Article pubs.acs.org/EF
Spray Combustion Characteristics of Waste Oil and System Analysis for an Energy System Using Microwave-Induced Nonequilibrium Plasma Tsuyoshi Yamamoto,*,† Ryosuke Imazato,† Takahisa Yamamoto,‡ and Ryo Tanaka§ †
Department of Chemical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Department of Mechanical Engineering, Gifu National College of Technology, 2236-2 Kamimakuwa, Motosu, Gifu 501-0495, Japan § Aichi Electric Company, Ltd., 1 Aichi-cho, Kasugai, Aichi 486-8666, Japan ‡
ABSTRACT: To utilize the waste oil containing the chlorine compound substance effectively, microwave-induced nonequilibrium plasma of the glow charge type was applied to spray combustion of the used electrical insulation oil containing the chlorine compound substance. The spray combustion experiment was conducted in a quartz reaction tube with microwaveinduced nonequilibrium plasma. Many O and H radicals are generated by the electron impact dissociation reactions O2 + e → 2O + e and CmHn + e → CmHn−1 + H + e (hydrocarbon CmHn is produced by vaporization of insulation oil) under nonequilibrium plasma, and many OH radicals are produced by the O + CmHn and O + H reactions. Because O and OH radicals play an important role in the combustion reaction and work as a combustion reaction promoter, the ignition and combustion of insulation oil are enhanced under nonequilibrium plasma. As a result, the flammability limit of combustion of insulation oil is expanded and the heat flow rate of the exhaust gas increases under nonequilibrium plasma, because the exhaust gas volume and flow rate of combustible gases such as H2 and CO are increased.
1. INTRODUCTION Waste oil, such as lubricant oil containing short-chain chlorinated paraffin and electrical insulation oil containing polychlorinated biphenyls (PCB), has a heating value that is at the same level as that of heavy fuel oil and is used in power plants using a combustion system to generate electricity throughout the world. However, high concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are produced during the combustion of waste oil containing the chlorine compound substance.1,2 Postcombustion synthesis of PCDDs and PCDFs is mainly related to three mechanisms as follows: pyrosynthesis, i.e., formation in the gas phase at a high temperature; formation from various precursors, such as chlorophenols or polychlorinated diphenyl ethers, which may be formed in the gas phase during incomplete combustion and combine heterogeneously and catalytically with fly ash; and de novo synthesis caused by heterogeneous reactions that require the presence of a chlorine source, oxygen in the exhaust gas, and heavy metals and carbon acting as a catalyst.3 The last two mechanisms are dominant in the temperature range of 573−723 K.4 There is much debate regarding the toxicity of these compounds, but there is accumulating evidence that they can have carcinogenic, reproductive, and developmental effects in humans.5,6 In the western world, waste oil is handled by the controlled combustion process to inhibit the production of PCDDs and PCDFs. In contrast, because Japanese environmental regulation is extremely strict, it is difficult to handle waste oil via combustion in Japan. To utilize waste oil containing the chlorine compound substance in combustion power plants in Japan, the chlorine compound substance must be more strictly decomposed and handled. © 2013 American Chemical Society
We have paid attention to the plasma technology, which is extremely useful in the treatment of hazardous industrial waste, as a processing method for the chlorine compound substance. Plasma is considered to be the fourth state of matter, consisting of a mixture of electrons, ions, and neutral particles, although overall it is electrically neutral. There are two types of plasma technologies used in the treatment of hazardous industrial waste: thermal plasma7 and nonequilibrium plasma.8 Thermal plasma is that in which electrons, ions, and neutral particles are under thermal equilibrium conditions at high temperatures. Because a fair amount of energy is required to create that condition, thermal plasma is an extremely costly method of processing. Meanwhile, nonequilibrium plasma is that in which only electrons are at a high temperature and ions and neutral particles are at a low temperature. Nonequilibrium plasma is a low-cost method for heating only electrons, which have a low heat capacity, as compared with thermal plasma. Flame is considered a form of plasma in the area of plasma science. There has been considerable research of ignition and combustion enhancement by thermal plasma9,10 or nonequilibrium plasma.11,12 Thermal plasma using a plasmatorch was applied to the combustion treatment of insulation oil containing 1.0 indicates the fuel-poor condition, and an oxygen ratio of 10 min. The analyses of dioxins were performed by the Nagoya City Environmental Science Research Institute.
