High-temperature solar chemistry for converting solar heat to chemical fuels

This paper reviews the recent developments on thermochemical conversion of concentrated solar high temperature heat to chemical fuels. The conversion has the advantage of producing long term storable energy carriers from solar energy. This conversion also enables solar energy transportation from the sunbelt to remote population centers. The thermochemical pathway is characterized by a theoretical high efficiency. However, there are solar peculiarities in comparison to conventional thermochemical processes—high thermal flux density and frequent thermal transients because of the fluctuating insolation—, and conventional industrial thermochemical processes are generally not suitable for solar driven processes. Therefore, the adaptation to such peculiarities of solar thermochemical processes has been the important R&D task in this research field. Thermochemical water splitting, steam or CO₂ gasification of coal, steam or CO₂ reforming of methane, and hydrogenetive coupling of methane, are industrially important, endothermic processes to produce useful chemical fuels such as hydrogen, synthesis gas and C₂-hydrocarbons, which have been examined as solar thermochemical processes. The technical developments and feasibilities to conduct these endothermic processes by utilizing concentrated solar radiation as the process heat are discussed here. My recent experimental results to improve the advanced solar thermochemical technologies are also given.     

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.     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

Mechanism of soot initiation in methane systems

Thermochemical and kinetic evaluations of the very rapid elementary radical reactions consuming the C₂H₂ produced in a chlorine catalyzed polymerization of CH₄ are presented. An earlier examination of the data and mechanism leading to C₂H₂ supports a methyl and chloromethyl recombination path to C₂ hydrocarbons. The relative yield of CH₃ and CH₂Cl depends on the excess of methane.

In the CH₄, system consumption of C₂ species to ultimately form benzene is shown to proceed by a stepwise addition of CH₃ radicals to C_nH_m species. When n is even the dominant species is an unsaturated polyolefin molecule. When n is odd the dominant species is a conjugated, unsaturated radical such as allyl, pentadienyl, benzyl, etc. Mono-olefins or saturated molecules are rapidly stripped to these species by radical catalyzed dehydrogenations. In the current system chloromethyl radicals are equivalent kinetically to methyl and play a dominant role. Their addition to unsaturated species produces chlorinated radicals that dechlorinate rapidly or recombine with chloromethyl to produce dichlorohydrocarbons that dehydrochlorinate very rapidly.

A very important reaction in the sequence is the isomerization of propenyl and chloropropenyl radicals to allyl and chlorallyl by a 1–3 H (or Cl) atom shift. Its high pressure Arrhenius parameters at 1300 K are estimated to be log [k(sec⁻¹)] = 13.7 − 37/θ = 13.7 - 37/0 where 0 = 2.303 RT in kcal/mol. It appears likely that benzene conversion to soot also proceeds via a CH₃/CH₂Cl radical, sequential addition mechanism.

Stoichiometry considerations applied to the product yield distribution support the role of methyl and chloromethyl predicted by the proposed mechanism. Ionic pathways are shown to be insignificant in the formation of aromatics.     

Thermodynamic calculation on energy densities of multi-electron transfer Mg/Al batteries

镁离子电池和铝离子电池因其高能量密度、地壳储量丰富、安全等优良特性有望成为下一代新型高能量密度储能体系,是未来二次电池研究的热点之一。本文采用热力学方法计算和分析了近300种镁离子和铝离子电池体系的理论质量能量密度、体积能量密度和电压。在所得数据的基础上,以目前商业化锂离子电池正极材料钴酸锂为对比参考,综合考虑质量能量密度、体积能量密度、标准电极电位、毒性、腐蚀性、易燃性、环境友好性等诸多因素,逐步筛选出符合条件的一系列镁离子正极材料（O₂、S、MnO₂、MoO₃、Fe₂O₃、Fe₃O₄、NiO、MoO₂、CuO、Cu₂O）和铝离子的正极材料（O₂、S、MnO₂、MoO₃、NiO、CuO、Cu₂O）。     

