Ultrafine carbon particles (d(sv) = 165 nm) are formed from the pyrolysis of shredded tyres in a fluidized bed reactor. The carbon particles affect the hydrodynamic properties of the fluidized bed itself, such as the minimum fluidization velocity U-mf, the elutriation rate, etc. This work investigates how humidity and temperature influence the elutriation rate constant K*(i) of the carbon particles, and how the presence of ultrafine carbon particles in a sand bed influence the minimum fluidization velocity.