Reduction Kinetics of Foreign-Ion-Promoted Ilmenite Using Carbon Monoxide (CO) for Chemical Looping Combustion

In a previous work by our group, a new kind of ilmenite oxygen carrier, namely, natural ilmenite particles impregnated with foreign ions (K+, Na+, or Ca2+), was reported as having a promoted reactivity. The objective of this work was to investigate the detailed reduction kinetics of the new foreign-ion-promoted ilmenite carrier. Ilmenite particles impregnated with different types of foreign ions, including K+, Na+, and Ca2+, at different loading ratios were investigated by thermogravimetric analysis (TGA) using CO as the reducing gas. The samples tested by TGA for kinetics included the initial ilmenite particles and particles extracted from the fluidized-bed reactor after 40, 70, or 100 continuous redox cycles, as was done in the previous work. The obtained kinetic parameters were applied for the design of a chemical-looping combustion (CLC) reactor. The findings confirmed that the impregnation of foreign ions can promote the reduction rate of ilmenite. The kinetic data obtained by TGA indicated that the 15 wt % K+-promoted ilmenite showed a similar to 6.85 times faster reactivity than the activated raw ilmenite after 40 cycles in the fluidized bed. Even when the number of cycles was increased to 100, the modified ilmenite still maintained a much higher reduction rate than the activated raw ilmenite. Although the reduction rate of the 15 wt 96 K+-promoted ilmenite decreased as the CO concentration decreased, it still remained higher than the reactivity of the activated raw ilmenite. A theoretical calculation with the kinetic parameters indicated that the required minimum solids inventory of the fuel reactor was 1670 kg/MWth for the 15 wt % K+ promoted ilmenite, which was approximately one-seventh that required for the activated raw ilmenite (11436 kg/MWth).