Thermal management of CO2 methanation with axial staging of active metal concentration in Ni-YSZ tubular catalysts

CO₂ methanation has attracted considerable interests as a promising approach to productively utilizing CO₂ and reducing emissions to realize a low-carbon society. One major difficulty with packed bed reactors for catalyzed CO₂ methanation is maintaining an optimal reactor temperature distribution. Although a high temperature increases the catalytic activity, it also leads to the formation of an inlet hotspot, which causes thermal runaway, unfavorable equilibrium products, and catalyst degradation. To address this, in this study, we proposed an approach to manage the temperature profile in CO₂ methanation reactors by increasing catalytic activity along the reactor length using different Ni composition catalysts (gradient-distributed Ni-YSZ catalyst). Ni-based tubular catalysts with different Ni compositions were prepared and stacked in order of ascending Ni content from the inlet to the outlet. The effect of gradient Ni compositions on the temperature profile was investigated based on both numerical simulations and experimental observations. The gradient-distributed Ni catalyst could successfully prevent hotspot formation at the inlet of the reactor compared to the highly active uniform catalysts. The use of the catalyst caused a small difference in the reactor temperature (of ~70 °C) and afforded a high CH₄ yield (~90%). The proposed approach using gradient-distributed catalysts could be a potential method to manage CO₂ methanation reactor temperature and to achieve high CO₂ conversion in compact reactors.     

Integration of sorption-enhanced steam glycerol reforming with methanation of in-situ removed carbon dioxide - An alternative for glycerol valorization

Coupling CO₂ desorption and methanation in the presence of hybrid materials offers a promising alternative to convert the CO₂ in-situ removed during sorption-enhanced steam glycerol reforming (SESGR) avoiding the high energy-intensive CO₂ sorbent regeneration. The all-inclusive integrated process exemplifies an option for glycerol valorization via consecutive SESGR and CO₂ conversion by catalytic hydrogenation. The dual-function catalyst performing successively, in the same reactor, SESGR and CO₂ desorption/conversion encompasses reforming/methanation catalyst (10%Ni5%Co) and dispersed nano-sized CaO on γ-Al₂O₃. Simultaneous CO₂ desorption/conversion integrated process is explored in a fixed-bed reactor via an unsteady-state, two-scale, non-isothermal model, highlighting the impact of key parameters on the process performance. At large adsorbent/catalyst mass ratio (2.0) and high CaO conversion (0.5) in preceding SESGR, CO₂ is released and hydrogenated for an extended period with extra hydrogen consumption, without a balance between CO₂ desorption and CO₂ hydrogenation rates. Increasing pressure (3.0 MPa) and gas velocity offers a match between these competitive reaction rates, resulting low CO₂ concentration in the exit stream. Desorption/methanation thermal behavior controls the magnitude of CO₂ low concentration period and the methanation efficiency.     

The influence of CO,CO2 and H2O on selective CO methanation over Ni(Cl)/CeO2 catalyst: On the way to formic acid derived CO-free hydrogen

For the first time the influence of CO, CO₂ and H₂O content on the performance of chlorinated NiCeO₂ catalyst in selective or preferential CO methanation was studied systematically. It was shown that the rate of CO methanation over Ni(Cl)/CeO₂ increases with the increasing H₂ concentration, is independent of CO₂ concentration and decreases with increasing CO and H₂O concentrations; the rate of CO₂ methanation is weakly sensitive to H₂ and CO₂ concentrations and decreases with increasing CO and H₂O concentrations. High catalyst selectivity was attributed to Ni surface blockage by strongly adsorbed CO molecules and ceria surface blockage by Cl, which both inhibit CO₂ hydrogenation.For the first time, selective CO methanation over Ni(Cl)/CeO₂ was studied for deep CO removal from formic acid derived hydrogen-rich gases characterized by high CO₂ (40–50 vol%), low CO (30–1000 ppm) content and trace amounts of water. Composite Ni(Cl)/CeO₂-η-Al₂O₃/FeCrAl wire mesh catalyst was demonstrated to be effective for this process at temperatures of 180–220°С, selectivity 30–70%, WHSV up to 200 L (STP)/(g∙h). The catalyst provides high process productivity, low pressure drop, uniform temperature distribution, and appears highly promising for the development of a compact CO cleanup reactor. Selective CO methanation was concluded to be a convenient way to CO-free hydrogen produced by formic acid decomposition.     

Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO₂ is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO₂ capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH₄ concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO₂ capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO₂ capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.     

The Ni/ZrO2 catalyst and the methanation of CO and CO2

The Ni/ZrO₂ catalyst is one of the most active systems for the methanation of CO to be employed in the hydrogen purification for PEMFC. This contribution aims to study the effect of ZrO₂ on the methanation of CO and CO₂. The catalytic behavior of Ni/ZrO₂, Ni/SiO₂, a physical mixture comprising Ni and ZrO_2, and a double-bed reactor were evaluated. The TPD of CO and CO₂, TPSR and the cyclohexane dehydrogenation reaction were carried out to describe the catalysts and the reactions. The high activity of Ni/ZrO₂ catalyst toward the methanation of CO is related to the presence of active sites on the ZrO₂ surface. The methanation of CO occurs on ZrO₂ due to its ability to adsorb CO and also because of the hydrogen spillover phenomenon. Apparently, the effect of ZrO₂ is less relevant for the methanation of CO₂. Ni/ZrO₂ is a very promising system for the purification of hydrogen.     

Methanation of CO2 over Ni/Al2O3 modified with alkaline earth metals: Impacts of oxygen vacancies on catalytic activity

This study investigates the impacts of the alkaline earth metal (Mg, Ca, Sr, Ba) additives on properties and performances of nickel catalysts for CO₂ methanation. The results show that addition of Mg, Sr, and Ba creates more pores while Ca addition leads to merge of small pores. The alkalinity of the catalyst increases with the addition of Mg, Ca, Sr or Ba, however, it does not necessarily enhance the catalytic activity. The degree of reduction of nickel species is another important factor affecting catalyst activity. Mg or Ca addition promotes the reverse water gas shift reaction to form more CO but not the methanation. In converse, with the addition of Sr or Ba, the activities for methanation increased drastically, especially in the low temperature region. In situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) studies show that *OH, *CO₃, *CO₂, CH_x, HCOO*, *CO and H₂CO* species are main reaction intermediates. Mg or Ca promotes the carbonate formation. Sr or Ba promotes *CO and H₂CO* formation, which are the important reaction intermediates in the conversion of CO₂ to CH₄. In addition, the Electron Paramagnetic Resonance (EPR) characterization shows that the catalyst modified with Sr species generates the oxygen vacancies that prevent electrons from being paired, forming a Lewis basic position. The oxygen vacancies generated are crucial for enhancing the catalytic activities for methanation at the low reaction temperatures.     

Development of a Co–Ni bimetallic aerogel catalyst for hydrogen production via methane oxidative CO2 reforming in a magnetic assisted fluidized bed

A novel nano-sized Co–Ni bimetallic aerogel catalyst was synthesized via the sol–gel process followed by the supercritical drying method. The catalyst exhibited at least 28% higher activity and 15% higher H₂ selectivity than those of a monometallic cobalt aerogel catalyst in methane oxidative CO₂ (Oxy-CO₂) reforming. Cold-model experiments revealed that channels were alleviated and the agglomerate size was reduced when the catalyst was applied in a magnetic assisted fluidized bed (MAFB). Owing to the improved fluidization quality of the catalyst in the MAFB reactor, CH₄ conversion was raised by 12% and 7% as compared with those in the fixed bed reactor and the conventional fluidized bed reactor, respectively. Furthermore, the catalytic performance was quite stable during a 50 h reaction. This stable performance can be illustrated both by the superior catalytic property of the Co–Ni bimetallic aerogel catalyst and the intensified gas–solid interaction in the MAFB reactor.     

