Effect of Coordinated Air-Pollution Control Devices (APCD) on Trace

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Effect of Coordinated Air-Pollution Control Devices (APCD) on Trace Elements Emissions in Ultra-low Emission Coal-Fired Power Plant Jiawei Wang, Yongsheng Zhang, Zhao Liu, Yongzheng Gu, Pauline Norris, Hong Xu, and Wei-Ping Pan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03549 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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Energy & Fuels

Co-effect of Air Pollution Control Devices (APCD) on Trace

1 2

Element Emissions in an Ultra-Low Emission Coal-Fired Power

3

Plant

4

Jiawei Wang a, Yongsheng Zhang a,1, Zhao Liu a, Yongzheng Gu a, Pauline Norris b,

5

Hong Xu a, Wei-Ping Pan a, b

6

a. Key Laboratory of Condition Monitoring and Control for Power Plant Equipment,

7

Ministry of Education, North China Electric Power University, Beijing, 102206, P.R.

8

China;

9

b. Institute of Combustion Science and Environmental Technology, Western

10

Kentucky University, Bowling Green, KY 42101, USA

11

Abstract: In this study, distribution, partitioning concentration, balance rate, and

12

removal efficiency of trace elements (TEs) in an ultra-low emission coal-fired electric

13

power plant (EPP) were investigated and evaluated. Results include determination of

14

the TEs in flue gas, coal, fly ash, bottom ash, gypsum, limestone, makeup water, wet

15

flue gas desulfurization (WFGD) wastewater and wet electrostatic precipitator (WESP)

16

wastewater. The results indicate that most of the TEs are distributed in the fly ash, with

17

a smaller portion in the bottom ash. The concentration of TEs in the flue gas gradually

18

decreases as the gas moves through the air pollution control devices (APCDs). Only a

19

small amount of TEs are emitted from the stack (Be: 0.039 µg/m3, Cd: 0.019 µg/m3,

20

Cr: 2.229 µg/m3, Ni: 0.350 µg/m3, Pb: 0.194 µg/m3, Sb: 0.017 µg/m3, Se: 0.307 µg/m3

1

Corresponding author E-mail: [email protected] 1

ACS Paragon Plus Environment

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1

and U: 0.029 µg/m3). The Se concentration in the liquid portion was high and thus

2

significantly contributed to the material balance. The low temperature economizer-

3

electrostatic precipitator (LTE-ESP) removal efficiencies for Pb, Sb and U are between

4

84~96%, but the removal efficiency of the LTE-ESP unit for Be is only 42.17%. The

5

WFGD removal efficiencies for Ni and Se are 69.39% and 82.12%, respectively. The

6

WFGD removal efficiencies for the other TEs are less than 33.33%. Both WFGD and

7

WESP contribute towards removing TEs. The APCDs were able to remove 99.97%,

8

99.82%, 99.86%, 99.35%, 99.98%, 99.96%, 99.48% and 99.97% of the Be, Cd, Cr, Ni,

9

Pb, Sb, Se and U, respectively, from the flue gas stream. The application of LTE-ESP

10

+ WFGD + WESP can effectively control the emission of TEs in an ultra-low emission

11

plant.

12 13

Key words: Trace elements; Distribution; Ultra-low emissions; Removal efficiency;

14

Coal-fired Power plant

15 16

1. Introduction

17

Trace elements (TEs), such as Be, Cd, Cr, Pb, Ni and Se, are a group of metal and

18

metalloid elements that have acute and chronic toxicity. Long-term absorption of TEs

19

can have a significant toxicological impact on the environment and on human health 1-

20

6.

21

In China, power plants use large amounts of coal. The power industry accounts for

22

about 46% of annual coal consumption 9. In 2010, coal consumption by coal-fired

Coal combustion industries are the main source of anthropogenic TEs emissions 7, 8.

2


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power plants was about 1.591x109 t. In China, total emissions of Cd, Cr, Ni, Pb, Sb and

2

Se are estimated at 13.34 t, 505.03 t, 446.42 t, 705.45 t, 82.33 t, 459.40 t, respectively

3

10.

4

Air pollution control devices (APCDs) are now widely used in power plants.

5

Traditionally, selective catalytic reduction (SCR) systems, electrostatic precipitators

6

(ESPs) and wet flue gas desulfurization (WFGD) systems are used for controlling

7

nitrogen oxide (NOX) emissions, particulate matter (PM) and sulfur dioxide (SO2),

8

respectively. APCDs also have a synergistic removal effect on TEs 11-18 .

