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.     

The influence of nickel content on the performance of hydrotalcite-derived catalysts in CO2 methanation reaction

A series of mixed oxides obtained by thermal decomposition of hydrotalcites containing different amounts of Ni and constant M^II+/M^III+ molar ratio were characterized by XRD, XANES, XES, H₂-TPR, CO₂-TPD, elemental analysis and low temperature nitrogen sorption technique. The results confirm the formation of periclase-like structured materials after thermal treatment, with nickel present as NiO in octahedral coordination environment. As proven by H₂-TPR with increasing Ni content the interaction between Ni and hydrotalcite matrix weakens, which has a positive influence on the catalytic performance. The catalysts containing different amounts of nickel (10.3, 16.2, 27.3, 36.8, 42.5 wt.%) showed at 300 °C a very good catalytic performance in carbon dioxide methanation, with CO₂ conversion of 36 (Ni10.3)–82% (Ni42.5) and CH₄ selectivity of 98–99%.     

Material constraints related to storage of future European renewable electricity surpluses with CO2 methanation

The main challenges associated with a growing production of renewable electricity are intermittency and dispersion. Intermittency generates spikes in production, which need to be curtailed when exceeding consumption. Dispersion means electricity has to be transported over long distances between production and consumption sites. In the Directive 2009/28/EC, the European Commission recommends sustainable and effective measures to prevent curtailments and facilitate transportation of renewable electricity. This article explores the material constraints of storing and transporting surplus renewable electricity by conversion into synthetic methane. Europe is considered for its mix of energy technologies, data availability and multiple energy pathways to 2050. Results show that the requirements for key materials and land remain relatively low, respecting the recommendations of the EU Commission. By 2050, more than 6 million tons of carbon dioxide might be transformed into methane annually within the EU. The efficiency of renewable power methane production is also compared to the natural process of converting solar into chemical energy (i.e. photosynthesis), both capturing and reenergizing carbon dioxide. Overall, the production of renewable methane (including carbon dioxide capture) is more efficient and less material intensive than the production of biofuels derived from photosynthesis and biomass conversion.     

Effect of water addition and stoichiometry variations on temperature profiles in an autothermal methane reforming reactor with Ni catalyst

Catalytic autothermal reforming of methane was studied over a commercial nickel catalyst as a function of feed flow rate, feed composition and oven temperature. Temperature profile of the catalyst bed was measured by IR thermography and product composition was measured with a continuous gas analyzer.     

CO2 methanation over Ni and Rh based catalysts: Process optimization at moderate temperature

H₂ was produced from aluminum/water reaction and reacted with CO₂ over Ni and Rh based catalysts to optimize the process conditions for CO₂ methanation at moderate temperature. Monometallic catalysts were prepared by incorporating Ni and Rh using nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and rhodium(III) chloride trihydrate (RhCl₃·3H₂O)as a precursor chemical. The preliminary study of the catalysts revealed higher activity and CH₄ selectivity for Rh based catalyst compared to that of Ni based catalyst. Further, Rh based catalyst was investigated using response surface methodology (RSM) involving central composite design. The quadratic model was employed to correlate the effects of variable parameters including methanation temperature, %humidity, and catalyst weight with the %CO₂ conversion, %CH₄ selectivity, and CH₄ production capacity. Analysis of variance revealed that methanation temperature and humidity play an important role in CO₂ methanation. Higher response values of CO₂ conversion (54.4%), CH₄ selectivity (73.5%) and CH₄ production capacity (8.4 μmol g^?1 min^?1) were noted at optimum conditions of 206.7°C of methanation temperature, 12.5% humidity and 100 mg of the catalyst. The results demonstrated the ability of Rh catalyst supported on palm shell activated carbon (PSAC) for CO₂ methanation at low temperature and atmospheric pressure.     

