Role of Oxalic Acid in Promoting Ignition and Combustion of Boron: an Experimental and Theoretical Study

Boron is an attractive fuel for propellants and explosives because of its high energy density. However, its combustion is inhibited by the oxide layer that covers the particles. The use of oxalic acid as an additive was shown to promote boron oxidation. In this study, the thermodynamic model FactSage 6.2 and a laser ignition facility were used to investigate the effect of oxalic acid on the burning characteristics of boron particles. The results of the thermodynamic analyses show that oxalic acid can reduce B₂O₃(l) production during boron combustion. This enables removal of the the oxide film and promotes the burning of boron. However, only at high temperatures (>1500 K) H₂O(g) (produced from H₂C₂O₄) can react with B₂O₃ and remove the oxide film. The evolution of boron combustion flame takes place in three stages: ignition, stable combustion, and extinction; the bright yellow color in the flame indicates boron ignition, the bright white color indicates boron combustion, and the bright green color is interpreted as BO₂ emission. Addition of oxalic acid into boron powders can significantly promote boron ignition and combustion. The ignition delay time of the resulting mixture is reduced by 42.4 %, the combustion intensity is raised by 16.7 %, and the combustion efficiency of boron is increased by 21.5 percentage points. The mechanism of action of oxalic acid on enhancing the combustion of boron was studied by scanning electron microscopy.     

Boron-nitride production by the condensation-combustion method

The thermal reduction of boron halide by alkali-metal vapors in the presence of ammonia is proposed for the production of boron nitride by condensation combustion. Combustion temperatures and equilibrium composition of the products are calculated. Self-propagating regimes of condensation combustion in an oxygen-free system are realized. A unit for producing condensed products with variation of the combustion parameters is designed. It is shown that the combustion products formed contain polycyclic structural fragments of boron nitride with characteristic sizes of 8.8·10^–10 m.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 4, pp. 80–85, July–August, 1996.     

Combustion of Boron Particles in Premixed Methane/Air Flames

Because of its high energy density, boron particles have been a subject of interest for the use as propellant in propulsion systems for many years. A cheap and fast opportunity to investigate multiphase reacting flows in such systems is offered by numerical simulations. Therefore, a detailed knowledge of the chemistry and kinetics of boron combustion in different gaseous surroundings is required. The main topic of this contribution is the experimental investigation of the influence of the equivalence ratio of reacting methane‐air mixtures on the combustion time of boron particles. Additionally, numerical calculations were performed using a combustion model proposed by Yeh et al. [1] and modified by Hussmann et al. [2, 3]. The experimental results show that for small particles there exists an optimal stoichiometry, at which the combustion time of boron particles is minimized. High equivalence ratios yield larger burning times because of a low concentration of oxygen. Low equivalence ratios are accompanied by low flame temperatures also leading to large burning times, because of a slow reactive evaporation of the boron‐oxide layer. With increasing particle size the burning process is dominated by the evaporation process. Numerical results are in a good agreement with the experiments for small particles. For larger particles, the predicted burning time is too high due to the fact that boron particles cannot be treated as a sphere as assumed in the original model. By implementing a sphericity factor good agreement with the experiments can be achieved.     

Effect of Carbon Dioxide on the Reactivity of the Oxidation of Boron Particles

Carbon dioxide produced in the primary combustion of propellants apparently affects the combustion of boron. In this study, the thermodynamic and kinetic properties of the combustion of boron particles in a CO₂ environment (solely or as a mixture with other gases) were investigated using thermogravimetric analyses. For the combustion of boron in an atmosphere containing 10 % oxygen, in addition to a large initial weight gain, a second increase in weight was observed when the temperature reached 1150 °C. However, when the combustion was carried out in pure CO₂ atmosphere, the sample lost weight at temperatures above 1300 °C. The above results indicated that the layer of boron oxide covering the boron particles had a significant effect on the combustion process. With a limiting concentration of O₂ (10 %), the initial temperature and effective activation energy slightly decreased as the content of CO₂ increased from 0 to 30 %. However, a further increase in the CO₂ content (50 %) increased the effective activation energy, indicating the inhibitory effects of CO₂ at higher concentrations. Furthermore, the weight and the rate of weight gain gradually increased with increasing CO₂ content. This behavior was attributed to the improvement in the diffusion of the oxidant. This study conclusively revealed that the inclusion of an optimal level of CO₂ in a reaction environment containing O₂ facilitated the combustion of boron particles.     

