396
Energy & Fuels 1995,9, 395-405
Preflame Oxidation Characteristics of Methanol K. W. Aniolek and R. D. Wilk* Department of Mechanical Engineering, Union College, Schenectady, New York 12308-2311 Received August 22, 1994@
The low-temperature slow oxidation of methanol was examined experimentally in a constant volume stirred reactor. Initial temperatures ranged from approximately 650 to 700 K. Initial pressures of 700 Torr and below were examined and the equivalence ratio was varied between 0.5 and 1.5. Temperature and pressure histories were obtained for methanol reacting in air and oxygen. Global indicators of reaction rate, reactivity, and autoignition tendency were derived from these data. The relationship among these indicators and the effects on these indicators of initial temperature, initial pressure, and fuel/oxidizer mixture ratio were examined. In general, it was found that the overall reaction rate increased with increasing temperature, pressure, and equivalence ratio. Correlations were developed from the data which represent these effects over the range of conditions examined. In addition, chemical measurements were made using gas chromatography to examine the reactant, stable intermediate, and product species histories and t o gain insight into the oxidation mechanism at the conditions studied. The results are in agreement with an earlier mechanism and indicate that the basic path of the low-temperature oxidation of methanol proceeds via CH30H CHzOH CHzO CHO -, CO (+ COz).
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Introduction Methanol is a leading candidate among the many alternative fuels being considered to extend and replace gasoline and distillate fuels. It is especially suitable for use as a transportation fuel in SI engines. The major advantages of methanol over conventional gasoline for this purpose are its high octane quality and lower emissions. In addition, it can be produced from nonpetroleum resources such as natural gas, coal, and biomass. Addressing the combustion and emissions-related problems associated with the utilization of methanol or any other transportation fuel requires a fundamental understanding of the combustion process. In many practical combustion systems, the fuel is oxidized over a range of pressures and temperatures and the characteristics of the combustion process change with these conditions. Ideally it is desirable to have a detailed understanding of the physical and chemical behavior of the fuel-oxidizer system which occurs a t any condition. It is impossible t o develop a single combustion experiment which could be exercised over a wide range of pressures and temperatures and yield the level of detailed information desired. Consequently, many different types of well-defined, bench scale experiments (shock tubes, flames, flow reactors, rapid compression machines, stirred reactors, and constant volume bombs) have been developed and are being used. Each of these isolates a particular combustion regime (range of temperature, pressure, and reactant concentration) for detailed study. The information from the various experiments is used to develop and validate combustion reaction mechanisms which can then be used to model combustion processes. It is desirable to have a comprehensive reaction mechanism, that is, one which is
* Author to whom correspondence should be addressed. Phone: (518) 388-6268.FAX: (518)388-6789. Abstract published in Advance ACS Abstracts, April 1, 1995. @
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valid over a wide range of conditions and confirmed by measured data from different experimental systems. In 1979, Westbrook and Dryer' developed a mechanism for methanol oxidation, applicable to temperatures in the range 1000-2200 K, pressures of 1-5 atm, and equivalence ratios of 0.05-3.0. This model has been applied in a number of studies on methanol to simulate the results (igntion delay times, flame velocities, and species concentrations) of many different experiments. In addition, new mechanisms were presented by Dove and Warnatz2 and most recently by Grotheer et al.3As new information on methanol oxidation became available, the Westbrook and Dryer mechanism was modified by Norton and Dryer,4 Egolfopoulos et al.,5 and most recently by Held and Dryer.6 There has been to date a significant amount of experimental work done to study the oxidation of methanol in the various combustion regimes. The hightemperature (2' > 1000 K) oxidation has been the subject of many studies. Shock tubes have been used in many of these. Cooke et al.7 measured ignition delay times and monitored CH and OH radicals. Bowmana measured characteristic reaction times, correlated them with initial reactant concentrations and temperature, measured concentrations of 0, OH, HzO, and CO, and (1)Westbrook, C. K.; Dryer, F. L. Combust. Sci. Technol. 1979,20, 125-140. ( 2 )Dove, J.E.; Warnatz, J. Ber. Bunsen-Ges.Phys. Chem. 1983,87, 1040-1044. (3)Grotheer, H.; Kelm, S.; Driver, H. S. T.; Hutcheon, R. J.; Lockett, R. D.; Robertson, G . N. Ber. Bunsen-Ges.Phys. Chem. 1992,96,13601375. (4)Norton, T.S.;Dryer, F. L. Combust. Sci. Technol. 1989,67,107129. ( 5 )Epolfopoulos, F.N.; Du, D. X.; Law, C. K. Combust. Sci. Technol. 1992,83,33--75. (6)Held, T.J.;Dryer, F. L. Twenty Fifth Symposium (International) on Combustion, [Pmceedingsl; - The Combustion Institute: Pittsburgh, PA, 1994,in press. (7)Cooke, D. F.;Dodson, M. G.; Williams, A. Combust. Flame 1971, 16,223-236. (8)Bowman, C. T.Combust. Flame 1975,25,343-354.
