Chain ignition of propane-air and pentane-air mixtures in a heated vessel

The spatial propagation of the chain ignition of propane-air and pentane-air mixtures with oxygen at a pressure of 1 atm and T = 600–800 K is studied. It is established that the features of the spatial propagation of chain ignition process are determined by the conditions of the reactor’s surface. It is shown that the site (or sites) of ignition are located on the surface of the reaction vessel; the flame front propagates from the site into the volume at a normal speed corresponding to the reactor temperature and the composition of the combustible mixture.     

Initial Pyrolysis Reactions in Unmodified and Flame-Retardant Cotton

Abstract

Thermogravimetric data were used to make Arrhenius plots for the vacuum pyrolyses of unmodified cotton and of cotton finished with various add-ons of THPOH-NH3 and THPS-urea-Na₂HPO₄ flame retardants. These plots show that all pyrolyses occurred in consecutive stages: The first and second initial stages, associated with the less ordered regions of the cotton fibers, and the main cellulose pyrolysis reaction, associated with cellulose crystallites. Cotton decrystallized by ball milling showed only the two initial pyrolysis stages. The second stage followed first-order kinetics. The first pyrolysis stage in unmodified cotton was characterized by a moderately low activation energy and by a large negative entropy of activation; the second stage showed a larger activation energy and a less negative entropy of activation. Mechanisms involving cellulose chain scission and chain unzipping are proposed for the first and second stages, respectively. Add-on of the two flame retardants had contrasting effects on the two initial pyrolysis stages. These effects are explained in terms of the way in which the flame retardants are deposited in the less ordered regions of cotton fibers.     

The role of oxygen in the ignition of polystyrene by a small flame

The ignition of slabs of high-impact polystyrene by a lean hydrogen–oxygen flat flame was studied. The ignition delays and inital rates of flame development after ignition are reported as functions of gas temperature and the separation between flame and polymer surface. The delays follow an Arrhenius-type expression with an activation energy of 98 ± 18 kJ mol^?1. The rates of flame development drop as the gas temperature increases. During long ignition delays the apparent heat transfer coefficient at the sample surface dropped from about 100 W m^?2 K^?1 to values close to that expected for a hot gas impinging at right angles on a cold surface. For short delays it was higher and more constant at about 100 W m^?2 K^?1. Although the surface temperature reached before ignition exceeded that required for nonoxidative pyrolysis, the polymer surface charred only when oxygen was present. It is concluded that both oxidative and nonoxidative pyrolysis contribute to the ignition of polystyrene.     

Ignition of low-density polyethylene slabs by a small flame

Slabs of low-density polyethylene (LDPE) were exposed to the wake of a lean hydrogen-oxygen flat flame. The ignition delay and initial flame velocity after the ignition were measured at several gas-air equivalence ratios and distances from the igniting flame. When ignition occurred, the surface temperature was far lower than that required for pyrolysis in the absence of oxygen. Small amounts of char formed on the polymer surface during the delay, consistent with the involvement of oxygen in solid-phase preignition processes. Plots of In(delay) versus 1/(absolute temperature) were linear and the activation energy was derived from the Arrhenius equation, 64 ± 10 kJ/mol. Initial rates of flame development decreased with increased separation between the polymer and the igniting flame, but unlike those reported for poly(methyl methacrylate), they were independent of the duration of the preceding delay except when the polymer was very close to the flame. The results are explained by a model in which both the ignition delay and the subsequent rate of flame development depend on the concentration of species associated with the chain-propagation steps of the combustion process.     

Synergism in combustion processes

The effect of CF₃H, CF₄, and N₂ on the ignition of methane–air mixtures has been investigated. The effect of trifluoromethane is due to its being involved in reaction chain termination. A nonadditive effect of trifluoromethane and nitrogen on the concentration limits of ignition and flame propagation in methane–air mixtures has been predicted and revealed. The synergistic effect arises from the exponential dependence of the rate of the chain process on the concentrations of the initial components.     

