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Oxides of nitrogen formed in high-intensity methanol combustion
O X I D E S O F N I T R O G E N F O R M E D IN H I G H - I N T E N S I T Y M E T H A N O L COMBUSTION S. SINGH,* W. GROSSHANDLER, P. C. MALTE, AND R. W. CRAIN, JR.
Thermal Energy Laboratory, Department of Mechanical Engineering, Washington State University, Pullman, Washington 99164
The oxidation of methanol in a jet-stirred reactor has been studied to characterize the mechanisms of methanol combustion and nitric oxide formation in a high-intensity, turbulent environment. F u e l / a i r equivalence ratios of 0.7 to 1.4 were investigated at a constant temperature of 1870 K and mean reactor residence times of ca. 2.5 msec. Total NO x was measured along with major combustion products, CO, CO 2, H 2 0 , 02, CH 4, and C e l l 2. Emission spectroscopy was used to determine oxygen atom concentration as well as relative levels of electronically excited radical species, OH*, CH*, and Cz*. Comparative measurements were repeated with methane as the fuel under similar reactor conditions. Measured NO~ for methanol was 8.5 ppm maximum at equivalence ratio 1.10. The ratio of methanol-NO~ to methane-NO~ varied between one-half for fuel-lean combustion to one-third for equivalence ratios at or above stoichiometric. Molecular O 2, atomic-O and calculated O H were essentially the same for both fuels. At fuel-rich conditions, methanol demonstrated higher carbon monoxide. Concentrations o f C H 4 and C z H~ were markedly higher with methane fuel, as were CH* and C2", while OH* was slightly higher. Evidence indicated the persistence of the C - - O b o n d in the methanol molecule during oxidation. The NOx, O-atom, and O~ concentrations indicated that the extended Zeldovich mechanism was insufficient to explain the difference in observed NO x for methanol and methane. It was hypothesized that hydrocarbon fragments played an important role in NO~ formation in the stoichiometric and rich mixtures; it was concluded that low methanol-NO x was due to low hydrocarbon levels.
Introduction Despite the abundance of practical knowledge about methanol as an alternate fuel for internal-combustion engines and other industrial applications, little experimental data are available on its combustion characteristics at temperatures of 1600 to 2500 K, the main temperature range of interest in combustion systems. Most of the combustion and pollutant emission work on methanol has been carried out in spark-ignition engines, either pure or in blends with gasoline. 1 Methanol oxidation at high temperatures has also been studied in stationary flames, 2 constant-pressure ducts, "~and shock tubes. 4'5 Bowman 5 extended the shock-tube investigations on methanol oxidation up to 2180 K for fuel-air equiva-
lence ratios between 0.375 and 6.0, and proposed a 28-step reaction mechanism. However, these onedimensional studies find only partial application to high-intensity, turbulent combustion processes characteristic of gas-turbine combustors and wallfired burners because of the forced mixing between fresh reactants and the burning gas and of possible turbulence/chemistry interactions. With these considerations in mind, a careful laboratory examination of methanol combustion, and attendant NO, formation, in a turbulent, recirculative, premixed flame was carried out.
Experimental Apparatus The experiments were performed in a jet-stirred reactor, shown in Fig. 1. Cast of zirconia, the reactor was a spherical segment of a Longwell reactor ~ as modified by Pratt, v with an internal volume of 50
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cm a. High intensity mixing and combustion were obtained by injecting jets of prevaporized, premixed reactants through a group of small sonic orifices drilled into a stainless steel feed tube. Methanol and methane were burned in the reactor. The operating conditions investigated and the measurements made at each condition are shown in Tables I and II, respectively. The nominal temperature was maintained constant at 1870 K over the range of equivalence ratios by diluting the fuel/air mixture with differing amounts of nitrogen. The thermocouple was Pt-Rh, 0.2 mm in diameter, coated with Y203 and BeO. The measured temperature was corrected for radiation heat losses from the thermocouple. For all gas analyses, a partially water-cooled 3 mm dia. quartz probe was used to remove a continuous stream of gas sample from the reactor. The sample flowed sonically through a 0.2 mm tip orifice; probe pressure was about 18 kPa (abs.). Standard infrared process analyzers, Beckman models 864 and 315B, were used for carbon dioxide and carbon monoxide detection, respectively. A Beckman model 951 chemiluminescent analyzer was used in measurements of nitric oxide and total oxides of nitrogen ([NOel = [NO] + [NOd]). Varian gas chromatographs, models 90-P3 and 1200, equipped with a thermal conductivity and flame ionization detector, respectively, were used to monitor the
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TABLE I Experimental conditions
Fuel/air equivalence ratio, 9 Temperature Pressure Residence time Reactor loading, ria/V
CHaOH (vapor)
CH 4
0.76-1.33 1870 K 93 kPa (abs.) 2.1-3.1 msec 77 kg/mZ-sec
0.70-1.40 1870 K* 93 kPa (abs.) 2.5-3.5 msec 64 kg/m3-sec
"For ~ = 0.70, the maximum temperature which could be attained was 1780 K. TABLE II Experimental measurements Quantity Temperature Species mole fraction: CO, C02 H20, Oz CH 4, CH3OH, C2H2 NO, NO~ O Emission intensity: O H ' , CH*, C2 ~
Technique Pt-Rh thermocouple Infrared process analyzer Gas chromatography, thermal conductivity detector Gas chromatography, flame ionization detector Chemiluminescence process analyzer Emission spectroscopy, path-integrated, in situ Emission spectroscopy, path-integrated, in situ
HIGH-INTENSITY METHANOL COMBUSTION remaining stable species. A quarter-meter Spex monochromator fitted with a 1P28A photomultiplier tube was used to detect radiative emission between 300 and 600 nm. The nominal oxygen atom concentration was determined by monitoring the path-integrated chemiluminescence at 380 nm of CO2" (~B2) formed during the overall reaction of CO with O. The change in intensity caused by adding small amounts of carbon monoxide to the reactor was directly related to the relative oxygen atom levels, s'9Absolute oxygen mole fraction was determined by calibrating the reactor/optical system at a reference condition for which O and OH levels were known from previous measurements.]~ There, absolute OH levels were found by absorption spectroscopy, which, with indicated partial equilibrium for the reaction OH + OH ~ H 2 0 + O, gave absolute O-atom mole fractions. A complete description of the apparatus and techniques is given in the dissertation by Singh. H
Reactor Characterization
General Methanol and methane were burned under essentially equivalent experimental conditions in the reactor. By holding the total mass flow rate through the reactor approximately constant for the two fuels (it was slightly greater for methanol because of the need for increased N 2 dilution), the gross fluid mechanical field of the reactor, the mean residence time of the reactor, and the characteristic diffusion time did not vary appreciably with fuel type. The gross thermal field was held constant by the technique of diluting the incoming reactants with varying amounts of N 2. Furthermore, the characteristic chemical kinetic times of CH 4 and CH 3OH were similar, based upon one-dimensional flame speed measurements done by other experimenters. ' z.a At the nominal residence time of 2.5 msec the reactor was intensely backmixed and chemically-rate dominated; however, it deviated somewhat from the desired homogeneous perfectly stirred behavior. A finite mixing influence was felt in the feed jet regions, as shown below. Nonetheless, given the repeatability of the reactor conditions, the similar gross thermal and fluid mechanical fields, and the similar characteristic times for mean-residence, diffusion, and chemical kinetics, it was possible to accurately compare the relative chemical processes for the two fuels.
spatial inhomogeneities. Figure 2 shows typical data, here taken for methanol combustion at a fuel/air equivalence ratio (qb) of one, plotted against the distance across the reactor. The line of the measurements, taken through the sampling port in the upper part of the reactor, passed through the central feed jet and the larger surrounding regions of intense recirculation; the line was in a plane coincident with the optical path shown in Fig. 1. The temperature, here uncorrected for radiation loss, made a dip of about 250 K at the vertical centerline of the reactor. This location corresponds to the incoming central feed jet, where the combustion was less complete. The surrounding recirculation regions were approximately homogeneous and more completely burned. Figure 2 shows a similar temperature scan for methane (dashed line). The spatial measurements of CO in methanol combustion, also shown in Fig. 2, exhibited an increase on the reactor centerline to a value 35% above the mean value. For the CO2, there was a 5% centerline dip below the spatially averaged value. The maximum temperature, located 15 mm within the reactor in the recirculation zone, was 1870 K when corrected for thermocouple radiation loss. This is treated as the nominal reactor temperature. The uncertainty in temperature due to uncertainties in the radiation correction was +30 K. The 1870 K
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temperature was 30 K below the adiabatic equilibrium temperature for methanol and 55 K below that for methane combustion. The flow field found in the reactor has been modeled by Wormeck ~2 for adiabatic combustion of methane at a throughput rate of 38 kg/m3-sec. Traversing across the flow field from the recirculation region to the jet centerline, along a path similar to that taken in Fig. 2, Wormeck's results at qb = 1.0 showed the following: 1) 150 K drop in temperature, 2) 40% increase in CO, 3) 40% increase in 02, 4) 100% increase in O-atom, and 5) 25% increase in OH on the jet centerline relative to the recirculation region. The temperature and CO predictions are in semi-quantitative agreement with Fig. 2. From these results, it is recognized that due to less than instantaneous mixing, the reactor feed jets exhibited a somewhat greater degree of combustion nonequilibrium and hydrocarbon pyrolysis, compared to the reactor whole; whereas, the large surrounding zones of intense recirculation exhibited a closer approach to equilibrium. Active species and pyrolysis fragments are, therefore, expected to be of somewhat higher concentration in the feed jets. The flow field is best characterized as a high-intensity recirculative flame, dominated by chemical kinetics, but influenced by finite mixing.
