In-Flight Mercury Removal and Cobenefit of SO2 and NO

The in-flight mercury removal performance of ammonium bromide impregnated activated carbon (NH4Br-AC) was evaluated in an entrained flow reactor (EFR)...
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In-Flight Mercury Removal and Cobenefit of SO2 and NO Reduction by NH4Br Impregnated Activated Carbon Injection in an Entrained Flow Reactor Qiang Zhou, Yu-feng Duan,* Chun Zhu, Min She, Jun Zhang, and Ting Yao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China ABSTRACT: The in-flight mercury removal performance of ammonium bromide impregnated activated carbon (NH4Br-AC) was evaluated in an entrained flow reactor (EFR) under simulated flue gas. The factors that affect in-flight mercury removal efficiency were explored. The optimum operating parameters were selected to be verified in the EFR under real flue gas, which was derived from the anthracite combustion in a 6 kW circulating fluidized bed (CFB) combustor. The coeffect of NH4Br-AC injection on SO2 and NO emission was also investigated. The results show that the in-flight mercury removal rate of raw activated carbon (R-AC) is significantly improved by the NH4Br modification. Greater sorbent feed rate, longer sorbent residence time, and smaller sorbent particle size are beneficial for improving the in-flight mercury removal rate. In the anthracite combustion flue gas, with the increase of sorbent residence time from 0.59 to 1.79 s, the in-flight mercury removal rate of NH4BrAC increases from 70.7% to 90.5%. Although the physisorption strengths of SO2 and NO are greater than that of gas-phase mercury, the increase of the Br group on the NH4Br-AC surface improves the mercury adsorption affinity. The reduction rates of SO2 and NO reach 30.6% and 38%, respectively, but the SO3 concentration in the flue gas increases 116% compared to the original emission concentration. The reduction of SO2 and NO in the flue gas is attributed to the chemisorption on the NH4BrAC surface and the oxidation by the injected O2 existing in the sorbent carrier gas, which promotes more SO3 and NO2 generation in flue gas.

1. INTRODUCTION Mercury is an environmentally toxic metal.1 Mercury emission to the atmosphere has attracted global concern because of its bioaccumulation, persistence, and toxicity.2−4 It is generally agreed that coal combustion is the largest anthropogenic source of mercury emission.5−7 Therefore, reduction of mercury emission from coal-fired power plants has been a major challenge.1,8 It is well-known that the mercury existing in the coal combustion flue gas is in the form of elemental mercury (Hg0), oxidized mercury (Hg2+) and particle-bound mercury (HgP).9−11 Most Hg2+ and HgP can be removed by air pollution control devices (PCDs), such as wet flue gas desulfurization (WFGD), electrostatic precipitators (ESPs), or fabric filters (FFs), but the Hg0 is directly discharged into the atmosphere with the flue gas because of its high volatility and insolubility.12−14 Therefore, it is important and necessary to transform the Hg0 into Hg2+ or Hgp, to reduce the mercury emission from coal combustion. To date, sorbent injection technology has been considered to be the most promising and effective control approach for reducing the mercury emission from power plants.15,16 Most previous research focused on developing the sorbent with low cost and high efficiency, such as modified activated carbon, fly ash, calcium-based sorbent and biomass sorbent.14 The previous sorbent evaluation was primarily concentrated in fixed bed reactor, to obtain the mercury breakthrough curve and simulate the mercury adsorption in FFs. However, research17 has shown that the in-flight mercury removal with short sorbent residence time reached approximately 90% of the total mercury removal in both duct and PCDs. Therefore, it is © 2015 American Chemical Society

