Methanol Oxidation and Its Interaction with Nitric Oxide - Energy

An experimental and theoretical study of the oxidation of methanol in the .... Combustion and emissions behaviour for methanol–gasoline blended fuel...
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Energy & Fuels 2001, 15, 724-729

Methanol Oxidation and Its Interaction with Nitric Oxide M. U. Alzueta,* R. Bilbao, and M. Finestra Department of Chemical and Environmental Engineering, Centro Polite´ cnico Superior, Maria de Luna, 3, 50015 Zaragoza, Spain Received November 14, 2000. Revised Manuscript Received March 8, 2001

An experimental and theoretical study of the oxidation of methanol in the absence and in the presence of NO has been performed. The experiments were conducted in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 700-1500 K. The influence of the temperature, oxygen concentration, and presence of NO on the concentrations of methanol, CO, CO2, and NO has been analyzed. A reaction mechanism based on the model of Glarborg et al.1 for hydrocarbons/NO interactions, updated in relation to the methanol reaction subset has been used for calculations. The results show that the oxidation regime of methanol for different air excess conditions is very similar in the absence of NO, but significant differences are observed when NO is present. The presence of NO implies a different behavior depending on the stoichiometry, in the way that such presence results in a inhibition of methanol conversion for richest conditions, while it promotes methanol oxidation for very lean conditions. The experimental results are analyzed in terms of detailed chemistry and the main issues are discussed.

Introduction Oxygenated hydrocarbons are nowadays considered as an alternative to gasolines for transport purposes. Among those, methanol which is the most simple alcohol may be of interest because of its different possible production sources. Methanol is a comparatively clean fuel because of, among others, the presence of oxygen in its composition and the lack of C-C bonds in its structure which limits the probability of an incomplete combustion. Different experimental studies concerning methanol oxidation have been reported until now. The oldest studies used static reactors and covered the temperature range of 600 to 900 K.2-6 Flow reactor studies covering a larger range of operating conditions have been reported for temperatures of 700 to 1100 K and pressures of 1-20 bar,7-10 while shock tube data cover the * Author to whom correspondence should be addressed. Fax: +34 976 761879. E-mail: [email protected]. (1) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1-27. (2) Fort, R.; Hinshelwood, C. N. Proc. R. Soc. London A 1930, 129, 284-296. (3) Bone, W. A.; Gardner, J. B. Proc. R. Soc. London A 1936, 154, 297-302. (4) Bell, K. M.; Tipper, C. F. H. Proc. R. Soc. London A 1956, 238, 256-265. (5) Cathonnet, M.; Boettner, J. C.; James, H. J. Chim. Phys. 1982, 79, 475-490. (6) Aniolek, K. W.; Wilk, R. D. Energy Fuels 1995, 9, 395-405. (7) Aronowitz, D.; Santoro, R. J.; Dryer, F. L.; Glassman. 17th Symp. (Int.) Combust.; The Combustion Institute: Pittsburgh, 1978; pp 633642. (8) Norton, T. S.; Dryer, F. L. Combust. Sci. Tecnol. 1989, 63, 107129. (9) Held, T. J.; Dryer, F. L. 25th Symp. (Int.) Combust.; The Combustion Institute: Pittsburgh, 1994; pp 901-908. (10) Taylor, P. H.; Cheng, L.; Dellinger, B. Combust. Flame 1998, 115, 561-567.

temperature range of 1200 to 2200 K.11-13 Furthermore, a significant number of studies on methanol flames have also been reported in the literature. Based on these studies, detailed reaction mechanisms for methanol conversion have been proposed,8,9,14,15 which cover a wide range of operating conditions. The aim of the present work is to extend the experimental database on methanol oxidation as well as on the interaction of methanol with nitric oxide, as well as to perform a detailed kinetic modeling study. The oxidation of methanol is studied under flow reactor conditions in the 700-1500 K temperature range, both in the absence and presence of nitric oxide for different air excess ratios for examining the sensitized oxidation of methanol. The results are analyzed in terms of a detailed chemical kinetic model. Experimental Section The experimental installation used in the present work is described in detail elsewhere 16,17 and only a brief description is given here. A quartz flow reactor following the design of Kristensen et al. 18 is placed in a three-zone electrically heated oven, ensuring a uniform temperature profile ((5 K) along the reaction zone. The reactor tube has a reaction zone of 8.7 mm (11) Bowman, C. T. Combust. Flame 1975, 25, 343-351. (12) Tsuboi, T.; Hashimoto, K. Combust. Flame 1981, 42, 61-67. (13) Cribb, P. H.; Dove, J. E.; Yamazaki, S. Combust. Flame 1992, 88, 186-194. (14) Westbrook, C. K.; Dryer, F. L. Combust. Sci. Technol. 1979, 20, 125-138. (15) Egolfopoulos, F. N.; Zhu, D. L.; Law, C. K. 23rd Symp. (Int.) Combust.; The Combustion Institute: Pittsburgh, 1990; pp 471-479. (16) Alzueta, M. U.; Oliva, M.; Glarborg, P. Int. J. Chem. Kinet. 1998, 30, 683-697. (17) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Iban˜ez, J. C. Energy Fuels 1998, 12, 1001-1007. (18) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1996, 107, 211-222.

