An experimental and numerical study of the thermal oxidation of chlorobenzene

Chemosphere 42 (2001) 703±717

An experimental and numerical study of the thermal oxidation

of chlorobenzene

a,

b

c

Brian Higgins ^*, Murray J. Thomson , Donald Lucas ,

d

Catherine P. Koshland , Robert F. Sawyer

d

a

California Polytechnic University, San Luis Obispo, CA, USA

b

University of Toronto, Canada

Lawrence Berkeley National Laboratory, Berkeley, CA, USA

c

d

University of California, Berkeley, CA, USA

Abstract

A combustion-driven ¯ow reactor was used to examine the formation of chlorinated and non-chlorinated species

from the thermal oxidation of chlorobenzene under post-¯ame conditions. Temperature varied from 725 to 1000 K,

while the equivalence ratio was held constant at 0.5. Signi®cant quantities of chlorinated intermediates, vinyl chloride

and chlorophenol, were measured. A dominant C±Cl scission destruction pathway seen in pyrolytic studies was not

observed. Instead, hydrogen-abstraction reactions prevailed, leading to high concentrations of chlorinated byproducts.

The thermal oxidation of benzene was also investigated for comparison.

Chemical kinetic modeling of benzene and chlorobenzene was used to explore reaction pathways. Two chloro-

benzene models were developed to test the hypothesis that chlorobenzene oxidation follows a CO-expulsion breakdown

pathway similar to that of benzene. For the temperatures and equivalence ratio studied, hydrogen abstraction by

hydroxyl radicals dominates the initial destruction of both benzene and chlorobenzene. Chlorinated byproducts (i.e.,

chlorophenol and vinyl chloride) were formed from chlorobenzene oxidation in similar quantities and at similar

temperatures to their respective analogue formed during benzene oxidation (i.e., phenol and ethylene). Ó 2001 Elsevier

1. Introduction

of hazardous emissions. Transient or upset conditions,

cold-wall impingement, poor atomization of liquid

waste, and rogue droplets are examples of poor oper-

ating conditions (Oppelt, 1986). Waste-incinerator upset

conditions can be replicated in a laboratory environment

with a ¯ow reactor. Flow reactor studies are useful be-

cause they have measurable boundary conditions, re-

peatable operation, and can be operated on a scale many

times smaller than a waste incinerator. For these rea-

sons, ¯ow reactors are used often to investigate and to

predict the reactions that occur in large-scale waste in-

cinerators.

Thermal oxidation of chlorinated hydrocarbons in an

incinerator is a common waste-disposal method. Under

ideal conditions, chlorinated hydrocarbons oxidize to

H₂O, CO₂and HCl. The HCl can be removed easily

with exhaust gas processing. Equilibrium concentrations

of parent and intermediate species are negligible (Yang

et al., 1987). However, in a poorly operating hazardous

waste incinerator, chlorinated hydrocarbons can escape

the ¯ame zone and exist in the post-¯ame region, where

chemical-kinetic limitations can lead to the production

Chlorobenzene decomposition is of interest as the

chlorine atom is bonded strongly to the benzene ring

(Tsang, 1990) and potentially contributes to dioxin

formation (Ritter and Bozzelli, 1990, 1994; Sommeling

et al., 1993). A number of studies have examined

^*Corresponding author. Present address: Mech. Eng. Dept.,

Cal. Poly., San Luis Obispo, CA 93407 USA.

E-mail address: bshiggin@calpoly.edu (B. Higgins).

PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 2 4 5 - 9

704

B. Higgins et al. / Chemosphere 42 (2001) 703±717

chlorobenzene reactions, focusing on the removal of the

chlorine atom. Tsang (1990), Ritter and Bozzelli (1990),

and Manion and Louw (1990) found that chlorine dis-

placement by atomic hydrogen dominates over chlorine

abstraction at post-¯ame temperatures. Ritter and

Bozzelli (1990) found that in an H₂/O₂reaction system,

O₂catalyzed Cl abstraction from chlorobenzene by H

atom. Martinez et al. (1995) suggests that HO₂attack is

a major pathway of chlorobenzene destruction in the

presence of H₂O₂. Tsang (1990) stated that chlorination

of the aromatic ring is not favored. Senkan (1993)

studied premixed, laminar chlorobenzene ¯ames and

detected chlorinated intermediates in both fuel-rich and

fuel-lean ¯ames. The formation of chlorinated byprod-

ucts under post-¯ame conditions has not been studied in

depth.

anisms have not developed completely for every im-

portant intermediate species.

This study uses ¯ow reactor experiments and chem-

ical-kinetic modeling to explore the reaction pathways of

chlorobenzene at moderate temperatures and fuel-lean

conditions. The study focuses on the analogies between

benzene and chlorobenzene oxidation. These two com-

pounds are separately injected into a ¯ow reactor; the

concentrations of these species and their decomposition

products are measured and compared. This experimen-

tal data is also used to validate the chemical kinetic

models. The modeling provides a more detailed exam-

ination of the injected compoundÕs reaction pathways.

2. Experimental data

A detailed understanding of chlorobenzene destruc-

tion is strengthened with insight gained from studying

benzene reactions and its decomposition products. Most

of the chemical-kinetic development, for aromatic

compounds, has been under pyrolytic conditions to in-

vestigate soot and polycyclic aromatic hydrocarbon

(PAH) formation. There are some notable exceptions.

Venkat et al. (1982) and Lovell et al. (1988) have studied

the oxidation of benzene in an adiabatic ¯ow reactor. A

number of chemical-kinetic benzene mechanisms have

been published (e.g., Bittker, 1991; Emdee et al., 1992;

Zhang and McKinnon, 1995; Marinov et al., 1996; Tan

and Frank, 1996). Oxidative benzene reactions are

generally understood. However, chemical-kinetic mech-

Intermediate species from chlorobenzene oxidation

were studied by injecting benzene (C₆H₆) or chloro-

benzene (C₆H₅Cl) into a non-isothermal, combustion-

driven ¯ow reactor. This reactor operated with peak

temperatures between 700 and 1200 K, an equivalence

ratio of 0.5, and residence times near 0.3 ms. Gas

composition was measured downstream, along the ¯ow-

reactor centerline, using extractive sampling and a

Fourier-transform infrared (FTIR) spectrometer.

The ¯ow reactor, schematically illustrated in Fig. 1,

was capable of independent control of residence time,

equivalence ratio, and peak temperature. Combustion

products were delivered to the ¯ow reactor from two

Fig. 1. Schematic representation of the combustion-driven ¯ow reactor. The external ¯ame is cooled and mixed with the internal ¯ame

upstream from the point of injection. Products are extracted along the centerline, 1.2 m downstream of the injection point.

B. Higgins et al. / Chemosphere 42 (2001) 703±717

705

sources. Temperature control was achieved by cooling

one of the combustion-product sources and mixing it

with the non-cooled source. By changing the relative

¯ow through the two combustion sources, the temper-

ature in the ¯ow reactor was varied without altering the

equivalence ratio. Each combustion source was operated

at an equivalence ratio of 0.92. An equivalence ratio of

0.5 was achieved by mixing secondary air at the point

where the combustion sources were mixed. The total

¯owrate was held constant at 350 slpm. The temperature

was held constant with an active-feedback-control sys-

tem, which varied the relative ¯owrate of the two com-

80 K at a rate of approximately 300 K/s for the ¯owrates

used in this study. The combined eect of mixing and

heat transfer to the wall produces a peak temperature in

the ¯ow that is located downstream of the point of in-

jection. This measured peak temperature is the temper-

ature used to plot the data in later ®gures.

