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Three different power-to-methane process chains with grid injection in two scales (1 MWel and 6 MWel) were analysed regarding their investment and operation cost. The process chains were based on biological or catalytic bubbling fluidised bed methanation in combination with proton exchange membrane or solid oxide electrolyser cells. A bottom-up techno-economic analysis showed a cost benefit of around 17–19% lower biomethane production cost for the bubbling fluidised bed technology as less than a third of the reactor volumes is required for catalytic methanation. This cost benefit is only given in combination with PEM electrolysis, as the high-temperature electrolyser stacks currently result in high investment cost. Based on electricity cost of 5 €-ct/kWhel and a plant size of 6 MWel, biomethane production cost of 13.95 €-ct./kWh for catalytic and 17.30 €-ct/kWh for biological methanation could be obtained, both including PEM electrolysis. A significant efficiency increase by integrating the heat of catalytic methanation reaction with the high-temperature electrolysis can be achieved; however investment cost have to decrease below 1000 €/kWel to obtain economically feasible production cost of biomethane. Under current economic and technological circumstances, CO2 methanation using the bubbling fluidised bed technology is the most cost effective.

The mechanism and kinetic of CO2 methanation reaction of 9.5 % Ni/Al2O3 catalyst is analysed under a wide range of operating conditions. Once the catalyst activity is stabilized, the influence of temperature, total pressure and space velocity is studied for kinetic characterisation. A data set comprising of 153 experimental runs has been used to develop a kinetic model capable to accurately predict the reaction rate. Ni/Al2O3 catalyst shows an apparent activation energy of 80.1 kJ mol(-1) in CO2 hydrogenation. Data obtained under differential mode adjust quite precisely to a power-law model with H 2 O inhibition, with a water adsorption constant of 3.1 atm(-1) and apparent orders of 0.24 and 0.27 for H-2 and CO2, respectively. Based on DRIFTS results, we propose for the first time the H-assisted CO formation route, which is compared with the more conventionally reported carbonyl route, and describe the corresponding reaction rate LHHW equation, resulting in notable improvement for mean deviation (D) of 7.0 % in our model related to that based on the carbonyl route (D = 20.1 %) usually suggested for catalysts with higher Ni loads around 20 %. The H-assisted CO formation route considers the formate species decomposition into carbonyls via H-assisted CO formation mechanism and further carbonyls hydrogenation into CHO as the rate determining step. Thus, the LHHW mechanism, in which carbonyls as well as formate species participate in CO2 methanation, is capable to reflect the kinetics of lowly-loaded Ni/Al2O3 catalyst with high accuracy under relevant process conditions (315-430 degrees C, 1-6 bar, H-2 to CO2 molar ratios between 1-16 and, different reagents and products partial pressures). Support for this study was provided by the Spanish Ministry of Economy and Competiveness (Project PID2019-105960RB-C21 and PID2019-105960RB-C22) and the Basque Government (Project IT1297-19). One of the authors (JAOC) acknowledges the Post-doctoral research grant (DOCREC20/49) provided by the University of the Basque Country. Other of the authors (AQ) also acknowledges University of the Basque Country by his PhD grant (PIF-15/351).

Power to gas (PTG) refers to the technology of converting power into energy-storage gas, which can absorb excess power when there is excess power and release energy-storage gas when needed. Based on the carbon dioxide (CO2) emission of Karamay City in Northwest China, this study designed a process flow of the CO2 absorption process, and the hydrogen and CO2 methanation process, in PTG technology. The results show that the efficiency of the CO2 absorption process was 91.5%, and the methanation efficiency was 77.5%. The heat recovery module was set during the process, and the total heat recovered was 17.85 MW. The cost of producing synthetic natural gas (SNG) in the PTG factory was 1782 USD/ton. In terms of cost, the cost of hydrogen production from electrolyzed water accounted for the largest proportion. In terms of product profit, the sale of pure oxygen was the largest part of the profit. At present, the carbon emission reduction index profit brought by SNG production accounted for a small proportion. In the future, with technological progress, industrial upgrading and the improvement in the carbon trading market, PTG technology is expected to become one of the ways to achieve carbon-emission-reduction targets.

