An experimental investigation of the combustion of technical tetrachlorobenzene in a laboratory scale incinerator

An Experimental Investigation of the Combustion of Technical Tetrachlorobenzene in a Laboratory Scale Incinerator ALAIN BISSONIER, CHRISTIAN CHAUVEAU, JEAN-LOUIS DELFAU and CHRISTIAN VOVELLE* LCSR (UPR 4211-CNRS), IC, avenue de la Recherche Scientifique 45071 Orle´ans Cedex 2, France

and YOLANDA DIAZ DE MERA

Departamento de Quimica Fisica, Facultad de Ciencias Quimicas Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain An experimental study of the combustion of technical 1,2,3,4-tetrachlorobenzene has been carried out in conditions closely similar to those in incinerators of industrial hazardous wastes. A non-premixed flame fed by a fuel spray was stabilized in a reactor heated electrically. Measurements of axial and radial temperatures and species concentration profiles in well-defined conditions have been systematically repeated to specify the influence exerted by key parameters on the efficiency of the thermal degradation process. The reactor’s wall temperature was varied from 1223 to 1363 K. Air injection was distributed between axial and peripheral injectors to change the swirl intensity. Two fuel injectors have been compared to assess the influence of atomization efficiency. A methane-air pilot flame helped to anchor the flame and to decrease the overall Cl/H ratio. The latter was also decreased by addition of water vapor. Results show that conditions leading to a complete consumption of the fuel can be associated with very limited carbon conversion into CO2 and chlorine conversion into HCl. © 2002 by The Combustion Institute

INTRODUCTION Many industrial hazardous wastes, especially chlorinated compounds, are treated by incineration. Usually, these compounds are liquid at ambient temperature and their combustion is performed in a non-premixed flame, fed by a spray obtained by atomization of the fuel jet. The flame is stabilized in a furnace made of refractory material to limit heat losses at the wall. Vaporization and mixing with air of the fuel droplets are favored by swirl generated by radial injection of one part of the air flow. The efficiency of the overall incineration process is quantified in term of the Destruction Removal Efficiency (DRE): DRE ⫽

冉

冊

Do ⫺ Df 100 Do

(1)

where Do and Df are inlet and outlet values of the waste flow rate.

* Corresponding author. E-mail: [email protected] COMBUSTION AND FLAME 129:239 –252 (2002) © 2002 by The Combustion Institute Published by Elsevier Science Inc.

Regulations impose DRE values higher than 99.99% for industrial waste incinerators. However, high values of the DRE only ensure that the initial waste is efficiently destroyed. For chlorinated compounds, it has been shown that products of incomplete combustion (PICs), some more toxic than the initial waste, can be formed [1, 2]. Hence, it is important to incinerate chlorinated hydrocarbons in conditions leading to their complete destruction with only the final products CO2, H2O, HCl, and Cl2 being formed. This crucial issue has motivated numerous studies aimed at specifying the formation mechanisms of PICs in well-controlled conditions: laminar premixed flames [3–9], diffusion flames [10], flow reactors [11–15]. The experimental results have been used to develop and validate detailed reaction models [6, 7, 16 –18]. Globally, modeling predictions are in fairly good agreement with experiments. However, this agreement refers to kinetically controlled conditions and only a few studies have considered the influence of physical phenomena such as fuel atomization and vaporization, mixing of the fuel vapor with air. Amongst the latter 0010-2180/02/$–see front matter PII S0010-2180(01)00372-8

240 Kramlich et al. [19, 20], correlated atomization quality and destruction efficiency of chlorinated compounds in turbulent spray flames. Koshland and Thomson [21] compared the thermal destruction of gaseous and liquid 1,1,1-Trichloroethane. Brower et al. [22] investigated the influence of incomplete turbulent mixing on the extent of reaction and PICs formation during chlorocarbon combustion. Delplanque et al. [23] studied the influence of injection configurations on the destruction efficiency of chlorobenzene. Millet [24] performed an experimental study of the combustion of dichloromethane in a pilot incinerator. Clack et al. [25], considered the effects of spray density on the vaporization of multicomponent chlorinated hydrocarbons and their thermal destruction. Law et al. [26 – 28] studied the rates of vaporization and combustion of chlorinated and multicomponent fuel droplets. All these studies point out the key role of atomization and vaporization of the fuel droplets for the overall efficiency of thermal degradation. In this work, we conducted an extensive study on the effects of the main adjustable parameters in industrial waste incinerators on the degradation efficiency and the formation of intermediate products during the combustion of technical 1,2,3,4-tetrachlorobenzene (tech-TCB). The certified purity (Aldrich) was 90% and gas chromatography revealed 1,2,3-trichlorobenzene as the main impurity. This information was taken into account for atomic balance calculations, where the initial composition was taken as C6H2.1Cl3.9. The selection of tech-TCB as a fuel allowed us to address problems related to the incineration of heavily chlorinated compounds, since its Cl/H ratio is 1.86. A laboratory-scale device was specially built to measure both temperature and species’ concentrations under controlled conditions of temperature, fuel atomization, residence time, overall equivalence ratio, chlorine content, air swirl. In addition to the above-mentioned parameters, hydrogen addition by using either a methane-air pilot flame or adding water vapor to the airflow was considered to vary the Cl/H ratio. Indeed, the addition of H2O plays a key role, as shown by its marked effect on the burning rate of chlorinated hydrocarbon droplets [26, 27]. All the experiments were per-

