PCDD / F and PCBz Emissions during Start-up and Normal Operation of a Hazardous Waste Incinerator in China

The PCDD/F emissions from incinerator start-up are a major contributor to the total amount of such emissions, as has been reported in studies of numerous municipal solid waste incinerators. However, very few studies have examined the start-up process at hazardous waste incinerators (HWIs). In this work we studied the emissions of PCDD/Fs and other pollutants, such as PCBz, at the stack during start-up and normal operations in a HWI. We found that the PCDD/F emissions during start-up were greater than during normal operations, and were comparable to the annual emissions during the normal combustion regime. The emissions of PCDD/Fs were highest during start-up when the temperature was around 500°C, reaching 59.5 ng/Nm (5.49 ng I-TEQ/Nm) when no APCDs were applied. The emission values of PCDD/Fs during start-up with the application of APCDs and during normal operations were very low, which indicates the importance of APCDs when starting up a HWI, as well as before feeding waste. The chlorination degree, the ratio of PCDFs/PCDDs and the congener profiles were also discussed during start-up and normal operations, with the results suggesting different formation mechanisms of PCDD/Fs. PCBz emissions are two or three orders of magnitude higher than those of PCDD/Fs, and 1,2,4,5-TeCBz was the best correlated PCBz used as a PCDD/F indicator in real HWI flue gas.


INTRODUCTION
In waste treatment, incineration has multiple advantages, such as volume reduction, energy recovery, pathogen elimination and chemical-toxicity destruction (Dempsey, 1993).Hazardous waste (HW) further refers to industrial hazardous waste, medical hazardous waste and household hazardous waste.With increasing hazardous waste production in China, from 8.3 million tonnes in 2000 (National Bureau of Statistics of China, 2001) to 14.3 million tonnes in 2009 (National Bureau of Statistics of China, 2010), more attention should be paid to the safe disposal of HW.However, stack emissions from hazardous waste incinerators (HWIs) are a serious concern, not to mention the harmful effect on workers at the incinerator plant and vicinity residents (Aramunt et al., 2005;Ferré-Huguet et al., 2005;Mari et al., 2007).
Large amounts of PCDD/Fs would be produced during transient combustion conditions from incinerators, even lasting long time after combustion conditions became well controlled (Zimmermann et al., 2001).Huge emissions of PCDD/Fs have been perceived in several municipal solid waste incinerators (MSWIs) after transient combustion conditions of start-up, shutdown or malfunctioning (Neuer-Etscheidt et al., 2006;Tejima et al., 2007;Wang et al., 2007a, b;Chen et al., 2008) and the temporary stack emission of PCDD/Fs resulting from start-up is between 0.8 and 3 times that of a whole year's normal operation.The formation of PCDD/Fs during start-up has been recognized as an important source of the PCDD/F emissions from modern MSWIs (Blumenstock et al., 2000).However, few studies have allowed estimating the importance of start-up for PCDD/F emissions in HWIs, since such HWIs suffer considerably from thermal shock during periodic shutting down.
This study compares the formation of PCDD/Fs and PCBz during start-up with that during normal operation conditions in a hazardous waste incinerator (HWI) in China.The PCDD/F contribution during start-up and normal operation is discussed and the correlation among representative PCBzs as surrogates for PCDD/Fs is evaluated.

Operating Conditions of the Experimental HWI
A series of experiments were carried out on a 50 tonnes/day HWI.Its flow diagram is presented in Fig. 1.The main combustion sections of the incinerator include a rotary kiln (solids, ash) and a secondary combustion chamber (burnout of flue gas).During normal combustion, shredded industrial hazardous waste is continuously fed into the kiln.
The flue gas is burned out and then cooled by heat exchange in a boiler and in a quenching tower.Dioxins and all other gas pollutants are controlled by the air pollution control devices (APCDs), including an acid scrubber, an activated carbon chamber, baghouse filter and alkaline scrubber.Consumption figures are: 96 kg/h of lime, 4.8 kg/h of powdered activated carbon and 768-960 kg/h of liquid alkali, respectively.Diesel oil is used as auxiliary fuel for starting up the incinerator.Flue gas was sampled at the stack when the temperature reading at the end of the rotary kiln reached 300 and 500°C (start-up 1 and 2), respectively.In both cases the APCDs were not yet put into use and the baghouse filter was by-passed.When the temperature at the end of the kiln reached 600°C (start-up 3) the hazardous waste feed was started with APCDs applied.The third sampling was carried out after two hours of hazardous waste combustion.Sampling under normal operation conditions was carried out after three days of hazardous waste combustion (normal 1 and 2).