0887-0624/95/2509-0395$09.00/00 1995 American Chemical Society
396 Energy & Fuels, Vol. 9, No. 3, 1995
compared this data with results from a 19-step mechanism which was developed. Natarajan and Bhaskarang and Tsuboi and HashimotolO obtained correlations for ignition delay times and assembled mechanisms to model their high temperature results. High-temperature methanol oxidation has also been studied in flames by Akrich et a1.,l1Vandooren and Van Tiggelen,12 and Pauwels et al.13 Methanol has been studied extensively at temperatures in the range 10001100 K. Most of this work is based on results obtained from the Princeton flow reactor by Aronowitz et al.14 and Norton and Dryere4J5A flow reactor was also used by Koda and Tanaka16 who studied the effects of the addition of NO2 on the ignition temperatures. The pressure used in most of the work listed above was either near atmospheric or subatmospheric. Three studies specifically focused on the autoignition process at high pressures. Seibers and Edwards17 measured ignition delay times and rates of pressure rise associated with the autoignition of methanol fuel sprays in a constant volume bomb at temperatures from 900 to 1500 K. Lee et a1.18 used a rapid compression machine in the range 750-1000 K t o obtain autoignition delay times and overall activation energies and compared their results t o calculated results using the mechanism presented by Egolfopoulos et al.5 Recently, Held and Dryer6 studied methanol oxidation experimentally in the range 752-1023 K, and 1-20 atm., conditionswhich are very relevant to autoignition. They also performed kinetic modeling of the results. Another area of interest is the low-temperature regime in which the preflame or preignition behavior of the fuel-oxidizer system is important. There are many interesting combustion phenomena associated with this regime. Chief among these are the occurrence of cool flames and a negative temperature coefficient in the rate of oxidation which have been exhibited by many hydrocarbons. In many practical and laboratory systems, the fuel can spend appreciable time at lower pressures and temperatures. This is important since the preignition reactions occurring in the fuel/oxidizer system lead up to and determine autoignition, which is an important mode of initiating the combustion process. Better descriptions of preflame or low-temperature processes are necessary in order to model more effectively the overall combustion process. Low-temperature (600-900 K) studies on methanol using static reactors have been conducted previously. Fort and Hinshelwoodlg measured the pressure rise for different methanol oxygen mixtures at a single temper(9) Natarajan, K.; Bhaskaran, K. A. Combust. Flame 1981,43,3549. (10) Tsuboi, T.; Hashimoto, K. Combust. Flame 1981,42,61-76. (11)Akrich, R.; Vovelle, C.; Delbourgo, R. Combust. Flame 1978, 32, 171-179. (12) Vandooren, J.; Van Tiggelen, P. J . Eighteenth Symposium (International) on Combustion, [Proceedings];The Combustion Institute: Pittsburgh, PA, 1981; pp 473-482. (13)Pauwels, J . F.; Carlier, M.; Devolder, P.; Sochet, L. R. Combust. Sci. Technol. 1989,64,97-117. (14)Aronowitq D.; Santoro, R. J.; Dryer, F. L.; Glassman, I. Seventeenth Symposium (International) on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, PA, 1979; pp 633-644. (15) Norton, T. S.; Dryer, F. L. Twenty Third Symposium (International) on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, PA, 1990; pp 179-185. (16) Koda, S.;Tanaka, M. Combust. Sci. Technol. 1986,47, 165176. (17) Siebers, D. L.; Edwards, C. F. SAE Paper 870588, 1987. (18)Lee, D. L.; Hochgreb, S.; Keck, J. C. SAE Paper 932755, 1993.
Aniolek and Wilk
ature, 710 K, and reported a few analyses of the products. Bone and Gardner20 analyzed the effects of surface-to-volume ratio and temperature on the pressure rise for two different methanol-oxygen mixtures. Bell and Tipper21,22studied the slow oxidation of methanol in oxygen at temperatures in the range 660740 K at subatmospheric pressures. Species measurements were made at 713 K and the kinetics were studied by analyzing the pressure histories of the reactions. Surface effects were examined and a reaction scheme was postulated to explain the slow oxidationlow-temperature mechanism. Most recently, Cathonnet et al.23 examined methanol oxidation and obtained species profiles for four different equivalence ratios at 823 K and 200 Torr total pressure. The current study examines experimentally the low-temperature slow oxidation of methanol in air and oxygen using a constant volume stirred reactor. A parametric investigation was conducted determine the effects of temperature, pressure, and equivalence ratio on the global indicators of reaction rate or autoignition tendency. In addition, some chemical species data were obtained and are presented. This research was intended to supplement and complement the previous work on methanol combustion by providing additional and updated information on the low-temperature oxidative process of this fuel. This data should be useful in validating comprehensive methanol mechanisms at lower temperatures so they can be applied to model the autoignition process.
