This paper develops an axisymmetric flow model to predict the flow field inside a directcurrent plasma torch reactor, where the magneto-hydrodynamic equations, including the continuity, momentum, energy and current continuity equations as well as turbulence transport equations are solved with a finite volume discretization in a segregated manner. The thermodynamic and transport coefficients of thermal plasma are obtained from the condition of local thermal equilibrium, whereas a kinetics model is employed to consider the production and destruction of species due to the chemical reactions in a decomposition process. Species transport phenomena arising in the decomposition process are described by the transport equations for different species. The perfluorocarbons decomposition characteristics in a well-type, non-transferred nitrogen torch reactor are analyzed by the proposed numerical model to disclose the influence of working parameters on the decomposition efficiency

Chau, S. W.

Chen, S. H.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/ppt.2017.2.198Plasma Physics and Technology 4(2):198–201, 2017 © Department of Physics, FEE CTU in Prague, 2017MODELING OF PERFLUOROCARBONS DECOMPOSITION INNITROGEN THERMAL PLASMAS.W. Chaua,∗, S.H. Chenba Department of Engineering Science and Ocean Engineering, National Taiwan University, 1, Sec. 4, RooseveltRd., 10617 Taipei, Taiwan, ROCb Physics Division, Institute of Nuclear Energy Research, 1000 Wenhua Rd., Longtan, 32546 Taoyuan, Taiwan,ROC∗ chausw@ntu.edu.twAbstract. This paper develops an axisymmetric flow model to predict the flow field inside a direct-current plasma torch reactor, where the magneto-hydrodynamic equations, including the continuity,momentum, energy and current continuity equations as well as turbulence transport equations aresolved with a finite volume discretization in a segregated manner. The thermodynamic and transportcoefficients of thermal plasma are obtained from the condition of local thermal equilibrium, whereas akinetics model is employed to consider the production and destruction of species due to the chemicalreactions in a decomposition process. Species transport phenomena arising in the decomposition processare described by the transport equations for different species. The perfluorocarbons decompositioncharacteristics in a well-type, non-transferred nitrogen torch reactor are analyzed by the proposednumerical model to disclose the influence of working parameters on the decomposition efficiency.Keywords: perfluorocarbons decomposition, nitrogen, thermal plasma, direct-current, plasma torch.1. IntroductionCompared with other methods to remove hazardousgases arising in modern industrial processing, thermalplasma delivers an advantage of removal efficiency,processing flexibility, operation safety and installationcost that attract the attention of recent researchers[1, 2]. Perfluorocarbon (PFC), such as CF4, C2F6, andNF3, can be first reduced into an atomic or molec-ular form of its chemical components in the high-temperature environment created by thermal plasmaflow. A further introduction of water into the reactingprocess can enhance the production of HF and CO2,which can be easily treated in the following processes.This paper is first to develops an axisymmetric flowmodel to predict the thermal plasma inside a direct-current torch reactor operating with nitrogen. Theperfluorocarbons decomposition characteristics in awell-type, non-transferred plasma torch reactor arethen analyzed by the proposed numerical model todisclose the influence of working parameters on thedecomposition efficiency.2. Thermal plasma modelThe following assumptions are adopted for a steadyand axisymmetric thermal plasma flow: The plasma isoptically thin, the plasma is assumed in local thermalequilibrium, the gravitational effect and the