(237k) Computational Fluid Dynamics Simulation of Lignocellulosic Biomass Transport in a Compression-Screw Feeder

There is continued interest in lignocellulosic biomass feedstocks for producing power, heat, and transportation liquid fuels in order to increase energy diversification and security. However, there are a few problems associated with the biomass production that must be resolved before biorefineries are considered economical. One of these problematic operations is the process of feeding biomass to reactor systems. Compression-screw feeders have been widely used as a way to fulfill this purpose. However, due to variability of feedstock properties and its complex challenges, production plants operate at less than 50% of their design capacity due to process upsets along with mechanical wear and failure. The objective of this study is to numerically investigate the screw-feeder operation and performance for various biomass physical properties and under different operating conditions. Therefore, the findings from this study will reduce the number of physical experiments needed to improve the plant design.

Continuum simulations of the rotating screw feeders present a unique challenge regarding the resolution of rotating interfaces along with baffle like features on the stator surface. We utilize the dynamic-mesh capabilities in the open-source CFD toolbox, OpenFOAM, where the screw is embedded in a cylindrical mesh that slides with respect to a stationary background grid for the stator surfaces. The biomass feedstock is modeled as a compressible Bingham fluid [1]. The non-Newtonian transport model includes a plastic viscosity as well as a density dependent yield stress component. We also modify the low Mach solvers in OpenFOAM to incorporate an equation of state compatible with pressure-density variations for different biomass feedstocks. A pilot-scale hopper/screw feeding system at NREL [2] is 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 at 2-10 rad/s in a conical throat which contains anti-rotational bars. The simulations will be used to determine important operational parameters such as the dependence of torque with respect to varying solids fractions and different biomass feedstocks.

[1] Stickel, Jonathan J., et al. "Rheology measurements of a biomass slurry: an inter-laboratory study." Rheologica acta 48.9 (2009): 1005-1015.

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