Enrichment characteristics, thermal stability and volatility of hazardous trace elements in fly ash from a coal-fired power plant

Fuel 225 (2018) 490–498

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Full Length Article

Enrichment characteristics, thermal stability and volatility of hazardous trace elements in fly ash from a coal-fired power plant

T

⁎

Shilin Zhaoa,b, Yufeng Duana, , Jincheng Lua, Shuai Liua, Deepak Pudasaineeb, Rajender Guptab, Meng Liua, Jianhong Lua a b

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton T6G 1H9, Canada

A R T I C LE I N FO

A B S T R A C T

Keywords: Coal-fired fly ash Hazardous trace elements Enrichment characteristics Thermal stability Volatility

Release of hazardous trace elements (HTEs) during re-use of coal-fired fly ash involving heat treatment process can cause health risks for on-site workers and contaminate the surrounding environment. Enrichment characteristics, thermal stability and volatility of five HTEs (Hg, As, Pb, Cr, Mn) in fly ash collected from a 660 MW coal-fired power plant were investigated. Brunauer-Emmett-Teller (BET), scanning electron microscope- energy dispersive spectrometer (SEM-EDS), and X-ray power diffraction spectrometry (XRD) were used to characterize pore structure and chemical composition of the coal-fired fly ash (CFA) samples. Results show that the coal belongs to bituminous, containing low content of S, Cl, and the HTEs analyzed. CFA samples have low content of Hg (0.04–0.20 mg/kg) and As (3.0–8.7 mg/kg), and high content of Mn (782.46–1115.85 mg/kg). Burnout degree and specific surface area result in the decreasing HTEs concentration in CFA samples when increasing the particle size. Relative enrichment index (REI) of HTEs in the raw CFA are in the order of Mn > Cr > Pb > 1 > As > Hg. Increase in particle sizes leads to decrease of the REI value, implies that HTEs are enriched more in finer fractions. Thermal stability of HTEs in CFA samples becomes weaker at higher temperature. The smaller the particle size is, the worse the thermal stability becomes. Volatilization fractions of Hg, As, Pb, Cr, and Mn in CFA samples at four temperatures (350 °C, 550 °C, 750 °C, 950 °C) range 88.35%–99.50%, 2.32%–23.17%, 4.08%–22.59%, 20.36%–34.43%, and 1.21%–23.43%, respectively. The derivative of the concentration-fitted curve to the temperature in the range of 20–900 °C at heating rate of 10 °C/ min indicates volatility of HTEs in CFA follows the decreasing order of Hg > As, Cr > Mn, Pb.

1. Introduction Hazardous trace elements (HTEs), namely Hg, As, Cr, Mn, Pb, Ba, etc., emitted from coal combustion can cause harm to the environment by contaminating air, water, soil and can cause human health hazards [1–3]. U. S. Clean Air Act Amendments (CAAA) listed 11 elements including Hg, As, Cr, Mn, Ni, Se, Be, Cd and Co as the key toxic air pollutants in 1990; in which Hg, As, Cr, Pb, Se and Cd were chosen as priority elements [4,5]. Some HTEs, like As, Pb, Hg, Ni, and Cd, have also been considered as prime environmental concerns by the European Union and the Canadian Environmental Protection Agency. In China, the State Council has approved the 12th five-year plan for comprehensive prevention and control of some HTEs pollution in 2011 [6]. With huge amount of coal consumption, coal-fired power plants for electricity generation are considered as one of the major anthropogenic emission sources of HTEs [7,8]. During coal combustion, some HTEs

⁎

existed in coal volatilize and react with surrounding environment including flue gas components and fly ash. With flue gas cooling, HTEs undergo homogeneous nucleation, condensation reaction, or adsorption. Finally, HTEs are emitted in gaseous and particulate forms in the flue gas, and part of them occur in bottom ash [9]. Field tests on coalfired power plants have shown that most HTEs, except Hg, are mainly distributed in fly ash removed from particulate collection devices such as electrostatic precipitator (ESP) or fabric filter (FF) [10–13]. This indicates that the particular attention should be given to the HTEs pollution during treatment of the coal-fired fly ash. The annual generation capacity of coal-fired fly ash in China, the USA and India in 2015 was about 580, 130, 190 million tonnes, respectively [14]. Based on the physical, chemical and mineralogical properties, coal-fired fly ash can be used for different purposes such as applied in soil amelioration, construction industry, ceramic industry, catalysis, environmental protection, depth separation, zeolite synthesis,

Corresponding author. E-mail address: [email protected] (Y. Duan).

https://doi.org/10.1016/j.fuel.2018.03.190 Received 10 November 2017; Received in revised form 13 February 2018; Accepted 30 March 2018 0016-2361/ © 2018 Published by Elsevier Ltd.

