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Partitioning of hazardous trace elements among air pollution control devices in ultra-low-emission coal-fired power plants Chenghang Zheng, Li Wang, Yongxin Zhang, Jun Zhang, Haitao Zhao, jinsong Zhou, Xiang Gao, and Kefa Cen Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017
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Partitioning of hazardous trace elements among air pollution control devices in ultra-low-emission coal-fired power plants Chenghang Zheng, Li Wang, Yongxin Zhang, Jun Zhang, Haitao Zhao, Jinsong Zhou, Xiang Gao*, Kefa Cen State Key Laboratory of Clean Energy Utilization, State Environmental Protection Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou 310027, Zhejiang, China
Abstract: In this work, the partitioning of hazardous trace elements among air pollution control devices in five ultra-low-emission coal-fired power plants was investigated. Results showed that most of the trace elements were enriched in fly ash at 58.0% to 93.3% (Hg), 75.2% to 95.3% (As), 78.2% to 94.9% (Cd), 79.4% to 96.6% (Se), 73.8% to 89.2% (Cr), and 86.5 to 99.5% (Pb). Low-low temperature electrostatic precipitator (LLT-ESP) and electric fabric filter (EFF) greatly increased the relative enrichment factors of Hg, As, and Se in fly ash up to 0.78 to 1.23, 0.85 to 1.04, and 0.83 to 0.99, respectively. In the wet flue gas desulfurization (WFGD) system, the concentrations of trace elements in fine fractions were much higher than those in coarse fractions. Large amounts of Hg (2.17 μg/kg to 168 μg/kg), Se (21.3 μg/kg to 357 μg/kg), and Cd (44.1 μg/kg to 839 μg/kg) in wastewater needed special treatment to satisfy the discharge standard. The wet electrostatic precipitator (WESP) system removed hazardous trace elements mainly by capturing fine particles in the flue gas, and a small amount of hazardous trace elements (0.2% to 26%) were retained in the washing water. The concentrations of Hg in the fine particulates captured by WESP were 16.8 to 60.1 times 1
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of those in the fly ash, which could reach up to 17.5 mg/kg. The application of selective catalytic reduction + LLT-ESP/EFF + WFGD + WESP could effectively control the emission of hazardous trace elements in coal-fired power plants. Keywords: Ultra-low emission, hazardous trace elements, partitioning, air pollution control devices
1. Introduction With the burgeoning economy, the growing demand for electricity in China relies heavily on coal-fired power plants. The toxic hazardous trace elements, such as Hg, As, Se, and Pb, released from coal combustion have received increasing attention due to their high mobility, bioaccumulation, and persistent harm to the health of people1, 2 . The Environmental Protection Agency finalized the first national Clean Air Act standards in December 20113 to reduce mercury and other toxic hazardous trace elements from coal-fired power plants. The Ministry of Environmental Protection of the People’s Republic of China regulated the emission limit of Hg for power generation for the first time in January 1, 20124, suggesting a reinforcement in the assessment and management of the mercury released from coal-fired power plants. The control of other hazardous elements has also gradually aroused attention. Forecasting environmental pollution
because
of
emissions
of
several
of
hazardous
the trace
elements from the thermal power industry is only possible in terms of the objective knowledge of heir speciation, concentrations conditions
in
the original fuel
5, 6
distribution,
and
accumulation
and the combustion byproducts 7-9.
The simultaneous removal of hazardous trace elements by installed air pollution 2
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control devices (APCDs) is one of the most promising technologies due to its high feasibility at no additional operation cost. Previous research indicated that APCDs have an enormous impact on the partitioning of hazardous trace elements, and the removal efficiency varies. The selective catalytic reduction (SCR) system can promote the transformation of Hg0 to Hg 10-13and subsequently increase the removal efficiency of mercury in the wet flue gas desulfurization (WFGD) system. Hazardous trace elements tend to be adsorbed on particulate matters (PMs) and subsequently captured by the electrostatic precipitator (ESP) and fabric filter (FF) with the decreasing flue gas temperature. The amount of mercury captured in different types of particulate removal equipment varies from 5% to 91% based on reported data14-17. For As, Cd, Se, Cr, and Pb, fly ash shows a high enrichment capacity 11, 12 with a removal efficiency of up to approximately 80% to 99%
18, 19
. The WFGD system can capture most of Hg2+ and
other hazardous trace elements. The total mercury removal efficiency can reach up to 70% to 95%20-22, and a limited amount of other hazardous trace elements may escape from the wet scrubber14, 23, 24. The hazardous trace elements removed by APCDs are distributed in solid residues and effluents, and this phenomenon has aroused concerns about the environmental risks from the solid residues25 and wastewater discharged from power plants26, 27. Therefore, the study on the distributions of hazardous trace elements in solid and liquid streams across APCDs is of great significance. The ultra-low-emission technology for coal-fired power plants was proposed and has been widely applied in China to satisfy the strict environmental regulations. Coalfired power plants are typically equipped with SCR + low-low temperature ESP (LLT3
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ESP)/electric FF (EFF) + WFGD + wet ESP (WESP). LLT-ESP is employed to lower the ESP inlet gas temperature below the acid dew point using a low-temperature economizer or hot media gas–gas heat exchanger, and the gas temperature is typically below 95 °C. The PM removal efficiency can be increased under this mode. WESP is usually installed downstream of the WFGD to capture ultrafine particulates and other pollutants. With the ultra-low-emission system, PM, NOx, and SO2 emissions can be limited to 5, 35, and 50 mg/m3, respectively. Previous studies indicated that ultra-lowemission reformation has significant effects on trace element emission control20, 28. However, few studies on the partitioning and removal mechanism of hazardous trace elements through ultra-low-emission APCDs are available. In this paper, we present a comprehensive analysis of the partitioning of typical hazardous trace elements (Hg, As, Cd, Se, Cr, and Pb) in solid and liquid streams in five typical ultra-low-emission coal-fired power plants. The concentrations and mass distribution of hazardous trace elements in solid and liquid streams across APCDs were obtained. The relative enrichment factors (REFs) in fly ash and bottom ash were calculated, and the partitioning patterns of hazardous trace elements in the WFGD and WESP systems were analyzed. The information on the mass flow of hazardous trace elements from this study provides new insights into the hazardous trace elements abatement capacity over ultra-low-emission APCDs and establishes a foundation for further control strategies for hazardous trace elements.
2. Materials and methods 2.1 Site description 4
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Five coal-fired power plants with typical ultra-low-emission systems were selected. The information on the tested coal-fired power plants are presented in Table 1. Four of the five tested plants are pulverized coal (PC) boilers with installed capacities that range from 660 MW to 1000 MW. The fifth one is a circulating fluidized bed (CFB) boiler that burns blended coal and waste sludge. The properties of the feed fuel during the sampling process are shown in Table S1 in the supporting information. The tested plants are equipped with SCR systems, and plant-5 employs a combined selective noncatalytic reduction (SNCR) + SCR system to control NOx. Cold-side ESP (CS-ESP) is applied in plant-3, LLT-ESPs are installed in plants-1, 2, 4, and EFF is installed in plant5 to remove the particles in flue gas with high efficiency. WFGD systems are installed to control SOx and simultaneously remove other pollutants. WESP is installed downstream of the WFGD to further remove the remaining particles, carryovers, and other pollutants. A WFGD and WESP integration system (WFGD&WESP) is applied in plant-3. In this integration system, the WESP was installed on top of the wet scrubber, into which the washing water of the WESP flows down directly. Table 1. Information of tested plants
Installed capacity
Boiler type
Coal type
APCDs
Plant-1
1000 MW
PC
Bita
SCR + LLT-ESP + WFGD + WESP
Plant-2
660 MW
PC
Bita
SCR + LLT-ESP + WFGD + WESP
Plant-3
660 MW
PC
Bita
SCR + CS-ESP + WFGD&WESP
Plant-4
660 MW
PC
Bita
SCR + LLT-ESP + WFGD + WESP
Plant-5
50 MW
CFB
Bita+sludge
SNCR-SCR + EFF + WFGD + WESP
a
Bit is bituminous coal.
2.2 Sample collection The operational parameters of the power plants during the sampling campaigns are 5
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presented in Table S2 in the supporting information. The sampling campaigns were carried out under continuous operating conditions, with the boiler loads maintained at approximately 70% to 100%. The APCD configurations and sampling points are shown in Figure 1. All the solid and liquid streams in and out of the system were collected. The input flows included feed fuel, limestone, and process water, and the output flows included fly ash, bottom ash, gypsum, wastewater, and sludge. WFGD slurry and WESP wastewater containing solid substances were also obtained. The WESP wastewater and the contained slag were discharged into the wet scrubber (as shown in Figure 1). Hence, they were not included in the output flows. The solid effluents in the WESP wastewater (slag in WESP) were essentially the fine particles captured by the WESP system. Solid and liquid streams were collected at each sampling point in triplicate and were rapidly sealed in sterilized bottles to keep the samples uncontaminated.
