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Partitioning and emission of hazardous trace elements in a 100MW coal-fired power plant equipped with SCR, ESP, wet FGD Shilin Zhao, Yufeng Duan, Chunfeng Li, Yaning Li, Cong Chen, Meng Liu, and Jianhong Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01608 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 2, 2017
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Energy & Fuels
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Partitioning and emission of hazardous trace elements in a 100 MW coal-fired
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power plant equipped with SCR, ESP, wet FGD
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Shilin Zhao, Yufeng Duan, Chunfeng Li, Yaning Li, Cong Chen, Meng Liu, Jianhong
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Lu
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Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education,
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School of Energy and Environment, Southeast University, Nanjing, 210096, China
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ABSTRACT:
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Hazardous trace elements (HTEs) emitted from coal combustion have caused great
9
harm on the environment and human health. Partitioning and emission of eight HTEs,
10
namely, Zn, Sb, Pb, Cd, As, Cr, Mn and Ba, were conducted on a 100 MW coal-fired
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power plant, which was equipped with SCR, ESP and wet FGD. US EPA Method 29
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was used to sample the eight HTEs in flue gas at the four sites before or after each
13
device, simultaneously. Feed coal, bottom ash, ash from ESP (ESP ash), limestone
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slurry, and desulphurization wastewater were collected at the same time. Results show
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that mass balance rates of eight HTEs of the whole system and each device are in the
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acceptable range of 70%-130%. The studied HTEs are mainly distributed in ESP ash
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with a relative mass distribution ratio of 86.23%-98.25%, followed by 1.65%-13.67%
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for bottom ash. Concentrations of Cd, Sb, As, Cr, Pb, Zn, Mn, Ba in flue gas at the
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inlet and outlet of SCR are 18.22, 78.60, 380.57, 1416.76, 3021.00, 3746.24, 5720.50,
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20355.09 µg/m3 and 18.02, 60.83, 358.42, 1418.04, 3023.00, 3753.47, 5596.90,
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20382.44 µg/m3, respectively, with a high particulate form proportion of
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99.39%-99.99%. Removal efficiency of the eight HTEs in flue gas by ESP + wet FGD
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is 99.78%-99.96% with that of 99.43%-99.95% for ESP. 18.10% of Sb, 22.25% of Ba,
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23.16% of Cr, 28.39% of Mn, 31.15% of As, 53.17% of Pb, 61.26% of Zn, 68.47% of
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Cd, in the flue gas is captured by wet FGD compared to their concentration at its inlet.
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Emission concentrations of Cd, Sb, As, Pb, Cr, Zn, Mn and Ba in flue gas to
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atmosphere are 0.03, 0.07, 0.15, 1.42, 3.11, 6.74, 7.82, and 8.11 µg/m3, with
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corresponding emission factors of 0.25, 0.61, 1.23, 11.53, 25.07, 54.93, 63.46, and
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65.90 mg/ (t coal), respectively. All the studied HTEs except Mn are prone to enrich
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in ESP ash than bottom ash.
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KEYWORDS: Coal-fired power plant; Hazardous trace elements; Partitioning;
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Emission; Enrichment characteristic
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1. INTRODUCTION
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Hazardous trace elements (HTEs) emitted from coal combustion have received 1
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worldwide attention because of their dangerous harm on the environment and public
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health
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total coal consumed was burned directly for power generation in 2007 4. It is
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estimated that the proportion will reach 65% in 2050 5. With the huge consumption of
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coal, coal-fired power plants are the main anthropogenic sources of HTEs, beside
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conventional air pollutants, such as SO2, NOx, PM 6, 7.
1-3
. Coal is the main primary energy in the world. In China, near 50% of the
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During combustion, HTEs are released from coal and distributed to bottom ash,
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fly ash, and flue gas 8. Thus, HTEs in flue gas can be divided into particulate (which
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are associated with particulate matter) and gaseous form. With great concern on the
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conventional air pollutant emission, most of the coal-fired power plants have been
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installed with cyclones, electrostatic precipitator (ESP), or fabric fibers (FFs) in China.
