The main purpose of a fast internal circulating fluidized bed gasifier is the steam reforming of solid organic matter, like biomass, to a nearly nitrogen-free syngas. The calorific value of this syngas is approximately three times higher than the gas from common air-driven gasifiers. This article deals with a study of the particle dynamics in a 1 MWt fast internal circulating fluidized bed plant and focuses on the design of the gasification reactor’s geometry. Superheated steam is used for the fluidization and gasification in the reactor. The gasification of solid fuels causes an increase in the volume flow of the fluidizing gas and at the same time also a change in the fluidization regime. Approaching a turbulent fluidization regime or even fast fluidization is not desirable. However, with the proper design of reactor, i. e., an appropriately conical bed angle, suitable gasification conditions in the form of a fluidizing regime can be achieved across the entire height of the bed. For the purposes of the experimental research, a semi-industrial unit was set-up. The process was designed and experimentally tested on a lab-scale, cold-flow model and scaled-up to a semi-industrial process. The guidelines for designing the geometry of the gasification reactor were set.