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and valuable metal recovery [15]. According to the annual comprehensive resource utilization report (2012) released by the National Development and Reform Commission (NDRC) of China, the generation and utilization amount of coal-fired fly ash was 540 and 367 million tonnes in 2011, respectively. In all the sections of utilization, the top three are cement (41%), brick and tiles (26%) and concrete (19%). However, when the coal-fired fly ash is used as a raw material in a production process like cement, brick and tiles, the high temperature treatment process is unavoidable [16,17]. It can lead to release of HTEs previously existed in coal-fired fly ash, which can pose toxic occupational health risks to on-site workers and neighbors. Huang et al. [18] investigated the effects of some vital flue gas components and mineral sorbents on the partitioning behavior of four major HTEs (Cd, Pb, Zn and Cu) which are often present in municipal solid waste (MSW), finding that presence of HCl promotes evaporation of the four HTEs while SiO2- or Al2O3- containing minerals may benefit for their control. Wu et al. [19] prevented the evaporation of HTEs (Pb, Zn and Cd) by controlling their chemical speciation in MSW incineration fly ash, results of which indicated phosphate chemical pretreatment resulted in the least evaporation of HTEs in the fly ash. Hu et al. [20] compared the CaO’s effect on the fate of HTEs (Pb, Cr, Zi, Cu, Zn and Cd) during thermal treatment of two typical MSW incineration ashes in China. It showed that conversion of Ca to aluminosilicates occurred especially at 1323 K, which promoted the HTEs immobilization and decreased their volatile fractions. In addition, Hu et al. [21] found that part of arsenic was stabilized in the ash matrix including some un-oxidized As(III) which remained stable even when the MSW incineration ash was heated at 1323 K. Nowak et al. [22] investigated the influence of chloride addition (zero to 200 g Cl/kg as added as CaCl2, MgCl2, or NaCl) on the trace elements (Pb, Cr, Cd, Zn, Ni and Cu) removal from MSW fly ash by treating their mixture in a muffle oven (800–1200 °C, 20 h) and a laboratory-scale rotary reactor (1000 °C, 60 min), results of which showed that Cd, Pb, and about 10% Cr were either present in ash in volatile form or also reacted with ash inherent Cl, while CaCl2 and MgCl2 were more effective for the evaporation of the most heavy metals than NaCl. All these above mentioned studies focused on release behavior of some trace elements with relatively low volatility such as Pb, Cr, As in the MSW incineration ash; in which the mineral compositions in MSW incineration fly ash and coal-fired fly ash are different, especially the Cl content. However, the studies related to the release of HTEs in coal-fired fly ash are very little, thus, there is a need to investigate on the release behavior of HTEs from coal-fired fly ash. HTEs can be classified into three groups according to the partitioning behavior, which has been recognized by many researchers [23,24]. Group 1: have high volatility and are mainly emitted in gaseous state, such as Hg, Br, Cl, etc. Group 2: volatilize and be prone to adsorb or deposit on the surface of small particles with the decrease in flue gas temperature, such as As, Pb, Cd, Cu, and Zn, etc. Group 3: do not vaporize and be equally distributed in bottom ash and fly ash, such as Cr, Mn and Co, etc. Thus, HTEs, namely Hg, As, Pb, Cr and Mn, representing these three groups were chosen in this study. Enrichment characteristics, thermal stability and volatility of HTEs in fly ash with three different particle sizes were investigated. The main objective is to understand the release behavior of HTEs in coal-fired fly ash, which can provide guidance for the effective control of HTEs during re-use of coalfired fly ash involving heat treatment process.

Fig. 1. Size distribution of the CFA0, CFA1, CFA2, and CFA3.