Experimental Results
Primary Combustion Products The primary combustion products are plotted in Fig. 3 over the range 0.7 < ~b < 1,4. A normalization factor was used for all concentration measurements to account for different nitrogen dilution rates, the results corresponding to the nitrogen present in air. At stoichiometric conditions, methanol required a 35% increase in nitrogen flow rate, relative to normal air; whereas methane required a 25% increase to maintain the temperature of 1870 K. The sample probe was located 7.5 mm inside the reactor, at the same elevation used for the preceding measurements. All gas samples were collected at this point (within the nearly homogeneous recirculation zone). At this location, the methanol produced 20 to 25% more water and carbon dioxide than did methane (when corrected for the higher nitrogen dilution in methanol combustion). This was primarily due to the 25% less air needed for complete oxidation of methanol, since the fuel contains oxygen. Thus, for a given equivalence ratio, although the carb o n / h y d r o g e n / o x y g e n ratio is the same for both methane and methanol, the amount of nitrogen associated with the required air is less in methanol oxidation. Carbon monoxide levels were 30% higher in methanol combustion for fuel-rich conditions, but approached the levels in methane combustion
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for qb < 1.0. There was essentially no difference in the measured 02 mole fractions for the two fuels. Unburned hydrocarbons appeared above equivalence ratios of 1.10. Acetylene was the most dominant hydrocarbon with a maximum value of 1650 ppm at qb = 1.40 for methane combustion. The acetylene levels in methanol combustion were much less, as shown in Fig. 3; a maximum of 40 ppm was recorded for ~b = 1.33, more than 15 times lower than the corresponding value from methane combustion. A methane mole fraction of 300 ppm was measured at d~ = 1.40 for methane combustion, while no methane was detected for methanol combustion. Neither methanol nor formaldehyde was detected in either system.
Radical and Excited Species Path-integrated oxygen-atom mole fractions, shown in Fig. 4, were essentially the same in methanol and methane combustion. The estimated experimental uncertainty in the relative O-atom mole fraction was +30%. All optical measurements were taken along the optical path shown in Fig. 1. From the oxygen-atom and water vapor measurements, the nominal hydroxyl radical could be calculated under conditions where partial equilibrium prevailed for the reaction OH + OH ~ H 2 0 + O. ~~The calculated OH showed no significant difference for methanol and methane combustion for 0.76 < ~b < 1.10. The O and OH results are not
HIGH-INTENSITY M E T H A N O L COMBUSTION
methanol systems could then indicate much lower levels of CH, C2, or both. No C2 ~ was observed in methanol combustion. Suggested pathways indicate that C2 ~ may be formed from C2 H~ hydrocarbon fragments through a carbon suboxide mechanism ]7 (C2 O + C ---) C~* + CO), or from single carbon fragments, CH~, such as CH + CH ~ Ca* + H a. The critical conclusion which can be made from these suggested pathways is that the methanol system must have contained lower concentrations of ground-state hydrocarbons, such as CHx and Ca H , , relative to the methane system in order to be consistent with the emission results. This is further supported by the gas sampling measurements of CH4 and C~ H e mentioned earlier.
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Figure 5 is a plot of nitric oxide and total oxides of nitrogen (NOt) measured in both the methane and methanol systems as a function of equivalence ratio. (As above, the sample probe was located 7.5 mm inside the reactor, in the recirculation region.) Peak NO x measurements fell on the slightly fuel-rich side for both fuels, as observed in previous stirred reactor measurements. ~~ The NO x levels in methane combustion were more than three times the NO, levels in methanol for equivalence ratios between 0.90 and 1.20. For ~b = 1.10, the NO x levels were 27.4 and 8.6 ppm for methane and methanol, respec-
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unexpected since the same energy releasing hydrogen/oxygen kinetic scheme is operational for CH4 and CH3OH. 5 The relative emission bands of OH* (310 nm), CH ~ (430 nm and 390 nm), and Ca ~ (Swan bands at 438-563 nm) were measured and compared for the two fuels. The OH* emission, which peaked at about ~b = 1.10, was about 50% higher for methane combustion than for methanol when corrected for nitrogen dilution. Emission due to CH* from methanol was an order of magnitude below that from methane, as shown in Fig. 4. The C2* results were most striking. No Cz ~ was detected in the methanol system; but for methane combustion, the total integrated C~* intensity was four times that of CH ~ The kinetic mechanisms which lead to the electronically excited states OH*, CH*, and Ca*, have been discussed but not fully established in the literature. TM~4'~ ~.t6.~7 Nevertheless, after studying the literature and the relative measurements in the present experiments, a few general remarks can be made. The literature suggests that OH* may be formed within the H2/O2 system. ~3Since the OH ~ is higher in methane combustion, this could possibly indicate that additional reactions involving hydrocarbon fragments, for example CH + O 5 --~ OH* + CO, ~3 lead to the higher chemiluminescence in methane combustion. CH* has been speculated to form in reactions of CH or C 2, such as Cz + O H --* CH* + CO, ]a and perhaps CH + OH + H --* CH* + Ha O. ~r The relative CH* levels in the methane and
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tively. As mentioned earlier, the reported mole fractions have been corrected for the effect of nitrogen dilution. However, no attempt was made to account for the first order dependence of thermal NO on N 2 concentration. If this were done, since higher nitrogen levels were present in the methanol system, the effect would be to enhance the difference in NO mole fraction between the two fuels. Relatively large NOx/NO ratios were observed in both fuels, particularly under fuel-lean conditions. Reactor NOz could have been real or a probe effect. Kramlich and Malte 19 have shown that the discrimination of NO from NOz may be unreliable for fuel-lean conditions due to reactions within the probe. However, the total NO~ measurement is undistorted by these probe effects. (Of course, care must be taken to prevent NO 2 loss to water condensate in the same system.) A traverse across the reactor for stoichiometric combustion indicated little spatial variation of NOx, as shown in Fig. 2. Total oxides of nitrogen showed a gradual decrease toward the reactor far wall which was due to the cooling effect of the sampling probe as it was inserted farther into the reactor. For qualitative comparison, NOz is also plotted in Fig. 2. The measured NO~ levels rose noticeably at the reactor center, which corresponded to the centerline of one of the incoming jets. The temperature at this point was 250 K below that of the surrounding gas, and computer predictions lz have indicated a maximum HO 2concentrationat this location. Under these conditions, the sudden rise in measured NO2 mole fraction could have reflected combustor NO2 due to NO + HO~--* NO~ + OH. 19
Methanol Oxidation The progressive oxidation of methanol is shown schematically in Fig. 6, along with a possible side reaction for the formation of nitric oxide. (The H J O z system and Zeldovich N/O system have been deleted since they are similar for both methane and methanol.) There are two primary paths for the formation of CO, one through CH z and the other through CH20. Methane is also oxidized along these two paths as indicated. Although all of the reactions are reversible, the initial reactions of the parent fuel molecules must proceed in the direction as shown. Notice that CH3 OH can proceed in two directions, to CH 3or CHz OH, while CH 4must proceed through the methyl radical. Although the C - - H bond is stronger than the C--O bond in methanol, there are three hydrogen atoms which can be abstracted to form CH~OH while only one bond breakage will lead to CH 3. A third reaction, through CH30, is likely to quickly proceed to CH~O. It appears from the experimental results that the preferred path to CO for methanol is through CHeOH rather than through CHa. If this were not
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FIG. 6. General outline of methanol and methane oxidation to carbon monoxide, with a side reaction to nitric oxide. so, then the hydrocarbon levels and chemiluminescence in methanol and methane combustion would not be expected to differ as much as the experiments indicated. This conclusion also supports the importance of the side reaction for NO formation through CH hydrocarbon fragments, and supports the much lower nitric oxide levels measured in the methanol system.
Discussion A primary result of this work is the significantly greater NOx concentration in methane combustion relative to methanol combustion at similar conditions. The pertinent question is whether this is a general chemical kinetic result, or simply an artifact of the present flame; and if it is a general result, what is the origin of the higher methane-NOx ? It is first important to recognize that the above probe and optical measurements were heavily replicated and repeatable from "day-to-day." The results for O-atom, 02 , and OH, which can be treated as nominal values indicative of the upper part of the reactor, were independent of fuel type. Also, the gross thermal and fluid mechanical fields, and mean residence times, were similar for the two fuels. A major difference between the two fuels was the greater hydrocarbon concentrations noted for methane combustion. The extended Zeldovich mechanism for NO formation is N2 + O--~ NO + N N + Oz---~NO + O N + OH--* NO + H. This mechanism, in concert with the time-steady
695
HIGH-INTENSITY METHANOL COMBUSTION field in the upper part of the reactor, should have produced similar NO levels for the two fuels, especially at near-stoichiometricconditions. The fact that this agreement did not occur means that the factor two-to-three higher N O measured for methane compared to methanol is not explained by this route. This conclusion continues to hold if the nitrous oxide mechanism is included, TM and it is not substantially changed if experimental uncertainties are considered. The magnitude of Zeldovich NO produced by the time-steady field was estimated at qb 0.8 by using the PSR model;* and was found to be about 10 ppm, in essential agreement with the experimental results for fuel-lean conditions and for methanol. (It shouldbe noted that we are assuming the NO~ measurements to be synonymous with the original NO formed in the reactor. The measured NO2 is treated as an artifact, resulting from original NO oxidation in the reactor or in the sample probe train.) The factor three higher NO in methane combustion is likely due to either, 1) turbulent fluctuations acting to nonlinearly reinforce the Zeldovich mechanism, or 2) an additional kinetic pathway to NO formation. It is well established that the Zeldovich mechanism is highly sensitive to temperature and stoichiometry, and therefore, turbulent fluctuations in these quantities could have reinforced the Zeldovich NO formation rate. z~However, given the experimental setup of similar gross thermal and fluid mechanical fields, and similar characteristic times, the effect should have been approximately similar for the two fuels. Furthermore, it can be argued that large fluctuations in temperature (to levels significantly above the measured temperature) and stoichiometry (to high oxidant levels), sufficient to cause a marked increase in the Zeldovich NO, should have been exceptional occurrences in the reactor. The only way in which the local stoichiometry could have varied would have been through selective diffusion of either fuel or oxidizer out of a turbulent eddy. This could not have occurred to any significant degree because of the short residence time available within the reactor. Likewise, the only way in which the final temperature of an eddy could have significantly increased would have been through selective diffusion of inert diluent out of the eddy, since the mean reactor temperature was already close to the adiabatic equilibrium temperature (see section "Reactor Characterization" ). Next, note the flat NO~ trace in Fig. 2; the NOx did not reflect the spatial gradients exhibited by CO and temperature, but was fairly homogeneous across the reactor. A flat NO, profile was measured for both methanol and methane at 9 = 1; this trend has also been noted in our previous research for
*T = 1870 K; O-atom taken from Fig. 4.
