Effects of increasing chlorine concentration in feedstock on the emission and distribution characteristic of dioxins in circular fluidized bed boiler

Field studies were conducted to study the emission and distribution characteristics of dioxins by elevating the chlorine concentration in feedstock in a circular fluidized bed boiler. The concentration and total equivalent quantity of polychlorinated dibenzo–p–dioxins and polychlorinated dibenzofurans (PCDD/Fs) in all flue gas, electrostatic ash, bag filter ash, and bottom ash samples under blank condition (i.e., feedstock was normal coal) and chlorine labeling condition (i.e., feedstock mixed with coal and chlorine-containing labeling agent) were analyzed. Results illustrated that the concentration of PCDD/Fs in all gaseous and ash samples increased with the addition of chlorine in feedstock, with the largest and least increment in dioxin concentration observed in electrostatic ash and flue gas. PCDDs were the predominate congeners in flue gas, accounted for 50.1–60.4% of the total PCDD/F concentration under chlorine labeling and blank conditions, while PCDD/F distribution changed from PCDD– to PCDF–predominate by increasing chlorine content in feedstock under all field test conditions: 46.6–92.9%, 34.0–76.1%, and 47.0–53.1% of PCDFs were distributed in electrostatic ash, bag filter ash, and bottom ash, respectively. Highly chlorinated PCDD/F congeners such as O8CDD/F and 1,2,3,4,6,7,8-H7CDD/F were the primary contributors to dioxin concentration in flue gas and bottom ash samples, whereas low-chlorinated 2,3,7,8-T4CDF and 1,2,3,7,8-P5CDF congeners became critically dominating in electrostatic and bag filter ash.


Introduction
Dioxin, generally referring to the 17 most toxic polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran (PCDD/F) congeners, comprises a class of carcinogenic, teratogenic, and mutagenic persistent organic pollutants (Bai et al. 2014). PCDD/Fs have been linked to several health risks, including immunotoxicity, neurotoxicity, reproductive toxicity, and endocrine disruption . Unintentional incineration of wastes in high-temperature industrial kilns could lead to PCDD/F formation (Huang et al. 2012).
China exhibits an increasing trend of the use of hightemperature industrial kilns such as cement kilns, multicomponent slurry gasifiers, and brick kilns to co-process hazardous and industrial wastes, since specific types of waste could be used as alternative fuel or raw materials during co-processing, hence reducing the consumption of fuel and raw materials. Incomplete statistics estimated that the ability of cement kiln co-processing hazardous wastes in China increased from approximately 1 million tons in 2014 to more than 8 million tons in 2020; meanwhile, the number of licensed cement kiln co-processing enterprises increased from less than 20 to nearly 120. Vigorous development of waste co-processing in high-temperature industrial kilns has aroused significant concerns on dioxins in China; research on the emission and distribution characteristics of PCDD/ Fs during co-processing has become widely investigated. In 2015, Liu et al. used cement kiln to co-process fly ash and found out low-chlorinated PCDFs, such as 2, 3,7,3,4,7, CDF, were identified as dominant contributors to the international total equivalent quantity (I-TEQ) of the PCDD/Fs in flue gas and particulates (Liu et al. 2015). Zhao et al. investigated that the alkaline condition and low copper content inside cement kiln could effectively suppress deacon reaction and thus created an insufficient chlorine content environment. In the absence of adequate chlorine, low-chlorinated PCDFs became predominantly ). Wang et al. studied 12 hazardous waste incinerators in China and observed that the chlorine content in feeding wastes played an important role in the formation of PCDD/F congeners, with the chorine content below a threshold value at 8000-11,000 mg/kg; high-chlorinated PCDDs became the most abundant congeners in flue gas (Wang et al. 2014). Yan et al. used multicomponent slurry gasifiers to co-process organic wastes and found out that PCDD/Fs showed reverse distribution characteristics in flue gas and solid samples; 86.6-94.2% of PCDDs were distributed in flue gas, whereas 76.5-93.1% of PCDFs were distributed in solid samples. They assumed that instantaneous decrease of temperature at quenching chamber may affect the ratio of PCDDs/PCDFs in flue gas and solid samples . The above researches illustrate that the distribution characteristics of dioxins in flue gas and solid samples differ kiln-by-kiln when co-processing wastes.
