PAH emission from the incineration of three plastic wastes 

Environment International v.27, i.1,  Jul01

Chun-Teh Li, Huan-Kai Zhuang, Lien-Te Hsieh, Wen-Jhy Lee and Meng-Chun Tsao
Department of Environment Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC
Received 8 May 2000; accepted 10 April 2001 Available online 27 June 2001.

Abstract

A batch-type, controlled-air incinerator was used for the treatment of polyvinyl chloride (PVC), high-density polyethylene (HDPE), and polypropylene (PP) plastic wastes. The concentration and composition of 21 individual polycyclic aromatic hydrocarbons (PAHs) in the raw wastes, flue gas (gas and particle phases), and ash were determined. Stack flue-gas samples were collected by a PAH stack-sampling system. Twenty-one individual PAHs were analyzed primarily by a gas chromatograph/mass spectrometer (GC/MS). The CO concentration correlated well with the total PAH (R2>.89), and thus can be used as a surrogate indicator for PAH emission. Excess amounts of air supply in the incineration of plastic wastes could decrease not only the concentration of the PAHs in the bottom ash but also the emission factor (EF) of the total PAH in the stack flue gas. Of the three plastic wastes, HDPE was found to have the highest mean EF of the total PAHs (462.3 mg/kg waste) from the stack flue gas. Incinerating PVC would result in a higher EF of PAHs (195.4 mg/kg waste) in the bottom ash. When PVC plastic wastes were incinerated, higher-ringed PAHs constituted a larger percentage in the bottom ash as compared to those from PP and HDPE plastics. By judging the output and input (O/I) ratio of the PAHs from the incineration trials of plastic wastes, the PAHs involved in incineration of three plastic wastes were almost entirely destroyed; and a low residual amount between 0.00018 and 0.00032 remained in the emission.

Author Keywords: PAHs; Emission factor; Plastic wastes; Incineration; Flue gas

1. Introduction

In recent decades, due to the worldwide use of plastic products, plastic wastes have come to be a major component of both industrial and municipal wastes. According to the estimation of investigations, the generation rate of plastic wastes is over 1,700,000 ton/year in Taiwan (U.S. EPA, 1991). In many countries, incineration is a traditional and available method in reducing the different plastic wastes. In view of both reduction and destruction, incineration is a valuable means of waste disposal with the advantage of being highly effective in reducing the volume of waste. Thus, the method or technology regarding incineration is becoming a more widespread concern. During the incineration process, air pollutants such as CO, NOx, SOx, particles, and polycyclic aromatic hydrocarbons (PAHs) are exhausted. Understanding the characteristics of both formation and emission of PAHs is necessary because it is proved that some PAHs are carcinogenic (Doll and Speizer). PAHs and their derivatives are widespread harmful compounds generated by incomplete combustion of organic material arising, in part, from natural combustion such as forest fires and volcanic eruptions, but for the most important part, from human activities (Bjorseth; Benner and Baek), such as industrial production, transportation, waste incineration, and so on.

Tuominen et al. (1988) found that the major factor of effecting PAH concentration in the city atmosphere was transportation exhaust. Masclet et al. (1986) showed that higher molecular weight PAHs (such as, fluoranthene, pyrene, coronene, and so on) were contributed mainly by mobile sources. Incineration of wastes is also a major source of PAHs exhausted. Kamiya and Ose (1987) investigated the mutagenic activity of fly ash and emission gases from municipal waste incinerators and found that mutagens, equivalent to 1700-3000 cars, were discharged from the incinerator into the environment. Chiang et al. (1992) investigated the incineration of typical urban solid wastes containing plastic components. Their results showed that the incineration temperature controlled in the combustion zone and gasification zone exerted marked effects on the species of PAHs in the particulate samples. Wheatley et al. (1993) studied that PAHs were exhausted from the combustion of selected municipal plastic wastes. They found that the changes in PAH emission were mainly attributable to the postcombustion gas temperature and residence time. In addition, their results indicated that the minimum emission of hydrocarbons was achieved at the highest gas temperature (1150°C) and residence time (2 s).

The main objective of this study was to investigate the correlation between air supply and PAH emission, the comparison of emission factor (EF) of three plastic wastes, and the output/input (O/I) mass ratios of these 21 individual PAHs from incineration trials of three kinds of plastic wastes. This information is not only required for PAH control, but is also useful for the impact assessment on both ambient air quality and health.