A. BISSONNIER ET AL.

Fig. 1. Apparatus.

formed in parametric conditions reproducing closely those of industrial incinerators. In particular, the combustion of tech-TCB has been systematically conducted with air in large excess (overall equivalence ratio was 0.52 to 0.69). EXPERIMENTAL METHODS Figure 1 shows the major features of the experimental device specially built for this study. The central part was a tubular quartz reactor (12 cm i.d., 80 cm long) with four windows located at 90° for visual or photographic observation of the flame and/or radial measurements of species’ concentrations or temperature. This quartz tube was heated electrically and insulated to reproduce the high temperatures reached in industrial incinerators as a result of heat transfer from the flame to the refractory wall. In this work, the quartz tube could be maintained at a maximum temperature of 1363 K. A non-premixed-flame burner could be moved vertically in the quartz tube. Its upper part consisted of a ceramic plate with an assembly of concentric tubes in the center and six peripheral air injectors. Figure 2 specifies the relative positions of the peripheral air injectors. Each one could be rotated to select a given injection direction in a horizontal plane. All the experiments reported in this work were with a 40° angle with respect to the radius of the ceramic plate. Figure 3 gives details of the central tubes used to atomize the liquid techTCB, as well as how air was injected axially and the small methane-air pilot flames stabilized.

INCINERATION OF TETRACHLOROBENZENE

241 TABLE 1 Geometric Properties of Liquid Tech-TCB Injectors Tech-TCB injection

Injector A B

Fig. 2. Position and orientation of the peripheral air injectors.

The liquid tech-TCB flowed through the central capillary and was atomized by a high velocity jet of N2 flowing through a concentric capillary. To change the atomization conditions, two sets of concentric tubes were used. Table 1 specifies their geometrical properties. One part of the air flow was injected axially through a tube 1.0 cm i.d. A small stainless steel piece held the fuel and nitrogen capillaries in the center of the air tube. A fourth tube stabilized a series of small conical premixed methane-air flames. These pilot flames played a dual role: (1) they acted as a flame holder for the main tech-TCB-

Fig. 3. Four jets tech-TCB spray-air burner.

Nitrogen injection

i. d. (mm) o. d. (mm) i. d. (mm) o. d. (mm) 0.20 0.52

0.43 0.80

0.66 1.16

1.00 1.50

air flame and (2) they decreased the overall Cl/H ratio. Because tech-TCB is solid at ambient temperature (m.p. 320.5 K), the fuel line was heated electrically up to 353 K. The liquid tech-TCB was delivered by a Gilson HPLC pump, which also controlled the mass flow rate. Initial experiments showed that solidification of the techTCB after stopping the electrical heating of the line led to an overpressure, which damaged the pump. To avoid this, trichlorobenzene (liquid at ambient temperature) progressively substituted the tech-TCB before stopping a series of experiments. At the beginning of the next series, trichlorobenzene was used during the transient initial period and was replaced by tech-TCB only when the fuel line was stable at 353 K. To add water, a controlled mass flow rate of liquid was delivered by a second Gilson pump. A water spray was formed by injection of the liquid jet into a high velocity air flow. Heating up to 433 K ensured a fast vaporization of the spray. Then, the water vapor-air flow was mixed with the axial and peripheral air jets. The flow rates of nitrogen (for fuel atomization), air (water atomization, coaxial and peripheral jets, pilot flames), and methane (pilot flames) were controlled by Brooks rotameters. Frequent calibrations with a soap bubble flow meter ensured an accuracy of 3%. The flow rates of tech-TCB and water were controlled by measuring a rate of mass loss over a given period of time. These calibrations had an accuracy of 5%. Two main series of experiments were performed to study the influence of selected parameters on the combustion of tech-TCB. The first was based on species’ analyses at the exit of the furnace, in a gas collection device specially designed to mix the combustion products. Gaseous samples withdrawn by a quartz probe flowed through a liquid nitrogen cooled trap, where the unburned fuel and others chloroben-

