Elizabeth M. Fisher

Inhibition of nonpremixed flames by phosphorus-containing compounds

Phosphorus-containing compounds (PCCs) are proposed as viable alternatives to current, ozone-destroying, flame-inhibiting agents. An opposed-jet burner apparatus was used to study the effectiveness of two low-vapor-pressure PCCs, dimethyl methylphosphonate (DMMP) and trimethyl phosphate (TMP), in extinguishing a nonpremixed methane-air flame. The global extinction strain rate was determined as a function of dopant loadings. Tests were also conducted using nitrogen as an inert additive for reference. Results demonstrate that these phosphorus-containing compounds are significant inhibitors of nonpremixed methane-air flames when introduced into the oxidizer stream, 40 times more effective than nitrogen on a molar basis. A novel technique for measuring the extinction strain rate while maintaining a constant dopant level in one gas stream was developed.

TEMPERATURE DEPENDENCE OF PHOSPHORUS-BASED FLAME INHIBITION

Abstract An investigation of the inhibition properties of Phosphorus-Containing Compounds (PCCs) in moderately strained (global strain rate of 300 s −1 ) non-premixed methane-N 2 /O 2 /Ar flames is presented. The effect of DMMP [dimethyl methylphosphonate, O=P(OCH 3 ) 2 (CH 3 )] on relative OH concentration profiles was measured by using quenching-corrected Laser-Induced Fluorescence (LIF) for the first time. LIF measurements indicate a reduction in the total OH present of 23% for a non-premixed methane-air flame doped with 572 ppm of DMMP. As the stoichiometric adiabatic flame temperature is increased via substitution of Ar for N 2 in the oxidizer stream, the measurements show a strong decrease in the magnitude of the OH reduction. Experimental results show reasonable agreement with computational predictions made using a kinetic model that has been proposed for DMMP decomposition and phosphorus-radical chemistry. Analysis of the computational results shows that the reactions involving phosphorus remove H and O atoms from the radical pool, thus weakening the flame. These reactions produce OH directly, but the rest of the mechanism responds to O and H reductions by reducing OH levels. The key reactions involved in this inhibition process are identified.

Gas-phase pyrolysis of diisopropyl methylphosphonate

Abstract Gas-phase pyrolysis studies of diisopropyl methylphosphonate (DIMP) in nitrogen have been conducted to gain insight into the decomposition behavior of organophosphorus chemical warfare nerve agents. Experiments were conducted in a quartz-lined atmospheric flow reactor between 700 and 800 K, at residence times ranging from 15 to 90 ms. Propylene, isopropanol, isopropyl methylphosphonate (IMP), and methylphosphonic acid (MPA) were identified as decomposition products of DIMP. FTIR spectrometry was used to quantify parent, propylene, and isopropanol mole fractions in the reactor. The proposed pyrolysis mechanism for DIMP comprises two stages. The first corresponds to the unimolecular decomposition of the parent into IMP and propylene. The second involves two competing pathways for the unimolecular decomposition of IMP, one leading to isopropanol and the very reactive methyl dioxophosphorane; the other to propylene once again and MPA. In the range of temperatures studied, an isopropanol to propylene mole fraction ratio close to 0.25 suggests a branching ratio of 1.5 between these two pathways in favor of propylene production. The Arrhenius expression for the unimolecular decomposition of DIMP was found to be: k[s −1 ]=10 (12.0±1.5) [s −1 ] exp (−36.7±4.9[ kcal.mole −1 ]/(RT)). Pyrolysis experiments with isopropyl and t -butyl acetates, which have well-known decomposition rates, were performed to illustrate the apparatus’ ability to produce valid chemical kinetic data. An investigation of the effects of surface to volume ratio on the DIMP decomposition process shows that wall reactions are significant in a 4 mm i.d. quartz tube, but less important in an 8 mm i.d. tube. Their effects are expected to be small in the 45 mm i.d. reactor.

