T. A. Bolshova

The chemistry of the destruction of organophosphorus compounds in flames—III: the destruction of DMMP and TMP in a flame of hydrogen and oxygen

The structure of a premixed H2/O2/Ar (0.26/0.13/0.61 by volume) flame doped with dimethyl methyl phosphonate (DMMP) stabilized on a flat burner at 47 Torr has been studied by molecular-beam mass spectrometry and modeling. Using previous experimental measurements, the mechanism for the destruction of trimethyl phosphate (TMP) in H2/O2/Ar flames was refined. The present experiments with Twarowski’s reaction mechanism for hydrogen, oxygen, and phosphorus and Werner and Cool’s mechanism for the destruction of DMMP, enabled updated kinetic mechanisms for the destruction of both DMMP and TMP in a flame to be developed. Based on the available thermochemical data and using the computer codes PREMIX and CHEMKIN-II, the computer modeling of the destruction of DMMP and TMP in a flame was achieved. Matching the experimental and calculated concentration profiles for all the species found in flames allowed the rate constants for the reactions of intermediates to be evaluated and refined. The final result is that the calculated and measured concentration profiles are in satisfactory agreement for DMMP, TMP, H2, O2, H2O, OH, O, H, PO, PO2, HOPO, and HOPO2. The results provide an understanding of important regularities of the destruction of organophosphorus compounds, used here as simulants of sarin in flames.

Inhibition and promotion of combustion by organophosphorus compounds added to flames of CH4 or H2 in O2 and Ar

Abstract Early in evaluating the destruction mechanisms of a number of organophosphorus compounds (OPCs), such as trimethyl phosphate (TMP), dimethyl methylphosphonate, and diisopropyl methylphosphonate, in connection with the disposal of chemical warfare agents, the promotion and inhibition effects of OPCs on stabilized flat flames of H2 + O2 were studied in detail. Because OPCs were demonstrated to be more effective fire suppressants than CF3Br (Halon 1301) and due to the need for replacing the currently used Halon 1301, further investigation of the effects of the OPCs on flames is of interest. Thus a lean flame of CH4/O2/Ar (0.078/0.222/0.7) with and without TMP added, stabilized on a flat burner at 0.1 bar, was studied by molecular beam mass spectrometry (MBMS) and computer modeling using PREMIX and CHEMKIN codes. An experimental study of this flame revealed that TMP increases the width of the reaction zone by inhibiting the flame.

Modeling the chemical reactions of ammonium dinitramide (ADN) in a flame

Ammonium dinitramide (ADN) is a new energetic material that can be used as an oxidizer in solid rocket propellants. In the last few years, a number of papers devoted to the study of ADN combustion mechanism have been published. Molecular beam mass-spectrometry and thermocouple measurements are here used to study ADN flame structure at pressures of 0.3 and 0.6 MPa. Measurements include species concentrations and temperature profiles. A general mechanism for describing the chemical reactions in an ADN flame (172 reactions and 31 species) is developed based on these experimental studies and literature data. The scheme includes a sub-mechanism (98 reactions and 22 species) for propellants combustion suggested by Yetter and Dryer. The latter is further supplemented by a set of 74 reactions, including 63 steps suggested by Lin and Park and the ADN dissociation reactions suggested by the authors of this paper. The correlation between the experimental and calculation results is satisfactory.

Thermal Decomposition of Ammonium Dinitramide Vapor in a Two-Temperature Flow Reactor

The thermal decomposition of ammonium dinitramide (ADN) was studied using a two-temperature flow reactor. The vapor pressure at temperatures of 80-140°C and the heat of vaporization of ADN were determined. From the results obtained a mechanism for the thermal decomposition of ADN was proposed that includes the ADN vaporization stage, i.e., the transition from the condensed to the gaseous state in the form of the molecular complex NH

The chemistry of the destruction of organophosphorus compounds in flames—IV: destruction of DIMP in a flame of H2 + O2 + Ar

3·HN(NO2)2 (ADN vapor) followed by dissociation into NH3 and HN(NO2)2. The composition of the products from the thermal decomposition of ADN vapor at temperatures of 160–900°C was determined by mass spectrometry. The effective rate constant was determined for the dissociation of ADN vapor at temperatures of 160–320°C. The thermal decomposition of ADN vapor in a flow reactor was modeled using the proposed mechanism and the reaction rate constant for ADN vapor dissociation. Key words: ammonium dinitramide, dinitramide, molecular beam mass spectrometry, thermal decomposition, two-temperature flow reactor.

An Experimental and Kinetic Modeling Study of Premixed Laminar Flames of Methyl Pentanoate and Methyl Hexanoate

Abstract Molecular beam mass spectrometry with electron impact ionization at 11–70 eV and an electron energy spread of ± 0.25 eV was used to study the structure of a premixed H2/O2/Ar (0.26/0.13/0.61) flame without any additives and with 0.14% of diisopropylmethylphosphonate (DIMP), stabilized on a flat-flame burner at 62 mbar. Stable species (H2, O2, H2O), as well as atoms and radicals (H, O, OH) were monitored, including phosphorus-containing compounds: DIMP and some intermediates of its destruction, phosphorus oxides and acids. The profiles of the mole fractions of most species, including those of atoms and free radicals were obtained. The calibration coefficients for some species were determined experimentally, and estimated for others. Isopropylmethylphosphonate was detected as a main primary phosphorus-containing product of the destruction of DIMP. It has been shown that bimolecular reactions with hydroxyl radicals and hydrogen atoms, rather than a unimolecular decomposition, provide the crucial initial steps in the destruction of DIMP. A detailed mechanism for the destruction of DIMP in H2/O2/Ar flames is suggested.

