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Insight of particulate arsenic removal from coal-fired power plants
Fuel 257 (2019) 116018
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Full Length Article
Insight of particulate arsenic removal from coal-fired power plants Hongyu Gong, Yongda Huang, Hongyun Hu , Biao Fu, Tongtong Ma, Shuai Li, Kang Xie, Guangqian Luo, Hong Yao ⁎
T
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Coal combustion fly ash Particulate arsenic removal Speciation Size distribution characteristics Electrostatic precipitator
Arsenic is easily volatilized as vapors during coal combustion and predominantly transferred into particulate forms. The removal of particulate arsenic together with fine fly ash particles is essential to control arsenic emission during coal combustion. In this study, fly ashes and different size fraction samples (PM2.5, PM10, and PM20) were collected or prepared from four Chinese coal-fired power plants, with objectives of studying the arsenic distribution and speciation in fly ash, and understanding the retention relationship between the removal of ash particles and particulate arsenic. The results demonstrated that the concentration of arsenic of the first two ESP (electrostatic precipitator) hoppers was at similar level, then followed by a substantial increase in the rear hoppers. And the particle size distribution of fine fly ash particles from various ESP systems presented essentially similar trend. Furthermore, particulate arsenic was mainly present as arsenates probably through the interactions between arsenic vapors and accessory Ca/Fe/Al-minerals. Arsenic associated with calcium compounds tended to be enriched in coarse particles, while arsenic bound with Fe/Al-compounds was enriched in fine particles. Meanwhile, arsenic associated with calcium sulfate compounds was easier to be captured by ESP compared to calcium silicate-bound arsenic. The interactions between arsenic vapors and various accessory minerals not only affected the size distribution characteristics of particulate arsenic, but also had a great effect on its removal efficiency of ESP. Specifically, based on the ash resistivity calculation, arsenic-bearing fine particles with certain amount of Fe had suitable conductivity and favored the capture of particulate arsenic by ESP.
1. Introduction Arsenic is one of the most volatile and potentially toxic elements to
⁎
be released from coal-fired power plants. Arsenic is primarily volatilized as As2O3 (g) during high-temperature combustion process [1,2], more than 80% of which is partitioned into the fly ashes afterward and
Corresponding author. E-mail address: [email protected] (H. Hu).
https://doi.org/10.1016/j.fuel.2019.116018 Received 26 June 2019; Received in revised form 12 August 2019; Accepted 13 August 2019 Available online 20 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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eventually captured by dust removal device [3]. It's well known that ESPs experience a minimum collection efficiency for particles of about 0.2–0.3 μm [4], which might carry particulate arsenic into the atmosphere. Therefore, it’s essential to study the size distribution characteristics of particulate arsenic and understand the retention relationship between fine ash particles and particulate arsenic. Most arsenic vapors (As2O3) underwent complicated interactions with the accessory coal minerals and were transferred into particulate forms during coal combustion. And the transformation of arsenic vapors to particulate arsenic was determined not only by the particle size distributions characteristics, but also by the compositions of the fly ash particles [5–7]. Wang et al. [8] studied the migration and transformation behavior of arsenic in a 320 MW coal-fired power plant equipped with EFF (Electrostatic fabric filter) units. It was found that the enrichment factor of arsenic in the baghouse ash was larger than that of the ESP ash, probably due to the smaller particle size of baghouse ashes. Fu et al. [9] reported that the concentration of arsenic in the gaseous size-segregated particulate matter (< 10 µm) increased with the decrease of particle size. On the contrary, some researchers found that arsenic was enriched in the coarse stoker ashes as a result of Fe(III)-As (V) formation in the post-combustion stage [10]. In addition, Zhao et al. [6] found that the concentration of arsenic in fine particles showed a trimodal distribution. The composition of fly ash had a great influence on the size distribution characteristics of particulate arsenic and the particulate arsenic tended to combined with larger fly ash particles with the addition of calcium-based sorbent. Typically, the mineralogy and morphology of fly ash are complicated, resulting in the complexity of particulate matter formation as well as the migration behavior of particulate arsenic. In laboratory experiments, many researchers found that calcium was effective for As2O3 (g) capture at high temperatures by forming arsenates [11,12]. Other inorganic compounds such as iron oxides might also promote As2O3 (g) capture by forming FeAsO4 [13]. Recently, Yang et al. [14] suggested that Al played a significant role on arsenic speciation transformation and showed good capacity on arsenic capture. Regarding fly ash from real coal combustion plants, most arsenic was found to present as As5+ through the interactions between Ca/Fe/Al-compounds and gaseous arsenic. Luo et al. [15] indicated that both calcium and aluminum had a great influence on the formation of arsenates. On the other hand, ESP was widely applied for the emission control of particulate heavy metals in coal combustion plants. The interactions between arsenic vapors and various accessory minerals not only affected the size distribution characteristics of particulate arsenic, but also had a great effect on its removal efficiency of ESP [16]. At present, most pulverized coal fired power plants in China are optimizing and revamping electrostatic precipitators to enhance the removal efficiency of fine particles (including various toxic trace elements). The objective of this research is to study the transformation characteristics of gaseous arsenic towards particulate arsenic and the removal efficiency (by ESP) of particulate arsenic. In detail, feed coals and their corresponding combustion byproducts collected from four
