The use of NiO as an oxygen carrier in chemical-looping combustion

The feasibility of using NiO as an oxygen carrier during chemical-looping combustion has been investigated. A thermodynamic analysis with CH4 as fuel showed that the yield of CH4 to CO2 and H2O was between 97.7 and 99.8% in the temperature range 700–1200 °C, with the yield decreasing as the temperature increases. Carbon deposition is not expected as long as sufficient metal oxide is supplied to the fuel reactor. Hydrogen sulfide, H2S, in the fuel gas will be converted partially to SO2 in the gas phase, with the degree of conversion increasing with temperature, but decreasing as a function of pressure. There is the possibility of sulfide formation as Ni3S2 at higher partial pressures of H2S+SO2 in the reactor. The reactivity of freeze granulated particles of NiO with NiAl2O4, MgAl2O4, TiO2 and ZrO2 sintered at different temperatures was investigated in a small fluidized bed reactor by exposing them cyclically to 50% CH4/50% H2O and 5% O2 at 950 °C. During the reducing period, the NiO initially reacted with the CH4 to form CO2 and H2O. However, there were always minor amounts of CO from the outlet of the reactor even at high concentrations of CO2, which was due to the thermodynamic limitations. Here, the ratio CO/(CO2+CO+CH4) was between 1.5 and 2.5% at 950 °C for the oxygen carriers with alumina based inert. A small amount of CH4 was released from the reactor at high degrees of oxidation of the NiAl2O4 and MgAl2O4-based carriers. As the time under reducing conditions increased, steam reforming of CH4 to CO and H2 became considerable, with Ni catalyzing this reaction. Whereas the ZrO2 particles showed similar behavior as the alumina-based carriers, the TiO2-based particles showed a markedly different reaction behavior, likely due to the complex interaction between NiO and TiO2.