Polychlorinated Dibenzo-p-dioxin and Dibenzofuran ( PCDD / F ) Emission Behavior during Incineration of Laboratory Wastes . Part 2 : PCDD / F Profiles and Characteristics of Output Materials

This study investigates the polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) profiles of output materials and the influence of Cl content of input materials during the incineration of laboratory waste. The specimens, namely bottom ash (BTA), first quenching tower ash (FQA), secondary quenching tower ash (SQA), baghouse ash (BHA), and stack flue gas (SFG), were sampled and analyzed using high-resolution gas chromatography/high-resolution mass spectrometry, fieldemission scanning electron microscopy (FE-SEM), ion chromatography, and X-ray diffraction (XRD). The Cl content was highest in FQA, followed by that in SQA, BHA, and BTA, and most of the Cl existed in soluble form, especially for FQA and SQA. The Cl mass was mainly distributed in FQA and BTA during incineration. The PCDD/F content of ash in each category was highly related to the Cl level in the input materials. The PCDD/Fs of all ashes and the particulate phase of SFG were mainly 7-Cl or 8-Cl PCDD/Fs, but those of the gas phase in SFG were mainly 4-6 Cl PCDD/Fs. In addition, an increase of Cl in the input materials increased the fractions of 7-Cl and 8-Cl PCDD/Fs. The XRD analysis results indicate that the main crystalline phase in fly ashes was NaCl. FE-SEM images show a porous granular morphology, which is consistent with the XRD analysis results.


INTRODUCTION
Laboratory waste, generated during educational or research experiments, has a complex composition and is often in tiny amounts.The waste often has high toxicity and unknown harmfulness.Laboratory waste is temporarily stored on campus and then transported to a treatment facility combustible laboratory solid waste (LSW), laboratory plastic waste (LPW), and organic LLW.LPW comprises mainly plastic containers for LLW made of polyethylene or polypropylene.LSW is mainly combustible waste from medical or biochemical experiments and usually has high Cl content.The plasma melting system is designed to treat sludge, toxic ash, and other wastes from the two former systems.This system, with an addition of cullet, transforms hazardous materials into stable slag using vitrification (Kuo et al., 2010).A previous study reported that the slag could be mixed with polyester resin to remake composites (Kuo et al., 2011).
At present, the most common method in Taiwan for disposing of combustible waste is incineration.In 2011, 6,355,422 tons of municipal solid waste and general industrial solid waste was treated using incineration, generating 1,357,558 tons of ash (TEPA, 2012).During the incineration of solid wastes, toxic pollutants, such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs), are generated (Wang et al., 2010;Du et al., 2013;Thacker et al., 2013;Zhang et al., 2013).Without air pollution control devices (APCDs), the flue gas may have a high PCDD/F concentration and cause secondary air pollution (Coutinho et al., 2006).Due to the stringent standard for PCDD/Fs, > 90% of PCDD/Fs is collected as fly ashes.Thus, fly ashes, especially from baghouse filter units, often have high PCDD/F levels (Vehlow et al., 2006).
The incineration of input materials with high Cl content generates a large amount of PCDD/Fs (Hatanaka et al., 2005).Laboratory waste often has a high level of Cl because chloride or hydrochloric acid is a common reagent in laboratory experiments.The fly ash and bottom ash generated during the incineration of input materials with high Cl content usually have a high level of PCDD/Fs and involved fly ashes needed final disposal (Gidarakos et al., 2009;Wu et al., 2011).
The present study, divided into two parts, focuses on the PCDD/F emission characteristics of the incineration of laboratory waste.Part 1 discussed the PCDD/F emission profiles during the incineration process obtained using chemical assays and bioassays.Part 2 focuses on the PCDD/F profiles of output materials and the influence of Cl content on PCDD/F composition.In addition, the surface and crystalline characteristics of ashes were investigated.

