Could slurry be an answer to hydrogen infrastructure challenge?
One company, Safe Hydrogen LLC, as part of a three-year Dept. of Energy project, has developed an innovative technique in which hydrogen is stored as part of a slurry, a pumpable mixture of a solid and a liquid. The slurry both stores and generates 99.999 percent pure hydrogen readily when ordinary tap water is added. Besides hydrogen, the reaction generates some heat and a recyclable hydroxide byproduct.
The slurry offers important handling and safety benefits because it is nonexplosive and less flammable than fuel oil. The company said its approach stores hydrogen more compactly--10 times more than compressed hydrogen, and two times more than liquid hydrogen. The technology could supply hydrogen to a car with a fuel tank only about 8 percent larger than the average gasoline tank with the same range. Both the "loaded" and depleted slurry are pumpable and easily adapted into today's gas station infrastructure. They can be stored and handled at ambient temperatures pressures, the company said.
Sig Tullmann, CEO of the newly formed company, highlighted its saving in storage and transportation costs. "Especially in our new security-conscious world," he said, "it saves security risks and costs by providing a nonexplosive and nonflammable stored hydrogen." He estimated the cost of hydrogen to the consumer, if this technology were rolled out on a large scale, would be about 40 percent less than what Europeans are paying today to power their vehicles with gasoline refined flora Middle East oil.
For the DOE demonstration project, lithium hydride was selected after analysis of several chemical hydrides. Because lithium hydride is a mono-metal hydride rather than a bi-metal hydride, the company felt it is easier to reduce a mono-metal hydroxide than to separate and reduce a multi-metal hydroxide. Also lithium hydroxide forms a monohydrate while many of the bi-metal hydrides form multi-hydrates when reacted with water. The lithium hydroxide hydrate decomposes when it is heated above the temperature of boiling water, but many of the bi-metal hydroxide hydrates do not decompose without being heated to quite high temperatures.
Lithium hydride is prepared as a slurry with light mineral oil and a dispersant. The light mineral oil suspends finely ground lithium hydride and the disper sant keeps the particles from settling out of the suspension. The mineral oil coating also protects lithium hydride from inadvertent exposure to water or moisture absorbed from the air. If dry hydride is allowed to come into contact with humid air, reaction between the moisture and the hydride will take place, hydrogen will be released and heat will build up until it ignites the hydrogen. When mixed with mineral oil, the hydride cannot absorb moisture rapidly enough to be a hazard. Also because mineral oil has such a high vapor pressure, the mineral oil actually prevents the ignition of the lithium hydride from open flames.
In a demonstration, the flame from a butane torch was touched to the surface of the slurry with no effect. Gasoline would have ignited under the same conditions. When the flame was held on the slurry for sufficient time, some of the mineral oil evaporated and burned; but the flame went out when the torch was removed. The slurry remains stable at normal temperature and pressure for long storage periods at atmospheric temperatures and pressures.
While the reaction can occur at ambient pressure and temperatures, the reaction of the slurry with water can be performed at elevated pressures. Additional power could be generated from the exhaust hydrogen/steam from the reactor and/or the exhaust hydrogen/ steam could be used to run an air compressor for a more compact fuel cell as well as reducing the size of the hydrogen generator's components.
A prototype mobile generator demonstrated that the system could produce hydrogen at a rate of up to 3 kg/ hr. Based on this generator design, an advanced design could provide a fuel with a gravimetric energy density of 3.361 kW-hr/kg and a volumetric energy density of 1.954 kW-h/L, assuming that the water from the fuel cell is condensed and used to produce hydrogen in the hydride/water mixer. The generator design includes storage vessels for the lithium hydride slurry and a small amount of water, slurry and water pumps, mixing reactor, hint exchanger, and hydroxide storage tank. The tractor is a tube with an auger/mixer running through it. Hydride slurry and water are pumped into the reactor at one end. The auger/mixer moves this mixture through the reactor and mixes it as it is being moved. 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. The development program included an experiment using hydride-based hydrogen in a Ford Ranger's internal combustion engine modified to operate on hydrogen.
In order to be acceptable in a fuel cell, the hydrogen must have very low concentrations of carbon monoxide, less than 10 ppm. Actual carbon monoxide measurements showed that levels were well below the tolerable levels of a PEM (proton exchange membrane) fuel cell. Also concentrations of oxygen, nitrogen, carbon dioxide, mineral oil, and hydrocarbons were measured and were low.
Lithium hydride prepared as a slurry at centralized plants would be pumped into tanker trucks or pumped through pipes to distribution centers where it will be loaded into vehicles or carried to storage vessels in homes, business, or industry. The hydroxide waste would be transported back to the regeneration plant where it would be separated from the mineral oil and the lithium hydroxide will be regenerated into lithium hydride.
The proposed regeneration process is a carbo-thermic reduction process based on the use of low cost carbon flora coal or biomass. Lithium hydroxide and carbon are fed into a radiant reduction reactor where they are heated to approximately 1900[degrees]F. During this reaction, hydrogen and carbon monoxide are released and lithium is melted. Carbon monoxide is put through a shift reaction to form carbon dioxide and hydrogen. The hydrogen is used to produce electric power and lithium hydride.
The objective is to have zero net carbon dioxide emissions from the regeneration plant by capturing the highly concentrated carbon dioxide stream leaving the plant for sequestration. Regeneration will be performed in centralized plants much like refineries using technologies synergistic with blast, aluminum reduction and glass furnaces. Cost analyses indicate the process should be competitive with hydrogen produced from natural gas and stored as a liquid or compressed gas.
Safe Hydrogen sees several applications for this slurry storage technology. While its widespread use in automotive fuel cells may be still years in the future, more inmediate applications include milliwatt sizes for portable computers and portable consumer electronics; kW sizes for remote and marine applications and standby or backup power for industrial, commercial, and residential applications; and MW sizes for commercial ships and for backup power generation at commercial and industrial sites. Safe Hydrogen said it offers a lower cost and vastly safer replacement for compressed or cryogenic hydrogen used in industry.
While the technology was demonstrated with lithium hydride, toward the end of the development program, magnesium hydride was evaluated and found to offer many performance advantages of lithium hydride and many addition handling and cost advantages over lithium hydride. For instance, it does not react readily at room temperatures so that if it is spilled into water, hydrogen will not be produced. Its by-product is magnesium hydroxide, or milk of magnesia. It is a much cheaper metal. Finally, most of what was learned about lithium hydride will still apply to a magnesium hydride slurry, the company said.*
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Bill Siru, PhD, PE, is an independent technical journalist betted in ,Can Diego, Calif.