Thermal destruction of CH3Cl under lean postflame conditions

C O M B U S T I O N A N D F L A M E 90:185-195 (1992)

185

Thermal Destruction of CH3CI Under Lean Postflame Conditions ELIZABETH M. FISHER* and CATHERINE P. KOSHLAND Departments of Mechanical Engineering (E.M.F.) and Biomedical and Environmental Health Sciences (C.P.K.), University of California, Berkeley The thermal destruction of CH3CI in the lean postflame region of a nonisothermal turbulent combustor has been studied experimentally and modeled numerically. Conditions were chosen to simulate the destruction of waste in the postflame zone of a hazardous-waste incinerator, the process believed to be responsible for the bulk of incinerator emissions. Residence times were between 0.23 and 0.28 s, and equivalence ratios were between 0.49 and 0.67. The maximum temperature in the combustor varied from 1215 to 1417 K; temperatures declined from the maximum by several hundred degrees centigrade over the reaction zone. The transition from poor to thorough destruction of CH3C1 was observed experimentally when Tmax = 1220 K. The major products of CH3C1 destruction were CO2, H20, and HCI. In addition to these compounds, the following byproducts were observed: C2H 4, C2H 2, C2H3C1, and CO. The maximum level of the C 2 species was less than 2% of the initial CH3CI level; CO was present at a maximum level of about 20 percent of the initial CH3CI level. Numerical modeling of chemical kinetics was performed using a mechanism based on that of Karra et al. [Combust. Sci. Technol. 60:45-62 (1988)]. The modeling underpredicted the destruction of CH3CI, but predicted the same species as byproducts. According to the model, the C 2 byproducts were formed by recombination of chloromethyl radicals, which, in turn, had been formed by H abstraction from CH3CI by radicals. Discrepancies between model and experiment may be due to inaccuracies in modeling the initial composition or temperature history, or may indicate that the mechanism must be modified before it can make accurate predictions for fuel-lean conditions.

INTRODUCTION The breakdown of CH3C1 (methyl chloride, chloromethane) has been the subject of a number of recent studies. Interest in the decomposition reactions of this compound arises in connection with both hazardous waste incineration and hydrocarbon synthesis. Halogenated compounds, including CH3CI, account for roughly one quarter of incinerable wastes [1]. Although CH3CI comprises a minor part of this quantity, its simple structure and relatively low toxicity have made it a popular choice in fundamental combustion studies related to incineration. Moderately high-temperature reactions of CH3CI also play an important role in proposed processes [2, 3] for converting methane into more valuable unsaturated hydrocarbons such as ethylene and acetylene.

* Corresponding author. Present address: Sibley School of Mechanical and Aerospace Engineering, Upson Hall, Cornell University, Ithaca, NY, 14853-7501. Copyright © 1992 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc.

Detailed chemical kinetic modeling [4-7] and measurements in premixed flames [8-10], diffusion flames [11], flow reactors [2, 3, 6, 7, 12-15], fluidized beds [16], and shock tubes [8, 17] have all contributed to the understanding of the decomposition of CH3CI, especially under fuel-rich conditions. Like its nonchlorinated analogue CH4, CH3C1 produces C 2 species as it breaks down. Modeling indicates that these intermediates are created via the r e c o m b i n a t i o n of methyl ( C H 3) and chloromethyl (CH2CI) radicals, which were formed from CH3C1 by abstraction reactions or bond cleavage. Measured levels of C 2 species vary considerably over the range of temperatures and equivalence ratios represented by previous studies. However, C2H4 and C2H 2 are observed in almost all cases, generally at levels accounting for a few percent of the carbon initially in the form of CH3C1. Information about operating conditions and byproduct yields for three flow-reactor studies are presented in Table 1. 0010-2180/92/$5.00

186

E.M. FISHER AND C. P. KOSHLAND TABLE1

PreviousFlow ReactorStudies

Reference

T(K)

Granada et al.

