Combustion of Mechanically Alloyed Aluminum‐Magnesium Powders in Steam

Mechanically alloyed Al ⋅ Mg powders with the mole fraction of Al varied from 0.47 to 0.9 were burned at atmospheric pressure in water vapor. The powders were carried by nitrogen through the center of a hydrogen‐oxygen diffusion flame. The particles ignited in the steam at approximately 2500 K, generated as the hydrogen‐oxygen flame product. Filtered photomultiplier tubes were used to capture the optical emission traces of individual particles as they burned. It was assumed that the measured durations of individual emission pulses are representative of individual particle burn times. Distributions of the burn times were obtained for each powder and correlated with respective particle size distributions to relate particle burn times with their sizes. Color temperatures corresponding to the particle emission signals were also obtained. It was observed that the burn times measured for alloys were more close to those of pure Al than Mg; for particles smaller than 2–3 μm, burn times for the alloys were shorter than for pure metal particles. The effect was strongest for the alloy with 50 wt‐% of Mg (Al_0.47Mg_0.54). Approximately, burn times, τ, as a function of particle size, d, could be estimated using a τ∼dⁿ law, where n increased from 0.72 to 1.05 as the mole fraction of Mg increased from 0.1 to 0.53. The particle flame temperatures varied between 2500 and 3100 K for all alloys except for Al_0.7Mg_0.3, for which the temperatures were somewhat lower. The measured flame temperatures were reasonably close to the adiabatic flame temperatures calculated for combustion of mixed elemental Al and Mg in steam.     

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.     

Particle emission from combustion of oat grain and its potential reduction by addition of limestone or kaolin

Second-rate cereals, unsuitable for food, can be used as fuel for small-scale production of heat and hot water. However, there are more problems related to cereals than to woody fuels. This work aims at characterising the particle emission from residential combustion of oat grain and its potential reduction by addition of limestone or kaolin with the fuel. Then, to a large extent, the potassium supplied by the fuel is expected to be found in coarse particles, leaving the boiler as bottom ash, instead of being emitted to the air in the form of submicron particles. Combustion experiments were performed on a residential boiler, using filter sampling and low-pressure impactors to measure the mass and number concentrations and size distributions of the emitted particles. The particles and the bottom ash were subsequently analysed for inorganic material. To check the combustion conditions and basic emissions from combustion of cereals, the flue gas was analysed with respect to gaseous O₂, CO₂, CO, NO_x, TOC (total organic carbon), HCl and SO₂. Furthermore, thermodynamic equilibrium analysis was used to support the experimental data. Finally, it is concluded that the particle emission can be lowered by supplying kaolin, while there was no effect of limestone.     

Condensed combustion products of aluminized propellants. IV. Effect of the nature of nitramines on aluminum agglomeration and combustion efficiency

The condensed combustion products of two model propellants consisting of ammonium perchlorate, aluminum, nitramine, and an energetic binder were studied by a sampling method. One of the propellants contained HMX with a particle size D ₁₀ ≈ 490 μm, and the other RDX with a particle size _{D 10} ≈ 380 μm. The particle-size distribution and the content of metallic aluminum in particles of condensed combustion products with a particle size of 1.2 μm to the maximum particle size in the pressure range of 0.1–6.5 MPa were determined with variation in the particle quenching distance from the burning surface to 100 mm. For agglomerates, dependences of the incompleteness of aluminum combustion on the residence time in the propellant flame were obtained. The RDX-based propellant is characterized by more severe agglomeration than the HMX-based propellant — the agglomerate size and mass are larger and the aluminum burnout proceeds more slowly. The ratio of the mass of the oxide accumulated on the agglomerates to the total mass of the oxide formed is determined. The agglomerate size is shown to be the main physical factor that governs the accumulation of the oxide on the burning agglomerate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 78–92, July–August, 2006.     

