Magnesium hydride slurry: A potential net-zero carbon dioxide emitting aviation fuel

Abstract

A potential sustainable aviation fuel that could also absorb airborne carbon dioxide is magnesium hydride (MgH2), which combusts to release magnesium oxide (MgO) and water. The MgO can react with CO2 and water in the engine plume or atmosphere to form magnesium carbonate (MgCO3), or magnesium bicarbonate (Mg(HCO3)2). This work describes initial results of a study to determine the potential of a slurry consisting of MgH2 and an appropriate surrogate hydrocarbon jet fuel. Thermodynamic calculations were performed to compare the thermal combustion performance of MgH2 slurries with n-dodecane (n-C12H26) at varying ratios using conditions at aircraft cruising altitude. The heat of combustion as well as the mass and volume of each fuel required to reach target equilibrium temperatures at a given fuel equivalence ratio and oxidizer (air) mass are compared. MgH2 combustion's lower heating value (LHV) is 33.8 % lower than n-C12H26 per unit mass, but 22 % higher per unit volume. Thermodynamic models show that 12–17 % less volume of the MgH2/n-C12H26 slurry is required to reach engine-relevant temperatures relative to n-C12H26 under typical engine operating conditions. Based in part on this result, the Breguet range equation shows that a hydrocarbon-MgH2 slurry fuel should achieve up to 8 % longer aircraft range than the same volume of the hydrocarbon fuel, though other aircraft changes required to accommodate the denser fuel could offset that range extension. This is also over 2.5 times the range of liquid ammonia, and 3.5 times the range of liquid hydrogen, if those fuels could be used in the same tanks. Feasibility of the CO2 absorption process and sustainable MgH2 production are also discussed.