3.1 Emission characteristics of PCDD/Fs
The composition and mass concentration of PCDD/Fs in flue gas under BC, LLC, and HLC are shown in Fig. 2. An increasing trend in the PCDD/Fs mass concentration was observed from BC (25.8 pg/m3) to LLC (26.1 pg/m3) and to HLC (35.3 pg/m3). Compared to BC, the PCDD/Fs mass concentration in flue gas under LLC and HLC only increased by 1.2% and 26.9%, respectively. By considering the amount of chlorine-containing labeling agent added and the total chlorine content under LLC and HLC, these increase amounts were considerably small, which could be attributed to the combustion efficiency of the CFB boiler and APCD functioning. Sufficient combustion of the CFB boiler and APCD operation could effectively remove dioxins in flue gas by reducing the formation of PCDD/Fs precursors such as polyaromatic hydrocarbons and chlorinated aromatic pollutants (Li et al., 2016; Hsu et al., 2021).
The results for the PCDD/Fs mass concentration in electrostatic ash, cloth bag ash, and boiler ash are shown in Fig. 3. In general, the PCDD/Fs content exhibited an increasing trend for all three ash types under BC, LLC, and HLC, except cloth bag ash under LLC, which declined indistinctively from 12.1 ng/kg to 10.7 ng/kg below BC levels. In contrast to the slight increase in the PCDD/Fs 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/Fs 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 impacts the dioxin concentration in electrostatic ash. By elevating the chlorine concentration in feedstock, the PCDD/Fs mass concentration in electrostatic ash could dramatically increase. Previous study has indicated that PCDD/Fs could potentially be synthesized within the ESP since the operating temperature of ESP was between 210–240°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/Fs 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/Fs mass concentration in cloth bag ash reached 12.1 ng/kg, 10.7 ng/kg, and 21.3 ng/kg. Compared to BC, a negligible decrease in the PCDD/Fs mass concentration in cloth bag 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/Fs 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; Li et at., 2011). However, the distributed control system (DCS) 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/Fs mass concentration in cloth bag ash under LLC should not be related to the memory effect but should be attributed to instantaneous operating condition (i.e., gas flow and feedstock feeding rate) fluctuations inside the CFB boiler during the sample collection period.
The PCDD/Fs mass concentration in boiler 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/Fs mass concentration in boiler ash could also increase. However, the increase in dioxin content in boiler ash was negligible, especially when considering that the chlorine concentration under HLC was 2.7 times higher than that under LLC, but the PCDD/Fs concentration only increased by 7.1%.
The gas flow and production rate of electrostatic ash, cloth bag ash, and boiler 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 flue gas, electrostatic ash, cloth bag ash, and boiler ash are listed in Table 1. Overall, by increasing the concentration of chlorine in feedstock, the PCDD/Fs emission factors of gaseous and ash samples could also increase. Under the three test conditions, flue gas exhibited the lowest PCDD/Fs 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/Fs emission factors from industrial boilers and wood-fired boilers. The emission factors of industrial boilers and wood-fired boilers ranged from 0.5 ng/kg to 74.6 ng/kg and 12.8 ng/kg to 588.2 ng/kg, respectively (Zhang et al., 2022). The emission factors of this study are comparable with those of previous studies.
Horizontal comparison of the PCDD/Fs mass concentration distribution of flue gas, electrostatic ash, cloth bag ash and boiler ash under the three test conditions (Fig. 4) revealed that flue gas achieved the lowest PCDD/Fs 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/Fs mass concentration distribution of both fly ash types (i.e., electrostatic and cloth bag ash), which increased from 69.5% (BC) to 78.8% (HLC).
