to Elemental Sulfur by the Coupling of Cold Plasma and Catalyst

may have economic benefits from the sale of sulfur. Elemental sulfur constitutes ... process, in which not only must the electron gun be powerful and ...
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Ind. Eng. Chem. Res. 2004, 43, 5000-5005

Direct Reduction of SO2 to Elemental Sulfur by the Coupling of Cold Plasma and Catalyst (I) Zhihui Ban,* Jinchang Zhang, Shudong Wang, and Diyong Wu Dalian Institute of Chemical Physics, CAS, 457 Zhongshan Road, Dalian 116023, People’s Republic of China

The direct reduction of SO2 to elemental sulfur in flue gas by the coupling of cold plasma and catalyst, being a new approach for SO2 reduction, was studied. In this process, CO2 can be disassembled to form CO, which acts as the reductant under the cold plasma. With the coupling of the cold plasma and the catalyst, sulfur dioxide was selectively reduced by CO to elemental sulfur with a byproduct of metal sulfate, e.g., FeSO4. In the present work, Fe2O3/γ-Al2O3 was employed as the catalyst. The extent of desulfurization was more than 80%, and the selectivity of elemental sulfur is about 55%. The effects of water vapor, temperature, and the components of simulated flue gas were investigated. At the same time, the coupling of thermogravimetry and infrared method and a chemical analysis method were employed to evaluate the used catalyst. In this paper, we will focus on the discussion of the catalyst. The discussions of the detail of plasma will be introduced in another paper. 1. Introduction Because recent environmental concern enforces tighter regulations for the emission of SOx, the treatment of sulfur dioxide has become a significant problem. Commercial technologies for the removal of SOx from flue gas are based on throwaway routes, in which alkali or alkaline-earth metals react with SOx to form metal sulfate. However, because of the pollution problems of sulfate disposal, other removal technologies such as adsorption and absorption are under development. In conjunction with this development, it is necessary to devise processes to treat sulfur dioxide released from the sorption process. The best choice probably is the reduction of sulfur dioxide to elemental sulfur, a material safe to handle, transport, and store. Moreover, it may have economic benefits from the sale of sulfur. Elemental sulfur constitutes only one-third of the volume of the equivalent CaSO4 byproduct and is completely innocuous. Therefore, no secondary pollution issues ensue from this approach. The use of catalysts for the direct conversion of SO2 to elemental sulfur has been explored extensively in the past.1-8 Various reductants have been used for SO2 reduction, including CO, H2, CH4, and carbon. For example, for reducing agent CO, the activity and selectivity of fluorite-type oxides, such as ceria and zirconia, for the reduction of SO2 were investigated at temperatures above 450 °C with 95% (dry gas condition) and 70% (wet gas condition) yields of elemental sulfur.3 However, most studies showed that the approach must be at >300 °C and oxygen-free. There are some difficulties in the process of flue gas desulfurization (FGD), e.g., in power plants. At the same time, the source of reductant is also a problem. The electron-beam irradiation process for the FGD process was expected to put up in the 1970s9 and the pulse corona process for the FGD process in the later 1980s.10,11 However, the energetic electron sources of the * To whom correspondence should be addressed. Present address: Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana 70148. E-mail: [email protected].

two approaches are different. A high-energy electron beam (400-800 keV) is produced through cathode emission and electric field acceleration in the former process, in which not only must the electron gun be powerful and continuously shielded but also its low energy efficiency must be difficult to solve. However, these defects are well avoided in the pulse corona process, in which high-energy electrons (mean energy up to 5-20 eV) are produced by means of pulse streamer corona discharge in the flue gas at ambient temperature, generating cold plasma (nonequilibrium plasma). At present, these technologies have to involve ammonia for oxidizing SO2 to (NH4)2SO4. Both the catalytic reduction and the pulse corona method processes have merits and shortcomings. Therefore, we need a novel approach to realize direct reduction of SO2 to elemental sulfur. The coupling of cold plasma and catalyst renders a novel approach. In this process, CO2 can be dissociated to form CO*,12,13 which acts as the reductant under the cold plasma. With the coupling of the cold plasma and the catalyst, sulfur dioxide can be selectively reduced by CO* to elemental sulfur. The main reaction is

2CO2 f 2CO* + 2O*

(1)

SO2 + CO* w [S] + CO2

(2)

