Emission reduction by means of low temperature plasma. Summary

The work performed during the project is summarised. In the project we focused on removal of nitrogen oxides NOx (NO, NO₂) and, in particular, on removal of nitrogen monoxide (NO) by injection of plasma-produced reactive agents. As reactive agents we tested ozone (O3), NH and NH2 radicals from ammonia (NH₃) and nitrogen atoms (N) generated in dielectric barrier discharges (DBDs). Hydrazine (N₂H₄) as a reducing agent and direct plasma treatment of the entire exhaust gas was investigated as well. We performed laboratory experiments on synthetic exhaust gases, modelling of the mechanisms and a demonstration of the technique on a test engine, a 30 kW combustion engine fuelled with natural gas.

We achieved the best results with ozone injection into the exhaust gas. This technique is based on oxidation of NO to N₂O₅ that is subsequently removed from the exhaust gas by a scrubber. In the laboratory experiments we obtained complete removal of NO by injection of an ozone/oxygen mixture from an oxygen (O₂) DBD into synthetic exhaust gases with compositions close to those of real exhaust gases.

Ozone injection was chosen for demonstration of NO_x removal from the exhaust gas of the test engine. We achieved 95 % NO_x removal with an oxygen DBD and 70 % NO_x removal with an air DBD. The running costs relating to the demonstration were estimated to be approximately 30 DKK per kg removed NO_x for the oxygen DBD, and 25 DKK per kg removed NO_x for the air DBD.

For a commercially available ozone generator, the total costs of NO_x removal from a 3 MW power plant were estimated to 27 DKK per kg removed NO_x for oxygen as the feed gas, and 21 DKK per kg removed NO_x for air as the feed gas.