Stepwise production of CO-rich syngas and hydrogen via solar methane reforming by using a Ni(II)–ferrite redox system

The methane reforming process combined with metal–oxide reduction was examined on iron-based oxides of Ni(II)–, Zn(II)–, and Co(II)–ferrites, for the purpose of converting solar high-temperature heat to chemical fuels of CO-rich syngas and reduced metal oxide as storage and transport of solar energy. It was found that the Ni(II)-doping effectively improves the reactivity of magnetite as an oxidant for methane reforming. A two-step cyclic steam reforming of methane, which produces CO-rich syngas and hydrogen uncontaminated with carbon oxides alternately in the separate steps, was successfully demonstrated by using a ZrO₂-supported Ni(II)–ferrite (Ni_0.39Fe_2.61O₄/ZrO₂) as a working material in the temperature range of 1073–1173 K. The produced CO-rich syngas had the H₂/CO ratio that was more suitable for methanol production than that produced by a conventional single-step steam reforming. This syngas production using the Ni_0.39Fe_2.61O₄/ZrO₂ as an oxidant was also demonstrated under direct irradiation by a solar-simulated, high-flux visible light in laboratory-scale fixed bed system. The directly-irradiated Ni_0.39Fe_2.61O₄/ZrO₂ particles acted simultaneously as good radiant absorbers and reactive chemical reactants to yield more than 90% of methane conversion to a 2:1 molar mixture of CO and H₂ under flux irradiation of 500 kW m⁻² in the residence time less than 1 s.     

CO2 reforming of methane combined with steam reforming or partial oxidation of methane to syngas over NdCoO3 perovskite-type mixed metal-oxide catalyst

CO₂ reforming with simultaneous steam reforming or partial oxidation of methane to syngas over NdCoO₃ perovskite-type mixed metal oxide catalyst (prereduced by H₂) at different process conditions has been investigated. In the simultaneous CO₂ and steam reforming, the conversion of methane and H₂O and also the H₂/CO product ratio are strongly influenced by the CO₂/H₂O feed-ratio. In the simultaneous CO₂ reforming and partial oxidation of methane, the conversion of methane and CO₂, H₂ selectivity and the net heat of reaction are strongly influenced by the process parameters (viz. temperature, space velocity and relative concentration of O₂ in the feed). In both cases, no carbon deposition on the catalyst was observed. The reduced NdCoO₃ perovskite-type mixed-oxide catalyst (Co dispersed on Nd₂O₃) is a highly promising catalyst for carbon-free CO₂ reforming combined with steam reforming or partial oxidation of methane to syngas.     

Zirconia-supported tungsten oxides for cyclic production of syngas and hydrogen by methane reforming and water splitting

ZrO₂-supported tungsten oxides were used for cyclic production of syngas and hydrogen by methane reforming (reduction) and water splitting (re-oxidation). The reduction characteristics of WO₃ to WO₂ and WO₂ to W were examined at various temperatures (1073–1273 K) and reaction times. Significant portions of the tungsten oxides were also reduced by the produced H₂ and CO. The extent of reduction by H₂ varied greatly depending on temperature and WO₃ content and also on the reduction of either WO₃ or WO₂, while that by CO was consistently low. When the overall degree of reduction became sufficiently high, methane decomposition started to proceed rapidly, resulting in considerable carbon deposition and H₂ production. Consequently, the H₂/(CO + CO₂) ratio varied from around 1 to higher than 2. During the repeated cyclic operations with a proper reduction time at a given temperature, the syngas and hydrogen yields decreased gradually while the H₂/(CO + CO₂) ratio remained nearly constant and the carbon deposition was negligible.     