An apparatus for kinetic-mechanistic studies of polymer burning and regression rates: Poly(methyl methacrylate)

An apparatus for determination of regression (weight-loss) rates before, at, and after ignition as a function of oxidant gas flow rates has been constructed and successfully operated with poly(methyl methacrylate). In addition, flame temperatures, gas evolution kinetics and composition can be ascertained. The apparatus allows a proper evaluation of diffusion processes and chemically controlled processes. The regression rate for PMMA is directly proportional to the air flow rate. At low air flow rates the process is diffusion controlled and the energy of activation is identical with the heat of vaporization of the monomer. There is a liquid monomer layer and on top of it a stagnant vapor layer having the equilibrium vapor pressure of the monomer. At higher flow rates the energy of activation increases steeply. The layers become thin and defective and eventually thermal depolymerization becomes rate determining. This depolymerization has an energy of activation between 30 to 40 kcal mol^?1. Experiments have also been carried out in closed quartz tubes. Explosion limits, ignition lags and energies of activation have been determined. The latter were evaluated by the method of Semenoff.     

1H NMR study of the kinetics of substituted aniline polymerization. I. Homopolymerization of 2‐methoxyaniline

We studied the kinetics of the oxidative chemical homopolymerization of 2‐methoxyaniline (OMA) in aqueous acid solutions by monitoring OMA depletion with ¹H NMR spectroscopy. We used the same semiempirical kinetic model used for aniline (ANI) homopolymerization to evaluate the experimental data. The reaction kinetics of OMA homopolymerization was similar to that of ANI, although we obtained longer induction and propagation times for OMA. This was attributed to steric hindrance of the bulky methoxy substituent during the coupling reaction. Furthermore, it was suggested that a lower OMA polymerization rate could also be related to a lower concentration of nonprotonated OMA molecules in the reaction solution due to a higher pK_a value for OMA than for ANI. This may also explain the lower OMA end conversion (90%) compared with that of ANI (96%). The OMA end conversion was not influenced substantially by reaction conditions; it was lower than 90% only when high acid or low oxidant (oxidant‐deficient oxidant/OMA ratio) concentrations were applied. Because the oxidant took an active part in polymerization, it markedly influenced the polymerization rate, especially the initiation rate. The OMA initiation and propagation rates increased with increasing oxidant and initial monomer concentrations and with the reaction temperature, but there was no uniform trend in the correlation between the homopolymerization rate and acid concentration. The activation energies of the OMA initiation and propagation were 57 and 10 kJ/mol, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2471–2481, 2001     

低温燃料化学特性对湍流预混火焰传播的影响

大分子碳氢燃料的低温化学反应及两阶段点火特性会显著影响火焰的分区及燃烧情况。本文采用数值模拟的方法探究了正庚烷/空气预混混合气在RATS燃具上的湍流火焰传播,与试验结果具有一致性。模拟使用的是44种物质,112步的正庚烷简化动力学机理。使用Open FOAM的reacting Foam求解器建立了简化模拟流道及出口的三维模型,模拟了在大气环境下,初始反应温度450–700 K、入口速度6 m·s~(-1)与10 m·s~(-1)、焰前流动滞留时间100 ms及60 ms、当量比φ=0.6的正庚烷/空气混合气湍流火焰燃烧情况。结果发现,标准化湍流燃烧速度与混合气初始温度以及流动滞留时间有关。在低温点火阶段,正庚烷氧化程度受到初始温度与速度的影响,燃料分解并在预热区中产生大量中间物质如CH_2O,继而会影响湍流火焰燃烧速度。随着初始反应温度的升高,湍流燃烧火焰逐渐由化学反应冻结区过渡到低温点火区;温度超过一定数值后,燃料不再发生低温反应,此时燃烧位于高温点火区域。     

Thermal degradation mechanism of flame retarded epoxy resins with a DOPO-substitued organophosphorus oligomer by TG-FTIR and DP-MS

