Combustion characteristics of nanoaluminum, liquid water, and hydrogen peroxide mixtures

An experimental investigation of the combustion characteristics of nanoaluminum (nAl), liquid water (H₂O_(l)), and hydrogen peroxide (H₂O₂) mixtures has been conducted. Linear and mass-burning rates as functions of pressure, equivalence ratio (Φ), and concentration of H₂O₂ in H₂O_(l) oxidizing solution are reported. Steady-state burning rates were obtained at room temperature using a windowed pressure vessel over an initial pressure range of 0.24 to 12.4 MPa in argon, using average nAl particle diameters of 38 nm, Φ from 0.5 to 1.3, and H₂O₂ concentrations between 0 and 32% by mass. At a nominal pressure of 3.65 MPa, under stoichiometric conditions, mass-burning rates per unit area ranged between 6.93 g/cm² s (0% H₂O₂) and 37.04 g/cm² s (32% H₂O₂), which corresponded to linear burning rates of 9.58 and 58.2 cm/s, respectively. Burning rate pressure exponents of 0.44 and 0.38 were found for stoichiometric mixtures at room temperature containing 10 and 25% H₂O₂, respectively, up to 5 MPa. Burning rates are reduced above ∼5 MPa due to the pressurization of interstitial spaces of the packed reactant mixture with argon gas, diluting the fuel and oxidizer mixture. Mass burning rates were not measured above ∼32% H₂O₂ due to an anomalous burning phenomena, which caused overpressurization within the quartz sample holder, leading to tube rupture. High-speed imaging displayed fingering or jetting ahead of the normal flame front. Localized pressure measurements were taken along the sample length, determining that the combustion process proceeded as a normal deflagration prior to tube rupture, without significant pressure buildup within the tube. In addition to burning rates, chemical efficiencies of the combustion reaction were determined to be within approximately 10% of the theoretical maximum under all conditions studied.     

Conjugated processes of bulk aluminum and hydrogen combustion in water-oxygen mixtures

The article deals with the investigation of the combustion of aluminum bulk samples in water vapor, an aqueous solution of hydrogen peroxide (33.1 wt%), and water-oxygen mixture at uniform heating of the reactor (1 K/min) up to 773 K. It is revealed that the major portion of hydrogen peroxide is decomposed directly in aqueous solution. The resulting oxygen, as well as oxygen added to the water, provided oxidation of only a part of hydrogen (≈25%) released during the complete oxidation of aluminum by water. The time dependences of the reactants’ temperature and pressure, as well as the temperature corresponding to the onset of the H₂ release were determined. The most intense oxidation of aluminum in water vapor was noted within the temperature range of 593–769 K, as well as in a hydrogen peroxide solution at 548–693 K, and in H₂O/O₂ mixture at 567–742 K. It is revealed that the oxidation of hydrogen with oxygen intensifies water oxidation of aluminum to a greater extent than it follows from the heat effects of H₂ oxidation. As a result of oxidation, a loose powder of aluminum oxide nanoparticles was obtained.     

Oxidation kinetics of micron-sized aluminum powder in high-temperature boiling water

A new efficient method of hydrogen, heat and aluminum oxide/hydroxide co-production is proposed. Only micron-sized aluminum powder (without any chemical activation) and usual water are used as initial reagents. For aluminum to be effectively oxidized, water is converted into the high-temperature boiling state that creates high pressure inside oxidation reactor. Paper describes the oxidation kinetics of aluminum micron powder in high-temperature boiling water depending on powder size and reactor temperature (pressure). Kinetic experiments were carried out using four types of aluminum powders with different dispersity. Due to kinetic experiments it was established that micron-sized aluminum powder intensively oxidizes in boiling water at temperatures above 230 °C. Aluminum powders with average particle size of 4.1 μm, 7.2 μm and 22.5 μm were fully oxidized at 296 °C, 308 °C and 350 °C respectively. Aluminum powder with average particle size of 77.5 μm after 10 min staying under about 364 °C was 95% oxidized. Reaction times for 4.1 μm, 7.2 μm and 22.5 μm aluminum powders decreased from 870 s at 237 °C to 33 s at 359 °C, from 800 s at 273 °C to 41 s at 355 °C and from 780 s at 286 °C to 66 s at 350 °C respectively. Aluminum oxidation product X-ray analysis showed that aluminum oxidized to aluminum hydroxide AlOOH - boehmite. Microstructure analysis showed that micron-sized aluminum powder oxidation product represents nano-sized aluminum hydroxide.     

