McGill University’s Jeffrey Bergthorson wears heavy goggles to shield himself from the 3,000 C flame of burning aluminum, a metal he is studying as a carbon-free fuel source. Photo Credit: Professor David Frost
When it comes to technological alternatives desiApplied Energygned to wean our economy off carbon-based energy sources, metal fuels definitely count among the underdogs. True, their venerable history dates back to when the Chinese first started to fool around with fireworks more than 1,000 years ago and they are a good choice for heavy lifting, as evidenced by the aluminized solid rocket boosters that regularly help send satellites and space shuttles into orbit. Nevertheless, they remain a notoriously expensive and cumbersome way of doing business and few would consider them a serious option for conventional commercial or even personal transportation.
Jeff Bergthorson is eager to change this perception. The McGill University mechanical engineering professor has assembled a research group dedicated to exploring the prospects of using metal fuels as an entirely carbon-free way of powering the combustion engines that sustain so much of our civilization. “Metals pack a lot of punch,” Bergthorson says. “They have a lot of energy per unit mass and specifically a lot of energy per unit volume.”
In contrast to other alternative fuels like hydrogen, the infrastructure for extracting and handling metals is largely established. Similar to fossil fuels, says Bergthorson, metals remain stable for extended periods, making them easy to store and transport. Above all, while they might be hard to refine the first time around, they combust into convenient, cost-effective oxides that are readily captured after use. “The key idea is that if you’re recycling it and that metal is going around and around the cycle, then you’re not paying for the metal,” Bergthorson says. “You’re renting the use of it and what you’re really paying for is the energy that’s stored inside.” If that energy comes from a carbon-free source, such as nuclear, solar or wind generation, then metal fuels begin to look like the lynchpin of a carbon-free economy.
Currently such processes are anything but carbon free, as when iron oxide is recycled into iron by burning it with coal in a blast furnace. Bergthorson is envisioning a new strategy that would take carbon out of the picture, instead reducing the iron oxide with hydrogen or by subjecting it to chemical looping combustion in a circulating fluidized bed. While there are no commercial systems for doing so, his group is already experimenting with approaches that could complete this step and link it with existing facilities, such as a coal-fired power plant adapted to burn metals.
Bergthorson and his colleagues described their work in a paper that appeared last December in Applied Energy but acknowledges that this concept catches many people off guard, especially if they are unfamiliar with closed-cycle power plants such as Stirling engines that could function indefinitely on a limited supply of metal fuel. Such engines have 18th-century origins of modern mechanization and Bergthorson insists that it is time to revisit these technological roots. “What we’re really talking about is a reimagining of the steam locomotives of the industrial revolution,” he says. “It’s a recyclable ‘coal’ that you burn to generate heat and use that heat to generate steam and use the steam to generate power.”