Dielectric Barrier Discharge Reactor for the Removal of NOx From Automotive Exhaust


 In this study, a self-made wire-cylinder dielectric barrier discharge (DBD) reactor was used to remove NOx. The influence of electrical and gas parameters (e.g. structure, voltage, and frequency) and temperature on the NOx removal rate was studied systematically while operating the DBD reactor with a high-voltage positive–negative double pulse power supply. The experimental results showed that following conditions led to the optimal NO conversion rate and NOx removal rate: voltage of ±12 kV, pulse frequency up to 60 Hz, oxygen concentration at 6%, reaction temperature at 300°C, and C2H2:NOx ratio at 1.5. Under these conditions, the NO conversion rate and NOx removal rate reached the highest levels of 76.4% and 31.2%, respectively. Additionally, when the process was run in conjunction with a La0.7 Sr0.3 Ni0.5 Mn0.2 Fe0.3 O3 catalyst, the reactor efficiency increased markedly, and the NO conversion and NOx removal rates increased to 94.93% and 74.97%, respectively. The findings of this study demonstrate that DBD reactor technology shows promise for the removal of NOx from automotive waste streams.


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Nitrogen oxide from automotive exhaust is widely recognized as an important air pollutant.  removal of NOx by DBD are focused on the following three aspects: the power supply, reactor 53 structure, and chemical composition. Among these, the influence of the power supply can be 54 studied through changes in the voltage type, power frequency, and so forth. The aim of this 55 study was to improve the NOx removal rate from automotive exhaust by optimizing the power 56 supply, reactor structure, and operational parameters of a DBD reactor. The study showed that 57 the reactor had a good NOx removal efficiency and the removal of NOx was greatly enhanced 58 in the presence of the catalyst. The experimental system can be divided into the following three parts: gas path system, 64 reaction system, and analysis system. The gas path system was composed of the gas cylinder, 65 mass flow controller (MFC), tube furnace, and so on. The reaction gas (N2, O2, NO, CH4, etc.) 66 was controlled by a mass flow meter in the MFC and was introduced into the reactor through 67 a stainless steel tube with a diameter of 3 mm. In our research, the reactor was placed in the 68 tube furnace after the reaction, and the gas was introduced into the instrument through a 6 mm 69 stainless steel pipe.

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The reaction system was composed of several parts. The diameter D of the high-voltage 71 discharge electrode of the barrel-type DBD reactor, which consisted of a threaded copper rod, 72 was 10 mm, the inner diameter d of the dielectric tube was 16 mm, and the quartz glass was

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An analysis system was used to measure the performance of the reactor. Specifically, the 79 NOx removal and NO conversion rate of the reacted gas were analysed by a nitrogen oxide 80 analyser, and the O3 production amount was analysed by an O3 analyser.

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The exhaust components of small displacement engines are related to the engine type and 82 operating status. Generally, the concentration of oxygen is approximately 2-11%, the 83 concentration of NOx is approximately 100-500 ppm, and the ratio of hydrocarbons (HCs) to 84 NOx is approximately 0-2. According to these values and the existing conditions in the 85 laboratory, in our study, we adopted the following values: oxygen concentration range of 2- consisted of a threaded copper bar, which had a diameter of 10 mm. In our research, the outside 94 of the reactor was wrapped with aluminium foil to serve as the ground electrode and the length 95 of the aluminium foil was adjustable; a copper wire was used for fixing the grounding. The two 96 ends of the dielectric tube were sealed with a tetrafluoro joint and connected to a three-way 97 valve to control the entry and exit of the reaction gas. The power supply had the following characteristics: AC SINGLE-PHASE 220 V 10%, 501 101 Hz; rated output power of 600 VA. The pulse polarity was positive and negative bipolar, and 102 the power supply output pulse peak voltages were as follows: positive pulse +5 kV to +50 kV, 103 continuously adjustable; negative pulse -5 kV to -50 kV, continuously adjustable. The pulse 104 repetition frequency was continuously adjustable from 0 to 200 Hz, with a pulse width ≤500 105 NS and pulse rise front ≤200 NS. The experimental procedure consisted of the following steps.

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(1) Connect the experimental instruments according to the flow chart of the experimental 111 system shown in Fig. 3 and then, check the instruments to ensure that they are working properly.

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Set the mass flow measurement range according to the gas atmosphere required for the 113 experiment.

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(2) Open the valve to let in nitrogen and check whether the air tightness in the air passage 115 is good. (4) Turn on the power supply and the digital oscilloscope, set up the discharge parameters, 120 adjust the voltage to the required value to carry out the discharge experiment, and record the 121 concentration of the nitrogen oxide, instantaneous voltage, and current of the oscilloscope after 122 the discharge is stable.

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(5) After completing a set of tests, adjust the mass flow meter to change the volume and 124 concentration of gas, and ventilate the gas in the air path. Repeat the above steps to start a new 125 test after the air path becomes stable.

