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Variation characteristics of final size-segregated PM emissions from ultralow emission coal-fired power plants in China
Environmental Pollution 259 (2020) 113886
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Variation characteristics of final size-segregated PM emissions from ultralow emission coal-fired power plants in China* Bobo Wu a, b, Xiaoxuan Bai a, b, Wei Liu a, b, Chuanyong Zhu b, c, Yan Hao a, Shumin Lin a, b, Shuhan Liu a, b, Lining Luo a, b, Xiangyang Liu a, b, Shuang Zhao a, b, Jiming Hao d, Hezhong Tian a, b, * a
State Key Joint Laboratory of Environmental Simulation & Pollution Control, School of Environment, Beijing Normal University, Beijing, 100875, China Center for Atmospheric Environmental Studies, Beijing Normal University, Beijing, 100875, China School of Environmental Science and Engineering, Qilu University of Technology, Jinan, 250353, China d School of Environment, Tsinghua University, Beijing, 100084, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2019 Received in revised form 25 December 2019 Accepted 26 December 2019 Available online 28 December 2019
The ultralow emission (ULE) retrofits for Chinese coal-fired power plants (CFPPs) are nearing completion. Large-scale and rapid retrofits have resulted in significant changes in the emission level and characteristics of particulate matter (PM). To investigate the variation characteristics of final three size fractions PM (PM2.5, PM102.5, PM>10) emissions, we conducted field tests at the outlets of wet flue gas desulfurization (WFGD) and wet electrostatic precipitator (WESP) by a pair of two-stage virtual impactors in eight representative ULE CFPPs. Our results indicate that, after WESP installations, the mass concentrations of final PM are significantly reduced and those of the final total ions and elements decrease as most individual chemical compositions are reduced. WESP presents an excellent removal performance for large particle sizes and high PM concentrations. SO2 4 is the major ionic component at both the outlets of WFGD and WESP, and its proportion in total ions is reduced to some extent through WESP. Furthermore, the average mass contents of SO2 4 and most elements in PM2.5 are significantly lower than those in PM102.5 and PM>10 whether at the WFGD-outlets or WESP-outlets. By comparison, chemical profiles of PM have substantially changed after ULE retrofits, and those after WFGD (e.g., sulfate, Zn, Pb, and Cu) have also changed relative to existing data. The end-tail emission factors (EFs) of PM2.5, PM10, and PMtotal under the typical ULE technical routes of WESP are calculated in time, and the corresponding EFs are in the range of 2.82e8.97, 15.7e27.6, and 38.6e61.7 g t1, respectively. We believe the latest detailed PM EFs and the associated chemical profiles provided in this study are more representative of the current emission situations of Chinese CFPPs. © 2019 Elsevier Ltd. All rights reserved.
Keywords: ULE CFPPs Variation characteristics Size-segregated PM Chemical profiles Emission factors
1. Introduction In recent years, a wide range of continuous and severe haze pollution has occurred frequently in China, prompting unprecedented concerns on human health from the Chinese government and worldwide public organizations (Li and Zhang, 2014; Ma et al., 2017; Zhang et al., 2012). Based on the results of numerous studies
* This paper has been recommended for acceptance by Eddy Y. Zeng. * Corresponding author. State Key Joint Laboratory of Environmental Simulation & Pollution Control, School of Environment, Beijing Normal University, Beijing, 100875, China. E-mail address: [email protected] (H. Tian).
https://doi.org/10.1016/j.envpol.2019.113886 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
on the main sources of haze causes and sectors of pollutants, coalfired flue gas pollution is regarded as one of the major sources (Li et al., 2017a; Liang et al., 2019; Qiao et al., 2018; Wang et al., 2016; Zhang et al., 2013). It is well known that China is the top producer and consumer of coal in the world, and approximately half of the coal burned is used for power generation by Chinese coal-fired power plants (CFPPs) to support the electricity demand of rapid industrialization and urbanization (Chen et al., 2007; Shiu and Lam, 2004; Tong et al., 2018). Consequently, large amounts of pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM) and its toxic and harmful chemical components (sulfates, nitrates, trace elements, etc.) are released into the environment (Liu et al., 2015; Tian et al., 2013; Tian et al., 2015; Zhao et al., 2017; Zhao et al., 2008), contributing to severe haze
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formation (Cheng et al., 2016; Zhao et al., 2013) and adverse impacts on air quality (Fu and Chen, 2017; Hao et al., 2007), human health (Amster et al., 2014; Burnett et al., 2014; Zheng et al., 2015) and climate change (Akimoto, 2003; Wang and Chen, 2016). Therefore, the Chinese government has always regarded CFPPs as a key regulatory source of air pollution for control (Li et al., 2019). According to the “Action Plan on Coal-fired Power Emission Reduction and Upgrading” issued by three ministries in 2014, coalfired power units with installed capacities over 300 MW are forced to comply with the ultralow emission (ULE) standards (stack concentrations of NOx, SO2, and PMtotal (total particulate matter) less than 50, 35, and 10 mg Nm3, respectively) by 2020 (NDRC, 2014). These standards are currently the strictest emissions standards for CFPPs in the world. In addition to the strict regulations on newly built CFPPs, traditional in-use CFPPs should also be retrofitted to meet ULE standards. Before the ULE retrofits, traditional mature air pollution control devices (APCDs) installed in CFPPs show remarkable performance in dealing with different air pollutants, but it is still difficult to meet the ULE standards (Yi et al., 2008; Zhao et al., 2010; Wang et al., 2011; Liu et al., 2015). Both NOx and SO2 can be effectively reduced by upgrading the original selective catalytic reduction (SCR) and wet flue gas desulfurization (WFGD) systems. However, it is very difficult to achieve the strict ULE standard of PM controlled by conventional technology of electrostatic precipitators (ESP) (Parker, 1997; Kim and Lee, 1999; Huang and Chen, 2002; Xu et al., 2016a). The performance of fabric filters (FF) and the composite ESP&FF for PM removal has been continuously improved. With the popularization of flue gas desulfurization in Chinese CFPPs, WFGD is widely installed after these primary dry precipitators (CEC, 2018), which is thought to facilitate further PM removal through scrubbing. However, previous field measurements have found that flue gas entrains new particles, which are mainly recirculated slurry, through WFGD (Li et al., 2017b; Meij and te Winkel, 2004; Wu et al., 2018). Therefore, the terminal emission concentrations of PM are generally over 15 mg Nm3 under the aforementioned combination of processes (Cao et al., 2017; Wu et al., 2018). During recent years, low-low temperature ESP (LLT-ESP) (Chen et al., 2019; Wang et al., 2015; Wang et al., 2019) and wet electrostatic precipitators (WESP) (Yang et al., 2017; Yang et al., 2018) have been widely installed in Chinese CFPPs as two important ULE technical routes, and those are effective in lowering PM emissions and achieving the ULE standards. With the wide implementation of ULE technologies, final emission characteristics of PM from CFPPs have changed accordingly. Several studies have conducted field tests on emission characteristics of PM from ULE CFPPs (Chen et al., 2019; Sui et al., 2016; Wang et al., 2019; Wu et al., 2018; Xu et al., 2016b). For the ULE technology of LLT-ESP, reducing the flue gas temperature is crucial in the reduction of PM (Wang et al., 2019). Wang et al. (2019) conducted field measurements in four CFPPs installed with LLT-ESP and WFGD, and found that the mass concentrations of PM2.5 (PM 2.5 mm in aerodynamic diameter) and PMtotal increased through WFGD. It can be seen that LLT-ESP, as primary dust dry precipitator, cannot solve the problem caused by the downstream WFGD. Chen et al. (2019) investigated the final emission characteristics of PM2.5 in four ULE CFPPs equipped with LLT-ESP and followed by WFGD, and found some of the chemical profiles were different from the previous studies. However, the final ULE technology of WESP installed after scrubbing towers is different from LLT-ESP, and which is an ideal choice to handle PM with various characteristics and eliminate the problems caused by pretreatment facilities. Sui et al. (2016) examined the emission characteristics of different particle size in an ULE CFPP equipped with an WESP. Xu et al., 2016b conducted field measurements at two 300 MW CFPPs to investigate the PM removal performance of