242 zenes (tri, penta, and hexachlorobenzene) were retained. These compounds were analyzed by GC, after heating the trap up to ambient temperature and adding a known amount of toluene or naphtalene. The trap could be by-passed to allow direct measurements of the gas phase composition. The sampling line was heated up to 423 K at a reduced pressure (⬍ 16 kPa) to avoid condensation. Gas analyses were performed either by FTIR (CO2, CO, HCl, and H2O), or GC (O2, N2, CO) and GC/MS (Cl2, COCl2). The gas collection device could be moved vertically to vary the residence time. The second series of experiments was aimed at obtaining more detailed information on the consumption of the chlorinated fuel. A quartz probe was inserted in one of the lateral windows of the quartz reactor, so that radial concentration profiles could be measured. By moving the burner vertically, a complete map of concentrations was obtained. Gas sampling was performed under low pressure to ensure efficient quenching of reactions. Gas bottles, initially evacuated by a mechanical pump were filled up to 16 kPa for GC analyses. The efficiency of quenching a reaction was not controlled in specific experiments. However, the symmetry of the profiles of mole fractions about the vertical axis of the reactor indicated a lack of perturbation when the sampling probe crossed the flame. FTIR analyses were conducted with the sampled gas continuously flowing through the cell at a constant pressure of 13.3 kPa. The sampling line and the cell were heated up to 423 K. Details on the FTIR, GC, and GC/MS are available [29]. Measurements of concentrations were complemented by temperature measurements with Platinum-Platinum 10% Rhodium thermocouples (diam. of the wires: 100 ␮m). They were introduced either from the top of the gas collection device to measure the temperature profile along the reactor’s axis or from one of the four lateral windows to measure radial profiles. All experiments reported below were performed with constant flow rates for the liquid fuel (2.97 ⫻ 10⫺4 mol s⫺1), overall air (1.29 ⫻ 10⫺2 mol s⫺1) and nitrogen to atomize the fuel (3.95 ⫻ 10⫺4 mol s⫺1). Except for specific studies of the influence of reactor wall’s temperature, this was kept at 1273 K. The residence

A. BISSONNIER ET AL. time depended on the temperature and also on the distance between the sampling positions and the burner. Most results reported below were obtained with the sampling probe 50 cm above the burner. With the reactor wall at 1273 K, these conditions corresponded to a residence time ⬇ 2 s. RESULTS AND DISCUSSION Progress of the Overall Combustion Reaction The progress of the degradation of tech-TCB in the reactor was expressed in terms of three different factors: the DRE, the carbon conversion and the chlorine conversion. The DRE gives a direct evaluation of the fuel consumption. Since only the initial flow rates are known in this work, the general expression given in the introduction was adapted to:

DRE ⫽

冉 冊 冉 冊 冉 冊 DTCB D N2

XTCB X N2

⫺

o

DTCB D N2

f

100

(2)

o

where DTCB and DN2 denote input flow rates of tech-TCB and nitrogen, respectively; XTCB and XN2 are the mole fractions of tech-TCB and nitrogen in the exhaust gas. The initial flow rate of N2 takes account of the various contributions: axial and peripheral air jets, pilot flames and fuel atomization. For the carbon conversion, it was assumed that every carbon atom introduced into the reactor will be converted into carbon dioxide:

冉 冊 XCO2

%CC ⫽

冉

X N2

f

6DTCB ⫹ DCH4 D N2

冊

100

(3)

o

A similar approach was adopted to compute a chlorine conversion, with HCl as the only final product:

%ClC ⫽

冉

冉 冊 XHCl X N2

冊

f

3.9DTCB D N2

100 o

(4)

INCINERATION OF TETRACHLOROBENZENE

243

TABLE 2 Influence of Reactor Wall Temperature, Pilot Flame and Addition of Water on the Combustion Efficiency of Tech-TCB Expressed as DRE, Carbon Conversion and Chlorine Conversion (Fuel Injector A). Reactor wall Temperature (K) 1363 1363 1363 1363 1273 1273 1223 1323

