Properties of Gas‐Generating Mixtures Related to Different Fuel and Oxidizer Compositions

This work focuses on solid energetic materials designed to produce high‐pressure gas for pressurizing or inflating devices. In small gas generators sodium azide is often used. Unfortunately, this chemical exhibits drawbacks concerning toxicity and yield of gas. Other classical gas‐generating agents are double base propellants. However, they deliver toxic and reactive gases and their combustion temperatures are high. In previous work a series of alternative gas‐generating compositions have been proposed, fuelled with double base propellants, azodicarbonamide, nitroguanidine or guanidine nitrate and oxidized with potassium nitrate or potassium perchlorate. They were theoretically and experimentally evaluated on a series of combustion properties, such as ignition delay, burning rate, vivacity, specific energy, etc. The purpose of this paper is to experimentally examine the gas production of the previously proposed compositions. The yield of gas is determined through static pressure measurements after a closed vessel test, while the composition of the combustion gases is investigated through gas analysis. The addition of an oxidizer causes a significant drop in the yield of gas, but avoids the formation of hazardous gases, such as H₂ and CO, in most of the studied cases. The only exception is the mixture of a double base propellant with potassium nitrate: potassium nitrate does not fully react with the double base propellant and therefore the formation of CO and H₂ is not prevented.     

Laser Ignition of Various Pyrotechnic Mixtures – an Experimental Study

Ignition of several pyrotechnic mixtures by diode‐laser was studied experimentally using a novel combustion chamber. The ignition delay times dependence on laser intensity could be fit by the expression t_ign=aI⁻ⁿ for all compositions, with I being the laser intensity at target and n=1.4–2.1. This is roughly in accordance with thermal ignition theories assuming a semi‐inert solid. Differences in ignition delay times did not depend on fuel alone or oxidizer alone. The temperature of oxidizer decomposition does not correlate with ignition delay time. Furthermore, the steady state combustion temperature, deduced from emission spectra of the composition products are not correlated with ignition delay time. It is proposed that chemical reactions, taking place in the gas‐phase or in the solid‐phase, play a significant role, but are not solely responsible for ignition delay time. The seemingly uncorrelated ignition delay results between pyrotechnics containing either the same fuel or oxidizer hamper the construction of a “unified theory” for laser ignition of pyrotechnic mixtures.     

Combustion Behavior of Highly Energetic Thermites: Nano versus Micron Composites

Combustion behavior of energetic composite materials was experimentally examined for the purpose of evaluating the unique properties of nano‐scale compared with traditional micron‐scale particulate media. Behavior of composite systems composed of aluminum (Al) and molybdenum trioxide (MoO₃) were studied as a function of Al particle size, equivalence ratio and bulk density. Samples were prepared by mechanically mixing individual fuel and oxidizer particles and combustion experiments included measurements of ignition and flame propagation behavior. Ignition was achieved using a 50‐W CO₂ laser and combustion velocities were measured from photographic data. Reaction kinetics were studied with differential scanning calorimetry (DSC). Results indicate that the incorporation of nano‐Al particles (1) significantly reduces ignition temperatures and (2) produces unique reaction behavior that can be attributed to a different chemical kinetic mechanism than observed with micron‐Al particles.     

Influence of the Metal Particle Size on the Ignition of Energetic Materials

A theoretical study of multi‐particle ignition uses a hot spot model which calculates the temperature evolution of individual hot spots in an energetic material. It indicated that ultra‐fine hot particles would be very effective in igniting energetic materials if impinging and penetrating the solid propellant. Igniting mixtures were prepared containing ultra‐fine Ti particles which react fastest compared with coarse particle mixtures, standard B/KNO₃ and black powder. The ultra‐fine particles, however, are obviously oxidized or gasified too fast as to reach the energetic material to be initiated and longer ignition delays are found mainly compared with coarse particle mixtures. An optimized mixture of coarse and ultra‐fine particles would give an improvement of ignition delay times.     