externalmagnetic field is ignored. The magneto-hydrodynamicequations consisting of the continuity equation, mo-mentum equation, energy equation, and current con-tinuity equation are given as follows:.Continuity equation∂(ρu)∂z+ 1r∂(ρrv)∂r= 0 (1)Momentum equation (axial-direction)∂(ρu2)∂z+ 1r∂(ρruv)∂r= −∂p∂z+ 2 ∂∂z[µ(∂u∂z)]+1r∂∂r[rµ(∂u∂r+ ∂v∂z)] + jrBθ (2)Momentum equation (radial-direction)∂(ρuv)∂z+ 1r∂(ρrv2)∂r= −∂p∂y+ 2r∂∂r[µr(∂v∂r)]+ ∂∂z[µ(∂u∂r+ ∂v∂z)]− µ(2vr2) + ρw2r− jzBθ (3)Momentum equation (azimuthal-direction)∂(ρuw)∂z+ 1r∂(ρrvw)∂r= ∂∂z[µ(∂w∂z)]+ 1r2∂∂r[µr3 ∂∂r(wr)]− ρvwr(4)Energy equation∂(ρuh)∂z+ 1r∂(ρrvh)∂r= u∂p∂z+ v ∂p∂r+ ∂∂z(k∂T∂z) + 1r∂∂r(rk∂T∂r) + j2z + j2rσ−SR + 52kBe(jz∂T∂z+ jr∂T∂r) + µ|γ˙|2 (5)198vol. 4 no. 2/2017 PFCs Decomposition Modeling in Nitrogen PlasmaTurbulence model∂(ρuK)∂z+ 1r∂(ρrvK)∂r= G− ρ+∂∂z[(µm +µtσK)∂K∂z] + 1r∂∂r[(µm +µtσK)∂K∂r] (6)∂(ρu)∂z+ 1r∂(ρrv)∂r= c1GK− c2ρ 2K+∂∂z[(µm +µtσ) ∂∂z] + 1r∂∂r[(µm +µtσ)∂∂r] (7)Current continuity equation∂∂z(σ∂φ∂z) + 1r∂∂r(σr∂φ∂r) = 0 (8)where ρ denotes the fluid density, u the velocity com-ponent in the axial direction (z), v the velocity com-ponent in the radial direction (r), w the velocity com-ponent in the azimuthal direction (θ), p the pressure,µ the apparent viscosity (= µm + µt), µm the fluidviscosity, jz the axial component of current density,jr the radial component of current density, Bθ theself-induced magnetic field in the radial direction, hthe enthalphy, SR the radiation loss, k the thermalconductivity, T the temperature, kB the Boltzmannconstant, e the electric charge, K the turbulent ki-netic energy,  the dissipation rate of K, G the energyproduction due to turbulence [3], µt the turbulentviscosity (= cµρK2/), φ the electric potential, σ theelectric conductivity, (cµ, σK , σ, c1, c2) the con-stants of turbulence model [4] and |γ˙| the magnitudeof shear rate. The current density jz and jr are de-termined by the Ohm’s law:jz = −σ∂φ∂z, jr = −σ∂φ∂r(9)The self-induced magnetic field Bθ is calculated ac-cording to the Biot-Savart law:Bθ =µ0r∫ r0jz ξ dξ (10)3. Kinetic modelThe transport equation to describe the neutral speciesarising in the destruction process is given as follows:∇ · (niU) +∇ · Γi = Ri,p + Ri,d (11)where ni denotes the density of the ith species, Uthe flow velocity, Γi the flux of the ith species, Ri,pthe production of the ith species, Ri,d the destructionof the ith species. The decomposition mechanism offluorinated compounds disclosed in [1] together withthe kinetic model for water plasma suggested in [5]is adopted in this paper to describe the perfluoro-carbons decomposition in nitrogen thermal plasma.The following table summarizes the decompositionmechanisms for CF4 considered in this study.No. Reactions1 CH2F2 + H → CH2F + HF2 CF4 + H → CF3 + HF3 CF4 → CF3 + F4 CH2F + F → CHF + HF5 CF3 + O → COF2 + F6 CF + O → CO + F7 CF2 + O → CO + 2F8 CF3 + H → CF2 + HF9 CF2 + H → CF + HF10 CF + H → CH + F11 CF2 + F → CF3 + M12 CF3 + F → CF4 + M13 CF2 + H2 → CH2F214 CF3 + H2 → CHF3 + H15 CF2 + O2 → COF2 + O16 COF2 + H → COF + HF17 COF2 + O → CO2 + F218 COF + O → CO2 + F19 COF + H → CO + HF20 COF + F → CO + F2Table 1. Decomposition mechanisms for CF4.4. Numerical methodA finite volume method [6] is employed to discretizethe governing differential equations on a Cartesiangrid and then a integral form of governing equationover an arbitrary control volume V is obtained:∫Sρϕ(U · n) dS =∫S(Tϕ · n) dS +∫Vρfϕ dV (12)where the control volume V is defined by the con-trol surface S with an outer normal vector n, ϕ thegeneralized variable, U the velocity vector, Tϕ thegeneralized surface force, fϕ the generalized sourceterm. Using the value of neighboring nodes to approx-imate each term appearing in the governing equations,a linearized equation is obtained for every governingequation. A SIMPLE-like algorithm is employed tosolve