~ 1. The logical explanation for this behavior is that the chemical kinetic time for NO formation in the region of the measurements was much longer than the diffusion time. That is, any NO formation in the region of the measurements occurred in a manner characteristic of a perfectly stirred reactor: long kinetic time compared to diffusion time. The measured NO~ homogeneity implies that the local rate of turbulent diffusion of NO must have balanced the local chemical formation rate of NO. This follows from the species conservation equation, treated as a field equation, time-averaged at a point, and neglecting molecular diffusion. The local rate of turbulent diffusion is
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where YNo is the mass fraction of NO and %) is the characteristic diffusion time. The local chemical formation rate is W-~-'No= YNo/~'k, where r k is the characteristic chemical kinetic time. With the turbulent velocity estimated as one-quarter the initial feed jet velocity . ~1(somc), . and with the turbulent scale estimated as the initial feed jet size (1 mm), the characteristic diffusion time is of order 10 -~ sec. The characteristic kinetic time is of order 10 -3 sec, and thus, the fluctuation of NO is small compared to the time-mean NO. With respect to this analysis it is interesting to note that the NO could have essentially formed elsewhere in the reactor, that is, within the lower region, and then merely undergone efficient diffusion in the upper portion of the reactor. A flat NO, profile would have resulted. In the lower part of the reactor, the effect of hydrocarbon pyrolysis, which is discussed below, could have influenced the NO formation. We now turn to the second explanation listed above for the higher NOx in methane versus methanol combustion: an additional kinetic pathway. It is suggested here, in the light of the above comments on fluctuation-NO, and given the experimental setup of similar reactor conditions for the two fuels, that the substantially higher NOx measured in methane combustion was due primarily to the difference in hydrocarbon pyrolysis between the two fuels, and the subsequent hydrocarbon fields. As verified above, methane gave significantly higher hydrocarbon levels, both stable species and radical fragments. An obvious kinetic pathway is the so-called "prompt NO" mechanism,~'~ wherein hydrocarbon fragments attack bimolecular nitrogen, producing atomic-nitrogen, cyanides, and amines, which subsequently oxidize to nitric oxide. Possible initial reactions are CH + N 2 ~ HCN + N and Cz + N2 CN + CN. The CN and HCN may then react through NH i intermediates to form NO or Nz. ~4,z~
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A schematic of this mechanism is shown in Fig. 6. This deduction, based on the experiments, that differences in the hydrocarbon fields promoted higher NO formation in methane combustion compared to methanol combustion, is a general effect. Of course, in other combustion systems the effect may be mitigated by specialized mixing patterns, and may be overtaken by Zeldovich NO at high temperatures and long residence times, i.e. thermal NO. The present reactor, operated at moderate temperature and at short residence time, severely curtailed Zeldovich NO formation (as shown above). Also, for the present reactor it is not precisely known if the hydrocarbon influence on NO formation was distributed uniformly throughout the reactor, or whether the influence was concentrated somewhat in the lower portion of the reactor. One suspects that the effect may have been stronger in the lower portion of the reactor due to enhanced pyrolysis in that region.
Conclusions The experimental evidence indicates that reactions in addition to the Zeldovich mechanism are essential to explain the greater formation of nitric oxide in high-intensity methane relative to methanol combustion. The low levels of nitric oxide in methanol combustion, when compared to the higher levels of nitric oxide in methane combustion, suggest that certain intermediate species were at significantly different concentration levels in the two fuel systems, Specifically, the experiments indicated that O and OH were at similar levels for the two fuels, whereas the hydrocarbon fragments were significantly greater in methane combustion, and thus, may have provided the additional pathways to NO formation via the so-called prompt NO mechanism. Measured levels of acetylene and methane were an order of magnitude larger in methane combustion for equivalence ratios greater than 1.10. The chemiluminescent intensity of CH* and C2 ~ were also much greater in the methane system. Although it is difficult to directly relate these emission measurements to ground-state concentrations of CH and C 2, most suggested pathways to the excitation of CH and C~ involve hydrocarbon fragments such as CH and C~H. Therefore, lower levels of CH ~ and C2 ~ in methanol combustion when compared to methane combustion, can be interpreted to indicate lower levels of hydrocarbon fragments. The initiation steps in methanol oxidation involve the rupture of either the C - - H bond or the C - - O bond. The former reactions lead directly to formaldehyde through the CH2OH radical, while the latter creates the methyl radical. Under the present experimental conditions, the measurements support
reactions through CH2 OH, in which the C - - O bond remains intact. In summary, high-intensity turbulent combustion of methanol, compared to methane, produces significantly lower NO,. This effect appears to be due to lower hydrocarbon fragment concentrations formed during methanol oxidation. Acknowledgments
The authors wish to acknowledge the support of the National Science Foundation, Grant No. 7320136-A02, and the Thermal Energy Laboratory at Washington State University.
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HIGH-INTENSITY METHANOL COMBUSTION
o f Chemical Physics 43, 3680 (1965). 15. PORTER, R. P., CLARK, A. E., KASKAN, W. E. ANt) BROWNE, W. E.: Eleventh S y m p o s i u m (International) on Combustion, p. 907, The C o m b u s t i o n Institute, 1967. 16. MALTE, P. C., ANDSINCH, S.: " ' C h e m i l u m i n e s c e n t Intensities for H y d r o c a r b o n - A i r C o m b u s t i o n in a Jet-Stirred L o n g w e l l Reactor," Paper no. 14, C a n a d i a n S e c t i o n / T h e C o m b u s t i o n Institute, May 1977. 17. PELTERS, J., LAMBERT, J. F., HERTOGHE, P., AND VAN T1GGELEN, A.: Thirteenth S y m p o s i u m (International) on Combustion, p. 321, T h e Comb u s t i o n Institute, 1973. 18. ENGLEMAN, V. S., BARTOK, W., LONGWELL, J. P., AND EDELMAN, R. B.: Fourteenth S y m p o s i u m
697
(International) on Combustion, p. 755, T h e C o m b u s t i o n Institute, 1973. 19. KRAMLICH, J. C. AND MALTE, P. C.: Combustion Science and Technology 18, 91 (1978). 20. GOULmN, F. : Combustion Science and Technology 9, 17, (1974). 21. MALTE, P. C., AND PRATT, D. T.: Combustion Science and Technology 9, 221, (1974). 22. FENIMOBE, C. P.: Thirteenth S y m p o s i u m (International) on Combustion, p. 373, T h e C o m b u s t i o n Institute, 1971. 23. IVERACH, D., BASDEN, K. S., AND KIROV, N. Y.: Fourteenth Symposium (International) o f Combustion, p. 767, The C o m b u s t i o n Institute, 1973. 24. HAYNES, B. S.: "'The F o r m a t i o n a n d Behavior of Nitrogen Species in F u e l Rich H y d r o c a r b o n
COMMENTS M. Destriau, Universitd de Bordeaux I, France. Which are the reactions p r o d u c i n g excited C H and C 2 in y o u r s y s t e m s ? May partly thermal excitation
Occur?
Authors" Reply. The reaction m e c h a n i s m s responsible for the f o r m a t i o n of electronically excited C H and C 2 have b e e n o p e n to speculation due to lack of direct a n d quantitative g r o u n d state concentration m e a s u r e m e n t s of C H and C 2 and their precursors in the same environment. Bleekrode and N i e u w p o o r t 1 have experimentally established the excitation of C H a n d C 2 as p r e d o m i n a n t l y chemiluminescent in nature. At least an order of m a g n i t u d e higher relative C H e m i s s i o n intensity from m e t h a n e c o m b u s t i o n c o m p a r e d to m e t h a n o l has been observed in the p r e s e n t study. Definite mechanistic c o n c l u s i o n s c a n n o t be made as to h o w C H * arises. However, if it is due to the reactions C2H + O - - ) C H ~ + CO C2H+O 2--)CH 9 +C 2
T h e C~* e m i s s i o n m e a s u r e d in this study from rich m e t h a n e c o m b u s t i o n w a s five times as s t r o n g as C H ~ emission. Since C~ e m i s s i o n w a s not seen in m e t h a n o l c o m b u s t i o n b u t C H ~ was, the kinetic m e c h a n i s m for C ~ formation m u s t be able to a c c o u n t for this fact. I f C ~ arises from the reactions i n v o l v i n g t w o h y d r o c a r b o n fragments (e.g., C H + C H ---) C~ + Ha) or from a C~H 2 molecule (C2H ~ + O ~ C 2 0 + Ha; C 2 0 + C---~ C~ + CO), then lower hydrocarb o n f r a g m e n t s m u s t exist in m e t h a n o l c o m b u s t i o n . T h i s is consistent w i t h the lower stable h y d r o c a r b o n levels m e a s u r e d in methanol c o m b u s t i o n c o m p a r e d to m e t h a n e .
REFERENCES
1. BLEEKRODE, R. ANt) NIEUWPOORT, W. C.: lournal o f Chemical Physics, 43, 3680 (1965) 2. PELTERS, J., LAMBERT, J. F., HERTOGHE, P., AND VAN TIGCELEN, A.: 13th S y m p o s i u m (International) on Combustion, p. 321, The C o m b u s t i o n Institute, 1973.
C 2 + O H - - ) CH* + CO then the l o w e r C H ~ e m i s s i o n from m e t h a n o l w o u l d indicate lower levels of C 2 h y d r o c a r b o n fragments, since [O], [ O H ] , and [02] levels were a b o u t the same in b o t h the systems. It has also been suggested 2 that C H ~ m a y arise due to the reaction C H + O H + H---) CH* + H 2 0 A p r e d o m i n a n c e of this reaction w o u l d m e a n that the lower C H ~ e m i s s i o n f o u n d in m e t h a n o l c o m b u s tion indicates l o w e r g r o u n d state C H concentration.
