J . Phys. Chem. 1987, 91, 4160-4165
4160
formed, PFeI'CH, and PFexVCF3,are much less stable than those derived from the addition of those radicals to PFe". Relevance to the Toxicity of Halogenated Alkanes. The difference in reactivity of these radicals with PFe"' has important bearing on the toxicity of the related halocarbons. The first step in the metabolism of halogenated alkanes is their reduction by ferrous cytochrome P450, leading to radicals and the ferric hemoprotein. Further, peroxyl radicals are formed and attack the fatty acids, which results in deleterious chain lipid peroxidation. The effect of oxygen concentration in the development of these reactions has been d i s c ~ s s e d . ~At J ~ high oxygen concentration the alkyl or haloalkyl radicals are converted to the peroxyl radicals near the ferric cytochrome P450, which is likely to scavenge them before they could diffuse through the membrane. Consequently, the damage on the membrane is greatly reduced. This hypothesis is supported by studies on biological system^.^',^^ This mechanism
of toxicity holds for CC14 and very likely also for CBr4. Indeed, the latter compound as well as 'CBr, and CBr302' radicals behaves similarly to CC14 and related radicals. The metabolism of CF,Br might be quite different. Firstly, as deduced from the reactivity with ferrous porphyrin, it is not expected to react rapidly with ferrous cytochrome P450 to yield 'CF, radicals.25 Secondly, if formed, the radicals should back-react very rapidly with the ferric cytochrome P450 and will not be able to induce lipid chain peroxidation. The oxidized cytochrome P450 formed in this reaction could be converted to the initial ferric form by enzymatic reduction. If any cytochrome P450 is destroyed in this process, the result will be less deleterious than lipid peroxidation. The toxicity of halogenated compounds can thus be evaluated from their reactivity toward biological reducing agents, such as ferrous cytochrome P450, and from the reactivities of the related haloalkyl and peroxyl radicals.
(31) De Groot, H.; Noll, T. Biochem. Biophys. Res. Commun. 1984,119, 139. (32) Noll, T.; De Groot, H. Biochim. Biophys. Acta 1984, 795, 356.
Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy.
Lean Premixed Laminar Methanol Flames: A Computational Study Jim 0. Olsson,* Ingrid B. M. Olsson, and Lars L. Andersson Department of Physical Chemistry, Chalmers University of Technology, S-412 96 Gothenburg, Sweden (Received: September 4, 1986)
Three experimental premixed laminar methanol flames at 40 Torr, measured in detail by Vandooren, Balakhin, and Van Tiggelen, were studied computationally. The equivalence ratios of the flames were 0.89 (l), 0.36 (2), and 0.21 (3). Flame 3 seemed to be the flame with the highest experimental accuracy. A time-dependent flame code with an implicit description for both chemistry and transport allowed extensive computations. Two kinetic mechanisms for methanol combustion were used to compute the species profiles. The first was based on the mechanism compiled by Westbrook, Dryer, and Schugh (WDS), but with 5 times the rate constant for the reaction HCO + M. The second mechanism was compiled by Dove and Warnatz (DW). Strikingly, the experimental CH20H maxima were much smaller than the corresponding computational maxima. The smallest differences were found in flame 3. In this flame the experimental CH20H maxima were about 70 and 20 times smaller than computational maxima found by using the WDS and DW mechanisms, respectively. The sensitivity analysis gave analogous results for the three flames. It was found that the CH20H profiles in the three flames were dominated by the reaction CH,OH + 0,. An increase in the rate constant of reaction CHzOH + O2decreased the CH20H maxima nearly linearly, leaving other species profiles almost unchanged. For this reaction a rate constant (WDS) of about 1.0 X 1014exp(-3019/T) cm3 mol-] s-l gave good agreement between the experimental and computational maxima for CHIOH in flame 3.
Introduction Methanol is a reference fuel in engine studies and its radicals, C H 2 0 H or C H 3 0 , are considered to be important in the ignition of other fuels.' Furthermore, many of the key reactions in methanol flames will also be important in other alcohol and hydrocarbon flames.2 Measurements of species in laminar methanol flames have previously been made by Akrich et al.,, Pauwels et al.,4 and Vandooren et al.s36 Andersson et al.7 studied experimentally and theoretically a stoichiometric low-pressure methanol flame. Dove, J. E.; Warnatz, J. Ber. Bunsen-Ges. Phys. Chem.1983,87, 1040. Westbrook, C. K.; Dryer, F. L. Prog. En. Combust. Sci. 1984, 10, 1. Akrich, R.;Vovelle, C.; Delbourgo, R. Combust. Flume 1978, 32, 171. Pauwels, J. F.; Carlier, M.; Sochet, L. R. J. Phys. Chem. 1982, 86,
4330.