纤维素温和条件下一步水热法制备甲烷的研究

生物质发酵法制备甲烷存在甲烷收率低、CO₂含量高等问题。本研究以纤维素为原料,在温和条件下采用水热催化转化的方法制备甲烷。对一系列催化剂进行了考察,发现Ru/C对该反应的催化活性最高。采用Ru/C催化剂进一步考察了一系列反应条件,结果表明,升高反应温度、延长反应时间、增加催化剂用量以及提高氢气初始压力对甲烷的生成具有促进作用。在1 MPa H₂、220℃、12 h反应条件下,甲烷碳摩尔收率最高,达88%,反应过程中无CO₂产生。采用TEM、BET、XRD和FT-IR等对催化剂进行了表征,结果表明,Ru/C催化剂的高催化活性可能与催化剂本身比表面积大、钌粒子颗粒小且分散均匀的特性有关。本研究采用的催化转化方法具有甲烷收率高、CO₂排放量小（<5%）、反应条件更为温和等特点。     

A total fuel cycle approach to reducing greenhouse gas emissions: Solar generation technologies as greenhouse gas offsets in U.S. utility systems

Greenhouse gas emissions in the electric utility sector occur not only at generation facilities, but also during upstream processes that support the construction and operation of energy facilities. A total fuel cycle approach is used to evaluate the potential greenhouse gas savings that could result from the deployment of solar generation technologies in utility systems in the United States. Total fuel cycle analyses were completed for several renewable and conventional generation technologies to estimate the total greenhouse gas emission contribution from each generation technology. These results are used to develop total fuel cycle emission rates for planned electric capacity additions in the U.S., and these rates are compared with the emission rates that would occur if solar technologies were substituted for fossil generation capacity additions. Current projections for solar technology deployment are low relative to total capacity additions. Hence, even doubling the planned additions of solar technologies produces less than a 1% reduction in annual CO₂ and CH₄ emissions from new generation. However, the total lifetime greenhouse gas savings from increased deployment of solar technologies can be substantial. Increasing planned solar deployment by only 25% up to the year 2010 can create up to six million tons of CO₂ savings over the lifetime of the solar installations.     

Analysis of biomimetic hierarchical porous structure regulating radiation field to improve solar thermochemical performance based on minimum Gibbs free energy

Solar energy is the source of energy required for the dry methane reforming (DMR). In the high temperature field induced by concentrated solar energy, the spatial distribution of radiation intensity has a significant impact on the solar thermochemical performance. Based on principle of minimum Gibbs free energy (G_min), the idea of regulating radiation field to match solar thermochemical energy conversion on-demand is proposed. To improve solar thermochemical conversion efficiency, biomimetic hierarchical porous structure is introduced as solar thermochemical reactor, which can optimize both the radiation field and temperature field. The analysis model of solar driven DRM is established, combined with user-defined functions (UDFs). The effects of pore diameter combinations along the direction of L and R on the reforming properties are studied. The results show that by designing biomimetic hierarchical porous structure, the temperature of solar thermochemical reactor can approach the thermodynamic temperature (1050 K), and finally the methane conversion is improved by up to 7%.     

Reduction of NiAl2O4 containing catalysts for steam methane reforming reaction

Reducibility of a NiAl₂O₄ containing catalyst was studied. On a measurement of NiAl₂O₄ concentration in a catalyst, a peak area ratio of NiAl₂O₄ in XRD analysis was verified to express the NiAl₂O₄ concentration. The reducibility of NiAl₂O₄ was confirmed to be dependent on the calcining temperature to form NiAl₂O₄, not dependent on the calcining time. The catalyst containing NiAl₂O₄ was ascertained to be reduced under convenient conditions to actual plant operations; H₂/N₂ = 30/70 at 1023K for 1 h + steam/CH₄ = 6 at 1023K for 17 h.     

