Application of Lif-technique to Atmospheric Pressureplasmas

The atmospheric pressure non-thermal plasma has been extensively studied for decomposing environmental pollutants such as SOx [1], NOx [2,3], and VOCs [4,5] (volatile organic compounds). Many researchers have been engaged in developing removal technique of those toxic pollutants with the non-thermal plasma. A high energy electron beam (EB) irradiation is also very effective to remove SOx, NOx in the combustion flue gas or various toxic VOCs in the air [6,7]. All flue gas from 220MW heavy oil power station at Nagoya is tested by that EB process. Discharge plasma is generated by various kinds of modes. The barrier discharge and the surface discharge can generate the non-thermal plasma easily by commercial frequency or high frequency (kHz order) high voltage at atmospheric pressure. The DC corona discharge can produce the non-thermal plasma with poor energy efficiency. The RF or millimeter wave plasma is medium state from the non-thermal plasma to the thermal plasma. The thermal plasma was also tested to decompose high density flon (CFC)[8]. A submicrosecond pulse corona can decompose NOx and dioxin in the flue gas. A packed-bed reactor was also studied with a little bit poor energy efficiency but with low emission of ozone. For practical usage of the non-thermal plasma as toxic gas removal technique, the improvement of the energy efficiency and preventing the production of hazardous byproducts are very essential. Recent good result is that the input discharge power of 20 J/l can decompose more than 90 % trichloroethylene (TCE) in air where TCE concentration is 100 or 1,000 ppm with the aid of the catalyst inserted in the plasma region [9]. To improve the energy efficiency, chemical reactions in the plasma must be clear but most reactions and parameters are still under the research. Computational analysis of those mechanisms has been done by some researchers [10,11]. Some parameters were observed experimentally [12]. They predicted reaction channels of gas removal and regarded some radicals as principal species in the reaction channels. Identification of those radicals (and molecules/atoms) in the plasma is necessary to analyze reaction channels and to determine the reaction rate. However, still only a few researches on direct observation of those radicals in the atmospheric pressure plasma because of the technological difficulties. Laser-Induced Fluorescence (LIF) technique is developed for those radical analysis. LIF is used for the radical analysis of the low pressure (low temperature plasma) in the semiconductor process [13~15], steady state combustion flame analysis[16~19] or atmospheric chemistry[20]. Recently, some researchers tried to introduce that LIF technique as the atmospheric pressure plasma diagnosis observing time and spatial distribution analysis of those radicals. Those discharge plasma is nanoseconds pulse and non-equilibrium conditions which cause difficulties of LIF application. The target radicals of those LIF experiments include N [16], O [13, 17, 20], OH [17, 18] and NOx [17, 19]. Recently the authors succeeded to observe two-dimensional distribution of OH radicals in various kinds of pulse discharge plasmas, such as equilibrium [21] or non-thermal plasma [22] as time variation. Other Japanese international group also observed NO distribution during DC corona [23] or ultra short pulse corona [24] in Japan. The authors also observed similar NO removal pattern by LIF. This paper introduces those radical distributions related with discharge plasmas for better understanding of plasma chemical reactions in Japan.