Investigation of Different NiO/NiAl2O4 Particles as Oxygen Carriers for Chemical-Looping Combustion

Journal article, 2009

Chemical-looping combustion is a combustion technology, where CO2 is separated from the rest of the flue gases without an energy-consuming gas-separation process. The combustion is performed in two reactors, with metal oxide particles circulating between them, transferring oxygen from the combustion air to the fuel. Particles of NiO, supported by NiAl2O4, have been reported earlier as excellent oxygen carriers for this process. The aim of the present investigation is to verify that commercially available raw materials can be used to produce oxygen carrier particles with properties suitable for the technology. A total of 36 oxygen carrier materials were prepared by freeze granulation and investigated with respect to parameters important for chemical-looping combustion. The reactivity of the particles was investigated in a small fluidized bed reactor by exposing them cyclically to CH4 and 5% O2 in N2, at 950 °C. Although defluidization occasionally occurred for some materials, it was clear that the gas conversion and the reactivity were generally high. An addition of Ca(OH)2 to the oxygen carriers increased the strength and thus reduces the risk of fragmentation and attrition in a chemical-looping combustion device. An addition of MgO enhanced the fuel conversion early in reduction, which seemed to be restricted because of the limited amounts of metallic Ni. An increased sintering temperature generally resulted in harder particles of higher density; however, the risk of defluidization seemed to increase for such particles. Carbon formation was only detected when the oxygen carriers were highly reduced and the fuel conversion was incomplete, i.e., at conditions not expected in a real chemical-looping combustion device. Two of the investigated particles, NOV1T1400 and NOV2T1400, displayed a combination of high reactivity and strength as well as excellent fluidization behavior and should be feasible for use in a chemical-looping combustion unit.