important and necessary to evaluate the in-flight mercury removal performance of a sorbent, but the evaluation cannot be completed in a fixed-bed reactor. The entrained flow test has been confirmed to be an effective method for evaluating inflight mercury removal and obtaining the optimal operating parameters of sorbent injection.18−23 Ghorishi et al.20 tested a commercial Cl-impregnated activated carbon in a flow reactor. They found that the in-flight mercury removal rate reached 80− 90% with the sorbent residence time of about 3−4 s and the initial mercury concentration of 86 ppb. Lee et al.22 developed a cost-effective sorbent from the petroleum coke, which was evaluated in a flow reactor. They found that the pyrolysis petroleum coke was a potential sorbent for in-duct mercury removal. Lee et al.17 explored a brominated activated carbon and a cupric chloride-impregnated activated carbon (CuCl2ACs) in a bench scale entrained flow reactor with the sorbent residence time of 0.75 s and the gas temperature of 140 °C. They found that the mercury oxidation performance of CuCl2ACs was better than that of brominated activated carbon, and the carbon content of a sorbent determined the in-flight mercury removal capability. Zhang et al.23 investigated the mercury removal performance of different modified fly ashes in an entrained-flow reactor. They found that the fly ash modified by HBr had the best in-flight mercury removal ability, and the fly ash particle size had a significant effect on the in-flight mercury removal. Received: August 20, 2015 Revised: November 4, 2015 Published: November 5, 2015 8118

DOI: 10.1021/acs.energyfuels.5b01903 Energy Fuels 2015, 29, 8118−8125

Article

Energy & Fuels

Figure 1. Schematic diagram of simulated flue gas-EFR.

Figure 2. Schematic diagram of CFB-EFR.

operating parameters were selected to be verified in the EFR under real flue gas, which was derived from the anthracite combustion in a 6 kW circulating fluidized bed (CFB) combustor. The coeffect of NH4Br-AC injection on the SO2 and NO emission was also investigated.

In 2011, the People’s Republic of China has issued very stringent regulations for controlling the emissions of Hg, SOx, and NOx from the coal-fired utilities. 24 In order to simultaneously remove the multipollutant in flue gas by utilizing one control device/approach, multipollutant emission control strategies have been proposed.25 However, the studies on the simultaneous removal of Hg, SOx, and NOx by sorbent injection are very limited. Chiu et al.25 have investigated the mercury removal performance of CuCl2-modified zeolite and the cobenefit of SO2 and NO removal in a fixed-bed reactor. They found that the CuCl2 modified zeolite had high mercury removal capability, and the SO2 and NO removal rate reached 38% and 73%, respectively. However, the feasibility of in-flight multipollutant removal by sorbent injection in an entrained flow reactor is still not clear and worthy to be further explored. In our previous work,26 the mercury removal performance of ammonium bromide impregnated activated carbon (NH4BrAC) has been evaluated in a fixed-bed reactor. The NH4Br-AC showed high mercury removal capability. The present study aimed to further explore the in-flight mercury removal performance of NH4Br-AC in an entrained flow reactor (EFR) under simulated flue gas. The factors affecting in-flight mercury removal efficiency were explored. The optimum

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation and Characterizations. The preparation method of the NH4Br-AC has been described in the precious work.26 1% (mass ratio) NH4Br solution was used to impregnate the R-AC sample. After the procedure of stirring, filtration and drying, the preparation of NH4Br-AC was completed. The characterizations of N2 adsorption/desorption and energy-dispersive X-ray spectroscopy (EDX) were conducted to better understand the NH4Br impregnation process. 2.2. Mercury Removal Tests. 2.2.1. In-Flight Hg0 Removal in EFR under Simulated Flue Gas. As shown in Figure 1, the EFR is consisted by several components: simulated flue gas generation system, sorbent injection system, sampling and testing system, sorbent collecting and gas purification system. The reactor is 20 m long with an inner diameter of 16 mm. The internal surface of the reactor was made of Teflon to prevent the sorbent and mercury adhesion. There are five gas-phase mercury sampling ports evenly distributed along the reactor. The Hg0 vapor in the flue gas was generated by two Hg0 permeation devices and pure nitrogen was provided as carrier gas with 8119

DOI: 10.1021/acs.energyfuels.5b01903 Energy Fuels 2015, 29, 8118−8125

Article

Energy & Fuels Table 1. Proximate and Ultimate Analysis of Anthracite proximate analysis

ultimate analysis

heat value

sample

FCar (%)