By injecting the benzene and chlorobenzene along the

centerline of the reactor, wall eects were reduced, but

this created a region of unmixedness that was not in-

cluded in the numerical model. This presents a limitation

in the utility of the model, and became the primary

reason for injecting both benzene and chlorobenzene.

The published mechanisms for benzene oxidation are

assumed to be, and are shown to be, sucient to

document the general trends observed in the benzene

experiments. By modeling both benzene and chloro-

benzene, only the relative dierences and similarities

between the two are important, and ®ndings are not

diluted by the limitations of the model. Secondly, the

important reactions are highly temperature dependent

and although the benzene and chlorobenzene are in-

jected in an unmixed fashion, by the time the maximum

temperature is reached, they are well mixed.

bustion sources to maintain

a ®xed temperature

upstream of the injection point. The control system was

automated with a computerized control system, ther-

mocouples, and mass-¯ow controllers. Details of the

experimental setup are given by Higgins (1995).

The ¯ow reactor was operated in a non-isothermal

mode. Heat transfer to the wall was augmented by a

counter ¯ow of air in the annular region between the

¯ow reactor and the water jacket. This enabled ecient

data acquisition as the ¯ow reactor quickly achieved

steady state. Since the ¯ow reactor was not isothermal,

the centerline temperature pro®le within the reactor was

experimentally measured for each condition. These

measured temperature pro®les are plotted in Fig. 2 for

the chlorobenzene injection case; the pro®les for benzene

are nearly identical. The temperature pro®les were used

as the thermal boundary condition for chemical-kinetic

modeling. Note that they exhibit a characteristic shape.

There is an initial increase in temperature, a point of

peak temperature, and ®nally a steady decrease in tem-

perature. The initial increase in the centerline tempera-

ture occurs during the mixing process of the injected

species with the main ¯ow of combustion products; de-

tails are discussed by Higgins (1995). The steady

decrease in temperature near the end of the ¯ow is due to

heat transfer to the wall. Temperatures drop from 50 to

Gas species compositions were measured using ex-

tractive sampling and a FTIR spectrometer. Species were

extracted from the centerline of the ¯ow reactor through

a quartz capillary probe, and were carried from the ¯ow

reactor to the FTIR, through heated Te¯on tubing. The

byproducts were measured with a Biorad FTS-40 FTIR

spectrometer coupled to an infrared analysis, long path

cell (0.6 m base path length, 14.4 m total path length). A

needle valve at the exit of the optical cell was used to

maintain a pressure of 71 Torr in the cell. With a vacuum

pump downstream of this valve and the capillary probe

upstream, the ¯ow through the cell was continuous.

Stable measurements were possible after about 20 min of

sampling. The FTIR was calibrated by injecting liquid or

gaseous species into an evacuated cell and adding ni-

trogen to 71 Torr. All calibrated species are expected to

have an error of 20% (Hall et al., 1991) except for hy-

drogen chloride. Hydrogen chloride was dicult to cal-

ibrate due to absorption to the optical cell walls. Errors

in the hydrogen chloride calibrations are assumed to be

less than 50% (Higgins, 1995). Although absolute

concentrations exhibit large errors, the concentration of

any one species has much less relative error.

A carbon balance for benzene ± or chlorobenzene ±

cannot be given. The amount of CO₂in the combustion

products was approximately 8% of the total ¯ow, and

benzene ± injected at 563 ppm ± only accounts for 4% of

the total carbon in the system. Since an FTIR is not able

to measure CO₂with an accuracy of 4%, a carbon

balance is not included.

The walls of the combustor were constructed with

stainless steel. Several experiments were run with a quartz

liner to address the possibility of catalytic reactions

Fig. 2. Measured centerline temperature for chlorobenzene

injection.

706

B. Higgins et al. / Chemosphere 42 (2001) 703±717

of chlorinated hydrocarbons on the stainless-steel sur-

faces. All stainless steel in the ¯ow path was eliminated

from the ¯ow reactor and the extractive sampling lines.

With the quartz liner, there were no observable dier-

ences in the gas species concentrations. Wall eects were

not observed since the turbulent mixing time within the

¯ow reactor was of the same order of time as the con-

vective ¯ow time. Thus, the chlorinated hydrocarbons

did not have sucient time to ¯ow from the centerline of

the ¯ow reactor to the wall and back to the centerline

where the gasses were sampled (Higgins, 1995).

product ¯ow was in chemical-kinetic thermodynamic

equilibrium at the temperature measured just upstream

of the point of injection.

Benzene was modeled using ®ve published chemical-

kinetic mechanisms. The mechanism of Tan and Frank

(1996) gave no signi®cant benzene oxidation in the

temperature range studied, which was not unexpected

since their mechanism was developed for a rich, near-

sooting ¯ame. The mechanism of Bittker (1991) had

results that were signi®cantly dissimilar to the experi-

mental results. Three are analyzed further in this paper:

Emdee et al. (1992), Zhang and McKinnon (1995), and

Marinov et al. (1996). Both the Zhang and McKinnon

(1995) and Marinov et al. (1996) mechanisms incorpo-

rated portions of the Emdee et al. (1992) mechanism.

The benzene mechanism of Zhang and McKinnon

(1995) was developed at low pressures and required

modi®cation. Speci®cally, the mechanism reaction rates

were modi®ed for atmospheric pressure using the origi-

nal references and the QRRK method (Dean, 1985). The

CHEMACT computer code (Dean et al., 1991) was used

for QRRK calculations along with the parameters given

by Zhang and McKinnon (1995). The molecular ther-

modynamic data of Zhang and McKinnon (1995) were

also used.

Species were sampled through a quartz capillary

probe at low pressure. Reactions in the probe can con-

vert radical species into stable species that are not

formed directly in the ¯ow reactor. Lovell et al. (1988)

and Brezinsky et al. (1990) propose that conversion

of phenoxy to phenol is common in probes. This reaction

C₆H₅O RH ! C₆H₅OH R

is aided by the large quantities of hydrogen from com-

bustion products. Likewise, this suggests that chlori-

nated phenoxy radicals will undergo the same hydration

process. Eectively, measured phenol represents both

phenol and phenoxy sampled from the ¯ow reactor.

Therefore, when a phenoxy radical was predicted by the

model, it was assumed to hydrate in the probe. In the

®gures, predicted phenoxy-radical concentrations were

added to the phenol concentrations. Likewise, predicted

chlorophenol and chlorophenoxy radical concentrations

were added together.

Chlorobenzene data was modeled using two aggregate

chlorobenzene-oxidation mechanisms. One was based on

the benzene mechanism of Zhang and McKinnon (1995)

and the other on Marinov et al. (1996). No rate constants

were adjusted to ®t the experimental data. The aggregate

mechanism contained reactions from six sources: (1)

parent benzene sub-mechanism (Zhang and McKinnon,

1995; or Marinov et al., 1996), (2) CO/H₂O/HCl reaction

sub-system of Roesler et al. (1992), (3) Cl/hydrocarbon

reactions (Senkan, 1993), (4) chlorinated C₁and C₂sub-

mechanism of Qun and Senkan (1994), (5) individual

chlorobenzene and chlorophenol reactions from various

literature sources (e.g., Mallard et al., 1998), and (6) re-

actions developed in the discussion section of this paper.

More details are provided in later sections. Whenever

possible, literature molecular thermodynamic data was

used. Otherwise, the computer program THERM (Ritter

and Bozzelli, 1991) was used to calculate thermodynamic

parameters of radicals and molecular species based on

the methods of Benson group additivity, and properties

of radicals based on bond dissociation.