SO2 + 2O* f SO42-

(3)

where [S] represents the various elemental sulfur forms (S2, S6, and S8) and CO* represents a product of CO2, which is activated by cold plasma. The process occurs at ambient temperature. A number of side reactions and intermediate products are, in general, possible in the SO2 + O* system. Byproducts, such as SO42-, may be formed under certain conditions. The catalyst under the coupling condition has not been studied before. In the present work, we report on the catalytic activity of the Fe2O3/γ-Al2O3 catalyst for the reduction of SO2 by coupling with the cold plasma. At the same time, the

10.1021/ie0498652 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004

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m2 g-1. The pore volume of the catalyst was about 0.6 cm3 g-1. The result of X-ray diffraction (XRD) shows that there was an active component on the surface of the fresh catalyst and the promoter component V2O5 was not found for its low concentration (shown in Figure 1). 3. Apparatus and Procedures

Figure 1. XRD results for fresh catalyst.

effects of water vapor, temperature, and the components of the simulated flue gas were also investigated. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. Bulk catalysts were prepared by the impregnation method using metal (Fe) nitrate and vanadate (V). This method provides well-dispersed metal oxides. The preparation process consists of the following steps: (1) using metal nitrate and vanadate to prepare the solution; (2) cooling the solution to ambient temperature; (3) immersing the prepared bulk Al2O3 (pretreated by the mixture of alcohol and water) into the solution for a few minutes; (4) burning the catalyst after immersion to move the inside alcohol and get a nonuniformed bulk catalyst; (5) heating the catalyst in static air for 2 h at 500 °C. In the prepared catalyst, the active component Fe2O3 is 10%, and there is 10% (V/Fe) promoter component V2O514 to activate SO2, which is known to promote SO2 oxidation. The typical surface area (BrunauerEmmett-Teller) of the thus prepared catalyst was 250

3.1. Catalyst Performance Evaluation. The reaction was carried out in a fixed-bed reactor [the 10 cm diameter × 100 cm length poly(tetrafluoroethylene) tube] heated externally in an electrical furnace (shown in Figure 2). The temperature of the catalyst bed was controlled to within 5 °C. The voltage was ∼34 kV. The feed gas contained 4-5% (v/v) O2, 11-14% (v/v) CO2, and 1800-2100 ppm SO2, with nitrogen as the balance gas. In some experiments, water vapor was added to the feed gas [∼8% (v/v)]. Unless otherwise specified, the gas flow rate was held constant at 500 L/h measured at standard temperature and pressure. Water vapor was introduced by continuously pumping deionized water into the preheating zone of the reactor, which is at the same temperature as the reactor. The gas mixture leaving the reactor was directed to three online infrared gas analyzers in series: SO2 (SOA-7000, Shimadzu), CO2 and CO (both CGT-7000, Shimadzu), NOx and O2 (both NOA-7000, Shimadzu) (shown in Figure 3). The used catalyst was analyzed by the Netzsch STA 449 C Jupiter, coupled with a Bruker FTIR-system VECTOR. For each experiment, 400 g of catalyst was used, except where otherwise specified. The conversion of SO2 is defined as follows:

conversion (%) )

(SO2)inlet - (SO2)outlet (SO2)inlet

× 100

(4)

where (SO2) represents the concentration of sulfur

Figure 2. Sketch of the reactor (a) and the cross section (b) for direct reduction of SO2 to elemental sulfur by coupling of cold plasma and catalysis (34 kV, 50 Hz; components of simulated gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 balance; weight of the catalyst, 400 g; flow rate of the inlet gas, 500 L/h).

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Figure 3. Experimental apparatus for the coupling process.

Figure 4. Activity of the catalyst at a temperature of 28 °C (34 kV, 50 Hz; components of simulated gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2balance; weight of the catalyst Fe2O3/γAl3O2, 400 g; flow rate of the inlet gas, 500 L/h).

dioxide. The subscripts, inlet and outlet, indicate the inlet and outlet positions of the reactor. 4. Results and Discussion 4.1. Catalyst Activity of SO2 Reduction in the Coupling Process. In the pulse-induced plasma process, not only oxidative species such as OH radical, HO2 radical, O, O3, O2-, O2*, etc., but also reductive species such as CO, H*, etc., are produced. At the same time, SO2 can also be excited. Therefore, when there is a suitable catalyst, the reduction process will be carried out perfectly. The reduction process without the plasma usually occurs at high temperatures (>300 °C) and hardly at ambient temperature. On the other hand, without the catalyst, the plasma desulfurization process has a lower conversion of SO2 (Figure 4). To find a catalytic system that is more effective at lower temperatures than has been reported so far in this new approach, various types of catalysts were investigated. The present catalyst shows high activity in the coupling process. Figure 4 also shows the conversion of SO2 as a function of the reaction time over Fe2O3/γ-Al2O3 at a temperature of 28 °C. The degree of desulfurization of SO2 is more than 80%, and the conversion of SO2 without the catalyst was only about 0.02%. The activity of the catalyst is very stable. Therefore, the coupling approach is a promising method for FGD; the catalyst is effective in the process.

