Optimizing the Elimination of Dioxin-Like Compounds (PCDD/Fs and PCBs) Through Effective Combustion Practices in the Flue Gas in the Medical Waste Incineration Facility

This chapter presents the impacts of temperature, O₂ levels, residence time, and the distribution of gas and solid-phase compounds on the destruction of PCDD/Fs and PCBs. Additionally, it discusses the fingerprint profile for PCDD/Fs congeners in stack emissions, the fingerprint profile of PCBs congeners in stack emissions and the emission factors and potential release of PCDD/Fs to the environment.

3.1 Effects of temperature in PCDD/Fs and PCBs destruction

The effect of temperature on the formation and destruction of PCDD/Fs and PCBs during MSWI treatment is demonstrated in Figure 1. At a temperature between 600℃ and 800℃, PCDD/Fs concentrations were high to identify their isomer compositions. It was observed that the destruction of PCDD/Fs above 900℃ was greater than its formation. This is consistent to the findings reported by Palmer et al. [18], Liu et al. [19], and Sørmo et al. [20]. Both PCDD/Fs were generated in solid phase at the temperature range of 300℃ however, it was noted that PCDFs were more predominant in both solid and gas phase than PCDDs. The samples taken from 300℃ and 600℃ were similar in isomer composition except that the highly chlorinated congener such as octa-chlorinated dibenzo-p-dioxin and octa-chlorinated dibenzofuran were observed to be more dominated. These findings align with those reported by Xia et al. [21]. The typical profile for PCDFs from combustion systems were more highly chlorinated species because of the Cl content present inside the reactor. Other studies such as Chen et al. [22]; Gunes et al. [23]; Lin et al. [24]; Wei et al. [25] have confirmed that PCDDs were likely to be formed in less temperatures between 180 and 280℃ through de novo synthesis. De novo synthesis is known to occur in a relatively low temperature range of 200 - 400℃. It involves the combustion of relatively innocuous chemical molecules, accompanied by simultaneous oxidation and chlorination at these lower temperatures [26]. It was also noted that the mechanism of PCDFs formation was slightly different from that of PCDDs and each formation was influenced by precursor concentration and chlorine content.

The study revealed interesting trends regarding the concentrations of PCDFs and PCDDs at varying temperatures. As the temperature increased from 300 to 500℃, there was a steady rise in both PCDD/Fs concentrations. However, a sharp increase in concentrations was observed as the temperature rose above 600℃. Both PCDDs and PCDFs exhibited similar trends concerning temperature increase but overall PCDFs were more dominant. Across the investigated temperature ranges, it was consistently observed that the total emission of PCDFs surpassed that of PCDDs, with a minor deviation noted at temperatures exceeding 900℃, this slightly contradicts of Qiu et al. [27], who measured higher concentration of PCDDs instead. At 600℃, the formation of both PCDDs and PCDFs was predominant especially for lower chlorinated congeners such as the 2,3,7,8-substituted tetra-chlorinated contributing up to 93% of the total PCDD/Fs due to favorable conditions in the combustion zones. However, as the temperature increased to 850℃, a decline in both PCDDs and PCDFs was observed with PCDFs showing slower reduction rates. At 900℃, PCDDs formation surpassed that of PCDFs but soon after, both compounds demonstrated a sharp decline indicating increased destruction rates. Shortly after this temperature point, both compounds demonstrated a sharp decline, indicating increased destruction rates. This pattern suggested a critical temperature threshold where the destruction of PCDDs and PCDFs became more efficient, possibly due to enhanced combustion conditions or increased residence time at higher temperatures. These findings align with those reported by Lin et al. [28] and Xing et al. [29]. Additionally, measurements at 670℃ revealed that the temperature at the inhibitor feeding point significantly influenced PCDD/F reductions, highlighting the importance of careful temperature control for efficient destruction. Ultimately, these findings emphasize the complex relationship between temperature and PCDD/Fs behavior underlining the need for well-designed and controlled processes to mitigate the PCDD/Fs effectively while minimizing their formation. Simultaneously, PCBs formation exhibited analogous patterns in relation to PCDD formation just in lower concentrations.

