Application of Non-thermal plasma technology on NOx removal

1.1 INTRODUCTION

Among different air pollutants emitted by internal combustion engines, nitrogen oxides are one of the major toxic gaseous emissions. Nitrogen oxides cause a lot of deleterious effects such as respiratory and cardiovascular diseases, mortality, acid rain, ground-level ozone formation (smog), photochemical smog, global warming, nose and eye irritation, visibility impairment, the formation of toxic products and water quality deterioration. Due to the mentioned adverse effects of NOx, the related legislative restrictions have become much more stringent in recent years. The abbreviation NOx usually refers to the summation of NO and NO2 in emission standards. One of the main sources of NOx is motor vehicles, especially diesel engines, which emit 2-20 times more NOx than gasoline engines. Up until now, several conventional technologies have been employed for NOx removal from exhaust gas such as three-way-catalyst, selective catalyst reduction (SCR), Two-Stage Fuel Injection and lime–gypsum method. However, these methods require strict operating conditions such as controlled reaction temperature and gas compositions. Furthermore, energy efficiency and the cost of conventional methods are still high.

Non-thermal plasma (NTP) technology has been introduced as a promising method for NOx removal, especially for emission reduction in diesel engines recently. It has great potential for emission treatment since it can operate stably at atmospheric pressure and even at low temperature. Electric discharge plasma has been widely studied as a low cost and high energy efficiency exhaust gas treatment method. NTP treatment of exhaust gases is effective through the introduction of plasma inside the exhaust gases. Vehicle exhaust gases, both diesel and gasoline, undergo chemical changes when exposed to the plasma. Logically, oxidation processes dominate in the plasma state. These reactions include oxidation of hydrocarbons, carbon monoxide, and nitrogen oxides.

NTP can be generated in several ways, such as through electrical corona discharges, radio frequency discharges, microwave discharges, dielectric barrier discharges (DBDs), and electron beams. Dielectric barrier discharge (DBD) reactors are used more often than other types of plasma reactors in environmental applications as a result of the easy formation of stable plasmas, homogeneous discharge, scalability, effectiveness and low operating cost. The basic design of a DBD reactor consists of a set of electrodes with at least one dielectric barrier between them, and therefore high enough electric field must be applied to cause a breakdown in the gas. This is the advantage of DBD than the other conventional plasma generators. A schematic of a regular DBD electrode is shown as follows: