Enhanced resistance to carbon deposition in chemical-looping combustion of methane: Synergistic effect of different oxygen carriers via sequence filling

Chemical-looping combustion (CLC) is a promising technology which can realize CO2 capture with low energy consumption. Carbon deposition is one of the main challenges in CLC of methane, which decreases the efficiency of combustion and CO2 capture. In this work, the carbon formation behaviors of four typical oxygen carriers (OCs) including NiO/ZrO2, Fe2O3/Al2O3, MnO2/ZrO2 and CuO/SiO2 were investigated, and a strategy for carbon elimination was proposed by arranging the four OCs with suitable sequences in fixed-bed reactor. It is found that carbon is easy to form on the NiO/ZrO2 OC, although it shows high CH4 conversion. The reduction of Fe2O3/ Al2O3 is a stepwise process and carbon will be generated in the deep reduction from Fe3O4 to FeO/Fe. MnO2/ ZrO2 OC exhibits poor activity for CH4 conversion, but it shows high resistance to carbon deposition. For CuO/ SiO2 OC, it shows high CH4 conversion (89.1%) and CO2 selectivity (97.6%) with almost no formation of carbon. When combing the four OCs in the CuNiFeMn sequence, high-performance CuO/SiO2 and NiO/ZrO2 OCs in front of the filling fixed-bed can oxidize most of CH4. CuO/SiO2 OC will oxidize CH4 to CO2 and H2O, then the formed CO2/H2O may gasify the carbon deposition on the NiO/ZrO2 OC to produce CO and H2. Thereafter, these reducible gases as well as the residual CH4 can be oxidized by the following Fe2O3/Al2O3 and MnO2/ZrO2 OCs with relatively poor activity. In this way, the synergistic effect among the sequenced OC mixtures can significantly enhance the CLC efficiency and resistance to carbon deposition.