Nickel-copper oxygen carriers to reach zero CO and H2 emissions in chemical-looping combustion

Abstract

Ni-based oxygen carriers allow to work at high temperatures (900-1100ºC) in a Chemical-Looping Combustion (CLC) process with full CH4 conversion, although thermodynamic restrictions result in the presence of CO and H2 in the gas outlet of the fuel reactor. On the other hand, Cu-based oxygen carriers allow complete fuel combustion to CO2 and H2O, but the operating temperature is limited due to the low melting point of the metallic Cu. The objective of this research was to analyze the behavior of several Ni-Cu oxygen carriers to reduce or avoid CO and H2 emissions during a CLC process working at high temperatures. Commercial Al2O3 and  Al2O3 were used as support to prepare by dry impregnation different oxygen carriers based on nickel and copper. The reactivity of these oxygen carriers was determined in a thermogravimetric analyzer (TGA). The effect of the mixture of NiO and CuO on the CO and H2 generation was analyzed in a fixed bed reactor, and the gas product distribution during reduction/oxidation reactions was studied in a batch fluidized bed reactor working with CH4 as fuel and diluted air for oxidation. The fluidization behavior of the oxygen carriers with respect to the attrition and agglomeration processes was also analyzed during the multi-cycle batch fluidized bed tests. The presence of CuO in the Ni-Cu oxygen carriers allows the full conversion of CH4 to CO2 and H2O in the batch fluidized bed reactor during the initial part of the reduction time, and this time depended on the CuO content of the oxygen carrier. TGA and X-ray diffraction studies indicated that CuO is used for the reduction reaction before NiO. In addition, it was observed that the presence of NiO stabilized the CuO and allowed to work at 950 ºC with Ni-Cu oxygen carriers with a high metal oxide utilization.