Fs Formation in Thermal Processes

Environ. Sci. Technol. 2001, 35, 1867-1874

Thermodynamic Modeling of PCDD/Fs Formation in Thermal Processes PENGFU TAN,1 IN ˜ AKI HURTADO, AND DIETER NEUSCHU ¨ TZ Lehrstuhl fu ¨ r Theoretische Hu ¨ ttenkunde, Rheinisch-Westfa¨lische Technische Hochschule Aachen, Korpernikusstrasse 16, D-52056 Aachen, Germany GUNNAR ERIKSSON GTT-Technologies, Kaiserstrasse 100, 52134 Herzogenrath, Germany

Three thermodynamic databases of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), derived using the Group Additivity approach and two computational molecular modeling methods, Modified Neglect of Diatomic Overlap (MNDO) and Parametrized Model 3 (PM3), respectively, combined with the Scientific Group Thermodata Europe (SGTE) database have been used to model the formation of PCDD/Fs in thermal processes, such as iron ore sintering process. The predictions using the three different databases are compared, and similar thermodynamic conditions of PCDD/Fs formation are found. The comparison of the calculated values with measured results obtained from industrial iron ore sinter plant indicates that the PCDDs and PCDFs found in practice are not in equilibrium with each other. While within each dioxin and furan homologue equilibrium between the isomers appears to be established in industrial processes, reactions between dioxins and furans seem to be kinetically inhibited. This view has been supported by assuming no reaction at all between PCDFs and PCDDs in the simulation. With this assumption, both PCDFs and PCDDs reached partial pressures between 600 and 800 K in the order of magnitude actually found in practice. Taking this restriction into account, the conditions for PCDD/Fs formation were calculated as a function of oxygen partial pressure; temperature; concentrations of carbon, hydrogen, and chlorine; and C/H ratio.

Introduction Knowledge of the thermodynamics of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) is very important in understanding the formation mechanism of PCDD/Fs in thermal processes. It would assist in the effective control or elimination of the PCDD/Fs emissions during the thermal combustion and reduction processes. Unfortunately, due to the large number of these compounds (as many as 212 compounds) and the extreme toxicity of certain isomers, experimental information on their thermodynamic properties is difficult to obtain. So far, only several experimental values of the enthalpies of formation for dibenzodioxin (DD), dibenzofuran (DF), * Corresponding author e-mail: [email protected]; telephone: +49-(0)241-80-5984; fax: +49-(0)241-8888-295. 10.1021/es000218l CCC: $20.00 Published on Web 04/03/2001

 2001 American Chemical Society

1-chlorodibenzodioxin (1-CDD), 2-chlorodibenzodioxin (2CDD), and 2,3-chlorodibenzodioxin (2,3-DCDD) are available (1-8). Experimental values of the ideal gas entropy obtained from calorimetric measurements are known for unsubstituted dibenzofuran (DF) only (3). In the absence of experimental data, some estimations and theoretical predictions on thermodynamic properties of PCDD/Fs have been reported (9-16). Shaub (9) has developed a group contribution method to estimate the heats of formation of polychlorinated dioxins using values derived from chlorinated phenols to represent the dioxin chlorination. In addition to the effect of replacing a hydrogen atom with a chlorine atom, Iorish (15) considered the ortho, meta, and para interactions between chlorine atoms and chlorine and oxygen atoms. Semiempirical calculations (17, 18), recent estimations by means of the additivity method (13, 14, 19), and predictions of relative concentrations of chlorinated dioxins and dibenzofurans (12, 14) indicate that Shaub’s model is unsatisfactory. Because of the strong oxygen-chlorine interactions suggested in this model, the value of the enthalpy of formation of 2,3,7,8tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) is found to be much lower (-345 kJ mol-1) than that estimated by other methods (between -160 and -190 kJ mol-1). Spencer and Neuschu ¨ tz (10) have applied the data published by Shaub (9) for 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD), together with data for the condensed phase provided by Rordorf (11), to predict concentrations of 2,3,7,8TCDD under various conditions of concentration and temperature. Unsworth (12) used a molecular orbital program, MOPAC, which implements four different semiempirical Hamiltonians (MNDO, AM1, MINDO/3, and PM3) to test the accuracy of the predicted values of ∆H°f, S°, and Cp at 298 K derived by the four different Hamiltonians for 12 chlorobenzenes by comparing them with experimental values. His work suggested that the MNDO method predicted the most reliable data. Thompson (13) has developed a group contribution method to estimate the gas-phase enthalpies of formation of PCDDs and PCDFs based on experimental data for chlorinated benzenes, quinones, hydroquinones, and phenols and simulated fuel-rich combustion products in the C, H, O, and Cl system including to PCDD/Fs. Saito and Fuwa (16) have evaluated the thermodynamic properties of PCDD/ Fs using a semiempirical molecular orbital method with the PM3 Hamiltonian and statistical thermodynamic correlation. The three thermodynamic databases of PCDD/Fs developed by Unsworth (12), Saito and Fuwa (16), and Iorish (15) are used in the present work. Unsworth (12) and Saito and Fuwa (16) used a semiempirical molecular orbital method with the MNDO and PM3 Hamiltonians, respectively. Iorish (15) employed Benson’s Group Additivity method.

