Assessment of emissions and removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) at start-up periods in a hazardous waste incinerator

Abstract

A study was conducted to observe the changes in polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) levels and congener profiles in the flue gas of a hazardous waste incinerator during two start-up periods. Flue gas samplings were performed simultaneously through Air Pollution Control Devices (APCDs) (including boiler outlet, electrostatic precipitator (ESP) outlet, wet scrubbers (WS) outlet, and activated carbon (AC) filter outlet) in different combustion temperatures during a planned cold (long) start-up and an unplanned warm (short) start-up. The results showed that PCDD/F concentrations could be elevated during the start-up periods up to levels 3–4 times higher than those observed in the normal operation. Especially lower combustion temperatures in the short start-ups may cause high PCDD/F concentrations in the raw flue gas. Assessment of combustion temperatures and Furans/Dioxins values indicated that surface-catalyzed de novo synthesis was the dominant pathway in the formation of PCDD/Fs in the combustion units. PCDD/F removal efficiencies of Air Pollution Control Devices suggested that formation by de novo synthesis existed in ESP also when in operation, leading to increase of gaseous phase PCDD/Fs in ESP. Particle-bound PCDD/Fs were removed mainly by ESP and WS, while gaseous phase PCDD/Fs were removed by WS, and more efficiently by AC filter.

Implications: This paper evaluates PCDD/F emissions and removal performances of APCDs (ESP, wet scrubbers, and activated carbon) during two start-up periods in an incinerator. The main implications are the following: (1) start-up periods increase PCDD/F emissions up to 2–3 times in the incinerator; (2) low combustion temperatures in start-ups cause high PCDD/F emissions in raw gas; (3) formation of PCDD/Fs by de novo synthesis occurs in ESP; (4) AC is efficient in removing gaseous PCDD/Fs, but may increase particle-bound ones; and (5) scrubbers remove both gaseous and particle-bound PCDD/Fs efficiently.

Introduction

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), which are known as “dioxins,” are a group of persistent organic pollutants originating from various anthropogenic activities such as waste incinerators, iron ore sintering, coal-fired power plants, electric arc furnaces, and open burning of rice straw (Li et al., 2011). Generation and release of PCDD/Fs have created great public concern due to their acute and chronic health effects on immune, nervous, endocrine, and reproductive systems, and potential carcinogenic effects (U.S. Environmental Protection Agency [EPA], 1994; Alcock et al., 1999; World Health Organization [WHO], 1999). Although the sources, pollution levels, and toxic effects of PCDD/Fs have been exhaustively investigated in many countries for many years, the research on PCDD/Fs is a relatively new subject in Turkey. PCDD/Fs became the subject of public discussions for the first time in Turkey in 1997, when the first hazardous and medical waste incinerator (IZAYDAS) started to operate in Kocaeli, the most industrialized region of Turkey.

Because the waste incineration is known as the major source of PCDD/Fs (Oh et al., 2002), many studies have focused on the fate of the PCDD/Fs in the incinerators, including their formation mechanisms, congener profiles, gas/particle partitioning, and removal by air pollution control equipment, etc. The majority of these studies were interested in the behavior of the PCDD/Fs during normal operation of the incinerators; however, some studies conducted in recent years have included the investigation of incinerator PCDD/Fs in the nonstationary combustion conditions (i.e., start-up and shut-down periods), which could cause unexpected PCDD/F emissions. These studies reported that PCDD/F emissions could increase to the levels much higher than those measured in the normal operation conditions, especially in the start-up periods, leading to a significant contribution to the annual PCDD/F emissions of the plant. Additionally, significant changes in the congener profiles were also reported during and after the start-up periods (Blumenstock et al., 2000; Neuer-Etscheidt et al., 2006, 2007; Tejima et al., 2007; Wang et al., 2007; Chen et al., 2008; Cunliffe and Williams, 2009). These high PCDD/F levels observed during the start-up period were attributed to improper and unstable combustion conditions (such as low temperatures), de novo synthesis of PCDD/Fs in the boiler and electrostatic precipitator (ESP), memory effects of PCDD/F formation from carbonaceous deposits in the combustion chamber and boiler, etc. In general, levels and congener profiles of PCDD/Fs during the start-up periods differ from one incinerator to another, because the start-up procedures, combustion conditions, and air pollution control systems are different in each plant. Therefore, the necessity for the studies aiming at a detailed investigation of the fate of PCDD/Fs during the start-up periods is clear for each incinerator, which will make the optimization of the start-up process to minimize the PCDD/F emissions possible.