Thermodynamic and experimental study of combined dry and steam reforming of methane on Ru/ ZrO2-La2O3 catalyst at low temperature

Analysis of the effect of adding small amounts of steam to the methane dry reforming feed on activity and products distribution was performed from thermodynamic equilibrium calculations of the system based on the Gibbs free energy minimization method. This analysis is supported by new insights from the direct experimental investigation of the influence of co-feeding with H₂O over a Ru/ZrO₂-La₂O₃ catalyst. Activity measurements were carried out in a fixed-bed reactor but using the operating conditions applicable in a Pd membrane reactor, that is, at maximum reaction temperature below 550 °C. Experimental results were in good agreement with thermodynamics predictions. It was observed that the addition of H₂O into the dry reforming feed strongly affects activity and products distribution. The co-feeding of steam resulted in increasing methane conversion and hydrogen yield but decreasing carbon dioxide conversion and carbon monoxide yield. At a given temperature, syngas composition (H₂/CO ratio) can be tuned by changing the amount of H₂O co-fed. Interestingly the stability of the Ru/ZrO₂-La₂O₃ catalyst was improved by adding steam to the dry reforming reactant mixtures.     

Complete removal of carbon monoxide in hydrogen-rich gas stream through methanation over supported metal catalysts

Complete removal of CO by methanation in H₂-rich gas stream was performed over different metal catalysts. Ni/ZrO₂ and Ru/TiO₂ were the most effective catalysts for complete removal of CO through the methanation. These catalysts can decrease a concentration of CO from 0.5% to $\text{20}\text{ppm}$ in the gases formed by the steam reforming of methane with a significantly low conversion of CO₂ into methane. Catalytic activities of supported Ni and Ru strongly depended on the type of supports, i.e. ZrO₂ for Ni and TiO₂ for Ru are suitable supports for the methanation of CO. The effect of catalytic supports on methanation of CO could be explained by particles sizes of Ni and Ru metal. Catalytic activity of supported Ru catalysts for the complete removal of CO through methanation became higher as particle sizes of Ru metal became smaller, while Ni metal particles with relatively larger diameters were effective for the reaction.     

Ceria–zirconia stabilised tungsten oxides for the production of hydrogen by the methane–water redox cycle

The generation of essentially pure hydrogen through a redox cycle using methane and then water has been investigated for a series of tungsten oxides stabilised by ceria–zirconia. The calcined starting materials were largely monoclinic WO₃ with CeO₂ and ZrO₂ undetectable by XRD. Samples containing 80% and especially 69% WO₃ showed additional XRD lines due to a phase of unknown composition. Temperature programmed reduction to 750 °C in methane converted samples containing WO₃ alone to WC together with small amounts of tungsten metal and a WC_1−x phase. Reoxidation in water at the same temperature then produced CO and H₂ in corresponding amounts. Under the same conditions the 69% WO₃ sample was reduced only as far as WO₂ and reoxidation yielded H₂ largely free of CO. The reoxidation product was not WO₃ but consisted of various non-stoichiometric oxides with composition WO_2+x (x = 0.72, 0.83, etc). The reduction–reoxidation cycle could be repeated many times without loss of hydrogen production efficiency.     

Steam enhanced carbon dioxide reforming of methane in DBD plasma reactor

Considering the inevitable high energy input to implement the CO₂ reforming of methane under high-temperature operation using conventional catalysis method, the low temperature conversion of CO₂ and methane in the coaxial dielectric barrier discharge (DBD) plasma reactor was investigated in this work. Steam was introduced to enhance the CO₂ reforming of methane with synergetic catalysis effect by cold plasma and catalyst. The experimental results showed that a certain percent of steam could promote the conversion of both CH₄ and CO₂. Meanwhile, the carbon deposition was evidently reduced compared with the dry reforming of methane. With the increase of steam input, the steam reforming occurred predominantly. As a result, the hydrogen volume percentage in the product gases increased. In this way, the products with different H₂/CO ratio could be achieved by changing the mole ratio of CH₄/CO₂/H₂O at the reactor inlet. In particular, when the mole ratio of H₂O/CH₄ increased to almost 3 corresponding to the pure steam reforming process, the conversion of CH₄ reached almost 0.95 and the selectivity to H₂ was almost 0.99 at 773K.     

