VOLUME 16, NUMBER 4
JULY/AUGUST 2002
© Copyright 2002 American Chemical Society
Articles Development of a New Dry-Desulfurization Process by a Non-Thermal Plasma Hybrid Reactor Heejoon Kim,* Akira Mizuno, and Yuhei Sakaguchi Department of Ecological Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 441-8580, Japan
Guoqing Lu and Masayoshi Sadakata Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Received July 26, 2001. Revised Manuscript Received January 25, 2002
To find out a new dry-type desulfurization process with high efficiency and cost performance, a hybrid-type reaction process combining pulsed streamer corona plasma and TiO2 catalyst was developed in order to oxidize SO2 to SO3. Experiments of both gas-phase reaction and surface reaction were performed to elucidate oxidation characteristics. Experimental results show that the oxidation fraction of SO2 to SO3 in the gas-phase reaction is below 5% at low temperatures (under 800 K). Similarly, when using TiO2 only as catalyst and applying the pulsed streamer corona plasma, respectively, the oxidation fraction is not increased significantly. Moreover, when simply combining the gas-phase reaction and the surface reaction, the oxidation fraction can reach and maintain only about 10% below 673 K. Contrary to the above two results, the oxidation fraction could be increased significantly by adding H2O of about 0.2 vol %. Hydrogen peroxide was demonstrated to have better oxidation promotion characteristics in comparison to the addition of H2O and H2O + plasma. The oxidation fractions of 60% and 90% can be achieved, respectively, by adding a very small amount of H2O2 and applying the pulsed streamer corona plasma in the reaction process. Finally, these experimental results confirmed our initial hypothesis that hydroxyl radicals (OH) enhance the oxidation of SO2 to SO3 in both the gas-phase reaction and the surface reaction on TiO2.
Introduction Associated with nuclear power generation reduction in some developed countries, coal-fired power generation will continue to play a key role in power supply in the world in the foreseeable future. SO2 emission and acid rain from coal combustion have become a serious problem in the world. On the other hand, the current wet-type gas desulfurization process has to consume a large amount of water and has high desulfurization cost. * Corresponding author.
However, water shortage in the world has attracted increasing attention. For example, in China, the largest coal consumption country in the world, about 76% of the energy requirement is provided by coal.1 In 1995 the total SO2 emission rose to 2.37 million tons, more than 90% of which was from coal combustion.2 Coalfired power plants and industrial boilers are responsible (1) Chen, C. H. Indoor Air Pollution and its Health Effects in China. Environ. Technol. 1992, 13, 301. (2) Kim, H. J.; Hasimoto, S.; Ona, S.; Matsui, K.; Sadakata, M. J. Jpn. Energy 1997, 76, 205.
10.1021/ef010190e CCC: $22.00 © 2002 American Chemical Society Published on Web 05/04/2002
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Figure 1. High-voltage square wave pulse generator circuit.