1.png

Figure 1. Formation of PCDD/Fs and PCBs at different temperatures

3.2 Effect of oxygen concentration

The O₂ concentration in combustion processes exerts a substantial influence on the intricate mechanisms of PCDD/Fs and PCBs formation and destruction. Figure 2 shows the dependence of the PCDD/Fs concentration on O₂ content in the carrier gas stream where at the temperature of 300℃, the PCDD/Fs and PCBs concentration reached its maximum formation. When O₂ concentrations in the surrounding gaseous medium was less than 1 vol.% in pyrolytic conditions, the PCDD/Fs and PCBs practically could not be formed. At the O₂ level between 2% and 4%, the formation of PCDD/Fs and PCBs started to show a slight increase however, the formation was still minimal. When the O₂ levels reached 6%, a significant reaction in the formation of PCDD/F and PCBs compounds was observed indicating a strong correlation between O₂ levels and the generation of PCDD/F. The maximum yield of PCDD/Fs and PCBs was observed when the O₂ content reached 8% which aligns with the de novo mechanism of formation. However, above 8% O₂ content PCDD/Fs and PCBs formation began showing a significant decline with almost no formation above 10% O₂ content. These findings align with those reported by Ren et al. [30], Yu et al. [31], Zhao et al. [32], and Wang et al. [33]. Nevertheless, when the O₂ levels were reduced below 2%, incomplete combustion and combustion starvation was observed leading to increased concentrations of CO. This suggests that the presence of O₂ plays a crucial role not only in the formation of PCDD/Fs and PCBs but also in the overall combustion efficiency and the formation of other combustion byproducts like CO. Adequate O₂ supply is crucial in promoting complete carbon combustion and reducing the formation of chlorinated intermediates, thus decreasing the likelihood of PCDD/F formation [34]. These observations highlight the significance of controlling O₂ levels during combustion processes to mitigate the formation of PCDD/Fs, PCBs and CO.

2.png

Figure 2. O₂ dependence in PCDD/Fs and PCBs formation process

Nganai and Lomnicki [35] suggested that O₂ concentration had no effect on the PCDD/Fs formation during de novo synthesis, but instead it’s the presence of metal oxide/ chlorides that influence the PCDD/Fs formation within the de novo synthesis model. In contrary, this study concluded that O₂ levels played a critical role in the formation of PCDD/Fs during de novo synthesis. The results of the present study are consistent with those of Zhang et al. [36], Potter et al. [37], and Xu et al. [38] indicating that both precursor formation and de novo synthesis play significant roles in the generation of PCDD/Fs within the post-combustion zone of a municipal solid waste incinerator. On the other hand, in oxygen-deficient conditions, chlorinated compounds may persist longer increasing the risk of PCDD/F generation. In terms of destruction, oxygen-rich environments as encountered in specialized incineration facilities enhance the efficient breakdown of PCDD/Fs, breaking them down into less harmful products. Nevertheless, maintaining a delicate balance is essential as excessively high O₂ concentrations may also lead to the formation of nitrogen oxides (NO_x), potentially facilitating the generation of more toxic PCDD/F congeners under specific circumstances [39]. Additionally, catalytic processes can offer an alternative means of destruction utilizing catalysts to facilitate the breakdown of PCDD/Fs at lower temperatures and with lower oxygen concentrations thereby promoting selective destruction pathways and minimizing the production of hazardous byproducts [35]. Overall, understanding the role of O₂ concentration was vital for managing and mitigating the environmental impact of PCDD/Fs in combustion processes. It is recommended that, O₂ level greater than nine percent (9%) be maintained for more efficient destruction of PCDD/Fs and PCBs, as indicated by the discussion above. At this concentration, the formation of PCDD/Fs begins to decline significantly, while still providing sufficient oxygen for efficient combustion processes and minimizing the risk of other harmful byproducts.