Laser particle size analyzer (Microtrac S3500, US) was used to obtain the particle size distribution of CFA0, result of which is shown in Fig. 1. Based on the size distribution, three kinds of particle sizes, namely < 50 μm (defined as CFA1), 50–100 μm (defined as CFA2), > 100 μm (defined as CFA3) were acquired by the mechanical screening method. The size distribution of CFA1, CFA2, and CFA3 is also shown in Fig. 1. 2.2. Determination of Hg, As, Pb, Cr and Mn in the CFA samples The mercury analyzer named Milestone DMA80 was used to determine concentration of Hg in the CFA, detection limit of which was 0.2 ppb. For determining As, Pb, Cr and Mn in the CFA, they were firstly digested in a mixture of acids (HNO3, HCl, HF, HClO4) and then measured by the inductively coupled plasma - mass spectrometry (ICP-MS, Agilent Technologies 7700x, US). 2.3. Thermal stability and volatility of HTEs in the CFA samples To explore the thermal stability of HTEs in CFA with different particle diameters, CFA1, CFA2, and CFA3 were placed in a muffle furnace at 350, 550, 750, and 950 °C for 4 h, respectively. To study the volatility of HTEs in CFA samples, CFA1, CFA2, and CFA3 were placed in a tube furnace with a heating rate of 10 °C/min. Then, samples were taken out at the temperature of 150, 250, 350, 450, 550, 650, 750, 850, 950 °C, respectively. The concentration of Hg, As, Pb, Cr and Mn in the three CFA samples under each situation was determined. 2.4. Relevant characteristics of the CFA samples To determine the unburned carbon (UBC) content in the CFA, the samples were dried at 102 °C for 8 h to remove moisture. The dried samples were then placed in a muffle furnace at 850 °C for 3 h. The ratio of the weight difference between the dried and burned samples to that of dried samples was defined as the UBC content [25]. In this study, the UBC contents of CFA0, CFA1, CFA2, and CFA3 are 11.76%, 15.59%, 11.43%, 9.65%, respectively, indicating the quality of CFA samples is in the order of CFA3 > CFA2 > CFA0 > CFA1 based on the burnout degree. The Brunauer-Emmett-Teller (BET) surface area of CFA samples was determined using an automatic specific surface area and pore analyzer (ASAP-2020M, Micromeritics, US). Combination of scanning electron microscope (SEM, Hitachi s-4800 microscope, Japan) and energy dispersive spectrometer (EDS, Thermo Noran SYSTEM7, US) was used to characterize pore structure and chemical element of the surface. X-ray power diffraction spectrometry (XRD, Shimadzu Corporation XD-3A, Japan) was used to detect the mineral composition. The XRD patterns

2. Experimental 2.1. Coal-fired fly ash (CFA) samples The coal-fired fly ash (defined as CFA0) was collected from ESP (the first ESP in series) of a 660 MW coal-fired power plant. This plant was equipped with selective catalytic reduction (SCR), ESP, wet flue gas desulfurization (WFGD), and wet electrostatic precipitator (WESP) along the flue gas path. The coal used belongs to the bituminous. The 491

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Table 1 Proximate and elemental analysis of the coal sample. Proximate analysis

LHV

Elemental analysis

Mar

Aar

Var

FCar

Qar,net

Car

Har

Oar

Nar

Sar

Clar

%

%

%

%

MJ/kg

%

%

%

%

%

%

12.37

16.40

27.60

43.62

21.37

56.24

3.66

10.07

0.90

0.37

0.011

ar: as received basis.

Concentration [mg/kg]

were recorded over a 2 h interval of 5–80° using Cu Kα radiation with a step size of 0.02°. In order to observe the changes in physical and chemical properties of fly ash sample surfaces, some typical conditions such as room temperature (RT), 550 °C, and 950 °C of the CFA samples with different particle sizes were chosen. 3. Results and discussion 3.1. Properties of the coal The proximate and elemental analysis of the coal sample is shown in Table 1. According to the Chinese classification method for coal used in boiler [26], this coal sample belongs to bituminous. The content of S in the coal is 0.37%, which is low based on sulfur content of 0.2%–8% in Chinese coal reported by Gao et al. [27]. The chlorine content in the tested coal is 0.011%, which is lower than the average value of chlorine for Chinese (0.022%) and American coal (0.0614%) [28]. Concentrations of HTEs in the coal sample are shown in Table 2, in which their average contents in coal of China and the world are also listed [29–32]. Except Mn, the average concentrations of As and Cr in Chinese coal are lower than that in the world’ coal, while that of Hg and Pb are opposite. Compared to the average value of the Chinese and world’s coal, concentrations of the five HTEs in the coal are relatively low.