Unsupported cobalt nanoparticles as catalysts: Effect of preparation method on catalytic activity in CO2 methanation and ethanol steam reforming

Cobalt nanoparticles (10–50 nm) have been prepared by different procedures. Materials produced by reduction of cobalt chloride and nitrate by NaBH₄ contain B impurities as borates or borides. They are very active in ethanol steam reforming at 673–773 K with up to 85% hydrogen yield at 773 K. B-free samples obtained by thermal decomposition of Co₂(CO)₈ is slightly less selective to hydrogen, due to its activity in ethanol cracking to methane which is probably poisoned by boron impurities on the other catalysts. B-containing samples are inactive in CO₂ methanation and have weak activity in the reverse water gas shift (RWGS) reaction to CO. B-free nanoparticles have high activity in both CO₂ methanation and RWGS. However, methanation activity is reduced fast by growth of encapsulating carbon species. These particles however also show quite stable activity in RWGS to CO, attributed to CoO impurities.     

Dual application of Ti-catalyzed Li-RHC composite for H2 purification and CO methanation

2LiH + MgB₂ composite doped with TiO₂ (Li-RHC-Ti) is employed with a two-fold purpose: hydrogen purification under a H₂–CO (0.1 mol%) mixture and CO methanation. Upon dynamic cycling under CO–H₂ mixture, hydrogen release curves display a quite stable amount of pure hydrogen of about 10 wt%, short release times of around 60 min, and minor degradation. Gas analysis by Fourier transform infrared spectroscopy (FTIR) after a thermal dehydrogenation process of MgH₂ and LiBH₄ under CO evidence the conversion of CO to CH₄. Li-RHC-Ti dehydrogenated under CO shows the simultaneous formation of CH₄, CH₃OH, and B(CH₃)₃ in the gas phase. X-ray powder diffraction (XRPD) and FTIR characterizations of the solid phases of Li-RHC-Ti after both H₂–CO mixture and CO interactions demonstrate the formation of MgO, LiBO_2, and HCOO⁻ species. Li-RHC-Ti acts as a hydrogen source and promoter for the CO conversion. Reaction pathways are proposed based on experimental results and equilibrium composition calculations.     

Techno-economic analysis of adiabatic four-stage CO2 methanation process for optimization and evaluation of power-to-gas technology

Owing to increasing demands for clean energy, caused by global warming, renewable energy sources have attracted significant attention. However, these sources can affect the reliability of electrical grids owing to their intermittency. Power-to-gas technology is expected to help address this issue. In this study, the CO₂ methanation process, which yields synthetic natural gas (SNG) via the synthesis of CO₂ and H₂ through proton exchange membrane (PEM) water electrolysis using surplus electricity generated from renewable energy, was evaluated and optimized based on techno-economic analyses. Requirements for the introduction of SNG produced through CO₂ methanation in domestic natural gas markets are presented by considering various scenarios. Results indicate that, even if the electricity costs, including system marginal price and renewable energy costs, are minimal, the costs for PEM water electrolysis and CO₂ methanation must be reduced by ~$550/kW and 25%, respectively, relative to current levels for the viable introduction of SNG in domestic markets.     

CO removal via selective methanation over the catalysts Ni/ZrO2 prepared with reduction by the wet H2-rich gas

The wet H₂-rich gas was used as reducing gas instead of the H₂/N₂ gas in the reduction step of the catalyst preparation. It is found that the selectivity for CO methanation over the catalysts 0.4Ni/ZrO₂ so-obtained was decreased in comparison to the case of the H₂/N₂ gas used as reducing gas. Even though, the samples with the different feed atomic ratios of Ni/Zr prepared by the impregnation method and the co-precipitation method, respectively, were evaluated with the wet H₂-rich gas both as reducing gas and as reactant gas. The catalysts Ni/ZrO₂-CP prepared by the co-precipitation method exhibited a high catalytic activity for CO removal at a lowered reaction temperature with increasing the Ni loading. Over the catalyst 3.0Ni/ZrO₂-CP, CO in the reactant gas could be removed to below 10 ppm at reaction temperatures of 220–260 °C with the selectivity higher than 50%. And the selectivity was kept at 100% during the 100 h test at 220 °C. The catalysts were characterized by XRD, XPS, XRF and the adsorption isotherm measurement. In addition, effect of water vapor in reactant gas was studied over the catalysts 0.4Ni/ZrO₂ with the wet H₂-rich gas and the dry H₂-rich gas as reactant gas, respectively, in the case of the H₂/N₂ gas fixed as reducing gas. It is seen that presence of water vapor in the reactant gas retarded methanation reactions of CO and CO₂ on the catalysts.     

