Combustion aluminum

As mentioned in the previous section, the condition for vapor phase combustion versus heterogeneous combustion may be influenced by pressure by its effect on the flame temperature (Tvol or Td) as well as by its effect on the vaporization temperature of the metal reactant (Th). For aluminum combustion in pure oxygen, combustion for all practical conditions occurs in the vapor phase. In air, this transition would be expected to occur near 200 atm as shown in Fig. 9.15 where for pressures greater than —200 atm, the vaporization temperature of pure aluminum exceeds the adiabatic flame temperature. This condition is only indicative of that which will occur in real particle combustion systems as some reactant vaporization will occur at temperatures below the boiling point... 

The combustor was ignited on propane. After a short period of operation on propane, aluminum flow was started and the propane flow was turned off. Once aluminum combustion was established, the oxygen flow was gradually decreased until only operation on H2O could be sustained. Typically, one to two minutes of aluminum firing on an O2/H2O mixture was required before the combustor was warmed up enough to make H2O only operation possible. Total duration of test runs was usually about 10 min. Preheating was not required for test runs using an 02/Ar oxidizer. 

Measurements of combustion temperatures, radiation intensity distributions in the range from 400 to 800 nm, and particle size distributions of combustion products have been made for the reaction of aluminum powder with both 02/Ar and H2O oxidizers in atmospheric dump combustors. The fraction of unburned aluminum in the combustion products was also determined for the H2O oxidizer case. An analytical study was performed to determine if the measurements are consistent with each other and with theory, and also to estimate the rate of heat loss from the combustion products. A Monte Carlo technique was used to determine the expected spectral energy distribution that would be emitted from a viewport located in the side of a combustion chamber containing products of aluminum combustion. 

Computational Determination of the Energetics of Boron and Aluminum Combustion Reactions... 

We have used the first procedure to calculate AH°(298 K) and AH°(2000 K) for 27 gas phase reactions that are examples of possible steps in boron and aluminum combustion. Our results are compared, in Table 8, to values obtained with the experimentally-based temperature-dependent enthalpy relationships given in the NIST tables [13]. The average absolute deviations overall are 1.6 kcal/mole at 298 K and 1.5 kcal/mole at 2000 K they are... 

We have determined the transition states and activation barriers at 298 K for a number of reactions that are likely to be involved in boron and/or aluminum combustion [23,30]. The structures of the transition states are shown qualitatively in Figure 2 the details of their geometries (bond lengths and angles, etc.) have been reported earlier [33,35,36]. The internal reaction coordinates were determined to verify that they lead to the desired products [41]. The CBS-QB3 computed activation barriers, AHact(298 K), and heats of reaction, AH(298 K), are in Table 10. While we have made no attempt to survey the literature pertaining to each of these processes, we do have some comments about certain ones of them. 

Therefore, we can synthesize aluminum nitride powders in a wide range of sizes - from nanosized to micron powders with various morphologies (spherical, laminar, thread- and needle-like) by changing aluminum combustion temperature, nitrogen pressure, and the initial mixture composition. Each size can be considered as optimum for specific appUcations. 

Aluminum combustion in water has been considered as a carbon-free source of hydrogen. The enthalpy of formation of aluminum oxide is —1675.7 kJ/mol ... 

Aluminum-containing propellants deflver less than the calculated impulse because of two-phase flow losses in the nozzle caused by aluminum oxide particles. Combustion of the aluminum must occur in the residence time in the chamber to meet impulse expectations. As the residence time increases, the unbumed metal decreases, and the specific impulse increases. The soHd reaction products also show a velocity lag during nozzle expansion, and may fail to attain thermal equiUbrium with the gas exhaust. An overall efficiency loss of 5 to 8% from theoretical may result from these phenomena. However, these losses are more than offset by the increase in energy produced by metal oxidation (85—87). 

If the temperature of a molten lead—calcium (tin)—aluminum ahoy is not kept sufficiently high, finely divided aluminum particles may precipitate and float to the top of the melt. These may become mixed with oxides of lead in the dross. The finely divided aluminum particles can react violently with the oxides in the dross if ignited. Ignition can occur if attempts are made to melt or bum the dross away from areas of buildup with a torch. The oxides in the dross can supply oxygen for the combustion of aluminum once ignited. 

Uses. Magnesium alkyls are used as polymerization catalysts for alpha-alkenes and dienes, such as the polymerization of ethylene (qv), and in combination with aluminum alkyls and the transition-metal haUdes (16—18). Magnesium alkyls have been used in conjunction with other compounds in the polymerization of alkene oxides, alkene sulfides, acrylonitrile (qv), and polar vinyl monomers (19—22). Magnesium alkyls can be used as a Hquid detergents (23). Also, magnesium alkyls have been used as fuel additives and for the suppression of soot in combustion of residual furnace oil (24). 

Furnaces used to bake anodes for prebake cells use the cooling anodes to preheat combustion air. Hot combustion gases from the baking 2one are used to preheat incoming anodes. Using these techniques, about 4.2 MJ/kg (1004 kcal/kg) of anode carbon or only 2520 MJ/1 (6.03 x 10 kcal/t) of aluminum is required to produce anodes. 

Reactivity. Bromine is nonflammable but may ignite combustibles, such as dry grass, on contact. Handling bromine in a wet atmosphere, extreme heat, and temperatures low enough to cause bromine to soHdify (—6° C) should be avoided. Bromine should be stored in a cool, dry area away from heat. Materials that should not be permitted to contact bromine include combustibles, Hquid ammonia, aluminum, titanium, mercury, sodium, potassium, and magnesium. Bromine attacks some forms of plastics, mbber, and coatings (62). 

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

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