1150 1100 1050 1000 950 900 850 800 750 700 80 70 60 50 40 30 20 10 0

CFA0 CFA1 CFA2 CFA3

0.24 0.20 0.16 0.12 0.08 0.04 0.00

Hg

Hg

As

Pb

HTEs

Cr

Mn

Fig. 2. Concentrations of HTEs in CFA0, CFA1, CFA2 and CFA3 samples.

3.2. Concentrations and enrichment characteristics of HTEs in the CFA samples From the concentrations of HTEs in CFA0, CFA1, CFA2 and CFA3 samples shown in Fig. 2, it can be found that their contents in CFA0 are between the maximum and minimum vales in CFA1, CFA2, and CFA3. Hg and As in CFA samples have a relatively low concentration (Hg: 0.04–0.20 mg/kg; As: 3.0–8.7 mg/kg), while content of Mn is relatively high (782.46–1115.85 mg/kg). As increasing the particle size, the concentrations of Hg, As, Pb, Cr and Mn in CFA samples decrease. The XRD results of the CFA samples are shown in Fig. 3. It indicates that the CFA samples are mainly composed of quartz, char, calcite, sillimanite, hematite, and magnetite, which is similar with results of others [33]. With changing of the particle size, the mineral compositions of the CFA samples seem unchanged. The SEM images and their corresponding EDS spectra of CFA samples are given in Fig. 4. It can be found that the CFA samples mainly contain Si, O, C, Ca, Fe, Al, etc., which shows a good agreement with the XRD results. From the SEM images of CFA1-3, CFA 1 contains the large numbers of spherical particles while CFA 2 and CFA3 contain few irregular shaped particles. At the high furnace temperature, the coal

Fig. 3. XRD results of the CFA1, CFA2, and CFA3 samples.

will firstly undergo pyrolysis and ignition. Accompanied by precipitation of volatile matter and the decomposition of minerals, the coke begins to burn and break into small particles. HTEs existed in the excluded minerals and coke volatize and react with surrounding atmosphere to form inorganic vapors. As the flue gas cooling, part of gaseous HTEs transform into sub-micrometer and agglomerated ash or adhere to fly ash through homogeneous nucleation, condensation, or adsorption. HTEs those not escaped from the minerals or coke will be retained in the particle, which eventually ends up in bottom ash or fly ash [9]. Because of the high combustion temperature, all the CFA samples have molten surface with less pore structures, which is confirmed by the BET results (total pore volume: 0.001–0.002 cm3/g). With the decreasing particle sizes of CFA samples, their BET specific surface area increases from 0.41 to 0.95 m2/g, which makes HTEs be condensed or adsorbed on the fly ash more easily. In addition, the UBC contents of CFA samples (CFA1: 15.59%, CFA2: 11.43%, CFA3: 9.65%) follows the order of CFA1 > CFA2 > CFA3, which indicate that burnout degree of the CFA samples is gradually improved with increase of particle sizes. It promotes the volatilization of HTEs in CFA with large particle size. Both of these factors result in the changes of HTEs concentration in the

Table 2 Concentration of Hg, As, Pb, Cr and Mn in coal sample [mg/kg]. Conc. Coal sample China [29,30] World [31,32]

Hg 69.41 × 10 0.19 0.10

−3

As

Pb

Cr

Mn

1.0 3.79 8.3

6.54 15.1 7.8

7.0 15.4 16

68.23 116.2 nd

Conc.: concentration; nd: no data. 492

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Si

1000 800 Al

600

O

400

Mg

Ca

C

200

Cl 0

K

Cl

0

2

Fe

Ti 4

Fe

6

KeV

8

800 (b) RT, CFA2 600

Si

400 O 200

Fe

Al Ca

Mg

C Cl 0

K

Ca

Cl

0

2

Fe

Ti

4

KeV

6

8

800 (c) RT, CFA3

Si 600

400

O

200

Al

Ca

C Cl

0

0

Cl 2

Ti

K 4

KeV

Fe 6

8

Fig. 4. SEM images and their corresponding EDS spectra of CFA samples (The magnification of (a), (b) and (c) is 5000× magnification, 500× magnification and 200× magnification, respectively. EDS spectra of (a), (b) and (c) were obtained from the whole area of their corresponding SEM images at 500× magnification).

