1.3 Plasma Technology in Environmental Protection

To find out the ignition methods of plasma that combine the advantages of both thermal plasma (high energy density) and non-thermal plasma (high electron temperature but low background temperature) is one of the significant challenges for the current plasma technologies. In other words, initiating non-thermal plasmas under atmospheric conditions provides many advantages, such as high energy density, high efficiency, easy operation, and low costs, as no vacuum equipment is needed, which is beneficial for the breakthrough for the large-scale applications of cold plasmas for industrial and environment-friendly energy purposes.

In the beginning of the 20th century, the gliding arc discharge was first used for the purpose of nitrogen-based fertilizer production in chemical applications[6]. In 1994, Czernichowski had demonstrated some successful applications of the gliding arc in laboratory and industrial chemical processes[4]. Since 2003, some gliding arc reactors patented in China have been developed in our laboratory with an aim of application for energy- and engineering-friendly environment[7-13].

Typical gliding arc discharges are produced between at least two electrodes and across the fast gas flow. As shown in Fig. 1.1, gliding arc discharge yields a typical non-thermal plasma that develops between at least two electrodes placed in a flat and fast gas flow. When the high-voltage DC or AC power source provides high enough voltage between the electrodes, the gas flow in the gap between the electrodes is broken down to form the arc. The arc is then pushed downstream by the gas flow and glides along the electrode surface until it quenches. After the decay of discharge, there is a new breakdown at the narrowest gap and the cycle is repeated. This periodical discharge evolves from an arc to a discharge, containing both quasi-equilibrium and non-equilibrium phases[4]. The main advantages of the gliding arc discharges are as follows[4]:

(ⅰ) The gliding arc discharges perform their own maintenance on the electrodes, preventing chemical corrosion and erosion.

(ⅱ) The electrodes do not need to be cooled; the electric energy is directly and totally transferred to the processed gas.

(ⅲ) Multielectrode systems can be installed easily in large gas lines.

(ⅳ) Any gas or vapor, including polluted air, can be directly processed. Moisture droplets, mists, and powders can be present. A gas of any initial temperature and pressure can be accepted.

Initially, the 2D flat geometry of the gliding arc discharge was developed in many chemical processes, but later, many applications preferred 3D cylindrical geometry. Based on the 2D or 3D geometry, some gliding arc reactors with new structures, such as rotating gliding arc, vortex gliding arc discharge, and gas-liquid gliding arc discharge, were also designed and developed[14-16]. Two or more electrodes (three phases; n phases; and parallel, serial, or mixed mounting) for gliding arc reactors can be developed for an industrial size plant with DC or AC power[4].

Based on the special construction of gliding arc, gliding arc plasma reactors can directly process different gases (air, water vapor, gas-liquid mixture, Ar, O2, N2, H2S, CO, CO2, hydrocarbons, and their mixtures) at a negligible pressure drop. Many applications, mostly for energy engineering and environment control, were successfully developed in laboratory and industrial scale reactors[4, 7-17]:

● Emission control of industrial volatile organic compounds (toluene, xylene, heptane, and tetrachloroethylene), NH3, phenols, formaldehyde, organic nitrates, diluted H2S, or mercaptans, etc.;

● Emission control of soot, polyaromatic hydrocarbons, SOx, and NOx;

● Complete or partial incineration of concentrated H2S or H2S + CO2mixtures;

● Conversion of natural gas into syngas (H2+ CO);

● Reforming of heavy petroleum residues;

● Decomposition and incineration of concentrated freons;

● CO2dissociation;

● Overheating of steam, oxygen, and other gases or flames;

● Ignition of propellants;

● UV generation;

● Decontamination of soil or industrial sands;

● Organic wastewater treatment;

● Activation of organic fibers or activated carbon; and

● Sterilization.

Inexpensive gliding arc discharges can considerably reduce the energy consumption of existing industrial processes relating to volatile organic compounds (VOCs) and sulfur-containing compounds. Up to 75%-80% of the electrical energy can be dissipated in the non-equilibrium zone of the gliding arc discharge. It is clear that this continuous, powerful, and atmospheric pressure discharge could be applied for energy engineering and environment control reactions with high efficiency and selectivity.

References

[1] Bogaerts A, Neyts E, Gijbels R, van der Mullen JJAM. Gas discharge plasmas and their applications. Spectrochim Acta B. 2002; 57(4): 609-658.

[2] Petitpasa G, Rolliera JD, Darmonb A, Gonzalez-Aguilara J, Metkemeijera R, Fulcheria L. A comparative study of non-thermal plasma assisted reforming technologies. Int J Hydrogen Energ. 2007; 32(14): 2848-2867.

[3] Tendero C, Tixier C, Tristant P, Desmaison J, Leprince P. Atmospheric pressure plasmas: a review. Spectrochim Acta B. 2006; 61(1): 2-30.

[4] Czernichowski A. Gliding arc applications to engineering and environment control. Pure Appl Chem. 1994; 66(6): 1301-1310.

[5] Fridman A, Nester S, Kennedy LA, Saveliev A, Ozlem MY. Gliding arc gas discharge. Prog Energ Combust. 1998; 25(2): 211-231.

[6] Naville AA, Guye CE, Guye P. Gas reactions at the temperature of the electric arc, French. Patent FR361827. 1905.

[7] Yan JH, Chi Y, Li XD, Jiang XG, Ma ZY, Wang F, Jin YQ, Du CM, Ni MJ, Cen KF. Gliding arc discharge plasma device for organic waste water treatment, China. Patent CN1557731A. 2004.

[8] Yan JH, Li XD, Chi Y, Ma ZY, Du CM, Bo Z, Cen KF. A volatile organic compounds gas treatment device, China. Patent CN2817959Y. 2005.

[9] Du CM. A rotary gliding arc discharge plasma device for volatile organic compounds gas treatment, China. Patent CN201131940Y. 2007.

[10] Du CM. A three phase gliding arc non equilibrium plasma device for water treatment, China. Patent CN201099636Y. 2007.

[11] Du CM. Non equilibrium plasma generating apparatus for spray disinfection sanitizer, China. Patent CN101156955B. 2010.

[12] Du CM. Non equilibrium plasma device for removing volatile organic compounds and generating hydrogen, China. Patent CN101279715 B. 2011.

[13] Du CM, Yan JH, Cheron B. Decomposition of toluene in a gliding arc discharge plasma reactor. Plasma Sources Sci T. 2007; 16: 791-797.

[14] Lu SY, Sun XM, Li XD, Yan JH, Du CM. Decomposition of toluene in a rotating glidarc discharge reactor. IEEE T Plasma Sci. 2012; 40(9): 2151-2156.

[15] Ren Y, Li XD, Yu L, Cheng K, Yan JH, Du CM. Degradation of PCDD-Fs in fly ash by vortex-shaped gliding arc plasma. Plasma Chem Plasma P. 2013; 33: 293-305.

[16] Du CM, Shi TH, Sun YW, Zhuang XF. Decolorization of acid orange 7 solution by gas-liquid gliding arc discharge plasma. J Hazard Mater. 2008; 154(1-3): 1192-1197.

[17] Du CM, Liu H, Xiao MD, Gao D, Huang DW, Li ZY, Chen TF, Mo JM, Wang K, Zhang CR. Adsorption of iron and lead ions from an aqueous solution by plasmamodified activated carbon. Ind Eng Chem Res. 2012; 51(48): 15618-15625.