Migration Behavior of Trace Elements at a Coal-Fired Power Plant with

Dec 12, 2016 - School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, People's Republic of China. §Xi'an Gerui Pow...
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Migration Behavior of Trace Elements at a Coal-Fired Power Plant with Different Boiler Loads Shilin Zhao,† Yufeng Duan,*,† Chenping Wang,† Meng Liu,† Jianhong Lu,† Houzhang Tan,*,‡ Xuebin Wang,‡ and Lituo Wu§ †

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ‡ School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China § Xi’an Gerui Power Technology, Limited, Xi’an, Shaanxi 710065, People’s Republic of China ABSTRACT: Trace elements (TEs) emitted from coal-fired power plants have done great harm on the environment, which has caused widespread concern in the world. In this study, the effects of boiler loads (100 and 75% output) on the migration behaviors of TEs (including Cr, Mn, Co, Ni, Cu, Zn, As, Ba, and Pb) were conducted on a coal-fired power plant with selective catalytic reduction (SCR), electrostatic precipitator (ESP), wet flue gas desulfurization (WFGD), and wet electrostatic precipitator (WESP). Simultaneous sampling of coal, bottom and fly ash, flue gas, and the byproducts from WFGD and WESP was performed. It shows that mass balance rate of TEs at the two loads is in the acceptable range. TEs mainly distribute in bottom and ESP ash in the coal-fired power plant at the two loads. In comparison to the 100% load, the amount of TEs in bottom ash increases at the 75% load. The overall removal rates of TEs through the whole system at the two loads are higher than 99.9%. Except Co and Ba, the overall removal rate at the 100% load is more than that at the 75% load. The ability of the ESP to capture particulate TEs at the 75% load is lower than that at the 100% output, while it is adverse for the gaseous TEs, apart from As. WFGD and WESP have the synergistic effects to remove the total TEs. TEs may re-emit in the desulfurization process. There is little impact on the enrichment of TEs in ESP and bottom ash when the load changes. The concentration of TEs emitted to the atmosphere is in the range of 0.01−1.76 and 0.01−2.32 μg/m3 at the two loads in the gaseous form, which is extremely low. At last, the further work in the removal of TEs is proposed. micrometer and agglomerated ash or adhere to fly ash through homogeneous nucleation, condensation reaction, or adsorption. TEs that did not escape from the mineral substances or coke will be wrapped in the particles, which will be in the bottom ash or fly ash. At the end, TEs in coal will be emitted in gaseous and particulate states in the flue gas and some of them distribute in bottom ash. The partitioning mechanisms of TEs during coal combustion can be found in Figure 1.11 Recently, to control some air pollutants, such as NOx, particulate matter (PM), and SO2, emitted from power plants, more and more air pollution control devices (APCDs), such as selective catalytic reduction (SCR), electrostatic precipitator (ESP), wet flue gas desulfurization (WFGD), etc., have been installed.2 However, these APCDs have the synergistic effect of TE removal to some extent. To better reduce TE emission in the coal-fired power plant, understanding the distribution of the TEs across the whole system comprehensively is a very important step. Deng et al.12 investigated the emission characteristics of Cd, Pb, and Mn from coal combustion at six coal-fired power plants in China. Megalovasilis et al.13 researched the behavior of TEs in the pulverized lignite, bottom ash, and fly ash of Amyntaio power station, Greece. López-Anton et al.3 studied the distribution of TEs from a coal that burned in two different Spanish power stations. Bhangare et al.14 determined the TE