Methane flow rate mol s⫺1 ⫺4

1.03 ⫻ 10 1.03 ⫻ 10⫺4 0 0 1.03 ⫻ 10⫺4 1.03 ⫻ 10⫺4 0 0

Water flow rate cm3 s⫺1 0 7.53 ⫻ 10⫺4 0 7.53 ⫻ 10⫺4 0 7.53 ⫻ 10⫺4 0 7.53 ⫻ 10⫺4

It is worth noting that if an experiment is to lead to 100% carbon conversion, such a value for the chlorine conversion requires addition of hydrogen to decrease the initial Cl/H ratio to a value lower than 1. The input values of the Cl/H ratio are computed as: Cl/H ⫽

(3.9 DTCB)o 100 (2.1 DTCB ⫹ 4 DCH4 ⫹ 2 DH2O)o (5)

Results of the analyses of gases and liquid in the gas collection device have been used to compute these three factors, for different flame configurations. Table 2 summarizes one part of the results and focuses on the influence exerted by the reactor wall’s temperature, the methaneair pilot flame and the addition of water vapor. All the experiments in this table were performed with tech-TCB injected through the small diameter injector to increase the DRE via a fast atomization of the fuel. Table 2 readily shows that all experiments, even those conducted without adding methane or water, lead to very high values of the DRE. On the other hand, both carbon and chlorine percent conversions can be limited to low values, corresponding to the presence of PICs in the exhaust gases. A closer examination of Table 2 shows that the conversions of carbon and chlorine are not sensitive to the same parameters. The former strongly depends on the reactor wall’s temperature, as shown by the high values at 1363 K. At lower temperatures, low values were obtained in experiments conducted without adding water.

Cl/H

Carbon conversion %

Chlorine conversion %

DRE

1.12 0.23 1.86 0.25 1.12 0.23 1.86 0.25

95.74 98.07 95.10 100.00 42.52 96.78 41.09 92.95

87.84 100.00 66.96 100.00 87.74 98.05 58.97 98.10

100.00 99.99 99.54 100.00 99.89 99.94 99.95 99.76

The lack of sensitivity with respect to the methane-air pilot flame can be explained by the low flow rate of methane, which does not permit Cl/H values lower than 1. The Cl/H ratio is the dominant parameter for the chlorine conversion. The four experiments adding water vapor (i.e., Cl/H ⬍ 1) lead to high conversions of chlorine, independently of the reactor temperature over the range (1223–1363 K) employed in this work. However, the two experiments with Cl/H ⫽ 1.86 at 1363 and 1223 K shows that when chlorine is in excess, the reactor wall temperature does exert a positive effect on the conversion of chlorine. The local measurements of temperature and the concentrations of gaseous species were examined to obtain more detailed information on the changes induced by variations of the controlling parameters. Amongst the latter, the temperature deserves special consideration, because in addition to the direct change obtained by varying the reactor wall’s temperature, it can be also affected by others parameters, especially the methane-air pilot flame and the proportion of peripheral air. These influences have been studied specifically by means of axial temperature measurements along the reactor. Influence of Parameters on the Flame Temperature Figures 4a and 4b compare the evolution of the temperature along the reactor’s axis for four values of its wall temperature and two different

244

A. BISSONNIER ET AL.

Fig. 5. Influence of the pilot flame on the axial temperature profile. E without the tech-TCB flame; } with the tech-TCB flame but without the pilot flame; F with both tech-TCB and pilot flames (methane flow rate: 1.04 ⫻ 10⫺4 mol s⫺1). Water flow rate: 7.71 ⫻ 10⫺4 mol s⫺1. Reactor wall temperature: 1273 K. 67% peripheral air. TCB injector: B.

Fig. 4. Influence of the reactor wall’s temperature on the axial temperature profile. % peripheral air: a) 67%; b) 91%. 䡬 1223 K; 䉬 1273 K; 䡺 1323 K; ● 1363 K. Flow rates (in mol s⫺1): Methane 1.04 ⫻ 10⫺4; water 7.71 ⫻ 10⫺4 fuel injector B.

distributions of the air flow. In these experiments, the fuel was injected through the large diameter capillary (injector B). Figure 4a shows that a substantial axial contribution leads to marked gradients in the temperature profiles, especially in the vicinity of the burner surface. These gradients are likely to be related to the helical structure of the flame, as noticed by visual observation. The latter also shows that the flame stabilized with a cooler reactor wall is highly unstable, and local extinction can explain the marked gap in the axial temperature profiles with respect to the others conditions. An increase in the proportion of air flowing through the six peripheral injectors led to considerably smoothers axial temperature profiles as shown in Fig. 4b. However, such an increase in the mixing intensity also results in a marked decrease in the maximum temperature, showing that more heat is transferred from the flame to the wall.