Nanocomposite Thermites with Calcium Iodate Oxidizer

Iodine bearing reactive materials and fuel additives are being developed to inactivate harmful aerosolized spores and bacteria by combined thermal and chemical effects. Nanocomposite thermites with aluminum and boron serving as fuels and calcium iodate as an oxidizer were prepared by arrested reactive milling. Both materials contained 80 wt % of calcium iodate. Morphology and particle sizes of the prepared materials were characterized using scanning electron microscopy (SEM). Both powders comprised particles finer than ca. 10 μm with fuel and oxidizer mixed on the submicrometer scale. Powders were exposed to room air to assess their stability. They were ignited as a thin coating on an electrically heated filament. Powders were injected in an air‐acetylene flame to observe combustion of individual particles. Pressed pellets for both prepared materials were prepared and ignited using a CO₂ beam. Al ⋅ Ca(IO₃)₂ oxidizes rapidly in room air, whereas no aging was detected for B ⋅ Ca(IO₃)₂. Ignition of Al ⋅ Ca(IO₃)₂ occurs around 1150 K, after both aluminum and calcium iodate melt. Ignition is accompanied by ejection of sintered particles undergoing microexplosions while they are combusting. Ignition of B ⋅ Ca(IO₃)₂ occurs between 600 and 700 K, before either of the components melt. Combustion is accompanied by the formation of a luminous halo above the material, suggesting a vapor‐phase reaction involving boron suboxides. Longer ignition delays are observed for the pellets of Al ⋅ Ca(IO₃)₂ heated by the CO₂ laser beam compared to similar pellets of B ⋅ Ca(IO₃)₂. Burn rates of B ⋅ Ca(IO₃)₂ pellets are nearly twice as fast as those of Al ⋅ Ca(IO₃)₂, primarily due to the lower ignition temperature for the boron‐based thermite. The flame temperatures obtained from the time‐integrated optical spectra are close to 2140 and 2060 K for Al ⋅ Ca(IO₃)₂ and B ⋅ Ca(IO₃)₂, respectively. Individual particles of B ⋅ Ca(IO₃)₂ injected into an air‐acetylene flame burn slower than similar Al ⋅ Ca(IO₃)₂ particles. Based on their better stability, lower ignition temperatures, shorter ignition delays, and longer burn times leading to a more gradual release of iodine, B ⋅ Ca(IO₃)₂ composites are suggested to be better suited as components of energetic formulations aimed to defeat stockpiles of biological weapons.     

Combustion kinetics of coke particles in a fluidized bed reactor

Combustion studies with a metallurgical coke have been carried out in batch experiments in an electrically heated fluidized bed reactor. Different experiments were carried out in air at temperatures ranging from 750°C to 950°C and coke particle diameters between 0.675 and 3.500 mm. The reactor outlet concentrations of O₂, CO₂ and CO were monitored continuously. A methodology for estimating the kinetic parameters of char combustion has been developed. From the response of gas outlet concentrations to the batch amount, a parameter to estimate the mean oxygen concentration in the bed was deduced. The overall rate constants at each burnoff stage were also obtained. Particle size of irregular coke particles at different stages of the combustion process were determined by means of image analysis technique. This makes it possible to evaluate the real importance of the external mass transfer resistance during combustion and to estimate the chemical rate constant. The results indicate that the coke particles studied react under Regime II in the size and temperature ranges analysed, though moving to Regime III as long as bed temperature and particle size increase. The estimated mean oxygen concentration around the char particles was in every case lower than that estimated by applying the two-phase theory. The dependence of the chemical rate constant on temperature can be described by the equation k_p[g/(cm² s atm O₂)] = 30 exp(− 22,340/RT_p), where the activation energy is expressed in units of cal/mol.     

Analysis of oxide and vanadate supports for catalytic hydrogen combustion: Kinetic and mechanistic investigations

Combustion synthesized oxide and vanadate compounds (CeO₂, Fe₂O₃, CeVO₄, and FeVO₄) were tested for catalytic hydrogen combustion. The compounds were characterized by X‐ray diffraction and X‐ray photoelectron spectroscopy. All the four compounds showed good activity and stability for catalytic hydrogen combustion and more than 95% conversion was observed over all the compounds within 500°C. The mechanisms for the reaction over the different classes of compounds (cerium‐based and iron‐based compounds) were proposed on the basis of spectroscopic observations. The main difference in the mechanisms was in the nature of adsorption of H₂ over the sites. The elementary processes for the reaction were proposed, corresponding rate expressions were derived, and the rate parameters for the reaction were estimated using nonlinear regression. Langmuir‐Hinshelwood and Eley‐Rideal mechanisms were also tested for the reaction and the proposed mechanism was compared with these mechanisms. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