Currently, high-temperature industrial kiln co-processing field studies are carried out in rotary kilns, mechanical grate furnaces, and gasifiers; studies on using boilers, especially circular fluidized bed (CFB) boilers, to co-process wastes are limited (Li et al. 2020;Yang et al. 2021). Compared to other industrial kilns, CFB boilers provide the advantages of a higher combustion efficiency, more stable combustion temperature, sufficient material residence time, and lower pollutant emissions (Zhou et al. 2021). Researches on PCDD/F emission and distribution in waste co-processing involving CFB boilers mainly focus on two aspects: firstly, the emission and distribution characteristics of PCDD/Fs in liquid, gaseous, and solid samples during co-processing (Ren et al. 2022;Zhang et al. 2022), and secondly, methods to reduce PCDD/F formation during co-processing (Amand and Kassman., 2013;Pongpiachan et al. 2016). However, there are limitations in studying the emission and distribution characteristics of dioxins in CFB boilers through co-processing wastes, since the chlorine content, dioxin precursor (such as polychlorinated biphenyl, polyaromatic hydrocarbons) contents, and dioxin-catalytic metal (such as Cu, Fe) contents differ from waste to waste, and the emission and distribution characteristics of PCDD/Fs for specific wastes are not suitable to represent a general characteristic of dioxins in CFB boilers. Previous studies revealed that chlorine plays an important role in dioxin formation; the increase of chlorine content in feeding wastes could lead to the deterioration of boiler combustion conditions, resulting in the increase of incomplete combustion dioxin precursors, and therefore promoting the formation of PCDD/Fs (Wang et al., 2003). From the perspective of researching emission and distribution characteristics of dioxins in CFB boilers, if only chlorine-containing labeling agents are added to the CFB boiler, the emission and distribution of PCDD/Fs in the boiler will be more intuitive and less interference from dioxin precursors and catalytic metals, compared with co-processing wastes.
In this work, field studies were conducted to reveal the emission and distribution characteristics of PCDD/Fs in flue gas and solid samples by gradually elevating the chlorine content in feedstock in a CFB boiler. The PCDD/F concentration in all emitted gaseous and solid substances, including flue gas, electrostatic ash, bag filter ash, and bottom ash under blank conditions (BCs) (i.e., only coal was added to the CFB boiler) and chlorine labeling conditions (i.e., both coal-and chlorine-containing labeling agents were added to the CFB boiler), were detected. The environmental risk of PCDD/Fs under all field test conditions was also analyzed. These data may have significant guidance to the development of CFB boiler co-processing technology.

Basic parameter and operating conditions of the CFB boiler
The CFB boiler used in these field studies started operating in 2014 in Sichuan, China, and composed of six cyclone separators, economizer, air preheater, and primary combustion chamber, with a maximum evaporation capacity of 1900 t/h. The air pollution control devices (APCDs) of the boiler included an electrostatic precipitator (ESP), a desulfurizing tower (DST), and a bag filter (BF). Specifically, the ESP and BF are set to remove particulate pollutants, and DST aims to control the SO 2 emission in flue gas. The operating parameters of the boiler were as follows: (1) the combustion temperature of the combustion chamber was above 830 °C (dense-phase zone) and 650 °C (dilute-phase zone); (2) the evaporation capacity of the boiler stayed stable at 770 t/h; (3) the export oxygen concentration in flue gas was strictly maintained at 6-8%.
A process flow diagram of the CFB boiler is shown in Fig. 1. Coal-and chlorine-containing labeling agents were added from coal bunker and transferred into the boiler through chain and belt conveyor. CaCO 3 was used as an internal desulfurizing agent to desulfurize flue gas inside the CFB boiler, and CaOH was used as an external desulfurizing agent to desulfurize flue gas in the DST. After passing through the BF, clean flue gas was discharged into the atmosphere through the flue gas stack. Electrostatic ash, bag filter ash, and bottom ash were stored in ash storage A, ash storage B, and bottom ash storage, respectively.

Experimental methods and materials
Field studies could be categorized into three conditions. BC refers to the addition of feedstock comprising only coal, whereas chlorine labeling conditions correspond to the addition of feedstock mixed with coal and chlorinecontaining labeling agent. To further investigate the relationship between the chlorine concentration and PCDD/F emission, chlorine labeling conditions were subdivided into low-chlorine labeling condition (LLC) and high-chlorine labeling condition (HLC).