Simultaneous production of two types of synthesis gas by steam and tri‐reforming of methane using an integrated thermally coupled reactor: mathematical modeling

In this work, tri‐reforming and steam reforming processes have been coupled thermally together in a reactor for production of two types of synthesis gases. A multitubular reactor with 184 two‐concentric‐tubes has been proposed for coupling reactions of tri‐reforming and steam reforming of methane. Tri‐reforming reactions occur in outer tube side of the two‐concentric‐tube reactor and generate the needed energy for inner tube side, where steam reforming process is taking place. The cocurrent mode is investigated, and the simulation results of steam reforming side of the reactor are compared with corresponding predictions for thermally coupled steam reformer and also conventional fixed‐bed steam reformer reactor operated at the same feed conditions. This reactor produces two types of syngas with different H₂/CO ratios. Results revealed that H₂/CO ratio at the output of steam and tri‐reforming sides reached to 1.1 and 9.2, respectively. In this configuration, steam reforming reaction is proceeded by excess generated heat from tri‐reforming reaction instead of huge fired‐furnace in conventional steam reformer. Elimination of a low performance fired‐furnace and replacing it with a high performance reactor causes a reduction in full consumption with production of a new type of synthesis gas. The reactor performance is analyzed on the basis of methane conversion and hydrogen yield in both sides and is investigated numerically for various inlet temperature and molar flow rate of tri‐reforming side. A mathematical heterogeneous model is used to simulate both sides of the reactor. The optimum operating parameters for tri‐reforming side in thermally coupled tri‐reformer and steam reformer reactor are methane feed rate and temperature equal to 9264.4 kmol h^?1 and 1100 K, respectively. By increasing the feed flow rate of tri‐reforming side from 28,120 to 140,600 kmol h^?1, methane conversion and H₂ yield at the output of steam reforming side enhanced about 63.4% and 55.2%, respectively. Also by increasing the inlet temperature of tri‐reforming side from 900 to 1300 K, CH₄ conversion and H₂ yield at the output of steam reforming side enhanced about 82.5% and 71.5%, respectively. The results showed that methane conversion at the output of steam and tri‐reforming sides reached to 26.5% and 94%, respectively with the feed temperature of 1100 K of tri‐reforming side. Copyright © 2013 John Wiley & Sons, Ltd.     

The mechanism characterizations of methane steam reforming under coupling condition of temperature and ratio of steam to carbon

To elucidate the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming process, the characterizations of methane steam reforming at different temperature and ratio of steam to carbon in term of distribution of H₂ and CO, and the elementary reaction rate were investigated. Meanwhile, the formation mechanisms of H₂ and CO via sensitivity analysis and reaction path analysis were obtained. The results showed that the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming were higher than that of individual factor. The effects of temperature on the methane steam reforming were higher than that of the ratio of steam to carbon. The adsorption and desorption reaction of CH₄ on the surface of Ni-based catalyst had the most obvious effect on the sensitivity of H₂, CH₄ and CO. Besides, the effects of adsorption and desorption reaction of H₂O on the sensitivity of H₂ were higher than that of CH₄ and CO. Hydrogen was generated by the desorption reaction of H(s) in the adsorbed state and from three generating paths: a) CH₄(s) dissociated directly or reacted with O(s) to form H(s); b) The dissociation reaction of H₂O(s) produced H(s); c) OH(s) dissociated directly or reacted with C(s) to form H(s). Carbon monoxide was generated from single path: CH₄(s)→CH₃(s)→CH₂(s)→CH(s)→C(s)→CO(s)→CO(g).     

Effect of CaO–ZrO2 addition to Ni supported on γ-Al2O3 by sequential impregnation in steam methane reforming