1253

Equivalence Ratio 5.5

Residence Time(s)

Destruction Efficiency (%)

0.25

32

[4]

Ho and Bozzelli [6, 19]

1073-1223

5

Hung and Pfefferle

900-1350

0.036

1

50 at 1123 K

0.016-0.0195

50 at 1310 K

[7]

Byproduct

Approximate Peak Bypoduct Level, as a Fraction of Initial CH3CI Level

C2H 2 C2H 4 CH 4 CO H2 C2H3C1 CO C2H 2 C2H 4 C~H3CI CO C ~H 3C1 C2H 2 CH 4 C2H 4 CH zCI ~ 1,2-C2H4C12

0.10 0.09 0.05 0.05 0.03 0.02 1.08 a 0.13 a 0.03 a 0.01 a 0.31 0.035 0.033 0.021 0.015 0.002 0.002

a CH3C 1 is not the only source of carbon, both CH3C1 and CH 4 are present initially, in the ratio 2:1.

The present study involves the breakdown of CH3CI in a new, fuel-lean environment: CH3C1 is injected into a combustor in the region downstream of a lean propane/air flame. This configuration was chosen to mimic conditions in the postflame zone of an incinerator, a regime that appears to play a crucial role in determining emissions levels [20]. "Failure modes" such as poor mixing, poor atomization, or flame quenching by cold surfaces, can occur in incinerators. When an incinerator operates under these nonoptimal conditions, part of the waste decomposes in the postflame region, where temperatures and radical levels are lower than in the flame. The correspondingly lower destruction efficiencies and higher byproduct levels may be responsible for the bulk of incinerator emissions. EXPERIMENT

The decomposition of CH3C1 was studied in a tubular turbulent combustor that has been used in previous studies involving C2H5C1 [21] and 1,1,1-C2H3C13 [22] destruction; the experimental apparatus and procedure have been described in detail elsewhere [21-24]. The corn-

bustor consists of an insulated stainless-steel tube, 5.1 cm in inner diameter and 3.8 m in length. Heating and an initial radical pool are provided by a lean propane/air flame stabilized on a screen near one end. A secondary air stream mixes into the combustion products; the temperature profile in the combustor is adjusted by varying this flowrate. Farther downstream, a mixture of CH3CI and nitrogen is injected through four quartz injectors. The injectors are sections of 2-mm-i.d. tubing with one sealed end and five 0.4-mm holes. They are arranged at 90-degree intervals at a particular axial location and oriented to inject the CH3C1/nitrogen mixture in the upstream direction, to promote mixing. CH3C1 makes up less than one percent of the total flowrate; the nitrogen flow was required to prevent significant pyrolysis of the CH3C1 in the injectors. On the basis of other chlorinated hydrocarbon decomposition studies in quartz reactors [25], it appears that the combustor diameter is large enough to make the contribution of wall reactions negligible. Downstream of the injection location, bare chromel/alumel thermocouples and quartz probes are used to measure temperature and composition profiles along the

T H E R M A L DESTRUCTION OF CH3CI

187

combustor centerline. Gas samples are extracted from the combustor through lengths of 3-mm-i.d. quartz tubing and pass through heated stainless-steel lines and into the longpath cell of a Fourier-Transform Infrared (FTIR) spectrometer. FTIR spectra were examined, and quantitative composition measurements were obtained for HCI, CO, CH3CI, C 2 H 2 , C 2 H 4 , and C 2 H 3 C 1 with detectability limits of 100, 20, 50, 1.5, 8, and 16 ppm, respectively. CO 2 and H 2 0 were observed in all spectra but not quantified. Spectra were examined for peaks caused by other species, including CH4, C2H5C1 , and CC120 (detectability limits 15, 130, and 7 ppm), but these species were not found. Measured moles of all species were normalized by the total number of moles elemental chlorine detected in the sample. As described elsewhere [21, 23], this corrected for the effects of non-instantaneous mixing of the chlorinated hydrocarbon/nitrogen mixture into the propane/air combustion products. After mixing was complete, chlorine balances were good to within 30%. Operating conditions, including flowrates, equivalence ratio, residence time, and Reynolds number, are listed in Table 2 for six experimental runs, labeled M-3 through M-8. The equivalence ratio is evaluated after the addition of secondary air and CH3C1; the flame equivalence ratio is 0.79 in all cases. Hazardous waste incinerators typically operate with overall equivalence ratios between 0.3 and 0.67 [1]. The maximum measured temperature is the temperature measured 11.5 cm upstream of the CH3C1 injectors. The residence time is calculated from temperatures and flowrates, as described below. For comparison, typical gas

