(753c) Combustion of Boron in Air- Acetylene Pre-Mixed Flame

Boron is an attractive fuel for propellants and explosives because of its high volumetric and gravimetric energy density. However, there are two principal reasons why it is difficult to recover the full energy of its oxidation in practical combustion applications: (1) Ignition is inhibited by a passivating layer of B₂O₃ and B(OH)₃ on the particle surfaces and (2) because of the very high boiling point of boron, it burns heterogeneously at a relatively low combustion temperature restricted by the boiling point of boron oxide (2130 K).  In addition, formation of the thermodynamically favorable B₂O₃ is delayed kinetically when HOBO and other relatively stable intermediate gaseous species are produced.  Finally, boron particles tend to agglomerate causing even longer ignition delays and burn times.   Despite multiple previous studies, however, burn times of fine boron particles in well-characterized environments are not well known.  This work is aimed to characterize combustion of boron powder injected into an air-acetylene flame.  It is further focused on improving boron combustion characteristics by functionalizing surface of commercial boron powders.  

The experiments use 95% pure commercial boron powder with particles in the broad range of 0.04– 40 µm, determined by low angle laser light scattering. Premixed acetylene- air flames with varying equivalence ratios are produced; boron powder is injected axially into the flame with a nitrogen jet.  The particle size distributions are determined using powder that exited from the injector and captured directly on to microscope slides.  Thus, agglomerated particles fed into the flame are observed directly and accounted for in the particle size distributions.  Powders are analyzed using scanning electron microscopy; particle sizes are corrected accounting for the fractal dimensions of the observed agglomerates. The average particle number-based diameter after correction is found to be 1.73 µm.  The measured burn times are correlated with the obtained particle size distributions to recover the effect of particle size on its burn time.  The environment in which the particles burned is characterized in detail using computational fluid dynamics. Preliminary results indicated that boron particles have a burn time, t_b and particle diameter, d correlation of t_b d^0.8 and t_bd^0.7 for equivalence ratios of 0.22 and 0.10 respectively, which is comparable to earlier studies, where coarser powders were used. The work also explores the removal of the inhibiting oxide layer from the commercial boron powder using acetonitrile as a solvent.  To protect clean boron surface, powders are additionally washed in toluene and hexane.   Results of thermo-analytical measurements quantifying the effect of powder surface functionalization on its oxidation kinetics will be discussed as well as preliminary results on combustion of the surface-functionalized powders.