CFA1-3. To evaluate the enrichment degree of HTEs in fly ash, relative enrichment index (REI) was used, which can be expressed as Eq. (1) [34].

REI = [HTEsCFA × A coal,ad]/[HTEscoal × 100]

(1)

where, HTEscoal and HTEsCFA represent the concentrations of HTEs in coal and CFA samples, respectively, mg/kg. Acoal, ad represents the ash content in the coal on air dried basis, %. The REI of HTEs for the CFA samples is shown in Fig. 5. For CFA0, the REI of the five HTEs follows the decreasing order of Mn > Cr > Pb > 1 > As > Hg. For Hg, the REI is only 0.45, far less than 1. It indicates that only little amount of Hg is enriched in fly ash and major portion is emitted in gaseous form during coal combustion, which is consistent with others [11,35]. It is obvious that REI of the five HTEs in CFA samples decreases with the increase in fly ash particle sizes. Fig. 5. REI of HTEs in CFA0, CFA1, CFA2 and CFA3.

3.3. Thermal stability of HTEs in CFA samples The volatilization fraction (η) is used to describe thermal stability of HTEs in CFA samples, which is defined as Eq. (2). 493

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100

100 CFA1 CFA2 CFA3

80

80 70

(As) [%]

(Hg) [%]

70 60 50

60 50

40

40

30

30

20

20

10

10

0

CFA1 CFA2 CFA3

90

90

350

0

950

750

550

0

350

Temperature [ C ] 100 90

90

CFA1 CFA2 CFA3

80

CFA1 CFA2 CFA3

80 70

(Cr) [%]

70 60

(Pb) [%]

950

750 0

Temperature [ C ]

100

50

60 50

40

40

30

30

20

20

10 0

550

10 350

0

950

750

550 0

Temperature [ C ]

350

550

0

750

950

Temperature [ C ]

100 90 CFA1 CFA2 CFA3

80

(Mn) [%]

70 60 50 40 30 20 10 0

350

550

0

750

Temperature [ C ]

950

Fig. 6. Volatilization fraction of HTEs in CFA samples varied with temperature.

indicates that Hg is unstable at certain temperatures. The volatilization fractions of As, Pb, Cr, and Mn in three kinds of CFA samples (CFA1, CFA2 and CFA3) at the four temperatures (350, 550, 750 and 950 °C) are 2.32%–23.17%, 4.08%–22.59%, 20.36%–34.43%, and 1.21%–23.43%, respectively. Mercury in coal is mainly associated with organic matter or sulfide. In the combustor with high temperature, it will emit from coal in the form of elemental mercury. In the range of 20–900 °C, it can be transformed into oxidized mercury or particulate mercury through reaction with flue gas components like O2, HCl, Cl2, SO2, NO, NH3, H2S, etc. or fly ash [36,37]. The temperature programmed decomposition desorption of mercury compounds confirms that HgS(red), HgS(black), HgSO4/HgO are the main mercury species in fly ash [11]. The thermal decomposition temperature of standard mercury compounds mixed with pretreated fly ash can be found in Table 3 [38]. It can be found that most mercury compounds in fly ash are decomposed into elemental mercury into atmosphere below 630 °C. The volatilization fraction of