power plants were designed to obtain the arsenic enrichment behavior. In order to clarify the speciation and partitioning behavior of arsenic in fly ashes during post-combustion stage of coal-fired power plants, each fly ash was further screened into different particle size (PM2.5, PM10, and PM20) to be characterized for studying the size dependence of arsenic content in fly ashes. Furthermore, the speciation of arsenic in various fly ash was differentiated to illustrate the pathway of gaseous arsenic into particulate arsenic. The effect of fly ash composition on the removal efficiency of particulate arsenic was also discussed. Based on the above research, the scientific basis and reference for strengthening the removal of particulate arsenic in coal-fired power plants were proposed. 2. Experimental 2.1. Materials In the present study, four coal samples and the corresponding fly ashes from four coal-fired power plants were collected. The specific information of these four plants is shown in Table 1. As listed in Table 1, Plant 4 (Pl.4) burned coal in a fluidized bed boiler, while others were pulverized coal-fired plants. Pl.1 was equipped with EFF (with a three-stage electrostatic precipitator followed by a two-stage fabric filter). Pl.2 and Pl.3 were equipped with a three-stage electrostatic precipitator. Pl.4 was equipped with a single-stage fabric filter. For the purpose of studying the distribution characteristics of arsenic in ash particles of different size from various ESP/FF systems, fly ashes from Pl.1 and Pl.2 were separated into PM2.5 (particle size less than 2.5 μm), PM10 and PM20 by a particulate matters resuspension device. This device adopted the principle of resuspension separation and impact cutting to achieve accurate classification of particle size. And the details regarding the separation process were described elsewhere [17]. 2.2. Analytical methods 2.2.1. Arsenic concentration determination Arsenic content in coal samples was determined according to Chinese Standard GB/T 3058-2008 [18]. The coal samples were first mixed with Aldrin and burned at 800℃, then the burned products were dissolved in hydrochloric acid. And finally the arsenic content was determined by atomic fluorescence spectroscopy (AFS). Fly ash samples were first mixed with acid (HNO3 + HCl) and then dissolved at 80℃. The solution was then sent for AFS analysis. The accuracy of the studied arsenic determination was calibrated by standard reference materials GBW11117 (coal) and GBW08401 (ash). The precision of the tests is estimated to be lower than 5%. 2.2.2. Chemical speciation analysis The valence of arsenic in fly ashes was determined by AFS coupled with high performance liquid chromatography (HPLC). Prior to the
Table 1 Specific information about the plants selected. Samples
Plant Plant Plant Plant a b c d
1 2 3 4
(Pl.1) (Pl.2) (Pl.3) (Pl.4)
Proximate analysis (%) Madd
Aad
Vad
FCad
7.08 4.43 2.07 2.81
35.81 29.30 6.96 51.4
30.60 13.00 45.86 18.06
26.51 53.27 45.11 27.73
Arsenic concentration (μg/g)
Furnace type
Dust removal device
43.28 3.498 0.896 8.290
Pulverized coal furnace
EFFa ESPb ESPb FFc
Electrostatic fabric filter. Electrostatic precipitator. Fabric filter. Air dry. 2
Fluidized bed
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HPLC-AFS analysis, two valence of arsenic (As3+ and As5+) were extracted from the ash samples with phosphoric acid and ascorbic acid [19]. Furthermore, the five-step sequential extraction procedures were used to differentiate the Ca/Fe/Al-bound arsenic in fly ashes [20,21]. Accordingly, arsenic in various phases, including non-specifically adsorbed arsenic (F1), specifically adsorbed arsenic (F2), calcite-bound arsenic (F3), amorphous and poorly crystalline Fe/Al-bound arsenic (F4) and well crystalline Fe/Al-bound (F5) arsenic were extracted and determined. Moreover, in order to confirm the possible arsenic capture behavior by accessory Ca/Fe/Al compounds in fly ashes, CaSiO3/CaSO4/Fe2O3/ Kaolin were selected to react with arsenic vapors at 900 °C. Afterward, arsenic speciation in the adsorbents was analyzed by the five-step sequential extraction procedures as described above.
Fig. 1. Mass distribution of arsenic in coal combustion products. (Others represent limestone, gypsum, sludge and waste water).
2.2.3. Calculation methods As shown in Eqs. (1) and (2), the relative enrichment index (REI) is adopted to study the partitioning behavior of arsenic in combustion byproducts (REI-1) and the enrichment characteristics of arsenic in different fly ashes from various dust removal systems (REI-2) [22–24]:
REI
1 = Mproduct CAs,product /Mcoal CAs,coal
(1)
REI
2 = CAs,fly ash A coal,ad/CAs,coal
(2)
Mproduct represents the mass of combustion residues, such as bottom ash, fly ash, waste water etc., kg; CAs, product represents the total As content in product, mg/kg; Mcoal represents the mass of coal, kg; CAs,coal represents the total As content in the coal, mg/kg; CAs,fly ash represents the total As content in fly ash, mg/kg; Acoal,ad represents the ash yield in the coal on air-dried basis. In this study, fly ash was considered to account for 80% while bottom ash was considered to account for 20% in pulverized coal-fired plants while the fly ash and bottom ash produced by fluidized bed boiler accounted for 60% and 40% in the whole ash generated by coal combustion. In order to study the relationship between the composition of fly ash and its resistivity, Eq. (3) is used to calculate fly ash resistivity with chemical composition and temperature [16].
Fig. 2. The variation of arsenic content in fly ashes collected from different hoppers of four power plants. Table 2 Analysis of size distribution of ESP/FF fly ashes collected from four power plants. Samples
lg = 2023.96 + [ 1.22lgNK+Na + Li
0.81(lgNK+Na + Li ) 2] +
Pl.1-ESP1 Pl.1-ESP2 Pl.1-ESP3 Pl.1-FF1 Pl.1-FF2 Pl.2-ESP1 Pl.2-ESP2 Pl.2-ESP3 Pl.3-ESP1 Pl.3-ESP2 Pl.3-ESP3 Pl.4
[0.31lgNSi + Al + 0.11(lgNSi + Al )2] + [0.75lgNCa + Mg + 0.14(lgNCa + Mg ) 2] + [ 0.82lgNFe
0.61(lgNFe ) 2]
45801/ T
1203lgT + 182(lgT)2 (3)
NK+Na+Li is the amount percent of K, Na, and Li atoms (%); NFe is the amount percent of Fe atoms (%); NCa+Mg is the amount percent of Ca and Mg atoms (%); NAl+Si is the amount percent of Al and Si atoms (%); and T denotes temperature (K). 3. Results and discussion
a
3.1. Enrichment characteristics of arsenic in different fly ashes
D(4,3)a (μm)
Volume (%) PM20
PM10
PM2.5
PM1
26.35 52.55 56.43 87.46 91.22 35.13 40.47 67.20 34.60 40.10 43.59 45.25
13.21 32.68 36.74 74.47 79.38 15.60 22.16 50.26 18.81 22.94 28.77 28.08
4.06 11.04 13.04 30.72 35.25 3.65 5.48 16.19 4.93 6.32 9.43 7.99
1.51 3.74 4.18 8.61 9.23 1.82 2.50 6.04 2.03 2.54 3.51 2.15
56.368 17.829 15.887 3.991 3.170 31.698 28.251 8.934 35.566 31.698 28.251 25.179
D(4,3), the bulk particle size obtained using the volume average method.
