Modeling hydrodynamic and biomass pyrolysis effects of recycled product gases in a bubbling fluidized bed reactor

Fast pyrolysis of biomass in a fluidized bed reactor is typically conducted in a nitrogen gas environment. Recycling product gas can improve the economics of operating such a system by reducing reliance on pure process streams, but much less is known about how recycling pyrolysis product gas may affect fluidization behavior and pyrolysis kinetics. Therefore, gas effects in a fluidized bed biomass pyrolysis reactor were investigated using engineering correlations, low-order models, and CFD simulations for N₂, H₂, CO, CO₂, and CH₄ carrier gas mixtures. Our findings reveal viscosity of a gas mixture can be significantly underestimated depending on the model and correlation. Furthermore, fluidization characteristics such as U_mf and gas-solid convective heat transfer can be greatly affected by the gas properties. By utilizing H₂ as the fluidizing gas (instead of N₂), while maintaining a constant fluidization ratio (U_s/U_mf), the bio-oil yields can be increased ∼5%. This is due to the lower density H₂ producing similar hydrodynamics as N₂ at higher gas flow rates. These higher flow rates result in shorter gas residence times, and as a result, less secondary reactions that convert bio-oil to light gases and char. Model results also suggest that bio-oil yield is not significantly affected by the type of carrier gas used, with bio-oil yield varying by ∼2% across different gas mixtures while maintaining constant flow rate. This indicates that recycled pyrolytic gas can be used as the carrier gas for biomass pyrolysis.