1. INTRODUCTION Trace element (TE, such as As, Cd, Mn, Pb, etc.) emission has caused serious damage to the public’s health and the environment.1−3 TEs can lead to the contamination of soil and water bodies and disease events. More than 30 severe poisoning cases related with TE pollution have occurred in China.4 Coal combustion from power stations is an important anthropogenic contributor for the TE emission.5 In China, about 45% of total coal consumed today is applied to the direct combustion by power plants.6 Although the content of TEs in coal is usually below 100 ppm,7 the large quantities of coal consumed will result in enormous emission of TEs.8 Therefore, some policies have been developed by some countries to reduce TE emission. In 1990, the Clean Air Act Amendments proposed by the United States Environmental Protection Agency (U.S. EPA) have stipulated that the potentially 11 kinds of toxic TEs produced from combustors and incinerators must be controlled.9 In February 2011, the State Council of China officially approved the 12th Five-Year Plan for comprehensive prevention and control of TEs.10 Relative emission standards are being developed for metal-based pollutants from the coalfired power plant.6 During coal combustion, the coal particles first go into pyrolysis and ignition. With the release of the volatile, the coke begins to burn and break into smaller particles. TEs that existed in the excluded minerals and the coke volatilize and then react with the surrounding environment to form inorganic vapors. As the flue gas cools, part of gaseous TEs transform into sub© 2016 American Chemical Society

Received: September 17, 2016 Revised: November 26, 2016 Published: December 12, 2016 747

DOI: 10.1021/acs.energyfuels.6b02393 Energy Fuels 2017, 31, 747−754

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Figure 1. Partitioning mechanisms of TEs during coal combustion.11

Table 1. Proximate and Elemental Analyses of the Coal Sample proximate analysis

LHV

elemental analysis

Mar (%)

Aar (%)

Var (%)

FCar (%)

Qar,net (MJ/kg)

Car (%)

Har (%)

Oar (%)

Nar (%)

Sar (%)

14.11

12.51

27.33

46.04

23.09

58.18

3.59

10.43

0.81

0.37

Table 2. Concentration of TE Contents in the Coal Sample (mg/kg) coal sample China17 world18 a

Cr

Mn

Co

Ni

Cu

Zn

As

Ba

Pb

12.8 15.4 16

147 116.2 nda

6.2 7 5.1

10.7 15 13

12.1 17.5 16

30.7 38 23

2.7 3.79 8.3

251.63 159 150

27.93 15.1 7.8

nd = not detected.

Figure 2. Simultaneous sampling locations in the tested power plant.19

concentration in the coal and ash samples collected from five different thermal power plants across India to know their distribution, enrichment, and partitioning behavior. However, the information about the concentration of TEs in the flue gas through the coal-fired power plants is not enough. In addition, there are few papers to study the effects of boiler loads on the TE distribution in the coal-fired power plant. In this study, a field test concerning the influence of boiler loads on the TE distribution was conducted on an ultralow emission coal-fired power plant, which was equipped with SCR,

ESP, WFGD, and wet electrostatic precipitator (WESP). Flue gas TE sampling was performed at the inlet or outlet of the each APCD simultaneously according to the U.S. EPA Method 29.15 Coal samples and combustion byproducts, such as bottom ash, ESP ash, flue gas desulfurization (FGD) gypsum, etc., were collected at the same time. The main contents include the following: (1) mass balance rate, (2) distribution of TEs in the coal-fired power plant, (3) removal efficiency of TEs across each APCD, (4) enrichment characteristics of TEs in ESP ash and bottom ash, (5) emission characteristics of TEs to the 748

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Energy & Fuels atmosphere, and (6) further work in the removal of TEs. The main purpose is to update or supplement the emission data of TEs in coal-fired power plants, which is expected to provide experimental and theoretical guidance for the TE control in coal-fired power plants.

Table 4. Operational Condition of the Power Plant during the TE Sampling mean produced electricity during the sampling hours load of the power plant input feed coal output fly ash output bottom ash limestone slurry density of the limestone slurry mass flow of the desulfurization wastewater mass fraction of the gypsum in desulfurization wastewater WESP fresh water WESP wastewater

