Coupled hydrodynamic and kinetic model of liquid metal bubble reactor for hydrogen production by noncatalytic thermal decomposition of methane

Industrial-scale implementation of liquid metal bubble reactors (LMBRs) to produce hydrogen by methane decomposition will require large gas holdups (e.g., 20–30 vol%) and elevated gas pressures (>20 bar) to allow for practical reactor sizes. A realistic reactor design must account for the coupling between reaction kinetics and hydrodynamic effects. The gas holdup is predicted from the superficial gas velocity with a drift flux model that was experimentally corroborated in gas-molten metal mixtures. Large superficial gas velocities (>0.40 m s⁻¹) are required to achieve gas holdups of about 25 vol% in liquid metal baths (LMBs). A noncatalytic kinetic model is developed to provide thermodynamically consistent decomposition rates at methane conversions approaching equilibrium. The coupled model optimizes the LMB dimensions (diameter and length) and the inlet pressure to minimize the volume of liquid metal when the hydrogen production rate, bath temperature, methane conversion, metal composition, and maximum gas holdup are specified. For example, 200 kt a⁻¹ of hydrogen can be produced in an LMBR containing at least 96.5 m³ of molten tin held at 1100 °C in a bath measuring 3.50 m in diameter and 14.3 m in length, with an inlet methane pressure of 57.8 bar resulting in an average gas holdup of 29.7 vol% and a methane conversion of 65%.     

Hydrogen production from methane using vanadium-based catalytic membrane reactors

The application of vanadium-based membranes as the hydrogen separation membrane for a catalytic membrane reactor system was investigated for the direct production of hydrogen from methane. The methane conversion and hydrogen production rates of the catalytic membrane reactor system with Pd-coated 100 μm-thick vanadium-based membranes were comparable with the reactor using 50 μm-thick Pd–Ag alloy membrane at all temperatures examined. The methane conversion rates of the catalytic membrane reactor with the Pd-coated vanadium-based membranes were approximately 35% and 62% at 623 K and 773 K, respectively. The hydrogen production rates were around 660  μmol min⁻¹ at 623 K, and reached over 1710  μmol min⁻¹ at 773 K. The relationship between the methane conversion rates and hydrogen permeation fluxes of the catalytic membrane reactor confirmed that the removal of hydrogen from the reaction site enhances the methane decomposition reaction. Further, the vanadium based membrane exhibited good stability against Fe in a hydrogen containing atmosphere.     

Semi-continuous hydrogen production from catalytic methane decomposition using a fluidized-bed reactor

Non-oxidative, catalytic decomposition of hydrocarbons is an alternative, one-step process to produce pure hydrogen with no production of carbon oxides or higher hydrocarbons. Carbon produced from the decomposition reaction, in the form of potentially valuable carbon nanotubes, remains anchored to the active catalyst sites in a fixed bed. To facilitate periodical removal of this carbon from the reactor and to make hydrogen production continuous, a fluidized-bed reactor was envisioned. The hypothesis that the tumbling and inter-particle collisions of bed material would efficiently separate nanotubes anchored to the active catalyst sites of the bed particles was tested and shown to be invalid. However, a switching mode reaction system for the semi-continuous production of hydrogen and carbon nanotubes by periodic removal and replenishment of the catalytic bed material has been successfully demonstrated.     

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.     

Thermocatalytic decomposition of methane for hydrogen production using activated carbon catalyst: Regeneration and characterization studies

A series of experiments was conducted to study the deactivation and regeneration of activated carbon catalyst used for methane thermocatalytic decomposition to produce hydrogen. The catalyst becomes deactivated due to carbon deposition and six decomposition cycles of methane at temperatures of 850 and 950 °C, and five cycles of regeneration by using CO₂ at temperatures of 900, 950 and 1000 °C were carried out to evaluate the stability of the catalyst. The experiment was conducted by using a thermobalance by monitoring the mass gain during decomposition or the mass lost during the regeneration with time. The initial activity and the ultimate mass gain of the catalyst decreased after each regeneration cycle at both reaction temperatures of 850 and 950 °C, but the amount is smaller under the more severe regenerating conditions. For the reaction at 950 °C, comparison between the first and sixth reaction cycles shows that the initial activity decreased by 69, 51 and 42%, while the ultimate mass gain decreased by 62%, 36% and 16% when CO₂ gasification carried out at 900, 950 and 1000 °C respectively. Temperature -programmed oxidation profiles for the deactivated catalyst at reaction temperature of 950 °C and after several cycles showed two peaks which are attributed to different carbon characteristics, while one peak was obtained when the experiment was carried out at 850 °C. In conclusion, conducting methane decomposition at 950 °C and regeneration at 1000 °C showed the lowest decrease in the mass gain with reaction cycles.     