increased. According to the particle size analysis (Table 2), the average particle size of fly ash showed a decreasing trend as the number of dust collector stages increased. The trend was not surprising because the coarser ash particles were preferentially collected in the front rows of ESP systems while finer ash particles, with more adsorption sites, were easier to escape from the front dust removal system and enrich in the back rows [27–29]. Therefore, the capture capacity of arsenic vapors by fine ash particles was higher, which attributed to the increase of arsenic content in the rear particulate control systems. Fig. 3 shows the value of REI-2 of arsenic in various fly ashes, further confirming the enrichment of arsenic in the back rows of the ash removal systems. Interestingly, as observed in Fig. 2, the concentration of arsenic in fly ash from ESP3 was significantly higher than that of ESP1 and ESP2
The mass distribution of arsenic in different ashes collected from four power plants is illustrated in Fig. 1. Based on the value of REI-1, arsenic was found to tend to enrich in fly ash rather than in bottom ash. Furthermore, arsenic distributed more in the bottom ash in fluidized bed boiler compared to the pulverized coal boiler. This might be attributed to a lower combustion temperature (750–950 °C) and a relatively complete reactions in the fluidized bed boiler, which would inhibit the vaporization of arsenic and promote the interactions between arsenic and minerals [25,26]. The concentration of arsenic in the ash samples collected from various EFF/ESP/FF systems is shown in Fig. 2. Generally, the arsenic content in fly ashes increased as the number of dust collector stages 3
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Fig. 3. Enrichment characteristics of arsenic in ash samples from various dust collectors.
Fig. 5. A comparison of arsenic distribution in sized fly ashes according to previous studies.
while ESP1 and ESP2 showed a slight difference regarding the arsenic content. As shown in Table 2, the percentages of PM2.5 in fly ashes from ESP3/FF were significantly higher compared with the first two rows, suggesting that the distribution of arsenic in different ESP fly ashes showed essentially similar trend. Therefore, the particle size distribution of each row ashes played an important role on the distribution of arsenic content. In order to study the relationship between particle size and arsenic distribution, arsenic content in sized fly ash (PM2.5, PM10, and PM20) was determined. The distribution of arsenic in the sized fine ash particles from Pl.1 and Pl.2 is shown in Fig. 4(a) and (b), respectively. According to Fig. 4(a), arsenic concentration in PM2.5, PM10, and PM20 segregated from different ESP/FF ashes showed great discrepancy,
probably as a result of the decreasing in the average particles size of PM2.5, PM10, and PM20 (Table 2). It was also found that the concentration of arsenic in PM2.5, PM10, and PM20 collected from the same ESP or FF hoppers was close to each other, indicating that arsenic content did not increase linearly as the particle size decrease. A summary of arsenic distribution in sized fly ashes from previous studies was comparatively shown in Fig. 5. The results revealed that, in some researches, the content of arsenic in PM2.5 segregated from fly ash was more than twice that of PM2.5–10, indicating arsenic was extremely enriched in PM2.5; whilst other studies observed no significant variations in arsenic content as the particle size increasing [6,8,9,32]. Zhao et al. [6] proposed that arsenic distributed in size-classified particulate matter peaking at 0.1 μm, 0.6 μm and 4 μm particles. And the presence of different arsenic peaks might be attributed to the interactions between arsenic and various minerals. Consequently, understanding the speciation of particulate arsenic helps the illumination of the participation mechanisms of arsenic towards the fly ashes. 3.2. Chemical speciation of arsenic in sized fly ash particles Fig. 6 shows the valence of arsenic in fly ashes sampled from various ESP/EFF units. Arsenic in all fly ashes was mainly present as arsenates (As5+), which was in agreement with previous studies [10,30,34,35]. The predominance of As5+ over As3+ indicated that most of the As2O3 (g) reacted with ash minerals (Al/Ca/Fe) by forming arsenates [14,35]. In addition, there was still a certain proportion of As3+ in fly ashes, which might be present as As (III)-glass [30]. Meanwhile, more fractions of As3+ were found in fabric filters (FF) ash, with small average
Fig. 4. The concentration of arsenic in sized fine ash particles (PM2.5, PM10, and PM20) from different ESP hoppers (a) Plant 1 (Pl.1), (b) Plant 2 (Pl.2).
Fig. 6. Proportions of As3+/As5+ in fly ashes from various dust collectors. 4
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Fig. 7. Arsenic partitioning in the adsorption products after arsenic reactions with CaSO4/CaSiO3/Fe2O3/Kaolin at 900 °C. (F1, non-specifically adsorbed As; F2, specifically adsorbed As; F3, calcite-bound As; F4, amorphous and poorlycrystalline Fe/Al-bound As; F5, well-crystallized Fe/Al-bound As).
Fig. 8. Arsenic partitioning in the sequential extraction procedures fractions of fly ashes from various plants. (F1, non-specifically adsorbed As; F2, specifically adsorbed As; F3, calcite-bound As; F4, amorphous and poorly-crystalline Fe/Albound As; F5, well-crystallized Fe/Al-bound As).
