The kinetic nature of sulfur’s chemistry in flames

Abstract Radi et al. recently presented the first quantitative multiplexed measurements of S2 and OH concentrations in flames using degenerate four-wave mixing (DFWM). Although the absorption-calibrated OH measurements were in agreement with expectations, very severe discrepancies were reported for the indirectly calibrated S2 densities. Magnitudes ranged up to 236-fold larger than predicted. The researchers have questioned the adequacy of previous flame work and the current kinetic model of the mechanisms of sulfur’s flame chemistry. The implications of their suggestion are extensive and severe, and their result unexpected. They have suggested that the kinetic modeling is either incomplete, or that an additional species such as NS may be responsible. As a consequence, a reexamination of their work and all previous studies in H2, C3H8, and CH3OH flames has been made. The resulting conclusions are that it will be very difficult to modify the kinetic model of sulfur in flames to encompass such results. Suggestions that other species such as NS may be playing a significant role are shown to have little merit. In addition, an analysis of potential roles for OCS, CS, CS2, and one recently suggested for HCS in fossil-fueled flames also indicates these to be very minor. A closer examination of the recent DFWM measurements implies various other disquieting aspects. One is that the reported S2 densities are essentially close to or above expected flame equilibrium values. Numerous independent measurements all agree that S2 concentrations are depressed by flame nonequilibrium, and increase with downstream time as flame radical concentrations relax towards their equilibrium values. In the present case, however, measured OH concentrations still are in the range of 42- to 10-fold above their equilibrium values. A second aspect of concern is that the measurements imply an insensitivity to S2, recording levels at several hundreds of ppm with some difficulty. Other DFWM measurements, for example, with CH, C2, CN, NO, and OH, all report sensitivities down to a few ppm 3 , 4 , 5 , 6 , 7 , 8 . Even CH3 can be measured in flames at levels of 65–70 ppm 9 , 10 . The fact that under saturation conditions, DFWM intensities fall off as the square of the concentration indicates the severity of these differences. On the other hand, the degree of theoretical understanding now is quite sound for DFWM, and the levels of approximation involved would not appear to conceivably introduce the magnitude of change that is required to bring this body of data together. The DFWM spectroscopy involved, per se, appears to be reasonable and valid. An indirect calibration method, however, is used to scale the S2 intensities, and has never been validated. One plausible explanation that is proposed herein relates to the very efficient collision free coupling that occurs between the S2(B3Σu−) and S2(B″3Πu) states. There is evidence in the literature that the latter long-lived state can act as a pseudometastable state. Instantaneous depletion of the pumped S2(X3Σg−, v = 2) into this state would modify the data by lowering the resultant values. If correct, this appears to be the first such reported interference with DFWM monitoring. A review of the current status of our understanding of the behavior of the major sulfur species in flames indicates that any possible need for modification should be only minor. Major remaining uncertainties, which cannot noticeably perturb the sulfur chemistry itself, center on the exact nature of the mechanisms by which sulfur modifies NOx formation and, to a lesser extent, to reexamine the validity of the exact mechanisms involved in the catalytic flame radical recombination cycles. Definitive studies of these have yet to be done.