Mercury speciation and removal across full-scale wet ... - Springer Link

JOURNAL OF COAL SCIENCE & ENGINEERING (CHINA)

DOI 10.1007/s12404-010-0116-7

pp 82–87

Vol.16 No.1

Mar. 2010

Mercury speciation and removal across full-scale wet FGD systems at coal-fired power plants WU Cheng-li1,2, CAO Yan2, DONG Zhong-bing1, CHENG Chin-min2, LI Han-xu1, PAN Wei-ping2 ( 1. School of Chemical Engineering, Anhui University of Science and Technology, Huainan

232001, China; 2. Institute for

Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green 42101, USA ) © The Editorial Office of Journal of Coal Science and Engineering (China) and Springer-Verlag Berlin Heidelberg 2010

Abstract The Ontario Hydro Method (OHM) recommended by the United States Environmental Protection Agency (EPA) was used to determine mercury speciation in the combustion flue gas across wet FGD systems. Four coal-fired units with wet FGD systems were chosen to evaluate mercury speciation and mercury removal efficiencies through these wet FGD systems. Chlorine content in coal had been suggested as a main factor that affects mercury speciation in flue gas. It is shown that the higher the chlorine concentration in coal is, the higher the percentage of oxidized mercury (Hg2+) is removed in wet FGD systems, which can increase overall mercury removal efficiencies through wet FGD systems. The selective catalyst reduction (SCR) system has a function of oxidizing elemental mercury (Hg0) to oxidized mercury. A higher percentage of oxidized mercury in the total vapor mercury at the FGD inlet is observed when SCR is in service. Therefore, higher overall mercury removal efficiencies through wet FGD are attained. Because of different wet FGD operating conditions, there are different mercury removal efficiencies in different units. Elemental mercury reemission took place when a fraction of oxidized mercury absorbed in the slurry is reduced to elemental mercury, and Hg0 is reemitted from stack, which results in decreases in mercury removal efficiencies through wet FGD systems. Keywords wet FGD, mercury speciation, mercury removal, field testing

Introduction Mercury emission is a global air pollution problem, and it attracts more and more stringent attention in the world. Of all the anthropogenic mercury emissions, coal combustion at coal-fired power plants has been reported as the largest mercury emission source, contributing about one third of total mercury emissions (Cao et al., 2005; Agarwal et al., 2006; Cao et al., 2007). Most recently, a federal court overturned the EPA’s rule controlling mercury emissions from coalfired power plants. It is likely that the EPA will require a more stringent control rule on mercury emission, Received: 2 April 2009 Tel: 86-554-6668459, E-mail: [email protected]

based on the explicit act by the U.S. Congress. In addition to sulfur dioxide control, wet FGD systems have the potential of providing reliable and cost-effective mercury control. The most important factor influencing mercury control emission by wet scrubbers is the forms of mercury in the flue gas. Vapor-phase mercury in flue gas can appear as elemental mercury (Hg0, metallic mercury vapor) or as oxidized mercury (Hg2+, vapor-phase species of various volatile compounds of mercury). Hg0 is nearly insoluble in water, whereas Hg2+ compounds are highly soluble (Pavlish et al., 2003; Renninger et al., 2004; Diaz-Somoano et al., 2007).

WU Chengli, et al. Mercury speciation and removal across full-scale wet FGD systems at

The speciation of vapor-phase mercury depends on coal type and other factors (Romero et al., 2006; Cao et al., 2008). Generally, bituminous coals produce a higher percentage of oxidized mercury than sub-bituminous and lignite coals. An empirical relationship exists between the chloride content of coal and the extent to which mercury appears in the oxidized form (Kellie et al., 2005; Agarwal et al., 2006; Zhou et al., 2007). FGD and SCR systems have also been shown to affect both the speciation of mercury in the stack and the amount of mercury removed in the air pollutant control devices (Renninger et al., 2004; Yang and Pan, 2007). Mercury speciation, gas and slurry composition, L/G, oxidation modes, operating temperature and slurry pH value affect mercury removal efficiency through wet FGD. Mercury speciation also impacts mercury removal across wet FGD systems with SCR or SCR by-passed. In this paper, four units equipped with SCR and ESP upstream of the wet FGD systems were chosen to study mercury speciation and mercury removal. Field tests of mercury measurement through wet FGD sysTable 1

83

tems were conducted. Effects of chloride in coal, SCR and wet FGD systems on mercury speciation and removal were investigated. The Ontario Hydro Method (OHM) was used to test mercury emission and speciation. OHM sampling and analyzing quality assurance/ quality control (QA/QC) procedures were followed all through the tests.

