This work deals with the coupling between high temperature steam electrolysis using solid oxide cells (SOEC) and carbon dioxide methanation to produce a synthetic natural gas (SNG) directly injectable in the natural gas distribution grid via a power-to-gas (P2G) pathway. An intrinsic kinetics obtained from the open literature has been used as the basis for a plug flow reactor model applied to a series of cooled multitube fixed bed reactors for methane synthesis. Evaporating water has been considered as coolant, ensuring a high heat transfer coefficient within the shell side of the reactor. A methanation section has been designed and optimized in order to moderate the maximum temperature within the catalytic bed and to minimize the catalyst load. Then, process modeling of a plant coupling high temperature electrolysis and methanation is presented: the main goal of this analysis is the calculation of overall plant efficiency (in terms of electricity-to-SNG chemical energy). Plant size has been set considering a 10 MW el SOEC-based electrolysis unit; heat produced from the exothermal methanation is entirely used for water evaporation before the steam electrolysis. A heat exchanger network (HEN) has been designed in order to reduce the number of components, resulting in an external heat requirement equal to 185 kW (≈1.9% of the electrolysis power). The SOEC-based power-to-gas system presented a higher heating value based efficiency equal to ≈86% (≈77% if evaluated on lower heating value basis).
Power to gas systems is one of the most interesting long-term energy storage solutions. As a result of the high exothermicity of the CO2 methanation reaction, the catalyst in the methanation subsystem is subjected to thermal stress. Therefore, the performance of a commercial Ni/Al2O3 catalyst was investigated over a series of 100 hour-long tests and in-process relevant conditions, i.e. 5 bar from 270 to 500 °C. Different characterization techniques were employed to determine the mechanism of the observed performance loss (N2 physisorption, XRD, TPO). The TPO analysis excluded carbon deposition as a possible cause of catalyst aging. The BET analysis evidenced a severe reduction in the total surface area for the catalyst samples tested at higher temperatures. Furthermore, a direct correlation was found between the catalyst activity decline and the drop of the catalyst specific surface. In order to correctly design a reliable methanation reactor, it is essential to have a kinetic model that includes also the aging kinetics. For this purpose, the second set of experiments was carried out, in order to determine the intrinsic kinetics of the catalyst. The kinetic parameters were identified by using nonlinear regression analysis. Finally, a power-law aging model was proposed to consider the performance loss in time.
The continuous increase in electricity production from renewable energy sources (RESs) introduces the intrinsic fluctuating characteristic of RESs in the electric power grid, causing nontrivial grid management issues (e.g., grid congestion). In this work, an innovative power to liquefied methane concept was developed, and process simulations for a 200 kWel demonstration plant were carried out. The proposed concept is based on water electrolysis to produce hydrogen, CO2 capture from air using solid adsorption materials, catalytic CO2 methanation, gas separation, and a single mixed refrigerant (SMR) liquefaction process. The gas separation unit produces an exhaust stream, rich in not only hydrogen and carbon dioxide but also methane, that is recycled to the methanation unit inlet. A thermodynamic analysis excluded the possibility of carbon deposition formation in the methanation reactor due to methane recirculation. The gas separation system was designed using a combination of temperature swing adsorption techniques (stream dehumidification) and membrane separation (CO2 separation). After a screening of different polyimide-type membranes, a two-stage layout was selected and dimensioned. Subsequently the liquefaction unit was developed, optimizing the SMR composition and pressures to minimize the total work required. Hence, the minimum work required for the liquefaction resulted in being 0.57 kWhel/kgLNG. Finally, the thermal integration was performed to minimize the external heat requirement. The heat produced by the electrolyzer and methanation unit is greater than the thermal energy requirement by the CO2 capturing unit during desorption. A process efficiency up to 46.3% (electric to chemical) resulted from the study. The process modeling results also evidenced that the impact of the gas pretreatment and liquefaction process on the plant energetics is 4% of the total power input.
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