05 Mar Recyclable metal fuels for clean and compact zero-carbon power
Metal fuels, as recyclable carriers of clean energy, are promising alternatives to fossil fuels in a future low-carbon economy. Fossil fuels are a convenient and widely-available source of stored solar energy that have enabled our modern society; however, fossil-fuel production cannot perpetually keep up with increasing energy demand, while carbon dioxide emissions from fossil-fuel combustion cause climate change. Low-carbon energy carriers, with high energy density, are needed to replace the multiple indispensable roles of fossil fuels, including for electrical and thermal power generation, for powering transportation fleets, and for global energy trade. Metals have high energy densities and metals are, therefore, fuels within many batteries, energetic materials, and propellants. Metal fuels can be burned with air or reacted with water to release their chemical energy at a range of power-generation scales. The metal-oxide combustion products are solids that can be captured and then be recycled using zero-carbon electrolysis processes powered by clean energy, enabling metals to be used as recyclable zero-carbon solar fuels or electrofuels. A key technological barrier to the increased use of metal fuels is the current lack of clean and efficient combustor/reactor/engine technologies to convert the chemical energy in metal fuels into motive or electrical power (energy).
High energy and power densities can only be provided through chemical or nuclear sources, with chemical energy, in the form of fuels burning with atmospheric air, being the most reliable for society to date. Alternative fuels or energy carriers are known as solar fuels, or electrofuels. A fuel can be defined as a reduced material, containing chemical energy through its oxidation potential, that can be burned or oxidized, usually with oxygen from the air, to yield useful energy on demand. A solar fuel is produced with solar energy as the dominant input, while electrofuels are produced primarily from electricity, during the reduction process to convert spent combustion/oxidation products back into reactive fuel.
Metals as zero-carbon, recyclable electrofuels
In the search for alternative electrofuels, the periodic table of the elements provides all available options. Since the fuel must be oxidized by oxygen from the air, only groups 1–14 are of interest. In addition, it is desired that the fuel has high specific energy, which means minimizing the nuclear mass for a given number of valence electrons; therefore, only elements within periods 1–4 are practical. Hydrogen and carbon sit at the upper left and right of this subset of the periodic table and are important fuel options, due to their chemical and physical properties that result from their valence numbers and nuclear mass. The other possible fuel elements are all metals or metalloids and are all referred to as metals for convenience in the rest of this review.
A largely-overlooked energy-carrier option is the use of metals as recyclable electrofuels, which can be reacted, or burned, with both water or air as oxidizers. Metals, among all elements, have the highest volumetric heat production when burned in air and, hence, are the most energetically-dense chemical fuels available. Some metals (metalloids), such as boron, show specific energies higher than all current options, when the mass of the necessary hydrogen-storage system is taken into account.
Metals as fuel additives in propellants and energetic materials
The high energy density of metals motivates their use as fuels within metallized solid rocket propellants, fireworks and pyrotechnics, liquid-hydrocarbon/metal-powder slurry fuels, explosives, and other energetic materials. Metals are also used as fuels within metal-water propellants, air-breathing ramjet engines and water-breathing propulsion systems for torpedoes and underwater vehicles, with only part of this work reflected in the open literature.
Metals as fuels within batteries
The high energy density of metals, along with their ability to take part in reduction/oxidation (redox) reactions, also motivates their use as the “fuel” within many batteries. Secondary batteries, such as Li-ion, are attractive for low power applications, such as cell phones and laptop computers, because they are conveniently recharged and have high round-trip charge/discharge efficiencies. Unfortunately, secondary batteries have low energy density due to the need to carry all reactants onboard, like a rocket. The low energy density means that current secondary batteries are not practical for high-power and long-duration applications.
Metal-air batteries aim to improve energy density by using the oxygen within the air as a freely available oxidizer, similar to all air-breathing engines. Metal-air batteries are typically primary batteries that must be “mechanically recharged”, or refuelled, where the spent anode, once discharged, must be replaced with a fresh one, while the oxidized spent anode must be collected for central recycling. The primary energy needed to recycle the metal anode would preferably be supplied by clean power sources; in this case, the resulting metal anode would be a low-carbon electrofuel.
Metal-air batteries are effectively a type of fuel cell where a metal fuel is electro-chemically oxidized by oxygen from the air. Aluminum-air batteries can achieve specific energies of 0.3 kW · h/kg and electrical efficiencies of 45%. This specific energy is significantly lower than the specific energy of pure aluminum metal. The low reaction rates within these batteries, resulting from the low temperatures of the reaction, low surface area of the anode material, and the bulky oxygen-reduction catalyst, lead to low power densities. Metal-air batteries have been deemed impractical for vehicle applications unless the reaction rates can be increased by two orders of magnitude.
Another serious problem for many metal-air batteries is that parasitic reactions of the metal anodes with water leads to a requirement that ultra-pure metals be used for the anode material, which increases costs and reduces the energy-cycle efficiency. These same corrosion reactions are exploited when metal fuels are reacted or burned with water.
