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.     

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.     

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.     

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.     

Step-by-step methodology of developing a solar reactor for emission-free generation of hydrogen

This study presents a methodology to develop a solar reactor based on the thermodynamics and kinetics of methane decomposition to produce hydrogen with no emissions. The kinetic parameters were obtained in the literature for two cases; methane laden with carbon particles and methane without carbon particles. Results show that there is significant difference in experimentally obtained and theoretically predicted methane conversion. The paper also presents a parametric study on the effects of temperature, pressure and the influence of inert gas composition, which is fed along with methane, on the thermodynamics of methane decomposition. Results show that there is significant effect of the inert gas presence in the feeding gas mixture on the equilibrium of methane conversion and product gas composition. Results also show that higher conversions are obtained when the carbon particles laden with methane. The step-by-step reactor design methodology for homogenous methane decomposition and the parametric study results presented in this paper can provide a very useful tool in guiding a solar reactor design and optimization of process operating conditions.     

Decomposition of limestone in a solar reactor

The thermal decomposition of limestone has been selected as a model reaction for developing and testing an atmospheric open solar reactor. The reactor consists of a cyclone gas/particle separator which has been modified to let the concentrated solar energy enter through a windowless aperture. The reacting particles are directly exposed to the solar irradiation. Experimentation with a 60 kW reactor prototype was conducted at PSI's 90m² parabolic solar concentrator, in a continuous mode of operation. A counter-current flow heat exchanger was employed to preheat the reactants. Eighty five percent degree of calcination was obtained for cement raw material and 15% of the solar input was converted into chemical energy (enthalpy).The technical feasibility of the solar thermal decomposition of limestone was experimentally demonstrated. The use of solar energy as a source for high-temperature process heat offers the potential of reducing significantly the CO₂ emissions from lime producing plants. Such a solar thermochemical process can find application in sunny rural areas for avoiding deforestation.     

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.     

Performance improvement of a solar volumetric reactor with passive thermal management under different solar radiation conditions

To alleviate the effect of solar radiation fluctuation on the solar volumetric reactor, phase change material (PCM) is applied to buffer the temperature vibration and improve the stability of thermochemical reactions. In this work, we analyzed the heat flow and distribution characteristics of the conventional double-walled volumetric reactor filled with PCMs (SVR1). We then proposed a novel solar volumetric reactor design (SVR2) to solve the problems of local high temperature, slow charging-discharging rate, and fluctuating methane conversion in various radiation conditions. The heat and mass transfer model coupled with thermochemical reaction kinetics was established to compare the performance of SVR1 and SVR2 under steady state, heat charging-discharging mode, and actual solar radiation fluctuation, respectively. The results show that compared to SVR1, the maximum temperature of SVR2 decreases by 106.3 K, and the minimum methane conversion rate increases from 77.4% to 93.6% under natural solar radiation fluctuation.     

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%.     

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.     

Design,simulation and experimental study of a directly-irradiated solar chemical reactor for hydrogen and syngas production from continuous solar-driven wood biomass gasification

The use of concentrated solar energy as the high-temperature heat source for the thermochemical gasification of biomass is a promising prospect for producing CO₂-neutral chemical fuels (syngas). The solar process saves biomass resource because partial combustion of the feedstock is avoided, it increases the energy conversion efficiency because the calorific value of the feedstock is upgraded by the solar power input, and it also reduces the need for downstream gas cleaning and separation because the gas products are not contaminated by combustion by-products. A new concept of solar spouted bed reactor with continuous biomass injection was designed in order to enhance heat transfer in the reactor, to improve the gasification rates and gas yields by providing constant stirring of the particles, and to enable continuous operation. Thermal simulations of the prototype were performed to calculate temperature distributions and validate the reactor design at 1.5 kW scale. The reliable operation of the solar reactor based on this new design was also experimentally demonstrated under real solar irradiation using a parabolic dish concentrator. Wood particles were continuously gasified at temperatures ranging from 1100 °C to 1300 °C using either CO₂ or steam as oxidizing agent. Carbon conversion rates over 94% and gas productions over 70 mmol/g_biomass were achieved. The energy contained in the biomass was upgraded thanks to the solar energy input by a factor of up to 1.21.     

A directly irradiated solar reactor for kinetic analysis of non‐volatile metal oxides reductions

A directly irradiated solar reactor was designed and built to develop kinetic analysis of metal oxides reductions present in many high‐temperature thermochemical processes. The reactions were monitored by measuring the oxygen concentration in a carrier gas stream. The pattern flow in the reactor cavity was determined by applying the dispersion law. The dimensionless group D/uL was calculated for flows of 9, 12, and 15 Nl/min, and room and operation temperature. It was found to be close to an ideal plug flow behavior in every case. The solar reactor was also thermally characterized, which involved the operation temperature measurement and the thermal efficiency obtaining. For an incoming power of 530 W, temperatures higher than 1400 °C were measured at the middle of the cavity (where the sample should be placed), and thermal efficiency of 41.6% was calculated. Then, a methodology for kinetic analysis was proposed and applied to a case study. It consisted of a combination of experimental results and a numerical model that reproduced the reactant sample performance. Kinetic was obtained for the layer of reactive particles that were directly heated by concentrated radiation. Kinetic data were fitted to diffusion‐controlled mechanism, and obtained kinetic parameters were E_a = 362 kJ/mol and A = 1.39?10⁹/s. Copyright © 2015 John Wiley & Sons, Ltd.     

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%.     

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.     

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.     

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.     

Optimizing fullerene synthesis in a 50 kW solar reactor

Fullerene mixture of C₆₀ and C₇₀ was produced in gram quantities in a solar reactor using partially (50 kW) the power of the 1 MW CNRS solar furnace at Odeillo. Fullerene yield was studied as a function of buffer gas (helium and argon), pressure (in the range 80–500 hPa) and gas flow rate. Mean Fullerene yield of 13.5% was measured with helium at 450 hPa and 10 sm³/h, at a carbon vaporization rate of 21 g/h. This mean value refers to three samples collected inside the experimental set up; the maximum yield for one single sample (inside the reactor) was 16%. The dramatic effect of pressure and gas flow rate on the process selectivity is correlated to the dilution number (number of carbon atoms versus number of buffer gas atoms) and to the results of a numerical simulation related to the temperature distribution in the annealing zone of the reactor.     

A novel reactor for large-area epitaxial solar cell materials

A novel vertical stagnation flow organometallic vapor phase epitaxy reactor was designed and fabricated for the growth of GaAs and AlGaAs for solar cell applications. The reactor had an inverted configuration to eliminate recirculation problems. The susceptor and gas inlet nozzle were closely spaced (about 1 cm) in order to achieve improvements in deposition efficiency, layer uniformity and abruptness of interfaces. A specially designed water-cooled inlet nozzle was used to maintain the nozzle surface at relatively low temperatures under all operating conditions. A computer model was formulated to study the various thermal processes in this reactor. The model used rigorous thermal boundary conditions which included thermal radiation effects. Simulated and experimental nozzle temperatures were compared for different susceptor temperatures, susceptor-nozzle distances, gas flow rates and reactor pressures. The maximum nozzle temperature was about 100 °C, which is sufficiently low to prevent premature decomposition of the reactants on its surface.     

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.