A study was conducted to observe the changes in PCDD/F levels and congener profiles in the flue gas of Izmit Clinical and Hazardous Waste Incinerator (IZAYDAS) during two start-up periods. The study also included the investigation of PCDD/F removal efficiencies of the air pollution control devices (APCDs) in the plant (i.e., electrostatic precipitator [ESP], dual wet scrubber [WS], and fixed-bed activated carbon [AC]). Flue gas samplings were performed simultaneously through APCDs (boiler outlet, ESP outlet, WS outlet, and AC outlet) at different combustion temperatures during a planned cold (long) start-up after a routine annual maintenance, and an unplanned warm (short) start-up after an obligatory maintenance. The samples were analyzed for 17 2,3,7,8-substituted PCDD/Fs in both gas and particulate phases.

Materials and Methods

The incinerator

The detailed information on IZAYDAS Incinerator was given in previous studies (Bakoglu et al., 2003; Karademir et al., 2003). In summary, the plant, with a capacity of 35000 t/yr, has a two-stage combustion system consisting of a rotary kiln and a vertical postcombustion chamber. The kiln is 12 m in length and 4.2 m in diameter, with combustion temperatures in the range of 950–1050 °C and a retention time of 2–2.5 hr for solids. The vertical postcombustion chamber is 12 m in height and 4.1 m in diameter. This chamber incinerates the gases produced from the rotary kiln, vapors from the ash quench chamber, and some liquid wastes. It has a retention time of about 2 sec at 1150–1250 °C, thus ensuring the complete destruction of hazardous organic compounds. The original air pollution control system of the plant consisted of an ESP and dual wet scrubbers (a venturi scrubber and a lime scrubber) in series. An additional fixed-bed AC unit was installed downstream of wet scrubbers in 1999, although PCDD/F emissions were measured below the national limit of 0.1 ng international toxic equivalents (I-TEQ)/Nm³ in several trial burns. PCDD/F removal efficiencies in ESP and WS were examined in Karademir et al. (2003), whereas those in AC bed were studied in detail in Karademir et al. (2004).

Sampling and analysis

Flue gas samples were taken from four sampling points (ESP inlet, WS inlet, AC inlet, and AC outlet) simultaneously in different combustion temperatures during two start-up periods. PCDD/F sampling, cleanup, and quantification were conducted in accordance with European Standard EN 1948:1–3. Isokinetic flue gas sampling was performed by the filter-condenser method with a sampling train consisting of a quartz fiber filtration medium, in line with a cooling device, glass cartridges, and an isokinetic sampler. Glass cartridges packed with XAD-2 Amberlite resin and supported by a polyurethane foam (PUF) plug were used to collect PCDD/Fs after condensation of the moisture in the gas. Sampling time was about 2–4 hr in each test, resulting in sampling volumes of about 1 Nm³ of flue gas. Following extraction and chromatographic cleanup, PCDD/F analysis of sample extracts was conducted by high-resolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) in a certified laboratory. Samples were spiked with ¹³C₁₂-labeled 2,3,7,8-substituted PCDD/F internal standards, and recovery ratios were reported as 75–90%. Toxic equivalents as 2,3,7,8-TCDD (TEQ) were calculated by using international toxicity equivalency factors (I-TEF).