for about 67% of the SO2 emission from coal combustion, the remainder is covered by domestic stoves and smallcapacity boilers.3 Likewise, at present some regions in the world are also facing a serious insufficiency of water. Accordingly, development of an economical and dry-type method for removing SO2 in flue gas from combustion facilities is urgently needed to mitigate the worldthreatening acid rain. Although some wet- and dry-type desulfurization processes such as calcium-gypsum, etc., have been developed and applied, the most economical process is a direct limestone injection method. However, the current dry-type process of direct limestone injection has a poor SOx-removal efficiency. In recent years, new dry-type desulfurization/denitrification processes based on a E-Beam Process4 and Pulsed Corona Plasma5,6 have attracted considerable attention. Among them, the nonthermal plasma process is considered to be one of the most effective methods for reducing SOx and NOx in industrial flue gas although its energy efficiency does not reach a suitable level since the nonthermal plasma can induce a host of radicals and excited-state molecules due to electron excitation. In this study, the oxidation reaction experiments of SO2 to SO3 were performed experimentally in order to find a new dry-type desulfurization process with high efficiency and better cost performance. A hybrid-type reaction process combining pulsed streamer corona plasma and TiO2 catalyst was developed. The hybridtype reaction process consists of two reaction stages: (i) the gas-phase reaction induced by plasma, and (ii) the surface reaction by radicals captured and/or formed on the catalyst surface. A series of experiments of both the gas-phase reaction and surface reaction were carried out. This paper discusses the mechanism of the oxidation reaction of SO2 to SO3 in the gas phase and on the catalyst surface and the effect of additives such as H2O and H2O2. Experimental Apparatus and Procedure Pulsed Streamer Corona Plasma Generator. A highspeed rotary spark gap switch was used as shown in Figure 1 to generate the fast-rising square-wave voltages.7 In this study, commercial AC (100 V; 60 Hz) was input and transformed up to 12.5 kV by a transformer. A high-voltage diode, a ballast resistor of 50 kΩ, and a capacitor (1200 pF) were lined to the (3) Yan, C. L. Development Report of China Energy, Beijing, China, 1994; p 17. (4) Kawamura, K.; Aoki, S.; Kimura, H.; Adachi, K.; Kawamura, K.; Katayama, T.; Kengaku, K.; Sawada, Y. Environ. Sci. Technol. 1980, 14, 287. (5) Masuda, S.; Nakao, H. IEEE/IAS Annual Meeting, 1986; p 1173. (6) Mizuno, A.; Clements, J. S.; Davis, R. H. IEEE Trans. Ind. Appl. 1986, 22, 516. (7) Kiyono, M. Characteristics and Application for TiO2 (in Japanese); Tokyo, Japan, 1991; p 54.
Kim et al. transformer. The high-voltage electrode was connected to a dc high voltage, then disconnected and grounded repeatedly according to the order of the spark gap switch rotation. The connection, disconnection, and grounding were completed instantaneously due to the high-speed rotation of the switch. The pulsed high-voltage generator of rotary spark gap (RSG) switch can produce fast-rising pulsed high voltage up to +12.5 kV. Schematic Apparatus and Procedure. In this study, the oxidation reaction experiments of SO2 to SO3 were performed in a laboratory-scale apparatus schematically shown in Figure 2, which consists of the mass flow control part, the reaction part with temperature controller, and the flue gas analysis part. The reactor was composed of a cylindrical quartz tube with 15 mm inner diameter, a high-voltage power supply equipped with the plus electrode inside the quartz reactor and minus electrode on the outside of the quartz reactor, and TiO2 catalyst pellets with 3 mm diameter. The reaction part was further divided into two parts: (1) the gas-phase reaction induced by plasma, and (2) the surface reaction with radicals captured and/or formed on the catalyst surface. The gas-phase reaction zone consists of a metal tube electrode (o.d. 3 mm, plus) located in the center of the quartz reactor and a copper thin-film ground electrode (10 cm long, minus) wound around the quartz reactor (Reaction Zone 1). This metal tube electrode was used for supplying additives such as H2O or H2O2. Reactant gases were separately supplied to the quartz reactor. The surface reaction zone is set up at a distance of about 30-100 mm (changeable) away from the metal tube electrode for controlling a gas-phase reaction or distinguishing the surface reaction on TiO2 catalyst from the gas-phase reaction induced by plasma. The surface reaction zone is packed with TiO2 catalyst pellets inside the quartz reactor, as shown in Figure 2. Gases excited by plasma will promote an oxidation of SO2 on the surface of TiO2 catalyst. The experimental conditions are summarized in Table 1. In all the experiments, the quartz reactor was packed with TiO2 catalyst pellets of 10 g and the reactor was heated to a predetermined temperature. The reactant gases were supplied into the quartz reactor with a predetermined fraction. The total flow rate of reactant gases was always kept at 1 L/min. To test the effect of water or hydrogen peroxide additions on the reaction efficiency, air saturated with water or hydrogen peroxide was supplied at various desired temperatures. In the experiments, the oxygen concentration was always fixed at 2.1 vol % to maintain excess oxygen conditions. The initial concentration of SO2 was 906 ppm (N2 basis). During the reaction, SO2 concentration was continuously sampled and analyzed with a SO2 meter (Shimadzu SOA-7000), and the time-concentration history of SO2 was obtained for the entire reaction process. The SO3 concentration was analyzed using ion chromatography (Dionex DX-120). The current wave and voltage were monitored with a digital oscilloscope (Tektronix TDS 350). The input power was measured by a power meter.