1

Experiment

1.1

Unit and coal sample Field tests of mercury concentration were performed through four full-scale wet FGD systems at coal-fired power plants. All of these four units equipped with SCR, cold-side electrostatic precipitators (ESP) and wet FGD, and bituminous coal was fired in all the units. Table 1 summarizes some information and characteristics of these units. Coal samples were collected at OHM sampling periods. Table 2 lists the proximate and ultimate analysis as well as Hg/F/Cl concentrations of the coal sample.

Summary of four full-scale units information

Parameters

Unit 1

Unit 2

Unit 3

Unit 4

Capacity (MW)

90

205

190

450

Boiler type

Cyclone

Tangential fired

Cyclone

Pulverized coal(PC)

Fuel type

Bituminous

Bituminous

Bituminous

Bituminous

PM control

Cold-side ESP

Cold-side ESP

Cold-side ESP

Cold-side ESP

NOx control

SCR

SCR

SCR

SCR

SO2 control/ oxidation mode

LSFO-WFGD

LSFO-WFGD

LSNO-WFGD

LSFO-WFGD

Note: LSFO-WFGD is the limestone forced oxidation wet FGD; LSNO-WFGD is the Limestone natural oxidation wet FGD.

Table 2 Units

Proximate analysis (%) Mad

Aad

Vad

Unit 1

6.83

10.48

33.01

Unit 2

8.85

10.70

Unit 3

4.75

Unit 4

3.85

Heat value (kJ/kg)

Coal samples analysis results at four units Ultimate analysis(%)

Content(mg/kg)

w(Cad)

w(Had)

w(Nad)

w(Sad)

w(Oad)

Mercury

Fluoride

Chloride

27 374.30

65.36

5.45

1.31

3.76

13.63

0.07

49.63

1 806.47

34.92

26 489.08

63.43

5.52

1.18

3.91

15.26

0.06

49.46

1 936.64

14.97

34.99

28 849.66

69.45

4.72

1.15

3.04

6.67

0.11

105.67

1 248.33

14.02

39.39

28 401.24

71.65

4.75

1.46

4.45

3.65

0.19

27.34

194.76

All the coal samples contained high sulfur (3.04%~4.45%), therefore wet FGD were equipped at all the four units. However, there were big differences in chloride and fluoride contents; coal samples fired in Units 1, 2, and 3 have high chloride contents (>1 000 mg/kg), of which coal in Unit 2 reach

1 936.64 mg/kg, but there was lower chloride content (194 mg/kg) in the coal of Unit 4. As for mercury concentration in coal, mercury concentrations in these coals from the four units were in the range of 0.06 to 0.19 mg/kg, but mercury concentration in coal at Unit 4 was 0.19 mg/kg which was the highest mercury

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Journal of Coal Science & Engineering (China)

content in coal samples. Mercury concentration in flue gas depended on mercury content in the coals combusted at the power plants, and mercury speciation through a series of air pollution control devices (APCD) was related to halogen concentration in coal, fly ash characteristics, configurations and operation conditions of APCD etc. 1.2 Sampling and analysis method OHM is the EPA reference method for measuring mercury speciation in flue gas that has been approved by ASTM (Method D6784), and OHM was used to determine vapor mercury speciation (elemental mercury and oxidized mercury) in this work. As shown in Fig.1, gas sample was withdrawn from the flue gas stream isokinetically. Oxidized mercury was collected in impingers containing a chilled aqueous potassium chloride solution. Elemental mercury was collected in subsequent impingers (one impinger containing a chilled hydrogen peroxide/nitric acid and three impingers

Fig.1

2

containing chilled potassium permanganate/ sulfur acid). The samples were recovered and digested using an automated mercury preparation system, then solutions were analyzed using a cold vapor atomic absorption spectroscope (CVAAS) (Leeman Lab Hydra, Teledyne Leeman Labs, NH). All the mercury analysis data were corrected to a dry basis and 3% oxygen basis in the flue gas to compare mercury concentration through all the wet FGD systems. Mercury measurements were performed at wet FGD inlet and stack at the four units, and the boilers kept running at almost full and stable load during OHM sampling time. EPA OHM sampling quality assurance/quality control (QA/QC) procedures were followed throughout the tests. OHM sampling at the FGD inlet and stack of the four units without SCR and with SCR were conducted at non-ozone season (October 1st—April 30th next year) and ozone season (May 1st—September 30th), respectively.