The first start-up, which was performed in October 2011, was a planned long (cold) start-up including routine maintenance of the combustion units and cleaning of APCDs. The inner surfaces of boiler, ESP, and WS were cleaned by sand blasting, compressed air, and washing, respectively. AC filter medium was renewed completely. The refractory bricks in the rotary kiln were replaced with refractory concrete. Therefore, the start-up procedure was performed in a long period (totally 124 hr) to protect that renewed material from a rapid increase of temperature. Two sets of flue gas sampling for PCDD/Fs (R1, R2) were made during the first start-up.

The second start-up procedure was applied in March 2012, after an obligatory shutdown of the plant. The problem was related to the accumulation of particles at the bottom of the vertical postcombustion chamber, which leads to a risk of clogging the passage from the rotary kiln to the postcombustion chamber. It is thought that as the particles carried by the flue gas flow from the rotary kiln enter the postcombustion chamber, a part of them settle to the bottom due to the disturbance of gas flow patterns in the first section (i.e., change of flow direction and gas velocity). The particles accumulated at the bottom (or within the bottom ash) then undergo a solidification process due to high temperatures in the chamber to form a concrete-like layer. As the thickness of this layer at the bottom increases with time, the risk of blocking the flue gas flow from rotary kiln emerges. Then, the plant is shut down necessarily to remove this tough layer by a drilling machine. Apart from the removal of this layer from the bottom of the postcombustion chamber, no other maintenance and/or cleaning process was applied to the plant equipment during that short start-up. Approximately 14,150 tons of hazardous waste were incinerated in total during the 5 months between the first and second start-up periods. Three sets of PCDD/F sampling (R3, R4, and R5) were performed in the second start-up.

One set of flue gas sampling included four simultaneous samplings (boiler outlet, ESP outlet, WS outlet, and AC outlet) of flue gas for PCDD/F analyses in gas and particulate phases, separately. Sampling sets were designed to have different combustion conditions (temperature and feeding) to obtain a clear understanding of the fate of PCDD/Fs during the start-up periods. Because ESP could be operated at the gas temperatures of about 180 °C, which requires a kiln temperature about 650–800 °C in turn, it was off-line during the first two sampling sets in the second start-up (R3 and R4). Therefore, flue gas samples were not taken from ESP outlet, i.e., WS inlet, in these sets. The details of the sampling sets are given in Table 1.

Table 1. The conditions related to flue gas sampling during start-up procedures

Start-up	Long (124 hr)		Short (12 hr)
Sampling set	Run 1	Run 2	Run 3	Run 4	Run 5
Feeding	Fuel oil	Waste	Fuel oil	Fuel oil	Waste
Sampling start time	104 hr	136 hr	4 hr	8 hr	24 hr
Rotary kiln temp. range (°C)	650–800	925–1050	250–550	550–925	925–1050
Rotary kiln temp. (°C) *	650	925	250	550	925
P.C. chamber temp. (°C) *	600	1100	230	445	1100
Boiler temp. (°C) *	180	190	45	100	190
ESP temp. (°C) *	170	180	35	80	180
WS temp. (°C) *	50	60	25	40	60
AC temp. (°C) *	55	60	20	35	60
Boiler outlet sampling	Yes	Yes	Yes	Yes	Yes
ESP outlet sampling	Yes	Yes	No	No	Yes
WS outlet sampling	Yes	Yes	Yes	Yes	Yes
AC outlet (stack) sampling	Yes	Yes	Yes	Yes	Yes
Note: *The temperatures measured at the beginning of sampling.

