Reaction kinetics of freeze-granulated NiO/MgAl2O4 oxygen carrier particles for chemical-looping combustion

The kinetics of reduction and oxidation of Ni based oxygen carrier particles with CH4 and O-2 have been investigated. The kinetic parameters were obtained from reactivity data using a thermogravimetric analyzer (TGA), where the freeze-granulated particles were tested using different reactant gas concentrations, temperatures, and particles sizes. The particles showed high reactivity during both reduction and oxidation at temperatures above 900 degrees C. The shrinking-core model for spherical grain geometry of the reacting particle with chemical reaction control was used to determine the kinetic parameters during both the reduction and oxidation reactions. The reaction order found was 0.4 and 1 for CH4 and O-2, respectively, while the activation energies found were 114 and 40 kJ/mol for reduction and oxidation reactions, respectively. The reactivity data and kinetic parameters were used to estimate the solid inventory needed in a chemical-looping combustion (CLC) system. The total solid inventory varies with the solid conversion at the inlet of the fuel and air reactor, and for the investigated particles, the minimum solid inventory was 22 kg/MWf. It was found that to operate fuel and air reactors of a CLC system at 950 and 1000 degrees C, respectively, using an NiO/MgAl2O4 oxygen carrier with a 50% active NiO content, the conversion variation between the two reactors should not exceed 0.18. The recirculation rate between the air and fuel reactors needed was 4.15 kg/(s MWf). The high reactivity of the NiO/MgAl2O4 both with methane and oxygen found in this work, together with the good fluidizing properties found in earlier studies, suggests that this is an excellent oxygen carrier for a CLC system.