New Dry-Desulfurization Process In this study, a method to produce sulfuric acid or ammonium sulfate as a valuable byproduct is considered in the design of the new dry-type desulfurization process as shown in Figure 3. The sulfuric acid process takes place through two reaction steps: (i) oxidization of SO2 to produce SO3, followed by (ii) reaction with H2O to form H2SO4.
SO2 + 1/2O2 f SO3
(1)
SO3 + H2O f H2SO4
(2)
The ammonium sulfate process takes place through two reaction steps: (i) oxidation of SO2 to produce SO3,
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Figure 2. Lab-scale apparatus for studying the oxidation reaction experiments of SO2 to SO3. Table 1. Experimental Conditions reaction temperature [K] gas flow rate [L/min] concentration [vol %] catalyst reactor
288, 373, 473, 573, 673, 773, 873, 973, 1073 SO2: 0.9; air: 0.1 SO2: 906 ppm (N2 basis); O2: 2.1%; H2O: 0.18%; H2O2: 0.055% TiO2 (diameter: 3.1 mm) cylindrical quartz tube (diameter: 10-15 mm)
and subsequently (ii) reaction with NH3 in the presence of H2O to form (NH4)2SO4.
SO2 + 1/2O2 f SO3
(3)
SO3 + 2NH3 + H2O f (NH4)2SO4
(4)
In these processes, the oxidation of SO2 is the key reaction, because this reaction does not progress no matter with or without catalyst at low temperatures (below 673 K) except when Pt is used as catalyst. Also, one purpose of this work is to find a catalyst to promote the oxidation of SO2 at low temperatures with some radicals by plasma. Results and Discussion To examine the SO2 oxidation fraction in each reaction, i.e., the gas-phase reaction with plasma, and the surface reaction with catalyst, each reaction was tested independently. To simplify the reaction analysis, the gas flow-rate was always maintained at 1L/min, the charging voltage of the capacitor at +12.5 kV, and the frequency at about 150. The artificial gas composition was fixed at 906 ppm SO2, and 2.1% O2 in basic gas of N2. Gas-Phase Reaction Induced by Plasma. Figure 4 shows the temperature dependence of the oxidation fraction of SO2 to SO3 with and without the pulsed
streamer corona plasma. The residence time of reaction gas changed from 0.2 s to 0.065 s, depending on the temperature change in the reaction system. During the reaction process without the pulsed streamer corona plasma, the oxidation fraction of SO2 to SO3 has a slight increase with increasing the temperature. Under 673 K, the oxidation fraction is below 2% and can be ignored. On the other hand, during the reaction process with the pulsed streamer corona plasma, the oxidation fraction is almost a constant, about 5%, below 873 K and then rises to 10% with increasing the temperature to 1073 K. The experimental results showed that the gas-phase reaction and the surface reaction due to quartz reactor inside wall could be ignored when the reaction system temperatures is under 900 K. Combining Gas-Phase Reaction and Surface Reaction. The oxidation fraction of SO2 to SO3 with TiO2 catalyst and the plasma is shown in Figure 5. During this reaction process, the reaction system temperature changed from about 293 K to 1073 K and the influence of the plasma and TiO2 catalyst on the oxidation fraction were investigated. Before each experiment was carried out, TiO2 pellets were baked for 2 h at 573 K in order to remove moisture on TiO2 pellets surface. The general trend of the oxidation fraction in the case of the TiO2 catalyst without pulsed streamer corona plasma shows a very slow increment with increasing temperature, and reaches only about 13% at 973 K. Therefore, the oxidation promotion by TiO2 catalyst only without H2O or H2O2 is considered to be negligible below 673 K. In the case of the combined process with the gasphase reaction by the plasma and the surface reaction with TiO2 catalyst, the oxidation fraction sustains always about 10% at temperatures below 673 K. And then the oxidation fraction decreases as the reaction temperature increases over 673 K. In the temperature
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Figure 3. New dry-type desulfurization processes.
