestration (CCS) and; (iii) Thermal dissociation of hydrocarbons into hydrogen and carbon (pyrolysis). For methane pyrolysis, a process based on a catalytic molten Ni-Bi alloy is described and used for comparative cost estimates. In the absence of a price on carbon, SMR has the lowest cost of hydrogen production. For low-CO₂ hydrogen production, methane pyrolysis is significantly more economical than electrochemical-based processes using commercial renewable power sources. At a carbon price exceeding $21 t^?1 CO₂ equivalent, pyrolysis may represent the most cost-effective means of producing low-CO₂ hydrogen and competes favorably to SMR with carbon capture and sequestration. The current cost disparity between renewable and fossil-based hydrogen production suggests that if hydrogen is to fulfil an expanding role in a low CO₂ future, then large-scale production of hydrogen from methane pyrolysis is the most cost-effective means during the transition period while infrastructure and end-use applications are deployed.     

Parametric study of the decomposition of methane using a NiCu/Al2O3 catalyst in a fluidized bed reactor

CO₂-free production of hydrogen via catalytic decomposition of methane (CDM) was studied in a fluidized bed reactor (FBR) using a NiCu/Al₂O₃ catalyst. A parametric study of the effects of some process variables, including catalyst particle size, reaction temperature, space velocity and the ratio of gas flow velocity to the minimum fluidization velocity (u_o/u_mf), was undertaken. A mean particle size of 150 μm allows optimization of results in terms of hydrogen production without agglomeration problems. The operating conditions strongly affect the catalyst performance: hydrogen production was enhanced by increasing operating temperature and lowering space velocity. However, increases in operating temperature, space velocity and the ratio u_o/u_mf provoked increases in the catalyst deactivation rate. At 700 °C, carbon was deposited as carbon nanofibers, while higher temperatures promoted the formation of encapsulating carbon, which led to rapid catalyst deactivation.     

Technoeconomic analysis for hydrogen and carbon Co-Production via catalytic pyrolysis of methane

Catalytic Methane Pyrolysis (CMP) is an innovative method to convert gaseous methane into valuable H₂ and carbon products. The catalytic approach to methane pyrolysis has the potential to decrease the required operating temperature for methane decomposition from >1000 °C to under 700 °C. In this work, a novel inexpensive catalyst is discussed that displays low operating temperatures, while still maintaining high reactivity and long proven lifetimes. The kinetics associated with the catalyst's performance are modeled and a correlation was developed for use with practical simulation tools. A techno-economic assessment was conducted applying experimentally determined kinetics for the CMP reaction with the specific catalyst. Two process concepts that utilize CMP using the novel catalyst are presented in this work. Optimizations were considered in these processes and the CO₂ emissions and cost of hydrogen production of the two optimized cases, CMP with H₂ combustion (CMP-H₂) and CMP with CH₄ Combustion (CMP-CH₄), are compared to that of the current industrial standard for hydrogen production, Steam Methane Reforming with carbon capture and sequestration (SMR-CCS). Both of the proposed concepts convert methane into gaseous hydrogen and valuable carbon products, graphitic carbon to carbon Nano fibers. The carbon price was treated as a variable to determine the sensitivity of hydrogen production cost to the carbon price. The analysis indicates that cost of hydrogen production is highly dependent on the recovery and sale of carbon byproducts. Based on Aspen modeling of these two concepts for large scale hydrogen production (216 tons/day), the cost of hydrogen production, without considering carbon sales, was estimated to be $<3.25/kg, assuming a natural gas price of $7/MMBTU and conservative catalyst cost of $8/kg. Assuming 100% recovery of carbon, the price can be reduced to $0/kg by selling the carbon at <$1/kg. A market assessment suggests that values of graphitic carbon and carbon fibers range from ~$10/kg and ~$25–113/kg, respectively. The cost of H₂ production via conventional SMR is ~$2.2/kg when accounting for the cost of CO₂ sequestration. The proposed processes produce a maximum of 0–2 kg CO₂/kg H₂ in contrast to the 10 kg CO₂/kg H₂ produced via conventional SMR-CCS. The process displays an enormous potential for competitive economics accompanied by reduced greenhouse gas emissions.