|Pouring the hydride slurry.|
Safe Hydrogen, LLC. of Lexington, Mass has been awarded $308,000 from the Massachusetts Renewable Energy Trust SEED Program (Sustainable Energy Economic Development), a division of the Massachusetts Technology Collaborative (MTC), to complete a three-year, $2.4-million DOE project designed to determine the functionality, cost and efficiency of Safe Hydrogen’s pumpable hydride slurry.
Safe Hydrogen has developed a pumpable magnesium-hydride-based fuel that releases hydrogen as needed.
A pumpable fuel rich in hydrogen would eliminate several key road blocks to wide spread adoption in transportation, including distribution infrastructure and storage safety and efficiency.
The slurry, both before and after yielding the hydrogen, is not flammable, safe to handle, easy to store and can use current pumps and tanks used for diesel fuel, gasoline or water. The slurry is reacted with water to produce the hydrogen required. The metal hydroxide byproduct is captured and recycled.
The slurry consists of a finely ground light-metal hydride, protected by mineral oil and suspended by dispersants to keep the particles from settling out of the suspension. (Safe Hydrogen originally developed the concept using lithium hydride, but is further developing it with magnesium.)
The oil forms a protective coating around the hydride particles that slows the movement of water toward the particle.
This protective coating allows the hydride to be safely handled and stored in the air without absorbing moisture from the air. It also slows the kinetics of the reaction allowing the development of reaction vessels to mix the hydride with water for releasing hydrogen.
A prototype on-board hydrogen generator developed earlier by Safe Hydrogen consists of storage vessels for the hydride slurry and a small amount of water, pumps for both the slurry and the water, a mixing reactor, a heat exchanger, and a hydroxide storage tank.
The reactor is a tube with an auger/mixer running through it. Hydride slurry and water are pumped into the reactor at one end, and moved and mixed by the auger/mixer. Excess water is evaporated, absorbing and carrying the heat of reaction out of the reactor with the hydrogen.
Hydrogen and water vapor are separated from the hydroxide product in the head of the hydroxide tank. The water vapor is condensed in the heat exchanger. Condensed water is returned to the water circuit and hydrogen is delivered to the fuel cell.
In the current project, which began in 2004, Safe Hydrogen is targeting developing a MgH2 slurry with an energy density of 3.9kWh/kg and 4.8kWh/L and a mixing system to use MgH2 slurry meeting 2kWh/kg and 1.5kWh/L system targets.
Slurry production and hydroxide recycling are also part of the project.
One unit of slurry, according to the company, carries the potential of generating twice as much volume of hydrogen (at about the same weight) as one unit of cryogenically cooled liquid hydrogen. Liquid hydrogen is a proven method of storing hydrogen, but it takes substantial energy to liquefy the hydrogen and there is continual boil off of hydrogen during storage. Slurry, on the other hand, is stored at normal temperature and normal pressure.