(43e) Computational Fluid Dynamics Simulation of Compressible Non-Newtonian Biomass in a Compression-Screw Feeder

Compression-screw feeders play a critical role in biorefineries to transport lignocellulosic-biomass feedstocks into pressurized thermochemical biomass-conversion reactors. The main challenge in the operation of compression-screw feeders is consistent flow of material, and in extreme cases jamming or blowback can shut down the feeding operation entirely. The focus of this paper is to numerically investigate the screw feeder at these challenging operating conditions and help with the optimization of the screw feeder design to avoid operation failure.

In this work a customized constitutive model was implemented in the open-source CFD package OpenFOAM [1] in order to simulate concentrated biomass as a highly viscous non-Newtonian fluid in the screw feeder. The biomass is modeled as a single-phase compressible Bingham fluid with a plastic viscosity as well as a density-dependent yield stress. The compressibility formulation (pressure-dependent density) and the density-dependent yield stress formulation in the governing equations follow from a recent study by Duncan et al. [2].

A pilot-scale hopper/screw feeding system at NREL [3] was used to compare the experimental observations with our simulation results. The auger is 280 mm long and tapered with outer diameter changing from 80 mm to 35 mm. The auger rotates from 10 to 60 rpm in a conical throat that contains anti-rotational grooves. The predicted torque and pressure increase at the exit agree with experimental data for biomass feedstocks with varying rheological properties and auger rotating speeds. The analysis of the stress forces helped to identify the critical conditions were excessive wear, jamming, and blowback could occur.

References:

[1] Weller, Henry G., et al. "A tensorial approach to computational continuum mechanics using object-oriented techniques." Computers in physics 12.6 (1998): 620-631.

[2] Duncan, Joshua C., et al. "Pressure-driven flow of lignocellulosic biomass: A compressible Bingham fluid." Journal of Rheology 62.3 (2018): 801-815.

[3] Sievers, David A., et al. "Online residence time distribution measurement of thermochemical biomass pretreatment reactors." Chemical Engineering Science 140 (2016): 330-336.