Reducing energy cost of NO_x production in air plasmas

Current worldwide fixed nitrogen utilization efficiency (NUE) is low, leading to significant environmental problems. An important aspect of this problem involves NH₃ released from bacterially degraded, N-containing organic waste such as animal manure. Acidifying organic waste with nitric acid traps NH₃ as NH₄NO₃, thus increasing the nitrogen content of the resulting organic fertilizer and reducing the environmental effects of NH₃ emission. Air plasmas have been proposed for this purpose through production of NO_x from air which can be readily converted to nitric acid. However, a major challenge remains in reducing the energy cost of NO_x production in air plasmas to make the technology economically competitive. Different types of discharges such as arcs, sparks, dielectric barrier discharges have been investigated for NO_x production but they are challenging to compare due to differences in their structure, geometry and excitation modes etc. Different discharges also utilize potentially different chemical reaction pathways to produce NO_x. As of yet, there are no general evaluation criteria to determine the qualities of an appropriate discharge configuration for energy-efficient NO_x production. In this study, we report on four types of discharges; dielectric barrier, glow, spark and extending arc discharge to examine NO_x production efficiency under different conditions. Based on our results and previously published results from the literature, we propose a dimensionless parameter to guide the design of appropriate plasma sources for reduced energy cost NO_x production.