(423a) Multiscale Modeling of Biomass Thermochemical Conversion in Fluidized Bed Reactors

Thermochemical conversion of biomass has recently gained a lot of interest as a potential renewable technology to provide both liquid and gaseous fuels from a variety of feedstock. However, design and scale-up of biomass thermochemical conversion reactors are mostly empirical processes, and require many pilot-scale reactor studies, which are both expensive and time consuming, and fail to provide the local and detailed quantitative information necessary to optimize the conversion process. Modeling and simulation tools are playing an increasingly important role for the design and optimization of the reactors, as they have the potential to provide detailed insight into the prevailing complex phenomena occurring during conversion. While significant progress has been achieved recently, predictive and reliable computational fluid dynamics (CFD) tools able to simulate the complex, reactive, multiphase flows found in fluidized-bed reactors are still lacking.

A major bottleneck in building predictive simulation capabilities for biomass thermochemical conversion is the adequate representation of the chemical processes, occurring both inside the biomass particle and in the gas phase. Particle-level chemical processes are coupled with intra-particle heat and mass transfer, occurring in a highly anisotropic porous structure. While intra-particle phenomena can affect the overall dynamics of the system, most CFD studies done at the reactor level neglect those and use a homogeneous sphere model to describe the biomass devolatilization. Secondary gas phase reactions are also most often neglected in reactor simulations, especially at the relatively low temperatures relevant for biomass pyrolysis processes.

This work aims at investigating the validity of those strong modeling assumptions, and characterizing quantitatively their impact on overall reactor performance. This is achieved by developing a one-dimensional intra-particle model for resolved simulation of single biomass particle devolatilization and a reduced model for the secondary gas phase reactions. The resulting models are integrated into a CFD solver specifically designed for the Lagrange-Euler simulations of multiphase flow. Simulations of pseudo-two-dimensional fluidized bed reactors are conducted with both the intra-particle and homogeneous particle models, and with and without secondary gas phase chemistry. A sensitivity analysis study of the results is performed to identify the controlling aspects.