Effects of entrainment and agglomeration of particles on combustion of nano-aluminum and water mixtures

The combustion of nano-aluminum and water mixtures is studied theoretically for a particle size of 80 nm and over a pressure range of 1–10 MPa. Emphasis is placed on the effects of entrainment and agglomeration of particles on the burning rate and its dependence on pressure. The flame thickness increases by a factor of ∼10, when particle entrainment is considered. This lowers the conductive heat flux at the ignition front, thereby reducing the burning rate. The pressure dependence of the burning rate is attributed to the changes in the burning time and velocity of particles with pressure. In the diffusion limit, the pressure exponent increases from 0 to 0.5, when the entrainment index increases from 0 to 1.0. A similar trend is observed in the kinetics-controlled regime, although the corresponding value exceeds the diffusion counterpart by 0.5. The kinetics-controlled model significantly over-predicts the burning rate and its pressure exponent, depending on the entrainment index. The present analysis suggests that nano-particles formed closely-packed agglomerates of diameter 3–5 μm, which may burn under diffusion-controlled conditions at high pressures.     

Experimental study of hydrogen production process with aluminum and water

This study reported a novel hydrogen production experimental set up, which utilizes the chemical reaction between aluminum and water to produce hydrogen. The developed experimental setup had an aluminum powder spraying subsystem integrated within the overall setup. The effectiveness of this hydrogen production experimental set up was improved using 149-μm aluminum powder, and nitrogen gas as the medium to facilitate the spraying of the aluminum powder. Furthermore, the study utilized sodium hydroxide as the reaction promoter. The various experimental conditions implemented during the testing process included changes in the water temperature and system inputs. The criteria used to evaluate the system performance were the hydrogen yield and hydrogen production rate. The tap water was able to achieve a full hydrogen yield due to its composition, however, the 50% increase in NaOH mass trial was able to achieve a higher yield of 97.15% and 95.44% for the 3g and 6g aluminum sample test respectively. Furthermore, seawater was found to achieve a yield of 58.8%, which can be considered a viable option for future testing. Furthermore, seawater's abundance also adds to its viability for future testing. Also, the study results showed that an increase in reaction temperature best facilitates a chemical reaction taking place. This was evident during the staring temperature of the water test for the 6g aluminum samples. For instance, the maximum hydrogen production rate for the 70 °C was 35.04 mL/s, while the smallest peak for hydrogen production rate was observed using the 40 °C as the starting temperature. The 40 °C test produced a maximum hydrogen production rate value of 27.99 mL/s.     

一种计算火焰与燃烧室之间几何关系的新模型

建立了一种用于计算火焰与各种几何形状的点燃式发动机燃烧室之间相互关系的计算模型，应用在发动机准维模型中能很好地满足火焰形状椭球化、火焰中心飘移等计算要求。能计算火焰前锋面积，已、未燃区体积和已、未燃区与燃烧室之间的传热面积等参数。通过有效性讨论，认为模型具有较好的精确性和适应性。     

Experimental investigation of solar assisted hydrogen production from water and aluminum

Sustainable production of hydrogen at high capacities and low costs is one the main challenges of hydrogen as a future alternative fuel. In this paper, a new hydrogen production system is designed and fabricated to investigate hydrogen production using aluminum and solar energy. Numerous experiments are performed to evaluate the hydrogen production rate, quantitatively and qualitatively. Moreover, correlations between the total hydrogen production volume over time and other parameters are developed and the energy efficiency and conversion ratio of the system are determined. Also, a method is developed to obtain an optimal and stable hydrogen production rate based on system scale and consumed materials. It is observed that at low temperatures, the hydrogen production volume, efficiency and COP of the system increase at a higher sodium hydroxide molarity. In contrast, at high temperatures the results are vice versa. The maximum hydrogen production volume, hydrogen production rate, reactor COP and system efficiency using 0.5 M NaOH solution containing 3.33 g lit^?1 aluminum at 30 °C are 6119 mL, 420 mL min^?1, 1261 mL H₂ per 1 g of Al, and 16%, respectively.     

