(131b) CFD Study of Lignocellulosic Biomass Gasification in a Fluidized Bed Gasifier: A Study of the Impact of Particle Radius on Devolatilization and Mixing

Authors: 
Stark, A. K., Massachusetts Institute of Technology
Altantzis, C., National Energy Technology Laboratory
Ghoniem, A. F., Massachusetts Institute of Technology

The Thermochemical conversion of biomass to drop-in ready fuels and chemicals offers an attractive option to displace petroleum and utilize waste lignocellulosic materials as well as dedicated energy crops. Further, thermochemical conversion technology has the benefit of being governed by a shared set of chemical pathways. Depending on the desired products, reactor conditions (temperature, pressure and availability of oxidant) and design parameters (geometry, freeboard height, etc) can be optimized for many different products from the production of hydrogen-rich syn-gas (gasification) to oxygenated bio-oil (fast pyrolysis).

The accuracy of the model of devolatilization of the virgin feedstock is tantamount in gasification modeling, since volatile products account for >75% of the mass of the devolatilization products on a dry biomass basis. Furthermore, it has been demonstrated that the pyrolysis process of biomass is controlled by both chemical and particle-scale transport processes making it dependent not only on temperature, but on the particle length scales as well.

In this work a fully reactive Computational Fluid Dynamic (CFD) investigation of biomass gasification is undertaken utilizing the open-source code, MFiX. A Multi-Fluid Model (MFM) is employed to represent the gas, sand and biomass fuels as interpenetrating Eulerian fluids. Unfortunately, when the reacting biomass particles are represented as an Eulerian continuum, individual particle-scale transport phenomena must be modelled in a simplified way, since individual particle histories are not available.

In order to implement this internal heat transfer limitation on conversion into account a shrinking core model of biomass devolatilization has been developed for both cylindrical and spherical wood particles with radiuses ranging from 500µm to 1.75cm. This model has been validated against a one-dimensional high-fidelity particle model coupling heat transfer with devolatilization kinetics, giving excellent agreement in the prediction of conversion times with respect to varying radii. Here, this model is employed in a CFD study of a 3-D lab-scale gasifier (diameter 7.4 cm). The influence of fuel particle radius on solids segregation is assessed and compared to non-reactive conditions. The mixing dynamics are considered in the context of reactor design – ideal locations for the fuel and oxidant injection ports are identified that yield optimal mixing of reactants through the bed.