__
( 5 ) Vandooren, J.; Balakhin, V. P.; Van Tiggelen, P. J. Arch. Combust. 1981, I , 229-242.
(6) Vandooren, J.; Van Tiggelen, P. J. Sym. (In?.) Combust., [Proc.],18th 1980, 1981, 473. (7) Andersson, L. L.; Christenson, B.; H@lund, A,; Olsson, J. 0.;Rosengren, L. G. Prog. Astronaut. Aeronaut. 1985, 95, 164.
0022-3654/87/2091-4160$01 .50/0
Recently, Olsson et a1.' carried out a complementary investigation studying, the effect of water addition to methanol-air flames. Detailed computational studies of methanol combustion have been made by Westbrook and Dryer.gs'o Their mechanism is designed for handling many fuels and different physical conditions and it has been extensively used.2 Recently, Dove and Warnatz' presented another methanol mechanism in a computational study of premixed laminar methanol flames. The latter study and the one by Olsson et a1.' emphasized that the C H 2 0 H reactions with O2and M are important in methanol combustion. Unfortunately, the corresponding rate constants are the least certain, in the mechanism. According to Warnatz," their uncertainty is a factor of 10. In contrast, the other rate constants in the methanol mechanism have an uncertainty of about a factor of 3 or less. (8) Olsson, J. 0.; Karlsson, L. S.;Andersson, L. L. J. Phys. Chem. 1986, 90, 1458. (9) Westbrook, C. K.; Dryer, F. L. Combust. Sci. Technol. 1979, 20, 125. (10) Westbrook, C. K.; Dryer, F. L. Combust. Flume 1980, 37, 171. (,l 1) Warnatz, J. In Combustion Chemistry, Gardiner, Jr., W. C., Ed.; Springer: New York, 1984.
0 1987 American Chemical Society
Lean Premixed Methanol Flames Vandooren et aL5q6studied experimentally in detail three different lean flames with equivalence ratios gf 0.89,0.36, and 0.21. They used a flat flame burner at 40 Torr and modulated molecular beam sampling followed by mass-spectrometric analysis. Concentration profiles for many species were presented, for example, CH20, CH20H, CO, and C02. Vandooren and Van CH30H, 02, Tiggelen analyzed their data using a classical kinetic approach. However, no one has performed detailed computer modeling2 of these experiments. Such a study could give new and needed information about the combustion chemistry of methanol and especially C H 2 0 H reactions.' Evaluation of mechanisms necessitates comparisons of computed and experimental species profiles. Particularly, the modeling of intermediate species is a very severe test of a mechanism.12 In contrast, computational predictions of experimental flame velocities is of marginal value in the validation of a mechanism' The present study has the following aims: (1) to compute species profiles for the conditions studied by Vandooren et al.596 using the Westbrook-Dryer-Sch~gh'~and the Dove-Warnatz' mechanism; (2) to determine the most important reactions in the kinetic mechanism by sensitivity analysis, and evaluate parts of methanol mechanisms sensitive to the experiments.
