Production of hydrogen and carbon by solar thermal methane splitting. II. Room temperature simulation tests of seeded solar reactor

In the design of a solar thermal methane splitting reactor seeded with powder particles, care must be taken to prevent destruction of the reactor window by contact with incandescent solid particles. The method of screening the window by application of the tornado flow configuration (Int. J. Hydrogen Energy 28 (11) (2003) 1187) inside the reactor enclosure was studied experimentally by reactor flow simulation tests at room temperature. A secondary effect of the tornado flow pattern was identified during these tests. Friction between the swift jet of gas leaving the reactor enclosure in the axial direction and the surrounding relatively quiescent gas engenders a weak stream of gas along torroidal streamlines. Solid particles entrained by this secondary stream are carried towards the reactor window. An efficient gasdynamic method is described by which the above-mentioned negative effect is counteracted and eliminated. A solar reactor model of a special design was developed in order to enable demonstration of the effectiveness of the proposed method. Carbon black powder seeding tests were performed with this reactor model. Powder seeding conditions under which the reactor window is perfectly screened were determined by experiment.     

Co-production of hydrogen and carbon black from solar thermal methane splitting in a tubular reactor prototype

This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen and carbon black (CB) as a high-value nano-material with the bonus of zero CO₂ emission. The work focused on the development of a medium-scale solar reactor (10 kW) based on the indirect heating concept. The solar reactor is composed of a cubic cavity receiver (20 cm-side), which absorbs concentrated solar irradiation through a quartz window by a 9 cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results in the temperature range 1740-2070 K are presented: acetylene (C₂H₂) was the most important by-product with a mole fraction of up to about 7%, depending on the gas residence time. C₂H₂ content in the off-gas affects drastically the carbon yield of the process. The effects of temperature and residence time are analyzed. A preliminary process study concerning a 55 MW solar chemical plant is proposed on the basis of a process flow sheet. Results show that 1.7 t/h of hydrogen and 5 t/h of CB could be produced with an hydrogen cost competitive to conventional steam methane reforming.     

Solar hydrogen production from the thermal splitting of methane in a high temperature solar chemical reactor

This study addresses the single-step thermal decomposition (pyrolysis) of methane without catalysts. The process co-produces hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. It is an unconventional route for potentially cost effective hydrogen production from solar energy without emitting carbon dioxide since solid carbon is sequestered.A high temperature solar chemical reactor has been designed to study the thermal splitting of methane for hydrogen generation. It features a nozzle-type graphite receiver which absorbs the solar power and transfers the heat to the flow of reactant at a temperature that allows dissociation. Theoretical and experimental investigations have been performed to study the performances of the solar reactor. The experimental set-up and effect of operating conditions are described in this paper. In addition, simulation results are presented to interpret the experimental results and to improve the solar reactor concept. The temperature, geometry of the graphite nozzle, gas flow rates, and CH₄ mole fraction have a strong effect on the final chemical conversion of methane. Numerical simulations have shown that a simple tubular receiver is not enough efficient to heat the bulk gas in the central zone, thus limiting the chemical conversion. In that case, the reaction takes place only within a thin region located near the hot graphite wall. The maximum CH₄ conversion (98%) was obtained with an improved nozzle, which allows a more efficient gas heating due to its higher heat exchange area.     

A pilot-scale solar reactor for the production of hydrogen and carbon black from methane splitting

A pilot-scale solar reactor was designed and operated at the 1 MW solar furnace of CNRS for H₂ and carbon black production from methane splitting. This constitutes the final objective of the SOLHYCARB EC project. The reaction of CH₄ dissociation produces H₂ and carbon nanoparticles without CO₂ emissions and with a solar upgrade of 8% of the high heating value of the products. The reactor was composed of 7 tubular reaction zones and of a graphite cavity-type solar receiver behaving as a black-body cavity. Temperature measurements around the cavity showed a homogeneous temperature distribution. The influence of temperature (1608K–1928K) and residence time (37–71 ms) on methane conversion, hydrogen yield, and carbon yield was especially stressed. For 900 g/h of CH₄ injected (50% molar, the rest being argon) at 1800K, this reactor produced 200 g/h H₂ (88% H₂ yield), 330 g/h CB (49% C yield) and 340 g/h C₂H₂ with a thermal efficiency of 15%. C₂H₂ was the most important by-product and its amount decreased by increasing the residence time. A 2D thermal model of the reactor was developed. It showed that the design of the reactor front face could be drastically improved to lower thermal losses. The optimised design could reach 77% of the ideal black-body absorption efficiency (86% at 1800K), i.e. 66%.     

Production of hydrogen and carbon by solar thermal methane splitting. IV. Preliminary simulation of a confined tornado flow configuration by computational fluid dynamics

The confined tornado flow configuration has been developed at the Solar Research Facilities Unit, Weizmann Institute of Science, as a means for protection of the window of a solar reactor from contact with incandescent solid particles in gas suspension in the reactor cavity.

