Boron Ignition Research Articles

Effects of rare-earth oxide catalysts on the ignition and combustion characteristics of boron nanoparticles

Boron is a potential choice as a fuel additive, especially for biofuels, because it has a high volumetric heating value among different practical metal additives. Although theoretically it can yield a great amount of energy upon complete combustion, its combustion is retarded by the initial presence of boron oxide which coats the particle surface. The present study deals with an experimental investigation of the ignition and combustion behaviors of boron and ball-milled samples of boron and rare earth oxide catalysts in an ethanol spray flame. Commercial boron nanoparticles and both commercial and synthesized ceria-based catalysts have been used for this study. A simple ball-milling technique was employed to prepare the catalyst coated samples, and this technique can be easily adapted for production of large batches. It was observed that the addition of nano-sized catalyst particles to boron nanoparticles improved the ignition characteristics (significant reduction in ignition delay) of the boron. Three different percentages of catalyst loading were tested to determine the effects of catalyst loading on both the ignition behavior of the boron and the primary combustion reaction. Analysis of BO2∗ emissions and of the exiting particles confirmed complete boron combustion in the combustor geometry. The nanoparticles containing ceria demonstrated improvements in both boron ignition (reduced ignition delay time) and (less significantly) the ethanol combustion.

Metal-particle ignition and oxide-layer instability

The use of metals as high-energy fuel additives is generally compromised by the appearance of a strongly protective oxide layer that covers the fuel surface. The previous work concentrated on the elimination of the oxide layer by a global, symmetry-conserving attack, using an admixture of aggressive chemical constituents in the ambient atmosphere, a strong flux of radiation, or strongly shearing gas flows designed to intensely strain the surface layer. This paper shows that symmetry breaking leads to a different approach to ignition. For a liquid oxide layer, destabilization can be obtained via the Marangoni effect associated with longitudinal surface stress, thus breaking the translational symmetry in longitudinal directions. The thermodynamic state of the layer is described in a thin-film model, which leads to a creeping-flow approximation. Ignition by way of the Marangoni effect is then shown to result from spreading of punctures and ruptures in the oxide layer, which is a prerequisite for layer thinning or even complete layer removal. Boron-particle ignition is selected to illustrate the theory, because the well-known difficulties with boron ignitability have greatly impaired the use of boron fuel in propulsion devices. It is shown that, for ambient temperatures below 1634 K, the oxide surface layer can be destabilized by way of punctures and ruptures, owing to the peculiar property of the boron oxide, namely, positive Marangoni numbers. Previous models of boron-particle ignition insisted on conservation of symmetry and expressly excluded the appearance of punctures and ruptures. Because of this constraint, a critical ambient temperature of 1900 K for boron ignition was obtained, so that the new value of 1634 K opens up a novel approach to ignition.

EFFECTS OF FLUORINE-CONTAINING SPECIES ON THE IGNITION AND COMBUSTION OF BORON PARTICLES: EXPERIMENT AND THEORY

The ignition and combustion of isolated boron particles in fluorine-containing environments were investigated both experimentally and theoretically. Boron particles (l-μm amorphous and 3-μm crystalline) were ignited and burned completely in the post-flame region of a multi-diffusion flat-flame burner, which provided a uniform zone of combustion products of CH4/NF3/O2 mixtures. In fluorinated environments, no clear distinction was observed to define a two-stage combustion process, a characteristic feature of boron oxidation without fluorine. At 1,780 K, boron ignition required a higher oxidizer concentration in non-fluorinated environments than in fluorinated environments. HF was found to increase the total burning times (tb) of boron particles; whereas F significantly reduced tb. A theoretical model was developed for simulating the combustion of an isolated boron particle in fluorine-containing environments. The oxide layer removal process was modeled using a reaction mechanism, which considers vaporization process of B2O3/(BO)n mixture and four global surface reactions of oxide layer with O2, H2O, F, and HF. The major products during the oxide removal process were found to be OBF, FBOH, HBO2, and BO2. The "clean" boron combustion model includes four global surface reactions of O2, H2O, F, and HF with boron. BF3, OBF, HBO2, and B2O2 are the major products during the "clean" boron combustion stage. Predicted tb are in good agreement with the measured data in the current study and other published experimental data in the literature. The calculated results show that both oxide layer removal and "clean" boron burning rates increase significantly in the presence of atomic fluorine.

Phase changes in boron ignition and combustion

The ignition of electrically heated boron filaments in air and argon/oxygen mixtures has been studied. Boron filament resistance, temperature, and emissions from the BO and BO 2 bands were monitored. Preliminary experimental data have also been obtained to characterize the phases formed inside burning boron particles produced and ignited from the same filament material by feeding a vibrating boron filament into an oxygen–acetylene flame. The liquid boron particles so formed and ignited burned in room air where their combustion was recorded using a high-speed video system. Samples of both filaments and particles quenched at different times during their combustion were analyzed using electron microscopy to characterize their internal structures and compositions. The filaments “burned” in two distinct stages. The onset of the first stage was characterized by a local minimum in the filament resistance, a sharp spike in boron oxide radiation emission, and a rapid rise in temperature. It occurred at a temperature of 1500 ± 70°C, independent of the preheating history and oxygen content (5–40%) in the gas environment. These data and changes in the filament physical characteristics suggest that a phase transition occurs in the filaments at this temperature and triggers stage one combustion. A transition from α to β rhombohedral boron structures has been reported in this temperature range. The burning boron particles exhibited periodic brightness oscillations that arise from asymmetric particle combustion associated with internal phase changes. Electron microprobe analyses indicated significant amounts of oxygen contained within both quenched filaments and particles. Additionally, quenched filament samples collected during the second stage of combustion exhibited large spherical voids. The present observations indicate that in-depth heterogeneous processes play important roles in boron ignition and combustion and that their elucidation will result in a more complete description of ignition and combustion phenomena that previously have been only incompletely understood.

