Environ. Sci. Technol. 2003, 37, 2431-2435
Influence of Metallic Chlorides on the Formation of PCDD/Fs during Low-Temperature Oxidation of Carbon S. KUZUHARA,* H. SATO, E. KASAI, AND T. NAKAMURA Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Sendai, 980-8577, Japan
Experimental study was conducted to clarify the formation behavior of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from carbonaceous materials through a de novo synthesis route. Samples were prepared by changing mixing method and composite state of metallic chloride in graphite powder in order to simulate the texture of “unburned carbonaceous particles”, i.e., soot, formed in thermal processes. Reagents of KCl, CaCl2‚ 2H2O, FeCl3‚6H2O, and CuCl2‚2H2O were used as chlorine sources and were added to graphite powder with different methods. The composite state of metallic chloride was varied by preliminary treatments: hand-mixing, mixed-grinding using a high-intensity ball mill, and preheating at different temperatures between 500 and 1100 °C. In the de novo experiment, reaction temperature and oxygen concentration of flowed gas were set at 300 °C and 2.5 mol %, respectively. During the experiment, formation rates of CO and CO2 and the formed amounts of organic chlorine and PCDD/Fs were measured. The results show a reasonable relation between the amount of formed organic chlorine and oxidation rate of carbon, and the order of the activity of metallic chlorines was obtained as KCl < CaCl2 < FeCl3 , CuCl2. Furthermore, it was found that the effect of the composite state of metallic chloride on the formation of PCDD/Fs significantly depends on the kind of metal. The results will give useful information to examine the formation mechanism of PCDD/Fs from unburned carbon particles in thermal processes.
TABLE 1. Composition of Graphite Reagent [G] and Its Ash component
[G]
ash content (%) volatile (%) C (%) Cu (ppm) K (ppm) Cl (ppm) N (ppm) S (ppm)
0.08 0.39 99.3 [KCl-mixed], and that of organic chlorine is [CuCl2mixed] . [FeCl3-mixed] > [CaCl2-mixed] > [KCl-mixed]. In any case, the sample [CuCl2-mixed] shows the largest values, while [KCl-mixed] sample shows the smallest. These results suggest that the amount of active chlorine, which is released from metallic chloride, chlorinated organic carbons, and/or edges of macro carbon structure, are dependent on the kind of coexisting chloride. In other words, CuCl2 is relatively unstable and tends to release active chlorine at a lower temperature. Furthermore, catalytic ability for low-temperature oxidation of carbon appears to be different among the metallic chlorides. Under the present experimental condition, the larger the rate of carbon oxidation, the larger the formed amounts of organic chlorine and PCDD/Fs. It suggests that there is a strong relation between oxidation of carbon and the formation of organic chlorides. Figure 3 shows the changes in the rates of total weight loss and carbon oxidation obtained by the TG analysis and measurement of CO and CO2 concentration of outlet gas during the de novo experiment, respectively. The heating
FIGURE 4. Time dependence of total weight loss and the amounts of organic chlorides detected in the gas traps and solid residue for [CuCl2-mixed]. period to 300 °C (i.e., the initial 15 min) is represented by the negative values in this figure. The peaks of their curve are quite different. There are two peaks in the heating stage for the rate of total weight loss. The first and second peaks appear at -13 and -3 min, respectively. The first peak is higher than second one and is corresponding to about 90 °C in the course of heating. It appears to attribute to the desorptions and reactions of adsorbed molecules and functional groups existing at the surface of graphite particles. On the other hand, the peak for the rate of carbon oxidation appeared at about 9 min. Later than 15 min, both curves show similar values. The amount of oxidized carbon during the experiment occupies about 80% of that of the total weight loss. Figure 4 shows the amounts of total weight loss and organic chlorine measured for the gas traps and for solid residue obtained for the sample [CuCl2-mixed] with time. The trends of changes in the total weight loss and organic chlorine in the gas traps agree well. The amounts of total weight loss and organic chlorine for the gas traps during heating stage (i.e., the