Research on the catalytic oxidation of Hg0 by modified SCR catalysts

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 43, Issue 5, May 2015 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2015, 43(5), 628634

RESEARCH PAPER

Research on the catalytic oxidation of Hg0 by modified SCR catalysts ZHAO Li*, HE Qing-song, LI Lin, LU Qiang, DONG Chang-qing, YANG Yong-ping National Engineering Laboratory for Biomass Power Generation Equipment, North China Electric Power University, Beijing 102206, China

Abstract: A series of metal oxides were employed to modify the commercial SCR catalyst, and the CeO2 modified SCR catalyst was selected and then subjected to detailed catalytic oxidation of Hg0 by using the simulated flue gas. The results indicated that the catalytic activity of the catalyst was increased remarkably after the CeO2 modification, and the highest catalytic oxidation of Hg0 was obtained from the modified SCR catalyst with 9% CeO2 loading, being 40% higher than that of the non-modified SCR catalyst. The BET and XRD analysis indicated that the surface area of the 1%–9% CeO2 modified SCR catalyst did not show significant changes as compared with the non-modified SCR catalyst, and the CeO2 was well dispersed on the catalyst surface without any aggregation. The flue gas condition had great effects on the Hg0 conversion. The catalytic oxidation of Hg0 would be significantly increased by HCl, and also increased as the increasing of the temperature in a certain range. The highest catalytic oxidation efficiency reached 95.11% at the optimal space velocity, temperature and flue gas components. In addition, the CeO2 doping did not affect the denitration efficiency of the SCR catalyst. Keywords: modified SCR catalyst; Hg0; catalytic oxidation

Coal-fired power plants are considered as one of the most important pollution sources of mercury emissions in China. The average mercury content of coal in China is 0.2 mg/kg, which is significantly higher than the world’s average coal value (0.13 mg/kg)[1]. In July 2011, the Ministry of Environmental Protection of the People’s Republic of China revised Emission Standard of Air Pollutants for coal-fired power plants, in which mercury emission was limited to be lower than 0.03 mg/m3 from January 1, 2015[2]. Therefore, to meet the new demands of mercury regulations, effective control technologies are urgently required following the technologies of dust collection, desulfuration and denitrification in coal- fired power plants. During coal combustion, mercury can exist in different forms, including (Hg0) elemental, (Hg2+) oxidized and (Hgp) particulate-bound mercury. Elemental mercury, which occupies over 70% of the total amount, is most difficult to remove because of its water insolubility and chemical inertness[3]. Oxidized mercury is water-soluble and can be easily captured onto ash. Therefore, it is an effective approach by utilizing existing air pollution control devices to remove mercury as co-benefit[4–6]. The SCR reactors have been widely

used to reduce NOx emissions. The traditional V2O5-WO3/TiO2 SCR catalyst has a synergistic catalytic effect on oxidation of Hg0. Through SCR system, catalytic conversion of Hg0 to its oxidized form followed by the capture of the oxidized mercury in the wet flue-gas desulfurization (FGD) has been considered as an effective way to remove Hg0 from coal-fired power plants[7–11]. However, research showed that the oxidation of Hg0 by traditional V2O5-WO3/TiO2 SCR catalyst was low due to many factors[12–14]. Currently, many new catalysts have been prepared and reported for high Hg0 oxidation efficiency, such as MnOx-CeO2/TiO2[15,16], MnOx-CeO2/γ-Al2O3[17], SiO2-TiO2-V2O5[18,19], Cr2O3/TiO2[20], Fe2O3/TiO2[21,22] and CuO/TiO2[23]. But these catalysts are different from traditional SCR catalysts in components, and traditional V2O5-WO3/TiO2 catalyst has been already used for around twenty years because of its high catalytic activity, thermal stability and economic viability. Therefore, in this study, several modified SCR catalysts were prepared by adding a series of metal oxides as the second assistant on the basis of the traditional SCR catalyst. The optimal modified SCR catalyst was screened and evaluated for the mercury catalytic oxidation under different flue gas conditions.

