Absorption Chemistry of Plasma Generated NOx Gas for Green Nitric Acid Production

Nitric acid (HNO3) is predominantly produced in large production plants using the Ostwald process. In view of its widespread application as synthetic fertilizer, small scale and local production becomes of interest. The chemical precursor of nitric acid is NOx gas which can be produced from air at percentage-level concentration using small-scale electrically powered warm plasma reactors. The absorption and disproportionation chemistry of concentrated NOx streams under pressure and in minimal amounts of water, as in the Ostwald process is well documented. However, the chemistry at lower NOx concentration levels, without pressurization and in presence of excess water, relevant for nitric acid production with NOx streams from warm plasma has received much less attention. In this work, the NOx absorption chemistry is investigated at pH values of the scrubber solution in the range of 1 to 5. The kinetics of NOx absorption are found to be strongly pH dependent, requiring up to hours of recirculation to reach steady state conditions. Once at steady state conditions, the NOx removal efficiency turns out to be rather pH independent. The formation of nitrous acid (HNO2) byproduct reaches a constant value based on a dynamic equilibrium of its formation and decomposition. Approaches to minimize undesired nitrite and nitrous acid byproduct formation are discussed.