The effect of the methane-air pilot flame can be assessed from examining Fig. 5. As expected, a marked increase of the temperature was observed close to the burner. At larger distances (⬎ 20 cm), the profiles tend to merge, in agreement with the small change of the specific heat release on adding methane (11.6 kJ g⫺1 for the flame stabilized without the pilot flame and 12.6 kJ g⫺1 for those with the pilot flame). The axial temperature profile measured when only air is flowing through the reactor has been plotted in Fig. 5. The decrease observed at 5 cm above the burner corresponds to a local perturbation due to the four lateral windows. Influence of Operating Conditions on Concentrations When the conversion of carbon is lower than 100%, CO is the only carbonaceous product formed in addition to CO2. A wider variety of chlorinated products is observed when the chlorine conversion does not reach 100%. In addition to HCl, chlorine (Cl2), phosgene (COCl2) and very small amounts of chlorobenzenes are formed. The influence exerted by changes in the operating conditions on the concentration of these intermediate products will be considered for carbonaceous and chlorinated species. Figure 6 illustrates the effect of the reactor wall’s temperature on the concentrations (ex-

INCINERATION OF TETRACHLOROBENZENE

Fig. 6. Influence of the reactor wall temperature on the outlet [CO2] (a) and [CO] (b). Proportion of peripheral air: 䊐 50%; ‚ 67%; { 91%. Methane flow rate: 1.04 ⫻ 10⫺4 mol s⫺1. No water added. Fuel injector: B.

pressed as mole %) of the major carbonaceous species in the exhaust gas. These results were obtained with the methane-air pilot flame, but without adding water vapor, so that the Cl/H ratio is higher than 1. In these conditions, a large fraction of the carbon is still present as CO in the exhaust gas at 1200 K. The three curves correspond to different distributions of the air flow between peripheral and axial jets. The higher [CO] obtained with 91% peripheral air is likely to result from the decrease in flame temperature observed with high proportions of peripheral air. A priori, it was expected that an increase in the mixing intensity by injecting peripheral air would favor the conversion of intermediate species to final products. For CO, it is clear that the decrease in flame temperature is dominant, so that an opposite trend is observed. The influence of the methane-air pilot flame

245

Fig. 7. Influence of the pilot flame on the outlet [CO2] (a) and [CO] (b). Proportion of peripheral air: 䊐: 50%; ‚: 67%. Reactor wall temperature: 1273 K. No water addition. Fuel injector: B.

was studied for only two distributions of the air flows (50% and 67% peripheral). It was not possible to stabilize the flame at higher peripheral proportions without the pilot flame. The evolution with the methane flow rate of [CO] and [CO2] has been plotted in Fig. 7a and 7b, respectively. These experiments were performed with the reactor wall at 1273 K and a small addition of water vapor, so that the Cl/H ratio was higher than 1, when the pilot flame was not established. The pilot flame exerts a very positive effect on the conversion of CO to CO2. However, at 1273 K, the complete disappearance of CO from the exhaust gas requires a minimum methane flow rate of 2.1 ⫻ 10⫺4 mol s⫺1, which is a substantial value compared to the molar flow rate of 2.97 ⫻ 10⫺4 mol s⫺1 for tech-TCB. Adding water vapor constitutes another way of reducing [CO] in the exhaust gas, as can be

246

A. BISSONNIER ET AL.

Fig. 9. Influence of the Cl/H ratio on the outlet [CO]. Open symbols: methane addition; filled symbols: water vapor addition. ⫹: tech-TCB-air flame. Reactor temperature: 1273 K. Fuel injector: B.

are limited. The addition of water compensates for this inhibition because its consumption by reaction with Cl atoms introduces a new source of OH radicals: Cl ⫹ H2O 3 HCl ⫹ OH

Fig. 8. Influence of water addition on the outlet [CO2] (a) and [CO] (b). Proportion of peripheral air: 䊐: 50%; ‚: 67%. Reactor temperature: 1273 K. Methane flow rate: 1.04 ⫻ 10⫺4 mol s⫺1. Fuel injector: B.