the coupling of dependent variables iteratively.For the current density distribution at cathode, anexponential profile is assumed as follows [7]:jc(ξ) = Jmax e−bξ (13)where Jmax denotes the maximum current density, ξthe decreasing direction of current density and theconstant b is determined by the system current I andthe area of hot spot A:I = 2pi∫Ajc(ξ) dA (14)5. Boundary conditionsIn the numerical modeling, the DC torch is simplifiedas a hollow cylinder with length of 89mm and ra-dius of 12mm, Fig. 1. One end of the hollow cylinder199S.W. Chau, S.H. Chen Plasma Physics and TechnologyFigure 1. Computational domain and the location ofelectrodes.is closed, while the other end is open for outgoingplasma. The inlet boundary conditions are specifiedat the annular gas inlet adjacent to the torch bottom.The flow rate of nitrogen is 180 slpm and a swirler de-sign additionally yields a tangential velocity of 20m/sat the gas inlet. CF4 and H2O are supplied togetherwith the working gas (nitrogen) in a concentrationof 104 ppm and 2·104 ppm, respectively. The workingcurrent of the simulated case is 145A, which resultsin a measured voltage drop of 194V between twoelectrodes. The location of cathode is at the centerof torch bottom due to the installment of a cylindri-cal cathode. Anode’s position is then fixed by themeasured voltage obtained in the experiment. Thearc root sizes of both electrodes are assumed 3mm.About fifty thousand cells are used to discretize thecomputational domain, where the grid points closeto arc root and gas inlet, as well as the torch wall,are clustered to resolve the steep gradient of fieldvariables, such as velocity, temperature and currentdensity.6. Numerical resultsFigure 2 depicts the plasma temperature and velocityprofile at the torch outlet, where the predicted voltagedrop is 191V. The exiting plasma flow is acceleratedby the self-induced magnetic field to deliver a bluntaxial velocity profile. Except in the region close totorch wall (r > 10mm), the nitrogen plasma can reachthe speed range above 45m/s along with a maximumvalue of 72m/s at the centerline. Due to the installa-tion of swirler at the gas inlet, a strong rotational flowin introduced into the hollow torch. The peak of tan-gential velocity is predicted for about 39m/s locatedat about one third of the torch radius (4mm). The po-sition of the maximum tangential speed clearly definesthe size of plasma core, which owns a relatively highvelocity as well as temperature when compared withits outer region. Inside the core region, the plasmajet can attain a flow velocity over 60m/s, whereasthe temperature profile is between 3300K and 7200K.Figure 3 shows the plasma characteristics along thecenterline. Because cathode is installed at the torchbottom, high current density and temperature as wellas significant axial velocity is expected in the vicinityof the torch bottom. The spike of current density ispredicted to be 62A/mm2, which gives a maximumtemperature of 26000K along the centerline. The ax-ial velocity along the torch axis strongly oscillates asthe plasma moves away from the torch bottom. Theflow quickly soars from -50m/s to 290m/s near thetorch end, and substantially loses its velocity magni-tude at approximately one-third of the torch length.With the help of nontrivial current density existingin the middle part of plasma torch, the plasma jetobtains an obvious velocity increase and exits fromthe torch at 90m/s. Figure 4 illustrates the plasmacharacteristics inside the plasma torch. The right partof the figure disclose the velocity distribution, wherea nontrivial vortex motion with the center locatedabout 25mm in front of the cathode is observed in theneighborhood of cathode. The temperature field aswell as the current density vector is given in the leftpart of the figure. Significant current density emergesin the