Optimization of Heat Transfer through Heat Carriers in Bio-H₂ Production Using CFD Simulation

Abstract

Hydrogen (H₂) is one of the most promising secondary energy resources expected to contribute to the prevention of global warming. Bio-H₂, which is derived from biomass feedstock, is more environmentally friendly than hydrogen synthesized from fossil fuels. In our indirect thermochemical processes with solid-gas reactions, the effective heat transfer, which is accomplished by circulation of alumina balls acting as heat carriers (HCs), is a critical issue, and it is necessary to achieve circulation of the heat transfer medium that maximizes production efficiency. In this study, the heat transfer performance of a small-scale indirect biomass gasification process to be promoted in the near future was investigated, focusing on heating of HCs using high-temperature gas to achieve optimum heat utilization in the preheating reactor. To continuously pyrolyze cedar feedstock in the subsequent pyrolysis reactor, HCs must be heated to the desired temperature in the preheater within a residence time. Even if high-temperature gas with sufficient calorific value flows into the preheater, if the heat transfer is not completed within the residence time, the HCs will not be heated to the target temperature and the sensible heat is discharged as tail gas. Therefore, we evaluated the operating conditions that promote the heat transfer during the residence time, focusing on the hot flue gas conditions.