The evolution of 100-µm aluminum agglomerates and initially continuous aluminum particles in the flame of a model solid propellant. II. Results

A quenching/sampling method was used to study the evolution of monodisperse 100-μm aluminum agglomerates and continuous particles in the flame of combustion products — composite propellant at a pressure of 0.7–8 MPa. The particle residence time in the flame was varied in the range 6–170 msec, whereas the calculated time of their combustion was ≈25 msec. The evolution of a burning particle is related to the consumption of metallic aluminum and the deposition of oxide in the form of a cap, which, after complete combustion of aluminum, is transformed to the final spherical oxide particle. The density of the final oxide particles was measured. The ratio of the diameters and masses of the initial metal particle and the final oxide particles was determined. Data on particle fragmentation during combustion were obtained by comparing the numbers of the initial metal particles and the final oxide particles. Considerable differences in the combustion behavior of the agglomerates and continuous particles of size 100 μm were not found. It was established that the smaller the size of the burning particle the less oxide is deposited on the particle and the more oxide is carried away in the form of oxide smoke. For 100-μm particles, the fraction of deposited oxide was found to be ≈0.1 of the total mass of oxide formed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 61–71, November–December, 2008.     

Olive cake combustion in a circulating fluidized bed

In this study, a circulating fluidized bed of 125 mm diameter and 1800 mm height was used to find the combustion characteristics of olive cake (OC) produced in Turkey. A lignite coal that is most widely used in Turkey was also burned in the same combustor. The combustion experiments were carried out with various excess air ratios. The excess air ratio, λ, has been changed between 1.1 and 2.16. Temperature distribution along the bed was measured with thermocouples. On-line concentrations of O₂, SO₂, CO₂, CO, NO_x and total hydrocarbons were measured in the flue gas. Combustion efficiencies of OC and lignite coal are calculated, and the optimum conditions for operating parameters are discussed. The combustion efficiency of OC changes between 82.25 and 98.66% depending on the excess air ratio. There is a sharp decrease observed in the combustion losses due to hydrocarbons and CO as the excess air ratio increases. The minimum emissions are observed at λ=1.35. Combustion losses due to unburned carbon in the bed material do not exceed 1.4 wt% for OC and 1.85 wt% for coal. The combustion efficiency for coal changes between 82.25 and 98.66% for various excess air ratios used in the study. The ash analysis for OC is carried out to find the suitability of OC ash to be used as fertilizer. The ash does not contain any hazardous metal.     

Influence of precursor morphology on the microstructure of silicon carbide nanopowder produced by combustion syntheses

Silicon carbide nanopowder was synthesized using the combustion-based approach. Combustion synthesis was performed in reduction type SiO₂–Mg–C system. Silicon oxide powders with different morphologies and average particle size were used as starting powders. It was shown that even micro-size silica allows formation of nano-size silicon carbide powder. However, the specific surface area of synthesized SiC particles increases with the decrease the size of the silicon oxide precursor. The mechanism of silicon carbide formation in the combustion wave is also discussed.     

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.     

Effect of particle size on flame retardancy of Mg(OH)2‐filled ethylene vinyl acetate copolymer composites

Four kinds of magnesium hydroxide (Mg(OH)₂) with different particle sizes are chosen and mixed with ethylene vinyl acetate copolymer (EVA) to investigate the effect of particle size on the flame retardancy of composites, which is evaluated by limiting oxygen index (LOI) testing, horizontal fire testing, and cone calorimeter. When Mg(OH)₂ filling level changes from 35 to 70 wt %, the composites filled with nano‐Mg(OH)₂ do not always possess the best flame retardancy, and among the composites filled with micro‐Mg(OH)₂, the composites filled with 800 mesh Mg(OH)₂ show the best flame retardancy; however, the composites filled with 1250 mesh presents the worst one. So the effect of particle size on the flame retardancy of micro‐Mg(OH)₂‐filled EVA is not linear as expected. All the differences are thought to result from both particle size effect and distributive dispersion level of Mg(OH)₂. To prepare the composites with better mechanical properties and flame retardancy, authors suggested that Mg(OH)₂ of smaller size should be chosen as flame retardant, and good dispersion of Mg(OH)₂ particles also should be assured. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4461–4469, 2006     