residence times in incinerators are 0.3-3 s [1]. The Reynolds number is evaluated at the maximum measured temperature; and thus is the minimum Reynolds number of the gases after CH3CI injection. Over the course of several experiments the cycling of the reactor between high and ambient temperatures led to the formation of flakes of stainless steel, deposited just upstream of the CH3C1 injection location. These deposits were present in all runs except M-4 and M-6. Centerline temperature profiles for the six runs are shown in Fig. la. For the four runs with deposits of stainless-steel flakes there are dips in the temperature just after the injection location. These may be due to flow disruptions from the deposits. However, the changes in flow or surface reactions caused by the flakes seem to have little impact on composition profiles, as can be seen by comparing results from runs with similar flowrates performed with (M-3 and M-5) and without (M-4 and M-6) the deposit of stainless steel flakes. With the assumptions of ideal-gas behavior, plug flow, and instantaneous mixing, linear or quadratic fits to the temperature data (omitting the unexplained dips and, in some cases, the last thermocouple location) were manipulated to convert axial distance in the combustor to time after CH3C1 injection. Figure lb shows the centerline temperatures converted into functions of time from injection. Typical temperatures in the post flame zone of hazardous-waste incinerators are 900-1500 K [1]. Radial as well as axial gradients occurred in the combustor. A limited number of measurements indicated that temperatures were uniform within 50°C over the inner 75% of the radius, and dropped by

TABLE 2 Operating Conditions RUN ID

Propane Flow (g/s)

Primary Air Flow (g/s)

Secondary Air Flow (g/s)

CH3CI Flow (g/s)

Nitrogen Flow (g/s)

Equivalence Ratio

M-3 M-4 M-5 M-6 M-7 M-8

0.263 0.263 0.263 0.263 0.263 0.263

5.21 5.21 5.23 5.21 5.21 5.21

3.70 3.67 2.72 2.72 2.09 1.30

0.0585 0.0585 0.0585 0.0585 0.0590 0.0585

0.022 0.022 0.022 0.022 0.022 0.022

0.49 0.49 0.55 0.55 0.60 0.67

Reo

Residence Time(s)

Mass Fraction Elemental C1 (%)

Tmax (K)

4800 4700 4200 4100 3700 3200

0.23 0.23 0.24 0.25 0.26 0.28

0.44 0.45 0.49 0.50 0.54 0.60

1212 1222 1257 1268 1345 1417

188

E.M. FISHER AND C. P. KOSHLAND I

I

I

I

I

I

I

1400

10 0

¥.

I

I C H Cl I

l

i

i

3

. . . . . . . . . .--~ . . . . . . . . . . . . . . . . . . . . . . . . O HCI

" O

~ 121111 o 0

~E 1111111

~

C2H3Cl

Q

e

~N

.-."-.". -. ". . . . . . . . . . . . . . . . . ~ 'sO"~

Q

"E 8OO

1o3i I 60

600 -20

I

I 140

I

I 220

I

I[ 300

I 0.05

Axlal distance from Injectors (cm)

I

1400

I

'

I

I

I

M-8

lOOO i.

I 0.05

I 0.10

I 0.2

I 0.25

0.3

|

S. 'u-6 £M<'5-i

! 0.00

I 0.15

Fig. 2. Measured composition profile, run M-3 (Tmax = 1215 K).

E

600 -0.05

f 0.1

Time (s)

(a)

1200

(~)C2H4

I 0.15

I 0.20

I 0.25

0.30

Tlme from CH3CI Injeclon (s)

(b) Fig. 1. Centerline temperatures 9a) as a function of axial distance from the injectors; (b) as a function of time from CH3CI injection.

