The CaBr₂ addition experiments were conducted in one 300MW boiler of #1 unit in the power plant, whose annul power generation is 6~8 billion kW∙h. This 4×300MW unit plant adopts first medium reheating natural circulation drum boiler under subcritical pressure, equipped with the pollution control facilities and installations, such as the electrostatic precipitator, SCR denitrification and desulfurization devices, etc.

Sampling points were set at SCR entrance and export as shown in Fig. (1). CaBr₂ was fed with a screw feeder to vibration coal feeder. The flue gas was extracted from sampling points for thirty minutes for mercury collecting. At SCR entrance and export sampling points, the Ontario Hydro Method (OHM) [24Standard, A. Standard test method for elemental, oxidized, particle-bound, and total mercury in flue gas generated from coal-fired stationary sources (ontario-hydro). Designation, 2008. D6784-02] was applied. The ash particles were permeated via a heated quartz filter membrane, and then passed through three adsorbing bottles in turn, which one is 1 mol/L KCl solution, one with 10% H₂O₂-5% HNO₃ solution and another one is 4% KMnO₄-10% H₂SO₄ solution. These solutions were used to adsorption for Hg²⁺ and Hg ⁰. And the absorption liquid was taken back to the laboratory immediately for digestion and finally was collected in Teflon bottles. The RA-915 type mercury analyzer of LUMEX company was used for measuring mercury content in all the samples.

Fig. (1) CaBr2 addition experiments schematic.

Serial numbers C1 to C6 represent condition one to condition six, respectively. In detail, there is no CaBr₂ addition in C1. From C2 to C6, the amount of CaBr₂ increases gradually and the specific dosage is shown in Table 1. Proximate analysis and mercury content of coal sample are shown in Table 2.

Table 1
CaBr₂ addition amount.

Table 2
Proximate analysis and mercury content of coal.

Single layer graphene model was used to simulate unburned carbon surface, which has been proven that it provides satisfactory results for relevant reaction mechanisms of carbon based surface [25Liu, J.; Cheney, M.A.; Wu, F.; Li, M. Effects of chemical functional groups on elemental mercury adsorption on carbonaceous surfaces. J. Hazard. Mater., 2011, 186(1), 108-113.
[http://dx.doi.org/10.1016/j.jhazmat.2010.10.089] [PMID: 21144653] , 26Chen, N.; Yang, R.T. Ab initio molecular orbital calculation on graphite: Selection of molecular system and model chemistry. Carbon, 1998, 36(7), 1061-1070.
[http://dx.doi.org/10.1016/S0008-6223(98)00078-5] ]. As shown in Fig. (2), seven-ring benzene cluster model of Zigzag was applied to simulate unburned carbon in flue gas in this paper, and the active sites of this model were not saturated by hydrogen. Density functional theory B3LYP method [27Pople, J.A.; Scott, A.P.; Wong, M.W.; Radom, L. Scaling factors for obtaining fundamental vibrational frequencies and zero point energies from HF/6–31G* and MP2/6–31G* harmonic frequencies. Isr. J. Chem., 1993, 33(3), 345-350.
[http://dx.doi.org/10.1002/ijch.199300041] ] was used to the optimization of configuration and calculation of energy. In detail, pseudopotential basis set SDD was used to mercury atom, and the Pople 6-31G(d) basis set was applied for non-metal elements (C, H and Br) [25Liu, J.; Cheney, M.A.; Wu, F.; Li, M. Effects of chemical functional groups on elemental mercury adsorption on carbonaceous surfaces. J. Hazard. Mater., 2011, 186(1), 108-113.
[http://dx.doi.org/10.1016/j.jhazmat.2010.10.089] [PMID: 21144653] ]. In this paper, all of the calculation was carried out by Gaussian 09 program [28Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A. Gaussian 09W, revision A. 02., 2009, ]. The adsorption energy was calculated by equation (1):

(1)

Fig. (2) Seven-ring benzene cluster model. Yellow and blue ball represent carbon and hydrogen atom, respectively.

Where Emodel+adsorbate is the total energy of adsorption complex of benzene cluster model and adsorbate, Emodel and Eadsorbate are the energy of benzene cluster model and adsorbate, respectively. Negative value of Eads represents the adsorption progress is exothermic, and higher negative values of Eads reflects the stronger adsorption capacity of benzene cluster model onto adsobate [29Gao, Z.; Ding, Y. DFT study of CO₂ and H₂O co-adsorption on carbon models of coal surface. J. Mol. Model., 2017, 23(6), 187.
[http://dx.doi.org/10.1007/s00894-017-3356-2] [PMID: 28500519] , 30Xu, H.; Chu, W.; Huang, X.; Sun, W.; Jiang, C.; Liu, Z. CO₂ adsorption-assisted CH₄ desorption on carbon models of coal surface: A DFT study. Appl. Surf. Sci., 2016, 375, 196-206.
[http://dx.doi.org/10.1016/j.apsusc.2016.01.236] ].

