Formation Pathways and Emission Characteristics of PCDD/Fs during Co-combustion Tests of Sewage Sludge in Coal-fired Power Plants

This study was conducted in a full-scale coal-fired power plant (350 MW), mainly investigating the co-combustion influence of sewage sludge (SS) on the formation and emission characteristics of polychlorinated dibenzo-p -dioxins and dibenzofurans (PCDD/Fs). The introduction of SS enhanced the formation of major air pollutants such as NO x , SO 2 , and dust, but their emission concentrations still met the ultra-low emission requirements. By adding 3% and 6% SS, the formation concentrations of PCDD/Fs in flue gas at the economizer increased from 0.1317 to 0.2651 and 0.6023 ng m –3 , respectively, and the corresponding I-TEQ concentrations increased from 0.0104 to 0.023 and 0.058 ng I-TEQ m –3 , respectively. The emission concentrations of PCDD/F were steadily reduced to 0.0347–0.0645 ng Nm –3 (0.0021–0.0034 ng I-TEQ Nm –3 ) at the stack outlet. In comparison, the increased co-combustion ratio of SS decreased the 2,3,7,8-PCDD/F concentrations from 0.0222 to 0.0197–0.0098 ng g –1 and caused the change of dominant component from PCDDs (72.8%) to PCDFs (57.4%–59.5%) due to the inhibition effect of S-based substances. During the co-combustion process, de novo synthesis was confirmed as the major formation route of PCDD/Fs. Cu and Cl in mixed fuel of coal and SS were highly correlated with the parameters related to PCDD/Fs during co-combustion, which should be strictly limited to prevent large regeneration amounts of PCDD/Fs during the co-combustion process. The results provide an essential reference for controlling PCDD/F formation in the co-combustion process of SS and coal.


INTRODUCTION
In China, the urban dry sludge output has reached 14.23 Mt, of which 13.77 Mt was disposed of in 2021 (MOHURD, 2022).Sanitary landfill is the major disposal way of sewage sludge (SS) in China, accounting for 29.3% in 2019 (Wei et al., 2020).However, it will cause increasing pressure on land as well as environmental risks.Co-combustion of SS and coal can be a promising method due to its advantages of volume reduction, resource utilization, and harmless treatment (Kuo et al., 2021).
Previous studies mainly focused on the physicochemical properties of coal and SS mixture, combustion characteristics, and the formation of conventional pollutants.Park et al. (2017) reported that the thermogravimetric (TG) curves of the mixture of coal and dried SS were similar to that of the coal sample.The introduction of small amounts of SS could facilitate the combustion of fixed carbon in coal (Wei et al., 2012).Other studies reported that NOx emissions were less affected by the blending ratio of SS, but alkaline metal oxides such as CaO and Fe2O3 could inhibit the formation and release of NO and N2O (Jin et al., 2016;Zhang et al., 2019).The emission of SO2 was affected by factors such as the sulfur content, as well as alkaline metal oxides (Jin et al., 2016;Zhang et al., 2019).
Due to the high contents of chlorine and heavy metals in SS, the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) is another concerning problem in the co-combustion process.Some researchers had preliminarily investigated the influence of co-combustion ratios of SS on PCDD/F formation.Zhang et al. (2019) carried out the co-combustion experiments of coal and pickling sludge in a pilot-scale drop-tube furnace and reported the effect of the co-combustion ratios of SS on the toxic equivalent was not significant, i.e., 0.0174-0.0872ng I-TEQ Nm -3 under the ratios of 2%-20%.Zhang et al. (2013) reported the PCDD/F emission characteristics of coal-fired power plants under co-combustion of 5%-20% dried SS and found that the toxic equivalent of PCDD/Fs increased as the dried SS amounts increased, from 7.00 to 32.72 pg I-TEQ Nm -3 .However, few studies have investigated the formation mechanisms of PCDD/Fs during the co-combustion of coal and SS, especially in full-scale coal-fired power plant.Our previous study first reported the formation characteristics of PCDD/Fs and the influence of different heavy metals during the co-combustion of SS and coal through lab experiments (Rao et al., 2021;Lin et al., 2022).Then, this study was conducted to investigate the formation and emission characteristics of PCDD/Fs and other major air pollutants, as well as the influence factors and pathways, through full-scale co-combustion tests of coal and SS.
In this study, full-scale co-combustion tests with different SS addition amounts were conducted to investigate the formation, emission, and removal characteristics of PCDD/Fs and major air pollutants.This study also revealed the formation pathways of PCDD/Fs and the correlations between key influence factors and PCDD/Fs.The results provide essential and practical references on the co-combustion of SS and coal, as well as the formation control of PCDD/Fs.

