Distribution and Speciation Transformation of Hazardous Trace

Datang Environmental Industry Group Co., Ltd, Beijing , 100097 , China ... the power plant is low-sulfur and low-chlorine bituminous coal with 3.851 m...
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Distribution and Speciation Transformation of Hazardous Trace Element Arsenic in Particulate Matter of a Coal - fired Power Plant Shilin Zhao, Yufeng Duan, Cong Chen, Hao Wu, Deye Liu, Meng Liu, Jianhong Lu, and Xiaobing Gu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00472 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018

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Distribution and Speciation Transformation of Hazardous Trace Element

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Arsenic in Particulate Matter of a Coal - fired Power Plant

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Shilin Zhao a, b, Yufeng Duan a, *, Cong Chen a, Hao Wu a, Deye Liu c, Meng Liu a, Jianhong Lu a,

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Xiaobing Gu d

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a

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Energy and Environment, Southeast University, Nanjing, 210096, China

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b

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Canada

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c

Jiangsu Provincial Center for Disease Control and Prevention, Nanjing 210009, China

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d

Datang Environmental Industry Group Co., Ltd, Beijing, 100097, China

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ABSTRACT: Particle matter (PM) emitted from coal combustion can cause great

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harm on human health and the environment. The existence of hazardous trace element

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arsenic (As) in PM intensifies its toxicity. In this work, four particle sizes of fly ash

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PM (PM10) before the electrostatic precipitator (ESP) in a

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600 MW coal-fired power plant was sampled by a Dekati Low Pressure Impactor

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(DLPI). The coal sample, bottom ash, ash from ESP (ESP ash) were collected

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simultaneously. Concentrations of total and valent As (As3+ and As5+) in the sample

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were determined by the inductively coupled plasma-mass spectrometry (ICP-MS) and

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high performance liquid chromatography (HPLC) coupled with ICP-MS, respectively.

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The morphological structure and chemical components of the PM surface were

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characterized by the scanning electron microscope (SEM) and X-ray fluorescence

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spectrometry (XRF). Results show that the coal used for the power plant is low-sulfur

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and low-chlorine bituminous coal with 3.851 mg/kg of As. Total As content in the

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ESP ash is 11.44 times of that in the bottom ash (only 1.637 mg/kg). Arsenic is prone

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to enrich in ESP ash while dissipate in bottom ash. With fly ash particle diameter

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decreasing, concentration of total As increases from 9.599 mg/kg to 20.088 mg/kg and

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 1H9,

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the corresponding relative enrichment index increases from 0.68 to 1.42. The main As

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form in coal-fired fly ash is As5+, which occupies 90.98%-98.63% of the total As

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amount. Toxic As3+ has a little higher ratio in bottom ash with value of 21.01%. More

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As physical adsorption and chemisorption active sites on the smaller fly ash PM with

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higher specific area contribute to increase in the concentration of total As and As5+ as

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the particle sizes decrease. Moreover, the transformation mechanism of As to PM

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during coal combustion was discussed.

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KEYWORDS: Coal-fired power plant; Fine particulate matter; Arsenic; Distribution;

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Speciation transformation

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1. INTRODUCTION

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Coal is a major primary source of energy in China, which accounts for 67.3% of

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primary energy consumption 1. Particle matter (PM), especially nanoparticles, emitted

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from coal combustion can enter human lungs and cause various respiratory diseases

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such as asthma, lung cancer

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can diffuse in environment and damage the body's immune system, emission of

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arsenic (As) and its compounds in coal combustion process has caused the widespread

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concern in the world

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plants, coal-fired power plant is known as one of the largest anthropogenic sources of

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PM and As

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pollutants, such as NOx, SO2, and PM, more and more coal-fired power plants has

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been installed with the air pollution control devices (APCDs), like selective catalytic

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reduction (SCR), electrostatic precipitator (ESP), or fabric fibers (FFs), and wet flue

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gas desulfurization (WFGD) in China 12. Although these APCDs have the synergistic

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control effects on emission of PM and As

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ESP or FFs for fine particles, especially the PM with diameter of less than 0.5 µm 10, 17.

2-6

. Due to the high toxicity and carcinogenicity, which

7-9

. With the large amounts of coal consumed in thermal power

10, 11

. Facing the increasingly stringent requirements on the conventional

13-16

, there is little removal efficiency of

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At high furnace temperature, arsenic escape from the coal in the vapor phase. With

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flue gas cooling, it is prone to be enriched in fine PM 18. Thus, it increases the toxicity

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of ultra-fine and nano-fine particles undoubtedly, which needs to be given enough

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attention 19-23. The combination of PM and As has been investigated by several authors

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24

10, 24-31

.

