Controlling the burn13 March 2009
Tom Shelley reports on the feasibility of fuelling cars with powdered magnesium and magnesium hydride
One of the ideas around for providing hydrogen for future fuel cell powered cars is to generate it in the car. This is possible by interacting powdered magnesium or magnesium hydride with steam produced from a flash boiler.
While powdered magnesium and magnesium hydride are not the easiest materials to handle safely, they are no more difficult than petrol, and far easier to transport than liquid or gaseous hydrogen. Fuel cells require very pure hydrogen. This means that if it is not to be derived from natural gas, it either has to be made by electrolysing water or chemically on the vehicle.
While the idea of generating hydrogen from magnesium and steam is far from new, the latest scheme has been invented by Andrew Kindler at the California Institute of Technology in Pasadena, for NASA's Jet Propulsion Laboratory. He says: "We did not put a hydrogen generator in a vehicle, but we demonstrated our concept in the laboratory using small scale chemical reactors."
US based Ecotality, which aims to develop and commercialise clean energy technologies, sponsored the 18 month research programme that saw Kindler and his team look at two approaches.
The first is to take steam at more than 330°C and supply it into a reactor full of magnesium powder, which produces hydrogen and heat. This feeds into a boiler to produce more steam. Surplus heat is then supplied to a thermoelectric converter of some kind. For each kg of hydrogen generated, 44.5kWh of heat would be produced, while only about 6kWh would be needed to boil the water, which would be supplied from the effluent water and condensate issuing from the fuel cell. The hydrogen would have to be cooled to 80°C before it could be supplied to the fuel cell.
The second approach is to take magnesium hydride, which can be made by reacting magnesium and hydrogen at 200bar and 500°C in the presence of a magnesium iodide catalyst, and reacting it either with steam, or water, and allowing the mixture to get hot. Once running at more than 330°C, the magnesium hydride decomposes in to its elements with the absorption of heat. The magnesium produced reacts with steam to produce more hydrogen plus surplus heat in the same reaction vessel. The output of hydrogen per kg of magnesium would be doubled. The residue would, in all cases, be magnesium oxide, which could be collected for re-conversion to magnesium.
Kindler says: "Is this practical? The chemistry works, but there are issues. One is the difficulty in dealing with powdered fuels. The other is the difficulty of storing the fuel in the reaction tank. This is the most desirable way to do it from the point of view of saving weight. On the other hand, how do you refill a partly used charge if the waste is mixed with the fuel?"
If you feed the fuel in to the reactor continuously, it allows the fuel to be kept separate from the waste. But the operation becomes very complicated from the point of view of requiring seals and weight is increased because you need a storage tank and a reaction tank.
When the magnesium was reacted in a chamber, the team used water injection to control the temperature of the burn. It was certainly not the uncontrolled burn you see in air where the magnesium becomes white hot.
"Magnesium hydride has the reputation of being pyrophoric," says Kindler. "Possibly when it is freshly made, this might be true. I handled magnesium hydride in the open air all the time; I never could initiate a fire. The hydrogen gas is not noticeably released until you reach 300°C. It won't happen easily on its own."
The magnesium hydride can burn when water is added, but again, it is not fierce under controlled injection conditions. "I can't see how either of these materials is more dangerous than petrol," says Kindler. "We are simply used to using and storing petrol and accept its risks. It is actually incredibly dangerous."
Although the technology has been found to be unfeasible for use in cars, Kindler says that under the brand name Hydrality, it is currently developing the technology for large scale utility applications.
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All electrics in the short term
While the major car companies have all put major amounts of effort into fuel cell powered cars, the challenges involved in reducing both the cost of fuel cells and hydrogen fuel has meant that in the short term, the green option will be battery powered electric cars and plug-in hybrids.
Ford is to introduce a new battery electric commercial van in 2010 and a new battery electric small car to be jointly developed with Magna International in 2011. A plug-in hybrid version will be introduced from 2012.
GM's offering is the Chevrolet Volt, to be launched in late 2010, which will run for up to 40 miles after a three hour charge on a 240V outlet or eight to nine hours when plugged into a 120V socket. It will use a lithium-ion polymer battery made by LG Chem in Korea. When the battery runs out, an on-board petrol engine will self engage to generate electricity so that the car starts as a battery powered vehicle, then turns into a hybrid.
Toyota has announced that it will launch an all electric car for city commuting by 2012, although it still considers petrol electric hybrids to be the main basis of its planned future products.
Ford will be assisted in its $20 million programme by a $10 million grant from the US Department of Energy towards the cost of research, development and demonstration of plug-in hybrid electric vehicles.
* Pure hydrogen for fuel cells could be generated onboard by reacting steam with metallic magnesium, or by reacting magnesium hydride with water/steam.
* The water would come from the fuel cell.
* Magnesium hydride would yield twice the amount of hydrogen per kg compared with metallic magnesium.
* While the chemistry works, and suitable reactors have been demonstrated, powder fuel handling problems have yet to be tackled.
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