Biofuels from Pyrolysis in Perspective: Trade-offs between Energy Yields and Soil-Carbon Additions

Coproduction of biofuels with biochar (the carbon-rich solid formed during biomass pyrolysis) can provide carbon-negative bioenergy if the biochar is sequestered in soil, where it can improve fertility and thus simultaneously address issues of food security, soil degradation, energy production, and climate change. However, increasing biochar production entails a reduction in bioenergy obtainable per unit biomass feedstock. Quantification of this trade-off for specific biochar-biofuel pathways has been hampered by lack of an accurate-yet-simple model for predicting yields, product compositions, and energy balances from biomass slow pyrolysis. An empirical model of biomass slow pyrolysis was developed and applied to several pathways for biochar coproduction with gaseous and liquid biofuels. Here, we show that biochar production reduces liquid biofuel yield by at least 21 GJ Mg(-1) C (biofuel energy sacrificed per unit mass of biochar C), with methanol synthesis giving this lowest energy penalty. For gaseous-biofuel production, the minimum energy penalty for biochar production is 33 GJ Mg(-1) C. These substitution rates correspond to a wide range of Pareto-optimal system configurations, implying considerable latitude to choose pyrolysis conditions to optimize for desired biochar properties or to modulate energy versus biochar yields in response to fluctuating price differentials for the two commodities.

Variation of chemically active and inert flame-suppression effectiveness with stoichiometric mixture fraction

The need to find alternative fire suppressants has motivated experiments to determine the mode of action of possible candidates. An opposed-jet burner was used to characterize the effectiveness of one possible alternative, dimethyl methylphosphonate (DMMP). Similar tests were done with an inert compound, argon, for comparison. Flame strength was characterized by the extinction strain rate. Experiments included both oxidizer-side and fuel-side doping of methane-nitrogen versus oxygen-nitrogen flames of various compositions. The stoichiometric mixture fraction (Zst) is varied systematically while holding the undoped extinction strain-rate constant, by changing the amount of diluent in the reactant flows. This moves the flame location with respect to the stagnation plane, affecting the fraction of a particular reactant stream that reaches the flame. Measured effectiveness, of fuel-side and oxidant-side doping versus Zst reflects this change in quantity of dopant reaching the flame. To account for this dependence on quantity, effectiveness was normalized by the amount of dopant calculated to reach the maximum temperature contour of the flame. Argons normalized effectiveness was found to be independent of Zst, of adiabatic flame temperature, and of whether the oxidizer stream or the fuel stream is doped. DMMPs normalized effectiveness, however, was observed to be significantly greater when introduced in the oxidizer, rather than fuel, stream. It also exhibits a marked dependence on adiabatic flame temperature, with lower values at higher temperatures.

Effects of dimethyl methylphosphonate on premixed methane flames

The impact of dimethyl methylphosphonate (DMMP) was studied in a premixed methane/oxygen/N2-Ar flame in a flat flame burner slightly under atmospheric pressure at two different equivalence ratios: rich and slightly lean. CH4, CO, CO2 ,C H 2O, CH3OH, C2H6 ,C 2H4, and C2H2 profiles were obtained with a Fourier Transform Infrared (FTIR) spectrometer. Gas samples, analyzed in the FTIR, were extracted from the reaction zone using a quartz microprobe with choked flow at its orifice. Temperature profiles were obtained by measuring the probe flow rate through the choked orifice. Flame calculations were performed with two existing detailed chemical kinetic mechanisms for organophosphorus combustion. DMMP addition caused all profiles except that of CH3OH to move further away from the burner surface, which can be interpreted as a consequence of a reduction in the adiabatic flame speed. Experimentally, the magnitude of the shift was 50% greater for the near-stoichiometric flame than for the rich flame. Experimental CH3OH profiles were four to seven times higher in the doped flames than in the undoped ones. The magnitude of this effect is not predicted in the calculations, suggesting a need for further mechanism development. Otherwise, the two mechanisms are reasonably successful in predicting the effects of DMMP on the flame.

