Coherent Evaluation of Energy Efficiency for Microwave Catalytic Reactors Based on Reaction Equilibrium

Microwave-driven catalytic processes have attracted the attention of many researchers and industries as these processes are powerful strategies for use in industrial electrification. However, undetectable hot spots inside the catalyst bed can be misleading in terms of the deceptive effects of microwaves on acceleration of catalytic reactions because of the lack of physicochemical knowledge about these processes. In this paper, the hot spots temperatures were estimated based on catalytic ammonia (NH3) synthesis/decomposition equilibrium. The reaction temperature estimated from the chemical equilibrium was more than 100 degrees C higher than the catalyst bed surface temperature measured with an infrared thermometer under microwave irradiation. Assuming a formation of very steep local high temperature fields, rigorous kinetic analysis led to an estimate that the active site fraction which reaches the high temperature was only about 10 % of total sites. Compared with a resistivity heating furnace, the energy input was reduced by more than 85 % under microwave irradiation. The temperature estimated by the chemical equilibrium was almost corresponding to the temperature estimated by an oscillation of extended X-ray absorption fine structure recorded under microwave irradiation. This paper presents a proposal for a method to estimate hot spots based on thermodynamic equilibrium, yielding a quantitative evaluation of energy consumption reduction caused by microwaves. Microwave heating of powder catalysts results in uneven temperatures of the catalyst. When the surface temperature (Tsurf) was 300 degrees C, the temperature of various elements was analyzed based on extended X-ray absorption fine structure (EXAFS; TEXAFS) of 420 to 440 degrees C. In ammonia (NH3) synthesis/decomposition equilibrium, the temperature involved in the reaction was defined by calculating the temperature from the equilibrium conversion ratio (Teq.), which was compared to normal heating and achieved energy savings of up to 85 %.image