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 by thermal methane splitting in a nozzle-type laboratory-scale solar reactor

A high-temperature solar reactor has been developed for co-producing hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. The approach is based on a single-step thermal decomposition (pyrolysis) of methane without catalysts and without emitting carbon dioxide since solid carbon is sequestered.In the tested reactor, a graphite nozzle absorbs concentrated solar radiation provided by a solar furnace. The heat is then transferred to the reactive flow. The experimental setup, first test results, and effect of operating conditions are described in this paper.The conversion of methane was strongly dependant on the solar furnace power input, on the geometry of the graphite nozzle, on gas flow rates, and on the ratio of inert gas-to-reactive gas. CB was recovered in the carbon trap, and maximum chemical conversion of methane-to-hydrogen and CB was 95%, but typical conversion was in the range 30–90%.     

Particle–gas reacting flow under concentrated solar irradiation

A transient heat transfer model is developed for a reacting flow of CH₄ laden with carbon particles directly exposed to concentrated solar radiation and undergoing thermal decomposition into carbon and hydrogen. The unsteady mass and energy conservation equations, coupling convective heat and mass transfer, radiative heat transfer, and chemical kinetics for a two-phase solid–gas flow, are formulated and solved numerically for both phases by Monte Carlo and finite volume methods using the explicit Euler time integration scheme. Parametric study is performed with respect to the initial particle diameter, volume fraction, gas composition, and velocity. Validation is accomplished by comparing temperatures and reaction extent with those measured experimentally using a particle-flow solar reactor prototype subjected to concentrated solar radiation. Smaller particles and/or high volume fractions increase the optical thickness of the medium, its radiative absorption and extinction coefficients, and lead to higher steady-state temperatures, reaction rates, and consequently, higher extent of chemical conversion.     

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.     

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.     

Enhancing efficiency of hydrocarbons to synthesis gas conversion in a counterflow moving bed filtration combustion reactor

An improvement is considered for the partial oxidation conversion of hydrocarbon gases to synthesis gas in a continuous non-premixed filtration combustion reactor with inert solid granular material flowing countercurrently to the gas flow. The reactor is supplemented with an additional heat exchanger, wherein the second reactant gas is preheated prior to supply to the middle part of the reactor. The composition of the gaseous products self-consistent with the temperature of combustion are assessed using approximation of established thermodynamic equilibrium in the products. The parametric domain for major control parameters, namely oxygen-to-fuel supply ratio, granular solid flowrate, and steam supply rate providing highly efficient conversion is determined. Calculations for the POX conversion of methane and a model biogas composition (50% methane, 40% carbon dioxide, 10% nitrogen) with air and steam are provided as examples. The calculations show that the process gives a possibility to substantially improve energy efficiency and provides a flexibility to control hydrogen yield through steam supply. The process provides a high chemical efficiency of conversion even with air used as an oxidant for conversion of low-caloric gases.     

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%.     

Heat transfer analysis of thermal disassociation of ZnO for solar hydrogen production

Hydrogen production by the two-step solar thermochemical cycle has high cycle efficiency, low cost, and a great development space. Of special interest is the solar thermochemical cycle based on ZnO/Zn redox reactions since its high theoretical hydrogen yield and relatively low endothermic reaction temperature. In this paper, a steady heat transfer model for thermal ZnO dissociation in a solar thermochemical reactor is developed, coupling conduction, convection and radiation with chemical reaction. Accuracy was evaluated by comparison of results obtained from other references. Based on the new proposed reactor, the model is adopted to analyze the operating parameter effect on the conversion rate and fluid feature inside the solar reactor. The results show that the mass flow rate of ZnO and aperture gas temperature have a positive relation with ZnO conversion rate, however, the diameter of particles and aperture gas velocity has an inverse relation with ZnO conversion rate under specific condition. The results will provide useful foundation for improving the solar-to-fuel conversion rate in the near future.     

