Optimization of working parameters for double-dielectric non-thermal plasma reactor and spectrography analysis of air discharge

Abstract: The working principal of selective catalytic reduction (SCR) system is that urea solution, whose mass fraction is 32.5%, is injected into exhaust pipe. NH3 and HNCO are generated and mixed with NOx under the condition of high-temperature exhaust. Finally, N2 and H2O are generated in the catalytic reduction reaction of NH3 and NOx. In the case that NO accounts for 90% or even more in NOx of the diesel engine exhaust, the standard SCR reaction plays a dominant role when using SCR system to deal with NOx of the diesel engine exhaust. This reaction can provide high reduction efficiency when the exhaust temperature is between 300 and 450℃. However, the reduction efficiency of NOx will drop rapidly at lower exhaust temperature. So how to improve the reduction efficiency of NOx at lower exhaust temperature is an urgent problem to be solved. Non-thermal plasma (NTP) technology combined with SCR system is one of the most effective means to solve this problem. Active substances, generated in NTP reactor, can oxidize the part of NO from diesel exhaust to NO2 and improve the conversion efficiency of NOx at lower exhaust temperature. The ideal working parameters selected in experiment, can not only increase the concentration of active substances, but also avoid combining main gas components in exhaust such as N2 and O2. A test system of a double-dielectric non-thermal plasma reactor was established to conduct the air discharge test. To study the effect of working parameters on the performance of double-dielectric non-thermal plasma reactor, the changing rules of the volume fractions of NO and NO2 with several parameters such as discharge voltage peak-peak value, discharge frequency and air flow were researched. The results showed that, discharge frequency had great influence on the volume fractions of NO and NO2. Reaction mechanism of air discharge in the NTP reactor changed as discharge frequency changed. The volume fractions of NO and NO2 grew linearly as discharge voltage peak-peak value increased, when discharge frequency was 7 kHz. When discharge frequency was 8 kHz, with the increase of discharge voltage peak-peak value, the volume fraction of NO kept constant at the start and then increased, and the volume fraction of NO2 increased linearly. When discharge frequency was 9 kHz, with the increase of discharge voltage peak-peak value, the volume fraction of NO maintained at low level, and the volume fraction of NO2 kept constant at the start and then increased. The increase of air flow led to the decline of reaction rate. Under the same discharge voltage peak-peak value, the volume fractions of NO and NO2 decreased as air flow increased. The change of air flow could not alter the reaction mechanism of air discharge, and the rule between the discharge voltage peak-peak value and the volume fraction of NO and NO2 did not change. Under the same discharge frequency, spectral intensity of discharge region increased with the increase of discharge voltage peak-peak value. Under the same discharge voltage peak-peak value, spectral intensity of discharge region decreased as discharge frequency increased in the range of 7-9 kHz. This study can provide the theory basis for using NTP to oxidize part of NO from diesel exhaust and increase the conversion efficiency of NOx in lower exhaust temperature range.