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Chemical Production Rates of Intermediate Hydrocarbons in a Methane/Air Diffusion Flame

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Chemical Production Rates of Intermediate Hydrocarbons in a Methane/Air Diffusion Flame Twenty-first Symposium(International) on Combustion/TheCombustion Institute, 1986/pp. 1057-1065 CHEMICAL PRODUCTION RATES OF INTERMEDIATE HYDROCARBONS IN A METHANE/AIR DIFFUSION FLAME J. HOUSTON MILLER Department of Chemistry George Washington University Washington, DC 20052 W. GARY MALLARD AND KERMIT C. SMYTH Center for Fire Research National Bureau of Standards Gaithersburg, MD 20899 The net production and destruction rates for acetylene, diacetylene, butadiene, and benzene have been determined from measurements of species concentration, temperature, and velocity in a methane/air diffusion flame. In addition, profile measurements of the methyl radical are used to compute profiles of hydrogen atoms. These results provide an initial evaluation of proposed models of hydrocarbon chemical growth routes in diffusion flames, and indicate that the vinyl radical plays a key role in the formation of benzene. Evidence is also presented for acetylene participation in surface growth processes on small soot particles. I. Introduction composition is not sufficient to predict the concentrations of intermediate hydrocarbons, The identification of the formation mecha- such as acetylene and benzene, and soot nism of large hydrocarbons produced during particles 17. Therefore, in this paper an analysis combustion is one of the most challenging is undertaken combining detailed species pro- problems in high temperature chemistry. De- file measurements, temperature, and convec- spite the complexity of chemical growth reac- tive velocity measurements to calculate produc- tions in flames, there have been significant tion and destruction rates for intermediate recent gains in our understanding. Extensive hydrocarbons. The experimental values for the profile measurements have led to a detailed net production/destruction rates are compared description of the chemical structure of pre- with estimated rates of chemical growth reac- mixed flames 1-13. These results have been tions in order to evaluate possible formation combined with the time-temperature history in routes of aromatic hydrocarbon species. one-dimensional flame systems to produce simple models for molecular growth. Modelling efforts have also been undertaken using shock II. Experimental results tube data 14-16. Most practical combustion systems are diffu- Detailed species profiles have been measured sion flames. Here, in contrast to shock tubes in an atmospheric pressure methane/air diffu- and premixed flames, our knowledge of the sion flame using both mass spectrometric and soot formation process is much less complete. optical techniques 17. A Wolfhard-Parker burn- This is due in part to the lack of data which are er was utilized, consisting of a central fuel slot required for a detailed modelling of diffusion and two adjacent slots for air. Two flame sheets flames and in part to the inherently more were formed at the fuel/air interfaces, and all complex interactions between transport and species profiles were symmetric about the chemical processes in these systems. burner centerline. Profiles of stable flame Recently, concentration measurements have species were collected with a direct-sampling been reported for molecular species, including mass spectrometer equipped with a quartz intermediate hydrocarbons, in. laminar meth- microprobe (orifice diameter ~140 ~.m). Cali- ane/air diffusion flames 17'18. Our work has bration of the major species (methane, oxygen, shown that a knowledge of the local chemical nitrogen, carbon dioxide, carbon monoxide, 1057 1058 COMBUSTION GENERATED POLLUTANTS water, and hydrogen) led to a mass balance 20 ' ' ' ' I .... I ' ' ' ' I .... across the flame (total mole fraction = 1.01 -+ 0.04). In addition, absolute concentrations of acetylene, butadiene, and toluene were deter- mined using a direct calibration procedure. Profiles of methylacetylene (and/or allene), vinylacetylene, diacetylene, triacetylene, ben- zene, and naphthalene were also measured and calibrated indirectly 17. In a related series of measurements 19, a quartz microprobe was modified so that iodine vapor could be introduced into the inside of the ! J/ t l/: ',ltt, t,'I,H/, ', quartz tube. Sampled methyl radicals react quantitatively in the til~ of the scavenger probe to form methyl iodide z~ which can then be detected in the mass spectrometer. The optical measurements included laser- -0!0''' "?17' ' ' 0 .... ; .... 