2. EXPERIMENTAL SETUP The experimental apparatus is schematically shown in Figure 1. The equipment specifications and experimental conditions are listed in Table 1 and 2, respectively. The experimental apparatus consisted of a microwave generator, a quartz reaction tube, a rotary vacuum pump, a double-pipe condenser, and a diaphragm pressure gauge. The dimensions of the quartz reaction tube were 45 mm (inner diameter) and 750 mm (length). The pressure in the quartz reaction tube was reduced by a rotary vacuum pump and the tube irradiated with a 2284
dx.doi.org/10.1021/ef302174z | Energy Fuels 2013, 27, 2283−2289
Energy & Fuels
Article
The emission spectra from the quartz reaction tube were analyzed at the observation port located 25 mm from the two fluid nozzles by a polychromator (Bunko-Keiki Co., MK-300). The emission spectra were analyzed near the visible area between 200 and 830 nm, and the emission spectra were measured at point 1024 with a focus on 520 nm. The exposure times are 0.2 s at intervals of 0.22 s in the case of O2 nonequilibrium plasma by glow charge, during which insulation oil is not burned, and the exposure times are 0.01 s at intervals of 0.029 s in the case of spray combustion of insulation oil with and without microwave nonequilibrium plasma. The emission spectra were measured 100 times per condition, and the results are mean values of 100 measurements. Combustion gas is exhausted out through the rotary vacuum pump. The flow rate of the exhaust gas was measured by a gas meter at the outlet port of the rotary vacuum pump, and N2, O2, CO, CO2, CH4, C2H2, C2H4, and C2H6 in the exhaust gas were detected by a gas chromatograph (Shimadzu GC-2014AT TCD; Molecular Sieve 13×, Porapak N) at the same position.
plasma is very safe. Furthermore, because the dioxin concentration in exhaust gas was 2.5 pg of TEQ/m3, we predict that most of dioxins is ejected as a mist. 3.2. Spectral Analyses of Oxygen Plasma. Figure 2 shows the emission spectra of O2 nonequilibrium plasma by
3. RESULTS AND DISCUSSION 3.1. Generation of Dioxins. The electrical insulation oil containing PCB (20 g/m3) was burned in the quartz reaction tube at an oxygen ratio of 0.84 under microwave-induced nonequilibrium plasma. Emissions of dioxins were sampled by gas washing the bottle of hexane with a dry ice-cooled condenser. The concentrations of toxic dioxins produced from the spray combustor are listed in Table 3.16 The sum of dioxins produced by spray combustion using nonequilibrium plasma was 180 pg of TEQ/m3. The environmental regulation of dioxins adopted in Japan is very strict, and the emission limit of dioxins is 5000 pg of TEQ/m3. The observed data in this experiment are 3.6% of the emission limit. It is shown that the spray combustion using microwave-induced nonequilibrium
Figure 2. Emission spectra of O2 nonequilibrium plasma in the glow charge.
glow charge before insulation oil spray. As shown in Figure 2, O radical is produced near the wavelength of 780 nm because of the electron impact dissociation reaction of oxygen12,22 O2 + e → O + O + e
On the other hand, although O2 nonequilibrium plasma lacks hydrogen atoms, OH radical is also detected near the wavelength of 310 nm. The hydrogen atom exists because water vapor remains in the quartz reaction tube. OH radical is mainly formed by the reaction of H2O and O radical O + H 2O → OH + OH
Table 3. Concentrations of Toxic Dioxins Produced by a Spray Combustor at an Oxygen Ratio of 0.84 under Microwave-Induced Nonequilibrium Plasma dioxin
concn (pg of TEQ/m3)
2,3,7,8-tetraCDF 1,2,3,7,8-pentaCDF 2,3,4,7,8-pentaCDF 1,2,3,4,7,8-hexaCDF 1,2,3,6,7,8-hexaCDF 1,2,3,7,8,9-hexaCDF 2,3,4,6,7,8-hexaCDF 1,2,3,4,6,7,8-heptaCDF 1,2,3,4,7,8,9-heptaCDF octaCDF 3,4,4′,5-tetraCB (#81) 3,3′,4,4′-tetraCB (#77) 3,3′,4,4′,5-pentaCB (#126) 3,3′,4,4′,5,5′-hexaCB (#169) 2′,3,4,4′,5-pentaCB (#123) 2,3′,4,4′,5-pentaCB (#118) 2,3,3′,4,4′-pentaCB (#105) 2,3,4,4′,5-pentaCB (#114) 2,3′,4,4′,5,5′-hexaCB (#167) 2,3,3′,4,4′,5-hexaCB (#156) 2,3,3′,4,4′,5′-hexaCB (#157) 2,3,3′,4,4′,5,5′-heptaCB (#189) total
18 NDa 24 NDa NDa NDa NDa NDa NDa NDa 0.13 0.86 130 0.069 0.063 4.6 1.5 0.11 0.28 0.73 0.13 0.34 180
(R1)
(R2)
The electrical insulation oil was sprayed into the quartz reaction tube under O2 nonequilibrium plasma and then automatically ignited and burned in the quartz reaction tube. It shut off the electrical insulation oil spray to the quartz reaction tube and reverted to the O2 nonequilibrium plasma state in the quartz reaction tube. 3.3. Characteristics of This Experimental Apparatus. Figure 3 shows the O2 mole fraction and flammability limit compared for spray combustion by nonequilibrium plasma in the glow charge and normal spray combustion under reduced pressure. The source of ignition generally becomes necessary for ignition of the insulation oil. However, when the atomized
Figure 3. O2 mole fraction in exhaust gases and flammability limit compared for spray combustion by microwave-induced nonequilibrium plasma in the glow charge and normal spray combustion under reduced pressure.