η = [HTEsCFA,RT × mCFA,RT−HTEsCFA,T × mCFA,T]/[HTEsCFA,RT × mCFA,RT] × 100%

(2)

where, η represents volatilization fraction, %; HTEsCFA, RT represents the concentration of HTEs in CFA samples at the room temperature, mg/kg; HTEsCFA, T represents the concentration of HTEs in the CFA samples after heat treatment at constant temperature (350, 550, 750 and 950 °C, respectively) for four hours in air; mCFA, RT and mCFA, T represent the mass of CFA samples at room temperature and the constant temperature of heat treatment, g. The volatilization fraction of HTEs in CFA samples varied with temperature is shown in Fig. 6. It shows that volatilization fractions of all the HTEs increase with the increasing temperature, which implies that HTEs have weak thermal stability at high temperature. Fly ash with smaller particle sizes has higher volatilization fraction of HTEs. For Hg, it has highest volatilization fraction (88.35%–99.50%) in all the three kinds of particle sizes compared with As, Pb, Cr, and Mn, which 494

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discussion. Arsenic in coal is mainly associated with pyrite or other sulfide, although it has more than 200 mineral forms [39,40]. It volatize during coal combustion and condense completely before emitting from coalfired power plants [41]. Thermodynamic calculations indicate that AsO2 is the stable form at the burning atmosphere of low chlorine at 800 °C while AsCl3 is the main speciation in the high chlorine-coal combustion [42]. At the temperature ranges of 400–800 °C, part of As can transform into AsCl3. In addition, As in flue gas can react with Fe or Ca to form FeAsO4 or Ca3(AsO4)2 on fly ash surface. Fest tests found that As was predominantly as As(V) in arsenate (AsO43−) species while the more toxic As3+ form accounted for 10% of the total arsenic in fly ash [43,44]. It indicates that As in fly ash contains the physical adsorbed As and some arsenates or arsenites, in which the physical

Table 3 Thermal decomposition temperature of standard mercury compounds mixed with pretreated fly ash [38]. Standard mercury compounds

High peak T (°C)

Start of peak T- end of peak T (°C)

HgBr2 HgCl2 HgS(black) HgO HgS(red) HgSO4

220 240 259 312 350 540

100–300 150–350 200–300 200–380 250–400 450–630

Hg in CFA samples at the temperature of 750 °C in this study is 97.18%–99.22%, which shows a good agreement with the above

0.0000

CFA1 CFA2 CFA3

0.20 C1=0.11768*e

-t1/213.26689

-t1/213.26723

+0.11768*e

CFA3 CFA2

-0.0002

-0.00692

-0.0004

0.15

dC/dt [Hg]

Concentration of Hg [mg/kg]

0.25

0.10 C2=0.0299*e

-t2/322.0854

-t2/322.09116

+0.0299*e

-0.00366

CFA1

-0.0006 -0.0008 -0.0010

0.05

-0.0012

0.00

C3=0.02269*e

-t3/335.52504

-t3/335.50107

+0.02269*e

-0.0014

-0.00322

0

0

Temperature [ C]

Temperature [ C] 10

8

0.000

6

-0.010

C1=1.53304*e

-t1/1995.35008

-t1/43.04583

+2.49852*e

+5.57021

5 4

C2=15064.11741*e

-t2/5.04631E7

-t2/43.26952

+1.21402*e

-15061.88542

-0.015 -0.020 CFA3

-0.025

3

CFA1

-0.030

2 1

CFA2

-0.005

7

dC/dt [As]

Concentration of As [mg/kg]

0.005

CFA1 CFA2 CFA3

9

C3=1.71714*e

-t3/54.80314

-t3/6.85983E7

+16913.09897*e

-0.035

-16911.32104

0

-0.040 0

0

Temperature [ C] 0.005

CFA1 CFA2 CFA3

-0.005 -t1/-33707.03247

C1=-108.4387*e

C2=23.16974*e

-t2/31895.11193

-t1/-34260.20617

-108.43895*e

-t2/31480.65498

+23.16974*e

CFA1

0.000

+286.7335

dC/dt [Pb]

Concentration of Pb [mg/kg]

Temperature [ C] 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

-26.00252

-0.010

CFA2 CFA3

-0.015 -0.020 -0.025 -0.030 -0.035

C3=2.33829*e

-t3/222.95661

-t3/3.2739E7

+6982.34444*e

-6967.75299

-0.040 0

Temperature [ C]

0

Temperature [ C]

Fig. 7. Concentration and concentration change rate varied with temperature (t1, t2 and t3 in the equations represent the temperature, which corresponds to CFA1, CFA2, and CFA3, respectively). 495