In addition, Atoms in Molecules (AIM) method [31Tavakol, H.; Mollaei-Renani, A. DFT, AIM, and NBO study of the interaction of simple and sulfur-doped graphenes with molecular halogens, CH₃OH, CH₃SH, H₂O, and H₂S. Struct. Chem., 2014, 25(6), 1659-1667.
[http://dx.doi.org/10.1007/s11224-014-0446-y] , 32Bader, R.F. A quantum theory of molecular structure and its applications. Chem. Rev., 1991, 91(5), 893-928.
[http://dx.doi.org/10.1021/cr00005a013] ] was used to analyze the type and strength of bond between unburned carbon model and adsorbates. AIM was evaluated by wave function analysis program of Multiwfn 3.2.1 [33Lu, T.; Chen, F. Multiwfn: A multifunctional wavefunction analyzer. J. Comput. Chem., 2012, 33(5), 580-592.
[http://dx.doi.org/10.1002/jcc.22885] [PMID: 22162017] ], and isosurface maps of adsorption complexes were plotted by VMD program in this paper [34Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual molecular dynamics. J. Mol. Graph., 1996, 14(1), 33-38, 27-28.
[http://dx.doi.org/10.1016/0263-7855(96)00018-5] [PMID: 8744570] ].

The concentrations of gaseous mercury at the SCR entrance and export in different experimental conditions are shown in Table 3. It is clearly observed that before entering SCR, mercury is mainly presented as Hg ⁰ in the flue gas, which has been confirmed by S. Niksa [35Fujiwara, N.; Fujita, Y.; Tomura, K.; Moritomi, H.; Tuji, T.; Takasu, S.; Niksa, S. Mercury transformations in the exhausts from lab-scale coal flames. Fuel, 2002, 81(16), 2045-2052.
[http://dx.doi.org/10.1016/S0016-2361(02)00156-4] ].

Table 3
Gaseous mercury concentration at SCR in different conditions.

In Table 2 it can be found that the mercury mass fraction is 46.8 ng/g, and mercury exists in three forms: elemental mercury, divalent mercury and particle mercury. During the combustion of coal, variant mercury species decompose and release gaseous mercury into flue gas. Lopez-Anton [36Lopez-Anton, M.A.; Yuan, Y.; Perry, R.; Maroto-Valer, M.M. Analysis of mercury species present during coal combustion by thermal desorption. Fuel, 2010, 89(3), 629-634.
[http://dx.doi.org/10.1016/j.fuel.2009.08.034] , 37Rumayor, M.; Diaz-Somoano, M.; Lopez-Anton, M.A.; Martinez-Tarazona, M.R. Mercury compounds characterization by thermal desorption. Talanta, 2013, 114(3), 318-322.
[http://dx.doi.org/10.1016/j.talanta.2013.05.059] [PMID: 23953477] ] demonstrated that HgCl₂, HgS, HgO and HgSO₄ are the main mercury species in coal-fired ash and gas. In detail, the thermal decomposition temperature of HgCl₂ is 70°C. There are two different forms of crystal structure, black cube HgS and red six party HgS, which decompose at the temperature of 170°C and 240°C, respectively, the component HgO and HgSO₄ decompose at the temperature of 200°C and 500°C. In addition, mercury species of Hg₂SO₄ and Hg₂Cl₂ also exist in ash and flue gas, but Hg₂SO₄ and Hg₂Cl₂ are unstable.

In condition one in Table 3, in which the gaseous Hg ⁰ and Hg²⁺ concentrations are 8.26 μg/m³ and 0.83 μg/m³, respectively, it means mercury mainly exists in Hg ⁰ form. In detail, the mercury component Hg₂SO₄ easily decomposes to Hg and HgSO₄, Hg₂Cl₂ easily decomposes to Hg and HgCl₂, as shown in equations (2) and (3):

(2)

(3)

Similarly, the HgCl₂, HgS, HgO and HgSO₄ in fly ash and flue gas decompose into element mercury and other substance at high temperature. Along with the temperature of flue gas decreases, the decomposition of almost mercury species is halted, and mercury oxidation dominates gradually. Due to the halogen and sulfur consist in flue gas, the homogeneous oxidation reaction of element mercury to oxidized mercury are occurred, such as shown in equations (4) to (7) [38Sliger, R.N.; Kramlich, J.C.; Marinov, N.M. Towards the development of a chemical kinetic model for the homogeneous oxidation of mercury by chlorine species. Fuel Process. Technol., 2000, 65-66(99), 423-438.
[http://dx.doi.org/10.1016/S0378-3820(99)00108-3] ].

