(592c) CFD Modeling of the Catalytic Pyrolysis of Biomass With Zeolite in Fluidized Bed

Fast pyrolysis of biomass using zeolites as the catalyst material is a direct method to improve the quality of bio-oil in the pyrolysis reactor. In this paper, catalytic pyrolysis of sawdust is simulated in a fluidized bed. Two different cases, one with zeolite catalyst and one without catalyst are modeled and compared.

The flow behavior in the fluidized bed is modeled by means of gas-solid two-fluid model with kinetic theory of granular flow for the granular phase. Two kinds of granular are used in the reactor, i.e. the catalyst and biomass particles. The momentum transfer between the solid phases can be modeled by the drag coefficient simplified for the kinetic theory by Syamlal(1987). Kinetic scheme for catalytic pyrolysis is based on the kinetic research by Atutxa et al.(2005). The kinetic experiment has been carried out to study the catalytic pyrolysis of sawdust with zerlite HZSM-5 in a conical spouted bed reactor at the temperature 400°C. The effect of space time is also incorporated in the kinetic mechanism.   

The predicted product yield distribution from catalytic pyrolysis is in good agreement with the experiment data of Atutxa et al. (2005). The gas yields, due to the cracking of tar, increase with an increasing amount of catalyst. Gas, tar and char products vary along the height of the reactor, which give a good prediction of the residence time of biomass into the bed. Along the height, concentration of tar increases from bottom of the riser result from the pyrolysis reaction of biomass and then reduced by the catalytic reaction to gas. Three regions can be divided to indicate the biomass heating, pyrolyzing and catalyzing process. The simulation also shows the velocity of two solid phases in the riser. The velocity of catalyst is higher than the sawdust because of the larger diameter. This velocity difference between them can enhance the collision occasions and promote the turbulence intensity and solids distribution in the riser, hence benefiting chemical reaction and consequently the final pyrolysis yields.