Figure 1. APCD configurations and sampling points
2.3 Sample analysis WFGD and WESP wastewater samples were filtered to obtain the liquid phase and the corresponding solid phase. Slurry samples from WFGD were obtained, with approximately 15% to 17% in solid phase, and stored in centrifuge tubes. We employed 6
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a simple size separation procedure by manually shaking the tubes and then allowing them to settle under quiescent condition29. Two parts of the slurry samples were observed, namely, the dark brownish-colored fraction that floats at the top (fine fraction) and the light-colored denser fraction that settled at the bottom (coarse fraction), which were separated using vacuum suction. All the solid samples and precipitation obtained from the liquid samples were dried in a desiccator to a constant weight at 45 °C before the test. The test methods used in this study for the chlorine content analysis and the proximate and ultimate analysis of feed fuel are listed in Table S3. The mercury in the solid and liquid samples was directly measured by a RA–915M mercury analyzer with a PYRO–915+ pyrolysis equipment. For other hazardous trace elements, the solid samples were first digested by a HF-HNO3 solution in a microwave oven. The recovered solution samples and the liquid samples were analyzed for As, Cd, Se, Cr, and Pb using inductively coupled plasma mass spectrometry with reported by other authors30-32. X-ray fluorescence was used to determine the contents of major oxides and minor elements in the solid samples. The particle size of the solid samples was determined by a Malvern Particle Size Analyzer (MASTERSIZER 2000). All samples were tested in triplicate, and the measurement errors were maintained within ±5%.
2.4 Mass balance of solid and liquid phases The mass balance was calculated by normalizing the concentration of each trace element in a given stream with the corresponding stream flow for the entire installation. The hazardous trace elements in the stack gas were not investigated in this study due to 7
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the high hazardous trace elements removal efficiency of the ultra-low-emission APCDs. The mass balance results obtained from our tests were within the acceptable error range of ±30%33, thus providing reliable raw data. The flows considered for the overall mass balance calculations are as follows: Output flowg Ci , g flowFA Ci , FA flowBA Ci , BA floww&s Ci ,w&s , Input flowL Ci , L flowC Ci ,C flowPW Ci , PW
,
Massbalance Output / Input ,
(1) (2) (3)
Where flowg, flowFA, flowBA, Floww&s , flowl, flowFF, and flowpw represent the mass flows of gypsum, fly ash, bottom ash, wastewater and sludge, limestone, fuel, and process water, respectively. Ci represents the mean concentration of a certain element in the corresponding sample.
3. Results and discussion 3.1 Partitioning of hazardous trace elements in fly ash and bottom ash The concentrations of the hazardous trace elements in the feed fuel and combustion products (bottom ash and fly ash) are presented in Table 2. The Hg and Cd contents in the feed fuel, which varied from 0.063 mg/kg to 0.141 mg/kg and 0.1 mg/kg to 0.77 mg/kg, respectively, were distinctly lower than the other hazardous trace elements. The sludge blended in the coal resulted in high contents of ash and most hazardous trace elements in the feed fuel of plant-5. The Hg, As, Cd, Se, Cr, and Pb in the fly ash captured by ESP and EFF ranged from 0.38 mg/kg to 0.535 mg/kg, 21.9 mg/kg to 39.8 mg/kg, 0.53 mg/kg to 2.32 mg/kg, 9.61 mg/kg to 19.5 mg/kg, 39.1 mg/kg to 1190 mg/kg, and 78.5 mg/kg to 130 mg/kg, respectively, which were generally higher than those in 8
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the bottom ash. Table 2. Mean concentrations of hazardous trace elements in feed fuel and ash Hg
As
Cd
Se
Cr
Pb
Feed fuel
mg/kg
Plant-1
0.073
5.29
0.15
1.32
15.6
13.6
Plant-2
0.129
6.94
0.21
3.12
19
13.1
Plant-3
0.141
5.34
0.1
3.36
12.6
19.6
Plant-4
0.063
3.96
0.12
2.15
6.7
20.8
Plant-5
0.09
9.4
0.77
5.09
394
24.7
Plant-1
0.515
39.7
1.14
9.61
131
130
Plant-2
0.535
39.8
0.94
13.8
75.3
78.5
Plant-3
0.374
21.9
0.53
13.3
58.5
128
Plant-4
0.291
22.9
0.66
12.57
39.1
163
Plant-5
0.38
28.5
2.32
19.5
1190
97
Plant-1
0.0081
6.13
0.97
3.36
106
77.8
Plant-2
0.0024
8.35
0.02
10.8
297
5.82
Plant-3
0.0001
5.02
0.08