Lagrangian characterization of multi-phase turbulent flow in a solar reactor for particle deposition prediction

Solar cracking of methane is a promising technology for emission free hydrogen production. One of the major problems affecting methane cracking solar reactors' performance is the carbon particle deposition on the window, walls, and at the exit. In present study, a Lagrangian particle dispersion model has been implemented for predicting the particle deposition on the window of a seeded solar thermal reactor. A three-dimensional Computational Fluid Dynamics (CFD) analysis using Discrete Phase Model (DPM) has been done for qualitative validation of the experimental observations. In order to evaluate the turbulent quantities in the solar reactor; RNG k–? model has been applied. Species transport has been solved by taking the gas for window screening as different from that used in the main flow. In addition, this paper presents a thorough parametric study predicting the particle deposition on reactor window for various flow configurations and flow conditions, which can be summarized as; (1) when the inlet flow angle is smaller, higher tangential velocities or swirl strength is obtained, (2) higher tangential velocities help in maintaining a stronger swirl, which keeps the screening flow close to the reactor window, (3) by increasing the main flow and the screening flow rates, the particle deposition on window is reduced, (4) when a lower density fluid is used as window screening gas, the particle deposition is reduced because the Taylor instabilities are avoided. The CFD work and the findings presented in this paper would be used as a guide in designing a solar reactor or improving the configuration of existing reactor.     

Metallic and carbonaceous –based catalysts performance in the solar catalytic decomposition of methane for hydrogen and carbon production

Solar catalytic decomposition of methane (SCDM) was investigated in a solar furnace facility with different catalysts. The aim of this exploratory study was to investigate the potential of the catalytic methane decomposition approach providing the reaction heat via solar energy at different experimental conditions. All experiments conducted pointed out to the simultaneous production of a gas phase composed only by hydrogen and un-reacted methane with a solid product deposited into the catalyst particles varying upon the catalysts used: nanostructured carbons either in form of carbon nanofibers (CNF) or multi-walled carbon nanotubes (MWCNT) were obtained with the metallic catalyst whereas amorphous carbon was produced using a carbonaceous catalyst. The use of catalysts in the solar assisted methane decomposition present some advantages as compared to the high temperature non-catalytic solar methane decomposition route, mainly derived from the use of lower temperatures (600–950 °C): SCDM yields higher reaction rates, provides an enhancement in process efficiency, avoids the formation of other hydrocarbons (100% selectivity to H₂) and increases the quality of the carbonaceous product obtained, when compared to the non-catalytic route.     

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.     

Hydrogen production from solar thermal dissociation of natural gas: development of a 10 kW solar chemical reactor prototype

This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen, as well as Carbon Black as a high-value nano-material, with the bonus of zero CO₂ emissions. The work focused on the development of a medium-scale solar reactor (10 kW) based on the concept of indirect heating. The solar reactor is composed of a cubic cavity receiver (20 cm side), which absorbs concentrated solar irradiation through a quartz window via a 9 cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results were as follows: methane conversion and hydrogen yield of up to 98% and 90%, respectively, were achieved at 1770 K, and acetylene was the most important by-product, with a mole fraction up to about 5%. The effect of the methane mole fraction in the feed gas, the residence time and the temperature on the reaction extent was analyzed. In addition to the experimental section, thermal simulations were carried out. They showed a homogeneous temperature distribution inside the cavity receiver of the reactor and permit to draw up a thermal balance.     

Residence time distribution and flow field study of aero-shielded solar cyclone reactor for emission-free generation of hydrogen

This paper provides a thorough analysis on the flow field and Residence Time Distribution (RTD) of our “aero-shielded cyclone solar reactor” designed to generate hydrogen from solar thermal methane cracking process. The analysis has been carried out based on the results from flow dynamics, and residence time distribution by using Computational Fluid Dynamics (CFD). Kinetics is taken from the literature and the reactor volume is estimated based on a plug flow reactor assumption. Residence time distribution characteristics are obtained by gas tracer injection method, and particle tracking method. Based on the results of our flow studies, “reactors in series model” is adopted to model the aero-shielded cyclone reactor. Path lines show that operating variables have significant effect on the flow behavior inside the reactor. Results show that thermo chemical properties of the gases have effect on the flow behavior which significantly affect the mean residence time in the reactor. Results also show that the residence time, spread of the tracer by variance, and the number of reactors in series are observed to be changed by change in the flow rate, type of screening gas, and methane mole fraction in the feed.     