Aar (%)

Var (%)

Mt (%)

C (%)

H (%)

O (%)

N (%)

S (%)

Cl (%)

Hg (mg/kg)

Qnet (MJ/kg)

coal

58.42

31.24

6.60

3.74

57.28

0.70

3.47

0.70

1.32

0.006

0.155

21.62

a flow rate of 1 L/min for each, which was adjusted by a mass flow controller. The purified air, produced from a compressor, is the main component of the simulated flue gas. The sorbent feed rate was controlled by a micro screw feeder and the sorbent was injected to the reactor by an injection probe with the sorbent carrier gas of 23 L/min. The sorbent was mixed with quartz sand before injection (mass ratio of 1 to 9), to reduce the sorbent wall adhesion and increase the accuracy of sorbent feed rate. The total flow rate of the simulated flue gas was set at 5 m3/h with the sorbent residence time approaching 2 s, which is similar to the actual situation in power plants. The gas-phase mercury concentration was online tested by VM3000 and EMP-2 mercury analyzers. Before the flue gas entered to the mercury analyzers, a cyclone and a filter were utilized to eliminate the effect of sorbent particle on the mercury analyzers and improve the test accuracy. After each test, high speed nitrogen was used to clean the reactor to reduce the effect of deposited sorbent on in-flight mercury removal. The in-flight mercury removal performance of a sorbent was evaluated by the in-flight Hg0 removal rate η, defined as follows. η=

C0in − C0g C0in

× 100%

proved the mesopores (2−50 nm) and macropores (>50 nm). As Figure 3 shows, the R-AC and NH4Br-AC have well-

(1)

is the Hg0 where Cin is the initial inlet Hg concentration, μg/m ; 3 concentration at different sampling ports, μg/m . 2.2.2. In-flight Mercury Removal in CFB-EFR. After the NH4Br-AC was tested in the EFR under simulated flue gas, the obtained optimum operating parameters were selected to be verified in the CFB-EFR. As Figure 2 shows, the CFB-EFR apparatus consists of three main parts, including a CFB combustion system, an EFR system, and a flue gas sampling and treatment system. The 2200 mm high CFB combustor is constituted of three sections: dense zone (height 400 mm and diameter 66 mm), transition zone, and dilute zone (height 1700 mm and diameter 79 mm). The composition of the EFR is the same as that in Figure 1, but the inner diameter of the EFR was expanded to 25 mm. The sorbent carrier gas is purified air with the flow rate at 2 m3/h. Mercury in the flue gas was sampled by using the Ontario Hydro Method (OHM). The mercury content in solids was detected by Leeman Laboratories Hydro II C, and the mercury concentration in sampling solutions was measured by Leeman Laboratories Hydro AA. The flue gas components of O2, CO2, CO, NO, and SO2 were tested online by ecomJ2KN analyzer. The SO3 concentration in the flue gas was measured by using the controlled condensation method. A benchscale fabric filter and an alkali tank were installed after the EFR to collect the solid particle and absorb the acid gases from the flue gas, respectively. The Chinese anthracite was selected to be combusted in the CFB combustor to produce stable mercury-containing flue gas. As Table 1 shows, the mercury content of the anthracite is 0.155 mg/kg. The chlorine content is 0.006%, belonging to low chlorine coal. The sorbent performance was evaluated by the in-flight removal rate ε, shown in eq 2. 0

ε=

0

Cin − Cg Cin

× 100%

3

Cg0

Figure 3. N2 adsorption/desorption curves of R-AC and NH4Br-AC.

