The Destruction of Nitric Oxide by Nitrogen Atoms from Plasma Jets

The reduction of NO by N generated from a stream of N 2 by a novel design of plasma jet is studied with particular reference to the energy requirement and the efficiency of the process. An analysis of the thermodynamics shows that the absolute maximum efficiency-in the absence of heat losses, recombination, etc- is about 68 per cent at a specific enthalpy of 1.2 MJ mol −1 of Na (approx. 8,500 K) giving a minimum requirement of approx. 0.9 percent of the heat of combustion released for every 1000 ppm NO to be removed from the products of combustion of a typical hydrocarbon. Similar calculations for argon/nitrogen mixtures show that, while the efficiency is always somewhat reduced, optimum values should be attainable at very much lower specific enthalpies for appreciable dilution by a monatomtc carrier gas. Experimental measurements of the proportion of NO destroyed from a stream injected at various points in the plasma flow and at various plasma velocities reveal efficiencies which are surprisingly high and remain so for appreciable distances downstream. They cannot be accounted for in terms of the mean specific enthalpy of the nitrogen, which is far too low, and of the calculated recombination rate of N, which is far too high. After considering the possible role of long-lived excited states of N 2 , it is concluded that these results are due to the thermal stratification of the plasma jet which heats a very small proportion of the gas to exceedingly high temperatures. The small pockets of very hot gases are recorded optically and monitored by ionization probes. Since most of the nitrogen is not heated to very high temperatures, efficiency is also considered in terms of the flow of N 2 .