up to 100°C near the wall. Temperature measurements were not corrected for radiative and conductive heat losses. Measurements with a suction pyrometer indicate that thermocouple readings differ from gas temperature by about 20°C at 1200 K, and by about 50°C at 1290 K [26]. CALCULATIONS Chemical kinetic calculations of the decomposition of CH3CI were performed using the CHEMKIN subroutines [27] with the SENKIN driver program [28]. Plug flow and instantaneous mixing were assumed. Temperatures as a function of time were input to the program; these temperature histories were derived from fits to the centerline temperature profiles as

described above. The reaction mechanism was a modified version [21] of a chemical kinetic mechanism developed by Karra et al. [4] to describe rich a t m o s p h e r i c - p r e s s u r e CH3C1/O2/Ar flames. The modification to distinguish between isomers of the chlorinated C2 species was introduced to improve predictions for byproducts of C2H4C1 decomposition in a previous study [23]. For the cases reported here, the modified and original mechanisms give qualitatively similar results. Thermodynamic data collected by Miller [29] were used. Initial composition was assumed to be the equilibrium composition of the mixture of propane/air combustion products and secondary air at the injector temperature, with CH3C1 added. RESULTS AND DISCUSSION: EXPERIMENT

Figures 2 through 7 show normalized composition histories measured at the combustor centerline. Symbols are experimental data; curves were added to show the trends in the data. Figures 2 and 3 show partial decomposition of CH3C1, with formation of HC1, CO, C2H2, C2H4, and C2H3C1. The C~ species are present at levels accounting for about 1% of the initial chlorinated compound. Note that the oxidation of CO to CO 2 does not appear directly in these figures because CO 2, present in large quantities from the propane/air combustion products, is not quantified. As the temperature increases (Figs. 4 and 5), the destruction

THERMAL

="

DESTRUCTION

100 ~ ' ~ ' I ~ - : -

|

OF CH3CI

'

|

~HsCl

189

!

10 0

='-, co C2H2 c...c,

_,. ,o-.

| 10~ f" .* I 0.05

I 0.1

I 0.15

I 0.2

~

=o

~

-

i ~

~

i

I 0.25

i HCI

.._CH3C I

I C2H3CI

l

A

16 =

e o

0

10-3

0

I 0.05

I 0.1

I 0.15

I 0.2

I 0.25

0.3

Time (s) Fig. 4. Measured composition profile, run M-5 (Tmax = 1257 K).

ill I I 10 0 ~ . _ _ . . ~ _ _ . . . O , . ~ - - C "

I -

I -

0.05

0.3

0

~i/..___,r.__.._ C,.H'

3CI

g ~ lO~ -~ ~,'--- 0 - . - . ,

10"1

q)

~

,

~I~20~C:

I

I

~

IO

HCl

A

10.=

Time (s) Fig. 3. Measured composition profile, run M-4 (Tm~~ = 1222 K).

10 0 i

I

0

¢=

c,.:,

0

/

tO m 4.p o 10"1 .? i 0 E

I

I 0 HCl

I 0.15

I 0.2

l 0.25

0.3

Time (s)

Fig. 6. Measured composition profile, run M-7 (To,,x = 1345 K).

efficiency and the quantities of CO and HCI increase. The levels of C 2 species are somewhat higher than in runs M-3 and M-4, and pass through maxima. At still higher temperatures (Figs. 6 and 7), destruction of CH3C1 is complete to the detectability limit by the end of the combustor. The byproducts observed in lower-temperature runs are observed here as well, but are for the most part destroyed before the combustor exit. Figure 8 summarizes the experimental resuits by displaying the composition at the combustor outlet, as a function of the maximum measured temperature for the run. Residence times at this point vary between 0.23 and 0.28 s. Figure 8a shows the major species: CH3CI , HC1, and CO. About 50% of the CH3C1 is

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e,, 0 7.,

I 0.1

. . . . . . c:..; . . . . $ . .

8

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0

E 10-2 ~ . _ . ~ _ _

t,o L. 0

0.05

0.1

;c,., 0.15

0.2

0.25

0.3

Time (s) Fig. 5. Measured composition profile, run M-6 (Tm~x = 1268 K).

_lk c o lO-, _ ~ - . . . . . .

@

10-~, 0

t o.05

I 0.1

I 0.15

I 0.2

I 0.25

0.3

Time (s) Fig. 7. Measured composition profile, run M-8 (Tmax = 1417 K).

190

E . M . FISHER AND C. P. KOSHLAND Q

1.o _o

o.e

j O

' q...