Flue Gas Sampling and Analysis
Flue gas samples from the stack were collected with an isokinetic sampler (KNJ, Korea) according to US EPA method 23a, as described in detail by Chen et al. (Chen et al., 2008).The PCDD/F samples were treated sequentially with Soxhlet extraction, sulphuric acid wash and clean-up procedures, and 13 C 12 -labelled EDF-4054, 13 C 12 -labelled EDF-4053 and 13 C 12 -labelled EDF-4055 standards were spiked before sampling of flue gas, extraction, and purification, respectively.All labelled standard solutions of PCDD/Fs were purchased from Cambridge Isotope Laboratory (Andove, MA, USA).The analysis was performed by highresolution gas chromatography with high-resolution mass spectrometry (HRGC/HRMS) (JMS-800D, JEOL, Japan).A DB-5MS column (60 m × 0.25 mm × 0.25 μm) was applied to separate the PCDD/Fs and determine their concentration, as presented in detail by Wu et al. (Wu et al., 2011).
PCBzs were analysed using GC-ECD (GC 6890N, Agilent, USA) with a DB-5 column (30 m × 0.25 mm × 0.25 μm).Pre-treatment and analysis of PCBz were depicted in detail by Yan et al. (2010).The temperature program for GC oven was as follows: initial temperature 80°C, held for 4 min; increased at 5 °C/min to 106°C, held for 0.5 min; increased at 8 °C/min to 250°C and held for 15 min.The chlorinated benzenes standards were from Aldrich (Milwaukee, USA).Pretreatment was according to the state standard method of HJ/T74-2001 and GB 7492-87, in China, consisting of extraction and a clean-up procedure (H 2 SO 4 treatment, multilayer silica gel column and florisil).Tetra-octa-chlorinated PCDD/Fs (the seventeen 2,3,7,8-substituted) congeners and tri-to hexa-CBz were measured.The mean recoveries of standards for PCDD/Fs range from 50 to 130%, which are all within the acceptable 25 to 130% range.The average recoveries of PCBzs are all above 80%.All derived PCDD/Fs and PCBz concentrations were normalized to dry air, 11% O 2 , 1.01 × 10 5 kPa and 237 K.

Combustion Conditions and Gas Pollutants Emissions
The temperature evolution in different sections of the HWI is shown in Fig. 2. It took about 20 hours for the temperature to reach 300°C at the end of the rotary kiln, then 4 hours to reach to 500°C and another 2 hours to attain 600°C.Hazardous waste was then fed to the kiln (i.e., 26 hours after the first start) and a temperature drop could be seen in all sections of the incinerator, particularly at the end of rotary kiln, due to the cooling effect of feeding the raw hazardous waste material into the pre-heated incinerator.Temperature then increased, following the start of combustion of hazardous waste and it was controlled at 950 and 850°C, respectively at the end of the rotary kiln and in the secondary combustion chamber.Under normal combustion conditions the temperature didn't change much in different sections of incinerator, except at the head of rotary kiln, which was due to the irregular feeding of waste.Temperature increase was slow and uneven during start-up, yet combustion seems stable.
The mean emission values of pollutants at the stack during start-up and normal operation are listed in Table 1 Temperature before start-up was lower than 600°C and diesel oil was combusted; hence, concentrations of gases such as CO 2 , NH 3 , SO 2 , HCl and NO x were low.Poor combustion conditions at low temperature generated high emissions of CO and particulate matter.Temperature rises in relation to hazardous waste combustion; as a consequence higher concentrations could be observed of NH 3 , SO 2 , HCl and NO x , even while APCDs were applied; yet, CO concentrations dropped significantly, suggesting suitable combustion conditions.Changes in HCl and CO concentrations during start-up and normal operation were proposed (Hasselriis, 1987)   start-up 1 and 2, with temperatures still limited to 300 and 500°C, respectively.The combustion of diesel oil only produced low concentration of HCl, which was ascribed to chloride deposits on tube walls and heat exchanger.However, these chlorides suffice to produce appreciable amounts of chlorinated compounds, including PCDD/Fs, while the HCl concentration in the flue gas was still quite low (Neuer-Etscheidt et al., 2006).The HCl concentration increased during normal operation, related to the release of chlorine in hazardous waste.CO concentration is a good indicator for the sum of the PAH (Blumenstock et al., 2000), which can be co-formed by similar mechanisms with PCDD/Fs (Blumenstock et al., 2000), and PAH memory effect like those of PCDD/Fs that release from APCDs and flue duct ash have also been observed (Chang and Lin, 2001;Zimmermann et al., 2001;Wang et al., 2007a;Li et al., 2011), so consequently CO concentration might be applied as good indicator of PCDD/F formation, despite the time delays affecting this link (Weber et al., 2002).CO concentrations were higher during start-up 1 and 2 when temperatures were still comparatively low, and then dropped significantly during start-up 3 and during normal operation, which implies wellcontrolled combustion conditions.