Effect of ruthenium addition on molybdenum catalysts for syngas production via catalytic partial oxidation of methane in a monolithic reactor

Hydrogen is mainly produced from hydrocarbon resources. Natural gas, mostly composed of methane, is widely used for hydrogen production. As a valuable feedstock for ‘Fischer–Tropsch’ (FT) process and ‘Gas to Liquids’ (GTL) technology, syngas production from catalytic partial oxidation of methane (CPOM) is gaining prominence especially owing to its more desirable H₂/CO ratio; relatively less energy consumption, and lower investment, compared to steam reforming processes (SMR), the leading technology.In the present study, effect of ruthenium (Ru) addition on molybdenum (Mo) catalysts for syngas production from methane (CH₄) via partial oxidation in a monolithic reactor was investigated. Mo based catalysts supported on Nickel (Ni) and Cobalt (Co) metal oxides and Ni-Co bimetallic oxides and their Ru added versions were developed, characterized, and tested for performance in a monolithic type reactor system. Catalyst activity was investigated in terms of H₂ and CO selectivity, CH₄ conversion; and CO₂ emission and it is concluded that addition of Ru over the structure led to increase in catalytic activity and reduction in carbon deposition over the catalyst surface.     

Effect of metal additives on the catalytic performance of Ni/Al2O3 catalyst in thermocatalytic decomposition of methane

Thermocatalytic decomposition of methane is proposed to be an economical and green method to produce CO_x-free hydrogen and carbon nanomaterial. In present work, 60 wt% Ni/Al₂O₃ catalysts with different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by co-impregnation method to investigate promotional effects of these metal additives on the activity and stability of 60 wt% Ni/Al₂O₃ and find out a really effective promoter for decomposition of methane. The catalyst was characterized by N₂ adsorption/desorption, X-ray diffraction, scanning electron microscopy, inductively coupled plasma optical emission spectrometer and hydrogen temperature programmed reduction. While metal additives (5 wt%) were added into 60 wt% Ni/Al₂O₃, the activity stability of 60 wt% Ni/Al₂O₃ was improved and the CH₄ conversion of 60 wt% Ni/Al₂O₃ was also improved except Zn addition. Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al₂O₃ significantly and the CH₄ conversion of 5 wt% Mn-60 wt% Ni/Al₂O₃ was ∼80%. Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al₂O₃ and the CH₄ conversion of 5 wt% Cu-60 wt% Ni/Al₂O₃ decreased from 61% to 45% after 250 min of reaction time. Carbon nanomaterials formed in the thermocatalytic decomposition process were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer and Raman spectroscopy. Carbon deposits consist of amorphous carbon and carbon nanofibers.     

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.     

Insight into the role of Fe on catalytic performance over the hydrotalcite-derived Ni-based catalysts for CO2 methanation reaction

A series of Fe modified hydrotalcite-derived Ni-based catalysts (Ni₃Fe_x-calc) were synthesized to evaluate the effect of Fe on CO₂ methanation performance over Ni₃-calc catalyst. The results showed that Ni₃–Fe_0.5-calc had superior catalytic activity with 78% CO₂ conversion rate at 200 °C. The addition of moderate amount of Fe can effectively improve the reducibility, enrich the medium basic sites of Ni₃-calc catalyst, and further facilitate the adsorption and activation of CO₂. This resulted in the outstanding low-temperature CO₂ methanation activity, as well as the enhanced resistance of carbon deposition. In-situ DRIFTS results indicated that the CO₂ methanation reaction mechanism involved a progressive hydrogenation of carbonate and formate species to methane route. The formate species was the main intermediates during CO₂ methanation. The introduction of Fe could significantly accelerate the hydrogenation rate of carbonates and formate species.     