10.1021/ef0002602 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/20/2001

Methanol Oxidation and Its Interaction with NO

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inside diameter and 200 mm in length. The gases are led to the reactor in up to four separate streams: a main flow containing nitrogen and water, and the injector tubes for the rest of reactants. The product gas is quenched by the addition of cooling air at the outlet of the reaction zone. The analysis of the product gas is performed by means of a FTIR (Fourier transform infra-red) spectrometer and a continuous NO analyzer. The estimated uncertainty of the measurements is (5% but not less than 10 ppm.

Reaction Mechanism The experimental results are analyzed in terms of a detailed chemical kinetic model for methanol conversion in the presence of NO. The chemical kinetic model used in the present study is based on the reaction mechanism developed by Glarborg et al.,1 to describe the interactions among C1/C2 hydrocarbons and nitric oxide, together with the modifications of the H/N/O subset according to the recent works of Glarborg et al. 19 and Miller and Glarborg,20 even though it has been updated according to the chemistry involving methanol and derivatives. The main reactions of interest are discussed below and the full mechanism can be obtained directly from the authors. Calculations are performed using Senkin,21 which runs in conjunction with the Chemkin library.22 The reverse rate constants were obtained from the forward rate constants. As shown below, the oxidation of methanol is mainly sensitive to the initial consumption steps, even though reactions involving the hydroxymethyl radical have also certain significance. Under the conditions of the present work, methanol reacts mainly with the radical pool to produce CH2OH:

CH3OH + H/O/OH/HO2 h CH2OH + ... with a minor reaction path producing CH3O resulting from the interaction of methanol with OH and H radicals. For the rate for the reaction of methanol with OH radicals, which is the predominant consumption route, we have adopted the value of Li and Williams 23 which implies a variable branching ratio for the CH2OH and CH3O channels, increasing the importance of the CH3O route as temperature increases, in concordance with previous investigations.15 For the reaction with H radicals, the value of Tsang 24 together with the branching ratio of Li and Williams 23 has been used. The reaction of CH3OH with HO2 radicals appears to be a key reaction for the present study in concordance with the findings of Norton and Dryer,8 and the uncertainty concerning its reaction rate is high. For this reaction we have considered the reaction rate obtained by Tsuboi and Hashimoto 12 which in the temperature (19) Glarborg, P.; Østberg, M.; Alzueta, M. U. Dam-Johansen, K.; Miller, J. A. 27th Symp. (Int.) Combust.; The Combustion Institute: Pittsburgh, 1998; pp 219-226. (20) Miller, J. A.; Glarborg, P. Int. J. Chem. Kinet. 1999, 31, 757765. (21) Lutz, A.; Kee, R. J.; Miller, J. A. Senkin: A Fortran Program for Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis; Sandia National Laboratories Report SAND87-8248, 1990. (22) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics; Sandia National Laboratories Report SAND89-8009, 1989. (23) Li, S. C.; Williams, F. A. 26th Symp. (Int.) Combust.; The Combustion Institute: Pittsburgh, 1996; pp 1017-1024. (24) Tsang, W. J. Phys. Chem. Ref. Data 1987, 16, 471.