3.3 Effect of residence time

Residence time appeared to have a major influence on the formation of PCDD/Fs and PCBs and destruction. Based on the observations, the window of time available for the formation of PCDD/Fs and PCBs to take place was from 0.5 s during the combustion process. During this time period both PCDD/Fs and PCBs concentrations started to show a steady increase indicating the initiation of their formation. This steady increase continued until around 1.0 s suggesting that the conditions during this duration were conducive to PCDD/Fs and PCBs formation. Following this initial period, PCDFs, PCDDs, and dl-PCBs displayed different concentrations at various residence times as demonstrated in Figure 3. As the residence time increases from 0.1 s to 0.9 s, the concentrations of PCDFs, PCDDs, and dl-PCBs exhibit some fluctuations, this is consistent with the findings reported by Ma et al. [40]. For PCDFs, there is a gradual increase from 3.2 ng I-TEQm³ at a residence time of 0.1 s to 3.8 ng I-TEQm³ at a residence time of 0.6 s, after which it remains constant. This suggests that an extended residence time has a limited impact on PCDF concentrations beyond a certain point. In the case of PCDDs, a similar pattern was observed, with an increase from 1.5 ng I-TEQm³ at a residence time of 0.1 s to 1.9 ng I-TEQm³ at a residence time of 0.6 s, followed by stability. This implies that the residence time has a moderate influence on PCDD concentrations. The concentrations of dl-PCBs also follow a comparable trend, rising from 3.1 ng I-TEQm³ at a residence time of 0.1 s to 3.7 ng I-TEQm³ at a residence time of 0.6 s and remaining steady thereafter. Similar to PCDFs and PCDDs, dl-PCBs concentrations are notably affected by changes in residence time up to a certain threshold. The results suggest that an optimal residence time may exist for minimizing the concentrations of PCDFs, PCDDs and dl-PCBs. Beyond this point, further increases in residence time may have limited additional benefits in reducing these pollutants.

The current study also demonstrated that the samples taken with the longest residence times shifted the congener pattern towards higher PCDD/F congeners. Flue gas chlorination reaction became more efficient as the reaction of PCDD/Fs and PCBs increases. Residence time in the post-combustion region was highly dependent on the temperature intervals. It was discovered that at the shortest residence time of 0.2 - 0.3 s the most dominant congeners were TCDD, PeCDD, PeCDF, HxCDF, this aligns with the findings reported by Yang et al. [41] but in contrary to what Peng et al. [42] reported. At the medium residence time of 0.3 - 0.4 s the most dominant congeners were found to be HxCDD, HxCDF, and HxCDF in consistent with Cheruiyot et al. [43]. While at the longest residence time of 0.5 - 0.8 s the more dominant congeners were OCDF, OCDD, similar to the findings reported by Peng et al. [42], Xiao et al. [44], and Zhao et al. [32]. The samples taken during combustion in the primary chamber also shown that the concentration of PCDD/Fs and PCBs in the gas phase depended mostly on the residence time and temperature profile. Subsequently, when the residence time in the temperature ranged from 250℃ - 450℃ was shortened, the amount of both PCDD/Fs and PCBs decreased. Due to the pressure gradient drop caused by the exhausters installed in the incinerator, gas flow was mostly vertical from top to bottom, and the gas velocity was about 0.3 - 0.5 m/s.

In addition to the observed trends in PCDD/Fs and PCBs concentrations, further analysis revealed interesting insights into the relationship between residence time and the formation of these pollutants. Specifically, longer residence times in the post-combustion region were found to enhance the efficiency of the flue gas chlorination reaction, resulting in a shift towards higher PCDD/F congeners. This phenomenon suggests that prolonged exposure to specific temperature intervals facilitated more extensive chlorination reactions, leading to the formation of higher chlorinated congeners. Moreover, the study also highlighted the impact of residence time variations on the composition of the gas phase during combustion in the primary chamber. It was observed that alterations in residence time within the temperature range of 250℃ - 450℃ had discernible effects on the concentrations of PCDD/Fs and PCBs. Notably, shorter residence times within this temperature range correlated with decreased pollutant concentrations. This trend may be attributed to changes in the combustion dynamics and the availability of precursor compounds under different residence time conditions [42]. The findings also indicated that variations in residence time led to shifts in the dominant congeners present, reflecting changes in the combustion kinetics and precursor availability. These findings highlighted the intricate interplay between residence time, temperature profiles, and pollutant formation mechanisms within incineration processes.