The Formulation Design and Performance Test of Gas Generators Based on Guanidinium Azotetrazolate

Six types of gas generators based on guanidinium azotetrazolate (GZT) were designed into six formulations having different oxidants: GZT‐LiNO₃ (1), GZT‐NaNO₃ (2), GZT‐KNO₃ (3), GZT‐Mg(NO₃)₂ (4), GZT‐Sr(NO₃)₂ (5) and GZT‐KMnO₄ (6), respectively. The properties of these formulations were investigated in terms from gas production, appropriate combustion temperature and nontoxic gaseous emission. REAL software calculation program [1] was used to calculate the combustion heat at constant pressure, combustion heat at constant volume and specific volume in standard state. It showed that gas generators based on GZT with nitrate salts as oxidant exhibited better performance. Thus its thermal behavior and combustion temperature were studied further and the experimental results were consistent with the theoretical calculation results. Therefore, it can be concluded that formulation 3 has comprehensive optimal performance: low moisture content, insensitivity to friction, heightened vacuum stability, high combustion heat and specific volume. Namely, formulation 3 exhibited the most promising indications of commercial application, such as using in air bags of motor vehicles.     

Combustion of Ti and Zr Particles with KNO3

This paper describes the thermochemical properties and combustion characteristics of pyrolants consisting of Ti/KNO₃ and Zr/KNO₃ mixtures. Differential thermal analysis (DTA) and thermogravimetry (TG) were conducted in order to elucidate the reaction processes of these pyrolants. The results of the experiments suggest that Ti particles react exothermically at 970 K with the decomposed gases of KNO₃, and Zr particles react also exothermically at 700 K with liquified product of KNO₃. Burning rates of the pyrolants were measured with a chimney type strand burner which was pressurized with argon gas. The burning rate of Ti/KNO₃ is more sensitive to pressure than that of Zr/KNO₃. The results indicate that a major exothermic reaction on the combustion wave of Zr/KNO₃ pyrolants is in the condensed phase. The burning rates of both pyrolants are dependent on the oxidizer/fuel ratio and initial temperature of the pyrolants.     

A temperature-profile Study of the Combustion of Black Powder and its constituent binary mixtures

An earlier thermo-analytical study of black powder, using small sample masses and slow heating rates, has been extended to an examination of the behaviour of black powder under the less-controlled conditions of ignition and combustion, by simultaneous measurement of temperature profiles and burning rates. Burning-rate against composition curves for various charcoal/KNO₃, mixtures (sulphurless black powder) and for charcoal/KNO₃, mixtures with various proportions of sulphur, were concave-down-type curves. The compositions of mixtures with maximum burning rates did not correspond with the compositions of mixtures with maximum enthalpy-of-reaction. Maximum temperatures of ∼1400°C were recorded. Burning rates were found to decrease with increasing particle size of the constituents: with increasing compaction of the mixtures, or when inert diluents or subsidiary fuels were added to the mixtures. Burning rates were also affected by moisture contents above 276, and failure of burning occurred at >15% moisture.     

Combustion gas and NO emission characteristics of hazardous waste mixture particles in a fixed bed

Experiments with fixed-bed incinerators were carried out to model the combustion characteristics and gas emission characteristics of hazardous waste mixture particles in a grate furnace. The results indicate that combustion can be divided into three stages: ignition, main combustion and combustion completion stage. According to the various concentrations of O₂, CO₂ and CO, the main combustion stage can be subdivided into pyrolysis gas combustion and char combustion. Primary air rate, moisture and particle size have significant effects on concentrations of combustion gases and NO. Bed height has no effect on CO₂ concentrations but does have an effect on other combustion gases and NO emissions.     

Relationship between the dust flame propagation velocity and the combustion mode of fuel particles

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dust flame of microsized metal particles with diameters d ₁₀ < 15 μm and particle concentrations from ≈10¹⁰ to 10¹¹ m^?3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d ₁₀ = 4 μm) in a laminar dust flame is controlled by oxidizer diffusion toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d ₁₀ = 5–15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional mode.     

The Effect of Additives on the Burning Rate of Silicon‐Calcium Sulfate Pyrotechnic Delay Compositions

The effect of fuel particle size as well as the influence of inert and reactive additives on the burning rate of the Si‐CaSO₄ composition was evaluated. The burning rate decreased with increase in fuel particle size, while the enthalpy remained constant. Addition of fuels to the base composition increased the burning rate, with an increase from 12.5 mm ⋅ s⁻¹ to 43 mm ⋅ s⁻¹ being recorded upon 10 wt‐% Al addition. Ternary mixtures of silicon, calcium sulfate, and an additional oxidizer generally decreased the burning rate, with the exception of bismuth trioxide, where it increased. The Si‐CaSO₄ formulation was found to be sensitive to the presence of inert material, addition of as little as 1 wt‐% fumed silica stifled combustion in the aluminum tubes.     