PCDD/F Emission at the Stack
It has been reported that PCDD/F emission during startup are a large source of emissions from MSWIs (Neuer-Etscheidt et al., 2006); also during start-up of the main engine of a ship the highest PCDD/F emission could be measured (Cooper 2005).The PCDD/F concentration in flue gas from the HWI during start-up operation was much higher than during normal operation, as shown in Fig. 3.The PCDD/F concentrations during start-up 1 and 2 were 24.7 and 59.5 ng/Nm 3 , with toxic concentrations of 1.56 and 5.49 ng I-TEQ/Nm 3 , respectively.These values are much higher than the national emission limit (0.5 ng I-TEQ/Nm 3 ).However, diesel oil combust only without APCDs working during start-up operation.Hazardous waste was fed at higher combustion temperature (start-up 3); yet, the PCDD/F concentration dropped to 3.01 ng/Nm 3 (0.345 ng I-TEQ/Nm 3 ) with APCDs applied, indicating that APCDs are also beneficial for restricting PCDD/F emissions during startup.During normal operation PCDD/F emissions were significantly lower, with 0.407 and 0.328 ng/Nm 3 (0.039 and 0.031 ng I-TEQ/Nm 3 ).Hence, the start-up of the HWI is essential in shaping PCDD/F emissions and APCDs are required before feeding waste.
Comparing the concentration of PCDD/Fs with those of gas pollutants, we could find that the concentration of PCDD/Fs showed a positive correlation with that of CO, but a negative correlation with those of other gas pollutants since they were greatly affected by waste feeding.Significant formation and emission of PCDD/Fs were seen while raising the temperature with diesel oil (start-up 1 and 2).Formation was due to deposits of chlorine originally presented on tube walls and heat exchanger, and to the formation of fresh soot during deficient combustion, which could accelerate the formation potentially (Hunsinger et al., 2003) since soot formed at high temperature is supposed to be highly active (Zimmermann et al., 2001).Grosso et al. (2009) found similar PCDD/F formation in a MSWI during start-up with oil.Peak PCDD/F formation was found around 500°C (start-up 2), in agreement with Wang et al. (2007b).Combustion conditions were inadequate during both start-up 1 and 2, and the tube wall and heat exchange temperature has not attained the temperature that most suitable for formation of PCDD/Fs during start-up 1, but it has attained the temperature during start-up 2. The APCDs were effective already during start-up 3. Still, the emission of PCDD/Fs during normal operation was one order of magnitude lower than during start-up 3. It is still unclear, however, in how far memory effects affected the time-scale of emission.
The evolution of other PCDD/F characteristics, such as average chlorination degree and PCDFs/PCDDs ratio, during start-up and normal operation might reflect differences in formation mechanism (Fig. 4).The mass-averaged chlorination degree decreased during start-up, even though hazardous wasted was combusted during start-up 3 only.The chlorine supplies were limited, with diesel oil used for heating; yet, large amount of incomplete combustion products (PIC) emerged, generating lower chlorinated PCDD/Fs (Tejima et al., 2007).Since peak lower chlorinated PCDD/Fs formation was found during start-up 2, the chlorination degree of PCDD/Fs decreased from start-up 1 to 2. However, the chlorination degree of PCDD/Fs continued to decrease when waste was fed during start-up 3, even with more chlorine source and less diesel oil, which was different with previous study (Neuer-Etscheidt et al., 2006).Two possible reasons might be used to explain this phenomenon.First, combustion had not sufficiently when waste was fed, PIC still existed.The second reason might be that the highly chlorinated PCDD/Fs with low vapor pressure easily adhered to the particles that were removed by bag filter when APCDs applied (Chang et al., 2002;Karadenir et al., 2004), so the chlorination degree of PCDD/Fs continued to decrease when waste was fed.
Precursor formation of PCDD/Fs is a main pathway during start-up (Hunsinger et al., 2002;Aurell and Marklund, 2009) since high amounts of PIC (including chlorophenols) are formed and formation of PCDF (especially low-chlorinated) is favoured (Nakahata and Mulholland, 2000).The decrease of PCDFs/PCDDs ratio during start-up was also attributed to precursor formation and beginning with start-up 3, the formation mechanism might turn to de novo synthesis with the ratio of PCDFs/PCDDs increasing from 1.24 to 1.77, owing to an increase of temperature in the combustion chamber and a reduction of incomplete combustion products when the incinerator operated stable.However, the APCDs was applied during start-up 3, it may also affect the ratio of PCDFs/PCDDs.Therefore, the mechanism of PCDD/Fs formation during each case is needed further study.
The PCDD/F emissions during an environmental monitoring campaign (after long time normal operation) showed no apparent difference with the PCDD/F emission during normal operation in our experiment.Environmental monitoring is once a year required by the government of China, making sure that the emission of PCDD/Fs satisfies the national emission standard (0.5 ng I-TEQ/Nm 3 ).The result suggests that the PCDD/F emission will be stabilized two days after starting hazardous waste combustion since the complete combustion and stable operation of APCDs.

