Techno-Economic Analysis of Carbothermal Reduction of Magnesia in a Reduced Pressure Hybrid Solar-Electric Reactor

A techno-economic analysis was performed on carbothermal reduction (CTR) of magnesia in a moving bed, reduced pressure, hybrid solar-electric reactor. Pretreatment unit operations were designed for a pelletized carbon-MgO feed into the primary CTR reactor. Post-reaction steps included high temperature condensation, vacuum distillation, and ingot casting. CTR reaction kinetics derived from laboratory-scale experiments were used to optimize temperature and pressure in the primary reduction reactor. An n^th-plant with 20,00 tonne-Mg/year capacity was designed under two base cases where the primary reduction reactor was heated externally using 1) Electrical resistance heating, 2) Hybrid solar-electric heating. The solar tower height and heliostat field were optimized according to actual daily irradiance and supplemented with electrical resistance heating when the total net solar flux was insufficient for heating and reaction after accounting for heat losses. Economic profitability was driven primarily by the cost of magnesia ($0.30-0.80/kg) and the total fixed capital investment (-20%, +100%,). Operational energy requirements and emissions per kilogram of magnesium product were 10 kWh and 4.0 kgCO_2-eq, respectively, for Case 1. Case 2 showed operational energy requirements and emissions of 8.8 kWh and 2.9 kgCO_2-eq. These results are below DOE targets of 27 kWh and 10 kgCO_2-eq and far below state-of-the-art Pidgeon specifications of 102 kWh and 37 kgCO_2-eq. The hybrid solar-electric case revealed a production cost of $1.96/kg-Mg using $150/m² for the heliostat field, or 20% cheaper than Pidgeon magnesium.