Received: 08-Jan-2015; Revised: 10-Mar-2015. Foundation item: Supported by National Basic Research Program of China (973 Program, 2015CB251501) and the Fundamental Research Funds for the Central Universities (2014ZD17). *Corresponding author: ZHAO Li. E-mail: [email protected]. Copyright  2015, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

ZHAO Li et al / Journal of Fuel Chemistry and Technology, 2015, 43(5): 628634

Fig. 1 Schematic of the simulated device 1–4: gas cylinders; 5: valve; 6: mass flow controller;

1 1.1

catalytic oxidation

10: heat insulator; 11: reactor; 12: thermostat; 13: mercury analyzer;

experimental conditions: reaction temperatures 350°C, 5% O2 and N2 as

14: computer; 15: exhaust absorbing device

balance gas, GHSV=20000 h–1

Experimental Catalyst preparation

Modified SCR catalysts were prepared by the wet impregnation method based on the commercial SCR (1V2O5-9WO3/TiO2, containing 1% V2O5 and 9% WO3) catalyst by adding another metal nitrate including Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, Fe(NO3)3·9H2O, Cu(NO3)2·3H2O and Ce(NO3)3·6H2O as the second promoter. The impregnating solution was prepared by using the mixture of the metal nitrate, NH4VO3 and 3(NH4)2O-7WO3-6H2O. The mixture was exposed to an ultrasonic bath for 4 h, dried at 110°C for 12 h and calcined at 550°C in air for 4 h. Powder form catalyst was obtained by grinding the composite and sieved to 20–40 mesh. Several modified SCR catalysts were prepared including 5Ni/1V2O5-9WO3/TiO2 (5%Ni/SCR), 5Mn/1V2O5-9WO3/TiO2 (5%Mn/SCR), 5Cu/1V2O5-9WO3/TiO2 (5%Cu/SCR), 5Fe/1V2O5-9WO3/TiO2 (5%Fe/SCR) and Ce/1V2O5-9WO3/TiO2（Ce/SCR） in which the amount of CeO2 was 1%, 5%, 9% and 13%, respectively. 1.2

Fig. 2 Effects of different doping metallic elements on the mercury

7: volume flow controller; 8: gas mixer; 9: mercury generator;

Experimental apparatus

The simulated device consisted of gas cylinders, a gaseous mercury generator, a catalytic reactor, an on-line mercury analyzer and an exhaust absorbing device (Figure 1). The mercury permeation tube was placed into a glass U-tube in a constant temperature water bath, carried by high-purity N2, and the gas flow was precisely controlled by mass flow controllers with the total flow rate of 0.25 L/min. The gas pipelines were displaced with heater band to prevent condensation of gaseous mercury. The concentration of Hg0 was measured online by a mercury analyzer (GMA3212, Shanghai Beiyu Analytical Instruments Co., LTD).

1.3

Characterization of catalyst

Brunauer-Emmett-Teller (BET) surface areas, pore volumes and pore diameters of the catalysts were analyzed by using the Autosorb1-iQ instrument (Quantachrome) at 77 K. X-ray diffraction (XRD) measurement was carried out with a Rigaku diffractometric using Cu K radiation ( = 0.15406 nm) in the range of 10°–80°. 1.4

Experimental method

In each test, 0.20 g of catalyst was loaded in a quartz reactor, and the total flow rate of the simulated flue gas was 0.25 L/min with Hg0 concentration of 187 g/m3, HCl 7.5 g/m3, O2 5% (v/v), SO2 827 g/m3 and N2 as a balance with temperature of 350°C and gas hourly space velocity (GHSV) around 20000 h–1. The Hg0 concentration was measured after adsorption balance, and the decline value of Hg0 concentration between inlet and outlet could be considered due to oxidation of Hg0. This method allowed calculating the oxidation of Hg0 at the SCR unit by the following formula: Hg0 –Hg0 η= in 0 out ×100% (1) Hgin

where Hg0in is the initial Hg0 concentration, g/m3, and Hg0out is the Hg0 concentration in the outlet stream, g/m3. In each test, the initial concentration of Hg0 was maintained at 187 g/m3 and the data were recorded after adsorption saturation.

