Applied Science and Convergence Technology 2019; 28(4): 93-100
Published online July 31, 2019
Copyright © The Korean Vacuum Society.
Juyoung Koa, Tae-Hee Kimb,*, and Sooseok Choia,b,*
aDepartment of Nuclear and Engineering, Jeju National University, Jeju 63243, Republic of Korea
bInstitute for Nuclear Science and Technology, Jeju National University, Jeju 63243, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Although SF6 gas has been used as insulation in electric power and transmission equipment, it is a very harmful greenhouse gas. SF6 can be injected inside the cement kiln, for pyrolysis and safe conversion, during cement production. As the temperature distribution inside a kiln is not uniform, the pyrolysis and conversion efficiency of SF6 are low. In this study, a kiln reactor combined with thermal plasma torches was applied to develop a high pyrolysis rate for SF6. A numerical analysis was conducted to investigate the variation of heat flow that thermal plasma causes inside the cement kiln. The simulations were conducted for kilns that did and did not include a thermal plasma torch. The results showed that the average temperature inside the kiln with thermal plasma was relatively low due to the gas-mixing effect of the latter; however, the peripheral temperature was stabilized. In addition, the temperature of the near kiln inlet increased. As a result, it is expected that the internal temperature of a kiln combined with thermal plasma technology will become uniform due to the gas-mixing effect of thermal plasma, and the SF6 pyrolysis efficiency will increase as the temperature of the local region rises.
Keywords: SF6, Decomposition and conversion, Numerical analysis, Cement kiln, Thermal plasma
Sulfur hexafluoride (SF6) has been widely used as a typical insulating gas in electric power distribution and transmission equipment. It is a chemically stable perfluorocarbon compound containing the fluorine element. It is also a considerably harmful greenhouse gas, which have a long lifetime in the atmosphere and a high global warming potential of 23,900 (CO2 = 1) [1–5]. SF6 must be treated to be converted into environmentally safe compounds as emission amounts are increasing with industry growth [6,7].
SF6 can be injected inside the cement kiln, for pyrolysis and safe conversion, during the cement production process. It is theoretically pyrolyzed above 2000 K ; therefore, a cement kiln with a temperature of around 2000 K is a suitable environment for SF6 pyrolysis [9–11]. Additionally, kilns contain a large amount of calcium oxide (CaO), which is an effective reactant for converting pyrolyzed SF6 radicals into fluorite (CaF2) [12,13]. CaO is produced by the decarboxylation of calcium carbonate (CaCO3), which is the main ingredient in cement. Therefore, SF6 can be converted to a safe material, without affecting the cement production process, by the reaction between CaO and SF6.
As the temperature inside a kiln is not uniformly distributed according to the internal space, however, pyrolysis and conversion efficiency are low [11–13]. Therefore, it is necessary to develop a technique by which to effectively maximize its disposal and energy efficiency.
The thermal plasma technique provides a region of high temperature, of over several thousand Kelvin, to enable SF6 pyrolysis . It also provides advantages such as a rapid reaction time and the presence of abundant reactive species [15–17].
This work investigated the use of a cement kiln combined with a thermal plasma torch as an efficient disposal technique for high-capacity SF6. Thermal plasma offers an additional heat source and improves the SF6 pyrolysis process. A numerical analysis was performed to investigate variation of the heat flow inside the cement kiln by thermal plasma. In this work, the simulations were carried out for kilns combined with and without a thermal plasma torch.
A DC nontransferred thermal plasma torch was used as an additional heat source for decomposing SF6 [18,19]. The thermal plasma characteristics inside the torch were simulated using the DC plasma torch in unstructured grid system code [20,21], which is a self-developed magnetohydrodynamic code [22–25]. In the present study, two-dimensional (2-D) numerical modeling of thermal plasma was carried out under usual assumptions [25–29]. The nontransferred arc plasma was assumed to be quasi-neutral, steady, optically thin, and at local thermodynamic equilibrium under atmospheric pressure . The gravitational force was considered negligible compared to the Lorentz force . As the plasma flow can be modeled in a manner similar to a fluid, the hydrodynamic movements were described using the Navier–Stokes equations in terms of cylindrical coordinates (
The computational domain for simulation of the thermal plasma torch are shown in Fig. 1. For the 2-D modeling of the torch, the plasma forming gas inlet, torch outlet, cathode wall, anode wall, and plasma fluid zone for free space plasma were designated as the boundary conditions. A numerical simulation of the thermal plasma jet was performed to investigate the arc discharging, flow, and electrical characteristics inside the torch. The operating conditions of the plasma torch are given in
A rotary kiln involves rotation of a cylindrical kiln to convert raw material to cement clinkers, however rotation of the kiln was disregarded in this study . The steady state continuity, momentum, and energy equations were employed to solve the flow field. The renormalization group (RNG)
The equations of continuity and momentum are shown in
The source term
The energy equation is given in
The terms on the right side of the energy equation are expressions of the energy transfer. The term
In this study, the radiation was modeled as the P-1 model, which is in charge of the radiation exchange between coal particles and gas.
Combustion processes of coal are first treated as devolatilizing. The coal particles are heated by the energy from the gas. After, the coal particles begin to devolatilize. The single-rate model was employed to solve the rate of devolatilization, assuming that the rate of devolatilization was defined using the one-step Arrhenius scheme and depends on the residual volatiles of the particles .
