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.     

Reaction of coal with nitrogen in a microwave discharge

In a microwave discharge in nitrogen, high-volatile bituminous coal produces gaseous products, namely hydrogen cyanide, acetylene, small amounts of cyanogen and lowmolecular-weight hydrocarbon gases, and nitrogen-containing compounds, in addition to the usual hydrogen and carbon oxides. The reaction of coal in the nitrogen discharge occurs in two stages: interaction of active nitrogen with the coal molecule to cause rapid volatilization of gaseous products, and slow gasification of residual char by active nitrogen. Various factors influencing product type, yield, and distribution are examined. Under conditions where the gaseous products can be quickly quenched or removed, as by trapping at a very low temperature, more than 42% of carbon in the coal can be converted mainly to hydrogen cyanide and acetylene, which together constitute about 90% of the total products.     

Surface modification of polypropylene film using N2O-containing flames

Contact-angle measurements and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize polypropylene (PP) films that were exposed to laminar premixed air: natural gas flames containing small quantities of nitrous oxide. During combustion, the nitrous oxide generates gas-phase nitrogen oxides that lead to the affixation of nitrogen-containing functional groups to the PP surfaces. Treatment of PP in nitrous oxide-containing flames also leads to an increase in surface oxidation and markedly improves wettability when compared with standard flame treatments. The chemical form of the nitrogen affixed to the PP surface is strongly dependent on the flame equivalence ratio. Fuel-lean flames tend to affix highly oxidized forms of nitrogen such as nitrate and nitro groups, while fuel-rich flames tend to affix less-oxidized nitrogen groups such as nitroso, oxime, amide, and amine. A computational model, SPIN, was used to elucidate the chemistry of the flame as it impinges upon the cooled PP surface. The SPIN modeling indicates that the principal reactive gas-phase species at or near the PP surface are O₂, OH, H, NO, NO₂, HNO, and N₂O. A number of possible reactions between these species and the PP can account for the formation of the various nitrogen functional groups observed.     

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.     

Evolution of toxic gases from heated plastics

A brief investigation has been carried out into the nature and quantity of the toxic gases evolved during the thermal decomposition of polyurethane, urea-formaldehyde, nylon and acrylonitrile in air and in nitrogen. The weight fractions of the polymers evolved as hydrogen cyanide are given, together with the lowest temperatures at which hydrogen cyanide, carbon monoxide, ammonia and nitrogen oxides are evolved. Apparent activation energies for the evolution of hydrogen cyanide and carbon monoxide have been determined. A brief discussion of the experimental data is given.     

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.     

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.     

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.     

The stability of turbulent hydrogen jet flames with carbon dioxide and propane addition

In the study the lift-off, blow-out and blow-off stability limits of hydrogen/propane flames and hydrogen/carbon dioxide flames were tested in three different mixing arrangements. The first was to premix hydrogen with carbon dioxide or propane to form a jet flame. The second was to add the gas as an annular jet around the hydrogen flame. The third was to inject into the centre of the hydrogen flame. Propane and carbon dioxide have the same density but create very different chemical kinetic changes when added to hydrogen flames. The results showed that when premixed with hydrogen, propane is more effective in flame lift-off and blow-out. The analysis of kinetic mechanisms revealed that the propane is the dominating fuel in determining the burning rate of the hydrogen/propane while carbon dioxide mainly acted to dilute the hydrogen/CO₂ mixture. Comparing the three mixing arrangements, the experiments showed that hydrogen flame can be effectively lifted or blown out when gases were in annular flow around the hydrogen flame. The isothermal mixing process of the co-flow configuration was discussed.     

Selective oxidation of hydrogen in rich hydrogen-methane-air flames

Numerical simulation studies of flame propagation using the tracer method have shown that in flames of rich hydrogen, methane, and air mixtures there is selective oxidation of the initial molecular hydrogen. The combustion products contain superequilibrium concentrations of water formed from the initial molecular hydrogen and hydrogen formed from methane in the reaction zone, resulting in superadiabatic temperatures of the flames considered. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 3–11, September–October, 2007.     