developed micropores. The pore volume of NH4Br-AC was reduced compared to that of R-AC, which is illustrated in Table 2. In general, the macropores are the entry portals to the sorbent particle, the mesopores serve as the transportation channels, and the micropores provide the activate sites for mercury adsorption.26,28 The EDX analysis proves that approximately 1.36% bromine was found on the NH4Br-AC surface, with no bromine existing on the R-AC surface, which indicates that part of the bromine groups remained on the NH4Br-AC during modification. 3.2. Sorbents Performance in Simulated Flue Gas. 3.2.1. In-Flight Hg0 Removal with Sorbent Injection. As Figure 4 shows, as the NH4Br-AC was injected into the reactor, the gas-phase Hg0 concentration dropped quickly. After 10 min, the Hg0 concentration in the flue gas reduced from 10.5 μg/m3 to 1 μg/m3 and reached a relatively stable state. The effect of RAC injection on the gas-phase Hg0 concentration showed a similar trend to that of NH4Br-AC injection, but the in-flight Hg0 removal capability of the R-AC was significantly lower than that of NH4Br-AC. When injection of R-AC and NH4Br-AC into the reactor was stopped, the gas-phase Hg0 concentrations in flue gas slowly increased and gradually approached the initial Hg0 concentration, but the final Hg0 concentrations were lower than the initial concentration. This is because there were few sorbent particles depositing on the inner surface of the reactor due to the van der Waals force and the surface static electricity.17 In the series of experiments, it was found that the mercury removal by the deposited NH4Br-AC occupied about 5−10% and the mercury removal by the deposited R-AC occupies 1.5−3%. The mercury removal by the deposited NH4Br-AC was greater than that of the deposited R-AC, which indicates that the elevation of mercury adsorption capability of a sorbent not only significantly promotes the increase of inflight mercury removal rate, but also increases the mercury removal by the deposited sorbent.

(2)

where Cin is the initial emission concentration, including Hg, NO, and SO2; Cg is the concentration at different sampling ports after sorbent injection, including Hg, NO, and SO2.

3. RESULTS AND DISCUSSION 3.1. Sorbent Physicochemical Characteristics. Research from Skodras et al.27 showed that the rise of adsorbed volume at low relative pressure indicated the micropores ( SO2 > NO > CO. The dipole moments of O2, CO2, and N2 are equal to zero, and mercury is a nonpolar substance.31 This illustrates that the order of physisorption strength is H2O > SO2 > NO > Hg. Among the main flue gas components, NO and SO2 have significant effects on the in-flight mercury adsorption. Research from Miller et al.32 showed that NO promoted mercury adsorption. This is because NO can react with O2 to form NO2 and O, which can oxidize Hg0 to form HgO. The oxidized mercury is easier to be adsorbed compared to that of Hg0.33 The reaction process is listed as follows. NO + O2 → NO2 + O

(6)

3.3.7. Mechanism of NO and SO2 Reduction. The process of NO reduction can be described by eq 9−eq 11. NO in the flue gas can react with O2 to form NO2, which can react with the adsorbed H2O and O on the NH4Br-AC surface to form adsorbed HNO3. The ammonium-containing groups on the NH4Br-AC surface can react with the adsorbed HNO3 to form adsorbed NH4NO3. In addition, the increase of O2 concentration in the flue gas from 6% to 10% can promote NO reduction and more NO2 formation.

Table 4. Dipole Moment of Main Gaseous Component31 dipole moment (debye)

2[Br]− + [CnH x Oy ‐] → [Br‐CnH x Oy ]

(9)

2NO2 + [H 2O‐] + [O‐] → 2[HNO3‐]

(10)

[HNO3‐] + [NH3‐] → [NH4NO3‐]

(11)

The reduction of SO2 in the flue gas can be attributed to three aspects. The first is the chemisorption of SO2 on the NH4Br-AC surface, which can be described by eq 12−eq 16. In general, part of SO2 in flue gas can react with O2 to generate SO3, given in eq 12. It is a reversible reaction, and metal oxide such as Fe2O3 in fly ash serves as catalyst.38 The SO3 concentration in flue gas usually accounts for 0.5−1.5% of the total sulfur in coal39 and is determined by the concentrations of SO2 and O2 in flue gas. The generated SO3 in flue gas can react with the adsorbed H2O on the NH4Br-AC surface to form H2SO4. At the same time, the SO2 in the flue gas can directly react with the adsorbed H2O and O groups to generate H2SO4. Then, the ammonium-containing groups on the NH4Br-AC surface can react with part of adsorbed H2SO4 to form (NH4)2SO4 or NH4HSO4, given in eq 15 and eq 16. The second is the capillary condensation of the adsorbed SO2 on the NH4Br-AC. Due to the NH4Br-AC having well-

