Time-of-flight SIMS analysis of polypropylene films modified by flame treatments using isotopically labeled methane fuel

The surface of polypropylene (PP) film was oxidized by exposure to a flame fueled by isotopically labeled methane (CD₄). The isotopic sensitivity of static secondary ion mass spectrometry (SIMS) was then used to gain new insights into the mechanism of flame treatment. SIMS analysis indicated that much of the oxidation of PP occurring in fuel-lean flames is not deuterated, while for PP treated in fuel-rich flames, some of the affixed oxygen is deuterated. These observations imply that O₂ is the primary source of affixed surface oxygen in fuel-lean flame treatments, but that OH may be a significant source of affixed oxygen in fuel-rich flame treatments. Hydroxyl radicals are primarily responsible for hydrogen abstraction in fuel-lean flames, while H is the primary active gasphase species in fuel-rich flames. SIMS also detected trace quantities of oxidized nitrogen groups affixed to the flame-treated PP.     

NO – oxygen scavenger or reaction intermediate in the decomposition of N2O?

Nitric oxide and nitrogen dioxide were found during the thermal desorption of surface species left on Fe-ferrierites after the decomposition of nitrous oxide. This demonstrates the formation of surface NOx species during N₂O decomposition. Repeated decomposition and subsequent desorption of surface species confirm the active role of surface NOx species. Addition of NO up to a fraction of 0.1 times the amount of N₂O increased the decomposition of nitrous oxide as well as the amount of surface NOx species. The use of nitrous oxide labeled with ¹⁸O demonstrated that the zeolite oxygens participate in the reaction and that the presence of NO enhances this participation.     

Experimental and numerical study of CH4/CH3Cl/O2/N2 premixed flames under oxygen enrichment

A comprehensive experimental and numerical study has been conducted to understand the influence of CH₃Cl addition on CH₄/O₂/N₂ premixed flames under oxygen enrichment. The laminar flame speeds of CH₄/CH₃Cl/O₂/N₂ premixed flames at room temperature and atmospheric pressure are experimentally measured using the Bunsen nozzle flame technique with a variation in the amount of CH₃Cl in the fuel, equivalence ratio of the unburned mixture, and level of oxygen enrichment. The concentrations of major species and NO in the final combustion products are also measured. In order to analyze the flame structure, a detailed chemical kinetic mechanism is employed, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. The flame speeds predicted by this mechanism are found to be in good agreement with those deduced from experiments. Chlorine atoms available from methyl chloride inhibit the oxygen-enhanced flames, resulting in lower flame speeds. This effect is more pronounced in rich flames than in lean flames. Although the molar amount of CH₃Cl in the methane flame is increased, the temperature at the post flame is not significantly affected, based on the numerical analysis. However, the measured concentration of NO is reduced by about 35% for the flame burning the same amount of methyl chloride and methane at the oxygen enrichment of 0.3. This effect is due to the reduction of the concentration of free radicals related to NO production within the flame. In the numerical simulation, as CH₃Cl addition is increased, the heat flux is largely decreased for the oxygen-enhanced flame. It appears that the rate of the OH + H₂ → H + H₂O reaction is reduced because of the reduction of OH concentration. However, the function of CH₃Cl as an inhibitor on hydrocarbon flames is weakened as the level of oxygen enrichment is increased from 0.21 to 0.5. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 103–111, November–December, 2006.     