2. Materials and methods

2.1. Incineration system

The incineration of plastic wastes was investigated by using a solid grate incinerator. This incinerator was produced and installed by Taiwan Lonesun, Taipei, Taiwan. It was equipped with a primary and a secondary combustion chamber, a stack, a control panel, an air-supply device, and an auxiliary fuel feeding system. The wastes were burned batchwise (5 kg/run) by being fed manually into the combustion chamber. The volumes of the primary and secondary combustion chambers were 0.34 and 0.68 m3, respectively. In general, the residence time of air flow going through the secondary combustion chamber was between 0.8 and 1.5 s and is averaged at 1.1 s. In this study, seven runs of the incineration experiment were investigated. During the incineration of high-density polyethylene (HDPE) and polypropylene (PP), the operational temperature was controlled at 450°C and 850°C for the first and second combustion chambers, respectively. During the incineration of polyvinyl chloride (PVC), the operational temperature was controlled at 400°C and 850°C for the first and second combustion chambers, respectively. The excess air ratios of these experimental runs were between 1.0 and 1.8.

2.2. PAH sampling system (PSS) for stack flue gas

After draft design, the PSS for flue gas was produced and installed by Bay-Ming, Taipei, Taiwan. This PSS is based on the modification of the U.S. EPA's sampling method 5 (MM5) (40CFR60) by Graseby (U.S. EPA, 1996). This PSS was equipped with a sampling probe, a cooling device, a glass cartridge, a pump, a flow meter, and a control computer. The flue gas was sampled isokinetically from the stack. In this study, the operational flow rates of the PSS were between 4.5 and 5.5 l/min and were averaged at 5.0 l/min.

A PSS with a tube-type glass fiber filter (cleaned by distilled deionized water and heated to 450°C) was used to collect particulate and particle-phase PAHs. A glass cartridge packed with XAD-16 resin and supported by a polyurethane foam (PUF) plug was used to collect the phase PAHs. After each sampling cycle, the sampling train was rinsed with n-hexane (Yang, 1998).

The glass fiber filters were weighed before and after sampling to determine the amount of particles collected. PUF plugs and resin were always stored and transported in clean screw-capped jars with Teflon cap liners. Glass fiber filters were transported to and from the field in a prebaked glass bottle and were wrapped with aluminum foil.

Breakthrough tests were investigated by three stages of XAD-16/PUF cartridge. Each stage of XAD-16 resin was analyzed individually and compared for the PAH mass collected in each stage. Three breakthrough tests were investigated in this study and no significant PAH mass was found to be collected in the third stage of XAD-16 resin. All the experiments were repeated to make sure that the results were reproducible (Yang, 1998).

2.3. Sampling from ambient air

In order to calculate the O/I mass ratios of PAHs, both the particle and gas phases of PAHs in the ambient air were collected on the day of incinerator operation by using a standard semivolatile sampling train (General Metal Works PS-1). Two sets of PS-1 samplers were operated simultaneously for 24 h during the sampling periods to collect the PAHs. Besides, sets of 24-h samples have been determined in this study. Before sampling, the PS-1 sampler with a glass filter and puff cartridge was pretreated according to the same procedure used for the PSS.

2.4. Plastic waste and ash samples

The plastic waste samples bought from a plastic shop in Tainan City were used for this study. Information about plastic waste is shown in Table 1 and Table 2. After weighing, 20 g of plastic waste and ash samples were placed in an oven (104-105°C, respectively) for the measurement of moisture content and another group of 20-g samples were extracted directly for the PAH analysis. Therefore, the measured PAH composition in the plastic waste and ash samples, respectively, is reported on the basis of dry weight.

Table 1. Components of three plastic wastes and liquid diesel (not included here)

Table 2. Element composition and heat content of three plastic wastes and liquid diesel (not included here)

2.5. Liquid diesel sample

The auxiliary fuel used for the incineration was liquid diesel (specific GRAVITY=0.80; heating VALUE=10,948 kcal/kg), which was bought from a gas station in Tainan City, Taiwan. Information about the diesel is shown in Table 2. A 10-ml sample of liquid diesel was cleaned up and reconcentrated for the PAH analysis.

2.6. PAH analysis

After final weighing (if needed), the PAH sample was placed in a solvent solution (the mixture of n-hexane and dichloromethane, vol:vol=500:500 ml, respectively), and extracted in a Soxhlet extractor for 24 h. The extract was then concentrated, cleaned up, and reconcentrated to exactly 1.0 or 0.5 ml. (Manchester and Yang).