Charged fraction and electrostatic collection of ultrafine and submicrometer particles formed during O2-CO2 coal combustion

The charged fraction of submicrometer and ultrafine particles generated during bench scale coal combustion and the subsequent penetration of particles through a cylindrical-wire electrostatic precipitator (ESP) in O₂-CO₂ and O₂-N₂ environments were measured. In all combustion environments, natural particle charging within the combustor was not adequate for high efficiency particle collection in the ESP; thus, corona generation was necessary for enhanced particle charging. With a positive applied potential, the corona inception voltage and voltage required to reach a given current level in the ESP in gas mixtures composed of O₂ and CO₂ were higher than those composed of O₂ and N₂, while similar voltages were needed for negative corona generation. In positive coronas, particle penetration through the ESP in O₂-CO₂ environments was 1-2 orders of magnitude higher than in O₂-N₂ environments. Little difference in particle penetration between O₂-N₂ and O₂-CO₂ was seen in negative coronas.     

CFD modelling of air-fired and oxy-fuel combustion of lignite in a 100 KW furnace

In this paper, a comprehensive computational fluid dynamics (CFD) modelling study was undertaken by integrating the combustion of pulverized dry lignite in several combustion environments. Four different cases were investigated: an air-fired and three different oxy-fuel combustion environments (25 vol.% O₂ concentration (OF25), 27 vol.% O₂ concentration (OF27), and 29 vol.% O₂ concentration (OF29) were considered. The chemical reactions (devolatilization and char burnout), convective and radiative heat transfer, fluid and particle flow fields (homogenous and heterogenous processes), and turbulent models were employed in 3-D hybrid unstructured grid CFD simulations. The available experimental results from a lab-scale 100 KW firing lignite unit (Chalmer’s furnace) were selected for the validation of these simulations. The aerodynamic effect of primary and secondary registers of the burner was included through swirl at the burner inlet in order to achieve the flame stability inside the furnace. Validation and comparison of all the combustion cases with the experimental data were made by using the temperature distribution profiles and species concentration (O₂, CO₂, and H₂O) profiles at the most intense combustion locations of the furnace. The overall visualization of the flame temperature distributions and oxygen concentrations were presented in the upper part of the furnace. The numerical results showed that the flame temperature distributions and O₂ consumptions of the OF25 case were approximately similar to the reference combustion case. In contrast, in the OF27 and OF29 combustion cases, the flame temperatures were higher and more confined in the closest region of the burner exit plane. This was a result of the quick consumption of oxygen that led to improve the ignition conditions in the latter combustion cases. Therefore, it is concluded that the resident time, stoichiometry, and recycled flue gas rates are relevant parameters to optimize the design of oxy-fuel furnaces. The findings showed reasonable agreement with the qualitative and quantitative measurements of temperature distribution profiles and species concentration profiles at the most intense combustion locations inside the furnace. These numerical results can provide useful information towards future modelling of the behaviour of pulverized brown coal in a large-scale oxy-fuel furnace/boiler in order to optimize the burner’s and combustor’s design.     

In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2 mixtures in drop-tube furnace

Experimental investigation of the combustion of an air-dried Victorian brown coal in O₂/N₂ and O₂/CO₂ mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO₂ in place of N₂ affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO₂ relative to N₂ is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O₂ fraction of at least 30% in CO₂ is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O₂/CO₂ occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO₂ quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO₂ gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O₂ mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O₂ in CO₂ was raised to a level twice that in air at the furnace temperature of 1273 K. Similar temperatures were achieved for burning char particles in 27% O₂/73% CO₂ and air. As this O₂/CO₂ ratio is lower than that for bituminous coal, 30-35%, a low consumption of O₂ is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal.     

Ignition,combustion, and agglomeration of encapsulated aluminum particles in a composite solid propellant. II. Experimental studies of agglomeration

The combustion characteristics of propellants containing AP, HMX, an energetic binder, and aluminum particles with various polymer coatings are studied at pressures of 0.15 and 4.6 MPa. It is found that the coatings influence the burning rate, the particle size distribution of condensed combustion products, and the completeness of aluminum combustion. It is shown that the agglomeration can be reduced by using aluminum with fluorine-containing coatings. The application of some coatings results in a reduction in the mass of the agglomerates with an insignificant increase in their size. The greatest effect was achieved when using aluminum coated with (CH₂=CH-CH₂-O)₂Si[OCH₂(CF₂-CF₂)₂H]₂ [bis(allyloxy)-bis(2,2,3,3,4,4,5,5-octafluoropentyloxy)silane]. For this coating, a size reduction is also observed for micron-size oxide particles. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 83–97, May–June, 2007.     

