Pulse-modulated microwave air plasma for nitrogen fixation as NO x

Abstract A pulse-modulated microwave air discharge operating at 2.475 GHz was applied for the conversion of nitrogen ( N 2 ) and oxygen ( O 2 ) into nitrogen oxides ( NO x ), including NO, NO 2 , and N 2 O 4 . The effects of pulse modulation frequency, duty cycle, flow rate and O 2 content were investigated for better understanding and optimization of the NO x productivity and the corresponding energy cost (EC). The results indicated that high-frequency pulses (10–100 kHz) had a minimal effect on NO x productivity and EC. However, the duty cycle significantly influenced EC, leading to a reduction of approximately 21% when decreased to 40% compared to full load. Meanwhile, the reduction in duty cycle considerably decreased the plasma temperature, lowering it from around 4900 K at 100% to around 2700 K at 40%. The flow rate also had a significant effect on both NO x productivity and EC; higher duty cycles and larger flow rates increased productivity, while lower duty cycles and higher flow rates achieved the lowest EC, with a minimum value of 2.98 MJ mol −1 . As the O 2 content increases from 10 % to 90 % , NO x productivity initially rises and then declines, while EC follows opposite trend. The maximum NO x productivity and minimum EC occur at O 2 content of approximately 40 % or 50 % , with the lowest EC reaching 3.03 MJ mol −1 . This study demonstrates the effective reduction of NO x EC under various conditions by adjusting duty cycle, flow rate, and