Experimental Facility and Techniques A schematic of the experimental system is shown in Figure 1. The system consists of a 2900 cm3stainless steel cylindrical mixing vessel into which the liquid fuel is injected via a syringe adapter and allowed to vaporize. The vessel was heated t o 403 K to ensure complete vaporization of the fuel. The oxidizer was then introduced and the contents given an approximate 20-30 min diffusion and mixing time to reach homogeneity. This mixing procedure was verified experimentally using gas chromatographic analysis. The fuevoxidizer mixture was then sent into a reaction vessel located in an insulated furnace. The furnace is 61 by 61 cm wide and 46 cm with 7.6 cm thick walls insulated with foam rigid-board insulation. A temperature controller regulated three electric resistance rod-type heaters to achieve the desired furnace temperature. The heaters were turned on well in advance of the experiments t o produce a stable furnace temperature; subsequently, the reaction vessel temperature did not vary by more than &1deg. The reaction vessel consists of an 870 cm3Pyrex cylindrical, upright, stirred vessel with a surface-to-volume ratio of 0.65 cm-l as shown in Figure 2. The tubing connecting the vessel to the system has a dead volume of 6.5 cm3 comprising 0.75% of the total reacting volume. Prior t o the experiments, the vessel was washed with a concentrated boric acid (H3B03) solution to aid in achieving reproducible results. A Type R platinudplatinum- 13% rhodium fine wire (0.002 in. diameter) thermocouple, coated with silica to prevent catalytic reactions, measured the temperature inside the reaction vessel (19) Fort, F.; Hinshelwood, C. N. Proc. R. SOC.London 1930,A129, 284-299. (20) Bone, W.A,; Gardner, J. B. Proc. R. SOC.London 1936,A154, 297-328. (21) Bell, K. M.; Tipper, C. F. H. Proc. R. SOC.London 1956,A238, 256-268. (22) Bell, K. M.; Tipper, C. F. H. Trans. Faraday SOC.1957,53,982990. (23) Cathonnet, M.;Boettner, J. C.; James, H. J.Chim. Phys. 1982, 79,475-478.
Preftame Oxidation Characteristics of Methanol
Energy & Fuels, Vol. 9, No. 3, 1995 397
I
KEY:
1
I
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Itlim
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Figure 1. Schematic of experimental system.
(2.2 cm from the center). A Setra Model 204 pressure transducer measured the reaction vessel pressure. The temperature and pressure signals were monitored on a stripchart recorder. The reaction vessel was specially designed to house a Pyrex stirrer to achieve a uniform mixture and temperature. A magnet, located below the reaction vessel, drives a glassencapsulated steel rod attached to the bottom of the stirrer as shown in Figure 2. The magnet was driven by a 9.2 V dc motor located beneath the furnace. The glass-to-glass surface contact between the stirrer shaft and the reaction vessel limited the stirrer to speeds of 150 rpm. A Hewlett Packard Model 5890 Series I1 gas chromatograph was used to analyze samples taken from the reaction vessel at prescribed times during the course of the oxidation process. A solenoid valve controlled by an interval timer was used to obtain a sample from the reaction vessel in a 1 s window of time. The sample entered a n evacuated line, heated to 90 "C, where it was quenched. Two portions (0.25 cm3 each) of this sample were then injected into the gas chromatograph by a gas sampling valve. The GC operated in a dual-column mode with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The FID side was fitted with in., 6 ft long Porapak N and Q packed columns in series, for hydrocarbon separation and detection, followed by a nickel catalyst assembly for methanization and detection of carbon monoxide and carbon dioxide. The TCD side was fitted with an '/a in., 6 ft long molecular sieve packed column to allow for analysis of 0 2 and Nz. The liquid methanol used in the experiments was of 99.9% purity. The oxidizers used included zero grade air (HzO 5 ppm, THC 1 ppm) and ultra high purity oxygen (99.993%
pure). A calibration standard mixture was used to quantify C1 and CZ gaseous species. Formaldehyde standards were obtained by vaporizing a solution of formaldehyde in water and injecting the vapor into the GC. An error analysis was performed on the calibration data for chemical species detected. Typical deviations from the mean included f 3 % for CH30H and 0 2 ; f 5 % for CO, COz, and f25% for CH20. In any experiment, precision and repeatability are of paramount importance. For Pyrex statidstirred reaction vessels, a foremost concern was the proper ageing of the vessel surface. The boric acid wash aids in this effort, but with any type of surface coating a number of runs must be performed before results become fully reproducible. For this work, approximately 200 runs were performed before vessel became adequately aged. The repeatability of the measured pressure and temperature profiles were used as a measure of reproducibility. Another problem which had to be considered was the possible adsoptioddesoprtion of methanol on the metal surfaces in the system. To guard against this, many preliminary runs were carried out which served t o saturate the surfaces with methanol. Then, during the actual data collection, pumping time to evacuate the system was kept to a minimum. In this work methanol was oxidized in both air and oxygen a t initial temperatures ranging from 648 to 705 K, initial pressures from 350 to 700 Torr, and equivalence ratios from 0.5 to 1.5.