(4)

(5)

(6)

(7)

Where the (g) represents gaseous phase. Hg ⁰ can be oxidized to HgCl by gaseous HCl and Cl₂, and then some HgCl be oxidized completely to HgCl₂. In addition, heterogeneous oxidation reactions between chlorine and element mercury on ash particle are shown in equations (8) to (11) [39Presto, A.A.; Granite, E.J. Survey of catalysts for oxidation of mercury in flue gas. Environ. Sci. Technol., 2006, 40(18), 5601-5609.
[http://dx.doi.org/10.1021/es060504i] [PMID: 17007115] ].

(8)

(9)

(10)

(11)

Where the (ad) represents substance adsorbed on ash or unburned carbon particle.

In addition, Hg ⁰ also can be oxidized by sulfur. Rubel demonstrated that a good correlation exists between sulfur and mercury capture. He proposed that the unburned carbon released from high sulfur coals have shown to have higher mercury adsorption capacity than that from low sulfur coals. H₂S and SO₂ play an important role in oxidizing mercury in flue gas, the detailed oxidation reaction as shown in equations (12) to (14) [40Morimoto, T.; Wu, S.; Uddin, M.A.; Sasaoka, E. Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H₂S. Fuel, 2005, 84(14-15), 1968-1974.
[http://dx.doi.org/10.1016/j.fuel.2005.04.007] ].

(12)

(13)

(14)

H₂S can be oxidized to S ⁰ by oxygen or SO₂ on ash or unburned carbon surface, and then adsorbed sulfur can capture Hg ⁰, and finally, HgS may adsorb on ash surface to form particle mercury or releases into flue gas to form gaseous oxidized mercury. To sum up, the massive element mercury and a little oxidized mercury released during the coal combustion, so the Hg ⁰ concentration is larger than Hg²⁺ concentration at the SCR entrance. Along with the temperature of flue gas decreased, the oxidation of mercury is dominant than decomposition, so the Hg²⁺ concentration is larger than Hg ⁰ concentration at the SCR export.

In each condition, there is an increase of Hg²⁺ and conversely, a decrease of Hg ⁰ in flue gas at SCR export compared with entrance, which means the oxidation of Hg ⁰ to Hg²⁺ taken place in the SCR. Almost all of the mercury leaving furnace is presented in element form in flue gas with high temperature, whose oxidation from the elemental form to divalent form takes place as the temperature decreases and gets promoted under the effect of halogen in the coal and SCR catalyst. Divalent mercury is more strongly adsorbed on the surface of both ash particles and SCR catalyst than element mercury [41Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and reduction of mercury by SCR DeNOx catalysts under flue gas conditions in coal fired power plants. Appl. Catal. B, 2014, 144, 486-497.
[http://dx.doi.org/10.1016/j.apcatb.2013.07.040] ]. As clearly shown in C1 in Table (3), the increase of gaseous divalent mercury amount in the flue gas is less than that of decrease of gaseous element mercury and the total gaseous mercury concentration through SCR, which the reasons may be that the gaseous Hg²⁺ adsorbed on unburned carbon and SCR catalyst surface. With the additive CaBr₂ has been added in C2, the Hg ⁰ and Hg²⁺ concentration decreased in SCR entrance which compared with C1. The Hg ⁰ concentration decreased which dues to the oxidation of Hg ⁰ to Hg²⁺ by CaBr₂, but the Hg²⁺ concentration did not increase but decrease, which the reasons may be that the CaBr₂ promotes the adsorption of Hg²⁺ on unburned carbon surface. In addition, the Hg²⁺ concentration in SCR export is higher than that in SCR entrance in C2, which means that the SCR catalyst can promote the oxidation of Hg ⁰ to Hg²⁺. From C2 to C5, as CaBr₂ addition amount increases, the proportion of Hg²⁺ increases as well both at SCR entrance and export, at the same time, the Hg ⁰ proportion decreases at SCR entrance, experimental results indicate that CaBr₂ can facilitate the oxidation of Hg ⁰ to Hg²⁺ in general. Especially, C5 reached the extreme point among these conditions and although more CaBr₂ was added in C6, the proportion of Hg²⁺ in C5 is larger than that of C6 both at SCR entrance and export, the reason of which is mainly that the CaBr₂ addition amount has reached a saturation value beyond and thus the oxidation is no longer enhanced [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] , 42Niksa, S.; Fujiwara, N. Predicting extents of mercury oxidation in coal-derived flue gases. J. Air Waste Manag. Assoc., 2005, 55(7), 930-939.
[http://dx.doi.org/10.1080/10473289.2005.10464688] [PMID: 16111132] ].

