Enhanced combustion performance and reduced NOx emissions during chemical looping ammonia combustion with Cu-Fe oxygen carrier

Chemical looping ammonia combustion (CLAC) is thought to be an innovative method for energy utilization of ammonia. Fabricating the efficient, cheap and environment-friendly oxygen carriers(OCs) is one of the key issues for CLAC and there is rare reports about it. The paper is desired to fill this gap. Four lattice doped Fe-based OCs named as Cu-Fe, Ce-Fe, Ca-Fe and Ni-Fe were desired and the corresponding CLAC performances (including the NH conversion efficiency /N selectivity/NOx emissions) are conducted. Results showed that the NH conversion efficiencies were all over 98 % while the NOx emissions were different at the optimized 900 °C. Compared with the FeO, the NOx emission of Cu-Fe, Ni-Fe, Ce-Fe and Ca-Fe were reduced by 99 %,89 %,85 %,81 %, respectively and CuFe2 (with the Cu:Fe mole ratio of 1:5) was further optimized as the best lattice doped Fe-based OCs(with a peak NOx emission of 100 ppm, NH conversion efficiency (99 %) and N selectivity (99.9 %)). XRD, XPS, NH-TPD and in-situ DRIFT studies were introduced for understanding the mechanisms. When Cu was doped into the FeO lattice (CuFeO-FeO solid solution), the acidic sites were enhanced and the crystal oxygen were activated on the surface of the CuFe2 OCs due to the interaction between Cu and Fe ions thus converting most of the NH. Furthermore, in-situ DRIFT study indicated that Cu-doped FeO (CuFe2) speeded up the DeH rate of ammonia to form -NH and -HNO, and promoted the coupling of -HNO and -NH to increase the selectivity of nitrogen formation. DFT calculations demonstrated that Cu doping significantly lowered the reaction energy barrier for NH* + HNO* →N* + HO* (reduced the reaction energy barrier by 1.865 eV), thereby promoting the conversion of HNO* to N₂. The consumption of HNO* consequently inhibited the HNO* → H* + NO* process.