Results Parametric Effects on the Overall Reaction Rate. For the case of methanol reacting in air for an initial total pressure of 700 Torr, measurable slow
398 Energy & Fuels, Vol. 9, No. 3, 1995
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Figure 4. Pressure profiles for CH30W02; 4 = 0.5, 0'2 = 676 K, PO = 700 Torr.
REACTION VESSEL
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Figure 2. Furnace and magnetic stirrer assembly.
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Figure 3. Temperature profiles for CH30W02; 9 = 0.5, 2'0 = 676 K, PO= 700 Torr.
oxidation was found to occur at initial temperatures above 665 K. Figures 3 and 4 present typical temperature and pressure histories for a reacting methanoloxygen mixture. These figures actually show three sets of measured profiles for this condition. These data were
taken at the specified condition at different times during the course of this study (several weeks) in order to verify the repeatability of the measurements. The temperature profiles represent a history of the net heat release of the reaction (the heat generated by the reaction minus the heat lost through the vessel walls). Upon admission into the reaction vessel the mixture required approximately 30 s to reach thermal equilibrium, compared to approximately 3-5 s for pressure equilibrium. The thermal equilibration time was much less than the overall reaction times for all of the mixtures studied. After a delay period, the temperature increases, slowly at first, as the rate of energy generated exceeds the rate of heat loss. The temperature eventually reaches a maximum and then decreases back to the initial temperature as the rate of heat loss overtakes the rate of heat generated and then decreases to zero. In general, the temperature rise associated with slow oxidation in this study were relatively low, most cases not exceeding 12 deg. The pressure profiles exhibit the characteristic Sshape. There is an initial period during which there is very little pressure rise followed by a rapid increase in pressure. This is representative of constant volume, nonadiabatic, slow oxidation of most hydrocarbon fuels and consistent with an autocatalytic reaction with degenerate or delayed branching (Minkoff and Tipper,24 M c K ~and ~ ,Wilk26 ~ ~ 1. At the pressure and temperature conditions of this study the reaction times were long, on the order of minutes. The initial stage where the pressure rise is very low corresponds to low heat release and little chemical conversion. As the mixture begins to react, the rate of pressure rise increases t o a maximum and then drops off, resulting in a net pressure increase for the reaction, hpf. The rate of pressure rise, dPldt, is indicative of the reaction rate. Quite often, pressure history data are used to obtain much information about the preignitiodautoignition behavior of a given fuel-oxidizer system. (24) Minkoff, G. J.; Tipper, C. H . H . Chemistry of Combustion Reactions; Butterworth &. Co.: London, 1962. (25) McKay, G. Prog. Energy Combust. Sci. 1977,3, 105-126. (26) Wilk, R. D. Ph.D. Thesis, Drexel University, 1986.
Energy & Fuels, Vol. 9, No. 3, 1995 399
Preflame Oxidation Characteristics of Methanol 800
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Figure 5. Typical pressure-time profile for slow oxidation. From the typical pressure profile in Figure 5, it is possible to define directly four parameters which are often used as indicators of the relative reactivity of a fuel-oxidizer system. The induction period, z, is defined as the initial period of slow oxidation in which there is no significant pressure rise. It is measured here by extending down the tangent to the inflection point in the pressure-time curve until it intersects with the line of initial pressure. It represents a characteristic time required for the reaction to generate a sufficiently large pool of active intermediates which could then lead to chain branching and accelerate the overall reaction. AFf is the net pressure rise associated with the reaction and is indicative of the extent of reaction. (dPldt),, is the maximum rate of pressure rise attained in the reaction. In the period following the induction period, during which the exponential rise in pressure occurs, the change in pressure with time can be modeled by
where A€' is the pressure rise at time, t, B is a constant, and 8 is termed the net branching f a ~ t o r . ~ The ~ -rate ~~ of change of pressure during this period is indicative of the overall rate of reaction and can be described by
The net branching factor has its origins in chain reaction theory27and is the ratio of active intermediates formed to those reacted in the system. It represents the balance between chain branching and chain termin a t i ~ n . ~ O ,Thus, ~l the higher 8, the more rapid the increase in pressure and the more prone the system t o autoignite. Data from the pressure histories for each (27)Semenov, N.N.Chemical Kinetics and Chain Reactions; Oxford University Press: London, 1936. (28)Pettre, M. Third Symposium on Combustion and Flame and Explosion Phenomena, Williams and Wilkins: Baltimore, MD, 1949; pp 397-404. (29)Drysdale, D. D. Combust. Flame 1971,17, 263-265. (30)Sokolik, A. S. Self-Ignition Flame and Detonation in Gases; Israel Program for Scientific Translations, Ltd: Jerusalem, 1963. (31)Dryer, F. L. The Phenomenology of Modeling Combustion Chemistry. In Fossil Fuel Combustion: A Source Book; Bartok, W., Sarofim, A. F., Eds.; Wiley: New York, 1991;Chapter 3.