A novel phosphorus-containing oligomeric flame retardant, poly(DOPO substituted hydroxyphenyl methanol pentaerythritol diphosphonate) (PDPDP) was synthesized and applied to flame retarded epoxy resins. The thermal degradation behaviors of flame retarded epoxy composites with PDPDP were investigated by thermogravimetric analysis (TGA), thermogravimetric analysis/infrared spectrometry (TG-FTIR) and direct pyrolysis-mass spectrometry (DP-MS) techniques. The identification of pyrolysis fragment ions provided insight into the flame retardant mechanism. The results showed that the mass loss rate of the EP/PDPDP composites was clearly lower than pure EP when the temperature was higher than 300 °C in air or nitrogen atmosphere. The results also suggested that the main decomposition fragment ions of the EP/PDPDP composite were H₂O, CO₂, CO, benzene, and phenol. The incorporation of PDPDP can reduce the release of combustible gas and induce the formation of char layer, hence the fire potential hazard was reduced.     

Ignition of Quiescent Lean Propane–Air Mixtures at High Pressure by Nanosecond Repetitively Pulsed Discharges

We present an experimental study of lean mixture ignition by nanosecond repetitively pulsed (NRP) discharges. The plasma is created in a lean propane/air mixture at pressure up to 10 bar and equivalence ratio 0.7, premixed in a constant volume vessel. We characterize the initial spark radius, the ignition kernel development and the flame propagation as a function of pressure (up to 10 bar) and the pulse energy (1–6 mJ per pulse). Comparisons with a conventional igniter show that better results are obtained with NRP discharges in terms of flame propagation speed, in particular at high pressure, due to the increased wrinkling of the flame front that is induced by NRP discharges.     

Interaction of a low-pressure gas flame and heterogeneous catalytic processes on the reactor surface

Conclusions (1) It was shown that the Semenov theory in various modes of chain termination quantitatively describes oxyhydrogen combustion near the first ignition limit with allowance for the interaction between the flame and the catalytic processes on the reactor wall. (2) Numerical modeling of oxyhydrogen ignition in the diffusion-controlled chain-termination mode detected the dependence of the rate “constant” for heterogeneous chain initiation on the model of the reaction in the gas. (3) For the first time, the change in the specific rate of heterogeneous chain initiation during a single ignition was determined. (4) method for controlling the low-pressure flame mode by affecting tubes far from the reactor was proposed and applied. Original Russian Text ? E.N. Aleksandrov, S.N. Kozlov, N.M. Kuznetsov, 2006, published in Doklady Akademii Nauk, 2006, Vol. 407, No. 5, pp. 630–633. Presented by Academician A.E. Shilov October 5, 2005     

正十二烷高温机理简化及验证

由于详细化学反应机理在模拟燃烧室燃烧时,计算量极大,很难被广泛运用。为了满足工程设计要求,采用替代燃料的简化机理进行计算不失为一种行之有效的方法。本文基于误差传播的直接关系图法和敏感性分析法对正十二烷180组分1962步高温机理(温度大于1100 K)进行简化,获得40组分234步化学反应机理。在温度为1100–1650 K,压力为0.1–4 MPa条件下,采用简化机理及详细机理对不同当量比、压力下着火延迟时间进行模拟,模拟结果与实验数据吻合得较好。通过对不同压力及温度下火焰传播速度进行模拟,验证了简化机理能够正确地反映正十二烷的燃烧特性。利用C_(12)H_(26)/OH/H_2O/CO_2等重要组分随时间变化的数据,验证了简化机理能够准确描述燃烧过程反应物消耗、基团变化、生成物产生的过程,并表明该机理具有较高的模拟精度。利用该简化机理对本生灯进行数值分析,结果表明该机理能够准确地反映火焰区温度和组分浓度的变化。紧凑的正十二烷高温简化机理不仅能够正确体现其物理化学特性,而且能够用于三维数值模拟,具有较高的工程运用价值和应用前景。     

Measurements of Propanal Ignition Delay Times and Species Time Histories Using Shock Tube and Laser Absorption