2. EXPERIMENTAL SECTION 2.1. Utility Boiler. The tested ultralow emission coal-fired power plant is a 660 MW tangential-fired, pulverized coal boiler, which is installed with SCR capable of achieving a NOx conversion rate of about 80−84%, ESP used for PM removal, WFGD with SO2 removal efficiency of approximately 96−98%, and WESP to remove ultrafine particles or aerosols. The main component of the SCR catalyst used in the plant is V2O5−WO3/TiO2, which is placed in a high-dust way with a honeycomb form. During the TE sampling, four ESP fields are in operation. In addition, the load of the power plant was kept at 100 and 75%, respectively. Coal samples used at the two loads are the same, of which the proximate and elemental analyses are shown in Table 1. According to the National Coal Classification Standard of China (GB/ T 5751-2009), the coal sample belongs to bituminous coal. The concentration of TEs in the coal sample is shown in Table 2. The average content of TEs in Chinese and world coal is also listed in this table. It can be inferred that the average concentration of TEs, such as Co, Ni, Cu, Zn, Ba, and Pb, in Chinese coal is higher than that of the world coal, which indicates the difficulty and importance in controlling TEs for coal-fired power plants in China. Except Mn, Ba, and Pb, the content of the remaining TEs in the tested coal samples is lower than the average value of Chinese coal. In general, different geological conditions of the origin of coal are the reason for their different contents of TEs.16 2.2. Sampling Procedures and Analytical Method. The sample collection locations in the 660 MW coal-fired power plant are shown in Figure 2.19 TEs in the flue gas were sampled at the inlet/ outlet of the SCR, ESP, WFGD, and WESP simultaneously with a sampling duration of 2 h for the individual output, following the U.S. EPA Method 29.15 All of the detailed information about the sampling procedures and analytical method can be found in our previous work.19 It should be noted that the concentration of TEs in flue gas was unified to 6% O2 for comparison, which is based on the O2 content at the different sampling sites, the value of which is shown in Table 3.

100% output 75% output

inlet of SCR

outlet of SCR

outlet of ESP

outlet of WFGD

outlet of WESP

3.49

4.20

4.50

4.93

5.57

4.93

5.57

6.47

6.73

7.39

660

495

% t/h t/h t/h t/h kg/m3 t/h

100 254.70 29.05 3.06 4.67 1117.98 3.40

75 200.35 15.47 5.95 4.61 1116.95 3.20

%

28

28

t/h t/h

12.40 10.47

11.27 10.26

sample, the error in the sample analysis, etc., it is generally acceptable for the TE mass balance rate to be between 70 and 130%.12,21,22 In this field test, the concentration of TEs in coal combustion byproducts at the two loads is shown in Table 5. With the combination of the information of Tables 1−7, the mass balance of TEs across each device can be calculated in the acceptable range. It indicates the reliability and validity of the field test data. 3.2. Distribution of TEs in the Coal-Fired Power Plant. The relative distribution ratio of TEs at the 100 and 75% loads are shown in Figures 3 and 4, respectively. TEs emitted to the atmosphere include particulate and gaseous states. ESP, WFGD, and WESP have great removal effects on the particles in the flue gas, which results in that all of the particulate TEs are captured. Therefore, the particulate TEs emitted to the atmosphere are zero. The content of gaseous TEs emitted to the atmosphere and the TEs removed by WFGD and WESP is little relatively. Both Figures 3 and 4 indicate that TEs are mainly distributed in the bottom ash and ESP ash across the coal-fired power plant. The total amount of TEs in ESP ash is higher than that in bottom ash. According to the migration behavior of TEs during coal combustion, they can be divided into non-volatile elements, moderately volatile elements, and highly volatile elements.23 In the high furnace temperature, almost all of the highly volatile elements and moderately volatile elements and part of the nonvolatile elements escape in the gaseous state with coal burning. TEs not emitted will exist in the small particles from the burst of pulverized coal, which will exist in bottom ash and fly ash eventually. In this study, the site for the sampling of TEs in the flue gas emitted from the furnace is located at the inlet of SCR, the temperature of which is less than 400 °C. It provides good temperature and time conditions for the conversion of gaseous TEs to the particulate state. With the help of ESP, most fly ash will be captured. Therefore, TEs mainly distribute in bottom ash and ESP ash. The total amount of TEs in ESP ash being higher than that in bottom ash is mainly because bottom ash accounts for little fractions of the ash in the coal. In addition, the concentration of the studied TEs in the ESP ash is more than that in the bottom ash, except Mn, under the two loads. In comparison to the 100% load, the total amount of TEs distributed in the bottom ash increases relatively at the 75% load. This is because the combustion temperature is lower for the 75% load, which causes incomplete combustion of pulverized coal and increases the share of bottom ash.

