The effects of using structurally less-stable raw materials for the support of a spray-dried oxygen carrier with high NiO content

An oxygen carrier for chemical looping combustion (CLC) requires high mechanical strength, high reactivity, and other physical properties suitable for fluidized-bed processes. Spray-dried NiO oxygen carriers prepared using structurally stable raw materials such as α-Al2O3 and MgAl2O4 as support have required a calcination temperature above 1400 °C to obtain sufficient mechanical strength applicable to fluidized-bed process, and their oxygen transfer capacity has been less than 10 wt%. Here, we present a spray-dried oxygen carrier prepared using high NiO content (70 wt%) and structurally less-stable raw materials (20 wt% γ-Al2O3, 5 wt% pseudoboehmite, and 5 wt% bentonite), which resulted in reducing a calcination temperature and enhancing the performances of the oxygen carrier. The prepared oxygen carrier showed excellent physical properties, especially high mechanical strength and spherical shape, suitable for fluidized-bed applications and high reactivity. The calcination temperature to obtain sufficient mechanical strength was lowered to 1100 °C. At the reaction temperature of 950 °C, the oxygen transfer capacities determined by a thermogravimetric analyzer were 13.9 and 12.4 wt% for the oxygen carriers calcined at 1100 and 1250 °C, respectively, indicating more than 82% of the total oxygen in the raw NiO was transferred to the fuel. The excellent physical properties were still maintained after ten cyclic redox tests in a bubbling fluidized-bed reactor. On the other hand, the oxygen carrier prepared with α-Al2O3 had sufficient mechanical strength when calcined at 1400 °C, and showed much lower performance in reactivity and physical properties. This work confirms that the use of high NiO content and structurally less-stable raw support materials composed of γ-Al2O3 mixed with a small amount of pseudoboehmite and bentonite can reduce the calcination temperature to obtain sufficient mechanical strength and improve the performance of a spray-dried NiO oxygen carrier for the CLC of gaseous fuels.