Natural Gas-Assisted NOx Abatement Using Chemical Looping Scheme

In this study, we explore a novel approach to nitrogen oxide (NOx) purification based on the principles of the chemical looping (CL) platform. This technique harnesses the reducing power of natural gas (CH4), instead of ammonia, to eliminate NOx in the flue gas, addressing environmental concerns such as ammonia slip in the state-of-the-art selective catalytic reduction process. The CL scheme involves utilizing nickel oxide (NiO) as the solid oxygen carrier to facilitate oxygen transfer from NOx to CH4 in two steps. In the first step (carrier oxidation), NOx in the flue gas reacts with reduced nickel (Ni), transferring its O to the carrier, thereby forming NiO. In the subsequent step (carrier reduction), CH4 reacts with the lattice oxygen of the oxidized carrier, generating a pure stream of CO2 and reducing NiO to Ni. We designed a high-performance carrier by dispersing NiO on an inert support to enhance the carrier's kinetics and thermal stability. We evaluated various support materials, and among them, alumina (Al2O3) and zirconia (ZrO2) demonstrated significantly superior reactivity. Notably, the Al2O3-supported carrier outperformed the ZrO2-supported carrier in NO purification and CH4 reduction and demonstrated stable performance over 10 redox cycles. Furthermore, fixed-bed experiments revealed 100% NO conversion during carrier oxidation, with optimal reaction kinetics achieved at lower space velocities, resulting in an overall high carrier utilization. Importantly, CO2 showcased no adverse effects during the NOx purification step. During the carrier reduction, complete combustion was favored, leading to CH4 conversion exceeding 95% and a CO2 selectivity of 90% at 400 °C. The CL NOx purification approach represents a promising strategy for mitigating NOx emissions.