2

Results and discussion

2.1 Performance of the catalysts under different conditions

ZHAO Li et al / Journal of Fuel Chemistry and Technology, 2015, 43(5): 628634 Table 1 Specific surface area and pore structure of the catalysts Catalyst

Specific surface area A/(m2·g–1)

Pore volume v/(cm3·g–1)

SCR

72.09

0.42

23.39

1%Ce/SCR

71.68

0.41

22.84

5%Ce/SCR

71.23

0.40

22.28

9%Ce/SCR

70.61

0.40

23.33

13%Ce/SCR

67.67

0.36

17.89

Fig. 3 XRD patterns of different Ce loaded catalysts

The Hg0 oxidation effects were investigated over the non-modified SCR, 5%Ni/SCR, 5%Mn/SCR, 5%Cu/SCR, 5%Fe/SCR and 5%Ce/SCR catalysts under four different experimental conditions (Figure 2). The results show that the Hg0 oxidation increases in varying degrees over the modified SCR catalysts in the presence of HCl and SO2. At 350°C, for example, high Hg0 conversion is observed from the 5% Ce/SCR, followed by 5% Cu/SCR, increased by 40% and 25% compared with the non-modified SCR catalyst. As observed in Figure 2, HCl could promote Hg0 oxidation obviously. In the presence of HCl, the oxidation over modified SCR catalysts is 4–6 times higher than the results without HCl. In addition, except for the CeO2 modified SCR catalyst, the presence of SO2 inhibits the Hg0 oxidation of the other modified catalysts, especially the 5% Ni/SCR catalyst. This is probably due to the large oxygen storage capacity and unique redox couple of Ce3+/Ce4+ with the ability to shift between CeO2 and Ce2O3 under oxidizing and reducing conditions, respectively[24]. The conversion process of the redox cycle couple of Ce4+/Ce3+ could produce some high reactive oxygen vacancy or bulk oxygen, which could improve chemisorption of catalyst for Hg0 and HCl in the flue gas, and thus improving the Hg0 oxidation efficiency. In addition, competitive adsorption of the SO2 with the Hg0 in the flue gas is decreased by adding CeO2 into catalyst, making the catalyst possessing good SO2-resistant performance[25,26]. According to the above results, CeO2 is chosen as the best doping metal to modify SCR catalyst.

Average pore diameter d/nm

The BET surface areas, pore volumes and pore diameters of the catalysts with different CeO2 doping amounts were measured and the results are shown in Table 1. Compared with the non-modified SCR catalyst, there is no obvious change in the specific surface areas, pore volumes and average pore diameters for the 1% Ce/SCR–9% Ce/SCR catalysts. However, with the increasing of CeO2 doping amount to 13%, the specific surface area of the 13% Ce/SCR catalyst is slightly reduced, while the average pore diameter is decreased by 23.5%. This is due to that CeO2 crystal cluster is formed on the surface of the catalyst by high loading amount. Figure 3 illustrated the XRD spectra of the CeO2 modified SCR catalysts. The results show that the main phase is anatase TiO2 phase for all catalysts and no rutile TiO2 phase is found. The CeO2 is dispersed well on the 1% Ce/SCR–9% Ce/SCR catalysts because no obvious CeO2 peak is observed in the crystalline structures. But with increasing Ce doping amount to 13%, a weak peak of cubic CeO2 crystallites is discerned (2θ=28.38o).These results indicate that the cluster of CeO2 crystallites could decrease the surface area of the catalyst, and further block the pores, resulting in the decrease of the pore sizes. The data of the 13% Ce/SCR catalyst in Table 1 also prove this point. In addition, there is no obvious peak of cubic WO3 and V2O5 crystallites, and this implys that the two components are in suitable proportions and well dispersion. 2.3