Processes of Incineration System
The incineration system has a capacity of 120 kg/hr.A process flow diagram is shown in Fig. 1 (taken from part 1).Solid input materials, LSW and LPW, are fed through a feeding unit and organic LLW is injected into the incinerator directly.The primary and secondary combustion chambers are operated at combustion temperatures of 900°C and 1150°C, respectively.The secondary combustion chamber, equipped with a diesel combustor, is used to decompose persistent organics (PCDD/Fs, polycyclic aromatic hydrocarbons (PAHs), and other organic pollutants), carbon monoxide (CO), and unburned matter in the stack flue gas (SFG).The combustion chambers are equipped with a series of APCDs, which are designed to remove air pollutants from the SFG.The first two units are the first and secondary quenching towers, which reduce the SFG temperature immediately to 600°C and 160°C, respectively, to avoid the formation of PCDD/Fs.The quenching mist contains 3% NaOH to remove acidic pollutants (HCl and H 2 SO 4 ).The SFG then passes through baghouse filters with the preinjection of activated carbon to remove PCDD/Fs and particulates.After a finally water scrubbing and reheating to > 100°C (to avoid the formation of white smoke), the SFG is emitted to the ambient air through a stack.
For the present study, incineration tests with three input materials, namely LPW, LLW, and LSW, were conducted to investigate the influence of input materials.Part 1 investigated the PCDD/F emission characteristics during incineration.In part 2, the PCDD/F profiles of ash/flue gas and the influence of Cl on the PCDD/F content of ash are further investigated.

Sampling and PCDD/F Analysis for Flue Gas and Solid Specimens
The sampling process for SFG followed the standard sampling procedure for PCDD/Fs in stack flue gas, NIEA A807.74C (issued by Environmental Analysis Laboratory EPA, Executive Yuan in Taiwan), which is the same as that described in part 1. Solid samples, input materials, bottom ash (BTA), first quenching tower ash (FQA), secondary quenching tower ash (SQA), and baghouse ash (BHA) were collected from the bottom ash pit and APCDs of the incineration system.The solid specimens were dried and then ground for further treatment and analysis.The PCDD/F analysis was conducted following the procedure given in US EPA Modified Methods 23 and 1613, as done in a previous study (Wang and Chang-Chien, 2007).

Preparation and Analysis of Solid Specimens
Solid samples, including BTA, FQA, SQA, and BHA, were all pulverized to the particle size that passed through a 00-mesh sieve (149 µm) to ensure uniform digest efficiency All powdery samples were held in Teflon vessels hermetically with an acid mixture, composed of 1, 5, and 10 mL of hydrofluoric, nitric, and perchloric concentrated acids, respectively.The specimens were digested using a microwave (MARS/MARS Xpress CEM microwave) at 400 W and an 60 kg/cm 2 limit at 200°C for 15 minutes.After cooling, boric acid was added to the digests to remove excess hydrofluoric acid.Then, the digests were diluted to exactly 25 mL, filtrated by a 0.8-µm mixed cellulose ester filter, and analyzed with inductively coupled plasma optical emission spectrometry (ICP-OES, VISTA-MPX, Varian).The concentrations of elements, including As, Ba, Cd, Cr, Cu, Hg, Mn, Na, Ni, Pb, Se, and Zn, were determined.In this experiment, each sample was analyzed three times to ensure good reproducibility.

Examination of Surface and Crystalline Characteristics
Before surface examination, ground solid specimens (particle size smaller than 74 µm) were adhered to a metallic plate and then coated with an Au film using an ion coating sputter system.The coated samples were scanned using field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7000).Scanning electron beams, accelerated with a voltage of 10 kV, were generated by a Schottky-type field-emission electron gun.The sample surfaces were then examined at 10,000x magnification.
X-ray diffraction (XRD, Geigerflex 3063) with Ni-filtered Cu-Kα radiation was applied to determine the crystalline phases of solid specimens.The solid specimens were ground to a particle size of <74 µm and analyzed by XRD in the 2θ range of 10 to 60° with an angular speed of 4°/min.The XRD analysis results were compared with the intensities and positions of Bragg peaks listed in the Joint Committee on Powder Diffraction Standards data files to identify the crystalline phases in the solid specimens.