The results of a computational fluid dynamics (CFD) simulation of a tornado flow confined in a simplified reaction chamber are compared in this paper with information about such a flow obtained by gas dynamics experimentation. All the information obtained by experiment was corroborated by CFD. Moreover, the CFD simulation brought to view some important unexpected features of the confined tornado flow, which are discussed in detail.     

Effect of seeding on hydrogen and carbon particle production in a 10 MW solar thermal reactor for methane decomposition

Solar methane decomposition reactors are a novel technology for the production of carbon neutral hydrogen; however, the impact of this technology depends greatly on the ability to co-produce carbon black particles of commercial grade in order to offset the cost of hydrogen production and, therefore, the control of the reactor is very important. To this end, the seeding of indirect heating concept reactors using the product particles themselves could be used to control heat transfer inside the reactor. In this work, a previously developed one-dimensional reactor – particle population model was used to simulate the effect of seeding on the hydrogen and carbon particle production rates in the absorber tubes of a 10 MW indirect heating concept solar reactor. It was found that seed particle feed rates less than 10% of the methane-contained carbon feed rate allowed the hydrogen and fresh particle production rates to be doubled while keeping the rate of carbon growth on the tube walls constant. It was also found that similar seed fee rates could be used to maintain the hydrogen and particle production rates constant, given variations in the absorber tube wall temperature within a 100 °C range, for example due to cloud passage. Furthermore, it was found that the size characteristics of the freshly produced particles were not affected at these seed feed rates. Thus, seeding could be an effective means for increasing and controlling the hydrogen and carbon particle production rates in industrial scale indirect heating concept solar methane decomposition reactors, while also reducing carbon growth on the walls of the absorber tubes.     

Solar thermal cracking of methane in a particle-flow reactor for the co-production of hydrogen and carbon

An experimental investigation on the thermal decomposition of CH₄ into C and H₂ was carried out using a 5 kW particle-flow solar chemical reactor tested in a solar furnace in the 1300–1600 K range. The reactor features a continuous flow of CH₄ laden with μm-sized carbon black particles, confined to a cavity receiver and directly exposed to concentrated solar irradiation of up to 1720 suns. The reactor performance was examined for varying operational parameters, namely the solar power input, seed particle volume fraction, gas volume flow rate, and CH₄ molar concentration. Methane conversion and hydrogen yield exceeding 95% were obtained at residence times of less than 2.0 s. A solar-to-chemical energy conversion efficiency of 16% was experimentally reached, and a maximum value of 31% was numerically predicted for a pure methane flow. SEM images revealed the formation filamentous agglomerations on the surface of the seed particles, reducing their active specific surface area.     

Production of hydrogen from methane decomposition using nanosized carbon black as catalyst in a fluidized-bed reactor

Nanosized carbon black (NCB) was employed as catalyst for methane decomposition to produce hydrogen in a fluidized-bed reactor. The carbon atoms of the surface defects of NCB act as active sites in this reaction. The activity of NCB is improved after more defects in the surface of NCB are generated after the treatment in nitric acid and calcination in nitrogen gas. The loading of small amounts of Ni and Co can obviously increase the initial activity of NCB, however, their activity deceases very quickly after the reaction begins due to the encapsulation of the corresponding metal particles inside amorphous carbon produced from methane decomposition. After reaction, the formed carbon was found to grow into carbon flakes and cover the surface of NCB. The investigation with TEM and SEM indicates that they may form from a new carbon crystallite, not build upon the existing hexagon layer in the surface defects of NCB.     

Dehydrogenation characteristics of lean methane in a thermal reverse-flow reactor

In order to study the dehydrogenation reaction mechanism of ultra-low concentration methane in a thermal reverse-flow reactor, the effects of the cyclic period (120s–240s), the lean methane volume flow (90 Nm³/h to 180 Nm³/h), and the methane concentration (0.2 vol% to 0.8 vol%) on the dehydrogenation performance were studied systematically by using a thermal reverse-flow experimental system. When the methane concentration is 0.2 vol%, the reactor can achieve self-heat maintaining operation. With the increase in the methane concentration, the width of the high-temperature zone, the exhaust gas temperature, the methane conversion rate, and the maximum temperature of the heat-accumulator bed increase. With the increase in the lean methane volume flow, the width of the high-temperature zone, the distance between the center of the high-temperature zone and the center of the reactor, the maximum temperature, the exhaust gas temperature, and the methane conversion rate increase. With the increase in the cyclic period, the exhaust gas temperature and the deviation of the high-temperature zone increase, but the methane conversion rate and the maximum temperature decrease slightly.     