Numerical Method In this study of burner-stabilized flat flames we make the normal assumptions (idealizations). The premixed ideal gas flows with a constant mass flow through a wide porous burner, inducing a laminar flow in the gas. The steady flame burns at a constant pressure with no temperature or concentration gradients parallel to the burner, i.e. the flame is one-dimensional,12 Time-Dependent Flame Code. The time-dependent flame code computes the concentrations as a function of distance from the burner for the flame species. A multicomponent diffusion model was usedI4 and the Lennard-Jones parameters used in the computation of the diffusion coefficients were taken from Kee et al.I5 A detailed description of the flame code has been given elsewhere.I4J6 We use the measured mass flow and the measured temperature as input parameters to compensate for the cooling of the flame by the burner.17 By using these parameters, starting profiles are computed by a purely kinetic calculation. Transport processes are introduced by starting on a coarse grid. Depending on the accuracy needed, the computations are continued to successively finer grids. The flame code has been extensively Calibrated against the steady-state code developed a t Sandia labor a t o r i e ~ . ~ ~The , ~ *codes have similar efficiency.l4 We have now improved the code further by introducing an implicit transport description instead of an explicit description. The implicit transport description allows a much longer time step on dense grids. The execution time is thereby reduced by a factor of about 10. Exact Sensitivity Analysis. The method of sensitivity analysis used in this study is similar to our previous approach.Ig We start with a reference state at steady state. The program perturbs each rate constant in turn. In this study we kept the equilibrium constant fixed as we normally do. For each perturbation the system relaxes to a new steady state. The relaxation time and the corresponding execution time depend on the degree of perturbation and will be related to the importance of the perturbed parameter. The computer program continues the computations until approximately the same time gradient
2500-
- - - -- -- -- ---------------- - - -
I_-----n
0.0
0.5
BURNER DISTANCE
(CM)
Figure 1. Temperature profiles measured by Vandooren et al.5 for the three flames studied by computations in this study. The equivalence ratios of the flames were 0.89 (l), 0.36 (2), and 0.21 (3), respectively. The experimental temperature profiles of the flames were used in the computations, but below 0.1 cm no measurements were made and the profiles were therefore extrapolated as shown.
is reached in the perturbed state as in the initial reference state. The changes in the concentration profiles give a sensitivity measure for the reaction rate constant perturbed. For each species ( i ) we compute sensitivities for a reaction rate (j):
si,= J I(Yi,new
-
Yi,ref)I
dz/J
dz
(2)
The sum over all species gives a total sensitivity measure for the system for a reaction rate (j).
Chemical Kinetics Mechanism Two different kinetic mechanisms were used. The first mechanism (see Table I) was based on the mechanism compiled by Westbrook et al.13 Their mechanism contains 32 species and 93 reaction pairs. Preliminary kinetics and flame computations showed, as e x p e ~ t e d ,that ~ ~ ~most . ~ of the hydrocarbons could be eliminated. CH, and C H 3 were included, mainly to ensure an ignition in the kinetic calculation (see below); their effects were small in the flame computations. The species left, with the exception of the inert Ar, can be divided into different groups: (a) H-0 species H, H2, 0, OH, 02,H 2 0 , H 0 2 , and H202. (b) C-0 species C O and C 0 2 . (c) C-0-H species HCO, C H 2 0 , C H 3 0 , C H 2 0 H , and C H 3 0 H . (d) C-H species CHI and CH3. The mechanism, see Table I, consists of 18 species involved in 55 pairs of forward/backward reactions. The back-reaction rate constants were taken as specified by Westbrook et al.13 According to our previous studies,*J9the rate constant of reaction 23, H C O + M, is multiplied by 5. To be noted, this value is about 2 times lower than the corresponding value recommended by Dove and Warnatz.' A recent determination of the rate constant for HCO + M, by Vandooren et a1.,20gave a 4 times higher value than our value. For the third bodies the description suggested by Dove and Warnatz' was used: M ' = 1.5O[CO2]
+ 0.75[CO] + 6.50[H20] + 1.00[H2] + 0.40[02]
(12) Dixon-Lewis,G.In Combustion Chemistry, Gardiner, Jr., W. C., Ed.; Springer: New York, 1984. (13) Westbrook, C.K.; Dryer, F. L.; Schug, K.P. Symp. (In?.) Combust., [Proc.], 19, 1982 1982, 153. (14) Andersson, L. L.;Olsson,J. 0.Combust. Sci. Technol. 1986,46,95. (15) Kee, R.J.; Warnatz, J.; Miller, J. A. Sandia National Laboratories Report, SAND80-8003, 1983. (16) Olsson, J. 0.;Andersson, L. L. J. Comput. Phys. 1985, 59, 369. (17) Smooke, M. D. J. Comput. Phys. 1982, 48, 72. (18) Smooke, M. D.; Miller, J. A.; Kee, R. J. Combust. Sci. Technol. 1983, 34, 79. (19)Olsson, J. 0.; Andersson, L. L. Combust. Flame 1987, 67, 99.