Carbothermal reduction of alumina: Thermochemical equilibrium calculations and experimental investigation

The production of aluminum by the electrolytic Hall–Héroult process suffers from high energy requirements, the release of perfluorocarbons, and vast greenhouse gas emissions. The alternative carbothermic reduction of alumina, while significantly less energy-intensive, is complicated by the formation of aluminum carbide and oxycarbides. In the present work, the formation of Al, as well as Al₂OC, Al₄O₄C, and Al₄C₃ was proven by experiments on mixtures of Al₂O₃ and activated carbon in an Ar atmosphere submitted to heat pulses by an induction furnace. Thermochemical equilibrium calculations indicate that the Al₂O₃-reduction using carbon as reducing agent is favored in the presence of limited amounts of oxygen. The temperature threshold for the onset of aluminum production is lowered, the formation of Al₄C₃ is decreased, and the yield of aluminum is improved. Significant further enhancement in the carbothermic reduction of Al₂O₃ is predicted by using CH₄ as the reducing agent, again in the presence of limited amounts of oxygen. In this case, an important by-product is syngas, with a H₂/CO molar ratio of about 2, suitable for methanol or Fischer–Tropsch syntheses. Under appropriate temperature and stoichiometry of reactants, the process can be designed to be thermo-neutral. Using alumina, methane, and oxygen as reagents, the co-production of aluminum with syngas, to be converted to methanol, predicts fuel savings of about 68% and CO₂ emission avoidance of about 91%, vis-à-vis the conventional production of Al by electrolysis and of methanol by steam reforming of CH₄. When using carbon (such as coke or petcoke) as reducing agent, fuel savings of 66% and CO₂ emission avoidance of 15% are predicted. Preliminary evaluation for the proposed process indicates favorable economics, and the required high temperatures process heat is readily attainable using concentrated solar energy.     

Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600–1000°C

The oxidation of HCN and NH₃ with CO, CH₄, or H₂ addition has been studied in the temperature range between 600 to 1000°C. In most of the tests 10% oxygen was used. The experiments were carried out under well-defined conditions in a flow tube reactor made of quartz glass. The effects of NO addition and oxygen level have been tested. To study the importance of O/H radicals in the reaction mechanism and to confirm previous studies, iodine was added in some tests. A detailed chemical kinetic model was used to analyze the experimental data. In general, the model and experimental results are in good agreement. The results show that under the conditions tested CO significantly promotes NO and N₂O formation during HCN oxidation. During NH₃ oxidation carbon-containing gaseous species such as CO and CH₄ are important to promote homogeneous NO formation. In the system with CH₄ addition, the conversion of HCN to N₂O is lower compared to the other systems. In the HCN/NO/CO/O₂ system NO reduction starts at 700°C and the maximum reduction of approx. 40% is obtained at 800°C. For the NH₃/NO/CO/O₂ system the reduction starts at 750°C and the maximum reduction is 50% at 800°C. Iodine addition shifts the oxidation of HCN, NO, and N₂O formation as well as NO reduction to higher temperatures. Under the conditions tested, it was found that iodine mainly enhances the recombination of the O-radicals. No effect on NO formation was found in the HCN/CH₄/O₂ system when oxygen was increased from 6% to 10%, but when oxygen was increased from 2% to 6% NO formation decreased. The role of hydrocarbon radicals in the destruction of NO is likely to become important at low oxygen concentrations (2%) and at high temperatures (1000°C).     

Catalytic thermochemical reactor/receiver for solar reforming of natural gas: Design and performance

The advantages of thermochemical conversion of concentrated solar energy using catalytic processes are discussed. The design of a solar volumetric thermochemical reactor/receiver (TCRR) with catalytic absorber, method for synthesis of catalytically activated ceramics, and preparation of catalytic absorber have been described. The prototype TCRR was tested in the high flux solar furnace at the DAC, Cologne by using the dioxide reforming of methane. The tests were performed to check the main concept of the TCRR design and catalytic absorber, to study the influence of solar flux distribution, the reagent flows and their ratio on the productivity or conversion, determine the reagent's conversion depending on the focal point disposition with respect to the absorber, and to study the efficiency of the thermochemical conversion. The chemical and total efficiencies of the CO₂–methane conversion were calculated using the experimentally measured concentrations of the reaction products. The highest overall efficiency achieved in these experiments was 30% with the Ni–Cr catalytic absorber.     

