Parametric Design and Testing of a Fluidized-bed Auto-thermal Biomass Gasifier

Considering its excellent gas-solid mixing, temperature uniformity and flexibility in handling various types of fuels, fluidized-bed gasification technology has been an especially unique mechanism for gasifying biomass for further synthesis of liquid fuels and generation of heat and power. However, lack of sufficient knowledge about the hydrodynamics of biomass particles and the problems associated with tar production have limited the design and operation of a fluidized-bed gasifier. This work focused on the parametric study of the gasification process to improve the performance of a fluidized-bed biomass gasifier. A laboratory fluidized-bed column (6.91 cm I.D) was set up to study the effect of biomass average particle diameter (d p); mass fraction (%); and static bed height/diameter ratio (H/D) on minimum fluidization velocity (umf) and fluidization quality. Inert silica sand (dp = 204 microm) was employed as bed material to facilitate the fluidization of biomass particles. Among the six average particle diameters of the two types of biomass investigated, better fluidizing behavior and good bed bubbling were obtained with 725 microm of mixed wood-dust (MWD) at 4% mass fraction. Poor fluidization behaviors were observed for Miscanthus at particle diameters from 450 to 925microm. Further cold flow fluidization was performed on 2%, 4% and 8% mass fraction of 725 microm of MWD at different H/D ratios. Results revealed that umf increased with increase in mass fraction of biomass. Increase in the quantity of biomass in the bed resulted in increase in the bed voidage and introduction of more irregularly-shaped particles; and this can be responsible for the increase in u mf. It was also observed that increase in H/D ratio has no pronounced effect on the umf. In the second part of this work, the result of the cold flow fluidization study was employed to improve the design and operation of a fluidized-bed gasifier. Design equations were set up and with the results of mass balance calculations, a distributor plate (500 microm grid hole diameter, and grid hole pitch of 0.43cm) was designed and fabricated. Experimental test on the gasifier with the fabricated distributor plate revealed a profound regime of uniform bed bubbling and indicates that the developed procedure for distributor plate design was adequate. A Thermodynamics equilibrium model was set up to predict syngas composition and results obtained showed that increase in equivalent ratio (ER) favors CO2 production at the expense of CO and H2.