Combustion characteristics of micron-sized aluminum particles in oxygenated environments

Experimental data describing combustion of micron-sized aluminum particles as a function of their size are limited. Often combustion characteristics are derived indirectly, from experiments with aerosolized powder clouds. In a recently developed experiment, micron-sized particles cross two laser beams. When each particle crosses the first, low-power laser, it produces a scattered light pulse proportional to the particle diameter. The second, powerful CO₂ laser beam ignites the particle. The optical emission pulse of the burning particle is correlated with its scattered light pulse, so that the combustion characteristics are directly correlated with the size for each particle. In this work, emission signatures of the ignited Al particles are recorded using an array of filtered photomultipliers to enable optical pyrometry and evaluate the molecular AlO emission. Processing of the generated data for multiple particles is streamlined. Experiments are performed with spherical aluminum powder burning in atmospheric pressure O₂/N₂ gas mixtures with the oxygen concentrations of 10%, 15%, and 21% (air). In air, the AlO emission peaks prior to the maximum in the overall emission intensity, and the latter occur before the maximum of the particle temperature. The temperatures at which particles burn steadily increase with particle size for particles less than 7.4 μm. For coarser particles, the flame temperature remains constant at about 3040 K. In the gas mixture with 15% O₂, the flame temperatures are observed to increase with particle size for the entire range of particle sizes considered, 2–20 μm. At 10% O₂, the flame temperatures are significantly lower, close to 2000 K for all particles. The intensity of AlO emission decays at lower oxygen concentrations; however, it remains discernible for all environments. The results of this study are expected to be useful for constructing the Al combustion models relaxing the assumption of the steady state burning.     

Mechanochemical activation of aluminum with gallams for hydrogen evolution from water

A new method has been developed for mechanochemical activation of aluminum: the metal is treated with a gallam, and the “alloy” is processed in a high-energy ball mill. Kinetic parameters of the reaction between activated aluminum and water (hydrogen evolution rate and yield) under standard conditions are presented. The dependences of the hydrogen evolution rate on the composition and amount of the gallam and on the reaction temperature are reported. The storage behavior of activated aluminum has been investigated.     

Synergistic hydrogen generation from aluminum, aluminum alloys and sodium borohydride in aqueous solutions

A new method to produce high purity hydrogen using reactions of aluminum and sodium borohydride with aqueous alkaline solutions is described. This process mainly consumes water and aluminum (or its alloys) which are cheaper raw materials than the borohydride. As a consequence, this process could be competitive for in situ production of hydrogen. Moreover, a synergistic effect has been observed in hydrogen production rates and yields combining aluminum or aluminum alloys with sodium borohydride in aqueous solutions. Good results have been obtained for powders of Al, Al/Si and Al/Co alloys. The development of this idea could improve yields and reduce costs in power units based on fuel cells which use borohydride as raw material for hydrogen production.     

Enhancement of hydrogen generation rate in reaction of aluminum with water

The aim of this investigation is to enhance hydrogen generation rate in aluminum–water reaction by improving the activity of aluminum particles and using the heat released during the reaction. This was accomplished by developing fresh surfaces by milling aluminum particles together with salt. Salt particles not only serve as nano-millers, but also surround activated particles and prevent re-oxidation of bare surfaces in the air. Therefore, the activated powder can be easily stored for a long time. Immersing the powder in warm water, the salt covers are washed away and hydrogen begins to release at a high rate until efficiency of 100% is achieved. The rate of reaction depends crucially on initial temperature of water. Hence, the mass of water was reduced to employ released energy to increase water temperature and, consequently, to increase hydrogen production rate. The optimum value of salt-to-aluminum mole ratio for achieving high activation, air-storage capability and 100% efficiency was obtained to be 2. When immersed in water, at initial temperatures of 55 and 70 °C, the powder lead to average hydrogen generation rate of ∼101 and ∼210 ml/min per 1 g of Al, respectively. To increase the rate of corrosion, three different alloys/composites of aluminum were prepared by mechanical alloying and activated with optimum salt-to-aluminum mole ratio. The alloys/composites formed galvanic cells after being immersed in water. In the case of aluminum–bismuth alloy, the average hydrogen generation rate increased to ∼287 and ∼713 ml/min per 1 g of Al, respectively.     