NaCl was used as a chlorine-containing labeling agent for two reasons. First, recent reviews suggest that chlorine may occur in an inorganic form (i.e., water-soluble alkali metal chlorides such as NaCl or KCl) in coal (Mazurek et al. 2021). Moreover, NaCl is less expensive and hypotoxic than other chlorine-containing agents, such as KCl or dichlorobenzene, which makes NaCl suitable for use in large amounts in field studies, both economically and safely. Chlorine concentrations under BC, LLC, and HLC are listed in Table S1. Under LLC, the content of chlorine was approximately 1.5 times higher than that under BC, while under HLC, the chlorine concentration was approximately 4 times higher than that under BC. By considering that the excessive addition of chlorine in feedstock could lead to an increasing emission of HCl in flue gas, and therefore, corrode the boiler combustion chamber, APCDs, and flue gas stack, and also based on the chlorine content in the coal used, as well as the amount of NaCl prepared for the field tests, the content of chlorine was set to be 1.5 times (LLC) and 4 times (HLC) higher that of BC.
Samples were collected at four sampling points: flue gas stack, ash storage A, ash storage B, and bottom ash storage. The desulfurization wastewater produced by the DST during boiler operating was recycled and not discharged into the environment; therefore, flue gas and ash samples were all emitted substances of the boiler. Flue gas samples were collected based on the HJ/T 77.2-2008 standard, and ash samples were collected based on the HJ/T 77.3-2008 standard. Each field test condition was implemented for at least 24 h to achieve a possible combustion stability (i.e., steady-state operation conditions, an uninterrupted air intake, and minimization of process upsets). Under each field test condition, three flue gas samples, two electrostatic ash samples, two bag filter ash samples, and two bottom ash samples were collected.

Quality control and assurance
PCDD/Fs in all gaseous and ash samples were analyzed on a Waters AutoSpec Premier spectrometer, which contained an Agilent 6850 N high-resolution gas chromatography (HRGC) coupled with a high-resolution mass spectrometry (HRMS). A 60-m DB-5MS silica capillary column with an inner diameter and film thickness of 0.25 mm and 0.25 μm, respectively, was used. The HRGC instrument was set up on splitless injection mode with an inlet temperature of 270 ℃ and a carrier gas flow of 1.0 mL/min. The initial oven temperature was 140 °C (held for 1 min), and then increased to 200 °C (held for 1 min) at a rate of 20 °C/min, then heated to 220 °C at a rate of 5 °C/ min (held for 16 min), to 235 °C at a rate of 5 °C/min (held for 7 min), and finally to 310 °C (held for 10 min) at a rate of 5 °C/ min. An internal dioxin standard was added, with the recovery rate ranged from 75 to 125%. The selected ion monitoring mode was used for the HRMS instrument, with a resolution higher than 10,000.
The limits of detection (LODs) of PCDD/Fs were defined as three times the signal/noise (S/N) ratio. Each PCDD/F concentration below the LOD was replaced with 0.5 × the LOD when calculating the I-TEQ. The I-TEQ of PCDD/Fs was defined as the PCDD/F concentration times the toxic equivalency factor (TEF) of PCDD/Fs. As shown in Table S2, the TEF recommended by the World Health Organization was used to calculate the I-TEQ (Van den Berg et al. 2006).

PCDD/F emission factor
As expressed in Eq. 1, the emission factor of PCDD/Fs in flue gas is defined as the mass of the compounds released into the atmosphere per unit mass of fuel and labeling agent consumed by combustion.
where E gas is the emission factor of flue gas in nanograms/ kilogram, C i is the mass concentration of PCDD/F congeners in nanograms/cubic meter, R gas is the volume flow rate of the dry flue gas under standard condition in cubic meters/hour, M coal is the mass of coal consumed per hour in kilograms/ hour, and M label is the mass of the labeling agent consumed per hour in kilograms/hour.