Ni catalysts supported on (CaO–ZrO₂)-modified γ-Al₂O₃ were prepared by sequential impregnation. The effects of varied CaO to ZrO₂ mole ratios at 0, 0.20, 0.35, 0.45, and 0.55 on the activity and stability of the modified Ni catalysts were studied. As a result of using CaO–ZrO₂ as a promoter, each catalyst contained CaO–ZrO₂ at only 5%. γ-Al₂O₃ used as support was modified by CaO–ZrO₂ before the deposition of nickel oxide. The addition of CaO–ZrO₂ at an optimum ratio was expected to improve the stability of Ni catalysts due to the decrease of carbon formation resulting from carbon gasification. All the fresh catalysts were characterized by ICP, XRD, BET surface area, TGA in H₂, and TPR before catalytic testing in steam methane reforming at 600 °C. The spent catalysts were examined by TEM and TGA to observe the catalysts deactivation. The identification of CaO–ZrO₂ phases indicated that CaO and ZrO₂ reacted with each other to be monoclinic solid solution ZrO₂, CaZr₄O₉, CaZrO₃, and CaO corresponding to the phase diagram of CaO–ZrO₂. The existence of CaZrO₃ for 0.55 mol ratio of CaO/ZrO₂ enhanced activity in steam methane reforming because oxygen vacancies in CaZrO₃ greatly preferred the water adsorption creating the favorable conditions for carbon gasification and, then, water gas shift. The prominence and continued existence of these two reactions on the Ni catalysts leads to the particular increase of H₂ yield. Moreover, the increasing amount of CaZrO₃ in the Ni catalysts significantly improved carbon gasification. However, the Ni catalysts with CaZrO₃ showed whisker carbon after catalytic testing; this carbon specie has not been tolerated in steam methane reforming. Therefore, these results significantly differed from the hypothesis.     

Efficiency assessment of solar redox reforming in comparison to conventional reforming

Solar redox reforming is a process that uses solar radiation to drive the production of syngas from natural gas. This approach caught attention in recent years, because of substantially lower reduction temperatures compared to other redox cycles. However, a detailed and profound comparison to conventional solar reforming has yet to be performed. We investigate a two-step redox cycle with iron oxide and ceria as candidates for redox materials. Process simulations were performed to study both steam and dry methane reforming. Conventional solar reforming of methane without a redox cycle, i.e. on an established catalyst was used as reference. We found the highest efficiency of a redox cycle to be that of steam methane reforming with iron oxide. Here the solar-to-fuel efficiency is 43.5% at an oxidation temperature of 873 K, a reduction temperature of 1190 K, a pressure of 3 MPa and a solar heat flux of 1000 kW/m². In terms of efficiency, this process appears to be competitive with the reference process. In addition, production of high purity H₂ or CO is a benefit, which redox reforming has over the conventional approach.     

Improvement of steam methane reforming via in-situ CO2 sorption over a nickel-calcium composite catalyst

Optimization of steam methane reforming (SMR) reaction by CO₂ sorption enhancement was investigated. In this study, the sorption-enhanced steam methane reforming reaction (SESMR) was conducted to maximize hydrogen production via suitable adjustments in the operating conditions of the reaction, which include the molar ratio of steam to CH₄, space velocity, and temperature. The reforming catalysts were prepared by a physical mixture of 20 wt% Ni/Al₂O₃ and CaO. The results reveal that there are significant differences in CH₄ conversion between the SMR and the SESMR from 18% to 108%; this conversion strongly depended on the reaction conditions. High-purity H₂ products (98.9%) with <0.1 ppmv CO were obtained by SESMR under the suitable conditions of 2600 cm³/g/h, steam/CH₄ molar ratio of 4 and 823 K. This implies that the high-quality H₂ produced through the SESMR process could be directly used for the proton-exchange membrane fuel cell.     

Effect of basicity of metal doped ZrO2 supports on hydrogen production reactions

The basicity of ZrO₂ support was successfully controlled by doping La and alkaline earth metals. La doping significantly increased weak basic sites, and medium and strong basic sites appeared by the further increase of La doping. Mg and Ca doping on La doped ZrO₂ increased weak and medium basic sites, respectively. Significant increase of total number of basic sites including strong basic sites was observed by Sr doping. In ammonia decomposition, turn over frequency (TOF) of ammonia over Ru supported ZrO₂ and metal doped ZrO₂ exhibited a good relation to the number of medium and strong basic sites. From TOF with different reaction temperature, it was supposed that the higher basicity is effective to recombinative desorption of nitrogen at lower reaction temperatures, whereas the number of medium basic sites would be effective at a high reaction temperature. Catalytic activity in dry reforming of methane was evaluated over Ru supported ZrO₂ and metal doped ZrO₂ as well. The catalytic activity was improved by metal doping to the supports. Both TOF of CO₂ and methane showed a good relation to the total number of basic sites as well as the number of medium and strong basic sites. It was interestingly found that H₂/CO was increased with an increase in the total number of basic sites. This implies the weak basic sites would have a positive impact on H₂/CO.     