Hydrogen production via CO2 dry reforming of glycerol over ReNi/CaO catalysts

The present work investigates the performance of Re-promoted Nickel-based catalyst supported on calcium oxide for glycerol dry reforming reaction. The catalysts were prepared using wet impregnation method and their catalytic performance was tested in a packed bed reactor with CO₂ to glycerol ratio (CGR) of 1–5, reaction temperature of 600–900 °C and gas hourly specific velocity (GHSV) of 1.44 × 10⁴–7.20 × 10⁴ ml g_cat⁻¹ s⁻¹. The optimum operating temperature for both Ni/CaO and ReNi/CaO is 800 °C, with the GHSV of 3.6 × 10⁴ mL g_cat⁻¹s⁻¹. The optimum CGR for Ni/CaO and ReNi/CaO is 1.0 and 3.0, respectively. At this condition, hydrogen gas is directly produced from glycerol decomposition and indirectly from water-gas-shift reaction. After 2 h at the optimum conditions, 5% ReNi/CaO gives optimal glycerol conversion and hydrogen yield of approximately 61% and 56%, respectively, while in comparison to 15% Ni/CaO, the conversion and yield are 35 and 30%, respectively. Characterization of the spent catalysts showed the existence of whisker carbon from the CO₂ hydrogenation and methanation processes. By comparing to 15% Ni/CaO, the addition of Re increases the acidic sites of the catalyst and enhanced the surface adsorption of OH group of the glycerol. The adsorbed glycerol on the catalyst surface would further react with the adsorbed CO₂ to yield gases products. Thus, the catalytic activity improved significantly.     

Optimization of a fluidized bed reactor for methane decomposition over Fe/Al2O3 catalysts: Activity and regeneration studies

Catalytic methane decomposition was investigated over 40 wt% Fe/Al₂O₃ catalyst in fluidized bed reactor (FLBR). After optimization of FLBR conditions in terms of catalyst bulk density, particle size, minimum fluidization velocity, and the catalyst bed height, the catalyst activity and stability tests were conducted by comparison with a fixed bed reactor (FBR). Although a similar stable methane conversion was obtained over both reactors, the pressure drop during 35 min operation of FBR was 9 times higher than that of FLBR, which indicated the possibility of continuous operation of methane decomposition process over FLBR. Further, the influence of the space velocity, feed dilution and regeneration on catalysts reactivity was studied in FLBR to conclude that a reaction condition of 12 L/g_cat∙h, feed of 20%H₂–80%CH₄ and CO₂-regeneration of deactivated catalysts may be favourable for operating methane decomposition in FLBR continually and effectively to provide stable hydrogen.     

Nano composite composed of MoOx-La2O3Ni on SiO2 for storing hydrogen into CH4 via CO2 methanation

The crucial problems for the catalysts of CO₂ methanation are the low activity at low temperature and deactivation caused by metal sintering. In order to overcome the problems or to improve the shortages, a new scheme has been put forward by loading LaNi_1-xMo_xO₃ with perovskite-type structure on SiO₂. After reduction, Ni nanoparticles, MoO_x and La₂O₃ would be all stay together and highly dispersed on SiO₂ (Ni/MoO_x-La₂O₃/SiO₂). The techniques of BET, XRD, H₂-TPR, HRTEM, ICP, H₂ chemisorption and XPS were used to characterize the prepared samples. Through effectively combining MoO_x which is active for the reaction of reverse water gas shift and Ni which can catalyze CO methanation, the resultant Ni/MoO_x-La₂O₃/SiO₂ catalyst exhibited pretty good performance for CO₂ methanation, especially showing very good resistance to metal sintering. NiLa₂O₃/SiO₂ catalyst without adding Mo was investigated for comparison. Since many metallic ions can enter into the lattice of a perovskite-type oxide, therefore, many combined catalysts for sequential reactions may be designed via this scheme.     

