The objective of this work is to develop a method of producing H-2 for use in hand-portable fuel cells eliminating the bulkiness and high pressures associated with storage tanks. Water, either as liquid or vapor, will react with solid hydrides such as NaBH4 to produce pure hydrogen. However, a number of limitations are inherent in the liquid-solid reaction. The insoluble hydrolysis products are extremely basic and high pH inhibits the reaction. A large excess of acid must be added to the mixture in order to force the reaction to completion, but is detrimental to the equipment. Furthermore, the liquid-solid reaction is inefficient on a weight basis because a large excess of the water-acid mixture must be used to obtain acceptable yields of hydrogen. Exploiting the vapor-solid reaction overcomes some of these limitations. An isothermal semi-batch reactor was constructed to test the concept. In each experiment the reactor was loaded with one gram of hydride and ambient pressure steam was metered through the reactor. A GC analysis of the product gas detected only hydrogen and water. The yield of hydrogen was measured and compared to the theoretical yield. The pH of the condensed, unreacted steam was tested and the percentage of excess water used was measured. A statistical analysis was conducted on the results in order to determine interactions between the parameters of flow rate and temperature. For some hydrides, nearly 100% yield of hydrogen was obtained without addition of any acid. Hydrogen yield depended strongly on temperature and, to a lesser extent, on flow rate of steam. The results and thermodynamic analysis suggest a conceptual hydrogen generation system in which the exothermic hydrolysis reaction is linked to an endothermic dehydriding reaction for the purpose of producing additional hydrogen. (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