The intent of using a non-isothermal, combustion-

driven ¯ow reactor was to simulate conditions expected in

the post-¯ame region of a hazardous waste incinerator

during a failure mode. Heat loss, oxygen concentration,

residence time, POHC concentrations, and turbulence

levels were designed to model those found in a large-scale

incinerator. This lab-scale ¯ow reactor was also designed

tohavecontrolledandmeasurableboundaryconditionsto

facilitate modeling. The balance of these features allowed

the measurement and prediction of a real-life situation

within the bounds and expectations of a laboratory mea-

surement system. Discrepancies in the plug-¯ow-reactor

assumption used for modeling are made up through the

direct comparison of data from two species (e.g., the more

studied benzene versus the less studied chlorobenzene).

Using the above approach, two chlorobenzene

mechanisms have been produced. Each mechanism con-

tains the original reactions of the parent benzene mech-

anism. The chlorobenzene mechanisms developed for

this paper are used for analysis only and have not been

tested against data from other temperature and mixture

regimes. The reaction rates used in this study are included

in Tables 1±3. Table 1 contains 37 reactions added to

both the Zhang and McKinnon mechanism and the

Marinov et al. mechanism. In Table 2, are the 35 reactions

3. Numerical model and mechanisms

Numerical modeling of the chemical processes in the

¯ow reactor was performed using the CHEMKIN

package (Kee et al., 1990) with a plug-¯ow-reactor

(PFR) assumption (Chen, 1992). Measured centerline

temperatures were used to constrain the temperature in

the PFR model. The inlet boundary condition for the

plug ¯ow reactor model assumed that the combustion

B. Higgins et al. / Chemosphere 42 (2001) 703±717

707

Table 1

Reactions present in both chlorobenzene mechanisms (in cal-K-gmole-cm-s units)

Reaction

A

n

Ea

Source

CO & HCl submechanism

Chlorinated C₁and C₂submechanism

Reactions of C₁through C₆hydrocarbons with

Cl and ClO

Roesler et al., 1995

Qun and Senkan, 1994

C₆H₅Cl+O C₆H₅+ClO

1.00E+08

3.00E+15

5.20E+15

2.60E+15

1.50E+13

2.00E+13

4.00E+12

2.00E+12

1.20E+12

6.00E+11

5.00E+12

8.40E+12

4.20E+12

4.35E+13

2.69E+13

4.05E+13

3.11E+13

2.14E+13

2.51E+13

1.50E+13

1.00E+13

1.00E+07

2

0

2

37600

Qun and Senkan, 1994

Ritter and Bozzelli, 1990

Martinez et al., 1995^a

Ritter and Bozzelli, 1990

Manion et al., 1988

Louw et al., 1973^a

Martinez et al., 1995^a

Martinez et al., 1995

Frerichs et al., 1989^a

Manion and Louw, 1990

Estimate

C₆H₅Cl C₆H₅+Cl

95499

110000

7500

6450

12000

12800

14000

13000

4763

8243

7500

40300

0

C₆H₅Cl o-C₆H₄Cl+H

C₆H₅Cl m-C₆H₄Cl+H

C₆H₅Cl p-C₆H₄Cl+H

C₆H₅Cl+H C₆H₆+Cl

C₆H₅Cl+H C₆H₅+HCl

C₆H₅Cl+H o-C₆H₄Cl+H₂

C₆H₅Cl+H m-C₆H₄Cl+H₂

C₆H₅Cl+H p-C₆H₄Cl+H₂

C₆H₅Cl+Cl o-C₆H₄Cl+HCl

C₆H₅Cl+Cl m-C₆H₄Cl+HCl

C₆H₅Cl+Cl p-C₆H₄Cl+HCl

C₆H₅Cl+OH C₆H₅OH+Cl

C₆H₅Cl+O C₆H₅O+Cl

C₆H₅Cl+O o-C₆H₄ClO+H

C₆H₅Cl+O m-C₆H₄ClO+H

C₆H₅Cl+O p-C₆H₄ClO+H

o-C₆H₄ClOH+H C₆H₅OH+Cl

m-C₆H₄ClOH+H C₆H₅OH+Cl

p-C₆H₄ClOH+H C₆H₅OH+Cl

o-C₆H₄ClOH+H C₆H₅Cl+OH

m-C₆H₄ClOH+H C₆H₅Cl+OH

p-C₆H₄ClOH+H C₆H₅Cl+OH

o-C₆H₄ClO+H C₆H₅O+Cl

m-C₆H₄ClO+H C₆H₅O+Cl

p-C₆H₄ClO+H C₆H₅O+Cl

o-C₆H₄Cl+H C₆H₅+Cl

Estimate

m-C₆H₄Cl+H C₆H₅+Cl

p-C₆H₄Cl+H C₆H₅+Cl

Estimate

ClC₅H₄+H C±C₅H₅+Cl

C₅H₄ClO+H C±C₅H₅O+Cl

C₄H₄Cl H₂CCCCH₂+Cl

C₄H₄Cl+OH H₂CCCCH₂+HOCl

C₄H₄Cl+O H₂CCCCH₂+ClO

C₄H₄Cl+H H₂CCCCH₂+HCl

C₄H₄Cl+Cl H₂CCCCH₂+Cl₂

Qun and Senkan, 1994

0

^aisomerization added.

added only to the Zhang and Mckinnon mechanism and

in Table 3 are the 22 reactions added only to the Marinov

et al. mechanism. More work is needed to verify the re-

actions with estimated rates. Great care should be taken

when using these rates, particularly for conditions not

representative of the conditions found in this study.

Fig. 3, normalized destruction pro®les for benzene and

chlorobenzene are plotted versus the peak ¯ow-reactor

temperature; the pro®les are quite similar. The ®rst

indication that both benzene and chlorobenzene are

reacting is seen between 800 and 850 K. Benzene ex-

hibits slightly higher reactivity, reaching 50% oxidation

at 950 K compared to 960 K for chlorobenzene. By

993 K, more than 97% of the injected benzene was

destroyed, converted to products or intermediates.

Likewise, by 1000 K, over 96% of the chlorobenzene

was destroyed.

4. Results and discussion

Benzene or chlorobenzene was injected to give initial

concentrations of 563 or 719 ppm, respectively. In

708

B. Higgins et al. / Chemosphere 42 (2001) 703±717

Table 2

Additional reactions present in the mechanism based on the benzene mechanism of Zhang and McKinnon, 1995 (in cal-K-gmole-cm-s

units)

