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Industrial & Engineering Chemistry Research, Vol.57, No.29, 9725-9736, 2018
High Iron and Calcium Coal Ash As the Oxygen Carrier for Chemical Looping Combustion
The potential of Zhundong's coal ash as a novel oxygen carrier (OC) in chemical-looping combustion (CLC) was evaluated in a thermogravimetric analysis (TGA) under both the CO/Ar and air atmospheres. A single reduction-oxidation cycle was carried out in the TGA reactor to determine the optimum reaction temperature, which was 1123 K. The cyclic stability and agglomeration tendency of ash also was evaluated by consecutive redox cycles. On the basis of the kinetic analysis, the apparent activation energy of ash was about 26-29 kJ/mol and the reduction process of ash was divided into three stages. The reaction control steps for these stages were gas diffusion, diffusion and kinetics, and kinetics, respectively. Besides, the reactions in the first and second stages corresponded to the reduction of Fe2O3 to Fe and CaSO4 to CaS. In the third stage, the reaction occurred between the CaS and CaSO4.The products of the equilibrium reaction between CO and ash were obtained using Factsage (software), while the transition products from the reduction of ash were determined by X-ray diffraction (XRD). The main reactants in ash were Fe2O3 and CaSO4. Because of sintering and particle agglomeration, the actual percentage of oxygen release (22%) was less than the theoretical value (26%). In addition, after some reactions CaSO4 was converted to MgOCa3O3Si2O4.