This work presents a unique thermochemical process for synthesizing magnesium manganese oxide-based zero carbon solid-state fuel, which can be used for long duration energy storage. High temperature (1450°C) heating of the processing furnace can be driven by either renewable electricity or concentrated solar power. The very simple fuel synthesis concept is based on a tubular falling bed reactor with countercurrent oxygen-depleted gas flow for complete heat recuperation. A sophisticated control strategy using pulsed width modulated gas flow and an L-valve maintains consistent magnesium manganese oxide (Mg-Mn-O) particle flow up to 1450°C. The measured extent of the thermal reduction reaction after cooling the Mg-Mn-O particles is more than 90% of the fully reduced state at equilibrium. The thermal to chemical efficiency (ratio of chemical energy stored to thermal energy input) and overall system efficiency are above 96% and 28%, respectively. Both, the solid particle reactor flow at the extreme temperature of 1450°C and the system efficiency of 28% are the highest reported for thermochemical fuels in the literature to date.