'
' 300
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Figure 6. Comparison of the pressure profiles for CH30W02 and CHsOWair mixtures; TO= 667 K,PO = 700 Torr.
reaction are used and the net branching factor is obtained from the slope of plots of In AF against time. These indicators were determined from the methanol experimental data and used to assess the effects of temperature, pressure, mixture ratio, and oxidizer on the reactivity or autoignition tendency of this fuel. Substituting oxygen for air enhanced significantly the reactivity of the system. Shown in Figure 6 are comparisons of the pressure histories for cases with oxygen and air. In going from air to oxygen, the net pressure rise for the reaction increases by a factor of 3 for 4 = 0.5 and by a factor of 1.5 for 4 = 1.25. The induction period is 1.8 and 1.2 times lower for equivalence ratios of 0.5 and 1.25, respectively. The maximum rate of pressure rise increases by 5.8 times for 4 = 0.5 and by a factor of 2.3 for 4 = 1.25. The net branching factor for the oxygen case is about twice that of the air at 4 = 0.5, and about 50% higher at 4 = 1.25. Since we are working with a constant volume system with fxed initial pressure, changing the oxidant from air to 02 and maintaining a constant equivalence ratio required that some of the N2 was replaced by methanol. Consequently, the effects demonstrated due to an increase in both 0 2 and methanol concentrations. The effects of varying initial temperature on the resulting temperature and pressure profiles are shown in Figures 7 and 8. Increasing initial temperature decreases the overall reaction time. For the case with initial temperature of 667 K, the system is nearly isothermal. The temperature rise increases with initial temperature to about 5 deg for the case with TO= 705 K. From the pressure profiles it can be seen that with increasing temperature, the overall reaction rate, as indicated by (Vldt),,, increases and the induction period decreases. The net pressure rise is nearly independent of initial temperature. The effect of initial temperature on the induction period and maximum rate of pressure rise are shown in Figures 9 and 10. The induction period is representative of a characteristic reaction time related to llkOv, where k,, is the overall reaction rate coefficient. Therefore, the variation of induction period (or ignition delay time for igniting systems) with initial temperature over
400 Energy & Fuels, Vol. 9,No. 3, 1995 705
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Figure 7. Effect of initial temperature on the temperature profiles for CHaOWair mixtures; 4 = 0.5, PO = 700 Torr.
lnltial Temperature (K)
Figure 9. Effect of initial temperature on the induction period for CHsOWair mixtures; PO= 700 Torr.
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Figure 8. Effect of initial temperature on the pressure profiles for CH3OWair mixtures; 4 = 0.5, PO = 700 Torr.
specific ranges of temperature for a specific mixture ratio is often expressed by an Arrhenius-type equation of the form z = C, exp(E,lRT,)
(3)
where E1 represents an overall activation energy of the preflame reactions.32 The equation can be linearized and a regression analysis performed on the data. The overall activation energy is obtained from the slope of the fitted line through the data plotted as In z against reciprocal initial temperature (Figure 11). This parameter is an indicator of how sensitive induction period is t o temperature. The overall activation energy values obtained are shown in Table 1. For comparison, shown in Table 2 are values obtained from other studies in (32) Cullis, C. F.; Foster, C. D. Proc. R.SOC.London 1977, A355, 153-165. (33) Dunphy, M. P.; Simmie, J. M. Combust. Flume 1991,85,48949R
(34) Nichols, R. J.
680
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SAE Paper 800258, 1980.
Initial Temperature (K)
Figure 10. Effect of initial temperature on the maximum rate of pressure rise for CHsOWair mixtures; PO = 700 Torr.
different systems at different conditions. It is interesting to note that the El values obtained in this study are comparable to those obtained from shock tube data at very different conditions including much higher temperatures and diluent concentrations. The variation of (Wldt),, with initial temperature can be expressed by a form very similar to that of the induction period: (df'ldt),,
= C2 exp(-EdRT,)
(4)
where E2 is an overall activation energy determined from the slope of the fitted line through the data plotted as ln(d?Ydt),,, against reciprocal temperature (Figure 12). E2 indicates how sensitive (dP/dt)m, is t o temperature. Values of E2 obtained in this study are given in Table 1. E2 values obtained in other studies are given in Table 2 for comparison. The Ez values obtained in this study are comparable to those obtained from the
Pregame Oxidation Characteristics of Methanol cc
Energy & Fuels, Vol. 9, No. 3, 1995 401 5
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Figure 11. Least-squares fit of induction period data for CH3OWair mixtures; PO = 700 Torr.
Figure 12. Least-squares fit of maximum rate of pressure rise data for CH3OWair mixtures; PO= 700 Torr.