3.png

Figure 3. PCDD/F and PCBs levels at different residence time

3.4 Gas and solid phase compound distribution

During the study, we observed significant variations in the emission behavior of PCDD/Fs and PCBs as the temperature increased. Initially, at 300℃, most PCDD/Fs were emitted in the solid phase suggesting that they are not fully vaporized and remain bound to particulate matter, but at 600℃, they tended to be trapped in filters and XAD resin, this shift suggests an increased volatilization and subsequent capture by these collection media (Figure 4). Conversely, higher temperatures led to a slight increase in PCDFs in the XAD-II resin, with more PCDFs being destructed at 900℃. PCBs, on the other hand, were found at higher concentrations in the gas phase between 600 and 900℃ compared to 300℃, consistent with previous findings by Guo et al. [45] and Hsu et al. [46]. Remarkably, PCBs in the gas phase were slightly more abundant than PCDD/Fs. As gas phase temperatures rose above 600℃, both PCDFs and PCBs exhibited a sharp increase, followed by a slower rise in PCDDs, as reported by Xiong et al. [47]. The transition temperature for PCDD/Fs to enter the gas phase occurred below 400℃, while for PCBs, it occurred above 800℃. Despite variations in waste types, combustion conditions, and furnace types, multiple studies have demonstrated strong correlations between PCBs and PCDD/Fs, consistent with the results presented by Ying et al. [48].

Some researchers have suggested that PCBs may act as precursor molecules for the formation of PCDD/Fs during the combustion process [9, 46, 49, 50]. Our study found that under normal operation conditions, the total PCDDs emission ranged from 0.1 to 3.45 ng I-TEQ/m³. However, during manual poking, which is regarded as unfavorable combustion conditions, a significant amount of dust deposition in the primary chamber resulted in a drastic increase in PCDFs concentration up to 5.24 ng I-TEQ/m³. As illustrated in Figure 4, the concentration of PCDD/Fs in the flue gas increased from the low-temperature zone of the incinerator to the outlet, and it further decreased significantly in the filter baghouse. However, concentrations of both PCDD/Fs and PCBs decreased at temperatures above 900℃ in the secondary chamber inlet. This contrasts slightly with the findings presented by Zhong et al. [51], who reported no changes at temperatures above 900℃. For comprehensive emission control, the destruction of PCDD/Fs occurred in the baghouse outlet (quenching tower) and in the cooling system as temperatures dropped to between 280℃ and 230℃. Overall, our results indicate that higher combustion temperatures generally lead to reduced concentrations of PCDDs, PCDFs, and PCBs, regardless of the collection method. The XAD method consistently recorded lower concentrations than the filter method for all pollutants and temperatures.

3.png

Figure 4. The distribution of PCDD/Fs, and PCBs at 300, 600, and 900℃ combustion

When comparing the concentrations of PCDFs across different combustion temperatures, it was observed that, the value was higher when burned at 600℃. This could be attributed to several factors: At 600℃, PCDFs exhibit increased volatility, leading to greater liberation from solid surfaces and enhanced transfer to the gas phase. However, simultaneous adsorption onto filter media and XAD resin occurred, resulting in higher concentrations being captured in these collection media. The increase in temperature from 300℃ to 600℃ accelerated the thermal decomposition of precursor compounds, leading to enhanced PCDF formation. As a result, more PCDFs were available for capture by the filters and XAD resin. This is consistent with the findings reported by Hsu et al. [46]. It was also observed that, the combustion process at 600℃ was more efficient in breaking down precursor molecules into PCDFs, leading to higher concentrations of PCDF compounds being released into the gas phase and subsequently captured by filters and XAD resin. Xiong et al. [34] suggested that, the kinetics of PCDF formation favors higher concentrations at 600℃ due to optimal reaction rates under these conditions, resulting in increased PCDF capture by filters and XAD resin. Overall, the combination of increased volatility, enhanced thermal decomposition, improved combustion efficiency, and favorable formation kinetics at 600℃ contributed to the largest PCDF filter value observed at this temperature during combustion.

