Energy & Fuels 2007, 21, 141-144
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Simultaneous Removal of NOx and SO2 by a Nonthermal Plasma Hybrid Reactor Heejoon Kim,* Jun Han, Ikuo Kawaguchi, and Wataru Minami Department of Ecological Engineering, Toyohashi UniVersity of Technology, Tempaku-cho, Toyohashi 441-8580, Japan ReceiVed February 16, 2006. ReVised Manuscript ReceiVed May 27, 2006
In previous research, a new nonthermal plasma hybrid reactor was developed for removing SO2 from flue gas. The high efficiency and cost performance were proven through experiments. However, the research on simultaneously removing SO2 and NOx has not yet been reported. The objective of this work is to find a high-efficiency multifunctional catalyst for simultaneous removal of SO2 and NOx from flue gas. In addition, we also studied the effects of temperature and discharge power on the SO2 and NOx oxidation fraction to find the optimum operation condition. The investigated catalysts are NiO/TiO2,V2O5/TiO2, and TiO2. The discharge power varies from 9 to 15 W, and the reaction temperature range is 200-450 °C. The experimental results show that NiO/TiO2 is the best among the three catalysts. Meanwhile, temperature and discharge power have a positive influence on SO2 oxidation in the case of using NiO/TiO2, while the effects on NOx and NO are negative.
Introduction Because of acid rain, photochemical smog, and other impacts on the environment and human health, the emissions of SO2 and NOx have been considered one of the most serious environmental problems in the world. Coal-fired power plants are the primary anthropogenic emission source of SO2 and NOx. In China, the largest coal consumption country, 90% of SO2 and 70% of NOx come from coal combustion.1 Hence, reducing or controlling the emission of SO2 and NOx during coal combustion is very urgent. At present, calcium gypsum (deSO2) and ammonia catalyst (de-NOx) methods are widely used for treating flue gas at coal-fired power plants. However, these methods have obvious disadvantages: low efficiency or low cost performance. Especially, these methods cannot simultaneously effectively remove SO2 and NOx.
Figure 1. New dry-type desulfurization process.
In recent years, a new dry-type desulfurization/denitrification process based on E-beam or pulsed corona plasma has attracted considerable attentions. Among them, the nonthermal plasma is regarded as one of the most prospective technologies. Tsukamoto and Namihira2 reported that 90% of NO and 50% of SO2 can be removed by the nonthermal plasma method. Lin and Gao3 also investigated the removal of NOx from flue gas by radical injection combined with plasma and NaOH solution scrubbing, the experimental results indicated that overall deNOx efficiency was as high as 81.2%. * To whom correspondence should be addressed. E-mail: kim@ eco.tut.ac.jp. (1) Lisheng, X. Experimental study and numerical simulation on high efficiency and low NOx combustion measures in large scale lean-coal boilers. The dissertation of Huazhong University of Science and Technology, China, 2000. (2) Tsukamoto, S.; Namihira, T. NOx and SO2 removal by pulsed power at thermal power plant. Pulsed Power Conference, Monterey, CA, 1999; pp 1330-1333. (3) Lin, H.; Gao, X. Removal of NOx from flue gas with radical oxidation combined with chemical scubber. J. EnViron. Sci. 2004, 3, 462-465.
Figure 2. Comparison of the removal rate with plasma and different additives.
In a previous study,4 we developed a new nonthermal plasma hybrid reactor for oxidizing SO2 in flue gas, and a great amount of experiments were carried out in the reactor. The outline of (4) Heejoon, K.; Akira, M. Development of a new dry desulfurization process by nonthermal plasma hybrid reactor. Energy Fuel 2002, 4, 803808.
10.1021/ef060067t CCC: $37.00 © 2007 American Chemical Society Published on Web 11/24/2006
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Kim et al. Table 1. Experimental Conditions temperature (°C) power (W) gas flow rate (L/min) concentration catalyst reactor additive
200-450 9-16 SO2, 1.5; NO, 1.5; air, 0.1 SO2, 380 ppm; NO, 180 ppm; O2, 7 vol % TiO2, V2O5/TiO2, NiO/TiO2 cylindrical quartz tube 2.5% H2O
Results and Discussion
Figure 3. Lab-scale reactor for studying the oxidation reaction of SO2 and NOx.