Measurement of Cl Content in Solid Specimens
An elemental analyzer (multi EA ® 5000 Analytik Jena) was applied to determine the total Cl amount in solid specimens.For ashes, Cl mostly existed in chloride form, which is often highly soluble, and thus the amount of soluble Cl was measured using the following procedure.The specimens (mass: 2 g) were extracted using 40 mL of deionized water with the aid of supersonic vibration for 2 hr in triplicate at least.The extracts were filtered by cellulose acetate filters and analyzed using ion chromatography (IC, DIONEX ICS-900).The Cl concentration for the final extraction was all lower than 5% of the total Cl concentration in previous extractions to ensure that the extraction efficiency for soluble Cl was higher than 95%.The ratio of soluble Cl to total Cl (S/T Cl ratio) was then determined to identify the characteristics of Cl in solid specimens.

Cl behaviors during Incineration Process
Table 1 (PCDD/F data taken from part 1) shows the composition and characteristics of the three input materials.The PCDD/F content of the input materials was about 0.1 ng I-TEQ/g (I-TEQ: international toxicity equivalent) or lower.The total Cl level of LSW (114,000 mg/kg) was much higher than those of LPW and LLW.The input materials were divided into low-Cl-content input materials (in runs 1 and 2) and high-Cl-content input materials (in run 3) for further discussion to elucidate the influence of Cl on the PCDD/F content of ash during the incineration process.The S/T Cl ratios were all approximately 0.1 or lower, indicating that Cl primarily existed in non-water-soluble form.The metal composition shows that the hazardous metal content levels were all lower than 10,000 mg/kg.Table 2 shows the metal composition and Cl content of ash.For BTA, the main metals were Na (30,200 and 28,400 mg/kg for LPW and LSW, respectively), Cu (2,320 and 5,800 mg/kg for LPW and LSW, respectively), Ni (2,710 and 2,880 mg/kg for LPW and LSW, respectively), and Cr (2,170 and 1,740 mg/kg for LPW and LSW, respectively).The main metal species in all fly ashes was Na, which was from the injection of quenching water (3% NaOH solution).For FQA, SQA, and BHA, the Na content ranged from 247,000 to 305,000 mg/kg, 199,000 to 278,000 mg/kg, and 73,200 to 208,000 mg/kg, respectively.Other metals were detected in trace amounts compared to that of Na.The total Cl content of ash was reasonably proportional to that of the input materials.The Cl levels of FQA were the highest among the ashes (that in run 3 was as high as 398,000 mg/kg).SQA and BHA had the next highest levels (333,000 and 318,000 mg/kg, respectively).The Cl levels of BTA were the lowest among the ashes in each run.This is probably due to chloride being vaporized into flue gas instead of staying in the residue during the incineration process (Abanades et al., 2002).
The S/T Cl ratios in the solid specimens show different patterns.For BTA, the S/T Cl ratios were 0.410 and 0.552, which were the lowest among the ashes.The S/T Cl ratios of FQA ranged from 0.881 to 0.945, which were the highest.The S/T Cl ratios of SQA and BHA ranged from 0.759 to 0.820 and 0.597 to 0.717, respectively.The patterns of S/T Cl ratios can be explained as follows.During the incineration process, the organic Cl in the waste might transform into metal chloride, which has a high boiling point, or unburned Cl-bearing residuals, which are not water soluble.These materials stayed in BTA, decreasing its S/T Cl ratios.During the incineration of waste containing Cl, organic Cl tends to form HCl via a reaction of H ions (Takasuga et al., 2007).HCl then vaporizes into the flue gas and reacts with the NaOH of quenching water to form NaCl. The formation of NaCl made the S/T Cl ratios for FQA the highest.After the removal of NaCl by the first quenching tower, the S/T Cl ratios in SQA reasonably decreased.The secondary quenching tower conducts a secondary scrubbing of NaCl, and thus further decreased the S/T Cl ratios in BHA.
Table 3 shows the total Cl mass distribution among the ashes.For low-Cl-content input materials, the Cl distributions were similar.For fly ash, the total Cl mass existed in FQA, BHA, and SQA, sequentially from high to low fractions.The mass fraction ratios among these ashes were roughly equivalent in the two runs.For run 3, the Cl mass was predominately distributed in BTA (36.3%), equally partitioned into FQA and BHA (~29%), and the residual fraction (5.8%) resided in SQA.