10 induced fluorescence,, multiphoton ionization, LATERAL POSITION, rnrn photodissociation, and Rayleigh scattering in- vestigations of the methane/air diffusion FIG. 1. Isothermal contours (solid lines) calculated flame 17'23. These studies have produced pro- from uncorrected thermocouple measurements and files of relative OH and methyl radical concen- streamlines (dashed lines) calculated from the velocity trations, as well as larger hydrocarbon species measurements for the methane/air flame. and small soot particles. Temperature profiles were measured with an 1.0 i .... r .... r .... uncoated 125 I~m diameter fine wire Pt/Pt- [CtHt] 10% Rh thermocouple. Corrections due to radia- o [C,H,]xS tion effects were estimated to be less than 7% at o [C=Ht] x3 the highest temperatures in this flame 17, while + lC4H=]xlO catalytic effects are expected to be small 24 and in g the opposite direction. For equilibrium and kinetic calculations discussed in Section IV, the x z original temperature data were corrected for the estimated radiation effect. Both the horizon- ~ 0.5 tal and vertical components of the convective velocity were determined using laser Doppler =, velocimetry. The gradients in concentration and =o temperature in this flame are steepest in the lateral direction. Therefore, profile data points were collected every 0.2 mm horizontally and every 2 mm in the vertical direction. 0.0 Figure 1 illustrates the temperature and -10 -5 0 5 10 velocity fields for this flame. Shown as solid LATERAL POSITION, mm lines are isothermal contours determined from the thermocouple profiles. Also shown are FIG. 2. Mass spectrometric profile measurements streamlines of convective velocity calculated of several minor species: acetylene, benzene, diacety- from the two measured velocity components. lene, and butadiene at a height of 9 mm above the The streamlines exhibit trajectories which origi- burner. nate in the lean region of the flame, cross the high temperature, primary reaction zones, and continue into the fuel-rich regions. III. Production Rate Calculation Figure 2 presents profiles collected for sev- eral intermediate hydrocarbons: acetylene, A. Procedure benzene, diacetylene, and butadiene. Peak con- centrations for these species are 6200, 800, 570, A primary goal of this work is to apply kinetic and 1 l0 parts per million, respectively, at this modelling to a methane/air diffusion flame; height. Profiles obtained for the other interme- that is, to compare the predictions of models diate hydrocarbons all exhibit concentration which have been used to describe premixed maxima in the same region of the flame. flame and shock tube results with our diffusion PRODUCTION RATES IN DIFFUSION FLAMES 1059 flame data (see Section IV, below). A viable first term in Eq.2 (concentration-driven diffu- model must not only account for the steady sion) dominates the second (thermal diffusion), state concentrations in species involved in and the contributions of thermal diffusion to chemical growth processes, but also for the mass transport can be ignored. The horizontal chemical flux of these species in the flame. To temperature gradients in our flame are steep in make this comparison, the net chemical pro- certain locations (2000-3000 K/cm), and thus dttction and destruction rates for individual thermal diffusion cannot be dismissed. To criti- species have been computed from concentra- cally evaluate the importance of thermal diffu- tion, temperature, and velocity measurements, sion, kr has been calculated as a function of as described below. temperature and composition for a variety of A laminar flame is a steady-state system, and binary mixtures. Our results indicate that the thus the value of any macroscopic variable does ratio kT/xi is insensitive to the concentration of not change with time at a particular spatial the minor component. Further, at high tem- location. Therefore, the net flux for each species peratures (> 1000 K), kr/xi is relatively indepen- into a given volume element due to convective dent of temperature. Therefore, it is possible to and diffusive transport must be balanced by a select a value of kT/xi which represents a "worst corresponding change in the species concentra- case" for the thermal diffusion contribution to tion due to chemical reactions: the calculated production rate for a given species in order to test the hypothesis that Ri = V[Ni (v + Vi)]. (l) thermal diffusion is not significant. (See next section) Here , Ri is the net chemical rate, Ni is the species concentration, v is the convective veloc- B. Production Rate Calculation Results ity, and Vi is the net diffusion velocity of the species into the local mixture. The rate of Figure 3 presents the diffusive (concentra- chemical production is computed by numerical tion-driven and thermal) and convective contri- differentiation of Eq. 1 using profile data at a butions to the calculated production/destruction particular height, H, as well as data from H --2- 2 rate for acetylene at a height of 9 mm above the mm. Due to extreme sensitivity to noise in this burner. Convection transports material across calculation, it
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