Table 1. Maximum Desulfurization of Different Components of Simulated Flue Gas (without Water Vapor, 28 °C, Fe2O3/γ-Al2O3, 34 kV, 50 Hz; Components of Simulated Gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 Balance) components of simulated flue gas

maximum desulfurization rate (%)

N2 + CO2 + SO2 N2 + CO2 + O2 + SO2

86.0 90.5

4.2. Influence of Components of Simulation Flue Gas. In the coupling process, there are two important components, O2 and CO2. The role of each component must be identified. Table 1 shows the results. When there were only N2, CO2, and SO2, the maximum desulfurization rate was 86.0%. It was probably attributed to the formation of CO and catalytic reduction of SO2. When O2 was added, the result was raised to 90.5%. One reason is that the presence of O2 could improve the modality of corona of discharge. Another reason is that SO2 was oxidized to SO3, which can be detected in the outlet gas. In the coupling process, both of the two reasons would give contributions to this effect. The fact is that the effect of O2 is smaller than that of CO2 in the process of desulfurization. 4.3. Influence of Temperature. Figure 5 shows the effect of temperature on the conversion of SO2. It can be seen that the conversion of SO2 decreased from 90.5% to 51.5% when the temperature increased from 28 to 95 °C. This temperature dependency of the conversion

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Figure 5. Effect of the temperature on the conversion of SO2 (34 kV, 50 Hz; components of simulated gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 balance; weight of the catalyst Fe2O3/γ-Al3O2, 400 g; flow rate of the inlet gas, 500 L/h).

Figure 6. Effect of water vapor on the process (34 kV, 50 Hz; components of simulated gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 balance; weight of the catalyst Fe2O3/γAl3O2, 400 g; flow rate of the inlet gas, 500 L/h).

of SO2 suggests that probably high temperature could affect the modality of corona of discharge because this kind of small temperature difference could not affect the reaction so much in a normal system. 4.4. Influence of Water Vapor. Some researchers15 have reported that water vapor may decrease the activity of the catalyst in SO2 reduction. Therefore, the presence of water vapor may affect the catalytic performance of the employed catalyst. The results in Figure 6 show that there is no significant change of ratio of

desulfurization when water vapor is added; at the same time there is a small increase of the ratio of desulfurization. We suggest that there are two possible reasons: one is the formation of a membrane of water on the surface of the catalyst, and SO2 can dissolve in it, which improves the reaction. The other is that H2O is ionized by an electric field and produces -OH and -H, which react with SO2. The above factors would counteract the possible decrease of the activity of the catalyst by the addition of water vapor. 4.5. Analysis of Products. 4.5.1. Coupling of Thermogravimetry (TG) and Infrared. The coupling of TG and infrared (Fourier transform, FTIR) was employed to identify the species on the used catalyst. The gases evolved from the sample during the TG analysis experiment are analyzed by a FTIR spectrometer. TG-FTIR coupling monitors the thermogravimetrical behavior and identifies the evolved gases during curing. The released gases can be referred to as the mass loss. Figure 7 shows the TG results of the used catalyst. Figure 8 shows the TG results of the fresh catalyst. Comparing the two figures, we can find that, at 150 °C in Figure 7, there is a peak in the differential curve that probably is the peak of H2O and elemental sulfur and, at 1000 °C in Figure 7, there is a small peak in the differential curve that may be the decomposition of sulfate. This peak of differential TG at 1000 °C is not significant, but in TG cure, there is a decrease at that temperature. In Figure 8, there is only one peak at 150 °C, which would be water. Figures 9 and 10 are the infrared analysis results of the tail gas of the TG analysis of the fresh catalyst and the used catalyst. Figure 9 shows that the tail gas of the fresh catalyst contained H2O only, which corresponds to the TG analysis result, while Figure 10 shows that the tail gas of the used catalyst containing not only H2O but also CO2 may also be SO2. SO3 was not detected. The existence of CO2 suggests that the catalyst can enrich CO2 to potentially make the enrichment of the intermediate product CO* on the surface of catalyst. 4.5.2. Chemical Analysis. The method of measuring SO42- and elemental sulfur is described in our earlier paper.14 For SO42-, we use CrO42- to replace it and analyze it with a spectrophotometer. For elemental

Figure 7. TG analysis of the used catalyst (34 kV, 50 Hz; components of simulated gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 balance; weight of the catalyst Fe2O3/γAl3O2, 400 g; flow rate of the inlet gas, 500 L/h).

