Enhanced methane conversion using Ni-doped calcium ferrite oxygen carriers in chemical looping partial oxidation systems with CO2 utilization

Chemical looping partial oxidation (CLPO) is a novel technology for converting methane into high quality syngas that can be further converted into liquid fuels. In the present work, Ni-doped Ca2Fe2O5 oxygen carriers are employed as looping media wherein the Ni-doping concentration is varied from 0 to 10%. Thermogravimetric performance tests are carried out where the doped/undoped carriers are subjected to methane reduction in the first half cycle and subsequently the reduced carriers are regenerated using CO2 or air in the second half. The cyclic redox performance of Ni doping on the characteristics and the stabilities of these oxygen carriers are also investigated by means of X-ray diffraction and scanning electron microscopy. Based on the oxygen carrier characterization, an unwanted phase, NiFe2O4, is formed beyond 5% dopant concentration which exhibited weak methane interaction and inhibited CO2 regeneration thermodynamically. Moreover, doping Ni at 0–5% exhibited increased reactivity across the temperature range of 750–1000 °C as compared to the undoped sample, with the 5% Ni doped sample showing a substantial improvement of 1149% over the undoped sample at 750 °C. This explains its potential in an adiabatic process where a temperature gradient is generally observed across the syngas generation reactor. Density functional theory (DFT) calculations further reveal the role of the Ni doping effect on methane partial oxidation and CO2 conversion, wherein adding the Ni dopant lowers the oxygen vacancy formation energy and increases the CO2 adsorption energy which is favorable for CO2 activation and splitting. The findings of this study provide a fundamental insight into the reactivity enhancement of calcium ferrite-based oxygen carriers and open new avenues for designing a novel chemical looping system for simultaneous syngas generation and CO2 utilization.