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100

CFA1 CFA2 CFA3

80

C1=11.0287*e

-t1/283.95872

-t1/41.44792

+38.41591*e

+52.75923

70

dC/dt [Cr]

Concentration of Cr [mg/kg]

90

60 50

C2=9.1324*e

-t2/216.25984

-t2/27.49254

+33.64718*e

+30.95126

40 30 20

C3=9.21346*e

-t3/387.64939

-t3/387.59979

+9.21346*e

+26.08474

10

0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40 -0.45 -0.50 -0.55 -0.60 -0.65 -0.70

CFA3 CFA1

CFA2

0

0

Temperature [ C]

Temperature [ C] 1300

1000

C1=5.36198E6*e

-t1/2.01424E7

-t1/3.46621

+45359.92464*e

0 -5

-5.361E6

-t2/3176.97197

-t2/81.91923

+127.75871*e

+261.70785

900 800

CFA3

-15

CFA1

-20 -25 -30

700 600

CFA2

-10

C2=586.41965*e

dC/dt [Mn]

Concentration of Mn [mg/kg]

1200 1100

5

CFA1 CFA2 CFA3

C3=65.3299*e

-t3/1008.948

-t/3/8.77524

+722.18816*e

-35

+644.48212

-40

500

0

0

Temperature [ C]

Temperature [ C]

(a) Concentration varied with temperature

(b) Concentration change rate varied with temperature Fig. 7. (continued)

the carbonate form will be decomposed and then react with hematite to generate jacobsite (MnFe2O4) [52]. Because of different existence forms in coal, Mn has various oxidized state in fly ash, such as Mn(II), Mn(III), and Mn(IV) [53]. Based on the distribution of Mn in coal-fired power plants, which has lager proportion in bottom ash than that of other HTEs [10,12,54], it indicates that part of Mn in fly ash can be more stable even at the high furnace temperature. Therefore, Mn in fly ash has good stability during the thermal treatment, the greatest volatilization fraction of which is 23.43%.

adsorbed As is easily released in heat treatment but part of the arsenates or arsenites are immobilized even at 1050 °C [21]. Thus, little amount of physical adsorbed As and unstable arsenates or arsenites in fly ash is the key reason for its relatively low volatilization fraction in the CFA samples at the four temperatures studied. Plumbum is associated with organics in coal, which mainly exists in the form of galena in Chinese coal [45,46]. In the combustion process, Pb associated with organic matters volatilize at around 850 °C. It occurs in the form of PbO (g) above 820 °C and be changed into PbO(s) with flue gas cooling from 820 °C to 730 °C. PbSO4(s) is the main speciation at temperature lower than 730 °C [47]. However, research finds that speciation of Pb in fly ash may be PbO [3]. Based on the melting point of PbSO4 or PbO higher than 886 °C, and boiling point of PbO being 1470 °C [48], it results in the volatilization fraction of Pb in CFA samples lower than 22.59% even at 950 °C. Chromium may be associated with organic matter, but most of it has strong relation with clay in coal. In the presence of sulfur, two hexavalent species [CrO2(OH)2, CrO3] are predicted to be stable in a notable amounts, and these species are not considered to be stable at the stack temperature. In pulverized coal combustion fly ash, the toxic Cr6+ is considered to be concentrated [49]. The melting points of CrCl3 and Cr2O3 are 631 °C and 1990 °C, respectively [48]. Though little research about the accurate speciation of Cr in fly ash is reported, the melting point of the Cr chloride or oxide reflects that it has strong stability in fly ash. In this study, the volatilization fraction of Cr in CFA samples at the four temperatures is not more than 34.43%. Manganese is associated with dispersed materials in coal, which mainly occurs in carbonate and residual form [50,51]. When burning,

3.4. Volatility of HTEs in the CFA samples Concentration of HTEs in CFA samples varied with temperature in the range of room temperature (20 °C)–900 °C at a heating rate of 10 °C/ min is shown in Fig. 7(a). It shows that concentrations of HTEs in CFA samples decrease as temperature increases. Except Cr, which has a certain fluctuation in its concentration value between CFA2 and CFA3 samples before 450 °C, concentrations of the other four HTEs, namely Hg, As, Pb, and Mn, decrease with the increase in particle size in the whole studied temperature range. As the temperature increases, concentrations of HTEs in CFA samples decrease slowly. To better describe the effects of temperature on HTEs concentration, equations to fit the concentrations of HTEs in fly ash at each temperature point and particle size were used, which are also shown in Fig. 7(a). The derivative of the fitted curve to the temperature, namely dC/dt, is calculated as shown in Fig. 7(b). The absolute value of the derivative can reflect the sensitivity of HTEs concentration in fly ash to the temperature, which can also indicate the 496