Figure 4. Oxidation fraction versus reaction temperature.
Figure 5. Oxidation fraction versus temperature with catalyst (TiO2) and plasma.
range under 673 K, the oxidation fraction of SO2 to SO3 in this combined process is several times larger than that in the surface reaction alone by TiO2 catalyst or in the gas-phase reaction alone by plasma. This phenomenon can be explained by the fact that the radicals such as OH and O created in the gas-phase reaction induced by the plasma improve the oxidation reaction on the TiO2 catalyst surface. However, at the high temperature range of from 673 K to 997 K, the reason of the decrement of the oxidation fraction with increasing temperature in the combined system as compared to the increment of the fraction only in the gas-phase reaction was not well understand. Above 997 K, the oxidation fractions in both the TiO2 catalyst alone process and the combined process are diminished. It might be assumed that this decrement reason is that the oxidation reaction
is an exothermic reaction, and a high temperature would push the oxidation reaction to move toward the opposite direction. Effect of Additives. It has been well-known that OH radicals play an important role in the oxidation reaction of SO2 to SO3. One of chain reactions by OH radicals can be expressed as follows:
SO2 + OH + M f HOSO2 + M
(5)
HOSO2 + O2 f HO2 + SO3
(6)
HO2 + SO2 f OH + SO3
(7)
On the other hand, OH radicals are easily formed when H2O adsorbs on the surface of TiO2 catalyst as shown
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Figure 6. Adsorption of H2O on TiO2 catalyst surface. Figure 8. Comparison of oxidation fraction between H2O and H2O + plasma.
Figure 7. Temperature dependence of oxidation fraction with H2O and H2O2 additions.
in Figure 6.7 Therefore, it can be concluded that H2O has some capabilities to promote the oxidation reaction of SO2 to SO3 on the surface of TiO2 catalyst. On the other hand, H2O2 is usually more reactive than H2O, so that it can be easily assumed that H2O2 has also some capabilities to promote the oxidation reaction of SO2 to SO3. Figure 7 shows the temperature dependence of oxidation fraction of SO2 to SO3 with TiO2 and H2O/ H2O2 addition. Despite addition of about 0.2 vol % H2O, the oxidation fraction of SO2 to SO3 is remarkably increased (by about twice) as compared to that without any addition of H2O. In this temperature range of 375 K to 575 K, the reaction temperature seems to have no significant influence on the oxidation fraction. This increment’s reason is that the addition of H2O produces OH radicals on the surface of TiO2 catalyst as shown Figure 6. Moreover, hydrogen peroxide is a strong oxidizing agent. Actually, Mizuno et al.8 reported that hydrogen peroxide affects the oxidation reaction of NO to NO2 in the plasma process. In this study, the oxidation promotion experiments of hydrogen peroxide for SO2 were also performed with temperature change as a basic parameter in nonplasma process. The experi(8) Mizuno, A.; Kamase, Y. IEEE Trans. On IAS 1989, 25, 54. (9) Kuroki, T.; Takahashi, M.; Okubo, M.; Yamamoto, T. International Conference on Phenomena in Ionized Gases, Nagoya, Japan, 2001; p 121. (10) Kim, H. H.; Tsunoda, K.; Katsura, S.; Mizuno, A. IEEE/IAS Annual Meeting, 1997; p 1937. (11) Xie, Y. S. The First International Symposium on Environmental Protection in The Asian Region, Research Report of Japan Society of the Promotion of Science Research of the Future Program, Tokyo, Japan, 1999, pp 1-8.
