Production of hydrogen from chemical hydrides via hydrolysis with steam

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 as sociated with storage tanks. Water, either as liquid or vapor, will react w ith solid hydrides such as NaBH4 to produce pure hydrogen. However, a numbe r of limitations are inherent in the liquid-solid reaction. The insoluble h ydrolysis 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 re action to completion, but is detrimental to the equipment. Furthermore, the liquid-solid reaction is inefficient on a weight basis because a large exc ess of the water-acid mixture must be used to obtain acceptable yields of h ydrogen. Exploiting the vapor-solid reaction overcomes some of these limita tions. An isothermal semi-batch reactor was constructed to test the concept. In ea ch experiment the reactor was loaded with one gram of hydride and ambient p ressure steam was metered through the reactor. A GC analysis of the product gas detected only hydrogen and water. The yield of hydrogen was measured a nd compared to the theoretical yield. The pH of the condensed, unreacted st eam was tested and the percentage of excess water used was measured. A stat istical analysis was conducted on the results in order to determine interac tions between the parameters of flow rate and temperature. For some hydride s, nearly 100% yield of hydrogen was obtained without addition of any acid. Hydrogen yield depended strongly on temperature and, to a lesser extent, o n flow rate of steam. The results and thermodynamic analysis suggest a conc eptual hydrogen generation system in which the exothermic hydrolysis reacti on is linked to an endothermic dehydriding reaction for the purpose of prod ucing additional hydrogen. (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.