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By the end of 2015, SO2 control devices and the selective catalytic reduction (SCR)
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devices used for NOx removal had occupied 91.20% and 94.54% of the installed
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capacity of coal - fired units, respectively 9. The use of these air pollution control
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devices (APCDs) in coal-fired power plants will influence the partitioning of HTEs in
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coal combustion by-products and accordingly change their way to atmosphere 11-14
10
.
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Studies
have shown that SCR benefits for elemental mercury oxidation, ESP or
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FF can capture over 99% of particulate mercury, wet FGD can remove 60%-95% of
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oxidized mercury but elemental mercury can re-emit in the desulfurization process,
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and the overall removal rate across SCR + ESP + wet FGD ranges from 43.8% to
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94.9%. Deng et al. 8 conducted a field test at six coal-fired power plants to study the
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emission characteristics of Cd, Pb and Mn from coal combustion, results of which
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showed that wet FGD combined with particulate control devices could remove
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94.9-98.2% of Pb, 98.9-99.9% of Cd and 99.9% of Mn in flue gas. Though Tian at
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al.15 summarized the average co-benefit removal efficiency of eight HTEs (namely,
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Hg, As, Se, Pb, Cd, Cr, Ni, Sb) by each APCD, such as ESP, FFs, WFGD and SCR, in
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coal-fired power plants, reference data for other HTEs is lacking compared to that of
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Hg. The papers for directly monitoring of HTEs’ concentration in flue gas along the
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APCDs in coal-fired power plants are not yet sufficient, which is a fundamental step
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to control the HTEs emission.
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In this study, US EPA Method 29 16 was used to sample eight HTEs, namely, Zn,
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Sb, Pb, Cd, As, Cr, Mn and Ba, in flue gas before or after SCR, ESP, and wet FGD
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simultaneously in a 100 MW small-scale coal-fired power plant. Feed coal, bottom
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ash, ash from ESP (ESP ash), limestone slurry, and desulfurization wastewater were 2
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collected at the same time. Partitioning and emission characteristics of the eight HTEs
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under 85% load in the power plant were investigated. The main objective is to explore
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the behavior of HTEs across the APCDs, including their release, transformation and
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removal, which can provide guidance for controlling HTEs’ emission in coal-fired
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power plants.
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2. EXPERIMENTAL SECTION
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2.1. Plant description
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The field test was carried out on a 100 MW coal-fired power plant, located in a
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chemical plant in China. The generated electricity from the power plant was only used
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for production of the chemical plant itself. For control of the conventional air
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pollutant emission, SCR and ESP were used to remove NOx and particle matters in
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flue gas, respectively. Wet FGD was equipped to capture SO2, which adopted the
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commonly used limestone-gypsum technology. The combination of SCR, ESP and
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wet FGD is the most popular APCDs in coal-fired power plants in China.
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2.2 Sampling
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In this study, the 85% load was investigated. The eight HTEs in flue gas
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including the particulate and gaseous form at the four sites before or after each APCD
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were sampled by US Apex instrument, which was consisted of a probe, a heated filter
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box, a set of glass impingers, a cumulative flow meter and a vacuum pump. The
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particulate HTEs were retained in the filter while gaseous HTEs could be absorbed in
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5% (v/v) nitric acid (HNO3) and 10% (v/v) hydrogen peroxide (H2O2) solution. All
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the procedures for sampling of the eight HTEs in flue gas were conducted strictly in
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accordance with US EPA method 29 16, in which a detail description could be found.
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For each test, sampling of the eight HTEs in flue gas lasted for one hour, and
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solid or liquid samples including feed coal, ESP ash, bottom ash, limestone slurry, and
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desulfurization wastewater were collected at 30-minute intervals. Then the same type
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of sample was gathered evenly on an equal weight basis. The sampling sites for the
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eight HTEs in the flue gas, solid and liquid are shown in Figure 1. To reduce
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uncertainties and obtain accurate data, field tests under the 85% load were carried out
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repeatedly. The operating data, such as the boiler output, the amount of feeding coal,
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and the amount and density of limestone slurry, was obtained from the online
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monitoring system of the power plant. The amount of ESP ash and bottom ash was
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calculated based on the mass balance of ash, and the flow rate of desulfurization
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wastewater was obtained by the mass conservation of the desulfurization system. 3
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Figure 1 Sampling sites in the coal-fired power plant
104 105 106 107
2.3 Analysis methods The proximate and elemental analysis of the coal used at the 85% load was determined according to the National Standard of China 17.