9

Previous work has measured the TE concentrations at power plants. Meij et al. 19

10

concluded that most of the TEs can be captured along with small fly ash particles in the

11

ESP and FGD. Guo et al. 5, 19-21 concluded that the concentration of TEs will increase

12

with lower flue gas temperatures or reduced fly ash particle size at a 300MW coal-fired

13

plant with ESP. Cheng’s results 11 indicated that the ESP could remove up to 97% of

14

available arsenic at a 795MWe coal-fired plant with SCR, ESP and FGD. Linak and

15

Wendt

16

coal combustion. TEs could exist in the vapor phase on sub-micrometer and super-

17

micrometer particles

18

micrometer particles and the WFGD could capture a portion of the super-micrometer

19

particles at nine coal-fired plants in China and the US. The remaining portion would be

20

released into the atmosphere 15.

22

suggested several possible mechanisms that govern the fate of TEs during

22.

Wang’s results

15

indicated that the ESP could capture sub-

21

China uses a large amount of coal for power generation. China has decreased its

22

emission requirements for PM, SO2 and NOx, from 30, 100 and 100 to 10, 35 and 3


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1

50mg/m3, respectively. These new standards are called ultra-low emissions (ULE)

2

standards and have been widely implemented 23-26. In order to reach ULE limits for PM,

3

SO2 and NOx pollutants et al., some new technologies have been installed in coal-fired

4

power plants, such as low temperature economizers (LTE) and wet electrostatic

5

precipitators (WESPs)

6

distribution and emissions has not been sufficiently studied.

7

23-27.

However, the impact of ULE technologies on TEs

LTEs can reduce the flue gas temperature and improve the particulate matter 28-32.

8

removal efficiency of the ESP

The LTE should also improve the TE removal

9

efficiency, since TEs are enriched on particulate matter

33-36.

WESPs are especially

10

good at removing fine particles 37. The concentration of TEs on fine particles is even

11

greater

12

available on TE concentrations in ultra-low emission power plants 12-15, 33, 34. However,

13

the effects of LTE and WESP on TE removal have not been sufficiently studied.

38, 39,

so WESP should also promote the removal of TEs. There is some data

14

TE concentrations in coal-fired power plants are very low. Low concentrations are

15

more difficult to measure accurately. Although several previous studies have included

16

mass balance calculations

17

considered. The inflow and outflow of liquid in WFGD and WESP have an impact on

18

TEs, but TE concentrations in the WFGD and WESP wastewater and the potential

19

influence on the TEs mass balance are unknown.

11, 15, 16, 40,

the portion of TEs in the liquid is usually not

20

This work studies the impact of APCDs (especially LTE and WESP) on TEs

21

partitioning and removal. Balance rate, distribution, partitioning concentration, and

22

removal efficiency were considered. The mass balance study includes the 4


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concentrations of TEs in the flue gas, coal, fly ash, bottom ash, gypsum and wastewater.

2

The purpose of this study is to determine the distribution and emission of TEs, and to

3

better understand the effect of different APCDs on TEs. This work should provide

4

fundamental data useful for controlling TE emissions in ULE power plants.

5

2. Materials and methods

6

Samples of coal, flue gas, fly ash, slag, limestone, gypsum, makeup water, WFGD

7

wastewater, WESP freshwater and WESP wastewater were collected from a 300MW

8

ULE bituminous coal-fired power plant located in north China. The removal efficiency

9

of NOx, SO2 and particulate matter (PM) is 80%, 98% and 99.97%, respectively, in the

10

power plant. The emission concentration of NOx, SO2 and PM is 17, 10 and 0.42 mg/m3,

11

respectively. Figure 1 and table 1 describe the APCDs for the power plant. This power

12

plant is equipped with a LTE to decrease the temperature of the flue gas to 90℃ before

13

entering the ESP. A WESP is used to improve the removal efficiency of fine PM.

14

15 16 17 18

Figure 1. Air-pollution control devices and mass balance range Table 1. Electric power plant data Units Capacity

Boiler

Fuel type

NOx

Particulate

control

Control

5


SO2 control

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Low NOx 300MW

PC

Bituminous coal

burner, SCR

Page 6 of 25

LTE, ESP, WESP

WFGD

1

Flue gas samples from the inlet and outlet of the APCDs were collected at the same

2

time. Flue gases were sampled according to EPA Method 29 20. The Ontario isokinetic

3

sampling equipment (APEX instruments) includes a sampling probe, filter, filter tank,

4

impingers and the master control box. The sampling process is shown in Figure 2.

