Ignition and combustion of aluminum in oxygen/nitrogen mixture streams

The ignition and combustion of a solid cylinder of AI was studied experimentally by using the stagnation region of impinging O-2/N-2 (20/80) mixture streams over a wide range of pressure and velocity of the streams. When an original oxide coating did not exist on the Al surface, the critical spontaneous ignition temperatures were lower than the melting point of Al2O3, decreasing with reduction in pressure and velocity of the streams. Whether surface reactions initially formed a protective Al2O3 film on the surface or not decided the ignition, leading to the development of a luminous diffusion flame. When the Al surface was covered with an original oxide coating, it was observed that ignition occurred in the gas phase at the instant of the breaking of the coating. During combustion, the Al surface remained clean or covered with porous deposits. AlO was produced in the gas phase through the reactions of Al vapor with O-2. The AlO had a peak concentration in the gas phase away from the surface. Al2O3 also condensed in the gas phase. N-2 in the mixtures was found not to affect the ignition and combustion process due to much less reactivity with Al. The combustion mechanism for the Al-O-2/N-2 system has been postulated, including the surface and gas-phase reactions producing Al vapor, AlO, Al2O, AlO2, O, and condensed Al2O3. The ignition mechanism is also discussed. Copyright (C) 1997 by The Combustion Institute.