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

Abstract

The feasibility of using NiO as an oxygen carrier during chemical-looping combustion has been investigated. A thermodynamic analysis with CH 4 as fuel showed that the yield of CH 4 to CO 2 and H 2O 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, H 2S, in the fuel gas will be converted partially to SO 2 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 Ni 3S 2 at higher partial pressures of H 2S+SO 2 in the reactor. The reactivity of freeze granulated particles of NiO with NiAl 2O 4, MgAl 2O 4, TiO 2 and ZrO 2 sintered at different temperatures was investigated in a small fluidized bed reactor by exposing them cyclically to 50% CH 4/50% H 2O and 5% O 2 at 950 °C. During the reducing period, the NiO initially reacted with the CH 4 to form CO 2 and H 2O. However, there were always minor amounts of CO from the outlet of the reactor even at high concentrations of CO 2, which was due to the thermodynamic limitations. Here, the ratio CO/(CO 2+CO+CH 4) was between 1.5 and 2.5% at 950 °C for the oxygen carriers with alumina based inert. A small amount of CH 4 was released from the reactor at high degrees of oxidation of the NiAl 2O 4 and MgAl 2O 4-based carriers. As the time under reducing conditions increased, steam reforming of CH 4 to CO and H 2 became considerable, with Ni catalyzing this reaction. Whereas the ZrO 2 particles showed similar behavior as the alumina-based carriers, the TiO 2-based particles showed a markedly different reaction behavior, likely due to the complex interaction between NiO and TiO 2.