PCDD/F Congener Profiles during Start-Up and Normal Operation
The congener profiles of PCDD/F and I-TEQ during startup and normal operation are shown in Fig. 5.The congener profiles during start-up favoured high-chlorinated PCDD/Fs, especially for HpCDD/Fs and OCDD/Fs (Fig. 5(a)).The contribution of OCDD/Fs patterns was lowest during startup 3 even though the temperature was higher than that during start-up 1 and 2. Precursor synthesis occurred at temperatures between 250 and 650°C, which favoured the formation of PCDDs, especially for the high-chlorinated ones.Large amount of chlorophenols were thought to be formed after feeding hazardous waste during start-up 3, and it might be the reason that congeners turned to low chlorinated ones and ratio of PCDFs/PCDDs increased at such stage (Hagenmaier et al., 1987).
As usual, 2,3,4,7,8-PeCDF takes up an important part in TEQ concentration both during start-up and normal operation (Fig. 5(b)), in line with earlier findings from flue gas, fly ash, ambient atmospheric air and soil in our research group (Chen et al., 2008;Yan et al., 2008;Xu et al., 2009;Du et al., 2011;Wu et al., 2011), and elsewhere (Wang et al., 2007a;Gao et al., 2009).Significant increase of the 2,3,4,7,8-PeCDF contribution could be seen during start-up 3. The phenomenon is mainly contributed to the formation mechanism favours of low chlorinated PCDD/Fs during start-up 3 process, and 2,3,4,7,8-PeCDF with high TEQ factor is favoured.However, other PCDD/F congeners don't seem to change significantly with respect to TEQ contribution when combustion is different.

Evaluation of PCDD/F Emission during Start-Up and Normal Operation
Even though start-up has been recognized as a source of amplified PCDD/F concentration in flue gas and fly ash from MSWIs, the main PCDD/F source is from fly ash and the extra contribution during start-up is negligible over the whole year (Chen et al., 2008).Nevertheless, this contribution is not negligible in all MSWIs.Trends in HWIs might be similar with the ones in MSWIs.Table 2 shows the PCDD/F emission during start-up and normal operation.The PCDD/F emission rate of one operation (from start-up operation to shut-down operation) was calculated as follows.
where E is the PCDD/F emission rate, C i is the PCDD/F concentration of each start-up or normal operation, V i is the flue gas rate of each start-up or normal operation, T i is the time of each start-up or normal operation, and T a is the total time of start-up or normal operation of one operation.
In this study, 32 hours for start-up operation (20 hours for start-up 1, 6 hours for start-up 2 and 6 hours for start-up 3) and 1440 hours (about 2 months) for normal operation.
As Table 2 shows, the PCDD/F emission during start-up was 423 μg/h (32.9 μg I-TEQ/h), that is much more than that during normal operation of 7.53 μg/h (0.718 μg I-TEQ/h), which means that APCDs application is necessary during start-up.The emission ratio of start-up/normal is 1.25 and toxic emission ratio is 1.02 for one operation, which is similar with studies in MSWIs (Neuer-Etscheidt et al., 2006;Tejima et al., 2007;Wang et al., 2007b;Chen et al., 2008).However, HWIs suffer more from shutting down because of maintenance and lack of raw materials, the emission of PCDD/F from start-up might be more serious.

