V. M. Shvartsberg

The destruction chemistry of organophosphorus compounds in flames -- I: Quantitative determination of final phosphorus-containing species in hydrogen-oxygen flames

Abstract This paper presents the results of a quantitative determination of the composition of final phosphorus-containing products (PO, PO 2 , HOPO, and HOPO 2 ) from the destruction of the organophosphorus compounds trimethyl phosphate (TMP) and dimethyl methylphosphonate (DMMP) in premixed hydrogen–oxygen flames. The flames were stabilized on a flat burner at 47 Torr and probed using molecular beam mass spectrometric techniques. Quantitative analysis of these species is difficult, due to problems with mass spectrometric calibrations. Also these compounds are unstable under normal conditions and are not readily available. To solve this problem a material balance equation for the element phosphorus has been used to analyze the results in stoichiometric, rich, and lean flames, doped with different amounts of TMP and DMMP. A system of linear nondegenerate material balance equations was solved using the Singular Value Decomposition (SVD) algorithm. The calculated calibration coefficients for the phosphorus species have allowed their mole fractions to be derived. How the concentrations of PO, PO 2 , HOPO, and HOPO 2 depend on the initial concentrations of DMMP or TMP and on the mixture’s composition has been studied. The measurements are compared to the results of thermochemical equilibrium calculations.

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.

The destruction chemistry of organophosphorus compounds in flames -- II: Structure of a hydrogen-oxygen flame doped with trimethyl phosphate

Abstract Molecular beam mass spectrometry with electron-impact ionization at 12.9–21 eV and an electron energy spread of ±0.25 eV was used to study the structure of a premixed H 2 /O 2 /Ar (0.26/0.13/0.61) flame without additives and with 0.2 vol % of the additive trimethyl phosphate (TMP), when stabilized on a flat-flame burner at 47 Torr. Stable components (H 2 , O 2 , H 2 O), atoms, and radicals (H, O, OH) were found, as well as organophosphorus compounds including TMP and some intermediates of its destruction. Using measured intensity profiles of all the flame species and their calibration coefficients, their mole fraction profiles, including those of atoms and free radicals, were measured. The calibration coefficients for some species were determined experimentally; others were estimated. The previously suggested mechanism of destruction of TMP in H 2 /O 2 /Ar flames is refined.

Hydrogen-oxygen flame doped with trimethyl phosphate, its structure and trimethyl phosphate destruction chemistry

Molecular beam mass spectrometry was used to study the structure of a premixed H 2 -O 2 -Ar(0.26-0.13-0.61) flame without additives, and with additive (0.2–0.4%) of trimethyl phosphate (TMP) stabilized on a flat-flame burner at 47–80 torr. The behavior of TMP in the flame and the influence of the additve on the flame structure have been studied. Mass spectra of samples taken from the flames and intensity profiles of peaks 1 (H), 16 (O), 17 (OH), 18 (H 2 O), 28 (CO), 32 (O 2 ), 40 (Ar), 44 (CO 2 ), 47 (PO), 63 (PO 2 ), and 64 (HPO 2 ). AMU, and of peaks 80, 95, 96, 98, 110, 111, 112, 126 and 140 (TMP) AMU have been measured as a function of the distance from the burner surface to the sampling probe, using a quadrupole mass spectrometer and electron impact ionization at 12.9–21 eV with a spread of electron energy±0.25 eV. Intensity profiles of masses 95, 96, 126, 111, and 112 pass through a maxima, and those of masses 80, 109, and 110 pass through two maxima. Such behavior of intensity profiles shows that the species responsible for these masses are intermediates or fragmentary ions of intermediates. Dimethyl phosphate, dimethyl phosphite, monomethyl phosphate, and monomethyl phosphite were proposed to be the intermediates. The profile of the temperature in the flame has been determined by using a Pt-PtRh (10%) thermocouple covered by Ceramabond 569. The promotion of the H 2 O 2 Ar flame by the TMP additive has been observed. The possible chemical mechanism of TMP destruction is presented.

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

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.

Promoting effect of halogen- and phosphorus-containing flame retardants on the autoignition of a methane–oxygen mixture

This paper presents a numerical and experimental study of the effect of flame-retardant additives on the autoignition of methane behind shock waves. It is shown that at a temperature of 1300–1900 K, the compounds CCl4, CF3H, and (CH3O)3PO not only do not suppress ignition but significantly reduce the induction time of methane–oxygen mixtures. A kinetic mechanism is proposed which relates the promoting effect to the reactivity of the pyrolysis products of the additives.

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

The search for reactive additives capable of reducing the combustibility of dimethyl ether is an important problem due to the widening use of ether as an alternative environmentally friendly motor fuel. This paper presents a numerical study of the autoignition chemistry of mixtures of dimethyl ether with air in the presence of atomic iron. Atomic iron, which is an effective inhibitor of premixed laminar hydrocarbon flames, was found to shorten the induction period. However, the additive affects only the first stage of the induction period. The mechanism of promotion of the low-temperature oxidation of dimethyl ether–air mixtures by atomic iron is the formation of hydroxyls in reactions involving iron compounds. Since the additive hardly changes the duration of the second stage of the induction period, it can be suggested that OH radicals play an insignificant role in the low-temperature oxidation of dimethyl ether at this stage.

Role of hydroxyl production and heat release in the two-zone fuel-rich adiabatic dimethyl ether/air flames at atmospheric pressure

Fuel-rich laminar adiabatic flames of premixed dimethyl ether/air mixtures at a high initial temperature and atmospheric pressure have been studied by numerical simulation and sensitivity analysis. These flames, having two heat release zones, are of great interest as an unusual and little-studied subject. We have investigated the chemical processes occurring in the two zones and analysed the mechanism of heat release in the flame. It has been found that the key reactions that have a significant influence on the flame speed are those involving dimethyl ether and the products of its incomplete oxidation. Calculation of the heat release rate confirms the presence of two heat release zones in the flame. A comparison of the reactions making a major contribution to the heat release with those significantly affecting the flame speed indicates that the main factor determining the flame speed is the formation of hydroxyls, rather than heat release. Analysis of the flame speed sensitivity shows that in the case of a two-zone structure of the flame, its speed is mainly determined by the reactions taking place in the low-temperature zone. That is, the cool zone with a higher temperature gradient is the leading one.

A skeletal mechanism for flame inhibition by trimethylphosphate

On the basis of a multi-step kinetic mechanism for flame inhibition by organophosphorus compounds including more than 200 reactions, a skeletal mechanism for flame inhibition by trimethylphosphate was developed. The mechanism consists of 22 irreversible elementary reactions, involving nine phosphorus-containing species. Selection of the crucial steps was performed by analysing P-element fluxes from species to species and by calculating net reaction rates of phosphorus-involving reactions versus the flames zone. The developed mechanism was validated by comparing the modelling results with the measured and simulated (using the starting initial mechanism) speed and the chemical structure of H2/O2, CH4/O2 and syngas/air flames doped with trimethylphosphate. The mechanism was shown to satisfactorily predict the speed of H2/O2/N2 flames with various dilution ratios, CH4/air and syngas/air flames doped with trimethylphosphate. The skeletal mechanism was also shown to satisfactorily predict the spatial variation of H and OH radicals and the final phosphorus-containing products of the inhibitor combustion. Further reduction of the skeletal mechanism without modification of the rate constants recommended in the starting mechanism was shown to result in noticeable disagreement of the flame speed and structure.