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The eight HTEs, namely, Zn, Sb, Pb, Cd, As, Cr, Mn, Ba, in pure liquid sample,
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such as the absorption solution of 5% (v/v) HNO3 and 10% (v/v) H2O2, were
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determined by an inductively coupled plasma-mass spectrometer (ICP-MS, US). The
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eight HTEs in solid samples such as coal, bottom ash, ESP ash, and gypsum were
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determined by the ICP-MS after digestion through a mixture of some acids (HNO3:
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HCl: HF = 3:1:1) in a microwave oven. For the eight HTEs in limestone slurry and
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desulfurization wastewater, the concentration was obtained by calculation based on
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their content in the filtered pure liquid and solid of limestone or gypsum. For
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desulfurization wastewater, the concentrations and mass distributions of Zn, Sb, Pb,
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Cd, As, Cr, Mn and Ba in gypsum and the pure wastewater are shown in Table 1.
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Table 1 Concentrations and mass distributions of the eight HTEs in gypsum and the
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pure wastewater HTEs Zn Sb Pb Cd As Cr Mn
Concentration Gypsum Pure wastewater mg/kg mg/L 452.06±10.24 0.98±0.01 8.59±0.12 0.38±0.01 5.80±0.09 34.86±0.89 130.91±3.12
6.29±0.09 0.04±0.00 0.06±0.00 0.06±0.00 0.85±0.01 1.53±0.02 6.12±0.09
Mass distribution Gypsum Pure wastewater % % 94.73±2.11 86.66±1.98 97.20±2.34 59.33±0.99 63.06±1.32 85.04±1.77 84.24±1.67
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5.27±0.12 13.34±0.21 2.80±0.07 40.67±1.15 36.94±1.08 14.96±0.22 15.76±0.24
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Energy & Fuels
Ba
149.15±3.11
0.36±0.06
99.04±1.88
0.96±0.01
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The burning weight-loss method was used to get the unburned carbon (UBC)
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content of bottom ash and ESP ash 18. The samples were placed in a muffle furnace at
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850 0C for 3 hours after being dried at 102 0C for 8 hours. Then, the proportion of
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difference between the burned and the dried samples to the initial weight of the
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sample was defined as the UBC. The content of O2 in flue gas at the four sites of flue
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gas HTEs sampling was determined by a multifunction flue gas analyzer (MRU vario
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plus, Germany), the data of which is shown in Table 2. All the data given in this paper
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is the average value, and concentration of the eight HTEs in flue gas has been unified
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to 6% O2, dry flue gas for comparison.
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Table 2 Content of O2 in flue gas at the four sites of flue gas HTEs sampling (%) Sites
SCR inlet
SCR outlet
ESP outlet
Wet FGD outlet
85% load
7.57
5.60
6.60
6.71
2.4 Quality assurance and quality control
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For determination of the eight HTEs in samples by the ICP-MS, all the reagents
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were excellent grade. It required that the concentration of the eight HTEs in a blank
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digestion solution should be less than the maximum value among the 5% of the
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sample test value, 5% of the regulatory limit and method detection limit. When
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normalized, it should firstly measure the standard solution with low concentration and
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then that with high concentration. Relative standard deviation (RSD) for the three
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repeated measurements of standard solution with high concentration was required to
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be no more than 5%, otherwise, the sampling system should be checked. At least five
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points must be adopted to obtain the calibration curve, and the correlation coefficient
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should be more than 0.995. After measuring 10 samples, it should be back to
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determine the concentration of the eight HTEs in a standard solution. The RSD must
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be less than 10%, otherwise the standardization should be repeated and the
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determination of the 10 samples should be done again.
