Effect of Bed Hydrodynamics on Microwave-Assisted Steam–Carbon Gasification for Hydrogen Production: A Comparison of Packed- and Fluidized-Bed Reactors

Microwave-assisted steam gasification offers a potential electrified pathway for hydrogen production, but the influence of reactor hydrodynamics on microwave-assisted gasification performance remains insufficiently understood. This study experimentally compares hydrogen production by steam gasification of activated carbon in microwave-assisted packed- and fluidized-bed reactors, with particular emphasis on the effects of bed configuration, reaction temperature, and steam flow rate on hydrogen production and thermodynamic performance. Activated carbon serves as the carbonaceous reactant and microwave susceptor. The two reactor configurations exhibited distinctly different behavior. In the packed-bed reactor, the maximum hydrogen yield of approximately 50% was obtained at 700 °C and a water flow rate of 0.0082 mL/min, with corresponding maximum energy and exergy efficiencies of 13.37% and 6.2%, respectively. The maximum hydrogen production rate of 0.00733 mg/s occurred at a water flow rate of 0.0164 mL/min and the highest carbon conversion was found to be 29.2% in the packed-bed reactor. In comparison, the fluidized-bed reactor achieved a lower maximum hydrogen yield of 31.6% at 700 °C and 0.0082 mL/min, while its maximum hydrogen production rate reached 0.0059 mg/s under the same conditions. Maximum energy and exergy efficiencies of 5.3% and 2.60%, respectively, were obtained in the fluidized bed at 750 °C and 0.0164 mL/min and the highest carbon conversion was found to be 33.8% for fluidized bed. Despite the enhanced gas–solid contacting and thermal uniformity generally associated with fluidization, the packed bed exhibited substantially higher hydrogen yield and thermodynamic efficiencies. The observed differences suggest that the conventional transport advantages of fluidization may be offset in microwave-assisted reacting systems by changes in microwave–particle interaction, steam residence time, and heat losses. The results demonstrate that reactor hydrodynamics and microwave energy deposition should be considered together when designing microwave-assisted gasification systems for hydrogen production.