Model of Cellular Instability of Flames of Ternary Mixtures

A simple phenomenological diffusive-thermal model of cellular instability of premixed flames of ternary mixtures is developed and presented. The model shows that preferential diffusion can alter stoichiometry of the mixture, i.e., the ratio of the fuel and oxidizer concentrations, and also its effective dilution by an inert. Key parameters of the model are estimated using numerical modeling of burning velocities. Laminar burning velocities are calculated for hydrogen-oxygen-nitrogen, methane-oxygen-nitrogen, and propane-oxygen-nitrogen mixtures. Conditions for the appearance of cellular instability in ternary mixtures are determined and compared with experiments. In good agreement with experimental observations, the diffusive-thermal instability is predicted in hydrogen flames with equivalence ratios φ ≲ 1.45, in lean methane flames with φ ≲ 1.02, and in rich propane flames with φ ≳ 1.03. The magnitude of the change in the local flame velocity due to preferential diffusion is evaluated. It is demonstrated that nitrogen diffuses faster than oxygen in hydrogen-air and methane-air flames, while oxygen diffuses faster than nitrogen in flames of propane and other heavier hydrocarbons. In mixtures of air with propane or heavier hydrocarbons, the transition between stable and unstable regimes is predicted in mixtures that are leaner than the mixture corresponding to the peak of the burning velocity curve, in agreement with experimental observations. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 14–22, September–October, 2005.     

Harmful by-products in selective catalytic reduction of nitrogen oxides by olefins over alumina

The selective reduction of nitrogen dioxide and nitrogen monoxide by olefins (ethene, propene) has been studied over two different -aluminium oxides in the temperature range 473–873 K. Nitrogen dioxide was reduced more effectively than nitrogen monoxide with both, ethene and propene, as a reductant. At temperatures exceeding 700 K, ammonia was formed as a by-product over one type of alumina. Concentrations in the range 30–40 ppm were determined for propene in combination with both, NO and NO₂, while no ammonia was produced with ethene as a reductant. In addition, significant formation of hydrogen cyanide up to 70 ppm was observed with propene over both aluminium oxides starting from either NO or NO₂. In contrast, hydrogen cyanide formation remained below 10 ppm with ethene as a reductant. Nitrous oxide formation did not exceed 10 ppm for all investigations. The results show that for alumina catalysts ethene is a more suitable reductant than propene due to its lower tendency to form undesired by-products.     

Catalytic Partial Oxidation. Part I. Catalytic Processes to Convert Methane: Partial or Total Oxidation

The presence of large reserves of natural gas has stimulated research to utilize methane, its principal component, as an alternative energy source and to convert it to other fuels and industrially important chemicals. The reserves of natural gas in the world are estimated to be 1.4 × 10¹⁴ Nm³, while new gas fields are being discovered every year. Although this natural gas is available under pressure for piping and transport, extensive research efforts have been directed to develop gas-to-liquid (GTL) technology for the conversion of remote natural gas reserves into high-added-value liquid products, such as methanol and synthetic fuels, that can be more easily transported. A further incentive for natural gas utilization originates from environmental concerns that drive the search for cleaner energy sources. Catalytic combustion of methane offers an attractive alternative to gas-phase homogeneous combustion since it can stabilize flames at lower fuel-to-air ratios, thereby lowering flame temperatures and reducing NO_x emission. Another alternative can be found in the conversion of natural gas into hydrogen, which can be used to generate electricity in fuel cells. Fuel cells have a much higher energy efficiency compared to current combustion-based power plants. Also, hydrogen is a much cleaner fuel than hydrocarbon feedstocks since the only product from hydrogen fuel cells is water.     

Interferences in the determination of nitrogen dioxide in a chemiluminescent analyser

A commercial chemiluminescent nitrogen oxides analyzer performed well in its nitric oxide mode when applied to the combustion products of polyurethane materials, but when test gases were passed through a thermal converter prior to measurement of nitrogen dioxide, serious, interferences was encountered from hydrogen cyanide and other nitrogen compounds.     

Skeletal mechanism of inhibition and suppression of a hydrogen flame by addition of trimethylphosphate

A skeletal mechanism of inhibition and suppression of H₂/O₂/N₂ by addition of trimethylphosphate was developed. It includes a mechanism of hydrogen oxidation (13 elementary steps involving 7 species) and two elementary reactions involving trimethylphosphate and its conversion products. This skeletal mechanism adequately predicts the burning velocity of flames with added inhibitor in the range of equivalence ratios studied, and can be used to model fire suppression.     

