Heat-release mechanism in ammonium dinitramide flame

To verify the adequacy of various models of heat release in ammonium dinitramide flame to real processes, chemical processes in products of thermal decomposition at a pressure of 10 torr and in ammonium dinitramide [ADN; NH₄N(NO₂)₂] flame at a pressure of 0.4 to 60 atm are numerically simulated. The calculations are performed on the basis of a detailed kinetic mechanism and boundary conditions correlated with experimental data, thermodynamic properties, and chemical composition of ADN. The kinetic mechanism includes submechanisms that describe high-temperature chemical processes in NH₃/N₂O/NO/NO₂/HNO₂/HNO₃ and NH₃/HN(NO₂)₂ mixtures, and the global stages of aerosol decomposition. Based on calculated and experimental data, the role of dinitraminic acid HN(NO₂)₂, aerosols, and ADN vapor in heat release in the ADN flame zone adjacent to the burning surface is estimated. The calculations predict that the main source of heat release in the cold flame zone at p ≥ 3 atm is dinitraminic acid incoming through the channel of dissociative evaporation ADN_liq → NH₃ + HN(NO₂)₂ from the burning surface. In the high-temperature flame zone, heat release is caused by the reaction that occurs in the NH₃/N₂O/NO/NO₂/HNO₂/HNO₃ mixture. At moderate pressures, the high-temperature and low-temperature zones are separated by an induction zone. The stage governing production of the OH radical, which plays an important role in combustion, in the induction zone is the reaction HNO₃ + M → OH + NO₂ + M. Because of a high activation energy of the stage, small temperature perturbations in the induction zone at low pressures lead to a finite change in the stand-off distance between the high-temperature flame zone and the burning surface. Therefore, small temperature perturbations in the induction zone, which are caused by admixtures in the sample or by heat transfer between the reacting gas and the ambient medium, may be responsible for disagreement between various experimental data and between experimental and calculated data on the stand-off distance between the high-temperature flame zone and the burning surface. In numerical calculations, the position of the high-temperature zone is effectively controlled by varying rate constants of elementary stages within admissible limits. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 64–76, September–October, 2007.     

HMX flame structure for combustion in air at a pressure of 1 atm

The chemical structure of HMX flame during combustion in air at a pressure of 1 atm was calculated using molecular beam mass spectrometric sampling. HMX vapor was recorded for the first time near the burning surface. A total of 11 species were identified in the HMX flame (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂, NO₂, and HMX vapor), and their concentration profiles were measured. The HMX combustion was unstable. The species concentration profiles exhibit periodic pulsations related to variation in the HMX burning rate. The HMX flame structure at various distances to the burning surface was determined using the average value of the burning rate. Two main zones of chemical reactions in the flame were found. In the first zone ≈0.8 mm wide adjacent to the burning surface, HMX vapor decomposes and NO₂, N₂O, and CH₂O react with each other to form HCN and NO. In the second zone ≈0.8–1.5 mm wide, HCN was oxidized by nitric oxide to form the final combustion products. The composition of the final combustion products was analyzed. The global reaction of HMX gasification at a pressure of 1 atm was established. Heat release values in the condensed phase calculated by the global gasification reaction and by the equation of heat balance on the burning surface (using literature data from microthermocouple measurements) were analyzed and compared. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 26–43, November–December, 2008.     

RDX flame structure at atmospheric pressure

The chemical structure of an RDX flame at a pressure of 1 atm was studied using probing molecular beam mass spectrometry. The flame was found to contain RDX vapor, and its concentration profile was measured in a narrow zone adjacent to the burning surface. In addition to RDX vapor, ten more species were identified (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂ and NO₂), and their concentration profiles were measured. Two main chemical-reaction zones were found in the RDX flame. In the first, narrow, zone 0.15 mm wide adjacent to the burning surface, decomposition of RDX vapor and the reaction of NO₂, N₂O, and CH₂O with the formation of HCN and NO occur. In the second, wide, zone 0.85 mm wide, HCN is oxidized by NO to form the final combustion products. The composition of the final combustion products was analyzed from an energetic point of view. The measured composition of the products near the burning surface was used to determine the global reaction of RDX gasification at a pressure of 1 atm. Values of heat release in the condensed-phase calculated by the global gasification reaction and by the equation of heat balance on the burning surface (using data of microthermocouple measurements) were analyzed and compared. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 49–62, January–February, 2008.     