D. R. Crosley, Ballistic Research Laboratory, USA. You have more spectroscopic d a t a - - t h a t on C 2 a n d C H - - t h a n you appear to make use of in c o m p a r i s o n w i t h the model. T h o u g h C 2 w a s not i n c l u d e d in y o u r reaction network, C H was. Are the c o n c e n t r a t i o n s of these species u s e f u l to ascertain the m e c h a n i s m s ? You replied that you c o u l d n ' t incorporate C a and C H because y o u d i d n ' t k n o w rate c o n s t a n t s for the excited states. But m a n y of the transient s p e c i e s - - C H , C2, O H , C N and N H - -
698
NO~, SOx
w h i c h were in your network can be probed in the g r o u n d states by laser excited fluorescence. H o w well do these concentrations confirm the model? Also, can you make use of relative concentrations as a function of equivalence ratio, rather than needing absolute values which are more difficult to measure ?
Authors" Reply. G r o u n d state concentration meas u r e m e n t s in our reactor of the radical species CH, C 2, C H and N H would be extremely u s e f u l in elucidating the excitation pathways, as well as the kinetic m e c h a n i s m for the formation a n d destruction of the pollutant species. E v e n t h o u g h accurate rate data are unavailable for most C 2 radical reactions, absolute concentration m e a s u r e m e n t s of Cz w o u l d permit evaluation of various e x p a n d e d reaction s c h e m e s and estimation of s o m e rate coefficients. Relative m e a s u r e m e n t s of g r o u n d state transient species could be u s e d to support the authors' explanation of the role of hydrocarbon fragments in the formation of nitric oxide. T h e s e could also be u s e d to ascertain the relative importance of reactions for f o r m i n g excited species, s u c h as those d i s c u s s e d in the reply to Prof. Destriau.
D. Aronowitz, University of Southern Calif., USA. 1) Because in methane c o m b u s t i o n there are more C a n d C 2 fragments than in m e t h a n o l c o m b u s t i o n a n d the C a n d C~ fragments could be i m p o r t a n t in forming NO~, C H a O H c o m b u s t i o n s h o u l d i n d e e d give less NO~ than C H 4 a n d this m a y explain the factor of 3 difference. 2) T h e paper by Aronowitz et al. in this s y m p o s i u m a n d one to be p u b l i s h e d shortly elsewhere by Westbrook a n d Dryer on a detailed C H 3 O H m e c h a n i s m , s h o u l d provide you with a good e n o u g h m e c h a n i s m to test your experimental data. 3) A l t h o u g h your results s e e m to support a C H 3 O H ---> CH~OH ~ CO route, it is important to realize that at 1800 K, the initiation reaction C H 3 O H + M --~ C H 3 + O H + M will become important, b u t its effect will be m a s k e d by the very rapid CH~ oxidation (at 1800 K, C H 3 r e c o m b i n a t i o n is considerably slower).
Authors'Reply. The authors are in agreement w i t h the point concerning the significant role played b y the C H a n d C 2 radicals in the formation of NO, as detailed in the paper. T h e more detailed m e c h a n i s m by Westbrook a n d Dryer will be a welcome addition, a n d its incorporation into our analysis will be investigated. T h e global m e c h a n i s m s u g g e s t e d by Aronowitz et al. in this s y m p o s i u m may or may not be directly applicable to our experiments, since the present work was performed at 1870 K and no u n b u r n e d m e t h a n o l
was detectable in the reactor. T h e flow reactor study was performed at lower temperatures a n d the global m e c h a n i s m was derived from the disappearance rate of methanol. T h e importance of the CH~OH radical relative to the methyl radical d u r i n g the initial stage of c o m b u s t i o n may, in fact, be explained from a statistical argument. Although the C - - H b o n d is stronger than the C---O b o n d in the parent fuel, there are three h y d r o g e n atoms w h i c h can be abstracted to form CH2OH, while only one bond breakage will lead to C H 3. T h e relative importance of these two initiation reactions m a y alter with temperature. However, all of the reported data in the present study at 1870 K support a m e c h a n i s m favoring the CH3OH ~ CH2OH (or C H 3 0 ) ---> CO route.
G. S. Samuelsen, University of California, lrvine, USA. E v e n though the overall time-mean temperature is maintained constant with the two fuels, the required diluent to achieve that condition will affect loeal transport properties. As a result, it is possible that local temperature fluctuations (which affect, if not dominate, the local NO x time-mean production rate in premixed systems) may explain some of the difference in total production rate observed. It might be possible to test this by (1) u s i n g different species for the diluent, a n d (2) assessing w h e t h e r the same difference in NO x p r o d u c t i o n rate is observed at lower time-mean temperatures.
C. T. Bowman, Stanford University, USA. Your m e a s u r e m e n t s s h o w significant spatial non-uniformities in temperature a n d in CO concentration in your reactor, i m p l y i n g that the fuel oxidation reactions are occurring u n d e r u n m i x e d conditions. What is the justification for the use of a perfectlystirred reactor c o m p u t e r code to model a n d interpret your experimental results? Authors" Reply. As s h o w n in the paper via measurements, and by Wormeck (Ref. 12 in paper) via computer modeling, the feed jets relative to their recirculative s u r r o u n d i n g s exhibited a greater degree of chemical n o n - e q u i l i b r i u m a n d hydrocarbon pyrolysis. T h e NO x, however, at least in the upper part of the reactor for d~ = 1, exhibited homogeneity, indicating a long chemical kinetic time compared to diffusion time. T h e PSR code, as a first order model, had s o m e validity for ascertaining the feasibility of various NO formation m e c h a n i s m s in our reactor.
HIGH-INTENSITY M E T H A N O L COMBUSTION
F. C. Gouldin, Corneli University, U.S.A. You have attributed the differences in NO~ emission for methane vs. methanol firing to prompt NO type reactions, even when the reactor is operating under lean conditions. Since emission levels for the two fuels vary by a factor of two or more, one is tempted to conclude further that most of the NO in either case is prompt NO related. Such a conclusion would be controversial and leads one to question your conclusions about prompt NO. Is it possible to attribute some of the differences in NO~ levels to unmixedness in your reactor? In order to maintain constant ~b, p, T and staytime in the reactor, when changing fuels, nitrogen must be added as a diluent. Some change, perhaps small, in NO x emissions due to the diluent is expected. Did you evaluate this effect quantitatively?
Authors" Reply. Doctor Gouldin's questions regarding the effects of unmixedness and Na dilution have been addressed in the paper. It was noted that the gross temperature and velocity fields for the two fuels were the same. Gaseous methanol has been reported to have a slightly higher flame speed than methane (Ref. 3 in paper); however, within experimental scatter it may be argued that the two fuels have essentially equal flame speeds (personal communication, C. T. Bowman). Therefore, in the experiments the characteristic times, i.e., stay time, kinetic time, and diffusion time, were essentially equal for the two fuels. The important difference between the two fuels was their respective hydrocarbon fields. Ground and excited state hydrocarbon concentrations were significantly higher for methane than for methanol. Since hydrocarbon pyrolysis reactions tend to be fast, unmixedness would be expected to have its greatest impact here. Indeed, Wormeck's model (Ref. 12 in paper) shows greatest spatial inhomogenieties for the hydrocarbon species, particularly in the early feed jet region. The unmix-
699
edness, however, doesn't change the basic conclusion of this study: hydrocarbon concentrations were greater for methane than for methanol, leading to greater NO formation. There are additional indications regarding the effect of unmixedness on the hydrocarbon field and NO x. We have consistently measured a smooth NOx profile in the upper region of the reactor (see Figure 2 in paper). This is somewhat unexpected, given the nonuniform temperature and (other) composition profiles, both measured and predicted. One explanation given in the paper for the smooth NO~ profile was NO~ formation early in the reactor (feed jets); thereafter the NO x simply "sloshed" around in the bulk of the reactor. Also, attempts to measure absolute CH by absorption spectroscopy in the upper reactor region gave CH ~ 1 ppm, perhaps too low of concentration to explain the observed (prompt) NO x. Finally, other measurements in the jet-stirred reactor (at "equal" conditions) have also shown a striking fuel dependence on observed NOx. Acetylene gave maximum NOx, being a factor ten greater than NO x for H a and H J C O fuels. Methane and propane gave intermediate NO x. The respective intensities of the hydrocarbon emission bands followed the same trend with fuel type. To maintain the gas temperature within the reactor at 1870 K the fuel/air mixture was diluted with differing amounts of nitrogen over the range of equivalence ratios. At stoichiometric conditions, methanol required a 35% increase in N 2 flow rate, relative to normal air, whereas methane required a 25% increase. Since NO~ production was directly proportional to the N 2 concentration, the higher dilution of the methanol would, in fact, artificially increase the NO~ measured. This effect was not corrected for; but if it were, the difference between the two fuels would become even greater. The simple dilution effect of the N~ on all concentrations, however, has been accounted for, and all the reported data have been adjusted to zero nitrogen dilution of the incoming reactants.
Thermal Energy Laboratory, Department of Mechanical Engineering, Washington State University, Pullman, Washington 99164
The oxidation of methanol in a jet-stirred reactor has been studied to characterize the mechanisms of methanol combustion and nitric oxide formation in a high-intensity, turbulent environment. F u e l / a i r equivalence ratios of 0.7 to 1.4 were investigated at a constant temperature of 1870 K and mean reactor residence times of ca. 2.5 msec. Total NO x was measured along with major combustion products, CO, CO 2, H 2 0 , 02, CH 4, and C e l l 2. Emission spectroscopy was used to determine oxygen atom concentration as well as relative levels of electronically excited radical species, OH*, CH*, and Cz*. Comparative measurements were repeated with methane as the fuel under similar reactor conditions. Measured NO~ for methanol was 8.5 ppm maximum at equivalence ratio 1.10. The ratio of methanol-NO~ to methane-NO~ varied between one-half for fuel-lean combustion to one-third for equivalence ratios at or above stoichiometric. Molecular O 2, atomic-O and calculated O H were essentially the same for both fuels. At fuel-rich conditions, methanol demonstrated higher carbon monoxide. Concentrations o f C H 4 and C z H~ were markedly higher with methane fuel, as were CH* and C2", while OH* was slightly higher. Evidence indicated the persistence of the C - - O b o n d in the methanol molecule during oxidation. The NOx, O-atom, and O~ concentrations indicated that the extended Zeldovich mechanism was insufficient to explain the difference in observed NO x for methanol and methane. It was hypothesized that hydrocarbon fragments played an important role in NO~ formation in the stoichiometric and rich mixtures; it was concluded that low methanol-NO x was due to low hydrocarbon levels.
