Numerical Simulation of Biogas Chemical Looping Reforming in a Dual Fluidized Bed Reactor

Climate changes call for both an increase in renewable sources utilization and the development of efficient energy conversion processes. Biogas, a product of the anaerobic digestion of organic matter, is a renewable gaseous mixture of CH4 (45%-65%), CO2 (55%-35%), H2O (2%-10%), and minor impurities. The amount of heat produced by biogas combustion directly depend on the CH4 content since all the other species behave as inert. Biogas upgrading (removal of CO2) and cleaning (separation of impurities) is consequently mandatory to increase its calorific power. Alternatively, biogas can be reformed into syngas, a gaseous mixture of H2 and CO that may further be exploited for hydrogen, power production, and biofuel syntheses. In the present work a biogas Chemical Looping Reforming (CLR) process is proposed and numerically studied. The proposed model couples a hydrodynamic simulation of a Dual Fluidized Bed (DFB) system equipped with non-mechanical valves for bed solids circulation with a 1D, dynamic and non-isothermal CLR model developed to determine temperature and oxidation degree of solids and gaseous species concentration at the exit of both Air and Fuel reactors. The DFB, consisting of a riser and of a bubbling fluidized bed (BFB) as Air and Fuel reactors respectively, was modelled as a combination of interconnected blocks (riser, cyclone, L-valve, BFB, loop-seal) after selection of constitutive equations. Nickel (II) oxide was selected as oxygen carrier. The effects on process performances of variations in biogas CH4/CO2 ratio and water content, as well as in the fuel reactor operating conditions, have been addressed.