Extinction of dispersed heterogeneous systems

We consider extinction of various dispersed systems. Isolated boron particles and boron particles in gases are studied. Stability analysis of steady-state thermal regimes of reacting heterogeneous systems for the case of two parallel reactions on the reaction surface using the Frank-Kamenetskii method gives extinction conditions in oxygen-containing media. Curves of the extinction particle size versus the ambient temperature, oxidizer concentration, and, for particles in gases, also versus the oxidizer-to-fuel ratio are plotted. Approximate analytical calculations showed that the extinction process can be most actively controlled by varying the combustion temperature: a decrease in the latter increases the extinction particle size and decreases the completeness of fuel combustion. It is shown that at low ambient temperatures the extinction particle size for suspensions is larger than that for isolated particles. This effect is caused by a decrease in the oxidizer concentration during combustion of suspensions. At high temperatures, the role of this factor weakens.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 6, pp. 12–19, November–December, 1996.     

Effect of Initial Oxide Layer on Ignition and Combustion of Boron Powder

Only few data exist for experimental studies on ignition and combustion of boron particles with initial oxide thickness. The oxidation, ignition and combustion characteristics including the onset temperatures, weight gain, apparent activation energy, emission spectra during combustion, and ignition delay time of crystalline boron powders with different initial oxide thickness (x₀) were studied by a laser ignition and thermogravimetric (TG) analyses. Simulations of the kinetics of oxide layer during boron ignition were conducted using a common model. The results indicated that the onset temperature was approximately 775 °C, independent of x₀. The total weight gain decreased with increasing x_0, whereas the weight gain at 775 °C did not change. The apparent activation energy was found to be insensitive to x₀ and had a constant value of about 210 kJ mol⁻¹. The intensity of the emission spectra gradually decreased while the ignition delay time increased with increasing x₀. Numerical simulation showed that the removal rate of oxide layer enhanced with increasing x₀. The experimental results revealed that the oxidation of boron powder was no diffusion‐controlled process at low temperatures. But the diffusion of oxygen could become important to the oxidation reaction at high temperatures     

Specific Features of Combustion of Cellulose Nitrate and Inert Filler Based Composites

Combustion of composites based on granulated cellulose nitrates and containing, as fillers, aluminum oxide, silicon carbide, carbon, boron nitride, sodium chloride, zinc oxide, and tungsten particles, is studied. Burning rates under atmospheric pressure, combustion-front temperatures, and mass losses after complete combustion of the composites are measured. The dependence of the burning rate on the size of silicon-carbide particles is obtained. Models of solid and laminar combustion are used to predict combustion characteristics, which are in good agreement with experimental data. Some specific features of combustion of composites with particular fillers are explained.     

含团聚硼富燃料推进剂的能量特性及燃烧性能

用最小自由能计算程序计算了含硼富燃料推进剂的能量性能,探讨了不同压力时硼粉的质量分数对富燃料推进剂能量性能的影响,采用靶线法和化学滴定法研究了富燃料推进剂的燃烧特性和燃烧残渣中硼粉的燃烧效率。结果表明,随着硼粉含量的增加,推进剂的能量增大;大粒径的团聚硼对富燃料推进剂的燃速和压强指数影响较大,随着团聚硼含量的增加,推进剂的燃速提高;含硼富燃料推进剂中的硼粉燃烧后单质硼和硼化物的摩尔比发生了明显的变化,无定形硼粉经团聚后燃烧效率明显提高。     