Reaction

A

n

Ea

Source

Benzene submechanism

C₆H₅Cl+OH o-C₆H₄Cl+H₂O

Zhang and McKinnon, 1995

Mulder and Louw, 1987 and

Zhang and McKinnon, 1995^a

Mulder and Louw, 1987 and

Zhang and McKinnon, 1995^a

Mulder and Louw, 1987 and

Zhang and McKinnon, 1995^a

Estimate

1.68E+12

8.42E+11

0

4491

C₆H₅Cl+OH m-C₆H₄Cl+H₂O

C₆H₅Cl+OH p-C₆H₄Cl+H₂O

4491

o-C₆H₄ClOH+OH o-C₆H₄ClO+H₂O

m-C₆H₄ClOH+OH m-C₆H₄ClO+H₂O

p-C₆H₄ClOH+OH p-C₆H₄ClO+H₂O

o-C₆H₄ClO+H o-C₆H₄ClOH

m-C₆H₄ClO+H m-C₆H₄ClOH

p-C₆H₄ClO+H p-C₆H₄ClOH

o-C₆H₄ClO ClC₅H₄+CO

2.95E+06

7.24E+47

7.41E+11

2.09E+12

3.00E+13

1.50E+13

3.00E+13

2.51E+11

1.20E+11

7.50E+06

1.00E+06

1.00E+07

1.00E+16

1.00E+07

3.89E+17

0.66E+12

0.33E+12

2

)1312

20190

43900

7470

2

Estimate

2

)9.7

0

Estimate

m-C₆H₄ClO ClC₅H₄+CO

0

Estimate

p-C₆H₄ClO ClC₅H₄+CO

0

o-C₆H₄Cl+O₂ o-C₆H₄ClO+O

m-C₆H₄Cl+O₂ m-C₆H₄ClO+O

p-C₆H₄Cl+O₂ p-C₆H₄ClO+O

o-C₆H₄Cl+O₂ 2CO+C₂H₂+CHClCH

m-C₆H₄Cl+O₂ 2CO+C₂H₂+CHClCH

p-C₆H₄Cl+O₂ 2CO+C₂H₂+CHClCH

o-C₆H₄Cl+O₂ 2CO+C₂H₂+CH₂CCl

m-C₆H₄Cl+O₂ 2CO+C₂H₂+CH₂CCl

p-C₆H₄Cl+O₂ 2CO+C₂H₂+CH₂CCl

o-C₆H₄Cl+O₂ 2CO+C₂HCl+C₂H₃

m-C₆H₄Cl+O₂ 2CO+C₂HCl+C₂H₃

p-C₆H₄Cl+O₂ 2CO+C₂HCl+C₂H₃

ClC₅H₄+HO₂ C₅H₄ClO+OH

C₅H₄ClO C₄H₄Cl+CO

0

Estimate

0

7470

0

Estimate

0

15002

0

Estimate

0

Estimate

0

Estimate

0

Estimate

0

Estimate

0

43900

0

5000

C₄H₄Cl+O₂ C₄H₃Cl+HO₂

0

Estimate

C₄H₃Cl+OH C₄H₂Cl+H₂O

2

2.00

C₄H₃Cl+OH H₂CCCCH+HOCl

C₄H₃Cl+O H₂CCCCH+ClO

C₄H₃Cl+M H₂CCCCH+Cl+M

C₄H₃Cl+Cl H₂CCCCH+Cl₂

CH₂CCl+C₂H C₄H₃Cl

44700

36400

100800

42800

2291

Qun and Senkan, 1994

Estimate

2.00

0.00

2.00

1.75

0

C₄H₂Cl+O₂ CHClCO+HCCO

C₄H₂Cl+O₂ CH₂CO+C₂ClO

0

Estimate

^aisomerization added.

5. Benzene oxidation results

quantities of CO₂and H₂O in the background com-

bustion-products ¯ow.

The ®rst phase of this study was to evaluate existing

benzene chemical-kinetic mechanisms against the ex-

perimental results. The experiments span a range of

temperatures and species concentrations dierent from

the data originally used to validate the mechanisms.

Benzene (C₆H₆) oxidation was observed between 850

and 1000 K. The results for C₆H₆oxidation are plotted

in Figs. 4±7. Measured byproducts include phenol

(C₆H₅OH), formaldehyde (CH₂O), ethylene (C₂H₄),

acetylene (C₂H₂) and carbon monoxide (CO). The ulti-

mate products of C₆H₆oxidation (i.e., CO₂and H₂O)

were observed but not quanti®ed due to the large

In Fig. 4, measured and predicted concentrations of

C₆H₆are plotted versus peak temperature. All the three

mechanisms predict that the start of C₆H₆oxidation

occurs above 900 K. Experimental data shows signs of

benzene oxidation at temperatures approximately 50 K

lower. Complete destruction of the parent species was

predicted equally by all the three mechanisms. The mech-

anism of Marinov et al. (1996) appears to ®t the data best.

Data for phenol (C₆H₅OH), the only detected aro-

matic byproduct, are plotted in Fig. 5. The numerical

results include both phenoxy (C₆H₅O) and C₆H₅OH as

described earlier. C₆H₅OH was only observed at tem-

B. Higgins et al. / Chemosphere 42 (2001) 703±717

709

Table 3

Additional reactions present in the mechanism based on the benzene mechanism of Marinov et al., 1996 (in cal-K-gmole-cm-s units)

Reaction

A

n

Ea

Source

Benzene submechanism

C₆H₅Cl+OH o-C₆H₄Cl+H₂O

Marinov et al., 1996

Mulder and Louw, 1987 and

Marinov et al., 1996^a

Mulder and Louw, 1987 and

Marinov et al., 1996^a

Mulder and Louw, 1987^a

estimate

1.90E+07

1.42

1454

C₆H₅Cl+OH m-C₆H₄Cl+H₂O

1454

C₆H₅Cl+OH p-C₆H₄Cl+H₂O

o-C₆H₄ClOH+OH o-C₆H₄ClO+H₂O

m-C₆H₄ClOH+OH m-C₆H₄ClO+H₂O

p-C₆H₄ClOH+OH p-C₆H₄ClO+H₂O

o-C₆H₄ClO+H o-C₆H₄ClOH

C₆H₄ClOm+H m-C₆H₄ClOH

p-C₆H₄ClO+H p-C₆H₄ClOH

o-C₆H₄ClO ClC₅H₄+CO

9.00E+06

2.95E+06

1.00E+14

7.40E+11

2.60E+13

3.00E+13

2.51E+11

6.00E+11

4.00E+11

3.00E+12

3.30E+13

6.60E+13

1.42

2

1454

)1310

0

2

Estimate

2

0

Estimate

0

Estimate

0

43850

6120

0

m-C₆H₄ClO ClC₅H₄+CO

0

Estimate

p-C₆H₄ClO ClC₅H₄+CO

0

o-C₆H₄Cl+O₂ o-C₆H₄ClO+O

m-C₆H₄Cl+O₂ m-C₆H₄ClO+O

p-C₆H₄Cl+O₂ p-C₆H₄ClO+O

ClC₅H₄+HO₂ C₅H₄ClO+OH

C₅H₄ClO C₄H₄Cl+CO

0

Estimate

0

Estimate

0

43900

0

Estimate

C₄H₄Cl+O₂ C₃H₂ClO+CH₂O

C₄H₄Cl+O₂ CHCHCHO+CHClO

C₃H₂ClO+O₂ COCl+CHOCHO

C₃H₂ClO C₂H₂+COCl

0

Estimate

0

33000

Estimate

C₃H₂ClO C₂HCl+HCO

0

^aisomerization added.

Fig. 4. Outlet concentration of C₆H₆(squares) for the injection

of C₆H₆. Modeled using the mechanisms of Emdee et al. (dotted

line), Zhang and McKinnon (dashed line), and Marinov et al.

(solid line).

Fig. 3. Normalized C₆H₆(®lled squares) and C₆H₅Cl (open

squares) outlet concentration for increasing peak temperature.

Data normalized to inlet concentrations predicted from ¯ow-

rate measurements.

within a factor of two and adequately described produc-

tion trends. The Zhang and McKinnon (1995) mechanism

also described the formation trends of C₆H₅OH and the

Marinov et al. (1996) mechanism predicted a lower tem-

perature for peak concentration of C₆H₅OH.

peratures where partial C₆H₆oxidation is observed.

Above 1000 K, where C₆H₆was fully oxidized, C₆H₅OH

was not observed. C₆H₅OH peaked near 960 K at a con-

centration of 201 ppm, or a 71% yield of the reacted C₆H₆.

Below 950 K, the conversion rate of reacted benzene to

phenol is between 90% and 100%. The Emdee et al. (1992)

mechanism predicted peak C₆H₅OH concentration

The trends for CH₂O, Fig. 6, followed that of

C₆H₅OH. Peak concentration occurred at the same

temperature (960 K) and at a slightly lower concentra-

710

B. Higgins et al. / Chemosphere 42 (2001) 703±717

Fig. 7. Outlet concentration of C₂H₂(squares) and C₂H₄(cir-

cles) for the injection of C₆H₆. Only C₂H₂was modeled, using

the mechanisms of Emdee et al. (dotted line), Zhang and Mc-

Kinnon (dashed line), and Marinov et al. (solid line). None of

the mechanisms predicted signi®cant C₂H₄.

