Power-to-Gas through High Temperature Electrolysis and Carbon Dioxide Methanation: Reactor Design and Process Modeling

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).