1.5
1.0
+ 0.40[N2] + 2.OO[CH3OH] (3)
For the other reactions we also followed Warnatz description regarding third-body efficiency and used a similar efficiency for all species. The second mechanism used was compiled by Dove and Warnatz.' Only minor modifications were made to the original mechanism. The DW mechanism specifies back rate constants for only a few of the reactions contrary to the WDS mechanism. In the cases where DW specifies both forward and backward rate constants only the forward rate constant was used. (20) Vandooren, J.; Oldenhove de Guertechin, L.; Van Tiggelen, P. J. Combust. Flame 1986, 64, 127.
4162
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987
Olsson et al.
I
0.154 c
0
0.05-
I
-
CH20 5
,
0.04-
+
I \ I
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c02
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a
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,
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1
"
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'
'
8
i
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~
0.5 1.0 BURNER DISTANCE
0.0
(CM)
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I
~
~
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+ 0
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+ 0.04-
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0
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I
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,
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,
,
,
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'
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8
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I
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I
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. ...
,_ _ _ - - -
-I
0
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0'. 5
1. o
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Figure 2. The flame (3) C H 3 0 H / 0 2 / H 2(10.9%, 85.9%, 3.2%) at 0.052 atm. Profiles of stable species (a, top) CHIOH, CO,; (b, middle) H2, CO; (c, bottom) H 2 0 , 02.Computed by us (solid lines) and from experiments by Vandooren et aL5 (broken lines). The mechanism is from Westbrook et aI.,l3 but the rate constant of reaction HCO + M is multiplied by 5 . Third body (M') efficiencies according to Dove and Warnatz.'
All back rate constants were computed from equilibrium constants derived from the J A N A F Thermochemical Tables. Computations of Methand Flames The measured mass flows, compositions, and the undisturbed (uncooled) temperature profiles (see Figure 1 ) from the experiments by Vandooren et aL5v6have been used as input parameters. The flow time was about 25 ms on the IO-cm grids used. On the final third grid, with a time gradient of 0.25 s-l, 45 points were used with about 22 of them in the flame zone. The conditions
,
0'. 0
,
,
,
I
0'. 5
1.5
1.0
BURNER DISTANCE
(CM)
Figure 3. Profiles of intermediate species in flame 3: (a, top) C H 2 0 and OH; (b, middle) 0 and C H 3 0 ; (c, bottom) C H 2 0 H and H. Computed by us (solid lines) and from experiments by Vandooren et aL5 (broken lines). Same conditions as in Figure 2.
above were used for both ordinary flame computations a s well as for the sensitivity analysis. Note that flames 1 and 2 interact with the burner and seem to have temperature gradients going into the burner (see Figure 1). In contrast, flame 3 is slower with an almost adiabatic temperature profile. Flame 3 was also found to be the simplest flame chemically, and thereby easier to extract kinetic information from, especially compared to the stoichiometric flame 1. Consequently, most of the results and all of the graphs in the paper concern flame 3. Comparison of Experimental and Computed Profiles. For flames 1 and 2, the profiles computed with the WDS mechanism start slightly before the corresponding experimental profiles, but
~
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 4163
Lean Premixed Methanol Flames TABLE I’
Dove and Warnatz forward rate
Westbrook et al. forward rate 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 42 43 44 45 46 47 48 49 50 51 52 53 54 55