WESP. Cao et al. (2017) investigated a wet dust removal system combined with a wet phase transition agglomerator and a WESP, and found that this system helped to meet ULE of particles from CFPPs by keeping PM emissions below 5 mg Nm3 under all tested conditions. However, these studies have not involved the emission characteristics of detailed chemical compositions in PM through WESP. In our previous studies, we have conducted field tests in four 300 MW level ULE CFPPs installed with WFGD and WESP, mainly studying the effects of WFGD on PM and ionic components (Wu et al., 2018). Furthermore, the chemical characterization of primary emission sources, referred to as the source profile, is of great importance in atmospheric PM source apportionment (Bi et al., 2019; Chen et al., 2019; Liu et al., 2018; Liu et al., 2014). CFPPs are one of the major sources of PM emissions in China (Ma et al., 2017), and the database of chemical profiles of end-tail size-segregated PM is still very limited since the initiation of the ULE retrofits. Bi et al. (2019) investigated and reviewed the characteristics of main primary source profiles of PM in China from 1987 to 2017, and pointed out that some chemical profiles reported in previous studies were currently out of date and needed to be updated immediately. Actually, the real-world complicated operation conditions, high cost and difficult field tests limit the collection of enough samples for analysis. Comprehensive research on the variation characteristics of final size-segregated PM and the associated chemical compositions emissions from ULE CFPPs installed with WESP based on field tests is still lack. Therefore, fill the gaps of emission characteristics of PM from local CFPPs under various ULE technical routes and update emission data become urgently for compiling and updating emission inventories and source apportionment. In this study, we conducted field measurements at the outlets of WFGD and WESP in eight full-scale and representative ULE CFPPs to investigate the variation characteristics of final size-segregated PM, including size distribution, chemical characteristics, profiles, and emission factors (EFs). 2. Experimental methods 2.1. Tested plants description To ensure that the tested CFPPs are representative, we have considered the key factors affecting PM emissions, including the boiler type, coal type, unit capacity, and the installed APCDs. In this study, eight typical Chinese CFPPs were selected for field tests to cover more different installed capacities and processing technologies. During the tests, all tested power plants operated normally without failure. Table S1 summarizes the detailed information of the eight tested boilers and APCDs. The eight tested boilers are all pulverized coal (PC) boilers with the steam flow rate at the range of 480e2209 t h1, and the tested units include one 165 MW, five 300 MW level, one 600 MW, and one 700 MW. Since the research purpose of this study is to investigate the variation characteristics of final PM emissions after WESP installation, the selected CFPPs are all installed WESP with typical combinations of APCDs (e.g. SCR þ ESP/FF/ESP&FF þ WFGD þ WESP). The schematic configuration of the typical ULE technical route with WESP installation and its working theory is illustrated in Fig. S1. To minimize the impact of coal types on the emissions of chemical composition in PM, all the eight tested CFPPs used bituminous coal. The main indicators of feed coal are analyzed without detailed chemical compositions analysis, and the detailed category of bituminous coal, and coal grade of calorific value, total sulfur, and ash used in the tested CFPPs are shown in Table S2, according to the national standards of “Commercial coal quality-coal for pulverized coal-fired boiler for power generation (GB/T 7562-2018)” (SAC, 2018). The average mass
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contents of volatiles (Vdaf), ash (Ad) and sulfur (St,d) in the feed coal are in the range of 19%e36%, 15%e34%, and 0.59%e1.60%, respectively. During the measurements, we investigated the data of continuous emission monitoring system (CEMS), which is used to monitor the final emissions from CFPPs (Zhang et al., 2019). The hourly average mass concentrations of PMtotal, SO2, and NOx at the eight tested CFPPs for 24 h are shown in Fig. S2, and they all meet the corresponding ULE standards. The average mass concentrations and the ranges of PMtotal, SO2, and NOx tested by CEMS are 2.04 (0.48e4.02) mg Nm3, 15.65 (8.96e28.60) mg Nm3, and 29.70 (9.14e40.05) mg Nm3, respectively.
2.2. Particulate matters sampling description A pair of two-stage virtual impactors is used to collect PM samples, which are divided into three levels of PM2.5, PM102.5 (PM between 2.5 mm and 10 mm in aerodynamic diameter), and PM>10 (PM 10 mm in aerodynamic diameter), and all three sizesegregated PM samples are collected on quartz filters (Pall Corp. USA). During the tests, three parallel samples were collected simultaneously at both the outlets of WFGD and WESP. The diagram of size-segregated PM sampling system and the sampling positions are shown in Fig. 1, and detailed description of this PM sampling system can refer to our previous study (Wu et al., 2018) or the studies of Jiang et al. (2014) and Wada et al. (2016). To ensure the samples collected are valid in high humidity flue gas and avoid water vapor condensation on the filters, the impactor is preheated for 15e20 min in flue gas with the same direction of sampling nozzle and the flue gas flow before sampling. The sampling time depends on the level of PM mass concentration in flue gas, usually 90e120 min to ensure the collected mass is enough for analysis. Average sampling flow rates of PM>10, PM102.5, and PM2.5 are 1.68 ± 0.25, 2.27 ± 0.17, and 20.39 ± 1.78 L min1, respectively, depending on the velocity of the flue gas at the test point.
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2.3. Chemical composition of PM analysis The quartz filters are baked in a muffle furnace for 4 h at 450 C to remove organic compounds before sampling. Then, the filters are weighted after 48 h of equilibration (humidity, 50%; temperature, 25 C) before and after sampling to calculate the average PM concentration. Finally, the weighed filters are placed in the refrigerator with the temperature of 4 C before chemical composition anal2 þ þ ysis. Major ions (anions: F, Cl, NO 3 , and SO4 ; cations: Na , NH4 , Kþ, Mg2þ, and Ca2þ) are analyzed by ion chromatography (Dionex ICS-2000, Thermo Fisher Scientific Inc., USA). See the detailed pretreatment process for collecting samples in our previous study (Wu et al., 2018). Then, the remaining filters are digested for elemental analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES, SPECTRO ARCOS EOP, SPECTRO Analytical Instruments GmbH, USA) (Gao et al., 2018). In this study, a total of 22 elements are measured, which are divided into major elements (S, Al, Mg, Fe, Ca, Na, K, Ti, and Ba) and trace elements (Se, Cr, Ni, Zn, Mn, Co, Mo, Cu, Pb, V, Sb, As, and Cd.) based on the mass concentrations. In the end, the results of PM mass concentrations and chemical compositions are all unified to 6% O2 for all the tested CFPPs for the sake of comparison. 3. Results and discussion 3.1. Variation characteristics of size-segregated PM through WESP The mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal at the outlets of WFGD and WESP for the eight tested ULE CFPPs are presented in Fig. 2. In general, the mass concentrations of the three size-segregated PM at the tested sites show PM>10 > PM102.5 > PM2.5. The average mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal at the WFGD-outlets are in the ranges of 0.82e7.43, 4.06e8.38, 5.12e23.42, and 12.95e32.62 mg Nm3, respectively, and those of the corresponding particles at the
Fig. 1. The diagram of size-segregated PM (PM2.5, PM102.5, PM>10) sampling system and the sampling positions.