Schematic of OHM sampling train

Results and discussions

Mercury speciation measurements were conducted through four wet FGD systems, and four sets of OHM testing were performed in the first three units, and three sets of OHM sampling were carried out at Unit 4. All of the OHM sampling were conducted at the FGD inlet and stack at these units simultaneously, and there were different oxygen concentrations at the FGD inlet and outlet at different units. In order to compare these data at the same basis, mercury concentration of OHM data was corrected at dry basis and 3% oxygen basis. Total vapor mercury concentration is formulated as HgT=Hg0+Hg2+, total mercury removal efficiency through wet FGD can be shown as η = T (Hg TFGD,inlet − HgStack ) / Hg TFGD,inlet . Elemental mercury reemission where elemental mercury concentrations increased through wet FGD occurred. Elemental mer0 − Hg 0FGD,inlet ) / cury reemission rate is η = (HgStack 0 Hg FGD,inlet . There were different mercury removal rates

in different wet FGD systems and different sampling seasons (ozone season and non-ozone season). 2.1 Effects of SCR and no SCR on mercury speciation at FGD inlet SCR was in service in the ozone season and was bypassed in the non-ozone season according to the EPA NOx emission regulation requirements. SCR systems had dual functions of reducing NOx emission and oxidizing elemental mercury in the flue gas (Cao et al., 2007). Total mercury concentration almost stayed stable across SCR but a significant percentage of elemental mercury was converted to oxidized mercury. A higher percentage of oxidized mercury in total mercury can be captured through downstream wet FGD systems. Mercury speciation and oxidized mercury fraction in the total mercury at the FGD inlet with SCR and without SCR at the four different units are shown in Fig.2. The first set of bars in the tested unit is mercury concentration at the FGD inlet without SCR,


and the second is the mercury speciation at the FGD inlet with SCR. About 31% to 64% of the total mercury concentration in the flue gas was oxidized mercury in the absence of the SCR system. The coals combusted in three units were high-chlorine bituminous coals (>1 000 mg/kg), while Unit 4 had lowchlorine coal content. Chlorine can enhance oxidization of elemental mercury, and higher chlorine coal can produce higher fraction of oxidized mercury in total vapor mercury. The high oxidation rate of Hg0 in the bituminous coal flue gases has been related to the composition of the fly ash in addition to the chlorine content. The presence of high iron oxide (Fe2O3) and calcium oxide (CaO) in coal fly ashes was an influential factor in determining the extent of Hg0 oxidation. Mercury oxidization efficiency was enhanced when SCR was in service, and oxidized mercury percentage in total mercury was from 63.66% to 92.59%. The mixing rates between vanadium oxides and other oxides in the catalyst, the pore size distribution, and catalyst deactivation with time, space velocity, temperature and flue gas composition affected mercury oxidation through SCR systems. Oxidization efficiencies were different from SCR systems at four units, and oxidized mercury fraction in the total mercury increased from 31.03% to 64.84% through SCR at Unit 1, 86.96% to 92.45% at Unit 2, 65.49% to 92.59% at Unit 3, and 63.66% to 85.73% at Unit 4, respectively. Oxidation extent was most evident through SCR at Unit 1, while it was not apparent at Unit 2. SCR systems have been developed into a cobenefit mercury speciation transformation technology at coal-fired power plants. Higher percentage of oxidized mercury in total mercury produced through the SCR system can be absorbed by wet FGD systems.

tional wet FGD systems because Hg0 was almost insoluble in water while oxidized mercury (mainly HgCl2) was extremely soluble. Nowadays, wet FGD systems have high SO2 removal efficiency 80%~98% at coal-fired power plants, and wet FGD systems have a function of capturing oxidized mercury. Mercury removal through wet FGD systems with SCR and without SCR was investigated. Mercury speciation and overall mercury efficiencies through wet FGD systems with and without SCR are shown in Fig.3. Each set of bars in the figure represents the average FGD inlet and stack gas-phase mercury concentration of four OHM runs. The number above the stack bar shows the average total mercury removal for a given test unit. Most of the oxidized mercury was captured by the wet FGD system with SCR and without SCR, and elemental mercury was not removed through wet FGD, but elemental mercury increased across wet FGD systems at some units.

