McGill University researchers are exploring a potentially important source of clean energy that has so far been largely overlooked: metal particles.
Odd as the concept may sound, the idea of burning metal powders is nothing new. They’ve been used for centuries in fireworks. Nowadays, the space shuttle’s solid-fuel booster rockets burn aluminum.
“Our idea is to take that rocket science and make it into clean tech,” says Prof. Jeffrey Bergthorson, Associate Director of the Trottier Institute for Sustainability in Engineering and Design at McGill.
Metals store energy during the refining process. As a result, they could be used to transport and stockpile energy, much in the way that fossil fuels are used now. And the by-products from burning metal powders could be recycled and reused as fuel.
“To mitigate climate change, the world needs to transition away from fossil fuels,” says Bergthorson, who directs McGill’s Alternative Fuels Laboratory. “Biofuels can be part of the solution, but won’t be able to satisfy all the demand. Hydrogen requires big, heavy fuel tanks and is explosive, and batteries are too bulky and don’t store enough energy for many applications. Our research over the past 10 years has identified metal fuels as the best low-carbon alternative to carbon fuels.”
In a 2015 study published in the journal Applied Energy, Bergthorson and five other McGill researchers, along with a European Space Agency scientist, laid out a novel concept for using tiny metal particles – similar in size to fine flour or icing sugar – to power external-combustion engines.
Unlike the internal-combustion engines used in gasoline-powered cars, external-combustion engines use heat from an outside source to drive an engine. External-combustion engines -- modern versions of the coal-fired steam locomotives that drove the industrial era --- are widely used to generate power from nuclear, coal or biomass fuels in power stations.
Using a custom-built burner, the McGill researchers demonstrated that a flame can be stabilized in a flow of tiny metal particles suspended in air. Flames from metal powders “appear quite similar” to those produced by burning hydrocarbon fuels, they reported.
Recyclable after combustion
The concept takes advantage of an important property of metal powders: when burned, they react with air to form stable, nontoxic solid-oxide products that can be collected relatively easily for recycling – unlike the CO2 emissions from burning fossil fuels that escape into the atmosphere.
Iron is also plentiful, making it a strong candidate for use in metal-fueled engines. Millions of tons of iron powders are already produced annually for the metallurgy, chemical and electronic industries. And iron is readily recyclable with well-established technologies.
“The idea is to close the loop – to use that same bit of iron, or that same bit of aluminum, over and over again,” Bergthorson says. “That’s what enables it to be sustainable.”
What’s more, combustion may not be the only effective way to tap the energy stored in metals. Another approach that has long intrigued scientists involves causing metal – particularly aluminum – to react chemically with water.
In aluminum-water reactions, the water molecule, composed of one oxygen atom and two hydrogen atoms, gets split apart. The released hydrogen gas is a fuel that can be burned in air or used in a fuel cell. The oxygen attaches to the metal atoms, forming an oxide, which can be recycled.
In the Alternative Fuels Laboratory, research led by PhD student Keena Trowell focuses on reacting aluminum particles with high-temperature water at high pressures. The result: stored energy is released in the form of heat and hydrogen.
Beyond transmission lines
Both techniques – burning metal powders or combining aluminum particles with water to produce hydrogen – could be used to extend clean electricity beyond the reach of transmission lines.
“When we make aluminum here in Quebec from aluminum ore, the primary input is renewable hydroelectricity,” Trowell says. “So you can think of that as storing hydroelectricity in the form of aluminum.”
Other technologies to generate clean electricity – primarily solar and wind power – are being developed rapidly. But “we can’t use that electricity for many of the things that oil and gas are used for today, such as transportation and global energy trade,” Bergthorson notes.
“The transition away from fossil fuels presents a unique opportunity for metal producers to expand into the energy market,” he adds.
Next step: building prototypes
While laboratory work at McGill and elsewhere has shown that use of metal fuels with heat engines is technically feasible, no one has yet demonstrated the idea in practice. So the next step toward turning the lab findings into usable technology will be to build a prototype burner and couple it to a heat engine.
Bergthorson’s lab aims to begin testing such a metal-engine prototype later this year. “The goal is to demonstrate that we can generate electricity from metal fuels without producing any carbon dioxide emissions. Once proven in the laboratory, the technology will be ready to scale up to become a commercial technology.”
In parallel, Bergthorson’s group is building a consortium to develop a prototype metal-water reactor technology. “Demonstration of efficient hydrogen production from coarse and safe aluminum will open the door to using this novel clean fuel in a variety of applications, from remote power generation to heavy duty transportation, such as shipping.”