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Fig. 2. The mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal at the outlets of WFGD and WESP for the eight tested ULE CFPPs.
WESP-outlets are in the ranges of 0.37e1.53, 0.79e4.42, 1.06e7.74, and 2.35e13.70 mg Nm3, respectively. The results based on field tests reveal that the mass concentrations of PMtotal after WFGD are generally exceeds the ULE limits of PM (10 mg Nm3), and those after WESP can meet the ULE standards (even less than 5 mg Nm3 in several tested CFPPs). After WESP installations, the mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal for all the tested CFPPs are further removed at the ranges of 27%e83%, 39%e83%, 36%e84%, and 39%e79%, respectively. However, we find the mass concentrations of PM analyzed by weight are slightly higher than those tested by CEMS (Fig. S2). For PM monitoring, CEMS mostly relies on optical principles such as transmission and scattering. The different measurement principles of these two methods may be one of the main reasons for the difference in their results. In addition, there are some potential influence factors, such as the artificial errors during sampling and equipment maintenance, complex test conditions and flue gas conditions. Furthermore, the mass concentrations of PM are reduced to a closer range after WESP. Normally, WESP presents a lower removal efficiency when the mass concentrations of PM in WFGD-outlets are at a lower level. For example, the average capture efficiency of WESP for PM2.5 is 31% (17%e47%) when PM2.5 mass concentrations are in the range of 0.46e0.99 mg Nm3 at the WFGD-outlets, whereas the average capture efficiency of WESP for PM2.5 is 69% (42%e84%) when PM2.5 mass concentrations are in the range of 1.04e9.46 mg Nm3 at the WFGD-outlets. Fig. 3 depicts the distribution of PM2.5, PM10-2.5, and PM>10 in the PMtotal at the outlets of WFGD and WESP, which can provide valuable data for updating and evaluating PM emissions from ULE CFPPs. In general, the proportions of PM>10, PM102.5, and PM2.5 in PMtotal decrease in sequence at each tested site, with the range of corresponding proportions being 44%e72%, 22%e45%, and 3%e26% at the WFGD-outlets, and 44%e67%, 15%e38%, and 7%e22% at the WESP-outlets, respectively. Along with the reduction of PM
through WESP, the proportion of three size-segregated PM in PMtotal changes accordingly for individual CFPPs. The proportions of PM>10 in PMtotal at the WESP-outlets in five (plant #1, #3, #4, #7, and #8) of the eight tested CFPPs are lower than those at the WFGD-outlets, as shown in Fig. 3. The PM>10 mass concentrations at WFGD-outlets of these five CFPPs are in the range of 9.15e23.42 mg Nm3, which are higher than the values of the other three CFPPs at the WFGD-outlets. As shown in Fig. 2, the capture efficiencies of PM>10 through WESP for the five CFPPs are generally higher than those of PM2.5 and PM102.5. Meanwhile, the proportions of PM2.5 and PM102.5 at the WESP-outlets are higher than those at the WFGD-outlets for the five CFPPs, except for one case (PM2.5 in Plant #4 with 83% capture efficiency). Based on the analysis above, WESP has better removal performance for large particle sizes and higher PM mass concentrations. By comparison, there are still big differences in the distribution of PM through WESP between this study and that of Sui et al. (2016). The results studied by Sui et al. (2016) based on field tests for one ULE CFPP showed that the PM2.5/PM10 ratio was close to 1 after WESP, indicating that PM102.5 were removed effectively, and most of the PM emitted to the environment will be PM2.5. However, the average mass ratios of PM>10, PM102.5, and PM2.5 are 4.6/2.5/1 and 3.8/2.1/ 1 at the outlets of WFGD and WESP in this study. The difference in the results may be caused by the different sampling systems, and an electrical low-pressure impactor (ELPIþ) was used in the study of Sui et al. (2016). Therefore, more test samples and multiple sampling methods are needed to further investigate the size distribution of PM for ULE CFPPs.
3.2. Variation characteristics of chemical compositions in PM through WESP The variation characteristics of ions and elements in PM2.5, PM102.5, and PM>10 through WESP for the eight tested ULE CFPPs
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Fig. 3. Percentage distribution of size-segregated PM at the outlets of WFGD and WESP for the eight tested ULE CFPPs of China.
are illustrated in Fig. 4a and Fig. 4b, respectively. As shown in Fig. 4a, the total ions mass concentrations decrease for most individual ion reduction through WESP before finally emitting into the atmosphere. The total mass concentrations of ions in PM2.5, PM102.5, and PM>10 decrease on average by 51%, 66%, and 66% from WFGD-outlets to WESP-outlets, respectively. At both the outlets of WFGD and WESP, sulfate is the major ionic component in total ions.
The average percentage of SO2 4 in total ions for PM2.5, PM102.5, and PM>10 is 64%, 66%, and 71% at the WFGD-outlets, respectively, and the corresponding percentage at the WESP-outlets is 58%, 62%, and 65%, respectively. It can be seen that the proportion of SO2 4 in total ions is reduced to some extent while the SO2 concentrations 4 decrease through WESP. As depicted in Fig. 4b, the mass concentrations of total elements decrease significantly through WESP,
Fig. 4. The mass concentrations of ionic species (a), major and trace elements (b) in size-segregated PM at the outlets of WFGD and WESP for the eight tested ULE CFPPs.
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with the average reduction of total elements in PM2.5, PM102.5, and PM>10 being 56%, 66%, and 64%, respectively. In addition, the total mass concentrations of ions and elements in PM>10, PM102.5, and PM2.5 decrease in sequence at outlets of WFGD and WESP.
3.3. Variation characteristics of chemical profiles of CFPPs through WESP In this study, the mass contents (mg g1) of chemical components at outlets of WFGD and WESP are calculated based on per unit mass of size-segregated PM, which will contribute to the studies on better understanding their variation characteristics and enriching the chemical profiles database of CFPPs after WESP installation. Fig. S3 and Fig. 5 depict the mass contents (mg g1) of individual ionic and elemental components per unit mass of PM at the outlets of WFGD and WESP. As expected, the mass content of most ionic and elemental components in size-fractionated PM change significantly across a wide range through WESP for the tested ULE CFPPs. The emissions of PM chemical profiles from different CFPPs also present significant differences, which may be affected by many factors, such as the chemical components in feed coal, combustion conditions, boiler load, APCDs configuration, and their performance for flue gas control process. The average mass contents of the total ions in PM2.5, PM102.5, PM>10 at WFGD-outlets (WESP-outlets) are 278 ± 102 (314 ± 108), 342 ± 123 (366 ± 169), and 296 ± 91 (324 ± 117) mg g1, respectively, and those of SO2 4 in the corresponding PM at WFGD-outlets (WESP-outlets) are 172 ± 94 (178 ± 77), 217 ± 93 (215 ± 114), and 205 ± 67 (208 ± 91) mg g1, respectively. It can be seen that the average mass contents of total ions and SO2 4 in three size fractions PM are in the order of PM102.5 > PM>10 > PM2.5. As depicted in Fig. S3, the mass contents of Naþ, Cl, and Mg2þ in PM2.5 at WESP-outlets are higher than those at WFGD-outlets for most samples. The mass content of Naþ in PM2.5 increases on average by 43% and may be affected to some extent by the alkaline solution (NaOH) added into the spraying and flushing liquid in WESP (Xu et al., 2016b). Total mass contents of the tested elements are 304 ± 78, 455 ± 203, and 558 ± 247 mg g1 in PM2.5, PM102.5, PM>10 at WESP-outlets, respectively. Since the average mass contents of most major and trace elements in PM2.5 are lower than those in PM102.5 and PM>10 at WESP-outlets, the total mass contents of major and trace elements in three sizesegregated PM all present an order of PM2.5 < PM102.5 < PM>10.
