C.P. Fenimore

Reactions of fuel-nitrogen in rich flame gases

Abstract Fuel-nitrogen fed as ammonia or as pyridine to rich flames is mainly present in the burnt gas in the forms HCN, NH3, NO and N2. The HCN decays in this region to form more NH3, and the NH3(or species equilibrated with NH3)undergoes two types of reactions: an oxidation to NO at rate R1, reaction with NO to form N2 at rate R2. The total remaining fuel-N not yet converted to NO or N2, i.e. RN = HCN + NH3, therefore decays in accordance with -d[RN] dt =R1 + R2 , and the simultaneous change in NO is -d[NO] dt =R1 − R2 , Empirically, these rates can be epxressed R 1 5×10 12 [ H 2 O ] 2 [ NH 2 ] [ H 2 e −20 kcal /RT mole cc,sec , R 2 = 9×10 12 [ NO ] [ NH 2 . R1 is not readily interpreted in terms of elementary reactions; R2 is the rate of NO + NH2 → N2 + H2O. The findings are not necessarily valid in the primary reaction zone where other processes also occur. To the extent that all the fuel-N added to rich flames attains NO and N2 in accordance with R1 and R2, however, the yield of NO is predictable a priori. The prediction agrees roughly with the yields observed.

Superequilibrium and thermal nitric oxide formation in turbulent diffusion flames

Abstract Measurements and modeling of the formation of superequilibrium radicals and nitric oxide in atmospheric pressure turbulent jet diffusion flames are presented which quantify the influence of superequilibrium on thermal NO x formation. Variation of fuel gas compositions (CO/H 2 /N 2 , CO/H 2 /CO 2 , and CO/H 2 /Ar) permits partial separation of chemical and fluid mechanical effects. Superequilibrium OH radical concentrations are measured by single-pulse laser saturated fluorescence and NO and NO 2 concentrations by probe sampling and chemiluminescent detection. Four different types of probes were used to quantify probe sampling effects. In turbulent reaction zones, virtually all of the NO x in the flame occurred in the form of NO but far downstream of the flame nearly half of the NO x occurred as NO 2 . Thermal NO x maximized near stoichiometric flame zones; the rich shift observed by others may be a probe sampling artifact. In turbulent CO/H 2 /N 2 jet diffusion flames, both measurements and a nonequilibrium turbulent combustion model show that superequilibrium decreases average temperatures by 250K, increases average OH concentrations by a factor of 4–6, and increases thermal NO x formation principally by broadening the range of mixture fraction (both rich and lean) where thermal NO x is formed. Calculated increases in thermal NO x due to superequilibrium in turbulent CO/H 2 /N 2 jet diffusion flames are factors of 2.5 at 1 atm and 1.4 at 10 atm. The two-scalar pdf model predicts that thermal NO x yield is independent of Reynolds number in disagreement with previous experimental reports.

Formation of nitric oxide from fuel nitrogen in ethylene flames

The yield of nitric oxide from small additions of various nitrogen compounds was measured in premixed ethylene flames of mixture strengch= 0.9 to 2.0 and of temperature = 1860 to 2250°K. The additives were pyridine, methacrylonitrile, methyl amine, or ammonia, and with any of these the observations satisfied the equation [ NO ] X = 1 · e x p ( − [ N ] + [ NO ] 2 X ) where [N]= nitrogen compound added, espressed as ppm NO if all N formed NO. [NO] = ppm NO formed from the additive. X = a parameter characteristic of the flame and independent of [N]. The parameter X is a normalizing factor for [N], the yield [NO]/[N]being the same in all flames at equal [N]/X. For small [N]/X. [NO] = [N]; for large [N]/X, [NO] = X and is independent of [N] The values of X are larger the hotter and the less fuel-rich the flame, and are consistent with the identification X = (2–3) [OH] where [OH] represents ppm hydroxyl radicals in the flame. A possible interpretation of the experiments is that all the additives gave the same nitrogenous intermediate which reacted 2–3 times faster with OH than with NO to form and to destroy NO respectively. The equation also fits test stand data obtained in an oil spray, turbulent diffusion flame at 5 atm pressure with an overail fuel/air wt ratio of 0.021. The yields of NO from fuel nitrogen agree with the equation if this particular diffusion flame possessed an X parameter approptrate to an adiabatic premixed flame of mixture strength= 1.5–1.6.

Flammability of polymers

The flammability of polymers has been measured by determining the oxygen content of the atmospheres just capable of burning them. The effects of the following three commonly used agents for reducing flammability have been compared in polyethylene. (1) A moderate degree of inhibition is conferred by antimony trioxide plus sufficient chlorine: the maximum effect requires about 0·01 antimony atoms per C 2 group in the polyethylene, and develops just as well when chlorine/antimony is six as when it is twenty. The full effect of the antimony does not develop when chlorine/antimony is two. (2) Various phosphorus compounds are all of similar effectiveness per atom of phosphorus added to polyethylene. (3) With sufficient substitution of hydrogen by halogen in polyethylene, a greater inhibition is achieved than is possible by (1) or (2). We have no evidence how (1) and (2) inhibit burning, but present evidence is that in (3) a large substitution of chlorine works mostly by affecting the degradation of the polymer rather than by interfering with gas phase flame reactions.