Result and Discussions

Concentrations

PCDD/F concentrations through APCDs in the tests are summarized in Figure 1. In the first test (R1), PCDD/F concentrations were relatively low in all the sampling points. Because only fuel oil (with chlorine content of 0.1%) was fed to a cleaned system in R1, these low PCDD/F concentrations could be expected in both the raw flue gas (boiler outlet) and treated gas through APCDs (other points). Low furan/dioxin (F/D) ratio in the boiler outlet (0.54; the only F/D ratio lower than 1 in all the tests) indicates the dominance of precursor pathway over surface-catalyzed de novo synthesis in the combustion units. Because the combustion temperatures of about 600 °C are quite higher than the temperature window of de novo synthesis, and suitable for the formation of some precursors (possibly from the fuel elements), precursor pathway for PCDD/F formation could take place in higher rates (EPA, 2000; Environment Australia [EA], 2001). Then, the PCDD/F concentrations showed an increase through ESP, which could be attributed to the temperature effect in ESP. This effect is related to de novo synthesis of PCDD/Fs because ESP operating temperatures (about 200 °C) remain within de novo synthesis temperature window. Additionally, it was reported that part of the synthesized PCDD/F on the fly ash may desorb to the gas phase and increase the outlet PCDD/F concentrations in ESP (Wang et al., 2007). An assessment of PCDD/F concentrations in gas and particulate phases before and after ESP showed that the increase in gas-phase PCDD/Fs was higher as compared with that in particulate phase generally, supporting the suggestions mentioned above. The increase in the F/D ratio through ESP (from 0.54 to 1.79) is an indication for de novo synthesis of PCDD/Fs, especially the heterogeneous synthesis (Wang et al., 2007; Chi et al., 2005). Finally, the memory effect in ESP should be noted, even after the cleaning of the equipment, which was probably caused by the particulate matter that could not be removed by the cleaning processes.

Figure 1. PCDD/F concentrations through APCDs in the start-up tests.

After ESP, PCDD/Fs were decreased to low levels by wet scrubbers (from 0.063 to 0.002 ng I-TEQ/Nm³, corresponding to a removal efficiency of 97%), which resulted mainly from the removal of particulate-phase PCDD/Fs. Finally, PCDD/Fs increased again in the exit gas after AC filter. This increase was due to the increase of high-chlorinated PCDD/Fs in the particulate phase, whereas gas-phase PCDD/Fs showed a slight decrease through AC unit in general. The increase in the particle-bound congener concentrations was attributed to the PCDD/F content of new AC material placed in the unit. PCDD/F analysis of this AC material was also performed for a mass-balance study in the incinerator. Taking into consideration that high-chlorinated PCDD/Fs were dominant in the AC material (1,2,3,4,6,7,8-HpCDD, OCDD, and OCDF constituted about 85% of total PCDD/F content), it was concluded that the addition of new AC material in particulate form into the flue gas during its passage through AC unit caused that increase of PCDD/Fs in the exit gas. Moreover, it was observed that fine particulate filter medium had accumulated on the sampling probe during the sampling period, which also proves the addition of AC material into the flue gas also.

In the subsequent test (R2) where feeding of waste started, PCDD/F concentrations in the raw gas (boiler outlet) increased to 0.7 ng I-TEQ/Nm³. Then, they decreased to the levels below the national emission limit of 0.1 ng I-TEQ/Nm³ by ESP, with a removal efficiency of 90% on I-TEQ basis. This decrease was achieved mainly by the removal of particulate-phase PCDD/Fs. Gas-phase PCDD/Fs, on the other hand, showed slight decreases in PCDDs and high-chlorinated PCDFs, but increases especially in low-chlorinated PCDFs in ESP, causing an increase in F/D ratio (similar to the previous test R1). After ESP, dual wet scrubbers also provided an additional 90% removal of PCDD/Fs both in particulate and gas phases. However, PCDD/F concentrations slightly increased in the last step (AC) again, mainly due to the increase of high-chlorinated PCDD/Fs in particulate phase. Because a similar case was observed in R1 also, it was attributed to the effect of new AC material.

The last three tests were carried out during the short start-up in March 2012 (5 months later than the first start-up procedure) after the obligatory shutdown of the plant. During the start-up period, ESP was not operated until the flue gas temperatures reached the normal operation temperatures of ESP (i.e., 180–200 °C), which required combustion temperatures of about 650–800 °C in turn. Therefore, ESP was off-line in the first two tests (R3 and R4) and flue gas was not sampled in the ESP outlet.