I

+..........;;c,

/

Q

°,

+o.+

_~

E 0.4

+ z

o.,_: x

,co •

0.0 1200

I 1250

CH3C I "T"" ~ ~i 1300 ~'350

1400

1450

Maximum temperature (K)

(a) 0.04 / o

I

0.03

'

'

'

'

I C2H2

0.02 ~-

°

0

/ " ' ~" ~ +.%.

-- 0.01 LAD' ~

a ",~C2H3CI

-

E o z

|C2H, ,<:~... i ..... '~ ...... I..+ 0.00 1200

1250

1300

1350

1400

1450

Maximum temperature (K)

(b) Fig. 8. Measured outlet concentration as a function of maximum temperature for the run. (a) major species; (b) minor species.

destroyed in run M-4, which has a maximum temperature of 1222 K. At its maximum, CO accounts for roughly 20% of the carbon initially present as CH3C1; the peak appears to occur at a slightly higher temperature than the point of 50% CH3CI destruction. The minor byproducts, C2H2, C2H4, and C2H3C1, are shown in Fig. 8b. They peak for runs with maximum temperatures in the range 1222-1257 K; at their maxima they are present respectively at 1.5%, 0.7%, and 1.7% of initial CH3C1 level. Because the reactor in the present study is nonisothermal, it is not possible to make a direct comparison of destruction efficiencies as a function of temperature with the other flow reactor studies listed in Table 1. However, destruction rates are clearly higher than in the experiment of Granada et al. [3], where only

32% CH3CI destruction was observed in an isothermal reactor at 1253 K for the same residence time as the present study. This difference may be due to higher initial radical levels in the present study, or it may indicate that the present oxidative environment provides more rapid destruction of CH3C1 than the oxygen-starved environment of the previous study [3]. It is interesting to compare the byproduct measurements among the flow reactor studies. C 2 H 4 , C 2 H 2 , C 2 H 3 C 1 , and CO, are observed in all four studies. In the present study and in one previous study [7], where equivalence ratios are less than one, C2H3C1 is more abundant than C 2 H 4 ; under rich conditions [3, 6, 19] the opposite is true. Additional compounds appearing in Table 1 were undetectable by FTIR (H 2) or probably below detectability limits (CHzC12 and 1,2-C2H4C12). The reaction mechanism provides an explanation for the difference in C 2 products, as discussed in the following section.

RESULTS AND DISCUSSION: CALCULATIONS As documented elsewhere [23], the composition profiles calculated by C H E M K I N agree poorly with the experimental results. Only representative results are presented here: Fig. 9 shows the calculated composition for run M-7. This corresponds to the experimental results

10 0 L

i

ICH3C I J

l

i

C O Cl

...-'"

lO'"

..°- ...............................

.o°" ~

o) O

fl°°

E

o°O.°'° . . . . . . . . .

CO C2H 2

......................

• ...............

~

CH 2

162

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i"'"

...

10 .3

2.,.:o

CH20

-" ......... '.'>.} ,,,,.c+c,, 0

0.05

.

0.1

0.15

0.2

.

.

.

.

.

0.25

0.3

Time (s) Fig. 9. Calculated composition profile, run M-7 (Tmax = 1345 K).

THERMAL DESTRUCTION OF CH3C1

191

presented in Fig. 6. All species included in the chemical kinetic mechanism are shown in Fig. 6, although radicals (CHzCI) and homonuclear diatomic species (H 2) could not have been detected experimentally. The degree of destruction of CH3C1 is much lower in the calculations than in the experimental results--comparable to the experimental results of run M-3, which had a maximum measured temperature 200°C lower. Figure 10 summarizes the computational results, and corresponds to Fig. 8. In Fig. 8, only the species observed experimentally are shown. As can be seen by comparing Figs. 8 and 10, the transition from poor to thorough destruction of CH3C1 occurs about 150°C higher in the calculations than in the experiments. The same intermediates are prominent in the two

m c 0

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: HCI

X

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\/ ..\

0.6 0

~ o.4 ~

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0.2

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0.0 1200

1250

1300

Maximum

1350

1400

temperature

1450

(K)