Among the major elements, average mass contents and ranges of S, Mg, and Ca in the size-fractionated PM present a decreasing trend through WESP except for Mg and Ca in PM2.5. Sui et al. (2016) pointed out that the limestone/gypsum particles were carried into the flue gas after FGD and removed by WESP subsequently. Xu et al., 2016b reported that WESP could effectively capture the new Ca-enriched particles formed in WFGD, and reduce the Ca content in the PM. Furthermore, the average mass contents and ranges of Al, Fe, Na, K, and Ba in three size-fractionated PM present an increasing trend through WESP. These trends are possibly affected by factors related to the circulating of solutions, low condensation temperature, adhesion, or the complex working conditions in WESP (Sui et al., 2016; Xu et al., 2016b). For the tested trace elements, the average mass contents of Cr, Ni, Co, Mo, V, and Sb increase through WESP. We also compare the chemical profiles of PM obtained in this study with the results reported by Bi et al. (2019), to investigate the variation characteristics after ULE retrofits. The results reported by Bi et al. (2019) are all PM2.5 after WFGD without WESP installation, and the ionic and elemental profiles are shown in Fig. S3 and Fig. 5. 2þ 2þ For ionic profiles, SO2 are the dominate species in 4 , Ca , and Mg CFPPs at the WFGD-outlets. By comparison, the latest mass con2þ tents of SO2 4 , Mg , and F in this study are higher, while those of þ 2þ Ca and NH4 are lower than the traditional ionic profiles of Bi et al. (2019). The increase in SO2 and F may indicate that the 4 entrainment of recirculated slurry by flue gas after WFGD is more pronounced, and the increase in Mg2þ results from the magnesiumbased WFGD in this study. Comparing with traditional APCDs, the upgrading of various facilities (e.g. SCR, ESP, and WFGD) under ULE retrofits may result in the reduction of Ca2þ and NHþ 4 in PM2.5. The mass contents of the remaining test ions (Naþ, Kþ, Cl, and NO 3 ) are similar to those reported by Bi et al. (2019). For elemental profiles, the average mass contents and range of most elements reported by Bi et al. (2019) are close to the results of this study. However, we find significant differences in the mass contents of several elements, such as S, Mg, Ca, Zn, Cu, and Pb, through a t-test analysis, which is a statistical method used to determine if there is a significant difference between the means of two groups. The elements 2þ of S, Mg, and Ca present similar trends with the ions of SO2 4 , Mg , 2þ and Ca . Moreover, the average mass contents of Zn, Cu, and Pb in this study are significantly lower than those reported by Bi et al. (2019).
Fig. 5. The mass contents (mg g1) of major elements (a) and trace elements (b) in per unit mass of size-segregated PM at the outlets of WFGD and WESP.
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Table 1 Emission factors (g t1) of PM2.5, PM10, and PMtotal based on different combination of APCDs from various studies. study
after APCDs
PM2.5
PM10
PMtotal
This study
FF þ WFGD ESP þ WFGD ESP&FF þ WFGD FF þ WFGD þ WESP ESP þ WFGD þ WESP ESP&FF þ WFGD þ WESP ESP þ WFGD ESP þ WFGD ESP þ WFGD ESP þ WFGD ESP þ WFGD þ WESP ESP þ WFGD þ WESP ESP&FF þ WFGD
13.3 ± 11.4 25.9 ± 20.5 6.24 ± 4.33 4.58 ± 1.73 8.97 ± 4.81 2.82 ± 1.19 14.7A (9.20A22.5A) 241 ± 93 38 ± 4 13 11 ± 4 1 3
67.0 ± 27.1 75.0 ± 32.1 69.9 ± 40.9 15.7 ± 5.09 27.6 ± 20.8 17.4 ± 10.5 21.0A (12.9A31.7A) 221 ± 88
111 ± 29.9 171 ± 52.5 247 ± 120 42.7 ± 9.05 61.7 ± 47.4 38.6 ± 16.8 23.1A (14.2A34.8A) 111 ± 44
16 12 ± 4 2 25
19 14 ± 5 3 42
Zhao et al. (2010) Wang et al. (2011) Xu et al. (2017) Li et al. (2018) Yang et al. (2018) Note: A, ash content in feed coal, %.
3.4. Comprehensive EFs of size-segregated PM based on field tests Since the mass concentrations of PM are further reduced by WESP, it is particularly urgent to update the existing PM EFs of ULE CFPPs in China. By combining field test data from WESP and survey results of related production information during the sampling period in eight ULE CFPPs, we calculated the EFs (g t1) of PM based on the mass of coal consumed under different combinations of APCDs (as listed in Table 1). The EFs of PM2.5, PM10, and PMtotal after WESP treatment combined with three typical dust collectors (ESP, FF, and ESP&FF) and WFGD are in the range of 2.82e8.97, 15.7e27.6, and 38.6e61.7 g t1, respectively. We also compared the results of this study with other previous studies (Li et al., 2018; Wang et al., 2011; Xu et al., 2017; Yang et al., 2018; Zhao et al., 2010). By comparison, we find that the EFs of PM have reduced significantly since the implementation of the ULE standards (as shown in Fig. S4). The reduction of PM is due to not only the installation of WESP but also the upgrading of upstream dust and SO2 capture facilities. However, despite the same combination of APCDs, differences in PM EFs still exist in various studies. Therefore, more field tests should be conducted to enrich the database of PM EFs. We believe the new updated EFs of PM in this study, which are summarized based on the field test results of eight typical CFPPs in the same period after the implementation of ULE, are more representative of the current emission levels of Chinese CFPPs. These EFs would benefit for updating emission inventories of Chinese CFPPs and result in better environmental management and policy making.
CFPPs with WESP treatment are developed in this study. By comparison, chemical profiles in PM after WFGD and WESP have indeed changed relative to existing data since the initiation of the ULE retrofits. To update the EFs in time, we have calculated the end-tail EFs of PM2.5, PM10, and PMtotal under the ULE technical routes of ESP/FF/ESP&FF þ WFGD þ WESP, and the corresponding EFs are in the range of 2.82e8.97, 15.7e27.6, and 38.6e61.7 g t1, respectively. We believe the new updated EFs of PM and their chemical profiles in this study are more representative of the current emission levels of Chinese CFPPs and can supplement and replace the outdated data in the relevant research. Acknowledgements This work was funded by the National Natural Science Foundation of China (21777008, 21377012, and 21177012), the Trail Special Program of Research on the Cause and Control Technology of Air Pollution under the National Key Research and Development Program of China (2018YFC0213202, 2016YFC0201501), and the National Key Scientific and Technological Project on Formation Mechanism and Control of Heavily Air Pollution (DQGG0209). We thank the editors and anonymous reviewers for their valuable suggestions and comments on improving this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.113886.
