Shock tube study on homogeneous thermal oxidation of methanol

C O M B U S T I O N A N D F L A M E 42: 61-76 (1981)

61

Shock Tube Study on Homogeneous Thermal Oxidation of Methanol TAKAO TSUBOI and KAZUNOBU HASHIMOTO Yokohama National University, Department of Mechanical Engineering, 156, Tokiwadai, Hodogaya-ku, Yokohama, 240 Japan

Homogeneous thermal oxidation of methanol-oxygen mixtures, highly diluted with argon, was studied in shock waves by following infrared emission of various species during the course of the reaction. The equivalence ratio # = 1.5 [CH3OH]/[O2] wasvaried from 0.2 to 2.0. The total density extended from i x l0 -5 to 2 x l0 -4 mol/cm3. For mixtures of methanol and oxygen between 0.05 and 1.0%, the followingexpressiondescribesthe inductionperiod: r i = 1.4 × 10- 1 6 [CHaOH]-O.I[o2]--O.19[Ar]--O .62

× exp (201 kJ/mol/RT) s. For very dilute mixtures, the experiments indicated a decrease of the power dependency of argon density. In order to obtain the mechanism for the oxidation of methanol, profiles of some species were obtained from the numerical integration of 57 reactions and were compared with the measured results. The calculated inductionperiods agreed well with the measured ones. The argon density strongly influences the inductionperiod in the mixtures with less than 1% CH3OH since the decompositionof H202 plays an important role during the inductionperiod.

INTRODUCTION

concentrations:

Because of d i m i n i s h i n g p e t r o l e u m resources, m e t h a n o l has been suggested as one of the substitutive fuels for internal c o m b u s t i o n engines. Pischinger [1] has r e p o r t e d that the characteristics of m e t h a n o l engines are as g o o d as (or even better than) those of gasoline engines. T h e u n d e r s t a n d i n g of the m e c h a n i s m of metha n o l o x i d a t i o n is one step t o w a r d u n d e r s t a n d i n g the details of ignition a n d c o m b u s t i o n of methanol. C o o k e et al. [2] r e p o r t e d on the ignition d e l a y times for a s t o i c h i o m e t r i c m i x t u r e of m e t h a n o l oxygen at t e m p e r a t u r e s between 1570 a n d 1870K in the pressure r a n g e 200--300 T o r r using shock t u b e techniques. B o w m a n [3] studied the oxida t i o n of m e t h a n o l b e h i n d reflected shock waves in the t e m p e r a t u r e r a n g e 1545-2180K a n d obt a i n e d an expression for the time ~ .... between s h o c k - h e a t i n g a n d the a t t a i n m e n t of the maxi m u m of the p r o d u c t of C O a n d O - a t o m

Ti, max

Copyright © 1981 by The Combustion Institute Published by Elsevier North Holland, Inc. 52 Vanderbilt Avenue, New York, NY 10017

=

2.1 X 10 - - i s [CHzOH] - ° - 1 [02] - 0 . 5 X exp (151.5 k J / m o l / R T ) s.

F u r t h e r m o r e , he p r o p o s e d a 19-reaction mechanism. Recently, A r o n o w i t z et al. 1-4] studied the o x i d a t i o n in an a d i a b a t i c , t u r b u l e n t flow r e a c t o r a n d o b t a i n e d for the d i s a p p e a r a n c e rate of m e t h a n o l an expression --d [CH30H ] / d t = 1011"53 [CH30H ] o . s i × exp ( - 2 2 9 k J / m o l / R T ) mol/cm s s

( $ < 1.0),

--d [CHaOH ] M t = 1031"54 [CHaOH ] 1.55 X exp ( - 5 1 5 k J / m o l / R T ) mol/cm 3 s

(~ > 1.0).

T h e y p r o p o s e d a 28-step e l e m e n t a r y reaction mechanism.

0010-2180/81/07061+16502.50

62

TAKAO TSUBOI and KAZUNOBU HASHIMOTO

Westbrook and Dryer 1-30] also studied the mechanism of methanol oxidation in a shock tube and in a turbulent flow reactor. Adding to these references, Bahn [32] discussed a reaction package of 100 chemical reactions for combustion of CH3OH as well as CH4, C2H6, and C2HsOH. Owing to the results of these previous studies, one can now understand the outline of the oxidation mechanism. The experiments described in this paper were planned to cover as large a range of conditions as possible in order to obtain the further informaation on the oxidation mechanism. Mixture ratio and total density were varied over a large range, while keeping the shock waves always well defined. In addition, the reaction should proceed as isothermally as possible in order to keep the shock waves one-dimensional. Data evaluation from the various concentration profiles should remain comparatively simple because not only the induction periods described here but also the species concentration profiles are used in order to have insight into the mechanism of methanol oxidation.

emission between 2.7 and 5.5 /am. A two-prism monochromator (Zeiss MM 12 with CaF2 prism) with a slit opening up to 2 mm served as the dispersion element. For the infrared region, a RPY 31 Mullard photodiode was used as a detector. Mixtures of methanol and oxygen in argon were prepared in steel containers and used after proper mixing time. The purity of the substances, which were used without further purification, was: 99.8~o CH3OH (liquid) from Junsei Chemical Co., Ltd., with 0.1~o H20, 20 ppm CH3COOH, 10 ppm CH3COCH3; 02 from Takachiho Chemistry Co., Ltd., with a dew point o f - 60°C; Ar from Nippon Sanso Co., Ltd., with 0.1 ppm 02, 0.5 ppm hydrocarbon and a dew point - 70°C.