(5) 34

However, research from Liu et al. showed that low NO concentration in flue gas promoted the mercury adsorption at 8123

DOI: 10.1021/acs.energyfuels.5b01903 Energy Fuels 2015, 29, 8118−8125

Article

Energy & Fuels developed pores, the high SO2 concentration in the flue gas may cause capillary condensation to occur in the mesopores of the NH4Br-AC. The third is the SO2 oxidation in the flue gas. The increase of O2 concentration in the flue gas from 6% to 10% promotes SO2 reduction and more SO3 formation.

(2) Zheng, Y.; Jensen, A. D.; Windelin, C.; Jensen, F. Review of technologies for mercury removal from flue gas from cement production processes. Prog. Energy Combust. Sci. 2012, 38, 599−629. (3) Liu, 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, 108−113. (4) Li, Y.; Murphy, P.; Wu, C. Y. Removal of elemental mercury from simulated coal-combustion flue gas using a SiO2−TiO2 nanocomposite. Fuel Process. Technol. 2008, 89, 567−573. (5) Padak, B.; Wilcox, J. Understanding mercury binding on activated carbon. Carbon 2009, 47, 2855−2864. (6) Scala, F.; Chirone, R.; Lancia, A. Elemental mercury vapor capture by powdered activated carbon in a fluidized bed reactor. Fuel 2011, 90, 2077−2082. (7) Zhou, Q.; Duan, Y. F.; Zhu, C.; Zhang, J.; She, M.; Hong, Y. G. Adsorption equilibrium, kinetics and mechanism studies of mercury on coal-fired fly ash. Korean J. Chem. Eng. 2015, 32, 1405−1413. (8) Li, H. L.; Li, Y.; Wu, C. Y.; Zhang, J. Y. Oxidation and capture of elemental mercury over SiO2−TiO2−V2O5 catalysts in simulated lowrank coal combustion flue gas. Chem. Eng. J. 2011, 169, 186−193. (9) Wang, Y. J.; Duan, Y. F.; Yang, L. G.; Zhao, C. S.; Shen, X. L.; Zhang, M. Y.; Zhuo, Y. Q.; Chen, C. H. Experimental study on mercury transformation and removal in coal-fired boiler flue gases. Fuel Process. Technol. 2009, 90, 643−651. (10) Martinez, A. I.; Deshpande, B. K. Kinetic modeling of H2O2enhanced oxidation of flue gas elemental mercury. Fuel Process. Technol. 2007, 88, 982−987. (11) Chi, Y.; Yan, N. Q.; Qu, Z.; Qiao, S. H.; Jia, J. P. The performance of iodine on the removal of elemental mercury from the simulated coal-fired flue gas. J. Hazard. Mater. 2009, 166, 776−781. (12) Rupp, E. C.; Wilcox, J. Mercury chemistry of brominated activated carbons−packed-bed breakthrough experiments. Fuel 2014, 117, 351−353. (13) Shu, T.; Lu, P.; He, N. Mercury adsorption of modified mulberry twig chars in a simulated flue gas. Bioresour. Technol. 2013, 136, 182−187. (14) Yang, H.; Xu, Z.; Fan, M.; Bland, A. E.; Judkins, R. R. Adsorbents for capturing mercury in coal-fired boiler flue gas. J. Hazard. Mater. 2007, 146, 1−11. (15) Prabhu, V.; Kim, T.; Khakpour, Y.; Serre, S. D.; Clack, H. L. Evidence of powdered activated carbon preferential collection and enrichment on electrostatic precipitator discharge electrodes during sorbent injection for mercury emissions control. Fuel Process. Technol. 2012, 93, 8−12. (16) Hu, C. X.; Zhou, J. S.; He, S.; Luo, Z. Y.; Cen, K. F. Effect of chemical activation of an activated carbon using zinc chloride on elemental mercury adsorption. Fuel Process. Technol. 2009, 90, 812− 817. (17) Lee, S. S.; Lee, J. Y.; Keener, T. C. Bench-scale studies of in-duct mercury capture using cupric chloride-impregnated carbons. Environ. Sci. Technol. 2009, 43, 2957−2962. (18) Serre, S. D.; Gullett, B. K.; Ghorishi, S. B. Entrained-flow adsorption of mercury using activated carbon. J. Air Waste Manage. Assoc. 2001, 51, 733−741. (19) Sjostrom, S.; Ebner, T.; Ley, T.; Slye, R.; Richardson, C.; Machalek, T.; Richardson, M.; Chang, R. Assessing sorbents for mercury control in coal-combustion flue gas. J. Air Waste Manage. Assoc. 2002, 52, 902−911. (20) Ghorishi, S. B.; Keeney, R. M.; Serre, S. D.; Gullett, B. K. Development of a Cl-impregnated activated carbon for entrained-flow capture of elemental mercury. Environ. Sci. Technol. 2002, 36, 4454− 4459. (21) Zhuang, Y.; Zygarlicke, C. J.; Galbreath, K. C.; Thompson, J. S.; Holmes, M. J.; Pavlish, J. H. Kinetic transformation of mercury in coal combustion flue gas in a bench-scale entrained-flow reactor. Fuel Process. Technol. 2004, 85, 463−472. (22) Lee, S. H.; Rhim, Y. J.; Cho, S. P.; Baek, J. I. Carbon-based novel sorbent for removing gas-phase mercury. Fuel 2006, 85, 219−226.

