Valeri I. Babushok

Numerical study of the inhibition of premixed and diffusion flames by iron pentacarbonyl

Iron pentacarbonyl (Fe(CO){sub 5}) is an extremely efficient flame inhibitor, yet its inhibition mechanism has not been described. The flame-inhibition mechanism at Fe(CO){sub 5} in premixed and counterflow diffusion flames of methane, oxygen, and nitrogen is investigated. A gas-phase inhibition mechanism involving catalytic removal of H atoms by iron-containing species is presented. For premixed flames, numerical predictions of burning velocity are compared with experimental measurements at three equivalence ratios (0.9, 1.0, and 1.1) and three oxidizer compositions (0.20, 0.21, and 0.24 oxygen mole fraction in nitrogen). For counterflow diffusion flames, numerical predictions of extinction strain rate are compared with experimental results for addition of inhibitor to the air and fuel stream. The numerical predictions agree reasonably well with experimental measurements at low inhibitor mole fraction, but at higher Fe(CO){sub 5} mole fractions the simulations overpredict inhibition. The overprediction is suggested to be due to condensation of iron-containing compounds since calculated supersaturation is suggested to be due to condensation of iron-containing compounds since calculated supersaturation ratios for Fe and FeO are significantly higher than unity in some regions of the flames. The results lead to the conclusion that inhibition occurs primarily by homogeneous gas-phase chemistry.

Influence of CF3I, CF3Br, and CF3H on the high-temperature combustion of methane☆

Abstract The effects of a number of flame retardants (CF 3 I, CF 3 Br, and CF 3 H) on the high-temperature reactions of methane with air in a plug flow reactor are studied by numerical simulations using the Sandia Chemkin Code. 1 The dependence of (a) the ignition delay and (b) time for substantially complete reaction as a function of temperature and additive concentrations are calculated. In agreement with experiments, the ignition delay can be increased or decreased by the addition of retardants. The reaction time is always increased by additives. The mechanism for these effects has been examined. It is concluded that the ignition delay is controlled by the initial retardant decomposition kinetics, which releases active species into the system. These species can either terminate or initiate chains. The reaction time is largely a function of the concentrations of the active radicals H, OH, and O that are formed during the combustion process. It is shown that their concentrations, particularly those of H atoms, are lowered in the presence of the retardants. We find that the chemical mechanism governing reaction time is very similar to that which controls the flame velocity and a correlation between decreases in flame velocity and H-atom concentration is demonstrated. The calculations suggest that relative reaction time and H-atom concentrations should be effective measures for the estimation of retardant effectiveness.

Inhibitor rankings for alkane combustion

Abstract The effect of hydrocarbon fuel type on the ranking of inhibitor effectiveness has been investigated through computer simulations. The approach involves carrying out sensitivity analysis on the detailed kinetics of the combustion of C1-C4 hydrocarbons. It is demonstrated that the main reactions determining burning velocities are the same. Similar suppressant rankings from the combustion of different hydrocarbon fuels are largely due to the reactions of a number of small radicals that are common to all of these systems. Inhibitor addition reduces the concentration of these radicals with the active agents being recycled by the common breakdown products of the fuel. Inhibitor effectiveness of additives in a variety of fuels was analyzed using experimental data on the effects of additives on burning velocity in small additive concentration ranges. An universal ranking of additive efficiency is presented. The results demonstrate that the active agents in practically all cases are the small inorganic compounds created from decomposition processes. Inhibition effectiveness of agents is at a maximum at low concentrations. At higher concentrations, saturation effects, brought about by the approach of active radicals to their equilibrium concentrations, lead to substantial decreases in the effectiveness of high efficiency suppressants in comparison with their effects at small concentrations. The results show that the probable maximum increase in total flame suppression effectiveness of high efficiency agents will not exceed one order of magnitude in molar fractions in comparison with the effect of halon 1301 (CF3Br).

