267766 Effect of Particle Size On Hydrogen Production by Reaction of Liquid Water and Aluminum Powders

Wednesday, October 31, 2012: 8:30 AM
Conference A (Omni )
Hongqi Nie1, Mirko Schoenitz1 and Edward L. Dreizin2, (1)NJIT, Newark, NJ, (2)Otto H. York Department of Chemical, Biological, and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, NJ

Abstract

Reaction of aluminum with water is of interest for hydrogen generation for a wide range of applications, from in-situ operated fuel cells to propulsion of underwater vehicles.  This reaction is extensively studied and various aluminum-based composite materials and alloys are under development with the aims to accelerate the rate of hydrogen production and enable controlled operation of the hydrogen-generating systems.  It would be particularly interesting to achieve the desired rate of hydrogen production at low operating temperatures, using liquid water as opposed to water vapor.  However, the understanding of the rate-limiting processes is lacking for the reaction of pure aluminum and liquid water.  Recent research by different authors showed, somewhat unexpectedly, that reaction dynamics are different for micron sized spherical aluminum powders with different size distributions.  This result can be interpreted considering evolution of aluminum hydroxide layers and possible changes in their integrity and diffusion resistance when the layer thickness becomes comparable to its radius of curvature.   In addition, porosity in such layers can develop when they are sufficiently thick; however, such thick layers may not necessarily grow on finer Al particles.  This work attempts to understand the effects of particle size on its reaction dynamics in liquid water using experimental data from time-resolved micro-calorimetry.  The experimental data tracking energy release caused by the Al/H2O reaction are processed using the actual measured particle size distributions for three different Al powders.   The reaction rate is assumed to be proportional to the particle surface; thus the entire heat flow measured by the calorimeter is partitioned among Al particles based on their size distribution.  Evolution of particle sizes and thickness of the grown Al(OH)3 layers are tracked during the micro-calorimetry experiment.  The results are processed using a simplified reaction model and diffusion coefficients are found for particles of different sizes and at different reaction stages.  Ideally, such coefficients should only depend on temperature, so that their observed dependencies on particle size and reaction progress serve to indicate where the modifications of the simplified reaction model are necessary.  Furthermore, reaction rates are compared for individual particles of the same size which are present in aluminum powders with different size distributions.  Once the correct reaction model is implemented, the reaction rates for such identical particles must coincide.  Additional comparisons and measurements will be presented and discussed, aimed at establishing a corrected reaction model for aluminum and liquid water.  Based on the developed reaction mechanism, a composition and morphology of an optimized, Al-based material for hydrogen generation will be proposed.  


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See more of this Session: Thermophysical Properties of Energetic Materials
See more of this Group/Topical: Particle Technology Forum