Metals as recyclable electrofuels
The high energy density inherent to reactive metals, which motivates their use as additives to propellants and energetic materials, or as anodes within batteries, also inspires their use as recyclable electrofuels. The concept of a metal-fuel cycle, in which metals are utilized as recyclable zero-carbon electrofuels is illustrated in Figure 1. In this concept, metals, typically as powders or sprays, are burned with air to produce heat for a heat engine or are reacted with water to produce heat and hydrogen that can be used in heat engines or fuel cells. Metal fuels can enable the high-power density of heat engines to be achieved without producing carbon dioxide or other pollutant emissions. Metal fuels can be refilled, and products emptied, rapidly, as we are accustomed to with hydrocarbon fuels. Metal fuels can be thought of as primary-battery anodes that carry no dead weight, and the metal engine is to the metal-air battery what the hydrogen engine is to the hydrogen fuel cell.
The key limitation to hydrocarbon electrofuels is the inefficiency in, and energy intensity of, the carbon-recycling process, making it difficult to close the fuel cycle. This difficulty arises because carbon dioxide is a gas under standard conditions. Recyclable electrofuels are required for sustainability. A fuel that produces a solid-phase combustion product that could be captured for recycling would likely provide a better life-cycle efficiency than solutions that use the atmosphere as both a sink, and source, of carbon.
Metal fuels produce metal oxides, which are typically solids under standard conditions and can be collected for recycling. Metal fuels can be recycled from the metal-oxide products, using clean primary energy sources and existing or advanced metal-reduction techniques an effectively infinite number of times while avoiding loss of material to the environment. Metal fuels are, therefore, electrofuel options consistent with the concept of a circular economy.
Metal fuel options
Out of all of the metals identified as possible energy carriers, silicon, aluminum, and iron are the 2nd, 3rd, and 4th most abundant elements in the Earth’s crust, respectively; the most abundant element is oxygen, typically in the form of an oxide of these metals. Iron is, by far, the most used metal today and is a promising electrofuel. Silicon, technically a metalloid rather than a metal, is another potential energy carrier, since it can be produced from sand (SiO2), water, and solar energy. Aluminum and magnesium are also very interesting as metal fuels due to their high energy density, relatively low costs, and the potential to recycle them with low carbon emissions. Boron and zinc have been considered as electrofuels that can produce hydrogen on demand for power production. Alkali metals, such as lithium and sodium, have also been considered as recyclable energy carriers. Other metals, such as titanium, could also be of interest for specific applications.
Recycling of the metal-oxide products
Metal fuels produce solid metal-oxide combustion products that can be separated from the combustion/reaction products and then be stored for later recycling, commonly termed smelting. Many current metal production and recycling technologies rely heavily on fossil energy inputs. For example, iron is primarily reduced using coke, a high-carbon distillate of coal. Aluminum is produced using the Hall–Héroult process, which can make use of clean primary energy but also requires the use of consumable carbon anodes to reduce the metal, resulting in the production of stoichiometric amounts of carbon dioxide. Metal oxides can, however, be reduced using clean primary energy sources in zero-carbon reduction processes, which is essential for metals to be useful as zero-carbon recyclable electrofuels.
Outlook and future research directions
Many important questions regarding metal-fuel combustion processes remain unanswered, as do questions concerning the energy-cycle efficiency and life-cycle environmental impacts and economics of metals as recyclable fuels. However, metal fuels can be an important technology option within a future low-carbon society and deserve focused attention to address these open questions. The Dry and Wet Cycles could both be operated at a range of scales from motive power for transportation to large-scale stationary power generation. Advances in clean primary power generation and development of efficient technologies to reduce metal oxides to reactive metals using minimal clean energy inputs are also topics of active research interest that are critical to a transition to a low-carbon global economy, since metals are a major component material for the manufacturing of everything from personal electronics to transportation vehicles and for the construction of buildings and infrastructure.
Technological developments for both the production of metal fuels and their utilization in power generation systems will further improve the prospects for using metals as sustainable energy commodities. While such a transition would require a significant and long-term investment, the overall investment in converting our power system from fossil to metal fuels should be lower than that for hydrogen, or other options, due to the ability to repurpose existing infrastructure, including metal smelters, rail cars, cargo ships, and coal or biomass power plants, to produce, transport, and consume metal fuels. Metal fuels should be seriously considered as electrofuel options for a future low-carbon economy since they show significant potential compared to the other proposed low-carbon options.
This is an excerpt of the journal article: Recyclable metal fuels for clean and compact zero-carbon power, by Jeffrey M.Bergthorson. Published: 18 June 2018 in Progress in Energy and Combustion Science, Volume 68, 169-196. DOI: https://doi.org/10.1016/j.pecs.2018.05.001 under a Creative Commons Attribution License (CC BY 4.0).