Environ. Sci. Technol. 2000, 34, 3920-3924
Formation of PCDD/Fs in Artificial Solid Waste Incineration in a Laboratory-Scale Fluidized-Bed Reactor: Influence of Contents and Forms of Chlorine Sources in High-Temperature Combustion TAKESHI HATANAKA,* TAKASHI IMAGAWA, AND MASAO TAKEUCHI National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba-shi, Ibaraki Japan 305-8569
The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) has been examined to investigate the influence of organic and inorganic chlorine sources and their contents in municipal solid waste incineration. A laboratory-scale fluidized-bed reactor with electric heating was used to control combustion condition identically. Combustion temperature was set to 900 °C, and the amount of air supplied was twice as much as the amount of theoretical air. Artificial wastes containing organic (polyvinyl chloride, PVC) or inorganic (NaCl) sources of chlorine at several levels and copper chloride (CuCl2‚2H2O) as a catalyst were prepared to define the waste composition and make it constant. The experimental setup had been carefully planned to suppress the effects of experimental conditions except the waste composition. Results of combustion experiments revealed that no PCDD/ Fs were detected in the absence of Cl sources and copper chloride, but PCDD/Fs formation was recognized in the cases with Cl and a catalyst. In our experimental conditions, both organic and inorganic chlorines affect PCDD/Fs formation obviously. As Cl content in the waste was increased, CO concentration in flue gas became higher, and more PCDD/Fs were formed in both series of experiments with PVC or NaCl. It seems that combustion conditions indicated by CO concentration are strongly related to PCDD/Fs formation during incineration. It cannot be said that there is a significant difference between the effects of PVC and NaCl on PCDD/Fs formation in the artificial solid waste incineration.
Introduction It is well-known that a municipal solid waste incinerator is a major emission source of polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/Fs) in Japan (1). Though the recent developments of dioxin control technologies on the incinerator systems have made it possible to reduce their release significantly, such emission is still a serious problem.To minimize it, it is indispensable to inhibit the formation itself through basic research. Many investigations have been * Corresponding author phone: +81-298-61-8228, fax: +81-29861-8209, e-mail:
[email protected]. 3920
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conducted to understand the formation mechanisms of PCDD/Fs, which were recently reviewed by Addink (2) and Tuppurainen (3). However, the formation mechanisms are still unclear, and many problems exist on the prevention of dioxin formation in the incinerator. There are many studies on the influence of waste composition (4-10). Especially, those of chlorine sources in the waste are a matter of interest because they may be directly related to PCDD/Fs formation. Some studies found a correlation between chlorine sources and PCDD/Fs formation while others did not (4, 10-25). Wikstrom et al. (23) have reported from experiments using a laboratory-scale fluidized-bed reactor that there is no correlation between the quantities of formed PCDD/Fs in the combustion process and the level of chlorine in the waste when the chlorine level is below 1%, but an increased formation rate is noted when the chlorine level exceeds 1%. They also have studied the influence of organic (polyvinyl chloride, PVC) and inorganic (CaCl2‚6H2O) chlorine in the waste on PCDD/Fs formation and showed that the PCDD/Fs formation is not influenced by the form of chlorine. Lenoir et al. (16) have studied the PCDD/Fs formation in the incineration plant of the refusederived fuel (RDF) with two chlorine sources (NaCl and PVC). No differences in PCDD/Fs emissions were found between the combustion experiments with or without NaCl in the waste. Only the addition of 3% PVC to polyethylene resulted in an increase in PCDD/F concentrations. Halonen et al. (22) have indicated that an increase in chlorine input does not significantly increase the amounts of highly chlorinated organic compounds, like PCDD/Fs, in incineration of aliphatic liquid mixed with inorganic chloride (NaCl) and organic chloride (tetrachloroethylene, C2Cl4) in the laboratory-scale pilot plant. Comparing organic chlorine to inorganic chlorine tests showed that more highly substituted PCDD/F congeners were formed when the organic chlorine was the additive. Despite these studies, the discussion has been kept up on the influence of forms (organic or inorganic) and levels of chlorine on PCDD/Fs formation. The main reason for the conflicting data found by various researchers is that PCDD/Fs formation is affected by many complicated factors in the combustion phenomena in incineration. For example, changes of fuel composition might exert different effects on PCDD/Fs formation for various incinerators or experimental facilities because different combustion conditions are caused by changes of fuel composition for various combustors. To further clarify the effect of chlorine sources, it is necessary to exclude the effects of many complicated factors in the combustion phenomena and to obtain more precious data. The aim of this study is to clarify the influence of forms and levels of chlorine sources in the waste on PCDD/Fs formation in municipal waste incineration. We have already reported the preliminary data using the same facility and methods (26). This study reports the limited experimental data of higher combustion temperatures as a first step. A small laboratory-scale fluidized-bed reactor with external heating was employed to control the combustion conditions closely. The smallness of the reactor could suppress radial distribution of combustion intensity in the reactor. External heating was used to keep the temperature of each section of the reactor constant. Usually a small combustion facility cannot continue the combustion with the heat from the fuel only because the heat loss from the reactor surface exceeds the heat release. The external heating also enables us to conduct an experiment with a very small amount of waste. To avoid uncertainty regarding waste ingredients, artificial 10.1021/es991258w CCC: $19.00
2000 American Chemical Society Published on Web 08/04/2000
TABLE 1. Properties and Elementary Composition of Artificial Waste, Based on Analysis of Dry Material Properties moisture (%) ash (%) calorific heat value (MJ/kg)
3.24 0.43 17.7
Elementary Composition (%) C H N S
47.62 6.07 0.68 0.07
TABLE 2. Cl Contents in Artificial Solid Wastesa fuel no.