particle size. The enrichment of As3+ in FF ashes might be associated with the increase of adsorption sites on the surface of fine particles and the chemical reactions between arsenic vapors and Al-compounds [33]. To study the interactions between arsenic and different minerals, the five-step sequential extraction procedures (SEP) were adopted. Prior to the SEP analysis, CaSiO3/CaSO4/Fe2O3/Kaolin, as common Ca/ Fe/Al-based components in fly ash for arsenic retention [6,11,14], were chosen to be the potential absorbents for gaseous arsenic, to verify the accuracy of this method. After being extracted by different reagents, the modes of arsenic occurrences in different adsorbents are illustrated in Fig. 7. As shown in Fig. 7, arsenic bound with Ca-bearing compounds was mainly extracted in the first two steps while Fe/Al-bound arsenic was mainly extracted in the last two steps. Furthermore, arsenic extracted in the first step (F1) was mainly the arsenic associated with CaSO4 while arsenic extracted in the second step (F2) was mainly Ca–Si-bound arsenic. In addition, the experimental results suggested that arsenic combined with Al/Fe-based absorbents was mainly present as stable compounds, which might be formed through heterogeneous reactions between gaseous arsenic and ash components [31,33,35,36]. Ca-based components, as main absorbent for arsenic retention reported in many publications [11,37,38], captured and transformed gaseous arsenic into unstable ion-exchangeable arsenic in fly ashes, which might be harmful to local environment during ash landfilling [39]. Figs. 8 and 9 show the percentage distribution of different arsenic speciation in bulk fly ashes and in sized fractions, respectively. According to Fig. 8, more than 60% of arsenic was associated with Fe/Alcompounds in fly ashes collected from Pl.1 while more than half of arsenic was combined with Calcium in ashes from other plants. The difference in arsenic speciation between various plants might be related to the proportion of Ca/Fe/Al-compounds in ash and the reactions between arsenic and ash minerals. Many researchers reported the influence of calcium in arsenic capture [11,12], however, this study found that aluminum also played a significant role in arsenic transformation, especially in Pl.1. In addition, Fe/Al-bound arsenic increased while Cabound arsenic decreased as the number of dust collector stages increased. It indicated that arsenic associated with calcium compounds tended to enrich in coarse particles, while arsenic bound with Fe/Alcompounds was prone to enrich in fine particles. In other word, arsenic combined with Fe/Al-compounds tended to escape from electrostatic precipitators compared to arsenic combined with Ca-compounds. Meanwhile, the fraction of arsenic extracted from the first step (F1) decreased as the number of dust removal stages increased, while the fraction of arsenic in F2 showed slight difference. It’s suggested that arsenic associated with calcium silicate compounds tended to escape from ESP and calcium sulfate-bound arsenic was easier to be removed,
Fig. 9. Arsenic partitioning in the sequential extraction procedures fractions of sized fly ashes (a) Pl.1, (b) Pl.2. (F1, non-specifically adsorbed As; F2, specifically adsorbed As; F3, calcite-bound As; F4, amorphous and poorly-crystalline Fe/Al-bound As; F5, well-crystallized Fe/Al-bound As).
relatively. As shown in Fig. 9, arsenic speciation in PM2.5/PM10/PM20 showed great discrepancy from the first field to the third field, suggesting that particle size of fly ash had a great influence on the speciation of particulate arsenic. The fraction of Fe/Al-bound arsenic in 5
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formation of particulate arsenic. Different minerals show great discrepancy on its resistivity, which is closely related to the proportion of Ca/Fe/Al [16]. Moreover, the ash removal efficiency of ESP systems is heavily depend on the resistivity of fly ash. Coal fly ash is mainly inorganically composed of aluminum-based compound (mullite), calcium-based compound (anhydrite) and iron-based compound (hematite) [6]. Different combination of these minerals will lead to both the difference of the Ca/Fe/Al content and the change of fly ash resistivity. Consequently, arsenic combined with Ca/Fe/Al-based minerals (as discussed in Section 3.2) shows different ash resistivity, which in turn affects the removal efficiency of particulate arsenic by ESP systems. In this study, the removal efficiency of arsenic in ESP was evaluated on the basis of the calculation of fly ash resistivity. The resistivity calculation model summarized by Zhen et al. [16] was adopted and the charging characteristics of different minerals was also discussed. The variation of ash resistivity versus the percent of Ca/Fe/Al atoms of anhydrite, hematite, and mullite was given in Fig. 10, respectively. Previous studies showed that the most suitable range of fly ash resistivity was at 104–5 × 1010 Ω cm [40,41]. However, if the fly ash resistivity was too high (> 1011 Ω cm), the conductivity of ash particles declined and the ash removal efficiency decreased [40,41]. According to Fig. 10, when NFe reduced to 6.9% or 17.7%, the resistivity of mullite or anhydrite would exceed 1011 Ω cm and the removal efficiency would decline. However, the resistivity of hematite would not change a lot when NAl/NCa varied over a large range. This calculation showed that arsenic associated with iron-bearing minerals was easier to be removed by electrostatic precipitator compared to Ca/Al-bound arsenic. 4. Conclusions The characteristics of arsenic distribution in various ash particles as well as the association relationship between accessory minerals and arsenic in the fly ash were investigated in this study. The results showed that arsenic was found to enrich in fly ash rather in bottom ash. Meanwhile, arsenic distributed more in the bottom ash in fluidized bed boiler compared to the pulverized coal boiler, which might be attributed to the lower combustion temperature and relatively complete reactions occurring in the fluidized-bed boiler. The concentrations of arsenic in the fly ash from the first two ESP hoppers were at similar level while were significantly lower than that of ash samples from rear hoppers. Furthermore, particle size as well as ash components both had a great influence on arsenic speciation. Arsenic associated with calcium compounds tended to enrich in coarse fly ashes whilst arsenic bound with Fe/Al-compounds tended to enrich in fine fly ashes. In addition, arsenic associated with calcium silicate compounds tended to escape from ESP and calcium sulfate-bound arsenic was easier to be removed, relatively. According to the calculation results of resistivity based on ash minerals (mullite, anhydrite, and hematite), it was suggested that arsenic associated with Fe-compounds was easier to be captured by electrostatic precipitators compared to Ca/Al compounds. Acknowledgments
Fig. 10. Calculation results of sensitivity of resistivity to Fe, Ca, and Al atoms of mullite, anhydrite and hematite at 363 K.
This work was supported by National Key Research and Development Project of China (2018YFB0605103) and Program for HUST Academic Frontier Youth Team (2018QYTD05). Professor Chungang Yuan and his group are thanked for providing fly ash separating technology support.
PM2.5/PM10/PM20 showed an increasing trend from the first stage to the last stage while the average particle size of fine particles decreased. It further confirmed that Fe/Al-bound arsenic tended to enrich in fine particles. Meanwhile, the fraction of arsenic associated with calcium in PM2.5/PM10/PM20 decreased as the number of dust removal stages increased, which was consistent with the phenomenon found in fly ash from various ESP/FF units (Fig. 8).