5

During sampling, the flue gas passes through the sampling probe, filter and the

6

impingers. The PM and TEs in the flue gas are collected by filter and acidified H2O2 in

7

the impingers. There was an insufficient amount of PM after the WFGD and WESP in

8

the flue gas, so the TEs in PM were not measured. The O2 concentration and

9

temperature in the flue gas at the five sampling locations are listed in Table 2. To

10

facilitate comparison with other researchers, the TE content was unified to a 6% O2

11

concentration in the flue gas at relevant sampling locations.

12

TEs in the samples were measured by Beijing Building Materials Testing Academy

13

Co., Ltd. The analyses of TEs in the coal, fly ash, slag, limestone and gypsum were

14

performed following Chinese method HJ 766-2015 solid waste–determination of

15

metals–inductively coupled plasma mass spectrometry (ICP-MS) (NexION 300X,

16

Perkin Elmer Company). Flue gas samples were analyzed using ICP-MS, according to

17

the sample pretreatment and analytical guidelines of EPA’s Method 29. Coal analyses

18

were performed following standard ASTM guidelines (moisture, ash and volatile

19

matter-ASTM D-5142-02a; sulfur-ASTM D-4239-02a). Fly ash major oxides and

20

minor elements were analyzed by inductively coupled plasma atomic emission

21

spectrometry (ICP-AES) (Prodigy, Leeman Laboratories Company), following 6


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Energy & Fuels

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standard ASTM D6349-01. Detailed analytical procedures and methods are listed in the

2

references 15.

3

Table 2. O2 concentration and temperature at the flue gas sampling location Location

Outlet of

Outlet of

Outlet of

ESP

WFGD

WESP

4.8

5.4

5.7

5.8

93

92

47

45

Inlet of LTE

Inlet of ESP

O2 (%)　

4.5

Temp (℃)

121

4

5 6

Figure 2. Schematic of the EPA 29 sampling train

7

3. Results and discussion

8

3.1 Characterization of coal

9 10

Proximate and ultimate analysis, fly ash composition and TEs concentration in the coal and limestone are shown in Tables 3-5:

11 12

Table 3. Ultimate/Proximate analysis of coal samples (Dry Basis) Proximate analysis (%)

13 14

Ultimate analysis (%)

V

A

FC

C

H

O

N

S

29.31

18.48

52.21

63.43

4.21

12.24

0.89

0.75

Table 4. Fly ash Major and Minor composition analyses (%) 7


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Al2O3 18.87

CaO 12.52

1 2

Fe2O3 7.10

Page 8 of 25

Major and Minor analyses Na2O MgO P2O5 SiO2 2.18 0.76 0.62 48.53

K2O 2.34

TiO2 0.81

BaO 0.07

MnO 0.13

SrO 0.65

Table 5. TEs concentration in the coal and limestone sample (µg/g) Be

Cd

Cr

Ni

Pb

Sb

Se

U

Coal sample

1.23

0.10

14.50

8.65

13.61

0.40

0.46

0.99

Limestone

0.18

0.50

9.81

19.12

2.62

0.34

0.18

0.22

China

41

2.11

0.25

15.40

15.00

15.10

0.84

2.47

2.43

World

42

1.60

0.22

16.00

13.00

7.80

0.92

1.30

2.40

3 4

As shown in Table 3, the coal has high volatility (29.31%), moderate ash content

5

(18.48%) and relatively low sulfur content (0.75%). As shown in Table 4, Al, Ca, and

6

Fe concentrations are 18.87%, 12.52% and 7.10%, respectively. As shown in Table 5,

7

Cr, Ni, and Pb concentrations in the coal are 14.5, 8.65 and 13.61 µg/g, respectively.

8

The Be, Cd, Sb, Se and U concentrations are 1.23, 0.10, 0.40, 0.46, and 0.99 µg/g,

9

respectively. The TE concentrations are less than the average level in China. The Pb

10

concentration is higher than the world average. The Cr, Ni and Pb concentrations are

11

relatively higher than the Be, Cd, Sb, Se and U concentrations. The differences in TE

12

concentrations can be ascribed to factors such as the coal formation process,

13

surrounding conditions, coal type, etc. 17. The Ni, Cd, Cr, Pb and Sb concentrations are

14

relatively high in the limestone.