Fig.3

Fig.2

2.2

Effects of SCR and no SCR on mercury speciation at FGD inlet

Effects of wet FGD systems on over-all mercury removal efficiency Elemental mercury was not removed in conven-

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Mercury speciation and removal through wet FGD with SCR and without SCR

Oxidized mercury was effectively captured through wet FGD systems, but overall mercury removal efficiencies depended on elemental mercury fractions in the flue gas at the FGD inlet and oxidized mercury conversion reaction through wet FGD. As shown in Fig.3, no increase of elemental mercury was detected in Units 1 and 3, but increase of elemental mercury was observed across wet FGD systems in Units 2 and 4, which resulted in a decrease of overall mercury removal efficiency through wet FGD systems. Higher overall mercury removal efficiencies were

Journal of Coal Science & Engineering (China)

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found when SCR was in service; total mercury removal efficiencies were from 30.89% to 77.39% through four wet FGD systems without SCR, while they were from 37.84% to 89.47% through wet FGD systems with SCR. The high levels of oxidized mercury through wet FGD combined with SCR system increased mercury removal efficiencies. By comparison of mercury removal efficiencies through wet FGD with and without SCR at the four units, increase of overall mercury removal efficiency at Units 1 and 3 was obvious, but it was not apparent at Unit 2 and Unit 4. Average total mercury removal efficiencies were 89% and 48% with and without SCR operation, respectively. Variations were possibly due to differences in L/G, pH value, and oxidation mode of wet FGD systems. Oxidation of elemental mercury through SCR is significant because FGD systems were more effective at removing oxidized mercury. 2.3 Elemental mercury reemission through wet FGD systems Capture of mercury by wet FGD systems is one method of meeting mercury emission regulations for coal-fired power plants when there is high proportion of oxidized mercury in the flue gas, and research is being conducted to optimize mercury capture through FGD systems. However, field testing had revealed that oxidized mercury captured by the FGD system can be chemically reduced into elemental mercury, and elemental mercury was emitted with flue gas from stack. More fraction of vapor Hg0 at stack was emitted than at the FGD inlet. Oxidized mercury percentages in total mercury were different at the FGD inlet when SCR was in service and bypassed, and how much oxidized mercury was converted into elemental mercury depended on the operating parameters of wet FGD, then elemental mercury reemissions were evaluated at the four units respectively. As shown in Fig.4(a), in the absence of the SCR system, elemental mercury increased through wet FGD systems, and elemental mercury concentrations were 0.86 and 1.21 µg/m3 at the FGD inlet and stack of Unit 2 6.02 and 9.62 µg/m3 at Unit 4. Elemental mercury reemission took place at Unit 2 and Unit 4, and Hg0 reemission rates at both two units were about 40% and 60%, respectively. No Hg0 reemission was detected at Units 1 and 3. As shown in Fig.4(b), When SCR systems were in service, there were higher elemental reemission rates of 107% and 257% across wet FGD systems at Units 2 and 4. No elemental mercury reemission also occurred across wet FGD systems at Units 1 and 3.

Fig.4

Mercury reemission through wet FGD without SCR and with SCR

Some ions exist that have a reduction capacity to reduce Hg2+ to Hg0 existed in the slurry, and Hg2+ initially absorbed in wet FGD slurry could react with sulfite to produce Hg0, therefore, sulfite ion was found to be a major driver for chemical reduction of oxidized mercury to elemental mercury that can result in reemission of elemental mercury from the wet FGD absorber. The reduction reaction of Hg2+ by sulfite ion such as HSO3- in weak acid condition was proposed as follows: HSO3− + H 2 O + Hg 2 + → Hg 0 + SO 24 − + 3H + . (1) Sulfite ion was from the absorption of SO2 in the absorber and its concentrations were related to absorbent type, liquid to gas ratio (L/G) in the scrubber, SO2 concentration at the FGD inlet and oxidation modes, etc. Some trace metal ions such as Cr3+, Fe2+, Pb2+, Sn2+, Ni2+, Mn2+ were also suggested to reduce Hg2+ into Hg0 in the wet FGD systems. These trace metal ion sources in the slurry were most likely from the limestone, makeup water, abrasion of equipment, fly ash entrained to the scrubber and volatile elementals in coal-fired flue gas. A general reaction for the reduction of Hg2+ by dissolved metal ions (divalent ion as example) was proposed: 2Me2 + + Hg 2 + → Hg 0 + 2Me3+ . (2) 2+ Redox reaction of Hg and trace metal ions occurred at wet FGD systems, and Hg2+ was reduced into Hg0, and divalent metal ions were oxidized into high


valence metal ions.