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Figure 8. TG analysis of the fresh catalyst.

Figure 9. Infrared analysis of the tail gas of TG from the fresh catalyst.

sulfur, first we oxidize it to form SO42-, and then we use a spectrophotometer to get the total amount of sulfur, which includes elemental sulfur and sulfate. The difference between them is the amount of elemental sulfur. The key step of the method is to avoid the effects of components of catalyst. Table 2 shows the results of chemical analysis for the products of SO42- and S on the used catalyst. The selectivity may be more than 55%. There are sulfates in the catalyst, which would be the reason for deactivation of the catalyst. The outlet gas of the process was also analyzed. Little SO3 and elemental sulfur were detected. This means that the

Figure 10. Infrared analysis of the tail gas of TG from the used catalyst (34 kV, 50 Hz; components of simulated gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 balance; weight of the catalyst Fe2O3/ γAl3O2, 400 g; flow rate of the inlet gas, 500 L/h). Table 2. Results of Chemical Analysis (34 kV, 50 Hz; Components of Simulated Gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 Balance; Weight of the Catalyst Fe2O3/γAl3O2, 400 g; Flow Rate of the Inlet Gas, 500 L/h; Reaction Time, 250 min) analysis results wt % (S/cat.) used catalyst

S

SO42-

0.086

0.070

main products were on the catalyst. The method of analysis of SO3 and elemental sulfur for the tail gas was almost the same as that mentioned above.

Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5005

reducing agent is on-site-generated rather than produced using normal reducing agents, which probably needs another small factory set up to generate them. The last thing is that the final product elemental sulfur could be sold. So, considering these factors, the final cost of this technique would be economical. We expect that this novel approach may give us a new way for solving the environmental problem. Acknowledgment The authors express grateful appreciation to Huazhong University of Science & Technology for their technology support and Guangdong Power Test & Research Institute for their financial support. Figure 11. Outlet concentration of CO during reaction (34 kV, 50 Hz; components of simulated gas, 2000 ppm SO2, 5% O2, 12% CO2, and N2 balance; weight of the catalyst Fe2O3/γ-Al3O2, 400 g; flow rate of the inlet gas, 500 L/h).

4.6. CO Analysis in the Outlet Gas. The formation of CO is an important step of the coupling process. Figure 11 shows the outlet concentration of CO. It proved the formation of CO in the process. The CO formed and activated by the cold plasma in the process was different from normal CO. The fact is that when normal CO was introduced, the effect of desulfurization was not significant. 5. Conclusions Elemental sulfur can be recovered from sulfur dioxide containing flue gas by the coupling of cold plasma and catalyst. The employed catalyst is not expensive because cheap metal is used. The conversion of SO2 is more than 80%, and the selectivity of elemental sulfur is more than 55%. The coupling process occurs via two steps: CO2 is dissociated to form CO*, which acts as the reductant under the cold plasma. Then under the coupling of cold plasma and catalyst, sulfur dioxide is selectively reduced by CO* to elemental sulfur. In the process, experimental results show that water vapor has no significant effect on the conversion of SO2, but high temperatures decrease the conversion of SO2. The results also show that the catalyst is very stable under experimental conditions. The coupling of cold plasma and catalyst in a fixed-bed reactor is a novel approach, so many further investigations for this technique to treat flue gases of power plants need to be done, such as residence times to treat a typical volumetric flow rate of flue gases, energy consumption, and regeneration of the catalyst. Some of them may be solved by engineering design. For the problem of energy cost, which is the same problem as those in other processes, the fact is that this technique uses high-voltage power, which is easy to get in power plants, instead of heating the flue gas to a higher temperature (>300 °C), and in the process, the

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Received for review February 17, 2004 Revised manuscript received April 26, 2004 Accepted May 23, 2004 IE0498652