Fig. (3) shows the variation of Hg⁰ and Hg²⁺ concentration decrease rate at SCR entrance alone with the increase of CaBr₂ addition amount. As shown in Fig. (3), with the additive CaBr₂ has been added, the gaseous Hg⁰ concentration decreased. In C2, CaBr₂ with addition level of 0.5 kg/h was added in flue gas, the reduction rate of gaseous Hg ⁰ concentration at SCR entrance is 70.7% comparing with that of C1. Similarly, the reduction rates of gaseous Hg ⁰ of C3, C4, C5 and C6 are 85.2%, 89.0%, 90.6% and 87.0%, respectively. Analyzing these reduction rates of different conditions, it can be found that the gaseous mercury concentration decrease rapidly along with the increase of CaBr₂ addition level in general, which indicates that additive CaBr₂ can oxidize Hg ⁰ to Hg²⁺, these findings agree with previous studies [43Ancora, M.P.; Zhang, L.; Wang, S.; Schreifels, J.; Hao, J. Economic analysis of atmospheric mercury emission control for coal-fired power plants in China. J. Environ. Sci. (China), 2015, 33(7), 125-134.
[http://dx.doi.org/10.1016/j.jes.2015.02.003] [PMID: 26141885] , 44Ma, J.J.; Yao, H.; Luo, G.Q.; Fang, X.; Liu, W.; Xu, M.H. Effect of sodium bromide on nitric oxide reduction and mercury oxidation during coal combustion. J. Eng Thermophys-Rus, 2010, 31(8), 1407-1410.]. Interestingly, along with the increase of CaBr₂ addition amount, the gaseous Hg²⁺ concentration is not increase but decrease at the SCR entrance, the reasons may be that the additive CaBr₂ contributes to more fly ash in flue gas, and more oxidized Hg²⁺ adsorb on ash particles, which makes the decrease of Hg²⁺ concentration at SCR entrance.

Fig. (3) Variation of Hg 0 and Hg2+ concentration decrease rate at SCR entrance versus CaBr2 addition amount.

Fig. (4) shows the variation of Hg²⁺ concentration increase rate at SCR export alone with the increase of CaBr₂ addition amount. As shown in Fig. (4), whether or not to add additive CaBr₂, the gaseous Hg²⁺ concentration all increased. In C1 which there is no CaBr₂ addition, the increase ratio of Hg²⁺ concentration between SCR export and entrance has reached to 74%, probably because that in C1, with quite little halogen in coal and the subsequent in flue gas, almost all the mercury oxidation is conducted in SCR while halogen contributes to mercury oxidation. Along with the increase of CaBr₂ addition amount, the increase rate of Hg²⁺ concentration also increases, and in C5 which the CaBr₂ addition amount is 15 kg/h, the increase rate reaches the maximum value, which indicates that the oxidation reaction is saturated. In C6, with the CaBr₂ addition amount continues to increase to 50 kg/h, the Hg²⁺ concentration increase rate has decreased to 82.4%.

Fig. (4) Variation of Hg2+ concentration increase rate at SCR versus CaBr2 addition amount.

The subsequent products such as CaO and Ca(OH)₂ may be produced because of CaBr₂ addition as shown in equation (15) to (16), and these subsequent products are effective of Hg²⁺ adsorbents. Flue gas leaving the furnace with high temperature, bromine is presented mainly as HBr [41Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and reduction of mercury by SCR DeNOx catalysts under flue gas conditions in coal fired power plants. Appl. Catal. B, 2014, 144, 486-497.
[http://dx.doi.org/10.1016/j.apcatb.2013.07.040] ], and as the gas temperature decreases, Br atoms are in a concentration [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ] and in the subsequent cooling process Br₂ is formed. Therefore in the flue gas leaving the furnace before entering SCR, HBr is the most speciation of bromine. In addition, the HBr released from the reaction of CaBr₂ and H₂O, the reaction as shown in equations (15) and (16).

(15)

(16)