Production of hydrogen by catalytic methane decomposition using biochar and activated char produced from biosolids pyrolysis

Catalytic methane decomposition (CMD) was studied by employing biochar and activated char of biosolids’ origin under different reaction temperatures and methane concentrations. Higher reaction temperatures and lower inlet methane concentrations were found to be favourable for achieving higher methane conversion. A maximum initial methane conversion of 71.0 ± 2.5 and 65.2 ± 2.3% was observed for activated char and biochar, respectively at 900 °C and for 10% CH₄ in N₂ within the first 0.5 h of experiment. Active sites from oxygen containing carboxylic acid functional groups and smaller pore volume and pore diameter were attributed to assist in higher initial methane conversion for biochar and activated char respectively. However, rapid blockages of active sites and surfaces of biochar and activated char due to carbon formation have caused a rapid decline in methane conversion values in the first 0.5 h. Later on, crystalline nature of the newly formed carbon deposits due to their higher catalytic activity have stabilised methane conversion values for an extended experimental period of 6 h for both biochar and activated char. The final conversion values at the end of 6 h experiment with biochar and activated char at 900 °C and for 10% CH₄ in N₂, were found to be 40 ± 1.9 and 35 ± 1.6% respectively. Analysing carbon deposits in detail revealed that carbon nanofiber type structures were observed at 700 °C while nanospheres of carbon were found at 900 °C.     

Natural Fe-based catalysts for the production of hydrogen and carbon nanomaterials via methane decomposition

Tierga and Ilmenite Fe-based ores are studied for the first time in the catalytic decomposition of methane (CDM) for the production of carbon dioxide-free hydrogen and carbon nanomaterials. Tierga exhibits superior catalytic performance at 800 °C. The effect of the reaction temperature, space velocity and reducing atmosphere in the catalytic decomposition of methane is evaluated using Tierga. The highest stability and activity (70 vol% hydrogen concentration) is obtained at 850 °C using methane as a reducing agent. Reduction with methane causes the fragmentation of the iron active phase and inhibits the formation of iron carbide, improving its activity and stability in the CDM. Hybrid nanomaterials composed of graphite sheets and carbon nanotubes with a high degree of graphitization are obtained. Considering its catalytic activity, the carbon quality, and the low cost of the material, Tierga has a competitive performance against synthetic iron-catalysts for carbon dioxide-free hydrogen and solid carbon generation.     

Deactivation of palm shell-based activated carbon catalyst used for hydrogen production by thermocatalytic decomposition of methane

Thermocatalytic decomposition of methane over activated carbon acting as a catalyst is proposed as a potential alternative for hydrogen production. However, over a certain duration catalyst becomes deactivated due to intensive carbon deposition.     

Some issues in modelling methane catalytic decomposition in fluidized bed reactors

This paper reports a model of fluidized bed thermo-catalytic decomposition (TCD) of methane. The novelty of the model consists of taking into account the occurrence of different competitive phenomena: methane catalytic decomposition, catalyst deactivation due to carbon deposition on the catalyst particles and their reactivation by means of carbon attrition. Comparison between theoretical and experimental data shows the capability of the present model to predict methane conversion and deactivation time during the process. The model demonstrates to be also a useful tool to investigate the role played by operative parameters such as fluidizing gas velocity, temperature, size and type of the catalyst. In particular, the model results have been finalized to characterize the attrition phenomena as a novel strategy in catalyst regeneration.     

IR spectroscopic measurement of hydrogen production kinetics in methane dry reforming

Dry reforming of methane (DRM) is a reaction that converts two greenhouse gases, CH₄ and CO₂, to syngas (H₂ + CO). Gas chromatography (GC) is almost exclusively used to evaluate catalyst performance. In order to measure the hydrogen production rate with GC, an inert gas with a constant flow rate is usually fed into the system as an internal standard. In this work, an IR spectroscopy-based method is used to achieve the same technical goal with much higher time resolution and much smaller measurement errors. IR measures the molar fractions of CH₄, CO₂, CO and H₂O in the reaction effluent. By applying general mass balance principle and the relevant reaction stoichiometries, H₂ production rate is successfully measured without an internal standard. The results are quite close to those obtained by GC with much higher time resolution, making it possible to observe fast reaction kinetics.     