4. Conclusion
References
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Variation characteristics of final size-segregated PM emissions from ultralow emission coal-fired power plants in China* Bobo Wu a, b, Xiaoxuan Bai a, b, Wei Liu a, b, Chuanyong Zhu b, c, Yan Hao a, Shumin Lin a, b, Shuhan Liu a, b, Lining Luo a, b, Xiangyang Liu a, b, Shuang Zhao a, b, Jiming Hao d, Hezhong Tian a, b, * a
State Key Joint Laboratory of Environmental Simulation & Pollution Control, School of Environment, Beijing Normal University, Beijing, 100875, China Center for Atmospheric Environmental Studies, Beijing Normal University, Beijing, 100875, China School of Environmental Science and Engineering, Qilu University of Technology, Jinan, 250353, China d School of Environment, Tsinghua University, Beijing, 100084, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2019 Received in revised form 25 December 2019 Accepted 26 December 2019 Available online 28 December 2019
The ultralow emission (ULE) retrofits for Chinese coal-fired power plants (CFPPs) are nearing completion. Large-scale and rapid retrofits have resulted in significant changes in the emission level and characteristics of particulate matter (PM). To investigate the variation characteristics of final three size fractions PM (PM2.5, PM102.5, PM>10) emissions, we conducted field tests at the outlets of wet flue gas desulfurization (WFGD) and wet electrostatic precipitator (WESP) by a pair of two-stage virtual impactors in eight representative ULE CFPPs. Our results indicate that, after WESP installations, the mass concentrations of final PM are significantly reduced and those of the final total ions and elements decrease as most individual chemical compositions are reduced. WESP presents an excellent removal performance for large particle sizes and high PM concentrations. SO2 4 is the major ionic component at both the outlets of WFGD and WESP, and its proportion in total ions is reduced to some extent through WESP. Furthermore, the average mass contents of SO2 4 and most elements in PM2.5 are significantly lower than those in PM102.5 and PM>10 whether at the WFGD-outlets or WESP-outlets. By comparison, chemical profiles of PM have substantially changed after ULE retrofits, and those after WFGD (e.g., sulfate, Zn, Pb, and Cu) have also changed relative to existing data. The end-tail emission factors (EFs) of PM2.5, PM10, and PMtotal under the typical ULE technical routes of WESP are calculated in time, and the corresponding EFs are in the range of 2.82e8.97, 15.7e27.6, and 38.6e61.7 g t1, respectively. We believe the latest detailed PM EFs and the associated chemical profiles provided in this study are more representative of the current emission situations of Chinese CFPPs. © 2019 Elsevier Ltd. All rights reserved.
Keywords: ULE CFPPs Variation characteristics Size-segregated PM Chemical profiles Emission factors
1. Introduction In recent years, a wide range of continuous and severe haze pollution has occurred frequently in China, prompting unprecedented concerns on human health from the Chinese government and worldwide public organizations (Li and Zhang, 2014; Ma et al., 2017; Zhang et al., 2012). Based on the results of numerous studies
* This paper has been recommended for acceptance by Eddy Y. Zeng. * Corresponding author. State Key Joint Laboratory of Environmental Simulation & Pollution Control, School of Environment, Beijing Normal University, Beijing, 100875, China. E-mail address: [email protected] (H. Tian).
https://doi.org/10.1016/j.envpol.2019.113886 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
on the main sources of haze causes and sectors of pollutants, coalfired flue gas pollution is regarded as one of the major sources (Li et al., 2017a; Liang et al., 2019; Qiao et al., 2018; Wang et al., 2016; Zhang et al., 2013). It is well known that China is the top producer and consumer of coal in the world, and approximately half of the coal burned is used for power generation by Chinese coal-fired power plants (CFPPs) to support the electricity demand of rapid industrialization and urbanization (Chen et al., 2007; Shiu and Lam, 2004; Tong et al., 2018). Consequently, large amounts of pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM) and its toxic and harmful chemical components (sulfates, nitrates, trace elements, etc.) are released into the environment (Liu et al., 2015; Tian et al., 2013; Tian et al., 2015; Zhao et al., 2017; Zhao et al., 2008), contributing to severe haze
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formation (Cheng et al., 2016; Zhao et al., 2013) and adverse impacts on air quality (Fu and Chen, 2017; Hao et al., 2007), human health (Amster et al., 2014; Burnett et al., 2014; Zheng et al., 2015) and climate change (Akimoto, 2003; Wang and Chen, 2016). Therefore, the Chinese government has always regarded CFPPs as a key regulatory source of air pollution for control (Li et al., 2019). According to the “Action Plan on Coal-fired Power Emission Reduction and Upgrading” issued by three ministries in 2014, coalfired power units with installed capacities over 300 MW are forced to comply with the ultralow emission (ULE) standards (stack concentrations of NOx, SO2, and PMtotal (total particulate matter) less than 50, 35, and 10 mg Nm3, respectively) by 2020 (NDRC, 2014). These standards are currently the strictest emissions standards for CFPPs in the world. In addition to the strict regulations on newly built CFPPs, traditional in-use CFPPs should also be retrofitted to meet ULE standards. Before the ULE retrofits, traditional mature air pollution control devices (APCDs) installed in CFPPs show remarkable performance in dealing with different air pollutants, but it is still difficult to meet the ULE standards (Yi et al., 2008; Zhao et al., 2010; Wang et al., 2011; Liu et al., 2015). Both NOx and SO2 can be effectively reduced by upgrading the original selective catalytic reduction (SCR) and wet flue gas desulfurization (WFGD) systems. However, it is very difficult to achieve the strict ULE standard of PM controlled by conventional technology of electrostatic precipitators (ESP) (Parker, 1997; Kim and Lee, 1999; Huang and Chen, 2002; Xu et al., 2016a). The performance of fabric filters (FF) and the composite ESP&FF for PM removal has been continuously improved. With the popularization of flue gas desulfurization in Chinese CFPPs, WFGD is widely installed after these primary dry precipitators (CEC, 2018), which is thought to facilitate further PM removal through scrubbing. However, previous field measurements have found that flue gas entrains new particles, which are mainly recirculated slurry, through WFGD (Li et al., 2017b; Meij and te Winkel, 2004; Wu et al., 2018). Therefore, the terminal emission concentrations of PM are generally over 15 mg Nm3 under the aforementioned combination of processes (Cao et al., 2017; Wu et al., 2018). During recent years, low-low temperature ESP (LLT-ESP) (Chen et al., 2019; Wang et al., 2015; Wang et al., 2019) and wet electrostatic precipitators (WESP) (Yang et al., 2017; Yang et al., 2018) have been widely installed in Chinese CFPPs as two important ULE technical routes, and those are effective in lowering PM emissions and achieving the ULE standards. With the wide implementation of ULE technologies, final emission characteristics of PM from CFPPs have changed accordingly. Several studies have conducted field tests on emission characteristics of PM from ULE CFPPs (Chen et al., 2019; Sui et al., 2016; Wang et al., 2019; Wu et al., 2018; Xu et al., 2016b). For the ULE technology of LLT-ESP, reducing the flue gas temperature is crucial in the reduction of PM (Wang et al., 2019). Wang et al. (2019) conducted field measurements in four CFPPs installed with LLT-ESP and WFGD, and found that the mass concentrations of PM2.5 (PM 2.5 mm in aerodynamic diameter) and PMtotal increased through WFGD. It can be seen that LLT-ESP, as primary dust dry precipitator, cannot solve the problem caused by the downstream WFGD. Chen et al. (2019) investigated the final emission characteristics of PM2.5 in four ULE CFPPs equipped with LLT-ESP and followed by WFGD, and found some of the chemical profiles were different from the previous studies. However, the final ULE technology of WESP installed after scrubbing towers is different from LLT-ESP, and which is an ideal choice to handle PM with various characteristics and eliminate the problems caused by pretreatment facilities. Sui et al. (2016) examined the emission characteristics of different particle size in an ULE CFPP equipped with an WESP. Xu et al., 2016b conducted field measurements at two 300 MW CFPPs to investigate the PM removal performance of
