Performance evaluation of zeolite sorbents for sorption-enhanced CO2 methanation in a lab-scale fluidized bed

The sorption-enhanced methanation (SEM) process, involving in situ steam removal, has been recently proposed in a chemical looping configuration based on dual interconnected fluidized beds. By exploiting this configuration, the methanation and the sorbent hydration reactions occur in one fluidized bed, while the regeneration of the exhausted sorbent takes place in the other one. In this work, SEM of CO 2 (with a stoichiometric feed ratio H 2 /CO 2 = 4) was studied in a batch lab-scale apparatus to test the performance of two commercial zeolites, 3A and 4A, as water sorbents. A traditional Ni/Al 2 O 3 catalyst for methanation was synthesized and used during the experimental campaign. The selected sorbents were tested in the temperature range 200–250 °C, showing similar H 2 O capture capacities. The choice of such a narrow range of investigated temperatures is due to the physical and chemical constraints imposed by water adsorption and methanation kinetics: the Ni-based catalyst is only active above 200 °C, whereas the sorbent H 2 O capture capacity strongly decreases with increasing temperature, and above 250 °C a significant worsening occurs. Despite the temperature issues, a clear enhancement of methane productivity compared to traditional methanation was observed under SEM conditions, for both sorbents. As expected, this effect decreased with increasing temperatures, due to the worsening of the physical H 2 O adsorption performance. Both zeolites showed a quite stable and repeatable behavior with an average enhancement ranging from 22 % to 63 % and 27 % to 66 % for zeolites 3A and 4A, respectively.