Both homogeneous and heterogeneous chemistry exist in Hg ⁰(g) oxidation. In the flue gas, according to what Stephen [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ] has found, Hg ⁰ is first partially oxidized by Br atom into HgBr and then oxidized into HgBr₂ by Br₂. Since the concentration of Br₂ is smaller than that of Br atoms in high temperature flue gas, homogeneous oxidation results in most mercury being presented as HgBr(g) with a small amount of HgBr₂. Gao [23Gao, Z.; Sun, L.; Lv, S.; Yang, P. Research on the effect of additives on mercury speciation in coal fired derived flue gas. Environ. Prog. Sustain, 2016, 35(6), 1566-1574.
[http://dx.doi.org/10.1002/ep.12379] ] proposed that in the flue gas leaving the furnace before entering SCR, most bromine is presented in HBr and a small part in Br atoms with tiny amount of Br₂. Stephen [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ] presented that Hg homogeneous chemistry with Br species is much faster than with Cl species because the Br atom concentrations at the furnace exit are three to four orders of magnitude greater, the dominant channels with Br are analogous to those for Cl, whereby a Br atom partially oxidizes Hg ⁰ into HgBr. In addition, mercury also oxidizes heterogeneously on unburned carbon with Br species. Raik [41Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and reduction of mercury by SCR DeNOx catalysts under flue gas conditions in coal fired power plants. Appl. Catal. B, 2014, 144, 486-497.
[http://dx.doi.org/10.1016/j.apcatb.2013.07.040] ] suggested that in the subsequent cooling process, the fuel bromine or added bromine is eventually transformed into HBr. Based on the DFT calculation, Padak’s [47Padak, B.; Wilcox, J. Understanding mercury binding on activated carbon. Carbon, 2009, 47(12), 2855-2864.
[http://dx.doi.org/10.1016/j.carbon.2009.06.029] ] results for Hg adsorption in the presence of chlorine, it was found that HgBr species are more stable on the unburned carbon surface than HgBr₂ species. Fujiwara [35Fujiwara, N.; Fujita, Y.; Tomura, K.; Moritomi, H.; Tuji, T.; Takasu, S.; Niksa, S. Mercury transformations in the exhausts from lab-scale coal flames. Fuel, 2002, 81(16), 2045-2052.
[http://dx.doi.org/10.1016/S0016-2361(02)00156-4] ] proposed that heterogeneous Hg ⁰ oxidation on unburned carbon is usually the essential inherent mechanism to oxidize Hg ⁰ in coal-derived flue gas. Bench-scale experiments [45Cauch, B.; Silcox, G.D.; Lighty, J.S.; Wendt, J.O.; Fry, A.; Senior, C.L. Confounding effects of aqueous-phase impinger chemistry on apparent oxidation of mercury in flue gases. Environ. Sci. Technol., 2008, 42(7), 2594-2599.
[http://dx.doi.org/10.1021/es702490y] [PMID: 18505002] ] and quantum chemistry theoretical studies [46Padak, B. Mercury reaction chemistry in combustion flue gases from experiments and theory., 2011, D] have indicated that heterogeneous mercury oxidation is at least 90% proportion of the overall oxidation in coal-fired flue gas. Heterogeneous reaction is mainly determined by adsorption on Unburned Carbon (UBC) surfaces, and the oxidation mechanism is as follows:

(17)

(18)

(19)

Where (g) and (ads) represents gaseous and adsorption state, respectively [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ].

Since most bromine is in HBr form in the high temperature gas, which is easily adsorbed on unburned carbon surface via equation (17). And then, gaseous element mercury is adsorbed on carbon surface through the interaction with Br(ads). In addition, HgBr(ads) may be oxidized further into HgBr₂(ads) by gaseous HBr. Padak [47Padak, B.; Wilcox, J. Understanding mercury binding on activated carbon. Carbon, 2009, 47(12), 2855-2864.
[http://dx.doi.org/10.1016/j.carbon.2009.06.029] ] confirmed that HgBr is more strongly and stably adsorbed on the unburned carbon surface than HgBr₂ by quantum chemistry calculation. Gale et al. [48Gale, T.K.; Lani, B.W.; Offen, G.R. Mechanisms governing the fate of mercury in coal-fired power systems. Fuel Process. Technol., 2008, 89(2), 139-151.
[http://dx.doi.org/10.1016/j.fuproc.2007.08.004] ] found that the Ca enhances mercury capture by preventing Hg²⁺ desorption from carbon surface. Therefore, due to homogeneous and heterogeneous chemistry, in the high temperature gas before entering SCR, mercury presented mainly in HgBr form, most of which is adsorbed on UBC surface.

According to the Deacon reaction: 4HCl+O₂→2H₂O+2Cl₂, when the flue gas enters SCR, Cl₂ is produced under the effect of the vanadium-based catalyst. There has been no specified reaction in bromine situation and it is predicted here that the concentration of Br₂ begins to increase in SCR as flue gas temperature decreases [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ]. Both the homogeneous and heterogeneous mercury oxidations exist in SCR. According to Hein [41Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and reduction of mercury by SCR DeNOx catalysts under flue gas conditions in coal fired power plants. Appl. Catal. B, 2014, 144, 486-497.
[http://dx.doi.org/10.1016/j.apcatb.2013.07.040] ], homogeneous mercury oxidation in SCR involves Hg ⁰(g) reacting with HBr to produce HgBr₂(g). Since mercury in the flue gas entering SCR is mainly in HgBr(ads) form, no big Hg ⁰(g) concentration variation is supposed to be detected. Mercury oxidation in SCR is mainly in heterogeneous chemistry [39Presto, A.A.; Granite, E.J. Survey of catalysts for oxidation of mercury in flue gas. Environ. Sci. Technol., 2006, 40(18), 5601-5609.
[http://dx.doi.org/10.1021/es060504i] [PMID: 17007115] ].