Modes of inhibiting polymer flammability

Evidence is presented that burning polymethyl methacrylate and polyoxymethylene do not react chemically with the gas around them, but merely vaporize in the heat of the surrounding diffusion flames. The pyrolysis products oxidize subsequently in the flames. Polyethylene probably burns similary, but Teflon may react directly with oxygen. The two-stage burning of polymers which gasify merely by heating might be inhibited at either stage—in the condensed phase, or in the flame—and the mode of action of some known inhibitors is investigated. It is shown that chlorine substituted in polyethylene inhibits by affecting the pyrolysis of the condensed phase, but the pair of reactants, antinomy plus a little chlorine, poisons the flame. Bromine is more effective than chlorine because bromine also poisons the flame.

Coagulation of soot to smoke in hydrocarbon flames

Abstract The soot cloud in a luminous diffusion flame contains carbon monoxide and other unburnt gases, and both soot and gases oxidize when additional oxygen diffuses into the flame. According to the literature, the relative oxidation rates satisfy d log ⁡ [ s o o t ] d log ⁡ [ C O ] ⋝ 0 ⋅ 5 × 10 − 6 D where D is the diameter of the soot particles in centimetres. Particles with D ⋍ 1 × 10−6cm (a size often observed in flames) should burn up with the gases. In order to survive the oxidation of the gases and give smoky flames, the particles must have considerably larger diameters. Plots of log [soot] versus log [CO], as both oxidized, were made for flames containing various mole fractions of soot. Mole fraction was defined as atoms of carbon precipitated as soot per gas molecule in the soot cloud. At sufficiently large mole fractions, the oxidation rate of soot decreased relative to the oxidation rate of CO until, eventually, soot survived to give smoky flames. This was taken as evidence for coagulation to larger particles the greater the mole fraction. Increasing the pressure increased the mole fraction of soot precipitated, and also increased the coagulation of a given mole fraction. A simple calculation suggests: (1) When the log/log plots just failed to give evidence for coagulation, soot particles did not exceed 2·2 × 10−6cm diameter. (2) When the flames first became smoky, the particles could have grown as large as 6 × 10−6cm diameter.

Phosphorus in the burnt gas from fuel-rich hydrogenoxygen flames

Abstract Green bands, attributed to HPO by Lam Thanh My and Peyron, are emitted from the burnt gas of fuel-rich flames containing a trace of trimethyl phosphate, but not from lean flames. Their intensity is proportional to [ phosphorus ]0·1±0·1 [ H ] 2 [ H 2 O ]0·5±0·5 [ H 2 ]0·1±0·5 exp {− (5±5) κcal/RT} where [phosphorus] represents the addition of trimethyl phosphate. The dependence on [H]2 is good to the nearest integral power. The uncertainty by the factor { [H 2 O] [H 2 ] } 0·5 reflects the small change in this quantity in most of the experiments. An interpretation is offered which is consistent with the identification of the emitter as HPO: that the flame intensity is proportional to [H] [PO], and that the phosphorus is present mostly as P2 molecules which are equilibrated according to: P +O-= P + PO P + OH = PO + H The flame result can also be interpreted by an intensity proportional to [H] [P] with the same equilibria among P2, PO and P. On either interpretation, the phosphorus must be considered to be mainly present as P2 molecules.

Destruction of NO by NH3 in lean burnt gas

Abstract In lean gas at 1100 T 3 is consistent with the mechanism NH 3 +OH→NH 2 +H 2 Owith k 6 of order 3 × 10 11 cc/mole·sec,where [NO] c is a critical concentration below which NO cannot be reduced by injecting NH 3 . The limiting [NO] c increases with temperature because [OH] increases-and this limits the control of NO emissions by NH 3 injection to temperatures which are not too high. The control is limited to temperatures which are not too low by the rate of formation of NH 2 -i.e., by the rate of (6) in the present work.

Decomposition of sulphur hexafluoride in flames by reaction with hydrogen atoms

Abstract A little SF 6 was added to low pressure flames of H 2  O 2  Ar , C 2 H 2  O 2  Ar , or H 2  N 2 O  Ar , and its rate of reaction and the concentrations of hydrogen atoms estimated by probing the reaction zones. The measurements are consistent with a decomposition controlled by the process H+SF 6 → k HF+SF 5 k=2×10 15 e−()30±5) kcal/RT cm 3 mole−1 sec−1 The subsequent reactions are unknown. On the grounds that SF 6 inhibits the fuel-rich H 2 -air flame, it is suggested that the subsequent steps do not regenerate the hydrogen atom which is consumed in the initial step.

Flame inhibition by methyl bromide

Abstract Burdon et al. proposed that at the flammability limit of methyl bromide-hydrogen-air mixtures, the generation of free radicals by (1), H+O 2 →OH+O, was cancelled by an equal occurrence of (2), H+CH 3 Br→CH 3 +HBr; and an approximate value of K 2 can be deduced on this interpretation. By probing flames burning on a cooled porous plate at low pressure, an independent estimate of k2 has been obtained, K 2 =1·4X10 13 cm 3 mole −1 sec −1 at 1900°K, which agrees within a factor of two with the k 2 from flammability limits. It is concluded that the interpretation of the latter was approximately valid, though doubtless not exact. Methyl bromide is not an effective inhibitor in nitrous oxide flames where [H] is only about the equilibrium concentration, but is in oxygen flames where radical concentrations are greater than equilibrium.The excess [H] is reduced in the presence of the inhibitor and a higher reaction temperature is necessary to maintain the same burning velocity. The methyl radical must be responsible for a large part of the effect, but hydrogen bromide also plays a role. In the course of the work, the rate constant was estimated for the reaction CH 3 + NO → CH 3 NO .The value is consistent with lower temperature results but does not prove whether the reaction is really bimolecular or termolecular in flames.