Catalyst

SO2 + O2 ←⎯⎯⎯⎯→ SO3

(12)

SO3 + [H 2O‐] → [H 2SO4 ‐]

(13)

2SO2 + 2[O‐] + 2H 2O → 2[H 2SO4 ‐]

(14)

2[NH3‐] + [H 2SO4 ‐] → [(NH4)2 SO4 ‐]

(15)

[NH3‐] + [H 2SO4 ‐] → [NH4HSO4 ‐]

(16)

4. CONCLUSION (1) The in-flight mercury removal rate of R-AC is significantly improved by the NH4Br modification. Greater sorbent feed rate, longer sorbent residence time, and smaller sorbent particle size are beneficial for improving the in-flight mercury removal rate. In the anthracite combustion flue gas, with the increase of sorbent residence time from 0.59 to 1.79 s, the in-flight mercury removal rate of NH4Br-AC increases from 70.7% to 90.5%. Although the physisorption strengths of NO and SO2 are greater than that of gas-phase mercury, the increase of Brcontaining groups on the NH4Br-AC surface improves the mercury adsorption affinity. (2) After the NH4Br-AC was injected into the anthracite combustion flue gas, the reduction rates of SO2 and NO reach 30.6% and 38%, respectively, but the SO3 concentration in the flue gas increases 116% compared to the original emission concentration. The reduction of SO2 and NO in the flue gas is attributed to the chemisorption on the NH4Br-AC surface and the oxidation by the injected O2 existing in the sorbent carrier gas, which promote more SO3 and NO2 generation in flue gas.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-83795652, fax: +86-25-83795652, E-mail address: [email protected] (Yu-feng Duan). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was funded by the National Nature Science Foundation of China (51376046, 51576044), the Graduate Student Research and Innovation Program of Jiangsu Province (KYLX_0115, CXZZ13_0093, KYLX_0184 and KYLX15_0071), the National Science, Technology Support Program of China (No. 2012BAA02B01-02), the Jiangsu Province United Creative Subject (BY2013073-10), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1505). We thank Dr. Hong Lu of the Illinois State Geological Survey of the University of Illinois at Urbana−Champaign for help with the manuscript, and the financial support from the China Scholarship Council (CSC).

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DOI: 10.1021/acs.energyfuels.5b01903 Energy Fuels 2015, 29, 8118−8125