Inhibition effectiveness of halogenated compounds

Abstract A numerical study of the inhibition efficiency of halogenated compounds was carried out for C 1 - 2 hydrocarbon-air laminar premixed flames. The inhibition efficiency of CF 3 Br, CF 3 I, CF 3 H, C 2 HF 5 , C 2 F 6 , and CF 4 additives was interpreted using an additive group method. In agreement with measurements, the calculated burning velocity decreased exponentially with increasing additive concentration over a wide concentration range. The inhibition parameter Φ proposed by Fristrom and Sawyer indicating inhibition efficiency was modified to take into account the exponential dependence of burning velocity on inhibitor concentration. The inhibition indices for halogen atoms and groups important in the inhibition process were determined for stoichiometric conditions. The physical and chemical effects of the additives were studied. With increasing additive concentration, the chemical influence of an inhibitor saturates and the physical influence increases. Therefore, use of a composite inhibitor composed of a mixture of an effective chemical inhibitor with a high heat capacity diluent may be beneficial. The contribution of physical and chemical components on inhibitor influence are estimated near entinction. A procedure for determination of a regeneration coefficient, which indicates an effective number of catalytic cycles involving inhibitor during the combustion process, is suggested. The regenation coefficient of HBr in stoichiometric methane-air flame with 1% CF 3 Br added is approximately 7.

Chemical limits to flame inhibition

Abstract This paper deals with the ultimate limits of chemical contributions to flame inhibition. Particular attention is focussed on the inhibition cycles which regenerate the inhibitor. This leads to the definition of an idealized “perfect” inhibition cycle. It is demonstrated that for such an inhibitor in a stoichiometric methane/air flame, additive levels in the 0.001–0.01 mole percent range will lead to a decrease in flame velocity of approximately 30%. This efficiency corresponds roughly to the observed behavior of metallic inhibitors such as iron pentacarbonyl which is known to be as much as 2 orders of magnitude more effective than currently used suppressants. This correspondence between the behavior of a “perfect inhibitor” and iron carbonyl leads to the conclusion that only gas-phase processes can account for its inhibitive power.

Flame Inhibition by Ferrocene and Blends of Inert and Catalytic Agents

The production of the fire suppressant CF3Br has been banned, and finding a replacement with all of its desirable properties is proving difficult. Iron pentacarbonyl has been found to be up to several orders of magnitude more effective than CF3Br, but it is flammable and highly toxic. Ferrocene [Fe(C5H5)2], which is much less toxic and flammable than Fe(CO)5, can also be used to introduce iron into a flame. We present the first experimental data and numerical modeling for flame inhibition by ferrocene and find it to behave similarly to Fe(CO)5. A ferrocene mole fraction of 200 ppm reduced the burning velocity of slightly preheated premixed methane/air flames by a factor of two, and the effectiveness dropped off sharply at higher mole fractions. For air with a higher oxygen mole fraction, the burning velocity reduction was less. We also present experimental data and modeling for flames with ferrocene blended with CO2 or CF3H. The combination of the thermally acting agent CO2 with ferrocene mitigated the loss of effectiveness experienced by ferrocene alone at higher mole fractions. An agent consisting of 1.5% ferrocene in 98.5% CO2 performed as effectively as CF3Br in achieving a 50% reduction in burning velocity. Likewise, four times less CO2 was required to achieve the 50% reduction if 0.35% ferrocene was added to the CO2. In contrast, addition of 0.35% ferrocene to the hydrofluorocarbon CF3H reduced the CF3H required to achieve the 50% reduction in burning velocity by only about 25%. Thermodynamic equilibrium calculations predict that the formation of iron/fluoride compounds can reduce the concentrations of the iron-species oxide and hydroxide intermediates which are believed to be responsible for the catalytic radical recombination cycles.