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Wang et al.

collected PM before an ESP of a 660 MW coal-fired power plant

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located in Nanchang by the Dekati Gravimetric Impactor (DGI), results of which

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showed that concentration of As, Cd, Cr, Pb increased with decreasing the particle

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size. Yu et al. 25 sampled PM at the inlet of ESP in a coal-fired power plant in Beijing

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through an Andersen 8-stage Stack Impactor with aerodynamic diameter separation to

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study the distribution of As, Pb, Cr, Cd, Ni, Co, Cu, Zn in fine PM. It was found that

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obvious enrichment of those 8 hazardous trace elements occurred in fly ash with small

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particle diameter, and concentration of most volatile As in smallest PM (Aerodynamic

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diameter: 0-0.71 µm) was 30 times of that in the largest PM (10.97-17.54 µm). Yi et al.

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26

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V, Zn, Mn, Fe in fine fly ash at the inlet and outlet of FFs in a 220 MW power plant

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by the Electrical Low Pressure Impactor (ELPI), finding that enrichment trends of all

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elements after FFs were basically the same as the inlet of it. Wang et al.

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investigated the enrichment of trace elements in PM entering and leaving the novel

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ESP of a 660 MW coal-fired power plant, results of which indicated enrichment

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factors of As, Cd, Cr, Pb in PM were 31.2-41.4, 20.1-26.8, 12.2-14.0, 4.8-6.0,

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respectively. However, the current studies were only focused on the enrichment

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characteristic of hazardous trace elements in PM. The content of different forms of As

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(As3+ is more toxic than As5+ 32) in PM, and the transformation mechanism of As to

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PM during coal combustion are seldom studied.

studied the size-classified relative enrichment factors of As, Hg, Se, Cd, Cr, Cu, Al,

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In this study, the PM at the inlet of ESP in a 600 MW coal-fired power plant was

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sampled by a Dekati Low Pressure Impactor (DLPI). The content of total As in

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different sizes of PM was detected by the inductively coupled plasma-mass

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spectrometry (ICP-MS). Valent As, namely As3+ and As5+ in PM was determined by

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the high performance liquid chromatography (HPLC) coupled with ICP-MS. The

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scanning electron microscope (SEM) and X-ray fluorescence spectrometry (XRF)

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were used to characterize the physical and chemical properties of the PM surface. The

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contents includes: (1) Enrichment characteristics of total As in the samples; (2)

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Content and proportion of valent As in the samples; (3) SEM, XRF analysis of the

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samples; (4) Transformation mechanism of As to PM in coal combustion process. The

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objective is to explore the distribution and speciation transformation of hazardous

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trace element As in PM, which hopes to provide a guidance for the effective control of

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PM with As.

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2. EXPERIMENTAL SECTION

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2.1. Plant Description, PM Sampling and Coal analysis

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The field test was conducted in a 600 MW coal-fired power plant located in

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Inner Mongolia. To control the conventional air pollutant emission, SCR and ESP

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were used to remove NOx and PM in flue gas, respectively. WFGD was adopted for

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the SO2 removal as well as the fine PM. The combination of SCR, ESP and WFGD is

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the most popular APCDs in coal-fired power plants in China. PM in flue gas at the

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inlet of ESP was sampled by a DLPI (Dekati Ltd, Finland), which has 13 stages (12

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channels) with 50% cutpoints ranging from 0.0281, 0.0565, 0.0944, 0.154, 0.258,

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0.377, 0.605, 0.936, 1.58, 2.36, 3.95, 6.6 to 9.8 µm (in aerodynamic diameter),

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respectively. To collect enough amount of the PM for As determination, the sampling

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was conducted twice. For each time, the PM sampling lasted for 3 hours with gas flow 4

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rate of 10 L/min. To avoid the interference of water vapor condensation in flue gas,

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the temperature of DLPI was kept above 100 °C. And working pressure of the device

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was 100 mbar. After the sampling, four particle sizes of PM (named PM10), which was with aerodynamic diameter of less than 1 µm, 1-2.5 µm,

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2.5-10 µm, larger than 10 µm, respectively, in the two sampling processes were mixed

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for the determination of As speciation and concentration.