Kinetic model of liquid B2O3 gasification in a hydrocarbon combustion environment: I. Heterogeneous surface reactions

AbstractAs part of an ongoing program to model hydrocarbon assisted boron combustion, a kinetic model has been developed to describe gasification of the ubiquitous boron oxide coating that inhibits particulate boron ignition. This model includes homogeneous gas phase oxidation reactions, multi‐component gas phase diffusion, heterogeneous surface reactions, and oxide vaporization. The kinetic processes are treated using a generalized kinetics code, with embeded sensitivity analysis, for the combustion of a one‐dimensional (particle radius), spherical particle.This article presents the heterogeneous surface reactions used to describe oxide gasification and presents selected model results for a spherical boron oxide droplet which illustrate the dependence of the oxide gasification rates on the ambient temperature and particle diameter.

An investigation of combustion of boron suspensions

A steady nitrogen jet transporting fine (0.04∼0.15 μ diameter) boron particles at low loading densities was injected coaxially into the hot combustion products of a flat-flame burner to study the kinetics of boron ignition and combustion. Three types of boron flame-plumes were found: a dark-yellow plume emitting, dark-brown smoke from its tip, at measured maximum flat-flame temperatures below 1800 K; a yellow plume surrounded in its lower part by green emission, between approximately 1800 K and 1900 K; and a bright-yellow plume entirely surrounded by bright-green radiation, above 1900 K. In these patterns, which were independent of the oxygen mole fraction in the product gas, over the range 0.08 to 0.80, the bright yellow was interpreted as boron ignition and the bright green as BO2 emissions from boron combustion products. Based on a theory for a one-step, Arrhenius, ignition process, a theoretical analysis of the jet flow was employed to extract overall, rate parameters from measurements of the height of the yellow plume as a function of the flat-flame temperature. The results were interpreted in terms of existing and new potential models for the ignition and combustion of boron particles.

Emissionspectroscopy of Boron Ignition and Combustion in the range of 0.2 m̈m to 5.5 m̈m

AbstractBoron particle combustion is retarded by initial presence of an oxide coating. In current boron ignition models, oxygen is assumed to desolve in the oxide and diffuse to the B/B2O3 interface for reaction. One method to observe the reaction of boron with its surrounding atmosphere is the time resolved emission spectroscopy we applied in the range of 0.2 m̈m to 5.5 m̈m for different burning processes.In one experiment boron powder was burned in oxygen atmosphere initiated by an efficient pyrotechnic device. The energy transfer by the hot gases led to a glowing phase of the boron particles which then changed to a high temperature combustion and ended in a further glowing phase. The two glowing phases emitted continuous emission spectra, while the burning phase emitted the bands of BO and BO2.Another experimental setup was used to feed boron particles in a hot oxidizing atmosphere provided by a propane/air flame which flew into a combustion chamber. Herein the reaction of boron was recorded with high speed cinematography and time resolved emission spectroscopy. The flame contained a small amount of background radiation and we could identify emitted bands of BO, BO2, HBO2, CO and CO2.

Direct Mixing and Combustion Efficiency Measurements in Ducted, Particle-Laden Jets

Using water-cooled probes and hot-gas valves, gas-particle samples were withdrawn from the secondary duct of an air-augmented laboratory burner. Using a boron-loaded propdlant, air/fuel ratio and secondary duct pressure were varied from 12/1 to 32/1 and from 82 to 127 psia, respectively. From sample analysis of chlorine, argon (air tracer), boron, and boron oxide, radial and axial profiles of air, gaseous fuel, participate fuel and percent of boron combustion were determined. Particles and gases mixed at significantly different rates. Measured gasphase mixing rates were comparable to model predictions, which assumed particles to be in equilibrium with gases. Boron combustion efficiency varied markedly with duct position, air/fuel ratio and secondary chamber pressure. Low boron combustion efficiency resulted principally from delayed ignition of the gaseous fuels after dilution by air reduced resulting gas-phase temperatures below the boron ignition temperature. This was especially true at low secondary duct pressures.

Boron Particle Ignition in Hot Gas Streams

A model of boron ignition treating the inhibiting effect of a boric oxide coating has been developed. Transient differential equations describing the generation and removal of the oxide and associated thermal effects along with heat transfer between the particle and surroundings have been derived, converted to difference form and programmed for computer solution for determination of ignition limits and ignition times. Predictions for particles studied by Macek in a flat-flame burner have been compared with experimental results—agreement is good for dry gas cases, but poor when the gas stream includes water. The program has been used to predict effects of initial oxide thickness, particle size, pressure, oxygen content, initial particle temperature, gas temperature and surroundings radiation temperature on ignition. In addition, the original differential equations have been treated by a stability analysis to determine ignition limits—excellent agreement is found between results obtained with the stability analysis and the numerical analysis.

Boron Ignition and Combustion in Air-Augmented Rocket Afterburners

Abstract Results of experimental studies of the afterburning of the exhausts of highly boron-loaded, fuel-rich propellants with air and results of boron single particle combustion studies have been examined for definition of limiting processes causing incomplete combustion of boron in typical afterburners, particularly at low afterburner pressure. This examination indicates that ignition of the boron particles, inhibited by the presence of a coating of boron oxide, is likely the controlling process. Transient differential equations describing the generation and removal of oxide and the temperature history of a particle have been derived, converted to difference equation form and programmed for computer solution to permit prediction of whether or not a given particle will ignite and, if it will, the time required for ignition. Predicted ignition times for particles studied by Macek in a flat-flame burner have been compared with his experimental results—reasonably good agreement has been obtained. The progr...