PCBz Emission and Its Relation to PCDD/F Emission
During start-up and normal operation the trends of PCBz emissions were similar with those of PCDD/F (Table 3), but the total PCBz concentration was two or three orders of magnitude higher than those of PCDD/Fs, which is in line with previous study (Zimmermann, 1996).The highest PCBz emission was similar with the behaviour of PCDD/F during   (Zimmermann et al., 2001) can be explained by co-formation along similar mechanism, or PCBz serving as precursors, i.e., reactants to form PCDD/Fs (Oh et al., 2007).Therefore, PCBz have been used as PCDD/F surrogate (Blumenstock et al., 2001).Among PCBz congeners, TrCBz, especially 1,2,3-TrCBz, were most significant during all experimental conditions during start-up and normal operation.However, in other studies, PeCBz or HxCBz were found to be dominating congeners in incinerator stack gas (Kaune et al., 1994).Those differences might be due to different types of incinerators and fuel, different cleaning devices, different sampling points and different ways to normalize the experimental data (Lavric et al., 2005).
In several studies (Öberg and Bergström, 1985;Kaune et al., 1998;Yan et al., 2010) higher chlorinated benzenes were indicated as PCDD/Fs or as I-TEQ surrogate; however, other research pointed out low chlorinated benzenes as best I-TEQ indicators (Blumenstock et al., 1999;Blumenstock et al., 2000), and the first indirect on-line measurement of the I-TEQ at a full scale industrial plant using REMPI-TOFMS has been performed relying on such correlation (Zimmermann et al., 1999).Therefore, the research of this paper has a certain reference value on the application of online monitoring technology in the future, especially in the hazardous waste incinerator.
Fig. 6 shows two congeners of PCBz, which are best correlated to PCDD/Fs and their I-TEQ, the correlations of total PCBz with PCDD/Fs and their I-TEQ are also depicted.It was found that higher chlorinated PCBz such as HxCBz was more correlated with PCDD/F concentration than lower ones, which might be due to the similar phy-chemical characteristic between higher chlorinated PCBz and PCDD/Fs that present the low vapour pressure (Blumenstock et al., 2000).1,2,4,5 TeCBz was the most correlated with PCDD/F I-TEQ concentration, which was also found by Pandelova et al. (Pandelova et al., 2006).It might due to lower chlorinated PCDD/F with high I-TEQ factors, especially 2,3,7,8-TeCDD, and the most correlated PCBzs were also the lower chlorinated ones such as 1,2,4,5-TeCBz, the similar temperature dependence of PCBz and PCDD/Fs in the formation was also an factor to the correlation (Wikström et al., 1998).The total PCBzs were both correlated well PCDD/Fs concentrations and their I-TEQ, and more fitness was found in I-TEQ.It is concluded that PCBz, especially 1,2,4,5-TeCBz are candidate surrogate in real hazardous waste incinerator flue gas at stack.

CONCLUSIONS
In this work, we studied the emission of PCDD/Fs and other pollutants such as PCBz at stack during start-up and normal operation in a HWI.The emission of PCDD/Fs were highest during start-up when temperature was around 500°C (start-up 2), with 59.5 ng/Nm 3 (5.49ng I-TEQ/Nm 3 ) when no APCDs applied.The emission values of PCDD/Fs during normal operation were very low and the application of APCDs was helpful for controlling the PCDD/F emission during start-up 3. The emission of PCDD/Fs during start-up was larger than normal operation for one operation, which denotes the importance of APCDs in hazardous waste incineration.The chlorination degree, the ratio of PCDFs/ PCDDs and the congener profiles were also discussed during start-up and normal operation, which suggest different formation mechanisms of PCDD/Fs.PCBz emission was two or three orders of magnitude higher than PCDD/F, and 1,2,4,5-TeCBz was the best correlated PCBz used as PCDD/F indicator in real HWI flue gas.

Fig. 2 .
Fig. 2. Combustion conditions of the HWI during start-up and during normal operation.

Table 1 .
Gas pollutants emission during start-up and during normal operation.

Table 2 .
Evaluation of PCDD/F emission during start-up and normal operation.

Table 3 .
PCBz emissions during start-up and during normal operation.