Mechanism of inhibition of hydrogen/oxygen flames of various compositions by trimethyl phosphate

Abstract Detailed chemical structures of stoichiometric and rich premixed laminar flames of methyl pentanoate and methyl hexanoate were investigated over a flat burner at 20 Torr and for methyl pentanoate at 1 atm. Molecular beam mass spectrometry was used with tunable synchrotron vacuum ultraviolet (VUV) photoionization for low pressure flames of both methyl pentanoate and methyl hexanoate, and soft electron-impact ionization was used for atmospheric pressure flames of methyl pentanoate. Mole fraction profiles of stable and intermediate species, as well as temperature profiles, were measured in the flames. A detailed chemical kinetic high temperature reaction mechanism for small alkyl ester oxidation was extended to include combustion of methyl pentanoate and methyl hexanoate, and the resulting model was used to compare computed values with experimentally measured values. Reaction pathways for both fuels were identified, with good agreement between measured and computed species profiles. Implications of these results for future studies of larger alkyl ester fuels are discussed.

Multistage Mechanism of Thermal Decomposition of Hydrogen Azide

The effect of the catalytic recombination reactions of H and OH− involving phosphorus-containing products of trimethyl phosphate (TMP) combustion on the burning velocity and the structure of H2/O2/N2 flames at atmospheric pressure has been investigated. An earlier mechanism for inhibition of rich hydrogen/oxygen flames by organophosphorus compounds has been tested and modified by comparing experimental data with the results of simulation. The sensitivity analysis of the calculated flame speed to the rate constants of chain branching reactions and chain termination reactions involving phosphorus-containing compounds has revealed significant specific features of the inhibition mechanism of hydrogen flames with various stoichiometries and dilution ratios. Unlike the inhibition efficiency of hydrocarbon flames, in which the reactions of H and OH− radicals with PO, PO2, HOPO, and HOPO2 play the key role, the inhibition efficiency of hydrogen flames at atmospheric pressure is determined by the interaction of hydrogen and oxygen atoms with TMP and with organophosphorus products of its decomposition in the low-temperature zone of the flame. The sensitivity analysis has demonstrated that, as the equivalence ratio (ϕ) or the dilution ratio is increased, the ratio of the chain branching rate to the rate of chain termination via reactions involving phosphorus compounds decreases. As a consequence, the efficiency of inhibition of H2/O2/N2 flames, as distinct from that of hydrocarbon flames, increases as ϕ is raised from 1.1 to 3.0 and as the mixture is progressively diluted with nitrogen.

A numerical study of the superadiabatic flame temperature phenomenon in HN3 flames

A kinetic mechanism for combustion of hydrogen azide (HN3) comprising 61 reactions and 14 flame species (H2, H, N, NH, NH2, NNH, NH3, HN3, N3, N2H2, N2H3, N2H4, N2, and Ar) was developed and tested. The CHEMKIN software was used to calculate the flame speed at a pressure of 50 torr in mixtures of HN3 with various diluents (N2 and Ar), as well as the self-ignition parameters of HN3 (temperature and pressure) at a fixed ignition delay. The modeling results of the flame structure of HN3/N2 mixtures show that at a 25–100% concentration of HN3 in the mixture, the maximum temperature in the flame front is 25–940 K higher than the adiabatic temperature of the combustible mixture. Analysis of the mechanism shows that burning velocity of a HN3/N2 mixture at a pressure of 50 torr is described by the Zel’dovich-Frank-Kamenetskii theory under the assumption that the burn rate controlling reaction is HN3 + M = N2 + NH + M (M = HN3) provided that its rate constant is determined at a superadiabatic flame temperature. The developed mechanism can be used to describe the combustion and thermal decomposition of systems containing HN3.

Autoignition mechanism of dimethyl ether–air mixtures in the presence of atomic iron

The phenomenon of superadiabatic flame temperature (SAFT) was discovered and investigated in a low-pressure HN3/N2 flame using numerical modelling. A previously developed mechanism of chemical reactions in the HN3/N2 flame at the pressure 50 Torr and the initial temperature T0 = 296 K was revised. Rate constants of several important reactions involving HN3 (HN3 (+N2) = N2 + NH (+N2), R1; HN3 (+HN3) = N2 + NH (+HN3), R2; HN3 + H = N2 + NH2, R4; HN3 + N = N2 + NNH, R5; and HN3 + NH2 = NH3 + N3, R7) were calculated using quantum chemistry and reaction rate theories. Modified Arrhenius expressions for these reactions are provided for the 300–3500 K temperature range. Modelling of the flame structure and flame propagation velocity of the HN3/N2 flame at p = 50 Torr and T0 = 296 K was performed using the revised mechanism. The results demonstrate the presence of the SAFT phenomenon in the HN3/N2 flame. Analysis of the flame structure and the kinetic mechanism indicates that the cause of SAFT is in the kinetic mechanism: exothermic reactions of radicals with hydrogen atoms occur in the post flame zone, which results in the formation of super equilibrium H2 concentrations. The flame propagation velocity is largely determined by the second-order HN3 decomposition reaction and not by the reaction of HN3 with H, as was previously assumed. Calculation of the flame propagation velocity according to the Zeldovich-Frank-Kamenetsky theory with the decomposition reaction as a limiting stage yielded a value that agrees with that obtained in numerical modelling using the complete reaction mechanism.