Fig. 5. Outlet concentration of C₆H₅OH (squares) for the in-

jection of C₆H₆. Modeled data plotted as the sum of C₆H₅OH

and C₆H₅O, using the mechanisms of Emdee et al. (dotted line),

Zhang and McKinnon (dashed line), and Marinov et al. (solid

line).

All the three benzene-oxidation mechanisms predict-

ed byproducts that were not measured. The Marinov

et al. (1996) mechanism predicted signi®cant concentra-

tions of 2,4-cyclohexadienone (C₆H₆O), benzoquinone

(OC₆H₄O), and 2,4-cyclopentadiene-1-one (C₅H₄O).

OC₆H₄O peaked at 66 ppm at 993 K. C₆H₆O and C₅H₄O

continuously increased with temperature, reaching 81

and 160 ppm at the highest temperature condition (1051

K). The Zhang and McKinnon (1995) mechanism pre-

dicted 2,4-cyclopentadiene-1-one (C₅H₄O) formation.

The C₅H₄O increased with temperature, forming 64 ppm

at the highest temperature condition (1051 K). The C₄H₄

radical was predicted to peak at 76 ppm at 1021 K. The

Emdee et al. (1992) mechanism also predicts high levels

of C₄H₄and C₅H₄O.

tion (173 ppm). The mechanism of Emdee et al. (1992)

did not include CH₂O. The CH₂O data were underpre-

dicted by the other two mechanisms, although both

preserved qualitative trends.

In Fig. 7, data for C₂H₄and C₂H₂are plotted. Peak

concentrations and concentration pro®les for both C₂H₄

and C₂H₂occurred at higher temperatures than was seen

for C₆H₅OH and CH₂O. At 980 K, concentrations

peaked at 33 and 15 ppm, respectively. The pro®les peak

at higher temperatures relative to C₆H₅OH, and the

species occur later in the destruction pathway than C₆H₆

and C₆H₅OH (discussed in the next section). The

Marinov et al. (1996) mechanism did the best job of

modeling C₂H₂concentrations, but none of the mecha-

nisms predicted signi®cant levels of C₂H₄.

For near stoichiometric conditions, Emdee et al.

(1992) present experimental data including both C₄and

C₅species as byproducts for benzene oxidation at an

equivalence ratio of 0.9. For our conditions, the Emdee

et al. mechanism predicted C₄and C₅byproduct species.

While many of the predicted C₄and C₅species are de-

tectable with an FTIR, concentrations above 1 ppm

were not detected in our experiments. This discrepancy

is probably due to the ¯ow of combustion products in

which the benzene is injected. That is, in our experiments

benzene is injected at low concentrations into the ¯ow of

fuel-lean combustion products. High concentrations of

H₂O and CO₂alter the chemistry by providing active

reacting partners and a source of radical species. De-

struction reactions of C₄and C₅species in a ¯ow of

combustion products are not well characterized in the

literature for these temperatures (ꢀ700 to 1000 K).

With the exception of lower temperatures, the de-

struction of C₆H₆was well predicted by all the three

mechanisms. This con®rms that much of the C₆chemistry

is valid for our experimental conditions. At low temper-

Fig. 6. Outlet concentration of CH₂O (squares) for the injec-

tion of C₆H₆. Modeled using the mechanisms of Emdee et al.

(dotted line), Zhang and McKinnon (dashed line), and Marinov

et al. (solid line).

B. Higgins et al. / Chemosphere 42 (2001) 703±717

atures (e.g., 800±900 K), a lack of benzene oxidation and

711

phenol production suggests that some minor changes in

the C₆chemistry is needed. The prediction of minor

species requires more work. Discrepancies in CH₂O,

C₂H₂, and C₂H₄concentrations point to incomplete

chemistry describing the cascade from C₆to C₂species.

6. Benzene oxidation destruction pathway (993 K)

The three mechanisms examined were developed se-

quentially, each expanding on the premise that aro-

matic-ring oxidation occurs through a CO-expulsion

breakdown pathway, reducing the number of carbons in

the ring (i.e., C₆! C₅! C₄) until it is no longer stable

and a linear species is formed. In general, each carbon-

removal step includes an oxygen-addition step and a

CO-expulsion step. The CO-expulsion step from a C₅

species breaks the ring, forming either a linear C₄species

or two C₂molecules.

Fig. 9. Destruction-pathway diagram of C₆H₆at 993 K, using

the mechanism of Marinov et al. Relative ¯ux through path-

ways are proportional to the arrow width. Species in bold were

experimentally measured.

The general description of C₆H₆oxidation can be

expanded upon and visualized with a reaction pathway.

In Figs. 8 and 9, the reaction pathways for the oxidation

of C₆H₆at 993 K are shown using the Zhang and Mc-

Kinnon (1995) mechanism (Fig. 8) and the Marinov et al.

(1996) mechanism (Fig. 9). In these ®gures, each reaction

path is indicated with arrows whose thickness is pro-

portional to the ¯ux through that pathway. A tempera-

ture of 993 K was chosen because that was the lowest

temperature at which nearly all of the benzene reacted.

In Fig. 8, the Zhang and McKinnon (1995) reaction-

pathway diagram, the dominant destruction pathway for

C₆H₆is through H abstraction by OH to form C₆H₅, with

a secondary path through O substitution to directly form

C₆H₅O. The C₆H₅reacts with molecular oxygen to form

either C₆H₅O or C₂products. The C₆H₅O expels CO to

form C₅H₅. The majority of the C₅H₅reacts with HO₂and

follows a similar expulsion cycle (i.e., oxidation, and CO

expulsion). The direct oxidation of C₆H₅to C₂products is

described by a global reaction. The inclusion of a global

reaction is unfortunate, but is required (to some degree)

since details of benzene oxidation at these concentrations

and temperatures are not complete. While Tan and Frank

(1996) suggest that the use of this global reaction path is

inappropriate, without the global reaction concentrations

of C₂species (e.g., acetylene and ethylene) are signi®-

cantly underpredicted. There is also a global reaction for

the destruction of C₅H₄O directly to C₂products.

For the Marinov et al. (1996) reaction-pathway

diagram (Fig. 9), the same general C₆chemistry is seen

and nearly all of the reaction occurs through the

C₆® C₅® C₄, CO-expulsion pathway. After C₆H₅and

C₆H₅O are formed, the subsequent breakdown of the

aromatic ring is dominated by dierent reactions than for

the Zhang and McKinnon (1995) mechanism. In partic-

ular, progression from C₆to C₅is described with funda-

mental reactions. The C₅H₄O reactions are grouped into

one global reaction producing C₂products, while reac-

tions of C₅H₅O are modeled with fundamental reactions.

Both mechanisms have the same fundamental C₆

chemistry, where breakdown of C₆H₆is predicted at low

temperatures due to an ecient chain-branching path-

way:

Fig. 8. Destruction-pathway diagram of C₆H₆at 993 K, using

the mechanism of Zhang and McKinnon. Relative ¯ux through

pathways are proportional to the arrow width. Species in bold

were experimentally measured.

