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

• Published in: Biomass & bioenergy 2024-05, Vol.184 (C), p.107172, Article 107172
• Main Authors: Wiggins, Gavin M., Oyedeji, Oluwafemi A., Mills, Zachary G.
• Format: Article
• Language: English
• Subjects: Bio-oil; Biomass; Fluidized-bed; Pyrolysis; Thermochemical
• ISSN: 0961-9534; 1873-2909
• DOI: 10.1016/j.biombioe.2024.107172

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 Umf 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 (Us/Umf), 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. •Correlations for gas mixture properties do not provide consistent results.•Secondary reaction kinetics for pyrolysis do not account for gas phase properties.•Bio-oil yields can increase 5% with low molecular weight fluidization gases.