Table 1. Overall Activation Energies for Methanol Oxidation: This Study
oxidizer/$ air10.5 Od0.5 air10.75 airIl.0 airll.25 Od1.25 airll.5
E1 (kcaVmo1)
E2 (kcaumol)
40.7 40.2 49.4 38.0 38.0 39.4 38.5
47.4 55.4 52.3 62.0 59.6 57.8 60.0
static reactor studies in which the conditions were comparable to in the present study. The overall activation energies obtained from the induction period data show little variation with equivalence ratio. The values obtained from the (dP/dt)m,, data show more variation. Also, consistent with the previous studies, the Eg values are larger than the E1 values. This indicates that (dP/dt)m, is more sensitive to changes in temperature than t. Also, since induction period decreases with increasing temperature and (dP/dt),, increases with increasing temperature, and both are exponentially related to temperature, one may expect some correlation between l/t and (d.Z'/dt)m,. This was found to be the case. There was actually a high degree of correlation between these two parameters. The case for 4 = 0.5 is shown in Figure 13. Net branching factors were determined from the pressure profiles for each condition by the methods described earlier. For a specified mixture ratio, the net branching factor increased with initial temperature. This is t o be expected based on the known effect of
O.OO0
0
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(d P/dt)max (tor rlmin)
Figure 13. Correlation of reciprocal induction period and maximum rate of pressure rise for CHsOWair; q5 = 0.5, PO = 700 Torr. Each point represents a reciprocal induction period and the corresponding maximum rate of pressure rise for a certain initial temperature.
temperature on chain branching. Other studies have shown that the net branching factor and induction period are inversely proportional to each other (to = const). This was also found to be the case based on the measured induction periods and calculated net branching factors from this study. Most of the data fit the form
Table 2. Overall Activation Energies (kcaYmol)for Methanol Oxidation: Other Studies
ref 7 8 9 10
33 18 19 20 21
34
system shock tube shock tube shock tube shock tube shock tube rapid comp. machine static reactor static reactor static reactor static reactor
oxidizer14 02 in 95% ArIl.0 0 2 in 95% Ar10.375-6.0 0 2 in 90% ArlO.5-1.5 0 2 in 98% ArlO.2-2.0 0 2 in 98% Arll.0 0 2 in 75% Ar + Ndl.0 OdO.8 Od3.0 Od1.2 air11.O
correlation parameter ignition delay reaction time ignition delay ignition delay reaction time induction period (dP/dt)max (dP1dt)max (dP1dt)mm ignition delay
E1 35.80 36.19 30.72 48.02 22.9 43.9 -
56.2
Ez -
62.5 53 45-53 -
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402 Energy & Fuels, Vol. 9, No. 3, 1995 800
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re = 4.0 t o within 12%. When the net branching factor was written in an Arrhenius-type form, 8 = C3 exp(-EdRTo), linearized and fit to the data, overall activation energies of approximately 40 kcaymol were obtained which are very close to the values obtained based on the induction period data. This indicates that the sensitivities of 8 and r to temperature are comparable. The effects of varying the fueVoxidizer mixture ratio on the reactivity are shown in Figure 14 where pressure profiles are shown for five different equivalence ratios at 667 K. The overall reaction rate increases and the induction period decreases with increasing equivalence ratio from lean t o rich. As was discussed relative to Figure 6, we were working with a constant volume system with a fxed initial pressure. Here the nitrogen/ oxygen ratio was fxed. Consequently, an increase in the equivalence ratio was obtained by increasing the initial fuel concentration and decreasing the initial oxidizer concentration simultaneously. Thus, from this it is difficult to draw any conclusions as t o the separate effects of methanol or oxidizer concentration on the reaction rate. However, previous studies on methanol oxidation in comparable systemsz1have found the maximum rate to be independent of oxygen concentration. On the basis of this, we were able t o attempt to correlate the rate parameters with initial fuel concentration using the following relationships: = aPfuelm
z = bP,,,"
40
(8)
Figure 14. Effect of equivalence ratio on pressure profiles for CHsOWair mixtures; TO= 667 K, PO= 700 Torr.
(dPfdt),,,
0
(6)
where Pfuelis the initial partial pressure of the methanol, a and b are constants which depend on the initial temperature, and m and n are exponents. When regression analyses were performed on the data, the values of m and n were found to be 1.5 and -0.5, respectively, and independent of initial temperature. Figure 15 shows the effect of fuel partial pressure on the induction period. Measured data are shown along with the best fits to the data at each initial temperature
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140
Methanol Partial Pressure (torr)
Figure 15. Effect of methanol partial pressure on the induction period. Symbols represent measured data; curves represent best fit t o
-2
60
-
50
-
5
= bPCH30H-0'5.
A L
b
U
E t 3
a a t n
40 -
L
z0)
30
-
J
A
/
0 TOr687K
A 20 40
60
To1678K
80
To=705K
100
120
140
Methanol Partial Pressure (torr)
Figure 16. Effect of methanol partial pressure on the net pressure rise for CHaOWair mixtures; PO= 700 Torr.