Inhibition of premixed methane-air flames by fluoroethanes and fluoropropanes

This paper presents experimental and modeling results for laminar premixed methane-air flames inhibited by the fluoroethanes C{sub 2}F{sub 6}, C{sub 2}HF{sub 5}, and C{sub 2}H{sub 2}F{sub 4}, and experimental results for the fluoropropanes C{sub 3}F{sub 8} and C{sub 3}HF{sub 7}. The modeling results are in good agreement with the measurements with respect to reproducing flame speeds. For the fluoroethanes, calculated flame structures are used to determine the reaction pathways for inhibitor decomposition and the mechanisms of inhibition, as well as to explain the enhanced soot formation observed for the inhibitors C{sub 2}HF{sub 5}, C{sub 2}H{sub 2}F{sub 4}, and C{sub 3}HF{sub 7}. The agents reduce the burning velocity of rich and stoichiometric flames primarily by raising the effective equivalence ratio and lowering the adiabatic flame temperature. For lean flames, the inhibition is primarily kinetic, since inhibitor reactions help to maintain the final temperature. The peak radical concentrations are reduced beyond that due to the temperature effect through reactions of fluorinated species with radicals.

Cosolvent-assisted Spray Pyrolysis for the Generation of Metal Particles

A cosolvent-assisted spray pyrolysis process was developed for the formation of phase-pure metal particles from metal salt precursors without the direct addition of hydrogen or other reducing gas. Generation of phase-pure copper and nickel particles from aqueous solutions of copper acetate, copper nitrate, and nickel nitrate over the temperature range of 450 to 1000 °C was demonstrated. Addition of ethanol as a cosolvent plays a crucial role in producing phase-pure metal powders. Results of a modeling study of ethanol decomposition kinetics suggest that cosolvent decomposition creates a strong reducing atmosphere during spray pyrolysis via in situ production of hydrogen and carbon monoxide.

Inhibition of Premixed Methane Flames by Manganese and Tin Compounds

Abstract The first experimental measurements of the influence of manganese- and tin-containing compounds (MMT, TMT) on the burning velocity of methane/air flames are presented. Comparisons with Fe(CO) 5 and CF 3 Br demonstrate that manganese and tin-containing compounds are effective inhibitors. The inhibition efficiency of MMT is about a factor of two less than that of iron pentacarbonyl, and that of TMT is about 26 times less effective, although TMT is still about twice as effective as CF 3 Br. There exist conditions for which both MMT and TMT show a loss of effectiveness beyond that expected because of radical depletion, and the cause is believed to be particle formation. Kinetic models describing the inhibition mechanisms of manganese- and tin-containing compounds are suggested. Simulations of MMT- and TMT-inhibited flames show reasonable agreement with experimental data. The decomposition of the parent molecule for the tin and manganese species is found to have a small effect on the inhibition properties for the concentrations in this work. The inhibition effect of TMT is determined mostly by the rate of the association reaction H + SnO + M ↔ SnOH + M, and the catalytic recombination cycle is completed by the reactions SnOH + H ↔ SnO + H 2 and SnOH + OH ↔ SnO + H 2 O. The inhibition mechanism by manganese-containing compounds includes the reactions: MnO + H 2 O ↔ Mn(OH) 2 ; Mn(OH) 2 + H ↔ MnOH + H 2 O, and MnOH + OH (or H) ↔ MnO + H 2 O (or H 2 ), and the burning velocity is most sensitive to the rate of the reaction Mn(OH) 2 + H ↔ MnOH + H 2 O.

Kinetic modeling of the laser-induced breakdown spectroscopy plume from metallic lead

We report initial results of a study aimed toward developing a computational fluid dynamics (CFD) model to simulate the laser-induced breakdown spectroscopy (LIBS) plume for the purpose of understanding the physical and chemical factors that control the LIBS signature. The kinetic model developed for modeling studies of the LIBS plume from metallic lead includes a set of air reactions and ion chemistry as well as the oxidization, excitation, and ionization of lead atoms. At total of 38 chemical species and 220 reactions are included in the model. Experimental measurements of the spatial and temporal dependence of a number of lead emission lines have been made of the LIBS plume from metallic lead. The mechanism of generation of excited Pb states in the LIBS plume is analyzed through kinetic modeling and sensitivity analysis. Initial CFD model results for the LIBS plume are presented.