Analysis of Thermal and Reaction Times for Hydrogen Reduction of Lunar Regolith

System analysis of oxygen production by hydrogen reduction of lunar regolith has shown the importance of the relative time scales for regolith heating and chemical reaction to overall performance. These values determine the sizing and power requirements of the system and also impact the number and operational phasing of reaction chambers. In this paper, a Nusselt number correlation analysis is performed to determine the heat transfer rates and regolith heat up times in a fluidized bed reactor heated by a central heating element (e.g., a resistively heated rod, or a solar concentrator heat pipe). A coupled chemical and transport model has also been developed for the chemical reduction of regolith by a continuous flow of hydrogen. The regolith conversion occurs on the surfaces of and within the regolith particles. Several important quantities are identified as a result of the above analyses. Reactor scale parameters include the void fraction (i.e., the fraction of the reactor volume not occupied by the regolith particles) and the residence time of hydrogen in the reactor. Particle scale quantities include the particle Reynolds number, the Archimedes number, and the time needed for hydrogen to diffuse into the pores of the regolith particles. The analysis is used to determine the heat up and reaction times and its application to NASA’s oxygen production system modeling tool is noted. Nomenclature A reactor cross section area (m) Aeff effective surface area of heater (m) Ar Archimedes number c molar concentration of hydrogen in particle (moles/m) cs molar concentration of hydrogen at reaction surface (moles/m) cx molar concentration of hydrogen at location x in reactor (moles/m) c0 molar concentration of hydrogen at upstream end of regolith bed (moles/m) Ceff effective specific heat (J/kg/K) Cpg specific heat of gas (J/kg/K) Cps specific heat of regolith (J/kg/K) D effective gas diffusion coefficient (m/s) E energy required to heat bed h heat transfer coefficient g gravitational acceleration (m/s) k equilibrium constant L length of regolith bed (m) Meff effective mass (kg)