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In the whole PM sampling process, the corresponding coal sample, bottom ash,

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and ash removed by ESP (ESP ash) were gotten about per hour. Then, the same kind

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sample was putted together evenly, which minimized errors in sample collection. The

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electricity generation capacity was maintained at about 480 MW. The proximate and

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ultimate analyses of the coal sample are shown in Table 1. Based on Chinese

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classification method for coal used in boiler, the coal belongs to bituminous coal. The

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chlorine content (0.01%) is lower than the average value in Chinese and American

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coal, which is 0.022% and 0.0614%, respectively 33. According to the concentration of

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sulfur in Chinese coal within the range of 0.2%-8% 34, the sulfur content (0.90%) in

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the coal sample is relatively low. Table 1. Proximate and elemental analysis of the coal sample a

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Proximate analysis (%)

Element analysis (%)

Qnet, ar

M

A

V

FC

MJ/kg

C

H

O

N

S

Cl

17.90

23.38

23.22

35.51

16.98

44.01

2.60

10.52

0.69

0.90

0.01

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Note: a: All the value is on the as received basis

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2.2. As Speciation and Concentration Determination

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Concentration of total As in solid samples, such as coal, PM, bottom ash, ESP

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ash was determined by the ICP-MS (Agilent Technologies 7700x, US) after digestion

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by the mixture of concentrated nitric acid (HNO3) and perchloric acid (HClO4).

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Because different As speciation has different toxicities, it is meaningful to know their

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content in the solid samples. Concentration of As3+ and As5+ can be determined by the

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HPLC (Shimadzu LC20-AB, Japan) coupled with ICP-MS (named HPLC-ICP-MS)

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in this work, which has been used in some studies successfully

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PM, bottom ash, ESP ash was digested by 10 mL of 0.1% (v/v) HNO3 to release the

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As completely. Then the solution was purified in a centrifuge at 7000 rpm for 10

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minutes to obtain the supernatant. The content of As3+ and As5+ in the clear solution

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can be directly detected by HPLC-ICP-MS. The accuracy of As3+ and As5+

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concentration in samples was determined through a mixed solution of GBW08667

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and GBW08666 as standard reference material. The chromatogram of As3+ and As5+

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gotten from the GBW08667 and GBW08666 is shown in Figure 1. In order to ensure

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the accuracy, all the measurements of As speciation were conducted in several times

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to achieve parallel results with relative standard deviation less than 5%.

35, 36

. About 0.01g of

22500 As3+

20000 75ppb

17500 15000

Intensity (cps)

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12500

As5+

10000

125ppb

7500 5000 2500 0 -2500 -100

0

100

200

300

400

500

600

700

800

Time (s)

137 138

Figure 1. Chromatogram of As3+ and As5+ based on mixed solution of GBW08667

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and GBW08666

140 141

2.3. Relevant Characterization Methods Burning weight loss method was used to obtain the unburned carbon (UBC) in 6

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the ESP ash and bottom ash. The samples were firstly dried in an oven at 102 °C for 8

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hours to remove moisture and then kept in a muffle furnace with 850 °C for 3 hours.

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The ratio of the mass difference between the dried and the burned sample to that of

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dried sample was defined as the UBC content 37. The SEM was used to acquire the

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physical structure of PM, ESP ash, and bottom ash on a Hitachi s-4800 microscope

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(made in Japan). The XRF (ARL ADVANT’ XP, Switzerland) was applied to

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determine the major elements and oxides on the surface of PM and ESP ash.

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3. RESULTS AND DISCUSSION

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3.1. Enrichment Characteristics of Total As in the Samples

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Concentration of total As in the coal determined by ICP-MS is 3.851 mg/kg,

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which is little higher than the average content in Chinese coal (3.79 mg/kg) while less

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than that in world’s coal (8.30 mg/kg) 38. Figure 2 gives the total As content in bottom

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ash, ESP ash, and PM, which ranges from 1.637 mg/kg (bottom ash) to 20.088 mg/kg

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(PM10, PM2.5-10, PM1-2.5, and PM10

PM2.5-10

PM1-2.5

PM10, PM2.5-10, PM1-2.5, and PM10

0.192

8.733

9.599

90.98

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PM 2.5-10

n.d.

11.982

12.365

96.91

PM 1-2.5

n.d.

13.431

14.606

91.95

PM 10

47

. The

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Energy & Fuels

PM 2.5-10

PM 1-2.5

PM 10

to PM 1-2.5 then decreases from PM 1-2.5 to PM

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to PM 1-2.5 then increases from PM 1-2.5 to PM

10

17.67

18.95

3.68

2.40

33.39

40.53

5.15

3.42

PM 2.5-10

18.05

18.62

3.24

2.04

34.10

39.84

4.54

2.91

PM 1-2.5

21.02

24.61

2.95

2.46

39.72

52.65

4.12

3.51

PM