Environ. Sci. Technol. 2006, 40, 2247-2253
Formation and Control of Toxic Polychlorinated Compounds during Incineration of Wastes Containing Polychlorinated Naphthalenes S H I N - I C H I S A K A I , * ,† TAKASHI YAMAMOTO,‡ YUKIO NOMA,‡ AND ROBERT GIRAUD§ Environment Preservation Center, Kyoto University, Yoshidahon-machi, Sakyo-ku, 606-8501 Kyoto, Japan, National Institute for Environmental Studies (NIES), 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan, and DuPont Engineering Technology, Wilmington, Delaware 19898
To estimate the potential impact on municipal solid waste (MSW) incinerator toxic equivalent (TEQ) emissions of treating wastes containing polychlorinated naphthalenes (PCNs), pilot-scale thermal treatment experiments were conducted. MSW (run 1) and MSW fortified with synthetic rubber belts containing PCNs (runs 2 and 3) were incinerated. Flue-gas and ash samples were analyzed for polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), coplanar polychlorinated biphenyls (co-PCBs), and PCNs. Final exhaust-gas WHO-TEQ emissions were all less than 0.1 ng/Nm3. Flue-gas TEQs were mainly from PCDFs (58-74%). When 2,3,7,8-tetrachlorodibenzo-p-dioxin relative potency factors (REPs) of specific PCN congeners from previous reports were used as estimated toxic equivalency factors to compute estimated PCN TEQs and total TEQs along with PCDDs, PCDFs, and co-PCBs, the contributions of PCNs to the total TEQs were small in ash samples and up to 28% in final exhaust gas. The TEQs in primary combustion flue gases increased through the formation of dioxins and PCNs and then decreased via secondary combustion, fabric filtration, and activated carbon adsorption. From this pilot-scale study, the incremental impact of incinerating PCN-containing wastes on annual TEQ emissions in Japan is estimated as 0.27 g of total TEQ.
Introduction Polychlorinated naphthalenes (PCNs) consist of 75 congeners containing between one and eight chlorine atoms per naphthalene molecule. The chemical properties of PCNs are quite similar to those of polychlorinated biphenyls (PCBs). PCN formulations such as Halowaxes were once produced and used for many applications such as capacitor impregnants, cable insulation, wood preservatives, carriers in dye production, machine oil additives, rubber product additives, and flame retardants (1, 2). However, the production and use of PCNs were phased out in many countries because of their toxicities (1-3) or contamination to the environment (4-6) or human tissues (7, 8). * Corresponding author phone: +81-75-753-7706; fax: +81-75753-7710; e-mail:
[email protected]. † Kyoto University. ‡ National Institute for Environmental Studies (NIES) § DuPont Engineering Technology. 10.1021/es052156a CCC: $33.50 Published on Web 03/07/2006
2006 American Chemical Society
Although PCNs were banned from import and manufacture in Japan beginning in 1979 (9), PCN wax was accidentally imported to manufacture Neoprene FB polymer (a specialty grade of chlorinated synthetic rubber) in recent years (10, 11). Approximately 25 t of Neoprene FB was used in the production of Neoprene FB belts and other finished products for the Japanese market during 1999-2001 prior to the end of Neoprene FB production and distribution. The annual average production of Neoprene FB belts in Japan was 200 t during this time. Yamashita et al. found PCNs in certain synthetic rubber belt products used in OA (office automation) products such as printers, confirming the use of Neoprene FB belts in such applications (11). These finished Neoprene FB products are destined to be discarded as municipal solid wastes (MSW) at the end of their useful lives. Because about 78% of the MSW in Japan is incinerated (12), most of such end-of-life rubber products containing PCNs will be or have been incinerated. Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), coplanar polychlorinated biphenyls (co-PCBs), and PCNs are well-known byproducts formed in thermal processes (13-19). In Japan, annual toxic equivalent (TEQ) emissions of PCDDs, PCDFs, and the 12 co-PCBs (collectively known as WHO-TEQ) from MSW incinerators were estimated as 5000 g of TEQ/year in 1997 (20). Because of the remarkable success of recent countermeasures to reduce dioxin emissions [including combustion control requirements (21)], estimated WHO-TEQ emissions from MSW incinerators in Japan were 71 g of TEQ/year in 2003 (20). Although there are many reports about the formation of PCNs during thermal processes, potential influences caused by the loading of wastes containing PCNs have not been described prior to those related to this study (22, 23). Recent studies have investigated the dioxin-like toxicity of PCNs, evaluating ethoxyresolfin-O-deethylase (EROD) activities or arylhydrocarbon- (Ah-) receptor-mediated effects via in vitro bioassays to determine potencies of specific PCN congeners relative to 2,3,7,8-tetrachlorodibenzo-pdioxin (2,3,7,8-TeCDD) (24-27). Several congeners such as 1,2,3,4,6,7-HxCN, 1,2,3,5,6,7-HxCN, and 1,2,3,4,5,6,7-HpCN have relatively higher relative potencies (REPs). Although these congeners are often found in incineration-related sources such as fly ash, their concentrations are rather small in PCN formulations (28, 29). Thus, congeners formed by thermal processes might be important for evaluating toxicities of PCNs in the environment. Therefore, we carried out thermal treatment experiments to investigate the formation, decomposition, and emission control performance of PCNs, PCDDs, PCDFs and co-PCBs during the MSW incineration of wastes containing PCNs. The overall formation and decomposition behaviors of PCNs, PCDDs, PCDFs, and co-PCBs are discussed in other reports (22, 23). In this article, we describe the behavior of specific toxic polychlorinated compounds at various stages of the thermal treatment process during the incineration of wastes containing PCNs and estimate the composition and distribution of contributors to TEQ emission and the incremental impact on annual TEQ emissions from MSW incineration in Japan.
Experimental Section Materials. Representative Neoprene FB finished products likely to be incinerated at the end of their useful lives, i.e., synchronous belts produced with Neoprene FB polymer (FB belts), were the primary PCN sources in these experiments. VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Experimental Conditions of Incineration Experiments run 1
run 2
run 3
MSW with typical level of FB belts 10000
MSW with high level of FB belts 10000
combustion sample
MSW
total MSW volume (g) MSW size (mm) MSW feed rate (kg/h) FB belt size (mm) FB belt addition to MSW (mg/kg) assumed PCN concentration (ng/g) temperature (°C) primary combustion secondary combustion outlet of secondary combustion gas flow rate (Nm3/h) air flow rate (m3/h) flue gas at bag exit O2 (%) CO (ppm) NOx (ppm) acid gas treatment HCl before (ppm) HCl after (ppm) total residues bottom ash (g) fly ash (g) sample volume kiln exit gas (Nm3) bag filter entry (Nm3) bag filter exit (Nm3) final exit gas (Nm3)
10000 8(d) × 25 2.5 2.5 3 × 1.5 5
2.5 3 × 1.5 150
34
48
454
840 900 764
840 900 748
838 900 755
40 22
40 22
40 22
10.3 0 43 NaHCO3 150 final exit gas > fly ash, presumably because of the carbon adsorption performance issue noted above. The compositions of the total TEQs across feed samples, flue-gas samples and incineration residues are shown in Figure 2. In feed samples, the majority of the total TEQ was from PCDFs (59-63%), and the contributions of PCDDs and co-PCBs were each less than 20%. The contributions of PCNs to the total TEQs in the feed samples were small in runs 1 and 2, but somewhat larger in run 3 (8.3%) because of the high mixing ratio of FB belt to MSW. The TEQ in the flue-gas samples and incineration residues was also mainly from PCDFs (58-74%). PCDDs contributed 24-36% of the total TEQs in the flue-gas and ash samples, except for the final exit-gas sample in run 1 (4.9%). Although the contribution of PCNs to the total TEQs in the final exit-gas samples was 1.4-28%, the contributions of PCNs to the total TEQs in the kiln exit flue gas and incineration residues were even smaller (below 1%). The contributions of co-PCBs to the total TEQs were also small. Previously, we examined mass balances of PCDDs, PCDFs, and co-PCBs across full-scale MSW incineration facilities in Kyoto, Japan (16, 32). The older facility had stoker-type furnaces and electrostatic precipitators for duct collection and was built in prior to 1980. The newer facility (constructed
