Synthesis and characterization of geopolymer oxygen carriers for chemical looping combustion

Highlights: • A novel oxygen carrier based on geopolymers and hematite produced. • No-toxicity and low-price of the developed materials. • Iron oxide content up to 41% by mass. • Appreciable response for CH₄ conversion and high rate index. • Good fluidization properties and resistance to abrasion/agglomeration. - Abstract: The present article reports on the first application of geopolymers for the production of oxygen carriers for chemical looping combustion. Granules with different properties and in typical sizes for fluidized bed applications were produced starting from geopolymer/iron oxide slurries. These slurries were prepared according to a previously-developed formulation, modified by adding iron oxides and pore-forming agents to obtain oxygen carriers with different micro- and macrostructures. The performance of these novel oxygen carriers was tested in thermogravimetric equipment, measuring a capacity very close to the theoretical value 1.3 after repeated cycles. Tests conducted in a laboratory-scale differential reactor gave rise to a lower O-carrying capacity (<1.0%), but rather high kinetics (i.e. rate index) by comparison with other materials and published data. The analysis on the sample (ESEM, XRD and MIP) provided a reasonable interpretation of the phenomena observed, attributable mainly to the influence of internal porosity. Fluidization tests, elutriation/attrition results and the lack of any sign of particle agglomeration proved the suitability of the synthesized granules for use in fluidized beds.