Recent Progress of Plasma-Assisted Nitrogen Fixation Research: A Review

Nitrogen is an essential element to plants, animals, human beings and all the other living things on earth. Nitrogen fixation, which converts inert atmospheric nitrogen into ammonia or other valuable substances, is a very important part of the nitrogen cycle. The Haber-Bosch process plays the dominant role in the chemical nitrogen fixation as it produces a large amount of ammonia to meet the demand from the agriculture and chemical industries. However, due to the high energy consumption and related environmental concerns, increasing attention is being given to alternative (greener) nitrogen fixation processes. Among different approaches, plasma-assisted nitrogen fixation is one of the most promising methods since it has many advantages over others. These include operating at mild operation conditions, a green environmental profile and suitability for decentralized production. This review covers the research progress in the field of plasma-assisted nitrogen fixation achieved in the past five years. Both the production of NOx and the synthesis of ammonia are included, and discussion on plasma reactors, operation parameters and plasma-catalysts are given. In addition, outlooks and suggestions for future research are also given.

Comparative study of NO removal in surface-plasma and volume-plasma reactors based on pulsed corona discharges

Nitric oxide (NO) conversion has been studied for two different types of atmospheric-pressure pulsed-corona discharges, one generates a surface-plasma and the other provides a volume-plasma. For both types of discharges the energy cost for NO removal increases with decreasing oxygen concentration and initial concentration of NO. However, the energy cost for volume plasmas for 50% NO removal, EC(50), from air was found to be 120 eV/molecule, whereas for the surface plasma, it was only 70 eV/molecule. A smaller difference in energy cost, but a higher efficiency for removal of NO was obtained in a pure nitrogen atmosphere, where NO formation is restricted due to the lack of oxygen. For the volume plasma, EC(50) in this case was measured at 50 eV/molecule, and for the surface plasma it was 40 eV/molecule. Besides the higher NO removal efficiency of surface plasmas compared to volume plasmas, the energy efficiency of surface-plasmas was found to be almost independent of the amount of electrical energy deposited in the discharge, whereas the efficiency for volume plasmas decreases considerably with increasing energy. This indicates the possibility of operating surface plasma discharges at high energy densities and in more compact reactors than conventional volume discharges.

A dual-use of DBD plasma for simultaneous NO(x) and SO(2) removal from coal-combustion flue gas

Dielectric barrier discharge (DBD) was investigated for the simultaneous removal of NO(x) and SO(2) from flue gas in a coal-combustion power plant. The DBD equipment was used in either a mode where flue gas was directed through the discharge zone (direct oxidation), or a mode where produced ozonized air was injected in the flue gas stream (indirect oxidation). Removal efficiencies of SO(2) and NO for both methods were measured and compared. Oxidation of NO is more efficient in the indirect oxidation, while oxidation of SO(2) is more efficient in the direct oxidation. Addition of NH(3), has lead to efficient removal of SO(2), due to thermal reaction, and has also enhanced NO removal due to heterogeneous reactions on the surface of ammonium salt aerosols. In the direct oxidation, concentration of CO increased significantly, while it maintained its level in the indirect oxidation.

Reaction pathways of NO oxidation by sodium chlorite powder

NO oxidation is an important prerequisite step to assist selective catalytic reduction at low temperatures (< 250 degrees C). If sodium chlorite powder (NaClO2(s)) can oxidize NO to NO2, the injection of NaClO2(s) can be simply adapted to NO oxidation. Therefore, we explored the reaction pathways of NO oxidation by NaClO2(s). Known concentrations of NO and NO2 in N2 balance were injected into packed-bed reactor containing NaClO2(s) at 130 degreesC. NaClO2(s) oxidized NO to NO2 which reacts again with NaClO2(s) to produce OClO. Comparison of experimental data with simulation results demonstrates that each NO2 molecule removed by the reaction with NaClO2(s) generated one OClO molecule, which also oxidized NO to NO2 with the production of ClNO and ClNO2. Using these results, we conclude that the oxidation of NO by NaClO2(s) occurred by two pathways. One is through the direct reaction of NO with NaClO(s). The other is through both the reaction of NO with OlCO produced by the reaction of NO2 with NaClO2(s) and the reaction of NO with ClO produced by the reaction of NO with OClO.