Modeling Sorption Enhanced Methanation in Dual Interconnected Fluidized Beds with Different Sorbents

In the framework of the energy transition, the possibility of combining the use of CO₂ from carbon capture techniques with a power-to-gas technology, exploiting the renewable electric energy surplus to produce synthetic methane from hydrogen, is gaining attention. Sorption enhanced methanation (SEM) may improve the performance of the process shifting the equilibrium composition towards a higher CH₄ production, enabling, for example, to reduce the operating pressure for methanation. The concept involves the removal of the produced steam using a selective sorbent. A novel dual interconnected fluidized beds (DIFB) SEM configuration was recently suggested to overcome fixed bed limitations such as the transient operation mode and the difficult temperature control.

CO₂ DIFB-SEM chemical looping was simulated in AspenPlus, investigating two possible sorbents: CaO derived from limestone and a commercial zeolite 3A. Two “FluidBed” blocks, including all significant reaction kinetics, simulated the methanator in a range of temperature from 250°C to 350°C and the regenerator at 450 °C, where the spent sorbent is continuously recirculated for dehydration. The analysis was carried out focusing on the outlet gas quality in terms of contaminants for the direct injection in the natural gas grid, introducing dimensionless parameters characterizing the gas and the solid sorbent feeds.

Results showed that the effect of the undesired CaO carbonation prevailed over the CaO hydration one; however, with a sub-stochiometric H₂ gas feed, an optimal condition was identified under which the outlet stream met the network specification. As for zeolites, due to the low adsorption capacity, the results suggested that only with a significantly higher amount of sorbent the SEM effect could be appreciable. These results highlighted the need for research on novel sorbents: selectivity towards water adsorption with respect to CO₂ appears to be crucial, suggesting that engineered zeolites might be preferable for this technology.