Using metal powders as a fuel source

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2016

Iron holds the most promise due to its availability, low cost, combustion properties and capacity to be recycled. 

 

KEY CONCEPTS
• Metal powders have potential to be used as fuels.
• The challenge is finding the right metal that can efficiently burn and also form big metal oxides upon combustion that can be collected and recycled.
• Iron is the most promising metal to be used as a fuel because of performance and economic considerations.

THE SEARCH FOR ALTERNATIVE FUELS TO PETROLEUM HAS LED RESEARCHERS to evaluate biofuels, hydrogen and batteries. Each of these options has shown promise but also has operational problems that have not been overcome.

For example, hydrogen is inherently very flammable and difficult to handle. Refueling with hydrogen can potentially lead to explosion hazards. In a previous TLT article, a safer hydrogen storage material known as ammonia borane was discussed (1). Ammonia borane is a solid material that has a hydrogen density of 19.6%. The article details a new process for converting ammonia borane to hydrogen using a ruthenium catalyst at elevated temperatures without forming byproducts that can be detrimental to the use of hydrogen as a fuel. Batteries are convenient but heavy and take up a lot of space to deliver the same energy as hydrocarbon fuels. 

An alternative option is the use of metals, which are the fuel within batteries, as recyclable fuels. Jeffrey Bergthorson, associate professor of mechanical engineering and associate director of the Trottier Institute for Sustainability in Engineering and Design at McGill University in Montreal, Quebec, Canada says, “Metal powders have been known to burn for a long time and are used in fireworks and as propellants. Two applications are in fireworks and as propellants. One very prominent application is the use of aluminum powder at a 16% concentration in the propellant for the space-shuttle solid rocket boosters.”

Research on metal powders has been ongoing at McGill for some time. Bergthorson says, “The group has been evaluating metal powders for use in military applications and also at investigating safety concerns regarding the potential combustion of powders used in industrial processes.”

There are two ways for extracting energy from metal powders. Bergthorson says, “One option is to react metals with water to generate hydrogen. This process is not efficient because the amount of energy captured in the hydrogen is less than the total usable energy stored in the metal. The problem is that, in most cases, the heat produced as a byproduct of the reaction is at low temperatures and is not usable for generating power. The second option is to directly burn the metals with air to produce heat for an engine, just like fossil fuels are used for today in everything from cars to coal-fired power plants.”

One idea that was proposed about 10 years ago was using metal powders as the fuel for an internal combustion engine. Bergthorson says, “We were approached by one of our research partners to explore the concept of using metal powders as a fuel for automobile engines. The problem with this approach is that metal oxides, which are the product of combustion, will coat the engine and lead to the fouling of pistons and abrasion of engine components.”

But metal powders can be used as a fuel in other engine types, such as external combustion engines. Bergthorson says, “In these systems, combustion is separated from the working parts of the engine. What is needed is a clean combustion system that can separate out the metal oxide combustion products and produce clean heat for the external combustion engine, which could be a Stirling or steam engine.”

METAL COMBUSTION PROPERTIES
The rationale for this approach is the potential for developing a metal fuel cycle, powered by clean energy. Bergthorson explains, “Metal powder will be oxidized into solid metal oxides and hydroxides during its use as a fuel in order to produce motive or electrical power on demand. The byproducts, which are oxides of the metal, can be converted back into metal fuel through the use of clean primary energy sources such as solar and wind power. In effect, this process is analogous to charging and discharging a battery, but the “combustible battery” is very compact and lightweight since only the metal fuel needs to be carried. The metal can be filled into a tank at refueling stations just like gasoline or diesel. In this way, the metal becomes a type of recyclable coal. This recyclable fuel can be traded globally, to move clean energy from where it is produced to where it is needed, and can be stored for long periods of time.”

While it has long been known that metals can be burned, our understanding of their combustion properties lags far behind that of our more-common fuels. In general, the rate of oxidation is limited by the ability of the oxidizers to physically get to the metal surface. 

Bergthorson says, “To be effective as a fuel, a metal must have a high surface to volume ratio either in the form of a particle or a droplet. Ideally, metal particles must have a diameter in the micron size range to efficiently burn.”

But the combustion process for each metal can differ. Bergthorson says, “Aluminum burns with air at 3,000 C but produces very small aluminum nano oxides that are difficult to collect and reprocess. Iron burns at 2,000 C, close to the temperatures of typical hydrocarbon flames, and produces bigger oxide particles that are easier to handle.”

Figure 2 shows a comparison between a hydrocarbon flame on the left and an aluminum flame on the right.


Figure 2. Metal powders such as aluminum can undergo combustion (see flame on the right) and are under evaluation as alternative fuels to hydrocarbons (see flame on the left). (Figure courtesy of McGill University.)

The ability to use a metal as a fuel is based on how combustion occurs. Bergthorson says, “A key factor is whether a metal will burn as a vapor or as a solid/liquid. For particles whose temperature of combustion is higher than the boiling point of the metal, the metal droplet will burn with a vapor-phase flame surrounding it similar to that for hydrocarbon fuels. In this case, very small metal oxides are formed that are difficult to capture. If the flame temperature is lower than the metal boiling point, the combustion process will occur heterogeneously as a gas-solid/liquid reaction at the surface of the metal particle/droplet. Metals such as iron that burn and produce a solid (porous) metal oxide on the surface of the particle will result in relatively large metal oxide products that are easier to capture for recycling.”

Another factor that the researchers are examining is burning velocity. Bergthorson says, “Burning velocity is a measure of how quickly the combustion reaction occurs. A faster rate is desired to facilitate flame stabilization in a compact combustion system. Metal powders will exhibit high burning velocities if they have small enough particle size and high enough concentrations. But more work is needed to optimize the parameters.”

Bergthorson says, “The most promising metal for use as a recyclable fuel at the moment is iron because it is readily available, has a low cost relative to other metals, produces combustion products that facilitate capture and can be recycled using green energy and existing technologies.” Additional information on the concept of using metal powder as a fuel can be found in a recent review article (2) or by contacting Bergthorson at jeff.bergthorson@mcgill.ca. 

REFERENCES
1. Canter, N. (2012), “More efficient access to hydrogen as a fuel,” TLT, 68 (1), pp. 14-15.
2. Bergthorson, J., Goroshin, S., Soo, M., Julien, P., Palecka, J., Frost, D. and Jarvis, D. (2015), “Direct combustion of recyclable metal fuels for zero-carbon heat and power,” Applied Energy, 160, pp. 368-382.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.