712

B. Higgins et al. / Chemosphere 42 (2001) 703±717

7. Chlorobenzene oxidation

C₆H₆ OH ! H₂O C₆H₅

C₆H₅ O₂! C₆H₅O O

A similar analysis is used to investigate chloroben-

zene (C₆H₅Cl) oxidation. The experimental results for

C₆H₅Cl oxidation (from 860 to 1000 K) cover conditions

not used to validate any existing mechanisms. Measured

byproducts include phenol (C₆H₅OH), three isomers of

chlorophenol (o-, m-, and p-C₆H₄ClOH), formaldehyde

(CH₂O), ethylene (C₂H₄), vinyl chloride (C₂H₃Cl),

acetylene (C₂H₂), and carbon monoxide (CO). The ul-

timate products of C₆H₅Cl (i.e., CO₂, HCl, and H₂O)

were observed but only HCl was quanti®ed. Molecular

chlorine (Cl₂) was not quanti®ed since it is not detect-

able with an FTIR spectrometer. All of the experimental

results for C₆H₅Cl oxidation are plotted in Figs. 10±15.

The experimental data are plotted with the results from

the modeling eort, using two aggregate chlorobenzene-

oxidation mechanisms.

These reactions converts one radical (OH) into two

(C₆H₅O and O). Standard H₂/O₂chemistry converts the

O back into OH and the cycle repeats. Equilibrium

concentrations of OH in the combustion-products ¯ow

eciently initiate this reaction pathway at the low tem-

peratures. Without this chain branching reaction, little

destruction of C₆H₆is predicted. This is the primary

reason the mechanism of Tan and Frank (1996) does not

predict C₆H₆destruction at low temperatures.

The source of CH₂O in both mechanisms is the oxi-

dation of products from the destruction of the aromatic

ring. The lack of good agreement in CH₂O is probably

not due to insucient C₂chemistry, but rather a lack of

fundamental chemistry describing the destruction of the

aromatic ring. The Zhang and McKinnon (1995)

mechanism used a global reaction step to produce C₂H₃,

which then reacts with O₂to form CH₂O. The Marinov

et al. (1996) mechanism formed CH₂O directly from the

reaction of C₄H₅with O₂. Better prediction of the aro-

matic ring destruction to C₂species is needed to better

understand the amounts of CH₂O, C₂H₂, and C₂H₄seen

in the experiments.

As seen in Fig. 4, the reactivity of chlorobenzene is

similar to benzene. Examination of the experimental data

reveals that the byproducts of chlorobenzene oxidation

are similar in concentration and temperature range to

benzene. For example, in Fig. 11 the peak concentration

summed for all phenolic compounds is very close in

magnitude and temperature as seen for benzene (Fig. 5).

Similar byproduct trends for chlorobenzene and benzene

are seen for CH₂O, CO, and C₂species.

While there were some obvious dierences in pre-

dicted peak concentrations and trends with temperature

among the three mechanisms, the qualitative description

of byproduct formation was generally good. There were

three notable exceptions: (1) the lack of ethylene (C₂H₄)

prediction, (2) the lack of a viable phenol (C₆H₅OH)

oxidation pathway other than through phenoxy radicals

(C₆H₅O), and (3) incomplete fundamental C₅and C₄

chemistry, as evidenced by C₁and C₂species predictions.

The qualitative dierence between benzene oxidation

and chlorobenzene oxidation is found solely in the

presence of chlorinated byproducts for chlorobenzene

oxidation, which are analogous to the non-chlorinated

byproducts measured for benzene oxidation. The lack of

detectable C₄and C₅species is noted for both the ben-

zene and chlorobenzene experiments.

Fig. 11. Outlet concentration summed for all phenolic species,

C₆H₅OH, o-, m-, and p-C₆H₄ClOH (squares) for the injection

of C₆H₅Cl. Modeled data is plotted as the sum of all phenol,

chlorophenol, phenoxy and chlorophenoxy isomers using ag-

gregate mechanisms based on the Zhang and McKinnon (thin

line) and Marinov et al. (thick line) mechanisms.

Fig. 10. Outlet concentration of C₆H₅Cl (squares) and HCl

(circles) for the injection of C₆H₅Cl. Model data is plotted for

C₆H₅Cl (solid lines) and HCl (dashed lines) using aggregate

mechanisms based on the Zhang and McKinnon (thin lines)

and Marinov et al. (thick lines) mechanisms.

B. Higgins et al. / Chemosphere 42 (2001) 703±717

713

Initial modeling eorts, using the mechanism of

Martinez et al. (1995), predicted only 5% destruction of

chlorobenzene at 1000 K. This mechanism also did not

include chlorinated intermediate species. Most chemical-

kinetic studies of chlorobenzene have focused on fuel

rich and/or pyrolytic conditions. The resulting destruc-

tion pathways relied heavily on a C±Cl scission to ini-

tiate reaction. The bond dissociation energy for the

C±Cl bond in chlorobenzene is about 15% less than the

C±H bond in benzene. When pure or inert-diluted

chlorobenzene is pyrolyzed, the weaker bond strength

results in C±Cl scission as the dominant route of uni-

molecular chlorobenzene decomposition. Further, in a

dilute atmosphere, chlorinated hydrocarbon byproducts

are reduced, since the liberated chlorine radical quickly

abstracts hydrogen to form HCl. The C±Cl scission step

is weakly chain branching and does not provide for the

oxidation of chlorobenzene observed at lower tempera-

tures (e.g., 1000 K), which requires a stronger chain-

branching mechanism to describe the measured results.

the three chlorophenyl radical isomers, ortho-, meta-, or

para-chlorophenyl.

Reactions were sought to describe this destruction

pathway. Using the NIST database (Mallard et al., 1998)

and literature sources, 17 reactions describing chloro-

benzene and chlorophenol reactions were found and in-

cluded in each aggregate chlorobenzene mechanism. The

reactions of chlorobenzene with OH, O and Cl were from

Louw et al. (1973), Mulder and Louw (1987), Manion

et al. (1988), Frerichs et al. (1989), Ritter et al. (1990),

Qun and Senkan (1994) and Martinez et al. (1995).

Unimolecular reactions of chlorobenzene were included

from Ritter et al. (1990) and Martinez et al. (1995).

Chlorophenol reactions were included from Mulder and

Louw (1987). Destruction of the chlorophenyl radicals

was assumed to follow the benzene analogy, and reaction

rates from the benzene mechanisms were used to estimate

the reaction rate of chlorophenyl radicals with molecular

oxygen to create chlorophenoxy radicals. Further reac-

tions include the addition of H to form chlorophenol,

and the expulsion of CO to produce chlorinated C₅

species. All of the chlorinated-C₆reactions track the in-

dividual chlorinated isomers, and the rates of the reac-

tions are approximated using the benzene reaction rates.

Once C₅species are created, only one chlorinated isomer

was assumed. Reactions were scaled to create chlori-

nated products in proportion to the number of possible

isomers of the reactants. For example, consider

8. Chlorobenzene oxidation via benzene analogy

Based on the above observations, the chain-branch-

ing destruction pathway seen for benzene is proposed to

exist for chlorobenzene oxidation as well. This is used to

develop chemical-kinetic modeling that adequately de-

scribes the experimental results. Two chlorobenzene

chemical-kinetic mechanisms are developed. One was

developed based on the benzene mechanism of Zhang

and McKinnon (1995) and the other based on Marinov

et al. (1996). Two mechanisms were created because it is

not the intent of this paper to develop a chemical-kinetic

mechanism; rather it is to test the idea that benzene and

chlorobenzene undergo analogous reactions under the

conditions of this study.

CH₂CHCHCH O₂! CH₂O C₃H₃O

or similarly:

C₄H₅ O₂! CH₂O C₃H₃O

The analogous chlorinated reaction would have two

outcomes:

C₄H₄Cl O₂! CHClO C₃H₃O

and

We hypothesize that chlorobenzene, under post-¯ame

oxidative conditions, reacts similarly to benzene. Spe-

ci®cally, (1) there should be an ecient chain-branching

initiation step to promote reaction at low temperature,

and (2) there should be a CO-expulsion pathway analo-

gous to the benzene mechanisms (Emdee et al., 1992;

Zhang and McKinnon, 1995; Marinov et al., 1996).