Introduction Despite the abundance of practical knowledge about methanol as an alternate fuel for internal-combustion engines and other industrial applications, little experimental data are available on its combustion characteristics at temperatures of 1600 to 2500 K, the main temperature range of interest in combustion systems. Most of the combustion and pollutant emission work on methanol has been carried out in spark-ignition engines, either pure or in blends with gasoline. 1 Methanol oxidation at high temperatures has also been studied in stationary flames, 2 constant-pressure ducts, "~and shock tubes. 4'5 Bowman 5 extended the shock-tube investigations on methanol oxidation up to 2180 K for fuel-air equiva-
lence ratios between 0.375 and 6.0, and proposed a 28-step reaction mechanism. However, these onedimensional studies find only partial application to high-intensity, turbulent combustion processes characteristic of gas-turbine combustors and wallfired burners because of the forced mixing between fresh reactants and the burning gas and of possible turbulence/chemistry interactions. With these considerations in mind, a careful laboratory examination of methanol combustion, and attendant NO, formation, in a turbulent, recirculative, premixed flame was carried out.
Experimental Apparatus The experiments were performed in a jet-stirred reactor, shown in Fig. 1. Cast of zirconia, the reactor was a spherical segment of a Longwell reactor ~ as modified by Pratt, v with an internal volume of 50
*Present address: Drexel University, Philadelphia, PA. 689
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cm a. High intensity mixing and combustion were obtained by injecting jets of prevaporized, premixed reactants through a group of small sonic orifices drilled into a stainless steel feed tube. Methanol and methane were burned in the reactor. The operating conditions investigated and the measurements made at each condition are shown in Tables I and II, respectively. The nominal temperature was maintained constant at 1870 K over the range of equivalence ratios by diluting the fuel/air mixture with differing amounts of nitrogen. The thermocouple was Pt-Rh, 0.2 mm in diameter, coated with Y203 and BeO. The measured temperature was corrected for radiation heat losses from the thermocouple. For all gas analyses, a partially water-cooled 3 mm dia. quartz probe was used to remove a continuous stream of gas sample from the reactor. The sample flowed sonically through a 0.2 mm tip orifice; probe pressure was about 18 kPa (abs.). Standard infrared process analyzers, Beckman models 864 and 315B, were used for carbon dioxide and carbon monoxide detection, respectively. A Beckman model 951 chemiluminescent analyzer was used in measurements of nitric oxide and total oxides of nitrogen ([NOel = [NO] + [NOd]). Varian gas chromatographs, models 90-P3 and 1200, equipped with a thermal conductivity and flame ionization detector, respectively, were used to monitor the
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TABLE I Experimental conditions
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CHaOH (vapor)
CH 4
0.76-1.33 1870 K 93 kPa (abs.) 2.1-3.1 msec 77 kg/mZ-sec
0.70-1.40 1870 K* 93 kPa (abs.) 2.5-3.5 msec 64 kg/m3-sec
"For ~ = 0.70, the maximum temperature which could be attained was 1780 K. TABLE II Experimental measurements Quantity Temperature Species mole fraction: CO, C02 H20, Oz CH 4, CH3OH, C2H2 NO, NO~ O Emission intensity: O H ' , CH*, C2 ~
Technique Pt-Rh thermocouple Infrared process analyzer Gas chromatography, thermal conductivity detector Gas chromatography, flame ionization detector Chemiluminescence process analyzer Emission spectroscopy, path-integrated, in situ Emission spectroscopy, path-integrated, in situ
HIGH-INTENSITY METHANOL COMBUSTION remaining stable species. A quarter-meter Spex monochromator fitted with a 1P28A photomultiplier tube was used to detect radiative emission between 300 and 600 nm. The nominal oxygen atom concentration was determined by monitoring the path-integrated chemiluminescence at 380 nm of CO2" (~B2) formed during the overall reaction of CO with O. The change in intensity caused by adding small amounts of carbon monoxide to the reactor was directly related to the relative oxygen atom levels, s'9Absolute oxygen mole fraction was determined by calibrating the reactor/optical system at a reference condition for which O and OH levels were known from previous measurements.]~ There, absolute OH levels were found by absorption spectroscopy, which, with indicated partial equilibrium for the reaction OH + OH ~ H 2 0 + O, gave absolute O-atom mole fractions. A complete description of the apparatus and techniques is given in the dissertation by Singh. H
Reactor Characterization
General Methanol and methane were burned under essentially equivalent experimental conditions in the reactor. By holding the total mass flow rate through the reactor approximately constant for the two fuels (it was slightly greater for methanol because of the need for increased N 2 dilution), the gross fluid mechanical field of the reactor, the mean residence time of the reactor, and the characteristic diffusion time did not vary appreciably with fuel type. The gross thermal field was held constant by the technique of diluting the incoming reactants with varying amounts of N 2. Furthermore, the characteristic chemical kinetic times of CH 4 and CH 3OH were similar, based upon one-dimensional flame speed measurements done by other experimenters. ' z.a At the nominal residence time of 2.5 msec the reactor was intensely backmixed and chemically-rate dominated; however, it deviated somewhat from the desired homogeneous perfectly stirred behavior. A finite mixing influence was felt in the feed jet regions, as shown below. Nonetheless, given the repeatability of the reactor conditions, the similar gross thermal and fluid mechanical fields, and the similar characteristic times for mean-residence, diffusion, and chemical kinetics, it was possible to accurately compare the relative chemical processes for the two fuels.
spatial inhomogeneities. Figure 2 shows typical data, here taken for methanol combustion at a fuel/air equivalence ratio (qb) of one, plotted against the distance across the reactor. The line of the measurements, taken through the sampling port in the upper part of the reactor, passed through the central feed jet and the larger surrounding regions of intense recirculation; the line was in a plane coincident with the optical path shown in Fig. 1. The temperature, here uncorrected for radiation loss, made a dip of about 250 K at the vertical centerline of the reactor. This location corresponds to the incoming central feed jet, where the combustion was less complete. The surrounding recirculation regions were approximately homogeneous and more completely burned. Figure 2 shows a similar temperature scan for methane (dashed line). The spatial measurements of CO in methanol combustion, also shown in Fig. 2, exhibited an increase on the reactor centerline to a value 35% above the mean value. For the CO2, there was a 5% centerline dip below the spatially averaged value. The maximum temperature, located 15 mm within the reactor in the recirculation zone, was 1870 K when corrected for thermocouple radiation loss. This is treated as the nominal reactor temperature. The uncertainty in temperature due to uncertainties in the radiation correction was +30 K. The 1870 K
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temperature was 30 K below the adiabatic equilibrium temperature for methanol and 55 K below that for methane combustion. The flow field found in the reactor has been modeled by Wormeck ~2 for adiabatic combustion of methane at a throughput rate of 38 kg/m3-sec. Traversing across the flow field from the recirculation region to the jet centerline, along a path similar to that taken in Fig. 2, Wormeck's results at qb = 1.0 showed the following: 1) 150 K drop in temperature, 2) 40% increase in CO, 3) 40% increase in 02, 4) 100% increase in O-atom, and 5) 25% increase in OH on the jet centerline relative to the recirculation region. The temperature and CO predictions are in semi-quantitative agreement with Fig. 2. From these results, it is recognized that due to less than instantaneous mixing, the reactor feed jets exhibited a somewhat greater degree of combustion nonequilibrium and hydrocarbon pyrolysis, compared to the reactor whole; whereas, the large surrounding zones of intense recirculation exhibited a closer approach to equilibrium. Active species and pyrolysis fragments are, therefore, expected to be of somewhat higher concentration in the feed jets. The flow field is best characterized as a high-intensity recirculative flame, dominated by chemical kinetics, but influenced by finite mixing.
Experimental Results
Primary Combustion Products The primary combustion products are plotted in Fig. 3 over the range 0.7 < ~b < 1,4. A normalization factor was used for all concentration measurements to account for different nitrogen dilution rates, the results corresponding to the nitrogen present in air. At stoichiometric conditions, methanol required a 35% increase in nitrogen flow rate, relative to normal air; whereas methane required a 25% increase to maintain the temperature of 1870 K. The sample probe was located 7.5 mm inside the reactor, at the same elevation used for the preceding measurements. All gas samples were collected at this point (within the nearly homogeneous recirculation zone). At this location, the methanol produced 20 to 25% more water and carbon dioxide than did methane (when corrected for the higher nitrogen dilution in methanol combustion). This was primarily due to the 25% less air needed for complete oxidation of methanol, since the fuel contains oxygen. Thus, for a given equivalence ratio, although the carb o n / h y d r o g e n / o x y g e n ratio is the same for both methane and methanol, the amount of nitrogen associated with the required air is less in methanol oxidation. Carbon monoxide levels were 30% higher in methanol combustion for fuel-rich conditions, but approached the levels in methane combustion
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for qb < 1.0. There was essentially no difference in the measured 02 mole fractions for the two fuels. Unburned hydrocarbons appeared above equivalence ratios of 1.10. Acetylene was the most dominant hydrocarbon with a maximum value of 1650 ppm at qb = 1.40 for methane combustion. The acetylene levels in methanol combustion were much less, as shown in Fig. 3; a maximum of 40 ppm was recorded for ~b = 1.33, more than 15 times lower than the corresponding value from methane combustion. A methane mole fraction of 300 ppm was measured at d~ = 1.40 for methane combustion, while no methane was detected for methanol combustion. Neither methanol nor formaldehyde was detected in either system.