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volatility of HTEs. For Hg, the absolute value of dC/dt decreases with increasing the particle sizes. It shows that Hg volatility becomes weaker in larger fly ash particles, which shows a good agreement with the above discussion of mercury thermal stability. For As and Cr, the absolute value of dC/dt decreases obviously before 200 °C, while the value changes little after that temperature. It indicates that low temperature has a significant effect on their release in fly ash while high temperature has little influence. This may be due to the fact that the desorption of the adsorbed As and Cr in fly ash occurs at low temperature, and compounds of As and Cr existed in fly ash has good thermal stability at high temperature. Except Pb in CFA3 sample, the value of dC/dt for Pb and Mn is almost zero after 80 °C, which shows that the increase of temperature has little influence on the volatility of Pb and Mn. Thus, volatility of the five studied HTEs in fly ash follows the order of Hg > As, Cr > Mn, Pb.

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4. Conclusion The coal sample belongs to bituminous with low S and Cl content, which also contains low concentration of the five HTEs. CFA samples have low concentration of Hg (0.04–0.20 mg/kg) and As (3.0–8.7 mg/ kg), and high content of Mn (782.46–1115.85 mg/kg). Burnout degree and specific surface area lead to concentrations of the studied HTEs in the CFA samples decrease with the particle size increasing. REI of the HTEs in CFA0 follows the descending order of Mn > Cr > Pb > 1 > As > Hg. With the increase of particle size, the REI value of the HTEs decreases. The HTEs’ thermal stability becomes weaker as the temperature increases. The smaller the particle size is, the worse the thermal stability becomes. Volatilization fractions of Hg, As, Pb, Cr, and Mn in CFA samples at the four constant temperatures (350 °C, 550 °C, 750 °C, 950 °C) are 88.35%–99.50%, 2.32%–23.17%, 4.08%–22.59%, 20.36%–34.43%, and 1.21%–23.43%, respectively. In the range of 20–900 °C with a heating rate of 10 °C/min, concentrations of HTEs in CFA 1–3 decrease slowly with the increase of temperature. The derivative of the concentration-fitted curve to the temperature shows volatility of HTEs in CFA follows the decreasing order of Hg > As, Cr > Mn, Pb. Notes The authors declare no competing financial interest. Acknowledgments This project was financially supported by the National Key Research and Development Program (2016YFB0600604-02), the National Natural Science Foundation of China (51376046, 51576044), the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1706), the Graduate Student Research and Innovation Program of Jiangsu Province (KYCX17_0072), and the financial support from the China Scholarship Council (CSC). References [1] Oliveira MLS, Ward CR, Izquierdo M, Sampaio CH, Brum IAS, Kautzmann RM, et al. Chemical composition and minerals in pyrite ash of an abandoned sulphuric acid production plant. Sci Total Environ 2012;430:34–47. [2] Ribeiro J, Taffarel SR, Sampaio CH, Flores D, Silva LFO. Mineral speciation and fate of some hazardous contaminants in coal waste pile from anthracite mining in Portugal. Int J Coal Geol 2013;109–110:15–23. [3] Tian HZ, Lu L, Hao JM, Gao JJ, Cheng K, Liu KY, et al. A review of key hazardous trace elements in Chinese coals: abundance, occurrence, behavior during coal combustion and their environmental impacts. Energy Fuels 2013;27:601–14. [4] U.S. Environmental Protection Agency (EPA). Clean Air Act Amendments of 1990; 1st Congress (1989–1990); U.S. EPA: Washington DC, 1990. [5] Swaine DJ. Why trace elements are important. Fuel Process Technol 2000;65–66:21–33. [6] Ministry of Environmental Protection of the People’s Republic of China (MEP). The 12th Five-year Plan for Comprehensive Prevention and Control of Heavy Metals

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