In the first test (R3), which was conducted in the combustion temperatures between 250 and 550 °C, PCDD/F concentrations were measured as 1.33 ng I-TEQ/Nm³ in the raw flue gas sampled from boiler outlet. It should be noted that almost all of the PCDD/Fs were in particulate phase in that “cold” flue gas (its temperature during the sampling was 45 °C). There is some evidence that these relatively high PCDD/F concentrations were caused by the fly-ash-catalyzed de novo synthesis of PCDD/Fs due to the chlorine content of particulate matter in the combustion units. First, lower combustion temperatures in the rotary kiln and postcombustion chamber were close to the temperature window for PCDD/F formation by de novo pathway. Then, it is known that de novo synthesis includes complex reactions between unburned carbon of fly ash and chlorine sources of metallic catalysts such as Cu, Fe, and Mn (Chin et al., 2012). On the other hand, Lu et al. (2007) found that NaCl and MgCl₂ were relatively effective in producing more PCDDs, whereas KCl, AlCl₃, and CaCl₂ generated more PCDFs during heterogeneous reactions occurring on fly ash. Inductively coupled plasma mass spectroscopy (ICP-MS) analysis of the fly ash from postcombustion chamber in IZAYDAS showed that they contain substantial amounts of Fe, Ca, Zn, Al, Mg, Mn, and Cu. Finally, the studies showed that de novo mechanism leads preferentially to the formation of PCDF, in contrast to most of the known precursor routes that generate PCDD. Therefore, high concentrations of low-chlorinated furans (especially 2,3,7,8-TCDF), which resulted in very high F/D values in the boiler outlet (about 13), support this de novo synthesis suggestion (Neuer-Etscheidt, 2006; Wang et al., 2007). 2,3,7,8-TCDF was the most significant contributor to the toxicity of the PCDD/Fs, in accordance with the study of Lu et al. (2007). Then, the sorption of gas-phase PCDD/Fs onto the particles could take place as the flue gas cooled along the boiler.

After the boiler, PCDD/F concentrations were reduced to 0.08 pg I-TEQ/Nm³ by the wet scrubbers that received an excess particle loading in the absence of ESP. This decrease in the PCDD/F concentrations corresponds to a removal efficiency of 94% based on I-TEQ, and is consistent with the previous data given in Karademir et al. (2003). Based on the gas- and particulate-phase concentrations of the congeners through WS, PCDD/F reduction was predominantly related to the removal of particulate-phase PCDD/Fs as expected, whereas the gas-phase concentrations did not change significantly. On the other hand, the opposite of this phenomenon was true for AC filter: substantial decreases in the gas-phase concentrations and insignificant changes in the particulate-phase concentrations (actually most of them were small increases). In total, PCDD/F concentrations were decreased to 0.05 pg I-TEQ/Nm³ by AC filter, indicating a removal efficiency of 44%.

The subsequent test (R4), which was conducted in combustion temperatures in the range of 550–925 °C with fuel oil feeding, produced the highest PCDD/F concentrations (2.30 pg I-TEQ/Nm³) in the raw flue gas taken from boiler outlet. Because the combustion temperatures both in the rotary kiln and postcombustion chamber were still within the optimum temperature window for PCDD/F formation during the sampling (about 450–550 °C; see Table 1), these high values could also be attributed to the surface-catalyzed formation of PCDD/Fs. Taking into account that F/D ratio decreased from 13 in R3 to 4.5 in R4, it could be suggested that de novo synthesis was still the dominant pathway, whereas the precursor pathway favoring the formation of PCDDs also took place to some degree simultaneously. Additionally, the fact that the formation of some precursors requires the temperatures about 500 °C or higher in the precursor pathway of PCDD/F formation supports this suggestion (EPA, 2000; EA, 2001). A comparison between the congener profiles in the raw flue gas in R3 and R4 showed that the concentrations of high-chlorinated PCDD/Fs increased substantially in R4 (they were 5–10 times higher compared with R3), whereas those of low-chlorinated congeners did not change significantly. Other observations in R4 were generally similar to those in R3. PCDD/Fs were partitioned mainly to the particulates with fractions higher than 95% in the boiler outlet. Then, PCDD/F concentrations were decreased to 0.13 pg I-TEQ/Nm³ by the wet scrubbers, corresponding to a removal efficiency of 94% based on I-TEQ. This decrease was achieved mostly by the removal of particle-bound congeners in the scrubbers. After that, the PCDD/F concentrations were lowered to 0.02 pg I-TEQ/Nm³ by AC filter (84% removal based on I-TEQ), but this time a significant removal of particulate-phase PCDD/Fs was observed in AC filter in addition to the removal of gas-phase PCDD/Fs.

