Dual fluidized bed gasifiers (DFBG) are effective in producing nitrogen-free syngas from biomass. However, to improve the gasifier performance, pressure drops and solid fractions within the DFBG system need to be controlled. In this study, the effects of varying the fluidizing velocity in the fast fluidized bed (FFB) on the pressure drops and the solid fractions in the system were investigated in a 100kW DFBG and in a dual fluidized bed cold model (DFCM). Based on the experimental results, empirical correlations were developed to predict the height-averaged solid fraction in the bottom section of the FFB. Accuracy and advantages of the correlations were analyzed. The correlation is useful for design and modeling of the DFBG systems where the height-averaged solid fraction is required to be determined.