Preparation and Properties of Boron‐Based Nano‐B/NiO Thermite

Boron particles have several major burning problems, such as incomplete combustion, poor ignitability, and a complex burning process in solid propellants. It is documented that the low ignitability and combustion efficiency of boron are caused by the oxidation of its surface. In order to improve the combustion efficiency of boron particles, a precipitation method was employed to prepare nanometer‐sized NiO and coat it on boron particles. The morphology and coating results of the B/NiO nanocomposite thermite were characterized using different approaches such as SEM, X‐ray Diffraction (XRD), and EDS. The results indicated that the boron particles were well distributed and coated completely by nanocomposite NiO. The B/NiO nanocomposite thermite reaction process was tested by TG‐DTA. The results showed that the reaction temperature of B/NiO particles is about 30 °C lower than that of boron particles. The B/NiO thermite and boron powder were added to Mg/PTFE propellant to be measured for their respective combustion performance. The results showed that the burning rate of the B/NiO‐Mg/PTFE propellant increased by 22.8–25.2 %, mass burning rate by 26.7–30.8 %, and combustion temperature increased by 8–56 °C compared to the B‐Mg/PTFE propellant. The above results indicate that NiO coating of boron particles has a significant effect on the combustion behavior and increases the combustion performance of the propellant compared with uncoated particles.     

典型尺寸燃煤颗粒富氧燃烧特性及燃烧本征动力学研究

利用微型流化床加热速度快、温度分布均匀以及气体近平推流等优势,在直径20 mm自动控温的微型流化床反应分析仪中研究了粒度分布为1.7~3.35 mm和0.12~0.23 mm两种典型尺寸燃煤颗粒在790~900℃温度范围内的富氧燃烧行为。通过快速响应过程质谱对燃烧产生的烟气进行实时监测,成功地识别和记录了粗颗粒燃烧过程中经历的挥发分燃烧和原位新生半焦燃烧两个主要阶段。挥发分析出速度最快,然后快速燃烧,而半焦燃烧速度较慢。相比之下,细颗粒燃烧的这两个阶段具有几乎相同的速率,因而相互耦合而难以区分。根据实验结果,挥发分析出和燃烧为快速反应,煤颗粒燃烧过程速率受原位新生半焦燃烧过程控制。进一步研究了挥发分和原位新生半焦燃烧动力学行为,获得其本征动力学的活化能分别为107.2和143.9 kJ/mol。     

Experimental Study of Combustion of a Cloud of Boron Particles in Air

Stabilization of a premixed boron—air flame at the Bunsen burner nozzle exit is studied. The normal burning velocity of the gaseous suspension of boron particles is determined by spectral zonal camera recording. Qualitative spectral analysis of the particle-gas flame showed the presence of fluctuation bands in the range of 508–600 nm due to the radiation from BO₂ molecules, which confirms gas-phase combustion of the boron particles in the gaseous suspension. Chemical, particle-size, and morphological analyses of the condensed combustion products of powdered boron in air showed the presence of submicron and micron particles with number mean diameters of 100 nm and 14 fum, respectively.     

Effect of distributed injection of air into the afterburning chamber of a ram-rocket engine on the efficiency of combustion of boron particles

A mathematical model of combustion of boron particles in a ram-rocket engine is developed. The boron combustion efficiency for one-stage and two-stage injection of air into the afterburning chamber is calculated. It is demonstrated that two-stage injection of air sometimes allows the time of complete combustion of boron particles to be significantly reduced (by a factor of 1.5–3); thus, the fuel combustion efficiency in the ram-rocket engine can be increased. The simulated results are consistent with available experimental data.     