Propanal is an aldehyde intermediate formed during the hydrocarbon combustion process. Potentially, the use of oxygenated biofuels reduces greenhouse gas emissions; however, it also results in increased toxic aldehyde by‐products, mainly formaldehyde, acetaldehyde, acrolein, and propanal. These aldehydes are carcinogenic, and therefore it is important to understand their formation and destruction pathways in combustion systems. In this work, ignition delay times were measured behind reflected shock waves for stoichiometric (Φ = 1) mixtures of propanal (CH₃CH₂CHO) and oxygen (O₂) in argon bath gas at temperatures of 1129 K < T < 1696 K and pressures around 1 and 6 atm. Measurements were conducted using the kinetics shock tube facility at the University of Central Florida. Current results were compared to available data in the literature as well as to the predictions of three propanal combustion kinetic models: Politecnico di Milano (POLIMI), National University of Ireland at Galway, and McGill mechanisms. In addition, a continuous wave‐distributed feedback interband cascade laser centered at 3403.4 nm was used for measuring methane (CH₄) and propanal time histories behind the reflected shock waves during propanal pyrolysis. Concentration time histories were obtained at temperatures between 1192 and 1388 K near 1 atm. Sensitivity analysis was carried for both ignition delay time and pyrolysis measurements to reveal the important reactions that were crucial to predicting the current experimental results. Adjustments to the POLIMI mechanism were adopted to better match the experimental data. Further research was suggested for the H abstraction reaction rates of propanal. In addition to extending the temperature and pressure region of literature ignition delay times, we provide the first high‐temperature species concentration time histories during propanal pyrolysis.     

Carbon black,multiwall carbon nanotubes,expanded graphite and functionalized graphene flame retarded polypropylene nanocomposites

Herein, we examine the influence of adding functionalized graphene (FG), distinct expanded graphites and carbon nanofillers such as carbon black and multiwall carbon nanotubes on mechanical properties, morphology, pyrolysis, response to small flame and burning behavior of a V‐2 classified flame‐retarded polypropylene (PP). Among carbon fillers, FG and multilayer graphene (MLG) containing fewer than 10 layers are very effectively dispersed during twin‐screw extrusion and account for enhanced matrix reinforcement. In contrast to the other fillers, no large agglomerates are detected for PP‐FR/FG and PP‐FR/MLG, as verified by electron microscopy. Adding FG to flame‐retardant PP prevents dripping due to reduced flow at low shear rates and shifts the onset of thermal decomposition to temperatures 40°C higher. The increase in the onset temperature correlates with the increasing specific surface areas (BET) of the layered carbon fillers. The reduction of the peak heat release rate by 76% is attributed to the formation of effective protection layers during combustion. The addition of layered carbon nanoparticles lowers the time to ignition. The presence of carbon does not change the composition of the evolved pyrolysis gases, as determined by thermogravimetric analysis combined with online Fourier‐transformed infrared measurements. FG and well‐exfoliated MLG are superior additives with respect to spherical and tubular carbon nanomaterials. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of gas composition and flame sheathing on the spatial velocity profiles of laminar analytical acetylene flames

A recently developed technique has been employed to spatially map the rise velocity pronies (horizontal and vertical) of commonly used laminar analytical flames and to determine the influence on the profiles of fuel-to-oxidant ratio and the presence of a flame sheath. The rise velocities for fuel-rich, lean, and stoichiometric flames were found to differ substantially, the entire profile being greatest for the fuel-rich condition and lowest for the fuel-lean flame. In addition, the change in rise velocity with the addition of a solid (quartz tube) or gas (N₂) sheath was studied. The flowing sheath, at several different flow rates, affected each rise velocity profile principally by altering atmospheric entrainment and thereby changing secondary combustion in the flame. In contrast, a solid quartz tube used as a sheath produces an additional increase in the entire velocity profile of each flame, since it constrains gas expansion to the direction of flame propagation. The degree to which the velocity of each flame is affected by either a flowing N₂ and quartz sheath is strongly influenced by fuel-to-oxidant ratio.     

