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

Abstract

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.