Atmospheric-pressure microwave plasma torch for energy-efficient NOx generation and direct production of high-concentration plasma-activated water for on-site fertilizer

• Developed microwave plasma-activated water (PAW) system for nitrogen fixation (NF) • System achieved the lowest energy cost among plasma NF methods • Stable, high-concentration PAW was produced and stored for six months • Dilution yielded 1,000-10,000 L of fertilizer with high energy efficiency • The approach is scalable for renewable-energy-driven fertilizer production Plasma-based nitrogen fixation (NF) is gaining prominence as a sustainable alternative to the Haber-Bosch process because it offers carbon-free operation, high energy efficiency, and potential for decentralized production. This paper reports the design and evaluation of a coupled microwave plasma torch-microbubble reactor system to enhance the efficiency of conversion from NO x to plasma-activated water (PAW) for high-concentration PAW production. Among the various plasma sources, atmospheric-pressure microwave plasma torches provide electrode-free, sTable discharges with high ionization rates, optimizing them for NO x generation. The effects of the N 2 /O 2 mixing ratio and specific input energy (SIE) were systematically investigated. At a N 2 /O 2 ratio of 1:1 and a SIE of 1,500 J/L, the system achieved an energy efficiency of 82 g (NO x )/kWh and energy cost of 1.86 MJ/mol (per mol of total NO x ), representing the lowest reported energy cost among atmospheric-pressure plasma-based NF technologies. Under the optimal condition, the system achieved the highest NO x production rate and directly produced highly concentrated PAW with a nitrate (NO 3 - ) concentration of 5 wt.%. After dilution to 100-1,000 ppm NO 3 - , the system generated 1,000-10,000 L of nitrate formulation with high energy efficiency (up to 543.4 L/kWh). The concentrated PAW maintained chemical stability over six months, and continuous operation for 16 h demonstrated operational durability. Collectively, the results highlight the feasibility of coupling microwave plasma-based PAW production with air separation units, enabling an on-site, renewable-powered nitrogen fertilizer supply.