Simulation of Biogas Upgrading by Sorption-Enhanced Methanation with CaO in a Dual Interconnected Fluidized Bed System

In this work, ASPENplus was used to simulate biogas upgrading by sorption-enhanced methanation in a dual interconnected bubbling fluidized bed configuration using inexpensive, abundant, and eco-friendly CaO to remove H2O from the reaction environment. The chemical looping scheme consisted of two reactors: a methanator/hydrator, where the catalytic reactions occurred on a catalyst with 20% Ni supported on alumina as well as the steam removal by CaO, and a regenerator, where the Ca(OH)(2) was dehydrated back to CaO. The simulations were carried out to identify possible reactant compositions (H-2 and biogas), CaO amount, and the methanation temperature able to produce an outlet gas matching the specifications for direct grid injection. When considering a stoichiometric gas feed ratio at the methanator inlet, the unwanted CaO carbonation worsened the process performance, subtracting CO2 from the desired methanation reaction. However, optimal conditions were found with hydrogen-lean gas feedings, balancing the limited H-2 amount with the capture of CO2 due to the sorbent carbonation. Thermodynamic considerations pointed out the possibility of solid carbon formation induced by sorption-enhanced methanation conditions, especially for H-2 sub-stoichiometric feedings.