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For the sampling of HTEs in flue gas, the RSD should be less than 34% when the
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concentration of HTEs in flue gas was less than 3 µg/m3. Otherwise, the value should
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be no more than 11%. The field tests were conducted three or more times to meet
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these requirements.
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3. RESULTS AND DISCUSSION
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3.1 Coal analysis
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The proximate and elemental analysis of the coal used at 85% load is shown in 5
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Table 3. According to the Chinese classification method for coal used in boiler, this
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coal belongs to lean coal. Flue gas components, such as hydrogen chloride (HCl),
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chlorine (Cl2), and sulfur dioxide (SO2), can affect the speciation of HTEs
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average content of Cl in Chinese coal is 220 mg/kg, and the value in American coal is
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614 mg/kg 22. The content of S in Chinese coal is in the range of 0.2%-8%, reported
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by Gao et al. 23. For the coal used in this work, the content of Cl (281 mg/kg) is a little
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higher than the Chinese average value, while the content of S belongs to the low
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sulfur coal.
. The
Table 3 Proximate and elemental analysis of the coal sample a
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Proximate analysis M %
A %
V %
FC %
7.09 21.30 11.45 60.16 160
19-21
Element analysis (%)
Qnet, ar MJ/kg 26.05
C %
H %
O %
N %
S %
Cl mg/kg
63.12 3.09 3.87 0.98 0.55
281
a
Note: : All the value is on as received basis
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Concentrations of the eight HTEs in the coal sample are shown in Table 4. The
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average content of the eight HTEs in the coal of China and the world is also listed in
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the table 24, 25. Except Mn, the average content of Zn, Pb, Cd, and Ba in Chinese coal
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is higher than that of world’s coal. The concentrations of Zn, Pb and Ba are higher
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than the average value of both Chinese and world’s coal. The different content of the
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eight HTEs in the coal may be due to their different coal formation process,
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surrounding condition, coal type, etc.
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Table 4 Concentration of the eight HTEs in the coal sample (mg/kg) Coal sample China 24 World 25
Zn
Sb
Pb
Cd
As
Cr
Mn
Ba
39.47 38 23
0.53 0.71 0.92
23.02 15.1 7.8
0.20 0.25 0.22
4.14 3.79 8.3
12.99 15.4 16
43.95 116.2 nd
173.25 159 150
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Note: nd: no data
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3.2 Mass balance rate and mass distribution
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The mass balance rate, which is defined as the ratio of the output to the input
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amount per unit time for a particular object, is used to verify the accuracy and
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credibility of field test data in this work. For the whole system, the input HTEs are
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that coming from feeding coal and limestone slurry; the output HTEs include that in
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bottom ash, ESP ash, desulphurization wastewater and that in flue gas emitted to
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atmosphere. For each device, namely, furnace, SCR, ESP, and wet FGD, the input 6
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HTEs is that in flue gas or limestone slurry entering the device; the output HTEs is
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that in flue gas, bottom ash, ESP ash, or desulfurization wastewater leaving the device.
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As shown in Figure 2, all the mass balance rates of the eight HTEs for the whole
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system and each device are in the range of 74.07%-128.53%. In general, some factors,
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such as the representativeness of the collected liquid or solid samples, error in
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determination analysis of the HTEs, and fluctuations in the feeding coal amount, can
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affect the mass balance rate. Thus, the mass balance rates of 74.07%-128.53% in this
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work are within the acceptable range, which is 70%-130%, as reported by others 10, 26. System Furnace SCR ESP Wet FGD
130 120 110 100
Mass balance rate / %
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Energy & Fuels
90 80 70 60 50 40 30 20 10 0
185 186
Zn
Sb
Pb
As
Cd
Cr
Mn
Ba
Figure 2 Mass balance rates of the eight HTEs of the whole system and each device
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The relative mass distribution proportion of the eight HTEs across the power
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plant, which is defined as the ratio of mass amount of HTEs in each part to the total
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mass amount including that in bottom ash, ESP ash, removed by wet FGD, and flue
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gas to atmosphere, is shown in Figure 3. The eight HTEs are mainly distributed in
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ESP ash, the relative mass distribution proportion of which is 86.23%-98.25%. Mass
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amount of the eight HTEs in bottom ash accounts for 1.65%-13.67%. The eight HTEs,
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which are removed by wet FGD and that in flue gas to atmosphere are little, the
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relative mass distribution proportion of which is 0.006%-0.214% and 0.037%-0.190%,
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respectively. This indicates that more attention about treatment and recycle for the
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studied HTEs should be paid to the ESP ash and bottom ash.