Radical and Excited Species Path-integrated oxygen-atom mole fractions, shown in Fig. 4, were essentially the same in methanol and methane combustion. The estimated experimental uncertainty in the relative O-atom mole fraction was +30%. All optical measurements were taken along the optical path shown in Fig. 1. From the oxygen-atom and water vapor measurements, the nominal hydroxyl radical could be calculated under conditions where partial equilibrium prevailed for the reaction OH + OH ~ H 2 0 + O. ~~The calculated OH showed no significant difference for methanol and methane combustion for 0.76 < ~b < 1.10. The O and OH results are not
HIGH-INTENSITY M E T H A N O L COMBUSTION
methanol systems could then indicate much lower levels of CH, C2, or both. No C2 ~ was observed in methanol combustion. Suggested pathways indicate that C2 ~ may be formed from C2 H~ hydrocarbon fragments through a carbon suboxide mechanism ]7 (C2 O + C ---) C~* + CO), or from single carbon fragments, CH~, such as CH + CH ~ Ca* + H a. The critical conclusion which can be made from these suggested pathways is that the methanol system must have contained lower concentrations of ground-state hydrocarbons, such as CHx and Ca H , , relative to the methane system in order to be consistent with the emission results. This is further supported by the gas sampling measurements of CH4 and C~ H e mentioned earlier.
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O, II,gb METHANE
FI~;. 4. Measured O-atom and relative emission intensities in jet-stirred reactor as a function of fuel/air equivalence ratio.
unexpected since the same energy releasing hydrogen/oxygen kinetic scheme is operational for CH4 and CH3OH. 5 The relative emission bands of OH* (310 nm), CH ~ (430 nm and 390 nm), and Ca ~ (Swan bands at 438-563 nm) were measured and compared for the two fuels. The OH* emission, which peaked at about ~b = 1.10, was about 50% higher for methane combustion than for methanol when corrected for nitrogen dilution. Emission due to CH* from methanol was an order of magnitude below that from methane, as shown in Fig. 4. The C2* results were most striking. No Cz ~ was detected in the methanol system; but for methane combustion, the total integrated C~* intensity was four times that of CH ~ The kinetic mechanisms which lead to the electronically excited states OH*, CH*, and Ca*, have been discussed but not fully established in the literature. TM~4'~ ~.t6.~7 Nevertheless, after studying the literature and the relative measurements in the present experiments, a few general remarks can be made. The literature suggests that OH* may be formed within the H2/O2 system. ~3Since the OH ~ is higher in methane combustion, this could possibly indicate that additional reactions involving hydrocarbon fragments, for example CH + O 5 --~ OH* + CO, ~3 lead to the higher chemiluminescence in methane combustion. CH* has been speculated to form in reactions of CH or C 2, such as Cz + O H --* CH* + CO, ]a and perhaps CH + OH + H --* CH* + Ha O. ~r The relative CH* levels in the methane and
35
I
~'-3
'
I
[
- - r - -
30 . ,.i.~,,~NOx. METHANE
r
O
2 .=
//
.IL.'"~_ \\
--
/
/
/I
IC
0,7
ir /
0.8
//~NO, METHANE ~',~
/
0.9
",, i,
1.0
EOUIVALENCE
/NO x,METHANOL\~
I.I
[.2
1.3
1.4
RATIO
F1G. 5. Oxides of nitrogen mole fraction from combustion of methane or methanol in jet-stirred reactor as a function of fuel/air equivalence ratio.
694
NOx, SO~
tively. As mentioned earlier, the reported mole fractions have been corrected for the effect of nitrogen dilution. However, no attempt was made to account for the first order dependence of thermal NO on N 2 concentration. If this were done, since higher nitrogen levels were present in the methanol system, the effect would be to enhance the difference in NO mole fraction between the two fuels. Relatively large NOx/NO ratios were observed in both fuels, particularly under fuel-lean conditions. Reactor NOz could have been real or a probe effect. Kramlich and Malte 19 have shown that the discrimination of NO from NOz may be unreliable for fuel-lean conditions due to reactions within the probe. However, the total NO~ measurement is undistorted by these probe effects. (Of course, care must be taken to prevent NO 2 loss to water condensate in the same system.) A traverse across the reactor for stoichiometric combustion indicated little spatial variation of NOx, as shown in Fig. 2. Total oxides of nitrogen showed a gradual decrease toward the reactor far wall which was due to the cooling effect of the sampling probe as it was inserted farther into the reactor. For qualitative comparison, NOz is also plotted in Fig. 2. The measured NO~ levels rose noticeably at the reactor center, which corresponded to the centerline of one of the incoming jets. The temperature at this point was 250 K below that of the surrounding gas, and computer predictions lz have indicated a maximum HO 2concentrationat this location. Under these conditions, the sudden rise in measured NO2 mole fraction could have reflected combustor NO2 due to NO + HO~--* NO~ + OH. 19
Methanol Oxidation The progressive oxidation of methanol is shown schematically in Fig. 6, along with a possible side reaction for the formation of nitric oxide. (The H J O z system and Zeldovich N/O system have been deleted since they are similar for both methane and methanol.) There are two primary paths for the formation of CO, one through CH z and the other through CH20. Methane is also oxidized along these two paths as indicated. Although all of the reactions are reversible, the initial reactions of the parent fuel molecules must proceed in the direction as shown. Notice that CH3 OH can proceed in two directions, to CH 3or CHz OH, while CH 4must proceed through the methyl radical. Although the C - - H bond is stronger than the C--O bond in methanol, there are three hydrogen atoms which can be abstracted to form CH~OH while only one bond breakage will lead to CH 3. A third reaction, through CH30, is likely to quickly proceed to CH~O. It appears from the experimental results that the preferred path to CO for methanol is through CHeOH rather than through CHa. If this were not
(r
COMBUSTION
MECHANISM
FOR
METHANOL
FIG. 6. General outline of methanol and methane oxidation to carbon monoxide, with a side reaction to nitric oxide. so, then the hydrocarbon levels and chemiluminescence in methanol and methane combustion would not be expected to differ as much as the experiments indicated. This conclusion also supports the importance of the side reaction for NO formation through CH hydrocarbon fragments, and supports the much lower nitric oxide levels measured in the methanol system.
Discussion A primary result of this work is the significantly greater NOx concentration in methane combustion relative to methanol combustion at similar conditions. The pertinent question is whether this is a general chemical kinetic result, or simply an artifact of the present flame; and if it is a general result, what is the origin of the higher methane-NOx ? It is first important to recognize that the above probe and optical measurements were heavily replicated and repeatable from "day-to-day." The results for O-atom, 02 , and OH, which can be treated as nominal values indicative of the upper part of the reactor, were independent of fuel type. Also, the gross thermal and fluid mechanical fields, and mean residence times, were similar for the two fuels. A major difference between the two fuels was the greater hydrocarbon concentrations noted for methane combustion. The extended Zeldovich mechanism for NO formation is N2 + O--~ NO + N N + Oz---~NO + O N + OH--* NO + H. This mechanism, in concert with the time-steady
695
HIGH-INTENSITY METHANOL COMBUSTION field in the upper part of the reactor, should have produced similar NO levels for the two fuels, especially at near-stoichiometricconditions. The fact that this agreement did not occur means that the factor two-to-three higher N O measured for methane compared to methanol is not explained by this route. This conclusion continues to hold if the nitrous oxide mechanism is included, TM and it is not substantially changed if experimental uncertainties are considered. The magnitude of Zeldovich NO produced by the time-steady field was estimated at qb 0.8 by using the PSR model;* and was found to be about 10 ppm, in essential agreement with the experimental results for fuel-lean conditions and for methanol. (It shouldbe noted that we are assuming the NO~ measurements to be synonymous with the original NO formed in the reactor. The measured NO2 is treated as an artifact, resulting from original NO oxidation in the reactor or in the sample probe train.) The factor three higher NO in methane combustion is likely due to either, 1) turbulent fluctuations acting to nonlinearly reinforce the Zeldovich mechanism, or 2) an additional kinetic pathway to NO formation. It is well established that the Zeldovich mechanism is highly sensitive to temperature and stoichiometry, and therefore, turbulent fluctuations in these quantities could have reinforced the Zeldovich NO formation rate. z~However, given the experimental setup of similar gross thermal and fluid mechanical fields, and similar characteristic times, the effect should have been approximately similar for the two fuels. Furthermore, it can be argued that large fluctuations in temperature (to levels significantly above the measured temperature) and stoichiometry (to high oxidant levels), sufficient to cause a marked increase in the Zeldovich NO, should have been exceptional occurrences in the reactor. The only way in which the local stoichiometry could have varied would have been through selective diffusion of either fuel or oxidizer out of a turbulent eddy. This could not have occurred to any significant degree because of the short residence time available within the reactor. Likewise, the only way in which the final temperature of an eddy could have significantly increased would have been through selective diffusion of inert diluent out of the eddy, since the mean reactor temperature was already close to the adiabatic equilibrium temperature (see section "Reactor Characterization" ). Next, note the flat NO~ trace in Fig. 2; the NOx did not reflect the spatial gradients exhibited by CO and temperature, but was fairly homogeneous across the reactor. A flat NO, profile was measured for both methanol and methane at 9 = 1; this trend has also been noted in our previous research for
*T = 1870 K; O-atom taken from Fig. 4.