Table 3. O2 Concentration in the Flue Gas at the Sampling Location (%) location

MW

2.3. Operational Conditions during the TE Sampling. During the TE sampling process, some relevant parameters can be obtained from the power plant, which are shown in Table 4. It provides the data for the mass balance calculation of the TEs.

3. RESULTS AND DISCUSSION 3.1. Mass Balance Rate. The mass balance rate can be acted as an effective way to evaluate the accuracy of the field test data,20 which is the ratio of output quality to that of the input per hour. For the whole system, the input is the feeding coal, while the output includes bottom ash, ESP ash, flue gas, and TEs removed by the WFGD and WESP. For the each device, the mass balance rate is the TEs entering the device divided by those leaving. As a result of fluctuations in power plant equipment operation, the representative of the selected 749

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Energy & Fuels Table 5. Concentration of TEs in the Coal Combustion Byproducts at the Two Loads 100% load

75% load

item

content

Cr

Mn

Co

Ni

Cu

Zn

As

Ba

Pb

ESP ash bottom ash limestone slurry (limestone) limestone slurry (water) gypsum WFGD wastewater WESP fresh water WESP wastewater ESP ash bottom ash limestone slurry (limestone) limestone slurry (water) gypsum WFGD wastewater WESP fresh water WESP wastewater

mg/kg mg/kg mg/kg μg/L mg/kg μg/L μg/L μg/L mg/kg mg/kg mg/kg μg/L mg/kg μg/L μg/L μg/L

87 47.9 12.0 0.7 27.5 7.2 0.5 21.3 81 69.2 11.5 0.5 26.9 1.9 0.5 18.5

1000 927 69.6 71.0 71.6 71500 3.6 268 679 702 58.3 11.2 77.8 50000 3.1 258

28.8 14.6 1.2 0.56 2.7 551 0.001 8.9 53.9 21.6 1.0 0.24 2.6 550 0.003 17.6

53.5 32 2.8 1.7 6.8 827 1.4 29.2 82.3 37.5 2.5 1.4 7.0 831 1.5 40.2

65.3 34.5 3.1 2.0 6.9 72.3 0.6 9.8 91.3 35.2 2.8 2.2 7.4 60.5 0.7 6.5

230.9 50 4.4 7 11.5 360 42 27.6 250.4 32.7 4.0 37 12.4 381 13 139

20.2 3.6 2.1 0.5 5.1 8.0 0.05 7.3 20.5 2.9 1.6 0.5 3.6 7.6 0.05 3.6

1530 1070 24.1 42.4 60.7 146 1.7 518 1599 1161 20.5 34.8 62.1 153 0.9 620.2

153 21.2 2.8 0.1 7.9 0.7 0.01 127.5 238.9 18.8 1.9 0.1 6.7 0.1 0.01 29.1

ηAPCD‐x ,TE = [TEp,APCD‐x ,in − TEp,APCD‐x ,out]/TEp,APCD‐x ,in p

(2)

× 100%

ηAPCD‐x ,TE = [TEtot,APCD‐x ,in − TEtot,APCD‐x ,out] tot

/TEtot,APCD‐x ,in × 100%

The concentration and removal rate of TEs across each APCDx at the two loads are shown in Tables 6 and 7, respectively. Removal rates of the total TEs through the whole system under the two loads are both higher than 99.9%. Except Co and Ba, the overall removal rate at 100% output is more than that at 75% output. Total TE removal rate ηTEtot across the ESP at the full load is in the range of 99.89−99.98%, while the value is in the range of 99.84−99.90% for the 75% load. It indicates that ESP makes a great contribution for the TE removal in the coalfired power plant. At the 75% output, the removal rate of particulate TEs across ESP is less than that at the full output. This is because the particulate TE concentration at the inlet of ESP is lower (except Cu, which are similar under the two loads), while the value is higher at the outlet at the load of 75%. In comparison to that at the 100% output, the capacity for the ESP to capture the gaseous TEs increases, except gaseous As, at the 75% load. It may be that the low flue gas temperature at the low load promotes the homogeneous nucleation or adsorption of the gaseous TEs on fly ash particles. From Tables 6 and 7, it can also conclude that WFGD and WESP have the synergistic removal effects for the total TEs. The concentration of the gaseous TEs increases when they go through WFGD under the two loads, except gaseous Ni and As, at the 100% output, which is not consistent with the results of Deng et al.12 Studies has shown that Hg remission occurs in the WFGD process, of which the influence factors can be the pH value, temperature, redox potential, concentration of sulfur and Cl ions, etc.24 In combination the Hg phenomenon and the test data, it may be due to the TE remission in the desulfurization process. Because little research about this aspect is reported and the concentration at the inlet/outlet of the WFGD is minimal, the mechanism of this phenomenon needs to be studied further by other methods. 3.4. Enrichment Characteristic of TEs in ESP Ash and Bottom Ash. The relative enrichment index (REI) can reflect