the removal process was shown in Figure 1. The experimental results demonstrated that SO2 can be effectively oxidized into SO3 when both the additive (H2O2) and catalyst (TiO2) were used in the reactor, and the efficiency was approximately 90% at 500 °C, as shown in Figure 2. Moreover, this system had a higher cost performance than other technologies (less than 1/10 of the operating cost).5 However, the operating cost increased with H2O2 injection. Meanwhile, H2O2 may endanger the health of the operator. In this paper, different catalysts (TiO2, V2O5/TiO2, and NiO/ TiO2) are compared to find a SO2 and NOx oxidation method with high efficiency and cost performance. In addition, the effect of the operating condition on the oxidation fraction of SO2, NO, and NOx is also discussed. Experimental Section The experiments are carried out in a laboratory-scale reactor, as shown in Figure 3. The simulating flue gases are generated using compressed gases (SO2, NO, N2, and dry air) and regulated by a mass-flow controller. Moisture is added to the gases by passing air through a water bath. The reactor consists of a cylindrical quartz tube with a diameter and length of 15 mm and 0.5 m. A small metal tube inside the quartz tube is used as a plus electrode, and a minus electrode is located on the outside of the quartz tube. The schematic diagram of a high-voltage square wave pulse generator is shown in Figure 4, and the detailed description of the generator can be found in ref 4. The metal tube electrode is also used for supplying an additive such as H2O or H2O2.The catalyst pellet is arranged at a place about 30-100 mm (changeable) away from the plus electrode, and the weight and diameter are about 5 g and 3 mm, respectively. The exhaust gases from the reactor are scrubbed by H2O, and then NO, NOx, and SO2 are monitored by an on-line gas analyzer. Before the experiment, the reactor is heated to the designated temperature and measured by a thermocouple. Experimental conditions are summarized in Table 1.
Figure 4. High-voltage square pulse generator.
The radicals such as OH and O created by plasma can improve the oxidation reaction, which induces in the significant increase of the SO2 and NO oxidation fraction. The mechanisms of the overall oxidation reaction are shown in Figure 5. On the basis of Figure 5, it can be concluded that the oxidation reactions of SO2 and NOx are controlled by the following factors: the concentration of SO2 and NOx, the concentration of the radical, the activity of the catalyst, and the reaction temperature. Effect of Discharge Power. As described above, the radicals created in the gas-phase reaction play an important role in oxidizing SO2 and NOx. At the same time, the discharge power of plasma has a direct relation with the concentration of radicals. Here, the effects of the discharge power on the oxidation fractions of SO2 and NOx with catalysts are experimentally studied, and the experimental results are demonstrated in Figures 6 and 7. Figure 6 demonstrates that the removal rate of SO2 has a linear relation with the discharge power in the absence of the catalyst. The oxidation fraction increases from 0 to 15% when the discharge power varies from 10 to 15 W. In the case of using the catalyst, the variation of the oxidation fraction also has a similar trend. As for NO, the variation of the oxidation-fraction-dependent discharge power undergoes two stages: the discharge power varies from 11 to 13 W, and the oxidation fraction increases with power enhancement; 13-15 W belongs to the second stage, where discharge power enhancement has only a slight influence on the oxidation reaction. A peak of the oxidation fraction of NOx versus the discharge power can be found in Figure 7. The maximum oxidation fraction and the optimum discharge power depend upon the catalyst. The best discharge power is 10 W for NiO/TiO2, whereas 12 and 13 W are best for TiO2 and V2O5/TiO2, respectively. When the discharge power is above the optimum value, the oxidation reaction will decrease. There appears to be two mechanisms that allow for the decreasing NOx oxidation fraction at high discharge power: one mechanism is that the dense streamer channels are formed and the oxidation of NO may be hampered because of an increase of the local gas
Nonthermal Plasma Hybrid Reactor
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Figure 6. Comparison of the oxidation fraction of SO2 with plasma and catalysts at 200 °C.