Steam reforming of ethanol over Ni/ZrO2 catalysts: Effect of support on product distribution

Ni catalysts supported on ZrO₂ with different crystalline phases and particle sizes were prepared to study the role of zirconia support in ethanol steam reforming for hydrogen production. Catalytic behavior of the catalysts was examined at relatively low temperature of 673 K with different contact times. The decrease in particle size of zirconia results in enhanced metal-support interaction, which accounts for the high activity of the catalyst. Regarding the impact of crystalline phase of zirconia on catalytic performance, tetragonal zirconia yields a higher activity in water gas shift reaction but a lower activity in methane steam reforming than that of monoclinic zirconia. Nevertheless, zirconia plays a secondary role in product distribution, especially at long contact times. Catalytic activity tests performed at elevated temperature demonstrated a high activity and stability of Ni/ZrO₂ catalyst for hydrogen production from steam reforming of ethanol.     

A study of the methanation of carbon dioxide on Ni/Al2O3 catalysts at atmospheric pressure

The hydrogenation of carbon dioxide producing methane and CO has been investigated over Ni/Al₂O₃ catalysts. The as prepared catalysts have been characterized by XRD and Temperature Programmed Reduction. Spent catalysts have been characterized by XRD and Field Emission SEM. Catalytic activity needs the presence of Ni metal particles which may form in situ if the Ni loading is higher than that needed to cover the alumina surface with a complete monolayer. If Ni content is lower, pre-reduction is needed. Catalysts containing very small Ni particles obtained by reducing moderate loading materials are very selective to methane without CO formation. The larger the Ni particles, due to higher Ni loadings, the higher the CO production. Cubic Ni metal particles are found in the spent catalysts mostly without carbon whiskers. The data suggest that fast methanation occurs at the expense of CO intermediate on the corners of nanoparticles interacting with alumina, likely with a “via oxygenate” mechanism.     

Ni/Al2O3 catalysts for CO2 methanation: Effect of silica and nickel loading

Samples containing from 1 to 33 wt.% of NiO on silica and alumina doped with silica (1 and 20 wt.% silica in the support) have been prepared and characterized by BET, XRD, FT-IR, UV–vis–NIR, FE-SEM, EDXS, and TPR techniques. Catalysts have been pre-reduced in situ before catalytic experiments and data have been compared with Ni/Al₂O₃ reference sample. Characterization results showed that SiO₂ support has a low Ni dispersion ability mainly producing segregated NiO particles and a small amount of dispersed Ni²⁺ in exchange sites. Instead, for the Si-doped alumina a “surface spinel monolayer phase” is formed by increasing Ni loading and, only when the support surface is completely covered by this layer, NiO is formed. Moreover, H₂-TPR results indicated that NiO particles are more easily reduced compared to Ni species. Low loading Ni/SiO₂ catalysts show high selectivity and moderate activity for RWGS (reverse Water Gas Shift) reaction, likely mainly due to nickel species dispersed in silica exchange sites, as evidenced by visible spectroscopy. High loading Ni/SiO₂ catalysts show both methanation and RWGS but evident short-term deactivation for methanation, attributed to large, segregated Ni metal particles, covered by a carbon veil. Ni on alumina -rich carriers, where nickel disperses forming a surface spinel phase, show high activity and selectivity for methanation, and short-term catalyst stability as well. This activity is attributed to small nickel clusters or metal particles interacting with alumina, formed upon reaction. The addition of SiO₂ in Al₂O₃ support decreases the activity of Ni catalysts in CO₂ methanation, because it reduces the ability of the support to disperse nickel in form of the surface spinel phase, thus reducing the amount of Ni clusters in the reduced catalysts.     