EXPERIMENTAL The experiments discussed here were performed in a brass shock tube of 100 mm internal diameter. The low-pressure section was 4265 mm long, while the high pressure section was 2020 mm long. The leak rate of the driven section was about 3 × 10-3 Torr/min. It was evacuated with a rotary pump to 10-2 Torr. The new test gas mixture was streamed through the driven section in order to clean out the residual gas. Then the test section was filled by introducing the gas approximately up to the pressure (P1) to be used in the experiment. The experiments were always done within 3 min after closing the valves. Shock velocities were measured by four platinum heat transfer gauges and an electronic counter to measure the time within 0.2~o. Attenuation usually remained below 2~,/m. The progress of the reaction behind reflected shock waves could be followed by measuring

RESULTS

Methanol Oxidation Measurements on methanol oxidation were performed at temperatures between 1200 and 1800K and at densities from 1 x 10 -5 to 2 x 10 -4 mol/ cm 3 behind the reflected shock waves. Mixture ratios are given in Table 1. The following wavelength ranges were used in order to monitor the concentration of various species during the reaction. (a)

Emission of CH3OH can be observed in a wavelength range from 2.4 to 2.9/am and from 4.8 to 5.2/am as well as from 3.1 to 3.9 tan. Around 3.4/am, the emission is rather strong. The signal remains constant for a certain time after passage of the shock wave, and then it goes to zero more or less rapidly, so that an induction period ~ can easily be assigned as in Fig. la. The time rm is the time for the main reaction, and it will not be discussed here. (b) Emission of H20 around 2.6/am. The 1-120 emission started a little before the end of the induction period defined as in Fig. la. (c) Emission of CO2 can be observed in a wavelength range from 4.1 to 4.8 /am.

METHANOL OXIDATION IN SHOCK WAVES Around 4.3 /am, the emission is rather strong. The CO2 emission started after the emission of CH3OH disappeared. (d) Emission of C O can be observed in a wavelength from 4.7 to 5.0 /fin. Around 4.9 tam~ the C O emission can be distinguished from the emission of CO2. The emission started around the end of induction period determined in (a) (see Fig. lb). (e) Emission of C H 2 0 can be observed from 3.5 to 3.9/am~. The time between the shock front and m a x i m u m C H 2 0 emission agreed well with the induction period after (a) (see Fig. lc).

63 activation energies were almost same for our experimental conditions: 201:+ 8 kJ/mol. The induction periods at a given temperature could be extracted and plotted as a function of total density from plots like Fig. 2. This is shown in Fig. 3a, b, c for various mixture ratios and mixture concentrations at a temperature of 1500K. The dashed line indicates the mixtures with constant methanol (and oxygen) densities. One can see that the influence of total density on the induction period is approximately constant, so that one can write ri "" [ A r ] - O . e 2

Though all these signals proved to be rather convenient for measuring induction periods, most of the data reported here were obtained by method (a). Figure 2 shows the temperature dependence of the induction periods for a mixture of 0.2% CH3OH and 1.5% 02 as a function of the reciprocal temperature for various total densities in the reaction zone. This means that along one line, the density of the reactants is also constant. The deviation of the density in each experiment could be corrected) The apparent

1 The deviation of the total density is corrected as follows: The temperature dependence of induction periods for the mixture with a constant mole fraction is obtained by changing the pressure in the low-pressure section of the shock tube. Due to the fact that the lower initial density is not completely compensated by a higher density ratio across the shock front, the total densities at 1600 K are about 20% lower than those at 1300 K. From this temperature dependence one obtains the induction period r i and the total density p at a given temperature by interpolation. These values, 1-i and p, are plotted in Figures. Then, the total density dependence of the induction periods for a mixture with a constant mole fraction is obtained at a given temperature (solid lines in Fig. 3a, b, c) and the power dependency a at various conditions is obtained using the correlation

1-f ~ [p]a. Then, induction periods 1-t,given at a given total density Pgtven are calculated from an experimentally determined induction period 1-Lexp at the total density Pexp USing the correlation 1"/,given = 1-i,exp [Pgiven/Pexp] a.

These corrections are done within a density deviation of 20%.

for total densities between 1 x 10 - s and 2 x 10 -4 mol/cm 3. For the mixtures with very low CH3OH concentrations, the influence of total densities tends to become smaller toward high total densities. In the mixtures with high reactant concentrations (above ~ 2 % CH3OH), the influence of total densities on the induction periods also seems to become somewhat small (see Fig. 3c). In order to know the influence of the oxygen (and methanol) concentration on the induction periods, the induction periods at a given temperature were plotted as a function of oxygen (and methanol) concentration. One example is shown in Fig. 4 for various methanol concentrations at a temperature of 1500K. The solid lines in Fig. 4 indicate mixtures with constant methanol and total densities. One can see that the induction periods increase with decreasing oxygen (also methanol) concentration and that the influence of oxygen concentrations on the induction periods is more dominant toward the low-methanol concentrations. This indicates that the power dependencies of methanol and oxygen are not constant and one cannot obtain the power dependencies of the methanol and oxygen concentrations in one form. However, they are approximately constant only in the mixtures with 0.05-1.0% CH3OH. For these mixtures, therefore, the correlation between induction periods and concentrations was obtained by plotting log r~ against l o g [ X ] by keeping the other

64

TAKAO TSUBOI and KAZUNOBU HASHIMOTO TABLE 1 Experimental Gas Mixtures Mixture No.