Numerical simulation of a tubular solar reactor for methane cracking

This study addresses the solar thermal cracking of methane for the co-production of hydrogen and carbon black as a medium to avoid CO₂ emissions from natural gas combustion processes. The objective of this work is to numerically simulate the transport processes of momentum heat and mass in an indirect heating solar reactor, which is fed with an argon-methane mixture. The reactor is composed of a cubic cavity receiver, which absorbs concentrated solar irradiation through a quartz window and a graphite reaction tube is settled vertically inside this cavity. A series of numerical experiments were carried out in order to gain a better understanding of the interaction between the several transport phenomena taking place. The simulations showed that, in general, when the temperature of the reaction chamber is higher than 2000 K, the methane conversion is practically 100%. To validate our simulation results we compared them with available experimental data obtaining good agreement. Moreover, our results clearly evidence that most of the reaction takes place at the bottom of the reactor, which is the zone with the highest temperature profiles. Therefore, we propose modifications in the reactor design to increase conversion. The results of this work can thus serve to improve design and control of solar reactors for light hydrocarbons.     

Experimental and numerical analysis of a methane thermal decomposition reactor

An indirectly heated tubular reactor is fabricated and used to study methane thermal decomposition conversion and determine kinetic parameters. A combined perfectly mixed reactor with bypass (CPMR) is proposed as an alternative to the traditional perfectly mixed and plug flow reactors. The CPMR model is used in order to account for buoyancy flow in the reactor. Results comparing the numerical predictions from all three models to experimental data show that buoyancy effects are significant in the reactor under study and also in most reactors in the literature. Including this effect might significantly improve the accuracy of the model predictions. The CPMR reactor model with a reaction rate constant of 5.43 × 10¹⁵ 1/s and an activation energy of 420.7 kJ/mol is capable of reproducing the obtained experimental data in this study and in the literature.     

Characterization of a thermochemical reactor for thermal solar energy

The main purpose of this work is to elucidate the thermochemical characteristics of a fluidized bed reactor to be used as a solar reactor in thermal energy storage. Zinc sulfate dissociation was studied over the temperature range from 973 to 1123 K. During the reaction problems such as non isothermisity of the bed and pressure drop changes with the reaction, were detected. It was shown that the fluidity increased with temperature and degree of dissociation, but the pressure drop amplitude increased exponentially with gas velocity and particle size when slugging is present in the bed.     

Heat transfer model and scale-up of an entrained-flow solar reactor for the thermal decomposition of methane

The solar thermochemical decomposition of CH₄ is carried out in a solar reactor consisting of a cavity-receiver containing an array of tubular absorbers, through which CH₄ flows and thermally decomposes to H₂ and carbon particles. A reactor model is formulated by coupling radiation/convection/conduction heat transfer and chemical kinetics for a two-phase solid-gas reacting flow. Experimental validation is accomplished by comparing measured and simulated absorber temperatures and H₂ concentrations for a 10 kW prototype reactor tested in a solar furnace. The model is applied to optimize the design and simulate the performance of a 10 MW commercial-scale reactor mounted on a solar tower system configuration. Complete conversion is predicted for a maximum CH₄ mass flow rate of 0.70 kg s⁻¹ and a desired outlet temperature of 1870 K, yielding a solar-to-chemical energy conversion efficiency of 42% and a solar-to-thermal energy conversion efficiency of 75%.     

Kinetic modeling of hydrogen production by thermal decomposition of methane

A kinetic model for the thermal decomposition of methane was developed by modifying a model for soot formation in combustion. The modifications consisted of adding five reactions representing surface chemistry on carbon particles. One kinetic parameter for the new surface reactions was fit to experimental data and the other parameters were determined by analogy to known gas-phase reactions. The modified model reproduces well the hydrogen concentrations at the beginning and end of recent experiments on the thermal decomposition of methane in the presence of carbon catalysts. The model predicts that increasing the pressure from 0.1 to 3.0 MPa will decrease hydrogen production with carbon catalysts by 48–60%. This prediction is robust to variations in the rate constants for the surface reactions and the rate constant for a key gas-phase reaction.     

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%.     

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.     

Impedance characterization of dye-sensitized solar cells in a tandem arrangement for hydrogen production by water splitting

The present work reports the photoelectrochemical characterization of a dye-sensitized solar cell (DSC) to assist water split in a photoelectrochemical (PEC) cell. Performance parameters were extracted from standard current–voltage characteristic (I–V) and the charge transfer phenomena occurring at different interfaces of the DSC were evaluated by electrochemical impedance spectroscopy (EIS). The DSC comprised the N719 dye and a robust electrolyte (1-propyl-3-methylimidazolium iodide in guanidinium thiocyanate additive). At 1 sun illumination the DSC yielded a short-circuit photocurrent density of 14.9 mA cm⁻², an open-circuit voltage of 0.797 V, a fill factor of 0.712 and an overall efficiency of 8.5%. Different PEC systems based on silicon-doped and undoped hematite photoelectrodes were considered. The required additional anodic bias necessary for actual water cleavage was supplied by two DSCs in series operating just under open-circuit voltage (1.56 V), allowing a conversion efficiency of about 1.12% for the silicon-doped hematite deposited by APCVD, 0.51% for the silicon-doped prepared by USP and 0.12% for the undoped hematite sample.