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Mass relative distribution proportion/ %
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HTEsg, stack
100.0
HTEsp,stack wet FGDremoval
99.8
ESP ash bottom ash
99.6 99.4 99.2
30 20 10 0
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Zn
Sb
Pb
As
Cd
Cr
Mn
Ba
198
Figure 3 Relative mass distribution proportions of the eight HTEs across the power
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plant
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3.3 Concentration and removal efficiency of HTEs across SCR, ESP and wet
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FGD
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HTEs in coal are usually bound to organic and inorganic matter 27. At the furnace
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temperature, some HTEs will be evaporated in flue gas during coal combustion. Some
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HTEs, which can’t escape from the burnout char particles, will go into fly ash and
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bottom ash. As flue gas cools down, HTEs will be in gaseous and particulate phase by
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homogeneous reactions, condensation, and adsorption. Concentrations of the eight
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HTEs across SCR, ESP, and wet FGD are shown in Table 5. At the inlet and outlet of
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SCR, concentrations of Cd, Sb, As, Cr, Pb, Zn, Mn, Ba in flue gas are 18.22, 78.60,
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380.57, 1416.76, 3021.00, 3746.24, 5720.50, 20355.09 µg/m3 and 18.02, 60.83,
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358.42, 1418.04, 3023.00, 3753.47, 5596.90, 20382.44 µg/m3, respectively. Cd has
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the minimum concentration while Ba has the maximum value, which may be due to
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their different contents in the coal. The proportion of the eight particulate HTEs is far
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more than that of their gaseous form, which is in the range of 99.39%-99.99%.
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Studies have proven that SCR catalysts are beneficial for Hg0 oxidation, in which
215
hydrogen chloride (HCl) and sulfur dioxide (SO2) have positive effects
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However, due to the unavailable valence of the eight HTEs by the current sampling
217
method, complexity of flue gas components, and extremely low concentration of
218
gaseous form, it is hard to explore the effects of SCR system on the studied HTEs.
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With great removal efficiency of more than 99.95% for the particulate matter, 8
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concentration of the eight HTEs at the outlet of ESP decreases dramatically with the
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value of 0.09-17.39 µg/m3. In addition to the conventional removal of SO2, wet FGD
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can capture the soluble and low volatile trace element compounds, as well as the
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ultrafine particles further. This leads to a decrease in concentration of the eight HTEs
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at the outlet of wet FGD, which is 0.03-8.11 µg/m3.
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Table 5 Concentrations of the eight HTEs across SCR, ESP, and wet FGD SCR HTEs
inlet
outlet
ESP
wet FGD
outlet
outlet
Conc.
Prop.
Conc.
Prop.
Conc.
Prop.
Conc.
Prop.