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Full Length Article
Insight of particulate arsenic removal from coal-fired power plants Hongyu Gong, Yongda Huang, Hongyun Hu , Biao Fu, Tongtong Ma, Shuai Li, Kang Xie, Guangqian Luo, Hong Yao ⁎
T
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Coal combustion fly ash Particulate arsenic removal Speciation Size distribution characteristics Electrostatic precipitator
Arsenic is easily volatilized as vapors during coal combustion and predominantly transferred into particulate forms. The removal of particulate arsenic together with fine fly ash particles is essential to control arsenic emission during coal combustion. In this study, fly ashes and different size fraction samples (PM2.5, PM10, and PM20) were collected or prepared from four Chinese coal-fired power plants, with objectives of studying the arsenic distribution and speciation in fly ash, and understanding the retention relationship between the removal of ash particles and particulate arsenic. The results demonstrated that the concentration of arsenic of the first two ESP (electrostatic precipitator) hoppers was at similar level, then followed by a substantial increase in the rear hoppers. And the particle size distribution of fine fly ash particles from various ESP systems presented essentially similar trend. Furthermore, particulate arsenic was mainly present as arsenates probably through the interactions between arsenic vapors and accessory Ca/Fe/Al-minerals. Arsenic associated with calcium compounds tended to be enriched in coarse particles, while arsenic bound with Fe/Al-compounds was enriched in fine particles. Meanwhile, arsenic associated with calcium sulfate compounds was easier to be captured by ESP compared to calcium silicate-bound arsenic. The interactions between arsenic vapors and various accessory minerals not only affected the size distribution characteristics of particulate arsenic, but also had a great effect on its removal efficiency of ESP. Specifically, based on the ash resistivity calculation, arsenic-bearing fine particles with certain amount of Fe had suitable conductivity and favored the capture of particulate arsenic by ESP.
1. Introduction Arsenic is one of the most volatile and potentially toxic elements to
⁎
be released from coal-fired power plants. Arsenic is primarily volatilized as As2O3 (g) during high-temperature combustion process [1,2], more than 80% of which is partitioned into the fly ashes afterward and
Corresponding author. E-mail address: [email protected] (H. Hu).
https://doi.org/10.1016/j.fuel.2019.116018 Received 26 June 2019; Received in revised form 12 August 2019; Accepted 13 August 2019 Available online 20 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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eventually captured by dust removal device [3]. It's well known that ESPs experience a minimum collection efficiency for particles of about 0.2–0.3 μm [4], which might carry particulate arsenic into the atmosphere. Therefore, it’s essential to study the size distribution characteristics of particulate arsenic and understand the retention relationship between fine ash particles and particulate arsenic. Most arsenic vapors (As2O3) underwent complicated interactions with the accessory coal minerals and were transferred into particulate forms during coal combustion. And the transformation of arsenic vapors to particulate arsenic was determined not only by the particle size distributions characteristics, but also by the compositions of the fly ash particles [5–7]. Wang et al. [8] studied the migration and transformation behavior of arsenic in a 320 MW coal-fired power plant equipped with EFF (Electrostatic fabric filter) units. It was found that the enrichment factor of arsenic in the baghouse ash was larger than that of the ESP ash, probably due to the smaller particle size of baghouse ashes. Fu et al. [9] reported that the concentration of arsenic in the gaseous size-segregated particulate matter (< 10 µm) increased with the decrease of particle size. On the contrary, some researchers found that arsenic was enriched in the coarse stoker ashes as a result of Fe(III)-As (V) formation in the post-combustion stage [10]. In addition, Zhao et al. [6] found that the concentration of arsenic in fine particles showed a trimodal distribution. The composition of fly ash had a great influence on the size distribution characteristics of particulate arsenic and the particulate arsenic tended to combined with larger fly ash particles with the addition of calcium-based sorbent. Typically, the mineralogy and morphology of fly ash are complicated, resulting in the complexity of particulate matter formation as well as the migration behavior of particulate arsenic. In laboratory experiments, many researchers found that calcium was effective for As2O3 (g) capture at high temperatures by forming arsenates [11,12]. Other inorganic compounds such as iron oxides might also promote As2O3 (g) capture by forming FeAsO4 [13]. Recently, Yang et al. [14] suggested that Al played a significant role on arsenic speciation transformation and showed good capacity on arsenic capture. Regarding fly ash from real coal combustion plants, most arsenic was found to present as As5+ through the interactions between Ca/Fe/Al-compounds and gaseous arsenic. Luo et al. [15] indicated that both calcium and aluminum had a great influence on the formation of arsenates. On the other hand, ESP was widely applied for the emission control of particulate heavy metals in coal combustion plants. The interactions between arsenic vapors and various accessory minerals not only affected the size distribution characteristics of particulate arsenic, but also had a great effect on its removal efficiency of ESP [16]. At present, most pulverized coal fired power plants in China are optimizing and revamping electrostatic precipitators to enhance the removal efficiency of fine particles (including various toxic trace elements). The objective of this research is to study the transformation characteristics of gaseous arsenic towards particulate arsenic and the removal efficiency (by ESP) of particulate arsenic. In detail, feed coals and their corresponding combustion byproducts collected from four
