The Study On The Degradation Of Naphthalene In Exhaust Gas Using Dielectric Barrier Discharges

Toxic compounds in the atmospheric environment have been widespreadly concerned becanse of adverse effects on human health and environment safe in recent years. Polycyclic aromatic compounds (PAHs) from incomplete combustion of hydrocarbon fuels, have been identified as carcinogenic and teratogenic. PAHs should be controlled before them emission into the atmosphere. Dielectric barrier discharge (DBD) due to its strong oxidization ability, non-selectivity, high carbon balance, as well as low temperature operation. It is a potential technology for controlling PAHs emission.Naphthalene is as a typical kind of PAHs. In this study, the degradation efficiency of naphthalene and selectivity of COx in a DBD reactor and catalyst packed DBD reactor were studied. Some important influence factors on degradation performance of naphthalene, such as discharge perameters and exhaust gas conditions, were investigated. In addition, intermediate products formed during non-completely chemical reaction were identified by GC-MS. Basing on the results of GC-MS, degradation pathways of naphthalene were proposed. Meanwhile, influences of packed materials, capaticy of TiO2loaded on diatomite and gas temperature on degradation performance of naphthalene have been taken into account in the packed DBD reactor.The research results are as follows:In the DBD reactor, energy density increases with increasing discharge voltage and discharge frequency. Highest energy density of the reactor can reach into350J/L. The degradation efficiency of naphthalene elevates obviously with increasing discharge voltage, while selectivity of COx during napathalene degradation decreases. Under the same discharge voltage, higher discharge frequency means higher energy injection into the reactor and higher degradation efficiency of naphthalene. The discharge is produced more difficulty as the gap bigger, which is disadvantage for naphthalene degradation. Degradation efficiency of naphthalene decreases with increasing initial concentration of naphthalene, while energy efficiency is just contrary. Energy efficiency can achieve21.1g/kWh at100ppm initial concentration of naphthalene. Long residence time has little promotive effection on degradation efficiency of naphthalene. However, more intermediate products can be completely oxidized. As a result, COx selectivity can be improved. Oxygen gas plays an important role during the degradation of naphthalene. Relative high degradation efficiency can be achieved at only 3%oxygen concentration, while selectivity of COx is just about30%in this case. Then, selectivity of COx can increase gradually as the oxygen gas concentration increases from3%to30%. Very little NOx is detected during the discharge, while O3is of high concentration. In addition, the size distribution and peak size concentration of aerosol produced during the discharge are0-100nm and107#/cm3, respectively. The peak size concentration of aerosol decreases with increasing oxygen gas concentration. Other by-products of naphthalene degradation are also detected except of COx, such as naphthoquinone and aliphatic compounds. Through the analysis of degradation by-products, degradation pathways of naphthalene were proposed in this study.In the DBD packed reactor, it can be found that discharge characteristics change because of the different packed materials. At the same energy density, peak value of discharge current decreases comparing with non-packed reactor. Diatomite as an adsorbent can prolong the residence time of naphthalene or intermediate products formed during the degradation reaction. And then the degradation efficiency and COx selectivity can be improved by19%and6.3%respectively. TiO2widely used as photocatalyst is loaded on diatomite, which can promote the reactive performance of naphthalene degradation. When the loaded capacity of TiO2increases, degradation efficiency of naphthalene increased gradually. Selectivity of COx firstly increases and then maintains at a certain value of78%with increasing the loaded capacity of TiO2. The selectivities of CO2and CO increase25.3%and5.9%respectively. Temperature of simulated exhaust gas has an important effect on the TiO2photochemical performance. Although the degradation efficiency of naphthalene only increases about8%when the temperature of exhaust gas rises from25℃to100℃. However, selectivity of COX elevates rapidly from45.3%to87.5%. Especially, CO2selectivity is improved by33.1%. Aerosol has also been found in the DBD packed reactor. Comparing with the DBD reactor, the size distribution of aerosol changes very little, but the peak size concentration is less. The size distribution of aerosol is still0-100nm. The peak size concentration is about105#/cm3.