PCDD/F Content and Profile of Output Materials
Fig. 2 shows the PCDD/F content of ash for various Cl content levels of input materials and ash.Fig. 2(a) indicates that the PCDD/F content of ash (for a given type of ash) with high-Cl-content input materials is much higher than that with low-Cl-content input materials.According to Fig. 2(b), no notable relationship was observed between the Cl content and PCDD/F content of ash.Previous studies reported  that PCDD/Fs mainly form in the temperature range of 250 to 350°C, which is the case in the first quenching towers (Hajizadeh et al., 2011;Jansson and Andersson, 2011).According to our previous study, metals which vaporized as the particulate phase reacted with Cl to form chloride in flue gas (Chang et al., 2012).During this stage, metal chloride plays an important role in catalyzing the formation of PCDD/Fs (Lu et al., 2007;Nganai et al., 2011).Therefore, the PCDD/F content of ash increased with increasing Cl level in the input materials, which was also reported in a previous study (Hatanaka et al., 2005).
In addition, a previous study reported that 40-50% of PCDD/Fs were removed by a scrubbing process, which agrees with the results given in part 1 of the present study (Hunsinger et al., 1998).Thus, the residual fraction of PCDD/Fs was mostly adsorbed in BHA.From the results shown in Table 3, the fraction of Cl residing in BTA and collected in quenching ash accounted for >70% of Cl.The Cl level in BHA was thus lower than that in quenching ash.
Therefore, BHA with a low Cl level had a high PCDD/F level.As previously mentioned, the formation of PCDD/Fs mainly occurred in the first quenching tower.However, quenching processes only removed about 40-50% of PCDD/Fs but 60-80% of Cl, and the residual fractions of PCDD/Fs and Cl gathered in BHA.Therefore, BHA had lower Cl content but higher PCDD/F content compared with those of quenching ash.

Surface and Crystalline Characteristics of Ashes
Figs. 6 and 7 show SEM images and XRD patterns, espectively, of BTA, FQA, SQA, and BHA.The major crystalline phases of BTA were NaCl and Na 2 SO 4 .The crystalline structure with sharp edges agrees with the results of XRD analysis.FQA and SQA had a porous granular structure.The XRD analysis indicates that the crystalline phases were NaCl, which agrees with the high S/T Cl ratios in FQA and SQA.
The salts formed by neutralization are very fine, as shown in Figs.6(b) and 6(c).In addition, NaOH may react with Ca ions to form Ca(OH) 2 , which further reacts with CO 2 in SFG.Thus, CaCO 3 (see XRD analysis results in Fig. 7(c)) formed.This crystalline phase has often been found in municipal solid waste fly ash (Kuo et al., 2012).
The major crystalline phase of BHA was also NaCl, but the level was much lower than that in quenching ash (Table 2).The NaCl was the residual fraction which passed through quenching towers and collected in baghouse filters.In addition to the porous granular structure, shown in Figs.6(b) and 6(c), polyhedra with sharp edges appeared.This may be the injected activated carbon or other particulates in SFG.

CONCLUSION
This study investigated the behavior of Cl during the incineration process of laboratory waste.The total Cl level

Fig. 2 .
Fig. 2. Effect of Cl content of materials on PCDD/F content: (a) input materials and (b) ashes.

Table 1 .
Characteristics of input materials in three runs.

Table 2 .
Metal and Cl content of output materials (unit: mg/kg).