As expressed in Eq. 2, the emission factor of PCDD/Fs in ash is defined as the mass of the compounds absorbed by ash per unit mass of fuel and labeling agent consumed by combustion.
where E ash is the emission factor of ash in nanograms/kilogram, and PR ash is the production rate of ash in kilograms/ hour. (1)

Emission characteristics of PCDD/Fs
The mass concentration of PCDD/Fs in flue gas under BC, LLC, and HLC is shown in Fig. 2. An increasing trend in the PCDD/F mass concentration was observed from BC (25.8 pg/m 3 ) to LLC (26.1 pg/m 3 ) and to HLC (35.3 pg/ m 3 ). Compared to BC, the PCDD/F mass concentration in flue gas under LLC and HLC only increased by 1.2% and 26.9%, respectively. By considering the amount of chlorinecontaining labeling agent added and the total chlorine content under LLC and HLC, these increases were considerably small, which could be attributed to the combustion efficiency of the CFB boiler and the functioning of APCDs. Sufficient combustion of the CFB boiler and APCD operation could effectively remove dioxins in flue gas by reducing the formation of PCDD/F precursors such as polyaromatic hydrocarbons and chlorinated aromatic pollutants (Li et al. 2016;Hsu et al. 2021).
The results for the PCDD/F mass concentration in electrostatic ash, bag filter ash, and bottom ash are shown in Fig. 3. In general, the PCDD/F content exhibited an increasing trend for all three ash types under BC, LLC, and HLC, except bag filter ash under LLC, which declined indistinctively from 12.1 to 10.7 ng/kg below BC levels. In contrast to the slight increase in the PCDD/F mass concentration in flue gas, electrostatic ash revealed a notable increase in dioxin concentration from BC (7.0 ng/kg) to LLC (24.7 ng/kg) and to HLC (68.0 ng/ kg). The PCDD/F mass concentration in electrostatic ash under LLC and HLC was 3.5 and 9.3 times, respectively, higher than that under BC. The chlorine concentration in feedstock significantly affects the dioxin concentration in electrostatic ash, by elevating the chlorine concentration in feedstock, the PCDD/F mass concentration in electrostatic ash increased dramatically. Previous study has indicated that PCDD/Fs could potentially be synthesized within the ESP since the operating temperature of ESP was between 210 and 240 °C, which was in the de novo synthesis temperature windows (Wang et al. 2007). The operational temperature of the ESP used in the field study remained below 180 °C, which could prevent PCDD/F regeneration. The prominent increase in PCDD/Fs under LLC and HLC could be explained by the effectiveness of the ESP in capturing particle and solid-phase PCDD/ Fs, especially congeners with a low chlorination degree, through absorption (Guerriero et al. 2009).
Under the three field test conditions, the PCDD/F mass concentration in bag filter ash reached 12.1 ng/kg, 10.7 ng/ kg, and 21.3 ng/kg. Compared to BC, a negligible decrease in the PCDD/F mass concentration in bag filter ash was observed under LLC, and the concentration almost doubled from LLC to HLC. Former studies have illustrated that the memory effect, defined as an unexpectedly high PCDD/F mass concentration in solid samples due to unstable combustion temperature and unusual APCD operation conditions, could occur in the BF during incineration (Chang and Lin., 2001;. However, the distributed control system of the CFB boiler revealed that no significant fluctuation in the combustion temperature and APCD operational parameters was observed during field testing, so the working efficiency of the ESP, DST, and BF could be considered consistent under the three test conditions, and the memory effect should occur under all test conditions rather than only under BC. Therefore, the decrease in the PCDD/F mass concentration in bag filter ash under LLC should not be related to the memory effect but could be attributed to instantaneous operating condition (i.e., gas flow and feedstock feeding rate) fluctuations inside the CFB boiler during sample collection period.
The PCDD/F mass concentration in bottom ash reached 6.6 ng/kg, 22.6 ng/kg, and 24.2 ng/kg under the three test conditions. By elevating the chlorine concentration in feedstock, the PCDD/F mass concentration in bottom ash could also increase. However, the increase in dioxin content in bottom ash was negligible, especially when considering that the chlorine concentration under HLC was 2.7 times higher than that under LLC, but the PCDD/F concentration only increased by 7.1%.