Hydrogen production from thermo-catalytic decomposition of methane using carbon black catalysts in an indirectly-irradiated tubular packed-bed solar reactor

The solar thermo-catalytic decomposition of methane using carbon black catalysts for CO₂-free hydrogen production is studied in a packed-bed reactor. The indirectly-irradiated reactor is based on a cavity receiver and a tube-type absorber in which a given load of particle catalyst is injected during on-sun operation, while enabling multiple refilling for catalyst replacement. Concentrated solar power is used as an external radiative source for supplying the high temperature process heat and for driving the endothermic reaction. The indirect irradiation via the intermediate opaque tubular absorber results in a more uniform heating of the whole reacting bed volume and thus an easier reaction temperature control and determination. Carbon particles are used for enhancing the rate of the heterogeneous decomposition reaction and the coupling of the reactor with a particle injection system is implemented to operate in semi-continuous mode with possibility of catalyst load renewal after deactivation.     

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.     

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.     

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%.     

CFD analysis on the influence of helical carving in a vortex flow solar reactor

Solar thermal cracking of natural gas is a promising technology, which has attracted researchers in recent years for its potential to lead to the development of CO₂ free hydrogen production process. However, experimental access to the reaction chamber of solar cracking reactors is a challenge due to the high temperature process as the instruments capable of measuring fluid flow cannot survive the medium inside the reactor. However, computational fluid dynamics (CFD) can provide an insight into the flow, where experimental access is limited or not possible. This paper presents a CFD analysis for directly irradiated solar thermochemical reactor to characterize the influence of flow behavior on the heat transfer and solar cracking process. The heat transfer by radiation from carbon particles is considered by providing global absorption and scattering coefficients in the computational domain obtained from Mie code. The flow field is based on RNG k–? model derived using renormalization group theory. This technique accounts for the effect of swirl on turbulence thereby enhancing accuracy for the swirl flows. Validation of the numerical results is carried out by making a comparison with the experimental results. Highlighting the effects of carving on the solar reactor walls, this study presents numerical analyses of solar reactor geometry for two cases; namely, when there is no vortex forming carving in the cavity, and when there is vortex forming helical carving. The results show that carving has significant influence on the flow behavior, however, it has very little effect on the outlet temperature. The numerical results also show that the radiative heat transfer mechanism is the dominant means of heat transfer compared to the effects of conduction and convection.     

Sensitivity analysis of the rapid decomposition of methane in an aerosol flow reactor

A one-dimensional, nonisothermal model is developed to describe the thermal dissociation of methane to hydrogen and carbon black occurring in a fluid-wall aerosol flow reactor. The model expressions are scaled and nondimensionalized to determine the minimum parametric representation of the system. The sensitivity of this thermal dissociation to three parameters (flow rate of carbon particles fed to initiate the reaction, carbon particle radius, and reactor wall temperature) is studied. The results of the study indicate that in order to achieve nearly complete conversion, high reactor wall temperatures must be maintained. In addition, micron-sized carbon black particles must be fed into the reactor to enhance the heat transfer to the gas phase. The actual flow rate of particles fed is not critical, as long as some flow rate of fine particles is maintained.     

Numerical analysis of the biomimetic leaf-type hierarchical porous structure to improve the energy storage efficiency of solar driven steam methane reforming

Due the energy resource comes from solar energy, resulting in a high working temperature, radiation field has a significant influence on the energy storage efficiency of the high temperature solar thermochemistry. In order to promote the solar energy conversion efficiency of solar driven steam methane reforming (SMR), the idea of regulate the radiation field to be in accordance with the energy conversion on-demand is proposed and the biomimetic leaf-type hierarchical porous structure solar thermochemical reactor is introduced, which can regulate the spatial distribution of solar radiation intensity and optimize the temperature field. Combined with thermochemical kinetics and Finite Volume Method (FVM), the numerical calculation model of the SMR reaction in a biomimetic solar thermochemical reactor is established to optimize the temperature field. The effects of different reaction conditions and reactor parameters on steam methane reforming hydrogen production are analyzed. The results show that methane conversion in the biomimetic leaf-type solar thermochemical reactor is increased by 4.5%.