Molecular-beam mass-spectrometric study of the flame structure of composite propellants based on nitramines and glycidyl azide polymer at a pressure of 1 MPa

A study was performed of the chemical and thermal structure of flames of model composite propellants based on cyclic nitramines (RDX and HMX) and an active binder (glycidyl azide polymer) at a pressure of 1 MPa. Propellant burning rates were measured. The chemical structure of the flame was studied using molecular-beam mass spectrometry, which previously has not been employed at high pressures. Eleven species (H₂, H₂O, HCN, N₂, CO, CH₂O, NO, N₂O, CO₂, NO₂, and nitramine vapor) were identified, and their concentration profiles, including the composition near the burning surface were measured. Two chemical-reaction zones were observed. It was shown that flames of nitramine/glycidyl azide polymer propellants are dominated by the same reactions as in flames of pure nitramines. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 48–57, November–December, 2006.     

Thermal Decomposition and Combustion of Ammonium Dinitramide (Review)

A comprehensive review of thermal decomposition and combustion of ammonium dinitramide (ADN) has been conducted. The basic thermal properties, chemical pathways, and reaction products in both the condensed and gas phases are analyzed over a broad range of ambient conditions. Detailed combustion-wave structures and burning-rate characteristics are discussed. Prominent features of ADN combustion are identified and compared with other types of energetic materials. In particular, the influence of various condensed- and gas-phase processes in dictating the pressure and temperature sensitivities of the burning rate is examined. In the condensed phase, decomposition proceeds through the mechanisms ADN → NH₄NO₃ + N₂O and ADN → NH₃ + HNO₃ + N₂O, the former mechanism being the basic one. In the gas phase, the mechanisms ADN → NH₃ + HDN and ADN → NH₃ + HNO₃ + N₂O are prevalent. The gas-phase combustion-wave structure in the range of 5–20 atm consists of a near-surface primary flame followed by a dark-zone temperature plateau at 600–1000°C and a secondary flame followed by another dark-zone temperature plateau at 1000–1400°C. At higher pressures (60 atm and above), a final flame is observed at about 1800°C without the existence of any dark-zone temperature plateau. ADN combustion is stable in the range of 5–20 atm and the pressure sensitivity of the burning rate has the form r _b = 20.72p ^0.604 [mm/sec] (p = 0.5–2.0 MPa). The burning characteristics are controlled by exothermic decomposition in the condensed phase. Above 100 atm, the burning rate is well correlated with pressure as r _b = 8.50p ^0.608 [mm/sec] (p = 10–36 MPa). Combustion is stable, and intensive heat feedback from the gas phase dictates the burning rate. The pressure dependence of the burning rate, however, becomes irregular in the range of 20–100 atm. This phenomenon may be attributed to the competing influence of the condensed-phase and gas-phase exothermic reactions in determining the propellant surface conditions and the associated burning rate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 6, pp. 54–79, November–December, 2005.     

Combustion Mechanism of Azide Polymer

The combustion wave structure and thermal decomposition process of azide polymer were studied to determine the parameters which control the burning rate. The azide polymer studied was glycidyl azide polymer (GAP) which contains energetic – N₃ groups. GAP was cured with hexamethylene diisocyanate (HMDI) and crosslinked with trimethylolpropane (TMP) to formulate GAP propellant. From the experiments, it was found that the burning rate of GAP propellant is significantly high even though the adiabatic flame temperature of GAP propellant is lower than that of conventional solid propellants. The energy released at the burning surface of GAP propellant is caused by the scission of N N₂ bond which produces gaseous N₂. The heat flux transferred back from the gas phase to the burning surface is very small compared with the heat generated at the burning surface. The activation energy of the decomposition of the burning surface of GAP propellant, E_s, is determined to be 87 kJ/mol. The burning rate is represented by r = 9.16 × 10³ exp(–E_s/RT_s) where r (m/s) is burning rate, T_s (K) is the burning surface temperature, and R is the universal gas constant. The observed high temperature sensitivity of burning rate is correlated to the relationship of (∂T_s/∂T₀)_p = 0.481 at 5 MPa, where T₀ is the initial propellant temperature.     

Structure of ultrahigh molecular weight polyethylene–air counterflow flame

The combustion of ultrahigh molecular weight polyethylene (UHMWPE) in airflow perpendicular to the polyethylene surface (counterflow flame) was studied in detail. The burning rate of pressed samples of UHMWPE was measured. The structure of the UHMWPE–air counterflow flame was first determined by mass spectrometric sampling taking into account heavy products. The composition of the main pyrolysis products was investigated by mass spectrometry, and the composition of heavy hydrocarbons (C₇—C₂₅) in products sampled from the flame at a distance of 0.8 mm from the UHMWPE surface was analyzed by gas-liquid chromatography mass-spectrometry. The temperature and concentration profiles of eight species (N₂, O₂, CO₂, CO, H₂O, C₃H₆, C₄H₆, and C₆H₆) and a hypothetical species with an average molecular weight of 258.7 g/mol, which simulates more than 50 C₇—C₂₅ hydrocarbons were measured. The structure of the diffusion flame of the model mixture of decomposition products of UHMWPE in air counterflow was simulated using the OPPDIF code from the CHEMKIN II software package. The simulation results are in good agreement with experimental data on combustion of UHMWPE.     

On the nature of the burning rate-controlling reaction of energetic materials for the gas-phase model

For model systems with known kinetics of elementary reactions (CH₃NO₂ and HN₃), temperature ranges are established in which the rate-controlling reactions are the initial endothermic decomposition of the starting material or the subsequent secondary reactions. Heat release in reactions of NO₂, NO, and N₂O with various fuels, such as CH₂O, CO, H₂, and HCN, is modeled to establish the kinetic parameters and nature of the rate-controlling reactions in gas flames of nitro compounds. It is shown that the activation energy of the heat-release reaction due to the interaction of NO₂ with a hydrocarbon fuel (which is characteristic of the first flame of nitro compounds) is in the range of 29–33 kcal/mole, depending on the type of fuel. According to the calculations performed, the activation energy of the rate-controlling heat-releasing process due to the deoxidation of NO and N₂O (which is typical of the second flame of nitro compounds) is 43–58 kcal/mole. In the range of high pressures, where the flames merge, the kinetic parameters of heat release are determined by the reactions of the most reactive nitrogen oxide NO₂. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 59–71, May–June, 2007.     

Multistage mechanism of thermal decomposition of hydrogen azide

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.     

Combustion mechanism of HMX

The combustion wave structure and thermal decomposition process of HMX were examined in order to elucidate the burning rate characteristics of HMX. The combustion wave can be divided into three zones: nonreactive solid-phase, surface reaction, and gas-phase reaction zones. Measurements with micro-thermocouples revealed that the heat flux produced in the surface reaction zone is approximately equal to the heat flux transferred back from the gas phase to the burning surface. Accordingly, the reaction process in the suface reaction and the gas phase zones plays a dominant role in the burning rate of HMX. The gas phase reaction zone consists of a two-stage reaction process: the first stage is the exothermic rapid reaction process between NO₂ and aldchydes, and the second stage is the exothermic slow reaction process between NO and N₂O and remaining fuel species. The luminous flame zone which is determined to be the second stage reaction process approaches rapidly the burning surface as pressure increases. However, the luminous flame reaction appears to be little responsible for the burning rate of HMX. Examinations of the quenched burning surface of HMX samples revealed that the burning surface melts and forms a noncrystallized intermediate material. The surface structure appears to be different from the structure of thermally degraded HMX samples which were obtained by a thermogravnmetric analysis.     