WESP. Cao et al. (2017) investigated a wet dust removal system combined with a wet phase transition agglomerator and a WESP, and found that this system helped to meet ULE of particles from CFPPs by keeping PM emissions below 5 mg Nm3 under all tested conditions. However, these studies have not involved the emission characteristics of detailed chemical compositions in PM through WESP. In our previous studies, we have conducted field tests in four 300 MW level ULE CFPPs installed with WFGD and WESP, mainly studying the effects of WFGD on PM and ionic components (Wu et al., 2018). Furthermore, the chemical characterization of primary emission sources, referred to as the source profile, is of great importance in atmospheric PM source apportionment (Bi et al., 2019; Chen et al., 2019; Liu et al., 2018; Liu et al., 2014). CFPPs are one of the major sources of PM emissions in China (Ma et al., 2017), and the database of chemical profiles of end-tail size-segregated PM is still very limited since the initiation of the ULE retrofits. Bi et al. (2019) investigated and reviewed the characteristics of main primary source profiles of PM in China from 1987 to 2017, and pointed out that some chemical profiles reported in previous studies were currently out of date and needed to be updated immediately. Actually, the real-world complicated operation conditions, high cost and difficult field tests limit the collection of enough samples for analysis. Comprehensive research on the variation characteristics of final size-segregated PM and the associated chemical compositions emissions from ULE CFPPs installed with WESP based on field tests is still lack. Therefore, fill the gaps of emission characteristics of PM from local CFPPs under various ULE technical routes and update emission data become urgently for compiling and updating emission inventories and source apportionment. In this study, we conducted field measurements at the outlets of WFGD and WESP in eight full-scale and representative ULE CFPPs to investigate the variation characteristics of final size-segregated PM, including size distribution, chemical characteristics, profiles, and emission factors (EFs). 2. Experimental methods 2.1. Tested plants description To ensure that the tested CFPPs are representative, we have considered the key factors affecting PM emissions, including the boiler type, coal type, unit capacity, and the installed APCDs. In this study, eight typical Chinese CFPPs were selected for field tests to cover more different installed capacities and processing technologies. During the tests, all tested power plants operated normally without failure. Table S1 summarizes the detailed information of the eight tested boilers and APCDs. The eight tested boilers are all pulverized coal (PC) boilers with the steam flow rate at the range of 480e2209 t h1, and the tested units include one 165 MW, five 300 MW level, one 600 MW, and one 700 MW. Since the research purpose of this study is to investigate the variation characteristics of final PM emissions after WESP installation, the selected CFPPs are all installed WESP with typical combinations of APCDs (e.g. SCR þ ESP/FF/ESP&FF þ WFGD þ WESP). The schematic configuration of the typical ULE technical route with WESP installation and its working theory is illustrated in Fig. S1. To minimize the impact of coal types on the emissions of chemical composition in PM, all the eight tested CFPPs used bituminous coal. The main indicators of feed coal are analyzed without detailed chemical compositions analysis, and the detailed category of bituminous coal, and coal grade of calorific value, total sulfur, and ash used in the tested CFPPs are shown in Table S2, according to the national standards of “Commercial coal quality-coal for pulverized coal-fired boiler for power generation (GB/T 7562-2018)” (SAC, 2018). The average mass
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contents of volatiles (Vdaf), ash (Ad) and sulfur (St,d) in the feed coal are in the range of 19%e36%, 15%e34%, and 0.59%e1.60%, respectively. During the measurements, we investigated the data of continuous emission monitoring system (CEMS), which is used to monitor the final emissions from CFPPs (Zhang et al., 2019). The hourly average mass concentrations of PMtotal, SO2, and NOx at the eight tested CFPPs for 24 h are shown in Fig. S2, and they all meet the corresponding ULE standards. The average mass concentrations and the ranges of PMtotal, SO2, and NOx tested by CEMS are 2.04 (0.48e4.02) mg Nm3, 15.65 (8.96e28.60) mg Nm3, and 29.70 (9.14e40.05) mg Nm3, respectively.
2.2. Particulate matters sampling description A pair of two-stage virtual impactors is used to collect PM samples, which are divided into three levels of PM2.5, PM102.5 (PM between 2.5 mm and 10 mm in aerodynamic diameter), and PM>10 (PM 10 mm in aerodynamic diameter), and all three sizesegregated PM samples are collected on quartz filters (Pall Corp. USA). During the tests, three parallel samples were collected simultaneously at both the outlets of WFGD and WESP. The diagram of size-segregated PM sampling system and the sampling positions are shown in Fig. 1, and detailed description of this PM sampling system can refer to our previous study (Wu et al., 2018) or the studies of Jiang et al. (2014) and Wada et al. (2016). To ensure the samples collected are valid in high humidity flue gas and avoid water vapor condensation on the filters, the impactor is preheated for 15e20 min in flue gas with the same direction of sampling nozzle and the flue gas flow before sampling. The sampling time depends on the level of PM mass concentration in flue gas, usually 90e120 min to ensure the collected mass is enough for analysis. Average sampling flow rates of PM>10, PM102.5, and PM2.5 are 1.68 ± 0.25, 2.27 ± 0.17, and 20.39 ± 1.78 L min1, respectively, depending on the velocity of the flue gas at the test point.
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2.3. Chemical composition of PM analysis The quartz filters are baked in a muffle furnace for 4 h at 450 C to remove organic compounds before sampling. Then, the filters are weighted after 48 h of equilibration (humidity, 50%; temperature, 25 C) before and after sampling to calculate the average PM concentration. Finally, the weighed filters are placed in the refrigerator with the temperature of 4 C before chemical composition anal2 þ þ ysis. Major ions (anions: F, Cl, NO 3 , and SO4 ; cations: Na , NH4 , Kþ, Mg2þ, and Ca2þ) are analyzed by ion chromatography (Dionex ICS-2000, Thermo Fisher Scientific Inc., USA). See the detailed pretreatment process for collecting samples in our previous study (Wu et al., 2018). Then, the remaining filters are digested for elemental analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES, SPECTRO ARCOS EOP, SPECTRO Analytical Instruments GmbH, USA) (Gao et al., 2018). In this study, a total of 22 elements are measured, which are divided into major elements (S, Al, Mg, Fe, Ca, Na, K, Ti, and Ba) and trace elements (Se, Cr, Ni, Zn, Mn, Co, Mo, Cu, Pb, V, Sb, As, and Cd.) based on the mass concentrations. In the end, the results of PM mass concentrations and chemical compositions are all unified to 6% O2 for all the tested CFPPs for the sake of comparison. 3. Results and discussion 3.1. Variation characteristics of size-segregated PM through WESP The mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal at the outlets of WFGD and WESP for the eight tested ULE CFPPs are presented in Fig. 2. In general, the mass concentrations of the three size-segregated PM at the tested sites show PM>10 > PM102.5 > PM2.5. The average mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal at the WFGD-outlets are in the ranges of 0.82e7.43, 4.06e8.38, 5.12e23.42, and 12.95e32.62 mg Nm3, respectively, and those of the corresponding particles at the
Fig. 1. The diagram of size-segregated PM (PM2.5, PM102.5, PM>10) sampling system and the sampling positions.
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Fig. 2. The mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal at the outlets of WFGD and WESP for the eight tested ULE CFPPs.
