Simulation of a sorption-enhanced methanation process with CaO in a dual interconnected fluidized bed system

In the framework of carbon capture and utilization technologies and bio-energy transition, catalytic methanation seems to be an interesting pathway for production of synthetic methane for energy storage. This fuel has the main advantage to be already largely used in the current society, with the existence of an extensive transportation and storage infrastructure and a good social acceptance. The production of synthetic methane is under scrutiny by the scientific community. Innovative concepts such as sorption-enhanced methanation have been recently proposed, to improve the performance of catalytic methanation in terms of CH4 yield and cost-effectiveness of the process.In this work, the simulation of a sorption-enhanced methanation process in a dual interconnected fluidized bed system was carried out in ASPEN plus using the 'fluid bed' block and using low-cost CaO (derived from lime-stone) as the sorbent for steam capture. The model consists of two bubbling fluidized beds operated, respectively, as methanator and regenerator under chemical looping conditions with the support of a Ni-based catalyst. The feeding gas, characterized by different concentrations of H2, CO2 and CO, enters the methanator where the catalytic hydrogenation reactions take place with the presence of CaO coming from the regenerator for the exploitation of sorption-enhanced methanation conditions. In the regenerator, the de-hydration reaction of the hydrated sorbent occurs. The optimal operating conditions were investigated in terms of gas feeding composition and recirculation of CaO between the methanator and the regenerator, to produce methane streams that are suitable for injection into the natural gas grid. Also, the effect on the system performance induced by metha-nation temperature was scrutinized.The results showed that the (unwanted) carbonation reaction of the CaO sorbent had a negative impact on methanation when a stoichiometric gas feeding was fed into the methanator. Conversely, with hydrogen-lean gas feeding, it was possible to find conditions suitable for direct injection of the outlet gas in the grid since the limited amount of H2 was properly balanced by CO2 capture by the sorbent. Moreover, a specific thermodynamic analysis highlighted the possibility of carbon formation induced by H2 sub-stoichiometric feeding and sorption -enhanced methanation conditions.