Thermodynamic Calculations In the present work, PCDD/Fs formation in one of the wind boxes of an iron ore sinter plant has been modeled. With the exception of the data for PCDD/Fs, all thermodynamic values for the substances taking part in or being formed during the sinter process were extracted from the SGTE (the Scientific Group Thermodata Europe) database (20). The species in the calculation are shown in Table 1. The equilibrium calculations were carried out using the commercial package ChemApp (21), which is based on a free energy minimization routine. VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. List of Chemical Components Included in the Calculations

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FIGURE 1. Effect of temperature on PCDD/Fs formation assuming equilibrium between dioxins and furans (PO2 ) 10-50 bar).

FIGURE 2. Calculated regions in which the partial pressures of 2,3,7,8-TCDD and 2,3,7,8-TCDF reach 10-13 and 10-11 bar, respectively, excluding equilibrium between dioxins and furans (C/H ) 4).

The measured elementary compositions (mole fractions) of the off-gas in one of the wind boxes (wind box no. 9 over 14) in an industrial sinter plant have been reported (22) to be 22.9% O, 3.7% C, 7.5% H, and 65.4% N. According to previous modeling work (22), only part of the hydrogen present in the gas phase appears to be chemically reactive with respect to dioxin formation because of kinetic restrictions. Furthermore, dioxins are only found to be stable in the

amounts measured during the sinter process when the oxygen partial pressure is assumed to be less than 10-30 bar (22). The calculated concentrations of PCDD/Fs are negligible if PO2 > 10-30 bar and if all of the hydrogen in the off-gas takes part in the reactions. In this work, we assume that only 40% hydrogen is chemical reactive in this calculations or the elementary compositions (mole fractions) of 22.9% O, 3.7% C, 3.0% H, and 69.9% N are used in the input. The VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Relations between the oxygen partial pressure and temperature allowing that the practical concentrations of PCDDs and PCDFs in a iron ore sinter plant are the mutually consistent equilibrium values.

FIGURE 4. Effect of temperature on the formation of PCDD and PCDFs excluding equilibrium between dioxins and furans (PO2 ) 10-50 bar). concentration of Cl is assumed to be 0.01%. The formation of solid carbon must be excluded from the calculations, as suggested in the previous simulation (10, 22), otherwise PCDD/Fs are not stable at all under any realistic condition.

Results and Discussion The effect of temperature on the PCDD/Fs formation in the gas phase are calculated using the three different databases (12, 15, 16), and shown in Figure 1. While during operation of the iron ore sinter plant, analyses of the waste gases after passing the filter system show negligible dioxin content, the dioxin levels measured in the wind boxes upstream the filter 1870

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system are significant (22, 40). Measurements indicate that partial pressures of toxic PCDD/Fs in the wind boxes are about 10-13-10-11 bar (23) and that the PCDD/Fs formation occurs preferentially in the temperature range between 550 and 750 K (24-27). Figure 1 shows that the PCDD/Fs partial pressures observed in the wind boxes are only obtained by calculation for the above temperature range between 550 and 750 K when PO2 is assumed to be 10-50 bar. The industrial measurements (23) show that the differences between the partial pressures of PCDFs and PCDDs are not larger than 2 orders of magnitude. However, according to Figure 1 the differences between the partial pressures of

FIGURE 5. Effect of oxygen partial pressure on PCDD/Fs formation excluding equilibrium between dioxins and furans (T ) 700 K).

FIGURE 6. Effect of chlorine concentration on PCDD/Fs formation excluding equilibrium between dioxins and furans (T ) 700 K, PO2 ) 10-50 bar). PCDFs and PCDDs predicted by the present calculations are larger than 10 orders of magnitude, which means that the PCDFs/PCDDs molar ratio is larger than 1010. This large difference has been equally obtained from each of the three databases. It implies that the toxic furans are by far more stable than the toxic dioxins, which is in sharp contrast to practical observation. To gain a clear understanding of the PCDD/Fs formation, stability ranges for dioxins and furans have been calculated in a plot log PO2 versus temperature with chlorine contents as the parameter and for a constant ratio C/H ) 4. The shaded ranges indicate log PPCDF > 10-11 bar and log PPCDD > 10-13 bar. The chlorine contents were chosen as 0.01%, 0.1%, and

1%. The results shown in Figure 2 are only those for 2,3,7,8TCDD and 2,3,7,8-TCDF for clarity’s sake, but the behavior of the other toxic PCDDs and PCDFs is very similar to those shown. Again, the different databases do not lead to any major deviations of the stability ranges. Dioxins and furans become “stable” only below PO2 ) 10-35 bar, are restricted to the temperature range from 500 to 800 K, and extend their stability range slightly with increasing chlorine content in the system. The stability of the furan is somewhat higher than that of dioxin. To understand the relation between oxygen partial pressure and temperature under which the calculated PCDFs/ PCDDs molar ratios correspond to the practical values, the VOL. 35, NO. 9, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Effect of carbon concentration on PCDD/Fs formation excluding equilibrium between dioxins and furans (T ) 700 K, PO2 ) 10-50 bar).

