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 NH3 conversion efficiency /N2 selectivity/NOx emissions) are conducted. Results showed that the NH3 conversion efficiencies were all over 98 % while the NOx emissions were different at the optimized 900 degrees C. Compared with the Fe2O3, 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 Febased OCs(with a peak NOx emission of 100 ppm, NH3 conversion efficiency (99 %) and N2 selectivity (99.9 %)). XRD, XPS, NH3-TPD and in-situ DRIFT studies were introduced for understanding the mechanisms. When Cu was doped into the Fe2O3 lattice (CuFe2O4-Fe2O3 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 Cu2+ and Fe3+ ions thus converting most of the NH3. Furthermore, in-situ DRIFT study indicated that Cu-doped Fe2O3 (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* -> N2* + H2O* (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.