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(1) Measurement of the discharge power 129 In the experiment, the discharge power was measured by the instantaneous power method 130 with the discharge instantaneous voltage; the instantaneous current and voltage were recorded 131 by an oscilloscope, and the effective energy e was obtained by using Origin software: where E is the pulse effective energy (j), T0 is the starting time of the pulse front at the 133 beginning of the pulse injection, T is the effective time of the pulse, and P is the effective power. (2) Calculation of the nitrogen oxide removal rate 136 The formula for calculating the NO conversion rate is shown in equation (3):

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(3) The formula for calculating the NOx removal rate is shown in equation (4): The formula for calculating the NO2 production quantity is shown in equation ( which will make subsequent discharges more difficult and weaken the discharge current. When

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O atoms and O2 are oxidized to O3, the O3 reacts with NO to produce NO2, but as the reaction 157 proceeds, the concentration of NO2 increases and the NO2 reacts with O atoms to produce NO, 158 which will reduce the removal efficiency of NO. The specific reactions are as follows: With an increase in voltage, the concentration of NO2 increases, which leads to a decrease

Effect of pulse frequency on the NOx removal rate
At a 2% oxygen concentration, Fig. 6 shows the effect of the pulse frequency on the NOx 166 removal rate. As can be seen from the diagram, the NO conversion rate and NOx removal rate 167 both increased when the pulse frequency increased from 30 to 60 Hz, and the values reached a 168 maximum of 42.85% and 23.24%, respectively, when the pulse frequency was increased from 169 60 Hz to 70 Hz; meanwhile, the NO conversion rate and NOx removal rate decreased to 36.12% 170 and 22.27%, respectively, when the pulse frequency was increased from 60 Hz to 70 Hz. In Fig.   171 6, the trend of NO2 was the same as that of the NO, and in Fig. 7 the highest production quantity when the gas flow rate was 1.3 L/min. Fig. 9 shows that the NO2 production decreased with an 188 increase in the gas flow rate, and the decrease was smaller after the gas flow rate reached 1.1 189 L/min, which was the same trend observed for the NO conversion. In order to achieve an 190 optimal pollutant gas treatment capacity, higher energy efficiency, and NO conversion rate, 1.1 191 L/min was selected as the experimental gas flow.

Effect of oxygen concentrations 194
The exhaust from an automobile contains relatively high concentrations of oxygen (2-10%).  (10) and (11): interacts with O2, through the following reaction equation Because the concentration of oxygen is much higher than the concentration of NO, this 211 contributes to improvements in the production quantity of NO in formula (12), and the quantity 212 is much higher than that in reaction (9); furthermore, NO will be produced continuously, 213 although NO will be reduced by 4 () NS and the NO will be oxidized to NO2 by O, O2, and 214 O3. However, as the concentration of oxygen increases gradually, reaction (12) produces too much NO to effectively remove NO. At low NOx concentrations, a large proportion of NO is 216 converted to NO2, and this process reaches a dynamic equilibrium. It can be seen from Fig. 12 217 that when the NOx concentration is 200 ppm, the amount of NO2 generated will be significantly 218 higher than 100 ppm.

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During the process of discharge, some O2 is oxidized to O3 by the action of high energy 220 electrons and active radicals, which contributes to the oxidation of NO, but the amount of O3 221 is not very large. As shown in Fig. 13, the ozone produced by the discharge at a NOx  NOx removal rate, and NO2 production. It can be seen from Fig. 16 that the NO conversion rate  increase in the ratio between the concentration of C2H2 and the initial concentration of NOx. with the addition of CH4 at the same proportion, the removal rate of NOx increased by 6%.

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After the addition of C2H2, the following reactions occurred in the reactor:  Fig. 19 shows the influence of different C2H2 additions on NO2 production. It can be seen 296 from the figure that the production of NO2 increased with the increase in the concentration ratio 297 of C2H2 to NOx, and the maximum production of NO2 was 74 ppm. Since the main reactions 298 after the addition of C2H2 are formulas (19) and (20), the production of HO2 with a strong 299 oxidation potential is less than that with CH4, so the production of NO2 is less than that with 300 CH4 gas. reactor and used it to remove NOx. The main findings are as follows.

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(1) The optimal voltage was ±12 kV, and the frequency was 60 Hz. The optimal gas flow 351 rate was 1.1 L/min, the optimal oxygen concentration was 6%, and the optimal initial 352 concentration of NOx was 400 ppm.

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(2) The NO conversion rate and NOx removal rate were improved by the addition of a HC 354 reducing gas. In the system of N2/NO/O2/CH4, the NO conversion rate and NOx removal rate 355 increased by 23% and 6%, respectively, compared with the condition in which reducing gas 356 CH4 was not added. In the system of N2/NO/O2/C2H2, after the addition of reducing gas C2H2, 357 the NO conversion rate and NOx removal rate were further increased by 14% and 6%,   The authors declare no competing interests.

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The authors hereby assure that this study did not involve any human trials.

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All the authors agreed to publish the study and did not object to their data being published 386 in the journal. Before submitting a paper to a journal, we have ensured that all authors agree to 387 publish their data.     562 Fig. 9. Effect of the gas flow rate on the NO2 production quantity.    Photograph of the quartz glass used in the dielectric barrier discharge reactor.