with n = -0.5. It should be noted that for this case the oxygen concentration also varied. The effect of initial fuel partial pressure on the net pressure rise is shown in Figure 16. As with the overall rate, the net pressure rise increases with the fuel partial pressure. However, the rate of increase drops off as the methanol concentration increases. The data also indicate that the net pressure rise is nearly independent of initial temperature, as seen previously in Figure 8. Figure 17 shows pressure histories for five different equivalence ratios at the highest initial temperature examined, 705 K. The lean and stoichiometric cases exhibit the characteristic S-shape associated with slow oxidation. However, for the two fuel-rich cases, the pressure increases more rapidly culminating in a spike in the pressure profiles, corresponding to the occurrence of hot ignition. Thus, in going from an equivalence ratio of 1.0 to 1.25,at an initial pressure of 700 Torr and an
Preflame Oxidation Characteristics of Methanol
I
1300
Energy & Fuels, Vol. 9,No. 3, 1995 403 750 I
1
II
1200
I100 n L
g
Y
?! a cn
cn
1000
900
E P
500
-
400
-
800
700 600
I
I
I . I . I . I . I . 1 0 100 200 300 400 500
L
350 0
200
400
600
Time (si) Time (si)
Figure 19. Effect of initial pressure on the pressure profiles
Figure 17. Effect of equivalence ratio on pressure profiles
for CHsOWair mixtures; q5 = 0.5, TO= 705 K.
for CH3OWair mixtures; TO= 705 K, PO = 700 Torr.
E
Y
?3!
c
E
d
E c
IU
700' 0
"
50
"
100
'
I
"
150
200
.
'
250
100
400
500
600
700
800
" 300 350
"
Initial Pressure (torr) Time
(si)
Figure 18. Effect of equivalence ratio on temperature profiles for CHsOWair mixtures; TO= 705 K, PO = 700 Torr.
initial temperature of 705 K, the system crossed the boundary between ignition and nonignition. Initially, it was thought that the pressure spikes could have been cool flames. Each of the pressure spikes resulted in a transient pressure rise of approximately 550 Torr which is much higher than that produced in a typical cool flame. Also, these cases were repeated and the reaction was observed visually. A bright orangeyellow flame accompanied the pressure increase and thus indicated a hot ignition. The richer of the two mixtures ignited slightly earlier. The net branching factors obtained for each of these cases were the highest of any of the cases investigated. Concurrent with the rapid rise in pressure accompanying the hot ignition for each case was the rapid heat release. This showed up as spikes on the temperature profiles (Figure 18). The pressure and temperature profiles including the spikes were found t o be reproducible. The temperature rise for these two cases (ap-
Figure 20. Effect of initial pressure on induction period for CHsOWair mixtures; q5 = 0.5, TO= 705 K.
proximately 130 "C)was much higher than that for all of the other cases investigated (approximately 12 "0. The data for these two cases were not included in the correlations. The effects of varying the initial pressure on the reaction are shown in Figure 19, which shows several pressure histones for an initial temperature of 705 K and 4 = 0.5. At this temperature, as the initial pressure increases by a factor of 2, from 350 to 700 Torr, the net pressure rise also doubles and the induction period decreases by a factor of 2 (Figures 20 and 21). Thus, for these parameters, we can write APf = PO and z = Po-'. The maximum rate of pressure rise is more sensitive t o changes in initial pressure and shows a higher order dependence: (dPldt),, Po2 (Figure 22). Although surface-to-volume ratio and diluent effects were not studied parametrically in the present work, these issues are worth mentioning here. It is wellk n o w n that rate of low-temperature hydrocarbon oxidation can be affected greatly by varying the vessel surface-to-volume ratio or by changing the diluent
404 Energy & Fuels, Vol. 9, No. 3, 1995
Aniolek and Wilk 0.10
2201 I
0.04 -.-. CH20.100 C02.10
0.02
100 80'
YO0
'
'
400
"
500
'
'
600
'
"
700
'
Figure 21. Effect of initial pressure on the maximum rate of pressure rise for CHsOWair mixtures; 4 = 0.5, TO= 705 K.
"
400
"
500
"
600
coo
0
1000
Time
initial Pressure (torr)
0' YO0
0.00
"
700
'
1500
2000
800
800
Initial Pressure (torr)
Figure 22. Effect of initial pressure on the net pressure rise for CHaOWair mixtures; 4 = 0.5, TO= 705 K.
concentration. The extent of surface and diluent effects is not constant throughout the reaction. Affected most is the initial stage of the process and the rate parameter affected most is the induction period. This effect is greater a t temperatures below about 625 K, which is below the temperatures of the current study. The effect of decreasing the surface-to-volume ratio or increasing the diluent concentration is t o decrease the heterogeneous termination of active species like HO2 which results in a decrease in induction period. Above about 625 K and especially at higher pressures, heterogeneous termination is less important. Bone and Gardne9O demonstrated that the effect of surface area on the overall rate of oxidation for methanol was much less than that for methane. While changing the surface-to-volume ratio and diluent level will affect r and 8, it should not alter greatly the effect of temperature on the preflame behavior. This is supported by the data in Tables 1and 2, which show comparable values of E1 and E2 for very different systems and reaction conditions. As will be discussed
(8)
Figure 23. Species concentration profiles for CHaOWair; q5 = 0.5, To = 678 K.
