Switchgrass ultimate stresses at typical biomass conditions available for processing

Biomass tensile and shear ultimate failure stresses were measured with the aim of identifying biomass “weakest mode of failure” or “natural fracture point” as a basis for future grinder designs. Switchgrass (Panicum virgatum L.) ultimate stresses were determined for Alamo and Kanlow varieties over ranges in maturity and moisture content. Alamo had greater ultimate tensile stress than Kanlow (P=0.0091), with mean values of 97.8 and 89.7 MPa, respectively. Alamo had greater ultimate shear stress than Kanlow (P=0.0091), with mean values of 20.5 and 17.9 MPa, respectively. Shear was the “weakest mode of failure”. Grinders that use knives, shear bars, and mechanical pinch points that apply opposed-sliding actions are expected to be more energy efficient. Mean ultimate tensile stress and shear stress were significantly different between switchgrass varieties. A survey of failure stresses for a range of biomass feedstocks is recommended for future study. Ultimate tensile stress increased two-fold as elapsed time after harvest increased from 2 to 386 h, with a corresponding (confounded) decrease in moisture content of 60–10% (wet basis (w.b.)). Future study should isolate whether the effect was due primarily to moisture or aging. Tensile-dominant size reduction should be conducted early in the harvest process and at a high moisture content to minimize energy consumption for grinding. Ultimate shear stress was relatively insensitive to switchgrass maturity, elapsed time after harvest, and moisture content.