µg/m3
%
µg/m3
%
µg/m3
%
µg/m3
%
Zng
17.44±0.55
0.47±0.01
17.13±0.52
0.46±0.01
15.69±0.51
90.23±2.93
5.46±0.21
81.10±3.12
Znp
3728.80±112.86
99.53±3.01
3736.34±113.21
99.54±3.02
1.70±0.06
9.77±0.35
1.27±0.04
18.90±0.59
Sbg
0.12±0.00
0.16±0.00
0.10±0.00
0.17±0.00
0.07±0.00
71.39±0.00
0.05±0.00
69.37±0.00
Sbp
78.48±2.45
99.84±3.12
60.73±1.91
99.83±3.14
0.03±0.00
28.61±0.00
0.02±0.00
30.63±0.00
Pbg
1.95±0.06
0.06±0.00
1.91±0.06
0.06±0.00
1.65±0.06
54.53±1.98
0.39±0.01
27.36±0.71
Pbp
3019.05±91.78
99.94±3.04
3021.09±91.24
99.94±3.02
1.38±0.05
45.47±1.65
1.03±0.04
72.64±2.82
Cdg
0.11±0.00
0.61±0.00
0.10±0.00
0.55±0.00
0.09±0.00
91.48±0.00
0.03±0.00
78.91±0.00
Cdp
18.11±0.67
99.39±3.68
17.92±0.67
99.45±3.72
0.01±0.00
8.52±0.00
0.01±0.00
21.09±0.00
Asg
0.04±0.00
0.01±0.00
0.03±0.00
0.01±0.00
0.03±0.00
13.39±0.00
0.03±0.00
16.61±0.00
Asp
380.53±12.32
99.99±3.24
358.39±13.87
99.99±3.87
0.19±0.01
86.61±4.53
0.13±0.00
83.39±0.00
Crg
4.55±0.21
0.32±0.01
4.06±0.19
0.29±0.01
3.39±0.12
83.78±2.97
2.59±0.08
83.24±2.58
Crp
1412.21±50.33
99.68±3.55
1413.98±50.56
99.71±3.57
0.66±0.02
16.22±0.49
0.52±0.02
16.76±0.64
Mng
9.57 ±0.31
0.17±0.01
8.34±0.32
0.15±0.01
8.17±0.32
74.89±2.93
5.76±0.25
73.70±3.20
Mnp
5710.93±173.22
99.83±3.03
5588.56±170.22
99.85±3.04
2.74±0.09
25.11±0.82
2.06±0.09
26.30±1.15
Bag
1.47±0.05
0.01±0.00
1.27±0.05
0.01±0.00
1.20±0.04
11.46±0.38
1.14±0.04
14.06±0.49
Bap
20353.62±667.32
99.99±3.28
20381.17±677.87
99.99±3.33
9.24±0.35
88.54±3.35
6.97±0.31
85.94±3.82
Notes: Zng: gas phase Zn; Znp: particulate Zn; Sbg: gas phase Sb; Sbp: particulate Sb; Pbg: gas phase Pb; Pbp: particulate Pb; Cdg: gas phase Cd; Cdp: particulate Cd; Asg: gas phase As; Asp: particulate As; Crg: gas phase Cr; Crp: particulate Cr; Mng: gas phase Mn; Mnp: particulate Mn; Bag: gas phase Ba; Bap: particulate Ba; Con.: concentration; Prop.: proportion. 10
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228
To better describe removal effects of APCD on the eight HTEs in flue gas, the
229
removal efficiency for ESP, wet FGD, and ESP + wet FGD is defined as follows.
230
ηHTEs(t)APCD = [HTEs(t)in, APCD-HTEs(t)out, APCD] / THEs(t)in, APCD × 100%
231
Where, ηHTEs(t)APCD represents the removal efficiency of APCD on the HTEs(t) in
232
flue gas, %; HTEs(t) represents the total concentration of gaseous and particulate
233
HTEs in flue gas, µg/m3; APCD represents the ESP, wet FGD, and ESP + wet FGD,
234
respectively; THEs(t)in, APCD and HTEs(t)out, APCD represent the HTEs(t) in flue gas at
235
the inlet and outlet of the APCD.