3.5 Optimal fingerprint profile for PCDD/Fs congeners in stack emissions

Seventeen (17) 2,3,7,8 -PCDD/Fs homolog patterns were used to generate distinct fingerprints representative of incineration plant emissions, as detailed in Table 1. Throughout the study, four sampling campaigns were conducted, denoted as SC1, SC2, SC3, and SC4. To ensure accuracy and reliability of the results, consistent sampling conditions were maintained across all campaigns. This included keeping the temperature, oxygen concentration, residence time, and waste type consistent throughout each sampling campaign. This rigorous approach minimized potential variables that could influence the emission behavior of PCDD/Fs and PCBs, ensuring reliable data for analysis and comparison. The analysis of the PCDD/Fs congener profiles in the stack demonstrated the understanding of PCDD/Fs destruction mechanisms using good combustion practices. The most dominant congener in SC1 was 2,3,4,7,8-PeCDF with a measured concentration of 6.035 ng I-TEQ/m³ measured at 900^oC. Following closely was the congener 1,2,3,7,8-PeCDD which exhibited a concentration of 3.72 ng I-TEQ/m³ at the same temperature interval. These results indicate that during SC1, when samples were exposed to the high temperature of 900^oC, 2,3,4,7,8-PeCDF was the most abundant congener of PCDF present, significantly surpassing the concentration of the other congeners, with 1,2,3,7,8-PeCDD being the second most prevalent. The findings are consistent with the results presented by Cai et al. [52] and Xia et al. [21]. In contrast to the dominant PCDF and PCDD congeners measured during SC1, octachlorodibenzo-p-dioxin (OCDD) and octachlorodibenzofuran (OCDF) were found to be much less prevalent in all samples at different temperature intervals. Both OCDD and OCDF reported concentrations of less than 0.003 ng I-TEQ/m³ indicating their less dominance under the conditions of the test. The low concentrations of OCDD and OCDF are significant because they are also members of the PCDD and PCDF family and can exhibit toxic properties similar to other congeners with higher concentrations. However, in this particular test, their concentrations were notably negligible compared to the more dominant congeners like 2,3,4,7,8-PeCDF.

The low concentrations of OCDD and OCDF during SC1 were attributed to several factors, such as temperature, residence time (0.8 s), O₂ concentration (10%) and the presence of certain metal catalysts. This aligns with the observation reported by Lin et al. [53]. The conditions during the incineration process did not favor the formation of OCDD and OCDF congeners leading to their low presence in the samples. This incineration facility is equipped with advanced emission control technologies such as scrubbers and filters to capture and remove hazardous pollutants from the exhaust gases. These technologies effectively captured and reduced the release of OCDD and OCDF into the stack. This contrasts with the findings of Qiu et al. [54], who indicated that bag filters alone are ineffective in reducing PCDD/F, emphasizing the need for a combination of technologies. OCDD and OCDF are known to be less volatile compared to some other PCDD/Fs [45]. The findings obtained during this campaign highlights the potential risks posed by PCDD/Fs when exposed to high temperatures emphasizing the importance of monitoring and mitigating their presence in various environmental settings to safeguard public and ecological health. The congener distributions observed in the current study are notably similar to those reported by Hsu et al. [46], Xia et al. [21], and Jiang et al. [55], who also measured concentrations in the ranges of 0.049 ng I-TEQ/m³ - 11.28 ng I-TEQ/m³ from incineration stacks.

