Nitrous oxide is known as a greenhouse gas and is suspected to participate in stratospheric ozone depletion. Among the identified anthropogenic sources, adipic acid production by nitric oxidation of cyclohexanol and/or cyclohexanon is one of the most important, another being nitric acid production itself. Several ways to eliminate N₂O exist or are in development, e.g. thermal destruction or catalytic decomposition.
 

Our aim is rather to transform as much as possible of a N₂O waste into NO and NO₂ (called NO_x) in order to re-use these oxides in Nylon plants where HNO₃ is needed for the oxidation to the adipic acid. The global endothermal process:

should be performed industrially, at rather low temperature (to limit N₂O decomposition), in oxidising atmosphere, in a sufficiently energy-rich medium.
 

A new environmental-friendly electrical assisted process has been found out to fulfil all these conditions when performed in so-called GlidArc reactor producing non-thermal plasma at atmospheric or higher pressures. The bench-scale GlidArc reactor consisted of a cylindrical 2.5 L steel vessel in which gliding electrical discharges were run. Inside, three pairs of the knife-shaped steel electrode blades were placed around a gas inlet nozzle. Before entering the reactor, the gas could be preheated in an annular path around the vessel. The electrodes were supplied from an AC 3-phase 50 Hz high-voltage power supply. Low-current arc discharges are formed between electrodes. Due to the fast flow of the operating gas (> 10 m/s) and the diverging form of the electrodes, the discharges "glide" along them in the direction of the gas flow until they extinguish. The lifetime of the single discharge is a few milliseconds. Several discharges are present at the same time because of the electrical phase shift at this multi-electrode system. This makes it possible to cover a large part of the inter-electrode space with discharges, increasing thus the efficiency of discharge interaction with the processed gas. Dry or wet N₂O/N₂/O₂/CO₂/NO₂ mixtures (close to the industrial off-gas composition, eventually enriched in oxygen) were processed. The molar percentage of N₂O in the mixtures was 32 to 51 %. The flow-rate of the operating gas through the atmospheric pressure reactor ranged from 1.3 to 4.9 m³(n)/h. The electric power) of the discharge was 0.62 to 1.2 kW. The temperature of the preheated operating gas could reach 355°C.
 

The N₂O in gas mixtures was both partially decomposed and transformed into NO_x in the gliding discharge. The total N₂O conversion rate was up to 60 % whereas up to 68 % of converted N₂O was transformed to NO_x. The NO_x concentration in the exit gas was as high as 8.9 % (in mass) when no NO_x was present in entry gas.
 
 

The experimental results were compared with theoretical calculation of NO_x formation rate in both thermal and non-equilibrium plasmas. The GlidArc discharge has higher efficiency of N₂O conversion with respect of both non-equilibrium cold and thermal plasmas! It can be related with the transient regime of the plasma generation where high values of the electron and vibrational temperatures provide a super-equilibrium oxygen and nitrogen atom production and relatively (with respect to cold plasma) high level of translational temperature and high level of vibrational excitation which permit to stimulate reactions of N₂O conversion into NO_x.

A new GlidArc II reactor and new modeling results show a significant process efficiency increase. The process of N₂O conversion stimulated by non-equilibrium plasma of the Glidarc II will consume a minimum of energy and is feasible for industrial application.