Characterization Of DC Plasma Jets Aimed At The Treatment Of Hazardous Waste

An original DC double anode plasma torch which provides a long-time and highly stable atmospheric plasma jet, either in double or single arc mode, has been devised in the purpose of hazardous waste treatment.The arc fluctuations and dynamic behaviour of the argon and argon-nitrogen plasma jets under different operating conditions have been investigated by means of classical tools, such as the statistic method, fast Fourier transform (FFT) and correlation analysis. In our experiments, the takeover mode is identified as the fluctuation characteristic of argon plasma jet while the restrike mode is typical in argon-nitrogen plasma dynamic behaviour. In case of pure argon, the FFT and correlation calculation results of electrical signals exhibit the only characteristic frequency of 150 Hz, which is originated from the torch power and independent of any change of the operating conditions. It indicates that the nature of fluctuations in an argon plasma jet is mainly induced by the undulation of tri-phase rectified power supply. By contrast, besides the same low frequency bulk fluctuation, the dynamic behaviour of the argon-nitrogen plasma jet at high frequency (4.1 kHz) is ascribed to the rapid motion of both arc roots on the anode surface. In addition, it is found that each arc root attachment is rather diffused than located at a fixed position on the anode wall in the argon plasma jet, while constricted arc roots occur when nitrogen is added into argon as the plasma working gas.The spectroscopic technique is performed to measure the emission spectra of different atmospheric plasma jets inside and outside the arc chamber. The chemical, thermodynamic and ionization states of the plasma source have been investigated by determining the electronic, vibrational and rotational temperatures as well as the electron density. The validity criteria of the local thermodynamic equilibrium (LTE) state in the plasma is also discussed. The results show that the measured electron densities are in good agreement with those deduced from LTE model, which indicates that atmospheric double arc argon and argon-nitrogen plasma jets are close to LTE state under our experimental conditions. Concerning the argon plasma jet working in the single arc mode, the axial evolution of the spectral line intensities and electron parameters can be ascribed to a strong engulfment of the ambient air at the plasma torch exit.The axial distribution of the heat flux of DC laminar argon plasma jets impinging on the plate surface has been measured by stationary method under different operating modes. Major factors that affect the heat flux fluctuations are also investigated using standard tools, such as FFT and correlation function. The available flux density of the argon plasma decreases from 100 to 10kW/m~2over around 20 cm along the jet axis. The fluctuations of the heat flux in argon plasma jets result from both the undulation of the tri-phase rectified torch power and the engulfment of the ambient air. The latter phenomenon has also been emphasized by a strong variation of the 0 I and NI line intensities between 20 and 40 mm downstream of the torch exit.Based on the above researches, a lab-scale MSWI fly ash vitrification system using this double anode plasma torch has been developed. The plasma arc technology based vitrification process is employed to convert toxic incinerator residues into harmless stable materials. The toxicity characteristic leaching procedure (TCLP) test has been performed to evaluate the chemical resistance of the vitrified slag. The amount of heavy metals leached is much lower compared with the regulatory limit, which reveals that the contaminants are sufficiently immobilized in the silicate matrix and the produced glass slag satisfies the environmental standards. In addition, the microstructure, crystalline phase, as well as the density and porosity of the slag are also investigated. Such glass-phase product with good quality has great potential to be used as a viable alternative for the construction applications. These promising results indicate that plasma arc vitrification process in our study is very effective and reliable for stabilization, detoxification and recycling of MSWI fly ashes.