1

2

3

4

5

6

CH30H [%1 02 [%] Ar [%1

0.010 0.075 99.915 0.2 8 0.20 0.30 99.50 1.0

0.050 0.375 99.575 0.2 9 1.00 1.50 97.50 1.0

0.20 1.50 98.30 0.2 10 0.0100 0.0075 99.9825 2.0

1.00 7.50 91.50 0.2 11 0.0500 0.0375 99.9125 2.0

2.00 15.00 83.00 0.2 12 0.20 0.15 99.65 2.0

0.010 0.015 99.975 1.0 13 1.00 0.75 98.25 2.0

7 0.050 0.075 99.875 1.0

variables constant. The result for z~ in seconds is ri = 1.4 X 10- i 6 [CH3OH] -0-i+-o-o5

X[O2] -0-19-+0.08 X [Ar] -0.62-+0.14 X exp (201 -+ 8 kJ/mol/RT) s. Figure 5 shows that the induction periods are made fit by this correlation formula for the mixtures with 0.05-1.0~o CH3OH.

suggests that the rates of unimolecular reaction of methanol could be estimated from the comparisons of measured and calculated disappearance rates of methanol (see Discussion). Nearly the same value, which was 264 kJ/mol, was obtained for the overall activation energies from the disappearance rates of the emission at 3.4/am. More precise results of the methanol decomposition will be published elsewhere. DISCUSSION

Methanol Decomposition In order to know the details of the initiation step of methanol oxidation, methanol decomposition was followed at a wavelength of 3.4/am. The emission decreases monotonously with time, and disappears completely at the end of the reaction. The rates of the disappearance of the emission increase with decreasing dilution by argon (see Fig. 6). This suggests that the observed decomposition was not only the unimolecular reaction of methanol, but also the reaction including other bimolecular reactions which followed after the unimolecular reaction. In order to obtain the rate constants of the unimolecular reaction of methanol, the mixtures were diluted with argon as highly as possible (until 0.01~o). The computer simulation showed the following: When the value kl,un i decreased by a factor of 2, the value of dl/dt at 3.4/am decreased by 25~o for the mixture with 0.01~ methanol at the total density of 5 x 10 -5 mol/cm 3 and at 1600K. This

Bowman reported that the induction periods r~..... obtained from the maximum of the product of the CO and O-atom concentration, show no dependence upon the argon density for mixtures of 0.75-4~o methanol and oxygen in argon. However, a strong influence of the total density was observed for our mixtures, which were more highly diluted with argon. This influence of the total density became smaller toward high methanol (and oxygen) concentration (i.e., toward Bowman's condition). This suggests that one or a few reactions that are pressure-dependent exist in the initial step and that toward low reactant concentration the reaction plays an important role in the induction period assigned by us, while the induction period assigned by Bowman has more bimolecular reactions. Therefore, the influence of the pressure-dependent reactions on the induction periods decreases. This expectation can also be pointed out from the power dependencies of reactants and from the apparent

METHANOL OXIDATION 1N SHOCK WAVES

....

°!.....

•

~.

65

~ ......... i "'~-4"~"

.......... l

T~~I,,XT;

~i ........... ~i ..........

..... !.~.

I_L T'--iT 'Rel~e.~edLw~ve i

:~''J'

Fig. la. Emission at 3.4 ~m (Ah = 0.12 pm): 0.2% CHsOH, 1.5% 0 2, 98.3% At; T 5 = 1381 K; P5 = 4.0 × 10- 5 mol/cmS. Attenuation of incident shock wave: 0.9 %/m.

Z

.pS SIAJ I1TI

~aiJ vii!

t 111!

" I1~1.

iii

iiii

ifll

n " "m /B l mnnanKdRe :1ei.-:ted wo ve .Fig. lb. Emission at 3.6 p m (AX = 0.I 15 #m): 0.2% C H 3 O H , 0.3% 0 2, 99.5% Ar; T 5 = 1345 K; P5 = 3.5 X 10 - 5 mol/cm8. Attenuation: 1.8 %/m.

~,

1 ,11.! TIll!

i!11

30 ~s ~ . : ~ + IT!

III

r'

.',.t.~-o ~',

r ii Ref ~ct~" Na,,e Fig. 1c. Emission at 4.9 , m (~xX = 0.085 ~m): 0.2% C H 3 O H , 0.3% 0 2, 99.5% At; T 5 = 1486 K;p 5 = 3.8 X 10 - 5 mol/cm3. Attenuation: 0.4 %/m.

66

T A K A O T S U B O I and K A Z U N O B U H A S H I M O T O

,o~ ~'= 0 , 2

o C H s 0 H = 0 , 2 Yo

02=1.,5 %

/~/

:

I

102L" ees=S,,lO-? ~%=1,,~o-4

i011' , 5.5

,

,~e5= 2xI0~,L' [mollcm -Jl7.5 104/T [K-l]

I

,

6.0

6.5

7.0

Fig. 2. Induction periods r i for a mixture with 0.2% CH3OH and 1.5% 09. at various total densities plotted as a function of reciprocal temperature: - - , o, e, o, O, • , measured values; - - - , calculated values.

!

I

I

I

I

10 3

,-q [ysl

"

" ' - ' ~ ~.,.~,, .,_~_

7

..,.I ~ e

:

fCH3OH=S.10-9

-"~,-Lo2=37s,,l~

-

-"KdX,.