15

3.2 TEs Emissions

16

Gas phase Cr, Ni, Pb, Se, Be, Cd, Sb, and U concentrations at different sampling

17

locations are shown in Figure 3. ALT represents after the LTE, BESP represents before

18

the ESP, AESP represents after the ESP, AFGD represents after the FGD, and AWESP

19

represents after the WESP. The concentrations of the trace elements in the flue gas

20

gradually decrease as the flue gas passes through the air pollution control devices, with 8


Page 9 of 25

1

the TE concentrations at the stack being very low. The concentrations of Cr, Ni, Pb and

2

Se are relatively high. The concentrations of Cr, Ni and Pb are 7 to 145 times higher

3

than the other TEs. The concentrations of Be, Cd, Sb and U are relatively low. The Se

4

concentration in the coal is similar to the Be, Cd, Sb and U concentrations, but Se is a

5

volatile element that easily escapes into the flue gas, so the Se concentration in the flue

6

gas is relatively high. The concentrations of Cr, Ni, Pb and Cd decreased significantly

7

in the electrostatic precipitator. The concentrations of Be, Se, Sb and U decreased

8

significantly after the LTE. This indicates that the temperature has a greater impact on

9

Be, Se, Sb and U. The lower temperature could promote the absorption of these

10

elements onto the fly ash, resulting in a reduction in the flue gas concentration. Only a

11

small portion of TEs are emitted from the stack (Be: 0.039, Cd: 0.019, Cr: 2.229, Ni:

12

0.35, Pb: 0.194, Sb: 0.017, Se: 0.307, U: 0.029 µg/m3, respectively).

13

The APCDs in the power plant were able to remove 99.97%, 99.82%, 99.86%,

14

99.35%, 99.98%, 99.96%, 99.48% and 99.97% of the Be, Cd, Cr, Ni, Pb, Sb, Se and U,

15

respectively, from the flue gas stream. This indicates that ultra-low emissions retrofits

16

have a strong ability to capture TEs. 14

18 19

8 6 4 2

ALT

BESP

AESP

AFGD

Be Cd Sb U

3

TE Flue gas Concentrations ( ug/m )

3

10

0

17

0.6

Cr Ni Pb Se

12 TE Flue gas Concentrations (ug/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.4

0.2

0.0

AWESP

ALT

BESP

AESP

AFGD

Sampling location

Sampling location

(a) Cr, Ni, Pb and Se

(b) Be, Cd, Sb and U

Figure 3. Concentration of TEs at different sampling locations 9


AWESP

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1

TE concentrations in the coal combustion byproducts are listed in Table 6. Most of

2

the TEs in the coal were retained in the fly ash. The Be, Cd, Cr, Ni, Pb, Sb, Se and U

3

concentrations in the fly ash are 4~6 times higher than the coal concentrations. The Be,

4

Cd, Cr, Ni, and U concentrations in the slag are 3~5 times higher than the coal

5

concentrations, while the Pb, Sb and Se concentrations in the slag are at the same level

6

as the coal concentrations. The eight TEs studied in this work are easily volatilized

7

during coal combustion. Following combustion, they mostly accumulate in the fly ash

8

and, to a lesser extent, in the bottom ash.

9

The concentrations of Be, Cd, Ni and U in the fly ash are similar to the

10

concentrations in the bottom ash. The concentrations of Cr, Ni, and Pb in the limestone

11

and gypsum are relatively high at this power plant.

12

In China, there are no concentration limits on TEs in solid and liquid coal

13

combustion by-products. TE concentration limits for soil and groundwater are listed in

14

Table 6 13, 43, 44. For the solid combustion by-products emitted from the power plant, the

15

Cr, Ni and Pb concentrations are lower than the environmental quality protection

16

standard limit, although the Cd concentration is not. The data indicates that most of the

17

TEs in the fly ash, slag and gypsum materials may not have a significant impact on the

18

soil. However, solid samples containing Cd might be of greater concern. Fly ash is

19

mainly used for building materials, road construction, agriculture and mineral

20

extraction in China. Previous results show that TEs are relatively stable in fly ash and

21

have little impact on the environment, so the TEs should be retained in the fly ash as

22

much as possible 12, 40. The concentrations of Cd, Cr and Pb in the WFGD wastewater 10


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Energy & Fuels

1

are below the standard limit. However, the concentrations of Be, Ni and Se in the

2

WFGD wastewater are almost 15~70 times higher than the groundwater concentration

3

limit. Thus, the emphasis should be on the treatment of desulfurization wastewater.