The produced Br₂ in SCR is a strong oxidant and can easily oxidize adsorbed HgBr into HgBr₂ which then gets desorbed. Also the remained HBr may react with HgBr(ads) to form HgBr₂(g) via equation (20) [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ] and therefore Hg²⁺(g) concentration in SCR increases.

(20)

On the other hand, two reaction mechanisms have been previously proposed for mercury heterogeneous oxidation on vanadium-based SCR catalyst surface. Senior [49Senior, C.L. Oxidation of mercury across selective catalytic reduction catalysts in coal-fired power plants. J. Air Waste Manag. Assoc., 2006, 56(1), 23-31.
[http://dx.doi.org/10.1080/10473289.2006.10464437] [PMID: 16499143] ] applied Eley-Rideal mechanism which indicates that adsorbed mercury on catalyst surface reacts with hydrogen halide in the flue gas, while Hein et al. [41Stolle, R.; Koeser, H.; Gutberlet, H. Oxidation and reduction of mercury by SCR DeNOx catalysts under flue gas conditions in coal fired power plants. Appl. Catal. B, 2014, 144, 486-497.
[http://dx.doi.org/10.1016/j.apcatb.2013.07.040] ] have mentioned Langmuir-Hinshelwood mechanism that Hg ⁰ and hydrogen halide both are adsorbed on catalyst surface firstly, and then start to react. When hydrogen halide concentration is much larger than element mercury, mercury oxidation on the SCR catalyst surface is mainly dominated by Langmuir-Hinshelwood mechanism because of the higher affinity of hydrogen halide with the active sites in SCR catalyst than mercury [50He, S.; Zhou, J.; Zhu, Y.; Luo, Z.; Ni, M.; Cen, K. Mercury oxidation over a vanadia-based selective catalytic reduction catalyst. Energy Fuels, 2008, 23(1), 253-259.
[http://dx.doi.org/10.1021/ef800730f] ]. Therefore compared to HBr in the flue gas, there are not much Hg ⁰ and Hg²⁺ adsorbed on SCR catalyst surface. Abad-Valle et al. [51Abad-Valle, P.; Lopez-Anton, M.A.; Diaz-Somoano, M.; Martinez-Tarazona, M.R. The role of unburned carbon concentrates from fly ashes in the oxidation and retention of mercury. Chem. Eng. J., 2011, 174(1), 86-92.
[http://dx.doi.org/10.1016/j.cej.2011.08.053] ] found that hydrogen halide binds itself to carbon particle surface prior to Hg, indicating that not much Hg²⁺(g) is adsorbed on unburned carbon surface. Through analyzing heterogeneous and homogeneous mercury oxidation and adsorption on the unburned carbon and SCR catalyst surface, it is shown that CaBr₂ addition contributes to an increase of element mercury transform into divalent mercury.

The dominant forms of Hg in coal-fired flue gas are Hg ⁰ and Hg²⁺ [52Niksa, S.; Fujiwara, N.; Fujita, Y.; Tomura, K.; Moritomi, H.; Tuji, T.; Takasu, S. A mechanism for mercury oxidation in coat-derived exhausts. J. Air Waste Manag. Assoc., 2002, 52(8), 894-901.
[http://dx.doi.org/10.1080/10473289.2002.10470829] [PMID: 12184687] ], and the fraction of Hg²⁺ in flue gas depends on the coal quality, boiler loads and so on. Hg ⁰ can be oxidized to HgCl and HgCl₂ by Cl₂ and HCl, respectively. HCl is the major Cl species under the condition of SCR operation [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ]. HCl is more stably adsorbed on SCR catalyst surface than Hg, which means that the HCl inhibits the mercury adsorption on catalyst surface [53Wilcox, J.; Rupp, E.; Ying, S.C.; Lim, D.; Negreira, A.S.; Kirchofer, A.; Feng, F.; Lee, K. Mercury adsorption and oxidation in coal combustion and gasification processes. Int. J. Coal Geol., 2012, 90, 4-20.
[http://dx.doi.org/10.1016/j.coal.2011.12.003] ]. Generally, element mercury on SCR catalyst oxidizes faster than that on unburned carbon under the condition of chlorine level is moderate or higher.