After the released volatile of the particle is completely burned, combustion of the char begins. To describe char burning, the kinetic/diffusion-limited rate model was used, as given in
In this study, a numerical analysis was conducted to examine variation of the heat flow inside the cement kiln by thermal plasma. For the two cases, simulations were conducted of the kiln combined with and without thermal plasma. The simulation domain and three-dimensional grid system for the numerical modeling of the cement kiln is shown in Fig. 3. The kiln length and diameter were 94 and 5.36 m, respectively. Coal along with coal carrier air, primary air, and central air were injected through the burner, and secondary air was entered around the burner. The diameter of the burner was 0.72 m. The boundary conditions of the kiln imposed primary and secondary velocity inlet, pressure outlet, and no-slip wall, respectively. The flow rate and boundary conditions for kiln operation are detailed in
Calculating the chemical equilibrium composition predicts the thermodynamic properties including decomposition temperature and recombination of SF6. The thermodynamic equilibrium composition was calculated for the temperature range of 300–6000 K under atmospheric pressure to estimate the decomposition temperature of SF6. The calculation was achieved using HSC chemistry version 9 (Outotec), which is designed for chemical reactions and equilibria calculations of mixed gas. The thermodynamic equilibrium composition of 1 mol SF6 in the given temperature range under atmospheric pressure is shown in Fig. 5. As a result, it was confirmed that SF6 began to dissociate into SF5, SF4, SF3, SF2, SF, and F2 at 1300 K, and completely dissociated into S and F atoms at over 4000 K. CaF2 was produced by the reaction of pyrolyzed SF6 and CaO according to the following chemical reaction inside the kiln:
To predict the re-decomposition temperature of CaF2, the thermodynamic equilibrium composition of CaF2 was calculated for the temperature range 300–6000 K under atmospheric pressure. The result is presented in Fig. 6. CaF2 began to pyrolyze at temperatures above 3000 K, indicating the possibility that CaF2 can be pyrolyzed when it is mixed with high-temperature thermal plasma, of several thousand Kelvin or more. Therefore, it is necessary to simulate the temperature distribution inside the kiln combined with the thermal plasma torch, which provides a region of high temperature over 10,000 K to prevent re-decomposition.
Figure 7 presents the temperature distribution inside the cement kiln. Figure 7(a) shows the temperature distribution inside the kiln on the
Figure 8(a) shows the temperature distribution inside the cement kiln combined with thermal plasma on the
A comparison of the velocity vector field near the inlet boundary of the kiln is shown in Fig. 9; (a) shows the vector field inside the kiln without thermal plasma, and (b) shows the vector field inside the kiln combined with thermal plasma. As in Fig. 9(b), the air which entered through the burner was observed to be quickly drawn to the center. Figure 10(a) shows the mole fraction of CO2 inside the kiln, and Fig. 10(b) shows the case-applied thermal plasma. Figure 10(b) confirms that the concentration of CO2 remarkably decreased and rapidly diffused to the periphery. Due to the rapid mixing of the air by thermal plasma, the reaction between the gas was accelerated, and it was predicted that coal burning occurred rapidly. The temperature profiles according to the radial direction at the points 10, 15, and 20 m along the central axis of the kiln are shown in Fig. 11. As the distance from the kiln inlet increases, it can be seen that the temperature changes inside the kiln combined with thermal plasma were less than those of the original kiln (without thermal plasma). The temperature inside the kiln was confirmed to become uniform as the high-temperature gas diffused to the surroundings rather than forming the local high-temperature region. The temperature distribution near the inlet boundary of the kiln for the two cases are compared in Figs. 12(a) and 12(b). It can be seen clearly from Fig. 12(b) that a high-temperature region of approximately 4000 K or more was locally formed up to 0.5 m as applied thermal plasma. It is expected that SF6 injected form the burner will be mixed in the high-temperature region and sufficiently pyrolyzed.
In this study, a numerical analysis was carried out to investigate variation of the heat flow inside a cement kiln by thermal plasma for the high-capacity pyrolysis of SF6. As a result of the thermodynamic equilibrium calculations, SF6 dissociates at a temperature range of ~1300–2000 K and completely pyrolyzes to each element at 4000 K. From the simulation results, it was confirmed that the average temperature of gas inside the kiln was approximately 1500 K. Therefore, it is predicted that SF6 will not completely decompose by only the temperature of the kiln. The average temperature inside the kiln combined with thermal plasma was analyzed to be relatively low due to the gas-mixing effect of the thermal plasma; however, the peripheral temperature was stabilized. In addition, it was confirmed that the temperature near the inlet boundary of the kiln increased due to the injected thermal plasma. It is expected that the high-temperature region formed locally by thermal plasma will increase the pyrolysis rate of SF6. As a result, the internal temperature of the kiln combined with thermal plasma should become uniform due to the gas-mixing effect of thermal plasma, and the pyrolysis rate of SF6 and the pyrolysis process efficiency increase as the temperature of the local region rises.
This project was supported by the “R&D Center for reduction of Non-CO2 Greenhouse gases (2017002430 003)” funded by Korea Ministry of Environment (MOE) as “Global Top Environment R&D Program” and the National Research Foundation of Republic of Korea (NRF-2015M2B2A9030393), and the present work is results of a study on the “Leaders in Industry-university Cooperation +” Project, supported by the Ministry of Education and National Research Foundation of Korea.
Operating condition of DC nontransferred thermal plasma torch.
|Data||Working gas (N2) (L/min)||Current (A)||Arc voltage (V)||Power (kW)|
Flow rate and boundary conditions for kiln operation.
|Boundary condition||Inlet velocity (m/s)||Temperature (K)|
|Flow rate of primary air||89.99||300|
|Flow rate of coal / coal feeding||38.00 / 4.17 (kg/s)||300|
|Flow rate of central air||1.447||300|
|Flow rate of secondary air||7.559||1300|