Nonthermal Plasma Processing for Air-Pollution Control: A Historical Review, Current Issues, and Future Prospects

Plasma is a partially or fully ionized gas consisting of various particles, such as electrons, ions, atoms, and molecules. For the quantitative description of plasma, the term of temperature is usually used. Thermal plasma is in a state where almost all its components are at thermal equilibrium. In nonthermal plasmas (NTPs), temperature (i.e. kinetic energy) is not in thermal equilibrium, and differs substantially between the electrons and the other particles (ions, atoms, and molecules). In this sense an NTP is also referred to as a ‘‘nonequilibrium plasma’’ or a ‘‘cold plasma’’. Because of the small mass of electrons, they can be easily accelerated under the influence of an electric field. The temperature of electrons typically ranges from 10 000 K to 250 000 K (1–20 eV). These highly energetic electrons produce free radicals from parent molecules in multi-step physical and chemical processes. These free radicals react with and decompose pollutants to yield decomposition products. However, since the typical concentration range of the pollutants of interest is in the order of several hundred parts per million by volume (ppmv), direct interactions between the electrons and pollutants can usually be ignored. Although there are a variety of methods for producing NTPs, such as corona discharge, pulsed corona, microwave, radio frequency (RF) plasma, ionizing irradiation, etc., this review only describes the method based on the NTP technology using electrical discharges at atmospheric pressure. There have been a considerable numbers of review articles and books both on the fundamental and the engineering aspects of NTP, therefore this review will describe slightly different aspects of NTP processing focusing on the historical development, current issues and future prospects. The most distinctive characteristic of NTP as a chemical process is its ability to induce various chemical reactions at Summary:This review describes the historical development, current status, and future prospects for nonthermal plasma (NTP) technology and its application as an air-pollution control. In particular, this review focuses on the cutting-edge technology of hybrid NTP, where it is combined with other methods such as wet processing, the use of adsorbents, and catalysis. Historical landmarks in the development of NTP technology and the current status of large-scale applications are discussed. The general characteristics of the combined system of NTP with catalysts are described in the context of the decomposition of NOx and benzene.