Since the unburned carbon can enhance mercury capture, different coal ranks with dissimilar unburned carbon content in flue gas have different results in mercury capture. The combustion results of anthracites coal and bituminous coal show that the unburned carbon in fly ash is anisotropic, unresolved and dense. On the contrary, the structure of unburned carbon in fly ash is isotropic and porous, which formed from subbituminous or lignite. The different characteristics of unburned carbon greatly affect the mercury capture, the UBC with anisotropic fused structure is more favorable for adsorbing mercury. Gale [48Gale, T.K.; Lani, B.W.; Offen, G.R. Mechanisms governing the fate of mercury in coal-fired power systems. Fuel Process. Technol., 2008, 89(2), 139-151.
[http://dx.doi.org/10.1016/j.fuproc.2007.08.004] ] found that much more mercury oxidation can be realized by blending coal, even though there was less chlorine. The reasons may be that the added bituminous coal yields more unburned carbon in the ash, which enhances mercury oxidation. In addition, low-rank coal with high concentration of calcium can promote mercury oxidation. Gale [48Gale, T.K.; Lani, B.W.; Offen, G.R. Mechanisms governing the fate of mercury in coal-fired power systems. Fuel Process. Technol., 2008, 89(2), 139-151.
[http://dx.doi.org/10.1016/j.fuproc.2007.08.004] ] proposed that calcium can enhance the HgCl adsorption on the unburned carbon surface and element mercury adsorption onto chlorinated-carbon sites.

Temperature affects mercury capture via the unburned carbon formation in the boiler with high temperature to the quenching environment of flue gas. In addition, homogeneous Hg oxidation by Br begins as the flue gas cools below 600°C [4Niksa, S.; Naik, C.V.; Berry, M.S.; Monroe, L. Interpreting enhanced Hg oxidation with Br addition at Plant Miller. Fuel Process. Technol., 2009, 90(11), 1372-1377.
[http://dx.doi.org/10.1016/j.fuproc.2009.05.022] ]. Subtle changes in boiler operation condition can potentially results in significant changes for unburned carbon properties [54Hower, J.C.; Senior, C.L.; Suuberg, E.M.; Hurt, R.H.; Wilcox, J.L.; Olson, E.S. Mercury capture by native fly ash carbons in coal-fired power plants. Pror. Energy Combust. Sci., 2010, 36(4), 510-529.
[http://dx.doi.org/10.1016/j.pecs.2009.12.003] [PMID: 24223466] ], and these changes may impact mercury capture in fly ash.

It is well known that chlorine can cause serious corrosion to boilers, heater exchangers and pipes surface as a result of complex chemical interactions between species like HCl, Cl₂, H₂O, O₂ and metal [55Huijbregts, W.; Leferink, R. Latest advances in the understanding of acid dewpoint corrosion: Corrosion and stress corrosion cracking in combustion gas condensates. Anti-Corros. Methods Mater., 2004, 51(3), 173-188.
[http://dx.doi.org/10.1108/00035590410533129] , 56Persson, K.; Broström, M.; Carlsson, J.; Nordin, A.; Backman, R. High temperature corrosion in a 65 MW waste to energy plant. Fuel Process. Technol., 2007, 88(11), 1178-1182.
[http://dx.doi.org/10.1016/j.fuproc.2007.06.031] ]. Similar to chlorine-associated corrosion, bromine may cause corrosion depending on the flue gas conditions. Element bromine is injected via the addition of CaBr₂, and HBr is the primary Br species in the gas phase and little Br₂ may be present at higher temperature. Both the HBr and Br₂ can react with metal to form metal bromides. The oxidation mechanisms can be revealed via the equation (21) to (26) [57Zhuang, Y.; Chen, C.; Timpe, R.; Pavlish, J. Investigations on bromine corrosion associated with mercury control technologies in coal flue gas. Fuel, 2009, 88(9), 1692-1697.
[http://dx.doi.org/10.1016/j.fuel.2009.01.013] ].

(21)

(22)

(23)

(24)

(25)

(26)

Stated thus, the CaBr₂ addition has caused corrosion to the metal of boiler, which makes the negative effect on the stable operation of heat exchange pipes. The present of HBr and Br₂ can thin the pipe walls and reduce the structural strength of metal pipes. Although the additive CaBr₂ can promote the mercury oxidation, the corrosion characteristic of HBr which formed from CaBr₂ also should be considered.

In order to gain further insight into the adsorption mechanisms of mercury and halogen on the unburned carbon surface, the reaction path of Hg⁰ on Zigzag carbon model which adsorbed single bromine atom has been studied, in addition, the adsorption complex of Zigzag···HgBr₂ has been optimized.

Fig. (5) The reaction path of Hg0 and C25H9-Br model. R, IM, TS and P represents reaction, intermediate, transition state and product.

According to the Equation (18), the generation path of adsorbed HgBr via gas Hg⁰ and Zigzag-Br model has been shown in Fig. (5). Gas element mercury chemically adsorbed on active sites which on the side of Br atom, and corresponding adsorption energy is 74.89 kJ/mol. In transition state structure of TS, the bond between active site and Br atom broken, and the Br atom closed to the adsorbed Hg atom. The structure of TS is not stable and corresponding energy is higher 114.25 kJ/mol than that of reactants. TS quickly converts into final product along with the Br atom far away from active sites. The total energy of final product is lower 66.08 kJ/mol than that of the reactants. The theoretic DFT results have verified the validity of Equation (18) and indicated that the adsorption of gas Hg ⁰ on carbon model which adsorbed Br is chemisorption.

