3.1. Emission Characteristics of Air Pollutants
To verify the air pollutant characteristics of plants, we first examined the O
2, CO, CO
2, NO
x, and SO
x concentrations from WHB and stack in accordance with the dioxin sampling (
Table 4).
The O
2 concentrations of the plants were 7.2% on average in the WHB and 8.0% in the stacks. The most effective and stable range of O
2 concentrations to control air pollutants in emissions such as CO is approximately in the range of 7–9% [
14]. Bio-2,3 and SRF-3 plants exhibited O
2 concentrations below the stable range during operation. In addition, in the plants using bio-SRF, the O
2 and CO concentrations were proportionally inversed. In the plants using SRF, the CO concentrations were the lowest in SRF-3; the O
2 concentrations were approximately in the range of 6–7%. SRF-1 and SRF-2 exhibited higher O
2 and CO concentrations than those of the other plants. However, both plants demonstrated lower CO
2 concentrations than those of the other plants. In this study, CO concentrations were high when O
2 concentrations were greater than 7–9%, while the CO
2 concentrations were low when the O
2 concentrations were greater than 7–9%. The primary cause of CO is due to incomplete combustion. When the O
2 concentration is above 7–9% in flue gas during combustion, incomplete combustion behavior occurs more frequently in the plants. The NO
x generation depends on fuel and thermal NO
x. Fuel NO
x is generated when a large quantity of nitrogen is included in feedstock, and nitrogen compounds produced from the fuel during combustion react with the O
2, resulting in Fuel NO
x. Thermal NO
x is generated when O
2 in the air oxidizes nitrogen during the high-temperature combustion reaction. SO
x is predominately generated by the sulfur component, including in fuel [
15]. The sulfur content of the SRF used in the plants is approximately 0.06 wt.%, which results in a small amount of SO
x emission [
16].
3.2. Concentration and Congener Pattern of Dioxin Emission in Incineration Plants
Table 5 indicates the toxic equivalents of PCDD/DFs in the plant WHBs and stacks. According to the Persistent Organic Pollutants Control Act [
17], if the capacity is larger than 2 ton/h, the limit of dioxin emission is 0.1 ng international toxic equivalency quantity (I-TEQ)/Sm
3. The results indicate that the concentration of PCDF is higher than PCDD in both the WHB and stack. In particular, the SRF-2 plant approximately presented 9.47 ng I-TEQ/Sm
3 of PCDD/DFs in WHB, which was higher than that of the other plants. On the other hand, the air pollutant control device of the SRF-2 plant showed a good reduction performance of PCDD/DFs emissions, which was approximately 2.6% of the emission limit. The average PCDD/DFs concentrations from WHB and stack of seven incineration plants were approximately 1.74 and 0.02 ng I-TEQ/Sm
3.
We compared the results of the PCDD/DFs congener pattern in WHB and a stack of three bio-SRF incineration plants (Bio-1, Bio-2, and Bio-3) in
Figure 3,
Figure 4 and
Figure 5. One nanogram of
37Cl
4-2,3,7,8-TCDD was used as the internal standard for sampling, and the average recovery rate of the reference material was 84.40%.
In the identification of the congener pattern in the WHB and stack of bio-SRF incineration plants, we found that 1,2,3,4,6,7,8-HpCDF exhibited the highest ratio (approximately 21%) in the WHB and the high ratio in the stack at Bio-1. In the case of 2,3,7,8-TCDF, it was shown in the third-highest ratio in WHB but a low ratio in the stack. In addition, Bio-2 showed the highest ratio of 1,2,3,4,6,7,8-HpCDF (approximately 21%) in the WHB. 1,2,3,7,8-PeCDF, 2,3,7,8-TCDF, and OCDD demonstrated high ratios in all bio-SRF incineration plants. In particular, the highest ratio of OCDD was displayed in the stack of Bio-1 and Bio-2. In Bio-3, the ratio of 2,3,7,8-TCDF was the highest in the WHB. The analysis concentration of 2,3,7,8-TCDF from the WHBs of Bio-1 and Bio-3 was similar. However, the ratios were different because the total congener emissions of the PCDD/DFs were higher in Bio-1.
Overall, the ratio of 1,2,3,4,6,7,8-HpCDF was high in all Bio-1, Bio-2, and Bio-3 stacks, and the ratio of OCDF was approximately 15% higher in Bio-3 than that in the other incineration plants. Both Bio-1 and Bio-2 have the same air pollutant control device, while Bio-3 does not install the SCR. An air pollutant control device might influence the ratio of OCDF. After the SCR treatment, the ratio of OCDF decreased [
18]. 1,2,3,4,6,7,8-HpCDF is generated when the chlorine in the ninth position of the OCDF is removed [
19], and it is more stable than other congeners [
20]. Therefore, the ratio of 1,2,3,4,6,7,8-HpCDF is high because the chlorination reaction is active, and the OCDF likely transforms into 1,2,3,4,6,7,8-HpCDF with large quantities of highly chlorinated dioxins.
In addition, PCDD/DFs congener pattern in WHB and stack of four SRF incineration plants (SRF-1, SRF-2, SRF-3, and SRF-4) was compared in
Figure 6,
Figure 7,
Figure 8 and
Figure 9. One nanogram of
37Cl
4-2,3,7,8-TCDD was used as the internal standard for sampling, and the average recovery rate of the reference material was 89.25%. SRF incineration plants, which installed the SCR, indicated high ratios of 1,2,3,4,6,7,8-HpCDF, and OCDF in the stack.
3.3. Reduction Rate of PCDD/DFs Congener Concentration in WHB and Stack
The reduction rate of each congener between the WHB and stack of Bio-2 and Bio-3 incineration plants presented more than 85% in all congener concentrations of PCDD/DFs in
Table 6. In contrast, in the case of Bio-1, the reduction rates of 1,2,3,6,7,8-HxCDD (approximately 71%), 1,2,3,4,6,7,8-HpCDD (approximately 61%), and OCDD (approximately 67%) were lower than those of the other plants. The difference in the arrangement of the air pollutant control device was that the Bio-1 incineration plant contains an SDR, and Bio-2 and 3 incineration plants contain a DR. Resultingly, using a liquid alkali reactant, unlike using a DR powder reactant, may affect the reduction rates of 1,2,3,4,7,8-HxCDD, 1,2,3,6,7,8-HxCDD, 1,2,3,7,8,9-HxCDD, 1,2,3,4,6,7,8-HpCDD, and OCDD. PCDFs had high reduction rates overall, but SRF-4 exhibited low reduction rates of 1,2,3,4,7,8-HxCDD, 1,2,3,6,7,8-HxCDD, 1,2,3,7,8,9-HxCDD, and 1,2,3,4,6,7,8-HpCDD. SRF-4 incineration plant involves dry venturi without the SCR in the process.