During SC2, the most dominant congeners in the stack were 2,3,4,7,8-PeCDF measuring 4.26 ng I-TEQ/m³, followed closely by 1,2,3,7,8-PeCDD measuring 1.98 ng I-TEQ/m³ both at 900^oC respectively. This differs slightly from the findings reported by Ying et al. [56] who reported a dominance of OCDD. As with other PCDD and PCDFs, these congeners are toxic environmental pollutants and can pose health and environmental risks. Understanding the distribution and levels of specific congeners like 2,3,4,7,8-PeCDF and 1,2,3,7,8-PeCDD is crucial for assessing the potential impact of emissions from industrial processes and guiding appropriate measures to control and reduce their release. In SC2, OCDD and OCDF were the least dominant congeners measuring less than 0.001 ng I-TEQ/m³ indicating their minimal presence in the stack emissions which was parallel with the findings reported by Fujimori et al. [57] and Ying et al. [48]. The composition of the materials being incinerated significantly impacted the formation of OCDD and OCDF during SC2. According to studies by Wang et al. [17], Zhu et al. [58], Zhang et al. [59], and Qiu et al. [54] more-chlorinated PCDD homologs contribute more to total PCDD concentrations in stack gas samples. On the contrary, the current study did not detect any variations in PCDF concentrations between the more chlorinated and less chlorinated homologs.

Out of all the campaigns conducted, it was precisely SC3 that exhibited superior performance in efficiently destructing a greater number of PCDD/Fs congeners. This remarkable outcome was achieved through the meticulous implementation of optimal combustion practices. It is important to note that the effectiveness of SC3 in reducing the PCDD/Fs congeners was unparalleled when compared to the other campaigns in the study. Among the measured congeners in SC3, 2,3,4,7,8-PeCDF was the most dominant with a concentration of 1.16 ng I-TEQ/m³, followed far behind by 1,2,3,7,8-PeCDD measuring 0.60 ng I-TEQ/m³. On the other hand, the least dominant congeners were OCDD, OCDF, and 1,2,3,4,7,8,9-HpCDF, all measuring less than 0.0001 ng I-TEQ/m³ respectively. Stringent regulations and emission standards for dioxins and furans in many countries including South Africa have prompted incineration facilities to adopt measures that minimize the formation and release of these compounds. It is important to note that the reasons for the lower concentrations of OCDD and OCDF during the incineration process depended on the operating conditions, technologies employed and the composition of the waste being incinerated. According to the findings, although most PCDF WHO₂₀₀₅-TEQ/PCDD WHO₂₀₀₅-TEQ ratios were less than 0.1 ng I-TEQ/m³, PCDFs slightly dominated PCDD/s congeners. Several congeners, including OCDD, OCDF and 1,2,3,4,7,8,9-HpCDF measured much lower concentrations during post-combustion zone indicating that de novo synthesis played a significant role in the low-temperature zone.

The results from SC4 indicate that among the congeners measured, 2,3,4,7,8-PeCDF was the most dominant exhibiting a concentration of 5.64 ng I-TEQ/m³ at 900^oC. Following closely was 1,2,3,7,8-PeCDD with a concentration of 3.06 ng I-TEQ/m³ also at 900^oC. In contrast, OCDD was the least dominant congener in this campaign as in other tests. Similar results have been reported in North China, Zhu et al. [58] who measured the concentration between 0.11 - 2.53 ng I-TEQ/m³ and 0.007 - 0.059 ng I-TEQ/m³, respectively. While the most dominant PCDD/F congeners in SC1 - 4 were 2,3,4,7,8-PeCDF and 1,2,3,7,8-PeCDD. However, TCDD, PeCDD, PeCDF, and HxCDF were dominant during the combustion phase which is in agreement with Liu et al. [8] and Ying et al. [48]. We compared the congeners profile in the incineration process with other industrial sources and found that they were all dominated by PCDFs as in the findings by Zou et al. [60] and Li et al. [9]. Xia et al. [21] reported a different pattern that the PCDD/Fs in stack gas were usually characterized by higher fractions of OCDF, 1,2,3,4,6,7,8-HpCDF, OCDD and 1,2,3,4,6,7,8-HpCDD. However, the dominant congener profiles in this study remained constant. Previous studies such as Zhu et al. [58], Qiu et al. [54], and He et al. [61] have shown that most PCDD/Fs were formed through de novo synthesis when the PCDF WHO2005-TEQ/PCDD WHO2005-TEQ ratio was >1. That coincided with our conclusion that PCDD/Fs were mostly formed through de novo synthesis during the incineration processes.