Stepwise production of syngas and hydrogen through methane reforming and water splitting by using a cerium oxide redox system

Stepwise production of syngas and hydrogen from ZrO₂-supported CeO₂ through methane reforming and water splitting was investigated in order to find proper operating conditions under which carbon deposition could be minimized. Recommendable operating temperature and time were 1073 K and 30 min for both the methane reforming and the water splitting. Even though the H₂/CO ratio during the methane reforming was maintained close to the desired ratio of 2, undesirable methane cracking occurred to a small extent and further reduction of Ce₂O₃ to metallic Ce by CH₄ and H₂ occurred to some extent. When the methane reforming-water splitting cyclic operations were repeated, the yields of syngas and hydrogen decreased considerably from the first cycle to the second cycle, but from the second cycle to the fifth cycle the gas yields were maintained nearly constant. As the CeO₂ content in the sample increased, the gas yields per mole of CeO₂ decreased but the gas productions per gram of sample increased.     

Hydrogen-rich syngas production and carbon dioxide formation using aqueous urea solution in biogas steam reforming by thermodynamic analysis

Biogas is a renewable biofuel that contains a lot of CH₄ and CO₂. Biogas can be used to produce heat and electric power while reducing CH₄, one of greenhouse gas emissions. As a result, it has been getting increasing academic attention. There are some application ways of biogas; biogas can produce hydrogen to feed a fuel cell by reforming process. Urea is also a hydrogen carrier and could produce hydrogen by steam reforming. This study then employes steam reforming of biogas and compares hydrogen-rich syngas production and carbon dioxide with various methane concentrations using steam and aqueous urea solution (AUS) by Thermodynamic analysis. The results show that the utilization of AUS as a replacement for steam enriches the production of H₂ and CO and has a slight CO₂ rise compared with pure biogas steam reforming at a temperature higher than 800 °C. However, CO₂ formation is less than the initial CO₂ in biogas. At the reaction temperature of 700 °C, carbon formation does not occur in the reforming process for steam/biogas ratios higher than 2. These conditions led to the highest H₂, CO production, and reforming efficiency (about 125%). The results can be used as operation data for systems that combine biogas reforming and applied to solid oxide fuel cell (SOFC), which usually operates between 700 °C to 900 °C to generate electric power in the future.     

Characterization and activity test of commercial Ni/Al2O3, Cu/ZnO/Al2O3 and prepared Ni–Cu/Al2O3 catalysts for hydrogen production from methane and methanol fuels

In this study, methane and methanol steam reforming reactions over commercial Ni/Al₂O₃, commercial Cu/ZnO/Al₂O₃ and prepared Ni–Cu/Al₂O₃ catalysts were investigated. Methane and methanol steam reforming reactions catalysts were characterized using various techniques. The results of characterization showed that Cu particles increase the active particle size of Ni (19.3 nm) in Ni–Cu/Al₂O₃ catalyst with respect to the commercial Ni/Al₂O₃ (17.9). On the other hand, Ni improves Cu dispersion in the same catalyst (1.74%) in comparison with commercial Cu/ZnO/Al₂O₃ (0.21%). A comprehensive comparison between these two fuels is established in terms of reaction conditions, fuel conversion, H₂ selectivity, CO₂ and CO selectivity. The prepared catalyst showed low selectivity for CO in both fuels and it was more selective to H₂, with H₂ selectivities of 99% in methane and 89% in methanol reforming reactions. A significant objective is to develop catalysts which can operate at lower temperatures and resist deactivation. Methanol steam reforming is carried out at a much lower temperature than methane steam reforming in prepared and commercial catalyst (275–325 °C). However, methane steam reforming can be carried out at a relatively low temperature on Ni–Cu catalyst (600–650 °C) and at higher temperature in commercial methane reforming catalyst (700–800 °C). Commercial Ni/Al₂O₃ catalyst resulted in high coke formation (28.3% loss in mass) compared to prepared Ni–Cu/Al₂O₃ (8.9%) and commercial Cu/ZnO/Al₂O₃ catalysts (3.5%).     

