Chemical-looping combustion with NiO and Fe2O3 in a thermobalance and circulating fluidized bed reactor with double loops

Abstract

For the chemical-looping combustion (CLC), readily available metal oxides (NiO, Fe2O3) for oxygen carriers and bentonite, TiO2, and Al2O3 for the supports of the looping materials were selected. The reactivity of the oxygen carrier particles was determined in a thermobalance reactor under the reducing (CH4) and oxidizing (O-2) conditions at 923-1223 K. The reactivity of NiO is higher than Fe2O3, and the particles supported on bentonite or Al2O3 produce higher reactivity than those on TiO2. The reactivity of the metal oxide particles increases with increasing temperature and the amount of MO. The obtained kinetic data of the NiO-Fe2O3/bentonite can be analyzed based on the modified volumetric and shrinking core models for the reduction and oxidation conditions, respectively. The CLC experiment was carried out in an annular shape circulating fluidized bed (CFB) reactor with double loops. To determine the optimum fuel gas velocity, the mixture of NiO and Fe2O3 (75:25) on a bentonite support was tested at 1123 K. The CH4 conversion was higher at lower velocities than that at higher ones, and the optimum CH4 gas velocity for complete combustion was found to be around 2-3 u(mf) (minimum fluidizing velocity). Combustion efficiency increases with increasing temperature, and the optimum reaction temperature was found to be around 1123 K. It was found that CO emission from the fuel reactor was negligibly small, and no H-2 emission was detected at the optimum conditions. From the oxidation reactor, NOx emission was also negligibly small, and CO2 emission was not detected.