Summary of the Incineration System
The tests were conducted in a typical coal-fired power plant with two 700 MW electric generating units in southern China, and the test load was 350 MW due to the peaking requirements.As shown in Fig. 1, the air pollution control devices system (APCDs) includes desulfurization, denitrification, and dust removal systems.The denitrification system includes low-nitrogen combustion in the furnace and selective catalytic reduction (SCR).The desulfurization system adopts wet flue gas desulfurization with limestone.And the dust removal system adopts an electrostatic fabric filter.The operating conditions of the coal-fired power plant and sampling program are shown in Table 1.The sampling pollutants contained NOx, SO2, dust, and PCDD/Fs.For flue gas, the sampling sites included economizer outlet, SCR outlet, and stack outlet.The ash samples were collected from the electrostatic fabric filter.Parallel samples were collected at each sampling site to ensure the reliability of data and results.
The proximate and ultimate analysis results of coal and SS are collected in Table S1.Significant differences between coal and SS could challenge a lot on the co-combustion conditions due to the high Mad (16.15%),Aad (51.27%), and Vad (31.08%), but low Cad (14.06%) and caloric value (2.38 MJ kg -1 ) of SS.Fortunately, the St,ad of SS (0.40%) is lower than coal (0.66%).

Sampling and Analysis
The PCDD/F samples of flue gas were collected by an intelligent PCDD/Fs sampler for flue gas (ZR-3720, Qingdao Junray Intelligent Instrument Co., Ltd., China) according to the Chinese standard of HJ 77.2-200877.2- (MEEPRC, 2008a)).The samples of PCDD/Fs in fly ash were collected according to the Chinese standard of HJ 77.3-20083- (MEEPRC, 2008b)).The pre-treatment and analysis of PCDD/Fs in samples were consistent with our previous studies (Chen et al., 2008;Ying et al., 2021) and the Chinese standards mentioned above.All experimental solvents were purchased from Mallinckrodt Baker Inc., USA.Each sample was spiked with 1 ng 13 C12-labelled PCDD/F standards and then was Soxhlet extracted by 250 mL toluene for 24 h.The Soxhlet extract was concentrated to 1-2 mL by a rotary evaporator.The next step was washing the extract with sulfuric acid until the color of the sample solution was visible.Then the extract was further purified through the multilayer silica gel column and alumina column.After cleanup procedures, the extract was concentrated to 20 µL by nitrogen-blowing and then spiked with 1 ng of 13 C12-labelled recovery standards before analysis.Finally, the samples were detected by high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) (JMS-800D, JEOL, Japan).
The recoveries of PCDD/F standards ranged from 28.2% to 119.2%, meeting the requirements of HJ 77.2-2008and HJ 77.3-2008(MEEPRC, 2008a, b).Moreover, the concentrations of the gas samples should be normalized to dry air containing 11% O2 under 101.3 kPa and 0°C.The theoretical number of PCDD and PCDF congeners (Number of chlorine substituents ≥ 4) was 136.However, due to the close peak times of some PCDD/F congeners, this study only detected and analyzed 38 PCDD congeners and 55 PCDF congeners.
The elemental compositions of fly ash were analyzed by an X-ray fluorescence spectrometer (XRF, Axios, Panalytical, Netherlands).The heavy metal content was determined by inductively coupled plasma emission spectrometry (ICP-OES, ICAP 6300, Thermo Fisher Scientific, USA).