power plants were designed to obtain the arsenic enrichment behavior. In order to clarify the speciation and partitioning behavior of arsenic in fly ashes during post-combustion stage of coal-fired power plants, each fly ash was further screened into different particle size (PM2.5, PM10, and PM20) to be characterized for studying the size dependence of arsenic content in fly ashes. Furthermore, the speciation of arsenic in various fly ash was differentiated to illustrate the pathway of gaseous arsenic into particulate arsenic. The effect of fly ash composition on the removal efficiency of particulate arsenic was also discussed. Based on the above research, the scientific basis and reference for strengthening the removal of particulate arsenic in coal-fired power plants were proposed. 2. Experimental 2.1. Materials In the present study, four coal samples and the corresponding fly ashes from four coal-fired power plants were collected. The specific information of these four plants is shown in Table 1. As listed in Table 1, Plant 4 (Pl.4) burned coal in a fluidized bed boiler, while others were pulverized coal-fired plants. Pl.1 was equipped with EFF (with a three-stage electrostatic precipitator followed by a two-stage fabric filter). Pl.2 and Pl.3 were equipped with a three-stage electrostatic precipitator. Pl.4 was equipped with a single-stage fabric filter. For the purpose of studying the distribution characteristics of arsenic in ash particles of different size from various ESP/FF systems, fly ashes from Pl.1 and Pl.2 were separated into PM2.5 (particle size less than 2.5 μm), PM10 and PM20 by a particulate matters resuspension device. This device adopted the principle of resuspension separation and impact cutting to achieve accurate classification of particle size. And the details regarding the separation process were described elsewhere [17]. 2.2. Analytical methods 2.2.1. Arsenic concentration determination Arsenic content in coal samples was determined according to Chinese Standard GB/T 3058-2008 [18]. The coal samples were first mixed with Aldrin and burned at 800℃, then the burned products were dissolved in hydrochloric acid. And finally the arsenic content was determined by atomic fluorescence spectroscopy (AFS). Fly ash samples were first mixed with acid (HNO3 + HCl) and then dissolved at 80℃. The solution was then sent for AFS analysis. The accuracy of the studied arsenic determination was calibrated by standard reference materials GBW11117 (coal) and GBW08401 (ash). The precision of the tests is estimated to be lower than 5%. 2.2.2. Chemical speciation analysis The valence of arsenic in fly ashes was determined by AFS coupled with high performance liquid chromatography (HPLC). Prior to the
Table 1 Specific information about the plants selected. Samples
Plant Plant Plant Plant a b c d
1 2 3 4
(Pl.1) (Pl.2) (Pl.3) (Pl.4)
Proximate analysis (%) Madd
Aad
Vad
FCad
7.08 4.43 2.07 2.81
35.81 29.30 6.96 51.4
30.60 13.00 45.86 18.06
26.51 53.27 45.11 27.73
Arsenic concentration (μg/g)
Furnace type
Dust removal device
43.28 3.498 0.896 8.290
Pulverized coal furnace
EFFa ESPb ESPb FFc
Electrostatic fabric filter. Electrostatic precipitator. Fabric filter. Air dry. 2
Fluidized bed
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HPLC-AFS analysis, two valence of arsenic (As3+ and As5+) were extracted from the ash samples with phosphoric acid and ascorbic acid [19]. Furthermore, the five-step sequential extraction procedures were used to differentiate the Ca/Fe/Al-bound arsenic in fly ashes [20,21]. Accordingly, arsenic in various phases, including non-specifically adsorbed arsenic (F1), specifically adsorbed arsenic (F2), calcite-bound arsenic (F3), amorphous and poorly crystalline Fe/Al-bound arsenic (F4) and well crystalline Fe/Al-bound (F5) arsenic were extracted and determined. Moreover, in order to confirm the possible arsenic capture behavior by accessory Ca/Fe/Al compounds in fly ashes, CaSiO3/CaSO4/Fe2O3/ Kaolin were selected to react with arsenic vapors at 900 °C. Afterward, arsenic speciation in the adsorbents was analyzed by the five-step sequential extraction procedures as described above.
Fig. 1. Mass distribution of arsenic in coal combustion products. (Others represent limestone, gypsum, sludge and waste water).
2.2.3. Calculation methods As shown in Eqs. (1) and (2), the relative enrichment index (REI) is adopted to study the partitioning behavior of arsenic in combustion byproducts (REI-1) and the enrichment characteristics of arsenic in different fly ashes from various dust removal systems (REI-2) [22–24]:
REI
1 = Mproduct CAs,product /Mcoal CAs,coal
(1)
REI
2 = CAs,fly ash A coal,ad/CAs,coal
(2)
Mproduct represents the mass of combustion residues, such as bottom ash, fly ash, waste water etc., kg; CAs, product represents the total As content in product, mg/kg; Mcoal represents the mass of coal, kg; CAs,coal represents the total As content in the coal, mg/kg; CAs,fly ash represents the total As content in fly ash, mg/kg; Acoal,ad represents the ash yield in the coal on air-dried basis. In this study, fly ash was considered to account for 80% while bottom ash was considered to account for 20% in pulverized coal-fired plants while the fly ash and bottom ash produced by fluidized bed boiler accounted for 60% and 40% in the whole ash generated by coal combustion. In order to study the relationship between the composition of fly ash and its resistivity, Eq. (3) is used to calculate fly ash resistivity with chemical composition and temperature [16].
Fig. 2. The variation of arsenic content in fly ashes collected from different hoppers of four power plants. Table 2 Analysis of size distribution of ESP/FF fly ashes collected from four power plants. Samples
lg = 2023.96 + [ 1.22lgNK+Na + Li
0.81(lgNK+Na + Li ) 2] +
Pl.1-ESP1 Pl.1-ESP2 Pl.1-ESP3 Pl.1-FF1 Pl.1-FF2 Pl.2-ESP1 Pl.2-ESP2 Pl.2-ESP3 Pl.3-ESP1 Pl.3-ESP2 Pl.3-ESP3 Pl.4
[0.31lgNSi + Al + 0.11(lgNSi + Al )2] + [0.75lgNCa + Mg + 0.14(lgNCa + Mg ) 2] + [ 0.82lgNFe
0.61(lgNFe ) 2]
45801/ T
1203lgT + 182(lgT)2 (3)
NK+Na+Li is the amount percent of K, Na, and Li atoms (%); NFe is the amount percent of Fe atoms (%); NCa+Mg is the amount percent of Ca and Mg atoms (%); NAl+Si is the amount percent of Al and Si atoms (%); and T denotes temperature (K). 3. Results and discussion
a
3.1. Enrichment characteristics of arsenic in different fly ashes
D(4,3)a (μm)
Volume (%) PM20
PM10
PM2.5
PM1
26.35 52.55 56.43 87.46 91.22 35.13 40.47 67.20 34.60 40.10 43.59 45.25
13.21 32.68 36.74 74.47 79.38 15.60 22.16 50.26 18.81 22.94 28.77 28.08
4.06 11.04 13.04 30.72 35.25 3.65 5.48 16.19 4.93 6.32 9.43 7.99
1.51 3.74 4.18 8.61 9.23 1.82 2.50 6.04 2.03 2.54 3.51 2.15
56.368 17.829 15.887 3.991 3.170 31.698 28.251 8.934 35.566 31.698 28.251 25.179
D(4,3), the bulk particle size obtained using the volume average method.
