Experimental Study On Plasma-enhanced Selective Catalytic Reduction Of Nitrogen Oxides

Due to the advantage of mature catalyst performance and strong resistance to sulfur poisoning, the selective catalytic reduction (SCR) with ammonia (NH3) as the reducing agent received much research interest, and is considered to be one of the most promising methods to meet the Euro V and above emission regulations within the specified time. However, the low NOx removal efficiency of SCR at low temperature condition is a big problem for diesel vehicle application. For this reason, partial pre-oxidation of NO into NO2 is needed to achieve the fast-SCR, which process is active at low temperature. Pre-oxidation of NO raise another challenge that the sulfur matters in the diesel exhaust easily devastate the precious metal catalysts usually used for NO oxidation.In order to solve the problems mentioned above, a new approach, plasma-enhanced selective catalytic reduction of NOx was proposed. In this paper, a novel SCR system combined with plasma reactor was established. Both simulated gas experiment and diesel engine bench test were investigated in this study which mainly consists of four sections as followed.(a) An experimental system of NH3-SCR for simulated gas was set up and on which, a series of experiments aiming to study the NOx removal efficiency of SCR system under different reaction conditions (component and concentration of gas mixture, reaction temperature and space velocity, etc.) were carried out. The results demonstrate that NOx removal efficiency was influenced by reaction temperature greatly. NOx removal efficiency increases with the increasing concentration of NO2 in the gas mixture, especially at low temperature. Peak NOx removal efficiency occurs when the ratio of [NO2]/[NO] reaches 1 at low temperature levels.(b) A dielectric barrier discharge (DBD) reactor was designed to partially oxidize NO to NO2. Several factors, including gas component, temperature and energy density were investigated on the NO pre-oxidation performance of DBD reactor with the presence of appropriate amount of O2. It was found that hydrocarbon species greatly affect NO pre-oxidation in plasma process by promoting NO coversion to NO2.(c) Removal of NOx in the system combing DBD reactor with SCR reactor was tested under different conditions (gas component, reaction temperature and energy density, etc.). The performance difference between SCR system and Plasma-SCR system was analyzed. The results reveal that with the addition of C3H6, the NOx removal efficiency can be improved significantly, but high C3H6 concentration leads to large amount of CO emission. The Plasma-SCR system has much higher NOx removal efficiency than that of fast-SCR process achieved by adding NO2 directly.(d) An engine test bench with Plasma-SCR system was setup based on YC6G diesel engine. NOx removal in both SCR system and Plasma-SCR system was tested under different conditions (engine speed, torque). At low temperature condition, NOx removal efficiency can be improved significantly by coupling SCR reactor with DBD reaction. However, the consumed energy in engine bench test is much higher than that of simulated gas experiment.