a
The concentration is below the detection threshold of the measurement method. 2285
dx.doi.org/10.1021/ef302174z | Energy Fuels 2013, 27, 2283−2289
Energy & Fuels
Article
Figure 4. Comparison of spray combustion by microwave-induced nonequilibrium plasma in the glow charge and normal spray combustion under reduced pressure for the exhaust gas flow rate of each chemical species: (a) H2, (b) CO, (c) CO2, and (d) C2H2.
part of insulation oil was emitted in unchanged form without reacting with oxygen. 3.4. Exhaust Gas Flow Rate of Each Chemical Species. Figure 4 compares the flow rates of H2, CO, CO2, and C2H2 between spray combustion by nonequilibrium plasma in the glow charge and normal spray combustion under reduced pressure. Here, because CH4, C2H4, and C2H6 were rarely detected in this study, the flow rates of these chemical species are not given here. The flow rates of H2, CO, and C2H2, which are combustible gases and are produced by pyrolysis of insulation oil, increased with a decrease in the oxygen ratio in both cases, and these flow rates in the case of nonequilibrium plasma were greater than those for the normal condition. Meanwhile, the flow rate of CO2, which was formed by the complete burning, was increased with an increase in the oxygen ratio in both cases, and the flow rate of CO2 in the case of nonequilibrium plasma was lower than that for the normal condition. We found that levels of combustible gases such as H2, CO, and C2H2 are increased and the level of CO2 is decreased by the assistance of microwave-induced nonequilibrium plasma in the spray combustion of electrical insulation oil. 3.5. Spectral Analyses of Spray Combustion by Nonequilibrium Plasma. Figure 5 shows typical emission spectra of spray combustion of insulation oil by nonequilibrium plasma in the glow charge as compared with normal spray combustion under reduced pressure. O radical and Hα radical were measured only in the case of spray combustion by nonequilibrium plasma. Because O radical and Hα radical were not detected in the case of normal spray combustion under reduced pressure, these radicals were referred from nonequilibrium plasma in the glow charge. O2 and hydrocarbon CmHn, which is produced by vaporization of insulation oil, were
electrical insulation oil was supplied to the oxygen plasma in the quartz reaction tube, it was automatically ignited by nonequilibrium plasma and burned in the quartz reaction tube without the source of ignition. In the case of spray combustion by nonequilibrium plasma in the glow charge, because nonequilibrium plasma promotes ignition and combustion, the insulation oil could be burned over the entire range examined in this work. On the other hand, in the case of normal spray combustion under reduced pressure, the insulation oil could be burned when the oxygen ratio was between 0.40 and 0.89. Because the flow rate of insulation oil was very low and the spray of insulation oil was slightly unsteady, the flammability limit of this spray combustor was very narrow under normal spray combustion. However, when the ignition and combustion in this spray combustor were assisted by microwave-induced nonequilibrium plasma, the flammability limit of this spray combustor was dramatically increased. It is found that microwave-induced nonequilibrium plasma has excellent enhancement characteristics with respect to ignition and combustion. Oxygen concentrations under both conditions of microwaveinduced nonequilibrium plasma and normal showed the same behavior in a measured range. When the oxygen ratio was >1.0, the oxygen concentration increased with an increase in the oxygen ratio. When the oxygen ratio was