Two chlorobenzene-oxidation mechanisms were as-

sembled following the CO expulsion steps seen for

benzene: initial hydrogen abstraction, followed by oxy-

gen addition, leading to CO expulsion (repeated twice).

The number of carbons in the ring is reduced (i.e.,

C₆! C₅! C₄) until the ring is no longer resonantly

stabilized and the resulting C₄ring breaks into linear

species. The chlorobenzene mechanism is complicated

by the lower symmetry in chlorobenzene. Hydrogen

abstraction from benzene creates a phenyl radical re-

gardless of which hydrogen was abstracted, while hy-

drogen abstraction from chlorobenzene creates one of

C₄H₄Cl O₂! CH₂O C₃H₂ClO

Each outcome was given a pre-exponential reaction-rate

factor equal to (2/5) and (3/5), respectively, to account

for the relative quantities of chlorinated isomers. This

procedure was repeated for every reaction in each parent

benzene mechanism. Reactions with Cl substitution by

H-atom attack have also been added for all chlorinated

aromatic compounds. All reactions used are included in

Tables 1, 2 and 3.

All of the original benzene chemistry is included in

the aggregate mechanisms so that if the C±Cl bond is

broken, the required chemistry is intact. Additionally,

once chlorinated C₂species are reached, the added sub-

mechanisms are capable of tracking the further evolu-

tion of these species.

714

B. Higgins et al. / Chemosphere 42 (2001) 703±717

In order for chemical-kinetic mechanisms to have

broad utility, the speci®c reaction rates for each chlori-

nated reaction should be correctly derived and/or ex-

perimentally determined. Many of the reaction rates

used in this analysis have been estimated using reaction

rates from published benzene mechanisms. Care should

be taken when using the rates in Tables 1, 2 and 3. The

value of this work is to establish the direction required

for the formulation of the reactions that are relevant to

fuel-lean thermal oxidation of chlorobenzene.

9. Chlorobenzene oxidation results

Fig. 13. Outlet concentration of o-C₆H₄ClOH (squares, left

axis), m-C₆H₄ClOH (circles, left axis), and p-C₆H₄ClOH (dia-

monds, right axis) for the injection of C₆H₆Cl. Modeled data

plotted as the sum of C₆H₄ClOH and C₆H₄ClO for each isomer,

using aggregate mechanisms based on the Zhang and McKin-

non (thin line) and Marinov et al. (thick line) mechanisms.

The thermal oxidation of C₆H₅Cl between 860 and

1000 K produced measured quantities of C₆H₅OH, o-,

m-, and p-C₆H₄ClOH, CH₂O, C₂H₄, C₂H₃Cl, C₂H₂,

CO, and HCl. Experimental and numerical-modeling

data, using aggregate mechanisms based on the benzene

mechanisms of (1) Zhang and McKinnon (1995) and of

(2) Marinov et al. (1996), are plotted in Figs. 10±15.

In Fig. 10, measured and predicted concentrations of

the injected species, C₆H₅Cl, are plotted versus peak

temperature. Overall, destruction of chlorobenzene is

predicted well by the mechanism based on Marinov et al.

(1996), although it slightly underpredicts chlorobenzene

destruction below 950 K. The Zhang and McKinnon-

based mechanism predicts chlorobenzene reaction only

above 970 K. Also plotted in Fig. 11 are HCl concen-

trations. At the highest temperature (1000 K), most of

the measured chlorine appears to have been converted to

HCl. One-to-one conversion of Cl from C₆H₅Cl to HCl is

dicult to determine since HCl was hard to measure

accurately and Cl₂was not detectable with an FTIR

spectrometer. At 1000 K, the Marinov-based mechanism

predicted that only 0.4% chlorobenzene was converted to

Cl₂, a maximum concentration of 3 ppm.

All of the phenolic compounds (o-, m-, p-C₆H₄ClOH

and C₆H₅OH) were summed and plotted in Fig. 11. The

numerical results also include the phenoxy and chloro-

phenoxy radicals, o-, m-, p-C₆H₄ClO and C₆H₅O. The

peak concentration of these species reached 205 ppm at

956 K. This equates to a yield of 71% of the reacted

chlorobenzene and occurs when about 60% of the

chlorobenzene were destroyed. These numbers are rela-

tively close to those found for the benzene oxidation

case. The Marinov-based mechanism accurately predicts

the temperature at which the phenolic compounds peak,

but the magnitude is underpredicted by a factor of two.

In Fig. 12, phenol (C₆H₅OH) concentrations are

plotted. Peak quantities of C₆H₅OH occur at 932 K with

a maximum concentration of 36 ppm. This occurs about

Fig. 12. Outlet concentration of C₆H₅OH (squares) for the

injection of C₆H₆Cl. Modeled data plotted as the sum of

C₆H₅OH and C₆H₅O using aggregate mechanisms based on the

Zhang and McKinnon (thin line) and Marinov et al. (thick line)

mechanisms.

Fig. 14. Outlet concentration of C₂H₄(squares), and C₂H₃Cl

(circles) for the injection of C₆H₅Cl. Neither mechanism pre-

dicted signi®cant C₂H₄or C₂H₃Cl concentrations.

B. Higgins et al. / Chemosphere 42 (2001) 703±717

715

ppm, C₂H₂at 61 ppm, C₂H₄at 8.3 ppm and C₂H₃Cl at

5.1 ppm. In Fig. 14, data for C₂H₃Cl and C₂H₄are

plotted. In Fig. 15, data for C₂H₂and CH₂O are plotted.

C₂H₃Cl was the only non-aromatic chlorinated hydro-

carbon detected. The three C₂species peak at nearly the

same temperature (980, 980, and 970 K), which is

slightly higher than the peak C₆H₅OH and CH₂O tem-

perature but less than that for peak of CO. Maximum

concentrations for C₂H₂and C₂H₄occur at nearly

identical peak temperatures as seen for C₆H₆.

Of the four minor byproducts, only C₂H₂and CH₂O

were predicted by the two mechanisms. C₂H₃Cl and

C₂H₄were not predicted at levels above 10 ppb. No

signi®cant concentrations of other stable chlorinated

species were predicted by the mechanisms. The Mari-

nov-based mechanism predicts C₂H₂well, but CH₂O

was underpredicted and with a higher peak temperature.

These results are likely to be due to the chlorinated C₅,

C₄and C₃chemistry. The poor prediction of C₂H₃Cl

from chlorobenzene oxidation is analogous to the poor

prediction of C₂H₄in benzene oxidation. Chlorinated C₅

and C₄compounds are required to explain the produc-

tion of vinyl chloride.

Fig. 15. Outlet concentration of C₂H₂(squares), and CH₂O

(circles) for the injection of C₆H₅Cl. Modeled data plotted for

C₂H₂(solid lines) and CH₂O (dashed lines) using aggregate

mechanisms based on the Zhang and McKinnon (thin lines)

and Marinov et al. (thick lines) mechanisms.