Vaporization behavior of boron from standard coals in the early stage of combustion

Boron-containing compounds have been listed as one of environmentally hazardous substances in Japan since 2001, and known to condense in coal fly ash particles during coal combustion and coal fly ash formation in coal-fired electric power stations. So far, the authors have revealed that the speciation of boron-containing compounds in coal fly ash particles is mostly a calcium orthoborate or pyroborate. In this research, the speciation of boron compounds in standard coals and their char generated by laboratory-scale combustion test has been investigated by using a microwave-assisted acid digestion method and a Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) in order to reveal the vaporization behavior of boron in standard coals during combustion at relatively low temperature. Three isolated peaks are observed in ¹¹B MAS-NMR spectra of standard coals, and all of them are attributed to four-oxygen-coordinated boron atom. Around 50% of boron vaporizes even though heating condition is 200 °C and O₂ = 25%, and the percentage of vaporization reaches higher value than 80% at 400 °C and O₂ = 25%. The remaining boron contents in ash components are relatively small, and it suggests that most of boron in standard coals exist with relatively volatile carbon contents, and they volatilize in the very early stage of coal combustion.     

Emissivity characteristics of boron and boric oxide at high temperatures

The spectral emissivity of boron at wavelengths λ = 0.45–6 μm during ignition and combustion was determined experimentally. The spatial resolution was not lower than 100 μm. Liquid boric oxide, boron filaments 100 μm in diameter, and artificially made agglomerates of amorphous boron particles 2 mm in size and ≈1.22 g/cm³ in density served as radiation objects. It was determined that, as the surface temperature increased from 1 490 to 2 800 K, the emissivity of boron in the examined range of wavelength decreased almost linearly in an interval of 0.35–0.20. At temperatures of 1400–2100 and wavelength of 0.65 μm, the absorption coefficient of liquid boric oxide was 2.2 ± 0.2 cm⁻¹.     

Study of fragmentation in cBN powders under ultra-high pressure

Studying the fragmentation law and refinement of cubic boron nitride powder under ultra-high pressure is crucial to producing a high-strength, high-density polycrystalline cubic boron nitride. In this paper, brown and black cBN crystalline powders with different micron sizes were selected as initial raw materials for an ultra-high-pressure simulation experiment. Single and mixed particles were extruded under 80MPa low pressure and 5.5GPa ultra-high pressure at ambient temperature for 1 min. The crushing behavior and particle size distribution of cBN powders with different particle sizes and ratios were investigated using a laser particle size analyzer and scanning electron microscopy. Results revealed no particle breakage or deformation at low pressure, and the compaction density was low. However, under ultra-high pressure, the cBN particles showed cracks, plastic deformation, and fragmentation, resulting in crushed fine particles filling in the voids of coarse particles, which led to a higher pressing density. Small-sized or mixed cBN particles with high density ratios were not easily crushed; the coarser the particle size, the more severe the ultra-high-pressure extrusion and crushing. The pressing density also declined, and brown cBN crystal particles with higher impact toughness demonstrated a lower particle breakage rate. The ultra-high-pressure crushing law should be considered and appropriate binders should be selected to improve the sintering performance of PcBN materials; ultra-high-pressure crushing of cBN powder contributes to cBN-cBN and cBN-M-cBN bonds under high temperatures and ultra-high pressure.     

Combustion of mixtures of ultrafine powders of aluminum and boron in air

Results of investigation of the combustion of mixtures of ultrafine aluminum and boron powders (the oxidizer is air) are presented. It is shown that the combustion proceeds in two steps, which differ in temperature. The addition of boron influences the concentrations of AlN, residual Al, and α-Al₂O₃ in the end products of combustion of mixtures of ultrafine powders of Al and B in air. For a fixed sample weight of 4 g, the maximum AlN content is observed in the combustion of an Al+20% B mixture of ultrafine powders, and the combustion temperature is also maximum in this case. When the sample weight is smaller than a certain critical value, the combustion proceeds in one step. Increasing the sample weight of the starting mixture of ultrafine powders of Al and B leads to an increase in the AlN content in the combustion products with simultaneous rise in the combustion temperature. A considerable part of the combustion products stabilizes as acicular polycrystals of micron and submicron sizes formed with participation of a gas phase during combustion. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 61–64, November–December 1999.     