Figure 3. Relative distribution ratio of TEs at the 100% load.

Figure 4. Relative distribution ratio of TEs at the 75% load.

3.3. Removal Rate of TEs Across Each APCD. To describe the removal situation of the TEs across each APCD or the whole system, the removal rates for the gaseous TEs, particulate TEs, and total TEs are defined, which can be found as eqs 1−3. ηAPCD‐x ,TE = [TEg,APCD‐x ,in − TEg,APCD‐x ,out]/TEg,APCD‐x ,in g

× 100%

(3)

(1) 750

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Energy & Fuels Table 6. Concentration and Removal Rate of TEs Across Each APCD-x at the 100% Load ESP

WFGD

TEs

in (μg/m3)

out (μg/m3)

ηTEg/ηTEp (%)

Crg Crp Mng Mnp Cog Cop Nig Nip Cug Cup Zng Znp Asg Asp Bag Bap Pbg Pbp

2.03 1544.07 2.44 18856.82 0.12 519.25 0.44 961.97 0.45 956.51 0.62 3962.67 0.21 353.91 5.63 28285.23 0.61 2637.22

0.34 0.32 0.37 2.54 0.01 0.15 0.69 0.1 0.54 0.49 0.31 1.76 0.02 0.04 1.15 4.02 0.26 0.93

83.25 99.98 84.84 99.99 91.67 99.97 −56.82 99.99 −20.00 99.95 50.00 99.96 90.48 99.99 79.57 99.99 57.38 99.96

ηTEtot (%) 99.96 99.98 99.97 99.92 99.89 99.95 99.98 99.98 99.95

WESP

out (μg/m3)

ηTEg/ηTEp (%)

ηTEtot (%)

0.41 0.16 0.81 1.8 0.02 0.05 0.26 0.1 0.83 0.12 1.39 0.42 0.02 0.04 2.06 2.76 0.68 0.28

−20.59 50.00 −118.92 29.13 −100.00 66.67 62.32 0.00 −53.70 75.51 −348.39 76.14 0.00 0.00 −79.13 31.34 −161.54 69.89

13.64 10.31 56.25 54.43 7.77 12.56 0.00 6.77 19.33

out (μg/m3)

ηTEg/ηTEp (%)

ηTEtot (%)

overall removal rate (%)

0.44 0.00 0.63 0.00 0.01 0.00 0.34 0.00 0.66 0.00 1.76 0.00 0.01 0.00 1.44 0.00 0.15 0.00

−7.32 100.00 22.22 100.00 50.00 100.00 −30.77 100.00 20.48 100.00 −26.62 100.00 50.00 100.00 30.10 100.00 77.94 100.00

22.81

99.972

75.86

99.997

85.71

99.998

5.56

99.965

30.53

99.931

2.76

99.956

83.33

99.997

70.12

99.995

84.38

99.994

Table 7. Concentration and Removal Rate of TEs Across Each APCD-x at the 75% Load ESP

a

WFGD

TEs

in (μg/m3)

out (μg/m3)

ηTEg/ηTEp (%)

ηTEtot (%)

Crg Crp Mng Mnp Cog Cop Nig Nip Cug Cup Zng Znp Asg Asp Bag Bap Pbg Pbp

2.47 860.68 3.08 7330.73 0.19 638.04 0.41 986.48 1.24 1004.58 3.8 2760.34 0.18 188.25 4.51 15656.99 2.1 2443.58