(a) 0.04

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=

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cases, but maximum levels differ by as much as a factor of 2. The chemical kinetic modeling results can be used to identify the most important reaction pathways for the conditions of interest. The relative importance of these pathways depends on the reaction mechanism. Figure 11 shows the rate of destruction of CH3C1 by particular reactions for run M-7. Only the five reactions with the highest absolute values of the rate of destruction of CH3C1 are shown. Note that all rates are positive, indicating that CH3C1 is being destroyed by these reactions. The most important reactions are hydrogen abstraction by the OH and CI radicals, forming CHzC1. The OH attack reaction has a maximum at the beginning of the run, when OH levels are at their initial equilibrium values. Later in the run, attack by Cl becomes more important. Considerably less prominent are the reactions of CH3C1 with the radicals O, HO 2, and H. The first two form CH2CI and the second forms CH 3. Each of these radical attack reactions contributes more than the unimolecular decomposition reaction does. Hence the destruction rate is determined by the levels of radicals as well as the temperature. The next step of the decomposition process involves the chloromethyl radicals (CHzC1) that are the main products of the initial radical attack. Figure 12 shows the contribution of individual reactions to destruction and formation of chloromethyl radicals. Reactions with negative destruction rates are sources of

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Maximum

i 1350

temperature

n 1400

1E-8 1450

(K)

(b)

Fig. lO. Calculated outlet concentration as a function of maximum temperature for the run. (a) major species; (b) minor species.

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I.

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p

~CH~CI+HO..-~-CH~CI+H~O.

CH 3CI+H - - ~ CH3+HCI

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0.05

0.10

0.15

0.20

0.25

0.30

Time (s)

Fig. ll. Rate of destruction of CH3CI: contribution of the five most important reactions.

192

E.M. FISHER AND C. P. KOSHLAND 7E-8

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,

he."

3E-8

~"

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The products of the recombination reactions decompose, forming C2H3C1 and C2H 4. C2H3C1 is created directly in reaction 1 and is formed from the product of reaction 3 by HCI elimination. C2 H4 is formed by/3-scission from the 2-chloroethyl radical created in reaction 2. Another channel exists for C2H 4 formation: when CH3CI is destroyed by H attack, the methyl radical is formed. This radical reacts with a chloromethyl radical to form ethylene as follows:

,

2CH CI--4.- CH

CICH CI

c.,c..o,--..-c.,o.,c.+(

-'~

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=

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~

0.05

0.10

CH_CI÷OH --~CH_CI+H O 0.15

0.20

0.25

0.30

Time (s)

(5)

CH 3 + CH2C1 = C2H 4 + HCI.

Fig. 12. Rate of destruction of CH2CI: contribution of the seven most important reactions.

decomposes primarily by HCI elimination to form C2H 2. C 2 H 4 is destroyed mainly by radical attack. The vinyl radical ( C 2 H 3) is formed; it reacts with molecular oxygen and is ultimately converted to CO and finally to CO 2. Figure 13 shows normalized sensitivity coefficients for CH3CI for the run M-7. These coefficients are defined as (Aj/Yi)*(o~yi/OAj), where Yi is the mass fraction of species i and Aj is the preexponential factor in the modified Arrhenius rate expression for reaction j. In this case, results are shown for the five reactions to which the mass fraction of CH3CI has the highest sensitivity. Note that, unlike the reactions featured in Fig. 10, these reactions need not involve CH3C1 as a reactant or product. The importance of radical levels in the determining destruction efficiency for CH3C1 is evident from the sensitivity ranking of reactions. The reactions with the highest negative sensitivity coefficients are those associated with C 2 H3C1

CH2CI. As expected, reactions involving radical attack on methyl chloride are the most important of these. Destruction of the chloromethyl radical occurs largely through recombination reactions, of which the most important are CHzCI + CH2CI = C2H3C1 + HC1,

(1)

CH2CI + CH2C1

(2)

= CHzCICH 2 +

C1,

CH2CI + CH2CI = CH2CICH2C1.

(3)

Next in importance is the reaction of the chloromethyl radical with oxygen: CH2C1 + 02 = C H 2 0 + C1 + O.