in the late 1990s) had stoker-type furnaces equipped with a secondary combustion chamber, a bag filter, and a catalytic denitrification system. The overall TEQ inputs were 1.5 and 1.74-3.17 µg of TEQ per metric ton of waste for the older and newer facilities, respectively. The overall TEQ outputs were 220 and 11.7 µg of TEQ per metric ton of waste for the older and newer facilities, respectively. Comparing the experimental results obtained in this study with the previously reported values shows that the overall input of total TEQs (16.8-17.9 µg of TEQ per metric ton of waste) here was about 10 times higher than that of a full-scale MSW incinerator. However, the overall output of total TEQs (6.49-11.5 µg of TEQ per metric ton of waste) here was about the same as that of the newer facility and about 10 times smaller than that of the older facility, commensurate with dioxin reductions achieved through techniques such as secondary combustion employed in the thermal treatment plant tested in this study and in modern full-scale MSW incinerators in Japan. Profile and Concentration Changes of Dioxin TEQs in the Incineration Process. Changes in the amount of TEQs for some PCDD, PCDF, and co-PCB congeners are shown in Figure 1 (lower left column). In the feed sample, 2,3,4,7,8PeCDF and 3,3′,4,4′,5-PeCB were the predominant TEQ contributors across the three experiments. In flue gases at the kiln exit, 2,3,4,7,8-PeCDF, 2,3,7,8-TeCDD, and 1,2,3,7,8PeCDD were the predominant TEQ contributors, whereas the contribution of 3,3′,4,4′,5-PeCB to the total TEQ decreased relative to the feed sample. On a TEQ basis, 2,3,4,7,8-PeCDF and 2,3,7,8-TeCDD in the kiln exit flue gas increased several hundred-fold compared to the feed samples in the three experiments, indicating their formation during kiln combustion. The increase in the amount of TEQs contributed by 3,3′,4,4′,5-PeCB in the kiln exit gas was about 10% of the TEQ amount contributed by PCDD/PCDF congeners. Although 2,3,4,7,8-PeCDF was the largest contributor to the TEQs in the final exit flue gas for each of the runs, the final exit flue-gas congener patterns in the three experiments did not resemble each other. In run 1, 2,3,7,8-TeCDF and 2,3,4,6,7,8-HxCDF were the other predominant TEQ contributors. In run 2, 2,3,7,8-TeCDD and 1,2,3,7,8-PeCDD were the other predominant contributors. In run 3, the other predominant contributors to the final exit flue-gas TEQs were 2,3,7,8-TeCDD and 2,3,7,8-TeCDF. In general, congeners with higher TEFs contributed more to the flue-gas TEQs at each stage of the thermal treatment process. Congener Profiles and Changes of PCN TEQs in the Incineration Process. Profiles of toxic (dioxin-like) PCN congeners in the feed sample, the flue-gas samples, and the incineration residues from run 3 are shown in Figure 3. 1,2,5,8-/1,2,6,8-TeCNs (nos. 38/40) and 1,2,3,5,7,8-HxCN (no. 69) were the predominant contributors to the estimated PCN TEQs in the run-3 feed sample; these are considered to be of formulation origin (Table 2). The formation of PCN congeners of combustion origin (17) during kiln combustion is evident from the dominant contribution of nos. 66/67 and 1,2,3,6,7-PeCN (no. 54) to the estimated PCN TEQs in the kiln exit flue-gas sample from this run. In the final exit gas, no. 73 was the predominant PCN TEQ contributor, followed by no. 66/67. In fly-ash samples, no. 73 was the predominant contributor except in run 1. In run 1, nos. 66/67 were dominant, followed by no. 73. In bottom-ash samples, the congener patterns were similar to those of kiln exit-gas samples. Changes in the estimated TEQ amounts for some PCN congeners are also shown in Figure 1 (right column). The estimated TEQ amounts of combustion mark congeners nos. 66/67 and 54 in the kiln exit flue gas increased several hundred times above their respective estimated TEQ amounts in the feed samples