Application of B MAS-NMR to the characterization of boron in coal fly ash generated from Nantun coal

Boron and its compounds are environmentally hazardous substance and are well-known condensed products that appear in coal fly ash during combustion of coal in coal-fired electric power stations. In a previous study, we suggested that boron in coal fly ash obtained from Nantun coal in China, identified as Ash-N, may exist on the surface of relatively large coal fly ash particles or as very fine particles generated by homogeneous nucleation. Although the characterization of boron in coal fly ash is important for its effective stabilization or removal, its detection is quite difficult because of its low concentration in coal fly ash and its light atomic weight. In the present work, solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) technique has been applied to reveal the local chemical structures of boron in Ash-N. In the ¹¹B MAS-NMR spectrum of Ash-N, two peaks which are attributed to a three-oxygen coordinated boron unit (BO₃) and a four-oxygen coordinated boron unit (BO₄) were observed with high resolution. We have estimated quadrupole parameters of the BO₃ unit in Ash-N using computer simulation, and we have fingerprinted these moieties with the parameters of borates. The result of the present analysis shows that calcium- or magnesium-bearing orthoborate or pyroborate are the most likely forms of boron in Ash-N.     

粒径对福建无烟煤在CFB锅炉中燃尽的影响

建立了福建无烟煤细颗粒燃烧模型,计算了其在容量35 t/h循环流化床锅炉炉膛内的燃尽时间和一次通过炉膛的停留时间,分析了不同粒径煤颗粒在不同燃烧温度和不同烟气流速时在CFB锅炉内的燃尽时间和停留时间的变化差异. 实验研究了福建无烟煤粒径对飞灰碳含量的影响及燃尽的影响. 结果表明,细煤颗粒的燃尽时间与停留时间均随粒径增大而增长,但燃尽时间增幅更明显,颗粒一次通过炉膛完全燃尽的临界粒径约为0.15 mm;粒径越大的颗粒其停留时间和燃尽时间对烟气流速和燃烧温度变化越敏感;无烟煤入炉粒径明显影响CFB锅炉飞灰含碳量,选用粒度为3~8 mm的偏粗颗粒为宜.     

Application of Amorphous Boron Granulated With Hydroxyl‐ Terminated Polybutadiene in Fuel‐Rich Solid Propellant

Amorphous boron powder granulated with HTPB, whose particle diameter could be controlled, was prepared by mechanical mill method. It was found that amorphous boron powder could be granulated with HTPB binder to form B‐HTPB particles, whose median particle diameter (d₅₀) and specific surface area are in the range of 125.0–431.0 µm and 0.02–0.1 m² g⁻¹, respectively. The B‐HTPB particles could be dispersed in the HTPB binder with relatively low viscosity compared with direct addition of amorphous boron powder to the HTPB binder. The experimental results showed that the content of boron particles in a fuel‐rich propellant could be increased by addition of B‐HTPB particles and the combustion characteristics of the fuel‐rich solid propellant could be improved.     

Removal of boron from coal fly ash by washing with HCl solution

Boron as an environmentally regulated substance is well known to condense in the coal fly ash generated from coal combustion plants. Since boron in the coal fly ash tends to elute into the soil easily, a technology for its stabilization or removal from fly ash is required. An acid washing process is proposed and studied as one of the candidate technologies for the removal of boron from coal fly ash. A laboratory-scale investigation is conducted on the dissolution behavior of boron in the coal fly ash in a diluted HCl solution. The dissolution of boron and alkaline species is considerably fast and exhibits a behavior different from that of aluminum and silicon, which are major components of the ash. From the kinetic model, it is expected that boron in the ash may mainly be in the form of alkaline or alkaline earth borates that are deposited on the surface of relatively large ash particles of alumino-silicate or may be precipitated as fine particles during coal combustion. This acid washing process is extended to a bench-scale plant and boron is successfully removed from the coal fly ash until its content is less than the regulation limit.