Effects of amalgam on hydrogen generation by hydrolysis of aluminum with water

Hydrogen generated by hydrolysis of metal aluminum with water exhibits some potential merits such as system simplicity, safety and controllability. However aluminum is prone not to react with water at low temperatures due to the passive oxide film formed on its surface. In the present investigation, mercury or zinc amalgam acting as a catalyst was employed to promote the aluminum hydrolysis reaction and the hydrogen evolution performance of the aluminum surface covered by mercury and zinc amalgam thin film was evaluated. The results showed that in the presence of mercury or zinc amalgam the hydrolysis of aluminum with water to generate hydrogen could occur at room temperature. The hydrogen evolution rate was strongly dependent on reaction temperature and the maximum hydrogen generation rate of 43.5 cm³ h⁻¹ cm⁻² was obtained at 65 °C for the case coated with zinc amalgam. Zinc amalgam showed a more pronounced effect on aluminum activation and hydrogen generation rate than mercury only. The apparent activation energy calculation showed that the aluminum hydrolysis induced by zinc amalgam has a lower value of 43.4 kJ mol⁻¹ than the case coated by mercury (74.8 kJ mol⁻¹). The X-ray diffraction results revealed that the byproduct is bayerite. The hydrolysis mechanism of aluminum in the presence of mercury or zinc amalgam was proposed based on the microscope observation. The aluminum hydrolysis reaction was found to take place at the mercury/water interface and the moving species in mercury or zinc amalgam thin film to sustain the hydrolysis reaction was aluminum particles.     

Combustion characteristics of fuel droplets with addition of nano and micron-sized aluminum particles

The burning characteristics of fuel droplets containing nano and micron-sized aluminum particles were investigated. Particle size, surfactant concentration, and the type of base fluid were varied. In general, nanosuspensions can last much longer than micron suspensions, and ethanol-based fuels were found to achieve much better suspension than n-decane-based fuels. Five distinctive stages (preheating and ignition, classical combustion, microexplosion, surfactant flame, and aluminum droplet flame) were identified for an n-decane/nano-Al droplet, while only the first three stages occurred for an n-decane/micron-Al droplet. For the same solid loading rate and surfactant concentration, the disruption and microexplosion behavior of the micron suspension occurred later with much stronger intensity. The intense droplet fragmentation was accompanied by shell rupture, which caused a massive explosion of particles, and most of them were burned during this event. On the contrary, for the nanosuspension, combustion of the large agglomerate at the later stage requires a longer time and is less complete because of formation of an oxide shell on the surface. This difference is mainly due to the different structure and characteristics of particle agglomerates formed during the early stage, which is a spherical, porous, and more-uniformly distributed aggregate for the nanosuspension, but it is a densely packed and impermeable shell for the micron suspension. A theoretical analysis was then conducted to understand the effect of particle size on particle collision mechanism and aggregation rate. The results show that for nanosuspensions, particle collision and aggregation are dominated by the random Brownian motion. For micron suspensions, however, they are dominated by fluid motion such as droplet surface regression, droplet expansion resulting from bubble formation, and internal circulation. And the Brownian motion is the least important. This theoretical analysis explains the different characteristics of the particle agglomerates, which are responsible for the different microexplosion behaviors that were observed in the experiments.     

Conversion of aluminum foil to powders that react and burn with water

In the US, the total amount of aluminum scrap and waste, including foil, is outpacing efforts to recycle it into conventional aluminum materials. It would be attractive to develop technologies for converting aluminum foil scrap and waste to useful products and energy carriers. The present paper focuses on the feasibility of converting foil to activated Al powders that chemically split water, releasing hydrogen. As a method for this conversion, high-energy ball milling of Al foil with sodium chloride is investigated, with removing NaCl from the obtained powder by dissolution in cold water or methanol. The powders are characterized using BET specific surface area analysis, laser diffraction particle size analysis, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The obtained micron-sized Al powders readily react with warm (35–80 °C) water. Hydrogen evolution is studied using water displacement, while solid byproducts are examined by X-ray diffraction and thermal analysis. The powders are also mixed with gelled water at various mass ratios and combustion of these mixtures is studied in argon environment. With increasing Al concentration, the combustion front velocity increases despite the decrease in the combustion temperature. The burning rate of the stoichiometric mixture of the activated Al powder with water is comparable with the values reported previously for the mixtures based on nanoscale aluminum, while the content of active Al in the obtained micron-sized powder is significantly higher.     