Optical Coal Particle Temperature Measurement under Oxy‐Fuel Conditions: Measurement Methodology and Initial Results

Coal combustion under oxy‐fuel conditions shows significant differences to combustion in air. Examinations on the single‐grain level give detailed insight into the combustion phenomena of ignition, volatile combustion, and char burnout and, therefore, provide the fundamentals for the development of large‐scale oxy‐fuel facilities. The combustion of a hard coal in a size fraction of d_p = 90–125 μm was investigated in a laminar flow reactor at a temperature of 1500 K. The gaseous fuel oxidizer contained 3 % O₂ by volume and CO₂ or N₂ as diluents. A third measurement in a CO₂‐rich atmosphere containing 9 % O₂ is also presented to show the influence of O₂ concentration. Particle temperatures were measured for three residence times with an imaging two‐color pyrometer.     

Investigation on the Reaction of Powdered Tin as a Metallic Fuel with Some Pyrotechnic Oxidizers

Thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) have been used to examine the thermal behavior of Sn+KClO₃, Sn+KNO₃, and Sn+KClO₄ pyrotechnic systems and the results were compared with thermal characteristics of individual constituents. TG curves for tin powder, heated alone in air, showed a relatively slow oxidation above 570 °C. From thermal results the decomposition temperatures of KClO₃, KClO₄, and KNO₃, in nitrogen atmosphere, were measured at 472, 592 and 700 °C, respectively. For the Sn+KNO₃ pyrotechnic system, the tin oxidation was completed within the range of 480 to 500 °C. Replacing KNO₃ with KClO₄ led to an increase of thermal stability of the pyrotechnic mixture. Among above‐mentioned pyrotechnic mixtures, Sn+KClO₃ has the lowest ignition temperature at about 390 °C. The apparent activation energy (E), ΔG^#, ΔH^# and ΔS^# of the combustion processes were obtained from the DSC experiments. Based on these kinetic data and ignition temperatures, the relative reactivity of these mixtures was found to obey in the following order: Sn+KClO₃>Sn+KNO₃>Sn+KClO₄.     

Coal gasification by a mixture of air and carbon dioxide in the filtration combustion mode

This paper presents the results of a numerical and experimental study of gasification of carbonaceous materials in the filtration combustion mode using mixtures of air with CO₂ as an oxidizer. The results obtained are compared with the results on gasification of carbonaceous materials by a steam–air mixture. It is shown that the replacement of steam in the gaseous oxidizer by an equal volume CO₂ leads to a marked reduction in the combustion temperature. The maximum calorific values of the product gas in coal gasification by a mixture of air and CO₂ are close to the values obtained for steam–air gasification.     

Investigation of a Two‐Stage Airbag Module with Azide‐Free Gas Generators

Airbags are used as a standard equipment of cars to strongly increase the safety of drivers and passengers on accidents. Recent developments include azide‐free formulations of the gas generator and ‘smart’ airbag combustion modules which enable a variable gas output. Guanidium azotetrazolate (GZT) is investigated as a fuel and Cu(NO₃)₂ċ2Cu(OH)₂ as an oxidizer and the resulting azide‐free formulation produces a gas output of about 500 l/kg. The use of V₆Mo₁₅O₆₀, as a catalyst reduces the amount of harmful gases below accepted limits. Various grains were produced, ignited in closed vessels, and the burning rates measured. The investigations of a two‐stage module with igniter units consisting of two squibs and boosters showed a broad variety of pressure‐times curves achievable by time delayed initiation. A simulation by a simplified model could even demonstrate the possibility to compensate the temperature dependency of the combustion by a delayed ignition. In static deployment and sled tests the prototype airbag module fulfilled the requirements.     

Manufacture of Granular Polysilicon from Trichlorosilane in a Fluidized‐Bed Reactor

Manufacturing of polysilicon by chemical vapor deposition from SiHCl₃ in a fluidized‐bed reactor was studied. The effects of reaction temperature, H₂/SiHCl₃ ratio, gas velocity, and seed particle loading were evaluated. The outlet gas composition was analyzed by gas chromatography. The physical features of the product particles were determined by scanning electron microscopy and laser particle size analyzer. Well‐grown product particles were obtained. The temperature and H₂/SiHCl₃ ratio significantly affected conversion, yield, and selectivity, which were less affected by gas velocity and seed particle loading at higher temperatures. The surface reaction kinetics determined the product yield only at lower temperatures, and thermodynamic equilibrium was approached at temperatures above 900 °C.     