(1)
236
The removal efficiency of the eight HTEs in flue gas across ESP, wet FGD, and
237
ESP + wet FGD is shown in Figure 4. ESP has great effects on the removal of the
238
eight HTEs in flue gas, which ranges from 99.43% to 99.95%. Wet FGD can remove
239
18.10% of Sb, 22.25% of Ba, 23.16% of Cr, 28.39% of Mn, 31.15% of As, 53.17% of
240
Pb, 61.26% of Zn, 68.47% of Cd, in the flue gas, respectively. The total removal
241
efficiency of ESP + wet FGD on the HTEs is in the range of 99.78%-99.96%, which
242
agrees with the results of Deng et al. 8. This indicates that APCDs in power plant are
243
beneficial for HTEs’ removal besides capture of conventional air pollutants. 100.0
ESP Wet FGD ESP+Wet FGD
99.8 99.6
Removal 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
99.4 99.2 99.0 60 50 40 30 20 10 0
244
Zn
Sb
Pb
Cd
As
Cr
Mn
Ba
245
Figure 4 Removal efficiency of the eight HTEs across ESP, wet FGD, and ESP + wet
246
FGD
247
3.4 Emission characteristic of HTEs in flue gas to atmosphere
248
Emission concentrations of the eight HTEs in flue gas to atmosphere are the sum
249
of their gaseous and particulate form at the outlet of wet FGD, which are in the scope
250
of 0.03- 8.11 µg/m3, as shown in Table 5. Though there is no certain emission 11
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251
standard for HTEs emitted from coal-fired power plants except mercury with value of
252
30 µg/m3 in China, the Integrated Emission Standard of Air Pollutants, enacted by the
253
National Environmental Protection Agency of China in 1996, specified the limits of
254
Pb, Cd, and Cr emitted for the pollution source established after January 1st, 1997 as
255
0.7, 0.85, and 0.07 mg/m3, respectively
256
concentration of Pb, Cd, and Cr is far less than the limits. Zhao et al.
257
average emission concentration of Cr, Ba, and Pb in a 350 MW coal-fired plant as
258
1.05, 4.28, and 0.21µg/m3, respectively, which is lower than the value in this study. It
259
may be mainly due to high content in the coal of this study.
30, 31
. It indicates that the emitted 32
gave the
260
Emission factor is commonly used for comparison between different power
261
plants or different conditions, which is defined as the ratio of ultimate HTEs emitted
262
to atmosphere to the amount of coal consumed (seen as Formula (2)) 33, EF = [HTE(t)stack /FC] × 1000
263
(2)
264
Where, EF represents emission factor, mg/ (t coal); HTE(t)stack is the total
265
emission mass amount of HTEs escaping from stack to atmosphere per hour, g/h; FC
266
represents mass amount of the feeding coal, t/h.
267
The emission factors of Cd, Sb, As, Pb, Cr, Zn, Mn, and Ba are also listed in
268
Table 6, which are in the range of 0.25-65.90 mg/ (t coal). Cd, Sb, and As have the
269
low emission factors of 0.25-1.23 mg/ (t coal), while Zn, Mn, and Ba have the high
270
value of 54.93-65.90 mg/ (t coal). The difference in the emission factors of the studied
271
HTEs may be due to their content and occurrence form in coal, coal combustion
272
temperature, removal efficiency of the APCD, the nature characteristics of the trace
273
element compounds themselves, etc. 26.
274
Table 6 Emission concentrations and emission factors of the eight HTEs in flue gas to
275
atmosphere
276 277
Zn
Sb
Pb
Cd
As
Cr
Mn
Ba
Conc. µg/m3
6.74 ±0.21
0.07 ±0.00
1.42 ±0.06
0.03 ±0.00
0.15 ±0.00
3.11 ±0.11
7.82 ±0.25
8.11 ±0.31
EF mg/(t coal)
54.93 ±1.82
0.61 ±0.01
11.53 ±0.52
0.25 ±0.01
1.23 ±0.03
25.07 ±0.78
63.46 ±2.08
65.90 ±2.61
Notes: Conc.: concentration
3.5 Enrichment of HTEs in ESP ash and bottom ash
278
Relative enrichment index (REI) can be adopted to assess the enrichment
279
characteristic of HTEs in bottom ash and ESP ash, which can be expressed in Formula 12
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280
(3) 34, 35. REI = HTEsash × Acoal, ad / HTEscoal (3)
281 282
Where, REI represents relative enrichment index of HTEs in ESP ash or bottom
283
ash; HTEsash represents the concentration of HTEs in bottom ash or ESP ash, mg/kg;
284
Acoal,ad represents the ash content in the feeding coal on air dried basis.