seen from examining Fig. 8. It was expected that the methane-air pilot flame and the addition of water vapor would improve the conversion of CO to CO2 by increasing the pool of radicals, especially H and OH. The key reactions consuming CO in the presence of chlorine and water have been studied [12, 30 –32] to elucidate the marked inhibition by chlorine of the conversion of CO to CO2. The formation of HCl by chlorine abstraction traps H atoms: RCl ⫹ H 3 HCl ⫹ R

(R1)

and reduces their availability for the main branching reaction: H ⫹ O2 3 OH ⫹ O

(R2)

As a consequence, the formation of OH and the main reaction consuming CO (R3): CO ⫹ OH 3 CO2 ⫹ H

(R3)

(R4)

To confirm the strong dependence of the conversion of CO to CO2 on the Cl/H ratio, the [CO] measured in the exhaust gas in all experiments performed at 1273 K is plotted against this parameter in Fig. 9. One plot is observed, independently of the means used to vary the Cl/H ratio (pilot flame or water addition) and of the air distribution. Figure 9 shows that addition of an H donor is an essential condition to ensure low [CO] when heavily chlorinated compounds are incinerated. However, even when [CO] is low in the exhaust gas, it can reach locally high values. Local concentration measurements, performed by moving radially the lateral sampling probe, were used to check this point. Results obtained with the small diameter fuel-injector are plotted as [CO] maps in Fig. 10 for three values of the air distribution. Figure 10 shows that the decrease in the temperature because of a high proportion of peripheral air leads to a [CO] as high as 5% close to the burner. With only 50 or 67% peripheral air, [CO] is very low, even in the flame. An increase in the diameter of the fuel injector markedly increases the droplets’ concentration in the first few centimeters above the burner. This is illustrated in Fig. 11, where two

INCINERATION OF TETRACHLOROBENZENE

247

Fig. 11. Influence of the fuel injector diameter on atomization efficiency. a: injector A; b: injector B. Reactor temperature: 298 K.

photographs taken with illumination of the fuel spray by a light sheet are compared. These photographs correspond to cold flow conditions. The effect of a change in the atomization conditions on the formation of CO was studied by measuring the axial concentration profile of CO. The results obtained with three proportions of peripheral air are plotted in Fig. 12. At 50% and 67% peripheral air, the small diameter

Fig. 10. Influence of the proportion of peripheral air on the local [CO]. a: 50%; b: 67%; c: 91%. Reactor temperature: 1273 K. Fuel injector: A.

Fig. 12. Dependence of [CO] on the fuel injector diameter. Filled symbols: injector A; open symbols: injector B. proportions of peripheral air: 䊐, ■ 50%; ‚, Œ 67%; {, } 91%. Reactor temperature: 1273 K. Flow rates (in mol s⫺1): Methane 1.04 ⫻ 10⫺4; Water 1.04 ⫻ 10⫺4.

248 injector leads to very low axial [CO]. With the large diameter injector, [CO] is high close to the burner and only reaches low values at 40 cm above the burner. With 91% air injected through the peripheral injectors, the influence of the fuel injector’s diameter is considerably reduced. Even with the narrow injector, the reduction in temperature induced by the intense mixing leads to [CO] ⬇ 1.0% at 20 cm above the burner. The influence of the adjustable parameters on the formation and consumption of the chlorinated products was also studied. In every case, HCl is largely dominant. Figure 13a shows that its outlet concentration does not depend on the reactor wall’s temperature in the range 1223 to 1363 K. This figure is for experiments without water vapor added and with the pilot flame fed by a small flow rate of methane, so that the overall Cl/H ratio is higher than 1 (1.16) and chlorine was detected in the exhaust gases. Figure 13b shows that [Cl2] increases with the reactor wall’s temperature. This effect is likely to result from the consumption of intermediate chlorinated hydrocarbons, especially chlorobenzenes and phosgene. In fact, a marked decrease in [COCl2] was observed with a hotter reactor wall, as shown in Fig. 13c. Hence phosgene and carbon monoxide exhibit the same temperature dependence. This result will be discussed later on. Figure 14 illustrates the influence of the methane-air pilot flame on the chlorinated products. These values refer to experiments carried out with a slight flow of water vapor added to the air jets, so that the Cl/H ratio exceeds unity only when the pilot flame is not established. This ratio decreases to 0.81 and 0.62 when the pilot flame is used with methane flow rates equal to 1.04 ⫻ 10⫺4 and 2.08 ⫻ 10⫺4 mol s⫺1, respectively. A marked decrease in [Cl2] and [COCl2] was observed when the pilot flame reduced the Cl/H ratio to less than 1.0. Simultaneously, [HCl] increased from 6 to 8%. A further increase in the methane flow rate could not produce additional changes, because all the chlorine atoms initially present in the fuel molecule had already been converted into HCl. Figure 15 shows the concentrations of the chlorinated products when more water vapor