Figure 1. Concentration of CH3OH, CO, and CO2 as a function of temperature for the conditions of set 2 in Table 1. Comparison between experimental data (symbols) and model predictions (lines). Table 1. Experimental Conditions (The experiments are conducted at constant mass flow, and thereby the residence time is dependent on the reaction temperature, as listed) expt

CH3OH (ppm)

O2 (ppm)

H2O (%)

NO (ppm)

λ

res. time (s)

set 1 set 2 set 3 set 4 set 5 set 6

225 325 321 450 450 330

206 429 3821 250 771 7256

1.75 1.87 1.78 1.16 1.10 0.94

0 0 0 505 416 3450

0.61 0.88 7.94 0.37 1.14 14.66

179/T(K) 180/T(K) 180/T(K) 188/T(K) 182/T(K) 185/T(K)

range of this work is an intermediate value among other reported values, such as those of Tsang 24 and Cathonnet et al.5 The CH2OH and CH3O formed from methanol give quantitatively CH2O. The most important reaction is that of CH2OH with O2, which has been extensively studied and significant discrepancies in the reaction rates are reported. For this reaction, we have used the recommendation of Baulch et al..25 The formed CH2O follows the reaction sequence CH2O f HCO f CO f CO2. Results and Discussion A study of the oxidation of methanol at atmospheric pressure in the temperature range 700-1500 K has been carried out. The experiments were conducted keeping a constant mass flow rate and varying the reaction temperature, resulting in a variable residence time as a function of temperature. In addition to temperature, the influence of other variables such as the excess air ratio, λ, and the NO concentration has been analyzed. Table 1 lists the conditions of selected experiments, all performed under highly diluted conditions. Reactants used are CH3OH, O2, H2O, NO, and nitrogen is used to balance. Figure 1 compares experimental and calculated results of methanol, CO, and CO2 as a function of temperature for the conditions of set 2, i.e., almost stoichiometric conditions, in the absence of NO. Symbols denote experimental results, and lines denote model (25) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just, Th.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys. Chem. Ref. Data 1992, 21, 411-734.

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Figure 2. Conversion of CH3OH as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1-3 in Table 1.

Alzueta et al.

Figure 4. CO2 concentration profiles during methanol oxidation as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1-3 in Table 1.

tendency: the onset of CO and CO2 formation is shifted to lower temperatures as the stoichiometry becomes leaner. Again, calculations agree well with experimental data, except for the maximum CO concentration which is underpredicted in all the cases. Figure 5 shows a reaction path diagram for the oxidation of methanol under the present conditions. The main reaction paths agree well with previous studies of the oxidation of methanol in the absence of NO, see for example the works of Westbrook and Dryer,14 Norton and Dryer,8 and Held and Dryer.9 The oxidation of methanol is initiated by reaction with the H/O radical pool, giving mainly hydroxymethyl radicals,

Figure 3. CO concentration profiles during methanol oxidation as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1-3 in Table 1.

calculations. The model matches well the experimental data. For the conditions of Figure 1, the conversion of methanol starts slightly below 1000 K. The onset of CO formation coincides with the decrease in methanol concentration, and CO peaks at around 1100 K. As the temperature is increased, methanol is completely oxydized to CO2, even under slightly rich conditions, due to the presence of oxygen in the methanol molecule. Apart from methanol, CO, and CO2, only formaldehyde is detected in the FTIR spectra, but it is not shown in the figures because the amount detected lies within the uncertainty of the measurements, i.e., (10 ppm. Figures 2 to 4 show the results of methanol, CO, and CO2 as a function of temperature for different air excess ratios, rich, almost stoichiometric and very lean conditions, in absence of NO. As seen in Figure 2, the oxygen concentration does not influence significantly the onset of methanol conversion, even though the oxidation of methanol is produced at a slightly lower temperature as the stoichiometry becomes leaner. The model agrees well with the experimental results. However, the oxygen availability has a slightly more pronounced effect on the concentrations of CO and CO2, keeping the same

CH3OH + H h CH2OH + H2

(1)

CH3OH + OH h CH2OH + H2O

(2)

CH3OH + O h CH2OH + OH

(3)

CH3OH + HO2 h CH2OH + H2O2

(4)

The reactions with H and OH radicals are the most important in all the conditions studied in this work, and the relative importance of them depends on the availability of oxygen, being the reaction with H radicals dominant only for the richest conditions. A small fraction of methanol produces also methoxy radicals by reaction with H and OH radicals,

CH3OH + H h CH3O + H2

(5)

CH3OH + OH h CH3O + H2O

(6)

Both methoxymethyl and methoxy radicals give quantitatively formaldehyde by bimolecular decomposition or reaction with molecular oxygen,

CH2OH (+ M) h CH2O + H (+ M)

(7)

CH2OH + O2 h CH2O + HO2

(8)

CH3O (+ M) h CH2O + H (+ M)

(9)

Table 2 shows a first-order sensitivity analysis for CO

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Figure 6. Conversion of CH3OH as a function of temperature for different air excess ratios in the presence of NO. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 4-6 in Table 1.