4

Liquid streams are treated by different waste water treatment technologies, such as

5

sedimentation tank, chemical precipitation, evaporation tank and flue evaporation.

6

These treatment technologies can help desulphurization wastewater discharge meet the

7

waste water release standard.

8

The Cd, Cr, Ni and Pb concentrations in the WESP wastewater are lower than the

9

standard limit. However, the concentrations of Be and Se in the WESP wastewater are

10

slightly higher than the standard. Thus, Be, Se, and Ni should be given special attention

11

during the wastewater treatment process.

12 13

Table 6. TE concentrations in coal combustion byproducts Solid samples (µg/g) Element

Fly ash

Gypsum

References standards

WFGD

WESP

Groundwater 43

Soil 44

wastewater

wastewater

(μg/L)

(µg/g)

Be

6.55

4.65

0.18

1.43

0.10

0.02

n.g.

Cd

0.50

0.42

0.34

5.50

0.10

10

0.30

Cr

76.9

52.08

8.94

10.53

1.79

50

200

Ni

37.11

34.31

25.63

773.09

30.79

50

50

Pb

69.18

16.13

3.19

5.76

0.14

50

300

Sb

2.16

0.63

0.97

1.26

0.59

n.g.

n.g.

Se

2.13

0.16

0.74

342.62

12.52

10

n.g.

U

5.27

4.39

0.32

3.88

1.22

n.g.

n.g.

an.g.

14

Slag

Liquid samples (μg/L)

is short for “not given”.

3.3 Balance of trace elements 11


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1

The measurement of TEs is more complicated than conventional air pollutants. The

2

accuracy and reliability of TEs measurement is typically evaluated by mass balance 11,

3

45.

4

and liquid. Balance of TEs is calculated using the following formula 11:

A general equilibrium constant, RI, is calculated by the measurement of gas, solid

5 6

RI =

𝑀𝑜𝑢𝑡

(1)

𝑀𝑖𝑛

where RI represents the recovery factor for TEs. A RI value of 1 indicates a perfect 21,

7

mass balance. An acceptable error for sampling is 70~130%

8

repeatability and precision of sampling and analysis. Min stands for the overall input of

9

TEs in coal (Mcoal). Mout stands for the overall output of TEs in flue gas (Mflue), Slag

10

in view of the

(Mslag), precipitator ash (Mfa), WFGD (MWFGD) and WESP (MWESP).

11

Min = Mcoal

(2)

12

Mout = Mflue + Mslag + Mfa + MWFGD + MWESP

(3)

13

where

14

MWFGD = Mgy + MWFGDW - Mlim - Mmw

(4)

15

MWESP = Mww - Mwf

(5)

16

Mgy represents the TEs content in the desulfurization gypsum. MWFGDW represents

17

the TEs content in WFGD wastewater. Mlim represents the TEs content in the limestone.

18

Mmw represents the TEs content in the makeup water. Mww represents the TEs content

19

in the WESP wastewater. Mwf represents the TEs content in the WESP freshwater.

20

TEs are released into the flue gas during combustion and are present in the form of

21

vapor and particulate phases, which can be ascribed to physicochemical interactions of

22

TEs with fly ash 2. Mass balance results (RI) for the TEs have been calculated and are 12


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shown in Table 7. RI is calculated with solid, liquid and gas. RI ＇is calculated with

2

solid and gas. The overall mass balance for TEs is about 80~108%. The calculation

3

using the solid, liquid and gas provided a better result (108%). This is due to the high

4

concentration of Se in the liquid portion which came from WFGD waste water. Thus,

5

the material balance must include contributions from solid, gas and liquid. TE

6

distributions in the power plant are shown in Figure 4. Most of the TEs are retained by

7

the fly ash, with a smaller portion in the bottom ash. Of the eight elements studied, Pb

8

concentrations are greatest in the fly ash and Se concentrations are the lowest in the fly

9

ash and the bottom ash. However, the content of Se is relatively high in the gypsum,

10

the WFGD wastewater, the WESP wastewater, and the flue gas. The U concentrations

11

in the bottom ash are the largest (8.43%), followed by Ni (8.26%) and Cd (8.19%). The

12

U concentrations in the water are low, especially in the WFGD wastewater (0.16%).

13

The Be content in the gypsum is the lowest (0.61%), followed by Pb (1.07%) and U

14

(1.31%).

