Outlook of Corona Discharge as DeNO_<i>x</i> Technology

This review explores the efficacy of corona discharge (CD) as a non-thermal plasma (NTP) technology for the reduction of nitrogen oxides (NOx) from combustion gases. The NTP is generated when a treated gas is subjected to a large electric field, resulting in the ionization of gas molecules without a significant thermal energy increase. NTP operates post-combustion at low temperatures, favoring NOx reduction through the generation of reactive species, such as O, O3, OH, and various nitrogen radicals. These species facilitate the conversion of NO to NO2, which can be further reduced or absorbed. The review analyses the effect of different gas compositions on the effectiveness of the CD process. Oxygen enhances NOx reduction by generating additional oxidizing species, while water vapor contributes through the formation of OH radicals and the dissolution of nitric and nitrous acids. The polarity of the applied voltage significantly impacts the ionization process and the extent of the plasma region, with negative polarity generally providing better results in terms of NOx reduction. Additionally, the use of pulsed corona discharge (PCD) shows higher energy efficiency compared to continuous direct current (DC) discharge due to optimized electron generation. The temperature influences the effectiveness of NTP, with higher temperatures potentially reducing the efficiency due to the decomposition of reactive species, like O3. The optimal residence time and initial NOx concentration are also crucial for maximizing reduction efficiency while maintaining energy efficiency. Overall, CD is presented as a promising DeNOx technology, particularly when optimized for gas composition, voltage polarity, and discharge nature.