a chain branching sequence which feeds the radical pool providing HO2 and OH radicals which contribute to extend the conversion of methanol, Figure 5. Reaction path diagram for methanol oxidation (in absence of NO). Table 2. Linear Sensitivity Coefficients for CO at Selected Temperatures for Sets 1-3. (The sensitivity coefficients are given as AiδYj/YjδAi, where Ai is the pre-exponential constant for reaction I and Yj is the mass fraction of the jth species. Therefore, the sensitivity coefficients listed can be interpreted as the relative change in predicted concentration for the species j caused by increasing the rate constant for reaction I by a factor of 2) reaction CH3OH + H h CH3OH + H2 CH3OH + H h CH3O + H2 CH3OH + OH h CH3O + H2O CH3OH + HO2 h CH2OH + H2O2 CH3OH + O2 h CH2OH + HO2 CH2OH + O2 h CH2O + HO2 CH4 + OH h CH3 + H2O CH3 + OH (+M) h CH3OH (+M) O + OH h O2 + H OH + H2 h H2O + H H + HO2 h H2 + O2 H + HO2 h OH + OH OH + HO2 h H2O + O2 HO2 + HO2 h H2O2 + O2 H2O2 + M h OH + OH + M H2O2 + OH h H2O + HO2

set 1 1050 K

set 2 1010 K

set 3 1000 K

-1.35 0.12 -0.38

-0.77 0.11 -0.08 12.10 0.84 -1.02

-0.02 0.05 -0.01 10.28 1.74 -0.34

0.19 0.77 1.35 -0.10 -0.58 0.43 -0.15 0.26 1.14 0.16

0.55 0.56

2.62

corresponding to the initiation conditions of the experiments corresponding to sets 1 to 3 in Table 1. The sensitivity analysis confirms that the predicted onset of CO formation is very sensitive to the methanol + radical reactions. It is interesting to note the importance of the reaction between methanol and the hydroperoxyradical, CH3OH + HO2 h CH2OH +H2O2, under the present conditions. The generation of H2O2 is a key issue for the oxidation of methanol. H2O2 is formed by the interaction of methanol with HO2, but also through OH radical recombination. Calculations show that as temperature increases, i.e., above approximately 1000 K, H2O2 starts

H2O2 + H h HO2 + H2

(10)

H2O2 + H h OH + H2O

(11)

H2O2 + OH h H2O + HO2

(12)

We have also studied the oxidation of methanol in the presence of nitric oxide. The use of methanol as a fuel may produce nitrogen oxides, through the thermal NOx formation mechanism, under combustion conditions. Once formed, nitrogen oxides may interact with methanol or its derivatives. NO may be reduced in reburning type reactions under rich conditions 26 or may favor the oxidation of methanol in a mutually sensitized oxidation process.10 Figures 6 to 8 show the results of carbon species obtained during the oxidation of methanol in the presence of NO, corresponding to the experiments of sets 4 to 6 in Table 1. Model calculations agree very well with the experimental findings. The results of methanol as function of temperature for different air excess ratios are shown in Figure 6. In contrast to Figure 2 (methanol conversion in absence of NO), the results of Figure 6 show a significant influence of the oxygen concentration in the presence of nitric oxide, with the onset of methanol oxidation being produced at lower temperatures as the stoichiometry becomes leaner. Under excess oxygen conditions and once the hydroxymethyl radical is formed, the reaction with O2, i.e., CH2OH + O2 h CH2O + HO2, becomes the main reaction channel of CH2OH consumption. The hydroperoxy radical formed in this reaction reacts with nitric oxide in the sequence,

NO + HO2 h NO2 + OH

(13)

NO2 + H h NO + OH

(14)

(26) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1997, 109, 25-36.

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Figure 7. CO concentration profiles during methanol oxidation as a function of temperature for different air excess ratios in the presence of NO. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 4-6 in Table 1.

Alzueta et al.

Figure 9. Comparison between CH3OH conversion data under rich conditions in absence and presence of NO, as a function of temperature. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 1 and 4 in Table 1.

of methanol. Under these conditions, the presence of NO acts catalyzing radical recombination as has been suggested earlier (i.e., Glarborg et al.,27 Bromly et al. 28), diminishing the radical pool concentration, thereby inhibiting methanol oxidation. Under the conditions of Figure 9, in the presence of NO, in addition to the interaction of NO with HO2, a new reaction channel becomes important, involving the reaction of NO with CH2OH:

Figure 8. CO2 concentration profiles during methanol oxidation as a function of temperature for different air excess ratios in the presence of NO. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 4-6 in Table 1.