Influence of Activation and the HDS Reaction on the Structure of Magnesium Fluoride-Supported Ru Catalysts Derived from Ruthenium Chloride Hydrate

The effect of water and reductants (CO and H₂) on the decomposition of NO _x stored on BaO/Al₂O₃ at 300 °C has been investigated. Water eliminates the initial rapid total uptake of NO₂ but has little effect on the subsequent formation of nitrates that is accompanied by evolution of NO. Water hinders liberation of NO₂ and NO during temperature-programmed decomposition of stored NO _x . Both CO and H₂ lower the temperatures required for decomposition through reduction of NO₂ to NO and N₂ thus restricting NO₂ readsorption. The rate of reduction is lower with H₂ than with CO.     

Selectivity-determining role of C3H8/NO ratio in the reduction of nitric oxide by propane in presence of oxygen over ZSM5 zeolites

The reaction of (NO + C₃H₈ + O₂) can result in selective formation of NO₂ over H-ZSM5, Cu,H-ZSM5, Ag,H-ZSM5, and Li,H-ZSM5 catalysts when the concentrations of NO and O₂ are 0.1 and 9%, SV > 60,000 h⁻¹ (typical for automotive exhausts), and C₃H₈/NO > 1. Despite stoichiometric excess of reductant hydrocarbon below this limit, the in situ formed NO₂ does not react with C₃H₈, thus conversion of NO to N₂ is negligible. NO can be reduced by C₃H₈ selectively to N₂ only when C₃H₈/NO ≧ 1. Contrary to many suggestions the reaction temperature, concentration of oxygen, space velocity, and type of exchange ions have minor influence on the selectivity for N₂. These parameters affect the rates of reactions (NO + ₂), (C₃H₈ + NO_x) and (C₃H₈ + O₂), therefore they also affect the production of N₂ in the HC-SCR process, but only when the ratio of C₃H₈/NO permits. The metal-exchanged zeolites were prepared in situ by solid-state ion exchange from H-ZSM5. Despite the low degree of copper exchange (63%), Cu,H-ZSM5 produces substantially more N₂ than H-ZSM5, Ag,H-ZSM5, or Li,H-ZSM5. However, the selectivity for N₂ is lowest over Cu,H-ZSM5, which also produces considerable NO₂ in the reaction of (NO + C₃H₈ + O₂) even at C₃H₈/NO ≧ 1. Contrary to prior findings, the catalytic activity of Cu,H-ZSM5 for the oxidation of NO by O₂ to NO₂ in absence of hydrocarbon was comparable to that of H-ZSM5 at high space velocities (2.3 l g⁻¹ min⁻¹). By replacing 30 and 40% of the protons of H-ZSM5 by Ag⁺ and Li⁺ ions in Ag,H-ZSM5 and Li,H-ZSM5, respectively, the catalytic activity for this reaction becomes negligible at temperatures ≧100°C. Some mechanistic consequences of these experimental observations are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Burning and cultivation effects on greenhouse gas emissions and nutrients in wetland soils from Saskatchewan,Canada