The gas flow and production rate of electrostatic ash, bag filter ash, and bottom ash of the CFB boiler under the three field test conditions are listed in Table S3, and the emission factors of PCDD/Fs based on mass concentration and toxicity concentration in gaseous and ash samples are listed in Table 1. Overall, by increasing the concentration of chlorine in feedstock, the PCDD/F emission factors of gaseous and ash samples could also increase. Under the three test conditions, flue gas exhibited the lowest PCDD/F emission factor ranging from 0.26 to 0.39 ng/kg, whereas the highest emission factor was observed for electrostatic ash, ranging from 0.70 to 6.83 ng/kg. Previous review concluded PCDD/F emission factors from industrial boilers and wood-fired boilers. The emission factors of industrial boilers and wood-fired  Horizontal comparison of the PCDD/F mass concentration distribution of flue gas, electrostatic ash, bag filter ash, and bottom ash under the three test conditions (Fig. 4) revealed that flue gas achieved the lowest PCDD/F mass concentration distribution, and by increasing the chlorine content in the applied feedstock, the mass concentration distribution of flue gas further decreased from BC (7.3%) to LLC (4.1%) and to HLC (3.1%). In contrast, the addition of chlorine notably impacted the PCDD/F mass concentration distribution of both fly ash types (i.e., electrostatic and bag filter ash), which increased from 69.5 (BC) to 78.8% (HLC).

Distribution characteristics of PCDD/Fs
The PCDD/F congener profile is usually considered a fingerprint pattern to analyze the formation and decomposition mechanism of PCDD/Fs in flue gas . In this study, 17 PCDD/F congeners were categorized into PCDDs and PCDFs for convenient analysis, and the distribution characteristics of flue gas under BC, LLC, and HLC are shown in Fig. 5. PCDD/Fs in flue gas were mainly dominated by PCDDs under BC and LLC, accounting for 60.4% and 50.2%, respectively, of the total PCDD/Fs mass concentration, whereas PCDFs increasingly dominated under HLC (50.1%). Highly chlorinated compounds, such as O 8 CDD/F, 1,2,3,4,6,7,8-H 7 CDD/F, and 1,2,3,4,7,8,9-H 7 CDD/F, were the primary contributors to the PCDD/F mass concentration under the three field test conditions. Under BC, O 8 CDD, 1,2,3,4,6,7,8-H 7 CDD, 1,2,3,4,7,8,9-H 7 CDF, and 1,2,3,6,7,8-H 7 CDF monomers accounted for 47.7% of the total mass concentration in flue gas, with the proportions of 20.2%, 14.7%, 6.6%, and 6.2%, respectively, while under LLC, PCDD/F monomers mainly included O 8 CDD, O 8 CDF, 1,2,3,4,6,7,8-H 7 CDD, and 1,2,3,4,6,7,8-H 7 CDF, with proportions of 16.2%, 9.3%, 8.6%, and 8.6%. Moreover, 1,2,3,4,6,7,8-H 7 CDD (17.6%), 1,2,3,4,6,7,8-H 7 CDF (12.9%), 1,2,3,6,7,8-H 6 CDF (11.6%) and O 8 CDD (11.3%) accounted for more than 51% of the total PCDD/F mass concentration in flue gas under HLC. In previous studies, a higher percentage of highly chlorinated PCDD/F congeners in flue gas was observed for wood-fired boilers (Bai et al. 2017;Moreno et al. 2016), multicomponent slurry gasifiers , and electric arc furnaces (Chiu et al. 2011  During combustion, dioxins could be synthesized along three different routes, including de novo synthesis (elementary reaction between carbon, hydrogen, oxygen, and chlorine) from 200 to 400 °C, catalyst-assisted coupling of precursors (catalytic reaction between PCDD/F precursors and transition metals) from 300 to 600 °C, and high-temperature gas-phase reaction (condensation, cyclization, hydroxyl substitution, and dichlorination reactions of short-chain chlorinated hydrocarbons) from 500 to 800 °C (Stieglitz and Vogg., 1987;Cains et al. 1997;Mckay., 2002). The ratio of PCDDs/PCDFs is commonly used to determine the formation mechanism of PCDD/Fs, and most researchers agree that de novo synthesis occurs at PCDD/PCDF ratio < 1; conversely, PCDD/Fs are produced through a precursor synthesis mechanism (Chen et al. 2018;Zhang et al. 2022). The ratios of PCDDs/PCDFs in flue gas under the three field test conditions were 1.53, 1.01, and 1.00, suggesting that precursor synthesis was the predominant PCDD/F formation mechanism. The stable and consistent combustion of CFB boilers could potentially control the concentration of chlorine and catalytic metals at a low level, and therefore, the homogeneous reaction of precursors at high temperatures becomes the major formation mechanism of PCDD/Fs in flue gas (Zhong et