resulting in a net increase of OH radicals which are the main responsible for methanol conversion, and thus producing its conversion at lower temperatures. It is also interesting to see in Figure 8 and for the leanest conditions studied, the profile of CO2. CO2 formation is seen to start at temperatures slightly above 900 K, even though its sharp increase is produced at temperatures of about 1100 K. This behavior, similar to the two-stage ignition behavior, can be attributed to the decomposition regime of H2O2, which starts to be significant at temperatures above 1100 K. Therefore, as the temperature is increased the reaction sequence eqs 10 to 12 becomes significant and contributes to the rapid conversion of methanol and the formation of CO and CO2, respectively. While for very lean conditions, a mutually sensitized oxidation of methanol and NO is observed, such effect is not seen for the rest of conditions studied in this work, i.e., almost stoichiometric and rich. For clarity, Figure 9 shows the comparison of methanol conversion results under rich conditions as a function of temperature both in the absence and in the presence of NO. Here, it is seen that the presence of NO does inhibit the oxidation

NO + HO2 h NO2 + OH

(12)

CH2OH + NO h CH2O + HNO

(15)

The latest reaction produces HNO, which initiates the following chain terminating sequence, responsible for methanol conversion inhibition,

HNO + M h H + NO + M

(16)

HNO + H h H2 + NO

(17)

HNO + OH h NO + H2O

(18)

In contrast, in the absence of NO, the hydroperoxy radical formed by the interaction of CH2OH and O2, is consumed by reaction with the radical pool,

HO2 + H h OH + OH

(19)

HO2 + HO2 h H2O2 + O2

(20)

but also with methanol when still present,

CH3OH + HO2 h CH2OH + H2O2

(4)

Finally, Figure 10 shows the results of NO versus temperature for the different stoichiometries studied. Only for the leanest conditions, λ ) 14.7, a significant variation in NO is seen, with a minimum for a temperature of 1000 K. The decrease in NO is due to its conversion to NO2. In the experiments, NO2 was identi(27) Glarborg, P.; Kubel, D.; Kristensen, P. G.; Hansen, J.; DamJohansen, K. Combust. Sci. Technol. 1995, 111, 461-485. (28) Bromly, J. H.; Barnes, F. J.; Muris, S.; You, X.; Haynes, B. S. Combust. Sci. Technol. 1995, 115, 259-296.

Methanol Oxidation and Its Interaction with NO

Figure 10. NO concentration profiles during methanol oxidation as a function of temperature for different air excess ratios. Comparison between experimental data (symbols) and model predictions (lines). The inlet conditions correspond to sets 4-6 in Table 1.

fied in the same range of temperature conditions as the NO decrease, but NO2 was not quantified in those experiments. Model calculations also predict such conversion to NO2 for the leanest conditions of Figure 10. It is also noticeable that no NO decrease is seen at the highest temperatures studied. Therefore, no reburn reactions appear to be important, which is attributed to the fact that at the highest temperatures of the present work, the conversion of methanol is completely produced and only CO2 is detected, and thus no hydrocarbon radicals are available for reburn type reactions. Conclusions The oxidation of methanol has been studied in a quartz reactor at atmospheric pressure and tempera-

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tures from 700 to 1500 K. The stoichiometry has ranged from rich to lean conditions and the effect of the addition of nitric oxide has been investigated. The results have been analyzed in terms of a detailed chemical kinetic model. In the absence of nitric oxide, oxidation of methanol is initiated above 1000 K. Little influence of the oxygen level is seen for the onset of methanol conversion, while a higher effect is observed on the CO and CO2 profiles. The conversion of methanol follows basically the sequence CH3OH f CH2OH f CH2O f HCO f CO f CO2, with a minor fraction proceeding through CH3O. The kinetic modeling predictions agree well with the experimental data. The addition of NO in the methanol oxidation experiments results in a significant importance of the oxygen level on the onset of methanol conversion, being produced at lower temperatures as the stoichiometry becomes leaner. Under very lean conditions, the presence of NO is responsible for the sensitized oxidation of methanol, while for rich conditions the NO presence acts inhibiting methanol conversion by catalyzing radical recombination. During methanol conversion in the presence of NO, only for lean conditions, an appreciable decrease in NO concentration is seen, resulting into the formation of NO2, thereby no net reduction of nitric oxides is observed.

Acknowledgment. The authors express their gratitude to projects QUI97-1112 and DGA-P061/99-T for financial support. The useful comments and contributions from C. Westbrook are fully acknowledged. EF0002602