In SC1, the total TEQ measurement was 19.40 ng I-TEQ/m³, while SC2 recorded a total of 13.37 ng I-TEQ/m³. The lowest TEQ reading of 3.85 ng I-TEQ/m³ was obtained during SC3, whereas SC4 yielded the highest measurement at 20.04 ng I-TEQ/m³. Higher proportions of PCDFs over PCDDs was observed in this study which was contradictory to Pan et al. [62] who reported higher proportions of PCDDs over PCDFs in the flue gases from CFB and MWI. Formation of low-chlorinated PCDDs (1,2,3,7,8-PeCDD, 1,2,3,4,7,8-HxCDD, 1,2,3,6,7,8-HxCDD and 1,2,3,7,8,9-HxCDD) possessed strong correlation and the chlorophenols were the important precursors of low-chlorinated PCDDs which were generated within the low chlorinated content [62]. The stack emissions presented certain similarities with an increasing chlorinated level, the content of the 2,3,7,8-PCDD congener increased but the content of 2,3,7,8-PCDF congener showed irregularities. This is consistent with the results presented by Dong [63] and Gul et al. [64]. Based on the results obtained from these campaigns, it appears that relying solely on good combustion practices is not sufficient to completely destruct PCDD/Fs emissions. Even with the implementation of optimal combustion techniques, certain congeners were still measured in abundance. To achieve a more comprehensive destruction of PCDD/Fs, a combination of combustion practices and additional measures such as dry sorbent injections is necessary. By implementing this complementary technique along with improved combustion practices, it becomes more feasible to achieve complete destruction and minimize the environmental and health risks associated with PCDD/F compounds.

Table 1. PCDDs, PCDFs stack emission measurement survey (concentrations reported in ng I-TEQ/m^-³)

In the current study, fingerprints of incineration plant emissions were established using twelve (12) homolog patterns of PCBs, as illustrated in Table 2. During SC1, the most dominant congener was #52 measuring 0.445 ng I-TEQ/m³ at 900℃ followed shortly by congener #44 reporting a measurement of 0.352 ng I-TEQ/m³ at 300℃. The least congeners were observed to be #110 measuring 0.011 ng I-TEQ/m³ at 300℃ and congener #153 reporting a concentration of 0.015 at 600℃. This differs slightly from the findings reported by Hsu et al. [46] who reported a dominance of #118 and #156. For most PCB congeners their concentrations decreased as the temperature increased. This is a common trend in PCB degradation studies as the temperature rises, the compounds undergone thermal degradation leading to a decrease in their concentration [41, 65]. Congeners such as #44 and #118, showed a relatively high concentration at 300℃ which decreased significantly at 900℃. While #153 and #110 had relatively low concentrations across all three temperatures indicating that these congeners were less prevalent in the sample and less stable under the conditions tested. Congeners #18 and #28 exhibited fluctuations in concentration as the temperature increased i.e. #18: 0.171 ng I-TEQ/m³ at 300℃, 0.205 ng I-TEQ/m³ at 600℃ and 0.117 ng I-TEQ/m³ at 900℃; #28: 0.236 ng I-TEQ/m³ at 300℃, 0.173 ng I-TEQ/m³ at 600℃ and 0.103 ng I-TEQ/m³ at 900℃. The decrease in ∑PCBs with increasing temperature suggested that higher temperatures were more effective at degrading these compounds and reducing their overall toxicity. These fluctuations can be attributed to the complex chemical reactions occurring at different temperature ranges, which may lead to variations in the formation and destruction rates of these congeners. Furthermore, these fluctuations emphasize the importance of understanding the temperature-dependent behavior of individual congeners to accurately assess their emissions and potential environmental impact. Such detailed analysis provides valuable insights into the mechanisms driving the formation and destruction of these POPs in combustion processes, aiding in the development of more effective mitigation strategies [30]. Additionally, the fluctuations observed in the concentrations of congeners #18 and #28 highlight the dynamic nature of the combustion process and the intricate interplay between various factors influencing the formation and destruction of these compounds. These fluctuations may be attributed to factors such as changes in reaction kinetics, thermodynamic equilibria, and the availability of precursor molecules at different temperature intervals [50]. Moreover, the presence of catalytic species or inhibitors in the combustion environment further modulated the behavior of individual congeners, contributing to the observed variations in their concentrations.

