Metal (Al, Mg)/water mixtures are of interest for hydrogen generation, propulsion and other applications [1-3]. Due to the highly exothermic metal-water reaction, such mixtures, upon ignition, exhibit self-sustained propagation of combustion wave with simultaneous release of hydrogen from water. In this work, we develop a mathematical model for this process, where the combustion wave structure includes a thin water-boiling front, a preheating zone with water vapor flowing through porous solid phase, and a wide zone of reaction between the formed water vapor and the solid phase. Note that the formation of a gaseous oxidizer (water vapor) during the process is a major feature of this model, which distinguishes it from previous developments for filtration combustion of metal powders. It is assumed that the combustion wave propagates uniformly, which allows us to develop the equations in a moving coordinate system. The modeling provides the propagation velocity and thermal profile of a combustion wave for different kinetic parameters and metal particle sizes. In parallel with modeling, experiments with stoichiometric Mg/water mixtures were conducted. The experimental results on the combustion front velocity for different Mg particle sizes confirm the model predictions.

References:

[1] Shafirovich, E., Diakov, V., and Varma A., “Combustion–assisted hydrolysis of sodium borohydride for hydrogen generation”, International Journal of Hydrogen Energy, Vol.32, 2007, pp. 207-211.

[2] Diakov, V., Diwan, M., Shafirovich, E., and Varma, A., “Mechanistic studies of combustion stimulated hydrogen generation from sodium borohydride,” Chemical Engineering Science, Vol. 62, 2007, pp. 5586-5591.

[3] Risha, G.A., Son, S.F., Yetter, R.A., Yang, V., and Tappan, B.C., “Combustion of nano-aluminum and liquid water,” Proceedings of the Combustion Institute, Vol. 31, 2007, pp. 2029-2036.