102 - o o o,,.c,~o,

: e o os~

\ % . ~ "-.c,~o,.~.,o' 6-,,.'~.., y,..

I

"CH30H

101 I

10-5

=5x10-7

i i II,,,, I 10-4 e5[mollcm s] (a)

METHANOL OXIDATION IN SHOCK WAVES

67

I

I 10 3

I

_T5:1500K cb=l.0

.--~r

~

l~s]

H3,0H=5x10-9 2 =7.5x10-9

10 2 -:O 0.0 1%CH3()H : o 0.05%

[ I

..

- e 0.2%

..

il.o%

..

,,,,

-

CH3OH=Ix10.7

101 10-5

i0 -l, (b)

,o3 _

I

T s = 1 500

Ti

[JJsl

e 5 [ mollcm3 ]

K 0 e

cb=0.2 f\

• •

.'x

0.01% C H 3 O H - 0.05% .. 0,2%

-

~.o%

..

2.0%

..

-

I

ICH30H = 5 x1~89[mol/cm3 ] O9=3.75x10"

10 2

'C~, CH3OH=2 x I 0-B

101 10-5

10 - 4

(c)

10-3

e 5 [m°llcm3 ]

68

TAKAO TSUBOI and KAZUNOBU HASHIMOTO

Ts = 1 5 0 0

'

=0.2

I

O 0.01%CH30H

e 0.05*/, .' 0 •

103 "ri [~sl

0.2% 1.o%

..

..

5x10" 9 [mollcm3J

~ C H 3 O H = 10 2

101

I

10-5

i

i

,I

....

10-4 (d)

i

,

,

1

e5 [mol/cm3]

l

Fig. 3. Experimentally obtained influence of total density on induction periods ri at 1500 K for equivalence ratios ~ = 2.0 (a), ~ = 1.0 Co), ~ = 0.2 (c), and calculated influence of total density on induction periods ~'i at 1500 K for equivalence ratio ~ = 0.2 (d). Figure 3c and 3d show how well the measured and the calculated values agree. Solid lines indicate mixtures with the constant mole fractions. Dashed lines were drawn by plotting the induction periods of the mixtures with a constant methanol (and oxygen) density using these solid lines.

activation energies: i.e., Bowman's induction period r~.... is assigned to later part of the reaction sequence than ~ of this work; therefore ~.max has more reaction steps. In this case the power dependency of initial oxygen concentration can become larger (Bowman, [02]-°5; this work, 1-O2]-°~9) and the apparent activation energy can become smaller (Bowman, EA.B = 151.5 kJ/mol; this work, EA = 201 kJ/mol). For the very low reactant concentration, the dependence of induction periods on argon density decreased again. This suggests that at least one of the pressure-dependent reactions is the decomposition of the intermediate product and that the reaction sequence of this intermediate product becomes less important toward low (very low) reactant concentration during the induction

period of the methanol oxidation. This species was H202, produced by the reaction HO 2 + HO 2 --. H202 + 02, as discussed in thelatter part of the discussion. As shown in Fig. 3a, b, c and Fig. 4, the power dependencies 1, m, n of the initial reactant concentrations give various values; therefore one has to use carefully the expression of form ri = A [Reactant] z [Oxygen] m [Argon]" × exp

(EA/RT),

which was applied for hydrocarbon oxidations by some of previous investigators l3, 5-9]. When the power dependencies depend strongly on the mixture ratios, one has to use another form, like Ref. [10], or one has to show them with figures.

METHANOL OXIDATION IN SHOCK WAVES

I

103

69

'l

T5=I OOIK ..... , 05:5 xl O-5mol/cm 3

"¢t

[~sl

102

-

101

-

I

i

10-2

,

,

, I,,,,I

h 10-1

,

= , I,,,,

1

100

,

,

, 1 ....

021%1

I

101

Fig. 4. Experimentally obtained influence of oxygen density on induction periods for constant methanol densitiesat 1500 K with a total density/)5 = 5 X 10--5 mol/ema.

The power dependency of methanol is inverse to and smaller than those of other hydrocarbon: i.e. ~',~,[CH3OH] -°-1 in the methanol oxidation, while ri,,,[CI-I4] +0.32(Ref. [9]), ~i~[C2I-I6]+0.~s(Ref. [7]),ri ~ [ ~ H s ] +0.57(Ref. [81) in the hydrocarbon oxidation. This small value of the power means that the induction period does not strongly depend on the methanol concentration. At wavelengths between 3.5 and 3.9 /an, the emission of formaldehyde can be observed in addition to the emission of methanol. This formaldehyde is probably produced from the CH2OH or CH30 radical. The temperature dependence of the rates of CH20-production d(Icrl2o/Icrl3Ol.i.t=oh.6um/dt'shows an apparent activation energy of 142+20 kJ/mol/in a mixture of 0.2% CH3OH and 0.3% 02 at a density of 3.3 x 1 0 -s mol]cm 3, and it coincides fairly well with the value of 166 +__15kJ/mol at a density of 5 x 10-5, obtained from the calculated rates d [CH20]/dt (see Fig. 7). In order to know the oxidation mechanism of methanol, the concentration profiles of some chemical species were obtained from the computer simulation and compared with experiment-

ally determined ones. Table 2 shows reactions used in the computer simulation. Though there were some less important reactions among them, all of them were used in the computer simulation in order to know the influence of each reaction on the profiles. The values of the rate constants for forward reactions were taken from the literature, and suitable values were established within the stated error limits to give better agreement with our experimental results. Rate constants for the reverse reactions were calculated from the rate constants for the forward reactions and the equilibrium constants obtained from the JANAF Thermochemical Tables [111 as well as from the literature [12]. Before the computation of methanol oxidation, the measured rates of methanol decomposition were compared with the calculated rates, using the same reactions in Table 2. The emission at 3.4 /an contains not only the emission of methanol but also those ofCH20, CI-L, C2I-I6,and C2I-L. The observed decay rates kA = d In I/dt ~it 3.4 #m were compared with kA' = d In {[CHaOH] + [CI-L] + [CH201 + [C2I-I6] + [C~I-L]}~dr of calculation. The dashed line in Fig.

