Simulation of biomass pyrolysis in a fluidized bed reactor: Independent sand, biomass and char granular phases

The Eulerian multiphase approach coupling reaction kinetic models is used to investigate a continuous rice husk fluidized bed pyrolyzer with the minimum fluidization velocity of 0.1 m/s. Considering the fact that the freshly fed biomass (terminal velocity = 0.613 m/s) and the pyrolyzed product char (terminal velocity = 0.33 m/s) show different hydrodynamic behavior, we initially consider four-phase modeling: mixgas, sand, biomass and char. The pyrolyzed products are initially detected about 0.0004 s–0.0005 s after the biomass feeding. The results confirm that char phase motion is independent to motion of other phases. The biomass entrance position determines the lowest position where the light char can possibly reach. At the fluidizing velocity of 0.3 m/s, the computational economic global kinetic model gives good char yield prediction, which deviates from the experimental result by 0.27%. The poorer bio-oil and syngas yield predictions are due to the underestimated secondary decomposition rate. At the highest fluidizing velocity of 0.4 m/s, the expanded bed pyrolyzes biomass effectively and gives the lowest unreacted biomass fraction; and the bio-oil's short residence time avoids the secondary decomposition by showing its highest fraction in the product. The highest bio-oil mass fraction of 45.5% is predicted at the reaction temperature at 500 °C.