Wetland fringe areas in prairie agricultural landscapes may be subjected to burning of vegetation in autumn followed by cultivation in spring. The objective of this study was to examine the greenhouse gas (CO₂, N₂O and CH₄) emissions and plant nutrient (NO₃, PO₄ and SO₄) supplies in wetland fringe soils as affected by simulated burning + cultivation, at field capacity and saturation moisture content. Using undisturbed soil cores collected from grassed wetland fringes at four sites in southern Saskatchewan, the impacts were examined over a 20-day period. The burning + cultivation treatment generally reduced CO₂ emissions, tended to increase NO₃–N availability, and had no consistent effect on N₂O emissions, or PO₄–P and SO₄–S supply. Production of CH₄ occurred only at one site, and only under saturated conditions. Compared to field capacity, saturation reduced CO₂ emissions and NO₃–N supply, tended to increase PO₄–P availability, and had no consistent effect on N₂O emissions and SO₄–S. The CO₂ emissions and SO₄–S were greater for soil cores with higher organic matter and salinity, respectively. The N₂O emissions were only occasionally related to soil NO₃–N supply rate.     

Synthesis of Pd–alumina and Pd–lanthana Suspension for Catalytic Applications by One-step Liquid Flame Spray

Catalytic materials of alumina and lanthana supported nanosized palladium particles (7 wt%) in a water suspension were prepared by Liquid Flame Spray (LFS) method. The particle production rate was 90 g/h, using liquid precursors containing Al(NO₃)₃ · 9H₂O, La(NO₃)₃ · 6H₂O and Pd(NH₃)₄NO₃ in water solution. In the LFS method, a turbulent, high-temperature (T_max ∼ 2,700 °C) H₂–O₂ flame is used. The liquid precursor is atomized into micron sized droplets by high velocity H₂ flow and introduced into the flame where the droplets will evaporate. The evaporated compounds decompose and the reaction product re-condenses into particulate material. Here, the nanosized particles are formed by gas-to-particle conversion and the micron sized particles via liquid-to-solid route. In this study, the produced particulate material was collected by thermophoresis along with condensing water into a suspension (nanoparticles in water) in a one-step process. Thus, the whole suspension was produced from the end products of the flame. According to TEM-EDS analysis, the particulate material contained micron sized porous aluminum oxide or lanthanum oxide carrier particles, coated by nanosized palladium particles (∼2–10 nm). The surfactant (Rhodasurf-La 42) was injected into the suspension just after collection to reduce agglomeration. Catalytic performance of the produced Pd–lanthana containing suspension was tested in laboratory with synthetic gases, in order to use it as a possible raw material for three-way catalyst (TWC). The suspension was used as Pd raw material in TWC washcoat and dispersed onto a metallic honeycomb.     

Numerical simulation of the effect of the addition of NO and NO<Subscript>2</Subscript> on a rich hydrogen flame using the tracer method

The effect of the addition of nitric oxides (NO and NO₂) on rich hydrogen-air flames was studied using the tracer method in numerical simulation. It is shown that the effects of these additives are not similar. Both oxides promote the formation of OH and H₂O in the low-temperature zone of the front. The addition of NO reduces the first maximum of the OH profile and the burning velocity. The addition of NO₂ increases the first maximum of the OH profile and does not change the burning velocity. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 19–25, May–June, 2009.     

Hydrazine Radiolysis by Gamma-Ray in the N2H4–Cu+–HNO3 System

Radiolysis of chemical agents occurs during the decontamination of nuclear power plants. The γ-ray irradiation tests of the N₂H₄–Cu⁺–HNO₃ solution, a decontamination agent, were performed to investigate the effect of Cu⁺ ion and HNO₃ on N₂H₄ decomposition using a Co-60 high-dose irradiator. After the irradiation, the residues of N₂H₄ decomposition were analyzed by Ultraviolet-visible (UV) spectroscopy. NH₄⁺ ions generated from N₂H₄ radiolysis were analyzed by ion chromatography. Based on the results, the decomposition mechanism of N₂H₄ in the N₂H₄–Cu⁺–HNO₃ solution under γ-ray irradiation condition was derived. Cu⁺ ions form Cu⁺N₂H₄ complexes with N₂H₄, and then N₂H₄ is decomposed into intermediates. H⁺ ions and H^● radicals generated from the reaction between H⁺ ion and e_aq⁻ increased the N₂H₄ decomposition reaction. NO₃⁻ ions promoted the N₂H₄ decomposition by providing additional reaction paths: (1) the reaction between NO₃⁻ ions and N₂H₄^●+, and (2) the reaction between NO^● radical, which is the radiolysis product of NO₃⁻ ion, and N₂H₅⁺. Finally, the radiolytic decomposition mechanism of N₂H₄ obtained in the N₂H₄–Cu⁺–HNO₃ was schematically suggested.     