Conversion of fuels containing nitrogen to oxides of nitrogen in hydrogen and methane flames

Nitrogen-containing compounds such as hydrogen cyanide, acetonitrile, acrylonitrile, pyridine, benzonitrile, ammonia and methylamine, which are typical of the products likely to be encountered during the decomposition of nitrogen-containing polymers in fires, have been introduced into hydrogen and methane flames burning in oxygen-argon atmospheres. There is a complete conversion of fuel nitrogen in all cases to oxides of nitrogen and molecular nitrogen. The relative conversion to oxides of nitrogen (as NO_x/N₂) increases as the injection rate of nitrogen-containing fuels is decreased. The relative yields of oxides of nitrogen tend to be similar with methane and hydrogen premixed flames and markedly greater than observed with hydrogen diffusion flame. In all cases the yield of oxides of nitrogen-containing products such as hydrogen cyanide can also present a toxic risk during the burning of nitrogen-containing polymers, particularly when high temperature are involved. The combustion of these products in flame zones cannot be assumed to alleviate the additional toxic risk because of their conversion to oxides of nitrogen.     

The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

The CuO/Al₂O₃ system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al₂O₃ is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N₂ or N₂O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N₂O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N₂ formation.     

The formation of NOx from nitrogen containing materials in diffusion flames

The conversion of fuel-N species to NO_x in diffusion flames has been studied by adding acetonitrile to the fuel flow for a methane–Oxygen–argon diffusion flame burning in excess oxygen. It is shown that the conversion is significantly lower than that obtained in a ‘corresponding’ pre-mixed flame, although the observed concentrations of NO_x are still much higher than the appropriate thermodynamic equilibrium concentrations. The effect of initial concentration of acetonitrile, flame temperature and amount of excess oxygen on the conversion can all be explained in terms of the basic structure of a diffusion flame. This enables molecular nitrogen to be formed in the reducing atmosphere which exists on the fuel side of the flame through reaction of cyanide radicals with nitric oxide. The latter diffuses back from the oxygen side of the flame where it is formed, but the overall result is that a proportion of the fuel-N is converted to molecular nitrogen before it can be converted to nitric oxide.     

Production of nitrogen compounds from molecular nitrogen in fuel-rich hydrocarbon-air flames

Measurements of HCN, NH₃ and NO concentrations in the burnt gases of a number of fuel-rich hydrocarbon-air flames have been made. Only HCN and/or NO leave the primary reaction zone of all flames studied, while NH₃ is formed later in the burnt gases. The behaviour of these species in the post-flame zone is qualitatively similar to that observed in flames seeded with fuel-nitrogen. The generation of nitrogen compounds from molecular nitrogen is obviously due not only to the Zeldovich mechanism but also to some as yet unidentified hydrocarbon-N₂ reaction. The characteristics of this reaction are that it begins to dominate in flames richer than φ = 1.2, and that the total amount of combined nitrogen generated increases only slowly as the flame is made richer, even though the distribution of this nitrogen amongst HCN, NH₃ and NO varies markedly.     

CH,NH, and NH2 Concentration Profiles in Methane/Air Flames Doped with N2O

Intracavity Laser Absorption Spectroscopy (ICLAS) and Cavity Ring-Down Spectroscopy (CRDS) were used to measure concentration profiles of the CH, NH, and NH₂ radicals in a low-pressure (30 Torr) stoichiometric methane/oxygen/nitrogen flat flame doped with a small amount of nitrous oxide (1.7%). Concentration profiles of the CH and NH radicals were measured by CRDS, whereas the NH₂ radical profile was measured by ICLAS. Temperature profiles were obtained using measured CRDS spectra of the OH radical. The radical absorption spectra were recorded with good signal-to-noise ratio. For the first time, absolute concentrations of NH and NH₂ were measured in flames of this kind. The obtained experimental profiles and peak amounts agree well with model predictions based on GRI-Mech 2.11 (Gas Research Institute Mechanism). The mechanism also describes well the observed decreasing CH concentration with addition of N₂O.     