20 K lower, with approximately one-sixth the yield of the

C₆H₆experiments. Both mechanisms poorly predicted

C₆H₅OH, indicating that they still lack important Cl-

abstraction or substitution reactions. It is doubtful that

the C₆H₅OH is underpredicted due to poorly estimated

reaction rates since all of the reactions leading to

C₆H₅OH were taken directly from literature sources. The

three isomers of chlorophenol are plotted individually in

Fig. 13 and have maximum concentrations of 30, 107,

and 38 ppm, respectively, occurring near 950 K (Note,

there are two meta- and ortho-substitution sites, but only

one para-substitution site). To ease comparison, the data

for C₆H₅OH (Fig. 12) and p-C₆H₄ClOH (Fig. 13, right

axis) are plotted with an ordinate scale that is twice that

of o-and m-C₆H₄ClOH (Fig. 13, left axis). By doing this,

the phenolic compounds are plotted in proportion to the

number of substitution sites. It is easy to see that

the experimental data for C₆H₅OH, p-C₆H₄ClOH, and

m-C₆H₄ClOH all have approximately the same trends

and the same peak quantities (per substitution site).

Additionally, plotting the numerical data in this manner

causes all to fall on top of one curve (for each mecha-

nism). This happens because the reaction-rate data were

scaled proportionally to the number of substitution sites.

The experimental data for o-C₆H₄ClOH diers from

the other phenolic compounds, with much lower con-

centrations. This can be explained by a combination of

geometric and electrophilic eects: the adjacent Cl atom

physically shields the hydrogen, and the electrophilic

nature of the Cl atom shortens the H bond length

making it stronger and less reactive. These eects have

not been considered when estimating the chemical-

kinetic rates; doing so should give results that are

qualitatively closer to the measure data.

10. Chlorobenzene oxidation destruction pathway (1000 K)

Fig. 16 shows the principal reaction pathways of

chlorobenzene using the Marinov-based chlorobenzene

mechanism. The principal reaction pathways from the

chlorobenzene mechanisms based on the work of Zhang

and McKinnon (1995) are essentially identical. Only the

signi®cant C₆chemistry is included in the ®gure. C₅±C₁

species show the same dominant trends seen for each

benzene mechanism used to develop the two aggregate

chlorobenzene mechanisms.

The two main pathways for initial C₆H₅Cl destruction

are through H abstraction by OH to form C₆H₄Cl and

through O substitution to directly form C₆H₄ClO. The

Fig. 16. Destruction-pathway diagram of C₆H₅Cl at 1000 K,

using the aggregate chlorobenzene mechanism based on Mari-

nov et al. Relative ¯ux through pathways are proportional to

the arrow width. Species in bold were experimentally measured.

Four minor byproducts were observed at peak con-

centrations less than 100 ppm, including CH₂O at 93

716

B. Higgins et al. / Chemosphere 42 (2001) 703±717

C₆H₄Cl undergoes molecular oxygen attack to form

C₆H₄ClO. Passing through C₆H₄ClO, CO is expelled to

form C₅H₄Cl, which follows a similar expulsion cycle

(i.e., oxidation, and CO expulsion) leading to products.

The C₆chemistry is the same as for benzene with the

following distinctions. Destruction of C₆H₅Cl is initiated

by the same chain branching seen for the benzene case.

Grouping the C₆reactions gives two dominant pathways:

to produce by the chlorination of combustion byprod-

ucts. This suggests that C₂H₃Cl is probably originating

from chlorinated C₅and C₄species, just as C₂H₄is

probably originating from C₅and C₄species in the

benzene case.

12. Conclusions

C₆H₅Cl O₂ OH ! H₂O C₆H₄ClO O

and

The results of an experimental ¯ow reactor study of

the thermal oxidation of benzene and chlorobenzene

have produced some interesting mechanistic conclu-

sions. Although much work has been published on

benzene chemical kinetics, most of it has focused on

conditions dierent to those used in this study. The

chemical-kinetic mechanisms of Emdee et al. (1992),

Zhang and McKinnon (1995), and Marinov et al. (1996)

did a suitable job of describing post-¯ame benzene de-

struction under thermal oxidation conditions. Most of

the measured byproducts were predicted and the trends

were usually adequate, however, the magnitude of the

predictions was typically beyond the experimental error

of the measurements. Although the chemical-kinetic

mechanisms have evolved and improved considerably,

trace byproduct prediction still requires more work be-

fore reliable results in real-life environments are realized.

Published chlorobenzene reaction mechanisms did a

poor job of predicting oxidation of the parent species for

the temperatures and species concentrations used in this

study. This is not unexpected since most studies focused

on pyrolytic conditions. Furthermore, literature sources

of reaction rates did not cover all reactions occurring in

this regime (e.g., chlorophenol destruction reactions)

and reaction rates were estimated using the benzene

mechanisms.

C₆H₅Cl O ! C₆H₄ClO H

The ®rst is chain branching and the second is chain

propagating. It is these reactions and the equilibrium

concentrations of OH in the combustion-products ¯ow

that result in ecient destruction of chlorobenzene at

low temperatures. Note that most of the rates for the

chain-branching, C₆-species reactions were taken from

literature sources and were not estimated from benzene

reaction rates. The majority of the estimated reaction

rates were for reactions not containing C₆species.

Chlorophenol, C₆H₄ClOH, was also formed. Due to

the relative stability of phenolic compounds, formation is

chain terminating and slows the destruction of C₆H₅Cl.

Similar to benzene, destruction of C₆H₄ClOH is only

predicted to occur back through C₆H₄ClO. The bond

dissociation energy of the phenolic hydrogen is still only

about 90% that of the ring hydrogen and direct destruc-

tion orcontinuedoxidation isprobable, butnotpredicted.

No signi®cant amount of dechlorination of the

chloroaromatic compounds is predicted. This is respon-

sible for the underprediction of phenol and indicates that

Cl-removal reactions are missing from the mechanisms.

Experimentally, chlorobenzene was slightly less re-

active than benzene. A destruction scheme was pro-

posed, analogous to benzene oxidation, where

chlorobenzene oxidation occurs through hydrogen ab-

straction followed by CO expulsion. Hydrogen-

abstraction reactions, leading to ecient, low-tempera-

ture chain branching, were essential for predicting the

destruction of chlorobenzene at 900 K. In addition, C±

Cl scission reactions and Cl substitution reactions were

not found to dominate at the temperatures and back-

ground species concentrations (combustion products)

used in this study. The analysis was complicated by the

reduced symmetry of chlorobenzene, which introduces

many more chlorinated isomers.

11. Summary of chlorobenzene oxidation

These results support the idea that the thermal oxi-

dation pathways of chlorobenzene are analogous to

those of benzene. As the reaction mechanism of benzene

becomes better understood, it is expected to also im-

prove the chlorobenzene mechanism. Until the C₅and

C₄chemistry in the benzene mechanism is ®rmly estab-

lished, chlorinated C₅and C₄chemical-kinetic destruc-

tion pathways will be dicult to determine accurately.

De®ciencies in the chlorobenzene mechanisms presented

here include the reaction rates for elementary chloro-

phenyl and chlorophenol oxidation reactions, detailed

chlorinated-C₅and -C₄chemistry, and the formation

pathways to directly produce chlorinated C₂species

from chlorinated C₄species.

Since the relative dissociation bond energies between

phenolic and ring hydrogens are nearly the same, con-

tinued reaction of phenol and chlorophenol is also ex-

pected, resulting in destruction routes of the phenolic

compounds that are dierent than the formation routes.

The lack of predicted C₂H₄from both benzene and

chlorobenzene and C₂H₃Cl from the chlorobenzene

The absence of vinyl chloride (C₂H₃Cl) and ethylene

(C₂H₄) prediction is analogous to the absence of C₂H₄

prediction in the benzene case. There is sucient sub-

chemistry in the model to account for C₂H₃Cl if it were

B. Higgins et al. / Chemosphere 42 (2001) 703±717

717

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With the lack of an active C±Cl scission reaction

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Article Doi

An experimental and numerical study of the thermal oxidation of chlorobenzene

DOI: 10.1016/S0045-6535(00)00245-9

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