Nonthermal plasma removal of CH4-NOx under LNG engine exhaust environment: experiment, mechanism and kinetic analysis

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To explore the mechanism underlying the removal of CH4 and NOx, which are typical emissions from LNG engines, through the nonthermal plasma method, a plasma chemistry model of CH4-NOx under the LNG engine exhaust environment was established in this study. Sixty key reactions affecting the conversion of CH4, CO, and NOx were determined through sensitivity analysis, and their pre-exponential factors were optimized using a genetic algorithm. The proposed model shows good performance in predicting the concentrations of CH4, CO, and NOx under the exhaust conditions of LNG engine. Then, reaction path analyses for evolution of CH4, CO, NO, and NO2 were performed under specific conditions. The results showed that CH4 mainly decomposed into CH2 that is subsequently converted into CO and CO2. CH2 is the main source for CO production. The concentration of NOx is determined by the oxidation reactions of N with O2, OH, and HO2. The specific concentration distributions of NO and NO2 were influenced by the oxidation–reduction reactions between them. Analyses of the time scales for the conversion processes of CH4, CO, and NOx were also conducted, and it was found that the reaction time scales of CH4 and CO were approximately 1 × 10−10−1 × 10−5 s, and that was approximately 1 × 10−9−1 × 10−4 s for NOx. Furthermore, during the overlapping period for the conversion of CH4, CO, and NOx, the NOx was dominant in the competition for O. This study provides a basis for the construction of a plasma catalytic chemistry model of CH4-NOx under LNG engine exhaust conditions.