Carbon deposition behavior of a Co–Ni aerogel catalyst in CH4 oxy-CO2 reforming using various types of reactors

Carbon deposition behavior of a Co–Ni aerogel catalyst in CH₄ oxy-CO₂ reforming is investigated using a deactivation method in three types of reactors including a magnetic field assisted fluidized bed (MAFB) reactor, a conventional fluidized bed reactor and a fixed bed reactor. The spent catalysts are analyzed by TG/DSC and FESEM. It is found that the reactor influences the amount as well as type of carbon species on the spent catalysts. The amount of carbon on the spent catalyst in the MAFB reactor is 11.7 wt.%, which is 10.8 wt.% and 2.6 wt.% less than those in the fixed bed and conventional fluidized bed reactors. Fluidization behavior analysis reveals that agglomerate and bubble size of the catalyst are obviously decreased with the application of the MAFB, which should be accounted for the enhanced carbon resistance of the reactor.     

Numerical studies of sorption-enhanced glycerol steam reforming in a fluidized bed membrane reactor at low temperature

In order to improve the hydrogen production efficiency by glycerol steam reforming, a membrane-assisted fluidized bed reactor with carbon dioxide sorption is developed to enhance the reforming process. Low-temperature operation in a membrane reactor is necessary considering the thermal stability of membrane. In this work, the sorption-enhanced glycerol steam reforming process in a fluidized bed membrane reactor under the condition of low temperature is numerically investigated, where the hydrotalcite is employed as CO₂ sorbents. The impact of operating pressure on the reforming performance is further evaluated. The results demonstrate that the integration of membrane hydrogen separation and CO₂ sorption can effectively enhance the low-temperature glycerol reforming performance. The fuel conversion above 95% can be achieved under an elevated pressure.     

Coal char supported Ni catalysts prepared for CO2 methanation by hydrogenation

Catalytic CO₂ methanation is a potential solution for conversion of CO₂ into valuable products, and the catalyst plays a crucial role on the CO₂ conversion and CH₄ selectivity. However, some details involved in the CO₂ methanation over the carbon supported Ni catalysts are not yet fully understood. In this work, commercial coal char (CC) supported Ni catalysts were designed and prepared by two different methods (impregnation-thermal treatment method and thermal treatment-impregnation method) for CO₂ methanation. Effects of the preparation conditions (including the thermal treatment temperature and time, the mass ratio of CC:Ni and the preparation method), as well as the reaction temperature of CO₂ methanation, were investigated on the catalyst morphology, reducibility, structure and catalytic performance. Fibrous Ni-CC catalyst is achieved and shows high CO₂ conversion (72.9%–100%) and CH₄ selectivity (>99.0%) during the 600-min methanation process. Adverse changes of the catalyst surface and textural properties, reducibility, particle size and morphology are the potential factors leading to the catalyst deactivation, and possible solutions resistant to the deactivation were analyzed and discussed. The CO₂ methanation mechanism with the CO route was proposed based on the oxidation-reduction cycle of Ni in this work.     

Design and test of a miniature hydrogen production reactor integrated with heat supply,fuel vaporization,methanol-steam reforming and carbon monoxide removal unit