initial 15 min) occupy 31.5% and 36.7% of their total values during the experiment, respectively. After being heated to 300 °C, the amount of organic chlorine remaining in the solid sample begins to decrease, although the value of that in the gas traps continues to increase. Therefore, the sums of organic chlorine in the gas traps and solid sample keep almost the same values throughout the experiment. The reason can be explained as follows: Organic chlorides formed on the surface of carbon particles are partly decomposed by oxidation and/or released from the surface to the gas stream; on the other hand, organic chlorides VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Amounts of PCDD/Fs detected in the gas traps and solid residue for (a) [KCl-mixed], [KCl-ground] and (b) [CuCl2-mixed], [CuCl2ground]. continuously form on the surface, and its rate is similar to that of disappearance by decomposition. A decrease in organic chlorine in the solid sample suggests that the number of adsorption sites for the organic chlorides decrease with time. Furthermore, it agrees with the phenomena that the rate of carbon oxidation decreases gradually with time (see Figure 3) because such sites generally are highly reactive. At the same time, however, the Cu and Cl content in the solid sample decrease by about 30 and 82%, respectively, after the de novo experiment for 120 min. Therefore, the catalytic effect of copper and/or its compounds on the carbon oxidation seems to be lowered. It may also affect to the formation of organic chlorides. Detailed analysis on the change in the surface property of graphite particles is necessary to clarify the changes in the reaction rates during the de novo experiments. The amount of chlorine combined with PCDD/Fs remained in the solid sample after the experiment is about 8.6 µg/g of sample. This value is surprisingly large (i.e., about 20% of the total organic chlorine remained). According to a qualitative analysis on the composition of the outlet gas, aliphatic chlorides having smaller molecular mass such as trichloroacetaldehyde, chloroform and carbon tetrachloride mainly form in the heating stage, from 170 to 280 °C (-10 to -5 min). On the contrary, in the holding stage at 300 °C, aromatic compounds such as CBz form. It suggests that, by prolonging the experiment time, aromatic compounds having larger molar mass tend to remain in the solid sample. Furthermore, it can be imagined that the release of aromatic compounds from the surface of carbon occurs with the oxidation of carbon. Both reactions are closely related, even the orders of their rates are extremely different. Effect of the Composite State of Metallic Chlorides on the PCDD/Fs Formation. Figure 5a shows the amounts of PCDD/Fs detected in the gas traps and remaining in the solid sample obtained for the samples [KCl-mixed] and [KClground]. Similarly, Figure 5b shows the values for the samples [CuCl2-mixed] and [CuCl2-ground]. The amounts of PCDD/ Fs formation except for the gas traps of the samples with CuCl2 addition increase by the mixed-grinding operation. The increased amount of the total PCDD/Fs formation by the mixed-grinding operation for CuCl2 addition is larger than that in KCl addition. On the other hand, in terms of the increased ratio, the total PCDD/Fs formation for KCl addition is larger than CuCl2 addition. The average interplanar spacing, d(002), obtained for the samples [KCl-mixed] and [KCl-ground] were estimated by X-ray diffraction (XRD) analysis. Their values are 0.335 and 0.336 nm, respectively. Similarly, that for the samples [CuCl2mixed] and [CuCl2-ground] is 0.335 nm. It does not change by the mixed-grinding operation. On the other hand, specific surface areas (SSA), measured by the BET method, for the samples [KCl-mixed] and [KCl-ground] are 7.9 and 5.8 m2/g, 2434