Effect of processes at the reactor surface on the low-pressure hydrogen f lame

The interaction of the low-pressure flame of a 2H₂-O₂ mixture with a quartz reactor surface was studied by the resonance fluorescence technique. The results confirmed the fundamental statement of N. N. Semenov’s theory concerning chain propagation in the gas and termination on the surface in the kinetic region of chain termination (quadratic decay in the heterogeneous negative chain interaction) and in the diffusion region (linear decay). The kinetic curves observed in the kinetic and diffusion chain termination regions on the wall were well matched using N. N. Semenov’s theory, taking into account the heterogeneous catalytic chain initiation and interaction processes occurring on the wall with a variable “rate constant.” The interaction of chains on the wall markedly retards ignition in the gas in the kinetic region and has almost no influence on chain propagation in the gas in the diffusion region of the heterogeneous chain termination. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1301–1308, August, 2006.     

Two novel heterobimetallic metal-organic frameworks for the enhanced catalytic thermolysis and laser ignition of CL-20

The study of multiple complex catalytic mechanisms is currently one of the great scientific issues for the application of high-energy solid propellants. Two novel heterobimetallic metal-organic frameworks (MOFs), Ba₄Pb₄(CH₃CO₂)₈ [(CH₆CO₂)₄Pb](CH₃CO₂)₄ (PbBa-MOF) and Ba₂Ni(CO₂H)₆(OH₂)₄ (NiBa-MOF), were prepared via the solvothermal method, and their structures and composition were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR) techniques and N₂ adsorption/desorption experiment. The thermal decomposition characteristics of the two MOFs and their catalytic performances on the hexanitro hexaazaisowurtzitane (CL-20) thermolysis were also studied by differential scanning calorimetr (DSC) and thermogravimetric-fourier transform infrared spectroscopy-mass spectrum (TG-FTIR-MS) methods. The results showed that the NiBa-MOF presented a lower initial decomposition temperature than the PbBa-MOF, and the difference of the MOFs structures affected the starting point of thermal decomposition. Compared with the pure CL-20, the thermolysis peak temperature and apparent activation energy (E_a) of the CL-20/PbBa-MOF mixture were decreased by 2.2 °C and 23.76 kJ?mol^?1, respectively. The E_a of CL-20/NiBa-MOF mixture was lower and 42.01 kJ?mol^?1, indicating the better catalytic activity of NiBa-MOF. The thermolysis catalytic mechanisms were studied by analyzing the transformation of gas products during the pyrolysis of mixtures. The effect of these two MOFs on the CL-20 thermolysis is primarily owing to the strong attraction of metal cations to electronics, bimetallic synergistic catalysis, and the release of active free radicals. Furthermore, the laser ignition and flame propagation features showed that these two MOFs reduced the minimum ignition power density and ignition delay time of the CL-20, and the flame becomes brighter and more luminous. The influence of the two MOFs on the flame bright spot of CL-20 based mixtures was described.     

Quenching of Flame Propagation with Heat Loss

The steady propagation of a planar laminar premixed flame, with a one-step exothermic reaction and linear heat loss, is studied. The corresponding travelling wave equations are solved numerically. The dependence of the flame velocity on the heat loss parameter is determined and compared with known results obtained by asymptotic expansion and other approximations. Due to the introduction of an ignition temperature the problem can be reduced to a bounded interval (of length L) and the graph of flame speed versus heat loss parameter can be parametrised by L. The numerical method is tested in the case of a step function nonlinearity when the exact solution of the differential equations can also be calculated.     

Isothermal waves of hydrogen oxidation

It is analytically shown that the chain ignition region is extended in the reaction of hydrogen oxidation due to the nonlinear interaction H + HO₂ = 2 OH. As a result, the reaction propagates isothermally under the isothermal conditions outside the ignition region, which is calculated by the scheme taking into account only linear reactions with respect to radicals, if the reaction is initiated by additives of hydrogen atoms. The ignition limits were calculated with allowance for the above interaction, and the mathematical modeling of propagation of the hydrogen oxidation reaction under the isothermal conditions was performed.