Article pubs.acs.org/IECR
Selenium-Assisted Reduction of Sulfur Dioxide by Carbon Monoxide in the Liquid Phase Bentao Yang,† Liyuan Chai,†,‡ Fangfang Zhu,† Xu Yan,†,‡ Kaisong Xiang,† Zhilou Liu,† Cong Zhang,† and Hui Liu*,†,‡ †
School of Metallurgy and Environment, Central South University, Changsha, China Chinese National Engineering Research Centre for Control & Treatment of Heavy Metal Pollution, Changsha, China
‡
ABSTRACT: Regenerable flue gas desulfurization (FGD) is highly attractive because of the transformation of sulfur dioxide (SO2) to high-value products. In this study, selenium-assisted SO2 reduction by carbon monoxide (CO) in the liquid phase [dimethylformamide (DMF) + water (H2O) mixture] is proposed and applied in the FGD process. This process realizes recycle of the elemental sulfur product under room temperature. Moreover, results show that both the selenium catalyst and DMF solvent have good stability and can be well recycled, implying that this process is economically feasible. Furthermore, the mechanism of selenium-assisted SO2 reduction by CO in DMF/H2O is investigated. Results show that the selenium catalyst dissolves in solution and forms Sen2− species, which are the active intermediates and promote formation of the sulfur product.
1. INTRODUCTION Contamination by sulfur dioxide (SO2) from coal-fired power stations, fossil fuel consumption, and smelting industries is a worldwide environmental problem, like PM2.5, acid rain, etc.1−3 The emission of SO2 will continue to increase with rapid urban growth in the following decades.4 To reduce SO2 emission, various flue gas desulfurization (FGD) technologies have been developed, including ammonia, catalytic oxidation, wet limestone, and lime scrubbing. However, traditional alkaline scrubbing technologies are still difficult to apply widely in China because of the poor reuse of desulfurization products. For example, only 3% of the calcium sulfate (CaSO4) product is well reused in China.5 The improper treatment of these desulfurization products will lead to new environmental concerns. To overcome this problem, the development of “regenerable” FGD is highly anticipated because it allows the recovery of sulfur resources as high-value byproducts, e.g., sulfuric acid, liquid SO2, and elemental sulfur.6 Among these byproducts, elemental sulfur is more desirable because it is easier to store and transport. In addition, sulfur is an important raw material for sulfur-related products and is widely used in different industrial applications, including sulfuric acid manufacturing, the sugar processing industry, and rubber manufacturing. Nowadays, sulfur is also widely used in the synthesis and processing of advanced materials for cathodes of rechargeable batteries and mercury capture.7−10 To date, the conversion of SO2 to elemental sulfur remains a hot topic for researchers. Technologies for the conversion of SO2 to elemental sulfur can be divided into two categories: dry (gas-phase) reduction and wet (liquid-phase) reduction.11 In dry reduction, SO2 is © XXXX American Chemical Society
reduced in the gas phase using reducing gases such as dihydrogen (H2), carbon monoxide (CO), methane (CH4), ammonia (NH3), and hot coke.12 The mixture gas is passed through a bed of catalysts, e.g., cobalt oxide, nickel oxide, molybdenum oxide, and tungsten oxide. Gas containing elemental sulfur, water (H2O), and CO2 is produced. Then, the CO2 is removed from the flue gas stream. When H2 is the reductant, the operating cost will be high. Using low-cost coal as the reductant leads to low quality of the sulfur product.13 Dry reduction of SO2 by CO will produce poisonous byproduct (COS). More importantly, these dry reduction processes require high reaction temperature (>430 K) to obtain complete conversion of SO2.14 Under such a high temperature, the product sulfur will be melted and will block the pipes, and thus the products are difficult to recycle. Compared with dry reduction, wet reduction of SO2 is more attractive with the advantages of facile conditions, easy recovery of the product, and no poisonous byproducts. More importantly, other pollutants [e.g., hydrogen chloride (HCl), NH3, nitrogen dioxide (NO2), and compounds of mercury and arsenic] in actual flue gas can also be removed by wet FGD, which can be operated by multipollutant control devices. These advantages also give low energy consumption and power expenses. In wet reduction, SO2 is absorbed in the liquid phase and then reduced to elemental sulfur. The common reduction process includes the photochemical reduction of SO2 by Received: Revised: Accepted: Published: A
December 6, 2016 January 30, 2017 February 3, 2017 February 3, 2017 DOI: 10.1021/acs.iecr.6b04718 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research magnesium tetraphenylporphyrin,15,16 catalytic reduction of SO2 using Cp*2Mo2S4 as the catalyst and hydrogen as the reductant,17 electrochemical reduction of SO2 in dimethylformamide (DMF),18 and Bio-FGD.19 Considering that CO is one of the waste gases generated from coal-fired power plants,20 wet reduction of SO2 by CO seems to be an alternative technology in terms of economy. However, to our knowledge, few researches investigate wet reduction of SO2 by CO although this dry reduction has been a well-known topic in the desulfurization field. Notably, CO can not only reduce SO2 to elemental sulfur but also promote the formation of HSe− from selenium solid under mild conditions.21 Our previous studies found that HSe− had a strong reductive ability and could reduce SO2 easily.22 In this study, inspired by the selenium-catalyzed reduction of organic compounds using CO,23−25 we suggest a new strategy to reduce SO2 by CO using selenium as the catalyst in the liquid phase. A common polar aprotic and nonvolatile solvent DMF and H2O mixture solution was used as the solvent. This solvent helped the reaction to occur under atmospheric pressure and room temperature.26−28 To our best knowledge, this is the first time that a selenium/CO/DMF/H2O reduction system has been applied in a desulfurization process. Here, the feasibility and stability of this process was investigated. Also, the conversion mechanism of SO2 to elemental sulfur was supposed via measuring the intermediates and active species. This study provides the theoretical basis for the optimization and practical application of the process in the future.