Cl source
Cl contents (%)
fuel no.
Cl source
Cl contents (%)
1 2-1 2-2 2-3 2-4
PVC PVC PVC PVC
0.02 0.18 0.45 0.64 1.24
3-1 3-2 3-3 3-4
NaCl NaCl NaCl NaCl
0.21 0.48 0.63 1.00
a
Fuels 2-1 to 3-4 contain 0.25% CuCl2‚2H2O.
FIGURE 1. Schematic diagram of experimental setup. solid waste was used, and its composition was clearly defined to eliminate composition variations of the waste supplied to the reactor. Furthermore, to ensure avoidance of a memory effect, each experiment was started using new sand for the fluidized material, and the whole inner surface of the reactor was washed out (27). The contaminated parts of the quartz reactor that could not be cleaned were replaced with new parts. This experimental setup makes it possible to generate more meaningful data than ever.
Experimental Section Figure 1 shows the schematic diagram of the experimental setup. The reactor was divided into a main combustion section that consisted of primary and secondary combustion zones and a post-combustion section. The primary combustion zone was the laboratory-scale fluidized-bed reactor, which had a diameter of 60 mm and a height of 300 mm. The fluidized material was silica sand of 100-140 µm, and the bed height was set to 100 mm. The secondary combustion zone was a freeboard section 30 mm in diameter and 1450 mm in height. All parts of the main combustion section coming in contact with the flue gas were made of quartz. Air was supplied to the primary and secondary combustion zones from a compressor. Temperature of each part of the reactor was controlled using electric heaters. Flue gas from the secondary combustor was cooled quickly by passing it through a narrow connecting pipe to the setting temperature of the post-combustion section. The post-combustion section was composed of three glass tubes, each 30 mm in diameter and 300 mm in length. The flue gas from the post-combustion section was led to NDIR (nondispersive infrared spectroscopy) and magnetic oxygen analyzers (HORIBA VIA-510 and MPA-510), and the concentrations of CO, CO2, and O2 were thus measured continuously. The artificial solid waste was synthesized to define the waste composition and make it constant. The base ingredients of the artificial waste were 45% unbleached pulp powder, 40% unbleached flour (purchased from Nisshin Flour Milling Co., Ltd.), and 15% wood powder. Unbleached flour represented kitchen waste. Properties and elementary composition of the artificial waste containing only the base ingredients are given in Table 1. The composition adds up to 54%. Since the waste has only 0.43% ash, it is estimated that most of the rest is oxygen. In addition to the base ingredients, an organic
TABLE 3. Experimental Conditions temperature primary combustion zone secondary combustion zone post-combustion section flow rate primary air secondary air fuel feed rate
900 °C 900 °C 350 °C 0.46 Nm3/h (λ ) 1.3) 0.26 Nm3/h (λ ) 0.7) 100 g/h
Cl source (PVC, degree of polymerization n = 1100, purchased from Wako Pure Chemical Industries, Ltd.) or an inorganic Cl source (NaCl, Wako) was mixed at several concentration levels. Copper chloride (CuCl2‚2H2O, 0.25%, Wako) was also mixed as a catalyst. These solid reagents were of special grade. All ingredients were ground separately, mixed mechanically, and then pelletized into particles in the range of 1-3 mm in diameter. These artificial wastes are designated as listed in Table 2, and analyzed Cl contents are also indicated in the same table. Fuel 1 is the reference sample that has the base composition but without any Cl sources and copper chloride. The other wastes have organic or inorganic Cl sources and 0.25% CuCl2‚2H2O (i.e., 0.1% Cu). Sampling was carried out for 4 h or more at three points: the outlet of the main combustion section (indicated as point A), the inlet of the post-combustion section (point B), and the middle of the post-combustion section (point C). The flue gas residence time was about 0.3 s from A to B and 1 s from B to C. Solids in the flue gas were trapped by a glass wool filter and Soxhlet extracted with toluene for 24 h. The flue gas sample was also collected in an ice-cooled water trap and a florisil (60-100 mesh, Wako) trap and was extracted with hexane. All solvents were of pesticide grade. The extracted samples were cleaned up by silica gel and activated carbon columns. PCDD/Fs in the samples were analyzed by gas chromatography-mass spectrometry (GC-MS) Hitachi M-80B or JEOL JMS-700 (28). In all experiments, the setup was assembled with new sand for the fluidized material. After each experiment, the quartz surface in contact with flue gas was washed out to avoid the effect of experimental order. In case the inner surface did not become clear enough, the contaminated quartz parts were replaced with new ones. Actually, the reactor was remade in most cases because Cu compounds VOL. 34, NO. 18, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Relation between Cl content in waste and total concentrations of tetra- to octa-chlorinated DD/Fs at sampling point C. FIGURE 2. Homologue profiles of PCDD/Fs in the experiment of fuel 2-3. adhered to it by reacting with the quartz surface of the reactor. The experimental conditions are listed in Table 3. The ratio of the primary to the secondary air had been explored to minimize CO emission. Consequently, it was decided that the excess air ratio (λ) be set to 1.3 at the primary and 0.7 at the secondary; in total, the air was supplied twice as much as the amount of theoretical air.