~ 1. The logical explanation for this behavior is that the chemical kinetic time for NO formation in the region of the measurements was much longer than the diffusion time. That is, any NO formation in the region of the measurements occurred in a manner characteristic of a perfectly stirred reactor: long kinetic time compared to diffusion time. The measured NO~ homogeneity implies that the local rate of turbulent diffusion of NO must have balanced the local chemical formation rate of NO. This follows from the species conservation equation, treated as a field equation, time-averaged at a point, and neglecting molecular diffusion. The local rate of turbulent diffusion is
U ~ - C~Xi
-
-
-
-
TD
where YNo is the mass fraction of NO and %) is the characteristic diffusion time. The local chemical formation rate is W-~-'No= YNo/~'k, where r k is the characteristic chemical kinetic time. With the turbulent velocity estimated as one-quarter the initial feed jet velocity . ~1(somc), . and with the turbulent scale estimated as the initial feed jet size (1 mm), the characteristic diffusion time is of order 10 -~ sec. The characteristic kinetic time is of order 10 -3 sec, and thus, the fluctuation of NO is small compared to the time-mean NO. With respect to this analysis it is interesting to note that the NO could have essentially formed elsewhere in the reactor, that is, within the lower region, and then merely undergone efficient diffusion in the upper portion of the reactor. A flat NO, profile would have resulted. In the lower part of the reactor, the effect of hydrocarbon pyrolysis, which is discussed below, could have influenced the NO formation. We now turn to the second explanation listed above for the higher NOx in methane versus methanol combustion: an additional kinetic pathway. It is suggested here, in the light of the above comments on fluctuation-NO, and given the experimental setup of similar reactor conditions for the two fuels, that the substantially higher NOx measured in methane combustion was due primarily to the difference in hydrocarbon pyrolysis between the two fuels, and the subsequent hydrocarbon fields. As verified above, methane gave significantly higher hydrocarbon levels, both stable species and radical fragments. An obvious kinetic pathway is the so-called "prompt NO" mechanism,~'~ wherein hydrocarbon fragments attack bimolecular nitrogen, producing atomic-nitrogen, cyanides, and amines, which subsequently oxidize to nitric oxide. Possible initial reactions are CH + N 2 ~ HCN + N and Cz + N2 CN + CN. The CN and HCN may then react through NH i intermediates to form NO or Nz. ~4,z~
696
NO~, SO s
A schematic of this mechanism is shown in Fig. 6. This deduction, based on the experiments, that differences in the hydrocarbon fields promoted higher NO formation in methane combustion compared to methanol combustion, is a general effect. Of course, in other combustion systems the effect may be mitigated by specialized mixing patterns, and may be overtaken by Zeldovich NO at high temperatures and long residence times, i.e. thermal NO. The present reactor, operated at moderate temperature and at short residence time, severely curtailed Zeldovich NO formation (as shown above). Also, for the present reactor it is not precisely known if the hydrocarbon influence on NO formation was distributed uniformly throughout the reactor, or whether the influence was concentrated somewhat in the lower portion of the reactor. One suspects that the effect may have been stronger in the lower portion of the reactor due to enhanced pyrolysis in that region.
Conclusions The experimental evidence indicates that reactions in addition to the Zeldovich mechanism are essential to explain the greater formation of nitric oxide in high-intensity methane relative to methanol combustion. The low levels of nitric oxide in methanol combustion, when compared to the higher levels of nitric oxide in methane combustion, suggest that certain intermediate species were at significantly different concentration levels in the two fuel systems, Specifically, the experiments indicated that O and OH were at similar levels for the two fuels, whereas the hydrocarbon fragments were significantly greater in methane combustion, and thus, may have provided the additional pathways to NO formation via the so-called prompt NO mechanism. Measured levels of acetylene and methane were an order of magnitude larger in methane combustion for equivalence ratios greater than 1.10. The chemiluminescent intensity of CH* and C2 ~ were also much greater in the methane system. Although it is difficult to directly relate these emission measurements to ground-state concentrations of CH and C 2, most suggested pathways to the excitation of CH and C~ involve hydrocarbon fragments such as CH and C~H. Therefore, lower levels of CH ~ and C2 ~ in methanol combustion when compared to methane combustion, can be interpreted to indicate lower levels of hydrocarbon fragments. The initiation steps in methanol oxidation involve the rupture of either the C - - H bond or the C - - O bond. The former reactions lead directly to formaldehyde through the CH2OH radical, while the latter creates the methyl radical. Under the present experimental conditions, the measurements support
reactions through CH2 OH, in which the C - - O bond remains intact. In summary, high-intensity turbulent combustion of methanol, compared to methane, produces significantly lower NO,. This effect appears to be due to lower hydrocarbon fragment concentrations formed during methanol oxidation. Acknowledgments
The authors wish to acknowledge the support of the National Science Foundation, Grant No. 7320136-A02, and the Thermal Energy Laboratory at Washington State University.
REFERENCES 1. MOST, J. W. AND LONGWELL, J. P.: "'SingleCylinder Engine Evaluation of Methanol--Improved Energy, Economy and Reduced NOT," Society of Automotive Engineers, Paper no. 750119, 1975. 2. DEWlLDE, W. AND VAN TIGGELEN,A.: Bul. Soc. Chim. Belges. 77, 67 (1968). 3. WISER,W. H., ANDHILL, G. R.: Fifth S y m p o s i u m (International) on Combustion, p. 553, Reinhold Publishing Co., 1955. 4. COOKE, D. F., DOBSON, M. G., AND WILLIAMS, A.: Combustion and Flame 16, 223 (1971), 5. BOWMAN,C. T.: Combustion and Flame 25, 343 (1975). 6. LONGWELL, J. P., AND WEISS, M. A.: Industrial and Engineering Chemistry 47, 1634 (1955). 7. MALTE, P. C., ANn PaATT, D. T.: Fifteenth S y m p o s i u m (International) on Combustion, p. 1061, The Combustion Institute, 1975. 8. SCHM1DT, S. C., MALTE, P. C., AND PRATT, I). T.: Experimental Diagnostics in Gas Phase Combustion Systems, Progress in Astronautics and Aeronautics, Vol. 53, p. 243, American Institute of Aeronautics and Astronautics, 1977. 9. BOWMAN,C. T.: Fifteenth S y m p o s i u m (International) on Combustion, p. 869, The Combustion Institute, 1975. 10. MALTE, P. C., SCHMIDT, S. C., AND PRATT, D. T.: Sixteenth S y m p o s i u m (International) on Combustion, p. 145, The Combustion Institute, 1977. 11. SINCH, S.: "A Study of Methanol Combustion and Oxides of Nitrogen Formation in a JetStirred Reactor," Ph.D. Thesis, Washington State University, Pullman, Washington, 1978. 12. WORMECK,J.: "Computer Modeling of Turbulent Combustion in a Longwell Jet-Stirred Reactor," Ph.D. Thesis, Washington State University, Pullman, Washington, 1976. 13. GAYDON,A. G.: The Spectroscopy o f Flames, Chapman and Hall, 2nd Edition, 1974. 14. BLEEKRODE, R. AND NIEUWPORT, W. C.: Journal
HIGH-INTENSITY METHANOL COMBUSTION
o f Chemical Physics 43, 3680 (1965). 15. PORTER, R. P., CLARK, A. E., KASKAN, W. E. ANt) BROWNE, W. E.: Eleventh S y m p o s i u m (International) on Combustion, p. 907, The C o m b u s t i o n Institute, 1967. 16. MALTE, P. C., ANDSINCH, S.: " ' C h e m i l u m i n e s c e n t Intensities for H y d r o c a r b o n - A i r C o m b u s t i o n in a Jet-Stirred L o n g w e l l Reactor," Paper no. 14, C a n a d i a n S e c t i o n / T h e C o m b u s t i o n Institute, May 1977. 17. PELTERS, J., LAMBERT, J. F., HERTOGHE, P., AND VAN T1GGELEN, A.: Thirteenth S y m p o s i u m (International) on Combustion, p. 321, T h e Comb u s t i o n Institute, 1973. 18. ENGLEMAN, V. S., BARTOK, W., LONGWELL, J. P., AND EDELMAN, R. B.: Fourteenth S y m p o s i u m
697
(International) on Combustion, p. 755, T h e C o m b u s t i o n Institute, 1973. 19. KRAMLICH, J. C. AND MALTE, P. C.: Combustion Science and Technology 18, 91 (1978). 20. GOULmN, F. : Combustion Science and Technology 9, 17, (1974). 21. MALTE, P. C., AND PRATT, D. T.: Combustion Science and Technology 9, 221, (1974). 22. FENIMOBE, C. P.: Thirteenth S y m p o s i u m (International) on Combustion, p. 373, T h e C o m b u s t i o n Institute, 1971. 23. IVERACH, D., BASDEN, K. S., AND KIROV, N. Y.: Fourteenth Symposium (International) o f Combustion, p. 767, The C o m b u s t i o n Institute, 1973. 24. HAYNES, B. S.: "'The F o r m a t i o n a n d Behavior of Nitrogen Species in F u e l Rich H y d r o c a r b o n
COMMENTS M. Destriau, Universitd de Bordeaux I, France. Which are the reactions p r o d u c i n g excited C H and C 2 in y o u r s y s t e m s ? May partly thermal excitation
Occur?
Authors" Reply. The reaction m e c h a n i s m s responsible for the f o r m a t i o n of electronically excited C H and C 2 have b e e n o p e n to speculation due to lack of direct a n d quantitative g r o u n d state concentration m e a s u r e m e n t s of C H and C 2 and their precursors in the same environment. Bleekrode and N i e u w p o o r t 1 have experimentally established the excitation of C H a n d C 2 as p r e d o m i n a n t l y chemiluminescent in nature. At least an order of m a g n i t u d e higher relative C H e m i s s i o n intensity from m e t h a n e c o m b u s t i o n c o m p a r e d to m e t h a n o l has been observed in the p r e s e n t study. Definite mechanistic c o n c l u s i o n s c a n n o t be made as to h o w C H * arises. However, if it is due to the reactions C2H + O - - ) C H ~ + CO C2H+O 2--)CH 9 +C 2
T h e C~* e m i s s i o n m e a s u r e d in this study from rich m e t h a n e c o m b u s t i o n w a s five times as s t r o n g as C H ~ emission. Since C~ e m i s s i o n w a s not seen in m e t h a n o l c o m b u s t i o n b u t C H ~ was, the kinetic m e c h a n i s m for C ~ formation m u s t be able to a c c o u n t for this fact. I f C ~ arises from the reactions i n v o l v i n g t w o h y d r o c a r b o n fragments (e.g., C H + C H ---) C~ + Ha) or from a C~H 2 molecule (C2H ~ + O ~ C 2 0 + Ha; C 2 0 + C---~ C~ + CO), then lower hydrocarb o n f r a g m e n t s m u s t exist in m e t h a n o l c o m b u s t i o n . T h i s is consistent w i t h the lower stable h y d r o c a r b o n levels m e a s u r e d in methanol c o m b u s t i o n c o m p a r e d to m e t h a n e .