increased. According to the particle size analysis (Table 2), the average particle size of fly ash showed a decreasing trend as the number of dust collector stages increased. The trend was not surprising because the coarser ash particles were preferentially collected in the front rows of ESP systems while finer ash particles, with more adsorption sites, were easier to escape from the front dust removal system and enrich in the back rows [27–29]. Therefore, the capture capacity of arsenic vapors by fine ash particles was higher, which attributed to the increase of arsenic content in the rear particulate control systems. Fig. 3 shows the value of REI-2 of arsenic in various fly ashes, further confirming the enrichment of arsenic in the back rows of the ash removal systems. Interestingly, as observed in Fig. 2, the concentration of arsenic in fly ash from ESP3 was significantly higher than that of ESP1 and ESP2
The mass distribution of arsenic in different ashes collected from four power plants is illustrated in Fig. 1. Based on the value of REI-1, arsenic was found to tend to enrich in fly ash rather than in bottom ash. Furthermore, arsenic distributed more in the bottom ash in fluidized bed boiler compared to the pulverized coal boiler. This might be attributed to a lower combustion temperature (750–950 °C) and a relatively complete reactions in the fluidized bed boiler, which would inhibit the vaporization of arsenic and promote the interactions between arsenic and minerals [25,26]. The concentration of arsenic in the ash samples collected from various EFF/ESP/FF systems is shown in Fig. 2. Generally, the arsenic content in fly ashes increased as the number of dust collector stages 3
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Fig. 3. Enrichment characteristics of arsenic in ash samples from various dust collectors.
Fig. 5. A comparison of arsenic distribution in sized fly ashes according to previous studies.
while ESP1 and ESP2 showed a slight difference regarding the arsenic content. As shown in Table 2, the percentages of PM2.5 in fly ashes from ESP3/FF were significantly higher compared with the first two rows, suggesting that the distribution of arsenic in different ESP fly ashes showed essentially similar trend. Therefore, the particle size distribution of each row ashes played an important role on the distribution of arsenic content. In order to study the relationship between particle size and arsenic distribution, arsenic content in sized fly ash (PM2.5, PM10, and PM20) was determined. The distribution of arsenic in the sized fine ash particles from Pl.1 and Pl.2 is shown in Fig. 4(a) and (b), respectively. According to Fig. 4(a), arsenic concentration in PM2.5, PM10, and PM20 segregated from different ESP/FF ashes showed great discrepancy,
probably as a result of the decreasing in the average particles size of PM2.5, PM10, and PM20 (Table 2). It was also found that the concentration of arsenic in PM2.5, PM10, and PM20 collected from the same ESP or FF hoppers was close to each other, indicating that arsenic content did not increase linearly as the particle size decrease. A summary of arsenic distribution in sized fly ashes from previous studies was comparatively shown in Fig. 5. The results revealed that, in some researches, the content of arsenic in PM2.5 segregated from fly ash was more than twice that of PM2.5–10, indicating arsenic was extremely enriched in PM2.5; whilst other studies observed no significant variations in arsenic content as the particle size increasing [6,8,9,32]. Zhao et al. [6] proposed that arsenic distributed in size-classified particulate matter peaking at 0.1 μm, 0.6 μm and 4 μm particles. And the presence of different arsenic peaks might be attributed to the interactions between arsenic and various minerals. Consequently, understanding the speciation of particulate arsenic helps the illumination of the participation mechanisms of arsenic towards the fly ashes. 3.2. Chemical speciation of arsenic in sized fly ash particles Fig. 6 shows the valence of arsenic in fly ashes sampled from various ESP/EFF units. Arsenic in all fly ashes was mainly present as arsenates (As5+), which was in agreement with previous studies [10,30,34,35]. The predominance of As5+ over As3+ indicated that most of the As2O3 (g) reacted with ash minerals (Al/Ca/Fe) by forming arsenates [14,35]. In addition, there was still a certain proportion of As3+ in fly ashes, which might be present as As (III)-glass [30]. Meanwhile, more fractions of As3+ were found in fabric filters (FF) ash, with small average
Fig. 4. The concentration of arsenic in sized fine ash particles (PM2.5, PM10, and PM20) from different ESP hoppers (a) Plant 1 (Pl.1), (b) Plant 2 (Pl.2).
Fig. 6. Proportions of As3+/As5+ in fly ashes from various dust collectors. 4
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Fig. 7. Arsenic partitioning in the adsorption products after arsenic reactions with CaSO4/CaSiO3/Fe2O3/Kaolin at 900 °C. (F1, non-specifically adsorbed As; F2, specifically adsorbed As; F3, calcite-bound As; F4, amorphous and poorlycrystalline Fe/Al-bound As; F5, well-crystallized Fe/Al-bound As).
Fig. 8. Arsenic partitioning in the sequential extraction procedures fractions of fly ashes from various plants. (F1, non-specifically adsorbed As; F2, specifically adsorbed As; F3, calcite-bound As; F4, amorphous and poorly-crystalline Fe/Albound As; F5, well-crystallized Fe/Al-bound As).