A gas chromatograph (GC) (Hewlett-Packard 5890A) with a mass selective detector (MSD) (Hewlett-Packard 5972), and a computer workstation was used for the PAH analysis. This GC/MS (mass spectrometer) was equipped with a Hewlett-Packard capillary column (HP Ultra 2 -- 50 m×0.32 mm×0.17 m), an HP-7673A automatic sampler, with injection volume at 1 l, splitless injection at 310°C, ion source temperature at 310°C, and an oven with ranges of 50-100°C at 20°C/min and 100-290°C at 3°C/min, held at 290°C for 40 min. The masses of primary and secondary ions of PAHs were determined using the scan mode for pure PAH standards. Qualification of PAHs was performed using the selected ion-monitoring (SIM) mode.

The concentrations of the following PAHs were determined: naphthalene (Nap), acenaphthylene (AcPy), acenaphthene (Acp), fluorene (Flu), phenanthrene (PA), anthracene (Ant), fluoranthene (FL), pyrene (Pyr), cyclopenta[c,d]pyrene (CYC), benz[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), perylene (PER), indeno[1,2,3,-c,d]pyrene (IND), dibenzo[a,h]anthracene (DBA), benzo[b]chrycene (BbC), benzo[ghi]perylene (BghiP) and coronene (COR).

PAH recovery efficiencies varied between 0.75 and 1.12 and were averaged at 0.845. The blank tests for PAHs were accomplished by using the same procedure as the recovery-efficiency tests without adding the known standard solution before extraction. Analyses of field blanks, including the glass fiber filter and PUF/XAD-16 cartridge found no significant contamination (GC/MS integrated area<detection limit).

2.7. Trial sampling

Prior to formal sampling, several trial burns were investigated to obtain optimum waste-feed amounts (kilograms), air-supply rates, duration time of incineration, and the sampling flow rate of PSS.

3. Results and discussion

3.1. Correlation between CO and total PAH concentration in the stack flue gas

In general, the CO and CO2 concentration can be used as a surrogate indicator for combustion efficiency. The formation of PAH is associated with incomplete combustion. Hence, the correlation between CO and total PAH concentration in the stack flue gas is worth discussing. Several previous investigations have indicated that the CO concentration does not correlate well with the total PAH concentration in the stack flue gas (Frenklach, 1990). However, the results in this study present a positive correlation. As shown in Fig. 1, three R2 values of the correlation between CO and the total PAH concentration were .93, .96, and .89 for the case of incinerating PVC, PP, and HDPE wastes, respectively. Although the incineration results from three different plastic wastes were not the same, three relative trends of the total PAH concentration (g/nm3) vs. CO concentration (ppm) were similar (Fig. 1A-C). With increasing CO concentration, the total PAH concentration increases linearly in the case of incinerating PVC, PP, and HDPE wastes. The trend in this study is consistent with the results reported by Li et al. (1995). Since the increase of CO concentration occurs with incomplete combustion, the total PAH concentration rises increasingly in the stack flue gas, especially from incineration of PVC during the experiment process.

Fig. 1. The correlation between CO and the total PAH concentration in the stack flue gas by incinerating PVC, PP, and HDPE wastes, respectively. (not included here)

3.2. Correlation between excess air ratio and PAH composition in the bottom ash

The good correlation between excess air ratio and PAH composition in the bottom ash by incinerating PVC, PP, and HDPE wastes, respectively, are shown in Fig. 2A-C. The relative trends of three plastic wastes were similar, especially the comparison between PP and PVC with the approximate PAH content level (1.92-2.16 g/g bottom ash). The R2 values of the three plastic wastes were .98, .93, and .91 for PVC, PP, and HDPE, respectively. Compared with PP and PVC, the trend of HDPE plastics was milder. This result indicated that increased air supply would reduce the PAHs in the bottom ash; however, this drop was not as evident as the effect found in the emission exhaust.

Fig. 2. The correlation between the excess air ratio and the PAH content in the bottom ash by incinerating PVC, PP, and HDPE wastes, respectively.  (not included here)

3.3. Correlation between excess air ratio and EF of the total PAH in the stack flue gas

The correlation between excess air ratio and EF of the total PAH in the stack flue gas is shown in Fig. 3. When the excess air ratio increased, the mean EF of the total PAH decreased linearly. In a comparison of the linear regression in the three different plastic wastes (Fig. 3A-C), PVC is the best. The R2 values of PVC, HDPE, and PP were .97, .88, and .71, respectively. The results also suggested that by providing more air supply, the EF of the total PAH in the stack flue gas could be reduced.