A. BISSONNIER ET AL.

Fig. 13. Influence of the reactor temperature on the outlet [HCl] (a), [Cl2] (b) and [COCl2] (c). Proportion of peripheral air: 䊐 50%; ‚ 67%; { 91%. Methane flow rate: 1.04 ⫻ 10⫺4 mol s⫺1. No water added. Fuel injector: B.

was added to the peripheral and axial air jets. In these experiments, flame stability requirements imposed the use of the pilot flame, with a methane flow rate of 1.04⫻ 10⫺4 mol s⫺1, so that the highest value of the Cl/H ratio, obtained without water added is only 1.16. The

INCINERATION OF TETRACHLOROBENZENE

Fig. 14. Influence of the pilot flame on the outlet [HCl] (a), [Cl2] (b) and [COCl2] (c). Proportion of peripheral air: 䊐 50%; ‚ 67%. Reactor temperature: 1273 K. water flow rate: 7.85 ⫻ 10⫺5 mol s⫺1. Fuel injector: B.

addition of water vapor increased [HCl] (see Fig. 15a) and decreases [Cl2] and [COCl2] concentrations (see Figs. 15b and 15c). A closer examination of the results shows that for chlorinated species, especially phosgene, an increase in the methane flow to the pilot flame reduced an outlet concentration more efficiently than

249

Fig. 15. Influence of water addition on the outlet [HCl] (a), [Cl2] (b) and [COCl2] (c). Proportion of peripheral air: 䊐 50%; ‚ 67%. Reactor temperature: 1273 K. Methane flow rate: 1.04 10⫺4 mol s⫺1. Fuel injector: B.

adding water vapor. Without the pilot flame and with a water vapor flow rate of 7.85 ⫻ 10⫺5 mol s⫺1, that is, Cl/H ⫽ 1.10, [Cl2] and [COCl2] concentrations ⫽ 0.01 % and ⫽ 0.15% (see Fig. 14b and c). With the pilot flame (methane flow rate: 1.04 ⫻ 10⫺4 mol s⫺1) and no water vapor

250

A. BISSONNIER ET AL. combustion of chlorinated aromatics have been identified by Tsang [38] in a comparative study of the consumption of chlorinated methanes, alkanes (with more than two carbon atoms per molecule), alkenes and aromatics. For the latter, the progressive abstraction of chlorine leads to benzene and phenol and then to the phenoxy radical. Hence, the aromatic ring ruptures in the decomposition of phenoxy and cyclopentadienyl radicals, as for non-chlorinated aromatics:

Fig. 16. Dependence of [COCl2] on the fuel injector diameter. Filled symbols: injector A; open symbols: injector B. proportions of peripheral air: 䊐, ■: 50%; ‚, Œ: 67%; {, } 91%. Reactor temperature: 1273 K. Flow rates (in mol s⫺1): Methane 1.04 ⫻ 10⫺4, Water 1.04 ⫻ 10⫺4.

added to the air jets; that is, Cl/H ⫽ 1.16, the corresponding values are [Cl2] ⫽ 0.008 % and [COCl2] ⫽ 0.024 %. Because it has been shown that the pilot flame markedly increases the temperature in the vicinity of the burner (see Fig. 5), it is likely that a thermal effect also contributed to the decrease in [COCl2], in addition to the decrease of the Cl/H ratio. Figure 16 illustrates the influence of the fuel injector diameter on the axial profiles of [COCl2]. The results are closely similar to those observed for [CO]. At 50% and 67% peripheral air, the smaller injector leads to negligible phosgene concentrations, whereas substantial concentrations are obtained with the larger injector. With 91% peripheral air, the cooling of the flame by the intense mixing is dominant and profiles of [COCl2] tend to be independent of the diameter of the fuel injector. The systematic study of the influence exerted by the main adjustable parameters in an incinerator on the overall progress of the degradation of tech-TCB shows that CO and COCl2 are the main intermediate products, whose concentrations can be significant in the exhaust gases. Detailed reaction mechanisms accounting for the formation of these products in the combustion of chlorinated aromatic compounds are not yet available. Most studies on chlorinated aromatics have focused on pyrolysis [34, 35] or aimed at elucidating the formation of dioxins and furans [36, 37]. The main routes for the

