Increasing energy efficiency in chemical looping combustion of methane by in-situ activation of perovskite-based oxygen carriers

CaMnO3-based perovskites are attractive for use as oxygen carriers in chemical looping combustion (CLC) because they are made from cheap materials (manganese, calcium). However, these materials show low reactivity with methane, and high oxygen carrier-to-fuel ratios (phi) are required to achieve complete fuel combustion. It is believed that energy efficiency in the chemical looping process would be improved if the reactivity of these materials were higher. In this work, the increase in reactivity of a perovskite (CaMn0.775Mg0.1Ti0.125O2.9-delta) material is evaluated during its in-situ activation under determined operating conditions in a 0.5 kWth CLC pilot plant. The combustion efficiency of methane over 60 h of continuous operation was analysed taking into consideration the effect of operating conditions on the activation process for increasing the reactivity of the perovskite. The material increased in reactivity as the phi ratio decreased to below 6, improving combustion efficiency. Moreover, the material was pre-activated by being fully reduced in the CLC plant. The conversion of methane with the pre-activated material was subsequently higher than the conversion achieved before the activation process, and almost complete combustion was achieved at a lower phi ratio. Characterization of the particles after the combustion tests showed good stability of the material with the operating time, although a negative effect on the crushing strength was observed when the particles were highly reduced. The impact of the perovskite material activation on the energy efficiency of the process is discussed.