Effects of CeO2 loading amount on Hg0 oxidation rate

Fig. 4 Effects of Ce loading on the mercury catalytic oxidation experimental conditions: reaction temperatures 350°C, 7.5 mg/m3 HCl, 827

2.2

Catalyst characterization

mg/m3 SO2, 5% O2 and N2 as balance gas, GHSV=20000 h–1

ZHAO Li et al / Journal of Fuel Chemistry and Technology, 2015, 43(5): 628634

Fig. 5 Effects of gas composition on the mercury oxidation without SCR catalyst

It could be seen from Figure 4 that the oxidation of Hg0 increases and reaches 95.11% with increasing CeO2 doping amount to 9%. However, with further increase of the loading amount to 13%, the Hg0 oxidation decreases slightly to 92.66%. This agrees well with the results described in BET and XRD characterization, and thus, the optimal loading amount of the CeO2 is 9%. The following experiments are then conducted to investigate the effects of gas component, temperature and space velocity on the Hg0 oxidation by using the 9%Ce/SCR catalyst. 2.4 Effects of gas components and HCl on the Hg0 catalytic oxidation In the absence of the SCR catalyst, the effects of gas components on the Hg0 oxidation are firstly investigated in Figure 5. 5% O2, 827 mg/m3 SO2 and 7.5 mg/m3 HCl are successively added into the gas circuit at a, b, c points. The total flow is remained constant and the temperature is fixed at

350°C. The result indicates that Hg0 concentration does not show significant change during the whole experimental process. This suggests that without the SCR catalyst, the gas components would not contribute to Hg0 oxidation. The effects of gas components on the catalytic Hg0 oxidation over the 9% Ce/SCR catalyst are shown in Figure 6(a). In the presence of either SO2 or O2, the catalytic oxidation efficiency is low, only 5.47% and 3.46%, while the efficiency slightly increases to 12.38% in the presence of both SO2 and O2. Moreover, when some HCl (10 mg/m3) is added, the oxidation efficiency rapidly increases to 91.34%. Based on the results, it could be concluded that HCl has a positive promoting effect on the Hg0 oxidation, which might be the oxidation of Hg0 by the gas-phase O2 or that the lattice oxygen would be very difficult in the absence of HCl, due to the high reaction activation energy[27]. As shown in Figure 6(b), with increasing HCl concentration, the Hg0 catalytic oxidation increases gradually. A number of experimental studies have confirmed that in flue gas, higher HCl concentration would result in higher Hg oxidation across SCR system[28–30]. Several mechanisms[31–33] have been proposed for Hg0 catalytic oxidation over SCR catalyst in the presence of HCl and the global reaction could be expressed as follows[34]. 2Hg+4HCl+O2 2.5

SCR catalyst

2HgCl2 + 2H2 O

(2)

Effects of temperature on the Hg0 catalytic oxidation

As shown in Figure 7, along with the temperature ranging from 290 to 380°C, the Hg0 oxidation increases steadily and reaches 86.59% at 380°C when using the 9% Ce/SCR catalyst. With further increase of temperature to 410°C, the oxidation decreases to 79.38%.

Fig. 6 Effects of gas composition and HCl on the mercury catalytic oxidation experimental conditions: (a): reaction temperatures 350°C, N2 as balance gas, GHSV=20000 h–1; (b): reaction temperatures 350°C, 0–12.5 mg/m3 HCl, 827 mg/m3 SO2, 5% O2 and N2 as balance gas, GHSV=20000 h–1