Sampling and Analysis
The average chlorination degree of PCDD/Fs (dc) explained the average number of chlorine substituents, which was calculated as follows: ( ) 4,5,6,7,8 Cj represented the concentrations of different PCDD/F homologues; nj represented the number of the substituted chlorine in the different PCDD/F homologues; C represented the total concentration of PCDD/Fs.
The removal efficiencies (RE) of NOx, SO2, and PCDD/Fs by the APCDs were calculated as follows: 100 CECO represented the concentration of NOx, SO2, or PCDD/Fs at the economizer outlet; CSTO represented the concentration of NOx, SO2, or PCDD/Fs at the stack outlet or SCR outlet.

Emission Characteristics of Flue Gas and Fly Ash
As shown in Table 2, the original concentrations of SO2 at the economizer outlet (ECO) first decreased (3%SS) and then increased (6%SS) as the co-combustion ratio of SS increased.On the one hand, lower content of St,ad in SS contributed to the lower concentration of SO2; on the other hand, the worst co-combustion condition of 3%SS caused the highest combustible residue of ash (1.60%-2.20%)and lower concentrations of SO2.The co-combustion characteristics were optimized under the 6%SS condition, which caused the higher concentrations of NOx and SO2, but the lowest combustible residue of ash (0.43%).The emission concentration of NOx at the inlet of SCR decreased after adding SS, which could be attributed to the inhibition effect of the alkaline metal oxides such as CaO and Fe2O3 on the generation and release of nitrogen oxides such as NO and N2O (Zhao et al., 2019).Benefiting from the efficient removal capacity of APCDs, the emission concentrations of NOx, SO2, and dust at the stack outlet under any co-combustion ratio of SS all met the ultralow emission requirements (NOx ≤ 50 mg Nm -3 , SO2 ≤ 35 mg Nm -3 , dust ≤ 5 mg Nm -3 ).
Proximate and ultimate analysis of fly ash collected from the electrostatic precipitator were conducted, and the results are shown in Table 3. Hydrogen was not detected in the three fly ash samples.The low carbon content (< 1.21%) and combustible residue (< 2.22%) indicated that the co-combustion process could achieve near-complete combustion, particularly under 6%SS conditions.The Aad was as high as 99.42% under 6%SS conditions, and the FCad and Vad were significantly lower than those under the other two co-combustion conditions.Based on these, higher co-combustion ratios of SS were practical in the future.The X-ray fluorescence equipment (XRF) results of fly ash are listed in Table S2.As the co-combustion ratio of SS increased, the contents of Fe, S, etc. gradually increased.Ion chromatography (IC) was used to measure the chlorine content in SS and fly ash (Table S3).The higher content of chlorine in SS (1201 mg kg -1 ) caused higher chlorine content in mixed fuel of coal and SS, which further resulted in the increased chlorine contents in fly ash, i.e., from 135 to 143 and 201 mg kg -1 .Table S4 summarizes the contents of heavy metals in fly ash.Overall, the contents of Fe, Cu, Zn, Cr, S, Cl, and other elements in the fly ash gradually increased with the increasing co-combustion ratio.Such higher output of heavy metals should be noticed and would limit the elevation of the co-combustion ratio.
Fig. 3 illustrates the proportion distribution of the PCDD/F homologues profiles.As shown in Fig. 3(a), the distribution patterns under three co-combustion conditions were similar, that is, the elevated co-combustion ratio of SS influenced less on the formation characteristics of PCDD/F homologues.The dominant PCDD/F homologue under three conditions was HxCDDs, accounting for 23%.And the proportions of PeCDDs, TCDFs, PeCDFs, and HxCDFs all exceeded 9%.The chlorination degree (dc) of PCDD/Fs at ECO was slightly increased from 5.57 (0%SS) to 5.64 (3%SS) and 5.74 (6%SS), respectively, which could be attributed to the decreased sulfur content in the mixed fuel of coal and SS (Table S1).Due to the removal effect of APCDs, the distribution patterns of PCDD/F homologues changed significantly.The highly chlorinated homologues (chlorination degree (dc) ≥ 6) were more easily removed, as the most efficient removal unit could be the electrostatic fabric filter in this coal-fired power plant.It mainly worked through the removal of fly ash from the flue gas, which can benefit the adsorption of the highly chlorinated homologues of PCDD/Fs (Li et al., 2004;Chi and Chang, 2005;Ryan et al., 2005).Therein, the lowly chlorinated homologues such as TCDFs at the STO showed a significant proportion elevation (˃ 20%) compared with those at the ECO under three co-combustion conditions.
The distribution pattern of PCDD/F homologue based on the I-TEQ concentrations (Fig. 3(b)) was very different from Fig. 3(a) due to the I-TEF difference among PCDD/F congeners, but the effect of the elevated co-combustion ratio of SS and the APCDs were similar.The dominant homologues of PCDD/Fs at ECO were PeCDDs (8%-10%), PCDFs (11%-15%), HxCDDs (33%-45%), and HxCDFs (29%) under three co-combustion conditions.It was found that the proportion of PeCDFs at ECO increased from 33% (0%SS) to 37% (3%SS) and 45% (6%SS), as the co-combustion ratio of SS elevated.The highly chlorinated PCDD/Fs were more easily removed by APCDs, which also caused the increased proportions of lowly chlorinated PCDD/Fs, particularly for the TCDDs (15%-30%).
Fig. 4 illustrates the removal efficiencies of PCDD/F homologues by APCDs.As the co-combustion ratio of SS elevated, the removal efficiencies of PCDD/F homologues except TCDFs showed an increasing trend, but the increment decreased.Among the five PCDD homologues and five PCDF homologues, their removal efficiencies increased first and then decreased with elevated substitution number of chlorine, and the peak values belonged to HxCDDs (78.2%),HpCDDs (92.9%-95.2%),and HxCDFs (74.4%-96.6%).Such removal pattern was similar to the APCDs equipped in MSW incineration plants (Lin et al., 2020) and caused the abovementioned changes in PCDD/F homologue distribution.