Pyrolysis of Triethyl Phosphate

The pyrolytic decomposition of triethyl phosphate (TEP) has been studied at one atmosphere, from 706 to 854 K, in an isothermal quartz-lined, 4.5-cm-ID turbulent flow reactor with residence times between 15 and 85 ms. TEP was initially present at levels between 30 and 125 ppm, in N2. CO was used as an internal standard for mixing. Fourier-transform infrared spectrometry and gas chromatography/mass spectrometry preceded by conversion to trimethylsilyl derivatives were used to measure TEP and its products in samples extracted through quartz sampling tubes. Ethene, ethanol, diethyl phosphate, monoethyl phosphate, and orthophosphoric acid were observed as products. A multistep decomposition mechanism has been proposed, involving parallel pathways producing ethanol and ethene. Overall uni-molecular rate parameters for TEP decomposition have been obtained. Wall reactions affect the ethanol-to-ethene ratio, and may also affect the TEP decomposition rate.

Extinction of Opposed Jet Diffusion Flames of Scramjet Fuel Components at Subatmospheric Pressures

The effect of atmospheric and subatmospheric pressure on the global extinction strain rate of opposed-jet diffusion flames of gaseous and vaporized liquid fuels in air is quantified. After ignition of a flame between the burner tubes in a vacuum chamber, extinction was approached by gradually reducing the chamber pressure, while the momentum flux balanced flowrates of the fuel and air jets were held constant. Results for methane, ethylene, and n-heptane/methane and n-heptane/nitrogen mixtures are presented and compared to predictions based on new pressure dependent chemical-kinetic mechanisms concurrently being developed for scramjet engine simulations. The extinction strain rates for all fuels are seen to monotonically increase with pressure in the subatmospheric range explored, and to depend on other factors such as reactant temperature and composition, and burner tube geometry. The numerical simulations are in qualitative agreement with the experimental results, but over predict the extinction strain rates by up to 30%.

Calculations of the effect of nitrogen vibrational kinetics on laminar flame temperature profiles

Abstract Calculations of several premixed and nonpremixed laminar flames have been performed using two chemical kinetics mechanisms: (1) a vibrational kinetics mechanism treating nitrogen as a number of distinct species corresponding to different vibrational energy levels, with reactions representing transitions between energy levels; and (2) a traditional mechanism that treats nitrogen as being in vibrational equilibrium. In the vibrational kinetics calculations, translational/vibrational and vibrational/vibrational energy transfers are included, as is the effect of collisions with CO2 and H2O on N2 vibrational excitation. Vibrational temperatures are calculated from the populations of various N2 species. For a stoichiometric, atmospheric-pressure premixed methane/air flame, the vibrational temperature is 40 K lower than the rotational/translational temperature in the region of high temperature gradient. The lag in filling upper vibrational energy levels of nitrogen also results in a lower effective heat capacity for the mixture in the vibrational kinetics case than in the vibrational equilibrium case. Rotational/translational temperatures exceed those calculated with the traditional mechanism by as much as 15 K in the region of steep temperature gradient. For diffusion flames over the range of strain rates investigated here, the effect of vibrational kinetics is much smaller. Sensitivity analysis indicates that, among the vibrational kinetics reactions, the initial vibrational excitation of N2 by CO2 and H2O has the greatest impact on the temperature results.

Effect of Oxycombustion Diluents on the Extinction of Nonpremixed Methane Opposed-Jet Flames

ABSTRACT This study investigates the differences between oxycombustion and conventional air combustion through methane flame extinction measurements and calculations. We report global extinction strain rates for methane with O2/N2, O2/CO2, and O2/CO2/H2O flames for a range of calculated stoichiometric adiabatic flame temperatures. Tests were performed in a subatmospheric opposed-jet burner, with pre-vaporized water. For a given oxidant diluent, global extinction strain rates increase strongly with increasing flame temperature and with pressure. At a given temperature, the effect of oxidant composition depends on whether dissociation is included in computing the stoichiometric adiabatic flame temperature. When dissociation is included, O2/CO2 and O2/CO2/H2O flames are roughly 30% harder to extinguish than O2/N2 flames. When dissociation is not included, the oxidant composition has a minimal effect. Calculations with the CHEMKIN suite of programs yield the same ranking of extinction strain rate for the different additives, when stoichiometric adiabatic flame temperature is matched with dissociation included.