ZHAO Li et al / Journal of Fuel Chemistry and Technology, 2015, 43(5): 628634

Fig. 7 Effects of reaction temperature on the mercury catalytic oxidation

The space velocity was an important factor for the catalytic reaction, which reflected the residence time of flue gas over catalyst surface. Figure 8 shows the effects of space velocity on the Hg0 catalytic oxidation over the 9% Ce/SCR catalyst. The Hg0 oxidation decreases with higher GHSV and shorter residence time. For example, the Hg0 catalytic oxidation approximately reaches 100% at a low space velocity of 1×104 h–1. However, during the operation of power plants, the GHSV was designed only for NOx reduction across the SCR system. Therefore, it was not feasible by reducing space velocity to increase Hg0 catalytic oxidation. Under appropriate GHSV, it is essential to increase the number of surface active sites of catalyst or to strengthen the adsorption capacity of Hg0 on catalyst.

experimental conditions: 7.5 mg/m3 HCl, 827 mg/m3 SO2, 5%O2 and N2 as balance gas, GHSV=20000 h–1

Fig. 8 Effect of space velocity on the mercury catalytic oxidation experimental conditions: reaction temperatures 350°C, 7.5 mg/m3 HCl, 827 mg/m3 SO2, 5% O2 and N2 as balance gas

According to the previously proposed Hg0 oxidation mechanisms over the SCR catalyst, a first mechanism suggested that Hg0 oxidation was via Deacon Processes[32], a second mechanism proposed Hg0 oxidation followed Langmuir-Hinshel-wood model[33] and a third mechanism suggested Hg0 oxidation reaction followed Eley-Rideal model[34]. These reaction mechanisms all showed that Hg0 catalytic oxidation over SCR catalyst relied mainly on chemical adsorption. In a certain temperature range, with the increase of temperature, the number of molecules with needed reaction activation energy was increased and chemical adsorption of Hg0 was enhanced, improving the Hg0 conversion. When reaction temperature exceeded a certain value, the Hg0 desorption occurred, resulting in low Hg0 oxidation[35]. 2.6 Effects of space velocity on the Hg0 catalytic Oxidation

2.7 Effects of the CeO2 modified SCR catalyst on denitration efficiency The primary function of the SCR catalyst was to reduce NOx. Hence, the denitrification efficiency of the non-modified SCR and 1%–13% Ce/SCR catalysts were evaluated under the experimental condition of 503 mg/m3 NO, 292 mg/m3 NH3, 733 mg/m3 SO2, 3.4% O2 and N2 as a balance at 350°C and space velocity of 1.19×104 h–1. The results show that the denitrification efficiency is 97.5%, 98.1%, 98.5%, 99.5% and 99.4% for the non-modified SCR, 1% Ce/SCR, 5% Ce/SCR, 9% Ce/SCR and 13% Ce/SCR catalysts, respectively. Therefore, it could be concluded that the CeO2 modified SCR catalyst could improve the catalytic oxidation of Hg0, while had no negative effect on the denitrification efficiency.

3

Conclusions

In this study, several metal oxides including NiO, Mn2O3, CuO, Fe2O3 or CeO2 were co-doped into the traditional V2O5-WO3/TiO2 catalyst and the Hg0 catalytic oxidation was evaluated over these modified SCR catalysts. The Hg0 catalytic oxidation was obviously improved by the CeO2 modified SCR catalyst. The BET and XRD analyses showed that there was an optimal CeO2 doping amount into the commercial SCR catalyst and the cluster of CeO2 crystallites would appear on the catalyst surface above a certain amount. The highest Hg0 catalytic oxidation was obtained by the 9% Ce/SCR catalyst, being 40% higher than that of the non-modified SCR catalyst. The presence of HCl increased the Hg0 oxidation because of the ability of chlorine to oxidize Hg0 into HgCl2. The Hg0 oxidation increased with the reaction temperature rising from 290 to 380°C and also increased along with the decline of the GHSV. The CeO2 modified SCR catalyst could improve the Hg0 catalytic oxidation while retaining high denitrification efficiency.

ZHAO Li et al / Journal of Fuel Chemistry and Technology, 2015, 43(5): 628634

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