Current Developments in Catalytic Methanation of Carbon Dioxide—A Review

The utilization of fossil fuel has increased atmospheric carbon dioxide (CO2) concentrations drastically over the last few decades. This leads to global warming and climate change, increasing the occurrence of more severe weather around the world. One promising solution to reduce anthropogenic CO2 emissions is methanation. Many researchers and industries are interested in CO2 methanation as a power-to-gas technology and carbon capture and storage (CCS) system. Producing an energy carrier, methane (CH4), via CO2 methanation and water electrolysis is an exceptionally effective method of capturing energy generated by renewables. To enhance methanation efficiency, numerous researches have been conducted to develop catalysts with high activity, CH4 selectivity, and stability against the reaction heat. Therefore, in this mini-review, the characteristics and recent advances of metal-based catalysts in methanation of CO2 is discussed.

Optimal Renewable Energy Distribution Between Gasifier and Electrolyzer for Syngas Generation in a Power and Biomass-to-Liquid Fuel Process

By adding energy as hydrogen to the biomass-to-liquid (BtL) process, several published studies have shown that carbon efficiency can be increased substantially. Hydrogen can be produced from renewable electrical energy through the electrolysis of water or steam. Adding high-temperature thermal energy to the gasifier will also increase the overall carbon efficiency. Here, an economic criterion is applied to find the optimal distribution of adding electrical energy directly to the gasifier as opposed to the electrolysis unit. Three different technologies for electrolysis are applied: solid oxide steam electrolysis (SOEC), alkaline water electrolysis (AEL), and proton exchange membrane (PEM). It is shown that the addition of part of the renewable energy to the gasifier using electric heaters is always beneficial and that the electrolysis unit operating costs are a significant portion of the costs. With renewable electricity supplied at a cost of 50 USD/MWh and a capital cost of 1,500 USD/kW installed SOEC, the operating costs of electric heaters and SOEC account for more than 70% of the total costs. The energy efficiency of the electrolyzer is found to be more important than the capital cost. The optimal amount of energy added to the gasifier is about 37–39% of the energy in the biomass feed. A BtL process using renewable hydrogen imports at 2.5 USD/kg H2 or SOEC for hydrogen production at reduced electricity prices gives the best values for the economic objective.

Electrochemically Enhanced Deposition of Scale from Chosen Formation Waters from the Norwegian Continental Shelf

Reservoir formation waters typically contain scaling ions which can precipitate and form mineral deposits. Such mineral deposition can be accelerated electrochemically, whereby the application of potential between two electrodes results in oxygen reduction and water electrolysis. Both processes change the local pH near the electrodes and affect the surface deposition of pH-sensitive minerals. In the context of the plugging and abandonment of wells, electrochemically enhanced deposition could offer a cost-effective alternative to the established methods that rely on setting cement plugs. In this paper, we tested the scale electro-deposition ability of six different formation waters from selected reservoirs along the Norwegian continental shelf using two experimental setups, one containing CO2 and one without CO2. As the electrochemical deposition of scaling minerals relies on local pH changes near the cathode, geochemical modelling was performed to predict oversaturation with respect to the different mineral phases at different pH values. In a CO2-free environment, the formation waters are mainly oversaturated with portlandite at pH > 12. When CO2 was introduced to the system, the formation waters were oversaturated with calcite. The presence of mineral phases was confirmed by powder X-ray diffraction (XRD) analyses of the mineral deposits obtained in the laboratory experiments. The geochemical-modelling results indicate several oversaturated Mg-bearing minerals (e.g., brucite, dolomite, aragonite) in the formation waters but these, according to XRD results, were absent in the deposits, which is likely due to the significant domination of calcium-scaling ions in the solution. The amount of deposit was found to be proportional to the concentration of calcium present in the formation waters. Formation waters with a high concentration of Ca ions and a high conductivity yielded more precipitate.

Frequency Response Analysis of the Unsteady-State CO/CO2 Methanation Reaction: An Experimental Study

The utilization of renewable electricity for power-to-gas (PtG) applications induces fluctuations in the H2 availability from water electrolysis. For subsequent methanation of CO or CO2 the unsteady-state operation of the respective reactor allows to minimize H2 storage capacities. However, the impact of temporal fluctuations in feed gas composition on the methanation reaction and the respective transient kinetics has not yet been fully understood. We investigated the methanation of various CO/CO2 (COx) feed gas mixtures under periodically changing gas compositions with emphasis on the effect of the frequency on the reactor response. We show that the frequency response of CH4 exhibits a characteristic hysteresis, which depends on the switching direction between COx-lean and COx-rich feeds and their composition. From the shape of the hysteresis we are able to conclude on the preferred COx species being hydrogenated to CH4 under respective conditions, which also provides mechanistic insights. By applying high cycling frequencies, the highly reactive species present under CO methanation conditions can even selectively be activated, which explains the higher reactivity compared to steady-state conditions reported, frequently.

Worrisome Exaggeration of Activity of Electrocatalysts Destined for Steady-State Water Electrolysis by Polarization Curves from Transient Techniques

Cyclic and linear sweep voltammetry techniques substantially misjudge the performance of water splitting electrocatalysts due to their transient nature that forbids the interface from reaching a steady-state. This misjudgment leads to the potentially detrimental yet unwittingly falsified data accumulation in the literature that requires immediate attention. Alternatively, sampled-current voltammetry (SCV) constructed from steady-state responses is advised to be widely adopted for screening electrocatalysts that are actually destined for steady-state operations. To show that this exaggeration is universal, a well-characterized activated SS, coprecipitated Co(OH)2, and Pt foil electrodes are studied for OER and HER in 1.0 M KOH. The results urge that it is time to adopt a relatively more precise alternative technique such as SCV.