Controlled hydrogen generation by reaction of aluminum/sodium hydroxide/sodium stannate solid mixture with water

Aluminum/water reaction system has gained considerable attention for potential hydrogen storage applications. In this paper, we report a new aluminum-based hydrogen generation system that is composed of aluminum/sodium hydroxide/sodium stannate solid mixture and water. This new system is characterized by the features as follows: the combined usage of sodium hydroxide and sodium stannate promoters, the use of solid fuel in a tablet form and the direct use of water as a reaction controlling agent. The factors that influence the hydrogen generation performance of the system were investigated. The optimized system exhibits a favorable combination of high hydrogen generation rate, high fuel conversion, rapid dynamic response, which makes it promising for portable hydrogen source applications.     

Hydrogen generation using activated aluminum/water reaction

The effective parameters for the Al-water reaction are studied. Salts such as sodium chloride (NaCl), potassium chloride (KCl) and barium chloride (BaCl₂) were added to Al particles and the mixture were alloyed via high-energy ball milling. It was observed that the reaction of Al/water is evolved and the reaction induction time declined significantly by the application of BaCl₂ as new modifier in comparison with NaCl and KCl. The X-ray diffraction patterns revealed that the other valuable reaction byproduct is AlOOH, which can be calcinated to gamma or alpha alumina. Microstructure of alloys were studied via FESEM and it was observed that as the time of ball milling increased the size of particles decreased. Increasing of the salt to Al powder ratio lead to increasing of hydrogen yield as well as hydrogen production rate. Effect of NaOH alkaline solution was also investigated and according to the results, solutions with higher concentration of NaOH generate higher amount of hydrogen.     

Investigating the explosion hazard of hydrogen produced by activated aluminum in a modified Hartmann tube

Metallic powders exposed to water are sources of hydrogen gas that may result in an explosion hazard in the process industries. In this paper, hydrogen production and flame propagation in a modified Hartmann tube were investigated using activated aluminum powder as fuel. A self-sustained reaction of activated aluminum with water was observed at cool water and room temperatures for all treatments. One gram of Al mixed with 5 wt% NaOH or CaO resulted in a rapid rate of hydrogen production and an almost 100% yield of hydrogen generation within 30 min. The flame structures and propagation velocity (FPV) of released hydrogen at different ignition delay times were determined using electric spark ignition. Flame structures of hydrogen were mainly dependent on hydrogen concentration and ignition delay time, likely due to different mechanisms of hydrogen generation and flame propagation. As expected, FPVs of hydrogen in the Hartmann tube increased with ignition delay time. However, the FPV of upward flame propagation was much larger than that of downward flame propagation due to the effect of spreading acceleration at the explosion vent. Once ignited, the FPV of upward flame propagation reached 31.3–162.5 m/s, a value far larger than the 7.5–30 m/s for downward flame propagation. Hydrogen explosion caused by the accumulation of wet metal dust can be far more dangerous than an ordinary hydrogen explosion.     

Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames

This paper aims to present modeling results of hydrogen/air combustion in a micro-cylindrical combustor. Modeling studies were carried out with different turbulence models to evaluate performance of these models in micro combustion simulations by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are Standard k-ε, Renormalization Group k-ε, Realizable k-ε, and Reynolds Stress Transport. A three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames. Performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NO_x profiles. Combustion reaction scheme with 9 species and 19 steps was modeled using Eddy Dissipation Concept model. Results obtained from this study were validated with published experimental data. Numerical results showed that two equation turbulence models give consistent simulation results with published experimental data by means of trend and value. Renormalization Group k-ε model was found to give consistent simulation results with experimental data, whereas Reynolds Stress Model was failed to predict detailed features of combustion process.