Revealing the role of internodal–nodal heterogeneity in wheat straw compression densification and pellet formation using an anisotropic DEM model

An anisotropic discrete element method (DEM) model of wheat straw was developed by incorporating internodal–nodal structural heterogeneity to improve the representation of its mechanical behavior and compression densification. Wheat straw was divided into two structural units, namely, internodal wheat straw (IWS) and node-containing wheat straw (NWS), for which corresponding DEM submodels were developed. The contact parameters were calibrated using the angle-of-repose test, whereas the bond parameters were independently calibrated using uniaxial compression and shearing tests in combination with central composite design–response surface methodology. The model was further validated using uniaxial compression, shearing, three-point bending, and open-die compression densification tests. The calibration error of the contact parameters was 4.14%. The relative errors of the four groups of bond parameters were 6.8%, 7.6%, 5.7%, and 6.3%, respectively. In the mechanical validation tests, the errors for uniaxial compression, three-point bending, and shearing of IWS and NWS were 2.2%/4.3%, 5.9%/3.1%, and 13.3%/1.7%, respectively. Results of open-die compression densification further indicated that the compressive load-bearing capacity followed the order NWS > IWS + NWS > IWS and that the three material combinations showed distinct differences in collapse pathways and final pellet morphology. The proposed heterogeneous DEM model and open compression validation framework provide an effective approach for elucidating the structure-dependent densification behavior and pellet formation characteristics of wheat straw and may support the efficient utilization of heterogeneous lignocellulosic biomass.