(137g) Discrete Element Modeling of Wedge Hopper Discharge of Loblolly Pine Chips

Forestry residues such as loblolly pine chips are a low-cost source of lignocellulosic biomass feedstocks for conversion into biofuels. Achieving efficient material handling operations (transport, feeding, etc.) of biomass feedstocks has been a challenge in the design and operation of a biorefinery. The efficiency of material handling is strongly influenced by the flowability of feedstocks: the propensity of a material to flow in proportion to applied stress. For Newtonian fluids, the flowability is in proportion to the inverse of the fluid viscosity. For biomass feedstocks, however, the flowability cannot be represented by a single material property like viscosity, but rather, by a collection of particle-scale attributes such as particle size distributions, shapes, density, moisture content, friction coefficient, and particle stiffness. Those attributes give rise to the bulk properties such as bulk stiffness and compressibility, internal angle of friction, cohesion, and unconfined yield stress, which collectively determine the flowability of biomass feedstocks. Feedstocks with poor flowability are more likely to cause process upsets such as clogging and jamming in material handling operations, resulting in increased downtime and higher operational cost, and consequently, lower economic value of biofuels.

In this work, we adopt a set of experiment-validated hysteretic nonlinear contact models for the discrete element method (DEM), which can capture the bulk behavior of flexible biomass particles. DEM-based particulate flow simulations of wedge hopper discharge of loblolly pine chips are performed. The DEM simulation-based study enables detailed and quantitative analyses of the hopper flow and clogging behavior based on fundamental particle physics. Wide ranges of critical processing parameters (CPPs) (of the hopper) and the critical material attributes (CMAs) (of milled loblolly pine) are considered in the analysis. Among various CPPs, hopper opening width and semi-inclination angle are the two key parameters that significantly influence the critical quality attributes (CQAs) of the hopper (i.e., discharge rate and continuity). The critical arching distance as a function of semi-inclination is predicated by DEM and compares well with experimental data and finite element method-based predictions. DEM results also show the interparticle motion resistance (a CMA) and the packing height (a CPP) have a strong influence on the critical arching distance. Findings from this work have practical implications for engineers to design and operate equipment involving biomass particle flow in biorefineries.