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 alpha-Al2O3 and MgAl2O4 as support have required a calcination temperature above 1400 degrees 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% gamma-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 degrees C. At the reaction temperature of 950 degrees 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 degrees 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 alpha-Al2O3 had sufficient mechanical strength when calcined at 1400 degrees 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 gamma-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. (C) 2012 Elsevier Ltd. All rights reserved.