19 December 2015

Metal makes for a promising alternative to fossil fuels

metal powders burning with air

Stabilized flames of different metal powders burning with air, compared to a methane-air flame (Credit: Alternative Fuels Laboratory/McGill University)

Clean fuels come in many forms, but burning iron or aluminum seems to be stretching the definition unless you ask a team of scientists led by McGill University, who see a low-carbon future that runs on metal. The team is studying the combustion characteristics of metal powders to determine whether such powders could provide a cleaner, more viable alternative to fossil fuels than hydrogen, biofuels, or electric batteries.

Metals may seem about as unburnable as it's possible to be, but when ground into extremely fine powder like flour or icing sugar, it's a different story. The simile is an apt one because the metal powders are similar to flour or sugar in more than particle size. Almost anything ground so fine will burn or even explode under the right conditions.

Grinding a powder so fine vastly increases the ratio between the surface area and the volume of the grains, so they burn very readily. In fact, they burn so readily that it's the reason why flour mills are so well ventilated. The slightest spark in floury air and a mill can blow up like a munitions dump. The same goes for sugar, metals, or even some types of rock.

This fact is already employed in a number of areas. Iron or aluminum, for example, can be ground up and turned into colorants for fireworks, solid rocket fuel powerful enough to lift a payload into orbit, or thermite that can burn hot enough to cut steel rails. What the McGill team hopes to do is harness this principle and turn it into a practical power source for everyday use.

The McGill team explains that metal powders aren't a primary energy source, like petroleum, but a storage medium for energy sources, such as nuclear or hydroelectric, which would be used to refine the metals into a pure, flammable form. The metal powders would be used for external combustion or heat engines, such as steam engines.

Under laboratory conditions, the team found that the flames produced from metal powders were quite similar to those of hydrocarbon fuels and they calculated that the energy and power densities of a metal-burning engine would be comparable to those of a conventional internal combustion engine.

The trick is to get the powders to burn in an even, steady flame. The team have mapped out a possible burner design that works by blowing air through a stream of metal powder. These combine and are injected into a combustion chamber. A cyclonic chamber separates the resulting metal ash out afterwards and clean nitrogen gas forms the exhaust, while the heat is used to run the engine.

The McGill team sees a number of advantages to using metal powders as fuel. Such powders would be transportable without the need for special tanks or cryogenic cooling. They are also much less bulky than hydrogen, and have higher energy density than batteries. Since they run heat engines, the technology can be scaled to be small enough for use in vehicles or large enough to run power plants.

Another plus is that metal powders are recyclable. As they burn, metal powders create stable, non-toxic solid-oxides that can be collected, refined back to pure metals, and used again with a minimum of carbon dioxide or other emissions.

If a metal powder engine does become practical, the McGill team says that iron will be the most likely candidate. Not only is it relatively cheap, but iron powders are already manufactured in the millions of tons for the metallurgy, chemical, and electronic industries. The main difficulty would be to ensure that iron refining is as carbon-neutral as possible.

The team is currently working on building a prototype burner that can be hooked up to a heat engine, as well as developing carbon dioxide-free recycling processes.

"We are very interested in this technology because it opens the door to new propulsion systems that can be used in space and on earth," says team-member David Jarvis, who is head of strategic and emerging technologies at the European Space Agency. "The shift away from fossil fuels for vehicle propulsion is a clear trend for the future. While not perfected and commercialized today, the use of low-cost metallic fuels, like iron powder, is a worthy alternative to petrol and diesel fuels. If we can demonstrate, for the first time, an iron-fueled engine with almost zero CO2 emissions, we believe this would then trigger even more innovation and cost reduction in the near future."

The team's research was published this month in in Applied Energy.

Source: McGill University, gizmag 


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