Solar thermochemical gasification of wood biomass for syngas production in a high-temperature continuously-fed tubular reactor

Biomass gasification is an attractive process to produce high-value syngas. Utilization of concentrated solar energy as the heat source for driving reactions increases the energy conversion efficiency, saves biomass resource, and eliminates the needs for gas cleaning and separation. A high-temperature tubular solar reactor combining drop tube and packed bed concepts was used for continuous solar-driven gasification of biomass. This 1 kW reactor was experimentally tested with biomass feeding under real solar irradiation conditions at the focus of a 2 m-diameter parabolic solar concentrator. Experiments were conducted at temperatures ranging from 1000 °C to 1400 °C using wood composed of a mix of pine and spruce (bark included) as biomass feedstock. This biomass was used under its non-altered pristine form but also dried or torrefied. The aim of this study was to demonstrate the feasibility of syngas production in this reactor concept and to prove the reliability of continuous biomass gasification processing using solar energy. The study first consisted of a parametric study of the gasification conditions to obtain an optimal gas yield. The influence of temperature, oxidizing agent (H₂O or CO₂) or type of biomass feedstock on the product gas composition was investigated. The study then focused on solar gasification during continuous biomass particle injection for demonstrating the feasibility of a continuous process. Regarding the energy conversion efficiency of the lab scale reactor, energy upgrade factor of 1.21 and solar-to-fuel thermochemical efficiency up to 28% were achieved using wood heated up to 1400 °C.     

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.     

Hydrogen adsorption on graphene sheets and nonporous graphitized thermal carbon black at low surface coverage

Behavior of hydrogen adsorption on nonporous carbon based materials was comparatively studied for selection of an efficient carrier for catalytic metals. Graphene sheets (GS) and graphitized thermal carbon black, which respectively has a specific surface area about 220 m²/g and 36 m²/g, were selected for adsorption equilibrium testes within temperature–pressure range from 77 K–87 K and 0–1 kPa. Henry law constants were employed to calculate the second virial constants and the limit isosteric heat of adsorption. The Weeks, Chandler and Andersen (WCA) perturbing scheme and the fundamental measure theory (FMT) were used to determine the interaction energy between solid atoms and hydrogen molecules. Adsorption potential well was determined by linear interpolation based on the Boltzmann distribution approximation. It shows that the potential well between hydrogen molecules and the GS, BP280 is respectively about 33.55 K and 31.97 K, suggesting that the bonding energy between the GS and hydrogen molecules is larger than that on carbon black.     

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.     

Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles

This paper presents a process analysis of ZnO/Zn, Fe₃O₄/FeO and Fe₂O₃/Fe₃O₄ thermochemical cycles as potential high efficiency, large scale and environmentally attractive routes to produce hydrogen by concentrated solar energy. Mass and energy balances allowed estimation of the efficiency of solar thermal energy to hydrogen conversion for current process data, accounting for chemical conversion limitations. Then, the process was optimized by taking into account possible improvements in chemical conversion and heat recoveries. Coupling of the thermochemical process with a solar tower plant providing concentrated solar energy was considered to scale up the system. An economic assessment gave a hydrogen production cost of 7.98$ kg⁻¹ and 14.75$ kg⁻¹ of H₂ for, respectively a 55 MW_th and 11 MW_th solar tower plant operating 40 years.     

Production of hydrogen and nano carbon powders from direct plasmalysis of methane

In this study, a single-stage, non-catalytic, dry methane plasmalysis process for producing mainly hydrogen and carbon powders using an atmospheric-pressure microwave plasma reactor is demonstrated. A high selectivity of H₂ and carbon powders (C₁), and a low required energy consumption of H₂ could be obtained simultaneously at a low-applied power and a high inlet concentration of CH₄ ([CH₄]_in), reaching 86.0% (selectivity of H₂), 50.7% (selectivity of C₁), and 6.7 eV/molecule–H₂, respectively, at 0.8 kW, [CH₄]_in = 20%, and 12 slpm when CH₄ gas was fed from the downstream of the cavity resonator. In addition, large amounts of nano carbon powders, which consisted of C atoms and had a graphite–rhombohedral structure with a particle size of about 50 nm, were produced. These particles had potential for using as a support for platinum catalysts in fuel cells.     

Atomic layer deposited thin film metal oxides for fuel production in a solar cavity reactor

Alumina thin film structures were produced by coating high surface area polymer particles via atomic layer deposition (ALD), using the polymer as a sacrificial template. Burnout of the polymer material left high surface area, high pore volume structures, with 15 nm wall thickness. Further deposition of up to 27 mol% Co and Fe was performed via ALD to produce high surface area CoFe₂O₄ particles for thermochemical water splitting. The ALD particles were thermally cycled in electrically heated lab reactors and on-sun using a concentrated solar, reflective cavity reactor. Surface area measurements of cycled ALD particles showed improved surface area retention as compared to bulk Fe₂O₃ nanopowders. Reaction rates as high as 15.2 and 9.8 μmol/s/g were observed, on-sun, for H₂O and CO₂ splitting respectively. Thermochemical cycling in a concentrated solar cavity reactor showed an order of magnitude increase in solar utilization efficiency between ALD particles and bulk Fe₂O₃ nanopowders.     

A technical note on application of internally finned tubes in solar parabolic trough absorber pipes

The heterogeneous incoming heat flux in solar parabolic trough absorber tubes generates huge temperature difference in each pipe section. Helical internal fins can reduce this effect, homogenising the temperature profile and reducing thermal stress with the drawback of increasing pressure drop. Another effect is the decreasing of the outer surface temperature and thermal losses, improving the thermal efficiency of the collector. The application of internal finned tubes for the design of parabolic trough collectors is analysed with computational fluid dynamics tools. Our numerical approach has been qualified with the computational estimation of reported experimental data regarding phenomena involved in finned tube applications and solar irradiation of parabolic trough collector. The application of finned tubes to the design of parabolic trough collectors must take into account issues as the pressure losses, thermal losses and thermo-mechanical stress, and thermal fatigue. Our analysis shows an improvement potential in parabolic trough solar plants efficiency by the application of internal finned tubes.     

Solar-powered hydrogen refuelling station for unmanned aerial vehicles: Design and initial AC test results

Fuel cell technology can offer environmental benefits (low noise and emissions) and also a competitive advantage over conventional power sources (better performance, low thermal signature, less vibration issues, etc) in small manned and unmanned electric air vehicles (UAVs). To develop an environmentally acceptable solution, the hydrogen fuel source must be produced on-site from renewable energy sources. This paper describes the development and testing of a fully operational small-scale demonstrator to generate and supply hydrogen for 2 to 3 daily fuel cell-powered UAV operations. The purity of the hydrogen delivered to the air platforms is ≥99.99%.     

NiAl coatings on carbon steel by self-propagating high-temperature synthesis assisted with concentrated solar energy: mass influence on adherence and porosity

NiAl have been produced by a self-propagating high-temperature synthesis (SHS). The power source that ignite the SHS reaction is concentrated solar energy. NiAl coatings are obtained in few seconds and the processes are economic and environment friendly. Three different NiAl mass are tested: 0.3; 0.6 y 1.7 g. Coating porosity and adherence to substrate depends on the NiAl mass. Pores are large in samples with 1.7 g while the other specimens have small pores. Coating adherence is better when the amount of reactive powder is larger.