NH3 and urea in the selective catalytic reduction of NOx over oxide-supported copper catalysts

The temperature-programmed activity of a series of oxide-supported (TiO₂, Al₂O₃ and SiO₂) Cu catalysts formed from two different Cu precursors (Cu(NO₃)₂ and CuSO₄) for the selective catalytic reduction of NO_x using solutions of urea as a reductant have been determined. These activities are compared to those found using NH₃ as a reducing agent over the same catalysts in the presence of H₂O and it is found that catalysts that are active for the selective reduction of NO_x with NH₃ are inactive for its reduction using solutions of urea. Poisoning of the surface by H₂O_ads is not responsible for all of this decrease in activity and it is postulated that the urea is not hydrolysing to form NH₃ over the catalysts but rather is oxidising to form N₂ or forming passivated layers of polymeric melamine complexes on the surface. The catalysts were characterised by temperature-programmed reduction while temperature-programmed desorption and oxidation of NH₃ and temperature programmed decomposition of urea are used to characterise the interaction of both reductants with the various catalysts.     

Experimental study of N<Subscript>2</Subscript> dilution on bluff-body stabilized LPG jet diffusion flame

The present paper reports the effects of N₂ addition and preheating of reactants on bluff-body stabilized coaxial LPG jet diffusion flame for two cases, namely, (I) preheated air and (II) preheated air and fuel. Experimental results confirm that N₂ addition to the fuel stream leads to an enhancement in flame length, which may be attributed to the reduction in flame temperature. The soot free length fraction (SFLF) also increases, which might be caused by the decrease in fuel concentration and flame temperature. The flame length and also the SFLF are observed to be reduced with increasing temperature of reactants and lip thickness of the bluff body. The NO _x emission level for all burner configurations are found to be attenuated with nitrogen addition, which can be attributed to the reduction of the residence time of the gas mixture in the flame. The emission index of NO _x (EINO _x ) also becomes enhanced with increasing lip thickness and reactant temperature due to an increased residence time and thermal effect, respectively. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 3–10, January–February, 2009.     

Combustion Aspects of Sodium Azide and its mixtures with potassium perchlorate and burning catalysts

Linear burning rate, thermal aualysis, temperature profile, flame structure and cryogenic burnability for the mixtures of sodium azide (SA) of different particle sizes (3.5 μm, 22 μm, and 67 μm), potassium perchlorate (KP) and with or without three kinds of burning catalysts (GeO₂, Er₂O₃, and Y₂O₃) have been investigated. The linear burning rates increase with the KP content up to 33Wt% for similar SA particle size. The temperature-time histories in the vicinity of burning surface were obtained with 20 μm Type K thermocouple embedded in a Strand. The burning surface temperaturres of neat SA and of the SA/KP mixtures are nearly 350°C and 350°C ∼ 550°C, respectively, while the existence of the decomposition surface at 250 °C and condensed layer was suggested with SA/KP mixtures. In visual observation for the flame structure, the front of luminous flame zone appers to be in contact with the condensed phase surface. For example, however, the temperature profile suggests that there exists finitc distance from decomposition surface to flame front in the order of 0.05 mm ∼ 0.1 mm at 1 MPa for SA/KP = 80/20. The differential thermal analysis indicates that the tested catalysts have retarding effect on SA combustion, but a positive effect on neat KP decomposition in spite of being impotent for the burning rate increase of the SA/KP mixture. It was also found that SA strands containing appropatiae fractions of KP can hurn even in liquid nitrogen.