15

Table 7. Mass balance of TEs in flue gas Element　

Be

Cd

Cr

Ni

Pb

Sb

Se

U

RI (%)

92

91

92

85

83

96

108

93

92

89

92

82

84

96

80

93

RI＇ (%)

16

13


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Page 14 of 25

1 2 3

Figure 4. Distribution of TEs in the power plant

3.4 The effect of LTE-ESP, WFGD and WESP on TEs removal

4

Coal-fired power plants are equipped with different APCDs. The devices effect the

5

transformation and removal of TEs. The TE removal efficiencies for LTE-ESP, WFGD,

6

and WESP are calculated using the formulas below:

7

η LTE = (C in, LTE -C out, LTE) / C in, LTE x 100%

8

η LTE-ESP = (C in, LTE-ESP -C out, LTE-ESP) / C in, LTE-ESP x 100%

(7)

9

η WFGD = (C in, WFGD-C out, WFGD) / C in, WFGD x 100%

(8)

η WESP = (C in, WESP-C out, WESP) / C in, WESP x 100%

(9)

10 11

(6)

ηLTE, ηLTE-ESP, ηWFGD, ηWESP is the % removal efficiency of the LTE, LTE-ESP, in, LTE,

C

in, LTE-ESP,

C

in, WFGD,

WFGD, and WESP. C

13

concentration at the flue gas inlet from the LTE, LTE-ESP, WFGD and WESP,

14

respectively. C out, LTE, C out, LTE-ESP, C out, WFGD, C out, WESP is the μg/m3 TE concentration

15

at the flue gas outlet from the LTE, LTE-ESP, WFGD and WESP, respectively. 14


C

is the μg/m3 TE

12

in, WESP

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

3.4.1 The effect of LTE-ESP on TEs removal

2

The inlet and outlet flue gas samples at the LTE-ESP were collected and analyzed

3

to investigate the effect of the LTE-ESP on the removal of TEs. The TE removal

4

efficiencies were calculated and are shown in Figure 5. LTE-ESP removal efficiencies

5

for Cd, Cr, Ni, Pb, Sb and U are quite high, between 68~96%. The Pb removal

6

efficiency was 96%. The LTE-ESP removal efficiencies for Be and Se are low, only

7

42.17% and 51.45%, respectively. The LTE removal efficiencies for Be, Sb, Se and U

8

are quite high, 36.14%, 41.79%, 30.98% and 54.86%, respectively. This indicates the

9

importance of temperature on Be, Se, Sb and U removal. Although the LTE-ESP

10

removal efficiencies for Cd, Cr, Ni and Pb are quite high, the corresponding LTE

11

removal efficiencies are low, 0.83%, 6.45%, 1.32% and 6.89%, respectively. The high

12

frequency electric source in the ESP could be encouraging the TEs to combine with the

13

fly ash, which is then removed by the ESP. The inlet temperature of the LTE is about

14

120℃, and the LTE reduces the temperature of the flue gas at the ESP entrance to about

15

90℃. The melting points for Be, Sb, Se and U containing compounds, such as Be(PO4)3

16

﹒ 3H2O, Sb2S5, SbBr3, SeO3, SeBr4, UO4 ·H2O and UO2(NO3)2 ﹒ 6H2O etc., are

17

between 96.6℃ and 135℃. Thus, the lower temperature in the LTE is related to the

18

decreased concentration of these elements in the gas phase. The major Se and Cr species

19

present in the fly ash samples were Se4+ and Cr6+. The major Sb species present in the

20

fly ash was Sb4+, or Sb3+ at temperatures below 177℃, and the mode of occurrence of

21

Sb is Sb2(SO4)3 (s)3. Clemens 46 reported that SeO2 will be trapped in the solid phases

22

at temperatures around 125℃. Se was mainly found to be selenite with a minor amount 15


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Page 16 of 25

1

of selenate 47. It’s noteworthy that the oxidizable fraction was relatively higher in fly

2

ash, which validates the theory that the selenium might be expected to condense as SeO2

3

at temperatures below 200°C

4

absorbed on the fly ash surface in the convective section by chemical reaction. Se tends

5

to react with iron oxides and calcium oxides in the fly ash 48, 49.

12.