Fig. 3. The correlation between the excess air ratio and the EF of the total PAH in the stack flue gas by incinerating PVC, PP, and HDPE wastes, respectively.  (not included here)

3.4. Comparison of EF of the total PAH among three plastic wastes

Fig. 4A and B shows the mean EFs of the total PAH in the stack flue gas were 320.5, 315.3, and 462.3 mg/kg waste for PVC, PP, and HDPE, respectively. The mean EF of the total PAH from the stack flue gas for HDPE was 1.5 times higher than that for PP and PVC. In the bottom ash, the mean EF of the total PAH was 195.4, 45.2, and 71.4 mg/kg waste for PVC, PP, and HDPE, respectively. The mean EF of the total PAH from the bottom ash for PVC is 4.33. and is 2.74 times higher than that from the bottom ash for PP and HDPE, respectively. This is due to the fact that incinerating PVC needed the lower combustion temperature.

Fig. 4. The comparison of total PAH EFs among incinerating three plastic wastes.  (not included here)

3.5. Distribution of PAH output mass 

The PAH mass discharged from the incinerator includes the mass from stack flue gas and that from ash residue. In this study, the mean output mass fraction of the total PAH was 78.8% and 21.2%, discharged from the stack flue gas and ash residue, respectively. The total PAH mass was emitted mainly by the stack flue gas. Fig. 5 shows the distribution of individual PAH mean output mass fractions by incinerating PVC, PP, and HDPE, respectively. Lower molecular weight PAHs -- Nap, AcPy, Acp, and Flu -- had dominant fractions of their mass discharged by the stack flue gas. Furthermore, it is clear that the distribution of the individual PAHs exhausted by incinerating PP and HDPE wastes was similar. It is associated with the fact that PP and HDPE plastic wastes were similar in both physical and chemical properties. When PVC plastic wastes were incinerated, higher-ringed PAHs constituted a larger percentage in the bottom ash as compared to those from PP and HDPE plastics. This is exactly the characteristic of PAHs exhausted.

Fig. 5. The distribution of individual PAH output mass fractions by incinerating PVC, PP, and HDPE wastes, respectively.  (not included here)

3.6. PAH O/I mass ratio

Table 3 shows the PAH O/I mass ratios from the incineration trials of three kinds of plastic wastes. The total PAH O/I mass ratios were between 0.00018 and 0.00032 and were averaged at 0.00024. During incineration of PP plastics, the BbF O/I mass ratio was the highest -- 0.05035. However, it was still quite low. As a whole, most of the PAHs have been destroyed during the incineration of the three plastic wastes, and the remains of PAHs that exist in the emission became rarer (Table 3). This is associated with the fact that the three plastic wastes are just suitable for incineration and the level of PAH content in those plastic wastes is not high. In this study, the result was similar to that from previous measurements (Bjørseth and Ramdahl, 1985).

Table 3. The input and output ratio of the PAHs from the incineration of three kinds of plastic wastes  (not included here)

4. Conclusions

(1) The result indicated that the CO concentration correlated well with the total PAH (R2>.89), and thus can be used a surrogate indicator for PAH emission. Despite the difference in the incinerating results of the three plastic wastes, the relative trend of the total emission concentration of PAH vs. CO concentration was similar.

(2) Excess amounts of air supply in the incineration of plastic wastes could decrease not only the concentration of the PAHs in the bottom ash but also the EF of the total PAH in the stack flue gas. The relationship between the amounts of PAH in the bottom ash of PP plastics and PVC plastics and the excess air ratios was similar. In HDPE plastics, the trend was milder.

(3) Of the three plastic wastes, HDPE was found to have the highest EF (462.3 mg/kg waste) of the total PAH from the stack flue gas. Incinerating PVC would result in a higher EF (195.4 mg/kg waste) of PAH in the bottom ash.

(4) Lower molecular weight PAHs -- Nap, AcPy, Acp, and Flu -- had dominant fractions of their mass discharged by the stack flue gas. The distribution of the individual PAH exhausted by incinerating PP and HDPE wastes were similar. When PVC plastics were incinerated, higher-ringed PAHs constituted a larger percentage in the bottom ash as compared to those from PP and HDPE plastics.

(5) By judging the O/I ratio of the PAH from incineration trials of plastic wastes, the PAHs of incineration of three plastic wastes were almost entirely destroyed, leaving a low residual amount, between 0.00018 and 0.00032, in the emission. The key points are that the plastic wastes themselves are suitable for incineration and the PAH content in the plastic wastes is not high.

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