(4)

The net rate of formation of the chloromethyl radical is small in comparison to the rates of creation and destruction. The rate expressions used for these reactions are listed in Table 3. Similar profiles for other species lead to the following outline of the important reaction pathways after chloromethyl recombination:

15

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C o

o 05 _>,

Modified Arrhenius Expression for the Rates of Selected Reactions3 k = A T " exp ( E / R T )

~ 0.0 C

~41.5

n

E

1 2 3 4

1.31E24 1.91E17 1.10E36 1.50E13

- 3.25 - 1.013 - 7.2 0

8172 9655 8600 30300

"~.%

"~..~

H+O 2"~'OH+O

°*~%.%

"~'...2. CH2?I-~CH2C'CH2+C'

-1.01

CH2C1+O2''~"

0

a Units: mol, cm, s, K, cal [4].

I

=CI-=" e l H3CI+HCi

// 2 CH2Cl-D-CH2CICH2CI ..~-***~s._ • • . • o. o. o . . . . . . . . . .

TABLE3

A

.~ . . . . . . . . "1. . . . ./'~CH

1.0

o O

Reaction Number

I

CH20+CI+O

Z -15 000

0.05

0.10

0.15

0.20

0.25

0.30

Time (s)

Fig. 13. Normalized sensitivity coefficients for CH3CI: reactions to which CH3CI level is most sensitive.

THERMAL DESTRUCTION OF CH3CI chain branching or chain propagation: reactions 4 and 2, which involve chloromethyl radicals, and the reaction H + 02 = OH + O, the most important branching reaction in hydrocarbon combustion. On the other hand, the reactions with the largest positive sensitivity coefficients are chain termination reactions: reactions 1 and 3, in which two chloromethyl radicals combine, forming the stable species C2H3C1 and HCI or 1,2-C2H4C12. Reactions 1-4, identified because of their high sensitivity coefficients for CH3C1, were the subject of further sensitivity calculations. Increasing the rate of either reaction 2 or reaction 4 by a factor of 2, or decreasing the rate of either reaction 1 or reaction 3 by a factor of 2, resulted in the destruction CH3CI in run M-7 to below experimental detectability limits. Because the uncertainty in the rate coefficient is probably greater than a factor of 2 for these reactions, this calculation indicates that better knowledge of crucial rates may bring the experimental and computational results into agreement. These reaction rates are currently under evaluation by others [7, 19]. The mechanism outlined above provides an explanation for a phenomenon noted in the discussion of experimental results: the effect of equivalence ratio on the relative C2H 4 and C2H3C1 yields. In lean environments, H atoms are much less abundant than OH radicals, and furthermore tend to react with 0 2 to create OH and O radicals. Thus C1 abstraction by H, creating CH 3 from CH3C1, occurs rarely, and C 2 formation is dominated by the recombination reactions of two chloromethyl radicals (reactions 1 through 3). In rich environments, on the other hand, H and thus CH 3 would be present at much higher levels. This would lead to a greater role for the reactions analogous to reactions 1-3, but involving a methyl radical and a chloromethyl radical, or two methyl radicals. All these analogous reactions would lead to the production of C2H 4 rather than C2H3C1. COMPARISON BETWEEN EXPERIMENTAL

RESULTS AND CALCULATIONS Several reasons for the discrepancy between experimental and computational results have

193 been investigated. Because initial radial levels are not well known and radical attack appears to dominate CH3CI destruction, sensitivity to initial radical levels was considered. Crude chemical kinetic modeling of the region between the propane/air flame and the CH3C1 injection location indicates that OH levels at the injection location are between 2 and 50 times higher than the equilibrium values assumed in the original calculations. Using these superequilibrium values lowered the calculated temperature of the transition from poor to thorough CH3CI destruction from about 1370 to about 1300 K. Another possible source of inaccuracy is the modeling of the temperature history experienced by the CH3CI. Corrections for thermocouple heat losses would bring measurement and calculation into better agreement, shifting the experimentally observed transition from poor to good destruction efficiency upward by about 25°C. Aside from this effect, it is unclear whether differences between the model and the actual temperature history should cause an underprediction or an overprediction of destruction efficiency. One would expect that the use of the centerline temperatures in the model would cause CH3C1 destruction to be overpredicted, since average temperatures are somewhat lower. Thus accounting for radial temperature variation would increase the discrepancy between experiment and calculations. On the other hand, the plug flow model fails to represent the initial travel of the injected species upstream into the hot combustion products. Both temperatures and residence times are underestimated for this crucial early stage of the destruction process. Finally, inaccuracies in the chemical kinetic mechanism or thermodynamic data may contribute to the disagreement. The mechanism, which contains many estimated rates, is based upon a mechanism developed to model rich flames. It is reasonable to expect that CH3CI destruction would be dominated by different reactions in the lean environment of the present study, and that the rich study might not provide a good test of the rates that dominate the lean study. Sensitivity analysis indicates that the choice of rates for reactions 1-4, which involve the chloromethyl radical, has a