285
The REI of the eight HTEs in ESP ash and bottom ash is shown in Figure 5. For
286
Mn, the REI in both bottom ash and ESP ash is higher than 1, which shows that it
287
enriches in bottom ash and ESP ash equally. For Pb, Sb, Ba, Cr, Zn, Cd, and As, the
288
REI of them in ESP ash is more than that in bottom ash, which indicates that these
289
HTEs are more enriched in ESP ash than bottom ash. The differences in the REI of
290
the eight HTEs in ESP ash and bottom ash have relations with the mineral
291
composition, surrounding temperature, and pore structure of ash, as well as their
292
existence form in coal and flue gas, etc. 17. For Mn, it is mainly bound to the dispersed
293
material with the most occurrences of carbonate and residual form in coal
294
coal combustion process, it is hard to escape into flue gas. Thus, it always appears in
295
residual particles, such as bottom ash and ESP ash.
35, 36
. In
1.2 Pb Mn 1.0
Ba
Sb
REI of ESP ash
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
Cr
0.8 As
Zn
Cd
0.6
0.4
0.2
0.0 0.0
298
0.4
0.6
0.8
1.0
1.2
REI of bottom ash
296 297
0.2
Figure 5 Relative enrichment index of the eight HTEs in bottom ash and ESP ash 4. CONCLUSIONS
299
The coal used in this work belongs to lean coal, which has little high Cl and low
300
S content. Mass balance rates of the eight HTEs of the whole system and each device
301
are in the acceptable range of 70%-130%. The eight HTEs are mainly distributed in
302
ESP ash with relative mass distribution ratio of 86.23%-98.25%, followed by
303
1.65%-13.67% for the bottom ash. Mass amount proportion of the eight HTEs 13
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304
removed by wet FGD and in flue gas to atmosphere accounts for very little.
305
Concentrations of Cd, Sb, As, Cr, Pb, Zn, Mn, Ba in flue gas at the inlet and outlet of
306
SCR are 18.22, 78.60, 380.57, 1416.76, 3021.00, 3746.24, 5720.50, 20355.09 µg/m3
307
and 18.02, 60.83, 358.42, 1418.04, 3023.00, 3753.47, 5596.90, 20382.44 µg/m3,
308
respectively. Removal efficiency of the eight HTEs in flue gas by ESP + wet FGD is
309
99.78%-99.96%, while that for ESP is 99.43%-99.95%. Wet FGD can capture 18.10%
310
of Sb, 22.25% of Ba, 23.16% of Cr, 28.39% of Mn, 31.15% of As, 53.17% of Pb,
311
61.26% of Zn, 68.47% of Cd, in the flue gas compared to that at its inlet, respectively.
312
Concentrations of Cd, Sb, As, Pb, Cr, Zn, Mn and Ba in flue gas emitted to
313
atmosphere are 0.03, 0.07, 0.15, 1.42, 3.11, 6.74, 7.82, and 8.11 µg/m3, respectively.
314
Cd, Sb, and As have low emission factors of 0.25-1.23 mg/ (t coal), but Zn, Mn, and
315
Ba have high value of 54.93-65.90 mg/ (t coal). Except Mn, which enriches in bottom
316
ash and fly ash equally, all the other studied HTEs are prone to enrich in ESP ash than
317
bottom ash.
318
319
Corresponding Author
320
Yufeng
321
86+025-83795652.
322
Notes
323
The authors declare no competing financial interest.
324
AUTHOR INFORMATION
Duan:
[email protected] Telephone:
86+025-83795652.
Fax:
ACKNOWLEDGMENTS
325
This project was financially supported by the National Key Research and
326
Development Program (2016YFB0600604), the National Natural Science Foundation
327
of China (51376046, 51576044), the Scientific Research Foundation of Graduate
328
School of Southeast University (YBJJ1706), and the Graduate Student Research and
329
Innovation Program of Jiangsu Province (KYCX17_0072). The authors would like to
330
thank the anonymous reviewers for their critical comments.
331
332
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