70

TAKAO TSUBOI and KAZUNOBA HASHIMOTO

to

(5

10-2 L.

<

o

0

d

O_3 0 o3 I 0

A [] ,

o- .L 5.5

6,0

6.5

C[%] H30H [mol~Scm 3] 1.0 (D l.O (~ l.O [ ] 0.2 L-I 0.2 ~i~ 0.2 A 2.0

1.0 0.2 0.05 l.O 0.2 0.05 l.o

lOxl0-5 lo 10 lO lO lO 10

&2.0

0.z

10

2.0

0.05 lO

~ ~ 1.0 (~ 1.0 (~ 1.0 ~]0.2 []0.2 E~0.2 2.0 /~2.0 A~2.0

7,0

C[%] H30H[molPl5cm3] 1.0 0.2 0.05 l.O 0.2 0.05 1.0 0.2 0.05

5xl0"5 5 5 5 5 5 5 5 5

~04/T [ (~

(~ ].0 C)I.0 O1.0 [~0.2 F~]0.2 [-]0.2 A 2.0 A~ 2.0 A 2.0

K-17'15

CH30H [%] [mo~lScm3] 1.0 0.2 0.05 l.O 0.2 0.05 1.0 0.2 0.05

Ixl0"5 I 1 l I 1 1 l 1

Fig. 5. Reduced induction periods as a function of reciprocal temperature.

METHANOL OXIDATION IN SHOCK WAVES

71

I

I

T=1600

K

exp cal l - E ) iD 0 . 2 % C H 3 0 H

10 4

_

0.05%

e e

~.

qL,-¢)/I

o.o,o,

~ /

- -

t~

<

..:¢ 10 3

/ I

I

I I I III

1 0-6

I

1

,

, I I I,

1 0-5

,

, I,,,,

1 0 -/" e 5 I mol/cm 3 ]

Fig. 6. Influence of total density on "overall" rate of methanol decomposition: ®, e, o, measured values; - - , ~, e, I, calculated values; , proposed rate constant of unimolecular reaction CHaOH.

-1°t'" 0 ~ -

I°-I

-" " L ~

9=5x10-Smol/c m3

- "" ~

"-."-

"'-0~

,o"2

no 1

5.0

1

I

I

I

I

6.5

I

I

I

7.0

I

I

I

7.5

I04/T [ K"11

Fig. 7. Temperature dependenceo£ the rates of CH20 production: 0, - - - measured values - - , calcuated valuesd[CHgO]/dt.

d(/CH20/ICHaOH,~O)3.6/4m/dt;

72

TAKAO TSUBOI and KAZUNOBU HASHIMOTO TABLE 2 Reaction Mechanism Rate constanta Reaction

1

2 3 4 5 6 7 8

9 10 11 12 13 14

15 16 17 18 19 20 21 22 23 24

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

CHaOH + (M) = CH 3 + OH + (M) p = 1 × 10--5 p = 5 X 10--5 p = 1 X 10 - 4 CHaOH + H = CH a + H 2 0 CHaOH + H = CH2OH + H 2 CHaOH + O = CH2OH + OH CHaOH + OH = CH2OH + H 2 0 CHaOH + HO 2 = CH2OH + H 2 0 2 CHaOH + CH 3 = CH2OH + CH 4 C H 2 O H + (M)= C H 2 0 + H + (M) p = I X 10 - 5 p = 5 X 10 - 5 p = 1 × 10 --4 CH2OH + 0 2 = C H 2 0 + HO 2 C H 4 + H =CH3 + H 2 CH 3 + O = C H 2 0 + H CH 3 + OH = C H 2 0 + H 2 CH 3 + 0 2 = C H 2 0 + OH C H 2 0 + (M) = C H O + H + (M) p = 1 × 10- 5 p = 5 x 10 - 5 p = 1 x 10 - 4 C H 2 0 + H = CHO + H 2 C H 2 0 + O = CHO + OH C H 2 0 + OH = CHO + H 2 0 C H 2 0 + HO 2 = CHO + H 2 0 2 CHO + M = CO + H + M CHO + H = CO + H 2 CHO + O = CO + OH CHO + OH = CO + H 2 0 CHO + 0 2 = CO + HO 2 CO+O(M)=CO 2+(M) p = 1 x 10--5 p = 5 x 10--5 p = 1 x 10 ---4 CO + OH = CO 2 + H CO + HO 2 = CO 2 + OH H 2 0 2 + M = OH + OH + M H 2 0 2 + O = HO 2 + OH H 2 0 2 + OH = H 2 0 + HO 2 HO2 + HO 2 = H 2 0 2 + 0 2 HO 2 + M = H + 0 2 + M HO 2 + H = OH + OH HO 2 + OH = H 2 0 + 0 2 H 2 + 0 2 = H + HO 2 02+M=O+O+M 0 2 + OH = O + HO 2 H + H + M = H2 +M H + OH + M = H 2 0 + M H + H 2 0 = H 2 + OH H +O 2 = OH+ O O + H 2 = OH + H