0.3 0.98 0.19 8.25 0 0.66 0.25 1 0.44 1.11 1.3 3.04 0.02 0.25 0.35 19.44 0.13 2.9

87.85 99.89 93.83 99.89 100.00 99.90 39.02 99.90 64.52 99.89 65.79 99.89 88.89 99.87 92.24 99.88 93.81 99.88

99.85 99.88 99.90 99.87 99.85 99.84 99.86 99.87 99.88

WESP

out (μg/m3)

ηTEg/ηTEp (%)

ηTEtot (%)

0.52 0.18 0.77 1.53 0.02 0.12 0.36 0.19 0.84 0.21 3.52 0.56 0.04 0.05 1.75 3.61 0.56 0.54

−73.33 81.63 −305.26 81.45 a 81.82 −44.00 81.00 −90.91 81.08 −170.77 81.58 −100.00 80.00 −400.00 81.43 −330.77 81.38

45.31 72.75 78.79 56.00 32.26 5.99 66.67 72.92 63.70

out (μg/m3)

ηTEg/ηTEp (%)

ηTEtot (%)

overall removal rate (%)

0.48 0.00 0.32 0.00 0.01 0.00 0.37 0.00 0.97 0.00 2.32 0.00 0.05 0.00 0.85 0.00 0.66 0.00

7.69 100.00 58.44 100.00 50.00 100.00 −2.78 100.00 −15.48 100.00 34.09 100.00 −25.00 100.00 51.43 100.00 −17.86 100.00

31.43

99.944

86.09

99.996

92.86

99.998

32.73

99.963

7.62

99.904

43.14

99.916

44.44

99.973

84.14

99.995

40.00

99.973

Not given.

the volatility of TEs and their affinity to fly ash or bottom ash,12 which has been used in a lot of studies.14,25 The definition of the REI can be expressed by eq 4. REI = [TEsashA ad,coal ]/[TEscoal × 100]

change has little impact on the enrichment of TEs in ESP ash and bottom ash. According to the volatility and migration behavior, TEs can be divided into three categories, which have been accepted by many researchers.23,27,28 Class I is the non-volatility elements, which mainly exist in large residue particles, such as bottom ash. Furthermore, they will distribute in fly ash and bottom ash equally. The elements in this class include Mn, Ba, etc. Class II is the moderately volatile elements, which emit in the furnace but are prone to deposit on the surface of particles with flue gas cooling. Elements such as Zn, Sb, Pb, Cd, As, etc. are included in this class. Class III is the elements with high volatility, such as Hg, Br, Cl, etc. It is hard for them to condense from the gaseous state. However, some TEs are put into different classes

(4)

The REI of the TEs in ESP ash and bottom ash at the two loads is shown in Figure 5. It can be found the REI of Mn in ESP ash and bottom ash is approaching at the two outputs, which indicates that Mn has a similar enrichment characteristic in ESP ash and bottom ash. Generally, Mn exists in coal in the form of ankerites, carbonates, and siderites, which can be one reason for its low volatility.26 The REI for other TEs in ESP ash is higher than that in bottom ash at two loads. It shows that the load 751