in the three experiments. In runs 2 and 3, where VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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from full-scale MSW incinerators are not available, the potential impact of treating FB belts in MSW incinerators is estimated here using experimental data from this study of treatment in a pilot-scale thermal treatment plant representative of full-scale modern MSW incinerators in Japan. As noted above, the high FB belt mixing ratio in run 3 had a clear effect on the input TEQs of this run (contributing 7% of the total TEQs in the feed), and the atypical performance across the activated carbon adsorption tower and the wet scrubber during run 2 apparently resulted in elevated final exit-gas TEQ emission levels not representative of the overall TEQ behavior in the incineration process during run 2. Hence, run 3 provides the best available basis for estimating the incremental TEQ emission impact of incinerating end-oflife FB belts mixed into MSW. With data from run 3, the incremental impact on annual MSW incinerator TEQ emissions can be estimated. First, the TEQ emissions per unit amount of FB belts for the MSW incinerator was computed to be 0.11 mg of estimated PCN TEQ per metric ton of FB belts or 1.36 mg of total TEQ per metric ton of FB belts, without discounting run 1 TEQ emissions solely related to the incineration of MSW. Second, these values were multiplied by the 200 t per year of FB belts discarded into MSW (23) to calculate the corresponding annual mass of MSW incineration TEQ emissions in Japan as 0.02 g of estimated PCN TEQ/year or 0.27 g of total TEQ/ year. Third, these values were compared to the 71 g of TEQ/ year national WHO-TEQ emission inventory reported (20) for MSW incinerators in Japan in 2003, indicating that the annual estimated PCN TEQs represented 0.03% of the dioxin emission inventory or that annual total TEQs represented 0.38% of the national dioxin emission inventory for MSW incinerators. This estimate is based on experimental results from steady-state incineration of a mixture of RDF and wastes containing PCNs (FB belts) in a pilot-scale thermal treatment facility. The very low average CO levels shown in Table 1 are lower than typical CO levels at MSW incinerators but are generally consistent with the performance of modern MSW incinerators in Japan operated in accordance with combustion control requirements (less than 100 ppm CO) in Japanese regulations (21). Although the potentially wide range in MSW composition across the many MSW incinerators might not be represented by the single RDF sample in this study, testing with a waste mixture containing 30 times the typical level of FB belts in MSW (run 3) appears to adequately consider the potential variability in the main source of PCN incineration input evaluated in this study. Although the facility in this study is much smaller in scale than actual MSW incinerators, it is representative of full-scale MSW incinerators in Japan by design, as evidenced by the system configuration and the TEQ levels in the final exhaust-gas samples. Most MSW incinerators use carbon adsorption, which is the same as in this study. Furthermore, even those MSW incinerators that do not have separate secondary combustion chambers do effectively have some secondary combustion volume above the grate (stoker) or above the bed (FBI).
Literature Cited FIGURE 3. Profiles of dioxin-like PCN congeners in feed sample, flue gases, and incineration residues in run 3. FB belts were mixed into the feed, the TEQ amounts of nos. 38/40 (formulation mark congeners) in the kiln exit gas decreased from their respective estimated TEQ amounts in the feed. These reductions demonstrate the decomposition of input congeners by primary combustion in the rotary kiln. Estimation of Annual TEQ Emissions from the Incineration of Wastes Containing PCNs. Because relevant data 2252
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Received for review October 28, 2005. Revised manuscript received January 5, 2006. Accepted January 25, 2006. ES052156A
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