2.3. Characterization. The inlet and outlet gases were analyzed by an ecom-J2KN (RBR, Germany) gas analyzer equipped with six gas sensors to measure O2, CO, CO2(IR), NO, NO2, and SO2. Prior to analysis, the gas was balanced by nitrogen at a constant gas flow rate of 1.6 L/min to reach the minimum gas flow rate (1.6 L/min) of the analyzer needed. The SO2 absorption efficiency can be obtained from the following formula: η = (C in − Cout)/C in × 100%
(1)
where η is the SO2 absorption efficiency and Cin and Cout are the inlet and outlet SO2 concentrations, respectively. UV−visible transmittance spectroscopies of the solution were recorded using a double-beam spectrophotometer (Hitachi, Tokyo, Japan). Commercial 0.1 mm spacing quartz cells were used and obtained from Gaoss Union, China. The quartz cells were fitted with a joint of Teflon to avoid touching with air. Spectra were taken in the wavelength range of 200− 800 nm (scan speed of 400 nm/min and data interval of 1 nm). The reaction solutions were pumped into the quartz cells every 10 min in the preparation of the reduction absorbent stage through the peristaltic pump (LongerPump, Hangzhou, China) and filtered using a filter (0.25 μm). After analysis, the solutions were pumped back to the reaction flask. Horizontal attenuated-total-reflectance Fourier transform infrared (HATR-FTIR) spectroscopy (Nicolet IS10, Thermo Scientific, USA) was used to investigate the polythionate species. The reaction solutions were sampled during the desulfurization stage at different times. The solution samples were filtered using a filter (0.25 μm) to obtain the filtrate without a catalyst. Elemental analysis for the recovered sulfur was identified by energy-dispersive spectroscopy (EDS) using an EDX-GENESIS spectrometer (EDAX, Ltd., USA). Laser Raman spectroscopy (LRS) was performed at ambient temperature on a Invia Raman spectrometer (Renishaw, England) with an argon-ion laser at an excitation wavelength of 532 nm to characterize the chemical situations of selenium. X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, USA) was used to analyze the surface species of selenium with an ESCALAB 250Xi spectrometer. The structures of selenium and sulfur were measured by a X-ray diffraction (XRD; Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.54 Å). A Mastersizer 2000 laser particle size analyzer (Malvern Instruments Ltd., Malvern, England) was used to measure the size of the selenium particles.
2. MATERIALS AND METHODS 2.1. Materials. The elemental selenium, sodium hydroxide (NaOH), and DMF were of reagent grade and were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. 5% CO and 3% SO2 in balanced nitrogen were supplied by Hunan Changsha High-Tech Gas Co. Ltd., Changsha, China. All of them were used without further purification. Ultrapure H2O was used and generated using a water purification system (Shanghai, China). 2.2. Catalytic Reaction. In a typical experiment, the procedure consisted of three sections. 2.2.1. Preparation of the Reduction Absorbent. The experiments were carried out in a 250 mL three-necked round-bottom flask. First, 80 mL of DMF, 16 mL of H2O, and 2.08 g/L selenium were added to the flask. The flask was put into a DF-101S water bath with a magnetic stirrer at a speed of 260 rpm. The temperature was maintained at 373 ± 1 K. Then 5% CO was continually bubbled into the flask for 90 min with a bubbling rate of 0.1 L/min. Finally, an olive-green solution was formed. The off-gas was absorbed by 1.0 mol/L NaOH and then the excess of CO was lit. 2.2.2. Desulfurization. After the absorbent was obtained, 3% SO2 was bubbled into the flask to replace the same volume of CO (0.1 L/min). The temperature was maintained at 298 ± 1 K, and the stirring rate was maintained at 260 rpm. The off-gas was absorbed by 1.0 mol/L NaOH. 2.2.3. Sulfur Separation. Subsequently, a qualitative filter paper (pore size ∼15 μm, Hangzhou Special Paper Industry Co., Ltd., Hangzhou, China) was used to separate selenium particles from the solution. The solution can be divided into an evaporation liquid (the mixture of DMF and H2O) and an evaporation residue (polythionates) by a vacuum rotary evaporation procedure (363 K). Finally, the polythionates were acidulated to obtain the sulfur particles.
3. RESULTS AND DISCUSSION 3.1. Desulfurization Process. The change of the SO2 absorption efficiency with time during the selenium-assisted reduction by CO in DMF/H2O is shown in Figure 1. For comparison, the SO2 absorption efficiencies versus time without adding selenium/CO/DMF are also presented. Here, the absorption ability of SO2 is mainly evaluated by the time that the absorption efficiency remains ∼100% at the initial process. Clearly, the SO2 absorption ability in the selenium-assisted reduction by CO in DMF/H2O is highest (curve 4). Specifically, the SO2 absorption efficiency by H2O drops to