Probe sampling measurements and modeling of nitric oxide formation in ethane + air flames

Burning velocity and probe sampling measurements of the concentrations of O₂, CO₂, CO and NO in the post-flame zone of ethane + air flames are reported. The heat flux method was used for stabilization of laminar, premixed, non-stretched flames on a perforated plate burner at 1 atm. Axial profiles of the concentrations of the major species were used to assess interaction of the flame with the burner surface and conversion of the sampling gases in the probe. Tests performed with the probes of different inlet diameters showed negligible CO–CO₂ and NO–NO₂ conversion within the experimental accuracy. Two kinetic models, the GRI-mech. 3.0 and in-house modified detailed reaction mechanism, were tested. Both kinetic mechanisms accurately reproduce laminar burning velocities and concentrations of the major species, CO, CO₂ and O₂, in these flames. Numerical predictions of the concentrations of NO in a post-flame zone of lean and stoichiometric flames are in good agreement with experiment when downstream heat losses to the environment were taken into account. The GRI-mech. 3.0 over-predicts the [NO] by about 30 ppm at the equivalence ratio of 1.4. The predictions of the in-house mechanism in rich flames are closer to the experimental data with an under-prediction of about 15 ppm. The influence of the assumed temperature gradient downstream the flame front on the calculated flame structure was also assessed.     

Comparison of the structures of methane-air and propane-air partially premixed flames

A comparison of flame structures between methane-air and propane-air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O₂, CO and CO₂ and those of the intermediate hydrocarbons like C₂H₆, C₂H₄, C₂H₂ and CH₄ (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C₂H₆ in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C₂H₄ and C₂H₂ are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO₂. Similar distributions of CO and CO₂ are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.     

Flame surface modification of polypropylene film

Contact-angle measurements, the ASTM standard wetting test for polyolefin films, and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize flame-treated polypropylene (PP) films. Two combustion models, STANJAN and PREMIX, were then used to determine the chemical and physical properties of the flames used to treat the PP films. Both the flame equivalence ratio and the position of the PP film in the flame are important variables in determining the extent of oxidation and improvement in wettability obtained by flame treating. The optimal equivalence ratio for the flame treatment of PP is 0.93, while the optimal luminous flame-to-film distance is 0-2 mm. Modeling of the combustion processes occurring in the flame provides evidence that the extent of treatment correlates closely with the concentrations of H, O, and OH radicals present in the flame. The extent of surface modification of the flame-treated PP does not appear to correlate with either the flame temperature or the concentraion of oxygen molecules. The mechanism of surface oxidation by flame treatment probably involves polymer-radical formation by O and OH, followed by rapid reaction of the polymer radicals with O, OH, and O₂.     

Experimental study of the structure of laminar premixed flames of ethanol/methane/oxygen/argon

The structures of three laminar premixed stoichiometric flames at low pressure (6.7 kPa): a pure methane flame, a pure ethanol flame, and a methane flame doped by 30% of ethanol, have been investigated and compared. The results consist of mole fraction profiles of CH₄, C₂H₅OH, O₂, Ar, CO, CO₂, H₂O, H₂, C₂H₆, C₂H₄, C₂H₂, C₃H₈, C₃H₆, CH₃-C CH (propyne), CH₂ C CH₂ (allene), CH₂O, and CH₃HCO, measured as a function of the height above the burner by probe sampling followed by on-line gas chromatography analyses. Flame temperature profiles have been also obtained by using a PtRh thermocouple. The similarities and differences between the three flames have been analyzed. The results show that, in these three flames, the mole fraction of the intermediates with two carbon atoms is much larger than that of the species with three carbon atoms. In general, the mole fraction of all intermediate species in the pure ethanol flame is the largest, followed by the doped flame, and finally the pure methane flame.     

Molecular-beam mass-spectrometric study of the flame structure of composite propellants based on nitramines and glycidyl azide polymer at a pressure of 1 MPa

A study was performed of the chemical and thermal structure of flames of model composite propellants based on cyclic nitramines (RDX and HMX) and an active binder (glycidyl azide polymer) at a pressure of 1 MPa. Propellant burning rates were measured. The chemical structure of the flame was studied using molecular-beam mass spectrometry, which previously has not been employed at high pressures. Eleven species (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂, NO₂, and nitramine vapor) were identified, and their concentration profiles, including the composition near the burning surface were measured. Two chemical-reaction zones were observed. It was shown that flames of nitramine/glycidyl azide polymer propellants are dominated by the same reactions as in flames of pure nitramines. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 48–57, November–December, 2006.     