al. 2020). Figure 6 reveals the distribution characteristics of PCDD/ Fs in electrostatic ash under BC, LLC, and HLC. Generally, the mass concentration of PCDD congeners increased with the increasing of chlorinated level under BC and LLC, but fluctuated from 2, 3,7,where 2,3,7,2,3,4,7, CDD showed the highest and lowest mass content of 1.7 ng/kg and 0.3 ng/ kg, respectively. PCDF congeners exhibited the opposite trend; an increase in the chlorinated level led to a decrease in the PCDF mass concentration. PCDD/Fs in electrostatic ash were largely dominated by PCDDs (53.4%) under BC, whereas PCDFs became significantly dominant under LLC (80.8%) and HLC (92.9%). Highly chlorinated compounds were the crucial PCDD/F congeners in electrostatic ash under BC, and the concentration proportion of O 8 CDD, 1,2,3,4,6,7,8-H 7 CDD, 1,2,3,4,6,7,8-H 7 CDF, and O 8 CDF monomers reached 27.7%, 11.4%, 10.4%, and 7.8%, respectively. In contrast, by increasing the chlorine concentration in the applied feedstock, low-chlorinated compounds became increasingly dominant in electrostatic ash under LLC and HLC. In addition, mass concentration proportion of 2,3,7,8-T 4 CDF (36.1%), 2,3,4,7,8-P 5 CDF (14.4%), and 1,2,3,7,8-P 5 CDF (7.9%) accounted for 58.3% of the total PCDD/F mass concentration under LLC and further contributed more than 78% to the total PCDD/F mass concentration under HLC, with proportions of 52.9%, 11.5%, and 14.3%, respectively. The ratio of PCDDs/PCDFs in electrostatic ash under BC was 1.15, illustrating that PCDD/Fs were mainly formed through precursor synthesis. However, a sharp reduction in the ratio of PCDDs/PCDFs under LLC (0.24) and HLC (0.08) was observed. By increasing the chlorine content in feedstock, the formation mechanism of PCDD/ Fs in electrostatic ash changed from precursor synthesis to de novo synthesis. The higher vapor pressure of PCDF congeners could provide a better desorption capacity than that provided by the vapor pressure of PCDD congeners with the same chlorination substituted number ). Therefore, more PCDFs could be released into electrostatic ash in the electrostatic precipitation process. The distribution characteristics of PCDD/Fs in bag filter ash under BC, LLC, and HLC are shown in Fig. 7. An increase in the chlorinated level led to an increase in the mass concentration of PCDD congeners but a significant decrease in the mass concentration of PCDF congeners in bag filter ash. PCDDs contributed to more than 66% and 56% of the total PCDD/F mass concentration in bag filter ash under BC and LLC, respectively, while PCDFs accounted for 76.1% of the total PCDD/F mass concentration under HLC. Moreover,O 8 CDD and 1,2,3,4,6,7, CDD were the predominant PCDD/Fs in bag filter ash under BC and LLC, with total proportions of 57.3% and 48.8%, respectively. In contrast, 2,3,7,8-T 4 CDF (43.6%) and 1,2,3,7,8-P 5 CDF (13.1%) became critical under HLC, and the proportions of O 8 CDD and 1,2,3,4,6,7, CDD dropped to only 16.9% and 2.8%, respectively. The PCDDs/PCDFs ratios in bag filter ash under the three field test conditions were 1.94, 1.29, and 0.31, indicating that by increasing the chlorine content in the feedstock applied to CFB boilers, the PCDD/F formation mechanism changed from precursor synthesis to de novo synthesis in bag filter ash. Figure 8 shows the distribution characteristics of PCDD/ Fs in bottom ash under BC, LLC, and HLC. In contrast to the distribution characteristics in electrostatic and bag filter ash, distribution of PCDD/Fs in bottom ash revealed a trend of increasing chlorinated level, and the mass concentration of both PCDD and PCDF congeners also increased under the three field test conditions. The PCDD/F mass concentration was mainly dominated by PCDDs in bottom ash under BC and LLC, accounting for 53.0% and 50.3%, respectively, of the total PCDD/F concentration, and PCDFs became increasingly dominant under HLC (53.1%). Highly chlorinated compounds were the predominant PCDD/F congeners in bottom ash under BC,LLC,and HLC,with O 8 CDD,1,2,3,4,6,7,O 8 CDF,and 1,2,3,4,6,7, CDF comprehensively contributing 59.5%, 74.7%, and 90.2%, respectively, to the total PCDD/F concentration. The PCDD/ PCDF ratio in bottom ash under the three test conditions reached 1.13, 1.01, and 0.89, illustrating that the formation mechanism of PCDD/Fs changed from precursor synthesis to de novo synthesis by increasing the chlorine content in the applied feedstock.