Energy storage solutions are a vital component of the global transition toward renewable energy sources. The power-to-gas (PtG) concept, which stores surplus renewable energy in the form of methane, has therefore become increasingly relevant in recent years. At present, supported Ni nanoparticles are preferred as industrial catalysts for CO2 methanation due to their low cost and high methane selectivity. However, commercial Ni catalysts are not active enough in CO2 methanation to reach the high CO2 conversion (>99%) required by the specifications for injection in the natural gas grid. Herein we demonstrate the promise of promotion of Ni by Mn, another low-cost base metal, for obtaining very active CO2 methanation catalysts, with results comparable to more expensive precious metal-based catalysts. The origin of this improved performance is revealed by a combined approach of nanoscale characterization, mechanistic study, and density functional theory calculations. Nanoscale characterization with scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDX) and X-ray absorption spectroscopy shows that NiMn catalysts consist of metallic Ni particles decorated by oxidic Mn2+ species. A mechanistic study combining IR spectroscopy of surface adsorbates, transient kinetic analysis with isotopically labeled CO2, density functional theory calculations, and microkinetics simulations ascertains that the MnO clusters enhance CO2 adsorption and facilitate CO2 activation. A macroscale perspective was achieved by simulating the Ni and NiMn catalytic activity in a Sabatier reactor, which revealed that NiMn catalysts have the potential to meet the demanding PtG catalyst performance requirements and can largely replace the need for expensive and scarce noble metal catalysts.

Titania-supported nickel catalysts (3, 6, 12 and 20 wt% Ni/TiO2) were prepared by a sol-gel method. The results showed that the CO2 conversion increased with increasing Ni content and 20 wt% Ni/TiO2 exhibited the highest CO2 conversion and CH4 yield. The results on XRD and EXAFS revealed unit cell expansion and lattice distortion, which indicated that Ni2+ was incorporated into the TiO2 lattice. Upon increasing the Ni content, the NiO phase was observed. The oxidation states of nickel and titanium were analyzed to be +2 and +4, respectively for fresh catalysts. After pretreatment by H-2, Ni2+ was converted to Ni-0 and this electronic state remained unchanged during the course of methanation while Ti4+ was kept unaltered for fresh catalysts and those during the reaction. The addition of Ni led to the formation of oxygen vacancy and NiO phase which might act as adsorption sites for CO2 and H-2, respectively. (C) 2020 Elsevier Ltd. All rights reserved.

The power-to-methane process using a solid-oxide electrolyser, followed by an exothermic methanation reactor, is effective for the production of an energy carrier (synthesis methane) that can be readily stored and transported at a large scale. A significant gain of the coupling is the steam generation from the heat produced by the methanation reaction for the more effective steam electrolysis (compared to liquid water electrolysis). This paper investigates, both theoretically and experimentally, the system integrability and operability of an evaporating water-cooled methanation reactor designed for thermal coupling with a solid-oxide electrolyser. The results show that the pressurized steam generation directly by the reactor's cooling system improves the flexibility of the system design. Under certain conditions, an increase in overall power-to-methane efficiency of more than 3 % can be obtained from the use of a superheater and a turbine located after the evaporation process. The parametric study of the methanation reactor indicates that decreasing the flowrate of the H2 and CO2 mixture, increasing the reaction gas pressure and increasing the cooling system pressure (saturation temperature) shifts the hot spot towards the inlet and improves the H2 conversion rate, which can reach above 97 %. It was determined that, under steady-state operation, the steam generation can be sufficiently stable for injection into the solid-oxide electrolyser; rapid pulsations with a standard deviation under 10% were measured. The calibrated 1D pseudo-homogeneous plug flow reactor model captures many of the trends, though, the assessment is limited without the exact kinetic model of the catalyst pellets.