The Formation Pathways of PCDD/Fs
The formation pathways of PCDD/Fs were revealed based on the full-scale co-combustion process, which will update the research status.As shown in Fig. 2, the PCDFs were the dominant component, and the ratios of PCDFs to PCDDs were higher than 1, namely 1.56 (0%SS), 1.38 (3%SS), and 1.63 (6%SS).On the one hand, these indicated that more PCDD/Fs were generated through de novo synthesis (Zhang et al., 2016;Ma et al., 2021).On the other hand, the precursors routes and/or chlorination routes also contribute a lot, as the ratios of PCDFs to PCDDs were not extremely as high as in the modeling experiments of de novo synthesis (Zhang et al., 2016;Zhang et al., 2017).Usually, the high-temperature homogeneous reaction mainly involved chlorinated precursors such as CPs and CBs, while the low-temperature heterogeneous reaction involved not only precursor synthesis but also de novo synthesis (Cunliffe and Williams, 2009;Nganai et al., 2014).And de novo synthesis was confirmed as the major PCDD/F formation pathway in the combustion process (Zhang et al., 2013;Lin et al., 2022).
DD/DF chlorination was also an indispensable formation pathway of PCDD/Fs (Weber and Hagenmaier, 1999;Tuppurainen et al., 2003).Some studies reported that DD/DF chlorination always followed the sequence of 2→8→3→7→1→4→6→9 positions, and the 2,3,7,8-substituted congeners could be pointed as the indicators of DD/DF chlorination (Luijk et al., 1992;Chen et al., 2018).Table 5 summarizes the 2,3,7,8-substituted congeners based on the Hagenmaier profile (Hagenmaier et al., 1994).The signal intensities of 2,3,7,8-substituted PCDDs and PCDFs in the flue gas at the economizer outlet increased with the elevation of the co-combustion ratio of SS.The average signal intensity of 2,3,7,8-substituted PCDD/Fs increased from 10.12% (0%SS) to 10.40% (3%SS) and 12.28% (6%SS), respectively.The slight elevation indicated the co-combustion of SS would enhance the chlorination reaction through the enhancement of the chlorine source or catalytic metal (Chen et al., 2021).But the general contribution of DD/DF chlorination was not high in comparison with the MSW incineration process (Ma et al., 2021).The major reason was that the chlorine content in the fed coal or SS was much lower than that in MSW (Rao et al., 2021;Lin et al., 2022).The low contents of metal catalysts, such as CuCl2, CuCl, FeCl3, etc., were another reason, which would be poisoned by SO2 (Shao et al., 2010;Chen et al., 2014).
Through the above analysis and discussion, the CP-route and DD/DF chlorination route contributed less to the formation of PCDD/Fs in the original combustion or the co-combustion conditions.Therefore, the de novo synthesis was confirmed as the major formation route of PCDD/Fs, which was consistent with the abovementioned ratio of PCDFs to PCDDs (˃ 1).In addition, the introduction of SS and the increased co-combustion ratio of SS enhanced dramatically the formation of PCDD/Fs (Fig. 2), which could most be resulted from the de novo synthesis and was consistent with the higher chlorine content in the mixture of coal and SS (Table S3).To confirm this, correlation analysis was introduced in this study.

