Modeling of an Autothermal Process for Integrated Carbon Dioxide Capture and Methanation by Magnesium Looping (MgO/MgCO3) and Renewable Hydrogen

Carbon dioxide (CO2) capture and conversion into fuels by means of renewable hydrogen (H2) is a key strategy to simultaneously reduce atmospheric CO2 emissions and increase the exploitation of renewable energies. In this work, a process for integrated CO2 capture and methanation is investigated. Magnesium looping cycle, relying on iterated magnesium oxide (MgO) carbonation and magnesium carbonate (MgCO3) calcination, is considered for the capture of CO2. The core feature of the process is its autothermal operation: The heat released by the methanation is exploited for the sorbent regeneration, thanks to the strong synergy existing between the two chemical reactions. A scheme of the overall integrated process is designed, and the performance of the system is evaluated through a purposely developed thermodynamic model relying on mass and energy balance equations. Data of MgO sorbents doped with alkali nitrates molten salts are considered. Model computations suggest that, in the base case considered, about 93% of the CO2 is captured from the flue gas and converted into a synthetic methane (CH4) stream with a purity of 91Û. The overall CH4 yield is 91%, and the process works autothermally: CO2 is captured and converted into CH4 without the need for external heat inputs. The sensitivity analysis reveals that the process resists well the variation of several operating parameters, preserving its autothermal operation in almost any case. The proposed process claims low energetic penalties and can represent a valid solution for thermochemical energy storage of renewable energy and for the development of a circular carbon economy.