194 l a r g e i m p a c t on t h e CH3C1 d e s t r u c t i o n efficiency. T h e s e rates, a d o p t e d u n c h a n g e d f r o m t h e m e c h a n i s m o f K a r r a et al. [4] w e r e estim a t e d o r o b t a i n e d f r o m Q R R K calculations. R e a c t i o n 4 in p a r t i c u l a r s e e m s w o r t h investig a t i n g m o r e closely, since it is unlikely to have b e e n i m p o r t a n t u n d e r the c o n d i t i o n s for which t h e original m e c h a n i s m was d e v e l o p e d .

CONCLUSIONS T r a n s i t i o n f r o m p o o r to t h o r o u g h d e c o m p o s i tion o f CH3C1 for 0.25 s r e s i d e n c e t i m e in a lean, p o s t f l a m e e n v i r o n m e n t was o b s e r v e d to o c c u r w h e n the m a x i m u m t e m p e r a t u r e o f the ( n o n i s o t h e r m a l ) run was a b o u t 1220 K. C O , C 2 H 3 C I , C 2 H 4 , a n d C 2 H 2 w e r e o b s e r v e d as b y p r o d u c t s , with t h e C : species a c c o u n t i n g for a few p e r c e n t o f t h e c a r b o n initially p r e s e n t as C H 3 C I , a n d with C O p r e s e n t at h i g h e r levels. C h e m i c a l kinetic m o d e l i n g p r e d i c t e d the s a m e b y p r o d u c t s , a n d i n d i c a t e d that the C 2 species w e r e f o r m e d by the following process: 1. H o r C1 a b s t r a c t i o n f r o m CH3C1 by radicals 2. R e c o m b i n a t i o n o f the resulting C H 2 C I o r C H 3 radicals. H o w e v e r , t h e m o d e l i n d i c a t e d that the transition f r o m p o o r to t h o r o u g h d e s t r u c t i o n o f C H 3 C I s h o u l d o c c u r for a run with m a x i m u m t e m p e r a t u r e a r o u n d 1370 K. T h e d i f f e r e n c e b e t w e e n m o d e l a n d e x p e r i m e n t m a y be d u e to i n a c c u r a c i e s in m o d e l i n g t h e t e m p e r a t u r e history a n d r a d i c a l levels, o r m a y b e d u e to the c h e m i c a l kinetic m e c h a n i s m . T h e m o d e l i n g results w e r e sensitive to the choice o f rates for t h e following r e a c t i o n s : CH2C1 r e c o m b i n a t i o n r e a c t i o n s , t h e r e a c t i o n o f CHzC1 with 0 2 , a n d t h e r e a c t i o n H + 0 2 = O H + O. B e t t e r d e t e r m i n a t i o n o f t h e r a t e s o f r e a c t i o n s involving CH2C1, especially t h e r e a c t i o n with 0 2, w o u l d g r e a t l y i m p r o v e m o d e l i n g c a p a b i l i t i e s for fuellean conditions. We t h a n k R d m i Cervera, S h u n c h e n g Lee, and D a v i d Wallenstein f o r help with the experiment, a n d Drs. D o n a l d Lucas, Robert Sawye~ Joseph Bozzelli, Lisa Pfefferle, a n d Selim S e n k a n f o r useful discussions. This research was supported by the N a t i o n a l Institutes o f E n v i r o n m e n t a l

E . M . F I S H E R A N D C. P. K O S H L A N D Health Sciences Superfund Research Program, grant n u m b e r P42 ES047050-01.

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OF CH3CI

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