The last test was performed after the combustion temperatures in the combustion units (rotary kiln and postcombustion chamber) reached the normal operation temperatures and the waste feeding started. The conditions in the tests R2 and R5 were similar (see Table 1) and they closely resembled the normal operation conditions. However, PCDD/F concentrations showed a different trend in R5. The concentrations in the boiler outlet were very low (0.03 pg I-TEQ/Nm³), but they increased to 0.14 pg I-TEQ/Nm³ after ESP, which was operated for the first time in the second start-up period. A short review of gas- and particulate-phase data showed that both gaseous and particle-bound fractions of PCDD/Fs increased in ESP, but the increase in the gaseous PCDD/Fs was much higher (about 30 times) than that in the particulate-phase PCDD/Fs (3 times). On the other hand, F/D ratio in the gaseous PCDD/Fs increased from 3.5 to 4.7 through ESP. Therefore, the increase in gaseous PCDD/Fs in ESP may also be related to the surface-catalyzed de novo synthesis of PCDD/Fs in the operation temperatures of ESP, which is a well-known phenomenon. On the other hand, the increase in the particle-bound PCDD/Fs could be attributed to the memory effect caused by the contribution of the fly ashes accumulated on ESP walls into the flue gas, because the congener profiles and F/D ratio in that phase were not changed significantly through ESP. Then, the PCDD/F concentrations were reduced to 0.10 pg I-TEQ/Nm³ by wet scrubbers, which resulted from 44% decrease in the gas phase and 59% decrease in the particle-bound PCDD/Fs. Finally, PCDD/Fs increased to 0.12 pg I-TEQ/Nm³ after the AC filter, i.e., in the exit gas, due to an increase in the particle-bound PCDD/Fs by 33%. Gas-phase PCDD/Fs, on the other hand, decreased by 89% through AC filter, indicating good removal efficiency related to gaseous compounds. All the particle-bound congeners except for 2,3,7,8-TCDD, 2,3,7,8-TCDF, and 1,2,3,7,8-PeCDF showed increase rates between 9% and 56% through the AC filter (especially high-chlorinated PCDDs had the highest increase rates), indicating some contribution of the particulate AC material into the flue gas. Finally, it should be noted that R5 was the only test producing PCDD/F concentrations higher than the national emission limit of 0.1 pg I-TEQ/Nm³ in the exit gas.

Congener profiles through APCDs and removal efficiencies

Average congener concentrations measured at the sampling points (including both gas and particulate phases) were given in Figure 2. Congener concentrations shown in Figure 2 represent the average results of five samplings performed at the outlet of the units, with the exception of ESP outlet where only three samplings were made (because ESP was off-line in R3 and R4). Absence of ESP led to high particulate matter and PCDD/F concentrations in WS inlet in these tests (PCDD/F levels were 10–40 times higher than the levels in normal operation), which caused relatively high PCDD/F concentrations subsequently in the WS outlet. Therefore, these high PCDD/F concentrations measured in the WS outlet in R3 and R4 elevated the average WS outlet concentrations to the levels close to the average ESP outlet levels (see Figure 2).

Figure 2. Average congener concentrations through APCDs (pg/Nm³).

