PLASMA ASSISTED CONVERSION of N2O to NO and NO2


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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 NOx) 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:

N2O + 0.5 O2 = 2 NO - 99.2 kJ/mole N2O

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 a 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, also 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 NOx in the gliding discharge. The total N2O conversion rate was up to 60 % whereas up to 68 % of converted N2O was transformed to NOx. The NOx concentration in the exit gas was as high as 8.9 % (in mass) when no NOx was present in entry gas.
 

The experimental results were compared with theoretical calculation of NOx 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 concentration of oxygen and nitrogen atom 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 NOx.

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 less energy and is feasible for industrial application.



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