Agglomeration of magnesium particles in magnesium-sodium nitrate combustion

Agglomeration phenomenon of magnesium particles during combustion of Mg NaNO₃ propellant has been studied. High speed photographs of combustion zones and the burning surface temperature data indicate that the metal particles form agglomerates on the burning surface in varying degree depending on the mass fraction of NaNO₃. It is found that the increase of oxidizer content increases the metal agglomeration and the agglomerate size depends on the initial particle size of the ingredients. An attempt has been made to predict the size of the agglomerates based on the consideration that the agglomerate size depends on the thickness of the molten oxidizer layer enveloping the metal particles in the condensed phase and surface heat flux providing local temperature environment to agglomerate the metal particles and to eject from the burning surface for the vapour phase combustion. The results were compared with the experimental data. The prediction describes fairly well the observed effects of the concentration and particle size.     

Combustion of Lithium Particles: Optical Measurement Methodology and Initial Results

Combustion examinations on the single‐grain level were carried out in order to get further fundamental insight into the ignition and combustion of lithium particles. Combustion of solid lithium particles in a defined size fraction was analyzed in a laminar‐flow reactor. The exhaust gases of a methane‐air flame provided the reactants O₂, CO₂, N₂, and H₂O for the lithium conversion. Two different atmospheres at various temperatures were investigated. A high‐speed camera system measured size and radiation intensity of burning particles. The results indicate that two different combustion phenomena occurred in lithium combustion. The first was identified as a homogeneously enveloping flame around the lithium particle and the second as a reaction zone next to the particle surface.     

Aerosol-Generating Pyrotechnic Compositions with Components Interacting in the Combustion Wave

Observations were performed of the burning surface of the pyrotechnic aerosol-generating KNO₃/melamine/iditol system. Its effectiveness in suppressing a diffusion hydrocarbon flame and the quantity of the combustion residue were measured. An extremal dependence of the flame suppression effectiveness on the KNO₃/melamine ratio was obtained. The maximum effectiveness corresponds to the disappearance of the melt from the burning surface. This is due to the formation of nonmelting organometallic compounds in the combustion wave. The maximum flame-suppression effectiveness is achieved when the entire metal of the oxidizer is involved in the reaction. Possible versions of such reactions are discussed. Examples of calculation of optimal compositions of this type are discussed.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 3, pp. 86–89, May–June, 2005.     

Filtration combustion of a carbon-inert material system in the regime with superadiabatic heating

Superadiabatic regimes of combustion of carbon mixed with an inert solid with filtration of the steam-air mixture are studied theoretically and experimentally. The temperature in the combustion wave and the composition of gaseous products are obtained as functions of the fraction of carbon in the fuel and the amount of steam in the gaseous oxidant. In the examined range of the control parameters, the maximum temperature in the combustion wave is shown to depend only slightly on the fraction of carbon in the mixture and the amount of steam in the oxidant gas. Simulations of filtration combustion of carbon with allowance for the kinetics of its oxidation are in good agreement with experimental results. The calculated combustion temperature coincides with that measured in experiments. In calculating the composition of the gaseous products, coincidence with experimental data is observed only for particular compositions with the mass content of carbon under 60%. As the fraction of the fuel exceeds 60%, the yield of CO and H₂ increases in experiments, though such a behavior is not predicted by the theoretical analysis. Hypotheses on the reasons for the disagreement in results are put forward and experimentally checked. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 30–38, May–June, 2008.     

Combustion characteristics of a direct-injection engine fueled with natural gas-hydrogen blends under different ignition timings

In this paper, combustion characteristics of a direct-injection spark-ignited engine fueled with natural gas-hydrogen blends under various ignition timings and lean mixture condition were investigated. The results show that the ignition timing has significant influence on engine performance, combustion and emissions. The time intervals between the end of fuel injection and ignition timing are very sensitive to direct-injection gas engine combustion. The turbulence in combustion chamber generated by the fuel jet maintains high and relatively strong mixture stratification is presented when decreasing the time intervals between the end of injection and the ignition timing, giving fast burning rate, high brake mean effective pressure, high thermal efficiency and short combustion durations. For specific ignition timing, the brake mean effective pressure and the effective thermal efficiency increase and combustion durations decrease with the increase of hydrogen fraction in natural gas. Exhaust HC concentration decreases and exhaust NO_x concentration increase with advancing the ignition timing while the exhaust CO gives little variation under various ignition timings.