Correlation Analysis
The abovementioned study showed that the co-combustion of SS in coal-fired boilers enhanced the formation of PCDD/Fs at the post-combustion zone.To better reveal the influence factors, this work analyzed the correlation relationship among the total concentrations of PCDD/Fs, the intensity of congener signals, and the chlorination degree in ECO flue gas and fly ash, as well as the contents of three trace heavy metals and chlorine in the mixed fuel under three combustion conditions.Fig. 6 shows the results of the principal component analysis (PCA) of the emission characteristics of PCDD/Fs and the key influence factors mentioned above.The variance explained by factor 1 was 97.16%, and the contents of Cu, Zn, and Cl were highly correlated with the parameters of PCDD/Fs.Cu and Cl were highly correlated with the concentrations and chlorination degree of PCDD/Fs during co-combustion (Lin et al., 2022).Zn was clustered with Cu and Cl only because the changing trend of its content was close to those of Cu and Cl content as the co-combustion ratio of SS increased.Previous studies illustrated that ZnO did not play a major role in promoting PCDD/Fs formation in the co-combustion process (Lin et al., 2022).In addition, Fujimori et al. (2011) reported that ZnCl2 can promote the formation of chlorinated aromatic compounds, while ZnO will inhibit it.Therefore, the contents of Cu and Cl in SS were necessary to be focused on to avoid large regeneration amounts of PCDD/Fs during the co-combustion process.

CONCLUSIONS
In the full-scale coal-fired power plant (350 MW), although the co-combustion of SS (≤ 6%) enhanced the formation of NOx, SO2, and dust, their emission always achieved the ultra-low emission requirements due to the effective removal capacity of APCDs.
Co-combustion of 3% and 6% SS increased PCDD/F formation in flue gas at ECO from 0.1317 to 0.2651 and 0.6023 ng m -3 , respectively, which were steadily reduced to 0.0347-0.0645ng Nm -3 (0.0021-0.0034 ng I-TEQ Nm -3 ) at stack outlet.In comparison, the elevated co-combustion ratio of SS decreased the 2,3,7,8-PCDD/F concentrations from 0.0222 to 0.0197-0.0098ng g -1 and caused the change of dominant component from PCDDs (72.8%) to PCDFs (57.4%-59.5%)due to the inhibition effect of S-based substances.In addition, the contents of Fe, Cu, Zn, and Cl in fly ash increased as the co-combustion ratio of SS elevated.
De novo synthesis was confirmed as the major formation pathway of PCDD/Fs in the co-combustion process.Cu and Cl in mixed fuel of coal and SS were highly correlated with the PCDD/Fs characteristics during the co-combustion process, which were necessary to pay attention to avoid large regeneration amount of PCDD/Fs during the co-combustion process.

Fig. 3 .
Fig. 3.The distribution of PCDD/F homologues based on (a) mass concentration and (b) I-TEQ concentration.

Fig. 5 .
Fig. 5.The (a) mass concentration and (b) proportion distribution of PCDD/F homologues in fly ash.

Fig. 6 .
Fig. 6.Principal component analysis results of PCDD/F characteristics and key elements.

Table 1 .
The operating conditions of the coal-fired power plant and sampling program.

Table 3 .
Proximate and ultimate analysis of fly ash (%).