Kinetic study of microwave heating-assisted chemical looping dry reforming of methane over magnetite

Dry reforming of methane carried out via the chemical looping concept and employing microwave heating is a sustainable syngas (a mixture comprising hydrogen and carbon monoxide) production technology. Understanding the intrinsic reaction kinetics of reduction and oxidation is essential for successful scale-up of this technology. By employing magnetite as a microwave absorber oxygen carrier, we investigated the reaction kinetics at bulk temperatures in the range of 650-800 degrees C for reduction and 500-650 degrees C for oxidation. Results indicated both reactions followed a phase-boundary controlled (contracting sphere) reaction mechanism. Upon developing the reaction kinetics based on solid temperature relevant to microwave-heated particles, we estimated the activation energy to be 85 kJ/mol for the reduction reaction and 22 kJ/mol for the oxidation reaction. By developing the reaction kinetic of the reduction under microwave heating and based on bulk temperature, we estimated 68 kJ/mol as the activation energy of the reduction reaction. Comparing these values with the activation energy of magnetite reduction by methane under conventional heating (around 90 kJ/mol) indicated that microwave irradiation apparently decreased the activation energy. Consequently, by developing the reaction kinetics based on an appropriate temperature, i.e., solid temperature, we demonstrated that microwave primarily had a thermal effect in our study, increasing the reaction rate constant, rather than a non-thermal effect, like altering the activation energy.