Results and Discussion No PCDD/Fs were detected in the experiment of fuel 1, which did not contain any Cl sources and copper chloride, but PCDD/Fs formation was confirmed in the experiments of the other wastes with Cl sources and copper chloride. Because the combustion experiments were conducted in an arbitrary order, the effect of experimental order could be ignored. Also, it was confirmed that there was no PCDD/Fs formation originating from the reactor design. The homologue profiles obtained in the experiment with fuel 2-3 are shown in Figure 2 as an example. The ordinate value is the PCDD/Fs concentration, which is calculated by dividing the summation of the amounts detected in solid and flue gas samples by the sampling gas volume. The averaged concentrations of CO, 84 ppm; CO2, 9.6%; and O2, 9.4% were obtained during the sampling period. Figure 2 indicates that a considerable amount of PCDD/Fs is already formed at the exit of the combustion section. A high PCDD/ Fs concentration at the combustor outlet like in Figure 2 was reported in a municipal solid waste incinerator using a fluidized-bed furnace (27). As the flue gas is cooled from 900 to 350 °C in the connection between the main combustion and the post-combustion section (from the sampling point A to B), the PCDD/Fs concentrations increase despite a very short residence time of about 0.3 s. PCDD/Fs are also formed in the following post-combustion section (B to C) at 350 °C, which is the temperature range in which de novo synthesis is highly promoted. After some experiments, particle matter was observed inside the glass tubes of the post-combustion section. In all experiments using the wastes with Cl and Cu, PCDFs were more dominant than PCDDs, and higher chlorinated PCDD/Fs were formed preferentially as is the case in Figure 2. The same trends were also observed in a fluidized-bed type incinerator (27). The PCDD/Fs profile like in Figure 2 is not unique to these experiments. However, the degree of chlorination is likely to be too high. The reason is that the strong chlorination catalyst CuCl2 is too effective. These distinctive homologue profiles are similar to that observed in a laboratory-scale experiment of de novo synthesis with excess Cu catalyst (28). 3922
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FIGURE 4. Relation between Cl content in waste and Clconcentration in water trap. Figure 3 shows the relation between Cl content in the waste and the total amounts of PCDDs (tetra- to octachlorinated dibenzo-p-dioxins, T4CDDs to O8CDD) and PCDFs (tetra- to octa-chlorinated dibenzofurans, T4CDFs to O8CDF) at the sampling point C. Similar profiles were also obtained at the sampling points A and B. As Cl content was increased, PCDFs show a rapid rise, and PCDDs slowly increase in both series of the wastes with PVC or NaCl. These results show that PVC is obviously one of Cl sources for PCDD/Fs formation. Also, it is valid to say that NaCl is another Cl source in these experimental conditions, which indicates that PCDD/Fs can form during incineration without the existence of PVC in the waste. Considerable formation of PCDD/Fs with NaCl can be explained in terms of the high conversion ratio to hydrogen chloride (HCl) in case NaCl coexists with silica at high temperatures (29). Cl- ion concentration in a water trap installed before the CO measuring instrument is plotted in Figure 4 against Cl content in the waste. The water trap for removing HCl had 150 mL of distilled water and was in contact with 500 mL/min of flue gas during the experiment. The Cl- concentration was measured by ion chromatography (Shimadzu, shim-pack ICA3 column, 4.6 mm i.d., 150 mm). As Cl content in the waste increases, the Cl- concentration in the trap becomes high except fuel 3-4 (Cl content: 1.00%, NaCl). It seems reasonable to suppose that Cl- ion detected in the water trap comes from HCl in flue gas. Hence it can be said that NaCl is converted to HCl, and increased NaCl in the waste causes the rise of HCl concentration in flue gas. In Figure 3, the total amounts of PCDD/Fs obtained in the experiments with PVC
