Decomposition of NOx with low-temperature plasmas at atmospheric pressure:: Neat and in the presence of oxidants, reductants, water, and carbon dioxide

The fundamental aspects of decomposition of NO with atmospheric pressure low-temperature plasmas have been systematically investigated in this study. A charge-coupled device detector has been employed to record the emission spectra of plasma-induced reactions for mechanistic analyses. Specific energy consumption for the process (kilojoules per mole of converted NO) has been defined and discussed in detail. Variables such as initial NO concentrations ([NO](0)), space velocities, and input voltages are all important in decomposition of NO using plasmas. Additives such as O-2, carbon, CO, CO2, water vapor, and hydrocarbons also produce significant effects. Effective direct decomposition of NOx of 250-10000 ppm has been achieved using novel all-quartz tubular plasma reactors, which show good activity and excellent stability,especially in the presence of water. The relatively lower initial activity and very good stability with all-glass reactors relative to metal-electrode reactors (which deactivate markedly) suggest that NO decomposition with plasmas in quartz reactors is noncatalytic while catalytic effects are involved with metal-electrode reactors. In addition, the all-quartz reactors have much higher energy efficiency for NO, decomposition than metal reactors. The kinetic equation of NO, decomposition can be expressed as -d[NOx]/dt = k[NOx](1/4) for a 1% NO/He system. An Arrhenius correlation has been established between the rate constant k and the measured voltage during plasma reaction. The addition of CO, ethane, or carbon species considerably increases the conversion of NO by reducing NO, into COx and N-2. The conversion of NOx may be enhanced by the addition of O-2 and is inhibited by the presence of water vapor or CO2. Conversion of NOx decreased on increasing the concentration of H2O or CO2. A mechanism based on the interaction between excited species of carrier gas and NO (comprising the participation of radicals such as N ., O ., OH, and H .) has been proposed on the basis of emission spectroscopic data.