Metal fuels production for future long-distance transportation through the carbothermal reduction of MgO and Al2O3: A review of the solar processes

The need for sustainable zero-carbon fuels to substitute the conventional fossil fuels in the transport field becomes unavoidable to overcome the depletion of petroleum reserves and global warming. Metal fuels are promising energy-carrying materials that can be regenerated through the carbothermal reduction of their oxide combustion products. Their use is economically beneficial and environmental-friendly when oxides are recycled using sustainable energy resources. Among various choices, aluminum/alumina and magnesium/magnesia couples are the best candidates for the combustion/reduction cycles. Thus, the following study reviews the advanced technologies established for the carbothermal reduction of alumina and magnesia using concentrated solar energy to achieve an efficient regeneration of the metal fuels. In conclusion, 68% Al and 96% Mg yields, of highly-pure stable micro-sized powders, were achieved by optimizing the solar reactor design and the reaction operating parameters (pressure, temperature, heating rate, carbon reducing agent, C/oxide content, C/oxide properties, catalysts, bentonite binder). Further investigations are needed to improve the aluminum regeneration by reducing the formation of unwanted Al-oxycarbides by-products.