A new kind of ilmenite oxygen carrier was prepared by impregnating the raw ilmenite particles with K 2 CO 3 , Na 2 CO 3 , or Ca(NO 3 ) 2 . The cyclic reduction reactivity of the new oxygen carrier was investigated in a fluidized bed reactor. It has been found that the addition of foreign ions can significantly promote the reduction reactivity of ilmenite. The effect of foreign ions on enhancing the reduction reactivity of ilmenite is in the sequence of K + > Na + > Ca 2+ . The effect of the loading amount of K + on increasing the ilmenite reactivity is in the sequence of 15 wt % K + > 10 wt % K + > 5 wt % K + . The reduction reactivity of the ilmenite impregnated with 15 wt % K + can be improved ∼8 times faster than that of the activated raw ilmenite. This reactivity reaches up to the same level of the synthetic Ni-based carrier. The modified ilmenite obtains a porous structure caused by the migration of K or Na ions. One possible explanation for the reactivity enhancement of ilmenite with the addition of foreign ions, especially K + and Na + , is the migration of K + or Na + . Another explanation may be the active principle of the alkali-rich phase formed between the foreign ions and titanium iron oxides, that is, K 1.46 Ti 7.2 Fe 0.8 O 16 or Na 2 Fe 2 Ti 6 O 16 . This study proves that the reduction reactivity of the natural ilmenite can be promoted significantly by impregnating ilmenite with K + .
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 ∼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 % 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).
Chemical looping combustion (CLC) is an advanced technology with inherent CO 2 capture in which a solid oxygen carrier circulates between an air reactor and a fuel reactor. For coal-fueled CLC, the existence of solid impurities requires the oxygen carrier not only to have good reactivity but also to be contaminant-resistant, low-cost, and readily available. Therefore, the development of cost-effective and well-performing oxygen carriers is very meaningful for the coal-fueled CLC process. Natural red mud, a byproduct from the aluminum industry, was found to function well as an oxygen carrier and has also been found to have in situ coal catalytic gasification behavior. A thorough study on the longterm cyclic performance of red mud with coal char in a fluidized reactor was conducted in this work. For the purpose of comprehensively understanding the functions of inert supports as well as the sodium content in red mud, the effect of various inert oxides (Al 2 O 3 , SiO 2 , TiO 2 , and CaO) and the addition of sodium was evaluated. It has been proven that inert supports, Al 2 O 3 , SiO 2 , and TiO 2 , have a positive effect on both the reduction and oxidation reactivity of iron-based oxygen carriers by developing a porous structure in the particle. Al 2 O 3 and SiO 2 show the ability to stabilize the reactivity of iron oxide with a gaseous reductant (CO), even under fluidized conditions. Both Al 2 O 3 and TiO 2 can assist in maintaining the mechanical strength of the oxygen carrier after many cycles in a fluidized-bed reactor. The addition of sodium (Na) to red mud does not exhibit much effect on the reactivity of OC with CO as the fuel. However, it can significantly enhance the char gasification rate due to its catalytic function. Additionally, interaction between the active iron oxide and inert supports or sodium in the form of red mud at high temperatures leads to the formation of spinel phases. The growth of spinel phases results in the reduction of the oxygen carrying capacity. However, it helps fix sodium as a relatively stable chemical compound (NaAlSiO 4 or NaFe 0.25 Al 0.75 O 2 ). Both inert supports and sodium in natural red mud play critical roles in the performance of red mud as an oxygen carrier from either physical or chemical aspects.
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