Analyses of hydrogen energy system as a grid management tool for the Hawaiian Isles

One of the objectives of the research project at Hawaii Natural Energy Institute (HNEI) is to demonstrate long-term durability of the electrolyzer when operated under cyclic operation for frequency regulation on an Island grid system. In this paper, a Hydrogen Energy System with an electrolyzer is analyzed as a potential grid management tool. A simulation tool developed with a validated model of the hydrogen energy system and Island of Hawaii grid model is presented and employed for this investigation. The simulation study uses realistic measured solar and wind power profiles to understand what optimal electrolyzer size would be required to achieve the maximum level of grid frequency stabilization. The simulation results give insight into critical information when designing a hydrogen energy system for grid management applications and the economic impact it has when operated as a pure grid management scheme or as a limitless hydrogen production system.     

An assessment of the impacts of renewable and conventional electricity supply on the cost and value of power-to-gas

Power-to-gas (P2G) is a promising enabling technology for more cross-sector integration but its high cost has so far been a key barrier to implementation. Electricity supply is the greatest contributor to the levelised cost therefore it is important to understand which technologies and strategies can minimise the cost and accelerate the deployment. In this study, a method is devised to evaluate the cost and value of combined systems comprising P2G and renewable energy technologies such as solar photovoltaics, wind and hydro as well as comparing to traditional electricity supply via the wholesale market. The proposed models are based on a temporal resolution of 1 h and include partial operation and ageing throughout the system's lifespan. Our analysis covers both distributed and centralised P2G systems producing hydrogen or methane as well as various value-adding services across different geographies. It is found that the capacity factor of a P2G system drives the economic case and therefore the electricity supply from hydropower plants is economically more attractive than electricity from wind and solar photovoltaic plants in this order. Under today's market conditions, it is highly advisable to combine local renewable supply with wholesale-based supply but interestingly, a 20% capital cost reduction in wind technology or a P2G system efficiency of 80% are break-even points for P2G systems producing hydrogen and connected to wind plants.     

Effect of Ca,Ce or K oxide addition on the activity of Ni/SiO2 catalysts for the methane decomposition reaction

To increase the activity and stability of Ni/SiO₂ catalysts, a series of Ni–Ca, Ni–K and Ni–Ce promoted catalysts were prepared by successive impregnations. The textural properties, reducibility and catalytic performance in the methane decomposition reaction were investigated. The catalyst containing 30 wt.% Ni and 30 wt.% cerium oxide greatly increased the conversion of methane (90% of equilibrium value) and improved the stability, whereas the Ni–K and Ni–Ca were less active and stable than the Ni/SiO₂ catalyst. The results suggest that Ce addition prevents the sintering of nickel particles during reduction process maintaining a random distribution between the silica and cerium oxide improving the distribution and migration of deposited carbon.     

Enhanced performance of Ni catalysts supported on ZrO2 nanosheets for CO2 methanation: Effects of support morphology and chelating ligands

A series of Ni/ZrO₂ catalysts was prepared by the impregnation method with modification of the morphology of ZrO₂ support as well as the impregnation procedure and tested for CO₂ methanation. The catalysts supported on the ZrO₂ nanosheets displayed superior catalytic performance as compared with that on ZrO₂ nanoparticles, which could be mainly attributed to the abundant oxygen vacancies promoting the adsorption and dissociation of CO₂ molecules as well as the high dispersion of Ni species. With the introduction of ethylenediamine (En) in the impregnation procedure, the resulting Ni-15En/ZrO₂-1.5 catalyst showed the optimal activity with CO₂ conversion of 86% significantly higher than Ni/ZrO₂-0 of 44% and Ni/ZrO₂-1.5 of 79% at 0.5 MPa and 300 °C. The excellent performance was attributed to increased moderately basic sites for CO₂ adsorption in ZrO₂ nanosheets, as well as the enhanced dispersion of nickel caused by the complexation of Ni ions with En, which inhibited the aggregation of nickel particles in the subsequent thermal treatments. In conclusion, the synergistic effects of the morphology of ZrO₂ nanosheets as well as the chelating behavior of En contributed to the enhanced performance of Ni-15En/ZrO₂-1.5 in the CO₂ methanation reaction. The strategy shows good prospects for controlling the size of active metals, especially those that were dispersed on the surface of the two-dimensional (2D) metal oxide materials.