WESP-outlets are in the ranges of 0.37e1.53, 0.79e4.42, 1.06e7.74, and 2.35e13.70 mg Nm3, respectively. The results based on field tests reveal that the mass concentrations of PMtotal after WFGD are generally exceeds the ULE limits of PM (10 mg Nm3), and those after WESP can meet the ULE standards (even less than 5 mg Nm3 in several tested CFPPs). After WESP installations, the mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal for all the tested CFPPs are further removed at the ranges of 27%e83%, 39%e83%, 36%e84%, and 39%e79%, respectively. However, we find the mass concentrations of PM analyzed by weight are slightly higher than those tested by CEMS (Fig. S2). For PM monitoring, CEMS mostly relies on optical principles such as transmission and scattering. The different measurement principles of these two methods may be one of the main reasons for the difference in their results. In addition, there are some potential influence factors, such as the artificial errors during sampling and equipment maintenance, complex test conditions and flue gas conditions. Furthermore, the mass concentrations of PM are reduced to a closer range after WESP. Normally, WESP presents a lower removal efficiency when the mass concentrations of PM in WFGD-outlets are at a lower level. For example, the average capture efficiency of WESP for PM2.5 is 31% (17%e47%) when PM2.5 mass concentrations are in the range of 0.46e0.99 mg Nm3 at the WFGD-outlets, whereas the average capture efficiency of WESP for PM2.5 is 69% (42%e84%) when PM2.5 mass concentrations are in the range of 1.04e9.46 mg Nm3 at the WFGD-outlets. Fig. 3 depicts the distribution of PM2.5, PM10-2.5, and PM>10 in the PMtotal at the outlets of WFGD and WESP, which can provide valuable data for updating and evaluating PM emissions from ULE CFPPs. In general, the proportions of PM>10, PM102.5, and PM2.5 in PMtotal decrease in sequence at each tested site, with the range of corresponding proportions being 44%e72%, 22%e45%, and 3%e26% at the WFGD-outlets, and 44%e67%, 15%e38%, and 7%e22% at the WESP-outlets, respectively. Along with the reduction of PM
through WESP, the proportion of three size-segregated PM in PMtotal changes accordingly for individual CFPPs. The proportions of PM>10 in PMtotal at the WESP-outlets in five (plant #1, #3, #4, #7, and #8) of the eight tested CFPPs are lower than those at the WFGD-outlets, as shown in Fig. 3. The PM>10 mass concentrations at WFGD-outlets of these five CFPPs are in the range of 9.15e23.42 mg Nm3, which are higher than the values of the other three CFPPs at the WFGD-outlets. As shown in Fig. 2, the capture efficiencies of PM>10 through WESP for the five CFPPs are generally higher than those of PM2.5 and PM102.5. Meanwhile, the proportions of PM2.5 and PM102.5 at the WESP-outlets are higher than those at the WFGD-outlets for the five CFPPs, except for one case (PM2.5 in Plant #4 with 83% capture efficiency). Based on the analysis above, WESP has better removal performance for large particle sizes and higher PM mass concentrations. By comparison, there are still big differences in the distribution of PM through WESP between this study and that of Sui et al. (2016). The results studied by Sui et al. (2016) based on field tests for one ULE CFPP showed that the PM2.5/PM10 ratio was close to 1 after WESP, indicating that PM102.5 were removed effectively, and most of the PM emitted to the environment will be PM2.5. However, the average mass ratios of PM>10, PM102.5, and PM2.5 are 4.6/2.5/1 and 3.8/2.1/ 1 at the outlets of WFGD and WESP in this study. The difference in the results may be caused by the different sampling systems, and an electrical low-pressure impactor (ELPIþ) was used in the study of Sui et al. (2016). Therefore, more test samples and multiple sampling methods are needed to further investigate the size distribution of PM for ULE CFPPs.
3.2. Variation characteristics of chemical compositions in PM through WESP The variation characteristics of ions and elements in PM2.5, PM102.5, and PM>10 through WESP for the eight tested ULE CFPPs
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Fig. 3. Percentage distribution of size-segregated PM at the outlets of WFGD and WESP for the eight tested ULE CFPPs of China.
are illustrated in Fig. 4a and Fig. 4b, respectively. As shown in Fig. 4a, the total ions mass concentrations decrease for most individual ion reduction through WESP before finally emitting into the atmosphere. The total mass concentrations of ions in PM2.5, PM102.5, and PM>10 decrease on average by 51%, 66%, and 66% from WFGD-outlets to WESP-outlets, respectively. At both the outlets of WFGD and WESP, sulfate is the major ionic component in total ions.
The average percentage of SO2 4 in total ions for PM2.5, PM102.5, and PM>10 is 64%, 66%, and 71% at the WFGD-outlets, respectively, and the corresponding percentage at the WESP-outlets is 58%, 62%, and 65%, respectively. It can be seen that the proportion of SO2 4 in total ions is reduced to some extent while the SO2 concentrations 4 decrease through WESP. As depicted in Fig. 4b, the mass concentrations of total elements decrease significantly through WESP,
Fig. 4. The mass concentrations of ionic species (a), major and trace elements (b) in size-segregated PM at the outlets of WFGD and WESP for the eight tested ULE CFPPs.
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with the average reduction of total elements in PM2.5, PM102.5, and PM>10 being 56%, 66%, and 64%, respectively. In addition, the total mass concentrations of ions and elements in PM>10, PM102.5, and PM2.5 decrease in sequence at outlets of WFGD and WESP.
3.3. Variation characteristics of chemical profiles of CFPPs through WESP In this study, the mass contents (mg g1) of chemical components at outlets of WFGD and WESP are calculated based on per unit mass of size-segregated PM, which will contribute to the studies on better understanding their variation characteristics and enriching the chemical profiles database of CFPPs after WESP installation. Fig. S3 and Fig. 5 depict the mass contents (mg g1) of individual ionic and elemental components per unit mass of PM at the outlets of WFGD and WESP. As expected, the mass content of most ionic and elemental components in size-fractionated PM change significantly across a wide range through WESP for the tested ULE CFPPs. The emissions of PM chemical profiles from different CFPPs also present significant differences, which may be affected by many factors, such as the chemical components in feed coal, combustion conditions, boiler load, APCDs configuration, and their performance for flue gas control process. The average mass contents of the total ions in PM2.5, PM102.5, PM>10 at WFGD-outlets (WESP-outlets) are 278 ± 102 (314 ± 108), 342 ± 123 (366 ± 169), and 296 ± 91 (324 ± 117) mg g1, respectively, and those of SO2 4 in the corresponding PM at WFGD-outlets (WESP-outlets) are 172 ± 94 (178 ± 77), 217 ± 93 (215 ± 114), and 205 ± 67 (208 ± 91) mg g1, respectively. It can be seen that the average mass contents of total ions and SO2 4 in three size fractions PM are in the order of PM102.5 > PM>10 > PM2.5. As depicted in Fig. S3, the mass contents of Naþ, Cl, and Mg2þ in PM2.5 at WESP-outlets are higher than those at WFGD-outlets for most samples. The mass content of Naþ in PM2.5 increases on average by 43% and may be affected to some extent by the alkaline solution (NaOH) added into the spraying and flushing liquid in WESP (Xu et al., 2016b). Total mass contents of the tested elements are 304 ± 78, 455 ± 203, and 558 ± 247 mg g1 in PM2.5, PM102.5, PM>10 at WESP-outlets, respectively. Since the average mass contents of most major and trace elements in PM2.5 are lower than those in PM102.5 and PM>10 at WESP-outlets, the total mass contents of major and trace elements in three sizesegregated PM all present an order of PM2.5 < PM102.5 < PM>10.