A study of salt-assisted solution combustion synthesis of magnesium aluminate and sintering behaviour

We synthesized nano MgAl₂O₄ with a ~80?nm particle size by salt-assisted solution combustion synthesis using LiCl as salt. Nano MgAl₂O₄ produced by conventional solution combustion synthesis commonly exhibits poor uniformity in terms of size with partially sintered particles and a high degree of agglomeration, leading to poor sinterability. It was found in this study that the use of the salt-assisted solution combustion method has successfully lowers the degree of agglomeration with uniform particle size and morphology, demonstrating superior sinterability. Conventional sintering in air atmosphere at 1550?℃ of MgAl₂O₄ obtained by salt-assisted solution combustion followed by calcination at 700–1100?℃ yielded up to 94% of relative density, while the conventional solution combustion method could not match this. In addition, using the spark plasma sintering technique, fully dense (over 99%) submicrometer (~340?nm) transparent polycrystalline MgAl₂O₄ with elevated mechanical properties (~16.6?GPa) was achieved. The salt-assisted solution combustion method could be effectively used for fully dense material, but can be further developed for various nano oxide materials where high dispersion with a low degree of agglomeration is preferred.     

Low temperature complete combustion of dilute toluene and methyl ethyl ketone over Mn‐doped ZrO2 (cubic) catalysts

Combustion of dilute toluene and methyl ethyl ketone over Mn‐doped ZrO₂ catalysts prepared using different precipitating agents, such as tetra‐alkyl ammonium hydroxides and NH₄OH, having Mn/Zr ratios from 0.05 to 0.67, and calcined at different temperatures has been thoroughly investigated. The Mn‐doped ZrO₂ catalyst shows high toluene or methyl ethyl ketone combustion activity, particularly when its ZrO₂ is in cubic form, when its Mn/Zr ratio is close to 0.2, and when it is prepared using tetra‐methyl ammonium hydroxide as a precipitating agent and calcined at 773 K. Copyright © 2005 Society of Chemical Industry     

Combustion synthesis of hollow carbon fibers

Mixtures of calcium carbide with different oxidizers, such as C₂Cl₆, C₆Cl₆, (C₂F₄) _n , and (CF) _n , were investigated. Reactions between these substrates in the presence of sodium azide are exothermic enough to proceed in a high-temperature self-sustaining regime. Combustion of tested mixtures was performed in the presence/absence of ferrocene (Fc) as an agent catalyzing the growth of nanostructures. Heat effects accompanying the reactions were measured and solid reaction products were analyzed. SEM and TEM observations revealed the presence of multi-walled hollow carbon fibers in combustion products formed in the CaC₂/C₂Cl₆/NaN₃/Fc mixture. Exfoliated graphite was observed in solid combustion products when (CF) _n was used as an oxidizer. In others systems, a soot-like morphology was found to be predominant.      

Effect of addition of potassium carbonate to aluminum powder on the grain size of Al2O3 nanoparticles formed in the laminar dusty flame

Results of studying the effect of K₂CO₃ additives on the grain size of the products of combustion of a gas suspension of Al particles (with the mean particle diameter of 4.8 µm) in a laminar diffusion flame are reported. An extreme character of the dependence of the mean size of Al₂O₃ particles on the additive concentration is experimentally observed. For the concentration of the K₂CO₃ additive equal to 0.5%, the mean diameter of Al₂O₃ particles is 30 nm; for the additive concentration of 5%, the mean particle size increases to 67 nm. It is demonstrated that the change in the mean size of Al₂O₃ particles as a function of the concentration of the readily ionized additive is caused by interaction of the dusty and ionic subsystems of the plasma of the combustion products in the reaction zone in the flame. At a high concentration of ions (above 10²⁰ m^?3), this interaction increases the rate of coagulation of Al₂O₃ particles.