A direct synthesis of nickel nanoclusters embedded on multicomponent mesoporous metal oxides and their catalytic properties for glycerol steam reforming to hydrogen

Nickel nanoclusters embedded in multicomponent mesoporous metal oxides (Ni–MMOs) are obtained at various support compositions by a single-step synthesis of Ni ion incorporated mesoporous metal oxides (NiO–MMOs) followed by selective reduction of the NiO to Ni metal clusters. The resultant Ni–MMOs catalysts displayed enhanced Ni dispersion with well-developed mesopore structures at various support composition, exhibiting superior catalytic properties when compared to a siliceous SBA-16-supported Ni catalyst prepared by a conventional impregnation method. Glycerol steam reforming conducted at 873 K on 1Ni–2Al₂O₂–2ZrO₂ and 1Ni–2SiO₂–2ZrO₂ catalysts exhibited considerably higher glycerol conversions over the 10 wt%-Ni/SBA-16 catalyst with similar Ni loading amount. This was primarily due to the enhanced Ni dispersion resulting from the direct synthesis process. The multicomponent mesoporous supports also significantly affect product selectivity, favoring higher hydrogen concentration in the product stream. The water–gas shift reaction appears to be positively affected by the 2Al₂O₂–2ZrO₂ and 2SiO₂–2ZrO₂ multicomponent metal oxide matrices, which facilitated the conversion of the CO produced by the glycerol reforming further to additional hydrogen. Direct single-step synthesis of Ni–MMO catalysts was effective in enhancing the dispersion of Ni nanoclusters, as well as variation of the support components of the mesoporous catalyst systems.     

Hydrogen production from glycerol dry reforming over Ag-promoted Ni/Al2O3

In recent times, glycerol has been employed as feedstock for the production of syngas (H₂ and CO) with H₂ as its main constituent. This study centers on dry reforming of glycerol over Ag-promoted Ni/Al₂O₃ catalysts. Prior to characterization, the catalysts were synthesized using the wet impregnation method. The reforming process was carried out using a fixed bed reactor at reactor operating conditions; 873–1173 K, carbon dioxide to glycerol ratio of 0.5 and gas hourly space velocity (WHSV) in the range of 14.4 ≤ 72 L g_cat⁻¹ h⁻¹). Ag (3)-Ni/Al₂O₃ gave highest glycerol conversion and hydrogen yield of 40.7% and 32%, respectively. The optimum conditions which gave highest H₂ production, minimized methane production and carbon deposition were reaction temperature of 1073 K and carbon dioxide to glycerol ratio of 1:1. This result can attributed to the small metal crystallite size characteristics possessed by Ag (3)–Ni/Al₂O₃, which enhanced metal dispersion in the catalyst matrix. Characterization of the spent catalyst revealed the formation of two types of carbon species; encapsulating and filamentous carbon which can be oxidized by O₂.     

Biomass to hydrogen-rich syngas via catalytic steam reforming of bio-oil

Hydrogen-rich syngas production from the catalytic steam reforming of bio-oil from fast pyrolysis of pinewood sawdust was investigated by using La_1−xK_xMnO₃ perovskite-type catalysts. The effects of the K substitution, temperature, water to carbon molar ratio (WCMR) and bio-oil weight hourly space velocity (W_bHSV) on H₂ yield, carbon conversion and the product distribution were studied in a fixed-bed reactor. The results showed that La_1−xK_xMnO₃ perovskite-type catalysts with a K substitution of 0.2 gave the best performance and had a higher catalytic activity than the commercial Ni/ZrO₂. Both high temperature and low W_bHSV led to higher H₂ yield. However, excessive steam reduced hydrogen yield. For the La_0.8K_0.2MnO₃ catalyst, a hydrogen yield of 72.5% was obtained under the optimum operating condition (T = 800 °C, WCMR = 3 and W_bHSV = 12 h⁻¹). The deactivation of the catalysts mainly was caused by coke deposition.