REFERENCES
1. BLEEKRODE, R. ANt) NIEUWPOORT, W. C.: lournal o f Chemical Physics, 43, 3680 (1965) 2. PELTERS, J., LAMBERT, J. F., HERTOGHE, P., AND VAN TIGCELEN, A.: 13th S y m p o s i u m (International) on Combustion, p. 321, The C o m b u s t i o n Institute, 1973.
C 2 + O H - - ) CH* + CO then the l o w e r C H ~ e m i s s i o n from m e t h a n o l w o u l d indicate lower levels of C 2 h y d r o c a r b o n fragments, since [O], [ O H ] , and [02] levels were a b o u t the same in b o t h the systems. It has also been suggested 2 that C H ~ m a y arise due to the reaction C H + O H + H---) CH* + H 2 0 A p r e d o m i n a n c e of this reaction w o u l d m e a n that the lower C H ~ e m i s s i o n f o u n d in m e t h a n o l c o m b u s tion indicates l o w e r g r o u n d state C H concentration.
D. R. Crosley, Ballistic Research Laboratory, USA. You have more spectroscopic d a t a - - t h a t on C 2 a n d C H - - t h a n you appear to make use of in c o m p a r i s o n w i t h the model. T h o u g h C 2 w a s not i n c l u d e d in y o u r reaction network, C H was. Are the c o n c e n t r a t i o n s of these species u s e f u l to ascertain the m e c h a n i s m s ? You replied that you c o u l d n ' t incorporate C a and C H because y o u d i d n ' t k n o w rate c o n s t a n t s for the excited states. But m a n y of the transient s p e c i e s - - C H , C2, O H , C N and N H - -
698
NO~, SOx
w h i c h were in your network can be probed in the g r o u n d states by laser excited fluorescence. H o w well do these concentrations confirm the model? Also, can you make use of relative concentrations as a function of equivalence ratio, rather than needing absolute values which are more difficult to measure ?
Authors" Reply. G r o u n d state concentration meas u r e m e n t s in our reactor of the radical species CH, C 2, C H and N H would be extremely u s e f u l in elucidating the excitation pathways, as well as the kinetic m e c h a n i s m for the formation a n d destruction of the pollutant species. E v e n t h o u g h accurate rate data are unavailable for most C 2 radical reactions, absolute concentration m e a s u r e m e n t s of Cz w o u l d permit evaluation of various e x p a n d e d reaction s c h e m e s and estimation of s o m e rate coefficients. Relative m e a s u r e m e n t s of g r o u n d state transient species could be u s e d to support the authors' explanation of the role of hydrocarbon fragments in the formation of nitric oxide. T h e s e could also be u s e d to ascertain the relative importance of reactions for f o r m i n g excited species, s u c h as those d i s c u s s e d in the reply to Prof. Destriau.
D. Aronowitz, University of Southern Calif., USA. 1) Because in methane c o m b u s t i o n there are more C a n d C 2 fragments than in m e t h a n o l c o m b u s t i o n a n d the C a n d C~ fragments could be i m p o r t a n t in forming NO~, C H a O H c o m b u s t i o n s h o u l d i n d e e d give less NO~ than C H 4 a n d this m a y explain the factor of 3 difference. 2) T h e paper by Aronowitz et al. in this s y m p o s i u m a n d one to be p u b l i s h e d shortly elsewhere by Westbrook a n d Dryer on a detailed C H 3 O H m e c h a n i s m , s h o u l d provide you with a good e n o u g h m e c h a n i s m to test your experimental data. 3) A l t h o u g h your results s e e m to support a C H 3 O H ---> CH~OH ~ CO route, it is important to realize that at 1800 K, the initiation reaction C H 3 O H + M --~ C H 3 + O H + M will become important, b u t its effect will be m a s k e d by the very rapid CH~ oxidation (at 1800 K, C H 3 r e c o m b i n a t i o n is considerably slower).
Authors'Reply. The authors are in agreement w i t h the point concerning the significant role played b y the C H a n d C 2 radicals in the formation of NO, as detailed in the paper. T h e more detailed m e c h a n i s m by Westbrook a n d Dryer will be a welcome addition, a n d its incorporation into our analysis will be investigated. T h e global m e c h a n i s m s u g g e s t e d by Aronowitz et al. in this s y m p o s i u m may or may not be directly applicable to our experiments, since the present work was performed at 1870 K and no u n b u r n e d m e t h a n o l
was detectable in the reactor. T h e flow reactor study was performed at lower temperatures a n d the global m e c h a n i s m was derived from the disappearance rate of methanol. T h e importance of the CH~OH radical relative to the methyl radical d u r i n g the initial stage of c o m b u s t i o n may, in fact, be explained from a statistical argument. Although the C - - H b o n d is stronger than the C---O b o n d in the parent fuel, there are three h y d r o g e n atoms w h i c h can be abstracted to form CH2OH, while only one bond breakage will lead to C H 3. T h e relative importance of these two initiation reactions m a y alter with temperature. However, all of the reported data in the present study at 1870 K support a m e c h a n i s m favoring the CH3OH ~ CH2OH (or C H 3 0 ) ---> CO route.
G. S. Samuelsen, University of California, lrvine, USA. E v e n though the overall time-mean temperature is maintained constant with the two fuels, the required diluent to achieve that condition will affect loeal transport properties. As a result, it is possible that local temperature fluctuations (which affect, if not dominate, the local NO x time-mean production rate in premixed systems) may explain some of the difference in total production rate observed. It might be possible to test this by (1) u s i n g different species for the diluent, a n d (2) assessing w h e t h e r the same difference in NO x p r o d u c t i o n rate is observed at lower time-mean temperatures.
C. T. Bowman, Stanford University, USA. Your m e a s u r e m e n t s s h o w significant spatial non-uniformities in temperature a n d in CO concentration in your reactor, i m p l y i n g that the fuel oxidation reactions are occurring u n d e r u n m i x e d conditions. What is the justification for the use of a perfectlystirred reactor c o m p u t e r code to model a n d interpret your experimental results? Authors" Reply. As s h o w n in the paper via measurements, and by Wormeck (Ref. 12 in paper) via computer modeling, the feed jets relative to their recirculative s u r r o u n d i n g s exhibited a greater degree of chemical n o n - e q u i l i b r i u m a n d hydrocarbon pyrolysis. T h e NO x, however, at least in the upper part of the reactor for d~ = 1, exhibited homogeneity, indicating a long chemical kinetic time compared to diffusion time. T h e PSR code, as a first order model, had s o m e validity for ascertaining the feasibility of various NO formation m e c h a n i s m s in our reactor.
HIGH-INTENSITY M E T H A N O L COMBUSTION
F. C. Gouldin, Corneli University, U.S.A. You have attributed the differences in NO~ emission for methane vs. methanol firing to prompt NO type reactions, even when the reactor is operating under lean conditions. Since emission levels for the two fuels vary by a factor of two or more, one is tempted to conclude further that most of the NO in either case is prompt NO related. Such a conclusion would be controversial and leads one to question your conclusions about prompt NO. Is it possible to attribute some of the differences in NO~ levels to unmixedness in your reactor? In order to maintain constant ~b, p, T and staytime in the reactor, when changing fuels, nitrogen must be added as a diluent. Some change, perhaps small, in NO x emissions due to the diluent is expected. Did you evaluate this effect quantitatively?
Authors" Reply. Doctor Gouldin's questions regarding the effects of unmixedness and Na dilution have been addressed in the paper. It was noted that the gross temperature and velocity fields for the two fuels were the same. Gaseous methanol has been reported to have a slightly higher flame speed than methane (Ref. 3 in paper); however, within experimental scatter it may be argued that the two fuels have essentially equal flame speeds (personal communication, C. T. Bowman). Therefore, in the experiments the characteristic times, i.e., stay time, kinetic time, and diffusion time, were essentially equal for the two fuels. The important difference between the two fuels was their respective hydrocarbon fields. Ground and excited state hydrocarbon concentrations were significantly higher for methane than for methanol. Since hydrocarbon pyrolysis reactions tend to be fast, unmixedness would be expected to have its greatest impact here. Indeed, Wormeck's model (Ref. 12 in paper) shows greatest spatial inhomogenieties for the hydrocarbon species, particularly in the early feed jet region. The unmix-
699
edness, however, doesn't change the basic conclusion of this study: hydrocarbon concentrations were greater for methane than for methanol, leading to greater NO formation. There are additional indications regarding the effect of unmixedness on the hydrocarbon field and NO x. We have consistently measured a smooth NOx profile in the upper region of the reactor (see Figure 2 in paper). This is somewhat unexpected, given the nonuniform temperature and (other) composition profiles, both measured and predicted. One explanation given in the paper for the smooth NO~ profile was NO~ formation early in the reactor (feed jets); thereafter the NO x simply "sloshed" around in the bulk of the reactor. Also, attempts to measure absolute CH by absorption spectroscopy in the upper reactor region gave CH ~ 1 ppm, perhaps too low of concentration to explain the observed (prompt) NO x. Finally, other measurements in the jet-stirred reactor (at "equal" conditions) have also shown a striking fuel dependence on observed NOx. Acetylene gave maximum NOx, being a factor ten greater than NO x for H a and H J C O fuels. Methane and propane gave intermediate NO x. The respective intensities of the hydrocarbon emission bands followed the same trend with fuel type. To maintain the gas temperature within the reactor at 1870 K the fuel/air mixture was diluted with differing amounts of nitrogen over the range of equivalence ratios. At stoichiometric conditions, methanol required a 35% increase in N 2 flow rate, relative to normal air, whereas methane required a 25% increase. Since NO~ production was directly proportional to the N 2 concentration, the higher dilution of the methanol would, in fact, artificially increase the NO~ measured. This effect was not corrected for; but if it were, the difference between the two fuels would become even greater. The simple dilution effect of the N~ on all concentrations, however, has been accounted for, and all the reported data have been adjusted to zero nitrogen dilution of the incoming reactants.