3

Conclusions

Mercury measurements were conducted through four full-scale wet FGD systems in the non-ozone and ozone season. Comparison of average mercury speciation and mercury removal efficiency was evaluated at the FGD inlet and stack by OHM sampling. (1) Both SCR systems and chloride content in coal combusted at power plants can promote oxidation of elemental mercury. When SCR systems were bypassed at non-ozone season, chloride mainly affected oxidation efficiency of elemental mercury. Oxidized mercury in total mercury at the FGD inlet at Unit 1 was lower than 31%, while there were high percentages of oxidized mercury from 63%~87% at Units 2 and 3. In the presence of SCR systems, oxidization efficiencies were different from SCR systems of the four units, and oxidized mercury fraction in the total mercury increased from 31.03% to 64.84% through SCR at Unit 1, 86.96% to 92.45% at Unit 2, 65.49% to 92.59% at Unit 3, and 63.66% to 85.73% at Unit 4, respectively. (2) Higher overall mercury removal efficiencies across wet FGD systems were attained at the units except Unit 4 when SCR was in service than they were without SCR. Total mercury removal efficiencies were from 30.89% to 77.39% through the four wet FGD systems without SCR, while they were from 37.84% to 89.47% across wet FGD with SCR. (3) Elemental mercury reemissions across wet FGD were observed at Units 2 and 4. Oxidized mercury was reduced to elemental mercury under the presence of sulfite ion and some transit metal ions with low valence in the wet FGD slurries. No Hg0 reemission was observed through wet FGD at Units 1 and 3. References Agarwal H, Stenge H G , Wu S, Fan Z, 2006. Effects of H2O, SO2, and NO on homogeneous Hg oxidation by Cl2. Energy & Fuels, 20(3): 1 068-1 075. Cao Y, Chen B, Wu J, Cui H, John S, Chen C K, Paul C, Pan W

87

P, 2007. Study of mercury oxidation by a selective catalytic reduction catalyst in a pilot-scale slipstream reactor at a utility boiler burning bituminous coal. Energy & Fuels, 21(1): 145-156. Cao Y, Duan Y F, Kellie S, Li L C, Xu W B, Riley J T, Pan W P, 2005. Impact of coal chlorine on mercury speciation and emission from a 100 MW utility boiler with cold-side electrostatic precipitators and low-nox burners. Energy & Fuels, 19(3): 842-854. Cao, Y, Gao, Z Y, Zhu, J S, Wang Q H, Huang Y J, Chiu C, Parker, B, Chu P, Pan W P, 2008. Impacts of halogen additions on mercury oxidation, in a slip-stream selective catalyst reduction (scr), reactor when burning sub-bituminous coal. Environment Science & Technology, 42(1): 256-261. Diaz-Somoano M, Unterberger S, Hein Klaus R G, 2007. Mercury emission control in coal-fired plants: The role of wet scrubbers. Fuel Processing Technology, 88: 259-263. Kellie S, Cao Y, Duan, Y F, Li LC, Chu P, Mehta A, Carty R, Riley J T, Pan W P, 2005. Factors affecting mercury speciation in a 100-MW coal-fired boiler with low-NOx burners. Energy & Fuels, 19(3): 800-806. Pavlish J H, Sondreal E A, Mann M D, Olson E S, Galbreath K C, Laudal D L, Benson S A, 2003. Status review of mercury control options for coal-fired power plants. Fuel Processing Technology, 82(2/3): 89-165. Renninger S A, Farthing G A, Ghorishi S B, Teets C, Neureuter J A, 2004. Effect of SCR catalyst, ammonia injection and sodium Hydrosulfide on speciation and removal of mercury within a forced-oxidized limestone scrubber. In: Combined power plant air pollution control mega symposium, Washington DC: Department of Energy, BR-1754: 1-10. Romero C E, Li Y, Bilirgen H, Sarunac N, Levy E K, 2006. Modification of boiler operating conditions for mercury emissions reductions in coal-fired utility boilers. Fuel, 85(2): 204-212. Yang H M, Pan W P, 2007. Transformation of mercury speciation through the SCR system in power plants. Journal of Environmental Science, 19(2): 181-184. Zhou J S, Luo Z Y, Hu C, Cen K F, 2007. Factors impating gaseous mercury speciation in post-combustion. Energy & Fuels, 21(2): 491-495.