H + 0 2 = 0 + OH H2 + 0 = H + OH H 2 0 + 0 = OH + OH H2O + H H2 + OH H202 + OH = H20 + HO2 H 2 0 + M’ = H + OH + M’ H + 0 2 + M’ = H02 + M’ H02 + O = OH + 0 2 H 0 2 + H = OH + OH H02 + H = H2 + 0 2 HO2 + OH = H20 + 0 2 H202 + 0 2 = HO2 + H02 H202+ M’ = OH + OH + M’ H202 + H = H02 + H2 0 + H + M’= OH + M’ O2 + M’ = 0 + 0 + M’ H2 + M’ = H + H + M’ CO + OH = C02 + H CO + HO2 = C02 + OH CO + 0 + M’ C02 + M’ CO2 + 0 = co + 0 2 HCO + OH = CO + H2O HCO + M’ = H + CO + M’ HCO + H = CO + H2 HCO + 0 = CO + OH HCO + H02 = CH2O + 0 2 HCO + 0 2 = CO + H02 CHzO + M = HCO + H + M CH2O + OH = HCO + H20 CH2O + H = HCO + H2 CHzO + 0 HCO + OH CH2O + HO2 = HCO + H202 CH4 + M CH3 + H + M CH4 + H CH3 + H2 CH4 + OH = CH3 + H20 CH4 + 0 = CH3 + OH CH4 + H02 = CH3 + H202 CH, + H02 = CHgO + OH CH3 + OH = CH2O + H2 CH, + 0 = CH2O + H CH3 + O2 = CH30 + 0 CH2O + CH3 = CH, + HCO CH3 + HCO = CH4 + CO CH3 + H02 = CH4 + 0 2 CH30 + M = CH20 + H + M CH30 + O2 = CH20 + H 0 2 CH3OH + M = CHg + OH + M CH3OH + OH = CH20H + H20 CH3OH + 0 = CH2OH + OH CH30H + H = CHZOH + H2 CH30H + H = CH3 + H20 CH3OH + CHI = CH20H + CH4 CH3OH + HO2 = CH2OH + H202 CHZOH + M = CH2O + H + M CHZOH + 0 2 = CH2O + H02
log A 14.27 10.26 13.53 13.98 13.00 16.34 15.22 13.70 14.40 13.40 13.70 13.60 17.08 12.23 16.00 15.71 14.34 7.1 1 14.18 15.77 12.44 14.00 14.16 14.30 14.00 14.00 12.60 16.52 12.88 14.52 13.70 12.00 17.15 14.10 3.54 13.20 13.30 13.51 12.60 14.11 13.68 10.00 11.48 12.00 13.70 12.00 18.48 12.60 12.23 13.48 12.72 11.26 12.80 13.40 12.00
b
Ea
0.00 10.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.30 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.08 0.00 0.00 0.00 0.00 0.00 0.00 0.50 0.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
16.79 8.90 18.35 20.30 1.80 105.00 -1.00 1.oo 1.90 0.70 1.oo 42.64 45.50 3.75 0.00 115.00 96.00 -0.77 23.65 4.10 43.83 0.00 19.00 0.00 0.00 3.00 7.00 8 1.OO 0.17 10.50 4.60 8.00 88.40 11.90 2.00 9.20 18.00 0.00 0.00 2.00 29.00 6.00 0.00 0.40 21.00 6.00 80.00 2.00 2.29 7.00 5.34 9.80 19.36 29.00 6.00
log A
b
Ea
16.39 18.30
0.00 -0.80
114.24 0.00
13.30
0.00
0.00
6.64
1.50
-0.74
13.70 14.85 14.30
0.00 0.00 0.00
0.00 16.81 0.00
12.48
0.00
0.00
13.00
0.00
1.70
14.00 13.00
0.00 0.00
25.04 7.19
‘The base mechanism used in this study is a subunit of the mechanism compiled by Westbrook et al.I3 Up to reaction 46, their numbering is used. The units are in cm3, mol, kcal, s. The most important change is that we use a 5 times greater value for the rate constant of reaction HCO + M’ (23). Furthermore, the rate constants for the third-body reactions are changed. To take into account the strong effect of water as a third body, the description of Dove and Warnatzl for third-body efficiencies in H2/02reactions and in HCO + M’ was chosen. For the most important reactions the table also shows the values from the second mechanism (DW) used in this study.
for flame 3 these computed profiles are slightly delayed compared with the experimental profiles (see Figure 2 and 3). The species profiles computed with the DW mechanism come about 0.5 mm earlier than profiles computed with the modified WDS mechanism in the three flames. The differences between experimental and computed species maxima depend strongly on the species in question. Figures 2 and 3 show some representative experimental and computational concentration profiles from flame 3. In flames 1 and 2, the experimental H2 maxima are higher than the computational maxima by factors of about 2 and 1.4, respectively. The experimental H 2 0 maxima are also higher, by factors of about 1.3, 1.15, and 1.05, respectively, than the computational maxima in
TABLE Ik Computed CH,OH Maxima Divided by the Experimental Maxima for Three Flames Using Two MechanismsoModified according to Table I and the Text flame equiv ratio WDS/exut DW/exut 1 0.89 135 35 2 3 a
0.36 0.21
124 70
33 19
WDS, Westbrook et al.;Ig DW, Dove and Warnatz.‘
flames 1, 2, and 3. Surprisingly high differences were found for the maxima of C H 2 0 H (see Table 11). The experimental maxima of CH,OH are consistently much smaller, up to a factor of 135
4164
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987
104
I
COTOH H+O +M'
80
CO+OH 100 4 v,. v,
1
80804
6 0;
Z W
60-
40-
N
40401
v,
0
2 0-
0I
I
1
20 30 40 5 0 60 REACTION NUMBER Figure 4. The total sensitivity of the system (see text). The results are given as percent of the sensitivity due to the most important reaction. Same conditions and mechanism as in Figure 2 and 3. The numbering up to reaction 46 is from Westbrook et al." The rate constants were multiplied by 3 for each reaction in turn and the system was allowed to relax to a new steady state.