Solar reforming of methane in a direct absorption catalytic reactor on a parabolic dish: I—Test and analysis

The concept of solar driven chemical reactions in a commercial-scale volumetric receiver/reactor on a parabolic concentrator was successfully demonstrated in the CAtalytically Enhanced Solar Absorption Receiver (CAESAR) test. Solar reforming of methane (CH₄) with carbon dioxide (CO₂) was achieved in a 64 cm diameter direct absorption reactor on a parabolic dish capable of 150 kW solar power. The reactor was a catalytic volumetric absorber consisting of a multilayered, porous alumina foam disk coated with rhodium (Rh) catalyst. The system was operated during both steady-state and solar transient (cloud passage) conditions. The total solar power absorbed reached values up to 97 kW and the maximum methane conversion was 70%. Receiver thermal efficiencies ranged up to 85% and chemical efficiencies peaked at 54%. The absorber performed satisfactorily in promoting the reforming reaction during the tests without carbon formation. However, problems of cracking and degradation of the porous matrix, nonuniform dispersion of the Rh through the absorber, and catalyst deactivation due to sintering and possible encapsulation, must be resolved to achieve long-term operation and eventual commercialization.     

High efficiency solar energy water splitting to generate hydrogen fuel: Probing RuS2 enhancement of multiple band electrolysis

The conditions under which an oxygen photocatalyst can improve multiple band gap (semiconductor) solar energy water splitting are probed. Recently, we provided evidence that previous models significantly underestimated the magnitude of H₂ fuel which may be generated by solar energy, and demonstrated a bipolar band gap solar system electrolyzing water at V_H2O

H₂O→H₂+1/2O₂; V_H2O>E°_H2O=E°_O2−E°_H2;E°_H2O(25°C)=1.229 V

at an unprecedented 18.3% solar energy conversion efficiency. Three conditions are shown in which oxygen photocatalyst addition can further improve this process; (i) a reduction in V_H2O; (ii) at V_H2O, capability to sustain electrolysis currentsgenerated photocurrents, and (iii) catalyst activation at hν_photo-O2>hν_{photo-bipolar}. We show that RuS₂ with 1% Fe is capable of meeting these conditions.     

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%.     

Hydrogen production by biomass gasification in supercritical or subcritical water with Raney-Ni and other catalysts

Gasification of peanut shell, sawdust and straw in supercritical or subcritical water has been studied in a batch reactor with the presence of a series of Raney-Ni and its mixture with ZnCl₂ or Ca(OH)₂. The main gas products were hydrogen, methane, carbon dioxide, and a small amount of carbon monoxide. Different types of Raney-Ni, containing different metal components such as Fe, Mo or Cr, have different influences on the gasification yield and hydrogen selectivity. The catalysis effect can be improved obviously by adding ZnCl₂ or Ca(OH)₂. Increasing the reaction temperature or adding ZnCl₂ and Ca(OH)₂ could improve the mass of H₂ in gas products and reduce the mass of CH₄ and CO₂ at the same time. The possible mechanism is that ZnCl₂ can decompose the biomass particle by accelerating cellulose hydrolyzation in high-temperature water, increasing more specific surface to admit catalysts, while Ca(OH)₂ can absorb CO₂ to produce CaCO₃ deposit, which can drop out from the reactant system, and which will drive the reaction to get more hydrogen. With respect to the biomass conversion to gas product and selectivity of H₂ at low temperature, the series of Raney-Ni has shown many advantages over other catalysts; thus, this kind of catalyst has great potential to be utilized in the hydrogen industry for the gasification of biomass.     

