Introduction
Coal combustion is one of the main sources of anthropogenic mercury emission around the world. Mercury and its compounds, especially methyl mercury species, are of high toxicity and bioaccumulation [1]. Three forms of mercury exist in coal combustion flue gas: elemental mercury (Hg0), oxidized mercury (Hg2+) and particle-bound mercury (HgP). Hg2+ can be easily removed by the wet flue gas desulfurization (WFGD) due to its water solubility. HgP can be captured along with the fly ash by the electrostatic precipitators (ESP). However, Hg0 is difficult to control because it is extremely volatile and water-insoluble [2]. Therefore, Hg0 is the dominant form of mercury that released to the atmosphere. One of the feasible ways to remove Hg0 is to use catalytic oxidation method to convert Hg0 to Hg2+.
The SCR catalyst was recognized as a promising catalyst for Hg0 oxidation, as it can be used for NO reduction simultaneously. In particular, the V2O5-based commercial SCR catalysts for NO removal have been reported as Hg0 oxidation catalysts [3], [4]. Since the activities of V2O5-based commercial SCR catalysts are significantly affected by flue gas temperature, the catalytic activities are unstable due to the load variation of the boiler. Meanwhile, as the catalysts are placed before the ESP, they are deactivated easily due to the high concentration of dust in the flue gas [5], [6]. It has been found that, relative to its effects on the efficiency of NO reduction, the catalyst deactivation could more significantly reduce Hg0 oxidation efficiency [7]. In this case, there is a need for a novel catalyst that could perform well for Hg0 oxidation at lower temperature, which can be located after the cold side ESP. Meanwhile, Huaneng group, a huge power generation company in China, has issued a strict NOx emission standard (less than 50μg/m3) for the coal-fired power plants. The existing commercial SCR catalysts cannot meet the emission standards. Therefore, to save the cost, the catalyst used for Hg0 oxidation at lower temperature was required to remove the NO escaped from upstream simultaneously.
Fortunately, the low toxic MnOx catalysts have been proved to be the co-benefit catalysts for NO and Hg0 removal at lower temperature, and they exhibit excellent performance under certain conditions [8], [9], [10]. However, the Hg0 oxidation efficiencies of these catalysts strongly depended on HCl concentration in the flue gas, a factor that cannot be controlled in the power plants. The catalysts exhibited excellent mercury oxidation activities at ∼250°C, which were not the typical flue gas temperature after the cold-side ESP. Considering the above limitations, Mn-based oxides could be modified to be used for Hg oxidation at lower temperature subject to less influence of HCl concentration.
The Mn-based perovskite oxides seem an attractive candidate for Hg0 oxidation due to its excellent thermal stability, superior redox properties and great versatility [11], [12]. The general formula of perovskite oxides is ABO3. The A-site ions fitted into the dodecahedral interstices are always large rare-earth or alkaline-earth metal ions. The B-site ions are always the transition metal ions which occupy oxygen octahedrons and play a dominant role in the catalytic process. It is generally believed that the properties of the B-site ions, the concentration of oxygen species, and the existence of lattice defects are responsible for the good catalytic activities of the perovskite-type oxides [13]. A-site cations in the perovskite oxides play a role of structure-stabilizing and partial substitution of A-site cations can modify the physicochemical properties of perovskite oxides. The incorporation of a low valence cation in the A-site decreases the valence that is compensated by the creation of oxygen vacancies and generation of more higher valence B-site cations. The most common Mn-based perovskite catalyst is LaMnO3, which has been investigated as a low temperature SCR catalyst recently [11].
Few researches investigated catalytic effects of LaMnO3 for Hg0 oxidation. Hence, some tests need to be carried out to investigate whether LaMnO3 can be served as an efficient catalyst for Hg0 oxidation. The abundant active oxygen species on the catalyst surface may be conducive for the formation of oxy-chloride or active chlorine species which are active for Hg0 oxidation. In addition, it has been reported that the perovskites exhibits better catalytic performance when the A-site ions La3+ are partially substituted by Sr2+ [14]. This is because partial substitution of La3+ with Sr2+ can generate mixed-valence manganese (Mn3+ and Mn4+) and more active oxygen species. The shift between Mn3+ and Mn4+ is beneficial for the generation of labile oxygen vacancies and bulk oxygen species [15]. The redox couple may also benefit the Hg0 oxidation due to the Mars-Maessen mechanism in the absence of HCl. It seems likely that the LaMnO3 modified by partial substitution of La3+ by Sr2+ could oxidize Hg0 with low concentration of O2 and HCl in the flue gas when low rank coals are burned.
Accordingly, we hypothesize that the La1−xSrxMnO3 (x=0/0.2/0.4/0.6) perovskite catalysts could exhibit remarkable catalytic performance on Hg0 oxidation with low concentration of O2 and HCl in the flue gas at low temperatures. In the present work, the Mn-based perovskite oxides were synthesized and characterized to investigate the catalytic activity of the oxides on Hg0 oxidation. In order to improve the catalytic activity of the LaMnO3, the La3+ ions were partially substituted by Sr2+ ions to increase the surface defects so as to generate more oxygen vacancies and active oxygen species. The catalytic performance of the oxides under conditions such as low concentration of O2 and HCl at low temperature was also investigated, and the Hg0 oxidation mechanism was identified. The effects of SO2, H2O, NO and NH3 on the Hg0 oxidation efficiency were studied. Hg0 oxidation efficiency under SCR atmosphere and the SCR activity of the selected catalysts (La0.6Sr0.4MnO3) were considered as well. It is demonstrated that the synthesized perovskite oxides may serve as an environmentally friendly catalyst for simultaneous removal of NO and Hg0 from coal combustion flue gas at low temperature.
Section snippets
Preparation of catalysts
The La1−xSrxMnO3 (x=0/0.2/0.4/0.6) catalysts were prepared by complexation method using citric acid as a complexation agent [11]. They are denoted as LSx, where L is the raw LaMnO3, S is the doped Sr, and x represents the mole ratio of Sr/(Sr+La). Stoichiometric amounts of lanthanum nitrate, strontium nitrate and manganese nitrate were dissolved in deionized water. Citric acid was separately dissolved in distilled water and added to the mixed solution. The amount of the citric acid was equal to
Characterization of catalysts
The N2 adsorption–desorption isotherms and pore size distributions of the LSx catalysts were presented in Fig.2(a)and(b), respectively. As seen in Fig.2(a), the catalysts showed N2 adsorption–desorption isotherms of type II with a hysteresis loop of type H3 in the relative pressure range of 0.7–1.0. This type of the isotherm was characteristic of macroporous materials [17] and this type of hysteresis loop indicated the formation of mesopores. Moreover, a certain amount of N2 were adsorbed
Conclusions
In this study, the catalytic effects of La1−xSrxMnO3 (x=0/0.2/0.4/0.6) on Hg0 oxidation were investigated. It was found that La0.6Sr0.4MnO3 exhibited the strongest catalytic activity for Hg0 oxidation at 100–200°C. HCl had a prominent enhancement effect on Hg0 oxidation in the presence of O2, and 10 ppmv HCl was sufficient for enhancing the oxidation process. The results suggest that the catalyst could be used in power plants burning low-chlorine coals. The catalyst could help oxidize the Hg0
Acknowledgments
This work is supported by the National Natural Science Foundation of China (51476064, U1261204) and the National Basic Research Program of China (2013CB228501). The support of the Analytical and Testing Center at the Huazhong University of Science and Technology is also appreciated. Mr. Ho Simon Wang has helped improve the linguistic presentation of the manuscript.
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