Ni doped carbons for hydrogen production by catalytic methane decomposition

Ni doped carbons were prepared from raw coal and direct coal liquefaction residue (CLR) by KOH activation with addition of Ni(NO₃)₂, and used for catalytic methane decomposition (CMD) to produce hydrogen. The catalytic activity of the Ni doped carbon for CMD was compared with those of metal catalysts (Ni/SiO₂ and Ni/Al₂O₃), coal- and CLR-based carbons, and Ni-carbon catalysts prepared by traditional impregnation and precipitation methods. The results show that the Ni doped carbon has higher and more stable activity than the metal and carbon catalysts at 850 °C. The preparation method for Ni doped carbons can make full use of the reducibility of the carbon composition and simplify the traditional synthesis process. The Ni content and the morphology of carbon deposits produced during CMD have a great effect on the catalytic activity of the Ni doped carbon.     

Ultra-pure hydrogen production via ammonia decomposition in a catalytic membrane reactor

In this work two alternatives are presented for increasing the purity of hydrogen produced in a membrane reactor for ammonia decomposition. It is experimentally demonstrated that either increasing the thickness of the membrane selective layer or using a small purification unit in the permeate of the membranes, ultra-pure hydrogen can be produced. Specifically, the results show that increasing the membrane thickness above 6 μm ultra-pure hydrogen can be obtained at pressures below 5 bar. A cheaper solution, however, consists in the use of an adsorption bed downstream the membrane reactor. In this way, ultra-pure hydrogen can be achieved with higher reactor pressures, lower temperatures and thinner membranes, which result in lower reactor costs. A possible process diagram is also reported showing that the regeneration of the adsorption bed can be done by exploiting the heat available in the system and thus introducing no additional heat sources.     

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.     

Evaluation of the economic impact of hydrogen production by methane decomposition with steam reforming of methane process

There has been considerable interest in the development of more efficient processes to generate hydrogen. Currently, steam methane reforming (SMR) is the most widely applied route for producing hydrogen from natural gas. Researchers worldwide have been working to invent more efficient routes to produce hydrogen. One of the routes is thermocatalytic decomposition of methane (TCDM) - a process that decomposes methane thermally to produce hydrogen from natural gas. TCDM has not yet been commercialized. However, the aim of this work was to conduct an economic and environmental analysis to determine whether the TCDM process is competitive with the more popular SMR process. The results indicate that the TCDM process has a lower carbon footprint. Further research on TCDM catalysts could make this process economically competitive with steam methane reforming.     

Hydrogen production by catalytic methane decomposition over yttria doped nickel based catalysts

Catalytic methane decomposition can become a green process for hydrogen production. In the present study, yttria doped nickel based catalysts were investigated for catalytic thermal decomposition of methane. All catalysts were prepared by sol-gel citrate method and structurally characterized with X-ray powder diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and Brunauer, Emmet and Teller (BET) surface analysis techniques. Activity tests of synthesized catalysts were performed in a tubular reactor at 500 ml/min total flow rate and in a temperature range between 390 °C and 845 °C. In the non-catalytic reaction, decomposition of methane did not start until 880 °C was reached. In the presence of the catalyst with higher nickel content, methane conversion of 14% was achieved at the temperature of 500 °C. Increasing the reaction temperature led to higher coke formation. Lower nickel content in the catalyst reduced the carbon formation. Consequently, with this type of catalyst methane conversion of 50% has been realized at the temperature of 800 °C.     

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.     

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.     

One-dimensional model of solar thermal reactors for the co-production of hydrogen and carbon black from methane decomposition

The solar thermal decomposition of methane is a promising route for the large scale production of hydrogen and carbon black with zero CO₂ emissions, however careful control of the reactor is required to ensure product particles of specific sizes. A one-dimensional model employing a sectional method is developed to simulate the evolution of polydisperse fresh and seed particle populations in an indirectly heated solar reactor. The model accounts for the homogeneous nucleation of fresh particles, the heterogeneous growth of the fresh and seed particles, particle coagulation, and the growth of carbon on the walls of the reactor from heterogeneous reaction and particle deposition. The heat transport mechanisms modelled include wall-gas convection, wall-particle radiation exchange, particle-gas convection and heat release from chemical reaction. The model is validated in terms of methane conversion against a 10 kW experimental solar reactor and used to extract kinetic parameters for the homogeneous and heterogeneous reaction paths. The model shows promise as a quick and simple tool for the design and control of industrial scale solar reactors.