Se has higher volatility. Gaseous oxides of Se are

6

The decreased temperature reduces the resistivity of the fly ash and decreases the

7

flow of the flue gas. This results in an increase in the particulate removal efficiency of

8

the ESP. At the same time, the gas temperature in the LTE may help TEs to condense

9

on PM, which are then captured by the ESP system. Many researchers 3, 50-55 report that

10

decreasing flue-gas temperatures and particle sizes correspond to higher TE

11

concentrations in the fly ash. A large portion of TEs are in the condensed form when

12

using a LTE-ESP, which makes them easier to remove. Thus, the ESP not only removes

13

particulate matter, but also removes a large amount of TEs in power plants. Highly

14

volatile elements, such as Se, are known to be concentrated in the submicron size fly

15

ash particles 56. These elements are captured by the ESP with lower efficiency than the

16

other TEs. Excluding Se, the remaining TEs studied in this work were mostly in the fly

17

ash. Pb and Sb are mostly removed by the ESP which indicates that the majority of the

18

Pb and Sb are associated with the particulate. Deng

19

vaporized at high combustion temperatures and subsequently condenses or adsorbs onto

20

the fly ash. Janoito 55 reported that Sb, Be, Cd, Cr, Pb, Ni and Se were present mainly

21

as Sb2O5, BeH4O6S, BeH8O8S, CdSO4, CdCl2, Cr2O3, PbSO4, NiSO4 and SeO2 in the

51

reported that most of the Pb is

16


Page 17 of 25

1

140−30 °C temperature range. Cd may exist as Cd, Cd(OH)2 and CdSO4 in oxidizing

2

conditions 57. Cr6+ and Cr3+ were the main valence states of Cr in ash 58.

ESP LTE

100

Capturing Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

80

29.55

49.57

60

40

89.84 6.02

68.33

62.4

20.47

77.27

54.86

20

41.79

36.14 0.83

0

Be

Cd

6.45

1.32

Cr

Ni

30.98

6.89

Pb

Sb

Se

U

Element

3 4 5

Figure 5. LTE-ESP TEs capturing efficiency

3.4.2. The effect of the WFGD system on TEs removal

6

Flue gas samples at the inlet and outlet of the WFGD system were collected and

7

analyzed to evaluate the TE removal efficiency of the WFGD system. The TE removal

8

efficiencies for the WFGD were calculated and are shown in Figure 6.

9

The removal efficiencies for Be, Cd, Cr, Pb, Sb and U were 12.50%, 18.92%,

10

26.17%, 21.89%, 33.33% and 23.38%, respectively. The removal efficiencies of Ni and

11

Se were 69.39% and 82.12%, respectively. The higher removal efficiencies for Ni and

12

Se could be explained by the elements being present in the form of a chloride, which

13

would be more soluble in water.

14

Cd is usually present as Cd2+ or Cd1+. Some researchers 34 have concluded that Cd

15

is mainly in the form of soluble compounds in the WFGD. Cd2+ can react with alkaline 17


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Page 18 of 25

1

substances, such as Ca(OH)2, to produce cadmium hydroxide (Cd(OH)2). Thus, WFGD

2

can move the Cd(OH)2 into the slurry.

3

Cr in the flue gas is mostly present as Cr2O3 and CrO3

13.

The WFGD doesn’t

4

remove much Cr from the flue gas. Xu et al. 54 concluded that most regulated TEs are

5

concentrated in submicron particles. Since the TEs bound to particulates are insoluble

6

in the WFGD system, they are prone to integrate into the slurry. In WFGD, most of the

7

Pb is present in the gypsum slurry and only a small portion is present in the flue gas 12.

8

Pb in the flue gas (mainly Pb2+) can dissolve in water or acid. Thus, the wet limestone-

9

gypsum method has a synergistic removal effect on the Pb in the flue gas.

10

In the environment, four species of selenium are possible, such as selenide (Se2-),

11

selenium (Se0), selenite (Se4+) and selenate (Se6+). Most of the available Se is removed

12

by the WFGD system and is present in the WFGD gypsum slurry. Se in the WFGD

13

gypsum and wastewater is primarily in the form of Se6+ and Se4+

14

chemically reacts with water to produce H2SeO3, and then combines with Ca2+ to

15

produce CaSeO3 in the WFGD slurry.

59.

Gaseous SeO2

16

The removal efficiency for U is 23.38% in the WFGD, indicating that using WFGD

17

in coal fired power plants might reduce the emission of U. The re-circulation of filtered

18

water increases the concentration of U and Se in WFGD water. U may chemically react

19

with carbonates to generate soluble uranyl complexes which may promote the presence

20

of U in the gypsum slurry.