This study presents a designed and tested integrated miniature tubular quartz-made reactor for hydrogen (H₂) production. This reactor is composed of two concentric tubes with an overall length of 60 mm and a diameter of 17 mm. The inner tube was designed as the combustor using Pt/Al₂O₃ as the catalyst. The gap between the inner and outer tubes is divided into three sections: a liquid methanol-water vaporizer, a methanol-steam reformer using RP-60 as the catalyst and a carbon monoxide (CO) methanator using Ru/Al₂O₃ as the catalyst. The experimental measurements indicated that this integrated reactor works properly as designed. The methanol conversion, hydrogen production rate and CO concentration were found to increase with an increasing methanol/air flow rate in the combustor and decreases with an increasing methanol/water feed rate to the reformer. The methanator experimental results indicated that the CO conversion and H₂ consumption can be enhanced by increasing the Ru loading. It was also found that the CO methanation depends greatly on the reaction temperature. With a higher reaction temperature, the CO methanation, carbon dioxide (CO₂) methanation, and reversed water gas shift reactions took place simultaneously. CO conversion was found to decrease while H₂ consumption was found to increase. At a lower reaction temperature both the CO conversion and H₂ consumption were found to increase indicating that only CO methanation took place. From the experimental results the maximum methanol conversion, hydrogen yield, and CO conversion achieved were 97%, 2.38, and 70%, respectively. The actual lowest CO concentration and maximum power density based on the reactor volume were 90 ppm and 0.8 kW/L, respectively.     

Methane autothermal reforming with CO2 and O2 to synthesis gas at the boundary between Ni and ZrO2

A series of different amount ZrO₂-promoted SiO₂ supported Ni catalysts were prepared and used for methane autothermal reforming with CO₂ and O₂ [MATR] to synthesis gas in a fluidized bed reactor. Pulse-injected surface reactions and in situ XRD characterizations disclosed that CO₂ dissociated exclusively at the boundary between Ni and ZrO₂. O species derived from CO₂ dissociation overflowed to metallic Ni and accelerated the activation of methane. Ni/5ZrO₂–SiO₂ catalyst with larger Ni–ZrO₂ boundary exhibited the best activity and stability for MATR even at an extremely space velocity 90,000 h^?1.     

CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification

For the solar thermochemical gasification of coal coke to produce CO + H₂ synthetic gas using concentrated solar radiation, a windowed reactor prototype is tested and demonstrated at laboratory scale for CO₂ gasification of coal coke using concentrated Xe light from a 3-kW_th sun simulator. The reactor was designed to be combined with a solar reflective tower or beam-down optics. The results for gasification performance (CO production rate, carbon conversion, and light-to-chemical efficiency) are shown for various CO₂ flow rates and ratios. A kinetics analysis based on homogeneous and shrinking core models and the temperature distributions of the prototype particle bed are compared with those for a conventional fluidized bed reactor tested under the same Xe light irradiation and CO₂ flow-rate conditions. The effectiveness and potential impacts of internally circulating fluidized bed reactors for enhancing gasification performance levels and inducing consistently higher bed temperatures are discussed in this paper.     

CO2 methanation in the presence of methane: Catalysts design and effect of methane concentration in the reaction mixture

The work in this paper evidences the viability of producing synthetic natural gas (SNG) via the methanation reaction tackling two fundamental challenges on methanation catalysis (i) the development of advanced catalysts able to achieve high CO₂ conversion and high methane yields and (ii) the unexplored effect of residual methane on the methanation stream. Both challenges have been successfully addressed using Ni/CeO₂-ZrO₂ catalysts promoted with Mn and Co. Mn does not seem to be a good promoter while Co prevents carbon deposition and secondary reactions. In fact, our Co-doped sample reached high levels of CO₂ conversion and CH₄ selectivity, especially at low reaction temperatures. In addition, this catalyst exhibits excellent catalytic behaviour when methane is introduced into the gas mixture, indicating its feasibility for further study to be conducted in realistic flue gases environments and methanation units with recycling loops. Furthermore, when methane is introduced in the reactant mixture, the Ni-Co/CeO₂-ZrO₂ sample is very stable maintaining high levels of conversion and selectivity. Overall our Co-doped catalyst can deliver high purity synthetic natural gas for long-term runs, promising results for gas-phase CO₂ conversion units.     