FIGURE 5. Influence of Cl content in waste on average CO concentration during sampling. show a good correlation with Cl content in the waste. It suggests high reliability of the data in these experiments, resulting from the features of the laboratory-scale reactor and the experimental method. Comparing the influence of Cl sources, it seems that the amounts of PCDD/Fs in the experiments with NaCl are slightly more than those with PVC in Figure 3. However, it is difficult to determine that more PCDD/Fs form inevitably in combustion with NaCl than with PVC from these results. For there is a possibility that an increase of sodium in the waste drops the melting point of ash in the fluidized bed. This may worsen the fluidized condition in the reactor and inhibit adequate combustion conditions. Actually, an agglutinative layer on the top of the used fluidized sand was observed after the experiments with NaCl. CO, O2, and CO2 concentrations, combustion temperature, and excess air ratio can be taken as the parameters that represent the combustion condition. Since O2 and CO2 concentrations, combustion temperature, and the excess air ratio were held nearly constant in the experiments, CO concentration is adopted as the representative parameter of the combustion condition. Figure 5 indicates the average CO concentration in the flue gas during the sampling period in each experiment. As Cl content in the waste was increased, the CO concentration rose in both series of the experiments. It is uncertain whether the increase of CO is directly caused by Cl increase or indirectly caused by the change of the combustion state due to Cl increase. At any rate, the increase of Cl supplied with either of PVC or NaCl clearly causes the higher CO concentration. In comparison with Cl sources, there is a steeper CO increase in the case of NaCl than PVC. From the experiments in a laboratory-scale pilot plant, Halonen et al. (22) reported that the CO + H2 concentration in inorganic chlorine (NaCl) tests was high as compared to organic chlorine (C2Cl4) tests. They explained that the difference between organic and inorganic chlorine tests resulted from the higher amount of water input in inorganic than in organic chlorine tests. However, there is no significant difference in water contents in the wastes with PVC or NaCl in our experiments. Since the agglutinative layer was observed on the top of the fluidized material after the experiment with NaCl, the increase of sodium in the waste could possibly obstruct the smooth fluidization of sand bed. Because fluidization of the bed material is strongly connected to the gasification of volatile matter in the solid fuel, the combustion condition might be directly affected by the increase of sodium content. Then, the higher CO concentration with NaCl than that with PVC could be explained in terms of the worse combustion condition. PCDD/Fs concentrations are replotted in Figure 6 against
FIGURE 6. Relation between average CO concentration in sampling period and total concentrations of tetra- to octa-chlorinated DD/Fs at sampling point C. the averaged CO concentration. PCDD/Fs formed in both series of experiments with PVC or NaCl seem to be proportional to the CO concentration, and a little scattered data in Figure 3 converge. The correlation coefficients between the CO concentration and the amount of PCDFs are 0.96 for PVC and 0.99 for NaCl. The difference of the PCDD/Fs concentrations between the experiments with PVC and NaCl in Figure 6 also decreases significantly from that in Figure 3. The correlation coefficients of both data for PVC and NaCl are 0.88 between Cl content and PCDFs in Figure 3 and 0.97 between CO and PCDFs in Figure 6. It could not be said that there is a significant difference between the contributions of organic and inorganic Cl sources to PCDD/Fs formation in our experimental conditions. This means that the influence of CO concentrations on PCDD/Fs formation is hardly different between the experiments with PVC and NaCl. The higher PCDD/Fs formation level with NaCl than that with PVC could be attributed to the higher CO concentration in the flue gas. On the other hand, the CO concentrations in the experiments with NaCl include the effect of the combustion condition resulting from the change of fluidization as mentioned above. Therefore, it is proper to say that Cl itself affects combustion reactions and changes the combustion condition. Cl increase would contribute to the deterioration of combustion condition resulting in formation of more PCDD/Fs. It is a fact that Cl contents in wastes play the role of a Cl source and exert a direct influence on PCDD/Fs formation. However, Cl effects on the combustion condition leading to PCDD/Fs formation were significant in our experimental conditions. PCDD/Fs formation in these experiments could be closely related to products affected by variations of combustion condition such as products of incomplete combustion.
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Received for review November 9, 1999. Revised manuscript received June 13, 2000. Accepted June 20, 2000. ES991258W