18


Page 19 of 25

100 82.12

90 69.39

80 Capturing Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

70 60 50 21.89

18.92

30 20

33.33

26.17

40

23.38

12.5

10 0

Be

Cd

Cr

Ni

Pb

Sb

Se

U

Element

1 2 3

Figure 6. WFGD TEs capturing efficiency

3.4.3. The effect of WESP on TEs removal

4

Flue gas samples at the WESP inlet and outlet were collected and analyzed to

5

evaluate the TE removal efficiency of the WESP. The TE removal efficiencies for the

6

WESP were calculated and are provided in Figure 7. The Be, Cd, Cr, Ni, Pb, Sb, Se and

7

U removal efficiencies for the WESP are 7.14%, 36.67%, 9.39%, 36.48%, 32.87%,

8

15.00%, 23.06% and 50.85%, respectively. The removal efficiencies of Cd, Ni, Pb and

9

U are relatively higher than the others. TEs are known to accumulate on fine particles.

10

It’s win-win for the WESP system to capture TEs, however, the proportion of TEs is

11

quite low in the total mass balance. The authors propose that the WESP removal

12

performance is closely associated to the size of the particle. Both WFGD and WESP

13

contribute towards removing TEs.

19


Energy & Fuels

60

50.85

Capturing Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

40

36.67

36.48 32.87

23.06 20

15 9.39

7.14 0

Be

Cd

Cr

Ni

Pb

Sb

Se

U

Element

1 2

Figure 7. WESP TEs capturing efficiency

3

In summary, different pollution control devices have certain effects on TEs in ULE

4

coal-fired power plants. The catalyst layer helps to reduce the NOx in the flue gas at the

5

ULE power plant and enhances the oxidation of TEs. TEs present in their oxidized

6

forms are easily captured by fly ash or dissolved in the water system. This effect is most

7

likely responsible for the increased TE removal efficiency of the LTE-ESP, WFGD and

8

WESP systems. As shown in figures 5 and 7, the capturing efficiency of Cd, Ni, Pb and

9

U was high, indicating that compounds containing these elements are not soluble in

10

water. Therefore, most of them are bound to particulate matter. As shown in figures 5

11

and 6, the capturing efficiency of Ni and Se is high, indicating that compounds

12

containing these elements exist in a form that is soluble in water. About 90% of the TEs

13

were present in the LTE-ESP, less than 9% of the TEs were in the slag, and less than

14

1% of the TEs were emitted to the atmosphere. The remaining portion was in the WFGD

15

and WESP. 20


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Energy & Fuels

1

4. Conclusions

2

The emission of TEs could be effectively controlled by the joint application of SCR

3

+ LLT-ESP + WFGD + WESP in ULE plants. Most TEs accumulate in the fly ash, with

4

a smaller portion in the bottom ash. The concentration of TEs in the flue gas decreases

5

gradually as the temperature decreases. A small portion of TEs are emitted into the

6

stack. The Pb, Sb and U removal efficiencies for the LTE-ESP unit are between

7

84~96%, but the Be removal efficiency is only 42.17%. Excluding Ni and Se, the

8

WFGD system removed less than 33.33% of TEs. The WFGD removal efficiencies for

9

Ni and Se are 69.39% and 82.12%, respectively. Both WFGD and WESP contribute

10

towards removing TEs. Overall, the APCDs in the power plant were able to remove

11

99.97%, 99.82%, 99.86%, 99.35%, 99.98%, 99.96%, 99.48% and 99.97% of the Be,

12

Cd, Cr, Ni, Pb, Sb, Se and U, respectively, from the flue gas stream.

13

5. Nomenclature

14

TEs = trace elements.

15

EPP = electric power plant.

16

WFGD = wet flue gas desulfurization.

17

WESP = wet electrostatic precipitator.

18

APCDs = air pollution control devices.

19

LTE-ESP = low temperature economizers-electrostatic precipitator.

20

SCR = selective catalytic reduction.

21

NOx = nitrogen oxide.

22

PM = particulate matter. 21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

SO2 = sulfur dioxide.

2

ULE = ultra-low emissions.

3

APH = air preheater.

4

ICP-MS = inductively coupled plasma mass spectrometry.

5

ICP-AES = inductively coupled plasma atomic emission spectrometry.

6

Acknowledgements

7

This work was supported by the National Key Research and Development Program

8

of China (No. 2016YFB0600205), National Natural Science Foundation of China

9

(51706069) and the Fundamental Research Funds for the Central Universities

10

(2017JQ002 and 2017MS014).

11

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Energy & Fuels

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