a

E

a

E

Ref. b

12.78 13.55 13.94 12.47 12.84 13.12 14.30 12.00 13.70

310 314 318 20 20 15 25 42 41

9.92 10.69 11.08 11.78 11.20 12.05 14.19 11.43 14.48

-82 -78 -73 44 127 30 110 -2 68

* * * 15 15 16 3* ** 14

10.12 10.75 11.02 13.00 14.30 13.78 12.60 11.95

121 121 121 0 55 0 0 50

9.47 10.10 10.37 12.53 12.82 14.92 14.10 11.96

5 5 5 80 52 292 300 274

3* 3* 3* * 13 13 13 13

11.84 12.40 12.55 13.52 13.62 14.00 12.00 15.02 14.00 14.00 14.00 12.90

301 301 301 18 17 17 33 124 0 0 0 0

10.11 10.67 10.82 12.41 12.15 13.48 11.02 15.12 14.73 14.37 15.34 13.18

-19 -19 -19 135 126 195 82 7 320 311 382 78

17 17 17 13 13 13 18 13 13 13 13 13

8.61 9.24 9.48 12.00 14.00 16.20 13.45 13.00 14.27 15.32 14.40 13.00 13.74 15.48 12.86 13.72 13.49 13.97 14.34 13.85

6 8 10 19 96 181 27 8 14 191 8 0 242 520 233 -36 -59 85 70 49

11.00 11.64 11.88 14.13 15.09 14.40 12.96 13.46 14.85 15.18 13.35 14.03 13.30 14.09 12.78 14.34 14.70 13.38 13.21 13.49

528 531 532 113 356 -23 86 137 187 -4 173 303 0.4 24 0 402 439 24 3 41

19 19 19 13 20 21 22 20 23 20 20 24 20 13 13 20 20 20 20 20

M E T H A N O L OXIDATION IN SHOCK WAVES

73

T A B L E 2 (Continued) Rate

Reaction

a

E

constanta ~7

a

Ref.b

42 43

O + H 2 0 = OH + OH CH 3 + CH 3 + (M) = C2H 6 + (M)

13.83

77

12.88

7

20

44 45 46 47 48 49 50 51 52 53

p = 1 × 10--5 p = 5 × 10-5 o = 1 × 10---4 CH3 + CHa = C2H 5 + H CH8 + CH3 = C2H4 + H2 C2H6 + H = Call 5 + Ha C2H6 + O = C2H5 + OH C2H6 + OH = C2H 5 + H20 C2H 5 = C2H4 + H C2H4 + M = C2H3 + H + M C2H4 + H = C2H3 + H2 C2H 4 + O = CH3 + CHO C2H 4 + OH = CH3 + CH20

11.58 10.97 10.88 14.90 16.01 14.30 13.95 13.19 13.58 17.58 13.80 12.70 13.00

-15 -45 -52 111 134 48 31 24 159 410 25 7 4

14.90 14.28 14.19 16.86 17.80 13.56 12.85 13.04 11.78 15.66 12.50 10.95 12.72

351 321 314 66 368 71 48 111 1 -33 19 181 70

25, 26 25, 26 25, 26 27 27 13 13 13 3 28 28 13 13

54 55 56 57

C2H3 + M = C2H 2 + H + M C2H 2 + O = CH 2 + CO C2H 2 + OH = CH3 + CO CH2 + O = CHO + H

14.90 13.30 13.30 13.48

132 13 13 0

14.63 13.13 13.58 13.60

-36 224 253 429

25 ** ** **

a Rate c o n s t a n t is given in an Arrhenius form: k = 10 a exp (-E/RT). Units: kJ, mol, cm3, s. b * indicates values obtained in this work. ** indicates estimated values.

6 indicates the m e a s u r e d rate kA, and the solid line indicates the calculated kA'. F r o m these c o m parisons the rates of u n i m o l e c u l a r reaction of m e t h a n o l were es t i m at ed as follows: p = 1 X 10 - 5 mol/cm a,

kl,un i

= 6.0 X 1019" exp ( - 3 1 0 p = 5 × 10 - 5 mol/cm a, = 3.5 X 1013 exp ( - 3 1 4 p=l

X 10 - 4 m o l / c m a,

= 8.7 X 10 l a exp ( - 3 1 8

kJ/mol/RT)s -1

kl,un i

kJ/mol/RT) s- 1 , kl,uni

kJ/mol/RT) s- x .

O u r k~,u,i values indicate that the rate c o n s t a n t s of m e t h a n o l d e c o m p o s i t i o n in o u r e x p e r i m e n t a l c o n d i t i o n s are in the transition region described in the u n i m o l e c u l a r reaction theory. Therefore, it seems i n a p p r o p r i a t e to c o m p a r e o u r values with those of A r o n o w i t z et al. [29], W e s t b r o o k a n d

D r y e r [30], and o t h e r references. H o w e v e r , the k~,univalue at 1600K and at P = 1 x l 0 -5 m o l / c m 3, which is the smallest total density we obtained, seems to exist near the l o w pressure region. If so, all these values m ay be c o m p a r e d , and o u r value 1 0 2.66 S- ! lies between the value o f A r o n o w i t z et al. ( 1 0 3"87) and the value of W e s t b r o o k an d D r y e r (10257). T h e r e a c t i o n that m ak es the rate of the d i s a p p e a r a n c e of the emission at 3.4/am increase with decreasing dilution of a r g o n is CHaOH + CH a -~ C H 2 0 H + CH 4

(7)

and the rate c o n s t a n t is k 7 = 5 × 10 l a exp ( - 4 1

kJ/mol[RT)

c m a / m o l s.