On the nature of the burning rate-controlling reaction of energetic materials for the gas-phase model

For model systems with known kinetics of elementary reactions (CH₃NO₂ and HN₃), temperature ranges are established in which the rate-controlling reactions are the initial endothermic decomposition of the starting material or the subsequent secondary reactions. Heat release in reactions of NO₂, NO, and N₂O with various fuels, such as CH₂O, CO, H₂, and HCN, is modeled to establish the kinetic parameters and nature of the rate-controlling reactions in gas flames of nitro compounds. It is shown that the activation energy of the heat-release reaction due to the interaction of NO₂ with a hydrocarbon fuel (which is characteristic of the first flame of nitro compounds) is in the range of 29–33 kcal/mole, depending on the type of fuel. According to the calculations performed, the activation energy of the rate-controlling heat-releasing process due to the deoxidation of NO and N₂O (which is typical of the second flame of nitro compounds) is 43–58 kcal/mole. In the range of high pressures, where the flames merge, the kinetic parameters of heat release are determined by the reactions of the most reactive nitrogen oxide NO₂. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 59–71, May–June, 2007.     

Effect of the addition of triphenylphosphine oxide,hexabromocyclododecane, and ethyl bromide on a CH<Subscript>4</Subscript>/O<Subscript>2</Subscript>/N<Subscript>2</Subscript> flame at atmospheric pressure

The chemical and thermal structure of a Mache-Hebra burner stabilized premixed rich CH₄/O₂/N₂ flame with additives of vapors of triphenylphosphine oxide [(C₆H₅)₃PO], hexabromocyclododecane (C₁₂H₁₈Br₆), and ethyl bromide (C₂H₅Br) was studied experimentally using molecular beam mass spectrometry (MBMS) and a microthermocouple method. The concentration profiles of stable and active species, including atoms and free radicals, and flame temperature pro.les were determined at a pressure of 1 atm. A comparison of the experimental and modeling results on the flame structure shows that MBMS is a suitable method for studying the structure of flames stabilized on a Mache-Hebra burner under near-adiabatic conditions. The relative flame inhibition effectiveness of the added compounds is estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the flame propagation velocity. The results of the investigation suggest that place of action of the examined flame retardants is the gas phase. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 12–20, September–October, 2007.     

Investigation of the synergistic effect of red phosphorus and magnesium hydroxide on the thermal degradation behavior and flame resistance of the intumescent fire-retardant polypropylene system

High fire-resistance polypropylene (PP) composites were prepared by using environment-friendly flame retardants including expandable graphite (EG), red phosphorus (RP), and magnesium hydroxide (MH). Synergism between EG, RP, and MH on the thermo-oxidation behavior and flame resistance of PP was found. The incorporation of MH and RP formed highly thermally stable mixtures of magnesium phosphates consisting of Mg₃(PO₄)₂, Mg(PO₃)₂, and α-Mg₂P₂O₇ at both amorphous and crystalline phases in the burning process. The mixture not only covered the surface of burning materials but also could reinforce the char structure of the PP/EG composites, thereby significantly enhancing the condensed phase flame retardant mechanism of the composites. Mass ratios of the flame retardants were also optimized to obtain the composite with the highest flame retardant efficiency. The result revealed that the combination of EG, RP, and MH in PP at MH/RP mass ratio of 3/2 with only a total additive content of 18 wt.% could make its limiting oxygen index (LOI) increase from 16.8% to 27.2% and the UL-94 rating was improved from none to V-0. In addition, the mechanical properties of the composites were improved via the surface treatment of MH and RP with calcium stearate and silicone oil, respectively.     