Calculation of Decomposition Products from Components of Gunpowder by using ReaxFF Reactive Force Field Molecular Dynamics and Thermodynamic Calculations of Equilibrium Composition

The major combustion products from munitions containing nitro‐based propellants are water, carbon monoxide, carbon dioxide, hydrogen, and nitrogen. In addition, compounds including hydrogen cyanide, ammonia, methane, nitrogen oxides, benzene, acrylonitrile, toluene, furan, aromatic amines, benzopyrene, and various polycyclic aromatic hydrocarbons are detected in minor concentrations. The literature shows that the thermodynamic prediction of the major decomposition products agrees fairly well with the measurements. However, poor agreement is found for the minor species. We have studied the thermal decomposition products of the main gunpowder ingredients. Each of the components nitrocellulose, nitroglycerine, and the nitrate ester stabilizers diphenylamine and ethyl centralite were thermally decomposed with ReaxFF reactive force field molecular dynamics and equilibrium thermodynamics. The molecular dynamics results for the major decomposition products from nitrocellulose were partly consistent with measurements. Compared to the thermodynamic calculations, the molecular dynamics simulations agreed considerably better with experimental results for minor species like hydrogen cyanide. The nitrate ester stabilizers are the main sources for ammonia and aromatic combustion products, whereas hydrogen cyanide is produced from nitrocellulose as well as from the stabilizers when gunpowder is combusted.     

湍流扩散火焰中氮氧化物排放的数值研究

对一系列具有不同Reynolds数的湍流甲烷射流扩散火焰中氮氧化物的排放进行了数值模拟,以考察湍流对氮氧化物排放的影响.采用一个双尺度的k-ε湍流模型计算速度场,将概率密度函数（PDF）方法和Lagrangian火焰面模型相结合求解标量场,燃料甲烷的氧化过程和氮氧化物的生成过程采用详细的GRI-Mech 3.0机理描述.将氮氧化物的计算结果和实验数据进行了比较,并探讨了Reynolds数、标量耗散和火焰面时间对氮氧化物排放的影响.发现计算结果和实验数据符合较好,计算模型正确预测了氮氧化物的生成量随着Reynolds数的增加而减少的趋势.     

Catalytic effect of submicron TiO<Subscript>2</Subscript> particles on the methane–air flames speed

A PIV study of a conical premixed methane–air Bunsen flame has shown that the inside of the cone has a complex gas-dynamic structure. In this system, the velocity of the gas flow entering the flame front varies in different parts of the flame cone and the stream tubes are not straight. The Landau–Markstein effect is discussed in the interpretation of the experimental data. A method of processing PIV measurement results is proposed that improves the accuracy of determining the burning velocity and allows a quantitative determination of the catalytic effect of submicron TiO₂ particles, which is proportional to the particle surface area. The relative increase in the burning velocity is 2% per each ≈0.01 cm²/cm³ (particle surface/gas volume) of the total specific surface area of the particles. The experimental data are well described by modeling using well-known literature data on the detailed mechanism of chemical reactions and the mechanism of catalytic oxidation of methane with oxygen on metal oxides.     

Composition of gaseous combustion products of polymers

The gaseous products generated by the flaming combustion of ten kinds of synthetic polymers and a kind of wood (cedar) under the same conditions (sample weight, 0.1 g; temperature, 700°C air flow rates, 50 and 100 l./hr) were quantitatively analyzed by infrared spectrophotometry, gas chromatography, and colorimetric tube method. The main hydrocarbons generated were methane, ethylene, and acetylene. The amount of acetylene generated by the flaming combustion of polymers was much larger than the amount of acetylene formed by pyrolysis at 700°C in nitrogen. Acetylene increased in quantity with increasing air. For nitrogen compounds, hydrogen cyanide was generated from every polymer containing nitrogen used, but ammonia was detected only for nylon 66 and polyacrylamide. Nitrogen monoxide and nitrogen dioxide were detected only in small amounts. Nitrous oxide was detected in the gaseous products generated by the nonflaming combustion of urea resin and melamin resin. It was also found that about 70% of the nitrogen in N-66 and PAA was converted into nitrogen gas (N₂) by combustion under the conditions described above.     

Inhibition of various hydrogen combustion regimes in air by Propylene and Isopropanol

This paper studies the effects of small additives of propylene and isopropanol on the hydrogen-air flame speed in the predetonation regime, deflagration-to-detonation transition, and burning rate. It is shown that the difference in the effects of these additives on the combustion is determined primarily by their ability to terminate reaction chains. In hydrogen flames, the additives are consumed as a result of their reactions with the active intermediate products of H₂ combustion in which these species are replaced by inactive radicals.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 3–14, January–February, 2005.