Yield Optimization and Scaling of Fluidized Beds for Tar Production from Biomass

Abstract

A numerical study is performed in order to evaluate the performance and optimal operating conditions of fluidized bed pyrolysis reactors used for condensable tar production from biomass. For this purpose, a previously validated biomass particle pyrolysis model is coupled with a detailed hydrodynamic model for the binary gas particle mixture. The kinetics scheme is based on superimposed cellulose, hemicellulose, and lignin reactions. Any biomass feedstock can be simulated through knowledge of its initial mass composition with respect to these three primary components. The separately validated hydrodynamic model is based on a three-fluid model (gas, sand, and biomass) derived from the kinetic theory of granular flows. Separate transport equations are constructed for each particle class, allowing for the description of such phenomena as particle segregation and for separate temperatures for each particle class. The model is employed to investigate the effect of various operating conditions on the efficiency of tar collection in fluidized bed reactors. Results indicate that, at fixed particle size, the operating temperature is the foremost parameter influencing tar yield. The biomass feed temperature, the feedstock, and fluidization velocity magnitude, all have minor impact on the yield. The particle diameter has a considerable influence on the short-time tar yield, but it is inferred that it may have a more moderate influence on the steady-state tar yield. For the range of fluidizing gas temperatures investigated, optimum steady-state tar collection is obtained for 750 K under the assumption that the pyrolysis rate is faster than the feed rate; the predicted optimum temperature is only slightly higher if this assumption is not satisfied. Finally, scale-up of the reactor is addressed and is found to have a small negative effect on tar collection at the optimal operating temperature. It is also found that slightly better scaling is obtained by using shallow fluidized beds with higher fluidization velocity.

Multi-fluid reactive modeling of fluidized bed pyrolysis process

“…The superficial gas velocity plays an important role in thermal cracking of tar in fluidized bed system. According to Lathouwers and Bellan (2001), the ratio of mean gas residence time to tar conversion time (τ) should be greater than 0.1 for conducting these reactions in a given fluidized bed reactor configuration. When the operational data (u¼ 2.6 u mf , T¼ 500 1C) of Xue et al (2012) are used with the kinetic data of Liden et al (1988), it gives τ to be only 0.099, which does not favor the thermal cracking of tar.…”

Biomass pyrolysis—A review of modelling, process parameters and catalytic studies

“…However, this model had drawbacks in terms of fulfilment of energy requirement of reactor and achievement of high temperatures of around 498 K for biomass feed. The model was used to optimize the tar yield as a function of operating parameters [123]. Papadikis et al [119] studied the pyrolysis of biomass using EulerEuler-Lagrangian approach [124,125] and compared different drag models, namely Gidaspow, Syamlal O'Brien, and Wen-Yu, based on their impact on heat transfer, degradation rate, product yields and char residence time during biomass pyrolysis [126].…”

Modeling the thermochemical degradation of biomass inside a fast pyrolysis fluidized bed reactor

“…Heat transfer at the particle surface is considered to occur by a combination of convective and radiative mechanisms. Particle–particle radiation must be included in fast pyrolysis studies to accurately predict the tar yield 28. The conductive interparticle heat transfer contribution compared to convection and radiation in fluidized beds is only of minor importance10, 11, 28 and has not been considered in this study.…”

“…">Secondary tar reactions in the gas phase are not taken into account. A first simple criterion for the significance of secondary tar reactions in the gas phase of the reactor is given by Lathouwers and Bellan 28. They proposed that the tar‐to‐gas conversion mechanism is of importance as long as where τ R is the mean gas residence time, τ t is the tar conversion time scale, A t = 4.28e +6 1/s and E t = 108.0 kJ/mol30.…”