Reduction Kinetics of Ilmenite Ore as an Oxygen Carrier for Pressurized Chemical Looping Combustion of Methane

Reduction kinetics of ilmenite ore as an oxygen carrier for methane chemical looping combustion under elevated pressures was studied using a pressurized thermogravimetric analyzer (PTGA). The reduction phase of the experiments was carried out in a mixture of methane and nitrogen with carbon dioxide and/or steam to simulate an actual combustion environment. The oxidation phase of the experiments was carried out with air. Effects of temperature (1123-1223 K), total pressure (0.6-1.6 MPa), methane partial pressure (0.23-0.64 MPa), and particle size (45-300,mu m) were studied. Tests were also carried out to examine the effect of higher redox cycle numbers on the performance of ilmenite ore. The results showed that increasing the total pressure reduced the rate of conversion during ilmenite ore reduction, e.g., when the total pressure increased from 0.9 to 1.6 MPa, the reduction conversion rate decreased by 6-14% depending upon the reaction temperature because the negative effect of the pressure was less pronounced at higher temperatures. However, this negative effect can be compensated for by increasing the methane partial pressure. Increasing the methane pressure while maintaining the same methane/carbon dioxide ratio and total pressure increased the rate of reduction of ilmenite ore without affecting the oxygen carrying capacity until 1198 K. A kinetic model based on a phase-boundary-controlled mechanism with a contracting sphere was developed in the Arrhenius form using these experimental data. The model shows activation energies of 28.2 and 76.4 kJ mol(-1) K-1 at 0.9 and 1.6 MPa, respectively, and it was able to reproduce the test results with a conversion ratio of up to 70%.