Figure 5. Mechanisms of the overall oxidation reaction.
temperature around the streamer channel; another mechanism is that the O3 generation is reduced at high temperature.6 Effect of the Catalyst. Zheng7 reported that the addition of a small quantity of metal oxide could change the structure of TiO2. Figures 6 and 7 also indicate the effect of adding different metal oxides into TiO2 on the oxidation fraction of SO2, NO, and NOx. As depicted in Figure 6, these metal oxides can obviously promote the oxidation fraction of SO2 in the nonthermal hybrid reactor when the power is lower than 15 W. Among the three catalysts, NiO/TiO2 is considered the best for SO2 oxidation. The effects of the three catalysts on SO2 oxidation follow the sequence: NiO/TiO2 > V2O5/TiO2 > TiO2. Although NiO/TiO2 is the best catalyst for oxidizing SO2, it is not the optimum catalyst for oxidizing NO and NOx, with the exception of the discharge power below 11 W. When the discharge power is above 11 W, the activities of other catalysts (V2O5/TiO2 and TiO2) increase sharply, V2O5/TiO2 becomes the best catalyst. Meanwhile, TiO2 is the best in oxidizing NO. Considering both SO2 and NOx oxidation efficiencies, NiO/TiO2 is regarded as one of the best multifunctional catalysts. Effect of the Temperature on the Removal Efficiency of NOx and SO2. In Figures 6 and 7, only the oxidation reactions of SO2 and NOx occurred at 200 °C are studied. It is wellknown that the temperature has an important impact on the catalytic reaction. Here, the experiments of oxidation reactions with the NiO/TiO2 catalyst at different temperatures and discharge powers are performed. In this study, the temperature (5) Lin, H.; Gao, X. The chemical kinetics analysis of removal of oxides of sulphur and nitrogen from flue gas with pulsed corona. J. EnViron. Sci. 1998, 3, 1-7. (6) Akira, M. NOx removal process using pulsed discharge plasma. IEEE Trans. Ind. Appl. 1995, 5, 957-963. (7) Zheng, Z. Progress of titanium dioxide based solid acid catalyst. Chem. Online 2003, 1, 1-8.
Figure 7. Comparison of the oxidation fraction of NO and NOx with plasma and catalysts at 200 °C.
Figure 8. Comparison of the effect of NiO/TiO2 on the SO2 oxidation at different temperatures.
range is 200-450 °C, and the discharge power varies from 9-15 W. Figure 8 shows that the temperature has a positive effect on the oxidation process of SO2 into SO3. When the temperature increases from 200 to 400 °C, the oxidation efficiency also raises from 10 to 16%. Nevertheless, the conclusion is only valid at temperatures below 400 °C. When the temperature is above 400 °C, the oxidation efficiency decreases with the temperature
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Figure 9. Comparison of the effect of NiO/TiO2 on the NO oxidation at different temperatures.
increment. It might be assumed that the reasons are the following: the oxidation reaction of SO2 into SO3 is an exothermic reaction, and high temperature will push the oxidation reaction toward the opposite direction; the concentration of the radical decreases with a temperature increase. Figures 9 and 10 demonstrate the oxidation fraction of NO and NOx versus the discharge power when NiO/TiO2 is used at 200-450 °C. It is observed that the oxidation fraction of NO decreases with an increasing temperature because the oxidation reaction is exothermic. In contrast to SO2, the higher the discharge power, the stronger the negative effect of temperature on the oxidation process. The difference of the oxidation fraction of NO between 200 and 400 °C is only 10% at 10 W, while the difference reaches 40% when the discharge power is raised to 15 W. Especially, the oxidation fraction decreases with the discharge power increment when the temperature is above 400 °C. The reason for the phenomena may be attributed to the formation of NO by chemical reactions 12 and 13.
N2 + e- f N + N-
(12)
2N + O2 f 2NO
(13)
As for NOx, the same conclusion that the temperature and discharge power have a negative effect on oxidation reaction is
Figure 10. Comparison of the effect of NiO/TiO2 on the NOx oxidation at different temperatures.
also drawn from Figure 10. The oxidation fraction will fall quickly with an increasing temperature until 400 °C. At 200300 °C, the change rate of the oxidation fraction versus temperature is about 5.6%/100 °C and decreases to around 1.5%/ 100 °C at 400-450 °C. It is also found that the oxidation efficiency of NOx falls with an increasing discharge power, and the conversion rate is even below 0. Conclusions Experimental investigations are conducted to study the simultaneous removal of SO2 and NOx from the flue gas in a nonthermal plasma hybrid reactor combined with different catalysts. After the removal efficiencies of three catalysts on SO2 and NOx are compared, NiO/TiO2 is considered as the optimum catalyst. The relations of the oxidation fraction with the discharge power and temperature are also discussed in this paper. The result shows that the temperature and discharge power have a positive effect on the oxidation process of SO2. In contrast to SO2, the oxidation fraction of NOx will decrease with the discharge power and temperature increment. EF060067T