Figure 9. Comparison of oxidation fraction with plasma and different additives.
mental results are shown in the same figure. In fact, the oxidation fraction of SO2 to SO3 from H2O2 addition overwhelms drastically that from H2O addition. The oxidation fraction increases to about 70% with increasing temperature in the range of from 300 K to 900 K, and does not appear in decrement tendency as the temperature increases in the range above 900 K. This result shows that hydrogen peroxide is more easily decomposed to produce OH radicals on the surface of TiO2 catalyst, when it is compared to H2O. As said in Figure 5, the radicals can promote the oxidation reaction of SO2 to SO3 and the plasma is able to create OH and O radicals in the gas-phase reaction. The effect of the plasma with an addition of H2O was investigated by experiments in combined process and compared with the effect of H2O addition without plasma. The experimental results are shown Figure 8. Above 473 K, the oxidation fraction is increased about 10% as compared to the case without. This increment almost equals the amount in process without H2O addition. Hydrogen peroxide might be more easily decomposed to produce OH radicals in the gas phase (including decomposition by ultraviolet ray)8 and on the surface of TiO2 catalyst, as shown in Figure 7, when it is compared with H2O. Figure 9 shows the oxidation promotion results when catalyst (TiO2), additives (H2O and H2O2), and plasma (12.5 kV) were applied. The oxidation fraction in the combined process with addition of H2O2 exceeds 80% over 573 K and amounts to 90%
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above 773 K. By comparing Figures 7, 8, and 9, we know that the increment of oxidation fraction is about 3035% when with addition of H2O2 and about 10% when with addition of H2O in the process applying the plasma. It might be assumed that hydrogen peroxide is more easily decomposed to produce OH radicals in the gas phase than that of H2O, when the plasma is applied. These results reveal that OH radicals can enhance the oxidation reaction of SO2 to SO3 in both gas-phase reaction process and surface reaction process on TiO2. Conclusion An experimental study has been carried out in order to find a new dry-type desulfurization process. In this study, the reaction system combining the pulsed streamer corona plasma and the TiO2 catalyst was tested in order to develop a high-efficiency and low-cost performance process. The experiments of both gas-phase reaction and surface reaction were performed. In the gas-phase reaction by the pulsed streamer corona plasma, the oxidation fraction of SO2 to SO3 is only below 5% at low temperatures under 873 K. Similarly, the oxidation fraction by the only TiO2 catalyst is almost negligible at temperatures below 873 K. When the gas-phase reaction and the surface reaction are combined, the oxidation ratio is almost constant at about 10% at temperatures below 673 K, and then
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decreases with increasing reaction temperature. However, the oxidation fraction of SO2 to SO3 can be increased remarkably by adding H2O. Moreover, the oxidation efficiency is increased with applying the plasma as compared to the case in which no plasma is used. The oxidation fraction by adding hydrogen peroxide reaches 60% and 90%, respectively, when plasma was not used and when it was used. These results reveal that the hydrogen peroxide is more easily decomposed to produce OH radicals in the gas phase and on the surface of TiO2 catalyst than that of H2O. The OH radical can enhance the oxidation reaction of SO2 in both cases, i.e., the gas-phase reaction and the surface reaction on TiO2. Acknowledgment. This work is supported by the Core Research for Evolutional Science and Technology. Note Added after Print Publication. Errors appeared in Figures 8 and 9 and their corresponding captions in the version of this article published on the Web 5/04/2002 (ASAP) and in the July/August 2002 issue (Vol. 16, No. 4, pp 803-808). The correct electronic version of the paper was published on 10/16/2002, and an Addition and Correction appears in the November/ December 2002 issue (Vol. 16, No. 6). EF010190E