FIGURE 8. Effect of concentration of H on PCDD/Fs formation excluding equilibrium between dioxins and furans (T ) 700 K, PO2 ) 10-50 bar). following equations may be taken into account for oxidation reactions occurring in the wind boxes:

2,3,7,8-TCDF + 0.5O2 ) 2,3,7,8-TCDD

(1)

1,2,3,7,8-PCDF + 0.5O2 ) 1,2,3,7,8-PCDD

(2)

The oxygen partial pressure can be calculated from the Gibbs energy of formation of reactions 1-7 and the measured PPCDF/ PPCDD ratio:

log PO2) ∆G°i/(9.567 T) - 2 log10(PPCDF/PPCDD) (8)

1,2,3,4,7,8-H6CDF + 0.5O2 ) 1,2,3,4,7,8-H6CDD (3) 1,2,3,6,7,8-H6CDF + 0.5O2 ) 1,2,3,6,7,8-H6CDD (4) 1,2,3,7,8,9-H6CDF + 0.5O2 ) 1,2,3,7,8,9-H6CDD (5) 1,2,3,4,6,7,8-H7CDF + 0.5O2 ) 1,2,3,4,6,7,8-H7CDD OCDF + 0.5O2 ) OCDD 1872

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The relation between oxygen partial pressure and temperature has been calculated using the above three databases and the measured values for the PPCDF/PPCDD (23) and are shown in Figure 3. The relations in Figure 3 represent the conditions under which the industrial values for the PPCDF/ PPCDD ratios in the wind boxes correspond to the mutually consistent equilibrium values.

FIGURE 9. Effect of the C/H ratio on PCDD/Fs formation excluding equilibrium between dioxins and furans (T ) 700 K, PO2 ) 10-50 bar). By comparing Figure 2 with Figure 3, we could not find the same conditions (the same oxygen partial pressure and the same temperature) where the PCDDs and PCDFs partial pressures reach the observed levels (23), 10-13 and 10-11 bar, and are at the same time in thermodynamic equilibrium with each other. Apparently, furans and dioxins in the offgases of industrial combustion processes are not in equilibrium with each other. To support this view, a calculation was made to obtain the concentrations of PCDD/Fs under the same conditions as before, except that in one case all the dioxins were omitted from the list of components involved, and in the second case, all the furans were excluded. In other words, any possible reaction between dioxins and furans was suppressed assuming that the formation of these two groups of compounds proceeds totally independent of each other. The partial pressures of the toxic dioxins and furans calculated for the set of conditions as in Figure 1 (PO2 ) 10-50 bar, 0.01% Cl), but now with the restriction described above, are plotted in Figure 4 as a function of temperature and the different databases. In the critical temperature range, 600800 K, both the PCDDs and the PCDFs attain partial pressures around 10-10 bar, i.e., their values are close to the observed ones and close to each other. Therefore, the assumption that dioxins and furans do not noticeably react with each other appears to be close to reality. Actually, Iino and Gullett (36) have pointed out that the formation mechanism of PCDFs is completely different from that of PCDDs. Some researchers (34, 37-39) have also suggested that PCDDs and PCDFs might form in separate regions of an incinerator. While within each dioxin and furan homologue, equilibrium between the isomers seems to be easily established (12, 14, 35), reactions between PCDDs and PCDFs must be kinetically inhibited. The concentrations of PCDD/Fs calculated with the above assumption are also very sensitive to the concentrations of carbon, hydrogen and the C/H ratio, as shown in Figures 5-9. The behavior of PCDD/Fs formation proved to be similar when the different databases were used. It has been observed that the amount of PCDD/Fs formed increased with increasing carbon (28, 29) and chlorine content (26, 30-33). With respect to these variables, the predicted trends are in good agreement with the observed data.

In our former work (35), the predictions of the toxic PCDD/ Fs isomer distribution using the three different databases were compared with measured values from the industrial incinerator, wood combustion systems, electric arc furnace, and iron ore sinter plant. The comparison (35) showed that the MNDO method predicts more reliable isomer distribution than the PM3 method and the Group Additivity method, though similar thermodynamic conditions of PCDD/Fs formation are found using the three different databases in this work.

Acknowledgments The authors thank Dr. John F. Unsworth (Shell Research Ltd., Thornton Research Center, Chester, U.K.) for making available his thermodynamic data on PCDD/Fs. The authors also thank Dr. Stephan Petersen (GTT-Technologies, Herzogenrath, Germany) for his useful suggestion. This work is funded under ECSC Agreement 7210-PD/069.

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Received for review September 18, 2000. Revised manuscript received January 22, 2001. Accepted January 30, 2001. ES000218L