C6H5O 3 C5H5 ⫹ CO

(R5)

C5H5 3 C2, C3 unsaturates

(R6)

Tsang [38] also reported that with high Cl/H ratios, the addition of chlorine atoms to unsaturated species is fast. These reactions form chlorinated C2 and C3 radicals involved in the combustion of small chlorinated alkanes. For the latter, several experimental and modeling studies carefully considered the formation of phosgene. Senkan et al. measured the concentrations of phosgene along stabilized premixed flames fed with various chlorinated fuels: C2HCl3 [18, 39 – 41], 1,2-C2H4Cl2/CH4 [42], CHCl3/CH4 [43], CCl4/CH4 [44]. These flames cover a wide range of equivalence ratios, from lean to rich, and Cl/H ratios either larger or smaller than unity. On the other hand, this group did not report phosgene in CH2Cl2/CH4/O2/Ar flames [33]. Detailed reaction mechanisms developed to reproduce the measured concentration profiles contain three reactions forming phosgene [44]: CCl2CH ⫹ O2 3 COCl2 ⫹ CHO

(R7)

C2Cl3 ⫹ O2 3 COCl2 ⫹ COCl

(R8)

CCl3 ⫹ O2 3 COCl2 ⫹ ClO

(R9)

Simultaneously, Koshland et al. [45, 46] studied the thermal degradation of chlorinated hydrocarbons injected in the burned gases of propane or methane flames. Phosgene was not observed in experiments involving the lightly chlorinated additives: CH3Cl (temperature range: 1215–1417 K) [45] and C2H5Cl (temperature range: 950 –1225 K) [46]. Injection in the combustion-driven reactor of chlorinated compounds with Cl/H ratio ⫽ 1.0: 1,1,1-C2H3Cl3 [47] and CH2Cl2 [48] produced phosgene. These

INCINERATION OF TETRACHLOROBENZENE experimental studies were complemented by modeling [47– 49]. The dominant route forming phosgene from trichloroethane involves, as in Senkan et al.’s work, the dichlorovinyl radical: 1,1 C2HCl2 ⫹ O2 3 COCl2 ⫹ HCO

(R10)

Whereas, the dichloromethyl radical is a precursor when phosgene is formed from dichloromethane: CHCl2 ⫹ ClO 3 COCl2 ⫹ HCl

(R11)

Both groups of workers use the same submechanism to describe the consumption of phosgene: COCl2 ⫹ X 3 COCl ⫹ XCl

(12)

COCl ⫹X 3 CO ⫹ XCl

(13)

COCl ⫹ M 3 CO ⫹ Cl ⫹ M

(14)

with X ⫽ Cl, H, O, OH. In experiments with a pure chlorinated hydrocarbon as fuel, the disappearance of phosgene is the main route leading to CO and this observation could explain the close similarity between [CO] and [COCl2] found in this work.

the reactor exit can only be achieved with a Cl/H ratio smaller than 1, even at the highest temperature. However, it has been shown that adding H atoms via the pilot flame is more efficient than adding water, so that an increase of the flame temperature reinforces the effect of the Cl/H ratio. The variations of [CO] and [COCl2] are consistent with the kinetic models developed in more fundamental studies Financial support from RE.CO.R.D. association (REseau COordonne´ de Recherches sur les De´chets) is greatly appreciated. We are also very pleased to acknowledge Les Ciments Franc¸ais, TREDI, Solvay France, Gaz de France, Rho ˆne Poulenc for their continuous interest throughout this study.

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CONCLUSION This work confirms that heavily chlorinated hydrocarbons can be thermally degraded in conditions giving not only very efficient destruction of the fuel, but also limited conversions of carbon and chlorine into the final products CO2 and HCl. With TCB as fuel, CO and COCl2 are the dominant intermediate species. Trace amounts of tri, penta and hexa-chlorobenzenes were also detected. Complete conversion of CO to CO2 was observed in the exhaust gases at the higher temperatures (1363 K) in this reactor, quite independently of other parameters, such as the Cl/H ratio or the diameter of the liquid fuel injector. At lower temperatures, the absence of CO in the exhaust gases requires the addition of H atoms to decrease the Cl/H ratio below unity. This result can be obtained either by increasing the flow rate of CH4 feeding the pilot flame or by adding water vapor. The complete disappearance of phosgene at

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Received 18 May 2001; revised 7 December 2001; accepted 7 December 2001