Enhancement of nitric oxide removal by ammonia on a low-rank coal based carbon by sulphuric acid treatment

This work presents a study of the effect of wet sulphuric acid treatment and gas-phase treatment with SO₂ + O₂ + H₂O on the catalytic activity of a low-rank coal-based carbon for the nitric oxide reduction with ammonia. Carbons were characterized by N₂ adsorption, TPD, and FTIR in order to assess how the surface chemistry and the texture of carbons change after the treatments. A great amount of oxygenated functional groups both CO₂ and CO evolving ones are produced by liquid-phase sulphuric acid treatment. However, the amount of those groups after gas-phase treatment with SO₂ + O₂ + H₂O is lower, in particular the CO₂ evolving groups. The catalytic activity of carbons was examined in a fixed bed reactor at 150 °C in a gas flow containing NO, O₂, N₂ and NH₃, the effluent concentration being monitored continuously during the reaction. The obtained results indicate that an appropriate balance between the type of oxygen functional groups and surface area available to the reactant gas are required to reach high levels of NO conversion.     

Laminar burning velocities of three C3H6O isomers at atmospheric pressure

Laminar flames of three C₃H₆O isomers (propylene oxide, propionaldehyde and acetone), representative of cyclic ether, aldehyde and ketone species important as intermediates in oxygenated fuel combustion, have been studied experimentally and computationally. Most of these flames exhibited a non-linear dependency of flame speed upon stretch rate and two complementary independent techniques were adopted to provide the most reliable burning velocity data. Significant differences in burning velocity were noted for the three isomers: propylene oxide + air mixtures burned fastest, then propionaldehyde + air, with acetone + air flames being the slowest; the latter also required stronger ignition sources. Numerical modelling of these flames was based on the Konnov mechanism, enhanced with reactions specific to these oxygenated fuels. The chemical kinetics mechanism predicted flame velocities in qualitative rather than quantitative agreement with the measurements. Sensitivity analysis suggested that the calculated flame speeds had only a weak dependency upon parent fuel-specific reactions rates; however, consideration of possible break-up routes of the primary fuels has allowed identification of intermediate compounds, the chemistry of which requires a better definition.     

Laminar burning velocities and Markstein numbers of syngas-air mixtures

In the currently reported work, three typical mixtures of H₂, CO, CH₄, CO₂ and N₂ have been considered as representative of the producer gas coming from wood gasification. Laminar burning velocities have been determined from schlieren flame images at normal temperature and pressure, over a range of equivalence ratios within the flammability limits. The study of the effects of flame stretch rate was also performed. Combustion demonstrates a linear relationship between flame radius and time for syngas-air flames. The maximum value of syngas-air flame speeds is observed at the stoichiometric equivalence ratio, while lean or rich mixtures have lower flame speeds. The higher is the syngas heat value the higher is the laminar burning velocity of the syngas mixture. Markstein numbers show that typical syngas-air flames are generally unstable. Karlovitz numbers indicates that typical syngas-air flames are little influenced by stretch rate. Based on the experimental data, a formula for calculating the laminar burning velocities of syngas-air flames is proposed. The magnitude of laminar burning velocity for typical syngas compositions is comparable to that of a simulated mixture comprising 5% H₂/95% CO and proved to be similar to methane, although somewhat slower than propane.     

Investigation of the structure of a CH4/N2-O2/N2 Counterflow diffusion flame using molecular beam and microprobe mass spectrometry

The possibility of using molecular beam mass spectrometry (MBMS) to study the structure of counterflow flames was shown by the example of a CH₄/N₂-O₂/N₂ flame. The thermal structure of the flame was studied, and the CH₄, O₂, and CO₂ concentration distributions were measured using a microprobe technique and MBMS. The results of the measurements performed by the two methods were compared. The MBMS technique was used to study the hydroxyl concentration distribution in the flame. The species concentrations and temperature profiles on the burner axis were calculated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 26–33, July–August, 2006.