A proposed apparatus for generating hydrogen by means of chemical reactions of magnesium and magnesium hydride with steam would exploit the same basic principles as those discussed in the immediately preceding article, but would be designed to implement a hybrid continuous/batch mode of operation. The design concept would simplify the problem of optimizing thermal management and would help to minimize the size and weight necessary for generating a given amount of hydrogen.
The apparatus would include a vessel, the interior volume of which would be divided into an upper and a lower tank (see figure). The upper tank would serve as a fuel-storage/feeder unit: It would include a bellows initially filled with MgH2 powder (the fuel), plus a mechanism that would include a rotating threaded outer pipe with metering windows and a non-rotating, non-threaded inner pipe with metering windows, for feeding the powder into the lower tank at a controlled rate. As the outer pipe was rotated, the widows in the pipes would alternately expose or occlude each other. The mechanism would be driven by an external motor via a magnetic coupling. The mechanism would also serve partly as a valve to prevent the undesired flow of steam from the reactor into the storage volume in that when powder was not being fed, the outer pipe would be rotated to a shaft angle at which the windows in the two pipes would occlude each other. The thread on the outer pipe would engage a threaded fitting on the bottom of the bellows, so that rotation of the outer pipe would compress the bellows as the powder was consumed.
The lower tank would serve as both a reactor and chamber for storing the solid waste end product (MgO) of the hydrogen-generating reactions. As the powder was fed from the upper tank to the lower tank and the bellows was compressed, the volume of the lower tank would grow, making room for the growing amount of waste material. Because fresh fuel would be dropped over the most recently reacted portion of the consumed fuel, it would always come in contact with the hottest part. There would be ample time for the fuel to react as nearly completely as possible because once the fuel was in the reactor, it would stay there. A thermally insulating layer (not shown in the figure) on the bottom of the bellows would reduce the undesired flow of heat from the reactor to the storage volume, thereby helping to suppress undesired decomposition of the MgH2 in the storage volume.
As described thus far, the apparatus would operate in a batch mode. The upper tank could be refilled with MgH2 powder from the top, and the MgO solid waste could be removed from the bottom. However, it would be necessary to interrupt operation during such refilling and emptying and during concomitant reverse rotation of the threaded outer pipe to reset the bellows to full storage volume. Thus, truly continuous operation would not be possible: The apparatus would operate in a quasi-batch, quasi-continuous mode.
This work was done by Andrew Kindler and Yuhong Huang of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive
Refer to NPO-46058, volume and number of this NASA Tech Briefs issue, and the page number.