F o r the a c t i v a t i o n energy, the value of G r a y an d H e r o d [14] was applied. T h e o b t a i n e d pree x p o n e n t i a l factor of this reaction was a b o u t 250 times larger t h a n the value of G r a y an d H e r o d

74

TAKAO TSUBOI and KAZUNOBU HASHIMOTO

[14] at temperatures between 1600 and 1800K. The reactions that influence the thermal decomposition of methanol for our experimental condition are 1, 2, 3, 5, 7, 8, 10, 14, 15, 19, 43, 46, 49, 51, 54. The reactions 8, 10, 43, 45, 49, 51, 54 produce the intermediate products that emit at 3.4/an, while the main decomposition sequence is CH3OH ~ CH2OH --~ CH20 -* CHO - ' CO. By using the rate constants of the above mentioned reactions, the concentration profiles of some species were calculated for the CH3OHO2-Ar system. The same mixture ratios as those used for the experiments tabulated in Table 1 were used for this calculation. Our proposed mechanism is for the most part similar to the simulation of Westbrook and Dryer [30], whose paper appeared when our experiments were in progress. Some rate constants, however, differ from their values as discussed later. As mentioned before, the emission observed at 3.4 /an contains the emission of CH20, CI-L, C2I-I6 and C2I-I4 besides the emission of CH3OH. The emission profiles at 3.4 ian were therefore compared with the concentration profiles of [CH3OH ] + ['CH20 ] + [CH4] + [,C2H6] + [C2H4], assuming that these species have the same emissivity at 3.4 gin. (Though this assumption is not evaluated, we expect that the difference of emissivities is within a factor of 2.) Dashed lines in Fig. 2 show the induction periods obtained for the mixture of 0.2~o CH3OH and 1.5~o 02 from computer simulations as well as experiments. Figure 3d shows the comparisons of the density dependence of the induction periods for the mixture of the equivalence ratio ¢ = 0.2 at 1500K. The obtained power dependencies on the induction periods and the apparent activation energy can be expressed as

From both expressions as well as from Figs. 2 and 3d, one can see that the oxidation mechanism of methanol is explained by the reactions given in Table 2. According to the Table 2, one can review the oxidation process of methanol as follows: The initiation reaction is the unimolecular reaction of methanol. Methanol decays in reactions 2-7. The rate constant k5 is much larger than that of Westbrook and Dryer [.30]. We could not make it so small as that of Westbrook and Dryer, in order to make the calculated induction periods match with the measured ones. Formaldehyde is produced in reactions 8 and 9,

ri ~ [CHaOH]-0"25-+0"05 [0~]-°"18-+°'°5 X JAr]-°.6-+°-1 X exp (204 + 10 kJ/mol/RT), while the experimentally determined expression is I-i ,-, [CHaOH]-O.11-+o.o5 [02]--0.19+0.08 X [At] -0.62+0.14 X exp (201 + 8 kJ/mol/RT).

CH20H+ (Ar)

~

CH20 + H + (Ar),

CH20H + 02 -~ CHzO+ HO 2,

(8) (9)

and formaldehyde oxidation occurs (reactions 14-23). Reaction 8 influenced the induction period ~ and reaction 9 influenced the time Zmof main reaction in our calculation. Therefore, one set of the rate constants for 8 and 9 are determined from the comparisons of the measured and the calculated ~ and rm. Our rate constant k8 at 1600K agrees with the value ofAronowitz et al. [4] within a factor of 2. However, it is about 1/10 smaller than that of Benson and O'Neal [.31] and 50 times larger than that of Westbrook and Dryer [30]. Our rate constant k9 at 1600K is 70 times larger than that of Aronowitz et al. Reactions 24-42 are the hydrogen and carbon monoxide oxidation. Reactions 43-57 are not main reaction sequences of methanol oxidation. However, the emission at 3.4 /an includes the emissions of C2I-I6, CH4, and C2I-I4 that are produced and removed in these reactions. Therefore, these reactions must be included when the methanol oxidation is studied by use of the emission at 3.4 btm. Furthermore, higher hydrocarbons and soot may be produced through these reactions. About these reaction sequences one can find more precise reactions in the mechanism given by Westbrook and Dryer [30] as well as in our mechanism. The reaction that influenced the dependency of the total density on the induction periods is the unimolecular reaction of H202 (reaction 27). This H202 is produced in reaction

METHANOL OXIDATION IN SHOCK WAVES

_

o~.,,.c.J,o.

'~

~:~soo,

0.3%02[

~o-7 ~

-

-

3 _

75

It-.i------1

~ •

~

t

I

100

I

200 Ipsl

Fig. 8. Calculated and measured concentration prof'fles for the gas mixture containing 0.2% CHsOH a n d 0 . 3 % O 2 a t 1500 K with p = 5 X 10--5 mol/cmS: - - , calculated values; - - - , measured values.

