According to researchers at McGill University, the use of metal as a fuel is not a novel concept as it has been used in simple applications such as propellants, explosives and in more complex applications such as rocketry (Julien and Bergthorson 2017). A step forward for humanity would be to find a fuel that could take the place of fossil fuels and ultimately remove our dependency on carbon. In order to achieve this, the research group states that metals from the periodic table of elements need to be selected based on specific criteria. One criterion focuses on the need for the selected element to be similar or better than other “energy carriers.” The main benefits of using metal as fuel includes the absence of waste products such as carbon dioxide and the formation of a metal oxide product that can be converted back into metal for reuse. There are two types of metal reactions that are of importance: metal fuels reacting with air and metal fuels reacting with water. The heat produced from these reactions can then be used to power engines and be retrofitted to existing systems. It is important to note that external combustion engines could be used with metal fuels reacting with air and internal combustion engines or fuel cells could be used with metals reacting with water. The design of the combustor must be scalable from laboratory to industry in order to have a wide range of applications and ultimately result in metals taking the place of hydrocarbons as fuel (Bergthorson et al. 2015; Bergthorson et al. 2017; Bergthorson 2018; Julien and Bergthorson 2017; Trowell 2018).
The Importance of a High Reaction Rate and Full Product Yield
Based on research from McGill University, metal fuel reactions should proceed at a high rate in order to be comparable to hydrocarbon fuels. Recent work suggests that metal water reactions should occur under high pressure and temperature with supercritical water to achieve desired results (Bergthorson 2018; Trowell 2018). The desired results being full yield of hydrogen and full oxidization of the metal sample to release as much heat as possible. There should be no metal remaining (Trowell 2018). The metal fuel would be in the form of a powder or spray (Julien and Bergthorson 2017). The use of nano-sized particles is not economically feasible due to associated costs. Instead, researchers use micron-sized particles with the largest being the equivalent size of a human hair (Trowell 2018). The mode of particle combustion is another important factor to consider when oxygen is the oxidizer. The favorable mode of particle combustion is the heterogeneous combustion which forms solid metal oxide products. Researchers note that the type of metal fuel, the engine design, and the starvation of the combustion area of oxygen, are all key factors to consider in order to achieve favorable particle combustion at the desired reaction rate (Bergthorson et al. 2015; Bergthorson 2018). In metal to water reactions, there is a necessity to prevent the metal from premature oxidation before the reaction starts. There is a naturally occurring thin protective metal oxide layer that forms around the bulk metal in the presence of air. Once the metal reactant is put under supercritical water conditions, the thin metal oxide layer is then reduced to the extent that the reaction can proceed successfully (Trowell 2018).
Metallic Hydrogen is also being studied in the lab
Hydrogen can exist in a liquid and metallic state on gas giant planets like Jupiter or Saturn deep below the surface. A 2017 study from Harvard used diamond anvil cells to form a vice in an attempt to create the right conditions in the lab to isolate a small sample. According to a video published by Seeker.com in 2018, “scientists claim they crushed a hydrogen sample to nearly 4.9 million atmospheres until it wasn’t just metallic, but solid as well.” Further work needs to be done to determine the stability of metallic hydrogen. For instance, is metallic hydrogen at a solid state even when the pressure is lifted and possibly a room temperature superconductor (Dias and Silvera 2017). Following the 2017 publication in Science, several comments have been added disputing the authors claims or suggesting further research be conducted for the benefit of the physics community.
Bergthorson, J.M., S. Goroshin, M.J. Soo, P. Julien, J. Palecka, D.L. Frost, and D.J. Jarvis. 2015. Direct combustion of recyclable metal fuels for zero-carbon heat and power. Applied Energy 160: 368-382.
Bergthorson, J.M., Y. Yavor, J. Palecka,W. Georges, M.J. Soo, J. Vickery, S. Goroshin, D.L. Frost, and A.J. Higgins. 2017. Metal-water combustion for clean propulsion and power generation. Applied Energy 186(1): 13-27.
Bergthorson, J.M. 2018. Recyclable metal fuels for clean and compact zero-carbon power. Progress in Energy and Combustion Science 68: 169-196.
Dias, R.P. and I.F. Silvera. 2017. Observation of the Wigner-Huntington transition to metallic hydrogen. Science 355 (6326), 715-718. Retrieved from DOI: 10.1126/science.aal1579
Julien P., and J. M. Bergthorson. 2017. Enabling the metal fuel economy: green recycling of metal fuels. Sustainable Energy Fuels 1: 615-625. Retrieved from DOI: 10.1039/C7SE00004A
Trowell, K. 2018. Replacing fossil fuels: metal-water reactions for clean power – Keena Trowell at SEDTalks! Trottier Institute for Sustainability in Engineering and Design, McGill University, Montreal, Quebec. YouTube. Retrieved from https://youtu.be/INqlPTHek7E
Seeker.com. 2018. Why Metallic Hydrogen Is the Holy Grail of High Pressure Physics. YouTube. Retrieved from https://youtu.be/FYOuQ84-6Z0