Academic Insight - February 2016

Metal powders could become the clean fuels of the future

Professor Jeff Bergthorson, Department of Mechanical Engineering, McGill University, Montreal 

I started the Alternative Fuels Laboratory when I joined the McGill University faculty in 2006. We have done a lot of work with biofuels, and have shown that they generally produce lower pollutant emissions than the fossil fuels they replace. But the total potential biofuel production won’t be enough to offset a significant fraction of current fossil-fuel demand. There is no doubt they will play a part in the future fuel mix but others will be needed. 

Other alternative fuels such as hydrogen and batteries come with their own drawbacks. Hydrogen has long been considered to be the top future fuel since it can be produced from water and burned cleanly in engines. However, it must be transported by big, heavy fuel tanks and is explosive. Batteries are great for mobile phones and other low-power applications but they are too bulky and don’t store enough energy for many applications.

At McGill we have long studied the combustion properties of metals, typically in powder form, for use in energetic materials, such as propellants or explosives, and for industrial safety concerns. However, in 2007 we came across a paper that suggested burning iron powders as a fuel for internal combustion engines. We immediately saw the potential for using metals as a recyclable fuel. 

The idea we have put forward takes advantage of an important property of metal powders: when burned, they react with air to form stable, non-toxic solid-oxide products that can be collected relatively easily for recycling – unlike the CO₂ emissions from burning fossil fuels that escape into the atmosphere.

We have studied two ways of getting the energy out of the metal fuel: the wet cycle, where the metal is reacted with water to produce hydrogen on demand, and the dry cycle, where the metal fuel is directly burned with air to produce heat that can drive an engine.  

Active metals, such as aluminium or magnesium, react well with water, while other metals, such as iron, look to be more appropriate for the direct combustion approach.

We have also demonstrated that flames can be stabilised in clouds of micron-sized metal powders suspended in air that appear very similar to hydrocarbon flames. This means that they can be used in compact engines just as fossil fuels are today.

Currently, most metal production is powered by fossil fuels, such as the coal used to reduce iron in a blast furnace. However, research has been ongoing to develop metallurgical techniques that can reprocess metals using clean electrical and thermal energy. Such techniques can enable solar, wind, hydroelectric, nuclear, or other clean energy sources to be used to recycle the metal fuels, storing the clean primary energy as the chemical energy in the metal fuel. This would mean the metal-fuel cycle will have absolutely no carbon dioxide footprint associated with it.

Metals could also compete on a commercial scale. It has a cost, per joule of energy, roughly comparable to the retail price of fossil fuels. Metals, such as aluminium and iron, are widely abundant and available and should be able to be recycled at low cost once in circulation.  

To achieve this vision, it is likely that the cost of fossil fuels must be increased, or we must move away from them to prevent further climate change.  

As it stands, our understanding of the physics involved in metal combustion remains far behind that of our more common hydrocarbon fuels, and there remains much to be learned. The next steps for us will be to develop an engine prototype that is fuelled by metal particles to prove the concept.