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0.08, 5.0, and 0.9 mg/m3, respectively. In comparison to this standard, the emission concentration of TEs in the power plant at the two loads is far less than the value. The European Commission (EC) also gives the value of certain TEs in the Air Quality Standards,30 which are listed in Table 8. The concentration of Ni and As under the two loads and Pb at the 75% output is a little higher than the value. It is worth noting that the Air Quality Standards are not the standards for coal-fired power plants; thus, the TE emission concentration here is very low. An emission factor is a representative value that associates the amount of a pollutant emitted to the atmosphere with the relevant activity.25 In this study, the emission factor of TEs is described as the ratio of the amount of a given metal via stack to the amount of coal burned, shown in Table 9. The emission factor of TEs for the 100% load is within the scope of 0.09− 15.25 mg/tonne of coal, while it is in the range of 0.09−20.07 mg/tonne of coal for the 75% output. Under both loads, Zn has the maximum emission factor, while Co has the minimum value. Deng et al.12 gave the emission factor of Mn and Pb in the five pulverized coal-fired power plants equipped with ESP/ FF + wet FGD and SCR + ESP + wet FGD. The average value for Mn and Pb is 413.9 and 1115.8 mg/tonne of coal, respectively, which is far more than the value in this work. It may be related to the coal, boiler load, type of APCDs, etc. 3.6. Further Work in the Removal of TEs. Many factors can affect the migration and transformation behaviors of TEs in the coal-burning process, such as the occurrence of TEs in coal, their evaporation characteristics, the flue gas components, the oxidation/reducing atmosphere, the condensation/adsorption between TE compounds and fly ash surfaces, etc.7,31 In addition, a reaction may occur among the TE compounds. The problems mentioned above result in the complexity of the TE migration mechanism during coal combustion. When the boiler loads change, the furnace temperature will also change. It will have impacts on the factors that affect the emission and distribution of TEs in coal-fired power plants eventually. To better know and remove the TEs in coal-fired power plants, a lot of research work should be done further. (1) Distribution and enrichment characteristics of TEs in coal, ash, and flue gas: Emission characteristics of TEs in a typical coal-fired power plant should be tested systematically, of which the focus should be put on the emission concentration in the flue gas, the occurrence and content of TEs in ESP ash, FGD gypsum, FGD wastewater, etc. The leachable potential of TE inorganic pollutants that retained in FGD gypsum and the impacts on the environment should be investigated, which can give the reference for the relevant emission standards to be set. The overall removal rate for the APCDs in the coal-fired power plant should be known. (2) Transformation mechanism of TEs across each device, including the furnace, SCE, ESP, WFGD, etc.: As a result of the uncertainty or error in the TE sampling of the field test, the

Figure 5. REI of TEs in ESP ash and bottom ash at the two loads.

in certain studies, such as Cr, Co, Cu, etc. It may be due to the difference in the form of TEs in coal, flue gas components, mineral composition of bottom ash and fly ash, coal type, etc.16 3.5. Emission Characteristics of TEs to the Atmosphere. The concentration of TEs emitted to the atmosphere at the loads of 75 and 100% is shown in Table 8, which is the concentration at the outlet of the WESP. According to Tables 6 and 7, it can be found that all of the emitted TEs are in the gaseous form. The emission concentration of TEs is in the range of 0.01−1.76 μg/m3 at the full load, while the value is between 0.01 and 2.32 μg/m3 at the 75% load. Except Mn, Co, and As, the concentration of other TEs at the 75% output is higher than that at the 100% output. There is no specific emission standard for the TEs emitted from the coal-fired power plant, except Hg, in China, of which the limit value is 30 μg/m3. However, the National Environmental Protection Agency enacted the Integrated Emission Standard of Air Pollutants in 1996,29 specifying the limits of Cr, Ni, and Pb as

Table 8. Emission Concentration of TEs to the Atmosphere at the Two Loads (μg/m3)

a

TEs

Cr

Mn

Co

Ni

Cu

Zn

As

Ba

Pb

100% output 75% output China29 EC30

0.44 0.48 80 a

0.63 0.32 a a

0.01 0.01 a a

0.34 0.37 5000 0.02

0.66 0.97 a a

1.76 2.32 a a

0.01 0.05 a 0.006

1.44 0.85 a a

0.15 0.66 900 0.5

Not given. 752

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Energy & Fuels Table 9. Emission Factor of TEs to the Atmosphere at the Two Loads (mg/tonne of Coal) TEs

Cr

Mn

Co

Ni

Cu

Zn

As

Ba

Pb

100% output 75% output

3.81 4.15

5.46 2.77

0.09 0.09

2.95 3.21

5.72 8.40

15.25 20.07

0.09 0.43

12.48 7.35

1.30 5.71

Jiaotong University for help during the field test and thankfully acknowledge anonymous reviewers for their critical comments.

mechanism should be studied in the laboratory-scale bench. Some research methods can be used, such as the chemical thermodynamics, chemical kinetics, quantum chemistry, etc. (3) Ways to control TEs in the coal-fired power plant: The removal effects for the main TEs by fly ash itself or additives put into the coal and the reaction mechanism between solid adsorbent and TEs should be explored. At the end, the integrated control technology for TEs and other contaminants that is highly efficient and low-input should be developed.