Among the major elements, average mass contents and ranges of S, Mg, and Ca in the size-fractionated PM present a decreasing trend through WESP except for Mg and Ca in PM2.5. Sui et al. (2016) pointed out that the limestone/gypsum particles were carried into the flue gas after FGD and removed by WESP subsequently. Xu et al., 2016b reported that WESP could effectively capture the new Ca-enriched particles formed in WFGD, and reduce the Ca content in the PM. Furthermore, the average mass contents and ranges of Al, Fe, Na, K, and Ba in three size-fractionated PM present an increasing trend through WESP. These trends are possibly affected by factors related to the circulating of solutions, low condensation temperature, adhesion, or the complex working conditions in WESP (Sui et al., 2016; Xu et al., 2016b). For the tested trace elements, the average mass contents of Cr, Ni, Co, Mo, V, and Sb increase through WESP. We also compare the chemical profiles of PM obtained in this study with the results reported by Bi et al. (2019), to investigate the variation characteristics after ULE retrofits. The results reported by Bi et al. (2019) are all PM2.5 after WFGD without WESP installation, and the ionic and elemental profiles are shown in Fig. S3 and Fig. 5. 2þ 2þ For ionic profiles, SO2 are the dominate species in 4 , Ca , and Mg CFPPs at the WFGD-outlets. By comparison, the latest mass con2þ tents of SO2 4 , Mg , and F in this study are higher, while those of þ 2þ Ca and NH4 are lower than the traditional ionic profiles of Bi et al. (2019). The increase in SO2 and F may indicate that the 4 entrainment of recirculated slurry by flue gas after WFGD is more pronounced, and the increase in Mg2þ results from the magnesiumbased WFGD in this study. Comparing with traditional APCDs, the upgrading of various facilities (e.g. SCR, ESP, and WFGD) under ULE retrofits may result in the reduction of Ca2þ and NHþ 4 in PM2.5. The mass contents of the remaining test ions (Naþ, Kþ, Cl, and NO 3 ) are similar to those reported by Bi et al. (2019). For elemental profiles, the average mass contents and range of most elements reported by Bi et al. (2019) are close to the results of this study. However, we find significant differences in the mass contents of several elements, such as S, Mg, Ca, Zn, Cu, and Pb, through a t-test analysis, which is a statistical method used to determine if there is a significant difference between the means of two groups. The elements 2þ of S, Mg, and Ca present similar trends with the ions of SO2 4 , Mg , 2þ and Ca . Moreover, the average mass contents of Zn, Cu, and Pb in this study are significantly lower than those reported by Bi et al. (2019).
Fig. 5. The mass contents (mg g1) of major elements (a) and trace elements (b) in per unit mass of size-segregated PM at the outlets of WFGD and WESP.
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Table 1 Emission factors (g t1) of PM2.5, PM10, and PMtotal based on different combination of APCDs from various studies. study
after APCDs
PM2.5
PM10
PMtotal
This study
FF þ WFGD ESP þ WFGD ESP&FF þ WFGD FF þ WFGD þ WESP ESP þ WFGD þ WESP ESP&FF þ WFGD þ WESP ESP þ WFGD ESP þ WFGD ESP þ WFGD ESP þ WFGD ESP þ WFGD þ WESP ESP þ WFGD þ WESP ESP&FF þ WFGD
13.3 ± 11.4 25.9 ± 20.5 6.24 ± 4.33 4.58 ± 1.73 8.97 ± 4.81 2.82 ± 1.19 14.7A (9.20A22.5A) 241 ± 93 38 ± 4 13 11 ± 4 1 3
67.0 ± 27.1 75.0 ± 32.1 69.9 ± 40.9 15.7 ± 5.09 27.6 ± 20.8 17.4 ± 10.5 21.0A (12.9A31.7A) 221 ± 88
111 ± 29.9 171 ± 52.5 247 ± 120 42.7 ± 9.05 61.7 ± 47.4 38.6 ± 16.8 23.1A (14.2A34.8A) 111 ± 44
16 12 ± 4 2 25
19 14 ± 5 3 42
Zhao et al. (2010) Wang et al. (2011) Xu et al. (2017) Li et al. (2018) Yang et al. (2018) Note: A, ash content in feed coal, %.
3.4. Comprehensive EFs of size-segregated PM based on field tests Since the mass concentrations of PM are further reduced by WESP, it is particularly urgent to update the existing PM EFs of ULE CFPPs in China. By combining field test data from WESP and survey results of related production information during the sampling period in eight ULE CFPPs, we calculated the EFs (g t1) of PM based on the mass of coal consumed under different combinations of APCDs (as listed in Table 1). The EFs of PM2.5, PM10, and PMtotal after WESP treatment combined with three typical dust collectors (ESP, FF, and ESP&FF) and WFGD are in the range of 2.82e8.97, 15.7e27.6, and 38.6e61.7 g t1, respectively. We also compared the results of this study with other previous studies (Li et al., 2018; Wang et al., 2011; Xu et al., 2017; Yang et al., 2018; Zhao et al., 2010). By comparison, we find that the EFs of PM have reduced significantly since the implementation of the ULE standards (as shown in Fig. S4). The reduction of PM is due to not only the installation of WESP but also the upgrading of upstream dust and SO2 capture facilities. However, despite the same combination of APCDs, differences in PM EFs still exist in various studies. Therefore, more field tests should be conducted to enrich the database of PM EFs. We believe the new updated EFs of PM in this study, which are summarized based on the field test results of eight typical CFPPs in the same period after the implementation of ULE, are more representative of the current emission levels of Chinese CFPPs. These EFs would benefit for updating emission inventories of Chinese CFPPs and result in better environmental management and policy making.
CFPPs with WESP treatment are developed in this study. By comparison, chemical profiles in PM after WFGD and WESP have indeed changed relative to existing data since the initiation of the ULE retrofits. To update the EFs in time, we have calculated the end-tail EFs of PM2.5, PM10, and PMtotal under the ULE technical routes of ESP/FF/ESP&FF þ WFGD þ WESP, and the corresponding EFs are in the range of 2.82e8.97, 15.7e27.6, and 38.6e61.7 g t1, respectively. We believe the new updated EFs of PM and their chemical profiles in this study are more representative of the current emission levels of Chinese CFPPs and can supplement and replace the outdated data in the relevant research. Acknowledgements This work was funded by the National Natural Science Foundation of China (21777008, 21377012, and 21177012), the Trail Special Program of Research on the Cause and Control Technology of Air Pollution under the National Key Research and Development Program of China (2018YFC0213202, 2016YFC0201501), and the National Key Scientific and Technological Project on Formation Mechanism and Control of Heavily Air Pollution (DQGG0209). We thank the editors and anonymous reviewers for their valuable suggestions and comments on improving this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.113886.
4. Conclusion
References
With the rapid implementation of ULE in Chinese CFPPs in recent years, results on the emission characteristics of sizesegregated PM and the associated chemical components through WESP are still scarce. We conducted field measurements at the outlets of WFGD and WESP in eight Chinese ULE CFPPs to fill the knowledge gap. Our results show that, after the installations of WESP, the emission characteristics of final PM and its chemical components have changed. The mass concentrations of PM2.5, PM102.5, PM>10, and PMtotal are all further reduced through WESP with the range of 27%e83%, 39%e83%, 36%e84%, and 39%e79%, respectively. The proportions of three size fractions PM at the outlets of WFGD and WESP are quantified, and show the order of PM>10 > PM102.5 > PM2.5. We find the WESP exhibit better removal performance for large particle sizes and higher PM concentrations. Meanwhile, the final emissions of total ions and elements decrease with most individual chemical composition being reduced after WESP installations. Furthermore, the chemical profiles of ULE
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