NiO supported on Mg–ZrO2 as oxygen carrier for chemical-looping combustion and chemical-looping reforming

Abstract

Oxygen-carrier particles consisting of 40 wt% NiO supported on 60 wt% Mg-stabilized ZrO2 were produced by freeze granulation and examined as oxygen carrier for chemical-looping applications. Firstly, the particles were examined by oxidation and reduction experiments in a batch fluidized-bed reactor. These experiments indicated very high reactivity with CH4 and low affinity for carbon formation. For highly oxidized particles the products were CO2 and H2O, while for reduced particles they were CO and H2. Secondly, the particles were examined by 40 hours of operation in a small circulating fluidized-bed reactor, using natural gas as fuel. For chemical-looping combustion, there was complete conversion of fuel into products with high selectivity towards CO2 and H2O. At 950 °C, a combustion efficiency of 99.3% was achieved, which is only 0.1% point below the theoretical maximum, i.e. thermodynamic equilibrium. For chemical-looping reforming, the conversion of fuel was 99.9% or higher, with high selectivity towards CO and H2. Operating at the desired process parameters, which was a fuel reactor temperature of 950 °C and an air ratio of 0.30, worked flawlessly. When only natural gas was used as fuel there was slight formation of solid carbon in the fuel reactor. Adding 30 vol% steam to the fuel removed the carbon formation. The particles retained their physical and chemical structure reasonably well during operation. Approximately 5% of the particles added to the reactor were lost as fines during the first hours of operation. Further, the porosity of the particles increased somewhat during operation. The density was 10% lower for used particles, compared to fresh.