Anthropogenic emissions of CO2 and CH4 in an urban environment

Regular observations of atmospheric mixing-ratios of carbon dioxide and methane in the urban atmosphere, combined with analyses of their carbon-isotope composition (δ¹³C, δ¹⁴C), provide a powerful tool for assessing both the source strength and source partitioning of those gases, as well as their changes with respect to time. Intense surface fluxes of CO₂ and CH₄, associated with anthropogenic activities result in elevated levels of these gases in the local atmosphere as well as in modifications of their carbon-isotope compositions. Regular measurements of concentration and carbon-isotope composition of atmospheric CO₂, carried out in Krakow over the past two decades, were extended to the period 1995–2000 and also to atmospheric mixing-ratios of CH₄ and its carbon-isotope composition. Radiocarbon concentrations (δ¹⁴C) in atmospheric CO₂ recorded at Krakow are systematically lower than the regional background levels. This effect stems from the addition of ¹⁴C-free CO₂ into the local atmosphere, originating from the burning of fossil fuels. The fossil-fuel component in the local budget of atmospheric carbon calculated using a three-component mixing model decreased from ca. 27.5 ppm in 1989 to ca. 10 ppm in 1994. The seasonal fluctuations of this component (winter–summer) are of similar magnitude. A gradually decreasing difference between the ¹⁴CO₂ content in the local atmosphere and the regional background observed after 1991 is attributed to the reduced consumption of ¹⁴C-free fuels, mostly coal, in southern Poland and the Krakow municipal area. The linear regression of δ¹³C values of methane plotted versus reciprocal concentration, performed for the data available for Krakow sampling site, yields the average δ¹³C signature of the local source of methane as being equal to −54.2‰. This value agrees very well with the measured isotope signature of natural gas being used in Krakow (−54.4±0.6‰) and points to leakages in the distribution network of this gas as the main anthropogenic source of CH₄ in the local atmosphere.     

Preliminary attempt to enhance the hydrogenation of carbon dioxide to methane on cobalt oxide catalyst by adding CuO---ZnO---Cr2O3

Hydrogenation of carbon dioxide (CO₂) was carried out on cobalt oxide catalyst (Co₃O₄) at atmospheric pressure. The hydrogenation proceeded even at 473K. Total conversion reached a maximum (ca 55%) at 573K. Methane (ca 50%) and small amounts of CO (3–5%) were produced during the hydrogenation. It is indicated that the reaction proceeds on partially reduced Co₃O₄. An attempt to enhance the hydrogenation of CO₂ was carried out by adding CuO---ZnO---Cr₂O₃ as a cocatalyst and as a result, the yield of methane was selectively increased.     

NiO–ScSZ and Ni0.9Mg0.1O–ScSZ-based anodes under internal dry reforming of simulated biogas mixtures

Solid oxide fuel cells (SOFCs) with NiO–ScSZ and Ni_0.9Mg_0.1O–ScSZ-based anodes were operated by directly feeding a fuel mixture of CH₄, CO₂ and N₂ (CH₄ to CO₂ ratio of 3:2). Stable operation under constant current load (200 mA cm⁻²) was achieved with a NiO–ScSZ type anode during 200 h operating hours at 900 °C. Less stable operation occurred with a Ni_0.9Mg_0.1O–ScSZ type anode. In the case of SOFC with Ni_0.9Mg_0.1O–ScSZ as the anode, the methane reforming activity was higher than that with NiO–ScSZ. This was explained by change in the microstructure promoting reforming reactions. However, the addition of MgO resulted in degradation of electrochemical performance due to increase in ohmic resistance of the anode material during operation.     

The electrochromic properties of hydrated nickel oxide films formed by colloidal and anodic deposition

Hydrated nickel oxide NiO_xH_y films were deposited onto indium tin oxide (ITO) coated glass by two methods (i) colloidal precipitation and (ii) anodic electrodeposition. The electrochromic properties of hydrated nickel oxide films were studied by transmittance measurements (UV/VIS/NIR), and Fourier transform infrared reflectance spectroscopy as a function of the key deposition parameters. The solar transmittance was calculated for films switched in both bleached and coloured states. The best results were achieved for films produced by anodic electrodeposition from stable solutions with solar transmittance T_s(bleached) = 0.82 and T_s(coloured) = 0.22. Corresponding optimum values for the films produced by colloidal precipitation were solar transmittance T_s(bleached = 0.82 and T_s(coloured) = 0.47. Fourier transform spectrophotometry was used for elucidating changes in hydration, hydroxylation and for the characterization of structural characteristics in the bleached and coloured states. It was found that free OH stretching vibration at 3647 cm⁻¹ corresponds to Ni(OH)₂ for both anodic and colloidal deposited films in the reduced (bleached) state. In the oxidised state hydrogen bonded OH at 3360 cm⁻¹ is observed.