Experimental study on the mechanism of mercury removal with Fe2O3 in the presence of halogens: Role of HCl and HBr
Graphical abstract
Introduction
The amount of mercury from human activities is increasing gradually, and mercury which emits from the combustion and gasification is toxic and dangerous to human beings and is regarded as a global pollutant because of its bioaccumulation [1], [2], [3]. Hg0(g) is more difficult to remove because of its thermodynamically stability at high temperature, and it’s easy to enter the global atmosphere cycle in vapor phase due to its high volatility and low solubility in water [1], [4]. The emission standard of air pollutants for thermal power plants in China [5], revised heavy metal protocol of United Nations Economic Commission for Europe (UNECE) [6] and the Minamata Convention [2] have revealed that the mercury emission has become a major environmental concern, which is received more and more attention. Thereupon, understanding the characteristics and transportation of elemental mercury is essential for effectively controlling the emission of mercury.
HgCl2, which formed by the reaction of elemental mercury and chlorine species, is generally considered to be the dominant oxidized mercury in coal combustion flue gas [4]. Meanwhile, more elemental mercury is expected to remain in the synthesis gas from coal gasification than combustion [7], [8], [9], [10], [11] and the halogen compound from coarse coal gas in gasifiers does serious harmer to the equipment.
One of the most effective ways to remove Hg0 is by sorbent injection into the flue gas. Over the past decades, various sorbents including active carbon [12] and impregnated activated carbon, calcium-based sorbents [13], [14], supported noble metal catalyst [15], [16], metal oxide [17] and SCR catalysts [18] have been considered to remove the mercury. However, there are still many limitations in wide temperature range, high removal efficiency, inexpensive cost, high recycling use, etc. Exploring an efficient sorbent in capturing elemental mercury is a tough challenge, while recognizing the mechanism of mercury oxidation is more crucial to the mercury removal. The α-Fe2O3 and γ-Fe2O3 [19] samples were used to study mercury oxidation in our previous and present researches because of its huge advantages of being easily accessed, separated, cheaper and non-secondary pollution [19], [20]. We have proposed the mechanism of mercury removal in the presence of H2S [20], [21] on Fe2O3 sorbents, which possessed an excellent performance and could probably be arranged before or after the power plant devices, however, a detailed analysis in the presence of HCl on Fe2O3 sorbents, including homogeneous and heterogeneous processes occurring in syngas, is currently still unrevealed.
Intensive experimental and theoretical studies have been carried out to gain an increased understanding of the mechanisms involved Hg0 removal in the presence of HCl. Hall and Schager proposed that the Hg0 reacted with HCl, which probably transformed into an oxidized form, HgCl2(g) [22], [23]. Sliger et al. investigated that the 50% equilibrium conversion to HgCl2 took place at approximately 675 °C in the presence of 500 ppm HCl [24]. Furthermore, Galbreath et al. interpreted that the mercury speciation was cycle in coal conversion systems and oxidized via 7 reactions, including Hg + 2HCl → HgCl2 + H2 reaction [4], [23], [25]. Contrary to the above opinions, Lu et al. held that Cl2 had a significant impact on mercury transformation, while HCl appeared little significance in mercury removal [26], [27]. Additionally, it was suggested that the direct elementary oxidation pathway of Hg by HCl would not occur due to the high energy barrier of the Hg + HCl → HgCl + H reaction [14], [28], [29], [30], [31]. On the theoretical side, Jenifer Wilcox predicted rate constants for the oxidation of mercury by hydrogen chloride using transition state theory [28], [32], and Guo et al. reported the adsorption mechanisms of different mercury species including Hg0, HgCl and HgCl2 on both α and γ phase Fe2O3 (0 0 1) surface based on density functional theory [33].
Bromine, because of the similarity of chemical character with chlorine, was considered to have a stronger performance on the Hg0 oxidation and adsorption in activated carbons and brominated carbons [34], [35], [36]. Thus, it’s essential to clearly discover the difference between bromine and chlorine on Fe2O3 in Hg0 conversion.
In this paper, based on the homogeneous reaction between Hg0 and HCl as well as heterogeneous reaction among Hg0, HCl and Fe2O3, a clear and detailed bench-scale experimental scheme was designed to clarify the mechanism of mercury oxidation on Fe2O3 surface with HCl. The potential oxidation pathway of Hg0 involving HCl, typically in a wide temperature range (80–780 °C) which occupied an important role, was discussed. We also carried on a series of contrastive experiments with SiO2 and analyzed the spent samples to elucidate mercury speciation after capturing Hg0. Additionally, the influence of HBr on mercury removal was also explored.
Section snippets
Sorbents characterization
The pure nano ferric oxides, including α-Fe2O3 (average diameter <30 nm, purity >99.5%) and γ-Fe2O3 (average diameter <20 nm, purity >99.5%) as well as Iron(III) chlorideanhydrous (⩾99.5%) were purchased from Aladdin Chemistry Co. Ltd. and used in our experiments. Silicon dioxide (SiO2, analytical reagent) was used as sorbent for a comparison [20]. The physical characters of these sorbents are shown in Table 1.
Apparatus and procedure
As shown in Fig. 1, the whole system is composed with four main parts: the feed gas
Hg0 removal experiments in the presence of HCl at different temperatures
A series of Hg0 removal experiments on α-Fe2O3, γ-Fe2O3 and SiO2 with HCl were performed to investigate the Hg0 oxidation at 80–680 °C respectively. According to Fig. 2, at 80 °C, in the absence and presence of HCl, the Hg0 removal efficiency of pure SiO2 is merely about 9%, displaying that SiO2 does not have the capacity to capture Hg0 even if the BET surface area is two times as large as Fe2O3. It can be concluded that in the oxidation of Hg0, physical characters are not the main factors, which
Conclusion
Comparative Hg0 removal experiments were performed to explore the Hg0 oxidation pathway on α-Fe2O3, γ-Fe2O3 and SiO2 in the presence of HCl at 80–780 °C respectively, meanwhile, the Hg0 removal experiments on HCl pre-treatment samples were performed as well. The mercury removal mechanism in the presence of HCl could probably be divided into three stages based on the increasing temperature. At low temperature, HCl overcomes low energy barrier to preferentially dissociate and adsorb on the Fe2O3
Acknowledgements
Financial support was sponsored by the National Key Basic Research and Development Program, The National Natural Science Foundation of China (NSFC) (Grant Nos. 51176058) and the partial funding from the Ministry of Science and Technology, China (2013CB228504).
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