in the next section, changing the surface-to-volumeratio and diluent level may affect the onset of autoignition. Species Measurements and Oxidation Mechanism. In an effort to begin to identify some of the stable chemical reaction intermediates and products associated with the preignition oxidation of methanol and reveal some information on the chemical mechanism occurring at these conditions, gas chromatographic analyses were performed on samples extracted from the reaction vessel. Chemical species measurements were made for the case TO= 678 K, PO = 700 Torr, and q5 = 0.5. Samples were acquired at different times during the course of the reaction. Since the sampling procedure removed a significant portion of the reacting mixture, a separate experiment had t o be run for each data point. This made the system reproducibility a very important issue as discussed earlier. The data are presented in Figure 23 in the form of species concentration profiles which show the evolution of the chemical reaction. The actual data points are shown along with the smoothed curves. The chemical species detected and measured included methanol, oxygen, nitrogen, carbon monoxide, carbon dioxide, and formaldehyde. The species profiles for this case indicate that there is a staged sequence in the methanol oxidation process. The methanol is consumed relatively early in the reaction. Concurrent with this is a buildup of formaldehyde, which is the principal reaction intermediate and which peaks at about the time of the maximum rate of fuel consumption. The formaldehyde is subsequently consumed and leads to a build up of carbon monoxide and, to a lesser extent, carbon dioxide. These were the principal reaction products. These results confirm the basic mechanism put forth by Bell and Tipper.21 In this mechanism, the conversion of the fuel begins with hydrogen abstraction by 0 2 producing hydroxymethyl:
+ 0,
-
+ €30,
(R1)
+ HO, - CH,OH + H,O,
(R2)
CH,OH
CH,OH
HO2 continues the primary chain: CH,OH
Energy & Fuels, Vol. 9, No. 3, 1995 406
PrefZame Oxidation Characteristics of Methanol
The hydroxpethyl is then oxidized to formaldehyde:
CH,OH
+ 0,
-
CH,O
+ HO,
(R3)
Formaldehyde has its labile aldehydic hydrogen atom abstracted by HO2 and 0 2 producing formyl:
+ HO, - CHO + H20, CH20 + 0, - CHO + HO,
CH,O
(R4) (R5)
The formyl radical is then oxidized to carbon monoxide:
CHO
+ 0, -.CO + HO,
(R6)
Carbon moxoxide was the most abundant reaction product measured. At the temperatures of these studies there was little conversion of CO to CO2. The C02 produced was most likely through the reaction with HO2:
CO
+ HO, - CO, + OH
(R7)
The highly exothermic reaction of CO with OH is more important at higher temperatures:
CO
+ OH - CO, + H
(R8)
At 700 K the ratio of rate constants kR7/kR8 is approximately 2000. Consequently, the experiments revealed little net heat release with the exception of the two igniting cases. At the conditions of this study the oxidation of methanol proceeds by an intermediate temperature mechanism where the primary chain carrier is HO2. The induction period observed a t a particular initial condition is controlled by the competition between HO2 termination on the surface of the vessel and HO2 reactions in the gas phase t o propagate the chain. At the lower temperature conditions of this study (below about 700 K) the H202 produced in reactions R2 and R4 is probably also heterogeneously terminated on the walls forming HzO and 0 2 . However, as the initial temperature increases, H202 dissociation in the gas phase becomes a source of reactive OH radicals:
H,O,
+ M - OH + OH + M
(R9)
When this step becomes important OH replaces HO2 as
the dominant radical and the system will autoignite. This was seen in the experiments in the two fuel-rich cases at TO= 705 K. In view of the processes involving HO2 and H202, it can be seen that the autoignition temperature can be affected by the surface-to-volume ratio of the reaction vessel and by the diluent concentration. Despite these effects on the onset of ignition, many previous studies have demonstrated that the product distribution and basic mechanism are not greatly influenced by these effects.
Conclusions The low-temperature slow oxidation of methanol was examined experimentally in a constant volume stirred reactor. Methanol displays the characteristic signs of an autocatalytic reaction with degenerate chain branching. In this respect the preflame behavior of methanol is similar to that of non-oxygenated hydrocarbons. As with methane, methanol requires relatively high initial temperatures to initiate measurable reaction. No cool flames or negative temperature coefficient were observed for methanol in this study, although transition from slow oxidation to hot ignition was observed to occur at 705 K for 4 1.0. Global indicators of reaction rate, reactivity, and autoignition tendency were derived from pressure and temperature histories. Overall activation energies obtained from correlations of the data are in general agreement with values obtained for methanol in a number of various other systems. In general, it was found that the overall reaction rate increased with increasing temperature, pressure, and equivalence ratio. Chemical measurements were made using gas chromatography t o examine the reactant, stable intermediate, and product species histories and to gain insight into the oxidation mechanism a t the conditions studied. The results are in basic agreement with an earlier mechanism and indicate that the basic path of the oxidation of methanol near 700 K proceeds via CH30H CHzOH CHzO CHO CO (- C02). Additional studies are currently underway to map out an ignition diagram for methanol, examine the effects of surfaceto-volume ratio and diluent concentration on the autoignition temperature, and perform chemical kinetic modeling of the preflame processes.
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EF9401642
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