particle size. The enrichment of As3+ in FF ashes might be associated with the increase of adsorption sites on the surface of fine particles and the chemical reactions between arsenic vapors and Al-compounds [33]. To study the interactions between arsenic and different minerals, the five-step sequential extraction procedures (SEP) were adopted. Prior to the SEP analysis, CaSiO3/CaSO4/Fe2O3/Kaolin, as common Ca/ Fe/Al-based components in fly ash for arsenic retention [6,11,14], were chosen to be the potential absorbents for gaseous arsenic, to verify the accuracy of this method. After being extracted by different reagents, the modes of arsenic occurrences in different adsorbents are illustrated in Fig. 7. As shown in Fig. 7, arsenic bound with Ca-bearing compounds was mainly extracted in the first two steps while Fe/Al-bound arsenic was mainly extracted in the last two steps. Furthermore, arsenic extracted in the first step (F1) was mainly the arsenic associated with CaSO4 while arsenic extracted in the second step (F2) was mainly Ca–Si-bound arsenic. In addition, the experimental results suggested that arsenic combined with Al/Fe-based absorbents was mainly present as stable compounds, which might be formed through heterogeneous reactions between gaseous arsenic and ash components [31,33,35,36]. Ca-based components, as main absorbent for arsenic retention reported in many publications [11,37,38], captured and transformed gaseous arsenic into unstable ion-exchangeable arsenic in fly ashes, which might be harmful to local environment during ash landfilling [39]. Figs. 8 and 9 show the percentage distribution of different arsenic speciation in bulk fly ashes and in sized fractions, respectively. According to Fig. 8, more than 60% of arsenic was associated with Fe/Alcompounds in fly ashes collected from Pl.1 while more than half of arsenic was combined with Calcium in ashes from other plants. The difference in arsenic speciation between various plants might be related to the proportion of Ca/Fe/Al-compounds in ash and the reactions between arsenic and ash minerals. Many researchers reported the influence of calcium in arsenic capture [11,12], however, this study found that aluminum also played a significant role in arsenic transformation, especially in Pl.1. In addition, Fe/Al-bound arsenic increased while Cabound arsenic decreased as the number of dust collector stages increased. It indicated that arsenic associated with calcium compounds tended to enrich in coarse particles, while arsenic bound with Fe/Alcompounds was prone to enrich in fine particles. In other word, arsenic combined with Fe/Al-compounds tended to escape from electrostatic precipitators compared to arsenic combined with Ca-compounds. Meanwhile, the fraction of arsenic extracted from the first step (F1) decreased as the number of dust removal stages increased, while the fraction of arsenic in F2 showed slight difference. It’s suggested that arsenic associated with calcium silicate compounds tended to escape from ESP and calcium sulfate-bound arsenic was easier to be removed,
Fig. 9. Arsenic partitioning in the sequential extraction procedures fractions of sized fly ashes (a) Pl.1, (b) Pl.2. (F1, non-specifically adsorbed As; F2, specifically adsorbed As; F3, calcite-bound As; F4, amorphous and poorly-crystalline Fe/Al-bound As; F5, well-crystallized Fe/Al-bound As).
relatively. As shown in Fig. 9, arsenic speciation in PM2.5/PM10/PM20 showed great discrepancy from the first field to the third field, suggesting that particle size of fly ash had a great influence on the speciation of particulate arsenic. The fraction of Fe/Al-bound arsenic in 5
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formation of particulate arsenic. Different minerals show great discrepancy on its resistivity, which is closely related to the proportion of Ca/Fe/Al [16]. Moreover, the ash removal efficiency of ESP systems is heavily depend on the resistivity of fly ash. Coal fly ash is mainly inorganically composed of aluminum-based compound (mullite), calcium-based compound (anhydrite) and iron-based compound (hematite) [6]. Different combination of these minerals will lead to both the difference of the Ca/Fe/Al content and the change of fly ash resistivity. Consequently, arsenic combined with Ca/Fe/Al-based minerals (as discussed in Section 3.2) shows different ash resistivity, which in turn affects the removal efficiency of particulate arsenic by ESP systems. In this study, the removal efficiency of arsenic in ESP was evaluated on the basis of the calculation of fly ash resistivity. The resistivity calculation model summarized by Zhen et al. [16] was adopted and the charging characteristics of different minerals was also discussed. The variation of ash resistivity versus the percent of Ca/Fe/Al atoms of anhydrite, hematite, and mullite was given in Fig. 10, respectively. Previous studies showed that the most suitable range of fly ash resistivity was at 104–5 × 1010 Ω cm [40,41]. However, if the fly ash resistivity was too high (> 1011 Ω cm), the conductivity of ash particles declined and the ash removal efficiency decreased [40,41]. According to Fig. 10, when NFe reduced to 6.9% or 17.7%, the resistivity of mullite or anhydrite would exceed 1011 Ω cm and the removal efficiency would decline. However, the resistivity of hematite would not change a lot when NAl/NCa varied over a large range. This calculation showed that arsenic associated with iron-bearing minerals was easier to be removed by electrostatic precipitator compared to Ca/Al-bound arsenic. 4. Conclusions The characteristics of arsenic distribution in various ash particles as well as the association relationship between accessory minerals and arsenic in the fly ash were investigated in this study. The results showed that arsenic was found to enrich in fly ash rather in bottom ash. Meanwhile, arsenic distributed more in the bottom ash in fluidized bed boiler compared to the pulverized coal boiler, which might be attributed to the lower combustion temperature and relatively complete reactions occurring in the fluidized-bed boiler. The concentrations of arsenic in the fly ash from the first two ESP hoppers were at similar level while were significantly lower than that of ash samples from rear hoppers. Furthermore, particle size as well as ash components both had a great influence on arsenic speciation. Arsenic associated with calcium compounds tended to enrich in coarse fly ashes whilst arsenic bound with Fe/Al-compounds tended to enrich in fine fly ashes. In addition, arsenic associated with calcium silicate compounds tended to escape from ESP and calcium sulfate-bound arsenic was easier to be removed, relatively. According to the calculation results of resistivity based on ash minerals (mullite, anhydrite, and hematite), it was suggested that arsenic associated with Fe-compounds was easier to be captured by electrostatic precipitators compared to Ca/Al compounds. Acknowledgments
Fig. 10. Calculation results of sensitivity of resistivity to Fe, Ca, and Al atoms of mullite, anhydrite and hematite at 363 K.
This work was supported by National Key Research and Development Project of China (2018YFB0605103) and Program for HUST Academic Frontier Youth Team (2018QYTD05). Professor Chungang Yuan and his group are thanked for providing fly ash separating technology support.
PM2.5/PM10/PM20 showed an increasing trend from the first stage to the last stage while the average particle size of fine particles decreased. It further confirmed that Fe/Al-bound arsenic tended to enrich in fine particles. Meanwhile, the fraction of arsenic associated with calcium in PM2.5/PM10/PM20 decreased as the number of dust removal stages increased, which was consistent with the phenomenon found in fly ash from various ESP/FF units (Fig. 8).
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