high-temperature core along the centerline.Figure 5 gives the radial density profile of variousspecies stemming from the decomposition mechanismof CF4, where low density products are neglected inthe profile plot. Water is relatively abundant (between200mole/cm3 and 300mole/cm3) and smoothly dis-tributed in the outer region of torch outlet (r>5mm),whereas it rapidly decline with the decrease of radialdistance. Similar behavior is observed for CF4, and atthe same radial position the density of CF4 is aboutone tenth of the water density. CF3 as well as CH2Fshows a very low concentration in the plasma core,whereas an exponential grow of density is expectedoutside the hot plasma core. In contrast, the predicteddensity of fluorine atom varies in the range between7mole/cm3 and 20mole/cm3 in the plasma core buta strong fall of concentration is predicted outside thehigh-temperature zone. The product group, i.e., HF,CO, and CHF, also deliver a hill-shape profile alongthe radial direction, where the peak density approxi-mately occurs near the radial boundary of the plasmacore. CHF3 is the only material given in the profileplot, which exhibits a consistent tendency of growingconcentration with r. The efficiency of CF4 decompo-sition η in the investigated torch arrangement is 97%,where η is defined as the ratio of the mass flux at thegas inlet to the mass flux at the torch outletη = m˙CF4 |inletm˙CF4 |outlet(15)7. ConclusionsThis paper proposes a numerical model to simulate theperfluorocarbons decomposition within a well-type,non-transferred nitrogen torch. An operation condi-tion of 180 slpm, 145A and 193V is predicted usingnitrogen as background gas, where water vapor andTetrafluoromethane are initially with a concentrationof 104 ppm and 2·104 ppm, respectively, at the gasinlet. The numerical simulation predicts the majorproducts stemming from the decomposition mecha-nism of CF4 are H2O, F, HF, CO, CHF , CH2F, CF3,200vol. 4 no. 2/2017 PFCs Decomposition Modeling in Nitrogen PlasmaFigure 2. Plasma temperature and velocity profile atthe torch outlet.Figure 3. Plasma characteristics along the centerline.CF4 and CHF3. The efficiency of CF4 decomposi-tion via a nitrogen plasma torch under the studiedoperation condition is estimated about 97%.AcknowledgementsThis study was supported by the Institute of NuclearEnergy Research, ROC (NL1050877).References[1] S.H. Narengerile and T. Watanabe. Decompositionmechanism of fluorinated compounds in water plasmasgenerated under atmospheric pressure. Plasma Chem.Plasma Process, 30(6):813–829, 2010.doi:10.1007/s11090-010-9259-y.[2] M.S. Lim, S.C. Kim, and Y.N. Chun. Decompositionof PFC gas using a water jet plasma. J. Mech. Sci.Tech., 25(7):1845–1852, 2011.doi:10.1007/s12206-011-0422-z.[3] S.W. Chau, S.Y. Lu, and P.J. Wang. Modeling ofaxis-symmetric steam plasma flow in a non-transferredtorch. Comp. Phys. Comm., 182(1):152–154, 2011.doi:10.1016/j.cpc.2010.08.011.[4] B.E. Launder and D.B. Spalding. The numericalcomputation of turbulent flows. Comp. Methods inFigure 4. Plasma characteristics inside the torch.Figure 5. Density profile at the torch outlet.Appl. Mech. Eng., 3(2):269–289, 1974.doi:0.1016/0045-7825(74)90029-2.[5] S.W. Chau, C.M. Tai, and S.H. Chen. Nonequilibriummodeling of steam plasma in a nontransferred direct-current torch. IEEE Tran. Plasma Sci., 42(12):3797–3808, 2014. doi:10.1109/TPS.2014.2365479.[6] S.W. Chau. PhD Disseration. Universita¨t Hamburg,1997.[7] K.C. Hsu, K. Etemadi, and E. Pfender. Study of thefree-burning high-intensity argon arc. J. Appl. Phys.,54(3):1293–1301, 1983. doi:10.1063/1.332195.201

Modeling of Perfluorocarbons Decomposition in Nitrogen Thermal Plasma

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Modeling of Perfluorocarbons Decomposition in Nitrogen Thermal Plasma

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