Fluidized beds can be used to gasify biomass in the production of producer gas, a flammable gas that can replace natural gas in process heating. Knowing how the fluidized bed hydrodynamics vary as reactor dimensions are scaled up is vital for improving reactor efficiency. This study utilizes 10.2 cm and 15.2 cm diameter fluidized beds with added side port air injection to investigate column diameter effects on fluidized bed hydrodynamics. Both inert (glass beads) and biomass (ground walnut shell and ground corncob) bed materials are used and the hydrodynamic differences with side port air injection are recorded. Minimum fluidization velocity is determined through pressure drop measurements. Time-averaged local and global gas holdup are recorded using X-ray computed tomography imaging. Results show that by varying the side port air flow rate as a percentage of the minimum fluidization flow rate, partial and complete fluidization is observed in both fluidized beds. Local gas holdup trends are also similar in both fluidized beds. These results will be used in future studies to validate computational fluid dynamics models of fluidized beds.