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4. CONCLUSION Under the two loads, the mass balance rate of the TEs is in an acceptable range. TEs are mainly distributed in the bottom ash and ESP ash in the coal-fired power plant, and the total amount of TEs in ESP ash is greater than that in bottom ash. In contrast to the 100% output, the amount of TEs in bottom ash increases at the 75% load. Overall removal rates of the TEs through the whole system at the two loads are higher than 99.9%. Apart from Co and Ba, the overall removal rate at the 100% load is more than that at the 75% load. The removal rate of particulate TEs across the ESP at 75% load is less than that at the 100% load, while the value for gaseous TEs is opposite, except gaseous As. Both WFGD and WESP have the synergistic removal effects for the total TEs. The remission of TEs may occur in the desulfurization process. Load change has little impact on the enrichment of TEs in ESP ash and bottom ash. All of the TEs emitted to the atmosphere are in gaseous form. The emission concentration of TEs is in the range of 0.01−1.76 and 0.01−2.32 μg/m3 at the two loads. In comparison to standards of China and the EC, the emission of TEs in the coalfired power plant is extremely low. At the end, some further work is presented to better know and remove the TEs in coalfired power plants.

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AUTHOR INFORMATION

Corresponding Authors

*(Y.F. Duan) E-mail: [email protected]. Tel.: +86-02583795652. Fax: +86-025-83795652. *(H.Z. Tan) E-mail: [email protected]. Tel.: +86-02982668703. Fax: +86-029-82668703. ORCID

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Shilin Zhao: 0000-0002-5318-2042 Notes

The authors declare no competing financial interest.

NOMENCLATURE TE = trace element SCR = selective catalytic (NOx) reduction ESP = electrostatic precipitator WFGD = wet flue gas desulfurization WESP = wet electrostatic precipitator PM = particulate matter APCD = air pollution control device REI = relative enrichment index ηAPCD‑x,TEg = removal rate for the gaseous TEs across each APCD or the whole system ηAPCD‑x,TEp = removal rate for the particulate TEs across each APCD or the whole system ηAPCD‑x,TEtot = removal rate for the total TEs across each APCD or the whole system TEg,APCD‑x,in = concentration of the gaseous TEs at the inlet of the APCD-x TEp,APCD‑x,in = concentration of the particulate TEs at the inlet of the APCD-x TEtot,APCD‑x,in = concentration of the total TEs at the inlet of the APCD-x TEg,APCD‑x,out = concentration of the gaseous TEs at the outlet of the APCD-x TEp,APCD‑x,out = concentration of the particulate TEs at the outlet of the APCD-x TEtot,APCD‑x,out = concentration of the total TEs at the outlet of the APCD-x APCD-x = ESP, WFGD, WESP, or ESP + WFGD + WESP TEtot = concentration of the total TEs in the flue gas TEg = concentration of the gaseous TEs in the flue gas TEp = concentration of the particulate TEs in the flue gas TEtot = TEg + TEp ηTEg = removal rate of the gaseous TEs ηTEp = removal rate of the particulate TEs ηTEtot = removal rate of the total TEs TEsash = concentration of TEs in fly ash or bottom ash Aad,coal = content of the ash in the coal (air-dried basis) TEscoal = concentration of TEs in coal REFERENCES

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ACKNOWLEDGMENTS This project was financially supported by the National Natural Science Foundation of China (51376046 and 51576044), the National Key Research and Development Program (2016YFC0201105, 2016YFB0600604-02, and 2016YFB0600203-03), the Fundamental Research Funds for the Central Universities, Graduate Student Research and Innovation Program of Jiangsu Province (CXZZ13_0093, KYLX_0115, KYLX_0184, and KYLX15_0071), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1505). The authors thank the Xi’an 753

DOI: 10.1021/acs.energyfuels.6b02393 Energy Fuels 2017, 31, 747−754

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DOI: 10.1021/acs.energyfuels.6b02393 Energy Fuels 2017, 31, 747−754