30. The main reaction sequence during the induction period is the reaction 5 --, 9 --, 30 ~ 27 ( ~ 5). One example of the calculated and the experimental concentration profiles is shown in Fig. 8.

The main reaction sequences of methanol oxidation are given in Fig. 9 in schematic form. The reaction sequences above the dashed line in Fig. 9 show the initiation reactions of thermal oxidation of methanol and the production of methane

Ar CH3OH~C2H6

.

--

.

.

.

.

.

\

CH30H

1 Ar.02 / i~]

.....

~--- =---J

time Fig. 9. Main reaction sequencesin methanol oxidation.

76

TAKAO TSUBOI and KAZUNOBU HASHIMOTO

a n d ethane. The reaction sequences below the dashed line show the chain reaction mechanism. The oxidation progresses from left to right. A t the b o t t o m of Fig. 9 one can see the time histories of the p r o d u c t i o n a n d the decay of some representative species.

15. Aders, W. K., and Wagner, H. Gg., Z. Phys. Chem. N. F. 74:224 (1971). 16. Lefevre, H. F., Meagher, J. F., and Timmons, R. B., Int. J. Chem. Kinet. 4:103 (1972). 17. Tsuboi, T., and Yoshine, T., Thermal Oxidation Mechanism of Ethane (in Japanese). Paper presented at the 56th Meeting of the Japan Society of Mechanical Engineers, Tokyo, April, 1979, N. 790-6, p. 15. 18. Lloyd, A. C.,lnt. J. Chem. Kinet. 6:643 (1967). 19. Wagner, H. Gg., and Zabel, F., Ber. Bunsengesell. Phys. Chem. 78:705 (1974). 20. Baulch, D. I., Drysdale, D. D., Home, D. G., and Lloyd, A. C., High Temperature Reaction Data Vol. I and III, Butterworths, London, 1972. 21. Meyer, E., Olschewski, H. A., Troe, J., and Wagner, H. Gg., Twelfth Symposium (International] on Combustion, The Combustion Institute, 1969, p. 345. 22. Gehring, M., Hoyermann, K., Wagner, H. Gg., and Wolfrum, J., Ber. Bunsengesell. Phys. Chem. 73: 956 (1969). 23. Nicolet, M.,Disc. Faraday Soc. 37:7 (1964). 24. Brone, W. G., White, D. R., and Smookler, G. R., Twelfth Symposium (International] on Combustion, The Combustion Institute, 1969, p. 557. 25. Tsuboi, T., Japan. J. Appl. Phys. 17:709 (1978). 26. Gl~nzer, K., Quack, M., and Troe, J., Sixteenth Symposium (International] on Combustion, The Combustion Institute, 1977, p. 949. 27. Roth, P., and Just, Th., Ber. Bunsengesell. Phys. Chem. 83:577 (1979). 28. Roth, P., and Just, Th., Ber. Bunsengesell, Phys. Chem. 77:1114 (1973). 29. Aronowitz, D., Naegeli,D., and Glassman, I., Z Phys. Chem. 81:2555 (1977). 30. Westbrook, C. K. and Dryer, F. L., Combust. Sci. Technol. 20:125 (1979). 31. Benson, S. W., and O'Neal, H. E., Kinetic Data on Gas Phase Unimolecular Reactions, NSRDS-NBS 21, p. 588, National Bureau of Standards, 1970. 32. Bahn, S. B., Theoretical Nitric Oxide Production Incidental to Autoignition and Combustion of Several Fuels Homogeneously Dispersed in Air under Some Typical Hypersonic Flight Conditions, NASA CR2455 (1974).

This work was partially supported by a Grantin-Aid f o r Scientific Research f r o m the Ministry o f Education.

REFERENCES 1. Pischinger, F., VDI-Berichte 224:59 (1974). 2. Cooke, D. F., Dodson, M. G., and Williams,A., Combustion and Flame 16:233 (1971). 3. Bowman, C. T., Combustion and Flame 25:343 (1975). 4. Aronowitz, D., Santoro, R. J., Dryer, F. I., and Glassman, I., Seventeenth Symposium (International) on Combustion, The Combustion Institute, 1979, p. 633. 5. Seery, D. J., and Bowman, C. T., Combustion and Flame 14:37 (1970). 6. Lifshitz, A., ScheUer, K., Burcat, A., and Skinner, G. B., Combustion and Flame 16:311 (1971). 7. Burcat, A.,Crossley, R. W., Seheller, K., and Skinner, G. B., Combustion and Flame 18:115 (1972). 8. Burcat, A., Lifshitz, A., Scheller, K., and Skinner, G. B., Thirteenth Symposium (International) on Combustion, The Combustion Institute, 1971, p. 745. 9. Tsuboi, T., and Wagner, H. Gg., Fifteenth Symposium (International) on Combustion, The Combustion Institute, 1975, p. 633. i0. Bowman, C. T., Combustion, Science and Technology 2:161 (1970). 11. Stull, D. R., and Prophet, H., JANAF Thermochemical Tables, 2nd ed., U.S. National Bureau of Standards Publication NSRDS-NBS 37, 1971. 12. Bahn, G. S., Approximate Thermochemical Tables for some C-H and C-H-O Species, NASA Report CR2178 (1973). 13. Tsuboi, T., Japan, J. AppL Phys. 15:159 (1976). 14. Gray, P., and Herod, A. A., Trans. Farad. Soc. 64: 2723 (1968).

Received 14 May 1980; revised 22 August 1980