10
0
I
v,.
Olsson et al.
I i
2 0
,
0
10
20
30
40
50
60
REACTION NUMBER I
I
I
8oi I 0
10
20
30
40
50
60
REACTION NUMBER Figure 6. The sensitivity of C 0 2 (a, top) and CH,O (b, bottom) to different reactions. Same conditions as in Figure 4.
10
0
50 REACTION NUMBER 20
30
40
60
2+M'
H20+M' v,
I 40 0
20
20 30 40 50 60 REACTION NUMBER Figure 5. The sensitivity of H2(a, top) and OH (b, bottom) to different reactions. Same conditions as in Figure 4. 10
0
in flame 1 using the WDS mechanism, than the corresponding computed maxima. Kinetic Sensitivity Analyses. In the sensitivity analysis, the reaction rate constants were increased and decreased by a factor of 3. According to Warnatz,'." such an uncertainty factor is a maximum uncertainty for all the rate constants in the mechanism used, with two important exceptions. The reactions C H 2 0 H + M and C H 2 0 H O2 have rate constants uncertain by a factor of 10.
+
Figure 4 shows the total sensitivity for an increase by a factor of 3 in the rate constants for flame 3. The results are given as percentages of the sensitivity due to the most important reactions. Flame 2 was only slightly more complicated than flame 3 and very similar to it. The stoichiometric flame 1 was more complex compared with the other leaner flames. Figures 5,6, and 7 show sensitivity spectra for some of the interesting species in flame 3: (H2, OH); ( C 0 2 , C H 2 0 ) ; and ( C H 2 0 H , C H 3 0 H ) . Also here the results are given as percent of the sensitivity of the most important reaction. The spectra of H, 0, and O H differed only by scale factors. The correlation coefficients between spectra of these species were 0.99. For C 0 2 the only significant reaction is C O + O H (18). The spectrum of C O was very similar to the C 0 2 spectrum. The sensitivity spectra for C H 2 0 H show that the reaction C H 2 0 H + O2 (55) is the most important one for that species. Effect of Rate Constant Changes on the CH20HMaxima. The C H 2 0 H profile was studied in detail for changes in the rate constants by a factor of 3 of the reactions dominating the CH20H sensitivity spectra (see Figure 7a): CO + OH (1 8). HCO + OH (22), H C O + M (23), H C O + 0 (25), CH3OH + OH (48), C H 2 0 H M (54), and C H 2 0 H O2 (55) (see Table I). The C H 2 0 H maximum changed less than 5% due to changes in the rate constants of these reactions, with reactions 48 and 55 as important exceptions. A factor of 3 increase in the rate constant of reaction C H 3 0 H + O H (48) increased the C H 2 0 H maximum by a factor of about 1.3. A corresponding decrease of the rate constant of reaction 48 gave a decrease in the C H 2 0 H maximum by a factor of 1.3. For the important reaction C H 2 0 H + O2 (55) an almost oneto-one relationship was found between changes of its rate constant and corresponding changes of the C H 2 0 H maximum. The computational maximum coincided with the experimental max-
+
+
The Journal of Physical Chemistry, Vol. 91, No. 15, 1987 4165
Lean Premixed Methanol Flames
CH 20H+0
0.051 i
i0.04
, / /
0
Z w
HCO+M
/ - -
__--. .. .. 4.
*
0.5
0.0
BURNER DISTANCE
HCO+M' HCO+OH
CH30H+OH
w 60
I
t
2oL *
0
0
10
-_. ......... ......... ......... 20 30 40 50 60
REACTION NUMBER Figure 7. The sensitivity of C H 2 0 H (a, top) and CH,OH (b, bottom) to different reactions. Same conditions as in Figure 4.