3.2 Distribution characteristics of PCDD/Fs
The PCDD/F congener profile is usually considered as a fingerprint pattern to analyze the formation and decomposition mechanism of PCDD/Fs in flue gas (Yang et al., 2017). 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 O8CDD/F, 1,2,3,4,6,7,8-H7CDD/F, and 1,2,3,4,7,8,9-H7CDD/F, were the primary contributors to the PCDD/Fs mass concentration under the three field test conditions. Under BC, O8CDD, 1,2,3,4,6,7,8-H7CDD, 1,2,3,4,7,8,9-H7CDF, and 1,2,3,6,7,8-H7CDF monomers accounted for 47.7% of the total mass concentration in flue gas, with proportions of 20.2%, 14.7%, 6.6%, and 6.2%, respectively, while under LLC, PCDD/Fs monomers mainly included O8CDD, O8CDF, 1,2,3,4,6,7,8-H7CDD, and 1,2,3,4,6,7,8-H7CDF, with proportions of 16.2%, 9.3%, 8.6%, and 8.6%, respectively. Moreover, 1,2,3,4,6,7,8-H7CDD (17.6%), 1,2,3,4,6,7,8-H7CDF (12.9%), 1,2,3,6,7,8-H₆CDF (11.6%) and O8CDD (11.3%) accounted for more than 51% of the total PCDD/Fs mass concentration in flue gas under HLC. In previous studies, a higher percentage of highly chlorinated PCDD/Fs congeners in flue gas was observed for wood-fired boilers (Bai et al., 2017; Moreno et al., 2016), MSGCs (Yan et al., 2021) 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 200–400°C, catalyst-assisted coupling of precursors (catalytic reaction between PCDD/F precursors and transition metals) from 300–600°C, and high-temperature gas-phase reaction (condensation, cyclization, hydroxyl substitution, and dichlorination reactions of short-chain chlorinated hydrocarbons) from 500–800°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/Fs 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 2,3,7,8-T4CDD to O8CDD under HLC, where 2,3,7,8-T4CDD and 1,2,3,4,7,8-H4CDD 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 O8CDD, 1,2,3,4,6,7,8-H7CDD, 1,2,3,4,6,7,8-H7CDF, and O8CDF 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-T4CDF (36.1%), 2,3,4,7,8-P5CDF (14.4%), and 1,2,3,7,8-P5CDF (7.9%) accounted for 58.3% of the total PCDD/Fs mass concentration under LLC and further contributed more than 78% to the total PCDD/Fs 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 (Wang et al., 2022). Therefore, more PCDFs could be released into electrostatic ash in the electrostatic precipitation process.
The distribution characteristics of PCDD/Fs in cloth bag 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 cloth bag ash. PCDDs contributed to more than 66% and 56% of the total PCDD/Fs mass concentration in cloth bag ash under BC and LLC, respectively, while PCDFs accounted for 76.1% of the total PCDD/Fs mass concentration under HLC. Moreover, O8CDD and 1,2,3,4,6,7,8-H7CDD were the predominant PCDD/Fs in cloth bag ash under BC and LLC, with total proportions of 57.3% and 48.8%, respectively. In contrast, 2,3,7,8-T4CDF (43.6%) and 1,2,3,7,8-P5CDF (13.1%) became critical under HLC, and the proportions of O8CDD and 1,2,3,4,6,7,8-H7CDD dropped to only 16.9% and 2.8%, respectively. The PCDDs/PCDFs ratio in cloth bag 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/Fs formation mechanism changed from precursor synthesis to de novo synthesis in cloth bag ash.
Figure 8 shows the distribution characteristics of PCDD/Fs in boiler ash under BC, LLC, and HLC. In contrast to the distribution characteristics in electrostatic and cloth bag ash, distribution of PCDD/Fs in boiler ash revealed a trend of increasing chlorinated level, the mass concentration of both PCDD and PCDF congeners also increased under the three field test conditions. The PCDD/Fs mass concentration was mainly dominated by PCDDs in boiler ash under BC and LLC, accounting for 53.0% and 50.3%, respectively, of the total PCDD/Fs concentration, and PCDFs became increasingly dominant under HLC (53.1%). Highly chlorinated compounds were the predominant PCDD/F congeners in boiler ash under BC, LLC, and HLC, with O8CDD, 1,2,3,4,6,7,8-H7CDD, O8CDF, and 1,2,3,4,6,7,8-H7CDF comprehensively contributing 59.5%, 74.7%, and 90.2%, respectively, to the total PCDD/Fs concentration. The PCDDs/PCDFs ratio in boiler 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.