As shown in Figure 2, congener profiles were generally similar through APCDs in the plant. Evereart and Baeyens (2002) reported that the congeners of HpCDD and OCDD among PCDDs, and those of PeCDF, HxCDF, and HpCDF among PCDFs, are dominant in the majority of large-scale incinerators. The congener distributions given in Figure 2 agree with this statement generally. Furthermore, a comparison of these congener profiles with those given in Karademir et al. (2003) for the normal operation conditions indicates that PCDD/F congener profiles do not change significantly in the start-up periods in the incinerator. Average F/D ratios were similar correspondingly (between 4.6 and 5.1), with the exception of ESP outlet with the average F/D ratio of 2.2, which was caused by the relatively high particle-bound PCDD concentrations (especially 1,2,3,4,6,7,8-HpCDD and OCDD) measured in R1 and R5. The particle-bound PCDD/Fs were dominant at all the sampling points. Their average fractions, which were about 95% in the boiler outlet, decreased to 70% after ESP and WS, and increased again to 90% in the exit gas after the removal of gaseous compounds in the AC filter.

Figure 3 shows the average removal efficiencies of APCDs related to PCDD/F congeners in gas and particulate phases separately, which were computed based on the average concentrations in the inlet and the outlet of APCDs. For gaseous fractions, the positive effect of ESP is clear, especially for the low-chlorinated PCDD/Fs, with the exception of 2,3,7,8-TCDD, which was measured below the detection limits generally. 2,3,7,8-TCDF, on the other hand, had a low removal rate (about 9%) on average in ESP, but it should be noted that its concentration in gas phase showed an increase through ESP in two out of three tests. The same statement is valid for the gaseous fractions of other congeners showing positive removal efficiencies in ESP (i.e., 1,2,3,7,8,9-HxCDF, 1,2,3,4,7,8,9-HpCDF, and OCDF), but their concentrations in gas phase were relatively low as compared with 2,3,7,8-TCDF. The scrubbers then displayed good removal efficiencies (between 33% and 92%) for gaseous concentrations of all the congeners. Results indicate that removal efficiencies of low-chlorinated congeners in gas phase are slightly higher than those of high-chlorinated ones in the scrubbers. AC filter, on the other hand, showed removal efficiencies higher than 50% for all the congeners (between 62% and 91%). It provided better removal efficiencies for the gaseous fractions of high-chlorinated congeners, unlike the scrubbers.

Figure 3. Average PCDD/F removal efficiencies of APCDs.

For the removal of particle-bound concentrations of PCDD/Fs, ESP and WS showed very high removal efficiencies, most of which were higher than 90%. Especially the performance of WS in removing PCDD/Fs in particulate-phase in R3 and R4 where ESP was off-line should be noted. Average removal efficiencies of WS for particle-bound congeners in R3 and R4 varied between 92% and 99%. AC filter, however, produced lower removal rates for particle-bound congeners on average, varying between −8% for 1,2,3,6,7,8-HxCDD and 44% for 2,3,7,8-TCDD. Actually, the majority of the congener concentrations in particulate phase increased slightly through the AC filter in three or four tests out of five tests, producing lower average removal rates. As mentioned earlier, the increase in the particle-bound congeners in AC was attributed to the contribution of particular AC material to the exit gas.

Nomenclature

APCD	=	Air Pollution Control Device
AC	=	Activated Carbon
ESP	=	Electrostatic Precipitator
IZAYDAS	=	Izmit Medical and Hazardous Waste Incinerator
PCDD	=	Polychlorinated Dibenzo-p-Dioxin
PCDF	=	Polychlorinated Dibenzofuran
PeCDD	=	Pentachloro Dibenzo-p-Dioxin
PeCDF	=	Pentachloro Dibenzofuran
HpCDD	=	Heptachloro Dibenzo-p-Dioxin
HpCDF	=	Heptachloro Dibenzofuran
HxCDD	=	Hexachloro Dibenzo-p-Dioxin
HxCDF	=	Hexachloro Dibenzofuran
OCDD	=	Octachloro Dibenzo-p-Dioxin
OCDF	=	Octachloro Dibenzofuran
TCDD	=	Tetrachloro Dibenzo-p-Dioxin
TCDF	=	Tetrachloro Dibenzofuran
WS	=	Wet Scrubbers

Acknowledgment

The study was conducted within the scope of a cooperation program between University of Kocaeli and IZAYDAS. The authors gratefully thank IZAYDAS for the financial support and collaboration.