imum for rate constants of 8.5 X loi3exp(-3019/T) and 2.0 X loi4exp(-3620/n using the WDS (see Figure 8) and the D W mechanisms, respectively. Interestingly, these rate constant expressions are within a factor of 2 equal to the expression 1.0 X 1014exp(-2520/T) determined by Vandooren and Van Tiggelen.6 Accuracy in the Determination of CH20H. The discrepancy found here for the C H 2 0 H maximum between measurements and experiments could be explained in four different ways. First, we could have made a computational error. However, a check with the Sandia-Yale flame code2I gave within a few percent the same result for the CH20Hmaximum as our flame code. Furthermore, an extrapolation of the computational results by Westbrook and Dryerlo for a stoichiometric methanolair flame at 0.1 atm to the conditions used in flame 1 give a similar high value about 8 X lW3 for the CH20H maximum as found with our flame d e using the same mechanism. Second, the experimenters could have detected CH30only and not CH20H. The experimental maxima of CHzOH have a value similar to the computational maxima of CH30using the WDS mechanism; C H 3 0 is not included in the D W mechanism. However, the experimental C H 2 0 H maxima is placed rather late in the flame with a similar position as the computational maxima of CH20H. In contrast, the computational maxima of CH30were found early in the flame ahead of CH20 and CH20H. Third, the cooling from the sampling cone and the associated water-cooled wall could be strong. Such cooling could (21) Smooke, M. D., to be submitted for publication.
1. 5
1.0 (CM)
Figure 8. The flame (3) C H 3 0 H / 0 2 / H 2(10.9%,85.9%,3.2%) at 0.052 atm. Computed profiles of CH20H with the same mechanism as in the previous figures (solid lines) and the rate constant of CH20H + 0 2 multiplied by 85 and from experiments by Vandooren et aL5 (broken lines).
disturb especially the measurements near the burner. For these positions the distance from the burner to the cooled wall is only about 3 cm. We found from our one-dimensional computations that a boundary positioned at that distance influenced the species concentration profiles in the flame zone significantly. However, a flat flame with a molecular beam sampling cone is a complex configuration without the ideal one-dimensional properties characteristic of a flat flame alone. Finally, the rate constant of the reaction C H 2 0 H O2( 5 5 ) could be much too low in both the D W and WDS mechanisms. The uncertainty in the rate constant of ( 5 5 ) by a factor of 10, together with our computations and the determination by Vandooren and Van Tiggelen6 1.O X 1014exp(-2520/7') cm3 mol-' s-', supports this view. The latter expression gives a value about 1.5 times higher than the value we found in this study. Furthermore, recent determinations of the rate constant for reaction 55 at low temperatures by Dobe et ah2' and Grotheer et al.23give a 10 or 100 times higher value compared to the DW and WDS mechanisms, respectively.
+
Conclusions The present study demonstrates a huge discrepancy between detailed flame experiments based on MBMS and flame computations regarding the C H 2 0 H maximum. One possible explanation is that the rate constant for the important CH20H + O2 reaction in the mechanisms used the Dove-Warnatz and the Westbrook-Dryer-Schugh mechanism is too low by factors of 20 and 85, respectively. Disturbances of one-dimensionality of the experimental flames could also contribute to the differences found between experiments and models. Our results stress the need for further studies of the combustion chemistry of the basic species: hydrogen, carbon monoxide, formaldehyde, methanol, methane, ethane, etc. (see also Kaiser et al.24). Acknowledgment. This work was financially supported by the Swedish Natural Research Council, the Swedish National Board for Technological Development, and AB Volvo. Registry No. C H 3 0 H , 67-56-1; CH20H, 2597-43-5 (22) Dobe, S.;Temps, F.; Bohland, T.; Wagner, H . Gg. 2.Naturforsch. A . 1985, 40A, 1289. (23) Grotheer, H. H.; Riekert, G., Meier, U.; Just, T. Ber. Bunsen-Ges. Phys. Chem. 1985,89, 187. (24) Kaiser, E. W.; Rothschild, W. G.; Lavoie, G. A. Combust. Sci. Technol. 1984, 41, 271.