Fig. (6) The adsorption configuration of HgBr2 on C25H9 model.

The optimization complex of HgBr₂ adsorbed on Zigzag carbon model surface is shown in Fig. (6). One of the Hg-Br bond broken, Br atom and Hg atom adsorbed on active sites, respectively. The adsorption energy of HgBr₂ on Zigzag carbon model surface is 325.46 kJ/mol, indicates that the adsorption is chemisorption. As shown in Fig. (4), the bond length of Br-C and Hg-C is 0.190 nm and 0.213 nm, respectively, the bond strength of Br-C may stronger than that of Hg-C. This adsorption configuration may be formed via the oxidation reaction of Br atom and Zigzag -HgBr complex.

Atoms in Molecules (AIM) theory has been successfully and widely used to study the properties of shared-shell (covalent) and closed-shell bonding [58Popelier, P.; Bader, R. Effect of twisting a polypeptide on its geometry and electron distribution. J. Phys. Chem., 1994, 98(16), 4473-4481.
[http://dx.doi.org/10.1021/j100067a040] , 59Tang, T.; Hu, W.; Yan, D.; Cui, Y. A quantum chemical study on selected π-type hydrogen-bonded systems. J. Mol. Struct. THEOCHEM, 1990, 207(3-4), 319-326.
[http://dx.doi.org/10.1016/0166-1280(90)85033-J] ]. This theory depicts that the Hessian matrix of the charge density can be well applied to evaluate characterize of a bond. The bond critical point (BCP) is the laplacian of the charge density ²ρ(r) , which is the sum of three eigenvalues of Hessian matrix. According to the criteria proposed by Carroll and Bader [60Carroll, M.T.; Bader, R.F. An analysis of the hydrogen bond in BASE-HF complexes using the theory of atoms in molecules. Mol. Phys., 1988, 65(3), 695-722.
[http://dx.doi.org/10.1080/00268978800101351] , 61Carroll, M.T.; Chang, C.; Bader, R.F. Prediction of the structures of hydrogen-bonded complexes using the laplacian of the charge density. Mol. Phys., 1988, 63(3), 387-405.
[http://dx.doi.org/10.1080/00268978800100281] ] based on charge density topology, there are some regulations can be used to analysis the bond characterize. First, the presence of BCP between two atoms indicates that the existence of bond path between these two atoms. Second, the value of charge density at BCP is positively correlated with the binding energy for hydrogen bond and covalent bond. Third, the positive and negative values of laplacian of the charge density at the BCP are indicative of closed-shell and covalent interaction, respectively. In addition, Cremer suggested that potential energy density V(r), Lagranian kinetic energy G(r) and total energy density H(r) (H = V + G) can be used to distinguish the covalent and closed-shell interaction. In detail, |V(r)| > G(r) and H(r) < 0 is indicative of covalent interaction, conversely, |V(r)| < G(r) and H(r) > 0 is indicative of closed-shell bonding.

The BCPs between HgBr and carbon model have been shown in Fig. (7a), which indicates that the bond paths existed between these interaction atoms. The results of charge density topological analysis have been shown in Table 4. In the configuration of HgBr adsorbed on carbon model surface, the sign of ²ρ(r) at both BCPs of Hg-C bond is positive, which indicates that the bonding between Hg atom and active site is covalent interaction. In similarly, the positive sign of ²ρ(r) of Hg-Br bond in this configuration is indicative of covalent bond. According to the Cremer’s criterion, the BCP of Hg-C and Hg-Br bonds with characteristic of |V(r)| > G(r) and H(r) < 0 as shown in Table 4, which demonstrated that these bonding are covalent interaction. The value of charge density ρ at the BCP of Hg-C₂ bond is larger than that of Hg-Br bond, demonstrating that the bonding strength of Hg-C bond is stronger than that of Hg-Br bond. The same analysis has been conducted to the configuration of HgBr₂ adsorbed on carbon model as shown in Fig. (7b), it can be concluded that the bonding of Br and C atom belongs to covalent interaction, and the Br-C bond is stronger than Hg-C and Hg-Br bonds. Analyzing the values of G, V and H, it can be also found that these bonds are covalent interaction.

Fig. (7) Electron density topologies for (a) HgBr-C25H9 complex, (b) HgBr2-C25H9 complex. Bond critical points (BCP) are shown in orange, paths are shown in blue.

Table 4
Topological parameters of the BCP of bond in adsorption configurations.