A pilot-scale thermal plasma gasification system (TPGS) for waste-to-energy (WTE) was experimentally investigated to evaluate NO x formation and mitigation during plasma operation using air and nitrogen. NO was the predominant species, mainly generated through the vibrationally enhanced Zeldovich mechanism under high specific energy input (SEI) conditions. NO x concentrations decreased from 2545 – 3363 ppm to 1043 – 1162 ppm for air plasma and from 4827 – 5009 ppm to 1676 – 2095 ppm for N 2 plasma downstream of the wet scrubber at an SEI of 684 – 1073 kJ/mol, with reduction efficiencies of 31% – 42% for NO and 75% – 81% for NO 2 . Long-term operation demonstrated stable performance and an acidification rate of the scrubbing solution equivalent to 2.9 neutralization cycles per day, requiring approximately 1.4 kg/day of NaOH. This experimental study shows the first results of pilot-scale TPGS under high SEI conditions and provides key parameters for effective NO x control in plasma-based WTE systems, in comparison with previous research at low SEI and laboratory scales. • A pilot-scale thermal plasma gasification system was demonstrated for NO x characterization. • NO formation was governed by the vibrationally enhanced Zeldovich mechanism under high SEI. • Wet scrubbing significantly reduced NO 2 and partially mitigated NO emissions (p < 0.05). • Experiments offer practical NO x control in plasma-based waste-to-energy systems.