In SC2 the most dominant congener was #170 measuring 0.493 ng I-TEQ/m³ at 300℃ followed closely by congener #101 with a measurement of 0.475 ng I-TEQ/m³ at 600℃. The least reported congeners were #170 measuring 0.003 ng I-TEQ/m³ and #138 measuring 0.022 ng I-TEQ/m³ both at 600℃ respectively. These findings align with the outcomes reported by Shen et al. [66] in the steel making electric arc furnace. Overall, most congeners exhibited a decrease in concentration as the temperature increased. This suggested that, in this SC higher temperatures also lead to a reduction in the concentration of these compounds due to thermal degradation. In contrary to Mila et al. [50], several congeners such as #101, #110, and #170 show relatively high concentrations compared to the other temperatures. This indicated that these congeners were less stable at this intermediate temperature and were more susceptible to degradation or transformation. Congeners such as #180, #44 and #138 had very low concentrations at 900℃. The ∑PCBs for SC2 was 7.8 ng I-TEQ/m³ which was higher than the corresponding value of 6.44 ng I-TEQ/m³ in SC1. This indicated that the PCB congeners in SC2 had a higher overall toxic equivalency compared to SC1 due to differences in the composition and concentration of congeners in the two samples.

In SC3 the most dominant congener observed was #110 demonstrating a concentration of 0.484 ng I-TEQ/m³ at 900℃ followed by congener #180 with a concentration of 0.462 ng I-TEQ/m³ at 600℃. Conversely, the less dominant congeners were #153 measuring 0.023 ng I-TEQ/m³ at 300℃ and congener #170 reporting 0.021 ng I-TEQ/m³ at 900℃. This is in line with the findings reported by Dat et al. [67] from the secondary copper plant, but differs slightly from the findings reported by Hombrecher et al. [68] during the silicone rubber production. Most congeners show a decrease in concentration as the temperature increases. This is consistent with the general trend that higher temperatures lead to the degradation of PCBs [30, 69, 70]. Congeners such as #18, #28, #52, #118, #180 and #110, exhibited their highest concentrations at 900℃. The ∑PCBs for SC3 was 9.27 ng I-TEQ/m³ higher than the corresponding values in SC1 and SC2. The data in SC3 provided further evidence of the complex behavior of PCB congeners under different temperature conditions. While the general trend of decreasing concentrations with increasing temperature was observed, the specific responses of individual congeners can vary significantly.

In SC4 the most dominant congener observed was #101 demonstrating a concentration of 0.49 ng I-TEQ/m³ at 300℃ followed by #180 with a measurement of 0.486 ng I-TEQ/m³ at 900℃. Conversely, the less dominant congeners were #52 measuring 0.022 ng I-TEQ/m³ at 300℃ and #118 reporting a concentration of 0.093 ng I-TEQ/m³ at 600℃. Most PCB congeners in SC4 exhibited a decrease in concentration as the temperature increases. This consistent trend suggested that higher temperatures promote the thermal degradation of PCBs [41, 43, 71]. Congeners #18, #28, #52, and #194, showed their highest concentrations at 900℃ as observed in SC3. Unlike SC3, congeners, such as #52 and #118 had lower concentrations at 600℃. This suggested that the conditions at 600℃ are not ideal for the stability of these congeners. The ∑PCBs for SC4 was 8.59 ng I-TEQ/m³ which was higher than in SC1 and SC2 but lower than SC3. The four SC showed differences in the specific behavior of individual congeners. Some congeners displayed higher concentrations at certain temperatures in one SC but not in others indicating that the composition and reactivity of congeners can vary between samples. The ∑PCBs varies among the SC, with SC3 having the highest value (9.27 ng I-TEQ/m³), followed by SC4 (8.59 ng I-TEQ/m³), SC2 (7.8 ng I-TEQ/m³), and SC1 (6.44 ng I-TEQ/m³).

Table 2. PCBs stack emission measurement survey (concentrations reported in ng I-TEQ/m³)