Combustion-impregnation preparation of Ni/SiO2 catalyst with improved low-temperature activity for CO2 methanation

Exploiting Ni-based catalysts with excellent low-temperature activity is significant for CO₂ methanation, which is a promising route to CO₂ utilization. In this work, a facile combustion-impregnation method was developed to prepare the SiO₂ supported Ni catalysts. Small Ni particles (around 6 nm) and massive Ni–SiO₂ interface could be obtained due to the “combustion” process. The H₂-temperature programmed desorption (H₂-TPD) revealed the existence of Ni–SiO₂ interface and confirmed the high Ni dispersion obtained by this method, which were vital for the activation of reactant. Moreover, more medium basic sites which were beneficial for the CO₂ activation could also be created. In comparison with the reference Ni/SiO₂ catalyst prepared by the conventional impregnation method, much higher CO₂ conversion (66.9%) and more superior selectivity to CH₄ (94.1%) were achieved with the Ni/SiO₂-Gly catalyst at 350 °C. Additionally, it was also found that glucose, citric acid and glycine were all effective fuels for this combustion-impregnation method, and the as-prepared catalysts all exhibited greatly improved low-temperature activity. Therefore, this work represents an important step toward developing Ni-based catalysts for CO₂ methanation by a promising wide-used method.     

Photo-induced CO2 reduction by hydrogen for selective CO evolution in a dynamic monolith photoreactor loaded with Ag-modified TiO2 nanocatalyst

Ag-promoted TiO₂ nanoparticles immobilized over the cordierite monolithic support for dynamic and selective photo-reduction of CO₂ to CO by the use of hydrogen has been investigated. Ag-loaded TiO₂ NPs synthesized by a facile sol–gel method were coated over the monolith channels by dip-coating method. The samples were characterized by XRD, Raman, FTIR, SEM, TEM, XPS, N₂ adsorption–desorption, UV–Vis and PL spectroscopy. The photo-activity test of Ag-modified TiO₂ NPs was conducted for dynamic photocatalytic CO₂ reduction with H₂ as a reductant via a reverse water gas shift (RWGS) reaction in a cell type and monolith photo-reactors. Using 5 wt. % Ag/TO₂ NPs, CO₂ was energetically converted to CO with a yield rate 1335 μmole g-catal.^?1 h^?1, a 111 fold-higher than the amount of CO produced over the pure TiO₂ catalyst. More importantly, photo-activity of Ag/TiO₂ catalyst for CO evolution can be improved by 209 fold using monolith photo-reactor than the cell type reactor under the same operating conditions. This enactment was evidently due to the efficient light harvesting with larger illuminated surface area inside monolith micro-channels and efficient charges separation in the presence of Ag-metal. The reusability of Ag/TiO₂ NPs loaded over the monolithic support showed favorable recycling capability than the catalyst dispersed in a cell reactor. A possible reaction mechanism for this observation has been discussed in detail.     

Insights into structure-performance relationship in radial flow fixed bed reactor for CO2 methanation

The extraction of petroleum and natural gas is often accompanied by a large number of associated gases, especially the high CO₂ content reservoirs facing the emission of a large amount of CO₂. CO₂ methanation is recognized as one of the suitable candidates for CO₂ utilization to reduce the emission of CO₂. Because of the highly exothermic nature of the reaction, however, it is very important to enhance the heat transfer process inside the reactor and inhibit the formation of hot spots. In the fixed bed reactor, the heat transfer in the radial direction is greatly limited compared with that in the axial direction. Thus, this work adopted the radial flow reactor to evaluate the CO₂ methanation process by the means of a numerical model based on OpenFOAM. Four types of radial flow reactor configurations, namely centrifugal Z-type, centrifugal Π-type, centripetal Z-type, and centripetal Π-type, were compared. The fluid flow, heat transfer, and reaction performances for these reactors were discussed under consistent operating conditions. Results show that the centrifugal Π-type structure has the most uniform flow field. In terms of heat transfer and reaction performance, the centripetal Z-type structure is the best among the four radial flow reactor configurations. These findings provide a theoretical basis and technical guidance for designing and developing radial flow reactors.