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FIGURE 6. Amounts of PCDD/Fs detected in the gas traps and solid residue for (a) [KCl-mixed], [KCl-preheated 600, 800, or 1100 °C] and (b) [CuCl2-mixed], [CuCl2-preheated 500 °C]. respectively, and those for [CuCl2-mixed] and [CuCl2-ground] are 5.1 and 4.1, respectively. It means that the mixed-grinding operation decreases SSA of the samples. During mixedgrinding, complicated phenomena may occur (e.g., graphite and metallic chlorines are processed repeatedly by crushing and granulation and make some complex composites). They may form lamellar structures of carbon and metallic chloride by the flattening and folding processes without changing average particle size (20). For the samples with KCl addition, such composite state induced by the mixed-grinding operation is important because the saturated vapor pressure of KCl is relatively small at the experimental temperature. On the other hand, CuCl2 has a larger vapor pressure and therefore greater volatility and mobility (22). Furthermore, since the formation of such composite proceeds in the size of nanometers, an activated interface can be formed at the interface between carbon and metallic chloride, and then C-Cl bond may form locally. Weber et al. (21) have pointed out the importance of the formation of the C-Cl bond on/in the carbon particles for the organic chlorine formation including PCDD/Fs. To examine the details, quantitative analysis is necessary for the C-Cl bond formation during mixed-grinding and its change during the de novo experiment. In addition, the unburnt carbon particles in fly ash and dust formed in the thermal processes, like solid waste incinerators, are composites with various chlorides. C-Cl bonds may already exist in such particles taking into account their formation process. Figure 6a shows the amount of PCDD/Fs detected in the gas traps and remaining in the solid sample obtained for the samples [KCl-mixed] and [KCl-preheated 600, 800, and 1100 °C]. The amounts of PCDD/Fs formation increase remarkably by the preheating operation. [KCl-preheated 800 °C] shows higher values of PCDD/Fs formation than [KCl-preheated 1100 °C] in both of gas traps and solid residue. It appears that the preheating operation near the melting point of chloride, 776 °C for KCl, promotes PCDD/Fs formation. The average interplanar spacing, d(002), obtained for both the samples [KCl-preheated 800 °C] and [KCl-preheated 1100 °C] is 0.335 nm. SSA of them are 7.1 and 4.1 m2/g, respectively. The results indicate that the structure of carbon does not change but that the graphite particles are coated by KCl melt and/or that condensed from its vapor. Therefore, the contact area between graphite particle and KCl may increase significantly. On the other hand, about 35-40% of Cl in the solid sample was lost by the preheating operation. Therefore, the molar ratio of K/Cl increases to 1.68 and 1.22 for the samples [KCl-preheated 800 °C] and [KCl-preheated 1100 °C], respectively, from the initial value of 1. It should be noted that excessive K contained in both samples can be a cause of local formation of the intercalated compounds of graphite such as KC24 and KC8 (23).
From the above results, the preheating temperature for the sample with CuCl2 addition was set at 500 °C, which is nearly its melting point (498 °C). The results are shown in Figure 6b. The amount of PCDD/Fs detected in the gas traps for [CuCl2-preheated 500 °C] is almost equal to that for [CuCl2mixed], while the value for solid residue is smaller. This is different from the results for the sample with KCl addition (see Figure 6a). The amount of contained Cu in the solid sample does not change during preheating, while the Cl content is significantly decreased by 46% due to the decomposition of CuCl2 into CuCl and Cl2. Formed Cl2 may partly contribute to the formation of the C-Cl bond on the surface of the graphite sample during preheating, while it obviously leads to a decrease in the amount of activated Cl available during the de novo experiment. It may reduce the effect of the preheating operation for the sample with CuCl2 addition. In terms of increasing the ratio of PCDD/Fs formation, a higher effect of the pretreatment (e.g., mixed-grinding and preheating) is given to the samples with KCl addition than those with CuCl2 addition. It is explained by higher volatility and mobility of CuCl2 at the temperature of de novo synthesis.
Acknowledgments The financial supports, Grants-in-Aid for Scientific Research of JSPS, Grants for Strategic Research of ISIJ, and Steel Industry Foundation for the Advancement of Environmental Protection Technology to the present study are greatly acknowledged.
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Received for review January 15, 2003. Revised manuscript received March 20, 2003. Accepted March 27, 2003. ES034041H
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