Environmental risk from PCDD/Fs
The I-TEQ values of PCDD/Fs in flue gas, electrostatic ash, bag filter ash, and bottom ash are listed in Table 2  ng/kg occur far below the limit value of 40 ng I-TEQ/kg, as per the GB36600-2018 standard in China, and lower than the national standards for dioxin control in Finland (500 ng I-TEQ/kg), Germany (1000 ng I-TEQ/kg), Japan (1000 ng I-TEQ/kg), the USA (1000 ng I-TEQ/kg), and New Zealand (1500 ng I-TEQ/kg) (CCME 2007). Therefore, considering the increase in the chlorine-containing labeling agent content in the applied feedstock to 1756.6 mg/kg, the environmental risk of dioxins in electrostatic ash, bag filter ash, and bottom ash could be considered relatively low.

Conclusions
The PCDD/F concentration in flue gas, electrostatic ash, bag filter ash, and bottom ash increased with increasing chlorine concentration in the feedstock applied to the CFB boiler, and a significant increase in the PCDD/F concentration was observed in electrostatic ash, which could be attributed to the effectiveness of the ESP in capturing PCDD/Fs through absorption. However, due to the sufficient combustion conditions inside the CFB boiler and APCD operation, the concentration of dioxins in flue gas was hardly affected by the chlorine concentration in feedstock. The emission distribution of PCDD/Fs in all gaseous and ash samples indicated that flue gas achieved the lowest dioxin emission distribution. Only 3.1-7.3% of PCDD/Fs was released into flue gas, while the addition of chlorine significantly impacted the PCDD/F emission distribution in both electrostatic ash and bag filter ash, accounting for 69.5-78.8% of the total PCDD/F concentration. The distribution characteristics of PCDD/Fs in flue gas, electrostatic ash, bag filter ash, and bottom ash revealed that PCDDs were the predominant congeners in flue gas, accounting for 50.1-60.4% of the total PCDD/F concentration, and highly chlorinated compounds were the primary contributors. However, by increasing the chlorine concentration in the applied feedstock, the PCDD/F distribution in electrostatic ash, bag filter ash, and bottom ash changed from PCDD-to PCDF-dominated distributions under all field test conditions, namely, 46.6-92.9%, 34.0-76.1%, and 47.0-53.1% of the PCDFs occurred in the ash samples. According to the distribution of dioxin monomers, lowchlorinated 2,3,7,8-T 4 CDF and 1,2,3,7,8-P 5 CDF congeners became critically dominant in electrostatic and bag filter ash, while highly chlorinated PCDD/F congeners remained dominant in bottom ash. The dioxin amount in all gaseous and ash samples remained far below the standard limit in China and other countries, indicating a low environmental risk of dioxins resulting from increasing the chlorine concentration in feedstock applied to CFB boilers. These results could provide evidence in waste identification for electrostatic ash, bag filter ash, and bottom ash after CFB boiler  Bottom ash (ng I-TEQ/kg) 0.49 ± 0.07 1.05 ± 0.20 0.39 ± 0.08 co-processing, and the distribution characteristics of PCDD/ Fs could potentially be used in developing dioxin suppression agents in inhabiting PCDD-and PCDF-predominant dioxins formation in gaseous and ash samples.

Declarations
Ethics approval and consent to participate Not applicable.

Competing interests
The authors declare no competing interests.